Solving the physical and economic limits of multi-patterning photolithography has become a critical priority for the semiconductor industry as physical scaling approaches sub-2nm process geometries. Researchers at the University of Missouri (Mizzou), supported by the National Science Foundation (NSF), have developed a novel approach that enables selective-area atomic layer deposition using light to directly pattern nanoscale semiconductor devices.

By exploiting photochemically-induced variations in molecular "stickiness", the team has demonstrated a fundamentally new form of light-guided Atomic Layer Deposition (ALD) that can create nanometer-scale transistors and interconnects with simplified process flows and reduced environmental impact.

1. Overcoming the Limits of Conventional Photolithography

Modern nanometer-scale chip manufacturing relies on extreme ultraviolet (EUV) photolithography and sophisticated area-selective deposition (ASD) masks to pattern silicon wafers with unprecedented precision. However, these approaches have significant limitations that become increasingly challenging as process geometries shrink below 5nm:

Etch Complexity & Yield Loss: Conventional photolithography requires sequential application, exposure, and removal of photoresist polymers followed by aggressive wet-chemical etching that introduces opportunities for defect generation at nanometer-scale dimensions.

Extreme Cost Escalation: As feature sizes shrink below 5nm, multiple patterning steps (e.g., SAQP, SALELE) are required to achieve the necessary resolution, exponentially increasing both mask costs and cycle time.

Environmental Impact: Chemical resist films and solvent developer waste create substantial hazardous material (hazmat) disposal challenges.

2. Photochemistry Enables "Sticky" Surface Modification

The Mizzou team has developed a novel approach to area-selective atomic layer deposition that exploits the ability of ultraviolet (UV) light to alter molecular bonding interactions at solid surfaces:

UV-Induced Molecular "Stickiness": Instead of depositing resist films on a crystal wafer, researchers use a low-power UV laser to expose specific regions of a semiconductor substrate to modify its surface chemistry.

Atomic Layer Deposition (ALD): Gaseous chemical precursors are then introduced into the ALD chamber where they selectively bind to UV-exposed sites on the semiconductor surface, forming monolayer-thick films with sub-nanometer control.

Pattern-Free Etch Process: The light-induced deposition replaces the complex photolithography-etch sequence with a single step that directly creates the desired transistor or interconnect geometry.

3. Manufacturing & Environmental Implications

The shift to light-assisted ALD provides compelling advantages across manufacturing yield, process complexity, and environmental impact:

Process Simplification: Eliminating multiple resist/etch cycles reduces cycle time by an order of magnitude for advanced node patterning.

Material Efficiency: Postdoctoral researcher Andreas Werbrouck notes that "by bypassing traditional photolithography, we're able to reduce both the consumption and hazmat disposal of chemical reagents."

Yield Enhancement: Light-based direct-write approaches eliminate edge-placement error (EPE) that becomes a critical yield limitation for gate-all-around (GAA) and CFET device architectures at sub-2nm nodes.

4. Foundry Adoption Strategy Enabled by NSF Funding

With support from the NSF microelectronics program, the Mizzou team is working to transition this light-activated patterning approach from the R&D lab to high-volume manufacturing:

Material Expansion: Researchers are exploring alternative UV-absorbing precursors that would enable patterning of novel metal alloys, high- dielectrics, and 2D transition-metal dichalcogenides (TMDs).

Manufacturing Integration: Existing foundry ALD tools could be retrofitted with modified optics to enable area-selective patterning without requiring a complete process flow overhaul.

Sources