Lithography Flashcards
What are the main stages of a thin film patterning process?
Photolithography - photoresist deposition, alignment exposure, development
What is a lithography process? How do we classify the lithography processes? Please compare them and comment on their applicability
Lithography - pattern transfer
Classified by radiation used ie photo, X-ray etc
Explain the photo-resist processing stages. Outline their purpose and comment on the factors that can affect the photo-resist quality at each stage
clean - remove impurities and dust
pre-bake and primer coating - promotes adhesion of PR
Photoresist spin coating - improve quality of coating
soft bake - evaporating most of solvents that absorb radiation and affect adhesion
alignment and exposure - determines feature size
development - soluble areas dissolved to form pattern
hard bake -evaporating all solvents, improving etch and implantation resistance
inspection - cannot rework after etch so much check
Please draw a simple diagram and outline the work principle of industrial equipment for photoresist processing (eg applying, baking, developing of photoresist)
see notes
Why do we need resist materials?
Restricts etchant or prevents it from attacking the underlying material where it remains in place after development
What are the main requirements for the resist materials
High resolution - thinner PR has higher resolution, lower etching and ion implantation resistance
High etch resistance
Good adhesion
Wider process latitude - higher tolerance to process condition change
What type of resist materials do you know? Please list the PR components and outline their purpose
Polymer - solid organic material, transfers designed pattern to wafer surface, changes solubility due to photochemical reaction when exposed to radiation
Solvent - dissolves polymers into liquid -> thinning PR, allow application of thin PR layers by spinning
Sensitisers - additives that control or modify photochemical reaction of resist during exposure determines exposure time and intensity
Additives - various added chemical to achieve desired process results, such as dyes to reduce reflection
Please compare the positive and negative photoresists
Negative - becomes insoluble after exposure, when developed unexposed parts dissolve, mostly polyisoprene type, exposed becomes cross-linked polymer with higher chemical etch resistance, unexposed dissolves in development solution
Positive - soluble after exposure, when developed the exposed parts dissolved, better resolution, eg novolac resin polymer, acetate type solvents, sensitisers cross-linked within the resin, energy from the light dissociates the sensitiser and breaks down the cross-links, exposed part dissolve in developer solution, image the same that on the mask, higher resolution, commonly used in IC fabrications
Which are the main performance metrics of a patterning process?
Resolution - minimum feature dimension hat can be transferred with high fidelity to a resist film
Registration - how accurately patterns on successive mask can be aligned with respect to previously defined patterns
Throughput - number of wafers that can be exposed/unit time for a given mask level
Explain the effect of the light diffraction, numerical aperture, and depth of focus on the image resolution during a patterning process
Light diffraction - optical lens collect diffracted light and enhance image
Numerical aperture - ability of lens to collect diffracted light
Depth of focus - range that light is in focus and can achieve good resolution of projected image
Please draw a simple diagram and outline the work principle of the equipment for photoresist alignment processing
Scanner-stepper
Please outline the purpose of a phase-shifting mask
Phase-shifting mask - destructive interference
Please list the exposure light sources, which are used in photolithography
Mercury lamp
Excimer laser
Fluorine laser
What is the next generation lithography process?
Extreme UV E-beam I-line DUV ArF
Explain the principle of the extreme UV, X-ray e-beam and ion-beam lithography processes outline their advantages and disadvantages
Extreme UV -
Xray - gold mask, 15nm resolution, high cost, photomask device
ebeam - below 100nm resolution, high cost, main use research
ion-beam - 1 um resolution, high cost, research
Draw a simple diagram outline the principle of work of a machine for e-beam lithography
SEM setup
Please compare the NGL processes which you know
table
