Forming Nanostructures by the Top-Down Approach

1. Forming Nanostructures by the Top-Down Approach Photolithography and Microelectronics: Limitations Nanolithography: Electron Beam Lithography Scanning Near-Field Photolithography Soft Lithography: Chemically Printing on Surfaces Scanning Probe Microscopies: Writing on Surfaces

2. – Learning Objectives Part 3 – Top-Down Approach CHM4M2 – Nanoscale Science – • After completing PART 3 of this course you should have an understanding of, and be able to demonstrate, the following terms, ideas and methods. • (i) Appreciate what is meant by top-down and bottom-up wrt the fabrication of nanostructures • (ii) Understand the process of photolithography as applied to the microelectronics industry. • (iii) Understand the limits of photolithography. • (iv) Understand the process of e-beam lithography. • (v) Understand the process on scanning near field optical lithography • (vi) Understand the process of dip-pen nanolithography. • (vii) Understand the process of nanooxidation on surfaces, induced by SPMs. • (viii) Understand that SPMs can not only image, but draw and move particles on surfaces.

3. What is Meant by Top-Down? We discussed in Part 2 the Bottom-Up approach to nanostructures: Whereby atoms were assembled into molecules, and molecules into nanostructures (i) by covalent bonds (dendrimers), and (ii) by noncovalent bonds (supramolecules). The alternative approach is from the Top-Down: 6 x 1023 atoms of silicon (28 grams) can be continually divided until only two remain! This Top-Down approach has been enormously successful, and has been the mainstay of the microelectronics industry for the last forty years…but they are far from reaching 2 atoms Using a process called photolithography, feature sizes of less than 200 nm (about 1000 silicon atoms laid side by side) are routinely made on silicon chips. Additionally, using this technology 3 billion transistors per second are made in the US alone! Lithography Definition: A method of printing from a metal or a stone surface on which the printing areas are not raised but made ink-receptive as opposed to ink-repellent.

5. 1 A laser beam writes the circuit pattern for a microchip on a layer of light sensitive polymer that has been spun coated on a thin layer of chromium supported on a glass substrate. The irradiated polymer is selectively removed by a solvent. The unirradiated polymer film is left on the chromium. UV Light Laser Beam Mask Lens The exposed chromium is then etched away, by a chemical reagent, whilst the chromium that is covered by the polymer is not etched away. When the chromium has been removed to expose the glass, the rest of the polymer is then removed by an organic solvent. 2 Glass Substrate Silicon Wafer with Layer of Photoresista Thin Chromium Layer When a beam of UV light is directed at the mask, the light passes through the gaps in the chromium. A lens shrinks the pattern by focussing the light onto a layer of photoresist on a silicon wafer. 3 4 The exposed parts of the photoresist are removed, and the exposed silicon is etched away with a chemical reagent, allowing the pattern to be transferred to the silicon, resulting in the silicon chip. Photolithography: The Basis of the Microelectronics Industry 3 1 2 Processes 1 and 2 results in the mask - the equivalent of a photographic negative 4 Silicon Chips

6. Limitations to Photolithography • The questions that need to be addressed in terms of nanoelectronics are, • can photolithography be used to create structures of less than 100 nm? and • if so what is the limit of miniaturisation? Presently, the photolithography process uses wavelengths of UV light of <250 nm. To create structures, with dimensions less than 250 nm, using a mask with features less than 250 nm, will lead to diffraction of the UV light which blurs the projected image. This problem has been overcome by various technological breakthroughs related to the design of the mask. However, making mask structures less than half the wavelength of the light being used results in the projected image being so diffracted that it will no longer be viable. Thus, structures of sub 200 nm have been achieved, and with refinements of the technology there is still some scope for miniaturisation.

7. An obvious answer to this problem is to use UV light of even shorter wavelengths. Indeed, this avenue of research is being investigated, but there are at least two problems that need to be overcome, if smaller wavelengths are used: (i) Conventional lenses are not transparent to extreme (short) wavelength UV. (ii) The UV irradiation energy is inversely proportional to the wavelength and thus the UV light damages the masks and lenses. As you can imagine there is a great deal of research effort involving chemists to design, synthesise and characterise new materials that can address these problems.

8. What you continually have to bear in mind is that the microelectronics industry want to keep using this photolithography process for as long as possible, as the cost associated with building new fabrication plants using other technologies are huge. However, at some point the microelectronics industry will have to bite-the-bullet, and adopt new technologies if they are going to have increased capacity and performance. There are several technologies that are currently under investigation. Two of which are. X-Ray Lithography Electron Beam Lithography At this point, in time these two technologies look as if they may be able to be developed to a scaleable process for manufacturing silicon chips. We shall discuss only electron beam lithography.

9. Electron Beam Lithography Inducing Cross-linking or Cleavage of Bonds Non-Specific Chemistry

11. 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. Negative Tone Electron Beam Lithographic Resist Serial Writing is very slow, compared to Photolithography Spin Coated 10 -100s nm

12. 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 The pattern is then doped with appropriate materials to create an active pattern, i.e. will conduct electrons 4 4

13. 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)

14. Good Resolution Positive Tone Resist Poor Negative Tone Resolution Resist Good Etch Durabilty Resist Poor Etch Durabilty Resist Neither materials have good sensitivity towards the electron beam to make them crosslink efficiently, and neither can make a high resolution (thin) and tall (good etch durabilty) structures.

15. New Materials Used as Negative Tone E-Beam Resist These materials were shown to have better sensitivities toward the electron beam, but the etch ratios were still poor.

16. Sensitivity enhanced Sensitivity enhanced. Crosslinking increased Next Generation Resists Large p-surface Introduced strained cyclopropane ring Resolution equals or surpassed PMMA Etch ratio much better than SAL 601 Sensitivity much better than previous medium molecular weight materials

17. 14 nm ScanningElectron Micrographs 100 nm 35 nm 20 nm R = Pentyl Scanning Electron Micrographs of Resist Patterns ‘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.

18. Electron Beam Lithography Inducing Chemical Transformations Specific Chemistry

19. Patterning: Direct-Beam Writing e b e a m NH2 A single molecular monolayer NO2

20. e - b e a m N O N O N O N O N O 2 2 2 2 2 S S S S S A u N H N H N O N O N O 2 2 2 2 2 S S S S S A u R R 1 1 O O H N H N N O N O N O 2 2 2 S S S S S A u Background: Chemical Nanolithography with Electron Beams W. Eck, V. Stadler, W. Geyer, M. Zharnikov, A. Gölzhäuser, M. Grunze, Adv. Mater. 2000, 12, 805-808. AFM micrograph in frictional mode. Excellent system as chemical reactivity between nitro and amino group is different. And furthermore…

21. 1.1 nm SAM on Si/SiO2 • Film Formation • Immerse Si/SiO2 into 5 mM/anhy. THF under Ar • (Sonication at 25°C) • Reaction times: 2 hours • Sonicate twice in fresh THF for 5 min • Rinse intensively with CHCl3, EtOH and UHP H2O • Dry under Ar • Film Characterisation: • Contact Angle (surface type) • AFM (roughness) • Elipsometry (thickness) • XPS (elemental composition) NPPTMS Procedure from: N. Tillman, A. Ulman, J.S. Schildkraut, TL. Penner, J. Am. Chem. Soc., 1988, 110, 6136-6144.

22. NH (399.6 eV) 2 NO ( 405.6 eV) 2 (e) 447 min (d) 273 min Intensity / arbitrary units (c) 163 min (b) 97 min 409 404 399 394 (a) 3 min Binding energy / eV XPS Chemical Modification SAM Thickness= 1.2  0.2 nm Calculated = 1.1 nm Secondary back scattered electrons initiate the chemistry

23. Confirming the Chemical Transformation: NO2 to NH2 • Immersion of the irradiated surface in a 10% TFAA solution in dry THF overnight • E-beam XPS • Immersion of the irradiated surface in a 10% TFAA solution in dry THF overnight

24. P. Mendes, S. Jacke, Y. Chen, S.D. Evans, K. Kritchley, K. Nikitin, R. E. Palmer, D. Fitzmaurice,J.A. Preece, Langmuir, 2004, 20, 3766-3768. SEM Image Patterning: Direct-Beam Writing e b e a m primary beam energy = 5 and 6 keV doses between = 25 and 300 µCcm-2

25. Scanning Near-Field Optical Lithography Shuqing Sun, Karen S. L. Chong, and Graham J. Leggett* J. Am. Chem. Soc., 2002,124, 2414

27. Au Background: Scanning Near Field Photolithography S N O M Planar Surface

28. Nanoscale Molecular Patterns Fabricated by Using Scanning Near-Field Optical Lithography Shuqing Sun, Karen S. L. Chong, and Graham J. Leggett J. Am. Chem. Soc., 2002,124, 2414

31. Generation of Nanostructures by Scanning Near-Field Photolithography of Self-Assembled Monolayers and Wet Chemical Etching Shuqing Sun and Graham J. Leggett* Nano Letters 2002, 2, 1223-1227

35. Conclusions

36. Soft Lithography

37. Soft Lithography: Chemically Printing on Surfaces We are all familiar with chemistry in a round bottom flask, where reagent A and reagent B are both dissolved up in a solvent, and they then react to form product C, which still remains in solution. But there is a fascinating area of chemistry which utilises chemistry taking place on surfaces. This type of chemistry is a very mature area of science, because the applications of modifying surfaces are huge. For instance, surfaces can be made water repellent, corrosion resistant, non-stick and chemical resistant. The application of surface chemistry in novel lithographic techniques is an area which is currently receiving a great deal of research, because the structures which can be created are on the nanometre scale and are literally only one molecule thick. These novel lithographic techniques rely upon the formation of what are referred to as Self-Assembled Monolayers or SAMs. The most popular SAMs are formed between a gold surface and alkyl thiols.

40. -H Gold Substrate Self-Assembled Monolayer Formation The result of SAM formation is a highly ordered two dimensional solid of the organic moiety, as a result of the sulfur atoms being bonded in the three centre hollow of the gold atoms. These stable ordered structures allow SAMs to literally be written onto surfaces.

41. PDMS Monomer PDMS Stamp A monomer of PDMS is poured over a master, which has been produced by photolithography (200 nm features) or even electron beam lithography (20 nms). 1 Master 2 The liquid monomer is cured, to form the rubbery solid PDMS polymer. 3 The PDMS stamp is peeled of the master. PDMS Stamp Inked with Thiols 4 The PDMS stamp is inked with a solution of the thiols, and pressed against a gold substrate. The thiols form a SAM on the gold surface only where the stamp has been brought into contact with the gold. 5 Gold Surface SAM of Thiol 50 nm This technique can produce structures down to 50 nm lateral dimension and only one molecule thick (about 1 nm!). Nano-Contact Printing 2 1 3 4 5

42. Scanning Probe Lithography

43. Moving Atoms One By One to Create Nanostructures There are a group of techniques referred to as Scanning Probe Microscopies (SPM), examples of which are the Atomic Force Microscope (AFM) and Scanning Tunnelling Microscopy (STM). They have quite literally revolutionised the way the atomic world is viewed, and in part have been responsible for the increased research activity in nanoscale science Indeed, the significance of these techniques was recognised with the award of the Nobel Prize in Physics to Rohrer, Binning and Gimzewski in 1986. http://www.nobel.se/physics/laureates/1986/index.html The SPMs allow atomic mapping of surfaces, such that individual atoms on a surface can be visuallised, or adsorbate molecules on the surface can be visuallised (see Nature 2001, 413, 619-621), Additionally, they can induce chemical reactions on a surface. Furthermore, molecules and atoms can be moved and positioned on a surface (see www.almaden.ibm.com/vis/stm).

44. We shall look in more details at SPMs in Part 4, but the following examples illustrate the power of these techniques for creating nanostructures by, (i) depositing molecules onto a surface (Dip Pen Lithograpghy), (ii) Inducing chemical reactions on a surface (NanoOxidation), and (iii) Moving individual atoms/molecules on a surface. Additionally, the examples show how surfaces can be imaged at the nano and sub nanoscale.

45. 10 nm Dip-Pen Nanolithography http://www.chem.northwestern.edu/~mkngrp/ Dip-Pen Nanolithography (DPN) is an new Atomic Force Microscope (AFM) based soft-lithography technique which was recently discovered in the labs of Prof Merkin. DPN is a direct-write soft lithography technique which is used to create nanostructures on a substrate of interest by delivering collections of molecules (thiols) via capillary transport from an AFM tip to a surface (gold)

46. Scientific American 2001

48. Potential Applications of Dip-Pen Nanolithography

49. Further Reading on DPN D. Piner, J. Zhu, F. Xu, and S. Hong, C. A. Mirkin, "Dip-Pen Nanolithography", Science, 1999, 283, 661–63. Hong, S.; Zhu, J.; Mirkin, C. A. "Multiple Ink Nanolithography: Towards a Multiple-Pen Nanoplotter," Science, 1999, 286, 523-525. Hong, S.; Mirkin, C. A. "A Nanoplotter for Soft Lithography with Both Parallel and Serial Writing Capabilities" Science, 2000, 288, 1808-1811.

50. 200 nm Writing by Inducing NanoOxidation on a Surface An AFM tip in a humid atmosphere, such that a water condensate gathers at the tip substrate surface, can be utilised to create a conducting medium when a bias is applied between the tip and substrate. This conduction initiates an electrochemical oxidation of the surface as the tip is moved across the substrate surface, and a line of oxide is drawn across the surface.