Patent GB-1294477-A (GB)
Title: Improvements relating to apparatus and methods for measuring the spreading resistance of a specimen
IPC: H01L21/66
Filed: 1969-12-23
Abstract: 1294477 Measuring semi-conductor resistivity INTERNATIONAL BUSINESS MACHINES CORP 23 Dec 1969 [26 Dec 1968] 62507/69 Heading G1U [Also in Division H2] In order to determine the spreading resistance or apparent resistance, of a semi-conductor specimen, two probes are brought into contact with the specimen, a third, 'spreading resistance' probe is brought into contact with it thereafter (when non reproducible disturbances caused by the first two probes have ceased), a current of known magnitude is applied to the specimen via one of the first two probes and the third probe and the voltage between the other of the first two probes and the third probe is measured. The spreading resistance is determined in accordance with this voltage and the current. The specimen 31, which may be a semi-conduct- or wafer containing PN junctions wherein the spreading resistance of the several semi-conductor layers is to be determined, is supported on a base 32, which can be adjusted in two horizontal directions at right angles and in the vertical direction. The three probes 28, 29, and 30, 29 being the spreading resistance probe, are supported by probe arms 25, 26, 27 which can be relatively positioned by assemblies 18, 19, 20, have adjustable weighting and can be raised and lowered relative to the specimen by vertical shafts 80, 81, 82 The shafts are operable by a cam plate assembly 84, to lower the probes onto the specimen in the required sequence, the 'spreading resistance' probe being applied last to the specimen. The apparatus has a metal lid 140 which may be closed during a test to exclude light, stray fields and acoustic noises, and the adjustment control (71) have a lid which may be closed during a test to prevent accidental adjustment of the position of the specimen. To enable a specimen to be tested, the handle 123, which is locked to the cam plate, is rotated to lift the probes and latched against spring action. This allows the lids to be opened, and the specimen placed and adjusted. When push button 103 is operated the latch is released and the spring action rotates the cam plate against the action of a dashpot so that the probes are gradually lowered onto the specimen. The cam plate control arm actuates a microswitch 119 when the probes are fully lowered, to initiate a test sequence. The microswitch 119 operates a switch 121 to enable capacitor 157 to discharge through relay 159 This momentarily closes the contacts 161, 162 to complete the supply circuit via contacts 164, 144, (closed by closing the lid) to the motor 163, which drives the assembly of cams 167-175. The first of these, 167, maintains the contacts 161, 162 closed throughout a test. Cam 175 operates the contacts 190, 184 to reverse the current supply to probes 28, 29 in each quarter cycle of a test cycle. The contact 176 is operated during each quarter cycle to remove a short circuit from the current source 48 so that current can flow through the specimen in opposite directions in both half cycles. The switches 193, 196 are operated so that the digital voltmeter 46 is coupled across the potentiometer 187 whose resistance is standardised, to indicate current to the specimen in the first half cycle, and between the probes 29, 30 in the second half cycle, to indicate a voltage related to spreading resistance. The voltage measurements are taken for each direction of current flow and the mean value determined. The two measurements should be roughly the same. A switch 199 actuates the digital voltmeter at an appropriate time in each quarter cycle, while switch 204 similarly activates a coupler 202 to a card punch 203, operating at a different time to the voltmeter to avoid noise interaction Switch 213 alternatively activates lamps 211, 212 to show a test is proceeding. The spreading resistance is determined from the mean of the observed voltage and current values and is carried out for a number of specimen positions by adjusting the positioning control 71. It is said that provided the probe 30 is more than 10 times the diameter of the spreading resistance probe 29, 99% of the effective spreading resistance will be directly under the probe 29. The force applied by the probes must be sufficient to make good contact but not to penetrate the layer whose resistance is to be determined. The force may be adjusted for different materials by adjusting the probe weights.