Magnetic materials in microfabrication technologies are increasingly being used to enable new MEMS devices and applications. We recently spoke with Al Sidman, Chief Technical Officer at Advanced MicroSensors, about the ongoing trends and developments that relate to magnetics and MEMS. In this interview, Al reviews the main applications for MEMS and non-MEMS magnetic sensors, emerging MEMS applications, developments with MEMS actuators based on magnetics, MEMS and CMOS integration challenges, and current market drivers.
MEMS Investor Journal: What are the main applications for magnetic sensors? What are the new and emerging applications?
Al Sidman: Magnetic sensors are used widely in consumer mobile (compass, switch) and gaming, automotive or industrial (non-contact position, torque, angle or current sensing) applications. Magnetic thin film recording read-write heads in disk drives and tape drives have a well-established track-record of performance and reliability. Today, silicon-based MEMS applications are still dominated by inertial sensors, inkjet devices and pressure sensors.
We see growing applications for higher-sensitivity magneto-resisitive (MR) devices. These include anisotropic MR (AMR), giant MR (GMR) and tunneling MR (TMR) devices in next-generation smart mobile (phones and tablets) and critical automotive and industrial applications (replacing micro switches, reed relay switches and potentiometers) as well as in medical markets (implantable devices, hearing aides and high sensitivity biosensors), and gaming as well as augmented-reality technologies. Other applications include niche magnetic MEMS micro-actuators at the high-end, improved performance integrated RF passives and consumer device sensors at the low-end.
Tri-axis magnetic compassing sensors will become very large segment in the coming years. Additionally, the combination of silicon-based MEMS and magnetic devices holds many possibilities including the integration of compassing, inertial, gyro and pressure sensors in multi-axis sensing systems.
MEMS Investor Journal: Which devices currently utilize magnetic materials? For MEMS devices specifically, why are magnetic materials needed and what are their primary functions?
Al Sidman: Thin film magnetic recording heads and compassing sensors dominate the non-moving magnetic sensor market. Other applications include automotive sensors, motor controllers, image stabilization systems, magnetic bio-sensors, implantable and non-implantable switches (reed relay replacements), vehicle and munitions detection.
In addition to non-moving magnetic sensors, there are magnetic MEMS actuator devices in development at a few locations across the globe. Applications include optical switching, scanning and attenuation, electrical switching and micro-relays, munitions, fluidic micro-pumping and micro-valving. In these devices, magnetic materials interact with on-chip or externally generated magnetic fields to induce motion.
Magnetic actuation offers low-voltage operation, large displacement at large gaps and potential bi-directional actuation. Magnetic actuation can be achieved with embedded inductors or with external magnets (Lorentz force actuators). Magnetic MEMS actuator devices can be built as discretes or post-processed atop ASIC wafers. Soft magnetic materials can be used for pull-actuators. Permanent magnet materials can be used to create push-pull (bi-directional) actuators.
MEMS Investor Journal: In the near future, do you envision new uses for magnetic materials in MEMS devices and applications? If so, what are these emerging uses and applications?
Al Sidman: Miniaturized 3D MEMS passives such as inductors and transformers with integrated high-frequency magnetic materials may provide improved quality (Q) factors, reduced footprint and the ability to make high performance integrated RF passive systems-on-chip. Meso-scale magnetic MEMS devices may span the realm of micro and macro actuators, providing compact actuators for robotics and military applications.
MEMS Investor Journal: Based on the current levels of production, in which specific MEMS, or MEMS related, applications are magnetic materials most heavily used? What are your expectations for the future?
Al Sidman: The highest growth application for magnetic devices is magnetic compassing for mobile electronics (smart-phones and tablets) and gaming. This application and its derivatives will dominate the market for years until other potential volume applications are proven and move into production.
MEMS Investor Journal: Have you seen examples of integrated MEMS devices where magnetic and non-magnetic sensors were combined on the same substrate?
Al Sidman: Yes. These applications are emerging. Sensors can be fabricated atop ASIC wafers. STMicroelectronics is currently offering an assembled module which combines co-packaged multi-axis motion, compassing and temperature sensing functions. As is the trend in microelectronics, more of these functions may be integrated into fewer chips or a single chip.
MEMS Investor Journal: What are the unique challenges with fabricating MEMS and magnetics devices? What are main differences from non-magnetic MEMS device fabrication?
Al Sidman: Both silicon-based MEMS and magnetic sensors and actuators often require 3D microstructures, sometimes with high aspect ratios. Both require control of fine-line features and stress. Magnetic sensors usually do not have moving features and can be characterized as solid state. As such, the packaging and integration is more standardized and therefore lower cost. MEMS, in its many forms, tends to present more post-wafer challenges. Control of the magnetic films and parameters, however, requires unique expertise, especially on the more advanced GMR (giant magnetoresisitive) and MTJ (magnetic tunnel junction) films and devices.
MEMS Investor Journal: What types of unique materials are used in MEMS related magnetics applications?
Al Sidman: Magnetic products, which include sensors and actuators, often utilize magnetically “soft” (magnetization direction easily changed) and “hard” (magnetization direction difficult to change) ferromagnetic alloys, anti-ferromagnetic alloys, as well as conductors and dielectric materials. Example materials may be NiFe or CoFe, CoPt, PtMn or IrMn alloys, Cu, Au, Ru, Ta, Pt, Rh and Al2O3, MgO or Si3N4.
Oftentimes, in magnetic sensors, these devices are formed with multi-layer stacks of very thin alternating material films. Magnetic actuators may utilize thicker layers of magnetic materials. Either type of device can be built on silicon as discretes, or as active circuitry (ASIC) post-processed devices with integrated drivers and signal processing. Advanced MicroSensors has experience in post-processing magnetic sensors and actuators atop CMOS and other active circuitry device wafers, using processes that are compatible with underlayer device circuits.
MEMS Investor Journal: What types of unique production equipment is used in MEMS and magnetics applications?
Al Sidman: There is substantial overlap between typical silicon MEMS, and magnetic sensor and actuator applications in the processing equipment. Magnetic products, such as compass sensors, rely on films with tightly controlled composition, thickness, magnetic and stress characteristics. Generally, there is more specialized deposition equipment, as the magnetic multi-layer stacks require tight uniformity and property control for layer thicknesses that can be as small as a few angstroms per layer. Specialized test equipment is required to measure individual layer and composite stack properties as a function of internal device-applied or externally-applied magnetic fields, oftentimes in multiple directional axes.
Our history traces back to design, development and captive volume production of thin film magnetic recording heads for Digital Equipment Corporation and Quantum Corporation hard disk drives. And from there, since 1999, as an independent provider of multi-channel thin film magnetic recording heads for several leading manufacturers of digital linear tape data storage drive. This experience base has lead directly to the development and production of MR and advanced MR sensors for a variety of applications.
Al Sidman is the Chief Technical Officer at Advanced MicroSensors, recently acquired by Plures Technologies. He can be reached via phone at 508.770.2792 or email at firstname.lastname@example.org. Advanced MicroSensors’ magnetic and MEMS development and wafer fabrication facilities are based in Shrewsbury, Massachusetts, 40 miles west of Boston. For more information on Advanced MicroSensors, please go to www.advancedmicrosensors.com.
Copyright 2012 MEMS Investor Journal, Inc.