2. 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

3. – 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.

4. 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 10 23 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.

6. Processes 1 and 2 results in the mask - the equivalent of a photographic negative Laser Beam Glass Substrate Thin Chromium Layer UV Light Mask Lens Silicon Wafer with Layer of Photoresista Silicon Chips 1 2 3 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. 4 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. 1 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 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 Photolithography: The Basis of the Microelectronics Industry

7. Limitations to Photolithography The questions that need to be addressed in terms of nanoelectronics are, (i) can photolithography be used to create structures of less than 100 nm? and (ii) 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.

8. 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.

9. 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.

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

12. 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 Negative Tone Electron Beam Lithographic Resist Spin Coated 10 -100s nm

13. 3 4 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 The pattern is then doped with appropriate materials to create an active pattern, i.e. will conduct electrons 4

14. 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)

15. Poor Negative Tone Resolution Resist Good 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. Good Resolution Positive Tone Resist Poor Etch Durabilty Resist

16. 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.

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

18. 14 nm Scanning Electron Micrographs of Resist Patterns R = Pentyl 100 nm 35 nm 20 nm ScanningElectron 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.

19. Electron Beam Lithography Inducing Chemical Transformations Specific Chemistry

20. Patterning: Direct-Beam Writing ebeamebeam NO 2 NH 2 A single molecular monolayer

21. S NO 2 S NO 2 S NO 2 S NO 2 S NO 2 S NO 2 S NO 2 S NH 2 S NH 2 S NO 2 S NO 2 S NO 2 S HN S HN S NO 2 R 1 O R 1 O e-beam Au Au Au Background: Chemical Nanolithography with Electron Beams W. Eck, V. Stadler, W. Geyer, M. Zharnikov, A. Gölzhäuser, M. Grunze, Adv. Mater. 2000, 12, 805-808. Excellent system as chemical reactivity between nitro and amino group is different. AFM micrograph in frictional mode. And furthermore…

22. Film Formation Immerse Si/SiO 2 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 CHCl 3, EtOH and UHP H 2 O 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. SAM on Si/SiO 2 1.1 nm

23. (a) 3 min (e) 447 min (d) 273 min (c) 163 min (b) 97 min NO 2 ( 405.6 eV) NH 2 (399.6 eV) Intensity / arbitrary units 394399404409 Binding energy / eV XPS Chemical Modification Secondary back scattered electrons initiate the chemistry SAM Thickness= 1.2  0.2 nm Calculated = 1.1 nm

24. Immersion of the irradiated surface in a 10% TFAA solution in dry THF overnight Confirming the Chemical Transformation: NO 2 to NH 2 Immersion of the irradiated surface in a 10% TFAA solution in dry THF overnight XPS E-beam

25. Patterning: Direct-Beam Writing ebeamebeam SEM Image primary beam energy = 5 and 6 keV doses between = 25 and 300 µCcm -2 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.

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

28. Au SNOMSNOM Background: Scanning Near Field Photolithography Planar Surface

29. 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

32. 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

36. Conclusions

37. Soft Lithography

38. 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.

41. Self-Assembled Monolayer Formation Gold Substrate -H 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.

42. 2 1 3 Nano-Contact Printing 54 1 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). 3 The PDMS stamp is peeled of the master. 2 The liquid monomer is cured, to form the rubbery solid PDMS polymer. 4 The PDMS stamp is inked with a solution of the thiols, and pressed against a gold substrate. 5 The thiols form a SAM on the gold surface only where the stamp has been brought into contact with the gold. This technique can produce structures down to 50 nm lateral dimension and only one molecule thick (about 1 nm!). PDMS Monomer PDMS Stamp Inked with Thiols Gold Surface SAM of Thiol Master 50 nm

43. Scanning Probe Lithography

44. 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).www.almaden.ibm.com/vis/stm

45. 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.

46. 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) Dip-Pen Nanolithography 10 nm http://www.chem.northwestern.edu/~mkngrp/

47. Scientific American 2001

49. Potential Applications of Dip-Pen Nanolithography

50. 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. Further Reading on DPN

51. 200 nm 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. Writing by Inducing NanoOxidation on a Surface

52. TiO 2

53. SiO 2

54. Carbon nanotubes have previously been used as tips in atomic force microscopes (AFM) for producing images. But now for the first time nanotube tips have been used as pencils for writing 10-nm-width structures on silicon substrates. Ordinary graphite pencils write by wearing themselves down, but this is not the case with nanotube pencils developed at Stanford. The robustness of the nanotube tips permits a writing rate- --0.5 mm/sec---five times faster than was possible with older AFM tips. NANOTUBE NANOLITHOGRAPHY. http://www.stanford.edu/group/quate_group/index.html

55. The way the nanotube writes is for an electric field, issuing from the nanotube, to remove hydrogen atoms from a layer of hydrogen atop a silicon base. The exposed silicon surface oxidizes; thus the "writing" consists of narrow SiO 2 tracks. The Stanford results should help the development of nanofabrication, since tip wear problems have been an obstacle to the use of probe microscopes in lithography and data storage at the nm size scale.

56. Writing by Moving Individual Atoms Or Molecules! The AFM and STM can be utilised to move atoms and molecules which have been adsorbed to a surface, either by physically pushing the atoms/molecules (AFM) or picking them up by electrostatic forces (STM) and positioning them at another point on the surface. Thus, these processes in principle allow the creation of nanostructures. The SPM is being used as a robotic arm on the nanoscale, but is controlled from our macro world, to position individual molecules! Is this the Top-Down or Bottom-Up? The following pictures illustrate the power of these techniques for controlling the positioning of atoms on the nanoscale.

57. The Surface of Platinium (STM)

58. Xe on Ni (STM)

59. Quantum Coral Fe Atom Ring on Copper Carbon Monoxide Man CO 2 on Platininum Surface

60. Summary: Top-Down Approach PHOTOLITHOGRAPHY is the mainstay of the microelectronics industry for creating patterns on a surface. However, the miniaturisation that can be achieved will hit physical barriers in the coming years. E-BEAM LITHOGRAPHY is one methodology that is being employed in research labs as the possible successor to photolithography, for creating patterns on surfaces with sub 100 nm features sizes. The main problems that need to be confronted to make this process a viable methodology are the requirements to (i) increase the speed of the serial process and (ii) to have materials that respond efficiently to the e-beam. It should be pointed out that there are other techniques that are also being investigated, such as X-ray lithography. NANOCONTACT PRINTING is a simple methodology for creating nanostructures on surfaces by chemically imprinting structures. This process, however, is too slow to be used in the electronics industry as a mass production technology, but could be used to build prototype or very specialised devices. Nanocontact printing has other potential uses in areas such as sensing or biological evaluations.

61. SCANNING PROBE MICROSCOPIES are being used in several ways for creating nanostructures: (i)A modified AFM tip with an optiocal probe is used as a nanoscale light pencil to induce chemical reactions on a surface (Scanning Near Field Optical Lithography SNP) These approaches allow the creation of films one molecule thick, and with several nanometres lateral dimension. (iii)An AFM tip induces a chemical oxidation at the surface, and This approach allows nanostructures of 10 of nms to be created, but in principle, should be able to produce smaller structures. Unfortunately, the process produces oxides, which are generally not very good conductors. However, we are only limited by the imagination of the chemist to use other reactions in this process! (iv) An STM positions individual molecules on surfaces. This approach is the ultimate limit of fabrication. The control of matter on this length scale is already shedding new light on basic quantum physics (quantum coral). However, its use in the electronics industry for creating structures is a long way off: The process is extremely slow and generally requires extremely low temperatures in order that the adsorbates stick to the surface. (ii)An AFM tip is used as a nanoscale pencil to either deposit a chemical that will react with the surface (Dip-Pen Lithography)