Direct-Write Polymer Nanolithography in Ultra-High Vacuum

Deposition of materials in vacuum is essential to minimizing defects in electronic circuits.

June 1, 2012

Naval Research Laboratory, Washington, DC

The deposition of materials in vacuum is the foundational technology for creating modern electronic circuits; a vacuum being essential both to preserve the cleanliness of the substrate and the deposited materials, and to minimize the creation of defects. Consequently, most deposition techniques, from thermal evaporation to atomic layer deposition, require a high level of vacuum, preferably ultra-high vacuum (UHV), to be used effectively.

(a) Schematic of the **tDPN Process**, which uses a heated scanning probe microscope tip to deposit polymer from a moving tip. (b) Leaving the tip in contact, deposition is started and stopped by turning the heat on and off as shown by writing “UHV.” The PDDT was written on SiO₂ in UHV. The height of each polymer line was 20 nm, while the polymer width was 150 nm.

While the suite of established vacuum deposition technologies is vast and capable of highly precise deposition, there are relatively few methods to perform additive lithography in a single deposition step. Additive lithography deposits only the material that is needed for the intended device in the correct position. This is in contrast to the standard practice where an entire film is generated, and the great majority of this film is then removed. In addition to the benefit of reduced material cost, additive techniques have further benefits, including the ability to create softer, heterogeneous structures — such as polymers — that would be contaminated or destroyed by the multiple requisite coating and removal steps associated with conventional “liftoff” lithography.

To date, additive lithographies such as inkjet, dip-pen nanolithography (DPN), and micro-contact printing have been limited to deposition under ambient pressures, and therefore cannot achieve the benefits of the controlled environment under vacuum. One type of additive lithography is scanning probe lithography (SPL), where sharp probes either guide the deposition of material to a substrate, or modify previously deposited films. In the case of DPN, the atomic force microscopy (AFM) probe can be used to write a wide range of molecular inks with resolutions down to 15 nm. However, in conventional DPN, writing depends on the intrinsic fluidity of the ink molecules or on the creation of ink fluidity using solvents. Unfortunately, inks and solvents that have sufficient intrinsic fluidity for DPN evaporate quickly in vacuum.

This work demonstrates that thermal dip-pen nanolithography (tDPN) can deposit polymer nanostructures from a heated AFM tip in a high-vacuum environment. In tDPN, the probe temperature may be varied precisely within microseconds over a temperature range of 1000 °C. The probe temperature controls the viscosity of the coated ink, allowing independent control over the overall deposition rate, and the ability to turn off and on deposition. Many different materials, nanoparticles, and SAM molecules have been deposited using this technique.

Thermal DPN closely mirrors the capabilities of conventional DPN, but with greater control over the ink flow. Critically, the heat from the probes enables the deposition of high-meltingpoint inks such as polymers that also have low volatility and so may be deposited under a vacuum.

The initial approach for depositing organic inks was to attempt DPN with octadecanethiol (ODT), a classic ink for DPN that reproducibly transfers to the substrate. However, it was found that the ink on the DPN tip would invariably evaporate in the load lock chamber (~10-7 Torr), leaving insufficient coverage for observable deposition. Evaporation is readily observed visually since the ink leaves a haze on the tip that is absent after placing in a load lock chamber. This observation was more rigorously examined by creating a sample that mimicked the DPN tip surface chemistry: A silicon oxide on a silicon chip that was coated by holding it over ODT in a scintillation vial heated to 65 °C for 30 minutes. This procedure produced an ODT film that was 20-nm thick (measured by ellipsometry).

After placing the chip briefly under vacuum in a load lock chamber (~10-7 Torr), no ODT film was detectable. Additional attempts with less volatile inks — such as eicosanethiol — yielded similar results, leading to the conclusion that typical inks used in conventional DPN cannot be used for DPN under vacuum.

While alkanethiols could not be deposited, it was found that heated probes would retain and deposit polymer in UHV. For this work, the polymer to be poly(3-dodecylthiophene) (PDDT) was chosen, a conducting polymer that has found widespread usage in organic electronics. The probe temperature was controlled by applying current through the probe heater. One of the advantages of UHV tDPN is the lower melting point of inks under UHV. Because the molar volume of PDDT is lower in solid form than in liquid form, thermodynamics indicate that its melting point should drop as the surrounding pressure is lowered. The lower deposition temperature also reduces the risk of thermal damage when applied to prefabricated devices.

The molecular structure of the written PDDT monolayer nanostructure films depends on the chemistry of the silicon surface. Oxide termination leads to polymer side chains aligning perpendicular to the substrate, whereas silicon termination leads to the polymer lying flat. The thickness of the deposited polymer is a function of the speed of the scanning probe, and may be controlled monolayer-by-monolayer.

This new UHV-compatible direct-write technique should be of value both for nanoscale lithography of polymer structures and for the study of molecularly ordered polymer nanostructures. This result would also open a new method of studying polymer-semiconductor surface interaction at a molecular level, which is useful to develop polymer-based electronics compatible with inorganic semiconductor technology.

This work was done by Woo-Kyung Lee, Arnaldo R. Laracuente, and Paul E. Sheehan of the Naval Research Laboratory; Minchul Yang of the US Patent and Trademark Office; William P. King of the University of Illinois Urbana-Champaign; and Lloyd J. Whitman of the National Institute for Science and Technology. NRL-0059