Back Side Illumination Image Sensor Ion Implantation Thinning
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Solution Overview
Problem
Existing back side illumination image sensors face issues with reduced image quality due to smaller photodiode sizes, increased manufacturing costs from using expensive SOI wafers, wafer edge thinning failures, plasma damage, and defects on the substrate surface, which affect the efficiency and performance of the image sensor.
Innovation Solution
The method involves forming an ion implantation layer on the front side of a substrate, defining a pixel region, bonding a second substrate with a metal line, removing the lower part of the first substrate using wet etching, and forming a microlens on the photosensitive device, all while using an epi wafer as the donor substrate to minimize costs and prevent defects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If back grinding process is applied to remove the rear side of the substrate, then the substrate thickness is reduced to fit the gap between external module and optical lens, but wafer edge thinning is generated causing chip failure in subsequent etch-back process
Solution Approach 1:
The patent applies preliminary action by performing ion implantation to form a weakened layer in the substrate before the thinning process. This pre-treatment allows the substrate to be selectively removed at the edges without causing uncontrolled thinning, thereby preventing chip failure in subsequent etch-back processes while achieving the required thickness reduction.
Solution Approach 2:
The patent applies local quality by creating a non-uniform weakened layer through ion implantation that is concentrated at the substrate edges. This localized modification allows selective removal of edge portions during thinning while preserving the integrity of the chip area, thus preventing wafer edge thinning failures without compromising overall substrate thickness requirements.
2Length of stationary object
If etch-back process is performed after backside grinding to complete thinning, then the substrate is thinned to required thickness, but plasma damage occurs at wafer center deteriorating sensor performance
Solution Approach 1:
The patent applies preliminary action by forming a weakened layer through ion implantation before the etch-back process. This pre-created vulnerable layer allows the etch process to proceed more easily and uniformly, reducing the need for aggressive plasma conditions that would cause damage to the wafer center and deteriorate sensor performance.
3Productivity
If photodiode area is reduced to increase the number of pixels, then the pixel density increases, but image quality is reduced due to reduced illumination area
Solution Approach 1:
The patent applies dimensionality change by transitioning from front-side illumination to back-side illumination architecture. This allows the light to enter the photodiode from the opposite side, enabling the photodiode to be positioned directly behind the lens aperture without obstruction from metal routing or other front-side structures. Consequently, image quality is maintained even with reduced photodiode area, as the full illumination area is utilized effectively.
4Ease of manufacture
If SOI wafer is used as donor wafer for backside thinning, then the thinning process can be performed, but manufacturing cost increases significantly
Solution Approach 1:
The patent applies this principle by replacing the expensive SOI wafer with a standard silicon wafer as the donor substrate. The ion implantation process creates a weakened layer that enables sufficient thinning for back-side illumination applications without requiring the costly SOI structure. This substitution dramatically reduces manufacturing costs while maintaining the feasibility of the thinning process.
Solution Approach 2:
The patent applies parameter changes by modifying the substrate structure through ion implantation to create a weakened layer with specific properties. This allows standard silicon wafers to achieve the required thinning characteristics previously only available with SOI wafers, thereby reducing material costs while maintaining process feasibility.
5Reliability
If metal line is formed on readout circuit and wafer-to-wafer bonding is performed, then electrical connection between readout circuit and photodiode is established, but bonding defects occur causing incomplete contact or short circuits
Solution Approach 1:
The patent applies this principle by removing the complex wafer-to-wafer bonding step entirely. Instead of bonding separate wafers containing readout circuits and photodiodes, the invention integrates both components on a single substrate. This eliminates the bonding interface that causes defects, ensuring reliable electrical connection without the risks of incomplete contact or short circuits associated with multi-wafer bonding processes.
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 stabilizes the substrate removal process, improves image quality by minimizing defects and light interference, and significantly reduces manufacturing costs by eliminating the need for expensive SOI wafers and complex bonding processes.
Implementation Method 1
forming an ion implantation layer by implanting ions into an entire front side of a first substrate
Implementation Method 2
performing wet etching to a back side of the first substrate; removing a lower part of the first substrate under the ion implantation layer
Data Source
AI summary
A method of manufacturing a back side illumination image sensor according to an embodiment includes: forming an ion implantation layer by implanting ions throughout the front side of a first substrate; defining a pixel region by forming a device isolation region on the front side of the first substrate; forming a photosensitive device and a readout circuit on the pixel region; forming an interlayer dielectric layer and a metal line on the front side of the first substrate; bonding a second substrate with the front side of the first substrate where the metal line is formed; removing a lower part of the first substrate under the ion implantation layer; applying wet etching to a back side of the first substrate after removing the lower part; and forming a microlens on the photosensitive device at the back side of the first substrate.