ABSTRACT
We have developed a new thin-film technology for fabricating single-crystal films of functional complex oxides for optical, microwave, and electrical applications. In this short article, we review this new thin-film technology, called crystal ion slicing (CIS), from a process, material, and device perspective. We illustrate the potential of the technique using the example of CIS lithium-niobate micron-thin films for a low-voltage tunable device for optical-polarization switching.

INTRODUCTION
Thin-film technologies have been a major driving force in the integration of dense, high-speed electrical circuits. A similar trend can be found in recent progress in photonic integrated circuits research. Thin-film systems are required to achieve multifunctional integration by forming a system that includes different types of materials; such a system can achieve better device performance by exploiting the best properties of a wide variety of thin-film materials. A general technique for fabricating these films allows one to obtain the best form of optical device integration, namely device-specific selection of the optimal materials for each function on the integrated platform. A central challenge in such an effort is to fabricate high-performance, i.e., rigorously single-crystal, thin films of complex oxides such as ferrites or ferroelectrics. To date, hetero-integration of these thin-film complex oxides has been hampered by reaction and lack of lattice matching with many substrates. Our approach to fabricating thin films relies on the use of ion slicing, a new form of ion-mediated thin-film liftoff. The ability to fabricate thin-film crystal forms has opened new device designs.

Fig. 1. Plot of simulated (TRIM) ion density for He+ ion implantation (red arrow) into lithium niobate at 3.8MeV (left). The highest concentration of the implanted ions is found in a very narrow layer, at a depth of 10mm. This ion-range depth determines the thickness of the resultant film. An electron micrograph of a chemical sliced sample is shown on the (right). The undercut occurs within the implanted layer.

THE TECHNIQUE OF CRYSTAL ION SLICING
Crystal ion slicing is a new approach to ion-mediated epitaxial liftoff for metal oxide materials [1]. This technique produces high-quality, single-crystal metal-oxide films for a wide range of optical materials such as lithium niobate (LiNbO₃), barium titanate (BaTiO₃), and yttrium iron garnet (YIG). Our approach, which is reminiscent but distinct from the Smart-Cut[2] method for silicon on insulator material systems, introduces a local stress-enhanced chemical slicing process along with a tailored choice of implantation species. This single-crystal thin-film technology produces thin-films with "bulk-crystal" optical, dielectric, and piezoelectric properties and thus provides an important new technique for obtaining high-quality thin films for optical and electrical applications.
The technique of ion slicing is shown schematically in Fig. 1. In the process, a bulk crystal or LPE, MBE or MOCVD grown layer of oxide is first exposed to a flux of high-energy low-mass ions, typically He+ or H+. For ion-implantation at high-energies, typically <3.8MeV, the heavily implanted region is very narrow (~ 0.5mm) and its depth may be accurately calculated by transport range of ions in matter (TRIM) simulations, Fig. 1 (left). A high implantation dose (typically 10¹⁶ -10¹⁷ ions/cm²) of high-energy ions results in a change in the reactive chemistry and a disruption of the crystal lattice structure of a deeply buried sacrificial layer. This stress in the sacrificial layer renders it labile to selective solution-phase etching, which enables the top undamaged thin-film to be "sliced off"; rapid thermal treatment prior to etching can enhance the process significantly. The sliced film may then be bonded to a thick substrate via one of a variety of bonding methods (such as direct wafer bonding or anodic bonding) for device integration and for fragile thin-film handling. Successes with both H+ and He+ ions have been reported, where the choice of ion species and slicing conditions depends on the specific crystal composition.

Table 1: Materials sliced into thin-films using CIS process

MATERIALS
Our studies of ion slicing have shown that this technique can be applied to a variety of materials including ferrites and ferroelectric crystals. A summary of these "sliceable" materials is given in Table 1. In several cases these are the first fully single-crystal thin films obtained in these materials. Although the details of the slicing parameters such as ion species, dose, and etch mechanism is carefully chosen for each material system, all rely on the basic step of deep ion-implantation mediation to achieve a universal method for thin-film fabrication. Because of its importance in many electro-optical applications, ion-sliced films of lithium niobate have been investigated extensively. Most importantly, the thin-film electro-optical coefficient (after ion-slicing process and post-slicing annealing) of these films was found to be as large as the corresponding bulk material coefficient.

Fig. 2. Photograph of electro-optical polarization switch; a bulk device before slicing (left) and a thin-film device placed on a silicon wafer as an electrode (right). The top tuning electrode is not shown.

DEVICES
We have shown that ion-sliced thin films are monocrystalline and thus retain their bulk optical, dielectric, magnetic, electro-optical, and crystallographic properties. Since the ion-slicing process does not involve extreme physical conditions, conventional micro-fabrication technologies can be combined with the slicing process; Table 1 shows a list of compatible processes, which have been demonstrated with ion slicing. Recently, ion-sliced lithium niobate thin films were fabricated into integrated electro-optical devices using the standard processes of photolithographic patterning, metalization, and waveguide fabrication [3].
We illustrated the use of CIS thin lithium niobate with an integrated electro-optical polarization switch that couples orthogonal polarization modes in a waveguide via the electro-optical coefficient, r₅₁. The basic configuration contains a straight Ti-indiffused waveguide and a pair of electrodes placed on the side of the waveguide. The electrodes have a grating structure to achieve the phase-matching condition required for efficient coupling. Alferness reported a tunable version of this device by adding another electrode on the same surface to apply an adjustable external-bias field [4]. In our demonstration we slice a standard device into thin-film form and sandwich it between two electrode plates to tune the conversion wavelength.

Fig. 3. Plot of tuning of the polarization-switching wavelength versus applied voltage (left). Thin-film devices have larger tuning capability due to the higher electric-field/optical mode overlap (right, bottom) than for a bulk device (right, top).

The fabrication process of the thin-film tunable E-O polarization switch is as follows. First, conventional Ti-indiffused waveguides are made on a bulk crystal. The waveguide fabrication parameters allow it to support both TE and TM modes. Subsequently an electrode pair is evaporated on the side of the waveguides by thermal deposition of Au/Cr. The electrode has a grating pattern, 22.5mm in pitch, which is calculated to achieve phase matching between TE and TM modes at l=1550nm. After completing the bulk device, shown in Fig.2 (left), ion slicing is carried out. Helium ions with energies of 3.8MeV are implanted through the pre-fabricated device, which create the implanted layer 10 microns from the surface. The top-surface electrodes are encapsulated during etching. The sliced device is then placed on a heavily doped silicon wafer that is used as a bottom electrode, Fig. 2 (right). Another doped silicon wafer with a thermally grown-oxide layer was placed on top of the sliced device to complete a tunable polarization switch.
To measure the tuning performance, the voltage applied to deposited electrodes was kept constant and the tuning voltage applied between the top and bottom silicon wafers was scanned from 0V to 120V. The resulting shift of the peak conversion wavelength was measured as the input light wavelength was varied, Fig. 3(left). Large tuning (>0.1nm/V) was found despite the presence of an air gap between the top electrode and the device surface, which reduces the effective tuning electric field. Deposited tuning electrodes will lead to a much larger tuning capability (>0.2nm/V). The improvement of the thin-film device tunability over a conventional bulk device can be attributed to larger overlap integral between the guided light and the tuning electric field, shown in Fig. 3 (right).
The single-crystal lithium niobate thin films have been used in a wide variety of applications, ranging from pyroelectric detectors to thin-film waveguides. Figure 4 shows examples of these applications. Each device has shown better performance than its corresponding bulk version. For example, the thin-film pyroelectric detector has higher sensitivity and lower noise due to improved heat confinement and domain engineering [5]. A thin-film electro-optical scanner, sliced from poled LiNbO₃, can deflect an optical beam using a much lower voltage than a bulk device. Figure 4 also shows a zeroth-order half-wave plate constructed from a CIS film perpendicularly integrated into a silica waveguide platform; this device illustrates a second example of the use of CIS films chip-scale, multi-material systems [6].

Fig. 4. Examples of demonstrated thin-film lithium-niobate applications. Photograph of a sliced freestanding lithium-niobate film (top, left). Schematic of a thin-film pyroelectric detector (top, right). Poled domains for a low voltage thin-film beam scanner (bottom, left). Schematic for integration of a zero-order half-wave plate (bottom, right).

CONCLUSION AND FUTURE DIRECTIONS
We have demonstrated the results of a new thin-film method to achieve rigorously single-crystal show in complex oxides. With this method, we have demonstrated prototype, integrated devices in freestanding and bonded thin films, which show improved performance or new capability over existing materials forms. Our current ion-slicing projects are focusing on applications in nonlinear optics, photonic-crystal devices, and narrow-band filters. We have also initiated an extensive study of the slicing conditions with the goal of enhancing out understanding of the basic materials physics and chemistry of the ion-mediated liftoff technique.
Finally, we wish to make grateful acknowledgment of several key collaborations and seminal contributions to our research on this project by Prof. Miguel Levy, Prof. Hassaram Bakhru and Sasha Bakhru, John Lehman, and Prof James Yardley. Work on this project was sponsored by AFOSR and DARPA.

REFERENCES
1. M. Levy, R.M. Osgood, Jr., R. Liu, E. Cross, G.S. Cargill III, A. Kumar and H. Bakhru, "Fabrication of Single-Crystal Lithium Niobate Films by Crystal Ion Slicing." Appl. Phys. Lett. 73, pp 2293 (1998)

2. M. Bruel, "Silicon on insulator material technology." Electron. Lett. 31, pp1201, (1995)

3. T. Izuhara, R. Roth, R.M. Osgood, Jr., S. Bakhru, H. Bakhru, "Low-voltage tunable TE/TM converter on an ion-sliced lithium niobate thin-film." Electron. Lett. 39, pp 1118 (2003)

4. R.C. Alferness, "Tunable electro-optic waveguide TE-TM converter/wavelength filter." Appl. Phys. Lett. 40, pp 861 (1982)

5. A. Radojevic, M. Levy, and R. M. Osgood, Jr., J. H. Lehman, C. N. Pannell, "Fabrication and Evaluation of a Freestanding Pyroelectric Detector made from Single-crystal LiNbO₃ Film." Opt. Lett. 25, pp 1657 (2000)

6. A. M. Radojevic, R. M. Osgood, Jr., M. Levy, A. Kumar, H. Bakhru. "Zeroth-Order Half-Wave Plates of LiNbO₃ for Integrated Optics Applications at 1.55 mm." Photonics Tech. Lett. 12, pp 1653 (2000)