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PZT Piezoelectric Films on Glass for Gen-X Imaging

Rudeger H.T. Wilke*a, Susan Trolier-McKinstrya, Paul B. Reidb, Daniel A. Schwartzb

aMaterials Research Institute, The Pennsylvania State University, University Park, PA, USA 16802; bSmithsonian Center for Astrophysics, Harvard University, Cambridge, MA, USA 02138

ABSTRACT

The proposed adaptive optics system for the Gen-X telescope uses piezoelectric lead zirconate titanate (PZT) films deposited on flexible glass substrates. The low softening transition of the glass substrates imposes several processing challenges that require the development of new approaches to deposit high quality PZT thin films. Synthesis and optimization of chemical solution deposited 1 μm thick films of PbZr0.52Ti0.48O3 on small area (1 in2) and large area (16 in2) Pt/Ti/glass substrates has been performed. In order to avoid warping of the glass at temperatures typically used to crystallize PZT films (~700 oC), a lower temperature, two-step crystallization process was employed. An ~80 nm thick seed layer of PbZr0.30Ti0.70O3 was deposited to promote the growth of the perovskite phase. After the deposition of the seed layer, the films were annealed in a rapid thermal annealing (RTA) furnace at 550 oC for 3 minutes to nucleate the perovskite phase. This was followed by isothermal annealing at 550 oC for 1 hour to complete crystallization. For the subsequent PbZr0.52Ti0.48O3 layers, the same RTA protocol was performed, with the isothermal crystallization implemented following the deposition of three PbZr0.52Ti0.48O3 spin-coated layers. Over the frequency range of 1 kHz to 100 kHz, films exhibit relative permittivity values near 800 with loss tangents below 0.07. Hysteresis loops show low levels of imprint with coercive fields of 40-50 kV/cm in the forward direction and 50-70 kV/cm in the reverse direction. The remanent polarization varied from 25-35 μC/cm2 and e31,f values were approximately -5.0 C/m2. In scaling up the growth procedure to large area films, where warping becomes more pronounced due to the increased size of the substrate, the pyrolysis and crystallization conditions were performed in a box furnace to improve the temperature uniformity. By depositing films on both sides of the glass substrate, the tensile stresses are balanced, providing a sufficiently flat surface to continue PZT deposition. The properties of the large area film are comparable to those obtained on small substrates. While sol-gel processing is a viable approach to the deposition of high quality PZT thin films on glass substrates, preliminary results using RF magnetron sputter deposition demonstrate comparable properties with a significantly simpler process that offers a superior route for large scale production.

Keywords: PZT thin films, glass substrates, piezoelectric, sol-gel processing, sputter deposition, low temperature crystallization

1. INTRODUCTION Correcting figure errors using piezoelectric or electrostrictive actuators is a demonstrated route to improve resolution in spaced based telescopes [1-3]. There are several approaches that can be employed. Bulk ceramics can be bonded to the mirror, either as discrete segments or a continuous surface [4], but difficulties associated with reproducibility in machining parts and contacting the mirror without inducing additional strains during bonding limit the effectiveness of this method. Alternatively, the actuating elements can be deposited as a conformal piezoelectric thin film. With the film uniformly coating the entire backside of the mirror, the individual elements are defined by the location and shape of the top electrodes, which can readily be defined through a lithographic process. For thin films, one can deposit the films on thick Si substrates and use standard processing techniques to thin the wafer down sufficiently to ensure flexibility [5]. Conversely, by directly depositing the piezoelectric film on flexible substrates, the synthesis becomes much simpler and more easily scaled to the dimensions envisioned by the Gen-X program (1 m2 panels).

In terms of material selection, PZT thin films have found extensive use as actuators and sensors in microelectromechanical (MEMS) systems due to their high piezoelectric response. A complete solid solution exists between PbZrO3 and PbTiO3, with a nearly temperature independent morphotropic phase boundary near the composition PbZr0.52Ti0.48O3 [6]. It is at this boundary between tetragonal and rhombohedral phases that the dielectric and piezoelectric properties of PZT and PZT thin films are enhanced [7].

Adaptive X-Ray Optics, edited by Ali M. Khounsary, Stephen L. O'Dell, Sergio R. Restaino, Proc. of SPIE Vol. 7803, 78030O · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.862233

Proc. of SPIE Vol. 7803 78030O-1

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Simulations using 1 cm2 rectangular electrodes on 1 m2 panels of 400 μm thick glass substrates have shown that the deflections required to correct long wavelength errors require strain of order 0.1%. With a transverse piezoelectric coefficient, e31,f, of -6 to -9 C/m2 in 1 μm randomly oriented films, PbZr0.52Ti0.48O3 provides sufficient actuation to achieve the targeted goals.

A wide range of deposition techniques are capable of producing dense, crack free, 1 μm thick films with high dielectric and piezoelectric properties. Some of the best piezoelectric coefficients have been reported for films grown by the chemical solution deposition (CSD) method [8]. In CSD, a precursor solution containing the appropriate stoichiometric ratio of cations is spun onto a wafer, and pyrolized to remove any organics. Then, the film is crystallized at high temperatures. The process is repeated until the desired thickness of the film is achieved. For conventional depositions on platinized Si, films are typically crystallized near 700 oC.

There are two main challenges to transferring this process to flexible glass substrates. First, the slumping temperature of the glass substrates chosen for the Gen-X telescope, Schott D263, is near 560 oC, considerably lower than the 700 oC typically used to crystallize PZT, as shown in Table 1. Second, during processing of the actuator stack, the stresses achieved can reach several hundred MPa [9]; the flexible nature of the substrates requires that theses stresses be controlled in order to prevent warping. We present here the details of synthesis and optimization of small area (1 inch x 1 inch) and large area (4 inch x 4 inch) sol-gel deposited 1 μm thick films of PbZr0.52Ti0.48O3 on Pt/Ti/glass substrates. Although the properties of the films are adequate, processing difficulties make the use of sol-gel impractical for large-scale integration. Preliminary results on sputter deposition of PZT show that comparable properties can be achieved with considerably easier processing. As a result, sputter deposition should ultimately prove successful for large active optics installations.

2. RESULTS AND DISCUSSION 2.1 Sol-Gel Deposition of PZT Films on Small Area Substrates

In order to test the feasibility of depositing PZT films on the flexible glass substrates, films were first deposited on small area substrates (1 in2), where the effects of wafer curvature would not be as pronounced, relative to 4 in x 4 in substrates. Pt was chosen as the bottom electrode and was deposited onto the glass substrates via RF/DC Magnetron sputtering using a Kurt J. Lesker CMS-18 sputter tool. In order to improve the adhesion to the glass, a Ti under layer was deposited prior to the Pt. The Pt/Ti layers were deposited at room temperature at a pressure of 5 mtorr. The Ti target was biased using an RF power supply at a power density of 4.4 W/cm2 and the Pt target biased with a DC power supply at 4.4 W/cm2. The CMS-18 is a multi-target system with a large (~120 mm) throw distance that leads to low deposition rates, with growth rates of approximately 1.0 Å/s and 2.4 Å/s for the Ti and Pt, respectively. The corresponding depositions were run for 300 s and 417 s to achieve target thicknesses near 300 Å and 1000 Å.

The PZT deposition is based upon the process developed by Budd, Dey, and Payne [10] and the parameters were adapted from previous work using Si substrates as described in detail elsewhere [11]. A 2-methoxy-ethanol (2-MOE) based solution was used. Lead acetate trihydrate was dissolved in 2MOE and dehydrated under partial vacuum at 105 oC. Titanium iso-propoxide and zirconium n-propoxide were mixed in 2MOE then added to the dehydrated lead powder. Following a 2 hour reflux at 115 oC, the solution was distilled at 105 oC. Upon cooling to room temperature, 12.5 vol % acetylacetone was added to stabilize the solution. Solutions of compositions PZT 52/48 and PZT 30/70 were made with a molarity of 0.4 M, where PZT x/1-x denotes the Zr/Ti ratio of the film. The PZT 52/48 and PZT 30/70 solutions contained 15 and 10 mol % excess Pb, respectively, in order to compensate for PbO loss during crystallization.

Prior to spinning on the solution, the surface of the wafer was coated with 2MOE spun on at 1500 rpm and allowed to air dry. The residual organics on the surface improve the wetting ability of the PZT solution. The solution was spun on at 1500 rpm and pyrolyzed in two steps, the first at 225 oC and the second at 400 oC. Due to the lower thermal conductivity of the glass as compared to Si, to facilitate complete burn-off of the organics and densification of the amorphous matrix, the pyrolysis times were extended from 60 s to 240 s.



Table 1. Physical Properties of Si Wafers and Schott D263 Glass.

In initial attempts to crystallize PZT 52/48, the films were heat treated in a Modular Process Technology Corporation RTP-600S Rapid Thermal Annealing Furnace (RTA). The samples were heated to 500 or 550 oC at a rate of 20 oC/s, and held at temperature for 60 s. This initial step was intended to create nuclei of the perovskite phase. To promote grain growth, the samples were then placed in a box furnace and annealed at 500 or 550 oC for 1 hour. Following this procedure, films made from 52/48 solution showed the presence of large amounts of a non-piezoelectric pyrochlore (or fluorite) phase (Figure 1). The most intense X-ray diffraction peak for this secondary phase occurs near 29.5 o; it is present for PZT 52/48 films annealed at both 500 and 550 oC. The large width of the peak is due to the fine grain nature of the pyrochlore. In sol-gel films, pyrochlore tends to form grains that are of order 10-15nm in size [12], which leads to the low intensity, broad x-ray peaks.

It has been demonstrated that the temperature required to crystallize the perovskite phase is a function of the Zr/Ti ratio for PZT films [13]. The higher the Ti content, the lower the required temperature. As a result, the perovskite phase can be grown at lower temperatures for Ti rich compounds. Although the piezoelectric properties are lower at these

Figure 1. X-ray spectra for PZT 30/70 and PZT 52/48 films annealed at 500 and 550 oC for 60 s. PZT 52/48 compositions show peaks (marked Py) from the pyrochlore (fluorite) phase. Peaks are indexed using

pseudocubic indices. Peaks marked with “W” arise from contamination of the X-ray source by tungsten.

Physical Property Si D263 Thickness (μm) 600 400

Melting/Softening Temp (oC) 1414 557 Thermal Expansion

Coefficient (x10-6/oC) 2.6 7.2

Young’s Modulus (GPa) 150 72.9

Density (g/cm3) 2.33 2.51



compositions, a thin (<100 A) Ti rich layer can be used as a seed layer to nucleate the perovskite phase, allowing the growth of MPB PZT at significantly lower temperatures [14]. The piezoelectric properties of the films will be governed primarily by the thicker film, making this two-step approach a feasible route to obtaining PZT 52/48 crystallized below the slumping temperature of the glass.

Using the process outlined above for film deposition with a PZT 30/70 solution, the (111) peak of the perovskite phase emerges at a crystallization temperature of 550 oC. There is also a broad peak near 22 o, which is consistent with the {100} family of peaks for perovskite PZT. Importantly, there is no evidence in the x-ray spectra of any secondary pyrochlore phase (Figure 1).

Using the crystallized PZT 30/70 perovskite as a seed layer, PZT 52/48 was subsequently deposited, with the 1 hour box furnace step performed after the deposition of every three layers. The thickness of the resultant film is then a function of the number of PZT 52/48 layers deposited on top of the approximately 800 A thick PZT 30/70 seed layer. Depositing 6, 9, and 12 layers of 52/48 yielded films with overall thicknesses of 0.65, 0.88, and 1.06 μm, respectively. The x-ray patterns (Figure 2) show polycrystalline perovskite phase with no evidence of pyrochlore. The peak splitting for the 1.06 μm thick film is consistent with a population of both a- and c- oriented domains for tetragonal PZT 52/48 [15]. The larger intensity of the (002) peak relative to the (200) suggests the PZT film is under compressive stress, with a calculated spontaneous strain of 1.9%, which is reasonable for a PZT composition close to the morphotropic phase boundary.

Figure 2. X-ray spectra for PZT 52/48 deposited on PZT 30/70 seed layers and annealed at 550 oC.

The electrical properties for the three different thickness films are shown in Figure 3. There is a monotonic increase in the relative permittivity with increasing thickness. A dependence of the dielectric and piezoelectric properties on film thickness has previously been observed for films crystallized on Si [16], and has been attributed, at least in part, to the pinning of ferroelectric domain walls near electrode interfaces [17]. The magnitude of the permittivity is lower than for films crystallized near 700 oC. The loss tangents remain essentially constant as a function of film thickness. Over the frequency range 1 kHz to 100 kHz, the 1.06 μm thick film exhibits a permittivity near 800 with a loss tangent of approximately 0.04. Figure 3b plots the polarization – electric field (P – E) hysteresis loops. All films exhibit slight imprint, with built in bias values of approximately 15 kV/cm. Internal bias is typically a result of the presence of Pb and O vacancies forming aligned defect dipoles, ′′ - .. [18] or asymmetric electrodes. The remanent polarization values are



larger than many PZT thin films on Si, ranging from 30-34 μC/cm2, presumably as a result of the compressive stresses (or at least lower tensile stresses) in the film.

Figure 3. (a) Relative permittivity (solid symbols) and loss tangent (open symbols) for PZT films on small area glass

substrates. (b) Polarization/electric field hysteresis loops.



2.2 Sol-Gel Deposition of PZT Films on Large Area Substrates

The feasibility of scaling sol-gel processing up to larger area substrates was tested on 4 in. x 4 in. glass substrates. The increased length scale of these larger substrates imposes additional challenges to the deposition process as substrate warping is magnified. For large area substrates, the flexible glass allows the stress to be relieved by bending the substrate. Consequently, the device will begin to bow, forming either a convex or concave structure, depending upon the stress state of the entire stack. Either shape cannot be used with conventional CSD processing as the spin coating will not yield layers of uniform thickness, and the film can no longer be uniformly heated on a hot plate during the pyrolysis or crystallization steps. All of these effects result in further warping of the sample in a positive feed-back loop that ultimately results in bending significant enough that the wafer can no longer be held on the vacuum chuck for the spin coating step. An initial attempt at depositing films without attempting to balance the stresses resulted in a wafer with a concave structure after the PZT crystallization step. This suggests that the total stress in the PZT/Pt/Ti stack is tensile, even though the X-ray results for films deposited on small area substrates indicated compressive stress in the PZT. This may indicate that the net stress is dominated by the Pt layer, where stress relief in room temperature sputter deposited Pt upon annealing at high temperatures leads to tensile stress upon cooling to room temperature due to thermal expansion mismatch with the substrate [19].

Figure 4. X-ray spectrum of large area PbZr0.52Ti0.48O3 film grown on a PbZr0.30Ti0.70O3 seed layer. The pyrolysis and crystallization of the 1.09 μm film on the 16 in2 substrate was carried out entirely in a box furnace to provide a uniform

temperature environment (see text).

Therefore, in depositing on larger area substrates, careful attention must be paid to balancing the stress to prevent excessive warping of the substrates. The simplest way to balance the stresses is to deposit films on both sides of the substrates. By depositing on both sides, alternating sides layer by layer, the stress on the wafer will never exceed that from an individual layer and the wafer curvature will be minimized.

Pt/Ti bottom electrodes were deposited at room temperature on both sides of the 16 in2 substrates. The depositions were carried out using the same procedure as the small area substrates described above. A PZT 30/70 seed layer was again used in order to nucleate the perovskite phase at 550 oC. The pyrolysis and crystallization steps were both performed in



the box furnace due to the difficulties in uniformly heating curved wafers. The samples were ramped to 400 oC at a rate of 10 oC/min, held for 30 min at 400 oC, then ramped to 550 oC at 10 oC/min and held for 60 min. Following the crystallization step, the furnace was shut off and allowed to cool to room temperature. In order to achieve a thickness of 1.09 μm, a total of 13 layers were deposited on each side.

The X-ray spectrum shows a randomly oriented single- phase perovskite film with no evidence of pyrochlore (Figure 4). The electrical properties are comparable to those obtained on the small area substrates, with permittivity and loss tangents of approximately 800 and 0.04 at 10 kHz (not shown). The polarization electric field hysteresis loop (Figure 5) for the film on the large area substrate again exhibits an internal bias of approximately 15 kV/cm, with remanent polarizations near 24 μC/cm2 and the maximum coercive field value is approximately 64 kV/cm.

These results indicate it is possible to deposit PZT films on flexible glass substrates using sol-gel processing. The electrical and electro-mechanical properties of the films are only slightly lower than those achieved on crystallizing films near 700 oC on Si substrates. The processing leads to an internal bias field that, if engineered correctly, can be used to stabilize the domain structure. This would aid in preventing the films from becoming depoled and will ultimately be of benefit to the final device.

Figure 5. Comparison of polarization/electric field hysteresis loops for sol-gel deposited films on small and large area substrates.

2.3 PZT Films by RF Magnetron Sputtering

Although the films deposited by the sol-gel procedure show promising electrical and electro-mechanical properties, the processing challenges complicate the scalability. An alternate route to depositing PZT films is through RF magnetron sputtering. There are several approaches to sputter deposition of PZT. Perovskite PZT can be grown on a sol-gel deposited seed layer [20], in an in-situ process at elevated temperatures where the correct crystalline phase is grown during the deposition [21], or by an ex-situ process where amorphous PZT is deposited at room temperature and then crystallized in an RTA [22]. As outlined above, using a sol-gel process complicates the deposition on the flexible glass substrates and a technique that relies entirely upon sputtering is preferred. With regards to an in-situ versus ex-situ



approach, the challenge in producing high quality PZT rests in controlling the Pb content of the final film [23]. By depositing at room temperature, one avoids difficulties in controlling the PbO overpressure that can develop at elevated temperatures as the walls of the chamber become coated with material from the target. To simplify the process, an initial attempt at 1 μm thick films was performed at room temperature from a 10% excess PbO target using parameters adapted from [22] and listed in Table 2. The films were then crystallized in the perovskite phase by annealing in an RTA at 550 oC for 1 minute in an oxygen ambient. During this initial development phase, depositions are all carried out on 1 in. x 1 in. substrates.

Table 2. Parameters for sputter deposition of 1 μm thick PZT films.

Parameter Value

Target to Substrate Distance 120 mm

Target Composition Pb1.1Zr 0.52 Ti 0.48O3.1

Substrate Temperature 25 oC

Power Density 2.0 W/cm2

Gas Composition Ar

Pressure 5 mtorr

Deposition Time 25,000 s

Table 3 gives a comparison of properties between the 1.06 μm thick films on glass and the corresponding values for 1 μm thick films on Si prepared by chemical solution deposition, as well as the films by sputtering. Included in the table are values of the transverse piezoelectric coefficient, e31,f, measured using the wafer flexure technique [24]. It can be seen that the pertinent parameters are slightly lower for all of the films on glass relative to those on Si. This is likely a result of the lower crystallization temperature. As can be seen from the table, the properties of the sputter deposited films are comparable to those of the films produced by the sol-gel process. The advantage of sputter deposition however, is that the stresses in the films can be tailored by sputter deposition conditions and should allow for scaling to large dimensions without the need of a sacrificial layer of PZT on the back side of the wafer. Indeed, an initial attempt at depositing on 4 in. diameter glass round has shown the compressive stress in a stack of PZT/Pt/Ti results in a radius of curvature of ~1.6 m, which is large enough to allow further processing of the wafers using standard lithography, etc. for depositing the pixilated array of top electrodes. Work is currently underway to control the Pb content to allow electrical measurements on 1 cm2 electrodes.

Table 3. Comparison of electrical and electromechanical properties of PZT films deposited on glass versus Si substrates. Included are data for films on glass grown by sol-gel processing and sputter deposition.

Parameter Sol-gel on Glass Sputter on Glass Sol-gel on Si εr (1 kHz) 825 810 1000-1200

tan δ (1 kHz) 0.03 0.03 0.02-0.05 Pr (μC/cm2) 30 28 20-30 Ec (kV/cm) 54 51 40-50 -e31,f (C/m2) 5.9 5.3 6-9



3. CONCLUSIONS Piezoelectric PbZr0.52Ti0.48O3 thin films have been deposited on flexible glass substrates by sol-gel processing and through RF magnetron sputter deposition. Crystallization into the perovskite phase was achieved at 550 oC. In the case of the sol-gel processed films, the use of a Ti rich PbZr0.30Ti0.70O3 seed layer was required to obtain ferroelectric PZT. In order to scale this process to larger area substrates, a sacrificial PZT layer was deposited on the back side of the wafer to balance the stresses induced during processing. In the case of sputter deposition, crystallization into the perovskite phase was achieved at 550 oC without a seed layer, and the radius of curvature in large area substrates was large enough to eliminate the need for a sacrificial layer. The electrical and electromechanical properties of films deposited by both processes are comparable and only slightly lower than values obtained on Si substrates, where crystallization temperatures near 700 oC are typically used. For large scale production, sputter deposition appears to be the more viable technique. Future emphasis will be on controlling the Pb content in sputter deposited films.

4. ACKNOWLEDGMENTS This work was supported by grants from the Moore Foundation and from an APRA grant via subtracts SV0-79020 and SV-79021 from the Smithsonian Astrophysical Laboratory. The authors also gratefully acknowledge the usage of facilities in the W. M. Keck Smart Materials Integration Laboratory and the National Nanofabrication Infrastructure Network site at Penn State.

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