United States Patent o5
`Aguilera, Jr. et al.
`
`[54]
`
`175]
`
`[73]
`
`[21}
`
`[22]
`
`[62]
`
`{51]
`[52]
`
`[58]
`
`[56]
`
`MULTI-SPECTRAL FILTER
`
`Inventors:
`
`John A. Aguilera, Jr., Santa Rosa;
`William M. Robbins, Kenwood;
`Richard P. Shimshock, Santa Rosa;
`Leroy A. Bartolomei, Santa Rosa,all
`of Calif.
`
`Assignee: Deposition Sciences, Inc., Santa
`Rosa, Calif.
`Appl. No.: 728,724
`Filed:
`Jul, 11, 1991
`
`Related U.S. Application Data
`Division of Ser. No. 490,043, Mar. 7, 1990, Pat. No.
`5,072,109.
`
`Int, CLS oo ec cccsscceseceesescssecereacereseeenes G02B 5/28
`WLS. Ch ecccccceeteeeeseesseceeesaes 359/587; 250/226,
`359/589; 359/590
`Field of Search............... 359/587, 589, 590, 568,
`359/576; 250/226
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`.
`
`2,769,111 10/1956 Sadowski et al.
`2,999,034
`9/1961 Heidenhain .
`3,410,626 11/1968 Magrath oo. eeeeeeee 359/587
`3,585,286
`6/1971 Macovski
`.
`3,619,041 11/1971 Willoughby.
`3,745,236
`7/1973 Karato ...ccceeccetssseeercens 359/576
`3,771,857 11/1973 Thomasson .
`3,981,568
`9/1976 Bartolome? cies 359/587
`4,756,602
`7/1988 Southwell et al.
`.
`4,764,670
`8/1988 Pace et al. wees 250/226
`8/1988 Matsudaira et al. 359/587
`4,765,715
`
`MUOCAAAA
`
`US005164858A
`5,164,858
`(11] Patent Number:
`[45] Date of Patent: Nov. 17, 1992
`
`.
`4,822,998 4/1989 Yokota etal.
`4,823,357 4/1989 Casey...cccceccssescsessorennveees 359/568
`4,827,118
`5/1989 Shibata et al.
`.
`4,952,025
`8/1990 Gunning, HI .
`4,956,555
`9/1990 Woodberry .
`§,038,041
`8/1991 Egan wic..eecsssseceseccceseeneens 359/589
`OTHER PUBLICATIONS
`
`Schultz, J. and Russel, D., ‘New Staring Sensors”’,
`Defense Electronics, pp. 44-50 Jul. 1984.
`
`Primary Examiner—Martin Lerner
`Attorney, Agent, or Firm—Malcolm B. Wittenberg
`
`ABSTRACT
`[57]
`An optical thin film filter for the spatial and spectral
`separation of two or moretransmitting bands of radiant
`energy. For a two bandfilter the filter has a substrate
`substantially transparent to radiant energy in the trans-
`mitted bands. On onesurfaceofsaid substrate is a multi-
`layer interference coating that transmits both wave-
`length bands of interest. On the other surface of said
`substrate (or on the same surface) are two sets of non-
`overlapping butted, parallel stripes with one set being
`alternately interspersed with the other. The widths of
`said stripes are varied to provide for precisely defined
`regions of spatial and spectral delineation. One set of
`stripes is capable of transmitting one of the bands of
`interest and reflecting all others, while the secondset of
`stripes is capable of transmitting at least one band differ-
`ent from the one band transmitted by said first set of
`stripes and reflecting all others. Each of the stripes in
`itself is a multilayer interference coating formed of a
`plurality of high and low index of refraction materials.
`
`31 Claims, 11 Drawing Sheets
`
`10
`
`
`
`
`
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`15
`
`APPLE 1070
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`APPLE 1070
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`IPR2022-01291
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`

`

`U.S. Patent
`
`Nov. 17, 1992
`
`Sheet 1 of 11
`
`5,164,858
`
`>
`
`FIG. 1
`
`
`
`FIG. 3
`
`2
`
`

`

`U.S. Patent
`
`- Nov. 17, 1992
`
`Sheet 2 of 11
`
`5,164,858
`
`
`
`WAVELENGTH (MICRONS)
`FIG. 4
`
`4
`WAVELENGTH (MICRONS)
`FIG. §
`
`4
`3
`WAVELENGTH (MICRONS)
`FIG. 6
`
`5
`
`5
`
`21M)
`
`0
`
`—1
`
`-2
`
`-3
`
`-4
`
`-5
`
`6 -
`
`7
`
`-8
`
`2
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`"2
`
`1 0
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`~1
`
`-2
`
`-3
`
`—4
`
`-5
`
`4 -
`
`7
`
`-8
`
`2
`
`LOGOFTRANSMITTANCE(%)
`
`
`LOGOFTRANSMITTANCE(%)
`
`LOGOFTRANSMITTANCE(%)
`
`3
`
`

`

`U.S. Patent
`
`Nov. 17, 1992
`
`Sheet 3 of 11
`
`5,164,858
`
` 2
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`5
`
`4
`3.
`WAVELENGTH (MICRONS)
`FIG. 7
`
`4
`3
`WAVELENGTH(MICRONS)
`FIG. 8
`
`5
`
`2 1 0
`
`—1
`
`~2
`
`-3
`
`~4
`
`-5
`
`4 -
`
`7
`
`-8
`
`2
`
`
`
`LOGOFTRANSMITTANCE(%)
`
`
`
`
`
`LOGOFTRANSMITTANCE(°%)°
`
`4
`
`

`

`U.S. Patent
`
`Nov. 17, 1992
`
`Sheet 4 of 11
`
`5,164,858
`
`100
`
`90
`
`80
`
`70
`
`60
`
`50
`
`40
`
`30
`
`20
`
`10
`
`0
`
`2
`
`4
`WAVELENGTH (MICRONS)
`FIG. 9
`
` 2
`
`5
`
`4
`3
`WAVELENGTH (MICRONS)
`FIG. 10
`
`100
`
`
`
`
`
`.TRANSMITTANCE(%)TRANSMITTANCE(%)
`
`
`
`
`
`
`
`TRANSMITTANCE(%)
`
`100
`
`90
`
`80
`
`70
`
`60
`
`50
`
`40
`
`30
`
`20
`
`2
`
`4
`3
`WAVELENGTH(MICRONS)
`FIG. 11
`
`5
`
`,
`
`5
`
`

`

`U.S. Patent
`
`Nov. 17, 1992
`
`Sheet 5 of 11
`
`5,164,858
`
`
`
`TRANSMITTANCE(%)
`
`
`
`TRANSMITTANCE(%)
`
`100
`
`90
`
`80
`
`70
`
`60
`
`50
`
`40
`
`30
`
`20
`
`10
`
`2
`
`100
`
`90
`
`80
`
`70
`
`60
`
`50
`
`40
`
`30
`
`20
`
`10
`
`2
`
`4
`3
`WAVELENGTH(MICRONS)
`FIG. 12
`
`4
`3
`WAVELENGTH (MICRONS)
`FIG. 13
`
`5
`
`5
`
`6
`
`

`

`U.S. Patent
`
`Nov. 17, 1992
`
`Sheet 6 of 11
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`5,164,858
`
`10
`
`7
`
`

`

`U.S. Patent
`
`Nov. 17, 1992
`
`Sheet 7 of 11
`
`5,164,858
`
`
`
`ZtOld
`
`Rie
`
`
`
`8
`
`
`

`

`U.S. Patent
`
`Nov, 17, 1992
`
`Sheet 8 of 11
`
`5,164,858
`
`S|RE
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`9
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`

`

`U.S. Patent
`
`Nov. 17, 1992
`
`Sheet 9 of 11
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`5,164,858
`
`
`
`FIG. 19
`
`10
`
`10
`
`

`

`U.S. Patent
`
`Nov. 17, 1992
`
`Sheet 10 of 11
`
`5,164,858
`
`100
`
`2
`
`4
`3
`WAVELENGTH (MICRONS)
`FIG. 20
`
`5
`
`
`
`TRANSMITTANCE(%)
`
`
`
`TRANSMITTANCE(%)
`
`
`
`TRANSMITTANCE(%)
`
`100
`
`2
`
`4
`3
`WAVELENGTH(MICRONS)
`FIG. 21
`
`5
`
`10090 WL
`
`WwW
`
`80
`
`70
`
`60
`
`50
`
`40
`
`30
`
`20
`
`10
`
`0
`
`2
`
`4
`3
`WAVELENGTH (MICRONS)
`FIG. 22
`
`11
`
`<7
`
`5
`
`11
`
`

`

`U.S. Patent
`
`Nov. 17, 1992
`
`Sheet 11 of 11
`
`5,164,858
`
`100
`
`90
`
`80
`
`70
`
`60
`
`50
`
`40
`
`30
`
`20
`
`100
`
`
`
`TRANSMITTANCE(%)
`
`
`
`TRANSMITTANCE(%)
`
`5
`
`5
`
`2
`
`,
`
`4
`3
`WAVELENGTH (MICRONS)
`FIG. 23
`
`2
`
`4
`3
`WAVELENGTH (MICRONS)
`FIG. 24
`
`12
`
`12
`
`

`

`5,164,858
`
`1
`
`MULTI-SPECTRAL FILTER
`
`RELATED APPLICATIONS
`
`2
`sion(s) of the pattern approach the thickness ofthefilter
`coating.
`As taught in U.S. Pat. No. 3,771,857, lift off is the
`common method for patterning hard multilayer inter-
`ference coatings. In this method for forming the pat-
`terned multilayer interference filter, spaced parallel
`stripes of a material such as photoresist or metal are
`formed ona substrate in thickness greater than the total
`physical thickness of the multilayer interference coating
`to be patterned. Coating materials are deposited on the
`stripes and on the surface to a depth whichis insuffi-
`cient to cover the side walls of the stripes so as to pro-
`vide a discontinuity in the multilayer coating to facili-
`tate removalof the stripes after the dielectric multilayer
`coating is deposited. The photoresist material is then
`etched away, thuslifting off the coating material carried
`by the stripes so that there remainsa first set of spaced
`parallel stripes of the multilayer coating on the sub-
`strate. This technique is repeated for additional multi-
`layer interferencefilter patterns.
`The problem with patterning very thick coatings, e.g.
`greater than 5 micrometers, using this method stems
`from the severe problems associated with obtaining
`thick, adherent, well defined patterns of a suitable mate-
`rial such as photoresist that is stable at the temperatures
`and pressures typically involved in multilayer thin film
`deposition processes. Since spectral filters of the type
`addressed herein are typically on the order of 15 mi-
`crometers to 20 micrometers thick, the production of
`such coatings, in patterns whose width approaches the
`coating’s thickness, requiring good coating uniformity
`out to the edge of the pattern, are nearly impossible to
`fabricate using conventional technology.
`In summary, optical systems employing more than
`one radiant wavelength band detection can be greatly
`simplified if a means can be foundto spatially and spec-
`trally separate the incoming radiant energy into the
`appropriate wavelength bandsof interest using a single
`refractive substrate element. This allows a moretightly
`packed focal plane array which minimizes cooling and
`mechanical support to the focal plane while reducing
`overall focal plane costs.This results in a higher per-
`forming and morereliable sensor.
`Therefore, there is a need for a new and improved
`methodfor fabricating multicolor, thin film wavelength
`discriminating optical filters due to the very low yields
`and resultant high cost of placing discrete optical dis-
`crimination filters over each discrete detector element
`and for fabricating a patterned coating which alterna-
`tively passes the different wavelength bandsofinterest.
`The multicolor, thin film wavelength discriminating
`optical filter of the present invention comprises .a sub-
`strate that is substantially transparent to radiant energy
`at the wavelength bandsof interest and has at least one
`surface for receiving coatings. A first coating capable of
`transmitting two or more spectral bands of interest
`while reflecting and/or absorbing all out of band wave-
`lengths (those wavelengths which are notof interest) is
`provided on one surface of the substrate. First and sec-
`ond photolithographic patterned coatings,
`typically
`parallel stripes, are also provided in the radiant energy
`pathway. Both photolithographic patterned coatings
`are capable ofreflecting all wavelength bands but one
`spectral band transmitted by the first coating. These
`photolithographic patterned coatings are provided on a
`surface of the substrate, in either of two alternate con-
`figurations: the opposite surface from the first coating,
`
`The present applicationis a divisional of U.S. applica-
`tion Ser. No. 07/490,043 filed on Mar. 7, 1991, now US.
`Pat. No. 5,072,109, which issued on Dec. 10, 1991.
`
`TECHNICAL FIELD OF THE INVENTION
`
`Therelates to the field of design and fabrication of
`multicolor (multiple wavelength band), thin film wave-
`length discriminating optical filters and their use in-
`connection with detectors or detector arrays.
`DESCRIPTION OF PRIOR ART
`
`Optical systems typically consist of various collecting
`and focusing optics, spectral discriminating filters, and
`detectors for measuring the radiant energy being col-
`lected and focused by the optical portion of the system.
`Increasing demands for improved signal-to-noise and
`signal processing capability are driving sensor systems
`to use staring mosaic arrays rather than a conventional
`scanning approach. Providing spectral
`filtering for
`these staring systems is complicated by the requirement
`that spectral discrimination be provided in discrete
`spatial regions. If the optical system is also required to
`collect radiant energy in more than one spectral band
`the optical system becomes very complex as every de-
`tector or detector array must have a discrete wave-
`length discriminating filter placed over it. Rather com-
`plex beam steering optics or dichroic beam splitters are
`also required to direct the radiant energy onto the ap-
`propriate group of detectors. These additions to an
`optical system add considerable weight and complexity
`to the system thereby reducing reliability and perfor-
`mance while increasing system costs.
`Someefforts to place discriminating filter coatings
`directly upon the active surface of the detectors have
`been attempted with very little real success. The biggest
`problem with this approachis one of imposing an intrin-
`sically lowyield thin film coating process upon an in-
`trinsically low yield, high value detector process. Asso-
`ciated yield and cost make this an impractical solution
`to the problem.
`With the advent of large mosaic detector arrays the
`problem of providing wavelength discrimination has
`become more acute. For single wavelength sensors a
`large single discriminating filter can be placed over the
`entire detector array. However, for multicolor arrays
`there has been no easy way to provide wavelength
`discrimination in discrete spatial zones to date. It is a
`formidable undertaking to place discrete optical dis-
`crimination filters over each discrete detector element
`due to the very smal! physical dimensions of such detec-
`tors, and the process and temperature limitations im-
`posed by the fragile detector elements. Previous at-
`tempts to build a patterned coating which alternatively
`passes the different wavelength bands ofinterest have
`met with limited success even for relatively simple filter
`requirements. For sophisticated filter requirements that
`require greater film thickness to produce precise spec-
`tral discrimination and the attendant
`lower coating
`yields, this approach becomes impractical due to the
`technical difficulty and high cost associated with pat-
`terning thick multilayer interference coatings. This
`problem is further aggravated by the size of the pattern
`relative to the thickness of the multilayer coating, with
`the difficulty increasing substantially as the dimen-
`
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`or the first coating is applied directly on top of the two
`lithographic patterned coating is provided on the sec-
`photolithographic patterned coatings. If desired, addi-
`ond surface of the substrate with each pattern being
`tional photolithographic patterned coatings, each capa-
`deposited parallel to the otherset of stripes.
`ble of transmitting at least one wavelength banddiffer-
`Another object of the inventionis to provide for ease
`ent from said first and second patterned coatings while
`of varying the stripe widths, or pattern of the optical
`reflecting all other wavelength bands transmitted by
`switch module, to precisely match the mosaic focal
`said first surface coating can also be provided where
`plane detector array without
`impacting the spectral
`three or more colors are required.
`performanceof the multicolorfilter.
`In general, it is an object of the present invention to
`Anotherobject of the invention is to providea filter
`provide a multicolor, thin film wavelength discriminat-
`of the above character in which the sets of parallel
`ing optical filter that comprises a continuous multicolor
`stripes are disposed on the samesurfaceof the substrate.
`(multiband transmitting) filter in series or combination
`Another object of the invention is to provideafilter
`with two or more reflecting (photolithographic pat-
`of the above character in whichthefirst surface contin-
`terned) filters in which the filters are formed by a plu-
`uous coating in conjunction with second surfacestripes
`rality of high and low index dielectric materials ar-
`provides good spectral performance.
`ranged in multiple layers (optical switch coating).
`Anotherobject of the invention is to provideafilter
`Another object of the invention is to provideafilter
`of the above character in which each set of parallel
`of the above character on a single surface of the sub-
`stripes is capable of transmitting at least one band differ-
`strate in whichsaidfilter is designed to transmit two or
`ent from the other sets of stripes while rejecting all
`other bands.
`more spectral bandsofinterest while reflecting and/or
`absorbing required out of band wavelengths (multicolor
`Another object of the invention is to provide a filter
`filter) and additional filters on the second substrate
`of theabove character in which each photolithographic
`surface each consist of a set of parallel, butted and non-
`pattern is capable of modifying the transmission of at
`overlapping stripes capable of transmitting at least one
`least one wavelength band different from the other
`color different from the other set of stripes while re-
`photolithographic pattern while reflecting and/or ab-
`flecting all other colors (optical switch coating).
`sorbing all other bands.
`Another object of the present invention is to provide
`Another object of the invention is to provideafilter
`a multicolorfilter of the above character on one surface
`of the above character in whichhigh stripe densities can
`be obtained.
`of the substrate in order to reduce the thickness and
`complexity of optical switch coatings that could be
`Another object of the invention is to provide a filter
`patterned on a second surface, or the optical switch
`of the above character in whichthe transition between
`coatingsfilter can be applied directly over individual
`stripes is minimized.
`detectors so as to provide a cost effective means for
`Another object of the invention is to provide a filter
`spatial and spectral separation of two or more wave-
`of the above character in which the patterned reflecting
`length bands of radiant energy.
`coatings are in patterns other than stripes.
`Another object of the invention is to provide a multi-
`Another object of the invention is to provide a fabri-
`colorfilter of the above character on the same surface
`cation method of the above character which is repeat-
`able.
`of the substrate and directly on top of patterned optical
`switch coatings so as to provide a cost effective means
`Another objectof the invention is to provide a fabri-
`for spatial and spectral separation of two or more bands
`cation method of the above character which has high
`of radiant energy.
`yield.
`Anotherobject of the invention is to provide a multi-
`Another object of the invention is to provide a fabri-
`colorfilter on one surface of the substrate that is modu-
`cation method of the above character which is robust
`lar in design with each module designed to match the
`and provides wide processing tolerances.
`refractive index of air at each interface between mod-
`Anotherobject of the invention is to provide an opti-
`ules.
`cal filter design methodology in which the coating de-
`Another object of the invention is to provide a multi-
`sign or designs do not use the conventional periodic
`color filter on one surface of the substrate that consists
`structure, but use a highly refined structure with indi-
`of a short wavelength transmission band module, a
`vidual layer thicknesses precisely determined and estab-
`medium or long wavelength transmission band module,
`lished to provide optimum spectral performance and
`and a common blocking (out of band reflection and/or
`manufacturability.
`absorption) band module.
`Another object of the invention is to provide an opti-
`Another object of the present invention is to provide
`cal element which minimizes cross talk between the
`a filter of the above character in which said multicolor
`different transmission bands.
`filter is applied directly to one or both surfaces of a
`Anotherobject of the invention is to provide a filter
`refractive element and each “optical switch coating”is
`of the above character in combination with at least one
`applied directly to an appropriate detector, said optical
`patterned or discrete reflecting filter and at least one
`switch coatings reflecting all but one wavelength band
`detector that detects only one band that is transmitted
`transmitted by the multicolorfilter.
`by the multiband filter.
`Another object of the invention is to provide a filter
`Additional objects and features of the invention will
`of the above character in whichall but oneof the trans-
`appear from the following description in which the
`mitted wavelength bandsis reflected by a discrete re-
`preferred embodiments are set forth in detail in con-
`flecting filter, with at least two discrete reflecting filters
`junction with the accompanying drawing.
`being mounted in a filter wheel such that individual
`SUMMARYOF THE INVENTION
`wavelength bandsofinterest can be provided byselect-
`ing the appropriate discrete reflecting filter.
`Another object of the inventionis to provide a filter
`of the above character in which more than one photo-
`
`This invention provides a solution to the spectral and
`spatial separation of two. or more radiant wavelength
`bands with a single refractive element.
`
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`5,164,858
`
`6
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`5
`In a particular embodiment, a first coating is depos-
`ited on the surface of an appropriate substrate providing
`wavelength discrimination of two or more discrete
`FIG. 1 is a plan view of a multicolor, thin film wave-
`radiant bands. A wavelength attenuation, or blocking
`length discriminating optical filter incorporating the
`coating which is common to all wavelength bandsis
`present invention.
`FIG. 2 is a sectioned edge view of a multicolor, thin
`also placed upon this same substrate surface. Upon the
`opposite substrate surface is provided at least one pho-
`film wavelength discriminating optical filter incorporat-
`tolithographic patterned coating which passes the ap-
`ing the present invention.
`propriate wavelength(s) of interest while:
`reflecting
`FIG.3 is a sectioned edge view of a multicolor, thin
`film wavelength discriminating optical filter where the
`and/or absorbing all other out of band wavelengths
`multicolor continuous coating is made up of three dis-
`transmitted by thefirst coating.
`crete thin film modules with the interface between mul-
`Analternative to providing the wavelength discrimi-
`tilayer coating stacks being matched to the index of
`nation coating on the opposite surface from the photo-
`lithographic patterned coating is to provide the wave-
`refraction of air so as to facilitate design and process
`analysis and control, and to allow minor changes in
`length discrimination coating directly on top of the
`spectral performance of a single wavelength band to be
`photolithographic patterned coating. With either of
`made without impacting the performance of the other
`these alternative techniques, the thickness of the coat-
`ings to be patterned can be kept to a minimum,greatly
`band(s).
`facilitating the coating patterning process. In addition,
`FIG. 4 is a plot showing the spectral performance on
`a log of transmittance scale of a typical multiband trans-
`the reflective “switch” type coatings typically have
`significantly higher coating yields than wavelength
`mitting coating.
`discrimination coatings because the tolerances for these
`FIG.5 is a plot showing the spectral performance on
`a log of transmittance scale of a patterned reflecting
`coatings are not as stringent as those required for wave-
`filter that transmits the longer wavelength band while
`length discrimination.
`reflecting and/or absorbing the shorter wavelength
`In another embodiment, the photolithographic pat-
`band.
`terned coatings are not required, and instead relatively
`simple separation coatings are placed directly upon the
`FIG.6 is a plot showing the spectral performance on
`a log of transmittance scale of a patterned reflecting
`detector surfaces themselves. As these coatings repre-
`sent a high yield process there is an overall acceptable
`filter that transmits the shorter wavelength band while
`yield resulting from the combined detector/separation
`reflecting and/or absorbing the longer wavelength
`band.
`.
`coating processes. The coated detectors are then used in
`combination with the multicolor filter coated substrate.
`FIGS. 7 and 8 are plots showing the spectral perfor-
`In another embodiment,discretefilters can be placed
`mance on a log of transmittance scale which can be
`in a filter wheel or some other suitablefilter holding and
`obtained from having the multicolor continuousfilter in
`combination with one or the other of the patterned
`indexing device. For this case the reflecting coatings
`can be designed to not only transmit one band while
`reflecting filters.
`FIG.9is a plot showing the spectral performance on
`reflecting and/or absorbing all others, but to also mod-
`ify the band that is to be transmitted.
`a zero to one hundred percent transmittance scale of a
`In another embodiment, the photolithographic pat-
`typical multiband transmitting coating.
`FIG. 10 is a plot showing the spectral performance
`terned coatings deposited on the substrate are not de-
`signed to transmit one wavelength band different from
`on a zero to one hundred percent transmittancescale of
`all other bands, but to modify a single wavelength band
`a patterned reflecting filter that transmits the longer
`different from the others while, reflecting and/or ab-
`wavelength band while reflecting and/or absorbing the
`sorbing all other wavelengths transmitted by the multi-
`shorter wavelength band.
`color continuous coating.
`FIG. 11 is a plot showing the spectral performance
`In another embodiment, a continuousfirst coating is
`on a zero to one hundred percent transmittance scale of
`a patterned reflecting filter that transmits the shorter
`deposited upon one surface of a substrate which is a
`wavelength band while reflecting and/or absorbing the
`wide band transmitting coating, characterized by short
`wavelength edge and long wavelength edge of the
`longer wavelength band.
`transmission band being one edge of two distinct and
`FIGS. 12 and 13 are plots showing the spectral per-
`different colors. The wide band continuousfirst coating
`formance on a zero to one hundred percent transmit-
`tance scale which is obtained when the multicolor con-
`also provides required out of band blocking. Upon a
`tinuousfilter is provided in combination with one or the
`second surface of the substrate, two photolithographic
`other of the patterned reflectingfilters.
`patterned coatings are deposited, capable of providing
`the second transmission edge for a single color while
`FIG.14 is a sectioned edge view of a multicolor, thin
`film wavelength discriminating optical filter in which
`reflecting and/or absorbingall other wavelengthstrans-
`mitted by the wide band continuouscoating.
`the continuous multi-spectral coating is deposited di-
`rectly on top of the patterned reflecting coatings.
`In another embodiment, a continuousfirst coating is
`deposited upon one surface of a substrate which is a
`FIG.15 is a simplified schematic perspective view of
`wide band transmitting coating, characterized by short
`a long waveiength reflecting coating applied directly on
`top of a short wavelength detector array.
`wavelength edge and long wavelength edge of the
`transmission band being one edge of two distinct and
`FIG. 16 is a simplified schematic perspective view of
`different colors. The wide band continuousfirst coating
`a short wavelength reflecting coating applied directly
`also provides required out of band blocking. Coatings
`on top of a long wavelength detector array.
`are alternatively provided on top of individual detectors
`FIG. 17 is a simplified schematic perspective view of
`which provide the transmission edge for one color
`a multi-spectral filter on an appropriate substrate in
`series with a detector array consisting ofa substrate that
`while reflecting and/or absorbing all other wavelengths
`supports alternating short and long wavelength detec-
`transmitted by the wide band continuouscoating.
`
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`5,164,858
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`7
`8
`tor arrays with integral short and long wavelength
`U.S. Pat. No. 3,771,857, except that in the present in-
`reflecting coatings.
`,
`vention the stripes are non-overlapping and parallel to
`FIG.18 is a simplified schematic perspective view of
`each other, not at an angle as described in the ’857
`a multi-spectral filter on an appropriate transparent
`patent. The fiducial mark 6 is on the surface of the
`substrate in series with a detector array consisting of a
`substrate 1 where the pattern is provided, and the sub-
`substrate that supports alternating patterned short and
`strate 1 is opaque to visible and ultra-violet energy
`long wavelength reflecting coatings on the top surface
`thereby negating the need foran antihalation coating to
`reduce second surface reflections as described in said
`and alternating long and short wavelength detector
`arrays directly opposite and aligned with the patterned
`USS. Patent. It is important to note that the material or
`coatings on the opposite surface.
`photoresist as taught in U.S. Pat. No. 3,771,857 must be
`FIG.19 is a simplified schematic perspective view of
`greater in thickness than the coating which is to be
`a multi-spectral filter on an appropriate transparent
`patterned in the present
`invention: according to the
`substrate in series with an aperture mask andafilter
`prior art, preferably two times the thickness of the coat-
`wheel with discrete short and long wavelength reflect-
`ing to be patterned but not so thick (typically less than
`ing coatings on appropriate substrates.
`the width of the stripe to be applied) as to reduce the
`FIG. 20 is a plot showing the spectral performance
`uniformity of the coating across the width ofthestripe
`on a zero to one hundred percent transmittance scale of
`by shadowing the coating vapor arriving at the sub-
`a multiband transmitting coating.
`Strate at incidence angles off normal.
`FIG. 21 is a plot showing the spectral performance
`After the first photoresist pattern is provided the
`on a zero to one hundred percent transmittancescale of
`short wavelength reflective coating 8 is deposited on
`a patternedreflecting filter that transmits and shapes the
`the surface 2. The short wavelengthreflective coating 8
`short wavelength side of the long wavelength band
`is designed so that it has optical impedance matching
`while reflecting and/or absorbing the shorter wave-
`layers closest to the substrate which match the index of
`length band.
`refraction of the germaniumorsilicon substrate 1 which
`FIG.22 is a plot showing the spectral performance
`can have an index of refraction of approximately 4.0 for
`on a zero to one hundred percent transmittance scale of
`germanium or approximately 3.45 forsilicon. On top of
`a patternedreflecting filter that transmits and shapes the
`the reflective coating additional optical
`impedance
`long wavelength, side of the short wavelength band
`matching layers are deposited such that the upperlayers
`while reflecting and/or absorbing the longer wave-
`of coating will match into an index of approximately
`length band.
`1.00 whichis the refractive index of the atmosphere or
`FIGS.23 and 24 are plots showing the spectral per-
`a vacuum such as space in whichthefilter is to be uti-
`formance on a zero to one hundred percent transmit-
`lized. A suitable design for a short wavelength reflec-
`tance scale which is obtained from having the multi-
`tive coating 8 is set forth in TABLE 1 below.
`color continuousfilter in combination with one or the
`It is also intended that othersuitable dielectric materi-
`otherof the patterned shaping/reflectingfilters.
`als can be used in the fabrication of the short wave-
`length reflective element. Absorbing materials such as
`DETAILED DESCRIPTION OF THE
`dyes can also be used to act as a short wavelength re-
`PREFERRED EMBODIMENTS
`flective element, where permitted by the specific appli-
`cation.
`
`FIGS. 1, 2, and 3 show a multicolor, thin film wave-
`length discriminating optical filter incorporating the
`present invention. As shown therein, the filter consists
`of a substrate 1 whichis substantially transparent at the
`spectral wavelengthsofinterest. In a preferred embodi-
`ment, it is germanium supplied by Eagle-Pitcher Indus-
`tries or silicon supplied by Silicon Castings. The sub-
`strate 1 is provided with two spaced parallel surfaces 2
`and 3 which are highly polished (B—B per MIL-F-
`48616 over the entire surface, for example) and are very
`flat (less than 3 visible fringes flat and less than 1 visible
`fringe irregular over a rectangular area ofthe part). The
`substrate 1 can have any desired size. For example,it
`can have a width of approximately 2.1 inches, a length
`of approximately 2.5 inches and a thickness of approxi-
`mately 0.1 inches.
`Parallel-stripe coating 5 is deposited on the surface 2
`and forms the alternating short wavelength and long
`wavelength reflectors 8 and 9 as hereinafter described.
`Aswill be noted, the coating 5 does not cover the entire
`surface 2 butis limited so that an outer border region 4
`of the surface 2 remains uncoated. A fiducial mark 6 is
`provided on the substrate 1 and is located on the surface
`2. The fiducial mark may be applied in any desired
`mannersuch asbyscribing, painting, etching, and the
`like. The fiducial mark 6 is utilized in aligning each
`parallel stripe with the appropriate detector type in
`operation.
`One method by which the coating § is formed on the
`substrate 1 to provide thestriped filter is as taught in
`
`TABLE1
`
`PHYSICAL
`THICKNESS
`(Micrometers)
`
`LAYER
`LAYER
`MATERIAL
`NUMBER
`Si
`Substrate
`0.2553
`SiO
`1
`0.1098
`Ge
`2
`0.3620
`SiO
`3
`0.1699
`Ge
`4
`0.3037
`SiO
`5
`0.1414
`Ge
`6
`0.3622
`SiO
`7
`0.1698
`Ge
`8
`0.3572
`Ssio
`9
`0.1679
`Ge
`10
`0.3572
`SiO
`11
`0.1583
`Ge
`12
`0.3630
`SiO
`13
`0.1689
`Ge
`14
`0.3814
`SiO
`15
`0.1649
`Ge
`16
`0.3481
`sio
`7
`0.1465
`Ge
`18
`0.3491
`SiO
`19
`0.1723
`Ge
`20
`0.3457
`siO
`21
`0.0108
`Ge
`22
`0.3077
`sio
`23
`Air—
`
`Ascan be seen from TABLE1, the short wavelength
`reflective coating 8 is formed by alternating layers of
`low and high refractive index materials. This structure
`
`16
`
`16
`
`

`

`9
`essentially forms a stop band (reflectance coating) at the
`shorter wavelengths of interest and forms a pass band
`(transmissive coating) at
`the longer wavelengths of
`interest. The low refractive index material can be a
`suitable material such as silicon monoxide (SiO) having
`a refractive index of approximately 1.9 and supplied by
`Cerac. The high refractive index material can be a suit-
`able material such as germanium (Ge) having a refrac-
`tive index of approximately 4.0 and supplied by Eagle-
`Pitcher Industries. After the short wavelength reflector
`is coated, the photolithographic pattern is formed by
`dissolving the photoresist pattern and lifting off the
`coating that deposited on top of the resist pattern. One
`method for accomplishing the lift off of the coating
`deposited on top of the resist pattern is taught in U.S.
`Pat. No. 3,771,857, the relevant portions of which are
`hereby i