JP3420617B2 - Etching method and method of manufacturing semiconductor device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an etching method and a semiconductor device manufacturing method.

[0002]

2. Description of the Related Art In recent years, gallium arsenide metal semiconductor field effect transistors (hereinafter referred to as Ga
It is called AsMESFET. ) Is widely used.

7 to 9 are process sectional views in the vicinity of a gate electrode in a GaAs MESFET. 7 to 9, 1 is a GaAs substrate, 2 is an etching mask formed of a SiO ₂ film or a resist film, 3 is a recess structure, and 4 is a gate electrode.

In this GaAs MESFET, in order to prevent the surface states existing on the surface of the GaAs substrate 1 from capturing electrons moving near the surface of the GaAs substrate 1, a planar type FE is used.
When forming T, as shown in FIG.
Is provided in a recess (hereinafter referred to as a recess structure) 3 provided on the surface of the GaAs substrate 1.

Conventionally, as a method of forming the recess structure 3, as shown in FIG. 7, after forming an opening in an etching mask 2 on a GaAs substrate 1, a GaAs substrate corresponding to the opening is formed by a wet etching method. The surface of 1 is selectively removed, and the recess structure 3 is formed as shown in FIG. After that, as shown in FIG. 9, the gate electrode 4 was formed in the recess structure 3 by a lift-off method or the like.

However, in recent years, with the increase in the diameter of the GaAs substrate 1 and the miniaturization of elements, the wet etching method is
Since the depth and width of the recess structure 3 in the As substrate 1 cannot be sufficiently obtained, a dry process such as a reactive ion etching method (hereinafter referred to as RIE method) having good uniformity in the GaAs substrate 1 is obtained. Etching methods have come into use.

In the dry etching method for the GaAs substrate 1, high frequency power is applied to a gas containing Cl as a constituent gas as a reaction gas to discharge the gas, dissociate the gas molecules, and generate Cl radicals or Cl ions. 1 reacts with Ga and As on the surface. And GaCl ₂ , GaCl ₃ , and AsCl ₃ (relative boiling point, 535 degrees Celsius)
Deg., 201 degrees Celsius, 130 degrees Celsius) to generate the GaAs substrate 1
The surface of is being etched.

Further, in recent years, GaAs MESFETs (hereinafter referred to as GaAs MODFETs) having a heterojunction structure by stacking AlGaAs layers and GaAs layers capable of operating at higher speed than conventional GaAs MESFETs.
Has been put to practical use. When forming this GaAs MODFET, only the GaAs layer grown on the AlGaAs layer must be removed by etching.

In the dry etching method, it has been conventionally known that only a GaAs layer grown on an AlGaAs layer can be easily and selectively removed by using a gas system as described below. Has been converted.

That is, in order to selectively remove the GaAs layer on the AlGaAs layer by the dry etching method, a gas containing Cl and F as constituent elements is used as the reaction gas, or Cl is contained as constituent elements. It can be realized by using a mixed gas in which a gas and a gas containing F as a constituent element are mixed.

The mechanism for selectively removing only the GaAs layer is considered as follows.

The GaAs layer is etched by Cl radicals or Cl ions, and the surface of the AlGaAs layer is finally exposed. When the surface of the AlGaAs layer is exposed, Al and F radicals react with each other to generate AlF ₃ on the surface. Since AlF ₃ has a very low vapor pressure (boiling point, 1291 degrees Celsius) and is a stable substance, it serves as a protective film on the surface of the AlGaAs layer. Therefore AlGa
The As layer is prevented from being etched by Cl radicals or Cl ions, etching is stopped at the surface of the AlGaAs layer, and as a result, only the GaAs layer can be selectively removed.

Conventionally, the above-mentioned selective etching is
CFCs such as CCl ₂ F ₂ and CHCl ₂ F, or CCl ₄ ,
Gases such as BCl ₃ and SiCl ₄ have been used by mixing CF ₄ and SF ₆ .

[0014]

As described above, with the recent increase in the diameter of the GaAs substrate 1 and the miniaturization of elements, in the wet etching method, the depth and width of the recess structure 3 in the GaAs substrate 1 are reduced. There is a problem that sufficient uniformity cannot be obtained.

Conventionally, a CFC-based gas such as CCl ₂ F ₂ , CHCl ₂ F or CHClF ₂ or a gas system in which CF ₄ or SF ₆ is mixed with a gas such as CCl ₄ , BCl ₃ or SiCl ₄ is used. In the case of the dry etching method, the high frequency power was applied to the gas before the GaAs
There is a problem that there is a time delay (hereinafter referred to as dead time) before the etching of the surface of the substrate 1 is started.

FIG. 10 shows the characteristics of the etching time and the etching depth when the GaAs substrate 1 is etched by the RIE method using a mixed gas of BCl ₃ and SF ₆ as an example of the conventional example. In FIG. 10, the vertical axis represents the etching depth and the horizontal axis represents the etching time.

In FIG. 10, Si is used as an etching mask 2.
Using O ₂ , the flow rates of BCl ₃ and SF ₆ are 30 cm ³ / min and 7.
5 cm ³ / min, the pressure was 50 mTorr, and the high frequency power of 13.54 MHz was applied by 40 W between the parallel plate electrodes for etching,
After that, SiO ₂ is removed by BHF and the etching depth of the GaAs substrate 1 is measured by a step gauge.

In FIG. 10, there is a dead time in the area indicated by 5, and after this dead time, etching is rapidly started. The etching rate in FIG. 10 is 5000.
A / min.

The cause of the dead time 5 is F dissociated by the high frequency discharge immediately after the start of the high frequency discharge.
Radicals react with Ga and vapor pressure is low (boiling point, 1000 degrees Celsius)
Degree) GaF ₃ is generated to protect the surface of the GaAs substrate 1. However, after the discharge started, the GaF ₃
Is removed by the sputtering action, so that the reaction abruptly starts and is etched with a time delay from the start of discharge.

Further, this dead time 5 depends on the GaAs substrate 1
However, there is a problem that the reproducibility is poor because it changes depending on the surface condition, particularly the degree of moisture adsorption and the presence or absence of a natural oxide film. Therefore, the reproducibility of the etching depth after etching is poor even if the gas pressure, the high frequency power and the etching time are constant.

11 and 12 show RI using the above-mentioned gas system.
It is a schematic diagram of a cross section after etching the GaAs substrate 1 by the E method. In FIG. 11, 6a is an undercut caused by etching, and in FIG. 12, 6b is an overhang caused by etching. In the above-described conventional gas system, the GaAs substrate 1 is laterally etched to be wider than the opening width in the etching mask 2 of the SiO ₂ film, and the undercut 6a is generated.

Further, since the dead time 5 has no reproducibility, the amount of the undercut 6a is poor in reproducibility as well as the etching depth.

Further, the recess structure 3 required for the GaAs MESFET or GaAs MODFET has a depth of several hundreds to several hundreds of angstroms and a pattern width of about 0.1 μm to 1.5 μm. It was difficult to form the dimensional recess structure 3 with good reproducibility.

Further, when the surface of the GaAs substrate 1 is viewed microscopically, since the dead time 5 at each location on the surface of the GaAs substrate 1 differs depending on the surface condition at each location, etching at each location is performed. There is a problem in that there is a time difference in the start, and the smoothness of the surface after etching is poor and unevenness is likely to occur.

When the GaAs substrate 1 is etched by the RIE method using BCl ₃ alone, there is no dead time 5 because GaF ₃ having a low vapor pressure is not produced. However, since F is not present, it is not possible to selectively etch only the GaAs layer on the AlGaAs layer. Also, as shown in Fig. 12, the overhang in which the lower pattern width is wider than the upper pattern width
There is a problem that 6b tends to occur.

When the GaAs substrate 1 is etched by the RIE method, the surface of the GaAs substrate 1 is damaged by the impact of ions incident on the surface of the GaAs substrate 1, and the carrier density and movement near the surface of the GaAs substrate 1 are caused. However, there is a problem that the device characteristics are deteriorated due to a decrease in the degree.

The present invention has been made in view of the above points, and eliminates dead time, improves reproducibility of etching depth and undercut, improves smoothness of the substrate surface, and improves device characteristics. The purpose is to improve.

[0028]

Means for Solving the Problems In order to achieve the above object, means taken by the present invention include a gas containing Cl as a constituent element and a gas composed of three elements of C, H and F. Applying high frequency power to the mixed gas, exposing the surface of the substrate made of GaAs to the plasma generated in the mixed gas,
The surface of the substrate is etched.

Further, instead of the substrate made of GaAs, the surface of a substrate formed by sequentially growing an AlGaAs layer and a GaAs layer on a semi-insulating substrate is exposed, and only the GaAs layer of the substrate is selectively etched.

Furthermore, cleaning the substrate after the etching with organic alkaline aqueous solution.

In the GaAs MESFET or GaAs MODFET, the gate electrode is formed after the surface of the substrate where the gate electrode is to be provided is etched by the above-described etching method to form a recess in the above-mentioned arrangement. Alternatively, in GaAs MESFET or GaAs MODFET, after the surface of the substrate is etched by the above method, the substrate is annealed at a temperature of 400 ° C. or higher and 600 ° C. or lower .

Further, the mixed gas containing a BCl ₃ and CHF ₃
Gas, and the partial pressure ratio of the CHF ₃ gas is
And 10% or more is 60% or less, also for cleaning a substrate after the etching with organic alkaline aqueous solution.

[0033]

In the present invention having the above-mentioned structure, CHF ₃ is an example of the gas consisting of C, H, and F, and this CHF ₃ is changed from the following formulas (1) to (6) by high frequency discharge. Dissociate.

CHF ₃ ← → CHF ₂ + F …… (1) CHF ₂ ← → CHF + F …… (2) CHF ← → CH + F …… (3) CHF ₃ ← → CF ₃ + H …… ( 4) CF ₃ ← → CF ₂ + F …… (5) CF ₂ ← → CF + F …… (6) H + F ← → HF …… (7) In the above equation (7), the HF generated on the right side is Since the dissociation energy is so large, the chemical equilibrium of Eq. (7) is almost biased to the right side. Therefore, in CHF ₃ dissociation, H-free F
The F radical concentration is lower than that in the state after the compound is dissociated. Therefore, it is difficult to generate GaF ₃ that causes dead time. As a result, Cl radicals or Cl ions generated by the dissociation of BCl ₃ react with GaAs immediately after the start of discharge, so that no dead time occurs.

Further, since the reaction of the formula (7) is biased to the right side, H and F on the right side of the formulas (1) to (6) are insufficient, so that the chemical equilibrium of the formulas (1) to (6) is insufficient. Also leans to the right side. Therefore, the concentration of CHF, CH, CF ₂ , CF, etc. becomes high. Since these components have two or more unpaired electron pairs, they polymerize to form a polymer film (hereinafter, polymer).

In the RIE method, the polymer film is not easily decomposed and deposited on the surface of the substrate because it is exposed to the irradiation of ions, but the side walls of the recesses dug by etching are not easily irradiated with ions. Therefore, the polymer film is deposited on the sidewall portion. For this reason, the side wall of the recess of the substrate during etching is protected by the polymer film, so that it is not laterally etched and a cross-sectional shape without undercut is obtained.

As described above, when the gas of the C compound containing H and F is mixed with the gas containing Cl such as BCl _{3 and the} like, excess F radicals are not generated, and there is no dead time.
Further, since the side wall protective film is formed, a vertical etching shape without overhang can be obtained. Moreover, since the plasma contains an appropriate amount of F, only the GaAs layer on the AlGaAs layer can be selectively etched. Further, the protective film of the polymer film of the side wall is removed by washing organic alkaline aqueous solution.

Further, since the etching method of the present invention can obtain a vertical cross-sectional shape with no dead time and no undercut, a GaAs MESFET or GaAsM is formed by the RIE method.
When the recess structure of the portion where the gate electrode of the ODFET is provided is used for forming, the in-plane uniformity of the etching depth is good and the recess width can be accurately controlled.

Further, the substrate after etching is 400 degrees Celsius or more.
By annealing above 600 ° C, the GaAs layer and A
Even in a GaAs MODFET having a heterojunction in which lGaAs layers are stacked, it is possible to recover the damage generated during etching without damaging the two-dimensional electron gas at the interface between the GaAs layer and the AlGaAs layer, and prevent the deterioration of device characteristics. .

[0040]

Embodiments of the present invention will now be described in detail with reference to the drawings.

1 to 3 are sectional views showing the steps of GaAs etching in one embodiment of the present invention. In FIG.
Reference numeral 7 denotes a substrate made of GaAs, or a substrate in which an AlGaAs layer and a GaAs layer are sequentially grown on a semi-insulating GaAs substrate.
Is an etching mask, and a SiO ₂ film is used here.
Reference numeral 8 is a polymer deposited during etching, and 9 is a cross-section of a pattern formed by etching, which is a recess structure which is a recess formed in an arrangement portion where a gate electrode is provided.

1 to 3, SiO ₂ is grown on the substrate 7 made of GaAs or on the substrate 7 in which an AlGaAs layer and a GaAs layer are sequentially grown on a semi-insulating GaAs substrate. still,
The mixed crystal ratio of AlAs and GaAs in the AlGaAs layer is AlAs: GaAs = 1:
It was 4.

[0043] The after forming a resist pattern of the pattern width of submicron on SiO _2, after a SiO ₂ using the resist pattern as a mask was etched by RIE using a mixed gas of CHF ₃ and O _2, ashing the resist Remove with
An etching mask 2 of SiO ₂ is formed.

In FIG. 2, SiO ₂ etching mask 2
The underlying substrate 7 is etched by using. A mixed gas of BCl ₃ and CHF ₃ was used as an etching gas, and etching was performed by a parallel plate type RIE device. The partial pressure ratio of CHF ₃ is 0% to 7
The pressure was 0%, the pressure was 30 mTorr to 100 mTorr, and the high frequency power of 13.56 MHz was 30 W to 100 W.

As the substrate 7, AlGaAs is formed on a semi-insulating GaAs substrate.
When a substrate in which a layer and a GaAs layer are sequentially grown is used, the etching rate sharply decreases when the AlGaAs layer is exposed, and is hardly etched to a depth beyond this.

In the cross-sectional shape of the pattern after etching, as shown in FIG. 2, deposition of the polymer 8 was observed on the sidewalls of the recess structure 9 of the SiO ₂ etching mask 2 and the substrate 7, and the recess structure 9 which is the etching bottom surface was formed. A slight amount of polymer 8 was also found on the bottom surface.

[0047] Therefore, the substrate 7 was etched an organic alkaline aqueous solution TMAH (Tetra Methyl Ammonium Hydroxi
This polymer 8 can be removed by washing with a 2.38% aqueous solution of de), and a cross-sectional shape of the recess structure 9 having an etching pattern can be obtained as shown in FIG. The undercuts and overhangs seen when etching by the RIE method in the conventional method do not occur at all.

FIG. 4 shows RI using a mixed gas of BCl ₃ and CHF _3.
The relationship between the etching time and the etching depth by the E method is shown. In FIG. 4, the vertical axis represents the etching depth and the horizontal axis represents the etching time. And BCl ₃ and CH
The flow rate of F ₃ are each 15cm ³ / min, a 10 cm ³ / min, the pressure is 30 mTorr. High frequency power is 50W at 13.56MHz. After etching the substrate 7 by the method shown in FIGS. 1 to 3, the etching mask 2 was removed with BHF, and the etching depth was measured with a step gauge. The solid line 10 is the etching characteristic for the substrate 7 made of GaAs, and as is apparent from FIG. 4, the dead time 5 as in the conventional case does not occur. Also,
The dotted line 11 in FIG. 4 shows the etching characteristics of the substrate 7 in which the GaAs layer of 1400 A is grown on the AlGaAs layer, and the etching rate (gradient of the graph) sharply decreases when the etching depth reaches the AlGaAs layer. The etching rate of GaAs is about 40 times that of AlGaAs. The etching rate of GaAs is about 20 times that of SiO ₂ .

Further, in FIG. 4, the slope of the graph of the solid line 10, that is, the etching rate decreases with the etching time. This is because the polymer 8 is deposited on the bottom surface of the etched portion as the etching proceeds. This phenomenon was not observed when the high frequency power was 100 W. This is because the increase in the high frequency power increases the energy of the ions incident on the etching surface and decomposes the deposited polymer 8.

FIG. 5 shows changes in the etching rates of GaAs and Al0.2Ga0.8As when the partial pressure ratio of CHF _{3 in} the mixed gas is changed. In FIG. 5, the vertical axis represents the etching rate, the horizontal axis represents the partial pressure ratio of CHF ₃ , 12 represents the change in the GaAs etching rate, and 13 represents the change in the Al0.2Ga0.8As etching rate. The pressure was 30 mTorr and the high frequency power was 50 W.

[0051] In FIG. 5, GaAs etch rate, the partial pressure ratio of CHF ₃ increases with increasing partial pressure ratio of CHF ₃ to 40%. Further, when the partial pressure ratio of CHF ₃ is 10% or more, the effect of protecting the side wall of the recess structure 9 by the polymer 8 appears. Then, when the partial pressure ratio of CHF ₃ exceeds 40%, the etching rate of GaAs begins to decrease, and when it exceeds 60%, it rapidly decreases. This is because the polymer 8 is deposited on the surface to be etched due to the increase in the partial pressure ratio of CHF ₃ .

On the other hand, the etching rate of Al0.2Ga0.8As is CH
It rapidly decreases with an increase in the F ₃ partial pressure ratio, and when the CHF ₃ partial pressure ratio is 10%, the etching rate for GaAs is reduced to about 1/2. When the partial pressure ratio of CHF ₃ is 40%, the etching rate becomes minimum, and when the partial pressure ratio of 40% or more, the etching rate slightly increases. This is because the self-bias voltage between the plasma and the substrate to be etched 1 increases as the partial pressure ratio of CHF ₃ increases, and the etching barrier layer such as AlF ₃ formed on the AlGaAs surface is removed by the sputtering action. Is. When the mixed crystal ratio of AlAs increases, the etching rate of AlGaAs decreases as compared with FIG.

Therefore, from the result of FIG. 5, BCl ₃ and CHF ₃
When a mixed gas system of and is used, it becomes clear that good etching characteristics can be obtained when the partial pressure ratio of CHF ₃ is 10% or more and 60% or less.

FIG. 6 shows that the length of the gate electrode formed by the selective etching method of the present invention in the recess etching step is 0.25.
In the μm GaAs MODFET, it shows the existence of different annealing temperatures for the degree of recovery of damage caused by etching by annealing. In FIG. 6, the horizontal axis is the annealing temperature, and the vertical axis is the Ga of which the gate electrode length is 0.25 μm with respect to the transconductance of a GaAs MODFET having a recess structure 9 formed by conventional wet etching and a gate electrode length of 0.25 μm.
It is the ratio of the transconductance (gm) of AsMODFET. Then, it was annealed in an Ar atmosphere for 30 minutes.

In FIG. 6, transconductance (gm)
Recovers more rapidly than 300 degrees Celsius, from 400 degrees Celsius to 60 degrees Celsius
GaAs MOD formed by wet etching up to 0 degree
Recovers over 95% of FET. However, the transconductance (gm) deteriorates at 700 degrees Celsius. This is A of AlGaAs layer
This is because l begins to diffuse and the density of the two-dimensional electron gas generated at the interface between the AlGaAs layer and the underlying intrinsic GaAs decreases. Therefore, annealing for damage recovery after etching by the RIE method needs to be performed at 600 ° C. or less.

[0056]

As described above, according to the present invention, a mixed gas of a gas containing Cl as a constituent element, for example, BCl ₃ and a gas containing C, H, F, for example, CHF ₃ is used. Since it is difficult to generate GaF ₃ by etching, it is possible to prevent the occurrence of dead time.

Further, since the protective film of the polymer film is formed on the etched portion such as the depression, undercut and overhang can be prevented.

Since the plasma contains an appropriate amount of F,
Only the GaAs layer on the AlGaAs layer can be selectively etched.

Further, since a vertical cross-sectional shape having no dead time and no undercut can be obtained, when the recess structure of the arrangement portion where the gate electrode of GaAs MESFET or GaAs MODFET is provided is formed by the RIE method, the etching depth is The in-plane uniformity can be improved, and the recess width can be accurately controlled. As a result, GaAs MESFETs and GaAs MODFETs with finer electrode structures than conventional
Can be formed with high accuracy.

Further, the substrate after etching is set to 400 ° C. or more.
By annealing above 600 ° C, the GaAs layer and A
Even in a GaAs MODFET having a heterojunction in which lGaAs layers are stacked, it is possible to recover the damage generated during etching without damaging the two-dimensional electron gas at the interface between the GaAs layer and the AlGaAs layer, and prevent the deterioration of device characteristics. .

[0061] Also, since the cleaning of organic alkaline aqueous solution of the substrate after the etching, it is possible to completely remove the protective film of the polymer film in the depression and the like.

[Brief description of drawings]

FIG. 1 is a process cross-sectional view when forming an etching mask in GaAs etching.

FIG. 2 is a process sectional view during etching in GaAs etching.

FIG. 3 is a process cross-sectional view after cleaning in GaAs etching.

FIG. 4 is a characteristic diagram showing the relationship between etching time and etching depth by the RIE method using a mixed gas of BCl ₃ and CHF ₃ .

FIG. 5: CHF by RIE method using a mixed gas of BCl ₃ and CHF ₃
FIG. 3 is a characteristic diagram showing the relationship between the partial pressure ratio of ₃ and the etching rates of GaAs and AlGaAs.

FIG. 6 is a characteristic diagram showing a relationship of a mutual conductance recovery ratio with respect to an annealing temperature.

FIG. 7 is a process cross-sectional view when forming an etching mask near the gate electrode of a conventional GaAs MESFET.

FIG. 8 is a process cross-sectional view after etching in the vicinity of the gate electrode of a conventional GaAs MESFET.

FIG. 9 is a process cross-sectional view after forming a gate electrode of a conventional GaAs MESFET.

FIG. 10 is a characteristic diagram showing a relationship between etching time and etching depth of GaAs etching by the RIE method using a conventional mixed gas of BCl ₃ and SF ₆ .

FIG. 11 is a cross-sectional view after etching a GaAs substrate by a RIE method using a conventional gas system.

FIG. 12 is a cross-sectional view after etching a GaAs substrate by a RIE method using a conventional gas system.

[Explanation of symbols]

2 Etching mask 7 substrate 8 polymer 9 Recess structure (dent)

Claims (7)

(57) [Claims] 1. A high frequency power is applied to a mixed gas containing a gas containing Cl as a constituent element and a gas composed of three elements of C, H and F, and the mixed gas is generated in the plasma generated in the mixed gas. , exposing the surface of the substrate made of GaAs, the surface of the substrate is etched, the substrate after etching an organic alk
An etching method characterized by cleaning with an aqueous solution . 2. High frequency power is applied to a mixed gas containing a gas containing Cl as a constituent element and a gas composed of three elements of C, H and F, and the mixed gas is generated in the plasma generated in the mixed gas. Exposing the surface of a substrate obtained by sequentially growing an AlGaAs layer and a GaAs layer on a semi-insulating substrate, selectively etching only the GaAs layer of the substrate, and then etching the substrate.
An etching method characterized by cleaning with an aqueous solution of Lucari . 3. A high frequency power is applied to a mixed gas containing a gas containing Cl as a constituent element and a gas composed of three elements of C, H and F, and the mixed gas is generated in the plasma generated in the mixed gas. A method for manufacturing a semiconductor device, which comprises exposing the surface of a substrate of a GaAs MESFET made of GaAs, etching the surface of the substrate, and then annealing the substrate at a temperature of 400 ° C. or higher and 600 ° C. or lower. 4. A high frequency power is applied to a mixed gas containing a gas containing Cl as a constituent element and a gas composed of three elements of C, H and F, and the mixed gas is generated in the plasma generated in the mixed gas. Exposing the surface of a substrate of a GaAs MODFET formed by sequentially growing an AlGaAs layer and a GaAs layer on a semi-insulating substrate,
A method of manufacturing a semiconductor device, which comprises etching only the GaAs layer and then annealing the substrate at a temperature of 400 ° C. or higher and 600 ° C. or lower. 5. The method for manufacturing a semiconductor device according to claim 3 or 4 , wherein the mixed gas is composed of a gas containing BCl ₃ and CHF ₃ . 6. The method of manufacturing a semiconductor device according to claim 5 , wherein the partial pressure ratio of CHF ₃ gas is 10% or more and 60% or less. 7. The method of manufacturing a semiconductor device according to any one of claims 3-6, a method of manufacturing a semiconductor device, which comprises cleaning a substrate after etching with organic alkaline aqueous solution.