Self Assembled Monolayers

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Self-assembled monolayers (SAMs) are attractive ultrathin organic films for patterning high resolution features on a number of technologically relevant surface substrates. SAMs are molecular assemblies that are formed spontaneously by the adsorption of a surfactant with a specific affinity of its headgroup to a substrate (Ulman 1991,1996). The attraction of these systems as ultrathin resists is driven primarily because SAMs eliminate depth of focus, transparency issues and can be prepared with a discrete number of well-defined chemical functional groups to permit further nanoscale materials attachment. Patterning SAMs can be achieved using a variety of lithographic techniques, which include (i) stamping or molding methods, i.e. soft lithography (Xie 1998), (ii) scanning probe microscopy (SPM)-based techniques such as 'scanning probe electrochemistry' (Sugimura 2002), 'dip-pen nanolithography' (Piner 1999), 'nanografting' (Xu 1997, Liu 2002) and 'nanoshaving' (Liu 2000), which utilise the mechanical interaction of an SPM tip with a surface as well as (iii) radiative sources including ultraviolet (UV)/visible light, X-ray and electron-beams (e-beam).

E-beam irradiation can either induce removal, crosslinking, damage, or modification of an internal and/or terminal functional group of SAMs (Process A-Scheme 1). Such systems can then be used in a second stage as general templates to direct the assembly of nanoscale components, to either the unirradiated (Processes B-Scheme 1) or the irradiated (Processes C-Scheme 1) regions, to create three-dimensional nanostructured surfaces.

Until recently, most work on e-beam irradiation of SAMs was focused on simple aliphatic and aromatic SAMs, and lateral dimensions down to 5 nm have been fabricated (Fig. 16.3) (Lercel 1996). E-beam irradiation of aliphatic SAMs (alkanethiols and alkylsilanes) has been shown to cause the loss of the orientational and conformational order (Zharnikov 1999, 2000), partial dehydrogenation, formation of C=C bonds, appearance of oxygenated functional groups, and partial desorption of the film fragments. Regarding aromatic SAMs (biphenylthiol on Au and hydroxybiphenyl on Si), electron irradiation induced primarily C-H bond scissions in the aromatic unit, with subsequently crosslinking between the neighbouring aromatic moieties. Structures with a lateral size below 20 nm have been fabricated. The increased etch resistance of the electron-irradiated aromatic SAMs make them a negative-tone electron resist for lithographic applications, whereas aliphatic SAMs, where electron-induced damage dominates over crosslinking, can be used as positive-tone e-beam resists.

Beyond the principal differences between aliphatic and aromatic SAMs, the molecular orientation and packing density also affect the extent of the irradiation-

induced modification (Frey 2002). Damage introduced into the film has also been found to be preferentially localised near the monolayer-vacuum interface, with the susceptibility to damage at this interface significantly increasing as the chain length increases (Olsen 1998). At the same time, the extent of the irradiation-induced desorption was found to be significantly enhanced by the introduction of sulphur-derived functionalities into the alkyl chain, as long as they were placed close to the SAM-vacuum interface (Heister 2004).

T SAM

T Modified SAM O Nanocomponents

Scheme 1. Schematic diagram illustrating precision chemical engineering by e-beam irradiation of SAMs (Process A-nanolithography), followed by selective self-organisation of self-assembled nanocomponents onto templated SAMs, i.e. nanoassembly. Nanoassembly can occur on the unirradiated (Process B) or irradiated areas (Process C)

T SAM

T Modified SAM O Nanocomponents

Scheme 1. Schematic diagram illustrating precision chemical engineering by e-beam irradiation of SAMs (Process A-nanolithography), followed by selective self-organisation of self-assembled nanocomponents onto templated SAMs, i.e. nanoassembly. Nanoassembly can occur on the unirradiated (Process B) or irradiated areas (Process C)

Fig. 16.3. AFM image of an array of ~ 5 nm dots generated in octadecylsiloxane SAM by a focused e-beam

Trimethylsilyl SAMs on Si/SiO2 can be removed by e-beam irradiation revealing the native SiO2, which can be used for adsorption of a second silane. This approach allowed the formation of a NH2-terminated SAM without an intermediate development step by adsorption of aminopropyltriethoxysilane to the exposed SiO2 surface (Krupke 2002, Liu 1999). Carbon nanotubes were shown to align well with the NH2 functionalised patterns (10-50 nm) on the surface. Similar templates have also been useful for attachment of a variety of materials relevant to biotechnology including 20 nm aldehyde-, and 40 nm carboxylic acid-functionalised polystyrene particles and biotin on silicon and gold substrates (Harnett 2001).

More interestingly, chemoselective processes promoted by e-beam irradiation have also been demonstrated. Grunze and co-workers (Eck 2000, Geyer 2001) demonstrated that nitro groups are selectively reduced to amine groups by irradiation of 4-nitro-1,1-biphenyl-4-thiol SAMs on gold substrates with low and high energy electrons, while the aromatic biphenyl layer is dehydrogenated and crosslinked. It has been suggested that the source of hydrogen atoms required for reduction of the nitro groups are generated by the electron-induced dissociation of the C-H bonds in the biphenyl moieties. Chemical patterns were generated by irradiation through a mask defining lines with a width down to 70 nm. Direct-write e-beam lithography achieved 20 nm resolution lines, which were suggested to be further reduced by using a highly focused e-beam (Golzhauser 2001). The templates were used for the surface immobilisation of fluorinated carboxylic acid anhydrides, rhodamine dyes and polystyrene brushes on the regions terminated by the NH2 groups.

A similar molecular transformation on nitro-terminated aromatic monolayers on Si/SiO2 substrate caused by low (Jung 2003, La 2002) and high energy electron beams has also been reported. Templates of reactive amino sites within the nitro-terminated monolayers were further employed to guide the assembly of 16 nm gold nanoparticles (Fig.16.4), to immobilise biotin and subsequently to bind Cy3-Tagged Streptavidin, and to initiate the ring-opening polymerisation of aziridine to form a hyperbranched polymer (Jung 2003). Cleavage of an internal disulfide group in a SAM of phenyl (3-trimethoxysilylpropyl) disulfide by high energy electrons has also been described (Wang 2003).

Fig. 16.4. AFM images of citrate-stabilised gold nanoparticles attached preferentially to the amino sites within the nitro-terminated monolayer

The resulting SH functionalised nanopatterns (35 nm) were further developed by reaction with N-(1-pyrene)maleimide. Brandow and co-workers (Dressick 2001) described an alternative e-beam patterning approach based on noncovalent entrapment of amine ligands in solvent-imprinted nanocavities on aromatic siloxane SAMs, and subsequent selective displacement of these ligands using high energy electrons. Amine ligands on the unirradiated regions were used to bind a catalyst capable of initiating the selective deposition of an electroless nickel film. Non-metallised features of 40 nm were generated in the nickel film.

Similar immobilisation systems have also been demonstrated by damaging/partially removing a functionalised monolayer, rendering that area of the monolayer inert for further attachment (Harnett 2000, Maeng 2003). NH2 patterned monolayers (80 nm) were used as templates for the site-selective immobilisation of palladium colloids, 20 nm aldehyde modified polystyrene spheres and 40 nm NeutrAvidin protein-coated polystyrene spheres onto the unirradiated areas of the monolayers (Harnett 2000). As discussed above, biomolecules and polymerisation of aziridine can be selectively immobilised on templates of reactive amino sites generated by e-beam irradiation on nitro-terminated monolayers (Jung 2003). Although demonstrated with micro scale resolution, a reverse system for immobilisation of these nanocomponents was shown to be more efficient. This system consisted of transforming an internal imine group of a benzaldimine monolayer into a nonhydrolyzable secondary amine group by low energy electron beams. Hydrolysis of the imine groups at the unirradiated regions yielded reactive primary amine functionalities for further modification (Jung 2003). This work clearly suggests that the efficiency of the chemoselective process promoted by the e-beam irradiation is crucial for the success of nanopatterning and subsequently building into the third dimension.

E-beam irradiation of a SAM exhibiting an initiator for atom transfer radical polymerisation has also been shown to create microscale structures of polymer brushes (Maeng 2003). For instance, the C-Br atom in a benzyl bromide moiety was selectively and homolytically cleaved by low energy e-beam through a mask, and the pattern was vertically amplified and transformed into patterned polystyrene and poly(methyl methacrylate) brushes at the unirradiated regions (Maeng 2003). E-beam irradiation has also been demonstrated to promote the fragmentation of furoxan on a furoxan imine monolayer to release nitrogen oxide and the concomitant generation of a carbon-carbon triple bond containing product on the surface (Kim 2003).

It is clear from the discussion above that e-beam lithography is capable of producing high-resolution patterns on SAMs with high reliability and precise control over the location of the feature. Moreover, the chemoselective reactions induced by e-beam irradiation widen the window of resist choice (Table 16.1). In contrast to an optical lithography system, the e-beam is not diffraction-limited, but it exhibits a low throughput. Nonetheless, since e-beam technology is commonly used in semiconductor manufacturing, several new parallel techniques/tools such as projection e-beam lithography (Harriott 1997), arrays of e-beam microcolumns (Chang 1996) and multibeam sources (Baum 1999, Pei 1999) have been developed.

Table 16.1. SAM systems exploited by e-beam irradiation

NANOLITHOGRAPHY

«S

NANOASSEMBLY

Subs/rale

SAM

Modified SAM

Resolution

Nanocomponents Irradiated Unirradiated

Au sl/slo; TiTiO,

-o

Disordered; De-kydrogenation: De sorption:

5 mil

SAM-»Bio-molecules; Carbon nano tubes: Polymer particles8

Au Si

-oo

Crosslinked

20 mil

-

-a

20 mil

Polymers; Organic moieties; Metal nanoparticles; Biomolecules; Dyes

Si/SiOn

-o

35 nm

Organic moieties

! -0

Entrap of 0

Si/SiO;

Displacement

40 nm

Metals

-a

Damage

80 nm

Metal and polymer nanoparticles

-d A

-O-Si-R—NH-v -d* \

J.i scale

Biomolecules; Polymers

Sir'SiO,

—O-Si—R—( -é ^

Damage

ji scale

Polymers

Si/SlOn

"0

—O-Si—R— -cf

-

-

aThese systems involve immobilisation of materials after formation of a second SAM on the patterned surface

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