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

aeonia at aeonia at
Mon Feb 25 17:34:22 PST 2008

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)"


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  

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.

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