investigation of inp etching mechanisms in a cl2/h2 ...€¦ · investigation of inp etching...

Investigation of InP etching mechanisms in a Cl2/H2 inductively coupledplasma by optical emission spectroscopy

L. Gatilova,a� S. Bouchoule,b� and S. GuiletLaboratoire de Photonique et de Nanostructures (LPN),CNRS, Route de Nozay, 91460 Marcoussis, France

P. ChabertLaboratoire de Physique et Technologie des Plasmas (LPTP), CNRS, Ecole Polytechnique, Route de Saclay,91128 Palaiseau, France

�Received 9 April 2008; accepted 15 December 2008; published 12 February 2009�

Optical emission spectroscopy �OES� has been used in order to investigate the InP etchingmechanisms in a Cl2–H2 inductively coupled plasma. The authors have previously shown thatanisotropic etching of InP could be achieved for a H2 percentage in the 35%–45% range where theInP etch rate also presents a local maximum �J. Vac. Sci. Technol. B 24, 2381 �2006��, and thatanisotropic etching was due to an enhanced passivation of the etched sidewalls by a silicon oxidelayer �J. Vac. Sci. Technol. B 26, 666 �2008��. In this work, it is shown that this etching behavioris related to a maximum in the H atom concentration in the plasma. The possible enhancement ofthe sidewall passivation process in the presence of H is investigated by comparing OESmeasurements and etching results obtained for Cl2–H2 and Cl2–Ar gas mixtures. © 2009 American
Vacuum Society. �DOI: 10.1116/1.3071950�
I. INTRODUCTION

Photonic device fabrication generally requires high-aspect-ratio etching of InP-based heterostructures. Wet etch-ing allows for obtaining minimal material damages, but theetching profile is difficult to control. A dry-etching processthat can produce highly anisotropic profiles and smooth side-walls free from undercuts or notches becomes necessarywhen key elements such as deeply etched facets and mirrors,deep ridge waveguides, ring resonators, or micropillars cavi-ties have to be defined. Inductively coupled plasma �ICP�etching of InP has been widely developed for this purpose inthe past years using Cl2 as the main etching gas. Pure Cl2atmospheres generally leading to significant undercuts,1,2 ad-ditive gas have been added to achieve anisotropic etching ofhigh-aspect-ratio patterns. Cl2 /CH4 /H2 chemistry has beenproposed to anisotropically etch InP-based heterostructureswith both electron cyclotron resonance and ICPtechniques,2–4 considering that CH4 could balance the re-moval of In and P elements and introduce some polymer-induced passivation.2,3 N2 is considered as a strongly passi-vating gas due to nitridation of the InP surface, andanisotropic etching has been reported in Cl2–N2 �Refs. 5 and6� gas mixtures. Rommel et al.1 proposed that the addition ofH2 to a standard Cl2 /Ar process balances the chemical andphysical etch, and that the etching of InP/InGaAsP wave-guide heterostructures with a high degree of anisotropy canbe obtained in this balanced regime when all epitaxial layersare approximately etched at the same rate. In a previouswork we showed that deep etching ��5 �m� of InGa�Al�As/InP heterostructures with smooth and vertical sidewalls could

a�Also at LPTP, CNRS–Ecole Polytechnique, Route de Saclay, PalaiseauFrance.

b�
Electronic mail: [email protected]
262 J. Vac. Sci. Technol. A 27„2…, Mar/Apr 2009 0734-2101/2009

be obtained in Cl2–H2 chemistry with no additive gas.7 Wehighlighted that the H2 percentage in the gas mixture was animportant parameter to control the anisotropy, in a similarway for both bulk InP and InGa�Al�As/InP samples. We alsoevidenced that using a Si wafer as the sample tray was es-sential to obtain smooth and vertical sidewalls. A high InPetch rate �900–1300 nm/min� together with a good selectivityover SiNx dielectric mask as well as a smooth and verticalprofile could be obtained in the 0.5–1 mTorr pressure rangefor an optimized hydrogen percentage of 35%–45%. Moredetailed studies, using ex situ energy dispersive x-ray spec-troscopy �EDX� coupled to transmission electron microscopy�TEM� revealed that this high etching anisotropy in Cl2–H2

plasma occurs due to the formation of a Si-rich passivationlayer on the InP sidewalls, when a Si wafer is used as thesample tray.8

In the present study we have used optical emission spec-troscopy �OES� and Langmuir probe measurements in orderto investigate the InP etching mechanisms in a Cl2–H2 in-ductively coupled plasma with a Si wafer used as the sampletray. This configuration corresponds to most commercial ICPetch systems having an electrode diameter of 4 in. or more,and used to etch InP samples with typical dimensions of 2 in.or less. The actinometry method has been employed to moni-tor the etch products �In, Si� and the reactive radicals �H, Cl�in the gas phase when the H2 percentage �H2%� is varied inthe gas mixture. It is evidenced that the etch rate evolutionwith H2% showing a local maximum in the 35%–45% range,is related to a maximum in the H relative concentration thatpartly compensates the decrease in Cl relative concentration.The H and Cl behaviors are compared to the results of asimple kinetic model, along with Langmuir probe measure-ments of the plasma parameters �electron density and elec-tron temperature�. The actinometry method is also used to
compare the Cl2–H2 chemistry with the Cl2–Ar chemistry.
262/27„2…/262/14/$25.00 ©2009 American Vacuum Society

263 Gatilova et al.: Investigation of InP etching mechanisms 263

Anisotropic etching of InP is hardly obtained with the latter,and we discuss the possibility of an enhancement of the pas-sivation of the InP sidewalls by silicon oxide depositionwhen hydrogen is added to the gas mixture.

II. EXPERIMENTAL SETUP AND DIAGNOSTIC

The study has been carried out in a Sentech SI 500 tripleplanar spiral antenna ICP etch system.7 The reactor chamberis made of aluminum. The ICP source �rf of 13.56 MHz� iscoupled to the plasma through an Al2O3 ceramic window.The InP samples transferred to the reactor chamber via aloadlock are deposited on a 4 in. carrier mechanicallyclamped above the rf-biased �13.56 MHz� electrode with anAl2O3 ceramic clamping ring. A 4 in. silicon wafer was usedas a carrier in all the experiments.

The InP samples of typical size 7�7 mm2 were pat-terned with a silicon nitride mask forming 2 �m widestripes and submicrometer diameter pillars in order to esti-mate the etch rate and check the etching profile. The plasmaconditions were fixed to those optimized in our previouswork:7 800 W ICP power, �140 V dc bias, 0.5 mTorr pres-sure, total gas flow rate of 28 SCCM �SCCM denotes cubiccentimeter per minute at STP�, and electrode temperature of150 °C. Only the Cl2 /H2 mixing ratio was varied. Thesamples were nonthermalized �i.e., not glued to the 4 in.carrier�.

The plasma phase was diagnosed using OES and Lang-muir probe measurements. An rf planar probe9 installed onthe reactor walls was used to measure the positive ion currentdensity. Electron density and electron temperature were mea-sured using a rf-compensated cylindrical Langmuir probe,which could be placed 5 cm above the wafer carrier andpositioned close to the reactor center. For OES measure-ments the optical emission of the plasma was collected by anoptical fiber coupled to the entrance slit of the Shamrock303i spectrograph, through a quartz window in the reactorchamber wall. Two diffraction gratings of 1200 lines/mmblazed at 300 and 500 nm were used in order to acquire thespectrum in 250–400 and 400–800 nm region, respectively.The spectral resolution was 0.06 nm. An optical filter wasused to avoid the overlapping of orders in the high-wavelength range.

The Cl and H atom relative concentrations were estimatedas a function of H2 percentage in the gas mixture using theoptical emission actinometry method. Argon gas was chosenas the actinometer. The default spectral lines used were theBalmer-� emission line �H��n=3�→H��n=2�� at 656.3 nmfor H, and the line at 754.7 nm for Cl. The atomic emissionlines corresponding to some reaction products have also beenrecorded: 288.16 nm for Si, 451.3 nm for In, and 253.6 nmfor P. The integrated intensities of all these lines have beennormalized by the intensity of Ar line at 750.4 nm. The 750.4nm Ar line is often chosen since it is virtually unaffected bythe potential contribution of metastable levels to the emis-sion from electron-impact excitation of the metastables. Themain advantage of actinometry is its simplicity, which makes
it easy to implement in a clean room environment mainly
JVST A - Vacuum, Surfaces, and Films

dedicated to III-V device processing. However, this tech-nique relies on many assumptions to be discussed, which isnot the case for more direct spectroscopic techniques such asabsorption spectroscopy10 or laser induced fluorescence�LIF�.11 We will thus discuss the two main conditions thathave to be fulfilled for actinometry to remain valid: �i� theproduction of excited species X� �H�, or Cl�� by electron-impact dissociative excitation of the parent molecule �H2, orCl2� must be negligible compared to the production of X� bydirect excitation of X, and �ii� the variation of the electron-impact direct excitation cross section for the species X �H orCl� should follow that of the electron-impact direct excita-tion of the actinometer �Ar�, so that the ratio of the excitationrate constants kdirectexc

X /kdirectexcAr remains unchanged in the

range of plasma parameters explored. This second point isparticularly important if the electronic temperature Te �or theelectron energy distribution function� varies in the experi-ments.

Let us first examine the first point. When considering H,the actinometry will be valid if the following is satisfied,

r0 =kexc-dissoc

H�

kexc-directH�

�H2��H�

� 1,

where kexc-dissocH�

and kexc-directH�

correspond to the rate constantsof the two reactions:

H2 + e → H + H��n = 3� + e �kexc-dissocH�

� ,

H + e → H��n = 3� + e �kexc-directH�

� .

Recently, Lavrov and Pipa12 revisited the calculation ofthe emission cross sections of H� and H� Balmer lines and ofthe corresponding rate coefficients for both direct and disso-ciative excitation by electron impact in plasma. For the H�

line and electron temperature values lying in the range from6 to 9.5 eV corresponding to our experimental conditions,the rate constant for direct excitation lies in the range from3.7�10−10 to 6.7�10−10 cm3 /s, and the rate coefficient fordissociative excitation lies in the range from 1.2�10−11 to

2.8�10−11 cm3 /s. The corresponding kexc-dissocH�

/kexc-directH�

ra-tio lies in the range from �0.032 to 0.041, consistent withprevious calculations by Rousseau et al.13 for lower Te val-ues. We have used the values of Lavrov et al. to calculate theratio r0 and to evaluate the validity of the actinometrymethod for the measurement of the relative concentration ofH.

The same discussion holds for Cl atoms. Due to a lack ofdata concerning the electron impact-excitation crosssection or excitation rate constant for the Cl level emittingat 754.7 nm, we have used the work by Malyshevand Donelly14 on the 792.46 nm line �energy above groundstate of the Cl level at 10.59 eV� to estimate the ratio

r0�= �kexc-dissocCl� /kexc-direct

Cl� � . ��Cl2� / �Cl��. For a gas pressure of1 mTorr pressure, with Te=4.6 eV �and Tg=600 K�,close to our experimental conditions, we obtain a ratio

Cl� Cl�
kexc-dissoc /kexc-direct=20�0.0051�0.01. We will use this av-


erage value to estimate r0� for the Cl level emitting at 754.7nm �energy level above ground state at 10.62 eV�.

Let us now consider the second point on the validity ofactinometry. The energy above ground state of the H, Ar, andCl levels considered in our measurements are of �12.1, 13.5,and 10.6 eV, respectively. As will be shown later, the electrontemperature typically varies from 6.5 to 9.5 eV when the H2

percentage is varied in the gas mixture. It is thus not obviousthat the ratio kdirectexc

X /kdirectexcAr �X=H, Cl� will remain ap-

proximately constant over the whole range of H2 percentageinvestigated. This issue has been addressed by comparing thenormalized intensities of different lines of species X, corre-sponding to excited states having different energy levelsabove ground state. For the case of H, the normalized inten-sity of the H� emission line �656.3 nm, 12.09 eV� was com-pared to the normalized intensity of H� �486.1 nm�, H�

�434.4 nm�, and H �410.2 nm� lines corresponding to energylevels of 12.75, 13.05, and 13.22 eV, respectively. In addi-tion, we used two different Ar lines for normalization: the750.4 nm emission line �level 13.5 eV� and the 811.5 nmemission line �level 13.07 eV�. For the case of Cl, we com-pared the following lines: Cl 639.9 nm �12.34 eV�, Cl 793.5nm �11.99 eV�, Cl 792.5 nm �10.59 eV�, Cl 754.7 nm �10.63eV�, and Cl 452.6 nm �11.94 eV�.

III. EXPERIMENTAL RESULTS

A. Etching mechanism: etch rate

We first checked that the addition of 10% of Ar �2.8SCCM� in the Cl2 /H2 gas mixture did not significantlychange the InP etch rate as well as the etching profile, asshown in Figs. 1 and 2. The changes in the InP etch rate andin the ridge profile with H2% �defined in the following asH2%= �H2

/total��100, where H2and total are the H2

and total flow rates, respectively, with total=28 SCCM� aresimilar with and without Ar addition, and are consistent withour previous observation.7 In pure Cl2 atmosphere the InPetch rate is high ��2300 nm /min�, but the correspondingetching profile, shown in Figs. 2�a� and 2�d�, presents anundercut that is unsuitable for the fabrication of photonic

FIG. 1. Positive ion current density �stars�, and InP etch rate measured in thecase of Cl2–H2 �circles� and Cl2–H2+10% of Ar �open squares� mixtures,as a function of H2%. The dashed lines are guide to the eyes �also valid forall the following figures�.

devices. The addition of a small H2% first strongly decreases

J. Vac. Sci. Technol. A, Vol. 27, No. 2, Mar/Apr 2009

the etch rate, then for H2% around 35%–45% the etch ratecurve exhibits a local maximum. A smooth and vertical ridgeprofile is observed in the same percentage window, as seen inFigs. 2�b� and 2�e�. Further increasing H2% ��55%� leads toa further decrease of the InP etch rate and results in a roughor “grassy” surface �Figs. 2�c� and 2�f��. The slight shiftobserved in the position of the etch rate curve when adding10% of Ar in the gas mixture as the actinometer compared tothe case of Cl2–H2 chemistry without Ar, may be related to adilution effect.

Figure 3�a� shows the normalized intensity of In line�IIn / IAr� as a function of H2% in the gas mixture. Its varia-tion follows the trend of the InP etch rate curve with a localmaximum around H2%�45%. Different Si lines in the 250–300 nm spectral range have also been detected during themeasurements; the normalized intensity of the 288.16 nmline is shown in Fig. 3�a�. The existence of Si lines mustresult from the etching of the 4 in. silicon wafer used as thecarrier in the experiments. The Si etch rate was measured asa function of H2% in the Cl2 /H2 /Ar mixture and data arereported in Fig. 3�b� �the InP etch rate is also reported forcomparison�. The Si line normalized intensity constantly de-creases with H2% as does the Si wafer etch rate. This sug-gests that Cl atoms are the principal Si etch agents. As aconclusion, In and Si emission lines can be considered asgood monitoring lines for etch rate in our conditions. The Pline at 253.6 nm could be detected in pure Cl2 plasma wherethe etch rate was high, but its intensity was very weak �smallresidence time, small sample size�, and this line almost dis-appeared with the addition of H2 in the mixture.

In order to explain the local maximum of the InP etch ratecurve, the intensities of Cl �754.7 nm� and H �656.3 nm�lines normalized by Ar �750.4 nm� have been measured as afunction of H2% and the results are presented in Fig. 4. Ascan be seen the ICl / IAr ratio decreases monotonously with

FIG. 2. Scanning electron microscopy �SEM� images of InP samples etchedin ��a�–�c�� pure Cl2–H2 mixture and in ��d�–�f�� Cl2–H2+10% of Ar, for a��a� and �d�� H2% of 0%, ��b� and �e�� 40%, and ��c� and �f�� 60%.

H2%. On the other hand, the IH / IAr ratio goes through a


maximum located around H2%=45%–50%, that is near thelocal maximum observed on the InP etch rate curve. Theincrease in the etch rate seems thus to be related to the in-crease in H atom concentration, which suggests that the Patoms of the InP surface are etched by hydrogen, via theformation of PHx, and that the etching by H atoms compen-sates for the decrease of the Cl atom concentration.

The same measurements have been performed for the H�,H�, and H lines using both the 750.4 and the 811.5 nm Arlines for normalization, and the results are reported in Figs.5�a� and 5�b�. The plasma parameters are the same as for Fig.4 except that no dc bias is applied to the electrode and no InPsample was introduced. It can be observed that the evolutionis fairly similar for all these lines corresponding to excitedstates having energy levels varying by more than 1 eV. In

FIG. 3. �a� Normalized emission line intensities for In atoms �black squares�and Si atoms �open squares� as a function of H2%. �b� InP �black squares�and Si �open squares� etch rates as a function of H2%.

FIG. 4. Integrated intensities of the Cl �754.7 nm� and H �656.3 nm� linesnormalized against the Ar line at 750.4 nm as a function of H2% with �open
symbols� and without �full symbols� an InP sample.

particular the energy level of the H line �13.22 eV� and811.5 nm Ar line �13.09 eV� are very close. We thus consid-ered that the variation of the normalized intensity of the H�

lines is a good indication of the variation of the relativeconcentration of H. Note that the maximum is around �50%of H2, that is slightly higher than in the case of Fig. 4. Thismay be due to the fact that no etching of Si/InP occurs �no dcbias�, or due to a change in the reactor walls conditioningbetween the series of measurements.

The same analysis has been done for Cl, with the resultshown on Fig. 6. A larger discrepancy between the differentCl lines is observed, however, the general trend indicates adecay of the Cl atom density as the H2% increases.

Cl and H atoms are principally generated by the dissocia-tion of Cl2 and H2 molecules, the rate constants of which

FIG. 5. Integrated intensities of the H�, H�, H, and H� lines normalizedagainst the Ar line at 750.4 nm �a� and 811.5 nm �b� as a function of H2%.

FIG. 6. Integrated intensities of the Cl lines at 639.9, 793.5, 452.6, 754.7,and 792.4 nm normalized against the Ar line at 750.4 nm as a function of
H2%.


depend on the electron temperature �or electron energy dis-tribution function� and electron density. In order to quantifythe production mechanisms, the electron density ne and theelectron temperature Te have been measured using the cylin-drical electrostatic probe. Figure 7 shows the results of theprobe measurements for ne and Te as a function of the H2

percentage. The electron density falls down from 6.5�1010 cm−3 for H2%=0%, to 1.5�1010 cm−3 at 80% of H2

in the mixture. These values are in good agreement withresults published in literature.15 The electron temperaturewas found to be of �6.5 eV in pure Cl2 plasma, also in goodagreement with published data. Malyshev et al.15 indeed re-ported an electron temperature of 5.5 eV measured by tracerare gas-optical emission spectroscopy �TRG-OES� in anICP reactor for 1 mTorr Cl2 plasma. In pure H2 plasma,Paunska et al.16 calculated an electron temperature of 9 eVfor a microwave H2 plasma discharge at 5 mTorr, whichtends to confirm that the electron temperature is high for H2

rich conditions.The electron density measurements suggest that the de-

crease in the ICl / IAr ratio is due to the simultaneous diminu-tion of the Cl2 concentration and electron density when H2%is increased. On the contrary, the IH / IAr ratio first increasesdue to the augmentation of H2%, but this augmentation be-comes gradually compensated by the decrease in the electrondensity. However, to explain the apparition and the positionof the observed H maximum, loss mechanisms have to beconsidered, and a simple kinetic model has been developed.

Kinetic model

Chlorine atoms are mainly formed by electron-impact dis-sociation, dissociative ionization and dissociative attachmentvia the following reactions:17

Cl2 + e→kdiss

Cl2

Cl + Cl + e �reaction�R1�� ,

kionCl

+

FIG. 7. Electron density �full symbols� and electron temperature �open sym-bols� measured as a function of H2%.

Cl2 + e→Cl + Cl + 2e �reaction�R2�� ,


Cl2 + e →kattach

Cl

Cl + Cl− �reaction�R3�� .

The rate constants for reactions �R1�–�R3� are, respectively,kdiss

Cl2 =3.8�10−8 exp�−3.824 /Te� cm3 /s; kionCl =3.88

�10−9 exp�−15.5 /Te� cm3 /s; kattachCl =3.69�10−10

�exp�−1.68 /Te+1.457 /Te2−0.44 /Te

3+0.0572 /Te4

−0.0026 /Te5�. For our measured electron temperature values,

the rate constants of reactions �R2� and �R3� lie in the rangeof 3.57�10−10–8.24�10−10 cm3 /s and 2.94�10−10–3.16�10−10 cm3 /s, respectively, that is two orders of magnitudelower than for �R1�: 2.11�10−8–2.59�10−8 cm3 /s. There-fore, �R2� and �R3� have been discarded in the calculations.

For energies below 12 eV the dissociation of H2 wasshown to proceed almost exclusively by direct excitation ofthe b 3�u

+ state:13

H2 + e → H2��b 3�u

+� + e → H + H + e �reaction�R4�� .

The rate constant of this reaction, kdissH2 , can be calculated

from our measured values of the electron temperature usingthe cross section given in literature18 and supposing that theelectron energy distribution function is Maxwellian. Wefound that kdiss

H2 varies in the range of 5.5�10−9–7.5�10−9 cm3 s−1.

One of the possible loss channels of Cl and H atoms canbe their consumption by the etching of InP or 4 in. siliconwafer. The experiments performed without InP samples inthe plasma �Fig. 4, full symbol� showed that the size of thesamples �7�7 mm2� was too small to induce a visiblechange in the normalized intensities of the Cl and H lines.The possible consumption of Cl and H by Si etching wasverified by repeating the same measurements without dcbias, that is in conditions where the etch rate is much lower.The evolution of the ICl / IAr and IH / IAr ratios with H2% re-ported in Fig. 8 are similar to that of Fig. 4 �with dc bias� andFigs. 5 and 6 �for different emission lines�: The H curveshows a clear maximum located around H2%�60%, whilethe Cl curve shows a continuous decrease. This allowed usnot to include the loss channel of atoms by etching in orderto simplify our kinetic model. We note that the position ofthe maximum in the H curve is slightly different in Figs. 5

FIG. 8. Integrated intensities of the Cl �754.7 nm� and H �656.3 nm� linesnormalized against the Ar line at 750.4 nm as a function of H2% without InPsample and without any dc bias.

and 8. We believe that this is due to different reactor condi-


tioning; the experimental data of Fig. 8 were recordedshortly after manual cleaning of the reactor �only few etchingruns performed�, while data of Fig. 5 were recorded when thereactor was heavily passivated due to a great number of ex-periments. As will be shown later, this can change the profileof the curve due to changes in the recombination probabilityof the H radicals at the reactor walls.

The loss rates of Cl2 and H2 due to pumping, kpumpCl2 and

kpumpH2 , are deduced with the plasma switched off since we

have

d�Cl2�0

dt=

QCl2

V− kpump

Cl2 �Cl2�0, �1�

d�H2�0

dt=

QH2

V− kpump

H2 �H2�0, �2�

where QCl2,H2are the flow rates of the Cl2 and H2 gases, and

�Cl2�0 and �H2�0 are the concentrations of chlorine and hy-drogen molecules in the reactor without plasma, respectively.The loss rates of Cl2 and H2 molecules due to the pumpingestimated from the Eqs. �1� and �2� at steady state arekpump

Cl2 =47 s−1 and kpumpH2 =37 s−1 for chlorine and hydrogen

molecules. The pumping rate for atomic chlorine kpumpCl was

considered to be equal to that for Cl2 molecules, since theeffective pumping speeds of all species with mass higherthan 14 amu are about identical with our pumping system, aschecked for Cl2, Ar, and N2. On the other hand, kpump

H2

=37 s−1 is lower than the pumping rate measured for Cl2,N2, Ar, and also He. It can therefore be anticipated that thepumping rate of H atoms kpump

H may be also lower than thatof H2 molecules, and kpump

H is expressed as kpumpH =xkpump

H2 withx�1.

The main paths considered for the heterogeneous recom-bination of Cl and H on the reactor walls are

Cl�g� + Cl�s� →kwall

Cl

Cl2�g� �reaction�R5�� ,

H�g� + H�s� →kwall

H

H2�g� �reaction�R6�� ,

where the symbol �g� stands for the gas phase, and �s�—foratoms adsorbed at the surface. At low pressure, the diffusionis fast such that the loss rate due to surface recombinationcan be estimated from kwall

H,Cl� SV�H,Cl

vth

4 , where S=5340 cm2

is the surface of the reactor, V=20 800 cm3 is reactor vol-ume, vth is thermal velocity, and �H,Cl is the recombinationprobability of H or Cl atoms. The thermal velocity was cal-culated considering a typical gas temperature Tg of 600K.17,19

At 0.5 mTorr, most of three-body recombination reactionsare negligible in the gas phase. However, in the case ofCl2–H2 plasma, the volume reactions involving the produc-tion and destruction of HCl may have to be considered, be-cause the second-order rate constants of these reactions are
relatively high. The three main reactions to be considered are

Cl2 + H→k1

HCl + Cl �reaction�R7�� ,

Cl + H2→k2

HCl + H �reaction�R8�� ,

HCl + H→k2�

Cl + H2 �reaction�R9�� .

�R7� and �R8� correspond to the chain reaction involved inthe production of HCl from hydrogen and chlorine with Clatoms acting as the chain carriers. �R7� is a fast reactionsince values of the second-order rate constant k1 around of2�10−11 cm3 molecule−1 s−1 have generally been reportedin literature.20 Berho et al.21 measured a rate constant of 2�10−11 cm3 molecule−1 s−1 at 300 K, and have derivedan estimation of k1 from 2�10−11 to 4�10−11 cm3

molecule−1 s−1 for a gas temperature varying from 300 to600 K. In an older paper, Albright et al.,22 reported measuredk1 values of 3�10−11 to 10�10−11 cm3 molecule−1 s−1 fortemperatures from 300 to 500 K, and a value of 13�10−11 cm3 molecule−1 s−1 could be extrapolated for a tem-perature of 600 K. The rate constants k2 and k�2 for reaction�R8� and reverse reaction �R9� are lower than k1 by morethan one order of magnitude.20,23 Typical values for k2 lie inthe range from 1�10−14 to 2.5�10−14 cm3 molecule−1 s−1

at 300 K, and from 7�10−13 to 15�10−13 cm3

molecule−1 s−1 at 600 K. The rate constant k�2 is typically ofthe same order of k2, since typical values lie in the rangefrom 5 to 13�10−13 cm3 molecule−1 s−1 at 600 K. The firstreaction to be considered in a kinetic model is consequentlyreaction �R7�.

We start our analysis by neglecting all the volume reac-tions, an assumption that shall be relaxed later. The analyti-cal expressions of Cl and H can be easily derived from �R1�,�R4�, �R5�, and �R6�, and read

�Cl� =2

1 +kpump

Cl2 + kwallCl

nekdissCl2

�1

kpumpCl2

�QCl2

V , �3�

�H� =2

x +kwall

H + xkpumpH2

nekdissH2

�1

kpumpH2

�QH2

V . �4�

The main unknowns here are the wall recombination coeffi-cients. Values of the recombination coefficient �Cl lying inthe range from 0.8 to 0.01 have been reported so far, andhave been shown to be strongly dependent on both the reac-tor walls state and the plasma mixture. Kota et al.24 reported�Cl values of �0.2, 0.1, and 0.02 in the 300–350 K tempera-ture range �typical temperature of the reactor walls� for an-odized aluminum, polysilicon, and quartz. Ullal et al.25 de-rived a recombination coefficient of 0.03 in a reactor withanodized Al walls passivated by a SiOClx redeposition layer.More recently Guha et al.26 measured �Cl in a reactor withanodized Al walls covered with an AlSiOCl passivation layer
resulting from the erosion of the quartz tube and found an


increase in �Cl with the Cl /Cl2 ratio from �0.01 to 0.1.Cunge et al.27 measured the Cl2 and Cl concentrations in anindustrial ICP reactor and derived a low �Cl value of 0.005for the case where the reactor walls were passivated by aSiOCl deposition layer. In our case the aluminum walls ofthe reactor are generally passivated by a SiOAlCl redeposi-tion layer resulting from the etching of the Si coverplate inthe Cl2-containing plasma, not far from the case of Ullal etal. and Cunge et al. except for a stronger Al incorporation �asestimated from ex situ EDX analysis performed duringmanual cleaning of the reactor�. The gas mixture is also var-ied, and it is likely that the �Cl value will vary with theplasma mixture due to competition between H, Cl, and evenCl2 adsorption,26 and due to other possible recombinationmechanisms of Cl atoms competing with the recombinationpath Cl�s�+Cl�g�→Cl2

�g�, such as H�g�+Cl�s�→HCl�g�, orSiCl�x−1�

�s�+Cl�g�→SiClx�g�. In the following calculations, we

will fix �Cl=0.03 �which corresponds to kwCl�170 s−1 with

Tg=600 K�.The recombination coefficient of H, �H, typically ranges

from 0.15 to 0.006 according to the literature.28,14 No experi-mental data are available on �H for the case of reactor wallspassivated by SiOCl layers to the authors knowledge. As forCl recombination, the value of �H may vary with the plasmamixture. Moreover, recombination mechanisms differentfrom the H�s�+H�g�→H2

�g� recombination path may take placein the Cl2–H2 plasma, such as H�g�+Cl�s�→HCl�g�. Kim etal.,29 for instance, calculated that the recombination coeffi-cient of H on a Si surface saturated with chemisorbed chlo-rine is of the order of 0.05 to 0.1. The impact of �H on the Hconcentration is illustrated in Fig. 9, where the evolution of�H� �Fig. 9�a�� and �Cl� �Fig. 9�b�� are calculated using sim-plified expressions �3� and �4� obtained without taking intoaccount HCl in the gas phase, and �H varied from 0.15 to0.006 �kw

H�3400–140 s−1, Tg=600 K�. It can be observedthat for �H typically of the order of or larger than 0.03, �i.e.,kw

H 685 s−1, compared to production rate nekdissH2 of the order

of 360–150 s−1�, the calculated �H� curve shows a maxi-mum which position is fixed to H2%�55%, that is close tothe experimental observation. This result is a preliminaryindication that the recombination coefficient �H is probablyhigher than 0.03 in our case.

It should also be noted that for the highest values of H2%�60%–90%� the electron temperature deduced from the cy-lindrical probe measurements shows a very fast increase bymore than 2 eV. This in turn leads to an increase in kdiss

H2 , sothat the production rate of H only slowly decreases�190–140 s−1� despite the continuous decrease in ne. Thisexplains why the calculated �H� curve is not symmetricaround its maximum for high �H values, with a slower de-crease in H concentration on the high H2% side of the maxi-mum.

We have verified for 0.5�x�1 that the position of the�H� maximum does not depend on x, which allows us to take

H H2
the value of kpump equal to kpump and to rewrite Eq. �4� as

�H� = A2z

1 +kwall

H + kpumpH2

ne�z�kdissH2 �z�

= A2z

1 +kwall

H + kpumpH2

K�z�

, �5�

where

z =H2%

100=

QH2

Qtotal,

and K�z�=ne�z�kdissH2 �z�. A is a constant given by

A =4.08 � 107Qtot

VkpumpH2

,

if Qtot is the total flow rate expressed in SCCM, V is in cm3,and kpump

H2 in s−1. The position of the �H� maximum, z0, givenby d�H� /dz z=z0

=0, reads

z0 = −

K�z0�2 � �1 +�kwall

H + kpumpH2 �

K�z�

��kwallH + kpump

H2 � �dK

dz�

z=z0

. �6�

In our case, kwH�685 s−1 if �H is assumed to be larger than

0.03 consistently with the apparition of a H maximum,H2 −1

FIG. 9. �a� H concentration normalized against the H concentration atH2%=60% calculated with the simple kinetic model as a function of H2%for �H=0.15, 0.10, 0.07, 0.05, 0.03, 0.015, 0.010, and 0.006 �left axis�. Theexperimental data IH / IAr of Fig. 8 are reported for comparison �blacksquares, right axis�. �b� Cl concentration calculated with the simple kineticmodel as a function of H2% �left axis�. The experimental data ICl / IAr of Fig.8 are reported for comparison �black squares, right axis�.

whereas ne�z�kdiss lies in the range of 360–150 s , thus


kwallH +kpump

H2 �K�z�=ne�z�kdissH2 , and z0 can be well approxi-

mated by

z0 � −K�z0�

�dK�z�dz

�z=z0

. �7�

Since ne varies from 6.6�1010 to 1.8�1010 cm−3 while kdissH2

is a more weakly varying function of z, a simplified form ofEq. �7� is

z0 � −ne�z0�

�dne�z�dz

�z=z0

. �8�

Equation �8� simply evidences that z0 is independent of thereaction rate constants �kwall

H , kpumpH2 , and kdiss

H2 � and is deter-mined mainly by the variation of ne and Te with H2%. Fromour experimental ne�z� curve �Fig. 7�, and our calculations ofkdiss

H2 from the Te data, we find that Eq. �7� is verified for z0

�0.56 �z0�0.53 using Eq. �8��, in very good agreementwith the measured data of Figs. 4, 5, and 8 exhibiting amaximum around 0.5–0.6 depending on the reactor condi-tioning. If kwall

H +kpumpH2 were of the same order as nekdiss

H2 , nomaximum would be observed, and the H concentrationwould continuously increase with H2%, as evidenced by thecalculations for low �H values.

In the case of this simple kinetic model the ratio r0 and r0�used to verify the validity of the actinometry method simplyread

r0 =kexc-dissoc

H�

kexc-directH�

�H2��H�

=kexc-dissoc

H�

kexc-directH�

kwallH + kpump

H

2nekdissH2

,

and

r0� =kexc-dissoc

Cl�

kexc-directCl�

�Cl2��Cl�

� 0.01kwall

Cl + kpumpCl

2nekdissCl2

.

The ratio for chlorine, r0�, is lower than 0.02 �with �Cl

=0.03� for all H2%, due to the strong dissociation of chlorine�dissociative excitation is therefore negligible�.

The r0 ratio calculated as a function of H2% for �H varied

FIG. 10. r0 ratio calculated with the simple kinetic model as a function ofH2% for �H=0.15, 0.10, 0.07, 0.05, 0.03, 0.015, 0.010, and 0.006.

from 0.15 to 0.006 is reported in Fig. 10. It can be deduced


that dissociative excitation is negligible for all H2% if �H

�0.05 ��kexc-dissocH�

/kexc-directH�

�� H2� / �H��0.15 for allH2%�. When �H�0.05, the actinometry measurement willclearly include both the H and H2 contributions, and there-fore will represent an overestimation of the real H concen-tration, particularly at high H2%. Consequently, we concludethat this effect cannot explain the discrepancy between theexperimental actinometry curve and the model. It seems thatanother loss mechanism exists for H that becomes very im-portant in the case of Cl2–H2 mixture for high H2%.

Finally the creation and loss terms of H and Cl due toreactions �R7� and �R8� can also be compared to the otherexisting loss/production terms by calculating the ratio r1, r2,r3 and r4 defined as

�loss of H: r1 =k1�Cl2�

�kpumpH + kwall

H �,

*creation of Cl: r2 =k1�H�

�2nekdissCl2 �

,

*creation of H: r3 =k2�Cl�2nekdiss

H2,

*loss of Cl: r4 =k2�H2�

�kpumpCl + kwall

Cl �.

Using k2=k�2=8�10−13 cm3 molecule−1 s−1, we found thatr3 and r4 are always lower than 0.06 for all �H valuesand �Cl=0.03. The r1 and r2 ratios are reported in Figs.

FIG. 11. �a� r1 and �b� r2 ratio calculated with the simple kinetic model andk1=4�10−11 cm3 molecule−1 s−1 as a function of H2% for �Cl=0.03 and�H=0.15, 0.10, 0.07, 0.05, 0.03, 0.015, 0.010, and 0.006.

11�a� and 11�b� as a function of H2% for k1=4


�10−11 cm3 molecule−1 s−1. Calculations of the dissociationrate of chlorine show that it is higher than 85% for any H2%in the gas mixture for �Cl=0.03. This is in a good agreementwith the experimental results on Cl2 dissociation obtainedunder similar plasma conditions �high ICP power, low pres-sure, and reactor walls covered by SiOClx layer�.14,25,27 Wesee from Fig. 11�a� that volume reaction �R7� can be ne-glected in this case as a destruction channel of H for �H

values �0.05. A similar result is obtained for the productionof Cl �Fig. 11�b��. For �H values in the range from 0.05 to0.03, although not being predominant, these loss/creationrates start to play a role. This effect is reinforced if higher k1

values are considered. For the high limit value k1=13�10−11 cm3 molecule−1 s−1, the creation/loss terms due to�R7� cannot be neglected in H and Cl equations as soon as�H�0.15. The effect of the volume reaction �R7� is furtherdiscussed in Appendix.

The presence of HCl in the plasma could not be checkeddirectly since the HCl emission lines are out of the range ofour spectrometer. However the continuum corresponding tovibration bands of HCl+ could be detected in the 300–370nm wavelength range. The emission spectrum recorded inthis spectral region for H2%=0% and H2%=70% is reportedin Fig. 12, and the observed emission is indeed attributed toHCl+. The intensity of the HCl+ emission normalized againstthe 750.4 nm Ar line is reported as a function of H2% in Fig.13. The HCl+ emission has been integrated in the 317–330nm spectral window marked in Fig. 12, in which no linesoriginating from other species exist. It can be observed thatthe normalized intensity continuously increases up to 70% ofH2 and then sharply decreases. It is also observed that whileno HCl+ emission is recorded for H2%=0%, a residual signalalways exists for H2%=90% that is when chlorine is sup-pressed from the gas mixture. This observation confirms thatthe reactors walls are covered by a passivation layer contain-ing Cl and that the surface reaction H�g�+Cl�s�→HCl�g�

should be taken into account in a more sophisticated model

FIG. 12. Optical emission spectra of the plasma recorded in the 300–370 nmwavelength range for H2%=0% �gray line�, and H2%=70% �black line�.The spectral window between the vertical dotted lines is used to calculatethe integrated intensity of the HCl+ emission in Fig. 13.

of plasma surface reactions with the Cl2–H2 chemistry,


while the hypothetic reverse reaction H�s�+Cl�g�→HCl�g�

does not exist in the reactor or only on a very short timescale ��1 min�.

The kinetic model presented here explains some featuresrevealed by the experiments; namely, a constant decay of thechlorine concentration and a maximum of the hydrogen atomconcentration near H2%=55%. However, the discrepancybetween experiments and the model is still large on the Hcurve. It seems clear that an additional mechanism has to betaken into account to explain the measurements. It is inter-esting to note that the measurement of HCl+ emission bandindicates that the HCl concentration continuously increasesup to a maximum around H2%=70%. For not negligible�HCl� concentrations in the gas phase, first the electron-impact dissociation of HCl will produce more H radicals,and second the secondary reactions �R8, 9� could also pro-duce more H radicals. It is thus possible that the HCl pro-duction may have a more complex effect on the H atomconcentration. Finally, it is worth noting that a better fit ofthe H curve could be obtained if the recombination coeffi-cient of H atoms were a function of H2%. A perfect math-ematical fit of the curve could for instance be obtained for �H

varying from 0.12 to 0.05 with H2% with a minimum valueachieved around H2%�55%. The recombination coefficient�H may indeed vary, although we cannot prove this assump-tion, as the value of surface recombination coefficients �generally strongly depends on the material, surface tempera-ture, and surface conditioning as discussed earlier in the text.The direct measurement of �H in our reactor and under ourplasma conditions is necessary to confirm such an assump-tion.

B. Etching mechanism: Passivation

As illustrated in Figs. 2 and 14, vertical and smooth etch-ing of InP can be obtained when the H2 percentage in theCl2–H2 gas mixture is around 40%. We have already re-ported that smooth and anisotropic etching of InP/InGa�Al�As heterostructures with no undercut nor notches atthe etched sidewalls could be obtained in a similar way as forbulk InP samples.7 Moreover, using a 4 in. Si wafer as a

FIG. 13. Intensity of the HCl+ emission band integrated in the 317–330 nmwavelength range and normalized against the Ar line at 750.4 nm as afunction of H2%.

sample carrier was essential to prevent lateral undercutting.


In a recent work8 we used EDX-TEM to analyze the passi-vation layer deposited on the sidewalls of InP micropillarsetched in Cl2–H2 chemistry. We evidenced that the aniso-tropic and smooth etching observed for H2% 35% is due tothe deposition of a silicon oxide layer on the pillar sidewalls,the origin of silicon being the etching of the 4 in. wafer inthe chlorinated atmosphere. We also suggested that the pres-ence of a small amount of oxygen in the plasma is necessaryfor the passivation layer to buildup, consistently with exten-sive studies carried out in the context of Si gate ICP etchingusing HBr /Cl2 /O2 chemistry for microelectronics.30 We as-sumed that oxygen should come from the sputtering of theinner parts of the reactor in our case where O2 was not in-tentionally added to the gas mixture. Finally the EDX-TEMex situ analysis showed that the silicon oxide layer was Sirich, with a Si/O ratio around 1/1 for the anisotropic Cl2–H2

process �i.e., H2%�35%�.8 Such a passivation layer was al-most not observed for the Cl2 chemistry without hydrogen,as evidenced in Fig. 14�f� showing a significantly undercutprofile. EDX-TEM analysis indeed evidenced that the siliconoxide was reduced to a thickness of �2 nm in this lattercase �that is typically at least five to seven times thinner thanin the case of anisotropic Cl2–H2 etching�, and could not actas an etch inhibitor to prevent the lateral etching of the InPmaterial.8

Anisotropic and smooth ICP etchings of InP/InGaAsPwaveguide heterostructures has already been reported byRommel et al.1 using the Cl2 /Ar /H2 chemistry. Starting froma Cl2 /Ar gas mixture with an Ar dilution of typically 40%–60%, the authors showed in this early work that H2 additioncould prevent undercut and notches at the interfaces betweenthe different InP and InGaAsP materials. From planar etchrate measurements performed for the different materials atvarious H2 flow rates, they concluded that this smooth etch-

FIG. 14. Typical SEM images of InP patterns etched in Cl2–H2 chemistrywith a H2% of �a� 25%, �b� 35%, �c� 50%, and ��d� and �e�� 60%. Thetypical profile obtained with H2%=0% is reported for comparison �f�.

ing was the result of an optimized regime where all materials


where approximately etched at the same rate, with the physi-cal component of the etching due to Ar balanced by theenhanced chemical etching of the V elements due to hydro-gen addition.1 Consistently with these early results, addingonly Ar �instead of H2� to Cl2 did not allow to obtain aniso-tropic etching in our conditions, as reported in Fig. 15. In-creasing the Ar percentage led to a regular decrease in thepositive ion current density, and of the InP and 4 in. Si waferetch rates �Fig. 15�a��, but a vertical and smooth profile wasnever obtained �Figs. 15�b� and 15�c��. EDX-TEM analysisof the sidewalls of InP micropillars etched with Cl2–Ar withan Ar dilution of �40% revealed that a silicon oxide layerwas deposited on the InP sidewalls, but that the net deposi-tion rate was lower than that obtained with the Cl2–H2

chemistry.8

Starting from an Ar dilution of �40% �corresponding to afixed Ar flow rate of 11 SCCM in our etching conditionswhere total=28 SCCM�, we added H2 to the gas mixture.The H2 and Cl2 flow rates were adapted to maintain a con-stant total flow rate of 28 SCCM. The InP etch rate measuredas a function of the relative H2 percentage defined as H2%=H2

/ �H2+Cl2

� where H2and Cl2

are the H2 and Cl2flow rates, respectively, is reported in Fig. 16�a�. The 4 in. Siwafer etch rate measured under the same conditions is also

FIG. 15. InP and Si wafer etch rates measured as a function of �a� Ardilution, and typical SEM images of the etching profiles obtained for an Ardilution of �b� 40% and �c� 70%. The other etching parameters are kept thesame as those used in the Cl2–H2 chemistry.

reported in the figure. The integrated intensities of the Cl


�754.7 nm� and H �656.3 nm� lines normalized against the Arline at 750.4 nm are reported in Fig. 16�b�, and the evolutionof the etching profile is illustrated in Fig. 17.

It is obvious that the etching behavior is very similar tothat obtained for Cl2–H2 without Ar addition. While an un-dercut profile is obtained for relative H2% lower than 25%,anisotropic etching is achieved when H2% is increased in the35%–50% range with a maximum in H concentrationreached for a relative H2% around 50%. Further increasingof the relative H2% ��55%� results in the development of agrassy surface. We conclude from these observations that thesame passivation mechanism occurs in Cl2–H2 and

FIG. 16. �a� InP and Si wafer etch rates, and �b� integrated intensities of theH �656.3 nm� and Cl �754.7 nm� lines normalized against the Ar line at750.4 nm as a function of the relative H2% in the Cl2–H2–Ar gas mixturefor a fixed Ar dilution of �40% �11 SCCM�. The other etching parametersare kept the same as those used in the Cl2–H2 chemistry.

FIG. 17. SEM images of InP pillars etched with the Cl2–Ar–H2 chemistrywith a fixed Ar dilution of �40% and for a relative H2% of �a� 0%, �b� 23%,and �c� 35%. The other etching parameters are kept the same as those used
in the Cl2–H2 chemistry.

Cl2–H2–Ar chemistries: H2 addition promotes the deposi-tion of the silicon oxyde layer on the InP sidewalls. Thenature of the sample carrier was not precised in Ref. 1, butwe believe that a similar passivation mechanism actuallytook place in these experiments, and that the major reasonfor smooth and anisotropic etching of the waveguide hetero-structures came from this passivation mechanism. Aniso-tropic etching was actually reported for Cl2 /Ar /H2 propor-tions ranging from 2/1/3 to 2/2/3, corresponding to a relativeH2% from 33% to 50%, that is very close to our results.

It appears from the above results that adding hydrogen tothe halogen etching gas �Cl2 in our experiments� promotesthe passivation of the InP sidewalls compared to Ar dilution,however, the exact role of hydrogen in the passivation pro-cess has still to be determined. Some experimental observa-tions might be in favor of an enhancement of the net depo-sition rate of the silicon oxyde layer in the presence ofhydrogen. First, we investigated adding a small amount��5%� of oxygen to the Cl2 atmosphere in order to en-hance the formation of silicon oxyde.8 We observed that aSiO2-like passivation layer was formed on the InP sidewallsin this case, but that the net deposition rate was still lowerthan in the case of the Cl2–H2 chemistry. Second, it wasreported in the domain of microelectronics in the context ofsub-100 nm Si gate etching that the addition of HBr �con-taining H� in the HBr /Cl2 /O2 mixture improves the processanisotropy,31 the exact physicochemical mechanism involvedin this improvement being not yet fully understood to ourknowledge. Third, anisotropic and smooth etchings werenever obtained with the Cl2–Ar chemistry, despite the factthat some plasma and etching characteristics could be veryclose to that of the Cl2–H2 anisotropic process. Indeed, an-isotropic etching of InP is presumably obtained when thedeposition rate of the Si-containing layer on the InP side-walls is not completely counterbalanced by the etch rate ofthis layer by Cl radicals via the formation of SiClx com-pounds. It is also expected that the passivation layer willmore readily grow on inert or slowly etched surfaces. Itwould then be reasonable to assume that the combination ofa high Si concentration and a moderate Cl radicals concen-tration in the plasma, together with a moderate InP etch rate,would favor sidewall passivation. Starting from this assump-tion, it is interesting to compare the etching results obtainedwith the Cl2–H2 chemistry for H2% in the 35%–50% range,to those obtained in Cl2–Ar chemistry for an Ar dilution inthe 60%–70% range. The InP etch rates are very similar inboth cases and around �900 nm /min if one compares theresults of Figs. 3�b� and 15�a�. Moreover, the Si etch rate arevery similar in both cases, around 90–100 nm/min �corre-sponding to an equivalent Si flux in the plasma of�1.2 SCCM� such that the Si concentration in the plasmashould be fairly similar. Finally, the Cl relative concentrationalso appears to be comparable as indicated by the Cl acti-nometry measurements reported in Fig. 18. However, astrongly undercut profile is obtained with Cl2–Ar �see Fig.
15�.


These simple observations indicate that hydrogen atomsplay a key role, and may have a more complex involvementin the passivation process. As an additional support to thisconclusion, it is worthwhile to note that for higher H2%�typically�57%� �ignoring in this discussion the fact thatthe InP etched surface becomes too grassy to make such aprocess suitable for photonic device fabrication�, a signifi-cant lateral undercut of the etched patterns is again observed,as for low ��25%� H2%. Rommel et al.1 also reported thatnotches were again developed at the heterostructure side-walls for Cl2 /Ar /H2 proportions above 6/9/9 �that is a rela-tive H2% 60% with total Ar dilution of 37.5%�. The 0%–25% and the �60% ranges are close to the regions where therelative H concentration in the plasma becomes to be re-duced as observed in Fig. 4 as well as in Fig. 5 and 8.

We propose the following mechanism to explain the en-hancement in the deposition of the passivation layer when H2

is introduced in the gas mixture and the H radical concentra-tion is increased. Some studies devoted in the past years tothe etching/deposition of Si:H,Cl layers in SiCl4–Ar–H2

plasma32 have shown that while the Si layer is etched inSiCl4 atmosphere or in gas mixtures where the H2 proportionis low, a switch to deposition is observed when the H2% isincreased. The same early studies have highlighted that thisdeposition mechanism is rather related to a surface recombi-nation mechanism than to a change in the gas phase �volumereactions�. Although these studies were carried out at a rela-tively high pressure compared to our case, we believe that asimilar mechanism can occur on the sidewalls of the III-Vetched patterns. The proposed surface reaction

Si�O�Clx�s� + H�g� → Si�O�Cl�x−1�

�s� + HCl�g�,

could, indeed, explain the enhancement in the deposition ofthe passivation layer and also the Si enrichment of this layerobserved in our case. We suggest that a similar mechanismmay occur with the HBr chemistry that we recently devel-oped for the ICP smooth and anisotropic etchings of both

33

FIG. 18. Comparison of the integrated intensity of the Cl �754.7 nm� linenormalized against the Ar line at 750.4 nm in the Cl2–H2 and in the Cl2–Argas mixtures. In the case of the Cl2–Ar chemistry, the intensity of the Clline was normalized by the intensity of the Ar line divided by the Ar flowrate. In the case of Cl2–H2 chemistry where 10% of Ar �2.8 SCCM� is usedas the actinometer, the intensities were normalized by the intensity of the Arline divided by 2.8.

InP-based and GaAs-based photonic heterostructures. A Si-


rich silicon oxide layer �with Si/O ratio up to 3/1� has indeedalso been observed on the InP sidewalls with this chemistry.Finally, a similar mechanism may occur during Si gate etch-ing in Cl2–HBr–O2 inductively coupled plasma that couldexplain the process anisotropy with HBr addition. Furtherstudies are required to investigate the respective roles of Oand H in this passivation layer deposition mechanism. Wehave indeed observed that the intentional addition of a smallamount of oxygen in the plasma changes the deposition layercomposition from Si-rich to more stoichiometric SiO2 forboth the HBr �Ref. 33� and Cl2–H2 chemistries.8

IV. CONCLUSION

We have used optical emission actinometry and Langmuirprobe measurements in order to investigate the InP etchingmechanisms in a Cl2–H2 ICP under low pressure �0.5–1mTorr� conditions. We have identified that the local maxi-mum observed in the etch rate curve when the H2 percentageis increased in the gas mixture, is closely related to the ap-parition of a maximum in H atom concentration in theplasma. This implies that the decrease in the InP etch rateexpected from the observed decrease in the Cl concentrationwhen H2% is increased, is partially compensated by the etch-ing of V elements �P� by hydrogen. This presumably occursdue to the formation of PHx etch products, and leads to abalanced regime �in the 35%–50% percentage window� inwhich the etch rate varies only weakly. A simple kineticmodel has been developed to identify the main plasma pa-rameters controlling the apparition and the position of the Hmaximum observed in the experimental curve. We showedthat this maximum appears in conditions where the H atomproduction rate remains lower than the total loss rate, indi-cating that the recombination coefficient of H atoms shouldbe higher than 0.03 in our case. Under such conditions, the Hconcentration maximum is nearly independent of the loss/productions mechanisms and is mainly imposed by the varia-tion of the electron density with H2% in the Cl2–H2 mixture,in reasonable agreement with the experimental data. More-over, the 35%–50% range also coincides with the anisotropicand smooth etching regime of both bulk InP or InP/InGa�Al�As heterostructures. We have previously evidencedthat this etching regime is due to the formation of a Si-richsilicon oxide layer on the etched sidewalls, with silicon com-ing from the etching of the 4 in. Si wafer used as the sampletray. By comparing the etching results and the actinometrymeasurements performed in the Cl2–H2 mixture and in aCl2–H2–Ar mixture with a high Ar dilution, that is close tothe early experimental conditions of Rommel et al.,1 weshowed that the etching behavior and the passivation mecha-nism are actually very similar for the two chemistries, andthat the H concentration is one of the key parameter of theetching process. We believe that the fact that the differentmaterials included in an InP-based heterostructure can beetched at an approximately similar etch rate with Ar addition,is only a second-order parameter to achieve anisotropic etch-ing. Finally, we found conditions in which the Cl atom con-
centration, the Si etch rate and the InP etch rate were similar


in Cl2–H2 and Cl2–Ar gas mixtures, yet leading to verydifferent etching anisotropy �lateral etching always presentwith for Cl2–Ar�. We therefore concluded that adding H al-lows for enhancing the passivation mechanism and we sug-gest a surface reaction that could explain this enhancement,although our diagnostics do not allow to rigorously provethis assumption.

ACKNOWLEDGMENT

L.G. was financially supported by Sentech InstrumentsGmbH, Berlin, Germany for this work.

APPENDIX

In a first step toward the integration of HCl in a morecomplete description of the Cl2–H2 plasma, the main reac-tion �R7� has been included in the kinetic model as

d�Cl�dt

= 2nekdissCl2 �Cl2� − �kwall

Cl + kpumpCl ��Cl� + k1�H��Cl2� ,

�A-1�

d�Cl2�dt

=QCl2

V− nekdiss

Cl2 �Cl2� − kpumpCl2 �Cl2� +

1

2kwall

Cl �Cl�

FIG. 19. H concentration normalized against the H concentration at �a�H2%=60% and �b� Cl concentration calculated with �H=0.05 and �Cl

=0.03 as a function of H2% for k1=0, 4�10−11, and 1.3�10−10 cm3 molecule−1 s−1 �left axis�. The calculated �H� values at H2%=60% are of 3.3�1012, 3.0�1012, and 2.6�1012 cm−3 for k1=0.4�10−11,and 1.3�10−10 cm3 molecule−1 s−1, respectively. The experimental IH / IAr

and ICl / IAr data of Fig. 8 are reported for comparison �black squares, rightaxis� in �a� and �b�, respectively

− k1�H��Cl2� , �A-2�


d�H�dt

= 2nekdissH2 �H2� − �kwall

H + kpumpH ��H� − k1�H��Cl2� ,

�A-3�

d�H2�dt

=QH2

V− nekdiss

H2 �H2� − kpumpH2 �H2� +

1

2kwall

H �H� �A-4�

and the steady-state solutions were numerically solved inorder to check the importance of the gas phase reaction in thekinetic. The calculations performed for k1=0.4�10−11, and1.3�10−10 cm3 molecule−1 s−1, and for wall recombinationcoefficients fixed to �Cl=0.03 and �H=0.05, are shown onFigs. 19�a� and 19�b�. When considering k1=4�10−11 cm3 molecule−1 s−1 the effect of �R7� is not verylarge and a plateau still appears from H2%=55% on the Hcurve. When considering the upper value of k1, the maxi-mum disappears, clearly underlying the lack of an efficientloss mechanism for H atoms at high H2% as discussed in thetext.

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investigation of inp etching mechanisms in a cl2/h2 ...€¦ · investigation of inp etching...

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