Coating processes – Interior of hollow article coating – Coating by vapor – gas – mist – or smoke
Reexamination Certificate
2000-08-17
2002-04-02
Barr, Michael (Department: 1762)
Coating processes
Interior of hollow article coating
Coating by vapor, gas, mist, or smoke
C427S255180, C427S255270, C427S255393, C427S314000, C423S349000
Reexamination Certificate
active
06365225
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
This invention relates to CVD reactors and to methods for chemical vapor deposition of bulk polysilicon directly onto the walls of the reaction chamber and on tube walls within the chamber. More particularly, it relates to production of bulk polysilicon by a chemical vapor deposition process where a removable thin wall tubular casing is used to construct the reaction chamber within a CVD reactor, and where additional removable middle and core tubes may also be employed; the tube walls providing additional surface area upon which the polysilicon is deposited, external and core heat sources providing a flatter thermal gradient and improved thermal efficiency.
2. Background Art
One of the widely practiced conventional methods of polysilicon production is by depositing polysilicon in a chemical vapor deposition (CVD) reactor, and is referred to as Siemens method. In this method, polysilicon is deposited in a CVD reactor on high-purity thin silicon rods called “slim rods”. Because of the high purity silicon from which these slim rods are fabricated, the corresponding electrical resistance of the slim rods is extremely high. Thus it is extremely difficult to heat this silicon “filament” using electric current, during the startup phase of the process.
Sometimes the slim rods are replaced by metallic rods that are more conductive and easier to heat with electrical current. This method is referred to as Rogers Heitz method. However, the introduction of metal into the chemical vapor deposition process introduces metal contamination. This contamination of the polysilicon yield is not acceptable in the semiconductor/microelectronics industry.
In the Siemens method, external heaters are used to raise the temperature of these high purity rods to approximately 400° C. (centigrade) in order to reduce their electrical resisitivity. Sometimes external heating is applied in form of halogen heating or plasma discharge heating. However in a typical method, to accelerate the heating process, a very high voltage, in the order of thousands of volts, is applied to the rods. Under the very high voltage, a small current starts to flow in the slim rods. This initial flowing current generates heat in the slim rods, reducing the electrical resistance of the rods and permitting yet higher current flow and more heat.
This process of sending low current at high voltage continues until the temperature of slim rods reaches about 800° C. At this temperature, the resistance of the high purity silicon rods falls very drastically and the high voltage source is switched to a low voltage source that is capable of supplying high current.
Referring to prior art
FIG. 1
, a CVD reactor consists of a base plate
23
, quartz bell jar
17
, chamber cover
24
, bell jar supports
16
, and heater
18
between the bell jar and the chamber cover. There is incorporated in base plate
23
, a gas inlet
20
and a gas outlet
21
, and electrical feedthroughs
19
. A viewing port
22
provides for visual inspection of the interior.
In the prior art polysilicon manufacturing process by CVD, the silicon slim rod structure is assembled in the form of a hair pin by having a cross rod
2
placed horizontally on two long, spaced apart, vertical rods
1
and
3
. The structure is mounted and connected so as to provide a current path between electrical feedthroughs
19
, generating the heat necessary for deposition to occur. During the CVD process, polysilicon deposit accumulates uniformly on the slim rods; the deposit being shown here partially removed to show the slim rod structure. Deposits of silicon on the reactor walls can occur if they become hot enough, so cooling of the reactor walls is sometimes employed to prevent this.
Different users employ different methods for joining the horizontal rod to the vertical rods. One method requires a groove or a key slot at the top of each vertical rod. A small counter bore or conforming fitment is formed on the ends of the horizontal rod so that it can be press fitted into the grooves to bridge the two vertical rods.
A typical prior art reactor consists of a complex array of subsystems. Two power sources are required, one power supply that can provide very high voltages and low current; and a second power supply that can sustain a very high current at relatively lower voltage. Also needed are the slim rod heaters and their corresponding power supply for preheating the slim rods. Another component is the high voltage switch gear. Moreover, the entire startup process is cumbersome and time consuming. Since the current drawn by the slim rods at around 800° C. is of a run away nature, the switching of the high voltage to low voltage needs to be done with extreme care and caution.
Also, through this electric current method for heating the slim rods, the rods become an interior heat source losing tremendous amounts of heat via radiation to the surroundings. There is significant energy loss inherent in the existing practice.
There is a plethora of prior art in the general area of reactors used for chemical vapor deposition, some employing intentional deposition on heated reactor walls. For example, Jewett's U.S. Pat. No. 4,265,859 is a system for producing molten polycrystalline silicon and replenishing the melt of a crystal growth crucible. The system includes a hot wall, muffle furnace reactor in which silicon is deposited in low density form on the wall and inner tube of the reaction chamber by delivering a gaseous silicon compound through the heated chamber, at nominally 1000 degrees Centigrade. After a certain amount of silicon has been deposited on the fused quartz chamber walls and inner tube of the reactor, the chamber temperature is raised higher, to about 1450 degrees Centigrade, to melt down the silicon deposit for recovery, letting it run molten out the bottom of the reactor into the melt crucible of the crystal growth part of the system. When the heat is reduced in the outflow trap, a silicon plug forms, re-sealing the reactor for the next cycle. This gas inflow/molten silicon outflow reactor operation is repeated cyclically without cooling or opening the reactor between cycles, to support the crystal growth operation. The hot quartz chamber wall of the Jewett reactor requires a fully encircling support, disclosed as graphite, to sustain wall integrity at high temperature. It is not used or useful for the production of bulk polysilicon ingots.
Gautreaux et al's U.S. Pat. No. 4,981,102 discloses a hot wall CVD muffle furnace reactor with a heated liner for collecting silicon deposits on the inner face, from a through-flow of silicon gas. The reactor can be cycled to high heat to melt the silicon for molten outflow, or opened via a large door on the reactor to remove the liner so that the deposited silicon can be removed from the inside surface of the liner for use as bulk ingots of polycrystalline silicon. The liner is disclosed to be a removable unitary or assemblage of liner components, fabricated or coated with molybdenum, graphite, silicon, silicon nitride or other materials. It is not known to be in commercial practice.
Jewett's U.S. Pat. No. 4,123,989 discloses a horizontal muffle furnace and method for producing silicon by CVD on the inside of a silicon tube emplaced horizontally in the through-flow furnace so as to define the reaction chamber. The silicon tube is reportedly supported on its sidewall within the muffle tube by graphite support rings, and sealed or at least supported securely for alignment at both ends to cooling head end caps through which the process materials are flowed. Water is circulated through the cooling heads to prevent deposition on the cooling heads. A muffle tube of quartz or other high temperature, non-contaminating material surrounds the silicon tube. The space between the muffle tube and the silicon tube may be held at an overpressure state with argon to assure no out gassing from the silicon tube chamber. A resistance heater system surrounds the muffle tube, and the assemb
Chandra Mohan
Gupta Kedar P.
Jafri Ijaz
Prasad Vishwanath
Talbott Jonathan A.
Asmus Scott J.
Barr Michael
G.T. Equipment Technologies Inc.
Maine Vernon C.
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