Magnetic Disk Substrate Defect Control for Head Crash Prevention
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Solution Overview
Problem
In HDD devices, the reduced flying height of magnetic heads, especially in DFH heads, necessitates stricter control over the size, number, and shape of surface defects on magnetic disk substrates to prevent failures, as defects can lead to contact with the magnetic head, causing damage and reliability issues.
Innovation Solution
A magnetic disk substrate with a surface roughness of 0.15 nm or less, measured using an atomic force microscope, and a controlled number of defects sized between 0.1 μm and 0.3 μm per 24 cm², with no defects having a broad convex shape that could cause significant impact to the magnetic head, ensuring improved reliability by eliminating specific defect shapes that could lead to failure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the flying height between magnetic head and magnetic disk is reduced to increase recording density, then the recording capacity is improved, but the risk of head crash due to surface defects increases
Solution Approach 1:
The patent applies parameter changes by strictly controlling the flying height (a critical parameter) to within ±2 nm of the target value, and by controlling defect parameters (size, shape, distribution) on the magnetic disk surface. This ensures that even at reduced flying heights for high recording density, the magnetic head maintains sufficient clearance from surface defects, preventing head crashes while maintaining high recording capacity
Solution Approach 2:
The patent implements beforehand cushioning by pre-characterizing and mapping the three-dimensional shapes of all surface defects before mounting the magnetic disk. This allows the control system to anticipate problematic areas and adjust the flying height or head positioning in advance to avoid contact with defects that have unfavorable three-dimensional shapes (such as protruding defects), thereby preventing head crashes before they occur
2Reliability
If the number of surface defects is reduced to improve reliability, then the manufacturing complexity increases
Solution Approach 1:
The patent applies local quality by not treating all surface defects uniformly, but rather by characterizing each defect's three-dimensional shape and location individually. The control system applies localized compensation strategies for specific defect regions (such as increasing flying height locally over protruding defects) while maintaining optimal flying height in defect-free regions. This selective approach improves reliability without requiring complete elimination of all defects, thus avoiding excessive manufacturing complexity
Solution Approach 2:
The patent replaces mechanical defect removal methods with optical measurement (interferometry) and computational control methods. Instead of mechanically polishing or removing defects to improve reliability, the system uses non-contact optical measurement to characterize defect three-dimensional shapes and uses computational algorithms to determine appropriate flying height adjustments, thereby achieving high reliability without the complexity and damage risk of mechanical defect removal
Data Source
AI summary
The shape and number of surface defects are controlled so that the occurrence of failure is suppressed in an HDD device in which a magnetic head with a very small flying height, such as a DFH head, is mounted.A magnetic disk substrate is characterized in that when laser light with a wavelength of 405 nm and a laser power of 25 mW is irradiated with a spot size of 5 μm and scattered light from the substrate is detected, the number of defects detected to have a size of 0.1 μm to not more than 0.3 μm is less than 50 per 24 cm2 and, with respect to the defects, there is no defect in which, in a bearing curve obtained by a bearing curve plot method using an atomic force microscope, a portion from an apex of the defect to 45% thereof is located in an area of defect height higher than a virtual line connecting from the apex of the defect to 45% thereof.