Semiconductor device manufacturing: process – Chemical etching – Vapor phase etching
Reexamination Certificate
2000-12-11
2004-06-29
Vinh, Lan (Department: 1765)
Semiconductor device manufacturing: process
Chemical etching
Vapor phase etching
C438S710000, C438S712000, C427S571000, C427S575000, C156S345420
Reexamination Certificate
active
06756311
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to methods for producing semiconductor devices and to semiconductor production apparatus.
2. Description of Related Art
In the production of semiconductor devices, plasma processes, which utilize plasma generated by exciting various process gases with electromagnetic waves, are widely used. For example, plasma-enhanced chemical vapor deposition (CVD) is used to form various films on surfaces of semiconductor substrates. Plasma etching is used to form patterns of various materials on surfaces of semiconductor substrates by etching films of the material in accordance with a mask pattern.
In plasma etching, a substrate having a film to be etched is placed in a chamber having a plasma source of various types, such as electron cyclotron resonance (ECR), inductively-coupled plasma (ICP) and reactive ion etching (RIE). A process gas suitable for the material of the film to be etched is supplied into the chamber, and plasma of the process gas is generated by introducing an electromagnetic wave of various frequencies, such as a microwave and a radio frequency (RF) wave of 400 kHz, 2 MHz, 13.56 MHz, 27 MHz, 60 MHz, or the like, into the chamber through the plasma source. The surface of the substrate is irradiated with various active species and ions generated in the plasma. A bias is also applied to the semiconductor substrate, and the energy of the ions that irradiate the substrate is controlled. Anisotropic processing of the film to be etched is achieved.
Hereinafter, gate etching will be described as an example of a typical conventional etching process.
In gate etching, polycrystalline Si (poly-Si) film on a substrate having an underlying gate oxide film is etched. In this process, finer gate dimensions and thinner gate oxide films are required. For example, in semiconductor devices with a design rule of 0.18 &mgr;m or less, the thickness of the gate oxide film is 3.5 nm or less. Further, when the thickness of a remaining gate oxide film is 2.0 nm or less in gate etching, the crystallinity of the silicon substrate may be disordered. Therefore, extremely high oxide film selectivity (poly-Si/oxide film etching rate ratio) must be ensured.
Usually, a two-step etching process including a main etching condition and an overetching condition is employed in gate etching. In conventional gate etching techniques, the oxide film selectivity in the main etching condition using a Cl
2
-O
2
-HBr based process gas is about 15. In the overetching condition, in which an O
2
-HBr based process gas is utilized, the oxide film selectivity is about 100. The typical thickness of the poly-Si layer is 200 nm when the gate is formed with a silicide/poly-Si bi-layer. The end-point is detected using optical emission spectroscopy, and an over etch is added with the main etching condition for about 10% of the time to detect the end point, and then with the overetching condition for about 50% of the time to detect the end point. About 1.3 nm and about 1.0 nm of the gate oxide film is etched during the overetchings with the main etching and the overetching condition, respectively. As a result, the thickness of the remaining oxide film is approximately 1.0 nm. Consequently, the problem of damage described above may occur in the semiconductor device. Accordingly, the oxide film selectivity must be improved.
Further, as described above, an end-point detection (EPD) technique is used to control the etching process. In the etching of a poly-Si layer, for example, optical emission from Si-containing species generated by the etching reaction is monitored. Because the intensity of the optical emission from the Si-containing species decreases when the poly-Si layer has been removed, the end-point can be determined by monitoring a change of the intensity of the emission. In conventional etching processes, however, the intensity changes continuously, and thus the end point can only be determined with a time resolution of several to more than ten seconds. That is, in the above-described poly-Si etching process, after the gate oxide layer has been partly exposed, several to more than 10 seconds may pass before the end point detection is performed.
Thus, in addition to an etching method with higher selectivity, a method to detect the end-point with a shorter time resolution is desired for the production of future semiconductor devices.
By increasing the oxygen partial pressure in the process gas, the oxide film selectivity in the main etching condition and in the overetching condition can be increased to about 20 or more and to 150 or more, respectively. However, when the partial pressure of O
2
is increased, the surface of the poly-Si film is oxidized as schematically shown in
FIG. 7A
, and the etching rate is reduced. In order to prevent the reduction in etching rate, the bias voltage applied to the substrate must be increased.
When the bias voltage is increased, the mask material is sputter etched, and the sputtered material and/or plasma generated product produced with the sputtered mask material is deposited on the sidewall of the etched patterns as schematically shown in FIG.
7
B. Because the deposited material on the sidewall acts as a mask, the cross-sectional shape of the etched pattern is tapered, and hence, the ability to control the shape of fine patterns is limited. Further, sputtered material and/or plasma generated product also deposit on the inner wall of the etching chamber. An excessive deposition of the material on the chamber wall may generate particles, and decrease the production yield.
In particular, when the mask layer includes patterns of different densities, the cross-sectional shape of the etched pattern may differ depending on the pattern density. That is, oxygen radicals are likely to diffuse in the vicinity of the isolated pattern, and hence, the surface of the poly-Si film at the vicinity of the isolated pattern is excessively oxidized. In addition, an excessive redeposition of etched material may occur in the isolated pattern. As a result, as shown in
FIG. 7C
, the sidewall of the isolated pattern is likely to be further tapered compared to the sidewall of the dense pattern.
In order to solve the problem of damage, a so-called digital etching method, such as described in U.S. Pat. No. 5,328,558, is proposed. This method comprises repeating a step of supplying activated species to a semiconductor substrate having a film to be etched to adsorb the activated species on a surface of a film, and a step of irradiating the adsorbed activated species with Ar ions. In this technique, however, different process gas compositions each for generating the activated species and Ar ions should be alternatively supplied for many times. In addition, a few atomic layers can only be etched away in one cycle. Therefore, the etching time is too much prolonged. Accordingly, the technique cannot be used in practice for mass production.
SUMMARY OF THE INVENTION
Accordingly, in consideration of the conventional problems described above, it is an object of this invention to provide methods for producing semiconductor devices with superior ability of controlling the process. It is also an object of this invention to provide a production apparatus, in which superior ability of controlling the process can be obtained.
According to one aspect of this invention, an exemplary method of producing a semiconductor device includes: placing a semiconductor substrate in a chamber; introducing a process gas into the chamber; producing an excited process gas by exciting the process gas using two electromagnetic waves; and processing a surface of the substrate using the excited process gas. The powers of the two electromagnetic waves change periodically and separately, keeping a first timing relationship with each other.
According to another aspect of this invention, an exemplary method of producing a semiconductor device includes: placing a semiconductor substrate in a chamber; introducing a process gas into the chamber; producing an excited process gas by exc
Kawasaki Microelectronics Inc.
Oliff & Berridg,e PLC
Vinh Lan
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