[1]3 1 4 9 4 2 8 0 3 uii'i'ted States Patent Office Patented Feb. 10, 1970 1 2 3,494,803 METHOD OF BONDING A SEMI-CONDUCTOR TO A METAL CONDUCTOR AND RESULTANT PRODUCT Leonard E. Avis, Baltimore, Md., assi-,Bor, by mesue as. sign ments, to Teledyne, Iiic., Los Angeles, Calif., a cor- 5 pora tion of Delaware Filed May 12, 1966, Ser. No. 549,538 Int. Cl. H01v 1128, 1114 U.S. Cl. 136-237 17 Claims This invention relates to formin- improved bonds be- 10 twee n semiconductor or thermoelectric elements and a com mon m.-tal conductor which have much better electrical and physical properties at high temperatures. More parti cularly the invention relates to an economical method of formin- an unusually strong, durable, low electrical 15 resis tance dovetail-transition bond between semiconduc tor elements or thermoelectric elements and metal conduct ors. Whe n rods of dissimilar thermoelectric compositions have their ends joined to form a continuous loop, two 20 ther moelectric junctions are established between the respec tive ends so joined. If the two junctions are maintaine d at different temperatures, an electromotive force will be created in the circuit thus formed. This effect is kno wn as the th.-rmoelectric or Seebeck effect, and may 25 be re.-arded as due to the charge carrier concentration grad ient produced by a temperature gradient in the two mate rials. The effect cannot be ascribed to either material alon e, since two dissimilar, thermoelectrically comple30 ment ary materials are necessary to obtain this effect. It is therefore customary to measure the Seebeck effect produce d by a particular material by forming a thermocoup le in which one circuit member or thermoelement cons ists of this material, and the other circuit member cons ists of a metal such as copper or lead, which has 35 negli ,-ible thermoelectric power. The thermoelectric pow er Q of a material is the open circuit voltage develo ped by the above thermocouple when the two junctions are maintained at a temperature difference of l' C. Whe n thermal energy is converted to electrical energy 40 by thermocouple devices utilizing the Seebeck effect, each devi ce may be r".arded as a heat engine operating betwee n a heat source at a relatively hot temperature TR and a heat sink at a relatively cold temperature Tc. The 45 limiti ng or maximum efficiency theoretically attainable from any heat engine is the Carnot efficiency, which is THTC T, 5 0 Thu s it is well known that the efficiency of Seebeck effe ct devices is increased by increasing the temperature diffe rence between the hot junction temperature TI, and the cold junction temperature Tc. It is convenient to oper ate such Seebeck devices with the cold junction at 55 roo m temperature or at as low a temperature as possible, but for any -iven cold junction temperature it fohows that hi.-h efficiency in the conversion of thermal energy to electrical energy reqiiires that the hot junction tem- 60 pera ture TH be as hi.-h as possible. Man y thermoelectric compositions which are useful at relatively low temperature cannot be operated at elevate d temperatures because they tend to break down phys ically or chemically react with the environment when 65 exp osed to relatively high temperatures. It is therefore nece ssary that hi,@hly efficient Seebeck devices utilize only thos e thermoelectric compositions which are stable at elev ated tempcratures. In the same manner all of the com ponents of a thermoelectric device in contact with the hi.-h temperature source, including the hot shoe bond 70 betw een the thermoelements, must operate effectively at the hi.-hest temperatures possible in order to (1) maintain the highest temperature difference possible between the hot junction temperature TH and the cold junction temperature Te; (2) - niaintain a continuous electrical circuit between the components for extended periods of time; and (3) maintain the lowest electrical resistivity in the components and in the thermoelectrical system as a whole in order to generate the large currents necessary for high heat conversion efficiency. In the discussion that follows and in the claims the comnlon conductor that connects the thermoelements on the end exposed to the heat source will be referred to interchangeably as the hot shoe and as the common conductor. The thermoelements may be made of any of the known formulations of thermoelectrically complementary materials used in this art. The term thermoelectrically complementary preferably refers to known Ntype and P-type semiconductor elements which form effective thermoelectric devices, but non semiconductor materials could be used. The composition of the thermoelements and the methods of doping semiconductor thermoelements materials are well known in this art. In most prior art thermoelectric devices the junction bonds between the metal conductor and the thermoeleetrically complementary elements physically and chemically decompose at high temperatures and the normally relatively high electrical and thermal resistance at the bonded junctions tend to rapidly increase to higher levels as the operating temperature increases. Such thermoelectrical devices do not operate satisfactorily for long periods of time. Conventional brazing or soldering materials contaminate the thermoelements, thereby substantially reducing their efficiency. When the metal conductor is directly bonded to a semiconductor, phase changes occur in the metal as the temperature enters different temperature zones and generally a chemical bond effected in one phase loses its effectiveness in another phase. Further, differences in expansion and contraction with temperature variations between the metal elements and semicondtictor elements cause such direct bonds to physically deteriorate very rapidly at high temperatures. Still further, when iron is bonded directly with a doped semiconductor it rapidly corrodes at high temperatures. A complete analysis of the causes of bond deterioration is rather complex due to the many materials which can be present at the bonding interfaces, especially if solders are used. However, it is certain that the main problem stems from the various materials being chemically and/or physically incompatible with each other. The higher the temperature of operation, the more severe the interaction and hence the degradation becomes. An important object of this invention is to provide an economical method for mass producing semiconductor and thermoelectric devices with durable bonds between the semiconductor thermoelectric elements and the common metal conductor which have good electrical, chemical and physical properties at relatively high temperatures. Another important object of this invention is to provide more efficient thermoelectric devices wh'ch have a lon@er life at high operating temperatures. Another important object of this invention is to provide a common conductor or hot shoe that combines the advantages of good electrical and thermal conduction of a metal shoe and the good long lasting bond of a semiconductor shoe. Other objects and advantages of this invention will be apparent to those skilled in the art after studying the followin.- description and drawings. In accordance with this invention it has been discov,-red that the use of a transition bond which makes a gradual composition transition from the metal con-iposition at the metal shoe end of the bond to the semicondtictor composi-
[2]3j494,803 3 tion or thermoelement composition at the thermoelement end of the bond and which is chemically and physically similar to the metal in the metal shoe and the semiconductor or other thermoclement composition in the thermoelement provides an efficient, stable high temperature bond between these dissirnilar elementmaterials. Further in accordance with this invention it has been discovered that semicoiiductor elements and thermoelements havin.- transition bonds of unusually high physical strength can be mass produced much more economically using a fluid-screen-to-metal bonding technique described below. Both the product and the method of forming the product are important features of this invention. In the drawings: FIGURE I shows a typical thermoelectric device in accordance with the principles of this invention; and FIGURE 2 is an enlar-ed view of a cross section of the transition bond which shows the dovetail joint between the fused metal screen and neutral semiconductor material. In a preferred embodiment of this invention shown in FIGURE 1 a metal mesh screen I iS fLIsed to a common conductor 2, which is the same or a similar type juetal as th.- metal composition of said screen. Embedded in and covering said screen I is a substantially neutral semiconductor material 3. Joined to the semiconductor material 3 are semiconductor thermoelements of N-type 4, and P-type 5. A difflision barrier 6 formed by cutting away a center section of semiconductor material 3 and screen I until the bare metal is visible completes the thermocouple device. The transition bond fori-ned by the metal screen I and the neutral semiconductor material 3 provides a gradual composition transition from the metal conductor shoe 2 to the semiconductor thermoelements 4 and 5. The transition layer comprises a semiconductor material layer 3, which is preferably neither P nor N-type but substantially neutral in nature, and a layer which comprises both substantially neutral semiconductor material 3 and the fused metal screen 1. While in the preferred embodiment illustrated in the drawin.-s, this latter layer is shown as comprising approximately 50% semiconductor material 3 and 50% flised metal screen 1, it will be apparent that the relative proportions of these materials therein may be altered to optimize performance Of various specific embodiments of the invention described herein. Additionally, the proportion of metal to semiconductor in contact with the metal shoe can be modified as desired. In this fused metal-neutral semiconductor layer a series of dovetail-like joints are formed. Together the composite strength of all of these dovetail-like bonds produces an unusually stron@ and lasting physical bond. FIGURE 2 shows an enlarged view of this dovetail feature and the transition bond. In a -preferred embodiment illustrated in the drawings, the screen I is made of iron fused to a common conductor 2, also made of iron. Other suitable materials may alternately be employed for these purposes such as, for instance, alloys of iron, stainless steel, molybdenum and other refractory metal and alloys thereof. Preferably the screen and conductor are made of the same metal, but different compatible metals could be used, such as an iron screen with a stainless steel common conductor. In a preferred embodiment illustrated in the drawin.-s, the neutral semiconductor 3, symbolized by SnTe forms a dovetail-like area that physically enhances the strength of the overall bond between the metal common conductor or shoe 2 and the semiconductor thermoelements 4 and S. It has been found that SnTe forms an excellent bond with iron; however, other materials may be utilized for the substantially neutral semiconductor material 3 as lon.- as they are compatible for use with the materials from which the other members of that device are formed. For instance, an undoped PbTe neutral semiconductor material 3 could be employed with a sgreen I and the common conductor 2 4 formed of iron and doped lead telluride thermoelectric elements 4 and 5. Also, in certain applications, it will be desirable to utilize a germanium telluride neutral semiconductor material with a molybdenum screen and common conductor. 5 In a preferred embodiment illustrated, the metal portion in the layer of the transition bond adjacent the common conductor 2 takes the form of a screen which produces a dovetail effect with the substantially neutral semiconductor material 3 present in the same layer. This 10 dovetail effect may take other forms in alternate embodiments of the invention and still effect the same function wbich is to produce a mechanical locking between the semiconductor material and the metal portions of the 15 layer to supplement the strength of the metallurgical bond therebetween. This same dovetail type of mechanical interlockin-. effect can be obtained by utilizin- a perforated metal plate or a sheet of expanded metal for the screen I or by fusing metal particles of various shapes 20 to the common conductor. This mechanical locking between a joining material and one of two elements to be joined to.-ether obviously has utility in conventional structures wherein a thermoelectric element is joined to a hot shoe without utilizin,- a neutral semiconductor layer. 25 Illustrative of this aspect, one prior art process for joining, a lead tellui-ide thermoelectric element to an iron hot shoe is to bond the two components together utilizing a brazin.- operation employing a lead tin alloy. The strength of the bond between the thermoelectric element and hot 30 shoe can be increased by initially fusin@, for instance, an iron screen to the iron shoe element prior to the brazing operation. Such a technique produces a dovetail joint between the screen and braze material thereby enhancing the strength of the bond between the thermoelectric ele35 ment and the shoe. The following example illustrates a preferred embodiment of the fused-screento-metal bonding techniqlie outlined above and preferred thermoelectric components. The numbers set forth in this exardple correspond with 40 those shown in the drawings. A 35 mesh iron screen I was fusion bonded under pressure to a 1/16" iron sheet 2 in a hydrogen furnace at 2100' F. for 48 hours. SnTe powder of less than 100 mesh was cold pressed into and over the bonded sheet at 25 t.s.i. to form the substantial neutral layer 3 of the transition 45 bond. Approximately one-half of the SnTe is embedded in the mesh of screen 1, the otber balf lies above the mesh. The size of the surface area of the composite slab may be varied as desired. The composite slab obtained may be cut into smaller pieces to make a number of smaller hot 50 shoes. The size of the hot shoe may be varied as desired. An N-type thermoelement 4, consisting of PbTe doped with Pbl2, was bonded directly to the SnTe layer by hot pressing at 1425' F., at 2 t.s.i. for 20 minutes. A P-type element 5, consisting of AgSbTe-GeTe, was bonded to the 55 SnTe layer in a similar manner with the addition of a small amount of Te at the interface, at 1180' F. 200 at 1.2 t.s.i. for 5 minutes. The addition of Te at the interface merely wetted the surface; the excess evaporated. Diffusion barrier 6 was made by cutting away a center 6o section of the transition bond, including the screen, until the bare metal was visible. This thermoelectric device was subjected to ei,@hty thermocycles in which the temperature of the hot shoe was varied cyclically between 200' and 1000' F. On 65 examination there was no visible evidence of physical or chemical decomposition of the bond. The specific example set forth above used powdered SnTe, but niolten SnTe could have been used with equal success and/or other materials compatible with the semi7o conductor thermoelements could have been substituted for SnTe. In either case the SnTe or its substitute is allowed to solidify on the screen, then it is remelted to form a fused bond with the thermoelements which pref75 erably have a higher melting point than the SnTe. The
[3]5 remelting step is important since it prevents the formation of entrapped -as bubbles under the thermoelectric elements. in place of the metal mesh screen one cotild substitulle a p,- rforated metal p@Iale and the specific mesh size of the screen or the perforated plate can be varied as desired. StibstiLutin.- loose f@,ised metal fibers for the screen reduces the dovetailed bond effect; slibstituting powdered metal would eliminate it. The metallic material used should be compat,.ble with the neutral somicondtictor or transition material that it is in direct contact with. However, it is also possible, in view of the dovetail bond effect, to use a fused metal mesh that is incompatible w,th the neutral seniiconductor material in the transition layer, but better results and stronizer bonds are obtained when compatible mesh and neutral semico,iduc'tor materials are used. When compatible materials, su--h as SnTe and iron mesh, are used the adhesion of these compatible materials greatly adds to the overall bond stren-th and serves as a corrosion protector and resists ch.-mical poisonin.@ of the various elements. The dovetail-transit@'@on bond of this invention has for the sake of illustration been discussed in regard to its use in thermoelectric devices: it should also be nwed that it can be used to bond semiconductor elements to metal elem@@nts in other types of s-- miconductor devices. It should also be reemphasized that the specific metal used in the condtictor or hot shoe and in the screen, and the specific neutral semiconductor and thermoelectric P and N type semicondLicLor rr@aterials used iii conjunction with the dovetailed-transition bond of this invention may readily be varied by one sk-illed in th,- art. Many adequale substitut--s are known. The snecife materials set forth in the above examples proved to be very durable and effective. Obviously other fusion forming temperattires and Dresstires could be used with the specific materials set :Corth in the example and the siiitable temperature ran-,Cs and pressure ranges that may be used will vary with the materials. Also, re-ardless of the field of use, it wi@ll be apparent the transition re-.ion should be kept as small, or narrow, as possible aid yet obtain the necessary or desired buffer zone function which, for instance, avoids ins' abil-@ty of the bond during wide variations in temperature and/or contamination. Normally the transition region, at most, represents a small percenta,-e of the total mass of materials bein.- bonded or laminated. What is
