We report on the development of thin-film multijunction thermal converters for the measurement of ac current. The materials and designs chosen for these devices have been optimized to provide high accuracy over a wide range of input levels and frequencies. The design details of the MJTCs will be presented, and their performance described.

Dry etching is employed in the fabrication of new planar, thin-film multijunction thermal converters (MJTCs) on quartz membranes and crystalline quartz chips at NIST. The use of crystalline quartz as a material for the membrane and for the chip improves the performance of the MJTC in the frequency range from 100 kHz to 100 MHz. Simulations of the ac-dc voltage transfer difference for a heater resistance of 400 Ω in the frequency range from 1 MHz to 100 MHz show a reduction in the ac-dc transfer difference of more than one order of magnitude in comparison with MJTCs fabricated on silicon chips.

The relationship between the characteristics of various thermoelements (TE's) as voltage or current converters and the overall ac-dc differences of a voltage range in a coaxial thermal voltage converter (TVC) set is described. An algorithm to predict the relationships between the ac-dc differences of individual voltage ranges with different TE's is presented, and a method for recharacterizing a TVC containing a replacement TE is given. The measured results show that for most applications a complete recharacterization of the TVC set is unnecessary.