Confocal Microscopy Applied to Metrology of Integrated Circuits.

The measurement of microscopic feature sizes (both lateral and vertical) is important in several fields of research. Conventional optical microscopes are of some use in measuring feature widths (but have slightly poorer resolution than a confocal microscope) but are not usable for measuring feature heights. Two types of scanning laser microscopes (a reflection confocal scanning laser microscope and a differential phase contrast scanning laser microscope) were investigated with respect to their ability to measure feature heights. A theory describing the axial and lateral resolution of an infinity-corrected confocal microscope (with finite area pinhole) has been formulated for the first time. I have shown that the infinity-corrected confocal microscope will have better resolution than the conventional (non -infinity-corrected) confocal microscope. A new theory for using Differential Phase Contrast (DPC) detection to measure surface height variations has been developed. The limits of the application of DPC microscopy to feature height measurement have been determined for the first time. These limits are a height of 0.4 μm for features whose surfaces have shallow slopes, and 0.1 μm for features whose surfaces have steep slopes. Features greater than 0.1 μm in height with steep-sloped edges were measured using a new sample tilting technique. The measurements were repeatable and accurate. It has long been accepted that confocal DPC imaging using a single detector pinhole is impossible to implement. In this thesis I show for the first time that it is not only possible but also practical to use this technique, and I present single pinhole confocal DPC calibration data and a confocal DPC image using a single detector pinhole. A series of measurements using chrome-on-glass calibration targets were performed to compare the resolution of non-confocal and confocal laser microscopes with the resolution of a non-confocal differential phase contrast laser microscope. The ability of confocal microscopes to reject light from out-of-focus regions is well established. This allows surface profile measurements to be made using confocal microscopes. However I have shown that when confocal microscopy is used to measure the depth of micron-wide trenches (in integrated circuits, for example), there is a shift in the apparent depth of the object. Occasionally there is even a profile inversion, where a trench appears to be an isolated ridge structure. A diffraction theory which includes all of the scattering within the trench is presented. This formalism is applied to the confocal imaging of trench structures and the profile inversion which has been seen experimentally is confirmed theoretically.