Advances in time gating of ultrasonic signals in scanning acoustic microscopes have been proposed to enable detailed nondestructive examination of bonds and other interfaces deep within the interiors of such micromachined objects as high-density integrated electronic circuits and microelectromechanical systems. The capability to perform such examinations could contribute significantly to ensuring ruggedness and long operational lifetimes for electronic circuits, sensors, and actuators that must withstand harsh environments.

A scanning acoustic microscope is usually operated in a C-scan mode, using a pulse-gated acoustic signal with carrier frequency between 50 and 100 MHz. The scan can reveal features at depths up to several millimeters. However, the bonds, interfaces, and other features that are of interest in an assessment of the structural integrity of a micromachined object typically have dimensions of the order of microns, and acoustic signals (pulse echoes) that could be used to analyze these features often appear with similar signals from other features superimposed on them. As a result, it is difficult to assess structural integrity of a feature of interest.

In C-scan acoustic microscopy as it has been practiced until now, the pulse echo from an area of interest is gate-peak-detected to produce the ultrasonic image of that area. When the detection gate interval is set to encompass the entire pulse envelope, the depth resolution of the image is compromised because of the superposition of pulse echoes mentioned above.

The proposed advances in time gating would increase the isolation of the echo signal of interest from the other superimposed signals. One of the proposed advances would exploit the observation that by limiting the duration of the detection gate to a single cycle of the carrier signal, the sharpness of the acoustic image can be increased greatly. If the gate interval contains the echo from a feature of interest, the structural integrity of the feature can be analyzed.

Other advances could include improved sensors and shaping of pulses to increase signal-to-noise ratios. Yet another advance would be the use of a phase-lock loop to track the peak of the pulse echo corresponding to a feature of interest; this would greatly increase the capability for examining complex structures.

This work was done by E. James Chern ofGoddard Space Flight Center. GSC-14092