JPH0213780B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to integrated circuit photomasks, and more particularly to photomasks, techniques for their manufacture, and methods of using such masks for projection printing.

Photomasks used in projection printing, which are separated from a resist-coated semiconductor wafer or the like during use, have a high-resolution metal pattern on a glass substrate with feature sizes approximately equal to 1 to 2 μm. Since pattern quality is very important to semiconductor manufacturing processes, care must be taken to produce photomasks that are virtually defect-free.

It is essential that photomasks be kept substantially defect-free if devices of acceptable quality are to be produced. In addition to particulate contamination, the various cleanings and treatments that are applied to photomasks gradually degrade the pattern quality. Furthermore, it has been found that static electricity is generated on the mask surface and arcs are generated between parts of the metal pattern, thereby deteriorating the quality of the pattern. If these effects are doubled by the six or more photomasks required to manufacture a semiconductor device, this results in significant production losses. Therefore, it is economically desirable to find a way to protect the mask surface from contamination and deterioration while maintaining acceptable resolution.

One method of protecting photomask patterns is as described in U.S. Pat. No. 3,906,133.
There is a simple way to apply a protective coating onto the mask. The patent describes providing an iron oxide mask layer on a transparent substrate having a nitrocellulose protective coating that is thicker than the height of the protrusions on the surface of a processed resist-coated wafer. However, the patent
It relates to contact printing in which a photomask is placed in close contact with a photoresist coated wafer. In projection printing, the photomask is spaced apart from the resist-coated wafer and the light passing through the photomask must be focused onto the resist coating by an optical device. It has been found that this type of protective coating does not have a uniform thickness and causes diffraction and aberrations in the focused projected print pattern, resulting in an unacceptable production quality.

The present invention includes a transparent base plate having a thin metal pattern, and patterning of the base plate by a refractive index matching material injected between the base plate and the cover plate.
This problem has been overcome by a photomask consisting of a planar transparent cover plate in intimate contact with the surface.

In another embodiment, the photomask includes a light-transmissive base plate having a metallized (metallized) and non-metalized portion on a major surface thereof, and a light-transmitting base plate disposed over and with at least the non-metalized portion of said major surface. They have coating materials with similar refractive indices.

A mask can also be formed by bringing a substantially flat cover plate into close contact with a base plate having a metal pattern. One or more drops of index matching fluid are applied to the edge of the cover plate and base plate interface and the assembly is heated to cause the fluid to flow between the cover plate and the patterned base plate. The interface is then sealed to retain the fluid between the plates.

Furthermore, the method of positioning a photomask between a light source and a photoresist-coated substrate allows for selective exposure of a substrate having a photoresist coating to radiation. In this case, the photomask consists of (a) a translucent base plate with a thin metal pattern on its surface, (b) a translucent cover plate in close contact with the patterned base plate, and (c) between the patterned base plate and the cover plate. It consists of an injected index matching fluid. A light source is then activated to emit light onto the patterned photomask to selectively expose the photoresist coated substrate.

An advantage of the mask of the present invention is that it has a longer life than prior art masks and requires less cleaning.

Also, the need for mask surface re-inspection is substantially reduced.

Furthermore, virtually no electrostatic arcing occurred between the metal parts of the mask.

Most importantly, selective exposure of the photoresist-coated substrate resulted in unexpectedly enhanced edge resolution and virtually defect-free photoresist patterns.

Next, details of the present invention will be explained with reference to the drawings.

2. Description of the Related Art In the manufacture of integrated circuits and the like, a light projecting device generally designated by the reference numeral 10 in FIG. 1 has been used. A semiconductor wafer 11 with a resist coating (not shown) is aligned perpendicularly to an ultraviolet light source 12 and a light concentrator 13 . This type of projection device 10 is Microline Model No. 240.
Manufactured by Parking Ellimar Company.

In operation, photomask 16 is placed between ultraviolet light source 12 and light concentrator 13 . photo mask 16
has a transparent substrate 17 made of, for example, fused silica and having a metal pattern 18 on its surface. The ultraviolet light emitted from the light source 12 passes through the non-metalized area 19 on the photomask 16 and is focused on the resist film on the semiconductor wafer 13, exposing and insolubilizing the resist film (e.g., negative acting - polymerizing with light). (harden) - when using resist). The soluble resist film is then washed away with a suitable solvent for further processing of the exposed surface. A plurality of photomasks 16 with different patterns 18
This process is usually repeated many times to form a desired circuit. Although the metal pattern 18 is not in contact with the resist-coated wafer 11, various cleaning and other treatments degrade the quality of the pattern. Furthermore, static electricity is generated on the mask surface and arcs are generated between parts of the metal pattern, deteriorating the quality of the pattern.

As mentioned above, the metallization pattern 1 on the mask 16
It has been found that the protective coating deposited on 8 cannot be used in projection printing equipment. Accordingly, a photomask 30 (see FIGS. 2 and 3) has been developed that has features that overcome the above deficiencies. The photomask 30 includes a transparent base plate 31 having a metal pattern 32 on its surface. A substantially flat transparent cover plate 33 is placed above the substrate 31. Further, the refractive index matching fluid 34 (see FIG. 3) is connected to the base plate 31 and the cover plate 3.
3, substantially eliminating interfacial fringes caused by light reflecting from two adjacent surfaces. Bonding material 36 at interface edge 37 between base plate 31 and cover plate 33
holds the cover plate in place on the base plate while enclosing the index matching fluid 34.

Both cover plate 33 and base plate 31 are made of similar optically transparent materials, and index matching fluid 34 has a refractive index as close as possible to the refractive index of the materials forming the base plate and cover plate. preferable. In a preferred embodiment, the cover plate 33 is fused silica with a refractive index of nD=1.458 and the base plate 3
1 is a transparent low expansion glass with an unknown refractive index. The refractive index matching fluid 34 is connected to the cover plate 3
3 and the metal used to form pattern 32 is chromium. The refractive index matching fluid 34 is an aliphatic hydrocarbon hydrogenated terphenyl, type AA phthalate ester manufactured by R.P. Cargill Laboratories.

Not only does the photomask 30 have a long life expectancy, but it also requires little cleaning and retesting is similar in frequency. Also, deterioration of the metal pattern on the photomask due to electrostatic arcing occurring between portions of the pattern is also eliminated. This technique unexpectedly enhances the edge resolution of photoresists. The reason for these better-than-expected results is not clear, but one theory is that the index-matching fluid fills the relatively rough areas of the glass substrate that are formed when metal patterns are manufactured using the well-known etching method. It is said that Therefore, effectively eliminating such rough surfaces reduces the scattering of light passing through them, which has heretofore caused slight blurring of the image.

Thus, the present invention also provides the use of a mask to deposit a thin layer of index matching material having a refractive index similar to that of the base plate at least on its non-metallized surface. The matching material is applied by spraying, vapor deposition, or the like, depending on the composition of the material, the patterned metal, and the base plate.

In the projection printing device 10 (see FIG. 1), the cover plate 33 on the base plate 31 affects both the first-order and third-order aberrations of the optical device.
Installation requires some adjustment. The first-order effect of the glass cover plate 33 is to move the parallel axis image plane by an amount Δf expressed by the following equation.

Δf=(1-1/n')' (1) where t and n' are the thickness and refractive index of the cover plate 33, respectively. This movement is taken into account when refocusing the printing device. The focus shift dimension is also important in keeping the thickness of the cover plate 33 within tolerance. For example, since the depth of focus of the projection printer used is about 10 μm for feature dimensions of 3 μm to 4 μm, the thickness of the cover plate 33 should be flat to within 15 μm (Δt = Δf/n'). There must be. The actual desired tolerance is much narrower than this, since allowance must be made for variations in the plane of the coated semiconductor wafer 11. In addition, not only must the major surfaces of the cover plate 33 be substantially flat and parallel, but its thickness must also be uniform and within the above tolerances. Further, the distance between the cover plate 33 and the surface of the base plate 31 must be made as uniform as possible.

The main high-order aberration caused by the attachment of the cover plate 33 is spherical aberration, but ray tracing reveals that it is similar to the specific printer used here.
It can be seen that the influence of this aberration for an optical device with a numerical aperture of 0.1667 is very small. for example,
If the thickness of the cover plate 33 is 0.5 mm, the light spot size due to spherical aberration is at its most focused state.
0.1 μm, and the thickness of the cover plate 33
When it is set to 1.0 mm, the light spot size becomes 0.2 μm. These light spot dimensions are similar to the diffraction-limited light spot dimensions.
It is small compared to 1.25 μm. Spherical aberration also moves the plane of best focus away from the parallel-axis focal plane, thereby shortening the back focal length.

The only other effect caused by the cover plate 33 is that it slightly weakens the contrast of the image.
This results from light being reflected from the surface of the cover plate 33 and illuminating the chrome pattern 32.
If there is no cover plate 33, the glass pattern 3
The contrast of the chrome on 2 is 100%. When 4% is reflected from the surface of cover plate 33 and 100% from the chrome, the contrast is 92%.
However, since the contrast of the photoresist is strong, even if the contrast is weakened in this way, it does not affect the printing process much.

In a particular embodiment, a substantially defect-free fused silica plate measuring approximately 11.1 cm x 11.4 cm x 0.16 cm was initially selected to form the cover glass (plate) 33. This plate was then brazed onto the support member surface with a flatness of 1/4λ, followed by lap finishing and polishing to 1μm.
I did it within. Furthermore, remove the board from the flat support member,
It was inverted and brought into close contact with a 1/4λ optical flat. Glue the board to the optical flat by applying RTV silicone rubber adhesive to the edges of the board to reduce the thickness.
0.101cm±0.0013cm, maximum of 4μm on opposing major surfaces
Lap and polish again until they are parallel to each other and have a maximum flatness of 4 μm. The surface quality of both major surfaces must be approximately 20/10, as stated in the U.S. Military Standard MIL-O-13830.

It is preferable to assemble photomask 30 in a "clean indoor" environment to ensure that the interface between base plate 31 and cover plate 34 is defect-free.
The refractive index matching material 34 is made of two millipoles.
GVHP02500 Filtered through a 0.2μ filter, approx.
Degassing in a vacuum for about an hour to evacuate any contained dissolved gases and also to wash and dry the base plate 31 and cover plate 33. Next, the cover plate 33 is placed on the base plate 31 (see FIGS. 2 and 3) and pressed gently to release any air that may have accumulated between them. Tuff-bond the four corners of the cover plate 33 to the base plate 31 with epoxy adhesive and leave until the adhesive is completely dry. The resulting assembly is then heated to approximately 70° C. and one or more drops of index matching fluid 34 are applied to the interfacial edge 37 of one side of the cover plate 33. A fluid 34 is similarly heated to penetrate between the cover plate 33 and the base plate 31 by capillary action, filling the unpatterned volume between the plates, and creating a flow of fluid between the metal pattern 32 and the cover plate. Forms a thin layer. Additional fluid 34 as desired
may be added to fill the volume between the cover plate 33 and the base plate 31. The assembly may be heated at 70° C. overnight (eg, about 15 hours) to completely fill the volume.

After heating is discontinued and the assembly has cooled to room temperature, excess fluid 34 is removed with a cloth lightly dampened with acetone.
Next, the interface edge 37 is sealed by applying a thin layer of RTV silicone rubber sealant.

In another embodiment, a small amount of index matching fluid 34 is placed in the center of the base plate or cover plate before the base plate 31 and cover plate 33 are brought together. Next, fluid 34 is applied for several hours to about 70 to
It is heated to 100° C. to penetrate between the base plate 31 and cover plate 33. As before, a thin film of fluid 34 remains between metal pattern 32 and cover plate 33. If air bubbles are generated between the base plate 31 and the cover plate 33, it is necessary to selectively apply pressure to remove them. When all the bubbles are gone, the interface is sealed as described above.

A preferred embodiment of a light projection device 10 achieved by the technique of the present invention is shown in FIG. A photomask 30, shown in detail in FIG. 3, is placed between the ultraviolet light source 12 and the photoresist coated substrate 11. The light condensing device 13 is spaced apart from the substrate 11. In operation, light source 12 is activated to emit light onto patterned photomask 30 and photoresist coated substrate 1 .
1 is selectively exposed. Such a projection printing process increases edge resolution and makes the photoresist pattern virtually defect-free.

The present invention can be summarized as follows.

1. A photomask comprising a light-transmitting base plate having a metallized and non-metalized portion on its major surface, and a coating material applied to at least the non-metalized portion of the major surface and having a refractive index substantially the same as that of the base plate.

2. A photomask consisting of a translucent base plate having a metal pattern, a translucent cover plate having substantially parallel main surfaces and in close contact with the patterned base plate, and a refractive index matching material interposed between the base plate. .

3. The photomask according to item 2, wherein the patterned base plate, cover plate, and matching material have substantially the same refractive index.

4. The photomask according to item 2, wherein the surface of the cover plate is flattened to at least 4 μm and the major surfaces are parallel to each other by at least 4 μm.

5. The photomask according to item 2, wherein the interface edge between the cover plate and the base plate is sealed with a sealant to hold a refractive index matching material between both plates.

6. A step of closely contacting a substantially flat light-transmitting cover plate and a light-transmitting base plate having a metal pattern, a step of dripping at least one drop of refractive index matching fluid on the edge of the interface between the cover plate and the base plate, and the base plate. , heating a cover plate and an index matching fluid to flow the fluid between the patterned base plate and the cover plate.

7 Heat the base plate and cover plate at approximately 70℃.
7. The method for manufacturing a photomask according to item 6, wherein the photomask is heated to a temperature in the range of 100°C to 100°C.

8. (a) a translucent base plate having metallized and non-metallized portions on its major surface; and (b) a coating material applied to at least the non-metalized portion of its major surface and having a refractive index substantially the same as that of the base plate. a photoresist-coated substrate comprising the steps of: placing a photomask between a light source and the photoresist-coated substrate; and activating the light source to emit light onto the photomask to selectively expose the photoresist-coated substrate. A method of selective exposure to synchrotron radiation.

9. A photomask comprising (a) a transparent base plate having a metal pattern, (b) a transparent cover plate in intimate contact with the patterned base plate, and (c) an index matching fluid flowed between the base plate and the cover plate. selecting the photoresist-coated substrate as the emitted light, comprising the steps of: placing the photoresist-coated substrate between the light source and the photoresist-coated substrate; and activating the light source to emit light onto the photomask to selectively expose the photoresist-coated substrate. method of exposure.

10. The exposure method according to item 9, wherein the patterned base plate, cover plate, and matching material have substantially the same refractive index.

11 A translucent base plate having a thin metal pattern, a translucent cover plate having substantially parallel major surfaces and in close contact with the patterned base plate, and a refractive index matching fluid injected between the patterned base plate and the cover plate. A photo mask consisting of.

12. The photomask according to item 11, wherein the cover plate surface is planarized to at least 4 μm and the major surfaces are parallel to each other by at least 4 μm.

13. The photomask according to item 11, wherein the interface edge between the cover plate and the base plate is sealed with a sealant to maintain a refractive index matching fluid between both plates.

14 A step of closely contacting a substantially flat light-transmitting cover plate and a light-transmitting base plate having a metal pattern, a step of dripping at least one drop of refractive index matching fluid on the interface edge of the cover plate and the base plate, and a step of dropping the base plate. , heating a cover plate and an index matching fluid to flow the fluid between the patterned base plate and the cover plate.

15. The photomask manufacturing method according to item 14, wherein the base plate and the cover plate are heated to a temperature in the range of about 70° to 100°C.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a projection printing apparatus according to the prior art, FIG. 2 is an actual size diagram of a photomask according to the present invention, FIG. 3 is a cross section of a photomask according to the present invention, and FIG. The block diagram shown in FIG. 5 and FIG. 5 are schematic diagrams of a projection apparatus showing a preferred embodiment of the present invention. Explanation of symbols of main parts, 10...Light projection device, 11
...Semiconductor wafer, 12...Ultraviolet source, 13...Concentrator, 16...Photomask, 17...Transparent substrate, 18
...metal pattern, 19...non-metallized area, 30...photomask, 31...transparent base plate, 32...
Metal pattern, 33... Cover plate, 34... Refractive index matching fluid, 36... Binding material, 37... Interface edge.

Claims (1)

[Claims] 1. A base plate having a pattern and a flat cover plate form an assembly in close contact with each other, and a droplet of a refractive index matching fluid is applied to an edge of the interface between the base plate and the cover plate, and heating a refractive index matching fluid so that the refractive index matching fluid penetrates by capillary action into the gap between the base plate and the cover plate, and sealing the interface edge with a sealing material. How to make a photomask. 2. The method of manufacturing a photomask according to claim 1, wherein the heating is performed at a temperature in the range of 70°C to 100°C.