JPH0410549A - Manufacture of field-effect transistor - Google Patents

Abstract

PURPOSE:To contrive to prevent the deterioration of a breakdown voltage by a method wherein source, drain and gate electrodes are formed, an oxide layer is formed on the surface of a semiconductor layer exposed between the source and drain electrodes by a heating treatment under an oxidizing atmosphere and a dielectric film is formed on the surface of the oxide layer. CONSTITUTION:An active layer 2 consisting of a conductive GaAs epitaxial layer is formed on a semi-insulative substrate 1 consisting of a GaAs semiconductor single crystal and source and drain electrodes 3 and 4, which are ohmic- junctioned to the layer 2, are formed. Then, a P-type resist film 5 is formed on the whole surface of the substrate 1. A metal layer 6 is formed on the film 5 and on an opening part 5', through which the layer 2 is exposed. The film 5 is dissolved are removed, the layer 6 other than the layer 6 on the opening part 5' is removed and a gate electrode 7 equivalent to the region of the opening part 5' is formed. Then, a heat treatment is performed on the electrodes 7, 3 and 4 and on the surface of the substrate 1 including the region of the layer 2 exposed between the electrodes 3 and 4 in an oxygen atmosphere. After that, a silicon nitride film 8 is formed on the whole surface of the layer 2 as a dielectric film for protection use by a plasma CVD method.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing a field effect transistor having a gate electrode forming a Schottky junction, and in particular,
The present invention relates to a method of forming a protective dielectric film (passivation film) on a compound semiconductor.

[Prior Art] Field-effect transistors (hereinafter referred to as FETs), which have a gate electrode that forms a Schottky junction on a compound semiconductor such as GaAs, have excellent high-speed operation and are often used as amplification elements in the microwave band. ing. It is necessary to form a protective dielectric film made of silicon oxide, silicon nitride, etc. on the surface of this FET in order to protect the semiconductor surface and electrodes.

Conventionally, the gate electrodes of these FETs are formed by forming a mask (made of a resist film or an insulating film such as silicon oxide) on the surface of a semiconductor with an opening corresponding to the gate electrode, and then depositing metal over the opening and the mask. It is formed by a lift-off method in which a film is formed, and then the metal film on the mask is removed by removing the mask, and the metal film is formed only in the openings. Then, the protective dielectric film is formed by a sputtering method or the like after forming the gate electrode by a lift-off method.

[Problems to be Solved by the Invention] However, it is known that when fabricated using the above conventional method, the breakdown voltage (gate-source electrode breakdown voltage) of the FET deteriorates due to the formation of a protective dielectric film. The present invention solves the above-mentioned drawbacks, and an object of the present invention is to provide a method for forming a protective dielectric film in which the breakdown voltage does not deteriorate.

[Means and effects for solving the problem] The present invention is based on the idea that deterioration of FET characteristics can be prevented by performing some kind of treatment on the gate electrode and semiconductor surface before forming a protective dielectric film. It is something.

The present invention provides a method for manufacturing a field effect transistor including a source electrode, a drain electrode, and a gate electrode forming a Schottky junction on a semiconductor, including a first step of forming the source electrode and the drain electrode; a second step of forming an electrode; a third step of forming an oxide layer on the surface of the semiconductor exposed between the source electrode and the drain electrode by heat treatment or plasma discharge treatment in an oxidizing atmosphere; The fourth step is to form a dielectric film on the surface.

Desirably, in the third step, the plasma discharge treatment or heat treatment is performed until the thickness of the oxide layer is saturated. Further, the plasma discharge treatment is performed in an atmosphere of oxygen or a compound gas forming an oxidizing atmosphere.

Although the effect of the present invention is not clear, it is thought that since the exposed semiconductor surface of the FET is covered with a stable oxide, stable operation is possible if a dielectric film is subsequently formed.

[Example] The manufacturing process of an FET which is an example of the present invention is as follows:
This will be explained below using Figures (a) to (c).

0 on a substrate 1 made of a semi-insulating GaAs semiconductor single crystal.
．． An active layer 2 is formed of a conductive GaAs epitaxial layer having a thickness of about 3 μm. A source electrode 3 and a drain electrode 4 are formed in ohmic contact with this active layer 2. Next, a positive resist film 5 made of an organic polymer is formed on the entire surface of the substrate l. An opening 5' having a width of 1 μm is formed between the source electrode 3 and the drain electrode 4 of this resist film 5 by ordinary photolithography. Then, a metal layer 6 is formed on the resist film 5 and on the opening 5' where the active layer 2 is exposed. (Figure 1(a)
) The resist film 5 is dissolved and removed, and the metal layer 6 other than the opening 5' is removed.
By removing the gate electrode 7, the gate electrode 7 corresponding to the area of the opening 5'' is formed. (Lift-off method) Heat treatment is performed on the surface of the substrate 1 including the exposed active layer 2 region between 4 and 4 in an oxygen atmosphere. The relationship between the heat treatment time and the thickness of the oxide layer (measured by ellipsometry) is shown in Figure 2.As is clear from the figure, the thickness of the oxide layer is 1.5nm, but by heat treatment for more than 1 hour, it becomes 1.5nm.
．． The film thickness is approximately 6 nm.

Even if the heat treatment is continued for a longer time, the film thickness remains saturated.
It can be seen that there is no increase. Note that the atmosphere at this time is not limited to the air, but may be any oxidizing atmosphere containing oxygen.

Thereafter, a silicon nitride film 8 (SiN) having a thickness of 1100 nm is formed on the entire surface of the active layer 2 by plasma CVD. This plasma CVD method is performed by heating the substrate 1 to 230° C. and reacting silane (S i H,) gas and nitrogen gas for about 8 minutes.

Note that as the protective dielectric film, in addition to the silicon nitride film, a dense insulating film such as a silicon oxide film can be used.

Silicon nitride film 8 on source electrode 3 and drain 11t'M4 is partially removed to form wiring metal 9.9'. (FIG. 1(C)) FIG. 3 shows the voltage/current characteristics between the gate and drain electrodes of the FET manufactured by the process of Example 1 as Example 1. In addition, a case where the heat treatment was not performed and the other steps were the same as in Example 1 was described as a comparative example.

As is clear from FIG. 3, in the comparative example, the FET was destroyed when a voltage of -16 V was applied and a current of 50 μA flowed. However, in Example 1, even if a voltage of -19 V or more was applied and a current of 1 mA or more was repeatedly passed, the FET was not destroyed.

Note that in Example 1 above, heat treatment is performed after forming the gate electrode by lift-off, but in another example (Example 2), plasma treatment can be performed in an oxygen atmosphere instead of this heat treatment. It is.

The plasma treatment is performed in oxygen gas using a barrel plasma device. This plasma treatment in oxygen gas is performed at a gas pressure of 0.5 torr and a substrate temperature of 50° C. for 20 minutes. In addition, for plasma treatment in a mixed gas of oxygen gas and carbon tetrafluoride (CF) gas, the gas ratio (oxygen gas/carbon tetrafluoride gas) is 5/1, and the mixed gas pressure is 0.5 torr.
, and the substrate temperature is room temperature for 5 minutes. FIG. 4 shows the relationship between the plasma treatment time and the thickness of the oxide layer (as measured by ellipsometry). As is clear from the figure, the film thickness of the oxide layer is 2 nm or less before plasma treatment, but by plasma treatment for 20 minutes or more in oxygen gas and 5 minutes or more in mixed gas, the thickness of the oxide layer is about 7 nm. Formed thickly. It can be seen that even if the plasma treatment is performed for a longer period of time, the film thickness is saturated and does not increase. Note that an oxidizing atmosphere gas such as oxygen gas or nitrous oxide (N, O) can be used as the atmosphere for the plasma treatment.

FIG. 5 shows the voltage/current characteristics between the gate and drain electrodes of the FET produced through the steps of Example 2 as Example 2. Further, a case where the plasma treatment was not performed and the other steps were the same as in Example 1 was described as a comparative example.

As is clear from FIG. 5, in the comparative example, the FET was destroyed when a voltage of -15V was applied and a current of about 200 μA flowed. However, in Example 2, the FET was not destroyed even when a voltage of -17 V or more was applied and a current of 1 mA or more was repeatedly passed.

Note that although the above embodiment uses a lift-off method using a resist film, it is also possible to use an insulating film such as a silicon oxide film instead of the resist film.

[Effects of the Invention] As described above, the present invention provides a method for manufacturing a field effect transistor including a source electrode, a drain electrode, and a gate electrode forming a Schottky junction on a semiconductor. A first step of forming the gate electrode, a second step of forming the gate electrode, heat treatment in an oxidizing atmosphere or plasma discharge treatment,
A third step of forming an oxide layer on the surface of the semiconductor exposed between the source electrode and the drain electrode, and a fourth step of forming a dielectric film on the surface are performed.

Therefore, in the field effect transistor according to the present invention, since the semiconductor surface and the gate electrode surface are coated with a stable oxide, it is possible to form a protective dielectric film that does not deteriorate the breakdown voltage.

[Brief explanation of drawings]

Figures 1 (a) to (c) are cross-sectional views for explaining one embodiment of the present invention; Figure 2 is a diagram showing the relationship between heat treatment time and oxide film thickness; Figure 3; is the FET made according to Example 1 and Comparative Example
Figure 4 is a diagram showing the relationship between heat treatment time and oxide film thickness, Figure 5 is a diagram showing the voltage and current characteristics between the gate and drain electrodes of FET
FIG. 3 is a diagram showing the voltage/current characteristics between the gate and drain electrodes of FIG. In the figure, 1...substrate, 2...active layer, 3...source electrode,
4...Drain electrode, 5...Resist film, 5'...
・Opening portion, 6... Metal layer, 7... Gate electrode, 8.
...Silicon nitride film, 9.9'...Metal for wiring. Diagram: Tanzu'°Kaku---■ Heat treatment time (hr) Tanzu

Claims (2)

[Claims] (1) In a method for manufacturing a field effect transistor including a source electrode, a drain electrode, and a gate electrode forming a Schottky junction on a semiconductor, a first step of forming the source electrode and the drain electrode, a second step of forming an oxide layer on the surface of the semiconductor exposed between the source electrode and the drain electrode by heat treatment or plasma discharge treatment in an oxidizing atmosphere; 1. A method for manufacturing a field effect transistor, comprising performing a fourth step of forming a dielectric film. (2) The method for manufacturing a field effect transistor according to item 1, wherein in the third step, the plasma discharge treatment or heat treatment is performed until the thickness of the oxide layer is saturated.