A TEMPERATURE CALIBRATION SUBSTRATE

Abstract: A temperature calibration substrate (10) for producing increased temperature measurement accuracy. The temperature calibration substrate includes cavity means (13) located below the surface of said substrate and thermocouple means (15) disposed in the cavity for measuring the temperature of the substrate. The cavity means includes a cavity opening, an inner perimeter, and a length. Heat transfer means is disposed in the cavity means between the thermocouple means and the inner perimeter of the cavity means for transferring heat from the substrate to the thermocouple means. The cavity means is shaped to allow the thermocouple means to lie in close proximity to the substrate, and the thermocouple wires (14) are positioned substantially adjacent the inner perimeter of the cavity means and traverse the length of the cavity means thereby enhancing heat transfer efficiency from the substrate to the thermocouple means (15).

Description: A TEMPERATURE CALIBRATION SUBSTRATE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates in general to sensor technology and more specifically to wafer temperature measurement and calibration for semiconductor device thermal fabrication processes. 2. Description of the Prior Art In the process of manufacturing semiconductor devices various reactions such as oxidation, annealing, and chemical or physical vapor deposition occur on the surface of material being processed. The physical properties of the material surface and surface films can vary greatly during such processing. Wafer temperature is a very influential parameter in controlling the physical properties of the material surface structure or films. As such, control of wafer temperature and uniformity of wafer temperature is a key parameter for achieving process control and uniformity. Wafer temperature control systems typically employ either of two wafer temperature sensing methods, namely non-contact method or contact method. Radiation thermometry or pyrometry is the principal method of direct non-contact temperature measurement but has several disadvantages. Chief among these disadvantages is the reliance on surface optical emissive properties which vary with temperature, doping level and film properties. There are several techniques of contact method temperature sensing. One common technique is the use of contact sensors which measure the temperature of a body that the wafer rests on such as a hot plate. However, in environments of moderate or high vacuum, the temperature difference between the body being sensed(hot plate) and the wafer is large enough to preclude accurate wafer temperature measurement. Another common technique is the use of a sensor which directly contacts the surface of the wafer. The major source of error associated with this technique is that there is often greater heat transfer between the wafer and the structure holdin the sensor in contact with the wafer than between the wafer and the sensor itself. The attachment of a thermocouple to a wafer is another method of contact temperature measurement. The attachment of a thermocouple to a wafer historically has been achieved either by bonding the thermocouple to the surface of th wafer or by imbedding it in the wafer. However, the impracticality of inserting thermocouples into production wafers limits the utility of thermocouple wafers in process control. O the other hand, the high accuracy of these thermocouple wafers allows them to be utilized to calibrate other contact and noncontact temperature sensors. More specifically, thermocouple wafers are used in equipment development, process diagnostics, i the qualification and requalification of process equipment and i system matching. While thermocouples bonded into wafers generally provide greater accuracy than other contact or noncontact sensors, there are sources of measurement error that must be controlled in orde to provide the highest measurement accuracy.. Poor heat transfer between the thermocouple junction and wafer causes inaccurate temperature measurement. Furthermore, the thermal mass of the thermocouple and bond material, and the limited thermal conductivity of the bond material can cause a lag between measured and actual temperature during thermal transients. Since thermocouple instrumented wafers have had limited commercial availability, an approach used by many semiconductor process and equipment engineers was to surface bond a thermocouple to a wafer. There were several limitations to this approach. One limitation was that the bond material had inadequate adhesion characteristics. As a result significant amount of bond material was utilized and this altered the absorptive and emissive characteristics of the wafer. Further, the bond material's thermal mass made its transient response lag the wafer's transient response significantly. In addition, the thermocouple was not adequately shielded from silicon and reacted to form suicides. Another approach to overcome some of these thermocouple wafer problems is disclosed in U.S. Pat. No. 4,787,551. In U.S. Pat. No. 4,787,551 a silicon ball is formed enclosing the bead of a type C thermocouple and the ball is then electron beam welded to the silicon wafer. However, there were limitations to this technique, namely inadequate cleanliness of the interface at the weld site. Furthermore, the poor bond strength resulted in a fragile assembly and caused a low production yield. Additionally, a secondary junction was created between the silicon ball and the thermocouple wires which decalibrated the thermocouple. In many processing systems this caused erroneous readings due to a region of steep temperature gradients near the wafer. Furthermore, the choice of thermocouple materials was limited to alloys of tungsten and rhenium because at high temperatures other TC materials react more rapidly with silicon to form suicides that propagate through the wire causing wire breakage. One approach to solving the problems associated with a thermocouple wafer was developed by the assignee of the present application. In this approach, a thermocouple was imbedded in a small blind re-entrant cavity and prevented from reacting with silicon by a ceramic film covering. The limitations of this approach were chiefly that the emissive and absorptive characteristics of the bond surface were different to that of the wafer surface. Additionally, the thermocouple's geometry in the cavity failed to maximize heat transfer into the thermocouple junction. Accordingly, improvements which overcome any and all of the problems are presently desirable. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an improved method and apparatus for measuring the temperature of a substrate. It is another object of the invention to provide an apparatus that limits the difference between emissive and absorptive characteristics of a bond surface and those of a substrate surface. It is a further object of the invention to provide is a thermocouple junction shaped in geometries that enhance heat transfer in the bond area. The present invention is a temperature calibration substrat for producing increased temperature measurement accuracy. The temperature calibration substrate includes cavity means located below the surface of said substrate and thermocouple means disposed in the cavity for measuring the temperature of the substrate. Heat transfer means is disposed in the cavity means between the thermocouple means and the inner perimeter of the cavity means for transferring heat from the substrate to the thermocouple means. The cavity means is shaped to allow the thermocouple means to lay in close proximity to the substrate, and the thermocouple means is positioned substantially adjacent the inner perimeter of the cavity means and traverses the length of the cavity means thereby enhancing heat transfer efficiency from the substrate to the thermocouple means. The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the following detailed description, serve to explain the principles of the invention: FIG. 1A and IB are a top view and side view, respectively, illustrating a temperature calibration wafer in accordance with the first preferred embodiment of the present invention. FIG. 1C is a top view illustrating a temperature calibration wafer in accordance with the first preferred embodiment of the invention. FIG. 2A and 2B are a top view and side view, respectively, illustrating a temperature calibration wafer in accordance with the second preferred embodiment of the present invention. FIG. 3A and 3B are a top view and side view, respectively, illustrating a temperature calibration wafer with a trench geometry re-entrant cavity in accordance with the third preferred embodiment of the present invention. FIG. 4A and 4B are a top view and side view, respectively, illustrating a temperature calibration wafer in accordance with the fourth preferred embodiment of the present invention. FIG. 4C is a cross-sectional view of a temperature calibration wafer in accordance with the fourth preferred embodiment of the present invention. FIG. 4D is a top view of a silicon cap and weld area of a temperature calibration wafer in accordance with the fourth preferred embodiment of the present invention. FIG. 5A and 5B are a top view and side view, respectively, illustrating a temperature calibration wafer in accordance with the fifth preferred embodiment of the present invention. DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to those embodiments. On the contrary, the invention is intende to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Referring to the drawings, FIG. 1A and 1C are top views of the first preferred embodiment of the invention. Wafer substrat 10 of FIG. 1A is an enlarged portion of wafer 1 shown in FIG. 1C. In the first embodiment, wafer substrate 10 contains within the substrate material a blind circular re-entrant cavity 13 with interior diameter 12 and cavity opening 16. A circular formatio of thermocouple wire 14 and thermocouple junction 15 is encapsulated in a re-entrant cavity 13. The encapsulation of thermocouple wire 14 and thermocouple junction 15 in the interio perimeter of a blind re-entrant cavity 13 provides near isothermal conditions in the region of thermocouple junction 15. FIG. IB is a side view illustrating thermocouple wire 14 projecting through cavity opening 16 and bond material 18. Bond material 18 is a ceramic and has minimal surface area in order to minimize the errors associated with the absorption and emissivit of infrared radiation by the bond material. The bond material 18 provides heat transfer means between wafer substrate 10 and thermocouple wire 14. Insulation coating 17 is formed on thermocouple wire 14 prior to the insertion of thermocouple wire 14 in re-entrant cavity 13. The insulation coating 17 sheaths and chemically isolates thermocouple wire 14 from the silicon of wafer substrate 10 and thereby prevents suicide formation at high temperatures. Thermocouple junction 15 is also enclosed and chemically isolated by insulation coating 17. Insulation coating 17 and bond material 18 may be the same or different materials. The process for forming the first preferred embodiment will now be described with reference to FIGS. 1A and IB. A hole is ground in wafer substrate 10 with a cutting tool, and the re¬ entrant cavity 13 is then ground using a high speed cutter. Thermocouple wire 14 is formed by welding two alloys of thermocouple wire by conventional techniques such as capacitive discharge welding. Typically, type K,R, S, and T thermocouples are fabricated and utilized in this invention. Next, thermocouple junction 15 and thermocouple wire 14 are sheathed in insulating coating 17. Insulating coating 17 is applied to thermocouple wire 14 and thermocouple junction 15 by the technique of dip coating. As will be apparent to those skilled in the art, other methods such as plasma spraying, chemical-vapor deposition (CVD) , or physical vapor-deposition (PVD) may alternatively be used. Additionally, alternatives such as small ceramic bobbins may also be used to isolate thermocouple wire 14 and thermocouple junction 15 from the silicon of wafer substrate 10. Thermocouple junction 15 along with thermocouple wire 14 are then inserted and seated in re-entrant cavity 13 of wafer substrate 10. Thermocouple wire 14 leads are projected through cavity opening 16. Cavity opening 16 is then filled with a bond material 18 consisting of a ceramic slurry. Bond material 18 is conformal to the geometry of re- entrant cavity 13 and cements thermocouple wire 14 and thermocouple junction 15 within wafer substrate 10. Further, bond material 18 has the same coefficient of thermal expansion a the silicon of wafer substrate 10 and is typically constituted from a mixture of Si02 and A1203. Referring to FIGS. 1A and IB, in this and all the following embodiments the diameter D of thermocouple wire 14 should be kep small with respect to the path length traversed in the various cavities described. The path length traversed is the distance from wire entrance into the cavity, shown as L2, plus the wire distance in the cavity to the junction of the thermocouple, show as Ll. A desirable aspect ratio of path length LI + L2 to wire diameter D should be greater than a ratio of 15 to 1. FIG. 2A and 2B are a top view and side view, respectively, illustrating the temperature calibration wafer in accordance with the second preferred embodiment of the invention. The elements in this embodiment are the same as those described in the first preferred embodiment, and depicted in FIG. 1A and IB with the exception of thermocouple wire 14. FIG. 2A depicts thermocouple wire 14 formed over a longer serpentine path which provides greater heat transfer efficiency in this embodiment. FIG. 2B shows thermocouple wire 14 sheathed by insulating coating 17 and projecting through bond material 18 in the same manner as described with respect to the first preferred embodiment. The process for forming the second preferred embodiment will now be described with reference to FIG. 2A. Thermocouple wire 14 is formed into a ribbon geometry prior to insertion into re- entrant cavity 13 to provide a greater contact area between the thermocouple wire 14 the substrate material. Re-entrant cavity 13 is formed and, thermocouple wire 14 and thermocouple junction 15 are bonded into it in the same manner as described with respect to the first preferred embodiment. Similarly, thermocouple junction 15 and insulation coating 17 are formed as described in the first preferred embodiment. Furthermore, wafer substrate 10 is formed and bond material 18 is utilized in the same manner as described with respect to the first preferred embodiment. A third preferred embodiment of the invention is depicted in FIG. 3A and 3B. In this embodiment wafer substrate 10 contains within the substrate material an elongated re-entrant cavity 35 with interior perimeter 30 and cavity opening 31. Thermocouple wire 34 projects through cavity opening 31 and bond material 36 as illustrated in FIG. 3A and 3B. Thermocouple junction 38 is the weld joining the two alloys of thermocouple wire 34. Both thermocouple wire 34 and thermocouple junction 38 are sheathed in insulating coating 32 which chemically isolates thermocouple wire 34 from the silicon of wafer substrate 10. Insulating coating 32 is formed on thermocouple wire 34 and thermocouple junction 38 prior to placement of thermocouple wire and thermocouple junction 38 in elongated re-entrant cavity 35. The function of the trench geometry of elongated re-entrant cavity 35 is to provide a longer interior path length and hence improve the heat transfer between the silicon of wafer substrate 10 and thermocouple wire 34. The process for forming the third preferred embodiment will now be described with reference to FIG. 3A. A cutting tool is placed on a region of wafer substrate 10 and wafer substrate 10 is ground to the requisite depth. The cutting tool is then traversed laterally to produce cavity opening 31 and the re¬ entrant cavity 35 in wafer substrate 10. Thermocouple wire 34 and thermocouple junction 38 are formed and inserted in re- entrant cavity 35 in the same manner as described with respect to the first preferred embodiment. Furthermore, insulating coating 32 and bond material 36 are both formed and utilized in the same manner as described with respect to the first preferred embodiment. FIG. 4A is a top view depicting a temperature calibration wafer in accordance with the fourth embodiment of the invention. Wafer substrate 10 contains within the substrate material an annular cavity 41 with outer diameter 40 and inner diameter 43. Thermocouple wire 44 and thermocouple junction 48 are located within annular cavity 41. Ceramic filler 45, depicted in cross- sectional view FIG. 4C, provides improved heat transfer between wafer substrate 10 and thermocouple junction 48. Insulating coating 42 sheaths and chemically isolates both thermocouple wire 44 and thermocouple junction 48. The Insulating coating 42 is formed on thermocouple wire 44 and thermocouple junction 48 prior to insertion of wire and junction into annular cavity 41. FIGS. 4B-4D illustrate the placement of cap 46 in the completed assembly. As shown in FIGS. 4B and 4D, the cap 46 is welded to substrate 10 in weld area 47 in accordance with this particular embodiment. The cap 46 covers a majority of annular cavity 41 and reduces the radiant surface area of ceramic filler 45. The reduction in the radiant surface area of ceramic filler 45 allows the surface emissive and absorptive characteristics of the annular cavity 41 region to more closely match those of the wafer substrate 10. This improves both the transient response and steady-state temperature measurement accuracy of the assembly. The process for forming the fourth preferred embodiment will now be described with reference to FIGS. 4A through 4D. Wafer substrate 10 is drilled with a core drill to form annular cavity 41 shown in FIG. 4A. Thermocouple wire 44, thermocouple junction 48, and insulating coating 42 are all formed and utilized in the same manner as described with respect to the first preferred embodiment. Thermocouple wire 44 and thermocouple junction 48, both sheathed in insulating coating 42, are seated in annular cavity 41. Annular cavity 41 is then filled with ceramic filler 45 which consists of a ceramic slurry. Cap 46 has similar or the same emissive and absorptive characteristics of wafer substrate 10 and is placed within cavity outer diameter 40 to cover the ceramic filler 45. The cap 46 is held in place by a bond material, or it is welded in place using conventional techniques such as laser welding, plasma welding or electron beam welding. The leads of thermocouple wire 44 project up through ceramic filler 45 and silicon cap 46 as illustrated in FIGS. 4B and 4D. Referring to FIG. 5A and 5B, a top view and side view, respectively, of a fifth preferred embodiment are shown. FIG. 5A illustrates a wafer substrate 10 with a cylindrical hole 52 and thermocouple wire 50. FIG. 5C shows insulating coating 51 which sheaths and chemically isolates thermocouple wire 50. Also illustrated in FIG. 5C is an elongated trench cavity 53 along the bottom side interior of wafer substrate 10. Thermocouple wire 50 runs along the length of elongated trench cavity 53 turns a right angle and projects up through cylindrical hole 52 and past the surface of. wafer substrate 10. Thermocouple junction 54 is the weld joining the two alloys of thermocouple wire 50. Cover plate 56 made of silicon is welded or bonded in stepped recess 55 on the bottom side of wafer substrate 10 to provide a bottom surface similar in emissivity and absorption to the silicon of wafer substrate 10. The advantage of this particular embodiment is that the radiant surface area of the insulating coating 51 is minimized and hence the surface emissivity and absorption in the region surrounding cylindrical hole 52 closely matches that of the silicon of wafer substrate 10. A further advantage of this particular embodiment is that the path length of thermocouple wire 50 is increased thus establishing improved thermal contact. The process for forming the fifth preferred embodiment will now be described with reference to FIGS. 5A through C. Cylindrical hole 52 shown in FIG. 5A and 5C is ground from the top surface of wafer substrate 10 through the bottom surface of wafer substrate 10 with a cylindrical cutting tool. Next, the cutting tool is moved to the bottom surface of wafer substrate 10 and placed in one cylindrical hole 52 to an appropriate depth. It is then traversed to the other cylindrical hole 52. Elongated trench cavity 53 is ground in this manner. Finally, the cutting tool is indexed and stepped recess 55 is ground on the bottom surface of wafer substrate 10. Thermocouple wire 50, thermocouple junction 54, and insulating coating 51 are all formed and utilized in the same manner as described with respect to the first preferred embodiment. Thermocouple wire 50 and thermocouple wire junction 54 are inserted in elongated trench cavity 53 from the bottom surface of wafer substrate 10. Each lead of thermocouple wire 50 is projected through a cylindrical hole 52 as shown in FIG. 5C. Cover plate 56 is then placed in stepped recess 55. Cover plate 56 is attached to the bottom surface of wafer substrate 10 by a bonding material or by using conventional welding techniques such as laser welding, plasma welding, or electron beam welding. A few preferred embodiments have been described in detail hereinabove. It is to be understood that the scope of the invention also comprehends embodiments different from those described, yet within the scope of the claims. Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention should be limited not by the specific disclosure herein, but by the appended claims.

Claims: What is claimed is: 1. A temperature calibration substrate for producing increased temperature measurement accuracy, comprising: cavity means located below the surface of said substrate, said cavity means having a cavity opening, an inner perimeter, and a length; thermocouple means disposed in said cavity for measuring the temperature of said substrate; and heat transfer means disposed in said cavity means between said thermocouple means and said inner perimeter of said cavity means for transferring heat from said substrate to said thermocouple means, said cavity means shaped to allow said thermocouple means to lay in close proximity to said substrate, said thermocouple means being positioned substantially adjacent said inner perimeter of said cavity means and traversing said length of said cavity means thereby enhancing heat transfer efficiency from said substrate to said thermocouple means. 2. A temperature calibration substrate as in claim 1 wherein said thermocouple means has a diameter and wherein the aspect ratio of said length of said cavity means is substantially greater than said diameter of said thermocouple means. 3. A temperature calibration substrate as in claim 2 wherein the aspect ratio of said length of said cavity means to said diameter of said thermocouple means is approximately fifteen to one. 4. A temperature calibration substrate as in claim 3 wherein said cavity opening is covered with a cap for covering said heat transfer means, and wherein said cap has the same emissive and absorptive characteristics of said surface of said substrate. 5. A temperature calibration substrate as in claim 4 wherein said thermocouple means comprises a thermocouple wire an a thermocouple junction, and wherein said thermocouple wire extends through the surface of said cap. 6. A temperature calibration substrate as in claim 5 wherein said heat transfer means includes: an insulation coating formed around said thermocouple means for preventing said thermocouple means from chemically reacting with said substrate; and a bond material formed between said insulation coating and said substrate, said bond material having the same coefficient of thermal expansion as said substrate.

Inventors: RENKEN WAYNE, RENKEN, WAYNE
Applicants: SENSARRAY CORP, SENSARRAY CORPORATION
Application year: 1995
Main classification: G01K13
IPC codes: G01K1, G01K7, G01K13