Gutenberg and New Top-Down Tools: Learning How to Make Things Small
18 Sep, 2007 01:44 pm
Projection lithography (photo lithography) is the preferred top-down tool for device fabrication. Projection lithography has been highly successful during the last thirty years, but improving projection lithography at scales smaller than 100nm has become increasingly difficult. Stamps have been used for centuries to decorate cloth and to print documents and books. One can use patterns produced using projection lithography as a stamp. Using thermoplast or transparent molds, nanoimprint lithography can produce features at the 20-30nm range today or one can use templates with sub 10nm features.
NanoPrinting: Stamps have been used for centuries to decorate cloth and to print documents and books. An expensively produced lithography pattern can be used as a stamp. Stamps can be used many times increasing the production efficiency. At first it is not clear whether stamping will work at small scales. But impressive progress during the last decade made imprint techniques a viable tool in already a number of applications. S.Y. Chou and his collaborators spin-coated a thermoplastic polymer, brought a mold into contact and heated the polymer above its glass transition and then cooled it down before separating the mold. The pattern of the thermoplastic is then transferred with reactive ion etching. This technique is labeled as T-NIL ( thermoplast nanoimprint lithography) or is also called hot embossing. If a transparent mold (silica) is used, it is possible to use photo curable resists. UV exposure solidifies the resist after the mold is brought into contact with the surface. This technique is referred to as P-NIL that stands for photo nanoimprint lithography and is also called cold embossing. Droplets of resist material can also be placed on the surface in a controlled way, known as step and flash lithography (S-FIL) (2). Another method consists of taking the mold or stamp with a superionic conductor such as silver sulfide. Applying a voltage to the stamp in contact with a metal surface, removes the metal from the surface and the pattern is generated electrochemically. Molds can be made wafer sized or they can be stepped. Imprint technology brings together high throughput, high resolution and low cost. Molds can have a topography that can reduce complexity in the fabrication process. The requirement for the material is less stringent then for UV lithography and the illumination sources and optics are less cost intensive. Precise position control gives the possibility to generate patterns smaller than the pattern size of the mold by multiple stepping. But molds might have defects or might be contaminated and need careful inspection including regular cleaning steps. T-NIL has, however the disadvantage that the thermal cycling might not be appropriate for deposition of multiple layers.
Electron beams: Molds with small critical dimensions can be fabricated by using electron beam lithography. Electron beam lithography is a high resolution top-down technique and is not diffraction limited. But e-beam lithography is not a parallel process and the sequential writing slows down the process. E-beam lithography is, however suitable for the production of molds. Multiple electron beam lithography is developed today but the throughput will still be small compared to intrinsically parallel processes such as projection and imprint lithography. NIL can produce features at the 20-30nm range today and is a potential tool for 32nm device fabrication.
Templates: It is possible to take nanoscale objects like nanotubes as templates. Taking nanotubes as template it was demonstrated that 2.4nm wide features can be produced (3).
Future applications: NIL is considered for production of patterned media as for DRAM and magnetic storage with future feature sizes <20nm. Patterns below the wavelength of light have profound influence of the optical properties of the solid and can be used to boost the efficiency of LEDís
We can say that ultraviolet imprint lithography is a nanopatterening method developed during the last 10 years which has capabilities to define geometries at the sub 10nm range, is close of being ready for production and comes at low cost compared to extreme UV projection lithography.
(1) S.Y. Chou et al., Science 272 (1996) 85
(2) Proc. SPIE-Int. Soc. Opt. Eng. 3676 (1999) 379
(3) F. Hua et al, Nano Lett, 4 (2004) 2467
(4) DJ Resnick et al J. Mat. Chem. 17 (2007) 3575