Double-Sided Polishing of Sapphire Substrates
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Solution Overview
Problem
Current methods face challenges in achieving the necessary surface flatness and minimizing edge roll-off and stress-induced bow in large sapphire optical substrates during double-sided polishing, which are critical for stringent transmitted wave-front and beam deviation requirements.
Innovation Solution
A double-sided polishing method using an upper and lower platen with adjustable apertures and rotating carriers to ensure simultaneous polishing of both surfaces, with controlled pressure and speed, and optional mechanical bending of platen surfaces to maintain flatness and prevent edge roll-off.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If double-sided polishing is used to improve productivity and reduce stress-induced bow, then manufacturing efficiency improves and stress distribution balances, but achieving necessary surface flatness and minimizing edge roll-off becomes difficult due to large panel-to-platen size ratio and extreme material hardness
Solution Approach 1:
The patent applies different polishing conditions to different regions of the substrate by implementing variable pressure distribution across the platen surface and using multiple polishing zones with different pad characteristics. This allows edge regions to receive reduced pressure to minimize roll-off while central regions maintain higher pressure for flatness control, resolving the contradiction between productivity and precision.
Solution Approach 2:
The patent employs dynamic adjustment of polishing parameters during the process, including real-time modification of platen rotation speeds, pressure application, and pad positioning based on measured substrate response. This dynamic control enables maintenance of optimal surface flatness while sustaining high productivity through adaptive process optimization.
2Device complexity
If conventional single-sided polishing is used to maintain simplicity, then process complexity remains low, but repeated flipping and reworking are required to meet transmitted wave-front error requirements
Solution Approach 1:
The patent combines both polishing surfaces into a single simultaneous operation by using a specialized double-sided platen assembly that contacts both surfaces of the substrate at once. This merging of operations eliminates the need for repeated flipping and reworking, reducing time loss while maintaining manageable process complexity through integrated tooling design.
3Productivity
If high pressure is applied to improve polishing rate, then productivity increases, but edge roll-off and surface distortion increase
Solution Approach 1:
The patent implements spatially variable pressure distribution where the magnitude and direction of applied force differ across the platen surface. Edge regions experience reduced or tangential pressure to prevent roll-off, while central regions receive higher normal pressure for efficient material removal, thereby maintaining both productivity and shape integrity.
Solution Approach 2:
The patent introduces controlled curvature or camber to the platen surfaces and pressure application profiles, using arched or non-uniform pressure distributions that naturally redirect forces away from edges. This geometric approach to force distribution maintains high overall pressure for productivity while protecting edge regions from excessive stress that causes roll-off.
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 method achieves high flatness and reduced edge roll-off across the entire window, meeting stringent optical performance criteria with final transmitted wave-front error less than 0.25 waves rms and beam deviation less than 5 arc-seconds, while avoiding stress-induced bow.
Implementation Method 1
The upper platen and the lower platen are rotated with respect to the carrier, and the carrier is rotated with respect to the upper platen and the lower platen to polish the optical substrate
Implementation Method 2
both surfaces are polished at the same time. Simultaneous removal of nearly equal amounts of material from both surfaces
Data Source
AI summary
Disclosed is a method and apparatus for simultaneously polishing both surfaces of an optical substrate. An upper platen and a lower platen, each covered with a polishing pad material and at least one carrier having an aperture for holding the optical substrate between the platens are provided. The location of the aperture of the carrier is set such that the center of the optical substrate is offset from the center of the carrier and at least a portion of the outer perimeter of the optical substrate extends outwardly beyond at least a portion of at least one of the outer perimeter and the inner perimeter of the platens. The platens are rotated with respect to the carrier, and the carrier is rotated with respect to the platens to polish the optical substrate. The location of the aperture of the carrier is adjustable.