Note: Descriptions are shown in the official language in which they were submitted.
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THIN FILM STRUCTURE FOR CERAMIC SUBSTRATES
Background of the Invention
The present invention relates to the field of
metallized substrates for communications applications
and, more particularly, to a means of tuning thin film
structures such as stripline filters, using automated
low-power laser trimming.
Metallization on non-conductive substrates such as
ceramics is well known in the art for such circuit compo-
nents as stripline filters. Such filters typically
include a configuration or pattern of conductive elements
on one surface of the ceramic plate, these elements
interacting with a relatively large area of ground plane
on the opposing substrate surface. For electrical
reasons, the ground plane is required to be a very low
conductivity layer, typically including a heavy (one mil)
layer of plated copper. Filters or other such components
may require removal of a portion of the conductive ground
plane for tuning or other adjustment. Such adjustments
have been made by abrasion; i.e., mechanically removing
small portions of the ground plane in the appropriate
areas, using a diamond grinding wheel or sand trimmer.
It will be obvious that such a method does not lend
itself to automated trimming procedures, where the cir-
cuitry is monitored as it is tuned and the monitoring
equipment controls an extremely precise laser beam.
Thick film resistors on substrates can be trimmed
precisely by low-power (1.8 kw input power) laser beams,
but that power is not sufficient to trim a heavily plated
ground plane.
Summary of the Invention
It is therefore an object of the present invention
to provide a means of trimming a ground plane or the like
with a low-power laser beam.
It is a particular obje t to provide such a means
utilizing standard process steps.
It is another object to provide additionally for
removal of all conductive layers on other portions of the
substrate.
These objects, and others which will become
apparent, are provided in a struc~ure and accompanying
method in accordance with the present invention. On a
lS non-conductive wafer, one or more conductive circuit
components are formed on a first surface. A conductive
layer or ground plane is formed on a substantial portion
of the second surface of the wafer opposite at least one
element of a circuit component which requires a fine
tuning or trimming adjustment. The ground plane will
consist of a thin conductive layer, such as copper
evaporated over titanium, a heavy conductive layer such
as plated copper, and a thin protective layer of a noble
metal such as gold. The heavy copper layer will have an
~5 aperture opposite at least a portion of the component to
be adjusted. Therefore, tuning or trimming can be accom-
plished by removing discrete areas of the thinner layers
within the aperture in the heavy layer, using a relative-
ly low-power laser beam, such as those used to t-rim thick
film resistors.
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More p~rticularly, there is pro~ided;
A tunable device for use in communications equipment
and comprising:
a non-conductive wafer;
conductive circuit elements formed on a first
surface of said wafer; and
a conductive element formed on a substantial
portion of the second surface of said wafer, positioned
opposite at least one of said circuit elements on the
first wafer surface, and including a first conductive
layer, a superimposed second conductive layer, the second
layer being substantially thicker than the first ~ayer
and having an aperture opposite the at least one circuit
element on the first wafer surface, and a third conduc-
ti~e layer superimposed on the exposed portions of the
first and second conductive layers and formed of a noble
metal.
There is also provided:
A method of tuning an apparatus for use in communica-
tions equipment comprising the steps of:
providing a non-conductive wafer;
providing on a first major surface of said wafer at least
one conductive circuit element;
providing on the second major surface of said substrate
a relatîvely large conducti~e area having generally a first
predetermined thickness and having, opposite the at least one
circuit element, a portion of a second predetermined thickness,
the second predetermined thickness being substantially thinnex
than the first predetermined thickness; and
evaporating appropriate portions of the thinner portion
of the large conductive area by means of a low-power laser
beam having sufficient energy to evaporate the thinner portions
of the conductive areas on the second major surface and insuf-
ficient energy to evaporate the thicker portions; thereby
altering the capacitance between the conductive elements on the
opposing sides of ~aid wafer~
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There is further provided:
~ method of tuning an apparatu8 for use in communi
cation8 equipment comprising the following steps:
ta~ providing a non-conductive wafer;
~a'l providing conductive circuit elements on a first
major sur~ace of the wafer;
(b~ evaporating a relatively thin layer o~ metal onto a
second major surface o~ the wafer~
Cc) covering said thin metal layer w~th a layer of photo-
sensitive material;
Cd~ positioning a ma5k o~er said photo-sensitive material;
Ce2 exposing at least one portion of said photo-sensitive
material to radiation through said mask;
L~2 develop~ng said photo~sensitive material;
(gl removing the uncured portions of said photo-sensitive
mater~al;
(hl plating a relatively heavy layer of metal on those
portions of the first metal layer not covered by cured
photo-sen~itive material;
Ci) removing the cured portions of the photo-sensitive
material;
(~1 plating a layer of noble metal over all exposed metal
surfaces; and
(s'l tuning the apparatus ~y evaporating appropriate por-
tions of those areas of metal lacking the relatively
heavy plated layer and thereby reducing the capaci-
tance between the opposing metal layers by means of
a lo~-power laser beam having sufficient energy to
evaporate the thinner portions of the conductive areas
on the ~econd major surface and insufficient energy
to eVaporate the thicker portions.
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Brief Description of the Drawing
Fig. 1 is a bottom view of a stripline filter sub
strate.
Fig. 2 is a cut-away, perspective view of a portion
of the substrate of Fig. 1, along the line 2-2.
Fig. 3 shows the successive steps in one preferred
embodiment of the invention.
Fig. 4 shows the successive steps in another embodi-
ment, which provides for areas of bare substrate.
Detailed Description of the Preferred Embodiment
For purposes of clarity, the existing thin film
process will be briefly described. On a non-conductive
substrate, a very thin layer of titanium is deposited
first to provide adhesion for the subsequent layers of
metal. Next, a thin layer of copper is evaporated over
the titanium. A photo-resist is applied and exposed to
ultraviolet light through a mask, the unexposed areas
resulting eventually in conductive areas. After the
resist is developed, the unhardened portions are stripped
off and a thick layer of copper is plated on the areas of
bare copper. A thin layer of gold is plated on over all,
then the hardened resist is stripped from the substrate.
Etching subse~uently removes all thin layers down to the
bare substrate in the desired areas.
As may be seen in the drawing, the invention is used
for the fine tuning (trimming) of a stripline filter on a
substrate 10 ~see Figs. 2-4) such as a ceramic wafer. In
this example, the visible surface of the substrate is
covered by a ground plane 12 having t-hree apertures or
"trim windows" 14. The other surface of the substrate 10
has on it ~as indicated by dashed lines in Fig. 1) ele-
ments 16A, 16B and 16C of the filter circuit.
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Stripline filters are well known in the art, andhave previously been constructed using a layer of heavy
copper throughout the ground plane. Tuning or other fine
adjustment has been accomplished by use of an abrasion
techni~ue using a diamond grinding wheel. No further
description of the circuit aspects of the filter will be
given here, since the present invention is only concerned
with the capability of fine tuning by the removal of
discrete areas of the ground plane with a low-power laser
beam.
In the cut-away, perspective view of Fig. 2, the
filter of Fig. 1 is shown cut along the line 2-2 with the
edges of circuit elements 16A, B and C visible on the
lower surface (normally considered the top side of the
filter), The ground plane 12 and windows 14 are on the
upper si~e of the ceramic wafer 10 in this view and it is
apparent that the copper layer within the window 14 is
thinner than it is in the adjacent areas. It is to be
noted that no attempt has been made to scale any of the
drawing figùres; instead, typical dimensions are given as
appropriate in the text.
Fig. 3 shows a series of progressive steps in
producing the ground plane only of one embodiment of the
invention. Fig. 3A shows the bare substrate 10 and Fiy.
3B shows a very thin (500 A) added layer 18 of deposited
titanium. In Fig. 3C, a thin (10,000 A) layer 20 of cop-
per has been evaporated over the titanium layer 18. In
Fig. 3D, a film 22 of a photo-resist (such as that sold
commercially as Riston~ 218R) has been added, and a mask
24 is shown in position to expose the area 14' of the
photo-resist film 22 for providing one window 14 (Figs.
1 and 2). After the photo-resist has been exposed to
ultraviolet light through the mask, and developed, the
unhardened areas are removed as shown in Fig. 3E. A
heavy (0.7 to 1.2 mils) layer 25 of copper is plated on
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all areas except area 14, as shown in Fig. 3F. The hard-
ened resist 22 is then stripped off, as shown in Fig. 3G.
As shown in Fig. 3H, a thin (5000 A) layer 26 of gold is
plated over all exposed copper, both thick and thin
layers. In the tuning operation, a low-powered laser
beam can be manually or automatically controlled to
remove as much of the thin layers of gold and copper in
any of the windows 14 as is required; e.g., for tuning a
stripline filter.
In Fig. 4 is seen the series of progressive steps in
an alternate embodiment which can provide both the trim
windows 14 as shown in Fig. 3 and areas of bare substrate
which may be desired for other purposes than tuning.
Figs. 4A-4D are the same as Figs. 3A-3D except that
the mask 24-l has an area 14' for providing the window or
aperture 14 and another area 28' for providing an area
28 (see Fig. 4M) of bare substrate. After the photo-
resist 22 has been exposed to ultraviolet light through
the mask 24 and developed, the unhardened resist is
removed as in Fig. 4E, leaving areas 22-l and 22-2. The
heavy copper layer 25 is then plated on ~Fig. 4F) follow-
ed by the thin layer 26 of gold (Fig, 4G). The hardened
areas of photo-resist 22 are then stripped off as shown
in Fig. 4H, and another layer 30 of the photo-resist is
applied over all (Fig. 4I). All areas except area 14' of
the photo-resist are exposed through a second mask 24-2,
then the photo-resist is developed. After the unhardened
resist is removed (Fig. 4J); i.e., the portion in the
area 14', that area is plated with gold (layer 26) as
shown in Fig. 4K. When the hardened resist is removed
(Fig. 4L), all exposed surfaces have been plated with
gold except for the area 28'. An etching process (Fig.
4M) will remove the thin copper and titanium layer to
provide bare substrate in the area 28.
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Thus there has been shown and described a means of
providing a tunable circuit element on a substrate using
only the standard process steps, and of tuning said
element with a low-power laser beam. It is intended to
cover all modifications and variations which fall within
the spirit and scope of the appended claims.
