characterization of barium strontium titanate films using xrd

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Characterization of Barium Strontium Titanate Films Using XRD

Thomas Remmel, Richard Gregory and Beth Baumert

Materials Research and Strategic Technologies Semiconductor Products Sector, Motorola Inc.

Mesa, Arizona

Abstract

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 1999ISSN 1097-0002, Advances in X-ray Analysis, Volume 41

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This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website – www.dxcicdd.com

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ISSN 1097-0002, Advances in X-ray Analysis, Volume 41

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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. 0 02 0.4 06 OS 1.0

BaTiQ SrTi03

x (SrTiO3) ---+

BST Target Analysis 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 x .z 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.


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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 x .s 2 2 2 e: A

I 74 76 78 80 82 84 86 2-Theta

(200)

(111) (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. This target was found to consist of a mixture of BST, BTO and ST0 phases.

BST Film Analysis

BST films of various compositions were deposited by MOD (Metal-Organic Decomposition)/spin-on techniques, MOCVD (Metal-Organic Chemical Vapor 2 Deposition) and RF magnetron sputtering 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

indicates reasonable agreement, as shown 2 theta

Figure 6. XRD patterns of spin-on Ba,Sr,TiO, films for x= 0 to 1.0.


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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

4.10

4.05

z

5 ': 4.00 g V 8 .r 9 cl 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 -

I ” ” 1’3 7 ’ I’, ” I ” I “‘I I ” ” II 7

0

n 0 0

v

n v n

0 O

n

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.


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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.

0.8 I o (200) BSTLSi02 -! i 2.0

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.

t! I I I I

300 400 500 600

BST Deposition Temperature (“C)

Figure 10. (110) and (200) FWHM for sputtered Ba,,,Sr,,,TiO, films on SiO, and Pt.

4.00 , , , , , , , , , , , , , . 0.7 \

1

2 3.98 -

3

8 I2 3.96 -

8 .a

3 3.94 -

\ ‘% \ \ \ \ ‘* ‘& \

’ ‘\ \ * ‘1 \ \ ‘0 \ ’

‘,@ ‘\ \ \ k \ \ l ’

- 0.6 -

z

- 0.5 2

9 - 0.4 2

B \ 2 - 0.3 -

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 decreased as post-deposition anneal

3.92 I. <, I I I I .1 0.2

500 600 700 800 900 1000

Anneal Temperature (“C)

Figure 11. Lattice parameter and FWHM of the (110) peak of a Ba,~,,Sr,,,,TiO, film after 30 minute post deposition anneals.

temperature increased and approached the value for bulk BST (about 3.92 A for this composition). Similarly, the FWHM values for the (110) peak decreased as the anneal temperature increased. This decrease in peak broadening could be a result of BST grain growth; however since the lattice parameter also changes with anneal, a more likely explanation might be decreased defectivity or a change in the film structure. Similar behavior was also observed for BST films on Pt after elevated temperature post-deposition anneal.

XRD scans of Ba&G-,,TiO, films as a function of BST film thickness are shown in Figure 12. These films were sputter deposited on Pt at 550°C. The change in lattice parameter as a function of thickness is shown in Figure 13, for Ba,,&,,TiO, films deposited on both Pt and SiO,. In the initial stages of sputter film deposition on Pt, it appears that the lattice parameter starts out at a value comparable to that of bulk BST (or, alternatively, close to the lattice parameter of Pt). However, as the thickness increases, the lattice parameter increases until reaching a value of about 4.00 A for films thicker than 400 A. On SiO,, the trend is less obvious, since BST films thinner


d-i.

20 25 30 3.5 40 45 50 55 60

2 theta

Figure 12. XRD patterns of various thickness Ba,,,Sr,,,TiO, films deposited on Pt.

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392t,““““““““‘,i 0 500 1000 1500 2000

BST Film Thickness (A)

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 flow ratios. The thicknes%es of these 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.

0.60 1”’ 0 ” ” 1 ” 1 ” ” 1 ‘I

3 0.50 B N “v z 0.40

Ft G 2 0.30

0.20~““““’ 0 5ocl 1000 1500

BST Film Thickness (A) 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

Figure 14. Ba,$&,,TiO, lattice parameters vs film thickness for SiO, and Pt underlying films.


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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

The existence of the .g

bulk Ba,,Sr,,,TiO,. 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.


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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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characterization of barium strontium titanate films using xrd

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