Patent GB-1334128-A (GB)
Title: Method and apparatus for positioner calibration system
IPC: G05D3/20
Filed: 1971-08-17
Abstract: 1334128 Photo-electric positioning apparatus COMPUTERVISION CORP 17 Aug 1971 [24 Aug 1970] 38575/71 Heading H4D [Also in Division G1] In an apparatus for positioning a first object, e.g. a boundary area 38 on a semi-conductor wafer 32, with respect to two reference points, e.g. the edges 41 of an aperture 40 in a mark 36, using a light source/photo-cell arrangement 42/44 as a position sensor, the respective photocell output levels when the object is situated at two positions oppositely situated and outside the range between the two reference points are used as reference levels. In Fig. 5, the boundary 38 divides the semiconductor wafer 32 from a target area 34 on the wafer whose reflectivity differs from that of the wafer. The wafer is movable in the x-direction with respect to the fixed mask 36. The photocell 44 output as the wafer 32 moves from left to right is shown in Fig. 6. The plateau value 58 corresponds to the boundary 38 being located below the mask 36. The sloping region 62 corresponds to the boundary moving from beneath one edge 41 to beneath the opposite edge. At the plateau value 60 the boundary is beneath the mask 36 on the opposite side. To position the boundary a certain proportion P of the distance W between the two windows, a four part sequence is used.- (a) The proportion is set on a potential divider (96, Fig. 7, not shown) within a calibrator 64, and a reference input 72 to a comparator 68 is set to a value estimated to be the proportion P between the plateau values VH and VL, which at this stage are unknown. The comparator compares the reference level with the photo-cell output level 70 to provide an output 74 whose sign depends on the relative magnitude of those two levels. This output is passed by a sequencer 76 to a stepping motion means 78 which produces left-right motion of the wafer according to the sign of the comparator output. The drive ceases when the comparator output is zero, when the position of the wafer corresponds to the initial estimate of the desired proportion. (b) The sequencer provides a drive signal to the motion means so as to step the wafer a distance W in one direction, hence driving the boundary 38 beyond one edge 41 so that the photo-cell output assumes one of its plateau values. The value is stored by the calibrator (92). (c) The sequencer drives the wafer 2W steps in the opposite direction, so that the photo-cell output is in the other plateau. This value is also stored in the calibrator (94). (d) The estimated signal on line 72 is replaced by the correct value, based on the two measured plateau levels, derived from the pre-set potentiometer (96) in the calibrator. The wafer is then driven to the desired position. To prevent the wafer oscillating about its final position, the comparator output may also be supplied to a counter (134, Fig. 11, not shown) which isolates the motion means from the sequencer after a pre-set number of changes of sign of the comparator output. Fig. 13 shows circuitry for an arrangement which allows two-dimensional positioning of a wafer with respect to a mask. The wafer contains two square target areas 14, Fig. 1A. The mask contains two corresponding sets of apertures (16), Fig. 12 (not shown), each set comprising four apertures forming the sides of a square. When the wafer is correctly positioned the apertures are symmetrically placed with respect to the targets. At each of the two detecting stations, two photo-cells X+, X- monitor movement in the the x-direction. Each photo-cell monitors the movement of a target edge relative to its corresponding aperture, as previously described. The outputs of the four photo-cells monitoring xmovement are combined in X calibrator 158. The y-detector outputs from one station are combined in a Y calibrator 160, while those from the other station are combined in a # calibrator 162. Logic circuitry 154, 156 is provided so that the # sequencer 214, which controls a rotational stepping motor 220, is only actuated when the outputs of the Y 1  and Y 2  processors, 150 and 152, are of a different sign. This indicates a rotational misalignment, and under these conditions the logic circuitry provides no output to the Y sequencer 198. Similarly, when the Y 1  and Y 2  processor outputs have the same sign, the Y sequencer is rendered operative while # sequencer is not actuated. The four-part positioning sequence runs as follows:- (a) Estimated levels are set on the reference lines 164-168 and the X, Y and # motors are actuated by their appropriate sequencers until all the target edges are located at points estimated to be half-way between the aperture edges. (b) The wafer is driven a full aperture width along both x and y axes simultaneously. All the resulting plateau output levels are stored in the appropriate calibrators. (c) The wafer is driven two aperture widths in the opposite sense along both x and y axes. The resulting plateau values are stored.  (d) Using the newly-derived reference levels as reference inputs to the X, Y, and # processors, the wafer is driven so that the targets assume a symmetrical position with respect to the mask.