In response to NASA's challenge to develop smaller and less costly spacecraft more quickly, JPL is integrating state-of-the-art microelectronics into space flight hardware, including development of microelectromechanical systems (MEMS). MEMS are micron scale components fabricated (typically from silicon wafers) using micro-machining techniques. MEMS are classed as either sensors or actuators. They perform the same functions as conventional macroscopic devices, but on a microscopic scale. For the purpose of the present study, investigations were limited to MEMS sensors. In particular, sensors which operate on electron tunneling technology are presented as an example of MEMS NDE (specifically the tunneling accelerometer and tunneling hydrophone). These sensors react to an input such as an energy or signal source and produce an electrical output proportional to the strength of the input via an electron tunneling phenomena. MEMS sensors have been made which react to and measure virtually all forms of energy including, pressure, force, magnetic and electrical field strengths, sound and light energy, temperature, and more. Because of their small size, MEMS have unique inspection requirements. This is compounded by the fact that many of the features of interest are hidden within the structure. In the case of conventional systems, inspection takes place after components of the system are fabricated and before those components are assembled. In addition, it is not known how microscopic defects affect the strength and operational lifetime of MEMS devices. Micron size defects which can be ignored on macroscopic devices may be critical when considering the quality and reliability of MEMS devices.

EXPERIMENTAL TECHNIQUES

Acoustic Microscopy. By using a diffraction limited single surface acoustic lens, acoustic microscopy is essentially simple: a high-frequency focused sound beam is reflected from, or passes through, the specimen. An image of the object is then constructed by scanning the lens, or the object, in a raster fashion. The contrast of the acoustic image depends on the physical properties of the sample and acoustic impedance (product of density and speed of sound). Resolution and typical applications range from high-resolution surface and near-surface imaging (<1 mm) to low resolution subsurface imaging (>1 mm). Although transducer frequencies range from 30 to 500 MHz, the greatest benefits from acoustic microscopy lie in the frequency range of 50 to 100 MHz. Surface resolution is typically above 10 mm, however, it is possible to image at depths of up to several millimeters. The micro-acoustic imaging system is able to reveal details not discernible with any other techniques.

Microfocus X-ray Microscopy. X-ray microscopy produces results similar to conventional radiographic techniques but on a microscopic level. The advantage of this technique is the ability to produce a microfocused beam. Conventional radiographic techniques generate x-rays from a thick target, typically tungsten, oriented at 30° or 45° angles to the electron beam source. X-ray images are produced with limited magnification. Through the use of a thin film target oriented normal to the electron beam source, samples may be positioned opposite the beryllium window thereby minimizing working distance and maximizing magnification. The microfocused beam (~3 mm) further increases resolution by increasing sharpness of the image as compared to that obtained using larger focal spot sizes. Geometric magnification for typical applications range from ~3X to 1000X with capabilities of extending beyond 2000X. For conventional transmission microfocus x-ray, tube voltages range from 10 to 225kV with focal dimensions from 3 to 200 mm. By manipulating the sample and viewing a real-time image, defects normally obscured in conventional 2-D background noise can be readily imaged.

RESULTS AND DISCUSSION

Results - Tunneling Accelerometer. The tunneling accelerometer was inspected using the acoustic microscopic technique known as C-SAM and is demonstrated in Figure 1. The plan view (left) clearly showed the diaphragm bridge with the two sensing diaphragms located in the center. Analysis of the differences in the cavity reflection indicated deviations in the structure which prevented more fluid from entering the cavity on the right. Adjusting the optics to focus on the cross section parallel to the Diaphragm Bridge/Sensor Wafer interface, the accelerometer was again inspected. Most notable was the presence of partial voids located under the legs of the Diaphragm Bridge, identified by "A" in Figure 1. Considering that the Diaphragm Bridge is attached to the Sensor Wafer via four adhesive bonds located near the base of each leg, the voids are most probably a result of residual stress in the gold coating applied to silicon Diaphragm Bridge.

Results - Hydrophone. A tunneling hydrophone was inspected using X-ray microscopy and is shown in Figure 2. Although relatively difficult to image at lower magnifications due to excessive x-ray transmission, higher magnifications allowed for close inspection. A profile inspection (left) of the top cantilever bridge which is silicon diffusion bonded to the lower sensing wafer identified regions of un-bonded silicon. While rather benign to the operation of the device, this region could pose further problems associated with crack initiation during excessive vibration. Isometric inspection of the same cantilever bridge noted the presence of a crack running through the thickness of the silicon wafer which was partially obscured by the gold plating of the parts.

Results indicate that existing technology is sufficient for imaging traditional aspects such as delamination and crack identification. However, packaged devices and very close component tolerances proved to be a challenge for current limits of both techniques.

CONCLUSIONS

The present study investigated nondestructive evaluation techniques of acoustic and x-ray microscopy on microelectromechanical systems (MEMS) typical of those planned for deep space applications. Real-time transmission microfocus x-ray and acoustic microscopy, conventional microelectronic industry NDE techniques, were evaluated to determine applicability to existing and future JPL MEMS designs. Features inspected included diffusion bonds, silicon wafer surface mismatch, tunneling tip metrology and wire bond comparisons. Results of acoustic and x-ray imaging techniques indicate these techniques have direct applications in MEMS fabrication processes. Details indicate that existing technology is sufficient for imaging traditional aspects, however, several issues still provide a challenge for current limits of both techniques.