Refrigeration – Low pressure cold trap process and apparatus
Reexamination Certificate
2002-07-30
2004-04-13
Doerrler, William C. (Department: 1754)
Refrigeration
Low pressure cold trap process and apparatus
C062S050500, C118S688000
Reexamination Certificate
active
06718775
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a phase separator for use in an ultra high vacuum system, for example, a molecular beam epitaxy (“MBE”) system and, more particularly, to a phase separator integrated into a cryogenic reactor chamber within the MBE system that facilitates the smooth flow of liquid nitrogen into and gaseous nitrogen out of the system.
BACKGROUND OF THE INVENTION
Ultra high vacuum systems are used in many manufacturing, scientific and other applications. Throughout this application, ultra high vacuum (“UHV”) systems are defined as those having base system pressures less than approximately 10
−8
Torr. One example of a system employing UHV is epitaxial crystal growth.
One such epitaxial crystal growth application employing UHV is molecular beam epitaxy (“MBE”). In MBE, thin films of material are deposited onto a substrate by directing molecular or atomic beams onto a substrate. Deposited atoms and molecules migrate to energetically preferred lattice positions on a heated substrate, yielding film growth of high crystalline quality and purity, and optimum thickness uniformity. MBE is widely used in compound semiconductor research and in the semiconductor device fabrication industry, for thin-film deposition of elemental semiconductors, metals and insulating layers.
Purity of the growing films depends critically on the operating pressure of the growth chamber and the residual gas composition. To ensure the high level of purity required, for example, by the semiconductor industry, the MBE growth chamber base pressure is necessarily in the ultra high vacuum range (UHV), typically less than 10
—10
Torr.
Furthermore, film growth rates, film composition and film doping levels depend critically on the operating temperature of numerous critical components of the growth system, for example, the source cells and the substrate carrier. To this end, MBE growth chambers often employ a liquid nitrogen filled cryogenically cooled shroud (“cryo-shroud”) surrounding interior elements and enclosing the active growth region. This cryo-shroud serves a multiplicity of purposes: 1) to enhance the level of vacuum in the UHV chamber by condensing volatile residual species not removed or trapped by the vacuum pumping system i.e. providing a cryo-pumping action, 2) to enhance the thermal stability and temperature control of critical growth reactor components, and 3) to condense and trap source material emitted from the effusion cells but not incorporated into the growing film.
The implementation of a liquid nitrogen filled cryo-shroud in an UHV system requires a phase separator that allows the escape of gaseous nitrogen generated by the vaporization of the liquid nitrogen as heat is absorbed by the cryo-shroud. The phase separator also enables a replenishing feed of liquid nitrogen into the cryo-shroud to maintain the desired operating temperature. A conventional implementation of such an external phase separator is shown in FIG.
1
.
As shown in
FIG. 1
, vacuum chamber
100
contains a cryogenic shroud
110
having a liquid nitrogen inlet
112
and a liquid nitrogen outlet
114
. A phase separator
120
is connected to inlet and outlet
112
,
114
via ports
132
,
134
and lines
122
,
124
, respectively. Liquid nitrogen at or below its atmospheric boiling point of 77.5° K (−195.5° C.) is introduced into phase separator
120
via inlet
142
and flows through port
132
and line
122
and enters cryo-shroud
110
via inlet
112
. As nitrogen in cryo-shroud
110
warms to the boiling point due to heat absorbed from vacuum chamber
100
, vapor forms within the body of the liquid and bubbles rise by gravity to the top of the cryo-shroud and ultimately out through outlet
114
, liquid-filled exhaust line
124
, port
134
and gaseous nitrogen escapes via exhaust
144
. The formation and flow of these vapor bubbles result in the turbulence and seething normally associated with boiling action, causing mixing effects with the liquid-state nitrogen and counteracting the natural tendency for colder, more dense liquid to settle into the lower portion of the cryo-shroud.
Several problems are associated with a conventional phase separator design. First, the small cross-sectional area of the exhaust line results in a flow restriction for the vapor bubbles and formation of a “frothing”, boiling region in the upper section of the cryo-panel. This region will be elevated in temperature above the liquid nitrogen boiling point, resulting in poor heat absorption from the adjacent cryo-shroud surface. Second, large pockets of gas can accumulate within the body of the cryo-shroud before ultimately breaking loose and flowing to the exhaust line, giving rise to local, temporary warming of the cryo-shroud surface at the location of the trapped gas pocket. Third, the configuration results in an operating pressure within the cryo-shroud considerably above atmospheric pressure. This causes an elevation of the liquid nitrogen boiling point and an overall rise in the operating temperature of the cryo-shroud. A temperature rise of even a few degrees can significantly degrade the cryo-pumping performance of the cryo-shroud. For example, the vapor pressure of carbon dioxide (CO
2
) increases exponentially with temperature from 10
−9
Torr at 72.1° K to 10
−7
Torr at 80.6° K. The limited surface area of the gas-to-liquid interface in the exhaust line enhances these problems.
In addition to the problems associated with conventional phase separator designs, cryogenic fluids are expensive, which raises the cost of the entire deposition process. The deposition process generates substantial amounts of heat, which causes the cryogenic fluid to evaporate very quickly, thus reducing the effectiveness of the overall cooling system. Moreover, once vaporized due to the extreme amount of heat generated during the deposition process, the cryogenic fluid is no longer useful. Therefore, substantial amounts of cryogenic fluid must be used to cool the system.
SUMMARY OF THE INVENTION
The present invention overcomes the above difficulties in part by adding a non-cryogenic cooling panel to the vacuum chamber. The cooling system within the vacuum chamber thus comprises a cryo-panel or cryogenic panel and a non-cryogenic panel. This non-cryogenic panel is filled with a circulating fluid with a greater heat absorption capacity than the cryogenic fluid in the cryo-panel. The non-cryogenic fluid functions to efficiently dissipate the large amounts of heat produced during the operation of the MBE system, thereby prolonging the supply of cryogenic fluid and reducing costs.
The present invention further overcomes the above[[-]]difficulties by integrating the phase separator for the cryo-panel within the vacuum chamber, thus eliminating the lines of relatively small diameter connecting the vacuum chamber to an external phase separator. According to the present invention, the cryo-panel disposed within a vacuum chamber, (e.g., an MBE reaction chamber), includes a cryogenic shroud region and a phase separator region. Liquid nitrogen is introduced into the cryo-panel via an inlet line. As the liquid nitrogen warms and vaporizes, nitrogen vapor rises within the shroud. The phase separator region within the cryo-panel provides a large area vapor-to-liquid interface held at near atmospheric pressure, ensuring that nitrogen vapor may escape the panel smoothly, without forming gas bursts, and with minimal turbulence and general disturbance of the liquid reservoir.
The upper end of the cryogenic panel containing the phase separator region preferably is vacuum jacketed. The liquid nitrogen feed mechanism is designed such that the liquid-to-vapor phase boundary is always held at a level within the region encompassed by the vacuum jacket. This prevents exposed external surfaces of the cryo-shroud from varying in temperature from the nominal 77.4° K associated with the internal liquid nitrogen bath, thereby optimizing its performance and thermal stability.
The flow of liquid nitrogen in
Colombo Paul E.
Priddy Scott Wayne
Applied EPI, Inc.
Doerrler William C.
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