MEEN 489-500 Nanoscale Issues in Manufacturing Lithography Lecture 1: The Lithographic Process 1
Discuss Reading Assignment 1 1 Introducing Nano 2 2 Size Matters 3 3 Interlude One-The Fundamental Science Behind Nanotechnology Electrons, Atoms, Ions, Molecules Metals, Ceramics, Polymers Biosystems Electrical Conduction and Ohm s Law Quantum Effects Optics 2
Nanofabrication - 2 principal approaches Top-Down Miniaturizing existing processes at the Macro/Microscale Traditional approach in industrial applications E.g. Lithography, backbone of computing systems Bottom-Up Assembling structures from the atomic/molecular level Novel approach, conceptually imitating nature E.g. chemical self-assembly 3
Lithography Lithography in Art How lithography works Materials used for lithography drawing Photolithography Photolithographic process 4
Lithography in Art Invented by Alois Senefelder in 1798 Used for book illustrations, artist's prints, packaging, posters etc. In 1825, Goya produced a series of lithographs. In the 20 th and 21 st century, become an important technique with unique expressive capabilities in the Art field 5
How Lithography started Lithography (Greek for "stone drawing") relies on the fact that water and grease repel Draw a pattern onto a flat stone surface with a greasy substance Paint the printing ink onto the stone While the stone background absorbs water, the greasy substance retains wet ink on top Press paper against the stone to transfer the pattern Positive! Repeatable! 6
Materials used for lithography drawing Litho crayons and pencils (containing wax, pigment, soap and shellac), conte crayons, pens and graphite pencils, etc. Bulls of Bordeaux by Goya 7
Lithography, to date Miniaturized computing circuits require mass manufacturing of small features push lithographic approach to new limits Some lithography approaches for manufacturing Optical lithography (including ultraviolet) X-Ray lithography Electron Beam lithography Ion Beam lithography Dip-Pen lithography 8
Optical/UV Lithography Workhorse of current chip manufacturing processes Limited by wave length of light employed Smaller features reduce wave length UV light Here is how it works 9
Photolithographic process Wafer cleaning Barrier layer formation Photoresist application Soft baking Mask alignment Exposure and development Hard-baking 10
Wafer cleaning Cleans wafer chemically to remove particulate matter on the surface (traces of organic, ionic, and metallic impurities) 11
Barrier layer formation After cleaning, silicon dioxide (a barrier layer) is deposited on the surface of the wafer 12
Photoresist application After the formation of the SiO 2 layer, photoresist is applied to the surface of the wafer Spin coating produces a uniform thin film There are two types of photoresist: positive and negative 13
(Photolitho_main_process.jpg) http://britneyspears.ac/physics/fabrication/photolithography.htm 14
Soft baking During this stage, most of the solvents are removed from the photoresist coating The photoresist coatings become photosensitive (imageable) after softbaking 15
Mask alignment Use a photomask - square glass plate with a patterned emulsion of metal film on one side The mask is aligned with the wafer to transfer the pattern onto the wafer surface. 16
Exposure Photoresist - exposed through pattern on the mask with a high intensity ultraviolet light. Three primary exposure methods: - contact - proximity - projection 17
(plithp2.gif) http://www.ece.gatech.edu/research/labs/vc/theory/photolith.html 18
Development At low-exposure energies, the negative resist remains completely soluble in the developer solution a a solvent mixture delivered the resist to the surface and the masked area did not chemically change. For positive resists, the resist solubility in its developer is finite even at zero-exposure exposure energy the masked area did not chemically change, the exposed area must become MORE soluble. 19
(plithp3.gif) http://www.ece.gatech.edu/research/labs/vc/theory/photolith.html 20
Hard-baking The final step in photolithographic process Harden the photoresist and improve adhesion of the photoresist to the wafer surface 21
References: http://www2.mmlc.nwu.edu/c303/levavy/lith1.html http://www.ece.gatech.edu/research/labs/vc/theory/pho tolith.html http://www.ece.gatech.edu/research/labs/vc/theory/pho tolith.html http://en.wikipedia.org/wiki/photolithography http://britneyspears.ac/physics/fabrication/photolithogra phy.htm 22
Assignment Review this PowerPoint set. 23