A class of failure analysis techniques has evolved over the last several years that uses a scanning laser beam to activate electrical characteristics of the test device. Optical Beam Induced Current (OBIC) was the first of the class. It was later followed by Light Induced Voltage Alterations (LIVA), Thermally Induced Voltage Alteration (TIVA), and Seebeck Effect Imaging (SEI).(1) 

The use of this class of techniques for failure analysis is primarily in fault isolation. TIVA, for example, locates ohmic shorts due to the high resistance change that occurs with thermal heating at most shorts.  SEI locates opens due to the electrical imbalance of the thermal electric effects at the open site.

A significant feature of this class of techniques is their ability to work from both the topside and backside of an integrated circuit (IC). Backside analysis is critical for "flip-chip" packages and devices with multiple metal layers that obscure visibility from the topside.

A basic system for performing the fault isolation techniques uses a laser scanning microscope (LSM) to sequentially scan a focused laser spot over the IC. Scanning can be performed from the front or backside, with near infrared (IR) lasers used for backside probing.

Some device preparation for backside scanning is also necessary. In addition to unzipping the package, thinning and polishing of the substrate is generally required. Use of an anti-reflection coating improves image quality and allows more of the laser beam to penetrate into the substrate (2).

As the focused laser beam passes over an IC, it causes changes in the device’s electrical characteristics through two effects, generation of photocarriers and heating. If the laser wavelength is chosen to be below the semiconductor bandgap(3) then only heating occurs.

Shorter wavelengths, above the bandgap, will produce both photocarriers and heating. However, photocarrier effects are orders of magnitude stronger and generally dominate any thermal signals. Both photocarrier generation and heating can cause changes in the circuit resistance (photoconductive and thermal-conductive effects) and cause currents to flow (photovoltaic and Seebeck effects).

For the photovoltaic and Seebeck effects, no external bias is needed to observe a signal. For the photoconductive and thermal conductive effects, application of a constant current to the IC causes small voltage alterations to occur across the device in response to the optically induced changes in the circuit resistance.

The main application of these techniques, to date, has been fault isolation. In some cases, faults are located by comparison of an image from a "good" sample to the image of a failed device. Comparison is not generally required for TIVA and SEI, which only produce signals at the fault, associated drivers and drivelines. The types of faults detected are determined by the detailed physics of the laser interaction.