Apparatus for holding and aligning a scanning electron microscope sample

Apparatus for holding and aligning a sample to be examined by a scanning electron microscope or the like includes an alignment device having base structure installable in the scanning electron microscope in a predetermined orientation. The alignment device also includes a holder for the sample which is mounted to the base structure for rotative movement about a rotation axis relative to the base structure. An adjuster is mounted on the base structure and can be manipulated to rotate the sample holder about the rotation axis. This alignment device is installed in a base holder and a video camera captures an image of the sample held by the sample holder. The image is displayed on a video monitor and the adjuster is then manipulated to rotatively align the sample to a desired orientation. The alignment device, including the sample, may then be removed from the base holder and installed in the scanning electron microscope with the sample being properly aligned.

BACKGROUND OF THE INVENTION
 This invention relates to sample alignment for a scanning electron
 microscope and, more particularly, to apparatus for obtaining the
 appropriate sample alignment prior to placement of the sample in the
 scanning electron microscope.
 During the manufacture of certain integrated circuit chips, a machine known
 as a photoresist stepper is utilized to define photoresist lines on the
 integrated circuit chip wafer. To insure that the stepper is working
 properly, the widths of the photoresist lines are measured to determine if
 they meet specifications. In order to do this, the stepper is caused to
 create an array of parallel lines on a wafer, the wafer is cleaved at a
 90.degree. angle to the lines to provide a cross sectional examination
 sample, and the sample is examined using a scanning electron microscope.
 To obtain proper measurements, the sample must be held in the scanning
 electron microscope so that the electron beam is precisely aligned
 parallel to the photoresist lines. If there is a misalignment, two
 problems are encountered. First, one edge of the image of each line will
 be very bright (edge blooming), making it impossible to accurately
 determine the positions of edges and therefore the widths of the lines.
 The second problem is that the apparent widths of the lines will be
 somewhat reduced due to being viewed at an angle. If these measurements
 are altered by even a small percent, the resulting product can be
 detrimentally affected. It would therefore be desirable to be able to
 accurately align a sample for examination in a scanning electron
 microscope.
 In the past, such alignment was effected after the sample was placed in the
 scanning electron microscope. Present high resolution scanning electron
 microscopes are not capable of providing easy and accurate alignment of
 the sample in all angular directions. It would therefore be desirable to
 have apparatus and a procedure for aligning such a sample prior to its
 placement in the scanning electron microscope.
 SUMMARY OF THE INVENTION
 The present invention provides apparatus by means of which a sample can be
 properly aligned prior to its placement in a scanning electron microscope.
 The inventive apparatus includes an alignment device adapted to be
 installed in the scanning electron microscope in a predetermined
 orientation. The alignment device includes a sample holder which is
 adapted to hold the sample. The sample holder is mounted to a base
 structure for rotative movement about a rotation axis relative to the base
 structure. An adjuster is mounted on the base structure for movement
 relative thereto. The adjuster is adapted to be manipulated selectively to
 rotate the sample holder about the rotation axis so that the sample is
 properly aligned, after which the alignment device can be installed in the
 scanning electron microscope while holding the properly aligned sample.
 In accordance with an aspect of this invention, the alignment device also
 includes a spring bearing against the base structure to resiliently bias
 the sample holder for rotative movement in a first direction about the
 rotation axis. The adjuster is adapted to be manipulated selectively to
 rotate the sample holder in a second direction about the rotation axis
 against the bias of the spring or to allow the spring to rotate the sample
 holder in the first direction.
 In accordance with another aspect of this invention, the sample holder
 includes a projecting member, the base structure includes a first wall,
 and the spring is positioned between the first wall and the projecting
 member. The base structure includes a second wall which is substantially
 parallel to the first wall and on the opposite side of the projecting
 member from the first wall. The second wall is formed with an internally
 threaded bore having a central axis transversely intersecting the
 projecting member. The adjuster includes a threaded rod threadedly
 extending through the threaded bore along the central axis to engage the
 projecting member.
 In accordance with yet another aspect of this invention, the alignment
 device further includes a pair of leaf spring members each secured to the
 base structure and the sample holder to mount the sample holder to the
 base structure. The pair of leaf spring members are spaced apart each on
 respective opposed sides of the base structure and the sample holder so
 that the line of force applied to the sample holder by the spring is
 transverse to both of the pair of leaf spring members to flex the leaf
 spring members and cause rotative movement of the sample holder relative
 to the base structure about the rotation axis parallel to both of the pair
 of leaf spring members.
 In accordance with a further aspect of this invention, there is provided a
 base holder adapted to hold the alignment device in a predetermined
 orientation. A video camera is mounted fixedly with respect to the base
 holder and is positioned to capture an image of a sample held by the
 sample holder when the alignment device is held by the base holder. A
 video monitor coupled to the video camera displays an image captured by
 the video camera. Accordingly, an operator can manipulate the adjuster and
 view the image of the sample on the video monitor to properly align the
 sample.
 In accordance with yet a further aspect of this invention, the alignment
 device further includes a shaft journalled for rotation on the base
 structure parallel to the rotation axis, and an adjustment block secured
 to the shaft for rotation therewith. The adjustment block includes a
 projection extending radially outward relative to the shaft and interposed
 between the spring and the adjuster. The base structure includes a
 rectilinear upstanding block and the shaft extends through the upstanding
 block. The sample holder is secured to a first end of the shaft on a first
 side of the upstanding block, and the adjustment block is secured to a
 second end of the shaft on a second side of the upstanding block.
 In accordance with still a further aspect of this invention, the alignment
 device further includes a pair of bearing shafts mounted on the base
 structure parallel to the rotation axis, and a pair of bearing wheels each
 mounted on a respective one of the pair of bearing shafts. For each set of
 a bearing shaft and a bearing wheel at least one of the bearing shaft and
 bearing wheel of that set is journalled for rotation. The sample holder
 includes a holder block having a support surface shaped as a cylinder
 segment having a center of curvature aligned with the rotation axis, and
 with the support surface engaging the pair of bearing wheels.
 In accordance with still another aspect of this invention, the adjuster
 includes an adjusting shaft journalled for rotation on the base structure
 parallel to the rotation axis, and a linkage connecting the adjusting
 shaft to the holder block so that rotation of the adjusting shaft moves
 the support surface on the pair of bearing wheels. The linkage includes an
 elongated band having its ends fixedly secured to the holder block, with
 the band having a central portion between its ends tightly wrapped around
 the adjusting shaft. Accordingly, rotation of the shaft in a first angular
 direction causes the holder block to rotate about the rotation axis in a
 second angular direction opposite to the first angular direction.

DETAILED DESCRIPTION
 According to the present invention, the wafer examination sample is placed
 in an alignment device which includes a sample holder and base structure,
 wherein the sample holder is adjustable with respect to the base
 structure. The alignment device is adapted for a subsequent installation
 in a scanning electron microscope in a predetermined orientation. When in
 the scanning electron microscope, the sample should be oriented so that
 the plane of the wafer is vertical and the photoresist lines are aligned
 vertically. Before the alignment device is installed in the scanning
 electron microscope, it is installed in a base holder, where the sample is
 imaged by a video camera and the image is displayed on a monitor. The
 video camera is aligned horizontally and is aimed at the photoresist lines
 on the sample. With the image of the photoresist lines displayed on the
 monitor, the angular orientation of the sample holder relative to the base
 structure is manually adjusted until the lines displayed on the monitor
 are vertically aligned. The alignment device is then removed from the base
 holder and installed in the scanning electron microscope, maintaining the
 vertical alignment of the photoresist lines.
 Referring now to FIG. 1, an embodiment of the inventive apparatus,
 designated generally by the reference numeral 10, includes a video camera
 12 mounted to a stand 14 and coupled to a video monitor 16. Also mounted
 to the stand 14 is a base holder 18, whose position can be varied by means
 of three goniometers 20, 22, 24 along three orthogonal axes so that the
 base holder 18 is appropriately aligned relative to the video camera 12
 and the sample will be in focus by the video camera 12.
 As shown in FIGS. 2-4, a first embodiment of the alignment device 26, which
 holds a sample 28 and which in turn is held by the base holder 18 (FIG.
 1), includes a sample holder portion 30 and a base structure portion 32.
 In the following discussion, the relative term "front" refers to that end
 of the alignment device which holds the sample and the term "rear" refers
 to the opposite end of the alignment device. The sample holder 30 includes
 a block 34 having a front-to-back bore 36 in which is inserted a plunger
 38 surrounded by a spring 39. At the forward end of the plunger 38, there
 is mounted a spring clip 40. The spring clip 40 is used to mount the
 sample 28 to the block 34. As shown in FIG. 3, the sample 28 has a
 plurality of parallel photoresist lines 42 on its forward-facing surface.
 It is the lines 42 which are to be aligned vertically. The base structure
 32 includes a base block having a pair of lower oppositely directed
 outwardly extending lateral flanges 44, 46 and a pair of upwardly
 extending parallel walls 48, 50. The walls 48, 50 are directly across from
 each other. The illustrated base structure 32, with the flanges 44, 46, is
 adapted for use with scanning electron microscope Model No. S4700
 manufactured by Hitachi, as well as other Hitachi scanning electron
 microscopes. If another manufacturer's product is used, the base structure
 would be modified to conform therewith.
 The sample holder 30 is mounted to the base structure 32 for rotative
 movement about a rotation axis 51 adjacent to the upper edge of the sample
 28, so that the lines 42 can be angularly adjusted into vertical
 alignment. To provide such rotative mounting, a pair of leaf spring
 members 52, 54 are provided, flanking the walls 48, 50. Each of the leaf
 spring members 52, 54 is bent into a flat bottomed U-shape and is secured
 to the base structure 32 by having its flat bottom placed beneath the
 block 56, which is then secured to the base structure main block by screws
 or the like (not shown) extending through both the block 56 and the leaf
 spring members 52, 54. The upper ends of the leaf spring members 52, 54
 are then secured to the elongated blocks 58, 60, which in turn are secured
 to the sample holder block 34. The sheets from which the leaf spring
 members 52, 54 are formed remain parallel to the axis 51, as are all their
 bend lines and centers of curvature.
 Extending below the sample holder block 34 is a projecting member 62,
 preferably in the form of a centrally located downward fin, or keel. A
 leaf spring 64 curved into a circular arc is secured to the wall 48, as by
 the screw 66 or the like, and bears against the fin 62, to resiliently
 bias the sample holder 30 in a counterclockwise direction about the axis
 51, as viewed in FIG. 3. To control this rotative movement of the sample
 holder 30, an adjuster is provided. This adjuster includes a threaded
 screw 68 threadedly extending through a threaded bore in the wall 50. The
 distal end of the screw 68 contacts the fin 62 opposite the point of
 contact of the spring 64. Accordingly, turning of the screw 68 in a
 clockwise direction moves the fin 62 to the left, as viewed in FIG. 3, to
 overcome the biasing effect of the spring 64 and rotate the sample holder
 30 in a clockwise direction about the axis 51. Conversely, turning the
 screw 68 in a counterclockwise direction allows the spring 64 to rotate
 the sample holder 30 in a counterclockwise direction about the axis 51.
 This structure provides an adjustability of approximately .+-.5.degree.
 for the lines 42 about a point (i.e., the axis 51) closely adjacent the
 upper edge of the sample 28.
 To align the lines 42 on the sample 28, the alignment device 26 is
 installed in the base holder 18. As shown in FIGS. 5-7, the base holder 18
 is provided with a pair of L-shaped members 70, 72 which provide a pair of
 channels adapted to slidingly receive therein the flanges 44, 46 of the
 base structure 32 of the alignment device 26. Each of the L-shaped members
 70, 72, has a respective open slot 74, 76 into which extends an end of a
 respective spring 78, 80, the other end of which is secured to the
 upstanding wall 82 of the base holder 18. Thus, when the flanges 44, 46
 are inserted in the channels of the base holder 18, they are engaged by
 the respective spring 78, 80 to maintain their positions.
 The base holder 18 is further provided with a lever 84. Part of the lever
 84 extends away from the base holder 18, while another part is received in
 a channel 86 in the wall 82. The lever 84 is pivoted about a screw 88
 installed in a bore 90 extending upwardly through the wall 82 from the
 bottom of the base holder 18. When the alignment device 26 is installed in
 the base holder 18, the rear of the base structure 32 is up against the
 lever 84, as best shown in FIG. 5. When it is desired to remove the
 alignment device 26 from the base holder 18, the part of the lever 84
 extending out of the base holder 18 is pushed to the left, as viewed in
 FIG. 5, to engage the base structure 32 and push the pair of flanges 44,
 46 outwardly from the channels formed by the L-shaped members 70, 72.
 The base holder 18 also includes a side wall 92 having a bore 94 axially
 aligned with the screw 68 of the alignment device 26 when the alignment
 device 26 is installed in the base holder 18. A screwdriver 96 is captured
 in the bore 94, as by C-clips 98 or the like and is free to rotate within
 the bore 90. The screwdriver 96 is aligned with the screw 68 and has a
 head which is complemental to the head of the screw 68 so that the
 screwdriver 96 is used to provide the angular adjustment of the sample 28.
 In operation, a sample 28 is placed in the alignment device 26 by
 depressing the plunger 38 to release the spring clip 40 and then placing
 the sample 28 below the spring clip 40 with the lines 42 being at the top
 of the sample 28 and being as close to vertical alignment as possible when
 viewed with the naked eye. This alignment should be within .+-.5.degree.
 of vertical. The alignment device 26 is then installed in the base holder
 18 by sliding the flanges 44, 46 into the channels formed by the L-shaped
 members 70, 72. An image of the lines 42 is then viewed on the monitor 16,
 these lines being denoted by the reference numeral 42' in FIG. 1. The
 screen of the monitor 16 is provided with a fixed reference mark 100,
 which a vertical line. The screwdriver 96 is then moved inwardly to engage
 the head of the screw 68 and the sample holder 30 is rotated about the
 axis 51 by turning the screw 68 until the images 42' are parallel to the
 reference mark 100. When the sample 28 has been manually adjusted in the
 alignment device 26 to within +5.degree. of vertical alignment, it has
 been found that the inventive apparatus allows for alignment to within
 +0.1.degree. of the desired vertical alignment. After this alignment is
 achieved, the lever 84 is pivoted to remove the alignment device 26 from
 the base holder 18. The entire alignment device 26, along with the sample
 28, is then installed in the scanning electron microscope to analyze the
 operation of the photoresist stepper.
 FIGS. 8-10 illustrate a second embodiment of an inventive alignment device,
 designated generally by the reference numeral 110. The alignment device
 110 is adapted for insertion into the base holder 18 in the same manner as
 the alignment device 26 and therefore includes base structure having
 similarly configured outwardly extending lateral flanges 112, 114. Thus,
 as shown, the alignment device 110 includes base structure 116 having a
 rectilinear standing block 118. The block 118 is formed with a
 through-bore 120 extending from the front to the back of the block 118. A
 rotary shaft 122 extends through the bore 120 for rotation therein, with
 spacers 124 mounted on the shaft 122, one on each side of the block 118.
 The shaft 122 has a flat portion 126 at its front end and a bore 128
 drilled through the shaft 122 orthogonally to the flat portion 126. A
 sample holder block 130 is secured to the flat portion 126 of the shaft
 122 by the screw 132 extending through the bore 128 and an internally
 threaded bore 134 at the top surface of the sample holder block 130. The
 sample 28 is held to the front of the sample holder block 130 by the
 spring clip 136 held at the front of the sample holder block 130 by the
 screw 138. The sample 28 is held so that its upper edge is closely
 adjacent the rotational axis defined by the center of the shaft 122.
 The rear end of the shaft 122 is formed with a flat portion 140 which is
 orthogonal to the flat portion 126. The flat portion 140 extends into the
 front-to-back bore 142 of the adjustment block 144, and a set screw 146
 extending through the internally threaded bore 148 is used to secure the
 adjustment block 144 to the shaft 122 for rotation therewith.
 Secured to opposed sides of the block 118 and extending rearwardly
 therefrom are side walls 150, 152. The side walls 150, 152 flank the
 downwardly extending projection 154 of the adjustment block 144. The
 projection 154 extends radially outward relative to the shaft 122. A
 spring 156, illustratively a helical compression spring, is secured to the
 side wall 150, as by a screw or the like (not shown), and a threaded
 adjustment screw 158 extends through an internally threaded bore in the
 side wall 152. The spring 156 and the adjustment screw 158 are opposed to
 each other and contact the projection 154 on respective opposite sides
 thereof.
 Use of the alignment device 110 is the same as use of the alignment device
 26 and no further explanation thereof is necessary.
 FIGS. 11-15 illustrate a third embodiment of an inventive alignment device,
 designated generally by the reference numeral 170. The alignment device
 170 includes base structure 172 having outwardly extending lateral flanges
 174, 176 so that it can be inserted into the base holder 18. The base
 structure 172 also includes a pair of upstanding transverse walls 178,
 180. The upper surface of each of the walls 178, 180 is substantially
 planar, in the same horizontal plane, with the exception of a central
 circular groove 182, 184, respectively. The grooves 182, 184 are aligned
 one with the other and are of the same size. The front wall 178 is formed
 with a pair of front-to-back bores 186 which are equally spaced laterally
 with respect to the groove 182. Similarly, the rear wall 180 is formed
 with a pair of front-to-back bores 188 which are equally spaced laterally
 with respect to the groove 184. Each of the bores 186 is aligned with a
 respective one of the bores 188 and is of the same size. A pair of bearing
 shafts 190 each extends through a respective pair of bores 186, 188 and
 has mounted thereon a respective bearing wheel 192 between the walls 178,
 180. For each set of bearing shaft 190 and bearing wheel 192, at least one
 of the bearing shaft 190 and the bearing wheel 192 of that set is
 journalled for rotation.
 The alignment device 170 also includes a sample holder having a holder
 block 194 which has a support surface 196 shaped as a cylinder segment
 having a center of curvature aligned with the top edge of the sample 28,
 which has the rotation axis of the holder block 194, as will be described.
 The sample 28 is held to the holder block 194 by an arrangement similar to
 that of the alignment device 26, with a spring loaded plunger 198
 extending through the holder block 194 to a spring clip 200.
 To effect rotation of the holder block 194, there is provided an adjusting
 shaft 202 journalled for rotation on the base structure 172. Specifically,
 the adjusting shaft 202 is positioned in the grooves 182, 184 and is held
 therein by the pillow blocks 204, which are secured to the walls 178, 180
 as by screws 205 or the like. A linkage is provided between the adjusting
 shaft 202 and the holder block 194. Illustratively, this linkage includes
 an elongated band 206, which may be formed of stainless steel, having its
 ends secured to the sides of the holder block 194, as by screws 208 or the
 like. A central portion of the band 206 is wrapped tightly around the
 adjusting shaft 202, as best shown in FIG. 12. For assembly, the holder
 block 194, with the adjusting shaft 202 and the band 206, is placed on the
 base structure 172, with the shaft 202 disposed in the grooves 182, 184
 and the support surface 196 of the holder block 194 resting on the bearing
 wheels 192. The pillow blocks 204 are then installed. Accordingly,
 rotation of the shaft 202 in a first angular direction causes the holder
 block 194 to rotate about its center of curvature (i.e., the rotation
 axis) in a second angular direction opposite to the first angular
 direction, as shown by the arrows in FIG. 12.
 Alignment of the sample 28 using the alignment device 170 is substantially
 the same as when using the aforedescribed alignment device 26, with the
 exception that the adjusting shaft 202 is accessed from either the front
 or the rear of the alignment device 170, instead of from the side of the
 alignment device 26.
 FIGS. 16 and 17 illustrate a fourth embodiment of an inventive alignment
 device, designated generally by the reference numeral 220. The alignment
 device 220 includes base structure 222 having outwardly extending lateral
 flanges 224, 226 so that it can be inserted into the base holder 18. The
 base structure 222 also includes a first upstanding wall 228 and a pair of
 upstanding walls 230, 232 which are spaced apart and orthogonal to the
 wall 228.
 The alignment device 220 also includes a sample holder having a holder
 block 234. The block 234 is pivotally mounted to the wall 228, as by a
 bushing 236 or the like, to afford rotation about the axis 238. Thus, the
 axis 238 is orthogonal to the wall 228 and parallel to the walls 230, 232.
 The sample 28 is held to the front of the sample holder block 234 by the
 spring clip 240 which is held at the front of the sample holder block 234
 by the screw 242. The sample holder block 234 has a portion of its
 periphery formed as an arcuate segment of a worm wheel, as denoted by the
 reference numeral 244. The worm wheel arcuate segment 244 is centered at
 the rotation axis 238.
 To effect rotation of the sample holder block 234, there is provided a worm
 gear 246 mounted to the walls 230, 232 for rotation without longitudinal
 motion. The worm gear 246 is parallel to the wall 228 and is intermeshed
 with the worm wheel arcuate segment 244. An end portion 248 of the worm
 gear 246 is accessible at the wall 230 and is formed to be complementary
 to the screwdriver 96 so that manipulation of the screwdriver 96 results
 in rotation of the worm gear 246 and subsequent rotation of the sample
 holder block 234 about the rotation axis 238.
 The alignment device 220 also includes a leaf spring 250 secured at one end
 to the wall 232, as by a screw 252 or the like. The other end of the leaf
 spring 250 bears against the sample holder block 234 to maintain a tight
 engagement between the worm wheel arcuate segment 244 and the worm gear
 246, to limit their relative freedom of movement.
 Use of the alignment device 220 is the same as use of the alignment device
 26 and no further explanation thereof is necessary.
 Accordingly, there has been disclosed improved apparatus for holding and
 aligning a scanning electron microscope sample. While several illustrative
 embodiments of the present invention have been disclosed herein, it is
 understood that various modifications and adaptations to the disclosed
 embodiments are possible, and it is intended that this invention be
 limited only by the scope of the appended claims.