Probe-based nanofabrication

Nanometer-scale direct-write 3D patterning using probes

Overview

Progress in nanotechnology is intimately linked to the existence of high-quality methods for producing nanoscale objects and patterns at surfaces. Scanning probe technologies are intrinsically capable of addressing real space with atomic resolution and have been used to fabricate nanoscale devices with exceptional quality.

Patterning
Figure 1 (a) AFM scan of a 8-nm-
deep pattern written into probe
resist using a patterning pitch of
29 nm. The gray-scale range is
12 nm. (b) SEM image of the
pattern transferred.

 
However, high-resolution patterning in combination with sufficient throughput remains challenging. We are studying two new resist materials that are highly responsive to the presence of a hot tip and react by local material desorption, thereby enabling in-situ metrology of the fabricated patterns.

One strategy is to use small molecules per se. These small molecules are bound into a glassy bulk state at room temperature by physical interactions (molecular glass). The material is reproducibly removed with nanometer-scale precision using write-pulse durations of 5 μs and tip heater temperatures of 300-500ˆC. A second approach is to use a polymer with low ceiling temperature, here a polyphthalaldehyde with a ceiling temperature of ~150ˆC (see Fig. 2a).

The backbone of the molecule is thermodynamically unstable, i.e., the molecule auto-unzips into its monomer constituents upon the breaking of a single bond. We found that a tip heated to ~700ˆC and pulled into contact for ~20 μs is sufficient to trigger the reaction and achieve a uniform patterning depth of 25 nm.

SMA actuators
Figure 2. Polyphthalaldehyde molecule. (a) AFM
image of a patterned 18×18 µm2 area using a
pixel size of 20 nm. Inset right: Close-up of the
area marked by the white box.
The patterning quality is excellent for both materials. The material is cleanly removed and no pileup or redeposition of material can be detected. Moreover, the material removal can be cumulated, thereby enabling the fabrication of 3D structure. The 2D and 3D structures created were transferred into silicon substrates using standard RIE technology. In addition, to enhance the aspect ratio, a three-layer transfer process has been developed, enabling vertical amplification of the written structures by a factor of 50 without significant loss of lateral resolution.

Using this new technology, it is possible to fabricate complex three-dimensionally textured substrates, e.g. for the guided and directed assembly of shape-matching objects. This technique also offers a cost-effective and competitive alternative to high-resolution electron-beam lithography in terms of both resolution and speed.

References

  1. P. Paul, AW Knoll, F. Holzner, U. Duerig,
    “Field stitching in thermal probe lithography by means of surface roughness correlation,”
    Nanotechnology 23(38), 385307, IOP Publishing, 2012, doi:10.1088/0957-4484/23/38/385307.
  2. P.C. Paul, A.W. Knoll, F. Holzner, M. Despont, U. Duerig,
    “Rapid turnaround scanning probe nanolithography,”
    Nanotechnology 22(27), 275306, IOP Publishing, 2011, doi:10.1088/0957-4484/22/27/275306.
  3. F. Holzner, P. Paul, U. Drechsler, M. Despont, AW Knoll, U. Duerig,
    “High density multi-level recording for archival data preservation,”
    Applied Physics Letters 99, 023110, 2011, doi.org/10.1063/1.3610490
  4. F. Holzner, C. Kuemin, P. Paul, J.L. Hedrick, H. Wolf, N.D. Spencer, U. Duerig, A.W. Knoll
    “Directed Placement of Gold Nanorods Using a Removable Template for Guided Assembly,”
    Nano Letters 11(9), 3957-3962, ACS Publications, 2011, DOI: 10.1021/nl202276q.
  5. A. Knoll, D. Pires, O. Coulembier, Ph. Dubois, J.L. Hedrick, J. Frommer, U. Duerig,
    “Probe-Based 3-D Nanolithography using Self-Amplified Depolymerization Polymers,”
    Advanced Materials
    , 22(31), 336-3365, Wiley Online Library, 2010.
  6. O. Coulembier, A. Knoll, D. Pires, B. Gotsmann, U. Duerig, J. Frommer, R. D. Miller, P. Dubois, J. L. Hedrick,
    “Probe-Based Nanolithography: Self-Amplified Depolymerization Media for Dry Lithography,”
    Macromolecules
    , 43(1), DOI: 10.1021/ma9019152.
  7. D. Pires, J. L. Hedrick, A. De Silva, J. Frommer, B. Gotsmann, H. Wolf, M. Despont, U. Duerig and A. W. Knoll,
    “Nanoscale 3D Patterning of Molecular Resists by Scanning Probes,” 
    Science
    , 22 April 2010: science.1187851v1-1187851. Abstract | Reprint