JP2000012529A - Surface machining apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the subtrenches by dividing a high frequency bias into an on and off-periods and specifying the max. energy of ions in a plasma incident on a sample in the on-period to set the output of a high frequency power source. SOLUTION: A microwave is fed into a vacuum vessel 104 through a waveguide 102 and entrance window 103 from a microwave power source 101 to generate a high density plasma 106, a high frequency bias power source 109 is connected to a sample table 108, the high frequency bias power source 109 is divided into an on and off-periods and applied repetitively as shown with a voltage waveform 110, and the energy of ions in the plasma 106 is set to a max. value of 1,100 eV or more to obtain a waveform without substrenches. In the on-off modulation the subtrench is completely suppressed and a flat plane is ahtained. As a result, it is possible to avoid etching shape defects and reduction of an antioxidative film selection ratio at the same time and to process the gate of a transistor.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface processing apparatus, and more particularly to a surface processing apparatus suitable for etching a sample such as a semiconductor element substrate using plasma.

[0002]

2. Description of the Related Art Conventionally, an apparatus utilizing plasma has been widely used for etching a sample such as a semiconductor element substrate.
For example, a plasma device called an ECR (Electron Cyclotron Resonance) system is known. 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 the same as the frequency of the microwave, plasma can be generated efficiently because of resonance. A high frequency bias 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.

[0003] Japanese Patent Application Laid-Open No. 6-151360 is known to improve the accuracy of this type of apparatus. According to the present technology, the selectivity between Si, which is a substance to be etched, and a base oxide film is increased by intermittently controlling a high-frequency bias applied to a sample by turning it on and off.

[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 as described above, an abnormal processing shape called a subtrench becomes a problem in the element etching process. In processing a gate portion of a semiconductor element, it is necessary to etch a material (generally, polycrystalline silicon) serving as a gate electrode while leaving only the electrode portion, and to leave an underlying gate oxide film. If even a part of the gate oxide film is lost, a leak current flows and the device does not operate. The sub-trench is a phenomenon that, in a line pattern corresponding to a gate electrode, an etching rate immediately below a line side wall is increased, so that an underlying oxide film is etched away only in the vicinity of a line.

[0005] An object of the present invention is to provide a surface processing apparatus capable of reducing the number of the sub-trench.

[0006]

The object of the present invention is to provide a vacuum vessel, a means for generating plasma in the vacuum vessel, a sample table provided in the vacuum vessel for placing a sample whose surface is to be processed by the plasma, In a surface processing device consisting of a high-frequency power supply for applying a high-frequency bias, the high-frequency bias is periodically divided into on and off periods, and the maximum value of the energy of ions in the plasma incident on the sample during the on period is 1100 eV. This is achieved by setting the output of the high frequency power supply as described above.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 An embodiment of the present invention will be described below with reference to FIGS. FIG. 1A is an overall configuration diagram of a plasma etching apparatus to which the present invention is applied. Microwaves are introduced from a microwave power supply 101 into a vacuum vessel 104 through a waveguide 102 and an introduction window 103. The material of the introduction window 103 is a material that transmits electromagnetic waves, such as quartz or ceramic. Vacuum container 1
An electromagnet 105 is provided around the magnetic field 04, and the magnetic field strength is set so that the microwave frequency and the electron cyclotron frequency are the same.
At GHz, the magnetic field strength is 875 Gauss. The high-density plasma 106 is generated by this magnetic field intensity. The sample 107 is set on a sample stage 108. In order to accelerate ions incident on the sample, the high frequency bias power supply 109
8 is connected. There is no particular limitation on the frequency of the high-frequency bias power supply.
The range of MHz is practical. FIG. 1B shows a voltage waveform 110 of the high frequency bias power supply 109. The output of the high frequency bias power supply 109 is repeatedly applied to the sample stage 108 separately for the ON and OFF periods. The ratio of ON in one cycle of ON / OFF is called a duty ratio.

Next, FIG. 2 shows the result of etching a fine pattern consisting of lines and spaces using this apparatus. The etching gas is Cl2 (185 sccm) and oxygen (15 sccm)
Was used. 0.8 Pa inside the vacuum vessel 14
And The output of the microwave power supply 101 was 400 W. The frequency of the high frequency bias power supply 109 is 800 KHz. The structure of the etched element is a silicon substrate 20
1, the thickness of the gate oxide film 202 is 4 nm, the thickness of the polycrystalline silicon layer 203 is 300 nm, and tungsten silicide (W
Si) 204, thickness 80 nm, mask (made of silicon nitride) 2
05 has a thickness of 200 nm and a space width of 0.35 μm
It is. FIG. 2 shows a continuous output of high frequency bias (60
W) (FIG. 2 (a)) and on-off modulation (300W, duty ratio 20
%) (FIG. 2 (b)). In FIG. 2, the high-frequency power is adjusted so that the etching rates of both the poly-Si and the oxide film become substantially the same,
At a continuous bias of 60 W, the poly-Si etching rate is 37
1 nm / min, oxide film etching rate 21.4 nm / min, selectivity 17.3, and poly-S
The i etching rate is 330 nm / min, the oxide film etching rate is 18.9 nm / min, and the selectivity is 17.4. As can be seen from FIG. 2, a sub-trench 205 occurs with a continuous bias (Vpp = 366 V). On the other hand, the on / off modulation bias (Vp
(p = 1424V), the sub-trench is completely suppressed and a flat bottom surface is obtained. It is considered that the subtrench is generated because ions with poor direction are reflected on the side wall of the groove and enter near the line on the bottom surface of the groove to locally increase the etching rate in this portion. Therefore, if the direction is improved by accelerating the ions to high energy by increasing Vpp, the reflection on the side surface is reduced and the subtrench is reduced. However, if Vpp is simply increased, there is a problem that the etching rate of the oxide film increases and the selectivity decreases.
Thus, when the high frequency bias applied to the sample is modulated on / off as in the present invention, the subtrench can be suppressed without reducing the selectivity.

Next, the relationship between ion energy and high frequency bias will be described. When a high-frequency bias is applied to the sample stage via the plasma, a DC potential is applied to the sample stage so that ions are drawn into the sample stage due to the action of trying to cause a current to flow between the ground (generally, the conductor wall is grounded) and the electrodes. (Hereinafter referred to as Vdc). The ions are accelerated by an electric field in which Vdc and a time-varying high-frequency bias are superimposed. The maximum energy obtained by an ion changes depending on whether the ion follows the temporal change of the high frequency bias. Generally, the density of the plasma used for etching is 1
It is 10 10 or more per cubic cm. At this density,
When the high frequency is less than 15 MHz, the ions cross the plasma sheath and reach the sample during the period when the high frequency bias is negative, that is, during a half cycle of the sine wave.
ax is substantially equal to a value obtained by adding Vdc to half of the voltage amplitude (Vpp / 2). Actually, there is a voltage drop in the electric circuit, etc., and when Vpp shown in Fig. 2 is 366V and 1424V, Emax is 300
eV and 1100 eV. Since the essential physical quantity that affects the shape of the etching is not Vpp but ion energy,
In order to obtain a shape without a sub-trench, the maximum value of the ion energy should be set to 1100 eV or more. When the frequency of the high frequency increases and the change in the voltage does not follow the movement of the ions, Emax gradually approaches Vdc. If the frequency is between 15 MHz and several tens of megahertz, the transition period will be a transition period.
That is all.

According to experiments, the duty ratio is 50%.
It was found that the effect of the on / off control was exhibited below.

Embodiment 2 FIG. 3 shows another device structure to which the present invention is applied. In this device, a plasma is generated by inductive coupling at a frequency in a so-called radio wave band (hereinafter referred to as rf) of several hundred kHz to several tens MHz. generate. The vacuum container 301 is made of a material that transmits electromagnetic waves, such as alumina and quartz. Around that, plasma 3
03 is wound. An rf power supply 304 is connected to the coil. A sample stage 308 is provided in the vacuum container 301, and a sample 307 is placed on the sample stage 308.
And a high frequency bias power supply 309 is connected. The vacuum vessel 301 has an upper lid 305, which may be an integral type.

In this apparatus, the high frequency bias power supply 309
Is turned on / off, and Emax during the ON period is set to 1100 eV or more, whereby the subtrench can be suppressed without lowering the selectivity.

In the device shown in FIG. 3, the effect is the same even if the electromagnetic coil 302 is installed on the upper lid 305.

Embodiment 3 FIG. 4 shows another apparatus structure to which the present invention is applied.
Plasma is generated by capacitive coupling of rf power. In a vacuum vessel 401, two electrodes 402 and 405 are arranged in parallel. An rf power supply 403 and a high-frequency bias power supply 406 are connected to the electrodes, respectively. The sample 404 is placed on an electrode 405 which also serves as a sample stage. The gas is introduced into the container through the introduction tube 408 from a hole opened in the electrode 402 facing the sample. Plasma 407 is generated between the two electrodes.

In this apparatus, the high-frequency bias power supply 406
Is turned on / off, and Emax during the ON period is set to 1100 eV or more, whereby the subtrench can be suppressed without lowering the selectivity.

Embodiment 4 As a means for further expanding the effects of the present invention, there is a method of dividing an etching process into a plurality of steps. In this case, the method was performed by the apparatus shown in FIG. FIG. 5 shows a temporal change in the cross-sectional shape when the material to be processed is processed in steps. The structure of the sample is a silicon substrate 5
01, 4 nm oxide film 502, 200 nm polycrystalline silicon 5
03, a 200 nm silicon nitride film 504 is deposited,
A fine pattern is processed on the silicon nitride film. The line and space spacing is 350 nm. Normally, a natural oxide film 505 is provided on the surface of the polycrystalline silicon 503. Here, the sample having this structure is processed in three steps for etching. FIG. 5A shows an initial state. In Step 1, first, the natural oxide film 505 is removed. The etching gas is Cl2 (200sccm) and the pressure is 0.8Pa
And The output of the microwave power supply 101 was 400 W. The frequency of the high frequency bias power supply 109 is 800 kHz. Since the purpose of this step is to remove a thin natural oxide film, the time is 5 seconds, and the output of the high frequency bias power supply 109 is continuously 60 W. FIG. 5B shows a cross section of the sample after the end of Step 1. Step 2 is etching of the polycrystalline silicon 503 occupying a major part of the processing, and is hereinafter referred to as main etching. Since the shape control in this step is the most important, the high frequency bias power supply 109 is turned on / off by applying the present invention, and the power is set to 300 W and the duty ratio is set to 20% as in the first embodiment. Vpp becomes 1424V. The etching gas was a mixed gas of 185 sccm of chlorine and 15 sccm of oxygen at a pressure of 0.8 Pa. Other conditions were the same as in step 1. The etching time was 35 seconds. FIG. 5C shows the cross-sectional shape after Step 2 is completed. A flat bottom surface and vertical side surfaces without sub-trench are obtained. The time of step 2 is set so that the polycrystalline silicon 503 slightly remains. In the last step 3, since the underlying thin oxide film 502 is exposed, the condition is changed to a condition where the selectivity is high even if the etching rate of polycrystalline silicon is reduced. Specifically, the power of the high frequency bias power supply 109 was set to 30 W continuously. Other conditions are the same as in step 2. Under these conditions, the etching rate of polycrystalline silicon is 100 nm / mi
n, the selection ratio is 50. Etching having a relatively high selectivity after completion of the main etch is called overetch. The etching time was 30 seconds. FIG. 5D shows a cross section after Step 3 is completed. By combining with an overetch having a higher selectivity, it is possible to process the polycrystalline silicon 503 without leaving the underlying oxide film sufficiently.

As described above, by applying the present invention to the processing of the main portion of the substance to be etched, a fine pattern can be processed without damaging a very thin underlayer.

Even if the material to be etched is a multi-layered structure of various materials, it is possible to perform processing without shape abnormality by increasing the number of steps and applying the present invention to the main etch.

Embodiment 5 Here, another embodiment of the step switching method will be described.
Switching from the main etching to the overetch can also be determined by a change in the intensity of the emission waveform of the substance to be etched. FIG. 6 shows a temporal change in the emission intensity of silicon atoms when the sample shown in FIG. 5 is being etched. When the etching of the polycrystalline silicon starts to end, the emission intensity 601 also decreases because the density of silicon atoms in the plasma decreases. In general, since the etching rate has some variation in the wafer, the emission waveform of the light emission intensity gradually decreases as the etching of the polycrystalline silicon in the wafer surface is gradually completed. There are mainly three switching methods. (1) is a method of switching as soon as the intensity of the emission waveform starts to decrease, and is switched at a point A in FIG. (2) is a method of differentiating the emission intensity twice and switching at the point where the curve becomes 0, that is, the inflection point. FIG. 6 also shows a second derivative curve 602 of the emission intensity 601,
Switch with B. In (3), switching is performed at the point C where the emission waveform lowering is completed. When to switch from A to B, it is necessary to select the time depending on the material to be etched. If the underlying material is very thin, the method (1) or the time just before the end of the etching described in the fourth embodiment is measured in advance. It is better to switch over a little before that. If there is enough time for the base material to come off thickly, the methods (2) and (3) are suitable because etch residues hardly occur.

In these examples, when oxygen gas is added to the etching gas with chlorine gas as shown in FIG. 7, the oxide film etching rate with respect to Vpp of the same value is as shown in FIG. 1 (b). Time Modulat
ion) bias method (TM bias). Vpp is a measure of the ion acceleration energy. That is, by providing an off-period for the bias in the gas to which oxygen is added, the etching rate of the oxide film is reduced even if ions of the same energy enter. Reactive products of oxygen, chlorine and silicon are selectively deposited on the oxide film to reduce the etching rate of the oxide film. The reason that the selectivity is improved by the TM bias is that reaction products are uniformly and firmly deposited on the oxide film during the bias off period, that is, during the period when high energy ions are not incident, and the oxide film etching reaction during the bias on period is performed. It is thought that it suppresses.

As described above, according to this embodiment, it is possible to process the gate of the transistor while simultaneously avoiding the abnormal etching shape of the polycrystalline silicon and the decrease in the selectivity to the oxide film.
Although the processing of the gate is mainly described in the present embodiment, the shape abnormality can be similarly suppressed in other portions or other materials.

[0022]

According to the present invention, it is possible to reduce the number of sub-trenches, and to simultaneously process abnormalities in the etching shape of polycrystalline silicon and a decrease in the selectivity with respect to an oxide film, thereby processing the gate of a transistor. There is.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of an apparatus configuration showing one embodiment of a surface processing apparatus of the present invention.

FIG. 2 is a cross-sectional view showing a surface processing state of a semiconductor sample.

FIG. 3 is a longitudinal sectional view of an apparatus configuration showing another embodiment of the surface processing apparatus of the present invention.

FIG. 4 is a longitudinal sectional view of an apparatus configuration showing still another embodiment of the surface processing apparatus of the present invention.

FIG. 5 is a cross-sectional view showing a surface processing state of a semiconductor sample in the apparatus of FIG.

6 is a diagram showing a light emission waveform at the time of the etching process in FIG.

FIG. 7 is a view showing a relationship between oxygen addition and etching of an oxide film in an etching process in the apparatus of FIG. 1;

[Explanation of symbols]

101 ... microwave power supply, 102 ... waveguide, 103 ... introduction window, 10
4,301,401… Vacuum container, 105… Magnet, 106,303,407… Plasma, 107,307,404… Sample, 108,308… Sample stage, 109,309,
406: High frequency bias power supply, 110: Voltage waveform, 201, 501 ...
Silicon substrate, 202,502 gate oxide film, 203,503 polycrystalline silicon, 204 WSi, 205,504 mask, 206 subtrench, 302 electromagnetic coil, 304,403 rf power supply, 305
Upper lid, 402, 405: electrode, 408: gas passage inlet, 505: natural oxide film, 601: emission intensity, 602: second derivative curve.

Claims (8)

[Claims] 1. A vacuum vessel, means for generating plasma in the vacuum vessel, a sample table provided in the vacuum vessel for placing a sample whose surface is to be processed by the plasma, and applying a high-frequency bias to the sample A high-frequency power supply for performing a high-frequency power supply, the high-frequency bias is periodically divided into an on and off period, and the maximum value of the energy of ions in plasma incident on the sample during the on period is 1100 eV or more. A surface processing apparatus wherein the output of the high-frequency power supply is set to be as follows. 2. The surface processing apparatus according to claim 1, wherein the voltage amplitude of the high-frequency bias is set to 1400 V or more when the frequency of the high-frequency power supply is 15 MHz or less, and 2200 V or more when the frequency is higher than 15 MHz. Maximum value of 1
A surface processing device characterized by 100 eV or more. 3. The surface processing apparatus according to claim 1, wherein an on-period of the high-frequency bias accounts for 50% or less of one on-off period. 4. The surface processing apparatus according to claim 1, wherein a repetition frequency of on / off of said high frequency bias is:
Surface processing equipment characterized by 100Hz to 10kHz. 5. The surface processing apparatus according to claim 1, wherein the process from the start to the end of the surface processing is divided into a plurality of steps, and the processing speed ratio of the base material for finishing the steps is relatively small. A surface processing apparatus, wherein the high-frequency bias is divided into an on period and an off period in at least one of the first half steps. 6. The surface processing apparatus according to claim 5, wherein said first step group and said second step group are determined and switched according to time. 7. A surface processing apparatus according to claim 5, wherein the first group of steps and the second group of steps are switched by detecting the intensity of light emitted by the substance to be etched in plasma. Processing equipment. 8. The surface processing apparatus according to claim 7, wherein the intensity of light emitted from the substance to be etched in the plasma is detected in the first and second step groups and the intensity starts decreasing. A surface processing device characterized by switching at the point of time.