Abstract:
A method of automatically carrying IC-chips, on a planar array of vacuum nozzles, to a variable target in a chip tester uses a set of laser distance sensors to align the vacuum nozzles with the target. Alignment occurs when certain combinations of distance and distance changes are sensed.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to methods of automatically aligning IC-chips (integrated circuit chips) with a target in a system where the target has a set of co-ordinates which are indeterminate. This invention also relates to apparatus for automatically performing such methods. 
   In the prior art, one particular method of the above type is described in U.S. Pat. No. 6,774,651. This patent &#39;651 issued on Aug. 10, 2004, and is entitled METHOD FOR ALIGNING AND CONNECTING SEMICONDUCTOR COMPONENTS TO SUBSTRATES. 
     FIG. 4  of patent &#39;651 illustrates one preferred apparatus which performs the patented method. This apparatus includes a platform assembly  22  which is moveable, and a chuck assembly  24  which is stationary and spaced-apart from the platform assembly  22 . 
   The platform assembly  22  includes a vacuum nozzle  38  which holds an IC-chip  10 . The vacuum nozzle  38  is attached to, and moved by, a hexapod which is comprised of six linear actuators  34 . This hexapod is able to move the IC-chip  10  parallel to three orthogonal axis, and is able to rotate the IC-chip  10  about those three axis. 
   The chuck assembly  24  holds a substrate  12  for the IC-chip  10 . Thus, the substrate  12  is the target with which the IC-chip  10  needs to be aligned. More specifically, the IC-chip  10  has a face  13  with one set of electrical contacts  14 ; the substrate  12  has a face  15  with another set of electrical contacts  16 ; and these two sets of electrical contacts need to be aligned with each other and connected together. 
   To perform the alignment process in patent &#39;651, the platform assembly  22  includes one height gauge  42  and one camera  44 ; and, the chuck assembly  24  also includes one height gauge  52  and one camera  50 . How these four components are used in the alignment process is described in patent &#39;651 at line 37 of column 7 to line 11 of column 8, which is quoted below. 
   “During an aligning and connecting process, the moving platform  32  can be moved such that the height gauge  42  is proximate to the substrate  12 , and is able to determine the distance between the height gauge  42  and the substrate  12 . This distance information can be converted into a signal, which can be optically or electrically transmitted to the controller  46 . The distance information gives a Z-axis coordinate for the substrate  12 .” 
   “Similarly, the moving platform  32  can be moved such that the camera  44  is proximate to the substrate  12  and can generate an image of the facing surface  15  of the substrate  12 . The visual image can be used to identify three reference points X 1 , X 2 , X 3  ( FIG. 3 ) on the facing surface  15 . The references points X 1 , X 2 , X 3  can be known features of the substrate  12 , such as the substrate contacts  16 , or can be dedicated alignment fiducials formed on the substrate  12 . This image is then converted into a signal which can be optically or electrically transmitted to the controller  46 . By noting the X-axis and Y-axis coordinates of the reference points X 1 , X 2 , X 3 , and the Z-axis coordinate obtained by the height gauge  42 , the position and orientation of the plane containing the reference points can be determined. This calculation can be performed by the controller  46  or can be performed by another computer (not shown) in signal communication with the controller  46 .” 
   “The position and orientation of the component  10  can be determined in a similar manner. Specifically, a camera  50  and a height gauge  52  are mounted on a base  48  of the chuck assembly  24 . Operation of the platform assembly  22  allows the component  10  to be placed proximate to the height gauge  52  to determine distance information and the Z-axis coordinate of the component  10 . Similarly, the component  10  can be placed proximate to the camera  50 , and a visual image can then be obtained and communicated to the controller  46 . The visual image can be used to identify the X-axis and Y-axis coordinates of at least three points Y 1 , Y 2 , Y 3  ( FIG. 3 ) on the facing surface  13  of the component  10 . Again the three points can be features such as the component contacts  14  or can be dedicated alignment fiducials. Using this information and the Z-axis coordinate from the height gauge  52 , the orientation and position of the component  10  can be calculated by the controller  46  or another computer in signal communication with the controller.” 
   From the above quote, it is clear that the alignment method of patent &#39;651 depends on obtaining two dimensional images from the cameras  44  and  50 , and digitally processing those images. In particular, reference points X 1 , X 2 , and X 3  need to be identified in the two dimensional image of surface  15  on the substrate  12 , and reference points Y 1 , Y 2 , and Y 3  need to be identified in the two dimensional image of surface  13  on the IC-chip  10 . Then, the X and Y co-ordinates of these reference points in the actual three dimensional system need to be determined from the two dimensional images. By comparison, with the present invention, no cameras are used and no digital images are processed. 
   Several alternative embodiments to the  FIG. 4  apparatus are also shown in  FIGS. 5-10  of patent &#39;651. However, each of those alternative embodiments still include the two cameras that were described above. 
   Also in the prior art, another method of automatically aligning IC-chips with a target is described in U.S. Pat. No. 6,587,743. This patent &#39;743 issued on Jul. 1, 2003, and is entitled PICK AND PLACE TEACHING METHOD AND APPARATUS FOR EMPLEMENTING THE SAME. 
     FIG. 1A  of patent &#39;743 illustrates a side view of one particular system which implements the claimed method. This  FIG. 1A  system includes a vacuum nozzle  15  which moves parallel to three orthogonal axis X, Y, and Z, and which also rotates around the Z axis. 
   As one step of the alignment process, the laser source  50  emits a laser beam in the X-Y plane, while the vacuum nozzle  15  is moved along the Z-axis through the laser beam. This step is described in patent &#39;743 at lines 1-8 of column 7, which is quoted below. 
   “The Z-axis initialization is described in  FIG. 2 . In preferred aspects, the laser align system is used to determined the Z=0 point. Z=0 is defined as that point at which the laser align unit transitions between being able to “see” nozzle  15  and being unable to see nozzle  15 . That is, the position is defined such that nozzle  15  blocks the laser align beam for all positive Z and does not block the beam for all negative Z.” 
   As another step of the alignment process, the laser source  50  emits a laser beam in the X-Y plane, while the vacuum nozzle  15  holds an electronic device (such as an IC-chip) which is rotated around the Z-axis in the laser beam. This step is described in patent &#39;743 at lines 38-50 of column 9, which is quoted below. 
   “Briefly, the laser align unit takes measurements as the device is rotated. For example, one or more sensors monitor which of one or more laser beams is interrupted during a rotation of the device. At any given time the image can be characterized by a width and a center position. The laser align unit identifies the four positions (corresponding to the four sides of the device) at which the image exhibits local width minima, and returns the center position associated with each of the four positions. Using these four center coordinates, the software is able to compute a correcting move for the X, Y, and angle coordinates.” 
   From the above two quotes, it is seen that the alignment method in patent &#39;743 depends on the emission of a laser beam by a source that is spaced-apart from a laser sensor, and the detection of when the emitted beam is broken by an object which is moved in a straight line or rotated between the source and the sensor. By comparison, with the present invention, no such breakage of a laser beam occurs. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention which is claimed herein is a method of automatically carrying IC-chips, on a planar array of vacuum nozzles, to a variable target in a chip tester. This method includes the following six steps. 
   Step one measures a set of distances, to a reference in the chip tester for the target, with a set of lasers that are attached to the planar array of nozzles. 
   Step two selects a direction and/or angle of movement as a function of the measured set of distances, using a control module in the chip tester. 
   Step three moves the planar array of nozzles, via a robotic arm in the chip tester, by an increment in the direction and/or angle selected by the control module. 
   Step four repeats the above steps one, two, and three until the control module detects that the distances measured by the lasers meet a predetermined criteria. 
   Step five stores control signals from encoders in the chip tester which identify the particular position of the robotic arm when the distances measured by the lasers meet the predetermined criteria. 
   Step six carries IC-chips on the planar array of nozzles while the control module uses the stored control signals to move the robotic arm to the particular position which is identified in step five. 
   With the above method, no images by any cameras are taken, and no digital processing of any images occurs. Also with the above method, no detection is made of when an object blocks or passes a laser beam, as the object is moved through the laser beam. 
   In one particular version of the present invention, the measuring step (step one) uses only three lasers to measure three distances perpendicular to the planar array of nozzles. 
   In another particular version of the present invention, the measuring step uses three lasers to measure three distances perpendicular to the planar array of nozzles and uses three additional lasers to measure three additional distances parallel to the planar array of nozzles. 
   In still another particular version of the present invention, the measuring step uses three lasers to measure distances perpendicular to the planar array of nozzles and only one additional laser to measure distances parallel to the planar array of nozzles. 
   In yet another particular version of the present invention, the measuring step uses three lasers to measure distances perpendicular to the planar array of nozzles and only two additional lasers to measure distances parallel to the planar array of nozzles. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of one particular chip testing system which automatically carries IC-chips, on a planar array of vacuum nozzles, to a variable target in accordance with the present invention. 
       FIG. 2  is a top view of several components in the chip testing system of  FIG. 1 , which shows those components when they are aligned by the present invention. 
       FIG. 3A  illustrates some of the components in  FIG. 2 , when those components are at an initial position where the components are not aligned. 
       FIG. 3B  illustrates the components of  FIG. 3A  at a first intermediate position before alignment occurs. 
       FIG. 3C  illustrates the components of  FIG. 3A  at a second intermediate position before alignment occurs. 
       FIG. 3D  illustrates the components of  FIG. 3A  at a third intermediate position before alignment occurs. 
       FIG. 3E  illustrates the components of  FIG. 3A  at a fourth intermediate position before alignment occurs. 
       FIG. 3F  illustrates the components of  FIG. 3A  at a final position where alignment occurs. 
       FIG. 4  shows one particular modification which can be made to the embodiment of the invention that is illustrated in  FIGS. 1 ,  2 , and  3 A- 3 F. 
       FIG. 5  shows another modification which can be made to the embodiment of the invention that is illustrated in  FIGS. 1 ,  2 , and  3 A- 3 F. 
       FIG. 6  shows a side view of still another modification which can be made to the embodiment of the invention that is illustrated in  FIGS. 1 ,  2 , and  3 A- 3 F. 
       FIG. 7  is a top view of the modification that is shown in  FIG. 6 . 
   

   DETAILED DESCRIPTION 
   The details of one particular chip testing system, which operates in accordance with the present inventory, will now be described with reference to  FIG. 1 . This  FIG. 1  chip testing system includes all of the components  10 - 24  which are interconnected as shown. 
   Components  10 ,  11 , and  12  in  FIG. 1  together comprise the frame of the chip testing system. Component  10  is the base of the frame; component  11  is the back of the frame; and component  12  is the top of the frame. 
   Components  13 ,  14 ,  15 , and  16  in  FIG. 1  together comprise a robotic arm in the chip testing system. Component  13  is a part of the arm which moves components  14 ,  15 , and  16  in the “X” direction. Component  14  is a part of the arm which moves components  15  and  16  in the “Y” direction. Component  15  is a part of the arm which moves component  16  in the “Z” direction. Component  16  is a part of the arm which tilts in the θ, β, and α directions. 
   The “X” direction is along a horizontal axis which is indicated by a horizontal arrow in  FIG. 1 . The “Z” direction is along a vertical axis which is indicated by a vertical arrow in  FIG. 1 . The “Y” direction is along a horizontal axis which is perpendicular to the “X” axis and the “Z” axis. The θ, β, and α directions are angular directions which respectively rotate about the “X” axis, the “Y” axis, and the “Z” axis. 
   Component  17  in  FIG. 1  is a planar carrier which is rigidly attached to component  16  of the robotic arm. This carrier  17  is moved to various positions by components  13 - 16  of the robotic arm. 
   Each of the components  18  in  FIG. 1  is a vacuum nozzle which is rigidly attached to the carrier  17 . Each of these vacuum nozzles  18  has an open end which is adapted to hold an IC-chip. The open end of all of the vacuum nozzles  18  lie in a single plane. In the  FIG. 1  chip testing system, only one row of four vacuum nozzles can be seen; however, a second row of four additional vacuum nozzles lies behind that one row and is hidden from view. 
   Each of the components  19   a ,  19   b , and  19   c  in  FIG. 1  is a laser distance sensor which is rigidly attached to the carrier  17 . Each of these laser distance sensors measure distance perpendicular to the plane where the open end of the vacuum nozzles lie. To measure these distances, each laser distance sensor emits a laser beam, receives a reflected beam, detects the time interval which occurs between the two beans, and calculates distance as a function of the detected time interval. 
   Component  20  in  FIG. 1  is a load station in the chip testing system. This load station  20  has a surface  20   a  which is completely planar, except that the surface has two steps. Only one step  20   b  can be seen in  FIG. 1 . The other step is perpendicular to step  20   b  and is hidden from view in  FIG. 1 . These steps have a significance which will be described later in conjunction with FIGS.  2  and  3 A- 3 F. 
   The planar surface  20   a  is used to temporarily hold the IC-chips  30  that are to be tested. These chips  30  are placed on the surface  20   a  (by a mechanism not shown) with the exact same center-to-center spacing as the vacuum nozzles  18  on the carrier  17 . 
   Component  21  in  FIG. 1  is a test station in the chip testing system. This test station  21  has a surface  21   a  which is completely planar except that the surface has two steps. Only one step  21   b  can be seen in  FIG. 1 . The other step is perpendicular to step  21   b  and is hidden from view in  FIG. 1 . Here again, these steps have a significance which will be described later in conjunction with FIGS.  2  and  3 A- 3 F. 
   Each of the components  22  is a socket for holding an IC-chip while the chip is tested. These sockets are attached to the planar surface  21   a  with the exact same center-to-center spacing as the vacuum nozzles  18  on the carrier  17 . 
   In addition to the load station  20  and the test station  21 , the chip testing system also includes an unload station which is hidden from view in  FIG. 1 . This unload station lies behind the load station  20  in the “Y” direction. The IC-chips  30  are moved to the unload station, by the robotic arm, after the IC-chips are tested at the test station. 
   Component  23  in  FIG. 1  is an electronic control module for the chip testing system. This control module  23  is electrically coupled to each of the components  13 - 16  of the robotic arm by one set of conductors  23   a . Also, the control module  23  is electrically coupled to each of the laser distance sensors  19   a - 19   c  by another set of conductors  23   b . Further, the control module  23  is electrically coupled to an operator interface  24  by yet another set of conductors  23   c.    
   One function which control module  23  performs is direct the movement of the carrier  17 . To move the carrier  17  to a particular position in the “X” direction, or the “Y” direction, or the “Z” direction, the control module  24  sends control signals on the conductors  23   a  respectively to components  13 ,  14 , or  15  of the robotic arm. Similarly, to tilt the carrier  17  at any particular angle in the θ direction, or the β direction, or the α direction, the control module  24  sends control signals on the conductors  23   a  to component  16  of the robotic arm. 
   A second function which control module  23  performs is sense the current position of the carrier  17 . To that end, each of the components  13 ,  14 , and  15  in the robotic arm include one linear position detector which generate encoded signals that respectively indicate the current position of the carrier  17  on the “X”, “Y”, and “Z” axis. Similarly, component  16  in the robotic arm includes three angular position detectors which generate encoded signals that respectively indicate the current angle of tilt of the carrier  17  in the θ, β, and α directions. All of these encoded signals are sent to the control module  23  on the conductors  23   a.    
   A third function which control module  23  performs is activate the laser distance sensors  19   a - 19   c . To do that, the control module  23  sends a command on the conductors  23   b  to the laser distance sensors  19   a - 19   c . In response, each of the laser distance sensors  19   a - 19   c  makes a distance measurement and generates signals which indicate the result. Those signals are then sent to the control module  23  via the conductors  23   b.    
   Under normal operating conditions, each group of IC-chips  30  which need to be tested is picked-up by the vacuum nozzles  18  at the load station as follows. First, the control module  23  sends control signals on the conductors  23   a  which direct the robotic arm  13 - 16  to a particular “load position” that has co-ordinates X 1 , Y 1 , Z 1 , θ 1 , β 1 , and α 1 . In this load position, the open ends of the vacuum nozzles  18  are aligned with and barely touch the IC-chips  30  on surface  30   a  of the load station. Then the control module  23  sends a control signal which opens a valve in a vacuum line that is connected to the vacuum nozzles  18 . Note that this valve and vacuum line are not shown in  FIG. 1  in order to simplify the drawing. 
   Similarly, under normal operating conditions, each group of IC-chips which is picked-up at the load station is subsequently put into the sockets  22  at the test station as follows. First, the control module  23  sends control signals on the conductors  23   a  which direct the robotic arm  13 - 16  to a particular “test position” that has co-ordinates X 2 , Y 2 , Z 2 , θ 2 , β 2 , and α 2 . In this test position, the IC-chips which are held by the vacuum nozzles  18  are aligned with and barely touch the sockets  22  on surface  21   a  of the test station. Then control module  23  sends control signals on the conductors  23   a  which direct the robotic arm to move by a predetermined distance in the −Z direction and thereby put the IC-chips into the sockets  22 . 
   However, the co-ordinates X 1 , Y 1 , Z 1 , θ 1 , β 1 , and α 1  of the load position, as well as the co-ordinates X 2 , Y 2 , Z 2 , θ 2 , β 2 , and α 2  of the test position, vary each time the  FIG. 1  chip testing system is manufactured. These variations are caused by the accumulation of multiple manufacturing tolerances which inherently occur in all of the components  10 - 24  in the  FIG. 1  system. 
   But now, in accordance with the present invention, the control module  23  has a “start-up” mode of operation wherein the control module  23  automatically teaches itself what the co-ordinates X 1  through α 1  are at the load position, and what the co-ordinates X 2  through α 2  are at the test position. Thereafter, the control module  23  operates in the “normal” mode wherein it uses the co-ordinates X 1  through α 1  and X 2  through α 2  to repeatedly—1) pick-up IC-chips at the load station and move them to the sockets in the test station, and 2) pick-up IC-chips after they are tested at the test station and move them to the unload station. All of the details of how the control module  23  teaches itself will now be described with reference to FIGS.  2  and  3 A- 3 F. 
   In  FIG. 2 , the carrier  17  is shown in a top view at the test position. Also shown in  FIG. 2  is a top view of the vacuum nozzles  18  which are attached to the carrier  17 , and a top view of the laser beams  19   a ′,  19   b ′, and  19   c ′ which respectively are emitted from the laser distance sensors  19   a ,  19   b , and  19   c  of  FIG. 1 . Further shown in  FIG. 2  is a top view of the planar surface  21   a  of the test station, a top view of the two steps  21   b  and  21   c  which are in surface  21   a , and a top view of the sockets  22  which are attached to surface  21   a.    
   Inspection of  FIG. 2  shows that at the test position, the vacuum nozzles  18  are precisely aligned with the centers of the sockets  22 . This alignment occurs when all three of the following conditions  1   a - 3   a  are met. Condition  1   a  is that the laser beams  19   a ′ and  19   b ′ begin to hit step  21   b . Condition  2   a  is that the laser beam  19   c ′ begins to hit step  21   c . Condition  3   a  is that the distances which the laser beams  19   a ′,  19   b ′, and  19   c ′ travel are equal to each other. 
   The above alignment occurs as long as the one subassembly of components  17 ,  18 , and  19   a - 19   c  is accurately fabricated, and the other subassembly of components  21   a - 21   c  and  22  is accurately fabricated. When that occurs, the manufacturing tolerances in all of the other components in the chip testing system of  FIG. 1  are irrelevant because those tolerances are cancelled out when the conditions  1   a - 3   a  are met. 
   Also, the alignment shown in  FIG. 2  can be achieved even though the sockets  22  at the test station  21  are hidden from the view of an operator who works at the operator interface  24 . This is because with the chip testing system of  FIG. 1 , the control module  21  automatically moves carrier  17  until the conditions  1   a - 3   a  are met. Then the control module  21  stores the co-ordinates X 2 , Y 2 , Z 2 , θ 2 , β 2 , and α 2  that are sent from the robotic arm components  13 - 16  to the control module. 
   In a similar manner, the vacuum nozzles  18  on the carrier  17  are precisely aligned at the load station  20  with the centers of the IC-chips  30 . This alignment is visually seen by simply changing the reference numerals  21   a - 21   c  and  22  in  FIG. 2  to reference numerals  20   a - 20   c  and  30  respectively. 
   The vacuum nozzles  18  are precisely aligned with the IC-chips  30  at the load position when the following conditions  2   a - 2   c  are met. Condition  2   a  is that the laser beams  19   a ′ and  19   b ′ begin to hit step  20   b . Condition  2   b  is that the laser beam  19   c ′ begins to hit step  20   c . Condition  2   c  is that the distances which the laser beams  19   a ′- 19   c ′ travel are equal to each other. The control module  23  in the  FIG. 1  chip testing system automatically moves the carrier  17  until the conditions  2   a - 2   c  are met. Then the control module  23  stores the co-ordinates X 1 , Y 1 , Z 1 , θ 1 , β 1 , and α 1  that are sent from the robotic arm components  13 - 16  to the control module. 
   One preferred process, which the control module  23  automatically performs in order to meet the conditions  1   a - 1   c  is shown in  FIGS. 3A-3F . In all of those figures, reference numeral  17 ′ identifies the triangular shaped plane which is formed by interconnecting the points where the laser beams  19   a ′- 19   c ′ exit the three laser distance sensors  19   a - 19   c . All of the other reference numerals in  FIGS. 3A-3F  identify previously described components. 
   Initially, control module  23  directs the robotic arm to a predetermined position where the plane  17 ′ is as shown in  FIG. 3A . This position is selected such that the laser beams  19   a ′ and  19   b ′ hit surface  21   a  at two points which are spaced apart from the step  21   b , and the laser beam  19   c ′ hits surface  21   a  at a third point which is spaced apart from the step  21   c.    
   The exact position of the laser beams  19   a ′- 19   c ′ in  FIG. 3A  relative to the steps  21   b - 21   c  is indeterminate, due to all of the cumulative manufacturing tolerances which are present in  FIG. 1  chip testing system. Similarly, the exact angular orientation of the plane  17 ′ relative to the planar surface  21   a  in  FIG. 3A  is indeterminate due to the manufacturing tolerances. Thus, in  FIG. 3A , the distances d 1 -d 3  measured by laser distance sensors  19   a - 19   c  are all different. 
   Next, control module  23  directs the robotic arm to move in small increments to a series of positions whereby the plane  17 ′ eventually becomes parallel to the planar surface  21   a . One point in this series is shown in  FIG. 3B . There, the distances measured by two of the laser distance sensors  19   a  and  19   b  are equal to each other. The last point in this series is shown in  FIG. 3C . There, the distances measured by all three of the lasers  19   a - 19   c  are equal to each other. 
   Each time control module  23  directs the robotic arm to move by an increment, the control module waits for that movement to be completed. Then, control module  23  sends a command to the laser distance sensors  19   a - 19   c  which causes them to take a distance measurement. Then, based on that distance measurement, control module  23  selects the liner direction and/or angular direction for the next incremental movement of the robotic arm. 
   Next, control module  23  directs the robotic arm to move in small increments such that the plane  17 ′ moves from the position shown in  FIG. 3C  to the position shown in  FIG. 3D . During this move, the plane  17 ′ remains parallel to surface  21   a . Also during this move, the plane  17 ′ is moved in a straight line towards the step  21   b  until one of the laser beams  19   a ′ or  19   b ′ start to hit that step. 
   Whether laser  19   a ′, or laser beam  19   b ′, will be first to hit step  21   b  is indeterminate. This is because the angular orientation of the plane  17 ′, relative to the step  21   b , is not known when the movement from the position shown in  FIG. 3C  begins. Thus,  FIG. 3D  illustrates just one example where the laser beam  19   a ′ is first to hit the step  21   b . To determine when the laser beam  19   b ′ begins to hit the step  21   b , control module  23  detects when a rapid change occurs in the distance that is measured by the laser distance sensor  19   b.    
   Next, control module  23  directs the robotic arm to move in small increments such that the plane  17 ′ moves from the position shown in  FIG. 3D  to the position shown in  FIG. 3E . During this movement, the plane  17 ′ again remains parallel to surface  21   a . Also during this movement, the plane  17 ′ is rotated until both of the laser beams  19   a ′ and  19   b ′ start to hit step  21   b . To determine when both of the laser beams  19   a ′ and  19   b ′ start to hit step  21   b , the control module  23  detects a rapid change in the distances that are measured by both of the laser distance sensors  19   a  and  19   b , when plane  17 ′ is moved parallel to surface  21   a  and perpendicular to a line through the laser beams  19   a ′ and  19   b′.    
   Lastly, control module  23  directs the robotic arm to move in small increments such that the plane  17 ′ moves from the position shown in  FIG. 3E  to the position shown in  FIG. 3F . During this movement, the plane  17 ′ again remains parallel to surface  21   a . Also during this movement, the laser beams  19   a ′ and  19   b ′ travel parallel to the step  21   b . The final position of  FIG. 3F  is reached when the laser beam  19   c ′ begins to hit the step  21   c . To determine when this occurs, the control module  23 , detects a rapid change in the distance that is measured by the laser distance sensor  19   b.    
   When plane  17 ′ reaches the position shown in  FIG. 3F , the control module  23  stores the co-ordinates X 2 , Y 2 , Z 2 , θ 2 , β 2 , and α 2  which it receives from the robotic arm components  13 - 16  of  FIG. 1 . Then, control module  23  moves the carrier  17  to a predetermined position over the load station  20  of  FIG. 1 . Then, control module  23  performs the steps of  FIGS. 3A-3F  at the load station and stores the co-ordinates X 1 , Y 1 , Z 1 , θ 1 , β 1 , and α 1 . Thereafter, the control module  23  operates in the normal mode wherein it repeatedly uses the stored co-ordinates X 1  through α 1  and X 2  through α 2 , as previously described. 
   One preferred method of operating an IC-chip tester, in accordance with the present invention, has now been described in detail. Next, several modifications to that method will be described. 
   As a first modification, the shape of steps  21   b  and  21   c  in the planar surface  21   a  of the test station, can be changed from the shape that is shown in  FIGS. 1 ,  2 , and  3 A- 3 F. For example, one particular change is shown in  FIG. 4 , and another particular change is shown in  FIG. 5 . 
   In  FIG. 4 , the planar surface  21   a  of the test station has only a single continuous step  21   d . This step  21   d  includes three edges  21   d - 1 ,  21   d - 2 , and  21   d - 3  which are oriented on surface  21   a  of the test station as shown. 
   Using step  21   d  of  FIG. 4 , the control module  23  aligns the vacuum nozzles  18  with the sockets  22  as shown in  FIG. 2 , by the following process. Initially, the plane  17 ′ is moved in small increments to the positions that are shown in  FIGS. 3A ,  3 B, and  3 C. Them the plane  17 ′ is moved in small increments from the  FIG. 3C  position to a position which is the same as that shown in  FIG. 3D  except that the laser beam  19   b ′ hits edge  21   d - 1 . Next, plane  17 ′ is moved in small increments to a position which is the same as that shown in  FIG. 3E  except that both of the laser beams  19   a ′ and  19   b ′ hit edge  21   d - 1 . Lastly, plane  17 ′ is moved in small increments to a position which is the same as that shown in  FIG. 3F , except that both of the laser beams  19   a ′ and  19   b ′ hit edge  21   b - 1 , and laser beam  19   b ′ also hits edge  21   b - 2 . 
   In  FIG. 5 , the planar surface  21   a  of the test station again has only a single continuous step  21   e . But this step  21   e  includes four edges  21   e - 1 ,  21   e - 2 ,  21   e - 3 , and  21   e - 4  which are oriented on surface  21   a  of the test station as shown. 
   Using step  21   e  of  FIG. 5 , the control module  23  aligns the vacuum nozzles  18  with the sockets  22  as shown in  FIG. 2 , by the following process. Initially, the plane  17 ′ is moved in small increments to the positions that are shown in  FIGS. 3A ,  3 B, and  3 C. Them the plane  17 ′ is moved in small increments from the  FIG. 3C  position to a position which is the same as that shown in  FIG. 3D  except that the laser beam  19   b ′ hits edge  21   e - 1 . Next, plane  17 ′ is moved in small increments to a position which is the same as that shown in  FIG. 3E  except that laser beam  19   b ′ hits edge  21   e - 1 , and laser beam  19   a ′ hits edge  21   e - 4 . Lastly, plane  17 ′ is moved in small increments to a position which is the same as that shown in  FIG. 3F , except that laser beam  19   b ′ hits both of the edges  21   e - 1  and  21   e - 2 , and laser beam  19   a ′ hits edge  21   e - 4 . 
   As a second modification, each of the steps in the previously described  FIGS. 1 ,  2 ,  3 A- 3 F,  4  and  5  can be an upward step in the +Z direction, or a downward step in the −Z direction. Also, each of those steps can have any height as long as the edge of the step can be detected by control module  23  from the signals which the control module receives from the laser distance sensors  19   a - 19   c . In one particular embodiment, each of the laser distance sensors was model 0ADM-1216460/S35A that is sold by Baumer Electric Corporation; and with it, the edge of a step which is only sixty-thousandths of an inch high is accurately detected. 
   As a third modification, each of the steps in the previously described  FIGS. 1 ,  2 ,  3 A- 3 F,  4  and  5  can be changed to a ramp which has an upward incline or a downward incline. However, as the slope of the incline decreases, the accuracy of detecting where the incline starts also decreases. 
   As a fourth modification, three additional laser distance sensors  19   d ,  19   e , and  19   f  are attached to the carrier  17 , as shown in  FIGS. 6 and 7 . These laser distance sensors  19   d - 19   f  measure distance in a plane which is parallel to the plane where the open end of the vacuum nozzles  18  lie. However, the laser distance sensor  19   d  measures distance in a direction that is perpendicular to the direction in which the laser distance sensors  19   e  and  19   f  measure distance. 
   The above perpendicular directions are illustrated in  FIG. 7 . There, the laser distance sensor  19   d  emits laser beam  19   d ′, whereas the laser distance sensors  19   e  and  19   f  emit respective laser beams  19   e ′ and  19   f′.    
   Also in this modification of  FIGS. 6 and 7 , two flat reference members  21   f  and  21   g  are attached perpendicular to the planar surface  21   a  of the test station. Laser distance sensor  19   d  measures the distance d 4  to member  21   f , whereas the two laser distance sensors  19   e  and  19   f  respectively measure distances d 5  and d 6  to the member  21   g . The steps  21   b  and  21   c  that were shown in FIGS.  2  and  3 A- 3 F, are eliminated. 
   Using the modification of  FIGS. 6 and 7 , the control module  23  aligns the vacuum nozzles  18  with the sockets  22  as shown in  FIG. 2 , by performing the following process. Initially, the carrier  17  is moved in small increments to the positions that are shown in  FIGS. 3A ,  3 B, and  3 C. Then the carrier  17  is moved in small increments from the  FIG. 3C  position to a position that corresponds to the position shown in  FIG. 3D  where the distance d 5 , as measured by the laser distance sensor  19   e , equals a first predetermined distance. Next, the carrier  17  is moved in small increments to a position that corresponds to the position shown in  FIG. 3E  where the distance d 5  and d 6 , as measured by the laser distance sensors  19   e  and  19   f , both equal the above first predetermined distance. Lastly, the carrier  17  is moved to a position that corresponds to the position shown in  FIG. 3F  where—1) the distances d 5  and d 6 , as measured by the laser distance sensors  19   e  and  19   f , both equal the first predetermined distance, and 2) the distance d 4 , as measured by the laser distance sensor  19   d , equals a second predetermined distance. 
   As a fifth modification, only four of the laser distance sensors  19   a ,  19   b ,  19   c , and  19   d  are attached to the carrier  17  as shown in  FIGS. 6 and 7 , and the remaining two laser distance sensors  19   e  and  19   f  are deleted. Also, in this modification, the reference member  21   g  in  FIG. 7  is replaced with the step  21   b  of  FIG. 2 . 
   Using this fifth modification, control module  23  aligns the vacuum nozzles  18  with the sockets  22  as shown in  FIG. 2 , by performing the following process. First, the carrier  17  is moved in small increments to the positions that are shown in  FIGS. 3A ,  3 B,  3 C,  3 D, and  3 E. Then, the carrier  17  is moved to a position which is the same as that shown in  FIG. 3F  except the distance d 4 , as measured by the laser distance sensor  19   d , equals a predetermined distance. 
   Similarly, as a sixth modification, only five of the laser distance sensors  19   a ,  19   b ,  19   c ,  19   e , and  19   f  are attached to the carrier  17  as shown in  FIGS. 6 and 7 , and the remaining laser distance sensor  19   d  is deleted. Also in this modification, the reference member  21   f  in  FIG. 7  is replaced with step  21   c  of  FIG. 2 . 
   Using this sixth modification, control module  23  aligns the vacuum nozzles  18  with the sockets  22  as shown in  FIG. 2 , by performing the following process. Initially, the carrier  17  is moved in small increments to the positions that are shown in  FIGS. 3A ,  3 B, and  3 C. Then the carrier  17  is moved in small increments from the  FIG. 3C  position to a position that corresponds to the position shown in  FIG. 3D  where the distance d 5 , as measured by the laser distance sensor  19   e , equals a first predetermined distance. Next, the carrier  17  is moved in small increments to a position that corresponds to the position shown in  FIG. 3E  where the distance d 5  and d 6 , as measured by the laser distance sensors  19   e  and  19   f , both equal the above first predetermined distance. Lastly, the carrier  17  is moved to a position that corresponds to the position shown in  FIG. 3F  where—1) the distances d 5  and d 6 , as measured by the laser distance sensors  19   e  and  19   f , both equal the first predetermined distance, and 2) the laser beam  19   c ′, from the laser distance sensor  19   c , begins to hit the step  21   c  as shown in  FIG. 3F . 
   To perform any one of the processes which have been described above, control module  23  preferable includes a programmable microprocessor and a memory. The memory stores instructions which the microprocessor sequentially executes and thereby performs all of the steps of the processes, as described above. 
   Several preferred processes which incorporate the present invention have now been described in detail. In addition, however, many modifications can be made to these details without departing from the gist of the present invention. Accordingly, it is to be understood that the present invention is not limited to just the details of the above described preferred processes, but is defined by the appended claims.