Dry resist (also called dry film resist) refers to photoresist materials applied as solid thin films rather than liquid solutions spun onto the wafer. This approach eliminates the traditional spin-coating process and offers potential advantages for certain patterning applications.

How Dry Resist Works

• Traditional Liquid Resist: A resist solution is dispensed onto a spinning wafer. Centrifugal force spreads it into a uniform film. The solvent evaporates during a soft bake, leaving a solid resist layer.
• Dry Resist Approaches:
• Dry Film Lamination: A pre-formed solid resist film is laminated onto the wafer surface under heat and pressure.
• Chemical Vapor Deposition (CVD): Resist material is deposited from vapor phase directly onto the wafer.
• Physical Vapor Deposition: Resist is evaporated or sputtered onto the wafer.

Why Dry Resist?

• Topography Coverage: Liquid spin-coating struggles with severe topography — resist pools in recesses and thins on elevated features. Dry film or CVD resist can achieve more uniform coverage over 3D structures.
• No Spin Defects: Eliminates defects associated with spin-coating: comets, striations, edge bead, and particles from dispensing.
• Ultrathin Films: CVD processes can deposit extremely thin resist films (sub-20 nm) with excellent uniformity — difficult to achieve by spin-coating.
• Material Flexibility: Some resist materials are not soluble in suitable solvents for spin-coating. Dry deposition enables new material options.

Applications

• High Aspect Ratio Structures: MEMS, through-silicon vias (TSVs), and 3D packaging with severe topography.
• Metal-Oxide Resists for EUV: Some metal-oxide resist formulations are deposited by CVD or sputtering rather than spin-coating.
• Wafer-Level Packaging: Thick dry film resists (tens of microns) for bumping and redistribution layer (RDL) patterning.
• Advanced EUV: Exploring vapor-deposited resist for ultrathin, uniform EUV resist layers.

Challenges

• Film Quality: Achieving the same defect density and uniformity as mature spin-coating processes is difficult.
• Process Integration: Different equipment, handling, and process flows compared to established spin-coat-based lithography.
• Adhesion: Ensuring good adhesion of dry film to various substrate materials without the solvent-surface interaction that helps spin-coated resist adhesion.
• Throughput: CVD-based resist deposition may be slower than spin-coating for thin films.

Dry resist is a niche but growing technology — its importance is increasing as 3D packaging demands increase and EUV resist development explores non-traditional deposition methods.