Method and apparatus for handling and testing wafers

Disclosed is a wafer handling and testing apparatus. The wafer handling and testing apparatus includes a support assembly, a wafer handling assembly, and a probe assembly. The support assembly is capable of supporting a wafer to be tested and is also capable of rotating the wafer for testing. The wafer handling assembly is arranged to move the wafer to and from the support assembly. The wafer handling assembly is capable of moving the along a first axis and a second axis. The first axis is preferably orthogonal to the second axis. The wafer probe is arranged to test the wafer when the wafer is placed on the support assembly.

BACKGROUND OF THE INVENTION
 The present invention relates to the handling of wafers, and more
 particularly to handling and testing semiconductor wafers in a wafer
 testing apparatus.
 The manufacture of integrated circuit (IC) chips begins with blank,
 unpatternd semiconductor wafers. These wafers undergo a number of
 sometimes critical process steps before being formed into the final IC
 chip form. A substandard wafer can affect the yield (i.e., number) of
 usable IC chips on a wafer. It is therefore desirable to have a machine
 for testing wafers to ensure that the wafers meet a desired standard to
 maximize wafer yield.
 Testing of the wafers typically involves an automated process utilizing
 automated wafer handling machines. In this process, the automated wafer
 handling machines continuously handle and test the wafers. The automated
 process tends to be more efficient than manual testing and handling of
 wafers since an automated process is typically faster, more precise, and
 less prone to contamination than a manual process.
 One of the major uses of the automated wafer handling machines is for
 testing or processing the wafers to determine or change certain wafer
 characteristics (such as by depositing a film or removing a wafer layer).
 For example, automated wafer handling machines are often used to determine
 the orientation of a wafer, which provides a standard reference against
 which the location and characteristics of test points on the wafer may be
 measured.
 A conventional art wafer handling machine has a four degree of freedom. In
 this machine, the wafer cassette moves up and down, the chuck rotates, the
 arm moves from left to right. However, one of the drawbacks of the
 conventional wafer handling machines is movement of the wafers within the
 wafer cassette. For example, when the wafer cassette moves up and down to
 allow a robot arm to remove a wafer from the cassette or place a wafer
 into the cassette, the wafers within the cassette may be subject to
 unwanted jarring. The vibrations caused by the jarring are potentially
 harmful due to the creation of particle contaminants.
 In addition, the conventional wafer handling machines typically include a
 motor for each degree of freedom for a total of four motors. Generally,
 moving parts in a machine or apparatus such as motors are more prone to
 failure and require more maintenance than non-moving parts. Further the
 use of such number of motors typically require complex and costly
 mechanisms that require more maintenance, which is undesirable in
 production environments.
 Thus, what is needed is an apparatus and method that can efficiently move
 and test wafers without moving the wafer carrier or cassette. In addition,
 what is needed is an apparatus and method that can move and test wafers
 using less number of motors so as to reduce the cost and maintenance
 involved with the motors.
 SUMMARY OF THE INVENTION
 The present invention fills these needs by providing an apparatus and a
 method for handling and testing wafers in an integrated system. It should
 be appreciated that the present invention can be implemented in numerous
 ways, including as a process, an apparatus, a system, a device, or a
 method. Several inventive embodiments of the present invention are
 described below.
 In one embodiment, the present invention provides a wafer handling and
 testing apparatus. The wafer handling and testing apparatus includes a
 support assembly, a wafer handling assembly, and a probe assembly. The
 support assembly is capable of supporting a wafer to be tested and is also
 capable of rotating the wafer for testing. The wafer handling assembly is
 arranged to move the wafer to and from the support assembly. The wafer
 handling assembly is capable of moving the along a first axis and a second
 axis. The first axis is preferably orthogonal to the second axis. The
 probe assembly is arranged to test the wafer when the wafer is placed on
 the support assembly.
 In another embodiment, an integrated wafer handling and testing apparatus
 includes supporting means, handling means, and testing means. The
 supporting means supports a wafer to be tested and is capable of rotating
 the wafer in an x-y plane. The handling means moves the wafer to and from
 the wafer support assembly and is capable of moving the wafer along a
 first axis and a second axis, which are orthogonal to each other. The
 testing means tests the wafer when the wafer is placed on the support
 means.
 In yet another embodiment, the present invention provides a method for
 handling a wafer from a stationary wafer carrier for testing. The
 stationary wafer includes a plurality of wafers to be tested. The method
 includes (a) selecting a wafer to be tested; (b) picking up the wafer; (c)
 moving the wafer along an x-axis and a z-axis to a support assembly for
 testing; (d) placing the wafer on the support assembly; (e) rotating the
 supporting assembly to place the wafer at a desired test position; and (f)
 testing the wafer at the desired test position to determine a wafer
 characteristic.
 Advantageously, the present invention efficiently moves and tests wafers
 without moving the wafer carrier or cassette by providing a wafer handling
 assembly that has two degrees of freedom along the first axis and the
 second axis. In addition, by enabling the wafer handling assembly to move
 along the second axis, a support assembly need not move along the second
 axis, thereby eliminating the need for a motor in some embodiments of the
 present invention.
 These and other advantages of the present invention will become apparent to
 those skilled in the art upon reading the following detailed description
 of the invention and studying the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 An invention for a method and apparatus of efficiently changing probe test
 heads that contact a substrate is disclosed. In the following description,
 numerous specific details are set forth in order to provide a thorough
 understanding of the present invention. It will be understood, however, to
 one skilled in the art, that the present invention may be practiced
 without some or all of these specific details. In other instances, well
 known process operations have not been described in detail in order not to
 unnecessarily obscure the present invention.
 FIG. 1A illustrates an elevational view of an integrated wafer handling and
 testing apparatus 10 in accordance with one embodiment of the present
 invention. The wafer handling and testing apparatus 10 is configured to
 move individual wafers from storage to a test position, upon which the
 individual wafer may be tested for a variety of characteristics before
 being moved back to storage. The wafer handling and testing apparatus 10
 includes a wafer handling assembly 12, a wafer support assembly 14, a
 wafer carrier 16, and a computer 18.
 The wafer carrier 16 contains a number of wafers 20 to be tested and
 provides access to the wafer handling assembly 12 for testing the wafers
 20. The wafer carrier 16 may be a wafer cassette, which holds a number of
 wafers in slots in an open, unsealed container. For example, the wafer
 cassette can be positioned so that its opening faces the wafer support
 assembly 14 and wafer handling assembly 12.
 In the alternative, the wafer carrier 16 may be a wafer pod, which is an
 enclosed and sealed container that prevents contaminants from reaching the
 held wafers. For example, the wafer pod can be implemented as a Front
 Opening Unified Pod (FOUP), which is an industry standard wafer pod that
 includes a door designed to be opened from the front of the carrier rather
 than the bottom of the carrier (such as in a SMIF Pod). It should be
 appreciated that the present invention may utilize wafers of any suitable
 size for testing including, for example, wafers with diameters of all
 sizes up to 300 nm.
 Supporting the wafer carrier 16 in the wafer handling and testing apparatus
 is a base plate 22 disposed on a reference surface 28. The wafer carrier
 16 may be attached to the base plate 22, however it is preferred if the
 wafer carrier 16 is removably disposed on the base plate 22. In this
 manner, other the wafer carrier 16 may be removed from the base plate 22
 so that other wafer carriers may also be mounted on the base plate 22 to
 test another set of wafers. Alternatively, the wafer carrier 16 can be
 removed from the base plate 22, unloaded and loaded with new wafers, and
 repositioned on the base plate 22.
 When the wafer carrier 16 is implemented as a sealed wafer pod, the wafer
 carrier 16 may be provided on one side of an interface panel which seals a
 clean environment required for the testing processing of wafers. That is,
 the base plate 22 for supporting the wafer carrier 16 is located on the
 exterior of the environment in which processing and testing of wafers is
 to be accomplished.
 The wafer support assembly 14 includes a base platform 24 having a
 plurality of legs 26, which are disposed on the reference surface 28. The
 reference surface 28 may be a ground surface or a structural base that
 provides support to the wafer handling and testing apparatus 10. In one
 embodiment, both the base plate 22 and the legs 26 of the base platform 24
 are securely disposed on the reference surface 28. Preferably, the wafer
 carrier 16 and the base plate 22 is arranged such that the wafers 20
 within the wafer carrier 16 are disposed above the level of the base
 platform 24 to facilitate access to all the wafers 20.
 The wafer support assembly 14 further includes a testing chuck 70 supported
 by the base platform 24 and a drive mechanism 72, located under the
 testing chuck 70. The testing chuck 70 can be a disc-shaped, wafer-shaped,
 or any other shape suitable for supporting a selected wafer 21 for
 testing. In accordance with one embodiment of the present invention, the
 chuck 70 is rotatable about a central z-axis and may be moved upwardly or
 downwardly parallel to a z-axis 56. The chuck 70 also includes a blade
 insert 74, which assists the wafer handling assembly in moving the
 selected wafer 21.
 The drive mechanism 72 is operative to rotate the chuck 70 about the
 central z-axis. The drive assembly 72 comprises a pulley 76, and a number
 of motors 78 and 80. The pulley 76 is connected to the chuck 70 by a shaft
 82 that extends through the base platform 24 through suitable bearings
 (not shown). The pulley 76 is connected to the motor 78 by a drive belt
 84. The motor 78 rotates the pulley 76 and thereby rotates the shaft 82
 and the chuck 70. The motor 78 is connected to a support 86 of the motor
 80.
 One end of the support 86 is provided with a threaded nut 88, which engages
 a lead screw 90 coupled to rotate in response to the motor 80. The
 threaded nut 88 may be an anti-backlash nut or a ball screw nut. The
 support 86 is connected to the shaft 82 by a bearing 92. The far end of
 the support 86 is secured to the motor 78 by a rigid coupling and secured
 to the guide shaft 94 by a sliding connection. The motor 80 is positioned
 on the reference surface 28.
 When the motor 80 rotates, the support 86 moves parallel to the z-axis 56
 and carries the shaft 82, the pulley 76, the motor 78, and the chuck 70 in
 the same direction. The bearing 92 of the support 86 allows the shaft 82
 to rotate freely while still being securely held by the support 86. It
 should be appreciated that other mechanisms may also be used to implement
 the movement parallel to the z-axis 56 such as a linear actuator. In
 addition, gears may impart rotational motion to the chuck 70.
 Alternatively, the testing chuck 70 can be moved parallel to x-axis 54 or
 y-axis 55. This movement can be implemented for example, by positioning
 parallel tracks on the sides of the base platform 24 and moving the
 support assembly 14 along the tracks using wheels or gears. The motors 78
 and 80 are preferably stepper motors or position servo motors controlled
 by the computer 18 through a bus 96. The computer 18 is configured to
 precisely rotate the motors 78 and 80 in either direction, thus allowing
 the chuck 70 to rotate and move along the z-axis in precisely-defined
 motions.
 The wafer handling assembly 12 includes a first carriage assembly 41, which
 is arranged to move along the x-axis 54 in a x-y plane. The first carriage
 assembly 41 includes a carriage 30, a guide shaft 34, a lead screw 36, a
 motor 38, and a plate 40. The guide shaft 34 is coupled to the plate 40
 and extends through a bore 58 in the carriage 30 to allow the carriage 30
 to slide along the guide shaft 34. The lead screw 36 is coupled to the
 motor 38 through the plate 40 and extends through a threaded bore 60 in
 the carriage 30. In this configuration, when the lead screw 36 is rotated,
 the carriage 30 moves along the x-axis 54 along the length of the guide
 shaft 34 and the lead screw 36.
 The motor 38 is preferably a stepper motor or position servo motor and is
 coupled via a bus 96 to the computer 18. The computer 18 controls the
 motor 38 precisely to position the carriage 30 along the x-axis 54. Other
 mechanisms may be used to move the wafer handling assembly 12 parallel to
 the x-axis. For example, the carriage 30 can be driven along the guide
 shaft 34 and the lead screw 36 by motor gears.
 The wafer handling assembly 12 also includes a second carriage assembly 43,
 which is movably mounted on the carriage 30 of the first carriage assembly
 41 to allow translation along the x-axis 54. The second carriage assembly
 43 is arranged to move along a z-axis 56 in a x-z plane and includes a
 carriage 32, a guide shaft 50, a lead screw 52, and a motor 48. The guide
 shaft 50 guides the carriage 32 along the z-axis. The guide shaft 50 and
 the lead screw 52 are oriented parallel to the z-axis 56, which is
 perpendicular to the x-axis. Preferably, the z-axis is a vertical axis.
 The guide shaft 50 extends through a bore 62 in the carriage 32 in the
 second carriage assembly and through a bore 66 in the carriage 30 of the
 first carriage assembly 41. Similarly, the lead screw 52 extends through a
 threaded bore 64 in the second carriage assembly and through a threaded
 bore 68 in the carriage 30 of the first carriage assembly. The motor 48 is
 coupled to rotate the lead screw 52. In this configuration, when the lead
 screw 52 is rotated, the carriage 32 moves along the z-axis 56 along the
 length of the shaft 50 and the lead screw 52.
 The motor 48 is preferably a stepper motor or position servo motor and is
 coupled to the computer 18 through the bus 96. The computer 18 controls
 the motor 48 to precisely position the carriage 32 in the z-axis 56. Other
 mechanisms may be used to move the carriage 32 parallel to the z-axis. For
 example, the carriage 32 can be driven along the guide shaft 50 and the
 lead screw 52 by motor gears, a hydraulic or pulley system, a slide or
 rail mechanism, or other system providing such translation.
 The wafer handling assembly 12 further includes a support arm 42 that is
 coupled to and moves in unison with the carriage 32. A wafer blade 44 and
 a probe assembly 46 are coupled to the support arm 42. The support arm 42,
 the wafer blade 44, and the probe assembly 46 may be moved along the
 z-axis 56 by the second carriage assembly 43 when carriage 32 is moved
 along the guide shaft 50. Similarly, the support arm 24, the wafer blade
 44, and the probe assembly 46 may be moved along the x-axis 54 by the
 first carriage assembly 41 when carriage 30 is moved along the guide shaft
 34. Therefore, the wafer blade 44 and the probe assembly 46 have two
 degrees of freedom attained when the motors 38 and 48 drive the carriages
 30 and 32 along the x-axis 54 and the z-axis 56.
 The wafer blade 44 may be implemented in a variety of ways including a
 vacuum pick, a spatula, or an end effector. An end effector is a flat,
 spatula-like implement used to support a wafer from underneath the wafer
 and move the wafer to a desired location. In some embodiments, the wafer
 blade 44 may include apertures that are coupled to a vacuum pump to cause
 a suction force that securely holds a wafer to the wafer blade 44.
 The probe assembly 46 is configured to test a wafer to determine its
 characteristics. In one embodiment of the present invention, the probe
 assembly 46 is coupled to the support arm 42 and extends out from the
 bottom surface of the support arm 42. In other embodiments, the probe 46
 can be coupled to other areas of the support arm 42, the wafer blade 44,
 or the carriages 30 and 32. For example, the probe assembly 46 can be
 positioned on a separate support arm mounted on the carriage 30.
 The probe assembly 46 includes a test head on the bottom portion of the
 probe assembly 46. The test head preferably contacts a wafer to make test
 measurements. In other embodiments, the test head does not contact the
 wafer, but is positioned to a desired distance above the wafer to perform
 tests, for example, using electromagnetic beams to determine wafer
 characteristics as is well known to those skilled in the art. The test
 head includes individual probe leads, which are designed to take test
 measurements on the wafer surface.
 In a preferred embodiment, the test head includes a four-point probe
 apparatus that includes four metal, spring-loaded probes that engage the
 surface of a wafer. A current is usually induced in the outer probes of
 the four probes, and a voltage is measured across the inner probes. Such a
 probe is designed to measure wafer resistivity and film thickness. In
 other embodiments, other types of test probes can be provided to test
 various characteristics of wafers.
 The wafer handling assembly 12 may also include an edge mapping sensor for
 sensing the edge of the selected wafer 21 when the support assembly 14 and
 the support arm 42 are positioned appropriately. Such sensor is amply
 described in U.S. Pat. No. 5,546,179, which is incorporated by reference.
 Edge mapping techniques are well known in the art and are described, for
 example, in U.S. Pat. No. 5,452,078, which is also incorporated by
 reference herein.
 The wafer handling and testing apparatus 10 is thus configured to in
 accordance with one embodiment of the present invention as described
 above. In accordance with one embodiment of the present invention, the
 carriage assemblies 41 and 43 move wafer blade 44 into the wafer carrier
 16 under one of the wafers 20. The wafer blade 44 is then moved along the
 z-axis 56 in an upward fashion to lift the selected wafer 21 out of the
 wafer carrier 16. The selected wafer 21 is then moved in the two degrees
 of freedom described above by the carriage assemblies 41 and 43 towards
 the wafer support assembly 14. The selected wafer 21 is brought to rest
 upon the testing chuck 70 (as shown) by sliding the wafer blade 44 into
 the blade insert 74. The wafer blade 44 may then move away from the
 testing chuck 70 without disturbing the selected wafer 21.
 It is the same manner that the movement of the carriages 30 and 32 allows
 the probe assembly 46 to move in either or both the x-axis 54 and the
 z-axis 46. After the selected wafer 21 is placed upon the testing chuck
 70, the carriage assemblies 41 and 43 are used to move the probe assembly
 46 into contact with the selected wafer 21. The testing chuck 70 may be
 rotated as described above to aid the positioning of the probe assembly 46
 to a specific location on the selected wafer 21. The probe assembly 46
 then measures the characteristics of the selected wafer 21 as described
 above, and transmits the test results to the computer 18 through the bus
 96.
 After testing the selected wafer 21, the wafer blade 44 may pick up the
 selected wafer 21 using the blade insert 74 to obtain a position beneath
 the selected wafer 21. The wafer blade 44 then places the wafer back into
 the wafer carrier 16, preferably into its original wafer slot. The wafer
 blade 44 may then be moved to align another wafer 20 for removal and
 testing. In one embodiment of the present invention, the motors 38 and 48
 operate may operate simultaneously to move the wafer blade 44 and the
 probe assembly 46 in the x and z-axes 54 and 56 at the same time. In
 another embodiment, the motors 38 and 48 operate only one at a time so
 that the wafer blade 44 and the probe assembly 46 move only along the
 x-axis 54 or the z-axis 56 at a time.
 The computer 18 controls the movement of the components of the wafer
 handling and testing apparatus 10 as explained above. The computer 18 can
 be any suitable controller device, such as an IBM-compatible personal
 computer based on a Pentium class or other microprocessor, a Macintosh
 computer, a workstation, or other computing device. A preferred embodiment
 of the present invention integrates the handling, mapping, and testing
 functions of the wafer handling and testing apparatus 10 at a single
 workstation. Thus, the wafer blade 44 is able to pick up and transport the
 selected wafer 21 to the platform and the probe assembly 46 is able to
 test the wafer on the platform.
 FIG. 1B is an elevational view of the wafer handling and testing apparatus
 10 depicting a test chuck 70 that does not move along a z-axis in
 accordance with one embodiment of the present invention. In place of the
 motor 80 that previously moved the test chuck 70 in FIG. 1A, the support
 assembly 14 of FIG. 1B includes a leg 96, which is disposed over the
 reference surface 28 to provide support to the support 86.
 Thus, the test chuck 70 is capable of rotating in response to the motor 78
 and remains stationary with respect to the z-axis throughout the operation
 of the apparatus. In this configuration, the wafer handling assembly 12
 assumes the function of moving and placing the selected wafer 21 on the
 chuck 70 by moving along the z-axis 56 as well as the x-axis 54. The
 apparatus 10 of FIG. 1B thus enables the wafer handling assembly to move
 along the second axis, a support assembly need not move along the second
 axis, thereby eliminating the need for a motor.
 FIG. 1C is an elevational view of the wafer handling and testing apparatus
 10 depicting a probe assembly 46 that is constrained to move only in
 parallel to the x-axis 54 in accordance with one embodiment of the present
 invention. Instead of having the probe assembly 46 on the support arm 42
 as previously shown in FIG. 1A, the wafer handling assembly 12 of FIG. 1C
 includes a support arm 98 attached to the carriage 30.
 In this arrangement, the support arm 98 moves in unison with the carriage
 30 along the x-axis 54 only. The probe assembly 46 is mounted on the
 support arm 98 for testing a wafer. Hence, the probe assembly 46 is
 constrained to move in a direction parallel to the x-axis 54 to be placed
 over a specified area of the wafer for testing. In this embodiment, the
 z-motion capability of the chuck 70 is used to compensate for the lack of
 movement of the probe assembly 46 along the z-axis.
 FIG. 1D is an elevational view of the wafer handling and testing apparatus
 10 combining the features of the test chuck 70 and the probe assembly 46
 of FIGS. 1B and 1C in accordance with one embodiment of the present
 invention. Specifically, the support assembly 14 includes the supporting
 member 96, which is disposed over the reference surface 28 to provide
 support to the support 86. In addition, the wafer handling assembly 12
 includes the support arm 98 attached to the carriage 30. The probe
 assembly 46 is mounted on the support arm 98 for testing a wafer.
 In this configuration, the wafer blade 44 in the wafer handling assembly 12
 performs the function of moving and placing the selected wafer 21 on the
 test chuck 70 by moving parallel to the z-axis 56 as well as the x-axis
 54. The placement of the probe assembly 46 over a specified area of the
 wafer for testing is performed by moving the carriage 30, the support arm
 98, and the probe 46 parallel to the x-axis 54 and by rotating the test
 chuck 70.
 FIG. 2A is a top view of the wafer handling and testing apparatus 10 of
 FIG. 1A. The guide shaft 34 and the lead screw 36 extending through the
 carriage 30 is coupled to the base plate 22 supporting the wafer carrier
 16 in one embodiment. The guide shaft 34 and the lead screw 36 allow the
 carriage 30 to move in a direction parallel to the x-axis. Preferably, the
 wafer blade 44 is aligned with the center of the wafers and with the
 center of the test chuck 70 in a direction parallel to the x-axis.
 FIG. 2B is a side view of the wafer handling and testing apparatus 10 of
 FIG. 1A. The wafer carrier 16 has been loaded onto the base plate 22 of
 the wafer handling and testing apparatus 10 for testing the wafers 20. The
 wafer handling assembly 12 is initially away from the wafer carriage 16.
 When a wafer is to be tested, the wafer handling assembly 12 moves in the
 direction of the x-axis and the z-axis toward the selected wafer in the
 wafer carrier 16.
 FIG. 3A shows a side view of the apparatus 10 of FIG. 1A after the wafer
 blade 44 has been positioned to pick up the selected wafer 20. To arrive
 at this position, the wafer blade 44 along with the other elements of the
 wafer handling assembly 12 (e.g., carriages 30 and 32, support arm 42) has
 moved in both the x-axis and the z-axis. The wafer blade 44 is inserted
 into the wafer carrier 16 just underneath the wafer 21, which is supported
 by a guide slot in the carrier.
 In some embodiments in which the wafer carrier 16 is an enclosed pod that
 seals the wafers from contaminants, the wafer blade 44 may be inserted
 through a small opening in an interface panel, as is described in
 co-pending U.S. patent application Ser. No. 08/920,210, filed Aug. 25,
 1997, by David Cheng. In many pods and other wafer carriers, the
 bottommost wafer in the wafer carrier 16 is typically the wafer that is
 first tested and/or processed, followed by each wafer positioned in the
 next higher slot of the carriers. After positioning the wafer blade 44
 under the selected wafer 21, the wafer blade 44 picks up the selected
 wafer 21 for moving the selected wafer 21 to the support assembly 14 for
 testing.
 FIG. 3B illustrates a side view of the wafer handling and testing apparatus
 10 depicting the positioning of the selected wafer over the chuck. The
 motors 38 and 48 control the rotation of the screws 36 and 52,
 respectively, to move the wafer handling assembly 12 until the selected
 wafer 21 is centered over the test chuck 70. The selected wafer 21 may
 then be placed on the test chuck 70. In one embodiment, the motor 80
 causes the test chuck 70 to move up to receive the selected wafer 21. In
 another embodiment, the motor 48 may cause the carriage 32 to move down to
 place the selected wafer 21 on the test chuck 70.
 FIG. 3C shows a side view of the wafer handling and testing apparatus 10
 testing the selected wafer 21. The motors 38 and 48 moves the carriage 32
 in the x-axis or the z-axis directions to place the carriage 32 in a
 position to allow the probe assembly 46 to test a specified area of the
 selected wafer 21. Preferably, the motors 38 and 48 move the carriage 32
 such that the probe assembly 46 on the support arm 42 is positioned
 directly over the test area of the selected wafer 21. In addition, the
 motor 78 may further rotate the test chuck 70 to place the test area of
 the selected wafer 21 under the probe 46 assembly. In one embodiment, the
 motor 80 may be used to move the test chuck 70 to place the specified test
 area of the selected wafer 21 under the probe assembly 46.
 After placing the specified test area of the selected wafer 21 under or in
 contact with the probe assembly 46, the probe assembly 46 performs one
 more tests to determine the characteristics of the wafer. The probe
 assembly 46 may be implemented as a test head that contacts the surface of
 the selected wafer 21 to be tested. For example, the test head may include
 individual probe leads, which are designed to take test measurements on
 the surface of the selected wafer 21.
 In one embodiment, the test head includes a four-point probe apparatus that
 includes four metal, spring-loaded probes that engage the surface of the
 selected wafer 21. A current is typically induced in the outer probes of
 the four-point probe and the voltage across the inner probes is measured.
 Such a probe is designed to measure wafer resistivity and film thickness.
 Four-point probes are well known in the art. Those skilled in the art will
 readily appreciate that the present invention may utilize other test
 probes to test various other characteristics of a wafer.
 FIG. 3D illustrates a side view of the wafer handling and testing apparatus
 10 of FIG. 1C performing a test on the selected wafer. As illustrated
 above, the probe assembly 46 is mounted on the support arm 98, which is in
 turn attached to the carriage 30. After placing the selected wafer 21 on
 the test chuck 70, the wafer blade 44 moves away (e.g. up) to enable the
 support arm 98 having the probe assembly 46 to maneuver into position for
 testing.
 In this configuration, because the carriage 30 is constrained to movement
 along the x-axis, the probe assembly 46 is also constrained to move in
 parallel to the x-axis. The motor 38 moves the probe assembly 46 into a
 desired testing position along the x-axis over the wafer. Preferably, the
 motor 78 also rotates the selected wafer 21 to place a specified area of
 the selected wafer 21 to be tested under the probe assembly 46. In
 addition, the motor 80, when provided, may move along the z-axis to
 position the selected wafer 21 under the probe assembly 46 for testing.
 After the tests have been completed, the wafer blade 44 is used to pick up
 the selected wafer 21 from the test chuck 70 and return the selected wafer
 21 to the wafer carrier 16. The wafer blade 44 is moved under the selected
 wafer 21 into the blade insert 74 by using the first and second carriage
 assemblies 41 and 43, as described above. The wafer blade 44 is then able
 to pick up the selected wafer 21 and transport it back to the wafer
 carrier 16, after which the wafer blade 44 may select and remove a
 different wafer 20 for testing in the same manner.
 FIG. 4 shows a flow diagram illustrating a method 400 for handling and
 testing a wafer in accordance with one embodiment of the present
 invention. The acts of method 400, in the preferred embodiment, are
 controlled by computer 18 using the motors of the wafer handling and
 testing apparatus, where the computer can follow program instructions or
 code to control the wafer handling and testing apparatus. Alternatively,
 some acts can be performed by manual or operator control.
 The method 400 begins at an act 402 where a wafer to be tested is selected
 from a wafer carrier. In act 404, the selected wafer is picked up with a
 wafer blade (e.g., end effector, wafer pick, etc.). Preferably, the wafer
 blade is moved into the wafer carrier underneath the selected wafer and
 moved upwards to pick up the wafer. When the selected wafer is lifted off
 of the walls of a slot in which the selected wafer is positioned in the
 wafer carrier, the selected wafer is supported only by the wafer blade.
 In an act 406, the wafer blade moves the selected wafer to a support
 assembly. In one embodiment of the present invention, the wafer blade has
 two degrees of freedom so that it is capable of moving the selected wafer
 along paths parallel to both the x-axis and the z-axis. The placement of
 the selected wafer on the support assembly may be accomplished by either
 moving the wafer blade down to the support assembly or by moving the
 support assembly up to the wafer blade to receive the wafer.
 In an act 408, the probe assembly is positioned over a desired position
 (r,.theta.) by using a first carriage assembly and a second carriage
 assembly to move a support arm, which supports the probe assembly. Then,
 in an act 410, the support assembly is rotated to place the wafer at a
 desired test position (.theta.) so that a specified area of the wafer is
 placed under the probe assembly. Act 410 may also involve centering and
 orienting the wafer, for example, by rotating the support assembly and the
 wafer so that the center of the wafer is aligned with the center of the
 support assembly.
 In an act 412, the test probe then tests the wafer at the specified test
 area to determine one or more wafer characteristics such as resistivity
 and wafer thickness. In one embodiment of the present invention, more than
 one area of the wafer may be tested. For example, the support assembly and
 the probe assembly may be rotated or moved to place another area of the
 wafer under the probe for testing. An act 414 then determines whether more
 positions on the wafer are to be tested. If more positions need to be
 tested, then method 400 returns to act 408.
 After the testing is conducted, the wafer blade picks up the wafer and
 places the wafer back in the wafer carrier, in an act 416. The wafer is
 placed, preferably in the original slot from which the wafer was removed.
 The method 400 then proceeds to act 418 to determine if more wafers need
 to be tested. If so, the method 400 proceeds back to act 402 to select
 another wafer for testing, otherwise, the method 400 is terminated.
 In summary, the present invention provides a wafer handling and testing
 apparatus that efficiently moves and tests wafers without moving the wafer
 carrier or cassette by providing a wafer handling assembly that is able to
 move a selected wafer with two degrees of freedom. The invention has been
 described in terms of several preferred embodiments. Other embodiments of
 the invention will be apparent to those skilled in the art from
 consideration of the specification and practice of the invention.
 Furthermore, certain terminology has been used for the purposes of
 descriptive clarity, and not to limit the present invention. The
 embodiments and preferred features described above should be considered
 exemplary, with the invention being defined by the appended claims.