OBIRCH & Thermal Laser Fault Isolation Lab

1. Backside Through-Silicon Infrared Stimulation

Modern high-density flip-chip packages and 3D stacked integrated circuits feature complex multilevel metal interconnects (up to 15 layers of copper dual-damascene lines) that obscure defects from frontside optical inspection. Silicon exhibits an indirect bandgap of approximately $1.12\text{ eV}$, making it opaque to visible light ($\lambda < 1000\text{ nm}$). By operating with a sub-bandgap infrared laser ($\lambda = 1340\text{ nm}$ or $1420\text{ nm}$), photon absorption across the bandgap is negligible, allowing the beam to penetrate $50\text{ to }300\,\mu\text{m}$ of polished backside silicon to stimulate embedded metallization and junctions directly.

2. Localized Thermal Constriction & Resistance Physics

When the focused laser beam hits a conductive metal line or defective via, optical absorption creates a localized temperature rise $\Delta T$. The electrical resistance of the conductor changes according to its temperature coefficient of resistance (TCR, $\alpha$):

In intact copper lines, $\alpha \approx +0.0039\text{ K}^{-1}$. When the laser encounters a void or microcrack, the reduced cross-sectional area acts as a severe thermal constriction with impeded heat dissipation, causing $\Delta T$ to surge by $3\times$ to $10\times$. This creates a sharp local resistance spike.

3. OBIRCH vs. TIVA Electrical Detection Modes

Depending on device failure characteristics, FA engineers select between two primary biasing regimes:

• OBIRCH (Constant Voltage): The circuit or net is biased at constant voltage $V_{\text{bias}}$. The laser-induced resistance change causes a current fluctuation $\Delta I = -V_{\text{bias}} \cdot \Delta R / R^2$, which is detected with an ultra-low noise transimpedance lock-in amplifier.
• TIVA (Constant Current): The net is driven with a constant current source $I_{\text{bias}}$. The resulting voltage alteration $\Delta V = I_{\text{bias}} \cdot \Delta R$ is measured directly. TIVA provides exceptional sensitivity for isolated lines and power rail shorts.

4. Solid Immersion Lens (SIL) Diffraction Breakthrough

In sub-5nm and 2nm nodes, metal pitch drops below $30\text{ nm}$, while conventional 1340 nm laser spot sizes are limited to $\sim 1.0\,\mu\text{m}$. By placing a hemispherical or super-hemispherical Solid Immersion Lens (SIL) made of silicon or gallium arsenide ($n \approx 3.5$) in direct mechanical contact with the polished die backside, the numerical aperture expands to $\text{NA}_{\text{eff}} = n_{\text{SIL}} \cdot \text{NA}_{\text{obj}} \approx 2.8\text{–}3.0$. This compresses the optical spot diameter down to $\sim 250\text{ nm}$, yielding the spatial resolution needed to isolate individual vias.