Surface Photovoltage Measurement of Minority Carrier Diffusion Lengths
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Solution Overview
Problem
Current SPV measurement methods for minority carrier diffusion lengths in high purity silicon wafers are impractical and imprecise due to low signal-to-noise ratios and long measurement times, especially when trying to determine steady-state diffusion lengths exceeding 1000 μm, as they require low modulation frequencies, which are inaccurate and inefficient for fast and precise contamination monitoring.
Innovation Solution
Measuring diffusion lengths at elevated light modulation frequencies under non-steady state conditions using a surface photovoltage measurement system that calculates steady-state diffusion lengths based on measurements at multiple frequencies, enabling fast and precise data acquisition and reducing the influence of surface lifetime and contamination effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If low modulation frequencies are used to achieve steady state condition for accurate diffusion length measurement, then measurement accuracy is improved, but measurement time increases and signal-to-noise ratio deteriorates
Solution Approach 1:
The patent changes the modulation frequency parameter from traditional low frequencies (required for steady state) to elevated frequencies (enabling non-steady state measurements). By measuring diffusion lengths at multiple elevated frequencies and using mathematical relationships to derive the steady-state diffusion length, the method achieves both fast measurement and accurate results without requiring long measurement times or low frequencies.
2Reliability
If low modulation frequencies are used for steady state measurement, then steady state condition is achieved, but signal-to-noise ratio decreases and measurement precision deteriorates
Solution Approach 1:
The patent performs preliminary measurements at multiple elevated frequencies where non-steady state conditions prevail. By collecting data at these higher frequencies with better signal-to-noise ratios, and then using mathematical relationships to calculate the steady-state diffusion length, the method obtains reliable results without actually operating at steady state conditions during measurement.
3Productivity
If elevated modulation frequencies are used for fast measurement, then measurement speed increases, but traditional steady state assumptions are violated
Solution Approach 1:
The patent changes the operational parameters from low frequencies (steady state) to elevated frequencies (non-steady state). By measuring at multiple elevated frequencies and applying mathematical relationships that account for the non-steady state conditions, the method achieves both high measurement speed and accurate diffusion length determination.
4Measurement precision
If very long signal monitoring time constants are used to increase signal-to-noise ratio at low frequencies, then measurement precision improves, but measurement speed decreases
Solution Approach 1:
The patent changes the frequency parameter to elevated values where sufficient signal-to-noise ratio can be achieved without requiring long monitoring time constants. By operating at these higher frequencies in the non-steady state regime and using mathematical relationships to derive steady-state diffusion length, the method achieves both high precision and fast wafer mapping capability.
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 allows for accurate measurement of low concentrations of metal contaminants like iron and other recombination centers, providing high precision and fast data acquisition, capable of mapping large wafers within minutes while minimizing the impact of surface contamination and ambient changes.
Implementation Method 1
Small signal ac-surface photovoltage (SPV) measurements of minority carrier diffusion lengths
Implementation Method 2
illuminating a wafer surface with periodically modulated monochromatic light beams of multiple different wavelengths
Data Source
AI summary
Surface photo-voltage measurements are used to accurately determine very long steady state diffusion length of minority carriers and to determine iron contaminant concentrations and other recombination centers in very pure wafers. Disclosed methods use multiple (e.g., at least two) non-steady state surface photovoltage measurements of diffusion length done at multiple (e.g., at least two) modulation frequencies. The measured diffusion lengths are then used to obtain a steady state diffusion length with an algorithm extrapolating diffusion length to zero frequency. The iron contaminant concentration is obtained from near steady state measurement of diffusion length at elevated frequency before and after iron activation. The concentration of other recombination centers can then be determined from the steady state diffusion length and the iron concentration measured at elevated frequency.