DE10345525B4 - A method of forming a pattern of features on a photomask - Google Patents

Abstract

A method of forming a pattern (10) of opaque or semitransparent structural elements (20, 20 ') on a photomask (1), the pattern (10) having an edge region (60) extending from a transparent surface (40) or larger surrounded transparent areas on the photomask, comprising the steps:
Providing a mask substrate (12) covered with at least one opaque (16) and / or semitransparent layer (14) and one photosensitive layer (18),
- exposing (28) the photosensitive layer (18)
a) with the pattern (10), wherein the pattern (10) periodically arranged and linear, rectangular or square formed structural elements (20, 20 '), each having an identical width (23) and a grid spacing (74) from an adjacent Have structural element of the pattern,
b) having a plurality of further structural elements (70) in an immediate vicinity of the edge region (60) of the exposed pattern (10), which together form another periodic pattern, the widths and spacings of adjacent structural elements being dependent on ...

Description

The The invention relates to a method of forming a pattern of opaque ones and / or semitransparent structural elements on a photomask, wherein the pattern has a border area that is completely transparent area or a surface, the larger transparent Has areas on which photomask is surrounded. The invention especially concerns the formation of periodic patterns.

often take the layers to be formed on a semiconductor wafer in a layer plane Structures only a comparatively small area fraction on the wafer. The generally for the structuring of the respective layer plane necessary mask For this purpose, either a high bright field component, i. a large share have transparent surfaces, if a positive resist is used on the wafer. Or the mask has a high dark field level if negative resistance is present on the wafer is used.

One High bright field component requires that larger areas on a mask blank for producing the bright field photomask have to be exposed which is longer Write times may result in mask exposure devices.

Yet There are also in the case that high demands on the Structure widths of the structures to be formed, applications, in which masks with high Hellfedanteil for the transmission of structures of Photomasks are used on wafers.

When An example is the formation of a contacting level for the connection of memory cells a memory cell array - about a read / write random access memory (DRAM) - to bit lines called. At the for Memory of this type existing high resolution requirements, d. H. in the case of a very dense configuration of structural elements Small structural widths, is for the generally periodic pattern of vias in same size a high degree of uniformity - beside the positional accuracy - in demanded each to be formed structure widths.

The Contact hole levels usually include a periodic pattern of lattice-shaped, representing the later contact holes Structural elements which are for the chosen one Wafer process variant on a mask with high bright field component as opaque surfaces are formed in a transparent environment. Other structural elements, the outside the memory cell array of the circuit to be formed are generally not along with the pattern on the contact hole plane mask educated. A mask for forming contact holes can therefore have a completely transparent, large-area, essentially only the periodic lattice of opaque structural elements enclosing Include edge area on the photomask.

One not limited only to the aforementioned example of the contact hole level problem is that the in the manufacture of the photomask performed processing so-called Loading effects shows. The processing of the mask comprises in particular etching steps, through which the structures exposed by means of a mask writer in an opaque or semitransparent underlying the exposed resist Layer transferred. Wet chemical, but especially dry etching processes, which one higher dimensional stability the structure transfer offer, are subject to the said Loading effects. The term "Loading" here is the of the area occupation (the proportion of opaque structures on the total surface area on the mask) dependent local Efficiency of the etching process - in particular of the dry etching process - to be understood. The local efficiency depends from the area occupancy within a characteristic range of the respective etching process from.

is for example, the area occupancy in the vicinity of a structural element particularly low, so can within the characteristic range of the etching process briefly to a local enrichment or depletion of etchant substance come, so that in In this area, the efficiency of the etching process comparatively is high or low. Subject to elsewhere on the same Photomask a structural element with the same structural dimensions like the etching process just considered, this structural element of surrounded by other opaque structural elements at close range, that's how it is here the efficiency of the etching process different. The structural element considered first becomes accordingly on its outer edges in changed Dimensions etched as last considered. The structural dimensions achieved by the structure transfer Accordingly, different fall out, so that the degree of uniformity of the processing is reduced due to the different area occupancy.

Using the example of the contact hole plane described above, which is designed as a bright field mask, it can be shown, for example, that with structural dimensions of 90 nm for the individual contact holes representing structural elements a potentially required uniformity of less than 10 to 12 nm (3-σ value of a variety of measured structural elements in the pattern) with currently existing process conditions with respect to the etching technique is not achievable. The near the edge region of the periodic pattern of contact holes In fact, structural elements are etched with too great an efficiency and therefore have systematically smaller structural dimensions. This can lead to higher line or contact resistances or even, at worst, even short circuits or non-closed contacts and thus errors in writing as well as the interpretation of the stored charge in a memory cell.

US 2001/0005619 A1 (Toshiba) describes a method for producing a photomask, in which a transparent substrate of a mask is covered with a chromium layer and a resist. The resist is exposed and developed. The developed structures are called patterns ( 11a ) of lines in the opaque chromium layer. In addition to this pattern, another structural element is formed on the substrate, which arises in the immediate vicinity of the pattern in the chromium layer and performs a dummy function to reduce the difference in area occupation and structure density between the pattern and an isolated structure.

In the US 5,278,105 A For example, a method is described in which dummy features are added as a periodic pattern pattern design to reduce localized etching loading effects. It is proposed to adapt the size of the individual dummy structural elements to those of the pattern.

In the DE 196 32 845 C2 For example, a halftone phase mask is described which has a transparent substrate and two light-shielding layers. The pattern of the second light-shielding layer corresponds to a plurality of cell patterns and a test mark pattern, respectively.

It is therefore the object of the present invention, a method to provide a photomask by which the Uniformity of Structural dimensions of structural elements significantly improved becomes. It is especially a task masks with patterns that a high bright field component with a strongly varying local area occupation own, with high quality to be able to produce.

• – Bereitstellen eines mit wenigstens einer opaken und/oder semitransparenten Schicht und einer photoempfindlichen Schicht bedeckten Maskensubstrates,
• – Belichten der photoempfindlichen Schicht a) mit dem Muster, wobei das Muster periodisch angeordnete und linienförmig, rechteckförmig oder quadratisch ausgebildete Strukturelemente aufweist, welche jeweils eine identische Breite und einen Gitterabstand von einem benachbarten Strukturelement des Musters besitzen, b) mit einer Vielzahl weiterer Strukturelemente in einer unmittelbaren Umgebung des Randbereiches des belichteten Musters, welche zusammen ein weiteres periodisches Muster bilden, wobei die Breiten und Abstände benachbarter Strukturelemente in Abhängigkeit von denjenigen der Strukturelemente des Musters ausgewählt werden, – Entwickeln der photoempfindlichen Schicht zur Bildung einer Resistmaske mit dem periodischen Muster und den das weitere periodische Muster bildenden weiteren Strukturelementen, – Durchführen eines Ätzprozesses zum Übertragen des periodischen Musters und der weiteren Strukturelemente in die wenigstens eine opake und/oder semitransparente Schicht, – Entfernen der photoempfindlichen Schicht, – Entfernen der in die opake und/oder semitransparente Schicht übertragenen weiteren Strukturelemente zum Bilden der transparenten oder transparente Bereiche besitzenden Fläche in der Umgebung des Randbereiches des periodischen Musters.

The object is achieved by a method for forming a pattern of opaque or semitransparent structural elements on a photomask, wherein the pattern has an edge area which is surrounded by a transparent area or a larger area having transparent areas on the photomask, comprising the steps:

• Providing a mask substrate covered with at least one opaque and / or semitransparent layer and a photosensitive layer,
• - Exposing the photosensitive layer a) with the pattern, the pattern having periodically arranged and linear, rectangular or square formed structural elements, each having an identical width and a grid spacing of an adjacent structural element of the pattern, b) with a plurality of further structural elements in an immediate vicinity of the peripheral area of the exposed pattern forming together another periodic pattern, the widths and spacings of adjacent structural elements being selected in dependence on those of the structural elements of the pattern, developing the photosensitive layer to form a resist mask having the periodic pattern and the further periodic pattern forming further structural elements, - performing an etching process for transferring the periodic pattern and the other structural elements in the at least one opaque and / or semitransparent Schich t, removing the photosensitive layer, removing the further structural elements transferred into the opaque and / or semitransparent layer, in order to form the transparent or transparent areas in the vicinity of the edge region of the periodic pattern.

At the edge region of a pattern of structural elements formed on a mask Already in the design stage further structural elements for a Exposure process attached, which over the entire extension of the pattern, d. H. especially in its edge region, a substantially constant area occupancy cause. The subsequent etching process for transmission the structural elements into an opaque or semitransparent layer, which inevitably shows proximity effects (proximity effects) therefore with constant efficiency over the pattern carried away.

The additionally imaged in the exposure process, other structural elements, since they are not at the design stage specific layout of the circuit, in a subsequent process again away. The further invention Structure elements can thus also as dummy structures, Sacrificial structures or temporary structural elements be referred to on the photomask. The downstream process of removing the temporary added Structural elements comprises a another resist or exposure process, which but preferably is performed with lower resolution requirements.

In a particularly advantageous Ausgestal tion the temporarily added structural elements are added to the edge region of the pattern in a structure arrangement identical to the pattern with equal element spacings and diameters. As a result, a particularly high homogeneity of the local area occupation and thus of the etching process is achieved.

In In another advantageous embodiment, the distance of the Border area of the pattern, d. H. especially the outermost Randes, of structural elements in the pattern of the other temporarily added structural elements in dependence of the characteristic range of the proximity effect of the etching process selected. The value of the characteristic range, for example, from advance conducted Experiments be known. After this determined or from a Database is extracted in the design data of the layout the circuit taken over the other temporary structural elements, wherein the distance of the edge region of the pattern from the other temporary structural elements less than the characteristic range is selected.

The actual Efficiency of the etching process corresponds an averaging of the area occupation in a radius of one position, which is the radius of the characteristic Range possesses. Since the distance is chosen smaller, thus parts of the additional temporary Structures into the perimeter. According to this embodiment the distance will be made slightly larger than with an immediate continuation of the pattern in the additional temporary Structural elements into it. The second resist or exposure process by means of Mask writer can be particularly relaxed in this embodiment Requirements for the adjustment accuracy are performed.

Basically it is at the additions to act on a single structural element, for example, the Pattern partially or completely framed and occupies a comparatively large area. It can do that even from the edge of the pattern to the edge of the exposure field. As one of the above embodiments shows, however, the addition may also exist in a variety of smaller structural elements, the surrounding the pattern partially or completely in a systematic way. The depth of additions, d. H. the width of the array of additional temporary features calculated from the edge of the pattern to the outer edge of the arrangement, which turned away from the pattern should, at least itself turn the Have dimension of the characteristic range of the etching process.

According to one Particularly advantageous embodiment, the pattern has a periodic Arrangement of structural elements. Here is the local area occupancy over wide Areas constant and breaks at the edge of the periodic pattern the transparent surfaces down. The loss to uniformity (Uniformity) in the case of the prior art is on the sidelines of the pattern very easily detectable. The application of the steps according to the invention therefore offers considerable advantages here.

According to one further advantageous embodiment has to be created Mask a high bright field component. The proximity effects occur here namely particularly clearly visible - again especially if the local area occupancy varies greatly.

typical Dimensions of the characteristic range are in the case of dry etching processes up to 1 millimeter.

The The invention will now be better understood with reference to an embodiment with the help of a drawing closer explained become. Show:

1 a section of a mask with a periodic contact hole pattern in the state after the exposure
(a) and after performing a dry etching process
(b) according to the prior art;

2 in four consecutive steps the production of in 1 shown mask based on cross-sectional profiles;

3 in plan view an intermediate state in the production of a mask according to the invention with the periodic contact hole pattern and an additional temporary structural element to avoid loading effects;

4 in seven consecutive steps the production of in 3 shown photomask using cross-sectional profiles.

5 in plan view an intermediate state in the preparation of a mask according to the invention with the periodic contact hole pattern and a plurality of additional temporary structural elements to avoid loading effects, which in turn are arranged in a periodic pattern.

1 shows a mask with high bright field component according to the prior art in plan view. In 1a is on a mask 1 a periodic pattern 10 with contact holes representing opaque structural elements 20 to see that in the shown detail of a transparent surface 40 is surrounded, which to the edge 5 a relevant exposure field or the chip area on the mask 1 zoom ranges. The in 1a shown state gives the mask 1 after carrying out the projection or exposure again, ie the structural elements 20 are still here after developing as sublime resist structures 18 on the exposed surface of the underlying absorber layer surrounding it 16 available. At the absorber layer 16 however, it may in particular also be a chromium layer, however, any other light substantially opaque to incident light is also conceivable, such as TaHF or else the semitransparent, phase-shifting MoSi, insofar as it defines shadowing areas with only low light transmission.

1b shows the state after performing a subsequent dry etching process 30 , Due to the proximity effect of the dry etching process 30 becomes the border area 60 of the periodic pattern 10 etched more strongly than the area inside the periodic pattern 10 , Owned the structural elements 20 after developing a width 22 of 90 nm - based on the wafer scale -, so have the structural elements 20 at the edge 60 of the periodic pattern 10 only one width after the dry etching process 23 of, for example, 70 nm. Structural elements 20 , inside the periodic Mu sters 10 are arranged are due to the dry etching process 30 However, dimensionally stable in the absorber layer 16 and a semitransparent layer disposed thereunder 14 Transferred (molybdenum silicide).

2 shows in four successive steps along a line AB, which in the plan view of 1a and b is schematically drawn, the conventional production of the mask 1 with the periodic pattern 10 that is from a transparent surface 40 is surrounded. It becomes first a transparent mask substrate 12 , for example of quartz or CaF, provided by the semitransparent layer 14 , the absorber layer 16 and a photosensitive resist 18 is covered ( 2a ). A photosensitive resist is also understood here as meaning paints which are sensitive to the rays of electrically charged particles (such as electrons or ions).

In a mask writer, such as a laser or electron beam writer, the areas to be removed become in the resist 18 in a projection step 28 illuminated ( 2 B ) and developed. A structural element 20 ' in the border area 60 of the periodic pattern 10 has the same width 22 like a structural element 20 inside the periodic pattern 10 on (90 nm as in 1 described).

A dry etching process 30 is applied to the thus created resist mask, so that the structural elements 20 in the resist layer 28 in the absorber and / or semitransparent layer 14 respectively. 16 be transmitted. Due to proximity effects 34 will that be in the edge area 60 lying structural element 20 ' slightly over-etched. The dry etching process 30 has a characteristic range 50 on, within which the efficiency of the etching process 30 determined by the local area occupancy ( 2c ).

2d shows the state after removal of the resist layer 18 , The outer structural elements 20 ' at the edge 60 now only have one width 23 of 70 nm. The tolerance limit of 10 to 12 nm for the uniformity or homogeneity of line widths of the structural elements 20 . 20 ' is thus considerably exceeded.

An inventive embodiment shows the representation of 3 with a plan view of the periodic pattern 10 and another additional structural element 70 , Shown is the too 1a analog state after the developer step. In the design data used to carry out the projection step 28 is converted into machine data is at a distance 72 from the structural elements 20 ' at the edge 60 of the periodic pattern 10 a periodic pattern 10 surrounding additional structural element 70 intended. Taking into account the distance 72 extends the depth of the structural element 70 from the edge area 60 of the periodic pattern 10 to the edge 5 the relevant exposure field on the mask 1 ,

Deriving from the use of the additional structural element 70 The resulting advantage is based on the sequence of manufacturing steps for the photomask 1 , in the 4 shown, clearly. The in 4a mask blank shown corresponds to the in 2a As shown in the prior art and has the semitransparent layer 14 , the absorber layer 16 and the resist 18 on.

4 shows the cross section along the line AB according to 3 , The additional structural element 70 will be at a distance 72 from the outermost structural element 20 ' of the periodic grid 10 formed such that this distance 72 is less than the characteristic range 50 the subsequent etching process 30 who in 4c is shown.

After removing the resist 18 are the latitudes 23 the outermost structural element 20 ' and the width 22 an internal structural element 20 in the periodic pattern 10 essentially identical. By depending on the characteristic range 50 Engineered choice of distance 72 become close effects 35 the dry etching process 30 avoided.

The additional structural element 70 is temporary and should not be transferred to a wafer-forming structure for the production of the integrated circuit. It is therefore a fully transparent surface 40 on the mask, just like in 1a is shown. For this purpose, the in 4e -G steps shown.

4e shows the application of another resist layer 19 , Outside the periodic grid 10 Subsequently, a large-area exposure in a projection step 29 carried out. After development, this is the additional structural element 70 uncovered ( 4f ).

Subsequently, the exposed layer stack of the additional element 70 z. B. etched away wet-chemically. After that also the resist layer will be 19 removed, so that the bright field mask 1 with structural elements 20 of the periodic grid 10 with a comparatively high degree of uniformity.

Another, in 5a illustrated embodiment provides that the periodic pattern 10 with regularly arranged, rectangular or square shaped structural elements 20 . 20 ' are formed, each having an identical width and a grid spacing from an adjacent structural element of the pattern, and a plurality of other temporary structural elements 70 in the photosensitive layer at a distance of one in the edge region of the periodic pattern 10 located structural element 20 ' which is more than half and less than twice the lattice spacing.

Furthermore, set the other structural elements 70 in another periodic pattern 75 the substantially same lattice spacing between adjacent elements 70 and having the same feature widths as those distances 74 and latitudes 23 the structural elements 20 . 20 ' of the periodic pattern actually to be transferred to the wafer 10 , In 5b becomes the size comparison of the characteristic range 50 of the near effect 35 the dry etching process 30 with the distance 72 of other structural elements 70 to be imaged structural elements 20 ' at the edge 60 of the pattern 10 and the grid spacing 74 between respectively adjacent structural elements 20 respectively. 70 schematically illustrated.

Optimally, the distance corresponds 72 then the distance 74 of the periodic pattern. The distance 72 may be based on process technology but still larger or smaller than the distance within the scope of the invention 74 of the periodic pattern.

1

photomask

5

edge the chip area

10

Template, preferably periodically

12

mask substrate

14

semitransparent layer

16

opaque Layer, absorber layer

18

photempfindliche layer

19

Further Photosensitive layer

20

structural elements (inside the periodic structure)

20 '

structural elements (in the border area of the periodic structure)

22

structure width of the impaired by proximity effects

structural element

23

structure width

28

exposure process

29

Another exposure process

30

Etching process, dry etching process

35

Proximity Effect the etching process

40

transparent area

50

characteristic Range of the proximity effect of the etching process

60

border area

72

distance

74

grid spacing in periodic structure

75

Further periodic structure

Claims (7)

Method for forming a pattern ( 10 ) of opaque or semitransparent structural elements ( 20 . 20 ' ) on a photomask ( 1 ), where the pattern ( 10 ) a border area ( 60 ), of a transparent surface ( 40 ) or a larger area having transparent areas on the photomask, comprising the steps of: providing one with at least one opaque ( 16 ) and / or semitransparent layer ( 14 ) and a photosensitive layer ( 18 ) covered mask substrate ( 12 ), - exposure ( 28 ) of the photosensitive layer ( 18 ) a) with the pattern ( 10 ), where the pattern ( 10 ) periodically arranged and linear, rectangular or square formed structural elements ( 20 . 20 ' ), each having an identical width ( 23 ) and a grid spacing ( 74 ) of an adjacent structural element of the sample, b) with a plurality of further structural elements ( 70 ) in an immediate vicinity of the edge region ( 60 ) of the exposed pattern ( 10 ), which together form another periodic pattern, the widths and spacings of adjacent structural elements being dependent on those ( 23 . 74 ) of the structural elements ( 20 . 20 ' ) of the pattern ( 10 ) to be selected, Developing the photosensitive layer ( 18 ) to form a resist mask having the periodic pattern ( 10 ) and the further periodic pattern forming further structural elements ( 70 ), - performing an etching process ( 30 ) for transmitting the periodic pattern ( 10 ) and the further structural elements ( 70 ) into the at least one opaque ( 16 ) and / or semitransparent layer ( 14 ), - removing the photosensitive layer ( 18 ), - removing in the opaque ( 16 ) and / or semitransparent layer ( 14 ) transferred further structural elements ( 70 ) for forming the transparent or transparent areas ( 40 ) in the vicinity of the edge region ( 60 ) of the periodic pattern ( 10 ). Method according to claim 1, characterized in that further structural elements ( 70 ) in the photosensitive layer at a distance ( 72 ) of one in the peripheral area ( 60 ) of the periodic pattern ( 10 ) structural element ( 20 ' ), which is more than half and less than twice the lattice spacing ( 74 ) is. Method according to one of claims 1 to 2, characterized in that - the step of etching ( 30 ) has a property that at a position on the surface of the photomask ( 1 ) within a characteristic range ( 50 ) the strength of material removal in the opaque ( 16 ) and / or semitransparent layer ( 14 ) of an area fraction within reach ( 50 ) in the step of exposing ( 28 ) structural elements ( 20 . 20 ' ), - the characteristic range ( 50 ) of the etching step ( 30 ) before exposure ( 28 ) of the photosensitive layer ( 18 ), - the other structural elements ( 70 ) in the photosensitive layer ( 18 ) at a distance ( 72 ) of one in the peripheral area ( 60 ) of the pattern ( 10 ) structural element ( 20 ' ) which is less than the characteristic range ( 50 ). Method according to one of claims 1 to 2, characterized in that the width of the structural elements ( 20 . 20 ' ) of the periodic pattern ( 10 ) is not more than 90 nanometers. Method according to one of the preceding claims, characterized in that the step of etching ( 30 ) is a dry etching process. Method according to one of the preceding claims, characterized in that the step of removing the at least one further structural element ( 70 ) the steps - applying a further photosensitive layer ( 19 ) on the photomask ( 1 ), - exposure ( 29 ) and developing the further photosensitive layer ( 18 ) for exposing the further structural element ( 70 ), so that the developed photosensitive layer ( 19 ) as a resist mask on the structural elements ( 20 . 20 ' ) of the pattern ( 10 ), - etching the opaque ( 16 ) and / or semitransparent layer ( 14 ) of the further structural elements ( 70 ) for the full-surface exposure of the mask substrate ( 12 ) outside the pattern ( 10 ), - removing the resist mask from the pattern ( 10 ). Use of the pattern formed by a method according to any one of claims 1 to 6 ( 10 ) formed photomask ( 1 ) for producing a contact hole layer plane in a memory chip.