Proceedings Article | 16 March 2009
KEYWORDS: Microscopes, Diffraction, Distortion, Calibration, Diffraction gratings, Reticles, Data modeling, Metrology, Error analysis, Optical lithography
As imaging requirements and limits continue to be pushed tighter and lower, it has become imperative that accurate and
repeatable measurement of the projection lens (PL) pupil be readily available. These are important for setup and
adjustment of the illumination distribution, measurement and optimization of the lens aberrations, and verification of
lens NA and transmission. Accurate testing of these items is critical during initial installation and setup of a
photolithography tool, but it continues to prove useful each time any projection lens pupil-image measurement is made.
The basic raw data from any such measurement is in the form of a pixelized 'image' captured by a projection lens pupil
microscope. Such images have typically been referred to as pupilgrams1, and many prior works have reported on their
application and analysis1,2,3.
Each of these measurements can be affected by errors in the measurement tool used. The error modes can be broadly
divided into two distinct groups: uncompensated transmission loss, and uncompensated distortion (or remapping) error.
For instance, in illuminator measurements, the first will yield intensity error and the second will yield image shape
mapping error.
These errors may or may not lie exclusively in the optics of the measurement tool. But, regardless of their source, they
will propagate through the analysis of the pupilgram images. For this reason, at minimum they must be measured and
judged for relative impact, if only to confirm that the errors do not change the conclusions or results.
In this paper, we will discuss and present methods for measuring the image distortion present in a PL pupil-image
microscope. These data are used to build a 'map' of errors vs. position in the lens pupil. The maps then serve as the
basis for image-processing-based compensation that can be applied to all subsequent microscope images.
A novel vitally important feature of the technique presented is that it calibrates the distortion of the microscope image
field using fundamental optical physics. This is achieved by imaging, capturing, and analyzing the diffraction pattern of
reticle gratings. Several different reticle pattern pitches are imaged and their diffraction patterns are captured by the
microscope. The image field of the microscope is then calibrated to the spacing of the diffraction orders, meaning that
the calibration is traceable through fundamental optics to the very precise reticle patterns. Details of this procedure,
including an example, will be presented.
