Examples of Microfabrication Processes

We present three detailed examples of microfabrication to help the readers better understand how a fabrication flow for making devices is constructed. We have endeavored to incorporate all the basic fabrication steps as well as many techniques described in this chapter. Note that these examples were chosen intentionally to better prepare readers for subsequent chapters about microlenses rather than explain conventional IC fabrication. For more insight into IC fabrication, readers are referred to many excellent books that focus on the subject. 

Step 1 The choice of substrate is not specified, but Si or glass will work. We intend to use electroplating to form the Ni stirrer. Hence, the first step is to sputter the seed layers for the electroplating. We sputter three layers of metal thin films in sequence (Ti, Cu, and Ti), each about 20 nm thick. The bottom Ti layer is to enhance the adhesion of the future electroplated Ni onto the substrate. The middle layer of Cu is the seed layer for Ni electroplating. 

Fabrication process flow of Ni stirrer in microfluidic channel. The left column shows the cross section and the right column the top view. Source Adapted from Agarwal, A.K., Sridharamurthy, S.S., Beebe, D.J. et al. 2005. Journal of Microelectromechanical Systems, 14(6), 1409-1412. With permission.) 

Step 2 We will need a mold to perform the electroplating. The thickness of this mold must exceed the desired height of the electroplated Ni stirrer (100 pm). Hence we choose to utilize liquid phase photopolymerization rather than a PR since it is easier to generate structures with heights on this order. 

We first tape the substrate at the edges with an adhesive spacer. The height of the adhesive spacer will also define the height of the mold, say 150 pm, satisfying fhe prerequisite of exceeding the height of the Ni stirrer. Then we cover the substrate with a piece of transparency on which patterns have been printed to serve as the photomask. The photomask along with the substrate and the adhesive spacers form a chamber. 

---