BACKGROUND 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.

SUMMARY 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 10-1° 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 Figure 1.

As shown in Figure 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 (CO2) 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.

The present invention overcomes the above-difficulties by integrating the phase separator containing 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, a cryogenic 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 cryogenic panel via an inlet line. As the liquid nitrogen warms and vaporizes, nitrogen vapor rises within the shroud. The phase separator region within the cryogenic 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 into the cryogenic panel will generally be intermittent, gated by a level sensor located within the phase separator and a fill mechanism. Following a significant time period of no flow, the feed lines will have warmed considerably and liquid nitrogen may vaporize as it flows from the bulk supply tank to the cryogenic panel. This can give rise to high velocity gas injection into the phase separator region and mixing effects between the vapor and liquid phases, until the delivery line has sufficiently cooled. Normal flow of liquid nitrogen into the system can have similar, although less severe, effects. Terminating the inlet line in a"shower head"arrangement that disperses the gas or liquid flow and directs it in a generally horizontal direction can minimize turbulence in the liquid nitrogen reservoir, and general disturbance of the vapor/liquid interface.

Optionally, to permit higher heat loads, a portion of the cryopanel can be filled with a second fluid with greater heat absorption capacity, such as water. In a preferred embodiment, this is accomplished by dividing the cryoshroud into upper and lower sections. The upper section contains liquid nitrogen and surrounds the phase separator region. The lower section is water-cooled, providing optimal dissipation of heat generated during the operation of the MBE system. A plurality of

thin curved fill heads and a plurality of radially placed holes in the main supply ring continuously or intermittently spray water into the closed lower portion. Those skilled in the art will appreciate that additional cooling fluids may be utilized without altering the inventive concept disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of an UHV system showing a conventional phase separator.

Figure 2 is a cross-sectional view of a first preferred embodiment of the invention showing an UHV system having an integrated phase separator.

Figure 3 is a cross-sectional view of another preferred embodiment of the present invention showing the"shower head"feature.

Figure 4 is a cross-sectional view of a portion of Figure 3 showing the "shower head"feature.

Figure 5 is a planar top view of the invention as shown in Figure 3.

Figure 6 is a cross-sectional view of a further preferred embodiment of the present invention showing the"split panel"feature.

Figure 7 is a top view of a portion of Figure 6 showing the plurality of water fill heads.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An integrated phase separator for an UHV system according to the present invention is shown in Figures 2 through 7. Throughout the drawings, like numerals are used to indicate like elements.

As shown in Figure 2, the UHV system includes vacuum chamber 10 with cryo-panel 20 disposed within the chamber. Cryo-panel 20 includes cryo-shroud region 22 and phase separator region 24. Liquid nitrogen at a temperature equal to or below its boiling point of 77° IZ is introduced into the cryo-panel through input port 26 and fill head 27. It is to be understood that input port 26 is connected to a liquid nitrogen supply (not shown) and a fill mechanism (also, not shown).

As the liquid nitrogen warms in cryogenic shroud region 22 and vaporizes, the gaseous nitrogen rises into phase separator region 24. Phase separator region 24 provides a vapor-to-liquid interface layer"I"held at near atmospheric pressure.

Interface"I"has a large surface area substantially equal to the cross-sectional area of cryo-panel 20. The vapor side"V"of interface"I"is coupled essentially directly to exhaust 28. Interface"I"ensures that the nitrogen vapor may escape the panel smoothly via outlet port 28, causing minimal turbulence and without forming gas bursts or frothing, as occurs in prior art external phase separators.

A level sensor 30 preferably is provided to facilitate maintenance of the desired level of liquid nitrogen within the cryo-panel. Level sensor 30 is coupled operatively to the liquid side"L"of interface"I"and provides a measurable signal indicating the present level of the interface"I".

Vacuum jacket 32 preferably is disposed around the upper end of cryo-panel 20 so that at least the phase separator region 24, and particularly interface"I", is enclosed therein. Vacuum port 34 is connected to a vacuum pump (not shown). An alternate method to produce a vacuum inside the vacuum jacket 32 is to place a cryogenically activated sorption material into the jacket and seal the pump port 34 before introducing liquid nitrogen into the cryoshroud region 22. As the sorption material cools down it absorbs gas molecules and creates a vacuum without the need for external pumping. The use of sorption materials, such as activated charcoal or sodium alumino-silicate, to perform a pumping function is known in the art.

Enclosing interface"I"within vacuum jacket 32 prevents the external surfaces of cryo-shroud region 22 from varying significantly from the nominal 77° K of the internal liquid nitrogen bath, thus optimizing thermal stability and performance. It is envisioned that vacuum jacket 32 may be constructed of several pieces. In this case, it may be desirable to install stabilizers, such as welded rods, to ensure structural rigidity despite extreme temperature variations resulting from cool- down and subsequent return to ambient cycles that occur during normal usage.

A further embodiment of the integrated phase separator according to the present invention is shown in Figures 3-5. In this embodiment, liquid nitrogen fill head 27 is replaced with liquid nitrogen"shower"fill head 36. As shown particularly in Figure 4, shower fill head 36 includes a plurality of fluid exit apertures 38 through which liquid nitrogen is introduced into cryo-panel 20. This arrangement disperses the liquid nitrogen in a generally horizontal direction, as shown in Figure 5, which minimizes disturbance of interface"I"and turbulence in the liquid nitrogen reservoir (the liquid side"L"of interface"I").

An additional further embodiment of the integrated phase separator according to the present invention is shown in Figures 6-7. The primary difference between this embodiment and the prior embodiments is that, in this embodiment, the cryo-panel is split into two portions so that two different fluids can be used for cooling the panel.

Figure 6 shows a cross-sectional view of the"split panel"feature employed in an MBE system. As shown in Figure 6, cryo-panel 200 has been split into a first cryo-panel portion 224 and a second cryo-panel portion 250. The first cryo-panel portion 224 surrounds substrate holder 225, which supports the substrates to be coated. The first cryo-panel portion, similar to the previous embodiments, has a

cryo-shroud region 234 and a phase separator region 236. At least the phase separator region 236 is preferably surrounded by a vacuum jacket 232. The second cryo-panel portion 250 includes a plurality of apertures 251 for receiving the effusion cells (not shown) used to deposit material on the substrate.

In this embodiment, the first cryo-panel portion 224 is preferably filled with liquid nitrogen, while the second cryo-panel portion 250 is preferably filled with water. Liquid nitrogen at a temperature equal to or below its boiling point of 77° K is introduced into the first cryo-panel portion through input port 226, and vaporized nitrogen is released through exhaust 228. Cold water in introduced into the second cryo-panel portion through water inlet port 253, and exits the system through water drainage port 252. In order to illustrate the manner in which water is introduced into the second cryo-panel portion, the inner wall of that portion has been cut away in Figure 7. As can be seen in Figure 7, water inlet port 253 is fluidly connected to a ring 256 that, in turn, is fluidly connected to a plurality of curved water fill heads 254. A plurality of water exit holes 257 are radially positioned along the ring.

Water exits the fill heads 254 through small holes (not shown) near the top of the fill heads and through the radially positioned holes 257 along the ring. This arrangement helps to maintain the circulation of water in the second cryo-panel portion.

The primary advantage of the embodiment of Figs. 6-7 is that it allows the cryo-panel to handle heavier heat loads in the area that it is needed, while substantially reducing the amount of expensive liquid nitrogen consumed by the system. The effusion cells generate substantial amounts of heat, which causes the portion of the cryo-panel in that region to heat up substantially. In the previous embodiments, this high heat load causes the liquid nitrogen to evaporate very quickly, thus reducing the effectiveness of the overall cryo-panel and consuming substantial amounts of liquid nitrogen. In the embodiment of Figs 6-7, in contrast, the water in the second cryo-panel portion is better able to absorb the heat load from the effusion cells, and limits the use of liquid nitrogen to the area surrounding the substrates, where it is most needed.

As mentioned above, one example of an UHV system wherein the present invention is particularly useful is molecular beam epitaxy ("MBE"). In this case, vacuum chamber 10 is a MBE system reaction chamber. Other examples wherein an integrated phase separator according to the invention may be deployed include surface analysis instruments, such as Auger Electron Spectrometry ("AES"), Dynamic Secondary Ion Mass Spectrometry ("DSIMS") and Low Energy Electron Diffraction ("LEED") microscopes wherein UHV is required to minimize sample surface contamination. Of course, it is to be understood that these examples are

illustrative only and other systems and environments may benefit from the use of the present invention.

The above is for illustrative purposes only. Modifications may be made, particularly with regard to size, shape and arrangement of elements, within the scope of the invention as defined by the appended claims.