JPH11233488A - Surface machining method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the drop of the etching speed of polycrystalline silicon and an etching remainder, and to hold a high selection ratio against an oxide film when etching a semiconductor through the use of a plasma. SOLUTION: In a surface machining by using a plasma, a processing from the start to the end is divided into a plurality of steps. The steps are divided into a first half whose speed ratio with a base substance terminating machining is comparatively small but machining speed is large, and a latter half whose machining speed ratio is comparatively large. High frequency voltage is repetitively applied in on/off periods with at least one of he steps in the latter half.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface processing method, and more particularly to a surface processing method suitable for etching the surface of a sample on which a semiconductor element or the like is formed using plasma.

[0002]

2. Description of the Related Art An apparatus widely used for etching a semiconductor element is an apparatus utilizing plasma. Here, one of them, ECR (Electron Cyclotron Resonance)
The prior art will be described using an apparatus called a system as an example. In this method, plasma is generated by microwaves in a vacuum vessel to which a magnetic field is externally applied. Since Lorentz force acts on electrons in the plasma due to the magnetic field, the electrons rotate around the lines of magnetic force (cyclotron motion). When the frequency of the rotation is equal to the frequency of the microwave, resonance occurs, and plasma can be generated efficiently. A high frequency voltage is applied to the sample to accelerate ions incident on the sample. A halogen gas such as chlorine or fluorine is used as a gas to be plasma.

In order to improve the accuracy of such a device,
The technique described in JP-A-6-151360 is known. This technology is a substance to be etched by intermittently controlling on-off of a high-frequency voltage applied to a sample.
The selectivity between Si and the underlying oxide film is increased.

[0004]

With the recent miniaturization of semiconductor devices, the thickness of a gate oxide film is reduced in a metal oxide semiconductor (MOS) transistor, and is reduced to 6 nm or less in a memory device of 256M or later. As the gate oxide film becomes thinner in this manner, a higher selectivity is required in the element etching process. However, when the selection ratio is increased, there is a problem that the processing speed is generally slow and an etch residue is liable to occur.

SUMMARY OF THE INVENTION An object of the present invention is to provide a surface processing method capable of suppressing a decrease in the etching rate of polycrystalline silicon and the occurrence of an unetched residue, and maintaining a high selectivity with respect to an oxide film in etching a semiconductor using plasma. To provide.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to divide a surface process into a plurality of steps from the start to the end of the surface processing, and the processing speed ratio between the base material and the base material which ends the processing is relatively small. This is achieved by dividing the device into two parts, a small first half and a relatively large second half, and applying the high-frequency voltage to the sample repeatedly in at least one of the second half steps during the ON and OFF periods.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1A is an overall configuration diagram of a plasma etching apparatus to which the present invention is applied. Microwave power supply 101
From the vacuum vessel 10 through the waveguide 102 and the introduction window 103
A microwave is introduced into 4. The material of the introduction window 103 is a material that transmits electromagnetic waves, such as quartz or ceramic. An electromagnet 105 is provided around the vacuum vessel 104, and the magnetic field strength is set so that the microwave frequency and the electron cyclotron frequency are the same. For example, if the frequency is 2.45 GHz, the magnetic field strength is 875 Gauss . The high-density plasma 106 is generated by this magnetic field intensity. Sample 1
07 is set on the sample stage 108. In order to accelerate ions incident on the sample, the high-frequency power supply 109
08. There is no particular limitation on the frequency of the high-frequency power supply, but usually the frequency is practically in the range of 200 kHz to 20 MHz. Voltage waveform 11 of high frequency power supply 109
0 is such that the ON and OFF periods can be repeatedly controlled as shown in FIG.

FIG. 2 shows a change over time in the cross-sectional shape when a fine pattern consisting of lines and spaces on a semiconductor surface is etched in two steps using the apparatus of FIG. The structure of the etched element is a silicon substrate 20
1 has a thickness of 4 nm, a polycrystalline silicon layer 203 has a thickness of 300 nm, and a resist 204 has a thickness of 1 nm.
In μm, the width of the space is 0.35 μm. 2A shows the initial state, FIG. 2B shows the state after step 1, and FIG. 2C shows the state after step 2. In step 1, first, polycrystalline silicon 2
03 is etched. Step 1 occupies a major part of the processing and is hereafter referred to as the main etch. Etching gas is Cl
The pressure was 0.4 Pa with 2 (75 sccm) and oxygen (5 sccm). The output of the microwave power supply 101 was 400 W. The frequency of the high-frequency power supply 109 is 800 kHz in this case. In this step, the output of the high-frequency power supply 109 is continuously set to 80 W because the purpose is to etch the polycrystalline silicon at high speed so that no etching remains. The etch rate of polycrystalline silicon increases with high frequency power. In addition, foreign matter that is difficult to etch, such as a natural oxide film on polycrystalline silicon, is used as a mask for the remaining etch, and the portion below the remaining etch residue. Such foreign matter is also removed by increasing the high frequency power. Therefore, in order to increase the processing speed so that no etch residue appears, a relatively high frequency power is required. However, if the high-frequency power is large, the selectivity with respect to the underlying oxide film becomes low. Therefore, if the etching is continued as it is, the underlying oxide film is etched, which leads to a defective element. In order to prevent this, it is only necessary to switch to a condition having a high selection ratio at a certain stage. Here, in step 2, on / off control was performed without changing the amplitude of the high frequency voltage. The ON / OFF repetition frequency was 1 kHz. Such etching having a relatively high selectivity after the main etch is hereinafter referred to as overetch. By performing on / off control of the high-frequency power supply in step 2, as shown in FIG. 2 (c), it becomes possible to process without leaving an etch residue and leaving a sufficient base oxide film.

The switching between step 1 and step 2 is performed by using Si
It is advisable to monitor the emission intensity of the atoms and switch the emission intensity when the emission intensity starts to decrease or at the inflection point of the emission intensity curve (point at which the second derivative becomes zero). If the underlying oxide film is thin, a method may be used in which the time when the etching of the polycrystalline silicon is completed is measured in advance and the switching is performed slightly before the time.

In order to increase the selection ratio, there is a method of simply lowering the power. However, when the thickness of the underlying oxide film 202 is reduced to 4 nm, it is necessary to further increase the selection ratio. If the power is simply reduced, the etching speed of the polycrystalline silicon is reduced, and the remaining etching tends to occur. High frequency power supply 1 as in the present invention
The on / off control of the output of 09 can increase the selectivity without decreasing the etching speed of the polycrystalline silicon. This is shown in FIG. The horizontal axis in FIG. 3 is the etch rate of the polysilicon and the vertical axis is the selectivity between the polysilicon and the oxide film. A point A in the figure is a value of continuous 80 W, a curve 301 is a case where the value of the continuous power is reduced, and a curve 302 is an on / off control with a constant voltage amplitude and the duty ratio (the ratio of the on period to one on / off cycle). ) Is reduced. When the high frequency is turned on / off, the selection ratio becomes higher at the same polycrystalline silicon etch rate as compared with the continuous power. The cause can be estimated as follows. When oxygen or a reaction product containing oxygen adheres, the etch rate of the oxide film decreases. On the other hand, the etching rate of polycrystalline silicon does not decrease to a certain extent in the amount of oxygen attached. When the high-frequency power supply is turned on and off, ions are not accelerated during the off period, and only oxygen or reaction products are deposited. Ions are accelerated and incident on the surface during the on-period, but are inhibited by the deposits during the off-period, and the etch rate of the oxide film is reduced to increase the selectivity.

FIG. 3 shows that the duty ratio can be reduced to 30% or less in order to further enhance the effect of the on / off control. On-off repetition frequency is 100Hz
Between 10 and 10 kHz is appropriate. When the frequency is lower than this, the effect of the on / off control gradually decreases. Also, if the repetition frequency is high, it becomes technically difficult to manufacture the high-frequency power supply 109.

The value of the power of the high-frequency power supply 109 required to obtain the same etching speed greatly varies depending on the type of the device and the frequency of the high frequency, but in any case, the etching speed of the oxide film is proportional to the duty ratio. Therefore, although the absolute value of the power shown in FIG. 3 changes when the device structure or the like changes, the relationship between the duty ratio and the selection ratio is satisfied, and the numerical value of 30% or less applies to any case.

In the above embodiment, the method of etching in two steps has been described. However, even when the material to be etched is a metal or a multilayer structure of a compound of metal and silicon, the number of steps is increased and the underlying portion is exposed. Immediately before or immediately after the high-frequency power supply is turned on / off, it is possible to suppress the remaining etch and to maintain a high selection ratio.
Further, depending on the structure of the sample, the overetched portion may be further divided into a plurality of steps.

[Embodiment 2] FIG. 4 shows another apparatus structure to which the present invention is applied. In this apparatus, inductive coupling is performed at a frequency of a so-called radio wave band (hereinafter referred to as rf) of several hundred kHz to several tens MHz. Generates plasma. The vacuum container 401 is made of a material that transmits electromagnetic waves, such as alumina and quartz.
An electromagnetic coil 402 for generating a plasma 403 is wound therearound. An rf power supply 404 is connected to the coil. A sample stage 408 is provided in the vacuum vessel 401, on which a sample 407 is placed, and to which a high-frequency power source 409 is connected. The vacuum vessel 401 has an upper lid 405, which may be an integral type.

In this apparatus as well, in this apparatus, the processing is divided into a plurality of steps, and the high frequency power supply 409 is turned on / off by overetching, so that the remaining etching can be suppressed and the selection ratio can be kept high. In the device shown in FIG. 4, the effect is the same even if the electromagnetic coil 402 is installed on the upper lid 405.

[Embodiment 3] FIG. 5 shows another apparatus structure to which the present invention is applied. In this apparatus, plasma is generated by capacitive coupling of rf power. Two electrodes 5 are placed in a vacuum vessel 501.
02 and 505 are arranged in parallel. Each of the electrodes
An rf power supply 503 and a high-frequency power supply 506 are connected. The sample 504 is placed on an electrode 505 serving as a sample stage. The gas is introduced into the inlet tube 5 through the hole opened in the electrode 502 facing the sample.
08 into the container. Plasma 507 is 2
Occurs between the electrodes.

In this apparatus as well, in this apparatus, the processing is divided into a plurality of steps, and the high-frequency power supply 506 is turned on and off by overetching, so that the remaining etching can be suppressed and the selection ratio can be kept high.

[Embodiment 4] Next, a method for realizing a higher selectivity by overetching will be described. FIG. 6 shows the relationship between the etching rate and the selectivity of polycrystalline silicon when the mask of the workpiece is not a resist but an oxide film. A point A is a value where the high-frequency power is 80 W continuously, a curve 601 is a value where the power is continuously reduced, and a curve 602 is a value where the voltage amplitude is constant and the duty ratio is reduced by on / off control. It can be seen that by performing the on / off control as compared with FIG. 3, the selection ratio is further increased by about 50% at the maximum. This is because the carbon atoms contained in the resist were excluded. As described above, when the bias is turned on and off, reaction products and oxygen atoms are deposited on the oxide film during a period in which the high frequency is off, thereby lowering the etch rate of the oxide film. Products also accumulate. It is known that these substances have a property of increasing the etching rate of an oxide film. For this reason, the effect of the on / off control is reduced. By using a material that does not contain carbon, such as an oxide film, for the mask material, the deposition of carbon during the bias off period is eliminated, and a higher selectivity can be obtained. Another example of a mask material that does not contain carbon is silicon nitride.

As described above, according to this embodiment, the processing time can be shortened by processing at a high speed until the underlying oxide film is exposed. In addition, by controlling on / off of the high frequency applied to the sample immediately before or immediately after the base is exposed, it is possible to maintain a high selection ratio while suppressing a decrease in the etching speed as compared with a case where continuous power is applied. Therefore, the remaining etch is suppressed.

[0021]

According to the present invention, it is possible to process a semiconductor element while suppressing a decrease in the etching rate or the occurrence of the remaining etching in the etching of polycrystalline silicon, and maintaining a high selectivity to an oxide film. effective.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram showing one embodiment of an apparatus for performing a surface processing method of the present invention.

FIG. 2 is a view showing a processing cross section of a semiconductor sample by the apparatus of FIG. 1;

FIG. 3 is a diagram showing a relationship between a polycrystalline silicon etch rate and a selectivity.

FIG. 4 is an overall configuration diagram showing another embodiment of an apparatus for performing the surface processing method of the present invention.

FIG. 5 is an overall configuration diagram showing still another embodiment of the apparatus for performing the surface processing method of the present invention.

FIG. 6 is a diagram showing a relationship between a polycrystalline silicon etch rate and a selectivity.

[Explanation of symbols]

101 ... microwave power supply, 102 ... waveguide, 103 ... introduction window, 10
4,401,501… Vacuum container, 105… Magnet, 106,403,507… Plasma, 107,407,504… Sample, 108,408… Sample stand, 109,409,
506: High frequency power supply, 110: Voltage waveform, 201: Silicon substrate,
202: oxide film, 203: polycrystalline silicon, 204: resist, 301, 302, 601, 602: curve, 402: electromagnetic coil, 404, 50
3 rf power source, 405 top cover, 502,505 electrodes, 508 gas introduction tube.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Tokuo Kure, Inventor 1-280 Higashi Koigabo, Kokubunji-shi, Tokyo Inside the Central Research Laboratory, Hitachi, Ltd. (72) Inventor Masayuki Kojima 5-20-1, Josuihonmachi, Kodaira-shi, Tokyo In-house Semiconductor Division, Hitachi, Ltd.

Claims (6)

[Claims] 1. A plasma is generated in a vacuum vessel, a high-frequency voltage is applied to a sample stage on which a sample to be surface-processed by the plasma is placed, and a plurality of steps are performed from the start to the end of the surface processing of the sample. And divide the steps into two parts: a first half in which the processing speed ratio with the base material for finishing the processing is relatively small, and a second half in which the processing speed ratio is relatively large. At least in one of the latter steps, the high-frequency voltage is turned on and off. Wherein the repetitive application is performed separately for each period. 2. The surface processing method according to claim 1, wherein the sample to be processed does not contain carbon as a main component. 3. A surface processing method according to claim 1, wherein the ratio of the ON period to one ON / OFF cycle in the high frequency voltage is 30% or less. 4. A surface processing method according to claim 1, wherein the frequency is changed from 100 Hz to 10 kHz. 5. The surface processing method according to claim 1, wherein the step group in the first half and the step group in the second half are determined and switched according to time. 6. The surface processing method according to claim 1, wherein the first group of steps and the second group of steps are switched by detecting the intensity of light emitted in the plasma by the substance to be etched. Method.