issue: March 2008 APPLIANCE Magazine
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In the midst of research, engineers uncovered a phenomenon that is now being used to create a whole new category of integrated circuits (ICs).
Using a patented passive polysilicon adjustable resistor technology, Microbridge Technologies Inc. of Montreal, Canada, has introduced what it claims is the world’s first rejustor-enhanced analog IC, the MBSTC-02. The chip integrates an amplifier and a precise conditioner for piezoresistive sensors that allow calibration and temperature compensation after final assembly. “That’s the key—after assembly,” notes Bob Frostholm, vice president of Marketing, Strategic Alliances and Business Development. “No more cumulative errors from assembly and manufacturing processes: Just a quick, precise adjustment that eliminates the need for digital signal conditioning and the inherent errors that come from digitization.”
Perhaps the most interesting aspect of the technology, however, is how it was discovered. “Like many great inventions, this one was somewhat accidental,” Frostholm admits. “Our founders were actually working on something else when they continued to observe changes related to temperature. Previously, this had been considered a detriment, and engineers went to great lengths to keep the silicon cool, often designing elaborate circuits and new processes.”
Microbridge’s founders, however, saw an opportunity. The engineers observed that when polysilicon resistor material is heated to a very high temperature (1000°C or more) and then quickly cooled, the crystal structure changes. “They noticed that these crystal changes cause the resistance to change,” Frostholm explains. “Heat it and cool it several times quickly, and it will change its resistance in a very controllable and predictable way. Think of this as similar to annealing steel (rapid heating and cooling to make it stronger). We are annealing the polysilicon to lower its resistance.”
The technology was then applied to what the company calls a rejustor. According to Frostholm, rejustors are passive adjustable resistors whose ohmic values can be field-adjusted to better than 0.1% tolerances. The devices are made using any standard CMOS, BiCMOS, or BCD process by intertwining two polysilicon resistors—a circuit element and a “heater.”
The resulting chip enables adjustment and verification to be performed in a single temperature cycle. “Applying energy to the heater raises the temperature of the rejustor to more than 1000°C,” Frostholm explains. “The heater is only used for a few milliseconds to effect a change in the resistor element. Then it is shut off. The cycle is repeated several times over approximately one second, until the desired resistance is achieved. Once the desired resistance is achieved, the heater is not used again—ever—unless you want to create a new resistance value.”
To continuously heat and cool the rejustor in a few hundred milliseconds, the company had to thermally isolate it from the rest of the silicon. “Using low-tech MEMS technology, we etch a noncritical cavity below the rejustor,” Frostholm says. “The cavity provides thermal isolation and low thermal mass, which accommodates fast heating for resistor adjustment. At the same time, it only requires low power to create heat in the range of 1000°C and any adjacent circuit elements are unaffected. Temperatures measured where this silicon bridge attaches to the bulk silicon again show only about a 5°C rise.”
By incorporating low-noise operational amplifiers with rejustors, the MBSTC-02 is said to provide precise signal conditioning with integrated temperature correction for use with sensors with a negative TC sensitivity. Applications include any piezoresistive bridge-type sensors, such as pressure sensors, strain gauges, humidity sensors, and accelerometers.
“One [application] that comes immediately to mind is flow sensors, [which are] becoming more ubiquitous in clothes washers and dishwashers,” notes Frostholm. “Depending on the load of the machine, it is desirable to accurately control the volume of water being consumed during each cycle.”
Comfort conditioning systems are another possibility. “HVAC systems continue to gain intelligence to monitor and manage internal environments,” Frostholm says. “Low-pressure pressure sensors used in HVAC systems offer an ideal application to incorporate this product.”