AVS 49th International Symposium
    Applied Surface Science Tuesday Sessions
       Session AS-TuP

Paper AS-TuP5
Thermally Isolated Support Membrane for Low-noise Si JFET Amplifiers

Tuesday, November 5, 2002, 5:30 pm, Room Exhibit Hall B2

Session: Topics in Applied Surface Science
Presenter: Y. Bae, Jet Propulsion Laboratory
Authors: Y. Bae, Jet Propulsion Laboratory
J. Podosek, Jet Propulsion Laboratory
M. Yun, Jet Propulsion Laboratory
J. Bock, Jet Propulsion Laboratory
K. Sukhatme, Jet Propulsion Laboratory
E. Jones, Jet Propulsion Laboratory
Correspondent: Click to Email
A thermally isolated JFET module was fabricated at JPL to work with a high-sensitivity Bolometer that operates at a low temperature, ~300mK. The JFET module provides 24 differential pairs of JFETs, which need be warmed to 130K to provide optimal noise performance, 7nV/rtHz. Each channel has 2 feedback resistors and source, drain, gate, etc. electrical leads. However, power from the JFET electronics is dissipated on the 10K helium vapor-cooled stage of the Herschel cryostat, and the dissipation needs to be minimized to not affect the hold time and temperature of the liquid-helium. Although the JFETs are capable of satisfying the noise and power requirements, the design of the thermal isolation stage dominates the thermal dissipation requirement and presents a significant design challenge. We have devised a novel suspended nitride membrane to provide thermal isolation while satisfying the vibration and thermal cycle environment of a cryogenic space borne instrument. The JFET module needs to meet Bolometer's setup requirements: 1) a voltage noise performance less than 7nV/rtHz/JFET and 2) heat dissipation less than 3.5mW/module. Siliconix U401 JFET differential amplifiers must be heated to 130K to meet the 7nV/rtHz noise requirement. The silicon nitride membrane acts as a 2-D heat flow, so minimizing its thermal conductance is best achieved by reducing the membrane thickness or perforating the membrane. We present tests of the thermal conductance of several membrane architectures, and have carried out through testing of a complete 1.8micron silicon nitride assembly populated with JFET dies. The complete JFET module was characterized in functions of the power and voltage noise. The module passed shock environment test, 15g 400 - 2500Hz sine sweep, and passed thermal cycling to 77K, however it exceeded the heat dissipation requirement. Our test data on thinner membranes indicates we can meet the dissipation requirement with a 1.0micron membrane.