In lithographic semiconductor device fabrication processes, it is imperative that a stepper precisely focus an image of a reticle on a semiconductor substrate or wafer. Where the image of the reticle is out of focus, a state also known as defocus, the structures of the resulting semiconductor device may be of the incorrect size and form. For example, the edges of the resulting structures may be relatively diffuse and indistinct, having rounded or undercut surfaces in lieu of a more desired, often rectilinear geometry. This state of defocus often leads to poor function and/or inoperability in the semiconductor device in question. Measurement of defocus is therefore an important means for allowing semiconductor device manufacturers to ensure that a stepper consistently focuses a reticle image on a wafer, thereby enabling larger and more profitable yields from the manufacturing process.
Another problem common to the formation of semiconductor devices is that of exposure defects. Where the exposure of a photo resist layer to light falls outside a range of acceptable light dosages, the features that are to be formed on the semiconductor substrate may be formed incorrectly. Accordingly, it is also important to identify exposure defects where they exist.
In addition to inspecting a substrate or wafer for exposure or defocus defects, it is important to inspect substrates and wafers for process or material related defects commonly referred to as “macro” defects. Macro defects are often defined as chips, cracks, scratches, pits, delaminations, and/or particles that appear on a substrate that have a dimension of about 0.5 u to 10 u in size. Such defects can easily cause a failure in a semiconductor device and can significantly reduce the yield of a manufacturer of such devices. Note that the sizes of macro defects may depart up or down from the size range stated above, which merely defines a nominal size of such defects.
Traditionally, macro defects have been inspected using dedicated inspection systems that have not been able to readily or reliably identify the presence of exposure or defocus defects. Exposure and defocus defects are usually identified using optical critical dimension (OCD) techniques on any of a number of precision metrology tools such as ellipsometers, reflectometers and scatterometers. It would be desirable to combine the functions of identifying the presence of exposure and defocus defects with inspection of substrates for macro defects wherein the same optical system is used for both functions.