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A Selective Etching Solution for Use with Patterned Self-Assembled Monolayers of Alkanethiolates on Gold

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A Selective Etching Solution for Use with Patterned Self-Assembled Monolayers of Alkanethiolates on Gold 2332 Reprintecl from Chemistrv of NIA'I'ERIALS.1995. 7. A Selective Etching Solution for Use with Patterned Self-Assembled Monolayers of Alkanethiolates on Gold Younan Xia, Xiao-NletZhao, Enoch Kim, and GeorgeNI. Whitesides* Departmentot''Chemislrl', Haruard (Jnit,ersity,Camhridge, Massachusetts 02138 Receit,edMay 2,3,1995. ReuisedMonuscript Receit;edSeptember 25, 1995! This paper describes a selective etching solution fbr use with patterned self-assembied monolayers (SAMs) of alkanethiolates on the surface of gold; it is also effective for SAMs on the surfaces of silver and copper. This etching solution use'sthiosulfate as the ligand that coordinates to the metal ions and ferricyanide as the oxidant. This etchant has a number of advantages relative to the system of cyanide ion in oxygen-saturated, alkaline water used in previous work. First. it is less toxic, Iess hazardous. and has smaller environmental impact. Second,using it. complete etching of bare gold can be achieved more rapidly (-B min versus -15 min for 200 A of Au). Third, etching of bare gold occur-sr"'ith fewer def'ects in the SAM-covered regions and generates features of gold w,ith higher edge resolution. The influence of the composition of the etching solution on the rate of etching was studied svstematicallv. Introduction The toxicitv and environmental impact of an etchant that contain-.free c1'anideion are obviouslv unattrac- Nlicrocontactprinting trrCPtr is a technique that tive. IIere,lve. report a more selectiveetching solution generatespatterned self-assembledmonolayers lSAtr{s t2 for use with SA\,{sof alkanethiolateson gcildthat uses of'alkanethiolates ttypically, hexadecanethiolate ) on the thiosulfate rather than c;ranide as the coordinating surfacesof goid,1silver,ll and copper,land of alkylsilox- ligand. and ferricyanide rather than O: as the oxidant. anes on hydroxyl-terminated surf'aces.r'Patterned SAI{s Ferrilferrtrcyanideare far less toxic than free cyanide- of alkanethiolates serve as nanomc.ter-thickresists in values of' LD;ri (ORL-MUS) are KC^ (8.5 mglkg),14 certain etching solutions: the one we i.rave.used most K,tFe(ClJ),:(2970 mg lhg),ta and K$e(CN),; t5000 mg I extensivelywhen gold is the underiving substrate has k91.":' beenaqueous KCN solution(pll'. 1-t.ICN ] ..,0.1M) This f'ern/f'errocvanideetchant is also superior to that saturated with 0z.6 Llsing this etchant. tl-repatterns containing CIN in rts selectivity: it is more rapid. in the SAMs can be transf'erredfaithfuih' into the thin generatesf'eq'er defects in the SAM-coveredregions and films of gold; the resulting patterns of gold can be used yieids microstructuresof gold with higher edgeresolu- subsequentlyas secondarvmasks for the etching (iso- tion. tropic or anisotropic) of' underlving la.versof silicon dioxide and silicon;' or ils functional elements (for Experimental Section example,for microerlectrodeseand diffractiongratingsi0). printing Materials. Au 199.999?),Ag t 99.9999q(),Cu(99.9999( ),Cr- Microcontact is renrarkablvconvenient fbr the (>99.99'i ,. - t.Tit99.99(i K:S:O'i, KSCN, KrFe(CN)c, KrFe(CN)0, fabricationof structuresrvith featuresizes of 1 zrmand andCII'rrCli, r, ,SH 'nvc.re obtained from Aldrich. KI. KOH.and larger: it can be r-rsed.rvith more difficulty and less K('N n'r're purt'liirsedfrom Fisher Scientific. Poll'(dimethyl- reliabilitv. to fabricatef'eatures with sizesdown to - 100 siloxane',rl'DlISt u'as obtainedfrom Dow Corning iS;zlgard nm.'"1tt l.r 1.84).Polished Sirl00trvafers (Cz, N/phosphorous-doped. 1- 10 Q cm. test gnrde.SEMI Std. flats)were obtainedfrom Silicon Sense. Inr'. ,Nashua. NH). I{exadecanetLiiolwas purified s Abstract publisheclin Arft'cuiccACS A6.s1rccls.Novcmber 1. 199ir. undcr nitrogr:n bv chromatographythrough silica gel. Gold (1) See.{or t,xample:Wilbur. .I. I-.:Kumar. A.: Kirn. ll.: \\hitcsides. iilms t200 ,\ thick) were preparedusing thermal evaporation G. N'1.Arft XIattr. f 994. 6. 600. (Varian 3118) onto silicon wafers rn'hosesurfaces had been (2) Rervicrvof'SANIs: \lhitesides, (i. NI.:Laibinis, E. E. Lortgtrtttir (-50 Gold. f 990, 6. 87. Dubois.L. H.; Nuzzo.R. G. Artrtu.Rer'. /)it.r'q. ('ltent. 1992. primed w'ith thin la.vc'r-sof chromrum A thickr. 1,:).4il7 iilt'". and copperfilm-q r500-1200 A thickr were prepared t3 t Xia, Y.; Kim. E.: \\rlritesides.(i. \I. ,J.ElectroL ltcni. Sor'.. in prerss. usirrge'-beam sputtering onto tita'rium-primed (- 15A ) silicon ('1) Xia. Y.: Kim. Ii.: \Irksich. !1.; \{tutesides.G. NL ('lrr'ni.tr7oter.. wafers. We chose200 A thick gold films for most of the current Submitted. work for three r€'Asons:(i)Gold films '200 A seemto behave r5r Xia. Y.; NIrksich.NL; Kim, E.: Whitesides.(i. N{.J. Ant. Chent. verS'similarh' in ferri/ferrocyanide etching solutions; thinner Sor'.1995, I17,9516. (6t Kumar. A.; Biebuyck, H.; Abbott. N. L.; \\hitesidc's.(;. NI. ./. fihns have more defects: 200 A is thereforea "standard thin Arn. Chent.Soc. 1992. 11J.9188. film".r4 tii) Gold is relativelyexpensive, and we wishedto use t7) Kendall,D. L. Attnu. Rt'r.Moter. Scr. 1979,9.:17:1. I)cterson. K. E. Proc. IEEE 1982.70.'120. {8) I{rm. E.: Kurnirr. A.:\Vhite'side's.(i. \'I. .1.ELectrocltern. Sor' 1995. i13r Wilbur. .I. L.: Kim. E.: Xia. Y.: Whilesides.G. NI.Adrr. Mater. 112,628. 1995. 7. 6.19. t9t Abbott,N. i,.: Rolison.I). R.;Whitesides, G. Nl.LctrLgnutir 1994. illt Mott'riol SrLfet.tDuta Sheets (CD-ROMr. Sigma-Aldrich- 8.2672. t'luka: rv'lilw;ruki.e.WI. 1994. (10) Kumar. A.; Riebur-ck.H.; \\tritcsidcs, (1. \f . Lortgmuir1994, (15t Moterial Safett'I)nta Sheets(CD-ROMr. NIDL Infbrmation 10. 1498. Svstcms.Inc.: WashingtonDC, 199,1. (11) Xia. Y.; \trhitcsides.G. NI..1. Ant. Chent.Stx. 1995./ 17.327,+. t16t Zhao. X.-NL; Wilbur, J. L.: \\hitesides, G. X,I.,unpublished (12) Xia. Y.: \\rhitesides.G. NI. Adu.Mater. 1995.7.,+7t. rcsults. 0897-4 756 I 9512807-2:13 2S09. 00/0 O 1995 American Chernical Socictv Monol,aversof Alkanethiolates on Gold Au/SAM No SAM 1""(arb. unit; + t (tinescan) €.3 a 3min c 3.0 r :t \- z.z ri2 I (UL. I o - o1 ol ? b 7 min 0.0 5.0 10.0 15.0 20.0 2s.0 3.0 etching time (min) 0.6 'Etr .E c 10 E .r.N iH 5 0.0 0.05 0.1 0.1s lK2s2o3l(M) r- 25 lum 20 I Figure 1. SEMs of gold films (200 A thicki that had been Frs patterned with SAMs of hexadecanethiolate by rrCP, and E etchedin the standard etching solutioncontaining SzO,r2 and --E or ferrilferrocyanide for different intervals of time. Note that the 10 density of pits in the SAM-coveredregions increased as etching time increased. The intensity of the secondary electrons (1.", shown schematically as a line scan in arbitrary units) can be used to monitor the etching processsemiquantitatively. The scansdrawn on the right side were along the lines marked in 0.5 1 1.5 the SEMs on the left side. tKoHl(M) 10 c'oldfilms that were as thin as possible. (iii)As shown in the iext. 200 A gold films are thick enoughto be used as substrates d for forming ordered SAMs of alkanethiolates, and these gold films were also thick enough to be used as masks for the etching of underlying layers of SiOz and Si. c Measurements of 1.". The intensity of secondaryelectrons FC (1-")was measured with a SEM (JEOL-6400)by setting the mode in the line scan. Samples of Figures 1 and 2a were Ioadedinto the SEN{ chamber at the same time; readings of 1.,.were taken under the sameconditions (spot size, accelerate voltage,filament current, contrast, and brightness). Microcontact Printing (pCP). PDMS stamps were fab- 0 0,04 0.08 0.12 ricated accordingto the published procedure.l'r0A solution [KrFe(CN)r](M) of hexadecanethiolin ethanol (-2 mM) was used as the "ink" for pCP. Microcontactprinting was carried out in a clean room Figure 2. (a) Dependenct' of' the irttt'ttstties of secondar-'t- ( < 100particles/ft'r). Gold films (200A thickt usedin this paper electrons (1.,,)from both bare golcl t't'gion. if-tlled circlesi and were also preparedin a cleanroom ( < 10 000 particles/ft'r) and SAM-covered regions (open cit'clt's,ort the dr.tra.tionof'etching were used as prepared immediately without any further (for 200 A thick gold filnrs, Tht' clillt't'er.rcebetrveen these tu'tr cleaning. After application of the hexadecanethiolsolution (by intensities was plotte'das tt't:inglt's.The' quantitr' 1r is delflned - cotton Q-Tipt to the PDMS stamp, the stamp was dried in a as the time for complt'te ertching.and r1: 11)as the etching stream of Nz for -1 min and then brought into contact with windor,r'.For the composition of the standard etching snlution the surface of gold. After -5 s, the stamp was separated used (K:SzO',/O.1\'{. KOH/1.0 M. KrFe(CNriv0.01 M. and It- carefully from the gold surface. FetCNt/O.OO1 trlr, the'e'tching rvindorv is -'1 min. (b. c, dt Etching of Gold. The composition of the standard ferri/ Dependence rtf'1r and 1yon the concentrations of KzS:Or, KOH, ferrocyanideetchant was KzSzO3(0.1M), KOH (1.0 M), I(rFe- and K;FetCNtr. respectiveiy. Gold films of 200 A thickness (CN)6(0.01M), and ItFetCNro(0.001 Ml. Coution:potassium were used. When varving the concentration of each component, ferricvanide is l.ight sensitiue. The photodecontposedproducts the concentrations of other components were kept unchanged contain free cyanide.la This etching soluti,onshould be stored at the values as in the standard etching solution.
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