Designing Your Own Device: Basic Design Rules

Here are the basic design rules for microfluidic devices fabricated at the Stanford Microfluidic Foundry. Following these rules for a design improves the chances that the devices will be easily manufactured and operational.
It is assumed that the reader has an understanding of the basic Microfluidic Valve Technology fabrication process.


For information on design strategies in microfluidic large scale integration, and various microfluidic control subcomponents, please refer to the following publication:

J. Melin, S.R. Quake, Microfluidic Large-Scale Integration: The Evolution of Design Rules for Biological Automation, Annual Reviews in Biophysics and Biomolecular Structure 36:213-31, 2007


Basic Design Rules

1. Please refer to the AutoCAD template file when starting your design.

It is recommended that you use this file since units and layers are already defined. Refer to Mask Design Rules for more detailed guidelines for your layout.


2. All devices must have standard Device Alignment Marks.

Device alignment marks are different from mask alignment marks and they are for aligning different PDMS layers to each other.  These alignment marks can be downloaded here for a standard two layer device. This set of alignment marks must be copied and placed on the left and right edge of the wafer layout. Subsets of crosses (alignment marks) should also be included near the most critical chip features, and marks at the periphery of the device (four corners).  If possible, a string of alignment marks along a chip edge is useful.

If your design requires multiple height molds, please read the guidelines and alignment mark requirements for Multiple Height Molds.


3. Standard input/output holes to access the flow and control channels punchers.

All punch locations must be clearly marked with a standard punch marker. The foundry can punch holes of the following sizes (note that 20 gauge is standard).


If other punch hole sizes are required, you must provide this hole puncher yourself.

Gauge #

Approximate Diameter

14

1.75mm

15

1.63mm

20

0.66mm

25

0.36mm

 

You must include standard hole markers where you want holes to be punched. These markers can be copied from the AutoCAD file std-punch-align-marks.dwg

 4. No structure (control or flow) can be fabricated having an aspect ratio lower than 1:10 (height : width).  

Structures with lower aspect ratios are prone to collapse.  If a feature is wider than this design rule permits, support posts must be added to ensure the aspect ratio is no lower then 1:10 in between posts (i.e. posts every 100um for a 10um high structure). If your device does not call for valves (i.e. it is a simple single layer device), it is not as prone to collapses and lower aspect ratio can be used.


5. All devices must be designed with a border indicating where the chip should be diced. 


6. Designs must incorporate sufficient tolerances to allow for easy layer/layer registration.  Designs must account for alignment tolerances of 30um.  This means that the device must be designed to function properly despite 30um errors in alignment in all directions. 


7. Crossovers of control lines and flow lines can be created without resulting in a valve at each crossover point. To achieve this, the control line should be designed narrower (15-30um) at these crossover points to ensure no membrane valve is formed.


8. The following table indicates a nominal valve area for various channel heights (both push-down and push-up designs) and an approximate closing pressure:

Push-down Valve

Push-up Valve

Valve Area

Flow Channel Height

Closing Pressure

Valve Area

Flow Channel Height

Closing Differential Pressure

100um x 100um

5um

10psi

100um x 100um

5um

5psi

100um x 100um

10um

10psi

100um x 100um

10um

5psi

100um x 100um

15um

10psi

100um x 100um

15um

5psi

N/A

20um

N/A

100um x 100um

20um

5psi

N/A

25um

N/A

150um x 150um

25um

5psi

N/A

30um

N/A

150um x 150um

30um

5psi

N/A

35um

N/A

150um x 150um

35um

5psi

N/A

40um

N/A

200um x 200um

40um

5psi

N/A

45um

N/A

200um x 200um

45um

5psi

 

Note: Push-down valves which seal reliably within a reasonable pressure range do not have flow channel heights exceeding 15um.


9. The following table summarizes some critical design rule information that is useful when designing your device:

Parameter

Value

Minimum overall chip thickness

3mm

Maximum overall chip thickness

7mm

Minimum flow channel height ( push-down valve )

5um

Maximum flow channel height ( push-down valve )

15um

Minimum flow channel height ( push-up valve )

5um

Maximum flow channel height ( push-up valve )

45um

Minimum possible feature width

15um

Maximum possible feature height (rectangular profile only)

150um

Nominal control channel height

10um or 25um

Minimum spacing between borders of adjacent devices

2mm

Minimum center-to-center spacing between punch holes (for 20 gauge)

1500um

Minimum center-to-center spacing between punch holes (for 15 gauge)

2000um

 

Chip Sizes

 

10. When designing control channels layout, control channels which are 10um tall should be spaced at least 15um apart. Control channels that are 25um tall should be spaced at least 40um apart.


11. Largest aspect ratios possible for rectangular structures is approximately 1:1 (Y:X, Y=height of feature, X=width of feature). For features which are very close together the largest aspect ratio is 1.5:1 (Y:X, Y=height of feature, X=spacing between features).


12. No more than 35 hole punches will be allowed per chip except for certain pre designed chips.