The standard microfluidic device using membrane valves includes designing a control mold which has control channels of height A and a flow mold which has flow channels of height B. However, some applications require channels and features to have various heights on the same mold. For example, a reaction chamber may need to be 100µm tall, however the tallest valvable flow channel is only 45µm, see Figure 1.

Figure 1. Schematic of a multi-height mold showing 3 layers of different heights.

The Stanford Microfluidics Foundry allows molds with up to 3 layers of complexity to be fabricated. Below are design rules which should be followed if your design requires more than a single layer height.

1) The most common sequence of multi-height molds are:

Layer 1: Rounded Cross-sectional Profile

Layer 2: Rectangular Cross-sectional Profile

Layer 3: Rectangular Cross-sectional Profile

2) Alignment marks are very important in multi-layer/multi-height molds. Use the attached AutoCAD file of alignment marks (hybrid_mold_align.dwg) if you are designing a 3 layer mold (layer names: Rounded Layer1, Rectangular Layer2, Rectangular Layer3). If you are designing a 2 layer mold, simply delete layer Rectangular Layer3.

3) When designing a mold having shallow channels connecting significantly taller features, ensure that the shallow channels are designed such that they can be fabricated before the taller features, i.e. shallow channels = Layer 1, taller features = Layer 2.

4) When designing a mold having features directly on top of another layer of features, ensure that enough alignment tolerance is given (i.e. 30µm in all directions).

5) When designing multi-level molds with significant differences in heights, special considerations must be taken. If your design resembles Figure 2 where Layer 2 is much taller than Layer 1, the PDMS membrane thickness may become very uneven resulting in non-functioning valves.

Figure 2. Schematic of short channels connecting tall features where valves are desired at the short channels.