U.S. patent application number 09/737596 was filed with the patent office on 2002-05-02 for nickel barrier end termination and method.
This patent application is currently assigned to Harris Ireland Development Company, Ltd.. Invention is credited to McLoughlin, Neil.
Application Number | 20020050911 09/737596 |
Document ID | / |
Family ID | 25387847 |
Filed Date | 2002-05-02 |
United States Patent
Application |
20020050911 |
Kind Code |
A1 |
McLoughlin, Neil |
May 2, 2002 |
Nickel barrier end termination and method
Abstract
A method of providing nickel barrier end terminations for a zinc
oxide semiconductor device with exposed body surfaces and end
terminal regions, in which the device is controllably reacted with
a nickel plating solution only on an exposed end terminal region
and thereafter provided with a final tin or tin-lead
termination.
Inventors: |
McLoughlin, Neil;
(Blackrock, IE) |
Correspondence
Address: |
L. Lawton Rogers, III
Suite 300
1401 Eye Street, N.W.
Washington
DC
20005
US
|
Assignee: |
Harris Ireland Development Company,
Ltd.
|
Family ID: |
25387847 |
Appl. No.: |
09/737596 |
Filed: |
December 18, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09737596 |
Dec 18, 2000 |
|
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|
08885859 |
Jun 30, 1997 |
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Current U.S.
Class: |
338/21 ;
338/20 |
Current CPC
Class: |
H01C 17/28 20130101;
H01C 1/142 20130101; H01C 7/18 20130101; H01C 7/112 20130101 |
Class at
Publication: |
338/21 ;
338/20 |
International
Class: |
H01C 007/10 |
Claims
What is claimed is:
1. A method of making a semiconductor device, the body of the
semiconductor device having an exposed zinc oxide surface and
nickel end terminations, the method comprising the steps of: (a)
providing a semiconductor body having electrically conductive
plates interleaved with zinc oxide layers; (b) providing a selected
nickel plating solution for an intended method of nickel plating;
and (c) controllably contacting an end of the semiconductor body
with the nickel plating solution in order to form a desirably thick
nickel barrier cap over the end of the semiconductor body without
forming a nickel barrier cap over the entire semiconductor
body.
2. The method of claim 1, wherein the temperature of the nickel
plating solution is uncontrolled and remains at approximately room
temperature.
3. The method of claim 1, wherein the pH of the nickel plating
solution is maintained between about 2 and about 6.
4. The method of claim 1, wherein contact between the semiconductor
body and the nickel plating solution is maintained for a period of
approximately 10 to 120 minutes.
5. The method of claim 4, where contact between the semiconductor
body and the nickel plating solution is maintained until the
thickness of the nickel barrier cap is between approximately 1 and
3 um.
6. The method of claim 1, further comprising the step of forming an
solderable contact by partially immersing the nickel barrier cap in
an acid solution comprising one or more of Alkyl-Tin,
Alkyl-Tin-Lead, Tin-Lead sulphuric acid, or Tin sulphuric acid with
a pH between about 3 to about 6 at room temperature.
7. The method of claim 6, wherein the immersion of the nickel
barrier cap in the acid solution is for a period of about 10 to
about 120 minutes.
8. The method of claim 7, further including the application to the
nickel barrier cap of a biasing current of approximately 0.3 to 2.0
A/dm.sup.2.
9. The method of claim 7, wherein the immersion of the nickel
barrier cap in the acid solution continues until a solderable
contact having a thickness of 3 to 6 um is formed.
10. The method of claim 1, wherein the nickel plating solution is a
room temperature solution comprising one or more of nickel
sulphate, dimethylamineborane, lactic acid, ammonium citrate, and
ammonia.
11. The method of claim 10, wherein the zinc oxide layers have a
resistivity in the range from about 10.sup.10 to about 10.sup.12
Ohms/cm.sup.2.
12. The method of claim 1, wherein the contact is by partial
immersion in the nickel plating solution.
13. The method of claim 12, including the further steps of applying
a termination material comprising silver and glass frit onto the
end of the semiconductor body; and firing the semiconductor body to
mechanically bond the termination material with the end of the
semiconductor body.
14. The method of claim 13, wherein the termination material is
essentially free of platinum and palladium; and wherein the
termination material is fired at a temperature between about
550.degree. and 800.degree. C.
15. The method of claim 13, wherein the nickel plating solution
includes one or more of (i) nickel sulphate or nickel chloride,
(ii) boric acid, (iii) a wetting agent, and (iv) a stress relieving
agent at a temperature of about 50.degree. to 70.degree. C.
16. The method of claim 15, including the further step of applying
a biasing current of about 0.3 to about 2.0 A/dm.sup.2 during
nickel plating.
17. The method of claim 16, wherein the biasing current is variably
dependent on the area of the end of the semiconductor to be
coated.
18. The method of claim 12, wherein the immersion depth of the
semiconductor body ia controlled to thereby selectively control the
distance that the barrier cap extends upwardly from the end of the
semiconductor body.
19. The method of claim 1, wherein the controllable contact is by
impregnated absorbent material.
20. A method of providing a semiconductor device having a body with
an exposed zinc oxide surface and electrically conductive,
solderable metal end terminations, the method comprising the steps
of: (a) providing a semiconductor body having electrically
conductive plates interleaved with zinc oxide layers; (b) applying
a termination material comprising silver and glass frit onto
opposing ends of the semiconductor body; (c) mechanically bonding
the termination material to the ends of the semiconductor body by
firing; (d) providing at a temperature of about 50.degree. to
70.degree. C. a nickel plating solution comprising one or more of
(i) nickel sulphate or nickel chloride, (ii) boric acid, (iii) a
wetting agent, and (iv) a stress relieving agent; (e) coating a
silver terminated end of the semiconductor body by selectively
partially immersing the end of the semiconductor body in the nickel
plating solution for a period of about 15 to about 120 minutes
while applying a biasing current of about 0.3 to 2.0 A/dm.sup.2 to
thereby form a desirably thick nickel barrier cap in contact with
the silver terminated end which extends a selected distance up the
body of the semiconductor device; (f) providing a final termination
solution of one or more of alkyl-tin, alkyl-tin-lead, tin-sulfuric
acid or tin-lead-sulfuric acid, having a pH from about 3 to about 6
and an uncontrolled temperature; and (g) forming a desirably thick,
electrically conductive, solderable contact end termination over
the nickel barrier cap by selectively partially immersing the end
of the semiconductor body into the final termination solution for a
period of about 10 to about 120 minutes while applying a biasing
current of about 0.3 to about 2.0 A/dm.sup.2.
21. The method of claim 20, wherein the pH of the nickel plating
solution is maintained between about 2 and about 6.
22. The method of claim 20, wherein the silver termination material
is provided free of platinum and palladium and is fired onto the
semiconductor body at a temperature between about 550.degree. and
about 800.degree. C.
23. The method of claim 20, wherein the partial immersion of the
semiconductor body in the nickel plating solution is continued
until the thickness of the nickel coating is between about 1 and
about 3 um.
24. The method of claim 20, wherein the solderable contact is about
3 to about 6 um thick.
25. The method of claim 20, wherein the distance that the barrier
cap extends from the end of the semiconductor body is controlled by
controlling the immersion depth.
26. The method of claim 20, wherein the biasing current is varied
as a function of the area of semiconductor to be coated.
27. A method of providing metal end terminations to a semiconductor
device without the use of a plating resist comprising the steps of:
(a) providing a semiconductor body having a zinc oxide exterior
with electrically conductive elements interleaved between ceramic
layers consisting principally of zinc oxide; (b) providing a nickel
plating solution comprising one or more of nickel sulphate,
dimethylamineborane, lactic acid, ammonium citrate, and ammonia at
room temperature; (c) positioning one end of the semiconductor body
a selectable distance into the nickel plating solution for a period
of about 15 to about 120 minutes to thereby form a desirably thick
nickel barrier cap over the end of the semiconductor body; (d)
providing a metal termination solution of either: alkyl-tin,
alkyl-tin-lead, tin-sulfuric acid, or tin-lead-sulfuric acid,
having a pH between about 3 to about 6; and (e) forming a metal
termination over the nickel barrier cap by partially immersing an
end of the semiconductor body into the metal termination solution
for a period of about 10 to about 120 minutes while applying
biasing current of about 0.3 to about 2.0 A/dm.sub.2.
28. The method of claim 27, wherein the pH of the nickel plating
solution is maintained between about 2 and about 6.
29. The method of claim 27, wherein the semiconductor body is
immersed in the nickel plating solution until the thickness of the
nickel coating is between about 1 and about 3 um.
30. The method of claim 27, wherein the nickel barrier cap is
coated with a solderable contact 3 to 6 um thick.
31. The method of claim 27, further including the step of providing
a silver fired termination on the end of the semiconductor body
prior to partial immersion in the nickel plating solution.
32. The method of claim 27, wherein the zinc oxide resistivity is
between about 10.sup.10 to about 10.sup.12 Ohms/cm.sup.2.
33. A method of providing a metal termination on a semiconductor
device comprising the steps of: (a) providing a semiconductor body
having a zinc oxide exterior with electrically conductive elements
interleaved between ceramic layers consisting principally of zinc
oxide; (b) providing at room temperature a nickel plating solution
comprising one or more of nickel sulphate, dimethylamineborane,
lactic acid, ammonium citrate, and ammonia; (c) impregnating an
absorbent material with the nickel plating solution; (d)
positioning one end of the semiconductor body in contact with
absorbent material for a period of about 15 to about 120 minutes to
thereby form a desirably thick nickel barrier cap covering that end
of the semiconductor body and a selected portion of the body of the
semiconductor device immediately contiguous thereto; (e) providing
a metal termination solution of one or more of: alkyl-tin,
alkyl-tin-lead, tin-sulfuric acid, or tin-lead-sulfuric acid having
a pH between about 3 and about 6; and (f) forming a desirably thick
metal termination over the nickel barrier cap by partially
immersing an end of the semiconductor body into the metal
termination solution for a period of about 10 to about 120 minutes
while applying a biasing current of about 0.3 to about 2.0
A/dm.sub.2.
34. The method of claim 33, wherein the semiconductor body is
maintained in contact with the absorbent material for a period
sufficient to form a nickel barrier thickness of about 1 to about 3
um.
35. The method of claim 33, including the further step of moving
the semiconductor body relative to the absorbent material.
36. A varistor comprising: a body of interleaved resistive plates
and zinc oxide layers having an external surface of zinc oxide free
of any passivation material; and nickel barrier caps on opposing
ends of the body, the nickel barrier caps terminating with
naturally formed edges.
37. The varistor of claim 36, further comprising a silver barrier
between the body and the nickel barrier.
38. The varistor of claim 36, where a nickel barrier cap is between
about 1 and about 3 um thick.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to nonlinear resistive
devices, such as varistors, and more particularly to methods of
making such devices using controllable plating techniques in which
the exposed end terminals of the device are plated with nickel
barrier terminations while the exposed semiconductor body remains
unplated.
[0002] Nonlinear resistive devices are known in the art, and are
described, for example, in U.S. Pat. No. 5,115,221 issued to Cowman
on May 19, 1992, and is incorporated by reference.
[0003] With reference to the prior art shown in FIG. 1, a typical
device 10 may include plural layers 12 of semiconductor material
with electrically conductive electrodes 14 between adjacent layers.
A portion of each electrode 14 is exposed in a terminal region 16
so that electrical contact may be made therewith. The electrodes 14
may be exposed at one or both of opposing terminal regions, and
typically the electrodes are exposed at alternating terminal
regions 16 as illustrated. The exposed portions of the electrodes
14 are contacted by electrically conductive end terminals 18 that
cover the terminal regions 16.
[0004] While an apparently simple structure, the manufacture of
such devices has proved complex. For example, the attachment of the
end terminals 18 has proved to be a difficult problem in search of
a simplified solution. Desirably, the terminal regions 16 may be
plated with nickel and tin-lead metals to increase solderability
and decrease solder leaching. The process parameters in plating
nickel to zinc oxide semiconductor bodies has proved particularly
vexing and has required complex solutions.
[0005] One method of affixing the end terminals 18 is to use a
conventional barrel plating method in which the entire device is
immersed in a plating solution. However, the stacked layers are
semiconductor material, such as zinc oxide, that may be conductive
during the plating process so that the plating adheres to the
entire surface of the device. Thus, in order to provide separate
end terminals as shown in FIG. 1, a portion of the plating must be
mechanically removed after immersion, or covered before immersion
with a temporary plating resist comprised of an organic substance
insoluble to the plating solution. However, the removal of the
plating or organic plating resist is an extra step in the
manufacturing process, and may involve the use of toxic materials
that further complicate the manufacturing process.
[0006] It has also been suggested that the metal forming the end
terminals 18 be flame sprayed onto the device, with the other
portions of the surface of the device being masked. Flame spraying
is not suitable for many manufacturing processes because it is slow
and includes the creation of a special mask, with the additional
steps attendant therewith. See, for example, U.S. Pat. No.
4,316,171 issued to Miyabayashi, et al. on Feb. 16, 1982.
[0007] It is also known to react a semiconductor body, having
electrically conductive metal end terminations, with phosphoric
acid to selectively form a phosphate on the semiconductor body
prior to providing end terminations using conventional barrel
plating. See, U.S. Pat. No. 5,614,074 issued to Ravindranathan on
Mar. 25, 1997.
[0008] As illustrated by the above known methods, a simplified
manufacturing process for the attachment of the end terminals 18
has proved to be a illusive.
[0009] Accordingly, it is an object of the present invention to
provide a novel method and device that obviates many of the prior
art problems.
[0010] Accordingly, it is an object of the present invention to
provide a novel method of manufacturing a semiconductor device by
controllably reacting an exposed zinc oxide semiconductor device
having an exposed end terminal region with a nickel plating
solution to form a nickel barrier end termination over the
semiconductor body end without plating the entire exposed
semiconductor device.
[0011] It is another object of the present invention to provide a
novel method of providing a semiconductor device by controllably
partially immersing an exposed semiconductor body having a silver
termination with a nickel plating solution while applying a biasing
current to form a nickel barrier cap extending a selected distance
up the exposed body of the semiconductor device.
[0012] It is yet another object of the present invention to provide
a novel method of providing a semiconductor body with a nickel
barrier cap without the use of a plating resist by positioning an
exposed end of the semiconductor body a selectable distance into a
nickel plating solution for a controlled period.
[0013] It is still another object of the present invention to
provide a novel method of providing metal termination of an exposed
semiconductor body by contacting an end of the semiconductor body
with an absorbent material impregnated with a nickel plating
solution.
[0014] It is a further object of the present invention to provide a
novel semiconductor device having naturally formed nickel
terminations over a body of resistive plates interleaved between
zinc oxide layers.
[0015] It is yet a further object of the present invention to
provide a novel method of directly nickel plating zinc oxide bodies
having a preferred zinc oxide volume resistivity for the plating
method selected.
[0016] It is still a further object of the present invention to
provide a novel method of manufacturing zinc oxide semiconductor
devices minimizing solder leaching by providing a platinum-free and
palladium-free silver termination and thereupon forming a nickel
barrier termination.
[0017] These and many other objects and advantages of the present
invention will be readily apparent to one skilled in the art to
which the invention pertains from a perusal of the claims, the
appended drawings, and the following detailed description of the
preferred embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a pictorial depiction of a prior art varistor.
[0019] FIG. 2 is a vertical cross section of an embodiment of the
device of the present invention.
[0020] FIG. 3 is a vertical cross section of another embodiment of
the device of the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0021] With reference now to FIG. 2, an embodiment of a nonlinear
resistive element 20 may include a body 22 having stacked zinc
oxide semiconductor layers 24 with generally planar electrodes 26
between adjacent pairs of layers 24. The zinc oxide layers 24 need
not be comprised of pure zinc oxide and may be comprised of a
ceramic consisting principally of zinc oxide. Each electrode 26 may
have a contactable portion 28 that is exposed for electrical
connection to nickel barrier end terminations 30 that cover
terminal regions 32 of the body 22 and contact the electrodes 26.
The exterior portion of body 22 not covered with the end
terminations 30 remain as exposed zinc oxide surface 38. Nickel
barrier end terminations 30 may be plated with layers 34 of
electrically conductive, solderable tin or tin-lead metal that form
electrically contactable solderable end portions for the resistive
element 20.
[0022] With further reference to FIG. 3, in another embodiment of a
nonlinear resistive manufactured using the method of the present
invention, element 20 includes body 22 having stacked zinc oxide
semiconductor layers 24 and generally planar electrodes 26 between
adjacent pairs of layers 24. Each electrode 26 may have a
contactable portion 28 exposed for electrical connection to a first
electrically conductive metal (preferably silver, platinum-free
silver, or palladium-free silver) end terminations 36 with nickel
barrier end terminations 30 thereupon, covering terminal regions 32
and extending a desired distance along body 22. As with the
embodiment illustrated by FIG. 2, nickel barrier end terminations
30 may be plated with layers 34 of solderable tin or tin-lead metal
that form final electrically contactable end portions for the
resistive element 20.
[0023] By way of example, in one embodiment the zinc oxide layers
24 may have the following composition in mole percent: 94-98% zinc
oxide and 2-6% of one or more of the following additives; bismuth
oxide, cobalt oxide, manganese oxide, nickel oxide, antimony oxide,
boric oxide, chromium oxide, silicon oxide, aluminum nitrate, and
other equivalents.
[0024] In a first embodiment of the method of the present
invention, body 22 may be provided conventionally, electrodes 26
hazing contactable portions 28 exposed for electrical connection at
terminal regions 32 with the remaining portions of body 22 being
exposed zinc oxide surface 38. Process parameter control to avoid
process boundary problems including: 1) plating not occurring, 2)
plating not uniformly covering terminal regions 32, 3) plating too
thick or thin; and 4) plating spread beyond the desired terminal
region 32 onto exposed zinc oxide surface 38, requires the
selection of nickel plating solution appropriate for an intended
method of nickel barrier end termination plating-electro-plating- ,
electroless plating, or brush plating. Having determined the method
of nickel plating, an end of body 22 controllable contacts the
nickel plating solution to form a desirably thick nickel barrier
end terminations 30 over terminal region 32. Complimentary
parameter processes selection, identification of nickel plating
solution, plating method, and controllable contact assures that
nickel barrier end terminations 30 uniformly cover terminal region
32 without extending undesirably along exposed surface 38 and while
avoiding unacceptable zinc oxide etching, which etching is known to
cause electrical leakage currents and mechanical weakness in the
final device.
[0025] With the appropriate parameter selection, the method of the
present invention desirable allows the temperature of the nickel
plating solution to remain uncontrolled such that the solution
remains at approximately room temperature. The pH of the nickel
plating solution may be maintained between 2 and 6. Contact between
semiconductor body 22 and nickel plating solution may vary from 15
to 120 minutes to allow the formation of end termination 30 with a
thickness between 1 and 3 um.
[0026] One embodiment of the present invention further includes
forming solderable contact 34 over end termination 30 by
controllably immersing the nickel termination 30 into a room
temperature solution containing one of Alkyl-tin, Alkyl-tin-lead,
Tin-Lead sulfuric acid, or tin sulfuric acid having a pH from 2 to
6. The partial immersion may vary in the range from 10 to 120
minutes to allow the formation of solderable contact 34 with a cap
thickness ranging from 3 to 6 um. Desirably, solderable contact
plating may include application of a biasing current of
approximately 0.3 to 2.0 A/dm.sup.2.
[0027] Another embodiment of the present invention is preferably
suited to electroless and brush plating methods for forming nickel
end terminations 30. For this embodiment, a nickel plating solution
comprising a room temperature solution of nickel sulphate,
dimethylamineborane, lactic acid, ammonium citrate, and ammonia may
be used in combination with semiconductor body 22 having zinc oxide
layers 24 with a resistivity in the range from 10.sup.10 to
10.sup.12 Ohms/cm. The pH of the nickel plating solution may be
maintained between 2 and 6.
[0028] For electroless plating, one end of semiconductor body 22 is
positioned a selectable distance into the nickel plating solution
covering that end of body 22 and allowing the plating solution to
travel up a portion of exposed zinc oxide surface 38. Maintaining
body 22 immersed for a period of 15 to 120 minutes provides for a
nickel cap between 1 and 3 um.
[0029] For brush plating, a suitable absorbent material is
impregnated with the nickel plating solution. One end of
semiconductor body 22 is placed in contact with the impregnated
absorbent material such that terminal region 32 completely contacts
the absorbent material. Pressure between body 22 and absorbent
material is maintained to allow formation of nickel end termination
30 on terminal region 32 and a desired distance along exposed zinc
surface 38. The contact period may vary between 15 and 120 minutes
to control termination 30 thickness and travel up surface 38.
Relative motion may be provided so that semiconductor body 22 moves
relative to the absorbent material.
[0030] In another embodiment of the present invention, particularly
suitable for electroplating, a first electrically conductive metal
end termination 36 is provided intermediate end termination 30 and
body 20 and further includes providing a nickel plating solution
comprising one of nickel sulphate or nickel chloride, boric acid, a
wetting agent, and a stress relieving agent with the plating
solution maintained at a temperature of 50.degree. to 70.degree. C.
First end termination 36 material may preferably comprise silver,
platinum-free silver, and/or palladium-free silver and glass frit.
The use of platinum-free and/or palladium-free silver reduces the
cost of device manufacture. The silver/glass frit material may be
conventionally applied onto opposing ends of body 20 and fired to
mechanically bond the silver/glass frit materials to terminal
regions 32 forming first end terminations 36. Firing temperatures
of 550.degree. to 800.degree. C. have provided favorable
results.
[0031] Body 20 with first end termination 36 is partially immersed
into the nickel plating solution for a period from 15 to 120
minutes while applying biasing current of 0.3 to 2.0 A/dm.sup.2.
Variously controlling immersion depth, immersion time, and biasing
current will control nickel barrier termination 30 thickness and
travel upward along exposed zinc surface 38.
[0032] Optionally, a final solderable termination may be provided
over nickel end termination 30 using a room temperature solution of
one of Alkyl-tin, Alkyl-tin-lead, Tin-Lead sulfuric acid, or tin
sulfuric acid. Solder plating solutions having a pH in the range of
approximately 3 to 6 have been suitable when layers 34 are formed
with an immersion period ranging from 10 to 120 minutes and a
biasing current of 0.3 to 2.0 A/dm.sup.2. In the present invention,
solder leaching is minimized without the use of more expensive
platinum or palladium by coating first end termination 36 with
nickel termination 30 so as to avoid silver leaching when the
varistor device is soldered to a board.
[0033] While preferred embodiments of the present invention have
been described, it is to be understood that the embodiments
described are illustrative only and the scope of the invention is
to be defined solely by the appended claims when accorded a full
range of equivalence, many variations and modifications naturally
occurring to those of skill in the art from a perusal hereof.
* * * * *