LorentzLorenz correlation for reactively plasma deposited SiN films
A. K. Sinha and E. Lugujjo
Citation: Applied Physics Letters 32, 245 (1978); doi: 10.1063/1.90006
View online: http://dx.doi.org/10.1063/1.90006
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Lorentz-Lorenz correlation for reactively plasma deposited SiN films
A. K. Sinha and E. Lugullo8 )
Bell Laboratories, Murray Hill. New Jersey 07974
(Received 13 September 1977; accepted for publication 29 November 1977)
Amorphous Si-N films were prepared by reactive plasma deposition from SiH4 and NH J in a radial flow
reactor at 275°C, and studied using 1.8-MeV 4He+ Rutherford backscattering analysis. The films had a
Si/N ratio ranging from 0.75 to 1.5. density from 2.8 to 2.1 gcm- J • and refractive index from 1.9 to 2.3.
These data have been correlated using the Lorentz-Lorenz equation, yielding a self-consistent set of
electronic polarizabilities aN and asi' The aSi is found to be a function of the Si/N ratio. indicating the
increasing presence of Si-H complexes in off-stoichiometric films.
PACS numbers: 68.SS.+b, 68.60.+q, 77.SS.+f
Plasma-enhanced chemical vapor deposition process
can be optimized to produce a relatively thick (1 Il)
crack -resistant Si -nitride passivation layer having
excellent step-coverage characteristics and good adhesion to both Al and Au metallization. 1 The deposition
process is capable of producing a wide variety of films
with, e. g., resistivity (at 2 x 10 6 V/ cm) ranging from
104 to 10 20 n cm and density ranging from 2.1 to 2.8
g cm- 3. We have investigated these films using Rutherford backscattering analysis and provide here a
Lorentz - Lorenz correlation between the film density,
composition, and refractive index. The estimated values
of electronic polarizabilities provide a valuable insight
into the nature of Si and N atoms in these films.
Amorphous Si -N films were synthesized in a radial
flow reactor 2 using a plasma-assisted reaction between
SiH 4 and NH3 in an Ar carrier gas. A distinguishing
feature of the present improved reactor is that it contains a glow-discharge-limiting shield around the substrate table/heater assembly which permits operation
at higher rf powers (> 100 W) without any premature
reaction of SiH 4 and NH3 underneath the substrate
table. An Al cathode is located 1 in. above the substrate table. A capacitively coupled plasma is obtained
using a Varian rf power supply (13.56 MHz) and an
impedance-matching network. One set of deposition
conditions was as follows: SiH 4 -1. 8%, NH3 - 2. 2%,
remainder Ar; total gas flow - 2.3 liters/min; pressure -0.95 Torr; substrate temperature -275°C; and
rf power ranging from 100 to 350 W.
p can be deduced;
1AEM
(2)
P==-~Atu E'
where A is Avogadro's number, M is the molecular
weight, and E is the molar stopping cross section. For
one of the samples studied, the above procedure gave
the Si/N ratio as 0.96 and the denSity p as 2.58 g cm- 3.
For thicker films (1 Il) deposited under identical conditions, weight gain measurements gave p as 2.65
g cm- 3•
Figure 1 shows the variation of various film properties as a function of the nominal rf power to the plasma.
The deposition rate (DR) was nearly constant at -175
A/min; the etch rate (ER) in 7: 1 buffered HF decreased
;;;
...'" ~...
en
2
III
Iii ~
0
<T
~
•
0
~
.
•
.-------,
-.- "
TENSILE
•
COMPRESSIVE
~
'~
-2
1.6
2.8
1.4
2.6
1.2 i1!
;;
2....
1-'
in ~
~~
.
z
0..
2.4
1.0~
2.2
08
0
)(
For Rutherford backscattering analysis, 3 films,
- 2000 A thick, on (111) Si were employed. An incident
beam of 1. 8-MeV 4He+ ions was used; the spectral background due to the Si substrate was intentionally suppressed by channeling along [110] direction. The Si/N
ratio (Ns1/NrJ in the film was obtained from the ratio
of the respective areas (ASI/AN) under the peaks, since
tJl!!
• 2.0
"'z
1.8
300
:z:
<J
i
:;;~ i
~:.c
III
E.R.
200
-
(1 )
<J 100
where aN/aSI (= O. 22) is the ratio of the Rutherford
scattering cross section of N and Si atoms. From the
width of the spectra (AE) and the measured film thickness tu (using the Talystep or ellipsometry) the denSity
a.:'" ~ 200
I!!~ :.
o:.s
°
-0
O.R.
0 _ _ _ _0 - - 0 _
°
150
100
200
300
RF - POWER (WATTS)
a)
Present address: Makerere University, Uganda, East
Africa.
245
Appl. Phys. Lett. 32(4). 15 February 1978
FIG. 1. Effect of nominal rf power (reflected power = 0) on
properties of reactively plasma deposited Si-N films.
0003-6951/78/3204-0245$00.50
© 1978 American Institute of Physics
245
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with increasing power above 200 W. The refractive index (n) also decreased with rf power, as did the Si/N
ratio; however, the latter tended to level off at Si/N
-0.75. With increasing rf power, the film density increased linearly; the stress changed from low tensile
to moderate compressive at rf power'" 300 W.
n~+2p-3Eo'
x
w
a 2.2
t=
2.0
~
IEw
a:
1.8
eVD FILMS
(3)
Using Eq. (3), and giving special weight to a large
number of measurements on films with Si/N - 1. 0, we
extracted the following set of apparent polarizabilities
Ci!SI and (\IN:
(CV )SIN"'3. 06 x 10- 24 cm- 3,
sl
(aN)SIN",O. 35X 10- 24 cm- 3•
Data on films with Si/N t- 1 could be rationalized assuming that Ci!SI was a function of the Si/N ratio, i. e. ,
1O-24 CVSI = 1. 67 + 1. 39(Si/N)
so that, for these films (0. 75 < Si/N < 1. 6), Ci!SI is in the
range (2.7-3. 9)x 10- 24 cm- 3, whereas aN is assumed
to remain constant at 0.35 x 10- 24 cm- 3 for all the films.
A family of empirically derived L-L curves for Si-N
films with various ratios of Si/N (0.7-1. 8) is shown in
Fig. 2. Also shown, near the bottom, are horizontal
bars indicating that the present films span a density
range between those of amorphous Si and CVD Si3N4
films. For clarity, only the experimental points derived from Fig. 1 are shown. The following conclusions
may be drawn from the L- L correlation plots: (1) For a
given film composition, the refractive index increases
with increasing density. (2) For a given density, the
refractive index increases with increasing Si/N ratio
in the film. (3) For a given refractive index, the density increases with decreasing Si/N in the film.
The presently estimated (10 24 Ci!.)'S for Si and N may
be compared with those known for neutral noble gases,
Appl. Phys. Lett., Vol. 32, No.4, 15 February 1978
2.4
w
>
where Ci!e is the average electronic polarizability of SiN and EO is the dielectric constant of space. The assumptions implicit in the use of Eq. (3) are as follows:
(1) the Si-N film is isotropic, (2) at optical frequencies,
the polarization mode is predominantly electronic, (3)
the contributions to M (and to a lesser extent to (\Ie)
from any H atoms in Si-N films may be neglected, (4)
a self -consistent set of electronic polarizabilities may
be assigned to Si and N atoms.
246
12
~
These and other data on density, refractive index,
and composition of present Si-N films have been correlated using the Lorentz-Lorenz (L-L) equation 4:
n 2 - 1 M _ 47T A(\Ie
1.4
2.6
1.6
I
PRESENT RPD FILMS(275oc!)(700-1100·d
I
AMORp,HOUS
I--ISI
1.4 '":-----::"-::---:L_-,J-_-"J_ _.l.-_..l...-_.....L---....I
1.8
2.0
2.2
2.4
2.6
2.8
DENSITY (g cm- 3)
3.0
3.2
FIG. 2. Lorentz-Lorenz correlation for reactively plasma deposited Si-N films.
He (0.20), Ne (0.40), Ar (1. 66), Kr (2.54), Xe (4.15),
and those for ions isoelectronic with Ne: Si 4+ (?),
A1 3+ (?), Mg2+ (0.10), Na+ (0.19), Ne (0.40), F- (0.96),
0 2- (2.74), and N3- (?). 4 Clearly, the present data do
not fit with those expected for Si4+ and N3- and one can
safely rule out any ionization in Si -N films. Moreover,
the estimated (\lSI (but not aN which is nearly equal to
11! N.) is considerably larger than that for the rare gas
(Ar) immediately following Si in the periodic table.
This suggests that the effective size of "Si" in RPD
Si-N is quite large and may really correspond to that
of a Si -H complex. 5 Moreover, a SI progressively increases for the lower-density Si-rich Si-N films, indicating an increasing presence of Si-H complexes in
the off -stoichiometric films.
The authors would like to thank T. E. Smith and
G. Quintana for technical assistance and J. M. Poate,
H. J. Levinstein, and R. S. Wagner for helpful
conversations.
IA. K. Sinha, Extended Abstracts, Electrochem. Soc. 76-2,
625 (1976); 76-2, 629 (1976).
2A.R. Reinberg, Extended Abstracts, Electrochem. Soc.
74-1, 19 (1974).
3W.K. Chu, J.W. Mayer, M.A. Nicolet, T.M. Buck, G.
Amsel, and F. Eisen, Thin Solid Films 17, 1 (1973).
4N. E. Hill, W. E. Vaughan, A. H. Price, and M. Davies,
Dielectrical Properties and Molecular Behavior (Van
Nostrand Reinhold, London, 1969), p. 236.
5W.A. Lanford and M.J. Rand, J. Electrochem. Soc. 124,
286C (1977).
A.K. Sinha and E. Lugujjo
246
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