MEASURING EFFECT OF NANOPARTICLES/DOPANTS AND PROCESSING VARIABLES ON THIN FILM AND BULK MATERIAL INDEX

The accuracy, precision and robustness of the Model 2010/M's refractive index measurement makes possible measurement of small changes caused by the addition of nanomaterials or chemical dopants to thin films and bulk materials that might otherwise be undetectable by other techniques.  The same accuracy and resolution make possible monitoring of small index changes due to processing variables such as curing regimens for polymers/epoxies and deposition temperature for inorganic thin films.

The 2010/M’s unparalleled index accuracy (+/-.0002) and resolution (+/-.0001) is due to the fact that, in the prism coupling technique, unless samples are strongly absorbing, almost no advance knowledge about the material (thin film or bulk) is required.  And, for a thin film, the measured refractive index is independent of the optical properties of the substrate if the substrate index is higher than the film index.  If substrate index is lower than film index, measured film index is only very weakly dependent on the substrate index (and substrate index is easily determined at the same time the film is measured). 

Ellipsometry measurements, on the other hand, are sensitive to the optical parameters (n and k) of the measured material and its roughness and, for a thin film, the optical parameters (n and k) of the substrate and its roughness.  Film measurement techniques that rely on spectrophotometry (interference vs wavelength) are often useful for thickness but lack the high accuracy and resolution for index because of sensitivities to dispersion (variation in index vs wavelength) in both the thin film and the substrate.

The robustness of the 2010/M’s index measurement is due not only to the fact that it required little, if any, advance knowledge about the sample to be measured but also to its tolerance of relatively poor sample quality including poor polish or surface roughness, significant amounts of optical absorption, and haze.  The system also easily measures flexible or non-flat samples with the only exception that rigid samples must either be reasonably flat over a roughly 1 cm x 1 cm area or have gently convex surfaces so the measuring prism can be brought into close contact with the sample surface.

In recent years, the 2010/M has been used extensively to monitor index changes resulting from the incorporation of nanoparticles into both thin films and bulk materials.  In some cases, the nanoparticles are added to deliberately raise or lower the index of the matrix material and in other cases they are added to alter some other material property and it is desired to determine the resulting change in the refractive index of the composite material.  References 1-10 give examples of uses of the 2010/M in characterizing index changes caused by the incorporation of nanoparticles.

Phosphorus or other dopants in silicon dioxide and other thin films are another application where the high accuracy and resolution of the 2010/M’s index measurement permits precise monitoring of dopants in inorganic materials.  Such films have been used for many years as both planarizing coatings and diffusion sources in the semiconductor industry and more recently have been used extensively in optical waveguide fabrication.  For phosphorus, the monitoring application is based on the fact that as phosphorus concentration in SiO2 increases, refractive index also linearly increases (C. Adams et al, "Measuring the phosphorus concentration in deposited phosphosilicate films", J. Electrochem. Soc., 126, 334 (1979))   For the typical process, a one percent increase in the weight percent of phosphorus (e.g., from three weight percent to four) increases index by roughly .003.  Since the Model 2010/M can theoretically routinely resolve index changes of .0001 (±.00005), a typical phosphorus resolution of ~.03 weight percent can be obtained. To be conservative, however, a practical resolution of 0.1-0.2 weight percent is a more reasonable expectation due to index variation caused by film constituents other than phosphorus.

For photoresists, polyimide, or other polymers, the Model 2010/M's ease in measuring thickness and index of relatively thick and optically absorbing films (including free-standing films) makes it an ideal tool for both production monitoring and R&D studies.

Journal articles referencing the Model 2010/M:

1. B. Cai, T. Kaino, and O. Sugihara, “Sulfonyl-containing polymer and its alumina nanocomposite with high Abbe number and high refractive index”, Opt. Mat. Exp., 5, 1210 (2015).

2. B. Can-Uc, R. Rangel-Rojo, H. Marquez, L. Rodrıguez-Fernandez, and A. Oliver,  “Nanoparticle containing channel waveguides produced by a multi-energy masked ion-implantation process”,  Opt. Exp., 23, 3176 (2015).

3. P. T. Chung, C. T. Yang, S. H. Wang, C, W. Chen, A.S.T. Chiang, C-Yi Liu, “ZrO2/epoxy nanocomposite for LED encapsulation”,  Mat.  Chem. and Phys. 136, 868  (2012).

4. T. Otsuka, and Y. Chujo, “Poly(methyl methacrylate) (PMMA)-based hybrid materials with reactive zirconium oxide nanocrystals”, Polym. J.. (2010) 42, 58 (2010).

5. M. Takeda, E. Tanabe, T. Iwaki, A. Yabuki, 2 and K. Okuyama, “High-concentration transparent TiO2 nanocomposite films prepared from TiO2 nanoslurry dispersed by using bead mill”, Polym. J., 40, 694, (2008).  

6. Y. Tan, R. He, C. Cheng, D. Wang, Y. Chen, and F. Chen, “Polarization-dependent optical absorption of MoS2 for refractive index sensing” , Sci. Rep., 4, 7523 (2014).

7. Q-Y Tang, J. Chen, Y.C. Chan, C.Y. Chung, “Effect of carbon nanotubes and their dispersion on thermal curing of polyimide precursors”,  Polym. Deg. and Stab., 95, 1672 (2010).

8. T. Uehara, M. Nakagawa, and O. Sugihara, “Preparation of UV-cured organic–inorganic hybrid materials with low refractive index for multilayer film applications”, Opt. Mat. Express, 3, 1351 (2013).

9. M. Wong, J. Guenther, L. Sun, J. Blümel , R. Nishimura , and H-J Sue, “Synthesis and fabrication of multifunctional nanocomposites: stable dispersions of nanoparticles tethered with short, dense and polydisperse polymer brushes in poly(methyl methacrylate), Adv. Func. Mat., 22, 3614 (2012).

10. H-J Yen, C-L Tsai, P-H Wang, J-J Lin and G-S Liou, “Flexible, optically transparent, high refractive, and thermally stable polyimide–TiO2 hybrids for antireflection coating”,  RSC Adv., 3 , 17048 (2013).

 11. C. A. Mack, D. P. DeWitt, B. K. Tsai, G. Yetter, “Modeling of solvent evaporation effects for hot plate baking of photoresist”, Proc. SPIE, 2195, 584 (1994).

12. M. Ree, T. J.Shin, Y. H.Park, S. I, Kim, S. H. Woo, C. K. Cho, C. E. Park, “Residual stress and optical properties of fully rod-like poly(p-phenylene pyromellitimide) in thin films: Effects of soft-bake and thermal imidization history,” J.  Polym. Sci. Part B: Polym Phys, 36, 1261-1273 (1998).