Abstract:
The arrangement of test points on an integrated circuit (IC) undergoing testing in a probe station often requires rotation of the IC or the probes when performing a series of tests. A chuck with indexed rotation promotes rapid rotation of the device under test to a new test position and increases the productivity of the probe station. The device under test is mounted on a device mounting member that is affixed to a shaft rotationally mounted in a base. A resilient seal supports the device mounting member and forms a sealed chamber over a substantial part of the area of the device mounting member. Applying vacuum or pressure to the sealed chamber urges the device mounting member and base toward contact. The support provided by the resilient seal over substantial portion of device mounting member&#39;s diameter promotes stability and consistent planarity of the device mounting member without regard to the orientation of the shaft to device mounting member.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a chuck for a probe station and, more particularly, to an indexing, rotatable chuck for securing a device under test in a probe station. 
     Integrated circuits (ICs) comprise micro-circuits chemically etched on semiconductor material or wafers. It is customary to manufacture several ICs on a single wafer and then separate the individual circuits after performance and functional testing in a wafer probe station. Probe stations are also used for testing the performance and function of an IC after the IC has been incorporated into a composite device. 
     Generally, a probe station comprises an environmental chamber containing a chuck for securing and positioning the device under test (DUT), one or more probes to connect test points on the DUT with instrumentation, and optics to assist the operator in locating and probing test points on the IC. The environmental chamber protects the DUT and the delicate probes from electrical and other environmental hazards. The chuck provides the mechanism for securing and positioning the DUT. The chuck may also include means to further control the local operating environment, such as heating and cooling capabilities and additional electromagnetic field isolation. To test a device, the probe station operator examines the device under a microscope and, using positioning mechanisms for the chuck and probes, brings a probe tip into contact with a test point on the DUT. The test points on ICs are customarily laid out along rectangular grid coordinates and may be tested with multiple probes on a probe card or by single probes in a north-south-east-west arrangement. Likewise, ICs in composite device are typically arranged along rectangular coordinates. 
     To facilitate co-location of the probe tip and the test point on the DUT, both the probe and the chuck may be capable of movement in several directions. The chuck is typically mounted on a movable stage providing horizontal (x and y axes) and vertical (z axis) translation. In addition, the stage may provide for rotation about the z axis or “theta angle” adjustment to facilitate parallel alignment of the probe tips and the test points on the IC. Typically, the mounting for the probe provides for x, y, and z movement of the probe tips with micrometer precision. 
     While test points are commonly arranged in a rectilinear grid arrangement on the IC, a sequence of tests may require probing pluralities of test points that not arranged along the same xy axis. Even if test points on an IC are laid out with efficient probing in mind, the test points for devices containing multiple ICs are likely not to be conveniently arranged. As a result, either the DUT must be rotated on the chuck or the probe card must be removed and rotated to reorient the probe tips between tests. In addition to the time and effort required to reorient the DUT or the probe card, reorientation of the probes may require time consuming re-calibration of the attached instrumentation. The time required to reorient the probe and test points can be reduced by providing for rotation of the chuck about the vertical (z) axis (theta rotation). 
     Rotational movement in the form of “fine” theta adjustment is typically provided in probe station chucks. The fine theta adjustment is used to ensure that an array of DUTs are aligned with the x and y axis of the probe station so that the probe can step from device to device without further adjustment. The fine theta rotation is typically limited to about plus or minus seven and one half degrees (±7.5°) and the rotation speed is relatively slow to facilitate alignment of the microscopic probe tips and test points. Therefore, the fine theta adjustment mechanism is not adequate or convenient for rotating the DUT through substantial angles, often 90 degrees or more, to accommodate reorientation of test points for a sequence of tests. 
     Roch, U.S. Pat. No. 3,936,743, HIGH SPEED PRECISION CHUCK ASSEMBLY, discloses a rotating chuck for a wafer probe station. The chuck comprises a platform having a stem portion arranged for rotation in a bearing in a housing bore. The chuck is rotated manually by turning an adjustment knob and attached worm gear. The worm gear engages a spur gear attached to the rotating platform of the chuck. Although this mechanism permits rotation of the surface of the chuck to facilitate reorientation of the DUT, the worm gear drive adds mass to the chuck increasing wear and tear on the positioning mechanism of the stage and making accurate positioning by the stage more difficult. In addition, the planarity of the mounting surface is dependent upon the perpendicularity of the supporting stem and corresponding bore in the support structure. Since positioning is performed while observing the DUT under a microscope, even slight deviation in planar orientation or planarity can result in a need to routinely refocus the optics while positioning the DUT for probing. The worm gear mechanism also increases the height of the chuck which may dictate that the stage, optics, and environmental chamber of the probe station be specially designed to accommodate the rotating chuck. Further, while the worm gear drive provides continuous rotation of the DUT for precise re-alignment, it does not provide the rapid and convenient positioning of the DUT to a new test position which is important to productive probe testing. 
     Boucher et al., U.S. Pat. No. 5,676,360, MACHINE TOOL ROTARY TABLE LOCKING APPARATUS, disclose another worm gear driven rotary table. This table is adapted for use with a dicing saw. The planar orientation of the surface of the table is established by the orientation of the shaft on which the table rotates relative to the top surface of the table. As a result, the bearings supporting the table for rotation are widely spaced increasing the height of the table. The table does incorporate a brake to lock the table in a selected rotational position. Fluid pressure urges a circular piston to bear on a ledge on the periphery of the rotating table. Since the piston is free to assume any position relative to the table&#39;s base, application of the brake does not stabilize the table or effect its planar orientation. Further, a more massive table is required to resist deflection resulting from application of the brake force on the table&#39;s periphery. 
     What is desired, therefore, is a compact rotating chuck featuring rigidity, low mass, and precise planarity while facilitating rapid and accurate rotation of the DUT through a substantial angle for sequential probing of IC test points. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the aforementioned drawbacks of the prior art by providing a chuck for a probe station comprising a base attached to the probe station; a shaft mounted for rotation in the base; and a device mounting member affixed to the shaft for rotation therewith and having a planar orientation relative to said base substantially independent of the orientation of the shaft to the device mounting member. A large diameter resilient seal between the base and the device mounting member supports the device mounting member independent of the alignment of the shaft promoting rigidity and consistent planarity. Further, this method of support reduces the length of the shaft and, consequently, the height and mass of the rotary chuck. In addition, the rotary chuck can include a releasable rotation stop to permit indexed rotation of the device mounting member to a new test position. In another embodiment, a chuck for a probe station comprises a base attached to the probe station and a device mounting member constrained by the base for rotation relative thereto. 
     The foregoing and other objectives, features and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is perspective illustration of an exemplary probe station and chuck. 
     FIG. 2 is a cross section of an indexing, rotatable chuck. 
     FIG. 3 is a cross section of an indexing rotatable chuck of alternative construction. 
     FIG. 4 is a fragmentary view of a cross section of an indexing, rotatable chuck arranged for braking by a pressurized fluid. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, in a probe station  10  a device under test (DUT) is mounted on a chuck  12  which is supported on a movable stage  14  mounted on a station base  16 . Probes (not illustrated) are supported over the chuck by a platen  18 . The probes are provided with controls for positioning along horizontal (x and y) axes and the platen  18  may be adjusted in the vertical (z) direction to bring the probes into contact with test points on the integrated circuit of the DUT. To facilitate location and positioning of the probes, the probe station  10  includes a microscope (not illustrated) mounted on a microscope mounting  20  attached to an optics bridge  22 . The probe station  10  may include an environmental housing (not illustrated) to protect the DUT and probes from dust and other environmental hazards. 
     To facilitate relative positioning of the DUT and probes, the stage  14  provides for translatory and limited rotational (theta) movement of the chuck  12 . In the probe station  10  illustrated in FIG. 1, horizontal translation is accomplished with an x-motor  24  and a y-motor  26  that drive linear actuators  28  and  30  to move the stage  14 . A similar mechanism (not shown) provides vertical translation of the chuck  12 . Rotation about the vertical axis or fine theta adjustment is provided by a theta motor  32  and attached linear actuator  34 . The fine theta adjustment is provided to facilitate parallelism of the probe tips and test points. Rotation is typically limited to approximately 15 degrees (±7.5°). Rotational speed is relatively slow to facilitate alignment of the microscopic probe tips and test points. One or more linear encoders  36  provide feedback to a control for the stage positioning motors. 
     Referring to FIG. 2, the chuck  12  of the present invention comprises a device mounting member  50  attached to a shaft  52  that is mounted for rotation in a bushing  54  installed in a bore in a base  56 . A retaining ring  57  secures the shaft  52  in the bushing  54 . The base  56  is secured to a planarization plate  58  which is, in turn, attached to the stage  14  of the probe station  10  as illustrated in FIG.  1 . The base  56  is attached to the planarization plate  58  by an arrangement of mounting screws  60  and a plurality of spring washers  62  retained by each of the mounting screws  60 . The spring washers  62  are interposed between the base  56  and the planarization plate  58  and exert a force to separate the base  56  and the planarization plate  58 . The planarity of the upper surface of the device mounting member  50 , relative to the structure of the probe station  10 , can be adjusted by loosening or tightening one or more of the base mounting screws  60 . 
     A handle (not illustrated) can be installed in the device mounting member  50  for convenient manual rotation of the device mounting member  50 . The base  56  and the rotating device mounting member  50  can be provided with corresponding markings to indicate the angle of rotation. Manual rotation provides rapid and reliable rotation of the device mounting member  50  but rotation could be powered by a motor and a suitable drive train (not illustrated). Positioning of a powered device mounting member  50  can be controlled by a known motor controller and rotary position feedback device. 
     In the embodiment of the chuck  12  in FIG. 2, a device under test (DUT)  64  is secured to a mounting fixture  66  by clamps  68 . The mounting fixture  66  is, in turn, secured to the device mounting member  50  by a dovetail that mates with a corresponding dovetail groove in the device mounting member  50 . 
     Other methods of securing the DUT  64  could be used. For example, wafers are often retained on the surface of a chuck by vacuum means. Apertures (not illustrated) provided in the upper surface of the device mounting member  50  can be connected to a vacuum source (not illustrated) through a rotating union (not illustrated) and passageways in the device mounting member  50  and shaft  52 . When the vacuum source is connected to the passageways, air pressure will restrain the DUT on the upper surface of the device mounting member  50 . 
     Since the test points of ICs are typically arranged along rectangular coordinates, rotation of the DUT in 90 degree or quadrature increments is commonly required during testing. The inventors concluded that rapid rotation of the DUT  64  to a precise, but approximate, theta angle followed by fine theta angle adjustment to align the probes and contact points would substantially improve the productivity of the probe station. The chuck  12  of the present invention includes a mechanical indexing apparatus to speed precise rotation of the device mounting member  50  to a new position. A rotation stop  71  is provided to releasably index or limit the rotation of the device mounting member  50 . A first example of a rotation stop  71  comprises a stop member  70  in the form of a manually operated indexing pin that slidably engages a bore in the device mounting member  50  and, in an extended position, engages one of a plurality of bores  74  or other surfaces of the base  56 . The indexing pin  70  is biased to an extended position by a spring  72 . To rotate the device mounting member  50 , the indexing pin  70  is retracted from the bore  74  in the base  56 , freeing the device mounting member  50  for rotation. The indexing pin  70  may be retained in a retracted position by a second motion, such as a quarter turn rotation of the pin, providing chuck rotation without indexing. While the rectilinear arrangement of test points on ICs makes rotation in 90 degree or quadrature increments convenient in many applications, indexing in other angular increments is possible by providing pin receiving bores  74  at other or additional locations in the base  56 . The rotation stop could take other forms comprising a stop member  70  movably mounted to one element, either the device mounting member  50 , the shaft  52  or the base  56 , and engaging another element of the chuck  12 . For example, the indexing pin stop member  70  could be slidably mounted in a bore in the base  56  and engage bores or surfaces in the device mounting member  50 . Alternative examples of a rotation stop, include a flip-up arm stop member or a spring loaded plunger or ball detent stop member engaging features of the device mounting member  50 , the shaft  52 , or the base  56 . For example, referring to FIG. 3, an alternative example of a rotation stop  103  releasably limiting rotation of the device mounting member  92  comprises a ball stop member  104  movable in a bore in the base  94  and urged into contact with an indentation in a surface of the device mounting member  92  by a spring  102 . 
     The rotatable chuck  12  of the present invention also includes a rotational braking and stabilization system. The system permits locking the device mounting member  50  in an infinite number of rotational positions. In addition, the system promotes stability and planarity of the DUT  64  during testing, increasing the productivity of the probe station by reducing required microscope adjustments. The stability and planarity of the device mounting member  50  are improved by supporting the device mounting member at widely separated points with a resilient member  76 , for example a large diameter o-ring The o-ring  76  is installed in an o-ring in an o-ring groove in the lower surface of the device mounting member  50  and bears on the upper surface of the base  56  exerting a separating force between the device mounting member  50  and the base  52 . During rotation, the device mounting member  50  is supported across a substantial portion of its diameter by the resilient, large diameter o-ring  76  producing a stable mounting for the DUT  64 . Further, the planar orientation of the device mounting member  50  is not dependent on the orientation of the mounting shaft  52  relative to the device mounting member. Widely separated bearings and a long shaft are not required, reducing the height of the chuck  21 . 
     Additional o-ring vacuum seals  78  and  80  are installed in o-ring grooves in the shaft  52  to seal between the shaft  52  and the bushing  54 . The o-ring vacuum seals  76 ,  78 , and  80  form a sealed chamber  82  between the device mounting member  50  and the base  56 . A passageway  84  in the base  56  connects the sealed chamber  82  to a pressure control comprising a vacuum source  85  and control valve  86 . When the control valve  86  is actuated, air flows from the sealed chamber  82  to the vacuum source  85  and air pressure acting on the device mounting member  50  urges the device mounting member  50  toward contact with the base  56 . In other words, the device mounting member  52 , the base  56 , and the seals  78 ,  80 , and  76  form an actuator responsive to changes in fluid pressure. Friction between the device mounting member  50 , the o-ring seal  76 , and the base  56  retards rotation of the device mounting member  50  and locks it into position. The resilient member, o-ring  76 , is compressed as the device mounting member  50  and base  56  are pressed together and the device mounting member  50  is supported over a substantial portion of its surface area when the brake is applied. The planar orientation of the upper surface of the base  56  is adjusted with the screws  60  that support the base on the planarization plate  58 . The planar orientation or planarity of the upper surface of the device mounting member  50  is determined by parallelism of the upper and lower surfaces of the device mounting member  50  and is not dependent upon the orientation of the shaft  52  relative to the device mounting member  50 . As a result, the deviation in the planar orientation of the mounting for the device under test is minimized by the ability to control the parallelism of the top and bottom surfaces of the device mounting member  50  and a relatively short shaft  52  can be used to mount the device mounting member  50  minimizing the height of the rotary chuck. Minimizing deviation of the planar orientation of the device mounting member  50  and, consequently, the DUT avoids frequent refocusing of the optics as the DUT is rotated or translated. Minimizing the height of the rotating chuck makes it possible to install the chuck  12  in a probe station designed for a non-rotating chuck. 
     Referring to FIG. 3, in an alternative construction the device mounting member  92  of the chuck  90  is supported directly by a surface of the base  94 . An o-ring seal  96  mounted in an o-ring groove in an extension of the base  94  journals the device mounting member  92  for rotation. During rotation the device mounting member  92  is supported by the base  94  across the diameter of the device mounting member  92  providing rigid support and stable planar orientation. A second o-ring  98  seals an annular volume between an upper surface of the device mounting member  92  and the extension of the base  94 . When vacuum is applied to the sealed annular volume through a passageway  100 , the device mounting member  92  is drawn upward toward the extension of the base  94 , compressing the o-ring  98 . As a result, braking force is applied over an annular area approximating the diameter of the upper surface of the device mounting member  92  and the planarity of the device mounting member is determined by the flatness of this surface. Approximate indexing of the rotation of the device mounting member is provided by a ball detent comprising a spring  102  that urges a ball  104  into contact with an indentation in the lower surface of the device mounting member  92 . In addition, a locating pin  106  having a conical tip is slidably arranged in a bore in the device mounting member  92 . The locating pin  106  is retained in a retracted position by a spring  108  that is retained by a snap ring  110 . The locating pin  106  is sealed by an o-ring  112 . When a vacuum is applied to the chamber formed by the device mounting member  92  and the base  94 , the locating pin  106  is drawn into contact with a conical shaped indentation  114  in the device mounting member providing final rotational alignment of the device mounting member  92 . 
     Vacuum is a convenient energy source for actuating the braking and stabilizing system because vacuum is often used to secure wafers and other DUTs on the chuck. However, increased fluid pressure can be used to actuate the braking and stabilization mechanism. As illustrated in FIG. 4, the large diameter o-ring seal  120  can be installed in a seal ring  122  affixed to the periphery of the base  56 . A second o-ring  124  may be used to seal between the seal ring  122  and the base  56 . The seals  91  and  90  in conjunction with seals on the shaft (not illustrated) form a sealed fluid chamber  126  between the device mounting plate  50  and the base  56 . When pressurized fluid is directed to the chamber  126 , the upper surface of the device mounting plate  50  is pressed upward against the seal  120  and seal ring  122 . With this arrangement, the planarity of the upper surface of the device mounting member  50  is determined by the flatness of that surface. 
     All the references cited herein are incorporated by reference. 
     The terms and expressions that have been employed in the foregoing specification are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims that follow.