Emerging nanopatterning

Transcription

1 Nanotechnology for engineers Winter semester Emerging nanopatterning Soft-lithography: Microcontact printing Nanoimprint Lithography Stencil lithography Dip-Pen lithography / Nanoscale dispensing Conventional Nanolithography Today 65 nm in IC production Cost of equipment Mio US$ Affordable only for mass products (IC s, memories) Complex Equipment Processes inflexible Bottleneck for Micro/Nanosystems outside semiconductor field MEMS, flexible substrates, sensors Bio and life science devices Need for alternative, low-cost lithography/patterning methods 2 1

2 Tools for writing: From macro to nano Liquid probes for dip pen lithography Nanoengineered pore in tip 35 nm Mirkin group Micromachined lever with integrated microchannel Ø ~ 75 nm Delivery of nanocrystals 75 nm dots 500nm CSEM Neuchatel 3 Tools for writing: From macro to nano Shadow printing / stenciling SiN Nanostencil Cave painting years ago Japanese stencil 200 nm 70 nm Au nanowire In-vacuo technique ( no contamination) Nanostencil EPFL 4 2

3 Material selection control Shutter control Sc an/d istance control Nanostencil Sample su rface Emerging nanopatterning methods (replication) Soft-lithography Nanoimprint lithography Nanostencil lithography 10 nm Candidate for Integrated Circuit 5 Emerging Nanopatterning Molecule Delivery Wet, soft-contact, 100 nm scale Thermo-mechanical Nano-imprint, embossing, hard contact, 10 nm scale Local Deposition Stencil, vacuum, no contact nm scale Single & Scanning de novo DipPen NADIS BioPlum Heated AFM AFM Nanostencil Parallel & Static Replication Softlithography NIL Membrane Nanostencil Si Aperture Substrate Evaporation Parallel & Scanning Adaptive massproduction Parallel NADIS/ BioPlum Millipede Smart Stencil Evaporation sources Beam collimator µ-shutter Aperture X-Y-Z Nano-patt ern scan 6 3

4 Outline Replication methods: Soft-lithography: Microcontact printing Nanoimprint Lithography Stencil lithography Dip-Pen lithography / Nanoscale dispensing 7 Soft-lithography Basic principle: Fabrication of replicating stamps/molds Use of elastomeric materials Material is transferred from stamp to substrate Fabrication of stamps: Mold fabricated by conventional technologies Filled and cured by a elastomer (Elastomer base + curing agent, e.g. PDMS) Inks: Biological molecules (Self Assembled Monolayer (SAM)) Proteins, enzyme Particles, diluted nanoparticles Metals 8 4

5 Microcontact printing (µcp) µcp is used for chemical patterning of surfaces and high resolution lithography. The transfer of ink from a relief structure to a target surface is a common process in ancient printing techniques. In µcp this principle is used to fabricate chemical patterns with micron-scale resolution on technological surfaces. 9 Microcontact printing High-resolution µcp: Scanning electron micrograph of a stamp with 60 nm dots. The corresponding gold dots fabricated by printing and etching were slightly broadened due to ink diffusion and substrate roughness. The gold pattern served as a mask to etch the bare regions 250 nm deep into the underlying silicon by reactive ion etching

6 Microcontact printing Examples of layered hybrid stamps: a) Scheme of trilayer stamp (hard backplane, elastomeric cushion, hard polymer) showing improved adaptation to an uneven substrate b) Trilayer stamp with 270 nm features c) Bilayer stamp with 5 µm features on a 125 mm glass plate d) Example of a two layer, thin film stamp composed of a 100 µm glass backplane and a 30 µm polymer film with 270 nm features. [From B. Michel, H. Schmid, Macromolecules 33 (2000) 3042]. 11 Microcontact printing Conformal contact between a soft stamp and a hard substrate. a) A stamp composed of a patterned elastomer and a flexible backplane b) The stamp adapts its protruding zones to the macroscopically uneven substrate and (inset) its microscopic roughness, whereas recessed zones do not touch the substrate. c) Dependence of maximal roughness amplitude for spontaneous formation of conformal contact on the roughness wavelength (l) for a stamp with a Young's modulus of 2.5 MPa and a work of adhesion of 0.1 J/m² (Sylgard 184, solid line). a modulus of 9 MPa and a work of adhesion of 0.03 J/m² (dotted line)

7 Biopatterning Microcontact printing (µcp) and microfluidic networks (µfn) are powerful techniques to pattern substrates with proteins. Ex. of applications of these techniques: a. Fluorescence from a patterned IgG monolayer on a glass slide created by µcp b. AFM image of a small stamped feature of antibodies on a silicon wafer c. A neuron and its axonal outgrowth on affinity-stamped axonin-1; d. Repetitive stamping of different proteins onto the same plastic substrate e. Water condensation pattern on micropatterned albumin forming droplets of ~2 µm in diameter f. Fluorescence micrograph of different proteins patterned by µfn 13 Biopatterning

8 Pros and Cons +Low cost + Large area + Curved substrate + Defects cause only local damage +Biotechnology + Plastic substrates + No photoreactive surface needed - Accurate? - Mechanical stability limits the design - Limited aspect ratio 15 Outline Replication methods: Soft-lithography: Microcontact printing Nanoimprint Lithography Stencil lithography Dip-Pen lithography / Nanoscale dispensing 16 8

9 Nanoimprint lithography (NIL) Imprint process: 1. Heat to T>T g 2. Bring stamp & sample into contact 3. Apply pressure 4. Cool down 5. Separation at T<T g 6. RIE to remove residual layer 7. Lift off or pattern transfer by RIE or use printed functional polymer film. Relatively simple process, min feature size is ~ 10 nm, low cost, scalable to wafer size. 17 History of Imprint 12th century metal type printing techniques were developed in Korea and in 1234 "Kogumsangjong-yemun" (Prescribed Ritual Text of Past and Present) Gutenberg introduced his press. 300 two-volume Bibles s compact disks (CD). 1996, NanoImprint Lithography, NIL, sub-10 nm feature size, high throughput and low cost. Rhind or Ahmes Papyrus ~ 1850 BC Density< 1kB/in

10 Main Areas of Impact Applications requiring pattern transfer over large areas with high throughputs. Cost-efficient applications (e.g. plastic electronics) Laboratory-scale exploratory work on nanotechnology. Patterning of functional materials 19 Minimum feature size trend 20 10

11 Throughput vs. Resolution Imprint process Left: adapted hydraulic press Right: OBDUCAT equipment NanoImprint NIL- 2-OB

12 10 9 Transition Imprint process Viscosity Glassy 10 6 Rubber elastic plateau Terminal Flow 10 3 T g Temperature 23 Imprint process T g Typical parameters used in NIL T 1 ( C) = 185 T 2 ( C) = 95 P (bar) = 30 Δt 1 (s) = 60 Δt 2 (s) = 160 Temperature & Pressure time cycles 24 12

13 Nanoimprint Process Model The area of contact is that of the elevated features. Distances over polymer is transported are short Localised flow 25 Nanoimprint Process Model Possibility of trapped air The more the mould penetrates into the polymer layer the larger the distances for and the material to be displaced. Complete filling. Further vertical motion requires much larger forces

14 Challenges in NIL a) Isolated structures: Insufficient flow b) Inhomogeneous pressure distribution c) Insufficient displacement of the polymer 27 Stamps Large features of a stamp prepared by UV lithography SEM micrographs of a silicon stamp produced by photolithography and dry etching. Left: lines with 10 µm period. Right: 200 nm lines with 800 nm period 28 14

15 Stamps Smaller feature stamps made by Cr lift off (Au forthcoming), focused ion beam (FIB), e-beam SEM micrographs of 60 nm high Cr stamp consisting of features with sizes of 50nm (left), 100nm (middle) and 400nm (right) with a 1:1 ratio of feature size to space between features. 29 Example: Nanoelectrodes SEM image a of Ni/Si stamp showing 90nm channel-length inter-digitated electrodes made by EBL and lift off, ready to print conducting polymers

16 Example: High density arrays of 50 nm dots Array of metal dots for e.g. magnetic memories, with density ~TB/in 2 31 NIL in mass production Pattern data file 100x 100x 1x EBL Master Imprint press 100x 100,000x 1 st generation blanks Ni platting baths 100,000x 1 st generation stampers 100,000,000x 1 st generation stampers imprint presses 2 nd generation blanks Ni platting baths 2 nd generation stampers Imprint presses End product 32 16

17 Outline Replication methods: Soft-lithography: Microcontact printing Nanoimprint Lithography Stencil lithography Dip-Pen lithography / Nanoscale dispensing 33 Nanostencil Lithography Advantages No resist coating No temperature steps No solvents Direct, local deposition Multiple-length-scales Full-wafer compatible Challenges Gap control Clogging Mechanical stability Alignment (multi-layers) 34 17

18 Fabrication of low stress SiN stencils a) LPCVD nm thick SiN b) Pattern definition in photoresist c) Pattern transfer into SiN d) Membrane window definition e) Membrane release by KOH etch Fabrication of nanoscale apertures in membrane can be done by: Focused Ion Beam Milling Electron beam lithography Laser interference lithography Nanoimprintlithography Deep UV lithography 35 Nanostencil lithography Nano-membrane Hair Nanostencil Nanostructure Low cost, flexible 70 nm nanowire Nanodevice Millions of nanostructures

19 Challenges Clogging Gap Geometry Stress Alignment 37 Membrane deformation: Stress The deformed membrane induces both an increased gap and an altered aperture shape. Deformation of a coated cantilever can be modeled by: Disequilibrium in strain distribution Equilibrium 38 19

20 Membrane stabilization by rims 39 Application of Si-supported membrane 2x2 mm 2 non stabilized stencil membranes after evaporation (50nm Cr) 2x2 mm 2 Si supported stencil membranes after evaporation (50 nm Cr) 40 20

21 Opportunities in nanostencil lithography Self assembled monolayers (SAM) MEMS Thin membranes CMOS 41 Nanostencilalignment Movie on Stencil Alignement! 42 21

22 Outline Replication methods: Soft-lithography: Microcontact printing Nanoimprint Lithography Stencil lithography Dip-Pen lithography / Nanoscale dispensing 43 Dip-Pen Nanolithography (DPN) A tip of an AFM operated in air is inked with a chemical of interest and brought into contact with a surface. The ink molecules flow from the tip onto the surface as with a fountain pen. The water meniscus that naturally forms between the tip and the surface enables the diffusion and transport of the molecules. Possible inks: Polymers Au DNA Antibodies D. S. Ginger, H. Zhang, and C. A. Mirkin, The Evolution of Dip-Pen Nanolithography, Angewandte Chemie - International Edition, vol. 43, no. 1, pp ,

23 Dip-Pen Nanolithography (DPN) 45 Dip-Pen Nanolithography (DPN) Thermally activated DPN probes on individually addressable cantilevers. Used for printing 1-octadecanethiol on Au surfaces. D. Bullen, S.-W. Chung, X. Wang, J. Zou, C. A. Mirkin, and C. Liu, Parallel dip-pen nanolithography with arrays of individually addressable cantilevers, Applied Physics Letters, vol. 84, no. 5, pp ,