Photo lithographyPhoto lithography

Size Scales Accessible to Nanofabrication Approach

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 …

• 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

Patterning process consists of mask design, mask fabrication and wafer printing.

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

Photoresist Applying

Spindle

PR dispenser nozzle

Chuck

Wafer

To vacuum pump

Photoresist Spin Coating

Spindle

To vacuum pump

PR dispenser nozzle

Chuck

PR suck back

Wafer

Optical Edge Bead Removal Exposure

Spindle

Chuck

Wafer

Photoresist

Light source

Light beam

Exposed Photoresist

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.

-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 ??

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.

Photolithography process

Lift off

The liftoff process for pattern transfer.

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