Metrology of Small Holes and Micro Features


Bala Muralikrishnan, Jack Stone and John Stoup

Engineering Metrology Group, Precision Engineering Division, NIST


Dimensional metrology of micro features and small holes (approximately 100 mm diameter) is critical in a variety of industries such as metal cutting (fuel nozzles, wire drawing dies), semiconductor (via holes, line width), biomedical (stents, syringes, microfluidic channels), communications (fiber optic ferrules) and consumer products (ink jet cartridges). Probing systems of traditional Coordinate Measuring Machines (CMM) are limited in their capabilities (holes of up to 500 mm diameter only) because of large contact forces and thick styli (thinner styli pose stiffness issues). In order to measure smaller holes, novel ultra low force probing technologies are required. To meet the growing need in the US for calibration and measurement services in this area, we have developed a new low force probing technique for CMMs. We refer to this as Fiber Deflection Probing and the principle is based on optical imaging of the probe stem. We use a thin glass stem (50 mm in diameter) with a microsphere (75 mm diameter) bonded on the end, as the probe. The deflections of this probe upon contacting a surface are detected by imaging the stem from two orthogonal directions in a plane perpendicular to the fiber axis, a few mm below the tip of the probe ball. The CMM is used as a fine positioning system; its original Movamatic probing system is removed from the ram to facilitate mounting the test artifacts. Our fiber probe with its associated optics and camera is mounted on the CMM’s table pointing upwards, and serves as the new low-force CMM probe.


Some key results and features of our technique are –

1.        Comparison diameter measurement of three known artifacts with other techniques indicate agreement to within 60 nm

2.        Measured internal geometry (diameter and radial form – form shown in Fig. 1) of a fiber optic ferrule

3.        Uncertainty analysis indicates extremely small measurement uncertainty of only 70 nm (k = 2) on diameter and 100 nm (k = 2) on radial out-of-roundness (smaller than any reported uncertainty for CMM measurements in the literature)

4.        Capability to measure large aspect ratio (5 mm deep in 100 mm holes – this is much deeper than other competing methods)

5.        Inexpensive probes ($100 each) that can be easily replaced as opposed to MEMS based flexures ($3000 each)

6.        Large over-travel protection of approximately 1 mm before probe damage

7.        Extremely low contact force (approximately 0.5 mN) to avoid part damage.

8.        Extendable to full 3D (end faces of blind holes) measurement with addition of 3rd axis of imaging

















Presenter Information: Bala Muralikrishnan, Division 821, Manufacturing Engineering Lab, Bldg 220, A105, MS 8211, Phone 301-975-3789, Fax 301-869-0822,, Not a member of Sigma Xi, Category: Engineering