Optical Polarized Reflectance Characterization of Thin Aerogel and Xerogel Films

J. L. Hostetler, D. Stewart, C. E. Daitch, C. S. Ashley, and P. M. Norris

Journal of Non-Crystalline Solids, 225, 19, (1998)

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Advancement of porous dielectric materials like aerogels and xerogels for thin film devices demand sample characterization. Profilometers have been used to determine thickness, while the index of refraction and other optical properties are often determined using conventional ellipsometry. Commercial ellipsometers can be unreliable due to the low surface reflectivity (< 1%) for aerogel and xerogel films. A simple interferometric method, termed the polarized reflectance technique for thickness and index (PRETTI), is used to simulatenously measure the index of refraction and thickness of thin aerogel and xerogel films of SiO2, TiO2, ZrO2, and composite materials with an accuracy of +-0.5%.

Refractive Index (n) and Thickness (d) Measurements for Thin Aerogel and Xerogel Films

(laser wavelength = 632.8 nm)

Film Type
Method
d (PRETTI)
d (Ellipsometry)
n (PRETTI)
n (Ellipsometry)
Solid Volume Fraction
SiO2-A (acid catalyzed)
Dip
110
120
1.136
1.090
0.32
SiO2-A (acid catalyzed)
Dip
412
401
1.046
1.082
0.11
SiO2-X (base catalyzed)
Spin
848
-
1.239
-
0.55
SiO2-X (base catalyzed)
Spin
225
-
1.248
-
0.57
SiO2-X (acid catalyzed)
Dip
157
150
1.387
1.429
0.86
TiO2-X (acid catalyzed)
Dip
64
64
2.183
2.172
0.85
ZrO2-X (acid catalyzed)
Dip
142
142
1.932
1.943
0.90
4C-X (acid catalyzed)
Dip
89
92
1.464
1.459
-
4C-X (acid catalyzed)
Dip
134
126
1.446
1.424
-

A = Aerogel, X = Xerogel

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