Modeling, Design, and Demonstration of Low-temperature Cu Interconnections to Ultra-thin Glass Interposers at 20 µm Pitch

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1 Modeling, Design, and Demonstration of Low-temperature Cu Interconnections to Ultra-thin Glass Interposers at 20 µm Pitch Tao Wang, Vanessa Smet, Makoto Kobayashi+, Venky Sundaram, P Markondeya Raj*, and Rao Tummala, Fellow, IEEE 3D Systems Packaging Research Center, Georgia Institute of Technology, Atlanta, USA + Namics Corporation, Niigata, Japan * raj@ece.gatech.edu Abstract This paper reports the first design and demonstration of a manufacturable 20 µm pitch Cu interconnection technology to ultra-thin glass interposers. Bonding is accomplished at temperatures below 200 oc without the need for solders. Manufacturability challenges such as substrate warpage, bump noncoplanarity and assembly throughput with low bonding times are addressed with this technology. The modeling and experimental results indicate that the ultra-fine pitch Cu interconnection offsets more than 3 µm non-coplanarity. Bonding interfaces were characterized to show that metallurgical bonding microstructure is formed even with a bonding time of 5 seconds, with superior electrical properties. A mechanism for low-temperature metallurgical bonding is proposed based on the characterization results. Introduction Off-chip interconnections are predicted to reach 20 micron pitch over the next few years to meet the miniaturization and performance demands by 2.5D and 3D stacked packages. Traditional flip-chip and copper pillar-solder cap interconnections, in spite of their many advantages due to liquid solder formation at the reflow temperatures, face challenges in meeting these pitch and power requirements. Therefore, solid-state bonding-enabled Cu interconnection technology is becoming a very important and necessary offchip interconnections technology. The key benefits with this approach are: (1) Solid-state bonding enables smaller bumps for fine-pitch interconnections without the risk of bridging; (2) Cu has high electrical and thermal conductivities bringing high-speed signal transmission and high power-density handling; (3) Reliability challenges are minimized because no unstable interfaces associated with intermetallics and solders are present; (4) Cu plating is a standard BEOL process with well-established infrastructure. In the last decade, Cu-Cu direct bonding has been one of the most extensively researched approaches to enable ultrafine pitch and high performance chip-to-chip and chip-tointerposer interconnections. However, the need for careful Cu surface preparations and removal of residual oxides, combined with high-temperature and long annealing times for interdiffusion and recrystallization created several manufacturing challenges. Further, these bonding approaches are not tolerant to substrate warpage and bump noncoplanarities that are invariably present during IC assembly onto interposers and packages [1~3]. Ultra-thin glass interposers present yet another challenge, due to their brittle nature. This paper comprehensively addresses these challenges resulting in a novel low-temperature Cu interconnection /14/$ IEEE process for high thermo-mechanical reliability and high power-handling applications. The assembly technology also accounts for substrate warpage and bump non-coplanarity, and yet achieves ultra-short Cu interconnections at ultra-fine pitch, at low temperatures, without solders. The low-temperature assembly process is performed below the Tg of low CTE laminates, minimizing warpage issues during assembly. Table 1 summarizes the key objectives of Georgia Tech program with this innovative copper interconnection technology. Table1. Objectives of the low-temperature interconnection technology at Georgia Tech Metric Parameters Pitch Height µm 10 µm Properties copper Resistance: < 10 mω per interconnection 1000 cycles of TCT (-55 ºC ~ 125 ºC), >192 hours of U-HAST (85% RH, 130 ºC) Reliability Electromigration: > A/cm2, 130 ºC Manufac. process Bonding temperature: < 200 ºC Bonding time: <5 seconds Non-coplanarity offset: > 3 µm Bonding environment: in air These objectives are accomplished with a patented process [5] consisting of ICs having 5~10 micron height Cu bumps, interposers or substrates with bonding pads having traditional ENIG (electroless nickel immersion gold) surface finish on both Cu bumps and Cu pads, pre-applying a unique Bstageable non-conductive underfill (BNUF) on the interposer or substrate, and thermocompression bonding to form the Cu interconnections. Thin Ni-Au layers were formed on the bonding surfaces as shown in Figure 1. The key benefits with this approach are: (1)Lowtemperature bonding without the need for solders, (2) Ultrahigh current-handling because of stable metallurgical interfaces; (3) High thermo-mechanical reliability because of pre-applied polymer adhesive; (4) High throughput by forming metallic bonding in less than 5 seconds. This paper describes the finite element modeling, fabrication and assembly of dies Electronic Components & Technology Conference

2 on glass interposers, as well as the characterization of the interfaces. Figure 1. Schematics of Cu bump to Cu pad bonding with NiAu surface finishes on both sides Finite Element Modeling The first step in studying 20 µm pitch Cu interconnections is to model the deformation of the Cu bump during thermocompression bonding. The diameter of Cu bump was chosen as 8 µm, slightly less than 10 µm, to accommodate the bump widening effects during thermocompression bonding, as shown in Figure 2. The Cu bump height in the model was 12 µm, slightly larger than 10 µm, to accommodate the bump collapse. The thicknesses of Cu pads on the silicon or interposer or substrate were designed to be 2 µm and 3 µm, respectively. In addition, a 10 µm-thick polymer was assumed on either sides of the glass, before the formation of copper pads. ultimate stress of 210 MPa. The true stress-true strain curve of the oxygen-free copper at 175 ºC and a strain rate of 10-4 was used. [6] The frictional coefficient between the Cu surfaces (bump and pad) was set to be 0.75 [7]. The viscosity of the laminated polymer is neglected here, and it is considered a purely elastic material, described by its Young s modulus of 0.3 GPa. The Young s modulus of glass is 77 GPa and the Poisson ratio equals to The modeling results are shown in Figure 3 and discussed next. Bump and pad deformation: Stresses concentrate at the edge of the contacting area between the bump and pad. Plastic deformation of Cu initially starts at the stress-concentration area and then spreads towards the bump center. This leads to metal plastic-flow along the radial direction as well as longitudinal direction, emulating the mechanism of bump collapse while forming Cu interconnections. Collapse is defined as the vertical displacement of on-chip copper pads. The three states captured from the bonding modeling results are shown in Figure 3, in which collapse = 2.16 µm in state (a), collapse = 4.14 µm in state (b), and collapse = 6.39 µm in state (c). Figure 3. Bump and pad deformation and stress contour at various steps during compression bonding Figure 2. Finite-element model for the thermocompression bonding of Cu bump to Cu landing pad Copper is considered an elasto-plastic material, described by its Young s modulus of 110 GPa, Poisson ratio of 0.34, and Even though the bump diameter would increase due to the lateral deformation of the metal, it does not exceed so as to result in bridging between the two adjacent bumps. In Figure 4, the bump diameter was plotted as a function of the top-pad collapse, indicating that the bump diameter would increase by less than 2 µm when it collapses by 6.3 µm. Based on the theory of time-independent plasticity, the bump and pad deformation, as well as the resulting collapse can be accomplished in much less than one second. Therefore, these two factors provide the capability of ultra-fast die bonding with ultra-fine pitch off-chip interconnections. Stress distribution on the on-chip Cu pads: Minimal stresses are desired on the on-chip dielectrics in order to avoid 285

3 dielectric failures. Figure 4 plots the maximum von Mises stress on the dielectrics with the collapse in the top pad. It can be seen that the stress reaches an upper limit of 205 MPa after the large plastic strain. After achieving the bonding state shown in Figure 3(c), the maximum stress imposed on silicon dielectrics is 130 MPa, and occurs at the outer edge of the top pad-surface. Figure 4. The evolution of bump diameter and maximum von Mises stress on silicon chip dielectrics with the top pad collapse Fabrication Silicon wafers were plated with a routing layer and Cu microbump layer, followed by ENIG surface finish. Dry-film photoresist lithography was used for semi-additive Cu electrolytic plating in both layers. Commercial electrolyte formulations from Atotech were used for the Cu plating and ENIG finish. The thickness of Ni varies from 200 to 500 nm, while that of Au is close to 100 nm. Regarding Cu bumps noncoplanarity, the average surface roughness (Sa) was 0.3 µm, while the maximum peak-to-valley height (Sz) was ~2 µm. Bump height variations within a single die was less than 3 µm. The fabrication procedures for glass interposers consist of polymer lamination on a thin glass substrate, electroless Cu plating, photoresist patterning, electrolytic Cu plating, and ENIG surface finish. Glass substrates with 100 µm thickness were used, which brings handling issues due to its native fragility. The laminated polymer was 10 µm thick ZIS-100 from Zeon Corporation. The Cu interconnection without solders does not lead to any solder-bridging or intermetallics formation. These attributes enable scaling to ultrafine pitch and high reliability. Both Au and Cu have FCC structures, and are the two most widely used metals in wire bonding, which requires excellent malleability and ductility properties. Au is chemically noble and can be deposited easily with either dry or wet process. Extra-high stress is not required to break down native oxides to form direct Au-Au bonding [8, 9]. Therefore, Au, in combination with Cu interconnections, is the optimum metal for direct bonding, compared to other alternatives including Al, Ni, Ti, and Pd [10-15]. A diffusion barrier layer must be deposited between Cu and Au to prevent the interdiffusion of Cu and Au atoms. Electroless-plated Ni layer was applied to act as the adhesion and solid-diffusion barrier layer between Cu and Au. The thickness of Ni layer can be adjusted to 200~500 nm, much less than 5.5 µm for standard C4 interconnections, due to the elimination of intermetallics issues. Electroless or electrolytic plating Au can also be applied prior to immersion Au if a thicker Au layer is required. The Ni and Au surface finish processing were applied on both Cu microbumps on silicon die and the outer surface of Cu layer on glass interposers. Assembly on Glass Interposer Assembly was performed with thermocompression bonding. All the thermocompression bonding experiments were performed with silicon dies on 100 µm thick glass interposers at a bonding temperature of 200 C. Prior to bonding, the BNUF or NCF was dispensed on the glass interposer and baked at low temperatures. The BNUF adhesive had a material formulation to prevent Cu oxidation on microbump and pad surfaces [16]. Figure 5 shows the images of glass interposers before and after die assembly. The alignment between the Cu microbumps and landing pads were checked with X-ray microscopy. The standard bonding profile for the Cu interconnection comprises of 365 MPa bump pressure for 60 seconds at 200 C. Other bonding conditions were also studied by adjusting the bump pressure from 90 MPa to 365 MPa, and the bonding time from 3 seconds to 10 min. Figure 5. Top: Glass interposers before and after die assembly; Bottom: X-ray microscopy of the die-to-glass interposer assembly 286

4 As described in the modeling section, lateral plastic deformation of Cu occurs with the interconnection collapse, resulting in an increase of the contact area between the microbump and the pad. This implies that the effective pressure applied on the contact area decreases until it reaches the strength of the strain-hardened metal. The deformation stops at this point. To observe the Cu microbump deformation after thermocompression bonding, the die was detached from the glass interposer after the assembly step. Figure 6(a) shows the SEM image of Cu microbumps prior to bonding, while Figure 6(b) shows the SEM image of Cu microbumps after bonding. The bump height decreased from 12.5 µm to 9.0 µm due to the bump deformation. Usually, thermocompression bonding process designed for fine-pitch and short interconnection height faces major challenges associated with non-coplanarity arising from bump height variations, interposer warpage and bond head tilt. Considering the deformation of Cu pad, the described Cu interconnection technology can address all the non-coplanarity challenges by offsetting more than 3 µm noncoplanarites but not introducing any lateral bridging problems. This attribute promises the described Cu interconnection technology for manufacturable applications as die-tointerposer and die-to-package interconnections in the ultrafine pitch era. attributed to low-temperature metallurgical bonding in technologies that are similar, such as friction-stir welding, solid-state welding and thermosonic bonding [17-18]. When high pressures are applied, the metal layers plastically deform under pressure at temperatures below 200 oc, due to their native malleability and ductility. In addition, plastic deformation takes place due to high stress concentration that occurs at the rough surface regions along the bonding interfaces, resulting in local plastic strain variation which helps to bring the top Au surface and the bottom Au surface into intimate contact, although no planarization process was performed on the bonding surfaces prior to bonding. Therefore, the seams and void sizes were much smaller than the root mean square values of plated bumps and pads. Interdiffusion or self-diffusion of gold is expected to occur during thermocompression bonding after the two Au surfaces were pressed to attain intimate contact. Self-diffusion speed of Au atoms is accelerated at elevated temperatures, resulting in a robust and continuous Au direct bonding layer, which shows high reliability performance. This has been confirmed in a separate thermal aging study by Kumbhat et al. [4]. Moreover, the outward diffusion of Au atoms into Cu was blocked by the Ni barrier layer, stopping the formation of Kirkendall voids arising from Au-Cu interdiffusion [19]. Figure 6. Cu microbump array: (a) before thermocompression bonding, (b) after thermocompression bonding 10 µm Bonding Interface Characterization and Mechanisms The cross-sectional SEM image of Cu interconnection after ion milling is shown in Figure 7. The top side shows the Cu bump on silicon die, and the bottom side shows the Cu landing pad on the polymer dielctric on glass interposer. Both the Cu bump and the Cu pad were plastically deformed under thermocompression. The 10µm- thick polymer lying under the Cu pad has sub-gpa modulus and therefore undergoes continuous viscoelastic deformation at the bonding temperature, leading to significant pad-deflection as well as bump-collapse. During thermocompression bonding, local metallic contacts are formed at the bonding interface. The surrounding underfill polymer further enhances the bonding strength of the joint and thermomechanical reliability performance. Metallic bonding happens due to solid state diffusion and by plastic deformation at the bonding interfaces. While the relative contributions of each factor requires more extensive modeling and TEM characterization, both factors are Cu Au Cu 1 µm Figure 7. Cross-sectional SEM images interconnection and the bonding interface 287 of the Cu

5 Experimental correlation between the bonding times and temperature for Au direct bonding has been illustrated by Tong [9]. Their work concluded that the activation energy EA for low-temperature (room temperature to 250 C) Au-Au bonding is estimated to be ~0.41 ev. There are 3 major kinds of self-diffusion mechanisms in pure gold: lattice diffusion, grain boundary diffusion, and surface diffusion. Activation energies for these 3 kinds of diffusion are 1.71 ev [20], 0.88 ev [21], and 0.40 ev [22], respectively. Following Arrhenius relation, excellent correlation was seen between the experimental low-temperature Au-Au bonding results [9] and surface self-diffusion of pure Au. Therefore, it can be surmised that interdiffusion is driven by surface and grain boundary migration processes that are limited by the surface self-diffusion of the metal atoms. For direct bonding between two polished atomically-flat metal surfaces, diffusion-induced grain boundary migration, along with the reduction of interfacial energy is the main reason that causes the bonding interfaces to diminish [23]. However, with two rougher metal surfaces, after the initial plastic deformation stage, surface diffusion bonding is the slowest step that dominates the bonding mechanism of bridging the bonding interfaces until they disappear. This implies that the activation energies of direct bonding with Au interfaces are determined by the Au surface self-diffusion. The combined effects of local plastic deformation at the interface, and gold interdiffusion results in good metallurgical bonding along the bonding interface. In conjunction with the polymer adhesive that bonds the chip and the substrate, this results in high thermomechanical and electromigration reliability as previously shown by Kumbhat et al. [4]. The bonding conditions such as load, temperature and cycle time determine the extent of metallurgical bonding and overall reliability. To achieve a manufacturable process with microscale roughness and non-uniform copper surfaces, bonding pressure is the most critical parameter to accommodate such non-planarities. The bonding pressure can be further lowered with better bump process control, softer and more reactive interfaces created with specific bump geometry and interface material designs. Conclusions A breakthrough in low temperature Cu interconnection technology, that also overcomes the warpage and noncoplanarity issues, to achieve solder-free bonding for highly reliable, ultra-fine pitch off-chip interconnections is presented. Ni-Au layers were applied to the copper bonding interfaces prior to the thermo-compression bonding. The bonding pressure at deformation temperature brings the two Au surfaces into intimate contact, followed by solid interdiffusion to achieve the metallurgical bonding. The thin continuous Au bonding layer and the nickel diffusion-barrier layer on top of thick Cu, lead to high current-handling performance. The novel Cu interconnection technology at 30 µm pitch enables 2.5D integration on glass interposers with unprecedented I/O density that is extensible to 20 µm pitch and below, high speed and low-loss signal transmission, and high powerdensity. Acknowledgments This research is supported by the Interconnections and Assembly (I&A), and Low-Cost Glass Interposer and Packaging (LGIP) focused programs at Georgia Tech PRC. The authors would like to thank the full members and supply chain partners for their funding and intellectual support. 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