Patent Application
20080230008
This invention relates to a substrate processing chamber. More particularly, the invention relates to control of plasma species and uniformity through pulsed VHF.
Plasma etching and reactive ion etching (RIE) have become important processes in precision etching of certain workpieces such as substrates in the fabrication of semiconductor devices. The differences between plasma etching and reactive ion etching, which generally can be carried out in the same equipment, typically result from different pressure ranges employed and from the consequential differences in mean free path of excited reactant species in a processing chamber. The two processes are collectively referred to herein as plasma etching. Plasma etching is a “dry etching” technique and has a number of advantages over conventional wet etching in which the workpiece is generally immersed in a container of liquid etchant material. Some of the advantages include lower cost, reduced pollution problems, reduced contact with dangerous chemicals, increased dimensional control, increased uniformity, improved etch selectivity, and increased process flexibility.
As integrated circuit densities increase, device feature sizes decrease below 0.25 micron while the aspect ratio (i.e., ratio of feature height to feature width) of the device features increase above 10:1. Improved precision of the etch process is required to form these small device features having high aspect ratios. Additionally, an increased etch rate is desired to improve throughput and reduce costs for producing integrated circuits.
One type of plasma etch chamber utilizes two parallel plate electrodes to generate and maintain a plasma of the process gases between the plate electrodes. Typically, a parallel plate plasma etch chamber includes a top electrode and a bottom electrode. The bottom electrode typically serves as a substrate holder, and a substrate (or wafer) is disposed on the bottom electrode. The etch process is performed on a surface of the substrate that is exposed to the plasma.
Typically, one or more of the electrodes are connected to a power source. In a particular parallel plate reactor, those electrodes are connected to high frequency power sources. The power source connected to the upper electrode is typically operated at a higher frequency than the power source connected to the lower electrode. This configuration is believed to decouple ion energy and ion flux on the substrate to avoid damage on the substrate.
Another parallel plate reactor has two power sources connected to a lower electrode. The power sources are each operated at different frequencies in order to control the etching characteristics resulting on a substrate being processed.
Yet another parallel plate reactor includes three electrodes. A first electrode is adapted to support a substrate and is connected to a low frequency AC power source. A second electrode is disposed in parallel relationship with the first electrode and is connected to ground. A third electrode (i.e., the chamber body) disposed between the first and second electrode is powered by a high frequency AC power source.
Another conventional apparatus provides a single powered electrode reactor. High and low frequency power supplies are coupled to the single electrode in an effort to increase process flexibility, control and residue removal. The single electrode reactor includes a multistage passive filter network. The network is intended to perform the functions of coupling both power supplies to the electrode, isolating the low frequency power supply from the high frequency power supply and attenuating the undesired frequencies produced by mixing of the two frequencies in the nonlinear load represented by the reactor.
The frequency applied to the electrode may be VHF. However, as the size of the substrate increases, plasma reactors have also become larger to the point where the size of the reactor is no longer negligible. In a plasma environment, the electromagnetic wavelength is reduced by approximately a factor of 5 from its free space wavelength, such that its quarter wavelength may approach the dimensions of the plasma chamber. As a result, the plasma density across the reactor may no longer be uniform. This standing wave phenomenon is becoming more pre-dominant as the free space excitation frequency increases, the wavelength decreases. Furthermore, the high frequency resulting in a high plasma density can reduce the skin depth that may become small compared to the size of the reactor gap. As a result, a skin effect may occur where maximum plasma heating may be seen at the edge of the discharge.
The disparity in density of the plasma in the chamber causes variations of the processing parameters in the chamber, which results in inconsistent or non-uniform processing of substrates. Therefore, there is a need for a parallel plate plasma etch system that can substantially maintain process uniformity in light of the electromagnetic effects occurring at high frequencies.
The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings.
The following description sets forth numerous specific details such as examples of specific systems, components, methods, and so forth, in order to provide a good understanding of several embodiments of the present invention. It will be apparent to one skilled in the art, however, that at least some embodiments of the present invention may be practiced without these specific details. In other instances, well-known components or methods are not described in detail or are presented in simple block diagram format in order to avoid unnecessarily obscuring the present invention. Thus, the specific details set forth are merely exemplary. Particular implementations may vary from these exemplary details and still be contemplated to be within the spirit and scope of the present invention.
A method and apparatus for processing a substrate is described. A capacitively coupled processing chamber has a first electrode and a second electrode. The second electrode may be used to support the substrate. The first electrode may be disposed above the substrate parallel to the second electrode. A high frequency power source is electrically coupled to either the first or second electrode to supply a first RF signal. A low frequency power source electrically coupled to either the first or second electrode to supply a second RF signal. The first RF signal is pulsed on and off so as to generate electron loss in the chamber so as to control spatial plasma uniformity in the chamber.
The process system 100 generally includes a chamber 102 having a cavity 103 at least part of which is a processing region. An opening (not shown) may be formed in a wall of the chamber 102 to facilitate substrate transfers into and out of the processing system 100. A bottom of the chamber 102 may include an outlet 130 for exhausting gases from the chamber 102. An exhaust system 132 may be attached to the outlet 130 of the bottom of the chamber 102. The exhaust system 132 may include components such as a throttle valve and a vacuum pump. Once the chamber 102 is sealed, exhaust system 132 may be operated to draw and maintain a vacuum within the cavity 103.
A top plate electrode 104 is disposed at an upper end of the chamber 102. In one embodiment, the plate electrode 104 may include a protective coating which prevents or reduces erosion of the material of the plate electrode 104 caused by the plasma in the chamber. The protective coating may comprise a material such as quartz, sapphire, alumina, SiC, SiN, and Si.
In one embodiment, the top plate electrode 104 may include a showerhead of a gas distribution system. In such a configuration, the top plate electrode 104 may be part of a lid assembly that is adapted to distribute gases into the cavity 103. Accordingly,
While the top plate electrode 104 acts as a top electrode of a parallel plate electrode plasma reactor, a substrate support 106 acts as a lower electrode. The substrate support 106 is disposed in the cavity 103 and may be any structure suitable for supporting the substrate 108 (e.g. a wafer or mask), such as an electrostatic chuck or a vacuum chuck. The substrate support 106 may include a support plate (not shown) defining a substrate supporting surface that is generally shaped to match the shape of the substrate 108 supported thereon. Illustratively, the substrate supporting surface is generally circular to support a substantially circular substrate, In one embodiment, the substrate supporting surface is thermally connected to a substrate temperature control system, such as a resistive heating coil and/or fluid passages connected to a beating or cooling fluid system.
The substrate support 106 may be connected to a low frequency RF power source 118 and a high frequency RF power source 116 for generating and maintaining plasma 128 in the chamber 102. In accordance with another embodiment, three different frequencies may be coupled to the cathode: a low frequency (LF), a medium frequency (MF), and a very high frequency VHF. The low frequency RF power source 116 may be connected to the supporting substrate 106 through a low frequency match network 122 and enhances ion assisted etching at the substrate 108. The high frequency RF power source 116, or a VHF power source may be connected to the supporting substrate 106 through a high frequency match network 120, or a VHF match network, and enhances dissociation of the process gases and plasma density. Those of ordinary skills in the art will recognize that each of the match networks 120, 122 may include one or more capacitors, inductors and other circuit components. The low frequency RF power source 118 may deliver RF power to the supporting substrate 106 at a frequency at or below about 20 MHz while the high frequency RF power source 116 may deliver RF power to the supporting substrate 106 at a frequency at or above 13.56 MHz. In one embodiment, the low frequency RF power source 122 delivers RF power to the supporting substrate 106 at a frequency between about 100 kHz and about 20 MHz while the high frequency RF power source 116, or a VHF power source delivers RF power to the supporting substrate 106 at a frequency between about 27 MHz and about 200 MHz. Preferably, the high and low frequencies do not overlap during operation. That is, the low frequency RF power source 118 is operated at a frequency below the frequency of the high frequency RF power source 116, or a VHF power source.
A periodic high frequency pulse source 112 may turn the RF output of high frequency RF power source 116 on and off periodically. When the periodic high frequency pulse source 112 turns high frequency RF power source 116 on, the amplitude of the RF voltage applied to the substrate support 106 is sufficiently high to generate an electromagnetic field to excite the gas from gas source 124 to a plasma state 128. When the periodic high frequency pulse source 112 turns high frequency RF power source 116 off, the amplitude of the RF voltage applied to the substrate support 106 is insufficient to excite the gas from gas source 124 to the plasma state 128.
A controller 110 is connected to the high frequency pulse source 112. The controller 110 sends a signal to high frequency pulse source 112 to control a duty signal of the high frequency RF source 116. Furthermore, the controller 110 may be used to control the period of time over which pulsing occurs and the period of time over which no pulsing occurs.
In accordance with another embodiment, the low frequency RF power source 118, and the low frequency match network 122 may be either connected to the top or bottom electrode.
In accordance with another embodiment, the high frequency pulse source 112, the high frequency RF power source 116, and the high frequency match network 120 may be either connected to the top or bottom electrode.
In accordance with one embodiment, the plasma uniformity can be optimized in the chamber by applying pulsed high frequency power sources to the electrode in the chamber. The high frequencies which are pulsed may range from about 27 MHz to about 200 MHz. The pulse period may be from about 1 usec to about 1000 usec. The duty cycle may be from about 1% to about 100%. The duty cycle may be used to control plasma density uniformity of the plasma in the chamber. The pulsed high frequency capacitive plasma produces a novel plasma environment that cannot be obtained with traditional continuous wave usage.
Although the operations of the method(s) herein are shown and described in a particular order, the order of the operations of each method may be altered so that certain operations may be performed in an inverse order or so that certain operation may be performed, at least in part, concurrently with other operations. In another embodiment, instructions or sub-operations of distinct operations may be in an intermittent and/or alternating manner.
In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.
