3-D x-ray microscopy aids failure analysis of complex packages

Xradia's 3-D x-ray microscope design achieves submicron resolution at working distances from millimeters to inches.

Ann R. Thryft, Contributing Technical Editor -- Test & Measurement World, 8/1/2011 12:00:00 AM

Isolating and characterizing defects in complex 3-D and stacked-die IC packages can be nearly impossible with traditional failure-analysis techniques, so device manufacturers have begun looking for new imaging methods to help improve product yield. Some of these new methods, such as IR (infrared) microscopy (Ref. 1), have recently moved from semiconductor failure-analysis and R&D labs onto the production floor. A newer technology, 3-D x-ray microscopy, is an advanced version of microCT (micro computed tomography) imaging. It originated in synchrotron x-ray facilities, moved to university research departments, and is now available for failure analysis.

Traditional nondestructive electrical tests are still the first tools manufacturers use for locating and characterizing defects caused by failures in 2-D packages with simple geometries, said Kevin Fahey, Xradia’s VP of marketing. They then often turn to nondestructive imaging techniques to get further characterization information.

“But 3-D packages have features buried in multilayer stacks, they introduce more interfaces, and the electrical path is longer and more convoluted,” he said. “New materials such as low-k dielectrics and lead-free solder metals complicate the problem by producing additional, smaller defect types that complicate package reliability.” Consequently, it’s getting harder to identify defects and locate them with electrical tests. As features like solder bumps and wire bonds shrink, small defects become more important, while defects that weren’t critical in two dimensions become critical in three dimensions.

Existing nondestructive methods such as traditional microCT or 2-D x-ray don’t have the resolution needed to find submicron defects buried in 3-D packages, said Fahey. Techniques like acoustic imaging have difficulty with the reflections off the interfaces. Destructive methods such as FIB/SEM (focused-ion-beam/scanning-electron microscope) imaging give the highest possible resolution but require cutting open the package. This destroys it, eliminating the possibility of further characterization, and it can also alter defects. And none of these methods can achieve the high resolution at large working distances required for tests in environmental chambers, time-dependent electromigration tests, and operational tests.

High resolution at large working distances are required for tests of 3-D packages, but increasing working distance usually decreases resolution in traditional microCT architectures. Courtesy of Xradia. (Ref. 2)

Xradia’s VersaXRM-500 3-D x-ray microscope design achieves submicron resolution at working distances from millimeters to inches. This is not possible in a traditional microCT flat-panel projection architecture, said Fahey. “To create a 3-D image, you must rotate the sample, take hundreds of images at different angles, and reconstruct them into a single image. For 2-D images, a flat sample works fine. For 3-D images, you have to move the sample’s rotation axis far enough from the x-ray source so the source doesn’t hit the sample when it rotates in the z direction. But increasing working distance usually decreases resolution in traditional architectures.”

The VersaXRM-500 3-D x-ray microscope, using an advanced version of microCT imaging, achieves submicron resolution at working distances from millimeters to inches. Courtesy of Xradia.

Xradia’s architecture improves on conventional microCT architecture by using a two-stage magnification-detection scheme, said Fahey. The x-ray divergent cone provides some geometric magnification, but most magnification is produced by high-resolution detector optics after a scintillator converts the x-ray image to a visual image. The optimized scintillator screen with selectable objectives, between the sample and the detector optics, lets operators change magnification as needed.

The VersaXRM can be used with other testing techniques or replace some of them, said Fahey. After traditional nondestructive testing identifies the general region of a sample where a defect is located, the VersaXRM can perform a virtual cross section to identify defects and locate them exactly. It can then perform high-resolution experiments on the sample. Fahey said the Versa­XRM system can also be used in concert with FIB/SEM and other cross-sectioning techniques. “We are synergistic with FIB/SEM tools. You can use our device to figure out where to cut with the FIB/SEM and then perform imaging with either the SEM or a TEM [transmission-electron microscope] or even an atom probe.”

Right now, the VersaXRM 3-D x-ray microscope is best suited for use as a point solution for nondestructive defect imaging and re-localizing, said Fahey. “Our next step is to implement in situ testing rigs and protocols for measuring packages in stressed and real-time environments. For example, these can be used for conducting electromigration studies in packages to see what makes them fail, or stress tests during operation to see how cracks propagate and measure those cracks.” 

REFERENCE

2. This diagram was not included in the print version of the article.