US2415841A

US2415841A - Conducting material and device and method of making them

Info

Publication number: US2415841A
Application number: US568190A
Authority: US
Inventors: Russell S Ohl
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1944-12-14
Filing date: 1944-12-14
Publication date: 1947-02-18
Anticipated expiration: 1964-02-18
Also published as: GB606626A; BE471046A; FR941322A; NL130727C

Description

Feb. 18, 1947. s, OHL 2,415,841

CONDUCTING MATERIALS AND DEVICE AND METHOD OF MAKING THEM Filed Dec. 14, 1944 2 Sheets-Sheet 1 FIG.

, FIG. 2

lNl/EN roe R. 5.. OHL

A T TORNE I Feb. 18, 1947. R, s, O HL v 2,415,841 I CONDUCTING MATERIALS AND DEVICE AND METHOD OF MAKING THEM Filed Dec. 14, 1944 2 Sheet s-Sheet 2 24 /2 FIG. 4 as L4 /6E L lNl EN 8 R5. 0

A TIORNEY I Patented Feb. 1%, rec? UN l TED STATS CQNDUCTING MATERIAL AND DEVICE AND METHOD @F MAKING THEM Application December 14, 19%, Serial No. 568,190

"1 Claims. (c1. 25c 31) ,nsi

This invention relates to electrical conducting the desired characteristics, and then subjecting and translating materials and devices and to the rectifier to a succession of alternate applimethods of preparing them. cations of its self-biasing voltage and a reverse The objects of the invention are to increase biasing voltage supplied from a direct current the stability of conducting and translating de- 5 source, the source of superposed alternating vices; to improve their operating characteristics, voltage being continuously applied to the rectiincluding their electrical efiiciency; and in other fier during the process. More specifically, with respects to realize improvements in these devices each application of the self-biasing voltage the and in the methods of preparing the materials of ions that are small relative to the openings in which they are made. the crystal lattice, notably the ions of the un- It has been the practice heretofore to make wanted impurities, are urged out of the surface rectifier elements of fused high-purity silicon layer and into the body of the crystal. The ions containing less than two-tenths of one per cent of the desired impurities, however, such as the of impurities. This small residue consists of a added aluminum, are sufficiently large in size to substantial number of specific substances which lodge within the lattice structure and remain are usually inherent in the high-purity material. pe ma ent y n the Surface layer- With each Also a method has been devised in which the application of the conditioning bias voltage the crystal rectifier elements taken from the fused direction of migration reverses, and both wanted ingot are subjected to an oxidizing heat treatand unwanted ions move from the cryst l body ment for th purpose of forming on th rectifiinto the available openings in the surface layer. cation surface of the element a thin high-im- Because of their smaller sizes the unwanted ions pedance layer which improves t arr er-banreadily reenter the surface layer, provided there dling capacity of the rectifier. When rectifiers i av ab Sp Cet e Same time by Selecting prepared in this manner are used in harmonic the proper value of the reverse bias voltage some generators r for other purposes whe e they are of the larger wanted ions are forced into the Subjected to alternating fields of relatively high lattice of h rf il n lay r Wh t y a intensity, it is noted that there is a substantial locked and h ld in fi t y. A s cc s ve rereduction in efilciency with ti e, In the rd versalsoccur fewer and fewer of the undesired they do not possess the desired degree of stai s fin r t nt r t e s fa lay r, bility, and their initial eiiiciency is likely to Whereas m n m of t e l r r n ed diminish rather rapidly after they are put into io s te and become locked, until finally h y use. occupy all available space therein.

This decrease in the efficiency of the rectifier One of the advantages f the r tifi r mais believed to be due to a migration, away from tcrial p pa in a d n wi h he pre nt the,surface layer nd int th body of t crystal invention is an increase in rectification efficiency. element, of the positive ions of the metallic im- The p c f the fi ed metallic ions in the purities that remain in the surface layer follow- Surface layer f the y a c ases the C011- ing the heat treatment, These i are urged in ductivity of the rectifier in the forward direction a direction away from the crystal surface by the Without mp ring its resistivity in the rever e continuous self-biasing voltage developed across direction; in fact the s nc to fiOW Of 0 1'- the re tifie nt t b th applied t ti rent in the reverse direction is somewhat greater. voltage. While the size of the ions, relative to Another advantage i a marked improvement the interstices in the molecular lattice of the in the Stability of the t fi r after being subsilicon, may be such that the steady bias voltage 'efited the foregoing treatmentdoes not cause th t migrate appreciably h 4.5 Other features and advantages of the invensuperposed alternating voltage agitates them and tion Will be understccdfrom e win depermits them to filter through the lattice toward tailed description.

the interior of the body. With this migrational In t drawings accompanying the specificaefiect in mind, applicant has discovered that he tion:

can exercise a selective control over the move- 1 illustrates a block taken from an ingot ment of the ions with respect to the surface layer 0f fused Silicon? of silicon and obtain an ultimate disposition 2 Shows a Slab out the block of 1;

thereof in which desired ions are fixed within the 3 discloses an assembled rectifier unit? surface layer and undesired ions are excluded 4 illustrates a ammatically a work cirtherefrom cuit including one of the rectifier units together In accordance with present invention With circuit means for treating the rectification therefore, this desirable effect is achieved by first Surfacebf the unit; a

preparing the rectifier element of silicon to Fig. 5 is a view great y e g d 0 t Crystal which is added a small per cent of some metal, and contact elements of the rectifier unit.

such as aluminum, capable of producing ions of Usually the purpose for which this rectification material is to be used, whether in rectifier units for high frequency systems or in electrical circuits and systems of other types, determines the optimum resistance value in each particular instance. It is, therefore, desirable, as a preliminary step in the process, to calculate the exact amount of additional material that is to be combined with the high-purity silicon before casting the ingot. Also it is desirable, as alluded to above, to choose an addition material which is capable of producing ions having characteristics, particularly as to size, bearing a definite relation to the physical structure of crystalline silicon. One such substance is the element aluminum. Any suitable procedure may be employed for determining the desired amount of aluminum to add to the silicon material prior to casting. One suitable method is described in the copending application of Scafi and Theuerer, Serial No. 545,854, filed July 20, 1944. In any event, the amount of addition material is very small and may correspond roughly to the percentage of some of the inherent impurities present in the high-purity silicon material, all of which together do not exceed .2 per cent. However, it should be understood that the present invention is not limited to any particular percentage of addition material nor to any particular material, although, as mentioned, it has been found that aluminum gives good results.

Having determined the desired amount of addition material, it is thoroughly mixed with the selected quantity of high-purity silicon, and the mixture is cast in a furnace to produce an ingot of substantial size. One suitable method of effecting the fusing and casting steps is described in detail in the application of H. C. Theuerer, Serial No. 517,060, filed January 5, 1944.

Following the casting of the ingot, a block I is cut therefrom, and from the block I a thin slab 2 is out, of suitable thickness, for example, for use as rectifier elements. Diamond saws are usually employed for this purpose, best results being obtained when the large surface of the slab is normal to the axis of the ingot.

The next step in the preparation of the slab 2 for use in rectifier units is to polish one of its large faces to a high finish. Any suitable method may be used for polishing the slab, one such method being described in detail in the application of Scafi' and Theuerer, Serial No. 545,854, filed July 20, 1944.

If desired, the next step that may be taken in the preparation of the slab 2 is to oxidize the polished surface thereof. To this end the slab is placed in a heat chamber where it is subjected to a temperature of about 1050 C., for a period of two hours. For further details of the heat treatment process reference is made to the application of R. S. Ohl, Serial No. 530,419, filed April 10, 1944. The effect of the heat treatment is to form a thin coating of vitreous oxide material over the polished surface of the slab 2 and ,to produce thereunder a thin layer of crystallized silicon material which is characterized by its high impedance and good rectification properties.

Following the heat treatment the back or unpolished side of the slab 2 is ground with an abrasive material to remove completely the vitreous oxide layer. The ground back of the slab is then plated with nickel, and the slab is cut into small wafers of dimensions suitable for use in the rectifier units.

The wafer 3 thus prepared is now ready for assembly in a rectifier unit, one type of which is iliustrated in Fig. 3. This rectifier comprises a cylindrical metallic shell or housing 4, a crystal assembly, and a contact assembly. The metallic cylinder 4, which also serves as an electrical shield, may be made either of steel or beryllium copper and may be given a thin coating of tin. crystal assembly comprises a solid cylinder of brass, plated with tin, if desired, to which the polished silicon wafer 3 is affixed. The wafer 3 is prepared in the manner above described and is then soldered with its nickeled surface to the brass cylinder 5. The oxidized layer covering the polished rectifier surface of the crystal is now removed by etching, for example, in a bath of hydrofluoric acid and water. The spring contact wire assembly of the rectifier comprises a metallic pin 6, preferably of nickel, an insulating cylinder 1, and a short length of tungsten contact wire 8. A section of the pin 6 is threaded or corrugated, and the insulating cylinder l is then molded around this section of the pin. Good results are obtained by the use of a molding material composed of a mixture of quartz powder and a highly pure phenolic resin. The contact Wire 8 is about .002 of an inch in diameter, and the contact end thereof is ground to a plane circular surface normal to the axis of the wire. The opposite end of the wire 8 is spot-Welded to the end of the pin 6, and the wire is shaped in a single loop with the free end bent at such an angle that only a portion of the circumference of the tip of the wire engages the surface of the silicon crystal 3. The crystal supporting cylinder 5 and the contact wire assembly, including the insulating cylinder i, are driven into opposite ends of the housing tube 4 where they are held by a force-fit securely in their required positions.

The assembled rectifier unit is now connected in a work circuit, such as the one illustrated in Fig. 4, where it is subjected to the final step in the process, namely, the step of conditioning the surface of the crystal for improved efiiciency and stability. Only the essential elements of the rectifier, the silicon wafer l2 and the contact wire 53 (magnified in size), are illustrated in Fig. 4. These same elements are again illustrated in 5 with still greater magnification.

it may be assumed, for example, that the circuit id is designed to generate a desired harmonic of the fundamental Wave developed in the source l5. If the third harmonic is desired, the source 15 is preferably arranged to offer a very small impedance to the harmonic wave, and the load circuit or device 16 is designed to offer substantially no impedance to the fundamental wave. The condenser I! connected in the generator circuit serves as a by-pass for both the fundamental and harmonic waves.

The circuit i8 is employed in the final step of the process of preparing the rectifier unit, namely for conditioning the rectification surface. This circuit includes a source of direct current l9, an adjustable resistor 20, a circuit-closing key 2|, a milliammeter 22 and a voltmeter 23.

When the rectifier unit, which has just been prepared and assembled as above described, is connected in the generator circuit 14, it produces waves of the desired harmonic frequency and also acts to rectify both the fundamental and harmonic waves flowing in the circuit [4. If it is assumed that the silicon element I2 is electropositive, the rectifying action of the unit will permit current to flow from the element l2 to the contact Wire i3. During the portion of each cycle that positive potential is applied to the element l2 and negative potential to the contact wire l3, rectified current flows in a counter-clockwise direction around the circuit 14 and through the silicon element l2 in the direction indicated by the arrow 2'. Because of the rectification properties of the unit substantially no current is permitted to flow in the opposite direction in the circuit l4 nor through the element [2. Therefore, a continuous voltage is developed across the rectifier the value of which is the resultant of the successive half-cycle alternating potentials applied to the rectifier in the non-conducting direction. Although this continuous voltage does not cause the fiow of current in the circuit [4, it does apply a force to the ions within the silicon element l2 which urges them to move in the direction indicated by the arrow ii, that is, in the direction from the rectification surface 24 of the crystal it toward the interior thereof.

The presence of ions in the surface layer of crystallized silicon 25 is illustrated in Fig. 5. It is believed that the heat treatment to which the silicon element I2 is subjected produces on the surface of the element a thn layer 25 of crystallized silicon of extremely high purity. However, the ions of those metallic impurities remaining in the surface layer 25 are subjected to the force of the continuous voltage developed across the rectifier in the circuit Id urging them to move downward out of the surface layer'25 and into the body of the element I2. In generating circuits, such as the one of Fig. 4, this developed voltage is relatively large, and the result is that the ions. which are constantly agitated by the superimposed alternating voltage, are forced through the interstices of the crystal lattice and into the interior parts of the element 12. This migrational effect is particularly true with respect to those element wh ch form ions relatively small with respect to the openings in the crystal lattice. In Fig. 5 both the smaller and the larger ions are illustrated, the smaller ions representing those impurities which inherently exist in the high purity material from which the element is cast and the larger ions representing the addition material, such as aluminum, purposely added to the silicon before casting.

It will be seen, therefore, from this explanation that the continued use of the rectifier element in the circuit 54 will result in the migration from the surface layer 25 into the body of the element of substantially all of the impurity elements which form the small ions. gration of ions out of the rectification surface has a decided effect upon the efficiency and performance of the unit, and it is, therefore, very desirable to prevent such a change in performance after the unit is installed for use. Furthermore, it is known to be desirable to have a substantial percentage of the proper material, such as aluminum, permanently located in the surface layer of the element, particularly for certain uses. To avoid this change of stability and eificiency and to fix the rectification surface 25, the final step of the process is performed upon the rectifier unit by means of the circuit l8.

It will be noted that the polarity of the battery I9 is such that positive potential will be applied to the silicon element l2 and a negative potential to the contact wire it when the key 2! is closed. Thus a direct biasing voltage is applied across the rectifier Which is opposite in polarity to the voltage of self -bias which is developed across the This mirectifier from thesource Hi. The voltage applied by the battery I9 is relatively large and serves to urge the ions in a direction toward the surface layer 25. Each time, therefore, the key 2| is closed the conditioning bias from the battery l9 causes relatively large numbers of the small ions to move readily in the lattice structure of the surface layer 25 together with substantial numbers of the larger ions, these larger ions being aided in their migration by the constant agitation produced by the superimposed alternating voltage. After the key 2! has been closed for a suilicient interval it is again opened, whereupon the self-biasing voltage causes the smaller ions to move back out of the surface and into the interior of the element. The larger ions, however, which have been forced into the lattice with considerable effort resist the return movement and in effect become lodged or locked in the interstices of the lattice structure. On the next closure of the key 24 the same process is repeated, both the smaller ions together with additional larger ions being forced from the body of the element up into the available openings in the crystal lattice. Again, when the key 2! is opened, the smaller ions escape readily and return to the body of the element, whereas the larger ions remain behind and become lodged in the surface layer. By repeating this step a sufficient number of times it is possible to fill the interstices in the surface layer with the desired number of large size ions of the addition material and to exclude the unwanted smaller ions, forcing them to remain below in the body of the silicon element. Thus the silicon rectification element is prepared with a surface having characteristics of stability, high efiiciency and excellent rectification properties particularly when used in circuits requiring a substantial flow of current.

While one type of assembled rectifier is illustrated herein, it will be understood that the conducting and translating material prepared in accordance with the method described may be incorporated in numerous types of assembled devices.

What is claimed is:

1. The method of making a. translating device for electric waves which comprises casting an ingot from a quantity of silicon of high purity, adding to said silicon before fusion a small percentage of a metal capable of producing ions which are comparable in size to the interstices in the lattice structure of substantially pure crystalline silicon, shaping an element of the cast material from said ingot, heat-treating said element to produce on the surface thereof a thin layer of substantially pure crystalline silicon, applying an electrical stress to ionize the added metal present in said element and to cause said ions to lodge in the interstices in said surface layer.

2. The method of making a translating device for electric waves which comprises casting an ingot from a quantity of silicon of high purity, adding to said silicon before fusion a small percentage of a metal capable of producing ions which are comparable in size to the interstices in the lattice structure of substantially pure crystalline silicon, shaping an element of the cast material from said ingot, heat-treating said element to produce on the surface thereof a thin layer of substantially pure crystalline silicon, subjecting said element to a potential which causes the ions of said added metal to migrate into the said surface layer of silicon and to lodge therein, and impressing on said element an alternating potential which agitatessaid ions and facilitates their migration.

3. The method of making a translating device for electric Waves which comprises casting an ingot from a quantity of silicon of high purity, adding to said silicon before fusion a small percentage of aluminum, shaping an element of the cast material from said ingot, heat-treating said element to produce on the surface thereof a layer of substantially pure crystalline silicon, subjecting said element to a potential which causes the aluminum ions to migrate into said surface layer of silicon to lodge therein, and impressing on said element an alternating potential which agitates said ion and facilitates their migration.

4. The method of making a conducting device for electric waves which comprises forming a body by fusin a quantity of high-purity silicon containing a small percentage of inherent metaliic impurities, adding to said silicon before fusion a small percentage of aluminum, shaping an element from said silicon body, heat-treating said element to produce on the surface thereof a thin layer of substantially pure silicon having a, crystalline structure with interstices comparable in size with ions of aluminum and substantially larger in size than the ions of said inherent metallic impurities, subjecting said element to a potential for ionizing the metals present in said element, alternately reversing the direction of said potential first to cause the migration of said into said surface layer where the larger of the added aluminum are trapped and held and second to cause the migration of the smaller unwanted ions out of said surface layer and into the body of said element.

The-method of making a conducting device for electric Waves which comprises forming a body by fusing a quantity of high-purity silicon containing a small percentage of inherent metallic impurities, adding to said silicon before fusion a small percentage of aluminum, shaping an element from said silicon body, heat-treating said element to produce on the surface thereof a thin layer of substantially pure silicon having a crystalline structure with interstices comparable in size with ions of aluminum and substantially larger in size than theions of said inherent metallic impurities, subjecting said element to a potential for ionizing the added aluminum and the inherent metallic impurities present in said element, applying said potential in one direction to cause the migration of said ions into the surface layer of silicon where the larger ions of the added aluminum are lodged, reversing the direction of the applied potential to cause the migration of the smaller ions of the unwanted impurities out of said surface layer and into the body of said element, and repeating the reversals of said applied potential until a desired number of aluminum ions have become lodged in said Surface layer and the unwanted ions have been excluded therefrom.

6. The method of making a translating device for electric waves which comprises casting an ingot from a quantity of silicon of high purity, adding to said silicon before fusion a small percentage of a metal capable of producing ions which are comparable in size to the interstices in the lattice structure of substantially pure crystalline silicon, shaping an element of the cast material from said ingot, heat treating said element to produce on the surface thereof a thin layer of substantially pure crystalline silicon, ionizing the added metal present in said element, and fixing said ions in the interstices in said surface layer.

"1. The method of making a conductive device for electric Waves that comprises forming a body by fusing a quantity of high purity silicon containing a small percentage of inherent metallic impurities, adding to said silicon before fusion 9. small percentage of metal, shaping an element from said body, heat treating said element to produce on the surface thereof a thin layer of substantially pure silicon having a crystalline structure with interstices comparable in size with ions of the added metal and substantially larger in size than the ions of said inherent metallic impurities, subjecting said element to electrical stress for ionizing the metals present in said element, and selectively controlling said stress to cause the migration of ions into and out of said surface layer, whereby ions of the added metal are fixed Within the surface layer and other ions are excluded therefrom.

RUSSELL S. OHL.

REFERENCES CITED FOREIGN PATENTS Country Date British Feb. 12, 1943 Number