WO2005040037A1

WO2005040037A1 - Large area fabrication method based on the local oxidation of silicon and/or different materials on micro- and nano-scale

Info

Publication number: WO2005040037A1
Application number: PCT/EP2004/011817
Authority: WO
Inventors: Massimiliano Cavallini; Fabio Biscarini; Paolo Mei; Ricardo Garcia
Original assignee: Consejo Superior De Investigaciones Cientificas
Priority date: 2003-10-20
Filing date: 2004-10-19
Publication date: 2005-05-06
Also published as: ITBO20030614A1

Abstract

Process for local oxidation of a surface to be oxidized on micro and nano scale, including: i) the application of a stamp, made of or covered with a conductive material, with motives in relief of sub-micrometer or nanometer size, on or over said surface to be oxidized, in an atmosphere of gas having a controlled content of electrolyte vapours and ii) the application of an electric voltage between the stamp, connected to the negative pole, and the surface to be oxidized, connected to the positive pole.

Description

LARGE AREA FABRICATION METHOD BASED ON THE LOCAL OXIDATION OF SILICON AND/OR DIFFERENT MATERIALS ON MICRO- AND NANO-SCALE Technical Field The present invention concerns a process for the local oxidation of surfaces of silicon, even covered by a layer of native oxide, and/or of other oxidable materials through spatial control on micro- and nano-scale of the dimension, position and shape of the oxidized region. Background Art Despite of the continuous development of new nano-manufacturing methods, applied also to organic materials, nowadays photolithography remains the workhorse in the microelectronics industry. Although photolithography is applicable on large areas (e.g. 16" wafers) and it is reliable for production, the costs of the infrastructures, instrumentation (sources), materials (resists, optics) and processing increase exponentially with decreasing of the dimensions of the fabricated objects, viz. with increasing resolution. Serial fabrication techniques, such as electron beam lithography and focussed ion beam milling, are reliable tools to fabricate features at the nanometre scale, however they are not suitable for large area fabrication aimed to production because of low throughput. Alternative techniques to fabrication that could be high resolution, applicable to large areas and sustainable in terms of cost have been developed based on the use of contact stamps (micro-contact printing, hitesides et al, United States Patent 5,900,160 May 4, 199 "Methods of etching articles via microcontact printing"); pressure stamping (nano-embossing), Chou, United States Patent 5,772,905 June 30, 1998 "Nanoimprint lithography"); lithographically controlled wetting or dewetting (Biscarini and Cavallini PCT Application PCT EP03/10242, September 15, 2003 "Method for manufacturing and controlling structures and patterns of soluble and colloidal substances by printing on the micrometer and nanometer scale and with reduction of the size of the stamp's features""). These processes are able to create masks with nanostructured features on silicon wafers, but only in a few number of cases they can be applied directly to the fabrication of silicon oxide nanostructures. In the past years IBM developed a parallel fabrication by local probes where the single tip of an AFM has been replaced by an array of tips independently controlled (10) (Binnig et al, United States Patent 6,835,477 November 10, 1998 "Mass Storage Application of Local Probe Arrays"). This technique offers remarkable resolution and accuracy but cannot be applied to the direct fabrication of silicon in its present form. Local oxidation, also termed anodic oxidation or nano-oxidation, of metallic surfaces and semiconductors (inorganic but also organic in nature) performed with a conductive tip of an atomic force microscope (AFM) is a versatile technology for the manufacturing of nanostructures and devices. The process of local electrochemical oxidation gives rise to relieves protruding from the native surface. These relieves can be subsequently etched out by exposure to hydrofluoric acid, forming pits with topographical depression with respect to the native surface. This can be regarded as a first step towards manufacturing of structures on silicon also in 3D. Local oxidation lithography by scanning probe microscopy (OX-SPL) has been successfully used for fabricating patterns with lines and/or dots with size controlled down to 10 nm. Writing information on a silicon surface with a density in excess of 1 terabit/square inch (1,2) has been shown. Other demonstrated applications of OX-SPLs are: manufacturing of micrograting (3), wave guides (4), single electron transistors (5, 6), quantum point contacts (7), superconductor interference devices (8, 9), inverted nanolattices (10), manufacturing of sites for anchoring bio-molecules to be used as receptors (11), templates for oriented growth of ultra-thin films of semiconductor molecules (12). The local oxidation with a stamp is a process compatible with the planar technology of microelectronics, and does not require either masks and lift-off. In terms of rate of manufacturing, the OX-SPL is comparable with electron beam lithography, but it has the advantage of being more sustainable in terms of costs. The OX-SPL has similarity with the conventional anodic electrochemical oxidation but with the important difference that the local process is confined by the meniscus of water that forms between tip and substrate(l, 13, 14). The process of local oxidation is entirely general and it does not depend on the specific geometry of the cathode. Silicon oxide formed by local oxidation appears as a relief on the surface even though it has been demonstrated that during its formation it also grow under the surface of the silicon. It turns out that chemical etching of the lithographically grown oxide leaves depressions at the same positions. OX-SPL is a serial process because it allows one to fabricate only one structure at a time and not a whole pattern simultaneously. Moreover, the performances of OX-SPL are limited by the scanning velocity of the tip that acts as a cathode, so that the writing speed is very low. The success of a manufacturing technology depends not only on the peculiar properties of the built material, but also on the efficiency , simplicity and most of all operational cost. Disclosure of the Invention Main goal of the present invention is to provide a process for local oxidation of surfaces so as to fabricate sub-micrometric or nano-metric structures on a large area of the surface. These structures have a different chemical composition with respect to the pristine regions of the surface (viz. not affected by the process), and they in general protrude out of the surface as mounds (hillocks). Chemical etching of the mounds yields pits, whose chemical composition may be similar to that of the pristine regions. Thus, the process results in the fabrication of a surface with spatial modulation of the chemical and/or topography across a large area. This process is efficient, simple and has a low cost. An aim of the present invention is to provide a process that allows to perform local oxidation of the silicon wafer surface, even covered by a layer of oxide, with spatial control on the nanometer scale and across an area of several squared centimeters at the least. Another aim of the present invention is to provide a process that can be repeated for a large number of cycles and thus can be upscaled into a manufacturing technology of industrial relevance. Still another aim of the present invention is to provide a process that allows us to obtain a complete pattern on a large area of a surface, in few seconds and with local feature size from a few to a few tens nm.. Another aim of the present invention is to provide a process that is robust and highly fault-tolerant with respect to possible local deviations from perfect contact and planarity. This goal, this aims and other aims that will become evident from the following description are reached by a method for the local oxidation of surfaces, able to fabricate structures with submicrometer or nanometer size on a large area according to the present invention., comprising the steps of : i) application of a stamp, made of or covered with a conductive material, with motives in relief of sub-micometer or nanometer size, on or over said surface to be oxidized, in an atmosphere of gas having a controlled content of electrolyte vapours and ii) application of an electric voltage between the stamp, connected to the negative pole, and the surface to be oxidized, connected to the positive pole. The voltage applied is preferably in the range between 10 mV and 500 V. Moreover, the voltage applied can be modulated in time and intensity. Furthermore a voltage of opposite sign with respect to the voltage indicated for step i) can be temporarily applied between said stamp and said surface, for instance by applying an alternate voltage. The electrolyte used in the process of the present invention is water as a source of hydroxyl ions OH^", but other electrolytes can be used. The requirement is that the electrolyte is able to form a meniscus between the protrusions of the stamp and the surface to be oxidized. The electrolyte can also be a mixture of water and one or more other electrolytes, particularly alcohol or alcohol derivatives, In the process according to the present invention, the stamp can be applied on the surface to be oxidized with the motives in relief (protrusions) in contact with the surface to be oxidized, or the stamp can be applied over the surface to be oxidized at a distance from the surface appropriate for enabling the formation of a meniscus of liquid between the protrusions of the stamp and the surface to be oxidized . The control of the separation distance between the stamp and the surface can be achieved by pre-fabricating spacers on the stamp or on the surface. The height of the spacers ranges from 1 nm to 10000 nm. Moreover, such a control of the separation can be achieved by moving the stamp or the substrate along the approaching direction by means of an actuator that can be mechanical, piezoelectric, or a combination of the two. The control of the separation can be made by optical imaging from the top using suitable reference markers, by optical interference, by mechanical cantilevers integrated on the stamp, by piezoelectric or capacitive sensors. In the process of the present invention, a meniscus forms spontaneously when the stamp is applied on or over the surface to be oxidized else can be formed just prior or during the application of the voltage. The stamp may be made of or coated with a polymeric conductive material, or the stamp may be made of a metal or coated with a metallic film. Furthermore, the stamp can be made of a dielectric polymeric material coated with a film of conductive material. The surface to be oxidized with the process of the present invention can be a metal surface, also passivated with an oxide or other thin film, with conductive or superconductive electrical properties. Moreover, the surface to be oxidized by the process of the present invention can be a semiconductor surface, as for instance a silicon surface also covered with an oxide thin film; it can also be an organic or polymeric semiconductor surface or thin film. Furthermore, the surface to be oxidized can be made of a mixed valence oxide with a perovskitic structure, like for instance a Ln-doped manganite surface, SrTi03, or a YBCO surface as two examples of technological relevance.

Ways for carrying out the Invention Here, the fabrication process is demonstrated on the scale of some square centimetres; moreover the process is carried out using stamps endowed with motives in relief (protrusions) that are placed in intimate contact with a surface to be oxidized, particularly a silicon oxide surface. Being the silicon oxide surface, native or thermal, non stoichiometric, the oxidation of the oxide thin film is possible. Similarly, it is possible to oxidise a variety of surfaces that have several oxidation states. "Placed (or applied) in intimate contact" as used herein means placed so that the motives in relief or protrusions of the stamp are in physical contact with the surface to be oxidized or placed at a distance allowing the formation of a meniscus of liquid between the motives in relief or protrusions of the stamp and the surface to be oxidized. The mechanism is similar to that of the OX-SPL where the electrochemical process occurs in correspondence of the water meniscus formed by capillary forces between the surface and the tip of an atomic force microscope. In the present invention, each protrusion in the stamp gives rise to a meniscus which then allows for the simultaneous occurrence of the oxidation process under each protrusion. This simultaneous multiplexing of a local oxidation process determines the parallel fabrication of the oxide features in the present invention. The stamp used for the demonstration of the invention is made of a polymeric material, therefore it is partially flexible under the application of a pressure and able to conformally adapt to the surface. In the example, the stamp is made of a polymeric replica of a recorded digital video disk (DVD), thus the protrusions of the stamp (viz. the parts in relief) replicate the whole set of structures (areols) present in the original DVD master. The polymeric stamp is then coated with a metallic film (Au) of thickness 100 nm deposited by high vacuum sublimation. The process is performed in a humidity controlled environment, by fluxing moist nitrogen in the environment where the process is performed. It is necessary to reach a value of the relative humidity >75% prior to place the stamp in contact with the surface. Water (or of another electrolyte or source of hydroxy-ions) determines the formation of the menisci connecting the protrusions of the stamp and the substrate. The electrolyte is actively involved in the electrochemical oxidation of silicon. By applying a bias voltage (typically 15V or 30 V) between the stamp (set to negative potential) and silicon (set to positive potential), the oxidation of the silicon is obtained yielding the transfer of the motives of the stamp into oxide structures correspondingly to the position of the protrusions of the stamp. The oxide structures that are fabricated on the substrate can exhibit a size smaller than the size of the motives of the stamp's protrusions because the effective area where the oxidation occurs depends on the electric field between the stamp's protrusions and the sample. Depending on the curvature of the protrusions a concentration of the electric field lines can occur in regions narrower in comparison to the physical size of the meniscus. This is an important feature that allows one to downscale the fabricated feature size with respect to the stamp protrusion's size. The process of the present invention is described according to the Figures. Figure 1 shows the schematic representation of the main steps of the process. a) Initial Stage: Stamp and silicon substrate are exposed for one minute to water moist-nitrogen flux in a closed chamber, resulting in a relative humidity RH>75% in the environment . b) Printing: the conductive stamp is placed in intimate contact with the substrate and a pressure of around 2.5 Kg/cm² is applied. A voltage between 10 mV and 500 V is applied for a finite time maintaining the silicon to positive bias. The time duration of this process determines the feature size both in terms of height and width. c) Detachment: the stamp is removed from the surface of the silicon. Examples In the following, an example of the application of the printing procedure according to the present invention is given. The process has been demonstrated by using the prototype of a press shown in figure 2 on. a piece of a silicon wafer with size 6 mm x 5 mm. A system of calibrated springs maintains the contact pressure constant across the whole plate and guarantees that the intimate contact between stamp and substrate is formed across the whole area. The proper value of relative humidity is reached by flushing dry nitrogen into a water-filled drexel bubbler, and it is measured by a calibrated hygrometer. Figure 3 shows an image taken with the atomic force microscope

(AFM) of the stamp used for the demonstration of the process: the stamp is made of a DVD coated with a Au film 100 nm tick , deposited by high vacuum sublimation. The stamp consists of sequences of protrusions whose length varies between 350 and 1500 nm, the full width half maximum is 260 nm and height is 90 nm above the minimum baseline. Section "b" shows the line profile along the white line drawn in the section "a." The substrate is made of a Si(l l l) wafer covered by native oxide (nominal thickness 1-2 nm). The substrate has been cleaned by sonication in acetone (2 minutes), isopropanol (2 minutes) and ultra pure water (2 minutes). The sample has been then analyzed by AFM in non-contact mode. In figure 4 the result of manufacturing on an area of 15μm X 15μm is shown. This image is representative of the sample morphology. The surface is covered by structures 95±5 nm wide and about 1 nm thick. These structures are present on the whole region in which the stamp and the sample were in contact. The characteristic distance between adjacent parallel lines is 740 nm, that matches the distance between sectors in the DVD and hence the periodicity of the stamp. It is well known that the oxidation of the silicon simultaneously occurs outside and below the surface. Changes in the fabricated nanostructures upon a variation of the pressure of the stamp within 1 Kg/cm (higher or lower) have not been observed. Section "b" is a zoom (5 μm x 5 μm) of the section

"a", while the section "c" shows the corresponding line profile. The increase of the time of voltage application determines the increase of the thickness of the electrochemically - grown oxide. Figure 5 shows the trend of the dependence of the thickness of the oxide versus the time of application of the voltage (20 V) on a Si(l l l) wafer covered with native oxide. In general in the process with stamp the growth rate of the oxide turns out to be slower than the corresponding process performed by OX-SPL (13, 15-18) and it follows a quasi- linear (or slightly sublinear) dependence with respect to the time of voltage application. This difference in the kinetics of local oxidation can be explained by the following arguments: i) The effective electric field under the stamp's protrusions is smaller in comparison to the case of the OX-SPL. This is due to the larger radius of curvature of the protrusions in comparison to the tip apex of an AFM (possibly there is an increase by one order of magnitude, viz. from 10 to 100 nm). Accordingly, the barrier for diffusion of the oxidising ions is larger in the case of the local oxidation with the stamp. ii) The process occurs in a regime of mechanical contact between stamp and substrate. It is known that also in OX-SPL the contact mode process is slower than the non contact mode one due possibly to the need of an extra mechanical contribution to the free energy against the elastic force exerted by the cantilever. A second example of use of the process is shown in figure 6 and is obtained by treating the oxide fabricated by the local process with the stamp in a diluted solution of hydrofluoric acid (HF) 10% in volume. HF removes the oxide but leaves Si untouched; etching replaces the structures of SiOx, that are in relief with pits of identical lateral dimension but with a different depth in comparison to the thickness of the protruding oxide. The holes show that local oxidation process has formed oxide grown under the surface of the native silicon oxide. A third and last example where the local oxidation here described can be used to fabricate devices is a thin film of a non-stoichiometric Ln_xMni._x0₃._x, where the fraction x of sites occupied by divalent Ln in place of Mn3+ imparts to the perovskitic oxide different magnetic AND electrical properties. These properties can be tuned by locally changing x by means of the process here described. In this manner it is possible to spatially define dielectric and conductive regions on the surface and construct, for instance, tunneling junctions or spin valve junctions that are relevant systems for spintronics. Similarly, it is possible to define conductive pathways separated by dielectric regions on diamond and diamond-like surfaces, as well as inorganic III-V semiconductors, that can be locally oxidised at the surface. The process can be relevant for defining low-dimensional transport and quantised conductance (e.g. trumpet junctions), as well as gated structures. Among possible fields of application of the process of the present invention there are: • nano-manufacturing of devices or device components (viz. gate electrode dielectric in field effect devices); • anisotropic or isotropic structuring of surfaces with relieves and channels for micro - and nano-fluidics; • fabrication of nano-structured surfaces to be used as templates for growth processes, deposition, masters for replica, hard stamps; • fabrication of nano-structured surfaces with spatial modulation of the chemical affinity (e.g. the oxide grown by electrochemical oxidation exhibits different chemical properties in comparison to the native oxide). • corrosion e/o modelling of the substrate; • manufacturing of periodic motives or bi-dimensional lattices for optics and photonics application. The disclosures in Italian Patent Application No. BO2003A000614 from which this application claims priority are incorporated herein by reference.

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Claims

CLAIMS 1. Process for local oxidation of a surface to be oxidized on micro and nano scale, including: i) the application of a stamp, made of or covered with a conductive material, with motives in relief of sub-micrometer or nanometer size, on or over said surface to be oxidized, in an atmosphere of gas having a controlled content of electrolyte vapours and ii) the application of an electric voltage between the stamp, connected to the negative pole, and the surface to be oxidized, connected to the positive pole. 2. Process according to claim 1 in which said voltage is between 10 mV and 500 V . 3. Process according to claim 1 in which said voltage is modulated in time and intensity. 4. Process according to claim 1 in which a voltage of opposite sign with respect to said voltage is temporarily applied between said stamp and said surface, particularly by applying an alternate voltage. 5. Process according to claim 1 in which said electrolyte is water or is a mixture of water and one or more other electrolytes, particularly alcohol or alcohol derivatives, suitable to form the meniscus between the motives of the stamp and the surface to be oxidized. 6. Process according to claim 1 in which the stamp is applied on the surface to be oxidized with the motives in relief in contact with the surface to be oxidized. 7. Process according to claim 1 in which the stamp is applied over the surface to be oxidized at a distance from the surface appropriate for enabling the formation of a meniscus of liquid between the motives in relief of the stamp and the surface to be oxidized , particularly at a distance ranging between 1 nm to 10000 nm 8. Process according to the claim 1 in which the stamp is made of or coated with a polymeric conductive material. 9. Process according to the claim 1 in which the stamp is made of a metal or coated with a metallic film. 10. Process according to the . claim 1 in which the stamp is made of a dielectric polymeric material coated with a film of conductive material. 11. Process according to the claim 1 in which the surface to be oxidized is a metal with conductive or superconductive properties. 12. Process according to the claim 1 in which the surface to be oxidized is a semiconductor, in particular silicon also covered by a thin film of native or thermal oxide or another passivating film. 13. Process according to the claim 12 in which the surface to be oxidized is an organic or polymeric semiconductor, or a thin film consisting of an organic, organometallic molecule, a coordination compound, or a polymer. 14. Process according to the claim 1 in which the surface to be oxidized is a mixed valence oxide with a perovskitic structure.