Patent GB-809644-A (GB)
Title: Improvements in or relating to the manufacture of semi-conductor bodies
IPC: C04B41/52
Filed: 1956-11-29
Abstract: <PICT:0809644/III/1> A body of silicon semi-conductive material is heated in an oxidizing atmosphere to form a passivating oxide film on the surface at least 1000 Angstroms thick, and before, after or simultaneously with this treatment it is heated in an atmosphere containing a conductivity-type determining impurity to diffuse the latter into the body.  The presence of the oxide film is said to prevent erosion and pitting of the surface at high temperatures. When the impurity is applied first this is done in a non-oxidizing atmosphere and at a lower temperature to give a shallow layer; further heating under oxidizing conditions causes the impurity to diffuse into the silicon.  Diffusion over selected areas only may be effected by removing the oxide film in these areas and choosing an impurity to which the film is relatively impervious.  The preferred oxidizing gas is water-vapour in small quantities in a carrier gas such as hydrogen or nitrogen; oxygen itself may, however, be used alone or with nitrogen or helium as carrier.  In Fig. 1 hydrogen enters at the left and bubbles through water held at a controlled temperature (e.g. 25-75 DEG  C.) so that there will be a known partial pressure of water in the issuing mixture.  A silicon wafer 14 previously etched in the usual way is mounted in the tube 11 and heated by coils 16.  Introduction of the impurity is carried out in a similar apparatus (Fig. 2, not shown) in which a heated boat containing a relatively volatile compound of the element in question is placed in the left-hand portion of the tube 11.  The gas entering the tube may or may not be oxidizing depending on the order of operations.  At a temperature of 1100-1400 DEG  C. and a duration of 10 minutes or more (preferably 30 minutes) a film mainly of silicon dioxide is formed of 1500-3500   in thickness.  In examples: (1) the film is made first and hydrogen bubbled through water, passed over heated gallium oxide and then over the wafer, giving a P-N-P structure; (2) an N-P-N body is made by using hydrogen or nitrogen which has bubbled through water and then passed over heated antimony or its oxide; the film is made simultaneously with the deposition of the impurity; (3) the silicon wafer is oxidized, one side covered with wax, the other etched and the wax removed; treatment with gas containing water and arsenic trioxide causes a penetration which is greater on the side from which the oxide film has been stripped; (4) in the method of (3) the wax is removed in strips so that the result is heavily N-type in these strips only; a further treatment with gas containing water and gallium oxide converts to P-type the portions between and beneath these strips since diffusion of gallium is unaffected by the presence of the oxide film; the centre of the wafer remains in its original N-type; (5) as in (4) with the removal of the film completely from one side, the result being P-N-P-N; (6) as in (3) using oxygen instead of water; the arsenic trioxide may be replaced by boron trioxide; (7) dry nitrogen containing arsenic trioxide is passed over P-type silicon at a relatively low temperature (1000 DEG  C.) to give a shallow layer; further heating in an oxidising film causes penetration of the arsenic and the oxide film is finally removed with hydrofluoric acid; (8) as in (7) the film being stripped off one side only and successive low temperature treatments given with arsenic trioxide and boron trioxide; a final heating in an oxidizing atmosphere causes diffusion, the boron penetrating deeper than the arsenic to form a P-N-P-N structure.  The process may be used in the manufacture of solar cells (see Specification 782,663, [Group XXXVI]), the rectifiers of Specification 782,664, [Group XXXVI], and the transistors of Specifications 809,642 and 809,643, [both in Group XXXVI].