02-a Wet etching - Caltech Micromachining Laboratory

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IV. Etching Technology ...'- ~:~Z~",
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B, Schwartz _:".~.'~~".."(.
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Bell Laboratories, Murrau Hill, New JerBe'j 07974 :;';;'-:~.{~
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Galamar Industries, Palo Alto, California 94304 .-',~ :.:";-:
ABSTRACT
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The etching of silicon in HNOa-HF based systems proceeds by a sequ.en- --.~ :,.-#~~~.
tial oxidation-fo.11o",:ed-by-dissol~tion process. ~ ~ose composition ~~~ns 7 .:.~~;:.~::;;:':
where the solution 15 very low m HNOa and nch m HF, the rate-limiting .' .: ~'~~.
process .is the oxidation step. Conseq~ently, electro~ concen~ration, surface '- ~-~: ~t'
,?rientatlon, crystal defects, and c~t,alYSIS by lower I?Xl.des ~f .nItrogen play .an -;::;.:1,
lInportant role. In those composItIons where ~ 15 m 1imi~ su?ply, dis- :..~~"'-
solution of the formed oxide is the rate-contro11mg step and diffusIon of the .::;.~.:
complexing fluoride species is the important factor. Therefore, cr,:stal ori- :::.::
entation and conductivity type independ.ence as well as hydrodynan1lc co~t;rol -);.;~,;'..
are the consequences. In order to mearongfully select an etching COmI;>oSltlon ,; :'"-' --f:,
to solve a specific processing problem, it is necessary to un.derstand this CO;m- .--""t.:-~~-:~
position-mechanism interaction. Corollary with the mec~m under.standing, -,.; -.;.:~:~'~)J
sample geometry effects have be~n follow~ .as a functIon of solution c«;,m- ., -;'.".;;
position. The HF-HNO3-H20 solution coml?osltion pl~e h~ been characte~ '.::OJ:.[~:
into various regions where the two haslC mechanlSms mteract and specific ,:,::;:'d:;~
procesing utilization is shown. Similar results a~ shown fo~ the system HF- .:~..;.:i
HNOa-HC2H302. In addition, a number of particular etching problems .are ::!;':",,?
posed, and solutions offered, that make. u~e of the~e composition characterl;U- :..,: ~'._~:. '.
tions, a.nd show how one can use theIr Information to solve other practical .'. :.~-~~-;~
processIng problems. ~';'"
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Most of the data on silicon etching that one can not controlled with respect to any partic.war c~ :-::~~:i, 1
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find and kinetics the literature of the involve dissolution studies process of the (1-4). mechanisms There plane. means that Also, the the exposed dice were surfaces used contamed as. receIved, saw damage. which :-:',:;i-" :'~~.~:-!-"
are a few examples of practical applications, but they All dice were etched three times, and each etch was ' -;.'. :--/
deal primarily with crystallographic aspects of the performed in a fresh portion of the same solution. '-.. ;~;:
etching, e.g., defect delineation (5) or anisotropic Since each etching period had been chosen to remov~ :.:::-::-1,;
crystal plane etching (6). Because of the sophistication 4 to 6 mils from the specimen, it may be assumed that ;.; ~l::~
of present-day silicon technology, it appears to be the work damage had been removed after the first : _.:~.::
desirable to be able to control the geometrical aspects etch. The third--.etch-was p~rformed with a few ~:-- .~:.t-,=~of
a silicon slice, e.g., from sharp, possibly peaked gramss=~ NaNQ;2~tO the ~~ssi-PJL ':-:.,:;'f;..,
corners and edges to smooth and rounded edges and a lC u oses (1). er eac etching step, the '-:~'.~~~:;'.;,
corners, merely by controlling the chemical etching sample was rins m istilled water, dried, and mea- .._",::;.::..~..
environments. It would, therefore, seem appropriate sured for dimensional changes with a micrometer to '- ,~ ~:,~
to present some d~ta on t,h.e geometrical effects ob- :t. 0.05 ~s, and then ~e geometry of the specimen -~J~;~ . '
-served on the etchIng of silicon rectangular parallel- was exammed under a IDlcroscope. -~.:, ,O;i:,:.;
e~ipeds in solutions of HNOa, HF,. and H2O with and Concentra~ed hydrofluoric acid, as normally SUPP1!ed. .:... "~-.~; ,
without H~2H302. 'I;'hese ob~ervatlons ~ere m~de ,at may vary m concentration from 48 to 52 weIght :~':'-':~"'~
the same tiJ?e as t~ preVIously ~u~lished kinetics percent (w/o). SimnA1'ly, nitric acid may vary betwe81.~
data were beIng obtain"ed (1,2), b';lt 1: 15 onlY,recently the limIts of 69 and 71%. In order to estab;1ish control~"iI!1
that many requests for geometnc info~tio~ h~ve over the composition of the etching solutions, it was"':.:4=1'!~made
us aware of the usefulness of disselDlnating necessary to standardize the stock -acids from which .;.~"'::~..
these configurational data. the solutions were to be made, This was done by .~;f~
In orde,!, ,to m.ake the picture more ~omplete, s°!De titrating samples of the acids against a standard sodium ;~:"~~:~1
of the ongmal Is~-etch-rate figures will also be m- hydroxide solution. Potassium acid phthalate was used ;. .4
cluded, for companson purposes, as the primary standard for the sodium hydroxide. .I
~_. The bottle of hydrofluoric acid analyzed first at '
p~
Lxt'golmental Proced~re .49.25%, and this concentration was accordingly se- -;
The technique used for the etchm~ an~ thlck~ess lected as the standard HF reagent concentration. All ..-
monitoring has been adequatel~ descnbed m prevlo~s subsequent bottles of hydrofluoric acid were adjusted '.'"
publications (1-3). However, m order to make thiS to 49.25 :t 0.05% by the addition of 60% HF or water.
paper useful, we will have to repeat- some of. the as dictated by the chemical analysis, In a similar ."' ,
experimental detail, but will stress only the sall~nt manner, the value 69.51 % HNOa was established as " '::.
features of the sample and solution preparatIon the standard nitric acid concentration and all sub- .::':
methods and h°v.: the geometric aspects were fo11o",:",ed. sequent stock bottles of nitric acid ~ere adj\,1s.ted to _:~ 1
The sample dIce were rectangular parallelepIpeds this value :t. 0.05% by the appropnate addItIon of .
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(0.125 X 0.125 X 0.025 in. on side) cut from n-t~pe water or 90% HNOa. In a second series of experiments .
silicon slices with resistivities about 2 {l-cm. The dice designed to cover a larger area of the composition . ~.
were cut so that the large-area ~u,rfaces were along range, 90.45 :t. 0.05% HNOa and 59.75 :t 0.05% HF -.: j.
the (111) plane while the remaInIng surfaces were were the standard reagents used. In a third series of .-~ '~':.f~
.Electrochemical Society Active Member. experiments, glacial acetic acid was used as a ~uent, .:I.:~!~~_~i
1 All of the daU used In this paper were obtained whfie the and the only water present was that found In the -.~
authors were employed at Hughes Aircraft Company. original HNOs or HF reagents. ..;;.':-
Key words: I1l1con, etching, geometry control. i' : =-"'( ,", ~
1903 .'.":~';~,'~
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