Atomic calligraphy - using MEMS to write nanoscale structures
(Nanowerk Spotlight) The difficulties associated with precisely manipulating nanomaterials to turn nanoscale structures into reliable functional devices – at a reasonable cost – is one of the key challenges that needs to be overcome in mass-manufacturing nanodevices (other than computer chips, which require massive amounts of capital investment).
One of the most restricting parameters in nanofabrication is the difficulty involved with controllably patterning materials at precise locations in a repeatable manner over relatively large areas. The traditional process of randomly placing nanomaterials on a substrate typically leads to highly variable performance of the resultant functionalized devices.
Conventional lithography methods that are used in computer chip manufacturing are not only very expensive and wasteful, they also are reaching physical limitations. To overcome these issues, researchers have been developing a range of alternative, resist-free nanopatterning techniques, among them dip pen nanolithography, oxidation nanolithography, or colloidal self-assembly (see: "3D nanolithography without the expensive hardware").
A novel microelectromechanical system (MEMS)-based mask writer has now been developed by a team of researchers at Boston University. The device allows to directly write structures at the nanoscale without the need to use photoresist, lift-off techniques or other complex and expensive approaches. The technique uses a MEMS plate with apertures drilled into it and a shutter so that one can, in effect, spray paint with atoms. With the shutter, the process can be turned on and off.
"Our results extend and build upon previous work," Bishop tells Nanowerk. "Other, previous techniques have used static stencils, AFMs with apertures on them, or dip pen lithography. Our approach helps mitigate some of the limitations of earlier approaches by allowing for arrays of devices, with more complex patterns using a wider range of materials than could be previously accommodated."
The researchers are confident that their approach opens the door to being able to build atomic scale devices using a cost-effective manufacturable process.
To fabricate their MEMS writers, the team uses a standard lithography batch method used in foundries. The chips are inexpensive single use devices enabling high flexibility and turnover.
False color SEM micrographs of the MEMS writer. (a) Electrostatic comb actuator (F = 0.924(nN)/(V2) × V2) and folded springs (k = 0.335 N/m). Preset leads enable electrical access. (b) Plate with apertures. Aperture diameter ranges from 0.05-2 µm. Tethers connect the plate to the folded springs. (c) Double plate design. Top plate functions as a high-speed shutter. (Reprinted with permission from American Chemical Society)
The individual writers, consisting of electrostatic comb actuators, folded springs and a central plate, sit on a 2.5 × 2.5 mm2 die stack of 600 nm silicon nitride and doped silicon handle 675 µm thick. The central plate is suspended over the substrate by four doubly folded flexure springs and tethers. The springs and tethers can be combined into a single device that can move laterally >10 µm in all four quadrants, using the substrate as an additional electrode enables z-axis pull in.
"Our design is similar to MEMS based nanopositioners described in the literature using both comb or piezoelectric actuation methods," notes Bishop. "The smallest feature or dot that can be patterned is defined by the aperture dimension. We use a focused ion beam (FIB) to mill an aperture in the plate of the writer. By leveraging the strengths of the scalable MEMSCAP PolyMUMPs process with the nanoscale resolution of a FIB we obtain MEMS devices 2 mm across with customized feature sizes below 50 nm."
Plate and shutter device. Optical image of (a) open shutter (VSH = 40 V) with two apertures visible and (b) closed shutter (VSH = 70 V) with one aperture visible. (c) Concentric circles drawn by a continuously open aperture. (d) Truncated concentric circles resulting from the opening and closing aperture. (e) Concentric circles side by side illustrate MEMS shutter functionality. (Reprinted with permission from American Chemical Society)
Bishop points out that this technology can be used to build electronic circuits and structures in situ out of materials typically not used in nanolithography.
"The integrated MEMS shutter makes it possible to control stochastically the number of atoms passing through the aperture down to of order one," he says. "Integrating this writer together with a MEMS based evaporator, as well as resonant sensors for deposition rate and temperature creates a cheap and versatile “'Fab on a Chip'. This will enable new mesoscopic experiments of quench condensed films, quantum dots, and single atom effects."