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Feature Articles

Optical Inspection Techniques Inspect MEMs Sensors in Air Bags

by Winn Hardin, Contributing Editor - AIA

Accelerometers were among the first microelectromechanical systems (MEMS) devices to find commercial application. These devices measure g-forces caused by movement, triggering air bag inflation when g-forces exceed set limits. The automotive industry is among the largest consumers of these devices for use in airbags, traction control and anti-lock braking systems. However, despite a mature and growing market, automated optical inspection of these components remains a relatively untapped market, forcing component manufactures to use inline electrical testing after the fact, off line sampling or to forgo inspection completely.

Airbag DeploymentGrowing market
Accelerometers come in two basic types: comb and diaphragm. With comb-type MEMs accelerometers (see picture), changes in g-forces cause the interleaved ‘fingers’ of the comb to bend, resulting in a measurable change in conductivity, also called a piezoelectric effect. Diaphragm type accelerometers have a thin silicon diaphragm over a charged plate, like the skin cover of a musical drum. As the distance between the flexible silicon layer and the charged plate change, so does the capacitance.

Like many MEMs devices, accelerometers have all the challenges of semiconductor wafer inspection plus a few more. Like microchips, MEMs devices are made on silicon wafers for the most part through an optical lithography or laser micromachining process. Inspecting processed wafers requires high resolution in the micron range across large area fields of view of up to 200 mm per wafer. Inspection needs to be conducted in a clean room environment, and be rugged against vibration while maintaining high throughput. In addition to these shared concerns with the microchip industry, features on MEMS device can have extremely high aspect ratios, meaning that the height of a feature can be many times its width. This poses depth of focus issues for high-resolution optical inspection techniques optimized for micron resolutions, forcing the highest resolution systems with submicron resolutions to take multiple cross sections of a device and ‘stack’ the images through image processing techniques to create a complete picture of the device under test.

Despite the challenges posed by MEMs devices, automated optical inspection (AOI) system suppliers are tackling the MEMs issue with a variety of approaches – after all the potential market is significant. According to a report from Venture Development Corporation (Natick, MA), automotive market will represent 5.2 percent of the overall $34.2 billion MEMs market in 2006 or more than $1.7 billion, up from $1.1 billion in 2001, a CAGR of 9.1 percent.

Microscopes and lasers
MEMs manufactures have three ways to inspect their devices prior to packaging: microscopy, laser scanning systems and interferometry.

Optics powerhouse, Carl Zeiss (Gottingen, Germany), offers the ‘‘DeepView’‘ wavefront modulator to extend the depth of focus for traditional white light microscopes. DeepView allows optical microscopes to extend their depth of field by up to 20 times traditional depths. An aspheric lens and phase plate on the wavefront modulator capture encoded optical signals both in front of and beyond the traditional focus plane. (See pictures below.) Dedicated image processors take the information from the phase plate sensor and use it to clarify the image and extend the depth of focus.

Comb Accelerometer      Comb Accelerometer
Tilted comb accelerometer With Carl Zeiss DeepView (left) and without DeepView (right).
Application photos from Daniel L. Barton, Sandia National Laboratories, Albuquerque, NM, USA.

Traditionally, scanning electron microscopes (SEM) and laser scanning microscopes (LSM) are the main method for producing off-line, extremely high-resolution images of a complete 3D MEMs structure. However, some MEMs devices do not react well to electron scanning or the vacuums required for SEM, while high-resolution LSM requires multiple images at consecutive depths of focus to image the entire 3D MEMs structure. Also To get 3D information using SEM, the operator must cleave (cross section) the device at the precise location to be measured and then use the SEM to measure in high-resolution 2D measurements from the side. This method destroys the sample, can damage the object to be measured specifically if the MEMS feature is a cantilever or any other mechanical detached or overhanging structure and provides the measurement only at the specific plane where the device was cross sectioned and not anywhere else.

Non destructive testing (NDT) experts such as Solarius Development Inc. (formerly NanoFocus Inc., Sunnyvale, CA) RVSI (Nashua, NH) offer high speed laser profilometers that can provide dense 3D surface maps of MEMs devices with submicron resolutions at production speeds. Speed improvements for systems like RVSI’s WS-2500 wafer inspection system compared to traditional laser scanning microscopy methods are supported by high speed, multi-tap TDI CCD sensors, dedicated image processing elements and high-precision wafer stages with repeatability of 0.1 micron. This enhanced sensing and processing capability allows the WS-2500 to collect up to a million 3D data points in less than a second without upper limitations on Z-axis height or structures with large aspect ratios and with minimum resolutions in all axis of less than a micron.

White light interferometry
Interferometry, a method to determine a point in space based on phase differences between a reflected probe beam and isolated reference beam, has long been used by various industries  as one of the most accurate ways to profile a surface. Unlike the laser-based devices previously described, interferometery provides nanometer level resolution. These systems initially used lasers, increasing the cost and complexity of the systems and at the same time having a limited measurement range, ~150 Nano meters. To overcome this limitation, white light interferometry is used, increasing the measurement range to millimeters using mechanical scanning but sacrificing the resolution that drops significantly (several tens of nanometers). While being a stable and reliable technology, it suffers from one additional drawback – the very high sensitivity to vibration. Companies such as Nano-Or Technologies Ltd. (Lod, Israel) have improved on traditional interferometry by eliminating the reference beam, using narrow band light source, special optics and sophisticated algorithms. The special optics used in conjunction with a CCD array provide an instant profile of the surface under test without having to mechanically scan the sample, as with laser profiling or conventional interferometry.. By using  a unique optical concept and analyzing the wavefront reflected from the object, Nano-Or’s CEO, David Bannitt says, the system is inherently immune to vibration, it maintains the single nanometer resolution and has a large measurement range (~200 micrometers), allowing the operator to charge the device during test and measure changes in actuation or movement without compromising the test.

‘‘Since our device is insensitive to vibration, one can activate and simulate dynamic measurements, meaning activating the device, probing it while performing the measurements,’‘ Bannitt explained. Furthermore, the balance of the structures -- measuring stress related twist and bow of the mechanical structure -- may be of interest as well for process control purposes [in accelerometer manufacture].’‘ (see pictures)

White light interferometric approaches are gaining acceptance as more fabs learn more about the simplified operation and rugged nature of these systems compared to older interferometric approaches. Proprietary optical elements allow the reconstruction of the surface profile out of the beam with no use of a reference beam and the speed the extracting of depth or z-axis data. ‘‘This can be an online device because it’s fast, simply to operate and easy to load and unload. Right now we can grab an image in less than half a second and computation takes a few more seconds, or 3 seconds per measurement.

As scientists develop new MEMs structures and more applications find their way to a micron-scale world, AOI suppliers will continue to improve upon existing inspection techniques, combining the strengths of multiple methodologies to deliver commercial-grade, micron-resolution inspection systems. RVSI and others are already combining laser profiling with optical 2D inspection provide high speed 3D inspection techniques for bump bounded wafers and MEMs devices. As the MEMs market stretches towards a $34 billion global market by 2006, more and more AOI suppliers will invest in ways to meet the specific challenges of high aspect ratios, throughput and resolution. Like the rest of the electronics industry, as MEMs devices proliferate, suppliers will have to face the forces of competition: namely increased functionality at lower cost. As with most other industries, these goals can only be achieved with stringent quality controls, and in electronics, that has meant some form of AOI.












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