Absolutely, your questions are **very relevant** to the microfluidic fabrication process! Below is a **comprehensive answer** for your PDMS-glass device fabrication, specifically when using negative photoresist to create the master molds. --- ## 1. Device Structure Overview From your image and description: - **Two-layer PDMS device** (Upper: Pressure chamber + cubes; Lower: microchannels + cell culture chambers), bonded to glass. - **Multiple features with different heights** (e.g., chambers, microchannels, inlet/outlet/cube structures). --- ## 2. **Mask Design and Mold Requirement** ### **Q1: Do I need two different molds?** **Yes.** - You need **two separate master molds**: - **Top mold** for the pressure chamber and cubes. - **Bottom mold** for the cell culture chamber and microchannels. ### **Q2: Do I need different masks/photoresists for the microchannels and cell culture chamber?** **Depends on height differences**: - If **microchannels** and **cell culture chambers** are of **different heights**, you have two options: 1. **Two-layer photolithography** on the same wafer (requires careful alignment, two masks, two photoresist steps). 2. **Design both features on a single mask** if they are the same height (much simpler). - **Different photoresists** are only required if you need **different feature heights** and the photoresists have different properties (which is uncommon; SU-8 negative resist is popular for all heights). ### **Q3: Do I need another mask/mold for the cubes on the pressure chamber?** - **If the cubes are the same height as the pressure chamber:** - **No extra mask/mold needed.** - Design the cubes as part of the pressure chamber mask. - **If the cubes are taller or shorter:** - **Multi-layer photolithography** on the same mold is needed, requiring a second mask and alignment. --- ## 3. **Detailed 2D Fabrication Process Flow** ### **A. Master Mold Fabrication (Using Negative Photoresist, e.g., SU-8)** #### **For Each Layer:** #### a) **Design Masks** - **Top Layer Mask:** Pressure chamber + cubes (drawn in CAD, exported as photomask). - **Bottom Layer Mask:** Microchannels, cell culture chambers, inlet/outlet holes. #### b) **Wafer Preparation** - Clean Si wafer (piranha, DI water, dry). - Bake to dehydrate (e.g., 120°C, 10 min). #### c) **Photoresist Coating** - **Spin-coat SU-8** (choose SU-8 type/thickness for required feature height). - Example: SU-8 2050 for ~50 μm, SU-8 2100 for ~100 μm. #### d) **Soft Bake** - According to SU-8 datasheet (e.g., 95°C for 10–30 min). #### e) **UV Exposure** - Align mask over resist. - UV expose (dose per datasheet). #### f) **Post-exposure Bake** - Per resist datasheet. #### g) **Development** - Immerse in SU-8 developer (PGMEA), rinse and dry. #### h) **Hard Bake (Optional)** - To improve mold durability. --- ### **B. Multi-layer Photolithography (if different heights/features on same wafer)** - **Spin/Expose/Develop** first layer (e.g., microchannels). - **Spin/Expose/Develop** second layer (e.g., cell chamber), using alignment marks. --- ### **C. PDMS Replica Molding** 1. **Mix and degas PDMS** (10:1 base:curing agent). 2. **Pour PDMS** onto mold(s), degas, and cure (e.g., 70°C, 1 hr). 3. **Peel off PDMS layer** from master mold. --- ### **D. Device Assembly** 1. **Punch holes** for inlet/outlet. 2. **Align and plasma bond** top and bottom PDMS layers together. 3. **Plasma bond** assembled PDMS device to a glass slide. --- ## **Summary Table** | Feature | Mask Needed | Mold Needed | Photoresist | Notes | |------------------------|-------------|-------------|-------------|-----------------------------------------------------------------| | Microchannels/chambers | Yes | Yes | SU-8 | One or two heights: use single or dual-layer photolithography | | Pressure chamber/cubes | Yes | Yes | SU-8 | Include cubes in mask if same height; else dual-layer needed | --- ## **References & Tips** - [SU-8 datasheets and microfabrication guides](https://www.microchem.com/PDFs/SU-82000DataSheet2020.pdf) - Multi-height structures benefit from dual-layer photolithography but require careful alignment. - For many designs, **all features of a single height on a single mask** is much simpler and more robust. --- **If you provide heights of each feature, I can suggest exact SU-8 types and spin conditions! Let me know if you want detailed mask design tips (e.g., CAD file setup).**