Thermal Conduction Switch for Thermal Management of Chip Scale Atomic Clocks

Abstract

We report the first use of a bimetallic buckling disk as a thermal conduction switch. The disk is used to passively alter the thermal resistance of the package of a chip scale atomic clock. A vertical-cavity surface-emitting laser (VCSEL) and a cesium vapor cell, contained in the clock, must be maintained at 70±0.1°C even under an ambient temperature variation of -40°C to 50°C. A thermal test vehicle has been developed to characterize a sample package with a thermal conduction switch and has been modeled using Finite Element Analysis (FEA). Three cases are presented for the temperature control of the test vehicle under different load placements and environmental conditions: 1) the center resistor in a vacuum package; 2) the center resistor packaged in air; and 3) the side resistor in a vacuum package. At 38°C, the switch snaps upward to reduce the thermal resistance. As a result, the heating power needed to maintain the same temperature is increased from 118 to 200 mW for Case 1. Such a significant change of the thermal resistance demonstrates the effectiveness of the novel thermal switch. However, the switch becomes less effective with air filling the gap, as in Case 2. More interestingly, the switch is not effective at all if the side resistor's temperature is to be controlled as in Case 3.

Thermally actuated buckling beam memory: a non-volatile memory configuration for extreme space exploration environments

“…Thermal buckling, where the beams are buckled through the creation of thermal stresses in the structure, is a viable alternative actuation method (Chiao and Lin 2000). While thermal buckling is often an undesirable artifact of thermal stress build-up during micro-fabrication (Liu et al 2003;Martin et al 2007), it is also an effect that can be exploited in device actuation (Chiao and Lin 2000;Laws et al 2008;Arya et al 2006;Matoba et al 1994).…”

Review of atomic MEMS: driving technologies and challenges

“…Figure 5a, b are the SEM pictures which show the serpentine thermal isolation legs and the vapor cavity. Smart thermal conduction such as thermal switch (Laws et al 2008) and variable thermal resistor (Kim et al 2008) are used to regulate the thermal conduction between the thermal isolation island and outer package according to the chips and ambient temperature variations.…”

Thermal management of fully LTCC-packaged Cs vapour cell for MEMS atomic clock

“…After preliminary studies, obtained at room temperature [12,14] and in order to evaluate the solutions required to contain the power budget (among which utilisation of waste power), more complicated devices utilizing waste heat were developed. They include, e.g., variable thermal resistors made of an array of suspended thin gold beams electrostatically actuated [15], or thermal switches made of a bimetallic buckling disk [16] in order to change, actively or passively respectively, the thermal resistance as a function of the ambient temperature. Although these solutions could allow to reach a wide range of temperature such as [−40 • C, 50 • C] with a limited power consumption (especially during transient regime), they strongly complicate the system and its fabrication, and are probably not yet highly reliable [15] or not yet miniaturised enough [16].…”

“…They include, e.g., variable thermal resistors made of an array of suspended thin gold beams electrostatically actuated [15], or thermal switches made of a bimetallic buckling disk [16] in order to change, actively or passively respectively, the thermal resistance as a function of the ambient temperature. Although these solutions could allow to reach a wide range of temperature such as [−40 • C, 50 • C] with a limited power consumption (especially during transient regime), they strongly complicate the system and its fabrication, and are probably not yet highly reliable [15] or not yet miniaturised enough [16]. Consequently, for now, it seems more efficient to well insulate the physics package without benefiting from all the waste heat, as it can be seen in a recent and extensive thermal study of a CSAC, free of thermal switch, for which a range [0 • C, 50 • C] was considered [17].…”

“…In addition to the range of ambient temperature, another important parameter is the target temperature (T set ). Indeed, depending on the alkali atoms or the buffer gas mixture composition, operating temperatures can be 70 • C [16,17], 75 • C [14,15] or 80 • C [12] and are always above the ambient temperature. Thereby, only warming, less power consuming than cooling, is required, so that, in steady state, the thermal flux always flows from inside (the warmest elements are the heaters) to outside, through the enclosure.…”