Integrated through -wafer optical monitoring of MEMS for closed -loop control

Abstract

Current trends in many microelectronic systems show an increased use of microelectromechanical systems (MEMS) to perform a variety of tasks. The increased market for MEMS has led to microsystem technologies being employed in physically demanding environments and safety critical applications. This creates the need for higher degrees of certainty in MEMS operation, especially in systems that contain drive components operating under time varying load conditions. Situations such as these give rise to the need for detailed knowledge of the operational states of MEMS over the lifetime of the device, as well as device fault detection. Accurately obtaining this information by a means decoupled from the system shows the potential to further enable both complex and simple MEMS, and allows for the application of closed-loop control. Preliminary through-wafer optical monitoring research efforts have shown that through-wafer optical probing is suitable for characterizing and measuring the behavior of lateral harmonic oscillators. This presentation will discuss research undertaken to establish integrated optical monitoring (IOM) for closed-loop control. Design of the optical microprobe setup, as well as device geometry, were completed to achieve a through-wafer optical signal with increased positional resolution and mechanical stability. Successful linear closed-loop control results achieved using the redesigned probe setup and devices will be presented. Increased displacement information in the optical output waveform is needed for the successful application of more robust, nonlinear control routines. Theoretical optical output field intensity studies are presented and compared with experimental output waveforms, showing a positional resolution of 2 µm using grating structures. Initial binary Fresnel diffractive optical microelement design layout, fabrication parameters, and testing results will be given as well for implementation of a fully integrated optical monitoring system.

Diagnosis of operational changes in microelectromechanical systems via fault detection

“…In this device, the length of the outer trusses is 4µm longer than that of the center truss. The deviation in the calculated spring constant due to difference is negligible [64].…”

“…The through-wafer optical monitoring system is a setup that was developed at West Virginia University to measure the motion of the MEMS lateral comb resonator device [64,70,71,72,73]. A schematic of this test setup is presented in Figure 3.3.…”

“…Schematic of the Through-Wafer Optical MonitoringSystem utilized at West Virginia University[64,71,72,73] The monitoring setup consists of an inferred (IR) source (1310nm wavelength), a set of bulk optics, a single mode optical fiber, and a low-noise IR detector. The MEMS die is positioned between the bulk lens and the optical fiber.…”

“…The 550µm silicon substrate and additional nitride layer that constitute the surface of the die is nearly transparent at IR wavelengths, while the polysilicon that comprises the MEMS structure exhibits a low level of intensity attenuation. It has been found that the percentage transmission through the Poly0 layer is 60% to 75% and through a Poly0-air-Poly1 interface is 50% to 60%[64]. After the beam passes through the grating, it is coupled into the optical fiber oriented just above the die.The fiber routes the signal to an IR photodetector that converts the power of the IR signal into a measurable output voltage.As the center stage of the device oscillates laterally, a corresponding modulation in the attenuation of the optical IR signal (seeFigure 3.4a) occurs due to the grating structure.…”

Optical detection of multiple faults of a MEMS based linear comb resonator

“…The MEMS LCR contains 1 Sandia.gov Image Gallery shuttle mass. Measurement of the mass position in this work is done optically using a near infrared through wafer setup as explained in [12]. A 1310nm wavelength laser is used, as silicon is transparent to this wavelength.…”