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Research

IRG1, Chemically Advanced Nanolithography

Leader: Paul Weiss
IRG Faculty and Seed Faculty: Anne Andrews, Dave Allara, Ronald Hedden, Mark Horn, Tom Mallouk, Evangelos Manias, Gary Taylor and Paul Weiss.

In IRG1, Chemically Advanced Nanolithography, we exploit self- and directed assembly and selective chemistry in combination with conventional and other nanolithography tools to push forward fabrication techniques of devices with dimensions on the order and larger than 10 nm. The two inter-related research themes of this IRG are molecular rulers and chemical patterning. The emphasis on self-assembled structures naturally connects IRG 1 with IRGs 2 and 3.

Recent highlights:

Advanced Processing and Mechanistic Chemical Studies Provide Enhanced Molecular Ruler Performance (Weiss, Horn, Allara). Recently, molecular rulers research has brought the technique much closer to the molecular resolution that was expected of structures formed by the multilayer molecular self assembly method. Two improvements were fabrication of Au/SiO₂ test patterns with variable gap sizes to interrogate materials and process problems, and use of a Au/Cr/SiO₂ substrate to improve Au adhesion. We introduced a Cr wet and plasma processing mask, used Cr as the daughter metal, used a negative resist-liftoff-resist bilayer resist that improved resolution and afforded better liftoff selectivity and employed a better liftoff reagent. Narrow 20 nm gaps were made, but the edges were rough. A 248 nm excimer laser annealing step was used to smooth the Au line edges via nano flow to ~5 nm roughness, but with considerable linewidth loss. Nanogate and nanowire device structures have been fabricated with minimum features in the 20-60 nm size regime. Important challenges remain as molecular dimension gaps are approached including line edge roughness (LER), critical dimension control and applications of multilayer assembly on other substrates.

At molecular dimensions the chemistry of multilayer assembly on Au reaches elevated importance and has been thoroughly studied (Allara) using FTIR spectroscopy, XPS, ellipsometry and AFM. The lift off spacer is a stack of (alpha,omega(-C16-mercaptoalkanoic acid on evaporated gold with subsequent monolayer stacks attached via Cu ion binding and with a final capping layer of the C16 alkanethiol. For the COOH base SAM layer\ Cu ion layer\ capping layer assembly, the capping layer formed at 50% yield as distinct islands. The capping layer can be filled in to some extent, but never completely at 100% coverage by retreatment with copper ions and re-immersion in the alkanethiol solution. For the COOH base SAM layer treated wih the Cu ion the adlayer forms with dominant S-Cu-S binding at the interface, but with a significant contribution of S-Cu-(carboxylate) binding. The Cu coverage never achieves a full monolayer, even after re-immersion in Cu ions and then in mercaptohexadecanoic acid. The work also showed that odd methylene chain mercapto acids have a different assembly chemistry that results in poorly organized stacks and higher sorption of contaminants. In this case, there is inherent imprecision in ruler spacing that leads to very rough surfaces on the molecular scale, although some extent of smoothing is possible by re-immersion processing. Future studies will determine the actual topography of the rulers at various stages of the multilayer assembly process using noncontact AFM and in collaboration with Weiss and Horn, metrology studies on ruler fabricated metal gaps using the JEOL 4500 in conjunction with AFM using 1 nm single wall carbon nanotube tips.

SAMs on Non-traditional Substrates for Lithographic Patterning and Device Fabrication (Allara). High quality close packed thiol SAM films are easy to obtain on Au due to the Au-S bond strength and the structure of the Au surface. This is not true for Si, Cu & Al where flawed thiol and carboxylate SAM assemblies are formed. For device reasons this is essential if SAMs are to be useful in molecular size and nanoscale patterning on semi-conductor substrates. Consequently, methods for self-assembly of extremely high quality SAMs on GaAs, InAs and InP were explored for application to nanoscale patterning. Considerable success was achieved for alkanethiolate and rigid-rod thiolates on GaAs. A protocall was extensively explored and developed for obtaining high quality SAMs on three of the crystalline faces of GaAs. Complete removal of the surface oxide and exclusion of water and oxygen in all the processing were essential. The SAM films were characterized by contact angles, IR, ellipsometry, XPS, and NEXAFS at Berlin and Cornell (CHESS). Two unique SAM properties were high SAM ordering only for alkane thiols longer than C18 and biphenyl thiols with a terminal methyl group.

The new methods produce the best properties ever reported for SAMs on GaAs substrates and the first example ever of forming highly ordered SAMs on III-V semiconductors. The films were highly reproducible in their characteristics and were formed under processing conditions compatible with lithographic and device applications. Potential applications include improving electronic devices and optoelectronic devices using GaAs and related III-V materials. Lithographic studies and application to spintronic devices on epitaxial GaAs are future projects.

Novel Chemical Patterning: Displacement Printing with SAMs (Weiss). For 20 years microscopic SAM patterns have been transferred from soft and hard silicone polymer template stamps onto Au substrates. Resolution and reproducibility have been limited by the transferred SAM molecules which diffused onto the unstamped Au surface. Recent studies of chemical patterning by diffusing different alkane thiols into a preformed SAM layer of another alkane thiol revealed that the diffusion rates were highly dependent on the alkane thiol structures using contact and dip-pen deposited monolayers. 1-adamantane thiol, which assembled into highly ordered close-packed SAMs on Au, was observed to have very low lateral diffusion rates, but moderate to high vertical exchange rates with other alkane thiols due to its lower Au-S bond energy. These results lead to the experiment of microcontact printing decanethiol into an adamantanethiolate SAM/Au. Stable 1 micron lines of decanethiolate and alternate 1 micron lines of adamantanethiolate, were successfully fabricated as determined by lateral force microscopy. A broad variety of experiments have been conducted with different SAM pairs, and displacement patterning is being investigated for high resolution contact displacement and very high resolution dip-pen lithography. Important questions are line edge roughness, pattern transfer, defects and patterning on other substrates. This exciting new method has been widely acknowledged since its initial publication, including a press release with the NSF.

A hybrid lithographic process combining the best properties of the rulers process and the concept of displacement patterning has recently been developed and is discussed in the molecular ruler nugget. This new technique may realize its full potential in chemical nanopatterning applications such as nanoreceptor formation in biochemistry.

Sculptured Thin Films (STFs) for Nanosensors and Devices (Horn. Lathtakia). STFs are assemblies of parallel curvilinear nanowires that can be fabricated to design using physical vapor deposition techniques, such as thermal and arc evaporation, sputtering, and pulsed laser ablation. Until quite recently, STFs did not possess transverse architecture, had low growth rates and had transverse areas that rarely exceeded 1 sq cm without significant loss of transverse uniformity. The combination of large thickness (greater than 3 micron), large-area uniformity (75 mm diameter) and high growth rate (up to 0.4 micron /min) in assemblies of complex-shaped nanowires on lithographically defined patterns has been recently achieved.

The ability to sculpture the nanowires from a wide variety of solid materials, coupled with micro- and macroscale 1D and 2D topographic substrates, points towards new types of photonic, fluidic and sensor devices. We are using nanowire assemblies to study how nanopatterning of surfaces influences protein adsorption and cell behavior. We are pursuing research in photonic bandgap engineering because complex periodic features can be lithographically etched on large-area substrates with lattice sizes greater than 50 nm. We are collaborating with civil engineers and microbiologists to create sensors for bioremediation, where patterning can facilitate bioreduction of FeOx or CrOx STFs, thus indicating the proper environment for reduction of hexavalent uranium or chromium. We were the first to demonstrate polymer STFs, the first to demonstrate circular Bragg phenomena in SnO_n, and the first to correlate changes in the circular Bragg phenomena in TiO₂ induced by nanoengineering the serial bi-deposition process and post deposition processing. In addition, we have promising results using TiO₂ STFs in a Gratzel-style solar cell application through research conducted by a MRSEC summer REU student.

Neurotransmiter Chip & Nanoscale Biomolecular Probes (Andrews, Weiss, Pishko). The goal of the second sensor project is to create surfaces that are optimally functionalized with dilute small molecule probes that can capture specific molecules from complex mixtures. Patterned multiplexed substrates will be used for functional proteomics (vide infra). In addition, such surfaces will be used to select RNA aptamers to detect the molecules for which they are selective (initially neurotransmitters). The probe molecules must be confined to ~5 nm distance at a total concentration of ~0.5%, a challenge both for placement and analytical measurement.

Surfaces have been prepared using three different neurotransmitters and related molecules-serotonin (5-HT), 5-hydroxytryptophan (5-HTP, the biological precursor of serotonin), and dopamine all bound to and diluted in a matrix of oligoethylene glycol tethers. Grazing angle FTIR, ellipsometry, QCM, fluorescence, and other techniques were used to characterize surface functionalization and binding. These surfaces have greatly improved biospecificity over those previously prepared because of optimization in self-assembly processing and the use of new methods for placing the neurotransmitter tethers. The surfaces were treated with serotonin, dopamine and tyrosine hydroxylase antibodies, recombinant serotonin receptor proteins (a membrane protein); and a range of notoriously sticky proteins such as fibrinogen and fibronectin. Initial results show excellent biospecificity in each category. Particularly noteworthy is the biospecific capture of membrane-associated serotonin receptors that natively recognize free serotonin in solution and which are notoriously difficult to capture for proteomics.

The long-term impact of this work will be to enable functional proteomics of the brain proteins that will be associated not only with their particular function (i.e., the neurotransmission systems of which they are a part), but also under what (genetic, environmental) circumstances and in what subregion they are expressed. Even broader impact will occur in the design and development of multiplexed micro- and ultimately nano-biosensors for neurotransmitter sensing in the brain. The impact of this work goes beyond developing a greater understanding of the brain to the improved development of tethered small molecule probes that will be useful in chemical warfare agent and other small molecule sensing applications.