MEMS enables electrically trimmable passive resistor

Heat the polysilicon
"Our basic discovery was that when you heat polysilicon to 1,000 degrees [Centigrade] and cool it very quickly, its crystal structure changes, and that changes its resistance in a very controllable and predictable way," said Landsberger. "Our invention was finding a way to heat the polysilicon very quickly without affecting the circuitry near it, and to control the changing resistance in a very controllable manner."

The company was named after the MEMS part of the invention, since a microbridge on the chip suspends the adjustable resistor above an air gap, thereby enabling the controlled heating and precise adjustment of the polysilicon resistor. "The MEMS part of the device--our microbridge--is what allows us to heat the adjustable resistor without affecting any nearby circuitry," said Landsberger. Any CMOS process using chemical vapor deposition can be used to create the microbridge and its integrated polysilicon resistors.

The approach works by placing two polysilicon resistors next to each other; one acts as the heating element for the second resistor, whose value is adjusted by the heating and annealing process. Consequently, the rejustor is a four-pin device, two for the heating resistor and the other two for the adjustable resistor itself.

As a series of millisecond-long pulses is applied to the heating resistor, which is electrically isolated from the adjustable one, its change in resistance is monitored in real-time. Once the desired resistance is achieved, the heating resistor need never be powered again, except in applications where the adjustable resistor needs to be periodically tweaked--for instance, to compensate for aging in the rest of the circuit.

"There are a few applications where you want to adjust the resistor to a different value occasionally, but for most applications it's a set-and-forget procedure," said Landsberger.

Other MEMS devices form moving mechanical structures, but the MEMS element in the rejustor prevents the heating resistor from affecting the surrounding circuitry by merely suspending the structure over an air gap formed by excavating the silicon beneath the two resistors. In this microbridge structure, heating the adjustable resistor to 1,000 degrees Centigrade elevates the temperature of the surrounding circuitry by only about 5 degrees.

"MEMS technology is historically used to fabricate mechanically moving elements, but to our knowledge this is the first time that MEMS has been used to positively affect the performance of a circuit," said Landsberger.

Microbridge provides a software suite to fine-tune the values of its resistors--either by setting them to a very precise value or by measuring some other parameter in the circuit and trimming the resistors' value until that other parameter's optimal value is reached.

Changing the characteristics of the pulses that heat the polysilicon adjusts resistor value in either direction--up or down. The temperature coefficient can also be adjusted by putting two rejustors in series, one with a positive temperature coefficient and one with a negative. In this way, it is possible to keep the total resistance of the two series resistors fixed, by adjusting one upward and the other one downward, resulting in variable compensation that nulls out the effect of temperature on a circuit.