Note: Descriptions are shown in the official language in which they were submitted.
1~58330
PROCESS FOR DEPOSITING CONDUCTIVE LAYERS ON SUBSTRATES
BACKGROUND OF INVENTION
Microelectronics or microcircuitry, such as integrated
circuits, hybrid microcircuits, or the like, commonly utilized
gold as a thin film electrical conductor material because of its
combination of low electrical resistivity, high oxidation and
other corrosion resistance, and bondability characteristics.
Gold films, however, when deposited directly on substrate or
circuit substrate surfaces such as glass or ceramics or onto
thin film resistors such as tantalum nitride exhibit very poor -
adhesion and are readily pulled apart from the substrate. In
order to provide good adhesion to such substrates, a thin layer
of a bonding metal having good adhesion to the substrate
materials, such as chromium, is deposited onto the substrate
prior to gold deposition and the gold then deposited over the
adhesion metal. The adhesion metal thus acts as the bond to the
circuit substrate while the gold, which does adhere well to the
adhesion metal, acts as the electrical conductor.
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It has been found that such chromium-gold metallizations
increase in resistance from the chromium leaving the underlayer
region and alloying with the gold film along grain boundaries
within the gold film when the circuits or substrates are heated
during processing, such as from lead bonding, resistor stabili-
zation, or the vapor deposition itself, or from high temperature
uses. This increased resistance from chromium diffusion is
often unavoidable because of the processing requirements and may
reach levels at which the circuit would be unusable. It has
further been found that a portion of the chromium which
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diffuses into the gold reaches the outer surface of the gold layer at w~.ich
surface it may spread so as to cGver all or a large part of he gold
metallization. This chromium, even though quite thin, may be oxidized during
use or during processing which may prevent or inhibit the bondin~ of leads
to the gold. Such oxidizing may occur, for example, in resistor stabilization
of the microcircuits,
Various attempts have been made to attempt to block this diffusion of
chromium through gold to maintain the inherent high electrical conductivity
of the gold and good adhesion characteristics including depositing an
intermediate electrical conductor layer between the chromium and the gold,
such as palladium, through which the chromium will not diffuse and which will
not itself diffuse excessively through the gold. Because of the cost and
complaxities involved in applying multiple vapor deposited metallization
layers~ the addition of a thi~d layer adds to the complexity, cost and time
of producing metallizations. Another approach which has been taken is to
remove by etching any material which has migrated to the gold surface, such
as the chromium or oxidized chromium, after all the heating processing steps
have been essentially completed. However, this does not eliminate the
increased resistivity caused by such diffusion and the lead bonding steps `~
which may occur later may cause additional diffusion and resistance increases.
SUMMARY OF INVENTION ;
In view of the above, it is an object of this invention to provide a
diffusion barrier which will prevent migration of one metal through another
metal layer.
It is â further object of this invention to prevent the migration of
chromium through an overlayer of gold into the main body of the gold and its
outer surface.
It will be understood that various changes in the details, materials and
arrangements of the parts, which are herein described and illustrated in order
to explain the nature of the invention, may be made by those skilled in ~he art
within the principles and scope of the invention as e~pressed in the appended
claims.
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The invention relates to a process for providing a diffusion barrier
within a metal deposition layer by interrupting the deposition of the layer
and oxidizing any material which has to that point partially diffused
through the layer and then continuing thè deposition to complete the
metal layer.
The invention comprises a process for depositing electrically
conductive layers to a desired thickness with a high peel strength on ~
ceramic or glass circuit substrates comprising vapor depositing while ~ -
subjected to a vacuum of about 10 7 Torr an electrically conductive adhesion
metal layer on the substrate, vapor depositing on the adhesion layer while
subjected to this vacuum a first layer of a good electrically conductive
metal having a low free energy of oxide formation to a thickness of about
one half or less of the total desired conductive metal thickness, then
oxidizing by backfilling with oxygen to a pressure of from about 10 3
to about 10-6 Torr any of the adhesion layer metal which has diffused
through the first layer to form an oxide barrier against further diffusion
of the adhesion metal layer, and thereafter while subjected to the first
mentioned vacuum vapor depositing a second layer of the first layer metal
over the first layer and over the oxidized adhesion layer metal wherein
the first layer is of sufficient thickness to yield a continuous film
after the oxidizing step.
DESCRIPTION OF DRAWING
Figs. la, lb, lc, ld and le illustrate by enlarged cross sections the
various steps of the process of this invention.
Fig. 2 illustrates somewhat diagrammatically and by greatly enlarged
cross section the grain boundary diffusion of chromium in gold and the ;;
mechanism by which this diffusion is blocked by the process of this ;
invention.
DETAILED DESCRIPTION
The present invention provides a diffusion barrier within the conductive
metal deposition layer of a thin film electrical microcircuit by permitting
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or enhancing partial migration or diffusion of a metal underlayer
therethrough and then blocking further migration by "capping" the then
existing diffusion. This is achieved by oxidizing the metal which has to
that point diffused through the conductive metal layer and thereafter
continuing the deposition of the conductive metal layer over the oxidized
"caps" and the initially deposited layers.
It has been found that many of the non-reactive metals, such as gold,
palladium or platinum, which exhibit very high electrical conductivities
together with very high resistance to attack from environmental and
atmospheric conditions, fcr example exhibit a low free energy of oxide
formation, also exhibit very low adhesive strengths when deposited on
ceramics, glass or thin film resistor materials like tantalum nitride.
At the same time, materials such as chromium, copper, titanium, nickel,
tungsten, molybdenum and alloys of these materials may exhibit very
high adhesive strengths to these substrate materials but are very
reactive in certain environments, particularly oxidizing environments,
or do not exhibit sufficiently low resistance to be used as electrical
conductors in thin film electrical microcircuits. Attempts have been
made to overcome these problems by initially depositing the reactive
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metal as an adhesion layer on the substrate material and then overlaying
an additional film or metal~ization of the non-reactive metal to act as a
protecting layer and principle electrical conductor.
In accordance with the present invention the adhesion layer metals
area preferably selected fr~m the group consisting of chromium, titanium, ;
nickel, copper, tungsten, molybdenum, and alloys thereof and the nonreactive
conductive layer metals are preferably selected from the group consisting
of gold~ platinum, and palladium.
The process of this invention is illustrated in Figs. la through le
in which a substrate 10 has deposited thereon appropriate metal layers
having a diffusion barrier therein which prevents or minimizes degradation
of the overall electrical conducitivity of the metal layers and also of the
normally good bondability of the metal layer outer surface. The substrate
10 may be formed from a suitable ceramic~ such as A1203, glass or the
like, with or without thin film resistor layers or other layers to which
the highly electrically conductive and non-reactive metal conductors do
not normally bond. Substrate 10 may then be suitably cleaned and outgased
in a manner well known in the art, such as by organic solvents and
de-ionized water, and placed in an appropriate evacuated space or chamber
in which depo9ition of the desired conductive layers may be provided. For
example, vapor depositions are typically carried out in vacuums at levels
of about 10-7 Torr to prevent oxidation and gas incorporation in deposited
metals.
For pruposes of this invention~ the process will be described using
a reactive~ chromium adhesion layer deposited directly onto the substrate
10 with a non-reactive~ gold layer deposited thereover since this
combination provides both good adhesion and high electrical conductivities
as well as good resistance to degrading environments and atmospheres. It -
is understood that other combinations and metal selections may be utilized
for these layers which will resuIt in the diffusion barrier described
hereinbelow. In addition~ the present invention will be described with
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respect to vapor deposition of the respective metal layers understanding
that other deposition methods such as sputter deposition, ion deposition,
and the like may provide similar results when properly utilized in accord-
ance with this invention.
With substrate 10 suitably cleaned and situated within an evacuated
deposition chamber, an adhesion layer 12 of chromium is vapor deposited over
substrate 10 in a continuous film to cover desired portions of substrate 10 ~ -
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and provide adhesive strength, such as at a thickness of from about 100
Angstroms to about 1000 Angstroms, as indicated in Fig. lb. A first layer
14 of gold, as shown in Fig. lc, is then vapor deposited over layer 12 to
a thickness of about one half or less of the total final desired gold
thickness, such as a thickness of from about 2000 Angstroms to about 15,000
Angstroms. The total desired gold thickness may be from about 10,000
Angstroms to about 60,000 Angstroms, and most often at about 30,000 Angstroms.
First gold layer 14 should have sufficient thickness to insure continuity
of the film after the oxidizing step to be described has been performed.
The layers 12 and 14 may then be heated to a temperature of from about
200C. to about 500C., generally about 350C., so as to cause the chromium
in layer 12 to diffuse through layer 14. It has been found that this
diffusion occurs preferentially along the grain boundaries in gold layer 14
which may form nodules or caps 16 on the outer surface of layer 14; the
diffusion may follow the most direct paths along grain boundaries as
indicated at 18 in Fig. 2 and Fig. ld. It is understood that the vapor
deposition process itself will result in heating of substràte 10 and layers
12 and 14 to an extent which may often induce this diffusion without further
application of heat to the substrate and layers. After this diffusion has
occurred from the vapor deposition heat or externally applied heat, oxygen
may be back-filled into the deposition chamber to a pressure of from about
10-3 to about 10-6 Torr while maintaining substrate 10 and layers 12 and 14
at this temperature so as to oxidize the chromium caps 16. After about 10
to 20 minutes, the oxygen may be pumped or removed from the chamber to
the vapor deposition vacuum level referred to above. The temperature
utilized for the diffusion is dependent upon the thickness of gold layer `
14, the thicker the layer the higher the temperature required, and the
heating continued through the oxidizing step to enhance oxidizing of caps 16.
The time period required to oxidize caps 16 is also dependent on the
temperature to which the structure is heated.
After oxidizing caps 16 which effectively block the grain boundaries
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in gold layer 14 through which the chromium is most readily '
diffused, a second and final gold layer 20 is deposited over
first gold layer 14 and caps 16, as shown in Fig. le. The -
second gold layer 20 is deposited generally to a thickness of
from about 28,000 Angstroms to about 15,000 Angstroms, depending
upon the desired conductivity level within layer 20. The '~
successive metal layers may be deposited from separate boats
having sufficient metal to provide the desired layer '~
thicknesses.
With chromium oxide caps in place over the preferred or ',~
preferential diffusion paths within the first gold layer 14, it
has been found that the second gold layer 20 has stabilized ` '
electrical conductivity and very little chromium, if any, is ' '
diffused into or through layer 20. The substrate 10 with its
chromium layer 12, and gold layers 14 and 20 may be
appropriately etched to form the desired electrical conductor
patterns and may be fur,ther processed to stabilize the resistor
patterns by oxidation thereof and by suitable thermal
compression bonding of leads without any degradation of adhesion
of the metal layers to the substrate or of the leads to the
metal layer, or to the electrical resistance thereof.
The following table illustrates the improved strengths and
electrical conductivities achieved using the process of this
invention by comparing a circuit structure prepared using '
oxygen back-fills of different amounts for different per~ of
time as compared to a circuit structure formed without the
diffusion barrier of this invention. ~'
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C 2 3 Average
Concentration Lead Peel
Depos ition after 2 hours Peel Strength Gold
Condition at 300C. Test Range Resistivity
(Arbitrary Units) (lbs) (lbs) ohm-cmxlO
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AES AD S AD S
Typical 1.0 to 1.82 1.9 0.8 1.28 0 3.87
Deposition to to
2.35 2.37
2 sackfill
lxlO Torr 0.28 2.8 2.9 2.44 1.41 2.89 ~;
18 min. at to to
350C. 3.26 3.16
2 Backfill
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5xlO Torr 0.1 - 3.0 - 2.78 2.51
10 min. at to
350C. 3.14
The terms AES refers to Auger electron spectroscopy, AD refers
to "as deposited" and S refers to resistors stabilized at 300C.
for 2 hours in air. It has been found that the quantity of
chromium oxide (Cr203) concentration on the outer gold surface
with the diffusion barrier produced by this invention is less
than 10%, and as law as 2.5%, of the chromium oxide
concentrations on the outer gold surfaces without the diffusion
20 barrier. In addition, the prior metal layers exhibited bond
strengths to gold leads of almost O to levels, at best, of about
one half of those achieved with the present invention. The
conductivities of the resulting gold layers with the diffusion
barrier was at least 38% better than the prior gold layers.
