1 Introduction

Laser scanning displays have recently attracted considerable attention as portable and low power consumption display systems for head-mount displays or mobile projectors. In such displays, microelectromechanical systems (MEMS) scanner mirrors are key components, because they determine the specifications of the display such as projection size and resolution (Urey 2002). A two-axis MEMS scanner mirror is scanned both horizontally and vertically by microactuators of electrostatic (Schenk et al. 2000, 2001), electromagnetic (Yalcinkaya et al. 2006; Miyajima et al. 2001), and piezoelectric (Filhol et al. 2005; Tani et al. 2006; Yamada and Kuriyama 1998; Asai et al. 2003) systems. Among these systems, the piezoelectric microactuator system is advantageous because it enables low voltage actuation, low power consumption, and large operating force (Tani et al. 2007; Iseki et al. 2010; Pan et al. 2010; Koh et al. 2010; Kobayashi et al. 2005; Yasuda et al. 2005). Conventional MEMS scanners, including piezoelectric MEMS scanner mirrors, are mainly composed of Si substrates, because well-established Si-MEMS microfabrication technologies are useful to miniaturize scanners.

For horizontal scanning, the mirror is vibrated by torsion bars connected to microactuators, which are operated at the resonant frequency (typically over 20 kHz) to satisfy the wide display area and high display resolution (Urey 2002). The Si torsion bars suffer large stress because of their fast and large tilting motion. Therefore, these bars sometimes break.

To improve the fracture toughness of MEMS scanners, metal-based scanner mirrors have been proposed (Pan et al. 2010; Gokdel et al. 2009; Park et al. 2007). Several advantages are expected from metal-based MEMS scanners, such as simple and low-cost fabrication processes as well as high fracture toughness due to the ductile properties of metal compared with brittle silicon. Park et al. fabricated high-speed and wide-angle scanner mirrors composed of piezoelectric Pb(Zr, Ti)O3 (PZT) thick films on stainless steel substrates by aerosol deposition (AD). A lamb wave was generated by the PZT thick films, and a large optical scanning angle of 21° was obtained at a resonant frequency of 15.7 kHz with a driving voltage of 60 V pp (Park et al. 2007).

The AD process is advantageous for its high deposition rate and low-temperature processing compared with conventional deposition processes such as sputtering (Akedo and Lebedev 2000). In contrast, sputtering is usually advantageous for the deposition of high-quality and high-density ferroelectric thin films (Suu et al. 1996). In addition, control of the orientation, which can enhance the piezoelectric properties of PZT thin films, is possible by optimization of sputtering conditions (Ledermann et al. 2003).

In a previous study, we fabricated PZT films on metal substrates of stainless steel or titanium (Ti) using radio frequency (RF) magnetron sputtering (Suzuki et al. 2006; Kanda et al. 2009). We demonstrated that pyrochlore-free PZT films with perovskite structure were grown on Pt-coated Ti substrates. Furthermore, the piezoelectric properties of PZT films on Ti substrates were almost comparable to those on other substrates such as Si or MgO (Kanda et al. 2009; Kanno et al. 2003).

In this study, we have developed a horizontal MEMS scanner mirror composed of a piezoelectric PZT thin film on a Ti substrate. We deposited the PZT thin film on microfabricated Ti substrate by RF sputtering and evaluated the performance of the horizontal laser scanner. Direct deposition of the piezoelectric PZT film on microfabricated Ti is advantageous not only to simplify the fabrication process of piezoelectric MEMS actuators but also to improve the toughness and reliability of the torsion bars. In this paper, we report on the design, fabrication, and evaluation of a metal-based piezoelectric MEMS scanner composed of a PZT film on a Ti substrate.

2 Scanner design

We designed a horizontal MEMS scanner mirror suspended by two pairs of torsion bars that were driven by piezoelectric unimorph cantilevers, as shown in Fig. 1. The dimensions of the scanner are listed in Table 1. When sinusoidal voltages with opposite phases are applied to the right and left sides of the actuators, they deflect in opposite directions to each other because of the transverse piezoelectric effect (Fig. 2). The torsion bars generate rotation torques, which allow the mirror to be tilted on the rotation axis A–A′. The resonant frequency and scanning angle were estimated by a finite element method (FEM) simulation (ANSYS ver.13). Because of the small thickness of the electrodes, we neglected the effect of the electrodes and used a simple double-layer model of PZT and Ti. Figure 3 shows the first resonance of the torsional mode at around 20 kHz, which corresponds to the resonance of the unimorph actuators. Because the tips of the actuators are connected to the mirror, a large deflection of the actuators causes a large scanning angle of the mirror.

Fig. 1
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Schematic illustration of the horizontal piezoelectric MEMS scanner mirror

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Table 1 Dimensions of the scanner mirror structure
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Fig. 2
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Scanning motion of the mirror when sinusoidal voltages with opposite phases are applied to the right and left sides of the unimorph actuators

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Fig. 3
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Contour figure of the first resonance of the torsional mode

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3 Fabrication

The fabrication process of the metal-based piezoelectric MEMS scanner is shown in Fig. 4. First, a 50-μm-thick Ti substrate was microfabricated into the shape of the scanner mirror by wet etching process of acid solution. Then, the Pt bottom electrodes and successive (Pb, La)TiO3 (PLT) seed layers of 20 nm in thickness were deposited on the Ti substrate by RF magnetron sputtering (Kanda et al. 2009). Next, a 2-μm-thick PZT film was deposited on PLT/Pt/Ti by RF magnetron sputtering.

Fig. 4
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Fabrication flow of the Ti-based piezoelectric MEMS scanner. a Microfabrication of the Ti substrate into the shape of the scanner mirror by wet etching, b deposition of the Pt bottom electrodes and the PZT thin film by RF magnetron sputtering, c deposition of the Pt top electrodes through shadow masks, d scanner fixed into position using screws and aluminum frames

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The sputtering conditions are shown in Table 2. The target composition of the PZT film was Zr/Ti = 53/47, which is close to the morphotropic phase boundary (MPB) composition. Using energy dispersive X-ray (EDX) analysis, we confirmed that the film had almost the same Zr/Ti ratio. The growth of the PZT thin film was conducted at a substrate temperature of about 625°C in an Ar/O2 mixed gas atmosphere.

Table 2 Sputtering conditions for the PZT thin film
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After PZT deposition, the Pt top electrodes were deposited through shadow masks. Finally, the horizontal MEMS scanner mirror was fixed into position using screws and aluminum frames. A photograph of the scanner is shown in Fig. 5. In this study, because PZT thin film was directly deposited on a microfabricated Ti metal substrate, we could create a piezoelectric actuator system by simple fabrication processes without microfabrication after PZT deposition.

Fig. 5
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Photograph of the Ti-based piezoelectric MEMS scanner

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4 Scanner properties

The measurement setup for the MEMS scanner is shown in Fig. 6. The mirror was supported by four torsion bars and rotated by connecting unimorph cantilevers. Because sputtered PZT films are preferentially polarized from the bottom electrode to the top electrode (Kanda et al. 2009; Kanno et al. 2003), negative unipolar sinewave voltage with opposite phases was applied to the right and left top electrodes. Light from a He–Ne laser irradiated the mirror, and the reflected light was projected onto a scaled metric screen. Then, the light spot swept along the screen and generated a one-dimensional scanning line for the horizontal axis. By measuring the width x of the scanning line, we calculated the optical scanning angle θ opt from the following equation:

$$ \theta_{\text{opt}} = 2{\text{Arc}}\tan \left( {x/2L} \right) $$
(1)

where L is the distance from the mirror to the screen.

Fig. 6
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Measurement setup of the display performance of the MEMS scanner mirror

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In addition to the optical scanning angle θ opt, other characteristics of the horizontal scanning mirror were determined. The vibration characteristics were measured from the displacement of the mirror edge using a laser Doppler vibrometer, and the mechanical tilt angle θ mech (=θ opt/4) was calculated. In addition to the scanner, simple unimorph cantilevers were also included at the center of the substrate. From the tip displacement of the cantilevers, the transverse piezoelectric coefficient d 31 was evaluated, and this value was used for the FEM calculation. The damping ratio ξ was determined by fitting the experimental results of the mirror tilt angle with the FEM calculation, and then the Q factor of the Ti-MEMS scanner was estimated.

5 Results and discussion

5.1 Crystalline structure

The crystalline structure of the PZT thin film was measured by X-ray diffraction (XRD). Figure 7 shows the 2θ/θ XRD pattern of the PZT thin film deposited on the Pt-coated Ti substrate. We could confirm strong diffraction peaks corresponding to perovskite PZT without a pyrochlore phase. This result indicates that the polycrystalline PZT film with perovskite structures was grown on the microfabricated Ti substrate by RF sputtering.

Fig. 7
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XRD pattern of the PZT thin film deposited on the Ti substrate

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5.2 Scanner properties

The transverse piezoelectric coefficient d 31 of the PZT film on the Ti substrate was determined from the static tip displacement of the PZT/Ti unimorph cantilevers. A vibration amplitude of 606 nm was obtained at 10 V pp with 50-μm-thick, 300-μm-wide, 1.5-mm-long unimorph cantilevers. In the calculation, we used the Young’s moduli of PZT and Ti from the literature (PZT 72.5 GPa and Ti 104 GPa; Berlincourt et al. 1960; Shackelford and Alexander 2000). The average d 31 was calculated to be −32.3 pm/V, which is almost consistent with the value we reported previously (Kanda et al. 2009).

Figure 8 shows the frequency response for the first resonance of the scanner mirror. We applied driving voltages from 2.5 to 15 V pp. In this study, the same driving voltage was applied to each unimorph actuator. The first resonance was clearly observed at around 25.4 kHz, which is consistent with that derived from the FEM calculation of 25.5 kHz. The mechanical tilt angle θ mech of the mirror increased with the driving voltage. The resonant frequency slightly decreased as the driving voltage increased from 2.5 to 5 V pp because of air resistance. On the other hand, a further increase in the voltage up to 15 V pp resulted in an increase in the resonant frequency, which might be due to the non-linear hard spring effect of the torsion bars in large shear stress (Filhol et al. 2005; Pan et al. 2010; Debray et al. 2004; Lee et al. 2011).

Fig. 8
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Frequency response for the first resonance mode of the scanner mirror under an applied sweeping voltage of 2.5–15 V pp

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The dependence of the optical scanning angle θ opt on the negative unipolar driving voltage is plotted in Fig. 9. The optical scanning angle θ opt at the first resonance increased with the applied voltage, and a scanning angle as large as 22° was obtained at 20 V pp. As shown in Fig. 9, the optical scanning angle θ opt proportionally increased with the applied voltage up to 7.5 V pp; however, this increase in the scanning angle decreased above this voltage. This behavior might be caused by the hard spring effect in which the stiffness of the torsion bars increases with the mirror deflection (Filhol et al. 2005; Miyajima et al. 2005).

Fig. 9
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Optical scanning angle of the horizontal scanner mirror as a function of the applied voltage for the first resonant frequency

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Using the transverse piezoelectric coefficient d 31 of the PZT film on the Ti substrate, we evaluated the damping ratio ξ and the Q factor of the Ti-MEMS scanner by fitting the experimental results of the tilt angle with the FEM calculation. The Q factor was estimated to be 285.

Figure 10 shows the laser scanning lines for the first resonance of the torsional mode. The distance from the mirror to the screen (L) was 78 mm. As can be seen from Fig. 10, the resonance motion created horizontal scanning lines. The scanning line in Fig. 10e, whose width was 32 mm, corresponded to a large scanning angle of 22° at 20 V pp. However, a distortion of the scanning lines was observed, which increased with the applied voltage, as shown in Fig. 10b–e. The first resonance mode can generate a large deflection of the mirror because of the large displacement of the actuators. The tilt angle strongly depends on the alignment of the rotation axis to the center line of the mirror. In this study, we mounted the Ti-based piezoelectric scanner using screws to clamp aluminum frames, and a misalignment might have caused the slight curvature observed in the scanning lines.

Fig. 10
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Photographs of the laser scanning lines from the Ti-based piezoelectric MEMS scanner in the first resonance of the torsional mode at the following applied voltages: a 0 V pp, b 2.5 V pp, c 5 V pp, d 10 V pp, and e 20 V pp

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Using the horizontal scanning properties, the number of resolvable spots N was estimated from the following equation (Urey 2002; Hane and Sasaki 2008):

$$ N = \theta D/a\lambda $$
(2)

where D is the mirror diameter, a is the shape factor of the mirror (a = 1.22 for a circle), and λ is the wavelength of the incident light. Using an optical scanning angle of 22°, a mirror diameter D of 1 mm, and an incident light wavelength of 635 nm, the horizontal resolution was calculated to be 496, and this value is sufficient for QVGA display resolution (320 × 240).

In this study, we demonstrated that a Ti-based piezoelectric MEMS scanner had a high performance compared to that of the conventional Si-based MEMS scanners (Urey 2002; Iseki et al. 2010). During the fabrication and measurement processes, no breaks or fractures in the torsion bars occurred, which implies that our metal-based MEMS scanner is tougher than Si-based MEMS scanners. Such toughness is advantageous for reliability over long periods of operation. In addition, we simplified the fabrication process, because our MEMS scanner was fabricated without conventional photolithography after deposition of the PZT film. In this study, because we focused on the applicability of the metal-based piezoelectric MEMS actuators, we adopted simple and basic design for the scanner. Therefore, higher display resolution will be easily achieved by simple optimization of the design. Considering the low cost of Ti substrate, the Ti-based MEMS scanner is a promising device for future applications to laser displays.

6 Conclusion

In this study, we fabricated a Ti-based piezoelectric MEMS scanner mirror and evaluated its horizontal scanning characteristics. A pyrochrore-free PZT film was deposited on a Pt-coated Ti substrate by RF magnetron sputtering. To fabricate our horizontal MEMS scanner, PZT film was directly deposited on Ti substrate that had already been microfabricated; therefore, we prepared a MEMS scanner without using conventional photolithography after PZT deposition. The PZT film on the Ti substrate showed sufficient piezoelectric properties, with a transverse piezoelectric coefficient of d 31 = −32.3 pm/V. We achieved a large optical scanning angle of 22° at a high resonant frequency of 25.4 kHz using a low driving voltage of 20 V pp. These horizontal scanning properties would be applicable for laser displays.