Al Bond Pad Research Articles

Exploration of Interfacial Materials Chemistry Control to Improve Cu Wire Bonding Reliability

Copper wire bonding, with its advantages of higher electrical conductivity and better mechanical strength, has replaced gold wire bonding as a proven cost-effective electrical interconnection solution for IC packaging for the past 15 years. Early development of Cu wire bonding required overcoming of several technical challenges including bond pad damage caused by copper’s hardness and brittleness relative to gold. A more chemistry related challenge of using Cu as bonding wire is its well-known reactivity with oxygen. Inert atmospheric envelope of forming gas surrounding bonding capillary was developed to prevent oxidation of Cu wire during electronic flame off to enable a strong bonding. Another more elusive materials-chemistry related reliability challenge, with a typical low ppm occurrence, has been the chloride-induced corrosion defects between the Cu wire and Al bond pad. The opportunistic low-level chloride contaminations can originate from various points of packaging manufacturing process flow and often render it un-trackable. In this talk, we present recent efforts to systematically control interfacial materials chemistry across Cu bonding wire, Cu/Al bimetallic contacts and CuxAly intermetallic compounds with the aim to eliminate corrosion defects and improve the overall bonding reliability. The current prevailing manufacturing solution is to utilize Pd-coated Cu bonding wire that can only partially mitigate the CuxAly intermetallic corrosion vulnerability. We utilized a real-time corrosion screening metrology to explore the underlying interfacial materials chemistry drives vigorous corrosion between Cu wire and Al bond pad when exposed to trace level of chloride contaminant. Combining with SEM, sensitive infrared spectroscopy and electrochemical characterization, our data show strategic surface modification on both Cu bonding wire and exposed CuxAly intermetallic can have a significant impact on reducing corrosion defect rates. The obtained mechanistic insights provide several new strategies enabled by a novel Cu-selective passivation coating technology to effectively mitigate Cu wire bonding corrosion defects. Implications on further improving overall Cu wire bonding reliability will be present based on these new approaches that have low-cost and packaging friendly advantages.

Influence of HCl Concentration on Corrosion Behavior between Au or Cu Bonding Wires and the Bond Pad for Semiconductor Packaging

Wire bonding, one of the methods for electrically connecting a semiconductor chip with a substrate, involves attaching thin metal wires to pads. It is the oldest electrical connection method that exhibits high compatibility with other processes. The metal wires used for electrical connection in wire bonding are mainly made of Au, Cu, and Ag. After the wire bonding, molding is performed using the epoxy molding compound (EMC). However, EMC inevitably contains ions such as halogen elements. In addition, it absorbs moisture due to its hydrophilicity, creating a corrosive environment with electrolytes. In this study, we evaluated the influence of hydrochloric acid concentration on corrosion behavior between Au or Cu bonding wires and sputtered Al bond pads. The electrochemical factors such as corrosion potential difference (ΔE), galvanic corrosion current density (ig), and anodic and cathodic Tafel slopes were found to influence galvanic corrosion behavior. Galvanic corrosion tendency in first bond and second bond areas of PCB unit specimen was confirmed.

Investigation of CuAl IMCs Corrosion in Chloride Environment and its Prevention Strategy

Abstract In most current wire-bonding applications, electrical interconnection is accomplished by fine Cu wires bonded to Al bonding pads microfabricated on an IC chip and external contact pins of the PCB board. Corrosion-related failure defects between the Cu wire and Al bond pad have been an ongoing un-trackable reliability issue plaguing the IC packaging industry for the past ten years, despite approaching ppb levels. Most prior studies hypothesized that intermetallic compounds (IMCs) like Cu9Al4, and CuAl2 were responsible for the observed acute wire-bond lift-off corrosion defects. Further studies sought to quantify the rates of corrosion of these IMCs and explore the effects of mitigation efforts of adding Pd, relevant to Pd-coated Cu wire-bonding. However, utilizing a novel real-time corrosion screening approach, we previously established that peripheral bimetallic contact between Cu ball-bonds and Al bond pads also plays a substantial role in the aggressive Al pad corrosion, induced by chloride ion penetration, which often leads to device failure. In this work, we further explored the role of IMCs corrosion in wire-bond lift-off failure utilizing fundamental electrochemical studies to quantify rates of galvanic-induced corrosion of IMCs within the broader context of an interconnected stack of Al bond pad, Cu-Al IMCs and Cu bonding wire. We also explored the use of a corrosion inhibitor to suppress the galvanic corrosion currents of Al bond pad, and Al-rich IMCs when electrically connected to Cu wire or Cu-rich IMCs.

High Robustness of Coated-Ag Wire Bonding

Formation of free air ball (FAB) within the range of certain processing parameters leads to excellent bondability (distributed intermetallic coverage (IMC) at the center and periphery of the bonded ball interface, bonded ball concentricity, etc.) on bonding to aluminum (Al) bond pad and Au/Ag plated surfaces. The wire revealed zero stoppage and 0.8M bond touchdowns when tested under laboratory conditions. Thermal aging of bonded interface showed absence of Kirkendall voids until 192h at 250°C where Al diffuses into Ag bonded ball up to a depth of 5.3µm, consuming the complete Al bond pad and thus transforming into silver-aluminide containing 5 to 12wt% of Al and rest silver (Ag). Traces of coated gold (Au) and alloyed palladium (Pd) are noticed at the bond interface.

Intermetallic Compounds at the Interfaces of Ag–Pd Alloy Stud Bumps With Al Pads

Stud bumping using a traditional wire bonding process followed by the severing of the wire material from the ball bond has been widely applied in flip chip packages. Traditionally, Au stud bumps were employed because of the physical properties of gold, but the mediocre reliability of Au with Al pads and the cost of gold make this material unfavorable for industrial needs. Copper stud bumps were also considered, but the intrinsic features of Cu, such as its high hardness and ease of oxidation, present no advantages over gold stud bumps. However, Ag-alloy stud bumps exhibit satisfactory bondability and have been proposed as the optimal substitute for Au stud bumps to ensure better reliability in soldering and better intermetallic growth kinetics with Al bond pads. In this article, the intermetallic growth of Ag stud bumps alloyed with 2%, 10%, and 12% Pd after high-temperature storage tests (HTSTs) is investigated. The results indicated that the Pd atoms remained immobile in the bump-side matrix without diffusing into the aluminum pad side. Alloying with higher Pd content enhanced the formation of the favorable Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Al intermetallic compound (IMC), whereas lower Pd content resulted in severe cracking due to the formation of Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al IMC. Compositional analysis showed that continuous layers of Ag-Al IMCs doped with large amounts of Pd formed at the interfaces close to the Ag stud bumps, but no Pd was detected on the Al pad side. The bonding strength increased after aging at 150 °C and 200 °C for 200 h, but it decayed drastically after aging at 250 °C for 200 h. The relationship between intermetallic growth and failure mode was established.

Mechanistic study of copper wire-bonding failures on packaging devices in acidic chloride environments

Microelectronic reliability requirements are tightening to a standard of near zero ppb defects due to the evolution of self-driving cars and wearable electronics. The successful transition from gold (Au) to copper (Cu) in wire-bonding introduces corrosion related reliability concerns to wire-bonded devices in integrated circuit (IC) packaging. This report studies the effect of bimetallic contact, Cu wire vs. aluminum (Al) bond pad, on the corrosion failure of Al bond pads when exposed to low ppm levels of chloride (Cl−) contamination. Real time corrosion screening was carried out on simulating Cu/Al micro patterns for time-dependent observation of corrosion progression in 5–20 ppm Cl− solutions at pH 5. Al in Cu/Al bimetallic couple corroded at accelerated rate compared to Al without bimetallic contact. Cathodic hydrogen evolution was found to be the key factor driving this aggressive Al bond pad corrosion under the influence of the peripheral Cu/Al bimetallic contact in acidic chloride solution. Chemically modifying the surface of Cu wires to prevent this H2 evolution reaction resulted in the inhibition of Al bond pad corrosion.

Comparative Study of Chloride and Fluoride Induced Al Pad Corrosion in Wire Bonded Packaging Assembly

Microelectronics are used in virtually all electronic equipment’s nowadays ranging from medical instruments, automobiles, computers, cell phones and home appliances. Wirebonding process which connects the chip to the lead frame in IC packaging plays a very important role in the commercial production of IC’s. Chip usually consists of Al bond pads with Cu/Au wires. Thin film Al bond pads are easily vulnerable to corrosion in presence of contaminants and moisture. Bond pad corrosion is one of the common modes of corrosion failure in wire-bonded devices IC packaging. Improper rinsing, packaging and passivation defects allows moisture and contaminants into bondpads resulting in severe corrosion. Most common source of this corrosion comes from the presence of halide ions contamination as (Cl-, F-, Br- etc.) However, the corrosion morphology and the severity varies from different ions contamination as the chemistry governing the corrosion differs from one ion to another. Our research is focused on key findings on corrosion of Aluminum bond pad due to chloride (Cl-) and fluoride (F-) contamination. Our central motivation is to identify this F- corrosion mechanism, so that it will help to develop a strategy to prevent it and study the comparison between Cl- and F- Corrosion using Micro-pattern corrosion screening technique, Wire-bonded device (WBD) and other characterization technique such as GC-MS, SEM and tafel. GC-MS reveals that H2 gas evolution is observed during Cl- corrosion of the WBD, whereas little or no H2 evolution is observed during F- corrosion. SEM and EDX reveals no presence of Cl- in the corroded areas of its corrosion, whereas strong F- signal is observed in F- corrosion of the bondpads revealing, that the corrosion products formed on the surface has very poor solubility showing that Cl- and F- has a complete different corrosion morphology. Striking contrasts in corrosion morphology observed during the Cl- and F- corrosion were explained in the present work and will lead to better understanding of factors influencing the bond pad corrosion leading to the wire bond failure in IC packaging devices. [Fig. 1] Figure 1

Explore High Bonding Reliability of Cu Wire Bonded Devices under Extreme Halide Contaminated Environments

Abstract Copper has rapidly replaced gold as the preferred wire bonding material in microelectronic packaging due to its lower cost and many performance advantages. However, halides induced corrosion-related failures need to be carefully controlled to ensure maximum bonding reliability. Literature reported corrosion studies mostly focus on intermetallic compounds as the corrosion vulnerability. Utilizing a novel corrosion screening approach, we established that the bimetallic contact between Cu balls and Al bond pad is the real driver for the observed heavy Al pad corrosion induced by chloride ions [1]. We identified, for the first time, H2 evolution was the coupling cathodic half reaction to the Al bond pad corrosion. With these improved mechanistic insights, we developed an effective corrosion inhibition strategy that utilizes chemical vapor deposition to selectively bond corrosion inhibitors to these critical Cu ball/Al pad interfaces. A new corrosion-screening platform was developed to evaluate the effectiveness of this inhibitor protection treatment. After intentionally loaded chloride contaminants on the die surface, the molded Cu wire bonded devices with selected inhibitor treatment were subjected to PCT stress tests. The subsequent failure analyses, showed selected inhibitor treatment is highly effective of preventing Al bond pad corrosion even under the massive attack of heavy Cl− ion contamination and extensive delamination.

Deriving lifetime predictions for wire bonds at high temperatures

We report a method to predict wire bond lifetimes. Wire bond resistance increase due to thermal stress can be modelled by the Arrhenius equation, permitting prediction of wire bond reliability in a product prior to qualification if sufficient experimental data is collected and statistically analysed. Activation energies for various experiment “legs” are determined by stressing a number of wire bonds at 3 different temperatures, then, using survival analysis techniques for accelerated failure time models, extrapolating results to use conditions. Example lifetime predictions for Cu and PCC wire bonds on Al pads are shown to illustrate the method, using a small number of test dies that incorporate four-wire resistance measurements of wire bond pairs, with two ball bond sizes and two different wire types, stressed at precise temperatures with in-situ resistance monitoring in the University of Waterloo's “mini-oven” reliability test systems. These results show that typical Cu wire bonds on Al bond pads can be highly reliable up to 163 °C without encapsulation or 155 °C with encapsulation, with estimated lifetimes of 12,000 h with <1 ppm failure rate, assuming a failure condition of 10% resistance change.

Effects of Bond Pad Thickness on Shear Strength of Copper Wire Bonds

Abstract Copper (Cu) wire bonding is now widely accepted as a replacement for gold (Au), however, its use in high reliability applications is limited due to early failures in high temperature and humid conditions. The Au to Cu wire transition is mainly driven by cost savings though there are other advantages to Cu such as better electrical and thermal conductivity, slower intermetallic compound (IMC) formation and reduced wire sweep during transfer molding. Some automotive, industrial and aerospace industries are still reluctant to adopt Cu wire bonded products due to perceived risks of wire and bond pad cracks, the potential for corrosion, and some lack of understanding about its reliability in harsh conditions. A wire bond is considered good if destructive sampling qualification tests and periodic monitors pass for the batch. Tests include wire pull strength, wire bond shear, IMC coverage, and thickness of bond pad aluminum (Al) remaining beneath the bond. Nondestructive inspections also verify acceptable ball diameter and Al “splash”. This paper focuses on the bond shear test and its contribution to Cu ball bond reliability assessment, especially when changing Al bond pad thickness. A new revision of the JEDEC Wire Bond Shear Test Method, JESD22-B116B, has just been released, to include Cu wirebonds for the first time. It helps to clarify shear test failure modes for Cu ball bonds. However, there are still questions to be answered through research and experimentation, especially to learn the extent to which one may predict Cu ball bond reliability based on shear test results. Pad Al thickness is not considered in the current industry standards for shear test. Yet bond pad Al thickness varies widely among semiconductor products. This research is intended to contribute toward improved industry standards. In this study, power and time bonding parameters along with bond pad thickness are studied for bond strength. Several wire bonds are created at different conditions, evaluated by optical microscope and SEM, IMC% coverage and bond shear strength. Similar bonding conditions are repeated for bond pads of 4um, 1um and 0.5um thickness.

Effects of Probe Marks on Shear Test of Copper Ball Bonds in Two Pad Aluminum Thicknesses

Abstract This paper continues the work of reference [1], evaluating shear test results of Cu ball bonds over a variety of probe marks in two different pad aluminum (Al) thicknesses (0.8μm and 3μm). The presence of invasive probe marks on thick Al bond pads lowers certain shear force results.. Lower values of shear force imply reduced Cu bond reliability. Physical factors are investigated relating to poor intermetallic (IMC) formation in the Cu wirebond and bond shear force. Optical microscopic image analysis of Cu bonds, bond contact areas and Al “splash” are studied for correlation with the shear test results. Percent IMC coverage of bond contact areas decrease when invasive probe marks are present beneath the bond, which in turn may reduce the shear force. Probe mark features are studied to discover the characteristics of greatest influence on % IMC coverage and shear test values in each of the pad metal thicknesses.

Wedge Bond Process Optimization Method for 1.5 mil Al Wire on Al Bond Pad

Modern wedge bonders have evolved since their early inception in 1957. This paper will review the common challenges process engineers face when selecting a wedge bond machine configuration and developing robust processes. Wedge bond cases presented will show the tradeoff between process inputs and the resulting bond shapes, bond appearance of black ring, burrs, pull results, etc. The purpose of this work was to optimize the process outputs: bond shape, black ring, burrs, and pulls on a die with aluminum bond pads. Process inputs included Force, Time, and Ultrasonic Level. An aluminum wafer was used to understand the basic relationship between process parameter inputs and outputs. The learning was then applied to a die with aluminum bond pads. Examples of non-compliance and compliance will be shown to help process engineers evaluate wedge bonds and make refinements. The case studied was for an aluminum bond pad/Al wafer and 1.5 mil aluminum wire interaction that creates burrs around the bond (wire to pad interface), black ring on the bond periphery (wedge tool to wire interface) and the resulting pulls. Both the graphical and numerical results of the case study have clearly demonstrated the relationship between the typical process inputs and outputs, particularly bond shape, burrs, black ring and pulls. The findings in this study will provide a general guideline and a troubleshooting reference for wedge bonding process development.

Characterization and Modeling of Fine-Pitch Copper Ball Bonding on a Cu/Low-k Chip

Cu ball bonding faces more challenges than Au ball bonding, for example, excessive deformation of the bond pad and damage of Cu/low-k structures, because of the much greater hardness of Cu free air balls. In this study, dynamic finite-element analysis (FEA) modeling with displacement control was developed to simulate the ball-bonding process. The three-dimensional (3D) FEA simulation results were confirmed by use of stress-measurement data, obtained by use of stress sensors built into the test chip. Stress comparison between two-dimensional (2D) and 3D FEA models showed the 2D plain strain model to be a reasonable and effective model for simulation of the ball-bonding process without loss of accuracy; it also saves computing resources. The 2D FEA model developed was then used in studies of a Cu/low-k chip to find ways of reducing Al bond pad deformation and stresses of low-k structures. The variables studied included Al pad properties, capillary geometry, bond pad design (Al pad thickness, Al pad coated with Ni layer), and the effect of ultrasonic bonding power.

Micro-Structural Studies of Thermosonic Cu-Al Bonding Interface

Thermosonic Copper (Cu) wire interconnection has been under an extensive research and development to replace expensive Gold (Au) wire material in the semiconductor industry. However, a reliability concern is raised due to void formation at the bonding interface of Copper wire-Aluminum bond pad (Cu-Al) after High Temperature Storage (HTS) annealing condition. It is believed that the Intermetallic Compound (IMC) layer growth and evolution lead to a volumetric shrinkage which in turn results in the void formation. Annealing conditions influence the development of the IMC at the bonding interface which is related to the bonding reliability. In this work, the effects of annealing toward the micro-structure and IMC growth at the bonding interface were evaluated using Scanning Transmission Electron Microscope equipped with Energy Dispersive X-ray analysis. In the as-synthesized sample bonded at 100°C, an inhomogeneous IMC formation dominated by grain boundary diffusion was observed. After High Temperature Storage of 1000 hours, the consumption of the Al bond pad resulted in the formation of irregular IMC layers. The variation of phases existed in a localized region was believed due to simultaneous growth of IMC by both grain boundary and volume diffusions. Moreover, the diffusion of Cu into Si was observed. This resulted in the formation of the mixture of Si + η phases in the affected sea region.

Surface Roughness Influence on O&lt;sub&gt;2 &lt;/sub&gt;and Ar RIE Gas Treated Aluminum Surface

This study reports on the preliminary investigations on the effect of Reactive Ion Etch (RIE) parameters on the surface characteristics of Al bond pad. Investigation is done employing Design of Experiment (DOE) method. Quantity of Oxygen, Argon, ICP power and BIAS power were varied to get 16 sets of recipes. This provides 16 samples with different combination of RIE parameters. Surface characteristics of the samples were analyzed using Atomic Force Microscopy (AFM).Data collected were in terms of Surface Roughness (RA), Peak Vs Valley (P-V) and Surface Root-Mean-Square Roughness (RMS). Result shows that combination of these RIE parameters does not vastly affect the surface characteristics of the Al bond pad.

Microstructural and mechanical analysis of Cu and Au interconnect on various bond pads

Effects of High Temperature Storage (HTS) and bonding toward microstructure change of intermetallic compound (IMC) at the wire bonding interface of 3 types of bond pad (Al, AlSiCu and NiPdAu) were presented in this paper. Optical and electron microscope analyses revealed that the IMC growth rate of samples under 175 and 200 °C HTS increased in the order of Al > AlSiCu > NiPdAu. Besides, higher HTS and bonding temperatures also promoted higher IMC thickness. The compositional study showed that higher HTS and bonding temperature developed rapid interdiffusion in bonding interface. In the mechanical ball shear test, a decrease of the shear force of Al and AlSiCu bond pads after 500 h HTS was believed due to poorly developed IMC at bonding interface. On the other hand, shear force degradation at 1000 h was due to excessive growth of IMC that in turn causes the formation of defects. For NiPdAu bond pad, increasing trend of shear force with HTS duration at 175 °C implied a good reliability of the Cu wire bonding. The rapid microscopic inspection on Cu wired Al bond pad under HTS 175 °C showed the IMC development from the periphery to the center of the ball bond. However, after 500 h voids started to develop until the crack was observed at 1000 h.

Further characterization of Pd-coated copper wire on wirebonding process: from a nanoscale perspective

The objective of this research is to investigate the nanomechanical properties of ultra-thin Pd-coated copper (Cu) wire (ψ = 0.6 mil) and nanotribology along the interfacial between free air ball (FAB) and aluminum (Al) bond pad during wirebonding process. Two major analyses are conducted in the present paper. In the first, the characteristic of heat affected zone and FAB for Cu wire has been carefully experimental measured. Nanoscale interfacial tribology behavior between Cu FAB and Al pad is examined by atomic force microscopy. Secondary, the dynamic response on Al bond pad and beneath the pad during wirebonding process has been successfully predicted by finite element analysis. Micro-tensile mechanical properties of Cu wire before and after electric flame-off (EFO) process have been investigated by self-design pull test fixture. Experimental obtained hardening constant in Hall–Petch equation has significantly influence on the localize stressed area on Al pad. This would result in Al pad squeezing (large plastic deformation) around the smashed FAB during impact stage and the consequent thermosonic vibration stage. Microstructure of FAB is also carefully investigated by nanoindentation instruments. A real-time secondary EFO scheme has been conducted to reduce the strength of Cu wire and increase the bondability. All the measured data serve as material inputs for the finite element model based on explicit software ANSYS/LS-DYNA. In addition, nanoscale bondability on Cu-Al intermetallic compound is simulated by molecular dynamics. A series of comprehensive parametric studies were conducted in this research.

Aluminum–germanium wafer bonding of (AlGaIn)N thin-film light-emitting diodes

Eutectic aluminum–germanium wafer bonding was used to fabricate (AlGaIn)N thin-film light-emitting diodes (LEDs). Wafer bonding was carried out on 2″ wafer level at a bond temperature of 470 °C using patterned Al bond pads on the GaN-on-sapphire LED epiwafer and plain Ge substrates. The microstructure of the joint formation was characterized via cross-section analysis using scanning electron microscopy and energy dispersive X-ray spectroscopy (EDX). Scanning acoustic microscopy was used to investigate the bond interface. The shear strength was determined to be 1–2 kN/cm2. The formation of a liquid Al–Ge phase is evident from cross-section analysis and optical microscopy. During solidification, Al and Ge are separated into distinct phases again, which is revealed by EDX. The obtained bond is not free of micro-voids, yet it is mechanically stable and suited for the fabrication of thin-film LEDs by removing the sapphire substrate via laser lift-off, which is also demonstrated.

Process integration of fine pitch Cu redistribution wiring and SnCu micro-bumping for power efficient LSI devices with high-bandwidth stacked DRAM

This paper deals with the process integration to produce a high performance processor with a large-scale and high-bandwidth DRAM chip stacked. Ten micrometres pitch Cu redistribution wiring has been implemented on 12 in. DRAM wafers to relocate the Al bond pads where [email protected] pitch SnCu bumps are formed for a large number of I/Os of the memory interface. A phenol-melamine based resin film which is curable at less than 250^oC can be used so as to avert detraction from memory retention yield as well as to insulate the Cu lines, leading to reduction of wafer warpage in an effort to enable fine pitch lithography. In a chip-on-chip joining process, the logic chip is interconnected with the DRAM chip with a less than [email protected] accuracy by mass reflow bonding of the SnCu micro-bumps on both of the chips, revealing the self-aligning effect of the solder bumps. No failures have been observed after reliability stressing on the packaged two-chip stack. The present process integration has been qualified for the mass producing line to provide some merchant LSI devices and will be viable to future generation devices with more chips stacked using through Si via technology.

An Investigation on Mechanical Behavior on Pt-Coated Copper Wire from a Nanoscale Perspective

The aim of present research is to investigate the nanomechanical properties of ultra-thin copper wire (o̸=0.6mil) and nanotribology along the interfacial between free air ball (FAB) and aluminum (Al) bond pad. Micro-tensile mechanical properties were conducted through wire pull test. Microstructure of Cu FAB has been carefully examined and nanoindentation was applied to obtained thin surface elastic modulus on FAB. Nanotribology and interfacial frictional behavior along smashed FAB and bond pad were measured by Atomic Force Microscopy (AFM). The interfacial coefficient of frictional force can be derived from a serial of calculations.