U.S. patent number RE33,859 [Application Number 07/608,506] was granted by the patent office on 1992-03-24 for hermetically sealed electronic component.
This patent grant is currently assigned to John Fluke Mfg. Co., Inc.. Invention is credited to I. Macit Gurol.
United States Patent |
RE33,859 |
Gurol |
March 24, 1992 |
Hermetically sealed electronic component
Abstract
A ceramic substrate supports a thin or thick film electronic
circuit hermetically enclosed by a vitreous glass covering sealed
to the ceramic substrate by a heat fused vitreous sealing glass.
The vitreous sealing glass is screened onto the vitreous glass
covering in a composition comprising a binder material and a
liquifier. The electronic circuit is trimmed by a laser beam
directed through the vitreous glass covering as one of the final
process steps after completion of those process steps which tend to
affect the resistivity of the resistive element; process steps such
as high temperature baking and soldering of component parts.
Inventors: |
Gurol; I. Macit (Seattle,
WA) |
Assignee: |
John Fluke Mfg. Co., Inc.
(Everett, WA)
|
Family
ID: |
27085785 |
Appl.
No.: |
07/608,506 |
Filed: |
November 2, 1990 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
779643 |
Sep 24, 1985 |
04725480 |
Feb 16, 1988 |
|
|
Current U.S.
Class: |
428/76; 174/564;
361/748; 428/209; 428/210; 428/426; 428/433; 428/901; 501/13;
501/69 |
Current CPC
Class: |
H01C
17/242 (20130101); H01L 23/295 (20130101); H01L
23/3121 (20130101); H01L 23/3142 (20130101); H05K
3/28 (20130101); H01L 21/702 (20130101); Y10T
428/24917 (20150115); H01L 2924/09701 (20130101); H05K
1/0306 (20130101); H05K 1/167 (20130101); H05K
2201/017 (20130101); H01L 2924/0002 (20130101); Y10T
428/24926 (20150115); Y10T 428/239 (20150115); H01L
2924/0002 (20130101); H01L 2924/00 (20130101) |
Current International
Class: |
H01C
17/22 (20060101); H01L 21/70 (20060101); H01L
23/28 (20060101); H01L 23/31 (20060101); H01L
23/29 (20060101); H01C 17/242 (20060101); H05K
3/28 (20060101); H05K 1/03 (20060101); H05K
1/16 (20060101); B32B 003/02 () |
Field of
Search: |
;174/52.2 ;361/397
;428/76,209,426,433,901,210 ;501/13,17,21,69,70 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1014735 |
|
Dec 1965 |
|
GB |
|
1015549 |
|
Jan 1966 |
|
GB |
|
1098752 |
|
Jan 1968 |
|
GB |
|
Primary Examiner: Ryan; Patrick J.
Attorney, Agent or Firm: Hauptman; Benjamin J.
Claims
What is claimed is:
1. A hermetically sealed device comprising:
a substrate providing a base for supporting electronic circuitry
thereon;
electronic circuitry disposed on said substrate; and
a vitreous glass covering disposed over and hermetically sealing
said electronic circuitry with said vitreous glass covering sealed
onto said substrate, wherein said vitreous glass covering has a
sealing temperature below about 380.degree. C. .Iadd.at pressures
above about 100 psi.Iaddend..
2. The invention as claimed in claim 1 wherein said vitreous glass
covering has a sealing temperature between about 370.degree. C. and
375.degree. C.
3. A hermetically sealed device comprising:
a substrate providing a base for supporting electronic circuitry
thereon;
electronic circuitry disposed on said substrate; and
a vitreous glass covering disposed over and hermetically sealing
said electronic circuitry with said vitreous glass covering sealed
onto said substrate, wherein the coefficient of thermal expansion
between said substrate and said vitreous glass covering is within
about two millionths (2.times.10.sup.-6).degree.C. .Iadd.wherein
said vitreous glass covering has a sealing temperature below about
380.degree. C. at a pressure above about 100 psi.Iaddend..
4. A hermetically sealed device comprising:
a substrate providing a base for supporting electronic circuitry
thereon;
electronic circuitry disposed on said substrate; and
a vitreous glass covering disposed over and hermetically sealing
said electronic circuitry with said vitreous glass covering sealed
onto said substrate, wherein said vitreous glass covering is
transparent to light at laser frequencies .Iadd.wherein said
vitreous glass covering has a sealing temperature below about
380.degree. C. at a pressure above about 100 psi.Iaddend..
5. A hermetically sealed device comprising:
a substrate providing a base for supporting electronic circuitry
thereon;
electronic circuitry disposed on said substrate; and
a vitreous glass covering disposed over and hermetically sealing
said electronic circuitry with said vitreous glass covering sealed
onto said substrate, wherein said substrate is selected from a
group consisting of alumina, beryllia, steatite, magnesium alumina
silicate, titanium dioxide, silicon carbide, and zircon; and said
vitreous glass covering is a uniformly dispersed mixture of silica,
soda ash, and lime combined with metal oxides from the group
consisting of boron, calcium, lead, lithium, titanium, and cerium
.Iadd.wherein said vitreous glass covering has a sealing
temperature below about 380.degree. C. at a pressure above about
100 psi.Iaddend..
6. A hermetically sealed device comprising:
a substrate providing a base for supporting electronic circuitry
thereon;
electronic circuitry disposed on said substrate;
a covering disposed over said electronic circuitry; and
a vitreous glass seal disposed around said electronic circuitry
fused to said covering and said substrate to hermetically seal said
electronic circuitry wherein said vitreous glass seal has a fusion
temperature below about 380.degree. C. at pressures above about 100
psi.
7. The invention as claimed in claim 6 wherein said covering is a
vitreous glass having a softening temperature at least 50.degree.
C. above the fusion temperature of said vitreous glass seal at the
same pressure.
8. The invention as claimed in claim 7 wherein the difference in
the coefficient of thermal expansion between said vitreous glass
covering and said vitreous glass seal is within two millionths
(2.times.10.sup.-6) per .degree.C.
9. The invention as claimed in claim 8 wherein said vitreous glass
covering is transparent to light at laser frequencies.
10. A hermetically sealed device comprising:
a substrate providing a base for supporting electronic circuitry
thereon;
electronic circuitry disposed on said substrate; and
a vitreous glass covering disposed over and hermetically sealing
said electronic circuitry with said vitreous glass covering sealed
onto said substrate, wherein said substrate is a ceramic comprised
of material selected from the group consisting of alumina,
beryllia, steatite, titanium dioxide, magnesium alumina silicate,
silicon carbide, and zircon .Iadd.wherein said vitreous glass
covering has a sealing temperature below about 380.degree. C. at a
pressure above about 100 psi.Iaddend..
11. The invention as claimed in claim 10 wherein said covering is a
ceramic comprised of material selected from the group consisting of
alumina, beryllia, steatite, titanium dioxide, magnesium alumina
silicate, silicon carbide, and zircon.
12. The invention as claimed in claim 11 wherein said substrate and
covering are of the same material and the coefficient of thermal
expansion of the material is within about two millionths
(2.times.1.sup.-6).degree.C. of the coefficient of thermal
expansion of said vitreous glass seal.
13. The invention as claimed in claim 6 wherein said vitreous glass
seal is a uniformly dispersed mixture of silica, soda ash, and lime
combined with metal oxides from the group consisting of boron,
calcium, lead, lithium, titanium, and cerium.
14. The invention as claimed in claim 13 wherein said covering is
of the same material as said seal.
15. A hermetically sealed device comprising:
a ceramic substrate providing a base for supporting electronic
circuitry thereon;
electronic circuitry deposited on said substrate;
a vitreous glass covering disposed over said electronic circuitry;
and
a vitreous glass seal disposed around said electronic circuitry
fused to said covering and said substrate to hermetically seal said
electronic circuitry, said covering being a separate member from
said seal.
16. A hermetically sealed device comprising:
a ceramic substrate providing a base for supporting electronic
circuitry thereon;
electronic circuitry deposited on said substrate;
a vitreous glass covering disposed over said electronic circuitry;
and
a vitreous glass seal disposed around said electronic circuitry
fused to said covering and said substrate to hermetically seal said
electronic circuitry, wherein said vitreous glass seal has a fusion
temperature between 370.degree. C. and 375.degree. C. at
substantially 200 psi.
17. The invention as claimed in claim 16 wherein said covering is a
vitreous glass having a softening temperature above 420.degree. C.
at substantially 200 psi.
18. The invention as claimed in claim 17 wherein the difference in
the coefficient of thermal expansion between said vitreous glass
covering and said vitreous glass seal is within one millionth
(1.times.10.sup.-6).
19. The invention as claimed in claim 18 wherein said vitreous
glass covering is a borosilicate glass and is transparent to light
at laser frequencies.
20. A hermetically sealed device comprising:
a ceramic substrate providing a base for supporting electronic
circuitry thereon;
electronic circuitry deposited on said substrate;
a vitreous glass covering disposed over said electronic circuitry;
and
a vitreous glass seal disposed around said electronic circuitry
fused to said covering and said substrate to hermetically seal said
electronic circuitry, wherein said substrate is a ceramic comprised
of alumina, and has a coefficient of thermal expansion within one
millionth (1.times.10.sup.-6) of the coefficient of thermal
expansion of said vitreous glass seal and two millionths
(2.times.10.sup.-6) of said vitreous glass covering .Iadd.wherein
said vitreous glass covering has a sealing temperature below about
380.degree. C. at a pressure above about 100 psi.Iaddend..
21. A hermetically sealed device comprising:
a substrate providing a base for supporting electronic circuitry
thereon;
a vitreous glass covering disposed over and heremetically sealing
said electronic circuitry with said vitreous glass covering sealed
onto said substrate, wherein electronic circuitry is a resistive
element tending to become unstable at temperatures above
380.degree. C. .Iadd.wherein said vitreous glass covering has a
sealing temperature below about 380.degree. C. at a pressure above
about 100 psi.Iaddend..
22. The hermetically sealed device as recited in claim 6, wherein
electronic circuitry is a resistive element tending to become
unstable at temperatures above 380.degree. C.
23. The hermetically sealed device as recited in claim 16, wherein
electronic circuitry is a resistive element tending to become
unstable at temperatures above 380.degree. C.
Description
TECHNICAL FIELD
The present invention relates to apparatus and method for making
electronic components and more particularly to hermetically sealing
a glass or ceramic covering to a glass or ceramic substrate by
sealing glass material which is heat fused to the covering and to
the substrate.
BACKGROUND OF THE INVENTION
Microelectronic technologies commonly utilize both thin and thick
film microcircuitry on a glass or ceramic substrate; thin films
being generally less than 5 microns in thickness (per Mil Std 883-C
and the International Society of Hybrid Microelectronics'
definition) and thick films being considerably thicker. These
films, comprising patterns of resistors, conductors, and sometimes
capacitors applied by conventional film processing techniques, may
include "add-on" discrete components such as transistors, diodes,
etc., which are attached to the conductor and/or resistor portion
of the pattern by solder, wire bonds, or other techniques. In the
event a resistor film package is desired, there discrete add-on
components are not included.
In thin film resistor technology, the resistor/conductor components
are often made of vacuum deposited Nichrome, a registered trademark
of Driver-Harris Co., comprising 80% by weight of nickel, 20% by
weight of chromium. Thin film patterns are generally photoetched to
be long and serpentine in order to provide a sufficient number of
squares to achieve the required resistance, whereas thick film
patterns are often rectangular, using various resistivity thick
films.
When a thin film resistor is deposited, it is often desirable to
temperature anneal the resistive element at high temperature in
order to adjust its temperature coefficient of resistance (TCR).
Typically, the resistive element/substrate is baked for an hour or
so at a temperature of approximately 350.degree. C. It is often
then desirable to subject the resistive element/substrate to a
stabilization bake where the resistive element is heated at a high
temperature over a period of several days so that it will retain
its desired resistive value when subjected to temperature cycling
in an operational circuit. After the stabilization bake, the
substrate is scribed into rows and columns so that it may be
separated easily into individual die pieces. Package leads are then
soldered to conducting "pads" located on the substrate surface, and
these package leads may also be secured to the substrate by an
epoxy. Other components, such as transistors, diodes, etc., are
incorporated as part of the circuit, if desired.
Thick film technology components are fabricated in a multitude of
steps beginning initially with the creation of a pattern or "mask"
which provides an outline for depositing the resistor/conductor
material onto the substrate. Commonly used mask materials include
etched metal and emulsion-screens. Emulsion-screens are typically
constructed of stainless steel woven mesh utilizing a mesh count of
about 40 to 156 per centimeter. The screens are coated with an
emulsion which is hardened into a predetermined pattern by exposure
to ultraviolet light, with the remaining non-hardened emulsion
removed from that portion of the screen where the pattern is to be
printed.
After the substrate is cleaned, typically by mechanical scrubbing
action or by ultrasonic treatment utilizing deionized water or a
proprietary soap, the substrate is blown off with nitrogen and the
surface is dehydrated by baking in an oven for a specified time
period.
After the substrate has been prepared, the screen is positioned
relative to the substrate and the composition to be printed onto
the substrate, often referred as an "ink", is applied to the
screen. The "active" materials present in the ink composition
depend upon the purpose for which the film is intended to be used.
The active materials may comprise electrochemical metals or alloys
for resistor films, or dielectric materials for insulating films.
The screen is positioned a precise distance above the substrate
defined as the "snap off distance". A mechanically operated
squeegee is moved at a predetermined velocity across the top of the
screen at a predetermined angle to push the ink composition through
the screen and onto the surface of the substrate. After the
squeegee has passed over a portion of the screen, the screen snaps
off the surface of the substrate returning to its original
position.
The consistency of the ink is important because it must have a
sufficiently low viscosity to flow through the screen and then to
settle onto the substrate filling in the gaps left by the screen,
yet it must be sufficiently viscous to retain its basic shape after
the screen has returned to its snap off position. An organic
vehicle is normally included as part of the ink composition to
provide the desired consistency. The flow characteristics of the
ink composition are quite complex and are generally a function of
the shear rate of the composition as it is pushed through the
screen. The ink is dried and then it is fired in an oven where the
organic vehicle and binders are burned off and the ink is bonded to
the substrate.
Attachment of the package leads to the electronic circuitry inside
the package is accomplished using thermal compression or ultrasonic
sealing of aluminum or gold wire leads. The small diameter gold or
alumina wire constitute a significant error factor in the overall
value of the resistive element, particularly when these leads are
applied to small value resistors.
In order to protect the thin or thick film electronic circuitry, as
well as to provide a means for heat dissipation, film networks are
packaged generally in metal, ceramic or plastic; plastic being the
most popular because of its low cost. If, however, the anticipated
operating environment of the package is projected to be severe, a
hermetic enclosure or coating is required to enclose the electronic
component in an inert, dry atmosphere. High temperatures and
humidity accelerate chemical processes such as oxidation, corrosion
and electrolytic action, which erode the metallic elements, whereas
moisture absorption creates mechanical stresses which vary the
resistance value of the resistive element.
The bonding materials normally used to provide severe environment
hermetic seals are gold-tin eutectic solder or a solder glass such
as PbO--ZnO--Pb.sub.2 O.sub.3 ; these solders are for sealing
primarily ceramic packages and are undesirable for precision
components because of AC coupling effects. Glass is generally not
used as a packaging material. This is because easy to seal glasses
do not provide complete hermetic sealing while complete hermetic
sealing glasses have high sealing temperatures which adversely
affect the electronic component.
As noted above, a principal concern in the fabrication of film
resistive elements is the maintenance of absolute, as well as
relative, values of the patterned resistors. Absolute accuracy is
defined as the difference between the actual value of the resistor
and the denoted value of the resistor; whereas relative accuracy,
which is critical when resistors comprise a voltage divider, is the
difference between the actual ratio of the resistor values and the
denoted ratio of the resistor values. Modern electronic instruments
often require absolute and relative resistor accuracies of several
parts per million.
Excellent absolute accuracy can be achieved by laser trimming the
resistor pattern. Conventionally, laser trimming is accomplished
prior to the final packaging of the resistive element. Packaging,
however, whether it be plastic or metal, can often affect the
precise resistor values achieved by laser trimming due to the
deposition of materials onto the resistive element, as well as the
high temperatures utilized in the packaging process.
Other conventional apparatus and methods include those described in
U.S. Pat. No. 3,845,443--Fisher, which discloses a glass coated
resistive thermometer comprising a resistive element supported on a
alumina substrate and covered with a glass precoat. The resistive
element and glass precoat are also coated with an alumina top coat
which is "welded" to both the glass precoat and the resistive
element.
In U.S. Pat. No. 3,926,502--Tanaka, et al, there is disclosed a
liquid crystal display cell comprising two glass substrates
disposed in a parallel, spaced apart relationship, and hermetically
sealed together along their edges by a layer of glass having a
melting point of about 450.degree. C., thereby forming a space
between the plates for receiving a liquid crystal substance.
In U.S. Pat. No. 3,412,462--Stutzman, et al, there is disclosed a
method of making hermetically sealed thin film modules wherein a
glass substrate blank is melted onto a metal substrate to form a
hermetic glass-to-metal seal.
In U.S. Pat. No. 4,207,604--Bell, et al, there is disclosed a
capacitive pressure transducer comprising a pair of disc-shaped
members held in an adjacent parallel relationship by a glass frit
fired to permanently fuse the two members in said relationship.
Although it has been recognized that glass materials may be fused
as "covering" directly to metal and alumina substrates to provide a
hermetic seal for electronic circuitry supported on the substrates,
it has not been heretofore possible to fuse vitrified glass to a
substrate at temperatures sufficiently low to avoid adversely
affecting the electronic circuitry. Vitreous glass is a
thermoplastic material which melts and flows at the same
temperature each time it is thermally processed. Devitrified glass
is a thermosetting material which crystalizes by surface nucleation
on a time-temperature relationship.
Devitrified glass has been used in substrates because its thermal
stability and chemical durability are improved over the original
glass. Further, it will fuse at much lower temperatures than
vitrified glass. Unfortunately, it is much more permeable to
moisture than vitrified glass.
Further, conventional apparatus and methods have not provided for
laser trimming of a hermetically sealed resistive element after
completion of those process steps which can affect the absolute
value of the resistive element.
SUMMARY OF THE INVENTION
Accordingly, it is a general aim of the present invention to
provide an improved hermetically sealed electronic component and a
method for making same. There is also provided a vitreous glass
sealing composition for screening a heat fusible sealing glass onto
the package components wherein the sealing composition exhibits
good screening characteristics while maintaining its
consistency.
The present invention further provides an improved electronic
component on a ceramic substrate capped by a covering hermetically
sealed by a vitreous glass sealing material.
In one of its more detailed aspects, the present invention provides
an improved hermetically sealed device which comprises a ceramic
substrate providing a base for supporting electronic circuitry
thereon and method of making same. The hermetically sealed device
also includes a vitreous glass covering to cover the electronic
circuitry, and an agent for bonding the transparent glass covering
to the substrate. The bonding agent provides a seal between the
covering and the substrate to hermetically enclose the electronic
circuitry. The bonding agent comprises a sealing glass composition
heated to a sufficient temperature under a sufficient pressure to
cause the sealing glass to fuse to the glass covering and to the
substrate. The difference between the coefficient of thermal
expansion of the substrate and the fused sealing glass is selected
to be within about parts per million per degree centigrade. The
sealing glass composition is fused to the substrate and to the
glass covering at a temperature below about 380.degree. C. during
which time the sealing glass is compressed between the glass
covering and substrate at a pressure of between 100 psi to about
1,500 psi.
In the preferred embodiment, the sealing glass is fused to the
substrate and to the glass covering at a temperature between about
370.degree. C. and about 375.degree. C. during which time the
sealing glass is compressed between the glass covering and the
substrate at a pressure of about 200 spi. At atmospheric pressure,
the sealing glass has an unacceptably high fusion temperature of
about 415.degree. C.
In an alternate embodiment, there is disclosed a hermetically
sealed device comprising a ceramic substrate providing a base for
supporting electronic circuitry thereon. The hermetically sealed
deice includes a ceramic covering to cover the electronic circuitry
and an agent for bonding the ceramic covering to the ceramic
substrate. The bonding agent provides a hermtic seal between the
covering and the substrate to hermetically enclose the electronic
circuitry. The bonding agent comprises a sealing glass composition
heated to a sufficient temperature to cause vitreous sealing glass
to fuse to the covering and to the substrate. The difference
between the coefficient of thermal expansion of the substrate and
the fused vitreous sealing glass is selected to be within about
parts per million per degree centigrade. The difference between the
coefficient of thermal expansion of the ceramic covering and the
fused vitreous sealing glass is also selected to be within about
parts per million per degree centigrade.
In another embodiment of the present invention, there is disclosed
a hermetically sealed device comprising a vitreous glass substrate
to provide a base for supporting electronic circuitry thereon. The
hermetically sealed device includes a glass material to cover the
glass substrate and the electronic circuitry. The glass material
comprises a vitreous sealing glass heated to a sufficient
temperature to fuse the sealing glass to the substrate. The
difference between the coefficient of thermal expansion of the
fused vitreous glass and the vitreous glass substrate is selected
to be within about parts per million per degree centigrade.
In another embodiment of the present invention there is disclosed a
method of making a hermetically sealed device comprising the steps
of providing a ceramic substrate for receiving electronic circuitry
thereon and providing a vitreous glass covering to cover the
electronic circuitry. The glass covering is joined to the substrate
with a sealing composition which includes a vitreous sealing glass.
The glass covering is fused to the ceramic substrate by heating the
sealing composition to a temperature below about 380.degree. C.,
and then compressing the sealing composition, during the heating
step, between the glass covering and the ceramic substrate at a
pressure between about 100 psi to about 1,500 psi. The sealing
composition comprises an effective amount of the vitreous glass
material in granular form, an effective amount of a cellulose
binder, and an effective amount of a liquifier selected from the
group consisting of pine oil or a dihydric alcohol having the
formula: ##STR1## wherein R is H or C.sub.1 to C.sub.3 alkyl. The
cellulose binder most preferably comprises hydroxypropyl cellulose,
and the liquifier most preferably comprises pine oil.
The electronic circuitry preferably comprises a thin film resistive
element. By covering the resistive element with a vitreous glass
covering which is transparent at laser light frequencies, a laser
beam may be directed through the glass covering, after the fusing
step, to trim the resistive element to a predetermined resistive
value. This results in precision resistor trimming because the
trimming operation is conducted after those process steps which can
affect the value of the resistive element.
In another embodiment of the present invention there is disclosed a
method of making a hermetically sealed device comprising the steps
of providing a vitreous glass substrate for receiving electronic
circuitry thereon, and covering the electronic circuitry and the
vitreous glass substrate with a sealing composition including
vitreous sealing glass material therein. The vitreous sealing glass
is fused to the glass substrate and to the electronic circuitry so
as to hermetically seal the electronic circuitry by heating the
sealing composition to the temperature below about 380.degree. C.
and compressing, during the heating step, the sealing composition
against the glass substrate at a pressure between about 100 psi to
about 1,500 psi.
While the present invention is susceptible to various modifications
and alternative forms, specific embodiments thereof have been shown
by way of example in the drawings and will herein be described in
detail. It should be understood, however, that it is not intended
to limit the invention to the particular forms disclosed, but on
the contrary, the intention is to cover all modifications,
equivalents and alternatives falling within the spirit and scope of
the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a top view of a resistor package including a glass
covering heremetically sealed to a substrate enclosing therein a
thin film resistive element;
FIG. 2 is a side view of the resistor package of FIG. 1;
FIG. 3 is a flow chart depicting a process for hermetically sealing
a glass covering to a ceramic substrate;
FIG. 4 is a top view of a resistor package including a glass
substrate supporting a thin film resistive element thereon wherein
the resistive element and substrate are covered by a fused glass
material.
DESCRIPTION OF THE INVENTION
The present invention is particularly adapted for hermetically
sealing a resistive element which is supported on a ceramic or
glass substrate with a glass or ceramic covering. By the term
"resistive element", it is meant to include an individual resistor
as well as a plurality of resistors which may comprise a thin or
thick film voltage divider network or the like. The following
invention will be described with reference to hermetically sealed
thin film resistive elements which is the preferred embodiment, as
well as with reference (i) to the processes used for making such
hermetically sealed resistive elements and (ii) to compositions
used to hermetically seal the resistive elements. It should be
understood, however, that the present invention has broader
applications to other hermetically sealed packages such as
integrated circuits, including monolithic bipolar and MOS (metal
oxide semiconductor) monolithic integrated circuits.
In FIG. 1 therein is shown the preferred embodiment of the present
invention comprising a thin film resistor package indicated at 10,
including a thin film resistive element 12 supported on a ceramic
substrate 14. Resistive element 12 is enclosed and hermetically
sealed by a vitreous glass covering 18 which is bonded to the
surface of substrate 14 by a heat fusible, vitreous sealing glass
20 (better shown in FIG. 2), located around resistive element 12.
Resistor package 10 includes package leads 24 projecting outwardly
from the edge of substrate 14 parallel to the plane thereof for
connecting resistor package 10 to an electronic circuit (not
shown). package leads 24 engage a conductive bonding pad 28 located
on the surface of substrate 14 which is connected to resistive
element 12 via conductor leads 26 which are supported on the
surface of substrate 14.
To provide a suitable base for supporting elemental metals forming
the thin film electronic circuitry of the present invention and in
particular resistive element 12, the ceramic substrate 14 is
selected to have the desirable thermal conductivity, dielectric
constant, electrical nonconductivity, surface smoothness and
mechanical strength, when subjected to temperature extremes from
about -55.degree. C. (minimum operational temperature) to about
380.degree. C. (the maximum fabrication temperature). The term
"ceramic" is meant to include those non-metallic inorganic
materials formed through heat processing which are typically used
in microelectronic component substrates and component parts. The
ceramic substrate 14 is generally selected from the group of
ceramics consisting of alumina, beryllia, steatite, titanium
dioxide, magnesium alumina silicate, silicon carbide, and zircon or
combinations thereof. Preferably, however, ceramic substrate 14 is
comprised of alumina or beryllia, and most preferably of alumina.
Preferably, the alumina composition of substrate 14 is greater than
90%; more preferably the alumina composition of substrate 14 is
greater than 96%; and most preferably the alumina composition of
substrate 14 is greater than 99%.
Glass covering 18 is a dispersed mixture of silica, soda ash and
lime, and often combined with metal oxides such, for example, as
boron, calcium, lead, lithium, titanium and cerium, depending upon
the specific properties desired, which are heated to a fusion
temperature and the cooled to a rigid state; preferably glass
covering 18 is made from Micro Sheet, a registered trademark of
Corning Glass Works, Glass Code 0211, and which is a vitreous
borosilicate glass. The glass of glass covering 18 is selected to
be transparent to laser energy to allow conventional laser trimming
of resistive element 12 in a manner to be described in greater
detail hereinafter. Additionally, it is preferable that glass
covering 18 be transparent to visible light to allow for normal
inspection of the resistive element 12.
The glass in the glass covering 18 is selected to have a
coefficient of thermal expansion substantially similar to the
coefficient of thermal expansion of substrate 14 through the
temperature extremes, and which has a softening temperature at
least 50.degree. C., above the fusion temperature of the vitreous
sealing glass 20 at the same pressure (e.g. 420.degree. C. at 200
psi). By "softening" temperature it is meant the temperature at
which the glass first exhibits loss of its structural rigidity; by
the term "fusion temperature" it is meant the temperature at which
the granules of the vitreous sealing glass 20 bond with each other
as well as to any material which they are in contact with.
The vitreous sealing glass 20, a ceramic glass consists of a
uniformly dispersed mixture of silica, soda ash and lime often
combined with metal oxides such as boron, calcium, lead, depending
upon the specific properties desired. The glass 20 has a low
melting temperature and, in its unfused state, the sealing
composition is granular in nature having particles of a specific
size. It belongs to that class of materials known as "glass frits"
which are commonly used to join materials such as ceramics and
metals when heated to the fusion temperature.
In order to reduce mechanical stresses between sealing glass 20,
glass covering 18 and substrate 14 caused by temperature extremes
which can result in package 10 losing its hermetic seal, the
difference in the coefficient of thermal expansion between fused
sealing glass 20 and glass covering 18, and between fused sealing
glass 20 and substrate 14 should be preferably within two
(millionths 2.times.10.sup.-6), and more preferably within one
(millionth 1.times.10.sup.-6). The total difference between the
glass covering 18 and the substrate 14 should not be greater than
four (millionths 4.times.10.sup.-6). The desired percentage of
alumina in substrate 14 is a function of the difference in
coefficients of thermal expansion between substrate 14 and fused
sealing glass 20, as well as a function of the ability of sealing
glass 20 to bond with substrate 14.
Referring now to FIG. 3 there is shown a flow chart depicting a
preferred process for fabricating resistor package 10 illustrated
in FIGS. 1 and 2. Starting materials include substrate 14 in block
100, glass covering 18 in block 102 and glass frit 21 in block 104.
To apply the glass frit 21 to covering 18 which is cleaned in block
106, a binder material is used to hold the granular particles of
glass together. A liquifier is included with the binder material to
reduce the viscosity thereof to form the sealing composition 23 in
block 108. The viscosity should be sufficiently low to allow the
sealing composition 23 of glass mixture, binder and liquifier, to
be deposited onto a screening apparatus and pushed through a
screen, prepared in block 110, onto covering 18. The liquifier
should not be so volatile as to evaporate between successive
applications of the sealing composition, yet it should be
sufficiently volatile to evaporate when raised to the fusion
temperature of sealing glass 20.
A preferred sealing composition 23 includes an effective amount of
a cellulose binder; more preferably the binder is hydroxalkyl
cellulose wherein the alkyl group contains from 1 to 4 carbon
atoms; and most preferably the binder comprises hydroxypropyl
cellulose.
The sealing composition is formed by adding, to an effective amount
of binder, an effective amount of a liquifier selected from the
group consisting of pine oil having a distilling range between
about 200.degree. C. and about 225.degree. C. and comprising
secondary and tertiary terpene alcohols, or dihydric alcohols
having the formula: ##STR2##
wherein R is H or C.sub.1 to C.sub.3 alkyl or more preferably the
liquifier is pine oil. The binder/liquifier mixture when combined
with the glass frit 21 results in a sealing composition 23 which
has excellent consistency when deposited on a screening device,
which flows through the screening device when subjected to a
pushing force, and which regains its consistency when deposited on
substrate 14 thereby maintaining the shape of the screen pattern.
So as not to adversely affect resistive element 12, the
liquifier/binder composition decomposes within one hour at firing
temperatures below 420.degree. C. and more preferably below
350.degree. C.
It should be appreciated that the amount of granulated glass
utilized will depend upon the area of substrate 14 desired to be
covered as well as the desired thickness of sealing glass 20. On
the other hand, the relative amounts of binder and liquifier will
depend largely upon the screen mesh size used in depositing the
composition. A larger screen mesh size normally requires a smaller
ratio of liquifier to binder, whereas a smaller screen mesh size
will require a larger ratio of liquifier to binder. However, when
utilizing a binder of hydroxypropyl cellulose and a liquifier of
pine oil with a screen mesh size of 165, the ratio in parts by
total weight of binder/liquifier to sealing glass is from about 1:2
to about 1:9 wherein the ratio of parts by total weight of
liquifier to binder is about 25:1 to 250:1; a preferred sealing
composition comprises a ratio in parts by total weight of
liquifier/binder to sealing glass of about 1125 to 166, wherein the
ratio of parts by total weight of liquifier to binder is about 165
to 1.
The sealing composition 23 is deposited on glass covering 18
utilizing a thick film screening process wherein a screen pattern
is prepared using conventional techniques described previously. The
sealing composition is deposited onto a screen positioned
horizontally above glass covering 18 and the pushed through the
screen onto glass covering 18 by a horizontally propelled squeegee
in a manner known to one of ordinary skill in the art. Adjustments
to squeegee pressure, horizontal velocity, and snap off distance
are made to obtain the sharpest possible pattern by observing the
lay down. It should be appreciated that the sealing composition 23
may be screened onto substrate 14 instead of glass covering 18,
although screening the sealing composition 23 onto glass covering
is preferable. Often substrate 14 will include a protective oxide
layer, such as silicon oxide when substrate 14 comprises an alumina
composition, so that sealing glass 20 is fused directly to the
substrate oxide layer.
In an exemplary embodiment of the present invention illustrated in
FIG. 1, a screen pattern is designed to deposit annular patterns of
sealing composition 23 onto a sheet of glass coverings, called a
die, so that when a covering 18 is bonded to a substrate 14 the
sealing glass 20 surrounds a resistive element 12 and covers a
portion of conducting leads 26. The thickness of sealing
composition deposited on glass covering 18 is dependent upon the
smoothness of the surface of glass covering 18 and substrate 14. It
is important that sufficient sealing glass 20 be deposited onto
glass covering 18 to fill in any imperfections or "valleys" which
may exist later when substrate 14 and glass covering 18 are joined
together. In addition, it is preferable to achieve a thickness of
sealing glass 20 such that when substrate 14 and glass covering 18
are compressed together a small gap filled with the sealing glass
20 exists between their respective parallel surfaces. In order to
achieve these desired results, the sealing composition 23 is
applied to the glass covering die in block 112 at a thickness of
about 6 mils, later when being fused, the sealing composition will
be compressed to approximately 3 mils when pressure is applied to
bring glass covering 18 and substrate 14 together. To deposit the
desired thickness of the sealing composition onto glass covering
18, it is preferable to use a double additive process wherein a
layer of sealing composition is deposited and dried, and a second
layer is deposited onto the first layer and then dried thereon. The
glass covering 18 are then fired to remove the binder and vehicle
from the sealing composition 23.
After cleaning substrate die in block 114, resistive element 12 is
added thereto in block 116. The term "added" is meant to encompass
both thin and thick film additive and subtractive processes such
as, but not limited to, vacuum deposition, screening, wet chemical
etching, dry chemical etching such as sputter etching, plasma
etching and ion beam etching, as well as other microelectronic
patterning processes known to those persons of ordinary skill in
the art.
After the glass covering die and substrate die have been scribed
and broken into individual glass covering 18 and individual
substrates 14 in blocks 118 and 120, respectively, glass covering
18 is placed against substrate 14, and sandwiching the sealing
glass 20 therebetween in preparation for fusion in block 122.
Resistive element 12, particularly if made of Nichrome, is
temperature sensitive such that temperatures above 380.degree. C.
may cause resistive element 12 to become unstable and to change
from its desired resistive value. Conventional vitreous sealing
glasses, including the sealing vitreous sealing glass 20, will not
fuse below 400.degree. C. It has been found, in the present
invention that compression of the vitreous sealing glass 20 between
glass covering 18 and substrate 14 for a period between 5 minutes
to 2 hours, and preferably 1/2 hour, at compression pressure
applied to glass covering 18 and substrate 14 between about 100 to
about 1,500 psi, and preferably at about 200 psi, causes fusion of
the sealing glass 20 to glass covering 18 and substrate 14 at
temperatures below 380.degree. C. At temperatures from about
370.degree. C. to about 375.degree. C. and about 200 psi it is
possible to obtain a moisture impermeable hermetic vitreous glass
seal between glass covering 18 and substrate 14. Because sealing
glass 20 is fused at temperatures similar to those used for
adjusting the temperature coefficient of resistance (TCR) of
resistive element 12, there is no need for a separate TCR adjusting
step as absolutely essential utilized in conventional resistor
fabrications.
Fused sealing glass 20 forms a hermetic seal across conducting
leads 26 to maintain the hermeticity of package 10. The air trapped
in the space defined by the opposing surfaces of glass covering 18
and substrate 14 and by sealing glass 20 is relatively dry so as
not to adversely affect resistive element 12 hermetically enclosed
therein. Sealing glass 20, although in contact with conductor leads
26, is non-conductive and therefore does not interfere with the
conductivity of leads 26. In addition, the glass-to-metal interface
between fused glass 20 and conductor leads 26 does not
significantly increase capacitive coupling when resistor package 10
is used in high frequency applications.
After joining glass covering 18 to substrate 14, a stabilization
bake step may be performed in block 124 to stabilize resistive
element 12 against changes in its resistivity when it is eventually
placed in a operational environment and cycled through various
temperatures. The stabilization bake is conducted at temperatures
between 100.degree. C. to 200.degree. C. which is sufficiently low
to avoid adversely affecting the resistive properties, such as TCR,
of resistive element 12.
After the stabilization bake, package leads 24 are soldered to pads
28 and then epoxied to substrate 14 in block 126. The number and
location of package leads 24 is a function of the intended use of
resistive package 10, however, it is understood that package leads
may be positioned on substrate 14 as illustrated in FIG. 1 to form
a single in-line package (SIP); or at opposite sides of substrate
14 to form a dual in line package (DIP); or at all sides of
substrate 14. Further, solderable pads may be proved for surface
mounting. In addition, package leads 24 may be mounted
perpendicular as well as parallel to the plane of substrate 14.
Connecting leads 26, which connect resistive element 12 to package
leads 24, are patterns of elemental metal deposited on substrate
14, the resistivity of which is controlled by the geometry of the
lead pattern as well as the material selected to form the leads.
Therefore, when resistive element 12 comprises a small value
resistor, the pattern chosen for connecting leads 26 is enlarged to
reduce the resistivity thereof and thus reduce any error introduced
into the value of resistive element 12. Preferably, connecting
leads 24 comprise a sandwich of metal depositions including an
initial Nichrome layer deposited onto substrate 14, and a nickel
layer deposited on top of the Nichrome layer. A gold layer may be
deposited on the nickel layer in order to achieve a very low
resistivity.
It can be appreciated that many of the previously discussed steps
for fabrication of resistive package 10, such as heat fusion of the
sealing glass to glass covering 18 and substrate 14, as well as
soldering of package leads 24 to bonding pads 28, tend to introduce
incremental changes into the value of resistive element 12 when
these processes are performed prior to precision trimming of
resistive element 12. The transparent glass covering 18 used in the
present invention, however, allows for laser trimming in block 128
of resistive element 12 as one of the final steps in the
fabrication of resistor package 12 and after completion of those
process steps which affect the value of resistive element 12 and
after completion of the stabilization bake process step.
In the present invention laser trimmings conducted after hermetic
packaging by directing the laser beam through glass covering 18 and
onto resistive element 12 to remove, as well known in the art,
various shunts which affect resistance. Greater trimming precision
is achieved in the present invention because those process steps
which affect the value of resistive element 12 have been completed
prior to laser trimming. It has been found in final testing in
block 130 that the removal of minute portions of resistive element
12 during the trimming operation in a hermetically sealed space has
little or not adverse effect on the resistance.
Where the covering is made from a non-transparent material, the
stabilization and laser trimming steps are performed prior to
fusing covering 18 to substrate 14 as would be evident to those
skilled in the art.
In another embodiment of the present invention, there is
illustrated in FIG. 4 a resistor package indicated at 32 including
a transparent vitreous glass substrate 34 supporting a thin film
resistive element 36. The substrate 34 is dispersed mixture of
silica, soda ash, and lime often combined with such metal oxides as
boron, calcium, lead, etc. depending upon the specific properties
desired; a typical example of which is Pyrex Glass Brand No. 7740,
a borosilicate glass and a registered tradmark of Corning Glass
Works.
The resistive element 36 is encapsulated in a vitreous glass
covering 38 which is identical to the composition of the sealing
glass which was heat fused to form fused sealing glass 20 (FIG. 2)
used in the fabrication of resistor package 10. This glass covering
38 is formulated in a sealing composition identical to that sealing
composition used in the fabrication of resistor package 10. The
resistor package 32 includes package leads 40 projecting outwardly
from the edge of substrate 34 parallel to the plane thereof for
connecting resistor package 32 to an electronic circuit (not
shown). Each package leads 40 engages a conductive bonding pad 42
located on the surface of substrate 34 which is connected to
resistive element 36 via respective conductor leads 44 which are
supported on the surface of substrate 34.
The process steps depicted in FIG. 3 for fabricating package 10 are
identical with those described in fabrication of resistor package
32 except: (i) the screen pattern used for depositing sealing glass
20 in an annular configuration is modified to provide a deposition
of glass coating 38 over the entire surface of resistive element 36
and a portion of the surrounding substrate 34; and (ii) there is no
separate covering 18 utilized in this embodiment, therefore glass
covering 38 is screened directly onto glass substrate 34 where it
is subjected to the time/temperature cycling parameters described
in the preferred embodiment in order to obtain a sealed vitreous
glass bonded with substrate 34. It has been found that pressure is
not an absolute requirement for this embodiment.
The stabilization bake process and laser trimming steps are
conducted as described previously for fabrication of resistor
package 10 although sequentially they could be performed prior to
deposition of the glass coating 38. When it is desired to laser
trim as one of the final process steps, the laser trim is
accomplished by directing the laser beam through the transparent
glass substrate 34. Therefore, the material of substrate 34 must be
transparent to the frequency of laser light being employed for
trimming.
As many possible embodiments may be made of the invention without
departing from the scope thereof, it is to be understood that all
matters set forth herein and shown in the accompanying drawings are
to be interpreted in an illustrative and not a limiting sense.
* * * * *