Soft Lithography PracticumSoft Lithography Practicum

MAE 165MAE 165Spring 2010Spring 2010

Professor MadouProfessor Madou

Tuesday Group: Tuesday Group: Jeff DraperJeff Draper

Amanda EvansAmanda EvansDavid SzetoDavid Szeto

ContentsContents

Background and introductionBackground and introduction Process of soft lithographyProcess of soft lithography Advantages and disadvantagesAdvantages and disadvantages Future developments in soft lithographyFuture developments in soft lithography

What is Soft Lithography?What is Soft Lithography?

Lithography = Greek for “to write in stone”Lithography = Greek for “to write in stone” Called “soft” because it is typically used on Called “soft” because it is typically used on

elastomeric materials such as PDMS elastomeric materials such as PDMS (polydimethylsiloxane)(polydimethylsiloxane)

The technique underwent rapid expansion The technique underwent rapid expansion from 1995-2005from 1995-2005

Uses photolithography to make a mold for Uses photolithography to make a mold for PDMS, which gets permanently bonded onto PDMS, which gets permanently bonded onto glass to make a microfluidic device.glass to make a microfluidic device.

Lithography ProcessLithography ProcessStep 1: Create PDMS MixtureStep 1: Create PDMS Mixture

PDMS stands for PDMS stands for PolydimethylsioloxanePolydimethylsioloxane

Tape around the edges of Tape around the edges of the maskthe mask

Follow a 10:1 weight ratio of Follow a 10:1 weight ratio of Silicon Elastomer Base to Silicon Elastomer Base to Silicon Elastomer Curing Silicon Elastomer Curing AgentAgent

– roughly 30g of base and 3g roughly 30g of base and 3g of curing agentof curing agent

A pipette is used to achieve A pipette is used to achieve desired weight of Silicon desired weight of Silicon Elastomer Curing Agent Elastomer Curing Agent

Mix the PDMS gel vigorously Mix the PDMS gel vigorously to ensure equality to ensure equality throughout. throughout.

– It should turn from a clear It should turn from a clear to a white colorto a white color

Lithography ProcessLithography ProcessStep 2: Pour onto Mold and De-GasStep 2: Pour onto Mold and De-Gas

Pour mixture onto the Pour mixture onto the photoresist moldphotoresist mold

The mask was premadeThe mask was premade– Made by photolithography. Made by photolithography.

The PDMS gel contained air The PDMS gel contained air bubbles which must be bubbles which must be removed through a vacuum removed through a vacuum or/and through just letting or/and through just letting it sit. it sit. – de-gas PDMS to protect de-gas PDMS to protect

integrityintegrity

Lithography ProcessLithography ProcessStep 3: Step 3: Cure the PDMSCure the PDMS

Cure the PDMS by heating it Cure the PDMS by heating it on a hot plate at 100C for on a hot plate at 100C for roughly 10 min. roughly 10 min. – Curing at a lower temperature, Curing at a lower temperature,

such as 60C, for much longer such as 60C, for much longer durations is idealdurations is ideal

To help ensure equal To help ensure equal heating, cover molds with tin heating, cover molds with tin foil to create a convection foil to create a convection oven-like affect. oven-like affect.

Once cured, pull it off of hot Once cured, pull it off of hot plate and let the PDMS coolplate and let the PDMS cool

Lithography ProcessLithography ProcessStep 4: Cut and Mount PDMSStep 4: Cut and Mount PDMS

After PDMS solidified and After PDMS solidified and cooled, take a sharp knife cooled, take a sharp knife and cut out PDMS parts.and cut out PDMS parts.– The desired part will be a The desired part will be a

negative of the mold.negative of the mold. Mount the part to a glass Mount the part to a glass

slide, punch holes into slide, punch holes into desired location for fluid desired location for fluid injection.injection.

The process is then The process is then completecomplete

Advantages/Disadvantages/ApplicationsAdvantages/Disadvantages/Applications

AdvantagesAdvantages– Low cost Low cost – Fast production Fast production – Easily accessible Easily accessible – Does well with small Does well with small

details details DisadvantagesDisadvantages

– PDMS is a soft structurePDMS is a soft structure– PDMS has a high thermal PDMS has a high thermal

expansionexpansion

Applications Applications – MEMS DevicesMEMS Devices– Sensors Sensors – Micoreactors Micoreactors – MicrofluidicsMicrofluidics