Microfabrication of Suspended Micronozzles for Coupling Microfluidic Device to Electrospray Mass Spectrometry

Yanou Yang

We describe the design and fabrication of two types of in plane micronozzles made of parylene and silicon oxide as potential electrospray tips. For both types of electrospray nozzles, lithographically defined microchannels are isotropically etched to the silicon wafer. These microchannels are later encapsulated with either parylene or silicon oxide. The overhanging silicon oxide microneedle array is formed by a wet chemical etch from the backside of the wafer using the deep boron-doped layer as the etch stop layer. The parylene microtubes are released by breaking the fragile SiO₂ overhang on top of the microchannel, resulting in millimeter long parylene microtubes projecting on the edge of silicon surface.

The fabrication of the first type of electrospray nozzles started with deep boron diffusion on a p-type (100) silicon wafer to form the highly boron-doped layer, the thickness of which decided the final outside depth of the overhanging needle. Eight microchannels were lithographically defined and isotropically etched to the doped layer. Plasma enhanced chemical vapor deposition (PECVD) silicon oxide was used to close the microchannels. The reservoirs for sample introduction and the microneedle tips were defined by photolithography and etched beyond the doped layer by deep reactive ion etching. The overhanging of the microneedle was achieved by wet etch from the backside of the wafer, and the etch process stopped when reaching the doped layer. . The rectangular cross section of the microneedle is 30mm wide and 14 mm high, same as the thickness of the doped layer. The overhanging length of the silicon dioxide microneedles was 100 mm with an opening around 2.5 mm x 5 mm. The array of the microneedles on a single silicon chip was then inserted into the end of a plastic device aligned with eight microfluidic channels.

The second type of electrospray nozzle was fabricated by releasing self-sealing parylene tubes formed in microchannel molds. The fabrication procedure involved lithographic definition and subsequent isotropic etch of silicon molds into which polymeric hollow microtubes were created. The reservoirs for sample introduction were also introduced in this step. Then a room temperature vapor-phase deposition of parylene was employed to fabricate centimeter-long, self-sealing tubes. The needle part was then defined by photolithography and formed by reaction ion etching. Finally the needle part was released by breaking the fragile SiO₂ overhang. Microtubes with different overhanging length (between 0.1 mm to 1 mm) can be simply made by breaking the chip in different locations along the protected microtube area. The orifice diameter of the fabricated parylene tube is 15 mm.

Figure 1: SEM images of the overhanging microneedles made of silicon dioxide. A and B are the same microneedle with different magnification.

Figure 2: SEM images of the suspended parylene micronozzles with different overhanging lengths. A and B: 100 mm overhanging length. C and D: 200 mm overhanging length.