MaterialsProcessing Method With Mask DesignMaterials The materials that we used for the fabrication of our device are Silicon, PDMS and SU-8. Silicon wafer is used as a substrate as it is cheap, and convenient for fabrication process like lithography. PDMS is a soft polymer that has properties like elasticity, conformality, optical transparency etc. Due to its conformal nature, devices made of PDMS can be integrated to materials like glass and silicon. So both reversible and irreversible sealing is possible. In this stage, we used PDMS to create layers, as standard lithographic process make fabrication of these layers possible. And when all the layers are stacked on the top of each other, PDMS easily conforms and makes stacking possible. SU-8 is a negative based epoxy photo-resist consisting of 8 epoxy groups. This photo-resist is photosensitive and forms a cross-linking reaction when exposed to light. During developing, the SU-8 coated regions are not removed. SU-8 is spin coated on a Si wafer, and after developing, can be used to create reverse mold patterns of micro channels, reservoirs and interconnects.Processing Method With Mask Design In our preliminary design, alignment between channels and interconnects was an issue. The top layer reservoirs and bottom layer reservoirs were aligned different, so assuming fluid flow in one channel, say input 1 causes liquid to flow through output 1 as well as through input 2&3 in the preliminary design. This causes overflow of liquid in the channels. The existing packaging has designed in such way that the top layer had connections with the layer on bottom. Thus the preliminary design was modified to make connections between the inlet as well as outlet reservoirs, to be consistent with existing package. We designed our modified masks and the modified version of the masks is given below. Modified MasksWith this particular design of mask sets, we have certain considerations. The considerations were - How many of the nine channel intersections should be used as interconnects between layers (3 or 9)?- Should the first channel layer be open or closed on the bottom surface (PDMS orPyrex bottom)?- Should a top layer channel be used or should the top remain open?- Should the size of the reservoir throughputs be the same size or smaller than the reservoirs?Through the use of golf tees and rubber bands, we created a three-dimensional model of the micro-fluidic device that makes us to again modify our design . We modified our mask design by re-routing the input, output channels, by deleting portions of channels, reservoirs and removing some interconnects from previous design. There was 9 interconnects in the previous design, but in the new design, we reduced it to 4 interconnects. The new mask sets are given below in fig. Mask 1: Bottom fluid layer Mask 2: Interconnect layer Mask 3:Top fluid layer Mask 4: Top order layerThe processing method of the modified design consists of the following: 1. Begin with four polished Si wafers.2. Spin SU-8 (negative photoresist) on Si wafer and pre-bake at 95°C.3. Align wafer with Mask 1 - (Figure ) and expose SU-8 to ultraviolet light. Post-bake at 95°C.4. Develop SU-8 in SU-8 developer and unexposed areas are removed. This creates Mold 1 from Mask 1. In the same way, Mold 2 is formed from Mask 2 Mold 3 from Mask 3 & Mold 4 from Mask 4. 5. After creating the molds, spin on the PDMS less than the vertical dimension ofSU-8 protrusions.- Dip the Si wafer in a sodium dodecyl sulfate(SDS) adhesion barrier and allow it to dry naturally.- Mix PDMS (Sylgard 184, Dow-Corning) 10:1 with curing agent.- Spin on PDMS.- Bake in box furnace for 2 h at 70°C. 6. Spin PDMS Layer 1 on Mold 1 (Bottom Fluid Layer), PDMS Layer 2 on Mold2 (interconnect layer), PDMS Layer 3 on Mold 3 (Top Fluid layer) and PDMS Layer 4 on Mold 4(top order layer). Make a total of four PDMS layers- two layers from the channel mold (1&3), one layer from the interconnect mold and one layer from top mold. Figure( )shows the PDMS Layers 1, 2, 3 & 4 .PDMS Layer 1: Bottom fluid layer PDMS Layer 2: Interconnect layer PDMS Layer 3: Top fluid layer PDMS Layer 4: Top order layer7. Delaminate and stack all four PDMS layers in the following order: Layer 1 (bottom fluid layer), Layer 2(interconnect layer), Layer 3(Top fluid layer) and Layer 4(top order). The final result of the stacked PDMS layer is shown on figure.Figure: Final Stacked PDMS layers with molds (Bryce stage 2 picture should be
View Full Document
Unlocking...