Backside Illuminated Image Sensor Wafer Level Processing
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Solution Overview
Problem
Conventional processing techniques for backside illuminated image sensors face challenges such as increased die size, higher cost, and decreased image quality due to the thinning or removal of the silicon substrate, as well as difficulties in the use and attachment of temporary carrier wafers and formation of interconnects between image sensor bond pads and solder balls.
Innovation Solution
A wafer level process for forming backside illuminated image sensors involves attaching a temporary carrier wafer, removing the substrate, forming color filter arrays on the backside surface, attaching a transparent cover sheet, and creating redistribution metal conductors and contact metallizations to facilitate efficient packaging without significantly increasing die size or cost.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the silicon substrate is thinned or removed to enable backside illumination, then fill factor and quantum efficiency are improved, but handling becomes difficult and processing complexity increases
Solution Approach 1:
A temporary carrier wafer is attached to the frontside of the image sensor wafer before substrate removal. This preliminary action provides mechanical support during the substrate thinning and removal processes, enabling handling of the fragile thin structure without increasing final device complexity
Solution Approach 2:
The temporary carrier wafer acts as an intermediary structure that facilitates substrate removal and provides handling support. After the substrate is removed and backside processing is complete, the temporary carrier wafer is removed, leaving no permanent additional structure in the final device
2Manufacturing precision
If conventional processing techniques are used for substrate removal and carrier wafer attachment, then backside illumination is achieved, but die size increases and cost increases
Solution Approach 1:
Multiple functions are merged into the temporary carrier wafer: it provides mechanical support during processing, serves as a handling substrate during substrate removal, and enables precise alignment for backside color filter formation. This consolidation achieves backside illumination quality without requiring additional separate structures that would increase die size
Solution Approach 2:
The substrate is thinned to specific thickness parameters (e.g., 5-50 micrometers) before removal, and the temporary carrier wafer is bonded at controlled parameters. These parameter changes enable substrate removal while maintaining structural integrity during processing, achieving backside illumination without excessive die size expansion
3Manufacturing precision
If the substrate is thinned or removed, then light sensitivity is improved, but the image sensor becomes more vulnerable to damage and harder to process
Solution Approach 1:
The temporary carrier wafer is attached beforehand to cushion and support the thin substrate structure during the vulnerable phases of processing. This protective support prevents damage during substrate thinning, removal, and handling, maintaining structural integrity while enabling the light sensitivity improvements from substrate removal
Solution Approach 2:
The temporary carrier wafer serves as an intermediary protective structure that absorbs mechanical stresses and prevents damage to the thin image sensor wafer during processing. It provides the necessary mechanical strength and handling capability without interfering with the optical performance of the thinned structure
Data Source
AI summary
A backside illuminated image sensor comprises a sensor layer having a plurality of photosensitive elements of a pixel array, an oxide layer adjacent a backside surface of the sensor layer, and at least one dielectric layer adjacent a frontside surface of the sensor layer. A color filter array is formed on a backside surface of the oxide layer, and a transparent cover is attached to the backside surface of the oxide layer overlying the color filter array. Redistribution metal conductors are in electrical contact with respective bond pad conductors through respective openings in the dielectric layer. A redistribution passivation layer is formed over the redistribution metal conductors, and contact metallizations are in electrical contact with respective ones of the respective redistribution metal conductors through respective openings in the redistribution passivation layer. The image sensor may be implemented in a digital camera or other type of digital imaging device.