This project allows small laboratories to develop an easy-to-use platform for the fabrication of precise multilayer microfluidic devices. The platform consists of a three-dimensionally printed microscope mask alignment adapter using which multilayer microfluidic devices with alignment errors of <10 µm were achieved.
This protocol is significant as it allows for the generation of a cost-effective and easy to use platform that aids in the production of precise multilayer microfluidic master molds. The main advantage of the technique is that it only requires the use of the 3D printed platform and standard laboratory equipment commonly found in laboratories that produce microfluidic devices. Visual demonstration of this protocol is critical to demonstrate how to customize and use the 3D printed microscope mask alignment adapter.
Obtain the dimensions of the tray of the available UV light emission system to be the upper bound for the dimensions of the wafer holder. Measure the diameter of the inner circular rim, the inner height of the UV light emission systems tray, the total width and length of the tray. Using a computer design application, apply these dimensions to customize the wafer holder to fit within the UV light emission systems tray.
Measure the length between the screws and the width of the screws on the available upright microscope stage that hold the slide holder in place. Using a computer design application, apply these dimensions to customize the magnetic holder to fit the available microscope to allow for easy and precise fixation of the MMAA to the microscope. Use a four inch silicon wafer with you appropriate photo resist to create the first layer of the master mold, ensuring that the thickness is greater than the subsequent layers for easy identification of the alignment markers.
Use a light colored marker pen to color the first layer's alignment markers on all four sides. Using the photo resist manufacturer's instructions, initiate the second layer of the master mold by spin coating the photo resist onto the wafer and performing the soft bake. Insert the coated wafer into the wafer holder of the MMAA and fix the coated wafer to the MMAA using tape.
Attach the wafer holder to the available upright microscope using the magnetic microscope fastener. Move the position of the MMAA using the X and Y direction knobs of the microscope stage until one of the colored alignment markers on the wafer is in view through the microscope lens. Remove the wafer holder from the microscope stage and insert the second layer photo mask into the wafer holder on top of the coated wafer.
Ensure that the first layer's colored alignment markers can be partially seen through the alignment markers on the photo mask and that the straight edge of the photo mask is superimposed with the straight edge of the silicon wafer. Attach the wafer holder back onto the microscope stage and attach the photo mask to a scissor lift through one of the side cutouts with tape. Use the scissor lift to adjust the Z direction position of the photo mask until it lies right above the coated wafer.
While keeping the photo mask still, look through the microscope lens and identify the first layer's colored alignment markers beneath the alignment markers of the photo mask using the X and Y direction knobs of the microscope stage to move the position of the MMAA. Adjust the position of the MMAA until the alignment marker on the photo mask is superimposed with the colored alignment marker on the first layer. Carefully apply a slight force to the photo mask and use tape to secure the photo mask in place on top of the coated wafer.
Detach the photo mask from the scissor lift and ensure all four alignment markers on the photo mask are in alignment with the four alignment markers on the first layer. Post alignment, carefully detach the wafer holder from the microscope stage. Insert the glass top plate on top of the wafer and photo mask to decrease the gap between the two pieces.
Place the entire wafer holder into the available UV light exposure system and perform exposure of the second layer. Remove the wafer holder from the UV light exposure system, then remove the coated wafer from the wafer holder and detach the photo mask from the wafer. Complete the post-bake and developing of the second layer following the photo resist manufacturer's instructions.
Retrieve the master mold and place it on the stage of the upright microscope to determine the gap distance between the first layer and second layer. Measure the distance by which the second layer is shifted and misaligned from the first layer on the microchannel structures. Use the upright microscope to determine whether the PDMS chip contains channel walls that are straight with clear device edges.
Additionally, check the PDMS chip for any possible defects that may hinder device functionality. Through the optimization and use of the MMAA, multilayer master molds with minimal alignment error were fabricated. This system and the described protocol were used for the alignment of the markers on the photo mask with the markers on the initial layer of the master mold.
The double layer SU-8 master mold for a microfluidic device with a herringbone pattern was fabricated and shown to have a gap distance of less than five micrometres between the two layers. The two layer master mold was then used to fabricate PDMS microchips. Scanning electron microscope images show that the microfluidic device with the herringbone pattern contains clear edges, straight channel walls, and well-aligned layers, which are essential for proper device functionality.
In addition, a four layer master mold with simple circular features was created using the MMAA to show successful alignment of a multilayer master mold. Profilometer data confirms the four distinct layers of the master mold. It is important to have patience and work slowly when aligning the first and second layers'alignment markers and while fixing the photo mask to the coated wafer.
This procedure can be used for the production of many different multilayer master molds, allowing researchers from smaller labs to explore more complex microfluidic device designs.
