Method of making a glass encapsulated diode

A glass encapsulated semiconductor diode and a method for glass encapsulation of a fusion to form a semiconductor diode is disclosed. The fusion comprises a body of semiconductor material having a PN junction therein and metal electrodes affixed to opposed major surfaces thereof. The fusion is encircled by a ring-shaped glass member with an inner surface of the ring-shaped glass member fused to an edge surface of the body of semiconductor material to form a protective layer overlying the PN junction. The ring-shaped glass member is formed and fused to the edge of the body of semiconductor material by placing the fushion and a prefabricated glass ring, preferably cut from stress relieved glass tubing, encircling the fusion in a furnace. A weight is applied to the upper surface of the prefabricated glass ring. An atmosphere comprising a predetermined mixture of nitrogen and water vapor is established in the fusion furnace and the temperature of the fusion furnace is increased and decreased in accordance with a predetermined program to cause the prefabricated glass ring to soften and fuse to the edge surface of the body of semiconductor material to form a protective layer, comprising the ring-shaped glass member, overlying the PN junction.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to semiconductor devices and more specifically to 
glass encapsulated semiconductor diodes and to a method of fusing glass to 
a body of semiconductor material to form an encapsulation providing 
passivation, a hermetic seal and environmental protection for a PN 
junction within the body of semiconductor material. 
2. Description of the Prior Art 
Prior art semiconductor diodes using glass fused directly to the 
semiconductor portion of the diode as the sole means of protecting the PN 
junction from the environment have been limited to relatively low current 
diodes. An example of such a diode is type UT4005 manufactured and sold by 
the Unitrode Corporation. It is also known in the prior art to encapsulate 
semiconductor devices in thermosetting resinous insulating material. 
Examples of such hermetically sealed diodes using resinous material are 
disclosed in Pat. Nos. 3,475,662, 3,476,987 and 3,476,988 as well as 
3,486,084. Thin glass protective layers are also available in the prior 
art to passivate large prior art semiconductor devices. Glass forming 
these layers was typically applied to the body of semiconductor as a 
slurry and the device and the powdered glass were heated to fuse the glass 
and form a protective glass layer. Glass layers formed using this 
technique were limited to thicknesses in the order of 20 to 30 microns. 
These thin layers are not sufficient to provide complete environmental 
protection for PN junctions and other circuit elements within the body of 
semiconductor material. 
SUMMARY OF INVENTION 
The diode which is the subject of this invention utilizes a body of 
semiconductor material which includes a PN junction formed by the 
interface of P and N conductivity type regions which respectively extend 
from the PN junction to opposed substantially flat major surfaces of the 
body of semiconductor material. The area of the PN junction is coextensive 
with the area of the body or semiconductor material. That is, the junction 
extends entirely across the body of semiconductor material and terminates 
at an edge portion thereof, the edge portion of the body of semiconductor 
material extending from one major surface to the other. Electrodes are 
affixed to the opposed major surfaces of the body of semiconductor 
material by soldering, brazing or other suitable techniques known to those 
skilled in the art. The area of the electrodes is substantially 
coextensive with the area of the surface of the body of semiconductor 
material to which they are affixed. The body of semiconductor material and 
the electrodes affixed thereto are for convenience referred to as a 
fusion. 
The fusion is encircled by an annular or ring-shaped glass member having 
inner and outer surfaces. The inner surface of the glass member is fused 
to the edge of the body of semiconductor material to form an encapsulation 
or seal which provides environmental protection for the PN junction. 
(Encapsulation is used to mean the formation of a protective layer 
overlying selected portions of the body of semiconductor material to 
prevent degradation of the characteristics of a PN junction within the 
body of semiconductor material.) The ring-like glass member is fused to 
the edge of the semiconductor body by assembling the fusion and a dense 
prefabricated glass ring in a jig such that the prefabricated glass ring 
encircles the fusion. (The term "dense" is used to define glass rings made 
from glass which has been completely melted to form rings substantially 
free of voids and having a smooth inner surface. Rings cut from commercial 
quality stress relieved glass tubing meet this criteria.) Pressure is 
applied between top and bottom surfaces of the prefabricated glass ring. 
The fusion and the prefabricated glass ring are heated in a controlled 
atmosphere comprising a mixture of nitrogen, or some other inert gas, and 
water vapor causing the prefabricated glass ring to soften and fuse to the 
edge portion of the body of semiconductor material to form a protective 
layer over the PN junction. (Inert is used to designate any gas which does 
not react in a detrimental way with any components of the diode, nitrogen 
for example.) The temperature of the fusion furnace is controlled to 
prevent detrimental stresses from developing due to dimensional changes of 
the fusion and the glass. 
By using dense glass rings, interfacial fusion between the body of 
semiconductor material and glass is attained at relatively low 
temperatures without the need for total melting of glass. This lowers the 
temperature required and reduces the glass shrinkage. Lowering the 
temperature required and reducing the skrinkage increases the thickness of 
glass layers which can be fused directly to semiconductor materials for 
the purpose of protecting PN junctions therein.

DESCRIPTION OF PREFERRED EMBODIMENT 
The subject matter of the invention is a diode 20 and the method of 
encapsulating a fusion to form a semiconductor diode. 
Diode 20 utilizes a fusion consisting of a body of semiconductor material 
22 (preferably silicon, separately illustrated in FIG. 2) and first and 
second electrodes 24 and 26 (separately illustrated in FIG. 3 and 4). 
Electrodes 24 and 26 are preferably a refractory metal such as, for 
example, molybdenum, tungsten, tantalum, base alloys and mixtures thereof. 
The body of semiconductor material 22 includes a PN junction 27 at the 
interface of P conductivity type region 28 and N conductivity type region 
30. The P and N conductivity type regions, 28 and 30, respectively extend 
from the PN junction 27 to upper and lower surfaces, 32 and 34, of the 
body of semiconductor material 22. Electrode 24 is preferably cup-shaped 
and includes a lower surface 33 which is affixed to the upper surface 32 
of the body of semiconductor material 22. Similarly, electrode 26 includes 
an upper surface 38 which is affixed to the lower surface 34 of the body 
or semiconductor material 22. Electrodes 24 and 26 may be affixed to the 
respective surfaces of the semiconductor body 22 by soldering, brazing or 
any other suitable technique known to those skilled in the art. The body 
of semiconductor material with the electrodes affixed thereto is referred 
to as a fusion. 
The fusion consisting of the body of semiconductor material 22 and 
electrodes 24 and 26 is encircled by a preferably coaxially positioned 
annularly shaped or ring-like electrically insulating glass member 40, 
which is separately illustrated in FIG. 5. (Coaxially positioned means 
that the fusion and annularly shaped glass member 40 have a common 
vertical axis of symmetry.) Inner surface 42 of ring-like glass member 40 
is fused to edge portion 44 of the body of semiconductor material 22 as 
well as to outer surfaces, 48 and 50, of the electrodes, 24 and 26. 
Ring-like glass member 40 preferably includes two regions, 40a and 40b. 
Region 40a is preferably a lead-aluminum-borosilicate glass such as IP745 
sold commercially by Innotech. Region 40b is preferably a zinc-silicate 
glass such as IP645 also sold commercially by Innotech. Alternatively, 
both regions, 40a and 40b, may also be type IP745 glass. These glasses are 
described in detail hereinafter. 
A fused junction is formed along the inner surface 42 of annular shaped 
glass member 40 and the outer edge portion 44 of the body of semiconductor 
material 22. This fused junction provides a hermetic seal or encapsulation 
protecting the PN junction 27. However, additional protection is provided 
by the fused junction between the inner surface 42 of ring-like glass 
member 40 and the outer surfaces, 48 and 50, of the electrodes, 24 and 26. 
Bottom electrode 26 extends beyond the lower surface of the annular shaped 
glass member 40. This permits contact to be made with the bottom electrode 
26 through a flat surface without interference by annularly shaped glass 
member 40. 
Embodiments of the diode 20 actually constructed have a voltage rating of 
600 volts, a current rating of 150 amps and the following dimensions: 
______________________________________ 
Dimension Identification 
Character Dimension in Inches 
______________________________________ 
A (FIG. 1) 0.625 
B (FIG. 1) 0.915 
C (FIG. 1) 0.068 
D (FIG. 1) 0.035 
E (FIG. 1) 0.040 
F (FIG. 1) 0.010 
G (FIG. 1) 0.480 
H (FIG. 2) 0.010 
I (FIG. 3) 0.010 
______________________________________ 
In the diodes actually constructed the glass member 40 was a lead 
borosilicate glass such as IP745 sold in Innotech Corp. Alternatively, 
diodes in which the glass member 40 included a first region 40a of lead 
borosilicate glass and a second region 40b of zinc borosilicate glass, 
such as IP660 also sold by Innotech Corp. have also been constructed. 
Either embodiment functions satisfactorily, however, the combination of 
the lead borosilicate and the zinc borosilicate glasses has been found to 
yield superior results. The lead borosilicate glass has a composition by 
weight of 36.+-.4% SiO.sub.2, 15.+-.3% B.sub.2 O.sub.3, 45.+-.3% PbO and 
3.+-.1% Al.sub.2 O.sub.3. The zinc borosilicate glass has a composition by 
weight of 55.+-.5% ZnO, 31.+-.4% B.sub.2 O.sub.3, 8.+-.2% SiO.sub.2, 
4.5.+-.1% CeO and approximately 1.0% Al.sub.2 O.sub.3. 
FIG. 6 illustrates a second diode 220 which may be constructed using the 
disclosed process. This diode 220 is the subject of patent application, 
U.S. application Ser. No. 891,090, filed Mar. 28, 1978, the assignee of 
which is the same as that of the present invention. The same reference 
numbers as used in FIG. 1 plus 200 are used to identify similar parts of 
the diode 220. For example, the diode 220, illustrated in FIG. 6, utilizes 
a fusion comprising a body of semiconductor material 222 having a PN 
junction 227 therein and electrodes 226 and 224 respectively affixed to 
the upper and lower surfaces 232 and 234 of the body of semiconductor 
material 222. The fusion is encircled by a ring-like glass member 240 
which is fused to the edge surface 244 of the body of semiconductor 
material 222 as well as to the edges 248 and 250 of the top and bottom 
electrodes 224 and 226. Ring-like glass member 240 may also include two 
regions similar to regions 40a and 40b (FIG. 1). However, only one region 
is shown to illustrate an alternate embodiment of the ring-like glass 
member 240. The ring-like glass member 240 is encircled by a ring-like 
metallic member 252 (separately illustrated in FIG. 7). The thermal 
characteristics of the ring-like glass member 242 and the ring metallic 
member 252 are selected such that ring-like metallic member 252 maintains 
the ring-like glass member 240 in compression. Suitable materials for 
ring-like metallic member 240 include Kovar, titanium and steel. Kovar is 
a trademark for an alloy consisting of 20% nickel, 17% cobalt, 0.2% 
manganese with the balance iron. Ceramics, including zircon (Z.sub.2 
SiO.sub.4), mullite (Al.sub.2 O.sub.3 2SiO.sub.2), porcelain, titanium 
(TiO.sub.2) and spinel (MgAl.sub.2 O.sub.4) are also usable. 
The process for constructing the two diodes 20 and 220 described above are 
very similar. Therefore, the preferred process for constructing the diode 
20, illustrated in FIG. 1, will first be described in detail and then the 
modifications for constructing the diode 220, illustrated in FIG. 2, will 
be discussed. 
The first step in constructing the diode 20 is to affix the bottom 
electrode 26 and the top electrode 24 to the body of semiconductor 
material 22 to form the fusion. In the preferred embodiment bottom 
electrode 26 is affixed to the body of semiconductor material 22 by silver 
soldering the top surface 38 of electrode 26 to the bottom surface 34 of 
the body of semiconductor material 22. Silver solders are available to 
permit this process to be carried out at a temperature ranging from 
800.degree. C. to 900.degree. C. The silver solder may be an alloy of 
lead, tin and silver. These solders are commercially available. 
The top electrode 24 is affixed to the top surface 32 of the body of 
semiconductor material 22 by soldering or brazing the bottom surface 33 of 
electrode 24 to the top surface 32 of the body of semiconductor material 
22 using aluminum. Suitable prior art processes are available for 
performing this operation at a temperature ranging from 500.degree. C. to 
550.degree. C. 
In general, electrodes 24 and 26 may be affixed to the body of 
semiconductor material 22 using any suitable process known to those 
skilled in the art. 
After the electrodes, 24 and 26, have been affixed to the body of 
semiconductor material 22 the edge 44 of the body of semiconductor 
material 22 is beveled to complete the fusion. The beveling is preferably 
carried out by sandblasting followed by a chemical polishing and etching 
in a mixture consisting of hydrofluoric, nitric and acetic acids. This 
polishing technique is well-known in the semiconductor industry and can be 
performed using commercially available etchants and equipment. 
Diode 20 is constructed from the fusion described above and first and 
second prefabricated glass rings, 54 and 56 (separately illustrated in 
FIG. 8). A single preformed glass ring can also be used. However, two 
preformed glass rings are preferred because this permits the glass 
overlying the PN junction 27 (FIG. 1) to be selected to optimize the 
protection of the PN junction 27 and the remainder of the glass to be 
selected based on its electrical insulation, thermal and mechanical 
properties. 
The first step in constructing the diode 20 is to clean the fusion and the 
prefabricated glass rings, 54 and 56, using the following procedure: 
A. Boil all the components in reagent grade trichloroethylene; 
B. Rinse all the components twice (one minute each time) in reagent grade 
trichloroethylene; 
C. Rinse all the components ultrasonically twice (one minute each time) in 
reagent grade acetone; and 
D. Dry in room air on filter paper. 
Following cleaning as described above, all the components are assembled in 
a jig as illustrated in FIG. 9. The jig utilizes a graphite base member 58 
having a recess 60 therein. The recess 60 is circular and has a diameter 
slightly larger than the diameter of the bottom electrode 26. This permits 
the fusion to be assembled in the jig such that the bottom electrode 26 is 
in the recess 60 in the base member 58. 
The second prefabricated glass ring 56 is placed in concentric relationship 
with the top electrode 24 of the fusion. The first prefabricated glass 
ring 54 is then positioned concentric with the fusion and the second 
prefabricated glass ring member 56. 
The base member 58 of the fixture includes two guide pins, 64 and 66. Each 
of the guide pins, 64 and 66, includes lower, middle and upper portions 
66a, 66b, 66c and 64a, 64b and 64c. Portions 64a and 64c of guide pin 64 
are smaller than portion 64b. Similarly, portions 66a and 66c are smaller 
than portion 66b. Each of the guide pins are positioned in a hole in the 
base member 58 such that the center portions 64b and 66b, are supported on 
the upper surface 58a of base member 58. 
A top plate 68 is then positioned as shown in FIG. 9. The fusion and the 
prefabricated glass rings, 54 and 56, as assembled in the jig and 
illustrated in FIG. 9, are then placed in a fusion furnace having an 
initial temperature in the range of 350.degree. C. and heated in a 
controlled atmosphere to fuse the prefabricated glass rings 54 and 56 to 
produce a ring-like glass member 40, consisting of two regions 40a and 
40b, as illustrated in FIG. 1. 
The preferred control atmosphere mentioned above consists of a mixture of 
nitrogen and water vapor having a total pressure of one atmosphere with 
the partial pressure of the water vapor being in the range of 10.sup.-3 to 
10.sup.-2 atmospheres. The required water vapor is achieved by mixing 
approximately 2 parts of dry nitrogen with one part of wet nitrogen and 
flowing the mixture through the furnace. Dry nitrogen is passed through 
one inch of deionized water in a bubbler to form the required wet 
nitrogen. After the required atmosphere has been established, the 
temperature in the furnace is increased and decreased in accordance with 
the time temperature chart illustrated in FIG. 10. 
As can be seen from FIG. 10, the temperature of the furnace is initially in 
the range of 350.degree. C. The temperature is increased to a temperature 
in the range of 700.degree. to 720.degree. C. in a time interval of 
approximately 25 minutes. This temperature is maintained for a period of 
approximately 20 minutes. The prefabricated glass rings, 54 and 56, become 
soft and begin to flow at a temperature below 700.degree. C. Wettability 
of glass for silicon and pressure due to top plate 68 causes the soft 
glass to flow evenly along the edges of semiconductor body 22 and the 
outer edges 48 and 50 of electrodes 24 and 26. The larger portions 64b and 
66b of guide pins 64 and 66 limit the downward motion of the top plate 68 
when the prefabricated glass rings, 54 and 56, soften and the glass flows. 
The height of the larger portions 64b and 66b of guide pins 64 and 66 
determines dimension "C" FIG. 1. The surface tension of the soft glass 
causes the outer edge of the glass to form the circular shape as 
illustrated in FIG. 1. Small substantially flat areas may also be formed 
due to the interfaces of the base member 58 and the top plate 68 with the 
molten glass. 
Additionally, it should be noted that the top electrode 24 is affixed to 
the body of semiconductor material 22 by brazing with aluminum. The 
silicon-aluminum alloy produced by this brazing melts below 600.degree. C. 
However, using the disclosed process the top electrode 24 remains attached 
and the melting of the silicon-aluminum alloy does not degrade the PN 
junction 27. 
Next, the furnace is cooled from approximately 720.degree. C. to a 
temperature in the range of 525.degree. C. in about 15 minutes. The 
furnace temperature is maintained in this range for approximately 10 
minutes followed by a reduction to a temperature in the range of 
480.degree. C. in about 15 minutes. A temperature of 480.degree. C. is 
maintained for 20 minutes followed by a reduction to 410.degree. in 15 
minutes. This temperature is maintained for approximately 30 minutes 
followed by a reduction of the furnace temperature to room temperature at 
a rate of approximately 10.degree. C. per minute. This thermal cycle fuses 
the prefabricated glass rings, 54 and 56, to form ring-like glass member 
40 and prevents the formation of possible harmful stresses therein. 
In selecting a glass for the prefabricated glass rings 54 and 56, it is 
important that the thermal expansion coefficients for the glass be matched 
to or greater than the temperature expansion coefficients for the 
semiconductor body 22. It should also be noted that the expansion 
characteristics of the glass with temperature are different from the 
contraction characteristics when the glass is cooled. All of these 
characteristics must be considered in selecting the glass and the 
temperature cycle for the fusion furnace. 
Glasses suitable for use in this invention should have a temperature 
expansion coefficient in the range of 4.0 to 6.0.times.10.sup.-6 
cm/cm/.degree.C and the glass for the first prefabricated glass ring 54 
which passivates the PN junction 27 should be substantially free of alkali 
ions. It is also preferable, although not required, that the thermal 
expansion coefficient of the second prefabricated glass ring 56 be 
slightly larger than the thermal expansion coefficient of the first 
prefabricated glass ring 54. In order to maintain the glass adjacent the 
PN junction 27 (FIG. 1) in compression. In addition; 
(1) the glasses must have structural stability, e.g., must not devitrify or 
go through detrimental phase separation during the fusion process; 
(2) the glass must have good chemical resistance to the environment and 
humidity; 
(3) the glass must have thermal expansion characteristics compatible with 
those of the fusion; 
(4) the glass adjacent the silicon semiconductor body 22 must wet and 
adhere thereto; 
(5) the glass must have a viscosity low enough to flow; 
(6) the glass must not chemically attack the surfaces of the semiconductor 
or the electrodes in a detrimental way; 
(7) the thermal characteristics of the glass must be such that stresses can 
be relieved at temperatures within the limitations of the diode; 
(8) the glass must have a fusion temperature below the degradation 
temperature of the device; 
(9) the finished device must be resilient against thermal shock, thermal 
cycling and have good mechanical strength. 
Glasses having a composition by weight of: 
______________________________________ 
Constituent Percent 
______________________________________ 
SiO.sub.2 32-40% 
B.sub.2 O.sub.3 
12-23% 
PbO 42-48% 
Al.sub.2 O.sub.3 
2-6% 
______________________________________ 
have been found to be suitable for the first prefabricated glass ring 54. 
In particular, a glass having a composition of 
______________________________________ 
Constituent % by Weight 
______________________________________ 
SiO.sub.2 36 .+-. 4% 
B.sub.2 O.sub.3 15 .+-. 3% 
PbO 45 .+-. 3% 
Al.sub.2 O.sub.3 3 .+-. 1% 
______________________________________ 
have been found to be particularly satisfactory. This glass is sold 
commercially by Innotech under type No. IP-745. 
The characteristics of the second prefabricated glass ring 56 are not as 
strenuous as those for the first prefabricated glass ring 54 in that the 
glass comprising the second prefabricated glass ring 56 can have more 
alkali ions present. It is also preferable that the second prefabricated 
glass ring 56 have a temperature expansion coefficient slightly larger 
than the first prefabricated glass ring 54 so that the glass adjacent the 
PN junction 27 (FIG. 1) is maintained in compression. A glass particularly 
suitable for the second prefabricated glass ring 56 has a composition by 
weight of: 
______________________________________ 
Constituent % by Weight 
______________________________________ 
ZnO 55 .+-. 5% 
B.sub.2 O.sub.3 31 .+-. 4% 
SiO.sub.2 8 .+-. 2% 
CeO 4.5 .+-. 1% 
Al.sub.2 O.sub.3 1.0% (approximately) 
______________________________________ 
This glass is sold commercially by Innotech under the type No. IP660. 
The prefabricated glass rings, 54 and 56, used to construct the diode 20 
have the following dimensions: 
______________________________________ 
Dimension Identification 
Character Dimension in Inches 
______________________________________ 
K (FIG. 8) 0.630 
L (FIG. 8) 0.100 
M (FIG. 8) 0.080 
N (FIG. 8) 0.500 
O (FIG. 8) 0.015 
______________________________________ 
In the preferred embodiment discussed above, the two prefabricated glass 
rings, 54 and 56, are fused to form the composite ring-like glass member 
42 illustrated in FIG. 1. The ring-like member 40 has two regions 40a and 
40b. The first region 40a is composed essentially of the IP745 glass while 
the second region 40b is comprised of the IP660 glass. Additionally, it 
should be noted that both of the preformed glass rings, 54 and 56, may be 
IP745 type glass. If only one type of glass is used a prefabricated glass 
ring 75 (illustrated in FIG. 11) having an appropriate cross-section may 
be constructed by sintering glass powder. However, in the diodes actually 
constructed, it has been found that the most successful combination is to 
have the first prefabricated glass ring 54 of IP745 glass while the second 
prefabricated glass ring 56 is IP660 type glass. Glass type IP745 is 
composed by weight of 36.+-.4% SiO.sub.2, 15.+-.3% B.sub.2 O.sub.3, 
45.+-.3% PbO and 3.+-.1% Al.sub.2 O.sub.3. Glass type IP660 is composed by 
weight of 55.+-.5% ZnO, 31.+-.4% B.sub.2 O.sub.3, 8.+-.2% SiO.sub.2, 
4.5.+-.CeO and approximately 1.0% Al.sub.2 O.sub.3. It has also been found 
glass the second prefabricated glass ring 56 can be eliminated. However, 
eliminating this ring reduces the mechanical strength of the diode. 
Superior results have also been achieved by utilizing prefabricated glass 
rings, 54 and 56, which are cut from stress relieved glass tubing. These 
superior results are believed to be related to the fact that prefabricated 
glass rings of this type have smoother interior surfaces and consistent 
prior thermal histories, i.e., they are all pulled from a melt. 
FIG. 12 is a curve illustrating the relationship between the partial 
pressure of the water vapor in the fusion furnace and the leakage current 
of finished diodes. For example, at a water vapor partial pressure of 
10.sup.-3.5 atmospheres the leakage current is in the range of 60 
milliamps. An order of magnitude decrease in the leakage current is 
achieved by increasing the partial pressure of the water vapor to 
10.sup.-2.5 atmospheres. As previously noted, the preferred water vapor 
pressure has been found to be in the range of 10.sup.-2 to 10.sup.-3 
atmospheres with a total furnace pressure of one atmosphere. 
If for some reason, it is desirable to operate the fusion furnace at a 
pressure other than one atmosphere, the partial pressures of the nitrogen 
and water vapor should be adjusted to maintain the proper ratio between 
the water vapor and the nitrogen. It is also possible to use other inert 
gases, argon for example, rather than nitrogen. 
A second diode 220, illustrated in FIG. 6, can be constructed using 
essentially the same process described above with reference to FIG. 1 
except that an outer ring member 252, (preferably metal or a metal alloy) 
is positioned concentric with the prefabricated glass ring 75 prior to 
fusing the glass. This embodiment, as well as the embodiment illustrated 
in FIG. 1, can be constructed using an alternate jig as illustrated in 
FIG. 9. For completeness of description, the process for constructing the 
diode 220 will be discussed with reference to the use of the alternate jig 
illustrated in FIG. 13 and an alternate prefabricated glass ring 75, 
illustrated in FIG. 11. 
The jig includes a graphite base member 70. In assembling the components of 
the diode 220 in the jig the fusion is positioned such that lower 
electrode 226 is in a first recess 72 in graphite base member 70. A second 
recess 74 is concentric with the first recess 72. The prefabricated glass 
ring 75 (FIG. 11) is positioned concentric with the fusion. Prefabricated 
glass ring 75 has an inner diameter larger than the outer diameter of the 
fusion such that the lower edge of this preform rests in recess 74. The 
metallic ring 252 is then positioned concentric with the prefabricated 
glass ring 75. 
A graphite support cylinder 76 having a recess 78 along its inner wall is 
then placed over the metal ring member 252. The larger recess 74 in the 
base member 70 and the recess 78 in the inner wall of support cylinder 76 
are such that the upper surface of the metal ring 252 is in contact with 
both the base member 70 and the support cylinder 76. 
A graphite pressure cylinder 80 having an outer diameter slightly smaller 
than the inner diameter than the support cylinder 76 and an inner diameter 
slightly larger than the outer diameter of the top electrode 224 is then 
positioned to overlie the prefabricated glass ring 75. A weight 82 is 
placed on pressure cylinder 80 to complete the assembly of the components 
in the jig. The combination of the pressure cylinder 80 and the weight 82 
is in the range of 20-110 grams. The jig may also support a plurality of 
diodes, however, each diode should be supported as discussed above. 
The components of the diode as assembled and illustrated in FIG. 13 are 
then placed in the fusion furnace and subjected to the controlled 
atmosphere and temperature cycle previously described with respect to 
diode 2 illustrated in FIG. 1. This causes the prefabricated glass ring 75 
to become soft and fuse to form ring-like glass member 240 of FIG. 6. 
Alternatively, preformed glass rings of the type illustrated in FIG. 8 
could have been used rather than the glass preform 75. Prefabricated glass 
ring 75 can be formed by sintering suitable powdered glass as previously 
discussed. 
An important factor in selecting the time temperature profile for the 
furnace to assure success of the process discussed above is a careful 
examination of the contraction characteristics of the glass as well as the 
temperature characteristics of the materials comprising the fusion. 
The jig illustrated in FIG. 13 can be used to construct the diode 20. 
Similarly, the jig illustrated in FIG. 9 may be used to construct the 
second diode 220. 
The thermal expansion of glass is largely determined by the nature of the 
vitreous network. Unlike the crystalline state encountered in metals and 
other materials, the vitreous state is not fixed and can vary continuously 
depending on prior heat treatment. In the case of borosilicate glasses, it 
may even depend on the previous melting history. For example, a rapidly 
cooled glass has a higher specific volume than the same composition cooled 
more slowly. Actually, there is no single path that glass follows during 
contraction. FIG. 14 demonstrates that IP-745 glass used in encapsulation 
the diodes 20 (FIG. 1) can be cooled from 515.degree. C. in various ways 
to give an entirely different contraction path. Glass type IP745 is a lead 
borosilicate glass having a composition by weight of 36.+-.4% SiO.sub.2, 
15%.+-.3% B.sub.2 O.sub.3, 45.+-.3% PbO and 3.+-.1% Al.sub.2 O.sub.3. The 
variations in the contracting paths, depending on the way the glass is 
cooled, are shown as a shaded area in FIG. 14. If the glass is stabilized 
for sufficient time, e.g., 20 minutes, at its deformation temperature of 
approximately 515.degree. C., it will shrink along a path defined by the 
upper boundary of the shaded area illustrated in FIG. 14. However, if the 
same glass is heated close to its deformation point and cooled rapidly, an 
entirely different vitreous state will be formed and high shrinkage occurs 
along a path defined by the lower boundary or the shaded area illustrated 
in FIG. 14. The behavior of glass on cooling is generally considerably 
different than that on heating and depends to a great extent on the method 
of annealing and thermal history. Upon cooling, glass will not follow the 
heating curve and normally will not shrink to its original volume. 
The silicon as well as various refractory metals and alloys thereof which 
might be considered for component parts of the diode have relatively well 
behaved temperature characteristics. That is, they tend to expand and 
contract along a single line as illustrated in FIG. 14. By contrast, the 
temperature characteristics of the glass are nonlinear and also not 
necessarily repeatable since various characteristics of the glass are 
dependent on its prior history, as discussed above. Therefore, the key to 
making the process described above work is to select a glass and the 
temperature cycle for fusing the glass such that harmful stresses are not 
developed during the process. 
The process described above results in the glass having a contraction 
characteristic within the upper area of those that are possible. Using 
this temperature cycle discussed above, it is practical to fuse thick 
(i.e. greater than 30 microns) glass layers directly to the outer surface 
of the fusion to form a layer protecting the PN junction. Using this 
cycle, it is also possible to incorporate the outer metal ring 252 into 
the structure with the outer metal ring 252 maintaining the fused glass 
slightly in compression. Even though the process described here does not 
require outer metal compression rings as an essential part of the glass 
encapsulation, such components, if desired for packaging or any other 
reason, can be incorporated into the process. Suitable metals for 
performing this function include Kovar, titanium and steel. Kovar is a 
trademark for an alloy consisting of 20% nickel, 17% cobalt, 0.2% 
manganese with the balance iron. This alloy is also sold under the trade 
name Fernico. A suitable steel is type 304. This steel has a composition 
by weight of 0.08% carbon (max.), 2.0% manganese (max.), 1.0 % silicon 
(max.), 19.0% chromium, 10% nickel with the remainder iron. Molybdenum has 
too low a temperature contraction coefficient resulting in the glass ring 
being placed in tension which is sufficiently high to rupture the glass. 
By contrast, nickel has a rather high temperature expansion coefficient 
resulting in the glass being placed in sufficient compression to cause the 
rupture of the diode. It should also be noted that outer rings of ceramic 
materials are also usable. For example, ceramics, including zircon 
(Z.sub.2 SiO.sub.4), mullite (3Al.sub.2 O.sub.3 2SiO.sub.2), porcelain, 
titanium (TiO.sub.2) and spinel (MgAl.sub.2 O.sub.4) are also usable. 
It should also be noted that thermal history for the glass tubing from 
which the glass rings are cut effects its thermal characteristics. Since 
commercially available stress relieved glass tubing is made from molten 
glass all of the prefabricated glass rings cut from such tubing can be 
considered as having substantially the same prior thermal histories. 
However, it is believed that slightly higher yields might be realized by 
carefully controlling the manufacturing process for these tubes. 
Additionally, the glass rings can be formed by sintering powered glass. 
However, as previously mentioned, superior results have been achieved by 
using rings cut from stress relieved glass tubing.