Wafer Alignment Features Using Contrasting Material Trenches
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Solution Overview
Problem
Conventional wafer fabrication processes lack precise alignment capabilities for back side wafer processing without incurring additional cost and complexity, as conventional steppers do not have infrared (IR) capabilities.
Innovation Solution
Forming trenches in the wafer from the top side, lining them with contrasting materials like oxide or nitride, and grinding the bottom side to expose these trenches, which serve as alignment references for precise alignment during back side processing using a conventional stepper or 1× aligner.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a stepper tool with IR capability is used to achieve precise alignment for back side wafer processing, then alignment accuracy is improved, but cost and device complexity increase
Solution Approach 1:
The patent creates optical copies (images) of alignment features on both the front and back surfaces of the wafer. By forming trenches on the front side, filling them with contrasting material, and grinding the back side to expose corresponding trenches, the alignment system uses optical imaging to project and align these features without requiring complex IR capabilities. The alignment tool captures images of the contrasting material in the trenches and uses image processing to achieve precise alignment.
Solution Approach 2:
The patent changes the optical parameters of the wafer structure by introducing trenches with contrasting material lining. This creates features with different optical properties (refractive index, absorption) that can be detected by conventional optical alignment tools. The contrasting material (such as oxide or nitride) provides strong optical contrast against the silicon, enabling precise alignment using standard optical imaging rather than requiring IR capabilities.
2Device complexity
If conventional steppers without IR capability are used for back side processing, then cost and device complexity are reduced, but alignment precision deteriorates to around 10 micrometers
Solution Approach 1:
The patent modifies the physical-optical parameters of the wafer by creating trenches with contrasting material lining. This structural modification enables conventional optical steppers to achieve precise alignment by providing high-contrast features that can be clearly imaged and measured. The contrasting material creates strong optical signals that improve the signal-to-noise ratio in alignment imaging, allowing sub-micron to micron-level alignment accuracy with standard optical tools.
Solution Approach 2:
The alignment features (trenches with contrasting material) are formed on the front side of the wafer before back side processing. This preliminary action ensures that the alignment references are already in place and can be used during subsequent back side processing steps. The trenches are etched, lined with contrasting material, and the wafer is then ground on the back side to expose corresponding trenches, creating ready-to-use alignment features.
3Measurement precision
If trenches are formed deeply through the wafer to expose alignment features on the back side, then alignment reference visibility is improved, but manufacturing complexity and time increase
Solution Approach 1:
The patent applies partial action by etching trenches that extend through the front side device layers and into the handle layer, but not completely through the wafer thickness. The trenches are etched to a sufficient depth to expose the contrasting material lining on the back side after grinding, but stop before penetrating entirely through. This partial etching approach provides adequate alignment reference visibility while avoiding the excessive time and complexity of complete through-etching.
Solution Approach 2:
The trenches are formed and lined with contrasting material on the front side before the back side grinding step. This preliminary preparation ensures that when the back side is ground to expose the trenches, the alignment features are already in place and ready for use. The sequence of operations (etch → line → grind → align) optimizes the process by preparing alignment features in advance.
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
Enables precise alignment (under 1-5 microns) for back side processing of wafers, such as MEMS and CMOS devices, without the need for IR-capable tools, by using the contrasting material as an alignment reference in a conventional stepper, thus improving alignment accuracy and reducing fabrication costs.
Implementation Method 1
the contrasting material lining the exposed trench provides an alignment reference for precise alignment of the wafer for back side processing the wafer
Implementation Method 2
grinding a bottom side of the wafer to expose the trench
Data Source
AI summary
A method for forming an alignment feature for back side wafer processing in a wafer fabrication process involves forming a trench into but not entirely through a wafer from a top side of the wafer; forming a contrasting material on surfaces of the trench; and grinding a bottom side of the wafer to expose the trench using the handling wafer to handle the wafer during such grinding, wherein the contrasting material lining the exposed trench provides an alignment reference for precise alignment of the wafer for back side processing the wafer.