Reversible Laser Annealing on YB ~ , CU , O , - ~ Thin Films

High-temperature YBn2Cuj07-s thin films were patterned by a laser writing technique based on the wmperL\ture increase i n the focus of a laser beam. The corresponding local increase of the oxygen diffusion velocity can be employed to write reversibly deoxygenated (semiconducting) and reoxygenated (superconduct~ n p ) patterns. if nitrogen or oxygen, respectively, is chosen as ambient atmosphere. By using different scan velocities r t IS possible ro decrease or increase T, and the critical current. The influence of the arnbien~ atrnosphere. the pat~erning velocity and the laser power density on the resulting pattern was investigated by different methods. like light microscopy, scanning acoustic near-field microscopy (SNAM), and electrical four-point dc measurements. I t could be proved that this planar palteming technique has fully reversible properties. INTRODUCTION onset tem~erature between 86 and 89 K. The thickness of thd films is between 200 and 300 nm. The patMore common patterning lechniques of HTS 'bin terns were wf i t rn an &-ion cw laser (* = 488 or films like wet chemical and plasma etching, ion mrll5 14 nm). During the process either pure nitrogen or ing, and laser ablation are based on the ablation of wu Rowing through the in which material. These methods allow one to produce a large number of well-defined patterns with identical geomeYBCO film was mounted; the cell was kept at an ambitry. If the material itself is not \,cry homoneneous. howent temperature. e;er, this is not sufficient for producingdevices with identical properties. RESULTS AND DISCUSSION An alternative patterning technique for YBa2Cu,0,-s thin films that does not only influence the geometry but also the superconducting propenies themselves was first suggested by Johannsen [I] and is based on the observation that the electrical and optical properties of YBa,Cu,O, are very sensitive to its oxygen content. In the orthorhombic phase Ii.e., 0 < 6 < 0.6), YBa2C~~,0 , s behaves like a metallic conductor concerning the electronic transporr properties, and, below 92 K, i t becomes superconducring. The tetragonal phase of'YBa2CuI0, (i.e., 0:6 c 6 < I ) behaves like a semiconductor. The oxygen content can easily be changed by focused laser heating of the material, thereby causing locally an increase of oxygen diffusion velocity in the YB~,CU,O,-~ film [2-6]. The direction of oxygen diffusion (i.e., into or out of the film) may be controlled by the application of either a reducing or oxidizing atmosphere. In this \Yay. oxygen-rich (superconducting) regions or depleted (semiconducting) ones can be patterned into the YB~:CU?O,-~ film by scanning the focused laser beam across the surface. The process is planar. noninvasive. reversible, does not require a patterninp mask. and does not contaminate the surface of the patterned film. EXPERIMENTAL TECHNIQUE For the experiments we used superconducting YBazCu,O, filnls that were prepared by reactive evaporatiori onto ( 100) MgO substrates ( I x I cm) 171. The c.-axis-oriented. epitaxial films have a typical In a first step, just simple lines were patterned by scanning the focused laser beam across the film surface. The optical reflection micrograph in Fig. 1 shows a line that was patterned in nitrogen atmosphere by scanning the focused laser beam (full width w = 16 pn at intensity I I I , = l l e ) across the film surface (laser power ~ 0 . 7 W) with a velocity of 25 p d s . The deoxygenated line written in nitrogen atmosphere is slightly brighter than the fully oxygenated surrounding film. The contrast is caused by the oxygen dependence of the optical constants of YBCO [8, 91, which yield a decreasing absorption coefficient and an increasing reflectivity in the visible pan of the spectrum when YBCO is losing oxygen. The lower absorption coefficient of oxygendepleted YBCO in contrast to the fully oxygenated one can also be observed in Fig. 2. In this case, a line was patterned in oxygen atmosphere in a YBCO film that was fully deoxygenated before. The deoxidation before was achieved by heating the whole film for 20 min in an argon atmosphere in an oven at 400°C. Hall probe measurements showed nonsuperconducting properties after this treatment. The laser patterning of a the reoxygenated line was done by moving the laser focus ( w = 16 pm. P = 0.7 W) with a scanning velocity v = I )Im/s over the film surface. To what extent this laser patterning method influences the topography the YBCO film was investigated with an acoustic near-field microscope (SNAM) [lo]. The essential part of an SNAM is the comer of a quartz Fig. 1. Optical reflecrion rnicro~rq-h of a linc tliar was patlerned in nitrogen atmosphere by scanning the focused laser beam (full width \v = 16 pm at intensir!. I / / , , = i t < ) ikcross tllc film surface (laser power -0.7 W) with a velocity of 25 pds. Fig. 2. OPIIC;II r(tHt'~rio~i 111icriyr.1ph of :I linc [hat \vus patterned in oxy_cen irtmosphcre by scanning the focused laser beam (full width w = 16 pm ;it intrn>iry I l l , , = I ' r . ) :!cross rht lilm surhce (laser power-0.7 W) with 3 velocity of 1 pds. TheYBCO film was fully deoxygcnatcd ill ;In OVCII b c ~ ~ r r s h:lrlJ. LASER PHYSICS Vol. 7 No. 2 1997 REVERSIBLE LASER ANNEALING ON Y B ~ , C U ~ O , ~ TWIN I X M S