United States Patent (19)
`Cheney, II et al.
`
`(54)
`
`(75)
`
`METHOD OF FABRICATING THN FILM
`SENSORS
`Inventors:
`
`(73)
`(21)
`22)
`51
`52)
`58
`
`(56)
`
`Paul S. Cheney, II, Beverly Hills;
`William P. Van Antwerp, Los
`Angeles, both of Calif.
`Assignee:
`MiniMed Inc., Sylmar, Calif.
`Appl. No.:
`212,961
`Mar. 15, 1994
`Fied:
`Int. Cl......................... B29C 65/00; B32B 31/00
`U.S. C. .................................... 156/268; 156/286;
`156/290
`Field of Search ..................... 156/286, 290, 306.6,
`156/297, 268, 60, 631, 632, 638; 428/901;
`128/637; 204/403; 29/846
`References Cited
`U.S. PATENT DOCUMENTS
`Bean, Jr. ............................. 156/297
`2,899,658
`8/1959
`4,104,099
`Scherrer ........................... 156/306.6
`8/1978
`4,897,173
`1/1990
`Nankai et al. ....................... 204/403
`5,108,819
`4/1992
`Heller et al. .
`5,196,088
`Soda .................................... 156/631
`3/1993
`Primary Examiner-Michael W. Ball
`Assistant Examiner-Richard Crispino
`
`USOO539 1250A
`Patent Number:
`11
`45) Date of Patent:
`
`5,391,250
`Feb. 21, 1995
`
`Attorney, Agent, or Firm-Kelly Bauersfeld & Lowry
`57
`ABSTRACT
`An improved method is provided for making thin film
`electrochemical sensors, such as subcutaneous glucose
`sensors used to monitor blood glucose levels in a dia
`betic patient. The fabrication method comprises placing
`a thin film base layer of insulative material onto a rigid
`flat substrate, with a curable adhesive interposed be
`tween the perimeter of the base layer and substrate to
`define a shallow cavity underlying a central portion of
`the base layer. The subassembly is subjected to heat and
`pressure to cure the adhesive, resulting in air expulsion
`from the cavity such that the central portion of the base
`layer is drawn into intimate contact with the substrate.
`Appropriate conductor elements for one or more sen
`sors are formed on the base layer as by conventional
`contact mask photolithography, and a thin film cover
`layer of insulative material is applied thereover with
`apertures in the cover layer exposing distal end sensor
`electrodes and proximal end contact pads. The insula
`tive cover and base layers are then cut along a line
`surrounding each finished sensor which is lifted and
`separated easily from the substrate.
`
`13 Claims, 4 Drawing Sheets
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`U.S. Patent
`U.S. Patent
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`Feb. 21, 1995
`Feb. 21, 1995
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`U.S. Patent
`U.S. Patent
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`Feb. 21, 1995
`Feb. 21, 1995
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`U.S. Patent
`U.S. Patent
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`Feb. 21, 1995
`Feb. 21, 1995
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`Feb. 21, 1995
`Feb. 21, 1995
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`U.S. Patent
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`U.S. Patent
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`5,391,250
`5,391,250
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`1
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`METHOD OF FABRICATING THN FILM
`SENSORS
`
`BACKGROUND OF THE INVENTION
`This invention relates generally to fabrication meth
`ods for producing thin film electrochemical sensors of
`the type used, for example, in subcutaneous or transcu
`taneous monitoring of blood glucose levels in a diabetic
`patient. More specifically, this invention relates to an
`improved fabrication method wherein thin film sensors
`are formed on a rigid flat substrate by contact mask
`photolithography or the like, without requiring attach
`ment of the sensors to the substrate.
`15
`Thin film electrochemical sensors are generally
`known in the art for use in a variety of specialized sen
`sor applications. Such thin film sensors generally com
`prise one or more thin conductors applied by photoli
`thography mask and etch techniques between thin lay
`ers of a nonconductive film material, such as polyimide
`20
`film. The conductors are shaped to define distal end
`sensor tips having an appropriate electrode material
`thereon, in combination with proximal end contact pads
`adapted for conductive connection with appropriate
`electronic monitoring equipment. In recent years, thin
`25
`film sensors of this general type have been proposed for
`use as a transcutaneous sensor in medical applications.
`As one example, thin film sensors have been designed
`for use in monitoring blood glucose levels in a diabetic
`patient, with the distal end sensor electrodes positioned
`subcutaneously in direct contact with patient blood.
`In accordance with known photolithographic fabri
`cation techniques, thin film sensors have been produced
`by sequential buildup of the sensor layers on a flat and
`rigid substrate, such as a glass plate. A base layer of
`35
`insulative material is formed on the substrate, typically
`by applying the base layer material onto the substrate in
`liquid form and thereafter spinning the substrate to yield
`the base layer of thin, substantially uniform thickness.
`These steps are repeated to build up the base layer of
`40
`sufficient thickness, followed by a sequence of photo
`lithographic mask and etch steps to form the conduc
`tors. A cover layer of insulative material is then applied,
`and the resultant sensors are stripped from the substrate.
`However, this stripping step must be performed care
`45
`fully in order to separate the finished sensors from the
`substrate without damage. Unfortunately, the initial
`step of spin forming the base layer on the substrate
`causes the base layer to be firmly adhered to the sub
`strate, such that separation of the finished sensors from
`50
`the substrate is both costly and time consuming, thereby
`undesirably increasing the manufactured cost of thin
`film sensors.
`There exists, therefore, a need for improvements in
`methods for producing thin film electrochemical sen
`55
`sors, particularly with respect to reducing sensor pro
`duction time and cost by facilitating removal of finished
`sensors from a flat substrate. The present invention
`fulfills these needs and provides further related advan
`tages.
`60
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`30
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`ment of the sensors to the substrate. In the invention, a
`base layer of insulative material is vacuum drawn into
`intimate contact with the substrate to provide a flat and
`stable surface upon which remaining components of the
`thin film sensors can be applied by photolithographic
`techniques. The finished sensors can then be removed
`quickly and easily from the substrate.
`In accordance with the preferred form of the inven
`tion, a thin film base layer of insulative material such as
`polyimide sheet is placed onto the flat and rigid sub
`strate, with a thin die-cut sheet or frame of a curable
`adhesive such as an epoxy resin disposed between the
`perimeter of the overlying base layer and the underly
`ing substrate, thereby defining a shallow cavity under
`lying a central portion of the base layer. This subassem
`bly is subjected to appropriate heat and pressure to cure
`the adhesive. Air and other gaseous constituents within
`the cavity are driven therefrom during this cure step, to
`create a vacuum within the cavity. As a result, the cen
`tral portion of the base layer is vacuum drawn into
`intimate seated contact upon the flat substrate. The
`cured adhesive provides a barrier to post-cure return
`flow of gases into the cavity, such that the central por
`tion of the base layer is retained on the substrate in a
`uniformly flat orientation.
`Subsequently, the remaining components of one or
`more thin film sensors are applied to the base layer,
`preferably by photolithographic mask and etch tech
`niques. For example, a thin conductive film is applied to
`the base layer by electrodeposition, sputtering, etc. The
`conductive film is appropriately masked and etched to
`define elongated conductor traces for one or more sen
`sors. The conductor traces are in turn covered by a thin
`film cover layer of insulative material, such as a
`photoimagable polyimide suitable for masking and ex
`posure to form apertures therein to expose distal end
`sensor tips and proximal end contact pads. Electrode
`chemistries are applied to the sensor tips, such as glu
`cose oxidase for use in monitoring glucose blood levels.
`The thus-formed sensors are removed from the sub
`strate by cutting the cover and base layers along a line
`surrounding each finished sensor, whereupon each sen
`sor can then be lifted and separated easily from the
`underlying substrate.
`Other features and advantages of the present inven
`tion will become more apparent from the following
`detailed description, taken in conjunction with the ac
`companying drawings which illustrate, by way of ex
`ample, the principles of the invention.
`BRIEF DESCRIPTION OF THE DRAWINGS
`The accompanying drawings illustrate the invention.
`In such drawings:
`FIG. 1 is an exploded perspective view illustrating a
`plurality of thin film electrochemical sensors formed on
`a rigid flat substrate;
`FIG. 2 is an enlarged cross sectional view taken gen
`erally on the line 2-2, of FIG. 1;
`FIG.3 is a perspective view illustrating application of
`a die-cut strip of curable adhesive to the perimeter of
`the substrate;
`FIG. 4 is an exploded perspective view showing
`placement of a thin film base layer onto the substrate;
`FIG. 5 is a somewhat schematic representation of a
`cure step for curing the adhesive interposed between
`the base layer and substrate;
`
`5
`
`10
`
`SUMMARY OF THE INVENTION
`In accordance with the invention, an improved fabri
`cation method is provided for producing thin film elec
`trochemical sensors. The method comprises the forma
`65
`tion of one or more sensors on a flat and rigid substrate,
`such as a glass plate, by means of contact mask lithogra
`phy or the like, but without requiring physical attach
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`FIG. 6 is an exploded perspective view, with portions
`adhesive strip 30 comprises an epoxy resin which may
`broken away, illustrating photolithographic mask and
`be impregnated with fiberglass, such as an epoxy resin
`etch steps for forming conductive sensor traces on the
`available from 3M Aerospace Division of Springfield,
`base layer;
`Mo., under the name AF-163-205T. Alternative adhe
`FIG. 7 is a perspective view illustrating application of 5
`sive materials may include ultraviolet curable adhe
`a thin film cover layer over the conductive sensor
`sives, etc. Moreover, if desired for improved adhesion
`between the base layer 16 and the adhesive strip 30, a
`traces;
`FIG. 8 is a schematic representation of a cure step for
`perimeter region of the base layer may be surface
`curing the cover layer;
`etched.
`FIG. 9 is an exploded perspective view illustrating
`The subassembly including the base layer 16 and
`10
`photoimaging steps for exposing portions of the con
`substrate 12 are subjected to a curing step as viewed
`schematically in FIG. 5. As shown, these components
`ductive sensor traces;
`FIG. 10 is an enlarged fragmented perspective view
`are placed in a press 34 and subjected to appropriate
`illustrating sensor contact pad and distal end electrodes
`heat and pressure to cure the adhesive material 30. Dur
`exposed through the insulative cover layer; and
`ing this cure step, air and other gaseous constituents are
`15
`FIG. 11 is a perspective view illustrating removal of
`expelled from the cavity 32, resulting in an effective
`each finished sensor from the substrate.
`vacuum therein which draws the central portion of the
`base layer 16 downwardly into intimate and uninter
`DETALED DESCRIPTION OF THE
`rupted surface-to-surface engagement with the flat
`PREFERRED EMBODIMENT
`upper surface 28 of the substrate 12. When the adhesive
`As shown in the exemplary drawings, an improved
`material 30 reaches a substantially cured state, return
`fabrication method is provided for producing thin film
`ingress of gaseous constituents to the cavity 32 is pre
`electrochemical sensors referred to generally by the
`vented, whereby the central portion of the base layer 16
`reference numeral 10 in FIG. 1. One or more sensors 10
`is retained in intimate engagement with the substrate.
`are formed on a rigid flat substrate 12, such as a glass
`FIG. 6 illustrates photolithographic steps for forming
`25
`plate. The sensors 10 are formed in a manner which is
`conductive sensor traces on the insulative base layer 16.
`compatible with photolithographic mask and etch tech
`More specifically, the base layer 16 is initially coated
`niques, but wherein the sensors are not physically ad
`with a thin film conductive layer 38 by electrode depo
`hered or attached directly to the substrate 12. Accord
`sition, surface sputtering, or other suitable process step.
`ingly, finished sensors 10 can be removed quickly and
`In the preferred form, this conductive layer 38 may be
`easily from the substrate by simple lift-off separation.
`provided as a plurality of thin film conductive layers,
`The electrochemical sensors 10 shown in the accom
`such as an initial chrome-based layer suitable for chemi
`panying drawings are particularly designed for use as
`cal adhesion to the polyimide base layer 16, followed by
`subcutaneous or transcutaneous glucose sensors for
`subsequent formation of thin film gold-based and
`monitoring blood glucose levels in a diabetic patient.
`chrome-based layers in sequence.
`Each sensor comprises a plurality of elongated thin film
`The conductive layer 38 is then covered, in accor
`conductors 14 formed between an underlying insulative
`dance with conventional photolithographic techniques,
`thin film base layer 16 (FIG. 2) and an overlying insula
`with a selected photoresist coating 40, and a contact
`tive thin film cover layer 18. Apertures 20 and 22 (FIG.
`mask 42 is applied over the photoresist coating 40 for
`10) are formed in the cover layer 18 to expose distal end
`suitable photoimaging, The contact mask includes one
`electrodes 24 and proximal end contact pads 26. In a
`or more conductor trace patterns 43, such as the plural
`glucose monitoring application, the thin film sensor 10
`ity of traces shown in FIG. 6 in closely nested relation,
`is placed transcutaneously so that the distal end elec
`for appropriate exposure of the photoresist coating 40,
`trodes 24 are in direct contact with patient blood or
`followed by an etch step resulting in a plurality of con
`extracellular fluid, and wherein the contact pads 26 are
`ductive sensor traces 44 remaining on the base layer 16,
`45
`disposed externally for convenient connection to a
`as shown in FIG. 7. In the illustrative sensor construc
`monitoring device (not shown).
`tion designed for use as a subcutaneous glucose sensor,
`The substrate 12 comprises a rigid and flat structure
`each sensor trace is shown to include three parallel
`suitable for use in photolithographic mask and etch
`conductors 14 corresponding with three separate elec
`processes. In this regard, the substrate 12 defines an
`trodes 24 as will be described.
`50
`upper surface 28 (FIG. 3) having a high degree of uni
`The conductive sensor traces 44 are covered by the
`form flatness. A polished glass plate may be used defin
`thin film cover layer 18 of insulative material. This step
`ing the smooth upper surface 28. Alternative substrate
`is shown in FIG. 7 by applying liquid-based insulative
`materials include, for example, stainless steel, alumi
`material 45 over the sensor traces 44, and then spinning
`num, and plastic materials such as Delrin, etc.
`the substrate 12 as indicated by arrow 46 to distribute
`55
`A thin layer film 30 of a curable adhesive, provided as
`the liquid material 45 as a thin film overlying the sensor
`shown in the form of a die-cut strip or frame, is applied
`traces 44 and extending beyond the marginal edges of
`in a closed loop pattern to the perimeter of the substrate
`the sensor traces in sealed contact with the base layer.
`12, as viewed in FIG.3. The base layer 16 is then placed
`This liquid material is then subjected to a suitable radia
`on the substrate 12, with a perimeter of the base layer 16
`tion and/or heat cure step as shown in FIG.8. Various
`in intimate seated contact upon the adhesive strip 30.
`cover layer materials may be used, with a preferred
`The thus-assembled components define a shallow cavity
`material comprising a photoimagable polyimide avail
`32 between a central portion of the base layer 16 and the
`able from OCG, Inc. of West Paterson, N.J., under the
`underlying substrate 12, with the adhesive strip 30 cir
`product number 7020.
`cumscribing the peripheral edge of the cavity 32. In a
`FIG. 9 shows a contact mask 47 for placement over
`65
`preferred form, the base layer 16 comprises a thin film
`the cured cover layer 18. The contact mask 47 has aper
`sheet of insulative material, such as polyimide having a
`tures 48 and 50 formed therein through which the
`photoimagable layer 18 can be exposed to an appropri
`film thickness on the order of about 0.003 inch. The
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`ate light source. The thus-exposed cover layer 18 is
`removing the sensor from the substrate by cutting
`subsequently processed to remove portions of the cover
`through the cover and base layers along a line
`layer 18, resulting in the apertures 20 and 22 (FIG. 10)
`circumscribing and spaced outwardly from the at
`which respectively expose the distal end sensortips and
`least one sensor element.
`proximal end contact pads 26 of each sensor trace.
`2. The method of claim 1 wherein said step of form
`Appropriate electrode chemistries defining the distal
`ing the at least one conductive sensor element com
`end electrodes 24 can be applied to the sensortips subse
`prises forming a plurality of conductive sensor elements
`quent to exposure of the sensor tips through the aper
`on the base layer, said cover layer forming step com
`tures 20, or alternately before application of the cover
`prising the step of forming the cover layer to cover said
`layer 18. In this illustrative sensor embodiment for use
`plurality of sensor elements and to extend beyond the
`as a glucose sensor, one of the sensor tips is coated with
`marginal edges of each sensor element in sealed contact
`an electrode chemistry including glucose oxidase to
`with the base layer, and further wherein said removing
`defining a working electrode. The other two electrodes
`step comprises the step of cutting through the cover and
`24 may be coated with other suitable chemistries, or left
`base layers along a line circumscribing each sensor
`uncoated, to define a reference electrode and a counter
`element.
`electrode for the electrochemical sensor.
`3. The method of claim 1 wherein said sensor element
`The finished sensors 10 are quickly and easily re
`forming step comprises photolithographic mask and
`moved from the substrate 12 by cutting along a line
`etch steps.
`surrounding 11 each sensor on the substrate. FIG. 11
`4. The method of claim 1 further including the step of
`shows the cutting step to include a laser cutting device
`applying a selected electrode chemistry to said at least
`52 used to cut through the base and cover layers 16, 18
`one conductive sensor element.
`along a line surrounding or circumscribing each sensor,
`5. The method of claim 1 wherein the at least one
`in at least slight outward spaced relation from the con
`conductive sensor element is elongated in shape, and
`further including the step of forming apertures in the
`ductive elements so that the sufficient interconnected
`base and cover layer material remains to seal the side
`cover layer to expose selected portions of the sensor
`25
`edges of the finished sensor. Since the base layer 16 is
`element generally at opposite ends thereof.
`not physically attached or adhered directly to the un
`6. The method of claim 1 wherein said cover layer
`derlying substrate 12, the sensors 10 can be lifted
`forming step comprises the steps of applying insulative
`quickly and easily from the substrate, without signifi
`material in liquid form onto the base layer subsequent to
`cant further processing steps or potential damage due to
`said sensor trace forming step, spinning the substrate
`30
`stresses incurred by physically pulling or peeling at
`with the base layer thereon to form the liquid insulative
`tached sensors from the substrate. The substrate 12 can
`material into a thin film, and curing the liquid insulative
`thereafter be cleaned and reused, or otherwise dis
`material to form the cover layer.
`carded.
`7. The method of claim 1 wherein said cover layer
`The present invention thus provides an improved
`forming step comprises forming the cover layer from a
`35
`photoimagable insulative material, and forming aper
`sensor fabrication method wherein thin film flexible
`electrochemical sensors are formed on a flat substrate,
`tures in the cover layer to expose selected portions of
`without requiring direct physical adherence to or at
`the at least one sensor element by photomasking and
`removing portions of the cover layer.
`tachment of the sensors to the substrate.
`A variety of modifications and improvements to the
`8. The method of claim 1 wherein the base layer is
`fabrication method of the present invention will be
`polyimide sheet material.
`apparent to those skilled in the art. Accordingly, no
`9. The method of claim 1 wherein the central portion
`limitation on the invention is intended by way of the
`of the base layer is unattached to the substrate.
`foregoing description and accompanying drawings,
`10. A method of making a thin film conductive sen
`except as set forth in the appended claims.
`sor, said method comprising the steps of:
`What is claimed is:
`assembling a base layer of insulative sheet material
`1. A method of making a thin film electrochemical
`onto a flat surface of a rigid substrate;
`sensor, said method comprising the steps of:
`retaining at least a central portion of the base layer in
`applying a curable adhesive in a closed loop pattern
`intimate surface contact with the flat surface of the
`to a flat surface of a rigid substrate;
`substrate, without directly attaching the central
`assembling a base layer of insulative sheet material
`portion to the substrate;
`with the substrate, with the adhesive interposed
`forming at least one conductive sensor element on the
`between the substrate and the base layer generally
`central portion of the base layer;
`at the perimeter of the base layer, so that a shallow
`forming a cover layer of insulative material to cover
`cavity is formed between the substrate and a cen
`the at least one sensor element and to extend be
`55
`tral portion of the base layer;
`yond the marginal edges thereof in sealed contact
`applying heat and pressure to the assembled base
`with the base layer;
`layer and substrate to expel air from the cavity and
`said sensor element forming step and said cover layer
`to position the central portion of the base layer in
`forming step being performed while the central
`intimate surface contact with the substrate, and
`portion of the base layer is retained in intimate
`60
`further to cure the adhesive to seal the cavity and
`surface contact with the flat surface of the sub
`thereby prevent air return flow into the cavity;
`Strate; and
`forming at least one conductive sensor element on the
`removing the sensor from the substrate by cutting
`base layer;
`through the cover and base layers along a line
`forming a cover layer of insulative material to cover
`circumscribing and spaced outwardly from the at
`65
`the at least one conductive sensor element and to
`least one sensor element;
`extend beyond the marginal edges thereof in sealed
`said base layer assembling step comprising applying a
`contact with the base layer; and
`curable adhesive in a closed loop pattern on the flat
`
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`surface of the substrate, so that the adhesive is
`the base layer into intimate surface contact with the
`interposed between the substrate and the base layer
`substrate.
`generally at the perimeter of the base layer, and
`12. The method of claim 10 wherein the at least one
`further wherein said retaining step comprises ap
`conductive sensor element is elongated in shape, and
`plying heat and pressure to the assembled base 5
`further including the step of forming apertures in the
`layer and substrate to press the central portion of
`cover layer to expose selected portions of the sensor
`the base layer into intimate surface contact with the
`element generally at opposite ends thereof.
`substrate and to cure the adhesive.
`13. The method of claim 10 wherein the base layer is
`11. The method of claim 10 wherein said retaining
`polyimide sheet material.
`step comprises vacuum drawing the central portion of 10
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`k
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