Laser Stimulated Seebeck Effect Imaging (SEI)
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Laser Stimulated Seebeck Effect Imaging (SEI) is one type of Thermal Laser Stimulation (TLS) techniques and has become a powerful technique in microelectronics failure analysis for defect localization especially on open conductors. For the similar reason to IR-OBIRCH technique, SEI is based on the use of a laser with 1.3μm wavelength. SEI shapes are not always that clear, making the interpretation difficult, mostly because of superposition of additional OBIRCH when the device is biased. It was proposed [1] that the Seebeck effect works well for detecting voids when no electrical bias is applied to a device.

Thermal gradient in conductors generates electrical potential gradient with a typical value on the order of µV/°C [2] that is known as thermoelectric power or the Seebeck Effect named from the work of Thomas Johann Seebeck. As shown in Figure 2840a, the thermoelectric Seebeck effect is based on voltage generation between two materials at different temperatures [1]. The Seebeck coefficient S= dV/dT [μV/K] gives the generated voltage per temperature step. SEI is usually performed at zero bias in constant current mode when the voltage is generated by laser stimulation. The heat energy converted from optical energy from IR laser induces an electromotive force in the local materials. The induced voltage between material 1 and material 2 can be described by,

          ΔV = (Q1-Q2)*ΔT = Q1-2*ΔT -------------------------------- [2840]

          Q1 and Q2 -- The Seebeck coefficient or thermoelectrical power of material 1 and 2,
          Q1-2 -- The relative thermo-electrical power of the junction materials,
          ΔT -- The temperature variation optically induced by the IR laser beam.

If an IC conductor has no shorts, the potential gradient produced by laser-induced local heating is easily compensated for by the transistor or power bus electrically and thus no ΔV signal is produced. However, if the conductor is electrically isolated from a driving transistor or power bus, the Seebeck Effect will change the potential of the conductor. This potential change in the conductor will modify the bias condition of the transistors whose gates are connected to the electrically open conductor. In their backside method, Cole et al. [3] had laser transmission efficiency of ~3% to 25% (1340 nm wavelength used) depending on the objective lens, resulting in thermal gradients on the order of 4 to 30 °C and thus creating potential variations in the 1's to 10's of µV.

Seebeck Effect pinciple

Figure 2840a. Seebeck Effect pinciple.

Figure 2840b shows the SEI setup. The amplifier is operated in AC-coupled mode so that only the variations in VDD are recorded to show defective locations.

SEI setup

Figure 2840b. SEI setup. OBIC stands for optical beam induced current.

The lasers with 1340 nm wavelength eliminate the LIVA (light-induced voltage alteration) signal because the energy of the photons is below that of the Si (silicon) indirect band-gap. The photocurrent effects from e-h (electron-hole) pair generation by the 1064 nm laser used in LIVA produce signals 1 to 2 orders of magnitude larger than the SEI and TIVA signals so that the 1064 nm laser should be avoided in SEI and TIVA measurements. Note that some lasers with 1340 nm wavelength still contain a weak intensity (e.g. < 1%) of light with 1047 nm wavelength; therefore, a long wavelength pass filter needs to be placed in the beam path in order to eliminate the unwanted LIVA signals.






[1] Koyama, T., et al., IRPS, IEEE, USA, p.228 (1995).
[2] N.W. Ashcroft and N.D. Mermin, Solid State Physics, Philadelphia, Saunders College, 1976, ch. 1, pp. 24-25.
[3] Cole, EI; Tangyunyong, P; Barton, DL, Backside localization of open and shorted IC interconnections, 36th Annual IEEE International Reliability Physics Symposium, (1998). DOI: 10.1109/RELPHY.1998.670462.




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