photo lithography - kocwelearning.kocw.net/contents4/document/wcu/2012/chungnam/... ·...
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Photo lithographyPhoto lithography
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Size Scales Accessible to Nanofabrication Approach
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Semiconductor processing
1. Ion Implantation
2. Oxidation
3. Thin Film-CVD
4. Photo Lithography
5. Dry Etch
6. Metallization
-MOSFETBulk FETGP-SOI FETDG-FinFETVertical MOSCNT-FET …
-MemoryFerroelectric RAMMagnetic RAMPhase-Change RAM …
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• Photo-litho-graphy: latin: light-stone-writing• Photolithography: an optical means for transferring patterns onto a substrate. • Patterns are first transferred to an imagable photoresist layer.• Photoresist is a liquid film that is spread out onto a substrate, exposed with a desired pattern, and developed into a selectively placed layer for subsequent processing.• Photolithography is a binary pattern transfer: there is no gray-scale, color, nor depth to the image.
Photolithography
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Patterning process consists of mask design, mask fabrication and wafer printing.
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Surface Cleaning• Standard degrease:
– 2-5 min. soak in acetone with ultrasonic agitation– 2-5 min. soak in methanol with ultrasonic agitation– 2-5 min. soak in DI water with ultrasonic agitation– 30 sec. rinse in DI water– spin-rinse dry for wafers;– nitrogen blow-off dry for small substrates
• Organic residues: oxygen plasma (strip 2-3 min 200-300 W)
• Hazards:– acetone is flammable– methanol is toxic by skin adsorption
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Photoresist Applying
Spindle
PR dispenser nozzle
Chuck
Wafer
To vacuum pump
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Photoresist Spin Coating
Spindle
To vacuum pump
PR dispenser nozzle
Chuck
PR suck back
Wafer
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Optical Edge Bead Removal Exposure
Spindle
Chuck
Wafer
Photoresist
Light source
Light beam
Exposed Photoresist
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Prebake (Soft Bake) • Used to evaporate the coating solvent and to densify the resist after spin coating.
• Typical thermal cycles:
– 90-100°C for 20 min. in a convection oven
– 75-85°C for 45 sec. on a hot plate
• Commercially, microwave heating or IR lamps are also used in production lines.
• Hot plating the resist is usually faster, more controllable, and does not trap solvent like convection oven baking.
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-positive PR은 쉽게 설명하면 빛이 노출된 부분이 화학적인 분해로 인해노광후 현상액에 씻겨나가는 PR.
-negative PR은 빛이 노출된 부분이 화학적으로 결합하여 노광후 빛이노출되지 않은 부분이 현상액에 씻겨나가는 PR.
- positive PR의 경우 AZ계열, negative PR의 경우 SU-8계열.
-positive PR의 최고의 장점은 현상후 removing이 잘되는게 장점.
- negative PR은 한번 노광하고 현상 후 왠만한 화학약품이나 열에 변형이 일어나지 않음.
장단점 ??
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Postbake (Hard Bake)• Firm postbake is needed for acid etching, e.g. BOE.
• Postbake is not needed for processes in which a soft resist is desired, e.g. metal liftoff patterning.
• Photoresist will undergo plastic flow with sufficient time and/or temperature:
– Resist reflow can be used for tailoring sidewall angles.
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Photolithography process
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Lift off
The liftoff process for pattern transfer.
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Characteristic Positive Negative
Adhesion to Silicon Fair Excellent
Relative Cost More Expensive Less Expensive
Developer Base Aqueous Organic
Minimum Feature 0.5 μm and below ± 2 μm
Step Coverage Better Lower
Wet Chemical Resistance
Fair Excellent
Positive & negative resist