edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 27 NO. 4 8 where SNR does not change as the probing beam power increases. CHALLENGES AND FUTURE PERSPECTIVE There are three primary challenges associated with TD Imaging. The first is the expansion of the light source wavelength. It is desirable to have a light source with selectable wavelengths because TR Coeff depends on the sample material and structure. The second is the scan noise. In laser scanning optics, scan noise occurs at low frequency, so reducing this noise is important. The last one is RIN of the light source. RIN is a parameter that limits the sensitivity of TD Imaging, therefore a more stable light is required. LI-OBIRCH LI-OBIRCH[16,17] is a variant of OBIRCH,[12] which is used for finding temperature sensitive defects by means of laser heating.[12] A typical setup for LI-OBIRCH is illustrated in Fig. 5. A constant bias voltage is applied to the sample, which, for this example, consists of three distinct layers. The bottom layer is directly heated by a laser beam and the upper layers are indirectly heated via heat diffusion, which causes a small perturbation in current. Current perturbation is monitored typically by an AC-sensitive transimpedance amplifier, whose output is fed into a lock-in amplifier.[17] The laser beam spot position is scanned by laser scanning optics. While the beam is scanned, the beam is pulsed or modulated at a certain frequency.[16,17] A reference signal which synchronizes to the modulation is fed to the lock-in amplifier. The lock-in amplifier will yield the amplitude (R) and phase (θ) of the frequency component corresponding to the reference signal as shown in Fig. 5. Another representation of LI-OBIRCH data is in phase (I) and quadrature (Q) format, which is defined as I = R cos cos θ and Q = R sin sin θ. In LI-OBIRCH, the focused laser beam spot on the sample is the heat source and it is modulated by pulsing the laser at a frequency. Temperature field, which can be approximated by Eq 2, is generated around the laser beam spot. In LI-OBIRCH, the velocity of laser beam spot is set slow compared to laser modulation. With this condition, one can approximate that the temperature field moves with the laser beam spot without distorting its form. When the laser beam is focused on sample surface (z = 0) and lateral location (x', y'), the temperature field is simply Tw(x - x', y - y', z). For this discussion, uniform and anisotropic material is assumed, which is not actually realistic. However, this approximation is valid as the local structure’s dimension is very small. OBIRCH sensitivity distribution, γ(x,y,z), denotes how much OBIRCH signal is generated when temperature at (x,y,z) changes by unit temperature. Using γ(x,y,z) and temperature field expression, LI-OBIRCH signal at modulation angular frequency w, w(x,y), is derived by: (Eq 9) Fig. 4 Comparisons of TD Imaging SNR vs. probing beam power among different wavelength HILs. Fig. 5 Simplified diagram of the LI-OBIRCH system and interpretation of data.
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