Method of producing a film coating by matrix assisted pulsed laser deposition

A film of a coating material is produced on a substrate by a pulsed laser deposition method in which the material that forms the coating material is first combined with a matrix material to form a target. The target is then exposed to a source of laser energy to desorb the matrix material from the target and lift the coating material from the surface of the target. The target and the substrate are oriented with respect to each other so that the lifted coating material is deposited as a film upon said substrate. The matrix material is selected to have the property of being more volatile than the coating material and less likely than the coating material to adhere to the substrate. The matrix material is further selected as having the property such that when the target is exposed to a source of laser energy, the matrix material desorbs from the target and lifts the coating material from the surface of the target. In another aspect of the invention, a method of making an improved chemical or biochemical sensing device that includes a chemoselective or bioselective coating on a substrate is carried out by coating the substrate by pulsed laser deposition.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates generally to the production of film coatings and more 
specifically to the production of film coatings of sorbent materials by 
matrix assisted pulsed laser deposition. 
2. Description of the Related Art 
Chemical sensors commonly use coatings of chemoselective materials to 
effect the detection of chemical analytes. Chemoselective materials are 
substances that are chosen for their ability to interact with specific 
chemical analytes. A typical chemical sensing device includes a substrate 
transducer, a thin film coating of a chemoselective material on the 
substrate and a means for detecting the interaction of the chemoselective 
material with a chemical analyte. In a surface acoustic wave (SAW) device, 
the substrate is typically a piezoelectric material that is used to 
propagate a surface acoustic wave between sets of interdigitated 
electrodes. In a SAW chemical sensor, the chemoselective material is 
coated on the surface of the transducer. When a chemical analyte interacts 
with a chemoselective material coated on the substrate, the interaction 
results in a change in a physical characteristic of the coating which 
results in a detectable change in the SAW properties such as the amplitude 
or velocity of the propagated wave. The detectable changes in the 
characteristics of the wave indicates the presence of the chemical 
analyte. SAW devices are described in numerous patents and publications 
including U.S. Pat. No. 4,312,228 to Wohltjen and U.S. Pat. No. 4,895,017 
to Pyke, the disclosures of which are hereby incorporated by reference. 
Other types of chemical sensors known in the art that use chemoselective 
coatings include bulk acoustic wave (BAW) devices, plate acoustic wave 
devices, interdigitated microelectrode (IME) devices, and optical 
waveguide (OW) devices, electrochemical sensors, and electrically 
conducting sensors. 
The operating performance of a chemical sensor that uses a chemoselective 
film coating is greatly affected by the thickness, uniformity and 
composition of the coating. For some chemical sensor technologies, the 
sensitivity of the sensor to a chemical analyte increases with 
increasingly thicker coatings. However, for some types of sensors, 
increasing the coating thickness has a detrimental effect on the 
sensitivity. In these types of sensors, only the portion of the coating 
immediately adjacent to the transducer substrate is sensed by the 
transducer. If the coating thickness is too thick, the outer layers of the 
coating material, that is, the layers farthest away from the substrate, 
are not sensed. These outer layers of coating material compete for the 
analyte with the layers of coating being sensed and thus reduce the 
sensitivity of the chemical sensor. Further, as the thickness of the 
chemoselective coating is increased, the time taken for an analyte to 
diffuse into the coating and come to thermodynamic equilibrium is 
increased and hence the time taken to reach an equilibrium sensing signal 
is increased. Thus, the thickness of the coating is a critical factor in 
the performance of real time monitoring chemical sensors, affecting the 
response time, recovery time and response magnitude of the sensor. 
Uniformity of the coating is also a critical factor in the performance of 
a sensor that uses a chemoselective coating. In this regard, it is 
important not only that the coating be uniform and reproducible from one 
device to another, so that a set of devices will all operate with the same 
sensitivity, but also that the coating on a single device be uniform 
across the active area of the substrate. If a coating is non-uniform, the 
response time to analyte exposure and the recovery time after analyte 
exposure are increased and the operating performance of the sensor is 
impaired. The thin areas of the coating respond more rapidly to an analyte 
than the thick areas. As a result, the sensor response signal takes longer 
to reach an equilibrium value, and the results are less accurate than they 
would be with a uniform coating. Further, in a chemical sensing device 
that uses acoustic wave energy in the detection of interactions between a 
chemoselective coating and analyte molecules, a non-uniform coating causes 
a greater amount of insertion loss of the acoustic signal than does a 
uniform coating. Insertion loss is caused by the loss of wave energy to 
the coating, for example, in the form of heat. This loss is exacerbated by 
irregularities of the coating surface. If insertion loss can be reduced by 
providing a more uniform coating, it would be possible then to increase 
the thickness of the coating without significantly impairing the 
operational ability of the device. Further, by reducing the insertion loss 
by producing a more uniform coating, a device may functionally operate 
with a larger dynamic operating range. An additional advantage to having a 
more uniform coating is that the coating is less likely than a nonuniform 
coating to delaminate from the substrate surface. 
Conventional methods to produce film coatings on substrates or chemical 
sensing devices involve dissolving the coating material in a volatile 
solvent and applying the solution to the substrate surface by pipetting or 
spray coating. The substrate surface may be rotated at high speed in a 
technique called spin coating. These techniques have several 
disadvantages. It is difficult with the spin coating or spray coating 
methods to control the coating thickness precisely, or to ensure that the 
coating is uniform from one batch to another. Spray coating provides no 
control over the uniformity of a coating over a substrate surface. Spin 
coating potentially provides a more uniform coating surface than does 
spray coating, but nevertheless this method has the disadvantage that the 
edges of the coating tend to be thicker than the interior. If a plurality 
of devices on a single substrate are coated in a single batch, the devices 
closer to the outer edge of the substrate will have a thicker coating than 
the devices closer to the center of the substrate. Further, the spin 
coating method is difficult to scale up. The spin coating method is also 
awkward, unwieldy and wasteful for coating large surfaces at one time 
because of the difficulty of spinning a large substrate and because of the 
loss of material off the edges of the substrate during the spinning 
process. Also, the spin coating method is poorly suited for coating 
discrete areas of a substrate while leaving other areas uncoated, as might 
be desired when, for example, several devices are to be coated in a single 
batch or when only the active area of a device is to be coated. Leaving an 
area of a substrate uncoated in a spin coating process requires the use of 
tape, which can introduce impurities on a substrate. Moreover, the spray 
coating and spin coating methods are not useful to create coatings of 
materials that cannot dissolved in a solvent and are poorly suited for 
creating multilayer coatings. 
Thermal evaporation under a vacuum is another method of creating a film 
coating. This method is usable only for compounds that do not decompose at 
the required operating temperature. 
More precise and accurate control over the thickness and uniformity of a 
film coating may be achieved by using pulsed laser deposition (PLD), a 
physical vapor deposition technique that has been developed recently for 
forming ceramic coatings on substrates. By this method, a target 
comprising the stoichiometric chemical composition of the material to be 
used for the coating is ablated by means of a pulsed laser, forming a 
plume of ablated material that becomes deposited on the substrate. 
Although the method is used primarily to create coatings of oxide ceramics 
such as ferroelectrics, ferrites and high T.sub.c superconductors, it has 
also been used to create organic or polymer coatings for various uses. 
U.S. Pat. No. 4,604,294 to Tanaka et al, U.S. Pat. No. 5,192,580 to 
Blanchet-Fincher and U.S. Pat. No. 5,288,528 to Blanchet-Fincher disclose 
methods of making organic or polymeric thin films by laser 
vapor-deposition. In these methods, certain bonds of the organic compound 
or polymer are photochemically broken, releasing low molecular weight 
fragments that condense and repolymerize on a substrate. Similar methods 
are also discussed in Ogale, S. B., "Deposition of Polymer Thin Films by 
Laser Ablation", in Pulsed Laser Deposition of Thin Films, Chrisey, D. B. 
and Hubler, G. K., Eds. John Wiley & Sons, New York, 1994, Chapter 25; 
Hansen S. G. and Robitaille, T. E., "Formation of Polymer Films by Pulsed 
Laser Evaporation" Appl. Phys. Lett. 52 (1), Jan. 4, 1988, 81-83; Kale et 
al. "Deposition of Amorphous Fluoropolymers Thin Films by Laser Ablation" 
Appl. Phys Lett. 62 (5), Feb. 1, 1993, 479-481; Kale et al, "Deposition of 
Polyphenylene Sulphide (PPS) Polymer by Pulsed Excimer Laser Ablation", 
Materials Letters 15 (1992) 260-263; and Kale et al "Degradation of 
Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x Thin Epitaxial Films in Aqueous Medium 
and Control of Degradation Using Polymer Overlayers Deposited by Pulsed 
Excimer Laser" Thin Solid Films 206 (1991) 161-164. A method of producing 
collagen thin films by laser deposition is disclosed in the commonly 
assigned U.S. patent application Ser. No. 08/655,788. The disclosures of 
the above patents, patent applications and publications are incorporated 
herein by reference. 
Another factor that affects sensor performance is the chemical composition 
of the film coating. For some applications, it is desirable that a film 
coating be created with a minimum of fragmentation, rearrangement, 
degradation or damage to the material being transferred. This is 
particularly true in the creation of chemoselective films for chemical 
sensing devices, since chemical selectivity for a particular analyte often 
depends on the precise arrangement of substituents on the chemoselective 
material. A drawback to using conventional pulsed laser deposition in the 
creation of film coatings is that direct ablation of the target can be 
stressful and damaging to fragile materials. Chemoselective polymers used 
as coatings in chemical sensing devices commonly contain sensitive 
functional groups that can be easily destroyed by bond scission processes 
or other unwanted reactions if they are exposed to too much energy or 
stress. 
Methods of ablating and ionizing large molecules for mass spectral analysis 
have been described. U.S. Pat. No. 4,920,264 to Becker, the disclosure of 
which is incorporated herein by reference, describes a method of desorbing 
large, nonvolatile, thermally labile molecules from a substrate by laser 
ablation by combining the large molecule with a solvent and freezing the 
mixture and then exposing the frozen mixture to laser radiation. The 
desorbed molecules are ionized and introduced into a mass analysis zone or 
are introduced into a liquid chromatography interface. U.S. Pat. No. 
5,118,937 to Hillencamp et al, the disclosure of which is incorporated 
herein by reference, describes a method of laser desorption and ionization 
of large biomolecules by combining the biomolecules with a matrix that 
absorbs laser light at a wavelength of 300 nm or greater and irradiating 
the specimen with laser light in the range absorbed by the matrix. The 
desorbed biomolecules are then ionized and introduced into a mass 
analyzer. U.S. Pat. No. 5,135,870 to Williams et al, the disclosure of 
which is incorporated herein by reference, describes a method of pulsed 
laser ablation and ionization of high molecular weight compounds by 
combining the compounds with a solvent, freezing the solution, creating a 
thin film of the frozen solution on a sample stage, and irradiating the 
sample stage to create a plume containing the high molecular weight 
compound. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a method 
of making a film coating on a substrate wherein the method allows the 
thickness of the coating to be controlled accurately and precisely. 
It is a further object of the present invention to provide a method of 
making a film coating on a substrate wherein the method produces a uniform 
coating. 
It is a further object of the present invention to provide a method of 
making a film coating on a substrate wherein the method allows a large 
area to be coated. 
It is a further object of the present invention to provide a method of 
making a film coating on a substrate wherein the method allows only 
selected areas of the substrate to be coated. 
It is a further object of the present invention to provide a method of 
making a multilayer film coating on a substrate. 
It is a further object of the present invention to provide a method of 
making a film coating of a chemoselective material by a modified pulsed 
laser deposition process in a manner so that the chemical composition and 
chemoselective properties of the chemoselective material is not destroyed 
or significantly damaged. 
It is a further object of the present invention to provide an improved 
method of making a chemical sensing device of a type that includes a film 
coating of chemoselective material, whereby the performance of the 
chemical sensing device is improved. 
These and other objects are accomplished by a method of producing a film of 
a coating material on a substrate comprising the steps of combining a 
coating material with a matrix material to form a target and exposing the 
target to a source of laser energy to desorb the matrix material from the 
target and lift the coating material from the surface of the target. The 
target and the substrate are oriented with respect to each other so that 
the lifted coating material is deposited as a film upon said substrate. 
The matrix material used in the method of the invention is selected to 
have the property of being more volatile than the coating material and 
less likely than the coating material to adhere to the substrate in 
vacuum. The matrix material is further selected as having the property 
such that when the target is exposed to a source of laser energy, the 
matrix material desorbs from the target and lifts the coating material 
from the surface of the target. 
The thickness of the film is accurately and precisely controlled by 
monitoring and controlling the number of laser pulses to which the target 
is subjected and by adjusting other operating parameters such as incident 
beam energy, pulsed laser rate, etc. The composition of matrix material 
and the energy density of the laser may be controlled to minimize the 
damage to functional groups of the coating material. Moreover, the process 
may be carried out in an enclosed space and the composition, temperature 
and pressure of gases contained therein may be controlled to minimize the 
damage to functional groups of the coating material in flight between 
ablation and deposition. 
A further aspect of the invention is an improved method of making a 
chemical or biochemical sensing device of the type that is made by the 
steps of coating the substrate with a film of a sorbent chemoselective or 
bioselective material and providing means to detect an interaction of the 
film of the sorbent chemoselective or bioselective material with an 
analyte, wherein the improvement is to carry out the step of coating the 
substrate by laser deposition of the chemoselective or bioselective 
material onto the substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In one aspect, the method of the present invention can be used in any 
instance where it is desired to form a film of coating material on a 
substrate. The method is particularly useful in instances where it is 
desired to form a uniform and adherent film of a chemoselective or 
bioselective material of controlled thickness on a substrate in a manner 
that minimizes damage to the chemoselective or bioselective material. 
The substrate of the present invention may be any solid surface of any 
shape, composition and orientation. 
The method of the present invention is particularly useful in instances 
where the substrate is a component of a chemical or biochemical sensing 
device of the type that includes a substrate, a film coating of a 
chemoselective or bioselective material on the substrate and a means for 
detecting the interaction of the chemoselective or bioselective material 
with a chemical or biochemical analyte. Examples of such devices include 
surface acoustic wave (SAW) devices, bulk acoustic wave (BAW) devices, 
plate acoustic wave devices, interdigitated microelectrode (IME) devices, 
optical waveguide (OW) devices and surface plasmon resonance devices. The 
composition of the substrate depends upon the type of device. For example, 
when the substrate is a component of a surface acoustic wave (SAW) device, 
the substrate typically includes a piezoelectric material. 
The coating material may be any material that one may wish to deposit onto 
a substrate. In a chemical or biochemical sensing device, the coating 
material will be chosen for its ability to interact selectively with a 
particular chemical or biological analyte. Criteria for selecting coating 
materials for chemical sensing devices are described in detail in McGill 
et al, "Choosing Polymer Coatings for Chemical Sensors", CHEMTECH, Vol 24, 
No. 9, pp 27-37 (1994), the disclosure of which is incorporated herein by 
reference. Examples of chemoselective materials include SXFA (poly(oxy 
{methyl [4-hydroxy-4,4,bis(trifluoromethyl)but- 1-en- 1-yl] silylene})), 
P4V (poly(4-vinylhexafluorocumyl alcohol). In both of these polymers and 
in other similar polymers, their chemoselectivity is derived from pendant 
functional groups that extend outward from the polymer backbone. A purpose 
of the method of the present invention is to create uniform, adherent 
coatings of these materials without disrupting or damaging the functional 
groups. Other examples of chemoselective materials include 
perfluoro-polyethers terminated with a variety of functional groups such 
as CF.sub.3 CH.sub.2 OH, polyethylene imines, polysiloxanes, alkylamino 
pyridyl substituted polysiloxanes, polytetrafluoroethylene, polysilanes, 
polyesters, polyvinylaldehydes, polyisobutylene, polyvinylesters, 
polyalkenes, zeolites, aerogels, porous carbon, metals, silicalites, clay 
materials, cellulose materials, polyanilines, polythiophenes, 
polypyrroles, fullerenes, cyclodextrins, cyclophanes, calixeranes, crown 
ethers, and organic dyes. Examples of biochemical selective materials 
include antibodies, antigens, DNA, RNA, proteins, oligopeptides, 
polypeptides, oligosaccharides, polysaccharides, and lipids. 
The matrix material is selected to have the property that when it is 
combined with the coating material to form a target and the target is 
exposed to a source of laser energy, the matrix material absorbs some of 
the laser energy in such a manner so that the matrix material is desorbed, 
thereby lifting or evaporating the coating material from the surface of 
the target. Preferably, only a thin layer on the surface of the target 
desorbs at any one time. The matrix material is also selected to be more 
volatile than the coating material and less likely than the coating 
material to adhere to the substrate in vacuum. The choice of a particular 
matrix material will depend on the choice of the coating material. Factors 
to be taken into consideration in selecting the optimum matrix material 
for a particular coating material include its ability to dissolve or form 
a collodial or particulate suspension with the particular coating 
material, its melting point, heat capacity, molecular size, chemical 
composition, spectral absorption characteristics, heat of vaporization 
(factors that affect its ability to desorb and lift the coating material 
from the target) and its reactivity or nonreactivity towards the coating 
material. Any laser-produced decomposition products of the matrix material 
should be nonreactive. Another consideration is any special ability a 
particular matrix material may have to impart protection to a particular 
coating material from damage during the lasing, desorption and transfer to 
the substrate. For example, a matrix material that absorbs laser energy at 
the same wavelength as an important functional group on the coating 
material may serve to protect the coating material from damage from 
exposure to the laser energy. Alternatively, a matrix material may be used 
that absorbs at a wavelength in a spectral region substantially outside 
that of the coating material. In this instance, the matrix material 
absorbs kinetic energy imparted from the laser to the coating material. 
Preferred matrix materials include water, aryl solvents, especially 
toluene, acetophenone and nicotinic acid, ketones, alcohols, ethers and 
esters. Examples of coating material-matrix material combinations that 
work well are SXFA 
(poly(oxy{methyl[4-hydroxy-4,4,bis(trifluoromethyl)but-1-en-1-yl] 
silylene})) (coating material) with t-butanol (matrix material) and P4V 
(poly(4-vinylhexafluorocumyl alcohol)) (coating material) with 
acetophenone (matrix material). 
The coating material and the matrix material may be combined to form a 
target in any manner that is sufficient to carry out the purpose of the 
invention. If the coating material is soluble to some extent in the matrix 
material, the coating material may be dissolved in the matrix material and 
then the solution may be frozen to form a solid target. The target may be 
kept frozen while the surface is being exposed to a source of laser energy 
during the deposition process. Alternatively, if the coating material is 
not soluble in a suitable solvent, the coating may be mixed with a matrix 
material to form a colloidal suspension or condensed phase. The matrix 
material can also include soluble or insoluble dopants, that is, 
additional compounds or materials that one may wish to deposit onto the 
film. 
The target may generally be in any shape suitable for being exposed to a 
laser beam (for example, a pellet, disc, cylinder or parallelepiped). 
Preferably, the target is a dense, cylindrical pellet. For substrates of a 
given size, the target should be about four times larger in area. 
Apparatus for carrying out the steps of exposing the target to a source of 
laser energy to desorb the coating material and matrix material and for 
depositing the coating material on a substrate can be conventional 
apparatus for pulsed laser deposition as is described in Pulsed Laser 
Deposition of thin Films, edited by D. B. Chrisey and G. K. Hubler (Wiley, 
New York, 1994) and in Cotell, "Pulsed Laser Deposition and Processing of 
Biocompatible Hydroxylapatite Thin Films" Appl. Surf. Sci. 69 (1993) 
pp140-148, the disclosures of which are incorporated herein by reference. 
The target may be mounted by any support means. The angle of incidence 
between the source of laser energy and the target can be any angle used in 
the art and is typically about 45.degree.. Typically, the target is 
mounted on a moving support means, such as a rotating and/or translating 
shaft, such that different portions of the target are in the center of the 
laser beam at different times, thereby extending the useful lifetime of 
the target and providing enhanced film uniformity. Typically, the target 
is rotated at a speed of about 0.05-0.5 revs/s. Such an arrangement allows 
for greater uniformity of deposition on the substrate. In addition to (or 
instead of) moving the target, the center of the laser beam can be moved 
to achieve similar effects. Larger substrate areas may be coated by 
rastering the center of the laser beam across the target surface. In these 
cases, targets having diameters &gt;0.75 inch would be preferred. 
Any suitable source of laser energy can be employed. In general, as 
discussed below, a pulsed laser, particularly a short pulsed laser, is 
preferred in accordance with the present invention. For example, an 
excimer laser (e.g., ArF, KrF, XeF or XeCl) can be used, an ArF excimer 
laser being especially preferred. Other short pulsed lasers, e.g., Nd-YAG 
or CO.sub.2, could be used. 
Lasers for use in accordance with the present invention generally emit 
light having a wavelength in the range of about 193 nm-1100 nm, an energy 
density of about 0.05-10 J/cm.sup.2 (typically about 0.1-2.0 J/cm.sup.2), 
a pulsewidth of about 10.sup.-12 -10.sup.-6 second and a pulse repetition 
frequency of about 0-1000 Hz. In general, energy density (fluence) affects 
morphology; higher energies tend to produce deposited films that have 
larger particles. 
The distance between the target and the substrate is typically within the 
range of about 3 cm-10 cm. More often, the distance between the target and 
the substrate is about 3 cm-5 cm. A particularly preferred distance is 
about 4 cm. In general, larger distances are more suitable for depositing 
on larger substrate areas. Distances of greater than 10 cm may be used if 
desired, for example, for depositing on larger surface areas. However, the 
target-substrate distance is also inversely related to the film thickness 
achieved for a given period of deposition. The substrate may be 
manipulated, such as by rotation or translation, during deposition to 
allow deposition on non-planar or irregularly shaped surfaces. If the 
coating material used in the practice of the invention is a polymer, a 
more uniform coating may be created by heating the substrate to a 
temperature above the glass transition or melting point of the polymer 
while the coating material is being deposited on the substrate. 
The target and the substrate are preferably positioned within an enclosed 
space or chamber, referred to as a "PLD chamber", having an environment 
whose temperature, pressure and chemical composition are controlled to 
enhance the deposition process and to minimize the likelihood of damage to 
the coating material. Suitable environments according to the present 
invention may include argon, and argon/water, oxygen, alkanes, alkenes, 
alkynes, alcohols or a mixture of these gases. A gas having the same 
chemical composition as the matrix material may be used. Other 
non-reactive or inert gases may be substituted for argon. For creating a 
gas/water environment, there may be provided a gas inlet port which passes 
through a bubbler before passing into a vacuum chamber. For creating a 
water-free environment, there may be provided a gas inlet port which 
passes directly into a vacuum chamber. If desired, the chamber atmosphere 
may be changed at the end of a deposition process to bring the substrate 
to atmospheric conditions. Gases present before and during the deposition 
process may be referred to as "make-up gases." The gases introduced into 
the chamber at the end of deposition may be referred to as "quenching 
gases." 
The background pressure within the chamber during deposition determines the 
rate at which the film of coating material is deposited on the substrate. 
Generally, the lower the pressure, the faster the deposition. As the 
pressure within the chamber increases, the number of collisions between 
the ablated coating material and the gas increases. These collisions slow 
the deposition process, resulting in thinner films. Generally, the 
background pressure within the chamber during a deposition may be about 0 
Torr-760 Torr. Typically, deposition is performed below atmospheric 
pressure. A preferred range for the total pressure within the chamber 
during deposition is about 0.05 Torr-50 Torr. A particularly preferred 
range is about 0.1 Torr to 1 Torr. 
The thickness of the deposited film is proportional to the number of laser 
pulses to which the target is exposed and, consequently, to the time of 
deposition for a given laser pulse rate. The film thickness may be 
selected by controlling the number of laser pulses depending on the 
purpose for which the finished article is to be used. A typical range of 
thicknesses for acoustic wave devices is from about 1 nm to about 10 
microns. The thickness of the film may also be controlled by selecting 
factors such as the laser energy density, the target temperature and the 
distance of the substrate from the target. 
In the manufacture of an acoustic wave device, the thickness of the coating 
material during the deposition process may be monitored by operating an 
acoustic wave device having an initial predetermined oscillating 
frequency. The change in the frequency of the acoustic wave as the coating 
material is deposited on the substrate may be monitored in real time and 
the deposition process halted when the frequency changes to a second 
predetermined level. 
The method of the present invention is particularly suitable for making 
film coatings on relatively large substrate areas (up to about six inches 
in diameter). To make numerous chemical sensing devices at one time, a 
single large substrate can be coated at one time and then the coated 
substrate can be subdivided into separate chemical or biochemical sensing 
device components. The substrate may be masked so that only predetermined 
areas of the substrate are coated. 
The method of the present invention may be used to make multi-layer devices 
by repeating the steps of producing the film coating on a substrate using 
different coating materials. 
FIG. 1 shows a typical apparatus 100 useful for carrying out the method of 
the present invention. Vacuum chamber 10 with a base pressure of about 
4.times.10.sup.-8 Torr encloses the apparatus. Excimer laser 12 is focused 
through lens 14 onto target 16 made of the coating material and the matrix 
material. Target 16 is affixed to rotating arm 18 which allows plume 20 to 
be precessed over substrate 22, thus covering a wider area than a fixed 
arm geometry would allow. Substrate holder 24 is electrically isolated and 
may be heated by means of quartz lamp 26. The temperature is monitored by 
thermocouple 28. Gas inlet port 30 allows the introduction of gases into 
chamber 10 in the direction of arrow 31. Bubbler 32 (optional) may be used 
when a water vapor environment is desired. 
In an alternative embodiment, the invention relates specifically to an 
improved method of making a chemical or biochemical sensing device by the 
steps of (a) providing a substrate (b) coating the substrate with a film 
of a chemoselective or bioselective material, the film having a uniform 
thickness over a substantial portion of the substrate and (c) providing 
means to detect an interaction of the film of the chemoselective or 
bioselective material with an analyte, wherein the step of coating the 
substrate with a film of chemoselective or bioselective material is by the 
technique of pulsed laser deposition as described above. The 
chemoselective or bioselective material may be combined with a matrix 
material as described above, but if the chemoselective or bioselective 
material is able to withstand the conditions for pulsed laser deposition, 
this step may be omitted. Examples of chemoselective materials that can be 
used in the formation of a chemical sensing device without combining the 
material with a matrix material include polyolefins such as 
polyisobutylene, polyhalogenated olefins, polyhalogenated ethers, in 
particular, polyepichlorohydrin, perfluorinated polymers, particularly 
polytetrafluoroethylene and porous inorganic materials such as activated 
carbon, clays, zeolites and aerogels. 
Having described the invention, the following examples are given to 
illustrate specific embodiments of the invention including the best mode 
now known to perform the invention. These specific examples are not 
intended to limit the scope of the invention described in this 
application. 
EXAMPLE 1 
A solution of SXFA 
(poly(oxy{methyl[4-hydroxy-4,4,bis(trifluoromethyl)but-1-en-1-yl] 
silylene})) in t-BuOH (tertiary butyl alcohol) was prepared at a 
concentration of 0.005 g of polymer/g of matrix material. This solution 
was poured into a cylindrical teflon lined lid and frozen in liquid 
nitrogen. The frozen SXFA(tBuOH) solution in the lid was transferred to 
the PLD chamber as the target. The open end of the lid was secured in such 
a position to expose the surface of the frozen SXFA(tBuOH) solution to the 
laser. The PLD conditions were as follows: 
______________________________________ 
Type of Laser KrF, 248 nm 
Laser Power(W) @ 10Hz 
0.20 
Energy Density (J/cm2) 
0.45 
Repetition rate(Hz) 
5 
Spot Size(cm.sup.2) 
0.045 
Laser Voltage(kV) 
14.7 (Constant Energy Mode:375mJ) 
Substrate NaCl Plate 
Substrate Temperature (.degree.C.) 
25.degree. C. 
Make Up Gas Water & Argon 
Make Up Gas Pressure(torr) 
0.050 
Substrate-Target Distance 
3 cm 
Number of Shots 21,000 
______________________________________ 
Water vapor was generated by bubbling argon through water maintained at 
about 25.degree. C. The argon stream was therefore nearly saturated with 
water vapor. The deposition was stopped at 20,000 shots and the repetition 
rate increased to 10 Hz and an additional 1000 shots added. A comparison 
of FIG. 2 (native SXFA) and FIG. 3 (PLD-deposited SXFA) shows that 
functional groups of native SXFA are preserved in the PLD process. 
EXAMPLE 2 
A solution of poly(4-vinylhexafluorocumyl alcohol) (P4V) in acetophenone 
(ACPH) was prepared at the concentration of 0.0056 g/g. This solution was 
poured into a cylindrical teflon lined lid and frozen in liquid nitrogen. 
The frozen P4V(ACPH) solution in the lid was transferred to the PLD 
chamber as the target. The open end of the lid was secured in such a 
position to expose the surface of the frozen P4V(ACPH) solution to the 
laser. The PLD conditions were as follows: 
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Type of Laser KrF, 248 nm 
Laser Power(W) @ 10Hz 
0.20 
Energy Density (J/cm2) 
NA 
Repetition rate(Hz) 
5 
Laser Voltage(kV) 
16.2 (Constant Energy Mode:375mJ) 
Substrate NaCl Plate 
Substrate Temperature (.degree.C.) 
25.degree. C. 
Make Up Gas Water & Argon 
Make Up Gas Pressure(torr) 
0.050 
Substrate-Target Distance 
3 cm 
Number of Shots 21,000 
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Water vapor was generated by bubbling argon through water. The argon stream 
was therefore nearly saturated with water vapor. The deposited coating 
appeared to add a tinted sheen to the surface. 
A comparison of the FT-IR spectra of native and PLD-deposited P4V showed 
that functional groups of the P4V were preserved in the deposition 
process. (data not shown) 
EXAMPLE 3 
A 2 port (launch and receive) 250 MHz SAW resonator was coated with a film 
of SXFA by the PLD method described above to create a coating having a 
thickness such that the frequency of the device was shifted by 235 KHz 
during the deposition process. The insertion loss of the device was 
determined to be 13 db. The insertion loss of a similar device created 
using conventional coating methods is determined to be between 16 to 20 
db. With no polymer coating these SAW devices typically exhibit insertion 
loss values of between 8 to 10 db. The theoretical value is about 6 db. 
The PLD coated device yields insertion loss values that are at about 50% 
less than using conventional coating techniques. This is advantageous 
because the gain required to operate the SAW device does not have to be as 
strong. More coating can be deposited without significantly impairing the 
operational ability of the device. 
EXAMPLE 4 
A flat clean 1 cm.sup.2 Si (111) substrate wafer was coated with 
polyepichlorohydrin (PECH) under the following conditions: 
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Type of Laser KrF, 248 nm 
Laser Power(W) @ 10Hz 
0.20 
Energy Density (J/cm2) 
0.5 
Repetition rate(Hz) 
5 
Laser Voltage(kV) 
14.8 (Constant Energy Mode:375mJ) 
Substrate Temperature (.degree.C.) 
25.degree. C. 
Make Up Gas N.sub.2 /H.sub.2 
Make Up Gas Pressure(torr) 
.05 
Substrate-Target Distance 
3 cm 
Number of Shots 29,000 
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The PECH Coated Si (111) wafer was analyzed with a J.A. Woolam Co. focused 
beam M-44.TM. ellipsometer and VASE.RTM. (variable angle spectroscopic 
ellipsometer). The thickness of the PECH coating in a linear direction was 
sampled across the 1 cm.sup.2 substrate. The coating thickness was 
determined to be 2087+/-48 Angstroms. 
EXAMPLE 7 
An uncoated 250 MHz surface acoustic wave (SAW) device was placed in a 
vacuum chamber at a pressure of about 50 millitorr at room temperature. A 
target of polyepichlorohydrin (PECH) was exposed to a Kr--F pulsed laser 
(248 nm) to transfer the PECH from the target to the active area of the 
surface acoustic wave device, which was placed 4 cm from the target to 
create a 32 nm film of the PECH on the device. Performance of the SAW 
device when exposed to dimethylmethylphosphorate and 
bis-2-chloroethylether gases compared with the performance of a SAW device 
created by the conventional means of spray coating with PECH in a solvent. 
The SAW device coated by pulsed laser deposition showed greater 
sensitivity, faster sensor signal response, and a more rapid recovery of 
the sensor signal to a baseline value upon exposure to clean air in 
comparison to the spray-coated SAW device. 
Obviously, many modifications and variations of the present invention are 
possible in light of the above teachings. It is therefore to be understood 
that, within the scope of the appended claims, the invention may be 
practiced otherwise than as specifically described.