JP3704423B2 - Surface treatment equipment - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a surface treatment apparatus for semiconductor elements, and more particularly to a surface treatment apparatus for etching a semiconductor surface using plasma.
[0002]
[Prior art]
An apparatus widely used at present for etching semiconductor elements is an apparatus using plasma. The present invention is applied to an apparatus using such 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. Electrons cause cyclotron motion by a magnetic field, and plasma can be generated efficiently by resonating this frequency with the microwave frequency. 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]
Japanese Patent Laid-Open No. 6-151360 is known mainly for the purpose of improving the accuracy of this apparatus. In the present invention, the selective ratio between Si, which is a substance to be etched, and the base oxide film can be increased by intermittently controlling the high-frequency voltage applied to the sample on and off.
[0004]
[Problems to be solved by the invention]
With the recent miniaturization of semiconductor elements, the problem of damage to the elements caused by plasma used in the process has become apparent. Specifically, the thickness of the gate oxide film of a metal oxide semiconductor (MOS) transistor is about 4 nm in a memory element of 256M or later. When the aspect ratio of processing (longitudinal and lateral dimension ratio) increases in addition to the thinning of the gate oxide film, damage caused by a phenomenon called electronic shading becomes a problem.
[0005]
Next, the electronic shading phenomenon will be described with reference to the drawings. FIG. 2B is a cross-sectional view of a semiconductor wafer exposed to plasma in an etching apparatus. FIG. 2A is a view of the resist pattern of FIG. 2B viewed from above. Although the sample in the figure is an element for damage evaluation, in an actual device, it mainly corresponds to the gate portion of the transistor. An element isolation oxide film 204 and a gate oxide film 203 are formed on a Si substrate 205, and a poly Si layer 202 and a resist 201 are formed in a comb shape thereon. During plasma etching, electrons 206 and ions 207 enter the sample. The ions 207 are accelerated by a high-frequency voltage applied to the sample and are incident perpendicularly on the sample surface. However, since the electrons 207 have a small mass, they have a large random velocity component and are decelerated by a bias. Incident in the direction.
[0006]
For this reason, as shown in FIG. 2B, in the processing of a groove having a high aspect ratio, ions can reach the groove bottom 208, but electrons are mainly captured by the side wall of the resist 201. Then, positive charges are accumulated in the gate oxide film 203 through the poly Si layer 202, and when this amount exceeds a certain value, the gate oxide film 203 causes dielectric breakdown, resulting in an element failure. As described above, the phenomenon in which electrons are not supplied into the fine groove due to the difference in directionality between ions and electrons is called electron shading.
[0007]
An object of the present invention is to provide a surface treatment apparatus that reduces damage to elements due to this electronic shading.
[0008]
The purpose is to apply a voltage to the vacuum vessel, a means for generating plasma in the vacuum vessel, a sample stage for placing a sample whose surface is treated by the plasma, and a sample. The voltage is achieved by a surface treatment apparatus in which the amplitude has a period of 20 V or less and a period of 300 V or more, and these periods are alternately repeated. Furthermore, this is achieved by repeating the amplitude of the voltage at a frequency of 100 Hz to 10 kHz. Furthermore, it is achieved when the period in which the amplitude is 300 V or more is set to one period of the repetition is 5% or more and 70% or less . Furthermore, this is achieved by the sum of the rise and fall times of the voltage when switching the amplitude of the voltage being 10% or less of the repetition period .
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Example 1
An embodiment of the present invention will be described below with reference to FIGS.
[0010]
FIG. 1A is an overall configuration diagram of a plasma etching apparatus to which the present invention is applied. In FIG. 1A, microwaves are introduced from a microwave power source 101 into a vacuum vessel 104 through a waveguide 102 and an 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 intensity is set to resonate with the frequency of the microwave. For example, if the frequency is 2.45 GHz, the magnetic field intensity is 875 Gauss. With this magnetic field strength, the cyclotron motion of electrons in the plasma 106 resonates with the frequency of the electromagnetic wave, so that the energy of the electromagnetic wave is efficiently supplied to the plasma and a high-density plasma can be formed. The sample 107 is set on the sample stage 108. A high-frequency voltage power source 109 is connected to the sample stage 18 in order to accelerate ions incident on the sample. There is no particular limitation on the frequency of the high-frequency voltage power supply, but in general, the frequency is practically in the range of 200 kHz to 20 MHz.
[0011]
FIG. 1B shows a voltage waveform 110 of the high frequency voltage power supply 109. In accordance with the present invention, the voltage amplitude is applied in divided periods of 20 V or less, in this case, when 0 V is displayed as “off”, the amplitude is 300 V or more (indicated as “on” in the figure).
[0012]
Next, FIG. 3 shows the result of measuring the dielectric breakdown rate of the gate oxide film 203 by etching the damage evaluation element having the structure shown in FIG.
[0013]
As the etching gas, a mixed gas of Cl2 (75 sccm) and oxygen (5 sccm) was used. The pressure inside the vacuum vessel 14 was 0.4 Pa. The output of the microwave power source 101 was 600W. The frequency of the high-frequency voltage power supply 109 is 800 KHz, it is not turned on and off, it is continuous, and its amplitude Vpp is used as a parameter. Thickness 4nm of elements in the form a gate oxide film 203 shown in FIG. 1, the thickness of 0.2 mu m of poly Si layer 202, it is in 1μm thickness of the resist 201, 1 mm length of the comb, comb number 100, the comb width and the inter-comb space were each 0.5 μm. In FIG. 3, the horizontal axis represents the amplitude Vpp and the vertical axis represents the dielectric breakdown rate of the gate oxide film. As can be seen from FIG. 3, the breakdown rate has a maximum value with respect to Vpp, and when Vpp is 20 V or less and 300 V or more, the breakdown rate is within 5%, which is not a problem in an actual device. However, when Vpp is 20 V or less, there is a problem that ions are not sufficiently accelerated and the etching rate is slow or the etching direction is poor. On the other hand, when Vpp is 300 V or more, there is a problem that the ion energy is too high and the selectivity of the etching rate between poly Si and the oxide film becomes small.
[0014]
Therefore, in the present invention, as shown in FIG. 1B, the ion acceleration voltage is applied to the sample by repeating the off period in which Vpp is 20 V or less and the on period in which Vpp is 300 V or more. With the repetition frequency of 100 Hz to 10 kHz, the dielectric breakdown rate was within 5% with the element structure shown in FIG. Furthermore, when Vpp at the time of ON was set to 450V, the dielectric breakdown rate became almost zero. In this method, the poly Si etching rate and the oxide film selection ratio can be adjusted by changing the ratio (hereinafter referred to as the duty ratio) that makes the ON period one cycle. A decrease can be avoided. The duty ratio should be 5% or more and 70% or less. For example, if etching is performed under the above conditions with Vpp at ON being 350 V and the etching is performed under the above-described conditions, the anti-oxide film is formed at a poly Si etching rate of 250 nm / min with a duty ratio of 50%. When the selection ratio was 25 and the duty ratio was 15%, a good value was obtained with an oxide film selection ratio of 180 at a poly Si etching rate of 180 nm / min.
[0015]
Furthermore, in combination with this method, it has been found that the plasma power can be reduced by reducing the microwave power in order to realize low damage etching. Specifically, the dielectric breakdown rate could be further reduced by setting the microwave power to 400W. Since the volume of the plasma generation portion (the space from the upper surface of the sample stage 108 to the lower surface of the introduction window 103) of the etching apparatus shown in FIG. 1 is 15000 cc, the microwave rf power per cc is 0.027 W / cc. Even if the volume of the plasma generation portion changes or the method of the etching apparatus changes, the effect of the present invention is further improved if the ratio of the power of the plasma generation power source to the volume of the plasma generation portion is 0.027 W / cc or less.
[0016]
In the known example, a method for on / off control of the high-frequency voltage applied to the sample is described, but there is no description about a method for reducing damage, and analogy of others in the same industry is impossible without the experimental data shown in FIG. is there.
[0017]
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 403 is made of a material that transmits electromagnetic waves, such as alumina or quartz. Around that, an electromagnetic coil 402 for generating plasma 410 is wound. An rf power source 404 is connected to the coil. A sample stage 408 is provided in the vacuum container 401, and a sample 407 is placed thereon, and a high frequency voltage power source 409 is connected thereto. The vacuum vessel 401 has an upper lid 405, but this may be an integral type.
[0018]
In accordance with the present invention, the high frequency voltage power source 409 was applied to the sample stage 408 by repeating an off period of Vpp 20 V or less and an on period of Vpp 300 V or more. Also in this apparatus, the dielectric breakdown rate by electronic shading measured by the damage evaluation element shown in FIG. 2 was suppressed to 5% or less, and the usefulness of the present invention was found.
[0019]
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.
[0020]
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 voltage 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.
[0021]
Even in this type of apparatus, the high frequency voltage power source 506 was repeatedly applied to the sample with an off period in which Vpp was 20 V or less and an on period in which Vpp was 300 V or more, thereby preventing damage due to electronic shading.
[0022]
Example 4
Next, an example of the waveform of the high frequency voltage applied to the sample will be described. When Vpp is changed from 20 V or less to 300 V or more, it takes time to rise and fall depending on the performance of the power supply, and a waveform as shown in FIG. 6 is obtained. Between the rise time t1 and the fall time t2, Vpp is between 20V and 300V, that is, a voltage causing dielectric breakdown. The proportion of this period in one cycle was measured to find no problem. As a result, it was found that the effect described in the present invention is obtained if the ratio of the total of t1 and t2 to the period T is 10% or less.
As described above, according to the above-described embodiment, it is possible to prevent the dielectric breakdown of the gate oxide film of the transistor by avoiding the decrease in the etching rate of poly Si and the selectivity to the oxide film.
[0023]
【The invention's effect】
As described above , according to the present invention, it is possible to provide a surface treatment apparatus that reduces damage to elements due to electronic shading.
[Brief description of the drawings]
FIG. 1A is an overall configuration diagram of an apparatus to which the present invention is applied.
(B) It is a voltage waveform figure of a high frequency voltage power supply.
FIG. 2A is a cross-sectional view of a sample for damage evaluation (an explanatory diagram of an electronic shading phenomenon).
(B) It is sectional drawing of a semiconductor wafer.
FIG. 3 is a relationship diagram between a high-frequency voltage amplitude and a gate oxide film dielectric breakdown rate.
FIG. 4 is an overall configuration diagram of an apparatus to which the present invention is applied.
FIG. 5 is an overall configuration diagram of an apparatus to which the present invention is applied.
FIG. 6 is a waveform diagram of a high-frequency voltage.
[Explanation of symbols]
101 ... Microwave power supply, 102 ... Waveguide, 103 ... Introduction window, 104,403,501 ... Vacuum vessel, 105 ... Magnet, 106,410,507 ... Plasma, 107,407,504 ... Sample, 108,408 ... Sample stand, 109,409,506 ... High frequency voltage power supply, 110,508 ... Gas introduction tube , 110 ... voltage waveform, 201 ... resist, 202 ... poly Si layer, 203 ... gate oxide film, 204 ... element isolation oxide film, 205 ... substrate Si, 206 ... electron, 207 ... ion, 208 ... groove bottom, 402 ... electromagnetic coil, 404,503 ... rf power source, 405 ... top cover, 502,505 ... electrode, 601 ... high frequency voltage waveform.

Claims (4)

A vacuum vessel, means for generating plasma in the vacuum vessel, a sample stage for installing a sample whose surface is treated by the plasma, and a power source for applying a voltage to the sample A surface treatment apparatus in which the voltage has a period in which the amplitude is 20 V or less and a period in which the voltage is 300 V or more, and these periods are alternately repeated. The surface treatment apparatus according to claim 1, wherein the amplitude of the voltage is repeated at a frequency of 100 Hz to 10 kHz. 3. The surface treatment apparatus according to claim 1, wherein a ratio of the period in which the amplitude is 300 V or more is set to one period of the repetition is 5% or more and 70% or less. 4. 3. The surface treatment apparatus according to claim 1, wherein the total rise time and fall time of the voltage when switching the amplitude of the voltage is 10% or less of the repetition period. 4.

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