Magnetic Disk Glass Substrate Side Wall Geometry Control
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Solution Overview
Problem
High track recording density in HDDs leads to unstable servo signal reading errors at the outer circumferential side edge of magnetic disks, particularly due to factors other than roundness, such as inclination and unevenness of the side wall surface, which are not adequately addressed by conventional measurement methods.
Innovation Solution
A magnetic-disk glass substrate with a side wall surface having a roundness of 1.5 μm or less and a cylindricity of 5 μm or less, featuring chamfered surfaces and a specific shape evaluation value, which reduces flutter and enhances tracking performance by minimizing air turbulence.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the track recording density is increased to achieve high storage capacity, then the storage capacity is improved, but servo signal reading stability deteriorates due to air turbulence and flutter at the outer circumferential edge
Solution Approach 1:
The patent applies local quality by differentiating the measurement and control requirements for different regions of the disk. Specifically, the side wall surface roundness is measured at multiple positions (central position and positions spaced 200μm apart in the thickness direction) with different tolerances. The central position requires roundness of 1.5μm or less, while positions 200μm away require roundness of 3μm or less. This localized quality control ensures that the critical central region has superior geometry to minimize air turbulence, while allowing more tolerance in peripheral regions.
Solution Approach 2:
The patent transitions from conventional two-dimensional roundness measurement (single plane) to three-dimensional cylindricity measurement. By measuring the side wall surface roundness at multiple positions along the thickness direction (central position and positions spaced 200μm apart), the patent evaluates the geometric accuracy in three dimensions. This dimensional expansion allows comprehensive control of the side wall surface geometry, ensuring smooth air flow through the gap between the disk and HDD inner wall, thereby reducing flutter and improving servo signal stability.
2Device complexity
If conventional roundness measurement methods are used, then measurement simplicity is maintained, but measurement precision is insufficient to detect side wall surface inclination and unevenness
Solution Approach 1:
The patent segments the side wall surface measurement into multiple discrete positions along the thickness direction. The central position and positions spaced 200μm apart are measured independently, with each position evaluated for roundness. This segmentation allows detection of local geometric deviations such as inclination and unevenness that would be averaged out in a single-plane measurement, thereby improving measurement precision without requiring complex measurement equipment.
Solution Approach 2:
The patent introduces the concept of cylindricity as an intermediary measurement parameter that bridges conventional roundness measurement and comprehensive geometric evaluation. By measuring roundness at multiple positions and evaluating the overall cylindricity, the patent creates a practical measurement system that detects side wall surface inclination and unevenness while remaining implementable with standard measurement equipment, thus balancing measurement precision with device complexity.
Data Source
AI summary
A magnetic-disk glass substrate capable of suppressing turbulence of air flow in a vicinity of an outer circumferential side edge portion of the magnetic disk and suppressing disk flutter is provided. This magnetic-disk glass substrate includes a pair of main surfaces, a side wall surface formed on an outer circumferential side edge surface, and chamfered surfaces interposed between the side wall surface and the main surfaces, respectively. The side wall surface has a roundness of 1.5 μm or less. A difference in radius between an inscribed circle and a circumcircle of a plurality of outlines of the side wall surface at a plurality of positions that include a central position of the magnetic-disk glass substrate in a thickness direction and are different from each other in the thickness direction is 5 μm or less.