OCT meets new application in wafer-polishing-pad inspection at GIST

Gwangju, South Korea and Fremont, CA--Researchers at the Gwangju Institute of Science and Technology (GIST) and Western Digital Corporation have applied optical coherence tomography (OCT) to a non-biomedical OCT application; namely, measurement of the surface structure of wet pads used in chemical mechanical polishing (CMP), a key process for global planarization of silicon wafers for semiconductors, LEDs, and magnetic heads. Because removal rate of wafer material is directly dependent on the surface roughness of a CMP pad, the structure of the wet pad surface along with the existence of polishing slurries can be quantified using ultrahigh-resolution full-field OCT (FF-OCT).

The FF-OCT setup used bright illumination from a spatially incoherent broadband light (FWHM, 220 nm) from a 100 W tungsten halogen lamp guided to the interferometer via a fiber bundle (7.0 mm diameter). With an iris and a series of lenses, a spatially uniform illumination was made across the field of view. The interference image formed with the beams retro-reflected from the sample arm and the reference arm was projected onto a silicon CCD camera through a 250 mm focal length achromatic doublet lens. A pair of identical microscope objectives were used in both arms, and the field of view (FOV) was 670 square microns. For obtaining tomographic images of the wet pad sample, the four-bucket integrating technique of phase-shifting interferometry (PSI) was used, permitting extraction of an en-face image with acquisition time of 0.2 s and a depth resolution of approximately 0.8 microns in water.

The FF-OCT intrinsic feature of sub-micron spatial resolution allowed the characterization of the pad surface texture even under slurry mixture. The roughness of the pad surface was measured with respect to the number of polishing usages, and the same measurements were made for the depth of the pad groove. The OCT measurements clearly showed that the roughness of the pad surface was maintained for up to 139 wafer polishing steps and gradually degraded. Whereas, the groove depth was rapidly reduced due to pad wearing of the polishing and conditioning, which implies that these roughness quantities enable properly controlled polishing conditions in CMP processes. We also found that our OCT measurements showed good correlation with commercial Wyko optical profiler measurements and SEM observations.

SOURCE: GIST and Western Digital Corp.; www.opticsinfobase.org/view_article.cfm?gotourl=http%3A%2F%2Fwww%2Eopticsinfobase%2Eorg%2FDirectPDFAccess%2F3E252245%2DF07F%2D5417%2D7BBE84D553A5A624%5F219343%2Epdf%3Fda%3D1%26id%3D219343%26seq%3D0%26mobile%3Dno&org=