Back-Illuminated Imager Fabrication Using SOI Substrates
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Solution Overview
Problem
The high cost and complexity of manufacturing back-illuminated image array devices stem from challenges such as low fill factor and sensitivity due to shadowing by metal bus lines, absorption by array circuitry, and the need for additional backside treatment, which increases fabrication costs and complexity, especially when incorporating color filters and micro-lenses.
Innovation Solution
A method using semiconductor-on-insulator (SOI) substrates with an epitaxial layer, alignment keys, and a buried oxide layer for precise thickness control and alignment, allowing for the integration of color filters, micro-lenses, and wire bonding, while being compatible with existing foundry processes and eliminating the need for special backside treatment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional front-illuminated imaging devices are used, then manufacturing cost is reduced, but fill factor and sensitivity deteriorate due to shadowing by metal bus lines and absorption by array circuitry
Solution Approach 1:
The patent inverts the traditional illumination approach by using back-illuminated imaging devices instead of front-illuminated ones. Light enters through the back surface of the substrate, allowing the front surface to be dedicated to imaging elements without shadowing from bus lines or circuitry. This inversion resolves the contradiction by eliminating the structural elements that cause shadowing while maintaining cost-effectiveness through standard fabrication processes.
Solution Approach 2:
The patent transitions from two-dimensional front surface illumination to three-dimensional back surface illumination. By moving the light entry point to the opposite surface and thinning the substrate, the patent creates additional spatial dimension for light paths, allowing simultaneous presence of transparent substrates and front-side circuitry without mutual interference, thereby achieving high fill factor while maintaining manufacturing feasibility.
2Manufacturing precision
If back-illuminated imaging devices are used, then fill factor and sensitivity are improved, but manufacturing complexity and cost increase due to additional backside treatment
Solution Approach 1:
The patent extracts and removes the opaque metal bus lines and array circuitry from the light path area by placing them on the front surface while using the back surface for illumination. The substrate is thinned to allow light transmission, and the harmful elements (metal lines, circuitry) are separated from the illumination path, eliminating their negative impact while simplifying the overall structure.
Solution Approach 2:
The patent changes the physical parameter of substrate thickness to enable backside illumination. By thinning the substrate to a specific thickness range (10-100 micrometers), the patent allows sufficient light transmission while maintaining structural integrity. This parameter change enables the transition to back-illumination without requiring complete substrate removal or complex additional treatments.
3Use of energy by moving object
If wafer thinning is performed for back-illuminated devices, then light transmission is improved, but yield deteriorates due to stress and thickness uniformity issues
Solution Approach 1:
The patent performs preliminary actions during the fabrication process to prevent yield deterioration. Alignment keys are formed in the epitaxial layer before thinning to ensure precise alignment during subsequent processing steps. The buried oxide layer is positioned at a predetermined depth to serve as a stopping layer during etching, ensuring uniform thickness control. These preliminary actions prevent stress and uniformity issues that would otherwise reduce yield.
Solution Approach 2:
The patent introduces an intermediary buried oxide layer at a predetermined depth in the substrate. This oxide layer serves as a mechanical stop and etch barrier during the thinning process, ensuring uniform thickness across the wafer. The intermediary layer prevents uncontrolled etching and stress concentration, thereby maintaining high yield while achieving the necessary light transmission properties.
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
This approach results in cost-effective, high-volume production of back-illuminated imagers with improved sensitivity and alignment accuracy, reducing fabrication costs and complexity by utilizing SOI substrates and precise alignment techniques.
Implementation Method 1
etching through the insulator layer and at least a portion of the bond pad region to the buried oxide layer
Implementation Method 2
A plurality of alignment keys are formed in the epitaxial layer. The color filters and micro-lenses are aligned to the alignment keys on the backside of the imager
Data Source
AI summary
A method for fabricating a back-illuminated semiconductor imaging device on a semiconductor-on-insulator substrate is disclosed. The substrate includes an insulator layer and an epitaxial layer overlying the insulator layer. A bond pad region is formed extending into the epitaxial layer to a surface of the insulator layer. A bond pad is fabricated partially overlying the bond pad region. At least one imaging component is fabricated partially overlying and extending into the epitaxial layer. A passivation layer is fabricated overlying the epitaxial layer, the bond pad, and the at least one imaging component. A handle wafer is bonded to the passivation layer. A portion of the insulator layer and a portion of the bond pad region is etched to expose a portion of the bond pad.