Improved dielectric properties of lead lanthanum zirconate titanate thin films on copper substrates

Materials Letters 64 (2010) 22–24

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Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

Improved dielectric properties of lead lanthanum zirconate titanate thin films on copper substrates M. Narayanan ⁎, B. Ma, U. Balachandran Energy Systems Division, Argonne National Laboratory, Argonne, IL 60439, United States

a r t i c l e

i n f o

Article history: Received 19 June 2009 Accepted 29 September 2009 Available online 6 October 2009 Keywords: Ferroelectric High energy density Sol–gel PLZT Thin film Perovskite

a b s t r a c t Thin films of lead lanthanum zirconate titanate (PLZT) were directly deposited on copper substrates by chemical solution deposition and crystallized at temperatures of ~ 650 °C under low oxygen partial pressure (pO2) to create film-on-foil capacitor sheets. The dielectric properties of the capacitors formed have much improved dielectric properties compared to those reported previously. The key to the enhanced properties is a reduction in the time that the film is exposed to lower pO2 by employing a direct insertion strategy to crystallize the films together with the solution chemistry employed. Films exhibited well-saturated hysteresis loops with remanent polarization of ~ 20 μC/cm2, dielectric constant of > 1100, and dielectric loss of < 0.07. Energy densities of ~ 32 J/cm3 were obtained at a field of ~ 1.9 MV/cm on a ~ 1 μm thick film with 250 μm Pt electrodes. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Recently, embedding of high-permittivity dielectric materials into printed circuit boards (PCBs) has been of great interest because it reduces the device footprint and manufacturing costs while improving device reliability. The major problem has been the incompatibility of the processing temperatures used to make the PWB and the ceramic dielectric layers. The polymers used in the PWB packaging cannot withstand the high temperatures required to process the ceramic dielectric layers. At present, the film-on-foil approach is the most attractive method for embedding surface-mounted components in the PWB [1–4]. In this approach, the dielectric material is first coated on thin base metal foils (nickel, copper) by chemical solution deposition (CSD) and then crystallized at high temperatures followed by the deposition of top electrodes, thus forming the capacitor structure. These ready-made film-on-foil capacitors can later be embedded into a PWB. Lead lanthanum zirconate titanate (PLZT) and lead zirconate titanate (PZT) materials deposited directly on copper metal foils are attractive candidates for these embedded capacitor applications. Deposition of these perovskite materials on copper has been plagued by problems with balancing the right processing conditions to maintain the correct oxidation states of both the oxide ceramic and the metal substrate. This is mainly due to the ease in forming a low-permittivity and linear dielectric copper oxide (Cu2O) layer in such processing conditions that degrades the dielectric properties of the resultant capacitor structures. Kingon and Srinivasan [1] reported that the oxygen partial pressure

⁎ Corresponding author. Tel.: +1 630 252 6856; fax: +1 630 252 3604. E-mail address: [email protected] (M. Narayanan). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.09.059

(pO2) must be strictly controlled within the thermodynamic processing window (pO2 ~ 10− 13–10− 17 atm) during crystallization to avoid the formation of copper oxide while maintaining the high quality and phase integrity of the perovskite material. Losego et al. [2] explained the importance of solution chemistry selection to avoid copper oxidation and micro-cracking in films made directly on copper substrates. Previously, we reported the formation of crack-free PLZT films on copper with acceptable dielectric properties by using 2-methoxyethanol (2-MOE) based solution chemistry with a much relaxed pO2 processing window, along with a traditional deposition strategy [3]. Although crack-free, device-quality films were fabricated on copper substrates previously, the dielectric properties were not comparable to films made on platinized silicon for the same PLZT composition. In this letter we report on using 2-MOE solution chemistry combined with a modified deposition strategy that results in thin films on copper which are crackfree, exhibit dielectric properties that are comparable to those of films deposited on expensive platinized silicon substrates, and permit less processing time than reported previously [3]. 2. Experiment We prepared Pb0.92La0.08Zr0.52Ti0.48O3 (PLZT 8/52/48) thin films by sol–gel synthesis using lead acetate trihydrate, titanium isopropoxide, zirconium propoxide, lanthanum nitrate hexahydrate, and 2-MOE. We used 20 mol% excess lead in the starting solution to compensate for the lead loss during the high-temperature crystallization. The details of the solution synthesis are reported elsewhere [4]. Copper substrates (99.8% pure, ESPI Metals) with a thickness of 0.5 mm were polished to a 1-μm finish (~0.4 mm final thickness) and ultrasonically cleaned in acetone and methanol prior to coating. A 0.5 M PLZT stock solution was spin coated

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onto the substrate at 3000 rpm for 30 s, and the resulting material was dried in a furnace at 250 °C for 10 min. The film was then pyrolyzed at 450 °C for 18 min under flowing N2 (99.999%, 850 sccm) by direct insertion. The spin, dry, and pyrolysis steps were repeated two more times, and then the sample was crystallized in a furnace maintained at 650 °C for 18 min in 850 sccm of flowing N2 (pO2 ~10− 7–10− 8 atm). This crystallization step, after three layers, is important to avoid cracks and realize thicker films. The entire process up to and including crystallization was repeated two more times to yield a film with a thickness of ~1 μm (nine layers). Platinum top electrodes (250 μm diameter and 100 nm thick) were then deposited on the film by electron beam evaporation using a shadow mask. Phase identification was carried out by x-ray diffraction (Bruker), while surface morphology was determined with an atomic force microscope (Digital Instruments D3100 SPM) running under tapping mode. Dielectric measurements were made with an HP 4192A impedance analyzer using an oscillator level of 0.1 V at 10 kHz. A Keithley 237 high-voltage source meter and Radiant Technologies Precision Premier II Tester were used to measure the leakage current (E ~ 100 kV/cm) and polarization–electric field (P–E) loops. 3. Results and discussion The major factors that contribute to the realization of high-quality, crack-free P(L)ZT thin films are choice of solution chemistry, pyrolysis and crystallization temperatures, and the processing strategy [5–7]. Depositing thin films on substrates, like copper, that are easily oxidized at higher temperatures (>250 °C) restricts the freedom to vary the above synthesis parameters. Previously, we reported on successfully employing the 2-MOE solution chemistry to fabricate PLZT thin films directly on copper substrates without the formation of deleterious copper oxide [3]. It was found that at oxygen pressures below those corresponding to the stoichiometric composition, the oxide loses oxygen to stay in thermodynamic equilibrium with the ambient condition [8]. The oxygen from the lattice diffuses out as neutral atoms, leaving behind its two electrons represented in the Kröger–Vink notation as: 1 •• 0 OO ↔ O2 ðgÞ + VO + 2e : 2 This reaction results in increased conductivity and degrades the dielectric properties of the film. The other important characteristic of the oxygen vacancy is the ability to inhibit domain wall movement, which increases the coercive field of ferroelectric materials. Although the films made previously [3] were of device quality, the dielectric properties were still not as good as films made on expensive platinum-coated silicon substrates. So, to further decrease the oxygen vacancy concentration, we reduced the time the film is exposed to such low pO2 conditions. Additionally, preparation of films with acceptable dielectric properties requires effective control of the densification and crystallization of the as-deposited films. With higher heating rates, a lower viscosity structure is retained to higher temperatures in the films derived by sol–gel synthesis. By heating rapidly, fewer M–O–M crosslinks are introduced into the structure at lower temperature, and the material is therefore capable of more extensive consolidation. Thus, the material densifies prior to the onset of crystallization (nucleation and growth) by processes such as continued condensation reactions and structural relaxation rather than solid-state sintering, resulting in a dense microstructure [7,9]. Therefore, we used rapid thermal annealing (RTA) by direct insertion to crystallize the samples and thereby control these thermodynamic processes effectively. X-ray diffraction patterns of the PLZT films made by RTA revealed only randomly oriented polycrystalline perovskite phase without any copper oxidation. The primary Cu2O peaks at ~29.2° and ~36.2° were not observed, indicating the lack of substrate oxidation. Fig. 1 shows the

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time relaxation for the current density measured at room temperature on a PLZT/Cu sample (~1-μm-thick PLZT) made by RTA with a constant bias potential of 10 V (corresponding to an applied electrical field ~100 kV/cm) across the top and bottom electrodes. The measurements were conducted by keeping the top Pt electrode positive and the bottom Cu electrode grounded. The films exhibit strong initial time dependence, indicating depolarization. The decay in dielectric relaxation current obeys the Curie–von Schweidler law [10], J = Js + J0 × t

−n

ð1Þ

where Js is the steady-state current density, J0 is a fitting constant, t is relaxation time in seconds, and n is the slope of the log–log plot. Fitting the data to Eq. (1), we found an n value of 0.96 and leakage current densities of ~3 × 10− 9 A/cm2. This value is comparable to PLZT films made on LaNiO3 (LNO) buffered nickel substrates processed in air reported previously [11]. The leakage current density was ~3 × 10− 9 A/cm2 (for E ~ 100 kV/ cm) for these RTA samples, compared with ~4 × 10− 9 A/cm2 (for E ~ 90 kV/cm) reported for films made by the traditional crystallization approach [3] processed at the same temperature (a reduction of ~25%). This decrease in leakage current indicates a decrease in the oxygen vacancy concentration. It should also be noted that the films reported earlier were only 345-nm thick and were crystallized just once (at 650 °C) after three layers had been deposited, whereas the samples reported here were crystallized every three layers to build a nine-layer film. Previously, we found that the leakage current density increased by ~65% when the crystallization temperature was increased from 650 to 700 °C [3]. Nevertheless, we concluded that both the crystallization temperature [3] and time under low pO2 atmospheres have a major effect on the oxygen vacancy concentration in the PLZT film made on copper, with temperature being dominant among the two. The inset in Fig. 1 shows the surface morphology of the PLZT film on copper measured with an AFM. The grains are uniformly distributed with an average grain size <100 nm, which is smaller than the 150– 200 nm observed for samples prepared by the traditional crystallization approach [3]. This difference indicates an improvement in the film microstructure and quality as predicted for higher heating rates. No microcracks or delamination was observed after crystallization of a three-layer-thick film by RTA and is partly attributed to the smaller

Fig. 1. Leakage current density as a function of time for a nine-layer (~ 1 μm) PLZT film on copper substrate for an applied field of ~ 100 kV/cm at room temperature. Inset shows the surface topography of the film on copper obtained by AFM.

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grain size and the planar compressive stress on the films, resulting from the higher thermal expansion coefficient of the copper substrate. Fig. 2 shows the dielectric response as a function of bias field for films crystallized at 650 °C directly on copper substrates. These films exhibit well-defined hysteresis, saturation at high field, and good dielectric tunability. A dielectric constant (k) of >1100, dielectric loss (tan δ) of <0.07, and dielectric tunability of ~70% were typically observed for these films. The measured dielectric values are comparable to those reported for PLZT on LNO buffered nickel (k ~ 1300, tan δ < 0.06) [11,12] and on Pt–Si (k ~ 1400, tan δ < 0.04) [6,12] substrates processed in air. This result is attributed to the improved microstructure attained by RTA. We believe that the processing strategy can be refined further to match the dielectric properties reported on LNO/Ni [11]. The inset in Fig. 2 shows the relatively flat response of the capacitance and loss when measured up to 100 kHz indicating the high quality of the film. Capacitance density of 0.87 μF/cm2 was measured for these 1 μm thick films at 10 kHz which is comparable to 0.97 μF/cm2 observed for films formed on LNO buffered nickel substrates [11]. Fig. 3 shows the measured P–E loops for a Pt/PLZT/Cu capacitor (~1 μm thickness and 250 μm diameter) at 20 to 100 V with an increment of 20 V. Films exhibited slim and well-saturated loops with remanent polarization (Pr) of ~20 μC/cm2, saturation polarization (Ps) of ~55 μC/cm2 (for an applied voltage of 100 V), and coercive field (Ec) of 0.055 MV/cm. These values are comparable to those reported for highquality ferroelectric films made on LNO buffered nickel [11]. The coercive field is slightly lower than that observed for the traditional crystallization approach [3] processed at the same temperature, suggesting an electrically softer material. Thus, we attributed the observed decrease in coercive field and leakage current with decreasing crystallization time to a decrease in the oxygen vacancy concentration in the film. These characteristics are consistent with results reported in the literature [13]. Using Origin 7.0 software, we calculated the energy density by integrating the area under the discharge section of the curve, shown by the shaded region in the inset of Fig. 3. An energy density value of ~32 J/cm3 was calculated for ~1 μm thick PLZT films on copper at an applied field of ~1.9 MV/cm. 4. Conclusions In summary, we report on improved processing methodologies using 2-MOE solution chemistry that not only produce PLZT films on

Fig. 3. P–E hysteresis loops of a nine-layer (~ 1 μm) PLZT film on copper under increasing fields (applied voltages of 20, 40, 60, 80 and 100 V). Inset shows the calculated energy density (shaded area) of ~ 32 J/cm3 (for E ~ 1.9 MV/cm) by integrating the area under discharge section of the loop.

copper with superior dielectric properties (closer to that of PLZT films on Pt–Si and LNO buffered nickel substrates) but also reduce the processing time significantly (than reported previously in [3]) in addition to the relaxed pO2 window. Device-quality PLZT thin films made on copper substrates exhibited very good dielectric properties. We measured dielectric constant (k) >1100, dielectric loss (tan δ) <0.07, capacitance density ~ 0.87 μF/cm2 (at 10 kHz) and leakage current density (J) ~ 3 × 10− 9 A/cm2 at room temperature. Energy density values of ~32 J/cm3 were calculated for an applied field of ~ 1.9 MV/cm. Electrical measurements suggest that the oxygen vacancy concentration was further decreased by reducing the time the film is exposed to low pO2 conditions at high temperatures. These high-quality film-on-foil capacitors can be used for high-power electronic applications. Future work includes the additional refinement of the processing time and temperature for the pyrolysis and crystallization steps to enhance the film quality and reduce the grain size further (50–70 nm range). Acknowledgements Work was funded by the U.S. Department of Energy, Office of Vehicle Technologies Program, under Contract DE-AC02-06CH11357. References [1] [2] [3] [4] [5] [6] [7]

[8] [9] [10] [11] [12] [13] Fig. 2. Room-temperature dielectric response as a function of bias voltage for a ~ 1-μmthick PLZT film on copper. Inset shows the capacitance and tan δ as function of frequency for the PLZT capacitor on copper.

Kingon AI, Srinivasan S. Nat Mater 2005;4:233–7. Losego MD, Jimison LH, Ihlefeld JF, Maria JP. Appl Phys Lett 2005;86:172906. Narayanan M, Kwon DK, Ma B, Balachandran U. Appl Phys Lett 2008;92:252905. Ma B, Kwon DK, Narayanan M, Balachandran U. J Electroceram 2009;22:383–9. Schwartz RW, Schneller T, Waser RCR. Chimie 2004;7:433–61. Schwartz RW, Bunker BC, Dimos DB, Assink RA, Tuttle BA, Tallant DR, Weinstock IA. Integr Ferroelectr 1992;2:243–54. Schwartz RW, Narayanan M. Chemical solution deposition—basic principles. In: Mitzi DB, editor. Solution processing of inorganic materials. New York: John Wiley & Sons; 2009. p. 33–76. Yang GY, Lee SI, Liu ZJ, Anthony CJ, Dickey EC, Liu ZK, et al. Acta Mater 2006;54:3513–23. Schwartz RW, Boyle TJ, Lockwood SJ, Sinclair MB, Dimos D, Buchheit CD. Integr Ferroelectr 1995;7:259–77. Jonscher AK. J Phys D Appl Phys 1999;32:R57–70. Ma B, Narayanan M, Balachandran U. Mater Lett 2009;63:1353–6. Ma B, Kwon DK, Narayanan M, Balachandran U. Mater Lett 2008;62:3573–5. Lee J, Ramesh R, Keramidas VG, Warren WL, Pike GE, Evans JT. Appl Phys Lett 1995;66:1337–9.