Yield Management

Acoustic Microscopy of Inter-Layer Dielectric Defects

With microelectronic devices becoming increasingly smaller, manufacturers have a harder time to discern what level of success there is in the internal manufacturing features. Tom Adams of Sonoscan discusses how acoustic microscopy is helping manufacturers ‘see’ any defects in inter layer dieclectrics.

All sorts of internal features inside semiconductor packages are accessible to acoustic microscopes, but the layers of circuitry beneath the face of the chip have, conventionally, never been thought of by engineers as acoustic targets. The layers are extremely thin, and anomalies within the layers are likely to be very tiny.

This situation has been altered dramatically by the introduction of low-k dielectric layers among the circuitry of advanced chips. The various dielectric materials being used (polyimide, polymers) differ sufficiently in their acoustic properties from adjacent materials to be imaged acoustically. In order to increase the speed of the chip, low-k dielectric materials are needed, and some of these materials are porous, meaning - in acoustic terms - that they are filled with tiny voids each of which will reflect ultrasound. Other dielectric materials, while not porous, differ sufficiently in their acoustic properties from materials above and below them to be imaged.

The dielectric materials are also physically fragile. During wafer dicing or later handling, the inter-layer dielectric (ILD) may crack or become delaminated. As a method of yield enhancement, it becomes worthwhile to image advanced chips to ensure that process steps are not damaging the dielectric.

Figure 1. Highresolution timedomain acoustic image of a flip chip, imaged through the back side of the silicon. Return echo signals are gated on the underfill layer below the ILD depth. Solder bump bonds (small dark dots) are visible, along with two areas (arrows) that might be defects at the underfill depth

The layers of circuitry on the face of the chip are usually not seen in acoustic images, in part because the layer would require higher image resolution, and in part because the investigator’s attention is not focused there. The pattern of the circuitry is sometimes visible incidentally when a flip chip is imaged, because the depth of interest often is at the boundary between the chip face and the underfill material.

If there is no crack or delamination in the dielectric layer, the layer may be visible acoustically because of its acoustic properties. In a porous dielectric material, for example, each of the individual pores acts as a very tiny reflector of the pulsed ultrasound. The porous dielectric layer as a whole may therefore look somewhat brighter (i.e., more acoustically reflective) in an acoustic image than the surrounding materials. A nonporous dielectric material will have different acoustic properties than adjacent materials, and will be highly reflective at the boundaries with those materials. The presence of a crack or delamination in the dielectric layer substantially changes the acoustic image, and at the same time provides a neat corroboration of a recent observation related to cracks, delaminations and other internal gap-type features.

Any gap in a sample reflects virtually all of the ultrasound pulsed into the sample, and the gap is therefore imaged very brightly. If a porous dielectric material has a crack extending across part of the material, the crack will be significantly brighter than the other areas of the dielectric.

The pulsed ultrasound is reflected only from interfaces within a sample, and not from the bulk of homogeneous materials. (This is why the bulk silicon of a chip is featureless and effectively transparent acoustically.) When ultrasound encounters an interface, the amplitude of the reflected echo signals depends on the density and the acoustic velocity of the two materials involved.

A gap in a material means that an interface exists between a solid material and the air or another gas that is filling the gap. Gases typically have very low densities and very low acoustic velocities, so the difference in the acoustic impedance (density x velocity) between the two materials is extreme. Nearly all of the ultrasound is reflected at the solid-gas interface. A trivial amount of ultrasound crosses the gap, and is again reflected at the far side of the gap where the same difference in acoustic impedance exists, although the order of the materials (gas-solid) is reversed. A few years ago a study was commissioned by Sonoscan to determine the minimum thinness of gaps that can be imaged acoustically. Some gaps are presumably so narrow that ultrasound is easily propagated across the gap, but until the study was performed, little information was available.

What the study showed was that gaps as thin as 100Å to 1000Å (0.01 micron to 0.1 micron) behaved the same as thicker gaps - they reflected virtually all of the ultrasound. The dielectric layers in advanced chips are typically between 0.5 micron and 2 microns thick. Cracks and delaminations in these layers are likely to be substantially thinner than the layers themselves, and 0.01 micron to 0.1 micron seems to be a reasonable range. The possibility exists, of course, that gaps thinner than 0.01 micron are also imaged acoustically.

Cracks and delaminations in low-k dielectric layers are thought to occur in several ways. During dicing of the wafer, thermal or mechanical stresses may be great enough to damage the dielectric. The risk of damage during dicing is significant; one major supplier of systems for sawing wafers has developed a multi-cut approach in an attempt to reduce stresses on the dielectric layers. Other process steps that can put enough stress on the die to damage the dielectric layer include probe testing, wire bonding, and, in the case of flip chips, attachment to solder bumps.

Figure 1 is the high-resolution 300 MHz acoustic image of a flip chip. In making acoustic images of flip chips, the transducer scans the backside of the chip. Ultrasound is pulsed downward to the depth of the solder bumps because the solder bumps themselves, and their attachment to the substrate below and to the die face above, are key features of concern. This is a time-domain acoustic image, where the return echo signals are defined by a time window.

Figure 1 shows (as small dark dot-like features) the bonds of the solder bumps to the face of the die. It also shows (arrows) two areas of suspected defects, but these defects are at the depth of the underfill layer and not within the chip itself. Figure 2 is the acoustic image of the upper right corner of the same flip chip, This is a frequency-domain image rather than a time-domain image. In the frequency domain, the return echo signals have been decomposed to produce multiple images that display internal features in different ways.

Running along the top of the image is a series of irregular bright features. These are cracks in the ILD. Two of them are marked by arrows. Because they are gap-type defects (cracks), they reflect ultrasound more brightly than the other features in the image. Elsewhere in the image, and more or less concentrated in a horizontal band just above the bottom of the image, are a large number of small features having the same brightness. These are numerous smaller cracks in the ILD.

Figure 2 . Acoustic image of the upper right corner of the same flip chip. This is a frequencydomain image, in which the return echo signals have been decomposed to produce multiple images. This image reveals features in the layers on the face of the chip instead of at the bump interface. Bright features (arrows) are cracks in the ILD; numerous smaller features having the same brightness level are smaller ILD cracks.

Figure 2 differs from Figure 1 in another way as well. Figure 1 is a time-domain acoustic image, meaning that it uses return echo signals from multiple frequencies on both sides of the nominal transducer frequency (300 MHz, in this case) to produce the pixels for the acoustic image. In Figure 2 Fast Fourier Transforms (FFTs) have been used to decompose the return signals into discrete frequencies and to produce roughly 30 singlefrequency images; the solder bumps do not appear in this image. Figure 2 is the FFT image that best displayed the ILD defects.

Acoustic imaging of the ILD can be carried out on both wafers and singulated die, and can be used through plastic encapsulants.

In some instances it is used as a means of qualifying a production process; for example, by imaging die both before and after singulation, it can be determined whether the singulation process is producing mechanical stresses that are causing defects in the ILD.