Abstract:
A sample mount and method is disclosed for securing a semiconductor wafer sample to a generic base of a scanning electron microscope. The mount has two opposing clamp members that move relative to one another in response to rotational input to a lead screw. By placing a sample between the clamp members and rotating the lead screw, the samples may be clamped for inspection. When inspection is complete, the lead screw may be rotated in the opposite direction to release the clamping hold on the sample. The clamp members are adjustable to hold varying thicknesses and numbers of specimens making up the sample. In one embodiment, both clamp members move symmetrically from a common origin. In yet another embodiment, one clamp member is fixed relative to the mount and the other clamp member moves relative thereto.

Description:
This application is a Continuation of U.S. application Ser. No. 09/237,283, filed Jan. 25, 1999 now U.S. Pat. No. 6,414,322 which is incorporated herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention pertains to electron microscopes. Specifically, this invention pertains to a sample mount for use with a scanning electron microscope. 
     BACKGROUND OF THE INVENTION 
     The semiconductor wafer fabrication process relies heavily on physical inspection processes to ensure product quality. Due to the minute size of the wafer features, highly specialized equipment is required. This equipment typically includes a variety of viewing instruments such as microscopes which permit a technician to accurately magnify and view specific features of the wafer sample. 
     For various reasons, conventional optical microscopes are ill-fitted for wafer inspection. For example, they are limited in their ability to resolve detail at a level sufficient to enable adequate wafer examination. Additionally, they are unable to achieve the required magnification levels. Furthermore, depth-of-field (i.e., the ability to keep objects at two different depths simultaneously in focus) is restricted, requiring the operator to constantly re-focus the microscope as different areas of the sample are inspected. 
     These drawbacks are eliminated by using a scanning electron microscope (SEM). Unlike an optical microscope, the SEM utilizes an electron beam to bombard the sample as it sits within a vacuum environment. Due to the characteristics of the electron beam (as opposed to the visible light source used in optical microscopes), resolution and magnification are significantly increased. Additionally, no depth-of-field problems exist with the SEM so surfaces at any depth can be examined without re-focusing. These advantages have made the SEM essential to the wafer inspection process. 
     Before viewing the wafer sample in the SEM, the wafer must be securely mounted. Typically, the microscope includes a movable base to facilitate specimen mounting. However, an appropriate sample mount is necessary to secure the sample to the base. The sample mount used varies depending on the wafer features to be inspected. For example, sample mounts are known for inspecting the face of the wafer while other mounts permit inspection of wafer edge features. The present invention is addressed to the latter and the remainder of this discussion is directed accordingly. 
     One apparatus is described by the Applicant herein in a co-pending, commonly assigned application entitled “Wafer Sample Retainer for an Electron Microscope,” filed on Dec. 01, 1997 having Ser. No. 08/980,932. 
     For semiconductor wafers, inspection of edge features is usually accomplished by securing several wafers together and mounting the sample in a vertical orientation relative to the SEM. The mount typically consists of a vertical member to which one or more wafer specimens are secured using a curable adhesive. Copper tape is then wrapped around the specimens and the mount to secure the sample. While such mounts have proven effective, drawbacks exist. For example, the application and removal of the tape adds additional steps to the inspection process. Additionally, the curable adhesive may require several hours to cure prior to inspection. Furthermore, periodic cleaning of the fixture may be required to remove adhesive residue. 
     Thus, there are issues concerning increased setup time with current semiconductor sample retaining devices. As wafer fabrication facilities continue to increase production rates, the total number of wafers inspected must also increase. As a result, there is a need for a sample mount that provides quick and effective mounting without the drawbacks inherent with adhesives. 
     SUMMARY OF THE INVENTION 
     A sample mount for an scanning electron microscope (SEM) is disclosed in which the mount comprises a first clamp member, a second opposing clamp member, and a lead screw operatively connected to both clamp members. Rotation of the lead screw varies the distance between the clamp members. A method for retaining a sample for examination in a SEM is also disclosed comprising securing a sample mount to a base, inserting a sample into the sample mount, and turning a lead screw in a first direction to move a first clamp member toward a second clamp member, thereby securing the sample therebetween. 
     The sample may be a single silicon wafer or a plurality of wafers. Various sample thicknesses may be accommodated by merely turning the lead screw to move the clamp members relative to one another. 
     The sample mount may be removably mounted to a base on the SEM. The SEM may further include a rail in which the base is adjustably positionable. The base may be positionable with a motor-driven screw. 
     In one embodiment, both clamp members are movable relative to the base. In another embodiment, one clamp member is fixed relative to the base and the other clamp member moves relative thereto. 
     The sample mount may comprise a retaining assembly having a plate and a clamp body removable attached to the plate. A first and second clamp member may be operatively connected to the retaining assembly whereby the clamp members are capable of securing a sample therebetween. A lead screw may be secured to the retaining assembly and operatively connected to the first and second clamp members, whereby rotation of the lead screw varies the distance between the clamp members. A thumb-wheel may be provided at one end of the lead screw to assist the operator in turning the screw. The retaining assembly, first and second clamp members, and the lead screw may all be removably secured to a base on the SEM. 
     In one embodiment, the lead screw may comprise a central threaded portion having a first threaded portion and a second threaded portion wherein the first threaded portion has a right-handed thread and the second threaded portion has a left-handed thread. The first clamp member is threadably engaged to the first threaded portion of the lead screw and the second clamp member is threadably engaged to the second threaded portion such that rotation of the lead screw in a first direction results in relative closure of the clamp members. Alternatively, rotation of the lead screw in a second direction results in relative separation of the clamp members. Regardless, the clamp members move relative to a common origin. 
     The present invention provides an improved sample mount that permits quick and efficient edge mounting of wafer samples within a SEM. Furthermore, mounting is accomplished without the use of messy adhesives and tapes. By avoiding the use of adhesives, the sample mount does not require the lengthy cure time often associated with adhesive mounts. Advantageously, inspection throughput is increased, preventing wafer inspection from becoming a production bottleneck. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention described herein will be further characterized with reference to the drawings, wherein: 
     FIG. 1 is a perspective view of a SEM showing a sample mount in accordance with one embodiment of the present invention; 
     FIG. 2 is a diagrammatic perspective view of one exemplary embodiment of the sample mount in accordance with the present invention; 
     FIG. 3 is an exploded perspective view of the sample mount of FIG. 2; 
     FIG. 4 is a partial section view taken along line  4 — 4  of FIG. 2 wherein the clamp body, the clamp plate, the clamp members, and the thumb-wheel are shown in section; 
     FIG. 5 is an enlarged partial side view of the sample mount of FIG. 2 showing the tab and recess; 
     FIG. 6 is a diagrammatic perspective view of another exemplary embodiment of the sample mount in accordance with the present invention; and 
     FIG. 7 is a partial section view taken along line  7 — 7  of FIG. 6 wherein the clamp body, the clamp plate, the clamp member, and the thumb-wheel are shown in section. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In the following detailed description of the embodiments, reference is made to the accompanying drawings which form a part hereof, and in which are shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
     FIG. 1 shows a perspective view of one embodiment of a sample mount  10  in accordance with the present invention as it would be installed in a generic scanning electron microscope (SEM)  12 . Referring to FIG. 2, a sample mounting assembly  11  includes the sample mount  10  and a base  14  to which the sample mount  10  is secured. The base  14  is positionable along a rail  16  and may be externally driven by a drive screw  18  operably connected to a drive motor  20 . Alternatively, the base  14  may be manually positionable by a variety of devices known in the art or by hand. By selectively engaging the drive motor, the base  14  may be located at a point along the rail  16  that provides optimal sample viewing. The sample mount  10  extends upwardly from the base  14  to retain a wafer sample  22  (shown in FIG. 4) for viewing under the SEM. The sample may consist of one or more semiconductor wafer specimens of equal or varying thickness. 
     With this brief overview, attention will now be focused on one exemplary embodiment of the sample mount  10  of the present invention. Referring generally to FIGS. 3-5, the sample mount  10  comprises a plate  24  and a clamp body  26 . The plate  24  and clamp body  26  are substantially equal in size and form a generally rectangular shape in the plan view. The clamp body  26  is formed from two identical half-sections  21  and  23 . Each half-section includes two threaded holes (not shown) which, when assembled, are in axial alignment with two of the four fastener holes  27  located in the plate  24 . When assembled, four flathead cap screws  29  (only one of which is shown in FIG. 3) secure the plate  24  to the half-sections  21 ,  23  of the clamp body  26 . 
     Referring particularly to FIG. 3, the plate  24  includes a pair of opposing tabs  28  extending upwardly from a top side. It is noted that the terms “up” and “down” are used throughout the specification only for descriptive purposes and are not intended to limit the scope of the invention in any way. Located on the upper surface of each tab  28  is a first convex radius  30  (best shown in FIG.  5 ). The purpose of this radius will become apparent shortly. The clamp body  26  has a pair of centrally located opposing recesses  32 . Extending between the recesses  32  of the clamp body  26  is a centrally located slot  34  having a slot width  36 . The recesses are designed to receive the tabs  28  when the plate  24  and the clamp body  26  are assembled. While the tabs are shown integral to the plate  24 , it should be noted that the tabs  28  and recesses  32  could be reversed (i.e., the tab  28  could be integral to the clamp body  26  and the recess  32  could be integral to the plate  24 ). Such an arrangement would still be within the scope of the invention. As shown in FIG. 5, each recess  32  further includes a second convex radius  38  substantially equal to the first convex radius  30 . The construction of tabs  28  and recesses  32  are such that, when the bottom plate  24  is assembled with the clamp body  26 , the interrelation of the first and second convex radii  30 ,  38  form opposing, generally cylindrical openings  40  having a retaining diameter  42 . The tabs are spaced apart by a tab separation  44  best shown in FIG.  4 . 
     Referring once again to FIG. 3, the sample mount  10  additionally comprises a first clamp member  46  and a second clamp member  48 . The clamp members  46  and  48  are T-shaped members having a first or upper generally horizontal portion  50  and a second or lower generally vertical portion  52 . The upper horizontal portions  50  form opposing specimen holding faces. Each lower vertical portion  52  has a width  54  slightly smaller than the slot width  36  so that the lower vertical portion  52  of the clamp members  46 ,  48  slidably nests within the slot  34 . Each lower vertical portion  52  also includes a threaded hole  56 ,  57  whose purpose will become apparent shortly. Thus, the holding faces  50  are held in a parallel relation to one another (best shown in FIG. 2) by the nesting relationship of the lower vertical portion  52  and the slot  34 . However, each clamp member  46 ,  48  can move toward or away from the other clamp member by merely sliding within the slot  34 . 
     As shown in FIG. 3, a lead screw  58  spans between the cylindrical openings  40  (see FIG.  5 ). The lead screw has a central threaded portion  60  having a minor thread diameter. At each end of the central threaded portion  60  is a first end portion  64  which is best viewed in FIG.  4 . In this embodiment, the first end portion  64  comprises a first non-threaded portion  64  having a first diameter  66  which is smaller than the minor diameter of the central threaded portion  60 . The central threaded portion  60  and the first non-threaded portions  64  together define a screw length  62  that is best viewed in FIG.  4 . In an alternative embodiment, the first end portion  64  is merely a continuation of central threaded portion  60  such that the central threaded portion  60  extends over the entire screw length  62 . Located immediately adjacent and outboard to each first non-threaded portion  64  is a second non-threaded portion  68  having a second diameter  70  which is smaller than the first diameter  66 . The second diameter  70  is slightly smaller than the retaining diameter  42  (see FIG. 5) such that each retaining diameter may receive and retain one end of the lead screw  58  by the second diameter  70 . The lead screw  58  is also restrained from longitudinal motion by the close fit of the first non-threaded portions  64  and the tabs  28 . That is, the screw length  62  is slightly smaller than the tab separation  44 , thus restricting lead screw motion along the axis of the lead screw. 
     Extending from one end of the lead screw  58  is an extended portion  72  of the second non-threaded portion  68 . The extended portion  72  is designed to operatively engage a thumb-wheel  74 . In the embodiment shown, the thumb-wheel is secured by a press fit to the extended portion  72  but other securing methods (e.g., threaded engagement, staking, set screw) are also within the scope of the invention. The thumb-wheel is preferably knurled on its outer diameter to permit easy turning by hand. While the embodiment shown utilizes the thumb-wheel  74 , other features that assist in turning the lead screw  58  (i.e., flathead or Phillips screw head, splined head, allen head, etc.) are also within the scope of the invention. 
     Referring still to FIGS. 3 and 4, the central threaded portion  60  of the lead screw  58  further comprises a left-hand threaded portion  76  and a right-hand threaded portion  78 . When assembled, the left-hand threaded portion  76  threadably engages the threaded hole  56  of the second clamp member  48 , which has a left-hand thread, while the right-hand threaded portion  78  threadably engages the threaded hole  57  of the first clamp member  46 , which has a right-hand thread. In this way, rotational motion of the lead screw  58  in a first direction causes the clamp members  46 ,  48  to move closer together while rotation of the lead screw in a second direction causes the clamp members to separate. The operator may, accordingly, change the distance between the clamp members  46 ,  48  by merely turning the thumb-wheel  74 . When the thumb-wheel is turned, the clamp members move toward or away from a common origin. Thus, regardless of variations in sample thickness, the centerline of the sample remains “centered” relative to the base. 
     The threaded holes  56 ,  57  and threaded portions  76 ,  78  are each 6-32 UNC threads (either left-handed or right-handed as described herein). However, those skilled in the art will realize that other thread sizes and standards are also within the scope of the invention. 
     Referring once again to FIG. 2, the sample mount  10  may also includes a pin  80  extending downwardly from the plate  24 . The pin  80  is received by a central opening  82  in the base  14 . A socket set screw (not shown) or similar fastener may be threadably engaged with the base  14  normal to the opening  82  to secure the pin  80  relative to the base  14 . 
     Having described the invention in detail, assembly of the sample mount  10  will now be described. First, the pin  80  may be pressed into the plate  24 , ensuring that it does not protrude beyond an upper surface  81  (see FIG. 4) of the plate. Next, both clamp members  46 ,  48  may be threaded onto the lead screw  58 . Each clamp member  46 ,  48  should be threaded to its center-most position (i.e., the clamp faces  50  should be “centered” with respect to the lead screw threaded portion  60 ). The clamp members and lead screw together may then be placed into either half-section  21  (or  23 ) such that second diameter  70  of one end of the lead screw  58  is retained within the second convex radius  38  and the first non-threaded portion  64  is in an abutting relation with an inside surface of the clamp body  26  (as generally shown in FIG.  4 ). The half-section  21  may then be loosely fastened to the plate  24  with two fasteners  29 . The other half-section  23  (or  21 ) may then be attached and similarly fastened to the plate  24  with the remaining fasteners  29 . Finally, half-sections  21 ,  23  may be aligned relative to one another and the fasteners  29  may be tightened to an appropriate torque value. At this point, the lead screw  58  is retained between the plate  24  and the clamp body  26  by the retaining diameters  42  and the clamp members  46 ,  48  are threadably engaged to the lead screw  58 . If not already installed, the thumb-wheel  74  may be attached to the extended portion  72  as shown in FIGS. 2 and 4. 
     In use, the sample mount  10  is attached to the base  14  by inserting the pin  80  into central opening  82 . The mount  10  may be secured with a set screw (not shown) or other conventional means (e.g., threaded or frictional engagement). The specimen sample  22  (as shown in FIG. 4) may be placed between clamp members  46 ,  48  and the thumb-wheel  74  may be rotated in the first direction. As the thumb-wheel is turned, the clamp members move toward one another until they contact the sample  22 . The clamping force applied to the sample  22  can be varied proportionally to the amount of torque applied to the thumb-wheel  74 . Adjustment of the sample location relative to the SEM can be made by selectively energizing the motor  20  to drive the base  14  within the rail  16 . 
     To release the sample  22  after inspection, the thumb-wheel  74  is simply rotated in the second direction, thereby releasing the samples from the clamp members  46 ,  48 . A second set of specimens may then be inserted. Due to the combination thread of the lead screw  58 , the clamp members move toward and away from a common center or origin. Thus, even if a sample is of different thickness than the previously inspected sample, the center of the sample is always in the same position relative to the base. Accordingly, movement of the base  14  along the rail  16  is minimized. 
     Another exemplary embodiment of the sample mount of the present invention is shown in FIGS. 6 and 7. Here, a sample mount  110  is shown. While similar in most respects to the sample mount  10 , the sample mount  110  differs in that one clamp member defines a fixed (relative to the clamp body) clamp face. That is, the clamp body  126  additionally includes an upwardly extending clamp face  148  such that the clamp body  126  is a generally L-shaped member. A centrally located slot  134  is disposed perpendicularly to the clamp face  148  and extends through the clamp body  126 . In opposing relationship to the clamp face  148  is a generally T-shaped clamp member  146  having a first or upper generally horizontal portion  150  and a second or lower generally vertical portion  152 . The upper horizontal portion  150  forms a specimen holding face similar to clamp face  148 . The lower vertical portion  152  slidably nests within the slot  134 . The clamp member  146  can move toward or away from the clamp face  148  by merely sliding within slot  134 . The lower vertical portion  152  also includes a threaded hole  157  through which a lead screw  158  passes as shown in FIG.  7 . 
     Referring to FIG. 7, the lead screw  158  is retained in a fashion substantially identical to that shown by the lead screw  58  in FIG.  4 . However, the lead screw  158  comprises a unitary threaded portion  160  instead of the dual thread of the lead screw  58 . Like the embodiment described in FIG. 4, the lead screw  158  is retained by a first non-threaded portion  164  and a second non-threaded portion  168  located at each end of the lead screw. A thumb-wheel  174  is attached to an extended portion  172  of one of the second non-threaded portions  168 . The thumb-wheel  174  is preferably knurled on its outer diameter to permit easy turning by hand. 
     When assembled, the threaded portion  160  threadably engages the threaded hole  157  of the clamp member  146 . Rotational motion of lead screw  158  in a first direction causes the clamp member  146  to move toward the clamp face  148  while rotation of the lead screw in a second direction causes the clamp member to move away from the face  148 . The operator may therefore change the distance between the clamp member  146  and the clamp face  148  merely by turning the thumb-wheel  174 . 
     As with the embodiment shown in FIG. 2, the sample mount  110  shown in FIG. 6 also includes a pin  180  extending downwardly from the plate  124 . Preferably, the pin  180  is received by the central opening  82  in the base  14 . A socket set screw (not shown) may be threadably engaged with a threaded hole (also not shown) in the base to secure the pin  180  relative thereto. 
     To assemble the embodiment shown in FIGS. 6 and 7, the clamp member  146  is slightly threaded onto the lead screw  158 . The opposite end of the lead screw may then be inserted into the slot  134  and the clamp member  146  slid toward the clamp face  148  until the opposite end of the lead screw  158  can also drop through the slot  134 . Unlike the embodiment shown in FIGS. 2-5, splitting of the clamp body  126  is not required. The clamp body  126  may then be attached to a plate  124  and the lead screw  158  positioned such that it is retained in a manner substantially identical to that discussed regarding the lead screw  58 . Four fasteners (not shown) secure the clamp body  126  to the plate  124 . The thumb-wheel  174  may then be secured to the extended portion  172  of the lead screw  158 . 
     In use, the sample mount  110  is inserted and secured to the base  14 . Sample  22  may then be placed between the clamp member  146  and the clamp face  148  and the thumb-wheel  174  may be rotated in the first direction. As the thumb-wheel is turned, the clamp member  146  moves toward the clamp face  148  until the sample  22  is sandwiched therebetween. The clamping force applied to the sample  22  can be varied proportionally to the amount of torque applied to the thumb-wheel  174 . Adjustment of the sample location relative to the SEM can be made by selectively energizing the motor  20  to drive the base  14  within the rail  16 . Alternatively, the mount  110  may be rotated within the base. 
     To release the sample after inspection, the thumb-wheel is simply rotated in the second direction, thereby releasing the sample  22  from the clamp member  146  and clamp face  148 . A second sample may then be inserted and aligned for inspection. 
     CONCLUSION 
     The present invention provides an improved sample mount that permits quick and efficient edge mounting of wafer samples within a SEM. Furthermore, mounting is accomplished without the use of messy adhesives and tapes. By avoiding the use of adhesives, the sample mount does not require the lengthy cure time often associated with adhesive mounts. Advantageously, inspection throughput is increased, preventing wafer inspection from becoming a production bottleneck. 
     In the embodiments herein represented, all components are made from 300 series stainless steel. This material was selected for its excellent corrosion and chemical resistance. Nonetheless, other materials are also considered to be within the scope of the invention. 
     Exemplary embodiments of the present invention are described above. Those skilled in the art will recognize that many embodiments are possible within the scope of the invention. Variations, modifications, and combinations of the various parts and assemblies can certainly be made and still fall within the scope of the invention. Thus, the invention is limited only by the following claims, and equivalents thereto.