Cooling heat exchanger

A cooling heat exchanger such as a cold trap for removing contaminants from a vapor stream. Vapor flows through a conduit having angled baffles therein. The baffles are aligned within the conduit so as to form an optically dense trap. A cold region is created by placing a cold finger of a cryogenic refrigerator in contact with a sleeve surrounding the conduit adjacent to the baffles. Insulation of the cold region to maintain cryogenic temperatures is accomplished by insulation within an insulating container surrounding the cold region.

BACKGROUND 
A cooling heat exchanger such as a cold trap is typically used in vacuum 
systems to remove gases having a relatively high temperature of 
solidification, or sometimes liquefaction, from a gas with a lower 
temperature of solidification. Generally, gaseous or particulate 
contaminants are removed from a vapor stream flowing through the cold 
trap. For example, a cold trap is often used to remove aluminum chloride 
in a reactive ion etching system. 
A cold trap captures contaminants by providing a cold surface in the flow 
path of the particle. Particles which strike the cold surface are 
immobilized by freezing onto or by adsorption into the cold surface. To 
insure that all of the contaminants collide with the cold surface, 
chevrons or simple baffle arrangements are used for obstructing the 
traveling path. 
In order to create a cold surface or cold region sufficient to trap 
contaminant gases, cryogenic temperatures are needed. Typically, a coolant 
such as liquid nitrogen or a dry-ice-acetone mixture is placed in contact 
with the region that is to be cooled. Using these types of coolants, 
however, require special hardware for circulating or replenishing the 
liquid coolant. 
For an efficient cooling system the coolant is insulated to prevent heat 
transfer to the ambient enclosure. Conventionally, a vacuum chamber 
surrounding the coolant is used for insulation. This, however, adds 
hardware to the system to create the vacuum as well as a concern for leaks 
in the vacuum chamber. Such systems can be expensive and cumbersome. 
Therefore, there is a need for a cold trap which can be used to remove 
contaminants more efficiently, economically, and conveniently. 
DISCLOSURE OF THE INVENTION 
The invention relates to a cooling heat exchanger used as a cold trap for 
removing contaminant gases, but is not limited to only that application. 
The embodiment described may also serve as a fluid heat exchanger. 
In the preferred embodiment the exchanger comprises a flow-through conduit 
having baffles located therein. In thermal contact with the baffles is a 
cold end of a cryogenic refrigerator. In order to produce cryogenic 
temperatures on the surface of the baffles, the heat load to be overcome 
by the refrigerator is minimized by placing a container around the conduit 
such that an insulating volume is created. 
Preferably, the conduit is made of low thermal conductivity material to 
reduce the conduction of heat from the ends of the conduit to the baffles, 
thus increasing the temperature differential. In order to uniformly 
distribute the cryogenic temperature to the baffles, it is preferred that 
a sleeve of high thermal conductivity material surrounding a center 
portion of the conduit be placed adjacent to the baffles such that it is 
in close thermal contact with them. Further, it is preferred that the 
insulating volume surrounding the conduit be filled with insulation such 
as foam or fiberglass. 
When a cooling heat exchanger is used as a cold trap, the exchanger must be 
serviced from time to time in order to clean the trap contaminates 
collected. In order to facilitate cleaning, it is preferred that a heating 
tape be wrapped around the sleeve to decrease the time necessary to warm 
the trap to ambient temperatures. It is also preferred that the baffles 
are angled to facilitate draining when the trap is rinsed clean.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is directed to an apparatus such as a cold trap 10 
for trapping contaminants produced by a reactive ion etching system. The 
system makes use of the cold trap 10 which has a cold surface for removing 
contaminants from gas as it flows from a ion etcher 12 toward a mechanical 
roughing pump. Other applications for this invention such as a fluid heat 
exchanger are also possible. 
Embodied in FIG. 1, is a typical reactive ion etching system. A vacuum is 
created in the etcher 12 by a turbo-molecular vacuum pump 16 through a 
valve 14. In the etcher, noxious contaminants may be produced and 
withdrawn by the pump 16. From the pump 16, the gases enter the cold trap 
10 where contaminant gases or particles are to be removed from the system. 
While flowing through the cold trap, contaminating vapor such as aluminum 
chloride gives up energy when it collides with a cold surface within the 
cold trap. As a result, the aluminum chloride condenses and collects on 
the cold surface thereby removing it from the flowing gases. The purified 
gases are withdrawn from the cold trap 10 by a mechanical roughing pump 
18. 
In FIG. 2, a longitudinal section of the cold trap is shown. The device 
includes a conduit 20 with baffles 22 located therein. On each end of the 
conduit there is a flange 24. The flange 24 facilitates coupling and 
decoupling of the conduit 20 to the system. Preferably, the conduit is 
made from a material having poor thermal conduction. This characteristic 
allows for a greater temperature gradient between the outside of the cold 
trap and the inner cold, cryogenic surfaces used for trapping 
contaminates. Further, it is preferred that the required conduit and the 
baffles are made of a non-reactive material such as 316L stainless steel, 
a low carbon alloy of steel. If a non-inert material were used, decay may 
result when gases are passed through the cold trap. 
The baffles 22 located within the cold region 27 of the conduit 20 are 
preferably flat plates which angle away from the input end 23 of the 
conduit 20. The surfaces of the flat plates are at cryogenic temperatures 
and are used for trapping vapor contaminates. Preferably, the baffles 22 
extend at least to the axial center of the conduit 20 to provide a trap 
which is optically dense or non-transparent when looking through it. Other 
means for providing an optically dense conduit such as making an s-curved 
tube or putting an elbow in the conduit are possible. By constructing an 
optically dense trap, gases flowing through the cold trap are forced to 
hit the cold surfaces of the baffles. If an optically dense conduit was 
not used, contaminant might flow through the conduit without striking a 
cold surface and, thus, flow through without being trapped. 
Surrounding the tube 20 adjacent to the baffles 22 is a sleeve 26. The 
sleeve 26 is used to create the cold trapping region 27 of the conduit 20 
and should be made from a material such as copper having high thermal 
conductivity. In thermal contact with this sleeve 26 is a cold finger 28 
from a closed cycle cryogenic refrigerator such as a Gifford-MacMahon or a 
Stirling refrigerator. Such a refrigerator typically includes a piston, 
which may be a displacer having a regenerative matrix therein, which 
reciprocates within the cold finger cylinder. Expansion of refrigerant gas 
such as helium in an expansion space at the furthest end of the cold 
finger reduces that end of the cold finger to cryogenic temperatures 
typically less than 130 K. From the sleeve, the cold finger 28 extends 
through an insulating container 34 which forms an insulating volume 30 
around the tube 20. 
During operation, the cold finger 28 conducts cryogenic temperatures 
generated by the refrigerator to the sleeve 26 through the tube 20, to the 
baffles 22. The long, thin tube 20 of low thermal conductivity provides a 
low conductance path from the refrigerator to the environment. The wide, 
short path between the sleeve and the baffles, on the other hand, is of 
good conductance despite the low thermal conductivity. The sleeve 26 
serves as a heat sink in close thermal contact, through the tube 20, with 
the baffles 22. With its high thermal conductivity, the sleeve uniformly 
distributes the cyrogenic temperatures of the refrigerator along the 
length of tube 20 in which gas is trapped. 
In the past, it was believed that in order to sustain cryogenic 
temperatures within the cold trap, a vacuum surrounding the cold region 
was required to reduce the heat load. I have found that the insulating 
volume 30 may also be filled with an insulating material such as 
fiberglass or expanded foam to obtain and sustain the cryogenic 
temperatures necessary for trapping. The advantage of insulating the cold 
trap in this manner is that it avoids the hardware and maintenance 
associated with conventional methods used to create a vacuum for 
insulation. Additionally, using fiberglass or expanded foam for insulation 
also eliminates the concern for producing vacuum containers free of leaks. 
Thus, a more efficient and economical cold trap is provided. 
From time to time, the cold trap must be removed and cleaned of 
contaminants which collect on the cold surfaces. In order to reduce the 
time it takes to warm the cold trap 10 to ambient temperatures a strip of 
heat tape 32 is wrapped around the sleeve 26. The heat tape may be an 
electrical device which conducts heat from wires which have been wrapped 
around and taped to the sleeve. Once the cold trap 10 has warmed to 
ambient temperature, the trap can be quickly removed and cleaned. 
Conventionally in ion etching systems, warm water is flushed through the 
cold trap 10 to remove any debris collected by the cold trap. With the 
baffles obliquely angled relative to the conduit, water is more easily 
drained from the cold trap after it has been washed. 
The present invention eliminates the need for a liquid coolant and reduces 
the hardware necessary to produce cryogenic temperatures used to trap 
contaminants in the conventional system. Thus, a more efficient means for 
cooling the cold region is possible. Also, the present construction is 
cheaper and easier to manufacture. 
While this invention has been particularly shown and described with 
reference to preferred embodiments thereof, it will be understood by those 
skilled in the art that various changes in form and details may be made 
without departing from the spirit and scope of the invention as defined in 
the appended claims. For example, the construction described above could 
be used as a heat exchanger.