WO1998018568A1

WO1998018568A1 - Method and apparatus for the production of thin films

Info

Publication number: WO1998018568A1
Application number: PCT/US1997/019360
Authority: WO
Inventors: Robert Herpst; Kenneth B. Cuthbert
Original assignee: Robert Herpst
Priority date: 1996-10-28
Filing date: 1997-10-27
Publication date: 1998-05-07
Also published as: US6254931B1; JP2002501425A; EP0948413A4; EP0948413A1; US6594008B1

Abstract

Method and apparatus (2) for forming thin films by the application of a film forming liquid to a substrate (124) at rest or rotating a speed up to 500 rpm and then rotating the film forming liquid on the substrate (124) at a speed and for a time sufficient to form the thin film. Such films can be used for analysis by spectrophotometric methods. Apparatus for forming such thin films is also disclosed.

Description

METHOD AND APPARATUS FOR THE PRODUCTION OF THIN FILMS

Related Applications

This is a Continuation Application of U.S. Serial No. 60\046,044 filed May 9,

1997 and U.S. Serial No. 60/029,516 filed October 28, 1996.

FIELD OF THE INVENTION

The present invention is directed to a method and apparatus for the

production of thin films produced from liquids that undergo phase transition to a

viscoelastic state such as polymers and resins which have been dissolved or

melted, the liquids are placed onto a substrate and the substrate is rotated in a

manner and under such conditions that a uniform thin film is produced which can be

readily removed from the substrate. Such thin films can serve as samples for

analysis by infrared spectroscopy or as carrier vehicles for samples to be analyzed

by infrared, x-ray fluorescence ("XRF") or other spectrographic methods and the like.

Films made in accordance with the present invention can also be used as

standards for calibration of Infrared and FTIR spectrophotometers and carrier films

suitable for XRF can also be used as calibration standards for XRF

spectrophotometers. BACKGROUND OF THE INVENTION

The formation of coated substrates is known in the art. In this technology, a

coating material is prepared in the form of a coating liquid and then adhered to a

substrate in a manner and under such conditions that the coating remains affixed to

the substrate. For example, Hiroyoshi U.S. Patent No's. 5,238,713 and 5,116,250

disclose a coating apparatus including a spinning chuck rotatably supported within

an enclosure which holds a substrate. The substrate is a semi-conductor wafer or

glass and the coating material is typically a photoresist masking material. The

purpose of applying the coating material to the substrate is to form a permanent

bond therebetween (i.e. to mask portions of the semi-conductor).

The coating operation is performed by applying a liquid coating material to the

top surface of the semi-conductive substrate which is being continuously rotated at

high speeds. The liquid coating material is typically applied by a nozzle or other

dispensing device. The liquid coating material is typically applied under continuous

flow conditions and then drawn outwardly by rotating the table upon which the

substrate rests at high speeds. The process relies upon centrifugal force to spread

the coating material outwardly to form a coating layer which strongly adheres to the

substrate. Such coatings are applied to protect the substrate (e.g. semi-conductor

wafers and glass) by remaining permanently bonded thereto. SUMMARY OF THE INVENTION

The present invention is directed, not to producing an adherent coating on a

substrate, but rather to producing a thin film that may, if required, be readily

removed from the substrate. Such thin films can be used, for example, as optical

samples for infrared spectroscopy or as carriers for XRF, infrared and other forms of

spectroscopy. The conditions under which the film is made make use of both

centrifugal force and centripetal force and thus the process clearly distinguishes

from those processes, like the references mentioned above, which produce

adherent coatings on substrates.

The present invention is specifically directed to methods and apparatus for

forming a thin film on a substrate which film can be easily removed so that it may be

used as a sample and/or a medium for spectroscopy. When used as a sample for

spectroscopy, the thin film must be optically transmissive in the spectral region of

interest. When used as a carrier or medium, the film must be non-absorbing or

neutral so that it is not detected by the spectrophotometer in the region where the

sample is absorbing or detected.

In another aspect of the invention, the thin films can be used as calibration

standards for XRF and other spectrophotometers. In another aspect of the present invention, films of polymers can be used as calibration standards for IR

spectrophotometers.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings in which like reference characters indicate like parts

are illustrative of embodiments of the invention and are not intended to limit the

invention as encompassed by the description of the invention herein.

Figure 1 is a partially exploded perspective view of an embodiment of an

apparatus for forming thin film in accordance with the present invention;

Figure 2 is a partial side view of the apparatus shown in Figure 1 ;

Figure 3 is a perspective view of a substrate assembly for receiving a film

forming liquid and the application of the liquid to the substrate;

Figure 4 is a cross-sectional side view of the support mechanism for holding

the substrate assembly within the apparatus;

Figure 5 is a plan view of one embodiment of a substrate assembly used to

form a thin film in accordance with the invention; Figure 6 is a partial side view of the substrate assembly shown in Figure 5;

Figure 7 is a plan view of another embodiment of a substrate assembly which

is particularly adapted for holding a slide or crystal to form a film thereon;

Figure 8 is a partial cross-sectional view of the substrate assembly shown in

Figure 7;

Figure 9 is a plan view of still another substrate assembly for forming a film in

accordance with the present invention;

Figure 10 is a partial cross-sectional view of the substrate assembly shown in

Figure 9;

Figure 11 is a perspective view of still another substrate assembly for use in

forming thin films in accordance with the present invention;

Figure 12 is a cross-sectional side view of the substrate assembly shown in

Figure 11 ;

Figure 13 is a plan view of a further substrate assembly for use in forming thin

films in accordance with the present invention; Figure 14 is a partial cross-sectional view of the substrate assembly shown in

Figure 13;

Figure 15 is a schematic cross-sectional side view of an embodiment of

another apparatus for forming a thin film in accordance with the present invention;

Figure 16 is a spectrum of a Nicolet NIST traceable polystyrene calibration

film known in the art in which peaks at 2925.1 cm^"1 and 690.6 cm^"1 exceed the "Y"

axis of the spectrophotometer; and

Figure 17 is a spectrum of a polystyrene film cast on a KBr crystal in

accordance with the present invention in which the peaks do not exceed the "Y" axis

of the spectrophotometer.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a method and apparatus for forming thin

films. The sample is intended to be removed or to remain stretched over the

aperture of a frame, or to be placed on a substrate which is transmissive. The thin

film can be removed, in all cases to facilitate analysis by spectroscopy or any other

technique. Such films can be employed in technologies where films of uniform

and/or reproducible thickness are desired or where such films can be used as a carrier or medium. As used herein, spectroscopy includes infrared, Fourier

transform infrared, near infrared, Raman and x-ray spectroscopy.

As used herein the term "a sample for spectroscopy" shall mean either a thin

film sample which is a sample or a sample contained within a thin film, any of which

can be analyzed by one or more of infrared (IR), Fourier transform infrared (FTIR),

near infrared (NIR), Raman, x-ray fluorescence (XRF) or other spectrographic

methods. As used herein the term "carrier" or "medium" shall mean that the thin film

contains, holds, supports, suspends or carries another material (e.g. metal) which

can be detected by spectroscopy. As also used herein the term "substrate" shall

mean a surface capable of receiving a film forming liquid and/or a frame in which the

liquid is suspended between opposed supporting surfaces (e.g. liquid is placed in an

opening defined by a cylindrical supporting structure).

Thin films can be made of any substance that can be made flowable (viscous)

which will undergo a phase transition to a viscoelastic state when rotated on a

substrate in the manner described herein. Such materials include, but are not

limited to, polymers, resins, monomers in liquid form, glues, adhesives, paints,

varnishes, lacquers, nail polish, hair spray, urethanes, polyurethanes, proteins,

animal tissues, cell samples and the like, alone or in combination with a suitable

solvent each of the above substances being referred to herein as a "film forming

liquid". A thin film of the present invention can be formed by placing the film forming

liquid on a substrate at rest or at speeds up to 500 rpm, preferably up to 200 rpm

and then rotating the substrate at a speed sufficient to form the thin film, typically up

to 5,000 rpm. In this method, centrifugal and centripetal forces are employed.

Centripetal force is created by the rotation of the substrate around the axis of a

shaft, and this inward directed force drives the film forming liquid towards the center.

Centrifugal force creates an outward pulling on the film forming liquid. At the same

time, cohesion creates on inward force on the film forming liquid while adhesion

causes the film forming liquid to stick to the substrate. These opposing forces result

in a tensioning as the film forming liquid undergoes the phase transition from the

liquid state to the elastic state to the solid state. At some point during the phase

transition the opposing forces cease to have any further effect on the transitional

formation of the film and the film ceases to change form. However, the cohesive

bond of the more solid molecules now holds the film together in a way that gives it

more structural integrity than a liquid and also makes the film sufficiently strong in

most cases to allow it to overcome the adhesive force of the film to the unlike

molecules of the substrate. Accordingly, the last step in the process is the

overcoming of the adhesive forces by peeling the film from the substrate which can

readily be accomplished in most cases in accordance with the present invention.

It is understood that in certain cases the film forming liquid will adhere so

strongly to the substrate that it cannot be removed. Substances such as monomers in liquid form which do not undergo phase transition to a solid also cannot be

removed. In these situations, the film must be cast on a transmissive substrate such

as an optical crystal or on a reflective surface so that analysis can be performed by

reflection. In another preferred embodiment of the invention, films that cannot be

removed from the substrate are cast on frames (i.e. a space defined by spaced

apart supporting structures) as discussed below and the frame is then placed on the

substrate and rotated at high speeds to regulate the thickness and consistency of

the film.

An apparatus for performing the methods of the present invention is shown in

Figures 1-4. Referring to Figures 1-4, there is shown an apparatus 2 for forming thin

films. The apparatus includes a rotatable substrate section 4 with a protective shield

assembly 6 thereover. The apparatus includes a housing 8 preferably having a

control panel 10. The control panel 10 enables control preferably automatically, of

the turning on and off of the rotation cycle, the length of time of the film forming

operation and/or the rotational speed of the substrate upon which the film is formed.

As shown specifically in Figure 2, operation of the rotational substrate section 4 is

performed by a motor 12 which is capable of rotating the substrate to speed equal to

or exceeding 500 rpm and typically up to 5,000 rpm or higher. Removal of vapors

as the liquid film forming substance undergoes phase transition to a thin film is made

possible by an exhaust system 14 which removes vapors through an exit vent 16. Removal of vapors through the 16 is made possible by providing for the

intake of air through a series of intake vents 17. The air is drawn through the vents

17 and travels upwardly in the direction of the arrows shown in Figure 2 into the

rotatable substrate section 4 and through openings 18 (see Figure 1 ). The air draws

vaporized materials from the film forming liquid into the exhaust system 14 through a

fan 19 or the like.

As previously indicated, thin films in accordance with the present invention

are produced by placing a quantity of a film forming liquid on a substrate at rest or

rotating at low speeds up to 500 rpm, preferably up to 200 rpm then rotating the

substrate at a speed sufficient to form the thin film (i.e. typically up to 5,000 rpm or

higher).

Referring again to Figure 1 , the thin film is formed in the rotatable substrate

section 4 of the apparatus 2. As best shown in Figure 3, the rotatable substrate

section 4 includes a panel 20 having openings 1 B for the intake of air as previously

described enabling air to circulate and to assist in removing the vapors through the

exhaust system 14 as shown in Figure 1. Within the panel 20 is a cylindrical well 22

having a sidewall 24 along the circumference thereof. Set within the well 22 and

below the level of the panel 20 is a rotatable substrate assembly 26 shown in detail

in Figure 4. The rotatable substrate supporting device 26 includes an opening 28 for

receiving a substrate assembly as described in detail hereinafter with respect to Figures 5-14. The supporting device 26 as shown in Figure 4 is secured to a

connecting or coupling device 30 which operatively connects the supporting device

26 and a substrate assembly secured therein to the motor 12 (see Figure 2) through

a shaft 32. The connecting device 30 is secured to the shaft 32 in a conventional

manner such as by screws 34 which enter the connecting device 30 through holes

36.

The supporting device 26 can be any shape and includes means to secure a

rotatable substrate, such as a cavity 140 defined by at least one wall 142 and a

bottom surface 144. The cavity is of sufficient dimensions to have a substrate

assembly secured therein. Vents 146 may be provided in the bottom surface 144 to

allow entrapped air to escape when the substrate is placed into the cavity 140 to

allow proper seating of the substrate.

Some means of attachment (e.g. a screw 141 insertable into a groove 143) is

provided to enable the supporting device 26 to be secured to the connecting device

30. The screw 141 fits into the hole 145 of the connecting device 30. In addition,

pins 147 attached to the connecting device 30 are insertable into openings 149 of

the supporting device 26. As shown specifically in Figure 4, the connection device

30 has a projection 148 which is operatively secured within a corresponding cavity

150 of the supporting device 26. As previously indicated a substrate assembly is secured within the supporting

device 26 to provide a substrate on which a thin film can be formed. Examples of

suitable substrates are shown in Figures 5-14.

Referring first to Figures 5 and 6, there is shown a first embodiment of a

substrate assembly 28 which is comprised of a lower section 38 and a base 40

which provides a surface 42 on which the film forming liquid can be deposited. The

lower section 38 is adapted to be secured within the supporting device 26 through

the lower section 38 and particularly by an O-ring 44 which may be made of any

flexible material such as rubber or the like.

As shown best in Figure 6, the surface 42 of the base 40 preferably has a

slightly concave shape. It will be understood that the surfaces can be planar,

concave or convex (not shown). A concave surface provides additional resistance

against the film forming liquid as it spreads out while the substrate assembly 28 is

rotated at high speeds. It will be understood, however, that for some film forming

liquids of generally higher viscosity, a planar surface may be employed as well as

specifically shown in Figures 9 and 10. In particular, the base 40 has a planar

surface 49 especially adapted for higher viscosity liquids.

A further embodiment of the substrate is shown in Figures 7 and 8. Referring

to Figures 7 and 8, there is shown a substrate assembly 50 having a lower section 38, a base 40 and a ring 44 essentially as described in connection with the

embodiment of Figures 5 and 6. In this embodiment, the base 40 is modified to

have a surface 52 having a well 54 therein for holding a removable substrate 56

such as a slide or crystal. The shape of the removable substrate 56 is circular as

shown specifically in Figures 7 and 8. However, the shape of the substrate 56 may

be modified to accommodate substrates having three or more sides. In particular,

the well 54 is provided with a cavity 58 for housing the substrate 56. Above the

position of the substrate 56 are opposed areas 60 which can be used to grip the

sides of the substrate 56 so that it can be readily removed from the cavity 58.

The rotatable assembly may also be provided with a substrate in the form of a

sample cup which itself defines a cavity and has a handle for easy removal from the

base of the rotatable assembly. Sample cups made of thin gauge aluminum are

commercially available such as from Fisher Scientific Co. Referring to Figures 11

and 12, there is shown a sample cup 70 having a surface 72 on which is placed the

film forming liquid. The surface may be planar or as best shown in Figure 12, may

be slightly concave to accommodate lower viscosity liquids or liquids which do not

readily wet the substrate. A handle 74 may be provided to facilitate removal of the

sample cup from the supporting device 26. The sample cup is preferably made of

thin gauge aluminum which can be easily cut away to provide ready access to the

thin film formed thereon. In addition, the bottom of the sample cup can be used as a

reflective substrate. The substrate of the present invention may be in the form of a frame which

may be in the form of a vial cap having a hole therein in which the film forming liquid

is suspended thereon or the like. Referring to Figures 13 and 14, there is shown an

embodiment of the invention in which the base 40 of a substrate assembly 80 has a

platform 82 having a pair of opposed supports 84A and 84B which are spaced apart

at a distance defining an opening 86 sufficient to suspend a quantity of the film

forming liquid therebetween. Beneath the opening 86 is a pathway 88 which

enables vapors to be removed from the film-forming area.

The apparatus 2 of the present invention is optionally provided with a safety

device to ensure that the film forming liquid does not splatter and come into contact

with the user. Such a protective shield would be highly desirable if the film forming

liquid has any toxic properties with regard to exposed areas of the body such as the

eyes.

Referring to Figure 1 , the protective shield assembly 6 comprises a lid 90

having a cylindrical extension 92 therefrom which fits over the rotatable substrate

section 4 and particularly over the rotatable substrate assembly 26. Contained

within the extension 92 is a plate 94 (e.g. a watch glass) which serves to protect the

lid 90 from splatter of the film forming liquid. To ensure proper positioning of the

plate 94, a cylindrical insert 96 having an outside diameter slightly smaller than the

inside diameter of the extension 92 is placed within the extension 92 and serves to maintain the position of the plate 94 within the extension and to facilitate removal of

the plate 94 for cleaning or replacement. The plate 94 and the insert 96 are secured

in place such as by the use of set screws 97 which are secured in openings 98 of

the extension 92 and openings 99 in the insert 96.

In a preferred form of the invention, the well 22 of the rotatable substrate

section 4 is provided with a removable sleeve 100 to protect the inside of the well

against splatter of the film forming liquid. The sleeve 100 is removable to facilitate

cleaning or replacement thereof.

In another embodiment of the invention, the deposition of a thin film on a

rotatable substrate is shown in a hemispherical housing. Referring to Figure 15,

there is disclosed an apparatus 110 comprised of a housing 112 having a

hemispherical shape including a cover 114 having an opening 116 therein. The

housing has a curvilinear lower portion 118 including at least one exhaust port 120.

A substrate assembly 122 extends parallel to the cover 114 and has an impeller 123

associated therewith to draw air into the housing to remove vapors from the housing.

A substrate 124 is provided on a base 126 which is operatively connected to a

rotatable shaft 128 which in turn is controlled by a motor designated by the numeral

130. Rotation of the shaft 128 causes a film forming liquid on the substrate 124 to

spread out and form a thin film when the shaft 128 is rotated at speeds of at least

500 rpm. Further details of the present invention will now be described below. The present invention is directed, not to producing adherent coating on a

substrate as in some prior art systems previously mentioned, but rather to producing

a thin film that may, if required, be readily removed from the substrate or cast over

an aperture so that no substrate contacts the center of the film. Such thin films can

be used, for example, as optical samples for infrared spectroscopy or as carriers for

XRF, infrared and other forms of spectroscopy. Conditions under which the film is

made make use of both centrifugal force and centripetal force. For example, the

thin-film forming liquid in the present invention is not applied under continuous flow

conditions, but rather in discrete units (e.g. droplets or doses). In addition, the film

forming liquid is applied to the substrate at rest or when the substrate is operating at

low speed conditions (i.e., under 500 rpm or less). Further, the film forming liquid is

deposited at the center of the substrate at the axis. Films cast using the present

invention display the unique property of radial symmetry. This property does not

occur in films that are spin cast using devices that employ only centrifugal force.

The film radiates from the axis towards the perimeter of the substrate. Furthermore,

only after deposition is completed is the substrate rotated at high speeds and with

the present invention films can be cast not only on a planar substrates, but over a

parabolic substrate or the aperture of a frame.

Films formed of polymers, resins and monomers are useful in Infrared (IR),

Near Infrared (NIR), Raman and XRF. First, thin films can be analyzed as samples

by use spectroscopy of IR, NIR and Raman spectroscopy. Because polymers, resins and monomers can often be made into transmissive films they can be readily

analyzed as samples using IR, NIR and Raman spectroscopy in the transmission

mode. This means that the sample is sufficiently transparent to the energy

employed in the spectrophotometer (e.g. infrared) that the energy can penetrate the

sample and the absorbance of energy caused by the excitation of the molecular

bonds of the sample can be readily detected.

Certain polymers do not cause detectable interference with infrared energy in

certain portions of the spectrum. For example, polytetrafluoroethylene is

nonabsorbing from 4,000 to 1 ,300 wavenumbers (cm^"1). For this reason, certain

polymers can be used as carriers or mediums for samples that absorb infrared

energy in the region where the medium is nonabsorbing. Typically, this has been

done in the past by applying a sample to the surface of an insoluble polymer film

such as polytetrafluoroethylene.

Polymers are also used as a medium or carriers for samples in XRF

spectroscopy because they are not detected by the instrument. Typically, an

inorganic sample (metal for example) is placed on a piece of polymer film to

suspend the sample within the instrument in a neutral medium. A sample suitable

for XRF that is dissolved in a carrier polymer film can also be used as a standard for

calibration of XRF spectrophotometers. Similarly, samples dissolved in a suitable

film can be used as standards in connection with other spectrographic methods. Thin films cast by the methods and with the apparatus of the present

invention can be substituted for pressed film samples and for carriers or mediums

used in all fields of spectroscopy. Because the cast films are easier to prepare than

pressed films, they can save hours of time in the lab. Furthermore, they are

considerably less expensive to use than films sold as carriers or medium films which

are die cut from extruded sheets. Because the films can be made of varying

thicknesses with little effort by varying the speed of rotation, varying the

concentration of the sample and by layering, and because they can be made of

varied layered substances, they offer a wide variety of uses both as samples of

multiple components and as carriers or mediums. Variations of film thickness can,

for example, vary sensitivity of the sample to detection by the instrument by

increasing the pathlength (the thickness of the sample) and thereby altering the

absorbance of the sample as dictated by Beer's Law.

Layering can be used to study the effect of ultraviolet radiation on a protective

film placed over another polymer, which has implications in food storage chemistry.

Layering also offers the potential to study a wide variety of substances in use in the

cosmetics industry, such as by layering hair spray over a protein that simulates

human hair. The apparatus and method of the present invention can provide the

integration of samples with the thin films. This can be of considerable importance

because it not allows the sample to be preserved, but also allows the sample to be

preserved in the same proportion, dispersion and intensity, thereby permitting replication of test results with complete accuracy. In addition, dispersion of the

sample within the transmissive carrier or medium can result in enhanced sensitivity

for some samples.

Highly absorbing polymers cast as films in the manner herein described can

be used to calibrate IR spectrophotometers if they exhibit the traits required for a

calibration "standard" — consistency, uniformity and reproducibility. Of course, the

spectra of these films must exhibit absorption peaks over enough bands through the

spectral region where spectrophotometer is designed to operate to assure that the

instrument is functioning properly over the entire spectral region of interest.

Calibration of the abscissa or X axis is a demonstration that the band positions

exhibited when the calibration standard is scanned in the instrument have remained

relatively constant in wavenumber values. Calibration of the ordinate or Y axis is a

demonstration that the optical performance of the instrument and the detector have

not degraded and also is useful for comparing the photometric quality of

performance of one instrument against another. Such ordinate standards are

referred to herein as ordinate photometric standards.

Polystyrene is now used by the National Institute of Standards (NIST) as the

standard of choice to perform abscissa scale calibration of the abscissa of IR

spectrophotometers under NIST standard 1921. It is understood, however, that

other polymers could be used, particularly if films were formed in a manner that would result in consistent film thicknesses such as casting films by means of the

invention described herein. But, polystyrene film has certain deficiencies as a

calibration standard that arise from (i) the way that it has been formed commercially,

and (ii) the fact that it is a stand alone film with a certain amount of surface

reflectivity.

The ability to vary film thickness and to make very thin films also makes the

peaks in spectra produced by the present invention more useful. For example, IR

spectrophotometer calibration standards are made from extruded sheets of

polystyrene. These films are used to calibrate the abscissa scale or "X" axis of the

spectrophotometer by determining whether bands appear in the proper location as

measured in reciprocal centimeters (cm^'1). Because extruded films are not available

in thicknesses less than 38 microns, calibration films made from them often show

peaks which exceed the ordinate scale or "Y" axis of the spectrophotometer

because they are relatively thick at about 38 microns (0.0015 inches). For example,

the bands at 2925 cm^"1 and 690.6 cm^"1 almost always show as peaks with apices

that extend beyond the range of the "Y" axis as shown for example in Figure 16.

Referring to Figure 16, there is shown a spectrum of a known standard in the

form of a Nicolet NIST traceable polystyrene calibration film. The absorbances

(shown as peaks) sometimes exceed the ordinate scale or "Y" axis of the

spectrophotometer as shown in Figure 16. Further, when absorbance is too high, the detail of the spectrum can be obscured as is evident if Figure 9 in the case of the

bands at 3082 cm^"1, 3060 cm ¹, 3025 cm^"1, 2925.1 cm ¹ (apex missing due to the

peak exceeding the "Y" axis) and 2849.6 cm^"1. In addition, the reflective surface of

the stand alone film can exhibit interference fringes which can cause the bands of

various samples to appear in slightly different locations or which can obscure

absorption peaks. Another deficiency is that the polystyrene films used now to

calibrate the abscissa scale or "X" axis of the spectrophotometer do not have any

utility for calibration of the ordinate scale or "Y" axis of the instrument as an ordinate

photometric standard. Yet another deficiency is that the film thickness of

commercially drawn polystyrene is inconsistent, which is one reason why such films

are not useful for "Y" axis calibrations, the other being that they are not readily

available in multiple thicknesses which are consistent. Many of these deficiencies

can be cured using films cast in accordance with the present invention.

In accordance with the present invention, not only can the thickness of film

made from polystyrene be varied, but they can be made very thin which keeps the

peaks within the "Y" axis. And by altering the thickness of the film, the absorbance

can be varied which allows use of films of different thicknesses to calibrate the

ordinate or Y axis of the spectrophotometer. Furthermore, casting the films on

transmissive substrates eliminates interference fringes that can partially obscure or

distort peaks of interest in a spectrum. Referring to Figure 17, there is shown a spectrum of a polystyrene film cast

on a KBr crystal in accordance with the present invention. The film can be made

thin enough so that the peaks stay within the "Y" axis.

It must be understood that in certain cases, it is impossible to form a

removable film due to the structure of the sample. Application of a fine powder of

KBr, KCI, NaCI or another nonabsorbing optical crystal material to the substrate

before the film is cast should result in the relatively easy removal of most strongly

adhering films. In those cases where a film cannot be removed it can be applied to

a reflective removable substrate for analysis by reflection techniques. In the

alternative, the film can be cast on a highly transmissive nonabsorbing substrate

such as a KBr, KCI, or NaCI optical crystal or over a frame and the crystal urethane

frame containing the film can be removed thereby allowing for analysis of the film

through the aperture within the frame.

In accordance with the present invention, the film-forming liquid or other film

forming material is dropped onto the substrate, which can be flat, concave or

convex, when the same is at rest or is rotating at no more than 500 rpm. The power

source must be capable of rotating the shaft at speeds up to 500 rpm in the first step

of the method. In particular, a viscous fluid evenly distributes itself on a flat (or

concave or convex) substrate. A fluid which is not completely homogeneous, such

as human tissue, distributes relatively evenly on a flat substrate. The use of concave platens is useful for samples with difficult wetting properties. For example,

polyvinyl alcohol with a ratio of solvent (water) that is somewhat higher than when

the polymer is at a saturation state will not adhere to some flat substrates at speeds

in excess of 1 ,500 rpm, but when a concave substrate with a radius of curvature of

about 12 inches is employed the same material will adhere at speeds as high as

5,000 rpm. The concave substrate may be a substrate assembly of the type shown

in Figures 5 and 6 or may be an aluminum weighing dish with a diameter of about

2", which is not only disposable but can also be used as a reflective background for

specular reflection testing on an IR spectrophotometer. Convex substrates are

useful for extremely thick samples which may be difficult to make into thinner films

on a flat substrate, but which can be made thinner than would otherwise be possible

when cast on a convex substrate.

The rotational velocity is constant after a brief period of acceleration. Any

changes that occur to the fluid as a consequence of an applied force happen over

time. Given any set of conditions, there is a fixed time interval from the point in time

where a force is applied until the point where the force no longer has an effect on

the condition of the fluid. Films formed on frames which are capable of reaching a

viscoelastic state may also be rotated in the same manner to regulate the film

thickness, provided that the rotation begins before the film becomes solid. The other force which is employed in the present invention takes the form of a

constant velocity stream of air molecules. The applied direction of this force is

perpendicular to the plane of rotation. As a steady current of air molecules moves

towards the surface of the fluid, the response is a collision of the air molecules with

the rapidly vaporizing solvent molecules which were used to form the solution of film

forming liquid. The solvent molecules are then conveyed away from the quickly

changing fluid.

The first step in the process if a solvent is necessary is to dissolve a film-

forming material such as a resin or polymer into a solvent or to otherwise make the

film-forming material viscous. Virtually any soluble resin or polymer can be used to

form a solution. Monomers in liquid form (which are already viscous and do not

require a solvent) as well as molten resins and polymers and other soluble or liquid

materials can also be used as can anything that can be converted to a viscous state

such as by the application of heat. For example, polyvinyl alcohol may be dissolved

in water, poly (vinylidene fluoride) - difluoroethane may be dissolved in

cyclohexanol, polystyrene may be dissolved in toluene, polycarbonate may be

dissolved in tetrahydrofuran and polyvinylacetate may be dissolved in anhydrous

methanol. The amount of the resin or polymer which is dissolved in the solvent

should be sufficient to provide a solution with a suitable viscosity. Typically, the

amount of the resin or polymer dissolved in a solvent is from 5% to 30% by weight.

The viscosity of the solution or other sample will affect the wetting properties of the sample and also the film thickness. The lower the viscosity and thinner the film that

can be made from it at the same rotation speed.

The resulting film forming liquid or other film forming material is then placed

on the substrate for an initial contact (wetting) phase while the substrate is at rest or

in motion at no more than 500 rpm. After the initial contact (wetting) phase the

substrate is then accelerated to a constant speed (e.g. a constant speed above 500

rpm and typically up to as much as 5,000 rpm) for a fixed time interval (typically from

about 60 to 180 seconds). As a result there is formed a thin film which can be

readily removed from the substrate (if made from a film forming liquid which

undergoes a phase transition), or applied to a removable transmissive substrate

such an optical crystal or, in rare cases, applied to a removable reflective substrate.

In a preferred embodiment of the invention an aluminum weighing dish is

used for a substrate, as it is disposable, has a reflective surface and has a slightly

concave bottom. Such an aluminum weighing dish can be used with means for

mounting it to the rotatable substrate as a surface upon which to cast a film that

either can or cannot be readily removed. If the film can not be removed, the dish

can then be used in a specular reflection accessory to obtain spectra of the sample

cast on the reflective bottom of the dish by the specular reflection technique or

internal reflection technique. The edge of the sample cup can be cut off with a

scissor or the like so that the bottom of the cup can be used as a flat surface. In addition, the substrate assembly can be altered to accept a frame on which

a film has been formed as described above for the purpose of rotating the frame to

control the thickness of the film thereon before the film becomes solid.

The film may be made by physical contact of the film forming liquid with a

solid body ("frame") which contains or surrounds a hole or opening. The film made

by this method is formed by using the forces of cohesion and adhesion. The

attraction of the molecules of the film forming liquid to each other — cohesion —

produces a tendency of the liquid molecules to pull towards the center and away

from the area where there is no liquid. This is the force that tends to make liquids

form droplets when they fall on a surface. On the other hand, the film forming liquid

also tends to want to stick to unlike molecules — adhesion. When energy is applied

to the surface of the film forming liquid such that the surface is stretched beyond its

minimum surface area, cohesion tends to make the liquid want to return to the

minimum surface area, while adhesion tends to make the liquid want to stick to the

neighboring solid frame. This stretching is referred to as surface tension.

A thin film is formed in accordance with the present invention by contacting all

sides of the frame with the film forming liquid in a manner that allows the surface

tension of the film forming liquid to form an unbroken film over the aperture. As the

phase transition from liquid to solid occurs, the cohesive bond between like

molecules becomes stronger and lessens the tendency (in the case of film forming liquids that reach a viscoelastic state such as polymers) of the film to break as it is

stretched. The aperture must be small enough that the surface tension induced by

stretching the film forming liquid over the frame and the aperture will not cause the

film to break until the phase transition has been sufficiently completed to strengthen

the film. Viewed in another way, the cohesive forces become stronger as the phase

transition approaches elasticity and the effect of the opposing forces on the film are

neutralized at some point during the phase transition. The frame can be metal or

any substance which is both rigid and whose integrity will be unaffected by the

composition of the film forming liquid. A standard vial cap having a circular hole in

the top to accommodate a septa can be used as a frame.

In another embodiment of the invention, the resin or polymer solution is

impregnated or mixed with a sample (usually an inorganic material) so that the film

can be used as a carrier of medium for XRF spectroscopy. The sample materials

that may be mixed or suspended in the film include metals, ions and halogens. For

example, a water-soluble polymer such as polyvinyl alcohol can be used as a

medium for XRF spectroscopy. The carrier material is effective because it is

invisible when tested in an XRF spectrophotometer and is capable of forming a thin

film that will withstand radiation. Such carrier films can also be used as standards

for XRF spectroscopy and other spectroscopic methods. Unlike standards that are

suspended in water, these standards will be permanent as there will be no water to

evaporate that will change the concentration of the standard. By way of example, PVA dissolved in water is placed on the substrate

rotating at a speed sufficient to place the PVA in a viscoelastic state. Before the

PVA becomes completely solid a sample will adhere to it. A non-water soluble

material in the form of a fine powder (e.g. metal ore) intended to be used as a

sample is evenly dispersed on another substrate and the PVA containing substrate

is then pressed on the second substrate where the sample sticks to the PVA.

Thereafter, the PVA film in which the sample has become impregnated, is then

removed from the first substrate for XRF analysis of the sample. In the alternative,

the sample is suspended or dissolved in the PVA before the PVA film is cast.

In yet another embodiment of the invention, the resin or polymer solution is

mixed with a material capable of analysis by IR, FTIR, Raman, NIR or other

spectrographic methods. For example, this method is useful for making a version of

a calibration standard for IR spectroscopy wherein a highly absorbing substance

such as Boron is mixed with the polymer, as discussed below as previously

described.

A useful standard for calibration of an IR spectrophotometer must be:

uniform, reproducible and permanent. Polymers, such as, but not limited to,

polystyrene cast using the present invention on KBr or other transmissive or

reflective substrates meet this standard. It is understood that other film forming

liquids that can be cast by this method into a permanent film could also be used as standards if they exhibited absorption peaks at appropriate locations on a abscissa

scale. The method of casting a film on a transmissive substrate for use as a

standard is, first, to apply a known concentration of film forming liquid such as a

dissolved polymer on a transmissive substrate. In the case of crystal such as KBr

the wetting characteristics are such that, in a preferred embodiment of the invention,

the crystal should be completely covered with the liquid polymer before rotation is

commenced. The crystal or transmissive substrate is placed on a substrate as

previously described. Thereafter, the substrate is rotated so that the transmissive

substrate resting on top of it also rotates. The film is formed when the film forming

liquid reaches a viscoelastic state.

The substrate must be rotated at the same reproducible speed for the same

reproducible cycle time each time a film that is intended to be a standard is made

and each film must be made from a sample of film forming liquid having the same

concentration of sample.

In a preferred embodiment of the invention for use as a standard the film

forming liquid is a polymer such as polystyrene and the polymer is dissolved in a

solvent in a known and reproducible concentration. The film forming liquid such as a

dissolved polymer is then allowed to dry, preferably in a desiccator jar, and then, in a

preferred embodiment of the invention, the second side is coated with the same concentration of film forming liquid in the same manner and the film is then allowed

to dry, again preferably in a desiccator jar.

In a preferred embodiment of the invention the film forming liquid is

polystyrene and the substrate is a KBr crystal. Casting the film on both sides of the

substrate will have the effect of sealing the substrate if the substrate might otherwise

change or be unstable over time. KBr, for example, is hygroscopic, and as it

absorbs moisture the amount of infrared energy that it will transmit decreases. By

sealing both sides of a KBr crystal with polystyrene or some other film forming liquid

that is not affected by moisture, the substrate becomes stable and is therefore useful

in a standard. In another preferred embodiment of the invention the film is cast on a

reflective substrate such as for use in specular or other reflectance modes of

analysis in connection with making standards, and in yet another embodiment of the

invention the film used for a standard is removed from the platen or other substrate

on which it is cast. Such reflective substrates may be specially coated glass as

shown in U.S. Patent No. 5,160,826.

Another means of using films cast by the method herein described as

standards is to add to the film making material a known amount of a foreign

substance, such as Boron, which can create a distinct peak at a known band and of

a reproducible intensity which will be useful to calibrate both the ordinate scale and

the abscissa scale of an infrared spectrophotometer. In another preferred embodiment of the invention, the crystal substrate (such as KBr) on which a film

intended to form the standard is cast is doped with a foreign substance such as

Boron. The film then creates the absorption peaks which are used as the abscissa

standard and the foreign substance creates the absorption peak or peaks that are

used as the ordinate photometric standard. Boron, for example, shows only a single

distinct peak within the spectral range of an infrared spectrophotometer at 1958.6

cm^"1. Conveniently, the Boron absorption peak appears at a spectral band where

polystyrene is non-absorbing, which allows the use of polystyrene as a coating and

an abscissa standard while the Boron is used as the ordinate photometric standard.

It is understood that other combinations of coating materials and dopant foreign

substances could be used in the same manner, as could doped films and layered

films of different substances. The advantage of doping the crystal or the film itself is

that the intensity of the absorption peak can be more precisely controlled and

consistently applied than can multiple peaks in a film such as a polymer.

Yet another means of using films cast by the method herein described is to

cast two or more films of different thicknesses, each of which, under Beers Law, will

have different absorbance because the pathlength (thickness) of each film is

different. By checking the consistency of the difference in the intensity or height of

the absorption peaks of these two or more films, the ordinate scale or "Y" axis of an

infrared spectrophotometer can be calibrated. At present ordinate photometric

calibration is quite crude and is typically done not with two standards, but with one. This is inadequate for a number of reasons. First, the standards used are not

sufficiently consistent in absorbance to produce consistent results. Further, there is

no basis to compare the standard to a fixed point, as the baselines in FTIR

spectrophotometers are not established by the use of a standard.

Using as an ordinate photometric calibration standard a single film cast of a

polymer or other substance with a consistent film thicknesses cast by means of the

invention will greatly enhance the accuracy of ordinate photometric calibration.

Using matched pairs of two films from the same substance of differing thicknesses

will eliminate much of the guess work and uncertainty associated with ordinate

calibrations. There is the resultant advantage of comparing two standards to each

other and these two standards can be produced so that they will not degrade over

time in the manner discussed above. Degradation in instrument performance due to

optical or detector deterioration can thus be detected. When only one standard is

used, it is never certain what is occurring because the instrument itself provides the

baseline for the test. With two standards the standards provide their own baseline.

There is a mathematical relationship between film thickness, viscosity and

substrate rotation speed that is unique to that mixture, making it possible to predict

film thickness based upon substrate speed. The faster the substrate is rotated, the

thinner the film up to the point that the film reaches a plastic state. In a preferred

embodiment of the invention, the speed of rotation is precisely controlled so as to facilitate not only variations in film thickness but also wetting of the substrate on

which the film is cast. Since the apparatus accommodates the wetting properties of

a variety of polymers and other film forming liquids, its utility is greatly enhanced. It

has been found that certain polymers will not adhere well (wet) the substrate at

some speeds. Therefore, to assure the utility of a coater in a variety of applications

which may involve many different substrates, variable speed control is a desirable

feature. In another preferred embodiment of the invention, control of the speed at

which the rotation speed ramps up from a static state to the operating speed is also

employed. Control of ramp speed provides optimal control of film thickness and also

allows adaptation to the wetting properties of more samples. In another preferred

embodiment of the invention, the controller used to regulate the speed is

microprocessor based so that the speed is reproducible from run to run thereby

facilitating reproducibility of film thicknesses from run to run. In yet another

preferred embodiment of the invention, the cycle time during which the shaft rotates

is precisely controlled to optimize the regulation and consistency of film thickness

from film sample to film sample. As noted below, use of concave and convex

substrates is also a wetting aid.

Claims

IN THE CLAIMS:

1. A method for forming a removable film on a substrate comprising:

a) applying a film-forming liquid to a substrate at rest or rotating at

speeds up to 500 rpm; and

b) rotating the film forming liquid on the substrate at a speed and

for a time sufficient for said thin film to form on said substrate.

2. The method of claim 1 comprising applying the film forming liquid to

the substrate at speeds up to 200 rpm.

3. The method of claim 1 comprising rotating the film forming liquid on the

substrate at speeds up to 5,000 rpm.

4. The method of claim 1 wherein the substrate has a concave, convex or

planar shaped surface.

5. The method of claim 1 wherein the substrate is in the form of a frame

having an opening on which is placed the film forming liquid.

6. The method of claim 1 further comprising dissolving the film forming

material in a solvent.

7. The method of claim 1 wherein the liquid forming liquid is selected from

the group consisting of polymers, resins, monomers, glues, adhesives, paints,

animal tissues, cell samples alone or in combination with a suitable solvent.

8. The method of claim 1 wherein the substrate is a transmissive

substrate.

9. A method of forming a sample for analysis comprising

a) applying a film forming liquid to a substrate at rest or rotating at

speeds up to 500 rpm;

b) rotating the film forming liquid on the substrate at a speed and

for a time sufficient for said thin film to form on said substrate.

c) placing the thin film in an analyzer to determine the composition

thereof or as a standard for determining the composition of other materials.

10. The method of claim 9 comprising applying the film forming liquid while

the film is rotating at a speed of up to 200 rpm.

11. The method of claim 10 wherein the analyzer is an infrared

spectrophotometer.

12. The method of claim 8 comprising providing a substance to be

analyzed to the film forming material, said film forming material being not detected

when placed in the analyzer.

13. Apparatus for forming a removable film on a substrate comprising:

a) a substrate assembly comprising a rotatable surface for

receiving a film forming liquid;

b) means for controlling the rate of rotation of the substrate to no

more than 500 rpm while the film forming liquid is applied to the substrate;

c) means for controlling the rate of rotation of the substrate at a

speed and for a time sufficient to form said thin film.

14. The apparatus of claim 13 wherein the rotatable surface is planar,

concave or convex.

15. The apparatus of claim 13 further comprising means for preventing

splatter of the film forming liquid.

16. The apparatus of claim 13 comprising a rotatable housing having an

opening therein for securing a substrate assembly, said substrate assembly

comprising a base having a surface thereon for receiving the film forming liquid and

a lower section operatively secured and rotatable by the rotatable housing.

17. The apparatus of claim 16 wherein the surface is planar, concave or

convex.