Infrared alignment is the alignment technique that uses infrared transmission through silicon to view frontside marks from the backside during lithography registration - it is widely used for front-to-back overlay in thinned-wafer processing.

What Is Infrared alignment?

• Definition: Optical alignment method leveraging silicon transparency at selected infrared wavelengths.
• Use Case: Registers backside masks to hidden frontside alignment targets.
• System Requirements: Needs IR-capable optics, calibrated mark recognition, and distortion correction.
• Thickness Dependency: Transmission quality depends on wafer thickness and material stack absorption.

Why Infrared alignment Matters

• Overlay Precision: Enables accurate backside pattern placement relative to device features.
• Yield Improvement: Reduces misalignment-driven electrical failures.
• Process Flexibility: Supports complex dual-side patterning without destructive references.
• Advanced Packaging Support: Critical for TSV reveal and backside contact modules.
• Metrology Confidence: IR visibility improves alignment verification on bonded stacks.

How It Is Used in Practice

• Mark Engineering: Design alignment marks optimized for infrared contrast and detectability.
• Optics Calibration: Compensate for refraction and distortion across wafer thickness variation.
• Overlay SPC: Continuously monitor IR alignment error and apply tool corrections.

Infrared alignment is a core enabler for dual-side lithography registration - infrared alignment allows precise backside processing in advanced wafer stacks.