Real Time, In-situ Curvature Measurements of Epitaxial Ybco Films on Mgo

We have implemented coherent gradient sensing (CGS) as an in-situ diagnostic in an MOCVD reactor to measure curvature and changes in curvature with varying temperature and O2 partial pressure on thin ̄lm YBCO on MgO (001) substrates. CGS is a novel, real-time, full̄eld optical technique that provides a direct map of the components of the curvature tensor across the surface of the wafer. We have examined a well-characterized, YBCO ̄lm (700 nm thick) on a MgO substrate (25 x 25 x 0.5 mm) with increasing temperature from 25 ±C to 760 ±C The curvature increases with temperature, although it is not always spatially uniform. By Stoney's equation, we relate the observed curvature to the thermal mismatch of the ̄lm and substrate where the maximum observed stress in orthogonal sample orientations is 2.2 and 1.0 GPa. For larger similarly grown wafers under varying oxygen partial pressures, we observe dramatic local changes in the CGS interferograms, which we relate to the tetragonal to orthorhombic phase transformation YBCO exhibits as it cools in oxygen.