Performing critical-dimension localization microscopy includes: subjecting a first dimensional member and a second dimensional member of a reference artifact to critical-dimension metrology, the first and second dimensional members, in combination, including a critical dimension and each independently providing optical contrast; determining a primary length of the critical dimension to be traceable to International System of Units meter; imaging in a calibrant optical field, by optical microscopy, the first dimensional member and the second dimensional member, the calibrant optical field disposed in an ocular optical field; determining, from the optical microscopy of the first dimensional member and the second dimensional member, a secondary length and a secondary length uncertainty of the critical dimension subjected to the critical-dimension metrology; and calibrating the calibrant optical field and the secondary length, to the primary length to establish traceability of the secondary length to the International System of Units meter to perform critical-dimension localization microscopy.
Critical-Dimension Localization Microscopy (CDLM) is a new calibration and measurement method that establishes SI-traceability of optical microscopy and enables subnanometer localization accuracy over a submillimeter field. NIST fabricated arrays of sub-resolution apertures in an opaque metal film and applied a primary method of critical-dimension metrology to characterize a small field of the aperture array of a few tens of micrometers, providing measurements of aperture spacing that are traceable to the SI. Illumination of these apertures forms a reference array for calibration of a widefield optical microscope. This microscope then enables CDLM, extending SI-traceability to localization of single emitters over large fields with high throughput, for example in the production of reference materials for other optical microscopes.
NIST compared measurements from two aperture arrays fabricated using two independently traceable electron-beam lithography systems to estimate fabrication accuracy and traceability, in lieu of applying a primary method of critical-dimension metrology. The results are shown in Figure 1, where NIST used CDLM to characterize aperture position errors resulting from different fabrication process parameters. CDLM achieved subnanometer accuracy over submillimeter fields and greatly increased the throughput of such measurements in comparison to other forms of critical-dimension metrology.
This methodology will enable CDLM for manufacturing metrology to perform quality control of reference materials with high throughput at low cost. Additionally, there is no other method that enables a widefield optical microscope to achieve SI-traceable subnanometer localization accuracy across a submillimeter field. Current practice for calibration of optical microscopes generally relies on inferior reference materials that are unsuitable for this.