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 Start with a wafer Deposit or grow layer of desired material Deposit or grow layer of desired material Spin coat photoresist Spin coat photoresist Pattern photoresist by exposure Pattern photoresist by exposure Etch layer underneath or deposit/grow new layer Etch layer underneath or deposit/grow new layer Remove photoresist Remove photoresist Repeat 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
Benefits of Micromirror Lithography Programmable “mask” Vs. permanent photomask Vs. permanent photomask No cost for prototype masks Vs. $millions per set Vs. $millions per set Instant mask production Vs. weeks to months per set Vs. weeks to months per set All of the above = Cheaper!