JPH04105319A - Selective etching method - Google Patents

Abstract

PURPOSE:To obtain an etching method of GaAs/AlGaAs system with high selection ratio, without using flon based gas, by etching an GaAs layer on an AlGaAs layer by using mixed gas which chains gas containing C and F as constituent elements, gas containing Si and Cl as constituent elements, and rare gas. CONSTITUTION:A GaAs layer on an AlGaAs layer is selectively etched by using mixed gas which contains gas containing C and F as constituent elements, gas containing Si and Cl as constituent elements, and rare gas. For example, when an enhancement mode HEMT is manufactured, a substratum in which an undoped layer 2, an N-type AlGaAs layer 3 and an N-type GaAs layer 4 are formed in order on a semiinsulative GaAs substrate 1 is prepared, and the N-type GaAs layer 4 is etched by using a resist pattern as a mask, thus forming a recess 5. In the above process, the etching condition of the N-type GaAs layer 4 is as follows; high frequency power density is 0.05W/cm<2>, CF4: SiCl4:<=e=19.7:3.45:29 (flow rate), and gas pressure is 45mTorr.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a selective etching method, particularly for AlGaAs.
A method of etching the GaAs layer on the layer with a high selectivity without using a fluorocarbon gas is developed.

[Summary of the invention]

The present invention relates to a GaAs layer (hereinafter referred to as GaAs/AIGaAs) formed on an AffGaAs layer.
It is called a system. ), C and F
Highly selective etching is possible without using a fluorocarbon gas by using a mixed gas containing a gas containing Si and Cl as constituent elements, a gas containing Si and Cl as constituent elements, and a rare gas as an etching gas. That is.

[Conventional technology]

Selective etching of the GaAs layer in the GaAs/AeGaAs system is an enhancement process.
This technique is required when forming a recess (four parts) that will be the formation site of a gate electrode in a mode HEMT (high electron mobility transistor).

For example, in order to manufacture a HEMT of the type described above, first an undoped GaAs layer (2) is deposited on a substrate as shown in FIG.
, n-type Aj containing n-type impurities by Si doping etc.
! A substrate is prepared in which a GaAs layer (3) and an n-type GaAs layer (4) containing n-type impurities are sequentially laminated. Next, using the resist pattern as a mask, proceed as follows.
By etching the type GaAs layer (4), a recess (5) as shown in FIG. 4(B) is formed.

Here, the doping concentration, thickness, etc. of the n-type AlGaAs layer (3) are set so that a HEMT with an appropriate dark voltage can be constructed by removing only the undoped GaAs layer (2). It is necessary to carry out rigorously with high selectivity. Conventionally, CCl2. Good etching has been achieved using a mixed gas containing a fluorocarbon gas such as F and a rare gas such as He or Ne. A gate electrode is formed in the recess (5), and n-type Ga is formed on both sides of the gate electrode.
Ohmic electrode (source/drain) on the As layer (4)
By forming , a HEMT is constructed.

[Problems to be Solved by the Invention] However, as is well known, fluorocarbon gases are considered to be the cause of the destruction of the earth's ozone layer, and their production and use are likely to be prohibited in the near future. Therefore, there is an urgent need to develop alternative gases and techniques for using them in the field of etching.

Therefore, the present invention has a high selectivity ratio without using fluorocarbon gas.
The purpose of this invention is to provide a method for etching a system.

[Means to solve the problem]

The selective enching method of the present invention is proposed to achieve the above-mentioned object, and uses a gas containing C and F as constituent elements, a gas containing Si and Cl as constituent elements, and a rare gas. Using a mixed gas containing A/! This method is characterized by selectively etching the GaAs layer on top of the GaAs layer.

[Effect]

The mixed gas used in the present invention can generate the same etching species as in the case of using a fluorocarbon gas by electric discharge, and can advance the etching of the GaA three layer by the same mechanism. However, when the etching of the GaAs layer progresses and the underlying AffGaAs layer is exposed, stable AlF2 with low vapor pressure is formed, so that the AI! ,
The surface of the GaAs layer is passivated and etching does not proceed. With such a mechanism, high selectivity is achieved.

None of the component gases constituting the above-mentioned mixed gas contain chlorine atoms and fluorine atoms in their individual molecules, so even if they are discharged outside the etching reaction system without being dissociated by discharge, the fluorocarbon-based gas remains As such, it does not cause environmental destruction. Therefore, the present invention is extremely effective as a measure to eliminate fluorocarbons.

〔Example〕

Hereinafter, preferred embodiments of the present invention will be described based on experimental results.

In the following series of experiments, CF, gas, 5iCj! Etching of GaAs layer using a mixed gas consisting of 4 gas and He gas, and GaAs/AIGa
We investigated As-based selective etching.

First, in the etching of the GaAs layer using the above-mentioned mixed gas, the dependence of the etching rate on the high frequency power density and the mixed gas composition was investigated.

That is, a GaAs substrate is set in a parallel plate type etching apparatus, and CF4 gas and 5iCj! , a mixed gas was supplied into the enching chamber with a total gas flow rate of 205 CCM and a He gas flow rate of 295 CClff, and the gas pressure was increased for 30 s.
+ Torr. Here, the high frequency power density is 0.0
5W/cm", 0.10W/cm", 0.
S i Cl in the total flow rate of CF, gas and 5iCffi-gas was varied in three steps of 20 W/cm”.
The etching depth was measured when etching was performed for 5 minutes while changing the gas flow rate in the range of 0.15 to 0.61.

The results are shown in Figure 11. In the figure, the vertical axis is the etching depth (
μm) and etching speed (8m7min), horizontal axis is 5i
Cl, flow rate/(CF, flow rate +5iCj!, flow rate). Assuming the formation of a recess in an enhancement mode HEMT, the thickness of the GaAs layer to be etched is approximately 0.1 μm at most, so if controllability and avoidance of underlying damage are taken into account, the etching rate should be 0.5 μm. 1~0
．． It is desirable to control the speed to about 2 μm/min. In the above experiment, when the high frequency power density was 0.05 W/cm2, the etching rate was approximately 0.2 μm/min.
The effect of mixed gas composition was relatively small. Experiments conducted at higher radio frequency power densities have generally shown that the enching rate is too high to be practical.

Next, we investigated the gas pressure dependence of the selectivity between the n-type GaAs layer and the n-type AlGaAs layer under a constant high-frequency power density and mixed gas composition. The high frequency power is 0.05W/cm based on the above experimental results.
”, and the mixed gas composition is CF,: S i Cl,
: He=19.7:3.45:29 (flow rate ratio). Here, the gas pressure is 301 lTorr, 45 mT
orr.

Etching was performed for 5 minutes while changing the pressure to 60 mTorr.

The results are shown in Figure 2. In the figure, the vertical axis is the etching depth (8
m), the horizontal axis represents the gas pressure (mTorr), and the black circle (・)
The plots 1 and 2 represent the etching rates of an n-type GaAs substrate, the white circles (○) represent the etching rates of the n-type AffGaAs substrate, respectively. n of the n-type GaAs substrate calculated from this experimental result
The selectivity for type AlCaAs substrates is as follows: gas pressure 30 m
Torr: 59.45mTorr: 131
．． It was 171 at 60 mTorr, which was a practically sufficient value. In addition, gas pressure 3Q raTorr
In this case, the etched surface became mirror-like.

This is considered to be due to the fact that the sputtering effect appears strongly in the low gas pressure region. Gas pressure 45 mTo
rr and 60 mTorr, n-type Ga
Roughness was observed on the etched surface of the As substrate due to adhesion of reaction products, but since the selectivity was maintained sufficiently high, it did not affect practical performance.

From the above experimental results, the mixed gas system of the present invention shows that GaAs/AffiGaAs can be produced with low power and little damage.
It was suggested that highly selective etching of the system could be performed. Therefore, as a next experiment, we actually etched the substrate for configuring the HEMT as shown in Figure 4 (A) above, and monitored the increase in etching depth as the etching time elapsed using a probe method. did. In this HEMT substrate, the layer thickness of the uppermost n-type GaAs layer (4) is about 0.03 μm. The conditions are high frequency power density 0
．． 05W/cm2, CFa: S 1cla: H
e=19.7:3.45:29<s amount ratio), and the gas pressure was 45 mTorr.

For comparison, a 0.3 μm thick n-type GaAs layer was grown on an intrinsic GaAs substrate by MOCVD (
Hereinafter, this will be referred to as an n-type GaAs/intrinsic GaAs substrate. ) was also etched in the same way.

The results are shown in Figure 3, where the vertical axis represents the etching depth (μ
m), the horizontal axis represents the etching time (minutes), and the white circle (0)
The plots with . are for HEMT substrates, and the plots with black circles (.) are for n-type GaAs/intrinsic GaAs substrates. From this figure, when etching a HEMT substrate, the etching depth is approximately 0.03 seconds after the start of etching.
After reaching μm, the etching depth hardly increased.

This is n-type Ga As N(4) and n-type AlGa
This shows that a sufficiently large selectivity with respect to the As layer (3) is achieved. On the other hand, n-type GaAs/intrinsic G
For aAs substrates, it is clear that etching progresses at a more or less constant rate, even though different layers are exposed as the etching progresses.

Note that the present invention is not limited to the above-described embodiments. For example, gases containing C and F as constituent elements include, in addition to the above-mentioned CF, CHF, C, F6. C3
F8. Various fluorocarbon gases such as CH2F, CH, F, and CBrFl can be used. Also, S
As a gas containing i and Cj2 as constituent elements, in addition to the above-mentioned 5iCl4, 5iHzCj! □, silane-based gas such as 5iHCl1 may be used. Furthermore, as the rare gas, He, which has both the Svantha effect and the cooling effect, was used in the above embodiment, but Ne, Ar, Kr, Xe, Rn
You may also use The gas pressure, mixed gas composition ratio, etc. can be changed as appropriate as long as they are within the range in which the above-described experiment was conducted.

Furthermore, the present invention
As long as the process requires As-based selective etching, the present invention is not limited to manufacturing HEMTs as described above, but can also be applied to, for example, processing of laser elements.

[Effect of the invention] As is clear from the above explanation, by applying the present invention, GaAs/A can be achieved without using fluorocarbon gas.
1 Ga As type etching can be performed with relatively low power and high selectivity. Therefore, the present invention will be extremely useful for processing HEMTs and laser elements in the future as well.

[Brief explanation of the drawing]

FIG. 1 is a characteristic diagram showing the dependence of the relationship between the etching rate and the composition of the mixed gas on high frequency power density in the etching of a GaAs substrate with a mixed gas consisting of CF, gas, S r Cfa gas, and He gas. FIG. 2 is a characteristic diagram showing the relationship between the selectivity between the n-type GaAs layer and the n-type AlGaAs layer and the gas pressure under a constant high frequency power density and mixed gas composition. FIG. 3 is a characteristic diagram showing the relationship between etching depth and etching time during etching of a HEMT substrate in comparison with that of an n-type GaAs/intrinsic GaAs substrate. Figures 4(A) and 4(B) show the enhancement
FIG. 4 is a schematic cross-sectional view illustrating etching of an n-type GaAs layer performed in manufacturing a mode HEMT, and FIG.
) is the substrate before etching, and Figure 4 (B) is the n-type GaAs.
Each layer represents an etched state. 1... Semi-insulating GaAs substrate 2... Undoped GaAs layer 3--- n-type AjMl; aAs layer 4--- n-type GaAs layer 5... recess FIG. 1 FIG. 2 FIG. 3 FIG. 4 Figure (A) Figure 4 (B)

Claims (1)

[Claims] A GaAs layer on an AlGaAs layer is selectively etched using a mixed gas containing a gas containing C and F as constituent elements, a gas containing Si and Cl as constituent elements, and a rare gas. Choose the etching method.