JP2001144031A - Producing method for semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a producing method for semiconductor device, with which the deposition of polymerized films on the surface of a wafer is suppressed in the case of etching an insulating film with a resist pattern as a mask and satisfactory alloy reaction can be provided between a metal depositing film and the surface of the wafer while unnecessitating the mask formation of a side in a following lift-off process. SOLUTION: By using a gas mixing the O2 (gas of a volume flow ratio >=8% and <=25% in a total flow and CF4 gas as an etching gas, the deposition of CF polymerized films on the surface of a wafer 22 is suppressed, the etching selectivity of a silicon nitride film 24 is improved, a metal film 23 on the wafer 22 is surely separated from a metal film 23 on a mask 21 in the case of depositing the metal film 23 backward from the side wall of the mask 21, and following lift-off can be easily executed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method of manufacturing a semiconductor device, and more particularly, to a method of manufacturing a semiconductor device suitable for forming an ohmic metal on a bottom surface of a contact hole by a lift-off method.

[0002]

2. Description of the Related Art VLSI (Very Large Scale Integrated)
circuit) requires anisotropic etching of the metal film. At present, both plasma etching and reactive ion etching have reached a sufficient level and have been put to practical use. However, when using uncommon metals, one attractive method is the lift-off method.

In the lift-off method, an inverted pattern is formed by using a lithography technique, and a metal film is deposited on a masked substrate. Then, the mask and unnecessary metal layers are lifted off. To ensure lift-off,
As shown in FIGS. 4 and 5, a known resist material having different photosensitivity in the thickness direction such that the shape of the mask 1 becomes a T-shape or an inverted taper type is used (FIGS. 4A and 5A). By using such a mask whose width is wider toward the upper part, the deposition of the metal film 3 on the mask side wall is suppressed, and the metal film on the mask 1 and the metal film on the substrate 2 are reliably separated after the deposition ( 4B and 5B) and subsequent lift-off using a solvent for dissolving the lithographic pattern, it is possible to reliably remove only the unnecessary metal film on the mask 1 and leave the metal film 3 only on the substrate 2 (FIG. 4B, FIG. 5B). 4C, 5C).

[0004] Naturally, it is necessary to clean the substrate before forming the metal film. In the most common cleaning method, HF (hydrogen fluoride) or its buffer is used for the silicon substrate. Can be HC for gallium arsenide substrate
1 (hydrochloric acid) solution is used. These solutions include silicon,
It functions to remove a thin oxide film remaining on the surface of the gallium arsenide substrate.

[0005] By using the above method, a metal film is left only in a necessary portion, and a sufficiently low contact resistance to the substrate can be obtained. In an actual process, as shown in FIG. 6, for example, these insulating films 4 are anisotropically etched by a reactive ion etching method using a resist 1 on a silicon-based insulating film 4 such as a silicon nitride film or an oxide film as a mask. (FIGS. 6A and 6B), it may be necessary to form the metal film 3 only on the semiconductor substrate 2 such as a silicon substrate or a gallium arsenide substrate exposed by etching (FIG. 6).
C).

For the etching of such an insulating film, CF is used.
Gases mainly composed of fluorocarbons such as ₄ or the like, fluorohydrocarbons such as CHF ₃ or a mixture thereof are widely used. However, when etching is performed using these gases, a CF-based polymer film 6 is deposited on the surface of the substrate 2 (FIGS. 6B and 6C).

[0007]

The polymer film 6 formed by the etching as described above can be removed by O ₂ (oxygen) plasma ashing or sulfuric acid / hydrogen peroxide treatment, but the resist 1 is also removed at the same time. Then, it is necessary to form a mask for lift-off again. However, due to misalignment of the mask pattern, a step is formed on the side wall of the interface between the resist and the insulating film, and the metal film is attached to the step during the subsequent deposition of the metal film. The problem that the film remains remains. Therefore, there is a demand for a technique for removing the polymer film 6 which causes an increase in the contact resistance between the metal film 3 and the substrate 2 while leaving such a resist mask 1.

In particular, when a polymer film is present at the interface between the substrate and the deposited film, as in the case of forming a low-resistance ohmic contact to a gallium arsenide substrate, the ohmic metal (Ni / Au / Ge) is formed.
In the alloy treatment performed after the lift-off of (e.g.), the polymer film present at the interface inhibits the alloy reaction between the gallium arsenide substrate and the ohmic metal (FIG. 6D), which causes a problem that an ohmic contact cannot be obtained. I will.

The present invention has been made in view of the above-mentioned problems, and suppresses the deposition of a polymer film on a substrate surface when etching an insulating film using a resist pattern as a mask, so that it is not necessary to form a mask again in a subsequent lift-off step. It is an object of the present invention to provide a method of manufacturing a semiconductor device capable of obtaining a good alloy reaction between a metal deposition film and the surface of a substrate while maintaining the same.

[0010]

In order to solve the above-mentioned problems, in a method of manufacturing a semiconductor device according to the present invention, a silicon-based insulating film formed on a surface of a semiconductor substrate is formed on the insulating film. Using a resist pattern as a mask, a contact hole is formed by dry etching using a mixed gas of a fluorocarbon-based gas and an oxygen gas as an etching gas, and then the mask is lifted off when a metal film is deposited on the bottom surface of the contact hole. It is characterized in that it is used as a mask.

That is, according to the present invention, a contact hole is formed in a silicon-based insulating film by dry etching using a mixed gas of a fluorocarbon-based gas such as CF ₄ or CHF ₃ and an oxygen gas as an etching gas. The deposition of the CF-based polymer film on the surface is suppressed, whereby the mask used for the etching can be used as a mask for the metal deposition film in the subsequent lift-off step, and the re-use for the lift-off is performed again. It is not necessary to form a mask. Further, a favorable alloy reaction between the substrate surface and the metal deposition film on the bottom surface of the contact hole can be obtained, and an increase in contact resistance can be prevented.

Here, when the silicon insulating film is a silicon nitride film, a mixture of oxygen gas having a volume flow rate ratio of 8% or more and 25% or less with respect to the total flow rate as an etching gas and carbon tetrafluoride gas (CF ₄ ) is used. If a gas is used, the etching selectivity of the silicon nitride film with respect to the resist mask is improved, so that the side wall of the contact hole recedes from the side wall of the resist, and when the metal film is deposited on the bottom surface of the contact hole, the metal deposited on the bottom surface of the contact hole is removed. Complete separation between the film and the metal film deposited on the upper surface of the mask is achieved, and subsequent lift-off can be easily performed.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A method for manufacturing a semiconductor device according to an embodiment of the present invention will be described below with reference to the drawings.
In this embodiment, gallium arsenide (G) is used as a semiconductor substrate.
a case where a substrate is used, a contact hole is formed in a silicon nitride film as an interlayer insulating film formed thereon, and then a metal film is deposited as ohmic metal on the bottom surface of the contact hole by a lift-off method.

First, a silicon nitride film 24 is formed on a substrate 22 by using a known film forming technique such as a CVD method or a sputtering method, and a predetermined resist pattern 21 is formed thereon by using a known lithography technique. (FIG. 1A). Next, using the resist pattern 21 as a mask, the silicon nitride film 2
4 is dry-etched to form a contact hole 25 (FIG. 1B).

The dry etching of the silicon nitride film 24 is performed by reactive ion etching (RI) conceptually shown in FIG.
E: Reactive Ion Etching) device. The etching chamber 10 includes an upper electrode 13 and a lower electrode 14, and introduces an etching gas (reactive gas) from a gas introduction hole 11 and exhausts the inside of the etching chamber 10 from an exhaust hole 12 to a predetermined degree of vacuum (for example, 2.5 Pa). The wafer W including the substrate 22, the silicon nitride film 24, and the resist mask 21 is placed on the lower electrode 14, and an RF (high frequency) power supply 15 supplies a power density of 5 W / cm ² or more to the lower electrode 14. As a result, plasma of an etching gas is formed between the upper electrode 13 and the lower electrode 14, and the silicon nitride film 24 is anisotropically etched. At this time, overetching of 20% in terms of a silicon nitride film was performed.

In this embodiment, a mixed gas (CF ₄ / O) of carbon tetrafluoride gas and oxygen gas is used as an etching gas.
₂ ) was used. Therefore, as shown in Table 1, the volume flow rate ratio of the oxygen gas to the total flow rate of the etching gas was changed from 0 to 25.
%, And etching was performed under each condition. For comparison, etching was performed with a conventionally used mixed gas of carbon tetrafluoride gas and hydrogen gas (CF ₄ / H ₂ ).

[0017]

[Table 1]

Next, AuGe (gold-germanium alloy) 160 nm (12 Wt% Ge) /
A metal film 23 made of 40 nm of Ni (nickel) was deposited. At this time, the resist mask 21 used as an etching mask is used as it is as a mask for lifting off the metal film 23. Thereafter, the metal film 23 on the resist mask 21 was removed together with the mask 21 from the substrate 22 by lift-off, and the metal film 23 was formed only in the contact hole 25 formation portion.

Thereafter, the substrate 22 is subjected to alloying at 450 ° C. for 30 seconds in an atmosphere of 4% H ₂ /96% N ₂ , and then an alloy reaction between AuGe / Ni and GaAs is performed by SEM (scanning) of the contact region. Cross-section observation using a scanning electron microscope).

As a result, when the silicon nitride film 24 is etched under CF ₄ / H ₂ conditions, AuGe / N
Although no alloy reaction between i and GaAs was observed and no ohmic contact was formed (corresponding to FIG. 6D), when the silicon nitride film 24 was etched under the CF ₄ / O ₂ condition, Good alloy reaction was confirmed (FIG. 1D). Further, with the increase in the flow rate ratio of O _2, the silicon nitride film 24 slightly retreated from the side wall of the resist mask 21 in a manner of side etching, so that the metal film 23 adhered to the side wall of the silicon nitride film 24. No (Fig. 1B, Fig. 1C).

This is because the addition of H ₂ to CF ₄ captures F and generates HF, thereby increasing the C / F ratio and facilitating the deposition of a CF-based polymer, that is, a polymer film. On the other hand, the addition of O ₂ to CF ₄ makes it possible to suppress the deposition of the polymerized film in order to remove C by the generation of CO and CO ₂ . Alloying properties can be improved without the intervention of a polymer film. Therefore, according to the present embodiment, since the deposition of the polymer film on the surface of the substrate 22 can be suppressed, the mask 21 used for etching can be used as a mask for the metal film 23 in the subsequent lift-off step. A good alloy reaction between the substrate 22 and the metal film 23 can be obtained, and an increase in contact resistance can be prevented.

Further, the polymerization film attached to the sidewall of the silicon nitride film 24 to function as a protective film from etching in the case of CF ₄ / H ₂ (corresponding to FIG. 6B), CF ₄ /
In the case of O ₂ , since the deposition of the polymer film was suppressed, the etching selectivity of the silicon nitride film 24 was increased, and the aspect of side etching as shown in FIG. 1B was confirmed. Accordingly, the silicon nitride film 24 is deposited during the subsequent deposition of the metal film 23.
The adhesion of the metal film 23 to the side wall of the contact hole 25, that is, the side wall portion of the contact hole 25 is suppressed, and the metal film 23 on the surface of the substrate 22 which is the bottom surface of the contact hole 25 and the resist mask 21
A reliable separating action from the metal film 23 on the upper surface is obtained, and subsequent lift-off is facilitated.

FIG. 2 shows that O _{2 with} respect to the total CF ₄ / O ₂ flow rate.
For reference, the etching rate of the silicon nitride film 24 and the uniformity of the etching rate in the wafer surface with respect to the flow rate (volume flow rate) ratio are shown for reference. The etching rate is O ₂
8.33% (CF ₄ / O ₂ = 22sccm / 2sccm) cm25.00% (CF ₄ / O
₂ = 18 sccm / 6 sccm), and the in-plane uniformity is as good as ± 1.6% or less.

Although the embodiments of the present invention have been described above, the present invention is, of course, not limited thereto, and various modifications can be made based on the technical concept of the present invention.

For example, in the above embodiment, CF ₄ / O ₂ gas was used as an etching gas, but CHF ₃ / O ₂ was used.
It is also possible to use a mixed gas of another fluorocarbon-based gas such as _two gases and an oxygen gas. However, in this case, H
Therefore, the volume flow ratio of the O ₂ gas needs to be higher than that of CF ₄ / O ₂ from the viewpoint of suppressing the deposition of the polymer film.

In the above embodiment, a silicon nitride film is described as a silicon-based insulating film, but the present invention can be applied to a silicon oxide film. Further, the composition of the ohmic metal is not limited to the above-described composition, and it goes without saying that the composition can be applied to other metals.

[0027]

As described above, according to the method of manufacturing a semiconductor device of the present invention, the deposition of a CF-based polymer film can be suppressed during dry etching for forming a contact hole.
A good alloying reaction between the substrate surface and the metal film at the bottom of the contact hole can be obtained while preventing the increase in contact resistance, while enabling the mask used for the etching to be used as a mask for the metal film in the subsequent lift-off process. I do.

According to the second aspect of the present invention, the etching rate of the silicon nitride film with respect to the resist is improved, and the side wall of the contact hole recedes from the side wall of the resist to exhibit a side-etching state. At the time of film deposition, a metal film deposited on the bottom surface of the contact hole and a metal film deposited on the upper surface of the mask are completely separated, and subsequent lift-off can be easily performed.

Further, according to the invention of claim 3, it is possible to obtain a sufficient etching effect on the silicon nitride film to obtain the above-mentioned effect.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view of a wafer for explaining a method of manufacturing a semiconductor device according to an embodiment of the present invention, wherein A is a resist mask forming step, B is a contact hole forming step, and C is a metal film deposition step. , D respectively indicate the lift-off process.

FIG. 2 is a diagram showing a relationship between an O ₂ gas flow ratio with respect to a total flow rate of CF ₄ / O ₂ gas, which is an etching gas used in an embodiment of the present invention, and an etching rate and in-plane uniformity of a silicon nitride film. It is.

FIG. 3 is a conceptual configuration diagram of a reactive ion etching apparatus.

FIG. 4 is a schematic cross-sectional view of a wafer for explaining an example of a conventional method of forming a metal film by a lift-off method.
Represents a resist mask forming step, B represents a metal film deposition step,
C indicates a lift-off step.

FIG. 5 is a schematic cross-sectional view of a wafer illustrating another example of a method of forming a metal film by a conventional general lift-off method;
A indicates a resist mask forming step, B indicates a metal film deposition step, and C indicates a lift-off step.

FIG. 6 is a schematic cross-sectional view of a wafer for explaining a method of manufacturing a semiconductor device according to a conventional example, where A is a resist mask forming step, B is a contact hole forming step, C is a metal film deposition step, and D is a lift-off. Each step is shown.

[Explanation of symbols]

21: resist mask, 22: substrate, 23: metal film, 2
4: silicon nitride film, 25: contact hole.

Claims (3)

[Claims] 1. A silicon-based insulating film formed on a surface of a semiconductor substrate, using a resist pattern formed on the insulating film as a mask and a mixed gas of a fluorocarbon gas and an oxygen gas as an etching gas. A method for manufacturing a semiconductor device, comprising: forming a contact hole by etching; and using the mask as a lift-off mask when depositing a metal film on a bottom surface of the contact hole. 2. The silicon-based insulating film is a silicon nitride film, and the etching gas has a volume flow rate ratio of 8 to a total flow rate.
2. The method according to claim 1, wherein the gas is a mixed gas of oxygen gas of not less than 25% and not more than 25% and carbon tetrafluoride gas. 3. The method according to claim 2, wherein the RF power density in the dry etching is 5 W / cm ² or more.