Gas separation: processes – Deflecting – Centrifugal force
C055S438000, C055S423000, C422S212000, C422S215000, C422S187000
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is related to devices for agglomerating, capturing, and retaining solid particles from a gaseous flow, and more particularly to apparatus and methods for agglomerating, capturing, and retaining powders from gaseous effluent flows from chemical vapor deposition chambers and processes.
2. State of the Prior Art
Semiconductor devices are often manufactured by processes that include depositing thin, solid films of semiconductor, conductor, and dielectric materials onto substrates by reacting one or more precursor chemicals in a reaction chamber in a manner that produces and deposits or grows the desired solid state thin film material on the substrate. Many of such chemical vapor deposition (CVD) processes are conducted in a vacuum, and many are conducted in ambient, i.e., atmospheric, pressure. However, regardless of the pressure conditions, it is critical in CVD processes to maintain the precursor chemicals in precise proportions to each other and to exclude contaminants in order to obtain the uniformity, morphology, and quality of deposited material necessary to meet semiconductor device quality specifications and performance criteria. To do so, it is typical in CVD processes to flow the constituent precursor gases in the proper proportions into and through the reaction chamber in much larger quantities than is expected to be reacted and deposited on the substrate, and it is typical to mix such constituent precursor gases with much larger quantities of a carrier gas and flowing the mixture through the reaction chamber. Inert carrier gases, which do not enter into the reactions, are often used to dilute precursor gases flowing through the reaction chambers to enhance proper proportioning and mixing of the precursors or to carry by-products of the reactions out of the reaction chambers before such by-products contaminate the films being deposited. Typically, significantly more amounts of carrier gases are used in CVD processes that operate at or near ambient pressure than those that operate in evacuated systems. However, in both vacuum deposition chambers and ambient deposition chambers, the flow of precursor gases with or without carrier gases through the reaction chamber is assisted by a gas pump or fan blower positioned downstream of the reaction chamber. The pump or blower is generally called a vacuum pump in evacuated CVD systems and fans or blowers in atmospheric pressure systems. The pump or blower is typically connected to the reaction chamber with a pipe, often called a foreline, which conducts effluent gases from the reaction chamber to the pump or blower. From the pump or blower, the effluent gases from the CVD process are directed to appropriate treatment, recovery, or disposal apparatus, depending on the toxicity, value, or other characteristics of the effluent gases.
In addition to the deposited thin films on substrates, by-products are often formed by the chemical reactions of the precursors in the CVD reaction chambers, and many of such by-products are unwanted. In many cases, powdery by-products are produced in CVD reaction chambers, which are not helpful and can be harmful. For example, in CVD processes that deposit thin films of silicon dioxide (SO
) on substrates, there is a substantial amount of silicon dioxide powder produced in the reaction chamber, too. Excessive gas-phase chemical reactions often lead to generation of more powdery by-products, which is especially prevalent for atmospheric pressure chemical vapor deposition (APCVD) and sub-atmospheric chemical vapor deposition (SACVD) precesses. To keep such powder particles from contaminating and adversely affecting the quality of the silicon dioxide thin films being deposited on the substrates, it is important to maintain a sufficiently large flow rate of carrier gas flowing through the CVD reaction chamber to sweep such powders along with the CVD reaction by-products out of the reaction chamber, which, as mentioned above, is the function of the pump or blower. However, silicon dioxide is a hard, crystalline substance similar to glass and can cause severe wear and damage to internal parts of pumps and blowers. It is not unusual for pumps and blowers, cost in the range of $30,000 to $50,000, to be rendered useless by such wear in only a few weeks of operation, thereby requiring replacement. In such circumstances, the cost of the pump or blower is itself sufficient reason for finding solutions, but the costs of idling an entire system while repairing or replacing a pump or blower is even more substantial—often in the range of $5,000 to $8,000 per hour. In other CVD processes, powders of boron oxide (B
), and phosphorous pentoxide (P
) cause similar problems, and there are many others.
Removal of solid particulates, including powders, from gaseous flows is not a new problem, and there are many known methods and apparatus for doing so. For example, porous filter elements or membranes in which the gas, but not the solid particles, flows through the pores are common. However, to achieve the particle removal efficiency that is required to protect pumps, blowers, and other downstream components in APCVD and SACVD systems, filter media with small pore sizes are often used. Unfortunately, such small pore sizes also lower trapping capacity due to rapid clogging of the small pores. Cyclone structures are also very common for separating solid particulates from gas flows on a continuous basis with no clogging problems. In a cyclone, the gas flow stream laden with particulate matter is directed circumferentially into the top of an inverted conical chamber, where it is forced into a spiral flow pattern. Since the solid particles have more mass than the gas molecules and are more dense than the gas, the particles have more momentum (mass×velocity) and inertia than the gas. Therefore, the centrifugal force of the particles in the spiral flow is greater than the centrifugal force of the gas molecules, which forces the particles to the conical chamber sidewall as the gas stream is drawn away from the sidewall to a gas outlet. As the particulate matter loses its kinetic energy during the spiral flow in the cyclone, it moves downwardly in the inverted conical chamber. However, the decreasing diameter of the inverted conical chamber, in combination with the principle of conservation of momentum, increases angular velocity of the particles and maintains the centrifugal force that separates the particles from the gas flow until the particles eventually drop out an opening in the bottom of the inverted conical chamber as the gas flow exits from a hole in the top of the conical chamber. However, such cyclone separators do not work well in forelines of CVD reaction chambers, because the gas flow rates from APCVD systems (approximately 300 SLM) and LPCVD systems (approximately 0.2 SLM are too low to achieve optimal operation of a cyclone particle separation device and the particles generated inside APCVD and SACVD reaction chambers are often too fine to be separated from gas efficiently in cyclone separators. In CVD chambers operating under a substantial vacuum, the pressure is so low that it is not possible for the gas flow to impart enough kinetic energy or momentum to the particulate matter to create an effective cyclone flow.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide an improved apparatus and method for trapping and removing particulate matter, especially powders, from gaseous effluent of CVD reaction chambers.
Another object of the invention is to provide an improved apparatus and method for accelerating particle formation in certain kinds of gas effluents from reaction chambers, such as TEOS, to increase efficiency of particle/gas separation.
Additional objects, advantages, and novel features of the invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by the
Mueller James M.
Faegre & Benson LLP
Hopkins Robert A.
MKS Instruments Inc.
Young James R.
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