Structuring of surfaces is at the heart of nano-technology and CMOS-electronics. For future CMOS technology, the quest to continue with Moore’s scaling is driven by the economic advantages of integrating more functionality on a given Si footprint and the performance gain that can be achieved by using overall smaller devices. According to the roadmap, high throughput lithography will have to cope with a 20 nm feature size in 2017 [1]. It is not clear at all, whether optical lithography can be extended towards this scale.

At the same time prototyping tools are required to help develop the next generation CMOS devices. Today, electron beam lithography (EBL) is a well established high-resolution technology used in many fields ranging from prototyping nanoscale devices in research to the fabrication of optical masks for CMOS fabrication. However, at scales of some tens of nanometers, EBL is difficult to control because of the proximity effect, i.e., the unwanted exposure of nearby areas by secondary electrons. Therefore, new beam methods are currently investigated, e.g. He⁺ beam lithography with strongly reduced proximity effects [2].
Alternatively scanning probe methods have shown that surface modifications can be achieved down to the atomic level [3] and have been used to fabricate nanoscale structures [4] and devices [5] of exceptional quality. In real-world applications, the production of nanoscale patterns and devices requires substantial throughput capabilities in combination with sufficient tip endurance to address areas on the order of (0.1-1 mm)² at high resolution. At a typical pixel pitch of 10 nm, this translates to 10⁸ - 10¹⁰ pixels to be written with a single tip. Therefore a highly sensitive patterning approach that is gentle to the tip is indispensable.

Patterning of resist by heated probes

Our preferred strategy is to use a polymer medium that fully volatilizes upon contact with a hot atomic force microscope (AFM) tip [7-9]. This enables a solvent-free, i.e. dry patterning approach without the need to expose the substrate and the resist to solvents, thus avoiding swelling induced distortions and cross-contamination of the sample. As schematically shown in Figure 1a our cantilever-style probe sensor is made from silicon and comprises a capacitive platform for exerting a loading force by electrostatic means and a resistive heater for heating the tip, which is integrated on top of the heater. A capacitive force pulse brings the tip into contact with the resist surface and the hot tip locally triggers the evaporation reaction. Two materials that react in the desired manner have been identified. Shown in Figure 1b is a phenolic molecule [6], which forms a molecular glass in the bulk of the material [7]. Six hydroxy groups are located at the periphery of the molecule and give rise to numerous hydrogen-bonding interactions in the bulk of the material, as inferred from the high glass transition temperature T_g of 126°C. The second material is a polyphthtalaldehyde (PPA) polymer [8, 9], which exhibits a low ceiling- (or decomposition-) temperature of ~ 150°C. In such a polymer, the breaking of a single bond induces the spontaneous depolymerization of the entire polymer chain [10, 11], a concept that was first demonstrated as a dry lithography approach in the early 80’s. Both materials provide the necessary stability at room temperature so that they can be imaged by the scanning probe and serve as etch masks for transferring the written patterns into the substrate of choice.

Figure 1: Patterning principle and materials.
a) The silicon cantilever has an embedded heater to locally heat the tip and thereby the resist in contact with the tip. The cantilever is pulled into contact on the timescale of one microsecond by applying an electrostatic force F_cap.
b) Chemical structure of the molecular glass and
c) the polymer used as resist materials, which are sensitive to the heat stimulus and react by spontaneous desorption of material.

Figure 2: Patterning results and transfer into silicon substrates. a) Pattern written 8-10 nm deep into an 18 nm thick molecular glass resist. Also shown are a comparison with the programmed pattern and (b) the same pattern transferred 400 nm deep into silicon. c) 25 nm tall and 4 μm wide nanoscale replica of the Matterhorn. d) AFM image of a fractal carpet pattern written 16 nm deep into a 50 nm thick polyphthalaldehyde film. e) SEM images of the pattern after transfer into silicon. The final silicon structures are 90 nm tall. f) 3D world map written into a 60 nm thick polyphthalaldehyde film. a), b), and c) adapted from [7]; d) and e) reprinted from [9]

Examples of patterns written using the two materials are shown in Figure 2 for the molecular glass and the PPA polymer resist in the top and bottom row, respectively. The AFM images in Figure 2a and 2d exemplify the quality of conventional 2D resist patterning. Patterning is uniform, resulting from well-controlled and reproducible single-pixel-writing events. Although a substantial volume of resist material has been removed from the surface, no pile-up or redeposition of material can be detected. The patterns were transferred into silicon substrates as demonstrated by the SEM micrographs shown in Figure 2b and 2e, using reactive ion etch methods described in detail in [7] and [9]. Using these transfer steps vertical depth amplifications factors of 50 and 6 (in 2b and 2e, respectively) have been achieved by exploiting the etch selectivity of the different materials. Furthermore, the well controlled single patterning events enable the fabrication of 3D structures. A replica of the Matterhorn created in a molecular glass film is shown in Figure 2c. It was made by consecutive removal of 120 molecular glass layers with defined thickness. Fine details of the original are reproduced in the nanometer-scale replica. The conformal reproduction proves that the final structure is a linear superposition of well-defined single patterning steps. For the PPA polymer, a linear force-depth relation in combination with a high writing efficiency render the material an ideal candidate for direct three-dimensional patterning. The world map shown in Figure 2e has been written in a single patterning step encoding the depth by a linear transformation of the world's elevation data to a force map. The image comprises 5 × 10⁵ pixels written at a pitch of 20 nm, and the writing of the entire relief was accomplished in 143 s.

The unique capability to create nanometer precise 3D structures makes the method a perfect technique for generating templates that can then be multiplied and printed by the technologies developed for nano-imprint lithography [12, 13]. We see potential applications in printing optics on chips [13], fabricating 3D nanomedical particles [14], or the creation of nanoscale 3D templates for shape matching self-assembly of objects such as nano rods or cubes [15].

References:

[1] International Technology Roadmap for Semiconductors, 2008 Update ( www.itrs.net/Links/2008ITRS/Update/2008Tables_FOCUS_B.xls ).
[2] J. Morgan, J. Notte, R. Hill and B. Ward, Microscopy Today, 14, 24 (2006).
[3] D. M. Eigler, E. K. Schweizer, Nature 344 , 524 (1990).
[4] see e.g. R. Garcia, R. V. Martinez, J. Martinez, Chem. Soc. Rev., 35, 29 (2006).
[5] A. Fuhrer, S. Lüscher, T. Ihn, T. Heinzel, K. Ensslin, W. Wegscheider and M. Bichler , Nature, 413, 822 (2001).
[6] A. De Silva, J. K. Lee, X. André, N. M. Felix, H. B. Cao, H. Deng and C. K. Ober, Chem. Mater., 20, 1606 (2008).
[7] D. Pires, J. Hedrick, A. De Silva, J. Frommer, B. Gotsmann, H. Wolf, M. Despont, U. Duerig and A. Knoll , Science, 328, 732 (2010).
[8] O. Coulembier, A. Knoll, D. Pires, B. Gotsmann, U. Duerig, J. Frommer, R. Miller, P. Dubois and J. Hedrick , Macromolecules, 43, 572 (2010).
[9] A. Knoll, D. Pires, O. Coulembier, P. Dubois, J. L. Hedrick, J. Frommer, and U. Duerig, Adv. Mater., 22, 3361 (2010)
[10] H. Ito and C. G. Willson, Polym. Eng. Sci 1983, 23, 331.
[11] H. Ito and C. G. Willson, Polym. Eng. Sci 1983, 23, 1018.
[12] M. Li, L. Chen and S. Chou, Appl. Phys. Lett. 2001, 78, 3322.
[13] K. Watanabe, T. Morita, R. Kometani, T. Hoshino, K. Kondo, K. Kanda, Y. Haruyama, T. Kaito, J. Fujita, M. Ishida, Y. Ochiai, T. Tajima, and S. Matsui, J. Vac. Sci. Technol. B 2004, 22, 22.
[14] J. P. Rolland, B. W. Maynor, L. E. Euliss, A. E. Exner, G. M. Denison, and J. M. DeSimone, J. Am. Chem. Soc 2005, 127, 10096.
[15] T. Kraus, L. Malaquin, H. Schmid, W. Riess, N. Spencer, and H. Wolf , Nature Nanotech. 2007, 2, 570.

[Released: October 2010]