Abstract:
In a workpiece process end point detection system, light is diffused and then light intensity or color is sensed. Optical noise is greatly reduced and more accurate end point detection can be made. A light emitter and a light sensor may be located within a workpiece process chamber. A housing around the light emitter and the light sensor seals out process fluids and also diffuses light passing through. The diffused light may be optically filtered before reaching the light sensor.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to processing of workpieces, such as semiconductor wafers. Semiconductor and similar microscale devices are typically manufactured by performing many separate steps on substrates or wafers. The workpieces are often coated or plated with multiple layers or films of different materials. Process chemicals, typically etchants in liquid form, are applied to the workpieces to selectively remove one or more layers. Often, a layer on the workpiece is patterned generally using photolithographic methods, and only portions of the layer are removed. 
   In workpiece processing, it is often important to determine the end point of a process. In a layer or film removal process, the end point is defined as the point at which all of the targeted layer has been removed, exposing an underlying layer beneath the targeted layer. Extensive processing after the end point can waste time and process chemicals, and even damage the workpiece in more extreme cases. On workpieces patterned with photoresist, continuing to process the workpiece beyond the end point may undercut the photoresist and decrease the critical dimension of the microscopic device features formed on the workpiece. On the other hand, if a process is stopped before the correct end point, the workpiece will not be fully processed. For example, a layer of material which must be removed to achieve proper manufacturing may partially remain on the workpiece, or remain on certain areas of the workpiece. As a result, the workpiece would then require re-work, or have to be discarded. 
   In the past, the process time of etching processes has been determined strictly by a specific predetermined time interval (e.g., 120 seconds) which is known to be sufficient to remove the film and also include some over-etch to insure complete processing. Etching process times have also been determined visually by observing the workpiece through a window in the process chamber and noting a color change as the underlying film is exposed. A human operator then adjusts the process time, process chemical flow rates, or other parameters to try to optimize the processing, independent of variations that may arise, such as changes in process chemical concentration, temperature, variations in film thickness and film quality, etc. 
   Various automated methods using sensors and computers have largely replaced visual end point detection by a human operator. These methods include using electrical, optical, or even chemical measurements. Optical techniques are advantageous as they can be fast, reliable, and easier to perform. With optical end point detection methods, light intensity and/or color is measured. The end point is reached when a predefined condition in the intensity profile is met. However, in cases where the intensity change is slight, for example where color change between films is subtle, or where the percentage of area cleared is low, electronic or optical noise can mask detection of the endpoint. In addition, some process chambers are made of plastic materials, to better resist corrosion by process chemicals. These plastic materials, including fluorine resin materials, are not necessarily opaque. As a result, stray light may penetrate into the chamber, making it more difficult to achieve accurate optical measurements. Reflection and diffraction of light in the process chamber by droplets of chemical process liquids may also create errors in optical measurements. As a result engineering challenges remain in the design of optical end point detection. 
   SUMMARY OF THE INVENTION 
   A novel end point detection system has now been developed which overcomes the drawbacks of existing systems and also provides additional advantages. In one aspect, the present end point detection system diffuses emitted light directed at the workpiece, and/or reflected light impinging on a light detecting element. By diffusing the light, optical noise (for example resulting from a spray of liquid inside the process chamber) is greatly reduced. Accurate end point detection is improved. 
   In another and separate aspect, a light emitter and a light detector are located within a workpiece process chamber. A housing around the light emitter and the light detector in the process chamber, is at least partially translucent, allowing sufficient light to pass through to perform end point detection. The housing may also prevent potentially corrosive process liquids or gasses from coming into contact with the light emitter and detector. Placing the light emitter and detector within the process chamber provides for accurate end point detection with a highly compact system. Precise positioning or alignment of optical elements is also not necessary, as the system does not rely on specular reflected light. 
   In another and separate aspect, in a method of end point detection, light from a light source in a process chamber is diffused and directed generally towards a workpiece. A process fluid, such as a liquid etchant, is applied to the workpiece, typically while the workpiece is spinning. The process fluid removes a film on the workpiece surface. This causes a change in one or more optical properties of the workpiece, such as reflectance and color. Light reflected from the workpiece is detected via a light detector also in the process chamber. The reflected light may be diffused before it is detected. An output signal from the light detector is used to determine a process end point. The light reflected off of the workpiece may optionally be filtered before it is detected. 
   The invention resides as well in subcombinations of the systems, components, and method steps shown and described. The invention may of course be practiced in other forms without necessarily achieving each of the advantages described. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a processing system. 
       FIG. 2  is a plan view of the system shown in  FIG. 1  (with the top enclosure surface removed for illustration). 
       FIG. 3  is a top perspective view of one of the processors shown in  FIG. 2 . 
       FIG. 4  is a bottom perspective view of the processor shown in  FIG. 3 . 
       FIG. 5  is a section view taken along line  5 - 5  of  FIG. 4 . 
       FIG. 6  is a section view taken along line  6 - 6  of  FIG. 4 . 
       FIG. 7  is an enlarged detail view of the lower section of the processor shown in  FIG. 6 . 
       FIG. 8  is an inverted perspective view of the end point detector shown in  FIGS. 6 and 7 . 
       FIG. 9  is an enlarged section view taken along line  9 - 9  of the  FIG. 8 . 
       FIG. 10  is an exploded perspective view of the light emitter shown in  FIGS. 7-9 . 
       FIG. 11  is an exploded perspective view of the light detector shown in  FIGS. 7-9 . 
       FIG. 12  is an enlarged section view of the upper end of the light detector shown in  FIGS. 8-11 . 
   

   DETAILED DESCRIPTION 
   The systems and methods described may be used to process workpieces, such as semiconductor wafers, flat panel displays, hard disk media, CD glass, memory and optical media, MEMS devices, and various other substrates on which micro-electronic, micro-mechanical, or micro-electromechanical devices are or can be formed. These are collectively referred to here as workpieces or wafers. Descriptions here of semiconductors, or the semiconductor industry or manufacturing processes, also includes the workpieces listed above, and their equivalents. 
   Turning now to the drawings, as shown in  FIGS. 1 and 2 , as one example, a processing system  20  includes one or more processors  24  within an enclosure  22 . A control/display panel  26  is typically provided with the processing system  20 , to allow system operators to monitor various operations, as well as entering or modifying system instructions and software. One or more computer controllers  28  is also typically provided with the processing system  20  for controlling various system operations. The computer controller  28  may be connected via data links to one or more other computers associated with the manufacturing facility. Workpieces or wafers  60  are typically moved to the processing system  20  in boxes or containers  30 . One or more robots  32  within the system  20  carry workpieces  60  between the containers  30  (or other input/output assembly or location) and one or more of the processors  24  (or between different processors). The system  20  may include various features described in U.S. Pat. No. 6,900,132 incorporated herein by reference.  FIGS. 1 and 2  show a representative system  20 . The size, shape, and arrangement of components in these Figures are not essential. Various other systems may also be used. 
   Turning now to  FIGS. 3-6 , a typical processor  24  is shown. Various types of processors  24  may be used, for example, spray acid processors, spray solvent processors, spray coating processors, etc. The processor  24  may also include a centrifugal swing arm spray component, such as described in U.S. Patent Publication No. 2004/0013797, incorporated herein by reference. 
   Referring to  FIGS. 3-6 , in the example shown, the processor  24  has a head  42  which may be lowered into engagement with a bowl  40 . The head  42  may be lifted away from the bowl  40 , and optionally inverted, to load or unload a workpiece, via a head lifter or elevator (not shown). The head  42  advantageously holds and spins a workpiece  60 . Although various workpiece holding designs may be used,  FIGS. 5 and 6  show a workpiece  60  held by fingers  62  supported on a rotor  64 . The rotor  64  may be rotated by a motor  66 . A process liquid or gas inlet  68  may optionally be provided to introduce a process liquid or gas onto the up-facing side of the workpiece  60  (typically the back or non-device side). 
   The bowl  40  may have various process liquid or gas inlets  44 , and a liquid drain assembly  46 . Spray nozzles  56  and/or flood nozzles may also be used. The use of liquid and/or gas nozzles, the number and types of nozzles, as well as the process liquids and gasses used, may vary with the design of the processor  24  and with the specific manufacturing steps to be carried out by the processor  24 . 
   Referring now to  FIGS. 6 ,  7 , and  8 , an end point detector generally designated at  50  is provided on or in the processor  24 . The end point detector  50  includes a light emitter assembly  84  and a light detector assembly  86 , which may be held in place by a mounting collar  82 . The mounting collar  82  may fit within an opening in the bottom of the bowl  40 , and may be sealed via an O-ring  104 . 
   Turning specifically to  FIGS. 7 ,  9 , and  10 , the light emitter assembly  84  may include one or more LEDs  92 , or other light emitting element, positioned within a housing or tube  89 . In the design shown, a cluster of four LEDs  92  is used. Single wavelength light sources can provide improved performance. The top end  85  of the tube  89  is closed, while the bottom end of the tube  89  may be left open. A spacer  93  and a sleeve  91  hold the LED  92  in position adjacent to the top end  85 . A wire fitting  110  is attached to the bottom end of the tube  89 . The tube  89  is advantageously made of a translucent material which is resistant to process chemicals, e.g., a material such as fluorine resins (Teflon). 
   Referring now to  FIGS. 7 ,  9 ,  11 , and  12 , an optical filter  94  may be positioned above a light sensor  96  in an end cap  95  having an open top end  97 . The end cap  95  containing the sensor  96  and filter  94  is positioned within a housing or tube  99 , similar to the tube  89 . The end cap  95  is secured in place adjacent to the upper end  85  of the tube  99 , shown in  FIG. 11 , by a sleeve  91 , when a wire fitting  110  is attached to the lower end of the tube  99 . Although the LED  92  and light sensor  96  are shown in the process chamber, adjacent to the workpiece, one or both of them may also be located elsewhere, and provided with an optical link (such as a fiber optic element) into the chamber. 
   As shown in  FIG. 9 , O-rings  102  seal the mounting collar  82  around the outside surface of the light emitter assembly tube  89  and detector assembly tube  99 . An opaque shield ring  88  may be provided at the upper end of the light emitter assembly  84 . Wire leads  106  extend up through the open bottom end of the light emitter assembly  84  to the LED  92 . Similarly, wire leads  108  extend up through the open bottom end of the detector assembly  86  to the light sensor  96 . The wire leads  106  and  108  connect directly or indirectly to the computer controller  28 . A purge gas outlet  98  may be provided adjacent to the top end of the detector assembly  86 , with a nozzle positioned to direct a spray of gas onto or over the top end  85  of the detector assembly  86  and/or the light emitter assembly  84 . A split positioning collar  90  may be loosened to allow for vertical positioning of the light emitter assembly  84  and the light detector assembly  86 , and then retightened to secure them into place. 
   As shown in  FIG. 5 , the top end  85  of the detector assembly  86  is positioned below the workpiece  60  by a dimension A which may be 5-25 mm, 10-20 mm, or about 15 mm. The upper end of the light emitter assembly  84  may be at the same vertical position, or higher or lower than the detector assembly  86 . 
   As shown in  FIG. 6 , the detector assembly  86  is located between the center and edge of the workpiece  60 . The center of the workpiece  60  is nominally on the center line/spin axis  70  of the head  42 . The dimension S in  FIG. 6  showing the position of the detector assembly  86 , is generally 20-80%, 30-70%, 40-60%, or about 50% of the workpiece radius R. 
   In use, a workpiece  60  is loaded into the processor  24 . Processing is carried out on the workpiece  60  by spinning the workpiece and by applying one or more liquid, gas, or vapor process chemicals. The LEDs  92  in the light emitter assembly  84  are switched on. The light emitted from the LEDs  92  is preferably directed upwardly in a direction substantially perpendicular to the workpiece  60 , as indicated by the dotted lines in  FIG. 5 , although the specific direction of emitted light is not essential. The end  85  of the tube  89  is sufficiently thin (typically 0.5-4 mm or 1-2 mm), and the tube material is transparent or translucent enough to allow sufficient light to pass through the end  85  and impinge on the workpiece  60 . The light is diffused as it passes through the tube end  85 . As the workpiece is processed, the reflectance or color of the workpiece changes. For example, where a metal layer overlying a dielectric layer is etched, the reflectance of the workpiece surface decreases (since the metal layer is more reflective than the dielectric layer). The color may also change. 
   Consequently, the amount of light and/or the color of light reflected from the workpiece changes as processing proceeds. Reflected light is detected by the detector assembly  86 . Reflected light passes through the end  101  of the tube  99 , through the filter  94  (if used) and to the sensor  96 . The tube end  101  acts to diffuse light entering the detector assembly  86 . The sensor  96  provides a voltage output which varies with the amount of light, and color of light impinging on the sensor  96 . The filter  94  is preferably selected to pass light of the same wavelength as the light emitted by the LEDs  92 . Accordingly, stray light, for example, ambient room light passing through the walls of the bowl  40 , is effectively filtered out. Electronic filters and/or signal processing may be used with, or instead of, the optical filter  94 . The LEDs may also be driven in an electronically chopped mode, to suppress noise from stray light sources. The signal from the sensor  96  is provided to the computer controller  28  through the wire leads  108 . The computer controller  28  then determines when processing is complete, based on the output from the sensor  96 . Specifically, the computer controller  26  processes the signal from the sensor  96 , using known techniques which may be based on initial calibration of the processor and the specific process steps used. 
   If necessary, purge gas may be sprayed onto the top end  85  of the detector assembly  86 , to remove excessive liquid droplets which may interfere with light entering into the detector assembly. The ends  85  and  101  of the light emitter and light detector assemblies may be curved to help to avoid any accumulation of liquid on them. 
   While the processor  24  is shown in an upright or vertical orientation, with the workpiece horizontal, the end point detector  50  may operate with processors having other orientations. In addition, while the end point detector  50  is shown in a single wafer processor, it may also be used in batch processing. 
   The optical filter  94  may optionally be replaced by electronic filters. The LEDs  92  may be selected based on the specific materials on the workpiece  60 . The difference in reflectivity of various films and layers typically changes with the wavelength of light. Accordingly, for processors  24  intended for processing workpieces  60  having specific films or layers, the LEDs may be selected so that the change in reflectance is increased. For example, copper is a good reflector of near infrared light, whereas Titanium/Tungsten is a poor reflector of near infrared light. For applications where a copper layer overlying a Titanium/Tungsten layer is etched, LEDs emitting at near infrared wavelengths may be selected over other types of LEDs, to increase the change in reflectance. This provides an end point detector  50  having a greater sensitivity. 
   The computer controller  28  may be programmed to correctly determine the end point of various different processes. When the end point is detected, processing will generally be continued for a predetermined amount of additional time. The computer controller  28  may run consecutive process steps. The computer controller  26  can identify end points of each of the steps, and then control the processor  24  to stop processing, continue processing for a specific interval, commence rinsing, or take other action. The computer controller may be programmed to monitor the light intensity profile as the workpiece is processed. Once a consistent pattern of intensity change is established, a detection routine may be used to determine the end point. An absolute reflectance threshold may be used. For example, determining that the end point occurs when absolute reflectance drops below 50%, 40%, 35% or 30%. Alternatively, the first derivative of the reflected light signal with respect to time may be calculated to look at change in reflectance, rather than absolute reflectance. This may be especially useful for transparent films where the signal can vary sinusoidally (due to Fresnel reflectance as the film becomes thinner). Here, the end point occurs where the derivative of the reflected signal stabilizes at zero (i.e., constant reflectance) after crossing that value several times during the etch process. 
   Since the light emitted from the LEDs  92  is diffused as it passes out from the light emitter assembly  84 , and as the light entering the detector assembly  86  is similarly diffused before reaching the sensor  96 , alignment and/or positioning of the light emitter and detector assemblies is not critical. 
   Thus, a novel system, processor, and end point detector, and corresponding methods have been shown and described. This invention, therefore, should not be limited, except to the following claims and their equivalents.