JP2863365B2 - Method of forming resist pattern - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art Conventionally, MESFETs and HEMTs have been used as low-noise amplifiers in the microwave band as represented by satellite broadcasting and satellite communication. To improve low-noise amplification characteristics, a gate length is required. It is indispensable to reduce the parasitic capacitance by shortening (the portion of the gate electrode in contact with the channel portion), and reduce the gate length, and provide a low-resistance gate electrode (a cross section of the electrode) in which a low-resistance portion is provided above the gate electrode. Is called a mushroom gate because it has a mushroom shape).

In order to form the mushroom gate, first, resists having low, high, and medium sensitivity to an electron beam are sequentially laminated on a semiconductor substrate, and a three-layer resist pattern is irradiated with the electron beam. By developing, the resist (lower layer) close to the substrate forms a pattern with a small line width suitable for the purpose of defining the gate length, and the second resist (middle layer) forms a pattern with a large line width. The uppermost resist (upper layer) is a pattern with a smaller line width than the middle layer resist, but forms a large line width for the purpose of forming the low resistance portion provided above the gate electrode, and forms the middle layer resist and the upper layer resist. In general, a method of forming an undercut resist pattern that is advantageous for lift-off and finally depositing a metal to be a gate electrode and lifting off is used. That. (Eg, IEDM Te
ch. Dig. , P613-616 (1983)) Further, it is necessary to further improve the low-noise amplification characteristics by further shortening the gate length and processing the low-resistance portion provided above the gate electrode to have a larger line width. . Therefore, as shown in Japanese Patent Application Laid-Open No. 02-275958, in the above-described three-layer resist process, a low-resistance portion is formed above the gate electrode and the gate length is further reduced by a method in which only development is performed in two stages. A method has been proposed.

[0004]

However, in the above-mentioned method, the controllability and stability of the opening of the upper resist are not sufficient.

[0005] First, the beam diameter of the electron beam is primarily intended to define the opening of the lower resist. Therefore, in the region corresponding to the opening size of the middle layer and the upper resist, the electron beam is used. There is little exposure contrast. In other words, in the method of increasing the opening size of the middle layer and the upper layer resist by using a more sensitive resist or using a more soluble developing solution, the size of the resist is increased by exposure to an electron beam. Essentially, controllability and stability cannot be sufficiently ensured.

Second, if the beam diameter of the electron beam is made smaller in order to make the opening size of the lower layer smaller, it becomes difficult to obtain a stable opening size of the middle layer and upper layer resist. Third, if the opening size of the middle layer is much larger than the opening size of the upper layer, the strength of the upper layer will be insufficient when depositing metal in the subsequent process, and the opening size of the upper layer will change during the deposition. There are also problems.

In the conventional method using a three-layer resist, strict control is required for all three layers, and all of these depend on the sensitivity of the resist. There is a problem that it is difficult to change the size, shape, and the like of the low resistance portion.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to enable precise control of the opening size of the middle and upper resist layers and to independently control the opening size of each layer. .

[0009]

In order to achieve the above object, the present invention comprises a resist whose sensitivity is selected so that the resist sensitivity of the middle layer is higher than that of the upper layer and higher than that of the lower layer. The upper, middle, and lower resist layers are sequentially laminated on a substrate, and these resist layers are irradiated with a predetermined electron beam pattern and developed, and the opening size of the middle layer has a cross-sectional shape larger than that of the lower and upper layers. In the resist pattern formed, after the development process, the portion of the upper resist projecting from the middle resist
By performing a heat treatment at or above the temperature at which the component softens, the opening in the upper layer resist is widened, and is equal to the opening in the middle layer resist.
Alternatively, it is characterized by being slightly smaller than that.

More specifically, a resist (lower layer) close to the substrate has a pattern formed with a desired small line width in contact with the channel portion of the gate electrode, and a second resist (middle layer) has a low resistance portion provided above the gate electrode. Is formed with a desired large line width or a slightly larger line width.

Here, the uppermost resist (upper layer) needs to have a pattern with a smaller line width than the middle layer resist, but its opening dimension does not need to be particularly controlled. here,
The resist constituting the upper, middle and lower layers is formed of a methacrylic acid-phenyl methacrylate copolymer resist.

In addition, the above-mentioned middle layer resist is subjected to a developing process.
High solubility in the developer regardless of the amount of electron beam exposure
The use of a solubility resist allows the opening of the middle layer resist.
The mouth dimensions controlled by the development time, the heat treatment step after the development processing, the middle register the opening dimension of the upper resist
It is characterized in that it is defined and prepared according to the opening size of the opening.

Further, when the resist pattern is irradiated with ultraviolet light having a wavelength of 180 to 350 nm before the heat treatment step, the amount of irradiation and the irradiation time are controlled to control the spread of the upper layer opening in the subsequent heat treatment step. It is characterized in that a change in the shape of other parts is suppressed.
The irradiation amount and the irradiation time of the ultraviolet ray are appropriately selected according to the change of the temperature and the time of the subsequent heat treatment step. Preferably, the ultraviolet ray intensity is 0.66 mW / cm ^2.
In this case, a time in which the resist is easily lifted off is selected in a time of about 5 to 15 minutes.

[0014]

In general, when a resist having a fine structure is kept at a temperature equal to or higher than the softening point, the resist is deformed so that the surface area of the pattern becomes small. In addition, the deformation of finer portions occurs quickly and significantly.

Therefore, in the present invention, the heat treatment causes the shape of the portion of the upper resist projecting from the middle resist to change rapidly and at the lowest temperature, and the desired resist shape is obtained.

By this heat treatment, as long as the opening size of the upper resist is a pattern having a line width smaller than that of the middle resist, the opening size of the middle resist or
It allows for self-aligned changes to somewhat smaller dimensions.

In addition, when the developing process is used together with the use of a developing solution that dissolves the middle layer resist regardless of the exposure to the electron beam, the opening size of the middle layer can be set by the developing time regardless of the exposure to the electron beam. As a result, the opening dimensions of the middle layer and the upper layer can be controlled independently of the electron beam diameter.

Furthermore, if the resist pattern is irradiated with ultraviolet light having a wavelength of 180 to 350 nm before the above heat treatment, a cross-linking reaction occurs between the molecules of the resist polymer. In other words, the change of the fine portion appears relatively larger.

Therefore, not only is the heat treatment temperature higher than in the case where no ultraviolet irradiation is performed, but if the shape change due to the heat treatment reaches a certain amount due to the irradiation amount of the ultraviolet light, the processing time is further extended. It was also found that even when the temperature was raised by about 10 ° C., the change in the resist shape further became extremely small. Therefore, the change in the resist shape during the heat treatment can be controlled by the irradiation amount of the ultraviolet light, and it is not necessary to set the precise heat treatment conditions.

[0020]

【Example】

<Example 1> Resist solution composed of the following three copolymers having different copolymerization ratios of methacrylic acid-phenyl methacrylate copolymer, resist for lower layer [low sensitivity; methacrylic acid component 25.4 mol%, methacrylic acid 74.6 phenyl component
Mol% (5% by weight methyl cellosolve acetate solution)], resist for middle layer [High sensitivity; 5.7 mol% of methacrylic acid component, 94.3 mol% of phenyl methacrylate component (8% by weight isoamyl ketone solution)], for upper layer A resist [low sensitivity; 20.0 mol% of methacrylic acid component, 80.0 mol% of phenyl methacrylate component (5% by weight ethyl cellosolve solution)] was prepared, and a 0.05 μm silicon nitride film was formed on the surface by plasma CVD. GaAs deposited
The s wafer was spin-coated with a lower layer resist at a rotation speed of 2300 rpm, and prebaked at 230 ° C. for 20 minutes. The thickness of the lower layer resist after baking was 0.12 μm.

Next, the intermediate layer resist is spin-coated at a rotation speed of 2000 rpm,
Pre-baking for minutes. The thickness of the resist for the middle layer after baking was 0.3 μm.

Next, the upper layer resist is spin-coated at a rotation speed of 5000 rpm,
Pre-baking for minutes. The thickness of the resist for the middle layer after baking was 0.1 μm.

The total thickness of the resulting three upper, middle, and lower resist layers was 0.52 μm, and no mixing was observed between the resists.

Next, an electron beam lithography system was used to apply three lines of resist on these substrates under the conditions of an acceleration voltage of 25 KV, an irradiation current of 0.8 nA, and a beam diameter of 0.03 μm. A line pattern was drawn under a condition of 0.8 nC / cm.

Next, a mixed solution of 75% of methyl isobutyl ketone and 25% of ethylcyclohexane was used at 23 ° C.
A development process was performed for 180 seconds, and a rinsing process was performed at 23 ° C. for 20 seconds using ethylcyclohexane to form a pattern on the wafer. When the cross-sectional shape of the formed pattern was observed with a scanning electron microscope, a resist pattern (FIG. 1) in which the opening size of the middle layer was larger than the opening size of the lower layer and the upper layer was obtained as shown in Table 1.

[0026]

[Table 1]

Next, each pattern was heated at 140.degree.
Heat treatment was performed at 45 ° C. and 150 ° C. for 10 minutes each, and the cross-sectional shape was similarly observed with a scanning electron microscope.

As a result, the heat treatment at 140 ° C. showed almost no change in the cross-sectional shape. In the heat treatment at 145 ° C., the cross-sectional shape was slightly angled as a whole, and the opening of the upper resist was widened (FIG. 2). In the heat treatment at 150 ° C., the protrusion of the upper resist from the middle layer disappeared, and the cross-sectional shape was greatly deformed. Under these conditions, it was found that the heat treatment had to be performed at a temperature of 150 ° C. or less. Table 2 shows the opening size of each layer at 145 ° C.

[0029]

[Table 2]

Using the resist pattern as a mask, the underlying silicon nitride film is etched with an aqueous solution of hydrogen fluoride, then aluminum is deposited in a thickness of 0.7 μm and lifted off with acetone to reduce the gate length of the mushroom. A gate could be created.

<Embodiment 2> A resist was applied and baked in the same manner as in Embodiment 1, and then, using an electron beam lithography apparatus, an acceleration voltage of 25 KV, an irradiation current value of 0.8 nA, and a beam diameter of 0.03 μm were used. Under these conditions, a line pattern was drawn under the condition of a line irradiation amount of 3.0 nC / cm.

Next, the wafer on which these electron beams have been drawn is subjected to a development treatment at 23 ° C. for 150 seconds using a mixed solution (low-solubility solvent) of 20% dimethylacetamide and 80% butyl cellosolve to form an upper layer and an intermediate layer. The upper surface of the lower layer was exposed by penetrating.

Thereafter, a mixed solution of 80% of methyl isobutyl ketone and 20% of ethylcyclohexane (a solvent that develops the lower resist and dissolves the middle resist regardless of electron beam exposure) at 23 ° C. for 30 seconds to 180 °
The developing process was performed by changing the developing time to seconds, and a rinsing process was performed at 23 ° C. for 20 seconds using ethylcyclohexane to form a pattern on the wafer.

The cross-sectional shape of the formed pattern was observed with a scanning electron microscope.
As shown in Table 3, the opening size of the middle layer changed, but the opening size of the lower layer hardly changed. In addition, the opening size of the upper layer varied but was very small.

[0035]

[Table 3]

Next, each pattern was heated at 140.degree.
Heat treatment was performed at 45 ° C. and 150 ° C. for 10 minutes each, and the cross-sectional shape was similarly observed with a scanning electron microscope.

As a result, as in Example 1, the heat treatment at 140 ° C. showed almost no change in the cross-sectional shape. In the heat treatment at 145 ° C., the cross-sectional shape was slightly angled as a whole, and the opening of the upper resist was widened. In the heat treatment at 150 ° C., the protrusion of the upper resist from the middle layer disappeared, and the cross-sectional shape was greatly deformed. Under these conditions, it was found that the heat treatment had to be performed at a temperature of 150 ° C. or less.

Table 4 shows the opening size of each layer at 145 ° C.

[0039]

[Table 4]

Using this resist pattern as a mask, the lower silicon nitride film is etched with an aqueous solution of hydrogen fluoride, then aluminum is deposited in a thickness of 0.7 μm and lifted off with acetone to form a mushroom gate having a short gate length. Could be created.

<Example 3> In the same manner as in Example 2, a resist was applied, baked, drawn with an electron beam and developed to form a pattern, and ultraviolet rays having a wavelength of 180 to 350 nm (light intensity: 0.66 mW / cm 2) were used. For 10 minutes at room temperature.

Next, each pattern was heated at 150.degree.
The heat treatment was performed for 0 minutes, and the cross-sectional shape was similarly observed with a scanning electron microscope.

As a result, almost no change was observed in the sectional shapes of the lower layer and the middle layer, and the expansion of the opening was observed only in the upper layer. (FIG. 3) Table 5 shows the opening dimensions of each layer.

[0044]

[Table 5]

Using this resist pattern as a mask, the underlying silicon nitride film is etched with an aqueous solution of hydrogen fluoride, and then aluminum is deposited to a thickness of 0.7 μm.
A mushroom gate with a short gate length could be formed by lifting off with acetone at ℃.

Further, 15
No further change in the cross-sectional shape was observed even after the heat treatment at 0 ° C. for 10 minutes was added.

Also, when heat treatment was performed at 160 ° C. for 10 minutes, there was no difference from the cross-sectional shape obtained by heat treatment at 150 ° C., indicating that this heat treatment step was also possible. That is, the usable temperature range in the heat treatment process is widened by the irradiation of the ultraviolet rays, which is advantageous in the process.

Further, a change in resist cross-sectional shape due to a difference in ultraviolet irradiation time was examined. When the ultraviolet irradiation time was 1 minute or less, the entire resist was deformed in the same tendency as when no ultraviolet irradiation was performed, but the degree was smaller than when no ultraviolet irradiation was performed.

When the ultraviolet irradiation time is 1 minute or more, almost no deformation of the middle layer and the lower layer is observed, and until the irradiation time is 5 minutes, the opening of the upper resist tends to be less likely to occur. Tended to open greatly.

Table 6 shows the change in resist shape after heat treatment depending on the irradiation time.

[0051]

[Table 6]

That is, regardless of the ultraviolet irradiation time,
A mushroom gate with a short gate length could be created.

[0053]

According to the present invention, it is possible to precisely control the opening size of the middle and upper layer resists and independently control the opening size of each layer, so that a desired shape can be easily formed. Becomes possible.

According to the present invention, when a low-resistance gate electrode having a mushroom-shaped cross section is formed, the size of the upper low-resistance portion can be freely set while reducing the size of the portion in contact with the substrate. The size and shape of the upper low-resistance portion can be changed independently of the opening size.

[Brief description of the drawings]

FIG. 1 is a cross-sectional shape after dissolving and spreading an intermediate-layer resist.

FIG. 2 is a cross-sectional shape of a resist whose upper layer has been expanded by heat treatment.

FIG. 3 is a cross-sectional shape of a resist in which an upper layer is widened by heat treatment after ultraviolet irradiation is performed on the shape of FIG. 1;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Upper layer resist 2 Middle layer resist 3 Lower layer resist 4 Silicon nitride 5 Semiconductor substrate

Claims (4)

(57) [Claims] The resist of the middle layer is made of a resist whose sensitivity is selected to be higher than the resist sensitivity of the upper layer and higher than the resist sensitivity of the lower layer. laminated, performs a predetermined electron beam pattern irradiation and development in these resist layers, in which to form a resist pattern having a cross-sectional shape opening dimension of the middle layer is larger than the lower layer and the upper layer, after the development processing, the Protruding from the middle layer resist of the upper layer resist
A method for forming a resist pattern, comprising: performing a heat treatment at a temperature higher than a temperature at which a portion is softened to widen an opening in an upper resist and make it equal to or slightly smaller than an opening in a middle resist. 2. The method according to claim 1, wherein the resist constituting the upper, middle, and lower three layers is a methacrylic acid-phenyl methacrylate copolymer resist. 3. The method according to claim 1, wherein the intermediate resist is used in a developing step.
High solubility that dissolves in the developer regardless of the amount of electron beam exposure
The opening size of the middle layer resist
3. The resist pattern according to claim 1, wherein the resist pattern is formed by controlling a method by a developing time , and defining a size of an opening of the upper resist by a size of the opening of the middle resist by a heat treatment step after the developing process. How to make. 4. The method according to claim 1, wherein the resist pattern is irradiated with ultraviolet rays having a wavelength of 180 to 350 nm before the heat treatment step.