Soft Lithography Practicum

MAE 165 Spring 2010 Professor Madou Tuesday Group: Jeff Draper Amanda Evans David Szeto

Contents

 Background and introduction  Process of soft lithography  Advantages and disadvantages  Future developments in soft lithography

What is Soft Lithography?

 Lithography = Greek for “to write in stone”  Called “soft” because it is typically used on elastomeric materials such as PDMS (polydimethylsiloxane)  The technique underwent rapid expansion from 1995-2005  Uses photolithography to make a mold for PDMS, which gets permanently bonded onto glass to make a microfluidic device.

     Lithography Process Step 1: Create PDMS Mixture PDMS stands for Polydimethylsioloxane Tape around the edges of the mask Follow a 10:1 weight ratio of Silicon Elastomer Base to Silicon Elastomer Curing Agent – roughly 30g of base and 3g of curing agent A pipette is used to achieve desired weight of Silicon Elastomer Curing Agent Mix the PDMS gel vigorously to ensure equality throughout. – It should turn from a clear to a white color

Lithography Process Step 2: Pour onto Mold and De-Gas    Pour mixture onto the photoresist mold The mask was premade – Made by photolithography. The PDMS gel contained air bubbles which must be removed through a vacuum or/and through just letting it sit. – de-gas PDMS to protect integrity

Lithography Process Step 3: Cure the PDMS    Cure the PDMS by heating it on a hot plate at 100C for roughly 10 min. – Curing at a lower temperature, such as 60C, for much longer durations is ideal To help ensure equal heating, cover molds with tin foil to create a convection oven-like affect. Once cured, pull it off of hot plate and let the PDMS cool

Lithography Process Step 4: Cut and Mount PDMS    After PDMS solidified and cooled, take a sharp knife and cut out PDMS parts.

– The desired part will be a negative of the mold.

Mount the part to a glass slide, punch holes into desired location for fluid injection.

The process is then complete

Advantages/Disadvantages/Applications 

Advantages

– Low cost – Fast production – Easily accessible – Does well with small details 

Disadvantages

– PDMS is a soft structure – PDMS has a high thermal expansion  Applications – MEMS Devices – Sensors – Micoreactors – Microfluidics