JPS6311773B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming an insulating film on a semiconductor substrate,
In particular, it relates to a method of forming an Al oxide film on a GaAs substrate.

By forming an insulating film on a GaAs substrate
Passivation of GaAs surface is
It has extremely important significance in forming gate insulating films of MOS field effect transistors and protecting the surfaces of various electronic devices and light emitting/receiving elements. Therefore, in recent years, this type of insulating film formation method has been actively researched, and the methods can be broadly divided into two methods: a method of directly oxidizing a part of GaAs, and a method of loading another insulating film. Examples of the former include thermal oxidation, anodic oxidation in an electrolytic solution, or plasma oxidation; CVD, sputtering, vacuum evaporation,
Examples of the latter include molecular beam evaporation and glow discharge. And, as the insulating material, SiO,
SiO ₂ , Si ₃ N ₄ , SiO ₂ /Si ₃ N ₄ , SiN, SiON,
_Al2O3 , _{SiO2 / Al2O3} , _PSG , GaN, _{Ta2O5 ,} etc. are used.

By the way, one of the most important properties required for this type of insulating film is that the interface state density between the insulating film and the semiconductor is sufficiently low, especially in MOS
When using it as a gate insulating film for FET,
This is essentially important, as a large interface state density results in a disadvantage that the mutual conductance becomes small. Furthermore, if the interface state is extremely large, surface inversion will not occur and the device will not be able to function as an inversion type MOS FET.

However, in the conventional insulating film forming method described above, a large amount of interface states exist at the interface between the insulating film and the semiconductor. In other words, in order to know the interfacial properties
There is a known method of fabricating MOS diodes and measuring their capacitance-voltage characteristics (hereinafter referred to as CV characteristics), but those manufactured using conventional methods cannot
It has been confirmed that the CV characteristics exhibit large hysteresis, and that the maximum value of the slope in the capacitance change region is small and the insulation value of capacitance change is also small, clearly indicating a large interface state density. In addition, if the interface state density is low enough to cause an inversion phenomenon, in the low frequency region where minority carriers respond,
Although it is known that MOS capacitance increases up to oxide film capacitance, conventional MOS diodes do not show any increase in capacitance. That is, there is a drawback that no reversal occurs.

As described above, the conventional insulating film forming method cannot sufficiently reduce the interface state density between the insulating film and the semiconductor, and is therefore difficult to apply to the actual manufacture of electronic devices and the like.

The present invention improves these conventional drawbacks, and its purpose is to provide a method for forming an insulating film on a GaAs substrate with an extremely low density of interface states. Briefly explaining the present invention, first, Al is vacuum-deposited to a desired thickness on a surface-treated GaAs substrate or semiconductor layer. Next, this is anodized using, for example, an electrolytic solution to convert Al into an oxide, which is an insulator. At this time, make sure that Al is completely anodized but GaAs is not anodized too much.
It is important to reach the interface with the Generally, under the same anodic oxidation conditions, the thickness of the Al oxide film is proportional to the anodic voltage, and the oxide film thickness can be controlled by the anode voltage.Using this, it is possible to maintain the anodic oxidation within the above range. can be controlled so that it is done. And finally GaAs and
By heat-treating the Al oxide, it is possible to form the Al oxide on GaAs while keeping the interface state density extremely low.

Next, examples of the present invention will be described in detail using specific numerical values.

GaAs substrate (P type with carrier concentration 6.6×10 ¹⁷ cm ^-3
After degreasing and cleaning the surface (GaAs) using a commonly used method and etching the surface,
Al was vacuum deposited to a thickness of 800 Å. Next, this was anodized using a mixture of tartaric acid, ethylene glycol, and water to change the Al into an oxide of Al, which is an insulator. The anode voltage at this time was set to 125V so that only Al was anodized. A constant current source was used for this anodic oxidation at a current density of 3 mA/cm ² .

Next, this was heat-treated at 400° C. for 30 minutes in a N ₂ atmosphere. Although the heat treatment temperature varies depending on the heat treatment time, it was confirmed that heat treatment for 30 minutes at a temperature of less than 300°C increases the interface state density and does not cause inversion. Also
It was also confirmed that heat treatment at temperatures higher than 500°C makes the insulating film more likely to cause dielectric breakdown when voltage is applied. Therefore, the practically preferable range is as follows:
The temperature is approximately 300℃~500℃.

An Al electrode was deposited on the Al oxide formed in the above manner using a metal mask, and the back side was
Fabricate a MOS diode by vapor depositing AuGeNi,
The results of measuring the CV characteristics are shown in the diagram of FIG. In the figure, the horizontal axis represents applied voltage and the vertical axis represents capacitance. As is clear from the figure, the CV characteristics of the MOS diode manufactured by the method of this example are in each frequency range (in the figure, 5Hz, 50Hz, 500Hz, 5KHz,
The hysteresis is small and the absolute value of capacitance change is large at 50KHz measurement results), indicating characteristics when the interface state density is extremely low. Furthermore, the capacitance increases to the insulating film capacitance at low frequencies with respect to the applied voltage in the forward direction, clearly showing an inversion characteristic.

FIGS. 2 and 3 show the CV characteristics of MOS diodes manufactured under the same conditions as the samples of the above embodiments, except that the anode voltages at the end of anodization were 85 V and 150 V. In the sample shown in Figure 2, the anode voltage is as low as 85V, so the oxidation of Al has not completed completely, and the structure has a metal sandwiched between the insulating film and the semiconductor, so the capacitance is low. Little has changed. Therefore, it cannot be used as a gate electrode of a MOS FET. In addition, the sample shown in FIG. 3 is anodized to the inside of GaAs due to the high anode voltage, and as a result, no increase in capacitance at low frequencies is observed and no inversion characteristics are observed.

As can be seen from the above results, in the present invention, it is important to know where to stop the anodic oxidation.
An insulating film with a low interface state density can only be obtained by carrying out anodic oxidation until the oxidation of Al ends. however,
According to experiments, if the anode voltage is in the range between the voltage value at which Al tends to oxidize and a voltage value about 1.1 times that voltage, that is, even if GaAs is slightly oxidized, It has been confirmed that the interface state density is suppressed to a sufficiently low level and is sufficient to cause inversion. Furthermore, since the rates of anodic oxidation of Al and GaAs are different, when the time change of the anode voltage is recorded on a recorder, the slope is different before and after the oxidation of Al is completed. Therefore, the end point of Al oxidation can be easily detected. Also, since the surface reflectance of Al and GaAs is different, interference colors are not shown when the oxide layer is Al underneath, but interference colors are shown when the bottom is GaAs. Therefore, the beginning of the appearance of interference colors indicates that the oxidation of Al has ended, and the end of the oxidation of Al can also be detected optically. For this reason, it is possible to control the stoppage of anodic oxidation more easily than expected.

The essence of the present invention is to anodize Al deposited on GaAs at a low temperature right up to the interface, and therefore is not limited to the anodic oxidation method described in the previous embodiment. It is also possible to achieve the object of the invention by other methods, such as anodization in plasma oxygen.

As explained above, the present invention includes a step of vacuum-depositing Al on GaAs, then anodizing until the oxidation of Al is completed, and then heat-treating, thereby obtaining inverted characteristics. As is clear from the above, there is an effect that the interface state density can be made extremely small. Therefore, it can be put to practical use in forming gate insulating films of MOS type field effect transistors or surface protective films of various electronic devices, and its industrial value is extremely large.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the CV characteristics of a GaAs MOS diode manufactured using the insulating film forming method of the present invention.
Figures 2 and 3 show only the final voltage of anodization.
FIG. 2 is a diagram showing the CV characteristics of a GaAs MOS diode manufactured differently from the case shown in the figure.

Claims (1)

[Claims] 1. Insulating a semiconductor substrate characterized by comprising the steps of vacuum evaporating Al on GaAs, anodic oxidation until the oxidation of Al is completed after the step, and heat treatment after the step. Film formation method.