Trimming Gets Ultra-Precise With MEMS-Based Passive Resistors

Resistor trimming is the most common method of optimizing sensor circuit performance for both discrete and monolithic IC sensors. It helps compensate for component tolerances, manufacturing variations, and the effects of temperature and aging. Typically, trimming is performed by using manual mechanical potentiometers, lasers, fusible passive resistor arrays, and electronically configurable resistor arrays that employ digital potentiometers or nonvolatile memory.

Because all of those methods have their pros and cons, Microbridge Technologies found a way to include their best features and minimize their disadvantages in the Rejustor. This technology forms the basis for a family of electronically adjustable micro-resistance elements that are compatible with CMOS, biCMOS, and microelectromechanical-system (MEMS) processes.

Before this technology was developed, there had been no practical and cost-effective means to even measure the temperature coefficient of resistance (TCR) of an individual component, let alone trim it. Rejustor technology electronically trims resistors at very high levels of accuracy. (A pair of resistors can be trimmed to within 0.1% to 0.002%, depending on a variety of factors.) On top of that, it can measure a pair of resistors' TCR to better than 2 ppm/K. All of these procedures are nonvolatile (there's no need to hold the adjustment), and multiple readjustments can be done. Rejustors can also be matched for their temperature coefficient against others, again using electrical signals.

Rejustors are fabricated on a standard silicon process that produces an IC wafer populated with electronic circuits. Next, a cavity is etched on part of the silicon wafer. Suspended over the cavity are a trimmable functional micro-resistance structure and an associated inter-digitated secondary (auxiliary) resistor structure (Fig. 1a). Because the micro-resistance elements are suspended over a cavity, their thermal isolation from the substrate below it is enhanced, enabling the device to reach 800°C to 1000°C. Such temperature levels combined with the resistors' low thermal mass allows for localized, controllable, and rapid thermal cycling.

The functional resistor is adjusted by locally heating the auxiliary resistor in cycles, gently changing the physical properties of the functional resistor with each cycle. The auxiliary resistor is powered only during the actual trimming step. It may be used just once (for a single adjustment) or hundreds or thousands of times for subsequent and repeated bidirectional trimming.

Adaptive algorithms rapidly heat and cool the functional resistor, so many cycles can be applied in a short period of time. The resistor can be trimmed to within 5 to 20 ppm or better. It also eliminates the need to use high-temperature furnaces and prevents damage to other circuits nearby on the chip. Trimming time varies from fractions of a second to several seconds, depending on the resistance range involved and the accuracy desired and whether or not TCR is to be trimmed as well. The Rejustor technology carefully balances its power-handling capability, the power level required to perform the trimming, and the area of silicon needed to manufacture an electronically trimmable device.

Resistors span a range of 500 Ω to several megohms and can be bidirectionally adjusted over a 30% range. They're rated for 1-mW power dissipation (a 30-mW version is in development) and can be cycled 5000 times. The trimming voltage required ranges from 2 to 12 V at 2 to 5 mA. The technology can also be used to measure and adjust the TCR of a pair of resistors. A heater element between the pair of resistors electronically adjusts the two at the same rate so their relative TCRs may be measured (Fig. 1b).

Rejustors come in eight- and 16-pin DIPs. Pricing for the dual 16-pin DIP version is $0.80 each in lots of 1000 to 10,000 units, dropping to $0.40 each in lots of 1 million and higher. Sample quantities are available now, with production quantities scheduled for the first quarter of next year. An evaluation kit is also available (Fig. 2).