JPH04323823A - Dry etching - Google Patents

Abstract

PURPOSE:To increase a selectivity to the base at the time of overetching in the dry etching of a silicon material layer using no fluorocarbon gas. CONSTITUTION:In overetching, bromine gas is used as etching gas and the RF bias of relatively higher frequency than required in the prior etchings is applied. As clearly known from the fact that the magnitude of interatomic binding energy is SiO>Si-Br, theoretically Br* does not etch SiO2 voluntarily. Due to the increase in frequency, the large-weight ion such as Br<+> loses an electric field follow-up property and ion injection energy into a wafer is reduced. Consequently, a selectivity to the base is increased due to both chemical and physical effects. This invention is specifically effective in gate processing, etc., which requires a high selectivity to a gate oxide film.

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 in the field of manufacturing semiconductor devices, and more particularly to a method for improving selectivity to a substrate when over-etching a silicon-based material layer.

0002

[Prior Art] As semiconductor devices become more highly integrated and performant as seen in VLSI, ULSI, etc. in recent years, etching of silicon-based material layers also requires high anisotropy, high speed, and high performance. There is a strong desire for technology that can achieve the various requirements of selectivity, low damage, and low contamination without sacrificing any of them. Among various processes for etching silicon-based material layers, gate electrode processing, which involves etching polycrystalline silicon, refractory metal silicide, polycide, etc., is a process that requires particularly high precision. In the manufacturing process of MOS-FET, in which the source/drain regions are formed in a self-aligned manner, the dimensional accuracy and cross-sectional shape of the gate electrode are important, such as the channel length, the dimensional accuracy of the sidewalls to realize the LDD structure, and the source/drain region. This is because it directly affects the formation region of the drain region, etc. Another important reason is that gate insulating films, which have become thinner and thinner in recent years, need to be processed with extremely high selectivity to the underlying layer.

Conventionally, fluorocarbon-based gases such as fluorocarbon 113 (C2 Cl3 F3) have been widely used as etching gases for etching silicon-based materials. Since fluorocarbon-based gas has F and Cl as constituent elements in one molecule, it is possible to perform etching by both radical reactions and ion-assisted reactions, and while protecting the side walls with carbon-based polymers deposited from the gas phase. Anisotropic processing can be achieved. However, as is well known, fluorocarbon gases have been pointed out 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, in the field of dry etching, there is an urgent need to find a substitute for fluorocarbon-based gas and to establish an effective method for using it. Furthermore, if the design rules for semiconductor devices become even smaller in the future, carbon-based polymers deposited in the gas phase may become a source of particle contamination, and in this sense, measures to eliminate fluorocarbons are desired.

Against this background, HBr has recently attracted attention as an etching gas for silicon-based material layers. For example, Digest of Papers 19
89 2nd Micro Process
On page 190 of the Conference, n+
An example of anisotropic processing of a polycrystalline silicon layer has been reported. Br has a large atomic radius and does not easily penetrate into the crystal lattice or grain boundaries of the silicon-based material layer, so it does not spontaneously etch the silicon-based material layer, but when assisted by ion irradiation, Br does not etch the silicon-based material layer. Can be etched. Therefore, Br can be said to be an advantageous etching species for realizing anisotropic processing.

[0005] HBr can be used to etch silicon-based material layers.
Another reason why it is supported as a gas is that it can theoretically have high selectivity with respect to a base made of silicon oxide (SiO2). This is compared to the interatomic bond energy of Si-O bond of 111 kcal/mol.
Br binding is as low as 88 kcal/mol, and Br*
This is based on the fact that SiO2 does not spontaneously etch. In particular, in the current situation where semiconductor devices are becoming more highly integrated and device structures are becoming more complex, and the surface level difference of wafers is increasing, it is necessary to perform a certain amount of over-etching to ensure uniformity of processing within the wafer surface. It has become essential. Therefore, in gate electrode processing in which a polycrystalline silicon layer, polycide film, or the like is etched on a thin gate insulating film, extremely high selectivity to the underlayer is required during overetching. In such cases, the use of HBr becomes advantageous.

However, even when HBr is used, there are cases in which a much lower substrate selectivity is obtained than theoretically expected. Monthly Semiconductor World 1990 1
On pages 81-84 of the monthly issue (published by Press Journal),
Considerations and countermeasures regarding this cause have been reported. According to the above paper, the cause of the low selectivity is carbon contamination in the etching system. In other words, by adsorbing carbon on the surface of SiO2, the interatomic bond energy increases to 257 kcal.
/mol, a large C-O bond is generated, which is Si-O
Since the bond is weakened, SiO2 becomes easier to be etched by Br. As a countermeasure against this, carbon contamination is thoroughly eliminated by increasing the purity of HBr gas, changing the materials of the constituent members of the RIE apparatus, and the like.

[0007]

[Problems to be Solved by the Invention] By the way, in recent dry etching equipment, 400kHz, 2MHz
The trend is to apply a relatively low frequency RF bias to the wafer. This is because chip areas are increasing as semiconductor devices become more highly integrated, resulting in larger wafer diameters, and single-wafer processing has become mainstream from the perspective of improving processing uniformity. This is related to the fact that there is a strong demand for an improvement in etching speed in order to ensure productivity equivalent to or higher than that of . In other words, R
This is because by reducing the frequency of the F bias, the ability of ions to follow the reversal of the electric field is improved, and large ion incident energy is obtained, thereby achieving high-speed processing. However, this tendency toward lower frequencies is disadvantageous from the viewpoint of ensuring a high selectivity with respect to the gate insulating film. In other words, SiO2 constituting the gate insulating film
Since it is originally etched by a mechanism with strong ionic properties, it will naturally be removed if the ion incident energy is increased during over-etching. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a method for dry etching a silicon-based material layer that does not deteriorate the selectivity to SiO2 underlayer even during over-etching.

[0008]

[Means for Solving the Problems] A dry etching method according to the present invention is proposed to achieve the above-mentioned object, and is for etching a silicon-based material layer while applying a relatively low frequency RF bias. This method is characterized by comprising an etching step and an over-etching step using an etching gas mainly composed of bromine gas while applying a relatively high frequency RF bias.

[0009]

[Operation] In general, when an RF electric field is formed in the plasma generation region in plasma etching, when the RF bias frequency is low, both ions and electrons can follow the reversal of the electric field, so some of the ions are incident on the etched substrate. do. However, as the RF bias frequency increases, it becomes impossible to sequentially track ions starting from the largest mass, and the amount of ions incident on the substrate to be etched decreases. Furthermore, when the RF bias frequency is increased, electrons can no longer be followed and vibrate in the plasma, colliding with gas molecules and generating many radicals and ions. However, heavy ions that cannot follow the reversal of the electric field have difficulty absorbing energy from the electric field, and are hardly incident on the substrate to be etched.

In the present invention, first, the relatively low frequency R
While applying F bias, the silicon-based material layer is etched by approximately the layer thickness (hereinafter, this is referred to as just etching). In this process, etching progresses at high speed, assisted by large ion bombardment. Next, over-etching is performed using a bromine gas under conditions in which the frequency of the RF bias is relatively increased to weaken the ion bombardment. Here, the fact that Br theoretically does not spontaneously etch the SiO2-based material layer is understood from the above-mentioned relationship in magnitude of interatomic bond energy. Furthermore, in the present invention, as the frequency becomes higher, the electric field followability of ions such as Br+ having a large mass decreases, and ion bombardment is effectively reduced. Therefore, according to the present invention, the selectivity to the underlying SiO2 material layer can be significantly improved based on both chemical and physical effects.

[0011]

[Examples] Specific examples of the present invention will be described below.

Example 1 In this example, the present invention is applied to gate electrode processing, and HBr
This is an example in which a polycrystalline silicon layer is etched to a just-etched state while applying a relatively low-frequency RF bias using HBr, and then over-etching is performed using HBr at a higher RF frequency. First, a wafer was prepared in which an n+ type polycrystalline silicon layer was formed on a single crystal silicon substrate via a gate oxide film made of SiO2, and a resist mask patterned into a predetermined shape was further formed. This wafer was set on a wafer mounting electrode of a magnetic field microwave plasma etching apparatus. The above-mentioned magnetic field microwave plasma etching system is equipped with two systems of frequency 2 MHz and 13.56 MHz as RF power sources connected to the wafer mounting electrode, and either one of the RF power sources can be selectively connected using a changeover switch. This is a possible configuration. In addition, a cooling pipe is installed on the wafer-mounted electrode, and an appropriate coolant is supplied and circulated from a cooling system such as a chiller connected to the outside of the device, thereby making it possible to cool the wafer at a low temperature. And so. Here, ethanol coolant was used to cool the wafer to about -50°C. In this state, HBr
Flow rate 50SCCM, gas pressure 1.3Pa (10mTorr)
), the polycrystalline silicon layer was etched (just etched) by approximately the layer thickness under the conditions of microwave power of 850 W, RF bias power of 50 W, and RF bias frequency of 2 MHz.

[0013] In this process, Br* generated from HBr
Etching progressed by a mechanism in which the radical reaction of Br+ was assisted by incident ions such as Br+. At this time, CBrx, which is a reaction product between the resist mask and Br, is deposited on the sidewall of the pattern on the low-temperature cooled wafer.
and SiB, an etching reaction product with low vapor pressure.
rx etc. were deposited and exerted a side wall protection effect. Furthermore, the migration of Br* on the wafer surface was also suppressed by cooling the wafer to a low temperature. These effects appeared synergistically, and a gate electrode having a good anisotropic shape was formed.

However, in the steps up to the above-mentioned just etching, the selectivity to the gate oxide film is about 20 because the RF bias frequency is low. therefore,
If over-etching is performed under these conditions, there is a high possibility that the gate oxide film will be removed in the region where the gate oxide film is exposed relatively quickly. Therefore, over-etching was performed by switching the frequency of the RF power source to 13.56 MHz. The conditions at this time include, for example, an HBr flow rate of 50S.
CCM, gas pressure 1.3 Pa (10 mTorr), microwave power 850 W, RF bias power 5 W (13
．． 56 MHz), and the wafer temperature was -50°C. Under these conditions, the ion incident energy was effectively reduced due to the contributions of both a reduction in the electric field followability of Br+ due to the lower frequency of the RF bias and a reduction in ion acceleration due to the lower power of the RF bias. As a result, the selectivity to the gate film was improved to about 50.

Example 2 In this example, the present invention is applied to gate electrode processing, and S2
After etching the polycrystalline silicon layer to a just-etched state while applying a relatively low frequency RF bias using F2, etching the polycrystalline silicon layer using S2 Br2 and R
This is an example in which over-etching was performed under conditions where the F frequency was increased. First, the same wafer as used in Example 1 above was set in a magnetic field microwave plasma etching system.
As an example, S2 F2 flow rate is 20SCCM, gas pressure is 1.
3Pa (10mTorr), microwave power 850W
, RF bias power 20W, RF bias frequency 2
The polycrystalline silicon layer was etched to a just-etched state under the conditions of MHz and wafer temperature of -50°C.

The above S2 F2 is one of the sulfur fluorides proposed by the applicant of the present application in a series of applications including Japanese Patent Application No. 198045/1999. S2 F
2 has a F/S ratio (F atom and S
F*, which has a small atomic radius and is extremely reactive, is generated in the plasma by discharge dissociation. This radical reaction due to F* leads to S+, SFx +
The etching reaction proceeded at high speed through a mechanism assisted by ions such as On the other hand, free S was simultaneously generated in the gas phase from S2F2, and this was deposited on the pattern sidewalls of the low-temperature cooled wafer surface, exerting a sidewall protection effect. As a result, a gate electrode having a good anisotropic shape was formed.

However, in the process up to the above-mentioned just etching, F* is the main etching species, so the selectivity to the gate oxide film is about 5 to 6 at most. This means that the value of the interatomic bond energy is 1 for the Si-O bond.
This can be understood from the fact that it is 11 kcal/mol, whereas the Si-F bond has a larger value of 132 kcal/mol. Therefore, if over-etching is performed under these conditions, there is a high possibility that the gate oxide film will be removed in the region where the gate oxide film is exposed relatively quickly. Therefore, next, over-etching was performed by switching the etching gas to S2 Br2 and increasing the frequency of the RF bias. The conditions at this time are, for example, S2 Br2
Flow rate 20SCCM, gas pressure 1.3Pa (10mTorr)
), microwave power of 850 W, RF bias power of 5 W, RF bias frequency of 13.56 MHz, and wafer temperature of -50°C. This improved the selectivity to the gate oxide film to about 50. The S deposited on the sidewalls of the pattern during the just-etching and over-etching processes described above could be easily sublimated and removed by heating the substrate to be etched to about 90° C. after the etching was completed. Therefore, no particle contamination occurred. In addition, in the above-mentioned just etching process, SF2, SF
4, each sulfur fluoride of S2 F10, or S3
Sulfur chlorides such as Cl2, S2 Cl2, and SCl2 may also be used.

Although the present invention has been described above based on two examples, the present invention is not limited to these examples. For example, the above-mentioned HBr , S2 Br2 as well as Br2 , SiBr4 , S3 Br2 , SB
r2 etc. can be used. Further, various additive gases may be mixed with the etching gas used in the present invention. For example, when N2 is added, side wall protection by reaction products can be expected to be strengthened, and H2
, H2S, or a silane-based gas that can supply H* and/or silicon-based active species into the etching system, excess halogen radicals can be captured,
The S deposition effect can be enhanced. Furthermore, for the purpose of obtaining sputtering effect, cooling effect, and dilution effect, He, A
A rare gas such as r may be added. Furthermore, the type of material layer to be etched is not limited to the above-mentioned polycrystalline silicon layer, but may also be a metal silicide layer, a polycide film, or the like.

[0019]

Effects of the Invention As is clear from the above description, the dry etching method of the present invention allows silicon-based material layers to be etched with high anisotropy, high speed, high selectivity, and without using fluorocarbon gas. Etching can be performed with low contamination. Furthermore, during over-etching, we use a bromine-based gas that does not spontaneously etch the SiO2-based material layer, and we also use conditions that increase the frequency of the RF bias, so the ion incident energy is effectively reduced and the S
It becomes possible to improve the iO2 base selectivity. Therefore, the present invention is designed based on fine design rules, and is suitable for manufacturing semiconductor devices having high integration and high performance, and is also extremely excellent as a measure against CFCs.

Claims (1)

[Claims] 1. A step of etching a silicon-based material layer while applying a relatively low frequency RF bias, and an etching gas mainly containing bromine gas while applying a relatively high frequency RF bias. A dry etching method comprising the step of performing over-etching using.