Controlling the thermal conductance of silicon nitride membranes at 100 mK temperatures with patterned metal features

Freestanding micromachined membranes are often used for thermal isolation in electronic devices such as photon sensors. The degree of thermal isolation plays an important role in determining device performance, and so the ability to suppress the thermal conductance of a membrane without increasing its size or decreasing its mechanical strength is of practical importance. We present a simple method that controllably reduces the thermal conductance of silicon nitride membranes by as much as 56% at temperatures near 100 mK. The thermal conductance suppression is achieved by depositing one additional metal layer patterned into islands or rings onto the membrane surface. Complex impedance and noise measurements of superconducting transition-edge sensors fabricated using this technique show that their noise performance is not degraded.