Advanced Direct Bond Technology

The manufacturing adoption of 3D technology has principally leveraged the improvement of existing technology platforms like adhesives, wirebonding, and bumping to achieve rapid adoption at low cost. Examples of this include die stacking with tiered wirebonding4, 5, and package-on-package4, 6 assemblies. Relentless demand for smaller form factors and increased functional density has resulted in the development of a through-silicon-via (TSV) interconnect technology platform suitable for implementation in the Back End.7 The deep vias that are characteristic of this technology platform increase cost and interconnect pitch resulting in limited scalability and preventing realization of the full cost and performance potential benefit of 3D technology.3D integration will enable the full potential cost and performance benefits of 3D technology to be realized by combining the highest lateral density of 3D interconnects and vertical density of heterogeneous stacking with wafer scale economics. This capability results in some specific requirements for the three 3D technology platforms. First, the interconnect technology platform will require a TSV to propagate an interconnection between stacked die or wafers to eliminate parasitics that result from other interconnect approaches like wirebonding. However, it is important that the TSV not intrude into the Back-End-of-Line (BEOL) multi-level metal interconnect stack to avoid complicating or compromising the BEOL interconnect routing or increasing the die size. Building this type of TSV in a wafer foundry has a number of advantages with regard to cost and process integration optimization.8 Availability of this technology platform is imminent as indicated by wafer foundry roadmap announcements regarding this type of TSV integrated into their wafer fabrication8 and process design kits. Second, the thinning technology platform will need to be compatible with very thin 2D layers, less than ten microns for some applications. A 3D process flow that thins 2D layers before bonding will require a thin wafer handling or temporary wafer bonding technology that has yet to be proven suitable for high volume semiconductor manufacturing. Alternatively, a bond technology that allows thinning after bonding allows use of an existing wafer thinning technology platform. Third, the bond technology platform needs to be compatible with a scalable areal density of vertical electrical interconnections to support pixilated applications like image sensors and displays that rely on reductions in pixel pitch to achieve reduced cost, footprint, or increased performance. This requires an electrical interconnection integral to the bond to avoid a TSV-based inter 2D layer interconnect that increases cost and limits scalability with a TSV etch and fill exclusion extending through the BEOL interconnect of one of the 2D layers.8 Another requirement for this technology platform is adequate bond strength and uniformity to meet the minimum 2D layer thickness requirements.8 It is further preferred for the bond strength and uniformity to be adequate to allow thinning after bonding to avoid thin die or wafer handling requirements. Furthermore, achieving this

bond strength and uniformity with a low thermal budget is required for heterogeneous applications involving 2D layers with significant coefficient of thermal expansion (CTE) mismatch. A fourth bond technology platform capability requirement is post-bond compatibility with BEOL and Back End fabrication process including lithography, backgrinding, CMP, via etch and fill, and thermal cycling that are required for a number of 3D process flows.This chapter provides a review of advanced direct bond technology and evaluates its suitability as a bond technology platform for 3D integration. It is shown with the aid of comparison to other bond technologies that advanced direct bond technology delivers optimum 3D interconnect lateral scaling, vertical scaling, bond strength and uniformity, thermal budget and post-bond fabrication capability with a low cost-of-ownership and supply chain synergy resulting in an ideal solution for the requirements of a bond technology platform for 3D integration. 9.2 DIRECT BONDINGDirect bonding refers to forming a bond directly between the surfaces of two materials without an intervening third material, for example an adhesive, to realize the bond10. The surface roughness of the materials is preferably sufficiently smooth to allow the formation of a high density of bonds between atoms on the respective surfaces. The resulting bond strength is a function of the density and type of inter-atomic bonds. The types of inter-atomic bonds can be one or a combination of ionic, covalent, or metallic and hence result in a bond with strength comparable to the materials that are bonded. For applications like semiconductors wherein the material has a form factor characterized by a wafer with a specified thickness, bow and warp; the values of these parameters are preferably sufficiently low to allow the bond strength to adequately deform the wafers to bring the material surfaces into intimate contact. High temperatures are typically required after the materials are placed into contact to achieve bond strengths adequate for semiconductor applications.11 High temperatures and pressures may also be used, for example to accommodate excessive surface roughness with fusion of the direct bonded materials. High bond strength can also be achieved without the use of high temperatures or pressures by placing sufficiently smooth and clean surfaces together in a non-contaminating vacuum environment.12, 13 These temperatures, pressures, and/or vacuum environments are generally not compatible with the requirements for a bond technology platform suitable for 3D integration. Furthermore, these environments would increase the cost-of-ownership (CoO) of this platform.