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Understanding scanning white light interferometry

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December 2009 Volume 2 Issue 4

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By Susan Woods

Contributing Editor

(224) 225-6120

Scanning white light interferometry (SWLI) is a versatile technology that provides a noncontact, 3-D method of measuring surface roughness. The interference microscopy technology combines an interferometer and microscope into one instrument.

Illumination from a white light beam passes through a filter and then a microscope objective lens to the sample surface. The objective lens is coupled with a beam splitter so some of the light is reflected from a reference mirror. The light reflecting back from the surface recombines with the reference beam. The recombined beams create bright and dark bands called “fringes,” which make up the interferogram. Fringes represent the object’s topography.

The pattern of these fringes is captured on a CCD camera array for software analysis. By obtaining several frames of intensity information for each point, the system can recreate the sample surface. The frames are passed through an algorithm to convert those intensity signals into height information.

Microscope-based white light optical profilers are capable of measuring a variety of surface types, including ground and polished surfaces, steps and films. They do this by mapping surface heights that range from subnanometers to microns across areas that range from microns to millimeters in a single measurement. This rapidly provides surface roughness, shape and waviness data.

When the required measurement areas are larger than the field of view, a stitching procedure can be employed that involves a number of partially overlapping measurements being combined into one surface profile. Stitching, however, requires that regions overlap, with the overlapping data aligning adjacent measurements. Because overlap regions are measured more than once, overall measurement time increases.

The primary applications for SWLI are surface measurement of MEMS (microelectromechanical systems) and semiconductors. Other applications include measuring machined surfaces, microfluidic devices, optics and fibers, ceramics, glass, paper, thin films and polymers.

Shown measuring a MEMS device, the NewView 7300 white light optical profiler from Zygo characterizes and quantifies surface roughness, critical dimensions and other topographical features without sample preparation. Photo courtesyZygo.

“Anywhere where it is important to understand vertical features over a small lateral scale is where it has a great application,” said Eric Felkel, product manager, Zygo Corp., Middlefield, Conn., a manufacturer of 3-D noncontact optical measuring instruments.

A schematic showing how SWLI works. Illustration courtesy Veeco Metrology.

SWLI optical profilers are used in development and production environments. “It is an instrument that works well in the research lab, the failure analysis lab [and] in production manufacturing,” Felkel added.

While other metrology solutions provide either high resolution or high speed, SWLI offers both. It combines noncontact measurement, repeatable 3-D surface measurement, high speeds and subnanometer resolution. SWLI is employed for surfaces with average roughness down to 0.1nm Ra and peak-to-valley heights up to several millimeters, and repeatability can be 0.1nm or better. (While the upper peak-to-valley height limit would most often be applicable to macro applications, it can be useful in certain micro applications.)

Pros and cons

SWLI offers many advantages over other methods, such as stylus profilometry and atomic force microscopy (AFM). Two principle advantages are SWLI’s high-speed and noncontact capabilities. The user can rapidly acquire a 3-D rendered surface to make measurements immediately.

On the other hand, “AFM is a very high-resolution technique, but it takes a lot of time to perform. It requires a very knowledgeable user,” said Patrick O’Hara, president and CEO of Ambios Technology Inc., Santa Cruz, Calif., a supplier of surface metrology instruments.

And stylus profilometry is a contact technique, which may not be desired. “When you drag a stylus across the surface and it touches the part, it has the potential to damage or even destroy the surface you are interested in characterizing,” Felkel said.

Another benefit of SWLI is that it is an area-based technique, wherein the sensor images the interference signal from an area of the part and communicates that signal to the camera. Other topography techniques only sense the surface at a single point or along a single line. “You are looking at an area field of view that is determined by the magnification of the microscope objective being used,” Felkel said. “You can look at low-frequency structures, such as surface form, waviness and steps, with a low-magnification lens and then switch to higher magnification and look at high-frequency features that contribute to roughness. Other ways of performing this type of surface characterization are not area-based, such as a triangulation sensor or a line scanner.”

When compared to techniques such as video microscopy, SWLI is advantageous because it does not depend on part color to obtain high-quality data. Even if the surface being measured reflects a small amount of light, that reflected light interferes with the light that reflects off of the reference surface. This interference signal is what defines the surface measurement data. Because SWLI uses the interference signal to measure the height of the surface—and not simply a raw camera image like a video system—it is possible to measure structures that visibly have little color contrast, but are easy to see by their topography.

Lastly, SWLI is an easy-to-learn technique and does not require sample preparation. “It is as simple as putting the sample under the microscope, focusing and measuring,” said Greg Maksinchuk, marketing product manager, Veeco Metrology Group, Tucson, Ariz., a provider of metrology and process equipment.

The principle disadvantage of SWLI, or any optical technique, is that it depends on the optical properties of the medium through which it is looking, whether it’s glass, air or, for semiconductor manufacturing, thin films.

“Each of those dielectric thin films has it own optical properties,” O’Hara said. “This can produce anomalous results, such as inaccurate film thickness or step measurements, due to the different optical properties of the films. Most instrument manufacturers have proprietary methods of overcoming these anomalies, but the ambiguity remains. However, AFM or stylus profilometry provides an unambiguous measure of the top surface or the step.”

Also, SWLI is typically focused on the part’s vertical resolution, or topography, and less on lateral resolution, such as the part’s geometric dimensions. SWLI tools do focus on high lateral resolution, but if the user is primarily interested in lateral information, other less-expensive measurement methods are available.

New developments

While measuring thin films can be a challenge, one of the most important improvements to SWLI technology is its ability to measure surface topography in the presence of an optical film. “If the surface you are measuring has a transparent film on it, you have two interference signals because you see the top and bottom of the film,” Felkel said, adding that now SWLI can be used to measure the top and bottom of the film and its thickness.

And manufacturers are improving this process to work with thinner and thinner films. “We work along two different paths,” Felkel said. “We develop software that can identify these interference signals as they get closer and closer together, and we design imaging systems to resolve these interference signals with higher fidelity.”

A specific example of a recent development in SWLI is the TTM (transmissive media module) from Veeco Metrology for measuring through microdevice packaging and other transparent media. “We have added the capability to look through transparent materials, like glass in environmental chambers, to measure a part that is under temperature control or humidity changes,” Maksinchuk said. “You can do measurements in real time while the device is being exposed to a harsh environment.” Applications include corrosion studies and MEMS devices used in extreme environments.

This image illustrates the remarkably high Z-axis resolution offered by the SWLI technique. These terraces represent single atomic steps in the HOPG (Highly Ordered Pyrolytic Graphite) crystal lattice. The atomic terraces, or plateaus, are only a few angstroms high. Image courtesy Ambios Technology.

As for future developments, Felkel mentioned energy applications, such as solar panels. SWLI would be used to help control the manufacturing process. “The solar panel itself can be as big as your desk, but [SWLI would measure] the busbars and fingers; those are things that you want to know the profile of to understand how much material you are putting down and to control the process,” he said.

In the area of MEMS applications, there is interest in efficiently making very bright LEDs. “A lot of that comes down to topography-based features,” Felkel added. “These include roughness and the height of various layers as you stack the LEDs.” µ

About the author: Susan Woods is a regular contributor to MICROmanufacturing and Cutting Tool Engineering magazines. E-mail: susanw@jwr.com.

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Telephone: (224) 225-6120. E-mail:  Susan Woods.