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

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

4. 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, (ii)Resist Material requirements, (iii)Resolution (iv)Etch durability (v)Sensitivity (vi)Resist problems (beam speading, beam penetration, pattern collapse, line edge roughness), (vii)Why use low molecular weight materials (viii)Design considerationss for fullerene and triphenylene derivatives, (ix)Overcoming the sensitivity problem (low energy methodology and chemical amplification), Learning Objectives

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

6. Silicon “Organic” eeeeee eeeeee eeeeee eeeeee eeeeee 12 The unirradiated “organic” is removed with an organic solvent, leaving the cross-linked insoluble network pattern. The electron beam initiates a chemical reaction in the organic material, either (i) leading to fragmentation to smaller molecular components, which are soluble in some solvent (positive tone resist), or (ii) crosslinking to form an insoluble network (negative tone resist). 1 2 Serial Writing is very slow, compared to Photolithography Spin Coated 10 -100s nm Electron Beam Lithographic Resist

7. 3 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

8. 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)

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

10. Poor Resolution Negative Tone Resist ~70 nm Good Etch Durabilty Resist 6:1 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. Good Resolution Positive Tone Resist ~10 nm Poor Etch Durabilty Resist 1:1 Resist Resolution and Etch Durability

11. Photolithography Resist Sensitivity 0 50 100 110100 Exposure Dose (  C/cm 2 ) Normalised Film Thickness (%) D1D1 D2D2 Contrast:  = |log 10 (D 2 /D 1 )| -1 Sensitivity=D 50% D 50% PMMA = 140  C/cm 2 (20 keV) SAL601 = 8  C/cm 2 (20 keV)

12. Polymer Disadvantages Beam Spreading Pattern Collapse Line Edge Roughness

13. Beam Spreading and Penetration electron scattering simulations 20 keV Electrons3 keV Electrons 1 keV Electrons PMMA Si 500 nm

14. Pattern Collapse SiO 2 Developer

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

16. Line Edge Roughness Negative Tone CrosslinkingChain Scission Positive Tone

18. 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

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

20. Calixarenes Cyclic oligomer around 1 nm in diameter Negative tone electron beam resist 6 A chemically amplified epoxidised derivative has been demonstrated 7 J-I. Fujita et al, Jpn. J. Appl. Phys., 36, 7769 (1997); H. Sailer, et al, Microelec. Eng., 73 - 74, 228 (2004)

21. Catechols Cyclic oligmer around 1 nm in diameter with 3 aromatic rings per molecule Chemically amplified positive tone electron beam resist 8 Various functional groups allow the solvent to be altered N. Kihara, et al, J. Photopolym. Sci. Technol., 11, 553 (1998)

22. Fullerenes & Derivatives Aromatic cage molecule around 0.7 nm in diameter 60 carbons per molecule Negative tone electron beam resist 9 Various functional groups allow the sensitivity and solubility to be altered T. Tada, et al, Jpn. J. Appl. Phys., 35, L63 (1996)

23. Molecular Glasses Non-planar (propeller shaped) molecule around 1 to 2 nm in diameter Negative or positive tone electron beam resist 9 Chemical amplification has been demonstrated 10 M. Yoshiiwa, et al, Appl. Phys. Lett., 69, 2605 (1996); T. Kadota, et al, Chem. Lett., 33, 706 (2004)

24. Triphenylene Derivatives Liquid crystalline molecule around 1 to 2 nm in diameter Negative or positive tone electron beam resist 10 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)

25. LMW Resist Properties ResistSensitivity (µC/cm 2 ) Resolution (nm) Etch DurabilityCasting Solvent / Developer Calixarene> 700< 10ModerateUsually chlorinated/Xylene Calixarene [CA]1040ModerateMCB/MIBK Catechol [CA]1090GoodMethoxymethyl propionate / Aqueous Base Molecular Glasses 300070/150 (+ve/-ve) -THF/TMAH:IPA or 2-methoxyethyl acetate MG [CA]225-THF/TMAH:IPA

26. 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

27. Large flat  -surface Ordering Introduced strained cyclopropane ring Crosslinking increased X Y OO O X Y O n n ORRO RO RO OR OR Molecular Design Considerations High carbon content Etch Durability? Large  -surface Enhanced sensitivity?

28. 14 nm Scanning Electron Micrographs of Resist Patterns (20keV Beam) Sensitivity ~ 1000 µC/cm 2 100 nm 35 nm 20 nm Scanning Electron Micrographs ‘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. 2.5 nm

29. 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  C/cm 2 at 20-30 keV PMMA a factor of ~10 lower Comparison

30. 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/cm 2 ; triphenylene - 880 µC/cm 2 ) are still much lower that polymer based materials. Possible Solutions Low Energy Electrons Chemical Amplification

31. 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)

32. 20 keV MF02-01A 473 µC/cm 2 MF03-01 970 µC/cm 2 20 keV 1 keV MF02-01A 473 µC/cm 2 21 µC/cm 2 MF03-01 970 µC/cm 2 65 µC/cm 2 SAL 601 ~10  C/cm 2 (20 keV)

33. 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.

34. Fine Patterning The pure epoxide has a sensitivity of 600 µC/cm 2, which improves to 15 µC/cm 2 when the photoinitiator is added (Ratio of derivative to PI - 2:1). i.e. 45 fold increase in sensitivity. How….? 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

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

36. 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/cm 2 and 30 nm resolution have been demonstrated. Epoxide functionalised chemically amplified triphenylenes have good sensitivities (15 µC/cm 2 ), and promising resolutions (45 nm).

37. 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

38. Selected E-Beam Papers