Part 3: 2 Potential Next Generation Radiative Methods For Nanostructuring Surfaces

1. Part 3: 2 Potential Next Generation Radiative Methods For Nanostructuring Surfaces Electron Beam Lithography Scanning Near Fileld Photolithography

2. Part 3i: E-Beam Lithography Polymers and Molecular Materials

3. Learning Objectives • After completing PART 3i of this course you should have an understanding of, and be able to demonstrate, the following terms, ideas and methods. • (i) The e-beam lithographic process, • Resist Material requirements, • Resolution • Etch durability • Sensitivity • Resist problems (beam speading, beam penetration, pattern collapse, line edge roughness), • Why use low molecular weight materials • Design considerationss for fullerene and triphenylene derivatives, • Overcoming the sensitivity problem (low energy methodology and chemical amplification),

4. Overview • What is a Resist? • Resist Requirements for EBL • Polymeric Resist • Introduction to LMW Resists • Fullerene and Triphenylene Resists • Low Energy Resists • Chemical Amplification • Conclusions

5. Electron Beam Lithographic Resist The electron beam initiates a chemical reaction in the organic material, either leading to fragmentation to smaller molecular components, which are soluble in some solvent (positive tone resist), or crosslinking to form an insoluble network (negative tone resist). 1 e e e e e e e e e e e e e e e “Organic” 1 Silicon 2 2 The unirradiated “organic” is removed with an organic solvent, leaving the cross-linked insoluble network pattern. Serial Writing is very slow, compared to Photolithography Spin Coated 10 -100s nm

6. A chemical etchant is employed to remove the exposed silica, and in so doing also etches the irradiated organic material, result in the pattern transfer to the silicon. 3 3

7. The Organic Material Requirements For a Negative Tone Resist · Must interact with the electron beam · Must cross-link to form a network · Must have a high sensitivity to the electron beam (energy efficiency) · The network must be insoluble · The network must have good mechanical strength · The network must be resistant to the etchant that is used to remove the silicon in the pattern transfer step (aspect ratio)

8. Polymeric Resists Historically resists have almost always been polymeric1 Polymers readily form smooth, amorphous films by spin coating PMMA [1] “Photoresist Materials: A Historical Perspective”, C. Grant Willson et al, SPIE, 3049, p 28

9. Good Resolution Positive Tone Resist ~10 nm Poor Resolution Negative Tone Resist ~70 nm Resist Resolution and Etch Durability Good Etch Durabilty Resist 6:1 Poor Etch Durabilty Resist 1:1 Neither materials have low sensitivity towards the electron beam to make them crosslink efficiently, and neither can make a high resolution (thin) and tall (good etch durabilty) structures, and are not mechanically strong.

10. D1 D50% D2 Sensitivity=D50% Contrast:  = |log10(D2/D1)|-1 Resist Sensitivity Photolithography PMMA = 140 mC/cm2 (20 keV) SAL601 = 8 mC/cm2 (20 keV) 100 Normalised Film Thickness (%) 50 0 1 10 100 Exposure Dose (mC/cm2)

11. Polymer Disadvantages Beam Spreading Pattern Collapse Line Edge Roughness

12. Beam Spreading and Penetration 20 keV Electrons 3 keV Electrons 1 keV Electrons PMMA Si 500 nm electron scattering simulations

13. Pattern Collapse Developer SiO2

14. 3 4 Line Edge Roughness Line edge roughness is affected by factors including lithographic noise, processing, and polymer molecular weight Image from www.tpd.tno.nl/smartsite910.html

15. Negative Tone Positive Tone Crosslinking Chain Scission Line Edge Roughness

17. Avoiding Polymer Problems Several of the problems with polymers seem to stem from the size of the molecules and low durability. Why not use smaller carbon rich molecules? Smaller molecules tend to crystallize rapidly after spin coating, giving a rough and unusable polycrystalline film There are some exceptions to this

18. Low Molecular Weight Resists • Amorphous Molecular Materials • Calixarenes • Catechols • Fullerenes and its Derivatives • Molecular Resists/Molecular Glasses • Oriented materials (Liquid Crystals) • Triphenylene Derivatives

19. Calixarenes • Cyclic oligomer around 1 nm in diameter • Negative tone electron beam resist6 • A chemically amplified epoxidised derivative has been demonstrated7 J-I. Fujita et al, Jpn. J. Appl. Phys., 36, 7769 (1997); H. Sailer, et al, Microelec. Eng., 73 - 74, 228 (2004)

20. Catechols • Cyclic oligmer around 1 nm in diameter with 3 aromatic rings per molecule • Chemically amplified positive tone electron beam resist8 • Various functional groups allow the solvent to be altered N. Kihara, et al, J. Photopolym. Sci. Technol., 11, 553 (1998)

21. Fullerenes & Derivatives • Aromatic cage molecule around 0.7 nm in diameter 60 carbons per molecule • Negative tone electron beam resist9 • Various functional groups allow the sensitivity and solubility to be altered T. Tada, et al, Jpn. J. Appl. Phys., 35, L63 (1996)

22. Molecular Glasses • Non-planar (propeller shaped) molecule around 1 to 2 nm in diameter • Negative or positive tone electron beam resist9 • Chemical amplification has been demonstrated10 M. Yoshiiwa, et al, Appl. Phys. Lett., 69, 2605 (1996); T. Kadota, et al, Chem. Lett., 33, 706 (2004)

23. Triphenylene Derivatives • Liquid crystalline molecule around 1 to 2 nm in diameter • Negative or positive tone electron beam resist10 • Various functional groups allow the liquid crystal nature and sensitivity to be altered A.P.G. Robinson, et al,J. Phys. D, 32, L75 (1999)

24. LMW Resist Properties

25. Fullerenes & Triphenylenes We have investigated two families of low molecular weight resists - fullerene derivatives, and triphenylene derivatives. • Original results for fullerenes and triphenylene derivatives • Low energy electron beam exposures of fullerene derivatives • Chemical Amplification of fullerene and triphenylene derivatives

26. R O O R Introduced strained cyclopropane ring O O R O O R O O Crosslinking increased X X n O R n R O Y Y Molecular Design Considerations High carbon content Etch Durability? Large p-surface Enhanced sensitivity? Large flat p-surface Ordering

27. 14 nm Scanning Electron Micrographs 100 nm 35 nm 20 nm Scanning Electron Micrographs of Resist Patterns (20keV Beam) Sensitivity ~ 1000 µC/cm2 2.5 nm ‘A Triphenylene Derivative as a Novel Negative/Positive Tone Resist of 10 nm Resolution A.P.G. Robinson, R.E. Palmer, T. Tada, T. Kanayama, M.T. Allen, J.A. Preece, and K.D.M. Harris, Microelectronic Engineering, 2000, 53, 425-428. ‘Multi-adduct Derivatives of C60 for Electron Beam Nano-Resists’ T. Tada, K. Uekusu, T. Kanayama, T, Nakayama, R. Chapman, W.Y. Cheung, L. Eden, I. Hussain, M. Jennings, J. Perkins, M. Philips, J.A. Preece, E.J. Shelley, Microelectronic Engineering, 2002, 61, 737-743.

28. Comparison Resolution equals or surpassed PMMA Etch ratio much better than SAL 601 Sensitivity much better than previous medium molecular weight materials Sensitivities of around 1000 mC/cm2 at 20-30 keV PMMA a factor of ~10 lower

29. Possible Solutions Low Energy Electrons Chemical Amplification The Sensitivity Problem The resolutions of both fullerene and triphenylene derivatives are comparable with other LMW materials, and the etch durabilities are extremely high. However, like most LMW resists the best sensitivities (fullerene - 370 µC/cm2; triphenylene - 880 µC/cm2) are still much lower that polymer based materials.

30. Low Energy Exposure Low energy electrons deposit more of their energy in the resist and less in the substrate. This leads to an increase in sensitivity. Image after D.F. Kyser et al, J. Vac. Sci. Technol, 12, 1305, (1975)

31. 20 keV MF02-01A 473 µC/cm2 MF03-01 970 µC/cm2 20 keV 1 keV MF02-01A 473 µC/cm2 21 µC/cm2 MF03-01 970 µC/cm2 65 µC/cm2 SAL 601 ~10 mC/cm2 (20 keV)

32. Chemically Amplified Triphenylenes An alternative two component crosslinking system, based on pendant epoxy groups and using the photoinitiator UVI-6976 (Triarylsulfonium hexafluoroantimonate salts) was developed.

33. Fine Patterning C5/Epoxy:C5/C0:PI (14:4:9) Film Line width = 44 nm Line dose = 0.8 nC/cm PEB 100 °C / 120 s Development in MCB for 20 s The pure epoxide has a sensitivity of 600 µC/cm2, which improves to 15 µC/cm2 when the photoinitiator is added (Ratio of derivative to PI - 2:1). i.e. 45 fold increase in sensitivity. How….?

34. ‘Photo’-Acid Generator Further Cross-Linking

35. Conclusions It is likely that the issue of polymer size will have to be addressed within the next 5 years, based on ITRS line edge roughness requirements. Several low molecular weight alternatives are approaching viability in terms of sensitivity, but at the cost of resolution, which must instead be maintained. Fullerene derivatives, with their extremely high etch durability are a good candidate for low energy applications. Sensitivities of 20 µC/cm2 and 30 nm resolution have been demonstrated. Epoxide functionalised chemically amplified triphenylenes have good sensitivities (15 µC/cm2), and promising resolutions (45 nm).

36. Thanks Dr Alex Robinson Dr H. Mohd Zaid Fran Gibbons Nanoscale Physics Research Laboratory University of Birmingham www.nprl.bham.ac.uk For use of some of their slides

37. Selected E-Beam Papers