TECHNICAL FIELD The present invention relates generally to systems and methods for heat- treating objects, such as substrates. More specifically, the present invention relates to a system and method for cooling an apparatus used for heat treating, annealing, and depositing layers of material on or removing layers of material from a semiconductor wafer or substrate.

BACKGROUND Thermal processing apparatuses are commonly used in a wide variety of industries including in the manufacture of integrated circuits (ICs) or semiconductor devices from semiconductor substrates or wafers. Thermal processing of semiconductor wafers include, for example, heat treating, annealing, diffusion or

driving of dopant material, deposition or growth of layers of material, and etching or removal of material from the wafer. These processes often require the wafer to be heated to a temperature as high as 350°C-1300°C before and during the process, and that one or more fluids, such as a process gas or reactant, be delivered to the wafer.

Moreover, typically the wafer must be maintained at a uniform temperature throughout the process, despite variations in the temperature of the process gas or the rate at which it is introduced into the process vessel.

A cross-sectional schematic view of a thermal processing apparatus is shown in FIG. 1. Referring to FIG. 1, a conventional thermal processing apparatus 20 typically consists of a voluminous process chamber or vessel 22 positioned in or surrounded by a furnace 24. Wafers 26 to be thermally processed are held in a cassette or boat 27 and placed in the process vessel 22, which is then heated by the furnace 24 to a desired temperature at which the processing is performed. For many processes the sealed process vessel 22 seals to a base plate 28 and is evacuated through a valve 29 prior to processing. Once the process vessel 22 has reached the desired pressure and temperature reactive or process gases are introduced to process the wafer 26.

There are several design challenges to meeting the requirements of thermal processing apparatuses. One problem is that deposition and etching processes are often highly temperature dependent. For example, in chemical vapor deposition (CVD) and physical vapor deposition (PVD) processes, the rate of deposition of material is highly dependent upon the temperature of the wafer and the surrounding process vessel.

Another problem arises from the effect of thermal stress on residues formed on surfaces of the process vessel caused by fluctuations of the process vessel

temperature during processing. These thermal fluctuations can occur, for example, when the furnace is de-energized or operated at reduced power during transport of the wafer in and out of the process vessel. Thermal fluctuations of the process vessel during processing can cause it to expand and contract such that material deposited thereon flakes off during processing and contaminate the wafer being processed.

This particulate contamination can render the wafer unusable, or require costly re- processing of the wafer.

Conventional temperature control systems include water-jacket re-circulating systems and forced-air cooling systems. Water-jacket systems re-circulate water through cooling channels that surround the process vessel. However, the cooling channels occupy space around the process vessel that is often needed for placement of other external components. In addition, because the cooling channels have to avoid the external components, localized hot spots often occur at locations bypassed by the cooling channels. Moreover, it is often difficult to obtain uniform heat transfer rates across the process vessel because of the difficulty of attaching cooling channels to complex shaped or complicated process vessel surfaces.

Forced air cooling systems are described in, for example, U. S. Pat. No.

5,160, 545, which is incorporated herein by reference. Forced air cooling systems, typically use a fan to blow air across the process vessel surfaces and through a heat exchanger for cooling. However, portions of the process vessel surface that are shielded from the cooling air by components become hotter than other unshielded portions. Moreover, because the primary mode of heat transfer is conduction through contact with gas molecules, forced air systems require large fans to provide flow rates sufficient to control large temperature fluctuations, such as the temperature changes caused by turning on and off the furnace. Large fans can

present difficulties in placement in or near a controlled environment, such as a clean room in which thermal processing apparatus is usually operated.

A forced-air system 30 for controlling the temperature of a process vessel 22 is shown in FIG. 1. In this system, heating elements 32 of the furnace 24 maintain the temperature stable during idle and run modes. Excess heat is dissipated by a fan or blower 34 that blows re-circulated air into an annular passageway or plenum 36 defined by an inner wall 38 of the furnace 24 and an outer surface 40 of the process vessel 22. Typically, in forced-air systems 30, as shown in FIG. 1, air or a cooling gas is injected near the lower end of the furnace 24 and flows up through plenum 36 over the surface 40 of the process vessel 22 and out to a heat exchanger 42 having cooling coils 44 through which a heat transfer fluid is passed. Although an improvement over water cooling, there are a number of shortcomings or problems associated with conventional forced air systems 30 including a cooling ability and response time that is limited by the volumetric flow capacity of the blower 34, and a temperature gradient across the wafers 26 being processed due to non-uniform cooling of the process vessel 22.

The first problem arises, because cold air is injected near the bottom of the process vessel 22 the wafers 26 disposed near a lower portion of a stack of wafers are cooled more quickly than those wafers located toward the top portion of the wafer stack. This results in non-uniform temperature distribution an non-uniform processing of the wafers 26. Moreover, because cold air is usually injected on one or a few sides of the process vessel 22, one side of the process vessel and the wafers 26 therein may be cooled more quickly than another resulting in a temperature gradient across the wafers caused by convective and conductive heat transfer of thermal radiant energy from the wafers to the process vessel. In addition, although

not shown in FIG. 1, portions of the process vessel surface 40 are often shielded from the cooling air by components on the base plate or attached to or near the process vessel. These shielded portions of the process vessel 22 can become hotter than other unshielded portions, again resulting in a temperature gradient across the wafers 26. Such temperature gradients are particularly undesirable when processing wafers 26 having a crystalline structure since excessive radial thermal gradient can produce slip dislocations in the crystalline structure of in the wafers.

The second problem with conventional forced-air systems 30, arises from the fact that cold air travels linearly from the bottom 28 of the plenum 36 to the top resulting in an ineffective cooling capacity. In particular, pushing the cold air to the top of the plenum 36 typically requires a large blower 34. Moreover, because the primary mode of heat transfer is conduction through contact with gas molecules, a large blower 34 is also required to provide volume or flow rates sufficient to effectuate appropriate heat exchange and to control large fluctuations in heat load, such as those caused by turning on and off the heating elements 32. This requirement for a large or high capacity blowers 34 leads to more expensive manufacturing costs for conventional forced-air systems 30. Large blowers 34 can present difficulties in placement in or near a controlled environment, such as a clean room in which thermal processing apparatus is usually operated. That is, the mechanical vibrations induced by a large blower 34 can cause flaking of deposits formed on the process vessel 22 surfaces or movement of the enclosed wafer 26, both of which are undesirable.

Accordingly, there is a need for a cooling system and method for quickly and uniformly cooling a process vessel of a thermal processing apparatus. It is further desirable that the cooling system and method be able to quickly control large

fluctuations in heat load. It is also desirable that the cooling system and method not require large fans or blowers located near the thermal processing apparatus that could induce the mechanical vibrations which can damage or interfere with processing of an enclosed wafer.

SUMMARY The cooling system of the present application provides a solution to these and other problems, and offers other advantages over the prior art.

In particular, a cyclonic cooling system and method are provided for cooling a thermal processing apparatus used for heating work pieces, such as semiconductor substrates or wafers, for performing processes such as annealing, diffusion or driving of dopant material, deposition or growth of layers of material, and etching or removal of material from the wafer.

In one aspect a cyclonic cooling system is provided for cooling a thermal processing apparatus used for processing a substrates held in a carrier at high or elevated temperatures. The apparatus includes a vessel for containing the substrate to be processed, and a heat source having a number of heating elements supply thermal radiation to heat the substrate. Typically, the vessel has a cylindrical portion and a wall made of a material that is thermally conductive or substantially transparent to thermal radiation, and the heating elements are distributed around and spaced apart from the vessel to form a plenum which extends from the vessel to the heat source. The plenum has an annular portion and an outer circumference defined by an inner limit of the heat source. Generally, the cooling system has a first port and a blower with an outlet coupled to the first port to supply a gas thereto. Gas is ejected from the plenum through a second port axially separated from the first port.

In the present cooling system, the first port is oriented to inject the gas into the plenum substantially tangential to the outer circumference thereof to initiate a vortex flow along the wall of the vessel.

In one embodiment, the cooling system is a closed-loop cooling system including an gas-to-fluid heat exchanger coupled to an inlet of the blower to supply cooled gas thereto, and coupled to the second port to receive gas ejected therefrom.

The heat exchanger is also coupled to a source of cooling fluid, such as a facility chilled water supply. Optionally, the cooling system further includes flow switching valves coupled between the inlet and outlet of the heat exchanger and the first and second ports to reverse direction of the vortex flow of the gas. In one version of this embodiment, the cooling system further includes a controller to control operation of the flow switching valves to alternate the direction of the vortex flow of the gas at least once during a cooling operation.

In another embodiment, the cooling system the heat source includes an insulator having an inner wall with a cylindrical portion that is radially separated from the vessel by the plenum, and is coaxial with the heating elements, the vessel and the plenum. In one version of this embodiment, the cylindrical portion of the inner wall of the insulator includes baffles to direct the vortex flow of the gas. The heating elements can be recessed or embedded in the insulator adjacent to the inner wall thereof, or attached to the inner wall of the insulator. Optionally, the baffles also direct the vortex flow of the gas along the wall of the vessel to cool the vessel.

In one version of this embodiment, at least some of the baffles also direct the vortex flow of the gas to cool the heating elements. In another version of this embodiment, the first port and baffles are oriented to cause the gas to rotate around the vessel at least once before being ejected from the plenum.

In another version of the above embodiment, the first port includes a duct extending through the insulator, and the duct is tapered from a first cross-sectional area adjacent to an outer wall of the insulator to a second, smaller cross-sectional area adjacent to the inner wall of the insulator to increase velocity of the gas injected into the plenum.

A method is provided for cooling an apparatus for thermally processing a substrate. As noted above, the apparatus generally includes a vessel for containing the substrate to be processed, and a heat source with a heating elements to supply thermal radiation to heat the substrate. The vessel has a cylindrical wall. The heating elements are distributed around and spaced apart from the vessel to form a plenum which extends from the vessel to the heat source. The plenum has an annular portion and having an outer circumference defined by an inner limit of the heat source.

Generally, the method involves: (i) injecting a gas through a first port oriented substantially tangentially to the outer circumference of the plenum; and (ii) ejecting the gas from the plenum through a second port axially separated from the first port to eject the gas from the plenum, whereby a vortex flow of the gas is initiated along the wall of the vessel to cool the vessel and/or the heating elements. Optionally, the steps of injecting the gas through the first port and ejecting the gas from the plenum include the steps of injecting the gas through the first port and ejecting the gas from the plenum through the second port to cause the gas to rotate around the vessel at least once before being ejected from the plenum.

In one embodiment, the apparatus further includes a blower with an outlet coupled to the first port to supply gas thereto, and a gas-to-fluid heat exchanger coupled to an inlet of the blower to supply cooled gas thereto, and to the second port to receive gas ejected therefrom. In this embodiment, the step of injecting gas

through the first port includes the step of operating the blower to supply gas to the first port, and the step of ejecting the gas from the plenum through the second port includes the step of receiving gas ejected therefrom in the heat exchanger. In another version of this embodiment, the apparatus further includes flow switching valves coupled between the heat exchanger and the first and second ports to reverse direction of the vortex flow of the gas, and the method includes the further step of operating the flow switching valves to alternate the direction of the vortex flow of the gas at least once during a cooling operation.

In another aspect, a cooling system is provided for cooling a heat source or furnace used in a thermal processing apparatus for processing a substrates held in a carrier at high or elevated temperatures. The apparatus includes a vessel for containing the substrate to be processed, and a heat source having a number of heating elements to heat the substrate. The vessel has a top wall and a side wall made of a material which is thermally conductive or substantially transparent to thermal radiation. The heating elements are distributed about and spaced apart from the vessel to form a plenum which extends from the vessel to the heat source. The heat source further includes an insulator disposed about the heating elements and separated from the vessel by the plenum. The insulator has a side portion with an inner wall and a top block abutting the side portion. Generally, the cooling system includes an actuator adapted to move the top block a predetermined distance from the side portion of the insulator to define a gap therebetween, thereby enabling a fluid introduced into the plenum to flow through the plenum and out through the gap to cool the heat source.

The actuator can include any suitable device capable of being operated to lift the top block, and can include mechanical devices such as pneumatic cylinders, hydraulic cylinders, solenoids, hoists or lead screws.

Optionally, the fluid is a cooling gas, and the apparatus further includes an injection port axially separated from the gap to introduce the cooling gas into the plenum. In one version of this embodiment, the apparatus further includes a blower coupled to the injection port to supply cooling gas thereto. In one embodiment, the cooling system is a closed-loop cooling system including a gas-to-fluid heat exchanger that is coupled to the plenum through the gap to receive heated cooling gas ejected therefrom, and coupled to the injection port through the blower to supply cooled cooling gas thereto. The heat exchanger being also coupled to a source of cooling fluid.

In another embodiment, the cooling system further includes a controller to control operation of the actuator to automatically move the top block to initiate flow through the plenum and out through the gap. Optionally, the controller and the actuator are adapted to move the top block to a first predetermined position to provide a first rate of cooling, and to a second predetermined position to provide a second rate of cooling.

BRIEF DESCRIPTION OF THE DRAWINGS These and various other features and advantages of the present cooling system will be apparent upon reading of the following detailed description in conjunction with the accompanying drawings and the appended claims provided below, where:

(FIG. 1 prior art) is a cross-sectional schematic view of a prior art thermal processing apparatus having a conventional forced-air cooling system; FIG. 2 is a cross-sectional schematic view of a thermal processing apparatus having a cyclonic cooling system according to an embodiment of the present invention; FIG. 3 is a perspective view of a portion of the thermal processing apparatus having a cyclonic cooling system according to an embodiment of the present invention; FIG. 4 is a cross-sectional side view of an insulator of the thermal processing apparatus of FIG. 3 showing a vortex flow of gas injected by the cooling system according to an embodiment of the present invention; FIG. 5 is a cross-sectional top view of a portion of an insulator of a thermal processing apparatus showing contour of the injection port and ejection port according to an embodiment of the present invention; FIG. 6 is a cross-sectional side view of a portion of the insulator of FIG. 5; FIG. 7 is a cross-sectional side view of an insulator of a thermal processing apparatus showing vortex flow of gas injected by the cooling system according to another embodiment of the present invention; FIG. 8 is flowchart showing an embodiment of a process for cooling an apparatus for thermally processing a substrate using a cyclonic cooling system according to an embodiment of the present invention whereby apparatus is quickly and uniformly cooled to a desired temperature; FIG. 9 is a cross-sectional schematic view of a thermal processing apparatus having a cooling system according to an embodiment of the present invention;

FIG. 10 is a cross-sectional schematic view of the thermal processing apparatus of FIG. 9 showing a top block in a raised position; FIG. 11 is a cross-sectional schematic view of a thermal processing apparatus having a cooling system according to another embodiment of the present invention; and FIG. 12 is flowchart showing an embodiment of a process for cooling an apparatus for thermally processing a substrate using an embodiment a cooling system having a movable top block according to the present invention.

DETAILED DESCRIPTION A system and method is provided for cooling a thermal processing apparatus used for thermal processing work pieces, such as semiconductor substrates or wafers.

By thermal processing it is meant processes that require or result in the work piece or wafer being heated to a desired temperature before and during processing.

For example, in manufacturing integrated circuits semiconductor wafers are heated as high as 1300°C. Thermal processing of semiconductor wafers can include, for example, heat treating, annealing, diffusion or driving of dopant material, deposition or growth of layers of material, such as chemical vapor deposition or CVD, physical vapor deposition (PVD) processes, and etching or removal of material from the wafers.

A thermal processing apparatus according to one embodiment will now be described with reference to FIG. 2. For purposes of clarity, many of the details of thermal processing apparatuses that are widely known and are not relevant to the present invention have been omitted. Thermal processing apparatuses are described

in more detail in, for example, commonly assigned U. S. Patent number 6,005, 225, which is incorporated herein by reference.

FIG. 2 is a cross-sectional view of a thermal processing apparatus for thermally processing a batch of semiconductor wafers, and having an embodiment of a cyclonic cooling system. As shown, the thermal processing apparatus 100, generally includes a process vessel 102 having a support 104 adapted for receiving a carrier or boat 106 with a batch of wafers 108 held therein, and heat source or furnace 110 having a number of heating elements 112 for raising a temperature of the wafers to the desired temperature for thermal processing. The thermal processing apparatus 100 further includes one or more optical or electrical temperature sensing elements, such as a resistance temperature device (RTD) or thermal couple (T/C), for monitoring the temperature within the process vessel 102 and/or controlling operation of the heating elements 112. In the configuration shown the temperature sensing elements are profile T/C 114A, 114B, that have multiple independent temperature sensing nodes or points (not shown) for detecting the temperature at multiple locations within the process vessel 102. The thermal processing apparatus 100 can also include one or more injectors 116 (only one of which is shown) for introducing a fluid, such as a gas or vapor, into the process vessel 102 for processing and/or cooling the wafers 108, and one or more vents or purge ports 118 (only one of which is shown) for introducing a gas to purge the process vessel and/or to cool the wafers. The thermal processing apparatus 100 may further include a vessel liner 120 inside the process vessel 102 to increase the concentration of processing gas or vapor near the wafers 108, and to reduce contamination of the wafers from flaking or peeling of deposits that can form on interior surfaces of the process vessel 102.

Generally, the process vessel 102 is sealed by a seal, such as an o-ring 122, to a platform or base-plate 124 to completely enclose the wafers 108 during thermal processing. Openings for the injectors 116, T/Cs 114A, and purge ports 118 are sealed using seals such as o-rings, VCR@, or CFs fittings. Gases or vapor released or introduced during processing are evacuated through an exhaust port 126 or foreline formed in a wall of the process vessel 102 (not shown) or in a plenum of the base- plate 124, as shown in FIG. 2, or by any other suitable technique. The process vessel 102 can be maintained at atmospheric pressure during thermal processing or evacuated to a vacuum as low as 4 millitorr through a pumping system (not shown) including one or more roughing pumps, blowers, hi-vacuum pumps, and roughing, throttle and foreline valves.

The process vessel 102 and liner 120 can be made of any metal, ceramic, crystalline or glass material that is capable of withstanding the thermal and mechanical stresses of high temperature and high vacuum operation, and which is resistant to erosion from gases and vapors used or released during processing.

Optionally, the process vessel 102 is made from an opaque, translucent or transparent quartz glass having a sufficient thickness to withstand the mechanical stresses and that resists deposition of process byproducts, thereby reducing potential contamination of the processing environment. In one version of this embodiment, the process vessel 102 and liner 120 are made from a quartz that reduces or eliminates the conduction of heat away from the region or processing zone 128 in which the wafers 108 are processed to the seal 122. In another version of this embodiment, a portion of the process vessel 102 near the seal 122 comprises an opaque quartz to reduce the transfer of heat from the process vessel to the seal 122

and to increase thermal efficiency of the apparatus by reducing the transfer of heat away from the processing zone 128 through sidewalls of the process vessel.

The batch of wafers 108 is introduced into the process vessel 102 through a load lock or loadport (not shown) and an access or opening in the process vessel or base-plate 124 capable of forming a gas tight seal therewith. In the configuration shown in FIG. 2, the process vessel 102 is a vertical reactor and the access utilizes a movable pedestal 130 that is raised during processing to seal with a seal, such as an o-ring 132 on the base-plate 124, and lowered to enable an operator or an automated handling system, such as a boat handling unit (BHU) (not shown), to position the carrier or boat 106 on the support 104 affixed to the pedestal.

The thermal processing apparatus 100, further includes a cyclonic cooling system 140 adapted to inject a cooling fluid into a generally annular space or plenum 142 which is defined by and extends from the process vessel 102 the furnace 110.

The annular portion of the plenum 142 has an outer circumference defined by an inner limit of the furnace 110 or heating elements 112. The cyclonic cooling system 140 is adapted to inject the cooling fluid substantially tangentially to the outer circumference of the plenum 142 to initiate a helical or vortex flow over the surface of the process vessel 102 to cool the vessel.

As shown, the cyclonic cooling system 140, generally includes a first opening or injection port 144 located at one end of the plenum 142 through which the cooling fluid is injected and a second opening or exhaust port 146 located at the other end of the plenum axially separated from the injection port through which the cooling fluid exits the plenum. Optionally, the cooling fluid is a cooling gas, such as air or nitrogen, and the cyclonic cooling system 140 further includes a pump, fan or blower 148 with an outlet 150 coupled to the injection port 144 to supply cooling

gas thereto. In one version of this embodiment, the cooling system is a closed-loop cooling system including an gas-to-fluid heat exchanger 152 coupled to an inlet 154 of the blower 148 to supply cooled gas thereto and to exhaust port 146 to receive gas ejected therefrom. The heat exchanger 152 is also coupled via cooling fluid supply lines 155 to a source of cooling fluid, such as chilled water or equipment cooling water supplied from the building or facility in which the thermal processing apparatus 100 is located.

In the embodiment shown, the cyclonic cooling system 140 further includes a thermo-couple probe 143, a housing for power cables 145 and a temperature indicator 147. The cooling gas is injected through a first opening or injection port 144 located at one end of the plenum 142 and rotates at least once around the process vessel 102 before exiting through a second opening or exhaust port 146 located at the other end of the plenum axially separated from the injection port.

Those of ordinary skill in the art will understand that the cooling gas can be made to rotate a number of times about the process vessel 102 depending on a variety of factors. These factors can include the flow velocity of the cooling fluid, e. g. , air, the process vessel 102 size, the temperature, and orientation and configuration of the injection port 144 and exhaust port 146.

This cyclonic cooling mechanism increases cooling efficiency while reducing the necessary amount of cooling flow, that is the volume of and flow rate of cooling gas passed through the plenum 142 in a given time period, thereby minimizing the size, cost, noise, and vibration of the heat exchanger and blower system.

According to another embodiment, a pair of flow switching valves 156,158, switches or alternates the flow direction, i. e. , alternating the injection of cooling gas

between top and bottom of the plenum 142. FIG. 3 is a perspective view of a portion of the thermal processing apparatus 100 having a cyclonic cooling system 140 as described above. Referring to FIG. 3, in a first flow path indicated by solid arrows 160, cooling gas is blown from the outlet 150 of the blower 148 (not shown in this figure) though a first flow switching valve 156, through an external trunk or duct 162 and into an injection port 144 located near a top of the furnace 110. After flowing over and cooling the process vessel 102 (not shown in this figure), the heated cooling gas is ejected or exhausted from an exhaust port 146 through another external trunk or duct 164, through a second flow switching valve 158 and back to the heat exchanger 152. In an alternative or second flow path indicated by dashed arrow 166, cooling gas is blown from the outlet 150 of the blower 148 (not shown in this figure) though the first flow switching valve 156, directly into a second injection port (not shown in this figure) located near the bottom of the furnace 110, without passing through the external duct 162. After flowing upward in a helical manner over the process vessel 102 (not shown in this figure), the heated cooling gas is ejected or exhausted from a second exhaust port 168 located near the top of the furnace 110, through the second flow switching valve 158 and back to the heat exchanger 152.

Note that other flow paths through the plenum 142 and other approaches of altering the direction of flow are possible without departing from the scope of the invention. For example, the direction of flow of the flow path indicated by arrows 160 can be reversed without repositioning of the flow switching valves 156,158, by reversing direction of the blower 148. However, to minimize the exposure of the blower 148 and an associated motor (not shown) to elevated temperatures, thereby

extending its'operating life, it is generally desirable that the blower be located on an outlet of the heat exchanger 152.

In yet another embodiment, the flow switching valves 156,158, and blower 148 are under control of a temperature controller (not shown) that operates the valves and blower to alternate the direction of cooling gas flow a number of times during a cooling cycle, thereby achieving a more uniform cooling axially along the height of the process vessel 102, and therefore across the stack of wafers therein, than possible with conventional unidirectional cooling systems.

Optionally, the temperature controller is a dynamic feed forward temperature controller that uses a theoretical model to predict the thermal response of the system which is used as a feed forward loop into the regular control methodology. In one version of this embodiment, the temperature controller includes a learning sequence that uses historical processing data to improve dynamic response and to reduce sensitivity to the variations in thermal load, caused for example by variation in the number of wafers 108 being processed.

In still another embodiment, shown in FIG. 4, an interior surface 170 of the furnace chamber 110, which defines an outer circumference of the plenum 142, can include a number of air-foils or baffles 172 to aid in directing the helical flow of cooling gas through the plenum. FIG. 4 is a cross-sectional side view of the interior of the furnace 110 of the thermal processing apparatus 100 of FIG. 3 showing a vortex or helical flow of cooling gas injected by the cyclonic cooling system 140.

The baffles 172 extend radially inward from the interior surface 170 of the furnace chamber 110. The baffles can have a radial length from a minimum of about 10 mm, to about 30 mm. The baffles 172 extend radially inward from the interior surface 170 to a point near but not abutting an exterior surface of the process vessel

102. This embodiment, has the advantage of isolating the process vessel 102 from vibrations induce by the blower during operation and facilitating the placement of the isolating the process vessel in the furnace 110 during assembly of the thermal processing apparatus 100 and/or. In one version of this embodiment, there is a clearance of at least about 8mm between the baffles 172 and the exterior surface of the process vessel 102. It will be appreciated that number and placement of these baffles 172 as well their orientation, particularly their angle with respect to the direction of flow, can significantly effect the number of complete revolutions cooling gas helically flow about the process vessel 102 will make before exiting the plenum 142.

Additional details of the injection ports 144 and exhaust ports 146 will now be described with reference to FIGs. 5 and 6. FIG. 5 is a cross-sectional top view of a portion of the furnace 110 showing contours of the injection port and exhaust port according to an embodiment of the present cooling system 140. FIG. 6 is a cross- sectional side view of the portion of the furnace 110 shown in FIG. 5.

Both the injection ports 144 and exhaust ports 146 are oriented tangential to the inner surface 170 of the furnace 102. In one embodiment, the injection port 144 comprises a shallow scoop shaped portion 174 having a radius that decreases for a point of entry into the plenum 142 to a point distal from the point of entry. The decreasing radius of the scooped shaped portion 174 gradual redirects the cooling gas from flow in a linear direction tangential to the plenum 142 to a laminar or substantially laminar helical flow within the plenum.

The exhaust port 146 can include a portion 176 having a substantially rectangular cross-section, thereby maximizing the cross-sectional area of the exhaust port for exhausting of cooling gas and reducing the complexity and expense of

fabricating the exhaust port.. Alternatively, the exhaust port 146 can also include such a scooped shaped portion 174, not shown in this figure, thereby enabling the heated cooling gas to maintain a laminar flow while being ejected or exhausted from the plenum 142. This alternative embodiment has the further advantages of enabling the exhaust port 146 to serve as an injection port 144 when flow is reversed as described above, and of avoiding the generation of turbulence, which can occur at high flow rates.

According to another embodiment, both the injection ports 144 and exhaust ports 146 have tapered portions 178,180, of smoothly changing cross-sectional area to alter the velocity or flow rate of cooling gas entering or leaving the plenum 142.

As shown, the tapered portion 178 of the injection port 144 has a decreasing cross- sectional area from a point near an entry to the injection port to a point near the plenum to increase the velocity of cooling gas entering the plenum. It has been found that increasing the velocity of cooling gas entering the plenum 142 above a minimum amount is desirable to initiate and maintain a helical flow. That is if the cooling gas is introduced at too low of a flow rate the cooling gas tends to move or flow upward disrupting the helical flow. This in turn can lead to localized hot spots on the process vessel 102 or uneven cooling in areas where the cooling gas does not flow over the process vessel surface. Preferably, to achieve an appropriate momentum of helical rotation of the cooling gas, cooling gas is injected at a speed of at least about 5 meters per second (mps), and more preferably at speeds of gas of from about 5 mps to about 30 mps.

The tapered portion 180 of the exhaust port 146 decreases the velocity of the cooling gas entering the heat exchanger 152, thereby increasing the length of time the heated cooling gas is in the heat exchanger and increasing cooling efficiency of

the heat exchanger. For example, the inside diameter of the tapered portion 180 can increase from about 2 inches at the inlet to about 3 inches at the outlet.

In an alternative embodiment shown in FIG. 7, the cyclonic cooling system 140 includes two injection ports 144A, 144B, axially located near a center of the furnace 110 and two exhaust ports 146A,! 46B located at either end to produce two co-axial helical flow paths rotating in opposite directions. This embodiment has the advantage of increasing the cooling near a center of the process vessel 102, which is frequently nearest to the process zone 128, thereby maximizing cooling of the wafers 108.

It will be appreciated that the cooling system 140 described above is particularly useful for cooling the thermal processing apparatus 100 and the wafers 108 therein to a pull temperature after processing of the wafers 108 in preparation for unloading of the wafers 108 and by the BHU.

Alternatively, the cooling system 140 can be operated for other purposes during processing of the wafers 108. For example, the cooling system 140 can be operated to provide reduced or varying degrees of cooling during processing, thereby affording a constant thermal load to the heating elements 112 and minimizing or eliminating localized hot spots or thermal transients.

An illustrative method or process for cooling the thermal processing apparatus 100 is described with reference to FIG. 8. FIG. 8 is a flowchart showing steps of a method for cooling the thermal processing apparatus 100 using the cyclonic cooling system 140 described above. In the method, cooling gas is injected through an injection port 144 oriented tangentially to the outer circumference of the plenum 142 to initiate a vortex or helical flow of cooling gas adjacent to the process vessel (Step 184). Cooling gas is then ejected from the plenum 142 through an

exhaust port 146 axially separated from the injection port 144 (Step 186).

Optionally, the cyclonic cooling system 140 further includes flow switching valves 156,158, and the method involves the further step of operating the flow switching valves to alternate the direction of the vortex flow of the gas at least once during the cooling operation (Step 188). Optionally, the step of injecting the cooling gas, step 184, includes injecting the gas through the injection port 144 in such a manner as to cause the cooling gas to rotate in a helix around the process vessel 102 at least once before being exhausted from the plenum 142.

In accordance with another aspect, the thermal processing apparatus 100, further includes a chimney cooling system adapted to cool a heat source or furnace 110 and/or the process vessel 102. In the embodiment shown in FIG. 9, the heat source or furnace 110 further includes an insulator 202 disposed about the heating elements 112 and having a cylindrical side portion 204 with an inner wall 206 that is coaxial with a side wall 208 of the process vessel 102, and a top insulation block or top block 210 abutting the side portion. Generally, the chimney cooling system includes an actuator 212 adapted to move the top block 210 a predetermined distance from the side portion 204 of the insulator 202 to define a gap (not shown in this figure) therebetween, thereby enabling a fluid introduced into the plenum 142 to flow up through the plenum and out through the gap to cool the furnace 110 and/or the process vessel 102. As above, the cooling fluid is generally a cooling gas, such as air or nitrogen.

The chimney cooling system includes a heat exchanger to remove heat from the fluid or gas circulated through the plenum 142. For example, in the embodiment shown in FIG. 9, chimney cooling system includes an integral gas-to-fluid heat exchanger formed by a number of cooling coils 216, and a housing or enclosure 218

on which the actuator 212 is mounted. Optionally, the gas-to-fluid heat exchanger 214 can further include additional cooling coils 220 external to the housing 218 to cool the housing or to serve as a heat radiating portion of a closed loop gas-to-fluid heat exchanger for the cooling coils 216 within the housing.

In one embodiment, shown in FIG. 10, the cooling coils 216 are adjacent to an outer surface 222 of the side portion 204 of the insulator 202, and extend above the side portion so that when the top block 210 is lifted to define the gap 224, heated fluid from the plenum 142 rises and flows out through the gap past the cooling coils 216 to cool the cooling fluid. The cooling gas from the plenum 142 is further cooled by the gas-to-fluid heat exchanger 214 in a second plenum 226 defined between the exterior of the insulator and the interior of the housing 218. The further cooling and condensing of the cooling gas in the second plenum 226 causes it to sink toward injection ports 228 near the bottom of the second plenum resulting in a natural circulation of cooling gas through the plenum 142 as shown by arrows 230. By natural circulation it is meant the initiation and/or sustaining of a flow of cooling gas through the plenum 142. If desired, the natural circulation of cooling gas through the plenum 142 can be supplemented by the use of a fan 229, blower, or other mechanical flow sustaining means.

The actuator 212 can include any suitable mechanical device capable of being remotely operated to lift the top block 210, and can include mechanical devices such as pneumatic cylinders, hydraulic cylinders, solenoids, hoists or lead screws. In the embodiment, shown in FIGs. 9-11 the actuator 212 includes a chain or cable hoist 232 driven by an electric motor 234. Referring to FIG. 10, the motor 234 is of a type that can be precisely controlled to vary the size of the gap 224

thereby varying the degree or rate of cooling provided to the furnace 110 and/or the process vessel 102.

In another embodiment shown in FIG. 11, the chimney cooling system is a closed-loop cooling system, the heat exchanger 214 is a separate compact heat exchanger 214 and the chimney cooling system further includes a blower 236 coupled to the injection port 228 to supply cooled gas thereto. Although this embodiment does not rely on natural circulation to initiate or sustain the flow of cooling gas through the plenum 142, it will be appreciated that the heating of the cooling gas as it rises up through the plenum does aid maintaining a sufficient and laminar flow of cooling gas through the plenum, thereby enabling use of a smaller blower than would otherwise be required.

In yet another embodiment, the chimney cooling system further includes a controller (not shown) for controlling operation of the actuator 212 to automatically move the top block 210 to initiate flow through the plenum 142 and out through the gap 224. Optionally, the controller and the actuator 212 are adapted to move the top block 210 to a first predetermined position to provide a first rate of cooling, and to a second predetermined position to provide a second rate of cooling. Depending upon the temperature from which the furnace 110 is to be cooled, the chimney cooling system can be operated to cool the furnace, and therefore the wafers 108 held therein, at one or more different rates during the cool down cycle. For example, the size of the 206 can be adjusted or throttled during the cool down cycle to provide cooling in the range of from about 10 degrees Celsius (C) per minute to about 100 degrees C per minute. Those of skill in the art will appreciate that these ranges are exemplary, and that other cooling rates (average or instantaneous) may be achieved depending on the particular application.

An illustrative method or process for cooling the thermal processing apparatus 100 with a chimney cooling system will now be described with reference to FIG. 12. FIG. 12 is a flowchart showing an embodiment of a process for cooling an apparatus 100 for thermally processing a wafer 108 using an embodiment a chimney cooling system having a movable top block 210. In the method, the top block 210 is moved a predetermined distance from the side portion 204 of the insulator 202 to define a gap 224 therebetween (Step 240). A cooling fluid is injected into the plenum 142 through an injection port 228 axially separated from the gap 224 (Step 242), and ejected from the plenum 142 through the gap 224 so as to induce a flow through the plenum thereby cooling the process vessel 102 (Step 244). Generally, the cooling fluid is a cooling gas, and the step of introducing a fluid into the plenum, step 242, involves injecting cooling gas into the plenum 142.

In one embodiment, the step of introducing a fluid into the plenum, step 242, is accomplished by forcing the cooling gas into the plenum 142 using a blower 236 coupled to the injection port 228. In one version of this embodiment, the chimney cooling system is a closed-loop cooling system including a gas-to-fluid heat exchanger 214 coupled to the plenum 142 through the gap 224, and coupled to the injection port 228 through the blower 236, and the step of introducing a fluid into the plenum, step 242, includes the step of supplying cooled cooling gas to the blower from the gas-to-fluid heat exchanger, and the step of ejecting the fluid from the plenum 142 through the gap 224, step 244, includes the step of receiving in the gas-to-fluid heat exchanger 214 heated cooling gas ejected fluid from the plenum through the gap.

Optionally, the movement of the top block is precisely controlled by a controller, and the step of moving the top block, step 240, includes the step of

moving the top block to a first predetermined position to provide a first rate of cooling, and the method includes the further step of moving the top block to a second predetermined position to provide a second rate of cooling (Step 246).

The foregoing description of specific embodiments and examples of the invention have been presented for the purpose of illustration and description, and although the invention has been illustrated by certain of the preceding examples, it is not to be construed as being limited thereby. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications, embodiments, and variations are possible in light of the above teaching. It is intended that the scope of the invention encompass the generic area as herein disclosed, and by the claims appended hereto and their equivalents.