Etching quartz microfluidics

Materials World magazine
,
1 Nov 2008

Synthetic fused silica (quartz) based microfluidic devices can now be etched deeper and for longer, say engineers at Dolomite-Microfluidics, based in Royston, UK. This will enable more effective fluid control and chemical analysis in bioscience and medicine.

The company has achieved a depth of up to 150µm by depositing a range of harder metal photoresist layers onto the quartz, which better survive etching of patterns/channels (the exact composition of the masks is proprietary). Alternative market solutions offer depths of about 20µm, says Dolomite.

Quartz is mechanically stronger and more chemically resistant to acids and bases than glass-based instruments, which means processing the material takes longer and is more expensive. However, traditionally, if you etch for longer the polymeric photoresist is degraded by the acid hindering the process, explains Engineering Manager Phil Homewood. The new masks do not reduce the cost of etching but ‘allow us to etch for twice as long while achieving a greater depth’.

The technique also creates channels with a plus or minus one-micron accuracy and a high optical smoothness – they have a surface roughness of Ra five nanometres.

Such devices offer advantages for bio-chemical analysis as quartz is chemically inert, UV transparent, non-auto-fluorescent and non-porous.

Homewood says, ‘The main application is in the detection of chemical markers that fluoresce in the presence of UV light. An example is green fluorescent protein, which is often used in genetics. Other applications include photochemical reactions and photo polymerisation’.

Dr Paul Galvin, Head of the NanoBio Systems Group at Tyndall National Institute, Cork, Republic of Ireland, can see the merits of this development. He notes, ‘People are getting around [the need for] low-auto-fluorescence and better signal-to-noise ratio by using polymer substrates which have a high quality and low background signal. But the advantage of quartz is that it is more robust to solvents’.

However, he adds, ‘One of the limiting factors that I still see is whether Dolomite has a complementary sealing process for an enclosed mircofluidic device. Thermo-sealing would be complicated as the melting temperature of quartz is higher than glass’.

The team at Dolomite acknowledges that ‘thermal bonding’ is ‘tricky’, but says it has overcome this (although, they are keen to keep the knowledge secret).

The company will use its technology to produce a new range of devices, while also hoping to increase etching depth further. ‘We can look at the thickness of the photoresist layers and the different combinations,’ concludes Homewood.

Further information: Dolomite