Abstract:
A dryer lid/robot collision prevention system which prevents inadvertent collision of a spin dryer lid with a transfer robot as the robot approaches and loads wafer-containing cassettes into the spin dryer. The system includes at least one, and typically, a pair of photoelectric sensors mounted on an inner wall behind the spin dryer lid. Each of the photoelectric sensors is electrically connected to a relay circuit which activates a cylinder sensor in the spin dryer in the event that the photoelectric sensors sense the normal position of the open spin dryer lid. The cylinder sensors activate a PLC module, which activates the cassette cradles in the spin dryer. Finally, the activated mode of the cassette cradles signals the transfer robot to approach and load the wafer cassettes into the cassette cradles. The system may include a bypass switch for bypassing the system, as desired.

Description:
FIELD OF THE INVENTION 
   The present invention relates to processes and equipment for cleaning and spin-drying semiconductor wafers in the semiconductor fabrication industry. More particularly, the present invention relates to a system for monitoring the position of a lid on a wet bench spin dryer to prevent inadvertent collision of the lid with a wafer transition robot during placement of wafers in the spin dryer. 
   BACKGROUND OF THE INVENTION 
   Generally, the process for manufacturing integrated circuits on a silicon wafer substrate typically involves deposition of a thin dielectric or conductive film on the wafer using oxidation or any of a variety of chemical vapor deposition processes; formation of a circuit pattern on a layer of photoresist material by photolithography; placing a photoresist mask layer corresponding to the circuit pattern on the wafer; etching of the circuit pattern in the conductive layer on the wafer; and stripping of the photoresist mask layer from the wafer. Each of these steps, particularly the photoresist stripping step, provides abundant opportunity for organic, metal and other potential circuit-contaminating particles to accumulate on the wafer surface. 
   In the semiconductor fabrication industry, minimization of particle contamination on semiconductor wafers increases in importance as the integrated circuit devices on the wafers decrease in size. With the reduced size of the devices, a contaminant having a particular size occupies a relatively larger percentage of the available space for circuit elements on the wafer as compared to wafers containing the larger devices of the past. Moreover, the presence of particles in the integrated circuits compromises the functional integrity of the devices in the finished electronic product. Currently, mini-environment based IC manufacturing facilities are equipped to control airborne particles much smaller than 1.0 μm, as surface contamination continues to be of high priority to semiconductor manufacturers. To achieve an ultraclean wafer surface, particles must be removed from the wafer, and particle-removing methods are therefore of utmost importance in the fabrication of semiconductors. 
   The most common system for cleaning semiconductor wafers during wafer processing includes a series of tanks which contain the necessary cleaning solutions and are positioned in a “wet bench” in a clean room. Batches of wafers are moved in sequence through the tanks, typically by operation of a computer-controlled automated apparatus. Currently, semiconductor manufacturers use wet cleaning processes which may use cleaning agents such as deionized water and/or surfactants. Other wafer-cleaning processes utilize solvents, dry cleaning using high-velocity gas jets, and a megasonic cleaning process, in which very high-frequency sound waves are used to dislodge particles from the wafer surface. Cleaning systems which use deionized (DI) water currently are widely used in the industry because the systems are effective in removing particles from the wafers and are relatively cost-efficient. Approximately 4.5 tons of water are used for the production of each 200-mm, 16-Mbit, DRAM wafer. 
   Conventionally, wafers cleaned using DI water are subsequently dried using spin dryers. In a spin drying device, a cleaned wafer is rotated at high speeds in order to remove water remaining on the wafer after a rinsing step, using centrifugal force and air flow. The spin drying device is capable of drying wafers at a high throughput. Conventional spin drying devices are broadly classified into three types: multi-cassette dryers, single-cassette dryers and single-wafer dryers. 
   A typical conventional wet bench spin dryer, such as a KAIJO (trademark) spin dryer for drying residual DI water from semiconductor wafers, is generally indicated by reference numeral  10  in  FIG. 1 . The spin dryer  10  includes a chamber  11  which contains a pair of cassette cradles  12 , each of which receives a wafer cassette  20 , containing multiple wafers  21 , from a robot arm  19  of a wafer transfer robot  18 . A hinge  15  pivotally mounts a lid  13  on the chamber  11 . The lid  13  is raised and lowered at the hinge  15  by actuation of a lid cylinder  14 . A plastic stabilization chain  16  functions to stabilize the lid  13  in an even plane as the lid  13  is opened. 
   In operation, the wafer cassettes  20  are initially loaded onto the robot arm  19  of the wafer transfer robot  18 , which raises the wafer cassettes  20 , as shown by the dashed lines, preparatory to positioning the wafer cassettes  20  for subsequent loading in the cassette cradles  12  of the chamber  11 , as shown in the solid lines. Simultaneously, the lid  13  is raised to expose the cassette cradles  12 . Normally, the lid  13  remains open, and the robot arm  19  lowers the wafer cassettes  20  unimpeded into the respective cassette cradles  12 . The lid  13  is then closed and the cassette cradles  12  are rotated in the chamber  11  to remove residual rinsing water from the wafers  21 . 
   As further shown in  FIG. 1 , one of the problems commonly encountered in operation of the spin dryer  10  is that the welding joints of the hinge  15  become brittle and weaken after repeated use. Consequently, the typically heavy stainless steel lid  13  falls, as shown by the dashed lines, or the lid  13  shifts away from the normal position, hitting the robot arm  19  and damaging the semiconductor wafers  21  in one or both of the wafer cassettes  20 . Another problem that frequently occurs is that the plastic stabilization chain  16  becomes distorted and fails to maintain the lid  13  in an even plane upon opening, thereby causing the robot arm  19  to strike the lid  13  during the cassette-loading operation. It has been found that over a three-year period, about 50 wafers were broken in the heretofore-described manner during the wafer-loading process for one spin dryer. Accordingly, a system is needed for ascertaining the position of a lid on a spin dryer and communicating this information to a wafer transfer robot to prevent the robot from transferring the wafers to the wafer-loading position in the event that the hinge or stabilization chain fails and the lid does not open properly. 
   An object of the present invention is to provide a system for preventing collision of a spin dryer lid with a wafer transfer robot as the robot loads wafers into a spin dryer. 
   Another object of the present invention is to provide a system for automatically halting transfer of wafer cassettes to a spin dryer in the event that the lid of the spin dryer fails to open properly. 
   Still another object of the present invention is to provide a dryer lid/robot collision prevention system which prevents wafer damage and scrapping. 
   Yet another object of the present invention is to provide a dryer lid/robot collision prevention system which ascertains the proper position of a spin dryer lid prior to activating robot-controlled loading of wafers into the spin dryer in order to prevent inadvertent collision of the lid with the robot. 
   A still further object of the present invention is to provide a dryer lid/robot collision prevention system which utilizes at least one photoelectric sensor to ascertain the position of a spin dryer lid prior to actuating a wafer transfer robot in the transfer of wafer cassettes containing wafers into the spin dryer. 
   SUMMARY OF THE INVENTION 
   The present invention is generally directed to a dryer lid/robot collision prevention system which prevents inadvertent collision of a spin dryer lid with a transfer robot as the robot approaches and loads wafer-containing cassettes into the spin dryer. The system includes at least one, and typically, a pair of photoelectric sensors mounted on an inner wall behind the spin dryer lid. Each of the photoelectric sensors is electrically connected to a relay circuit which activates a cylinder sensor in the spin dryer in the event that the photoelectric sensors sense the normal position of the open spin dryer lid. The cylinder sensors activate a PLC module, which activates the cassette cradles in the spin dryer. Finally, the activated mode of the cassette cradles signals the transfer robot to approach and load the wafer cassettes into the cassette cradles. The system may include a bypass switch for bypassing the system, as desired. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a side view of a typical conventional wet bench spin dryer for semiconductor wafers, illustrating typical robot-actuated loading of wafer cassettes into the spin dryer; 
       FIG. 2  is a side view of a wet bench spin dryer in implementation of the present invention; 
       FIG. 3  is a front view of an inner wall of a spin dryer system, with diagonally-spaced high end and low end photoelectric sensors mounted on the wall in implementation of the present invention; 
       FIG. 4  is an electrical schematic diagram of an illustrative embodiment of the system of the present invention; 
       FIG. 5  is an electrical schematic diagram of a relay circuit of the present invention; 
       FIG. 6  is an electrical schematic diagram of a bypass switch of the present invention; 
       FIG. 7  is a side view of a wet bench spin dryer for semiconductor wafers, illustrating a normal open position for the spin dryer lid and robot-actuated loading of wafer cassettes into the spin dryer in implementation of the present invention; 
       FIG. 8  is a side view of a wet bench spin dryer, illustrating termination of the wafer cassette-loading operation due to failure of the spin dryer lid in implementation of the present invention; and 
       FIG. 9  is a side view of a wet bench spin dryer, illustrating termination of the wafer cassette-loading operation due to inadvertent tilting of the spin dryer lid in implementation of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring initially to  FIG. 2 , a wet bench spin dryer  25  suitable for implementation of the present invention is shown. The wet bench spin dryer  25  may be a conventional KAIJO (trademark) spin dryer which is commonly used to dry semiconductor wafers (not shown) using centrifugal motion after the wafers are rinsed in a DI water bath. The spin dryer  25  includes a chamber  26  which contains a pair of cassette cradles  27 , each of which receives a wafer cassette (not shown), containing multiple wafers (not shown), from a robot arm  34  of a wafer transfer robot  33 , as hereinafter described. A hinge  30  pivotally mounts a typically stainless steel lid  28  on the chamber  31 . The lid  28  is raised and lowered at the hinge  30  by actuation of a lid cylinder  29 . A plastic stabilization chain  82  stabilizes the lid  28  in a level plane as the lid  28  is opened. An inner wall  31  of the spin dryer system typically extends vertically behind the spin dryer  25 . 
   Referring next to  FIGS. 2–4 , the collision prevention system  45  of the present invention, an electrical schematic diagram of which is shown in  FIG. 4 , includes a high end photoelectric sensor  46  and a low end photoelectric sensor  47  which are mounted on the inner wall  31 , typically in diagonally-spaced relationship to each other, above the closed lid  28  of the spin dryer  25 . As hereinafter described, the high end photoelectric sensor  46  senses shifted positioning of the lid  28  caused by formation of fractures in the welding joints of the lid  28 , whereas the low end photoelectric sensor  47  senses deformation of the plastic stabilization chain  82  that functions to stabilize the lid  28  in an even plane during opening. The high end photoelectric sensor  46  and the low end photoelectric sensor  47  may each be an OMRON (trademark) model E3S-LS20C4S. As shown in  FIG. 2 , when fully opened, the lid  28  extends at least as high as the high end photoelectric sensor  46 . As shown in  FIG. 4 , the high end photoelectric sensor  46  is electrically connected to a relay circuit  48   a  [OMRON (trademark) model MY2N-D2 DC 24V] through high end signal wiring  54 . In like manner, the low end photoelectric sensor  47  is electrically connected to a relay circuit  48   b  [OMRON (trademark) model MY2N-D2 DC 24V] through low end signal wiring  55 . The relay circuit  48   a  and relay circuit  48   b  are each electrically connected to a cylinder sensor  76  (LS 200 ,LS 201 ) through cylinder sensor wiring  56  ( FIG. 5 ). The cylinder sensor  76  is, in turn, electrically connected to a PLC module (ID 219 )  72  through module wiring  73 , and the PLC module  72  is electrically connected to an equipment computer (EC)  75  for operating the spin dryer  25 , through EC wiring  74 . Finally, the equipment computer  75  is electrically connected to the wafer transfer robot  33  through robot wiring  38 . 
   As further shown in  FIG. 4 , 24 volt wiring  59  and 0 volt wiring  60  are connected to a terminal box  50  through terminal box input wiring  61 . The terminal box  50  is connected to the high end photoelectric sensor  46  through high end sensor wiring  51  and to the low end photoelectric sensor  47  through low end sensor wiring  52 . High end index lamp wiring  63  connects the terminal box  50  to a bypass switch  69 , through a typically 12-volt, DC high end index lamp  64 . Low end index lamp wiring  66  connects the terminal box  50  to the bypass switch  69  through a typically 12-volt, DC low end index lamp  67 . Switch wiring  65  connects the terminal box  50  to the bypass switch  69 . Accordingly, the terminal box  50 , the high end index lamp wiring  63 , the high end index lamp  64 , the bypass switch  69  and the switch wiring  65  define a circuit for the high end index lamp  64 , whereas the terminal box  50 , the low end index lamp wiring  66 , the low end index lamp  67 , the bypass switch  69  and the switch wiring  65  define a circuit for the low end index lamp  67 . Bypass wiring  70  connects the bypass switch  69  to the relay circuits  48   a  and  48   b , respectively. Relay wiring  57  connects the 24 volt wiring  59  and the 0 volt wiring  60  to the relay circuit  48   a  and the relay circuit  48   b . An electrical schematic diagram for the bypass switch  69  is further shown in  FIG. 6 . While the relay circuit schematic is indicated by reference numeral  48   a  in  FIG. 5 , the structure and function of both the relay circuit  48   a  and the relay circuit  48   b  will hereinafter be described with respect to  FIG. 5 . 
   Referring next to  FIGS. 4–8 , in use of the collision prevention system  45 , the lid  28  of the wet bench spin dryer  25  is initially raised to facilitate placement of a pair of wafer cassettes  35 , each containing multiple wet semiconductor wafers  36 , in the respective cassette cradles  27  in the chamber  26  of the spin dryer  25 . Accordingly, as shown in  FIG. 7 , under normal circumstances, the lid  28  is raised, typically by automated actuation of the lid cylinder  29 , from the closed position indicated by the dotted lines to the fully-open position indicated by the solid lines, to facilitate placement of the wafer cassettes  35  into the cassette cradles  27  by automated actuation of the wafer transfer robot  33 . As the lid  28  is raised to the fully-open position, the high end photoelectric sensor  46  and the low end photoelectric sensor  47  each senses a change in intensity of the light  41  entering the respective sensors  46 ,  47 , due to the raised lid  28 . Accordingly, each of the sensors  46 ,  47  transmits a voltage signal of 0 volts to the corresponding relay circuit  48   a  and  48   b . As shown in  FIG. 5 , the output voltage signal of 0 volts from the sensors  46 ,  47  are transmitted to point  13  on each of the respective relay circuits  48   a  and  48   b . This causes the incoming 24-volt signal at point  14  from the relay wiring  57  to close the circuit between points  5  and  9  on each relay circuit  48   a  and  48   b , at which point the relay circuits  48   a ,  48   b  energize the cylinder sensor  76  through the cylinder sensor wiring  56 . An LED (not shown) may be included in each relay circuit  48   a  and  48   b , in which case the LED is illuminated upon closing of the circuit. The energized cylinder sensor  76  actuates the PLC module  72  which, in turn, signals the equipment computer  75  to energize the cassette cradles  27  in the spin dryer  25 . The equipment computer  75  may be programmed to energize the cassette cradles  27  after a predetermined period of time, such as 90 seconds, for example. Finally, the equipment computer  75  actuates the wafer transfer robot  33  to initiate and sustain the cassette-loading process, at which time the robot arm  34 , carrying the wafer cassettes  35 , approaches the now-open chamber  26  of the spin dryer  25 , as indicated by the solid lines in  FIG. 7 , and lowers the wafer cassettes  35  into the respective cassette cradles  27 . Finally, the lid  28  is closed and the wafers  36  are dried in the spin dryer  25  according to process parameters known by those skilled in the art. After the drying process is completed, the lid  28  is again opened and the robot  33  removes the wafer cassettes  35  from the chamber  26  of the spin dryer  25  for further processing of the wafers  36  at a separate processing station. 
   As shown in  FIG. 8 , in the event that the lid  28  fails to open or opens and then falls back to the closed position on the chamber  26 , or the position of the lid  28  becomes shifted as may occur in the event of partial or complete failure of the welding joints of the hinge  30 , the high end sensor  46  senses the closed or shifted position of the lid  28  due to the difference in light which is received by the sensor  46  when the lid opens normally. Furthermore, in the event that the plastic chain  82  becomes distorted in such a manner that the lid  28  is in danger of opening in an uneven plane, as shown in  FIG. 9 , the low end sensor  47  senses the deformation of the chain  82 . Accordingly, in either case a 24-volt output signal is transmitted from either or both of the sensors  46 ,  47  to the respective relay circuits  48   a ,  48   b . Consequently, the relay circuits  48   a ,  48   b  maintain an open circuit between points  5  and  9  shown in  FIG. 5 , such that the cylinder sensor  76  is not actuated. Consequently, the PLC module  72  fails to signal the equipment computer  75  to energize the cassette cradles  27  in the spin dryer  25 , and thus, to actuate the wafer transfer robot  33  to initiate and complete the wafer cassette-loading process. Because the robot arm  19  of the robot  18  remains idle at the final rinse bath (not shown) from which the wafers  21  are removed and does not approach the chamber  26  of the spin dryer  25 , potential collisions which may otherwise occur between the robot  33  and the lid  28  in the event that the cassette-loading procedure is initiated and continued, are prevented. The equipment computer  75  may signal the closed or inadequately opened position of the lid  28  audibly, such as by use of a buzzer, and may further indicate the closed status of the lid  28  by displaying an appropriate visual indication. 
   As shown in  FIGS. 4 and 6 , the bypass switch  69  includes a low end lamp switch  78  and a high end lamp switch  79 . The low end lamp switch  78  normally completes the circuit between the terminal box  50 , the low end index lamp  67  and the bypass switch  69  to illuminate the low end index lamp  67 . Similarly, the high end lamp switch  79  normally completes the circuit between the terminal box  50 , the high end index lamp  64  and the bypass switch  69  to illuminate the high end index lamp  64 . In this switch configuration, both the high end photoelectric sensor  46  and the low end photoelectric sensor  47  remain functional and the collision prevention system  45  is enabled. By switching the low end lamp switch  78  to establish electrical communication between the switch wiring  65  and the bypass wiring  70 , the low end photoelectric sensor  47  is bypassed and the low end index lamp  67  is extinguished. Similarly, by switching the high end lamp switch  79  to establish electrical communication between the switch wiring  65  and the bypass wiring  70 , the high end photoelectric sensor  46  is bypassed and the high end index lamp  64  is extinguished. When both the low end lamp switch  78  and the high end lamp switch  79  are positioned to establish electrical communication between the switch wiring  65  and the bypass wiring  70 , both the low end photoelectric sensor  47  and the high end photoelectric sensor  46  are bypassed and the collision prevention system  45  is disabled. 
   Referring next to  FIG. 9 , in the event that the lid  28  of the spin dryer  25  begins to tilt or inadvertently close during the wafer cassette-loading operation, the collision prevention system  45  is capable of terminating approach of the robot  33  toward the spin dryer  25  and loading of the wafer cassettes  20  into the cassette cradles  12 . For example, in the event that the lid  28  tilts 2–5 mm out of a normal plane for the lid  28 , the high end photoelectric sensor  46 , the low end photoelectric sensor  47 , or both sensors  46 ,  47  sense the abnormal position of the lid  28  and transmit a 24-volt output signal to the relay circuit  48   a  and/or  48   b . Accordingly, the relay circuit  48   a  and/or  48   b  reverts from the closed to the open configuration, thereby turning off the cylinder sensor  76 . This transmits a signal to the PLC module  72 , which signals the equipment computer  75  to turn off the cassette cradles  27  and terminate further cassette-loading operation of the wafer transfer robot  33 . 
   While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.