US2921245A

US2921245A - Hermetically sealed junction means

Info

Publication number: US2921245A
Application number: US766032A
Authority: US
Inventors: Jr Clarence L Wallace; Edward J Diebold
Original assignee: International Rectifier Corp USA
Current assignee: Infineon Technologies Americas Corp
Priority date: 1958-10-08
Filing date: 1958-10-08
Publication date: 1960-01-12
Anticipated expiration: 1977-01-12

Description

Jan. 12, 1960 c. L. WALLACE, JR., ETAL HERMETICALLY SEALED JUNCTION MEANS Filed Oct. 8, 1958 F76 4-0 4/ 42 F/G 2 INVENTOR.

amen/cs 1.. mun- .12. EDWARD J. 0158040 BY Ms Mrs-Au ATIvEAIEKS United States Patent 2,921,245 HERMETICALLY SEALED JUNCTION IVIEANS Clarence L. Wallace, Jr., Topanga, and Edward J. Diel'rold, Palos Verdes Estates, Calif., assignors to International Rectifier Corporation, El Segundo, Calif., a corporation of California Application October 8, 1958, Serial No. 766,032

15 Claims. (Cl. 317-234) This invention relates to rectifiers for alternating electric current.

It is well known that a rectifier containing a crystal of silicon which is free from internal stresses, from structural defects, and from surface contamination has very valuable rectifying properties. These properties can rarely be fully utilized because inherent in conventional rectifiers is the likelihood of either contaminating the crystal in manufacture, or stressing the crystal when the device is in use, in such a manner that the rectifying capacity thereof is seriously reduced.

Contamination of the surface of a silicon crystal need not be great in order to almost completely destroy the ability of the crystal to operate as a rectifier. Only very low concentrations, of molecular order, are enough to cause an excessive increase in reverse current. It has been found that even the most carefully conducted surface treatment operations tend to leave harmful impurities on the crystal, and that the more the crystal is treated the more impurities are deposited on the crystal.

In addition because silicon rectifiers are particularly suited for rectifying heavy currents, they ordinarily are put in installations Where the crystal and its supporting structure are subjected to large temperature fluctuations, and these fluctuations cause undesirable mechanical stresses. These stresses tend to destroy the rectifying ability of the device.

Patented Jan. 12, 1960 ice structure, thereby relieving lateral expansion forces in the device. A flexible diaphragm interconnects the stud and the insulating ring, thereby relieving axial forces in the device.

A related feature of this invention resides in the method of assembling this rectifier, whereby the elements described above may be assembled in a furnace in stacked It is an object of this invention toprovide a rectifier in which, in the entire course of its manufacture, the crystal single manufacturing step. During the time the crystal is exposed, it is surrounded and protected by a very pure protective atmosphere, whereby a minimal amount of impurities can reach the crystal. Furthermore, the materials surrounding the crystal at the time of exposure are not of a type which would tend to contaminate it.

Still another object of the invention is to provide a structure in which the crystal is largely isolated from structural stresses which result from temperature changes resulting from the passage of high currents through the rectifier. Still another object of this invention is to provide a rectifier which can be made of easily and inexpensively fabricated parts, and in which the manufacturing opera- :tions are simple, and do not require complex or complicated procedures for minimizing contamination of the crystal.

This invention is carried out by installing a rectifying element comprising a semi-conductor such as crystal of silicon upon a base plate which provides one terminal for relationship with bonding solders and the like between them, and brazed together in the presence of a protective atmosphere. By this means, the silicon crystal is exposed to operations only once during the assembling process, and contamination is minimized by the protective atmosphere. Because the resulting structure is hermetically sealed by this operation, no additional contaminants ever reach the crystal.

The above and other features of this invention will be fully understood from the following detailed description and the accompanying drawings, of which:

Fig. 1 is a side elevation partly in cut-away cross-section of a rectifier according to the invention;

Fig. 2 is a cross-section taken at line 22 of Fig. 1;

Fig. 3 is a fragmentary cross-section showing another means of installing a silicon wafer in the rectifier structure of Fig. 1;

Fig. 4 is a side elevation partly in cut-away cross-section showing two of the rectifiers of the type shown inFig. I mounted in tandem and connected to supporting structure; and

Fig. 5 shows another means for mounting a plurality of rectifiers of the type shown in Fig. 1.

Fig. 1 illustrates a rectifier assembly 10 which includes a rectifier element 11. The rectifier element is enclosed in a surrounding structure and is mounted to a base plate 12 which forms a part of the surrounding structure.

The rectifier 11 includes a semi-conductor shown as a silicon wafer 13. This wafer consists essentially of pure intrinsic silicon, and is bonded to base plate 12 by a layer 14 of a solder which includes N-type impurities. N-type impurities are defined as metallic elements which are negative compared to elements in the fourth column of the periodic table of the elements, preferred elements for this purpose being selected from the fifth column, for example, arsenic, antimony and bismuth. The solder comprises a low concentration of these relatively negative elements in a carrier. Thee carrier consists of a metal which has a relatively stable outer electron shell, examples being gold, copper and silver. As is well known, these carrier metals serve as a bonding agent between metal plates. Under heat treatment some of the N-type impurities will migrate into the scilicon wafer so as to form a surface of N-type silicon with a boundary between what will become N-type silicon, and intrinsic silicon. A preferred N-type solder consists essentially of silver and a minor amount of antimony.

A layer 15 of a silicon-aluminum alloy serves to bond a metal disc 16 to the wafer 13. Layer 15 is an alloy of silicon and aluminum. The aluminum is present to provide a P-type impurity for the silicon crystal. A P-type impurity is defined as a metal which is more positive than the elements in the fourth column of the periodic table of the elements. Aluminum is a P-type metal. Gallium, indium and thallium are several other examples. These and alumium fall in the third column of the periodic table of the elements which column contains the preferred P-type metals for this use. As is well known, under heat treatment, the P-type impurity migrates into the intrinsic silicon, and results in the formation of a surface layer of P-type silicon with a boundary between the intrinsic silicon and the silicon with the P-type impurity.

As a result of the introduction of N-type impurities to one side of the crystal, and P-type impurities to the other, a rectifying crystal having the physical composi- 3 tion of N-ty-pe, intrinsic, and P-type silicon in contiguity in that order is obtained, which is suitable for rectifying alternating electric current. For the purpose of definition, the rectifying element 11 is defined as comprising wafer 13 and the two solder layers 14 and 15. .i

.The metaldisc 16-is preferably made of the;samematerial as the base plate so as to minimize stresses on the wafer 13 which could resultfrom.heatingtherectifier if the baseplate 12, disc 16, and a wafer l3sattached to them were to have different .thermal coefiicients of expansion. Molybdenum is the .preferred metal for both the base plate 12 and the disc 16, :because the thermal expansion of molybdenumclosely matches .that of silicon. The rectifier element is not internally stressed to any significant degreeuwhen the rectifier is heated up with this'arrangement, but it is pointed out that both :solder bonds aremore flexible-than the plates and the disc on either side, so that shear stresses between the plate and disc and the crystal are largely taken up by the solder, and are not transmitted tothe crystal.

An elongated stud 17 is bonded to the disc 16 by a layer 18 of silver solder. This stud has. ascircularsection throughout its length, and terminates in a Tod-like upper section 20. An advantage of this shape is;that it can easily be formed on a header by :a cheap fabricatron process.

A tubular element 21 is ring-shaped and:has.la U- shaped-radical cross-section. This section'has a flatjcentral bight 22 (which is thereforena fiatannulus) and a pair of depending

legs

23, 24. It isevidentthat these two legs are both tubes which have their free ends depending downwardly .from the :central bight. mThe lower ends of these legs areattached by solder rings 25;. 26 to the base plate 12, with the rectifier element attached to-the base plate, inside the tubular. element. 6

An insulator ring 27 iswplaced atop the tubular element 21. Itsupperand lower surfaces are metalized so that'the ring can receivesoldered connections. wiThe ring is preferably made of a refractory material :which does not dissociate. at high temperature, such asaluminum oxide. Thelower end of the insulating ring is bonded by a solder ring 28 to the central bight of the tubular member 21.-.An upper solder ring 29 :is. provided for joining the outer-periphery of adiaphragm 30 to-the upper surface ofthe ring.

Diaphragm 30 has acentral web 32 with an opening 33 at the center. There is a small turned-down flange 34 in the diaphragm adjacent the opening .which'provides withthe stud a depression-for receiving solder 35 that bonds the diaphragm 30 tothestud.

A convenient means "for connecting the rectifier'into a circuit is to .provide a metal cup 36 which is inverted so that its rim 37 fits into an annular-depression 38 in the diaphragm 30. Solder 39 may be used to' attac'h the rim of the cup to the diaphragm 30-. The -cu-p has an opening 40 at its central-portion for receiving a grommet 41 which serves to attach a fiexible terminal "lead 42 to the cup. .A cooling fin 43-ma'y be'bonded to the base plate by a layer 44 of solder, and the fin can then be used at the other term'inal of the-rectifier assembly, instead of the base plate, if desired.

In the construction shown inFig. l, the-order of polarity upward from the 'baseplate is N-intrinsic-P. It may be desired to-reverse this-polarity,:andprovide the order P-intrinsic-N. This is-achieved by substituting in place of the rectifier element 11 in Fig. 1, a rectifier element 45, shown in Fig. 3.

. together. The. solder fort-he joints will be provided as discs or rings, as appropriate. Then graphite weights are applied to hold the assembly together, and the weighted assembly is transferred to an oven having a hydrogen gas or inert gas atmosphere. The temperature is elevated so that the soldersy'rnelt to make their joints, and so.that the. Retype. and N-type impurities infuse into the silicon. The structure is then cooled and the rectifier is ready for use.

It will be noted that theatmosphere insidethering is that of the furnace,- and :that after the solderwbonds have been hardened, the same atmosphere remains in the hermetically sealed rectifier... .This'atmosphere protected the wafer from oxidation during the heating process, and afterass'ernbly, the surrounding structure isolates the wafer from any source of contamination. Furthermore, the metals within the structure are not the dissociative type, nor is the material of ring 27, No contaminants are likely to reach the water at all. Therefore=thiszmeans oft-manufacture. exposes the wafer'only once, and then itis not exposed .to any source of likely contamination.

The deviceshown-can be adapted to many :suitable structural applications. For example, inFig. 4,.a: rectifier 51 of the type shown in Fig. 1 has a terminal-lug 52 soldered thereto, instead of-the fiexible lead shown in'Fig. 1.

A second rectifier 53 of the'type shown in Fig. 1 has its cup soldered to a cooling fin 54.attached {to the upper rectifier 51, while its own fin 55-is soldered to a hexagonal-headed stud 56 having a thread 57zthereon. AsvshoWm-the stud can be threaded into .a-tapped:hole

58 in a bus bar 59. Persons skilled in the artwillapp'reciate-how stacks of-rectifiers such asthat shown in Fig--14 can be made up of any desired length -and how thevarious fins may be used for electrical terminals in structures such as bridge circuits and the like. The structure of Fig. 4 can, forexample, be used as oneha-lf of a full-wave rectifier.

As an additional example, in Fig. 5 a pair ofrectifiers ofthe type shown in Fig. 1 are-illustrated connected in series. A spacer 60 holds the two finsapart, whi1e a nut and bolt assembly 61 holds the fins in -tight assembly, so as to properly space and firmly mount the two rectifiers. The terminal lead 62 of a rectifier 63 -is connected by-a nut and bolt assembly 64 to the fin 65 of another rectifier 66. The terminal lead 67 of rectifier'66 provides one-terminal of series connection shown Whileanother lead 68 may be attached to the fin 69 ofrrectifier 63, to complete provisions for electricalconnections.

It will be observed that the silicon wafer is substantially protected against stresses in all directions because of-the construction of the A device. With''- respect to lateral stresses, the solder layers on eachside-of the wafer of'semi-conductor material have less resistancetoshear loadsthan the plates or other structures to which they bond the wafer. Therefore, when the base plate, silicon wafer, or stud heats up and causes lateral expansion in theregion of the wafer, anydifierential movement will be taken up by the soft soldered bonds and little any force will beexerted directly upon the silicon wafer.

Lateral forces resulting from-the expansion of the baseplate-are also taken up by the tubular: member Rectifier element 45 includes a'silicon "wafer 46 which 21. It is well known that the end of a tube can be expanded or contracted quite easily and without significantly distorting the-tube ata distance significantly-removed from: the distorted end. Accordingly, when thelends of the tubular legs 23, '24 are expanded or contracted; little if any of this movement is communicated to thenbight 22 so that the-lateral expansion or-contra'ctionoftthe base plate. has nosubstantial eifect on the over-allstructure, the tubular member acting to relieve them.

It-'is to be un'de'rstood that other types ofsiliconsjunctionsbouldw-have -beenw provided, such ascommon P-N junctions wherea-crystaldopedwith one type of--impurity is contacted by another type of impurity. The type of junction is completely variable, because the bonding materials can have either type of impurity therein, or even no impurity at all, and the wafer itself can be either intrinsic silicon, or either type silicon.

With respect to difierential axial movements, the flexible diaphragm 30 acts as a weak link in the system and can easily absorb any differential movement between the outer portion of the structure and the inner portion.

It will therefore be seen that this rectifier comprises an inexpensive structure which is easily manufactured from conventional parts and which inherently provides an easy means of manufacturing a clean, uncontaminated rectifier in which the full capabilities of a semi-conductor such as a silicon crystal in power rectification can be utilized.

This invention is not to be limited by the embodiments shown in the drawings and described in the description, which are given by way of example and not of limitation, but only in accordance with the scope of the appended claims.

What is claimed is:

1. A hermetically sealed rectifier comprising: a conductive base plate, a rectifying element conductively mounted on said base plate; a stud conductively mounted on said rectifying element; a tube mounted to said base plate; an insulating ring mounted to said tube, said ring and tube surrounding said rectifying element; and a diaphragm extending between and connected to the stud and to the insulating ring, the diaphragm making a fluid-tight seal between the ring and stud, the base plate, tube and ring completing, with the diaphragm, a fluidtight enclosure for the rectifying element. a

2. A rectifier according to claim 1 in which the tube and ring are substantially circular cylinders.

3. A hermetically sealed current rectifier comprising: a base plate; a rectifying element mounted to the base plate in conductive contact therewith; a stud mounted to said rectifying element on the other side thereof from the base plate; a channel member having a pair of legs and a central bight, the channel member being attached by its legs to the base plate around the rectifying element, the bight being spaced from the base plate by the legs; an insulating ring mounted to the web of the channel member on the other side thereof from the base plate and surrounding the rectifying element; and a diaphragm extending between and connected .to the stud and to the insulating ring.

4. A hermetically sealed rectifier comprising: a base plate with a flat surface; a semi-conductor wafer with a flat surface; a first metallic bond conductively mounting said wafer to the base plate in surface-to-surface relationship, the metallic bond having a shear strength less than that of the base plate and wafer; a tubular member attached to said base plate, surrounding the wafer; an insulating ring mounted to the tubular member on the opposite side thereof from the base plate; a stud conductively mounted to the wafer on the other side thereof from the base plate; and a flexible diaphragm interconnecting the insulating ring and the stud.

5. Apparatus according to claim 4 in which the insulating ring is made of refractory material, and the remainder of the rectifier is made of metal.

6. Apparatus according to claim 4 in which the tutular member has a U-shaped cross-section formed by a central bight and two depending legs, the legs being tubular, and attached to the base plate.

7. Apparatus according to claim 6 in which the insulat ing ring is made of refractory material, and the remainder of the rectifier is made of metal.

8. Apparatus according to claim 4 in which the first metallic bond includes an N-type impurity, and in which the stud is mounted to the wafer by a metal plate to which it is bonded, a second metallic bond bonding said metal plate to the other side of the wafer and including a P-type impuritv.

9. Apparatus according to claim 4 in which the first metallic bond includes an N-type impurity, said first bond mounting the wafer to the base plate by a first metal plate, said first plate being conductively bonded to said base plate, and in which the stud is mounted to the wafer by a second metal plate; a second metallic bond bonding said second metal plate to the other side of the wafer, and including a P-type impurity.

10. A hermetically sealed rectifier comprising: a. base plate with a flat surface; a semi-conductor wafer with a fiat surface; a first metallic bond conductively mounting said rectifying element to the base plate in surface-tosurface relationship, the first metallic bond having a shear strength less than that of the base plate and wafer; a tubular-member attached to said base plate, said tubular member being generally cylindrical and having in crosssection a central bight and two tubular legs extending from the same side of said bight, said tubular legs being attached to said base plate; an insulating ring mounted to the bight of the tubular member; a stud conductively mounted to the rectifying element on the other side of the base plate; and a flexible diaphragm interconnecting the insulating ring and the stud and forming a fluid-type barrier therebetween; said stud being mounted to the rectifying element by means of a metal plate interposed therebetween which is bonded to the stud and to the wafer; and a second metallic bond bonding the metal disc to the wafer, said base plate, tubular member, insulating ring, diaphragm, and stud being continuous structures, continuously joined to form a fluid-tight structure.

11. Apparatus according to claim 10 in which the first metallic bond includes an N-type impurity and in which the second metallic bond includes a P-type impurity.

12. Apparatus according to claim 10 in which the first metallic bond includes a P-type impurity and the second metallic bond includes an N-type impurity.

13. Apparatus according to claim 10 in which the outer periphery of the diaphragm has an annular groove therein, and in which a cup having a rim is inverted over the diaphragm with its rim conductively bonded in the annular groove whereby a first electrical connection can be made through the cup, diaphragm, stud and metal plate to the rectifying element, the base plate providing a second connection.

14. Apparatus according to claim 13 in which a terminal lead is connected to the cup.

15. Apparatus according to claim 13 in which a cooling fin is attached to the base plate in heat-transfer relationship therewith.

References Cited in the file of this patent UNITED STATES PATENTS 2,756,374 Colleran et al July 24, 1956 2,861,226 Lootens Nov. 18, 1958 2,864,980 Mueller et a1. Dec. 16, 1958