Rama R. Goruganthu

Controlled silicon thinning for design debug of C4 packaged ICs

Design debug through circuit modification on flip chip packaged ICs requires controlled thinning of substrate silicon. A laser-based tool and methodology for rapid controlled removal of silicon is described. This etching process uses a focused laser beam for localized melting of silicon in a chlorine atmosphere. At the high temperatures produced by the laser heating, chlorine reacts with silicon and forms volatile SiCl/sub 4/, which is gaseous. A novel method based on optical beam induced current (OBIC) was implemented for determining in situ endpoint for this backside thinning process. Specifics with respect to both the origin and magnitude of the OBIC signal are presented along with a model which relates the OBIC signal to the remaining silicon thickness. Details of the implementation of the endpoint signal are discussed. Through use of this methodology, controlled silicon thinning is successfully demonstrated where a localized Si area on an IC was thinned down to 15 /spl mu/m of silicon. The paper also addresses concerns regarding trench planarity and surface roughness. Furthermore, the paper describes an implementation of OBIC endpoint detection on a commercially available focused ion beam (FIB) tool, specifically designed for use with flip-chip packaged ICs. This allows for controlled silicon etching in preparing the trench floor for subsequent FIB processing such as milling to internal nodes. A discussion regarding the performance of the FIB OBIC implementation and results are also presented.

Control of localized access to circuitry through the backside using focused ion beam technology

Complex logic chips are almost exclusively assembled in flip chip packaging. This type of assembly complicates traditional debug and circuit modification techniques. Development of new applications and new equipment now enable precise access to the circuitry of flip chip parts. One such application and the equipment developed are being introduced in this article. The technique makes use of optical beam induced current (OBIC) as a way to measure the amount of silicon that is left covering active areas of a flip chip circuit after a trench has been milled in the bulk silicon using a focused ion beam (FIB) system. The apparatus is all contained in one system thus enhancing the throughput of such work. When accessing the circuitry of flip chip parts, it is crucial to be able to locally remove silicon from the backside to within a few microns of the circuitry. This is necessary in order to preserve the integrity of the part and allow access to the circuitry for probe point creation or circuit modification usin...