In addition to lithographic techniques like photolithography and electron beam lithography used commonly in producing nanotechnology related devices, a rapidly emerging technique is soft lithography.
Soft lithography is so called because it utilises cast moulded stamps made from flexible materials.
The process begins with the creation of a master. The master is made by etching a blank – normally a silicon wafer – with a negative photoresist. This gives a raised pattern of nanometer sized features on the silicon wafer that corresponds with the required channels in the polymer stamp.
A liquid polymer is then poured on top of the silicon wafer mould. The polymer is commonly a resin like PDMS (poly(dimethylsiloxane)) or fluorosiliconeheat. The polymer is heat cured and peeled off the mould.
The mould can now be used in a number of ways. These various alterations to the process determines the sub type of soft lithography.
Types of Soft Lithography
Micromoulding in Capillaries (MIMIC)
The stamp can be brought into contact with a solid substrate and capillary action used to add a polymer material to the channels. The polymer is cured and the stamp removed leaving a pattern with features as small as 1 µm.
Near-Field Phase Shift Lithography
Near-Field Phase Shift Lithography uses a transparent PDMS phase mask. The mask is placed on a photoresist and exposed to light. The relief on the mask shift the light phase and allows features between 40 and 100 nm to be produced on the photoresist.
A prepolymer or ceramic precursor is added to the stamp which is placed on a substrate. After curing the stamp is removed leaving a pattern with features down to 250nm.
Solvent-Assisted Microcontact Moulding (SAMIM)
In Solvent-Assisted Microcontact Moulding (SAMIM) the stamp is coated with a solvent and then placed on a polymer photoresist matched to the solvent. The solvent causes the polymer to swell and spread into the features of the stamp. The resulting photoresist canhave features as small as 60 nm.
In replica moulding the original master is not required to be a negative of the final piece. After producing a PDMS stamp of the original master, the stamp then is used as a secondary master and another stamp made from it. In this manner the original master cannot be damaged or degraded as multiple copies are made.
In microcontact printing the stamp is ‘inked’ with selected chemicals, normally alkanethiols. The stamp is then pressed onto the substrate and removed leaving a 1 molecule thick layer with features down to 300 nm.
Advantages of Soft Lithography
Soft lithography is a low cost production method that allows for the creation of three-dimensional patterns at room temperatures and pressures. Additionally, because the stamp is flexible the substrate material need not be perfectly flat. It can be flat curved, spherically curved or in some instances contain surface features or roughness.
Applications of Soft Lithography
Soft lithography can be used for the production of:
• lab-on-chip systems
• microelectromechanical systems (MEMS)