Analysis of Low-K Dielectric in Flip Chip Devices Using Acoustic Micro Imaging

Evaluation of flip chip devices for defects in the solder joint attach or underfill voids and delaminations using acoustic micro imaging (AMI) has become a matter of routine. However, the move presently is to utilize low-K dielectrics in the active layers on the chip surface. Low-K materials provide a significant increase in performance of the devices but the materials tend to be intrinsically porous and brittle. These properties can contribute to the creation of defects during the manufacturing process or in subsequent life cycle testing of the devices. The properties of low-K dielectrics also contribute to challenges when evaluating the devices using AMI.

AMI (Acoustic Micro Imaging) is a non-destructive test method that utilizes high frequency ultrasound in the range of 5 MHz to 500 MHz. Ultrasound is sensitive to variations in the elastic properties of materials and is particularly sensitive to locating air gaps (delaminations and voids). There is a direct relationship between frequency and resolution in AMI. Higher frequencies have shorter wavelengths and therefore provide higher resolution. Lower frequencies, which have longer wavelengths, provide better penetration of the ultrasound energy through attenuating materials, thicker materials or multiple layer assemblies. Generally a compromise is found between sufficient resolution and maintaining satisfactory penetration and working distance for a given application. More recently methods such as Fourier Domain imaging have been used to improve the resolution/detectability of features in acoustic images.

The low-K materials have proved to be somewhat attenuating to high frequency ultrasound. When evaluating flip chip devices using AMI the ultrasound is introduced through the back side of the silicon chip to reach the inner chip surface and subsequent underfill and substrate levels. Even though the active layers on the surface of a silicon chip are very thin the attenuation must be taken into account when deciding what frequency to use for evaluation. Higher frequencies are preferable for analysis of the active layers. However, selecting for the active layers in the image can minimize the appearance of defects and characteristics of the underfill (which are below the active layers in a flip chip device) in some cases compromising the detection of the flaws. Recent studies have yielded experience in working with low-K dielectrics. Various AMI methods have been used to detect and distinguish defects in the active layers from voids and delaminations in the underfill and further efforts are focused on extracting more precise depth information within the active layers.

This paper will present a brief background on flip chip devices with low-K dielectric and provide examples of analyses using AMI to detect internal features and defects in the devices.