The lowly resistor has been given a makeover. And if
your job has anything to do with precision, you should
pay attention. Microbridge recently announced its
MBT-303-A eTC Rejustor, which is 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 temperature coefficient (TC) can be adjusted independently. Microbridge's resistor-trimming technology relies on annealing poly-silicon resistors rather than laser trimming—and that's where
the story starts to get 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 high-temperature stability problems with poly-silicon films used in standard CMOS processes.
At one point, they were working with a sensor that needed a
very well-matched pair of thermally isolated resistors. But they remembered that those poly-silicon resistors "got unstable" at
high temperatures. After reviewing old data, they realized a
consistently repeatable relationship existed between temperature and time (duration) for heating the poly-silicon 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 feedback-based 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.