Abstract:
A temperature probe has a light conductor for optically transmitting temperature information to a pyrometer. The light conductor has a first portion which is adapted to capture temperature information and a second portion which is connected to the pyrometer. The probe also has an enclosure for protecting the second portion of the light conductor. The enclosure in turn has a passageway for housing the second portion of the light conductor and an opening for projecting the first portion of the light conductor from the passageway to the outside of the enclosure. Additionally, a seal is provided in the passageway adjacent the opening to encapsulate the second portion of the light conductor inside the passageway.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a probe, and more particularly, to a probe for making temperature measurements of a semiconductor substrate. 
     Many semiconductor device manufacturing processes require a tight control of temperatures associated with a semiconductor wafer or substrate during processing to increase device performance and yield as well as to ensure process repeatability. In certain processes, if temperature differentials in the wafer rise above 1-2° C./cm at 1200° C., the resulting stress may cause slips in silicon crystals and may destroy potential semiconductor structures on the wafer. To avoid damage to the substrate and undesirable process variations, a precise temperature monitoring device for the substrate is needed. 
     One method for determining substrate temperature applies the principles of pyrometry. Pyrometers, or devices based on pyrometry, exploit the general property that objects emit radiation with a particular spectral content and intensity that is characteristic of their temperature. By measuring the emitted radiation, the object&#39;s temperature can be determined. In systems that incorporate pyrometers, a thermal reflector is positioned near the substrate to create a virtual black body cavity between the reflector and the substrate. Additionally, a temperature probe with a light pipe is used to sample radiation in the cavity through an aperture in the reflector. The sampled intensity is passed through an optical transmitter to the pyrometer where it is converted to temperature information. Further, to increase the precision of the temperature monitoring process, the emitted radiation intensity can be monitored via a plurality of temperature probes and pyrometers which monitor the localized regions of the substrate and perform appropriate conversions to obtain temperature. Temperature readings from various probes and pyrometers can be used for real-time control of heating elements in the rapid thermal processing (RTP) of substrates. 
     Conventional temperature probes typically use sapphire light pipes that pass through conduits which extend from the backside of a base of a process chamber through the top of a reflector. Although expensive, sapphire light pipes have relatively small scattering coefficients and tend to have greater transverse light rejection. These capabilities provide more accurate and localized measurements. Additionally, as sapphire is inert, light pipes made of sapphire do not suffer out-gassing problems. However, as sapphire light pipes are small (about 0.125 inch in diameter), they are relatively fragile components that can be easily chipped during handling. Chipped sapphire light pipes transmit less light to the pyrometers, resulting in inaccurate temperature readings which can adversely impact the operations of the processing equipment. 
     As costs associated with replacing chipped probes can quickly become a significant portion of the operating expenses, a durable, cost-effective temperature probe that can operate in a high temperature processing chamber is needed. 
     SUMMARY 
     A temperature probe provides a light conductor having first and second portions and an enclosure housing the light conductor. The enclosure provides a passageway for housing the second portion of the light conductor and an opening connecting the passageway to the exterior of the enclosure. The opening is adapted to project the first portion of the light conductor from the passageway. The enclosure also has a seal in the passageway for encapsulating the second portion of the light conductor in the passageway. 
     In one aspect, the light conductor is a pure silica fiber optic cable. In another aspect, the light conductor is a multi-mode fiber optic cable with a silica core and a cladding exterior. The cladding is stripped to form the first portion, while the cladding exterior remains on the second portion. Thus, for either aspect, the first portion is an exposed silica portion while the second portion may be the silica core or the cladding exterior. 
     In another aspect of the invention, a high temperature epoxy is used to seal the passageway. In yet another aspect, one or more O-rings may be mounted in the passageway to insulate the passageway from the exterior of the enclosure. Additionally, a ferrule may be positioned between the second portion and the walls of the enclosure to provide the sealing function. 
     Among the advantages of the invention are the following. The temperature probe is more durable as only a short segment of the light transmitter extends from the opening while the rest of the light conductor is protected inside the enclosure. Because the short segment is structurally supported by the enclosure, it is rendered more rigid and less prone to chipping. Further, the seal insulates the second portion inside the enclosure from the environment of the process chamber. The resulting temperature probe is less expensive, easier to handle and more rugged. 
     Other features and advantages will be apparent from the following description and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional side view of an RTP system; 
     FIG. 2 is an enlarged cross-section side view showing details of a probe of FIG. 1; 
     FIG. 3 is an enlarged cross-section side view showing details of another embodiment of the probe of FIG. 1; and 
     FIG. 4 is an enlarged cross-section side view showing details of a probe mounted in the system of FIG.  1 . 
    
    
     DESCRIPTION 
     In the following description, the term “substrate” is intended to cover broadly any object that is being processed in a thermal process chamber and the temperature of which is being measured during processing. The term “substrate” includes, for example, semiconductor wafers, flat panel displays, and glass plates or disks. 
     FIG. 1 shows an RTP system with a plurality of temperature probes  126 A,  126 B and  126 C. The RTP system includes a process chamber  100  for processing a substrate  106 . The substrate  106  is mounted inside the chamber  100  on a substrate support structure  108  and is heated by a heating element  110  located directly above the substrate. The heating element  110  generates radiation  112  which enters the process chamber  100  through a water-cooled quartz window assembly  114  above the substrate  106 . The backside of the quartz window  114  is ideally coated with an inert material which is transparent to thermal radiation in all but this narrow band of wavelengths, thereby reducing the likelihood that the heat source will introduce stray radiation into the reflecting cavity. 
     Beneath the substrate  106  is a reflector  102  which is mounted on a water-cooled, stainless steel base  116 . Reflector  102  may be made of aluminum and may have a highly reflective surface coating  120 . The underside of substrate  106  and the top of reflector  102  form a reflecting cavity  118  for enhancing the effective emissivity of the substrate. 
     The temperatures at localized regions  109  of substrate  106  are measured by the plurality of temperature probes  126 A,  126 B and  126 C. The temperature probe  126 A is positioned within a conduit  124 A so that its uppermost end is flush with or slightly below the top of the reflector  102 . The other end of the temperature probe  126 A is connected to a flexible optical fiber  125 A that transmits sampled light from the substrate  106  to a pyrometer  128 A. The remaining temperature probes  126 B and  126 C are similarly connected via optical fibers  125 B and  125 C through conduits  124 B and  124 C to pyrometers  128 B and  128 C, respectively. The output of the pyrometers  128 A,  128 B and  128 C are sensed by a digital controller  150 , which in turn drives the heating element  110  to appropriately adjust the temperature in the chamber  100 . In the described embodiment, each of the pyrometers  128 A- 128 C has a narrow bandwidth (e.g. about 40 nm) located at about 950 nm. 
     Referring back to the reflector  102 , the highly reflective multi-layered coating  120  is formed on top of the reflector  102 . The bottom layer of the coating  102  is a thin layer of gold, which is deposited onto the surface of the reflector body. Gold is preferred because it has a reflectivity of about 0.975 in the infra-red wavelength range of interest (i.e., about 950 nm). To further enhance the reflectivity of the gold layer, a quarter-wave stack is formed on top of the gold layer. The quarter-wave stack is made up of alternating dielectric layers which have different indices of refraction and has a thickness equal to one-quarter of the wavelength to which the pyrometer is most sensitive (e.g., one-quarter of 950 nm). If gold is an unacceptable material for reflecting purposes, other reflecting materials may also be used. Other types of suitable coatings are disclosed in U.S. application Ser. No. 08/845,931, filed Apr. 29, 1997, entitled “REFLECTOR HAVING A METALLIC BONDING LAYER FOR A SEMICONDUCTOR PROCESSING CHAMBER” and U.S. application Ser. No. 08/697,633, filed Aug. 28, 1996, entitled “REFLECTOR FOR A SEMICONDUCTOR PROCESSING CHAMBER”, both of which are assigned to the assignee of the present invention and hereby incorporated by reference. 
     The top layer of the multi-layered structure is a passivation layer, which prevents the gold of the reflecting layer from possibly contaminating the RTP chamber. The passivation layer may be made of silicon dioxide, aluminum oxide, silicon nitride, or any other acceptable material that will passivate the reflecting layer without degrading its reflective properties at the wavelength of interest. 
     The separation between the substrate  106  and reflector  102  may be approximately 0.3 inch (7.6 mm), thus forming a cavity which has a width-to-height ratio of about 27. In processing systems that are designed for eight-inch (300 mm) silicon wafers, the distance between the substrate  106  and reflector  102  is between 3 mm and 9 mm, and ideally between 5 mm and 8 mm. Moreover, the width-to-height ratio of cavity  118  should be larger than about 20:1. If the separation is made too large, the emissivity-enhancement effect that is attributable to the virtual black body cavity that is formed will decrease. On the other hand, if the separation is too small, e.g., less than about 3 mm, then the thermal conduction from the substrate to the cooled reflector will increase, thereby imposing an unacceptably large thermal load on the heated substrate. Since the main mechanism for heat loss to the reflector or reflecting plate will be conduction through the gas, the thermal loading will depend up the type of gas and the chamber pressure during processing. 
     During thermal processing, the support structure  108  can be rotated. Thus, each of probes  126 A- 126 C can sample the temperature profile of a corresponding annular ring area on the substrate  106 . The temperature indications associated with each probe  126 A- 126 C may be corrected according to the individual sensitivity to variations in emissivity associated with the probe location using methods such as that disclosed in U.S. patent application Ser. No. 08/641,477, entitled “METHOD AND APPARATUS FOR MEASURING SUBSTRATE TEMPERATURES”, filed on May 1, 1996, assigned to the assignee of the present invention, and hereby incorporated by reference. 
     The support structure which rotates the substrate includes a support ring  134  which contacts the substrate  106  around the substrate&#39;s outer perimeter, thereby leaving all of the underside of the substrate  106  exposed except for a small annular region about the outer perimeter. The support ring  134  may have a radial width of approximately one inch (2.5 cm). To minimize the thermal discontinuities that will occur at the edge of substrate  106  during processing, the support ring  134  may be made of the same, or similar, material as the substrate, e.g., silicon or silicon carbide. 
     The support ring  134  rests on a rotatable tubular quartz cylinder  136  that is coated with silicon to render it opaque in the frequency range of pyrometers  128 A- 128 C. The silicon coating on the cylinder  130  acts as a baffle to block out radiation from external sources that might contaminate the intensity measurements. The bottom of the quartz cylinder is held by an annular upper bearing race  141  which rests on a plurality of ball bearings  137  that are, in turn, held within an stationary, annular, lower bearing race  139 . The ball bearings  137  may be made of steel and coated with silicon nitride to reduce particulate formation during operation. An upper bearing race  141  is magnetically-coupled to an actuator (not shown) which rotates the cylinder  136 , the support ring  134  and the substrate  106  during thermal processing. 
     The support ring  134  is designed to create a light tight seal with the cylinder  136 . Extending from the bottom surface of the support ring  134  is a cylindrically shaped lip which has an outside diameter that is slightly smaller than the inside diameter of the cylinder  136 , so that it fits into the cylinder  136 , as shown, and forms a light seal. On the inside region of the support ring, there is a shelf for supporting substrate  106 . The shelf is a region around the inside circumference of the support ring that is lower than the rest of the support ring. 
     A purge ring  145  which is fitted into the chamber body surrounds the cylinder  136 . The purge ring  145  has an internal annular cavity which opens up to a region above upper bearing race  141 . The internal cavity is connected to a gas supply through a passageway. During processing, a purge gas is delivered to the chamber through the purge ring  145 . 
     The support ring  134  has an outer radius that is larger than the radius of the cylinder  136  so that it extends out beyond the cylinder. The annular extension of the support ring beyond the cylinder  136 , in cooperation with the purge ring  145  located below it, functions as a baffle which prevents stray light from entering the reflecting cavity at the backside of the substrate  106 . To further reduce the possibility of stray light reflecting into the reflecting cavity, the support ring  134  and the purge ring  145  may also be coated with a material that absorbs the radiation generated by heating element  110  (e.g., a black or grey material). 
     Referring now to FIG. 2, details of one of the temperature probes  126 A- 126 C deployed in FIG. 1 are shown. In FIG. 2, the representative temperature probe  126 A houses a light conductor  214  with first and second end portions  216  and  218 , respectively. The light conductor  214  has a diameter of about 0.4 inch. In one embodiment, the light conductor  214  can be a one millimeter wavelength, multi-mode, fiber optic cable available from 3M Corporation of West Haven, Conn. The multi-mode fiber optic cable has a core made of quartz silica and a cladding made of a sheath of a low-temperature polymer. Although the silica core can withstand the high temperature of the chamber  100 , exposing the polymer to the high temperature environment of the chamber  100  would result in an effect known as “clouding”, where the polymer degrades via evaporation and renders the fiber optic cable unusable. 
     To protect the fiber optic cable against the high temperature associated with the operation of the chamber  100 , the cladding of the fiber optic cable is stripped away to expose the fiber optic core. Specifically, about 0.1 inch of the cladding material is removed to expose the core material of the fiber optic cable. This core thus forms the first end portion  216 , which is eventually inserted through one of conduits  124 A- 124 C to capture temperature information from localized regions  109 . Further, the second end portion  218  remains protected in an enclosure  201 , as described below. In this manner, only the silica core is exposed to collect high intensity radiation from the heated substrate  106  while the polymer sheath is protected from the high temperature. 
     The enclosure  201  which houses the light conductor  214  is made up of a tip  202 , a body  208  and a tail  210 . The first end portion  216  of the light conductor  214  is adapted to be housed in the tip  202 , which has an inner passageway  204  where the first end portion  216  is received. The tip  202  is approximately 0.1 inch long with an outside diameter of approximately 0.08 inch. Further, the inner passageway  204  has a diameter of approximately 0.04 inch. 
     The tip  202  protrudes from the body  208 , which may be made of stainless steel. The body  208  may have an outside diameter of approximately 0.3 inch and a length of approximately 0.9 inch. Further, the body  208  may have a body passageway  209  which is linearly aligned with the inner passageway  204 . At the interface with the inner passageway  204 , the body passageway  209  may have a diameter of approximately 0.04 inch, while at the other end, the body passageway  209  has a diameter of about 0.06 inch. The increase in diameter for the passageway  209  provides flexibility in inserting and mounting of the light conductor  214 . 
     Once the light conductor  214  has been inserted and mounted nearly flushed against the end of the tip  202 , a sealing system is provided to protect the interior of the chamber  100  from contamination. An O-ring  219  may be mounted outside the junction between the tip  202  and the body  208 , or an O-ring  222  may also be mounted in the passageway  209 . Additionally, a ferrule  224  can be positioned at the beginning of the passageway  212  to provide another seal. The O-ring  219  and ferrule thus prevent contaminants from reaching the chamber  100  during operation of the equipment. Additionally, the O-ring  219  and ferrule  224  prevent contaminants from disrupting the optics associated with the light conductor  214 . 
     The O-rings and ferrule are made of Viton™ and are available from Bay Seal Company of Hayward, Calif. Alternatively, in place of the O-rings  219  and/or  222  and the ferrule  224 , a high temperature epoxy, such as EPO-TEK 370, available from Epoxy Technology, Billerica, Mass., may be used to fill the tip passageway  204  containing the cable  214  to secure the first portion  216  of the cable  214 . Moreover, the epoxy can be applied to the passageway  209  to secure the cable  214  within the body  208 . 
     Further, the body  208  is connected to the tail  210  with the passageway  212 . The passageway  212  houses the second end portion  218  of the light conductor  214 , which is eventually connected to one of the pyrometers  128 A,  128 B or  128 C. 
     Referring now to FIG. 3, a second temperature probe  220  is shown. In this embodiment, an all silica fiber optic cable  230  is enclosed in a tip  240  with a suitable high temperature epoxy. The silica fiber optic cable  230  is further protected by a housing  250 . The all silica fiber optic cable, such as a WFGE 1000/1100 HPN fiber assembly, is available from CeramOptec Inc. of East Longmeadow, Mass. As the all silica fiber optic cable does not have a polymer cladding which can degrade at high temperatures, the cable  230  only needs to be mounted inside the tip  240  and the housing  250  to protect it against physical damage. In FIG. 3, the temperature probe tip  240  is connected to a body  224 , which in turn is connected to a tail portion  226 . The tip  240 , body  224  and tail  226  are connected via passageways  221 ,  228  and  229 , respectively. 
     During assembly, the cable  230  is initially inserted through the tail  226  and the passageway  221 , and is fitted flushed against the open end of the tip  240 . Next, the fiber  230  may be secured by injecting a high temperature epoxy, as discussed above, into the passageway  221  of the tip  240 . Further, a low temperature epoxy may be injected into the passageway  228  of the body  224 . Alternatively, as discussed above, suitable O-rings and ferrule may be used in place of the epoxy to insulate the chamber  100  from contaminants. 
     Turning now to FIG. 4, the mounting of the temperature probe of FIG. 2 or  3  in the chamber  100  is illustrated. In FIG. 4, a temperature probe  126 A is inserted through a passage  307  that extends from the backside of the stainless steel base  116  through the top of the stainless steel base  116 . Further, a tip  202  of the probe  126 A passes through a countersink  304  and a reflector passage  302  in the reflector  102 . A locking nut  320  securely clamps the probe  126 A to the base  116 . An end portion  330  of the probe  126 A provides a connection to the flexible optical fiber  125 A that transmits light to the pyrometer  128 A. 
     Although fiber optic cables are used, light pipes can also be made from any other suitable tubular material having a highly polished reflective inner surface. Further, the light pipes can be made of any appropriate heat-tolerant and corrosion-resistant materials, such as quartz, that can transmit the sampled radiation to the pyrometer. 
     Other embodiments are within the scope of the following claims.