Dear Sameer, you could think about using Si and prepare microchannels by etching (wet and dry) and lithographie technologies --> with this your fabrication cost will be high, but the final reslution and reproducibility will be good. You could also work with patterned Si wafers and a subsequent hot embossing in of the structures in polymers like PMMA or others. On the other hand you could work with typicall low cost polymers using laser micormachining together with laminating technologies. Check  our institute website for some more input:  http://www.enas.fraunhofer.de/en/core_competences/polymer_based_technologiesformicroandnanosystems.html

Frank Roscher: Thanks for support.

what are the constraint must be keep in mind during manufacturing of microchannels.

It all depends what you want to put in your microchannels.  Also, you need to bear in mind the resources that you have available- clean rooms, cash to contract it out etc.  For silica or silicon nitride, the processing will be familiar to the microelectronics manufacturers and there are well-established reproducible processes, usually wet or dry etching with HF.  Polymers are easy to prototype yourself, but the most widely-used- PDMS- is highly compliant, unable to resist pressure, permeable to gases and hydrophobic.  Thermoset polyester is alsmost as easy to work with in its part-cured form, but can be set hard by further heating, is rigid and in our hands was good to at least 90 bar pressure.  Hot embossing of PMMA (perspex), as mentioned above, can also be done in the lab using hot water and G-clamps.  Most polymers can also be micro-milled (which is cheap and good to around a micron) or, if you're feeling extravagant, laser micromachined.

A reasonable approach once you think you've got the design right, is to make a negative master tool using whatever process works best and use this to cast everyday "working" tools or moulds using PDMS.

Now a days SU-8 based microfluidic devices are getting popularity. Photopatternability and CMOS compatibility makes SU-8 an interesting option for microfluidic applications