BACKGROUND OF THE INVENTION
This invention was made with government support under Contract No. MDA 972-92-C-0054 awarded by Advanced Research Projects Agency (ARPA). The government has certain rights in this invention.
The present invention relates to processes for inspecting photomasks to detect defects. More particularly, the present invention relates to an automatic inspection system for detecting defects in photomasks.
Advances in capacity in semiconductor chips have generally been the result of decreases in the size of features on a chip. The lateral dimensions of features are generally defined by photolithographic techniques in which a detailed pattern is transferred to a photoresist by shining light through a photomask or reticle.
In recent years, phase shifting masks have been developed to improve photolithographic processes. Phase shifting masks increased image contrast and resolution without reducing wave length or increasing numerical aperture. These masks also improve depth of focus and process latitude for a given feature size.
With phase shift photolithography, the interference of light rays is used to overcome the problems of defraction and improve the resolution and depth of optical images projected onto a target. With this technology, the phases of the exposure light at the target is controlled such that adjacent bright areas are preferably formed 180° out of phase with each other. Dark regions are thus produced between the bright areas by destructive interference even when defraction would otherwise cause these areas to be lit. This technique improves total resolution at the target.
Another method that has been developed to produce masks for use in the fabrication of semiconductors containing small features is optical proximity effect correction (“OPC”). In this method, changes are made to the binary mask's layout so that it will print more clearly. Because of the limited resolution of the current photolithographic tools (i.e., steppers), the patterns defined on the photomask are transferred into the resist on the wafer with some distortions referred to as optical proximity effects. The main consequences in term of line width control are: corner rounding, difference between isolated and semi-isolated or dense patterns, lack of CD linearity or where small features print even smaller than their expected size compared to large features, and line end shortening where the length of a line having a small line width becomes smaller than its expected size.
Moreover, optical proximity effects are convoluted with subsequent processing step distortions like resist processing, dry etch proximity effects and wet etch proximity effects. In order to achieve a sufficient line width control at the wafer level, the mask designs are corrected for proximity effects, namely re-entrant and outside serifs are used to correct rounding and the edges of the patterns are moved to correct line width errors. Another technique consists in adding small, non-printing features, referred to as subresolution features, in order to correct line width errors. In some cases, these features can also improve the process latitude of the printed resist patterns.
Printable defects in photomasks and reticles have historically been a source of defects that have reduced die yields. With current photolithographic techniques, printable defects in the photomasks, are repeated many times over the surface of a semiconductor wafer and can result in substantial yield losses. Accordingly, it is important to detect and correct as many defects as possible in the photomasks.
Defects in photomasks can arise from many different sources. For example, certain defects such as bubbles, scratches, pits and fractures can be contained in the raw glass substrates. Defects can also be formed in the chrome layer by particulate inclusions, pin holes or voids, and excess material.
As advances have been made in photomask design such as phase shifting and OPC, it has become harder to detect defects in the photomasks. However, defect detection and correction has become increasingly important. Previously, masks were checked by exposing and developing an image on a resist layer on a plain quartz wafer. The resulting pattern was then inspected. However, there was no die-to-database inspection with this system.
Automatic photomask defect detection systems have been developed and are commercially available. These include systems such as the KLARIS system by KLA Instruments Corp. and the Chipcheck system by Cambridge Instruments. Inspection tools such as KLA and Orbot systems are also available for die-to-die inspection of the printed image on wafers. These systems are limited by the fact that the inspection is performed at 1× (versus 4× or 5× for most advanced reticles). The maximum allowable defect size is smaller and a complete inspection is not possible in the case of a single die reticle as die-to-database capability is not available on these systems.
In the KLA system, light is transmitted through the photomask and detected by a CCPD image sensor. This image is then compared to the image from a database or compared to the image from another die on the mask. If one comparison of a die to the database is performed, the remaining comparisons on the mask can all be die-to-die inspections that relate back to the initial comparison.
These prior art systems are generally limited to basic mask designs and have limited capability of checking advanced designs such as those containing optical proximity effect corrections and phase shifting layers.
Because of the importance in detecting and correcting photomask defects, it would be a significant advancement in the art to provide an automatic process for detecting defects in advanced photomask designs. Such a process is disclosed and claimed herein.
SUMMARY OF THE INVENTION
The present invention provides an automatic process for detecting printable defects in masks. The invention is particularly useful in analyzing advanced photomask designs such as those which include optical proximity effect corrections and phase shifting layers.
In a preferred embodiment, a mask design is generated from a binary mask layout. The mask design is then used to generate a photomask such as by suitably etching a chrome layer on a quartz plate. The present invention provides a process for detecting any defects that are formed in the photomask. In a preferred embodiment, an aerial image of the photomask is generated. This is then compared with a simulated image of the binary mask layout which has been adjusted to account for expected distortions and corner rounding caused by image processing of the mask and wafer. Any discrepancies between the aerial image and the simulated image are likely due to defects in the photomask.
In a second preferred embodiment, the aerial image of the photomask is compared with a simulated aerial image of the mask design. Again, any discrepancies between these two images are likely due to defects in the photomask.