Patent GB-1228754-A (GB)
IPC: H01L21/00
Filed: 1967-05-26
Abstract: 1,228,754. Semi-conductor devices. ASSOCIATED SEMICONDUCTOR MFS. Ltd. 16 May, 1968 [26 May, 1967], No. 24762/67. Heading H1K. In the manufacture of a transistor the N+ (or P +) type emitter region 32 is initially provided at a surface 23 of an N(P) type body 22, and the P(N) type base region 31 is subsequently formed by ion implantation into the surface 23 through the region 32. The annealing step required after implantation to reduce the damage to the crystal lattice caused by ion bombardment may be carried out at a sufficiently low temperature to eliminate the possibility of the " emitter dip " effect. In the Si transistor illustrated the emitter region 32 is produced by diffusion of phosphorus derived from phosphine through an oxide mask into an N-type epitaxial layer 22 on an N+ type substrate 21. The oxide mask is then removed over the area to be impregnated with ions and a fresh thin oxide coating 29 is deposited and densified by heating. An aluminium layer is evaporated over this coating, and is shaped by etching to provide a mask which overlies only the remnants of the first oxide mask 24. Boron ions are then implanted to form the base region 31. An R.F. ion source fed with boron trichloride is used, the beam being analysed magnetically to filter off unwanted contaminant ions. Ion energies increasing or decreasing in the range 10-130 kev are used, the axis of incidence of the beam being arranged to be within 7 degrees of a (111) axis of the crystal. After implantation the remaining aluminium mask is removed and the semi-conductor body is annealed for 30 mins. at 600-800‹ C. in dry N 2 . This causes a small adjustment of the junctions. Aluminium electrodes 36, 38 are then applied in a conventional manner. In the embodiment of Figs. 9-12 (not shown) a relatively thin oxide layer (65) is formed in an aperture in a thicker oxide layer (64) over the area ultimately to be implanted with ions. A small aperture (66) is opened through the layer (65) and the emitter region is diffused therethrough, a thin phosphosilicate glass coating (69) simultaneously forming in the aperture (66). The aluminium mask (70) is then applied as described above, and ion implantation and annealing are carried out to form the base region (72). An anodic oxidation step may be included prior to implantation to render the insulating covering of the aperture (66) as thick as the remainder of the oxide layer (65), and thereby to ensure a uniform thickness for the base region. The initial formation of the emitter region may, according to a further alternative, be by implantation of phosphorus ions using phosphorus trichloride as the source. The zone to be thus implanted may first be rendered amorphous by bombardment with ions which do not affect conductivity, thus minimizing channeling of the phosphorus ions. After phosphorus implantation through an aluminium mask, but prior to implantation of the boron ions to form the base region, 30 min. annealing stage at 900‹ C. is performed. Prior art methods are described in which the base region is first produced by diffusion or ion implantation and the emitter region is then formed by ion implantation. The subsequent high temperature anneal necessary to reduce damage in the emitter region produces the undesirable " emitter dip " effect. Ge and GaAs are also referred to as suitable semi-conductor materials.