Evaluation of BCB Bonded and Thinned Wafer Stacks for Three-Dimensional Integration

Abstract

A critical issue associated with the implementation of wafer-level three-dimensional ͑3D͒ integration is to achieve excellent bonding and thinning performance without degrading mechanical and electrical characteristics of integrated circuit ͑IC͒ chips in the 3D wafer stack. In this work, some mechanical and electrical impacts of wafer bonding and thinning processes used to fabricate 3D ICs are evaluated using patterned wafers with two-level copper interconnect test structures that included either silicon dioxide or porous low-k interlevel dielectrics ͑ILDs͒. Benzocyclobutene ͑BCB͒ ͑Cyclotene 3022-35 from Dow Chemical͒ is the adhesive used, and thinning consists of grinding, chemical mechanical polishing, and wet etching. Three procedures used to evaluate the integrity of BCB bonded and thinned wafer stacks are discussed: ͑i͒ optical inspection of the bonding interface using glass wafers with a coefficient of thermal expansion that is close to that of silicon wafers in order to check for voids, defects, and uniformity after each bonding and thinning process; ͑ii͒ four-point bending tests to quantify bond strength and to identify the weak bond interface, and ͑iii͒ electrical tests of the patterned wafers after two bonding and thinning processes and subsequent BCB removal by plasma ashing to expose the contact pads. These procedures evaluated the impacts of processing of wafer stacks without the need for interwafer interconnect processing. Some negative mechanical and electrical impacts were observed for interconnect structures that include a porous low-k ILD, while no significant changes were observed for interconnect structures with oxide ILD.Silicon back-end-of-the-line ͑BEOL͒ technology has advanced significantly in the last decade with the advent of copper/low-k damascene patterning technology, which extended the interconnect bottleneck by approximately three generations of scaling. 1,2 However, as minimum feature size goes deeper into the submicrometer regime, global interconnects will limit the performance of gigascale integrated systems even with damascene-patterned copper/low-k technology. Three-dimensional ͑3D͒ system integration is one approach to improve interconnect performance, with several analyses having demonstrated that 3D system integration can greatly reduce global interconnect delay. 1 Moreover, monolithic wafer-level 3D system integration promises increased functionality over conventional planar integrated circuits ͑ICs͒, while maintaining the cost advantage of monolithically fabricated interconnects. [1][2][3][4] One approach to 3D system integration, using low-k dielectric polymeric adhesives for wafer bonding and copper damascene processing for fabricating interwafer vias to connect the devices on stacked wafers, is discussed in this paper. 2,5-8 Benzocyclobutene ͑BCB͒ ͑Cyclotene 3022-35 from Dow Chemical͒ was selected to be the bonding adhesive, from among several low-k dielectric polymers, because of its high bond strength and high fraction of bonded area ͑nearly 100% on 200 mm wafers͒. 2,...

Void-free BCB adhesive wafer bonding with high alignment accuracy

“…among the commonly-used bonding adhesives like SU-8 (Iliescu et al 2006), polyimide (Bayrashev andZiaie 2003), and photoresist (Su et al 2001), benzocyclobutene (BcB) has emerging as a widely-used bonding adhesive in the applications of three-dimensional (3-D) integration (Kwon et al 2008;Zhang et al 2006;Kim et al 2013;Ohba et al 2010;lu et al 2003) and MeMS/microsensors (Dragoi et al 2008;Seok et al 2012;Zhou et al 2009;McMahon et al 2008). In MeMS/microsensor applications, BcB has been used as a permanent or temporary bonding material for wafer transfer or heterogeneous integration of MeMS/microsensors and integrated circuits (Brault et al 2010;lapisa et al 2011).…”

3-D Integration and Through-Silicon Vias in MEMS and Microsensors

“…Mainstream bonding adhesives include benzocyclobutene (BCB) [144]- [147], polyimide [148]- [150], epoxy resin [151], [152], as well as many other polymers [153], [154]. BCB has become a widely-used bonding adhesive in 3-D integration and MEMS [13], [155], [156] due to its good chemical resistance, high bonding strength, low out-gassing, good thermal stability, ease of patterning, and desirable dielectric properties.…”

Review of wafer-level three-dimensional integration (3DI) using bumpless interconnects for tera-scale generation

“…An organic adhesive such as benzo-cyclo-butene (BCB) polymer [15] with a thickness of approximately 5 µm was used to permanently bond the wafer. BCB adhesives start to polymerize with increasing temperature and are solidified at temperatures of 200-250°C.…”