Moving Ribbons
Key to the model 2200's operation is an array of parallel aluminum-covered silicon-nitride (SiN2) microribbons suspended above an air gap (Fig. 3a). The ribbons are built in a sacrificial layer and configured as alternate static and dynamic elements spaced less than 0.5 µm apart. Each ribbon is 200 to 300 nm thick, about 500 µm long, and about 10 µm wide.
Located above a ground plane, the ribbons form an air gap. These films provide the spring-like restoration force that counterbalances the electrostatic bias voltage applied to the dynamic ribbons. Multiple GLV elements are ganged together to create a linear array of 2, 10, 100, or more than 1000 elements to form the complete GLV device.
When a bias voltage of about 10 to 20 V is applied to the dynamic ribbons, they're deflected electrostatically. Meanwhile, the static ribbons hold taut and aren't deflected. The bias voltage is applied via drive electronics, and the user communicates through a dual-port RAM control interface.
In the normal state, when no bias is applied, all the mirrors are undeflected, and incoming incident light reflects off the aluminum surfaces. This is known as the specular state.
After the bias voltage is applied to the dynamic ribbons, incident light is diffracted in direct proportion to the amount of applied voltage, and thus the amount of ribbon deflection. This is called the diffraction state.
This analog control of the ribbons' positions enables fine position control and more than 40 dB of contrast levels. Ribbon spacing design is such that 1/4 wavelength (1/4 λ) is obtained upon deflection for maximum specular reflection (Fig. 3b).
The possibility of building large arrays of inline GLV elements provides a seamless design that can be used to actuate and control light signals in a DWDM system with no "blind spots." Because the ribbons are low-mass elements that are deflected very small distances, submicrosecond speeds are possible. Hundreds and thousands of these GLV arrays can be produced on standard semiconductor wafers (Fig. 4). This means they can be mass produced at low cost.
Conventional ICs need not worry about surfaces sticking together, as there are simply no moving parts. But in MEMS ICs, stiction (the sticking together of a moving surface with another moving or stationary surface) is a major problem that affects the reliability of MEMS ICs.
Although he won't disclose the details, Bob Monteverde, product marketing manager of Silicon Light Machines, says, "We have developed a proprietary step in our CMOS manufacturing process that minimizes stiction problems. This is key in the final step of releasing the MEMS ribbons above the sacrificial layer. Fatigue and wear problems also are minimized." The company is so sure about the reliability of the GLV elements that it reports individual units have undergone testing of more than 6 × 1012 switching cycles with no failure mechanisms.
Other Applications
This proprietary GLV technology has already been demonstrated for high-resolution displays and computer-to-plate print applications. Other potential applications include high-speed, small 1-by-2 optical switches (to replace today's mechanical devices) and modulators of low-bandwidth signals (typically 100 kHz to 1 MHz) on top of a DWDM signal.
GLV technology is presently in use in high-resolution display and imaging systems where its high efficiency, wide dynamic range, precise analog attenuation, rapid switching speeds, and high reliability attributes are crucial. Sony Corp., a licensee of the technology, is presently using it in high-definition TV (HDTV) and e-cinema applications. And, Evans & Sutherland uses GLV technology for high-end flight-simulator applications.
Price & Availability
The 2200 DGE subsystem, including the control electronics, optics module, and optical circulator, costs roughly $10,000, depending on volume and performance requirements. First samples are available now.
Silicon Light Machines, 385 Moffet Park Dr., Sunnyvale, CA 94089; Bob Monteverde, (408) 541-4996; www.siliconlight.com.