Influence of Ion Bombardment on the Nucleation and Growth of Plasna Deposited Amorphous Silicon
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INFUfflNCE OF ION BOMAR[MNr ON THE NUCLEATION AND (GROWTH OF PLASNA DEPOSITED AMORPtK)US SILICON
A.M. ANTOINE and B. DREVILLON Equipe Synthese de Couches Minces pour l'Energ~tique Ecole Polytechnique, 91128 - PALAISEAU (France)
(ER 258),
LPNHE,
ABSTRACT Real-time spectroscopic phase modulated ellipsometry is used to study the influence of ion bombardment on the growth of plasma deposited amorphous silicon (a-Si:H). The influence of the relative ion flux impinging on the substrate is discussed by comparing a-Si:H films deposited in RF and multipole discharges. A nucleation process is observed in the early stage of the growth of a-Si:H deposited on chromium substrates. When the ion bombardment is strong (multipole plasma) a coalescence of the island structure is observed leading to the formation of a high density film with a smooth interface.
D?1DUTON It has been extensively shown that the growth processes of plasma deposited amorphous silicon (a-Si:H) are strongly influenced by the preparation conditions [1]. Fast real-time spectroscopic phase modulated ellipsometry (SPME) has been used to study the influence of ion bombardment during film growth [2,3]. The a-Si:H films were deposited in a low pressure multipole DC discharge (PSiH < 5 mTorr) in which the ion bombardment can be very strong : the refative Si flux going onto the substrate in an ionized form can reach up to t+/tot = 0.8, the ion bombardment energy E. being controlled electrostaticafIy. In this case,
moderate ion bombardmnt energy (Eion = 50-100 eV) induces a very efficient rearrangement at the surface of the growing sample, producing high density films with a smooth surface. In contrast, when the ion bombardment energy is low (Eion < 20 eV) the initial growth can be described by a nucleation phenomenon, the incomplete coalescence of the nuclei lgading to the formation of a surface roughness on a scale of -1 00 A [2]. In a 13.56 MHz RF discharge ( PSIH > 30 mTorr), the relative ion flux *+/o decreases by about one drder of magnitude resulting in a weaker efect of the ion bombardment on the film density
[4]. The present report is concentrated on a detailed study of the influence of the positive ion flux on the growth of a-Si:H by systematically comparing SPME measurements performed on films deposited in multipole and RF discharges.
EXPIMEAL DETAILS The plasma reactors used to produce a-Si:H films are either the widely used RF (13.56 MHz) plasma capacitor system or the low pressure multipole device, both fed with pure Sill4 , the pressure being 30 mTorr and 4 mTorr respectively. Experimental details concerning both discharges can be found elsewhere [5,6]. The ion bombardment during film growth is analyzed with the electrostatic analyzer previously described [5,6]. In
the
multipole
discharge
the
ion bombardment energy closely follows the
Mat. Res. Soc. Symp. Proc. Vol. 75. ' 1987 Materials Research Society
334
value of the substrate bias Vb, the ion flux being quasi-monoenergetic. In contrast, a considerable broadening of the ion energy dis
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