Thermal Laser Stimulation
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Thermal Laser Stimulation (TLS) for IC failure analysis can be categorized by three mapping techniques: Optical Beam Induced Resistance Change (OBIRCh), Thermally Induced Voltage Alteration (TIVA), and Seebeck Effect Imaging (SEI). Signatures obtained on metallic elements are significant, but little data have been obtained on signatures of semiconductors elements.

In those measurements, a laser with a wavelength of 1.34 µm is normally employed to avoid creating electron-hole pairs [1]. The light has a lower energy than the Si band gap (Eg = 1.12 eV) so that the probability of electron-hole pair generation is low and absorption occurs as conversion of light into heat. Note that electron-hole pairs induce photo-current that is strong noise against the TLS signal. These optical properties allow performing frontside and backside analysis through Si thickness.

TLS process can be composed of scanning a focused IR laser beam on the IC front or back surface and modifying its electrical properties; therefore, temperature gradients are generated and localized in the conductive elements of the circuits. The local resistivity variation can be given by,

         Δρ = ρ0*TCR*(T-T0) = ρ0*TCR*ΔT -------------------------------- [2837a]

where,
          ρ0 -- The resistivity at ambient temperature,
          TCR -- The local temperature coefficient of resistance,
          T -- The local temperature during laser scanning.

ΔT indicates the temperature variation optically induced by the IR laser beam. Therefore, the IR laser stimulation techniques can detect and localize defect into the IC device.

Figure 2837 shows the sensitivity comparison of IC current increase in constant-voltage biasing mode to IC voltage decrease in constant-current biasing mode for a CMOS ACIC. It is necessary to note that some shorts or openings dissipate more power than the others.

Sensitivity comparison of constant voltage and constant current biasing under the same power stimulus

Figure 2837. Sensitivity comparison of constant voltage and constant current biasing under the same power stimulus. [2]

Thermal Laser Stimulation (TLS) setup and image acquisition

The integrated current/voltage amplifier is normally coupled with a voltage/current source that allows stimulating electrically the DUT (device under test). The variations of high current or voltage can be measured with the constant voltage mode (OBIRCh) or with the constant current mode (TIVA), while weak level variations can only be detected with other highly sensitive, constant voltage mode. Reflected laser images correspond to a predefined area of the DUT continuously scanned by the IR laser beam. The precise location of the defects is obtained by superposing the TLS signal on the laser-reflected images taken with the infrared laser-scanning microscope (LSM).

In detail, the electrical signal is detected and amplified with an TLS amplifier. Analog signal is then coded with certain bits (e.g. 212) and transformed into digital values with acquisition card. TLS signal amplitude can be displayed on screen in grey scale (e.g. from 0 to 4095) in a certain spatial resolution (e.g.1024 x 1024). Maximal positive and negative variations are displayed respectively in different colors (e.g. white and black, or red and green).

Frontside TLS investigations require laser getting access to the defective layers through all the metal layers, while backside analysis requires more silicon thinning for a best observation (≤ 100μm).

The laser power is sometimes reduced substantially so that the artifacts can be minimized. From instance, in this way, streaking effect can be removed and thus, the defects can be accurately localized.

 

 

 

 

 

 

[1] Chunlei Wu, Masuda Motohiko, Winter Wang, Grace Song, Jinglong Li, Joe Yu et al., “A Novel and low-cost Method to Detect Delay Variation by Dynamic Thermal Laser Stimulation,” IRPS 2011, pp. 765–769.
[2] 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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