Abstract:
Multiple sensors may be attached to a substrate to measure conditions at different points. Positioning the sensors accurately may be achieved using a template to establish sensor position. A pre-fabricated kit including a template, sensors and a cable assembly may be easily transported so that sensors may be attached in the field.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to positioning sensors with respect to substrates such as flat panel display glass substrates.  
         [0002]     Processing of substrates, such as flat panel display (FPD) substrates, printed circuit boards (PCBs) or silicon wafers, involves subjecting substrates to various process conditions such as high temperatures, radio frequency (rf) plasma, chemical etching and ion bombardment. To produce good quality and high yield products, these process conditions must be maintained to a very high degree of uniformity across the surface of the substrate. Modern flat panel display substrates are large thin sheets of glass. Such large substrates make uniformity particularly difficult to maintain. In order to obtain good uniformity, process conditions must be measured to determine the variation across a substrate so that the variation may be reduced. For example, the temperature at different points on a substrate may need to be known in order to adjust process conditions to achieve better temperature uniformity. One way to measure such conditions is by using a substrate that has sensors attached to it.  
         [0003]     Sensors may be mounted to a substrate to form an instrumented substrate that provides data on process conditions. Several sensors may be distributed on one or more surfaces of the substrate, or in recesses within the substrate, to create an array of sensors that monitors process conditions at various points. Such arrays of sensors are described by Renken et al., in U.S. Pat. No. 6,325,536, which patent is herein incorporated by reference in its entirety. Process conditions such as temperature, pressure, gas flow rates, chemical concentration, ion current density, position and acceleration may be monitored using such an array of sensors. Sensors may also be used for accurately measuring the physical location and orientation of a substrate.  
         [0004]     Sensors may be placed in recesses in the surface of a substrate and cemented in place. This provides a strong attachment to the substrate. For example a temperature sensor may be embedded in this manner to ensure a strong attachment and good thermal contact with the substrate. Cement, or potting compound, may be used to retain the sensor in the recess and to provide good thermal contact with the substrate. Attaching a sensor in this manner is generally done in a dedicated facility by trained technicians. A substrate may be sent to such a facility by a customer to be instrumented in this manner. The recesses are created and the sensors embedded at the facility. Then, the instrumented substrate is sent back to the customer. For smaller substrates such as silicon wafers, an instrumented substrate may simply be shipped to a customer by regular mail or courier. However, FPD substrates are large and fragile and require special packaging and handling.  
         [0005]     FPD substrates are rectangular with dimensions that may be greater than 2 meters. However, they are generally 1 millimeter thick or less. They are made of glass, which is brittle and is extremely fragile. This makes shipping difficult and expensive. Typically, a substrate is protected with impact resistant packaging material and is then enclosed within a shipping container such as a wooden crate. Even with such precautions, FPD substrates may be damaged in transit. After the substrate is instrumented with sensors at the facility and returned to the user, it is individually packaged with specially designed container. Transporting such large substrates may also be costly and time consuming, especially where international shipping is required.  
       SUMMARY OF THE INVENTION  
       [0006]     A sensor positioning system uses a template to establish the locations of sensor units being attached to a substrate. Such a template may be formed with cutouts in the desired locations of a flexible film of desired size to hold sensor units. Senor units may be inserted in the cutouts and held in place by adhesive tape. A cable assembly connecting to the sensor units may be run to an appropriate connector. An adhesive layer on the bottom surface of such a template allows it to be stuck to the substrate surface. A release layer protects the adhesive layer until it is used. When the template is aligned to a substrate and stuck in position, sensor units are individually bonded in place. Then, the template is removed. Such a system may be assembled and tested in a dedicated facility by trained technicians. It may then be rolled up and shipped easily. Using such a system to instrument a substrate does not require specialized skills or tools and may be done in a short time. The system allows a customer to instrument a substrate on-site without the cost, delays or risk of shipping a substrate.  
         [0007]     Another sensor positioning system uses visual alignment of sensor units to markings on a template. The template is placed behind the substrate where the markings are clearly visible through the substrate. A sensor kit contains sensor units with sensor leads wound around bobbins for shipping. Individual markings on the template identify individual sensor units so that each sensor unit may be placed at its predetermined location. Sensor units may be temporarily held in position until all the sensor units are positioned. Then, sensor units are individually bonded in position.  
         [0008]     Sensor units include sensors embedded in a chip of material and cemented in place. The chip material used may be the same material as the substrate or something similar to it. Embedding sensors in a chip means that it is not necessary to embed sensors in the substrate. Thus, the sensor units may be surface mounted to a substrate without the need to form recesses in the substrate. Sensor units may include recesses of different shapes including rectangular, circular and spiral shapes. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1A  shows a sensor positioning system that uses a template having cutouts to position individual sensor units.  
         [0010]      FIG. 1B  shows a sensor positioning system having wire clamps for one sensor lead or shared by multiple sensor leads, and having multiple sections of release film.  
         [0011]      FIG. 1C  shows a sensor unit within a cutout in a template in detail.  
         [0012]      FIG. 2A  shows a cross section of a sensor unit attached to a template.  
         [0013]      FIG. 2B  shows a sensor unit with bonding material applied.  
         [0014]      FIG. 2C  shows a sensor unit being bonded to a substrate.  
         [0015]      FIG. 2D  shows a sensor unit and substrate after template removal.  
         [0016]      FIG. 3A  shows a sensor unit having a rectangular recess.  
         [0017]      FIG. 3B  shows a sensor unit having a circular recess.  
         [0018]      FIG. 3C  shows a sensor unit having a spiral recess.  
         [0019]      FIG. 4  shows a sensor positioning system using visual alignment of sensor units.  
         [0020]      FIG. 5  shows a flowchart for fabrication of a system shown in  FIGS. 1A and 1B .  
         [0021]      FIG. 6  shows a flowchart for installation of an apparatus shown in  FIGS. 1A and 1B .  
         [0022]      FIG. 7  shows a flowchart for fabrication of a system shown in  FIG. 4 .  
         [0023]      FIG. 8  shows a flowchart for installation of the apparatus of  FIG. 4 . 
     
    
     DETAILED DESCRIPTION  
       [0024]      FIG. 1  shows one embodiment of a sensor positioning system  1  used to position sensor units  20   a -  20   j  on an FPD substrate (not shown). A template  10  is provided that is made of a thin sheet of a material. The material of template  10  can be a plastic or rubber material, for example Silicone, Mylar, polyethylene, polypropylene, polyester or Teflon. Template  10  may be flexible, thin and lightweight and should be particle free and non-contaminating. Template  10  may be transparent and may be colorless or may be colored so that it is easier to see for alignment. The dimensions of template  10  are selected to match the dimensions of a substrate that is to be instrumented. In this example, template  10  has the dimensions of an FPD substrate. However, in other examples, template  10  could have other dimensions. For example, a round template with a diameter of 300 mm could be used for 300 mm silicon substrates. One advantage of a thin flexible template is that it can be rolled up and may be easily transported in this rolled up configuration. Also, such materials are not easily damaged in transit.  
         [0025]     An aligning bracket  11  is provided to position template  10  with respect to a substrate. Bracket  11  is a solid part that provides a right angle to align a corner of template  10  with a corner of a substrate. Precise positioning of the sensors is desired and this depends on precisely aligning the template to the substrate. Aligning bracket  11  is simple to use and can provide the required level of alignment (less than 1 mm of error). Alignment may also be done by visually aligning template  10  with a substrate.  
         [0026]     Template  10  is coated with an adhesive coating  12  on one side. Adhesive coating  12  is comprised of an adhesive material that allows template  10  to be easily unstuck from a surface. Adhesive coating  12  does not form a permanent bond to a surface. It provides a tacky coating so that once adhesive coating  12  is pressed in contact with a smooth flat surface, template  10  does not slide across that surface. Adhesive coating  12  does not significantly adhere to sensor leads such as sensor lead  30   a . However, template  10  may still be removed easily by pulling in a direction perpendicular to the surface. Adhesive coating  12  does not leave significant material on the surface it contacts. Thus, a substrate that is contacted by adhesive coating  12  remains clean and does not have significant residue from adhesive coating  12 . An example of an adhesive that may be used for adhesive coating  12  is T-release made by Fralock. Template  10  is formed from a continuous sheet of material having a few cutouts such as  21   a - 21   j  for positioning. However, a template may also have openings other than those for positioning. Additional openings may be provided to reduce the weight of the substrate. Openings also prevent the formation of air bubbles during application of the template. Adhesive coating  12  may extend across the entire surface of template  10  or may cover portions of the surface forming tacky spots that hold template  10  in place.  
         [0027]     Release layer  15  covers adhesive coating  12 . Release layer  15  may be a plastic film. Release layer  15  protects adhesive coating  12  until it is used to adhere template  10  to a surface. Release layer  15  is easily removed.  FIG. 1A  shows a portion  6  of release layer  15  being peeled back from template  10 . Adhesive coating  12  is exposed where portion  6  of release layer  15  is peeled back. Release layer  15  may be peeled back by hand. Alternatively, a release layer tool may be used to remove release layer  15 . An example of such a tool is a roller that rolls up release layer  15  as the roller is rolled along the surface of template  10 .  
         [0028]     The configuration of sensor units  20   a - 20   j  is described with respect to an exemplary sensor unit  20   a . Cutout  21   a  extends through template  10 . Cutout  21   a  is made to accommodate sensor unit  20   a  so that the position of the sensor unit  20   a  is accurately established with respect to template  10 . There may be a gap between the edges of cutout  21   a  and sensor unit  20   a . However, the gap is generally small (less than 1 mm) so that the position of sensor unit  20   a  is tightly constrained within cutout  21   a .  
         [0029]     Tape  25   a  keeps sensor unit  20   a  within cutout  21   a . Tape  25   a  is on the opposite side of template  10  from adhesive coating  12 . Tape  25   a  overlies sensor unit  20   a  and portions of the surface of template  10  that are opposite adhesive coating  12 .  
         [0030]     Sensor lead  30   a  extends from sensor unit  20   a , through template clamp  40  and flat cable assembly  50 , to connector  60 . Sensor lead  30   a  consists of electrically insulated, flexible wire. Template clamp  40  is located within a cutout in template  10  and has a surface that is flat to allow it to be attached to a substrate surface. Template clamp  40  may be attached to a substrate, thus holding sensor lead  30   a  so that sensor lead  30   a  runs along the surface of a substrate and does not become kinked or snagged when the substrate is moved. Template clamp  40  also provides strain relief for sensor leads  30 . Sensor lead  30   a  passes through template clamp  40  and flat cable assembly  50  to connector  60 . Any tension in sensor lead  30   a  caused by flat cable assembly  50  or connector  60  is taken by template clamp  40 , not by sensor unit  20   a . This may be important where sensor units comprise relatively delicate structures. Template clamp  40  has a large flat surface so that good adhesion to a substrate is possible. Flat cable assembly  50  allows sensor leads  30  to pass from a process chamber to a connector that is external to the process chamber even where there is a large pressure difference between process chamber and the exterior (e.g. where process chamber is under vacuum). Sensor lead  30   a  may be made up of multiple wires. The number of wires in sensor lead  30   a  depends on the type of sensor unit  20   a . Typically, sensor lead  30   a  has two wires.  
         [0031]     Wire clamp  19   a  is between sensor unit  20   a  and template clamp  40 . Wire clamp  19   a  attaches sensor lead  30   a  to a substrate so that sensor lead  30   a  runs along the surface of the substrate where it is less likely to be damaged or become tangled or kinked.  FIG. 1A  shows an individual wire clamp between each of sensor units  20   a - 20   j  and template clamp  40 . Wire clamp  19   a  may consist of a chip of material with a channel in it. A sensor lead is bonded into the channel. The chip has at least one flat surface that may be bonded to a substrate surface.  
         [0032]      FIG. 1B  shows a sensor positioning system  2  in which wire clamps are shared by multiple sensor leads. For example, clamp  19   c  is shared by sensor leads  30   c  and  30   d . This is an alternative to the individual wire clamps for each lead shown in  FIG. 1A . However not all wire clamps in  FIG. 1B  are shared. For example, clamp  19   a  is only attached to sensor lead  30   a . In the embodiment of  FIG. 1B , the sensor positioning system  2  includes a release layer that is made up of sections  16 - 18 . This allows a portion of template  10  to be aligned and adhered first. Thus, section  16  of release layer  15  could be removed first and a portion of template  10  could be adhered before section  17  is removed.  
         [0033]      FIG. 1C  shows a more detailed view of the configuration of exemplary sensor unit  20   a  within cutout  21   a . The dotted line shows the outer perimeter of cutout  21   a . Sensor unit  20   a  is located within cutout  21   a  and is held in place by adhesive tape  25   a . Sensor lead  30   a  extends from sensor unit  20   a.    
         [0034]      FIG. 2A  shows a cross-sectional view of sensor unit  20   a  and portions of template  10  on substrate  90 .  FIG. 2A  shows template  10  after alignment of template  10  to substrate  90 . Template  10  is adhered to substrate  90  by adhesive coating  12 . Release layer  15  has been removed at this point so that adhesive coating  12  is in direct contact with substrate  90 .  
         [0035]     Sensor unit  20   a  includes a sensor  22   a  and chip  26   a . Sensor  22   a  is cemented into a recess  24   a  in chip  26   a  so that there is good thermal contact between sensor  22   a  and chip  26   a . Sensor lead  30   a  extends from sensor unit  20   a  between substrate  90  and template  10 . Chip  26   a  may be made from a variety of materials based on the desired properties. Here, sensor  22   a  is a temperature sensor, so thermal properties are important. In order to provide an accurate measurement of the temperature of the substrate, it is desirable to have the thermal properties of chip  26   a  close to those of substrate  90 . This is especially true where radiant heating is used and where the use of materials with different emissivities could affect heating and thus affect temperature readings. Therefore, chip  26   a  may be made of the same material as substrate  90 . Where conductive heating is used, it may be desirable to make a chip from a material that has a higher thermal conductivity than the substrate material in order to minimize temperature differences between the substrate and the chip.  
         [0036]     Overlying sensor unit  20   a  and portions of template  10  is an adhesive tape  25   a . Adhesive tape  25   a  holds sensor unit  20   a  in place within cutout  21   a . Adhesive tape  25   a  is easily removable from both template  10  and sensor unit  20   a . Alternatively, sensor unit  20   a  may be held in position by a mechanism such as a clip.  
         [0037]     Sensor unit  20   a  is shown having a gap around it separating sensor unit  20   a  from the sides of cutout  21   a . In practice, this gap is very small (one millimeter or less). The size of the gap is determined by the relative size of cutout  21   a  compared to sensor unit  20   a . It is desirable to keep the gap small so that the position of sensor unit  20   a  is established with a high degree of precision. The location of an individual sensor  20   a  is important because the reading from sensor  20   a  may be used by a software program to model values at other points. For example, temperature sensors located at known points on a substrate surface may be used to give a temperature map for the whole substrate by interpolation. Temperature uniformity values may also be obtained from analysis of the distribution of temperature readings. However, for such analysis to be accurate the positions of the sensors are important. This is especially true close to the edge of the substrate where there may be greater temperature differentials. The position of an individual sensor unit is determined by the position of the cutout that contains the sensor unit and the alignment of the template that contains the cutout. When a template is accurately aligned to a substrate, a sensor unit may be positioned to within +/−1 millimeter.  
         [0038]      FIG. 2B  shows sensor unit  20   a  in a raised position so that is clear of cutout  21   a . Sensor unit  20   a  may be raised by lifting adhesive tape  25   a  from template  10  and peeling back adhesive tape  25   a  from the end that is away from sensor lead  30   a . Sensor lead  30   a  remains attached to sensor unit  20   a  so sensor unit  20   a  may not be completely removed from template  10 . However, sensor lead  30   a  has sufficient slack to allow sensor unit  20   a  to be rotated from a horizontal position to a point where the lower surface  29   a  of sensor unit  20   a  is exposed. When lower surface  29   a  is exposed, a bonding material  27  is applied to it. Bonding material  27  may be a liquid cured to a solid. Ceramic cement may be used for temperatures above 400 degrees Centigrade, epoxy for low temperatures and silicone for mid-range temperatures of less than 300 degrees Centigrade. Bonding material  27  may be provided as part of a sensor positioning kit. Alternatively, a pressure sensitive double-sided adhesive tape may be used. The double-sided tape is covered by release layer  15  until release layer  15  is removed. Double-sided tape is limited to operating temperatures of less than about 250 degrees Centigrade.  
         [0039]      FIG. 2C  shows sensor unit  20   a  after bonding to substrate  90 . The bonding step involves replacing sensor unit  20   a  within cutout  21   a  and pressing sensor unit  20   a  against substrate  90 . Having an adequate gap between sensor unit  20   a  and cutout  21   a  facilitates lifting and replacing sensor unit  20   a . An inadequate gap makes manipulation difficult and may result in sensor unit  20   a  being bonded incorrectly. For example, sensor unit  20   a  may not be pushed to be in good contact with substrate  90  if a portion of template  10  is caught under part of sensor unit  20   a . A colored surface may be placed behind substrate  90  during positioning of template  10  and bonding. A dark colored surface (e.g. dark blue) provides a background that makes it easier to see bubbles formed between template  10  and substrate  90  and also makes it easier to see bonding material  27 . Obtaining good contact between sensor unit  20   a  and substrate  90  requires that bonding material  27  be a good thermal conductor and that the layer of bonding material  27  between sensor unit  20   a  and substrate  90  be thin. To obtain accurate results from sensor units  20   a - 20   j , it is desirable that individual sensor units  20   a - 20   j  are separated from substrate  90  by a similar thickness of bonding material  27 . In order to achieve this, particles of uniform size may be introduced into the bonding material. The thickness of bonding material  27  may be established by the particles in bonding material  27 . Bonding material  27  is generally cured for a period of time (e.g. 30 minutes) before it forms a strong bond. When bonding material  27  is fully cured then template  10  may be removed. Template  10  may be peeled back from substrate  90  to leave sensor units  20   a - 20   j  attached to substrate  90 .  
         [0040]      FIG. 2D  shows sensor unit  20   a  and substrate  90  after removal of template  10  and adhesive tape  25   a . Sensor unit  20   a  is bonded to substrate  90  by bonding material  27  so that there is good thermal conduction and the temperature of sensor unit  20   a  remains very close to the temperature of substrate  90  even during rapid temperature change. Bonding material  27  may be three bond, rescore  905  or similar commercially available bonding materials. Having a bonding material with high thermal conductivity reduces the variation in temperature readings caused by variation in bonding thickness. One way to achieve high thermal conductivity is to add a material that has high thermal conductivity to a bonding material. Examples of such a material are diamond powder, silver or aluminum powder. Sensor  22   a  is cemented in recess  24   a  so that sensor  22   a  accurately measures the temperature of chip  26   a  and substrate  90 . A portion of sensor lead  30   a  is also cemented in recess  24   a.    
         [0041]      FIG. 3A  shows sensor unit  20   a  in more detail. The perspective in  FIGS. 3A  is at 90 degrees from that of  FIG. 2D . The perspective in  3 A corresponds to a view from below in  FIG. 2D .  FIG. 3A  shows recess  24   a  in chip  26   a . Recess  24   a  extends from one end of chip  26   a , in a straight line, to near the opposite end of chip  26   a . Recess  24   a  is open at one end and is open along the bottom of chip  26   a . Sensor  22   a  and a portion of sensor lead  30   a  are located so that sensor  22   a  is within recess  24   a  and a portion of sensor lead  30   a  remains in recess  24   a . Recess  24   a  is filled with cement so that sensor  22   a  and the portion of sensor lead  30   a  within recess  24   a  are bonded to chip  26   a . There is a thin layer of cement between the sensor and the lower surface of the sensor unit  22   a  so that good thermal coupling is maintained between a substrate and sensor  22   a . Thus, sensor  22   a  is surrounded by cement, which in turn is surrounded by chip  26   a , except below and in the direction of sensor lead  30   a . When sensor unit  20   a  is bonded to a substrate as shown in  FIG. 2D , the bottom of sensor unit  20   a  is covered by the substrate and recess  24   a  is enclosed in all directions except in the direction of sensor lead  30   a . Because recess  24   a  is not enclosed in the direction of sensor lead  30   a , heat transfer in this direction may affect the accuracy of sensor  22   a . Sensor lead  30   a  includes electrically conductive portions that may also provide good heat conduction. Thus, it may be advantageous to keep a portion of sensor lead  30   a  cemented within recess  24   a  to reduce conduction of heat by sensor lead  30   a . Cementing a portion of sensor lead  30   a  also provides a strong mechanical connection so that sensor lead  30   a  and sensor  22   a  do not easily separate from chip  26   a.    
         [0042]      FIG. 3B  shows an alternative sensor unit  120 . In this example, a circular recess  124  in chip  126  is provided. Sensor  122  is at one side of circular recess  124  and portions of sensor lead  130  extend from sensor  122  on either side of circular recess  124  and extend outward from circular recess  124  opposite sensor  122 . This configuration provides increased mechanical strength and thermal conduction between sensor  122 , sensor lead  130  and chip  126 .  
         [0043]      FIG. 3C  shows another sensor unit  220 . In this example, a spiral recess  224  in chip  226  is provided. Sensor  222  is located at or near the center of spiral recess  224 . Sensor lead  230  extends from sensor  222  within spiral recess  224  and extends from sensor unit  220 . The spiral configuration may retain a longer portion of sensor lead  230  within spiral groove  224  than the configurations shown in  FIG. 3A and 3B . This configuration has two advantages. Firstly, it provides improved mechanical strength for the bond between sensor lead  230  and chip  226 . Secondly, it reduces the heat transfer effect of sensor lead  230  on sensor  222 .  
         [0044]     Sensor units  20   a ,  120  and  220  shown in  FIGS. 3A-3C  are alternative configurations that are principally used for temperature sensing. Sensors  22   a ,  122  and  222  may be thermocouples, thermistors, RTDs or other temperature sensors. However, sensors other than temperature sensors may also be used. Possible sensors include pressure sensors, gas flow rate sensors, humidity sensors and radiation dose sensors. Sensor units may be adapted to the process variable that they measure. Sensor units for various applications may be formed using a uniform chip size. A chip of uniform size and shape is well adapted to use with a template because cutouts in a template may then be of uniform size and production of both templates and chips may be automated. Also, having a sensor embedded in a chip provides some protection for the sensor and for the sensor lead that attaches to the sensor. A chip provides uniform attachment between sensors and the substrate. This may give more accurate measurements. Sensors of different types may be used in an array so that, for example, both temperature and pressure are measured by the same array. A single chip may contain more than one sensor so that an individual sensor unit may measure more than one process condition.  
         [0045]      FIG. 4  shows a sensor positioning system  403  employing visual alignment of sensor units. Substrate  490  is being instrumented in  FIG. 4 . Substrate  490  is placed in a position that provides access to the front surface  491  of substrate  490  that is to be instrumented. A template  410  is placed against the opposite surface  492  of substrate  490 . Template  410  has visible markings on its surface. Template  410  may simply be a sheet of paper that is the same size as substrate  490 . Template  10  may be rolled up or folded for transportation. Template  410  may be aligned to substrate  490  using an alignment tool or by visually aligning a corner of template  410  with a corner of substrate  490 . Visible markings are located at positions on the surface of template  410  that correspond to the desired positions of sensor units on substrate  490 . The markings may be simply darkened areas on a white surface.  FIG. 4  shows one such marking  470   a . Substrate  490  is a glass FPD substrate. Thus, marking  470   a  may be seen through substrate  490 .  
         [0046]     As part of sensor positioning system  403 , a sensor kit  411  is provided that includes sensor leads  430 . Sensor leads  430  run together from connector  460 , through flat cable assembly  450  and through template clamp  440 . From template clamp  440 , sensor leads  430  extend separately to bobbins  480 . Sensor leads  430  are individually wound around bobbins  480 . Sensor leads  430  terminate at sensor units (not shown).  
         [0047]     To use sensor positioning system  403  to instrument a substrate, template  410  is first aligned as described above. An individual sensor lead  430   a  is selected and is unwound from its corresponding bobbin. Sensor lead  430   a  terminates at sensor unit  420   a . The correct alignment mark  470   a  is located on template  410 . Sensor unit  420   a  is aligned to alignment mark  470   a  and is taped in position using an adhesive tape. This is repeated for sensor leads  430  until all sensor units are in position. Template clamp  440  is also aligned to a corresponding alignment mark on template  410  and taped in position. Ensuring that sensor units are positioned accurately to their corresponding alignment marks and are not mixed up is important to accurately analyze any data collected. Also, it is important that the sensors units are accurately placed with respect to their alignment mark. Either swapping sensors or inaccurately positioning sensors may cause errors in calculations based on sensor readings. An alignment mark  470   a  and a sensor unit  420   a  may be numbered or marked in some way to show that they correspond to each other. Sensor leads  430  may be trimmed to the correct length so that sensor units may not easily be misplaced. However, it is important that sensor leads  430  have sufficient slack to allow for thermal expansion of substrate  410  during heating. To facilitate this, sensor leads  430  may be wound into loops to keep sensor leads  430  from hanging loosely from substrate  410  while providing some slack.  
         [0048]     When the sensor units and template clamp  440  are positioned correctly they may be bonded in position in a process that is similar to the process described with respect to  FIGS. 2A-2D  above. An individual sensor unit  420   a  is lifted from substrate  490  by peeling back the adhesive tape holding it in position. Bonding material is applied to the bottom of the sensor unit  420   a  and sensor unit  420   a  is repositioned and pressed in place. The bonding material is allowed to cure and then the adhesive tape is removed. Template clamp  440  is also bonded in place in this manner. Additional wire clamps may also be bonded to substrate  490  to ensure that sensor leads  430  do not hang loose from substrate  490 .  
         [0049]      FIGS. 5-6  give a flowchart of the process of fabricating and installing the systems shown in  FIGS. 1A and 1B .  FIGS. 7-8  give a flowchart of the process of fabricating and installing the system shown in  FIG. 4 .  
         [0050]     Although the various aspects of the present invention have been described with respect to certain specific illustrative embodiments thereof, it will be understood that the invention is entitled to protection within the full scope of the appended claims.