Method of making a semiconductor device

Abstract: A method of manufacturing a semiconductor device capable of manufacturing a semiconductor device by using an inexpensive apparatus such as an i-line stepper with a high throughput and a high yield at a low cost is provided wherein the method includes a step for forming a first layer (3) of an insulative film on a semiconductor substrate (1), a step of coating a second layer (4) of a resist for a gate patterning on the first layer (3) of the insulative film, a step of forming a pattern having a desired gate length on the second layer (4) of the resist, a step of transcribing a gate pattern (5) in the first layer (3) of the insulative film by an anisotropic etching and a step of patterning a cap of a mushroom-type gate electrode (7) by coating a third layer of a resist (6) for forming the electrode (7).

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a method of making a semiconductor device, particularly to a method of making a gate electrode of a field-effect transistor. DISCUSSION OF BACKGROUND Generally, it is necessary for improving high-frequency characteristic of a field-transistor (FET) to reduce a gate resistance (Rg), a source resistance (Rs) and a source-gate capacitance (Cgs) and to promote a mutual conductance (gm). It is effective for reducing the source-gate capacitance (Cgs) and promoting the mutual conductance (gm) to shorten the length of the gate. Further, it is effective to provide a mushroom structure of the gate to prevent the gate resistance from increasing by shortening the length of the gate. Conventionally, a FET including a mushroom type gate electrode having the length of the gate in a sub half micrometer order has been realized by an electron beam (EB) exposure device etc. An outline of manufacturing steps are shown in Figure 13 and Figure 14. In these Figures a two-stage exposure is performed by using an electron beam 23a and an electron beam 23b of which intensities are different to form a post 27a and a cap 27b of a mushroom-type gate electrode 27 in a resist 22 for electron beam coated on the surface of a semiconductor substrate 21. Next, the resist 22 is developed, a mushroom shape 23 is formed in the resist 22 on which a gate metal is deposited by evaporation and the resist 22 is lifted off by which the mushroom-type gate electrode 27 is provided as shown in Figure 14. However, the conventional EB exposure device is provided with a poor throughput and a high apparatus cost and therefore, the product cost is enhanced. A method to form a mushroom-shaped gate electrode after the formation of a gate pattern in a photoresist is described in JP-A-6 104 285. A Γ-shaped gate electrode is known eg from EP-A-0 591 646. SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to provide a method of making a semiconductor device capable of manufacturing with a high throughput and a high yield at a low cost by an inexpensive apparatus such as an i-line stepper. It is another object of the present invention to provide a method of making a semiconductor device having an improved element characteristic capable of accurately forming a gate electrode by reducing a dispersion of dimensions by rendering a thickness of a first layer of an insulative film formed in a Schottky gate electrode forming region approximately equal to thicknesses of a source electrode and a drain electrode in an ohmic contact. In the following the means of solving the problems are described. According to a first aspect of the present invention, there is provided a method of making a semiconductor device having a Schottky gate on a semiconductor substrate according to appended claim 1. The method includes: 1. a step of forming a first layer of an insulative film on a semiconductor substrate, 2. a step of coating a second layer of a resist for performing a gate patterning on the first layer of the insulative film, 3. a step of forming a gate pattern having a desired gate length on the second layer of the resist, 4. a step of transcribing the gate pattern in the first layer of insulative film by an anisotropic etching, and 5. a step of coating a third layer of a resist for forming a mushroom-type gate electrode and patterning its cap. According to a second aspect of the present invention, there is provided a method of making a semiconductor device having a Schottky gate electrode on a semiconductor substrate as set out in appended claim 1. It includes: 1. a step of forming a first layer of an insulative film on a semiconductor substrate, 2. a step of coating a second layer of a resist for a gate patterning on the first layer of the insulative film, 3. a step of forming a gate pattern having a desired gate length on the second layer of the resist, 4. a step of transcribing the gate pattern on the first layer of the insulative film by an anisotropic etching, and 5. a step of coating a third layer of a resist for forming a Γ-type gate electrode of which post is proximate to a side of a source electrode and of which eaves is provided on a side of a drain electrode and patterning the eaves dislocated on the side of the drain electrode in the third layer of the resist. In-the present invention exposure is performed on the first layer of the insulative film that has been flattened and therefore, a sub half micrometer gate electrode can stably be formed with a high throughput. Further, it is not necessary to remove the second layer of the resist in later steps and therefore, the steps can be shortened and the manufacturing cost can be reduced. The mushroom-type gate electrode or the Γ-type gate electrode can be formed by a single vapor deposition and therefore, the steps can be shortened, the cost for used material can be restrained and the manufacturing cost can be reduced. Further, the cap of the mushroom-type gate electrode or the eaves of the Γ-type gate electrode is separately exposed and therefore, the cap or the eaves can be formed offset from the post of the gate electrode, the interval between the source and the gate can be narrowed while maintaining low the gate resistance (Rg) and the source-gate capacitance (Cgs) and the source resistance (Rs) can be reduced. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a view showing a step of forming an an ohmic electrode on a semiconductor substrate illustrating a first embodiment (Figure 1 through Figure 8) of the present invention; Figure 2 is a view showing a step of forming a first layer of a resist and flattening it; Figure 3 is a view showing a step of forming a second layer of a resist; Figure 4 is a view showing a step of exposing a gate pattern on the second layer of the resist and developing it; Figure 5 is a view showing a step of transcribing the gate pattern in the first layer of the resist while etching the second layer of the resist; Figure 6 is a view showing a step of reflowing the resist for rounding corners of the gate pattern; Figure 7 is a view showing a step of coating a third layer of a resist for forming a cap of a mushroom-type gate electrode and exposing and developing it; Figure 8 is a view showing a step of forming the mushroom-type gate electrode by depositing a gate metal by evaporation and by lifting off the first layer of the resist, and third layer of the resist and unnecessary metals thereon; Figure 9 is a view showing a step of exposing and developing a gate pattern deviated on the side of a source electrode of a second layer of a resist illustrating a second embodiment (Figure 9 through Figure 12) of the present invention; Figure 10 is a view showing a step of etching the second layer of the resist and at the same time transcribing the gate pattern on a first layer of a resist under an anisotropic etching condition; Figure 11 is a view showing a step of coating a third layer of a resist for forming a Γ-type gate electrode and exposing and developing an opening dislocated on the side of the drain electrode with respect to the gate pattern; Figure 12 is a view showing a step for forming the Γ-type gate electrode 7 in which an eaves is formed on the side of a drain electrode by depositing a gate metal by evaporation and by lifting off the first layer of the resist, the third layer of the resist and unnecessary metals thereon; Figure 13 is a view showing a step of exposing by an electron beam a mushroom-type gate electrode in a resist for the electron beam in a conventional manufacturing method (Figure 13 and Figure 14); and Figure 14 is a view showing a step of forming the mushroom-type gate electrode by depositing a gate metal by evaporation and lifting off the resist. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An explanation will be given of a first embodiment of the present invention in reference to Figure 1 to Figure 8 as follows. As shown in Figure 1, numeral 1 designates a semiconductor substrate and an activated layer (not shown) is formed on the surface of the semiconductor substrate 1. A source electrode 2a and a drain electrode 2b are formed on the semiconductor substrate 1 on which the activated layer has been formed, in an ohmic contact therebetween. As shown in Figure 2, a first layer of an insulative layer 3 of a resist having a thickness that is equal to or more than those of the source electrode 2a and the drain electrode 2b is spincoated in a gate electrode forming region between the source electrode 2a and the drain electrode 2b and is flattened. Further, the first layer of the insulative film 3 is etched back by reactive ion etching (RIE), a milling apparatus etc. As shown in Figure 3 a second layer of a resist 4 for gate patterning is spincoated. In this case, the film thickness of the second layer of a resist 4 is set to a film thickness showing an excellent exposure sensitivity for an i-line stepper. Further, to avoid mixing of the first layer of the resist 3 with the second layer of the resist 4, a treatment by baking at high temperatures, for example, 200°C or by CF4 plasma may be performed on the first layer of the insulative film 3. As shown in Figure 4 a gate pattern 5 is exposed on the second layer of the resist 4 and is developed. As shown in Figure 5 the gate pattern 5 is transcribed in the first layer of the insulative film 3 while etching the second layer of the resist 4 under an anisotropic etching condition. With regard to the etching condition an anisotropic etching utilizing accumulated substances on the side wall may be used. For example, it is an etching using a fluorocarbon group gas by RIE. As shown in Figure 6 a resist reflow (heating in an oven at 200°C for 30 minutes) is performed to round corners 3b of the gate pattern 5 of the first layer of the insulative film 3. These steps are performed to avoid disconnection by rounding a portion of a mushroom-type gate electrode in connection with a stand and a cap thereof (which corresponds to the corner portions 3b which are to be formed). As shown in Figure 7 a third layer of a resist 6 is coated for forming the cap of the mushroom-type gate electrode. An opening 6a is formed by performing exposure and development. Eaves 6b may be formed by chlorobenzene treatment or a multi-layer resist method to facilitate the lifting-off. As shown in Figure 8 a gate metal is deposited by evaporation and the first layer of an insulative film 3 and the third layer of a resist 6 and unnecessary deposited metals thereon are lifted off thereby forming a mushroom-type gate electrode 7. Further, in the step illustrated in Figure 2 of the first embodiment, a silicon nitride (SiNx) film or a silicon oxide (SiO2) film having a thickness approximately equal to those of the source electrode 2a and the drain electrode 2b may be formed in the gate electrode forming region as the first layer of the insulative film 3 composed of a single layer or a multi-layer structure by a film forming means such as sputtering, vapor deposition, chemical vapor deposition (CVD) etc. Next, an explanation will be given of a second embodiment in reference to Figure 9 through 11. Generally, an interval between a source and a gate is often made narrower than that between a drain and the gate to improve noise characteristic. In this case, in a gate electrode having the mushroom-type structure the interval between a one-side portion of a cap of the mushroom-type gate electrode and a source electrode becomes narrow whereby the source-gate capacitance (Cgs) may not be negligible. Accordingly, this embodiment solves the problem by providing a Γ-type gate electrode in which a source side portion of a cap of the mushroom-type gate electrode is dispensed with and an eaves is provided only on the side of the drain. The steps illustrated in Figure 1 through Figure 3 in the first embodiment are the same as in the second embodiment and therefore, the explanation will be omitted while attaching the same notations to the elements and an explanation will be given to the steps thereafter. As shown in Figure 9 exposure and development of a gate pattern 15 is performed at a portion of the second layer of the resist 4 that is deviated to the source electrode 2a. As shown in Figure 10 the second layer of the resist 4 is etched and at the same time the gate pattern 15 is transcribed in the first layer of the insulative film 3 under an anisotropic etching condition. As shown in Figure 11 a third layer of a resist 16 is coated for forming a Γ-type gate electrode. Further, an opening 16a that is dislocated to the side of the drain electrode 2b with respect to the gate pattern 15 is exposed and developed. Eaves 16b may be formed by a chlorobenzene treatment or a multi-layer resist method to facilitate the lifting-off. As shown in Figure 12 a gate metal is deposited by evaporation. Further, the first layer of the insulative film 3 and the third layer of the resist 16 and unnecessary deposited metals thereon are lifted off thereby forming a Γ-type gate electrode 17 having an eaves 17a on the side of the drain electrode. In the present invention a sub half micrometer gate electrode can stably be formed with a high throughput since exposure is performed on the flat first layer of the insulative film. Further, it is not necessary to remove the second layer of the resist in later steps and therefore, the steps can be shortened and the manufacturing cost can be reduced. Further, a mushroom-type gate electrode or a Γ-type gate electrode can be formed by a single step of vapor deposition and therefore, the steps can be shortened, used material can be saved and the manufacturing cost can be reduced. Further, the cap of the mushroom-type gate electrode or the eaves of the Γ-type gate electrode is separately exposed and therefore, the interval between the source and the gate can be narrowed and the source resistance (Rs) can be reduced while maintaining low the gate resistance (Rg) and the source-gate capacitance (Cgs) by offsetting the cap or the eaves with respect to the post of the gate electrode.

Claims: A method of making a semiconductor device having a Schottky gate electrode on a semiconductor substrate (1) comprising the steps of: forming a first layer (3) of an insulative film on the semiconductor substrate (1); forming a gate pattern (5) having a desired gate length in the first layer (3) of the insulating film; coating a third layer (6) of a resist; forming an opening (6a) in the third layer (6) of a resist such that the opening (6a) is placed on the gate pattern (5); and forming a mushroom-type gate electrode (7) and patterning a cap portion of the mushroom-type gate electrode (7) by depositing a gate metal on the third layer (6) of a resist and in the opening (6a); characterized in that a source electrode (2a) and a drain electrode (2b) is formed on the semiconductor substrate (1) before forming the first layer, the first layer (3) being formed between the source electrode (2a) and the drain electrode (2b) so that the first layer (3), the source electrode (2a) and the drain electrode (2b) form a flat surface; and the step of forming a gate pattern (5) comprises the steps of: coating a second layer (4) of a resist for a gate patterning on the first layer (3) of the insulative film; forming the gate pattern (5) having a desired gate length on the second layer (4) of a resist; and transcribing the gate pattern (5) in the first layer (3) of the insulative film by an anisotropic etching. A method of making a semiconductor device having a Schottky gate electrode on a semiconductor substrate (1) comprising the steps of: forming a first layer (3) of an insulative film on the semiconductor substrate (1); forming a gate pattern (15) having a desired gate length in the first layer (3) of the insulating film; coating a third layer (16) of a resist; forming an opening (16a) in the third layer (16) of a resist such that the opening (16a) is placed on the gate pattern (15); and forming a Γ-type gate electrode (17) in said opening (16a) whereby the post is proximate to a side of a source electrode (2a) and of which eaves (17a) are provided on a side of a drain electrode (2b) and patterning the eaves (17a) dislocated to the side of the drain electrode (2b); characterized in that the source electrode (2a) and the drain electrode (2b) are formed on the semiconductor substrate (1) before forming the first layer (3), the first layer (3) being formed between the source electrode (2a) and the drain electrode (2b) so that the first layer (3), the source electrode (2a) and the drain electrode (2b) form a flat surface; and the step of forming the gate pattern (15) comprises the steps of: coating a second layer (4) of a resist for a gate patterning on the first layer (3) of the insulative film; forming the gate pattern (15) having a desired gate length on the second layer (4) of a resist; and transcribing the gate pattern (15) in the first layer (3) of the insulative film by an anisotropic etching. The method of making a semiconductor device according to Claim 1 or 2, wherein the first step comprises: a step of forming a first layer (3) of an insulative film comprising one selected from the group consisting of a silicon nitride film (SiNx), a silicon oxide film (SiO 2 ) and a single layer or a multi-layer structure of a resist on the semiconductor substrate (1).

Inventors: YOSHIDA KAZUHIRO, YOSHIDA, KAZUHIRO, Yoshida, Kazuhiro, c/o Murata Manufac. Co., Ltd.
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1995
Main classification: H01L21
IPC codes: H01L29, H01L21