Abstract:
The present invention describes systems and methods for exposure and removal of material on the periphery of a substrate. The system includes an emitting radiation or exposure source, preferably a guide such as an optical assembly and an emitter, an edge detector, a transport, which comprise rotating and radial mechanisms, and a substrate support. The optical assembly directs radiation from the exposure source to the wafer or emitter. The transport supports the emitter and the edge detector about the substrate, and a tracking exposure head preferably supports the emitter and edge detector. As the emitter moves along the periphery of the substrate, the edge detector sends signals to control systems, which process signals, and command the transport to adjust the position of the emitter when necessary to expose the periphery of the substrate. In another embodiment, the system provides an exposure source chamber, a process chamber, and optionally a control chamber to isolate particle and thermal problems. In another embodiment, the wafer is in a rotating drum, which supports a radial mechanism, supporting a tracking exposure head holding the emitter and edge detector. The drum and tracking exposure head have guidance components and stopping mechanisms.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to systems and methods for exposing material on the periphery of a substrate. 
     In the manufacture of integrated circuits (ICs) and semiconductor devices, the process of spin coating is used to apply photoresist (resist) on semiconductor wafers. Spin coating places a wafer on a chuck, dispenses resist on the wafer, and spins the wafer to distribute a thin film of resist over the surface of the wafer. The centrifugal force distributes resist onto the periphery of the wafer, but some ICs and devices cannot be made in the periphery. In fact, wafer handlers, pick-and-place machines and wafer cassettes may contact the periphery, causing resist to peel off and contaminate the wafer. 
     To prevent these problems, systems have been designed to expose and remove the resist on the periphery. One system includes a chuck for rotating a wafer located below a non-rotating fiber optic emitting exposing light at the periphery of the wafer. A non-rotating sensor detects the wafer edge and sends signals to memory of a control system after traversing the edge of the entire wafer, a process referred to as wafer mapping. After wafer mapping, the control system sends signals based on the stored signals to a radial transport arm, which moves the fiber optic in the radial direction to control the depth of exposure on the wafer. 
     This system has several problems. The rotating chuck assembly can generate airborne particles, which contaminate the wafer, and heat, which impacts the wafer temperature uniformity. Wafer mapping requires additional time and thus reduces throughput. Further, failure to isolate the wafer from the heat generated by the lamp of the exposure source assembly and from the controller subjects the wafer to unnecessary heat and particle contamination. 
     An ideal system today should be able to accurately expose the periphery of different diameter wafer such as 200 mm and 300 mm with notches, clips, flats and/or edge irregularities, expose a predetermined width, such as from 0.5 mm to 10 mm from the wafer edge, have high throughput such as 100 or more wafers per hour, and make sharp transitions from exposed to non-exposed resist without varying the temperature uniformity in a contamination free environment. It should be easy to maintain, operate, manufacture, reliable, and compact. 
     SUMMARY OF THE INVENTION 
     The invention provides systems and methods of exposing the periphery of a substrate. In an embodiment, the system includes an exposure source coupled to a guide directing radiation to a wafer, an edge detector, a transport, a support, and a control system. The guide includes a fiber optic and/or an optical system such as a catadioptric optical assembly. The transport moves the guide along the periphery of the substrate during exposure. The transport may achieve this by radial and rotating mechanisms. In one embodiment, the rotating mechanism supports the radial mechanism supporting the guide and the edge detector. The edge detector sends signals to a control system, which processes the signals and controls the transport to adjust the position of the emitting guide when necessary to expose the periphery. 
     In other embodiments, the system includes an exposure source chamber, a process chamber, and a control chamber to isolate the substrate from particulate and thermal contamination. The rotating mechanism may include a drum to house the substrate and support the radial mechanism. The radial mechanism preferably supports a tracking exposure head holding the edge detector and the guide emitter adjacent the substrate. The rotating and the radial mechanisms move the emitter and the edge detector subject to guidance and stopping mechanisms. A fiber optic assembly preferably directs radiation from an exposure source to the wafer. The edge detector provides signals to control the tracking exposure head to adjust for edge irregularities. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the system, which removes enclosure walls to illustrate the components in the exposure, process, and control chambers of the system. 
     FIG. 2A is an elevation view illustrating the system of FIG.  1 . 
     FIG. 2B is a schematic of the exposure source assembly in an inner enclosure in the exposure source chamber. 
     FIG. 3 is a perspective view of the system illustrating the inner enclosure housing the exposure source assembly shown in FIG. 2B, and a guide leading from the source to the emitter supported by a tracking exposure head located adjacent a rotating drum. 
     FIG. 4 is a perspective view of the rotating drum shown in FIG. 3, with details regarding components mounted on the rotating drum. 
     FIG. 5 is a perspective view of an embodiment of the tracking exposure head. 
     FIG. 6 is a perspective view of an embodiment of the top portion of a radial mechanism. It illustrates the slides, the rails, the linear encoder and motor used to move the tracking exposure head. 
     FIG. 7 is a perspective view of the floor of the process chamber including the guide and stopping mechanisms for the rotating mechanism. 
     FIG. 8 is a perspective view of the system of FIG. 1 showing air and wire ducts. 
     FIG. 9 is an alternate embodiment of the guide for delivering radiation from an exposure source to a wafer. 
     FIG. 10 is a block diagram illustrating an embodiment of the control system. 
     FIG. 11 is a block diagram of the r-axis module of the control system. 
     FIG. 12 is a schematic of the optical system employed in the embodiment shown in FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a system  10  to expose and remove radiation-sensitive material on the periphery of a substrate. For brevity, we describe exposure of resist on semiconductor wafers, but other applications are possible. The system  10  includes a mounting plate  14 , with a passage  15 , and a pair of L-shaped brackets  16  (one shown) to secure the system  10  to a rigid frame (not shown), and a leveling arm  65 . A set of module holding and level arms  18 ,  20 , and  22  level the system  10  to a load/unload robot, not shown. An exposure source chamber  24  houses an exposure source assembly  38  (FIG.  2 B), a process chamber  26  houses a wafer  12  during exposure, and a control chamber  28  houses a control system. 
     FIG. 1 shows the exposure source chamber  24 , the process chamber  26 , and the control chamber  28  as stacked from top to bottom, but this arrangement is not essential. Each chamber preferably includes access for service and maintenance. The three-chambers isolate heat generated in the exposure source chamber  24  and in the control chamber  28  from the process chamber  26 . 
     The exposure source chamber  24  contains an inner enclosure  30 , protecting the exposure source assembly  38  (FIG. 2B) and providing a safety barrier to touching the exposure source assembly  38  when the access door is removed. If the exposure source assembly  38  contains an ultraviolet (UV) light source, the inner enclosure  30  also encloses UV radiation for operator safety. The inner enclosure  30  also helps manage thermal and particulate contamination. Process requirements determine what exposure source is used. For example, mercury lamps, such as mercury xenon and excimer lasers can expose the resist on wafers. More specifically, a suitable exposure source for deep ultraviolet resist centered on 248 nm is the Hypercure 200 manufactured by Hologenix in Huntington Beach, Calif. A suitable inner enclosure  30  and exposure source assembly  38  can be obtained as a single unit from Optical Associates, Inc. (OAI) of Milpitas, Calif. (e.g., part no. 0860-0041-01). 
     The exposure source chamber  24  has a floor  32  with a passage  34  leading to the process chamber  26 . Cooling gas such as air is drawn from the process chamber  26  through the passage  34  to cool the inner enclosure  30  and prevent particulate from falling through passage  34  into the process chamber  26 . A duct  140  (FIG. 8) also pulls gas such as air from the process chamber  24  and is connected to the exhaust  142  from control chamber  28 . 
     FIGS. 1 and 2A illustrate a transport for moving an emitter  41  emitting radiation and an edge detector  43  along the periphery. The emitter  41  is part of and/or compatible with a fiber optic and/or an optical system as will be discussed below. As shown in FIGS.  2 A and  3 - 4 , the transport includes a rotating mechanism and a radial mechanism. The rotating mechanism includes a drum  42 , including an upper cover  44 , a lower cover  46 , and a wall  48  with a wafer passage  49 , to reduce particle contamination on the wafer  12 . As the rotating mechanism rotates, the emitter  41  rotates about the periphery of the wafer  12 . Although a rotating drum is the preferred, FIGS. 1 and 2A show the rotating mechanism can be just the upper cover  44  or a rigid non-contaminating structure capable of controlled rotation adjacent the wafer  12 . The radial mechanism  56  transports the emitter  41  and an edge detector  43  over the wafer  12  in the radial direction, but is located over the upper cover  44  to reduce particle contamination of the wafer  12 . In the preferred embodiment a tracking exposure head  40  supports the emitter  41  and the edge detector  43 . 
     The control chamber  28  includes a pair of parallel guides  62 ,  64  for rollout of an electronics tray  66  supporting a main control system  68  including a computer preferably but not necessarily capable of interfacing with standard storage and peripherals and input devices such as a mouse, a keyboard and/or keypad, and standard output devices such as a display and/or a printer. The control chamber  28  includes an exhaust duct  142  to remove heat generated by the main control system  68 . The process chamber  26  includes a wafer passage  136  for loading and unloading the wafer  12  on and off the chuck  50  supported by a base  51 . 
     FIG. 2A is an elevation view of the system  10  shown in FIG.  1 . FIGS. 2A-2B illustrate that the fiber optic assembly  36  terminates in a roller and thrust bearing assembly  72  in the inner enclosure  30 . The fiber optic assembly  36  spins in the roller and thrust bearing assembly  72  when a rotating mechanism such as drum  42  (FIGS. 3-4) rotates. FIG. 2A shows the fiber optic assembly  36  leads down to the emitter  41  on the tracking exposure head  40  adjacent the wafer  12 . 
     The upper cover  44  supports an r-axis servo controller printed circuit board (PCB)  74 , a charge coupled device (CCD) counter PCB  76 , an amplifier PCB  78 , and an amplifier resonator  79 . OAI provides a suitable servo controller PCB  74  (e.g., OAI part no. 080-0005-01) and a CCD counter PCB  76  (e.g., OAI part no. 0860-0003-01). The CCD counter PCB  76  incorporates a 13-micron pitch CCD such as OAI part no. 5500-007-01, which communicates with a CCD sensor chip  59 , which is supported on the tracking exposure head  40 . One suitable sensor chip  59  is part no. 08600002-02 made by OAI. Amplifier PCB  78  and amplifier resonator  79  function as a motor amplifier driving the radial mechanism motor  56 . 
     Nanomotion, Ltd of Yokneam, Israel makes a suitable amplifier PCB  78  (e.g., OAI part no. 4900-116-01) and a suitable amplifier resonator  79  (e.g., OAI part no. 4900-118-01). 
     The dedicated controller PCBs  74 ,  76 , and  78  locations on the upper cover  44  reduce the number of lines from the main computer  68  to the radial mechanism  56 . The CCD counter PCB  76  communicates with a CCD sensor  59  on a PCB supported by the tracking exposure head  40 . Each controller PCB  74 ,  76 , and  78  preferably includes a conventional particulate cover not shown. 
     FIG. 2A shows a rotary motor  80  for rotating the drum  42 . A drum and cable tray  82  located below the lower cover  46  stores a flat cable (not shown), which carries power and control lines from the control chamber  28  to the motor  80  as well as to the controller PCBs  74 ,  76 , and  78 . The flat cable is arranged in a spiral in the drum rotary motor and cable tray  82  to permit the flat cable to wind up and unwind as the drum  42  rotates without stressing the flat cable. The flat cable exits the tray  82  and is secured to the cylindrical wall  48  of the rotating drum  42  and to the controller PCBs  74 ,  76 , and  78 . 
     FIG. 2B illustrates details of the exposure source assembly  38 . The exposure source assembly  38  includes a lamp  86 , e.g., the mercury lamp mentioned earlier, in a UV optimized reflector  88  and supported by an inner enclosure frame  91 , partially shown. One suitable lamp  86  is OAI part no. 3800-162-01. A lamp intensity probe  90 , next to the reflector  88  and the frame  91 , monitors the exposure energy output of the lamp  86  so it can be replaced before output falls below requirements. One suitable probe  90  is the OAI part no. 0860-0030-01, preferably traceable to and calibrated at the United States National Institute of Standards and Technology (NIST). NIST traceable means NIST documents the calibration and the equipment used to calibrate the probe  90 . The reflector  88  is preferably a detachable part of the exposure source assembly  38 . Thus, in this preferred embodiment, replacing the reflector  88  will not require replacing the lamp  86 . The reflector  88  focuses radiation from the lamp  86  in a cone shape  184  whose apex  186  aligns to fill the source end of the fiber optic assembly  36  within the roller bearing  72 . 
     FIG. 3 is a perspective view of the system  10  with the enclosure walls removed to illustrate components of the exposure source chamber  24  and the process chamber  26 . FIG. 3 shows the inner enclosure  30 , the floor  32  separating the exposure source chamber  24  and the process chamber  26 , the fiber optic assembly  36 , the rotating drum  42 , the tracking exposure head  40 , the emitter  41 , the radial mechanism  56 , the controller PCBs  74 ,  76 , and  78 , the resonator module  79 , the upper cover  44 , the rotating drum wall  48 , the wafer passage  49 , the rotary drum and cable tray  82 , the floor  96  separating the process chamber  26  from the control chamber  28 , the rotary limit switch guide wire  120 , the block follower  114 , the arm follower  116 , the block follower guide  118 , and the guide block  126 . The components in the lower portion of the process chamber  26  will be further discussed in connection with FIG.  7 . 
     FIGS. 4-6 show details of the components of the system  10 . FIG. 4 illustrates that the upper cover  44  supports the controller PCB  74 ,  76 , and  78 , and the radial mechanism  56 , including a photo sensor  98  and slide stop block  100 , to prevent the L-shaped plate  52  from sliding too far in operation in accordance with control signals from the photo sensor  98 . The photo sensor  98  also determines the home position of the radial mechanism  56 . The home position and the functions of the controller PCB  74 ,  76 , and  78  will be further described below in connection with FIGS. 10-11. 
     FIG. 5 is a perspective of the tracking exposure head  40 , which supports the emitter  41  (FIG. 1) and the edge detector  43 . The tracking exposure head  40  includes a bracket  54  attached or integral and perpendicular to the L-shaped plate  52 . The bracket  54  supports the edge detector  43  including a light emitting diode (LED)  58  facing the CCD sensor  59 . The CCD sensor  59  captures radiation from the LED  58  not covered by wafer  12  to detect the edge of the wafer  12  as will be described. A suitable LED  58  is OAI part no. 080-0018-01. 
     A crash sensor  61  detects obstructions in the path of the tracking exposure head  40 , and triggers a protection logic  164  as discussed below in connection with FIG. 11, which commands the motor  108  to stop further radial movement of the tracking exposure head  40  until the obstruction is cleared. 
     FIGS. 4-6 show the radial mechanism  56  is attached to the L-shaped plate  52  of the tracking exposure head  40 . As shown in FIG. 5, the radial mechanism  56  itself includes a fixed slide base  102 , mounted on the upper cover  44 , and a slide carriage  104  with outer cross roller bearing rails  105  and  107 , sliding on parallel inner cross roller bearing rails  110  and  112  of the fixed slide base  102 . A linear encoder  106 , a piezo-ceramic linear motor  108  supported by an encoder-motor mating bracket  109 , and an encoder tape (not shown) bonded to the side of the movable slide carriage  104  and adjacent to the linear encoder  106  provide controlled radial movement of the tracking exposure head  40  with respect to the wafer  12 . One suitable radial mechanism  56  is OAI part no. 0860-0010-01. 
     The radial mechanism  56  transports the tracking exposure head  40  supporting the emitter  41  into position for exposure of the periphery of the wafer  12 , and the edge detector  43  comprising the CCD sensor  59  and the LED  58 . The linear encoder  106  measures the position of the sliding carriage  104  of the radial mechanism  56 , which indicates the position of the CCD sensor  59 . The CCD sensor  59  signals are a function of the linear position of the slide carriage  104  and the tracking exposure head  40 . For example, if the motor  108  moves the radial mechanism  56  10 mm toward the center of the wafer  12 , the CCD sensor  59  will see 10 mm more of the wafer  12 , that is, the wafer  12  will block some calculable amount of light from the LED  58  from being captured by the CCD sensor  59 . 
     FIG. 7 is a perspective view of the components used to rotate, guide and stop the rotation of a rotating mechanism such as the drum  42 . In one embodiment, the components are mounted on the floor  96  of the process chamber  26 . The floor includes a hole  130  for the motor  80  (FIG. 2A) to rotate the drum  42  (FIG. 3) as well as a passageway  132  for the electrical cables such as those associated with the motor  80 . As shown in FIGS. 3 and 7, the block follower  114  pivots at the lower cover  46  (FIG. 2A) of the drum  42  and at the pivot point of an arm follower  116 , which in turn pivots on a block follower guide  118 . The block follower guide  118  includes an aperture, which permits it to slide with respect to a guide wire  120 . The guide wire  120  includes a reflective sensor  122 , held by a reflective sensor bracket  124 , which detects when the block follower guide  118  is adjacent with the sensor  122 , which provides a control signal to stop the motor  80  rotating the drum  42 . The opposite ends of the guide wire  120  include a reflective sensor  127 , held by a reflective sensor bracket  129 , which detects when the block follower guide  118  is adjacent with the sensor  127 , which provides a control signal to stop the motor  80  rotating the drum  42 . The system also provides a hard stop  128  to backup the sensor  127 . 
     FIG. 8 is a perspective view of the system shown in FIGS. 1-2 with an enclosure wall  134  with a wafer passage  136 , the inner enclosure  30 , the drum  42 , a wire duct  138 , and the duct  140  providing passage for electrical utilities and heat removal from the exposure source chamber  24  and control chamber  26  to the exhaust  142 . 
     An alternative guide shown in FIGS. 9 and 12 includes a light reflective assembly  200 , which delivers light (e.g., UV) from a lamp house-integrating sphere  166  through an optical system to the wafer  12 . The integrating sphere  166  gathers energy from a lamp such as a 200-Watt mercury lamp (not shown), which is inserted into the integrating sphere  166  through hole  202 , and preferably disposed at the focal point of the integrating sphere  166 . This helps compensate for the optical transmission losses from the lamp to the wafer  12 . As shown by FIGS. 9 and 12, the light from the integrating sphere  166  travels through a light shield  230  and is collimated by an upper lens  168 . A reflecting mirror  170  reflects the collimated light, and is curved to narrow the light to a line beam. The line beam travels through the bellows  172  on its way to a conventional housing  176  containing a conventional filter and a beam turning mirror  232  whose orientation can be adjusted by turning a set of mirror knobs  177  and  179  and/or a lower lens  234 . The bellows  172  expands and contracts as the tracking exposure head  40  (FIG. 1) moves across the wafer  12  in the radial direction. As shown, the beam turning mirrors  170  and  232  and the lenses  168  and  234  redirect the light into a light column  182  formed by an aperture  236  in the bottom of the housing  176  to expose the resist on the wafer  12 . One suitable light reflective assembly  200  is OAI part no. 0860-055-01. CVI of Livermore, Calif. provides a suitable upper lens  168  (e.g., CVI part no. RCX 49.0-38.1-UV245-390), lower lens  234  (e.g., CVI part no. RCX 40.0-19.1-UV245-390 coated), and beam turning mirrors  170  and  232  (e.g., CVI part no. TLM 248/365-45 UNP-RW 50.0-30.0-5.0) having an UV-enhanced dielectric on their reflector front surface. 
     FIG. 10 is a block diagram of an embodiment of the control system, which includes four modules: a system supervisor  144 , an r-axis module  146 , a theta-axis module  148 , and a coordination module  150 . The system supervisor  144  performs high-level control functions, while modules  146 ,  148 , and  150  perform low-level motion control functions. The system supervisor  144  receives recipes from an external host such as the SVG Pro Cell Host Computer through, for example, RS-232 serial communications, and directs the low-level modules  146 ,  148 , and  150  to perform the exposure routine specified by the recipe. The low-level modules  146 ,  148 , and  150  perform real-time control functions so that communication between the system supervisor  144  and the low-level modules  146 ,  148 , and  150  is not time-critical. Consequently, conventional serial communication can be used between the system supervisor  144  and the other modules  146 ,  148 , and  150  reducing wiring complexity. The system supervisor  144  is preferably implemented with a single-board computer  68  (FIG.  1 ), so that it is compact, easy to program and may employ standard computer peripherals such as a hard drive and floppy drive if desired. 
     The details of the operation of the motion control system are explained in connection with FIGS. 10-11. The theta-axis module  148  is responsible for control of the drum  42 , which rotates the emitter  41  providing the exposure beam around the wafer  12 . The theta-axis module  148  is constructed from a conventional rotary stage, which includes the motor  80 , a rotary encoder and control electronics. 
     The coordination module  150  is responsible for coordinating the motion of the r-axis controller with that of the theta axis. The coordination module  150  monitors the theta-axis position by connecting to the theta-axis rotary encoder, which generates a series of pulses indicating the theta-axis position for the r-axis controller. One pulse is generated each time the theta-axis rotates through a small angle. The coordination module  150  also captures the theta-position when the r-axis controller detects a notch or flat. This allows the system supervisor  144  to determine the notch or flat location by reading the captured theta position. 
     The primary function of the r-axis module  146  is to control the radial mechanism  56  to move the emitter  41  for the appropriate exposure depth during travel along the edge of the wafer  12 . The r-axis module  146  controls the power delivered to the motor  108  to move the emitter  41  in the radial direction. The exposure beam position is determined with the linear encoder  106 , and the LED  58  and the CCD sensor  59  detect the wafer edge position so that the measured wafer edge position is relative to the exposure beam position rather than an absolute position. The LED  58  is used for illuminating the CCD sensor  59  rather than radiation from the emitter  41  so the edge position may be determined without exposing the wafer  12 . The CCD sensor  59  is preferably not directly opposite the exposure beam either because the exposure beam may be too intense for the CCD sensor  59 . In one embodiment, the CCD sensor  59  leads the exposure beam by a small offset such as 8 mm. Consequently, the immediate data from the CCD sensor  59  corresponds to a position ahead of the emitter  41 .The embodiment uses a memory such as a first-in-first-out (FIFO) buffer  156  (FIG. 11) to store incoming data from the CCD sensor  59  until the exposure beam reaches the same point on the periphery of the wafer  12 , which correlates with the CCD data. The coordinating signal generated by the coordination module  150  governs the shifting of data through the FIFO buffer  156 . The FIFO buffer  156  is shifted one position with each pulse of the signal. By calibrating the theta-axis distance between pulses, the data leaving the FIFO buffer  156  can be matched with the immediate position of the emitter  41 . 
     Since the CCD sensor  59  moves with the tracking head  40 , the CCD data is dependent on the immediate position of the tracking exposure head  40 . For example, if the tracking head  40  moves inward in the radial direction 10 mm, the CCD sensor  59  will see 10 mm more of the wafer  12 , that is, the wafer  12  will further block the amount of light from the LED  58  captured by the CCD sensor  59 . The invention compensates for this dependency by subtracting the radial position, as measured by the encoder  106 , from the CCD data. The resulting data is independent of the position of the tracking exposure head  40 . This compensated data is then stored in the FIFO buffer  156  as previously described. 
     FIG. 11 illustrates the functionality of the r-axis servo controller PCB  74 . The PID compensator  162  represents a closed loop position controller to drive the r-axis module  146  to the position specified by the ref position signal. This is achieved using conventional PID compensation techniques, which determine the instantaneous motor command by combining the instantaneous position error, with the integral of the error, and the derivative of the error. 
     Ref position is the desired position for the motor  108 , e.g., the Nanomotion motor, at any point in time. It is generated by the controller according to the current mode of operation. For a simple point-to-point move, ref is based on the desired acceleration, velocity, final position and time. For edge tracking, ref position is based on wafer edge position and desired exposure depth. The controller uses the ref position to determine the output to the motor  108 . Ideally the motor would always be exactly at the ref position, but in practice it deviates due to the inertia of the motor/load, controller limitations, external forces on the motor, and other factors. 
     Before a command is sent to the motor  108 , crash protection logic  164  checks crash sensor  61 , a photoelectric break beam detector, OAI part no. 0860-0044-01 to determine if there is an obstruction in the path of the exposure tracking head  40 . If a crash is impending, the control system uses the signal to command the motor  108  to inhibit movement in that direction to avoid damage to the system  10 . 
     The remaining blocks in the FIG. 11 are responsible for generating the ref position based on the commands received from the system supervisor  144 . The command sequencer  158  receives high-level commands from the system supervisor  144  by conventional serial communications such as RS-232. The command sequencer  158  executes the high-level commands, such as find and track wafer edge or move to home position, by switching between three basic modes of control. The modes are tracking, move and hold, and each of these generates the ref position signal in a different manner. The hold mode maintains a constant reference position so that the position of the motor  108  is held constant. The hold mode is typically used when the r-axis reaches its final destination. 
     The tracking mode is used when the radial mechanism  56  must maintain a fixed distance from the edge of the wafer  12 , such as when exposing the edge of the wafer  12 . The tracking mode uses data from both the CCD sensor  59  and the linear encoder  106  to generate the ref position. 
     The output of the CCD sensor  59  is not linear with respect to wafer position because the angle of incidence of illuminating light from the LED  58  is not constant over the length of the CCD. The CCD sensor output is made linear by nonlinear conversion  152 . Since the CCD measurement is not taken at the point of exposure, the data is sent though the FIFO buffer  156  as previously described. The position coming out of the FIFO buffer  156  corresponds to the absolute wafer edge position at the point of exposure. The desired depth is then subtracted from the edge position to give the desired position for the exposure beam from the emitter  41 . Finally, the ref position is set equal to this desired exposure position, so the linear motor  108  is moved to the proper exposure position. 
     The third mode of control is the move mode. This is used to move the radial mechanism  56  to a desired destination. For example, when the exposure of the wafer  12  is complete, the system supervisor  144  commands the r-axis module  146  to move away from the wafer edge to an idle position. In order to obtain a smooth, controlled move to the final destination, the reference position is updated over time according to a trapezoidal velocity profile block  160 . This means the ref position is changed in small increments to reach the final position, rather than abruptly changing the ref position. A trapezoidal velocity profile has characteristics of a constant acceleration up to a maximum velocity, a constant velocity during translation, and a constant deceleration to the final position. The trapezoidal profiler block  160  in the lower-left of FIG. 11 is responsible for generating the time-varying ref-position according to the final position, velocity and acceleration specified by the system supervisor  144 . 
     The notch/flat detector  154  detects a notch or flat on the wafer  12  so that its position can be determined. The r-axis module  146  uses the CCD measurements and the coordinating pulses from the coordination module  105  to calculate the curvature of the wafer edge. The corner of a notch or flat has a relatively large curvature, so the notch/flat detector  154  detects the notch or flat of the wafer  12  based on a curvature calculation. When a notch or flat is detected, the r-axis module  146  sends a signal to the coordination module  150 , which immediately captures the current theta-position of the rotating drum  42 . 
     Some applications require that the exposed periphery not dip at a notch of the wafer  12 , which happens if a constant exposure depth is maintained all the way around the wafer  12 . Since the edge of the wafer  12  is not mapped, the r-axis module  146  should preferably skip over the notch without needing to know its location. When doing clips, the notch position may be determined before any exposure, but this is not typically done. However, since the CCD sensor  59  preferably leads the exposure beam from the emitter  41  by a small known distance, that is, an offset, the notch can be detected a short time before it is exposed. Once the notch is detected, tracking (i.e., edge detecting) continues up to a point just before the start of the notch. At this point, tracking is suspended and the radial mechanism  56  is held stationary while the drum  42  rotates. The r-axis module  146  is configured to hold a constant depth as the exposure beam passes over a notch. Once the exposure beam passes over the notch, the r-axis module  146  resumes tracking the edge of the wafer  12  so that the remainder of wafer  12  is exposed with a constant depth. 
     Some applications require small regions, called clips, to be exposed for wafer handling purposes. There are typically three to six clips distributed around the edge of wafer  12 , at known distances from the notch or flat. The following refers to a notch, but a flat can be treated similarly. Exposing the clips begins by determining the location of the notch or flat. With the lamp of the exposure source assembly  38  turned off, the drum  42  rotates in accordance with the theta-axis module  148 , while the r-axis module  146  tracks the wafer edge. When the CCD sensor  59  passes over the notch or flat, the notch or flat position is captured, and the drum  42  with the tracking exposure head  40  moves back to the home position. Next, the lamp of the exposure source assembly  38  is turned on and the drum  42  with the emitter  41  is rotated to expose up to the beginning of the first clip. The recipe provides information to determine the start and stop position of each clip with respect to the notch or flat. Prior to turning on the exposure source of the exposure source assembly  38 , the apparatus scans the wafer edge to determine notch location with respect to the home position. The apparatus can then use the recipe to determine the start/stop positions of clips with respect to the home position. 
     With the rotation stopped, the radial mechanism  56  moves the emitter  41  to the clip exposure depth and the rotation continues through the length of the clip. The recipe specifies a clip size (either in an angle or arc length). This is converted to a rotational distance for the rotary motor  80 . The rotary motor  80  is simply commanded to move this distance to expose the clip. The radial mechanism  56  moves the emitter  41  back out to the depth of the periphery where there are no clips and the process continues until the entire periphery of the wafer  12  is exposed. When the exposure is completed, the exposure source is turned off and the emitter  41  is rotated back to the home position. 
     Alternatively, the position of the notch can be captured and the emitter  41  can be turned on to expose the periphery including clips immediately after and the emitter  41  returned to the home position and the remaining periphery with any clips exposed up to the notch or flat.