JPH04269756A - Resist film suitable for lift-off working - Google Patents

Abstract

PURPOSE:To obtain such a resist film useful for formation of a mushroom-shape gate electrode, which can be developed in an alkali soln. and has high sensitivity and high heat resistance. CONSTITUTION:The above resist film has a three-layer structure comprising a lower layer, intermediate layer comprising water-soluble polymer, and upper layer. The lower layer consists of a positive resist (A) of alkali developing type which responds to electron beams. The upper layer consists of a positive resist (B) of alkali developing type and may be same as or different from (A). It is preferable that (A) contains three components of novolac resin, bis(t- butylphenyl) iodonium fluoromethyl sulfonate, and 2,2-bis(t-butoxy carbonyl oxyphenyl) propane and that the intermediate layer has an antistatic effect. This resist film is useful for microfabrication of semiconductor elements.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to electron beam lithography, and more particularly to a resist film useful for forming mushroom-shaped gate electrodes of semiconductor devices.

0002

[Prior Art] Conventionally, field effect transistors (FETs)
In order to improve the high-frequency characteristics of semiconductor devices, the gate width is becoming smaller. However, when the gate width is 0.5 μm or less, the electrical resistance of the gate electrode becomes high and the high frequency characteristics deteriorate. In particular, for GaAs-based substrates, it is common to process gate electrodes by lift-off, but in order to reduce the resistance of gate electrodes due to miniaturization, attempts are being made to process gate electrodes with a so-called mushroom-shaped cross section. ing. Several methods are known to date for forming gate electrodes having such shapes. for example,
Electrode stacking method, which first forms the lower part of the electrode that defines the gate width, and then forms the upper low resistance part, a method of stacking two or more types of resist and performing electron beam writing, and a method of penetrating into a single layer resist. different focused ion beams (FIB)
There are methods of irradiating, etc. However, the method of laminating electrodes has the disadvantage that the process is extremely complicated. Further, the method using FIB requires a special drawing device and the method of overlapping is complicated. Compared to these methods, the method of laminating two or more types of electron beam resists has the advantage that a normal electron beam lithography apparatus can be used, lamination is easy and highly accurate, and the process is simple.

0003

[Problems to be Solved by the Invention] However, conventional laminated resists mainly use methacrylate resists, and when resists are laminated, resist intermixing tends to occur, heat resistance is insufficient, and sensitivity is low. However, there was no resist with high sensitivity, high heat resistance, and good pattern forming ability. Furthermore, since conventional laminated resists are developed with organic solvents, they tend to degrade the working environment, and a resist that can be developed with water or an aqueous solution has been desired. The purpose of the present invention is to solve the drawbacks of conventional laminated resists, namely, easy intermixing of resists, insufficient heat resistance, low sensitivity, and development with organic solvents, and develop with alkaline aqueous solutions. An object of the present invention is to provide a resist film useful for forming a mushroom-shaped gate electrode, which has features of high sensitivity and high heat resistance.

0004

[Means for Solving the Problem] To summarize the present invention, the present invention relates to a resist film having a three-layer structure. The present invention is characterized in that a neutral polymer is used as an intermediate layer, and an alkali-developable positive resist (B, however, B may be the same as or different from A) is used as an upper layer.

More specifically, the present invention relates to a resist for lift-off processing, in which an alkali developable resist capable of forming a positive pattern by electron beam irradiation and development is disposed in the lower and upper layers. However, it is a laminated resist in which a water-soluble polymer is arranged in the middle layer, and the lower layer resist is characterized by lower sensitivity than the upper layer resist. Furthermore, the positive resist mainly contains a resin containing many aromatic rings, such as novolac resin or phenol resin, and therefore has high heat resistance. In particular, since high resolution is required for the lower resist, it is desirable to use a chemically amplified resist. Conventional electron beam-sensitive positive resists containing novolac resins and diazonaphthoquinone photosensitizers have a disadvantage of low resolution, and cannot be used for processing gate electrode widths in the sub-half micron range. On the other hand, electron beam sensitive chemically amplified resists often contain metal as an acid generator, but are characterized by high resolution. In particular, metal-free chemically amplified resists have been developed recently, and because they have high resolution, they are particularly suitable for the lower layer resist of the present invention. On the other hand, since high resolution is not required for the upper layer resist, it may be a chemically amplified resist or a conventional resist that is not chemically amplified. However, in order to form a mushroom-shaped resist pattern, it is desirable that the upper resist has higher sensitivity than the lower resist, and that the sensitivity is significantly different.

The sensitivity of chemically amplified resists varies greatly depending on the prebaking conditions, and post exposure bake heat treatment (post exposure bake) to advance chemical amplification before development is required.
, hereinafter abbreviated as PEB), the sensitivity and resolution vary greatly. By changing the composition of the upper and lower resists, it is possible to meet the requirements for sensitivity and resolution, or by setting the pre-bake of the lower resist to low-sensitivity conditions and the pre-bake of the upper resist to high-sensitivity conditions. It is also possible to form a multilayer resist film using the same resist solution by setting . Furthermore, as a method of lowering the sensitivity of the lower resist layer, it is also effective to add an appropriate amount of a base that traps an acid to the lower resist layer to adjust the sensitivity. Also,
Chemically amplified resists have the characteristic that a poorly soluble layer is formed on the resist surface depending on the material composition of the resist and the baking conditions, which tends to result in an overhang-like pattern. Since the pattern shape can be controlled by changing the material and baking conditions, if such a chemically amplified resist is used as the upper layer resist, it has the advantage that it is possible to easily form an overhanging pattern suitable for lift-off.

Conventional chemically amplified positive resists had the disadvantages of containing metals and being subject to large changes over time, but these were overcome by the metal-free, high-resolution positive resist developed by the present inventors. Mold resist (patent application Hei 2-235564)
This problem can be solved by using the following method, which is further useful in the present invention. The positive resist includes novolak resin (C), bis(t-butylphenyl)iodonium trifluoromethylsulfonate (D)
, and 2,2-bis(t-butoxycarbonyloxyphenyl)propane (E). More specifically, the novolac resin contains less than 20% p-cresol as a cresol structure, and the weight fraction is 0.03≦D≦0.08, 0.12≦E
A composition in which ≦0.25 and C+D+E=1 has particularly high resolution, and is preferable for the lower layer resist of the present invention, which requires high resolution.

[0008] The intermediate layer is used to avoid intermixing of the upper resist layer and the lower resist layer. The coating solvent for the upper and lower layers is an organic solvent that is immiscible with water, such as 2-ethoxyethyl acetate, methyl isobutyl ketone, ethyl lactate, or propylene glycol monomethyl ether. Therefore, the intermediate layer is coated using a solvent, such as water, that does not dissolve the upper and lower resist layers. In this case, it is particularly preferable that the intermediate layer has an antistatic effect. When forming a pattern using electron beam writing on a silicon substrate, charge-up on the substrate is often not a problem, but on insulating substrates such as SiO2 or GaAs, the pattern is easily deformed due to charge-up, making drawing difficult. Preventing charge-up is a serious issue since positional accuracy is reduced. Conventionally, a metal conductive film such as aluminum was deposited on the resist surface, but recently methods of applying an organic conductive film have been actively researched. The upper and lower resist layers used in the present invention are insulating materials and have no antistatic effect. Therefore, if a material with a charge-up prevention function is used in the intermediate layer, there is no need to deposit an antistatic metal film or apply an organic conductive film, resulting in a simple process with high performance. It has the advantage of being able to form patterns with precision. Since water is used as the coating solvent, the electrical conductivity of the intermediate layer is higher than that of ordinary insulating materials, but the residual moisture in the film changes depending on the pre-baking conditions, and the electrical conductivity changes accordingly. . Therefore, materials that improve electrical conductivity can be used as intermediate layer materials, such as organic salts such as ammonium chlorate and ammonium p-toluenesulfonate, organic acids such as p-toluenesulfonic acid and acetic acid, or materials with carboxyl groups or When a water-soluble polyether having a sulfone group is added, an antistatic effect can be effectively exhibited.

[0009]

[Examples] The present invention will be explained in more detail with reference to Examples below, but the present invention is not limited to these Examples.

Example 1 Ortho-novolak resin
73.5 parts by weight 2,2-bis[p-(t-butoxycarbonyloxy)phenyl]propane

20 parts by weight Bis[p-(t-butyl)phenyl]iodonium trifluoromethylsulfonate 6.
2-Ethoxyethyl acetate 17% containing 5 parts by weight of 3 components
The solution was spin-coated onto a silicon substrate and prebaked at 100° C. for 90 seconds on a hot plate. Film thickness is 0.3
It was μm. As an intermediate layer, a polyvinylpyrrolidone aqueous solution was spin-coated to a thickness of 0.1 μm. After that, the 3rd
Using a 25% solution of the components, the film thickness is 1μ as the upper layer resist.
It was applied to m. Prebaking was performed at 65° C. for 120 seconds under a nitrogen atmosphere. After drawing with an electron beam at an accelerating voltage of 30 kV, PEB was performed at 85° C. for 3 minutes. The lower resist opening was drawn at 26 μC/cm 2 and the upper resist opening was drawn at 14 μC/cm 2 . 2 with a 1.5% aqueous solution of tetramethylammonium hydroxide (TMAH).
Developed for 1 minute and rinsed with water for 30 seconds. It was possible to form a pattern with openings in the lower layer having a diameter of 0.2 μm and openings in the upper layer having a diameter of 0.9 μm.

Example 2 In Example 1, polyvinyl alcohol was used as the intermediate layer. Pattern formation was possible in the same manner as in Example 1. However, perhaps because polyvinyl alcohol easily gels with acid catalysts, the intermediate layer tends to remain on the film.

Example 3 Ortho-novolak resin
73.5 parts by weight 2,2-bis[p-(t-butoxycarbonyloxy)phenyl]propane

20 parts by weight Bis[p-(t-butyl)phenyl]iodonium trifluoromethylsulfonate 6.
5 parts by weight triphenylamine

Using a solution of 2-ethoxyethyl acetate containing 1 part by weight, the lower resist layer was coated to a thickness of 0.3 μm.
Prebaked at 0°C for 90 seconds. The intermediate layer was formed by spin coating an aqueous solution of polyvinylpyrrolidone. As the upper layer resist, WX242 (Olin Hunt) was used at 0.7 μm.
It was applied to a thickness of m. Prebaking was performed at 85°C for 90 seconds. 35 μC/c as lower resist opening by electron beam lithography
m2, 18μC/cm2 as upper layer resist opening
exposed to light. PEB was performed at 65° C. for 3 minutes and developed. The pattern opened up nicely.

Example 4 In Example 3, an intermediate layer was formed using a polyvinylpyrrolidone aqueous solution to which 5% ammonium chlorate was added. Pattern formation was carried out on a GaAs substrate, and it was confirmed that the pattern could be formed well and that the antistatic effect of the intermediate layer was effective.

Example 5 In Example 3, an intermediate layer was formed using a polyvinylpyrrolidone aqueous solution to which 5% p-toluenesulfonic acid was added. A pattern was formed on a GaAs substrate, and it was confirmed that the antistatic effect of the intermediate layer was effective.

[0015]

Effects of the Invention The positive resist film of the present invention is sensitive to high-energy rays, has higher sensitivity and resolution than the conventional ones, and has excellent heat resistance. A pattern suitable for wiring processing can be easily formed, and it is useful for microfabrication of semiconductor devices.

Claims (1)

[Claims] Claim 1: An alkali-developable positive resist (A) sensitive to electron beams is used as a lower layer resist, a water-soluble polymer is used as an intermediate layer, and an alkali-developable positive resist (B, where B is A).
(which may be the same as or different from) is disposed as an upper layer.