Hydrostatic Pad Pocket Design for Wafer Grinding
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Solution Overview
Problem
Double side grinding of semiconductor wafers often results in nanotopology degradation due to misalignment of hydrostatic pad and grinding wheel clamping planes, leading to undesirable surface features like center-marks and B-rings, which cannot be removed by subsequent processing and affect the ability to print miniaturized circuitry.
Innovation Solution
A hydrostatic pad design with a reduced total pocket surface area ratio to working surface area, featuring radially opposed pockets that provide a fluid barrier and minimize clamping pressure at the peripheral edges of the grinding wheels, allowing the wafer to conform to movement without sharp bending and reducing localized stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If hydrostatic pads with large pocket surface area are used to provide strong clamping force, then the wafer is held firmly during grinding, but misalignment causes sharp bending and high localized stress at peripheral edges
Solution Approach 1:
The patent applies local quality by creating a free region at the peripheral edges of the hydrostatic pad where no clamping pressure is applied. This localized modification allows the wafer to conform to grinding wheel movement without sharp bending at the edges, while maintaining firm clamping in the central region. The free region is specifically positioned at the periphery to eliminate localized stress without compromising overall clamping effectiveness.
Solution Approach 2:
The patent segments the hydrostatic pad surface into two distinct functional zones: a central clamping region with pockets that provide firm holding force, and a peripheral free region that allows movement conformance. This segmentation enables different parts of the pad to perform different functions - the center maintains wafer position while the periphery accommodates grinding wheel misalignment without causing stress.
2Manufacturing precision
If grinding wheels are positioned with clamping planes aligned, then uniform grinding is achieved, but alignment must be frequently adjusted during operation
Solution Approach 1:
The patent applies beforehand cushioning by designing the hydrostatic pad with a free region at the periphery that anticipates and accommodates grinding wheel misalignment before it causes damage. This free region acts as a buffer that absorbs the effects of alignment deviations, allowing the system to maintain uniform grinding without frequent adjustments even when minor misalignments occur during operation.
3Stability of the object's composition
If rigid clamping is applied to hold wafer position, then wafer stability is improved, but nanotopology degradation increases due to friction and stress
Solution Approach 1:
The patent applies flexible shells and thin films by allowing the wafer to flex slightly within the free region at the periphery of the hydrostatic pad. This flexibility enables the wafer to conform to grinding wheel movement without experiencing high friction or stress that would cause nanotopology degradation, while the central clamping region maintains overall position stability.
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 solution effectively minimizes nanotopology degradation, ensuring uniform grinding and reducing the formation of center-marks and B-rings, resulting in improved surface quality with average peak to valley variations less than 12 nm, maintaining this quality over extended grinder operation without frequent alignment adjustments.
Implementation Method 1
The hydrostatic pads beneficially produce a fluid barrier between the respective pad and wafer surface for holding the wafer without the rigid pads physically contacting the wafer during grinding
Implementation Method 2
This reduces damage to the wafer that may be caused by physical clamping and allows the wafer to move (rotate) tangentially relative to the pad surfaces with less friction
Data Source
AI summary
A hydrostatic pad for use in holding a semiconductor wafer during grinding of the wafer by grinding wheels. The pad includes hydrostatic pockets formed in a face of the body directly opposed to the wafer. The pockets are adapted for receiving fluid through the body and into the pockets to provide a barrier between the body face and the workpiece while still applying pressure to hold the workpiece during grinding. The hydrostatic pads allow the wafer to rotate relative to the pads about their common axis. The pockets are oriented to reduce hydrostatic bending moments that are produced in the wafer when the grinding wheels shift or tilt relative to the hydrostatic pads, helping prevent nanotopology degradation of surfaces of the wafer commonly caused by shift and tilt of the grinding wheels.