Glasses for Lithography and Lithography for Glasses
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Glasses for lithography and lithography for glasses Miroslav VLCEK Department of General and Inorganic Chemistry Faculty of Chemical Technology University of Pardubice, 532 10 Pardubice Czech Republic 1 Goals of IMI-NFG: •International Colaboration with Research Trust on 6 new Functionalities •Multimedia Glass Education delivered across the boundaries •Outreach/Networking Glass Lecture Series: prepared for and produced by the International Material Institute for New Functionality in Glass An NSF sponsored program – material herein not for sale Available at www.lehigh.edu/imi 2 Questions • What is lithography? What is glass? • Can glass be photosensitive? • Can glass be selectively etched/featured? If yes, how and what is the resolution limit? • Can a glass be applied in lithographic process and vice versa can lithography be applied to structure glasses? 3 Lithography – what does it mean? in ancient Greek: lithos = stones graphia = to write discovered by Alois Senefelder (Prague, Bohemia currently Czech Republic) in 1796 • oil-based image painted on the smooth surface of limestone • nitric acid (HNO3) emulsified with gum arabic burns the image only where surface unpainted and gum arabic sticks to the resulting etched area. • printing – water adheres to the gum arabic surface and avoids the oily parts, oily ink used for printing is doing exactly opposite, positive http://sweb.cz/galerie.litografie/ image is transferred on paper 4 „Technical“ understanding of term lithography these days: formation of 3-D relief images in a film on the substrate with the aim of transferring them subsequently to the substrate Microlithography – pattering method which allows features smaller than 10 μm to be fabricated Nanolithography – pattering on a scale smaller than 100 nm Contact and/or proximity lithography – photomask in direct contact with structurised resist-coated substrate and/or small gap between them Maskless lithography - no mask is required to generate the final pattern – examples: electron beam lithography – final patterns are created from digital representation, computer controls scan of an electron beam across a resist-coated substrate interference lithography 5 What lithography involves? - an exposure (irradiation) source - a mask and/or computer controled scan of suitable beam across resist-coated substrate - a resist itself - know how of a series of fabrication steps that would accomplish pattern transfer from the mask to resist and subsequently to substrate on which device is fabricated 6 How resists work? resist – radiation sensitive material, where chemical reactivity of exposed parts is modified relative to unexposed parts Etchant – agent (solvent, gas) which preferentially etches the exposed parts the unexposed parts positive etching negative etching original patterns are thus transferred into the resist after that substrate is patterned in resist-notcovered regions only 7 all resist removed from corrugated substrate positive etching negative etching resist SiO2 Si mask exposure etching of resist etching of SiO2 resist removal 8 Most important parameters of any resist Sufficient sensitivity to some radiation and proper technology of selective etching (simpler is better) Resistent to agents applied for substrate etching High resolution – nano better Easy to be deposited – homogenous in properties and thickness 9 What is glass? Glass – solid matter which is produced when the viscous molten material cools very rapidly to bellow its glass transition temperature and there is not sufficient time for atoms to form regular crystal lattice Silica based glasses – most common type of glasses about 70 % by weight of SiO2 soda-lime glass (≈ 30 % Na2O + CaO) borosilicate glass (≈ 10 % B203) lead crystal (at least 24 % of PbO) brittle, under compression can withstand a great force, chemically quit resistant, stable 3D compact structure, strong Si-O bonds obsidian – natural glass 10 http://www.galleries.com/minerals/mineralo/obsidian/obsidian.htm 11 chandelier in Capital, Washington New York – Trump Tower and Times Square 12 But go back a little bit to science 13 Chalcogenide glasses - nonoxide glasses O replaced by S, Se or Te - significantly lower Tg than oxide glasses - transmission in IR Ge S - high refractive index (≈ 1,8 – 3,2) 40 60 - !!! sensitive to different radiation!!! As10Ge30S60 As Ge S 20 20 60 Rel I As Ge S 30 10 60 As40S60 5101520 I. D. Aggarwal, J. S. Sanghera Journal of Optolectronics and λ (μm) Advanced Materials Vol. 4, No. 3, September 2002, p. 665 - 678 14 R. Ston, M. Vlček, H. Jain: J. of Non-Cryst. Solids 326&327 (2003) 220 – 225 Tailoring the properties M. Vlcek, A.V. Stronski, A. Sklenar, T. Wagner, S.O. Kasap Journal of Non-Crystalline Solids 266-269 (2000) 964-968 Adopted from A. Feltz:Amorphous Inorganic Materials and Glasses, VCH, 1993, Berlin, Germany 15 M.Vlček, A.V. Stronski, A. Sklenář, T. Wagner, S.O. Kasap: Journal Non-Cryst. Solids 266-269 (2000) 964-968 Tailoring the properties Fig. 5. a Absorption coefficient as a function of the photon energy for the three chalcogenide glassy compositions, opt As40S40Se20 (this work), As40S60 and As40Se60. b Determination of the optical gap, Eg , in terms of the Tauc law E. Marquez, J.M. Gonzales-Leal, R. Prieto-Aleon, M. Vlcek, A. Stronski, T. Wagner, D. Minkov Appl. Phys A 67 (1998) 371 16 !!! CHG sensitive to different radiation!!! What is the reason of sensitivity of CHG? generally – all amorphous materials - thermodynamically metastable exposure to suitable radiation can cause transformation in their structure or reaction with the environment (O2, metal, ....) → optical and physico-chemical properties including chemical resistance are influenced 17 Classification of radiation induced processes in amorphous chalcogenides Structural changes: - changes of local atomic configuration - polymerization – creating new bonds - phase changes, including crystallization Physico-chemical changes: - decomposition - photo-vaporization - photo-dissolution of certain metals - thermoplastic changes All these processes can result in changes of optical and physico-chemical properties 18 exposure with suitable radiation can change optical properties (T, R, n, α ...) M. Vlček, C. Raptis, T. Wagner, A. Vidourek, M. Frumar, I.P. Kotsalas, D. Papadimitriou: Journal Non- Cryst Solids192-193 (1995) 669-673 19 Exposure with suitable radiation can change chemical resistance What does it mean „suitable radiation“? band gap light (≈ 1 – 2.3 eV) UV or even visible light e - beam flux of ions X –ray.... both dry and wet etching can be applied Wet etching – all photoinduced processes can be applied Dry etching – usually photo-dissolution of certain metals is applied20 DRY ETCHING Plasma of ionized gases used to blast away atoms from the surface of the sample. (Also known as plasma etching) harsh conditions in plasma requires hard photoresist ! including: • high contrast of pattering • resistance to aggressive, ionied gases www2.ece.jhu.edu/faculty/andreou/495/2003/LectureNotes/DryEtching.pdf Certain metals usually added to CHG photoresist – Why? combine photostructural and compositional changes from photodiffusion of metal (mainly Ag) in ChG is the solution !!!21 • High contrast of resist pattering wanted Ag diffuses transversally only, no lateral diffusion • resistent to plasma etching gas • resistance increases due to formation ternary Ag-As-S glass but in exposed parts only As35S65 Ag diffuses into As-S step like - depth of diffusion - function of exposure dose Drawbacks – two more steps: • deposition of Ag 2 glass forming regions • removal of excess Ag from unexposed Ag ≈AgAsS As-S As-S 2 22 All Dry Process or combined process CHG ¾deposition of As-S Ag ¾deposition of Ag mask ¾exposure (vertical transfer of Ag into As-S) Ag-As-S ¾removal of excess Ag from unexposed parts by dry/wet Ag-As-S etching ¾dry/wet etching of As-S Ag-As-S ¾dry/wet etching of substrate ¾dry/wet removal of Ag-As-S layer from exposed parts23 bilayer photoresist Ag + As (and/or Ge) based chalcogenide glass exhibit excellent resolution, high contrast and good resistance to dry etching by CF4 (+ O2) www2.ece.jhu.edu/faculty/andreou/495/2003/LectureNotes/DryE tching.pdf Sensitization - evaporation of Ag 200 W Hg lamp, 60 mW/cm2 excess Ag removed in HNO3-HCl-H2O 0.5 Torr CF4 gas, 100 W rf power etching rates: undoped 55 nm/sec Ag photodoped 0.15 nm/sec 24 A. Yoshikawa Appl.Phys.Lett. 36(1) 107 Patterning Options for dry etching Different sources !!! UV or visible light e - beam X – ray beam 25 Dry etching Negative dry etching of Ag-As2S3 bilayer resist by CF4/O2 A. Kovalskiy, M. Vlcek, H. Jain, A. Fiserova, C.M. Waits, M. Dubey Development of chalcogenide glass photoresists for gray scale lithography Journal of Non-Crystalline Solids 352 (2006) 589–594 26 Dry etching of shaped structures - Ag diffuses into As-S glass in step like fashion - depth of diffusion - function of exposure dose Profilogram demonstrating the change of Optical Profiler image demonstrating the possibility etching depth with gradual variation of of smooth shaping with lens-like mask by photoinduced Ag diffusion into As S film with transparency of mask fragments. 2 3 following dry etching (reverse image, depth of etching 200 nm). CF4 as the etchant gas, with pressure of 100 mTorr, an electrode power of 110 W, CF4 flow rate of 100 sccm and an etching time of 2 min 27 A. Kovalskiy, H. Jain, J. Neilson, M. Vlcek, C.M. Waits, W. Churaman, M. Dubey On the mechanism of gray scale patterning of Ag-containing As2S3 thin films Journal of Physics and Chemistry