JPH06188226A - Dry etching - Google Patents

Abstract

PURPOSE:To perform an etching of an Si material layer using a Br*, which is capable of attaining a high selection ratio to a base SiOx material layer, without generating residues due to a natural oxide film. CONSTITUTION:In a process for forming a transfer electrode pattern of a horizontal register of a CCD, before a second layer polycrystalline Si layer 5 having a natural oxide film 6 on its surface is etched, the mixed gas produced in the mixing ratio of NH3: NF3=5:1 is discharged in a down flow etching device and the surface of a wafer is covered with a condensed layer 8. This layer 8 has a composition of (NH4)2SiF6 and when the wafer is heated, F etching species are discharged and the film 6 directly under the layer 8 is removed by reaction. This process is an isotropic chemical reaction and the film 6 does not exist even on high step parts. After the removal, when the layer 5 is immediately etched with HBr gas, a second transfer electrode 5a can be formed without generating etching residues.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dry etching method applied to the manufacture of semiconductor devices and the like, and particularly suppresses the generation of etching residues caused by a natural oxide film, and further, silicon oxide (SiO _x ; The present invention relates to a method of etching a silicon (Si) -based material layer while maintaining extremely high selectivity with respect to an x = 2) -based underlayer material layer.

[0002]

2. Description of the Related Art With the recent progress in high integration and high performance of semiconductor devices as seen in VLSI, ULSI, etc., high anisotropy, high speed and high anisotropy are achieved even in etching Si-based material layers. There is a strong demand for a technology that simultaneously achieves various requirements such as selectivity, low damage, and low pollution.

Among various processes for etching the Si-based material layer, polycrystalline Si, refractory metal silicide,
The processing of gate electrodes of MOS transistors and the processing of transfer electrodes of CCDs for etching polycide and the like are processes that require particularly high precision. Moreover, in recent years, the thickness of the gate oxide film forming the underlayer of the Si-based material layer has been thinned to around 10 nm in recent years due to the demand for high-speed operation. Maintaining is also an important issue.

Needless to say, the technology that has played a leading role in anisotropic processing is RIE (reactive ion.
Etching). However, in RIE, since accelerated reactive ions collide with the substrate to ensure high anisotropy by a mechanism that promotes reaction with active species, it is essentially difficult to obtain high selectivity to the substrate. . Therefore, conventionally Si
Attempts have been made to simultaneously achieve high anisotropy and high selectivity in dry etching of a system material layer.

For example, Proceedings of
 Symposium on Dry Procedures
S, (1988), II-4, S for etching gas.
iCl _FourAnd do not cool the wafer to around -30 ° C.
There is a report of a technique for performing etching. this is,
Cl as the main etching species of the Si-based material layer from the vapor phase^*
Is supplied simultaneously with the supply of Si,
By competing Si deposition and sputter removal,
SiO underlayer_xImprove selectivity for system material layers
It is what However, the amount of Si particles generated
, Process reliability, reproducibility, and
Practicality remains questionable from the viewpoint of maintainability.

The 36th Joint Lecture on Applied Physics (Spring Annual Meeting 1989), Proceedings Proceedings p. 571, the abstract number 1p-L-5 includes CFC113 (C ₂ Cl
₃ F ₃ ) and other sedimentary chlorofluorocarbons (CF
C) A technique of etching a polycide film using a gas has been reported. This is to reduce the incident ion energy required for anisotropic processing and improve the selectivity by utilizing a carbon-based polymer for sidewall protection. However, in order to prevent anisotropy deterioration due to excess radicals during overetching, it is necessary to deposit a large amount of carbon-based polymer, which is likely to increase particle contamination. Further, the CFC 113 corresponds to a so-called specific CFC and cannot be used in the near future, so that the process itself loses practicality.

From this background, what is regarded as the mainstream of etching of Si-based material layers in the future is a method using a bromine (Br) -based etching species. For example, Diges
tof Papers, 1989 2nd Micro
Process Conference, p. In 190, an example in which anisotropic etching of an n ⁺ type polycrystalline Si layer is performed by RIE using HBr is reported. Br
Has a large atomic radius and does not easily penetrate into the crystal lattice or the grain boundaries of the Si-based material layer, so that, for example, Br ^* alone does not cause spontaneous etching of the Si-based material layer. However, high anisotropy can be achieved in the presence of the ion assist mechanism.

Another reason why Br is supported as an etching species for the Si-based material layer is that theoretically high selectivity can be secured for the SiO _x- based material layer. This is because when the interatomic bond energies are compared, the Si—O bond (464 kJ / mole) is the Si—Br bond (368).
It is larger than kJ / mole). However, this high selectivity is greatly reduced by the interposition of carbon that exerts a reducing action on SiO _x . Therefore, the 36th Joint Lecture on Applied Physics (Spring Annual Meeting 1989), Proceedings Proceedings p. 572, Abstract No. 1p-L-7, and Monthly Semiconductor World (Published by Press Journal)
January 1990 issue, p. 81-84, a process example in which carbon contamination is thoroughly removed from the etching reaction system is also reported.

[0009]

As described above, since the Si-based material layer is etched by the Br-based etching species, there is no risk of particle contamination, and the CFC-based gas is not necessary. If it can be eliminated, it is an extremely effective process. However, since a resist mask is used as an etching mask in the actual process,
It is not possible to completely eliminate carbon from the etching reaction system. Therefore, in practical use, the incident ion energy is lowered, the wafer during etching is cooled, and the selectivity for the etching mask is improved, and the carbon supply amount from here is reduced as much as possible to address this problem. There is. Due to these measures, 1 against SiO _x materials
It has become possible to achieve a high selection ratio as high as 00 or more.

However, in the above process, an unavoidable problem has now emerged due to the high selection process. That is the presence of a natural oxide film existing on the surface of the Si-based material layer. Selectivity to SiO _x type material layer is 10
When the temperature rises to 0 or more, the natural oxide film formed on the surface of the Si-based material layer immediately acts as an etching mask due to the residual moisture existing in the etching chamber, which causes etching residues. Especially when the Si-based material layer is etched on a substrate having a large step, fence-like etching residues called stringers are likely to occur. This problem will be described with reference to FIG. 2 by taking the etching of the second-layer polycrystalline Si layer in the CCD manufacturing process as an example.

FIG. 2A shows a state of the wafer before etching. To briefly explain the steps so far,
The surface of the single-crystal Si substrate 11 is thermally oxidized to form a gate oxide film 12 made of SiO ₂ , and the first-layer polycrystalline Si layer 13 is patterned on the gate oxide film 12. The surface of the formed pattern of the first-layer polycrystalline Si layer 13 is thermally oxidized to form the interlayer insulating film 14, and then the second-layer polycrystalline Si layer 15 is deposited on the entire surface of the wafer by the CVD method or the like. The resist mask 17 is patterned into a predetermined shape by photolithography. The surface of the second-layer polycrystalline Si layer 15 is covered with the natural oxide film 16. This is because when the wafer is exposed to the atmosphere after completion of CVD, or when the resist material is applied, the surface is not exposed to moisture. It was formed due to contact.

Generally, in dry etching of a Si-based material layer, a natural oxide film called breakthrough is removed before the target etching. However, the second layer polycrystal S
Since the i layer 15 has a large surface step reflecting the pattern of the first-layer polycrystalline Si layer 13, even if a breakthrough is performed under normal conditions, as shown in FIG. On the side wall surface of the portion, the native oxide film 16a remains without being completely removed. This is because the incidence of ions is originally small on the side wall surface of the pattern, and the apparent film thickness of the native oxide film 16 when viewed along the normal line direction of the wafer is extremely large.

Thus, the natural oxide film 16a is formed on the side wall surface of the step.
When the second-layer polycrystalline Si layer 15 is etched while leaving the pattern, as shown in FIG.
Fence-shaped residue 15b remains on gate oxide film 12 at the time when 5a is formed. This is because the portion of the native oxide film 16a on the side wall surface of the step that has an apparently large film thickness functions as an etching mask.

The residue 15b causes a short circuit of the gate wiring and deteriorates the step coverage (step coverage) of the interlayer insulating film. Therefore, for example, overetching is performed under the condition that the radical reaction is the main component. It will need to be removed. However, it is difficult to perform this over-etching while maintaining high selectivity with respect to the SiO _x -based material layer, and normally, as shown in FIG. 2D, the gate oxide film 12 and the interlayer insulating film 14 are formed. Will be eroded.

As described above, in the etching of the Si-based material layer, in many cases, the natural oxide film to be removed and the underlying material layer that should not be removed have the same chemical composition, so that the residue It is difficult to achieve both suppression and substrate selectivity.

Therefore, the present invention is a dry etching method for a Si-based material layer capable of suppressing the generation of etching residues due to a natural oxide film and maintaining high selectivity with respect to the underlying SiO _x -based material layer. The purpose is to provide a method.

[0017]

The dry etching method of the present invention is proposed in order to achieve the above-mentioned object, and a silicon-based material layer having a natural oxide film on the surface layer is condensed with fluorine atoms. The method includes a step of covering with a layer, a step of removing the natural oxide film by heating and reacting with the condensed layer, and a step of etching the silicon-based material layer immediately after removing the natural oxide film.

The present invention also provides that the condensation layer comprises NH ₃ and N
It is formed by the depositable substance generated in the etching reaction system by the discharge dissociation of the mixed gas with F ₃ .

The present invention further uses a Br-based chemical species as a main etching species in the etching of the Si-based material layer.

[0020]

In order to prevent the generation of the etching residue due to the natural oxide film in the high step portion as described above, the inventor of the present invention is isotropic and gentle in advance before etching the Si-based material layer. We considered that it is necessary to remove the natural oxide film by a chemical reaction and focused on the condensed layer containing fluorine atoms.

The condensed layer is a thin film deposited on the surface of a wafer under a specific discharge condition, and is a reaction product of etching reaction.
It is composed of decomposition products of etching gas, or reaction products of both of them. If the incident ion energy is set to be sufficiently low under the above discharge conditions, the condensed layer can cover the surface of the wafer almost uniformly regardless of the step. If the upper surface of the wafer is a natural oxide film on the surface of the Si-based material layer, the condensed layer is formed in contact with this natural oxide film.

In the present invention, the condensation layer containing a fluorine atom is used, and the fluorine-based etching species generated from the condensation layer when the wafer is heated is used to form a natural oxide film (SiO _x) immediately below. ) Is removed. This removal is based on an isotropic chemical reaction,
The natural oxide film is uniformly removed without being restricted by the shape of the wafer surface. As a result, the clean surface of the Si-based material layer is exposed, and the etching in the subsequent step proceeds smoothly without generating a residue.

If the surface of the clean Si-based material layer is left to stand, it may come into contact with residual oxygen in the etching reaction system, or in some cases oxygen in the atmosphere during transportation, so that a natural oxide film is again formed on the surface. Since it grows, the Si-based material layer is etched immediately after removing the natural oxide film. In other words, the above wafer heating must be performed immediately before the etching in the next step.

By the way, specifically, the above-mentioned condensed layer can be formed by a depositable substance generated in the etching reaction system from the NH ₃ / NF ₃ mixed gas under discharge dissociation conditions. This technology is described in IEICE Technical Report p. 35-36, S
It was inspired by the content reported in DM 89-48 (1989). According to this paper, the mixing ratio of NH ₃ and NF ₃ is 5: 1 in a normal downflow etching apparatus.
When microwave discharge is performed using the mixed gas described above, a condensed layer containing fluorine atoms is formed. Moreover, it is reported that when this condensed layer is formed on a silicon substrate covered with a natural oxide film and then heated to 100 ° C. in vacuum, the condensed layer is sublimated and at the same time the natural oxide film is decomposed and removed. ing.

In this condensed layer, the etching reaction product SiF ₄ due to F ^* of the natural oxide film further reacts with the discharge reaction product NF ₄ F in the gas phase to form (NH ₄ ) ₂ Si.
It is believed that the compound of composition F ₆ is deposited.

The necessity of thorough removal of the natural oxide film as described above has arisen because the subsequent etching of the Si-based material layer is an ultrahigh selection process. This ultra-high selection process is specifically an etching using a Br-based chemical species that does not spontaneously etch the SiO _x -based material layer in the absence of the ion assist mechanism. According to the present invention, it is possible to achieve both high selectivity for the underlayer and elimination of the influence of the natural oxide film, and it is possible to truly enhance the practicality of etching with the Br-based chemical species.

[0027]

EXAMPLES Specific examples of the present invention will be described below.

[0028]Example 1 In this embodiment, the present invention is applied to transfer of a horizontal transfer register of CCD.
It is applied to the formation of electrode patterns and has a natural oxide film on the surface.
The surface of the second-layer polycrystalline Si layer₃/ NF ₃Mixed moth
Wafer after being coated with a condensed layer produced by the discharge of
Heat to remove the natural oxide film, and then remove HBr gas.
It is an example in which the second-layer polycrystalline Si layer is etched by using this.
This process will be described with reference to FIG.

FIG. 1A is a schematic cross-sectional view of a wafer used as an etching sample in this embodiment. A gate oxide film 2 obtained by thermally oxidizing the surface of a single crystal silicon substrate 1 is used. First layer consisting of a polycrystalline Si layer through
Transfer electrode 3 is formed, and the second transfer electrode 3 is formed by the surface oxidation of the first transfer electrode 3 via the SiO ₂ interlayer insulating film 4
A second-layer polycrystalline Si layer 5 is laminated, and a resist mask 7 which is an etching mask for the second-layer polycrystalline Si layer 5 is formed at a position facing the first transfer electrode 3. In the second-layer polycrystalline Si layer 5, there is a step that follows the pattern of the first transfer electrode 3, and the surface thereof has a moisture content when the atmosphere is released after film formation or during the resist coating step. The native oxide film 6 has grown due to the contact with.

Next, this wafer was set in a downstream type etching apparatus for generating plasma by microwave discharge, and a condensed layer was formed under the following conditions as an example. NH ₃ flow rate 1000 SCCM NF ₃ flow rate 200 SCCM Gas pressure 133 Pa Microwave power 1 kW (2.45 G
Hz) This process proceeded under the condition that charged particles were not incident on the wafer, and as shown in FIG. 1B, the condensed layer 8 containing F atoms was deposited on the entire surface of the wafer. At this time, at least in the vicinity of the interface between the natural oxide film 6 and the condensed layer 8, the etching product SiF ₄ due to F ^* of the natural oxide film 6 and the discharge reaction product NF ₄ F in the gas phase react with each other (N
Compounds of H _{4) 2} SiF ₆ having the composition is formed.

Next, this wafer was set in an RF bias application type magnetic field microwave plasma etching apparatus. This etching apparatus has a multi-chamber structure in which an etching chamber and a buffer chamber are connected by a vacuum load lock mechanism, and a wafer can be transferred between the chambers without exposing to the atmosphere. The etching chamber is evacuated to a high vacuum by a cryopump having excellent water removal characteristics. The wafer stage in the buffer chamber has a heating mechanism so that the wafer placed on the wafer stage can be heated to about 100 ° C.

First, the wafer is loaded into the buffer chamber,
Heated to about 100 ° C. Due to this heating, the natural oxide film 6 is condensed by the action of the F-based chemical species supplied from the condensation layer 8 except for the region shielded by the resist mask 7 as shown in FIG. 1 (c). It was removed immediately with layer 8. Since this process is based on an isotropic chemical reaction, the natural oxide film 6 did not remain on the step side wall surface of the second-layer polycrystalline Si layer 5.

Next, the above-mentioned wafer was transferred to an etching chamber using a vacuum load / lock mechanism, and the second-layer polycrystalline Si layer 5 was etched under the following conditions as an example. HBr flow rate 50 SCCM Gas pressure 1.3 Pa Microwave power 850 W (2.45 GH
z) RF bias power 10 W (2 MHz) Wafer temperature 0 ° C. (using ethanol-based coolant) In this process, Br ^* becomes the main etching species, and the SiO ₂ interlayer insulating film 4 and the gate oxide film 2 as the base are etched. 1
The second layer of polycrystalline Si while maintaining a high selectivity of more than 00
The etching of the layer 5 progressed, and the second transfer electrode 5a was formed. At this time, SiB which is an etching reaction product
Since a part (not shown) of r _x is deposited on the side wall surface of the pattern on the wafer kept at a low temperature, the sectional shape of the second transfer electrode 5a is slightly tapered. However, since the natural oxide film 6 was completely removed in advance in the etching region, no stringer was generated.

Example 2 This example is an example in which the same second layer polycrystalline Si layer 5 as in Example 1 was etched using a Br ₂ / SiBr ₄ mixed gas. The structure of the wafer used as the etching sample in this embodiment is as shown in FIG. 1A, and the formation of the condensed layer 8 and the removal of the natural oxide film 6 are performed in the same manner as in the first embodiment. It was

Next, in order to etch the second-layer polycrystalline Si layer 5, the above-mentioned wafer, from which the natural oxide film 6 has been removed, was transferred to the etching chamber. However, for the high vacuum exhaust system of the etching chamber used here, an ordinary turbo molecular pump without a cryopanel was used. Therefore, the removal level of water is lower than that of the first embodiment, and there is a slight possibility that the natural oxide film may be regrown.
The etching was performed under the following conditions as an example.

HBr flow rate 50 SCCM SiBr ₄ flow rate 5 SCCM Gas pressure 1.3 Pa Microwave power 850 W (2.45 GHz) RF bias power 10 W (2 MHz) Wafer temperature 0 ° C. (using ethanol-based refrigerant)

The above gas composition has been previously proposed by the present inventor in Japanese Patent Application No. 4-163794, and is automatically set during the transition from the etching of the polycrystalline Si layer 5 to the exposure of the underlayer. to self-control etch versus SiO _x selectivity is improved (self-con
It is intended to realize controlled etching). The concept of this etching is based on the proceedings of the 51st Annual Meeting of the Japan Society of Applied Physics (Autumn Meeting 1990),
p. 464, presentation number 26p-ZF-6.

Although a small amount of SiBr ₄ is contained in the above gas composition, this is also an etching reaction product at the time of etching the second-layer polycrystalline Si layer 5. That is, during the etching of the second-layer polycrystalline Si layer 5, the amount of SiBr ₄ present in the etching reaction system is large. In such cases, SiO _x
Is typically removed in the form of Si ₂ OBr ₆ (hexabromodisiloxane; melting point 118 ° C.). This is extremely convenient for etching the second-layer polycrystalline Si layer 5 while removing a natural oxide film that may grow a little, but in fact, it is possible to prevent the occurrence of stringers. did it.

Further, when the etching of the second-layer polycrystalline Si layer 5 is almost completed and the underlying SiO ₂ interlayer insulating film 4 and the gate oxide film 2 begin to be exposed, the reaction product SiBr ₄
Of SiB in the etching reaction system due to a decrease in the supply amount of
The total amount of r ₄ has decreased sharply. Therefore, the selectivity with respect to SiO _x is automatically increased, and etching can be completed while maintaining high selectivity with respect to the underlying SiO ₂ interlayer insulating film 4 and the gate oxide film 2.

The present invention has been described above based on the two embodiments, but the present invention is not limited to these embodiments. For example, as the compound capable of supplying the Br-based chemical species, in addition to the above-mentioned HBr and SiBr ₄ , B
r ₂ , BBr _3, etc. can be used. A rare gas such as He or Ar may be appropriately added to the etching gas for the purpose of obtaining a sputtering effect, a dilution effect, a cooling effect, or the like.

In addition to the above, it goes without saying that the structure of the sample wafer, the etching apparatus used, the deposition conditions of the condensed layer, the etching conditions and the like can be changed as appropriate.

[0042]

As is apparent from the foregoing description, since the natural oxide film before etching the Si-based material layer according to the present invention can be thoroughly removed, SiO _x system the etching The masking effect of the natural oxide film can be eliminated even when the treatment is performed under the condition that the ultrahigh selection ratio is maintained for the material. The above ultra-high selection process is 16M
It is almost certain that this will be realized by using a Br-based chemical species in the production of next-generation semiconductor devices such as DRAM and 64MDRAM, but the present invention is extremely useful in putting such an ultra-high selection process into practical use in a mass production line. This is a useful technique.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing an example of a process in which the present invention is applied to formation of a transfer electrode pattern of a horizontal register of a CCD in the order of steps, and (a) is a second-layer polycrystalline Si.
A state where a resist mask is formed on the layer, (b) a state where the surface of the wafer is covered with a condensation layer, (c) a wafer is heated and the natural oxide film in the etching region and the condensation layer are reacted and removed. And (d) shows a state in which the etching of the second-layer polycrystalline Si layer is completed without generating stringers.

FIG. 2 is a schematic cross-sectional view for explaining a problem of conventional etching by taking a transfer electrode pattern formation of a horizontal register of a CCD as an example. FIG. 2A is a diagram showing a resist layer formed on a second-layer polycrystalline Si layer. The state where the mask is formed, (b) the state where the natural oxide film in the surface layer portion of the second-layer polycrystalline silicon layer is removed in the etching region by breakthrough,
(C) shows a state in which stringers are generated during the etching of the second-layer polycrystalline Si layer, and (d) shows a state in which the gate oxide film and the like are eroded as the stringers are removed.

[Explanation of symbols]

1 ... Single-crystal Si substrate 2 ... Gate oxide film (SiO ₂ ) 3 ... First transfer electrode (first-layer polycrystalline Si layer) 4 ... SiO ₂ interlayer insulating film 5 ... the second layer polycrystalline Si layer 5a ... second transfer electrodes 6 ... natural oxide film (SiO ₂₎ 7 ... resist mask 8 ... condensation layer

Claims (3)

[Claims] 1. A step of coating a silicon-based material layer having a natural oxide film on a surface layer with a condensed layer containing fluorine atoms, a step of removing the natural oxide film by heating and reacting with the condensed layer, And a step of etching the silicon-based material layer immediately after removing the natural oxide film. 2. The dry etching method according to claim 1, wherein the condensed layer is formed of a depositable substance generated in an etching reaction system by discharge dissociation of a mixed gas of NH ₃ and NF _3. . 3. The dry etching method according to claim 1, wherein a bromine-based chemical species is used as a main etching species in the etching of the silicon-based material layer.