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Smallest Density Inhomogeneities in Quartz Glass

Photolithography requires lenses made of quartz glass of the highest homogeneity. PTB has developed a new method to determine deviations in density based on a simple concept. Detection of smallest density differences in quartz glass is now possible with this new method.

The flotation method allows measuring smallest differences in the density of quartz glass samples. The sample volumes are approximately 1 cm3.

With the increasing miniaturization of integrated electronic components, the precision requirements for the optical components in photolithography are growing as well: Quartz glass lenses of highly precise shape and extremely homogeneous refractive index are required for the objectives. The refractive index of high-purity quartz glass is essentially determined by the structure of the glass and is correlated with the density rhoQ of the glass. For quartz glass, a difference in the density Delta rhoQ of 10-5 rhoQ corresponds to a difference of the refractive index of 1,5 · 10-6. Such minute differences can hardly be detected by optical means. Therefore PTB was asked to develop a more precise detection method by direct density measurements. Only samples of roughly 1 cm3 were available. For samples of that size small differences in density can only be detected with a flotation method. Two (or more) samples are contained simultaneously in a liquid, the density of which rhoFl is very similar to the density rhoQ of the samples. By varying the pressure inside the liquid the density rhoFl can be adjusted to match the density of each sample rhoQ, i. e. rhoFl = rhoQ. According to this special case of ArchimedesÕ principle the sample, for which the equation holds - and only that sample - will float freely in the liquid.

The flotation apparatus varies the liquid pressure by adjusting two parameters: the hydrostatic pressure (exerted by an additional vessel connected with the measuring vessel) and the temperature. A small time constant of temperature equilibrium is obtained with special cuvettes. The detection limit for samples of cuboidal shape amounts to 1 · 10-6 rhoQ. First measurements on five quartz glass samples yielded relative density differences of up to 3 · 10-5 (corresponding to differences in the refractive index of 5 · 10-6). The measurement uncertainty achieved was 5 · 10-6 rhoQ (k = 2).

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