ELSEVIER Microelectronic Engineering 29 (1995) 293-296

MICROET.~-C~rROI~C ENGINEERING

Investigation of Pb(Zr0.70Ti0 30)O3 thin f i lms of' different textures on Ti/Pt

Keith Brooks, Radosveta Klissurska, Pedro Moeckli and Nava Setter

Laboratoire de CSramique, Ecole Polytechnique FadSrale de Lausanne CH 1015 Lausanne, Switzerland

Pb(Zr0.70Ti0.30)O3 rhombohedral thin films of (100), (110)/(111) and (111) textures were prepared on Si wafers with TUPt electrodes. Microstructures and textures were characterized by XRD, SEM, TEM, and AFM. Ferroelectric, dielectric and fatigue properties were measured. Dielectric constants of 1050, 880, and 920 were obtained for the (100), (110)/(111), and (111) films. Remanent and saturation polarizations increased in the order (100), (110)/(111), (111) at high fields. Coercive field values of 27, 43 and 31 kV/cm were measured for the (100), (110)/(111), and (111) samples, respectively, at 300 kV/cm. Similar fatigue characteristic were observed for all films at 300kV/cm, showing an initial increase in switched polarization followed by a decline at 107 - 108 switching cycles. Observed differences in the measured properties are explained based on allowed polarization directions for given textures, and film microstructure.

1. INTRODUCTION 2. EXPERIMENTAL.

Rhombohedral Pb(ZrxTil.x)O3 (PZT) thin films have been studied for memory applications due to low coercive field values compared to morphotropic and tetragonal compositions [1-4]. Zr rich compositions are reported to be more difficult to crystallize than Ti-rich PZT films due to increased stability of the transient pyrochlore, often leading to rosette type microstructures [5]. Ex-situ crystallization of sol-gel films is influenced by perovskite nucleation energy and pyrochlore stability [6-9]. Bellur et al.[10] investigated morphotropic films on Pt/Si and MgO substrates of random and (001) texture, respectively. Squarer hysteresis was reported for the (001) films. Larsen et al.[11] observed that columnar grains are required to obtain fastest switching.

In this study, annealing profiles were used to obtain crystallized Pb(Zr0.7Ti0.3)O 3 films of d i f ferent t ex tu res (columnar grains of defined orientation with respect to the s u b s t r a t e n o r m a l and r a n d o m orientation in the plane of film). Dielectric, switching and fatigue properties of (100), (110/(111) and (111) textured films are reported.

PZT films were prepared by a modified sol-gel method, [12] using a 15% Pb excess solution. Thin films o f - 0 . 3 5 ~tm thickness were prepared on Si substrates with Si02 (1000 nm), Ti (10 nm), and Pt (100 nm) layers by spin coating. Rapid thermal anneal ing (RTA) profiles, Fig. 1, were chosen to achieve different film textures [7,12].

8OO

soo

,co 200

7oo*c

r / (111).,.(11o) (loo),~ r / (111}

0 c - - - . * ~ l ~ l _ , ~ 0 20 1 3 $ 7 9 11

Time (s//rain)

Figure 1. RTA annealing profiles.

The method of Harris [13] was used to quant i fy the film textures . In teg ra ted intensities (obtained by Gaussian or Cauchy profile fits) of the perovskite diffraction peaks for each film sample were compared to

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294 K. Brooks et al. / Microelectronic Enginee14ng 29 (1995) 293-296

those of a sol-gel derived powder to calculate the oriented fraction of grains, Table I. Er rors in the t ex tu r e de te rmina t ion originate from a reduction in the number of m e a s u r a b l e peaks due to s u b s t r a t e interferences (221 and 300 with the Si 400 peak, and 320 and 321 with the Pt 222) and the very low intensities or absence of peaks from planes other than h00, hh0 and hhh due to the sharpness of the texture. The (100), (110)/ (111) and (111) f i lms were approximately 97, 13/75, and 98% textured, respectively.

Table 1 Quantitative texture results from XRD (%) First and second order reflections, h=1,2, as indicated and 211 and 310 reflections.

PZT Film Sample Peak (i00) (I00)+(Iii) (111) h00 97.0 9.2 < 1 hh0 < 1 12.9 <I hhh 3.0 75.0 98.6 211 2.9 <I 310 <I

The micros t ructure of the PZT film surfaces was investigated by scanning and t ransmission electron microscopy (SEM/ TEM), and atomic force microscopy (AFM), Table 2.

Table 2 Microstructural characteristics

PZT film sample (100) (100)+ (111)

Feature (111) Grain size

(~m) 0.1-0.3 .05-0.1 0.3-0.6 (SEM)

Roughness =30 nm <10 nm =30 nm (AFM)

Pyrochlore yes yes no (SEM)

The dielectric properties of the films were characterized at 25 ° C at 10 kHz and 50 inV. Switching character is t ics as a function of field (at 50kV/cm increments) were measured us ing commercial t e s t

equipment. For each increment, f i rs t a positive (+ on top electrode) and then negative loop were acquired. The following characterist ics were extracted: remanent and saturation polarizations, and average coercive field (( I +Ec [ + [ -Ec [ )/2). Fatigue was measured at 150 and 300 kV/cm (or approximately 5 and 10 volts) at 30 kHz.

3 . RESULTS AND DISCUSSION

TEM analysis indicated that all films were co lumnar , (grain b o u n d a r i e s perpendicular to subs t ra te surface) and regions of pyrochlore on the film surfaces were a few nanometers in thickness.

Dielectric property data are given in Table 3. The value of the dielectric constant obtained for the (110)/(111) oriented sample of 876 compares with that of approximately 840 reported by Watanabe [1]. There is a clear difference in the tan ~ values, with the (100) sample showing a loss of ~ 5 %, being nearly twice that of the random and (111) cases. The highest dielectric constant value, 1074, was obtained for the (100) textured film.

Table 3 Dielectric properties.

PZT film sample (100) (111) + (111)

Property (110) Dielectric constant 1074 876 922

(£) Standard deviation 139 58 47

of~ Dielectric

loss, 0.049 0.0265 0.0292 (tan a)

Virgin hysteres is loops (50 Hz) are shown in Fig. 2. The (110)/(111) film shows a strong asymmetry . The polar izat ion values are observed to increase in the order (100), (110)/111), (111). The allowed polarization directions (with respect to the plane of the film) are -35 ° (100) texture (all

K. Brooks et al. / Microe/ectronic Engineering 29 (1995) 293-296 295

directions are equivalent), 35 and 90 ° for (iii) texture, and 0 and 54 ° for (ii0) grains. Saturation polarization, remanent polarization and average coercive field as a function of field are shown in Figure 3. At saturating fields, the polarizations increase in the order (100), (110)/(111), (111), with the last two being similar at high fields. This can not be explained solely from microstructural considerations (i.e. presence of pyrochlore) as the (100) and (110)/(111) films have similar quantities of pyrochlore, and similar grain sizes. Considering the allowed polarization directions, the difference between the (100) and (111) films at high field is expected to be approximately 30 %, whereas the measured d i f ference is 32 % (at 450 kV/cm). Considering the average coercive fields, the lowest value was obtained for the (100) film. It can be reasoned tha t since only one equivalent polarization direction is allowed for this film, the coercive field will be less than in the other two cases.

. . . . . . . . . ( 1 0 0 )

--(111) . . . . . (110)/(111)

6O

0

S -2o

4 0

-60 - I I I I t i -300-200-100 0 100 200 300

Electric field (kV/cm)

Figure 2. Virgin hysteresis loops

Fatigue curves for 150 and 300 kV/cm field is shown in Fig. 4. In all cases an increase in Pr is observed during initial fatiguing, being more pronounced in the (110)/(111) and (111) films. Also, in each case, degradation of the polarization begins between 106 and 108 cycles. In each case the

60

)°

O0 1

(a) o," . . o !

O 0 • • • • • o O A A •

_.,tm'" " l " * (III)

• (I00) - • (110)/(111)

I i i i i i l i i I

1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 600 Applied Field (kV/cm)

20

~ 1 0

5

0

(b) . t 2 t "

O & O|m mmmm

o O m

m 4

OA

t t t i t t i i I I I

0 100 200 300 400 500 600

Applied Field (kV/cm)

i 70 60 (c) , ,

:, "+ 20 " = = = • • •

°1o I ' " 0 i i I I I I I I I I I

0 100 200 300 400 500 600 Applied Field (kV/cm)

Figure 3. Field dependence of (a) saturation and (b) r emanen t polarizat ions and (c) average coercive field (symbols consistent throughout).

296 K. Brooks et al. / Microelectronic Engineel4ng 29 (1995) 293-296

35 .o

30 o~

2S

10 t~

O~ 5 , n i l t I ' i l i i , l i i , i , - ' I , w -J , , l i i , , i l l p r o l l , i , l i l , i , i

O00 102 104 106 108 1010

Switching cycles

Figure 4. Fatigue of different textured films at 300 (open symbols) and 150 kV/cm (filled symbols). Symbols consistent with Fig. 3

non-switching component was essentially constant to 108 cycles. Shepherd reported s imi lar fa t igue profiles for sol-gel Pb(Zro.5oTio.so)O 3 thin films [2]. Further investigation is needed to understand the relationship between texture and fatigue.

4. CONCLUSIONS In t h i s work , r h o m b o h e d r a ]

Pb(Zr0.70Ti0.30)O3 thin films of (100), (110) /(111) and (111) textures were investigated. It was found that despite some microstructural differences, the hysteresis saturation characteristics could be linked to the film textures and moreover to the allowed polarization directions existing for the given textures.

This work indicates that some advantage can be gained by controlling the orientation of PZT thin films used in ferroelectric memory devices. In as much as the or ienta t ion dictates the possible polarization directions through the crystal l a t t i ce , i n s i g h t in to the domain configurations can be achieved through such investigations.

ACKNOWLEDGMENTS

The authors gratefully acknowledge I. Reaney (TEM analysis), and J. Castano (SEM). This work was sponsored by the Swiss Federal Government under the Swiss Commission for the Encouragement of Research (CERS), program M2S2.

REFERENCES

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3. W.H. Shepherd, Mat. Res. Soc. Syrup. Proc. 200 (1990) 277.

4. Vinay Chikarmane , J iyoung Kim, Chandra Sudhama, Jack Lee, AI Tasch and Steve Novak, J. of Electronic Materials, 21 (1992) 503.

5. Leo N. Chapin and Sharon ,~ Myers, Mat. Res. Soc. Symp. Proc. 200 (1990). 153-158.

6. Ian M. Reaney, Keith Brooks, Radosve t a K l i s su r ska , Czezlaw Pawlaczyk and Nava Setter, J. Am. Ceram. Soc., 77 (1994) 1209.

7. Keith G. Brooks, Ian M. Reaney, Radosveta Klissurska, Y. Huang, L. Bursill, and N. Setter, J. Mater. Res. 9 (1994) 2540.

8. Bruce A. Tuttle, R.W. Schwartz, D.H. Doughty, and J.A. Voight, Mat. Res. Soc. Symp. Proc., 200 (1990) 159.

9. C.K Kwok and S.B.Desu, Mat. Res. Soc. Symp. Proc., 200 (1990) 159.

10. Kashyap R. Bellur, H.N. Al-Shareef, S.H. Rou, K.D. Gifford, O. Auciello, and A.I. Kingon, Proc. ISAF'92, IEEE. (1992) 448.

11. P .K.Larsen, G.L.M. Kampsch6er , M.B. van der Mark and M. Klee, ISAF'92, IEEE. (1992) 217.

12. Keith G. Brooks, in Growth and A p p l i c a t i o n s of T h i n F i l m s , Prometheus, Prague, 1994, pp. 143-53.

13. G.B. Harris, Phil. Mag., 43 (1952) 113.