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 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°Cto50°C. A thermal test vehicle has been developed to characterize a sample package with a thermal conduction switch. Three cases are presented for the temperature control of the test vehicle under different load placements and environmental conditions: (1) a heating load with a good conduction path to the switch in a vacuum package; (2) the same loading as in Case 1, but packaged in air; and (3) a heating load insulated from the switch, in a vacuum package. At 38°C, the switch snaps upward to reduce the package’s thermal resistance. As a result, the heating power needed to maintain a constant temperature, 63.9±0.1°C, is increased from 118to200mW for Case 1. Such a significant change of the thermal resistance demonstrates the effectiveness of the thermal switch. However, the switch becomes less effective with air filling the gap, as in Case 2, and the switch is not effective at all if the heating load does not have a good conduction path to the switch as in Case 3. The steady state response of this novel thermal conduction switch has been well characterized through experimentation and finite element analysis.

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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.

The power dissipation for chip-scale atomic clocks (CSAC) is one of the major design considerations. 12 mW of the 30 mW power budget is for temperature control of the vertical-cavity-surface-emitting laser (VCSEL) and the alkali-metal vapor cell. Each of these must be maintained at 70+/−0.1°C even over large ambient temperature variations of 0–50°C. Thus the physics package of a CSAC device, which contains the vapor cell, VCSEL, and optical components, must have a very high thermal resistance, greater than 5.83°C/m W, to operate in 0°C ambient temperatures while dissipating less than 12 mW of power for heating. To create such a high level of insulation, the physics package is enclosed in a gold coated vacuum package and is suspended on a specially designed structure made from Cirlex, a type of polyimide. The thermal performance of the suspended physics package has been evaluated by measuring the total thermal resistance from a mockup package with and without an enclosure. Without an enclosure, the thermal resistance was found to be 1.07°C/m W. With the enclosure, the resistance increases to 1.71°C/m W. These two cases were modeled using finite element analysis (FEA), the results of which were found to match well with experimental measurements. A FEA model of the real design of the enclosed and suspended physics package was then modeled and was found to have a thermal resistance of 6.28°C/m W, which meets the project requirements of greater than 5.83°C/m W. The structural performance of the physics package was measured by shock-testing, a physics package mockup and recording the response with a high-speed video camera. The shock tests were modeled using dynamic FEA and were found to match well with the displacement measurements. A FEA model of the final design, not the mockup, of the physics package was created and was used to predict that the physics package will survive a 1800 g shock of any duration in any direction without exceeding the Cirlex yield stress of 49 MPa. In addition, the package will survive a 10,000 g shock of any duration in any direction without exceeding the Cirlex tensile stress of 229 MPa.

Power dissipation of chip-scale atomic clocks is one of the major design considerations. The largest power dissipation is for temperature control of the vertical-cavity surface-emitting laser (VCSEL) and cesium vapor cell. For example, the temperature of the VCSEL and Cs cell have to both be at 70±0.1°C or there will be frequency shift which will ruin the lock of the clock. These temperatures have to be maintained even under a large temperature variation such as −40°C to 50°C. There are three major thermal designs to consider: a) micro-heaters to fine-tune the temperatures of VCSEL and Cs cell, b) use of waste heat from other units to heat the system when outside temperature is low, and c) use of a thermal switch to release any extra waste heat when ambient temperatures are high. These three thermal designs have been incorporated in to a thermal test vehicle, which will be used to develop a thermal management design for the clock. This paper describes the proposed clock design, creation of the thermal test vehicle and development of a bimetallic snap based thermal conduction switch. The switch has been demonstrated to change thermal resistance from 52.9±2.8 K/W when the switch is open to 19.5±1.1 K/W with the switch closed.