Plasma vacuum pumping cell

Pumps – Electrical or getter type

Reexamination Certificate

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Details

C417S050000, C315S111710, C315S111810

Reexamination Certificate

active

06422825

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to systems for maintaining extremely low gas pressures during execution of industrial and scientific processes, particularly in regions which are continuously receiving a fresh supply of gas.
Many types of industrial and scientific processes are performed in a region which is evacuated to a very low pressure, of the order of several milliTorr (mT). Processes of this type include deposition and etching operations performed on semiconductor wafers with the aid of a plasma. In systems for carrying out such processes, a plasma is generated in a processing region which contains a processing gas maintained at a low pressure in the range of 1-100 mT, and frequently less than 10 mT. The gas will be ionized in the plasma and the resulting ions can be accelerated toward the wafer by suitable electric fields. During the course of the process, processing gas must be pumped out of the region at a high rate with a minimum of contamination by foreign materials, such as oil, that may be contained in the pumping equipment, and materials resulting from the processing itself, while fresh processing gas is supplied to the region.
The conventional technology employed to create low pressure levels of the order indicated above generally utilizes either one of the following two fundamental mechanisms: (1) increasing the momentum of gas molecules (hereinafter, references to “molecules” will be understood to encompass both atoms and molecules in those contexts where reference to both types of particles would be more technically correct) in a preferred direction and exhausting the gas through a valve or a baffle structure which inhibits reverse flow of gas; or (2) condensing the gas on specially prepared surfaces. Mechanism (1) is usually implemented with some type of piston, blower, or rapidly moving vanes which impart directed momentum to the gas by employing rapidly moving mechanical structures or streams of pumping molecules, such as molecules of mercury or readily condensable pumping oils. Mechanism (2) is commonly used in systems with low to moderate throughput requirements.
Turbomolecular pumps utilize mechanism (1) and are provided with rapidly spinning discs which impart directed momentum to gas molecules by colliding with those molecules. This mechanism is most effective for gas pressures which are sufficiently low that the mean-free-path of the molecules is longer than the dimensions of the pumping structures.
To establish low pump inlet pressures, of the order of 1-100 mT, employed in industrial plasma processes, it is presently the nearly universal practice to employ turbomolecular pumps as the first stage of a compound pumping system intended to pump large quantities of processing gas.
It has been found that the quality of processing operations of the type described above, and thus the quality of the finished semiconductor device, is dependent in large measure on the purity and composition of the processing gas and that these parameters can best be controlled if the flow rate of fresh gas into the processing enclosure is relatively high. The quality of the results produced by plasma assisted etching and deposition processes could be significantly enhanced if the gas throughput, or rate of flow of gas into and out of the processing region, could be increased to a level between 3 and 5 times that presently utilized.
Although there are currently available high speed turbomolecular pumps which can achieve throughputs of the order of 5500 liters per second, at low inlet pressures, the highest capacity pumps that are currently available are also extremely expensive, as well as being less reliable than smaller pumps.
Moreover, even a throughput of 5500 liters per second has been found to be less than the optimum value for performing processes on wafers having a diameter of 200 mm, while achievement of optimum processing results on larger diameter wafers would require even higher gas throughput. In general terms, the gas throughput required to achieve a certain processing result in terms of quality is proportional to the area of the substrate.
In addition, effective control of gas flow must allow for gas species that have a tendency to become attached to solid surfaces within the system. Such species include, for example, carbon compounds that are polymerized either by electrons or protons in the plasma. A plasma electron or proton flux can easily affix such materials to solid surfaces. Such materials may be subsequently released from the surfaces, perhaps in a modified form. The quality of any of the plasma assisted processes of the type described above is dependent on the extent to which polymerized or otherwise modified materials can be prevented from being deposited on the substrate surface and this, in turn, depends on the extent to which such materials can be prevented from forming and/or remaining in the processing region.
Gas molecules which remain in the processing region for any significant time can be deposited on the substrate in a chemical form which is resistant to subsequent etching processes. As a result, these molecules will form defects on the substrate surface.
In view of the possible occurrence of such phenomena, it is apparent that the shorter the residence time of gas molecules in the processing region, the higher will be the quality of the product resulting from a series of etching and/or deposition processes.
In addition to the vacuum pumping technologies that have been used in connection with processing operations of the type described above, pumps using a plasma as an active element have been proposed. Plasma vacuum pumps would be capable of pumping a variety of gasses, including hydrogen and helium, with relatively high efficiencies, and are relatively immune to damage by solid or corrosive materials.
The operation of plasma vacuum pumps involves transforming three-dimensional flow of a neutral gas into one-dimensional flow guided by a magnetized plasma which may be magnetically compressed and guided through suitable baffle structures. Momentum can be imparted to the plasma as a result of various electromagnetic interactions and can be imparted to the neutral gas through collisions between molecules of the neutral gas and moving ions which have been accelerated and have greater momentum than background gas.
However, the potential benefits of using plasma vacuum pumps in plasma processing systems has not heretofore been realized to any significant extent. In particular, no solution has been proposed which combines efficient plasma generation with the creation of magnetic fields compatible with the plasma processing operation to be performed and suitable for channeling the plasma, as well as with a suitable mechanism for effecting pumping at pressures in the range which is of importance in such plasma processing operations. In this connection, there have been no proposals which take into account the difficulties created by the ability of a plasma to shield its interior region from low frequency external electric fields.
The possibility of employing a plasma vacuum pumping in plasma processing systems has been described, for example, in U.S. Pat. No. 4,641,060, which is issued to Dandl on Feb. 3, 1987. This patent discloses a plasma vacuum pump which does not employ any moving mechanical parts and which is capable of producing high pumping rates at gas pressures of less than 1 mT. A primary mechanism underlying this plasma vacuum pump is a magnetically guided flow of plasma ions and electrons through simple tubular baffle structures that restrict the flow of neutral gas molecules back into the region which is to be maintained at a low pressure. The pump disclosed in this patent appears capable of functioning effectively with magnetized plasmas at pressures below an upper limit determined by the spontaneous formation of electrostatic potentials that block the flow of plasma ions. “Magnetized plasmas” as used herein is a plasma in which the electron flow is magnetized, i.e. the electrons circulate around the magnetic

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