Method of providing levelling and focusing adjustments on a semiconductor wafer

An adjustment method for photolithography. First, a semiconductor wafer is successively set in one of cell positions. In each of the cell positions, a laser beam is directed to the surface of the wafer and light reflecting off the wafer surface is detected and analyzed to determine a vertical offset position of the wafer at each cell position. Data representing the vertical offset position is stored in a memory and the process is repeated until the offset position data are derived from all cell positions. Thereafter, tilt angles of the wafer at all cell positions are determined from the stored offset position data, and angle data representing the determined tilt angles are stored in a memory. The wafer is then set in one of the cell positions, and the angle data is read from the memory corresponding to the set cell position and the wafer surface is horizontally aligned. The offset position data is read from the memory corresponding to the set cell position and the wafer surface is vertically moved to the focal point, and the process is repeated until the wafer is set to all cell positions.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates generally to photolithography for integrated
 circuits and more specifically to a method of providing levelling and
 focusing adjustments on a semiconductor wafer and a photolithography
 apparatus using the method.
 2. Description of the Related Art
 In a known photolithography process, a chip pattern is printed in each of a
 plurality of rectangular cells defined on a semiconductor wafer. Since the
 wafer is of a circular shape, those cells located at or near the periphery
 of the wafer may be left unexposed to the light of chip pattern. However,
 if a positive photoresist is applied to the wafer, the unexposed portion
 of the photoresist survives after the exposed portion is etched away by a
 solution. For this reason, the peripheral region of the wafer that is
 coated with a positive photoresist is also exposed to the chip pattern.
 However, due to the inherent curvature of the wafer near its
 circumference, the light modulated by the chip pattern is not precisely
 focused on the peripheral cells, causing some of the peripheral region of
 the positive photoresist to be left over as undesired remnant material.
 According to another photolithography process, several chip patterns are
 printed on a single mask (or reticle) and the wafer is exposed to all chip
 patterns of the mask. According to this technique, only those cells where
 chip patterns are appropriately printed are shipped as marketable products
 and others are discarded. However, the problem is that the peripheral
 cells are not exposed to precisely focused light.
 Therefore, a need does exist to precisely control the surface of a wafer at
 all cell locations for appropriate levelling and focusing purposes.
 SUMMARY OF THE INVENTION
 It is therefore an object of the present invention to provide an adjustment
 method for a semiconductor wafer which precisely aligns the surface of the
 wafer at all cell locations and precisely brings the wafer surface to the
 focal point of impinging light.
 According to one aspect of the present invention, there is provided an
 adjustment method for a semiconductor wafer. The method comprises the
 steps of (a) setting the wafer in one of a plurality of cell positions,
 (b) directing a laser beam to the surface of the wafer and detecting light
 reflecting off the surface of the wafer, (c) analyzing the detected light
 and determining therefrom a vertical offset position of the wafer and
 storing offset position data representing the vertical offset position
 into a first memory, (d) repeating steps (a) to (c) until a plurality of
 the offset position data are stored in the first memory, determining a
 plurality of tilt angles of the wafer at the plurality of cell positions
 from the stored plurality of offset position data, and storing a plurality
 of angle data representing the determined tilt angles in a second memory,
 (e) setting the wafer in one of a plurality of cell positions, (f) reading
 the angle data from the second memory corresponding to the set cell
 position and horizontally aligning the wafer surface according to the read
 angle data, and (g) reading the offset data from the first memory
 corresponding to the set cell position and vertically moving the wafer
 surface to the focal point according to the read offset position data.
 Steps (e) to (g) are repeated until the wafer is set to all cell
 positions.
 According to a second aspect, the present invention provides an adjustment
 method for a semiconductor wafer, comprising the steps of (a) setting the
 wafer in one of a plurality of cell positions, (b) directing a laser beam
 to the surface of the wafer at the one cell position and detecting light
 reflecting off the surface of the wafer, (c) analyzing the detected light
 and determining therefrom a vertical offset position of the wafer at the
 one cell position and storing offset position data representing the
 vertical offset position into a first memory, (d) repeating steps (a) to
 (c) until a plurality of the offset position data are stored in the first
 memory, determining, from the stored plurality of offset position data, a
 plurality of tilt angles of the wafer at a plurality of cell positions
 close to the periphery of the wafer, and storing a plurality of angle data
 representing the determined tilt angles in a second memory, (e) setting
 the wafer in one of a plurality of cell positions, and (f) determining
 whether or not the set cell location is close to the periphery of the
 wafer. At step (g), if the set cell location is not close to the
 periphery, the angle data is read from the second memory corresponding to
 the set cell position and the wafer surface is horizontally aligned
 according to the read angle data, and the offset data is read from the
 first memory corresponding to the set cell position and the wafer surface
 is vertically moved to the focal point according to the read offset
 position data. At step (h), if the set cell location is close to the
 periphery, a plurality of laser beams is directed to the surface of the
 set cell position of the wafer and beams reflecting off the surface are
 detected, the detected beams are then analyzed to determine a plurality of
 vertical offsets of the set cell position and a tilt angle of the wafer at
 the set cell position is determined, and the wafer surface is horizontally
 aligned according to the determined tilt angle and the wafer surface is
 vertically moved to the focal point according to one of the vertical
 offsets. Steps (e) to (h) are repeated until the wafer is set to all cell
 positions.
 According a third aspect, the present invention provides a photolithography
 apparatus comprising a mask support system for supporting a mask having a
 chip pattern and imagewise-modulating incident light with the chip
 pattern, a projection lens system for projecting the modulated light onto
 a focal point, a wafer support system for adjustably and horizontally
 movably supporting a semiconductor wafer, the system being controlled to
 successively set the wafer in one of a plurality of cell positions, a
 position sensing system for directing a laser beam to the surface of the
 wafer at each of the cell positions and detecting light reflecting off the
 surface of the wafer, position analyzer circuitry for successively
 analyzing a signal representing the reflecting light of each the cell
 positions and determining therefrom a plurality of vertical offset
 positions of the wafer at the plurality of cell positions, and angle
 calculation circuitry for determining a plurality of tilt angles of the
 wafer at the cell positions from the plurality of vertical offset
 positions. Control circuitry is provided for controlling the wafer support
 system to successively set the wafer in one of the cell positions,
 horizontally aligning, at each cell position, the surface of the wafer
 according to the tilt angle determined for the cell position and
 vertically moving, at each cell position, the surface of the wafer to the
 focal point according to the vertical offset position determined for the
 cell position.
 According to a fourth aspect, the present invention provides a
 photolithography apparatus comprising a mask support system for
 horizontally movably supporting a mask having a chip pattern and
 imagewise-modulating incident light with the chip pattern, a shield member
 having a slit for allowing a portion of the modulated light to pass
 therethrough, a projection lens system for projecting the light passing
 through the slit onto a focal point, a wafer support system for adjustably
 and horizontally movably supporting a semiconductor wafer, the system
 being controlled to successively set the wafer in one of a plurality of
 cell positions, a position sensing system for directing a laser beam to
 the surface of the wafer at each of the cell positions and detecting light
 reflecting off the surface of the wafer, position analyzer circuitry for
 analyzing a signal representing the reflecting light of each the cell
 position and determining therefrom a plurality of vertical offset
 positions of the wafer at the plurality of cell positions, and angle
 calculation circuitry for determining a plurality of tilt angles of the
 wafer at the cell positions from the plurality of vertical offset
 positions. Control circuitry controls the wafer support system to
 successively set the wafer in one of the cell positions, horizontally
 aligns, at each cell position, the surface of the wafer according to the
 tilt angle determined for the cell position, vertically moves, at each
 cell position, the surface of the wafer to the focal point according to
 the vertical offset position determined for the cell position, and
 controls the wafer support system and the mask support system to
 simultaneously move the wafer and the mask in opposite horizontal
 directions each time the wafer is set in each cell position to linearly
 scan the set cell position of the wafer with the light focused by the lens
 system.

DETAILED DESCRIPTION
 Referring now to FIG. 1, there is shown a scanning beam photolithography
 apparatus according to the present invention. The apparatus includes a
 light source 100, which produces sufficient optical energy for creating
 chip patterns on the surface of a semiconductor wafer. Light rays from
 source 100 are focused through a condenser lens 101 and reflected off the
 surface of a mirror 102 and directed downwards through a condenser lens
 103 to illuminate the surface of a mask (or reticle) 105.
 Mask 105, which is printed with a chip pattern, is firmly secured to the
 lower surface of a transparent mask support member 104. Mask support
 member 104 is movable in a direction parallel to the X-axis of the X-Y
 coordinate plane of the apparatus by means of an actuator 120 when a wafer
 is exposed to light through the chip pattern.
 Below the mask 105 is a stationary opaque member 106 having a slit 107 that
 extends in a direction normal to the direction of movement of the mask
 105. In the illustrated embodiment, the slit 107 extends along the Y-axis.
 The light rays incident on the mask 105 are modulated imagewise by the
 chip pattern and a portion of the modulated light is allowed to pass
 through the slit 107 and focused by a projection lens system 108 into a
 thin line beam. The optical axis of the projection lens system 108 is
 normal to the X-Y plane, and so it serves as the Z-axis of a
 three-dimensional space.
 A semiconductor wafer 109 is firmly mounted on a wafer support 110, which
 is secured to a base member 111 by means of a set of actuators 121, 122
 and 123 spaced 120 degrees apart from each other. Actuators 121 to 123 are
 individually operated to move the base member 110 in the direction of the
 Z-axis to adjust the tilt angle of the wafer 109 in order that the surface
 of each cell (or chip) of the wafer is aligned to the X-Y plane.
 An actuator 124 is provided to vertically move the base member 111 to bring
 the surface of the wafer 109 to the focal point of the line beam. Another
 function of the actuator 124 is to horizontally move the base member 111
 stepwise by a distance corresponding to one cell in the X and Y directions
 to set the wafer 109 in a desired cell position on the X-Y plane. With the
 wafer being set in the position of a desired cell, the actuator 124 is
 further operated simultaneously with the actuator 120 to move the wafer
 109 and the mask 105 in mutually opposite directions along the X-axis, so
 that the wafer is linearly scanned by the focused line beam and the chip
 pattern of the mask is printed within the boundary of the desired cell.
 A laser source 130 is provided to project a set of five laser beams at an
 angle to the surface of the wafer 109. As clearly shown in FIG. 2, the
 five laser beams are directed respectively to five spots on a cell, one at
 the center (L1) of the cell and the others at corner points (L2 to L5) of
 the cell.
 A photodetector 131 is mounted on a position opposite to the laser source
 130 to detect light beams reflecting off the wafer surface and supplies
 signals indicating the angles and intensities of the reflecting beams to a
 Z-position analyzer 132, where the signals are analyzed and the vertical
 offset positions of the individual spots L1 to L5 of a cell are determined
 with respect to the reference Z-axis position.
 The outputs of the Z-position analyzer 132 are received by a sequence
 controller 140. A Z-position memory 141 is connected to the sequence
 controller 140 to store offset position data of the individual cells of
 the wafer in a manner as will be described. Tilt angle calculation
 circuitry 142 is provided for calculating a tilt angle .theta. of each
 cell by using all offset position data supplied from the memory 141. The
 tilt angle determined for each cell is stored in an angle memory 143.
 Sequence controller 140 is programmed to sequentially control the various
 elements of the apparatus including light source 100 and laser source 130,
 and driver circuitry 150, 151 and 152, which serve as interfaces between
 the controller and the actuators 120 to 124.
 Wafer 109 is partitioned into a matrix of rows and columns to define
 rectangular cells 400 as shown in FIG. 4A and the location of the
 individual cell is represented by a pair of X-and Y-axis positions. This
 is achieved by operating the actuator 124 via driver 152. The wafer is
 first set in a predetermined X-Y position so that one of the cells is
 brought to the focal point of the projection lens system 108. The wafer is
 then moved to the next column and the process is repeated, so that all
 cells are traversed as indicated by the arrowheads beside a meandering
 dotted line 401 in FIG. 4A.
 According to a first embodiment of the present invention, only one of the
 vertical offset signals from the Z-position analyzer 132 is utilized. The
 operation of the sequence controller 140 of this embodiment proceeds
 according to the flowchart of FIG. 3.
 When a semiconductor wafer 109 is placed on the base member 110 and a mask
 105 is secured to the mask support member 104, the operation of the
 sequence controller 140 begins with step 201 in which the sequence
 controller operates the actuator 124 and sets the wafer in a desired X-Y
 cell position where a replica of the chip pattern will be printed.
 The routine proceeds to step 202 to energize the laser source 130 to emit
 laser beams to the wafer in the set cell position. Light reflecting off
 the wafer are analyzed by the Z-position analyzer 132 to produce data
 representing the vertical offsets of the cell at spots L1 to L5 are
 determined. Sequence controller 140 selects only one position data that is
 derived from the center spot L1 and stores the selected data into the
 position memory 141.
 At step 203, the sequence controller determines whether offset position
 data are derived from all cells and stored in the position memory 142. If
 not, the routine returns to step 200 to operate the actuator 124 again to
 set the wafer to the next cell position. In this way, a plurality of cells
 are defined on the surface of the wafer 109 and the individual Z-axis
 offset position data Z(.sub.xy) are derived from all cells and stored in
 the position memory 141. In FIG. 4b, the offset position data Z(.sub.xy)
 of all cells 400 are represented by vertical bars 402.
 When the offset position data of all cells are stored in the position
 memory 141, the routine proceeds from step 203 to step 204 to read all
 offset position data from the memory 141 and instructs the tilt angle
 calculation circuitry 142 to determine the tilt angles of all cells of the
 wafer from the read position data and store data representing the
 individual tilt angles into the angle memory 143.
 The routine then proceeds to step 205 to operate the actuator 124 to set
 the wafer in a predetermined X-Y position in a manner similar to step 200.
 At step 206, the controller reads tilt angle data from the angle memory
 143 that corresponds to a cell in the current X-Y position. By using the
 read angle data, the sequence controller 140 operates the actuators 121,
 122 and 123 that the surface of the cell is aligned to the horizontal
 reference plane and hence normal to the optical axis of the projection
 lens system 108.
 As shown in FIG. 5A on a rather exaggerated scale, the surface of the wafer
 109 may be warped so that its circumference is spaced a distance from the
 flat surface of the wafer support member 110. If this is the case, the
 approximate surface 500 of the cell 400 will be tilted at an angle .theta.
 to the reference horizontal plane 501 as shown in FIG. 5B. The angle
 adjustment at step 207 compensates for this angle error by tilting the
 wafer support so that the cell surface 500 is brought into alignment with
 the horizontal plane as shown in FIG. 5C.
 The routine then proceeds to step 208 to read the Z-axis offset position
 data from the position memory 141 that corresponds to the cell in the
 current X-Y position. By using the read offset position data, the sequence
 controller 140 operates the actuator 124 to move the base member 111
 according to the read position data so that the surface of the cell is
 brought to the focal point of the projection lens system 108 (step 209).
 At step 210, the sequence controller 140 energizes the light source 100 to
 focus an imagewise-modulated line beam onto the wafer surface and operates
 the actuators 120 and 124 to move the mask 105 and the wafer 109 in
 opposite directions along the X-axis. As a result, the focused line beam
 is scanned across the surface of the cell, printing the chip pattern of
 the mask in a reduced scale onto the cell.
 Decision step 211 is then executed to check to see if steps 205 to 210 are
 performed on all cells of the wafer. If not, the routine returns to step
 205 to repeat the process. Otherwise, the sequence controller proceeds to
 the end of the program.
 Since the direction of scan on the wafer surface is normal to the length of
 the slit 107 and hence to the length of the line beam, no out-of-focus
 condition occurs even if the wafer is warped about an axis parallel to the
 length of the line beam. In other words, since the slit 107 extends in the
 Y-axis direction, it is sufficient that the wafer surface is parallel with
 the slit 107 even if it is not fully parallel with the shield member 106.
 Therefore, the levelling is satisfactorily achieved by tilting the base
 member 110 about an axis parallel to the X-axis so that the wafer surface
 is parallel to the slit 107.
 A modified embodiment of the present invention is incorporated in the
 flowchart of FIG. 6, where the same reference numerals are used to
 designate steps corresponding in significance to those in FIG. 3.
 In the flowchart of FIG. 6, step 204 of FIG. 3 is modified as step 801, and
 steps 802, 803 and 804 are additionally provided.
 When the offset position data of all cells are stored in the position
 memory 141 (step 203), the routine proceeds to step 801 to read all offset
 position data from the position memory and instructs the tilt angle
 calculation circuitry 142 to determine the tilt angles of those cells
 located near the periphery of the wafer and store data representing these
 tilt angles into the angle memory 143.
 Following the execution of step 801, the wafer is set in a desired cell
 position at step 205. The routine then proceeds to decision step 802 to
 determine if the cell location is near the periphery of the wafer.
 If the location of the current cell is near the wafer periphery, the
 routine proceeds from step 802 to step 206 to read corresponding tilt
 angle data from the angle memory and the levelling is provided by using
 the read angle data, followed by steps 207 to 209 where the focusing is
 achieved by using the stored corresponding offset position data.
 If the current cell is not located near the wafer periphery, the decision
 at step 801 is negative, and the routine proceeds to step 803 to energize
 the laser source 130 to direct all laser beams to spots L1 to L5 of the
 cell. The detected laser beams are analyzed by the position analyzer 132
 to produce a plurality of vertical offset position data for the current
 cell. At step 804, the sequence controller 140 uses all the offset
 position data from the position analyzer 132 to determine the tilt angle
 and selects the vertical offset position of the cell at the center spot
 L1, and provides the levelling and focusing of the cell according to the
 currently determined tilt angle and to the selected vertical offset
 position. The routine proceeds from step 804 to step 210 for exposing the
 current cell to the scanned line beam.
 Since the number of cells for which angle and offset position calculations
 are determined is significantly smaller than the previous embodiment,
 large capacity memories are not required to hold position and angle data.
 In addition, since all data derived from the spots L1 to L5 are used for
 the cells located inwards of the wafer periphery, precision levelling and
 focusing operations can be achieved for the inner cells.