No abstract

In June 2007, Intel announced a new pixelated mask technology. This technology was created to address the problem caused by the growing gap between the lithography wavelength and the feature sizes patterned with it. As this gap has increased, the quality of the image has deteriorated. About a decade ago, Optical Proximity Correction (OPC) was introduced to bridge this gap, but as this gap continued to increase, one could not rely on the same basic set of techniques to maintain image quality. The computational lithography group at Intel sought to alleviate this problem by experimenting with additional degrees of freedom within the mask. This paper describes the resulting pixelated mask technology, and some of the computational methods used to create it. The first key element of this technology is a thick mask model. We realized very early in the development that, unlike traditional OPC methods, the pixelated mask would require a very accurate thick mask model. Whereas in the traditional methods, one can use the relatively coarse approximations such as the boundary layer method, use of such techniques resulted not just in incorrect sizing of parts of the pattern, but in whole features missing. We built on top of previously published domain decomposition methods, and incorporated limitations of the mask manufacturing process, to create an accurate thick mask model. Several additional computational techniques were invoked to substantially increase the speed of this method to a point that it was feasible for full chip tapeout. A second key element of the computational scheme was the comprehension of mask manufacturability, including the vital issue of the number of colors in the mask. While it is obvious that use of three or more colors will give the best image, one has to be practical about projecting mask manufacturing capabilities for such a complex mask. To circumvent this serious issue, we eventually settled on a two color mask -comprising plain glass and etched glass. In addition, there were several smaller manufacturability concerns, for example a "1X1" glass pillar (an isolated 0 phase pixel) were susceptible to collapse under the stress of mask processing, and therefore these had to be constrained out of the final configuration. A third key element was defining the objective function. We experimented with a large number of choices and eventually settled on a form that allows us to trade-off fidelity and contrast. A fourth key element was the optimization algorithm. The number of possible configurations for a trillion pixels present on our final product mask is greater than the number of total elementary particles in the known universe, so finding the proverbial needle in this haystack was difficult to say the least. We chose a mixture of stochastic and direct descent algorithms to find an arrangement that meets the demands. While we have not proved we are close to the absolute global minimum, we conducted several experiments to suggest this is the case. A fifth key element, and a large one at that, was scalin...

This paper introduces a novel concept, "comeless phase-shifting', that eliminates the need for the use of chrome to form patterns in optical lithography. Chromeless phase-shifting uses 180° phaseshifters on transparent glass to define patterns. The method relies on the destructive interference between phase-shifters and clear areas at the edges of the phase-shifters to define dark or opaque areas on the mask. Gratings sufficiently small will produce sufficient interference to completely inhibit the transmission of light (these gratings are thus named dark-field gratings). The combination of these effects makes it possible to form a wide range of patterns, from line-space patterns to isolated bright or dark areas.In this study, the lithography simulators SPLAT and SAMPLE were used to understand the principles behind this new scheme, and to verify various pattern designs. Simulation and experimental results are presented to demonstrate the concept. tional chrome mask. A phase-shift layer delays the light from a pattern so that it arrives 1800 out of phase with the light through a clear area. By careful placement of the phase-shifting material, the light from the phase-shifted and non-phase-shifted areas can be made to interact coherently, thus producing a

Basic studies of projection printed images are presented to identify the types of patterns which are most susceptible to residual aberrations and to establish test structures which may be used to monitor the presence of critical types of residuals. These effects are explored by including arbitrary lens optical path difference (OPD) aberration functions in a twodimensional optical image simulation program associated with SAMPLE. The lens aberration function is expressed either in Zernike polynomials or a series expansion. The intensity is calculated from Hopkin's transmission cross -coefficient formulation with a self -checking algorithm. A catalog of results is presented here for the dominant primary aberrations of coma and astigmatism for a fixed maximum OPD of 0.4 A,. Contact holes are shown to be much more susceptible to astigmatism than coma and the traditional checkerboard test pattern is verified as a sensitive diagnostic pattern. An alternative structure consisting of thin lines with a short break is shown to be even more sensitive to astigmatism and useful for distinguishing it from coma. A further improvement in sensitivity is obtained through the use of small nonprintable defect -like features in proximity to regular features which coherently interact with the blurred image of the feature. A test target of this type is recommended for monitoring coma.Best Focus + 0.5 ¡m N X O COMA -0.20 (0.75,0), OPD in 0.0X