Intro to Lithography Flashcards
Lithography
Transforms complex circuit diagrams into pattern w/c are defined on the wafer in a succession of exposure and processing steps to form a number of superimposed layers of insulator, conductor and semiconductors materials
How many lithography steps are required to fabricate a packed IC?
typically 8-25 steps
semiconductor manufacturing processes
design, wafer prep, front-ed processes, photolithography, etch cleaning, thin films, ion implantation, planarization, test and assembly
Die cut and assembly
good chips are attached to a lead frame package
soft lithography
uses photolithography to make a mold for PDMS (polydimethylsiloxane), w/c gets permanently bonded onto glass to make a microfluidic device; fast and cheap prototyping method
2 types of lithography
photolitography and soft lithography
type of lithography that makes a mold on a silicon wafer using UV light to etch a design
photolithography
type of lithography that uses the mold to make a chip from PDMS polymer
soft lithography
a unique comb. of properties resulting from the presence of an inorganic siloxane backbone and organic methyl groups attached to silicon
PDMS (Poly(dimethylsiloxane))
why are PDMS fluids iat room temp?
b/c they have very low galss transition
how can PDMS be readily converted into solid elastomers?
by crosslinking
Properties/Benefits of PDMS
- Optical: transparent, optical detection from 240-1100 nm
- Electrical: insulating, breakdown voltage (2x10^7 V/m); allows embedded surfaces, intentional breakdown to open connections
- Mechanical: elastomeric, high youngs module (-70 kPa); conforms to surface when not polymerized, reversible deformation
- Thermal: insulating, thermal conductivity of 0.2 W/(mk); can be used to insulate heated solutions
PDMS device applications
configurable gradient generator, valving via electrical breakdown of PDMS, cell patterning through surface modifications
Design saved in a DXF file format and sent to the printing company
SolidWorks Design
Fabrication of the mold (Photolitography) steps
- clean silicon wafer and dry
- spincoat a thin layer of SU-8 pthotoresist
- soft bake to evaporate the solvent
- align the mask
- flood exposure to UV light
- hard bake to cross-link the exposed portions of the photoresist film
- develop to wash away the non-exposed area of the photoresist
- rinse with isopropanol and dry with nitrogen gun
Rapid prototyping
- a system of channels is designed in a CAD program
- a commercial printer uses CAD file to produce a high-res transparency
- the transparency is used as a photomask in contact photolithography to produce a master
- master consists of a positive relief of photoresist on a silicon wafer and serves as a mold for PDMS
- liquid PDMS pre-polymer is poured over the master and cured for 1h @ 60C
- PDMS replica is peeled from the master
- replica is sealed to a flat surface to enclose the channels
soft lithography techniques
- replica molding (REM)
- micro-contact printing
- micro-transfer molding
- micro-molding in capillaries
Replica Molding
- Master mold is replicated in PDMS by casting & curing PDMS pre-polymer
- PDMS mold is oxidized in O2 plasma for 1 min & exposed to fluorinated silane for 2h to make a surface w/ low adhesion to PDMS
- PDMS is cast against this negative replica, cured and peeled off
- Makes positive replica of the original master
micro-contact printing
- uses PDMS stamp to form patterns of self-assembled monoplayers (SAM) on the surfaces of the substrates
- PDMS stamp is coated with an ink of the molecules and pressed onto the solid surface
- inking creates SAM on the solid surface
- use of elastomers allows the micropatterned surface to come into conformal contact with the surfaces over large areas
micro-transfer molding
- PDMS stamp is filled with pre-polymer
- excess prepolymer is removed
- press the rubber stamp against the surface
- cure polymer
- peel off stamp
micromolding in capillaries
- push PDMS stamp against the substrate
- prepolymer is applied to access holes in the mold (vacuum assisted)
- prepolymer fills the channels using capillary forces
- cure the polymer
- peel off the PDMS mold
Lithography Process
- Create PDMS Mixture
- Pour onto Mold and de-gas
- Cure the PDMS
