Patent GB-1480562-A (GB)
Title: Detecting a registration mark on a target
IPC: G06T1/00
Filed: 1975-01-08
Abstract: 1480562 Detecting registration marks INTERNATIONAL BUSINESS MACHINES CORP 8 Jan 1975 [28 Jan 1974] 818/75 Heading G1A [Also in Divisions H3 and H5] The Specification describes a method of detecting the position on a target of a registration mark defined by at least one pair of edges located on the target, which may be a semi-conductor wafer and which is irradiated by an electron beam substantially as described in Specification No. 1438737. In the present invention, the registration mark is located by moving a beam of charged particles, e.g. electrons, over the area of the target having the registration mark in a plurality of line by line scans, including making a first scan over the area to cause a signal baseline voltage to be within a predetermined range, e.g.  0À5V, in accordance with the material of the area having the registration mark, making a second scan over the area of the target having the registration mark to ascertain the signal baseline voltage and the highest positive and negative peak signals produced by the beam passing over the first and second edges respectively of the registration mark, then selecting a percentage, of the positive peak signal in comparison with the signal baseline voltage detected on the second scan to produce a positive threshold signal for all scans after the second scan; and similarly selecting a percentage of the negative peak signal compared with the signal baseline voltage to produce a negative threshold signal for all scans after the second scan; then producing a first signal when one of the positive or negative threshold signals is crossed by a signal produced by the beam passing over one of the pair of edges of the registration mark during a third scan, producing a second signal when the other threshold signal is measurement, the first and second averaged signals in conjunction with the location of the beam durcrossed during the third scan, making a plurality of additional scans e.g., eighteen to average the ing the scan in which each of the first and second signals is produced to locate the position of the registration mark on the target. Thus, the threshold voltages are correlated to the peak signals to be expected from the registration mark in the specific area of the semi-conductor to be scanned by the beam, since varying surface levels on the wafer produce significantly varying signal amplitude levels. The beam scans in both X and Y directions on successive routines. The registration marks 42, Fig. 9, comprises X-scan marks 44 and Y-scan marks 43, and may be grooves or raised portions. Each registration mark 42, occupies the corner of four adjacent chips 40 Fig. 8 formed from overlapping fields 39. The reflected electrons are detected by PIN diodes 45, 46, Fig. 2 for X-scan, a further pair of diodes, and associated threshold position detection circuitry for Y-scan, not shown, are provided. Gain control 50 is initially set to a predetermined range for the particular material. On the first scan voltage comparators 65 and 66 detect the signal baseline voltage output from differential amplifier 49. If this is greater than plus or minus 0À5V respectively, AND gates 72, or 81 respectively are enabled to pass clockpulses from X-counter 73 to up/down counter 76 which counts into A to D converter 77 which produces an output on lines 78, and 79 to alter the gain of preamplifier 47 to bring it within the desired limits. On the second line scan, peak detectors 54 and 56 are enabled to store the positive and negative signal peaks as the beam passes the 1st and 2nd edges of each registration mark respectively. Sample and average circuit 55 has been reset during the 1st scan, and on the second scan averages the signal baseline voltage to produce an output 57 which is combined with outputs 59 and 61 from peak detectors 54 and 56 via potentiometers 58 and 60 respectively to feed positive and negative threshold voltages which are a percentage of the peak signals as set on potentiometers 58 and 60 to positive voltage comparator 52 and negative voltage comparator 53. These threshold voltages are thereby set in accordance with the level and surface condition of the material forming the registration mark 42, and are maintained during the subsequent eighteen scans of the X-scan cycle. The positive and negative output pulses denoting leading and following edges of the registration mark grooves 44, or 43 for Y-scans, from comparators 52 and 53 are passed via OR gate 127 to gate 130 which allows transmission of X-counter 73 clockpulses through feedback channel 131 to computer 19, which then determines the registration mark positions as an average value obtained from the eighteen scans and hence the X-coordinates of the chips 40 on wafer 41. The process is repeated for Y-coordinates using registration mark grooves 43.