20 August 2001 Bremsstrahlung emission and absorption in electron projection lithography
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Abstract
Electron projection lithography (EPL) uses 100keV electrons to minimize the effects of beam blur, minimize forward scattering, and expose high aspect ratio resist patterns. When these electrons undergo collisions in the substrate, they decelerate and emit Bremsstrahlung photons. The Bremsstrahlung photons have a continuum spectrum with high energy, and absorption of these photons might damage device layers already present during the electron lithography step. A simulator that uses a Monte Carlo approach to track electron trajectories in this substrate calculates the Bremsstrahlung spectrum and its spatial distribution. The Bremsstrahlung temporal spectrum and spatial distribution was calculated with a model selected after reviewing various approaches described in the literature. The density of the Bremsstrahlung photons absorbed by the film stack of device layers is then calculated. Bremsstrahlung absorption by thin oxide layers on silicon is small compared to the SiO2 bond density. For gate dielectrics containing high atomic number metals such as HfO2, the absorbed flux of photons generated by the electron exposure is about one order of magnitude larger than for SiO2 gate dielectrics. Because the gate dielectric for future CMOS devices is only a few nanometers in thickness and the pixel exposure time is only a fraction of a millisecond, few of the photons generated by electron exposure are absorbed in the gate dielectric. The effects of absorption in other layers is negligible. The calculated number of charges in the gate dielectric that result from photon absorption is not large enough to shift the flat band voltage by more than ~1 mV. For the cases simulated with this model, electron projection lithography is not expected to cause significant effects on CMOS gate dielectric properties, even for metal oxide dielectrics.
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Scott Daniel Hector, Jonathan L. Cobb, Vladimir Ivin, Mikhail V. Silakov, George Babushkin, "Bremsstrahlung emission and absorption in electron projection lithography", Proc. SPIE 4343, Emerging Lithographic Technologies V, (20 August 2001); doi: 10.1117/12.436640; https://doi.org/10.1117/12.436640
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