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In this paper, the effect of SiN pull-back process for shallow trench isolation (STI) is investigated by measuring DRAM array's refresh time (Tref) and yield as sensitive monitors. The SiN pull-back is performed by using H3P04 solution after trench etch (i.e. before liner oxidation). For comparison, DRAMS were fabricated by using various isolation methods including LOCOS, conventional STI, and poly-buffered STI (PB-STI). The SiN pull-back process is known for reducing "divot" around the top corner in conventional STI. Both LOCOS and PB-STI can result in "divot" free. It is also known that 'Ldivot" will degrade the inverse narrow width effect of pass transistor and result in "double hump". In our study, SiN pull-back in STI indeed eliminates "double-hump'' in Id-vg curves of pass transistors. The SiN pull-back also can result in better data retention of DRAM than if without pull-back, but comparable to LOCOS and PB-STI. The optimized window of SiN pull-back in this study is 1 Onm to 40 nm with best yield at 15nm (slightly better yield than LOCOS and PB-STI).

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An optimization program has been developed for designing combined focusing and dual-channel deflection systems with low aberrations for electron beam lithography. The program can handle any combination of magnetic and electrostatic lenses and deflectors, and uses the damped least squares method for minimizing a weighted sum of squares of aberrations, subject to specified physical constraints. The program has been used to optimize the design of several types of lithography system. The results show that, in many cases, the program can yield designs with extremely low aberrations. For example, a system with one magnetic lens and two deflectors has been found which, at the corners of a 5 mm scan field with 5 mrad aperture, has only 0.19 μm overall aberration before dynamic corrections; by adding an electrostatic retarding field, this can be reduced to 0.12 μm. By adjusting the strengths and positions of the deflectors, several of the aberrations can be individually eliminated, including field curvature. It is also shown that electrostatic subfield deflectors can be added without significantly increasing the aberrations, and a pure electrostatic focusing and deflection system for ion beam lithography has been designed which is limited almost entirely by axial chromatic aberration.

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