Reusable wafer support for semiconductor processing

A wafer holding device supports a wafer during semiconductor processing. The device has a planar disk member with a raised supporting edge on which the wafer sits. Cooling gas passes through an aperture in the disk member to contact the bottom surface of the wafer. An O-ring, which sits on or near the raised supporting edge, is used to maintain an airtight seal between the wafer and the wafer holding device. Pegs or a ridge on the supporting edge fix the rotational position of the wafer. The wafer is secured to the device using a ring member, with holes for the ridge or pegs, placed on top of the wafer. At the bottom of the disk member is an annular projection that is held by a robotic transfer mechanism during transport into the process chamber. The projection fits processing equipment designed to hold standard-sized wafers.

FIELD OF THE INVENTION
 This invention relates generally to a support for holding and transporting
 silicon wafers in a vacuum chamber during semiconductor processing. More
 particularly, it relates to a support used during very deep or
 through-wafer etching.
 BACKGROUND ART
 Semiconductor wafers are a fundamental component of the electronic
 industry, and improving the quality and efficiency of their processing is,
 therefore, highly desirable. Wafers are processed in sealed vacuum
 chambers, where they are supported in a processing machine by a chuck,
 through which cooling gas flows to contact the bottom surface of the
 wafer. The wafer, therefore, has a different pressure environment on its
 top and bottom surfaces.
 For procedures in which holes are etched nearly or completely through the
 wafer, termed very deep to through-wafer etching, the etch must be stopped
 before it reaches the chuck. Holes that reach the bottom of the wafer
 expose the chuck to the process environment, damaging the chuck and
 contaminating the chamber. To prevent this, a thin film layer is placed on
 the bottom surface of the wafer to stop the etch. This solution causes
 further problems, because the thin film sticks to the chuck when heated.
 The thin films used also do not adequately support the wafer structure
 during and after very deep to through-wafer etching, causing wafers to
 cleave or shatter during processing.
 An additional constraint introduced by very deep to through-wafer etching
 arises during removal of the wafer from the chuck. Typically, pins rise
 out of the chuck to lift the wafer and a spatula reaches underneath the
 wafer to move it out of the process chamber. Deep holes are problematic if
 they are in the path of the pins or if they significantly decrease the
 wafer's structural integrity.
 A current solution is to use a backing wafer adhered to the process wafer
 with a thin film, for example photoresist, sandwiched between the two
 wafers. The backing wafer system, however, introduces further problems.
 Physical pressure on the process wafer and elevated temperatures are
 needed to effect adhesion, complicating processing and introducing
 significant potential for contamination. When the wafer is inserted into
 the processing chamber, air bubbles between the process and backing
 wafers, poor quality adhesion, or delaminating of the process wafer at
 high temperatures cause the wafers to break violently. Wafer breakage is a
 catastrophic and costly event, requiring operators to shut down and clean
 the process equipment.
 What is needed is a device, such as a wafer holder, that can be used to
 protect the chuck and support the substrate during through-wafer etching
 without using a backing wafer.
 Existing wafer holders are not designed for very deep to through-wafer
 etching and do not address all of the requirements outlined above. In
 general they are fixed to the chuck and themselves need to be protected
 from contamination by the process environment.
 In U.S. Pat. No. 4,213,698, Firtion et al. disclose an apparatus for
 holding a workpiece during semiconductor processing. Their device creates
 a planar holding face on the ends of many closely-spaced pins. The
 apparatus is not applicable for very deep to through-wafer etching and
 does not allow for cooling gas flow through the device to the wafer.
 Hattori describes a substrate carrier in U.S. Pat. No. 4,646,418. The
 carrier is designed to minimize operator handling of the substrate and is
 not used during processing.
 A workpiece carrier for heat transfer under vacuum conditions is described
 by Wagner et al. in U.S. Pat. Nos. 5,033,538 and 5,180,000. The carrier
 contains a complex system of channels and grooves through which a heat
 transfer gas flows. The channel system is highly complicated, and the
 carrier is not suitable for through-wafer etching.
 Finally, in U.S. Pat. No. 4,846,452 Geneczko discloses a rotational chuck
 assembly for finely controlling the rotational position of a wafer on a
 chuck. The assembly is very mechanically complicated, and is actually part
 of a chuck, not a separate wafer support used to protect a chuck.
 OBJECTS AND ADVANTAGES
 Accordingly, it is a primary object of the present invention to provide a
 wafer holding device for transporting a wafer into and out of a processing
 chamber. An advantage of this device is that it supports the substrate and
 protects the chuck and process chamber from damage from wafer chips and
 sticky material on the wafer's bottom surface. An additional advantage of
 the present invention is the ease with which it is loaded and unloaded
 from the chuck, compared with a wafer with many deep holes.
 It is a further object of the invention to provide a wafer holder that is
 reusable.
 It is an additional object of the invention to provide a means for
 positioning the wafer in a predetermined orientation in the process
 chamber, facilitating repeatable processing.
 Another object of the present invention is to provide a wafer holder that
 is compatible with existing processing equipment designed to hold and
 transport a wafer without a support. Specifically, the present invention
 fits into a robotic arm and into a chuck.
 Allowing for efficient wafer cooling is a further object of the present
 invention. When a backing wafer is used, the process wafer is cooled
 through the backing wafer. An advantage of the present invention is that
 the process wafer is separated from the cooling gas by only the thin film
 layer.
 Another object of the present invention is to accommodate wafers of any
 size and shape.
 Finally, the present invention accomplishes these objects without
 significant additional wafer handling or processing steps.
 SUMMARY
 The present invention provides a supporting device for holding, supporting,
 and transporting wafers in a processing chamber during semiconductor
 processing. In the preferred embodiment, the size and shape of the device
 are such that it holds a standard silicon wafer. The device is supported
 by the processing chuck in a position usually occupied by the wafer
 itself. To withstand the processing environment, the device, except for
 the deformable material to be described later, can be fabricated of metal.
 The invention also provides for a wafer-processing system incorporating
 the holding device.
 The device includes a base, preferably disk shaped, with top and bottom
 surfaces and a perimeter edge. The bottom surface of the wafer is
 supported by the base. In one embodiment of the invention, the base
 contains an aperture through its top and bottom surfaces, preferably
 passing through the center of the base. Cooling gas flows from the chuck
 through the aperture to contact the bottom surface of the wafer.
 A continuous, raised supporting edge at the perimeter of the base creates a
 recession well from the top surface of the base and the inner wall of the
 supporting edge. The wafer sits on this raised edge, allowing the cooling
 gas to flow through the aperture into the recession well. In the preferred
 embodiment, the edge has a planar top surface parallel to the top surface
 of the base.
 On or near the raised edge is a resilient, deformable material used to
 maintain an airtight seal between the wafer and the wafer holding device
 to isolate the cooling gas from the processing environment. In the
 preferred embodiment, this material is an O-ring. The O-ring can be placed
 in an O-ring groove on the planar top surface of the edge or, preferably,
 inside the recession well, contacting the inner wall of the edge.
 The device also contains a means for positioning the wafer in a
 predetermined position on the raised edge. This can include, but is not
 limited to, at least three pegs or a raised ridge at the perimeter of the
 raised supporting edge. The pegs or ridge contact the flat portion and at
 least part of the curved portion of the wafer edge to fix its position.
 A portion of the device is held by a robotic transfer mechanism for
 transporting the device into and out of the processing chamber and for
 positioning the device on the chuck. This portion can be an annular
 protrusion on the bottom surface of the base. In the preferred embodiment,
 the annular protrusion is shaped like a standard wafer to fit into a
 mechanical arm designed to move wafers.
 Finally, the device contains a means for securing the wafer to the base. A
 ring member with a hole, preferably centrally located, is placed over the
 device and rests on the wafer. In the preferred embodiment, the ring
 member is annular and has a ridge at the circumference of its bottom edge
 for fitting around the edge of the base. As necessary, the ring member
 contains holes to accommodate the pegs or other means for positioning the
 wafer.
 The holding device is incorporated into a standard wafer-processing system.
 Inside a vacuum processing chamber are a processing tool, a chuck
 containing cooling gas flow channels, and a robotic mechanism for
 transporting the holding device into the chamber and positioning it on the
 chuck. The system also contains means for creating and maintaining the
 vacuum and means for controlling the cooling gas flow in the chuck. The
 robotic mechanism contains an arm designed to fit standard silicon wafers,
 but in this system it fits the holding portion of the device. The device
 is secured to the chuck either electrostatically or mechanically. With
 electrostatic clamping, the wafer, ring member, and base are held together
 using screws or clamps. Alternately, mechanical clamps used currently hold
 the wafer on the device and also the device on the chuck.

DETAILED DESCRIPTION
 FIGS. 1A and 1B show a backing wafer system 11 used in the prior art. A
 wafer 10 is bound to a backing wafer 14 by a thin film 12, which is
 usually photoresist. A wafer-processing system of the prior art, shown in
 FIG. 2, contains a process chamber 16, a processing tool 20, a chuck 18,
 and robotic transfer means 22. Robotic transfer means 22 hold and
 transport backing wafer system 11 and position it on chuck 18 for
 processing.
 Although the following detailed description contains many specifics for the
 purposes of illustration, anyone of ordinary skill in the art will
 appreciate that many variations and alterations to the following details
 are within the scope of the invention. Accordingly, the following
 preferred embodiment of the invention is set forth without any loss of
 generality to, and without imposing limitations upon, the claimed
 invention.
 The main features of the invention are shown in FIGS. 3A-C. A disk member
 24 contains a raised supporting edge 26 for supporting a wafer, a holding
 portion 30 that is held by robotic transfer means 22 of FIG. 2, and means
 for positioning the wafer 28. In FIG. 3A, positioning means 28 is shown as
 a pin for aligning with a known point on the wafer. In FIG. 3C, the device
 has an aperture 33 extending through a disk member 32 and a holding
 portion 34. A cooling gas flows through aperture 33 from below holding
 portion 34 to reach the bottom surface of the wafer.
 In general, the device is used with a cooling gas and aperture 33, in which
 case it includes a means for maintaining an airtight seal between the
 holding device and the. wafer. Alternate embodiments of a raised
 supporting edge 36, 40, and 44 of different shapes and of correspondingly
 shaped deformable materials 38, 42, and 46 are shown in FIGS. 4A-C.
 A preferred embodiment of the invention is shown in FIGS. 5A-D. The wafer
 is supported by a planar top surface 56 of raised supporting edge 36. An
 O-ring 52 rests on the top surface of disk member 32 and contacts an inner
 wall 50 of raised supporting edge 36. When the wafer is secured to planar
 top surface 56 and O-ring 52 is compressed, an airtight seal between the
 wafer and the device is created.
 An annular protrusion 57, the preferred embodiment of holding portion 34
 shown in FIG. 3C, depends from the bottom surface of disk member 32. In
 the most preferred embodiment, annular protrusion 57 is the size of a
 standard silicon wafer, for fitting robotic transfer means 22, shown in
 FIG. 2.
 Pegs 54 and 55 position the wafer with its flat portion against pegs 55.
 Only one rotational orientation of the wafer is permitted by the
 arrangement of pegs 54 and 55. To secure the wafer to disk member 32, an
 annular ring 58, shown in top and bottom views in FIGS. 5C and 5D,
 respectively, is placed over the wafer. Annular ring 58 has holes 60 for
 fitting over pegs 54 and 55 and a raised rim 62 on its bottom surface for
 fitting around disk member 32.
 FIGS. 6A and 6B show an alternate embodiment of the invention. Instead of
 pegs 54 and 55 of FIG. 5A, the wafer is fixed in position using a raised
 ridge 64. An annular ring 66, shown in FIG. 6B, with raised rim 68 on its
 bottom surface, is placed over the wafer to secure the wafer to disk
 member 32. Annular ring 66 is similar to annular ring 58 of FIGS. 5C and
 5D, but lacks holes 60. Many alternate means for positioning the wafer may
 be imagined, each with correspondingly shaped rings for securing the wafer
 to the device. A further possible embodiment is shown in FIG. 7A-B, in
 which a bar 70 is positioned against the flat portion of the wafer and
 pegs 54 restrict the wafer position. A corresponding angular ring 72 with
 a cutout portion 76 to accommodate bar 70 and a raised rim 74 is shown in
 FIG. 7B.
 Depending on the structure of the surrounding process equipment, a modified
 annular protrusion 78, shown in FIG. 8, can have an inner diameter larger
 than aperture 33 to accommodate flow of a cooling gas.
 FIGS. 9 and 10 are front schematics of two possible embodiments of a
 wafer-processing system incorporating the present invention, a wafer
 holding device 84. The systems are similar to the prior art system shown
 in FIG. 2, except that robotic transfer means 22 now hold wafer holding
 device 84 rather than backing wafer system 11. Process chamber 16, chuck
 18, processing tool 20, and robotic transfer means 22 are identical to
 those used in the prior art system. Also shown in FIGS. 9 and 10 are
 channels 82 inside chuck 18 for flowing gas to wafer holding device 84.
 Two methods are currently used to clamp wafer backing system 11 to chuck
 18, namely mechanical and electrostatic. Both methods can be used to clamp
 wafer holding device 84 to chuck 18. The mechanical method of FIG. 9 uses
 weight mechanisms 86 to secure wafer holding device 84 to chuck 18.
 Referring to FIG. 5, weight mechanisms 86 also compress O-ring 52 to
 maintain an airtight seal between the wafer and wafer holding device 84.
 When electrostatic clamping is used, clips 88 secure annular ring 58 and
 the wafer to disk member 32.
 It will be clear to one skilled in the art that the above embodiment may be
 altered in many ways without departing from the scope of the invention.
 For example, instead of a ring member, the wafer can be secured to the
 disk member using clips. To support irregularly sized and shaped wafers,
 the supporting edge can also be irregularly shaped. Accordingly, the scope
 of the invention should be determined by the following claims and their
 legal equivalents.