Research results from the Opto electronics Division of the National Institute of Standards (NIST; Boulder, CO) indicate that Maker fringe mapping of lithium niobate (LiNbO3) wafers may provide an effective prescreening method to weed out defective semiconductor substrates before they undergo costly production processes. Maker fringes, the oscillations of second-harmonic-generation intensity within a crystal as a function of transmitted pump-laser beam angle of incidence, demonstrate patterns of spacing and visibility dependent upon crystal characteristics such as thickness, birefringence, and indices of refraction at the pump and second-harmonic wavelengths.
The NIST researchers evaluated the sensitivity of this well-known feature of nonlinear optics as an indicator in LiNbO3 crystals of uniformity in birefringence, thickness, and internal strain, said NIST researcher Norman A. Sanford.1 "The use of Maker fringes to evaluate nonlinear response of materials has been around for a long time," Sanford said. "We didn`t invent that, but we are some of the first to use Maker fringes to look at uniformity in terms of birefringence and photoelastic strain."
The NIST researchers examined a large number of commercially produced LiNbO3 samples in the form of 1-mm-thick, x-cut wafers with diameters of either 76 or 100 mm. A 1064-nm CW Nd:YAG laser provided the pump beam, which was polarized parallel to the y axis of the sample. The beam diameter on the surface of each sample was about 40 µm. And the depth of beam focus was adjusted to about 2 mm to provide a uniform pump intensity througout the sample thickness.
An automated mounting system varied the angle of incidence between the pump beam and the face of the crystal by rotating the sample about its vertical axis, which was also its y axis. In this way, Maker fringe patterns were collected in a grid-like fashion over an entire wafer and stored in a computer for subsequent analysis.
Based on analysis of Maker fringe scan data taken from many wafers, the researchers found that variations of 10-5 in the index of refraction could be resolved. Further, the presence of photoelastic strain in the samples was revealed by measuring the presence of a strong polarization component of the Maker fringe pattern that would corrupt a weaker polarization component.
Most of the cost in building integrated optical components from LiNbO3 undoubtedly comes from the labor involved in photolithography, processing, and finishing, Sanford said. So the ability to test and screen unprocessed wafers before these steps are performed could be valuable to manufacturers.
Results obtained so far indicate a role for Maker fringe analysis "as a means to examine the process uniformity of diffusion methods for waveguide device fabrication," Sanford said. "More research is required, however, to correlate these effects with crystal growth, wafer preparation, device fabrication, device performance, and manufacturing yields of finished devices."
The researchers also noted that even though the Maker fringe analysis was performed entirely in transmission through the bulk of the samples, artifacts induced by surface treatments or the weak outdiffusion of lithium from near the surfaces of the samples was easily detected.