Accurate determination of composition and bondingprobabilities in plasma enhanced chemical vapour deposition

amorphous silicon oxideJ.A. Moreno *, B. Garrido, J. Samitier

Department d'Electr�onica, Universitat de Barcelona (EME-UB), Mart�õ i Franqu�es, 1, 08028-Barcelona, Spain

Abstract

A method to improve the accuracy in a compositional and microstructural analysis has been applied to the plasma

enhanced chemical vapour deposition (PECVD) silicon oxides. Thin layers were deposited on silicon by PECVD under

di�erent preparation conditions (power, gas ¯ow ratios, etc.). Annealings at di�erent temperatures were carried out in

order to determine their e�ect on the microstructure. Stoichiometry and hydrogen content were obtained from the

infrared absorption bands measured by Fourier transform infrared spectroscopy (FTIR). A correction factor for band

areas was calibrated to deconvolve the e�ect of the multiple internal re¯ection on infrared spectra. A thermodynamic

model was used to determine the bonding probabilities for the di�erent units (H, O, Si, OH) attached to the silicon

bonds in a centred tetrahedra. Ó 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction

The plasma enhanced chemical vapour deposition

(PECVD) silicon oxides are widely used for low thermal

budget processes in silicon technology. Their main ap-

plications are in isolating dielectrics, passivation layers

and masking oxides. The PECVD also allows deposition

on the predeposited aluminium layers by using low

temperatures.

The mechanical and electrical properties of the

PECVD layers depend on the microstructure of their

network. In order to investigate the in¯uence of the

processing conditions on stoichiometry and micro-

structure, the PECVD silicon oxide ®lms were deposited

on silicon, varying the power, gas ¯ow ratio and post-

deposited temperature annealing. An analysis of the

composition and structure was carried out by Fourier

transform infrared spectroscopy (FTIR) and ellipsome-

try (EL).

Stoichiometry was obtained from the infrared spectra

which are sensitive to the variations of the refractive

index and thickness of the oxide [1,2]. This fact was

studied and a correction factor to the band areas was

obtained. Local bonding in the silicon tetrahedra was

explained in terms of their tendency towards thermo-

dynamic equilibrium. Several silicon oxides deposited by

other techniques (atmospheric pressure CVD and low

pressure CVD) have been added to the study in order to

compare and complete the analysis.

2. Experiment

Several layers were deposited by the PECVD over a

four inch n-doped á1 0 0ñ silicon wafers at a substrate

temperature of 300°C and a pressure of 200 mTorr.

Reactant gases were N2O and silane with the ¯ow ratio

R (N2O/SiH4) varying between 10 and 32. The deposi-

tion power was changed from 20 to 30 W. After depo-

sition, two pieces of each sample were submitted to a

rapid thermal annealing during 20 s with an atmosphere

of pure oxygen at temperatures of 950°C and 1150°C,

respectively. Infrared measurements were performed in a

BOMEM DA3 spectrometer in a transmission mode in

the range 4000±400 cmÿ1 and at a resolution of 2 cmÿ1.

The refractive indexes and thicknesses were measured

with a Rudolph Auto EI IV ellipsometer at 633 nm.

Microelectronics Reliability 40 (2000) 609±612

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* Corresponding author.

0026-2714/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 2 6 - 2 7 1 4 ( 9 9 ) 0 0 2 7 9 - 6

3. Ellipsometric measurements

The Ellipsometric measurements of thickness and

refractive index were carried out in several points of all

the samples and averaged. The samples with the lower

values of R have greater values of the refractive index.

This increase in refractive index is due to a greater

concentration of the Si±Si bonds relative to the Si±O

bond concentration. Hence, an enrichment of the silicon

content is obtained when R decreases, as expected.

During annealing, two processes compete to change

the thickness of the layer. On the one hand, viscoelastic

relaxation leads to a densi®cation of the oxide and, thus

results in a decrease in its thickness. On the other hand,

as the annealing atmosphere was pure oxygen, this was

incorporated during the thermal process. According to

these two e�ects, we ®nd that the thickness of the stoi-

chiometric samples decrease with annealing, while

thickness of the oxides with a lower oxygen content

growth.

4. Composition of di�erent samples

The parameters of the infrared absorption bands are

also used to extract information about the structure and

composition of the silicon oxide ®lms. Nevertheless,

these parameters have to be used with some care because

they are not intrinsic properties of the materials and

hence, they depend on the measurement conditions, ®lm

thickness and refractive index [1,2]. As stated in [1], we

will refer them hereafter as ``geometrical e�ects''.

In order to deconvolve the geometrical e�ects in the

infrared spectra from the intrinsic structural and com-

positional modi®cations, we have computed theoretical

transmittance spectra by varying the ®lm thickness and

band intensity and constructed a correction factor C for

the band areas. Fig. 1 shows the correction factors for

impurity of the impurity bands (weak) and for the ma-

trix (strong). If N is the bond concentration, L the

correction to local ®eld, n the ®lm refractive index and athe absorption coe�cient and taking into account the

sum rules of the dielectric function [3,4], the correction

factor C has to be applied as follows:

NL2f � 2:0594� 1015nCZ

a�x�dx;

where the dimensional parameter f is the normalised

oscillator strength, a measure of the intensity of the in-

frared response of the vibration. It plays the same role of

the conversion factors from band areas to bond densities

calibrated by some authors [5,6]. The absorption coef-

®cient is calculated according to the formula

a�x� � ÿ 1

dln

TT0

� �;

where d is the ®lm thickness in cm and T and T0 are the

transmittances of the sample and the substrate.

By using calibrations obtained from the literature

[7,8] after it was corrected for geometrical and local ®eld

e�ects and by ®tting measured infrared spectra to a

speci®c dielectric function model [1] we obtained these

parameters for the di�erent bands present in the silicon

oxides:

fH � 0:023 fOH � 0:153; fSiO � 0:077:

The resulting bond concentrations are presented in

Fig. 2. As expected, oxygen content increases with ¯ow

ratio R in all samples. It is always greater in oxides

annealed at higher temperatures as the annealing at-

mosphere was oxygen. For the as-deposited oxides hy-

drogen content decreases when R grows and for R > 22

it is similar to that of the annealed samples. The hy-

drogen content of the annealed samples is constant and

low irrespective of the R value. The O±H content is low

because these bonds are not present in the plasma.

The Si±O bond concentration obviously increases

with R. We can observe that the annealed samples with

R P 22 reach Si±O bond concentrations similar to that

of the thermal oxide, while the as-deposited samples

have concentrations of about 72% corresponding to the

thermal oxide. Hence, an annealing process is necessary

after the PECVD deposition to obtain silicon oxides

with a stoichiometry similar to the thermal ones.

5. Local bonding in silicon oxides

Following the Yin and Smith procedure [9], by

minimisation of the free energy G with respect to the

fraction yO of the oxygen atoms that are bonded only

to silicon atoms we obtain the following equivalent

Fig. 1. Correction factor for band areas wherein n is the re-

fractive index, d is the ®lm thickness in cm and x0 is the band

position in cmÿ1.

610 J.A. Moreno et al. / Microelectronics Reliability 40 (2000) 609±612

reaction between bonds Si±Si�O±H! Si±O� Si±H,

with an enthalpy DE � E�Si±O� � E�Si±H� ÿ E�Si±Si�ÿE�O±H�. This equation must not be considered as the

synthesis reaction of the oxide, but a reaction of the

hydrogen bond exchange between the oxygen and silicon

atoms.

When the temperature T increases, the reaction

constant Kn tends to unity, so the bond distribution is

almost random. For the low values of T, a negative sign

of DE leads to the preferential formation of the Si±O

and Si±H bonds.

Fig. 3 shows the reaction energy DE corresponding to

the values of the bond concentrations obtained for our

oxides (Fig. 2). The PE is the PECVD oxide described in

this work, where D, A, and B stands for the as-deposited

and annealed at 950°C and 1150°C, respectively. For

comparison, we have also included here the silicon ox-

ides which have been treated with the same calculation

techniques: UN are the atmospheric pressure chemical

vapour deposition (APCVD), PL are the more PECVD

oxides and ®nally, LTO are the low thermal low pressure

chemical vapour deposition. Some of the UN and PL

samples were annealed at 900°C and 1100°C. The da-

shed line denotes annealed samples with the following

formula:

DE � ÿ0:13ÿ 0:014 exp x=0:46� �:

All the samples exhibit negative values of DE. It re-

sults in a preference for the formation of Si±O and Si±H

bonds with a detriment to the Si±Si and the O±H bonds.

This tendency is enhanced for those samples with greater

oxygen content. In another words, oxides with low

stoichiometries have a greater tendency to randomness.

One important fact here is the variation of the reac-

tion energy with the oxide composition due to its in¯u-

ence in the electronegativity and energy. More work is

needed to study the di�erent contributions to energy in

order to achieve a good understanding of the oxide

composition dependence of the reaction energy.

The annealed samples have been denoted with solid

symbols. High temperature treatments increase the

mobility of the species composing the oxide, so it is

expected that these samples will be nearer to the ther-

modynamic equilibrium. Therefore, nearness to the

dotted line of Fig. 3 is a measure of the proximity to the

thermodynamic equilibrium. All of the PECVD oxide

energies are near the equilibrium one, but, of course,

annealed oxides have in all cases greater absolute values

than the as-deposited oxides. The increase of the energy

with temperature treatment corroborates that the oxide

tends to Si±O and Si±H formation if there are no kinetic

limitations.

Low temperature LPCVD oxides (LTO) have a

random behaviour and are far from equilibrium. These

oxides are expected to su�er great structural changes if

submitted to high temperature treatments.

6. Conclusions

The Si±O, Si±H and Si±OH bond concentrations in

the PECVD silicon oxides were calculated from the in-

frared spectra by applying a correction factor to their

band areas. From these results, we can conclude that it is

Fig. 3. Bond reaction energy DE versus stoichiometric ratio x

(O/Si).

Fig. 2. Si±O and Si±H bond concentrations versus ¯ow ratio.

J.A. Moreno et al. / Microelectronics Reliability 40 (2000) 609±612 611

possible to obtain silicon oxides with low hydrogen

content by using values of the ¯ow ratio R (N2O/SiH4)

between 23 and 32 without the need of a thermal an-

nealing.

A thermodynamic model of the local bonding has

been calculated according to the reaction

Si±Si�O±H! Si±O� Si±H

for bond interchange. The absolute values of the reac-

tion energy for the annealed oxides are always greater

than the ones of the corresponding as-deposited oxides.

The annealed samples are expected to be near the ther-

modynamic equilibrium because high temperatures in-

crease the species mobility. Consequently, the ®tted line

to the DE values of the annealed samples can be used as

a reference for predicting the oxide stability under

thermal changes in the subsequent processes. Nearness

to this line means that the oxide is near equilibrium and

that it will be quite insensitive to the thermal postpro-

cessing. All the PECVD oxide are near the equilibrium

line. The LTO oxides exhibit lower absolute values of

the reaction energy, resulting in an almost random be-

haviour of the local bonding. These oxides are far from

equilibrium due to the low synthesis temperature. The

behaviour of the APCVD (UN) oxides is similar to that

of the PECVD samples.

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