Low Power Chip Temperature Regulator Using Variable Thermal Resistance

Many modern electric devices and systems need stable operating temperature to ensure constant device characteristics. For examples, commonly used voltage references have 20 ppm/°C of voltage change rate over the commercial temperature range, and a MEMS silicon resonator shows 30 ppm/°C of temperature coefficient of resonant frequency if there is no active temperature control. Many electronic systems are being developed for handheld or other miniature battery-powered applications where the power budget for the device is tightly constrained. However, to achieve stable operating temperature in the electronic device, high power consumption is often necessary. For devices like the chip-scale atomic clock, heating to near 90 °C is required for normal operation. Our research is focused on developing variable thermal resistors (VTR) and thermal isolation structures in order to realize a temperature regulator consuming zero or low power. The variable thermal resistor adds or removes additional heat conducting paths between the ambient and the hot surface of the device chip in response to ambient temperature changes. As long as the intended device temperature is higher than ambient, the device temperature can be kept at the intended temperature because the VTR changes the thermal resistance. Since the power consumption of our candidate device chip was relatively low (~50 mW), a thermal isolation support structure was also needed to elevate the device temperature at the range of pre-programmed temperature.

Chip temperature is kept at a target value while the ambient temperature varies if the size of heat path also varies.

Variable Thermal Resistor (VTR)

1^st generation VTR: thermally activated VTR

We employed an array of thermal bimorph cantilevers between the package and the hot electronic chip (device) inside of the package. A Copper layer is deposited on the bottom side of the Silicon cantilevers in order to make the cantilevers bend up when temperature increases. We assigned a unique length to each cantilever such that each cantilever makes initial contact with the hot chip at a specific ambient temperature. Hotter ambient temperature causes more cantilevers to make contact with the chip.

Pros: zero powered, pre-programmable, simple circuit

Cons: big thermal contact resistance, big contact force requirement (over 40mN)

Array of bimorph actuators is placed between package and hot device.	The copper layer is deposited on the bottom side of cantilevers in order to make the cantilevers bend up when temperature increases.	The bimorph actuators are actuated by ambient temperature change and make contact with the hot device surface to change the thermal resistance between the device and the package. Every bimorph has a different length to initiate contact to the device at a different temperature.	Fabricated array of cantilevers.

2^nd generation VTR: electro statically activated VTR

To get the required force necessary to overcome the thermal contact resistance, we needed another type of actuator. The electrostatic actuator was a good candidate to produce large contact forces with low power consumption. Even if this is not passive type actuation, this approach consumes significantly less power than heater type active temperature controllers. The basic principles are similar to the previous design: using electrostatic forces, 1 micron thick gold suspended beams attach to or detach from the heated device to keep the device at a target temperature while ambient temperature varies.

Variable thermal resistor; electrostatic actuators, and pyrex isolation posts. When the electric potential between the beams and the top electrode is applied, the beams make contact with the top electrode die, reducing thermal resistance.

Preliminary data from the second generation VTR. By applying potential, we could see 3°C temperature change.

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