In wafer inspection systems which utilize two dimensional imaging, the inspection speed is determined, among other things, from parameters including field of view size, and time between imaging sequential images. Generally speaking, a larger field of view, or a shorter time between sequential images will increase the inspection speed.
Decreasing the time between imaging may be complicated and expensive. For instance, decreasing the time between images can require very fast detectors (much faster above normal 30 Hz detectors), fast illumination (for example, repetitive laser with hundreds of pulses per second), and a fast stage or other suitable components for generating relative motion between the wafer and imaging components to change which portion(s) of the wafer are in view for imaging.
A more preferable approach in some circumstances is to enlarge the field of view. However, when fine resolution is required (pixel size in the wafer plane is below 0.5 microns), the detector must contain a numerous pixels. For example, using 0.2 micron pixel, and a conventional commercial detector with 2K×2K pixels, the field of view is only 0.4 mm×0.4 mm. An enlarged field of view may also require a faster stage or other suitable components for providing relative motion between the imaging components and the wafer.
The image view can be increased by using multiple two dimensional detectors to obtain an image, with the image divided amongst the detectors. Some currently-existing systems split an image before the focal plane of the other optics used to obtain the image using, for instance, beam splitters and/or mirrors. See, for instance, U.S. patent application Ser. No. 10/345,097, filed Jan. 15, 2003, and published as U.S. Patent Application Publication No., 20040146295 which are each incorporated by reference in their entireties herein. However, splitting an image by a mirror or other element(s) before the focal plane may be problematic in some instances. The problems may include, for example, reductions in intensity and/or non-uniform intensity.
FIG. 9 illustrates an example wherein the intensity in some parts of a split image is reduced when some rays are reflected back from the mirror and do not actually reach the focal plane, since the actual splitting of the image occurs prior to the focal plane. As shown in FIG. 9, three rays (R1, R2 and R3) from the imaging optics 18 of an inspection system reach point A in the focal plane FP18 of the imaging optics if no splitting mirror is used (i.e., if the mirror shown in FIG. 9 is disregarded, all three rays reach point A). However, when the splitting mirror comprising reflective planes 902 and 904 is used, only two rays (R2 and R3) reach the detectors 908-1 and 908-2 in the split focal plane. The top ray (R1) is reflected back from the mirror.
FIG. 9 also illustrates an example of non uniform intensity that may result from splitting. The intensity reduction is position dependent—a given portion of the image that is closer to the splitting point will have a reduced intensity relative to a portion of the image far from the splitting point. In FIG. 9, point B′ gets only about half of the rays (i.e. rays generally emanating from the bottom half part of the imaging optics), while point A′, for example, gets more (about two thirds: from ray R2 to R3).
An example hypothetical intensity distribution in detector 908-1 and 908-2 imaging a uniform input image (I and II) is shown in FIG. 10. The image is darker at points closer to the splitting point, with denser cross-hatching representing progressively darker portions of the image (becoming darker from left to right in 908-2 and right to left in 908-1).
The angular distribution of the image is not preserved when an image is split in this manner. For a wafer inspection system, the angular distribution of the scattered or reflected light from the wafer contains information regarding the wafer characteristics. Using splitting mirrors before the focal plane changes the angular distribution since it blocks a range of ray angles and thus may result in reduced inspection accuracy.
When splitting by beam splitters, some of the rays (usually 50%) are reflected from the beam splitter while the rest of the rays are transmitted. This way does not break the uniformity or the angular distribution, but the intensity is reduced by 50%. When using more than one splitter to split an image into more than two portions, the intensity can be reduced even more.