Characterization of Barium Strontium Titanate Films Using XRD - ICDD

Copyright (C) JCPDS-International Centre for Diffraction Data 1999
38
Characterization of Barium Strontium Titanate Films Using XRD
Abstract
Thomas Remmel, Richard Gregory and Beth Baumert
Materials Research and Strategic Technologies
Semiconductor Products Sector, Motorola Inc.
Mesa, Arizona
Barium Strontium Titanate (BST) materials, both in target (bulk) and thin film (6OOA-
12OOA) form, were characterized using x-ray diffraction (XRD). Film structure as a function of
composition, processing parameters and underlying substrate was determined. Differences in the
lattice parameter between bulk and sputtered film form were observed, with the lattice parameter
being significantly larger for the films. In addition, the BST lattice parameter was observed to vary
as a function of thin film deposition temperature, being larger at lower deposition temperatures.
These differences in BST lattice parameters are investigated in detail with a view toward better
understanding the phenomena.
Introduction
Barium Strontium Titanate (BST) films are under investigation as a material for advanced
integrated circuit applications. Specifically, BST is of interest for use as the charge storage cell in
DRAMS (Dynamic Access Random Memories)“” because of its high dielectric constant, which can
reach values of 20000 in bulk ceramic B ST’ ‘. A cross-sectional view of a typical BST capacitor
structure is shown in Figure 1.
Ba,+Sr,TiO, has the perovskite structure ABX, which is shown in Figure 2. The high
dielectric constant results from a displacement of the Ti ion from the center of the oxygen
octahedron. Ba,-,Sr,TiO, exhibits complete solid solubility over all compositions12, with a cubic
structure at room temperature for 0.3 I x I 1, becoming tetragonal for 0 I x I 0.3. Lattice
parameters ranging from 3.905 A for SrTiO, to a=3.994 A and c=4.038 A for BaTiO, are reported
for bulk BST . For DRAM applications, the cubic form of BST is preferred, with higher
dielectric constants attained close to x=0.3. The tetragonal distortion of BST is associated with the
paraelectric-to-ferroelectric transition, which is close to room temperature for the composition
Bao,7Sro,3Ti0,. The Curie temperature decreases with increasing Sr content, as shown in Figure 3.
Figure 1. Cross-sectional view of BST charge
storage capacitor
Figure 2. The structure of Ba,Sr,TiO,.

Copyright (C) JCPDS-International Centre for Diffraction Data 1999
39
Experimental
BST films were characterized
using a full arsenal of analytical
techniques. This paper focuses on the
results of XRD analysis. All
diffraction data reported here were
obtained with a Rigaku rotating anode,
CuKa source, using Bragg-Brentano
geometry. Operating conditions were
typically 50kV, 2OOmA.
BST Target
Analysis
0 02 0.4 06 OS 1.0
BaTiQ
SrTi03
x (SrTiO3) ---+
Figure 3. Curie temperature of Ba,.,Sr,TiO, as a
function of stoichiometry.
Several BST sputter targets were characterized via XRD to determine structure and
uniformity. Shown in Figure 4 is the XRD pattern of a Bao,sSro,,TiO, target from supplier A. The
target appears to be comprised of a single phase of BST. As seen in the higher order peaks, the
peak positions corresponds very closely with the those expected for the nominal stoichiometry.
From the XRD pattern, the stoichiometry was estimated to be Ba,,,,Sr,,,,TiO,, which agreed very
well with chemical analyses.
- 5
cd
2-a
.$=
2
3
c
k?
p;?;
x
2
9
.z
x
2
B
3
i2
2
k
74 76 78 82 84 86
20 50 60
2-Theta
Figure 4. XRD pattern of Ba,,$r,.,TiO, target from supplier A.

Copyright (C) JCPDS-International Centre for Diffraction Data 1999
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On the other hand, XRD analysis of another target from supplier B, shown in Figure 5,
reveals a mixture of BST phases, including the nominal Ba,,,Sr,,TiO,, along with traces of
BaTiO, (BTO) and SrTiO, (STO). The existence of the secondary phases of BTO and ST0 is
evidenced by the shoulders at the base of the peaks.
- 3
cd
h
.s
8
%
22
!&
b4
2
9
.s
x
2
2
2
e:
A
I 74 76 78 80 82 84 86
2-Theta
(111)
(200)
(211)
(100)
I h I
(220)
/n, I\\ (3 10) (222)
20 30 40 50 60
2-Theta
70 80 90
Figure 5. XRD pattern of Bao,$ro,5Ti0, target from supplier B.
to consist of a mixture of BST, BTO and ST0 phases.
This target was found
BST Film
Analysis
BST films of various compositions
were deposited by MOD
Decomposition)/spin-on
(Metal-Organic
techniques,
MOCVD (Metal-Organic Chemical Vapor
Deposition) and RF magnetron sputtering
2
0
onto (100) Si substrates. Shown in Figure ‘g
6 are the diffraction scans from a series of 2
nominal 2000A thick sol-gel/spin-on 2
Ba,Sr,TiO, films. A shift in the 2-theta
peak positions as a function of BST film 2
composition is apparent in these scans.
Comparison of the lattice parameter
calculated from the spin-on films with those
reported in the literatureI for bulk BST 20 25 30 35 40 45 50 55 60
2 theta
indicates reasonable agreement, as shown
Figure 6. XRD patterns of spin-on Ba,Sr,TiO,
films for x= 0 to 1.0.

Copyright (C) JCPDS-International Centre for Diffraction Data 1999
41
in Figure 7. Similarly, analysis of
MOCVD films of Ba,,Sr,,TiO, yielded
lattice parameters similar to those of the
bulk material. However, review of the
literature’3-‘5 and in-house analysis of
sputtered Ba,-,Sr,TiO, films indicates lattice
parameters much larger than those expected
for bulk material. This anomaly for
sputtered BST films has been reported by
other investigators and has been explained
as being due to “nonequilibrium, highly
distorted states” within the films13. Shown
in Figure 7 is a summary of literature and
in-house measurements of lattice parameter
as a function of stoichiometry for
Ba,-,Sr,TiO,. Note that the sputtered BST
films were deposited at 55O”C, on a variety
of crystalline subsbrates, and va@ed in
thickness from 600A to over 3000A. The
films were polycrystalline and typically
randomly oriented.
XRD scans of sputtered deposited,
6OOA thick Ba,,Sr,,TiO, films as a
function of deposition temperature are
shown in Figure 8. These films were
deposited on Pt which had been sputter
deposited on oxidized Si wafers.
Improvement in the degree of BST film
crystallinity as deposition temperature is
increased is evident. (Also apparent is the
high degree of preferred orientation of the
underlying Pt film). A shift in the BST
peak positions toward higher two-theta
angles with increasing deposition
temperature can be seen in the diffraction
scans.
Lattice parameters (in the growth
direction) for the BST films shown in Figure
8 were calculated from the peak positions
(assuming a cubic structure) and are plotted
in Figure 9 as a function of deposition
temperature. Results from similar analysis
of Ba,,Sr0,,Ti03 films deposited at various
temperatures directly onto SiO, are also
shown in Figure 9. Note that the lattice
parameter for BST films deposited on SiO, is
larger than for those deposited on either Pt or
Ir. This was unexpected but might be
explained by the differences in the degree of
constraint placed upon the BST lattice by the
substrate. SiO,, being amorphous, would
likely place less constraint compared to the
crystalline Pt or Ir (whose structures are
z
4.10
4.05
5
': 4.00
g
V
.r 8
cl 9 3.95
3.90
Figure 7. Ba,$r,TiO, lattice parameters vs.
composition, taken from the literature and this
work.
20 25 30 35 40 45 50 55 60
%:-theta
Figure 8. XRD patterns of Ba,,Sr,,TiO,
films deposited at various temperatures on Pt.
4.10 F-
4.08 1
3 4.06 -
3
2 4.04 -
E
g 4.02 -
8
‘g 4.00 -
0
I ” ” 1’3 7 ’ I’, ” I ” I “‘I I ” ” II 7
n 0
v
n
0
v
n
0 O
cl t i
3.98
t Bulk Value = 3.95A 1
3.96 t.,,.l,,‘,l,“,I,‘,,l,,,,I.,,,‘,,,,’..,,’
300 350 400 450 500 550 600 6.50 700
BST Deposition Temperature (“C)
Figure 9. Ba,,,Sr,,TiO, lattice0 parameters vs
deposition temperature for 600A BST films on
various substrates.
n

Copyright (C) JCPDS-International Centre for Diffraction Data 1999
42
cubic and whose lattice parameters are
slightly smaller than BST).
Results from measurement of the
FWHM of the (110) and (200) peaks for
the same Ba0,,Sr0,,Ti03 films as a function
of sputter deposition temperature are shown
in Figure 10. Data for the (110) peak are
shown for Ba&iq,,TiO, deposited on both
Pt and SiO,, whereas only (200) data is
shown for BST on SiO,, due to
interference between the BST (200) and Pt
(200) peaks.
Decreasing ( 110) and (200) peak
widths with increasing deposition
temperature are indicative of increased grain
size in the BST films. However the finding
that the (200) BST peaks are about three
times wider than the (110) peaks was
unexpected. The increased Peak
broadening may be indicative of distortion
in the lattice (due to defectivity), or the
existence of a second phase. Argon is
known to be incorporated into these films
during sputter deposition (this was verified
using Rutherford Backscattering). It is
speculated that excess oxygen, due a
similar mechanism of neutral ion
bombardment, may also be incorporated
into these films. Finally, oxygen vacancies
might explain the lattice distortion.
The effect of post sputter-deposition
anneal on sputtered BST films is shown in
Figure 11. Ba,,,,Sr,~,,TiO, films, 500A
thick, that had been deposited on SiO,,
were annealed in air for 30 minutes at
temperatures ranging from 650°C to 950°C.
The lattice parameter of the BST films
0.8 I
t! I I I I
300 400 500 600
BST Deposition Temperature (“C)
o
(200) BSTLSi02
-! i 2.0
Figure 10. (110) and (200) FWHM for sputtered
Ba,,,Sr,,,TiO, films on SiO, and Pt.
4.00 , , , , , , , , , , , , , . 0.7
\
2 3.98 -
3
8
I2 3.96 -
8
.a
3 3.94 -
\
\ ‘%
\ \
\
‘*
\
‘&
’ ‘\
\ * ‘1 \ \ ‘0
\ ’
‘,@ ‘\
\ \ k
\ \
l ’
1
- 0.6 -
z
- 0.5 2
9
- 0.4 2
B
\ 2
- 0.3 -
3.92 I.

Copyright (C) JCPDS-International Centre for Diffraction Data 1999
43
d-i.
20 25 30 3.5 40 45 50 55 60
2 theta
392t,““““““““‘,i
0 500 1000 1500 2000
BST Film Thickness (A)
Figure 12. XRD patterns of various thickness
Ba,,,Sr,,,TiO, films deposited on Pt.
Figure 13. Ba,,Sr,.,TiO, lattice parameters vs
film thickness for SiO, and Pt underlying
films.
than 200 A appeared to be amorphous. These results are consistent with the constraining effect of
the substrate, as discussed earlier.
The change in FWHM of the (110) peak as a function of thickness for Ba&Sr,,,TiO, films
deposited on both Pt and SiO, is shown in Figure 14. The trend of decreasing peak width with
increasing film thickness agrees with the rationale that grain size (in the growth direction) increases
with film thickness. Grain sizes, calculated from the (110) peak broadening using the Scherrer
formula16, are on the order of the film thickness, but decrease as a percentage of film thickness as
the thickness increases.
Sputter deposiied BSOT films appear to be randomly oriented, w$h a hint of (110) preferred
orientation in the 400A-600A thick range. Above thicknesses of lOOOA, the films begin to exhibit
(111) texturing. With the exception of the very early stages of film growth, the substrate (Pt, Ir
and Si02) appears to have little effect on the texture of the films.
The effect of sputtering gas composition on the structure of the BST films was also
investigated. Shown in Figure 15 are XRD patterns of Ba,,,Sr,,TiO, films deposited on SiO,
under various argon:oxygen (Ar:O,) gas
0.60
flow ratios. The thicknes%es of these
1”’ 0 ” ” 1 ” 1 ” ” 1 ‘I
films were nominally 600A, with the
exception of the lO$l:O Ar:O, film, which
was about 1300 A. Judging from the
positions of the (110) peak in Figure 15,
the lattice parameter does not appear to
vary much as a function of Ar:O, gas
ratio. However, the diffraction pattern
of the thicker, 100:0 Ar:O, film indicates
that the texture of these films varies as a
function of thickness, as noted
previously.
An interesting feature evident in
the XRD pattern of the film deposited
with 50:50 Ar:O, gas ratio is the
existence of a doublet in the (200) peak.
This XRD pattern is shown in more
3
B
0.50
“v N
z 0.40
Ft
G
2 0.30
0.20~““““’
0 5ocl 1000 1500
BST Film Thickness (A)
Figure 14. Ba,$&,,TiO, lattice parameters vs film
thickness for SiO, and Pt underlying films.

Copyright (C) JCPDS-International Centre for Diffraction Data 1999
44
detail in Figure 16, where a doublet is also
seen in the (100) peak. The position of the
higher 2-theta (100) and (200) peaks in
these doublets agrees very well with a
lattice parameter of 3.95 A, equal to that of .g
bulk Ba,,Sr,,,TiO,. The existence of the
doublets seems to indicate that either the
sputtered BST film is comprised of multiple 2
phases, or the film is not cubic, but exhibits 6
some lattice distortion.
It is possible to fit the diffraction
scan shown in Figure 16 with a tetragonal
structure having elongated a and b
directions, as shown by the stick patterns.
Although the peak positions agree very
well, the (200) and (002) intensities are
reversed from that expected of a randomly
oriented, conventional tetragonal structure
where oa. To attain the proper intensity
ratios, either the tetragonal structure is of
the form where a>c or the BST film is not
fully random in orientation.
20 25 30 35 40 45 50 55 60
2-theta
Figure 15. XRD patterns of Ba,,,Sr,,TiO, films
deposited using various Ar:02 gas flow ratios
20 25 30 35 40 45 50 55 60
2-theta
Figure 16. Detailed XRD pattern for 6OOA Ba,,,Sr,,,TiO, film deposited on SiO, using 50:59
argon:oxygen gas flow ratio. Stick patterns represent positions of tetragonal phase of a=b=4.06 A
and c=3.95 A.

Copyright (C) JCPDS-International Centre for Diffraction Data 1999
45
Conclusions
XRD has proven to be an invaluable tool in the development of BST for DRAM
applications. Characterization of BST films as a function of the numerous process parameters has
revealed insight into the structure of the films and their dependence upon process conditions.
Sputtered BST films were found to have lattice parameters larger than MOD/spin-on, MOCVD and
bulk BST. This discrepancy was even greater for films sputtered onto SiO,, and was found to
decrease as a function of post-deposition anneal. Abnormal width and shape of the (200) peak
appears to point toward distortion of the BST lattice in sputtered films, perhaps of the tetragonal
form.
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