U.S. patent application number 12/855661 was filed with the patent office on 2010-12-02 for electrode passivation.
This patent application is currently assigned to DELAWARE CAPITAL FORMATION, INC.. Invention is credited to Jeffrey C. Andle, John H. Bradshaw, Reichl B. Haskell.
Application Number | 20100304012 12/855661 |
Document ID | / |
Family ID | 37640271 |
Filed Date | 2010-12-02 |
United States Patent
Application |
20100304012 |
Kind Code |
A1 |
Andle; Jeffrey C. ; et
al. |
December 2, 2010 |
Electrode passivation
Abstract
A coating providing high abrasion and chemical resistance
composed of a barrier layer from vanadium, molybdenum, niobium,
tantalum and the like, and an outer layer of diamond-like carbon.
The coating is especially applicable for acoustic wave device (AWD)
based sensors, and for passivating an electrode such as an
electrode deposited on the AWD sensing area. The coating provides
excellent mechanical and acoustical characteristics for coating
acoustic wave devices allowing the sensor to operate in harsh
environments.
Inventors: |
Andle; Jeffrey C.;
(Falmouth, ME) ; Haskell; Reichl B.; (Nashua,
NH) ; Bradshaw; John H.; (Atkinson, NH) |
Correspondence
Address: |
SALTAMAR INNOVATIONS
1 Mathewson Road
Barrington
RI
02806
US
|
Assignee: |
DELAWARE CAPITAL FORMATION,
INC.
Wilmington
DE
|
Family ID: |
37640271 |
Appl. No.: |
12/855661 |
Filed: |
August 12, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11814167 |
Jul 17, 2007 |
7800285 |
|
|
12855661 |
|
|
|
|
Current U.S.
Class: |
427/8 ; 427/122;
427/125 |
Current CPC
Class: |
H01L 41/0477 20130101;
G01N 29/32 20130101; H01L 41/0533 20130101 |
Class at
Publication: |
427/8 ; 427/122;
427/125 |
International
Class: |
C23C 16/52 20060101
C23C016/52; B05D 5/12 20060101 B05D005/12; C23C 16/14 20060101
C23C016/14 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 20, 2006 |
US |
PCT/US06/15537 |
Claims
1. A method of passivating an electrode, the method comprising the
steps of: depositing a barrier layer onto said electrode or a
portion thereof, the barrier layer comprising at least one metal
selected from the group consisting of tantalum, niobium, vanadium,
molybdenum, or a combination thereof; and, depositing an abrasive
resistant layer comprising Diamond-Like Carbon (DLC) onto said
barrier layer.
2. A method according to claim 1 further comprising the step of
depositing an adhesion layer disposed between said electrode and
said barrier layer.
3. A method according to claim 2, wherein said adhesion layer
comprises zirconium.
4. A method according to claim 2, wherein said adhesion layer
comprises titanium.
5. A method according to claim 2, wherein said adhesion layer
comprises a metal selected from a group consisting of chromium,
vanadium, niobium, tantalum, or a combination thereof.
6. A method according to claim 1, wherein the DLC is boron doped
diamond.
7. A method according to claim 1, wherein the electrode is
deposited on an acoustic wave device.
8. A method according to claim 1, further comprising the step of
depositing at least one chemically selective probe onto said
abrasive resistant layer.
9. A method according to claim 1, further comprising the step of
employing the abrasive resistant layer as a working electrode in an
electrochemical measurement.
10. A method according to claim 1, wherein said electrode comprises
platinum.
11. A method according to claim 1, wherein said electrode comprises
a metal selected from a group consisting of, gold, silver, copper,
aluminum, or a combination thereof.
12. A method according to claim 1, wherein said electrode comprises
palladium.
13. A method according to claim 1, wherein said electrode comprises
ruthenium.
14. A method according to claim 1, wherein said electrode comprises
rhenium.
15. A method according to claim 1, wherein said electrode comprises
osmium.
16. A method according to claim 1, wherein said electrode comprises
iridium.
17. A method according to claim 1, wherein said electrode comprises
carbon.
18. A method as claimed in claim 1, wherein a carbide alloy
interface is formed between the barrier layer and the abrasive
resistant layer.
19. A method of passivating an electrode comprising the steps of:
depositing an adhesion layer comprising titanium, zirconium, or a
combination thereof, over at least a portion of the electrode;
depositing a barrier layer comprising niobium, tantalum, or a
combination thereof, onto said adhesion layer or a portion thereof;
and, depositing an abrasive resistant layer of diamond like carbon
(DLC) onto said barrier layer.
20. A method as claimed in claim 19, wherein: said electrode
comprises gold or platinum; and, said barrier layer having a
thickness of between 25-300 nm.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of, and claims the right of
priority to, U.S. patent application Ser. No. 11/814,167, filed in
the US Jul. 17, 2007, presently allowed, which in turn claims the
right of priority to PCT application No. PCT/US06/15537 filed Apr.
20, 2006. The above mentioned applications are incorporated herein
by reference in their entirety.
FIELD OF THE INVENTION
[0002] This application is directed to passivation of electrodes,
and more particularly for passivation of electrodes by utilizing
Diamond Like Carbon (DLC) material.
BACKGROUND OF THE INVENTION
[0003] The need to operate sensors in harsh environments is common.
Examples include sensors, switches, and the like, which are
designed to operate in abrasive environments, in corrosive
chemicals, and the like. Oftentimes the long term sensor operation
is hindered by the continued contact the environment. The problem
is particularly severe in the field of electro acoustic sensors,
also known as Acoustic Wave Devices. Thus the present
specifications will relate to the application of a coating to
mitigate the problems associated with such operations, primarily by
using the AWD sensor. The skilled in the art will recognize the
applicability of the coating to other devices.
[0004] Piezoelectric sensors are well known. They are used for
sensing material properties such as viscosity and density, for
detecting the presence of certain materials in an environment, for
measuring purity of fluid substance, and the like. Structures known
for acoustic sensing range from the simple crystal resonator,
crystal filters, acoustic plate mode devices, Lamb wave devices,
and others. Briefly, these devices comprise a substrate of
piezoelectric material such as quartz, langasite or lithium
niobate, or thin films of piezoelectric material, such as aluminum
nitride, zinc oxide, or cadmium sulfide, on a non-piezoelectric
substrate. The substrate has at least one active piezoelectric
surface area, which is commonly highly polished. Formed on the
surface are input and output transducers for the purpose of
converting input electrical energy to acoustic energy within the
substrate and reconverting the acoustic energy to an electric
output signal. These transducers may consist of parallel plate
(bulk wave) or periodic interdigitated (surface-generated wave)
transducers. Each sensor has at least one sensing area which is
exposed to the environment being measured. The interaction between
the surface and the environment causes measurable changes in the
electrical characteristics of the sensor. The sensors may be used
for sensing density, viscosity, and other such physical
parameters.
[0005] Piezoelectric devices are generally manufactured from hard,
crystalline materials. However, even those surfaces change when
exposed to certain chemicals or abrasives. Piezoelectric based
sensors are very susceptible to changes in the sensing area. Thus
their use was so far limited to environments that will not damage
such surfaces. Damage may be chemical such as etching, or
mechanical such as abrasion. Therefore the usability of such
sensors in environments like oil wells measuring of characteristics
of drilling mud or oil, sensing characteristics of ink, melted
polymers, and similar abrasive materials was heretofore limited as
the sensors will suffer from high variability over time. Other
environments which can harm such sensors are chemically reactive
materials such as strong acids and bases used in polymer
processing, pulp and paper processing, and other industrial and
chemical processes. Furthermore, the need for a conductive
electrode or shield layer in the most desirable sensor topologies
introduces a further susceptibility to chemical attack and/or
abrasion as virtually all metals have one or more chemical
susceptibility and/or are soft, abrasion prone materials. A further
common requirement to liquid phase sensing is that the sensing
surface be sufficiently smooth. Many liquid phase AWD sensors
require nanometer or lower average roughness.
[0006] The electrodes that are deployed in the most desirable
sensor topologies are commonly made of gold (Au) deposited by any
convenient method. In many cases the electrode have to be
electrically insulated from the environment.
[0007] Insulating surfaces, devices, electrodes, and the like, from
a hostile environment is commonly done by coatings the likes of
plastic, glass or similar materials. However such coatings often
interfere with the operation of the sensor. For example in an AWD
type sensor, the coating must have acoustic qualities that will not
significantly impede the sensor operation, as well as the desired
hardness, toughness, electrical characteristics, and the like.
Plastics and glass incur excessive damping and are not always
chemically resistant, nor are they sufficiently hard.
[0008] Diamond Like Carbon, or DLC hereinafter, is well known
coating material. DLC enjoys high hardness and therefore high
resistance to abrasion, it may be smoothly applied, and generally
provides an excellent coating layer whose thickness may be tailored
to need. DLC is deposited using common methods such as vaporizing,
ion implanting, and the like.
[0009] Certain materials do not adhere well to each other. DLC
suffers from poor adhesion to materials like gold, platinum,
silver, most oxides, and many other materials, especially
piezoelectric materials commonly used in AWD sensors and the like.
Therefore, while the DLC clearly provides the required abrasion
resistance, providing adequate adhesion between a DLC layer and a
piezoelectric material or an electrode deposited on such
piezoelectric material presents a problem. In other cases,
different coating materials exhibit undesirably large ion migration
problems which adversely effect electrical or acoustic
characteristics of the desired coating.
[0010] Implantable medical devices have been demonstrated with
relatively thick (in the order of several to several hundred
microns) coatings of DLC, on thin (a few nanometers) adhesion
layers, as shown in "EXPERIMENTAL STUDIES ON DIAMOND-LIKE CARBON
AND NOVEL DIAMOND-LIKE CARBON-POLYMER-HYBRID COATINGS" Mirjami
Kiuru, PhD Dissertation, University of Helsinki (2004). Typical
substrates for such coatings are refractory metal parts such as
titanium hips. When these coatings are applied to semiconductor
devices the films are extremely unreliable. These coating films
often suffer from delaminate and in some cases cause breakage of
the silicon substrates.
[0011] Therefore, there exists a continuing and yet unresolved need
for a sensor capable of continued operation in broad range of
chemical, thermal and mechanical harsh environments.
[0012] It is therefore an object of the present invention to
provide a coating that will provide the above mentioned
characteristics when applied to a sensing area of a sensor. Since
no single material offers all of the ideal properties, many
applications will ultimately require a series of layers, relaxing
the requirements of the individual layers but further requiring
compatibility amongst the layers. Preferably the coating should be
sufficiently smooth to address fluid-phase sensor applications.
[0013] Many such acoustic sensors utilize an electrode on the
sensing surface, and the coating provided by the present invention
is particularly beneficial to such sensors. The skilled in the art
will recognize that the term electrodes may relate to ground
electrodes, transducers (especially in the case of actively driving
or driven electrodes), other structures that cause perturbation or
reflection in the piezoelectric crystal, or other conductors that
either carry electrical energy or deliver it to a predetermined
point.
[0014] An additional objective of the present invention is to
provide the sensor with a smooth (preferably nanometer scale)
sensing surface with excellent abrasion resistance, excellent
chemical resistance and the ability to withstand temperature
extremes from -50.degree. C. to +350.degree. C. Suitable coatings
include alloys of silicon-aluminum oxynitride (SiAION) including
the extremes of silicon nitride, aluminum oxide and the like,
amorphous boron nitride, amorphous and nanocrystalline carbon,
boron carbide (including the like of boron doped diamond and boron
doped DLC), and .beta.-C.sub.3N.sub.4. All of these materials offer
abrasion resistance and thermal stability with varying degrees of
chemical resistance.
[0015] Of these coatings, so-called diamond-like carbon (DLC) is
found to offer the best properties of surface smoothness, chemical
resistance and abrasion resistance. While there is considerable
debate as to an exact definition of diamond-like, for the purposes
of the present invention it shall be taken to imply all films with
a molar percentage of greater than 75% carbon and having mixed
chemical bonding state of graphitic (sp.sup.2) and diamond
(sp.sup.3). It should be noted that the term also extend to various
modification of diamond-like carbon, such as boron-doped diamond
(BDD), carbon-rich refractory metal carbides, and the like.
[0016] Diamond-like carbon is well-adhered to metals that form
carbides, such as tungsten, molybdenum, tantalum, niobium,
vanadium, hafnium, zirconium, titanium, and chromium in increasing
order of typical adhesive strength. These metals are traditionally
used in so-called adhesion layers. The adhesive strength of these
metals is in direct proportion to their ability to inter-diffuse
and alloy with adjacent materials.
[0017] Inter-diffusion of the adhesion layer into the DLC film is
undesirable since it leads to unstable film properties and poor
aging characteristics. It is especially desired that the underlying
metal have low mobility in the carbon and that metal will have low
mobility into the carbon, a condition that may become critical at
high temperatures.
[0018] Therefore by way of example, while titanium and zirconium
offer excellent adhesion, they are excessively mobile in carbon and
vice versa at high temperatures. On the other hand tungsten is an
excellent barrier metal (has low mobility and prevents other atoms
from diffusing into or through it); however tungsten has the
poorest adhesion. Niobium and tantalum are the preferred metals for
a barrier/adhesion layer under diamond-like carbon.
[0019] Niobium and tantalum offer good chemical resistance and
excellent adhesion of the outermost DLC layer. Both materials are
sufficiently conductive for shielding but are inadequate for many
electrode requirements. In these cases an innermost layer of a
thermally stable material with chemical resistance and high
conductivity is required. Although aluminum is frequently employed
as an electrode material it is chemically active and highly mobile
at elevated temperatures. The preferred metals are gold and
platinum, although silver and palladium are also acceptable for
some preferred embodiments. Platinum has the most favorable
properties while gold is more commonly employed. Ruthenium,
Rhodium, Rhenium, Osmium and Iridium may also prove desirable in
specific applications.
[0020] Applying the coating to a piezoelectric material presents
yet another problem stemming from the extremely high film stresses
associated with DLC and the significant mismatch of thermal
expansion between DLC and such metal or piezoelectric materials as
are commonly employed. The preferred embodiment of the present
invention therefore utilizes application of a relatively thin DLC
coating (of less than 1 .mu.m) and a thicker adhesion/barrier metal
system than is traditionally employed (of about 200 nm).
[0021] The adhesion of tantalum to gold or platinum and of the
electrode to the piezoelectric substrate can be improved through a
thin adhesion layer of chromium or titanium. The most preferred
embodiment would consist of a thin (Circa 10 nm) titanium,
zirconium or chromium layer, a conductive (50-200 nm) gold or
platinum electrode layer, another thin (Circa 10 nm) titanium,
zirconium or chromium layer, a barrier layer (25-300 nm) of niobium
or tantalum, and a surface of DLC (10-500 nm).
[0022] The exact composition of the DLC is a matter of choice.
Prior art in thin film development suggest numerous dopants or
surface treatments ranging from several parts per million to
several percent of nitrogen or various metals. Details of thin film
may be found in "Synthesis and Evaluation of TaC:C Low-Friction
Coatings", Daniel Nilsson, Ph.D. Dissertation, ACTA Universitatis
Upsaliensis (2004). Fluorine dopants have been considered from
trace levels through 67 atomic percent (perfluoroalkanes). The
abrasion and chemical resistance of such films appear best when the
atomic percent carbon is maximized.
[0023] Films with only 33% carbon include the limit of hydrocarbons
and fluorocarbons (e.g. Teflon.RTM.) and have no abrasion
resistance. Films in the range of 33% to 66% carbon are
characteristic of carbide alloys. These films are extremely hard
but are not as chemically resistant or as thermally stable as DLC.
In many cases the surfaces are not as smooth as true DLC. Thus only
films containing approximately 67% or higher percentage carbon in
their bulk are specifically considered as "DLC" herein. Our
experience with tantalum-doped and fluorine-doped films indicates
that the film should preferably have in excess of 90% carbon and
the most preferred embodiment has in excess of 97% carbon.
[0024] While chemical resistance and passivation generally favor an
insulating layer as is obtained by pure carbon DLC, certain sensor
applications using carbon electrodes in electrochemical sensors,
notably electrochemical-AWD hybrids, require a conducting DLC
layer. Boron doped (.about.10.sup.19 cm.sup.-3, consistent with
silicon doping levels, up to 1% B; 99% C) diamond is a suitable
material for use in electrochemical and/or acoustic sensors, and is
considered as a type of DLC.
[0025] A further object of the invention includes the ability to
attach chemically selective probes to the DLC surface.
[0026] Thus in one aspect of the present invention there is
provided a coated Acoustic Wave Device (AWD) based sensor, the
sensor having at least one piezoelectric plate having two opposing
faces, one of said faces having a sensing area, the sensor having a
coating applied to the sensing area, the coating characterized by:
a first electrode disposed on the sensing area; a barrier layer
disposed over the first electrode, the barrier layer comprising at
least one metal selected from the group consisting of tantalum,
niobium, vanadium, molybdenum, or a combination thereof; and an
abrasive resistant layer comprising Diamond-Like Carbon (DLC)
disposed over the barrier layer.
[0027] Optionally the coating further comprises an adhesion layer
disposed between the barrier layer and the electrode. Further
optionally another adhesion layer may be disposed between the
electrode and the sensing face. The adhesion layers may be
titanium, zirconium, chromium, vanadium, niobium, tantalum,
molybdenum, or a combination thereof.
[0028] Preferably the first electrode comprises a metal selected
from a group consisting of platinum, palladium, gold, silver,
copper, aluminum, osmium, iridium, or a combination thereof.
Optionally the DLC may be boron doped diamond. Further optionally,
chemically selective probes are coupled to the DLC.
[0029] In a preferred embodiment of the present invention there is
provided a coated acoustic wave device sensor as described above,
having a second electrode disposed on the face opposite the face
comprising the sensing area, forming a parallel plate resonator
with said first and second electrodes. Optionally, a third
electrode is disposed on the face opposite the face comprising the
sensing area, wherein the first and second electrodes form an input
parallel resonator, acting as an input transducer, the first and
third electrode also form an output parallel resonator, acting as
an output transducer; wherein the input and output resonators being
sufficiently close to couple acoustic energy from the input
resonator into the output resonator, for forming a multi-pole
coupled resonator filter.
[0030] In another aspect of the present invention there is provided
a method of passivating an electrode, the method characterized by
the steps of: [0031] depositing a barrier layer onto said
electrode, the barrier layer comprising at least one metal selected
from the group consisting of tantalum, niobium, vanadium,
molybdenum, or a combination thereof; and, [0032] depositing an
abrasive resistant layer comprising Diamond-Like Carbon (DLC) onto
said barrier layer.
[0033] Optionally the method further comprises the step of
depositing an adhesion layer between the electrode and the barrier
layer. Most preferably the electrode is coupled to an acoustic wave
device based sensor. Optionally, the method further comprise the
step of coupling chemically selective probes to the DLC layer.
SHORT DESCRIPTION OF DRAWINGS
[0034] The summary above, and the following detailed description
will be better understood in view of the enclosed drawings, which
depict details of preferred embodiments. It should however be noted
that the invention is not limited to the precise arrangement shown
in the drawings and that the drawings are provided merely as
examples.
[0035] FIG. 1 depicts a simplified elevation cross section view of
an acoustic sensor utilizing a coating in accordance with the most
preferred embodiment of the invention.
[0036] FIG. 2. depicts a process for coating a sensor in accordance
with the preferred embodiment of the invention.
DETAILED DESCRIPTION
[0037] An important objective of the present invention is to
provide AWD and other sensors with a smooth (nanometer scale)
surface with excellent abrasion resistance, excellent chemical
resistance and the ability to withstand a wide range of
temperatures, with the range from -50.degree. C. to +350.degree. C.
being most preferred. While an aspect of the invention relates to
coating piezoelectric material directly, a preferred embodiment
relates to coating one or more electrodes deposited on the
piezoelectric material. FIG. 1 depicts a cross-section of such
preferred sensor 75.
[0038] The most preferred sensor geometries require electrodes of
excellent electrical characteristics on the sensing area. In those
geometries an innermost electrode layer 30 of a thermally stable
material with chemical resistance and high conductivity is
deposited on the sensing face 65 or a portion thereof. Silver and
palladium are examples that offer such characteristics, but the
preferred embodiment uses gold or platinum. Platinum has the most
favorable properties while gold is more commonly employed. Other
candidates include ruthenium, rhodium, rhenium, osmium, and iridium
which have the requisite thermal stability and varying
environmental stability and conductivity properties. Notably, the
electrode layer need not extend to the entire sensing area, and the
electrode layer is not necessarily used as an electrode, i.e. it is
not necessarily electrically connected to any parts of the sensor
circuitry.
[0039] Diamond-like carbon is well-adhered to metals that form
carbides, such as tantalum (Ta), niobium (Nb), vanadium (V),
hafnium (Hf), zirconium (Zr), titanium (Ti), tungsten (W),
molybdenum (Mo) and chromium (Cr) by way of non-limiting example.
However it does not adhere well to gold, platinum, oxides, or most
piezoelectric materials. Therefore, the present invention
contemplates an intermediate barrier layer of carbide forming
metal, such as those described above. The barrier layer 40 lies
between an outer layer 50 and the sensing face 65 of the base
material 10 to be coated, and if an electrode layer is used, the
barrier layer overlies the electrode layer. In addition to
providing good adhesion and enhanced protection to the electrode
layer and/or the piezoelectric material, the barrier layer also
acts as a matching medium to match the thermal expansion and film
stresses of the DLC with the layers below the barrier layer.
[0040] For clarity, in these specifications the term outer layer
will relate to a coating layer of high abrasion material as
described above. It is important however to realize that other
coating layers, either of DLC or other materials, may be overlaid
on top of the `outer layer` 50, and the term `outer` should be
construed broadly only as relating to the layer interfacing with
the barrier layer 40 away from the base material 10. This interface
may be direct or indirect as described below.
[0041] To obtain the best coating stability, the underlying barrier
layer preferably has low mobility in the carbon, and the carbon has
low mobility into the barrier layer. This is an important
consideration at high temperatures. Therefore, niobium and tantalum
are the preferred metals for a barrier layer under diamond-like
carbon. The use of vanadium (V) is also contemplated as a barrier
metal since it is in the same column of the periodic table and
molybdenum (Mo) is contemplated since refractory metal properties
often track along a diagonal rather than vertically. A thin carbide
alloy interface which is stable against diffusion over wide
temperature ranges, is formed between these metals and the DLC
layer. These metals are less reactive with carbon than are
titanium, zirconium and chromium but are more adherent than
tungsten.
[0042] An additional benefit stemming form the use of niobium or
tantalum is that the volume of NbC and TaC are comparable to the
sums of the volumes of the metal and carbon. In contrast, titanium,
zirconium and chromium form carbides with shorter bond lengths than
the bulk metal and bulk carbon, causing an evolving dimensional
change as the metal and carbon inter-diffuse and carbide is formed.
The preferred embodiment therefore enjoys dimensional stability in
the thin film over long periods of time at high temperatures. It is
noted that in certain embodiments a niobium or tantalum layer may
act as an electrical shield, however it generally does not provide
sufficient and stable electrical conductivity to act as good
electrodes. This effect relates to the partial oxidization of the
metal as it is deposited and the associated film resistance.
[0043] The adhesion of the intermediate layer 40 to the electrode
layer 30, and of the electrode layer to the base material 10 may be
improved by depositing an optional thin adhesion layer of chromium,
zirconium or titanium. The adhesion layer 20 may be deposited only
between barrier layer 40 and electrode 30, or as shown by adhesion
layer 21 between the electrode and the sensing area 65, or
preferably both.
[0044] Thus most preferred embodiment would consist of a thin
titanium layer 21 deposited to the sensing area 65 to act as
adhesion layer. Preferably the adhesion layer 21 is of a thickness
in the range 3-30 nm, and more preferably in the range of 5-15 nm,
with about 10 nm being the most preferred embodiment. A conductive
gold or platinum film 30 is deposited on top of the adhesion layer
21 to form the electrode. Preferably the electrode 30 is of a
thickness in the range 10-300 nm, and more preferably in the range
of 50-200 nm, with about 50-150 nm being the most preferred
embodiment. Another thin titanium adhesion layer 20 is located on
the electrode layer, and is preferably of similar characteristics
as adhesion layer 21. A barrier layer 40 of niobium or tantalum is
located on the adhesion layer 20. Preferably the barrier layer 40
is of a thickness in the range 25-300 nm, and more preferably in
the range of 50-250 nm, with about 150 nm being the most preferred
embodiment. An outer layer 50 of DLC is placed on top of the
barrier layer. Preferably the outer layer 50 is of a thickness in
the range 10-500 nm, and more preferably in the range of 50-250 nm,
with about 150 nm being the most preferred embodiment. Notably,
adhesion layers 20 and 21 are optional.
[0045] The exact composition of the DLC is a matter of technical
choice. Common experience in thin film development suggest numerous
optional dopants or surface treatments ranging from parts per
million to several percent of nitrogen or various metals. Fluorine
dopants have been considered from trace levels through to 67 atomic
percent (perfluoroalkanes) with only low (<20%) concentrations
being practical. Boron doping up to 1% is especially of interest to
provide a conducting version of a DLC coating known as boron-doped
diamond. Such films are especially applicable to the integration of
electrochemical methods to an AWD.
[0046] The unterminated dangling bonds of the carbon film will
react with hydrogen in the air (from acidic moisture) to become
passivated. If desired other layers may be attached to the outer
DLC layer. Prior to this passivation the surfaces will react with
bromo-, iodo-, and chloro-functional molecules including
iodo-fluoroalkane, chloro-silanes, and bromo-perfluoropolyethers,
by way of non-limiting example. Silane chemistry is a well known
method of introducing a wide diversity of functional surfaces
including carboxyl and amine groups. The use of peptide bonds to
attach bioreceptors for biochemically selective sensing or polymer
films for chemically-selective sensing is then a well-know
extension.
[0047] In particular, functional chloro-silanes
(CISi(CH.sub.3).sub.2-R) react with a hydrogen-terminated carbon
surface by forming a C--Si bond and evolving HCI. The functional
group, R, is then used in a myriad of well known chemical synthesis
steps. While not as chemically or abrasion resistant as the DLC
surface, the covalently attached monolayers are quite robust.
[0048] In one preferred embodiment chemically selective probes are
attached to the DLC surface. It is possible to terminate the
freshly deposited DLC film with functional chemical groups
including amine and carboxyl groups, allowing further chemically
specific layers to be attached. Thus, by way of example, using a
commonly available compound such as CI(CH3)2--Si--(CH2)2NH2 to
attach -NH2 to the surface, it is then possible to use peptide bond
formation to attach a bioreceptor. Succinnic acid anhydride is used
to form --NH--C.dbd.O--(CH2)2--COOH surface functional groups.
These acid residues may then be joined to --NH2 residues on a
protein (antibody, antigen, enzyme) or synthetically attached to a
DNA or peptide nucleic acid (PNA) probe. The final coupling stage
is catalyzed using a water-soluble carbo-diimide. Alternatively the
carboxyl groups can be reacted with amines in a polymer film having
a preferential absorption of a target measurand. One such example
is to select a polymer having preferential absorption of
chlorinated hydrocarbons for environmental sensing. Another example
is a hydrophilic coating for measuring water content in gasoline
and other fuels.
[0049] Sensors in general, and liquid phase sensors in particular
are often used in harsh environments such as crude oil slurries and
drilling mud for in well measurements, abrasive inks, acids, and
the like. Long time stability of such sensors depends largely on
the capability to keep the smoothness and consistent
characteristics of the sensing area or areas. The utilization of
the coating as described in conjunction with an AWD type sensor
offers significant advantages as it allows the sensor to operate in
areas which heretofore lacked easy continuous coverage of the
sensed parameters. By way of example, uses enabled or improved by
an AWD sensor coated in accordance with the teachings of the
present invention include monitoring "cutting fluids" in
metal-working applications, process control in the manufacture of
titania and other slurries, oil production applications including
drilling mud, paper coating processes, highly caustic chemical
processes, and the like. While the benefits are most notable for
harsh environments such as those described above, less demanding
sensing applications will also benefit from longer sensor lifetime,
including for example automotive sensors such as engine oil
monitoring.
[0050] A preferred process of creating a coating is shown in FIG.
3. Firstly, the substrate--either the sensor sensing area, or any
other surface to be coated, is cleaned 300. Typically this step is
carried out utilizing solvent washes and water rinses, followed
preferably by an oxygen/argon plasma etch in a vacuum.
[0051] Optionally, a first adhesion layer 21 comprising of chromium
(Cr) or titanium (Ti) is then deposited 305 on the surface.
Zirconium (Zr) is also applicable and hafnium (Hf), niobium (Nb)
and tantalum (Ta) are suitable in some cases, but chromium is most
prevalent for moderate temperatures, and titanium is considerate
most suited for elevated temperatures.
[0052] An electrode 30 is then deposited 310 over the first
adhesion layer, if such layer is used. As described, for a
conductive electrode there are numerous choices including platinum
(Pt), palladium (Pd), gold (Au), silver (Ag), copper (Cu) and
aluminum(AI), as well as more exotic conductors such as ruthenium
(Ru), rhodium (Rh), rhenium (Re), osmium (Os), and iridium (Ir).
Aluminum (Al) is the least expensive material but is sometimes
undesirable as it is chemically reactive and has high diffusion
into adjacent materials. Platinum (Pt) and palladium (Pd) are the
most stable selections and gold (Au) is the most commonly used and
is the preferred embodiment for all but the harshest environments
and temperatures.
[0053] Note that the electrode region may be selectively deposited
using various lithography methods to only cover a specific pattern
on the surface. Such patterns may include transducers,
electrochemical electrodes, circuits, antennae and the like.
[0054] Optionally a second adhesion layer 20 may be deposited 215
between the electrode and the barrier layer, as was described for
the first adhesion layer 21, however in some applications the
vanadium, molybdenum, niobium or tantalum of the barrier layer
adhere directly to the electrode layer.
[0055] As described, a barrier layer 40 is deposited 220 over the
electrode and if applicable the second adhesion layer, to isolate
the electrode layer 30 and/or adhesion layer 20 from the outer DLC
layer 50. Commonly tungsten (W) and platinum (Pt) are used as
barrier metals. However both tungsten and platinum suffer from poor
adhesion to DLC. Furthermore, the use of an adhesion layer of
titanium or chromium between tungsten and DLC is unacceptable at
higher temperatures due to long term inter-diffusion. Therefore the
preferred embodiment of this aspect of the invention utilizes
niobium or tantalum as barrier layer. Vanadium and molybdenum may
also be suitable.
[0056] A DLC layer is then deposited 225 over the barrier layer, to
provide the desired mechanical characteristics. Proper selection of
process allows the barrier metal surface to provide excellent
direct adhesion of the DLC by forming a thin (typically circa 5
nm), stable transition region of carbide alloy.
[0057] In the most preferred embodiment, the layers are thus in
order starting in the base material 10, a first titanium adhesion
layer 21 about 3-30 nm thick, a gold or platinum electrode layer 30
about 10-300 nm thick, a second titanium adhesion layer 20 about
3-30 nm thick, a tantalum barrier layer 40 about 25-300 nm thick
and the final DLC layer about 10-500 nm thick.
[0058] In yet another aspect of the invention the coating is
applied to passivate at least one electrode deposited on material
such as a printed circuit board or other material exposed to
corrosive, explosive or other harsh environment. Further
application of the coating according to the present invention is
for providing coatings for tools, and the like.
[0059] The following is a list of materials that are preferred for
practicing the invention.
[0060] It should be noted however that the list relates only to
preferred materials and should not be construed as limiting in
nature.
[0061] Adhesion promoters for optional adhesion layers 20 and 21:
titanium, zirconium, chromium, vanadium, niobium, tantalum.
[0062] Conductive electrode layer 30: platinum, palladium, gold,
silver, copper, aluminum. Alloys of the above are also widely used,
as well as osmium, iridium and the like, and the invention extends
thereto.
[0063] Barrier layer 40: Tantalum, niobium, vanadium,
molybdenum.
[0064] The skilled in the art will recognize that the preferred
embodiments provided above are provided by way of non-limiting
examples and that the teaching of these specifications will allow
the skilled to produce a variety of materials to answer specific
requirements for which the coating or the coated device are
required to meet. Similarly, the skilled in the art will recognize
that while the specifications are primarily directed to sensors,
and even more particularly oftentimes to AWD type sensors, the
coating described is highly applicable to many other applications
that may benefit from the mechanical strength and smoothness of a
DLC layer, and yet suffer from poor adhesion characteristics to
DLC. Thus the invention should be viewed as extending to such
embodiments and similar derivatives of the teaching provided
herein. Various material combinations answering the specific needs,
as well as different coating thicknesses will likely be applied to
by the skilled artisan to obtain specific characteristics. However
the invention extends to such modifications as well.
[0065] While there have been described what are at present
considered to be the preferred embodiments of this invention, it
will be obvious to those skilled in the art that various other
embodiments, changes, and modifications may be made therein without
departing from the spirit or scope of this invention and that it
is, therefore, aimed to cover all such changes and modifications as
fall within the true spirit and scope of the invention, for which
letters patent is applied.
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