Getting the Small Picture

Hank Hogan

Anadigics Inc. of Warren, N.J., is a semiconductor company that helps provide the gift of gab — and data, too. Its gallium-arsenide integrated circuits are used in both wired and wireless telecommunications systems, and although its products operate in the radio portion of the spectrum, they could benefit from improvements in light microscopy.

Peter Tomic, a senior failure-analysis engineer at the firm, said that he and his co-workers often place hair-thin probes on a device to take electrical measurements of specific spots within a circuit. This involves putting the probe on a metal trace, a thin strip with a width measured in tenths of microns. He would like to have an automated setup consisting of a microscope, probes and cameras to help with this delicate task and to overcome a microscope limitation that hampers probe placement.

“The cameras would look at the probes in a side view,” Tomic explained. “Generally speaking, metallurgical microscopes with enough magnification to see the features and region of interest don’t have enough depth of field.”

Demonstrating advances in noncontact surface profiling, a Zeiss laser scanning microscope using specialized software enables the visualization of an array of solder joints.

Although his ideal system doesn’t exist, the latest microscopy innovations offer hope — and help — for this and other industrial tasks. Products from companies such as Carl Zeiss MicroImaging of Thornwood, N.Y., Leica Microsystems Inc. of Bannockburn, Ill., and Nikon Instruments Inc. and Olympus America Inc., both of Melville, N.Y., are increasingly automated, integrated with other systems and networks, and digital in nature. Optical improvements also are being developed and deployed. The combination promises to improve performance and drive down price, making it easier and less expensive to bring the very small into focus.

To illustrate the effects of various illumination techniques using a Nikon microscope, the left side of a metallurgical sample of scratches was illuminated using differential interference contrast, and the right side, bright-field episcopic illumination.

The better to see you

Today’s optical microscopes are the product of hundreds of years of scientific and engineering progress, but that doesn’t mean that optical advances have stopped. The depth-of-field difficulty cited by Tomic is one that bedevils more than semiconductor manufacturing. It affects any process, such as machining or medical device assembly, where the distance from top to bottom is significant. Another example of this depth challenge arises during the inspection of the small gears and mechanical elements of microelectromechanical systems, or MEMS.

Zeiss’ DeepView system uses hardware and software to combine different planes into a single view with greater depth of focus.

To help overcome such issues, Zeiss introduced DeepView, a combination of hardware and software that extends the depth of field. This process takes advantage of wavefront modulation and proprietary software. The system can be used at various magnifications and with different optics.

“This new technique effectively increases the depth of field of that optic up to 20 times,” said Dan King, assistant product manager for materials microscopy at Carl Zeiss MicroImaging.

Earlier last year, the company introduced two other optical microscopy innovations: circular differential interference contrast and a variation, total interference contrast. The former makes use of circularly polarized light, which eliminates the contrast blackouts created by having light polarized along an axis. The latter, an interferometric technique, allows step-height measurements of films and coatings and provides other roughness parameters.

In an ink-jet sample on paper, the left side view was imaged using a bright-field lens, and the right, a dark-field lens.

A definite edge

Accurate step-height measurements using this approach require a definite edge so that readings on either side of the step can be taken. The technique also needs a fairly flat and uniform sample surface. According to company literature, its measurement range is from 50 to 5000 nm. Unlike other height-profiling and interferometry techniques, this approach can be used with any objective and at any magnification.

Such improvements in cataloging the third dimension aren’t confined to one company. Alwyn Eades is a professor of materials science and engineering at Lehigh University in Bethlehem, Pa., and president of the Microscopy Society of America, an organization that encompasses both optical and electron microscopy. In his research, he employs electron microscopes but is aware that optical instrument makers have significantly improved surface-height profilometry.

The MM-60u microscope with Emax metrology software is seen in this typical Nikon measuring workstation.

“By integrating computers more fully with such instruments, they have become much more automatic and a lot easier to use,” he said. He added that, if you have the money to buy the instrument, it’s going to be much easier to get surface profiles than it used to be. Before, you needed a specialist.

The themes of greater computerization, increased automation and easier use show up in other areas of microscopy. For example, as in real estate, the key to surface inspection and other aspects of the industrial use of microscopes is often location, location, location. This is not the location of the microscope itself, but of the precise position of the area being inspected.

In fabricating and packaging a semiconductor circuit or medical device, for example, both the design of the product and experience may indicate that inspection and quality checks need to occur at specific spots. The challenge in a manufacturing environment becomes one of quickly locating these known problem areas and rapidly screening them. Ideally, this must be done without resorting to specialists or highly trained individuals.

To help achieve this, microscope manufacturers are revving their motors — literally. The stage and optics of microscopes are increasingly being motorized and put under computer control. Michael Metzger, department manager for Nikon Instruments, noted that the automation of microscopes is increasing because motorized movement brings definite advantages. He said that the company applies precision staging to determine where a defect is and to go back and look for defects in the same area in future products.

With such an arrangement, the inspection system can quickly move a unit into position and present an automatically focused image to an inspector. The inspector can speedily classify a device as good or bad. Such automation slashes inspection time and reduces the vagaries of human intervention.

Nikon’s precision movement technology helps power its VMH 300 vision system, enabling the instrument to achieve 0.01-μm positional resolution. This is greater than what can be done using the company’s measuring microscopes, built to gauge microscopic dimensions. This exacting capability has been used, according to Metzger, by various semiconductor and other companies to achieve a competitive advantage. The companies, not surprisingly, do not want their names revealed for fear of losing that edge.

The trend toward automation also is affecting other microscope suppliers. Zeiss’ King noted that a quarter of all microscope stands now sold are motorized, giving them the ability to automatically adjust the distance to a specimen. What’s more, that percentage is growing.

Another aspect of the same trend is the increasing use of digital cameras and interfaces. This enables the easy transmission and manipulation of images and is part of the overall movement away from film and toward digital image capture and storage.

Many microscope makers also produce digital cameras. Nikon, for instance, is introducing a 12-megapixel camera this month — a resolution greater than previously available. “We are able to match or exceed the resolution of the human eye in our digital representations now,” Metzger said.

For microscopes, this digital trend has several implications. One is that they all must interface with a camera, which sometimes means adding lenses and other elements to correct for distortions and other optical problems. Another is that microscopes will more often sport some sort of interface to hook up to the wider network. This will be necessary for both control and data exchange.

Crossover products

There is, however, a third impact. The use of software to run microscopes leads to crossover products, devices that are primarily intended for other uses but that could be part of an industrial or surface inspection arsenal. Olympus has produced a low-cost product that not only has some industrial uses, but also is an educational and biomedical tool. The company’s MIC-D system includes a built-in digital camera, an interface and software controls. It is an inverted microscope with a light source that can be rotated through transmitted light to reflected light. The system also can work with polarized light.

The Olympus MIC-D captured these chip images.

Because of its construction, the MIC-D is not modular. The 640 x 480-pixel image sensor cannot be swapped out, and the magnification of the microscope is limited to 2553. Achieving that requires a zoom lens. Thus, the device does not offer the inspection power of a higher-end microscope. However, the company’s scientific equipment group technical marketing specialist, Peter Dimitruk, said the MIC-D does have some unique attributes. He said it hybridizes characteristics of stereo, upright and inverted microscopes.

As an example of this melding of different microscopes and technologies, Dimitruk said that the MIC-D easily can move from 20x to 255x magnification. The ratio of high to low magnification is greater than 12.8:1, a tough number to achieve with good image quality. Nonetheless, the system can pull this off because of the design characteristics it shares with Olympus’ higher-end microscopes.

For many applications, the magnification range and the illumination capabilities of the new device will be more than adequate. For instance, manufacturers have used the microscope to look at plastic welds in polarized light to determine the heat-penetration depth.

In another example of a crossover product, Leica Microsystems recently introduced a fluorescent combination attachment, the Fluor Combi, for its MZFL III fluorescence stereomicroscope. Primarily intended for use in the life sciences, the device can be useful in product inspection. The attachment enables users to switch from a stereo objective and stereo imaging to a high-resolution compound objective, effectively bumping magnification up into the 400x or 800x range. That power is needed for such tasks as examining cracks and surface defects. The fluorescence can be useful in spotting unwanted intruders in the manufacturing process.

The Leica MZFL III microscope captured this view of a polymer resin grain under UV light.

“With the fluorescence, you can look for trace residues of different photoresists or other contaminants that tend to jump up at you under fluorescence imaging,” said Douglas Giszczynski, senior marketing manager for stereomicroscopes at Leica. The company likens such inspection to searching for a glowing needle in a haystack, he said.

He also noted that his company uses some of the microscope technology in its macroscopes. These devices, such as the M420, offer motorized focusing and a 6:1 maximum to minimum zoom ratio. Macroscopes are intended for the somewhat coarse inspections of fairly large objects, but they offer magnification ranges similar to those of stereomicroscopes.

The same Leica microscope and UV light enabled this image of carbon fiber.

Despite all of these advances in microscopes, optical surface inspection does have its limits, and there may be no economical way around them. Advanced Micro Devices Inc. of Sunnyvale, Calif., is another company that makes integrated circuits, but they are of the mainstream CMOS variety. Bryan Tracy, manager of the materials technology development department, said the company builds its products on advanced CMOS technology. He pointed out that its semiconductor manufacturing meets or exceeds the feature size and other requirements spelled out in the International Technology Roadmap for Semiconductors prepared by International Sematech, a research consortium based in Austin, Texas.

Tracy said that Advanced Micro has been removing optical microscopes from its production line and replacing them with electron microscopes. That’s because the latest manufacturing technology is at the 0.13-μm node, with 0.2-μm lines separated by 0.2-μm spaces. That puts the feature size well below the wavelength of light and makes the optical inspection of surface features carved into conducting and insulating layers extremely difficult and perhaps effectively impossible.

As Tracy explained, “That type of a feature is really just too small for even ultraviolet. Even a deep-UV microscope is pretty limited.