Statistical optics and optical elements for microtechnologies: Partial coherence, lithography and microlenses
The scientific problems treated in this thesis are expressed within the framework of
statistical optics and are generated out of the optical lithography industry. Optical
lithography uses partially coherent light, i.e. light with a random wavefront, to
increase the performance of the lithographic process.
One of the goals of this work has been to explore ways to numerically simulate
the behavior of partially coherent radiation. In this work a method is introduced
that decomposes the simulation of a partially coherent field into a simulation of
several coherent field, thus enabling the use of existing efficient numerical methods
for coherent fields.
Although the degree of partial coherence is an important property of light, it is
cumbersome to measure and characterize. In this work an inverse method is presented,
where the degree of partial coherence can be retrieved, using a numerical
algorithm, from a number of simple intensity measurements. An inverse method
for the design of microlenses with short focal lengths under coherent illumination
is also introduced.
One particular problem in optical lithography, and other industrial processes,
is to produce a uniform illumination over a surface using an unstable partially coherent
light source. Recently, an often applied solution has been to use diffractive
optical elements. In this thesis an analysis of the efficiency of this approach is made
for different types of diffractive optical elements.
Furthermore, a previously little-recognized, yet fundamental phenomenon referred
to as “dynamic speckle”, is introduced. It is found that dynamic speckle
may have a detrimental effect on the accuracy of optical lithography since it limits
the uniformity of the deposited energy for pulsed partially coherent sources.