Lithography

MAIN TYPES OF LITHOGRAPHY: * Photolithography * Electron beam lithography –X-ray lithography –Focused ion beam lithography –Neutral atomic beam lithography * = most common, we will talk about these! Lithography = “Photoengraving” - Transfer pattern into reactive polymer film (“resist”) - Use resist to replicate pattern into thin film or substrate

“The invention of photolithography is arguably as important as that of the wheel, bronze, or movable type in terms of its impact on society. It is, however, a technology that is specialized for use in microelectronics.”

PHOTOLITHOGRAPHY What is it? A transfer of the pattern using an optical technique. Resist = photoresist, a photoactive polymer STEPS: 1.Coat substrate with resist 2.Mask 3.Expose with light 4.Develop (dissolve exposed (POSTIVE) OR unexposed areas with chemicals (NEGATIVE)) 5.Etch unprotected areas or deposit layer of metal 6.Strip resist (take it off)

Fun with Playdough…

The maximum resolution (minimum size) of individual features is limited by diffraction… Diffraction: bending of light - around an edge - through a slit - past an object (edges)

Awkward Cornell scientist showing photolithography…ridiculous music. I apologize.

Electron Beam Lithography E-beam: –Finely focused beam of electrons (few nm diameter) –Electrons deflected accurately and precisely to “write” pattern without mask Resolution –Diffraction not an issue λ < 1 A (0.1 nm) –Scattering (is an issue) Forward (in resist layer) Backwards (substrate)  It’s more expensive and WAY SLOWER

Electron Beam Lithography Electron scattering E beam loses energy as it enters a solid Electrons have elastic and inelastic collisions Leads to broadening of beam, exposure sites, images Depends on velocity of electrons, atomic number and density of substrate and resist

Scanning Electron Beam Lithography How it works… –Electron source (gun) Thermionic –Heat material until electrons emitted from surface Field emission –Use electric field to extract electrons from the tip of a very sharp point of tungsten –Electron column (forms beam) –Mechanical stage (moves around under beam) –Control Computer Controls subsystems and transfers pattern data

Scanning Electron Beam Lithography Mechanical Stage –Moves substrate through the deflected e-beam

Soft Lithography 2 Common Soft Lithography Techniques: Microcontact printing Nanoimprint Alternative to photolithography…it’s not used with metals or semiconductors (hence the name “soft”) Printing of Self-Assembled Monolayers (SAMs) Molding of liquid precursors

Microcontact Printing “Stamp” made –Pour liquid polymer into a mold to make a “stamp” –Mold often made by photolithography “Ink” the stamp –Dip into solution so SAM formed on surface of stamp Stamp the substrate –Place the inked stamp on a substrate –SAM transferred to substrate in specific pattern

Your turn… Pick a stamp… Draw out the process that would be required to make the shape of your stamp as a divot on a silicon wafer with an oxide layer. Imagine you poured the rubber material that makes up your stamp into the divot to create your stamp. Ink it and stamp it!

Microcontact Printing You can even roll out patterns since it’s soft!

Summary Photolithography –Is reaching size limits (diffraction, etc.) Soft Lithography –Is relatively new –Used in fabrication of nanostructrures & nanodevices