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Thermal Trimming Revolutionizes the Resistor

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.

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.

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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Let’s consider in more detail what you can do with these things. Figure 2 shows the TC-offset versus offset characteristics of a generic Microbridge eTC divider. Offset, the deviation of the divider output voltage VIN X (R1/(R1+R2)), is measured in mV/V. TC-offset is the temperature coefficient of that divider output voltage, measured in µV per Kelvin (K) per volt of divider input voltage. Microbridge’s eTC adjustment software makes it possible to pick target values for offset and TC-offset as a point within the region delineated by the parallelogram in the figure.

Consider a case in which the pre-correction divider output voltage is 5% (50 mV/V) below its designed value and in which the divider exhibits an undesired positive 75 µV/VK temperature variation. To make output temperaturestable for the nominal voltagedivider resistor values, Microbridge allows the starting point for offset and TC to be moved from the point represented by the dot in Figure 2 to the center (0, 0) of the plot.

To show how the MBT-303-A might be used, consider the passive bridge in Figure 3. Ignore, for the moment, that the voltage divider on the right consists of a pair of Rejustor resistors, and consider all of the resistors to be ideal. In that case, if the resistance ratios on either side of the bridge were matched, the voltages at the two junctions, A and B, would be equal. Because the resistors were ideal and manifest no TC offset, the voltages at A and B would remain equal regardless of the temperature of their environment. That’s the theory. In the real world, there would, of course, be some voltage offset and some TC offset, which must be addressed.

For demonstration purposes, Microbridge’s founders built up two bridge circuits like the one shown in Figure 3 with an eTC Rejustor divider on the right-hand side. Then they measured their characteristics before and after compensation. Figure 4a presents the results, with each original sample showing an offset and one sample showing a positive TC and the other a negative TC. In addition, Figure 4 shows measurements after compensation was applied.

For additional “fine tuning,” compensation can be iterative. The “compensated” results (the more or less congruent data points in the topmost characteristics in Figure 4a) are an improvement, but they can be made better. They represent the results of compensation targets based only on the initial, pre-compensation measurement of offset and TC-offset depicted in the lower curves.

For higher precision, Microbridge measured offset and TC-offset again to calculate new targets and repeat the calibration process. Figure 4b uses a finer scale than Figure 4a to re-plot the results of the initial compensation on one of the test samples from the “blue diamond” trace along with the results of a second compensation on the same sample (red squares).

The above example involves a sensor bridge. But the application could just as well be an operational-amplifier circuit, where trimmable elements are used to adjust amplifier gain and and gain control better than 0.1% are readily attainable.

Although the MBT303-A is a discrete component, Microbridge’s technology can be incorporated in CMOS ICs. In terms of the fabrication process, one to three Rejustor-specific dopant implant masks may be required to tailor the resistor-poly film. Typically (but not necessarily), the functional resistance element and the heater resistor that receives the electrical adjustment signals are made in separate resistor film layers.

At the end of the fabrication process (for example, after the bond-pads are opened), the microstructures are released by a bulk-silicon etch process, leaving them suspended over a cavity. This provides the thermal isolation and low thermal mass that permits localized, controllable, and rapid thermal cycling of resistance elements embedded in the microstructures. If permanent protection is required for the microstructures, say, to protect them during plastic packaging, wafer-scale capping is applied prior to dicing.

What does Microbridge’s technology do that other technologies don’t? The company says existing solutions all have drawbacks, such as limited temperature range, limited maximum frequency of operation, the need for power and ground, limitations to 8- or 10- or 12- bit accuracies, the need for a skilled technician with a screwdriver or laser to perform the adjustment, and more. The range of applications includes:

• Compensating for unit-to-unit variability in other electronic components, such as actuators and active ICs in a system.
• Adjusting circuit parameters (voltage, current, offset, gain, frequency-response, etc.) as late as possible in a manufacturing flow after all of the manufacturing- induced or assembly-induced variances are already present.
• High-frequency circuits in which a digital potentiometer is impractical.
• Any time lasers are impractical, either because it’s too late in the manufacturing flow or for other reasons.

Available in a 16-lead quad flat no-lead (QFN) or eight-pin smalloutline IC (SOIC) package, the MBT-303-A is currently sampling. It costs $1.67 in lots of 1000.

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