GROWTH CHARACTERIZATION OF a Si:H FILMS BY MULTIPLE ANGLE OF INCIDENCE SPECTROSCOPIC ELLIPSOMETRY

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GROWTH CHARACTERIZATION OF a Si:H FILMS BY MULTIPLE ANGLE OF INCIDENCE

SPECTROSCOPIC ELLIPSOMETRY

J. Perrin, B. Drevillon

To cite this version:

J. Perrin, B. Drevillon. GROWTH CHARACTERIZATION OF a Si:H FILMS BY MULTIPLE

ANGLE OF INCIDENCE SPECTROSCOPIC ELLIPSOMETRY. Journal de Physique Colloques,

1983, 44 (C10), pp.C10-247-C10-251. �10.1051/jphyscol:19831052�. �jpa-00223509�

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Colloque CIO, supplément au n°12, Tome 44, décembre 1983 page C10-247

GROWTH CHARACTERIZATION OF a Si:H FILMS BY MULTIPLE ANGLE OF INCIDENCE SPECTROSCOPIC ELLIPSOMETRY

J. Perrin and B. Drevillon

Equipe de Recherche : Synthèse de couches minces pour l'Energétique, L.P.N.I1.E. , Ecole Polytechnique, 91128 Palaiseau Cedex, France

RESUME - Les processus de croissance et les propriétés optiques de couches minces de silicium amorphe hydrogéné (a-Si:H) déposées à partir de la décomposition du silane par plasma sont étudiés in situ par ellipsométrie en temps réel et ellipsomé- trie spectroscopique à trois angles d'incidence. On montre que le dépSt d'ions SiHn

accélérés (20-100 eV) favorise une croissance homogène et une densification de la couche alors que le dépôt d'ions non accélérés ou de radicaux neutres engendre un processus de croissance non homogène, où l'indice de réfraction et la densité de la couche dépendent de son épaisseur.

ABSTRACT- The growth mechanism and the optical properties of hydrogenated amorphous silicon (a-Si:H)films, deposited from silane plasma decomposition, are studied in situ by real time and spectroscopic ellipsometry at three angles of incidence. Depo- sition of accelerated (20-100 eV) SiH ions is shown to favour homogeneous and isotropic film growth and film densification. On the contrary deposition of non accelerated ions and/or neutral species results in an inhomogeneous growth mechanism with thickness dependent film index and density.

INTRODUCTION - The optical and structural properties of hydrogenated amorphous silicon a-Si:H films deposited from silane plasma decomposition cannot be simply related to their hydrogen content, ranging from 3-5 to 30 atomic percent, essentially determined by the substrate temperature during the film growth. We have shown recently that fast real time ellipsometry after deposition associated with measurement of the hydrogen content by '5N nuclear reaction and I.R. vibrational spectroscopy of the Si:H bonding modes are powerful tools to analyze the combined effects of substrate temperature, deposition rate, ion flux and ion bombardment energy on the optical, chemical and structural properties of a-Si:H films deposited in low pressure silane multipole dc discharges (1) - (3). We present here an improved analysis of the growth process of a-Si:H films using multiple angle ellipsometry.

EXPERIMENTAL

Silane multipole dc discharges sustained by 60-70 eV primary electrons have been extensively described elsewhere (4), (5). We consider here two plasma regimes which correspond to different deposition conditions.

i) low pressure (< 0.5 mTorr) plasmas where positive ions SiH (n = 2,3) coming from the plasma" represent &Q% of the total Si flux onto the sample (^<J>+/<J>t:ot-=0-8) ii) high pressure (^ 4 mTorr) plasmas where the ionic species are highly polymeri-

zed and represent only 20% of the total Si flux (ij>+/<f =0.2)

° "">

Depending on the disgharge current, the film thickness growth rate can reach 4 A s at 0.5 mTorr and 20 A s-' at 4 mTorr. The substrate temperature Tg range from 100 to 350°C.

The energy of ions E^ impinging on the substrate is measured by an electrostatic e- nergy analyzer (3). On a metallized substrate which can be d.c. polarized with res-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831052

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ClO-248

JOURNAL DE

PHYSIQUE

pect to the plasma E. can be varied from 0 to 100 eV. On an insulating substrate such as fused

silica'^.

is defined by the floating potential of the sample which is typically -45 eV at10.5 mTorr and -25 V at 4 mTorr.

The spectroscopic ellipsometer (1.8 < hy < 5 eV) equipping the deposition appara- tus is based on a MORVUE piezo-optical birefringence modulator at a frequency of 50 kHz. A 5ast analog to digital conversion of the detected signal using a micro- processor system (6) allows a real time Fourier analysis resulting in the capabi-

lity to measure every 5 msec one set of ellipsometric angles $ and

A.

Three angles of incidence

4

are available : 62'5, 70'5 and 81'5 and two experimental procedures are used to analyse the growth process and optical properties of a film deposited in given plasma conditions

i) real t h e ellipsometry at a fixed wavelength (A = 550 nm) is performed at the three angles of incidence on three samples deposited at the same substrate temperature and potential and with identical plasma conditions (the reproducibility of the growth rate is expected to be about 10% due to the precisions on the pressure and the discharge current). Experimental trajectories in the ($, A) plane for @I = 62"5 and 81.5' obtained during the deposition of a a-Si:H film on a fused silica substrate at Ts = 120'~ for a silane pressure of 4 mTorr and a discharge current Id = 400 mA are represented respectively in Figs. I-a and 1-b. A set of N experimental (I), A) points recorded at regular time intervals are then compared to a theoretical calculation assuming an isotropic and homogeneous film growth on the substrate with a constant deposition rate vd. The parameters of the fit (vd and the real and imaginary parts of the complex refractive index

%

= n - ik) are obtained by minimizing the

error function ; aS1:H I S ~ O P

where p = tan $ exp i A and p is the number of parameters. In order to derive quantitati-

,,,

ve com arisons of the minimized error func- D

tion 6min between different films, experimen-

P :

tal tra~ectories are fitted up to the same

:

^12B

final thickness and with the same number of

experimental points (N P\, 140). Further re- z

. . .

6 I D 16 10 22 26

PSI 30 3. J0

ductions of the error function 62:- can be

attempted by using more sophisti"c'fed models .SI:W / 0102

with increasing number of parameters such as introduction of a density deficient overlayer

of constant thickness at the growing film PHI-01.1

plasma interface.

ii) spectroscopic ellipsometry at the three 200.

angles of incidence on the save sample, after deposition of about 7000 A a-Si:H allows

,,,

the measurement of the effective dielectric constant

2

= of the film, as a function of the photon energy E

E~ spectra of films deposited under various ulasma conditions at the same substrate temperature Ts = 120°C and measured at

4

= 70°5 are represented in Fig. 2. These spectra can be characterized by the value of the maximum E max related to the density of the material, tie energy ~osition of the maximum Em

corresponding to the average Si-Si an8 Si-Ii bond energy and also the optical gap Eo ahd the parameter y which both describe the low

Fig. 1

-

Real time ellipsometry measurements at two angles of incidence on a-Si:H films deposited on Si02 substra- tes at Ts = 120°C in the same plasma condition : p(SiHq) = 0.5 mTorr, Id = 400 mA. One point is measured e- very 0.11 sec.

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lation ; 30 ₁ E

,

⁼^y^{(E - Eo)}

RESULTS AND DISCUSSION

In previous real time ellipsometry experiments at one angle of incidence (@ ? 70'5) $'

(I) - (3), it has been shown that the best adequationbetween the experimental ($, A) trajectories and the simplest model of an homogeneous and isotropic a-Si:H film growth at a constant deposition rate is obtained under low pressure plasma conditions 0

when the depositing plasma species are 3 4 5

mostly S ~ H + (n=2, 3) ions (@+/@tot = 0.8) E(eV) and when tge substrate is negatively po-

larized with respect to the plasma so that Fig. 2 - c2 spectra of 7000 A thick the ion energy is about 20 to 100 eV. In a-Si:H films deposited at ~ ~ = 1 2 0 ~ C such conditions ti2. is about On (a) from low pressure plasma :

the contrary fi2.

m:g

one or two orders of @+/@tot= 0,8

,

Ei = 45 eV,

magnitude largF:n(10-3 to either at vd = 1 A $-I; (b) from high pres- low pressure = 0.8) when ions are sure plagma:l@+/@tot = 0.2; Ei=25 eV;

not accelerated or at high pressure vd = 10 A s-

(@+/atot = 0.2). In the latter case 6iin is very sensiteve to the substrate temperature or tge dep sition rate and decreases down to IOf for the lowest deposition rates (% I A seC-') and the highest Ts(% 350'~). There is also a strong correlation between 6iin values and E2 spectra. The lower is 6iin the larger is czmax and the lower is y as shown in Fig. 2. This indicates that growth homogeneity favoured by ion bombardment and/or high Ts and low vd results in film densification. The specific influence of ion bombardment energy compared to Ts and vd on Emax and Eo and on Si-H bonding modes has been discussed elsewhere (3).

In order to improve the interpretation of film growth process, real time ellipsometry at three angles of incidence have been performed on a-Si:H films growing on fused silica substrate at Ts = 120°C for two extreme plasma gonditions :

i) p(SiH4) = 0.5 mTorr; $+/@tot = 0.8; Ei = 45 eV; vd

-

^A^s-I

ii) p(SiH4) = 4 mTorr; @+/@tot = 0.2; E; = 25 eV; vd 9 A s-I

Results of the fit for the homogeneous and constant deposition rate growth process model are presented in Table I. The values of diin are found independent of the angle of incidence which confirms the effect of ion bombardment energy on the film growth homogeneity. The values of vd, n and k are also independent of $ within 10%

reproducibility on the deposition conditions and the 90% confidence interval on each best fit parameter. The results presented in Table I have been obtained by, fitting the experimental ($, A)trajectory up to a final film thickness of 1000 A.

When the fit is performed for different film thicknesses, the values of vd, n and k obtained for theolow pressure plasma condition remain almost constant (except for the first 200 A) at any angle of incidence as expected from the low

$Gin

^"a-

lues. On the contrary for high pressure plasma condition the yalues of timLn and of the fitting parameters are thickness dependant below 1000 A . The variations of timin, vd, n and k as a function of the film thickness covered by the fit with the 2 sameonumber of experimental points are represented in Fig. 3 for @ = 62.5Oand 81.5 : It is worth noticing that the same variations are observed for both angles of incidence although experimental trajectories in the ( ~ , A) plane are very different for = 62.5" and @ = 81.5' (see Fig. 1). We can exclude then that such effects could be due to possible systematic errors in the measurements (6).

As evidencgd from Fig. 3 the film growth process involves three steps, For the first 200 A, vd increases up to a maximum value. Between 200 ando500 A , vd is decreasing while both n and k increase up to a maximum. Above 500 A a-statlonnary growth regime is gradually established where the apparent refractive index of the film is decreasing down to a steady state value. In a previous study (1) we showed

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C10-250 JOURNAL DE PHYSIQUE

Table 1 : Best fitting parameters for real time ellipsometry at three angles of incidence on growing a-Si:H films deposited in two different plasma conditions.

Fig. 3 : Variations of Amin, 2 vp, n, k determined from the f ~ t of experimental ($, A) trajectories as a function of the total film thickness covered by the fit. White symbols for C$ = 62.5" and black symbols for 41 = 81.5'.

that this stationnary regime can be described

by a growth model where the bulk a-Si:H mate-

k

rial is growing with a density defigient overlayer of constant thickness (% 100 A ) . This u imprcvement of the model results in a s i nifi- cant reduction by a factor of three of

4

Such a model does not accgunt however for the

,.,

growth of the first 1000 A. The main point to clarify is the mechanism leading to the formation of the density deficient overlayer. In

the case of CVI) deposition of a-Si on Si N ,9

3 4 substrate at 620°C, Theeten (7) has evidenced by real time ellipsometry that the initial growth step can be described by nucleation of hemispheres of constpnt refractive index ¹⁶ distant from about 100 A and followed by

coalescence. In the present case of plasma deposition it has been shown that such a model does not account for the growth of the first 200 ( I ) if the refractive index of the

hemispheres is taken to be equal to the vaAue 500 nm

determined for the a-Si:H film after 1000 A FIW THICKNESS ( A )

deposition. However the increase of the ef-

fective refractive index of the film up to 500 A , as shown in Fig. 3 , indicates that the initial growth mechanism could be apparented to a nucleation phenomenon but with a refractive index for the hemispheres much higher than the apparent refractive index in the stationnary growth regime. Above 500

1

the drastic reduction of n and k is attributed to chemical and/or microstructural modifications during the coalescence resulting in the formation of a density deficient overlayer for the subsequent film growth. The thickness and the chemical or structural composition of this overlayer is expected to be mainly dependant on the competition between the deposition rate vd which determines the mean residence time of depositing species at the top ot the film and the substrate temperature Ts which controls the film relaxation via thermally activated chemical rearrangements. We have evidenced indeed from real time ellipsometry that when the deposition is suddendly interrupted, the recorded ($, A) points depart from the spiral shape curve which indicates a relaxation of the film overlayer (See Fig. 4). As expected such an effect is not observed for deposition with accelerated ions at low pressure where the film growth appears to be very homogeneous. Further experiments are needed to study the influences of Ts and vd on the mean relaxation time of the film.

E 2 spectra recorded on the same 7000 thick a-Si:H film at the three angles of

inc~dence are found identical within the experimental

recision

of the measurements.

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when the plasma is suddendly cut off after deposition of a-Si:H

1 2 . film from high pressure condition.

D

2 4 . 1 24.3 24.5 2 4 . 7

PSI

REFERENCES

(1) J. PERRIN et B. DREVILLON, Acta Electfonica 24 (198111982) 239

(2) J. PERRIN, B. DREVILLON and M. TOULEMQNDE, proceedings of the Fifth Internatio- nal Conference on Collective Phenomena (The New York Academy of Sciences, 1983) (3) B. DREVILLON, J. PERRIN, J.M. SIEFERT, J. HUC, A. LLORET, G. de ROSNY and

J.P.M. SCHMITT, Appl. Phys. Lett. 42 (1983) 801

(4) B. DREVILLON,J. HUC, A. LLORET, J.?ERRIN, G. de ROSNY and J.P.M. SCHMITT, Appl. Phys. Lett.

21

(1980) 646

(5) J. PERRIN, J.P.M. SCWITT, G. de ROSNY, B. DREVILLON, J. HUC and A. LLORET, Chem. Phys.

2

(1982) 383

(6) B. DREVILLON, J. PERRIN, R. MARBOT, A. VIOLET and J.L. DALBY, Rev. Sci. Instrum. - 53 (1982) 969

(7) J.B. THEETEN, Surf. Sci.

96

(1980) 275