Lecture 9 Unconventional Nanotechnology & Nanopatterning (~2 lectures) Scanning Probe Lithography (done) Soft-Lithography & Nanoimprint (Today)

Discussions/Questions Last Homework Discussions/Questions

What instrument would you modify? Name one major limitation. Homework/Test questions Name: Draw a conceptual picture of an Electric Scanning Probe Lithographic Instrument. Describe how it works? What instrument would you modify? Name one major limitation.

Understand the basic function of an electric scanning probe lithography ESPL instruments and draw a picture of an ESPL showing the basic elements (label at least 6 elements that are essential to pattern a surface). Can and Atomic Force Microscope be converted? and if yes what is needed for the conversion to work? Can an STM Microscope be converted to expose and electron beam sensitive resist? and if yes what is needed for the conversion to work? What is the difference in the feedback to operate an AFM when compared to an STM? What does this mean for the samples?

H+ H+ OH- OH- water meniscus you have modified an atomic force microscope by connecting an electrical lead to apply a voltage bias to the semiconducting AFM tip. The sample is titanium and you would like to form titanium oxide underneath the tip. Do you need to apply a positive or negative potential to the tip with respect to the sample to oxidize the sample? Do you expect that all metals can be oxidized? Are the voltages going to be different what is a minimal voltage that you expect using a first order estimate and looking at: water meniscus H+ H+ OH- OH- http://en.wikipedia.org/wiki/Table_of_standard_electrode_potentials

Exposure of Electron Sensitive Resists. You have modified and AFM to expose "e-beam" sensitive resists. What is the price of such an instrument? ........ $ AFMs cost ~300k, SEMs cost > 1M. What resolution (line widths) do you expect based on previous results (see results)? ........ What is the throughput in time per cm2 sized area? Hint: Maximum line speeds today are ~ 100 um/s. Assume that you were to try to expose the entire surface of a cm2 sized area with a single tip/beam. How does the throughput compare with traditional single cell e-beam lithographic systems? ........

Explain how KFM can measure the local surface potential by filling in appropriate words: KFM detects localized ................ forces between the tip and the sample to measure the surface potential of the sample. A feedback is used to adjust the DC tip ................. until the ............... force is zero. At this point the DC tip ........... is equal to the local surface ............

How do you record charge patterns and surface potentials without touching the sample? KFM Kelvin Probe Force Microscopy --- A quick excurse to a special AFM to map electrical charges and potentials Name two physical effects that will cause a variation in the surface potential?

Homework/Test questions Name: top view Element Work Function(eV) Aluminum 4.08 Beryllium 5.0 Cadmium 4.07 Calcium 2.9 Carbon 4.81 Cesium 2.1 Cobalt Copper 4.7 Gold 5.1 Iron 4.5 Lead 4.14 Magnesium 3.68 Mercury Nickel 5.01 Niobium 4.3 Potassium 2.3 Platinum 6.35 Selenium 5.11 Silver 4.73 Sodium 2.28 Uranium 3.6 Zinc You have IC circuit structure that contains a number of different materials with different electron affinities. The structure has a Al, Au, and Si regions as illustrated. Au Questions: Is there a electrostatic potential difference between the different regions even if you do not apply an external bias to the films? YES/NO If YES which material is most positive? (Au, Al, Si) which film is most negative? (Au, Al, Si) which is in the center? (Au, Al, Si) What is the potential difference between the Au and the Al? I indicated a 1 um in diameter half circle. Would you expect an electrostatic field along this line? YES/NO If YES how big would this field be? E= ... V / m Au Au Al side view Au Au Si Si 4.52eV

Surface Charge Double Layer Relationship. 1 2 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ Surface Charge Double Layer Relationship. 3 4 Given is the picture with an insulating charged layer that is 100 nm thick and charged to 100 elementary charges per 100 nm x 100 nm area. The layer extends in x-y direction. The charge sits on an insulator with er = 15. Underneath the insulator is a gold film. Questions: Is the picture missing image charges in the gold substrate? (YES/NO) The field above the surface at location 1 and 2 is constant/not-constant? The field above the surface at location 1 and 2 is zero/non-zero? The field inside the insulator at location 3 is zero/non-zero? The field inside the conductor at location 4 is zero/non-zero? Calculate the potential drop that is caused by the surface charge? delta V = ........ Volts Imagine an AFM tip above the surface. What potential do you need to apply to the tip to null out the attractive force? ......... Volts. You now replace the substrate with a semiconductor. Do you expect a larger or smaller voltage for the same amount of charge?

Electric Scanning Probe Lithography

Throughput vs. Resolution traditional $$$ unconventional (inexpensive, $0-$100k) unconventional (inexpensive, $0-$100k) traditional (>$2000k) traditional (available at most top 20 research universities $2000k) "cheap" < $200k

Electric Nanocontact Lithography Science 291, 1763 (2001). Proc. Nanostructured Materials 2004  Exposure times 10 seconds

George M Whitesides and His Group at Harvard Acknowledgements: A large part of the following slides have been provided by Dong Qin Center for Nanotechnology University of Washington who gave a talk on Soft-Lithography at the Nanotechnology BootCamp in 2006. George M Whitesides and His Group at Harvard

Parallel Techniques: Patterning with micro and nanocontacts... Rapid Prototyping Investment Return of Investment (ROI)

Hot Embossing, Injection Molding, Casting on a nanoscale... Old concepts with new names? Hot Embossing, Injection Molding, Casting on a nanoscale...

Emerging nanopattering methods (replication) pioneer (Whitesides) pioneer (Steven Choi) IBM Early adaptors (IBM and HP)

The Father of Soft-Lithography Soft lithography represents a new conceptual approach to the fabrication and manufacturing of new types of micro- and nano-structures at low cost

Definition of Soft-Lithography Soft-Lithography is the collective name for a set of new techniques: micro-contact printing (µCP), replica molding (REM), micro-molding in capillaries (MIMIC), micro-transfer molding (µTM), solvent assisted embossing (SAE), electric nanocontact lithography (ENL), nanotransfer lithography, micro and nanofluidics, etc. Everything that uses a patterned elastomer (soft mask) as a stamp, mold, or mask to generate patterns and structures instead of a rigid photomask.

Soft Mask Fabrication using PDMS PDMS: Polydimethylsiloxane Frankel & Whitesides

PDMS Stamp Two important properties: no adhesion to the substrate and no side chemical reactions taking place So, it is important that it has low interfacial energy and good chemical stability PDMS has good thermal stability and durability (can be used many times)

Demonstrate what conformal contacts mean. Soft-Lithography The key to soft lithography are elastomers that form conformal contacts provide a piece of PDMS. • Polydimethylsiloxane (PDMS, silicone) – Si-based organic polymer: (CH3)2SiO unit – Elastomer after curing: chemically inert, durable – Passes gas easily, not liquid – Good thermal stability (~186oC in air) – Optically transparent down to 300 nm – Isotropic and homogeneous – Good adhesion on Si and glass (surface modifiable) Demonstrate what conformal contacts mean. • Curing process – 10:1 ratio of PDMS mix, PDMS oligomer and cross-link agent (Sylgard184, Dow Corning) is cast on the mold and cured for 2 hour at 80oC in an oven.

Xia & Whitesides, Angew. Chem. Int. Ed. Engl. 1998, 37, 551.

Microcontact Printing Exploration of µCP for chemical patterning of surfaces and high-resolution lithographies. The transfer of ink from a relief structure to a target surface is a common process in classical printing techniques. In µCP this principle is used to fabricate chemical patterns with micron-scale resolution on technological surfaces. http://zurich.ibm.com

Microcontact Printing (µCP) Self-Assembled Monolayers (SAMs) Relies on self-assembled monolayers (SAMs) Formed by substrate (AU or Ag) immersion in ligand containing solution Thickness can be determined by changing methylene groups on alkyl chain

Microcontact Printing (µCP) PDMS stamps are wetted with ink (hexadecanethiol in ethanol) Stamps are placed on top of SAMs for a short time (10-20s) Ink transfers to the substrate and forms hexadedecanethiolate to generate patterns SAMS must be autophobic and they must form rapidly with high order

Microcontact Printing (µCP) PDMS stamps are wetted with ink (hexadecanethiol in ethanol) Stamps are placed on top of SAMs for a short time (10-20s) Ink transfers to the substrate and forms hexadedecanethiolate to generate patterns SAMS must be autophobic and they must form rapidly with high order

"Inks" /SAMs Substrate Molecules Au Ag Cu Pd GaAs InP Alkanethiols (RSH) and Alkyldisulfides (RS-SR’) Glass, Mica, Si/SiO2 HO-Terminated Polymer Alkylsilanes, RSiCl3 or RSi(OEt)3 Ag2O, Al2O3 Alkylcarboxylic Acids (RCOOH) ZrO2 Alkylphosphates (RPO3) Pt Alkylamines, Alkylisonitriles

Interfacial Engineering Copyright of Whitesides

Selective Attachment of Cells Chen & Whitesides et al, Science. 1997, 276, 1425.

Formation of CuSO4 Crystals Qin & Whitesides et al, Advanced Materials, 1999, 11,1433.

Patterns of Silver and Silicon Xia & Whitesides, Angew. Chem. Int. Ed. Engl. 1998, 37, 551.

Kumar & Whitesides et al, Langmuir, 1995, 11, 825

Microcontact Printing PDMS Stamp PDMS + Ink Si + Au 2. Gold Etch 3. Si etch 1. Print Si + Au + Ink High-resolution µCP: (a) Scanning electron micrograph of a stamp with 60 nm dots. The corresponding gold dots (b) fabricated by printing and etching were slightly broadened due to ink diffusion and substrate roughness. (c) The gold pattern served as a mask to etch the bare regions 250 nm deep into the underlying silicon by reactive ion etching. http://zurich.ibm.com

Microcontact Printing on Curved Substrates Rogers & Whitesides et al, Science, 1995, 269, 664; Adv. Mater. 1997, 9, 475

Problems Gold and silver are not compatible with microelectronic devices based on silicon (SAMs). This is a slight problem if the purpose of these is for the microelectronic world

Microcontact Printing of Proteins Nanotech User Facility at the University of Washington

(anti-Goat IgG – Alexa 488 and 594) Patterned Proteins (anti-Goat IgG – Alexa 488 and 594) Dong Qin

Dip-Pen Lithography (PDN) Compare with Serial Writing Techniques such as: Dip-Pen Lithography (PDN) Chad Mirkin at Northwestern University

Chad Mirkin at Northwestern University