Devise a process flow with written steps and corresponding cross$-$ section images for making a microchannel mixing device out of PDMS$.$ Figure $3\mathrm{.}61$ shows the internal flow path through the micro$-$ channel; in other words, the device should contain a channel with the flow path drawn below. Also, draw each photomask necessary to produce the structure and indicate where each mask should be used in the process. Include any necessary alignment marks on the photomasks to achieve accurate registration between process layers. Briefly explain the function of each photomask.

FIGURE $3\mathrm{.}61$

Internal flow path through the microchannel.