Substrate Carrier Using Proportional Thermal Fluid Delivery System

Abstract

To provide a substrate carrier using a proportional thermal fluid delivery system.SOLUTION: A temperature adjustment system using proportional flow control of a wafer support includes heat exchangers 160 and 162 that provide, to a fluid channel 120 of a substrate carrier 122, a thermal fluid, which is for controlling the temperature of the substrate carrier during substrate processing, and receive the thermal fluid from the fluid channel. A proportional valve 106 controls the rate of flow of the thermal fluid from the heat exchangers to the fluid channel. A temperature controller 102 receives a measured temperature from a thermal sensor 164 of the substrate carrier and controls the proportional valve in response to the measured temperature to adjust the rate of flow.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] TECHNICAL FIELD Embodiments of the present invention relate to the microelectronics manufacturing industry, and in particular to a temperature controlled support for supporting a workpiece during plasma processing. [Background technology]

[0002] In the manufacture of semiconductor chips, silicon wafers or other substrates undergo a wide variety of processes in various processing chambers. In these chambers, the wafers may be exposed to plasma and chemical vapor processes, metal processes, laser etching processes, and various deposition and acid etching processes to form circuits and other structures on the wafer. Silicon wafers may be held in place during these processes by electrostatic chucks (ESCs), carriers, pedestals, or various other structures. ESCs hold the wafer by generating an electrostatic field and clamping the backside of the wafer to the flat or puck surface of the chuck. Other carriers use vacuum pressure, adhesives, or other techniques.

[0003] As manufacturing techniques advance for plasma processing equipment, such as those designed to perform plasma etching of microelectronic devices, the temperature of the wafer during processing becomes more important. ESCs have been designed to provide specific thermal profiles across the surface of the substrate (sometimes called the workpiece). ESCs have also been designed to precisely regulate the temperature of the workpiece.

[0004] The ESC uses liquid cooling to absorb and remove the plasma output heat from the chuck. In the ESC, liquid may also be used to heat the chuck. This allows for a wider processing window for different processes and plasma conditions. Heat exchangers are used to heat or cool the liquid before it is pumped through the chuck. Valves then control the flow rate and mix of hot and cold fluids pumped through the chuck. The more precise the thermal performance, the more precise the structures that can be created on the wafer. Summary of the Invention

[0005] A substrate carrier using a proportional thermal fluid delivery system is described. In one embodiment, the apparatus includes a heat exchanger that provides thermal fluid to and receives thermal fluid from a fluid channel of the substrate carrier for controlling a temperature of the substrate carrier during substrate processing. A proportional valve controls the flow rate of the thermal fluid from the heat exchanger to the fluid channel. A temperature controller receives temperature measurements from a thermal sensor of the carrier and controls the proportional valve in response to the temperature measurements to adjust the flow rate.

[0006] Embodiments of the present invention are illustrated by way of example, and not by way of limitation, in the accompanying drawings in which: [Brief description of the drawings]

[0007] [Figure 1] FIG. 2 is a diagram of a temperature regulation system using proportional flow control of the wafer support, in accordance with one embodiment of the present invention. [Diagram 2] FIG. 13 is a diagram of an alternative configuration for temperature regulation using proportional flow, according to one embodiment of the present invention. [Diagram 3] 2 is an alternative diagram of a plasma processing apparatus for a workpiece using a pedestal and proportional valve fluid control system in accordance with one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Although numerous details are set forth in the following description, it will be apparent to one skilled in the art that the present invention may be practiced without these specific details. In order to avoid obscuring the present invention, well-known methods and devices are sometimes shown in block diagram form without detailed description. Throughout this document, the term "embodiment" or "one embodiment" means that a particular feature, structure, function, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases "in an embodiment" or "in one embodiment" in various places in this document does not necessarily refer to the same embodiment of the present invention. Furthermore, particular features, structures, functions, or characteristics may be combined in any suitable manner in one or more embodiments. For example, a first embodiment and a second embodiment may be combined if the particular features, structures, functions, or characteristics associated with the two embodiments are not mutually exclusive.

[0009] As used in the description of this invention and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term "and / or" as used herein refers to and includes any and all possible combinations of one or more of the associated listed items.

[0010] The terms "coupled" and "connected," along with their derivatives, may be used herein to describe a functional or structural relationship between components. It should be understood that these terms are not intended to be synonyms for each other. Rather, in specific embodiments, "connected" may be used to indicate that two or more elements are in direct physical, optical, or electrical contact with each other. "Coupled" may be used to indicate that two or more elements are in direct or indirect (through other intervening elements) physical, optical, or electrical contact with each other and / or that two or more elements cooperate or interact with each other (e.g., are causally related, etc.).

[0011] As used herein, the terms "over," "under," "between," and "on" refer to the relative location of one component or layer of material with respect to another component or layer when looking at physical relationships. For example, in the context of layers of material, a layer disposed above or below another layer may be in direct contact with the other layer or may have one or more intervening layers. Furthermore, a layer disposed between two layers may be in direct contact with the two layers or may have one or more intervening layers. In contrast, a first layer "on" a second layer is in direct contact with the second layer. Similar distinctions are made in the context of assembling components.

[0012] The temperature of a substrate support, such as an electrostatic chuck (ESC) or vacuum chuck, can be controlled using a proportional fluid delivery system. A pulsed fluid delivery system rapidly cycles the opening and closing of a fluid valve. The duty cycle of the opening determines the amount of fluid that flows to the chuck, which is not as accurate or reliable as a proportional control system. The flow rate provided by a proportional value meets the cooling requirements of plasma processes with high bias RF power. These include those processes used to etch high aspect ratio structures. Without a closed loop temperature control system, the temperature of the chuck will gradually increase as high RF power is introduced during wafer processing. The described closed loop system provides steady state temperature control that improves etch profile performance during wafer processing.

[0013] FIG. 1 is a diagram of a temperature conditioning system for a wafer support such as an ESC or vacuum chuck as described above. The system has a hot fluid source 108 and a hot fluid return 109. The return delivers the fluid back to a heat exchanger 160, which heats the fluid to a predetermined temperature or by a regulated amount, and provides the fluid under pressure to a supply line 108. The fluid temperature may be fixed or may be adjusted based on a control signal. Similarly, the system has a cold fluid source 110 and a cold fluid return 111. A second heat exchanger 162 receives the returning cold fluid and cools it to a predetermined temperature or by a regulated amount. The first and second heat exchangers may be a single unit or two separate devices. The heat exchangers may be thermostatically controlled or may be designed to add a fixed amount of heat or cooling, with or without any particular control. The temperature of the coolant 120 circulated through the substrate carrier 122 is controlled in part by the temperature of the supplied hot 108 and cold 110 fluids, but also by the flow rates and mix of the hot and cold fluids, as described below. The system provides a fluid output line 120 that is pumped to the carrier 122, and a return line 124 for returning fluid from the carrier.

[0014] The hot fluid supply is provided to a three-way valve 126, which opens or closes the flow of hot fluid from the heat exchanger. Similarly, the cold fluid supply is provided to an identical three-way valve, which opens or closes the flow of cold fluid from a second heat exchanger. Fluid provided through this valve is provided to the substrate carrier 122 in the carrier supply line 120 for heating or cooling the substrate carrier 122. A further three-way valve 128 is connected to the hot return line 109 and the cold return line 111. Fluid from the substrate carrier can return to this valve 128 through the carrier return line 124 and back through this valve to the respective heat exchanger from which it came. These three-way valves are operated under the control of a temperature controller 102, which is connected to these two valves.

[0015] The temperature controller 102 receives a temperature signal from a thermal sensor 164, such as an optical temperature probe, attached to a substrate within the process chamber or substrate carrier 122 to directly or indirectly determine the temperature of the substrate. The controller opens and closes hot and cold valves based on the received temperature to control the flow rate through the substrate carrier 122. The controller 102 uses the temperature of the probe 164 to determine an error from a temperature set point and generates an output voltage using, for example, a PID (proportional integral derivative) controller.

[0016] The controller 102 determines the flow rate and cooling medium to be used in the substrate carrier fluid channel 120, either hot fluid 108 or cold fluid 110. In one embodiment, the fluid provided by the cold heat exchanger is about 0° C. and the fluid provided by the hot heat exchanger is about 55° C. Depending on the current temperature of the carrier, one of these is delivered to the fluid channel. If the temperature of the carrier is above a set point, the fluid from the cold cooler is used. If the temperature is below a set point, the fluid from the hot cooler is used. The fluid delivery system uses the variable flow valve 106 to control the temperature control medium 120 within the proper temperature range. The flow delivery system also controls temperature rise and overshoot during plasma processing.

[0017] The temperature controller 102 generates a valve control signal, e.g., an analog voltage, to the pressure regulator 104 to control the flow rate. The pressure regulator generates a pressure based on the valve control signal and provides the pressure to the flow control valve 106. The flow control valve opens or closes a passageway from the heat exchangers 160, 162 to the substrate carrier 122 in an amount that depends on the pressure provided. In this way, the flow rate of the heating or cooling fluid through the carrier is controlled by the flow control valve 106. As a result, the flow control valve controls the amount of heating or cooling provided to the substrate carrier. The higher the flow rate, the more heat is transferred by the fluid to or from the carrier. The carrier is thermally coupled to the substrate in the processing chamber. As a result, the higher the flow rate of the cold fluid, the more heat is removed from the carrier. The higher the flow rate of the hot fluid, the more heat is transferred to the carrier.

[0018] In the illustrated embodiment, the temperature controller 102 is coupled to an electronic air pressure regulator 104. Such regulator is provided with a CDA (compressed dry air) input 112 at a controlled pressure (typically constant pressure). The regulator adjusts the input CDA to produce a precise air pressure signal 168 in response to an electrical control signal 166 from the thermal controller. If the regulator is provided with a stepless analog signal, it may provide a regulator air pressure that is also steplessly variable. This regulated air pressure control pressure is applied to the top of the pressure regulating valve 106.

[0019] In one embodiment, the CDA is applied to an air supply solenoid valve of the regulator 104. When the input control voltage 166 rises, the solenoid valve opens. The pressure released through the air supply valve is measured by a sensor and fed to the control circuit. The air supply valve is adjusted until the measured pressure lines up with the pressure represented by the control signal. An exhaust solenoid valve is provided to relieve pressure in the regulator back to the supply CDA.

[0020] The pressure regulating valve 106 provides proportional fluid control in response to a control pressure 168 from the pressure regulator 104. This allows for stepless control of the fluid flow 120 from the three-way valve 126 through the carrier 122. This valve moves much less compared to a pulsed valve. This results in fewer repairs and longer hardware life. In addition, there is a smoother response to temperature changes. By using a variable flow rate that does not include continuous fluid off times during plasma processing, there is a smoother temperature response and reduced temperature oscillations.

[0021] In one embodiment, the pressure regulating valve has an upper chamber with an upper diaphragm in the valve body. The upper diaphragm moves in response to air pressure supplied to the upper chamber. The upper diaphragm in the upper chamber is connected through a shaft to a lower valve gate or diaphragm in a lower chamber of the valve body. The upper diaphragm moves against a spring in response to air pressure to move the lower gate or diaphragm by the connecting shaft. The lower chamber receives fluid from the three-way valve 126 at the inlet and then varies the flow rate to the outlet based on the position of the gate or lower diaphragm. A control feedback loop may be used using a flow meter 172 to provide the desired flow rate through the proportional valve.

[0022] A hot fluid return valve 140 allows heating fluid to flow from the hot supply to the hot return, bypassing the substrate carrier 122. A similar cold fluid return valve 142 allows cooling fluid to flow directly from the cold supply to the cold return without passing through the carrier. When either the hot or cold supply is not being supplied to the carrier by the settings of the three-way valves 126, 128, the hot fluid return valve 140 and the cold fluid return valve 142 allow fluid to flow through the corresponding heat exchanger without flowing through the carrier. This allows the corresponding heat exchanger to achieve a steady state temperature and maintain a supply of fluid at the desired temperature.

[0023] As shown, these fluid bypass valves 140, 142 may also be proportional valves controlled by a regulated CDA (compressed dry air) source 112 or an unregulated on / off CDA. These valves may also be controlled by a temperature controller that works in conjunction with a three-way valve to provide the desired flow.

[0024] The temperature conditioning system may also include various pressure sensors 132, 134, 136, 138 and a flow meter 172. These may be coupled to a system controller 170, which controls the temperature controller 102, the heat exchangers 160, 162, the substrate carrier 122, and other aspects of the process performed on the substrate. Pressure sensors 132, 136 on each of the hot and cold supplies allow the operation of the heat exchangers to be measured. Pressure sensors 134 on each CDA input allow the CDA compression to be measured and adjusted appropriately.

[0025] A flow meter 172 in the fluid lines 122, 124 through the carrier allows the flow rate to be measured. If desired, the measured flow rate can then be adjusted using the proportional valve 106. The pressure in the lines can also be measured with a pressure sensor 138 and adjusted using a proportional valve. In the example shown, the flow rate and pressure are measured in the return line 124 from the carrier. This allows the operation of the system through the carrier to be checked while it is in operation. Any losses in flow or pressure can be observed and the effect of changing the position of the proportional valve can also be measured.

[0026] The described temperature regulation system allows the temperature of fluid flowing through channels in the substrate carrier to be controlled, for example, within a range of 0°C to 55°C. A substrate carrier may have more than one thermal fluid channel. In this case, the system of FIG. 1 may be replicated to support each of the additional channels. A single hot heat exchanger and a single cold heat exchanger may be used to provide fluid to more than one channel, since mixing into and flow through the pressure regulating valve 106 may be controlled independently.

[0027] FIG. 2 shows, as an example, a configuration for a substrate carrier such as ESC 280 with an outer thermal fluid channel 282 and an inner thermal fluid channel 284. The outer and inner channels can be used to compensate for different conditions at different locations on the substrate. For example, for plasma processing of a wafer, the outer edge of the wafer tends to heat up faster than the inner edge of the wafer because the outer edge of the wafer has more surface area exposed to the chamber. In such an example, more cooling fluid can be provided to the outer fluid channel than the inner fluid channel. The wafer can thereby be maintained at a more consistent temperature across its surface. The principles of FIG. 2 can be applied to other types of ESCs or other types of substrate carriers where independent control is used for more than one fluid channel.

[0028] The cold supply heat exchanger 202 provides cooled thermal fluid to the cold supply line 206. The cold supply is pumped through a bypass valve 218 to a cold return manifold 220. The cold supply is returned to the cold heat exchanger through a cold return line 210. The bypass valve 218 may be controlled using a pressure sensor 216. When pressure in the cold supply line 206 is high, the pressure sensor opens the bypass valve, relieving pressure in the line. When pressure in the cold supply line is low, the bypass valve is closed to ensure sufficient pressure to support operation of the system. Another sensor is provided by a cold flow meter 222 in the cold return line. This other sensor, in combination with the pressure sensor 216, may be used by a thermal controller (not shown) or a system controller (not shown) to ensure proper operation of the system.

[0029] The bypass valve 218 may be a diaphragm valve that may be manually operated or controlled by an external controller. The bypass valve may be partially self-regulating based on pressure or controlled by a pressure sensor. A bypass valve for the cold temperature and another bypass valve 226 for the hot return line 212 help ensure smooth flow transitions and continuous flow during all modes of operation.

[0030] Similarly, the hot supply heat exchanger 204 provides heated thermal fluid to a hot supply line 208. The hot supply line is fed through a hot bypass valve 226 controlled by a pressure sensor 224 to feed the hot supply back to a return manifold 228. The hot thermal fluid in the return manifold is fed to the hot return line back to the hot supply heat exchanger 204. The hot return line also has a flow meter 230 to measure the flow from all sources to the return line.

[0031] The low and high temperature thermal fluid supplies are fed to a first three-way valve 240, which selects low temperature or high temperature thermal fluid, or both, for the outer cooling channel 282. The same low and high temperature thermal fluid supplies are also fed to a second three-way valve 242, which selects low temperature or high temperature thermal fluid, or both, for the inner cooling channel 284 of the ESC 280. The flow rate of the output from each of the three-way valves is controlled by a respective proportional valve 260, 262, such as the pressure regulating valve described above. However, as in the example of FIG. 1, any other type of proportional valve may be used. Each proportional valve has a respective valve controller 250, 252, such as the pressure regulator described above. The pressure regulator is coupled to a CDA source 286 and a control signal (not shown) from the thermal controller. The two proportional valves provide independent control of the flow rate through each cooling channel. With the configuration of FIG. 2, the heat exchangers are shared, while both the temperature and flow rate of the thermal fluid through each cooling channel are independently controlled.

[0032] Flow- and temperature-controlled fluid is provided from the outer channel proportional valve 260 via outer channel supply line 270 to the outer channel 282 of the ESC 280. The fluid flows through the outer channel of the ESC to the outer return line 274. Similarly, flow- and temperature-controlled fluid from the inner channel proportional valve 262 is provided via inner channel supply line 272 to the inner channel 284 of the ESC 280. The fluid flows through the inner channel of the ESC to the inner return line 276.

[0033] The ESC return lines 274, 276 are pumped back to the heat exchangers to be cooled or heated depending on the fluid source. The outer return line 274 passes through a flow meter 258 and then through a pressure sensor 254 to a three-way valve 244. The three-way valve delivers the return fluid to the return manifolds 220, 228 of the heat exchangers 202, 204 from which the thermal fluid came. Similarly, the inner return line 276 passes through a flow meter 268 and then through a pressure sensor 256 to another three-way valve 246. This three-way valve is also connected to the return manifolds 220, 228 to return fluid from the inner channels to the corresponding heat exchangers 202, 204.

[0034] All four three-way valves 240, 242, 244, 246 may be controlled by a temperature controller to ensure that thermal fluid is delivered to the supply and return lines as desired. If one of the heat exchangers is not connected to any supply line, the corresponding bypass valve 218, 226 may be opened. Flow and pressure sensors throughout the system allow the system to be monitored at various points at all times to ensure correct operation.

[0035] 3 is a schematic diagram of a plasma etching system 10 including a chuck assembly 42 according to an embodiment of the present invention. The plasma etching system 10 may be any type of high performance etching chamber known in the art, such as, by way of example and not limitation, an Enabler®, DPS II®, AdvantEdge™ G3, EMAX®, Axiom®, or Mesa™ chamber (all manufactured by Applied Materials, Inc., California, USA). Other commercially available etching chambers may similarly utilize the chuck assemblies described herein. Although the exemplary embodiment is described in connection with the plasma etching system 10, the chuck assemblies described herein may also be adapted for use in other processing systems (e.g., plasma deposition systems, etc.) used to perform any plasma manufacturing process.

[0036] Referring to FIG. 3, the plasma etching system 10 includes a grounded chamber 5. Process gas is supplied to the interior of the chamber 5 from one or more gas sources 29 connected to the chamber through a mass flow controller 49. The chamber 5 is evacuated via an exhaust valve 51 connected to a high volume vacuum pump stack 55. When a plasma power is applied to the chamber 5, a plasma is formed in the processing region above the workpiece 10. A plasma bias output 25 is coupled to the chuck assembly 42 to excite the plasma. The plasma bias output 25 typically has a low frequency between about 2 MHz and 60 MHz, and may be in the 13.56 MHz range, for example. In an exemplary embodiment, the plasma etching system 10 includes a second plasma bias output 26 operating in the 2 MHz range connected to an RF match 27. The plasma bias output 25 is also coupled to the RF match and is also coupled to the lower electrode through an output conduit 28. A plasma source 30 is coupled to the plasma generating element 35 through another match (not shown) to provide a high frequency source power for inductively or capacitively exciting the plasma. The plasma source power 30 may have a higher frequency than the plasma bias power 25, such as between 100 MHz and 180 MHz, for example within the 162 MHz range.

[0037] The workpiece 10 is loaded through the opening 15 and clamped to the chuck assembly 42 within the chamber. The workpiece 10, such as a semiconductor wafer, may be any wafer, substrate, or other material used in semiconductor processing techniques, and the invention is not limited in this respect. The workpiece 10 is disposed on a top surface of a dielectric layer or puck of the chuck assembly. The dielectric layer or puck is disposed above a cooled base assembly 44 of the chuck assembly. A clamping electrode (not shown) is embedded within the dielectric layer. In a specific embodiment, the chuck assembly 42 may include two or more different fluid channel zones, such as an inner channel 41 and an outer channel 99. Each channel 41, 99 may be independently controllable to the same or different temperature set points.

[0038] A system controller 70 is coupled to a variety of systems that control the fabrication process within the chamber. The controller 70 may include a temperature controller 75 that executes a temperature control algorithm (e.g., temperature feedback control), which may be software or hardware, or a combination of both software and hardware. The system controller 70 also includes a central processing unit 72, a memory 73, and an input / output interface 74. The temperature controller 75 outputs control signals that affect the rate of heat transfer between the chuck assembly 42 and heat sources and / or heat sinks external to the plasma chamber 5 for the various fluid channels 41, 99. The temperature controller 75 may be coupled to one or more temperature probes 43, which may be in or on the substrate carrier, may be coupled to fluid supply lines, or may be in any other desired location.

[0039] The thermal fluid zones may include separate, individually controlled thermal fluid heat transfer loops with separate fluid controls and controlled based on zone-specific temperature feedback loops as described above. In the exemplary embodiment, the temperature controller 75 is coupled to a first heat exchanger (HTX) / cooler 77 and may be further coupled to a second HTX / heater 78 and more heat exchangers (not shown) as desired depending on the particular embodiment. The flow rate of the heat transfer fluid or coolant through the conduits in the chuck assembly 42 is controlled by a proportional valve system 86 as described above.

[0040] The proportional valve system 86 is controlled by the temperature controller 75 to independently control the flow rate of thermal or heat transfer fluid to each of the various fluid channels. The temperature controller may also control the temperature set point used by each heat exchanger to cool or heat the thermal fluid, so that each heat exchanger can bring the thermal fluid for each coolant channel to a different temperature before returning it to the fluid channel.

[0041] The heat transfer fluid may be a liquid, such as, but not limited to, deionized water / ethylene glycol, a fluorinated coolant such as Fluorinert® from 3M or Galden® from Solvay Solexis, Inc., or any other suitable dielectric fluid, such as those containing inert perfluoropolyethers. Although this document describes an ESC in the context of a plasma processing chamber, the ESCs described herein may be used in a wide variety of chambers for a wide variety of processes. Depending on the specific embodiment, another substrate carrier may be used in place of the ESC.

[0042] It should be understood that the above description is intended to be illustrative and not limiting. For example, while the flow diagrams in the drawings show a particular order of steps performed in certain embodiments of the invention, it should be understood that such order is not required (e.g., steps may be performed in a different order, some steps may be combined, some steps may overlap, etc., in alternative embodiments). Moreover, many other embodiments will become apparent to those of ordinary skill in the art upon reading and understanding the above description. While the invention has been described with reference to specific examples, it will be recognized that the invention is not limited to the described embodiments, but can be practiced with modification and alteration within the spirit and scope of the appended claims. The scope of the invention should therefore be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

Claims

1. 1. A method for processing a workpiece, comprising: generating a plasma containing gas ions in a plasma chamber; applying the plasma to a workpiece on a workpiece holder; adjusting a temperature of the workpiece through the thermal fluid channels using heat exchangers providing thermal fluid to and receiving the thermal fluid from thermal fluid channels of the workpiece holder, a low temperature supply heat exchanger providing cooled thermal fluid and a high temperature supply heat exchanger providing heated thermal fluid; and wherein the thermal fluid in the thermal fluid channel is, during substrate processing, a proportional valve for controlling the flow of the thermal fluid from the heat exchanger to the thermal fluid channel; a temperature controller receiving temperature measurements from a thermal sensor in the workpiece holder and controlling the proportional valve in response to the temperature measurements to regulate the flow rate of the thermal fluid; a three-way valve that determines whether the heated thermal fluid from the hot supply heat exchanger or the cooled thermal fluid from the cold supply heat exchanger flows to the proportional valve; and controlling a temperature of the workpiece holder using The method, wherein the cooled thermic fluid and the heated thermic fluid comprise a polyether.

2. 2. The method of claim 1, wherein controlling the flow rate includes the temperature controller providing an analog voltage signal to a pressure regulator, and the pressure regulator providing fluid pressure to the proportional valve in response to the analog voltage signal to control the proportional valve.

3. 3. The method of claim 2, wherein the proportional valve is an analog flow valve having a passage that opens and closes in an amount dependent upon the fluid pressure.

4. 3. The method of claim 2, wherein the analog voltage signal is stepless and the fluid pressure is stepless.

5. The method of claim 2 , wherein the fluid pressure provided to the proportional valve is supplied by compressed dry air.

6. 2. The method of claim 1, further comprising receiving a fluid pressure from a pressure sensor and a fluid flow rate from a flow meter in the thermal fluid channel, the temperature controller controlling the proportional valve in response to the pressure sensor and the thermal fluid channel.

7. 2. The method of claim 1, wherein the proportional valve comprises a pressure regulating valve, the proportional valve comprising a valve body having an upper chamber and a lower chamber, and an upper diaphragm in the upper chamber connected to a lower diaphragm in the lower chamber.

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