2. 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

3. 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

4. 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

5. 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

6. Scattering vs Energy

7. GDSII pattern demonstrating the proximity effect Proximity effect

8. 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 14-19 March, 1999; p. 22) Proximity effect - simulation

9. (D. F. Kyser and N. S. Viswanathan, "Monte Carlo simulation of spatially distributed beams in electron-beam lithography", J. Vac. Sci. Technol. 12(6), 1305-1308 (1975)) Proximity effect - Energy

10. 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

11. 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

12. (Mark A. McCord, Introduction to Electron-Beam Lithography, Short Course Notes Microlithography 1999, SPIE's International Symposium on Microlithography 14-19 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

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

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

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

16. 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

17. + 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

18. 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

19. (Mark A. McCord, Introduction to Electron-Beam Lithography, Short Course Notes Microlithography 1999, SPIE's International Symposium on Microlithography 14-19 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

20. (Mark A. McCord, Introduction to Electron-Beam Lithography, Short Course Notes Microlithography 1999, SPIE's International Symposium on Microlithography 14-19 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

21. 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