Abstract:
A temperature sensor utilizing optical temperature measuring techniques is constructed to make firm contact with a surface whose temperature is being measured, an example application being the monitoring of semiconductor wafers or flat panel displays while being processed. A cap is mounted near but spaced apart from an end of a lightwave guide, with a resilient element that applies force of the cap against a surface whose temperature is being measured as the cap is urged toward the optical fiber end. An optical temperature sensing element, such as luminescent material or a surface of known emissivity, is carried within the cap. A bellows with a closed end conveniently serves as both the cap and the resilient element. An alternative temperature measuring device installs an optical temperature sensing material within a test substrate behind an optical window, and then views the sensing material through the window.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to optical temperature measuring techniques, and, more specifically, to devices and techniques for measuring the temperature of a surface of an article by contacting its surface during processing. 
     BACKGROUND OF THE INVENTION 
     There has been a great deal written about various optical temperature measuring techniques, both in patents and the technical literature, as well as many commercial products utilizing this technology. In one aspect of this technology, a luminescent material is used as a temperature sensor because certain aspects of its luminescence are temperature dependent. Typically in the form of a sensor at the end of a fiber optic cable, the luminescent material is excited to luminescence by sending excitation radiation of one wavelength to the sensor through the optical fiber, and the resulting luminescence at a different wavelength is photo-detected after passing back along the optical fiber. The detected signal is then processed to determine the temperature of the luminescent material in the sensor. Basic concepts of luminescent temperature sensing, as well as many different forms of sensors, are described in U.S. Pat. No. 4,448,547. The measurement of the decay time of the luminescence after termination of an excitation pulse, as a measurement of temperature, is described in U.S. Pat. No 4,652,143. Commercial products adopted the decay time measurement technique as a good measurement of temperature. 
     Applications of these luminescent sensor measurement techniques are numerous, including the measurement of surface temperature. U.S. Pat. No. 4,752,141 describes an elastomeric luminescent sensor at the end of an optical fiber that deforms as it is urged against a surface being measured in order to establish good thermal communication. An alternative use of a thin non-metallic disc with a layer of luminescent material between it and the end of an optical fiber is also described. One advantage and focus of luminescent temperature measurement techniques has been for applications in environments having strong electric and/or magnetic fields and the like, where metal sensors cannot be relied upon to provide accurate results since the metal is heated as a result of the field. 
     Another optical temperature measuring technique relies upon the infrared emissions of a black-body sensor, or one having the characteristics of a black-body. An example of such a system, generally used to measure higher temperatures than measured with luminescent sensors, is described in U.S. Pat. No. 4,750,139. The sensor is a black-body emitter formed at the end of an optical fiber. U.S. Pat. No. 5,183,338 describes several forms of a fiber optic sensor that includes both luminescent and black-body temperature measuring elements. Each of the foregoing identified patents is expressly incorporated herein in its entirety by this reference. 
     There are also many other optical temperature sensing techniques that have been described in patents and the literature, as well as being used commercially. But the luminescent and black-body techniques have generally been preferred over those others. 
     SUMMARY OF THE INVENTION 
     Briefly, an optical temperature sensor is formed on an end of an optical radiation wave guide, such as an optical fiber, which may use a luminescent, black-body or other existing optical temperature sensitive material, in a form especially adapted for measuring the temperature of a surface, particularly, by way of example, a surface of a semiconductor wafer, flat panel display or other substrate being processed within a processing chamber. A rigid support for the temperature sensitive material, such as a cap in which the material is held on an inside, is positioned adjacent the wave guide end with a space normally between them. A resilient element urges the cap away from the wave guide end, at least in response to the cap being pushed toward the wave guide end when the sensor being brought into contact with the surface being measured. This provides very close contact with the surface, resulting in good thermal transfer and equilibrium between the surface and the temperature sensitive material. There are several useful forms of the temperature sensitive material support and resilient element, which may be formed as one or made separate. One specific form uses a closed end bellows element that both provides a cap having the temperature sensitive material on its inside and the resilient element in its side walls, the optical fiber being inserted into the bellows from an open end. 
     The sensors of the present invention have application in a number of different environments, including very hostile environments. Sensors of the present invention operate within a chamber having a high vacuum, a chemical bath chamber, environments with extreme hot or cold, or those with high levels of radio frequency, microwave or other electromagnetic radiation bands. 
     A primary reason for wanting to know the temperature of a substrate being processed is to allow its temperature to be set or varied as desired by making suitable adjustments to the processing equipment. According to another aspect of the present invention, instead of measuring the temperature of the actual substrates being processed, however, a test substrate or other object being processed is provided with at least one optical temperature sensor formed in a surface of the substrate. The test substrate is then periodically or occasionally positioned in the processing chamber in place of an actual substrate being processed, during which time the processing equipment is calibrated to provide the desired substrate temperature during processing. Such a sensor preferably includes optical temperature sensing material imbedded into a small area of the surface behind a protective transparent window. The sensor is viewed by directing optical radiation through the window. 
     Additional aspects, features and advantages of the present invention are included in the following description of exemplary embodiments thereof, which description should be taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a general schematic diagram that shows a processing chamber in which temperature sensor of the present invention may be used; 
     FIG. 2 is a cross-sectional view of a general form of a surface temperature sensor; 
     FIG. 3 shows the temperature sensor of FIG. 2 in contact with a surface being measured; 
     FIG. 4 is a cross-sectional view of a first specific example surface temperature sensor; 
     FIG. 5 is a cross-sectional view of a second specific example surface temperature sensor; 
     FIG. 6 is a cross-sectional view of a third specific example surface temperature sensor; 
     FIG. 7 is a cross-sectional view of a fourth specific example surface temperature sensor; 
     FIG. 8 illustrates a modification of any of the temperature sensors of FIGS. 2-7 to include a first form of an infrared emitter as the temperature sensor; 
     FIG. 9 illustrates a modification of any of the temperature sensors of FIGS. 2-7 to include a second form of an infrared emitter as the temperature sensor; 
     FIG. 10 shows a form of package for any of the temperature sensors of FIGS. 4-9; 
     FIG. 11 shows one example use of a temperature sensor according to any of FIGS. 2-10; 
     FIG. 12 shows another example use of a temperature sensor according to any of FIGS. 2-10; 
     FIG. 13 illustrates a test substrate with a luminescent temperature sensor built into a surface; 
     FIG. 14 is a sectional view of FIG. 13, taken at section A—A thereof; and 
     FIG. 15 shows one way of optically coupling with the substrate sensor of FIGS. 13 and 14 in order to read its temperature. 
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The surface temperature techniques and sensors of the present invention may be used in a wide variety of environments and applications. The temperature of surfaces on any of a large number of types of objects may be measured. These measurements can be made while the object is being subjected to some processing where knowledge of the temperature of its surface is desired, or otherwise. The example application described herein is the measurement of the temperature of the surface of substrates during one or more steps of processing to form integrated circuits and/or visual display elements such as liquid display devices (LCDs) thereon. The substrate is either a semiconductor wafer or that of a flat panel display, in the examples described. 
     Referring to FIG. 1, a general vacuum processing chamber  11  formed by an enclosure  13  is schematically illustrated. A substrate  15  being processed within the chamber is supported horizontally or vertically by a structure appropriate for the substrate and type of processing, the support in this case being a chuck  17  upon which the substrate rests in a horizontal position. The substrate  15  is typically heated in some fashion, a radiant heater  19  being shown. In some processes, the chuck  17  is cooled by circulation of water or some other coolant through it from an outside water supply  21  that includes refrigeration to cool the water. A vacuum pump  23  lowers the pressure within the chamber  11 . Many processes involve the introduction of one or more gasses into the chamber  11 , an external supply  25  of such gas(es) being shown. Specific processing elements  27  within the chamber  11  vary depending upon the process being performed. Chemical vapor deposition (CVD) and physical vapor deposition (PVD), such as sputtering and vaporization, are among the processes wherein the temperature measurement techniques of the present invention have application. Substrates are loaded into and unloaded from the chamber  11  through a load lock  29 . 
     In the example of FIG. 1, the chuck  17  is provided with a temperature sensor  31  that contacts an underside of the substrate  15 , when lying on the chuck, to measure the temperature of the contacted surface. An optical signal of the sensor  31  is coupled to a photdetector  35  by an optical communication medium  33  which can be a wave guide in the form of an optical fiber, other form of light pipe or a hollow wave guide. An electrical signal output of the photodetector  35  is received by a measuring circuit card or instrument  37  to provide an output signal  39  of the measured temperature. This signal can be used for a number of purposes, such as to drive an indicator (not shown) that provides a human operator with the temperature information that enables he or she to make adjustments to the heater  19  or other aspects of the processing. Alternatively, the signal  39  can be used by a control system (not shown) of the processing chamber in a feedback loop to control the heater  19  or other processing element. 
     The optical temperature measuring element of the sensor  31  may be a luminescent material that has some aspect of its excited electrical radiation highly temperature dependent. Measurement of the decaying characteristics of the luminescent radiation output signal is usually preferred, as described in the patents discussed in the Background section above. When a luminescent sensor is employed, an excitation source  36  and beam splitter  34  are added to the configuration of FIG.  1 . An alternative sensor element is a non-luminescent surface of known emissivity that emits electromagnetic radiation with a magnitude proportional to its temperature, as previously described. Other potential optical temperature measuring techniques include monitoring the frequency of the band edge of a semiconductor element, the absorption of incident radiation by an element of temperature dependent transmission and the color of a material that changes with temperature. 
     A general form of sensor  31  is illustrated in FIGS. 2 and 3. A light wave guide  41 , such as an optical fiber, other form of light pipe or hollow wave guide, is held fixed within the chuck  17 . A cap  43  of material having a high degree of thermal conductivity is positioned within an aperture  45  and held by a resilient element  47  a distance away from an end of the wave guide  41 . The cap  43  normally extends a short distance above the upper surface of the chuck  17 , as shown in FIG. 2, but the resilient element  47  that holds the cap in that position has a strength that is matched to the weight of the substrate  15  so that the cap is urged downward into the opening  45  when the substrate  15  is laid on the chuck  17 . A substrate contacting end of the cap  43  has at least a significant portion of its surface formed in a mating shape to that of the surface being measured in order to form a close contact with that surface. That shape in this case is planar. The cap  43  is also allowed to rotate within some limit with respect to the fixed wave guide in order to facilitate its mating surface being orientated in close contact with the substrate surface as the substrate is lowered onto the chuck  17 . 
     The cap  43  may be made of a very thin heat conducting metal, such as nickel, whose substrate contacting end does not deform in shape during normal use. In this general example, the cap  43  has a cylindrical shape in plan view, a cross-sectional side view being shown in FIGS. 2 and 3. In addition to serving to contact the substrate, the cap  43  is a carrier of the optical temperature sensing element. In FIGS. 2 and 3, this element is a layer  49  of luminescent material that is attached to an inside surface of the cap  43 . An optically transparent cover  51  is usually used to seal the luminescent material layer  49  from out-gassing that can result from use in a very low pressure chamber. Gasses escaping from the luminescent material can interfere with the processing. The cover  51  may be made from sapphire, for example, since it is a very stable and inert material. Similarly, a sapphire cover  53  may be attached to the end of the wave guide  41  to prevent out-gassing of the wave guide materials. However, if the wave guide is itself made of sapphire, this is not necessary. Although the cap form of the carrier for the luminescent material is preferred, alternate carrier shapes are also possible. 
     Four different specific embodiments of the sensor generally shown and described with respect to FIGS. 2 and 3, are shown in FIGS. 4-8, wherein the same reference numbers are used for corresponding elements. Each sensor is shown in the form of a cartridge having an outer housing  55  with an outside shape that is suitable for its intended application. The entire unit is then inserted into a mating aperture of the chuck  17  or other element in which it is installed. The outside shape of the housing  55 , and thus the mating aperture of the chuck  17 , can be cylindrical (as shown), square or any other suitable shape. The housing preferably has an outwardly extending flange  57  that positions the sensor within the chuck in an axial direction. 
     In the embodiment of FIG. 4, the cap  43  is shaped to provide a ledge  59  against which a spring  61  (the resilient element  47 ) urges the cap upward. That same ledge also abuts a ledge  63  around the opening in the housing  57  through which the cap  43  extends, thereby constraining maximum movement of the cap  43  out of the housing. When the substrate  15  pushes against the end surface of the cap  43 , the cap is pushed downward into the opening  45  against the force of the spring  61 . In order to make sure that the cover  51  does not touch the end cover  53  of the optical fiber when the cover is pushed into the opening by the weight of a substrate, and thus limit its travel, the distances are made sufficient so that this does not occur. A void exists between the covers  51  and  53  at all times. 
     A difference with the embodiment of FIG. 5 is that the resilient element is formed as part of the cap. The cap  43 , instead of cylindrically shaped side walls, includes integral fingers  61 ,  62  and  63  that bend to cause their lower terminations to spread horizontally as the cap is pushed downward into the opening  45  when urged against a substrate surface. When not pushed downward, these fingers  61 ,  62  and  63  hold the surface contacting end of the cap  43  above the upper surface of the housing flange  57 . 
     The embodiment of FIG. 6 also uses a cap that has the resilient element formed in its side walls. In this case, the side walls are a bellows that allows the exposed end of the cap  43  to be pushed into the opening  45 . An end  65  of the integral cap structure is conveniently made to fit onto a mating boss formed as part of the housing  55 . The two mating surfaces may be held together by a layer of glue between them. The shape of the mating surfaces may be cylindrical (as shown), square or any other shape that is suitable for a particular application. The end of the wave guide thus extends into the interior of the bellows, again with space between the wave guide and the inside of the cap being maintained even when the cap is pushed downward by contact with the surface being measured. Such an open end bellows cap element, made of nickel, that is suitable for this application is available from Servometer Company. The housing  55  is preferably machined or molded as a single piece from polyamide-imide, this material being available from the General Electric Company. When the cap  43  is of a unitary, gas impermeable structure, and its open end is sealed to the housing  55 , the covers  51  and  53  may be omitted. 
     In the embodiment of FIG. 7, the fingers of the embodiment of FIG. 5 are extended substantially horizontally and provided with folds similar to those of the bellows in the embodiment of FIG.  6 . The ends of these fingers are attached by glue to the housing  55 . As the top surface of the cap is pushed downward, as with the bellows of the FIG. 6 embodiment, the folds of the fingers move closer together but return to their uncompressed state shown in the drawings when that force is removed from the cap. 
     FIGS. 8 and 9 show a modification of the sensors of FIGS. 2-7 where a blackbody surface is substituted for the luminescent material layer  49  as the temperature sensor. This is desirable when the range of temperatures being measured is higher than that which can be measured by luminescent materials. In FIG. 8, a layer  71  of material of a known, controlled surface emissivity is applied to an inside surface of an end of the cap  43 ′. This emissivity is preferably made to be high, in a range of 0.8 to 1.0, where 1.0 is the emissivity of a black body. The layer  71  can most simply be a paint that is applied to the inside of the cap. Alternatively, the layer  71  is omitted if the material of the cap  43 ′ is selected to have a known emissivity of its surfaces that is high enough for practical use. 
     In the sensor of FIG. 9, the inside surface of the cap  43 ″ is altered to include a number of cavities  73 , preferably conical in shape, that simulate the emissivity of a black body. With either of the sensors of FIGS. 8 or  9 , it is the intensity of emissions of the surface in the infrared range that are detected by the photo-detector  35  of FIG.  1  and measured by the system  37 . The excitation source  36  and beam splitter  34  of FIG. 1 are not used. The intensity is proportional to the temperature of the surface that is emitting the infrared radiation. 
     A preferred form of a cartridge sensor according to any one of FIGS. 4-9 is shown in FIG. 10. A sensor  81  includes an outer housing  55 ′ like the housing  55  of FIGS. 4-7 but with threads  83  added to a portion of an outside surface. An opening in the chuck  17  is preferably configured to be completely filled by the sensor  81  and includes threads on an inside surface that mate with the threads  83  to firmly hold the sensor  81  in place within the chuck  17 . The threads are replaced with smooth mating surfaces, which are then glued together, when used within vacuum chambers in order to avoid pockets between the threads which can hold gases. Or, if threads are used in a vacuum application, the housing  55 ′ is sealed to the chuck  17  at its top surface to prevent the escape of such trapped gases into the processing chamber. An opening  85  extends through the chuck  17  from the opening receiving the sensor  81  as a conduit for the wave guide  41 . Rather than extending that wave guide continuously through the chuck  17 , however, it is terminated to form a short stub extending from the bottom of the sensor  83 . Another wave guide  87 , preferably in the form of an optical fiber, is inserted into the opening  85  to optically communicate with the wave guide  41  and extend to the detection and measurement equipment. A lens, as shown, is attached to the mating ends of each of the wave guide  41  and optical fiber  87  in order to more efficiently couple radiation between the two. An advantage of the configuration of the sensor  81  is that it can easily be installed and replaced in the chuck  17 . 
     The general form of the sensors described is shown in FIGS. 2 and 3 to operate with the substrate being carried directly by an upper surface of the chuck  17 . The temperature sensors described above also have other applications. In FIG. 11, for example, the substrate is held above the surface of the chuck  17  by posts  91  and  92 . A sensor  81 ′, like the sensor  81  but without the upper flange, extends above the chuck surface to position the sensor cap  43  above the dashed line that represents the lower surface of the substrate  15  when carried by the posts. The cap  43  is then pushed downward by the weight of the substrate when carried by the posts  91  and  92 , to make firm contact with the underside of the substrate. 
     FIG. 12 illustrates use of two or more sensors  81  to additionally provide support for the weight of the substrate  15 . In this case, the resilient element within the sensor is made stronger than before so that the cap  43  is not pushed within the housing of the sensor. Some small degree of compression of the resilient element and rotation of the cap are desired in order to make firm thermal contact with an underside of the substrate. 
     A different form of luminescent temperature sensor is shown in FIGS. 13 and 14. A test substrate  101 , preferably in the shape of a semiconductor wafer, flat panel display, or other substrate being processed, includes a temperature sensor  103  built into a substrate surface. A layer  105  of luminescent material is sealed within a recess of the substrate by an optically transparent window  107  made of an appropriate material such as sapphire. Excitation radiation is passed through the window to the luminescent material, and resulting temperature dependent luminescent radiation passes back through the window. 
     Interrogation of the sensor  103  occurs by positioning appropriate optics to communicate with it while the substrate  101  is positioned within the processing chamber  11  (FIG. 1) in the same manner as substrates that are being processed. An example is shown in FIG. 15, where the test wafer  101  is held by posts  91 ′ and  92 ′ above a chuck  17 ′. An optical fiber  33 ′, or other appropriate wave guide, terminates in an upper surface of the chuck  17 ′. The sensor  103  is within the field of view of the optical fiber  33 ′ when the test wafer  101  is properly positioned on the chuck  17 ′. Although use of posts  91 ′ and  92 ′ is shown, the wafer can be supported by the upper surface of the chuck  17 ′ with the sensor  103  being very close to, or in contact with, the end of the optical fiber  33 ′. In the course of processing a large number of substrates, such a test substrate is occasionally substituted for a substrate being processed in order to occasionally calibrate the substrate heating system within the chamber. 
     Although the various aspects of the present invention have been described with respect to exemplary embodiments, it will be understood that the invention is to be protected within the full scope of the attached claims.