On-chip sensing stabilizes micromirror

For telecommunications systems to transition from 10-Gbit/second to 40-Gbit/s rates, signal paths using all-optical switches are becoming increasingly attractive. The central component of many all-optical switches is a movable mirror. Although engineers are developing mirrors without position-sense capability, the assembly and fiber-alignment costs are expected to result in relatively high prices. Position sensing, used in a feedback control loop that includes the high-voltage actuation circuitry and an external digital signal processor, allows a factor-of-10 reduction in the mirror's natural settling time to a new position. This control loop will also give more protection against shock and vibration than an open-loop system that actuates mirrors, but neither receives nor acts on position information.

Engineers in Analog Devices Micromachined Product Division have developed mirrors with capacitive position sensing integrated on-chip with the microelectromechanical systems structure. The focus is on the methods and results of the capacitive-sensing capability, which we consider an essential factor in reducing the cost of all-optical switches. This technology offers potential for the design of products employing all-optical switches with improved insertion-loss performance and capable of high port counts.

Integrated optical microelectromechanical system technology combines mirrors, high-voltage CMOS for electrostatic deflection and low-voltage CMOS for on-chip capacitive position sensing. This technology could significantly lower the cost of all-optical switching systems, ranging from 2 x 2 switches to large arrays.

The use of capacitance variation to measure mirror position provides a number of advantages over other methods. It allows the use of standard CMOS processing-rather than expensive custom processes-for devices that use the technique. Capacitive sensing is relatively stable over temperature, while other measurement methods, such as piezoresistive, can require complex connectivity to remove first-order temperature effects. Capacitive sensing also lends itself to on-chip circuit techniques that can add to this stability.

One way to accomplish angular position measurement around one axis is to form a capacitive divider between the mirror itself and two sense electrodes beneath the structure. As the mirror rotates around an axis that is parallel to the dividing line between the two electrodes, one capacitance value increases and the other value decreases. This differential capacitance can be used to measure the mirror rotation. In actual practice, of course, the electrodes are designed to sense multiple axes of rotation.

In a simplified version of a circuit to measure the differential capacitance, opposing voltage pulses are applied to each of the sense electrodes, and any differential capacitance will result in excess positive or negative charge on the mirror node itself. This charge is converted into a voltage by an on-chip amplifier. The voltage can provide a quantitative measurement of the mirror's angular position if the capacitive relationship is known.

By integrating the amplifier circuitry on the chip, greater position-measurement precision is possible than can be provided with off-chip detection. Parasitic capacitances and cross-coupling paths associated with bond wires can easily mask the tiny positional capacitances that are being measured. The much smaller parasitic capacitances associated with on-chip interconnections result in a dramatic increase in signal integrity.

A three-layer process-using silicon-on-insulator bonding-is used to fabricate the mirror, a silicon sacrificial layer and a circuit device layer on the same substrate. Analog Devices has successfully fabricated and tested the mirrors and position-sense circuitry.

The proprietary optical iMEMS process consists of a three-layer process based on silicon-on-insulator bonding, a silicon sacrificial layer and a circuit device layer on the same substrate. MEMS structures and precision high- and low-voltage BiCMOS circuitry exist on the same substrate. Source: Analog Devices Inc.