Back-Side Illuminated CMOS Image Sensor Substrate Thinning
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Solution Overview
Problem
Conventional back-side illuminated CMOS image sensors face challenges in achieving uniform substrate thinning, leading to total thickness variation (TTV) issues that affect light penetration and photodetector performance due to imprecise etching and uncontrollable implantation.
Innovation Solution
A method is introduced where an implantation region with non-uniform doping concentration is formed on the semiconductor substrate before etching, using an etchant with an etch rate inversely proportional to the doping concentration to reduce TTV, thereby improving the uniformity of substrate thinning and photodetector performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional etching is performed without prior implantation, then the etching process is simpler and faster, but the substrate thickness uniformity deteriorates due to uncontrollable etching rates
Solution Approach 1:
An implantation step is performed before etching to modify the substrate surface properties. This preliminary action creates a controlled interaction between the implantation ions and the substrate, which subsequently regulates the etching rate and improves thickness uniformity across the substrate surface.
Solution Approach 2:
The implantation process changes physical parameters of the substrate surface (such as surface energy, roughness, or chemical composition) to control the etching rate. By adjusting implantation conditions (ion type, energy, dose), the etching uniformity can be precisely controlled without changing the etching chemistry itself.
2Reliability
If the substrate is thinned to allow light penetration, then photodetector performance improves, but total thickness variation increases affecting manufacturing precision
Solution Approach 1:
The implantation process creates local variations in substrate properties that correspond to different regions requiring different etching rates. By spatially controlling the implantation distribution, the etching process achieves uniform thinning across the entire substrate, ensuring consistent light penetration and photodetector performance while maintaining tight thickness tolerances.
3Manufacturing precision
If uniform doping concentration is used during implantation, then the implantation process is simpler, but the etching rate cannot be controlled to reduce TTV
Solution Approach 1:
Instead of using uniform doping concentration, the invention employs non-uniform implantation patterns where the ion dose or energy varies across different regions of the substrate. This asymmetric approach allows different areas to etch at different rates, compensating for inherent substrate variations and achieving uniform final thickness despite the complex implantation profile.
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 significantly reduces the total thickness variation over the back-side of the semiconductor substrate, enhancing light penetration and photodetector performance by up to 50% compared to conventional methods.
Implementation Method 1
an implantation region having a non-uniform doping concentration is formed on the semiconductor substrate before etching
Implementation Method 2
etching the back-side of the semiconductor substrate to reduce the thickness of the semiconductor substrate between the photodetectors and the back-side of the semiconductor substrate, after forming the implantation region, using an etchant that has an etching rate that is a function of doping concentration
Implementation Method 3
Light incident on a back-side of a thinned semiconductor substrate, travels through a thin layer of semiconductor material to a plurality of photodetectors located within a front-side of the semiconductor substrate. The incident light generates carriers within the photodetectors which are transferred to back-end metal wiring levels that provide the carriers as an electric signal
Data Source
AI summary
The present disclosure relates to a method of forming a back-side illuminated CMOS image sensor (BSI CIS). In some embodiments, the method comprises forming a plurality of photodetectors within a front-side of a semiconductor substrate. An implant is performed on the back-side of the semiconductor substrate to form an implantation region having a doping concentration that is greater in the center than at the edges of the semiconductor substrate. The back-side of the workpiece is then exposed to an etchant, having an etch rate that is inversely proportional to the doping concentration, which thins the semiconductor substrate to a thickness that allows for light to pass through the back-side of the substrate to the plurality of photodetectors. By implanting the substrate prior to etching, the etching rate is made uniform over the back-side of the substrate improving total thickness variation between the photodetectors and the back-side of the substrate.