Excimer Laser Liftoff of Epitaxial Pb(Zr,Ti)O 3 Thin Films and Heterostructures
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Liftoff techniques demonstrated thus far have been based on selective etching, e.g. utilizing a sacrificial YBa 2Cu 3 O7-8layer to separate SrTiO 3 films from a SrTiO3 growth substrate.7 Similarly, light ion implantation into single crystal garnets8 and LiNbO 39 has been used to generate a buried damage layer with an enhanced etch rate. These selective etching liftoff techniques achieve the principal goal of enabling integration, but with limitations in speed, due to the required lateral transport of etching and products. Furthermore, immersion of the heterostructure in the etchant generally precludes the bonding of the heterostructure to the receptor substrate prior to liftoff. In this paper, we report the intact lift-off and transfer of epitaxial perovskite heterostructures to virtually any receptor substrate. The process begins with bonding of the perovskite heterostructure to the receptor substrate prior to liftoff, followed by excimer laser irradiation-induced separation of the heterostructure from its transparent sapphire or MgO growth substrate. The process is similar to the laser liftoff (LLO) process developed for GaN/sapphire,'°- 3 although the detailed mechanism of substrate separation from the perovskite heterostructure has not been conclusively established. EXPERIMENTAL (Pb,La)(Zr,Ti)0 3 (PLZT) films of several compositions were grown by pulsed laser deposition (PLD) on c-plane sapphire and 100-oriented MgO substrates. Typical growth conditions were a substrate temperature of 600°C in 200 mTorr of flowing 02, and a laser
filmi
[rowth ubtrt
PLZT
bonding agent -
I eepo substrrat] 2. Bonding of electroceramic heterostructure to silicon, glass or polymer substrate (e.g., low-temperature Pd-In metallization)
1. Growth of epitaxial electroceramic thin film on compatible, UV-transparent substrate (e.g., MgO or sapphire)
EZYJ 3. Selective decomposition of interface 4. Separation of oxide growth substrate by nsec UV irradiation through transparent growth substrate (e.g., 38 ns pulse from KrF excimer laser at 248 nm) Figure 1. Overview of bonding and laser liftoff process.
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fluence of 2-2.5 J/cm 2 . In certain cases the films were deposited on a thin (-50 nm) sacrificial layer of PbTiO 3 deposited under similar conditions. When necessary, epitaxial conducting films of LaSrO. 5 Co 0.503 (LSCO) were included as contact layers, having been deposited in situ at 640'C in 300 mTorr of flowing 02. The heterostructures were then bonded to a silicon receptor wafer using transient liquidphase Pd-In bonding (see Ref 13) or to an elastomer (Gel-PakTM) by van der Waals bonding. Separation of the films from their oxide growth substrates was achieved by irradiation with a single pulse from a KrF excimer laser (Lambda Physik, X=248 nm). This is unlike the case for laser liftoff of GaN/sapphire, where the surface tension of liquid Ga holds the substrate in place during laser processing. Furthermore, the thickness of the MgO substrates was < 0.5 mm so as to be sufficiently transparent at the particular wavelength uti
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