© International Microelectronics And Packaging Society The International Journal of Microcircuits and Electronic Packaging, Volume 23, Number1, First Quarter 2000 (ISSN 1063-1674) Intl. Journal of Microcircuits and Electronic Packaging 146 Metrology of Thin Layers in IC Packages Using an Acoustic Microprobe: Bondline Thickness Sridhar Canumalla and Bryan P. Schackmuth Sonoscan, Inc. 2149 E. Pratt Boulevard Elk Grove Village, Illinois 60007 Phone: 847-437-6400 ext. 235 Fax: 847- 437-6400/1550 e-mail: scanumalla@sonoscan.com Abstract A nondestructive technique is demonstrated for the metrology of adhesive layers that are considerably thinner than a single wave- length of ultrasound in that material. The acoustic microprobe technique discussed in this publication is capable of measuring the thickness and elastic properties of a thin layer sandwiched between two relatively thicker layers. Automated measurement over a 50 μm diameter spot are possible with this technique. The particular case of a silicone adhesive layer between an aluminum lid and a die in a Flip Chip package is discussed in this work. The thickness is estimated via an algorithm that minimizes the averaged residual error between the experimental response and theoretical prediction of a transfer function. The thickness of the Sylgard 577 adhesive was measured down to about 5 μm using transducers with nominal center frequencies of 50 and 100 MHz. The average error of the acoustic microprobe technique was verified by destructive sectioning to be approximately 0.5 μm. The minimum measurable thick- ness limit of the acoustic microprobe technique is several times better (smaller) than that of time based techniques using the same transducer. Key words: Acoustic Microscopy, Thickness Measurement, Sub-Wavelength, and Adhesive Bond. 1. Introduction The thickness of bond layers encountered in an IC package can play an important role in controlling the reliability of the package. For example, consider the thermally enhanced Flip Chip package shown schematically in Figure 1. The thickness of the adhesive layer between the lid and the die could have a direct influence on the thermal resistance due to the relatively low thermal conductivity of the adhesive. This in turn has a strong bearing on the heat removal rate and on the successful operation of the device. In addition to the thickness, the elastic properties of the adhesive could also influence the build up of thermal stresses. Aluminum Lid Si Adhesive Figure 1. Thermally enhanced Flip Chip package. Thus, there is a need to measure the thickness and elastic properties of thin layers accurately and nondestructively both for purposes of process monitoring and quality assurance. Ultra- sonic techniques have been used successfully for thickness mea- surement and material characterization in several applications primarily since they are nondestructive in nature and can yield reliable results for simple geometries. Acoustic microscopy tech- niques, in particular, are attractive for IC packaging applications Ceramic substrate