How many fully enclosed layers are needed for freely moving parts?
At least 2
Types of lithography
- electron beam
- laser
- proton beam
- x-ray
- soft
- stereo
negative resist
deposited soluble, responds to exposure by becoming insoluble (stays where the light shines, negative material height where mask is)
positive resist
deposited insoluble, responds to exposure by becoming soluble. (leaves where the light shines - positive material height where mask is)
serial process
carried out in series one step at a time
parallel process
all steps (or parts) are undertaken at the same time
aspect ratio
height : width
spin casting thickness is based on:
viscosity of resist and spin speed
Photo litho basic steps
1) resist applied on substrate, and prebaked to remove solvents
2) masks are aligned to substrate
3) exposure
4) post exposure bake
5) development washes away soluble material
6) resist can be used as a mask to pattern the underlying layer
7) resist is stripped to reveal the patterned underlying film
bright field mask
features are opaque
dark field mask
features are transparent
exposure system types
- contact aligner
- proximity aligner
- projection aligner
contact aligner
- mask and wafer in direct contact
- 1:1
- contact can damage mask
- high resolution
proximity aligner
- mask and wafer separated by a few micron gap
- 1:1
- lowered resolution due to increased diffraction
- better mask lifetime due to no contact
projection aligner
mask projected using high presicion options
5:1, - 10:1
lower mask tolerance and defect requirements
What two ways can photoresist be used
subtractive, or additive patterning
How to create a re-entrant profile for sucessful lift off?
- chemical treatment of resist with toluene prior to development to decrease solubility of top portion of resist
- bilayer resist can be created with differing solubility
- non-columnar light source can be used to expose the resist
raster scan
left to right, top to bottom
vector scan
beam traverses only the image areas
electron beam lithography
uses a focused beam of electrons to expose the resist and draw the pattern
laser lithography
uses a focused laser beam to expose the resist and draw the pattern
- can use either raster or vector method
- lower res than e-beam but generally less $
maskless lithography
uses a DLP to project images onto wafer
- speed depends on feature size but generally faster than laser lithography
proton beam lithography
- can be used the same way as electrons but has benefits because protons have greater mass and dont scatter.
- can get high AR
- need particle accelerator
x-ray lithography
- x-rays used as light source
- very high AR
expensive, requires synchrotron
