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3-D Microfabrication for In-Plane Optical MEMS Inertial Sensors



OBJECTIVE: MEMS inertial sensors using electromechanical readout approaches have achieved tactical grade performance in small inexpensive form factors. The objective of this effort is to demonstrate three-dimensional microfabrication approaches to achieve micro-optical inertial sensors capable of going beyond tactical grade while maintaining the form factors typical of current MEMS sensors.

DESCRIPTION: Army missile systems are continuing to become smaller and less expensive. In addition, these systems face ever increasing threats to GPS availability. These factors are driving advancement in small and inexpensive, yet high-performance, inertial sensor technology. MEMS technology has demonstrated small form factors for relatively inexpensive inertial components that are suitable for tactical grade operation. However, as missile systems decrease in size and must operate for extended periods of time in GPS-denied environments, future small inertial sensors must demonstrate increasingly higher levels of performance. MEMS inertial sensors utilizing electrostatic, piezoelectric, and magnetic proof mass displacement readout approaches have achieved success in both commercial- and defense-related applications. However, there is a desire for improved performance suitable for navigation-grade applications. This program proposes the development of technology that could yield the next generation Micro-Electro Mechanical Systems (MEMS) navigation-grade inertial sensor.The majority of MEMS inertial sensors utilize electromechanical readout of the motion of a micromachined proof mass. It is anticipated that these “traditional” MEMS pickoff techniques (capacitive, magnetic, piezoelectric, etc.) will not be able to achieve the required performance levels. Optical readout of mechanical displacements has demonstrated high levels of resolution in macro-scale applications including precision movement and placement systems. In addition, optical techniques are common in high performance inertial sensors such as fiber optic gyros and ring laser gyros. Incorporating optical readout approaches into MEMS acceleration devices may yield sufficient resolution to achieve navigation-grade performance. However, the integration of micro-optical components within MEMS devices suffers from multiple issues. Hybrid integration of micro-optical components is commonplace for electro-optic devices such as tunable lasers, modulators, detector arrays, and other complex optical systems. However, for MEMS components with released microstructures, the required microassembly processes suffer from the need to perform component alignment, place and contain adhesives, and attach components to movable structures. In addition, hybrid microassembly is not amenable to wafer-scale processing, leading to difficulties in controlling final component cost. Monolithic integration of micro-optical components within MEMS inertial sensors addresses these issues by providing benefits such as self-alignment and wafer-level fabrication. However, the fabrication of micro-optical components, such as lenses, mirrors, and beam splitters, directly within MEMS components has been limited primarily to the creation of out-of-plane features. These features can be used to realize out-of-place optical MEMS inertial sensors through approaches such as wafer stacking. However, there are few options for fabricating in-plane micro-optical features that can realize in-plane MEMS inertial sensors. The goal of this program is to develop approaches to realize in-plane MEMS inertial sensors that utilize monolithically fabricated micro-optical components for precise proof mass position sensing, thereby enabling high levels of inertial sensor performance without increasing potential form factors and costs.3-D microfabrication approaches can include such items as sidewall micromachining to achieve vertical high-quality micro optical surfaces, multi-level waveguide fabrication approaches to guide light between sources, detectors, and various micro-optical surfaces on the MEMS structure, self-assembled structures to create micro-optical functionality after chip fabrication, amongst other techniques. The performance goals for this effort are 1) range ≥ ±60 g, 2) Bias Instability ≤20 μg, 3)Scale factor stability ≤50 ppm, 4) Volume < 5 in³.

PHASE I: Conduct a design study with detailed fabrication and model development for each component of an in-plane optical MEMS inertial sensor. Predict in-plane optical surface quality for the selected 3-D microfabrication processes. Estimate optical MEMS inertial sensor performance based on potential optical quality. Perform proof-of-principle experiments to investigate 3D microfabrication process performance.

PHASE II: Develop and deliver a functional prototype MEMS inertial sensor with a 3D microfabricated optical pickoff. Characterize the resolution of proof mass displacement measurement. Characterize inertial sensor performance specifications.

PHASE III: Deliver a fully functional MEMS inertial sensor with a 3D microfabricated optical pickoff. Additionally, documentation verifying inertial sensor performance characteristics shall be included with each device delivered. Reported inertial performance characteristics should include scale factor, scale factor error, and bias instability. Measures of angle random walk and velocity random walk shall be included in the inertial sensor performance characteristics for gyroscopes and accelerometers respectively. The inertial sensor technology developed in this effort can be applied to commercial aviation, aerospace, and maritime guidance systems. The optical pickoff technology developed in this effort can also be applied to non-inertial microsystems such as telecommunication integrated optics modules, active alignment systems in microassembly approaches, and nanopositioning devices.

KEYWORDS: MEMS, Optical Inertial Sensor, Displacement Measuring Interferometry


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