early stage of pilsed-laser growth of silicon microcolumns and microcones in air and SF6

Ž.Applied Surface Science 154–1552000647–658www.elsevier.nl r locate r apsuscEarly stages of pulsed-laser growth of silicon microcolumns andmicrocones in air and SF 6Douglas H.Lowndesa,),Jason D.Fowlkes b ,Antonio J.PedrazabaSolid State Di Õision,Oak Ridge National Laboratory,Oak Ridge,TN,37831-6056,USAbDepartment of Materials Science and Engineering,The Uni Õersity of Tennessee,Knox Õille,TN,37996-2200,USAReceived 1June 1999;accepted 14July 1999AbstractDense arrays of high-aspect-ratio silicon microcolumns and microcones are formed by cumulative nanosecond pulsed excimer laser irradiation of single-crystal silicon in oxidizing atmospheres such as air and SF .Growth of such surface 6microstructures requires a redeposition model and also involves elements of self-organization.The shape of the microstruc-tures,i.e.,straight columns vs.steeply sloping cones and connecting walls,is governed by the type and concentration of the Ž.oxidizing species,e.g.,oxygen vs.fluorine.Growth is believed to occur by a ‘‘catalyst-free’’VLS vapor–liquid–solid mechanism that involves repetitive melting of the tips of the columns r cones and deposition there of the ablated flux of Si-containing vapor.Results are presented of a new investigation of how such different final microstructures as micro-columns or microcones joined by walls nucleate and develop.The changes in silicon surface morphology were systemati-Ž.cally determined and compared as the number of pulsed KrF 248nm laser shots was increased from 25to several thousand in both air and SF .The experiments in air and SF reveal significant differences in initial surface cracking and pattern 66formation.Consequently,local protrusions are first produced and column or cone r wall growth is initiated by different processes and at different rates.Differences in the spatial organization of column or cone r wall growth also are apparent.q 2000Elsevier Science B.V.All rights reserved.Keywords:Silicon;Pulsed laser;Ablation;Deposition;Columns;Cones;Whiskers;Nonequilibrium growth;Surface modification1.Introduction:formation of micron-scale silicon columns and conesRepetitive pulsed-laser irradiation of materials at relatively low laser energy densities,E ,of less than d 1J r cm 2produces gradual changes in surface topog-raphy and the development of ‘‘laser-induced peri-)Corresponding author.Tel.:q 1-423-574-6306;fax:q 1-423-576-3676.Ž.E-mail address:vdh@ D.H.Lowndes .Ž.Žw x odic surface structures’’LIPSS 1;see the series w x .of three papers by Young et al.2a,2b,2c .Such periodic structures have been widely observed in Žmetals,ceramics,polymers and semiconductors see w x .2Ref.3.At the higher E values of 1to 5J r cm d Ž.typically used for pulsed-laser deposition PLD of thin films,columnar or conical structures are formed in laser ablation targets,with the cones or columns w x pointing along the incident laser-beam direction 3,4.Earlier studies have established connections between the laser wavelength,polarization,and incidence an-gle and the spatial period of the near-surface ripple0169-4332r 00r $-see front matter q 2000Elsevier Science B.V.All rights reserved.Ž.PII:S 0169-43329900369-4本页已使用福昕阅读器进行编辑。

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()D.H.Lowndes et al.r Applied Surface Science154–1552000647–658 648w xstructures2a,2b,2c,while more recent experiments have pointed out the importance of using either aŽ.w x short pulse duration picosecond or femtosecond5Ž.w xor a short deep-UV laser wavelength6for preciseŽand efficient material removal drilling holes or cut-.ting trenches.The conical structures produced by sequential laser irradiation have shapes that vary from circular conesŽw xto straight,high aspect ratio columns see7–10and .w xbelow as well as irregular cone clusters3.Most of the models used to explain the development of coni-cal structures assume that they are formed by prefer-ential remoÕal of material surrounding the cones, with some sort of‘‘cap stone’’present to prevent erosionlocally.The presence of impurities resistant w xto ablation11,or surface modification of polymersw xto produce an ablation-resistant carbon layer12,or surface segregation conducive to a transparent coat-w xing13,all are models proposed to explain coneformation on targets irradiated in vacuum or low-Ž.pressure-1Torr ambient gases.However,we recently reported experiments in which arrays of tall,slender silicon microcolumns were formed by cumulative nanosecond pulsed-ex-w x cimer laser irradiation of a silicon wafer in air7,8.Ž.For example,1000pulses of KrF248nm radiation in air produces;20-m m tall Si columns with both the average column diameter and their mean separa-tion being;2m m,as shown in Fig.1.Experiments were carried out to reveal a succession of surface topographical changes as the number of laser pulses was increased and,through these,it was possible to identify the main features of the mechanism by which high aspect ratio silicon microcolumns are w xgrown7,8.These experiments demonstrate that any explanation of microcolumn formation by cumula-tive pulsed excimer laser irradiation clearly requires()a redeposition not simply erosion model,and also involves elements of within the ar-ray of columns.We note that Sanchez et al.also observed the formation of similar arrays of SiŽw xcolumns in air see Refs.9,10and the Discussion.section below.Additional experiments that we carried out re-cently in various ambient gases have revealed that anŽ. oxidizing atmosphere e.g.,oxygen or fluorine must be present for columns or cones to form at all under our experimental conditions.These experiments alsoŽ.Fig.1.Top SEM image of Si microcolumns formed after10002Ž.Ž. laser shots in air at E s3J r cm scale bar s100m m.BottomdŽDroplets formed at the tips of silicon microcolumns scale bar s10 .m m.show that the shape of the microstructures, e.g.,straight columns or steeply sloping cones connected by walls,is governed by the type and concentrationŽof the oxidizing species e.g.,oxygen-containing air.vs.fluorine-containing SF.Based on these results it6was proposed that,once a sufficient number of laser pulses have succeeded in nucleating some initialŽ.surface features see below,then silicon columns or cones will grow rapidly by a‘‘catalyst-free’’VLS Ž.vapor–liquid–solid growth mechanism.This mech-anism involves both the re-melting of the tips of the columns r cones and the deposition there of the in-tense ablated flux of Si-containing vapor that isw xproduced with each laser pulse7,8.The outline of this paper is as follows.We begin by briefly reviewing earlier results for the formation of fully developed but distinctly different arrays of densely packed micron-scale silicon columns or cones r walls,when cumulative pulsed-laser irradia-tion of silicon is carried out in air or SF,respec-6tively.We then present results of new experiments intended to investigate how such different final sur-face microstructures actually nucleate and develop.本页已使用福昕阅读器进行编辑。