Glass Substrate Hole Profile Control for HDD Track Misregistration
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Solution Overview
Problem
High-speed rotational and increased recording density in HDDs lead to track mis-registration (TMR) issues due to variations in circularity, concentricity, and thickness of glass substrates, causing magnetic heads to lose track of servo information, which conventional definitions and parameters fail to adequately address.
Innovation Solution
A glass substrate with an inside hole featuring parameters such as skewness and kurtosis within specific ranges, and an amendment concentricity calculated from the profiles of the outer and inner circumferential surfaces, ensuring stable disk balance and TMR characteristics by evaluating the distribution of irregularities and relative positions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional definitions of circularity and concentricity are used to evaluate glass substrates, then manufacturing simplicity is maintained, but measurement precision and manufacturing precision deteriorate, leading to track mis-registration at high rotational speeds
Solution Approach 1:
The patent changes the parameters used to evaluate circularity and concentricity. Instead of using conventional parameters (circularity as difference between maximum and minimum radii, concentricity as offset between centers), it introduces new parameters: circularity is evaluated using root mean square (RMS) of radial deviations, and concentricity is evaluated using RMS of radial deviations of both outer and inner circumferential surfaces. This parameter transformation enables more precise measurement that accounts for distributed irregularities rather than just extreme values, thereby resolving the contradiction between measurement precision and evaluation complexity.
2Manufacturing precision
If glass substrates are produced with tight dimensional tolerances for circularity and concentricity, then manufacturing precision improves, but productivity decreases due to increased inspection and process control complexity
Solution Approach 1:
The patent replaces complex mechanical measurement and control systems with a simplified evaluation methodology. By substituting the conventional measurement approach (based on extreme value differences) with an RMS-based statistical approach, it enables more efficient data processing and evaluation. This substitution allows for rapid assessment of circularity and concentricity using standard measurement data already collected during manufacturing, reducing the need for additional complex measurement equipment and process steps, thereby maintaining high manufacturing precision while improving productivity.
3Stability of the object's composition
If the inside hole shape is made perfectly circular to prevent run-out, then stability of rotation improves, but manufacturing precision deteriorates due to difficulty in achieving perfect circularity
Solution Approach 1:
The patent changes the acceptance criterion for inside hole circularity from requiring perfect geometric circularity to accepting shapes that meet RMS-based circularity and concentricity specifications. By transforming the evaluation parameter from extreme value difference to root mean square of distributed deviations, it allows for practical manufacturing tolerances while ensuring rotational stability. The new parameter accounts for the overall distribution of irregularities around the hole perimeter rather than demanding perfect uniformity, making the specification achievable with conventional manufacturing processes while still preventing run-out issues.
Data Source
AI summary
With regard to a glass substrate 1 according to the present invention, a value of an amendment concentricity AC that has taken into consideration Sk and/or Ku calculated from a shape profile over the whole circumference of an inside hole, or the skewness is within a predetermined range. The glass substrate for a magnetic recording medium can stably read servo information including track information stored on a magnetic disk when the glass substrate is used for an HDD.