Basic Technology

A new evaluation technology: HS-CMR Method (Hi Speed-Current Modulating Resistivity Method)

Unique technology for measuring effective carrier number
by dynamic resistivity measurement.

Conventionally, reflected microwave photoconductivity decay method (μ-PCD), which measures minority carrier lifetime on a substrate surface, has been employed as a quality evaluation method of Si crystal substrate. The method has been used for shipping inspection of Si crystal substrates for solar cells by Si crystal manufacturers and acceptance inspection of crystal substrates by solar cell manufacturers.

However, there was a problem that correlation could not be obtained between average value or maximum value (or minimum value) of this measured value and energy conversion efficiency of solar cell. In order to know the solar cell characteristics of Si crystal substrate, there is only way to manufacture a solar cell and measure its energy conversion efficiency.
However, since much cost and time are spent to manufacture solar cells, currently, it is decided whether or not to ship or purchase Si crystal substrates only by average value, maximum value (or minimum value) or resistivity of their lifetime value. As a result, quality dispersion and defect rate have been increasing in the solar cell manufacturing industry.

The HS-CMR method is new crystal quality measurement technology developed from the Institute for Materials Research, Tohoku University. The HS-CMR does not use carrier lifetime mapping or diffusion length mapping that has been used in the past, and has been designed by comprehensively considering various problems caused by the thermal processes of semiconductor crystal wafers and subsequent device manufacturing processes.

The HS-CMR method uses the four-point probe method to measure the resistivity while changimg the current with our own proprietary algorithm. With this method, the HS-CMRmethod can measure the “effective resistivity”. The effective resistivity is a value that reflects the total number of minority carriers and majority carriers that are not trapped by crystal defects or impurities. The HS-CMR method can obtain a single quality factor that accurately reflects the quality of the wafer by                                                                                                                comprehensively analyzing the transition of the resistivity and the effective resistivity.

                                                                                         The measured values obtained by the HS-CMR method can be used to improve and                                                                                           develop crystal growth and device processes.

Band Structure of Polycrystal Wafer and Single-crystal Wafer

The band structure of polycrystal wafer is spatially discontinuous, and the band structure of single crystal wafer tends to change periodically.
In order to measure the quality of the wafer, it is necessary to measure entire wafer rather than average value of partially collected data.