Refrigeration system and method for cooling a susceptor using a refrigeration system

The refrigeration system according to the invention includes a vacuum jacketed pressure vessel which contains both a spray heat exchanger and a shell and tube heat exchanger. The spray heat exchanger includes one or more nozzles for directing a first refrigerant at a heat exchanger element conveying a second refrigerant. The first refrigerant is sprayed in a fine mist from the nozzles onto the heat exchanger element conveying the second refrigerant. The first refrigerant then evaporates and cools the second refrigerant within the heat exchanger element. The shell and tube heat exchanger is a counter current heat exchanger which includes an inner tube for conveying the second refrigerant to the spray heat exchanger, and an outer tube surrounding the inner tube for conveying the first refrigerant recovered from the interior of the vacuum jacketed pressure vessel out of the pressure vessel. The spray heat exchanger recovers the heat of vaporization of the primary refrigerant, while the shell and tube heat exchanger recovers the enthalpy remaining in the evaporated first refrigerant.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a refrigeration system and a method for 
cooling objects using the refrigeration system, and more particularly, the 
invention relates to a refrigeration system which operates with a primary 
refrigerant and a secondary refrigerant to provide a more efficient 
cooling system capable of cooling to cryogenic temperatures. 
2. Description of the Related Art 
The manufacture of semiconductor wafers involves various steps, such as 
plasma etching of the wafers, which require a wafer processing apparatus 
to be cooled. During wafer processing, the wafer is cooled by cooling a 
portion of the wafer processing apparatus called a susceptor which is 
positioned beneath the wafer. The susceptor is cooled by passing a cold 
fluid refrigerant through channels within the susceptor. The temperature 
to which the susceptor is cooled affects the rate of plasma etching or the 
etch rate of the wafer. 
Manufacturers of semiconductors have expressed interest in being able to 
change the etch rate in a step-wise fashion, for example, to use a high 
etch rate to etch a wide area of the wafer and then change to a lower etch 
rate to reduce the width of the etch. To achieve this step-wise change in 
etch rate the temperature of the susceptor must be changed quickly. A 
series of step-wise temperature changes can be provided by controlling a 
cooling system with a control system having a preset temperature program. 
Cooling systems which are known for use in the semiconductor industry and 
which can achieve the cooling required for these step-wise changes in etch 
rate include very large mechanical cooling systems, typically using freon. 
However, these mechanical cooling systems suffer from the drawback that 
they tend to be too large for use in the electronics industry where space 
is at a premium. An additional disadvantage of freon cooling systems is 
their poor efficiency at low temperatures. The conventional freon cooling 
systems also do not accurately produce the desired temperatures at the 
susceptor, especially when a control system having a preset temperature 
program is used. 
SUMMARY OF THE INVENTION 
In accordance with the present invention a refrigeration system is 
presented which uses a first refrigerant and a second refrigerant to 
provide stable cooling power at temperatures down to cryogenic. The 
inventive refrigeration system provides a large increase in efficiency and 
a decrease in size over conventional mechanical refrigeration systems 
using freon. 
One aspect of the invention relates to a refrigeration system comprising a 
vacuum jacketed pressure vessel containing both a spray heat exchanger, 
and a shell and tube heat exchanger. The spray heat exchanger, which 
recovers the heat of vaporization of a liquid, includes at least one 
nozzle for directing a first refrigerant, and a refrigerant conveying 
system for conveying a second refrigerant. The at least one nozzle directs 
the first refrigerant toward the refrigerant conveying system so that the 
first refrigerant contacts and cools the refrigerant conveying system, and 
thus cools the second refrigerant flowing within the conveying system. The 
shell and tube heat exchanger, which recovers the enthalpy available in 
the first refrigerant sprayed from the at least one nozzle of the spray 
heat exchanger, includes an inner tube for conveying the second 
refrigerant to the refrigerant conveying system of the spray heat 
exchanger, and an outer tube surrounding the inner tube for conveying the 
first refrigerant recovered from the spray heat exchanger. 
In accordance with another aspect of the present invention, the 
refrigeration system comprises a vacuum jacketed pressure vessel, a 
primary liquid refrigerant delivery system which includes at least one 
spray nozzle mounted within the vacuum jacketed pressure vessel, and a 
secondary refrigerant circulation system which circulates a secondary 
refrigerant through the vacuum jacketed pressure vessel to an object to be 
cooled. The at least one spray nozzle is positioned to spray primary 
liquid refrigerant at a portion of the secondary refrigerant circulation 
system. The sprayed primary liquid refrigerant evaporates and cools the 
secondary refrigerant within the secondary refrigerant circulation system. 
A gas recovery system is provided within the vacuum jacketed pressure 
vessel for recovering the evaporated primary refrigerant which is used to 
precool the secondary refrigerant. 
In accordance with another aspect of the present invention, a method of 
cooling an object involves the use of a refrigeration system comprising a 
combination of a spray heat exchanger and a counter current heat exchanger 
contained within a vacuum jacketed pressure vessel. The method includes 
the steps of providing a primary refrigerant and a secondary refrigerant 
in liquid form, cooling the secondary refrigerant with the spray heat 
exchanger by spraying the primary refrigerant onto a manifold containing 
the secondary refrigerant, wherein the evaporation of the primary 
refrigerant cools the secondary refrigerant, recovering the evaporated 
primary refrigerant in a counter current heat exchanger, precooling the 
secondary refrigerant with the evaporated primary refrigerant in the 
counter current heat exchanger, and circulating the secondary refrigerant 
to cool the object. 
In accordance with a further aspect of the present invention, a method is 
presented for cooling a susceptor in a semiconductor wafer fabrication 
process. The method includes the steps of providing a primary refrigerant 
and a secondary refrigerant in liquid form, cooling the secondary 
refrigerant with a spray heat exchanger by spraying the primary 
refrigerant onto a manifold containing the secondary refrigerant, wherein 
the evaporation of the primary refrigerant cools the secondary 
refrigerant, recovering the evaporated primary refrigerant in a shell and 
tube heat exchanger, precooling the secondary refrigerant with the 
evaporated primary refrigerant in the shell and tube heat exchanger, and 
controlling the flow of at least one of the primary and the secondary 
refrigerants to cool the susceptor to a desired temperature.

These drawing figures are merely illustrative and are not intended to be 
drawn to scale. 
DETAILED DESCRIPTION 
The refrigeration system according to the present invention employs a first 
refrigerant and a second refrigerant to provide refrigeration which is 
more efficient at lower temperatures than a conventional freon 
refrigeration system and a system which is much smaller than conventional 
systems. The refrigeration system includes a combination of a spray heat 
exchanger 20, and a shell and tube heat exchanger 30 mounted within a 
single vacuum jacketed pressure vessel 10. 
FIG. 1 illustrates a schematic diagram of the fluid flow and control 
systems of the present invention. The primary elements of the 
refrigeration system include vacuum jacketed pressure vessel 10, spray 
heat exchanger 20, shell and tube counter current heat exchanger 30 and a 
control unit 40. A primary refrigerant in liquid form, which is preferably 
liquid nitrogen, is delivered from a pressurized refrigerant source 12. 
The heat exchanger according to the present invention is described herein 
as employing a primary refrigerant of liquid nitrogen. However, it should 
be understood that other primary refrigerants may be used, depending on 
the desired application, without departing from the scope of the 
invention. For example, primary refrigerants such as nitrogen argon, 
krypton, neon, and mixtures thereof may be used. The primary refrigerant 
may also include some amount of impurities. 
The liquid N.sub.2 from refrigerant source 12, controlled by a solenoid 
valve 14, enters spray heat exchanger 20 where the N.sub.2 is sprayed 
through one or more nozzles (not shown in FIG. 1). The N.sub.2 is sprayed 
through the nozzles which cause the N.sub.2 form a fine mist which impacts 
a secondary refrigerant conveying system. The N.sub.2 then evaporates and 
the heat of vaporization of the N.sub.2 is used to cool a secondary 
refrigerant in the conveying system. The structure and operation of spray 
heat exchanger 20 will be described in more detail below with reference to 
FIGS. 2-4. 
The N.sub.2 in gaseous form within vacuum jacketed pressure vessel 10, 
which has been evaporated by spraying through the nozzles of spray heat 
exchanger 20, is recovered and enters shell and tube heat exchanger 30. 
The gaseous N.sub.2 entering shell and tube heat exchanger 30 is still 
cold after the evaporation in the spray heat exchanger. The enthalpy 
available in this cold N.sub.2 gas can be recovered by precooling the 
secondary refrigerant flowing in an opposite direction through shell and 
tube heat exchanger 30. The structure and operation of shell and tube heat 
exchanger 30 will be explained further below. The N.sub.2 gas exiting 
shell and tube heat exchanger 30 is controlled by a solenoid valve 16 and 
is exhausted at N.sub.2 gas exhaust 28. 
According to the present invention, a closed secondary refrigeration system 
is provided in which the secondary refrigerant is circulated to cool an 
object, such as a susceptor used in a semiconductor wafer fabrication 
process. The secondary refrigerant may be propane, ethanol, propylene, 
methane, ammonia, n-butane, iso-butane, mixtures thereof, or any other 
suitable refrigerant. In the closed secondary refrigeration system, the 
secondary refrigerant is returned from the object being cooled by a 
refrigerant return 32. The secondary refrigerant is circulated by a pump 
18 to shell and tube heat exchanger 30 in which it is precooled by the 
N.sub.2 gas. From shell and tube heat exchanger 30 the secondary 
refrigerant passes to spray heat exchanger 20 where the final cooling is 
performed. The secondary refrigerant is then delivered to a refrigerant 
feed 34 where it is used to cool an object such as a susceptor. 
The flow of the secondary refrigerant through the closed system is 
preferably controlled by three control valves located at the exit of spray 
heat exchanger 20 which include a by-pass valve 22, a solenoid valve 24, 
and a small modulating valve 26. Small modulating valve 26 is used to 
provide moderate decreases in temperature and/or maintenance of 
temperature of the susceptor. Solenoid valve 24 is opened to provide 
greater decreases in temperature of the susceptor in a shorter time than 
is possible with modulating valve 26. Finally, by-pass valve 22 allows the 
secondary refrigerant to be recirculated through heat exchangers 20 and 30 
to achieve relatively constant refrigerant flows, thus allowing for more 
constant refrigerant temperatures. The use of by-pass valve 22, in 
combination with either modulating valve 26 or solenoid valve 24, allows 
the system to achieve very low temperatures in a short duration of time. 
The refrigeration system according to the present invention, using liquid 
N.sub.2 as a primary refrigerant, can achieve secondary refrigerant 
temperatures in the range of -190.degree. C. to 20.degree. C. The system 
of the present invention has a large cooling capacity which provides the 
ability to achieve cryogenic temperatures while having a small foot print 
(in other words, the system does not require as much floor space as a 
conventional system). 
The heat exchanger according to the present invention preferably includes a 
control system which incorporates a plurality of pressure sensors P and 
temperature sensors T, each of which is connected to a control unit 40. 
Control unit 40 controls valves 14, 16, 22, 24, 26 and pump 18 according 
to the information received from pressure sensors P and temperature 
sensors T and information input by the user. Control unit 40 may also be 
programmed to carry out a preset temperature program to achieve step-wise 
control of the temperature of an object. 
Schematic side views of the substantially cylindrical vacuum jacketed 
pressure vessel 10 are illustrated in FIGS. 2 and 3. Spray heat exchanger 
20 is preferably positioned in a center portion of vacuum jacketed 
pressure vessel 10 while shell and tube heat exchanger 30 is preferably 
disposed in a serpentine manner around the interior perimeter of vacuum 
jacketed pressure vessel 10. For clarity, shell and tube heat exchanger 30 
is illustrated in FIGS. 2 and 3 as having only a single vertical segment. 
However, as can be seen in FIG. 4, shell and tube heat exchanger 30 
includes a plurality of vertical segments which are connected by curved 
end segments. 
As illustrated in FIG. 2, the primary refrigerant, which is preferably 
pressurized liquid N.sub.2, enters the vacuum jacketed pressure vessel 
through an inlet pipe 42. The liquid N.sub.2 is sprayed from four nozzles 
44 which are located on both sides of a heat exchanger element 46. The 
liquid N.sub.2 which is sprayed in a fine mist onto heat exchanger element 
46 evaporates, thereby, cooling heat exchanger element 46 which in turn 
cools the secondary refrigerant which is flowing through heat exchanger 
element 46. Nozzles 44 are preferably arranged to spray a fine mist of 
droplets over the entire or substantially entire surface of heat exchanger 
element 46. The number of nozzles 44 and the nozzle size may be varied to 
tailor the refrigeration output for a particular application. An increased 
number and size of the nozzles provides for more rapid cooling of the 
second refrigerant and an overall high cooling capacity for the system. 
The cold N.sub.2 gas within vacuum jacketed pressure vessel 10 is exhausted 
through shell and tube heat exchanger 30 where the enthalpy of the cold 
N.sub.2 gas is recovered. Shell and tube heat exchanger 30 includes an 
inner tube 48 and an outer concentric tube 50 surrounding inner tube 48. 
The cold N.sub.2 gas enters shell and tube heat exchanger 30 through an 
inlet 64 near the bottom of vacuum jacketed pressure vessel 10. The cold 
N.sub.2 gas passing through outer concentric tube 50 cools the secondary 
refrigerant which is passing through inner tube 48 of shell and tube heat 
exchanger 30. The primary refrigerant is exhausted from shell and tube 
heat exchanger 30 through a N.sub.2 exhaust 52. Exhaust 52 may be 
configured to exhaust N.sub.2 gas at pressures of 30 psi to 100 psi. This 
exhausted N.sub.2 gas may be used in other applications. 
The secondary refrigerant enters vacuum jacketed pressure vessel 10 through 
a pipe 54 which is connected to inner tube 48 of shell and tube heat 
exchanger 30. The secondary refrigerant is precooled as it passes through 
inner tube 48 of shell and tube heat exchanger before it enters the bottom 
of heat exchanger element 46. 
Heat exchanger element 46, as illustrated in FIGS. 6 and 7, includes an 
upper manifold 56, a lower manifold 58, and a plurality of tubes 60 
extending between the upper and lower manifolds. As illustrated in FIG. 7, 
tubes 60 are positioned in a staggered manner on both sides of a plate 62. 
Plate 62 serves to separate the streams of N.sub.2 from nozzles 44 
provided on either side of heat exchanger element 46. Plate 62 also 
increases the surface area of heat exchanger element 46, improving heat 
transfer. Heat exchanger element 46 and a cold refrigerant exit pipe 47 
form a refrigerant conveying system. 
As illustrated in FIG. 4, shell and tube heat exchanger 30 winds up and 
down around the periphery of the inside of vacuum jacketed pressure vessel 
10. Shell and tube heat exchanger 30 includes a plurality of vertical 
segments which are connected to lower curved segments 80 and upper curved 
segments 82 which are depicted with broken lines in FIG. 4. This 
arrangement provides a heat exchanger which is extremely compact because a 
large number of segments can be disposed within vacuum jacketed pressure 
vessel 10. The length of shell and tube heat exchanger 30 may be varied by 
changing the number of vertical and curved segments. The appropriate 
length of shell and tube heat exchanger 30 will depend on the amount of 
enthalpy available to be recovered from the cold N.sub.2 gas. Spray heat 
exchanger 20 is preferably arranged in the center of the vacuum jacketed 
pressure vessel 10 with nozzles 44 positioned perpendicular to and spaced 
from heat exchanger element 46. As shown in FIG. 4, nozzles 44 may be 
positioned between two of the vertical segments of shell and tube heat 
exchanger 30. 
FIG. 5 illustrates a test stand used to test the present invention. The 
test stand represents an example of an apparatus which is used for 
semiconductor fabrication. The test stand includes an insulator 66, a 
susceptor 68, and a wafer support 70. A dummy wafer 72 and a resistance 
heater 74 were positioned on the test stand for purposes of testing the 
present invention. Susceptor 68 is provided with cooling tubes including 
an inlet 76 and an outlet 78 through which the second refrigerant is 
circulated to cool susceptor 68. 
The heat exchanger of the present invention was connected to the test stand 
illustrated in FIG. 5 to determine the temperatures which could be 
achieved on susceptor 68. FIG. 9 illustrates a graph of the temperature of 
various locations on susceptor 68 versus time. Susceptor 68 was first 
heated with resistance heater 74, having a heating capacity of 2 kW, and 
temperatures were maintained between 80.degree. C. and 90.degree. C. by 
operation of modulating valve 26 which allowed secondary refrigerant to 
circulate through susceptor 68. Susceptor 68 was then further cooled by 
opening solenoid valve 24 and allowing the secondary refrigerant to 
circulate at a higher rate through the susceptor. The curves T.sub.2, 
T.sub.3, T.sub.4, and T.sub.5 represent the temperature of various 
locations on the susceptor over time. As illustrated in FIG. 9, the 
temperature of susceptor 68 dropped 30.degree. C. in about 11 seconds. Due 
to its high efficiency and quick response time, the refrigeration system 
of present invention can advantageously replace conventional refrigeration 
systems in most applications where either low temperatures, accurate 
temperatures, or quick response times are required. The system of the 
present invention is particularly advantageous for use in systems where 
the use of temperature programs with preset temperature changes is 
required. 
As illustrated in FIG. 8, the refrigeration system according to the present 
invention is much more thermodynamically efficient than a conventional 
refrigeration system using freon. In particular, when cooling from 
20.degree. C. to -20.degree. C. is required, the conventional 
refrigeration system has a relative thermodynamic efficiency of between 8% 
and 9%. Whereas, the system of the present invention, when cooling from 
20.degree. C. to -20.degree. C., has a relative thermodynamic efficiency 
of about 88%. For applications where cooling to temperatures of 
-40.degree. C. and lower is required, the conventional mechanical systems 
have extremely poor thermodynamic efficiencies. In addition to the great 
increase in thermodynamic efficiency provided by the present invention, 
the present invention is up to approximately one tenth the size of the 
conventional refrigeration system which would be necessary to achieve the 
same amount of cooling. 
While the invention has been described in detail with reference to a 
preferred embodiment thereof, it will be apparent to one skilled in the 
art that various changes can be made, and equivalents employed without 
departing from the spirit and scope of the invention.