Abstract:
Methods, apparatuses, and systems are presented for positioning a sensing head relative to a workpiece, involving a control unit operative to provide a plurality of control signals to iteratively control positioning of the sensing head relative to the workpiece, a plurality of air injectors disposed and fixedly connected on a periphery of the sensing head, each of the air injectors capable of being independently controlled to eject a gas between the sensing head and the workpiece to create an air bearing and affect positioning of the sensing head relative to the workpiece in response to at least one of the control signals, and a plurality of sensors providing a plurality of feedback signals to the control unit, the feedback signals containing information relating to positioning of an optical imaging sensing head relative to the workpiece.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to test equipment and in particular to a system for testing using noncontact electro-optical imaging of a flat panel device such as a liquid crystal display. 
   Diagnostic sensor placement requirements are extremely high. A flat sensor plate that is part of a sensing head which measures approximately 8 cm on each side must be placed parallel within 3 um of a flat workpiece, such as an LCD glass panel. The gap distance between the workpiece panel and sensor plate needs to be a selectable value between 7 um and 30 um and preferably between 10 um and 25 um with a tolerance of +/−0.5 um. Component hardware used to position the sensing head cannot encroach upon the clear 8 cm square aperture of the sensing head because the sensing head produces information that is read by an optical array (a CCD camera) focused on the clear aperture. 
   The sensing head must be able to maintain the required gap position without contacting the glass panel even when added attracting electrostatic forces resulting from a high voltage applied between the sensor plate of the sensing head and panel are present. 
   The sensing head must be quickly separable from the panel surface to a gap of greater than 75 um to permit translation of the elements without contact between the panel and the sensor plate as the sensing head is moved over the panel to another site. Once the sensing head arrives at the new site, the gap must be quickly reduced to the low gap position to allow the sensing head to acquire data. 
   Sensor placement above the panel must compensate for the variation of panel surface height from the sensor datum. 
   SUMMARY OF THE INVENTION 
   The invention presents methods, apparatuses, and systems for positioning a sensing head relative to a workpiece, involving a control unit operative to provide a plurality of control signals to iteratively control positioning of the sensing head relative to the workpiece, a plurality of air injectors disposed and fixedly connected on a periphery of the sensing head, each of the air injectors capable of being independently controlled to eject a gas between the sensing head and the workpiece to create an air bearing and affect positioning of the sensing head relative to the workpiece in response to at least one of the control signals, and a plurality of sensors providing a plurality of feedback signals to the control unit, the feedback signals containing information relating to positioning of an optical imaging sensing head relative to the workpiece. 
   In one embodiment, a system is provided wherein a plurality of high accuracy air injectors are disposed along the edges of a sensor plate of a sensing head to form an air bearing and a plurality of high displacement air injectors are also disposed along the edges of the sensor plate to form an air bearing, each independently controlled, with the sensing head having sensors coupled in a feedback loop through a mapper which iteratively adjusts relative separation of the sensor plate and a flat panel workpiece to the desired positional accuracy through digital to analog converters supplying control signals to analog amplifiers controlling orifices. Translation is effected after the high displacement air injectors are activated, with the combination of flow of air from the air bearing outlets along the edge of the sensor plate and the translation in x and y of the flat panel being operative to air brush sweep the surface of the flat panel. 
   Translation of the LCD glass panel is effected after the high displacement air injectors are activated, with the combination of flow of air from the air injector outlets along the edge of the sensor plate and the translation in x and y of the flat panel being operative to air brush sweep the surface of the flat panel. 
   The placement of the air injectors to the side of the sensing head is important. Air leakage path between the surface of the air injector and the surface of the sensor plate is to be minimized. A means is provided for sealing the air leakage path between the air injector and the corner radius of the sensor plate edge. 
   In addition, edge placement of the of the injectors fulfills the requirement of sweeping the particulates out of the path of the advancing sensor, thus reducing or eliminating sensing head and panel abrasion damage. 
   The invention will be better understood by reference to the following detailed description in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of the system according to the invention. 
       FIG. 2  is a perspective view of the top of a sensing head according to the invention. 
       FIG. 3  is a perspective view of the face of the sensing head according to the invention. 
       FIG. 4  a schematic block diagram of a single transducer and air injector circuit with feedback control. 
       FIG. 5  is a block diagram of a specific embodiment of an electronic modules in a system according to the invention coupled to sensors, loads and a computer system (not shown). 
       FIGS. 6A-6D  are schematic diagrams of the air injector positions, coordinates of image statistics sensing regions, virtual sensor positions, and LVDT sensor positions. 
       FIG. 7  is a cross section of a corner of a prior art sensing head structure. 
       FIG. 8  is a cross section of a corner of a sensing head and air injector structure according to the invention. 
   

   DESCRIPTION OF EMBODIMENTS 
   Referring to  FIGS. 1 ,  2 ,  3  and  4 , a system  10  according to the invention with a sensing head  12  is positioned in x, y and z relative to a flat panel workpiece  14  mounted on a translation platform  16  of a table  18 . The translation platform is movable in x and y by positioning stepper motors  20 ,  22 . The sensing head  12  is suspended between an optical head  24  and the workpiece  14  on a cantilever spring  26  and is movable along the z direction (up and down relative to the workpiece) and in rotation about the x-axis and the y-axis (in the plane of a sensor plate  38  of the sensing head  12 ). However the sensing head  12  cannot translate along the x-axis (transverse to the cantilever spring  26 ) and can move only slightly along the y-axis with rotation about the x-axis at the base of the cantilever spring  26  and cannot rotate about the z-axis. The tolerances are extremely tight since the resolution of motion is comparable to within a few orders of magnitude of the wavelength of light. 
   The optical head  24  senses illumination through a CCD array  28  reflecting illumination from a light source  30  redirected through a partially reflective mirror  32 . An optical imaging surface  36  of the sensor plate  38  of the sensing head  12  is translatable relative to optics  34  to focus reflected light onto the CCD array  28 . 
   From (three) positions (L 1 , L 2 , L 3 ,  FIG. 6C ) a set of corresponding (three) linear voltage displacement translators (LVDT)  40  sense the distance D ( FIG. 1 ) between a point on the housing  42  of the optical head  24  and a point on the sensing head housing  44  and thus provides a measure of the distance between the CCD array  28  and the optical imaging surface  36 . 
   Imaging statistics at selected positions (S 1 , S 2 , S 3 , S 4 ,  FIG. 6B ) in the sensed image extracted from the reflected light of surface  36  yield readings of intensity, which can be translated into a small displacement distance d ( FIG. 1 ) between the workpiece  14  and the voltage sensing surface  46  of the sensor plate  38 . (The conversion of voltage to an optically sensible image is a modulation, so the sensing head is also often called a modulator.) 
   Spacing of the sensor plate  38  from the workpiece  14  is controlled by two different types of air injectors  50 - 52  and  53 - 55 , all mounted on the sensing head  12  along the side edges of the sensor plate  38 . A high accuracy, close positioning air injector set  50 - 52  comprises a plurality of first injector outlets  56 - 58  along the plate edge  60  whose single orifices  62 - 64  per outlet are controlled closely by amplifiers  66 - 68 . The orifices are choke flow valves wherein the pressure differential Pout/Pin is &lt;0.5 so that linear voltage change converts to a nearly linear air flow change. A high displacement air injector set  53 - 55  comprises a plurality of second injector outlets  70 - 72  along the plate edge  60  whose air source is via a solenoid valve  81  switching air to the second injector outlets  70 - 72  substantially simultaneously to lift the sensor plate  38  to be clear of any obstructions. 
   The valve orifices  62 - 64  have a diameter of about 100-250 um and the outlets  56 - 58  have a diameter of about 750 um. The high flow outlets have a diameter of about 750 um. 
   The sensing head  12  utilizes edge-fed air injectors, such as air injectors  53 - 55 , as contrasted to the center-fed air injectors of prior known air bearing designs. The spacing of the gap is sufficiently close that air serves as an adequate damper to prevent inertial oscillation of the sensing head when position is changed. One configuration is shown in FIG.  3 . Air injected at opposing edge locations into the gap between the sensor plate surface and panel workpiece  14  maintains the correct gap between the sensor plate  38  and panel workpiece  14  according to the required tolerances (˜1 um to 30 um +/−0.5 um). Control of this gap of distance d is achieved by controlling the volume of air flow into the sensor plate/panel workpiece interface at opposing edges where three injector outlets  56 ,  57 ,  58  are flush mounted to the sensing head  12  with the face of the outlets being substantially exactly at the same height as the sensor plate  38 . The amount of air flow to each injector is determined by information (image data related to luminosity) from image statistic sensors (S 1 , S 2 , S 3 , S 4 ,  FIG. 6B ) in the floating sensing head  12  and/or typically three LVDT sensors  40  mounted at three peripheral positions (L 1 , L 2 , L 3 ,  FIG. 6C ) to measure separation of the optical head from the sensing head. An analog signal to digital converter set  78  (three) provides readings in a feedback loop through a mapper (a programmable CPU)  80 . Other feedback signals from the CCD array  28  provide image statistics to the mapper  80 . The mapper  80 , without knowledge of the exact positions of the image statistics sensors or of the air injectors, but being responsive to the feedback information, iteratively adjusts relative separation of the sensor plate  38  and a flat panel workpiece  14  to the desired positional accuracy through (three) digital to analog converters  82 - 84  supplying control signal amplifiers  66 - 68  controlling orifices  62 - 64 . Precise location of image statistics sensors and air injectors is not critical, as will be explained. Gap indexing is reliably achieved by increasing the amount of air metered into the sensor plate/panel interface using the high flow outlets. The increased air volume causes the sensing head to quickly hop to a gap of greater than 75 um above the panel. The high volume air is applied through the separate set of high flow outlets to the air injector-sensing head interface. This eliminates the requirement for reacquiring the low flow air setting at the next site. 
   The sensor plate height d is automatically regulated to the correct position above the panel by software of the mapper  80  controlling the volume of air injected into the air injector orifices. Irregularities of workpiece panel surfaces are accounted for by adjusting the airflow though each edge-mounted air injector as required to maintain the needed gap. Lateral movement of the sensor plate  38  over the panel  14  surface is inhibited via the cantilever suspension system where each or a pair of parallel leaf springs  26  is wide compared to thickness so that there is high stiffness in the x and y directions parallel to the sensor plate  38  and thus the panel  14 . Other restraint systems are possible. 
   It is important to note that the desired gap is thus achieved for a wide variety of sensor plate orientations and surface profiles. 
     FIG. 4  a schematic block diagram of a single transducer and air injector circuit with feedback control. Mapper  80  sends a digital control signal to digital to analog converter  82 , which sends an analog control signal to E/P transducer  65 , which could be incorporated into valve orifice  62  but is shown here as a separate block controlling valve orifice  62 . Air flowing from valve orifice  62  is fed to air injector  50 , which is attached to sensing head  12 . The position of sensing head  12  relative to workpiece  14  is sensed by CCD array  28  and LVDT sensor  40 , represented here as one functional block which forwards position information signals to mapper  80 . 
     FIG. 5  is a block diagram of a specific embodiment of electronic modules  90  in a system  10  according to the invention coupled to sensors, loads and a computer system (not shown). Shown is a conditioning subsystem  92  connected with LVDT(s)  40 . The LVDT(s)  40  may send measurement signals to the conditioning system  92 . The conditioning system  92  may send conditioned measurement signals to a digitizer system  78 , which transforms the conditioned measurement signals to digitized conditioned measurement signals using an analog signal to digital converter set. DAC/amplifiers  98 ,  99  drive proportional air valve controller (PAVC)  102 , which adjusts air valves  66 - 68  (not shown) associated with air tubes connected to the sensing head housing  44 . Air supply at about twice the highest expected pressure of the output is supplied to the adjustable valves. 
   In the CPU, the software provides the functions of gathering image statistics from the N image statistic sensors (typically 4) S 1 , S 2 , S 3 , S 4 , which are transformed to measure the three dimensions of movement z, θx and θy (a.k.a. virtual sensors V 1 , V 2 , V 3 ), which is then used to adjusted the control air flow of the high accuracy, close positioning air injectors P 1 , P 2 , P 3 . 
   In a specific embodiment of three high accuracy, close positioning air injectors disposed at positions P 1 , P 2 , P 3  ( FIG. 6A ) around the sensing head, the sensing head position is controlled by adjusting the three high accuracy, close positioning air injector settings via feedback from N image statistics sensor values. Subsequent transformations are applied to these image statistics sensor values to yield three virtual sensor values. The virtual sensor value units are microns and are comparable to the (interpolated) LVDT sensor values. The virtual sensor space may also viewed as:
         (height, rotation about x-axis, rotation about y-axis).       

   Hence, the mapping of R^3 to R^n (pressure space to image statistics sensor space) is transformed into a differentiable, non-singular map from R^3 to R^3 (pressure space to virtual sensor space). 
   When the differential image statistics sensor values are out of tolerance, the low flow air injector settings are iteratively adjusted using a variation of Newton&#39;s method, specifically:
         1) Calculate a close approximation of the derivative of the map by individually varying each low pressure setting by a small amount and measuring the virtual sensor values. This yields a 3×3 matrix.   2) Apply the inverse of this 3×3 matrix to the virtual sensor (vector) differential value (delta), which yields a pressure (vector) differential value (delta).   3) Adjust the current pressure settings by this pressure differential.   4) Repeat steps 1 through 3 until the virtual sensor values are within the desired tolerance.       

   It has been found that this procedure has several advantages over known techniques for sensing an output for feedback:
         1) It is based on a simple intuitive mathematical model.   2) The map is differentiable and non-singular so its derivative may be represented by a 3×3 invertible matrix.   3) There is much less dependence on actual geometry. As a consequence, it is almost irrelevant as to where the air injectors are located (e.g., it does not matter that air injectors are symmetric only on y-axis), and there is great flexibility on number and location of the image statistics sensor values (which here requires four or more “symmetrically balanced” samples from the image).   4) The virtual sensor space and the LVDT space are in the same units (microns) and hence are comparable.   5) No pre-calibration is required. (The option is nevertheless available to use previously collected derivative data in order to more quickly make small adjustments as required).       

   The LVDT sensors are a common type of position sensor. The primary purpose of the LVDT sensors is to define and reproduce a defined focus position (the center of the depth of field of the camera optics). However, they are also used in the following contexts:
         1) As a backup sensor system and to increase the efficiency of the auto-gapping algorithm, namely the sensing head to panel gap positioning algorithm.   2) To detect positional anomalies and to do safety limit checks during an inspection.   3) To characterize the mechanical response of the various components of the sensing head, air injectors, controlling orifices, etc.   4) System diagnostics and calibration (e.g. the amount of time it takes for the sensing head to settle after the high flow injectors are turned off. This determines when it&#39;s ok to start image acquisition at each site)   5) To obtain fine grained positional data; which is information for algorithm development and tuning.
 
Notation used in  FIGS. 6A-6D  is as follows:
   p˜pressure(s)   s˜image statistics sensor values   v˜virtual sensor values (˜microns; at fixed offset from LVDT values)   1˜LVDT values   S˜map from pressure space to image statistics sensor space   V˜map from image statistics sensor space to virtual sensor space   V(S( ))˜composite map from pressure space to virtual sensor space   D(V(S( )))˜the first derivative of this composite map   D(V(S( ))): (dp1, dp2, dp3)→(dv1, dv2, dv3)       

   Several mappings are obtained, as indicated schematically: 
                        
 
   [pressure space] [image statistics sensor space] [virtual sensor space]
                        
 
S( ):=Implicitly defined function; where the pressure settings indirectly determine image statistics sensor values.
 
Mapping is according to the following equations, using the referenced notation:
 
V( ):=(s1, s2, s3, s4)→(s1′, s2′, s3′, s4′)
 
((s1′+s2′+s3′+s4′)/4, (s1′+s2′−s3′−s4′)
 
˜(z, dZx, dZy)
 
→(z, z+dzx, z+dzy)
 
:=(v1, v2, v3)
 
   This assumes exactly four sensor regions. 
   The first transformation (si→si′) yields micron units. 
   The resulting (v1, v2, v3) virtual sensors are in micron units which are at a fixed (vector) offset from the LVDT sensors. 
   L(z):=Map from the low pressure space to adjusted LVDT space (depends on Z-stage position). 
   It is important that the face of the sensing head structure of the edge of the sensing head  12  be flush. 
   Referring to  FIG. 7 , the prior art beveled edge  170  is shown with a silver epoxy paint  172  of uncontrolled large thickness. Referring to  FIG. 8 , the placement of the air injectors  50 - 55  to the side of the sensing head  12  is important. Air leakage path between the surface of the air injector ( 50 - 52 ) and the surface of the sensor plate  38  is to be minimized. In  FIG. 8 , the bevel is omitted in favor of a small chamfer  174  over which a silver coating  176  is deposited between the ITO coating  178  and the gold plating  179  of the contact. The air injectors  50 - 52  are flush (along an orthogonal edge) with the sensor plate  38 . The silver coating is less than the thickness of the polymer dispersed liquid crystal (pdlc) forming the sensor plate  38  and binds to the ITO coating  178  on the Mylar(r) polyurethane substrate  181 . The means provided for sealing the air leakage path between the air injector  50  and the coatings on the small chamfer  174  of the sensor plate  38  edge is an appropriate dielectric casting material  182  filling the void. 
   The invention has been explained with reference to specific embodiments. Other embodiments will be evident to those of ordinary skill in the art. It is therefore not intended that this invention be limited, except as indicated by the appended claims.