US2419561A - Crystal contact of which one element is mainly silicon - Google Patents

Description

April 29, 19 7 D. E. JONES ET AL I CRYSTAL CONTACT OF WHICH ONE ELEMENT IS MAINLY SILICON Filed Aug. 10, 1942 a v I w. U M h ,7 v 3 FIG. I.

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Patented Apr. 29, 1947 CRYSTAL CQNTACT OF WHICH ONE ELEMENT IS MAINLY SILICON Douglas Eniield Jones, Harrow, Charles Eric Ransley, Sudbury, .lohn Walter Ryde, London W. 14, and Stanley Vaughan Williams, Kenton, England, assignors to-The General Electric Company Limited, London, England Application August 10, 1942; Serial No. 454,290 In Great Britain August 20; 1941 11 Claims.

This invention relates to electrical crystal contacti devices of the type comprising a semiconducting: crystal and a metal (usually a metal point) in contact with it. Such contacts can be used as rectifiers of alternating current or as mixers, that is to'say as non-linear impedances to which are applied an oscillation of frequency f1 and another oscillation-f. frequency f2, so that oscillations are produced having frequencies (pfiiqfz) where p and q are integers.

Objects of the invention are to provide improved-processes for the preparation of the'crystal in such contact devices, and crystal contact elements andv wave-signal translating devices embodying such crystals that are more uniform or more efi'icient or both than those of the said type ..know-n= hitherto. The uniformity aimed at means that theelectrical characteristic of the contact should be as nearly as possible independent of the position on the crystal of a metal point. forming the otherelement of the contact. Efiiciency generally requires a high ratio of impedance in one direction (reverse impedance) to impedance in the other direction (forward impedance). It may also require that the absolute values of the forwardimpedance should be high or, alternatively,that they should be low. It may also require that the forward direction should be that'in which positive charge flows from the crystal to the metal (positive contact) or that it should be that in which negative charge flows from the crystal to the metal (negative contact).

When-the-contact is used'as a mixer; efficiency requires-that the signal-to-noise ratio should be relatively high. Efficiency also requires that the performance of the" contact judged by any of theforegoing criteria should deteriorate as little as possible with use or with age.

Silicon is one of the semi-conductors most widely used in crystal contacts. that the said objects of the invention can be attained by introducing into substantially pure silicon a small proportion of. aluminium. or beryllium or bothof these metals which form asolid solutionwith silicon and have extremely stable oxides. theratio of the number of atoms of the additive metal to 1 the number of atoms of silicon in the product: is not greater thanl/50. Impurities, such as'iron, which are often present in commercial silicon are generally undesirable and shoulcl'be' removed before the additive metal is introduced.

Crystal contact elements-resulting from the herein disclosed" process are the'subl'ect matter We have found The term small implies thatofdivisional application Serial No. 736,0'94;-filed- March 20'; 1947, and entitled Crystal contacts of which one elementis mainly silicon.

If the starting material is the commercial silicon at present available, the introduction of: the-selected additive metal must be preceded b y'a; purification; This purification can be efiected by subjecting commercial silicon to the-process-- described by N; P. Tucker in the Journal of the Iron and Steel Institute, vol. CXV(1), p. 412-;- 1927. This process removes only those impurities that are not contained in thesilicon crystals; but occur as an additional-phase, usually in the interstices between the' si1icon -crystals.-- Commercial silicon is usually'prepared by-cooling a melt slowly; thenpractically all the deleterious impurities are in the interstices-andcan be "re-'- moved by Tucker-s process. If that-processfails with any sample of silicon, it isprobably because the silicon has beenquenched rapidly; then' the silicon should be re-melted and cooled slowly before it is I subjected to the Tucker process:

The additive metal is preferably introduced-"by melting 'a mixture-ofthe:pure"silicon and the additive. During'this process care must be taken not to introduce undesired impurities; Those that might be derived from the air "(asmal1 quantity of oxygen is not'necessarily deleterious) can be avoided by melting in'a suitable atmosphere; the most suitable we have found is avacuum, that is to say residual gas at-a pressure of not more" than 0.001 mm. of mercury. But it is difliculu or perhapsimpossible, to findany crucible to contain the melt that does not react slightly with the melted silicon. This difii-- culty can be overcome by'using a cru'cib1e"con'- sisting of or lined With pure berylliaq' forthen' the only impurity introduced from" the" crucible is beryllium,- which is not harmful but beneficial. It might be possible" to introduce from the crucible all the additive'metal" characteristic of the invention; but in view of" patent" application Serial No.446;3l9, filed'June 8, 1942, We exclude from the scopeof this invention any process-of. manufacturing the mainly-silicon element ofacrystal contact which comprises only the steps mentioned in the claims of the specification ofthat" application. If therefore the first stepin the 'processconsists in grinding relatively im pure silicon to a-fine powder and treating it with chemicalreagents soas to produce a relativelypure product, the process will not lie Withinthe scope of the present invention;unlesspartat leastofmthe additive metal is derived" from a' source other than a container in which th silicon is melted. But it is to be understood that the introduction of additive from the container is always to be taken into account when the amount of additive to be introduced from other sources is considered. Whether additive is or is not introduced from the container, that otherwise introduced may be aluminium or beryllium or a mixture of the two.

In order that the resulting crystals may have the best qualities, it is generally necessary and always desirable that, after the introduction of the additive, the crystals should be subjected to an appropriate treatment modifying their surface. One part of this process is a controlled oxidation; this part can be omitted only if the necessary oxidation has been effected during the melting or the cooling of themelt. We have occasionally succeeded in producing satisfactory results by this method; but it is difiicult to control, and we greatly prefer to remove any surface oxide from the cooled melt with hydrofiuoric acid and then to heat th product in a definitely oxidising atmosphere. Further We prefer not to attempt to produce the optimum surface oxidation by the heating alone; it is usually better to oXidise the surface somewhat more than is required and then to remove some (but not all) the layer of oxide with hydrofluoric acid.

Other surface treatments are also permissible and often desirable. Thus it is usually preferable to polish the fragments of the melt, or at least that part of them which is to contact with the metal point, before the controlled oxidation. Again, crystals are often soldered into metal capsules; then it is desirable to plate with metal that part of the crystal that is to lie in the capsule, in order that the solder may adhere to it.

One method of performing the invention will now be described by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows the completed crystal contact device;

Figure 2 shows a silicon fragment temporarily mounted in readiness for a step in the process of preparation; and

Figure 3 illustrates the step of mounting the same element in a capsule.

Referring to Figure 1, the completed crystal contact device consists of a ceramic tube In capped at its ends respectively by a metal capsule l I and a bush [2. The crystal body 13 is set in the capsule H by means of a low-melting alloy l4. One or more metallic contact elements [5 carried by a metal rod 16 co-operate with a prepared face I! of the crystal body, the rod l6 being held fast in the bush l2.

The starting material is commercial silicon which has been ground and passed through a 200 mesh sieve; the material contained the following metals in the following amounts reckoned as oxides: (.Al2O3-I-ZIO2) 1.01%, FezOs 0.96%, TiO'2 0.021%, MnO 0.08%, BaO 0.03%, CaO 1.13%, CuO 0.002%, MgO less than 0.002. In step 1 this material was purified by the aforesaid method described by Tucker. The resulting material had the following composition: (AlzOa+ZrOz) 0.03%, F820 less than 0.005%, T102 less than 0.0005%, MnO less than 0.001%, BaO less than 0.003%, CaO 0.008%, CuO 0.001%, MgO less than 0.002%, In step 2, the purified material is melted in vacuo (pressure less than 0.001 mm. of mercury) in a pure beryllia crucible, together with aluminium in an atomic proportion of 1 Al to 400 Si. The

. 4 beryllia was fired to 1400 0., ground and'washed with nitric acid before being made into a crucible.

In a further step 3 a fragment of the cooled melt has a plane part of its surface highly polished by any of the processes customary among metallurgists, for example grinding with emery of increasing fineness and finally polishing with a1umina or magnesia or both. In a further step 4 the whole surface of the fragment is then dipped for 5 minutes into commercially pure 40% hydrogen fluoride diluted with an equal quantity of water. The main purpose of this treatment with hydrofluoric acid is to prepare th polished surface for the subsequent oxidation; accordingly it is not necessary, though it is usually convenient, that the unpolished part of the surface should be clipped.

In a further step 5 the necessary surface oxidation is performed. For this purpose the body resulting from step 4 is placed on a flat silica tray (together possibly with other bodies at the same stage of preparation) and introduced into a silica tube furnace filled with air at a region thereof maintained at 1050 C. There is a definite optimum temperature for this treatment, the temperature should be observed, e. g. by a th'ermocouple, and kept as near to 1050 C. as possible. The body is maintained in the said region for 2 hours and then allowed to cool. An atmosphere of oxygen at a controlled pressure may be substituted for the air in the furnace; then the time of heating will depend on the pressure of the oxygen.

In a further step 6 the treatment given in step 4 is repeated. Now part of its object is to prepare the unpolished part for step '7; accordingly the unpolished part must be dipped. In the process now being described in detail the dipping of the polished part is equally necessary, in order that part of the oxide layer formed in step 5 should be removed; but, as explained heretofore, it might possibly not be in a modification of the process. After the clipping, the body is washed with water.

In a further step 7 a metallic layer is deposited on some or all of the unpolished surfaces. The layer must not be deposited On the polished surface; accordingly, as shown in Figure 2, this is first protected by coating the flat end of a rod is with a layer of adhesive l9, which may be the material known as monostyrene, and pressing the polished surface ll of the body l3 against this layer. The exposed unpolished surface is then coated with copper by a method described by Bedel in Comptes Rendus, vol. 192(1931) page 802. It consists merely in dipping the surface for 5 to 10 seconds into a solution of cuprous oxide in 20% hydrofluoric acid; the copper layer deposited may be subsequently thickened by electrolysis. molybdenum by this process; accordingly the rod [8 may be conveniently of tungsten or molybdenum. Ihe coated surface is then washed and dried. The body can then be mounted in the metal capsule ll by means of solder or some other suitable low-melting alloy intervening between the metal layer and the capsule. For this purpose the rod I8 is inserted in the tube l0 so that the crystal enters the molten alloy M in the capsule H, which is temporarily placed on an end of the tube. After the alloy has set, the rod [8 is detached from the body and the polished surface of the body cleaned from the adhesive by washing with a suitable solvent, such as benzene or ethyl acetate. The mounted crystal body is then ready Copper is not deposited on tungsten or to lthe :axis. .of the wire.

for final :assembly into :the structure-shown in "Figured.

:usually be obtained, but then the forward'im- L pe'dance isgreater. The contact will usually stand :without appreciable deterioration the passage zacross ityforl a period of to '60 secs, of acurrentlimiting currentyof 40 to 80-ma. Comparable average figuresi for the commercial silicon that was thesstarting, material, -polished as described "hereinbefore 1 but not treated otherwise, :are forward impedance l80 ohms, ratio of reverse toforward impedance 5, limiting current 5- ma.

If alowertforward impedance is required, the 5 pressure between crystal .and wire 'maybe increased to -50 gm. weight. A forward impedance in the neighbour-hood' of 100 ohms can then be obtained, but the averageratio of reverse to forward impedanceis likely to be 10-20, and the limiting current 30-60 ma.

If a high limiting current is not so important as th impedance characteristic, the aforesaid sheared tungsten point may be replaced by a fine tungsten point obtained by the well known process of etching with Langmuirs solution.

One advantage of crystals prepared according to the invention is that they are so uniform that it is often possible to locate two metal points (whiskers) in parallel on the same crystal without adjusting them separately, as shown in Figure 1, and yet to obtain in this way an improved characteristic. The improvement is usually not so much in the impedance as in the limiting current that the contact will stand without deterioration. No novelty is claimed for the bare suggestion that whiskers on the same crystal may be connected in parallel; but, so far as we are aware, no disclosure has been made before how crystals might be prepared that are at once of high effipractically useful.

We claim:

1. In the manufacture of an electrical crystal contact device of the kind in which the semiconducting crystal contact element is mainly silicon, the production of said silicon contact element by a process which includes the step of introducing into silicon that is substantially free from the metallic impurities usual in commercial silicon a determinate small quantity of at least one of the two metals aluminium and beryllium, which additive metal forms a solid solution with the silicon.

2. In the manufacture of an electrical crystalcontact device of the kind in which the semiconducting crystal contact element is mainly silicon, the production of said silicon contact element by a process which includes the step of introducing into silicon that is substantially free from the metallic impurities usual in commercial silicon, a quantity of at least one of the two metals aluminium and beryllium, said quantity being such that the ratio of the number of atoms of said additive metal to the number of atoms of-siliconsinithe pro duct isnot greater than 1 1/50.

*"3. In themanufacturemf an electrical crystal contact devicegof the kind i in which lthe semiconducting crystal contact element is mainly si1icon,,theproduction of saidsilicon'contact element by a process which includes the-steps vof adding to solid silicon that is substantially free from the metallic impurities usual in commercial silicon'a determinate small quantity-of atleast one of the two metalsaluminium and beryllium also in solid form, and thereafter meltingthemixture. b

4. In themanufacture of an electrical crystal zcontactndevice of the kind in :which the semiconducting crystal contact element is mainly ,silicon, the'production of said silicon contact element by a-process which includes the steps of adding to solid silicon that is substantially free from the metallic impurities usualin commercial silicon a 1 determinate small quantity of J at least .one of the 'two metalsaluminium and beryllium also insolid form, and thereafter :meltingthemixture ina container at least the interior surfacesof the walls of which are composed of pure'beryllia.

5. In the manufacture of an electrical crystal contactdevice of the kind in -whichthe semi- --conducting crystal contact element is mainly silicon, the production of saidsiliconcontact element'bya processiwhi'ch includes the steps of introducing into: silicon vthat. is substantially free from the metallic impurities usual in commercial silicon a determinate small quantity of at least one of the two metals aluminium and beryllium, which additive metal forms a solid solution with the silicon, and subjecting to a controlled process of surface oxidation the body of silicon into which ciency and also so uniform that the suggestion is said additive metal has been introduced.

6. In the manufacture of an electrical crystal contact device of the kind in which the semi-conducting crystal contact element is mainly silicon, the production of said silicon contact element by a process which includes the steps of introducing into silicon that is substantially free from the metallic impurities usual in commercial silicon a determinate small quantity of at least one of the two metals aluminium and beryllium, which additive metal forms a solid solution with the silicon, treating with hydrofluoric acid a surface of the body of silicon into which said additive metal has been introduced, thereafter heating said body in an oxidising atmosphere, and treating said surface again with hydrofluoric acid so as to remove some but not all of the oxide layer formed during said heating,

7. In the manufacture of an electrical crystal contact device of the kind in which the semiconducting crystal contact element is mainly silicon, the production of said silicon contact element by a process which includes the steps of introducing into silicon that is substantially free from the metallic impurities usual in commercial silicon a determinate small quantity of at least one of the two metals aluminium and beryllium, which additive metal forms a solid solution with the silicon, polishing part at least of the surface of the body of silicon into which said additive metal has been introduced, and subjecting to a controlled process of surface oxidation said polished part.

8. In the manufacture of an electrical crystal contact device of the kind in which the semiconducting crystal contact element is mainly silicon, the production of said silicon contact element by a process which includes the steps of introducing into silicon that is substantially free from the metallic impurities usual in commercial silicon a determinate small quantity of at least one of the two metals aluminium and beryllium, which additive metal forms a solid solution with the silicon, and coating with a metallic layer part of the surface of the body of silicon into which said additive metal has been introduced.

9. In the manufacture of an electrical crystal contact device of the kind in which the semiconducting crystal contact element is mainly silicon, the production of said silicon contact element by a process which includes the steps of introducing into silicon that is substantially free from the metallic impurities usual in commercial silicon a determinate small quantity of at least one of the two metals aluminium and beryllium, which additive metal forms a solid solution with the silicon, polishing part of the surface of the body of silicon into which said additive metal has been introduced, affixing the flat end of a rod by means of an adhesive to said polished surface part, dipping said body into a solution of cuprous oxide in hydrofluoric acid, removing said rod from said body, and cleaning said surface part free from adhesive.

10. In the manufacture of an electrical crystal contact device of the kind in which the semi-conducting crystal contact element is mainly silicon, the production of said silicon contact element by a process which includes the steps of introducing into silicon that is substantially free from the metallic impurities usual in commercial silicon a determinate small quantity of at least one of the two metals aluminium and beryllium, which additive metal forms a solid solution with the silicon, coating with a metallic layer part of the surface of the body of silicon into which said additive metal has been introduced, and mounting said body in a metal holder to which said metallic layer is soldered. I

11. In the manufacture of an electrical crystal contact device of the kind in which the semi-con;- ducting crystal contact element is mainly silicon, the production of said silicon contact element by a process which includes the steps of introducing into silicon that is substantially free from the metallic impurities usual in commercial silicon a quantity of additive metal such that the ratio of the number of atoms of said additive metal to the number of atoms of silicon in the product is not greater than 1/50, polishing a part of the surface of the body of silicon so produced, treating said polished part with hydrofluoric acid, thereafter heating said body in an oxidising atmosphere, and

treating said'surface again with hydrofluoric acid so as to remove only some of the oxide layer formed by said heating.

DOUGLAS ENFIELD JONES. CHARLES ERIC RANSLEY.

JOHN WALTER RYDE.

STANLEY VAUGHAN WILLIAMS.

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