Soft Lithography is a microfabrication process in which a soft polymer (such as polydimethylsiloxane (PDMS) ) is cast onto a mold that contains a microfabricated relief or engraved pattern. Using this technique, membrane microvalves can be produced. This membrane microvalve is the fundamental component which enables liquids to be controlled on-chip and is the key to realizing microfluidic large scale integration.
A basic microfluidic device is composed of two elastomer layers. One layer contains channels for flowing liquids (flow layer), and the other layer contains channels that deflect the membrane valve into the flow channel and stop liquid flow when pressurized with air or liquid (control layer).


 Master Molds

Molds containing the relief of the desired microfluidic circuit are made using conventional photolithography. This entails first designing your desired microfluidic network in a CAD program and printing it onto a transparency film using a very high resolution printer. Next, an appropriate photosensitive polymer (photoresist) is spun onto a silicon wafer and ultraviolet light is exposed to the wafer through the overlaying mask. Finally, the wafers are developed to reveal the transferred microfluidic network pattern on the silicon wafer. Note: one mold is made for the control layer and one mold is made for the flow layer.


Photoresists and Geometry of channels:

A photoresist is a light-sensitive material used to form a patterned coating on a surface.

Photoresists are classified into two groups: positive resists and negative resists.

  • A positive resist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes soluble to the photoresist developer. The portion of the photoresist that is unexposed remains insoluble to the photoresist developer.
  • A negative resist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes insoluble to the photoresist developer. The unexposed portion of the photoresist is dissolved by the photoresist developer.

We use SU 8 series negative photoresist to create a rectangular type channels since after hard baking features have rectangular profile.
To make rounded channels we commonly use AZ  and SPR positive photoresists and they have  rounded profile after hard baking.


Single layer molds vs. multi layer molds:

Most commonly used are molds that have one layer of photoresist and all features are the same height.
Multi layer molds are made in cases when it is necessary to have features with different heights.

mold layers

Schematic of a multi-height (layer) mold showing 3 layers of different heights.

 

In this case, a second layer of photoresist is applied to the first one, and all the same basic mold making steps are repeated except exposure. Before exposing, it is necessary to align the first layer with the mask of the second layer.
In order to precisely position the mask of the second layer with the first layer mold, both masks for layers one and two should have alignment marks on them.
For a three layer mold the same steps are applied, and all 3 masks for those layers must have alignment marks in order to work.



PDMS Devices (Chips)

 

Types of devices

Push up

Push down

pushup_section.bmp

pushdown_section.bmp

Control lines pass under the flow channels. Pneumatic pressurization of the control line causes a membrane to deflect up into the flow structure, sealing the channel. Deep reaction chambers may be integrated into the flow layer (upwards). Control lines pass over the flow channels. Pneumatic/hydraulic pressure in the control lines flattens the membrane valve downwards to create a seal.
glasspushup_labled.JPG glasspushdown_labled.JPG

 

Steps to make devices: Push up device Push down device
Making Control layer Spinning PDMS on control mold to form a thin layer and bake Pour PDMS onto wafer to form a thick layer and bake
Making Flow layer Pour PDMS onto wafer to form a thick layer and bake Spinning PDMS on control mold to form a thin layer and bake
Aligning layers Align flow on control layer Align control on flow layer
Bonding layers Bake both layers Bake both layers
Bonding device to a substrate Bond the device to a substrate to seal the control layer Bond the device to a substrate to seal the flow layer

The following figure shows the basic fabrication process for this two-layer device (courtesy Dr. Carl Hansen):

processoverview_cropped.jpg
When a control channel and a flow channel cross, if the area of the intersection is large enough, a valve is created. The thin membrane separating the two channels deflects into the flow channel when the control channel is pressurized, creating a complete seal. The following picture shows a typical valve in the closed state (courtesy Dr. Carl Hansen):

http://thebigone.stanford.edu/foundry/images/index_clip_image002.jpg
Marc A. Unger, Hou-Pu Chou, Todd Thorsen, Axel Scherer, and Stephen Quake, “Monolithic Microfabricated Valves and Pumps by Multilayer Soft Lithography,” Science, vol. 288, no. 7, pp. 113-116, April 2000.
David C. Duffy, J. Cooper McDonald, Olivier J.A. Schueller, and George Whitesides, “Rapid Prototyping of Microfluidic Systems in Poly(dimethylsiloxane),” Analytical Chemistry, vol. 70, no. 23, pp. 4974-4984, December 1998.

 

SOURCE:  http://www.stanford.edu/group/foundry/Microfluidic%20valve%20technology.html

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