JPS5949689B2 - Anodizing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for anodizing semiconductor wafers.

In general, a Schottky barrier field effect transistor (FET) is formed on an epitaxial layer with a high impurity concentration grown on a semi-insulating CaAs crystal substrate, and a low impurity concentration layer is formed on a crystal substrate with a lower impurity concentration than the crystal substrate. In the manufacturing process of semiconductor devices such as lead-type implant diodes in which a layer with a higher impurity concentration than a layer with a lower impurity concentration is sequentially formed by an epitaxial method, the thickness of the high impurity concentration region of the epitaxial layer is controlled to be thin and uniform. In this case, anodizing method is applied.

For example, since the thickness of an n-type epitaxial layer formed by a vapor phase method on a semi-insulating GaAs crystal substrate varies widely within a wafer, the thickness of a Schottky barrier FET formed on the n-type epitaxial layer is The variation in pinch-off voltage increases. In addition, a thick layer with a low impurity concentration (
In the case of a lead type implant diode in which a thin n-layer with an appropriate impurity concentration is sequentially formed by an epitaxial method, it is difficult to control the thickness of the n-layer to be thin and uniform. The variation in avalanche breakdown voltage increases. In order to reduce variations in the pinch-off voltage and avalanche breakdown voltage, it is necessary to make the thickness of the epitaxial layer having a high impurity concentration thin and uniform. Conventionally, in an anodic oxidation method for making an epitaxial layer having a non-uniform thickness distribution uniform, the same anodic oxidation circuit has been used both in a dark state and in a light irradiation state. However, it has been found that, for example, when a variable current source circuit is used, there is a problem when uniformly reducing the thickness of the epitaxial layer under light irradiation. That is, especially when the impurity concentration of the epitaxial layer is relatively low, the epitaxial layer that has been made uniform by the dark anodic oxidation method is thick. Therefore, unless the epitaxial layer is removed to a considerable thickness by anodic oxidation under light irradiation, the GaAs
-Cannot be used for devices such as FETs. By the way, when it is desired to remove the epitaxial layer thickly, the anodic oxidation time becomes extremely long and often takes about several tens of minutes to an hour. As the oxidation time becomes longer, the etching effect of the oxide film in the anodic oxidation solution cannot be ignored. In particular, the degree of the etching effect is more remarkable in the light irradiation state than in the dark state. Therefore, the anodic oxidation method using only a variable current source circuit not only requires a lot of man-hours when forming the epitaxial layer thinly and uniformly, especially when the impurity concentration of the epitaxial layer is low. The drawback was that it was difficult to accurately control the film thickness. On the other hand, when using a constant current source circuit,
There was a problem with the anodic oxidation method in the dark. That is, when the thickness of the epitaxial layer, which has a non-uniform thickness distribution, is made uniform by dark-state anodic oxidation, if a constant current source circuit is used, current will no longer flow in the thin portion of the epitaxial layer. Accordingly, current is concentrated in the thicker portion of the epitaxial layer. Therefore, the current density is high in the thick portion of the epitaxial layer, and crystal defects contained within the epitaxial region are exposed to the surface. Further, at the moment when the entire surface of the epitaxial layer reaches a certain uniform thickness, a high voltage is applied to the oxide film, causing dielectric breakdown in the thin portion of the oxide film.

Therefore, a high voltage is directly applied on the epitaxial layer, exposing crystal defects on the surface of the epitaxial layer. In other words, in the dark state anodization method using a constant current source circuit, crystal defects are not formed on the epitaxial layer. The drawback was that the defects were exposed. An object of the present invention is to provide an anodizing method that eliminates the above-mentioned drawbacks.

The present invention provides high impurity concentration regions grown on a semi-insulating semiconductor crystal substrate, or semiconductor wafers in which a low impurity concentration region and a high impurity concentration region are sequentially formed on a semiconductor crystal substrate having an extremely high impurity concentration. In a method of controlling the thickness of an impurity concentration region to be thin and uniform using an anodizing method, the surface of the high impurity concentration region is anodized using a variable current source circuit in a dark state, and the generated oxide film is removed. After uniformizing the thickness of the high impurity concentration region, the surface of the high impurity concentration region is anodized using a constant current source circuit under light irradiation, and the generated oxide film is removed. It is characterized by uniformly reducing the thickness of the impurity region.

According to the present invention, the following four effects can be obtained.

First, since a constant current source circuit is used in the anodization method under light irradiation, the growth rate of the oxide film hardly changes as the oxide film thickness increases. As a result, the time required for the anodic oxidation process is significantly reduced. Second, in the case of uniform oxidation of the epitaxial layer under light irradiation, the current density does not change even if the oxide film thickness increases by using a constant current source circuit. Since the composition of the oxide film does not change due to current density, the specific resistance value of the oxide film does not change. Therefore, since the voltage drop per unit length in the oxide film is constant, the thickness of the oxide film can be accurately determined from the voltage drop, making it easier to control the thickness of the epitaxial layer. Thirdly, since the anodic oxidation time is greatly shortened by using a constant current source circuit under light irradiation, the etching effect of the oxide film caused by the anodic oxidizing solution can be almost ignored. Therefore, it is possible to precisely control the thickness of the epitaxial layer of the epitaxial wafer. Fourth, compared to the anodization method that uses a constant current source circuit in the dark state and in the light irradiation state, exposure of crystal defects on the surface of the epitaxial wafer is prevented by using a variable current source circuit in the dark state. be able to. The present invention will be explained by examples.

Impurity concentration 2X1 [17C
r1L-3, thickness approximately 0.29-0.37μMO) n-type G
A GaAs epitaxial wafer with an aAs layer grown by an epitaxial method is prepared.

After the wafer surface is cleaned and cleaned, it is set in an anodizing apparatus. FIG. 1 is a diagram illustrating a method of anodizing a semiconductor wafer in a dark state.

1 is a constant voltage source, and the voltage applied during anodization is adjusted by a variable resistor 2.

The applied voltage when anodic oxidation begins is 120V. A container 4 placed in a dark box 3 is filled with an anodizing solution 5 (a mixture of water, tartaric acid, and ethylene glycol), and a Pt electrode 6 and a GaAs epitaxial wafer 7 are set therein. . When an oxide begins to form on the GaAs epitaxial wafer, the voltage drop in the oxide film begins to increase, and the amount of voltage drop is measured by a voltmeter 8.
When the voltage drop reaches approximately 70V, the current value measured by the ammeter 9 sharply decreases. At that time, the dark anodic oxidation process is stopped. The GaAs epitaxial wafer 7 taken out from the melter 4 has a non-uniform oxide film with a thickness of approximately 0.03 to 0.15 μm (film thickness of GaAs consumed in the oxide film; 0.03 μm to 0.15 μm).
02 to 0.1 μm) is formed. FIG. 2 is a cross-sectional view of a GaAs epitaxial wafer anodized in the dark.

After removing the oxide film 12 with concentrated hydrochloric acid, the GaAs epitaxial wafer is washed with water several times. In this way, the thickness of the epitaxial layer 11 is made uniform to about 0.27 μm. Next, the GaAs epitaxial wafer is anodized again under light irradiation. FIG. 3 is a diagram illustrating a method of anodizing a semiconductor wafer under light irradiation.

A GaAs epitaxial wafer 7 is set in an anodic oxidation solution 5, and a spotlight 13 is used to uniformly irradiate the surface of the wafer 7 with light at 2 mA using a constant current source 1.
Anodic oxidation is performed at a current density of /Cd.

When the voltage drop reaches 60V, the anodic oxidation is stopped. By this anodic oxidation, an oxide film of about 900 A is formed on the GaAs epitaxial wafer. FIG. 4 is a cross-sectional view of a GaAs epitaxial wafer anodized under light irradiation.

When the oxide film 14 is removed with concentrated hydrochloric acid, a uniform epitaxial layer 1 having a thickness of about 0.18 μm is formed on the semi-insulating substrate 10.
1 is formed. As described in detail above, according to the present invention, the thickness of the epitaxial layer having an impurity concentration higher than that of the semiconductor substrate can be controlled to be thin and uniform, and it is possible to control the thickness of the epitaxial layer to be thin and uniform. It is highly effective in manufacturing semiconductor devices such as type impact diodes.

[Brief explanation of the drawing]

Figure 1 is a diagram explaining the method of anodizing a semiconductor wafer in the dark, and Figure 2 shows GaAs anodized in the dark.
FIG. 3 is a cross-sectional view of an epitaxial wafer. FIG. 3 is a diagram explaining a method of anodizing a semiconductor wafer under light irradiation. FIG. 4 is a cross-sectional view of a GaAs epitaxial wafer anodized under light irradiation. . 1... Constant voltage source, 2... Variable resistance,
3...Dark box, 4...Container, 5...
...Anodic oxidation solution, 6...Platinum electrode, 7...
・・GaAs epitaxial wafer, 8・・・・
·voltmeter. 9... Ammeter, 10...
GaAs substrate, 11...Epitaxial layer.
12, 14...Oxide film, 13...Spotlight.

Claims (1)

[Claims] 1 In a method for uniformly controlling the thickness of a high impurity concentration region formed on a low impurity concentration semiconductor region using an anodizing method, the surface of the high impurity concentration region is anodized using a variable current source circuit in a dark state. The process of removing the oxide film produced by oxidation and the process of removing the oxide film produced by anodic oxidizing the surface of the high impurity concentration region under light irradiation using a constant current source circuit are combined. Characteristic anodic oxidation method.