Patent GB-951256-A (GB)
Title: Improvements in and relating to the treatment of bodies
IPC: H01L21/00
Filed: 1960-05-26
Abstract: 951,256. Measuring thickness electrically. PHILCO CORPORATION. May 26,1960 [May 26, 1959], No.18652/60. Heading G1N. An apparatus for altering the thickness of a selected region of a solid body comprises means for developing an electrical quantity representative of the initial thickness, means for storing this quantity, and means responsive to the stored quantity for altering the thickness as a function of the quantity. In the arrangement described for reducing selected regions of a series of germanium wafers to a predetermined thickness by electrolytic etching each wafer 17 carried on a belt 7 (Fig.1) is moved between a pair of sensing prongs held apart by the oval shaft 20. In response to operation of switch 18 the shafts of motors 19 and 10 rotate through 90 degrees and 180 degrees respectively to connect capacitor 28 to terminals 24, 25 and to allow the sensing prongs to grip the wafer. The prongs form part of a thickness sensing circuit 22 which produces a direct voltage the polarity and magnitude of which correspond to the sense and extent of deviation from a predetermined thickness. A direct voltage corresponding to the predetermined thickness is set up in circuit 23 and added to the output of circuit 22 to produce a voltage proportional to thickness which is fed to capacitor 28. On arrival of the wafer at the next operating station (Fig.2) switch 41 operates to rotate the shaft of motor 10 to a further 180 degrees so that capacitor 28 is connected to contacts of relay 61 (Fig.3). Relay 45 is momentarily energized via the contacts of relay 42 and completes a latching circuit for itself and energizing circuits for relays 82 and 79 via contacts 47,49 and the contacts 77 of polarised relay 75. Relays 52 and 79 operate to turn on the jet of etchant 35 and to illuminate the region to be etched via lamp 32. After a time delay set by components C 1 , R 1  relay 54 is energized and through the intermediary of relay 57 applies a potential between the jet of etchant 35 and a base tab 12 on the wafer to initiate the etching process. At the same time relay 61 operates to connect capacitor 28 across the H.T. supply, between the grids of a pair of matched amplifying valves V 1 , V 2 . The circuit is preset by potentiometer 76 so that the currents in valves V 3 , V 4  are unbalanced to operate polarised relay 75 when the grid of V 1  is at the same potential as or more negative than that of V 2 . Consequently relay 75 remains energized when relay 61 operates and only releases when capacitor 28 has discharged and starts to recharge in the opposite sense. When relay 61 releases after a time proportional to the original voltage on capacitor 28 the circuits of relays 45, 52, 79 and 57 are opening to terminate the etching process. The sensing circuit 22 comprises two differential transformers which are fed with a 5 kc./s. current. Cores coupled to the probes move to alter the primary-secondary coupling. The two outputs are amplified, converted to direct current and combined. The transformers are adjusted to give a zero output for the desired wafer thickness. A signal representative of such a thickness is added to the combined output.