Patent Publication Number: US-2007095812-A1

Title: In-situ wafer parameter measurement method employing a hot susceptor as a reflected light source

Description:
RELATED APPLICATIONS  
      This is a continuation of U.S. patent application Ser. No. 10/202,498, filed Jul. 23, 2002, which claims benefit of U.S. Provisional Patent application No. 60/307,423, filed Jul. 23, 2001. 
    
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
      Not applicable.  
     TECHNICAL FIELD  
      This invention relates to radiometric temperature measurement systems (also known as “pyrometers”) and more particularly to a method employing measuring the change in hot susceptor radiation reflected by a target medium as it is moved into contact with the hot susceptor.  
     BACKGROUND OF THE INVENTION  
      Pyrometer-based temperature measurement systems have a long development history. For example, even before  1930 , U.S. Pat. Nos. 1,318,516; 1,475,365; and 1,639,534 all described early pyrometers. In 1933, U.S. Pat. No. 1,894,109 to Marcellus described a pyrometer employing an optical “lightpipe.” In 1955, U.S. Pat. No. 2,709,367 to Bohnet described a pyrometer in which sapphire and curved sapphire lightpipes are used in collection optics. In 1971, U.S. Pat. No. 3,626,758 to Stewart described using quartz and sapphire lightpipes with a blackbody sensor tip. Then in 1978, U.S. Pat. No. 4,075,493 to Wickersheim described a modem flexible fiber optic thermometer.  
      In the 1980s, U.S. Pat. No. 4,348,110 to Ito described electronic improvements to pyrometers, such as an integrating photo-detector output circuit. Then U.S. Pat. Nos. 4,576,486; 4,750,139; and 4,845,647, all to Dils, described further improvements to electronics, fiber-optics, sapphire rods, and blackbody emission temperature measurements.  
      In the 1990s, many patents issued that describe the use of pyrometers in semiconductor processing. For example, in 1990, U.S. Pat. No. 4,956,538 to Moslehi described using fiber optic lightpipes for wafer temperature measurements in rapid thermal processing (“RTP”) applications. In 1992, U.S. Pat. No. 5,154,512 to Schietinger described using a fiber optic thermometer with wavelength selective mirrors and modulated light to measure semiconductor wafer temperatures. In 1998, U.S. Pat. No. 5,717,608 to Jenson described using an integrating amplifier chip and fiber-optics to measure semiconductor wafer temperatures, and U.S. Pat. No. 5,815,410 to Heinke described an infrared (“IR”) sensing thermometer using an integrating amplifier. Then in 1999, U.S. Pat. No. 5,897,610 to Jensen described the benefits of cooling pyrometers, and U.S. Pat. No. 6,007,241 to Yam described yet another fiber optic pyrometer for measuring semiconductor wafer temperatures.  
      As one can see from these prior patents, pyrometer systems are commonly used for measuring the temperature of semiconductor silicon wafers housed within a process chamber while forming integrated circuits (“ICs”) on the wafer. Virtually every process step in silicon wafer fabrication depends on the measurement and control of wafer temperature. As wafer sizes increase and the critical dimension of very large scale ICs scales deeper into the sub-micron range, the requirements for wafer-to-wafer temperature repeatability during processing become ever more demanding.  
      Processes such as physical vapor deposition (“PVD”), high-density plasma chemical vapor deposition (“HDP-CVD”), epitaxy, and RTP can be improved if the wafer temperature is accurately measured and controlled during processing. In RTP there is a special importance to temperature monitoring because of the high temperatures and the importance of tightly controlling the thermal budget, as is also the case for Chemical Mechanical Polishing (“CMP”) and Etch processes.  
      As wafer sizes increase, the cost of each wafer increases geometrically, and the importance of high quality in-process temperature monitoring increases accordingly. Inadequate wafer temperature control during processing reduces fabrication yields and directly translates to lost revenues.  
      In addition to conventional pyrometry, the most common in-situ temperature sensing techniques employed by semiconductor processing wafer fabs and foundries also includes thermocouples and advanced pyrometry.  
      Thermocouples are easy to use, but their reliability and accuracy are sometimes questionable because of measurement delays. Thermocouples are only accurate when the wafer is in thermal equilibrium with its surroundings and the thermocouple is contacting or embedded in that environment. Otherwise, the thermocouple reading might be far from the correct wafer temperature. For example, in PVD applications, while the thermocouple embedded in the heated chuck (susceptor) provides a temperature measurement that resembles that of the wafer, there are large offsets between the wafer and the thermocouple. These offsets are a function of gas pressure and heat transfer. Despite delays, thermocouples often provide a good measurement of the hot susceptor temperature.  
      In conventional optical pyrometry, a pyrometer deduces the wafer temperature from the intensity of radiation emitted by the wafer. The pyrometer typically collects the radiation from the wafer through an interface employing a lens or a quartz or sapphire rod. Such interfaces have been used with PVD, HDP-CVD, RTP, and Etch. While conventional optical pyrometers are often superior to the use of thermocouples, there are measurement inaccuracy problems caused by background light, wafer transmission, emissivity, and signal-to-noise ratio.  
      Advanced pyrometry offers some satisfactory temperature monitoring solutions for semiconductor wafer production applications. “Optical Pyrometry Begins to Fulfill its Promise,” by Braun, Semiconductor International, March 1998, describes advanced pyrometry methods that overcome some limitations of conventional pyrometry. As such, optical pyrometers and fiber optic thermometers employing the Planck Equation are now commonly used for in-situ semiconductor wafer measurement. However, numerous problems and limitations are still encountered when measuring wafer temperature using “Planck” radiation (light) emitted by the wafer. There are numerous problems when measuring wafers at temperatures below about 400° C.: 1) minimal signal levels generated by the photo detector because the very small amount of radiation emitted by the wafer; 2) the wafer is semi-transparent at low temperatures and long wavelengths (greater then 900 nm); and 3) the background light is often larger than the emitted wafer signal and causes large errors when it enters the collection optics. Moreover, the often unknown emissivity of the object being measured increases the difficulty of achieving accurate temperature measurements.  
      What is still needed, therefore, is an advanced pyrometer system and measurement method that provides accurate and repeatable temperature measurements of an object, such as a semiconductor wafer, down below 400° C. and ideally to about room temperature without contacting the object being measured.  
     SUMMARY OF THE INVENTION  
      An object of this invention is, therefore, to provide a method for performing non-contacting temperature measurements of target media, such as semiconductor wafers.  
      The measurement method of this invention takes advantage of the fact that the susceptor (wafer chuck or wafer holder) is at a known temperature and, therefore, emits a known amount of light (radiation). The hot susceptor emission is well-known because of the known temperature of the susceptor and the Planck equation. The known amount of light is used to determine the wafer reflectivity by measuring how much of the light reflects off the wafer. Wafer emissivity is then calculated by applying Kirchoff&#39;s law (1 minus reflectivity equals emissivity), which is valid because of the wide field-of-view of the radiometric system lightpipe employed and the near hemispherical emission pattern from the hot susceptor.  
      Because of the well controlled geometry and known optical conditions, the wafer surface roughness can also be calculated from the change in reflected intensity. The amount of reflected light changes as a function of wafer roughness and illumination angle. The illumination angles change because the geometry changes as the wafer is lowered toward the hot susceptor. Wafer roughness is determined by running a set of test wafers each having a different roughness and plotting the light levels as a function of distance and roughness. After the test wafers have been run, the in-situ wafer roughness can be determined in real time.  
      An advanced pyrometer system suitable for use with this invention has reduced optical losses, better background radiation blocking, improved signal-to-noise ratio, and improved signal processing to achieve improved accuracy and temperature measurement capabilities ranging from about 10° C. to about 4,000° C.  
      The pyrometer system includes collection optics that acquire radiation and directly couples it to an optional filter and/or a photo detector. The collection optics may include lens systems, optic lightpipes, and flexible fiber optics. The preferred collection optic is a yttrium-aluminum-garnet (“YAG”) light guide rod. The photo detectors are formed from silicon, InGaAs or, preferably, doped AlGaAs having narrow bandpass detection characteristics centered near 900 nm.  
      The system further includes an amplifier that acquires and conditions signals as small as 10 −16  amperes for detection and measurement. A signal processor converts the amplified signal into a temperature reading. This processing is a combination of electrical signal conditioning, analog-to-digital conversion, correction factors, and software algorithms, including the Planck equation.  
      In a preferred measurement method, a cool semiconductor wafer is moved into position a distance spaced apart from a heated susceptor. As it is moved into position, the cool semiconductor wafer emits a relatively small amount of emitted radiation, which is preferably, but not necessarily sensed by a radiometric system of this invention. The emitted radiation increases when the semiconductor wafer is heated during subsequent lowering toward the hot susceptor. Before lowering the semiconductor wafer, the emitted radiation from the hot susceptor that is reflected by the semiconductor wafer as reflected radiation provides a baseline radiation measurement for comparing with measurements taken during the subsequent downward motion of the semiconductor wafer.  
      When the semiconductor wafer is moved into the process chamber and above the hot susceptor, the emission (light) from the susceptor is reflected off the wafer and into the radiometric system. The amount of susceptor emission is known from its temperature and the Planck equation. The amount of reflected light is measured by the radiometric system in real time as the distance between the susceptor and the wafer diminishes, that is, as the wafer and/or the hot susceptor are moved toward one another and eventually into contact. The change in reflected light level under the well known and controlled geometric conditions, provides the necessary parameters for determining wafer reflectivity, emissivity, and roughness.  
      Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof that proceed with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a combined pictorial and corresponding schematic block diagram of a pyrometer system suitable for use with this invention.  
       FIG. 2  is a simplified electrical block diagram of the electronic circuitry portion of the pyrometer system of  FIG. 1 .  
       FIG. 3  is a simplified pictorial view of a prior art optical pyrometer employing a first lens for collimating radiation through a filter and a second lens for focusing the filtered radiation on a silicon detector.  
       FIG. 4  is a simplified pictorial view of an optical pyrometer suitable for use with this invention employing a single lens for focusing radiation on a wavelength selective AlGaAs detector.  
       FIG. 5  is a simplified pictorial view of a prior art pyrometer system employing optical fiber cables to couple emitted radiation to detectors.  
       FIG. 6  is a simplified pictorial view of a pyrometer system suitable for use with this invention employing direct coupling of emitted radiation to detectors.  
       FIG. 7  is a sectional side view of a prior art light guide rod and detector mounting system in which the optical faces of the light guide rod and detector are recessed within threaded housings making cleaning difficult.  
       FIG. 8  is a sectional side view of a light guide rod and detector mounting system suitable for use with this invention in which the optical faces of the light guide rod and detector are flush to the edges of their respective housings and, therefore, easy to clean.  
       FIG. 9  is a graphical representation of a radiation transmission response as a function of wavelength for a reflective filter suitable for use with this invention.  
       FIG. 10  is a simplified schematic pictorial view of a pyrometer system suitable for use with this invention employed in a typical semiconductor process temperature measurement application.  
       FIG. 11  are graphs representing the transmission of radiation through a silicon wafer as a function of wavelength and temperature.  
       FIGS. 12A and 12B  is graphs representing respectively the optical density and transmittance as a function of wavelength and radiation incidence angles of a short wavelength pass filter suitable for use with this invention.  
       FIG. 13  is a graph representing the absorption coefficient of various detector materials as a function of wavelength.  
       FIG. 14  is a graph representing the photo sensitivity of various detector materials as a function of wavelength.  
       FIG. 15  is a graph representing photo sensitivity versus wavelength as a function of photo detector temperature.  
       FIG. 16  is a set of graphs representing wavelength shift as a function of temperature for typical infrared interference filters.  
       FIG. 17  is a simplified pictorial schematic diagram representing a semiconductor wafer temperature measurement method of this invention employing radiation emitted by a hot susceptor and reflected by the semiconductor wafer.  
       FIG. 18  is a simplified pictorial elevation view of an exemplary semiconductor wafer processing apparatus suitable for carrying out the temperature measurement method of this invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       FIG. 1  shows a radiometric system  10  that is suitable for use with the measurement methods of this invention and includes collection optics  12  for acquiring emitted radiation  14  from a target medium, such as an object  16 , which is preferably a semiconductor wafer. Collection optics  12  direct radiation  14  to a wavelength selective filter  18  and a photo detector  20 . Collection optics  12  may alternatively include rigid or flexible fiber optic light pipes and/or a lens system for measuring the temperature of predetermined areas on object  16 . The target medium may include gases, plasmas, heat sources, and other non-solid target media. While radiometric system  10  is preferred, virtually any pyrometer may be employed with the measurement methods of this invention.  
      Wavelength selective filter  18  selects which wavelengths of radiation  14  are measured. A preferred embodiment of filter  18  includes a hot/cold mirror surface  22  for reflecting unneeded wavelengths of radiation  14  back toward object  16 . Skilled workers will recognize that filter  18  and hot/cold mirror surface  22  should be housed to maintain them in a clean and dry condition.  
      Photo detector  20  converts radiation  14  into an electrical signal. Photo detector  20  can be a high efficiency solid-state detector device formed from silicon, InGaAs or a specially doped AlGaAs material having a narrow bandpass detection characteristic centered near or around 900 nm. Detector  20  is described in more detail with reference to  FIGS. 13 and 14 .  
      Radiometric system  10  further includes an amplifier  24  that receives the small electrical signal from photo detector  20  and amplifies the signal to a level suitable for further processing. Amplifier  24  allows measuring electrical signals as small as 10 −16  amps.  
      Radiometric system  10  further includes an analog-to-digital converter (“ADC”)  26  for converting the amplified electrical signal into a digital signal and a signal processor  28  for processing the digital signal into a temperature reading. The processing includes software algorithms employing the Planck equation.  
      Radiometric system  10  generates a high-speed digital output signal  30 , which can be viewed as temperature measurements on a personal or host computer running conventional user software or, preferably, a Windows®-based user software product named TemperaSure™, which is available from Engelhard Corporation, located in Fremont, Calif.  
      Radiometric system  10  further includes a generally tubular housing  32  that encloses at least photo detector  20 , amplifier  24 , ADC  26 , and signal processor  28 . Housing  32  is preferably at least about 2.54 cm (1 inch) in diameter and at least about 10.16 cm (4 inches) long. Of course, the shape and dimensions of housing  32  may vary to suit different applications.  
       FIG. 2  shows a block diagram of electronic circuitry  40  portions of radiometric system  10 , which circuitry is preferably included on a printed circuit board (not shown) that fits within housing  32 . Electronic circuitry  40  utilizes significantly smaller components and arrays them in a highly compact format such that the overall instrument size is reduced dramatically from prior pyrometers. This form factor enables direct coupling of photo detector  20  and electronic circuitry  40  to collection optics  12  and, therefore, eliminates the undesirable fiber cable often found in prior optical thermometers. Eliminating the fiber cable in semiconductor temperature measurement applications reduces optical losses and signal variations.  
      Electronic circuitry  40  preferably includes photo detector  20  and an array of IC chips for amplifying and integrating (or averaging) the electrical signal generated by photo detector  20 . Electronic circuitry further includes two or more temperature sensors  42  and  44  to monitor ambient temperatures of components, such as photo detector  20 , amplifier  24 , wavelength selective filter  18 , and a timing circuit  46 .  
      Compensating target temperatures based on information gained from sensors  42  and  44  accounts for deviations in component performance having differing temperature-dependent physical behaviors. For example, amplifier  24  gain changes with temperature as do the characteristics of photo detectors, analog to digital converters, timing oscillator crystals, and reference voltage or current sources. It is also beneficial to use an internal temperature sensor to monitor and compensate for the temperature of objects within the pyrometer system that occupy any part of the field of view (“FOV”) of the photo detector.  
      Electronic circuitry  40 , in combination with the techniques described herein, increases the signal-to-noise ratio of radiometric system  10  and allows temperature measurements to be made down to about 10° C. by measuring object emissions at or near 1,650 nm, and down to about 170° C. by measuring object emissions at slightly shorter than 1,000 mm. These conditions provide signal levels that have heretofore been too weak to measure accurately.  
      By comparison, the temperature measuring limits of prior optical radiometers operating at wavelengths shorter than 1,650 nanometers, with +5 degrees of noise, and a 1 Hz sampling bandwidth, is approximately 50° C. with a cooled/un-cooled indium gallium arsenide detector (“InGaAs”); or about 300° C. with a cooled/un-cooled silicon detector at 900 nm.  
      It should be noted that while the minimum temperature measuring limit is reduced by only a factor of two for the InGaAs detector and by a factor of about 1.6 for the silicon detector, the signal reduction at the detector is approximately a factor of 50 for the InGaAs detector and a factor of 3,000 for the equivalent silicon detector. This invention has enabled these minimum temperature measurement reductions through reducing optical losses, reducing or eliminating factors that cause signal level variations, and electronic signal processing improvements.  
      Regarding improvements that reduce optical losses,  FIG. 3  shows a prior art optical pyrometer  50  employing a first lens  52  for collimating radiation  14  through a wavelength selective filter  54  and a second lens  56  for focusing filtered radiation  58  on a conventional silicon detector  60 . Wavelength selective filter  54  transmits a desired radiation wavelength and blocks unwanted wavelengths. For example, long wavelength blocking filters block light at long wavelengths while transmitting short wavelengths of light. Unfortunately, filters do not transmit the desired radiation wavelengths with 100 percent efficiency, which causes optical losses that adversely affect the measurement system sensitivity. Moreover, filters work best with collimated light, which usually requires multiple lenses to collimate the light through the filter and then focus it on the detector. The multiple lenses further reduce the amount of light that reaches the detector.  
      In contrast,  FIG. 4  shows an optical pyrometer  70  that employs a single lens  72  for focusing radiation  14  on a wavelength selective AlGaAs detector  74 . The wavelength selective filtering achieved by AlGaAs detector  74  has a rapidly diminishing response as wavelength increases, enabling a measurement system having increased sensitivity because the losses associated with filter  54  and second lens  56  are eliminated. Also the angled light does not effect the wavelength sensitivity of the AlGaAs detector.  
      Miniaturization of the detector/electronics system and direct coupling to the light capturing source further increase the measurement sensitivity of the pyrometers suitable for use with this invention.  
       FIG. 5  shows a typical prior art pyrometer system  80  that employs a lens assembly  82  or a quartz or sapphire light guide rod  84  for collecting radiation  14  and propagating it onto an optical fiber or fiber bundle  86  for conduction to a detector  88 . Light guide rod  84  or lens assembly  82  interfaces with the high temperature environment of the object. Optical fiber  86  isolates detector  88  and associated electronics  90  from electrical noise and heat and provides mechanical flexibility for placing detector(s)  88  in a convenient location. While this arrangement provides mechanical convenience, the following factors associated with using optical fibers  86  in semiconductor applications reduce their ability to accurately transmit radiation  14  to detector(s)  88 :  
      1. If a single flexible fiber is employed to propagate radiation  14  from light guide rod (lightpipe)  84  to detector  88 , then there will be large (˜80%) optical losses due to the difference in index of refraction and the fact that the flexible fiber is usually smaller in diameter than the lightpipe. If, instead, a fiber bundle is employed to propagate the radiation from the light guide rod (lightpipe) to the detector, significant loss of optical signal strength will still result due to the mismatched index of refraction and the fill factor of the bundle (the spaces between fibers) being less than 100 percent.  
      2. Because of the limited availability of glass types from which to make optical fiber  86 , it is nearly impossible to achieve a numerical aperture that is equivalent to the index of refraction of light guide rod  84 .  
      3. Unless optical fiber  86  includes an antireflection coating, reflection losses will exist at the glass-to-air interfaces at the ends of optical fiber  86 . The reflection losses are exacerbated if the index of refraction is raised in an attempt to capture all of the light from light guide rod  84 .  
      4. Because optical fiber  86  can only contain radiation that is traveling over a limited range of angles, radiation  14  that is captured by lens assembly  82  or light guide rod  84  and propagated into optical fiber  86  will have a variable loss if optical fiber  86  is flexed.  
      5. As optical fiber  86  is heated or cooled, its transmission characteristics change causing transmitted signal variations.  
      6. When employing an optical fiber cable, errors are easily introduced at both ends of the cable: first, through misalignment of the cable ends when they are connected to the light guide rod  84  and photo detector  88 , and secondly due to imperfect cleaning of the two surfaces. Moreover, when optical fiber  86  is attached and removed from the radiation collection system or detector  88 , alignment changes can occur causing variations in the transmitted light.  
      By way of comparison,  FIG. 6  shows that in this invention, the losses and signal variations associated with optical fibers are eliminated by eliminating optical fiber(s)  86  and directly coupling detector  20  to light guide rod  12  and/or detector  74  to lens  72  of respective pyrometers  10  ( FIG. 1 ) and  70  ( FIG. 4 ). To accomplish this in a mechanically effective way, the detector and supporting electronics are miniaturized as shown in  FIG. 1  to fit into space-constrained locations.  
      As the device geometry of ICs becomes ever smaller, the measurement of lower temperatures becomes more critical for these processes. As temperatures decrease, the amount of radiation emitted by the wafer also decreases. Therefore, the radiation transmission efficiency of light guide rods coupled to detectors become ever more critical to accurate temperature measurements.  
      Moreover, as the price of IC&#39;s decreases, extreme cost-reduction pressure has been placed on semiconductor equipment manufacturers. Given the high cost of sapphire, the current state-of-the-art material for fabricating light guide rods, alternative materials have long been sought after for making light guide rods.  
      Accordingly, the pyrometer suitable for use with this invention includes an improved light guide rod material for reducing the optical losses encountered when employing optical pyrometry in, for example, semiconductor processing applications. This improved material is formed of aluminum oxide single crystal, the preferred type being YAG, which provides increased light transmission characteristics, resulting in improved low temperature measurement capabilities. A suitable alternative light guide rod material is yttrium aluminum perovskite (“YAP”). Recent processing improvements have allowed manufacturing YAG and YAP in rod lengths and form factors suitable for use in optical pyrometry applications.  
      YAG retains many of the benefits of sapphire, in that it is very similar in hardness (MOHS hardness of 8.2 vs.  9  for sapphire), melting point (1,965° C. vs. 2,050° C. for sapphire), and ability to withstand thermal shock. These unexpected benefits make YAG ideally suited for fabricating light guide rods  12  and  84  used in temperature sensing for semiconductor applications.  
      While YAG has been used for other optical applications such as in lasers, hitherto it could not be grown long enough and was usually doped, so it has never been considered as a potential light guide rod material. However, the increased demand for YAG for other applications resulted in major manufacturing advances, with producers now able to grow it in lengths up to one meter. This recent development and additional new research in un-doped YAG has led to the unexpected discovery of many properties that make YAG ideally suited for use in semiconductor applications. For example, when compared to sapphire, the YAG material: 
          reduces optical losses because of its higher index of refraction and better crystal structure;     reduces or eliminates factors which cause variations in the signal level due to lack of uniformity from light guide rod to light guide rod;     is less affected by surface contamination;     provides tighter machining tolerances;     reduces the light guide rod&#39;s thermal conductivity; and     as opposed to sapphire, YAG is easier to machine into round rods because of its crystal structure.        

      Regarding reduced optical losses, YAG has a higher refractive Index, resulting in better radiation transmission. When fabricating light guide rods, a ferrule is attached to the light guide rod as a means of securing the rod to the pyrometer. An O-ring is also typically attached to the rod to provide a seal between the rod and the wafer-processing chamber into which the rod is inserted. However, when these parts contact the rod, radiation can be scattered at the contact points. Care must be taken, therefore, in selecting materials with a high refractive index to prevent radiation from scattering at the contact points. Accordingly, only sapphire and quartz rods and fibers have been used in prior semiconductor applications. While these materials provide a high refractive index, a consistent problem (particularly with quartz) has been that radiation is still scattered at the ferrule and O-ring contact points, thus reducing the light guide rod&#39;s transmission capabilities.  
      An improvement therefore would be to employ a material having characteristics similar to sapphire or quartz but with a higher refractive index to reduce the amount of scattered radiation at the contact points. Because YAG has a higher refractive index (1.83 at 632.8 nm) than sapphire or quartz, it is less sensitive to radiation losses at the contact points and is, therefore, ideally suited as an improved light guide rod material.  
      When fabricating light guide rods, it is important to obtain highly polished rod sides to prevent radiation from scattering out of the guide rod sides. Because quartz is a soft material it is difficult to prevent scratches on the sides of quartz rods. On the other hand, because sapphire is such a hard material, it is difficult to polish out all the scratches produced on sapphire rods during their manufacture.  
      YAG is harder than quartz but not quite as hard as sapphire, making it ideally suited for fine side polishing, thereby preventing radiation from scattering from the sides of YAG rods.  
      When fabricating IC&#39;s (which now have device geometries as small as 0.11 microns), it is critical that the IC manufacturing equipment be uniform from tool to tool. Consequently, each component of a semiconductor-fabricating tool must maintain a very high level of uniformity, including a high level of uniformity among light guide rods. YAG provides several unexpected benefits for providing such uniformity, which could not be achieved with sapphire or quartz. These benefits include: 
          YAG has no birefringence, so it provides more uniform light collection;     YAG is an isotropic material, so it eliminates problems with growth misalignment and/or machining misalignment that are common to sapphire; and     YAG can be machined more easily to a tolerance as low as ±0.0001 inches, whereas sapphire can only be machined easily to a tolerance of ±0.001 inches.        

      The accuracy of pyrometers can be improved by preventing unintended heat from reaching the detector. When using light guide rods for transmitting radiation from the wafer to the detector, the light guide rod can itself become hot and conduct heat from the process chamber in addition to radiation from the wafer, causing temperature measurement errors. Consequently, the light guide rod should have a low level of thermal conductivity.  
      Fortunately, YAG has a lower level of thermal conductivity than sapphire. An additional benefit is that lower temperature epoxies can be used for securing ferrules to the YAG rods, and O-rings having lower heat resistance can be used.  
      Another series of improvements for facilitating the measurement of low temperatures is the reduction or elimination of factors causing signal level variations. A number of such factors have been identified, and techniques to improve or eliminate them have been developed as described below.  
      When using a light guide rod as a radiation collection system, the rod-to-detector coupling efficiency may be reduced by foreign matter that accumulates on the optical faces of the rod and detector. In particular, foreign particles can be deposited on the surfaces when the rod and detector are disconnected. In addition, the mechanical movement associated with connecting and disconnecting the rod deposits debris on the interface surfaces. This debris may adversely affect the measurement system calibration.  
       FIG. 7  shows a mounting system for prior art light guide rod  84  and detector  88  in which an optical face  92  of light guide rod  84  and an optical face  94  of detector  88  are recessed within a threaded housing  96 . This configuration makes cleaning of optical faces  92  and  94  difficult and ineffective.  
      In contrast,  FIG. 8  shows a mounting system for light guide rod  12  and detector  20  in which optical faces  100  of light guide rod  12  and detector  20  are flush to the edges of their respective housings  102  and  104  and are, therefore, closely coupled. The flush mounting facilitates easy and effective cleaning of optical faces  100 . The close coupling also improves rod-to-detector optical coupling and, thereby, reduces signal transmission variations.  
      Referring again to  FIG. 1 , when taking temperature measurements with radiometric system  10 , it is important to block undesirable wavelengths of radiation  14  to reduce errors introduced by heat build-up in filter  18  and detector  20  and to prevent damage to photo detector  20  caused by the undesired wavelengths. Undesired wavelengths of radiation  14  are typically blocked by using filters. Two improved ways of blocking undesired wavelengths are:  
      When a blocking filter, such as filter  18 , performs its function by absorbing radiation, the absorbed energy causes filter  18  to increase in temperature, which changes the blocking characteristics of filter  18 , altering the response of the measurement system, and resulting in temperature measurement errors. These errors can be prevented by introducing an additional blocking system for impeding undesirable wavelengths of radiation  14 .  
      A preferred way of accomplishing this additional blocking is to place reflective hot/cold mirror surface  22  coating on filter  18 . Hot/cold mirror surface  22  preferably causes minimal change in the spectral characteristics of filter  18  in the desired wavelengths yet transmits wanted wavelengths of radiation  14  while reflecting undesired wavelengths as undesired radiation  120 .  
      Reflecting the undesired radiation  120  back through collection optics  12  (light guide rod or lens) to the location being measured on object  16  is advantageous for the following reasons: the temperature of object  16  is not significantly altered because much of radiation  14  is returned to object  16 ; and filter  18 , photo detector  20 , and the associated electronics are more stable because they are not unduly heated by radiation  14 .  
       FIG. 9  shows a preferred response curve  130  for hot/cold mirror surface  22 . Hot/cold mirror surface  22  passes at least 70 percent of radiation  14  at about 900 nm and reflects substantial amounts of undesired radiation  120  at wavelengths above about 1,200 nm. Skilled workers will understand that hot/cold mirror surface  22  can be formed from a variety of suitable metallic and dielectric materials.  
      The response of a detector to radiation  14  and the electrical noise level it generates is a function of its operating temperature. Radiation wavelengths incident on the detector may not produce an electrical signal, but they may alter any existing signal by changing the detector temperature. In particular, short wavelength radiation may permanently alter the response characteristics of the detector. This radiation damage is prevented in part by the above-described hot/cold mirror surface  22  and also by filter  18 , which further blocks unwanted radiation wavelengths from the detector. An advantage of the hot/cold mirror is that it prevents UV damage and IR heating, which causes a shift in the wavelength response of the photo detector and also causes electrical noise.  
       FIG. 10  shows a pyrometer system  140  suitable for use with this invention employed in a typical semiconductor processing application. A major application of pyrometer system  140  is measuring the temperature a silicon wafer  142  as it is heated in a processing chamber  144  by high-power lamps  146  or plasma (not shown). Lamps  146  are typically mounted on the opposite side of silicon wafer  142  from light collection optics  12 .  
       FIG. 11 , shows graphs  150  representing the transmission of radiation through a silicon wafer as a function of wavelength and temperature. Graph  150  shows that silicon wafer  142  is transparent to radiation beyond a wavelength of about 1,000 nm. Therefore, it is important to block radiation beyond 1,000 nm to prevent detector and filter heating that would cause temperature measurement errors.  
      A common technique for achieving wavelength blocking is employing a short wavelength pass filter, which is fabricated by vacuum evaporation of optical materials having varying indices of refraction. By stacking a series of such materials, typically alternating high and low indices or refraction, a coating is produced that reflects or absorbs radiation over a limited range of wavelengths. To achieve blocking over a broad range of wavelengths, it is necessary to place successive stacks on top of each other such that each stack blocks a different wavelength range.  
       FIGS. 12A and 12B  represent the respective optical density and transmittance versus wavelength and radiation incidence angle of a short wavelength pass filter that us suitable for use with this invention. Skilled workers will understand how to make such a filter. As shown in the graphs, this technique is most effective if the radiation is incident to the filter over a range of angles less than about 27 degrees. However, if the radiation is incident over a wide range of angles, e.g., up to about 55 degrees, the wavelength blocking characteristics are altered.  
      A suitable short wavelength pass filter, therefore, includes a blocking coating that includes as a design parameter the numerical aperture of the light guide rod or optical fiber that propagates the light from the sample to the detector.  
      Another embodiment of a pyrometer suitable for use with this invention employs gallium aluminum arsenide (“AlGaAs”) and other wavelength-selective detector materials in place of band pass filters.  
       FIGS. 13 and 14  represent the respective absorption coefficient and photo sensitivity of various detector materials as a function of wavelength. Conventional pyrometer detectors utilize either InGaAs or silicon detectors. InGaAs detectors are sensitive to radiation wavelengths as long as 2,700 nm, which makes blocking very difficult. Silicon detectors are nominally insensitive to wavelengths longer than 1,300 nm, however the photo sensitivity of silicon diminishes with longer wavelengths.  
      An aspect of this invention, therefore, is to utilize a detector material having a photo sensitivity that diminishes rapidly at wavelengths at which silicon wafers begin to transmit radiation. A preferred detector material is AlGaAs, which has a photo sensitivity that peaks at 900 nm and diminishes by about three orders of magnitude at 1,000 nm. Alternatively, detectors materials such as GaP, GaAsP, GaAs, and InP are suitable for use as wavelength-selective detectors at wavelengths less than 1,000 mm.  
      The photo detector materials for wafer temperature measurements are chosen for photo sensitivity around the optimum wavelengths for measuring silicon, GaAs, and InP wafers. In particular, the material is chosen for sensitivity at wavelengths shorter than the 1,000 nm (bandgap for silicon wafers), yet as long as possible to provide a maximum amount of Planck Blackbody Emission without significant sensitivity to radiation transmitted through the wafer.  
      The photo detector suitable for use with this invention is made from AlGaAs, a tertiary compound, and is doped to optimize its photo sensitivity around 900 nm. This detector material is advantageous because it is insensitive to radiation wavelengths transmitted through a silicon wafer, and to much visible ambient light. It is also advantageous because it has a narrow wavelength detection sensitivity, minimizing the need for an additional wavelength selective filter. A suitable detector is manufactured by Opto Diode Corporation, located in Newbury Park, Calif.  
      Of course, in situations where sharper cutoff is desired, the detector can be combined with a filter to achieve a wavelength selectivity compounding affect. In these situations, it is also easier to design and manufacture band pass filters that are matched for use with the particular detector material.  
      The ability to eliminate the filter altogether (along with the ability to use a simple band-pass filter when one is required) further allows the detector to be spaced much closer (0.25 mm verses 2.54 mm) to the light pipe, enabling collecting about ten times more radiation. The close spacing also provides better low temperature measurement performance, e.g., the ability to measure 200° C. compared to 350° C. with a traditional band-pass filter and a silicon broad band detector.  
      As shown in  FIG. 15 , detector photo sensitivity changes with temperature, which causes output current variations that correspond to temperature measurement errors. Prior methods for dealing with this problem are to: 
          1) not correct for the error and simply specify a lower accuracy/repeatability specification;     2) use a band pass or cutoff filter to attenuate the detector wavelength selectivity skirts, thereby eliminating most of the spectral shifting variations; and     3) calibrate errors out by taking a set of measurements at various ambient and target temperatures and use the resulting data to extrapolate correction data.        

      Method 1 is clearly unacceptable for precision measurements.  
      Method 2 works well, although there are some remaining fluctuations caused by spectral shifts in the filter and detector. This method also significantly reduces the ability to measure lower temperatures because infrared wavelengths of interest are attenuated by the filter.  
      Method 3 also works well but is limited to the calibrated range of temperatures and is only relevant to systems of a similar configuration. The accuracy of this method is also limited by the conditions under which the data are taken and diminishes with higher target temperatures because of the difficulty of making accurate blackbody furnace measurements at these temperatures. In addition, this method is time consuming, limited in flexibility, and is not based on first principles of physics, making it prone to inaccuracies.  
      An improved method is to employ correction data generated from detector photo sensitivity curves as a function of wavelength, such as the curves shown in  FIG. 15 . A detector that is representative of the detectors used in a particular instrument model, is characterized with a monochromator at various ambient temperatures, such as −20, −10, 0, 10, 20, 25, 30, and 40 degrees C., to generate a set of data. The data are then used to generate scale factor correction data for detector current vs. temperature using the Planck equation and integrating the area under the spectrum curve vs. target temperature.  
      The data entered into the software are ambient dependent detector spectrum curves, minimum theoretical target temperature, maximum theoretical target temperature, and one actual predetermined target temperature.  
      This same correction method can be used for correcting for other optical components, such as optical filters that vary with ambient temperature.  FIG. 16  shows a set of graphical data representing wavelength shift as a function of temperature for typical infrared interference filters. Suitable correction data can be extracted from such data.  
       FIG. 17  shows a semiconductor wafer  160  undergoing an in-situ temperature measurement method employing radiometric system  10  of this invention. In-situ semiconductor wafer measurements are common place in IC fabrication facilities around the world. There are, however, numerous technical problems with measuring the temperatures of production wafers, such as semiconductor wafer  160 , when the Planck Equation is used to calculate its temperature from radiation emitted by a “hot” wafer.  
      For a wafer having a temperature above about 300° C., these technical problems include the unknown emissivity of semiconductor wafer  160 , and measurement errors caused by reflected background radiation. The unknown wafer emissivity causes large errors in temperature measurement because typical semiconductor wafer emissivities range from about 0.1 for metal films like copper to about 0.9 for oxides of certain thickness. Semiconductor wafer emissivity is a strong function of film type and thickness for both single- and multi-layer films deposited on both the front and backside of the semiconductor wafer  160 . Emissivity is also a function of the measurement wavelength and radiation collection angles employed by radiometric system  10 .  
      A preferred wafer temperature measurement method of this invention addresses sources of measurement error caused by unknown emissivity and reflected background radiation in processing applications that include a heated susceptor. Many semiconductor processing tools include one or more heated susceptors, which are commonly referred to as chucks, wafer holders, workpiece supports, or hot plates. Susceptors such as heated susceptor  162  are often manufactured from graphite that is typically coated with either silicon carbide or boron nitride. Susceptors may also be manufactured from aluminum, aluminum nitride, and silicon. The manufacture of susceptors, such as hot susceptor  162  is tightly controlled because its parameters directly impact the processing of semiconductor wafer  160 . For example, hot susceptor  162  has a tightly controlled surface texture, finish, and coating(s) to control among other things, contamination, heat transfer, and gas flow.  
      The temperature of hot susceptor  162  is also tightly controlled during processing of semiconductor wafer  160 , typically by employing closed loop feedback from sensors, such as a thermocouple  172  or a second radiometric system  174 , either of which is coupled to a CPU  176 . Other suitable temperature measuring devices include resistance temperature devices, platinum resistance thermometers, thermisters, and optical thermometers.  
      The semiconductor wafer temperature measurement method of this invention takes advantage of the tight control of the surface conditions and temperature of hot susceptor  162 , which tight control provides known and reproducible radiation emissions from hot susceptor  162 . The known amount of radiation emitted by hot susceptor  162  is employed as a stable radiation source for making precise reflectance measurements of semiconductor wafer  160 .  
      Collection optics  12  of radiometric system  10  is positioned in and sensing radiation through an opening  164  in hot susceptor  162 . Hot susceptor  162  emits emitted radiation  166 , which reflects off semiconductor wafer  160  as reflected radiation  168  that enters collection optics  12 , and is sensed by radiometric system  10 . When semiconductor wafer  160  is initially loaded in a processing chamber, it is relatively cold and, therefore, emits very little radiation. At this time, while semiconductor wafer  160  is separated from hot susceptor  162  by a gap  170 , most of the radiation sensed by radiometric system  10  is reflected radiation  168  originating from hot susceptor  162 . Semiconductor wafer  160  is then moved toward hot susceptor  162 , while radiometric system  10  makes multiple real-time measurements of reflected radiation  168 . Because the amount of reflected radiation  168  varies as gap  170  diminishes toward zero, radiometric system  10  senses information indicative of the reflectance and roughness of semiconductor wafer  160 . Semiconductor wafer  160  typically comes to rest on hot susceptor  162  as shown in dashed lines.  
      A process tool, typically a robot, has a fixed geometry and moves semiconductor wafer  160  toward hot susceptor  162  in a very reproducible manner. This makes it practical to calculate the amount of emitted radiation  166  by using the Planck Blackbody equation, then based on this result, to calculate the reflectivity of semiconductor wafer  160 . The emissivity of semiconductor wafer  160  can then be calculated using Kirchhoff&#39;s  1860  radiation law, which is expressed as: 
 
1 −R=ε,   (1) 
 
 where R is the reflectivity, and E is the emissivity. 
 
      Using Kirchhoff&#39;s law provides nearly 100 percent accurate and valid results because hot susceptor  162  is a very uniform and diffuse emitter, thereby illuminating semiconductor wafer  160  in a nearly hemispherical (all angles) manner, which is required for proper application of the law. Skilled workers understand that actual semiconductor wafers require only about a 50° total cone angle for reliable emissivity calculations when employing Kirchhoff&#39;s law.  
     EXAMPLE  
       FIG. 18  shows a semiconductor wafer processing apparatus  180  suitable for carrying out the temperature measurement method of this invention. A horizontal transporter  182  moves semiconductor wafer  160  by its peripheral margins into position above and spaced apart from hot susceptor  162  by the distance of gap  170 , which typically ranges from about 2.54 cm (1.0 inch) to about 0.0254 mm (0.001 inch). Note that horizontal transporter  182  does not substantially block the surface of wafer  160  from hot susceptor  162  or radiometric system  10 . As wafer  160  is moved horizontally into position, cool semiconductor wafer  160  emits some emitted radiation  184 , which is sensed by radiometric system  10 . Emitted radiation  184  is initially small and increases when semiconductor wafer  160  is heated during subsequent lowering toward hot susceptor  162 . Before lowering semiconductor wafer  160 , emitted radiation  166  from hot susceptor  162  that is reflected by semiconductor wafer  160  as reflected radiation  168  provides a baseline radiation measurement for comparing with measurements taken during the subsequent downward motions of semiconductor wafer  160 . Hot susceptor  162  typically has a predetermined temperature in a range from 70° C. or less to about 1,300° C.  
      A vertical transporter  186  lifts semiconductor wafer  160  off horizontal transporter  182 , which moves out from under semiconductor wafer  160 . Vertical transporter  186  then commences moving semiconductor wafer  160  toward hot susceptor  162 , which movement time ranges from a fraction of a second to a few second. As semiconductor wafer  160  moves downward, its reflected emission  168  is measured by radiometric system  10  in real time as a function of diminishing gap  170 . This relationship is employed to calculate the effective reflectivity of semiconductor wafer  160 . This calculation employs the well-known relationship shown below in Eq. 2, which relates the effective or apparent emission to substrate emission when a narrow gap exists between a workpiece (e.g., wafer) and an object (e.g., susceptor).  
               ɛ   A     =               ⁢     E   +       (     1   -   E     )     ·   R   ·     
     ⁢     BBemissionRation   ⁡     (     λ   ,     T   WORKPIECE     ,     T   SUSCEPTOR       )               1   -       (     1   -   E     )     ⁢   R                 (   2   )             
 
 where, ε A  is the effective workpiece emissivity, E is the real emissivity of the susceptor, R is the real reflectance of the workpiece, and BB is the blackbody emission from the hot susceptor. 
 
      This information may also be employed to characterize the roughness of semiconductor wafer  160 , which roughness influences the directionality and, therefore, the intensity of radiation collected by collection optics  12 . Gap  170  diminishes as semiconductor wafer  160  is lowered onto hot susceptor  162  (as shown in dashed lines). Therefore, the amount and angular components of radiation received by collection optics  12  also changes. This change is dependent on the reflectivity and roughness of semiconductor wafer  160 , which can be employed to calculate (Eq. 1) the emissivity of semiconductor wafer  160  and, thereby, its temperature.  
      Skilled workers will recognize that portions of this invention may be implemented differently from the implementations described above for preferred embodiments. For example, the description above applies primarily to temperature measurements of target media, but also applies to various forms of light measurements. Skilled workers will also recognize that this invention is not limited to the advanced pyrometer described above, but can also be used to complement standard single-, dual-, or multi-wavelength pyrometry. Notably, the target media may include semiconductor wafers undergoing any of epitaxial growth processing, chemical vapor deposition, plasma assisted chemical vapor deposition, and physical vapor deposition. The measurement methods are also usable for steel undergoing galvanneal processing, and for use in aluminum sheet processing.  
      It will be obvious that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. Accordingly, it will be appreciated that this invention is also applicable to temperature measurement applications other than those found in semiconductor wafer processing. The scope of this invention should, therefore, be determined only by the following claims.