WO1999048139A2

WO1999048139A2 - Apparatus for reducing heat loss

Info

Publication number: WO1999048139A2
Application number: PCT/US1999/003979
Authority: WO
Inventors: Charles D. Shaper; Khalid El-Awady; Hooman Bolandi; Peter Knoot
Original assignee: Applied Materials, Inc.
Priority date: 1998-03-18
Filing date: 1999-02-23
Publication date: 1999-09-23
Also published as: WO1999048139A3

Abstract

An apparatus for reducing heat loss around the perimeter of a material substrate during thermal cycling is provided. The apparatus for reducing heat loss comprises a perimeter apparatus for surrounding the substrate. The apparatus rests in the same plane as a material substrate during thermal cycling and can be made of the same material as the substrate. Additionally, the apparatus can be one continuous piece or can be several perimeter pieces. To sense the temperature of a substrate during thermal cycling, a representative temperature can be obtained by embedding sensors in an auxiliary heater or in a heat exchanger, both of which are components of a thermal cycling module. A relatively flat sensor can be embedded between film layers in a foil heater. Additionally, sensors can be embedded inserted into a passageway in a heat exchanger so that they are flush with the each exchanger's top surface. The wires to the sensor can then be enclosed in casing that also removably secures the sensor in the heat exchanger.

Description

APPARATUS FOR REDUCING HEAT LOSS

FIELD OF THE INVENTION This invention relates generally to apparatuses for sensing the temperature of material substrates during thermal cycling and for reducing heat loss at the perimeter of the substrate during thermal cycling. More particularly, it relates to an apparatus for reducing heat loss and facilitating temperature measurement and thermal cycling modules employing sensors for measuring temperatures during thermal cycling.

BACKGROUND OF THE INVENTION Certain stages of semiconductor manufacturing require baking the semiconductor substrate material, such as a wafer, and subsequently chilling it. For example, the photoresist processing state of semiconductor manufacturing requires such baking and chilling, or thermal cycling. In order to produce high quality wafers suitable for present integrated circuit applications, the temperature of the wafer during thermal cycling must be precisely controlled with respect to both the temporal temperature profile of the baking and chilling cycles and to the uniformity of the temperature across the substrate.

Conventional methods for baking and chilling wafers involve first baking the wafer at a temperature ranging typically between 70°C and 250°C for a period of time ranging typically between 30 seconds and 90 seconds. After baking the wafer, the wafer is mechanically moved to a cold plate where it is chilled to a temperature ranging typically between 0°C and 30°C. Recent developments for baking and chilling wafers involve thermal cycling modules where a wafer is baked and chilled on one plate, thus eliminating the need to move the wafer between plates. The thermal cycling modules can include numerous heating zones as well.

For conventional methods of baking and chilling wafers, the temperature of the plate or of the wafer could be determined by simply placing a temperature probe near the plate, placing thermocouples along the plate, or placing a thermocouple in the middle of the baking or chilling plate. With a thermal cycling module, however, the temperature of the plate and wafer change over time and temperature measurements are used to initiate various heating and cooling stages. This is particularly true where a multizoned thermal cycling module is used. Accordingly, when using a thermal cycling module, the sensors must be used in a closed control loop, and designs incorporating control sensors in the thermal cycling module without compromising or invading the wafer are required. The present invention provides apparatuses for thermal cycling of material substrates where the apparatus incorporates sensors that noninvasively determine the temperature of the substrate over time.

In addition to incorporating sensors into a thermal cycling module, it is also desirable to minimize heat loss near the perimeter of the substrate when baking, chilling or making a temperature transition. It is desirable to maintain a uniform temperature across the substrate during the thermal cycle; however, heat is lost around the perimeter of the substrate due to the extra lateral side of the wafer and the additional convective heat loss at the edge of the substrate. Accordingly, it is desirable to find ways to reduce the additional heat loss around the edge of the substrate. The present invention provides an apparatus for reducing heat loss at the perimeter of a substrate during the thermal cycling process.

Further features and advantages of the invention will be apparent from the following description and drawings.

SUMMARY OF THE INVENTION The present invention provides an apparatus for preventing thermal loss from the perimeter or lateral sides of a substrate during the baking and chilling phases of photoresist processing and embodiments of thermal cycling modules incorporating sensors for continuous noninvasive substrate temperature sensing during the baking and chilling phases. The perimeter apparatus comprises a continuous piece of thermally conductive material that surrounds the perimeter of the substrate when the substrate is placed in thermal contact with a thermal cycling module. The apparatus can be a solid continuous ring and can be made of the same material as the substrate. The apparatus sits on pins so that it is in the same plane as the substrate and can be fixedly attached to the thermal cycling module with a clamping piece that is placed above the apparatus. The thermal cycling module can also embody sensors for noninvasive sensing of the substrate's temperature during the baking and chilling phases. One embodiment of a thermal cycling module embodying noninvasive sensors comprises sensors fixedly attached to either the perimeter apparatus or to perimeter pieces that are fixedly attached around the perimeter of the substrate in the same manner as the perimeter apparatus. One perimeter piece or several perimeter pieces may be used, and the perimeter pieces are in the same plane as the substrate.

Another embodiment of a thermal cycling module with noninvasive sensors comprises an auxiliary foil heater where platinum wire resistant temperature detectors are imbedded between layers of the heater. The foil heater has a bottom and first upper layer of insulating film with electrically conductive wires, coils or lines disposed between the bottom and first upper layers. A second upper layer of insulating film is placed above the first upper film layer, and the resistant temperature detectors are disposed between the first and second upper film layers. The resistant temperature detectors are extremely thin, and the foil heater remains relatively flat with the sensors imbedded in it.

Finally, another embodiment of a thermal cycling module with noninvasive sensors comprises a heat exchanger with sensors imbedded therein. The heat exchanger defines holes or passageways that extend from the heat exchanger's bottom surface to its top surface. A sensor can then be removeably secured in the hole. The sensor can have a casing that is threaded to cooperatively engage threads in the holes defined by the heat exchanger. This allows the sensor to be adequately secured in the heat exchanger yet easily removed for replacement or repair.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages, features and design of the invention will become apparent from the following detailed description of the invention and the accompanying drawings in which like reference numerals refer to like elements and in which: Fig. 1 is a perspective view of a thermal cycling module; Fig. 2 is a side view of a thermal cycling module; Fig. 3 is a top view of a wafer positioned above a thermal cycling module with an embodiment of the perimeter apparatus of the present invention;

Fig. 3A is a vertical cross-section view of a manner of securing the perimeter apparatus to the thermal cycling module of the present invention; Fig. 4 is a top view of a wafer positioned above an embodiment of a thermal cycling module with sensors attached to perimeter pieces according to the present invention;

Fig. 5 is a side view of a wafer positioned above a thermal cycling module with sensors imbedded in the auxiliary heater according to the present invention; and Fig. 6 is a vertical cross-section view of a thermal cycling module with sensors imbedded in the heat exchanger according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION For conventional methods of baking and chilling wafers, the temperature of the plate or of the wafer could be determined by simply placing a temperature probe near the plate, placing thermocouples along the plate, or placing a thermocouple in the middle of the baking or chilling plate. With a thermal cycling module, however, the temperature of the plate and substrate change over time and temperature measurements are used to initiate various heating and cooling stages. This is particularly true where a multizoned thermal cycling module is used. Accordingly, when using a thermal cycling module, the sensors must be used in a closed loop, and means for incorporating control sensors in the thermal cycling module without compromising the substrate are required. The present invention provides apparatuses for thermal cycling of material substrates where sensors are incorporated to noninvasively determine the temperature of the substrate over time. In addition to incorporating sensors into a thermal cycling module, it is also desirable to minimize the heat loss near the perimeter of the substrate when baking, chilling or making a temperature transition. It is desirable to maintain a uniform temperature across the substrate during the thermal cycle; however, heat is lost around the perimeter of the substrate due to the extra lateral side of the wafer and the additional convective heat loss at the edge of the substrate. Accordingly, it is desirable to find ways to reduce the additional heat loss around the edge of the substrate. The present invention provides an apparatus for reducing the heat loss around the edge of a substrate during the thermal cycling process.

Referring to Figs. 1 and 2, a thermal cycling module can comprise a heat exchanger 14 and an auxiliary heater 12, although the auxiliary heater is not required. Both the heat exchanger 14 and auxiliary heater 12 are in thermal contact with a material substrate 10, such as a wafer. The heat exchanger can be comprised of a thermally conductive material and can have fluid flowing through it to cause its overall temperature to change. The auxiliary heater 14 can be any type of heater, such as an array of thermoelectric devices or a foil heater. The auxiliary heater 14 can be designed to have several heating zones, which will be controlled by a control circuit. The auxiliary heater 14 is powered by a variable power source.

As is shown in Figs. 1 and 2, the lateral edge 16 or perimeter of the substrate is exposed to the surrounding air, and consequently, extra heat will be lost from the lateral edge 16. Additionally, across the plate more heat will transfer from the heat exchanger and/or auxiliary heater to the substrate at its center than will at its perimeter due to "edge effects." Heat is transferred from the heat exchanger and/or auxiliary heater to the substrate by convection. At the center of the substrate, more heat will be transferred to the substrate than at the perimeter because at the perimeter, the heat will escape out around the substrate's edge. Similarly, extra heat is lost from the lateral sides of the substrate due to convection. Accordingly, heat loss will always be greater at the perimeter of the substrate. It is therefore desirable to artificially extend the perimeter of the substrate.

Referring to Fig. 3, to artificially extend the perimeter of the substrate, a perimeter apparatus 30 surrounds the substrate 10 and is in close proximity to the perimeter of the substrate 10. The perimeter apparatus 30 is preferably comprised of the same material as the substrate 10 undergoing thermal cycling. It is also preferred that the loss prevention apparatus 30 be a continuous piece. It can be, however, broken into several pieces that surround the substrate 10. By placing the perimeter apparatus 30 in close proximity to the substrate 10, the heat loss from the lateral edges of the substrate 10 will be diminished. First, instead of losing heat by convection to the surrounding air, the substrate will lose heat by conduction to the perimeter apparatus 30, which is a much less efficient manner of heat transfer. Additionally, the apparatus will reduce "edge effects" and prevent heat from escaping out from around the apparatus edge or perimeter. It is believed that the apparatus 30 will reduce the heat lost near the perimeter up to thirty percent.

Referring to Fig. 3A, the apparatus 3^'0 is preferably held in place on the thermal cycling module with clamping pieces 32. The apparatus rests on pins 34 to elevate it to the same height as the substrate 10 during thermal cycling. Then the clamping piece is placed above the apparatus 30, near its outer perimeter. The clamping piece 32 can have a nodule 36 that creates a light contact between the clamping piece 32 and apparatus 30. At the opposite end from the nodule 36, if one is used, the clamping piece 32 is removeably attached to the heat exchanger. In a preferred embodiment, it is attached with a screw 38 and nut 39, or similar connector. The apparatus 30 should be placed with enough of an air gap 31 between it and the substrate 10 so that the substrate can easily be placed above the heat exchanger 14 and/or easily removed. The apparatus 30 will prevent the substrate 10 from shifting due to vibrations. Similarly, the clamping pieces 32 and pins 34 will prevent the apparatus 30 from shifting or moving due to vibrations.

In addition to preventing extra heat loss around the perimeter of the wafer, it is desirable to be able to sense the temperature of the wafer throughout thermal cycling. The heat exchanger and/or auxiliary heater are controlled based on sensor readings corresponding to the temperature of the wafer. For example, a thermal cycling module can be controlled by a feedback control loop to regulate the substrate temperature during the thermal cycling. The feedback control loop includes a multivariable feedback controller with which sensors are in electrical contact. The sensors send to the controller electrical signals representative of substrate temperatures corresponding to substrate regions. Based on the sensor signals, a microprocessor in the controller calculates control signals and sends them to power the auxiliary heater or to control the heat exchanger. This enables the thermal cycling module to precisely cycle through, for example, a typical thermal cycle where a substrate starts at 20° C, is quickly ramped up to 150° C and held for 40 seconds, is then chilled to 20° C and held awaiting substrate removal. During each phase of the thermal cycle, sensors provide information regarding the approximate substrate temperature, thus enabling more precise control throughout the cycle. The sensors, however, cannot be placed in direct contact with the substrate. Thus, it is desirable to place substrates near the substrate in positions that accurately represent the substrate temperature.

Referring to Fig. 4, sensors 42 can be placed on perimeter pieces 40 located along the perimeter of the substrate 10. The perimeter pieces 40 are preferably semicircular if the substrate is circular. Additionally, it is preferable that the perimeter pieces 40 comprise the same material as the substrate 10, although that is not necessary. It is believed that the best correlation between the sensor 42 reading and the substrate 10 temperature will be achieved if the perimeter piece 40 is the same material as the substrate 10. The perimeter piece 40 is placed above and in thermal contact with the heat exchanger 14 and/or the auxiliary heater and is preferably located in the same plane as the substrate 10. There can be several perimeter pieces 40 surrounding the substrate 10, a few pieces 40 as shown in Fig. 4, or simply one piece 40 located along the perimeter of the substrate 10 depending on how many sensor readings are desired and on whether the perimeter pieces 40 will also be used to minimize heat loss around the perimeter of the substrate 10, as described above with reference to Fig. 3. The sensor 42 can be adhered to the center of each perimeter piece 40 or at some other location on the piece 40. It is preferred that the center be adhered generally near the center of the perimeter piece 40. Any type of sensor can be used. It is preferred, however, that a resistant temperature detector (RTD) sensor using a platinum wire be adhered to the perimeter piece with adhesive, as is generally known in the art.

The perimeter pieces 40 can be secured to the thermal cycling module in the same manner as the perimeter apparatus 30 is secured, as shown in Fig. 3A. It is preferred that clamping pieces 32 with nodules 36 are placed above the perimeter pieces 40. The perimeter pieces 40 rest on pins 34 to place the perimeter pieces 40 in the same plane as the substrate 10. The clamping pieces 32 are removeably attached at one end to the heat exchanger 14 with a screw 38 and nut 39 or other similar connector.

Referring to Fig. 5, sensors can also be imbedded in the auxiliary heater of the thermal cycling module. In particular, sensors can be imbedded in a foil heater 50. Fig. 5 shows a foil heater 50 having three layers of film 56, 57 and 58. The heating element 52 is located between a bottom film layer 56 and an upper film layer 57. The heating element comprises lines, wires or coils made of electrically conductive material such as tungsten or nickel-chromium for conducting electrical current through the foil heater. Sensors 54 can then be positioned between the upper film layer 57 and a second upper film layer 58. It is preferred that the sensors 54 are relatively flat so as to maintain a flat surface of the foil heater 50. For example, a platinum wire RTD sensor can be used. The film layers are preferably formed from an insulating material, such as KAPTON®, manufactured by E.I. DuPont de Nemours of Wilmington, Delaware. Alternative insulating layers include silicon rubber, mica, or NOMEX®, also manufactured by E.I. DuPont de Nemours. The heating element 52 and sensors 54 are attached to the film layers 56, 57 and 58 with adhesive, as is generally known in the art. Any number of sensors 54 may be used between the upper film layer 57 and second upper film layer 58. Preferably, there will be one sensor 54 positioned within each zone of the foil heater 50. Referring to Fig. 6, sensors can also be imbedded in the heat exchanger itself. In order to imbed the sensor 60 in the heat exchanger 14, holes or passageways must be placed through the heat exchanger at desired locations. The holes or passageways extend from the bottom surface 67 of the heat exchanger 14 through to the top surface 66 of the heat exchanger. The sensor 60 can then be inserted through the hole with the top surface 68 of the sensor 60 being flush with the top surface 66 of the heat exchanger 14 and the sensor wires 64 extending down through the bottom of the hole. The sensor wires 64 can be encased in a casing material 62. The casing material can then be threaded and the hole in the heat exchanger 14 be cooperatively threaded to facilitate connecting the sensor 60 and sensor casing 62 to the heat exchanger 14. This enables the sensor 60 to be removeably secured in the heat exchanger 14. The sensor casing 64 is preferably made of TEFLON®, manufactured by Dupont® and the sensor is preferably a RTD sensor. Any type of sensor and casing material may be used, however.

Claims

WHAT IS CLAIMED IS:

1. An apparatus comprising a continuous solid piece of thermally conductive material dimensioned for placing generally around the perimeter of a substrate during the baking and chilling phases of photoresist processing.

2. The apparatus of claim 1 wherein said continuous solid piece is comprised of a silicon material.

3. The apparatus of claim 1 wherein the continuous solid piece is a circular ring.

4. The apparatus of claim 1 further comprising at least one sensor fixedly connected to said continuous solid piece.

5. An apparatus comprising a plurality of solid perimeter pieces of thermally conductive material dimensioned for placing generally around the perimeter of a substrate during the baking and chilling phases of photoresist processing.

6. The apparatus of claim 5 wherein said solid perimeter pieces are comprised of a silicon material.

7. The apparatus of claim 5 wherein said solid perimeter pieces are semicircular.

8. The apparatus of claim 5 further comprising at least one sensor fixedly connected to said solid perimeter pieces.

9. A thermal cycling module for baking and chilling a material substrate comprising a heat exchanger and a perimeter apparatus to place generally around the substrate, said perimeter apparatus being in thermal contact with said heat exchanger and said perimeter apparatus comprising a continuous solid piece of thermally conductive material.

10. The thermal cycling module of claim 9 further comprising a clamping system to secure said perimeter apparatus to said heat exchanger, said clamping system comprising a plurality of pins fixedly attached to said heat exchanger to place under said perimeter apparatus and a clamping piece having a first end and a second end, said first end being in physical contact with and placed over said perimeter apparatus, said second end being fixedly attached to said heat exchanger.

1 1. A thermal cycling module for baking and chilling a material substrate comprising: a heat exchanger; at least one solid perimeter piece of thermally conductive material in thermal contact with said heat exchanger; and at least one sensor fixedly connected to said solid perimeter piece.

12. The thermal cycling module of claim 1 1 wherein said solid perimeter piece is comprised of a silicon material.

13. The thermal cycling module of claim 1 1 wherein said sensor is a RTD.

14. The thermal cycling module of claim 12 wherein said sensor is a Thermal Couple.

15. The thermal cycling module of claim 1 1 further comprising a clamping system to secure said perimeter piece to said heat exchanger, said clamping system comprising a plurality of pins fixedly attached to said heat exchanger to place under said perimeter piece and a clamping piece having a first end and a second end, said first end being in physical contact with and placed over said perimeter piece, said second end being fixedly attached to said heat exchanger.

16. A thermal cycling module for baking and chilling a material substrate comprising: a heat exchanger; an auxiliary heater in thermal contact with said heat exchanger; and at least one sensor fixedly connected to said heating element.

17. The thermal cycling module of claim 16 wherein said auxiliary heater comprises an upper film layer, a bottom film layer, and a conductive element interposed between the upper film layer and bottom film layer.

18. The thermal cycling module of claim 17 wherein said auxiliary heater further comprises a second upper film layer and wherein said sensor is interposed between said upper film layer and second upper film layer.

19. The thermal cycling module of claim 17 comprising a plurality of sensors and wherein said auxiliary heater further comprises a plurality of heating zones and at least one of said sensors is fixedly connected to said heating element within each zone.

20. The thermal cycling module of claim 19 wherein said auxiliary heater further comprises a second upper film layer and wherein said sensors are interposed between said upper film layer and second upper film layer.

21. The thermal cycling module of claim 18 wherein said sensor is a platinum wire resistant temperature detector.

22. The thermal cycling module of claim 20 wherein said sensor is a platinum wire resistant temperature detector.

23. A thermal cycling module for baking and chilling a material substrate comprising: a heat exchanger having a top surface and a bottom surface and defining a passageway between said top surface and said bottom surface; and at least one sensor disposed in said passageway defined by said heat exchanger.

24. The thermal cycling module of claim 23 wherein said sensor comprises RTD.

25. The thermal cycling module of claim 24 wherein said sensor further comprises a sensor casing fixedly connected to said sensor to facilitate fixedly connecting said sensor in said passageway defined by said heat exchanger.

26. The thermal cycling module of claim 25 wherein said sensor casing comprises RTD.