Abstract:
The present invention is directed to optimization of the optical detection system for the use of optical emission spectroscopy in end-point detection. The optimization specifically addresses the needs of a radiant heated wafer system in a downstream process chamber environment. The present invention maximizes signal light from relevant reactions, maximizes signal-to noise and signal-to-background ratios, utilizes very small diagnostics access, collects light from the region of most intense light emission from endpoint processes, collects light from representative parts of an entire wafer with just one diagnostic access port to ensure complete end-point, and eliminates light signals from sources other than the wafer.

Description:
FIELD OF THE INVENTION 
     The present invention concerns the field of optical monitoring of integrated circuit fabrication. 
     BACKGROUND OF THE INVENTION 
     During the fabrication of integrated circuits, a desired circuit pattern for a given layer of the integrated circuit is etched into a dielectric film. To accomplish this, a photoresist material is disposed on the area of the dielectric film where etching is not desired. An etching gas, which is chemically reactive with the dielectric material and less chemically reactive with the photoresist, is generated in a plasma. The plasma is then supplied to the surface of the dielectric being etched, causing the etching gas to diffuse into the surface of the dielectric film. The etching gas chemically reacts with the dielectric film to form a volatile by-product. The volatile by-product is then desorbed from the surface of the dielectric film and diffuses into the bulk of the etching gas. 
     After the pattern is etched into the dielectric layer, the photoresist that was used to define the metal circuit pattern on the dielectric layer is removed. Any post-etch residues including sidewall polymer deposition also must be thoroughly removed or stripped from the underlying layer. One dry process used to strip photoresist and photoresist residues from the dielectric layer is known as ashing. The process of ashing is similar to the etching process. The gas used for ashing, however, is more chemically reactive with the photoresist than with-the dielectric. The ashing gas chemically reacts with the photoresist to form a volatile by-product. The volatile by-product diffuses into the bulk of the ashing gas. After the ashing process is complete, the etched pattern is filled with copper or other conductive material. 
     Optical emission spectroscopy has previously been used to determine the end point of the etching process by providing information about the etching gas and the by-product of the etching gas and dielectric material. The technique relies on the change in the emission intensity of characteristic optical radiation from the dielectric by-product in the plasma. Excited atoms or molecules emit light when electrons relax from a higher energy state to, a lower energy state. Atoms and molecules of different chemical compounds emit a series of unique spectral lines. The emission intensity for each chemical compound within the plasma depends on the relative concentration of the chemical compound in the plasma. A typical optical emission spectroscopy apparatus operates by measuring the emission intensities of the reactive etching gas and the by-product of the etching gas and the dielectric. The emission of the by-product decreases and finally stops when an endpoint is reached. The optical emission spectroscopy apparatus senses the declining emission intensity of the by-product to determine this endpoint. 
     It is very important to accurately determine the endpoint of stripping, etching, or residue-removal processing of wafers for semiconductor devices. Accurate endpoint detection improves throughput and minimizes damage to other wafer layers. Over-ashing and under-ashing produce undesirable patterns in the integrated circuit wafer. It is difficult to accurately determine endpoint, because process chambers for semiconductor wafer processing offer very little diagnostic access. Optical emission spectroscopy (OES) has been used to determine endpoint measurements, but is inaccurate because of poor optical access to the region of the wafer. If radiant wafer heating lamps are used to promote the ashing process, the light used to heat the wafer interferes with the light emitted at the wafer surface, used to determine endpoint. 
     In a radiantly heated wafer process chamber the lamps used to heat the wafer emit broadband, blackbody light. The intensity of the broadband, black-body lamp emission may be several orders of magnitude greater than the emission by the reactant by-product of the coating and the plasma being monitored to detect end-point. The broadband, black-body light is most intense during a ramp phase when the lamps are on at full power. The stray light from the lamps becomes a critical issue during ramp phase. Both stray light outside the spectrometer but associated with the optics leading into the spectrometer and stray light which makes its way into the spectrometer make monitoring of the reactant by-product to determine endpoint more difficult. Stray light enters the optics monitoring system, because dirt and coating films, deposited on the optics during the ashing process, and imperfections in the optics scatter stray light into the spectrometer. Additionally, the far wall reflects and emits stray light into the field of view of the optics. The light emitted by the volatile by-product is in part reflected and diffused out of the field of view of the spectrometer by the coating on the optics and optics imperfections. Unwanted lamp light is in part diverted into the field of view of the spectrometer by the coating and dirt on the optics and imperfections in the optics. Coating and dirt on the optics reduces the by-product light, which reaches the spectrometer input and scatters lamp light into the spectrometer input. The increase in lamp light and decrease in volatile by-product light which enter the spectrometer result in a reduced signal-to-noise or signal-to-background ratio, which degrades the performance of the optical detection system. 
     Spectrometers are designed to direct light into a particular path for each wavelength for measurement. To accomplish this, spectrometers include numerous internal surfaces, off which the light either reflects, refracts, or is transmitted. These internal surfaces have surface imperfections and become dirty causing a small percentage of the lamp light, which enters the spectrometer, to be diffusely scattered. When lamps are used to radiantly heat the wafer a small percent of the high intensity lamp light leads to a moderately high level of randomly scattered light inside the spectrometer. Some lamp light, although at wavelengths other than the wavelength of the light emitted by the by-product, enters wavelength channels of the spectrometer designed to measure light emitted by the volatile by-product. Stray light inside the spectrometer is background or noise, scattered off the imperfect interior surface of the spectrometer. The stray light in the spectrometer results in a reduced signal-to-noise or signal-to-background ratio. 
     There are special problems with downstream process chambers with regard to the number of available OES signals and accessing the volume from which maximum signal is produced by the ashing and etching reactions. Often, there is both broadband light from the radiant heating lamps and the plasma itself which varies during the ashing and the etching processes, which make it more difficult to measure the light emitted by the wafer. Special cases of low ash rate processes, resulting in weak signals, also make endpoint detection more difficult. 
     The design of a standard etcher is distinct from the design of a downstream asher in two respects. First, the pressures associated with etchers are much lower. The lower pressure of etchers allows molecular flow, where given molecules of the reactant by-product move freely around the process chamber and bounce off numerous chamber walls many times, creating a substantially uniform distribution of by-product within the reaction chamber. Secondly, the plasma fills the process chamber and remains in the process chamber. For these two reasons, there is often nearly uniform signal strength from the reactant by-product in etchers. 
     The pressure in a downstream plasma asher is increased. Applicants have observed a transitional region of laminar viscous flow slightly above the wafer surface due to the increase in pressure. The reactant by-product of the coating being removed and the plasma is contained near the wafer surface by the laminar flow. The by-product being monitored is formed on or near the surface of the wafer and is constrained in this region by the laminar flow of the plasma. The region of a downstream plasma asher which is useful in monitoring by-product signal is the region extending from the wafer surface to slightly above the wafer surface. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an optically monitored wafer processing system that includes a wafer processing chamber, a wafer treatment apparatus and receiving optics. The wafer processing chamber includes a support for positioning one or more wafers. The wafer treatment apparatus includes means for routing wafer treatment material, such as a plasma, into the processing chamber for removing coating from one or more wafers. The receiving optics are mounted to the chamber in al position to monitor concentrations of the reactant by-product of the coating and the wafer treatment material on or above the surface of the wafer. The optics measure the reactant by-product on or above the surface of the wafer, where the by-product is concentrated. 
     The optically monitored wafer processing system may include a viewing dump. The viewing dump may be mounted, in the interior of the wafer processing chamber or to the exterior of the wafer processing chamber, adjacent to a window. In one embodiment of the invention, the viewing dump may comprise a narrow port or slot in the chamber wall opposite the optics, a blackened cover and a diamond shaped deflector within the box. The black box and deflector intercept background light within the plasma reaction chamber. The viewing dump increases the signal to background and signal to noise ratios of signals generated by the wafer processing system. Unwanted light from the wafer processing chamber is absorbed by the viewing dump rather than being detected by the optics monitoring system. 
     In one embodiment of the invention, a polarizer be mounted to the wafer processing chamber to filter signals emitted by the reactant by-product of the coating and the wafer treatment material. The polarizer removes background light and to a lesser extent removes the signal light of the by-product. Although the polarizer reduces desired signal, it has the benefit of increasing the signal to background and signal to noise ratios, because background and noise are removed to a much greater extent. 
     The receiving optics of the optically monitored wafer processing system can be configured in a linear array. Configuring the receiving optics in a linear array allows each fiber optic fiber of the array to be located near the surface of the wafer and gather light from points on and near the surface of the wafer. In this configuration, the entire line of sight of each fiber optic fiber is near the surface of the wafer. Configuring the receiving optics into a linear array maximizes the desired signal, because the line of sight of each fiber optic fiber extends through a region where the desired signal is concentrated and the background and noise are minimized. 
     In the preferred embodiment, the optics monitoring system includes a low pass filter. The low pass filter is placed in front of the lens, between the lens and the fiber optic fibers or between the fiber optic fibers and the spectrometer. The low pass filter selectively prevents light from passing through it to the lens, fiber optic fibers or the spectrometer. A low pass filter may be selected, which prevents light with wavelengths other than the wavelength of light emitted by the by-product of coating and plasma. 
     The receiving optics may also be configured in a fan shaped array. As with the linear array, each fiber optic fiber is located hear the surface of the wafer and gathers light from a point near the surface of the wafer. In the fan shaped array, the lines of sight of the receiving optics cover a greater area above the wafer. By monitoring a greater area above the wafer, a more statistically accurate signal is gathered. 
     The receiving optics may be arranged to monitor a uniform distribution of points on the surface of the wafer. The receiving optics may be held by a holding block with an orifice to position each optic at an appropriate distance and tangle from a receiving lens to define focal points uniformly distributed about the surface of the wafer. By monitoring a uniform distribution of points on the surface of the wafer, a larger area of the wafer is monitored, which assures more uniform endpoint detection. 
     These and other advantages and features of the invention will become better understood from the following detailed description of an exemplary embodiment which is described in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic plan view depicting an optical system for endpoint detection of the present invention; 
     FIG. 2 is a schematic front elevational view of an optical system for endpoint detection of the present invention; 
     FIG. 3 is a perspective view of a microwave plasma asher; 
     FIG. 4 is a front section view of a microwave plasma asher; 
     FIG. 5 is a schematic front elevational view of a process chamber of the microwave plasma asher; 
     FIG. 6 is a plan view of a process chamber of the microwave plasma asher; 
     FIG. 7 is an enlarged sectional view of a portion of the process chamber including an optics port attached to the process chamber; 
     FIG. 8 is an sectional view of a process chamber including an optics port attached to the process chamber; 
     FIG. 9 is a front sectional view of a process chamber having an optics port and a viewing dump; 
     FIG. 10 is a chart which represents the relationship between distance above the wafer and intensity of signal of a particular wavelength light; 
     FIG. 11 is a schematic view of the microwave plasma asher, which depicts the flow of plasma within the process chamber; 
     FIG. 12A is a schematic of optics having a line of action that extends above a wafer; 
     FIG. 12B is a schematic of unfocussed optics and a wafer; 
     FIG. 12C is a schematic of optics focused for viewing a region above the surface of a wafer; 
     FIG. 12D is a schematic of optics focussed at the surface of a wafer; 
     FIG. 13A is a schematic of optics focused above the surface of a wafer; 
     FIGS. 13B and 13C are schematics of optics focussed above the surface of the wafer with shorter focal lengths than those of FIGS. 12C and 12D; 
     FIG. 14 is graph which represents the relationship between the intensity of 309 nm light and time for two different reaction chambers; and 
     FIG. 15 is an enlarged perspective sectional view of a viewing dump. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIGS. 3 and 4 in particular, there is depicted a microwave plasma asher which is generally designated by reference numeral  20  and is suitable for practicing a method of photoresist and/or post etch residue removal by treatment with a plasma. The illustrated plasma asher  20  includes a plasma generating chamber  21  and a plasma reaction chamber  22 . The plasma generating chamber includes a microwave enclosure  24 . The microwave enclosure is a rectangular box which is partitioned into lengthwise sections having plasma tube  32  passing there through. Each partition has an opening through which the plasma tube passes. Each section is fed with microwave energy. Thus, each section appears to be a relatively short cavity to the incoming microwave energy, promoting the formation of modes having azithumal and axial uniformity. Outer tube  34  surrounds the plasma tube inside the cavity. The outer tube is slightly separated from the plasma tube and air under positive pressure is fed between the two tubes to provide effective cooling of the plasma tube. Tube  34  is preferably made of sapphire. Other plasma tube materials such as quartz or alumina coated quartz can be used. 
     An iris plate covers the open side of the microwave structure and is effective to feed microwave energy into adjacent sections. The plate is a flat metallic pate having irises, through which the microwave energy is fed. 
     Microwave traps  46  and  48  are provided at the ends to prevent microwave transmission. Such traps may be of the type disclosed in U.S. Pat. No. 5,498,308, which is incorporated herein by reference. Air seals/directional feeders are provided for admitting cooling air and feeding it to the space between the concentric tubes. 
     Magnetron  56  provides microwave power which is fed through coupler  58  to a waveguide supplying TE 10  mode, having mutually perpendicular sections  60  and  62 . The length of waveguide section  62  is adjustable with moveable plunger. At the bottom plate of waveguide section  62  is an iris plate, which couples microwave energy into partitioned microwave structure  24 , through which the plasma tube  32  extends; thus a plasma is excited in the gas flowing through the plasma tube. 
     Referring again to FIG. 3, it is seen that end cap  70  abuts microwave trap  48 , and fitting  74  having a central orifice for admitting gas to the plasma tube extends into the end cap. The gas supply is regulated by an external flow box (not shown). 
     The plasma tube is supported at this end by “o” ring in the end cap. The outer tube  34  is supported at its ends by abutment against microwave traps  46  and  48 . A spacer is present to provide the proper spacing in relation to the process chamber. The other end of the plasma tube is located in end member  78 , and has an orifice for emitting gas into the process chamber. 
     The plasma reaction chamber  22  includes wafer support pins  90  and  92 , which support a wafer  98  to be processed. A chuck (not shown) may alternatively be used. Heating may be accomplished by an array of tungsten halogen lamps  144  positioned below the wafer. Preferably, the substrate is heated from about 80° C. to about 350° C. during ashing. More preferably, the substrate is stepwise heated by incrementally increasing the temperature. Heating has been found to increase the reaction rate of the plasma with the photoresist and/or post etch residues and consequently, increase throughput. One or more baffle plates may be present above the wafer to promote even distribution of the plasma to the surface. Additionally, the reaction chamber also includes an optical detection system  110 . The optical detection system  110  optically detects an emission peak having a particular wavelength range that corresponds to the reaction by-product between the plasma and the photoresist. 
     FIG. 3 depicts a section of an exterior view of the plasma asher  20 . The reference numerals in FIG. 4 correspond to those, which are used in the other Figures. 
     Preferably, the microwave enclosure  24  is dimensioned to support the rectangular TM 110  mode and the enclosure  24  may have a square cross section. The dimensions of the cross sections are such that the TM 110  mode is resonant. The length of each section is less than λ g /2 where λ g  is the guide length within the cavity of the TE 104  mode. 
     In operation, the semiconductor wafer  98  with photoresist and/or post etch residues thereon is placed into the reaction chamber  22  on wafer support pins  90  and  92 . The wafer is preferably heated to accelerate the reaction of the photoresist and/or post etch residues with the plasma. The pressure within the reaction chamber is reduced. Preferably the pressure is maintained between about 1 torr to about 5 torr. A wafer treatment gas, such as, an excitable gas mixture of the hydrogen bearing gas and the fluorine bearing gas is fed into plasma tube  32  of the plasma generating chamber  21  via a gas inlet. The fluorine bearing gas is preferably less than 10 percent of the total gas composition. Each section is fed with microwave energy to excite a plasma in the plasma tube, which plasma is comprised of electrically neutral and charged particles. The charged particles are selectively removed prior to the plasma entering the reaction chamber. The excited or energetic atoms of fluorine (atomic fluorine) and hydrogen (atomic hydrogen) are fed into the reaction chamber and react with the photoresist and/or post etch residue. 
     A reaction by-product evolves and is monitored optically by the optical detection system  110 . The reaction by-product gas is continuously swept away by a gas sweep within the reaction chamber. An optical detection system  110  (FIGS. 1 and 2) detects the emission peak of by-product. Once the emission peak reaches a lower threshold the removal of photoresist and/or post etch residues is complete and the plasma is turned off. Alternatively, the plasma may be run for a predetermined time. This has been found to be particularly useful for removing post etch residues from the substrate wherein the photoresist remaining after etch is minimal. It has been found that the emission peak is not easily detected if a sufficient quantity of photoresist is not present. It is believed that a detectable emission peak is primarily derived from the reaction between the plasma and the photoresist. The vacuum is then released and the processed wafers are removed from the reaction chamber. Once the wafer is withdrawn, a deionized water rinse is then used to remove any remaining residue on the stripped wafer. 
     One exemplary embodiment of the invention is directed to a wafer processing system such as a plasma asher  20  having a plasma reaction chamber  22 , a plasma source  21 , and an optics monitoring system  112 . The wafer reaction chamber  22  includes a support having pins  90 ,  92  for positioning a wafer within an interior region of the reaction chamber  22 . The plasma source  21  routes wafer treatment material into the reaction chamber  22  and removes a coating from an exposed surface  136  of the wafer  98 . The monitoring system  112  has receiving optics  140  having a lens  116  focused at a location on or slightly above the surface of the wafer. The lens  116  is mounted to the reaction chamber  22  for monitoring concentrations of a reactant by-product of the coating and the wafer treatment material. 
     In the exemplary embodiment, the lens  116  transmits light from the reaction chamber  22  to fiber optic cables configured so that individual fiber optic fibers are in a linear array  118 . A typical array may have 600.010 inch diameter fibers. Ideally, the linear array would be dispersed into a fan shape or arranged to view light originating from regions spaced about the entire surface of the wafer. The lens  116  may be a bi-convex lens positioned outside the chamber or within a recessed port  126  in the chamber. The radius of curvature of the lens  116  is the same on both sides and in the exemplary embodiment of the invention the lens is 1.5 inches in diameter and has a 29.8 mm radius of curvature. The lens may be made by CVI Laser Corp. or Esco Products. A low pass filter  117  is positioned between the lens and the fiber optic fibers to intercept unwanted light of wavelengths other than the wavelength of the by-product signal. The optical access port  126  in the process chamber wall should be rectangular shaped, which is wider than it is high. The optical access ports  126  should recess the optics to prevent them from becoming dirty or coated with by-product. 
     The preferred array of fiber optic fibers  138  of the optical detection system is a quartz fiber optic cable  138  coupled to a spectrometer  131  having an input slit  132 . Light in the field of view of the optics monitoring system  112  passes through the lens  116  and into the array  118  of fiber optic fibers. The received light travels through the fiber optic fibers to the spectrometer input slit  132 . After passing through the input slit  132 , the light is separated into its wavelength components by a grating  133  inside the spectrometer  131 . The spectrometer includes a charge coupled device which categorizes the signals so that the spectrometer can analyze each of the separated wavelength components of the light to determine the characteristics of the light from a line of sight  152  of the optics monitoring system  112 . See FIG.  12 A. Two suitable spectrometers are model # PC2000 spectrometer available from Ocean Optics, Inc and model # SD1024 spectrometer available from Verity Instruments, Inc. 
     A viewing dump port  127  on the far side of the reaction chamber  22 , opposite the optics monitoring system  112 , allows a viewing dump  128  to be mounted either recessed in the reaction chamber or outside the process chamber. The viewing dump port size, shape and location are such that all unwanted light, that could be emitted from or reflected off the far wall and enter the optics monitoring system, is allowed to enter the viewing dump port  127 . The viewing dump  128  is sized to intercept any light which might enter the optics monitoring system  112 . 
     Measurement of the signal emitted by the volatile by-product is optimized by careful selection of the focal point  130  of the optics monitoring system  112 . The plasma  114  reacts with the surface  136  of the coating to form a volatile by-product, which emits a signal. A volatile by-product profile exists just above the wafer  98 , which may be measured by optical emission spectroscopy (OES) to determine when the ashing is complete. The volatile by-product emission profile is concentrated near the wafer surface  136 . The monitored volatile by-product signal is maximized by focusing the optics monitoring system  112  at a point near the surface, near the far edge  124  of the wafer and locating the optics monitoring system so that its entire line of sight  154  is in the concentrated volatile by-product profile. The resulting line-integral or line-of-sight averaging method, in addition to monitoring the area near the focal point, also samples chords above the wafer, extending from the lens  116  to the focal point  130  and from the focal point to the viewing dump port  127 . 
     FIG. 10 shows the intensity of a 309 nm line due to the signal emitted by the volatile by-product of the plasma and the coating. The intensity is greatly peaked just 0.1 inches above the wafer  98 . The viscous laminar flow characteristic of the plasma flow depicted in FIG. 11 leads to smooth flow contours which keeps the by-product of the coating and plasma confined to a layer just above the wafer surface  136 , peaked near the surface. The by-product profile is not peaked directly on the surface, but at approximately 0.1 inches above the surface, due to the fact that the line of sight for the viewing optics integrates along a path above the wafer. By placing and focusing the optics monitoring system, the by product produced near the focal point, as well as by-product along the entire line of sight of the optical monitoring system are monitored. 
     The signal intensity is not only dependent on the focal point  130  location above the wafer  98 , but also path length above the wafer. Since the OES is an integral measurement along some path length, the signal intensity increases as the path length 156 increases. 
     FIGS. 12B-12D show how an originally unfocussed optics monitoring system  112  can be optimized by properly locating the fiber optic fibers  138  with respect to the lens  116 . FIG. 12B depicts an unfocused optics monitoring system with the viewing ends  160  of the fiber optic fibers  118  located at an unfocussed position. A focal point  130  is defined by adjusting a horizontal distance d h  between lens and the fiber optic fiber ends  160 , as shown in FIG.  12 C. The distance from the focal point  130  to the lens  116  can be adjusted by changing the horizontal distance d h . Referring to FIG. 12D, the focal point  130  can be positioned closer to the wafer surface  136  by moving the fiber optic fiber ends  160  a vertical distance d v  from the viewing beam axis  152  of the lens  116 . The optimal focal point is at a location just above the wafer at the far side of the wafer. By optimizing the focal point  130  of the optics monitoring system, the emission signal is nearly doubled. 
     A linear array  118  of fiber optic fibers  138  within a fiber optic cable allows for a more complete sampling of the wafer  98 . The present invention uses a linear array  118  of fiber optic fibers to view the region of the wafer. For example, a single layer linear array using the  60  fiber optic fibers, nominally 9-10 micron diameter each fiber optic fiber can be made from an original circular bundle of such fiber optic fibers. Other linear arrays may be constructed using N rows and M columns of fiber optic fibers, creating a rectangular type bundle. A rectangular type bundle concentrates the receiving optical fibers at a position above the wafer at which the maximum emission strength occurs. 
     A circular array of fiber optic fibers views a large circular pattern within the reaction chamber  22 . In the case of the round bundle of fiber optic fibers, the viewing optics views not only the wafer  98  and the region above the wafer, but also the lower side of a baffle plate shown in FIG.  11 . Using a circular bundle of fiber optic fibers has the consequence of diluting the signal strength by not viewing the regions in the reaction chamber  22  from which the by-product signal is most intense. The circular bundle also views noise and background sources of light, which ultimately make the processing of the signals more difficult. 
     The use of a linear array  118  of fiber optic fibers has advantages in addition to viewing the desired by-product signal where it is concentrated. The linear array of fiber optic fibers sample a larger area of the wafer and therefore provide increased assurance that there is complete removal of coating, assuring more reproducible identification of the completion of the etching or ashing. The shape of the field-of-view of the linear array of fiber optic fibers is compatible with the viewing dump  128 . 
     An additional advantage of a linear fiber optic fiber array is that the vertical extent required for the optics monitoring system ports  126  is greatly reduced. When a viewing dump  128  is used in conjunction with a linear array of fiber optic fibers, the vertical extent of the viewing dump port  127  is also reduced. The reduced size of the ports results in a reduced perturbation to the internal geometry of the plasma reaction chamber  22 . Possible effects on the dynamic flow of plasma through the reaction chamber  22  are reduced, resulting in an improved uniformity of the ashing process of the surface  136  of the wafer. The elongated port geometry is perpendicular to the plasma flow. The short elongated geometry of the ports also results in a more symmetric plasma flow, since the extent of the port in the direction of the plasma flow is small. 
     In the exemplary embodiment, a fan-shaped distribution of fiber optic fibers  138  may be used in place of the linear array to further enhance the sampling of the wafer. Instead of a parallel configuration of fiber optic fibers, the fiber optic fibers can be spread into a fan-shaped array so that more of the area of concentration of by-product signal is sampled. If the array is correctly fanned out and has an appropriate number of fiber optic fibers, the entire two-dimensional area of the wafer surface  136  can be sampled. Assurance of complete coating removal is obtained by sampling a majority of the wafer surface  136 . Furthermore since the wafer  98  often ashes in an axially symmetric fashion, the fan-shaped array of fiber optic fibers will be sensitive to situations in which the outer edge  124  of the wafer  98  ashes last. Conventional methods of optically viewing the reaction chamber  22  are very insensitive to situations where the edge of the wafer ashes last. 
     By sampling a larger area of the wafer an increased assurance of complete removal of the coating is obtained. The shape of the field-of-view of a fan shaped array of fiber optic fibers is also compatible with a viewing dump. 
     In another embodiment of the invention, true two-dimensional sampling of the wafer surface may be obtained by configuring the array of fiber optic fibers to sample the wafer discretely. The array  118  of fiber optic fibers can be arranged to focus on and sample discrete locations on the wafer  98 , rather than utilizing the line-integral for line-of-sight averaging method of surveying different chords above the wafer. True two dimensional sampling can be accomplished by determining the exact position required for the receiving end of each fiber optic fiber to sample a uniformly distributed array off points covering the two-dimensional surface of the wafer. The correct position of each fiber optic fiber may be maintained by a holding block. The holding block is machined to accept the fiber optic fibers, with a narrow hole drilled to position each fiber optic fiber corresponding to the fiber optic fiber&#39;s appropriate angle and distance from the lens  116 . By employing true two-dimensional sampling, the fibers sample a larger representative area of the wafer and therefore provide increased assurance that there is more complete removal of coating, which assures more reproducible identification of the end of the ashing or etching process. The shape of the field-of-view of an array of fiber optic fibers, arranged for two-dimensional sampling, is compatible with the use of a viewing dump. 
     In the exemplary embodiment, a low pass filter  117  prevents light of undesirable wavelengths from entering the fiber optic fibers  138 . The wavelengths of the light emitted by the by-product of the plasma and the coating ranges from approximately 300 nm to 450 nm. The radiant heating lamps  144 , depending on power level, can produce light with wavelengths ranging from slightly below 300 nm to well above the 1000 nm range. The preferred low pass filter  117  only allows light in the 300-450 nm range to pass through. The low pass filter  117  may be a broad-band low-pass filter. A low-pass filter that prevents light with wavelengths greater that 450 nm from passing through may be used, since there is very little emission below 300 nm. The low pass filter prevents the bulk of the unwanted lamp light from entering the spectrometer  131  and potentially scattering into the detector at the output of the spectrometer  131 . The low pass filter allows the lower-amplitude light with wavelengths of the by-product light to enter the spectrometer and be processed. 
     The low pass filter  117  may be a thin, approximately ¼ inch thick, glass disk with a diameter of approximately inch. The low pass filter is inserted into the light path just before the fiber optic fibers leading to the spectrometer  131 . The low pass filter may also be inserted just before the spectrometer, or just before the lens  116 . 
     The low pass filter  117  is created by depositing numerous thin film layers on the surface of a glass disk to obtain the desired transmission and reflectance properties. For example, a BG3 glass substrate of glass may be covered by approximately 70 layers of dielectric and metal films to achieve the desired bandwidth. 
     In the exemplary embodiment, a viewing dump  128  is used to reduce background and signal noise. The viewing dump  128  prevents light emitted from the far wall  142  and reflected off the far wall  142  of the plasma reaction chamber  22  from entering the optics monitoring system. Even if the optics monitoring system  112  is focused on the wafer surface  136 , light rays may still originate from the far wall  142 , pass through the focal point  130 , and terminate in the fiber optic fibers  138 . Unwanted light may originate from the wall if the wall is slightly covered with coating and coating by-products resulting from previous or present etching and ashing processes. Unwanted light may also be reflected off the wall and into the field-of-view of the optics monitoring system. Unwanted, reflected light is particularly abundant in radiantly heated systems, which utilize lamps to heat the wafer  98 . 
     Light emitted by and reflected off the chamber wall opposite the optics monitoring system  112  reduces the signal-to-background, and thereby increases the demands on-the hardware and software processing systems to process a larger dynamic range of signals with high amplitude resolution. FIG. ( 14 ) shows the time evolution of the 309 nm signal during an entire process for a first process chamber and the process chamber of the current invention. The plot of the signal monitored in first chamber is indicated as  162  and the signal monitored in the inventive chamber is indicated as  166 . The first high emission signal levels are due to the lamp light during the high lamp power ramp phase, and the second (not as high) high levels are due to the ashing during the microwave operation phase. Residual unwanted light signals can be seen after approximately 100 seconds. The residual signal is either due to the lamps  144 , ashing signal originating from the wall  142 , or plasma light. FIG. ( 14 ) shows that for points focussed near the far edge of the wafer, there is a residual signal after the ashing process is complete due to signals emanating from the far wall  142  of the process chamber. 
     A substantial amount of lamp light is diffusely scattered off the far wall  142  of the process chamber  22 . Although the percentage of incident light on the far wall  142  that gets scattered into the field of view is small, the lamp light is so intense that a significant amount of lamp light may be viewed by the optics and measured by the spectrometer  131 . During operation of the asher, the coating from the wafer  98  may be deposited on the far wall  142 . Subsequent ashing processes remove the deposits on the wall as well as the coating on the wafer, causing a by-product signal to be emitted from the far wall. Plasma reacting with coating on the far wall causes the wall reflectivity to change over time. The viewing dump prevents most of the light emitted by and reflected off the far wall  142 , which would have otherwise been integrated by the optics monitoring system, from entering the field of view  155 . 
     The viewing dump  128  prevents light scattered off the far wall  142  from entering the field of view  155  of the optics  140 . The viewing dump works by removing the far wall locally so that no coating is deposited on the far wall, in an area that can be viewed by the optics monitoring system and capturing light that enters the viewing dump  142  so that it is not viewed by the optics monitoring system  112 . Depending on the design of the remaining portion of the viewing dump  142 , the port  127  or slot removed from the far wall may or may not lead to the outside of the chamber. Preferably, the viewing dump port  127  does not have complicated geometry where dirt and coating can accumulate. 
     By using a viewing dump  128 , the lamplight, which enters the optics monitoring system is reduced to 1.6% of its original value. The dump  128  is a broadband device, which eliminates broadband lamp and plasma light to improve the signal to background for the entire charge coupled device detector or other spectrometer used. The viewing dump  128  reduces demands on the hardware and software, making extraction of the relevant signals easier. The viewing dump  128  serves as a “black hole” for the optics monitoring system on the other side of the wafer  98 . 
     The viewing dump  128  can be made from any material which prevents light from emanating from the far wall  142 . One viewing dump is a stack of razor blades welded together. Standard utility razor blades, made from stainless steel may be used to produce an effective viewing dump  128  with a large angle of effectiveness for the incoming light. The blades are virgin stainless steel, and there is no need to coat or paint the blades. The viewing dump  128  made of razor blades presents a light absorbing surface. The viewing dump  128  appears black when viewed from angles, ranging from perpendicular to the front of the viewing dump to angles exceeding 60 degrees from perpendicular. Remaining unwanted light signal, not absorbed by the viewing dump  128 , is due to light scattering from the port  126  holding the window and lens  116  or from light scattered diffusely off of the surface of the wafer  98 . 
     In the exemplary embodiment, the viewing dump  128  comprises a slot or port  127  in the far wall of the reaction chamber  22 , a black box  176  cover and a reflective light capturing device  170 , which may be diamond shaped and reflective. In the exemplary embodiment, one or more of the viewing dump components are anodized. Preferably, the viewing dump components are anodized to a dark color, such as black. Light, which could enter the line of sight of the optics monitoring system  112 , passes through the port or slot into the black box  176 . The light, once in the viewing dump is prevented from reentering the reaction chamber  22  by the light capturing device  170 . The exemplary viewing dump may also have a black window in front of the vie wing dump port  127  and a black over  180 . The window and the cover prevent coating from adhering to the other viewing dump components. The viewing dump may also be any combination of windows with black covers, black covers, black dump boxes, black dump box with light capturing devices, windows with razor blade dumps, and razor blade dumps. 
     It should be apparent to those skilled in the art that there are a number of options for installing the viewing dump  128 . One preferred method is to install the viewing dump  128  inside the reaction chamber, within the confines of a viewing dump port  127  or port extension. Another option is to install the dump  128  outside of the chamber, with a window between the viewing dump and the reaction chamber  22 . Mounting the dump  128  on the outside of the chamber has the advantage of keeping the dump clean and not allowing it to participate in any reactions. One of the advantages of the razor blade type of viewing dump is that the surface condition of the dump blades does not affect the performance of the dump  128 . Even if the dump becomes dirty or covered with coating or coating residues, the light absorption properties of the dump will not degrade. The viewing dump  128 , made from reflective virgin stainless steel, becomes more effective as dirt or corrosion reduces its reflectivity. 
     The preferred viewing dump port  127  or slot is small to minimize flow disturbance of the plasma. A slot, which is compatible with the linear fiber optic array  118 , such as, a slot with a height, which is two times the height of the viewing beam  148  at the far wall  142 , may be,used. A slot, which is twice as high as the viewing beam  148  at the far wall  142 , allows for a margin of error. For example a viewing dump port  127  with a height of ¼ inches may be used for an array of fibers that views a ⅛ high band at the far wall. A larger slot will result in marginally better optical results, but does not justify the additional disturbance of the plasma. The viewing dump, which utilizes a slotted port, allows the optical components of the viewing dump to maintain deposition free surfaces for a longer period of time. 
     In the exemplary embodiment, a bi-convex type lens is used to couple and focus the light from the process chamber into the fiber optic fibers or directly into the spectrometer. The outermost surface of the optics may be a window in front of the lens, or the lens itself, if a protective window is not used. The lens  116  or window  150 , behind which the lens sits is recessed away from the inside of the process chamber  22 . When the outer surface of the optics  140  becomes dirty, light emitted by the by-product is prevented from entering the optics and unwanted lamp light is defracted or deflected off the dirt or coating into the optics, resulting in decreased signal to background and signal to noise ratios. The recessed lens or window, is much less prone to becoming covered with dirt or coating. Further, a narrow port, through which the optics  140  view the process chamber, is used to reduce the likelihood that coating and other contaminants will cover the outermost optical surface of the optics monitoring system  112 . Recessing the outermost surface of the optics  140  substantially reduces the amount of coating which adheres to the window or lens. The signal to noise and signal to background ratios are increased by recessing the optics  140 , because the desired by product light is allowed to enter the optics, while unwanted lamp light is not detracted in. Additionally, the time between required cleaning of the outer lens or window is reduced substantially by recessing the optics. 
     A cylindrical lens can be employed in place of a bi-convex lens to spread the line of sight horizontally across the wafer  98  so that more of the area of concentration above the wafer is sampled. Use of a cylindrical increases the reproducibility of the endpoint determination. The cylindrical lens may be employed with either a circular bundle of fiber optic fibers or a linear array of fiber optic fibers. 
     The exemplary embodiment of the invention, uses a polarizer  122  in conjunction with the lens  116  to reduce background and noise signals. At the relevant wavelengths (e.g., 309 nm) for the etching and ashing processes, a thin film polarizer usually has relatively large absorption, resulting in reduced signal strengths. However, if there is an adequate signal so that part of the signal can be sacrificed, then a thin film polarizer can be used. If the polarizer  122  is a thin film polarizer, it may be inserted between the lens and the window. If signal strength cannot be sacrificed, then a different type of polarizer may be used, which consumes more space along the optical, axis, but does not reduce the signal as significantly. For example, broadband polarizing beam splitter cubes or other components utilizing blocks may be used. If the polarizer  122  is large, it could be positioned between the lens  116  and the array of fiber optic fibers,  118 . 
     In the exemplary embodiment, viewing optics  140  are focussed on the wafer surface. By focussing the optics  140  on the surface of the wafer at an angle such that the entire line of sight  154  is in the concentrated area of the by-product, signal strength is improved. The distance between the line of sight of the optics and the wafer surface which is acceptable depends on a number of parameters, such as the minimum spot size of the focal point  130 , the angle of viewing with respect to the plane of the wafer surface, the distance between the focal point  130  and the edge  124  of the wafer, the flow rate of the gas through the process chamber, and the particular process being performed. 
     Referring to FIG. 14, a graph of signal intensity, measured by the spectrometer, is plotted over the operational time of the asher. The prior art signal is labeled  162  and the inventive signal is labeled  166 . The graph shows that the unwanted stray light, which is most prevalent during ramp is greatly reduced by implementation of the current invention. The graph also shows that the desired by-product signal is greatly increased by implementation of the present invention. 
     Referring to FIG. 10, for a typical focal spot size of 0.1 inch diameter, one achieves a strong by-product signal if the focal point is within 0.4 inch of the wafer surface. The peak intensity is obtained within 0.2 inch of the wafer surface. The signal decreases as the wafer surface is approached. The decrease in signal intensity near the surface is due to the fact that the viewing optics are viewing nearly tangential to the wafer surface and the optics are integrating along the tangential line-of-sight. By-products are swept along above the surface of the wafer, radially outward, by the laminar flow of plasma. The concentration of line-integrated by-product is higher slightly above the wafer surface due to the viscous drag of the laminar flow at the wafer surface. 
     Preferably, the optics monitoring system  112  views the wafer  98  from an angle corresponding to the Brewster angle for the material the wafer is made from. For typical wafer materials, such as silicon, the angle between the viewing optics and a focused region on the wafer surface would be 16.2 degrees. The optics monitoring system would ideally be focused on the wafer, surface  136  at a far side  124  of the wafer  98  relative to the optics monitoring system  112 . The feasibility of viewing the wafer at the Brewster angle is dependant on the aspect ratio of the reaction chamber  22 . For certain chambers, for example, the vertical dimension of the reaction chamber  22  may be too small to allow the lens  116  to focus on the far side of the wafer at the Brewster angle. 
     In the preferred embodiment, the light entering the optics monitoring system  112  is polarized to increase the signal to noise and signal to background ratios. The wafers  98  used in semiconductor processing tools are typically made of the dielectric materials, such as silicon. The properties of the dielectric material determines the angle at which incident light is reflected. The reflective properties depend on the polarization of the incoming light. This phenomena is applied to improve the signal-to-background of the optical detection system  110 . 
     As endpoint is approached a thin layer of remaining coating is removed relatively quickly. Therefore, the only material that needs to be monitored with regard to reflective properties is the dielectric material of the wafer  98  itself. The dielectric constants of various coating materials do not need to be considered. 
     For typical silicon wafers, the reflectivity is fairly high. However, the reflected light can be polarized by adjusting the viewing angle to the Brewster angle θ B . For ac given material, the Brewster angle θ B  is defined as: 
     
       
         θ B =tan −1 (n 2 /n 1 ) 
       
     
     θ B  is the incident angle relative to the normal to the surface. n 2  is the index of refraction of the material from which the light is incident, essentially the vacuum of the process chamber. n 1  is the index of refraction of the material onto which the light is incident, the silicon or other wafer material. At the Brewster angle θ B , all light with polarization of the electric field which is in the plane of incidence is transmitted into the second material, with no reflected component. The plane of incidence is defined as the plane which contains both the incident and reflected or refracted rays of light. 
     In the one embodiment, the optics monitoring system views the wafer from the Brewster angle θ B . Any light reflected by the wafer material that is monitored by the optics monitoring system is composed of only one polarization. A polarizer  122  is included in the optics monitoring system  112  to remove the remaining polarized component of the reflected light. The wafer material will have removed one component of the light incident on the wafer, and the polarizer  122  removes the remaining component. At endpoint, when all of the coating is removed, no light is measured by the spectrometer, because the wafer viewed at the Brewster angle θ B  removes one component of the light reflected off the wafer and the polarizer removes the remaining component. While the polarizing system also reduces the desired signal from the by-product, the background is reduced by a greater magnitude, yielding an increased signal-to background ratio. 
     The calculated optimum angle for silicon wafers is approximately θ B =73.8 degrees, which translates into an angle of 16.2 degrees between the optics and the surface of the wafer, resulting in a 3.44-to-1 ratio of the distances between horizontal and vertical between the focal point  130  and the optics monitoring system  112 . An aspect ratio of 3.44-to-1 is quite compatible with the construction of a typical reaction chamber  22 . 
     FIGS. 13A-13C show how the focal length L of the selected lens affects the viewing beam  148  of the optics monitoring system  112 . By selecting a lens with a short focal length, the viewing beam is narrowed allowing light only along the path where the emission signal is the strongest. Using a lens having a short focal length reduces the signal strength, because the viewing beam  148  is narrower. However, signal-to-background is increased because the amount of background monitored by narrower beam is also reduced. FIG. 13C demonstrates the results of implementation of short focal-length. 
     FIG. 13A shows the original configuration, with the lens  116  and fiber optic fibers  118  on the left and the wafer located at the bottom of the figure. The wafer is shown slightly below the rays depicting the viewing beam  148 . In FIG. 13A one can see that the viewing beam axis must be located well above the wafer in order to not cut off rays near the edges  124  of the wafer. Shorter focal-lengths are presented in FIGS. 13B And 113C. A lens with a shorter focal length allows redirection of the rays before they spread much, which maintains a beam-like quality of the viewing beam  148 . A narrower viewing beam can be lowered closer to the wafer surface  136  and thereby receive light from the regions just above the wafer surface  136  where the by-product signal is the strongest. 
     It is appreciated that while the invention has been described with a degree of particularity it is the intent that the invention include all modifications from the disclosed design falling within the spirit or scope of the appended claims.