IR Emission Microscopy Provides New Failure Analysis Tool

Alexander E. Braun, Associate Editor -- Semiconductor International, 5/1/1999

Although photo emission microscopy is a powerful failure analysis technique, it has been handicapped by traditional CCD-based systems' low sensitivity, resulting from spectral response limitations. This is a drawback in the inspection of flip-chip and other backside devices, because while silicon becomes transparent at about 1.07 µm, that is where CCD response drops to zero.

A.N. Zaplatin and F.J. Low, of Infrared Laboratories Inc. (Tucson, Ariz.), have designed an infrared microscopy system that seems to solve this spectral limitation, providing a new inspection technique for finding both thermal shorts and normally undetectable 'hot carrier' emission sites. This system is being developed into a production-worthy version.

The researchers reasoned that since the hot carrier emission's intensity, produced by different elements of semiconductor chips, increases toward the IR region where CCD arrays stop responding, a detector with high quantum efficiency beyond 1.0 µm should increase sensitivity. By using a Rockwell International mercury-cadmium-telluride (MCT) IR array cooled to liquid nitrogen temperature, they found that a much higher portion of the emission is detected, compared to that detected by a CCD. Even frontside observations benefited significantly.

The difference in sensitivity is even more obvious for backside observations because of the silicon filter effect. Only a small fraction of the signal photons is collected by a CCD camera. By using the IR array it is possible to get 10 to 100 times or better improvement in sensitivity. Even when dealing with highly doped samples (10¹⁹/cm³), useful images through silicon up to 400 µm thick can be obtained, although generally 200 to 250 µm is enough. Traditional CCD-based system users must thin samples down to 100 or 70 µm to obtain usable images.

The IR FPA's spectral response provides the ability to detect thermal emission. Since the IR array has a 0.8 - 2.5 µm spectral response, large fluxes of photons are registered by the detector even with room-temperature samples. This capability can be used to identify hot spots or to measure the temperatures of different regions of the chip. The tool's thermal sensitivity proved better than that of FMI or liquid crystals. These new procedures are not time-consuming and no special sample preparation or special skills are required.

The IR microscope has three basic applications: hot carrier emission detection and imaging, hot spot detection and thermal mapping.

Hot carrier emission detection and imaging is used for front and backside failure analysis. The MCT array's advantage is detecting weaker emissions difficult and often impossible to image with older, cooled silicon CCD arrays. The advantage is greater for backside emissions because the intrinsic silicon transmission spectrum rises abruptly at 1.07 microns where cooled silicon CCD arrays no longer detect photons. The detector's high sensitivity helps detect and locate emissions even from weak emitters such as p-channel transistors.

The system's spectral range allows not only hot carrier emission detection, but also hot spot detection - heat generated by currents passing through resistance (electrical shorts). This offers a safe, sensitive and fast alternative to liquid crystals.

The third mode for the system is chip surface thermal mapping, which shows temperature distribution over the die's surface.

Working with longer wavelengths reduces spatial resolution, since spatial resolution is determined by:

where l is wavelength (about 1.2 µm for hot carrier emission mode), and NA is numerical aperture (about 0.4 for long working distance microscope objectives), the resolution obtained is about 1.8 µm. This is inadequate for chip inspection; shorter wavelengths cannot escape silicon and IR images are not as sharp as UV ones. A way around this is to use the centroid location algorithm. The algorithm, used with CAD navigation software, allows the pinpointing of defect locations with submicron accuracy. If known emission sources are available for alignment, illuminated images are unnecessary.

An IR system's ultimate sensitivity can be achieved through design of the camera's cryomechanical portion. Cooling to LN₂ temperature is required to shield the detector from room-temperature thermal background. A well-designed nitrogen fill system allows 24-hour-a-day operation for six to eight months, and vacuum must be restored twice a year.