JP2612933B2 - Method of forming resist pattern - Google Patents

Description

The present invention relates to a method for forming a resist pattern.
In particular, it is used for manufacturing low resistance gate electrodes for GaAs MESFETs and AlGaAs / GaAs HEMTs.

(B) Conventional technology Conventional GaAs MESFETs and AlGaAs / GaAs HEMTs have been used as low-noise amplifiers in the microwave band as represented by satellite broadcasting and satellite communications. To improve low-noise amplification characteristics, However, it is indispensable to shorten the gate length to reduce the parasitic capacitance. However, simply reducing the gate length increases the gate resistance, which does not improve the device characteristics, but rather deteriorates. A low-resistance gate electrode in which only a portion in contact with a channel is shortened and a low-resistance portion is provided above a gate electrode is used. Such a gate electrode is called a mushroom gate because of its mushroom cross section.

This mushroom gate is formed by sequentially laminating low-, high-, and low-sensitivity resists on a semiconductor substrate to form three layers and developing (etching) with one developer (solvent). The resist near the substrate (lower layer) forms a pattern with the desired line width, the second resist (middle layer) forms a pattern with a large line width, and the top resist (upper layer) is smaller than the middle layer resist. It is manufactured by forming a line width pattern and forming an undercut shape advantageous for lift-off with the middle resist and the upper resist. As the three-layer resist, a polymethyl methacrylate (PMMA) and a methyl methacrylate-methacrylic acid copolymer may be used, or (IED
M, Dig, Tech.Papers p613-616 (1983)), changing the copolymer composition of methacrylic acid-phenyl methacrylate copolymer to change the sensitivity to electron beams, and using a mixed solvent of methyl isobutyl ketone and ethylcyclohexanol. A method of developing (etching) has been proposed.

(C) Problems to be Solved by the Invention However, resists formed by changing the copolymer composition ratio of methacrylic acid-phenyl methacrylate copolymer on different substrates are appropriately laminated in three layers, and the amount of electron beam irradiation is increased. Was changed and developed (etched) with one developing solution (solvent), and the opening size was measured. As shown in Table 1, the minimum opening size depending on the type of the substrate, The opening size ratios of the upper resist and the lower resist were greatly different, and the opening sizes of the three resist layers could not be controlled. It is considered that the reason why the opening dimensions of the upper resist and the lower resist greatly differ depending on the substrate is that the distance and the ratio of reflected electrons reaching the resist surface differ depending on the substrate, and the exposure state of the upper resist differs. Also,
The lower opening size of the lower resist differs depending on the substrate.
It is considered that the state of electron beam reflection on the substrate surface is different,
Since the amount of reflected electrons is reduced on the substrate covered with the insulating film, it is necessary to increase the amount of electron beam irradiation necessary for dissolving the upper and middle layer resists. As described above, under an electron beam irradiation condition that is excessive for the lower-layer resist, the opening size of the lower-layer resist changes due to a slight difference in the development (etching) time and the development (etching) solution temperature, which causes a problem of poor reproducibility.

Furthermore, as shown in Table 2, The aperture ratio between the upper resist and the lower resist differs greatly depending on the beam diameter of the electron beam.If the beam diameter is small, the aperture dimensions of the upper resist and the lower resist are almost the same,
There is a problem that a mushroom gate with low resistance cannot be manufactured.

An object of the present invention is to solve the above-mentioned problems, and the size of the opening of each of the three upper, middle, and lower resist layers on the substrate depends on the material of the substrate used and the electron beam diameter. An object of the present invention is to provide a method for forming a resist pattern that can be controlled without being affected by the influence and can form, for example, a mushroom-shaped low-resistance gate electrode.

(D) Means for Solving the Problems According to the present invention, an upper / middle methacrylic acid-phenyl methacrylate copolymer resist layer having low, high, and low sensitivity to an electron beam is provided on a substrate.・ Laminating the lower three layers in order, irradiating these resist layers with an electron beam in a predetermined pattern, and then using a solvent having low solubility for the resist layer, the upper layer and the middle layer corresponding to the pattern are formed. Pre-etching, after that, by etching again the pre-etched portion of the upper layer and the middle layer using a solvent having high solubility for the resist layer and etching the lower layer, the opening size of the middle layer is smaller than that of the lower layer and the upper layer. A method of forming a resist pattern, comprising forming a resist pattern having an etched cross section that is large and has an opening dimension of an upper layer larger than that of a lower layer. It is.

In this invention, three layers of upper, middle, and lower layers of a methacrylic acid-phenyl methacrylate resist layer having low sensitivity, high sensitivity, and low sensitivity to an electron beam are sequentially laminated on a substrate. As the substrate, a compound semiconductor such as GaAs, AlGaAs, InP, or the like, and an insulating film such as silicon nitride or silicon oxide formed on these compound semiconductors can be used. The methacrylic acid-phenyl methacrylate copolymer resist having low sensitivity to the electron beam is usually 10
A copolymer containing 〜40 mol% of a methacrylic acid component and 90〜60 mol% of a phenyl methacrylate component is dissolved in a solvent such as methyl cellosolve acetate or ethyl cellosolve so as to have a concentration of usually 2 to 3% by weight. This solution is applied on the substrate by, for example, spin coating or the like, dried, prebaked, and usually 0.1
It can be formed as a lower layer with a thickness of about 0.2 μm. This low-sensitivity resist usually exhibits a glass transition temperature of 130 to 150 ° C., and is usually at least 250 μc / cm ² in order to be soluble in a dimethylacetamide / butyl cellosolve mixed solvent having a volume ratio of 20/80. This resist requires electron beam irradiation. The methacrylic acid-phenyl methacrylate copolymer resist having high sensitivity to the electron beam is usually a copolymer containing 4 to 8 mol% of a methacrylic acid component and 96 to 92 mol% of a phenyl methacrylate component,
For example, usually 5 to 8% by weight in a solvent such as isoamyl ketone.
The solution is applied to the lower layer by, for example, spin coating or the like, dried, pre-baked, usually to a thickness of 0.2 to 0.3 μm, and formed as an intermediate layer. it can. This highly sensitive resist is a resist which can be made soluble in the above-mentioned mixed solvent by irradiation of an electron beam of usually 50 μc / cm ² or less.
Further, a methacrylic acid-phenyl methacrylate copolymer resist having low sensitivity to an electron beam can be formed as an upper layer on the middle layer so as to have a thickness of usually 0.05 to 0.1 μm.

In the present invention, after irradiating these resist layers with an electron beam in a predetermined pattern, the upper layer and the middle layer corresponding to the pattern are pre-etched using a solvent having a low solubility for the resist layer. The pre-etched portions of the upper layer and the middle layer are again etched again using a solvent having high solubility for the resist layer, and the lower layer is etched so that the opening size of the middle layer is larger than that of the lower layer and the upper layer and the opening size of the upper layer. Forms a resist pattern having an etching cross section larger than that of the lower layer.
The electron beam is used to convert the resist layer in a region intended to be etched (dissolved) so as to be soluble in a predetermined solvent, and irradiates the resist layer in a predetermined pattern. Can be converted to be soluble in a solvent described below by lowering the molecular weight of the methacrylic acid-phenyl methacrylate copolymer. The resist layer irradiated with an electron beam in a predetermined pattern is first pre-etched using a low-solubility solvent, and then etched using a high-solubility solvent so that the opening size of the middle layer is lower than that of the lower and upper layers. And a resist pattern having an etching cross section in which the opening size of the upper layer is larger than that of the lower layer can be formed. This low-solubility solvent can be used, for example, by diluting a polar solvent such as an amide-based or sulfoxide-based solvent with a cellosolve-based or alcohol-based solvent, usually in a ratio of 1: 7 to 1:10. As the polar solvent, for example, amide solvents such as dimethylformamide, dimethylacetamide, N-methylpyrrolidone, and hexamethylphosphamide, dimethylsulfoxide, and the like can be used. Examples of the cellosolve-based solvent include ethyl cellosolve, diethyl cellosolve, cellosolve acetate, butyl cellosolve, dibutyl cellosolve, butyl cellosolve acetate, phenyl cellosolve, and hexyl cellosolve. Examples of the alcohol solvent include methanol, ethanol, n-propanol, and isopropanol. The pre-etching dissolves the middle layer after the low solubility solvent etches the upper layer according to the irradiation pattern of the electron beam, but even after the middle layer is dissolved, the lower layer is hardly etched, and the opening size of the lower layer is affected. Do not give. Further, since the lower layer is exposed, the lower layer can be etched with good reproducibility. As the high solubility solvent, for example, a single or mixed solvent such as a ketone type, a fatty acid ester type, an aromatic type solvent can be used, and a solvent such as an alcohol type or a hydrocarbon type is usually used in an amount of 20 to 40% by volume. You may mix and use. Examples of the ketone-based solvent include acetone, methyl ethyl ketone, and methyl isobutyl ketone. Examples of the fatty acid ester-based solvent include ethyl acetate and amyl acetate. Examples of the aromatic solvent include monochlorobenzene, toluene, and xylene. The alcohol-based solvent,
For example, methanol, isopropanol and the like can be mentioned. Examples of the hydrocarbon solvent include hexane and ethylcyclohexane. The highly soluble solvent is used for etching the pre-etched portions of the upper layer and the middle layer again and for etching the lower layer, and dissolves the middle layer regardless of the amount of electron beam irradiation. The effect of the difference in electron beam irradiation on the solubility in the upper, middle, and lower layers is pure, but the lower layer <upper layer≪middle layer acts to increase the solubility in this order. it can.

As described above, after the treatment with the solvent having low solubility and the solvent having high solubility, rinsing is usually performed. Examples of the rinsing liquid include alcohols, hexane, and aliphatic hydrocarbons such as ethylcyclohexane. be able to. By passing through the development step consisting of these low-solvent, high-solubility solvents and rinsing, the upper layer experiences two etchings, so that the opening size of the upper layer is wider than that of the conventional etching method. The opening size of the upper layer can also be controlled by the etching time using a low-soluble solvent and the etching time using a highly-soluble solvent. In addition, when the opening size of the upper layer is increased to dissolve the solvent in the middle layer with a low-solubility solvent, etching that does not affect the opening size of the lower layer and that the minimum opening size differs depending on the substrate on which the resist is based has been observed. As a result, a resist pattern having an etched cross section in which the opening size of the middle layer is larger than that of the lower layer and the upper layer and the opening size of the upper layer is larger than that of the lower layer can be formed.

In the present invention, for example, a conductive substance is deposited on the etched concave portion of the resist pattern by, for example, an electron beam evaporation method or the like to form a mushroom-shaped low-resistance ME.
An SFET gate electrode can be manufactured.

(E) Action A low-solubility solvent penetrates the upper layer and the middle layer irradiated with the electron beam to expose the upper surface of the lower layer corresponding to the intended opening size of the lower layer. The region of the middle layer, which cannot be dissolved by the low-solubility solvent, is dissolved to widen the opening size of the upper layer and the middle layer, and the lower layer is etched without being affected by the fluctuation of the amount of irradiation of the electron beam due to reflection of the substrate.

(F) Example A resist solution comprising the following three copolymers having different copolymerization ratios of methacrylic acid-phenyl methacrylate copolymer, lower layer resist (low sensitivity); 25.4 mol% of methacrylic acid component, phenyl methacrylate Ingredient 74.6 mol% (5% by weight
Methyl cellosolve acetate solution) Resist for middle layer (high sensitivity); methacrylic acid component content
5.7 mol%, phenyl methacrylate component 94.3 mol% (8
Wt% isoamyl ketone solution) Upper layer resist (low sensitivity) Methacrylic acid component content 2
0.0 mol%, phenyl methacrylate component 80.0 mol% (5
Wt% ethyl cellosolve solution), a substrate Si, a GaAs wafer, and a GaAs wafer with a 1000Å silicon nitride film deposited on the surface by plasma CVD. Prepare a lower layer resist on each substrate at a rotation speed of 2000 rpm. Spin coating was performed, and prebaking was performed at 230 ° C. for 30 minutes. 0.2μm lower layer resist thickness after baking
Met. Next, the intermediate layer resist was spin-coated at a rotation speed of 1000 rpm, and prebaked at 230 ° C. for 30 minutes. The thickness of the middle layer resist after baking is
It was 0.5 μm.

Next, the upper layer resist was spin-coated at a rotation speed of 5000 rpm, and prebaked at 230 ° C. for 30 minutes. The thickness of the upper layer resist after baking was 0.1 μm. The total thickness of the resulting three upper, middle, and lower resist layers was 0.8 μm, and no mixing was observed between the resists. The lower, middle and upper resist layers have low solubility (dimethylacetamide / butyl cellosolve = 20/80) and high solubility (methyl isobutyl ketone / ethylcyclohexane = 80/80) with respect to the amount of electron beam irradiation when forming each layer. 20) The residual film ratio after dissolution by the solvent was measured, and it was found that the film had the characteristics shown in FIGS. 1 and 2.

Next, an acceleration voltage of 25 KV and an irradiation current value of 1 × 10 ^{−9 were} applied to the three resist layers on these substrates using an electron beam lithography apparatus.
A, under the condition of a beam diameter of 500 mm, from a radiation dose of 1.5 nc / cm ²
The amount of electron beam irradiation was changed under the condition of 2.5 nc / cm ² to draw a line pattern.

Next, these electron beam-drawn wafers are developed at 23 ° C. for 180 seconds using a mixture of dimethylacetamide (20%) and butyl cellosolve (80%) (low solubility solvent) to penetrate the upper and middle layers. The upper surface of the lower layer is exposed to a small size corresponding to the opening size of the lower layer. Thereafter, development processing is performed at 23 ° C. for 120 seconds using a mixed solution (highly soluble solvent) of 80% methyl isobutyl ketone and 20% ethyl cyclohexane, and further, 23 ° C. and 30 ° C. are performed using ethyl cyclohexane.
A second rinsing process was performed to form a pattern on the wafer. When the cross-sectional shape of the formed pattern was observed with a scanning electron microscope, the opening size of each resist layer was almost the same regardless of the type of substrate and the amount of electron beam irradiation as shown in Table 3. It was confirmed that an open state was obtained.

(G) Effects of the Invention According to the present invention, the opening dimensions of each of the three layers of upper, middle, and lower resist layers on the substrate are controlled without being affected by the material of the substrate to be used or the electron beam diameter. For example, it is possible to provide a method for forming a resist pattern capable of forming a mushroom-shaped low-resistance gate electrode.

[Brief description of the drawings]

FIG. 1 and FIG. 2 are graphs showing the residual film ratio after dissolving each resist with the amount of electron beam irradiation of the resists for the upper, middle and lower layers used in the embodiment of the present invention.

Claims (1)

(57) [Claims] An upper, middle, and lower three layers comprising a methacrylic acid-phenyl methacrylate copolymer resist layer having low sensitivity, high sensitivity, and low sensitivity to an electron beam are sequentially laminated on a substrate. After irradiating the resist layer with an electron beam in a predetermined pattern, the upper layer and the middle layer at a position corresponding to the pattern are pre-etched using a solvent having a low solubility for the resist layer. By re-etching the pre-etched portions of the upper layer and the middle layer using a highly soluble solvent and etching the lower layer, the opening size of the middle layer is larger than the lower layer and the upper layer, and the opening size of the upper layer is larger than the lower layer. A method of forming a resist pattern, comprising forming a resist pattern having an etched cross section.