Litho-Freeze-Litho-Etch (Lfle)

Litho-Freeze-Litho-Etch (LFLE) is an advanced multi-patterning technique that creates dense patterns by performing two separate lithography exposures on the same layer, with a "freeze" step in between to protect the first pattern from being disrupted by the second exposure.

How LFLE Works

• First Litho: Apply photoresist, expose with the first pattern, and develop to create pattern A.
• Freeze: Chemically treat (cross-link) the developed resist pattern to make it insoluble in the developer chemistry used for the second exposure. This "freezes" pattern A in place.
• Second Litho: Apply a second resist layer over the frozen first pattern. Expose with the second pattern (shifted by half-pitch) and develop to create pattern B.
• Etch: Both patterns A and B are now present on the wafer and are transferred into the underlying material in a single etch step.

The Freeze Step

• The critical innovation is the ability to render the first resist pattern chemically resistant to the second lithography process.
• Early approaches used thermal cross-linking agents or surface treatment chemicals.
• The first pattern must survive: (1) second resist coating (spin-on), (2) second exposure bake, and (3) second development — all without distortion.

Advantages

• Pitch Doubling: Creates features at half the pitch achievable by a single exposure — effectively doubling pattern density.
• Design Freedom: Both exposures are independent lithography steps, allowing more complex pattern combinations than spacer-based methods.
• No Spacer Process: Avoids the film deposition and etch steps needed for SADP (self-aligned double patterning).

Challenges

• Overlay: Two separate exposures must align to each other with sub-nanometer accuracy. Overlay errors directly become pattern placement errors.
• Freeze Process Control: The freeze must be complete and uniform — incomplete freezing causes pattern degradation.
• CD Control: Both exposure/develop cycles must produce well-controlled feature widths.
• Throughput: Two exposures per layer halve throughput compared to single exposure.

LFLE vs. Other Multi-Patterning

• SADP (Self-Aligned Double Patterning): Uses spacers — self-aligned, better placement but limited pattern freedom.
• LELE (Litho-Etch-Litho-Etch): Etches each pattern separately — avoids freeze but requires two etch steps.
• LFLE: One etch step, good design flexibility, but depends on freeze quality.

LFLE was explored as a potential multi-patterning solution for nodes beyond ArF immersion, though EUV lithography ultimately reduced the need for complex multi-patterning in most leading-edge applications.