Electron scattering in resist and substrate Proximity effect Resist interactions (positive /negative/chemically amplified resists, resist contrast) Dose definition Influence of beam energy (penetration depth) Resolution limits Outline

Properties very often scattering under small angles small-angle hence very inelastic (color symbolizes kinetic energy) generation of secondary electrons with a few eV kinetic Energy Forward scattering events Forward scattering

Properties occasionally scattering under large angles large angle hence mainly elastic (color symbolizes kinetic energy) high kinetic energy, range of the primary electrons Backscattered electrons Back scattering

Secondary electrons with few eV kinetic energy are responsible for most of the resist exposure Hence forward scattering within the resist is responsible for exposure And backscattering is responsible for exposure apart from incidence Resist exposure

Scattering vs Energy

GDSII pattern demonstrating the proximity effect Proximity effect

Simulation of electron trajectories: 1.5 µm resist thickness on a silicon wafer 50 trajectories, at 25 keV beam energy. (Mark A. McCord, Introduction to Electron-Beam Lithography, Short Course Notes Microlithography 1999, SPIE's International Symposium on Microlithography March, 1999; p. 22) Proximity effect - simulation

(D. F. Kyser and N. S. Viswanathan, "Monte Carlo simulation of spatially distributed beams in electron-beam lithography", J. Vac. Sci. Technol. 12(6), (1975)) Proximity effect - Energy

Positive resists usually work by polymer chain scission; exposed polymer becomes more soluble Negative resists usually work by cross linking between polymer chains; exposed polymer becomes insoluble Resist interaction

Chemically amplified resists, are modified during exposure. However the actual exposure takes place during the post exposure bake, when the acids are activated. HEAT Resist interaction

(Mark A. McCord, Introduction to Electron-Beam Lithography, Short Course Notes Microlithography 1999, SPIE's International Symposium on Microlithography March, 1999; p. 22) Resist contrast = Slope in resist remaining thickness log(Dose) D0D0 D1D1 D0D0 D1D1 Contrast  = [log 10 (D 1 )-log 10 (D 0 )] -1 PositiveNegative Resist contrast

High contrast: + Steeper side walls + Greater process latitude + Better resolution (not always) + Less sensitivity to proximity effects Low contrast: + 3d lithography Resist contrast

I beam = beam current T dwell = dwell time [µAs/cm²] s = step size Dose

Increase of dose with voltage for all resists Graph gives general behavior Dose vs Voltage

10 kV 20 kV 30 kV Areas100 µC/cm²200 µC/cm²300 µC/cm² SPLs300 pC/cm600 pC/cm900 pC/cm Dots0.1 pC0.2 pC0.3 pC (developer: MIBK + IPA, 1:3) The above values are good starting points. The best way to get optimum results is to perform a dose scaling: SPLs 0.5 – 5, Dots 0.1 – 10 Dose – PMMA 950K

+ Small scattering in resist + Small proximity effect 100 keV + Small beam damage + Small sample heating + Best electron-optical performance (classical columns) 20 keV + No beam damage + No proximity effect + High throughput (high resist sensitivity) 2 keV – High beam damage – Strong sample heating – Scattering in thick resist – Strong proximity effect – High scattering in resist – Needs very thin resists Beam Energy

Y. Lee, W. Lee, and K. Chun 1998/9, A new 3 D simulator for low energy (~1keV) Electron-Beam Systems Depth vs Beam Energy

(Mark A. McCord, Introduction to Electron-Beam Lithography, Short Course Notes Microlithography 1999, SPIE's International Symposium on Microlithography March, 1999; p.63) Beam resolution Thick resists (forward scattering) Thin resists (~0.5nm by diffraction, de Brogli wavelength) Resist limits Polymer size (~5-10nm) Chemically amplified resists (acid diffusion ~50nm) Resolution limits

(Mark A. McCord, Introduction to Electron-Beam Lithography, Short Course Notes Microlithography 1999, SPIE's International Symposium on Microlithography March, 1999; p.63) Secondary electron range (~5-10nm) In practice, the best achievable resolution in polymer resists is about 20nm, with inorganic resists (currently impractical for most applications) 5nm. Resolution limits

Ultra high resolution in PMMA (45nm thickness): 16nm line width in resist Possibilities?? 6 nm Ultra high resolution in HSQ (19 nm thickness) 6 nm line width in resist