MEMs Inspection Pushes Limits of Automated Vision

Microelectromechanical systems (MEMs) on their silicon chips can detect sudden shifts in pressure and movement; perform as micron-sized light switches; and even save a human life by guarding against disease.

If MEMs isn’t in your lexicon, it soon will be. It’s almost impossible to live in the USA and not own a device that uses MEMs devices. Ink jet printers use them in their print heads, hard drives in their read heads, automobiles in their airbags, and cell phones in their signal processing circuits. Movie theatres will soon go digital, using micron-sized arrays of moveable mirrors to put color on the screen, and drug delivery and disease detection – especially in biological warfare – may come from a wristband rather than a physician. MEMs devices are even going inside our bodies, little robotic watchdogs that can watch for disease or inspect the walls of your arteries.

This explosion of applications is translating to hard currency. According to John A. Gordon, Senior Analyst at Venture Development Corp. (Natick, MA), revenues from MEMs and microstructure devices will reach $34 billion by 2006 on sales of 10.4 billion units, up from $17 billion in 2002 from 3.4 billion units. Commercial success creates competition and competition pushes quality while emphasizing a need for efficient manufacturing processes – in other words machine vision systems to capable of providing quality control on the scale of thousandths to millionths of an inch.

In the trenches at high resolution
Despite the incredible variety of applications and device designs (raised cantilevers, microchannels, diaphragms, and rotating mirrors to name a few), MEMs devices do share some similarities. MEMs are manufactured in clean room environments using many of the same processes used to make microchips. MEMs features are typically 10 to 1000 times larger than the smallest features on a microchip. However, like new semiconductor chip scale packages, MEMs are 3D structures. Many MEMs devices have large aspect ratios where the height of a structure may be many times its width, posing depth of field imaging problems unparalleled in the microchip industry. For example, a single ink jet nozzle on a printer can be millimeters deep, but only a few tens of microns across.

Like semiconductors, MEMs have led to automated inspection systems that use a variety of approaches to answer the needs of specific steps in the manufacturing process, from high resolution laser triangulation to the marriage of interferometry and optical imaging. Devices used in critical applications such as accelerometers may require 100 percent backend inspection at lower resolutions while inspection that targets process control, R&D/prototypes and production development might benefit high-resolution sampling of wafers under manufacturer.

Electroglas’s QuickSilver line of inspection equipment typically operates at resolutions greater than 1 micron for 100% backend wafer inspection of MEMs devices. ‘‘We haven’t seen much submicron resolution [requirements] yet, but I’m sure that we’ll get there,’‘ said Electroglas product manager, Darren James. ‘‘The key requirement for this final [backend] quality check is for 100% inspection at production speeds. Submicron inspection is typically done at key process steps in the fab and only on a sample basis because of the time required to inspect at these high magnifications…At submicron you’re looking at a … $9 million system – a system with extremely high resolution. But when you get to that high resolution, you have to profile the wafer as well because the depth of field is so small.’‘

Electroglas’ Quicksilver uses a Dalsa () 8-tap TDI linear CCD sensor connected to multiple image processors through optical fiber to expedite the transfer of data from linear array into image processors and an ‘‘ultra-high capacity buffer’‘ to hold the high resolution images. A scan of a 30mm2 die at 2 microns produces a 250 MB image, ‘‘…so you have to get all of that data out of the camera some how. The faster you ship the data [out of the sensor], the faster the throughput and that’s the key for commercial applications,’‘ James said.

Process control
Although there is no substitute to 100 percent automated inspection for semiconductors, even the largest microchip fabs cannot afford to inspect every manufacturing step. Prototypes may require the highest resolution 3D data to refine a particular etching step for a pressure sensor or accelerometer where almost all of the silicon has been removed to create a diaphragm. However, aligning that step to subsequent etching steps may only require 2D coordinates.

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Traditionally, inspection systems that use laser triangulation such as Robotic Vision Systems Inc.’s (RVSI, Canton, MA) laser inspection modules yield 3D data with the highest repeatability and accuracy. For some applications, laser scanning may not deliver the required throughput. In those cases, companies like RVSI and Nano-Or Technologies Ltd. (Lod, Israel) use imaging systems capable of high-resolution area measurements.

Nano-Or uses a proprietary optics with a white light microscope to create stable vibration insensitive interferometric map of the 3D surface of a MEMs wafer. The system uses a standard VGA monochromatic CCD camera with frame grabber and PC host to capture the image and measure phase differences per pixel, which leads to 3D surface maps with nanometers resolution and accuracy in the Z axis and micron resolution and field of view defined by the microscope objective. By using proprietary algorithms with off-the-shelf components, Nano-Or’s 3Dscope2000 system is an inexpensive ($100-$150k) solution capable of capturing a full field of view in less than 1 sec and generate a 3D mapping in 3 seconds, according to CEO David Banitt.

The Nano-Or’s 3Dscope2000 system also helps reduce vibration concerns during prototyping or probing of MEMs devices. The system uses a single light source unlike most interferometric systems that require a reference beam in addition to the probe beam. Using a single beam to probe the MEMs surface reduces the systems sensitivity to vibration. ‘‘MEMs are often host to wires for actuation. All that movement doesn’t disturb our inspection system,’‘ Banitt said.

NanoVia (Londonderry, NH) takes a similar approach, using a Nomarski incident-light differential interference contrast prism to differentiate between materials with slightly different indexes of refraction, such as an optical waveguide in a silicon substrate used in fiber optic telecommunications. The system uses a Pulnix (Sunnyvale, CA) camera and Matrox (Dorval, Canada) Meteor-II frame grabber with Delta Tau (Chatsworth, CA) multiaxis PMAC motion-control platform. By sending the light through a holographic filter to flatten the light wave, the system delivers very high resolution images that easily differentiate between materials with index of diffraction (such as polymers and silicon) that only vary by 0.03 percent. ‘‘They would be almost invisible under normal microscope measurement,’‘ explained NanoVia’s director of R&D, Todd Lizotte. ‘‘By utilizing this technique you can see waveguides in bulk material, grab it with a CCD and machine vision software’‘

For some applications, step height or surface maps are not enough. To create a MEMs pressure sensor, most of the silicon is removed, living a thin layer or diaphragm. Virginia Semiconductor Inc.’s (Fredericksberg, VA) Optical Micrometer for Micromachined Substrates (OMMS) shines coherent infrared right up through the wafer under test. The system measures the amount of light absorbed and determines the thickness of the sample to 0.25 micron across a 5mm2 area for materials up to 200 micron deep based on the absorption coefficient of the material within 5 seconds. Because it is a direct imaging application instead of interferometric measurement, the system also provides a standard image for defect identification in addition to the absolute thickness measurements.

Although commercial MEMs have been around for 20 years or more, a quick canvas of new devices shows that the technology is quickly ascending the acceptance curve. As industries from electronics to medicine continue down the road of integration through miniaturization, MEMs devices will continue to proliferate in unimagined ways. Machine vision development will have to match that pace, leaning on advanced optical techniques and image processing to meet the specific concerns facing this revolutionary technology.