Photomask Absorption Layer Interference Measurement for Refractive Index

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Solution Overview

Problem

The refractive index of the absorption layer in photomasks used in advanced lithography processes is critical for precise pattern transfer, but existing methods lack accurate measurement techniques, leading to potential errors in exposure patterns due to variations in optical properties.

Innovation Solution

Incorporation of test and reference patterns in the absorption layer, utilizing double-slit interference measurements to determine the complex refractive index by comparing interference patterns formed by light waves propagating through trenches of varying depths and widths, allowing for precise measurement of the absorption layer's optical properties.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If conventional measurement methods are used for the absorption layer, then the manufacturing process is simple, but the measurement precision of the refractive index is insufficient leading to exposure pattern errors

Engineering Contradiction:

Improverefractive index measurement precisionVSAvoidmeasurement structure complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The measurement structure is segmented into distinct functional components: test patterns (with varying trench depths and widths), reference patterns, and calibration regions. This segmentation allows for systematic measurement of refractive index by comparing interference patterns from different trench configurations, thereby improving measurement precision while maintaining manageable complexity through modular design.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Interference patterns serve as an intermediary medium between the absorption layer's optical properties and the measurable quantities. By introducing test and reference patterns that generate interference patterns when illuminated, the patent enables indirect but precise measurement of the refractive index, transforming an otherwise difficult-to-measure property into observable interference fringes that can be analyzed quantitatively.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Manufacturing precision

If the absorption layer optical properties are not accurately measured, then the measurement process is quick and simple, but the exposure pattern accuracy deteriorates

Engineering Contradiction:

Improveexposure pattern accuracyVSAvoidmeasurement time

Core Design Contradiction:

Manufacturing precisionVSLoss of time

Solution Approach 1:

The test and reference patterns are pre-formed into the absorption layer during manufacturing, enabling refractive index measurements to be performed at any stage without requiring additional processing steps. This preliminary incorporation of measurement structures allows for quick, non-destructive measurements that ensure exposure pattern accuracy without adding significant time to the manufacturing process.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The absorption layer itself serves the dual function of both the functional layer for pattern transfer and the measurement medium. By embedding test and reference patterns directly within the absorption layer, the structure performs self-measurement of its own optical properties, eliminating the need for separate measurement devices or processes and thereby maintaining manufacturing efficiency while improving exposure accuracy.

Inventive Principle:

Principle #25Self-service

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

Enables accurate determination of the refractive index, ensuring consistent and precise exposure patterns in photoresist layers, thereby improving the manufacturing process of semiconductor integrated circuits.

Implementation Method 1

an absorption layer 140 formed on the capping layer 130

Methodology Applied

Scientific EffectAbsorption: Absorption (EM radiation)

Implementation Method 2

the complex refractive index by comparing interference patterns formed by light waves propagating through trenches

Methodology Applied

Scientific EffectRefraction: Refraction

Implementation Method 3

utilizing double-slit interference measurements to determine the complex refractive index by comparing interference patterns formed by light waves propagating through trenches of varying depths and widths

Methodology Applied

Scientific EffectInterference: Interference

Data Source

PatentUS12578652B2Photomask and methods for measuring and manufacturing the photomask

Publication Date: 2026.03.17 TAIWAN SEMICONDUCTOR MANUFACTURING CO LTD

AI summary

A method includes forming a test pattern and a reference pattern in an absorption layer of a photomask structure. The test pattern has a first trench and a second trench, the reference pattern has a third trench and a fourth trench, the test pattern and the reference pattern have substantially the same dimension in a top view, and the second trench is deeper than the first trench, the third trench, and the fourth trench. The method further includes emitting a light beam to the test pattern to obtain a first interference pattern reflected from the test pattern, emitting the light beam to the reference pattern to obtain a second interference pattern reflected from the reference pattern; and comparing the first interference pattern with the second interference pattern to obtain a measured complex refractive index of the absorption layer.