• Identifying the source of the contamination
  
  As an example, the lifetime image of a wafer is shown which was tempered in a RTA-reactor after handling by the contaminated gripper of a TXRF tool. Due to the locally drastically reduced lifetime the characteristic patterns are discernible.

  

  Picture 2: Life cycle picture

  
  
  

• Application for process control
  
  The control of oxygen precipitates is an essential factor in today's processes (e.g. when generating so-called denuded zones). The lifetime image on the right shows the map of a CZ-wafer following a thermal process. The low mean lifetime is most probably caused by oxygen precipitates that effectively reduce the lifetime. Furthermore, rotationally symmetric rings can be discerned caused by different oxygen concentrations in the bulk material (rotation-induced striations).

  

  Picture 3: Life cycle picture

  
  
  

• Identifying the contaminant
  
  Impurities create recombination centers in silicon reducing the lifetime of the carriers. A decisive factor is the effectiveness of the recombination center varying specifically for different contaminants as the number of the present carriers changes. The number of carriers is determined by the so-called injection level, i.e. the ratio of the concentration of the laser-generated excess carriers to the doping concentration. In the case of ELYMAT the injection level can be altered by varying the laser intensity. As an example for the availability of this effect for identifying contaminants, the picture on the right shows the variation of the lifetime in dependence of the injection level (injection level spectroscopy ILS) for two intentionally contaminated p-doped wafers. The shape of the lifetime curve for the different contaminants and the good correspondence between measured values and theoretical curves are clearly visible.

  

  Picture 4: Diagram

Specifications

• ELYMAT II

  Semiconductor laser used:	670 nm (10 mW) and 905 nm (40 mW)
  Wafer diameter:	150 mm, 200 mm, 300 mm

  
  
• ELYMAT I

  Semiconductor laser used:	670 nm (10 mW) and 820 nm
  Wafer diameter:	100 mm (125 mm, only 100 mm range)

  
  
• ELYMAT I+II

  Measurement range:	approx. 70-2000 µm (BPC), approx. 10-100 µm (FPC)
  Resistivity range:	approx. 0,1-1000 Ohm*cm
  Lateral resolution:	down to 1 mm (using special zoom mode for single sections down to 0.1mm); at 300 mm: down to 2 mm
  Edge exclusion:	approx. 15% of the wafer radius
  Measurement time:	approx. 6 min for 200mm wafer at a resolution of 1mm (measurement time proportional to area); additionally, approx. 5 min for loading and unloading of the wafer
  Manufacturer:	GeMeTec, Munich

Principle

The ELYMAT-technique (Electrolytical Metal Analysis Tool) offers the opportunity of determining the lifetime of the minority carriers and diffusion length. A laser beam scans across a silicon wafer immersed in an electrolytic cell with an applied voltage and the resulting diffusion current is measured. The extraction of the current takes place on the wafer backside (BPC-mode, see picture) or on the wafer front side (FPC-mode).

In the case of lattice defects in the semiconductor bulk material (e.g., metal contamination), the diffusion current decreases due to partial recombination of the diffusing carriers. Therefore, an approximate analytic determination of the diffusion length can be carried out basing on this so-called photocurrent.

Using both measurement modes, i.e. measuring on both sides of the wafer, provides supplementary information on whether the contamination is present near the surface or whether it is distributed homogeneously throughout the wafer bulk material. Furthermore, applying lasers with different penetration depths can in principle separate the influence of surface and bulk recombination. Finally, measuring at different laser intensities allows major contaminants such as iron to be identified (injection level spectroscopy, short ILS).

Picture 1: Principle

Contact:
Mathias Rommel
Tel.: 09131 / 761-108
Fax: 09131 / 761-360