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Projection Photolithography-Liftoff Techniques for Production of 0.2-Pm Metal Patterns

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Projection Photolithography-Liftoff Techniques for Production of 0.2-Pm Metal Patterns IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-28, NO. 11, NOVEMBER 1981 1375 Projection Photolithography-Liftoff Techniques for Production of 0.2-pm Metal Patterns MARK D. FEUER AND DANIEL E. PROBER, MEMBER, IEEE GLASS FILTER Abstract-A technique whichallows the useof projection photo- \ yPHOTORESIST lithography with the photoresist liftoff process, for fabrication of sub- micrometer metal patterns, is described. Through-the-substrate (back- \II a projection) exposure of the photoresist produces the undercut profiles necessary forliftoff processing. Metal lines andsuperconducting microbridges of 0.2-pm width have been fabricated with this technique. I Experimental details and process limits are discussed. II OBJECTIVE IMMERSION OIL ECENT DEVELOPMENTS inmicrolithography have SUBSTRATE - (d) R made possible the production ofa variety of devices with PHOTORESIST %- I-l submicrometer (submicron) dimensions [ 11 , [2] , offering the (a) advantages of higherspeed and packing density.For many Fig. 1. Schematicdiagram of back-projectionand metal-liftoff pro- Josephson-effect devices in particular, submicron dimensions cedure. (a) Exposure system, employing a Zeiss optical microscope. Image of the mask is projected through the substrate, which is shown are essential for achieving optimalperformance over awide in sideview. (b) Schematic contours of constant exposure intensity. range of operatingconditions [3] . Forsubmicron pattern (c) After photoresist development and metallization. (d) After liftoff. transfer, liftoff processing [ 11 generally has better resolution thanwet-chemical etching. Liftoff processing may also be after results achieved withthe back-projection technique preferred over the alternatives of chemical and plasma etching are presented. for films which are difficult to etch, where the etching process Thetechnique we have developed, through-the-substrate can cause chemical or physical damage to the patterned film, exposure, involves projecting the image of a mask through or where resist masking for the etching process will lead to the back of a transparent substrate onto the photoresist,which contamination problems for subsequent use of the patterned is onthe front side. Optical absorption in thephotoresist film. leads to an undercut exposure profile, whchi.s preserved after This paperreports an opticalprojection technique which development. This exposure method is shown in Fig. 1. achieves undercut photoresist edge profiles necessary for the The exposure system is like that of Palmer and Decker [5] . liftoff process ’ [l] . With the projection technique we have A Zeiss Photomicroscope with a type 11-C epi-illuminator is developed, excellent liftoff results and yield are obtained even used. The microscope is adjusted for Koehler illumination, for dimensions <OS pm, and metal lines and device patterns with the photomask in the field stop. A lOOX Oel Aufl. Pol as narrow as 0.2 pm have been produced [4] . These are as (strain-freeachromat) NA 1.25 oil-immersnon objective is small as any patterns produced with W-optical techniques. used.’ An objective aperture of 1 mm is used for best resolu- While electron-beam lithographic techniques [l], [2] are tion. This improves the image contrast by introducing partial more general and have somewhatbetter resolution on solid coherencein the illumination. The objective aperture was substrates,the optical technique describedhere is far less chosenempirically for best visual contrast.The coherence complex. Thissimplicity andthe low cost and rapid turn- factorwith this aperture is u 0.5. With the lOOX objective around possible makethis optical technique well suitedfor used, thediameter of the projected field is -150 pm,and production of individual experimental devices. Other optical the linear reductionof the mask,pattern is 43X. Non-oil- techniques have recently been developed which achieve under- immersionachromat objectives of lowermagnification and cut resist profilessuitable forliftoff processing.These are “Epiplan HD”planachromats also have yielded satisfactory based onmultiple-layer resists. These othertechniques will results. (Resolution is best forthe oil-immersionlenses, be discussed and compared to the back-projection technique however.) Thestandard 15-W incandescent illuminator is used with a red-glass filter,Corning CS2-60,for focusing Manuscript received April 1, 198l;revised June 22, 1981. This work and alignment (see below), and with a blue-glass filter, Corning was supported by the National Science Foundation under Grants ENG- 7710164 and ECS-7927165. M. D. Feuer was with Becton Center, Department of Engineering and Applied Science, Yale University, New Haven, CT 06520. He is now Immersion oil is Cargille Type A,Cargille Laboratories, Cedar Grove, with Bell Laboratories, Murray Hill, NJ 07974. NJ 07009. The immersion oil mustbe removed prior to photoresist D. E. Prober is with the Section of Applied Physics, Yale University, development. Removal is by wiping the oil off the surface, and then New Haven, CT 06520. dipping in trichlorethylene one or more times. 0018-9383/81/1100-1375$00.75 0 1981 IEEE 1376 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-28, NO. 11, NOVEMBER 1981 Photoresist processing is similar tothe manufacturer’s recommendations.* The basic steps are photoresistexposure and development, metal film evaporation, and metal film lift- off (see Fig. 1). A pre-exposure bakein air at 90°C for 15 min is used; no post-exposurebake is used. Development is for 30 s in stirred AZ developer* diluted 1 : 1 in deionized water. Iflarge-area patterning of the photoresist layer is required, contact exposure anddevelopment is carried outprior to projection exposure. Alignment of the microscope projection exposure to <OS pm is possible.Liftoff of the undesired portions of the metal film is accomplished by dissolution of thephotoresist in acetone,often with ultrasonicagitation. With the resist processing procedures described, theliftoff process itself is very reliable, with a yield of >90 percent. To achieve successful liftoff,particularly for the smaller structures, vertical orundercut photoresist edge profiles are essential. This is achieved with the back-projection technique, at least for the resist thicknesses >0.25 pm used. Absorption by 0.3 pm of unexposed photoresist is -25 percent [6],and this causes the fully exposed region to be widest where the light first enters the photoresist. Schematic contours of con- stant exposure intensity are shown in Fig. l(b). (Interference effects are here neglected.) If thedevelopment process had infinite contrast, these intensity contours would be produced in the developed photoresist pattern. In practice, even though thedevelopment process has only moderatecontrast [6], vertical or undercut edge profiles are obtained. Electron micrographs of photoresist edge profiles for three photoresist samples, prior to metallization, are shown in Fig. 2. The inset shows the pattern projected; the gap between the resulting photoresist “fingers” is 0.3 to 0.4 pm. Fig. 2(a)-(c) are side views of each photoresist pattern, viewed at 85” from u thesubstrate normal. In Fig. 2(a), thephotoresist film is Fig. 2. Photoresist edge profiles. The scanning-electronmicrographs 0.25 p thick,and showsvertical walls. Such athickness were taken at glancing incidence (i.e., nearly parallel to the substrate). (a) Through-the-substrate exposure of thin, 0.25-pmphotoresist and profile can be used for liftoff of thin metal films, <750 yields roughly vertical edge profiles. (b) Through-the-substrate expo- thick. In Fig. 2(b), the photoresist film is thicker, 0.5 pm sure of 0.5-pm photoresist yields clearly undercut edge profiles. (c) thick. This profile shows dramaticundercutting, and some Conventional top-surface exposureof thin, 0.25-pmphotoresist yields sloping edge profiles, which would not be suitable for liftoff process- (unexplained) raggedness. Such a profile is typicalfor this ing. 0.1-pm size scale is shown. Inset: Mask pattern. The photoresist thickne~s.~Finally, Fig. 2(c) showsa conventionalfront structures remainwhere mask is dark.The size of the gap at the (top)exposure of a thinphotoresist film. As expected,no center of the mask is 9 pm; this would be projected to 0.2 pm in the absence of diffraction effects. undercutting results, and liftoff is unreliable with such front exposures even for thin metalfilms. CS5-58, forexposure. The blue exposurefilter transmitsat Diffractioneffects set the lowerlimit onpattern dimen- wavelengths around 400 nm.Exposure times are typically sions. For a lens with a numerical aperture NA ,the minimum 40 s for a thin (0.3-pm) layer of thepositive photoresist used, resolvable feature hasdimensions given approximately by Shipley AZ1350B.2 Standard microscope cover glasses, type [71 PI No. 13, which are 0.17mm thick, are used as substrates. h (Most objectives designed for use with cover glasses are de- dmin= - 2NA signed forthis thickness. With oil-immersionlenses, the thickness is less critical.) The correct stage height for pattern 31n the discussion of resist edge profiles we have not included inter- reproduction is determined by scanning electron microscope ference effects, which are often of importance in front exposures on inspection of a test series of patterns exposed at various stage reflecting substrates. As seen in Fig. 2, interference effects appear to heights. For the 1OOX lens, the correct height is within 0.5 be small. This is due to the broad-band radiationused for exposure, the p smaller index mismatch
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