Laser polishing of vidicon s glass substrates

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1 Lser polishing of vidicon s glss substrtes Sidorenko S.P., Fedosovsky M.E., Veiko V.P. 3, Golubok A.O. 3, Levichev V.V. 3, Chuiko V.A. 3, Ykovlev E.B. 3, Shkhno E.A. 3 Joint stock compny «Dikont», joint stock compny «Electron», 3 Sint Petersburg Stte University of Informtion Technologies, Mechnics nd Optics. URGENCY OF THE OBJECT AND INNOVATIVE APPROACH Mny micro- nd nnoelectronic, micro- nd nnooptic devices include different films nd lyers deposited t the surfces of dielectric substrtes. Properties of these films nd lyers sufficiently depend on properties of the substrte surfce. Therefore substrte surfce preprtion before lyers nd films deposition is very importnt. One of the min prmeters of the surfce is its roughness tht influence on such prmeters of thin lyers s specific resistnce, electric strength, chemicl resistnce etc. By mechnicl tretment of the surfce its roughness is defined by the size of polishing powder grin. Tht is why preprtion of the surfce with low roughness is lborious one. Besides, preliminry mechnicl grinding with corse-grined powder cuses ppernce of ner-by-surfce crcked lyer, which cn be removed by deep mechnicl, chemicl or flme polishing. Lser polishing is sort of lst one. The problem of lser polishing ws repetedly discussed,. 0.6 μm СО -lser is the only cceptble one, s ll glsses well bsorb rdition t this wve length. The regime of continuous rdition is used in most cses, for which typicl durtion of influence by surfce scnning is severl milliseconds. Though the beneficil effect ws obtined (in before mentioned nd other ppers), the technique is not wide spred. In our opinion the reson of this is tht there re problems connected with phenomen specific for lser polishing: hydrodynmicl wves in the softening lyer nd thermomechnicl tensions in the substrte. Both of them re ggrvted by the three-dimensionl loclity of lser ction. This sitution cn be chnged by using of short nnosecond lser pulses - in this cse the heted lyer of substrte decreses nd the problem becomes not so criticl. For instnce, receiving plte of vidicon is the glss substrte with conductive, photosensitive nd high-resistnce lyers deposited on it. Thickness of CdSe photosensitive lyer with impurities is μm, working voltge supplied to the trget is 0 5 V, in these conditions electric intensity cn rech 0 kv/mm in the CdSe lyer. Such hevy processing environment of vidicon trget requires high-level homogeneity of photosensitive nd conductive lyers nd high qulity of the glss substrte surfce t which the lyers re being deposited. Defects nd imperfections of the work surfce of glss substrte cuse ppernce of lrge size defects in In O 3 conductive lyer deposited t it. Those defects become crystlliztion centers in CdSe lyer. Insufficient electric strength of crystllized regions result in ppernce of electricl escpe in them nd brekdown of the photosensitive lyer by working voltge supply to the pickup plte. As result those defect regions become insensitive to luminous flux nd blnk spots rise t the vidicon trget. Number of reject trgets cn rech tens of percent. For yeild incresing it is necessry to improve qulity of the glss substrtes surfce. We tried to solve this problem by preprtion of glss substrte for film deposition using 0 ns pulse СО-lser irrdition.. PHYSICAL FUNDAMENTALS OF LASER POLISHING By 0.6 μm wve length lser ction t the surfce of glss plte its ner-by-surfce lyer is softened. Thickness 4 of the softened lyer is hm = t, is glss therml diffusion, t is durtion of lser ction. In our cse = 0 cm /s nd t = 0 s, therefore h m = μm. Pressure of surfce tension in the softened lyer is in inverse proportion to rdius of curvture of the surfce, so there is grdient of surfce tension forces t the rough surfce. Hydrodynmic flow of the softened lyer is the result of tht, so it smoothes the surfce 3. One of the min fetures of lser polishing process is the method of lser scnning long the smple surfce. The most commonly used prmeters re: ) scnning bem of continuous rdition in the form of round spot or stripe; ) point-to-point irrdition of the whole surfce by periodic pulse ction t some re (usully squre or rectngulr), tht is trnsferring long the surfce step-by-step (similrly to imge multipliction in microelectronics). Lser Applictions in Microelectronic nd Optoelectronic Mnufcturing VII, edited by Michel Meunier, Andrew S. Holmes, Hiroyuki Niino, Bo Gu, Proc. of SPIE Vol. 70, 70I 009 SPIE CCC code: X/09/$8 doi: 0.7/.8747 Proc. of SPIE Vol I-

2 By scnning lser bem (Fig. ) grdient of surfce tension forces is rised due to the temperture grdient, tht cuse the wve of softened glss. Fig.. Scheme of glss polishing process by the use of scnning lser bem. Height of this wve Δ h depends on geometric prmeters of the softened lyer h m nd l (see Fig. ), viscosity μ, temperture of softening T m, surfce tension σ nd its temperture dependence, temperture t the wve crest T : hm dσ Tm Δ h= T ln m () μl dt T nd its velocity is defined s 3 hdt V = w μ dz () If scnning speed V s is slightly less thn wve velocity V w or equl to it, the surfce is smoothed to tomiclly flt one nd remins plne. If the velocities do not stisfy these conditions, the surfce either hs no time to become smooth or becomes wvy. The investigtion of hydrodynmic flow of softening glss showed, tht the mximum llowble lser bem 3 scnning speed v= Vmx in regime of surfce smoothing cn be decresed by lser power incresing V ~ mx p for point source (point lser spot) nd V ~ mx p for lrge scle source (lrge scle lser spot). By lser power incresing the size of softening glss region lso increses, tht cuses decresing of its current velocity. Therefore vlues of lser tretment prmeters providing surfce smoothing re situted in the nrrow bnd bounded by bove mentioned dependence V ( ) mx p on the one hnd nd by glss softening condition on the other hnd. This condition determines the vlue of lser bem scnning speed V S, by which softening temperture is chieved. Scnning speed V S decreses with p decresing. Moreover, the chosen lser tretment prmeters vlues hve to provide bsence of glss evportion. By pulse lser polishing (regime ) with fixed bem (see Fig. ) surfce smoothing is the result of pressure of surfce tension tht is in inverse proportion to rdius of curvture of the surfce. The surfce hs to be softened t depth greter then vlley depth. Another importnt thing is tht locl heting of the glss cuses thermomechnicl tensions tht cn led to glss surfce dmge (bursting, peeling). For those tensions decresing it is necessry to het s smll lyer of the glss plte s possible (but not less thn irregulriries height), to do it homogeniously nd up to the temperture tht is slightly lower thn glss-trnsition temperture 4. Proc. of SPIE Vol I-

p suce level Fig.. The scheme of the process of lser polishing of glss by fixed bem. 3.

Pulse durtion is 0 ns. Sufficiently lrge lser intensity mde it possible to use up to.5 mm effective dimeter of irrdited zone.

Smple ws trnsferred by stepping motor with verge speed from to 0 mm/s. 'ii b Fig.3. Instlltion for lser tretment of glss discs () with heting device (b).

3 p suce level Fig.. The scheme of the process of lser polishing of glss by fixed bem. 3. EXPERIMENTAL INVESTIGATION OF THE LASER POLISHING OF THE GLASS BY FIXED BEAM For investigtion of lser glss polishing regimes n instlltion on bse of СО -lser «OLYMP 50G» ws mde. Pulse durtion is 0 ns. Sufficiently lrge lser intensity mde it possible to use up to.5 mm effective dimeter of irrdited zone. Fine djustment of lser power nd dimeter of lser bem is possible. The plte ws processed in series by x mm squre zones, nd the process cn be repeted severl times. Smple ws trnsferred by stepping motor with verge speed from to 0 mm/s. 'ii b Fig.3. Instlltion for lser tretment of glss discs () with heting device (b). Choice of tretment prmeters ws the min tsk of the prcticl prt of the work. The problem ws tht it ws necessry to select the optiml combintion of severl interdependent prmeters: preheting temperture, lser power, lser bem dimeter, pulse repetition rte, velocity of tble movement, number of irrditing cycles. 0 glss pltes were processed in different regimes. Proc. of SPIE Vol I-3

4 After visul exmintion nd investigtion of topogrphy of processed surfce by the method of tomic force microscopy optiml regimes were chosen. Using these regimes qulity of the processed surfce ws cceptble (smples No., 4, 5, 0 in Tble ). Number of smple Tble. Optiml regimes of lser tretment of vidicon substrtes. Preheting temperture, о С Power, W Bem dimeter, mm Velocity of tble moving, mm/s Quntity of psses Results of tretment Homogeneous tretment Melting trces Melting trces Melting trces I nvestigtion of qulity of surfce ws crried out by tomic-force microscopes NT-VDT NTEGRA nd Р47. Scnning re vried from х μm to 50х50 μm. Resolution of surfce relief in verticl direction by mximum mgnifiction ws 0.0 nm. Results of investigtion of topogrphy of glss discs surfces by the method of tomic force microscopy using Р47, NT-VDT NTEGRA devices re given in Tble. Tble. Smple number Point number 3 Mx. overfll cross the field R mx, nm 50х50 0х0 0х0 μm 5х5 5х5 7,9 8,5 7,8 μm 6, 8, 8, 6,5 6,3 7,3 μm Roughness cross the field R, nm 50х50 0х0 0х0 μm 5х5 5х5 0,5 0,5 0,57 μm 0,5 0,59 0,63 μm 0,45 0,4 0, ,0-7, -6,5 6,97 7,9-58,8-69, -6,5 3,6 9,7 5,0, 3, 3,6 4,5 0,93,, 0,4 0,37 0,94,7,0 0,6 0, 0,6 0, 0,5 0,33 0, ,4 6, 3,9,5,5,6,9,4, 0,5 0,4 0,36 0, 0, 0,0 0,9 0,8 0, Typicl surfce profiles before nd fter lser processing re given in Fig. 4 nd Fig. 5. Proc. of SPIE Vol I-4

5 p DX=, 0 pm A D-, 9 AngIe= Pt/Ic, pm Fig.4. Mechnicl polishing results, smple 4. R mx = 44.6 nm, R =.07 nm. Fig.5. Lser polishing results, Rmx =,09 nm, R= 0,nm. Proc. of SPIE Vol I-5

Incresing of lser power in some cses resulted in formtion of hollows hving crters like shpe with the dimeter up to µm nd the depth up to 60 70 nm (see Fig. 6).

6 Incresing of lser power in some cses resulted in formtion of hollows hving crters like shpe with the dimeter up to µm nd the depth up to nm (see Fig. 6). nm p,, pm b Fig. 6. Lser polishing with the power higher then optiml one ) loction of crters t the glss surfce, b) profile of crters t the glss surfce, scnning surfce re is 50х50 μm, R mx = 7, nm, R =,09 nm. 4. CONCLUSIONS Anlysis of investigtion results let us mke following conclusions:. Using of СО lser short pulses llows to chieve good results of lser polishing of substrtes for film deposition, including such criticl prts s photosensitive lyers of vidicons. By this method one cn reduce or eliminte fissuring of the lyers fter mechnicl tretment nd essentilly decrese surfce roughness. Both fctors promote vidicon qulity incresing. Nevertheless the process is rther difficult s it is multiple-fctor one.. Using optiml lser power even out vlue of roughness ( R ) observed ll over the surfce t the level nm with mximum chnge of relief ( R mx ) no more thn 4-0 nm, tht is n order of mgnitude better thn results of mechnicl polishing ( R mx = 44.6 nm, R =.07 nm). 3. By lser power incresing t some exmples formtion of hollows t the processed surfce in the form of crters with depth up to nm nd dimeter up to µm ws observed. This effect cn be of interest s cretion of micro/nnostructure of glss surfce. 4. The process cn be further optimized by perfection of the equipment nd stbiliztion of prmeters. 5. ACKNOWLEDGEMENTS This work is supported by RFBR grnts nd Authors thnks D. Orlov for the help in the design of the pper. 6. REFERENCES. Lqurt F., Lupon N., Armengol J.Opticl glss polishing by controlled lser surfce-het tretment. Applied Optics v P Veiko V.P., Shhno E.A., Ykovlev E.B. Hydrodynmicl processes t the glss surfce by lser bem scnning. Proceedings SPIE, v.3093, Nonresonnt Lser Mtter Interction (NLMI 9), 997, p , Proc. of SPIE Vol I-6

7 3. V.P.Veiko Shkhno E.A., Shkol A.T., Ykovlev E.B. Hydrodynmicl processes t glss surfce t lser heting. Izvesti RAS, phys. ser.,v. 6, 7, 997, с V.P.Veiko, E.B.Ykovlev. Physicl fundmentls of lser forming of microopticl components. Opt. Eng., v.33, N, 994, p Proc. of SPIE Vol I-7

Some parameters of varicaps with gradient base area based on Shottky barrier

Some parameters of varicaps with gradient base area based on Shottky barrier ISSN: 35-38 Vol. 4, Issue, December 7 Some prmeters of vricps with grdient bse re bsed on Shottky brrier Mmtkrimov O.O., KuchkrovB.Kh. Rector, Nmngn engineering-technology institute, Kosonsoy str.,7, Nmngn,