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' -