United States Patent (19)
`Ghezzo
`
`54 METHOD OF ETCHING INDUM TEN OXDE
`75 Inventor:
`Mario Ghezzo, Ballston Lake, N.Y.
`73) Assignee: General Electric Company,
`Schenectady, N.Y.
`May 2, 1975
`(22 Filed:
`(21
`Appl. No.: 573,843
`
`52 U.S. Cl...................................... 156/8; 96/36.2;
`156/17; 252/79.2
`5ll Int. C.’............................................ C23F 1/02
`58 Field of Search ................ 96/36.2, 38.3; 156/8,
`156717; 29/580; 250/21 J; 204/129. 1, 192;
`252/79. , 79.2
`
`56)
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`2,371,529
`3, 21,852
`3,348,987
`
`References Cited
`UNITED STATES PATENTS
`3/1945 Loose ................................ 252/79.2
`2/1964 Boyd et al......................... 427,191 X
`Of 1967 Stark et al. ........................... 156117
`OTHER PUBLICATIONS
`Journal of the Electrochemical Society, A Chemical
`
`3,979,240
`11,
`(45) Sept. 7, 1976
`
`Polish for Snx Ploxi-x Te by J. Edward Coker, vol.
`116, No. 7, 7/1969, p. 1021.
`
`Primary Examiner-William A. Powell
`Attorney, Agent, or Firm-Julius J. Zaskalicky; Joseph
`T. Cohen; Jerome C. Squillaro
`
`57
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`ABSTRACT
`
`A method of etching a desired pattern in a layer of in
`dium tin oxide formed on a substrate of a semiconduc
`tor or an insulating material in which the desired pat
`tern is formed of a hardened photoresist on the layer
`of indium tin oxide and thereafter the substrate and
`the indium tin oxide layer with the hardened resist
`thereon is immersed in a solution of concentrated hy
`drobromic acid for a time sufficient to etch away the
`indium tin oxide unmasked by the hardened
`photoresist.
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`13 Claims, 3 Drawing Figures
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`M2
`MV////W 77MWOX1/AAF
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`SO/AS7747A
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`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1030, IPR2021-01160 Page 1 of 5
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`U.S. Patent Sept. 7, 1976
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`3,979,240
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`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1030, IPR2021-01160 Page 2 of 5
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`METHOD OF ETCHING INDIUM TIN OXEDE
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`This invention relates to the etching of patterns of
`conductive electrodes in a deposited layer of indium tin
`oxide in the course of providing transparent electrodes
`for semiconductor devices.
`Indium tin oxide, particularly in which are provided
`large concentrations of indium oxide in relation to the
`tin oxide, for example, 90% indium oxide to about 10%
`tin oxide, is not only transparent but also has low resis
`tivity. Such a combination of properties makes indium
`tin oxide highly suitable for use as electrode material in
`photosensitive semiconductor devices and arrays of
`such devices, such as disclosed in U.S. Pat. No.
`3,805,062 and also disclosed in patent application Ser.
`No. 573,842 filed May 2, 1975, both of which are as
`signed to the assignee of this application. The use of
`indium tin oxide for the electrodes of such devices
`enables improvements in both sensitivity and spectral
`response to be obtained. Successful incorporation of
`conductive electrodes of such a material in a semicon
`ductor device requires a suitable method for patterning
`or etching the layers of indium tin oxide. In addition, it
`is important to be able to closely space the transparent
`electrodes of indium tin oxide in order to provide high
`packing density and consequently high resolution in the
`image sensing arrays of such devices.
`In the etching of indium tin oxide several conditions
`must be met to obtain satisfactory etching action. The
`etch mask utilized must not lift from the surface of the
`indium tin oxide layer during the etching thereof. The
`etch used must be able to etch through relatively thick
`layers of indium oxide without so impairing the adhe
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`sion bond of the mask to the indium tin oxide layer as
`to cause a lifting thereof and impairing the resultant
`pattern formed in the indium tin oxide layer. The etch
`utilized must also be relatively inert with respect to the
`substrate on which the indium tin oxide layer is formed.
`It is also desirable in the formation of conductive elec
`trodes such as row or column stripes or lines that the
`edges of adjacent lines be relatively straight so that the
`conductive lines or stripes can be closely spaced to
`provide high packing density of devices in the array.
`Etches suggested in the prior art such as in an article by
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`D. B. Fraser and H. D. Cook, entitled “Highly Conduc
`tive Transparent Films of Sputtered Indium Tin Oxide'
`in Journal of the Electrochemical Society, Solid-State
`Science and Technology, October 1972, pg. 1374, have
`proved unsatisfactory in respect to satisfying these
`criteria.
`Accordingly an object of the present invention is to
`provide an improved method of etching layers of in
`dium tin oxide.
`Another object of the present invention is to provide
`a method of etching patterns of electrodes in a rela
`tively thick layer of indium tin oxide formed on a vari
`ous semiconductive and insulating substrates to close
`tolerances.
`In carrying out the invention in one illustrative em
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`bodiment thereofthere is provided a substrate compris
`ing an insulating material such as silicon dioxide on
`which a relatively thick layer of indium tin oxide has
`been applied. A coating of a photoresist material is
`applied over the exposed surface of the layer of indium
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`tin oxide and a hardened portion is formed therein of
`the pattern desired. The substrate with the hardened
`pattern of photoresist on the indium tin oxide layer is
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`immersed in a solution of concentrated hydrobronic
`acid for a time sufficient to etch away the indium tin
`oxide unmasked by the hardened photoresist. Thereaf
`ter the hardened photoresist is removed from the layer
`of indium tin oxide.
`The novel features which are believed to be charac
`teristic of the present invention are set forth with par
`ticularity in the appended claims. The invention itself
`both as to its organization and method of operation
`together with further objects and advantages thereof
`may best be understood by reference to the following
`description taken in connection with the accompanying
`drawings wherein
`FIG. 1 is a perspective view of a body which includes
`a suitable substrate on which a layer of indium tin oxide
`has been formed.
`FIG. 2 is a plan view of the body of FIG. 1 after
`processing in accordance with the present invention to
`form a pair of closely spaced conductive stripes of
`indium tin oxide thereon.
`FIG. 3 is a sectional view of FIG. 2 taken along sec
`tion lines 3-3.
`Referring now to FIG. 1 there is shown a composite
`body 10 including a substrate 11 on which has been
`deposited a layer 12 of indium tin oxide. The substrate
`may constitute an elemental semiconductor material
`such as silicon or germanium or a compound semicon
`ductor material such as gallium arsenide or gallium
`phosphide. The substrate may also comprise an insulat
`ing material such as silicon dioxide, silicon nitride and
`aluminum oxide. The substrate may also comprise a
`plurality of materials, for example, including an under
`lying layer of silicon on which is formed a layer of an
`insulating material such as silicon dioxide on which in
`turn a layer of indium tin oxide has been sputtered to a
`suitable thickness.
`FIG. 2 shows a pair of conductive electrodes 15 and
`16 constituting a desired pattern formed in the layer of
`indium tin oxide in accordance with the process of the
`present invention. The electrodes or stripes are shown
`in the form of straight stripes with adjacent edges
`closely spaced. In accordance with the present inven
`tion such edges can be formed closer than 1 micron in
`layers of indium tin oxide with appreciable thickness,
`that is, of the order of 0.5 microns. FIG. 3 is a sectional
`view through stripe 16.
`The present invention will be described in connec
`tion with a specific example. A layer 12 of indium tin
`oxide 0.5 micron thick is sputtered on a substrate 11 of
`silicon dioxide to form body 10 of FIG. i. The sputter
`ing is accomplished in r-fsputtering apparatus in which
`the target electrode is a disc of pressed and sintered
`indium oxide and tin oxide in the proportion of about
`90% indium oxide and 10% tin oxide and in which the
`atmosphere is argon. The body is annealed at about
`400°C for 10 minutes to densify the indium tin oxide
`and also to improve the conductivity thereof. Thereaf
`ter the body is suitably prepared for the application of
`a photoresist material thereto, for example, Waycoat
`negative resist made by Philip A. Hunt Chemical Corp.
`of Palisades Park, New Jersey. The preparation in
`cludes prebaking the wafer at 125°C for about a hour.
`Next, the resist having suitable viscosity of about 43
`centipoise is applied to the surface of the indium tin
`oxide and is spun for a short period of time to form a
`uniform coating of the resist of about 4 to 5 microns
`thereon. The body with the resist thereon is appropri
`ately cured by placing in another oven maintained at a
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`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1030, IPR2021-01160 Page 3 of 5
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`temperature of 80°C for 20 minutes. A dark photomask
`as well, however, the rate of etch through the indium
`having a pair of transparent portions corresponding to
`tin oxide would be less than with a more concentrated
`the electrodes 15 and 16 thereon is juxtaposed to the
`solution.
`surface of the cured resist. Ultraviolet light is next
`While a thickness of 0.5 micron of indium tin oxide
`directed onto the resist through transparent portions of
`was used in explaining the method of the invention, it
`will be appreciated that this is not a limiting thickness.
`the mask for a sufficient period of time and of sufficient
`intensity to cause hardening or polymerization of the
`While the composition of the indium tin oxide in con
`portions of the resist exposed to ultra-violet light. After
`nection with which the process was described had
`exposure the resist is put through a developing process
`about 90% indium oxide and 10% tin oxide, it will be
`in which the unhardened or unpolymerized portions of
`understood that the process will work for compositions
`the resist are completely removed in a sequence of
`of indium tin oxide in which the indium tin oxide is
`steps which includes developing, rinsing, cleaning and
`included in a lesser percentage than 90% in the com
`drying. After the developing sequence the body is run
`posite material, although a substantially greater per
`through a post bake step at 120°C for % hour to fully
`centage of the indium tin oxide should be indium oxide.
`polymerize the hardened portions of the photosensitive
`While the process has been described in connection
`with the application of the indium tin oxide to silicon
`resist and also to enhance the adhesiveness of the layer
`of resist to the substrate. The foregoing steps of apply
`dioxide, it should be understood that the process is
`equally applicable to the direct application of the in
`ing resist and developing a hardened pattern are well
`known to those skilled in the art.
`dium tin oxide to an elemental semiconductor such as
`In accordance with the present invention the body
`silicon as well as germanium and also it is equally appli
`with the hardened pattern of resist thereon is immersed
`cable to the application of the indium tin oxide to other
`in a solution of concentrated hydrobromic acid for a
`insulating layers such as silicon nitride, mixtures of
`time sufficient to etch away the indium tin oxide which
`silicon dioxide and silicon nitride and aluminum oxide.
`is now exposed and which is unmasked by the hardened
`Also, the invention is equally applicable to bodies in
`resist. I have found that with a concentrated hydro
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`which the indium tin oxide is applied to a semiconduc
`bromic acid solution of 48% hydrogen bromide and
`tor substrate such as gallium arsenide and gallium phos
`phide.
`with an indium tin oxide layer thickness of about 0.5
`micron that all of the exposed indium oxide is etched
`While the etching of indium tin oxide has been de
`away without any lifting of the hardened resist. With
`scribed as taking place at room temperature, it will be
`the 48% concentrated solution of hydrobromic acid the
`understood that the etching may be accomplished at
`etching action occurs in about 20 minutes. The hydro
`higher temperatures as well. While photoresist integrity
`bromic acid had imperceptible effect on the silicon
`and adhesion is not as severely affected by higher tem
`perature etching, the etch rate is increased at higher
`dioxide substrate. Thereafter the hardened resist is
`removed in a suitable photoresist stripper such as
`temperatures. Accordingly, use of higher temperatures
`Chem Strip made by Mallinckrodt Chemical Company
`is preferred for etching thicker layers of indium tin
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`of St. Louis, Missouri. After suitable rinsing the body is
`oxide, for example layers of the order of 1 micron.
`now ready for subsequent processing.
`While the invention has been described in a specific
`In accordance with the present invention it was found
`embodiment it will be appreciated that modifications
`that the layers of indium tin oxide about 0.5 micron
`may be made by those skilled in the art and it is in
`thick applied to a substrate of silicon oxide and pro
`tended by the appended claims to cover all such modi
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`cessed as described above to produce stripes, such as
`fications and changes as fall within the true spirit and
`electrodes 15 and 16, had straight edges with devia
`scope of the invention.
`tions from a straight line of less than 0.3 micron
`What I claim as new and desire to secure by Letters
`thereby enabling a close spacing of edges of adjacent
`Patent of the United States is:
`electrodes or stripes of indium tin oxide to within less
`1. A method of etching a first pattern in a layer of
`than 1 micron.
`indium tin oxide on a surface of a substrate comprising
`In the process described above in which a concen
`applying a coating of photoresist material over the
`trated solution of 48% hydrobromic acid was utilized
`exposed surface of said layer of indium tin oxide,
`on a silicon dioxide substrate and in which Waycoat
`forming in said coating a hardened pattern identical
`photoresist was applied as indicated in a desired pat
`to said first pattern and exposing said layer of in
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`tern on the indium tin oxide layer, it was found that
`dium tin oxide unmasked by said hardened pattern,
`good adherence between the patterned photoresist and
`immersing said substrate with said hardened photore
`the indium tin oxide layer was obtained for at least 20
`sist thereon in a solution of hydrobromic acid for a
`minutes.
`time sufficient to etch away indium tin oxide un
`While the invention has been described in connec
`masked by said hardened photoresist,
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`tion with a negative photoresist material such as Way
`removing said hardened photoresist from said layer
`coat negative photoresist, the invention is equally ap
`of indium tin oxide.
`plicable to the use of other negative photoresists and
`2. The method of claim 1 in which said photoresist
`positive photoresists as well such as positive photoresist
`material is a positive photoresist material.
`AZ 13J05 supplied by Shipley Company of Newton,
`3. The method of claim 1 in which said photoresist
`Mass. Positive photoresist materials are materials
`material is a negative photoresist material.
`which are applied in essentially hardened or polymer
`4. The method of claim 1 in which said solution of
`ized form and the action of ultra-violet light thereon is
`hydrobromic acid is a concentrated solution.
`to depolymerize the photoresist material which may
`5. The method of claim 4 in which said concentrated
`then be readily removed by the developing process.
`solution comprises aqueous solution of about 48% hy
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`While a highly concentrated solution of hydrobromic
`drogen bromide by weight.
`acid was used in the process described above it will be
`6. The method of claim 1 in which said substrate with
`readily apparent that less concentrated solutions work
`said hardened photoresist thereon is immersed in a
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`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1030, IPR2021-01160 Page 4 of 5
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`3,979,240
`S
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`Solution of concentrated hydrobromic acid for less than
`conductivity thereon prior to the application of said
`coating of photoresist thereto.
`20 minutes at substantially room temperature.
`1. The method of claim 1 in which said substrate
`7. The method of claim 1 in which said layer of in
`comprises an elemental semiconductor from the class
`dium tin oxide is less than 1 micron in thickness.
`of silicon and germanium.
`8. The method of claim 1 in which the major constit
`12. The method of claim 1 in which said substrate
`uent by weight of said indium tin oxide is indium oxide.
`comprises a compound semiconductor material from
`the class of gallium arsenide and gallium phosphide.
`9. The method of claim 8 in which said indium tin
`13. The method of claim
`in which said substrate
`oxide comprises about ninety per cent by weight of said
`comprises an insulating material from the class of sili
`indium tin oxide.
`con dioxide, silicon nitride, mixtures of silicon dioxide
`10. The method of claim 1 in which said layer of
`and silicon nitride and aluminum oxide.
`indium tin oxide is annealed to densify and increase the
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`Petitioner STMICROELECTRONICS, INC.,
`Ex. 1030, IPR2021-01160 Page 5 of 5
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