Integrated signaling devices of a proposed type would utilize thermal coupling to transfer digital signals between electronic circuits that are required to be kept electrically isolated from each other. The proposed devices would be implemented as silicon-on-insulator complementary metal oxide/semiconductor (SOI CMOS) integrated circuits and could therefore be readily integrated with other SOI CMOS devices. For some applications, the proposed devices could be an attractive alternative to conventional optocouplers, which have not been amenable to integration with CMOS devices because of a lack of silicon-based integrated sources of light.
The thermal-coupling concept offers a major advantage at the outset: There are several possible methods for on-chip conversion of electrical signals to thermal ones, and there already exists a significant body of published work on the conversion of thermal signals to electrical ones.
Though optimization of design can be expected to entail significant effort with attention to minute details, the basic principle of operation and design concept (see figure) is straightforward. The device would include an input circuit (e.g., an amplifier), through which an input digital signal pulse would be applied to a resistor to generate a pulse of heat. The thermal pulse would be conducted from the resistor, through an electrically insulating barrier, to a thermal sensor.
The resistive heater could be made of polycrystalline silicon. The thermally conducting, electrically insulating barrier would likely be made of a thin layer of SiO2. A simple thermal lens could be made from a suitably patterned surface layer of aluminum. (A more complex thermal lens might also include micromachined, embedded metallic heat pipes to decrease the thermal-response time.) The thermal sensor could be a silicon p/n diode.
A possible disadvantage is that the characteristic times of thermal propagation of signals could limit the speed of such a device. The propagation delay, coupling efficiency, and degree of electrical isolation of the thermal-transducer portion of the device are related to a combination of several geometrical factors. For example, as the distance between the resistor and the thermal sensor increases, the speed and the coupling efficiency decrease. The shape of the lens also exerts a significant effect on the coupling efficiency and speed.
Another major concern is that of input power. High instantaneous input power would translate to a larger signal and faster input/output coupling, but there is also a need to conserve energy and minimize spurious heating. A smarter alternative would be to design an input driver circuit that would produce (1) a shorter higher-power electrical pulse to generate a rapidly rising thermal pulse, followed by (2) a longer, lower-power pulse for maintaining the thermal pulse. The higher-power initial input pulse would thus ensure the rapid triggering of the output circuit via the thermal sensor, and the subsequent longer, lower-power input pulse would guarantee the triggered state of the output circuit. The combination of the two power pulses would result in less power demand than would the use of simple high-power pulses. It would be necessary to select an optimal combination of power levels and pulse durations as a compromise between minimizing average power consumption and maintaining the integrity of signals.
A third major concern is that the thermal signal would include a varying ambient-temperature component. It would be necessary to design the output circuit to reject this component and rapidly convert the rest into an equivalent electrical signal. Two key parameters for designing this circuit are the resolution of the thermal detector and its conversion speed. It may be necessary to amplify the output of the thermal sensor and feed the amplified signal as input to an output driver with digital hysteresis.
This work was done by Mohammad Mojarradi of Caltech for NASA's Jet Pro-pulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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