02-a Wet etching - Caltech Micromachining Laboratory

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Vol. 123, No. 1% ,'" .'. -CHEMICAL ETCHING OF SILICON' " " .,1t"'1--2~}-
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~ 0 lower portion of each plot, the' constant etch-rate traduction -of a catalyat, and the BQ etch mil
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contours of both systems run identical course. over
a considerable range of compositions. In ~ upper
to be quite useful in device work. .
--1-n summary, the behavior of the water d
~d
tnd
portion of each plot, however, the contours diverge
immediately upon the addition of diluent. The note-
Acetic acla-ailu{ed etches based on HF arid
_qualitatively verv similar J~~is governed
, u
ate
worthy feature here is that the contours for the acetic
acid system run parallel with lines of constant nitric
considerations. The differences are DrimJ .
ta Ive an -leranc
) a acid concentration and the plot in general bears a f~r acetic acid than for water.~his does not
nd
striking resemblance to the high concentration acid
system shown in Fig. 5rThe eQuivalent etching systems
the acetic acid diluted system is less critical
water diluted system, with respect to composi
eo HF HNO3 + diluent are seen to show- a much-merely that a greater amount of diluent may
te ~ter tolerance tor acetic acid than tor water as the ~ before the system becomes critical with r
the diluent. diluent or composition. Except in the high H
ha
?~~~n this section it was shown that in the
high HF region the etch rates are affected by the oxiboth
systems are initially indifferent to the ad
diluent, and the etching behavior is unaltered
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dizing ability of the nitric acid and the reaction is
critical with respect to the coupling of the generated
com}:osition is reached where the direction of
tours begins to change. Then feedback m
a c
catalyst. Th tolerance s stem for ace ic acid
-is thus i!:! ~~c~rd ~jt!! the known elUlanc= oxidizing
come into play and the behavior of the sol
comes erratic. It is significant that the etc'
e power of nitrIC acid in acetic aclQ over tnatof nitric acteristics reflect the critical depen.denc:e of.
xtr acI In e a I Ion oes rate on the local catalyst concentration m
.r no re uce the oxidizing power of the nitric acid until so that a favorable geometry is obtained at the
no a fairly large amount of the diluent has been added. of erratic or unpredictable behavior of the
vol Therefore, the rate contours remain parallel with lines Any attempt to make the etchant noncritical,
.1 s of constant nitric acid over a considerable range of an increase in temperature or the excessive
.added diluent. of catalyst, will simultaneously produce. a
q The same explanation is valid in the high HNO3 the geometry ot the specimen to that ch
d region. The sudden dependence of the etch rates on of a diffusion-limited mechanism.
ly
; S
the amount of added diluent has been associated with
a transition from an oxidation-independent mechanism
In the high HF region, the solution is criti
respect to the addition of water, because th
co to an oxidation-dependent mechanism. The effe r..it f amount of diluent is already contained in
s. the acetic acid is primarily to defer the onset of centrated stock HF. The use of acetic aci
oxidation-dependent mechanism until
amount of this diluent has been added.
a very e region may serve some purpose in making
less critical, and some useful sizing etches
The striking similarity of the water diluted etching
system and the acetic acid diluted system, coupled with
found here. It is surprising that the commo
containing acetic acid, such as CP4, were n
~ the nature consistency of the diluent, of the provides etch rates an additional irrespective argument of the ulated addition in of this acetic region, acid but seems in the to region serve wn
in favor of the diffusion-governed mechanism pre- purpose. (The amount of acetic acid in CP4 .
viously proposed.,~he fa,ct tha~. t!!e et£hrates-are
affected b the enhanc ctivity of the nitr"
not less than critical.)
It is often said that acetic acid moderates
in acet cid can only be ex a on basis th action and slows the rate. This is not true in
!!!~remffusion limited-:oor- r-e-a:-ctlvit): 20~ nit~ic acid region, and in. ~h.e high hydr°fi.u
The acetic acid diluent etch composition plane may
also be subdivided into general areas having different
regIon. How~ver,.m the vIclmty of the maxI
rate, the acetIc acId d°.es have the alleged effec
geometric etching characteristics (see Fig. 9). The It. seems on the basIs of the small amo~nt
principal difference between the two systems is that mat~on we h~ve present~d th~t ~y etching
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the areas in the acetic acid system are more extensive. attainable with an acetIc acId diluted etch
" .duplicated with a water diluted etch. The
In FIg. 9, we have IndIcated several commonly used diluent is thus a matter of individual preferenc
~ etches. The BJ etch and the CP4 etch are seen to be
~ located in area C, and they should have similar properties.
On the basis of our interpretation of the reaction Applications .,
ck (~~~~~~~niS~ in this area, we would venture that ~he A process specification for the fabrication of a semi- -' :
..romlne .the CP~ .formula seryes no u~eful function conductor device usually includes one or more ~>-~: '.
e etching of Sllicon to obtain a ~pecIfic geometry. ical etching steps. The etching operation is employed :';::r~-:-
The sa~e comment may be made WIth respect to the for the attainment of certain objectives such as the' .:~;~
~m ~he Purdue etches. There may be factl?rs, reduction of reverse current, machining to size, or. the .:.. .;;'"
""1h~,which we ~ave not ev.a1uated, such as.resulting removal of unwanted material. The diversity of cirelectrIcal
propertIes of devIces etched WIth these cumstances under which the etching is performed,
,
{ I ~!~:!o~~~ :~~~. ~g_~t__~h~~_~nee~. for the. addit!,:,.e' coupled with the infinite variability of the etching
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~~ ""~~1JOlnt of .v~ew. ?OWev~r. the use of an a~~ formulas has created a fertile field for the sorcerer
~ \ c tive in sur}, ,.nmpn~itinn~ ii po;ntl°i~ ~ and alch~mist to practice their arts. It was inevitable
The 111 etch is interesting because it lies in a region that some excellent etching procedures should have
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where the resultant surfaces are specular, but where
the corners tend to peak. It should produce better
been discovered in the course of time. However, these
procedures were applicable only to the attainment of
geometry retention than the BJ etch and it should be limited objectives with specific devices. Every situamore
reliable in the absence of addea catalyst than its tion became a separate problem which had to be coped
-E neighbor, the BQ etch. The latter is very critical, with with on a hit-or-miss basis, without the guidance of
respect to slight changes in composition, and with re- scientific principles, In such a state of affairs, a method
spect to the silicon surface. Infinitesimal composition for the selection of the optimum etch to accomplish a
changes such as the addition of one drop of acetic acid given objective appeared to be extremely valuable.
to 20 cm3 of etchant, or the use of 48.5% HF instead A key to the selection of an etch is an understanding
of 48.9% HF to prepare the solution, have been ob- of the etching characteristics of a solution as a function
-.served to cause the second etch rate (i.e., etching of a of the composition. The physical factors that influence
-work-damage-iree surface) to vanish. It has been the etching behavior have aready been discussed. It
observed that the etch rate in the BQ composition may may now be desirable to recapitulate some of the
have practically any value between zero and the di.fIu- concepts developed above and show how they can be.
K sion-limited value for that composition. However, these applied to the formulation of an etching solutioD
disadvantages may possibly be overcome by the m- having certain specific properties.
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