Patent GB-1081472-A (GB)
Title: Improvements in or relating to methods of providing separated metal layers side by side on a support
IPC: C23C14/04
Filed: 1965-04-15
Abstract: 1,081,472. Etching. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. April 15, 1965 [April 21, 1964], No. 16247/65. Heading B6J. [Also in Division H1] A first metal layer is applied to a support, an etching resist mask is applied to the layer, the metal is etched away where unmasked and also to a small distance beneath the mask by undercoating, and a second metal layer is deposited on the support where unmasked, so that the two metal layers are separated by a gap at the position of the undercut. A monocrystal germanium body 1, Fig. 1, of antimony-doped N- type material 2 has formed on its upper and under sides a P-type layer 3 by diffusion of indium. The upper side is covered with a resist lacquer and the P-type layer removed from the under surface by etching. The lacquer is removed, and arsenic diffused to form an N-type layer 4 on the upper side. An N-type layer of reduced resistivity is also formed on the under side. A thin silver layer 5 is deposited on the layer 4. An etch-resist lacquer is applied to the under side, and the layer 5 is covered with a photo-sensitive lacquer, which is locally illuminated over a series of circular areas 6, Fig. 2, except for a semi-circular area 7 in each area 6. The areas 6 are arranged in rows parallel to the length of the body 1. The photo-sensitive lacquer is treated with potassium hydroxide to remove the exposed portions. The body is then treated with nitric acid to remove the unmasked portions of the layer 5, and also the marginal portions beneath the mask by undercutting. A further etching is carried out with hydrofluoric acid and hydrogen peroxide to remove the layer 4 where not covered by the silver. After rinsing, drying and removing the lacquer from the underside of the body, first indium and then aluminium is deposited to form layers separated from the silver. The remainder of the lacquer 8 is removed with acetone, which also removes indium and aluminium deposited on the lacquer. The remaining indium and aluminium forms an alloy with the germanium. The assembly is used to form transistors, of which the portions (12), Fig. 5, of the layer 4 are the emitter regions and the layers (11) the base contact layers. The underside of the body is again covered with lacquer and the upper side with a photo-sensitive lacquer layer, which is then exposed except in the circular areas covering the layers (10) and the emitter regions (12), the exposed portions being removed with potassium hydroxide. The body is then treated with nitric acid to remove the uncovered portions of the silver layer 5 and with hydrofluoric acid and nitric acid to remove the unmasked portions of the layers 4 and 3 and some of the body 2. The body is then divided into separate transistors, and the layers (10) (silver) and (11) of each transistor are provided with connections in the form of gold wires (15, 16) (Figs. 7 and 8), the transistors being mounted on nickel tabs using gold as a solder. If the etching is prolonged to ensure the removal of all the layer 4 locally, then instead of indium and aluminium, the layers (11) may be formed by depositing a little indium and then silver. In another example (Figs. 9-19), an aluminium layer (22) is deposited on a glass plate (21) and covered with a photo-sensitive lacquer layer (23), and the lacquer is exposed and the exposed part removed with potassium hydroxide to leave portions shaped as shown at (25, 26, 27, 28, 29) (Fig. 12). The plate is then etched with orthophosphoric acid to remove the unmasked aluminium, and a marginal portion under the mask by undercutting. Chromium and then gold are then deposited, by use of a mask, in rectangular areas (40, 41, 42) each covering one of the narrow lacquer portions (29). Acetone is then used to remove the lacquer (25, 26, 27, 28, 29), the gold portions (42) being removed therewith. The remaining aluminium layer is shown at (35, 36, 37, 38, 39) and the gold at (40, 41) in Fig. 16. The aluminium is now subjected to an anodic electrolytic treatment in a bath of borax and boric acid with a contact attached to the strip (35) and a platinum electrode immersed in the bath, to form an aluminium oxide layer (50) on the aluminium (Fig. 17). Cadmium selenide is then deposited in square layers (51) (Figs. 18 and 19) covering part of the gold (40, 41) and the intermediate aluminium (39), leaving parts (60, 61) of the gold (40, 41) exposed. A plurality of field-effect transistors is thus formed in which the layers (40, 41) are the input and output electrodes and the strip (39) is the gate electrode, insulated from the semi-conductor cadmium selenide by the oxide layer (50). The exposed gold portions are used to make electrical connections to the source and drain electrodes, and the aluminium portions (37, 38) for connection of the gate electrode. The gold could be applied to the glass first, covered with a resist and etched, and the aluminium deposited, the etchant being potassium cyanide and hydrogen peroxide. The insulating layer (50) could be applied by evaporation.