July 10, 1962 B, MANNING 3,043,667 PRODUCTION OF ULTRA-PURE SELICON OR GERMANIUM Filed Oct. 31, 1960 3 Sheets-Sheet N 220 eaded sao 162 o o F.G. 6 3. 1. | 6ao.4%726/24a/ s. 77%WG/ ape/a/ay1 9/4 /64 Gaa. 404A2/V FIG. I azawad "WiiW4 B%0/.-- 2.2 77(42.45 Oag A/AX-S July 10, 1962 B. MANNING 3,043,667 PRODUCTION OF ULTRA-PURE SILICON OR GERMANIUM Filed Oct. 31, 1960 3. Sheets-Sheet 2 in SS INVENTOR, ABAA/WAAA M/4///////4 2AMWAS July 10, 1962 B, MANNING 3,043,667 PRODUCTION OF ULTRA-PURE SILICON OR GERMANIUM Filed Oct. 3l, 1960 3. Sheets-Sheet 3 2 4. 8 as7 s i.\ \ INVENTOR. AtAA/VAAA) M/4//W/WG B 'cC-4- 2. 92 - . Z 775"/0231. a Maas 3,043,667 United States Patent Office Patiented July 10, 1962 2 adsorbing impurities from the tetrahalide by passing a 3,043,667 solution thereof through purified silica gel, recovering PRODUCTION OF ULTRA-PURE SELICON the tetrahalide from the solution, Zone purifying the OR GERMANUM tetrahalide by passing a mass thereof slowly past alternate Bernard Manning, Waltham, Mass, assignor to the melting and cooling zones, and reducing the tetrahalide to United States of America as represented by the Secre elemental silicon or germanium by passing hydrogen gas tary of the Air Force into a retort containing the halide heated to an elevated Filed Oct. 3, 1960, Ser. No. 66,369 temperature, said gas and halide vapor passing into a 2 Claims. (C. 23-223.5) column having freely suspended therein a silicon or This invention relates to a process for the preparation 10 germanium crystalline body. The lower portion of the of semiconductor materials and more particularly to a body is heated electronically by induction to the reduc process for effecting the production of ultra-pure ele tion temperature of said halide with the result that the mental metals having semiconductor characteristics. An reduced, ultra-pure metal is deposited on said heated elemental metal having semiconductor characteristics is portion. understood to be a metal having the necessary electron 5 In a preferred embodiment, the halide is silicon or hole structure which enables it to be used in transistor germanium tetraiodide, the sublimation is conducted applications such as silicon and germanium. Specifically, under a vacuum at between 90° C. to 110° C., but pref this invention is concerned with a method of removing erably 100° C. The silicon body is heated to a tem impurities from the tetrahalides of silicon and germanium perature of between 600° C. to 850 C. The combina followed by reduction of the respective tetrahalide to 20 tion of these steps has been found to be especially ad obtain an ultra-pure elemental metal. vantageous in producing silicon or germanium of the Semiconductor metals to be suitable for transistor highest purity. applications must be extremely pure since even minute This invention may be better understood by reference quantities of impurities affect its electrical characteristics. to the accompanying drawings wherein like reference For example, transistor grade elemental metals now avail 25 numerals designate like elements throughout the several able are reported to be about 99.98 percent pure. Such views. purity is difficult to obtain and has made metals suitable FIGURE 1 is a flow chart; for transistor applications relatively expensive. FIGURE 2 is a front view of chromatographic ad Silica, having a minimum amount of certain impurities, sorption apparatus; is the usual starting material in the production of ultra 30 FIGURE 3 is a vertical cross-section of zone-purifi pure, transistor grade silicon. In general, the pure cation apparatus; silicon obtained heretofore has been prepared by reducing FIGURE 4 is a front view of alternative zone-purifi the selected silica to silicon, leaching the silicon with cation apparatus; acid, washing, converting to silicon tetrachloride, which FIGURE 5 is a vertical cross-section of reduction ap is then fractionally distilled and reconverted to silicon 35 paratus; and metal by the vapor phase reaction of silicon tetrachloride FIGURE 6 is the absorption spectra chart of a chloro and zinc. The purified silicon thereby produced is fur form silicon tetraiodide solution before and after chroma ther purified by melting, crystal pulling, and single crys tography. tal growth techniques. Recently a combination of float Referring to the drawings, FIGURE 1 illustrates the ing zone purification and reaction with water vapor 40 steps of the preferred embodiment of the present inven have been used to further purify the silicon. Necessarily, tion. Steps 1, 2 and 3 are well known in the art and are techniques which involve melting of silicon metal must included herein only to illustrate the preparation of the be carried out at temperatures above about 1430' C. silicon tetraiodide. The iodide thus produced is quite These temperatures are sufficiently high to volatilize impure, containing other metallic iodides, silica, and is minute portions of the oven components and to leach 45 discolored with iodine which remains with the product containers. Thus the purity of the final product is ad despite distillation. It is to be understood that although versely effected due to the resultant introduction of unde the discussion of this invention, presented in detail here sired impurities. Further, such techniques are expensive inafter, is limited to the purification of silicon, the process and are difficult to carry out and control. So disclosed is equally applicable to the purification of It is the principal object of this invention to circumvent 50 germanium. the above described limitations of the prior art by pro The major portion of the impurities, referred to here viding a novel method for the production of ultra-pure, tofore, are removed by vacuum sublimation, step 4 of transistor grade metals. FIGURE 1. This step can be carried out in any suitable A further object of this invention is to provide a method apparatus and has been successfully performed in a of producing ultra-pure transistor metals which does not 55 vacuum desiccator and in simple test tube type sublimers. require melting the metal at high temperatures. The maximum yield with the minimum impurity, occurs A still further object of this invention is to provide a when sublimation is conducted at 100° C. at a pressure purification method which is relatively simple to carry not exceeding 0.4 mm. mercury. The silicon tetraiodide out and control. should be finely crushed and evenly spread in a thin A still further object of this invention is to provide a layer so that the more volatile impurities such as iodine method of producing ultra-pure silicon and germanium 60 are initially removed. Large particles and sample masses suitable for use in transistor applications. permit these impurities to sublime continuously with the Another object of this invention is to provide a method silicon tetraiodide which are then partially reabsorbed in of purifying the tetrahalides of silicon and germanium. the silicon Sublimate. For the same reason high vacuum Still another object of this invention is to provide an pumping rates are more effective than slower rates. The economical method of producing ultra-pure silicon and Surface on which the sublimate condenses should be germanium which lends itself to the use of simple chemi heated to about 60° C. to yield a purer product in larger cal apparatus. crystals. In accordance with this invention, it has been found By this Sublimation technique, crude silicon tetraiodide that improved silicon and germanium purification is containing so much iodine that a portion of the material achieved by a method which comprises the steps of 70 was liquid has been rendered colorless. A solid residue, vacuum subliming the respective tetrahalide, selectively principally silica, is left after sublimation which is either 3,048,687 3 4. white or colored depending on the purity of the starting of 400° C. in a quartz container to dehydrate the sample material. Distilled silicon tetraiodide gives a fluffy, white thus producing a purified silica gel. silica residue. The condensing surface of the sublimer Referring again to FIGURE 1, step 6 in the process may also be advantageously seeded by leaving a few crys comprises a zone-purification of the previously purified tals from the previous sublimation. If the condensing silicon tetraiodide at the melting point of 120 C. Ap surface is clear and contains only a few seed crystals, paratus suitable for this purpose is illustrated in FIGURE crystals as large as 0.7 cm. on a side will form. As will 3 and comprises a tube 40 around which are coiled an be hereinafter shown, vacuum sublimation is an important alternate series of heaters 42 and coolers 44. The sample step in this purification process. of partially purified iodide is placed in a quartz, tube 46 The next purification step, step 5 of FIGURE 1, com 0. and sealed in a Pyrex container 48 containing an inert prises chromatography or selective adsorption of further atmosphere. The container 48 is suspended by a thread impurities from the sublimed silicon tetraiodide. Appara 50, preferably quartz, and is thereby slowly drawn up tus suitable for this process is illustrated in FIGURE 2, through the tube 40. As the solid iodide sample passes This apparatus comprises a boiling pot 20 having three the heating elements 42, a band of sample is melted and openings, a vapor delivery tube 22 insulated with an 5 then resolidified as it reaches and passes the cooling ele asbestos wrapping, a condenser 24, a reservoir 26 filled ments 44. As the sample is slowly drawn up through the with silicon tetraiodide, a column 28 filled with purified tube 40, liquid layers 52 slowly proceed to the bottom of silica gel 30, and a gas purging system comprising an the tube 46 carrying and concentrating the impurities at inlet 32 and an outlet 34.