Photomask Design for Thermal Expansion Reduction

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Solution Overview

Problem

The semiconductor photolithography process is affected by thermal expansion of the photomask, leading to position deviations in pattern formation on silicon wafers, particularly during the zero layer formation, due to varying influences from different machines.

Innovation Solution

Designing a photomask with a reverse tone and increasing the open ratio from less than 25% to 75% or more, reducing the area of the light shielding layer, thereby minimizing the impact of thermal expansion during exposure-development processes.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Object-affected harmful factors

If a photomask with a large-area light shielding layer is used, then the photomask can effectively shield light during exposure, but the thermal expansion of the photomask increases, causing position deviations in pattern formation

Engineering Contradiction:

Improvelight shielding effectivenessVSAvoidpattern position accuracy

Core Design Contradiction:

Object-affected harmful factorsVSManufacturing precision

Solution Approach 1:

The patent inverts the conventional photomask design by using a reverse tone photomask with high open ratio (75% or more) instead of the traditional low open ratio design. This inversion reduces the light shielding layer area to minimize thermal expansion while maintaining effective light shielding through the reversed tone pattern, thereby resolving the contradiction between light shielding effectiveness and pattern position accuracy.

Inventive Principle:

Principle #13The other way round (Inversion)

Solution Approach 2:

The patent changes the key parameter of open ratio from less than 25% to 75% or more, fundamentally altering the photomask design. This parameter change reduces the light shielding layer area, minimizing thermal expansion effects and improving pattern position accuracy while maintaining adequate light shielding through the reversed tone configuration.

Inventive Principle:

Principle #35Parameter changes

2Manufacturing precision

If the open ratio of the photomask is increased to reduce thermal expansion, then the area of the light shielding layer decreases, but the light shielding effectiveness may be compromised

Engineering Contradiction:

Improvepattern position accuracyVSAvoidlight shielding effectiveness

Core Design Contradiction:

Manufacturing precisionVSObject-affected harmful factors

Solution Approach 1:

The patent applies reverse tone design where the photomask pattern is inverted compared to conventional designs. This allows the photomask to achieve high open ratio (75% or more) while maintaining effective light shielding, as the reversed tone pattern ensures that light is blocked where needed and transmitted where required, resolving the contradiction between reduced thermal expansion and maintained shielding effectiveness.

Inventive Principle:

Principle #13The other way round (Inversion)

3Productivity

If a KrF laser light source is used for exposure, then the alignment mark can be effectively exposed, but the thermal expansion of the photomask varies significantly between different machines, affecting subsequent layer patterns

Engineering Contradiction:

Improveexposure efficiencyVSAvoidpattern position consistency

Core Design Contradiction:

ProductivityVSManufacturing precision

Solution Approach 1:

The patent applies preliminary anti-action by designing the photomask in advance with reverse tone and high open ratio to preemptively counteract the thermal expansion effects caused by KrF laser exposure. This preliminary design adjustment compensates for the known thermal expansion issue before exposure occurs, ensuring consistent pattern positioning across different machines while maintaining high exposure efficiency.

Inventive Principle:

Principle #9Preliminary anti-action

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

This approach significantly reduces the size variation of the zero layer, enhancing the accuracy and precision of the semiconductor photolithography process by minimizing the influence of thermal expansion on the photomask.

Implementation Method 1

a light shielding layer disposed on the transparent substrate

Methodology Applied

Scientific EffectLight shielding: Absorption (EM radiation)

Implementation Method 2

exposure-development may be performed by using a KrF laser light source

Methodology Applied

Scientific EffectPhotopolymerization: Photopolymerisation

Data Source

PatentUS11143973B2Method for designing photomask

Publication Date: 2021.10.12 POWERCHIP SEMICON MFG CORP

AI summary

A method for designing a photomask includes calculating an open ratio of an initial photomask to determine whether the open ratio of the initial photomask is less than 25%, and then changing a design of the initial photomask in response to determining the open ratio is less than 25%, such that a changed photomask has a reverse tone to the design of the initial photomask, and an open ratio of the changed photomask is 75% or more. The method can solve the issue caused by thermal expansion of the photomask.