Microwave Dielectric Tuning and Losses in Epitaxial Lift-Off Thin Films of Strontium Titanate

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265 Mat. Res. Soc. Symp. Proc. Vol. 603 © 2000 Materials Research Society

EXPERIMENT Thin-film Growth All our STO films were deposited by laser ablation from single crystal targets, via KrF 248 nm, 10 nsec. optical pulses, focussed to a surface energy density on the target of 1.5 J/cm2. The film growth process is substantially the same as is used in many laboratories during the growth of high temperature superconductor films. Briefly, we begin with single crystal substrates of (001) lanthanum aluminate (LAO) or (110) neodymium gallate (NGO). Both have served well as substrates for growth of high-quality STO in our lab [5, 8]. Substrates are mounted on a stainless steel heater block with silver paste. The substrate is then heated to 850 C in preparation for film growth. When preparing a STO film for epitaxial lift-off, we first grow a 'sacrificial' or release layer. This layer is chosen to have two properties. First, it must serve as a good epitaxial match for STO growth. Second, it must have distinct chemical differences from STO so that it may be chemically etched after growth to release the STO film. We have used both YBa 2Cu 3O7 -. (YBCO) and PrBa 2Cu 3 0 7., (PrBCO) as sacrificial layers. Our early efforts used 400 nm thick release layers. Most recently, we have prepared 50 nm release layers. We have found no systematic benefits to the use of either release material, or release layer thickness. Both are grown at typical YBCO deposition conditions that yield superconducting YBCO films with transition temperatures above 91 K (850C and 100 mTorr of molecular oxygen in our system). After growth of the release layer, we immediately deposit STO films at 850 C and 600 mTorr oxygen, to a thickness of 500 nm. Deposition thickness is monitored in situ with HeNe laser ellipsometry and is regularly verified, post deposition, via profilometer. Finally, the chamber is back-filled with 500 Torr of pure oxygen and the film is cooled to room temperature over a 2 hour period. Ellipsometric monitoring of the bilayer film during the cool down process indicates that the underlying release layers fully oxygenate. For bilayers with YBCO release layers, we have verified via a mutual inductance probe that the YBCO films superconduct above 89 K. X-ray diffraction on a subset of the films shows substrate peaks, (001) peaks from the release layer (these peaks obscure the predominantly (001) STO peaks), and a very small contribution (roughly 0.04%) of STO (110) and (220) peaks. Capacitor fabrication and lift-off process After film growth, we first fabricate capacitor structures and then perform the lift-off process. Our capacitors are coplanar, inter-digital structures. Electrodes are 20 nm Ti followed by thicker 1 gnm Au. Photolithographic patterning of the electrode structure is done using either NR8-l000 or NR8-3000 (Futurex, Inc.) photoresist. Metallization is performed by thermal evaporation followed by liftoff in acetone. After electrode fabrication, individual capacitors are diced from the wafer. A finished coplanar capacitor is s