Thursday, 16 March 2006 - h. 15:00
Aula Seminari (A34)
SQUID readout of a superconducting bolometer array for APEX
The purpose of the Large Bolometer Camera (LABOCA) project is to develop and install a 300 element bolometer array for a wavelength of 870 µm at the APEX radio telescope (Atacama Pathfinder Experiment) in Chile.
Superconducting Transition-Edge Sensor (TES) bolometers with SQUID readout will be used for this camera. The superconducting film for the TES is a proximity bi-layer of Mo and the alloy Au-Pd. The bi-layer’s critical temperature was chosen to be of about 500mK for an
optimum operation in a 3He cryostat. The TES is biased with constant voltage in order to obtain a strong electro-thermal feedback. The bolometers were fabricated on a Si substrate with a 1 µm thick SiN membrane.
A thin Ti film was used as absorber. Radiation is coupled to the bolometers via a horn array, which also acts as a high-pass filter for the desired wavelength band. The readout SQUIDs are integrated with the bolometers on the wafer. The signals from the readout SQUIDs are amplified with second-stage SQUIDs mounted separately in the vacuum on
the 1.4 K cold plate of the cryostat. Time-division multiplexing is used to reduce the system complexity and costs. Directly coupled SQUID electronics were developed to readout the amplifier SQUIDs and to provide a feedback for flux-locked-loop operation with multiplexed
The integration of the readout SQUIDs with bolometers on one chip will be used for LABOCA. Adaptation of both technologies to each other was performed. The test chips with TES bolometer and integrated readout SQUIDs show proper operation of SQUIDs at temperatures down to 300mK.
The main parameters of the bolometer bi-layer films also correspond to the design values. The optimisation of the thermal design of bolometers is in progress.
The current state of the system development will be presented.