Using Synchrotron Radiation Microtomography to Investigate Multi-scale Three-dimensional Microelectronic Packages

For this study synchrotron radiation micro-tomography, a non-destructive three-dimensional imaging technique, is employed to investigate an entire microelectronic package with a cross-sectional area of 16 x 16 mm. Due to the synchrotron's high flux and brightness the sample was imaged in just 3 min with an 8.7 µm spatial resolution.

This experiment was designed to utilize synchrotron radiation microtomography, which is a non-destructive three-dimensional imaging technique, in order to investigate a complex multi-level sample. The sample being imaged here is an entire microelectronic package that has a cross-sectional area of approximately 17 by 17 millimeters. However, the features we're interested in resolving range in length scale from micrometer to millimeter.

The main advantage of this technique is that it can non-destructively evaluate the microelectronic package at the micrometer scale with fast data acquisition time. The maker tomography beam line at the Advanced Light Source in Berkeley, California, has a setup which can be tailored to optimize resolution and image quality based on a sample's properties, such as volume and density. However, the sample size is limited to a maximum allowable field of view of 36 by 36 millimeters.

This method can help answer key questions in the semiconductor field. For example, it can be used to analyze electronic packages and identify failures through a reliability test in process development, as well as to provide experimental flexibility or how a x-ray source can quickly detect defects in complex next generation microelectronic packages. Prepare the sample for the scan by mounting it on a sample holder designed to fit in the beam line's rotational stage.

For samples that do not have a custom mount, adhere the sample to a post or drill chuck with clay or wax. Before loading the sample on the rotational stage inside the hutch, there is an offline mock rotation stage that is used to align the sample. Visual inspection of the center of rotation is usually sufficient for the alignment.

Mount the sample attached to the sample holder inside the hutch. Once the sample has been mounted in the hutch, two orthogonal centering motors allow positioning of the sample with respect to the center of rotation. Select the magnification for the scan based on the sample size and feature size of interest.

Since the sample scanned here is 22.6 millimeters in the longest direction, select the 1X lens with the PCO point 4, 000. This combination gives the largest sample field of view. The resulting pixel size is 8.7 microns.

Set the x-ray energy, or switch to a polychromatic beam using the beam line control computer. To get the best quality image, base the energy selection on targeting an approximately 30%transmission, which can be measured on the data acquisition computer. In general, percent transmission increases with increasing energy.

For the microelectronic package, select white light due to the thickness and material of the package. When using white light mode, add two to four metal aluminum and copper filters in line with the x-ray beam to filter out the lower energy x-rays. For this sample, use two copper sheets with a total thickness of approximately 1.2 millimeters.

Next, verify that the stage's center of rotation is aligned with the camera's center. To check that the sample is aligned, rotate it through 180 degrees using software on the beam line control computer, and visually observe the change in sample location by viewing the radiographs on the computer. Control changes to alignment on the same computer.

Set the sample to detector distance for the scan. The camera is on a translational stage that can move horizontally, and is used to change the sample to detector distance. When the distance increases, the face contrast contribution also increases.

Input the desired angular range and the number of images to collect over that range. The more angles selected, the longer the scan times and larger the data set size. For this study, use 1, 025 angles over 180 degrees during data acquisition.

After selecting the scanning mode and the number of bright and dark field images as described in the text protocol, verify that the sample is translated far enough so that it is not present in the bright field image in order to avoid large defects in the reconstructed images. Here, acquire 15 dark field images and 15 bright field images. After determining if tiling is necessary, execute run scan on the data acquisition computer.

The scan will run automatically based on the inputted settings. Shown here is a 3D rendering of an entire field programmable gate array system in package imaged with 8.7 micron resolution and a scan time of three minutes. A zoomed in view of a region of the package shows one corner of the field programmable gate array substrate and the circuit board interconnections.

A 3D volume rendering of the three different interconnect levels shows the entire system in package with an 8.7 micron resolution. Here, a 3D reconstructed image of the vertically scanned CPU dye package with first level interconnect and mid-level interconnect solder connections is shown. A zoomed in region of a 2D reconstructed slice shows a mid-level interconnect solder ball with a large center void and cracks caused during intentional thermal stress testing.

This movie shows tomography images of the microelectronic package imaged in the horizontal orientation. The 3D volume rendering of the 16 by 16 square millimeter package shows it from different perspectives. Here, the movie pans through the different cross-sectional views to show internal information from within the package.

The ability of tomography to accommodate large sample sizes with faster through put time, especially compared to table top CT systems, is of the utmost importance to the semiconductor industry. This technique enables non-destructive quantification of cracks, voids, delamination, defects, and much more. This method is very useful at providing insight into solder joint interconnects for the microelectronics industry.

However, it can also be applied to a wide range of material systems such as metal alloys, composites, biomaterials, organics, and additively manufactured components. Although there is a wide range of materials and volumes that can be imaged using synchrotron radiation microtomography, there are limitations due to the available energy range at the ALS synchrotron facility. Specifically, high density materials are constrained to very thin sample sizes due to the need to get sufficient x-ray transmission through the sample.

One of the most critical steps during the experimental setup is the stable mounting and focusing of the optics. These steps are vital to obtaining quality images that can be used for quantification of the data. Specifically, even a slight movement of the sample causes artifacts in the reconstructed image, and the focusing causes a deterioration in resolution.

To avoid the issues with image quality, reconstruct a test image which can take place simultaneously while the next sample scans. While attempting this type of experiment, it is important to modify your setup depending on your sample properties and to talk to your beam line scientist about optimizing your experimental procedures. The high resolution capability of synchrotron radiation microtomography provides valuable information for both failure analysis and assembly process development.

The application of synchrotron 3D x-ray CT to a microelectronic package opens up a wide range of possibilities in equality and reliability of 3D microelectronic packages, including reliability tests, inspection of failures in complex packages. It also provides directions for the development of next generation lab scale 3D x-ray CT.