University Ph.D. Oral Examination : Hyeun-Su Kim at CISX Aud, Feb. 29th 8:30am

Stanford University Ph.D. Oral Examination

Hyeun-Su Kim
Department of Mechanical Engineering
Adviser: Thomas W. Kenny

February 29st, 2008
8:30AM (Refreshments served at 8:15AM)
CIS-X Auditorium

"A Variable Thermal Resistor (VTR) for Low Power Temperature
Regulation of Chip-Scale Atomic Clock (CSAC)"

Abstract:

It is essential for a chip-scale atomic clock (CSAC) development to
make an efficient thermal isolation structure as well as to use low
power for temperature controlling due to the low-power requirements
(near 30mW total power consumption in the presence of -40~50°C of
ambient temperature variation). There have been several efforts on the
thermal management of a CSAC to keep the temperature of a vapor cell
in a CSAC at 75°C with low power using thermal isolation structures;
however, this is the first study that shows the ability of thermal
resistance change in multiple stages to control a CSAC temperature in
a range of ambient temperature variation. We developed a variable
thermal resistor (VTR) for the purpose of changing the thermal
resistance of a CSAC package according to the ambient temperature
variation.

The current VTR is comprised of thermal isolation structures
(polyimide posts) and an array of ten electrostatic actuators
(suspended gold beams). The top silicon die which has ground electrode
for vertical electrostatic actuator is separated by three 30um tall
polyimide posts. Ten 1.6um thick gold suspended beams are placed
between the top and the bottom dies while they keep the distance to
the top die 5±1um at zero bias. When 100V of electrical potential is
applied to the gold beams, they bend up and make contact with the top
die. As a consequence, the thermal resistance of VTR decreases. The
0.5um SiO2 passivation layer on the top die prevents electrical
contact between ground electrode and the gold beams.

In addition to the current design of VTR, we also discuss passive
actuation type VTR which is actuated by the ambient temperature
variation. An improved thermal resistance measurement method we have
developed is also introduced and evaluated.

The current version of VTR demonstrates thermal resistance variation
from 200°C/W to 1200°C/W with 100±20°C/W of resolution. As a room
temperature thermal switch that deals with low heat load, VTR may
serve as a high thermal resistance package solution for any low- power
and high-temperature electric device.