Surface treatment method and system

#This invention relates to the treatment for the surface of a substance being treated by use of plasma, and particularly to a method of supplying a bias voltage to a substance of which the surface is to be treated.

#FIG. 2 shows the most typical conventional biasing method called the RF biasing. A substance 1 to be etched is connected through a capacitor 2 to a high-frequency power supply 3. A sine-wave voltage as shown in FIG. 3 is applied through the capacitor 2 to the substance 1. At this time, since the amount of electrons which a plasma 4 supplies is dozens of times larger than that of ions, negative charges are accumulated on the substance-side of the capacitor 2. Because of the capacitor charges, a voltage shifted in the negative direction as shown in FIG. 4 appears on the substrate. The positive ions for etching are accelerated by this negative voltage and perpendicularly incident on the substrate, thus making it possible to etch the substrate in an anisotropic shape.

#Another idea is disclosed in JP-A-56-13480 (laid-open on Feb. 9, 1981) and JP-A-6-61182 (laid-open on Mar. 4, 1994) in which a pulse-waveform voltage is used as a bias voltage. However, the importance of the duty ratio and repetition frequency of a pulse wave has never been considered so far except that the present invention regards them as important.

#FIG. 1 is a construction diagram of the surface treatment system for use in supplying a pulse bias, according to the present invention.

#FIG. 2 is a construction diagram of the conventional etching system for use in supplying an RF bias.

#FIG. 3 is a diagram of the supplied bias voltage wave appearing when the RF bias is supplied as in the prior art.

#FIG. 22 shows a micro-wave etching system for processing polycrystalline silicon for gate in which the pulse bias of the invention is used. In this system, a microwave generated from a magnetron 18 is introduced through a waveguide 19 into a discharge tube 20, and a high density plasma can be generated therein by the electron cyclotron resonance (ECR) between the introduced microwave and the magnetic field produced by a coil 21. In this system, the potential of this plasma is prevented from being much changed when the pulse is supplied, by grounding the plasma by an earthed electrode 22 of which the superficial area is four times or above as large as that of the substance 1 being treated. The substance 1 being etched is formed of a Si wafer of 6-inch size of which the surface was thermally oxidized, a polycrystalline silicon film deposited on the surface and a resist mask formed on the poly (polycrystalline) silicon film. This substance 1 is connected through an insulator 23 for electrostatic chucks which has an electrostatic capacitance of 30 pF/cm2 to a constant voltage source 24 for electrostatic chucks and the pulse voltage source 17. In this invention, the pulse voltage source is required to have a high rising-up speed in order to generate a pulse voltage of a high frequency, low duty ratio and several hundred-V level. This rising-up speed is normally expressed by a value called through rate. The through rate is obtained by dividing the risen-up voltage by the rising-up time when the step voltage is generated, and it is peculiar to the pulse voltage generator. FIG. 23 shows minimum through rate of the pulse generator which can provide the given duty ratio and the given repetition frequency of the pulse voltage. In order to generate a pulse voltage of repetition frequency 400 KHz or above, duty ratio of 5% or below and several hundred-V level, it is necessary to use a pulse voltage source having a through rate of 8×102 V/μ sec or above. In order to generate a pulse voltage of repetition frequency 1 MHz or above and duty ratio of 1% or below, the through rate of the pulse voltage generator is necessary to be 104 V/μ sec or above. The repetition frequency and duty ratio of the pulse which can be generated by the commercially available wide-band arbitrary-wave generator are 100 MHz or below and 0.1% or above, respectively. Thus, the pulse voltage source 17 in this embodiment is formed of this wide-band arbitrary-wave generator, 25 and a high-speed wide-band power amplifier 26 of through rate 105 V/μ sec. The signal from the wide-band arbitrary-wave generator 25 is amplified by the high-speed wide-band power amplifier 26 to produce a pulse of repetition frequency 10 MHz, duty ratio 1% and several hundred-V level. FIG. 24 shows one example of the pulse waves which can be generated by this power supply. When a high-frequency pulse is generated, the pulse waveform is not always rectangular as shown in FIG. 24. In addition, the pulse is not linearly risen up. Therefore, the pulse width, the throughrate and the duty ratio-that is the function of the pulse width cannot be defined precisely. In this patent specification, the half-width of the full maximum of the pulse is defined as the pulse width, and the pulse width divided by the repetition frequency is defined as the duty ratio. Also, the through rate of the pulse is defined at the maximum gradient of the rising-up portion of the pulse.

#The electron density and electron temperature were measured by the triple probe method in order to examine the state of the generated plasma in this system. The measured electron density and electron temperature were respectively 1011/cm3 and 3 eV which are the same as the conditions of the above-mentioned simulation.

#The polycrystalline silicon fine pattern formed on the silicon dioxide film was etched by using chlorine plasma in this system. At this time, if the height of the pulse is too small, the voltage for accelerating electrons is reduced so that the electrons cannot be forced to anisotropically impinge on the fine pattern. Therefore, the pulse height is necessary to be at least 20 V. However, if the pulse voltage is too large, the energy for accelerating the ions is increased to deteriorate the selectivity of the polycrystalline silicon relative to the silicon dioxide film. Therefore, the pulse height is desired to be 2 KV or below. In this embodiment, the pulse voltage was fixed to 100 V, and first the notch size was examined with the repetition frequency set to 10 MHz and with the duty ratio changed. FIG. 25 shows the results. From the figure, it will be seen that the notch size becomes the minimum when the duty ratio is in the range from 0.5% to 5%. Particularly in the pattern which is not conductive to the silicon substrate (hereinafter, called FG pattern), the notch size is zero when the duty ratio is in this range. In the pattern which is conductive to the substrate (hereinafter, called SG pattern), the notching does not completely disappear. We found that the incomplete disappearance of the notching in the SG pattern is ascribed to the negative DC voltage supplied from the voltage source 24 for electrostatic chucks. FIG. 46 shows the mechanism of generating this difference. When a negative voltage of, for example, −500 V is supplied as a voltage for electrostatic chucks, there appears on the rear substrate 13 a potential which is about 20 V lower than that on the surface of the underlying dioxide film 9. Therefore, since the potential on the SG pattern which is conductive to the substrate is about 20 V lower than that on the surrounding dioxide film, the positive ions 5 are deflected to the SG pattern side to be easy to obliquely impinge. The obliquely impinging ions promote the generation of the notching. Thus, when a voltage of +500 V or above is supplied from the voltage source by changing the polarity of the voltage source for electrostatic chucks as shown in FIG. 43, the SG pattern is held at a potential which is 20 V higher than that on the surrounding dioxide film 9. Accordingly, it is hard for the positive ions to obliquely impinge on the SG pattern. The same measurement as in FIG. 25 was made by use of this system. In this case, when the duty ratio was in the range from 0.5% to 5%, the notching completely disappeared not only from the FG pattern but also from the SG pattern.