02-a Wet etching - Caltech Micromachining Laboratory

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"...-~-: 1804' .' -J. Electrochem. SoC.: SOLID-STATE SCIENCE AND TECHNOLOGY December ,".:: , :." '- --. ~_£~~~~, : ~ -.The etching was done in a Teflon beaker 1 in. in -: : ~ ."-:'~;'-.:.- -diameter and 2112 in. deep. Agitation was provided by ~ ::.' an electric stirrer equipped with a polyethylene paddle. ;j ,;;-=.~'~ The dice were etched one at a time in 10 cm3 of :! .6 ..,1:':~: '. solution, and duplicate results were obtained by re- ~ ~;:-~~' ,~'- peating the en~ire experiment. To quench the reaction ItJ -:. 1:;'.: at the proper tIme, a large volume of water was poured into the beaker. The etching technique was later mod- ~ 12 :I: '::','!':; .;~. ified when the ~igher concentration reagent~ we:e used. Here the dIce were etched two at a tIme, In ~ "j':. -20 cm3 of solution, while encased in a small perforated ~ ". ':. polyethylene basket. The basket and dice were agitated ~ for the required time in the solution, then immersed z: in a beaker of cold water to quench the reaction. The I- agitating action consisted of approximately 150 vertical strokes per minute, with the etching jig rotated ". betv.:een th~ thumb and middle finger as rapidly as 15 20 25 possIble d~Ing the up and do":",n strokes. TIME (SECONDS) All etchIng was performed In a constant temperature (A) bath regulated at 25.00. =- 0.02.C. Unfortunately, .: .the heat transmission of polyethylene and Teflon is .~: i:';: so poor that the temperature of the bath could not be realized in the .etch solution, and even after long Vi = :';.:;: .periods of equilibration the maximum initial e~h ~ 12 :::; ~J temperature was only 24.70.C. .AS; a further Ca:n- ..,.;; ~ lequence of the poor heat transInlSSlOn of the etching ~ z ":": ~.~~ container, the more rapid etches were run under essentially adiabatic conditions. ~ (.) 8 ~~;;,::: It was realized early in the study that in some etch ~ ..:0,: ., ':::', compositions the rate of attack on the die would be so high t~at if t~e specimen were left in the solution for ~ 4 ~:,.,,;.,~. one mlnute, eIther the sample would be etched away x: '.~:,,::.-i:Ompletely or it would be so thin that it would be. I- ,:_.:' :j:' '. was very made difficult to choose to handle. etch For time this for reason each composition an attempt 90 120 150 :",-:..such that only 4 to 6 mils of material would be re- TIME (SECONDS) ,4:::' moved. The etch rate was then linearly extrapolated (8) of "', ,' c L - loft d f t. f d .- II . :~;;.~'; '.:, to tobtain 1 t... the 1 11 min t value. ted b The Fi justification 1 h th for h the F... 1 C hang - ~ rn ~n~.s peas a unc Ion 0 ,. ~-.,: -:~.:, ..', .ex rapo a IO~ IS 1 us. ra y g. w ~re e.c ange posure time: (A) solution composed of 55% HF. 45% HNO3; (I) ::". .In two sam.ple different thickness etch 15 composItIons. plotte;~ as a It fu~ctIon.of 15 qwte tlme.for posslble solution composed of 30% HF . 70% HH03. ~:~-' .that for compositions where the etch rate is very high, ~~,--'.the linear relation between the amount of material -The solution composition field was subdivided into -:,~:" removed and the time is not valid for the entire a number of evenly spaced points representing dif- .f ."..:.- minute, since the reaction is known to be strongly ferent compositions. The amounts of each reagent .~:j.:~:,: ~c exothermic and to proceed under adiabatic conditions. corresponding to each composition were then calc:-u- ~';~,'. Nevertheless, it is believed that by limiting the etch lated, and the solutions were prepared by puttIng ;~J,~;~ ~. time to minimize the effect of changes in experimental together the required amounts of reagents. The hydro- 7;.:.:;; -: conditions, one could obtain numbers by linear extrap- fluoric acid was weighed out directly to =- O.Olg. but i",:~~ olation which are reasonably valid. The change in the weights of nitric acid and water were converted to ~:~' ".: concentration of the etch system during the experiment volumes and measured out in a burette to: 0.01 cm:J. ,~:..'{:~:.- was also considered an important factor. However, -.~.;;:.. if one allows only about 0.005 in. of silicon to be etched Experimental Results ~'::t'~':~' --away. during the operation, t~ would. .correspond to Figures 2 and 3 represent the iso-etch-rate contours :",:~.'~:";.: reactIng only about ~.4 mequlV. of SIlIcon, and the determined for the normal and high concentratioD '~;.'~;:'~' ;"";:-;c' '; change action would of concentration be most of 10;0, the which reagents is a due negligible to re- !-o
l eaN ;,..~;;1.,: -.~~.~ ~ Vol. ~, o.~. '.:J~-~."~:;~ ..'.,~...';,~.:,."'-""Q~~~IA --".~~-,:",;',::£~ CAL.'. "';i~~~::'" ETCHIN~ DF.SILICO~ ,'.' c-~', ~t':: ;;:'~..:~~""'::.r.. ~~. '- ,~~j~':';""~:i-.7 "i;--i,.:..",l9.Qb,;:---~.:., .~.,.,~" ..'~ high H NOf/-CATALYZED RATES faces -CATALYZED RATES 80 Hz (~46%) HF (59 Fig. 3. Curyes of constont rate of change of die thickness (mils per minute) as 0 function of etchant composition in the 60% HF. ~ 9O~o HNO3 system; the effect of added catalyst (NaN~) is shown 35 os the dashed lines. 40 reagents [to obtain the etching rate on either of the (Ill) faces, divide the listed value by 2], with added . water as the third vertex. Figures 4 and 5 are the same data, illustrating the shape of the etch-rate surface as a function of solution composition. The various I regions and the mechanisms involved in the dissolution process were adequately covered previously (1-3). ..We shall just note that in the region around the HF vertex the reaction is primarily oxidation-reduction F' 5 controlled, and in the region around the HNO3 vertex, Ig. . the reaction is primarily diffusion controlled. A con- the 60% sequence of the diffusion-governed kinetics is that different all etch at crystallographically the same rate. For oriented two widely surfaces different' should - . etch compositions, the rate of attack on the (Ill) plane has been found to be identical to the rate of attack on the (110) plane. In preferential etch systems (e.g., 0 PEAKED CORNERS NaOH), the (Ill) is the slowest etching plane and the 6 EDGES (110) plane is one of the fastest etching planes. m SQUARE CORNERS Figures 6 and 7 are plots of the same solution com- ~ 6 EDGES position fields, respectively, but in the latter two m ROUNDED ~N figures are shown the observed geometry effects on the 6 EDGES initially rectangular parallelepipeds.3 In these two figures it is seen that the geometry of the etched I Although solid tines are drawn from which sharp d!8COntinu. Itles could be inferred, this Is not the C&.se. Tbere are ended regIons in going from one area to another, but this would make Wustration well-n1ib imposaible. ~, ..etchant ~ c ,I co HF (492 H~(6951%) Hz Fig. 6. 0 PEAKED EDGES 6 15 (0.6) CORNERS 20 ~ SQUAREDGES 8 EO! CORNERS m ROUNDED EDGES 6 CORNERS -CONVEX -SHAPED LENS H2O ,. Fig. 4. Etch-rate surface as a function of etchant composition in Fig. 7 ~ 48% HF-70% HNOa system. the etch ~ --~,:.- ;"~-
Page 5 and 6: Table 9.1 Compositions of Commonly
Page 7: U! 's=-!,.- - r--- "~ t' 1--, ' Tab
Page 10 and 11: &~s t*c,k: ;] The H2SOc-H2O2-H2O Sy
Page 12 and 13: New Journal of Physics 5 (2003) 100
Page 16 and 17: .-.~~C.Tocnem. ~~.: ~LJD-STATE SCIE

02-a Wet etching - Caltech Micromachining Laboratory

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