Glass-encapsulated semiconductor device containing cylindrical stack of semiconductor pellets
BACKGROUND OF THE INVENTION 
     This invention relates to a glass-encapsulated semiconductor device and more particularly to a glass-encapsulated semiconductor device in which the same encapsulating glass can be used independent of the number of the laminated semiconductor pellets. 
     The glass-encapsulated semiconductor device in which semiconductor pellets are laminated and bonded together and to a pair of electrodes, in which the exposed edges of the pn junctions of the semiconductor pellets are surface-passivated and in which an encapsulating sintered glass layer is coated for hermetically sealing the pellets on the laminated pellets over the length from one electrode to the other, has a fatal problem that the encapsulating glass layer or the semiconductor pellets are sometimes broken in the process of fabrication or during use. 
     The breakage of the glass layer or the pellets is caused by thermal stress generated due to the difference in thermal expansion among different members. 
     In the case where only one or two pellets are used, the effect of soldering material layers, the total thickness of which is rather small, upon the average thermal expansion coefficient of the laminated pellets can be neglected even when the thermal expansion coefficient of the soldering material is greater than that of the semiconductor pellet. Accordingly, the thermal expansion coefficient of the encapsulating glass can be approximated to that of silicon. 
     On the other hand, in the case where the number of the laminated pellets is as large as thirty or so, the total thickness of the soldering material layers cannot be neglected so that there is a need for selecting in such a case an encapsulating glass having a thermal expansion coefficient approximating to the average thermal expansion coefficient of the laminated column of semiconductor pellets and soldering material layers. 
     This means that the quality of the encapsulating glass should be selected depending upon the number of the laminated pellets. 
     It is very difficult to select a suitable glass for encapsulating semiconductor pellets, having an appropriate thermal expansion coefficient without degrading its function of surface passivation. 
     In U.S. Pat. No. 3,913,127, in case where the number of the laminated pellets is large, it has been proposed to approximate the average thermal expansion coefficient of the laminated column of semiconductor pellets to the encapsulating glass by varying the respective thicknesses of the pellets and soldering material or inserting spacers. However, it was not necessary to change the material of the glass but necessary to adjust the respective thickness of the semiconductor pellets, soldering material and spacers. 
     SUMMARY OF THE INVENTION 
     One object of this invention is to provide a glass-encapsulated semiconductor device in which the same glass can be used for encapsulation independent of the number of the laminated semiconductor pellets. 
     Another object of this invention is to provide a glass-encapsulated semiconductor device in which there is used an encapsulating glass capable of surface-passivating the exposed edges of the pn junctions of the semiconductor pellets in the optimal condition, independent of the number of the laminated semiconductor pellets. 
     According to this invention, there is provided a glass-encapsulated semiconductor device comprising a rectifier unit, which includes a pair of electrodes each with an external lead wire attached to an end thereof, a plurality of semiconductor pellets each having a rectifying pn junction exposed at the periphery of each of said pellets, an electric-conductive spacer and a plurality of soldering material layers mechanically and electrically bonding and connecting said pellets and said spacer to each other, said pellets, said spacer and said soldering material layers being laminated together in the form of a stack with said pair of electrodes being disposed in contact with the soldering material layers at opposite ends of said rectifier unit, a protective glass layer for passivating the pn junctions of said pellets and for protecting said rectifier unit against mechanical ambient stress and having a prescribed coefficient of thermal expansion within a range between a coefficient of the thermal expansion of said pellets and a coefficient of the thermal expansion of said soldering material layers, surrounding and contiguous to the exposed surfaces of said pellets, of said spacers and of said soldering material layers, said glass layer extending from the periphery of one of said electrodes to the periphery of the other electrode, said conductive spacer having such a thickness that the thermal expansion coefficient of said spacer is equal to the average thermal expansion coefficient of a virtual laminated column of said semiconductor pellet alone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows in longitudinal cross section a glass-encapsulated diode as an embodiment of this invention. 
     FIG. 2 shows a simplified model of the glass-encapsulated diode shown in FIG. 1, prepared for facilitating the explanation of the diode. 
     FIG. 3 shows a simplified model of a conventional glass-encapsulated diode, prepared for the facility of explanation. 
     FIG. 4 graphically shows the relationship between the thickness of conductive spacer used in this invention and the average thermal expansion coefficient which is the mean of the thermal expansion coefficients of the silicon pellets and the conductive spacer. 
     FIG. 5 shows in longitudinal cross section a glass-encapsulated diode as another embodiment of this invention. 
     FIG. 6 shows a simplified model of the diode shown in FIG. 5, prepared for the facility of explanation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, reference numeral 10 generally indicates a glass-encapsulated diode for high voltage use, according to this invention. 
     An electrode lead 11 consists of an electrode section 11a of molybdenum or tungsten and a lead section 11b of copper or copper alloy coupled to the electrode section 11a by fusing. Another electrode 12 has the same chemical composition and mechanical structure. 
     Between the electrode sections 11a and 12a of the electrode leads 11 and 12 opposed to each other are stacked in lamination a plurality of silicon pellets 13a, 13b, . . ., 13n having a p-n-n+ or p+-p-n+ structure and two electrically conductive spacers 14a and 14b, the silicon pellets and the spacers being bonded to one another by soldering material 15. 
     In this figure, seven silicon pellets are shown provided for example, but in practice the number of the pellets can be chosen depending on the breakdown voltage required. 
     As indicated by dotted line with respect to the silicon pellet 13n, the pn junction of each silicon pellet 13a, 13b, . . . or 13n appears exposed in the periphery of the corresponding pellet. 
     The body of the lamination of the silicon pellets 13a, 13b, . . ., 13n and the conductive spacers 14a and 14b are covered by a layer 16 of sintered glass over the length from one electrode 11 to the other 12. 
     The glass-encapsulated diode having such a structure as described above can be fabricated, for example, through the following process. In this case, suppose each silicon pellet has a p-n-n+ structure. 
     A plurality of n-type silicon wafers are prepared and desired pn junctions are formed in them through diffusion of impurity. Layers of soldering material are applied through, for example, vacuum evaporation method on both sides of each of the thus processed silicon wafers and on both sides of each of the metal plates which are chosen according to this invention and serve as conductive spacers. 
     The silicon wafers and the metal plates are stacked in lamination in such a manner that the metal plates are located at both of the ends of the lamination. Then, the silicon wafers and the metal plates are bonded together by heat treatment. 
     The laminated structure is cut by, for example, a wire saw to have desired dimensions. 
     As a result, the silicon wafers are turned into silicon pellets 13a, 13b, . . ., 13n and the metal plates into conductive spacers 14a and 14b. 
     The lamination of the silicon pellets 13a, 13b, . . ., 13n and the conductive spacers 14a and 14b is disposed between and bonded to electrode leads 11 and 12 prepared separately. 
     Glass slurry of a mixture of distilled water and glass powder (frit) having a desired thermal expansion coefficient and surface passivation function, is applied to the lamination over the length from one electrode 11 to the other 12. A glass-encapsulated diode as shown is produced by heat treatment. 
     The conductive spacers 14a and 14b are disposed at both of the ends of the lamination, i.e. in contact with the electrode leads 11 and 12, for the following reasons. 
     First, in the sintering process after the application of glass slurry, air bubbles are liable to be generated in the bonding surface between the electrode and the end of the laminated column of silicon pellets, i.e. in the portion enclosed by a circle A shown in FIG. 1, and if the conductive spacers 14a and 14b are provided between the electrode 11a and the pellets 13a and between the electrode 12a and the pellet 13n, such air bubbles are prevented from reaching the pn junctions of the silicon pellets 13a and 13n so that the breakdown voltage is improved. 
     Secondly, in the case where the electrodes 11a and 12a have a thermal expansion coefficient different from that of the silicon pellets 13a and 13n, the conductive spacers, which has a thermal expansion coefficient intermediate between the thermal expansion coefficients of the electrodes and the silicon pellets, decrease the thermal stress due to the difference in thermal expansion coefficient. 
     Now, a concrete description will be given to the thicknesses of the conductive spacers 14a and 14b to be determined depending upon the thermal expansion coefficients and thicknesses of the silicon pellets 13a, 13b, . . ., 13n and the layers 15 of soldering material. 
     FIG. 2 shows the silicon pellets 13a, 13b, . . ., 13n disposed to the left and the conductive spacers 14a and 14b to the right for the sake of explanation. 
     The lamination of the silicon pellets 13a, 13b, . . ., 13n and the conductive spacers 14a and 14b is divided by a division line L into two groups G 1  and G 2  for facilitating the determination of the thickness of the conductive spacer. The group G 1  consists of the silicon pellets and the associated layers of soldering material whose number is the same as that of the silicon pellets, and the group G 2  includes the conductive spacers and the remaining layers of soldering material. 
     In the following explanation, the numbers of the silicon pellets and the conductive spacers should be thought of as arbitrary so as to give a general explanation suitable to the cases where the numbers differ according to the purpose in application. This will be seen to be reasonable since in practice the silicon pellets 13a, 13b, . . ., 13n as well as the conductive spacers 14a and 14b are of the same structure. It is also supposed that the soldering material layer 15 1  in the group G 1  of the silicon pellets is different in quality from the soldering material layer 15 2  in the group G 2  of conductive spacers. 
     The average thermal expansion coefficient α 1  in the group G 1  of the silicon pellets can be calculated as follows. ##EQU1## where α S  is the thermal expansion coefficient of a silicon pellet, α A  the thermal expansion of the soldering material layer 15 1 , t S  the thickness of a silicon pellet, t A  the thickness of a soldering material layer 15 1 , and N S  the number of the silicon pellets. 
     The average thermal expansion coefficient α 2  in the group G 2  of the conductive spacers can also be obtained as follows. ##EQU2## where α P  is the thermal expansion coefficient of a conductive spacer, α R  the thermal expansion coefficient of a soldering material layer 15 2 , t P  the thickness of the conductive spacer, t R  the thickness of the soldering material layer 15 2 , and N P  the number of the conductive spacers. 
     When α 1  equals α 2 , the average thermal expansion coefficient of the laminated column of the silicon pellets and the conductive spacers becomes constant, that is, the same encapsulating glass can be used, independent of the number of the used silicon pellets. 
     The equation α 1  =α 2  leads to the required thickness t P  for a conductive spacer such that ##EQU3## where α O  =(α S  t S  +α A  t A )/(t S  +t A ). 
     Usually, aluminum, aluminum-silicon alloy etc. having a larger thermal expansion coefficient than the silicon pellet are used as soldering material so that α R  &lt;α O . In order for the expression (3) above to be of positive value the thermal expansion coefficient α P  of the conductive spacer must be such that 
     
         α.sub.P &lt;α.sub.O                               (4) 
    
     here, if the soldering materials 15 1  ad 15 2  used in the groups G 1  and G 2  of the silicon pellets and the conductive spacers are of the same quality and have the same thickness, than α A  =α R  and t A  =t R . It follows therefore that ##EQU4## 
     The above expression (5) gives the average thermal expansion coefficient of a silicon pellet and a neighboring layer of soldering material and if the laminated column of silicon pellets and conductive spacers uses such conductive spacers as having a thermal expansion coefficient smaller than the average thermal expansion coefficient and having a thickness determined by the above expression (6), the thermal expansion coefficient of the laminated column can be made constant independent of the number of the silicon pellets used in lamination. 
     FIG. 3 shows for the purpose of comparison laminated column of silicon pellets 23 with the associated layers of soldering material 25. The average thermal expansion coefficient α 3  in this laminated column having no conductive spacer, proves to depend on the number N S  of laminated silicon pellets 23, as seen from the following expression. ##EQU5## where α S  is the thermal expansion coefficient of a silicon pellet, α A  the thermal expansion coefficient of a layer of the soldering material, t S  the thickness of a silicon pellet, t A  the thickness of a soldering material layer, N S  the number of the laminated silicon pellets, N A  (N A  =N S  +1) the number of the soldering material layers used for bonding. 
     As understood from the foregoing description, it is not the electrode leads, but the conductive spacers that must be taken into consideration in calculating the average thermal expansion coefficient, as seen from the above given expressions (1) and (2). If, however, the electrode section of the electrode lead has a cross sectional shape equal to that of the silicon pellet, it should be taken into consideration in such calculation. Accordingly, such an electrode section as having the same cross sectional shape as that of the silicon pellet, the encapsulating glass sintered thereover and even having its associated lead section, can be regarded as a conductive spacer. Such electrode section therefore serves as both an electrode and a conductive spacer. Namely, even if the electrode section and the conductive spacer are made of the same metal, no difficulty is caused since they are definitely divided. 
     Now, a concrete example of the conductive spacer according to this invention will be given. 
     The thickness t S  of a silicon pellet is determined depending on the breakdown voltage born by the pellet and the thickness of a layer of soldering material on the bonding strength required. 
     In this example, the following values are employed with respect to silicon pellets and soldering material layers. 
     
         t.sub.S = 0.25 mm 
    
     
         t.sub.R = 0.01 mm 
    
     
         α.sub.S =3.5×10.sup.-6 °C.sup.-1 . . . silicon 
    
     
         α.sub.R =25.7×10.sup.-6 °C.sup.-1 . . . aluminum 
    
     
         α.sub.P =4×10.sup.-6 °C.sup.-1 . . . molybdenum 
    
     When a single conductive spacer was used, the thickness t P  thereof was 1.2 mm. 
     FIG. 4 shows the relationships between the average thermal expansion coefficient of the laminated column having silicon pellet 13 and a conductive spacer 14 and the thickness of the spacer 14, the number N S  of the laminated silicon pellets 13 being 1, 5, 10, 20 and 50. 
     Since the encapsulating glass should have a thermal expansion coefficient equal to the average thermal expansion coefficient of the laminated column of silicon pellets and a conductive spacer, the average thermal expansion coefficient shown in FIG. 4 can be regarded as the thermal expansion coefficient of the encapsulating glass to be used. 
     FIG. 4 shows that if a conductive spacer having a thickness of 1.2 mm is used alone, an encapsulating glass having a thermal expansion coefficient of 4.37×10 -6  °C -1   should be used independent of the number of laminated silicon pellets. 
     The average thermal expansion coefficient α 4  of the laminated column including the conductive spacer, i.e. the thermal expansion coefficient of the encapsulating glass, is given by the following expression. ##EQU6## 
     In the case where on the basis of an already existing glass-encapsulated diode a similar glass-encapsulated diode having a different number of silicon pellets and using the same encapsulating glass is newly fabricated, it is the qualities and the thicknesses of the soldering material and the conductive spacer that should be carefully controlled. 
     Since the thickness and the quality of soldering material must be limited in view of the electric characteristic of the diode, the thickness and the quality of the conductive spacer should be suitably chosen to cope with the change in the number of the laminated silicon pellets. In that case, the number of spacers to be used, the thickness and the quality (especially thermal expansion coefficient) of the spacer are determined according to the above expressions (6) and (7). 
     The mechanical destruction of the silicon pellets and the encapsulating glass does not always take place due to the thermal stress caused by the difference in thermal expansion coefficient, but the silicon pellets or the encapsulating glass will be broken only if the thermal stress exceeds the tensile strength of the pellet or the glass. 
     Accordingly, the thickness and the thermal expansion coefficient of the conductive spacer need not strictly satisfy the expressions (6) and (7) but have only to meet them approximately if the thermal stress is less than the tensile strength. 
     The above expression (6) assures the free choice of the number of the conductive spacers to be used. Moreover, the spacers are harmonized in thermal expansion with the encapsulating glass and they can be located at any position in the laminated column. 
     FIG. 1 shows an example in which the two conductive spacers 14a and 14b are disposed in the vicinity of the electrode sections 11a and 12a respectively. However, if a plurality of conductive spacers are used and inserted among the silicon pellets 13a, 13b, . . ., 13n, then the conductive spacers serve as heat radiators for the heat generated by the silicon pellets. As a result, the range of allowable operating temperatures for the diodes can be increased. 
     The above expressions (6) and (7) are used to concretely determine the thickness and the quality of the conductive spacer and the grounds for this, i.e. the gist of this invention, is as described above to make the average thermal expansion coefficients of the group G 1  of silicon pellets and the group G 2  of conductive spacers roughly equal to each other within a range of the change in thermal stress for which the silicon pellets or the encapsulating glass is prevented from breaking. Accordingly, in the case where a plurality of conductive spacers are used, they need not have the same quality and the same thickness and in addition to this layers of soldering material need not have the same quality and thickness. The quality and thickness of one spacer or soldering layer may differ from those of another to a certain extent if necessary. 
     In the case where many conductive spacers are used serving as heat radiators, thicker spacers should be disposed in the middle of the laminated column to improve heat dissipation since more heat is liable to be accumulated in the middle of the column than in the vicinity of the electrode leads. 
     FIG. 5 shows a glass-encapsulated diode 30 as another embodiment of the glass-encapsulated semiconductor device according to this invention. 
     Leads 31 and 32 of, for example, copper having a thermal expansion coefficient much greater than that of silicon or glass, are formed in double header configuration. The surfaces of the first headers 31a and 32a and the second headers 31b and 32b are covered by coating films not adhesive to glass such as nickel films, but such films are not shown in FIG. 5. 
     The leads 31 and 32 are disposed in such a manner that their first headers 31a and 32a oppose to each other and the laminated column of silicon pellets 33a, 33b, . . ., 33n and conductive spacers 34a and 34b fastened together by soldering material 35 are interposed between and bonded to the first headers 31a and 32a by soldering material 37. The soldering material 35 is hard solder and the soldering material 37 is soft solder. A sintered glass capsule 36 is formed in the same manner as in the case of the diode 10 in FIG. 1, but since the surfaces of the headers of the leads 31 and 32 are coated with the films not adhesive to glass, the encapsulating glass 36 does not break due to the difference in thermal expansion coefficient. 
     Due to the thermal contraction of the leads 31 and 32, the encapsulating glass 36 is strongly compressed between the first headers 31a and 32a and between the second headers 31b and 32b so that the hermetical sealing of the silicon pellets can be assured even if the encapsulating glass 36 is not adhesive to the leads 31 and 32. 
     The reason why the soldering material 37 to bond the conductive spacers 34a and 34b to the leads 31 and 32 is of soft solder, is as follows. Namely, the melting point of the soft solder is much lower than that of the hard solder and the soft solder remains in the phase of liquid until the temperature of the diode in the cooling process after the sintering of the encapsulating glass 36 has lowered to a small value so that thermal stress, which may be caused due to the difference in thermal expansion coefficient between the leads 31 and 32 and the conductive spacers 34a and 34b, can be prevented from affecting the completed diode. 
     Therefore, the effect of the soldering material 37 upon the thermal expansion can be neglected so that the conductive spacers 34a and 34b are designed without consideration of the effect of the material 37. Namely, the soft solder 37 has no influence upon the average thermal expansion coefficient of the laminated column while all the layers of the soldering material in the example shown in FIG. 1 appreciably affect the average thermal expansion coefficient of the laminated column. 
     FIG. 6 shows a model similar to that shown in FIG. 2, useful in understanding the function of the conductive spacers 34a and 34b. Silicon pellets 33a, 33b, . . ., 33n and conductive spacers 34a, 34b are divided into two groups by a division line L 2 . One group G 3  is laminated column of silicon pellets 33 bonded together by soldering material 35 1  and the other group G 4  is a lamination of conductive spacers 34 bonded together by soldering material 35 2 . The difference of the model in FIG. 6 from the model in FIG. 2 is that no soldering material is applied between the groups G 3  and G 4 . 
     The average thermal expansion coefficients α 11  and α 12  of the groups G 3  and G 4  are given by the following expressions. ##EQU7## where the same symbolism is employed as in the expressions (1) and (2). 
     In order to enable the same glass to be used for encapsulation independent of the number of the laminated silicon pellets 33, the average thermal expansion coefficients of the groups G 3  and G 4  must equal each other, as taught by this invention. In such a case, i.e. α 11  =α 12 , the thickness of the conductive spacer is obtained as follows. ##EQU8## where α O  =(α S  t S  +α A  t A )/(t S  +t A ). 
     Provided that α R  &gt;α O  as in the expression (4), then the condition such that 
     
         α.sub.P &lt;α.sub.O                               (14) 
    
     has to be satisfied to make the value of the expression (13) positive. 
     Moreover, if the soldering materials 35 1  and 35 2  in the groups G 3  and G 4  are of the same quality and have the same thickness, i.e. α A  =α R  and t A  =t R , then it follows that ##EQU9## 
     The expression (11) represents the average thermal expansion coefficient of the assembly of a silicon pellet and a soldering material layer and if a conductive spacer having a thermal expansion coefficient smaller than the average thermal expansion coefficient and a thickness determined according to the expression (15), is used, the average thermal expansion coefficient of the laminated column of the silicon pellets and conductive spacers can be made constant, that is, the same glass can be used for encapsulation, independent of the number of the silicon pellets. In the embodiments described above, the silicon pellets are exclusively mentioned, but it is easily understood that this invention can also be applied to the cases where germanium or other semiconductor pellets are employed. 
     Moreover, the semiconductor pellet is not limited to that which has a single pn junction but a semiconductor pellet having more than one pn junction such as a punch-through diode having a pnp or npn configuration, can equally be used in this invention. 
     As described heretofore, according to this invention, the same encapsulating glass can be used independent of the number of the laminated silicon pellets by inserting some conductive spacers in the laminated column. 
     Especially, since the same glass can be used for encapsulation, the function of surface passivation for the exposed edges of the pn junctions remains unaltered even though the number of the laminated silicon pellets is changed so that the function of surface passivation can be kept optimal. Further, the nuisance in selecting a suitable quality of the glass used for encapsulating can be eliminated even when the number of the laminated silicon pellets is changed, so that the fabrication of a semiconductor device of the type described above can be much facilitated.