Probing tiny features

If a manufacturer is unable to accurately measure its parts, it can’t guarantee they will perform to specifications. The challenges to achieving accuracy are amplified when determining microscale dimensions with a coordinate-measuring machine that has a probe too large to access tiny geometrical features.

These features require a smaller-diameter probe tip, but applying a smaller tip requires an even smaller-diameter stylus, which, in combination with high probing forces associated with conventional systems, causes measurement errors and unacceptably high levels of uncertainty because of stylus bending, explained Hans Ott, sales manager for probe maker IBS Precision Engineering, Eindhoven, the Netherlands. He added that the smaller the probe tip, the higher the contact stress between it and the part surface, increasing the risk of part damage.

Conventional probe tips are made of ruby, and such spheres are widely available in diameters down to about 0.3mm, according to IBS. Producing smaller ruby spheres is difficult, primarily because it’s problematic to attach the sphere to the stylus, Ott noted. For larger spheres, a probe manufacturer drills a hole in the sphere to fix, or glue, it to the stylus. “You can imagine what a problem that is if you try to do it on a sphere the size of diamond grain,” he said.

Nonetheless, affixing tinier spheres to rigid stylii is possible. Saphirwerk Industrieprodukte AG, Brügg, Switzerland, for example, reportedly bonds 0.12mm-dia. ruby spheres onto 0.08mm-dia. tungsten-carbide styli to form measuring probes.

Going even smaller, IBS offers its Triskelion ultraprecision tactile probe system with a 70µm-dia., or 0.07mm-dia., monolithic tungsten-carbide probe tip, which the company developed to measure small holes, such as those in the latest-generation of fuel injectors, according to Ott.

IBS Precision Engineering offers its Triskelion ultraprecision tactile probe system with a sphere as small as 70µm in diameter. Photo courtesy IBS Precision Engineering.

He noted that another off-putting challenge when measuring with such small spheres is calibrating them to determine roundness error. “The 70µm tip is not commonly used,” Ott said. “We have had some applications where even smaller probes were required, but we have not yet seriously looked at them.”

Fibrous metrology

For those applications, Werth Inc. recently expanded on its metrology technology and introduced the 3D WFP Werth fiber probe, an optical/mechanical fiber sensor with a sphere diameter as small as 20µm. With this probe, light from an LED travels down the glass fiber and illuminates the sphere. The optical system measures the position of the sphere, which is located directly in the focal plane of the optics, explained Jeff Bibee, vice president of sales and marketing for Werth, Old Saybrook, Conn. With the previous 2½-D version, the probe accesses a feature to be measured, the sphere contacts the part and—instead of sending an electronic signal to the CMM to indicate deflection occurred the way a conventional probe does—the sphere’s X- and Y-axis positions are optically measured, and the location of the focal plane determines the Z-axis.

“Think of the sphere as a marker for an optical measurement,” Bibee said.

He pointed out that the new 3-D fiber probe provides direct measurement in all three axes of measurement, monitoring deflection in all directions. The 3D WFP is mounted via flexible coils, and a laser distance sensor monitors the deflection in the Z-axis while the optical sensor monitors the X- and Y-axis positions as before.

In addition, by dragging the probe along a surface while a camera with a high-refresh rate captures images to measure probe position, an end user can also perform surface scanning. While scanning, the possibility exists that the probe tip could stick in a spot that’s relatively rough, in microscale terms. To prevent any slip-stick action, a piezo head induces a slight vibration to keep the sphere from sticking, Bibee explained.

Another concern when measuring small, delicate features is that too much contact force from a conventional probe will deflect or damage the part by visibly scratching sensitive surfaces. According to Bibee, Werth’s probe exerts only 1µN of force. “It’s actually negligible,” he said. “The fiber probe will not deflect or damage the part at all.”

That low contact force also enables the fiber probe to scan rubber membranes and other soft parts without deflecting or damaging them, an issue with conventional touch probes, Bibee added.

The 3D WFP Werth fiber probe is a mechanical fiber sensor. Photo courtesy Werth.

Because it’s integrated into a “complete system,” including the CMM, software, controller and scanning camera, Bibee noted that the 3D WFP can only be used in Werth CMMs. According to Werth, a CMM with the 3D WFP provides low probing uncertainties via direct evaluation of the part feature’s position.

Inferring the position

The National Institute of Standards and Technology, Gaithersburg, Md., also developed a fiber probe for micromeasurement applications, but rather than directly evaluating the position of a feature like the Werth probe, NIST’s glass fiber probe infers the position of the deflected probe tip with indirect evidence obtained from the probe’s stem, or stylus, by taking a picture of the stem that’s slightly above the tip, explained Bala Muralikrishnan, NIST research engineer. “We’re imaging a point that’s maybe 10mm from the tip,” he said, adding that the technique isolates the measurement from disturbing influences that might be present when imaging inside a hole, such as reflections and diffraction.

“It’s a slow but extremely accurate measurement, with 100nm or less uncertainty.”

Measurement precision is enhanced because that light passing through the fiber is focused by the fiber—acting as a cylindrical lens—to form a bright line against a dark background, noted NIST physicist Jack Stone. “When this line of light is reimaged onto the camera, the position of the high-contrast image can be found with great precision,” he said.

The probe has a 75µm-dia. sphere and a 50µm-dia. stylus and can measure features as small as 80µm, according to Muralikrishnan.

He added that an alternate method of operation, where measurement is performed with the probe’s stylus in contact with a part instead of the sphere, is beneficial when measuring knife-edge apertures for radiometry. This technique avoids potential errors that would occur if the knife edge contacted slightly off the equator of the sphere, Muralikrishnan explained. “The main problem of using a sphere on a knife-edge aperture is if the part is warped, there would be roll-off on the sphere, where we would be measuring at different points—either above or below equator—on the sphere, leading to large errors,” he said.

Although an indirect approach is potentially subject to some errors that do not affect the direct method, Stone noted that NIST hasn’t found any clear evidence these errors cause significant problems.

He explained that the level of uncertainty rises not because the measurement tool uses an indirect or a direct approach, but because of the feature quality being measured. “If a hole has a rather irregular shape with a bad surface finish,” Stone said, “then it’s obviously more difficult to achieve high accuracy than if it’s a good-quality hole.”

Another issue when measuring microparts with a CMM is the presence of dust or dirt between the probe and part surface, Stone noted. “By virtue of the fact that the measurement is low force, if the probe runs into a piece of dust, it can’t push it out of the way,” he said, adding that NIST filters the air flowing through its metrology lab but it’s not a clean-room environment.

In addition to potentially being held by a surface’s peak and valleys, microprobes tend to become electrostatically attracted to a part’s surface or snagged by the capillary forces created because of adsorbed water on that surface, Stone explained. He noted that overcoming the inaccuracy-causing clinginess when scanning requires pulling the probe off the surface or vibrating it with a piezo device.

Doing the wave

To eliminate sticking, and to measure and scan features, InsituTec Inc., Concord, N.C., developed the MicroTouch sensor, which NIST is integrating with the same CMM it uses for measuring with other fiber probes. With MicroTouch, a quartz crystal oscillator vibrates a sphere-less, 7µm-dia., 3.5mm-long fiber probe 32,000 times per second to create a 30µm-wide, mechanical standing wave in the fiber, according to Shane Woody, InsituTec’s CEO. “It’s like taking a rope where you hold one end and I hold the other, and one person shakes the rope to generate the wave,” he said.

Similar to the static probes, Woody noted that the vibrating fiber exerts a low force (less than 100 nanonewtons) on the part surface and measures features with high aspect ratios (up to about 100:1). Unlike the static variety, the oscillating version is 1-D, meaning it is only sensitive in one direction. To achieve 2½-D surface roughness measurement and scanning capability, the fiber probe attaches to a scanning head that rotates the fiber at up to 60 rpm, which is probably too fast for a micropart, according to Woody.

“Our fiber probe is kind of like a profiler,” he said. “We would not refer to it as a 3-D probe in and of itself. We typically don’t use it in a touch-trigger mode.”

The scanning head must be retrofitted to a CMM, enabling the sensor’s controller to “speak” to the CMM’s controller. According to Woody, InsituTec’s goal is to have a turnkey solution in a couple years, noting that the company does not build CMMs. “We make an enabling technology for the CMM to do things it hasn’t done before.”

Woody added that the company is also working on scaling the fiber probe down by an order of magnitude, meaning to 700nm in diameter. That would allow scanning the sidewalls of deep, narrow channels, which are commonly found in the MEMS industry. “There is nothing currently available for measuring that,” he said. “We don’t need a sphere because we’re always measuring at a single point at the very end of the fiber, which then opens up these huge opportunities to scale down our fiber probe.” µ

About the author: Alan Richter is senior editor of MICROmanufacturing. Telephone: (847) 714-0175. E-mail: [email]alanr@jwr.com[/email].