Development of series SQUID array with on-chip filter for TES detector

A cold preamplifier based on superconducting quantum interference devices (SQUIDs) is currently the preferred readout technology for the low-noise transition edge sensor (TES). In this work, we have designed and fabricated a series SQUID array (SSA) amplifier for the TES detector readout circuit. In this SSA amplifier, each SQUID cell is composed of a first-order gradiometer formed using two equally large square washers, and an on-chip low pass filter (LPF) as a radio-frequency (RF) choke has been developed to reduce the Josephson oscillation interference between individual SQUID cells. In addition, a highly symmetric layout has been designed carefully to provide a fully consistent embedded electromagnetic environment and achieve coherent flux operation. The measured results show smooth V-phi characteristics and a swing voltage that increases linearly with increasing SQUID cell number N. A white flux noise level as low as 0.28 mu phi (0)/Hz(1/2) is achieved at 0.1 K, corresponding to a low current noise level of 7 pA/Hz(1/2). We analyze the measured noise contribution at mK-scale temperatures and find that the dominant noise derives from a combination of the SSA intrinsic noise and the equivalent current noise of the room temperature electronics.