Extended Abstracts of the l6th (1984 International) conference on solid state Devices and Materials, Kobe, 1gg4, pp.277-2g0 SIMS Analysis and Simulation of Reactive lon Etching E.Koyama, T.Nishioka, r.lrlashiko, Dl.yoneda and s.Kawazu LSI RtD Laboratory, Irtitsubishi Electric Corp. 4-1 Uizuhara Itani llyogo, 664 Japan B-4-2 Experinental 'results The anomalously etched pattern which is beautifully symmetrical -in shape is shown in Fig.2. Surface undulation form- ed after the reactive ion etching of poly-silicon represents the anomJlous pattern. Surface residues looked like dendr ites are observed in an SEl,t (Scanning Electron Microscope) picture as shown in Fig.3. The distri-bution of these residues seem to be related to the present symmetrical pattern, that is the residues distribute preferentially at the "petal" site in the symmetrical pittern. High magnif ication SEtt picture indi- cates that the residues act as masks against the RIE as shown in Fig.4. The surface of the residues is flal and the thickness is about 35 nm. - _ This paper deals sith a sr[s ana_rysis and a physical sinulation of an anomalous reactive idn etctring of poly silicon for understanding the physical environnents of the prls-na- Processing._ The sYnn_etriE -perturbation of the pi"=-ri cylinder affected the s-urface _nigration or iiltt conpounds resulting in synnetricarly tocarizld anonalous etchi;t- ;; poly sllicon. Introduction Plasma assisted etching is one of the key elements of the VLSI manufactur- ing technology. When a semiconductor device feature reaches a I pm dimension, reactive ion etching (RIE) tias become es- sential for the fine pattern definition. However, the plasma process depends on a nultitude of parameters and investiga- tions into plasma technology as applied to semiconductor device f abr icat i-on so far are non-quantitative or semi- quantitabive in nature. Correspondingly, a better under- standing of.the physics and chemistry of plasna etching is strongly required to solve commonly observed reproducibility and reliability problems associated witi plasma ,processes. The present paper deals with an analysis of an anomilous reactive ion etching of poly silicon and ? physical simulation of the phenomenon for the better understanding of the physical environments of th; plasma processes. Exper iment The present work was done in a parallel plate reactor used in the RIE mode. The 13.56 MHz rf-power was applied to the lower electrode. nar t-i-ally isotropic etching. of poly-silicon in C2Ci2 Fq gas was carried out at I4 pa. fne- d i ame ter of the uppe r and lower electrodes were L7 -Cn and t 4 crl r respectively. The distance between the electrodes was 4 cm. The diagram of the apparatus is shown in f ig.I. SID{S ( Seggndary Ion Mass Spectroscopyl analysis utilizing newly equipped ab-ubfe *icro channel plate was done -fbr high sensitive observations of surface resid[es. FrG.1. The diagram the present of the s tudy. Hllar t' Loucr Elaotrodc RIE apparatus used in
Extended Abstracts of the l6th (1984 International) conference on
solid state Devices and Materials, Kobe, 1gg4, pp.277-2g0
SIMS Analysis and Simulation of Reactive lon Etching
E.Koyama, T.Nishioka, r.lrlashiko, Dl.yoneda and s.Kawazu
LSI RtD Laboratory, Irtitsubishi Electric Corp. 4-1 Uizuhara Itani
llyogo, 664 Japan
B-4-2
The anomalously etched pattern whichis beautifully symmetrical -in
shape is shown in Fig.2. Surface undulation form-ed after the
reactive ion etching ofpoly-silicon represents the
anomJlouspattern. Surface residues looked likedendr ites are
observed in an SEl,t(Scanning Electron Microscope) picture asshown
in Fig.3. The distri-bution ofthese residues seem to be related to
thepresent symmetrical pattern, that is theresidues distribute
preferentially at the "petal" site in the symmetrical
pittern.
High magnif ication SEtt picture indi-cates that the residues act
as masksagainst the RIE as shown in Fig.4. Thesurface of the
residues is flal and thethickness is about 35 nm.
- _ This paper deals sith a sr[s ana_rysis and a physicalsinulation
of an anomalous reactive idn etctring of polysilicon for
understanding the physical environnents of theprls-na- Processing._
The sYnn_etriE -perturbation of the pi"=-ricylinder affected the
s-urface _nigration or iiltt conpoundsresulting in synnetricarly
tocarizld anonalous etchi;t- ;;poly sllicon.
Introduction
Plasma assisted etching is one ofthe key elements of the VLSI
manufactur-ing technology. When a semiconductordevice feature
reaches a I pm dimension,reactive ion etching (RIE) tias become
es-sential for the fine pattern definition. However, the plasma
process depends on anultitude of parameters and investiga-tions
into plasma technology as appliedto semiconductor device f abr icat
i-on sofar are non-quantitative or semi-quantitabive in
nature.
Correspondingly, a better under-standing of.the physics and
chemistry ofplasna etching is strongly required tosolve commonly
observed reproducibility and reliability problems associated
witiplasma ,processes. The present paperdeals with an analysis of
an anomilousreactive ion etching of poly silicon and ? physical
simulation of the phenomenon for the better understanding of
thephysical environments of th; plasma processes.
Exper iment
The present work was done in aparallel plate reactor used in the
RIE mode. The 13.56 MHz rf-power was appliedto the lower electrode.
nar t-i-allyisotropic etching. of poly-silicon in C2Ci2Fq gas was
carried out at I4 pa. fne- d i ame ter of the uppe r and
lowerelectrodes were L7 -Cn and t 4 crl rrespectively. The distance
between theelectrodes was 4 cm. The diagram of theapparatus is
shown in f ig.I. SID{S (Seggndary Ion Mass Spectroscopyl
analysisutilizing newly equipped ab-ubfe *icrochannel plate was
done -fbr high sensitiveobservations of surface resid[es.
FrG.1. The diagram the present
of the s tudy.
Hllar t' Loucr Elaotrodc
RIE apparatus used in
Fig.2. An5malous symmetric pattern appeared af- ter the reactive
ion etching of poly-siIicon.
From the SItrlS (Secondary Ion Mass Spectroscopy) analysis it is
found that the materials of the residues are mostly light elements
as weLl as iron as shown in the ion image of Fig.5. The volatility
of the residues is limited, and the materiaLs may form compopunds
such as FeF2 , Fe2 03 , or FeCIa of which vapor pressures are much
lower than that of poly-silicon.
Nucleus generation in an evaporated thin filn has well been
studied. It is concluded that some kinds of surface defects such as
Si oxides, surface con- tamination and adsorbed gaseous molecules
act as stepping-stones for aggregation of evaporated particles and
also these defects work as nucleation sites if the probability of
states exceed a certain threshold level. Stab1e clusters grow
surrounding these act,ivated sites.
Fig.4. Cross sectional view of the residues ob- served by the high
magnification SEM. The residues act as masks against the RIE.
Fig.3. SE!4(Scanning Electron t'licroscope) picture of surface
residues looked like dendrite.
Similar phenomenon can be expected in the present case directly
after the start of plasma etching of poly-Si by C2 C12F4 gas. The
shapes of the residues as shown in Fig.3 suggest the diffusion
limited aggregation (DLA) of the Fe compounds: calculated shapes of
residues based on the DLA are apparently sinilar to those observed
as in Fig.3. It is clear, therefore, that the residues are created
by the fast lateral surface migration of particles.Nucleus for the
Fe cluster formation should be activated when the plasma pressure
exceeds some critical point. The aggregation of Fe compound
particles is affected by the plasma pressure distribution. If the
in- tensities of plasma pressure vary syn- metrically, the growth
of the Fe com- pounds would also vary as the symmetrical
pattern.
Fig.5. Ion image of iron (M/e= 56) SIl,lS (Secondary Ion Mass
equipped with double micro
observed with Spectroscope ) channel plate.
Plasma simulation
To nake clear the mechanisn of theanomalous pattern generation,
distribu_Eton of plasma pressure is calculatednext. We consider a
helical magneticf ield in th:. plasna cylinder ci-iiyingcurrent in
the reactive chamber. Thepl-asma cylinder expressed in;
cyiinori_cal coordinates (r-,? ,zl is app#x-initea magne tohydr
odyn am i ca I Iy .
Eguation which describes the pertur_bed helical flow is expressed
as
_ . The equilibrium condition ofcan be expressed as
V p= (t: * jt, t" * G'l)
plasma
(71
where p is ttre total plasma -pressure, jand j denores unperrurbed
inti--il;;;iu"acurrent densities., respectively..--- --- Now I can
be ajsuned to be verylarge in an act-ual jf **u ehamber, andhense
is considered'i"- u" .;;;i;- l"ro.In thi s case , following
approxirnationscan be made:
h+2|,^/e, p+l tI -+ t r0 -,+! rf -{ r 1 = tJ"n (€ r )
4i* (t'/p 2o(s/prli= o
S - f (r) cos(mgl
{=t-tZtl ll/2r, (:/Ft P/art+t;/rzl (a'/oe.lpr.i
l"i#:i'ni.3nl'e'-',<;: d. '= 2frh 'vv
T.h.n_Eqs.(5) and (6) canthe form as
Br = 0 Bg =B o 2l^/s Jr (Er) 83 =Bq (l+2F,( (Jo (e r) _1) I
related relation
sin(nf, )cos(ng )
satisfies the following
(L/r ) (rf , /pl'+ ( E'/p -ZaE/rz _mz/r r 1 f=0 (3)
The prirnes denote derivatives withrespect to t , and m an order.
Thesolut ions of -Eg
( 3 ) u." elpressed in termsof Bessel's furictl"n,
rn nqs. (8) alg $) ,a/ag=O/er is assumed tobe zero and I j,B] -is'
srnalr to t" -;;i"._ted. Then unperturbed plasma pi"""i,r" pcan be
expressed as
p(r)=po* Bly"2p/ee (2,u-€l (Jo (tr)-r) (rop
Each component .of. y p (F is the perturbedprasma pressure) ii
exp?essed as forrows.
aila r=Bo lrrzl^Jelg,,il+Jr JJin'+ [-rntr, Jn/ (2r]r'+ 2J l Jl/(e
rl I lcos (rng)
L/r ai/n, =-r. /tto 2FJt2JJt Jn/(€r)2 + J,'Jn/(€r)l sin(nglThe
distribution of magneLic field
In th. unperturbed and- perturbed statecan be written in the
form
Bn=0 Ig = 9o !{r+h(Jr (erl-er/Zll BE = Bo (l+tr(J, (€rl_1) )
g' =- 3/.t f ^ (r) sin (n o )pv =-6/.F !i(r) cosir e iBs =as/p f*
(r) cos ir a i
where the quantity h isparameters € and o( by the
eEh z=-r,o /po 2/a J d 1J,' el E rl I sin (n f )
Here, Fo is .the magnetic constant ofvacuum and h is a constant.
Terms insidethe btackers I 1 are smalr ";;;;"j;; to(JrtJrl+J, Jt,)r
ahd can be n.gI;;t"aresulting in the following relati5n:
aimr=Bo /y,Zy't1Jf J;+J, .rl. ) cos (nf) (t2t
h= 2/t (t(/^-o() +2dl^)
Total plasma pressure p described as
P=Po +s| b, pr/i ( (zl.q (Ja (a r ) _tl +327s"t, iarl.ri'ierl cos
(nf) ) (t3)
Here Po and Bo €lf€ constants.
279
Fig.6. Calculated contour map of plasma-Pressure intensities
expected in the RIE chamber '
Comparing the radial and azimuthal distribulion in the anomalous
fringe with the calculation it is concluded that the actual
anomalous pattern is characterized by the perturbed term of the
total plasma Pressure.- Furthermore perturbation of plasma should
vary with time bY a certain frequeney w. So the plasma Pressure
which chaiacterize the anomalous pattern can be written as
F=e, /y"ilt'g, (tr)JJ (€r)cos(n9)sin(urt) (u)
The pattern generation should be influen- ced much more with larger
plasma density, and the effective plasma pressure which
characterizes the anomalous pattern is finaly expressed as
i.fi = lilst, t,,,t).1
Sumnary
The perturbation of the Plasma cylinder affected the surface
migration of iron compounds resulting in symmetri- cally localized
masks against the plasma etching. The physical environments of the
plasma processing especially concerning the plasma distribution has
thus been made clear, and a more sophisticated plasma etching
system may be constructed through these kinds of PhYsical
considerations.
Acknowledgment
The authors are indebted to Dr.H.Oka and Dr.H.Nakata for their
encouragement- Thanks are also due to T.Yasue for help- ful
discussion.
(rs1
The order m should be integers' -