Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a process for depositing
electrical circuit components, particularly conductors and resistors on
an insulating substrate. The invention relates also to microcircuits
including components or interconnects formed using such a process.
The common techniques for deposi~ing such circuit
components are the so-called thick and thin film processes.
Conventional screen-printed thick film methods are
capable of resolving conductor line spacing as small as 5 mil (125
microns) using mesh screens and approximately 3 mil (75 microns) using
more expensive screens and metal masks. However, the current design
trends in electronic packaging techniques require complicated geometry
and fine line resolution which are beyond the limits of conventional
screen printing technology. Recently a number of photolithographic
adaptations of these techniques have been developed to satisfy the new
demands but they require many additional processing steps with
corresponding increases in processing and equipment costs. High
resolution is attainable using thin film techni~ues but only at
additional expense incurred at having to develop and etch
photo-resistivè materials.
A medium film process is now proposed which produces
films intermediate thin and thick films in terms of resolution and, to
some extent, process steps. The process is loosely based on a
colography9 a known method of applying powdered material to solid
substrates to obtain multi-coloured images on ceramic materials, United
States Patent 3,637,385, Hayes et al, issued on January 25, 1972. In
this imaging process, a solid, positive-acting or negative-acting
radiation sensitive oryanic layer having a thickness of Ool to 40
microns is exposed to actinic radiation in an image
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receiving manner. As a result of selective exposure-related hardening
or softening, the organic layer is divided into particle receptive and
particle non-receptive regions. A layer of free~flowing powder
particles having a diameter along at least one axis of at least 0.3
microns up to 25 times the thickness of the organic layer is then
applied to the layerO While at a temperature below the melting points
of the powder and of the organic layer, the particles are physically
embedded as a monolayer in a stratum at the sur-face of the light
sensitive layer to yield images having portions varying in density in
proportion to the light exposure of each portion. Subsequently,
non-embedded particles are removed from the organic layer to develop an
image.
The processing conditions, especially the relat;onship
between powder size and organic layer thickness, described in the above
patent to produce multi-coloured monolayer images on a ceramic
- substrate were found to be unsuitable for adaptation of the me~hod to
fabrication of conductor interconnect and resistor patterns for hybrid
micro-electronic applications.
In order to achieve desired predictable electrical
characteristics, the resin should be appreciahly softer than that used
in colography so as to ensure the de-position of a multilayer; in
addition the resin should have a thickness T in the range of 5-20
microns with a powder particle si2e P in the range 0.5-10 microns where
T is not less than P. It is recogni~ed that powder particles will
rarely be perfectly spherical. A reference made in this specification
to a powder particle having a particular diameter indicates that the
particle has this diameter along one axis at leastO
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Conductor and resistor powders are preferably prepared by
firing a quantity of a conductive or resistive thick film ink or paste
and then pulverizing the fired material to a powder of the desired
particle size. Typically thick Film inks include about 5% glass and
each particle produced has a like glass content. The glass content of
the solid component of a thick film ink or paste may, however, vary
from 0 to 90% depending on the nature of the conductor or resistor and
the nature of ~he substra~e material.
Another way of achieving the powder is by dry mixing
finely divided resistive or conductive material together with finely
divided frit and then adding a charge of organic adhesive to produce
consolidated particles of size 0.5 to 10 microns from the finely
divided material.
Following removal of non-embedded particlesg ~he
substrate is fired at a temperature at which the organic layer is
burned off and the powder is sintered to create a homogeneous
electrical film.
Embodiments of the invention will now be described by way
oF example with reference ~o the accompanying drawings in which;-
Figures la to ld are sectional views, not to scale, of
part of a microclrcuît substrate showing stages in a medium film
process according to the invention;
Figure 2 is a block schematic representation of a
sequence of medium film process steps;
Figure 3 is a sectional view to an enlarged scale of a
substrate supporting a resin coating having embedded powder.
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Referring in detail to Eigure 1, a substrate 10 is coated
with a thin layer of an organic photo-sensitive resin 12 (Figure la)
which is then exposed to UV light through a positive photo mask (Figure
lb)o The exposed portion o-F the region 12 cross-links~ hardens and
becomes particle non-receptive. A powder 16 consisting of particles of
fused metal and glass is then applied to the substrate surface and
becomes physically embedded in the soft or unexposed portions 18 o-f the
light sensitive film 12 (Figure lc). Excess powder is removed and
process completed by -firing the substrate 10 at a medium temperature to
drive off t~le organic resin 12 by oxidation and, at a high temperature,
tc. bond the powder particles 16 to each other and to the substrate 10
by sintering and fusion mechanisms (Figure ld).
In the following sections, the materials used and the
processing steps are described in more detail.
MATERIALS
1. Substrates
The medium film technique has been used successfully on
substrates of glass, ceramic and porcelain--on-steel. Generally,
ceramic and porcelain on-steel substrates are cleaned using a process
developed for thick film substrates (P.G. Creter and. D.E. Peters,
Proceedings ISHM ~1977) p. 281), with subsequent ultrasonic cleanirg in
FreonR solvent follc.wed by firing at h;gh temperature (900C
for ceramic and 600C for porcelain-on-steel). Glass substrates are
cleaned us;ng processes developed for thin films (L. Maissel, R. &lang,
"Handbook of Thin Film Technology"~, McGraw-Hi7.1 Inc., New York (1970)
Chapter 6.). The use of contaminated substrates can result in poor
film adhesion and the formation of blisters.
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~ The Resi n
.
An exalnple of photoresist resin is as supplied by Ferro
Corporation (Cleveland, Ohio) under specification no. RV3566E. It is a
positive acting, light sensitive organic polymer. The surface
properties of this polymer can be varied between a soft particle
receptive condition and a hard particle non-receptive condition upon
expos~re to UV light. A photo-catalyzed hardening mechanism such as
photo-polyrnerization, photo-crosslinking or phcto-oxidation occurs. A
plasticizer and/or a photo-activator agent is added to the
photo-sensitive organic polymer for adjusting its powder receptivity
and sensitivity to UV light. The surface receptivity of exposed and
unexposed portions of the polymer film depend on such parameters as the
size of the powder particles, the thickness of the polymer layer, the
UV exposure time, the temperature and humidity of the environment and
the amount of force used in applying the powder. Other resins which
can be used in th~ medium film process are disclosed in the United
States Patent 3,731,831, issued on January 30~ 1973 to Hayes et al,
United States Patent 3,676~121, issued on July 11, 1972 to Jones et al,
and United States Patent 3,723,123, issued on March 27, 1973 to Jones
et al. The resins include resins which are negative acting, the resin
layer being initially particle non~receptive but being softened and
made particle receptive by irradiation, As these a~orementioned
patents indicate, resins can be softened and made more particle
receptive by the addition of a plasticizer or softener. Such resins
can also be made more photosensiti~e by the addition of a
photo-activator. Colography requires a resin which is relatively hard
so that powder particles do not adhere merely on contact with a
particle receptive region. To obtain a monolayer the powder must be
applied to the resin surface and then forcibly embedded. The resin for
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electrical circuit fabrication must be appreciably softer and tackier.
Initially, on spreading powder on the particle receptive areas a
monolayer adheres to the resin surface. Subsequent use of force both
presses powder particles down closer to the substrate and builds powder
layers above the level initially occupied by the resin surface thereby
forming a multilayer.
3. The Powders
a) Conductor Powders
.
Medium film powders required for the fabrication of
conductor interconnects in hybrid microcircuits consist of particles
which have sub-particles of metal or metal alloy and glass. The
metallic component of the powder determines the characteristics of the
conductors for microelectronic applications. These characteristics
include line cross-section, conductivity, solderability, bondability
and compatibili~y with resistor materials in the hybrid microcircuit.
The mos-t commonly used metallic conductors in hybrid microelectronics
are Au, Pt-Au, Au-Pd, Ag and Ag-Pd. Au and its alloys exhibit
excellent properties for microelectronic applications, but are the most
expensive materials. Ag and its alloys are the most widely used
conductor materials for most industrial and commercial hybrid
microelectronic applications. Medium film powders base on these
conductors may be readily prepared using the methods discussed below.
In addition, powders based on cheaper conductors such as copper, have
been made and successFully applied to substrates using a process
according to the inventionO
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Most conductor powders contain glass to pro~ide cohesion
of conductor particles and adhesion of these particles to the
substrate. Almost any low melting point glass can be used, but
alkali-free glasses3 such as high lead glasses are preferableu The
properties of the glass frit are crucial to the characteristics,
compatibility and performance of the conductors in hybrid
microcircuits. Thuss iF the thermal expansion coefficient of the glass
is significantly different from that of the substrate, cracks and
blisters develop in the conductor during firing. Also adhesion at
conductor/resistor interfaces is inFerior if ~he glass in the conductor
powder is not compatible with the glass in the resistor powder. Dry
mixed powders tend to show non-uniform distribution of glass frit in
metallic powder thus contributing further to the problems of poor
adhesion and blister formations. Satisfactory powders may be prepared
from proprietary thick film conductor inks by burning off the organic
vehicle in the ink and fusing the metal and glass frit at temperatures
betwen 400 and 700C. The resulting glassy mass is then pulverized to
obtain homogeneous powders. The medium film conductors prepared from
these powders are found to be reproduciblle, exhibit good adhesion to
ceramic substra~es, and are naturally compatible with thick film
resistors used in hybrid microcircuits.
Using this method, Au and Au alloy powders can be
prepared from commercially available inks such as Dupont Au-9791 and
Pt/Au-9596 or Engelhard Au A-3360 and Pt/Au A-3395. The powder is
prepared by stirring the conductor paste thoroughly, pouring the
conductor paste into a platinum boat and thoroughly air drying to
remove the organic solvents. Subsequently, the dried paste is fired in
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a -furnace at a peak temperatllre oF 400C and with an air flow
sufficient to completely oxidize all organic components during the
firing cycle. Following firing, the glassy matrix is removed from the
platinum boat and is ground using a mill until the particles can be
sieved to 0.5 to 10 microns. A similar process can be used to prepare
Ag and Ag alloy powders from commercial thick film pastes such as
Dupont Pd/Ag-9061 and Engelhard Pd/Ag A-3809 except for the firing
step. A firing temperature of 700C is required in the case of Pd-Ag
thick film paste to improve the properties of the conductor since
powders prepared by Firing the paste at lower temperature produce
conductive films which are thin, porous and exhibit high resistivity.
So-called fritless thick film inks have recently been
developed and from them, corresponding solid powder equivalents can be
prepared for use in the medium film process. In these materials,
cohesion is provided by chemical bonding.
An alternatiYe ~o fabricating the powder by pulverizing a
fused mass of metal particles and glass Frit is to thoroughly dry mix
very small particles of glass and metallic powder and then bring them
up to the required powder particle diameter (0,5 to 10 microns) by
adding an organic bonding agent. A lecithin based gum has been found
; suitable for this purpose. This method is especially suitable for
producing powder particles using malleable metals such as gold s;nce
any attempt to pulveri~e a gold-glass matrix often resul~s in thin
flakes of material rather than particles approximating to sphericity,
~he latter being the ideal shape for the medium film process. It will
be appreciated that in composition, though not in physical nature, a
particle produced by this process may be practically identical with one
produced by fusing and pulverizing.
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b) Resistor Powders
Another common component required ~or hybrid
microcircuits is a resistor. Commercially available bismuth ruthenate
thick film resistor pastes such as Dupont Biron 1400 series or
Engelhard Rely-Ohm series are converted to powders for medium film
applications using a process similar to that described for the
preparation o~ conductor powders. In the case o~ Pd/Ag conductors~ it
is necessary to fire the resistor paste at 700C to lock the ruthenate
particles in the glass component o~ the resistor. The ~ired paste is
then ground to a particle size of 0.5 to 10 microns. The measured
sheet resistivity of medium films produced ~rom these resistive powders
is Found to be close to screen printed thick film resistors. As in
thick film~ the exact resistance values of medium film resistors can be
adjusted by laser trinming. The medium ~ilm process is capable of
producing 10 mil sq. resistor elements which are too small for
~abrication by screen printing thick film tehcniques. These small
resistors coulds for example, be used as heating elements in moving or
fixed head thermal printers.
The powders produced from thick ~ilm pastes retain most
of the properties of screen printed thick -films such as conductivity or
resistivity, adhesion, solderability, bondability and compatibility
with thick ~ilm components. However, the medium film components
exhibit better line resolution of less than 2 mils (S0 microns). Such
line resolution currently cannot be obtained by thick film screen
printing technology, hut only by expensive thin film techniques~
Although medium and ~hick film tehcniques use about the
same amount of material in producing conductor interconnects and
~5596~
resistors, the powders can be manuFactured more cheaply than thick film
inks since in the former, no organic vehicle is necessary and flow
properties are not critical.
Processing Steps
1. Resin Coating
Four possible coating techniques for applying a uniform 5
to 20 microns medium film resin to various substrates are spinning,
spraying, roller coating and dip coating. Whichever technique is
chosen depends on such factors as substrate size and the nature of the
resin9 but spraying is probably the most versatile of these techniques.
Especially for coating large area substrates up to 12"
square, an air sprayer at a pressure of 30 psi is used. The resin is
diluted with a thînner (1-1-1 trichloroethane) in a ratio of 1:2
(resin:thinner). A shiny uniform coating is obtained for a 4"-6"
separation distance between the substrate and nozzle. A pebbly
appearance occurs if the sprayer is held at a large distance whereas
running edges are formed with a short separation.
It should be mentioned that in contrast to the colography
patent 3,637,385 mentioned previously, which cites a resin thickness of
0~1 to 40 microns with an optimum thickness of 0.5 to 2.5 microns~ the
thickness of the light sensitiYe resin for making medium film
microcircuits should be wi~hin the range of 5-20 microns, the optimum
thickness being dependent on the powder and the substrate material as
will be described presently. If resin thickness is less than 5
microns, the powder particles subsequently applied may be only
partially embedded and may be ripped off when subsequently removing
non-embedded powder. If resin thickness is greater than 20 microns,
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bulk resin may be left under the subsequently applied powder and powder
particles may be dislodged as the resin volatilizes on firing. In the
first case voids result and in the second the film blisters and
fragments.
2. Resin Exposure
The most effective radiation for the photo~polymerization
of the medium film resin RV3566E is in the 365nm region. The optimum
exposure time is determined by experiment with the particular powder to
be deposited. For hybrid micro-electronic applications, the exposure
time must be from ~-40 seconds~ the actual exposure time depending on
resin thickness, suhstrate, and powder materials used. In general, if
the exposure time is less than 5 seconds3 the powder is found
subsequently to be retained on the apparently exposed resin regions.
The electrical properties of the resulting medium film circuit
consequently become uncontrollable. On the other hand, exposure times
- longer than 40 seconds causes poor edge definition and a decrease in
the desired geometrical dimensions of the printed elemen~s. A typical
exposure time for 10 micron thick resin is eight seconds using a powder
X d nsity of 41mw/cm2 from a Tamarack UV exposure system with a
1000 watt Hg lamp for ceramic and porcelain steel substrake. However,
glass substrates, owing to increased reflection of UV light, require
additional exposure time.
~ Mask-substrate separation must be kept as small as
possible commensurate with not touching the coated substrate. If the
photomask touches the substrateg it either lifts off resin or roughens
its surface, creating voids and/or poor adhesive areas. A large
separation produces poor edge definition. A separation distance of 4
mil (100 micron) is usually satisfactory.
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It is preferable that resin coating and exposure be
carried out in a clean yellow room. However, this is not necessary
provided substrates are shielded from UV light and the powder
application is followed within one hour of exposing the medium film
resin to UV radiation.
3, Powder Application
A soft pad or Fine brush can be used to distribute the
medium film powder over the substrate coated with the selectively
exposed resin. The pad or brush should not be so stiff as to score the
film nor so loose fibered as to contaminate the resin. The quantity oF
the powder is not critical provided there is an excess available beyond
that required for full development of the pattern. The development of
the image seems to depend primarily on particle-to-particle interaction
rather than pad-to surface or brush-to-brush Forces. A circular
buffing motion is used to apply the powder to unexposed resin.
Excessive mechanical -force is avoided since this may result in loss of
resolution on subsequent firing.
AFter the development of the image, excess powder is
removed by firmly wiping the sur-Face with a clean brush or pad or is
blown off using an air gun. The powdeY particles must be well embedded
into the unexposed resin otherwise when wiping the surface, conductor
or resistor particles will be ripped out oF the unexposed layerO In
additiong unlike the colography requirements of a surface monolayer
nt I ~ r e cl p ~ ~ v, ~ :1 s / ~
discussed in U.S. patent No. 3,637,385,~the particle size and resin
thickness must be so related as to ensure a multilayer oF powder
extending through the resin layer to the suhstrate as shown in Figure
3. The multilayer requirement must be satisfied in order to obtain a
..
conductor or resistor layer of uniform thickness. Dispersal throughout
the resin layer must be achieved in order that subsequent firing does
not produce voids in the conductor or the resistor layer.
4. Firing
The substrate, patterned with the medium film powder, is
fired in air in a thick film conveyor furnace. During firing the
organic resin is burned out completely. As can be imagined, the
resulting gases affect the powdered distribution less if the powder is
distributed throughout the thickness of resin than if it forms a
monolayer shield overlying a bulk layer of resin. Firing is then
continued at a higher temperature at which the glass content of the
powder softens to wet the substrate and sinter the metal particles
together in a glassy matrix. Ag and Au conductors have been fired at
600C on glass and porcelain steel substrates and at 850C on ceramic
substrates, the substrates being kept at a peak temperature for 5 to 15
minutes.
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