Micromirror Lithography David Chen EECS 277
Overview • What is Lithography? • What are Micromirrors? • Successful Research • Future
What is Lithography? • It is the backbone of nearly all semiconductor device fabrication processes. • It allows for high precision and tiny devices. • It allows for mass production. • For the most part, it removes human error from the manufacturing process, giving high yield production.
What is Lithography? The procedure for lithography is as follows: • Start with a wafer • Deposit or grow layer of desired material • Spin coat photoresist • Pattern photoresist by exposure • Etch layer underneath or deposit/grow new layer • Remove photoresist • Repeat
What is Photoresist? • Light sensitive material • Changes molecular bond after exposure to certain wavelengths of light • Used to make planar patterns determined by photomasks • Protects layers underneath during etching • Can be easily removed after each process
Example of Lithography • A familiar device: CMOS inverter • Link • 7 different photoresist patterns • 7 different photomasks • Very simple device
Masks Cost How Much?!? • Costs per mask-set • 65nm ~ $1million • 45nm ~ $2.2million • 32nm ~ $4million • 28nm ~ $8million • Bearable for high volume manufacturers • Outrageous for small companies or prototyping • Masks also deteriorate after use!
Why are masks so expensive? • Very high resolution structures require expensive processes such as electron beam etching or laser etching (also lithography) • Unique pattern means masks are custom made per client order • Need an alternative • What other things produce high resolution images… • AKA high definition images… • AKA HD…
We Already Have the Technology! • Made famous by Texas Instrument’s DLP (digital light processing) televisions • DMD- digital micromirror device • Project high resolution images
What Are Micromirrors? • MEMS (Micro-electro-mechanical systems) • Array of millions of mirrors on a tiny chip • Each mirror can be independently controlled • On/Off states
TV vs. Maskless Lithography • Even though they’re HD, TV pixels are still way too big for lithography • We are trying to pattern photoresist on the sub-micron scale! • Need a HUGE lens to focus the array to a smaller scale • Making a huge lens is hard.
One Solution from MIT: ZPAL • Using an equal number of array of diffracting lenses fabricated by lithography, each beam of light can be focused individually. • Feature sizes can be much smaller than the beam width!
Results from test • Well defined structures • Low roughness • Features on spot size scale • Done using UV light
Future Improvements • Smaller wavelength light • EUV (extreme UV) 120nm-10nm • X-Ray 10nm-1pm • Higher refractive mediums • Liquid immersed fabrication • = Smaller spot size
Benefits of Micromirror Lithography • Programmable “mask” • Vs. permanent photomask • No cost for prototype masks • Vs. $millions per set • Instant mask production • Vs. weeks to months per set • All of the above = Cheaper!
Resources • http://www.eetimes.com/showArticle.jhtml;jsessionid=ONC4WJBQYCVL0QSNDLRSKHSCJUNN2JVN?articleID=211100224 • http://jas.eng.buffalo.edu/education/fab/invFab/index.html • http://www.eetimes.com/showArticle.jhtml;jsessionid=ONC4WJBQYCVL0QSNDLRSKHSCJUNN2JVN?articleID=211100224 • http://nanoweb.mit.edu/zpal/Presentations/SPIE_2005_web.pdf • http://nanoweb.mit.edu/zpal/Presentations/EIPBN-2004-Alpha.pdf • http://nanoweb.mit.edu/zpal/Presentations.html • http://nanoweb.mit.edu/zpal/Presentations/EIPBN-2004-SSP.pdf