Carbon Concentration Evaluation in Image Sensor Silicon Wafers
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Solution Overview
Problem
Current methods for measuring carbon concentration in silicon wafers, such as FT-IR, SIMS, and electron beam irradiation+low-temperature PL, are inadequate for detecting low carbon levels in the surface layer of image sensors due to low sensitivity and limited detection depth, particularly in the 1 to 2 μm photodiode region.
Innovation Solution
A method involving ion implantation of a predetermined element into a silicon wafer under specific conditions (1.1×10^11×[atomic weight of the implanted element]−0.73<implantation amount (cm^−2)<4.3×10^11×[atomic weight of the implanted element]−0.73) followed by low-temperature PL measurement to enhance the emission intensity of CiCs composites, allowing for high-sensitivity carbon concentration measurement in the surface layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If FT-IR method is used to measure carbon concentration, then measurement can be performed on entire wafer depth, but sensitivity is low and surface layer cannot be evaluated
Solution Approach 1:
The measurement is segmented into depth-specific evaluation by using ion implantation to create localized CiCs composites at controlled depths. By adjusting implantation energy, the measurement focuses on specific depth ranges (including surface layer 1-2μm) rather than averaging across the entire wafer depth, enabling surface layer carbon concentration to be evaluated separately from bulk material.
Solution Approach 2:
Ion implantation serves as an intermediary mechanism to enhance carbon detection. The implanted ions create point defects that react with carbon to form CiCs composites, which act as intermediaries that amplify the optical signal for carbon detection. This intermediary process significantly improves detection sensitivity compared to direct FT-IR measurement.
2Measurement precision
If SIMS is used to measure carbon concentration distribution, then depth distribution can be measured, but lower detection limit is high and very low carbon analysis is difficult
Solution Approach 1:
The detection sensitivity is improved by changing the measurement parameters through low-temperature PL measurement conditions. By performing measurement at low temperatures (e.g., 77K) and optimizing excitation conditions, the detection limit is reduced to approximately 0.002 ppma, which is significantly lower than SIMS detection limit of 0.05 ppma.
3Measurement precision
If electron beam irradiation+low-temperature PL method is used, then sensitivity is favorable and lower detection limit is low, but CiCs composites are formed in entire depth direction and shallow region cannot be measured
Solution Approach 1:
The measurement depth range is segmented by controlling ion implantation energy to create CiCs composites at specific depths. By adjusting implantation energy parameters, the method can selectively measure carbon concentration in shallow regions (1-2μm surface layer) separate from deeper regions, overcoming the limitation of bulk measurement in conventional electron beam irradiation methods.
Solution Approach 2:
Ion implantation is performed as a preliminary action before PL measurement to create localized CiCs composites at desired depths. This preliminary localization of carbon defects enables subsequent PL measurement to probe specific depth regions, particularly the shallow surface layer region that would otherwise be inaccessible.
4Measurement precision
If conventional ion implantation+PL method is used, then carbon concentration can be measured, but implantation conditions are not optimized for high sensitivity measurement
Solution Approach 1:
The implantation conditions are optimized by establishing specific parameter relationships: implantation amount is set to 1.1×10^11×[atomic weight of implanted element]−0.73 to 4.3×10^11×[atomic weight of implanted element]−0.73 ions/cm², and implantation energy is set to 0.5 MeV×[atomic weight of implanted element]−0.17 to 2.0 MeV×[atomic weight of implanted element]−0.17. These optimized parameters maximize CiCs composite formation efficiency and detection sensitivity.
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
Enables high-sensitivity measurement of carbon concentration in the 1 to 2 μm surface layer of image sensors, improving the selection of wafers with favorable image sensor characteristics and monitoring carbon contamination throughout the process.
Implementation Method 1
ions of a predetermined element are implanted into a silicon wafer
Implementation Method 2
a carbon concentration is quantified by the low-temperature PL method from an intensity of an emission caused by the composites
Data Source
AI summary
A method for evaluating a carbon concentration where ions of a predetermined element are implanted into a silicon wafer, and then a carbon concentration is measured by a low-temperature PL method from an emission intensity of a CiCs composite, where the ions are implanted under implantation conditions of 1.1×1011×[atomic weight of the implanted element]−0.73<implantation amount (cm−2)<4.3×1011×[atomic weight of the implanted element]−0.73, and the carbon concentration is evaluated. A method for evaluating a carbon concentration makes it possible to measure with high sensitivity, a carbon concentration in a surface layer of 1 to 2 μm, which is a photodiode region in an image sensor.