Forming Nanostructures by the TopDown Approach Photolithography and Microelectronics: Limitations Nanolithography: Electron Beam Lithography Scanning NearField Photolithography Soft Lithography: Chemically Printing on Surfaces Scanning Probe Microscopies: Writing on Surfaces
CHM4M2 – Nanoscale Science –
– Learning Objectives Part 3 –
TopDown Approach
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 topdown and bottomup 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 ebeam lithography.
(v)
Understand the process on scanning near field optical lithography
(vi)
Understand the process of dippen 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.
What is Meant by TopDown? We discussed in Part 2 the BottomUp 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 TopDown: 6 x 1023 atoms of silicon (28 grams) can be continually divided until only two remain! This TopDown 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 inkreceptive as opposed to inkrepellent.
Photolithography: The Basis of the Microelectronics Industry 1
2
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. 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.
3
1
UV Light Laser Beam
Lens
2 QuickTime™ and a Photo - JPEG decompressor are needed to see Thin this picture. Silicon Wafer with
Processes 1 and 2 results in the mask the equivalent of a photographic negative
3
4
Mask
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.
Glass Chromium Substrate Layer
Layer of Photoresista
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.
Silicon Chips
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.
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.
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 bitethebullet, 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. XRay 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.
Electron Beam Lithography Inducing Crosslinking or Cleavage of Bonds NonSpecific Chemistry
Negative Tone Electron Beam Lithographic Resist Serial Writing is very slow, compared to Photolithography
Spin Coated 10 100s nm
e e eee eeee eee ee e
1
“Organic” 1
Silicon
2
The unirradiated “organic” is removed with an organic solvent, leaving the crosslinked insoluble network pattern.
2
The electron beam initiates a chemical reaction in the organic material, either (ii) leading to fragmentation to smaller molecular components, which are soluble in some solvent (positive tone resist), or (iii) crosslinking to form an insoluble network (negative tone resist).
3
3
4
The pattern is then doped with appropriate materials to create an active pattern, i.e. will conduct electrons
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.
4
The Organic Material Requirements For a Negative Tone Resist
∙
Must interact with the electron beam
∙
Must crosslink 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)
Composite of Novolac Resin, Acid Generator and Cross Linking Agent SAL601
Poor Negative Tone Resolution Resist
Good Etch Durabilty Resist
Me n COOMe PMMA
Good Resolution Positive Tone 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.
New Materials Used as Negative Tone EBeam Resist Me O
O O OMe
5
J. Fujita, Y. Ohnishi, Y. Ochiai, S. Matsui Appl. Phys. Lett., 1996, 68, 1297
O
N
O
N N
N O
O
O
M. Yoshiiwa, H. Kageyama, Y. Shirota, F. Wakaya, K. Gamo, T. Takai Appl. Phys. Lett., 1996, 69, 2605
T.Tada, T. Kanayama Jpn. J. Appl. Phys., 1996, 35, L63
These materials were shown to have better sensitivities toward the electron beam, but the etch ratios were still poor.
Next Generation Resists Large πsurface
Introduced strained cyclopropane ring
RO
Sensitivity enhanced. O Crosslinking increased O X Y
X Y
OR
PMMA
O O n
Sensitivity enhanced
RO n
OR OR
RO
Resolution
20 nm
14 nm
Etch Ratio
6
6
20 nm (10 nm) <1 (<1)
Resolution equals or surpassed PMMA Etch ratio much better than SAL 601 Sensitivity much better than previous medium molecular weight materials
Scanning Electron Micrographs of Resist Patterns RO
14 nm
OR
ScanningElectron Micrographs RO
OR OR
RO
R = Pentyl ‘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, 425428. ‘Multiadduct Derivatives of C60 for Electron Beam NanoResists’ 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, 737743.
100 nm 35 nm
20 nm
Electron Beam Lithography Inducing Chemical Transformations Specific Chemistry
Patterning: DirectBeam Writing
e b e a m A single molecular monolayer
NH2 NO2
Background: Chemical Nanolithography with Electron Beams W. Eck, V. Stadler, W. Geyer, M. Zharnikov, A. Gölzhäuser, M. Grunze, Adv. Mater. 2000, 12, 805-808.
ebeam N O2 N O2 N O2 N O2 N O2
AFM micrograph in frictional mode. S S S S S Au N O2 N H2 N H2 N O2 N O2
S Au
Excellent system as chemical reactivity between nitro and amino group is different.
S
S
S
S
R1
R1 O O N O2 N O2 N O2H N H N
And furthermore… S Au
S
S
S
S
SAM on Si/SiO2
Film Formation Immerse Si/SiO2 into 5 mM/anhy. THF under Ar (Sonication at 25°C) Reaction times: 2 hours
NPPTMS O2N
O Si(OMe)3
NO2 NO2 NO2 NO2 NO2 NO2
1.1 nm
Sonicate twice in fresh THF for 5 min Rinse intensively with CHCl3, EtOH and UHP H2O Dry under Ar Film Characterisation:
O
O
O
O
O
O Contact Angle (surface type)
O Si O Si O Si O Si O Si O Si O O O O O O O Si Si Si Si Si Si
Procedure from:
AFM (roughness) Elipsometry (thickness) XPS (elemental composition)
N. Tillman, A. Ulman, J.S. Schildkraut, TL. Penner, J. Am. Chem. Soc., 1988, 110, 6136
XPS Chemical Modification
SAM Thickness= 1.2 ± 0.2 nm Calculated = 1.1 nm NH 2 (399.6 eV)
Intensity / arbitrary units
NO 2 ( 405.6 eV) (e) 447 min (d) 273 min (c) 163 min (b) 97 min (a) 3 min
409
404 Binding energy / eV
399
Secondary back scattered electrons initiate the chemistry
394
Confirming the Chemical Transformation: NO2 to NH2 NH2
• Immersion of the irradiated surface in a 10% TFAA solution in dry THF overnight
CF3 O
C
NH
O O
Si
O Si
O
O
O
Si/SiO2
Si/SiO2
NO2
O
Si
• Immersion of the irradiated surface in a 10% TFAA solution in dry THF overnight
Intensity (arbitrary units)
• E-beam
XPS
F (1s)
O
O Si/SiO2
700
695 690 685 Binding energy (eV)
680
Patterning: DirectBeam Writing
P. Mendes, S. Jacke, Y. Chen, S.D. Evans, K. Kritchley, K. Nikitin, R. E. Palmer, D. Fitzmaurice, J.A. Preece, Langmuir, 2004, 20, 37663768.
SEM Image e
primary beam energy
NO2
b e a m
= 5 and 6 keV doses between = 25 and 300 µCcm-2
NH2 5 µm
Scanning NearField Optical Lithography
J. Am. Chem. Soc., 2002, 124, 2414 Shuqing Sun, Karen S. L. Chong, and Graham J. Leggett*
Background: Scanning Near Field Photolithography CO2H
SH
S N O M
SO3
SH
Planar Surface Au
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
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
Conclusions
Soft Lithography
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, nonstick 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 SelfAssembled Monolayers or SAMs. The most popular SAMs are formed between a gold surface and alkyl thiols.
SelfAssembled Monolayer Formation
H
S H
Gold Substrate
S Au 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.
NanoContact Printing 1
2 3
2
1 PDMS Monomer
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). The liquid monomer is cured, to form the rubbery solid PDMS polymer. The PDMS stamp is peeled of the master.
3 PDMS Stamp
Master
4
5 PDMS Stamp Inked with Thiols
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.
Gold Surface
SAM of Thiol
This technique can produce structures down to 50 nm lateral dimension and only one molecule thick (about 1 nm!).
50 nm
Scanning Probe Lithography
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, 619621), 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).
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.
DipPen Nanolithography DipPen Nanolithography (DPN) is an new Atomic Force Microscope (AFM) based softlithography technique which was recently discovered in the labs of Prof Merkin. DPN is a directwrite 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)
http://www.chem.northwestern.edu/~mkngrp/
10 nm
Scientific American 2001
Potential Applications of DipPen Nanolithography
Further Reading on DPN
D. Piner, J. Zhu, F. Xu, and S. Hong, C. A. Mirkin, "DipPen Nanolithography", Science, 1999, 283, 661–63.
Hong, S.; Zhu, J.; Mirkin, C. A. "Multiple Ink Nanolithography: Towards a MultiplePen Nanoplotter," Science, 1999, 286, 523525.
Hong, S.; Mirkin, C. A. "A Nanoplotter for Soft Lithography with Both Parallel and Serial Writing Capabilities" Science, 2000, 288, 18081811.
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.
200 nm
TiO2
SiO2
NANOTUBE NANOLITHOGRAPHY.
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 10nmwidth 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/secfive times faster than was possible with older AFM tips.
http://www.stanford.edu/group/quate_group/index.html
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 SiO2 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
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 TopDown or BottomUp? The following pictures illustrate the power of these techniques for controlling the positioning of atoms on the nanoscale.
The Surface of Platinium (STM)
Xe on Ni (STM)
Quantum Coral Fe Atom Ring on Copper
Carbon Monoxide Man CO2 on Platininum Surface
Summary: TopDown 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. EBEAM 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 ebeam. It should be pointed out that there are other techniques that are also being investigated, such as Xray 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.
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) (ii)
An AFM tip is used as a nanoscale pencil to either deposit a chemical that will react with the surface (DipPen Lithography)
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.