You Nits – a different way to think about nano-lithography
Once in a while, it is fun to go and listen to what the IC guys are up too…its jaw dropping stuff. The trouble with reeling off the capability in industry standard units is that it is impossible to put in any perspective. Units are important, or as my physics teacher used to say when we made a mistake “You nits”. Translation…in England a nitwit is a stupid person !
The 22 nm node for memory involves patterning 22 nm lines and spaces at 200 wafers per hour. One way to gain some perspective is to think about it terms of the smallest useful unit – an atom (I will herby define a British Standard Atom as 0.3 nm diameter) . Basically they are imaging a feature roughly 50 atoms wide, positioning it to within 10 atoms, with light with a wavelength equivalent to 500 atoms. All the while, the reticle stage is moving at 2 giga (10^9) atoms a second, accelerating at least 2 times gravity, and achieving an area defect density of 4 parts per trillion (10^-12) in the final pattern.
At SPIE Advanced Lithography, Directed Assembly Features (DSA) were shown down to 18 atoms (6nm) diameter and imprinted features on patterned media down to 21 atoms (7nm) diameter. So this is roughly 5,000 atoms in a cylindrical spot of similar height. In DSA, the length of a single block molecule is around 500 atoms, so there are 10 molecules in each feature. The shot noise from adding or loosing a single molecule is 10% !! For a molecule in the wrong place, the line edge roughness will be around 0.7 nm. In contrast, the monomers in the Molecular Imprints low viscosity ink jettable materials are around 10 atoms, so there are 50 molecules per feature and the shot noise will be 2% and the roughness as small as it can be – 1 atom. It is worth noting that 6 atom wide nanotube features have imprinted in research.
It looks like you have to have continuous features at this size range for the polymer features to survive. Most of the small features are lines which have decent structural stability. Molecular Imprints showed that isolated pillars are unstable, but holes in a continuous film are stable and then are image reversed to create a pillar in a patterned media.
With 10- 50 – lets pick 30 molecules to play with in a single feature, what are the other implications for resists ? If it’s a 2 component mixture that’s 15 of each type and you will have a binomial distribution of 15 events – 1 sigma that corresponds to a 5% change in molecular ratio, 3x that in solubility or 15% – with hopeless variation in line width. I think that this means that molecules must all be linked. As it turns out James W. Thackeray from Dow Electronic Materials showed some impressive 10 nm features in a chemically amplified resist with the photosensitizer bound to the resin at Advanced Lithography this year.
About the Author
Mike Watts has been patterning since 1 um was the critical barrier, in other words for a longtime. I am a tall limey who is failing to develop a Texas accent here in Austin. I have a consulting shingle at www.impattern.com.
My blog “ImPattering” will focus on the latest developments in the business and technology of patterning. I am particularly interested in trying to identify how the latest commercial applications evolve.