JPH05275391A - Reactive ion etching method - Google Patents

Abstract

(57) [Abstract] [Purpose] Stable and precise gate pattern of polycide thin film required for constructing a high-speed and highly integrated semiconductor integrated circuit while maintaining high selective processability for the gate oxide film. It is possible to form. In forming a gate pattern of a polycide thin film, an ECR plasma formed by electron cyclotron resonance by mixing oxygen gas with chlorine gas is used as a reactive plasma whose etching characteristics can be controlled by the physical properties of each thin film. After the etching is performed until the silicon thin film is exposed, the supply of oxygen gas is stopped, and the silicon thin film is etched and over-etched using ECR plasma containing only chlorine gas.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an etching method for forming a fine pattern of various materials on a sample substrate in the production of electronic devices such as semiconductor integrated circuits.
In particular, by using ECR plasma generated by electron cyclotron resonance with microwaves, a highly precise fine pattern of a polycide thin film composed of a silicide thin film which is an alloy of refractory metal and silicon and a silicon thin film can be enhanced without damaging the base. The present invention relates to an ion etching method for forming with a selective ratio.

[0002]

2. Description of the Related Art Higher performance and higher integration of electronic devices such as semiconductor integrated circuits have been actively studied. MOS (Me
tal Oxide Semiconductor) In the formation of gate metal patterns for devices, a compound thin film (silicide thin film) of a refractory metal and silicon is used as the upper layer in place of the silicon thin film that has been widely used as a gate electrode for high-speed devices. The use of a two-layer thin film (polycide thin film) in which the lower layer is a silicon thin film is being studied. In order to form such an electrode using a multilayer film in a fine and highly accurate manner,
There is a demand for an etching method capable of highly accurately and stably forming a fine pattern of a semiconductor material or a metal material in consideration of the physical properties of the material.

In the gate pattern formation of this polycide thin film, when reactive plasma formed from one kind of etching gas (single gas or mixed gas) is used,
Since the silicide thin film and the silicon thin film have different reactivities, undercuts, etching residues and the like occur, so that it is difficult to obtain a highly precise fine pattern having a vertical shape as a polycide thin film. Furthermore, since there is a thin gate oxide film (silicon oxide; about 100Å) under the gate metal material that is the basis of device characteristics, the gate electrode and gate oxide film are selected so that the gate oxide film is not damaged when pattern formation is completed. Since it is necessary to give sufficient properties to perform etching, it is difficult to form a gate electrode using a polycide thin film with a reactive plasma formed from one type of etching gas.

Conventionally, in the polycide gate pattern formation, an etching method by a reactive ion etching method (RIE) utilizing an ion bombardment reaction and an attachment reaction for suppressing etching (undercut) of the pattern side wall has been used. .. This method has an advantage that a highly accurate gate pattern can be formed. For example, as reactive gases, sulfur hexafluoride (SF ₆ ) and Freon 115 (C ₂ F ₅ C
The following publications for the case of using l) mixed gas,
That is, SEClark, J.-K.Tsung and JWMarolf, "Depos
ition and Patterning of Tungsten and Tantalum Poly
cides "SolidState Technol., vol.27 (4), p.235, 198
It is described in detail in 4. Fluorocarbon (CF ₂ etc.) formed in the reactive plasma is used as a reaction species of the above-mentioned adhesion reaction because it easily causes a polymerization reaction.

However, in the reactive ion etching method, since the ion energy is as high as 100 eV or more, the etching rate of the gate oxide film by sputtering is high, and the presence of fluorocarbon causes the etching of the gate oxide film by the ion bombardment reaction. Since it is accelerated, it is difficult to obtain sufficient etching selectivity. Furthermore, as a measure against global environmental problems, it is necessary to use a gas type that does not contain Freon and is subject to specific CFC regulations.

In contrast to the above-mentioned two etching methods, the ECR reactive ion etching method disclosed in Japanese Patent Laid-Open No. 60-120525 is used especially for forming a gate pattern in which the gate electrode film is a silicon thin film. This made it possible to perform highly precise etching with high selectivity. Regarding the ECR reactive ion etching method, the following publications, T. Ono, M. Oda, C. Takahashi and S.
Matsuo, "Reactiveion stream etching utilizing elec
tron cyclotron resonance plasma "J.Vac.Sci.Techno
l., Vol. 4, p. 696, 1986. In the above ECR reactive ion etching method,
Reactive plasma utilizes electron cyclotron resonance (ECR) discharge using microwave to generate 10 ^-4 to 10 ^-2 Torr (1
ECR efficiently generated at a low gas pressure of 0 ^{-2 to 1} Pa)
It is plasma, and the ion energy is as low as about 20 eV because the above ECR plasma is drawn out to the etching surface as a plasma flow using a divergent magnetic field.
Further, as an etching gas, a halogen gas such as chlorine gas, which has relatively low reactivity of neutral radicals with respect to silicon but facilitates etching by ion bombardment action of low energy ions, is used. To do. The ECR reactive ion etching method, which has the above characteristics, can control the etching rate of a silicon oxide thin film to an extremely low level and perform highly accurate etching without undercutting, and has a high performance of a gate oxide film thickness of 100 Å or less. This is an effective etching method for forming a silicon thin film gate pattern of a MOS device.

However, when the above-mentioned ECR reactive ion etching method using chlorine gas is applied to the etching of a polycide gate thin film, for example, for tungsten polycide, tungsten silicide contains tungsten that easily reacts with chlorine radicals. As a result of the formation of tungsten chloride, some undercut occurs, and since tungsten chloride is relatively low in volatility, etching residue is likely to occur, so it is difficult to perform highly accurate etching stably. It was

[0008]

As described above, the present invention has been made to solve the problem that the polycide gate electrode cannot be formed with high precision and a large etching selection ratio with respect to the gate oxide film. The purpose of this is to control the etching characteristics in the gate pattern formation of the polycide thin film, which is necessary for constructing a high-speed and highly integrated semiconductor integrated circuit, in consideration of the physical properties of the material to be etched, thereby achieving high accuracy. It is an object of the present invention to provide a reactive ion etching method capable of stably forming a fine pattern with high selectivity for a gate oxide film.

[0009]

Mixing of oxygen gas is an effective method for suppressing the above etching residue in the ECR reactive ion etching method which can expect an ion bombardment effect of moderately low energy. In a reactive plasma formed from a mixed gas of chlorine gas and oxygen gas, the etching characteristics depend on the volatility of chloride oxide of the material to be etched. In the above ECR reactive ion etching method,
For example, molybdenum, which is a refractory metal with physical properties similar to tungsten, can mix an equal amount or more of oxygen gas with chlorine gas without lowering the etching rate, but a single crystal showing almost the same etching characteristics as a silicon thin film. 5 for silicon
Low as low as%. Regarding the etching characteristics mixed with the above-mentioned oxygen gas, the following publications are referred to respectively, the former is the above-mentioned article by T. Ono et al. And the latter is Takahashi, Matsuo, “EC.
Role of reaction product (SiClx) in R ion flow etching ", Proc. Of the 51st Autumn Meeting of Applied Physics (Autumn),
26p-ZF-6, p.464, 1990.

Further, with respect to the silicide thin film which is an alloy of refractory metal and silicon, it is estimated that the etching characteristic by the mixed gas of chlorine gas and oxygen gas is an etching characteristic intermediate between the above-mentioned simple substance of refractory metal and single crystal silicon. Therefore, in the etching of the polycide thin film, an etching method that optimizes the mixing ratio of oxygen gas in the etching of the silicide thin film and the silicon thin film is required.

Further, in the etching of the silicide thin film, by sufficiently mixing oxygen gas, oxygen radicals are stably adsorbed by the ions without being sputtered by the ions on the side wall of the pattern where almost no ions are incident, so that the oxidation reaction occurs. As expected, it can be expected that the generation of undercut due to chlorine radicals will also be suppressed. The present invention has been made based on the above knowledge.

According to the present invention, in forming a gate pattern of a polycide thin film, EC formed by electron cyclotron resonance by mixing oxygen gas with chlorine gas as reactive plasma whose etching characteristics can be controlled by the physical properties of each thin film.
After the silicide thin film is etched using R plasma until the silicon thin film is exposed, two-step etching is performed in which the supply of oxygen gas is stopped and the ECR plasma of chlorine gas alone is used to etch and over-etch the silicon thin film. By adopting it, it has the following features. First, regarding the etching of the silicide thin film, the chloride oxide of the refractory metal contained in the thin film is formed to increase the etching rate and stabilize the etching reaction to eliminate the etching residue. Since the thin oxide film layer is formed on the side wall of the non-existing pattern due to the oxidation reaction by the oxygen gas, it is possible to reduce the amount of undercut caused by chlorine radicals and improve the pattern forming accuracy. Secondly, E formed by mixing the above-mentioned chlorine gas with sufficient oxygen gas
Even if the CR thin film is continuously irradiated with the CR plasma after the etching of the silicide thin film is completed, the etching is almost completely stopped by the oxidation of the silicon thin film. Therefore, the etching of the silicide thin film of the first step to the etching of the silicon thin film of the second step is performed. It has a feature that it can be accurately switched to. Thirdly, in the etching of the second step, in addition to the etching of polycrystalline silicon, overetching is also performed after the gate oxide film is exposed, so that the high selectivity of the ECR reactive ion etching method with respect to the gate oxide film can be achieved. Has available features. A fourth feature is that since a deposition reaction by fluorocarbon is not used, a process for removing the deposited fluorocarbon, for example, an ashing process can be omitted among the processes required for forming the device.

Further, according to the present invention, by controlling the sample temperature during etching to about 0 ° C., it becomes possible to effectively control the amount of undercut using the oxidation of the pattern side wall by oxygen gas. The conventional technique is that ECR plasma with low ion energy formed by mixing chlorine gas as an etching gas with oxygen gas as a control gas is used as a reactive plasma. This is different from the prior art in that two-step etching is used, which is changed depending on whether the silicon thin film is etched or not, and the sample temperature is lowered to such an extent that the undercut of the silicide thin film can be controlled by mixing oxygen gas.

The structure of the present invention is as follows. That is, in a reactive ion etching method for a polycide thin film composed of a silicide thin film which is an alloy of refractory metal and silicon and a silicon thin film, a mixed gas of chlorine gas as an etching gas and oxygen gas as a control gas is microwaved. Reaction characterized by performing reactive plasma formation by exciting by electron cyclotron resonance using power and magnetic field, and performing etching by controlling the mixing ratio of the mixed gas in each thin film unit forming the polycide thin film It has a constitution as a reactive ion etching method, or has a constitution as a reactive ion etching method characterized in that in the reactive ion etching method, the sample temperature during etching is kept at about 0 ° C. It is a thing.

[0015]

EXAMPLE An example in which the present invention is applied to etching of a tungsten polycide thin film using tungsten silicide containing tungsten as a refractory metal is shown in FIGS.
It will be described with reference to the drawings. FIG. 1 is a schematic diagram of the apparatus configuration of the ECR reactive ion etching method. ECR plasma as a reactive plasma has a low gas pressure of, for example, 5 x 10 ^-4 due to electron cyclotron resonance using a microwave of 2.45 GHz and a magnetic field of 875 G generated by a magnetic coil in the plasma chamber.
The magnetic coil is generated by Torr and is further drawn to the sample stage as a plasma flow by the divergent magnetic field simultaneously formed in the etching chamber and used for etching. In order to use the above-mentioned reactive ECR ion etching method in the present invention, chlorine gas and oxygen gas are introduced as the reactive gas, and the temperature of the sample stage is controlled by a coolant of about 0 to 50 ° C.

FIG. 2 shows the dependence of the etching rate on the mixing ratio of oxygen gas in etching single crystal silicon and tungsten silicide thin films with chlorine gas. Single crystal silicon exhibits almost the same etching characteristics as a silicon thin film. Single-crystal silicon is etched at an oxygen gas mixture ratio of about 10% or less, and above that, oxidation occurs and the etching stops, whereas the tungsten silicide thin film is etched to a mixture ratio of about 20%. The above-mentioned etching characteristics are, for example, S
The formation of volatile chloride oxides such as i ₂ OCl ₆ and WOCl ₄ , especially tungsten chloride is the chloride WCl ₆
It can be explained by its higher volatility compared to. The data on the volatility of the above chlorine compound of silicon and the above chlorine compound of tungsten are given in the following documents, namely, DRLide, editor-in-chief, CRC Handboo
k of Chemistry and Physics, p.6-66, p.6-67, 1991,
SAShchukarev and GJNovikov, Zh.neorg. Khi
m., Vol. 1, p357, 1956. Since the tungsten silicide thin film is an alloy of tungsten and silicon, the etching characteristics show intermediate characteristics, and the amount of oxygen gas that can be mixed is larger than that of single crystal silicon. The mixing rate of oxygen gas increases the etching rate of single crystal silicon by about 20%, while that of tungsten silicide thin film increases by about 40%.

FIG. 3 shows an application example of two-step etching in which a mixed gas of chlorine gas and oxygen gas is used for the tungsten silicide thin film layer and a single gas of chlorine gas is used for the silicon thin film layer. 2 is a result of monitoring light emission from a chlorine compound of silicon in the reactive plasma of FIG.
In the figure, ts is the etching time of the first step of the tungsten silicide thin film, tp is the just etching time of the silicon thin film, t1 is the just etching time of the tungsten polycide thin film (however, t1 = ts + tp for the two-step etching). , T2 are overetching times, and t3 is tp + t2 (etching time of the second step). A clear light emission signal is detected in both the first step and the second step during the etching, and the light emission signal level is lowered with the completion of the etching of the tungsten silicide thin film, and the etching is stopped at the timing when the surface of the silicon thin film is exposed. You can see that. Further, it can be confirmed that the silicon oxide is exposed in the silicon thin film layer as well as the etching is completed, but the etching is stopped because the high etching rate selectivity is imparted. In this application example, there is a waiting time because the reactive gas is exhausted and re-introduced when proceeding from the first step to the second step, but it is possible to ensure that the silicon thin film is stopped only by stopping the supply of oxygen gas. Since the etching can be restarted,
By adopting the reactive gas control method of detecting the completion point of the etching of the step and continuously stopping the oxygen gas, the etching becomes pseudo one step and the total etching time can be greatly shortened. Moreover, if a light emission signal from a chlorine compound of silicon is used as a detection signal, only 1
The etching state can be easily monitored over the entire etching time only by using the seed detection signal.

FIG. 4 shows etching characteristics when the above-described two-step etching is applied to the tungsten polycide thin film. The sample has a structure in which a tungsten polycide thin film is formed on a silicon oxide thin film, and a mask pattern is further formed thereon. The just etching time is the time from the start of etching until the tungsten polycide thin film is completely removed and the silicon oxide thin film is exposed, and it can be easily detected by detecting the amount of light emitted from the chlorine compound of silicon generated by etching. Can be measured.
The undercut amount was observed as a cross-sectional shape of the tungsten polycide thin film etched based on the mask pattern using a scanning electron microscope, and was measured as a difference between the width of the mask pattern and the width of the tungsten polycide pattern. .. Note that the sample for measuring the undercut amount continues to be etched for about 30% of the just etching time even after just etching (overetching). The just etching time decreases as the oxygen gas mixing ratio increases, and becomes almost constant at a mixing ratio of about 8% or more. The etching rate is about 8
The increase is about 30% when the oxygen gas is mixed with 10% oxygen gas, which can be explained by the dependency of the oxygen gas mixture ratio on the etching rate of the single crystal silicon and the tungsten silicide thin film. The amount of undercut was measured in the tungsten silicide thin film layer (a) immediately below the mask pattern, the boundary between the tungsten silicide thin film and the silicon thin film (b), and the silicon thin film layer (c) immediately above the silicon oxide thin film. When gases are mixed, a, b, and c become almost equal, and it is easier to obtain the vertical shape of the tungsten polycide thin film than etching with chlorine gas alone. It is presumed that the characteristics related to the undercut amount are also due to the fact that the etching of the tungsten silicide thin film layer is stabilized by the mixing of an appropriate oxygen gas and the etching residue is eliminated as in the case of the etching rate.

FIGS. 5 and 6 show the results of evaluating the amount of undercut with respect to the etching time of the second step and the amount of overetching in the above-described two-step etching. The undercut amount is described as a negative value when the pattern width (a, b, c) of the tungsten polycide after etching is larger than the mask pattern. Fig. 5 is an application example in which the microwave power is 400 W and about 8% of oxygen gas is mixed, and Fig. 6 is the application in which the microwave power is increased to 600 W and the mixing ratio of oxygen gas is increased to about 15%. Here is an example. Overetching amount α (%) is t
It is 2 / t1 × 100. The undercut amount of the tungsten silicide layer directly under the mask pattern, a, and the undercut amount of the boundary between the tungsten silicide thin film and the silicon thin film, b, are almost constant at an overetching amount of about 20%, while they are directly above the silicon oxide thin film. The undercut amount, c, of the silicon thin film layer is about 35%, and the increase is almost saturated and constant. Furthermore, when the over-etching amount is about 35%
The values of a, b, and c are almost the same, and the etching shape is vertical. At this time, when the oxygen gas mixture of about 8%, the pattern formation accuracy is reduced by about 0.05 um, while it is about 15%.
In the case of mixing, the decrease in accuracy can be suppressed to about 0.03 um. It is presumed that the improvement of the pattern formation accuracy of the tungsten polycide is mainly due to the increase of the mixing ratio of oxygen gas, which strengthens the oxidation effect on the pattern side wall and suppresses the occurrence of undercut due to chlorine gas.

FIG. 7 shows the relationship between the above-mentioned undercut amount and the sample table temperature, that is, the refrigerant temperature. The reactive gas is only chlorine gas. The undercuts, a, b, of the tungsten silicide layer decrease as the sample table temperature decreases, and the undercuts, c, of the silicon thin film layer, 10 ℃
It will tend to decrease below. As for the tungsten polycide thin film, the undercut amount is significantly reduced and the verticality of the shape after etching is increased when the sample stage temperature is 10 ° C or less. Furthermore, it is easily expected that the amount of undercut can be kept extremely small when the sample table temperature is around -10 ° C. Therefore, highly precise pattern formation of a tungsten polycide thin film can be expected by stabilizing the etching reaction using the above oxygen gas mixture and oxidizing the side wall while setting the sample stage temperature to about -10 ° C.

[0021]

As described above, according to the present invention,
In patterning tungsten polycide thin film,
ECR formed from mixed gas of chlorine gas and oxygen gas
By taking advantage of the difference in the ratio of oxygen gas that can be mixed between the etching of the tungsten silicide thin film and the etching of the silicon thin film with respect to the plasma, the etching of the tungsten silicide thin film can be performed with an oxygen gas at a level not less than that at which the etching of the silicon thin film is stopped. Two-step etching is performed by mixing and then etching of the silicon thin film using only chlorine gas, while performing the two-step etching while maintaining the sample stage temperature at about 0 ° C. A vertical and highly precise pattern of the side thin film can be stably formed by imparting high selectivity. Further, in the above-mentioned etching method, one of chlorine compounds of silicon existing in plasma is
By detecting the species, the etching completion point of the tungsten silicide thin film and the etching completion point of the silicon thin film,
That is, the end point of etching as the tungsten polycide thin film can be easily detected. The etching completion point of the tungsten silicide thin film can be used as a control signal when proceeding from the first step to the second step etching, and if the supply of oxygen gas is controlled by this control signal, even in two-step etching. High-speed processing is possible without requiring waiting time.

Further, the present invention can be applied not only to the tungsten polycide thin film, but also to the etching of a polycide thin film containing a refractory metal having physical properties similar to tungsten, such as molybdenum polycide.

[Brief description of drawings]

FIG. 1 is a diagram showing an apparatus configuration of an ECR reactive ion etching method.

FIG. 2 is a diagram showing an effect of mixing oxygen gas in etching a tungsten silicide thin film and single crystal silicon using chlorine gas.

FIG. 3 is a diagram showing an effect of mixing oxygen gas on a just etching time and an undercut amount of a tungsten polycide thin film.

FIG. 4 is a diagram showing an application example of two-step etching in accordance with light emission of a chlorine compound of silicon in plasma.

FIG. 5 is a diagram showing a state of occurrence of undercut due to an overetching amount in two-step etching.

FIG. 6 is a diagram showing a state of occurrence of undercut due to the amount of overetching in two-step etching.

FIG. 7 is a diagram showing the relationship between the undercut amount and the temperature of the sample table.

[Explanation of symbols]

ts Tungsten silicide thin film first step etching time tp Silicon thin film just etching time t1 Tungsten polycide thin film just etching time However, for two-step etching, t1 = ts + tp t2 overetching time t3 tp + t2 (second step etching time) )

Claims (1)

[Claims] 1. In a reactive ion etching method of a polycide thin film composed of a silicide thin film which is an alloy of refractory metal and silicon and a silicon thin film, a mixed gas of chlorine gas and oxygen gas is used with microwave power and magnetic field. The reactive ion etching method is characterized in that a reactive plasma is formed by being excited by electron cyclotron resonance, and etching is performed by changing the mixing ratio of the mixed gas in each thin film unit forming the polycide thin film.