Presently the most common method to fabricate rigid stamps (usually in silicon) and masters required for replica molding of the soft stamps is photolithography (see the protocol in ref.(1)). This process, widely used for microfabrication, is based on the selective removing of parts of a film upon the exposition to light (usually UV). This is achieved thanks to some materials known as photoresists, that are light-sensitive, meaning that, according to their composition, they become soluble or insoluble to their developing solution upon illumination. The first family is called positive photoresist and the second negative.
The light is sent through a photomask that shadows the parts of the film that, according to the kind of resist used, must be removed or preserved. Different kind of masks can be used but their common feature is the ability to shadow as much as possible some parts of the photoresist below (contrasting power), the most used are transparent foils with pattern drawn by an ink non-transparent to UV-light or highly reflecting foils (usually metallic) where some portion have been removed by laser engraving. The current limits of photolithography is approximately 250 nm and the minimum feature size is 100 nm (2). These values are important on the one hand to define the ranges of fabrication of soft stamps for LCW but on the other hand for the forthcoming comparison with the feature sizes and resolution that LCW can achieve. The masks are usually drawn using CAD software and then transferred either by common printers on transparent foils, where the marks will represent the shadowed areas, or by laser engraving of highly reflective metallic sheets where the removed part will corresponding to the illuminated path. The photoresist film, upon illumination is then developed in the appropriate solution leaving on the surface only the features that will be used as master for replica molding or directly as stamp.
ELECTRON BEAM LITHOGRAPHY
When the resolution limits required by the process are below those of photolithography (diffraction limit), electron beam lithography (EBL) is usually chosen for fabricating the masters. This technique is in fact able to achieve resolution of 20-30 nm in lateral size (see the protocol in ref.(1)), because it make use of an electron beam to locally alter the chemical properties of a material (resist) that will be then removed in a development step. Beside the illustrated resolution advantage one must remind that EBL is an expensive technology and it is a serial technique, thus it is much slower than photolithography.
Replica molding (RM) is the most common way of fabricating the elastomeric stamp for soft lithography and LCW and it is one of the most important tools for these techniques. RM is based on the reticulation of the elastomeric precursors onto the master that is then removed, upon curing, by peeling it off. It is worth mentioning that the LCW is not limited to the elastomeric materials as other soft-lithographic methods and it makes wide use of rigid and metallic materials as it is shown other sections of this protocol. Nevertheless replica molding of the elastomeric materials still plays a crucial role because, making use of well known materials such as Polydimethylsiloxane (PDMS), it allows tuning the surface properties of the stamp by liquid or plasma chemical treatments.
Elastomeric stamps made of PDMS are deformed easily under the effect of capillary adhesion. The attractive force, exerted by the solution trapped between the substrate and the stamp, may involve sagging of the PDMS stamp and poor patterning in the region where the PDMS displaced the solution. The capillary force may be estimated according to De Gennes,(3) and needs to be considered during the design of the spacer and the dimensioning of the full PDMS replica. In this case, an effective parameter is the aspect ratio AR, defined as the ratio of width over thickness of the window delimited by the spacer. If we put PDMS Young modulus E ≈ 460 kPa, contact angle of solution θ = π/6, and surface tension γ = 0.02 N/m (organic solvents), we derived the critical value for AR that will induce sagging for various distances between the substrate and the PDMS replica, by finite element calculation (Figure 1). The empirical relationship can be summarized with a linear abacus for fast design, providing that the overall dimension of the stamp is exceeding the window defined by the spacer.
Nanoimprint lithography (NIL) is another important technique useful to fabricate tools (i.e. master for RM and stamps) for LCW. NIL consists of a physical (morphological) deformation of a thermoplastic material in a temperature and pressure controlled printing process. A silicon stamp fabricated by a conventional lithography such as EBL or photolithography is usually employed. In NIL the thermoplastic material (usually a polymer) is deformed by pressing the stamp into the polymer at a temperature above the polymer's glass-transition temperature. The polymer is then cooled down below the glass-transition temperature and the stamp is removed.
VACUUM SUBLIMATION OF METALS
One of the advantages of LCW is the versatility with respect to the material of the stamp used for patterning. In this frame, a role is played also by metallic stamps, some of them (gold for instance) can be functionalized using thiol based Self Assembled Monolayers (SAM) to tune their surface properties. These stamps are usually fabricated by vacuum sublimation of a thin metallic layer on a master previously realized by one of the above mentioned techniques. For some substrates such as silicon or mica an adhesive layer (usually made of chromium or titanium) has to be previously evaporated on the underlying material to favour the stability of the metallic film.
COMMERCIAL AVAILABLE MASTER/STAMPS
Commercial metallic grids commonly used for Electron Microscopy experiments have proven to be very suitable for as a stamp for LCW (See Fig 2). When the grid comes as a large sheet that must be cut, it is crucial to cut it with very sharp scissors to avoid large deformations of the areas near the cut. For the most kind of patterning inexpensive masters such as Compact Disc (CD) Digital versatile disk (DVD) or diffraction gratings are commercially available. This kind of devices are made of: Blank CD, parallel stripes 1.5 µm pitch, 500 nm width and 220 nm depth. Blank DVD, parallel stripes with 750 µm pitch, 300 nm width and 110 nm depth. Written disks contain a pattern o doth and line with the same size containing an information in digital (binary) code. Figure 3 shows typical AFM images CD and DVD masters.