Replication moulding

An alternative microfabrication technique is based on replication moulding. An elastomer is patterned by curing on a micromechanical mold. This technique is termed soft lithography , and is used to make blazed grating optics, stamps for chemical patterning and microfluidics devices. Soft fithography is essentially a subtractive method. 

Sha B, Dimov S, Griffiths C, Packianather MS (2007) Investigation of micro-injection moulding factors affecting the replication quality. J Mater Process Technol 183 284—296... 

The nanoimprint process utilises a patterned, 3D mould (template or stamp) to define patterns by embossing a soft polymer or liquid material. Once the material has completely filled the template cavities, it is hardened, using either a thermal or photochemical process, and the template is removed. The hardened imprinted polymer is an inverse 3D replication of the template mould. NIL uses a stamp or template to imprint or emboss a pattern into a polymer. The 3D polymer structures themselves may be used to create the desired nanostructures or alternatively, the polymer structures may be used as a protective mask to selectively protect a substrate during a subsequent process e.g. etching or deposition. 

The original motivation to develop the LIGA technology was to make micronozzles for the refinement of uranium isotopes [ 1 ]. These micronozzles have minimum feature of micrometers, but the overall sfructure is of millimetres. Fig. 2a is one of such micronozzles in PMMA resist. Fig. 2b is the electroplated nickel sfructure from the resist mould and Fig. 2c is the plastic replication of the micronozzle. 

Once the PMMA deep structures are made by X-ray lithography, the electroplating of metal and replication from the metal mould insert are all standard processes. The most commonly used metal for electroplating is nickel, because electroplating of nickel is a mature technique and nickel has the necessary hardness as a structure material for mould insert or functional microcomponents. 

In order to evaluate a material for its chemical stability it is necessary to test it under conditions which as nearly as possible replicate those likely to be encountered in service. One fairly simple test that can be applied to most plastic materials as part of an initial screening process, involves immersing moulded discs or bars for seven days in a variety of liquids at room temperature. This test uses a number of acids, alkalis and salt solutions of varying concentrations and a series of solvents with a range of polarities. At the end of the test the samples are examined for changes in dimensions, weight, colour and other physical properties. 

Biosensors made of plastic materials emerged because of their favourable properties cost effectiveness, suitability for mass replication based on injection moulding, excellent stability providing inexpensive, high-throughput, and large-scale devices with enviromnentaUy friendly disposability. 

The commercial introductimi of Uquid ciystal polymers did not occur until 1984 since liquid crystal polymers could not be injection moulded till then (Tai-Shung 2001). Nowadays, liquid crystal polymers can be melt processed on conventional equipment at fast speeds with excellent replication of mould. Some of these LCPs, which constitute as simple conunon displays, are found in calculators or mobile phones. However, the numerous and increasingly sophisticated applications, relying uprm the use of liquid crystalline materials, require a complexity of superior properties to achieve improved devices performance. Currently, LCPs play a dominant role in a large part of the display technology. 

---