Glass Substrate Edge Surface Roughness for SN Ratio

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Solution Overview

Problem

Magnetic-disk glass substrates with flying heights of 1 nm or less experience a decrease in signal-to-noise (SN) ratio due to corrosion-derived substances from the glass edge surfaces, which attach to recording and reproduction elements, hindering recording and reproduction processes.

Innovation Solution

A magnetic-disk glass substrate with an alkali metal component and a mirror-finished edge surface, where the roughness percentage in the bearing ratio curve of roughness cross-sectional areas is 40% or more after etching, is developed to prevent corrosion by minimizing latent cracks and surface defects.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Quantity of substance

If the flying height is reduced to 1 nm or less to increase recording density, then the storage capacity increases, but corrosion-derived substances from glass edge surfaces attach to recording elements causing SN ratio degradation

Engineering Contradiction:

Improverecording densityVSAvoidSN ratio

Core Design Contradiction:

Quantity of substanceVSReliability

Solution Approach 1:

The patent applies preliminary action by performing edge surface polishing and etching treatments on the glass substrate before the magnetic disk is assembled and used. The etching process (using HF acid or vapor) is conducted in advance to remove latent cracks and corrosion-prone regions from the edge surfaces, preventing corrosion-derived substances from detaching later during operation at low flying heights.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent applies preliminary anti-action by proactively eliminating the source of corrosion through edge surface treatment before the harmful effect can occur. By removing latent cracks and smoothing the edge surfaces in advance, the patent prevents the detachment of corrosion-derived substances that would otherwise contaminate the recording elements during low-flying-height operation.

Inventive Principle:

Principle #9Preliminary anti-action

2Ease of manufacture

If conventional polishing methods are used on edge surfaces, then manufacturing is simpler, but latent cracks remain causing corrosion and minute particle attachment

Engineering Contradiction:

Improveedge surface processingVSAvoidcorrosion and particle attachment

Core Design Contradiction:

Ease of manufactureVSObject-affected harmful factors

Solution Approach 1:

The patent introduces an intermediary substance (etching solution, specifically HF acid or vapor) between the glass edge surface and the final product. This intermediary etching process selectively removes material from the edge surfaces, eliminating latent cracks and smoothing the surfaces without requiring complex mechanical polishing equipment, thus resolving the contradiction between manufacturing simplicity and corrosion prevention.

Inventive Principle:

Principle #24Intermediary (Mediator)

Data Source

PatentUS11024335B2Magnetic-disk glass substrate

Publication Date: 2021.06.01 HOYA CORPORATION

AI summary

A magnetic-disk glass substrate containing an alkali metal component as a glass composition includes a pair of main surfaces and an outer circumferential side edge surface that is a mirror surface. The outer circumferential side edge surface includes a surface with a roughness percentage that is 40% or more when a bearing ratio of a roughness cross-sectional area is 50%, in a bearing ratio curve of roughness cross-sectional areas obtained when a surface roughness of the outer circumferential side edge surface obtained by etching the outer circumferential side edge surface by 2.5 μm is measured.