Integrated CMOS X-ray Sensor with Scintillating Waveguides
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Solution Overview
Problem
Current digital X-ray imaging devices face challenges in achieving high resolution and sensitivity due to the trade-off between scintillating material thickness and image resolution, and the complexity and cost of attaching discrete scintillating wave-guide structures to separate image sensors, leading to reduced performance and increased production costs.
Innovation Solution
Integrating scintillating material into wave-guide structures fabricated within a CMOS image sensor, where deep pores or stacked via openings are formed to align with photodiodes and filled with scintillating material using low-temperature methods, such as ForceFill technology, to create an integrated X-ray image sensor with improved resolution and reduced costs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the scintillating material is made thicker to increase X-ray absorption and sensitivity, then the sensitivity is improved, but the lateral resolution becomes lower due to isotropic emission of secondary photons
Solution Approach 1:
The scintillating material is divided into multiple thin layers, each coupled with corresponding photodiodes. This segmentation allows each layer to maintain thin thickness for good resolution while the stack of layers provides sufficient total thickness for high sensitivity. The light from each layer is collected by dedicated photodiodes, preventing cross-talk and maintaining resolution.
Solution Approach 2:
The patent transitions from a single thick scintillating layer to a three-dimensional stacked structure with multiple thin layers. This dimensional approach allows the system to achieve both high sensitivity (through multiple layers) and high resolution (through thin individual layers with dedicated photodiode coupling).
2Measurement precision
If discrete scintillating wave-guide structures are used to improve lateral resolution, then the resolution is improved, but the device complexity and manufacturing cost increase due to mechanical attachment requirements
Solution Approach 1:
The scintillating layers and photodiodes are merged into a single integrated structure where multiple thin scintillating layers are directly coupled with corresponding photodiodes in a stacked configuration. This eliminates the need for separate mechanical attachment of discrete components, reducing device complexity and manufacturing cost while maintaining high resolution.
Solution Approach 2:
The stacked structure serves multiple functions simultaneously: it provides wave-guiding for light collection, maintains mechanical integration with the photodiode array, and achieves both high resolution and sensitivity. The same structure that provides resolution also provides the integration benefit.
3Measurement precision
If discrete scintillating wave-guide structures are attached to separate image sensors, then the resolution is improved, but the production cost increases
Solution Approach 1:
The scintillating layers and photodiodes are merged into a single integrated structure that can be manufactured as one unit using standard semiconductor fabrication processes. This eliminates the need for separate assembly steps and mechanical attachment, significantly reducing production cost while maintaining the high resolution benefits of wave-guiding structures.
Solution Approach 2:
The scintillating layers are formed and integrated with the photodiodes during the preliminary fabrication stages using standard semiconductor processes. This preliminary integration eliminates the need for costly post-fabrication assembly and alignment operations, reducing overall production cost.
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 integrated approach results in a high-resolution, high-sensitivity digital X-ray sensor with reduced production costs and improved reliability, eliminating the need for separate scintillator plates and complex alignment processes.
Implementation Method 1
The scintillating material has a higher absorption coefficient than silicon, and generates a large number of secondary optical photons in response to absorbing one X-ray photon
Implementation Method 2
The CsI columns (needles) confine the generated secondary optical photons due to the total internal reflection at the interface between the CsI needles and the surrounding air
Data Source
AI summary
An X-ray image sensor having scintillating material embedded into wave-guide structures fabricated in a CMOS image sensor (CIS). After the CIS has been fabricated, openings (deep pores) are formed in the back side of the CIS wafer. These openings terminate at a distance of about 1 to 5 microns below the upper silicon surface of the wafer. The depth of these openings can be controlled by stopping on a buried insulating layer, or by stopping on an epitaxial silicon layer having a distinctive doping concentration. The openings are aligned with corresponding photodiodes of the CIS. The openings may have a shape that narrows as approaching the photodiodes. A thin layer of a reflective material may be formed on the sidewalls of the openings, thereby improving the efficiency of the resulting waveguide structures. Scintillating material (e.g., CsI(Tl)) is introduced into the openings using a ForceFill™ technology or by mechanical pressing.