JP4061691B2 - Surface processing method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a surface processing method, and more particularly to a surface processing method suitable for etching a surface of a sample on which a semiconductor element or the like is formed using plasma.
[0002]
[Prior art]
An apparatus widely used for etching semiconductor elements is an apparatus using plasma. Here, the prior art will be described taking an example of an apparatus called an ECR (electron cyclotron resonance) system, which is one of them. In this method, plasma is generated by microwaves in a vacuum container to which a magnetic field is applied from the outside. Since the Lorentz force acts on the electrons in the plasma due to the magnetic field, the electrons rotate around the magnetic field lines (cyclotron motion). When the rotation frequency and the microwave frequency are the same, resonance occurs and plasma can be generated efficiently. In order to accelerate ions incident on the sample, a high frequency voltage is applied to the sample. A halogen gas such as chlorine or fluorine is used as the plasma gas.
[0003]
In order to improve the accuracy of such an apparatus, a technique described in Japanese Patent Laid-Open No. 6-151360 is known. In this technique, the selection ratio between Si, which is a substance to be etched, and the base oxide film is increased by intermittently controlling on / off of the high-frequency voltage applied to the sample.
[0004]
[Problems to be solved by the invention]
With the recent miniaturization of semiconductor elements, the thickness of the gate oxide film is reduced in MOS (metal oxide semiconductor) transistors, and it is 6 nm or less in memory elements of 256 M and later. As the gate oxide film becomes thinner in this way, a higher selection ratio is required in the element etching process. However, when the selection ratio is increased, there is a problem that the processing speed is generally reduced and etching residue is likely to occur.
[0005]
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 generation of residual etching and maintaining a high oxide film selectivity in etching a semiconductor using plasma. It is in.
[0006]
[Means for Solving the Problems]
In the surface processing, the surface processing is divided into a plurality of steps from the start to the end of the surface processing, and the first half in which the processing speed ratio with the base material to finish the processing is relatively small and the second half in which the processing is relatively large. This is achieved by dividing it into two and applying it to the sample repeatedly in at least one of the latter steps, dividing the high frequency voltage into on and off periods.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
[Example 1]
Embodiments of the present invention will be described below with reference to the drawings.
[0008]
FIG. 1A is an overall configuration diagram of a plasma etching apparatus to which the present invention is applied. A microwave is introduced into the vacuum vessel 104 from the microwave power source 101 through the waveguide 102 and the introduction window 103. The material of the introduction window 103 is a substance that transmits electromagnetic waves, such as quartz or ceramic. An electromagnet 105 is installed 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. . A high-density plasma 106 is generated with this magnetic field strength. The sample 107 is set on the sample stage 108. A high frequency power source 109 is connected to the sample stage 108 to accelerate ions incident on the sample. There is no particular limitation on the frequency of the high-frequency power supply, but normally the frequency is practically in the range of 200 kHz to 20 MHz. The voltage waveform 110 of the high-frequency power source 109 can be repeatedly controlled between on and off periods as shown in FIG.
[0009]
FIG. 2 shows a change in cross-sectional shape over time when a fine pattern composed 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 that the thickness of the gate oxide film 202 on the silicon substrate 201 is 4 nm, the thickness of the polycrystalline silicon layer 203 is 300 nm, the thickness of the resist 204 is 1 μm, and 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, the polycrystalline silicon 203 is etched. Step 1 occupies the main part of the processing and is hereinafter referred to as main etch. The etching gas was Cl2 (75 sccm) and oxygen (5 sccm), and the pressure was 0.4 Pa. The output of the microwave power source 101 was set to 400W. In this case, the frequency of the high-frequency power source 109 is 800 kHz. Since the purpose of this step is to etch polycrystalline silicon so that there is no etching residue, the output of the high-frequency power supply 109 is continuously 80 W. The etch rate of polycrystalline silicon increases with high frequency power. Etch residue is masked by a foreign substance that is difficult to etch, such as a natural oxide film on polycrystalline silicon, and the lower portion becomes the etch residue. Such foreign matter is also removed when the high frequency power is increased. Therefore, in order to increase the processing speed so that no etching residue occurs, a certain amount of high frequency power is required. However, if the high-frequency power is large, the selection ratio with the base oxide film becomes low. Therefore, if the etching is carried out as it is, the base oxide film is etched, leading to device defects. In order to prevent this, it is only necessary to switch to a condition with a high selection ratio at a certain stage. Here, on / off control was performed in step 2 without changing the amplitude of the high-frequency voltage. The on / off repetition frequency was 1 kHz. In this way, etching having a relatively high selectivity after the main etching is hereinafter referred to as overetching. By performing on / off control of the high-frequency power source in step 2, as shown in FIG. 2 (c), it becomes possible to process without leaving an etching residue and leaving a sufficient base oxide film.
[0010]
The switching between Step 1 and Step 2 is preferably performed by monitoring the emission intensity of Si atoms, and when the emission intensity starts to drop, or at the inflection point of the emission intensity curve (the point where the second derivative becomes 0). Further, when the base oxide film is thin, it may be possible to measure the time when the etching of the polycrystalline silicon is completed in advance and switch it slightly before that.
[0011]
In order to increase the selection ratio, there is also a method of simply lowering the power. However, if the base oxide film 202 is as thin as 4 nm, the selection ratio needs to be increased. If the power is simply lowered, the etching rate of the polycrystalline silicon is lowered and the etching residue tends to occur. When the output of the high-frequency power source 109 is controlled on and off as in the present invention, the selectivity can be increased without reducing the etching rate of the polycrystalline silicon. This is shown in FIG. The horizontal axis in FIG. 3 is the etching rate of polycrystalline silicon, and the vertical axis is the selectivity between the polycrystalline silicon and the oxide film. In the figure, the point A is a continuous 80 W value, and the curve 301 is a continuous power value that is reduced, and the curve 302 is turned on and off with a constant voltage amplitude, and its duty ratio (ratio of the ON period in one on-off cycle) ) Is reduced. When the high frequency is controlled to be turned on / off, the selectivity is higher than that of continuous power even at the same polycrystalline silicon etch rate. This 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 when the amount of attached oxygen is reduced. When the on / off control of the high-frequency power source is performed, 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 hindered by deposits during the OFF period, and the etch rate of the oxide film is reduced, thereby increasing the selectivity.
[0012]
It can be seen from FIG. 3 that the duty ratio should be 30% or less in order to further increase the effect of the on / off control. The on-off repetition frequency is suitably between 100 Hz and 10 kHz. When the frequency was lower than this, the effect of the on / off control was gradually reduced. Further, when the repetition frequency is high, it is technically difficult to manufacture the high-frequency power source 109.
[0013]
Note that the power value of the high-frequency power supply 109 necessary to obtain the same etch rate varies greatly depending on the type of device and the frequency of the high frequency, but in any case, the oxide film etch rate is proportional to the duty ratio. Accordingly, although the absolute value of the power in FIG. 3 changes when the device structure or the like changes, the relationship between the duty ratio and the selection ratio holds, so the numerical value of the duty ratio of 30% or less is applicable in any case.
[0014]
In the above embodiment, the etching method is described in two steps. However, even if the material to be etched is a metal or a multilayer structure such as a compound of metal and silicon, the number of steps is increased and immediately before or immediately after the underlying portion is exposed. In addition, by controlling on / off of the high frequency power source, it is possible to suppress the etching residue and at the same time maintain a high selection ratio. Depending on the structure of the sample, the overetched portion may be further divided into a plurality of steps.
[0015]
[Example 2]
FIG. 4 shows another apparatus structure to which the present invention is applied. In this apparatus, plasma is generated by inductive coupling at a frequency of a so-called radio wave band (hereinafter referred to as rf) of several hundred kHz to several tens of MHz. The vacuum vessel 401 is made of a material that transmits electromagnetic waves, such as alumina or quartz. Around that, an electromagnetic coil 402 for generating plasma 403 is wound. An rf power source 404 is connected to the coil. There is a sample stage 408 in the vacuum vessel 401, a sample 407 is placed on it, and a high-frequency power source 409 is connected thereto. The vacuum vessel 401 has an upper lid 405, but this may be an integral type.
[0016]
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 controlled to be turned on / off by overetching, so that the remaining etching can be suppressed and the selection ratio can be kept high.
In the apparatus shown in FIG. 4, the effect is the same even if the electromagnetic coil 402 is installed on the upper lid 405.
[0017]
Example 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. In the vacuum vessel 501, two electrodes 502 and 505 are arranged in parallel. An rf power source 503 and a high frequency power source 506 are connected to the electrodes, respectively. The sample 504 is placed on an electrode 505 that also serves as a sample stage. The gas is introduced into the container through the introduction tube 508 from the hole opened in the electrode 502 facing the sample. Plasma 507 is generated between the two electrodes.
[0018]
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 controlled to be turned on / off by overetching, so that the remaining etching can be suppressed and the selection ratio can be kept high.
[0019]
Example 4
Next, a method for realizing a higher selection ratio by overetching will be described. FIG. 6 shows the relationship between the etching rate of polycrystalline silicon and the selectivity when the mask of the workpiece is an oxide film instead of a resist. Point A is a value at which the high frequency power is continuously 80 W, curve 601 is a value when power is continuously reduced, and curve 602 is a value when the duty ratio is reduced by on / off control with a constant voltage amplitude. It can be seen that the selection ratio is further increased by about 50% by the on / off control as compared with FIG. This is because the carbon atoms contained in the resist are eliminated. As described above, when the bias is controlled to turn on and off, reaction products and oxygen atoms are deposited on the oxide film during the period when the high frequency is off, reducing the etch rate of the oxide film. Product is also deposited. These materials are known to have the property of increasing the etch rate of the oxide film. For this reason, the effect of on / off control is lowered. By using a material that does not contain carbon, such as an oxide film, as the mask material, there is no adhesion of carbon when the bias is off, and a higher selectivity can be obtained. Other mask materials that do not contain carbon include silicon nitride.
[0020]
As described above, according to this embodiment, the processing time can be shortened by processing at 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 substrate is exposed, it is possible to maintain a high selection ratio while suppressing a decrease in etch rate as compared with the case where continuous power is applied. Accordingly, etching residue can be suppressed.
[0021]
【The invention's effect】
According to the present invention, it is possible to process a semiconductor element while suppressing a decrease in etch rate or occurrence of etching residue in etching of polycrystalline silicon and maintaining a high selectivity to oxide film.
[Brief description of the drawings]
FIG. 1 is an overall configuration diagram showing an embodiment of an apparatus for carrying out 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 selection ratio.
FIG. 4 is an overall configuration diagram showing another embodiment of an apparatus for carrying out the surface processing method of the present invention.
FIG. 5 is an overall configuration diagram showing still another embodiment of the apparatus for carrying out the surface processing method of the present invention.
FIG. 6 is a graph showing the relationship between the polycrystalline silicon etch rate and the selectivity.
[Explanation of symbols]
101 ... microwave power supply, 102 ... waveguide, 103 ... introduction window, 104,401,501 ... vacuum vessel, 105 ... magnet, 106,403,507 ... plasma, 107,407,504 ... sample, 108,408 ... sample stage, 109,409,506 ... high frequency power supply, 110 ... voltage waveform, 201 DESCRIPTION OF SYMBOLS ... Silicon substrate, 202 ... Oxide film, 203 ... Polycrystalline silicon, 204 ... Resist, 301, 302, 601, 602 ... Curve, 402 ... Electromagnetic coil, 404, 503 ... rf power source, 405 ... Upper lid, 502, 505 ... Electrode, 508 ... Gas introduction pipe.

Claims (1)

While generating plasma in the vacuum vessel using a power source for generating plasma, a high frequency voltage is applied to the sample table using a high frequency power source connected to the sample table and is incident on the sample installed on the sample table. In a surface processing method for accelerating ions to perform surface processing of the sample,
The process from the start to the end of the surface processing of the sample is divided into two steps, Step 1 and Step 2. In Step 2, the high-frequency voltage applied from the high-frequency power source is repeatedly applied in ON and OFF periods. The surface processing method characterized in that the ON period is 30% or less of the ratio of the ON and OFF periods.

Images