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Stress control of reactively sputtered thick NbN film on Si wafer changing the location of the substrate Si wafer against the Nb target on a magnetron cathode
We have been developing a superconducting NbN thin film coil in a spiral trench on a Si-wafer using MEMS technology. Connecting the coils on the different wafers using waferbonding process, a cylindrical wafer stack is to be formed as a unit of a compact SMES. The critical current density of our NbN film was measured to be around 1100 A/mm2. We measured critical current Ic of 47 mA for the previously fabricated coil of the film thickness tf = 0.5 μm. Ic in the spiral coil increases with tf. However, if we make the NbN film thicker, the film is apt to have higher lateral force caused by tensile or compressive stress which can cause peeling of the film from the Si substrate. It is well known that the stress of the sputtered thin films can be controlled from tensile to compressive stress by controlling the bombardment of high energy particles including argon atoms backscattered from the target surface. Based on this knowledge, a specially designed sputter-deposition apparatus was fabricated in which the substrate can be located not only at the different target-to-substrate distances but also at several different lateral distances from the central axis of the target (off-axis lateral shift). Using this apparatus, various stress conditions could be realized which contributed to fabrication of thick NbN film spiral coil in the trench. The film stress was calculated from bending analysis of the substrate Si wafer by stylus method using Stoney's formula. The maximum compressive stress of 2.5 GPa was measured. By an off-axis lateral shift, tf could be increased from 0.5 to 1 μm. By increasing sputtering gas pressure from 0.7 to 2 Pa, the compressive stress could be mitigated and tf could be further increased from 1.0 to 3 μm. Up to now, we measured Ic of 220 mA for a NbN spiral coil at tfaround 3 μm. More detailed adjustment of the deposition condition will bring further increase in tf, and hence Ic into sight.