JPS5949690B2 - Anodizing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is mainly applied to the case where the thickness of a semiconductor epitaxial layer grown on a semi-insulating semiconductor crystal substrate using a semiconductor such as gallium arsenide (GaAs) is controlled to be thin and uniform. The present invention relates to an anodizing method.

Generally grown on a semi-insulating GaAs crystal substrate,
In the manufacturing process of GaAs semiconductor devices such as Schottky barrier field effect transistors (FETs) in which the active region is formed in a semiconductor epitaxial layer with a higher impurity concentration than the crystal substrate, the thickness of the epitaxial layer is thinned and uniform. The anodic oxidation method is applied when controlling the

For example, the thickness of an epitaxial layer grown by a vapor phase method on a semi-insulating GaAs crystal substrate has large variations within the wafer, resulting in variations in the pinch-off voltage of Schottky barrier FETs formed on this epitaxial layer. growing. In order to reduce variations in the pinch-off voltage, it is necessary to make the thickness of the epitaxial layer thin and uniform. Conventionally, an anodic oxidation method has been proposed as a method for thinning and making an epitaxial layer having a non-uniform thickness distribution thinner and more uniform.

In this anodic oxidation method, the thickness of the epitaxial layer is made roughly uniform by continuing the steps of forming an anodic oxide film in a dark state and removing the formed anodic oxide film, and then anodic oxidation is performed in a light irradiation state. By removing the anodic oxide film, an epitaxial layer with a uniform thickness and a good surface condition was obtained. However, there was a problem in that the epitaxial layer was slightly etched when anodic oxidation was started in this light irradiation state. This etching is several hundred λ
Although the degree is relatively small, especially in the process of manufacturing devices such as Schottky barrier FETs, it not only has a large effect on the pinch-off voltage, but also makes it impossible to achieve reproducibility in pinch-off voltage control. This was also a problem. An object of the present invention is to provide an anodic oxidation method that allows precise control of the thickness of an epitaxial layer.

Based on the inventor's basic research for making the present invention, etching of the epitaxial layer with an anodic oxidizing solution that occurs immediately after the start of anodizing depends on the type of anodic oxidizing solution (pH value), the presence or absence of light irradiation, the initial current density, etc. It turns out that it is highly dependent. First, regarding the anodic oxidation solution, we used a mixture of ethylene glycol, water, and tartaric acid, and... the value was 2 + 0.
．． It was found that a composition ratio of 5 is good. ...If the value is small, the etching amount of the epitaxial layer will increase rapidly, and if the value is too large, the anodic oxide film thickness will change greatly depending on the value. It becomes difficult to control the epitaxial layer.

Furthermore, the amount of etching by the anodic oxidizing solution is so small that it can be ignored in a dark state, and becomes particularly large in a light irradiation state. Furthermore, it has been found that the amount of etching is strongly dependent on the initial current density (whether a variable current source or a constant current source). That is, when the initial current density is small,
This etching effect is significant and large. For example, when a GaAs epitaxial layer grown on a semi-insulating crystal substrate is anodized using the above-mentioned anodic oxidizing solution under light irradiation conditions with an initial current density of 0.5 mA/CfL, the etching time is about 40 mA/CfL. The etching lasts approximately 3000 λ of the GaAs epitaxial layer. on the other hand,
If the current density becomes too large, not only will crystal defects be exposed on the epitaxial surface, but the thickness of the anodic oxide film formed will not be uniform within the wafer. Therefore, high current density is not a desirable condition when the epitaxial layer is used for device manufacturing. According to the invention, semi-insulating GaA
Ga with non-uniform thickness distribution grown on s-crystal substrate
In the anodic oxidation method that controls the As epitaxial layer to be thinner and more uniform than the depletion layer width at the time of avalanche failure, the anodic oxidation method is performed in a dark state, and the resulting anodic oxide film is removed to form the epitaxial layer. After the thickness of the layer is roughly made uniform, the thickness of the epitaxial layer is made uniformly thin and thin by forming an anodic oxide film and removing the anodic oxide film using a light irradiation anodic oxidation method. In the step of obtaining a good surface condition, the anodic oxide film is formed in the light irradiation state mainly containing ethylene glycol, water and tartaric acid...value 2+0.
In addition to using No. 5 anodic oxidation solution, the initial current density was 3+1 m.
An anodic oxidation method is obtained which is characterized in that it is carried out under A/Cd conditions.

According to the present invention, the following three effects can be achieved.

First, as a result of optimizing the initial current density in the anodization method under light irradiation, the thickness of the GaAs epitaxial layer grown on the semi-insulating crystal substrate can be controlled uniformly and accurately. This made it easier to obtain an epitaxial layer of the required thickness, and the yield was improved. especially,
When the impurity concentration of the epitaxial layer is high, the effect of optimizing the initial current density is large. Second, by finding the optimal initial current density, we were able to reduce the density of crystal defects exposed in the epitaxial layer. Third, by determining the optimum initial current density, the thickness of the anodic oxide film formed under light irradiation was made more uniform. Next, specific embodiments of the present invention will be described with reference to the drawings.

The impurity concentration of 2×10−3 and approximately 0.29 to 0 was epitaxially grown on a GaAs semi-insulating crystal substrate 1.
．． Ga with thickness non-uniformity in the range of 36μ
A case will be described in which the thickness of the As epitaxial layer (corresponding to 2 and 3 in FIG. 1) is made uniform to about 0.12 μm.

First, the epitaxial wafer was subjected to a pre-cleaning process (triclene boiling and methyl ethyl ketone ultrasonic cleaning 5 times each).
After that, an anodic oxidation process is carried out in the dark using an anodic oxidation solution (pH value 2) consisting of ethylene glycol, water and tartaric acid, and the remaining epitaxial layer 2 is coated as shown in Fig. 1. An oxide film 3 is formed. In addition,
When the voltage drop across the anodic oxide film reached 60V, the anodic oxidation process was stopped. If oxide film 3 is removed, approximately 0
．． An epitaxial layer 2 having a thickness of 27μ has been formed. The epitaxial layer 2 thus obtained is relatively uniform in thickness, but contains many surface defects. Note that this process is repeated multiple times when the amount to be removed is large. Next, the next step of anodic oxidation is started while uniformly irradiating the surface of the epitaxial layer 2 with light.

A constant current source circuit is used as the positive oxidation circuit, and the initial current density is set to approximately 2.5 mA/(1-J).When the voltage drop in the oxide film increases by 100 compared to the initial voltage, the anode The oxidation circuit is opened and the anodic oxidation is stopped. At this time, an oxide film 4 is formed on the remaining epitaxial layer 7 as shown in FIG. If removed inside, the epitaxial layer will be approximately 0.12μ
The initial objective was achieved as the thickness was made uniform and a better surface condition was obtained. According to the present invention, since the film thickness of the epitaxial layer 2 etched immediately after the start of anodic oxidation is so small that it can be ignored, the thickness of the oxide film 4, and therefore the oxide film G consumed during 4
It has been found that the thickness of the aAs epitaxial layer 7 can be precisely controlled.

The embodiments of the present invention have been described above using an anodic oxidation method, a GaAs epitaxial wafer having a specific impurity concentration and thickness, and a specific oxide film etching solution. Without being limited to the above embodiments, the present invention is applicable to an anode under good light irradiation conditions, which is applied when controlling the thickness of a GaAs epitaxial wafer grown on a semi-insulating crystal substrate to be thin and uniform. Needless to say, this applies to all oxidation methods.

[Brief explanation of the drawing]

1 and 2 are cross-sectional views at each step for explaining an embodiment of the present invention.

Claims (1)

[Claims] 1. A first step of anodizing the surface of the semiconductor epitaxial layer in a dark state to convert it into a first anodic oxide film and removing the first anodic oxide film, and the epitaxial layer subjected to the first step. While the surface is irradiated with light, an anodic oxidizing solution with a pH value of 2 ± 0.5 is used, and the initial current density is 3 ± 1 mA/c.
and a second step of forming a second anodic oxide film by performing anodic oxidation under conditions of m^2.