WO2007072247A2 - An improved lift-off technique suitable for nanometer-scale patterning of metal layers - Google Patents

An improved lift-off technique suitable for nanometer-scale patterning of metal layers Download PDF

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Publication number
WO2007072247A2
WO2007072247A2 PCT/IB2006/054468 IB2006054468W WO2007072247A2 WO 2007072247 A2 WO2007072247 A2 WO 2007072247A2 IB 2006054468 W IB2006054468 W IB 2006054468W WO 2007072247 A2 WO2007072247 A2 WO 2007072247A2
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Prior art keywords
layer
lift
forming
mask layer
mask
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PCT/IB2006/054468
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French (fr)
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WO2007072247A3 (en
Inventor
Ronald Dekker
Francois Neuilly
Vijayaraghavan Madakasira
Stacey N. Serafin
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Koninklijke Philips Electronics N.V.
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Publication of WO2007072247A2 publication Critical patent/WO2007072247A2/en
Publication of WO2007072247A3 publication Critical patent/WO2007072247A3/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/027—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34
    • H01L21/033—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34 comprising inorganic layers
    • H01L21/0331—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34 comprising inorganic layers for lift-off processes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y10/00—Nanotechnology for information processing, storage or transmission, e.g. quantum computing or single electron logic
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/027—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34
    • H01L21/0271—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34 comprising organic layers
    • H01L21/0272—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34 comprising organic layers for lift-off processes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/027—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34
    • H01L21/033—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34 comprising inorganic layers
    • H01L21/0332—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34 comprising inorganic layers characterised by their composition, e.g. multilayer masks, materials
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/027—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34
    • H01L21/033—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34 comprising inorganic layers
    • H01L21/0334—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34 comprising inorganic layers characterised by their size, orientation, disposition, behaviour, shape, in horizontal or vertical plane
    • H01L21/0337—Making masks on semiconductor bodies for further photolithographic processing not provided for in group H01L21/18 or H01L21/34 comprising inorganic layers characterised by their size, orientation, disposition, behaviour, shape, in horizontal or vertical plane characterised by the process involved to create the mask, e.g. lift-off masks, sidewalls, or to modify the mask, e.g. pre-treatment, post-treatment
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer
    • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic System or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
    • H01L21/302—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to change their surface-physical characteristics or shape, e.g. etching, polishing, cutting
    • H01L21/306—Chemical or electrical treatment, e.g. electrolytic etching
    • H01L21/308—Chemical or electrical treatment, e.g. electrolytic etching using masks
    • H01L21/3083—Chemical or electrical treatment, e.g. electrolytic etching using masks characterised by their size, orientation, disposition, behaviour, shape, in horizontal or vertical plane
    • H01L21/3086—Chemical or electrical treatment, e.g. electrolytic etching using masks characterised by their size, orientation, disposition, behaviour, shape, in horizontal or vertical plane characterised by the process involved to create the mask, e.g. lift-off masks, sidewalls, or to modify the mask, e.g. pre-treatment, post-treatment

Definitions

  • the present invention relates to a method form forming a metal layer with a predefined lateral shape at a predefined position on a semiconductor substrate.
  • the invention also relates to a method for fabricating a device comprising a such a metal layer.
  • Patterning metal layers can be performed by etching.
  • An alternative technique that can be applied if etching is not possible is to perform a metal lift-off.
  • Figs. IA) to D) show a process flow of a known lift-off procedure for the formation of a metal layer with a predefined lateral shape.
  • the illustrated processing starts from a semiconductor substrate 100 that is covered with a resist bi-layer 102.
  • the resist bi- layer 102 consists of a first resist layer 104 and a second resist layer 106.
  • the first and second resist layers 104 and 106, respectively, are made from different resist materials.
  • the material of the first resist layer 104 has a higher development speed than that of the second resist layer 106.
  • an opening 108 is formed in the first and second resist layers 104 and 106 by light exposure through a mask and subsequent developing. Due to the different developing speeds of the two resist layers 104 and 106, a larger recess 110 is formed in the first resist layer 104. The recess reaches underneath the second resist layer 106.
  • the intermediate result of this processing is shown in Fig. IB).
  • metal layer 112 is deposited on the resist bi-layer 102. After the metal deposition, metal layer 112 covers layer sections 114 and 116 of the resist bi-layer 102. A section 118 of metal layer 112 is deposited directly on the substrate in recess 110 and with a lateral shape that is defined by opening 108 in the second resist layer 106. This intermediate state of the known metal deposition technique is shown in Fig. 1C).
  • a lift-off is performed by dissolving the resist bi-layer 102, thus lifting-off all metal layer sections other than metal layer 118 in the desired lateral shape.
  • a resist bi-layer 102 instead of using a resist bi-layer 102, it is also known to apply and pattern a lift-off resist that, after developing, exhibits a cross sectional recess profile, that continuously narrows from the semiconductor substrate surface to the top of the resist layer.
  • the purpose of the recess is in both cases to provide an initial working surface for exposure to a dissolving agent.
  • a method for forming a metal layer with a predefined lateral shape at a predefined position on a semiconductor substrate comprises the steps of depositing a layer stack comprising a first lift-off layer and a mask layer on the first lift-off layer, wherein the material of the first lift-off layer is selectively removable from the layer stack and allows a formation of an opening, and the material of the first mask layer allows a formation of an opening with a sub-micrometer extension;; forming an opening, which
  • - has the predefined lateral shape in the first mask layer, - forms a first recess in the first lift-off layer and laterally reaches underneath the first mask layer, and which
  • the method of the invention provides a novel lift-off process that is suitable also for patterning metal layers with lateral extensions in the deep sub-micrometer range.
  • the method uses a layer stack that comprises a first lift-off layer, and a first mask layer on, that is, abutting the first lift-off layer for defining the desired lateral shape of the metal layer prior to its deposition.
  • the method of the invention is based on the recognition that directly depositing a metal layer and subsequently patterning the metal layer by lithography is not feasible in the deep sub-micron range for the following reasons: first, the metal that is present on the semiconductor surface makes it very difficult to use the required very advanced lithography tools for deep sub-micrometer dimensions. Secondly, experiments have shown that during sputter-etching of the metal layer, such as a gold layer, metal atoms are implanted in the semiconductor surface below, resulting in an uncontrolled growth of nanowires. Use of metals like gold is therefore avoided where possible, especially in high-end cleanroom environments in order to avoid incorporation into the semiconductor, which can cause leakage currents.
  • the metal layer such as a gold layer
  • the metal layer is deposited only after the desired lateral shape has been defined at the desired lateral position on the semiconductor surface.
  • Deep sub-micron patterns can be defined in the layer stack of the lift-off layer and the first mask by advanced lithography techniques known in the art.
  • the novel lift-off technique further avoids undesired implantation of metal atoms in the semiconductor surface region.
  • the method of the invention uses a novel lift-off technique.
  • the step of forming an opening for the definition of the lateral shape of the metal layer comprises forming a recess that laterally reaches underneath the first mask layer, before the metal is deposited on the semiconductor surface.
  • the recess in the first lift-off layer has a larger lateral extension than the opening in the first mask layer on top of it.
  • lateral directions shall be understood as being parallel to the main semiconductor surface and perpendicular to the stacking direction of the deposited layer stack.
  • layer stack is used herein to summarize the layer sequence comprising the first lift-off layer and the first mask layer, and further layers, as will be explained in further detail in the context of preferred embodiments below. In the following, preferred embodiments of the method of the invention will be described.
  • the preferred material for the first lift-off layer is silicon dioxide
  • the preferred material for the first mask layer is silicon nitride.
  • these materials could also be used in combination with a semiconductor surface made from a different semiconductor material, such as GaAs, GaN, InP, InGaAs, SiGe, to mention a few examples.
  • first lift-off and the first mask layer Other materials can be selected for the first lift-off and the first mask layer, as long as the material of the first lift-off layer is selectively removable from the layer stack and allows a formation of an opening. Note that selective removability of the first lift-off layer from the layer stack implies that the and the material of the first mask layer is stable during removal of the first lift-off layer. The material of the first mask layer must, in addition, allow a formation of an opening with a sub-micrometer extension.
  • first lift-off layer aluminum can be used for the first lift-off layer, and TiW for the first mask layer. Suitable selective wet- and dry-etching processes for these materials are well known in the art. The choice of materials for the first lift-off layer and the first mask layer also depends on the availability of suitable etchants. Therefore, other materials for the first lift-off layer and the first mask layer can be found by the development of new suitable etchants. Polymers, such as PMMA (PolyMethyl MethAcrylate) could also be used for the first lift-off layer. PMMA can be patterned by etching and removed by heating.
  • PMMA PolyMethyl MethAcrylate
  • the opening in the first mask layer is formed by dry etching.
  • a dry- etching processes can be precisely controlled to achieve lateral extensions in the deep submicron range.
  • techniques such as laser ablation, light-assisted wet etching, reactive ion etching and plasma etching could be applicable.
  • Another preferred embodiment of the method of the first aspect of the invention employs wet-etching for removing the first lift-off layer.
  • the step of forming the opening comprises covering the first mask layer with a first resist layer, forming a first resist opening of the predefined lateral shape at the predefined position in the first resist layer, dry-etching the mask layer in the first resist opening, and removing the first resist layer.
  • the recess is then preferably formed by a step of wet-etching the first lift-off layer through the opening in the first mask layer.
  • the semiconductor surface is made of silicon or an alloy containing silicon
  • use of hydrofluoric (HF) acid is preferred for wet etching.
  • HF is for instance suitable for wet-etching a silicon dioxide lift-off layer.
  • a silicon surface requires a careful pre-treatment prior to the metal deposition.
  • the use of hydrofluoric acid for forming the first recess in the first lift-off layer and exposing the semiconductor surface does not only accomplish the required etching, but also provides a cleaning step of the surface and a hydrogen passivation.
  • diluted HF is used in this step.
  • an alternative embodiment employs a step of dry-etching a part of the first lift-off layer in the formation of the recess, and uses the described wet-etching only for the final etching phase that exposes the semiconductor surface and provides the cleaning and surface passivation.
  • a basic embodiment, hereinafter also referred to as the first preferred embodiment that employs the principle of the invention involves depositing the first lift-off layer directly on the semiconductor surface. This embodiment is particularly useful for defining closely spaced metal features, such as contacts, on the semiconductor surface.
  • the second preferred embodiment of the invention further comprises: a step of depositing a second lift-off layer on the semiconductor surface before depositing the first lift-off layer, wherein the material of the second lift-off layer is selectively removable and allows a formation of an opening; as a part of the step of forming the opening, a step of forming an opening in the shape of the predefined lateral structure at the predefined position also in the second liftoff layer; - a step of forming a lift-off grid by forming an array of holes in the first mask layer (304) and in the first lift-off layer, which holes laterally reach underneath the first mask layer, and stopping the hole formation on the second lift-off layer; after the step of removing the first lift-off layer, a step of removing the second lift-off layer, thus also lifting off metal layer sections deposited thereon.
  • the deposited layer stack consists of three layers.
  • the second lift-off layer is deposited directly on the semiconductor surface and is covered by the first lift-off layer and the first mask layer.
  • Two lift-off sub-steps are performed in order to provide a fast removal of the layer stack and of undesired metal layer sections deposited thereon.
  • the lift-off grid an array of holes in the first mask layer, provides a widespread, large-area working surface in the first lift-off layer in a wet-etching step (the first liftoff sub-step) that leaves only the second lift-off layer and metal layer sections deposited thereon, because the material of the second lift-off layer is the same as that of the first mask layer, and therefore stable in this wet-etching step.
  • the second lift-off sub-step is performed by removing the second lift-off layer, thus lifting off the remaining metal layer sections deposited thereon, and leaving only the metal layer deposited directly on the semiconductor surface in the desired lateral shape.
  • the material of the second lift-off layer is the same material as the first mask layer on the semiconductor surface before depositing the first lift-off layer.
  • processing complexity is kept low.
  • no additional device and material resources for layer deposition and for suitable removal techniques are required in comparison with the first preferred embodiment.
  • the step of forming the opening preferably comprises the steps of covering the first mask layer with a first resist layer, forming a first resist-opening in the resist layer with the predefined lateral shape at the predefined position, dry-etching the first mask layer, the first lift-off layer and the second lift-off layer in the first resist-opening, and removing the first resist layer.
  • the step of forming the lift-off grid in the second embodiment is performed after the step of removing the resist layer and before the step of depositing a metal layer. It preferably further comprises - depositing a second resist layer on the first mask layer; forming an array of second resist-openings at desired positions for forming the array of holes; forming the holes by dry-etching the first mask layer and the first lift-off layer in the second resist-openings, stopping on the second lift-off layer; removing the second resist layer laterally extending the holes to form recesses that laterally reach underneath the first mask layer by wet-etching the first lift-off layer.
  • the second resist layer will hereinafter also simply be referred to as the "resist layer".
  • the step of removing the first lift-off layer is preferably preformed by wet-etching the first lift-off layer through the holes of the lift-off grid, thus lifting off the first mask layer an metal layer sections deposited thereon.
  • the second preferred embodiment which implies depositing a second lift-off layer on the semiconductor surface, of the same material as the first mask layer
  • a third preferred embodiment which will be described in the following.
  • This third embodiment is based on the recognition that, in the second embodiment, the step of forming the opening also involves opening the second lift-off layer and thus exposing the semiconductor surface. Since this opening step is typically performed by dry-etching, the semiconductor surface is likely to be disturbed. A wet-etching of the second lift-off layer to avoid this problem would have the disadvantage of being slow, and therefore not practical.
  • the third embodiment which forms an alternative to the second embodiment, therefore further comprises, based on the method of the invention, a step of depositing a second lift-off layer on the substrate.
  • the material of the second lift-off layer is selectively removable from the layer stack and allows a formation of an opening.
  • the material of the second lift-off layer is made of the same material as the first lift-off layer.
  • a second mask layer is deposited on the second lift-off layer.
  • the material of the second mask layer allows a formation of an opening with a sub-micrometer extension
  • the second mask layer is preferably made of the same material as the first mask layer.
  • the material of the second mask layer can be selected to be a different material based on the criterion that it allows a formation of an opening with a sub- micrometer extension by dry-etching.
  • the complete layer stack of this embodiment consists of four layers instead of three that were used in the second preferred embodiment, and instead of two, that were used in the first preferred embodiment.
  • a lift-off grid is formed in the first mask layer and in the first lift-off layer, stopping the hole formation on the second mask layer.
  • the step of forming an opening in the layer stack comprises in the present embodiment a step of forming an opening in the shape of the predefined lateral shape at the predefined position also in the second mask layer, stopping on the second lift-off layer. This saves the semiconductor surface from disturbances, which in an embodiment that uses dry-etching for forming the opening would be caused by this aggressive dry-etching step.
  • the second mask layer modifies the lift-off sequence.
  • An additional step of forming a second recess in the second lift-off layer that laterally reaches underneath the second mask layer is performed, and the second of the two lift-off sub-steps, in comparison to the second embodiment, now is performed by removing a layer of the same material as the first lift-off layer instead of layer having the same material as the first mask layer.
  • this processing can be much faster by using HF acid, as explained before, which in addition cleans and passivates the exposed semiconductor surface.
  • the second lift-off layer is preferably deposited on the substrate as a thermal silicon dioxide layer by exposing the semiconductor surface to oxygen.
  • the first lift-off layer is in this embodiment, and in the any embodiments that use silicon dioxide for the first lift-off layer, preferably deposited by low-pressure chemical vapor deposition (LPCVD) using a tetraethylorthosilicate (TEOS) precursor.
  • LPCVD low-pressure chemical vapor deposition
  • TEOS tetraethylorthosilicate
  • the step of forming the opening preferably comprises covering the first mask layer with a first resist layer, forming a first resist-opening in the resist layer with the predefined lateral shape at the predefined position, dry-etching the first mask layer, the first lift-off layer and the second mask layer in the first resist-opening, stopping on the second lift-off layer, and removing the first resist layer.
  • the step of forming the lift-off grid in the third embodiment is preferably performed after the step of removing the resist layer and before the step of depositing a metal layer, and further comprises depositing a (second) resist layer on the first mask layer; forming an array of resist-openings at desired positions for forming the array of holes; forming the holes by dry-etching the first mask layer and the first lift-off layer in the resist-openings, stopping on the second lift-off layer; removing the resist layer; laterally extending the holes to form first recesses that laterally reach underneath the first mask layer, and forming a second recess in the second lift-off layer, laterally reaching underneath the second mask layer, by wet-etching the first and second liftoff layers.
  • a prominent application of the method of the invention is the deposition of metal layers with a width in range of less then 100 nm, preferably only between 30 and 50 nm.
  • gold nanowires can be deposited with high precision using the invention, which will be described in further detail in the following description of further preferred embodiments with reference to the figures.
  • the method of the invention provides a novel lift-off method that is can be used also for other than the particular applications described herein.
  • a method for fabricating a device is provided. The method includes a step of forming the metal layer according to the method of the first aspect of the invention, or one of its embodiments.
  • the advantages of the method of the second aspect of the invention correspond to those described for the method of the first aspect of the invention.
  • Examples of application fields of the device of the invention include electronic integrated-circuit devices, including, in particular, nanocircuits, and photonic devices.
  • the method can be used for fabricating devices that comprise a metal layer with a predefined lateral shape at a predefined position.
  • the method of the present aspect of the invention may also comprise a step of removing the metal layer at a later stage.
  • the metal layer can for instance be used as a catalyst for the growth of semiconductor nanowires, particularly with the vapor- liquid- so lid method, as known per se in that art. After growth of the nanowires the metal layer is preferably removed. Instead of being the catalyst itself, a catalyst such as a gold layer may be deposited on the metal layer.
  • the metal layer can be used for the generation of structures with deep-submicron extension, such as interconnects in integrated circuits with channel length of 90 nm, 65 nm or even less.
  • Figs. IA) to D) show a lift-off technique for depositing a patterned metal layer on a semiconductor surface according to the prior art.
  • Figs 2 A) to D) show different processing phases during a first embodiment of a lift-off method according to the invention for depositing a metal nanowire.
  • Figs. 3 A) to G) show different processing phases during a second embodiment of a lift-off method according to the invention for depositing a metal nanowire.
  • Figs. 4 A) to H) show different processing phases during a third embodiment of a lift-off method according to the invention for depositing a metal nanowire.
  • Figs. 2A) to D) show different processing phases during first preferred embodiment of a lift-off method according to the invention for depositing a metal nanowire.
  • a silicon wafer 200 is provided.
  • a first lift-off layer in the form of a silicon dioxide layer is deposited by LPCVD using a TEOS precursor.
  • a thin first mask layer in the form of a silicon nitride layer 204 (typical thickness: 50 nm) is deposited on the silicon dioxide layer 202.
  • the resulting LPCVD layer stack 206 which is formed by the layers 202 and 204 corresponds to the layer stack of the first lift-off layer and the first mask layer in the previous description and in the claims.
  • a silicon dioxide layer is sometimes also referred to in short as an oxide layer herein, and, similarly, a silicon nitride layer is sometimes also called a nitride layer.
  • a desired deep-submicron pattern is transferred to the silicon nitride layer 204 by dry-etching.
  • a gold wire of 30 to 50 nm width is to be deposited.
  • a corresponding opening 208 is formed in the nitride layer by dry-etching after advanced high-resolution lithography steps, which are known in the art and not shown in the figures.
  • a first resist layer (not shown) is deposited on the silicon nitride layer 204, followed by the formation of a resist opening with the lateral shape of the desired opening 208 before the dry-etching is performed in the first resist opening.
  • a recess 210 is formed in the silicon dioxide layer 202 by etching through the opening 208.
  • This etching step is performed either completely as a wet-etching step or as a combination of a dry-etching step and a subsequent wet-etching step.
  • the wet- etching agent is diluted hydrofluoric acid (HF).
  • HF hydrofluoric acid
  • a metal layer 214 is deposited.
  • a gold layer is either evaporated or sputtered.
  • the gold layer 214 covers the first mask layer on top of the previously deposited layer stack 206, and a section of the silicon surface 212 in recess 210 under the opening 208. This way, a thin gold layer 216 in the form of a nanowire with the width of opening 208 is deposited on the silicon surface 212.
  • the intermediate state after the deposition of gold layer 214, 216 is shown in Fig. 2 C).
  • a wet-etching of silicon dioxide layer 202 is performed beginning in recess 210.
  • the etchant removes the silicon dioxide layer without damaging the deposited metal pattern 216.
  • the silicon nitride layer with deposited metal layer sections 214 is lifted off from the wafer, as shown in Fig. 2 D.
  • the desired gold nanowire pattern has been completed at the desired position underneath the opening 208.
  • the processing of the present embodiment is especially suited for depositing closely spaced metal patterns.
  • the distance that needs to be underetched during the release-etch becomes so large that it will result in very long etching times that are economically unfeasible.
  • a modified processing is preferred, as will be explained with reference to Fig. 3 below.
  • Figs. 3 A) to G) show different processing phases during a second embodiment of a lift-off method according to the invention for depositing a metal nanowire.
  • a LPCVD-layer stack 306 comprising a second lift-off layer in the form of a bottom silicon nitride layer 318, a first lift-off layer in the form of a silicon dioxide layer 302 and a first mask layer in the form of a top silicon nitride layer 304 is deposited on a silicon wafer 300, Fig. 3A).
  • the bottom silicon nitride layer is the "second lift-off layer”
  • the silicon dioxide layer is the “first lift-off layer”
  • the top silicon nitride layer is the "first mask layer”. All three layers are deposited by LPCVD, and the silicon dioxide layer 302 is deposited using a TEOS precursor.
  • the critical deep sub-micron pattern is defined by a high-resolution lithography process.
  • the top silicon nitride layer 304 is covered with a resist layer (not shown) an opening is formed in the resist layer with the predefined lateral shape of the nano wires at the predefined position, and the complete LPCVD layer stack is dry-etched in the resist opening, down to the semiconductor surface 312.
  • the resulting opening 308 in the LPCVD layer stack 306 is illustrated in Fig. 3 B).
  • a lift-off grid is formed in the LPCVD layer stack 306 by etching, preferably dry-etching a two-dimensional array of holes.
  • Fig. 3 C Four such holes 320 to 326 are shown in Fig. 3 C).
  • the distance between the holes of the lift-off grid is between 1 and 2 micrometers, as measured between the closest-lying edges of the holes (indicated by double arrow 328 in Fig. 3 C).
  • the holes of the lift-off grid are formed only in the top nitride layer 304 and in the oxide layer 302.
  • the etching process for forming the lift-off grid is stopped at the bottom silicon nitride layer 318.
  • the intermediate processing stage after this step is shown in Fig. 3 C).
  • the silicon dioxide layer 302 is selectively wet-etched, to create recesses
  • a metal layer gold in the present embodiment, is deposited on all exposed surfaces.
  • Metal layer sections on the top silicon nitride layer 304 as indicated by reference labels 338 to 348, metal layer sections in recesses underneath the openings 320 to 326, as indicated by reference labels 350 to 356, and, as desired, the metal layer section 316 on surface section 312 of the silicon wafer.
  • a first lift-off sub-step is performed by wet-etching oxide layer 302, beginning in the recesses 330 through 336. This removes top silicon nitride layer 304 and metal layer sections 338 through 348 deposited thereon. Only the bottom silicon nitride layer 318 and the metal layer sections 350 to 356, and the desired metal wire 316 remain on the semiconductor surface, as shown in Fig. 3 F).
  • bottom nitride layer 318 is removed by wet-etching, thus also removing the remaining undesired metal layer sections 350 to 356 and leaving only the desired nanowire pattern 316 on the surface of silicon wafer 300.
  • the described process is particularly well-suited for isolated metal patterns, which are at a great distance from each other on a semiconductor surface. It is noted, however, that the described processing of this embodiment exposes the silicon surface 312 in the opening 308 to the etchant of the dry-etching process performed between the stages shown in Figs. 3 A) and 3 B). This may disturb the silicon surface. An additional drawback of the described processing is that the wet-etching of the second lift-off step performed to reach the final stage shown in Fig. 3 G) is slow. A modification of the processing of Fig. 3 that solves these problems will be described in the following with reference to Fig. 4.
  • Figs. 4 A) to H) show different processing phases during a third preferred embodiment of a lift-off method according to the invention, again using as an application example, without limitation, the deposition of a metal nanowire.
  • the description will focus on the differences of the processing in comparison to the embodiment of Fig. 3.
  • a four- layer stack 406 is deposited in the present embodiment.
  • the layer stack starts with a second lift-off layer in the form of a thermal silicon dioxide layer 458 with a thickness of, e.g., 30 nm.
  • a LPCVD layer stack comprises a second mask layer in the form of a bottom silicon nitride layer 418 having a thickness of, e.g., 20 nm.
  • the LPCVD layer stack further comprises a first lift-off layer in the form of a LPCVD silicon dioxide layer 402 of, e.
  • the thermal silicon dioxide layer 458 is the "second lift-off layer”
  • the bottom silicon nitride layer 418 is the "second mask layer”
  • the LPCVD silicon dioxide layer 402 is the "first lift-off layer”
  • the top LPCVD silicon nitride layer 404 is the "first mask layer”.
  • the dry-etching step is stopped on the thermal silicon dioxide layer 458, cf. Fig. 4 B). This way, a contact of the etchant with the surface of silicon wafer 400 is avoided, thus maintaining an undisturbed surface for later metal deposition.
  • a lift-off grid is formed by (dry-)etching holes 420 to 426 down to the bottom silicon nitride layer 458.
  • the thermal oxide layer is protected in the opening 408 during this step by covering the opening with a resist layer (not shown), in which the openings 420 to 426 are exposed.
  • recesses 430 to 436 are formed in the LPCVD silicon dioxide layer 402 by wet-etching.
  • the thermal oxide layer is etched to expose a surface section 412 of the silicon wafer and to form a further recess 460 underneath bottom silicon nitride layer 418, see Fig. 4 D).
  • This processing makes sure that the surface section 412 is cleaned and passivated before the following metal deposition step, which is shown in Fig. 4 E.
  • the lift-off sequence, shown in Figs. 4F) through H) is also modified in comparison with the second embodiment.
  • each of the embodiments has advantages in particular application cases. Therefore, the selection of a processing depends on the metal structures to be deposited. If closely-spaced metal structures are to be deposited, the embodiment of Fig. 2 provides a simple and effective method. If, however, the spacing between the metal structures is larger, the embodiments of Figs. 3 and 4 are preferred.

Abstract

The present invention relates to a method for forming a metal layer (216) with a predefined lateral shape at a predefined position on a semiconductor substrate (200). The method of the invention provides a novel lift-off process that is suitable in particular for patterning metal layers with lateral extensions in the deep sub-micrometer range, such as gold nanowires. The method uses a layer stack that comprises a first lift-off layer (202), such as silicon dioxide, and a first mask layer (204) on the first lift-off layer for defining the desired lateral shape of the metal layer prior to its deposition. Silicon nitride is an example of a suitable mask-layer material. An opening (208) is formed in the layer stack which has the predefined lateral shape in the first mask layer, forms a first recess (210) in the first lift-off layer and laterally reaches underneath the first mask layer (204), and which exposes a semiconductor surface section (212) at the predefined position. The metal layer (214) is deposited on the exposed semiconductor surface section and on previously deposited layer sections. Finally, the first lift-off layer is removed by etching, thus lifting off the first silicon nitride layer and metal layer sections deposited thereon.

Description

An improved lift-off technique suitable for nanometer-scale patterning of metal layers
The present invention relates to a method form forming a metal layer with a predefined lateral shape at a predefined position on a semiconductor substrate. The invention also relates to a method for fabricating a device comprising a such a metal layer.
Patterning metal layers can be performed by etching. An alternative technique that can be applied if etching is not possible is to perform a metal lift-off.
Figs. IA) to D) show a process flow of a known lift-off procedure for the formation of a metal layer with a predefined lateral shape. The illustrated processing starts from a semiconductor substrate 100 that is covered with a resist bi-layer 102. The resist bi- layer 102 consists of a first resist layer 104 and a second resist layer 106. The first and second resist layers 104 and 106, respectively, are made from different resist materials. The material of the first resist layer 104 has a higher development speed than that of the second resist layer 106.
For depositing a metal layer with a predefined lateral structure at a predefined position on the semiconductor substrate 100, an opening 108 is formed in the first and second resist layers 104 and 106 by light exposure through a mask and subsequent developing. Due to the different developing speeds of the two resist layers 104 and 106, a larger recess 110 is formed in the first resist layer 104. The recess reaches underneath the second resist layer 106. The intermediate result of this processing is shown in Fig. IB).
Subsequently, a metal layer 112 is deposited on the resist bi-layer 102. After the metal deposition, metal layer 112 covers layer sections 114 and 116 of the resist bi-layer 102. A section 118 of metal layer 112 is deposited directly on the substrate in recess 110 and with a lateral shape that is defined by opening 108 in the second resist layer 106. This intermediate state of the known metal deposition technique is shown in Fig. 1C).
Finally, a lift-off is performed by dissolving the resist bi-layer 102, thus lifting-off all metal layer sections other than metal layer 118 in the desired lateral shape.
Instead of using a resist bi-layer 102, it is also known to apply and pattern a lift-off resist that, after developing, exhibits a cross sectional recess profile, that continuously narrows from the semiconductor substrate surface to the top of the resist layer. The purpose of the recess is in both cases to provide an initial working surface for exposure to a dissolving agent.
The described standard resist-based lift-off procedures have the drawback that even though advanced lithography equipment is available for fabricating features in the deep nanometer range, that is, for instance in a range between 30 and 50 nm, there are no suitable resist systems for use with such an advanced optical lithography technique.
It is therefore an object of the present invention to provide a method for forming a metal layer with a predefined lateral shape at a predefined position on a semiconductor substrate, which is suitable also for defining features in the deep sub- micrometer range .
According to a first aspect of the invention, a method for forming a metal layer with a predefined lateral shape at a predefined position on a semiconductor substrate comprises the steps of depositing a layer stack comprising a first lift-off layer and a mask layer on the first lift-off layer, wherein the material of the first lift-off layer is selectively removable from the layer stack and allows a formation of an opening, and the material of the first mask layer allows a formation of an opening with a sub-micrometer extension;; forming an opening, which
- has the predefined lateral shape in the first mask layer, - forms a first recess in the first lift-off layer and laterally reaches underneath the first mask layer, and which
- exposes a semiconductor surface section at the predefined position; depositing a metal layer on the exposed semiconductor surface section and on previously deposited layer sections; - removing the first lift-off layer, thus lifting off the first mask layer and metal layer sections deposited thereon.
The method of the invention provides a novel lift-off process that is suitable also for patterning metal layers with lateral extensions in the deep sub-micrometer range. The method uses a layer stack that comprises a first lift-off layer, and a first mask layer on, that is, abutting the first lift-off layer for defining the desired lateral shape of the metal layer prior to its deposition.
The method of the invention is based on the recognition that directly depositing a metal layer and subsequently patterning the metal layer by lithography is not feasible in the deep sub-micron range for the following reasons: first, the metal that is present on the semiconductor surface makes it very difficult to use the required very advanced lithography tools for deep sub-micrometer dimensions. Secondly, experiments have shown that during sputter-etching of the metal layer, such as a gold layer, metal atoms are implanted in the semiconductor surface below, resulting in an uncontrolled growth of nanowires. Use of metals like gold is therefore avoided where possible, especially in high-end cleanroom environments in order to avoid incorporation into the semiconductor, which can cause leakage currents.
With the method of the invention, these problems can be avoided. The metal layer is deposited only after the desired lateral shape has been defined at the desired lateral position on the semiconductor surface. Deep sub-micron patterns can be defined in the layer stack of the lift-off layer and the first mask by advanced lithography techniques known in the art. The novel lift-off technique further avoids undesired implantation of metal atoms in the semiconductor surface region.
As mentioned before, the method of the invention uses a novel lift-off technique. For the purpose of metal lift-off, the step of forming an opening for the definition of the lateral shape of the metal layer comprises forming a recess that laterally reaches underneath the first mask layer, before the metal is deposited on the semiconductor surface. The recess in the first lift-off layer has a larger lateral extension than the opening in the first mask layer on top of it. After depositing a metal layer that covers a semiconductor surface section in the opening (at the desired lateral position) and previously deposited layer sections, the layer stack prepared in this way allows the actual lift-off step, that is, removing the first lift-off layer. The removal can be performed in several ways, which are described below as embodiments of the method of the invention. Removal of the first lift-off layer, in consequence, also removes the first mask layer and the metal layer as well, exposing the corresponding sections of the semiconductor surface, except for the metal layer section that was deposited directly on the semiconductor surface in the opening of the layer stack.
It is noted that lateral directions shall be understood as being parallel to the main semiconductor surface and perpendicular to the stacking direction of the deposited layer stack.
The term layer stack is used herein to summarize the layer sequence comprising the first lift-off layer and the first mask layer, and further layers, as will be explained in further detail in the context of preferred embodiments below. In the following, preferred embodiments of the method of the invention will be described.
In silicon processing technology, the preferred material for the first lift-off layer is silicon dioxide, and the preferred material for the first mask layer is silicon nitride. However, these materials could also be used in combination with a semiconductor surface made from a different semiconductor material, such as GaAs, GaN, InP, InGaAs, SiGe, to mention a few examples.
Other materials can be selected for the first lift-off and the first mask layer, as long as the material of the first lift-off layer is selectively removable from the layer stack and allows a formation of an opening. Note that selective removability of the first lift-off layer from the layer stack implies that the and the material of the first mask layer is stable during removal of the first lift-off layer. The material of the first mask layer must, in addition, allow a formation of an opening with a sub-micrometer extension.
As an alternative example, aluminum can be used for the first lift-off layer, and TiW for the first mask layer. Suitable selective wet- and dry-etching processes for these materials are well known in the art. The choice of materials for the first lift-off layer and the first mask layer also depends on the availability of suitable etchants. Therefore, other materials for the first lift-off layer and the first mask layer can be found by the development of new suitable etchants. Polymers, such as PMMA (PolyMethyl MethAcrylate) could also be used for the first lift-off layer. PMMA can be patterned by etching and removed by heating.
Preferably, the opening in the first mask layer is formed by dry etching. A dry- etching processes can be precisely controlled to achieve lateral extensions in the deep submicron range. However, techniques such as laser ablation, light-assisted wet etching, reactive ion etching and plasma etching could be applicable.
Another preferred embodiment of the method of the first aspect of the invention employs wet-etching for removing the first lift-off layer.
In one embodiment, the step of forming the opening comprises covering the first mask layer with a first resist layer, forming a first resist opening of the predefined lateral shape at the predefined position in the first resist layer, dry-etching the mask layer in the first resist opening, and removing the first resist layer.
The recess is then preferably formed by a step of wet-etching the first lift-off layer through the opening in the first mask layer. If the semiconductor surface is made of silicon or an alloy containing silicon , use of hydrofluoric (HF) acid is preferred for wet etching. HF is for instance suitable for wet-etching a silicon dioxide lift-off layer. Experiments have shown that a silicon surface requires a careful pre-treatment prior to the metal deposition. The use of hydrofluoric acid for forming the first recess in the first lift-off layer and exposing the semiconductor surface, does not only accomplish the required etching, but also provides a cleaning step of the surface and a hydrogen passivation. Preferably, diluted HF is used in this step.
Note that only the final etching stage that exposes the semiconductor surface benefits from the use of (diluted) hydrofluoric acid. While it is possible to employ this wet- etching using hydrofluoric acid for forming all of the first recess in the first lift-off layer, an alternative embodiment employs a step of dry-etching a part of the first lift-off layer in the formation of the recess, and uses the described wet-etching only for the final etching phase that exposes the semiconductor surface and provides the cleaning and surface passivation.
A basic embodiment, hereinafter also referred to as the first preferred embodiment that employs the principle of the invention involves depositing the first lift-off layer directly on the semiconductor surface. This embodiment is particularly useful for defining closely spaced metal features, such as contacts, on the semiconductor surface.
In the following, a second preferred embodiment will be described that is useful for defining isolated metal features. Here, the distance that needs to be underetched during the release-etch for lifting off the first mask layer and metal layer sections deposited thereon is rather large. Using the embodiment described above would result in very long etching times.
Therefore, to solve this problem, the second preferred embodiment of the invention further comprises: a step of depositing a second lift-off layer on the semiconductor surface before depositing the first lift-off layer, wherein the material of the second lift-off layer is selectively removable and allows a formation of an opening; as a part of the step of forming the opening, a step of forming an opening in the shape of the predefined lateral structure at the predefined position also in the second liftoff layer; - a step of forming a lift-off grid by forming an array of holes in the first mask layer (304) and in the first lift-off layer, which holes laterally reach underneath the first mask layer, and stopping the hole formation on the second lift-off layer; after the step of removing the first lift-off layer, a step of removing the second lift-off layer, thus also lifting off metal layer sections deposited thereon. In this embodiment, the deposited layer stack consists of three layers. The second lift-off layer is deposited directly on the semiconductor surface and is covered by the first lift-off layer and the first mask layer. Two lift-off sub-steps are performed in order to provide a fast removal of the layer stack and of undesired metal layer sections deposited thereon. First, use is made of the lift-off grid newly introduced in the second preferred embodiment. The lift-off grid, an array of holes in the first mask layer, provides a widespread, large-area working surface in the first lift-off layer in a wet-etching step (the first liftoff sub-step) that leaves only the second lift-off layer and metal layer sections deposited thereon, because the material of the second lift-off layer is the same as that of the first mask layer, and therefore stable in this wet-etching step. After that, the second lift-off sub-step is performed by removing the second lift-off layer, thus lifting off the remaining metal layer sections deposited thereon, and leaving only the metal layer deposited directly on the semiconductor surface in the desired lateral shape.
Preferably, the material of the second lift-off layer is the same material as the first mask layer on the semiconductor surface before depositing the first lift-off layer. This way, processing complexity is kept low. In particular, no additional device and material resources for layer deposition and for suitable removal techniques (such as etchant resources) are required in comparison with the first preferred embodiment.
In the second embodiment, the step of forming the opening preferably comprises the steps of covering the first mask layer with a first resist layer, forming a first resist-opening in the resist layer with the predefined lateral shape at the predefined position, dry-etching the first mask layer, the first lift-off layer and the second lift-off layer in the first resist-opening, and removing the first resist layer.
Preferably, the step of forming the lift-off grid in the second embodiment is performed after the step of removing the resist layer and before the step of depositing a metal layer. It preferably further comprises - depositing a second resist layer on the first mask layer; forming an array of second resist-openings at desired positions for forming the array of holes; forming the holes by dry-etching the first mask layer and the first lift-off layer in the second resist-openings, stopping on the second lift-off layer; removing the second resist layer laterally extending the holes to form recesses that laterally reach underneath the first mask layer by wet-etching the first lift-off layer.
It is noted that the second resist layer will hereinafter also simply be referred to as the "resist layer". In the processing of this variant of the second preferred embodiment, the step of removing the first lift-off layer is preferably preformed by wet-etching the first lift-off layer through the holes of the lift-off grid, thus lifting off the first mask layer an metal layer sections deposited thereon.
The second preferred embodiment, which implies depositing a second lift-off layer on the semiconductor surface, of the same material as the first mask layer , can be further optimized in a third preferred embodiment, which will be described in the following. This third embodiment is based on the recognition that, in the second embodiment, the step of forming the opening also involves opening the second lift-off layer and thus exposing the semiconductor surface. Since this opening step is typically performed by dry-etching, the semiconductor surface is likely to be disturbed. A wet-etching of the second lift-off layer to avoid this problem would have the disadvantage of being slow, and therefore not practical.
The third embodiment, which forms an alternative to the second embodiment, therefore further comprises, based on the method of the invention, a step of depositing a second lift-off layer on the substrate. The material of the second lift-off layer is selectively removable from the layer stack and allows a formation of an opening. Preferably, in contrast to the second preferred embodiment, the material of the second lift-off layer is made of the same material as the first lift-off layer.
In addition, a second mask layer is deposited on the second lift-off layer. The material of the second mask layer allows a formation of an opening with a sub-micrometer extension The second mask layer is preferably made of the same material as the first mask layer. However, in principle the material of the second mask layer can be selected to be a different material based on the criterion that it allows a formation of an opening with a sub- micrometer extension by dry-etching.
Thus, the complete layer stack of this embodiment consists of four layers instead of three that were used in the second preferred embodiment, and instead of two, that were used in the first preferred embodiment.
Like in the second embodiment, a lift-off grid is formed in the first mask layer and in the first lift-off layer, stopping the hole formation on the second mask layer. However, the step of forming an opening in the layer stack comprises in the present embodiment a step of forming an opening in the shape of the predefined lateral shape at the predefined position also in the second mask layer, stopping on the second lift-off layer. This saves the semiconductor surface from disturbances, which in an embodiment that uses dry-etching for forming the opening would be caused by this aggressive dry-etching step. Furthermore, the second mask layer modifies the lift-off sequence. An additional step of forming a second recess in the second lift-off layer that laterally reaches underneath the second mask layer is performed, and the second of the two lift-off sub-steps, in comparison to the second embodiment, now is performed by removing a layer of the same material as the first lift-off layer instead of layer having the same material as the first mask layer. When using silicon dioxide in the first and second lift-off layers, and silicon nitride in the first and second mask layers, this processing can be much faster by using HF acid, as explained before, which in addition cleans and passivates the exposed semiconductor surface.
In this third embodiment, if silicon or a silicon alloy is used as a substrate material, the second lift-off layer is preferably deposited on the substrate as a thermal silicon dioxide layer by exposing the semiconductor surface to oxygen. The first lift-off layer is in this embodiment, and in the any embodiments that use silicon dioxide for the first lift-off layer, preferably deposited by low-pressure chemical vapor deposition (LPCVD) using a tetraethylorthosilicate (TEOS) precursor. The different oxide formation methods for the first and second silicon oxide layers result in different etching speeds. The TEOS oxide etches faster then the thermal oxide layer.
In the third embodiment, the step of forming the opening preferably comprises covering the first mask layer with a first resist layer, forming a first resist-opening in the resist layer with the predefined lateral shape at the predefined position, dry-etching the first mask layer, the first lift-off layer and the second mask layer in the first resist-opening, stopping on the second lift-off layer, and removing the first resist layer.
The step of forming the lift-off grid in the third embodiment is preferably performed after the step of removing the resist layer and before the step of depositing a metal layer, and further comprises depositing a (second) resist layer on the first mask layer; forming an array of resist-openings at desired positions for forming the array of holes; forming the holes by dry-etching the first mask layer and the first lift-off layer in the resist-openings, stopping on the second lift-off layer; removing the resist layer; laterally extending the holes to form first recesses that laterally reach underneath the first mask layer, and forming a second recess in the second lift-off layer, laterally reaching underneath the second mask layer, by wet-etching the first and second liftoff layers.
A prominent application of the method of the invention is the deposition of metal layers with a width in range of less then 100 nm, preferably only between 30 and 50 nm. For instance gold nanowires can be deposited with high precision using the invention, which will be described in further detail in the following description of further preferred embodiments with reference to the figures. However, the method of the invention provides a novel lift-off method that is can be used also for other than the particular applications described herein. According to a second aspect of the invention, a method for fabricating a device is provided. The method includes a step of forming the metal layer according to the method of the first aspect of the invention, or one of its embodiments.
The advantages of the method of the second aspect of the invention correspond to those described for the method of the first aspect of the invention. Examples of application fields of the device of the invention include electronic integrated-circuit devices, including, in particular, nanocircuits, and photonic devices.
In particular, the method can be used for fabricating devices that comprise a metal layer with a predefined lateral shape at a predefined position. However, the method of the present aspect of the invention may also comprise a step of removing the metal layer at a later stage. The metal layer can for instance be used as a catalyst for the growth of semiconductor nanowires, particularly with the vapor- liquid- so lid method, as known per se in that art. After growth of the nanowires the metal layer is preferably removed. Instead of being the catalyst itself, a catalyst such as a gold layer may be deposited on the metal layer. In an alternative embodiment, the metal layer can be used for the generation of structures with deep-submicron extension, such as interconnects in integrated circuits with channel length of 90 nm, 65 nm or even less.
In the Figures: Figs. IA) to D) show a lift-off technique for depositing a patterned metal layer on a semiconductor surface according to the prior art.
Figs 2 A) to D) show different processing phases during a first embodiment of a lift-off method according to the invention for depositing a metal nanowire. Figs. 3 A) to G) show different processing phases during a second embodiment of a lift-off method according to the invention for depositing a metal nanowire. Figs. 4 A) to H) show different processing phases during a third embodiment of a lift-off method according to the invention for depositing a metal nanowire.
Figs. 2A) to D) show different processing phases during first preferred embodiment of a lift-off method according to the invention for depositing a metal nanowire. A silicon wafer 200 is provided. On a main surface of the silicon wafer 200, a first lift-off layer in the form of a silicon dioxide layer is deposited by LPCVD using a TEOS precursor. Subsequently, a thin first mask layer in the form of a silicon nitride layer 204 (typical thickness: 50 nm) is deposited on the silicon dioxide layer 202. The resulting LPCVD layer stack 206, which is formed by the layers 202 and 204 corresponds to the layer stack of the first lift-off layer and the first mask layer in the previous description and in the claims. A silicon dioxide layer is sometimes also referred to in short as an oxide layer herein, and, similarly, a silicon nitride layer is sometimes also called a nitride layer.
In a next step a desired deep-submicron pattern is transferred to the silicon nitride layer 204 by dry-etching. In the present embodiment, a gold wire of 30 to 50 nm width is to be deposited. For this, a corresponding opening 208 is formed in the nitride layer by dry-etching after advanced high-resolution lithography steps, which are known in the art and not shown in the figures. In short, for forming the opening 208 in the first silicon nitride layer 204, a first resist layer (not shown) is deposited on the silicon nitride layer 204, followed by the formation of a resist opening with the lateral shape of the desired opening 208 before the dry-etching is performed in the first resist opening. After the opening 208 has been formed, the resist layer is removed. Next, a recess 210 is formed in the silicon dioxide layer 202 by etching through the opening 208. This etching step is performed either completely as a wet-etching step or as a combination of a dry-etching step and a subsequent wet-etching step. The wet- etching agent is diluted hydrofluoric acid (HF). The wet-etching is performed down to the wafer surface 212 and therefore provides for a proper passivation of the silicon surface. The intermediate state of the processed wafer after this step is shown in Fig. 2 B).
In a next step a metal layer 214 is deposited. In the present embodiment, a gold layer is either evaporated or sputtered. The gold layer 214 covers the first mask layer on top of the previously deposited layer stack 206, and a section of the silicon surface 212 in recess 210 under the opening 208. This way, a thin gold layer 216 in the form of a nanowire with the width of opening 208 is deposited on the silicon surface 212. The intermediate state after the deposition of gold layer 214, 216 is shown in Fig. 2 C).
Next, a wet-etching of silicon dioxide layer 202 is performed beginning in recess 210. The etchant removes the silicon dioxide layer without damaging the deposited metal pattern 216. This way, the silicon nitride layer with deposited metal layer sections 214 is lifted off from the wafer, as shown in Fig. 2 D. The desired gold nanowire pattern has been completed at the desired position underneath the opening 208.
The processing of the present embodiment is especially suited for depositing closely spaced metal patterns. However, in case isolated metal patterns are to be deposited, the distance that needs to be underetched during the release-etch becomes so large that it will result in very long etching times that are economically unfeasible. For such applications, a modified processing is preferred, as will be explained with reference to Fig. 3 below.
Figs. 3 A) to G) show different processing phases during a second embodiment of a lift-off method according to the invention for depositing a metal nanowire. In this second preferred embodiment, a LPCVD-layer stack 306 comprising a second lift-off layer in the form of a bottom silicon nitride layer 318, a first lift-off layer in the form of a silicon dioxide layer 302 and a first mask layer in the form of a top silicon nitride layer 304 is deposited on a silicon wafer 300, Fig. 3A). In the language of the previous description and the claims, the bottom silicon nitride layer is the "second lift-off layer", the silicon dioxide layer is the "first lift-off layer" and the top silicon nitride layer is the "first mask layer". All three layers are deposited by LPCVD, and the silicon dioxide layer 302 is deposited using a TEOS precursor.
In a next step, see Fig. 3B), the critical deep sub-micron pattern is defined by a high-resolution lithography process. For this purpose, the top silicon nitride layer 304 is covered with a resist layer (not shown) an opening is formed in the resist layer with the predefined lateral shape of the nano wires at the predefined position, and the complete LPCVD layer stack is dry-etched in the resist opening, down to the semiconductor surface 312. The resulting opening 308 in the LPCVD layer stack 306 is illustrated in Fig. 3 B). After this dry-etching step, a lift-off grid is formed in the LPCVD layer stack 306 by etching, preferably dry-etching a two-dimensional array of holes. Four such holes 320 to 326 are shown in Fig. 3 C). The distance between the holes of the lift-off grid is between 1 and 2 micrometers, as measured between the closest-lying edges of the holes (indicated by double arrow 328 in Fig. 3 C).
The holes of the lift-off grid are formed only in the top nitride layer 304 and in the oxide layer 302. The etching process for forming the lift-off grid is stopped at the bottom silicon nitride layer 318. The intermediate processing stage after this step is shown in Fig. 3 C). Next, the silicon dioxide layer 302 is selectively wet-etched, to create recesses
330 to 336 from holes 320 through 326, respectively. As illustrated in Fig. 3 D), the recesses laterally reach underneath the top silicon nitride layer 304, which, like the bottom silicon nitride layer 318 is not modified in the wet-etching process. The wet-etching is performed using diluted HF, which cleans and passivates the exposed silicon surface section 312. In a next step, as shown in Fig. 3 E) a metal layer, gold in the present embodiment, is deposited on all exposed surfaces. Three different kinds of surfaces are exposed during the metal deposition step: Metal layer sections on the top silicon nitride layer 304, as indicated by reference labels 338 to 348, metal layer sections in recesses underneath the openings 320 to 326, as indicated by reference labels 350 to 356, and, as desired, the metal layer section 316 on surface section 312 of the silicon wafer.
Next, a first lift-off sub-step is performed by wet-etching oxide layer 302, beginning in the recesses 330 through 336. This removes top silicon nitride layer 304 and metal layer sections 338 through 348 deposited thereon. Only the bottom silicon nitride layer 318 and the metal layer sections 350 to 356, and the desired metal wire 316 remain on the semiconductor surface, as shown in Fig. 3 F). In a subsequent second lift-off sub-step, bottom nitride layer 318 is removed by wet-etching, thus also removing the remaining undesired metal layer sections 350 to 356 and leaving only the desired nanowire pattern 316 on the surface of silicon wafer 300.
The described process is particularly well-suited for isolated metal patterns, which are at a great distance from each other on a semiconductor surface. It is noted, however, that the described processing of this embodiment exposes the silicon surface 312 in the opening 308 to the etchant of the dry-etching process performed between the stages shown in Figs. 3 A) and 3 B). This may disturb the silicon surface. An additional drawback of the described processing is that the wet-etching of the second lift-off step performed to reach the final stage shown in Fig. 3 G) is slow. A modification of the processing of Fig. 3 that solves these problems will be described in the following with reference to Fig. 4.
Figs. 4 A) to H) show different processing phases during a third preferred embodiment of a lift-off method according to the invention, again using as an application example, without limitation, the deposition of a metal nanowire. The description will focus on the differences of the processing in comparison to the embodiment of Fig. 3.
Instead of a three-layer stack, a four- layer stack 406 is deposited in the present embodiment. The layer stack starts with a second lift-off layer in the form of a thermal silicon dioxide layer 458 with a thickness of, e.g., 30 nm. On top of the thermal silicon dioxide layer 458 a LPCVD layer stack comprises a second mask layer in the form of a bottom silicon nitride layer 418 having a thickness of, e.g., 20 nm. The LPCVD layer stack further comprises a first lift-off layer in the form of a LPCVD silicon dioxide layer 402 of, e. g., 150 nm thickness, and a first mask layer in the form of a LPCVD silicon nitride layer 404 of, e. g., 50 nm thickness, which are subsequently deposited to complete the layer stack 406. In the language of the claims and the previous description, therefore, the thermal silicon dioxide layer 458 is the "second lift-off layer", the bottom silicon nitride layer 418 is the "second mask layer", the LPCVD silicon dioxide layer 402 is the "first lift-off layer", and the top LPCVD silicon nitride layer 404 is the "first mask layer".
In a following step of defining and dry-etching an opening 408 of a critical dimension in the deep sub-micrometer range the dry-etching step is stopped on the thermal silicon dioxide layer 458, cf. Fig. 4 B). This way, a contact of the etchant with the surface of silicon wafer 400 is avoided, thus maintaining an undisturbed surface for later metal deposition.
The following processing resembles that described with reference to Fig. 3. A lift-off grid is formed by (dry-)etching holes 420 to 426 down to the bottom silicon nitride layer 458. The thermal oxide layer is protected in the opening 408 during this step by covering the opening with a resist layer (not shown), in which the openings 420 to 426 are exposed.
Next, recesses 430 to 436 are formed in the LPCVD silicon dioxide layer 402 by wet-etching. At the same time, the thermal oxide layer is etched to expose a surface section 412 of the silicon wafer and to form a further recess 460 underneath bottom silicon nitride layer 418, see Fig. 4 D).
This processing makes sure that the surface section 412 is cleaned and passivated before the following metal deposition step, which is shown in Fig. 4 E. The lift-off sequence, shown in Figs. 4F) through H), is also modified in comparison with the second embodiment. The second of the two lift-off sub-steps, in comparison to the second embodiment, now is performed by removing the thermal silicon dioxide layer 458 instead of a silicon nitride layer. Using HF acid, this processing can be much faster, and in addition cleans and passivates the exposed semiconductor surface.
The foregoing description disclosed three preferred embodiments of the method of the invention with reference to Figs. 2, 3, and 4. As explained, each of the embodiments has advantages in particular application cases. Therefore, the selection of a processing depends on the metal structures to be deposited. If closely-spaced metal structures are to be deposited, the embodiment of Fig. 2 provides a simple and effective method. If, however, the spacing between the metal structures is larger, the embodiments of Figs. 3 and 4 are preferred.
The disclosed lift-off methods could also be applied to the patterning of non- metallic structures, given that the material to be patterned withstands the etching procedures involved. However, as mentioned before, other suitable combinations of etchants and materials can be selected or developed that allow implementing the method of the invention.
Reference numerals given in the following claims shall not be construed as limiting the scope of the claims.

Claims

CLAIMS:
1. A method for forming a metal layer (216; 316; 416) with a predefined lateral shape at a predefined position on a semiconductor surface, the method comprising the steps of: depositing a layer stack (206; 306; 406) comprising a first lift-off layer (202; 302; 402) and a first mask layer (204; 304; 404) on the first lift-off layer, wherein the material of the first lift-off layer is selectively removable from the layer stack and allows a formation of an opening, and the material of the first mask layer allows a formation of an opening with a sub-micrometer extension; forming an opening (208, 210;308, 310; 408, 430, 460) which - has the predefined lateral shape in the first mask layer (204; 304; 404),
- forms a first recess (210; 310; 430) in the first lift-off layer and laterally reaches underneath the first mask layer, and which
- exposes a semiconductor surface section (212; 312; 412) at the predefined position; - depositing a metal layer (216; 216; 416) on the exposed semiconductor surface section and on previously deposited layer sections ; removing the first lift-off layer (202; 302; 402), thus lifting off the first mask layer (204; 304; 404) and metal layer sections (214; 338; 438) deposited thereon.
2. The method of claim 1 , wherein the opening in the first mask is formed by dry etching.
3. The method of claim 1 or 2, wherein the first lift-off layer is removed by wet- etching.
4. The method of claim 4, wherein the step of forming the opening (208, 210;308, 310; 408, 430, 460) comprises a step of wet-etching the first lift-off layer (202; 302; 402) through the opening in the first mask layer using hydrofluoric acid.
5. The method of claim 1, comprising, before the step of removing the first liftoff layer, a step of dry-etching a part of the first lift-off layer (302; 402) through the opening (308; 408) in the first mask layer (304; 404).
6. The method of claim 1 , wherein the first lift-off layer (202) is deposited on the semiconductor surface.
7. The method of claim 1, further comprising a step of depositing a second lift-off layer (318) on the semiconductor surface before depositing the first lift-off layer (302), wherein the material of the second lift-off layer is selectively removable and allows a formation of an opening; as a part of the step of forming the opening (308, 310), a step of forming an opening (310) in the shape of the predefined lateral structure (316) at the predefined position also in the second lift-off layer (318); - a step of forming a lift-off grid by forming an array of holes (330, 332, 334,
336) in the first mask layer (304) and in the first lift-off layer (302), -which holes laterally reach underneath the first mask layer, and stopping the hole formation on the second lift-off layer (318); after the step of removing the first lift-off layer (302), a step of removing the second lift-off layer (318), thus also lifting off metal layer sections (350) deposited thereon.
8. The method of claim 7, wherein the material of the second lift-off layer is the same material as that of the first mask layer.
9. The method of claim 7, wherein the step of forming the lift-off grid is performed before the step of depositing a metal layer, and further comprises depositing a resist layer on the first mask layer; forming an array of resist-openings at desired positions for forming the array of holes (330, 332, 334, 336); - forming the holes by dry-etching the first mask layer (304) and the first lift-off layer (302) in the resist-openings, stopping on the second lift-off layer (318); removing the resist layer; laterally extending the holes (330, 332, 334, 336) to let them laterally reach underneath the first mask layer (304) by wet-etching the first lift-off layer (302).
10. The method of claim 1, further comprising before depositing the first lift-off layer, (402) a step of depositing on the semiconductor surface a second lift-off layer (458) and of depositing a second mask layer (418) on the second lift-off layer (458), wherein the material of the second lift-off layer is selectively removable from the layer stack - and allows a formation of an opening, and the material of the second mask layer allows a formation of an opening with a sub-micrometer extension -; a step of forming a lift-off grid by forming an array of holes (430, 432, 434, 436) in the first mask layer (404) and in the first lift-off layer (402), stopping the hole formation on the second mask layer (418); as a part of the step of forming an opening, a step of forming an opening in the shape of the predefined lateral shape at the predefined position also in the second mask layer (418), and a step of forming a second recess (460) in the second lift-off layer (458) that laterally reaches underneath the second mask layer (418); and after the step of removing the first lift-off layer (402), a step of removing the second mask layer (418) in sections not covered with the metal layer (450), and, after that, a step of removing the second lift-off layer (458), thus lifting off remaining sections of the second mask layer and metal layer sections (450) deposited thereon.
11. The method of claim 10, wherein the material of the second mask layer is the same material as that of the first mask layer, and the material of the second lift-off layer is the same material as that of the first lift-off layer.
12. The method of claim 1, wherein the material of the first lift-off layer (202;
302; 402) is silicon dioxide and the material of the first mask layer (204; 304; 404) is silicon nitride.
13. The method of claims 11 and 12, wherein the semiconductor surface (412) contains silicon and the first and second lift-off layers are silicon dioxide layers, and wherein the second lift-off layer (458) is formed on the semiconductor surface by exposing the semiconductor surface to oxygen, and wherein the first lift-off layer (402) is deposited on the second mask layer by low-pressure chemical vapor deposition using a tetraethylorthosilicate precursor.
14. The method of claim 10, wherein the step of forming the lift-off grid is performed before the step of depositing a metal layer, and further comprises depositing a resist layer on the first mask layer; - forming an array of resist-openings at desired positions for forming the array ofholes; forming the holes (420) by dry-etching the first mask layer and the first lift-off layer in the resist-openings, stopping on the second lift-off layer; removing the resist layer; - laterally extending the holes to form first recesses (430) that laterally reach underneath the first mask layer, and forming a second recess (460) in the second lift-off layer, laterally reaching underneath the second mask layer (418), by wet-etching the first and second lift-off layers.
15. The method of claim 12, wherein the step of removing the first lift-off layer
(404) is performed by wet-etching the first lift-off layer through the recesses (430) of the liftoff grid, thus lifting off the first mask layer (404) and metal layer sections (438) deposited thereon.
16. The method of claim 15, wherein the step of removing the second mask layer
(418) is performed by wet-etching, thus lifting off metal layer sections (450) deposited thereon.
17. The method of claim 1, wherein depositing the metal layer (216; 316, 416) includes depositing gold.
18. A method according to claim 1, 5, or 10 for forming a metal structure (216; 316, 416) with a width of 30 to 50 nm at a predefined position on a semiconductor surface.
19. A method for fabricating a device, including a step of forming the metal layer according to the method of claim 1.
PCT/IB2006/054468 2005-12-22 2006-11-28 An improved lift-off technique suitable for nanometer-scale patterning of metal layers WO2007072247A2 (en)

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