In lieu of laser trimming or trim pots, high-temperature annealing allows independent fine-tuning of resistance and temperature coefficient.

by Don Tuite, Electronic Design

The lowly resistor has been given a makeover. And if your job has anything to do with precision, pay attention. Microbridge announced its MBT-303-A eTC Rejustor—a series-resistive voltage divider in a small IC-style package (Fig. 1).

Both resistors can be independently trimmed to any value between 21 and 30 kV with 0.01% precision. Also, the temp coefficient (TC) can be adjusted independently. Microbridge’s resistor trimming relies on annealing polysilicon resistors rather than laser trimming—and that’s where the story gets interesting.

THE AHA! MOMENT
Essentially, a Rejustor comprises a thermally isolated poly film resistor and an adjacent power resistor, which is pulsed in a controlled fashion, briefly raising the temperature of the Rejustor resistor.

Thermal trimming of resistors isn’t new. Several Japanese companies were working on it some 20 years ago, but those resistors weren’t thermally isolated. On the other hand, thermally isolated microstructures aren’t new either. Honeywell offers flow sensors and infrared detectors based on bulk-micromachined thermally isolated microstructures.

This is where Microbridge steps in. Microbridge’s founders were pursuing potential sensor applications that would use thermally isolated microstructures to raise chemically sensitive films to several hundred degrees Celsius. Along the way, they encountered hightemperature stability problems with polysilicon films used in standard CMOS processes.

At one point, the founders were working with a sensor that needed a very well-matched pair of thermally isolated resistors. However, they remembered that those polysilicon resistors “got unstable” at high temperatures. After reviewing old data, they came to realize a consistently repeatable relationship existed between temperature and time icon structure and how much its resistance changed.

They started manually applying short voltage pulses (touching the resistor terminal with a wire coming from the power supply) while watching the no-flow output. What they achieved was the first rudimentary in-circuit sensor offset trim, which ultimately led to the eTC Rejustor.

In those experiments, human observation provided the feedback during the pulse sequence. That now has been automated after a great deal of experimentation. In the manufacturing process, which can be applied to ICs as well as to discretes such as the MBT-303-A, localized annealing directly changes sheet resistance.

At the end of the CMOS process (for example, after the bond-pads are opened), the microstructures are typically released by a bulk-silicon etch process, leaving them suspended over a cavity. This offers enhanced thermal isolation and low thermal mass, which enables localized, controllable, and rapid thermal cycling of the resistance elements embedded in the microstructures.

PHYSICS
At very high temperatures (several hundred degrees Celsius, far out of normal electronics operating ranges), typical resistor materials exhibit the instability observed by Microbridge’s founders. The Rejustors are thermally isolated portions of common resistive films placed adjacent to highly localized and electrically controllable heat sources.

Material properties such as room-temperature resistivity and TC can be manipulated by careful control of the heating and cooling schedule. Thermal isolation means that only a few tens of degrees Kelvin per miliwatt are dissipated in the microstructure. Also, because the thermal mass being heated is small, rapid heating and cooling are possible, permitting a software-controlled feedbackbased adjustment algorithm.

The most important point about these developments is that it’s possible to adjust resistance and TC to independent targets. Hence, Microbridge called the result an “eTC Rejustor.” Unlike conventional TC-controlled components, no extra temperature sensor is needed because the eTC Rejustor is its own temperature sensor as well as the adjustment device.

This simplifies a number of vexing production problems for analog engineers. For instance, amplifier offset and TC offset can be compensated in the analog domain, right at the source. No lookup-table, analog-to-digital converter (ADC), or digital-to-analog converter (DAC) is needed, and the lack of a stepwise mixed-signal interface implies zero quantization noise.

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