Calibrating acoustic micro-imaging systems

A calibration wafer permits you to calibrate an acoustic micro-imaging system. These calibration lines are for feature sizes at the 25-µm to 100-µm level.

The ultrasonic transducer in an AMI system scans a semiconductor package while alternately pulsing ultrasound into the package and receiving the return echoes. Both of these events occur several thousand times a second and produce the pixels from which the acoustic image is made.

Ultrasound pulsed into the package reflects from internal interfaces—where the molding compound is bonded to silicon, for example. The strongest reflections come from an interface between a solid and a gas, such as at a delamination between molding compound and silicon. A solid-gas interface is characteristic of delaminations, cracks, and voids—the vast majority of internal packaging defects that create field failures.

A delamination covering half of a die face can easily be imaged at low frequency. But imaging a tiny 20-µm void nestled beside a flip-chip solder bump that is itself only 60 µm in diameter requires high resolution. To ensure that the transducer and the operating system are functioning optimally and that the acoustic image you see best represents the internal features, you can employ a calibration wafer such as the one in the figure, a 2-in. glass/silicon anodically bonded sandwich with features of various sizes etched into the silicon. The range of feature sizes permits the wafer to be used to calibrate AMI systems having transducers that operate at 100 MHz or above and produce resolution down to the 5-µm level.

Etched into the silicon portion of the calibration wafer are groups of lines and dots of successively smaller sizes. Bonding the glass layer onto the silicon wafer creates the solid-gas interfaces that make the lines and dots visible acoustically.

The smallest feature groups (for example, three parallel lines each a few microns wide and spaced only a few microns apart) give you a known standard against which to measure a system. You can select and image a pattern group on the wafer to determine both the AMI system's detection (the ability of the system to image a feature of a certain size) and resolution. A low-frequency transducer, for example, might image a group of three fine lines as a single feature. It is detecting the lines, but not resolving them. A high-frequency transducer should be able to resolve the individual lines in the group.

The difference between detection and resolution is important in specific applications. If a flip chip has a tight group of small underfill voids, then a low-frequency transducer may image the group as a single feature. But a high-frequency transducer will resolve the individual voids and facilitate the search for the cause of this anomaly.

The calibration wafer is useful when you are aiming for compliance with an ISO 9000 program, where thorough documentation of system functions and capabilities is essential. It is also a candidate for selection by a joint IPC/JEDEC task group that is currently considering the adoption of a test sample as a calibration tool for AMI systems.