MEMS create 3-D inspection challenges

Microphones, accelerometers, pressure sensors, and many other products take advantage of a microelectromechanical system, or MEMS, crafted on a wafer of silicon. The diversity of MEMS devices presents vendors and users of inspection system with a sizeable challenge: Unlike integrated circuits that all look somewhat similar, each MEMS device has its own peculiarities and structures that require a unique inspection “recipe.”

For example, an ink-jet printer cartridge uses a MEMS to electrically heat the ink and spray it onto paper through a precision opening—the jet. If process steps produce malformed, partially blocked, or fractured jets, a cartridge will not work properly, but only careful inspection of a MEMS wafer will detect these problems. Optical inspections also can measure the jet openings and identify blocked jets, foreign material in jets, and misshaped orifices.

This cross-section diagram of a generalized MEMS device indicates the locations and types of possible defects that require optical inspection. Courtesy of Camtek.

“We look at semiconductor wafers for discoloration, cracks, and other defects to determine if a problem occurred during the fab processes,” explained O'Reilly. “But with accelerometer MEMS wafers, we must look at the sensor electronics and at the micromachined mechanical beams to ensure the etching processes 'released' the beams, that they have uniform dimensions, and that the dimensions meet our requirements.”

The bottoms and sides of the channels and plateaus in a MEMS sensor all require inspection, so a vision system must change its focus to capture images of defects at all levels. Courtesy of Freescale Semiconductor.

“Many MEMS devices contain 3-D structures. Most inspections of these structures can be performed by taking one single image. To inspect a structure for a 1-micron defect, for example, a camera may have to perform inspections at different heights due to its limited depth of focus at very high magnifications,” noted Pieter Vandewalle, director of sales and marketing at ICOS Vision Systems. “Besides, test engineers may want to measure 3-D structures on the die as well. The capability of some 3-D metrology tools lets test engineers take one image and get a 3-D profile of a complete MEMS die. Then, a customer can decide what dimensions to measure and how to use the dimensional information. They might need an absolute height or perhaps they only need to know a component's height is less than x microns. These types of requirements differ greatly from customer to customer.”

Because MEMS rely on precisely formed mechanical components, measurement of critical dimensions plays a key role in inspections. Pressure-sensor and accelerometer MEMS devices manufactured by Freescale Semiconductor rely on precision springs. “We must measure critical dimensions of the springs, but when we used traditional wafer-inspection equipment, the MEMS die looked huge by comparison,” said Hemant Desai, Freescale's MEMS development manager. “Typical MEMS springs may measure two microns or larger, but the usual semiconductor inspection 'tools' measure characteristics on the order of 500 angstroms, or half a micron. Thus, we had to adapt the tools so we could use them to measure critical dimensions on large MEMS structures.”

After a saw cuts through the glass covers on MEMS devices, an inspection shows the jagged glass edges as well as defects on or in the glass covers. Courtesy of Camtek.

In addition to mating a MEMS wafer and a cover wafer to protect individual sensors, Analog Devices puts some MEMS sensors in open-cavity ceramic or plastic packages. A lid hermetically seals the sensor inside. “You constantly hear about particle contamination within the MEMS industry,” said O'Reilly. “During MEMS production, we worry about micron- and submicron-size particles. No one wants those particles in airbag sensors. So, when we use open-cavity packages, we inspect for residues or particles on the lid and in the package cavity before we insert a clean die. Those particle inspections are critical for hermetic packaging, and they are the most intensive inspections we do. Simultaneously, we perform metrology measurements that determine die tilt, die rotation, and alignment of the package and the die. We inspect every device during these steps.”

Not all defects or process variations carry the same weight in a MEMS device. “Engineers might see slight changes in the characteristics of MEMS ink jets,” explained Rudolph's Roy. “The etching process might not offer the same repeatability as expected from a semiconductor-processing line. Perhaps a MEMS feature 'moved' slightly or its shape changed slightly. A standard inspection system would sense these small changes as defects, while a customer might find them within spec for the type of product a MEMS device will drop into. Hence, an inspection system should have the flexibility to prioritize defects by regions of interest.”

He added, “When you have two operators examine the same group of wafers, they will agree on many defects but disagree on some small quantity. So, engineers should not expect an inspection system with automatic defect-classification [ADC] software to offer 100% accuracy. We recommend customers use ADC software to automatically classify the defects everyone agrees on, which reduces manual-inspection expenses. Then, customers will use ADC capabilities to classify a small set of 'questionable' defects as a consistent referee or to give equipment operators an easy way to manually review defects.”

The thicknesses of deposited polysilicon layers can add to inspection challenges. “Yet, thickness is a critical factor in determining how well a MEMS device will work,” explained Freescale's Desai. “But when you deposit polysilicon to a depth of two to five microns, the surface gets rough and it complicates thickness-measurement methods that rely on optical or ellipsometry techniques. A rough surface has a low optical 'signal-to-noise ratio,' which decreases measurement accuracy. We had to put a lot of effort into developing techniques that improved the accuracy of thickness measurements.” Equipment vendors also have techniques that improve on polysilicon thickness measurements.

“Vertical, or bright-field, illumination produces good images of flat surfaces,” noted Udi Efrat, strategic marketing manager at Camtek. “Angled, or dark-field illumination, accentuates structures and a mechanical defect such as a scratch. Because MEMS devices contain a mix of surfaces and 3-D structures, the inspection system should balance bright-field and dark-field illumination.” For best results, an inspection system should provide programmable light settings based on the MEMS device being inspected. The combination of the two light sources produces good contrast between defects and the background.

Roy explained that because features and surfaces on MEMS vary so widely, his company developed an illumination toolkit. “The kit lets test engineers select from 10 types of filters and light sources so they can adjust the inspection system for a variety of applications.”

ICOS tackles inspection of surface defects in two ways. “First, we can use dark-field illumination, colored lights or oblique illumination from ring lights,” said Vandewalle. “Each material reflects colors differently, so by using one color for illumination, our system can highlight specific materials in an image and de-emphasize others. In this way, users can more easily focus on a gold layer, for example, to determine that it is evenly distributed and not damaged.” Second, ICOS provides algorithms that distinguish between defects and the optically “noisy” background of a rough surface.

Making things even more interesting, some wafers need their back surface inspected as well. “Consider a pressure-sensor wafer, which has a thick base layer of perforated silicon bonded to its back surface,” said Camtek's Efrat. “The orifices in the base layer should align with the center of each sensor die, and orifices must be open to conduct pressure to the membrane. The orifices should be smooth and have precise dimensions so they properly fit and seal to the sensors. The inspection system will verify the orifices meet specifications and report deviations. And the inspection system must match any defects on the back surface to the corresponding MEMS dice on the front side.”

When you plan to inspect MEMS wafers, be sure to address how inspection equipment will handle them. “A standard inspection system uses a vacuum chuck to hold wafers in place,” explained Efrat. “But if the silicon substrate has small holes through it, as in the case of the base wafer for MEMS pressure sensors or the windowed glass substrate for an optical MEMS device, a regular vacuum chuck will not work. The inspection system vendor should produce a special chuck or an adaptor to secure a perforated wafer only around its perimeter.

“To inspect an upside-down wafer of pressure sensors, for example, you must protect the MEMS devices on the front side from rubbing against the chuck's surface. We supply a chuck with a recess and apply vacuum only along the wafer's perimeter only to hold it in place without damaging the MEMS structures.”

Inspecting MEMS wafers and dice requires you to think about new types of defects, critical dimension measurements, inspections of different horizontal levels, issues with surface effects, and new ways to handle wafers. Unfortunately, MEMS designers often don't think about inspection until they are ready to manufacture a device. Thankfully, vendors have given the topic much thought and can help you determine how to properly inspect your MEMS wafers.