| 摘要 |
1,177,759. Semi-conductor devices. R.C.A. CORPORATION. 14 Aug., 1967 [13 Dec., 1966], No. 37331/67. Heading H1K. Known methods of diffusing dopant into selected regions of a semi-conductor wafer through windows in a masking layer provided on the surface of the wafer suffer from the disadvantage that in practice the masking layer contains pin hole defects so that undesired regions of the wafer are also doped. This disadvantage is overcome according to the invention by providing a masking layer on a surface of the wafer, forming windows over the regions to be doped, depositing a source of dopant on the masking layer and on the exposed surface portions of the wafer, selectively removing the source from the masking layer and from any pin holes present in the layer but not from the surface portions exposed by the windows, and then diffusing the dopant into the wafer. Preferably the source of dopant is deposited at a temperature sufficiently low to prevent the dopant from diffusing into the wafer, but if diffusion does occur at this stage the diffused regions of the wafer beneath the pin holes can be subsequently removed by etching. In a preferred embodiment a silicon wafer is provided with a thermally grown silicon dioxide masking layer about 11,000 A. deep, or a layer of silicon nitride and windows are formed in the layer by a photo-lithographic and etching process. The dopant source is then deposited, and this may comprise the dopant itself in elemental form (e.g. Al, As, Sb), an oxide of the dopant (e.g. B, P), a glass containing the dopant (e.g. borosilicate or phosphorous silicate glass) or silicon dioxide or nitride containing any of the previously mentioned dopants or Ga. The source may consist of a layer 2000 Š-5000 Š thick. The selective removal of the source is then performed with a photo-lithographic and etching process using a photo-mask which is the negative of the photomask used in the window forming process. Suitable etchants are hydrofluoric acid for boron or phosphorous oxide, and a mixture of 8 parts (by weight) of ammonium fluoride, 2 parts hydrofluoric acid and 15 parts deionized water which attacks the vapour deposited dopant-containing silicon dioxide layer about three times faster than the thermally grown silicon dioxide masking layer. If necessary, any diffused regions beneath the pin holes can be etched away with nitric acid containing 1% hydrofluoric acid. The wafer is finally heat treated to diffuse the dopant into the regions beneath the windows. Examples of the times and temperatures required for particular dopants are given in the specification. |
