Certification of SRM1960: Nominal 10 μm Diameter Polystyrene Spheres ("Space Beads")

Abstract

Experimental, theoretical, and calculational details are presented for the three independent micrometrology techniques used to certify the mean diameter of Standard Reference Materisd 1960, nominal 10 μm diameter polystyrene spheres ("space beads"). The mean diameters determined by the three techniques agreed remarkably well, with all measurements within 0.1% of each other, an unprecedented achievement in the dimensional metrology of microspheres. Center distance finding (CDF), a method based on optical microscopy, gave a value of 9.89 ± 0.04 μm, which was chosen to be the certified mean diameter. The supporting measurements were done using metrology electron microscopy (MEM) and resonance light scattering (RLS). The MEM technique, based on scanning electron microscopy, yielded 9.89±0.06 μm for the mean diameter of the microspheres in vacuum, while the RLS value was 9.90 ±0.03 μm for the microspheres in liquid suspension. The main peak of the diameter distribution for SRM 1960 is nearly Gaussian with a certified standard deviation of 0.09 μm, as determined by CDF. Off the main peak, there are about 1% oversized particles and a negligible amount of undersized particles. The report gives a detailed description of the apparatus, the experimental methods, the data-reduction techniques, and an error analysis for each of the micro-metrology techniques. A distinctive characteristic of this SRM is that it was manufactured in microgravity aboard the NASA space shuttle Challenger and is the first commercial product to be made in space.

Keywords: Electron microscopy; length; light scattering; metrology; microgravity; micrometrology; microscopy; microspheres; particle sizing; particles; polystyrene spheres; reference materials; sizing; standards.

Fig. 1

Photo of NIST Standard Reference Material 1960 showing a vial of the SRM, the certificate, and the package.

Fig. 2

The certificate which comes with SRM 1960. The certificate which comes with SRM 1960 (reverse).

Fig. 2

The certificate which comes with SRM 1960. The certificate which comes with SRM 1960 (reverse).

Fig. 3

Scanning electron microscope photomicrograph of the SRM 1960 microspheres.

Fig. 4

The center distance finding (CDF) technique.

Fig. 5

CDF disordered assemblies.

Fig. 6

Microscope calibration curves for image magnification and distortion.

Fig. 7

Diameter distribution of the SRM 1960 spheres, as determined using CDF.

Fig. 8

Scatter in the microscope magnification.

Fig 9

Schematic diagram of the metrology electron microscope (MEM) system.

Fig. 10

Schematic diagram of the bright-field imaging mode in the MEM.

Fig. 11

MEM intensity profile of an SRM 1960 microsphere.

Fig. 12

Schematic of the resonance light scattering (RLS) apparatus.

Fig. 13

RLS spectra of SRM 1960 microspheres in liquid suspension (I_‖ and I_⊥.).

Fig. 14

Calculated RLS spectra for 89°, 90°, and 91°.