Abstract:
A method for detecting physical interference with desired transport of an article. The method includes the step of detecting an operative acoustic signal representing the structure-borne sound pattern of an article during said article transport, and detecting the presence of interference based on the acoustic signal. A system for performing the method includes a transport device adapted to transport the article through a predetermined path and an acoustic sensor in structure-borne acoustic contact with the transport device and capable of producing an acoustic signal indicative of physical interference.

Description:
TECHNICAL FIELD 
     The present invention relates generally to the detection of physical interference during transport of an article and, more specifically, to a structure and method for detection of robot handling errors. 
     BACKGROUND OF THE INVENTION 
     Robotic devices are used in many processes for transport of articles in predetermined paths. For instance, robots are used in semiconductor manufacturing to perform tasks such as selecting semiconductor wafers from carrier cassettes, placing them in process chambers, moving them between process chambers, and placing them back into cassettes for transport to a subsequent operation. These robots must operate at very high speeds to assure that tool throughput is not limited by wafer transport, but must also place wafers at their desired location within fractions of a millimeter in three-dimensional space. Because of these requirements, robots used in semiconductor manufacturing are taught the locations for picking up, transporting, and depositing wafers, and are expected to consistently maneuver precisely among those locations repeatedly for months at a time. 
     If wafers are misplaced by even a small amount, several handling errors are possible. These errors may become evident by some type of physical interference during wafer transport. The wafer may scrape or hit a tool or cassette surface, resulting in either broken wafers or scratched wafer surfaces that ruin any chips at the scratched location. In addition, the robot may cause impact collisions between the wafer and tool surfaces that are too slight to break wafers, but are sufficient to nick the wafer edge. These nicked wafers are then highly likely to break during high-stress process steps such as the polish or heat treatment steps. Finally, the robot itself may be the source of physical interference via rubbing at a joint or on a tool surface, resulting in elevated levels of foreign material particulate matter that may lead to decreased wafer yield and lower productivity. 
     No real-time method is available to determine if misaligned wafer placement is causing scratches or collisions, or to determine if the robot is rubbing against a surface and causing particulate generation. Periodic foreign material checks will not catch an intermittent problem or find a problem that is just beginning. Because often the same type of robot is used on multiple types of process tools, it is difficult to determine which robot is responsible once a problem has been identified. This difficulty is exacerbated if the problem is intermittent. 
     Several systems are known for sensing or preventing robot handling errors, including the use of force sensors, strain gauges, or limit switches on the robot arm. Such systems are capable of detecting large or forceful collisions. They cannot detect, however, the very slight physical interference produced by gently scratching or nicking the wafer on a tool surface, or by rubbing at the robot joints. 
     It is also known to mount active acoustic or light-radiation devices on robots to actively generate a “visual” map of the robot environment, from which the robot makes navigation decisions and avoids collisions. Such systems require a great deal of resources dedicated to active monitoring and mapping. They still may not detect slight physical interference, however, such as rubbing of parts of the robot not within the field of “vision.” 
     The use of magnetic fields created by placing magnetic strips in the robot arm and in the tool area, and using a field sensor to detect abnormalities in a previously characterized field pattern, has also been used to sense collisions. Such a pattern may not detect slight physical interference between wafers and work surfaces, however, because the relationship between the robot and the tool may be essentially the same for a non-interfering motion and a slight interfering motion. Also, the specific set-up required for mapping every robot motion and every robot geometry is time-consuming and expensive. 
     In view of the shortcomings of the known systems, there remains a need for an improved structure and method for detection of robot handling errors. 
     SUMMARY OF THE INVENTION 
     To meet this and other needs, and in view of its purposes, the present invention provides a method and associated system for using an acoustic sensor, such as an accelerometer, to detect the acoustic signal produced by physical interference with desired transport of an article being moved by a transport device through a predetermined path, such as a wafer being handled by a robot in a workstation. Specifically, the present invention provides a method for detecting physical interference with desired transport of an article, the method comprising: 
     a) detecting with an acoustic sensor an operative acoustic signal representing a structure-borne sound pattern of the article during transport; and 
     b) detecting the presence of physical interference based on the acoustic signal; 
     wherein the acoustic sensor is in structure-borne acoustic contact with the article at least during the physical interference. 
     The invention also comprises a method for detecting physical interference with desired transport of an article when the article is moved by a transport device through a predetermined path, the method comprising: 
     a) storing a reference acoustic signal; 
     b) detecting a subsequent operative acoustic signal representing the structure-borne sound pattern of an article during its transport through the predetermined path; 
     c) comparing the operative acoustic signal with the reference signal; and 
     d) detecting any differences between the reference signal and the operative signal and using the detected differences to determine the presence of interference during transport. 
     Step (a) may further comprise detecting a baseline acoustic signal representing the sound pattern generated during the transport of a sample article through the predetermined path without any physical interference and storing the baseline acoustic signal as the reference acoustic signal. 
     The method steps for detecting such physical interference may be performed by a system comprising: 
     a) a transport device adapted to transport the article through a predetermined path; and 
     b) an acoustic sensor capable of producing an acoustic signal indicative of the physical interference, wherein the acoustic sensor is in structure-borne acoustic contact with the article at least during the physical interference. 
     The present invention also comprises a program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine to perform method steps for detecting physical interference with desired transport of an article when the article is moved by a transport device through a predetermined path, the method steps comprising: 
     a) inputting and storing a baseline acoustic signal representing the structure-borne sound pattern generated during the transport of a sample article through the predetermined path without any physical interference; 
     b) inputting a subsequent operative acoustic signal representing a structure-borne sound pattern of an article during its transport through the predetermined path; 
     c) comparing the operative acoustic signal with the baseline signal; and 
     d) detecting any differences between the baseline signal and the operative signal and using the detected differences to identify the physical interference. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures: 
     FIG. 1 is a schematic illustration of a typical semiconductor wafer processing tool that uses a robotic arm for moving wafers along predetermined paths to predetermined locations; 
     FIG. 2 is a schematic illustration of a wafer holding cassette typically used with the tool of FIG. 1 for holding wafers; and 
     FIG. 3 is a schematic illustration of a displacement curve derived from the output of an accelerometer mounted on a robotic arm while the arm performed motions with and without collisions. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawing, wherein like reference numerals refer to like elements throughout, FIGS. 1 and 2 illustrate a specific embodiment useful in handling wafers in a semiconductor manufacturing process. The wafers are picked up and transported between predetermined locations along predetermined paths using a robotic arm. 
     FIG. 1 shows an example of a robotic tool  10  typical of those that may be used with the present invention. In this case, the tool shown is a Rapid Thermal Processing tool, such as that used in silicon wafer processing for annealing dopants or for other heat-reactive processes in the manufacture of integrated circuit chip wafers. The articles handled are silicon wafers. 
     The robotic arm  12  of tool  10  as shown in FIG. 1 comprises an article-grasping portion  14  having a suction disk  16  at its terminal end. The suction disk is connected to a vacuum source that allows the article-grasping portion to firmly hold wafers from underneath using suction. The article-grasping portion  14  is rotationally attached to a first arm section  18 , allowing 360° rotation in a single plane. First arm section  18  is in turn rotationally attached to a second arm section  18 ′, also allowing 360° rotation in a single plane parallel to the plane of motion of article-grasping portion  14 . A piston  20  provides vertical movement of robotic arm  12 . This series of connected arm sections gives robotic arm  12  a full range of rotational and translational movement within the work area. The structure of a robot to be used with the present invention may have any number of attached parts, however, comprising any number of degrees of freedom of movement. 
     As shown in FIG. 2, a cassette  50  holds a plurality of wafers  52 , typically on the order of twenty-five wafers in an entire cassette. The wafers are held on fingers  54  that support each wafer on its opposite edges, thus leaving the center portion  56  free. Each set of fingers  54  creates a slot  58  into which a single wafer  52  fits. Such a cassette is held in one of the cassette stands  22  or  22 ′ shown in FIG.  1 . Sometimes a full cassette may be placed on cassette stand  22  and an empty cassette on stand  22 ′, or vice versa. Other times, only a single cassette is used. 
     The robotic arm  12  takes wafers one at a time from a cassette held in one of the cassette stands  22  or  22 ′, typically starting at the topmost wafer in a cassette and working downward. To pick up a wafer with article-grasping portion  14 , robotic arm  12  moves article-grasping portion  14  into the center portion  56  of cassette  50  underneath a wafer  52  in the space between adjacent wafers. Air flows into suction disk  16  until contact is made with a wafer  52 , thus creating a vacuum seal between suction disk  16  and a wafer (not shown). If no wafer is present, air flow will continue through suction disk  16  even when robotic arm  12  is in a position where a wafer should have been contacted to block the air flow, thus indicating to the robot controller that there is no wafer in that slot. In such case, robotic arm  12  moves to the next slot. 
     After securing wafer  52  on article-grasping portion  14 , robotic arm  12  lifts the wafer vertically to clear contact with fingers  54 , and then retracts the wafer from cassette  50 . The wafer is then transferred to alignment station  24 . In the alignment station, the wafer is set down on a collection of pins (not shown), and the suction is released between the wafer and the suction disk  16  of article-grasping portion  14 . The article-grasping portion  14  is lowered so that only the pins support the wafer. At the alignment station, the wafer is rotated and translated by an alignment system to align a designated mark (not shown) on the wafer to a predetermined proper position. Once the alignment has been satisfied, the robot again secures the wafer by lifting it from underneath using suction disk  16  as previously described. 
     The robotic arm  12  then moves the wafer to heat chamber  26 . The door  28  of the chamber opens, and the robotic arm inserts the wafer. The wafer is set down inside the heat chamber and released by article-grasping portion  14 . The article-grasping portion retracts, and door  28  closes. Inside the heat chamber, the wafer sits upon pins (not shown) within a heat transfer ring (not shown) that contacts the wafer periphery. 
     At the end of the heat-processing step, the robotic arm secures the wafer, removes it from the heat chamber, and moves it to and sets it down upon a cooling station  30 . At cooling station  30 , the wafer rests on pins  32  while air blows through manifold  34 . When the wafer is cool, robotic arm  12  again picks it up and transfers it to a cassette  50 . 
     The return cassette may be the same cassette at the same cassette stand  22  or  22 ′ that held the wafer before processing, or it may be a different cassette on the opposite stand from the one that originally held the wafer. Often, the process may start with an empty cassette on one stand and a full one on the other, and the processing of wafers proceeds until all the wafers have been transferred from the full cassette to the empty one. 
     During the processing of a single wafer at one portion of the tool, other wafers may be simultaneously processed. So, for instance, while one wafer is being cooled at cooling station  30 , another may be in heat chamber  26 , and still another may be in the alignment station  24 . 
     Generally, during the transport of an article along a predetermined path by a transport device, such as the movement of a semiconductor wafer by robotic arm  12  between the various processes described above, some sort of physical interference with the desired transport may occur. As used in this specification, the term “physical interference” encompasses, but is not limited to, the following: collisions between the article and other objects, including parts of the transporting device; parts of the transporting device rubbing together; dropping of the transported article; collision or rubbing of any part of the transporting device with or against any adjacent structure; improper contact between the transporting device and the article; and improper operation due to accumulation of debris on or in the vicinity of the transporting device. In the particular application in which a robotic arm is used to grasp and transport a semiconductor wafer, physical interference includes the semiconductor wafer rubbing against the slots in the cassette, hitting a processing chamber wall, or the robotic arm touching the wafer in an undesired manner. Physical interference may be identified by a resulting acoustic signal differing from the expected acoustic signal for a given operation. 
     In most cases, the physical interference is slight in the sense that it does not pull excess current and shut down the robotic arm or otherwise register a problem by existing feedback control techniques. Because the wafers are very fragile and very complex on a microscopic scale, however, even the slightest nick or scratch on the surface of the wafer may render it unusable in part or in whole. Furthermore, rubbing of parts in the tool chamber creates foreign material, so even if the wafer itself is not directly hit, it may still be damaged by the presence of foreign material on its surface. 
     Therefore, the present invention comprises “listening” for vibrations that may indicate physical interference or a collision, preferably using an acoustic sensor sensitive to sound or vibration and attached to either the tool, the robotic arm, or even the outside of the tool chamber. The sensor may be a single-, dual-, or three-axis accelerometer such as a model ADXL150EM-3 manufactured by Analog Devices, Inc. of Norwood, Mass., or may be a sensitive microphone or other type of vibration detector. Such devices are capable of detecting even minute disturbances that may indicate physical interference. 
     Because the various structural parts of the system, including the wafer itself, are acoustically conductive, such a sensor may provide reliable information from a location virtually anywhere in the system. As long as the sensor is mounted where a structure-borne (as opposed to airborne) acoustic signal is conducted to the sensor from the point of physical interference, the structure-borne acoustic signal will reach the sensor. To increase sensitivity, however, sensors may be located closest to where physical interference is most likely. 
     The transport of an article by a transport device through a predetermined path, such as movement of a wafer by the robot in its standard motions through the processing steps within the tool, produces characteristic vibrations detected by the acoustic sensor or sensors and converted into an acoustic signal. Preferably, the present invention includes comparing an operative acoustic signal with a reference signal, such as a stored baseline acoustical signal. Storage, comparison, and input of the signals generated by the acoustic detector may be performed by a computer, a logic controller, or any program storage device that can be read by a machine, tangibly embodying a program of instructions that can be executed by the machine to perform the requisite method steps. 
     The baseline signal may be the result of detecting and storing the signal generated during a calibrating step, during which an article is transported along the predetermined path from a first location to a second location in the absence of any physical interference. The absence of physical interference may be assured by properly preparing the path and transport device and by actually observing the operation to determine the absence of any interference with the transport process. The acoustical signal generated through this operation is then stored to form the baseline signal. 
     Preferably, more than one calibrating acoustical signal will be generated and the baseline will be the result of averaging such signals. In such case, a statistical analysis may be performed on the resulting calibration acoustical signals to determine acceptable statistical limits to acceptable deviations from the average baseline against which to compare the operative signals during actual manufacturing use of the transport device. The development of such statistical limits permits quick and automated identification of unacceptable deviations from a normal signal. The identified unacceptable deviation may then trigger a proper response, ranging from simple activation of an alarm to a complete shut down of the operation. 
     The acoustical signal typically is a time-dependent signal whose duration is preferably coextensive with the duration of the article transport process. Although in its simplest form the acoustical signal may only have an amplitude which varies with time, preferably it will include both amplitude and frequency components. Still according to the present invention, information regarding the particular sounds generated by particular interferences is developed and stored, optionally, as part of the baseline acoustical signal. In this case, the type of physical interference can be determined by comparing the operative is signal amplitude and frequency spectrum to the frequency spectra in the stored baseline acoustical signal to identify the type of interference encountered. 
     A programmable controller or other program storage device, such as a computer, may perform the steps of inputting and storing the baseline and operative structure-borne acoustic signals, as well as comparing the signals and detecting differences between the signals. Deviations from the baseline acoustic signal may be a step change caused by a suddenly developing problem, or a gradual change, such as from an accumulation of deposited foreign material on system components. In a situation where the generation of the foreign material may for some reason escape detection, its effect on acoustic performance may be the factor that brings it to the attention of the control system. 
     Several preferred options for mounting the acoustic sensor on the robot itself are indicated by stars A, B, and C in FIG.  1 . These suggested locations are not limiting; sensors could be placed in any number of locations, including locations on various parts of the semiconductor processing tool. Such locations may be on the heat chamber door  28 , at cooling station  30 , at alignment station  24 , on the outside of the tool enclosure, or on the cassette  50 , and the like. In such locations, the acoustic sensor will be in structure-borne acoustic contact with the wafer only during the moment of actual contact or physical interference. Any combination of sensor locations may be used. Placement somewhere on robotic arm  12  itself is advantageous, however, because a sensor in such a location is especially sensitive to physical interference involving the wafer at the end of the robotic arm. 
     Similarly, any physical interference from parts of the robot rubbing together, or even from a wafer falling off the article-grasping portion  14  can be easily detected by a sensor on robotic arm  12  itself. Thus when the acoustic sensor is placed, for instance, on the robotic arm in location A, B, or C, it is in structure-borne acoustic contact with the surface of the robotic arm and the surface of the wafer being held by the robot. As robotic arm  12  moves wafer  52  relative to the various objects that comprise tool  10 , the surface of the wafer or the robotic arm itself may contact any of those objects, creating a vibration transmitted through the structure and detected by the acoustic sensor. 
     The acoustic sensors transmit signals to the robotic arm control device, and the robot motion may be halted whenever physical interference is detected, signifying a robot error. Alternately, the robot control device may merely log the errors for review later. Preferably, some combination of these options can be chosen so that errors classified as relatively minor or isolated are merely logged, but major or repeated errors shut the tool down until it can be inspected. 
     Referring again to FIGS. 1 and 2 for the specific tool configuration described above, review of the process reveals many places where physical interference of various forms may occur and b e detected as part of the operative acoustic signal. The difference between the operative acoustic signal with such interference and the baseline acoustic signal recorded without such interference indicates a robot error. 
     The motion of robotic arm  12  itself may produce rubbing at the joints between adjacent arm sections  18 ,  18 ′ or article-grasping portion  14 . Piston  20  may begin rubbing as it extends and retracts. Rubbing of parts against one another may create foreign material that can damage the wafer integrity. In performing the motions required to move a wafer, a non-article-grasping portion of the robot may strike another portion of the tool, such as the heat chamber  26 . Again, foreign material may be created. 
     The step of picking up a wafer may cause physical interference. Naturally, the first moment of contact between a wafer  52  and article-grasping portion  14  produces an expected contact. This desirable contact is also part of the baseline acoustic signal, however, and is thus not identified as a difference from baseline. As robotic arm  12  lifts wafer  52  out of cassette  50 , it may lift too far and produce physical interference, or it may not lift far enough and scrape fingers  54  with the wafer as it retracts. As article-grasping portion  14  enters the cassette it may rub against a wafer or the cassette wall. 
     As robotic arm  12  moves the wafer to alignment station  24 , it may bump the wafer against another portion of the tool, or it may hit the walls as it enters the station, creating physical interference. Physical interference may come from the robotic arm setting the wafer down too hard on the pins. On the way to a station or at any time while the wafer is being transported, the wafer may fall off article-grasping portion  14 , thus creating an undesirable scraping contact as it slides off. 
     As robotic arm  12  moves wafer  52  into heating chamber  26 , door  28  may not completely open, thus causing a nicking or scraping contact between the wafer and the door. In the system described, the aperture created by the open door is typically only one-half inch wide, thus not allowing much room for error. The door itself may close improperly if parts are rubbing together. Rubbing parts anywhere in the tool system create potential foreign material. The chamber door may be in the form of a valve that produces a characteristic vibration when the valve contacts the valve seat upon closing. The normal vibration is part of the baseline acoustic signal, so any departure from the baseline acoustic signal in the operative acoustic signal may be regarded as emanating from physical interference. 
     Physical interference potentially detected by an acoustic or vibration sensor is not limited to what has been listed above. There may be other events detectable with respect to the specific process equipment described, and furthermore there may be events specific to other process equipment not described. The use of an acoustic or vibration sensor to detect robotic errors may be used not only for the equipment described and related to the Rapid Thermal Processing tool, but for equipment related to robotic processes in any aspect of semiconductor wafer processing. Such equipment may include, but is not limited to, the implanter, wet bench, clustered processing tool, furnace, inspection equipment, bay-to-bay automated product transport, within-bay product transport, Chemical Mechanical Polishing (CMP) equipment, lithography equipment, dry strip equipment, sputter equipment, electroplating equipment, and the like. 
     Similarly, the use of robots, robotic arms, or other robotic handling devices is not limited to the semiconductor processing industry. Therefore, the use of vibration or acoustic sensors to detect robot errors is applicable to any industry in which robots are used. Furthermore, the need to detect undesired contact is not limited to processes involving robots. Other mechanical, or even non-mechanical transport devices, may need to detect physical interference with desired transport of an article being transported by a the transport device. This need may be solved by placing an acoustic sensor on a surface in acoustically conductive contact with either the transporting device or with the object likely to cause the physical interference, in accordance with the present invention. 
     EXAMPLE 
     The following example is included to more clearly demonstrate the overall nature of the invention. This example is exemplary, not restrictive, of the invention. 
     A dual-axis micro-machined accelerometer, model ADXL250 manufactured by Analog Devices, Inc. of Norwood, Mass., was attached to a robotic arm and its signal output attached to a computer readout using ADXL202EB-232 Demo Software, also by Analog Devices, Inc. The robotic arm performed a sliding motion without a collision, and then performed the same motion with a collision. Referring now to FIG. 3, there is illustrated the output curve from the accelerometer during these motions. 
     Output curves  70  and  72  represent the displacement signal in the X ( 70 ) and the Y ( 72 ) direction produced by the software from the double integral of the acceleration as sensed by the accelerometer and input to the computer. The curve shows the signal response to robotic motions without a collision  74  and  74 ′ and the same robotic motion with a collision  76  and  76 ′. As shown, the displacement curves clearly show peaks  78  during colliding motions  76  and  76 ′ that are not present during non-colliding motions  74  and  74 ′. To induce the collision, the robotic motion was performed at faster velocity, resulting in the compressed time scale of the signals from the motions with a collision. Periods of rest with no motion (and no collision) show a flat line  75 . In an industrial application, slight differences in the signals during non-colliding motions  74  and  74 ′ can be accommodated with signal processing filters to avoid false collision indications. 
     Although illustrated and described herein with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.