1/56 Institut de Microelectrònica de Barcelona Escuela de verano de Jaca July 2011 Concepts and principles of optical lithography Francesc Pérez-Murano Institut de Microelectrònica de Barcelona (CNM-IMB, CSIC) [email protected]2/56 Institut de Microelectrònica de Barcelona Escuela de verano de Jaca July 2011 1 cm 10 cm 100 um 1 mm 1 um 10 um 10 nm 100 nm 1 nm 0,1 nm mà Gra de sorra Diàmetre cabell humà Bacteries Molècula de DNA Distància interatòmica Oblia Xip Micromotors Circuit integrat Transistor MOS Dispositius quàntics Estructures atomiques Microelectrònica Nanotecnologia 3/56 Institut de Microelectrònica de Barcelona Escuela de verano de Jaca July 2011 Nanotechnology, D. M. Tennant . AIP/Springer, New York, 1999 Nanolithographies Nanolithographies 4/56 Institut de Microelectrònica de Barcelona Escuela de verano de Jaca July 2011 Summary Concept of optical lithography Resists Part ii Associated processes Part ii Miniaturization Limits optical lithography Part i
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Institut de Microelectrònica de Barcelona
Escuela de verano de Jaca July 2011
Concepts and principles of optical
lithography
Francesc Pérez-Murano
Institut de Microelectrònica de Barcelona (CNM-IMB, CSIC)
5. Develop6. Hard Bake7. Inspection8. Etch9. Resist Strip10. Final Inspection
* Some processes may include a Post-exposure Bake
Ten Basic Steps of Photolithography
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Institut de Microelectrònica de Barcelona
Escuela de verano de Jaca July 2011
1. Surface Preparation (HMDS vapor prime)
Dehydration bake in enclosed chamber with exhaust
Clean and dry wafer surface (hydrophobic)
Hexamethyldisilazane (HMDS)
Temp ~ 200 - 250C Time ~ 60 sec.
HMDS
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HEXAMETHYLSILIZANE (HDMS)Dehydration
Adhesion promotion by HDMS
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Institut de Microelectrònica de Barcelona
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2. Photoresist Application
Wafer held onto vacuum chuck
Dispense ~5ml of photoresist
Slow spin ~ 500 rpm Ramp up to ~ 3000 -
5000 rpm Quality measures:
time speed thickness uniformity particles & defects vacuum chuck
spindleto vacuum
pump
photoresist dispenser
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Institut de Microelectrònica de Barcelona
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Resist spinning thickness T depends on: Spin speed Solution concentration Molecular weight (measured by
intrinsic viscosity) In the equation for T, K is a
calibration constant, C the polymer concentration in grams per 100 ml solution, the intrinsic viscosity, and the number of rotations per minute (rpm)
Once the various exponential factors (, and ) have been determined the equation can be used to predict the thickness of the film that can be spun for various molecular weights and solution concentrations of a given polymer and solvent system
Metal ContentShelf lifeToxicityStability to process variations
Photoresist Material Parameters (requirements)
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Positive tone photoresist: DQN
Resin (N) / sensitizer(DQ)
N: phenolic Novolak resin: low molecular weight polymer. Forms the resists films properties. It dissolves in presence of water.
DQ (Photoactive siazoquinone ester) Photosensitive, insoluble in aqueous solution. Prevents the resin to be dissolved
Upon exposure to light, the dizaoquinones photochemically decompose
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Institut de Microelectrònica de Barcelona
Escuela de verano de Jaca July 2011
Example: AZ 1500 Photoresists
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Escuela de verano de Jaca July 2011
Positive tone photoresist: PMMA
PMMA: poly(methylmethacrylate)
Chain scission under DUV exposition
Also suitable for electron-beam lithography
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Escuela de verano de Jaca July 2011
Example: nano-PMMA
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Escuela de verano de Jaca July 2011
Negative tone photoresist
Resin: Cyclic Synthetic rubber (non radiaton sensitive, strongly soluble in the solvent)
PAC is a bis-arylazide. Upon exposure, it dissociates into nitrene and N2. The nitrene reacts with the rubber molecules so that a cross linking between resin molecules occurs, becoming unsoluble.
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Example: AZ-N4035
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Institut de Microelectrònica de Barcelona
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O
CH2
CH
CH2O
Epoxy based negative photoresistNegative photoresists becomeinsoluble in developing solutionswhen exposed to optical radiation
SU-8 is a commercial name for a fixed formulation. Any variation of thisformulation becomes a very similar resist, but as it is not exactly SU-8, thevariations are called epoxy based resists.
C CH3H3C
CH2
C CH3H3C
CH2
C CH3H3C
CH2
C CH3H3C
O O O
OOOO
CH2 CH2 CH2 CH2
CH2CH2
O
CH2 CH2
CH CH CH CH
CHCH CH CH
CH2 CH2 CH2 CH2
CH2CH2CH2CH2
O O O O
O O O O
• On exposure the PAG generates a strong acid
• Protons attack oxygen on some epoxides
• Crosslinking occurs during PEB resulting in an insoluble very dense polymer network