Preparation of Grids for Far Infrared Filters Reinhard Ulrich Capacitive grids and similar structures, to be used as reflectors in far ir interference filters, are prepared from a 1-, thick copper layer supported by a 2 . 5 -, thick dielectric film. The desired pattern is obtained by simple photolithographic contact printing of a suitable negative. The essential steps of this prepara- tion and the generation of some relevant patterns are described. Introduction Metal mesh and related structures have recently been shown to be suitable elements for the construction of far ir filters.'- 3 Whereas electroformed metal mesh is commercially readily available, 4 - 6 preparation in the laboratory is of interest for the other structures (ca- pacitive squares and resonant grids'), since these are not available in all desired patterns and on the necessary thin substrate. Therefore, some experiences in the preparation of these grids are reported here, although all single steps themselves may be routine in other con- texts. The grids consist of a thin, perforated layer of copper and are supported by a 2 . 5 -A thick PTER* film. This combination of materials has proved to be quite satis- factory concerning the optical performance at wave- numbers up to at least 100 cm-' and the good repro- ducibility in the preparation of the grids. The me- chanical strength of the grids is excellent; they are much tougher than metal mesh. In brief, the preparation starts with the vacuum deposition of a thin conducting layer of Cu on the PTER substrate. This primary layer is electroplated up to the required thickness (several skin depths) of approximately 1 u. The copper is then coated with light sensitive lacquer, and a photo- graphic negative of the desired pattern is contact- printed on it. After development, the grid is com- pleted by etching away the copper in the unexposed areas. Compared with the direct vacuum deposition of the metal through a mask, 3 the described method permits thicker Cu layers (i.e., lower losses), and allows the production of many identical grids from one nega- tive. The author was with The Ohio State University when this work was done; he is now with Bell Telephone Laboratories, Inc., Holmdel, New Jersey 07733. Received 3 September 1968. * Polyethylene terephthalate resin (equivalent to Mylar). These films were kindly supplied under the trade name HOSTAPHAN RE 2.5 by Kalle A. G., 6202 Wiesbaden-Biebrich, Germany. Preparation of the Cu Layer The PTER film, cut to pieces of proper size, is sand- wiched between paper and heated for 30 min to approxi- mately 150'C. This results in some preshrinkage and avoids excessive stress during a heat treatment that is necessary later. The PTER film is then glued with epoxy cement to suitable brass rings for support and handling. The primary Cu layer is vacuum deposited under standard conditions: pressure < 10-5 mm Hg, no glow discharge, approximately 0.5 g Cu of an electric wire evaporated from Mo-boat, 25 cm distance from target, Cu layer almost completely opaque. During this deposition, the substrate film is stretched over a slightly convex (radius of curvature 2 m) and highly polished aluminum plate [see Fig. 1 (a) ]. A number of springs keep the film stretched even if it should soften slightly. Backing it in this way is im- portant for a successful vacuum deposition of the Cu, since the heat capacity of the metal plate prevents ex- cessive heating of the substrate by the thermal radiation from the boat. Thus, wrinkling and melting of the thermoplastic film is avoided. For the same reason, the evaporator boat should be heated so highly that the deposition is completed in 1 min or less, and a shutter should protect the substrate film from radiation during the initial stage of evaporation. Cleaning the substrate before the Cu deposition is a problem because it is so thin and attracts dust electro- statically. It was found that the PTER film can be used without cleaning, as it comes from the manu- facturer, if touching the critical area with fingers is avoided. The electroplating process yields a smooth, uniform Cu layer without any precleaning if it follows im- mediately (< 1 h) after the Cu deposition. The sub- strate remains on the backing used during the evapora- tion. It is attached now at the bottom of the electro- plating cell [Fig. 1 (a) ], and the electrolyte 7 (188 g/liter of CuS04 5H20 and 74 g/liter of H2S0 4 ) is poured into the cell. This method avoids problems encountered some- times when the primary Cu layer is simply dipped into February 1969 / Vol. 8, No. 2 / APPLIED OPTICS 319
Transcript
Preparation of Grids for Far Infrared Filters
Reinhard Ulrich
Capacitive grids and similar structures, to be used as reflectors in far ir interference filters, are preparedfrom a 1-, thick copper layer supported by a 2 .5 -, thick dielectric film. The desired pattern is obtainedby simple photolithographic contact printing of a suitable negative. The essential steps of this prepara-tion and the generation of some relevant patterns are described.
IntroductionMetal mesh and related structures have recently been
shown to be suitable elements for the construction of farir filters.'- 3 Whereas electroformed metal mesh iscommercially readily available,4-6 preparation in thelaboratory is of interest for the other structures (ca-pacitive squares and resonant grids'), since these are notavailable in all desired patterns and on the necessarythin substrate. Therefore, some experiences in thepreparation of these grids are reported here, althoughall single steps themselves may be routine in other con-texts.
The grids consist of a thin, perforated layer of copperand are supported by a 2.5-A thick PTER* film. Thiscombination of materials has proved to be quite satis-factory concerning the optical performance at wave-numbers up to at least 100 cm-' and the good repro-ducibility in the preparation of the grids. The me-chanical strength of the grids is excellent; they are muchtougher than metal mesh. In brief, the preparationstarts with the vacuum deposition of a thin conductinglayer of Cu on the PTER substrate. This primarylayer is electroplated up to the required thickness(several skin depths) of approximately 1 u. The copperis then coated with light sensitive lacquer, and a photo-graphic negative of the desired pattern is contact-printed on it. After development, the grid is com-pleted by etching away the copper in the unexposedareas. Compared with the direct vacuum depositionof the metal through a mask,3 the described methodpermits thicker Cu layers (i.e., lower losses), and allowsthe production of many identical grids from one nega-tive.
The author was with The Ohio State University when this workwas done; he is now with Bell Telephone Laboratories, Inc.,Holmdel, New Jersey 07733.
Received 3 September 1968.* Polyethylene terephthalate resin (equivalent to Mylar).
These films were kindly supplied under the trade nameHOSTAPHAN RE 2.5 by Kalle A. G., 6202 Wiesbaden-Biebrich,Germany.
Preparation of the Cu Layer
The PTER film, cut to pieces of proper size, is sand-wiched between paper and heated for 30 min to approxi-mately 150'C. This results in some preshrinkage andavoids excessive stress during a heat treatment that isnecessary later. The PTER film is then glued withepoxy cement to suitable brass rings for support andhandling. The primary Cu layer is vacuum depositedunder standard conditions: pressure < 10-5 mm Hg,no glow discharge, approximately 0.5 g Cu of an electricwire evaporated from Mo-boat, 25 cm distance fromtarget, Cu layer almost completely opaque.
During this deposition, the substrate film is stretchedover a slightly convex (radius of curvature 2 m) andhighly polished aluminum plate [see Fig. 1 (a) ]. Anumber of springs keep the film stretched even if itshould soften slightly. Backing it in this way is im-portant for a successful vacuum deposition of the Cu,since the heat capacity of the metal plate prevents ex-cessive heating of the substrate by the thermal radiationfrom the boat. Thus, wrinkling and melting of thethermoplastic film is avoided. For the same reason,the evaporator boat should be heated so highly that thedeposition is completed in 1 min or less, and a shuttershould protect the substrate film from radiation duringthe initial stage of evaporation.
Cleaning the substrate before the Cu deposition is aproblem because it is so thin and attracts dust electro-statically. It was found that the PTER film can beused without cleaning, as it comes from the manu-facturer, if touching the critical area with fingers isavoided.
The electroplating process yields a smooth, uniformCu layer without any precleaning if it follows im-mediately (< 1 h) after the Cu deposition. The sub-strate remains on the backing used during the evapora-tion. It is attached now at the bottom of the electro-plating cell [Fig. 1 (a) ], and the electrolyte7 (188 g/literof CuS04 5H20 and 74 g/liter of H2S0 4) is poured into thecell. This method avoids problems encountered some-times when the primary Cu layer is simply dipped into
Fig. 1. (a) For electroplating, the PTER film forms the bottomof the electrolytic cell. For a uniform current density atthe cathode, the primary Cu layer is connected to the batterythrough six contacts along its edge (only one shown). The sup-porting aluminum plate also serves as a cool backing duringthe vacuum deposition of the primary Cu layer. (b) For theapplication of KPR, the Cu-coated PTER film is stretched
over the edge of a spinning disk.
the electrolyte. In that case, the Cu layer may getcontaminated while passing through the surface of theelectrolyte. The electrolyte cell and the Cu anodeshould have the same cross section as the area to beplated so that the current density is uniform. With 15mA/cm2 , a 1-i Cu layer is deposited in approximately 3min. The plating is done at room temperature. Fromthe completed Cu layer, the electrolyte is immediatelywashed off by rubbing it with a cotton pad under hotrunning water. While the film is still wet, the backingplate is removed and the backside of the film is cleaned,too, if necessary. Rinsing with distilled water anddrying follow.
Next, the film is baked for 5 min in an oven at 1500C.This heat treatment is essential for achieving a goodbonding of the Cu layer to the PTER. The bondingmust withstand the action of the solvents used in thesubsequent development and etching processes. Theheating yields an excellent bonding strength that suc-cessfully passes the adhesive tape test at the completedgrids. To some extent the bond improving heatingoccurs already during the vacuum deposition of the Cu.In the interest of a uniform quality, it seems advanta-geous, however, to avoid this as far as possible and tokeep the deposition and the heat treatment separate.
The Photolithographic ProcessFor cleaning and for coating with the light sensitive
lacquer, the Cu-plated film is stretched over the holder,Fig. 1(b). It supports the film only at its edge, and itcan be spun by a motor at up to 1800 rpm. An oxidelayer that may have formed on the Cu during the heat
treatment is washed off with acetic acid (20%). Thesurface is rinsed with distilled water, and dried by spin-ning the holder. Kodak Photo Resist KPR3, thinned1:1 with Kodak Ortho Resist Thinner, is poured on theCu layer (while at rest) until it is completely covered.Spinning the film then for about 1 min removes exces-sive lacquer and dries the remaining thin KPR layer.This should show now a uniform interference color whenobserved under the recommended gold fluorescentsafety light.8
For the contact printing, a negative of the desiredgrid is required. If available, a complementary grid2
may serve as such a negative (e.g., metal mesh for thepreparation of capacitive squares). Instead, a photo-graphic plate with the proper pattern may be used (bothfor capactive squares and for more sophisticated grids).In printing, an intimate contact between the KPR layerand the negative is imperative in order to avoid diffuse-ness of the shadow image by edge diffraction at thedetails of the negative. This contact can be achievedby an air cushion (Fig. 2). It consists of a short tubewith thin PTER windows glued to both ends. Thepressing window is 2.5 gu thick, the rear window 12.5 .For copying a metal mesh, this is stretched flat over thering R [Fig. 2(a)]. The KPR-coated substrate isstretched similarly over a plane, polished aluminumplate. This plate and the ring R are clamped individu-ally against the flange of the air cushion. When pres-surized, the latter presses the mesh firmly against theKPR layer. In this way, the influence of any residualdust particle, which prevents direct contact betweenmesh and KPR, is restricted to a small area. Theexposure is made through the air cushion.
When copying a pattern from a photographic plate,the PR-Cu film is stretched over the ring R [Fig.2(b) ]. The air cushion is used now to press the Cu filmwith the PR layer filmly against the photographicemulsion, and the exposure is made through the plate.The height of the air cushion, measured from the flange
(a) I I I i i LIGHT
PRESSPR
WINDOWS -RING R
METAL MESHKPR[Cu
(b) }LIGHT
, PHOTOGRAPH IC............ ~PLATELCuW I N D OW S t TPTER
RING R
-- AIR PRESSURE
Fig. 2. Contact printing a negative on the KPR layer. Anair cushion provides the necessary intimate contact. In (a),a metal mesh serves as the negative. In (b), the negative is a
Fig. 3. (a) The cross pattern is generated by the superpositionof a pattern of capacitive squares (photographic plate) and a pat-tern of inductive squares (metal mesh). The capacitive patternis shifted through (, ) with respect to the inductive one.(b) The pattern resulting for the displacement vector (2a/g,
2a/g) forms half of a pattern of square rings.
to the outer surface of the pressing window, must beslightly (approximately 50 ) less than the thickness ofthe ring R. This prevents the hard edge of the aircushion from touching the fragile mesh or the Cu film.
The KPR is exposed with the light of a 100-W fluores-cent lamp whose glass bulb with the fluorescent coatinghas been removed in order to utilize directly the bluelight of the inner mercury arc tube. The light isapproximately collimated by a glass lens (f = 15 cm;diameter 7.5 cm; distance from KPR layer 75 cm) inorder to minimize penumbral diffuseness of the shadowimage. The exposure time is determined empirically, atypical time being 3 min.
Development of the KPR in Kodak Ortho ResistDeveloper KOR and spray-rinsing with a fresh 2:1mixture of KOR and alcohol follow the lines given in theinstructions 8 for the KPR. The film is dried on thespinner. The KPR now covers only those areas of theCu which received light during the exposure. In theremaining areas, the Cu surface is open to the attack ofthe etchant in the final step of preparation.
Etching the Grid,A freshly prepared solution of approximately 50 g
FeCl3 *6H20 per liter of water serves as the etching bath.The grid-to-be is immersed completely in the solution atroom temperature ad is agitated steadily. If this isdone in a glass dish illuminated from underneath, thefinal stages of the etching process can be well observed.As soon as the grid appears uniformly bright it is with-drawn and is immediately (to avoid excessive under-cutting) washed in water. The wet grid can be in-spected now through a microscope, and the etchingmay be continued, if necessary. After thoroughly rins-ing in hot water and in distilled water, the preparationof the grid is completed by drying it on the spinyber.
Pattern GenerationFor the preparation of grids of capacitive squares,
metal mesh is available as master pattern in a variety ofdimensions.4 6 Woven wire cloth is suitable to a lesserdegree since it frequently has trapezoidal rather thansquare openings and is usually considerably less uniform.The use of a photographic negative instead of the metal
mesh as negative was found preferable for the prepara-tion of capacitive squares if the grid constant is g 2 100 A,since the plate is not so fragile and delicate as the mesh.The photographic plate for this purpose is prepared byfirst contact printing the metal mesh on a Kodak HighResolution Plate in essentially the same way as shownin Fig. 2(a). The resulting photographic capacitivesquares are then reversed to the required photographicinductive squares by contact printing them on a secondplate. Finer grids (g < 50 ,u) of capacitive squares,however, were copied directly from metal mesh. Theomission of the intermediate photographic step yields abetter definition and fewer faults. By this directcopying method, capacitive squares as fine as g = 25 Lohave been prepared whose density of faults is negligiblefor application in ir filters.
Metal mesh may also serve as a master for the genera-tion of the cross patterns of the resonant grids of Ref. 1.A plate with photographic capacitive squares is super-imposed on the metal mesh [Fig. 3(a)]. The axes of thetwo patterns are aligned parallel, and the capacitivepattern is displaced by a lattice vector (a, a) with respectto the mesh. Contact printing this combination yieldsthe pattern of capacitive crosses, and reversing thesein a second contact print produces the inductive crosses.'Both patterns (g = 102 /,u total area 10 X 12.5 cm 2)have been generated first on photographic plates, whichthen served as negatives for the final preparation of their grids. Figure 4 is a microphotograph of a com-pleted grid of inductive crosses. The slight asymmetryof the crosses is caused by imperfect registration of thetwo constituting patterns.
For a good registration, the lattices of the two pat-terns must be exactly identical, including any distor-tions. In order to achieve this, the photographic capaci-
Fig. 4. Microphotograph of a grid of inductive crosses. Thegrid constant is g = 102 /t. In the bright areas, the copper has
tive squares had first been directly printed from themetal mesh. The superposition was then adjusted sothat each single capacitive square was partially overlay-ing the individual inductive square from which it hadbeen generated. Ideally, the described method shouldresult in a pattern of crosses of dimensions a = b [seeFig. 3(a)]. The copying and especially the undercut-ting during the etching process, however, have oppositeinfluences on a and b, as is apparent from Fig. 4, whereb 2a. Some control of a and b is possible by changingthe conditions of exposure and etching. It should benoted here that in this preparation no special precau-tions had been taken against dust. The resulting pin-holes, though quite numerous (103 cm-2 ), are of anaverage size of less than 5 A. No apparent influence ofthese holes on the optical properties of the grids wasobserved. For the preparation of the finer (g < 100 ,)grids, however, operation under dust-free conditions isstrongly recommended.
By a similar technique, a pattern of square rings wasalso prepared whose ir optical properties are similar tothose of the crosses. Figure 3(b) shows the same super-position of photographic capacitive squares on a metalmesh as Fig. 3(a), but now the displacement vector is(2a/g, 2a/g). Contact printed on a photographicplate, this combination produces an L pattern. Thecapactive squares are then shifted to the position (- 2a/g,- 2a/g), and a second exposure is made on the same
plate. This results in a 'I pattern which combines withL's to the pattern of square rings.
This technique of multiple exposures through one ortwo masks that are shifted between the exposures per-mits the generation of many more patterns, all starting,e.g., from metal mesh. This technique is simpler thanthe step-and-repeat technique used conventionally forrepetitive pattern generation. It allows the fast genera-tion of comparatively very large patterns. Its resolu-tion is only moderate (approximately 5 i), however,since it is limited by the inaccuracies of the mechanicalcontact required for sharp contact printing.
This work was done under a fellowship of the Grad-uate School of The Ohio State University, Columbus,Ohio, which is gratefully acknowledged.
References1. R. Ulrich, Appl. Opt. 7, 1987 (1968).2. R. Ulrich, Infrared Phys. 7, 65 (1967).3. G. MX. Ressler and K. D. Moeller, Appl. Opt. 6, 893 (1966).4. W. Heimann, Physikalisch Technische Werkstaetten, Wies-
baden-Dotzheim, Germany5. Sumitomo Shoji, Kaisha Ltd., Osaka, Japan.6. Buckbee Mears Co., St. Paul, Minnesota, U.S.A.7. F. A. Lowenheim, Ed., Modern Electroplating (John Wiley &
Sons, Inc., New York, 1963).8. An Introduction to Photofabrication Using KPR (Publication
P-79, Eastman Kodak Company, Rochester, New York,1966).
Photographed by T. Vogl at the lens design conference organized by R. E. Hopkins and held in the offices of D. S. Grey Associates ofWaltham, Mass., are V. Carpenter (left), J. Shean, J. Hoagland, D. Grey, R. E. Hopkins, J. Buzawa, and P. Sands.
