Surface Science 486 (2001) 213±225
www.elsevier.nl/locate/susc
Silicon nitride chemical vapor deposition from
dichlorosilane and ammonia: theoretical study of surface
structures and reaction mechanism
Alexander A. Bagatur'yants a, Konstantin P. Novoselov b,
Andrei A. Safonov a,b, J. Vernon Cole c, Matthew Stoker c, Anatoli A. Korkin c,*
a
c
Photochemistry Center, Russian Academy of Sciences, ul. Novatorov 7a, Moscow, 117421 Russia
b
AOZT Soft-Tec, Nakhimovskii Pr. 34, Moscow, 117218 Russia
Digital DNA Laboratories, Semiconductor Products Sector, Motorola, Inc., A-Z09-M360, 2200 W. Broadway Road, Mesa, AZ 85202,
USA
Received 8 December 2000; accepted for publication 26 March 2001
Abstract
The structure of a Si3 N4 ®lm and the mechanism of Si3 N4 ®lm growth along the [0 0 0 1] crystal direction during
chemical vapor deposition have been examined using ab initio MP2/6-31G** calculations. The silicon nitride (0 0 0 1)
surface and deposited (chemisorbed) species on this surface were described using cluster models. It was found that the
dangling bonds of chemically bound Si and N atoms on the bare surface are relaxed to form additional p bonds or
Si@N surface double bonds. Energies of reaction and activation energies were calculated and the process of Si3 N4 ®lm
growth was analyzed. It was found that the removal of chemically bound hydrogen from the surface is the rate-controlling step of the deposition process. Ó 2001 Elsevier Science B.V. All rights reserved.
Keywords: Ab initio quantum chemical methods and calculations; Models of surface chemical reactions; Silicon nitride; Chemical
vapor deposition modeling; Models of surface kinetics; Surface chemical reaction; Clusters
1. Introduction
Silicon nitride is a material of great technological importance because of its electronic and optical properties (high dielectric constant and large
band gap), mechanical strength and hardness, and
exceptional thermal and chemical stability. There-
*
Corresponding author. Tel.: +1-480-655-3171; fax: +1-480655-5013.
E-mail address: [email protected] (A.A. Korkin).
fore, silicon nitride ®lms are widely used in microelectronics, in solar cells and for mechanical and
other applications [1±8]. The properties and quality of these ®lms are determined by their structure
and stoichiometry, which, in turn, strongly depend
on deposition conditions such as temperature,
pressure, and gas-phase composition [2]. Therefore, a comprehensive understanding of the deposition process could facilitate the development
of a reliable method for obtaining silicon nitride
®lms with the desired properties. Such an understanding cannot be achieved without knowledge of
the surface structure of a growing ®lm and the
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 1 0 5 0 - 0
214
A.A. Bagatur'yants et al. / Surface Science 486 (2001) 213±225
mechanism of chemical reactions that can proceed
at the surface.
The crystal structure of silicon nitride is
well documented [9±14]. Several papers have
been published on the nitridation of Si(1 0 0) and
Si(1 1 1) surfaces [15±24]. In particular, it was
shown [23,24] that the nitridation of a Si(1 1 1)
surface with NH3 gives rise to the formation of
b-Si3 N4 with the Si3 N4 (0 0 0 1) surface. Atomic
layer selective deposition of Si3 N4 on Si(1 0 0) was
studied in Refs. [25,26]. Stoichiometric ®lms were
deposited on Si(1 0 0) substrates by the atomic
layer controlled growth of Si3 N4 through selflimiting surface reactions between SiCl4 and NH3
[26]. The early stages of nitridation of the (2 1)
reconstructed Si(1 0 0) surface with NH3 (NH3
adsorption and initial decomposition) have also
been studied theoretically by DFT B3LYP calculations [27].
Empirical force ®elds have been developed for
silicon nitride and used for predicting its bulk
properties such as equilibrium lattice parameters,
phonon dynamics, thermodynamic properties, etc.
[28±33]. Recently, large-scale molecular dynamics
simulations have been performed to study the
structure, dynamics, and mechanical behavior of
cluster-assembled Si3 N4 and the silicon/silicon
nitride interface [34±37]. Several ab initio calculations have been performed to determine the
properties and band structure of bulk silicon nitride [38±43].
Chemical vapor deposition (CVD) from a
mixture of dichlorosilane (DCS) and ammonia is
one of the commonly used methods for obtaining
silicon nitride ®lms [8]. In our recent theoretical
studies we have shown that at lower temperatures
the one-step bimolecular reaction between DCS
and ammonia, which leads to SiN bond formation
and HCl elimination, dominates over the two-step
decomposition±insertion reaction path [44,45]. We
also reported preliminary results on the surface
chemistry of silicon nitride, including a proposed
CVD mechanism with estimated kinetic parameters [44,45]. In this work, we present the results of
a detailed ab initio cluster simulation of the silicon
nitride surface structures and surface reactions
related to silicon nitride CVD from DCS and
ammonia.
2. Computational details
Quantum chemical calculations were performed
using the GAMESS [46] and G A U S S I A N 9 8 [47]
program packages. Geometry optimization was
performed using the 6-31G** basis set with the
Mùller±Plesset second-order perturbation theory
(MP2).
Atoms in silicon nitride clusters were divided
into two groups: ``active'' (positions optimized) and
``inactive'' (®xed coordinates). The active group
included selected surface atoms (``active sites'') and
any chemisorbed groups or gas-phase molecules
reacting with the surface. The inactive group included boundary crystal atoms chemically bound
to the active-site atoms and hydrogen atoms saturating the broken bonds of the boundary atoms.
The hydrogen atoms were placed along the deleted
crystal Si±N bonds and ®xed at standard Si±H or
respectively).
N±H distances (1.46 and 1.01 A,
Vibrational frequencies were calculated for all
the stationary points found on the potential energy
surfaces. For each case we con®rmed that the
transition states corresponded to true saddle points
(only one imaginary frequency), and that the vibrational mode corresponding to this imaginary
frequency actually described the motion from the
reagent(s) to the product(s). The vibrations of the
inactive atoms of surface clusters were eliminated
by setting their masses to 105 amu. All the energies
given below include zero-point energy (ZPE) corrections calculated at the MP2/6-31G** level, unless otherwise speci®ed.
Although structural parameters for the various
transition states were calculated, this paper focuses
primarily on the reaction energetics. A detailed
discussion of the geometrical structures of the
transition states is beyond the scope of this paper,
the corresponding data may be obtained upon
request from the authors.
3. Results and discussion
3.1. Surface structure and relaxation
Depending on the process conditions, Si3 N4
CVD on a silicon surface may produce either
A.A. Bagatur'yants et al. / Surface Science 486 (2001) 213±225
amorphous or crystalline ®lms with di€erent degrees of crystallinity [10±13,23,24]. The crystal
structures of both a- and b-modi®cations of Si3 N4
consist of nearly planar (0 0 0 1) layers (four and
two layers per unit cell for a- and b-Si3 N4 , respectively) [9±14].
215
In our modeling study, we constructed surface
clusters assuming that the growing ®lm corresponds to a b-Si3 N4 (0 0 0 1) surface (see Fig. 1). A
bare b-Si3 N4 (0 0 0 1) layer consists of seven-atom
fragments (islands). An island has the same stoichiometry as the ®lm and contains the central
Fig. 1. (a) Schematic view of two upper crystal layers of b-Si3 N4 (0 0 0 1) and (b) a seven-atom Si3 N4 island resting on three neighboring
islands of the preceding layer. Atoms of the upper and lower layers are displayed as light gray and dark gray circles, respectively; large
circles designate silicon atoms and small circles designate nitrogen atoms.
216
A.A. Bagatur'yants et al. / Surface Science 486 (2001) 213±225
nitrogen atom (Nc ), three silicon atoms bound to
Nc , and three terminal nitrogen atoms (Nt ) bound
to the silicon atoms. Each Si or Nt atom has bonds
connecting it with the upper and underlying layers
and with atoms lying in the same layer (two for Si
and one for Nt ). All the Si±N bonds converging at
an Nc atom lie in the same plane. The neighboring
islands in the same layer are linked to each other
by bridges extending through the adjacent layers.
The shortest of these are diatomic Si±Nt bridges.
Note that similar seven-atom islands with the same
topology are the basic elements of crystal layers in
the a-Si3 N4 structure as well, though their geometrical structures in the a- and b-modi®cations of
Si3 N4 are slightly di€erent (see, for example, Ref.
[13]).
Each SiNt pair on a bare b-Si3 N4 (0 0 0 1) surface has two dangling bonds, which combine to
form a p bond or surface dimer, similar to those on
the Si(1 0 0) surface (see, e.g., Ref. [48]). We call
these dimers diatomic active sites (DASs). A sevenatom island on the surface consists of three of
these SiNt surface pairs linked through the central
nitrogen atom.
The simplest cluster describing a DAS (see Fig.
2a) has a single SiNt surface pair of atoms as an
active part of the cluster (single-DAS cluster
model, 1-DAS). The nearest crystal environment
(inactive part) was constructed as described above
in the computational section. The applicability of
small 1-DAS-based models was analyzed by comparison with calculations for a larger cluster (Fig.
2b). This cluster includes three DAS fragments
bound to the central Nc atom in a seven-atom
surface fragment as described above (seven-atom
surface model, 3-DAS). Its 1st and 2nd coordination spheres were constructed in the same way as
in the 1-DAS model.
The relaxed geometries of the bare and Hterminated Si3 N4 (0 0 0 1) surfaces and the hydrogenation energies of surface Si@N bonds were
calculated using both 1-DAS and 3-DAS models
(see Table 1). The calculated lengths of the hydrogenated H±[Si±Nt ]±H and formally double
Si@Nt surface bonds are in the range 1.73±1.74
and 1.61 A,
respectively, for all the clusters
A
under consideration. That is, the p-component of
the double Si@Nt surface bond reduces the bond
Fig. 2. (a) 1-DAS and (b) 3-DAS clusters used in calculations.
Atoms that belong to the active part of the clusters are indicated in bold letters.
Note for comparison
length by about 0.12±0.13 A.
that the lengths of the Si±N bonds lying in the
(0 0 0 1) plane in the bulk a- and b-Si3 N4 crystals
[9±14]. According
fall in the range of 1.72±1.74 A
to X-ray structural data for (t-Bu)2 Si@N±Si(t-Bu)3
[49,50] and (t-Bu)2 Si@N±Si(t-Bu)2 Ph [51], the
lengths of the single and double SiN bonds are (to
rethe second decimal place) 1.57 and 1.70 A,
as
spectively, with the same di€erence of 0.13 A
was found in our calculations.
In the symmetrical bare 3-DAS and fully hydrogenated (3-DAS)H6 clusters, the calculated
lengths of the single Si±Nc bonds (1.75 and 1.76 A,
respectively) are very close to each other. However,
in the unsymmetrical (3-DAS)H2 and (3-DAS)H4
clusters, the ±Si±Nc bonds adjacent to the single
A.A. Bagatur'yants et al. / Surface Science 486 (2001) 213±225
217
Table 1
and hydrogenation energies (DE
Geometrical relaxation (bond lengths R and shifts Dza along the vertical c-axis, for surface atoms, A)
(H2 )b , kcal/mol) of 1-DAS and 3-DAS clusters, modeling the Si3 N4 (0 0 0 1) surface
System
R
Nc ±Si±
1-DAS
(1-DAS)H2
3-DAS
(3-DAS)H2
(3-DAS)H4
(3-DAS)H6
1.72
1.80
1.78c
1.76
Dz
Nc ±Si@
1.69
1.75
1.73c
1.71
Si±Nt
1.74
1.73
1.74c
1.74
Si@Nt
Nc
1.61
0
0
0.07
0.07
0.08
0.07
1.61
1.61c
1.61
DE (H2 )
±Si@
0.25
0.24
0.26c
0.27
±(H)Si±
0.20
0.01
0.02c
0.01
Nt @
0.19
0.10
0.09
0.09
(H)Nt ±
0.13
0.04
0.04
0.04
66.9
70.9
70.1
69.2
a
The positive sign indicates upward from the bulk crystal position.
Geometry optimization was made with excluding the p-polarization functions on boundary hydrogen atoms and the ®nal energy
was evaluated with the full 6-31G** basis set at these geometries.
c
Average of two slightly di€erent values.
b
Si±Nt bonds are considerably longer than the @Si±Nc
bonds adjacent to the double Si@Nt bond. This
can be explained by the p-conjugation e€ects between a double Si@Nt bond and the Nc 2pz (p) lone
pair. The lengths of the @Si±Nc and ±Si±Nc bonds
in the small 1-DAS cluster are shorter (by 0.04±
than the lengths of the corresponding
0.06 A)
bonds calculated for the larger 3-DAS-based
clusters. This is evidently due to the geometry
restrictions (the ®xed position of the Nc atom) in 1DAS. Still, we can see that the hydrogenation
e€ects on the bond lengths of the 1-DAS cluster
correspond most closely to the results for the (3DAS)H4 cluster with only one unsaturated Si@Nt
bond. Thus, the 1-DAS-based model agrees rather
well with the Si±Nt and Si@Nt bond lengths in the
larger 3-DAS-based models. The vertical relaxation of the surface atoms is well described by their
shifts Dz along the vertical c-axis (orthogonal to
the (0 0 0 1) crystal plane) in reference to the unrelaxed crystal positions. The calculated vertical
displacements are rather large (see Table 1), which
re¯ects the fact that the quasi-two-dimensional
seven-atom fragment of the crystal surface layer is
relatively non-rigid in the vertical direction. In all
the 3-DAS-based clusters, the central Nc atom is
shifted up, which may be explained by the absence
of the upper crystal layer. The neighboring unsaturated Si atom (designated as ±Si@) is shifted
down, resulting in a pyramidalization of the Nc
atom. If a SiN bond neighboring to Nc is saturated, the corresponding saturated Si atom (desi-
gnated as ±(H)Si±) remains almost intact. Finally,
the Nt atom is displaced up, and this displacement
is larger for the unsaturated Si@Nt bonds. The
e€ect of geometry restrictions in the 1-DAS cluster
on the vertical relaxation is mainly revealed for the
vertical displacement of the ±(H)Si± atoms.
The calculated energies of the successive hydrogenation of 3-DAS and the hydrogenation of
1-DAS are also given in Table 1. The successive hydrogenation energies obtained for 3-DAS
lie within (71±69) kcal/mol and only slightly decrease in absolute value as the cluster becomes
more saturated. These values indicate that the hydrogenation energy of each Si@Nt bond is nearly
independent of the state of the adjacent SiN
bonds. The hydrogenation energy of the Si@Nt
bond calculated for the small 1-DAS cluster is
lower than the values calculated for the 3-DAS
clusters, though it is rather close to the hydrogenation energy of the last Si@Nt bond in the
(3-DAS)H4 cluster. This means that geometry relaxation e€ects are stronger for the unsaturated
Si@Nt bond than for the saturated one (the
structure of the latter deviates less from the ideal
crystal positions).
Finally, we may infer that the 1-DAS cluster
describes relatively well both the bond length
changes and the reaction energy of an isolated
Si@Nt double bond on the silicon nitride surface.
Therefore, we will primarily use the 1-DAS cluster
as a basis for constructing the various surface
structures.
218
A.A. Bagatur'yants et al. / Surface Science 486 (2001) 213±225
Table 2
Energies (kcal/mol) of intermediate complexes Ec , activation energies Ea , reaction energies Er , and reaction energies for gas-phase
analogues of surface reactions Eg ph for addition reactions of gas-phase molecules to the bare surface
Number
Surface reaction
SI.1
SI.2b
SI.3
SI.4a
SI.4b
H2 ‡ [Si@N] ˆ H[Si±N]H
HCl ‡ [Si@N]ˆ Cl[Si±N]H
NH3 ‡ [Si@N] ˆ H2 N[Si±N]H
SiH2 Cl2 ‡ [Si@N] ˆ Cl[Si±N]SiH2 Cl
SiH2 Cl2 ‡ ‰Si@N] ˆ H[Si±N]SiHCl2
a
b
Ea
Ec
0.2
13.4
34.9
8.1
8.1
19.2
10.2
27.1
4.2
4.1
Er
61.0
90.0
77.5
86.7
84.7
Eg
ph
a
47.1
68.8
61.0
67.2
66.2
The surface part [SiN] was formally replaced with the H2 SiNH group.
N±Cl bond formation is unfavorable and is not considered in this paper.
3.2. Gas±surface reactions on the bare surface:
dissociative chemisorption of H2 , HCl, NH3 , and
SiH2 Cl2
The calculated reaction energies Er , energies of
formation of the intermediate reaction complexes
Ec (non-dissociative adsorption), and activation
energies (transition state energies) Ea for chemisorption of H2 , HCl, NH3 , and SiH2 Cl2 on the
1-DAS cluster are presented in Table 2. In the
chemical formulas below, we will indicate explicitly only the active atoms of surface clusters for
short. The ``surface'' part of active cluster atoms
will be enclosed in square brackets, and the surface
groups chemisorbed on a cluster will be indicated
outside the brackets. Thus, 1-DAS (see Fig. 2a)
will be designated as [N@Si], whereas a 1-DASbased cluster with R1 and R2 chemical groups
chemisorbed on it will be designated as R1 ±[N±Si]±
R2 or simply R1 [N±Si]R2 .
Hydrogen chemisorption (reaction SI.1) is the
least exothermic reaction. Two paths of SiH2 Cl2
adsorption and dissociation on 1-DAS yielding
the Si±Cl or Si±H bonds are energetically nearly
equivalent ( 86.7 and 84.7 kcal/mol, respectively). For reactions SI.2±SI.4, we found relatively
stable intermediate complexes. The hydrogen molecule does not form a stable complex. Its reaction
with the surface Si@N bond is the only one whose
transition-state energy is above the energy of the
initial state, that is, the separated H2 and 1-DAS.
The transition state energies for reactions SI.2±
SI.4 are lower than the energies of the separated
reagents. This means that for all these reactions
with the bare surface, there are barrierless reaction
paths for the formation of the chemisorbed addition products.
We have computed gas-phase analogues of reactions SI.1±4 by formally replacing the 1-DAS
cluster with the H2 SiNH moiety (see Table 2). The
dissociative chemisorption on 1-DAS is more
exothermic than the corresponding reactions with
H2 SiNH because of combined strain and substituent e€ects in the 1-DAS cluster. We also
calculated the reaction energies Er for the dissociative chemisorption reactions of aminochlorosilane
SiH2 (NH2 )Cl, a product of the bimolecular gas
phase reaction: SiH2 Cl2 ‡ NH3 ! SiH2 …NH2 †Cl ‡
HCl [44,45].
Er
(SI.5a) SiH2 (NH2 )Cl + [Si@N] ˆ ClH2 Si(HN)±
[Si±N]±H
(SI.5b) SiH2 (NH2 )Cl + [Si@N] ˆ Cl±[Si±N]±
SiH2 (NH2 )
(SI.5c) SiH2 (NH2 )Cl + [Si@N] ˆ H±[Si±N]±
SiH(NH2 )Cl
(SI.5d) SiH2 (NH2 )Cl + [Si@N] ˆ H2 N±[Si±N]±
SiH2 Cl
84.4
83.5
83.6
83.7
Chemically, reactions SI.5a, SI.5b, and SI.5c are
analogues of reactions SI.3, SI.4a, and SI.4b, respectively, whereas reaction SI.5d combines features of reactions SI.3 and SI.4. The energies of
reactions SI.5a±SI.5d are similar within 1 kcal/mol
and close to the average value of reaction energies
for reactions SI.3, SI.4a, and SI.4b.
We used the results of quantum-chemical calculations for reactions SI.1±SI.4b in order to estimate the equilibrium composition of the Si3 N4
surface exposed to a mixture of DCS and NH3 .
A.A. Bagatur'yants et al. / Surface Science 486 (2001) 213±225
219
Table 3
Equilibrium constants Ki …T † (cm3 /mol) for reactions between gas-phase molecules and a diatomic active site on the surface for di€erent
temperatures
Number
T (K)
Surface reaction
800
SI.1
SI.2
SI.3
SI.4a
SI.4b
H2 ‡ [Si@N] ˆ H[Si±N]H
HCl ‡ [Si@N] ˆ Cl[Si±N]H
NH3 ‡ [Si@N] ˆ NH2 [Si±N]H
SiH2 Cl2 ‡ [Si@N] ˆ Cl[Si±N]SiH2 Cl
SiH2 Cl2 ‡ [Si@N] ˆ H[Si±N]SiHCl2
7.0 10
1.6 1021
1.0 1018
2.9 1019
5.4 1018
The equilibrium constants Ki for reactions SI.1±
SI.4b were calculated from the partition functions
Qads for the groups on the surface and the rotational and vibrational partition functions for the
free gas molecules Qgr;v and for the diatomic active
sites on the surface QDAS using the relationship:
3=2
2p
h2
Qads
DEr
exp
Ki ˆ
m i kB T
Qgr;v QDAS
kB T
The calculated equilibrium constants are given in
Table 3 for temperatures from 800 to 1200 K.
Next, we calculated the surface coverage with
the corresponding chemisorbed groups using the
equations:
hi ˆ
Ki ‰Xi Š
P
;
1‡
Ki ‰Xi Š
iP1
h‰Si@NŠ ˆ
1‡
1
P
900
14
Ki ‰Xi Š
iP1
where [Xi ] is the concentration of the ith gas-phase
component, hi is the coverage for the corresponding surface group, and h‰Si@NŠ is the fraction
of vacant surface sites. For simplicity, we considered only equilibria described by reactions SI.3,
SI.4a, and SI.4b.
The results for several mixtures of DCS and
NH3 with partial pressures of DCS and NH3 respectively of (a) 0.1 and 1.0 Torr, (b) 1.0 and 10.0
Torr, and (c) 1.0 and 1.0 Torr are presented in Fig.
3. The surface is almost completely covered by
chemisorbed groups at temperatures below 1300
K. For mixtures with a DCS:NH3 ratio of 1:10 the
equilibrium concentrations of ±NH2 and SiH2 Cl
groups chemisorbed at the surface become equal at
T ˆ 1000 K, whereas the equilibrium concentration of SiHCl2 groups remains substantially
1000
12
9.8 10
3.9 1018
5.7 1015
9.1 1016
1.9 1016
1100
11
3.3 10
3.3 1016
9.4 1013
9.3 1014
2.1 1014
1200
10
2.0 10
6.8 1014
3.3 1012
2.2 1013
5.5 1012
2.1 109
2.7 1013
2.1 1011
1.0 1012
2.7 1011
lower over the entire temperature range. For
the 1:1 mixture, the chemisorbed SiH2 Cl groups
dominate over the entire range of temperatures
considered here. This result explains why a large
excess of NH3 is normally used in the CVD process (see, eg., Refs. [8,44]). We may also infer that
the fraction of vacant surface sites remains negligibly small at typical CVD conditions, implying
that ®lm growth is limited not by chemisorption
but by reactions between chemisorbed species and
other gas-phase or surface species.
3.3. Gas±surface reactions and surface reactions
between surface groups
Within the model of ®lm growth along the
crystal c-axis (perpendicular to the Si3 N4 (0 0 0 1)
surface) assumed in this work, each seven-atom
surface island on the perfect Si3 N4 (0 0 0 1) surface
grows independently of the other surface islands,
bridging three neighboring seven-atom islands in
the previous layer (see Fig. 1b). According to the
stoichiometry of the overall process …3SiH2 Cl2 ‡
4NH3 ˆ Si3 N4 ‡ 6HCl ‡ 6H2 †, six HCl and six H2
molecules must be desorbed for each constructed
seven-atom island. In Fig. 1, it is also seen that 12
new Si±N bonds must be formed during the
growth of a seven-atom surface island.
Each Si or N atom incorporated into the
growing ®lm must undergo a series of reactions
which increase the number of the Si±N bonds
connecting it with the bulk. A silicon atom sequentially changes the number of bonds with
the bulk from 1 (chemisorbed Si, [s]±SiX3 ), to 2
(bridging Si, [s]@SiX2 ), to 3 (surface Si [s]BSiX),
220
A.A. Bagatur'yants et al. / Surface Science 486 (2001) 213±225
Fig. 3. Equilibrium concentrations of various surface groups as functions of temperature for DCS and NH3 gas mixtures with partial
pressures of (a) 0.1 and 1.0 Torr, (b) 1.0 and 10.0 Torr, and (c) 1.0 and 1.0 Torr, respectively.
and ®nally to 4 in the bulk. A nitrogen atom
changes its number of Si±N bonds from 1 to 3.
According to the results presented above, the
growing surface is almost completely covered by
chemisorbed surface groups (chemically terminated surface). Under real conditions, all combinations of chemisorbed surface groups may occur
on a growing Si3 N4 surface. In a monolayer of
chemisorbed groups on an ideal Si3 N4 (0 0 0 1)
surface, the following groups (combinations) are
conceivable: H±[Si±N]±H, Cl±[Si±N]±H, NH2 ±[Si±
N]±H, Cl±[Si±N]±SiH2 Cl, H±[Si±N]±SiHCl2 , and
NH2 ±[Si±N]±SiH2 Cl. These groups may partici-
pate in further reactions with gas-phase molecules
and with neighboring surface groups to form a
new seven-atom island of the next surface layer.
The formation of closed un®nished fragments of
the new island (bridges linking two of three islands
serving as a base for the new one, see Fig. 1b) is a
necessary step in this process. Therefore, along
with reactions of surface groups with gas-phase
molecules, we consider bridge closure reactions
and reactions of such un®nished bridged structures.
Two chemically di€erent types of reactions may
result in the formation of a new Si±N bond at the
A.A. Bagatur'yants et al. / Surface Science 486 (2001) 213±225
221
Table 4
Activation Ea and reaction Er energies for various surface reactions (kcal/mol)
Number
Surface reaction
Ea
Er
Reactions with surface atoms
SII.1
H[N±Si]±Cl ‡ NH3 ˆ H[N±Si]±NH2 ‡ HCl
SII.2
H[N±Si]±H ‡ NH3 ˆ H[N±Si]±NH2 ‡ H2
SII.3
H[Si±N]±H ‡ SiH2 Cl2 ˆ H[Si±N]±SiH2 Cl ‡ HCl
SII.4
H[Si±N]±H ‡ SiHCl ˆ H[Si±N]±SiH2 Cl
SII.5
H[Si±N]±H ‡ SiH(NH2 ) ˆ H[Si±N]±SiH2 NH2
SII.6
H[N±Si]±H ‡ SiHCl ˆ H[N±Si]±SiH2 Cl
SII.7
H[N±Si]±H ‡ SiH(NH2 ) ˆ H[N±Si]±SiH2 NH2
25.1
49.4
11.6
9.4
19.3
0.9
11.9
10.3
7.5
0.0
64.9
54.1
43.9
29.0
Reactions of chemisorbed surface groups
SIII.1
Cl[Si±N]±SiH2 Cl ‡ NH3 ˆ Cl[Si±N]±SiH2 ±NH2 ‡ HCl
SIII.2
H[Si±N]±SiHCl2 ‡ NH3 ˆ H[Si±N]±SiHCl±NH2 ‡ HCl
SIII.3
H[N±Si]±NH2 ‡SiH2 Cl2 ˆ H[Si±N]±NH±SiH2 Cl ‡ HCl
SIII.4
Cl[Si±N]±SiH2 Cl ˆ Cl[Si±N]±(HSi:) ‡ HCl
SIII.5
Cl[Si±N]±SiH2 Cl ˆ Cl[Si±N]±(ClSi:) ‡ H2
SIII.6
H[Si±N]±SiHCl2 ˆ H±[Si±N]±(ClSi:) ‡ HCl
13.2
21.5
7.6
72.2
76.9
72.1
8.5
6.4
0.3
63.1
30.6
55.7
Reactions of bridge closure
SIV.1
{Si}±NH2 ‡ fNg±SiH2 Cl ˆ fSig±NH±SiH2 ±fNg ‡ HCl
SIV.2
fSig±NH2 ‡ fNg±SiH2 Cl ˆ fSig±NH±SiHCl±{N} ‡ H2
SIV.3
fSig±NH2 ‡ fNg±SiH2 (NH2 ) ˆ fSig±NH±SiH±(NH2 )±fNg ‡ H2
18.4
44.7
41.5
5.1
22.6
18.5
Reactions with bridged Si atom
SV.1
H[N±Si](H)(H) ‡ NH3 ˆ H[N±Si](H)(NH2 ) ‡ H2
SV.2
H[N±Si](H)(H) ˆ H[N±Si:] ‡ H2
SV.3
H[N±Si:] ˆ [N@Si]H
SV.4
H[N±Si](H)(Cl) ˆ H[N±Si:] ‡ HCl
SV.5
H[N±Si](H)(NH2 ) ˆ H[N±Si:] ‡ NH3
38.2
76.9
77.0
70.8
78.6
17.4
28.9
13.5
56.6
46.3
surface: (1) reactions between saturated Si and N
centers with elimination of either HCl or H2 and
(2) addition reactions that involve a silylene-type
center. Among reactions of the ®rst-type, those
leading to desorption of HCl are energetically
more favorable [44,45].
The calculated results for various types of surface reactions are presented in Table 4. We classi®ed these reactions according to the sequential
steps of the ®lm growth. Most of the reactions
presented in Table 4 were calculated using 1-DASbased models. However, for the bridge closure
reactions SIV.1±SIV.3, we used reduced models in
which only the reacting surface groups were included in the active part of the clusters. In these
cases, the base surface atoms were ®xed at their
crystalline positions and terminated with hydrogen
atoms as described above in Section 2. The corresponding clusters are shown in Fig. 4. In the
designations of these reduced clusters, we indi-
cated the ®xed base surface atoms in braces (see
Table 4).
First, we consider reactions SII.1±SII.7 of gasphase molecules with the Si3 N4 surface terminated
with H and Cl atoms. The main trends are similar
to those found previously for analogous gas-phase
reactions [44,45]. The substitution reaction with
the displacement of HCl (SII.1) has a lower activation energy than similar reaction with elimination of H2 (SII.2). On the other hand, elimination
of H2 is exothermic, whereas the elimination of
HCl is endothermic. Insertion of silylenes into the
N±H bond is characterized by a higher activation
barrier than their insertion into the Si±H bond, but
the reaction is more exothermic in the ®rst case.
Insertion reactions of Si±H(NH2 ) have higher barriers and are less exothermic than the corresponding reactions of SiHCl. The insertion of
silylenes into the Si±H bond will produce nonstoichiometric defects with Si±Si bonds. Hence, the
222
A.A. Bagatur'yants et al. / Surface Science 486 (2001) 213±225
Fig. 4. Cluster models for the interaction between ±NH2 and ±SiH2 Cl groups on neighboring seven-atom islands on the Si3 N4 surface
that can participate in the reaction of bridge closure: (a) the surface groups before interaction and (b) the transition state for the
reaction of Si±N bond formation with the elimination of HCl. Fixed boundary surface SiH3 and NH2 groups are enclosed in the
rectangular box.
presence of silylenes in the gas phase (especially,
SiHCl) likely results in a nonstoichiometric silicon
rich ®lm.
The substitution reactions of surface groups
with the elimination of HCl (reactions SIII.1±
SIII.3), are characterized by relatively low activa-
tion barriers, similar to their gas-phase analogues
[44,45]. The formation of silylene centers on the
surface through reactions SIII.4±SIII.6 is characterized by rather high activation barriers.
Reactions SIV.1±SIV.3 in Table 4 are bridge
closure reactions. Fig. 4 illustrates the bridge clo-
A.A. Bagatur'yants et al. / Surface Science 486 (2001) 213±225
223
results for reaction SV.3 indicated that the :Si±N
silylene and Si@N double-bond structures are
separated by a high activation barrier.
3.4. Implications for the mechanism of Si3 N4 ®lm
growth from DCS and NH3
Fig. 5. Clusters modeling bridged (a) saturated ±H[N±Si]H2 ±
and (b) sylilene ±H[N±Si:] groups linking two neighboring
seven-atom islands on the surface. Fixed boundary surface SiH3
and NH2 groups are enclosed in the rectangular box.
sure process for reaction SIV.1. As in similar gasphase reactions and surface reactions described
above, HCl desorption has a lower barrier than
desorption of H2 .
The ®nal section of Table 4 shows the results
obtained for surface reactions at bridged surface
sites. The clusters used in reactions SV.1±SV.3 are
illustrated in Fig. 5. Reaction SV.1 is analogous to
reaction SII.2. In the case of the bridged site, the
geometrical restrictions for this substitution reaction are less rigid leading to a lower activation
barrier and higher exothermicity of the reaction at
the bridged site. The formation of silylene type
structures at the surface (reactions SV.2±SV.5) is
associated with high activation barriers and hardly
contributes to the entire deposition process. The
Let us now consider the construction of the
seven-atom island resting on three neighboring
islands of the underlying layer as a sequential
process of forming Si±N bonds. Twelve Si±N
bonds should be formed in order to complete such
an island: six bonds linking the new island with the
underlying islands and six bonds within the sevenatom island itself. According to stoichiometry, this
process requires that three SiH2 Cl2 molecules and
four NH3 molecules are consumed from the gas
phase (3SiH2 Cl2 ‡ 4NH3 ˆ Si3 N4 ‡ 6HCl ‡ 6H2 ).
Since seven gas phase molecules must participate in the full cycle of construction of a new
seven-atom island, seven new Si±N bonds will be
formed through gas±surface reactions and only
®ve Si±N bonds will be formed via reactions
between surface groups. However, only six HCl
molecules are eliminated from the surface in this
process. This simple count indicates that either one
of the gas-phase molecules must react with the
surface with the elimination of H2 or one H2
molecule must be removed from the surface
through direct dissociation from a ±SiH2 ± group
or from a @SiH±NH± diatomic site. We have demonstrated above that the abstraction (desorption)
of an H2 molecule from a surface ±SiH2 Cl group
(reaction SIII.5) or from a bridged SiH2 group
(reaction SV.2) to form a silylene surface
group requires a very high activation barrier of
77 kcal/mol. Of the gas±surface reactions involving the elimination of H2 , the lowest activation
energy is encountered in the reaction of a gasphase NH3 molecule with a bridged Si center (reaction SV.1), which has an activation barrier of
38 kcal/mol. Thus, we may infer that the removal
of one H2 molecule from the surface, either
through a gas-phase reaction with H2 elimination
or through the desorption of an H2 molecule from
the surface, must be the rate-determining step of
the overall Si3 N4 ®lm growth. Our results also
demonstrate that the deviation from stoichiometry
224
A.A. Bagatur'yants et al. / Surface Science 486 (2001) 213±225
observed in the Si3 N4 ®lms deposited from a
mixture of NH3 and SiH2 Cl2 are expected to be
primarily associated with SiHx and NHx (x ˆ 1, 2)
groups buried in the ®lm because of the high activation energy of H2 desorption from the surface
of the growing Si3 N4 ®lm. On the other hand, the
deviations from stoichiometry associated with the
formation of Si±Si bonds via reactions with either
gas-phase or surface silylene groups are less probable.
from the surface is the rate determining reaction
step in the silicon nitride ®lm growth process.
Acknowledgements
The authors would like to thank Dr. Edward
Hall and Dr. William Johnson of Motorola for
their support and encouragement. This work has
been supported by Motorola's Digital DNA laboratories.
4. Conclusions
The mechanism of Si3 N4 ®lm growth was examined by ab initio MP2/6-31G** calculations
using the cluster approach. The surface of a growing
®lm was considered to be structured like the
Si3 N4 (0 0 0 1) crystal plane. It has been found that
the dangling bonds on the bare surface are relaxed
to form diatomic >Si@N± surface groups. The
is considercalculated Si@N bond length 1.61 A
ably shorter than typical lengths of crystalline Si±
and the surface atoms of
N bonds (1:74±1:76 A),
these diatomic groups are signi®cantly displaced
from their bulk crystalline positions. The >Si@N±
surface groups are combined by three bonds
through a central N atom to form a pattern of
isolated quasi-planar seven-atom islands at the
surface. Examining the sequential hydrogenation
of separate >Si@N± groups in such an island has
indicated that each group can be considered as an
independent surface site. The following reaction
steps have been considered in the ®lm growth: (1)
reactions between NH3 and chemisorbed chlorosilyl groups or between SiH2 Cl2 and chemisorbed
NH2 groups with HCl elimination, (2) reactions
between chemisorbed chlorosilyl and amino groups
with HCl elimination, (3) reactions between NH3
and surface SiH2 and SiH groups with H2 elimination, (4) reactions between chemisorbed NH2
and NH groups and chemisorbed SiH2 and SiH
surface groups with H2 elimination, and (5) direct
H2 elimination from chemisorbed SiH2 groups
with the formation of bridged silylene groups (6)
H2 desorption from the surface HSiNH group and
formation of a Si@N surface dimer: diatomic active site. It is concluded that elimination of H2
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