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Microwave detected photoconductivity

The advanced method MDP is suited for defect investigation and mapping of wafers and ingotsWith its extraordinary sensitivity, resolution and speed MDP enables injection dependent measurements as well as mapping with a very high resolution.

MDP – microwave detected photoconductivity

The novel method MDP is well suited for both, defect investigation by e.g. injection dependent minority carrier lifetime measurements, as well as mapping of wafers or even bricks for inline metrology. It exceeds his competitors µPCD (microwave detected photoconductivity decay) and QSSPC (quasi steady state photo conductance) in terms of sensitivity, resolution and speed.

The photoconductivity, which is closely related to the diffusion length is measured by microwave absorption during and after the excitation with a rectangular laser pulse. Figure 1 displays the measurement principle for MDP and MD-PICTS measurements.

  • generation of free carriers

  • traps are filled with carriers

  • recombination of free carriers

  • thermal reemission of trapped carriers

  • temporally shifted recombination of reemitted carriers

A microwave with about 10 GHz is generated in a frequent stable microwave-generator and split into a reference and measurement part. With an attenuator the power can be adjusted and ranges typically from 1 to 100 MW. The sample is situated just outside the cavity and is part of the measurement system. A special iris in the cavity-wall allows the microwave field to penetrate the sample. Thus, the complex dielectric constant of the sample influences the resonant frequency and the loss properties of the cavity. Microwave absorption by excess charge carriers is detected with an IQ-detector. The sample is placed on an x-y-table, allowing theoretically every sample size and to move the sample in the x-y-plane. For temperature dependent MD-PICTS measurements the sample has to be part of a cryostat system, so that the sample size is currently limited, but apart from that  it is principally the same measurement system.

  • Fig. 1: Energy scheme of the measurement principle
  • Fig. 2: Exemplary signal
  • Fig. 3: setup for MDP and MD-PICTS

The high detection sensitivity enabled by this technique allows the application even of weak laser pulses with an intensity of µW to mW and with unlimited pulse duration. Hence it is possible to measure in a non- or steady state regime and to continously vary the pulse length from 100 ns to several ms. The resolution of this system is only limited by the diffusion length of the sample.

Besides advantages in speed and sensitivity, a major advantage of MDP is the ability to measure photoconductivity and minority carrier lifetime simultaneously. Accordingly more parameters can be extracted from each measurement, like diffusion length, mobility and even trapping dynamics.

  • Fig. 4: Exemplary lifetime map of a mc-Si wafer
  • Fig 5: Exemplary photoconductivity map of a mc-Si wafer

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