Glass Substrate Center Area Flatness via Segmented Polishing

Solution Overview

Problem

Conventional glass substrates for mask blanks fail to achieve sufficient flatness at the center area, leading to inadequate pattern overlay accuracy in semiconductor processes due to polishing pad hardness issues.

Innovation Solution

A glass substrate with a rectangular main surface featuring a quadrangular peripheral frame and a square center area, where the surface morphology is expressed using Legendre polynomials to maintain flatness below 20 nm, combined with a polishing process using a porous resin layer with a 100% modulus between 5 MPa and 14 MPa to prevent degradation of flatness.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If a hard polishing pad is used in rough polishing step, then polishing efficiency is improved, but flatness of center area deteriorates

Engineering Contradiction:

Improvepolishing efficiencyVSAvoidflatness of center area

Core Design Contradiction:

ProductivityVSManufacturing precision

Solution Approach 1:

The polishing process is divided into three distinct steps (rough polishing, intermediate polishing, and finish polishing), each using polishing pads with progressively softer properties. This segmentation allows the hard pad to efficiently remove material in the rough polishing step while subsequent softer pads restore and maintain flatness in the center area during intermediate and finish polishing steps.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The rough polishing step using a hard polishing pad is performed first to pre-remove material and establish the basic surface geometry. This preliminary action prepares the surface for subsequent polishing steps that will refine and correct the flatness, particularly in the center area where circuit patterns will be formed.

Inventive Principle:

Principle #10Preliminary action

2Manufacturing precision

If a soft polishing pad is used in fine polishing step, then flatness is improved, but polishing efficiency deteriorates

Engineering Contradiction:

Improveflatness of center areaVSAvoidpolishing efficiency

Core Design Contradiction:

Manufacturing precisionVSProductivity

Solution Approach 1:

The polishing process is divided into three distinct steps (rough polishing, intermediate polishing, and finish polishing), each using polishing pads with progressively softer properties. This segmentation allows the hard pad to efficiently remove material in the rough polishing step while subsequent softer pads restore and maintain flatness in the center area during intermediate and finish polishing steps.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The rough polishing step using a hard polishing pad is performed first to pre-remove material and establish the basic surface geometry. This preliminary action prepares the surface for subsequent polishing steps that will refine and correct the flatness, particularly in the center area where circuit patterns will be formed.

Inventive Principle:

Principle #10Preliminary action

3Manufacturing precision

If local polishing is conducted after global polishing, then flatness of center area is improved, but process complexity increases

Engineering Contradiction:

Improveflatness of center areaVSAvoidprocess complexity

Core Design Contradiction:

Manufacturing precisionVSDevice complexity

Solution Approach 1:

The polishing pads are designed with spatially varying properties, particularly in the radial direction. The nap layer has different thicknesses at different radial positions, creating locally optimized polishing characteristics. The center portion of the pad has different properties than the peripheral portion, allowing simultaneous global material removal and local flatness control in a single polishing operation.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The invention merges the functions of global material removal and local flatness control into a single polishing step by using a polishing pad with radially varying nap layer thickness. This combines what would traditionally require separate global and local polishing operations into one unified process, reducing overall process complexity.

Inventive Principle:

Principle #5Merging (Combining)

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

The solution improves the flatness of the center area and enhances pattern overlay accuracy by maintaining low and high-order component flatness within 20 nm, reducing deformation and improving exposure accuracy.

Implementation Method 1

a hard polishing pad is used in a rough polishing step whereas a polishing pad having a porous resin layer (referred to as nap layer) is used in a fine polishing step

Methodology Applied

Scientific EffectAbrasion: Abrasion

Data Source

PatentUS10146125B2Glass substrate for mask blank, mask blank, photomask, and method for manufacturing glass substrate for mask blank

Publication Date: 2018.12.04 AGC INC

AI summary

A glass substrate for a mask blank includes a rectangular-shaped main surface on which a film having a circuit pattern is to be formed. The main surface includes a quadrangular peripheral frame and a square-shaped center area defined by excluding the frame. The center area has a longitudinal length of 142 mm and a lateral length of 142 mm. A surface morphology of the center area is expressed by{z⁡(x,y)=∑k=0N1⁢∑l=0N2⁢akl⁢Pk⁡(x)⁢Pl⁡(y)Pk⁡(x)=12k⁢k!⁢dkdxk⁡[(x2-1)k]Pl⁡(y)=12l⁢l!⁢dldyl⁡[(y2-1)l].A flatness of a sum of compositing all of aklPk(x)Pl(y) is less than or equal to 20 nm when a sum of k and l is greater than or equal to 3 and less than or equal to 9. The flatness is less than or equal to 20 nm when a sum of k and l is greater than or equal to 10 and less than or equal to 30.