Residual thermomechanical stresses in ultrathin chip stack technology

ABSTRACT The aim of this work is to analyze the thermo-mechanical stresses evolution produced during the fabrication sequence of themulti-level UTCS structure. Several non-linear material models have been taken into account during the process modeling.We have therefore resorted to the Finite Element Method for the evaluation of such thermo-mechanical stresses that appearsin the manufacturing and stacking process. These efforts are made to optimize the product and process design.Keywords: residual stress, FEM, chip stack, creep, BCB, copper, interconnects, MCM 1. INTRODUCTION UTCS will deliver a new, very dense, 3D stacking technology for semi-conductor chips. This very dense, ultra thin stackingtechnology is of interest to all electronics industries where size and weight are important for product acceptance. Anotheradvantage for industry is the possibility of using standard chips form different vendors. The proposed new, dense stack willbe based on photosensitive Benzocyclobutene (onwards BCB). The procedure is as follows: the chips are thinned down to1 0-15 im and then, using planarisation techniques as used in semiconductor processing, the 3D stack is formed on a siliconsubstrate by depositing layers of dielectric, onto which a metallisation copper structure is patterned. The thinned chips areplaced on top of each dielectric layer and the vertical interconnection is realised with metal vias.This technology has required the development of a thinning technology applicable to standard fmished silicon chips thatachieve a fmal thickness in the order of 10-15 tm using chemical and mechanical procedures. Another challenging pointthat required some improvements was the development of transport, attachment and bonding solutions for the very thinsilicon chips that are used in the 3D stack. Also the existing planarisation techniques used in this stacking methodology hadto be duly modified.During the sequential built-up of the UTCS structure the constituent materials suffer thermal cycling, see 1 for a detailedthermal and technological description. The differences in the coefficients of thermal expansion (CTE) between dissimilarmaterials inevitably generate thermal residual stresses. These stresses may exceed the strength of the films resulting incracking, the interface may fail resulting in delamination or the excessive warping may hinder further processing. Is the aimof this work the evaluation of this potential source of problems for the UTCS technology by making use of the fmiteelement method2. Despite many works3'4'5 have been devoted to the analytical study of thermal stresses in multilayeredassemblies, the mathematical intricacies which easily arise, do only allow very simple geometries, and in consequence areoflittle applicability in a complex structure such as the UTCS.To be able to study these residual stresses we use a parametric process-modelling framework to simulate the evolution ofstresses and strains as the structure is sequentially fabricated6'7. This is in contrast to the usual approach of many researcherswho employ a "frozen-view" model starting from the geometry of the fmal configuration. In the "frozen-view" model, theentire structure is assumed to be stress-free at the curing temperature of the polymer and then cooled down to the roomtemperature. This procedure provides a better understanding of the process because the material layers are deposited atdifferent temperatures, and therefore, not all layers are stress-free at the same temperature. Furthermore, residual stressesmay develop plastic deformations at intermediate process steps, a feature that can not be captured by "frozen models".