.

Y

Mechanical and Microstructural

Evaluation of Barium Strontium Titanate

Thin Films for Improved Antenna

Performance and Reliability

by C. W. Hubbard, C. G. Fountzoulas,

M. W. Cole, and S. Sengupta

ARL-TR-1930 April 1999

Approved for public release; distribution unlimited.
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Army Research Laboratory

Aberdeen Proving Ground, MD 21005-5069

Mu-lx-1930 April 1999

Mechanical and Microstructural

* Evaluation of Barium Strontium Titanate

Thin Films for Improved Antenna

Performance and Reliability

C. W. Hubbard, C. G. Fountzoulas, M. W. Cole, and S. Sengupta

Weapons and Materials Research Directorate, ARL

.
Abstract

Ferroelectric barium strontium titanate (Ba,_,Sr,TiO, [BSTO]) films of l+m nominal

thickness were deposited on single crystals of sapphire and electroded substrates at substrate

temperatures varying from 30” C to 700” C. The microstructure of the thin films was columnar

at all substrate deposition temperatures. The film microhardness showed a trend toward

increased hardness with substrate temperature. Furthermore, with the elevation in substrate

temperature, there was a parallel increase in film-substrate adhesion measurements, while the

cohesion measurements were not influenced by substrate temperature.

ii
1.

2.

3.

4.

5.

Table of Contents

m

1)

List of Figures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V

List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Distribution List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Report Documentation Page. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..*............................ 13

. . . 111
.

INTENTIONALLY LEFT BLANK.

iv
List of Figures

Figure Pane

1. Scanning Electron Photomicrogra ph of a Fracture Cross Section of a BSTO Film

cr, /Tm = 0.15). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2. Transmission Electron Micrograph of a BSTO Film Illustrating Columnar

Grain Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3. Structure Zone Model for Coatings Produced by Sputtering (Thorton 1977)...........

4. Column Size of BSTO Films vs. T,/T~....“.............................................................~

5. Kuoop Microhardness of BSTO Films vs. T,/Tm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6. Cohesion Failure Load of BSTO Films vs. TJTm ,.........,.,........,...,....,.....................

5

7. Adhesion Failure Load of BSTO Films vs. Ts /Tm.................................................... 8

List of Tables

Table lPag;e

1. Column Diameter, Cohesion, and Adhesion Failure Load and Knoop

Microhardness of BSTO Films vs. TJTm................................................................. 6

V
INTENTIONALLY LEFT BLANK.

vi
1, Introduction

The Army has an active interest in phased-array antennas. Jn terms of performance,

phased-array antennas offer significant advantages over their mechanical counterparts, namely,

higher speed, increased accuracy, and higher reliability. Traditionally, phased-array antennas

have been constructed using ferrite phase-shifting elements. Due to circuit design requirements,

these antennas are large and costly (Sengupta 1996). Future Army requirements will demand

antennas that are smaller and lighter with higher performance and greater long-term reliability.

Thus, new materials are key elements in realizing this goal and must be developed, evaluated,

and optimized for the application. To this end, a barium strontium titanate (Bar_,Sr,TiOs

[BSTO]), phase-shifter device employing a planar microstrip construction has been demonstrated

as a possible candidate (L. C. Sengupta et al. 1995). Results of this work have clearly shown

BSTO to be a reasonable candidate for phase-shifter applications. In an effort to optimize the

performance of the material (i.e., to increase the operating frequency [>30 MHz] and lower

voltage requirements), thin-film fabrication of BSTO is critical. Thin films of BSTO deposited

via pulsed-laser deposition (PLD) have been demonstrated in previous work (S. Sengupta et al.

1995). These films have exhibited excellent electronic properties, including tunable dielectric

constants and low electronic loss (Sengupta et al. 1994). However, there has been little

investigation into the microstructural and mechanical aspects of these films. Previous work has

demonstrated that the microstructure of thin films has a significant influence on device electrical

performance and reliability (Jkawa 1993; Cole et al. 1998). Furthermore, it is postulated that

thin-film microstructure has a strong influence on its mechanical properties (Garsgm, Lavernova,

and Mokhov 1997). The mechanical properties, including internal stresses and adhesion, are

important factors affecting the mechanical integrity and reliability of a device constructed of

these thin films. Though a film may show superior electronic properties, it is the mechanical

properties, along with electrical properties, which permit the film to be useful in a device such as

a phased-array antenna. This paper presents the results of an investigation into the effects of

substrate temperature during deposition on the mechanical properties as well as microstructure of

BSTO thin films.
2. Experimental

Films of 1-p nominal thickness were deposited via PLD onto single-crystal sapphire and

electroded substrates. Prior to deposition, the substrates were ultrasonically cleaned in an

isopropyl bath. A 248~nm KrF laser was employed for the deposition of BSTO thin films from

source targets. The energy of the laser during deposition was 400 mJ per pulse, with a pulse

frequency of 10 Hz and pulse width of 20 ns. The chamber pressure was held at 50 mtorr of

oxygen. The laser spot was mechanically scanned across a rotating target to ensure film

uniformity and avoid cratering in the target. Target-to-substrate distance was 75 mm. The films

were deposited at a constant rate. The temperature of the substrate was varied from 30” C to

700” C via a resistance heater on the substrate mount. Temperature was monitored by a

thermocouple imbedded in the heater.

The Knoop microhardness on the free surface of these coatings, uncorrected for substrate

hardness effects, was measured using a 0.25-N applied load and a dwell time of 15 s. Even at

this low load, the maximum indenter penetration far exceeded the critical value of l/l0 of the

coating thickness that is considered to be sufficient for the substrate not to have a significant

effect on hardness values. However, since all substrates were identical, all measurements were

internally consistent. Thus, while the hardness numbers cannot be taken as absolute, if taken as a

set, they do illustrate a trend of increased hardness as substrate temperature increases.

The cohesion and adhesion values of the various coatings were evaluated primarily with a

Centre Suisse d’Electronique et de Microtechnique (CSEM)-Revetest (CSEM CH-20007,

Neuchstel, Switzerland) automatic scratch apparatus. The apparatus employed a diamond stylus

with a radius of 200 p. The testing procedures for this experiment are described in Kattamis

et al. (1993); Bhansali and Kattamis (1990); Kattamis (1993); and Steimamr, Tardy, and

Hintermarm (1987). The sample translation speed was held constant at 5 mm/min, and the

loading rate at 5 N/min; hence, the load gradient was dUdx = 1 N/mm. The cohesion failure

load, Lc, is the minimum crack initiation load within the coating, and the adhesion failure load,

2
LA, is the minimum load at which the crack causes massive delamination at the coating/substrate

interface.

The fracture cross sections of the films were observed by scanning electron microscopy

(SEM) to obtain a rough idea of the microstructure. Cross-sectional transmission electron

microscopy (TEM) was performed on the BSTO films to confirm the film’s columnar

microstructure. Selected-area electron diffraction (WED) was used to confirm the film’s

crystallinity. An Amray 1820 SEM with an accelerating voltage of 20 kV was used for surface

analysis. The JEOL 3010 transmission electron microscope with scanning attachment and

operating voltage of 300 kV was used to perform the cross-sectional TEM and SAED.

3. Results and Discussion

The microstructure of the BSTO thin film, at various substrate temperatures, was evaluated

by SEM and TEM. Figure 1 shows a fracture cross section of the BSTO film substrate. From

Figure 1) the columnar structure of the films can be classified as being in the Thornton (1977)

structural zone 2.

Figure 2 is a cross-sectional TEM micrograph of BSTO. The figure illustrates the dense

polycrystalline nature of the film and the columnar grain structure. There was no evidence of

microporosity at the boundaries or in the grain interiors. Also, note the different orientations of

the columnar grains. This can be attributed to the uneven surface of the substrate. Figure 2

further supports the point that these films are of the Thornton structural zone 2 type. Zone 2

consists of dense columnar grains with growth as the result of surface recrystallization and

surface diffusion. It is well known that the substrate temperature, T,; the ambient gas pressure;

and the energy of any incoming ions influence the growth conditions and, therefore, the film

structure produced under low-pressure conditions (Movchan and Demchishin 1969; Thornton

1977; Fountzoulas and Nowak 1991). A structural classification system that has gained the

broad acceptance for thin films produced by physical vapor deposition (PVD) process has been

3
Figure 1. Scanning Electron Photomicrograph of a Fracture Cross Section of a BSTO Film

(T,rr,= 0.15).

Figure 2. Transmission Electron Micrograph of BSTO Film Illustrating Columnar Grain

Structure. Note That the Dark Lines Are Added to Aid in Delineating the

Columnar Grain Boundaries (T.,/r, = 0.49).

presented by Movchan and Demchishin (1969). They proposed three zones to describe the

microstructures that can develop in films produced by vacuum evaporation as a function of

T,/T,, where T, is the absolute substrate temperature and Tm is the absolute melting temperature

,

4
of the deposited material. Thornton (1977) elaborated on the approach of Movchan and

Demchishin (1969) extending it to typical sputtering. Thornton (1977) also concluded that the

structure and physical properties of films produced by sputtering could be represented as a

function of T,/T,, in terms of four zones as shown in Figure 3, each with its own characteristic

structure and physical properties. The general features of Thornton’s model (1977) were based

on the examination of 25 to 250~p-thick coatings deposited at argon pressures of 1.33 x 10m4

(1 mtorr) to 3.9 x 10s4 Pa (30 mtorr), using cylindrical-post and hollow-cathode magnetron

sputtering sources. Fountzoulas and Nowak (1991) further elaborated on the approach of

Movchan et al. (1969) and Thornton (1977), extending

performed to confirm the crystalline structure of the films.

are indicative of a polycrytstalline film.

them to ion plating. SAED was

The SAED image showed rings that

Figure 3. Structure Zone Model for Coatings Produced by Sputtering (Thornton 1977).

The column size measurements as a function of the ratio of the absolute substrate

temperature, T, , and absolute film melting temperature, Tm, (2,004 K) are shown in Table 1 and

Figure 4. For T, /T, between 0.15 and 0.19, the average column diameter increased from 190 nm

to 230 nm. For T,/T, between 0.19 and 0.39, the average column size remained practically

constant At Ts/Tm = 0.49, where the film is fully crystalline, the column size increased

dramatically to 2,007 nm. The increased column diameter can be attributed to the increase of

.

intercolumnar diffusion at higher temperatures.

5
Table 1. Column Diameter, Cohesion, and Adhesion Failure Load and Knoop

Microhardness of BSTO Films vs. T,R,

LA (N) k (N) Knoop

Ts/Tm Thickness Column Size (Adhesion Load) (Cohesion Load) Microhardness

(nn.0 (nm) (GPa)

0.15 1,000 190 16.06 11.63 1.3

0.19 2,200 230 18.75 13.75 1.5

0.29 1,000 250 38.22 13.65 5.7

0.39 1,000 220 22.68 12.96 6.6

0.49 3.000 2.007 24.08 12.32 7.0

Figure 4, Column Size of BSTO Films vs. TJTm.

The Knoop microhardness of the BSTO films, uncorrected for the substrate hardness effect,

ranged between 1.3 GPa and 7 GPa. Table 1 and Figure 5 show Knoop microhardness values as

a function of T,/T,. The film microhardness increased with an increasing T,/T, ratio and

increasing film crystallinity. The highest Knoop microhardness was obtained at the highest

temperature (i.e., Ts /Tm = 0.49).

Measured average values of cohesion failure load, L,-, and adhesion failure load, LA, are

listed in Table 1. The cohesion failure load of the films remained fairly constant, independent of

6
Figure 5. Knoop Microhardness of BSTO films vs. T,/T,

the substrate temperature (Figure 6). The average cohesion failure load of the film was about

13 N. The adhesion failure load of the films increased with increasing TJT, ratio (Figure 7).

However, for reasons currently not understood, for T,/T, = 0.25, the adhesion failure load

exhibited a peak at a ratio of 38.22 N.

Figure 6. Cohesion Failure Load of BSTO Films vs. Ts/Ilm.
Figure 7. Adhesion Failure Load of BSTO films vs. TJIY,,,.

4. Summary

The structure, crystallinity, cohesion and adhesion failure loads and microhardness of BSTO

thin films produced by PLD were correlated with the substrate temperature. For the entire

temperature range, 278-973 K, the films were columnar. The column size increased with

increasing substrate temperature. For T,/T, I 0.29, the films could be categorized as being in

the transition region, T, of the Thornton structural zone model. When T,/T, > 0.29, the

microstructure corresponds to the Thornton structural zone 2 category. SAED confirmed the

crystalline nature of the fihns deposited on the electroded substrates at a temperature of 973 K.

The cohesion failure load was constant for the entire substrate temperature range. The adhesion

and microhardness increased with increasing substrate temperature.

8
5. References

.

.

Bhansali, K. J., and T. Z. Kattamis. “Quality Evaluation of Coatings by Automatic Scratch

Testing.” Wear, vol. 141, p. 59, 1990.

Cole, M. W., S. Sengupta, S. Stowell, C. W. Hubbard, and E. Ngo. ‘The Influence of

Microstructure on the Electronic Properties of Thin Films of Barium Strontium Titanium

Oxide Composites.” Journal of Integrated Ferroelectrics, vol. 21, pp. 469-473,1998.

Fountzoulas, C., and W. B. Nowak. “Influence of Ion Energy and Substrate Temperature on

Structure of Copper, Germanium, and Zinc Films Produced by Ion Plating.” Journal of

Vacuum Science and Technology, vol. A9, no. 4, p. 2128, July/August 1991.

Garsgin, A., Y. Lavernova, and E. Mokhov. “Influence of Impurities and Intrinsic Defects on

Physicomechanical Properties of Silicon Carbide Single Crystals.” Ceramics International,

vol. 23, p. 409, 1997.

lkawa, H. Ceramic Transactions. Vol. 32, p. 19, 1993.

Kattamis, T. Z. “On the Evaluation of Adhesion of Coatings by Automatic Scratch Testing.”

Journal of Adhesion Science and Technology, vol. 7, p. 783, 1993.

Kattamis, T. Z., K. J. Bhansali, M. Levy, R. Adler, and S. Ramalingam. “Evaluation of the

Strength and Adherence of Soft Cobalt-Base and Hard Tin Coatings on 4340 Low-Alloy

Steel.” Material Science Engineering, vol. A161, p. 105, 1993.

Movchan, A., and S. V. Demchishin. Physical Met. Metallogr. Vol. 28, p. 653, 1969.

Sengupta, L. C., S. Stowell, E. Ngo, M. E. O’Day, and R. Lancto. “Barium Strontium Titanate

and Non-Ferrous Oxide Ceramic Composites for Use in Phased-Array Antennas.”

Integrated Ferroelectrics, vol. 8, pp. 77-88,1995.

Sengupta, S., D. P. Vijay, L. C. Sengupta, and S. B. Desu. “Investigation of Characteristics of

Ferroelectric Thin Films Deposited by Pulsed Laser Ablation.” MRS Symposium on Smart

Materials, vol. 36, p. 545, 1995.

.

Sengupta, S., L. C. Sengupta, D. P. Vijay, and S. B. Desu. “Thin Films of Novel Ferroelectric

Composites.” Journal of Integrated Ferroelectrics, vol. 13, pp. 239-245, 1996.

.

Steinmann, A., Y. Tardy, and H. E. Hintermann. Thin Solid Films. Vol. 154, p. 333,1987.

Thornton, J. A. Annual Review of Material Science. TA401-A7, vol. 7, p. 239, 1977.

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REPORT DOCUMENTATION PAGE Form Approved

OblB No. 0704.0188

2. W. Hubbard, C. G. Fountzoulas, M. W. Cole, and S. Sengupta

REPORT NUMBER

J.S. Army Research Laboratory

4T’IN AMSRL-WM-MC ARL-TR-1930

Aberdeen Proving Ground, MD 21005-5069

il. SUPPLEMENTARY NOTES

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

Approved for public release; distribution is unlimited.

13. ABSTRACT(Maximum 200 words)

Ferroelectric barium strontium titanate (Ba,_,Sr,TiO, [SSTO]) films of 1-urn nominal thickness were deposited on

single crystals of sapphire and electroded substrates at substrate temperatures varying from 30” C to 700” C. The

microstructure of the thin films was columnar at all substrate deposition temperatures. The fii microhardness showed

a trend toward increased hardness with substrate temperature. Furthermore, with the elevation in substrate temperature,

there was a pamIle increase in fii-substrate adhesion measurements, while the cohesion measurements were not

infhrenced by substrate temperature.

14. SUBJECT TERMS 15. NUMBER OF PAGES

thin film, BSTO, ferroelectrics, phased-array antennas, microstructure, * 15

mechanical properties 16. PRICE CODE

17. SECURITY CLASSIRCATION lb SECURITY CLASSIRCATION 19. SECURITY CLASSlFlCATlON 20. LIMITATION OF ABSTRACT

OF REPORT OF THIS PAGE OF ABSTRACT

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