Characterization of PLZT thin film prepared by photochemical deposition using photosensitive metal-organic precursors

Microelectronic Engineering 71 (2004) 215–220 www.elsevier.com/locate/mee

Characterization of PLZT thin film prepared by photochemical deposition using photosensitive metal-organic precursors Hyeong-Ho Park a, Woo Sik Kim a, Jun-Kyu Yang a, Hyung-Ho Park a,*, Ross H. Hill b a

Department of Ceramic Engineering, Yonsei University, 134 Shinchon-Dong, Seodaemun-Ku, Seoul 120-749, South Korea b Department of Chemistry, Simon Fraser University, Burnaby, BC, Canada V5A 1S6 Received 5 August 2003; received in revised form 27 October 2003; accepted 23 November 2003

Abstract The electrical properties of lanthanum doped lead zirconate titanate (PLZT) thin films prepared by photochemical metal-organic deposition (PMOD) using photosensitive starting precursors have been characterized. PLZT films with various La concentration were prepared by PMOD on Si(1 0 0) for observing the image of self-patterned PLZT film or on Pt(1 1 1)/Ti/SiO2 /Si(1 0 0) for ferroelectric properties measurement. Even though PLZT film with 0 mol% of La, strictly PZT, showed an asymmetric behavior in polarization-voltage (P –E) relation, PLZT film by doping La showed symmetric behavior in P –E relation. The amelioration of electric and ferroelectric properties with increased substitution of La in PLZT film was observed, especially with 3 mol% La doped PLZT film, the most characteristic P –E hysteresis loop in the point of imprint property was obtained with comparatively large remnant polarization, 30 lC/cm2 at 15 V. Also, capacitance and leakage current behavior of PLZT film were revealed to be sensitive to the contents of La. Ó 2003 Elsevier B.V. All rights reserved. Keywords: PZT; PLZT; Photochemical reaction; Self-patterning; UV exposure

1. Introduction For the application of film to system devices, various thin film deposition techniques such as chemical vapor deposition, sputtering, sol–gel process, etc. have been investigated [1–3]. After the

*

Corresponding author. Tel.: +82-175380815; fax: +8223655882. E-mail address: [email protected] (H.-H. Park).

deposition, a patterning process should be carried out. However one of the associated issues is to develop dry etching process without producing the etching damage. During dry etching, possible changes in the physical and electrical properties of film due to reactive free radicals and ions in the plasma have been reported [4–6]. So another type of patterning technology, which does not induce the degradation of film properties is required. Recently, there has been an increasing interest on photochemical metal-organic deposition (PMOD) as a

0167-9317/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2003.11.004

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possible thin film forming technology [7]. PMOD has been used to deposit many materials including metal oxides of Cr, Co, Cu as well as mixed metal oxides [8,9]. Furthermore, PMOD lithographically produces patterned structure without using photoresist and dry etching [10]. That is, after exposure to UV, development of the film can be directly performed by removing unexposed area by rinsing in a solvent without dry etching process. A number of papers have reported on the effects of doping on the ferroelectric properties of PZT film [11,12]. One of alternatives to improve the ferroelectric properties is doping with A (Pb)-site donors like La3þ . La doping of PZT film is known to improve dielectric properties and leakage current [13,14]. Therefore PLZT has attracted considerable attention as a potential candidate material for use in non-volatile memories and other micro-electromechanical system (MEMS) application [15,16]. However the effect of La doping on the ferroelectric properties of PLZT film by PMOD is still unknown. In this work, a direct patterning by PMOD using UV exposure was applied to PLZT film and the ferroelectric properties of PMODed PLZT film were investigated.

washed with hexane for removing the unexposed area of the film to UV. The pre-annealing of patterned film at 400 °C for 10 min and the final anneal treatment at 650 °C for 30 min were done by direct insertion in a furnace under O2 atmosphere. The image of the patterned film was observed using scanning electron microscopy (SEM). Phase formation and crystallization were checked using X-ray diffractometer (XRD). For the measurement of electrical properties, a 370 nm thick PLZT film was prepared by repeated five times coating. The repeating procedure includes the pre-annealing of UV-exposed film, i.e., all steps except final anneal treatment at 650 °C. Capacitance–voltage (C–V ) and current–voltage (I–V ) characteristics were obtained using HP4284 and HP4145B (Hewlett Packard), respectively. The ferroelectric hysteresis loop and polarization fatigue property were measured using RT66A (Radiant Technology) ferroelectric tester.

3. Results and discussion XRD patterns of PLZT films coated on Pt(1 1 1)/Ti/SiO2 /Si substrate are given in Fig. 1. Well-crystallized PLZT films containing various mol concentrations of La could be obtained after final anneal treatment at 650 °C. The diffraction

2. Experimental procedure The precursors used for the photochemical production of PLZT film were 2-ethylhexanoates of lead(II), lanthanum(III), and zirconyl(IV) and titanium-isopropoxide. The composition ratio of Zr/Ti was fixed as 52/48 and 10% of extra Pb was added for the compensation of Pb-loss during high temperature anneal [17]. PLZT precursor dissolved in hexane was spin-coated at 2000 rpm for 30 s on two types of substrates. One is chemically cleaned Pt(1 1 1)/Ti/SiO2 /Si(1 0 0) substrate for the monitoring of ferroelectric properties and the other is pSi(1 0 0) wafer for the patterning of PLZT film by direct image forming procedure. For the completion of photochemical reaction, spin coated PLZT films were exposed to UV light with 365 nm wavelength for 30 min. The power of UV lamp was 1000 W. After the exposure, the film was

Fig. 1. X-ray diffraction spectra of PLZT films annealed at 650 °C with various La concentrations.

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patterns of PLZT films show that only perovskite phase was obtained for the crystal structure of PLZT films. In Fig. 1, the effect of mol concentration of La on the growth orientation of the film is clearly shown. With 0 mol% of La, strictly PZT, only (1 1 1) diffraction of the film was observed with (1 1 1) of substrate Pt due to (1 1 1) plane matching between Pt and PZT [18,19]. This highly preferred orientation meant that the nucleation and growth of PZT (PLZT with 0 mol% of La) was strongly affected from Pt substrate. However, with increasing La concentration, the relative intensities of (1 0 0) and (2 0 0) diffraction increased with reduced grain size while the relative intensity of the (1 1 1) orientation decreased [20,21]. This implies that in PLZT film, the nucleation is dominant than grain growth due to doping with La and then Pt substrate affects less on the growth orientation of PLZT film. Also, it should be considered that the (1 0 0) oriented perovskite lattice has the lowest growth activation energy [22]. This is why a self-patternable PLZT film does not show highly preferred (1 1 1) orientation after anneal for the crystallization of film. Fig. 2 shows the dielectric constant–voltage plots with applied voltage ranging from )10 to 10 V. With 0 mol% of La, the dielectric constant is approximately 700, and due to a shift of the loop to the left, asymmetric behavior was found in the

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C–V measurement result. The low dielectric constant and asymmetric C–V result could be attributed to the (1 1 1) preferred orientation of the film and domain pinning by space charge originated from the accumulation of oxygen vacancies at the PZT/Pt electrode interface. However, with the addition of La, a symmetric C–V behavior and the increase of dielectric constant were found. The introduction of La3þ is expected to lead to an excess of e
, which can compensate O vacancies [23]. As a consequence, domain mobility is greatly enhanced as well as higher dielectric constant than PZT and symmetric behavior in C–V can be obtained. I–V characteristics of 370 nm thick PLZT films annealed at 650 °C for 30 min under O2 atmosphere are shown in Fig. 3. With 0 mol% of La, when applied with negative bias, the leakage current density varied in the range of about 10
7 –10
4 A/cm2 . In case of positive bias, it was in the range of about 10
7 –10
5 A/cm2 . At the same bias voltage, it was revealed that the leakage current density obtained with positive bias is one order lower than that with negative bias. The major leakage current source of crystalline PZT – related compounds has been known to be Pb or O vacancies created during high temperature anneal for the

1 PLZT (La 0 mol %) PLZT (La 1 mol %) PLZT (La 3 mol %) PLZT (La 5 mol %)

0.1 2.0

800

1.6 700 1.4 600 1.2 500 1.0 0.8

PLZT PLZT PLZT PLZT

0.6 -10

-5

5

0.01 1E-3 1E-4 1E-5 1E-6

400

(La 0 mol %) (La 1 mol %) (La 3 mol %) (La 5 mol %)

0

Dielectric constant

Capacitance (nF)

1.8

Cur rent Density (A/cm2)

900

1E-7 300

1E-8 10

Voltage (V) Fig. 2. Dielectric constant of PLZT films annealed at 650 °C with various La concentrations.

-1000

-500

0

500

1000

Applied Field (kV/cm) Fig. 3. I–V characteristics of PLZT films annealed at 650 °C with various La concentrations.

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crystallization [24]. In our case, the voltage polarity dependence in the I–V characteristic might be related with asymmetric distribution of space charge density in the PLZT film [25,26]. However, according to the increase in La concentration, the I–V characteristic did not show the voltage polarity dependence. From the above results, it could be said that the defect dipole concentration was reduced by doping with La. This is why the leakage current density was improved according to the increase in La concentration. P –E hysteresis loops of PLZT films with varying applied voltage (8, 10, and 15 V) are shown in

Fig. 4. With 0 mol% of La, the remnant polarization was 34 lC/cm2 with 15 V of applied voltage while the values of coercive field were )151 and 108 kV/cm with a shift to the left. The voltage shift related to imprint could be attributed to the defect dipoles of oxygen and Pb vacancies at the interface, inducing the internal field [27]. This asymmetric behavior was also found in the C–V and I–V characteristics. However, with increasing La concentration, P –E hysteresis became symmetric. The measured Pr values of PLZT films with La 1, 3, and 5 mol% concentrations at 15 V were 31, 30, and 9 lC/cm2 , and Ec values were 140, 128, and

60

60

La 0 mol %

La 1mol % 2

Polaization (µC/cm )

40

2

Polaization (µC/cm )

40

20

0 15 V 10 V 8V

-20

-40

20

0 15 V 10 V 8V

-20

-40

-60

-60

-400

-200

0

200

400

-400

60

0

200

400

60

La 3 mol %

La 5 mol %

40

2

Polaization (µC/cm )

40

2

Polaization (µC/cm )

-200

Applied Field (kV/cm)

Applied Field (kV/cm)

20

0

15 V 10 V 8V

-20

20

0 15 V 10 V 8V

-20

-40

-40

-60

-60 -400

-200

0

200

Applied Field (kV/cm)

400

-400

-200

0

200

400

Applied Field (kV/cm)

Fig. 4. P –E hysteresis loops of PLZT films annealed at 650 °C with various La concentrations.

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107 kV/cm, respectively. The introduction of La3þ dopant would reduce Pr and Ec values of the films on the basis of the relative ionic radii, which are  for Pb2þ and 1.22 A  for La3þ [28]. Because 1.32 A the crystallographic structure of PLZT film was transformed from ferroelectric state to paraelectric state with increasing La concentration, Pr and Ec values were decreased [29]. From the above results, it could be said that the asymmetry behavior was improved with the defect dipole alignment by the reduction of the defect dipole concentration on doping La. Fig. 5 illustrates the change of polarization of PLZT film with a number of switching cycle through a bipolar square wave of 10 V at a frequency of 500 kHz. After 109 cycles, the remaining remnant polarizations of the films were about 3%, 19%, 34%, and 37% for 0 mol%, 1 mol%, 3 mol%, and 5 mol% of La concentration. With 0 mol% of La, the degradation of polarization was rapidly enhanced according to the increase in the number of switching cycles. As shown in Figs. 2–4, it could be said that the presence of pinning domains at the electrodes mainly causes this degradation of polarization property with switching cycle. As expected, with increased La concentration, the switching endurance characteristics were promoted due to the reduction of defect dipole such as oxygen vacancy near the interface causing the

DP/DPo (µC/cm 2)

1.0

PLZT (La 0 mol %) PLZT (La 1 mol %) PLZT (La 3 mol %) PLZT (La 5 mol %)

0.5

219

Fig. 6. SEM micrograph of self-patterned PLZT film with 3 mol% of La.

electronic charge trapping and domain wall pinning. The possibility of self-patterning of PLZT film by removing the area unexposed to UV using hexane was examined and the result is given in Fig. 6. In the SEM micrograph, the bright area corresponds to the PLZT film with 3 mol% of La and dark area to Si substrate, respectively. For self-patterning of the PLZT film with around 350 nm thick, the film was exposed for 30 min to complete a micro-structural rearrangement including local network bonding. Surely, exposure time to UV for developing an image should depend on film thickness, UV power, distance between film and UV, and so on. From this micrograph, we knew that self-patterning of PLZT film could be done nicely. From all the results, self-patterned PLZT film is suitable for MEMS application.

0.0

4. Conclusions -0.5

-1.0

-1

10

0

10

1

10

2

10

3

10

4

10

5

10

6

10

7

10

8

10

9

10

10

10

Cycles Fig. 5. Fatigue characteristics of PLZT films annealed at 650 °C with various La concentrations.

PLZT films were successfully formed using photosensitive starting precursors by PMOD and self-patterned without using a photoresist and dry etching procedure. Self-patterning of the film could be performed after UV exposure and removal of unexposed area of the film by rinsing with hexane. With 0 mol% of La, strictly PZT, it

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was revealed that defect-dipole responsible for the asymmetric behavior of P –E and C–V might be the major source of the leakage current. According to increased La concentration, it could be said that oxygen vacancy would tend to reduce the trapped charge near the film/electrode interface. Therefore for PLZT films prepared by PMOD, La doping plays a role in the development of electrical property including fatigue property. It was confirmed that self-patterning process enabled the fabrication of micro-patterned system without a photoresist and dry etching procedure. Acknowledgements This work was supported by Grant No. R012000-000-00244-0 from the Basic Research Program of the Korea Science & Engineering Foundation. References [1] R. Singh, S. Alamgir, R. Sharangpani, Appl. Phys. Lett. 67 (1995) 3939. [2] T.-F. Tseng, K.-S. Liu, T.-B. Wu, I.-N. Lin, Appl. Phys. Lett. 68 (1996) 2505. [3] S.B. Majumder, B. Roy, R.S. Katiyar, S.B. Krupanidhi, Appl. Phys. Lett. 79 (2001) 239. [4] C. Soyer, E. Cattan, D. Remiens, M. Guilloux-Viry, J. Appl. Phys. 92 (2002) 1048. [5] J.K. Lee, T.-Y. Kim, I. Chung, S.B. Desu, Appl. Phys. Lett. 75 (1999) 334. [6] C.W. Chung, C.J. Kim, Jpn. J. Appl. Phys. 36 (1997) 2747. [7] M. Gao, R.H. Hill, J. Mater. Res. 13 (1998) 1379. [8] C.W. Chu, R.H. Hill, Mater. Chem. Phys. 43 (1996) 135.

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