Bond Pad Surface Research Articles

Wire Bonding: The Ultrasonic Bonding Mechanism

Abstract Wire bonding is a welding process. During both ball and wedge bonding, wire and bond pad are massively deformed between the bond tool and the anvil of the bond pad or substrate. The dominant variables affecting deformation are ultrasonic energy, temperature, bond force and bond time. Deformation exposes new surface material that is clean and has not been exposed to atmospheric contamination and oxidation. As the new wire and bond pad surfaces mix, they form diffusion couples that grow and transform into the intermetallic weld nugget. The initial mixing is not at equilibrium in that it does not initially form the compounds described by the equilibrium phase diagram, but temperature and time very quickly allows diffusion to relax the initial mixture into the equilibrium phase diagram compounds. This paper will discuss the mechanisms behind the formation of ball and wedge bonds.

Environmentally Hardening IC Die Previously Assembled in Plastic Packages through Die Removal, Bond Pad Replating and Reassembly into Hermetic Packages

ABSTRACT Many integrated circuits (ICs) will operate well above their maximum rated temperature of +70°C or +125°C, but are often not packaged appropriately to reliably endure temperatures above +150C. Specifically, the original gold or copper bonds on the aluminum die bond pads are prone to Kirkendall or Horsting voiding, particularly at temperatures greater than +150°C. Also the mold compounds used in plastic packaging for IC assembly can degrade at these elevated temperatures. In some cases, commercial demand for higher temperature reliability can justify a separate offering of ICs assembled in hermetic, ceramic packages from the original component manufacturer (OCM). However, in most cases, the market demand is deemed insufficient. Global Circuit Innovations (GCI) has developed a high-yielding process, which can remove a semiconductor die (i.e., computer chip) from a plastic package, remove the original bond wires and/or ball bonds, plate the aluminum die bond pads with Electroless Nickel, Electroless Palladium, and Immersion Gold (ENEPIG), and then reassemble the now improved semiconductor die into a hermetic, ceramic package. Device Extraction, ENEPIG die bond pad plating and Repackaging (DEER) provides an improved die bond pad surface such that works well with either gold or aluminum bond wires in applications up to +250°C without mechanical or electrical connectivity degradation. GCI routinely exposes sample devices to +250°C bakes with 100% post bake yields so as to continuously ensure that any device processed with the DEER technology will reliably perform in high-temperature environments. Although the oil and gas industry has already expressed significant interest in the DEER process, with excellent lifetest and production application results demonstrating dramatically increased component lifetimes at elevated temperatures, this technology can also be leveraged for any application exposing ICs to harsh environments. Not only is the high-temperature reliability dramatically increased, but also the new hermetic, ceramic package protects the IC from a variety of elements and environments (i.e., corrosives and moisture).

BGA substrate outgassing negative impact study on Cu wire bonding

PurposeThe purpose of this paper is to attempt to study the failure mechanism of BGA (ball grid array) Cu wire bond ball lift and specifically focused on substrate outgassing’s impact on Cu wire bonding quality and reliability.Design/methodology/approachThe Galvanic corrosion theory has been widely adopted in explaining the failure mechanism of Cu ball bond lift issue during reliability test or field application in the presence of moisture. In this study, ion chromatography was performed on BGA substrate halogen analysis. EDX (energy-dispersive X-ray spectroscopy) was also used to detect the contaminant’s element at the bottom surface of a window clamp. Further FTIR (Fourier transform infrared spectroscopy) analysis verified that the contamination is from substrate outgassing during wire bonding. A new window clamp design proved effective in reducing the negative impact from substrate outgassing during wire bonding.FindingsThe solder mask in a fresh substrate contains a chlorine element. The chlorine can be detected in the BGA substrate outgassing during wire bonding by FTIR and EDX analyses, which have a negative impact on the Cu wire bonding. The window clamp with a larger opening can reduce the negative impact of the Cu wire bonding from the BGA substrate outgassing.Research limitations/implicationsBecause of the limitation of time and resources, bonding pad surface contamination from substrate outgassing and its correlation with Cu bonding ball lift failure after reliability test will be studied in depth later.Originality/valueThe BGA substrate outgassing has negative impacts on Cu wire bondability. A window clamp with a larger opening can reduce the negative impact from substrate outgassing.

Effects of ball bond diameter on wire bond reliability for automotive application

Purpose – The purpose of this paper is to investigate the reliability of wire bonds with three varying ball bond diameters, which are ball bonded with three different sizes of gold wires in light-emitting diode (LED) package under high-temperature environment. In automotive applications, “lifted ball bond” issue is a potential critical point for LED device reliability, as the wire bonds are usually stressed under high operating temperature during their lifetime. Moreover, the reliability of wire bonds in recent LED production has fallen under scrutiny due to the practice of reducing wire diameters to cut down production costs. Design/methodology/approach – Three gold wires with sizes of 2, 1.5 and 1 mm were ball bonded on the LED chip bond pad via thermosonic wire bonding method to produce three different ball bond diameters, that is, 140, 120 and 100 μm, respectively. The reliability of these wire bond samples was then studied by performing isothermal aging at 200°C for the time interval of 30, 100 and 500 hours. To validate hypotheses based on the experimental data, COMSOL Multiphysics simulation was also applied to study the thermal stress distribution of wire bond under an elevated temperature. Findings – Experimental results show that the interfacial adhesion of wire bond degrades significantly after aging at 200°C for 500 hours, and the rate of interfacial degradation was found to be more rapid in the wire bond with smaller ball bond diameter. Experimental results also show that ball bonds randomly elongate along an axis and deforms into elliptical shapes after isothermal aging, and ball bonds with smaller diameters develop more obvious elongations. This observation has not been reported in any previous studies. Simulation results show that higher thermal stress is induced in the wire bond with the decrease of ball bond diameter. Practical implications – The reliability study of this paper provides measurements and explanation on the effects of wire diameter downsizing in wire bonds for automotive application. This is applicable as a reliability reference for industries who intend to reduce their production costs. Other than that, the analysis method of thermal stresses using COMSOL Multiphysics simulations can be extended by other COMSOL Multiphysics users in the future. Originality/value – To resolve “lifted ball bond” issue, optimization of the bond pad surface quality and the wire bond parameter has been studied and reported in many studies, but the influence of ball bond diameter on wire bond reliability is rarely focused. Moreover, the observation of ball bonds randomly elongate and deform more into elliptical shape, and ball bond with smaller diameter has the highest elongation after isothermal aging also still has not been reported in any previous studies.

Investigation of oxygen followed by argon plasma treatment on LED chip bond pad for wire bond application

Purpose – The purpose of this paper is to investigate the efficiencies of argon (Ar), oxygen (O2) and O2 followed by Ar (O2→Ar) plasma treatments in terms of contaminant removal and wire bond interfacial adhesion improvement. The aim of this study is to resolve the “lifted ball bond” issue, which is one of the critical reliability checkpoints for light emitting diodes (LEDs) in automotive applications. Design/methodology/approach – Ar, O2 and O2→Ar plasma treatments were applied to LED chip bond pad prior to wire bonding process with different treatment durations. Various surface characterization methods and contact angle measurement were then used to characterize the surface properties of these chip bond pads. To validate the improvements of Ar, O2 and O2→Ar plasma treatments to the wire bond interfacial adhesion, the chip bond pads were wire bonded and examined with a ball shear test. Moreover, the contact resistance of the wire bond interfaces was also measured by using four-point probe electrical measurements to complement the interfacial adhesion validation. Findings – Surface characterization results show that O2→Ar plasma treatment was able to remove the contaminant while maintaining relatively low oxygen impurity content on the bond pad surface after the treatment and was more effective as compared with the O2 and Ar plasma treatments. However, O2→Ar plasma treatment also simultaneously reduced high-polarity bonds on the chip bond pad, leading to a lower surface free energy than that with the O2 plasma treatment. Ball shear test and contact resistance results showed that wire bond interfacial adhesion improvement after the O2→Ar plasma treatment is lower than that with the O2 plasma treatment, although it has the highest efficiency in surface contaminant removal. Originality/value – To resolve “lifted ball bond” issue, optimization of plasma gas composition ratios and parameters for respective Ar and O2 plasma treatments has been widely reported in many literatures; however, the O2→Ar plasma treatment is still rarely focused. Moreover, the observation that wire bond interfacial adhesion improvement after O2→Ar plasma treatment is lower than that with the O2 plasma treatment although it has the highest efficiency in surface contaminant removal also has not been reported on similar studies elsewhere.

Characterization of copper precipitates on aluminum copper bond pads formed after plasma clean and de-ionized water exposure

Semiconductor bond pads made from aluminum and small percentages of copper is susceptible to galvanic corrosion. In galvanic corrosion, the cathode (copper precipitate) is usually protected by the aluminum oxide that covers the surface of aluminum which acts as the anode. However, when the aluminum oxide thickness is reduced by plasma cleaning, the precipitates can be exposed. When exposed precipitates come in contact with de-ionized water, galvanic corrosion takes place. Therefore, though plasma cleaning in general is supposed to improve semiconductor bond pad surface in preparation for package level interconnection, adding the plasma clean step just before a process with de-ionized water can cause bond pad corrosion through the galvanic reaction between the exposed precipitate (cathode) and the surrounding aluminum (anode). This paper aims to investigate the mechanism of corrosion and characterize corroded bond pads by using wire bond ball shear method.

Comparison of Radio Frequency and Microwave Plasma Treatments on LED Chip Bond Pad for Wire Bond Application

The efficiencies of argon radio frequency [Ar(RF)] and argon microwave [Ar(MW)] plasma treatments were compared in terms of contaminant removal and wire bond interfacial adhesion quality in this paper. The efficiency in contaminant removal was analyzed by applying Ar(RF) and Ar(MW) plasma treatments with operating frequencies of 13.56 MHz and 2.45 GHz, respectively, onto the light-emitting diode chip bond pad prior to the wire bonding process. Surface characterization results show that Ar(MW) plasma treatment is able to remove the bond pad surface contaminant and improve the bond pad surface roughness more effectively than Ar(RF) plasma treatment. Moreover, contact angle measurement results show that bond pad samples after Ar(MW) plasma treatment have higher surface free energy compared with Ar(RF) plasma treatment. To study the improvement of wire bond interfacial adhesion after Ar(RF) and Ar(MW) plasma treatments, the bond pad samples were wire-bonded via the thermosonic wire bonding method and examined with the ball shear test. The ball shear test results show that Ar(MW) plasma treatment leads to a better improvement of wire bond interfacial adhesion compared with Ar(RF) plasma treatment.

CO2 SPRAY CLEANING AND OSEE NON-CONTACT INSPECTION FOR WIRE BOND PAD PREPARATION

Surface pad contamination is a major cause of poor performance for wire bonding operations. Examples of the wide range of contaminants that can degrade wire bond pull strength include, for example:Halogens and hydrocarbons: plasma etching, epoxy outgassing (dry processing), photoresist strippers, cleaning solvents.Contaminants from plating operations: thallium, brighteners, lead, iron, chromium, copper, nickel, hydrogen.Sulfur compounds: packing containers, ambient air, cardboard and paper, rubber bands.Miscellaneous organic contaminants: epoxy outgassing, photoresist, general ambient air (poor storage).Miscellaneous inorganic compounds: sodium, chromium, phosphorous, bismuth, cadmium, moisture, glass, vapor, nitride, carbon, silver, copper, tin.Human sources of contamination: skin particles, hair, sweat, spittle, mucus, cosmetics, hand lotions, facial make-up and fibers from clothing. As can be seen, there are many types of surface contaminations that may challenge a wire bonding operation, all of which must be removed to insure reliable and strong bonds. In this regard, conventional precision cleaning processes for high reliability surface pad preparation typically involve multiple steps, chemistries, and equipment to accomplish complete decontamination. Moreover, conventional cleaning methods are sometimes non-selective for the surface contaminants and substrates. For example, conventional vacuum plasma using Ar/O2 is typically used to clean bond pads. Vacuum plasmas are usually performed off-line, taking up to 30 minutes to complete, and are non-selective for the organic contamination. The entire organic substrate (i.e., PCB) is etched away during the plasma cleaning process to remove the bond pad contamination. During treatment, secondary organic surface contaminations (plasma treatment by-products from reacted substrate) are produced which can re-contaminate bonding surfaces. Advanced carbon dioxide (CO2) spray cleaning technology provides various methods for consistently preparing bond pads for critical wire bonding operations. A patented hybrid CO2 particle-plasma spray is presented in this paper that has demonstrated efficacy for selectively treating bond pad surfaces to remove a wide range of challenging surface contaminations. Moreover, a novel non-contact surface inspection technology called Optically Stimulated Electron Emission (OSEE) - developed to address surface cleaning and inspection issues that led to the 1986 Challenger Spacecraft explosion - is used to measure the effectiveness of the new CO2 surface cleaning processes. Statistically significant studies have been performed to determine the effectiveness of the selective CO2 particle-plasma surface treatment process for preparing bond pads for gold ribbon bonding operations. One such study compared and contrasted the performance of this new single-step CO2 surface treatment method with that of a conventional multi-step solvent-plasma method. The two treatment methods were used to prepare the surface of a metalized ceramic wafer that simulated bond pad surfaces and treatment areas representative of an actual high-reliability electronic board. The test results of this evaluation demonstrated that the CO2 particle-plasma surface treatment process is statistically similar to or sometimes better than a solvent-plasma hybrid cleaning process. CO2 spray cleaning was determined to be better for some types of contaminants as well – and in particular more relevant mixed-contaminant challenge tests. The CO2 cleaning process demonstrates a lower defect-per-million (DPM) level and an improved CpK. Finally, in this study OSEE surface quality analysis was performed before and after surface cleaning. OSEE analysis provided a reliable non-contact means of determining the proper level of surface pad preparation.

Non-Destructive Top Layer Bond Pad Cross Section

Contaminated and corroded aluminum bond pads can lead to a number of reliability failures. Poor wirebondability, false failures at electrical probe due to high contact resistance, opens on RDL layers due to incomplete sputter, and various early life failures during JEDEC testing can all be attributed to excess oxidation and corrosion on aluminum bond pads. Understanding what layers exist on the bond pad surface is critical. Oxides, hydrates, xyfluorides and various soups of materials can be confirmed from combinations of Auger, SIMMS and FTIR but there is a chance the die can be damaged and the spot sizes can be problematic picking up information outside the bond pad target. Layer analysis of the pad can also be difficult since sputtering rates vary based on the contaminants on the pad and any thermal processing. How does one evaluate bond pads without a destructive test? Is there a process for accurate bond pad evaluation while minimizing die damage? Sure. The cycle time and costs of these tests can also be a concern particularly for a customer wanting a quick response from failure analysis. A better method is to bond the die with a gold stud bump followed by selectively etching away the gold. Etching the gold highlights the intermetallics and shows how much of the gold alloyed with aluminum versus blocked oxides. Voiding can be demonstrated, that might be misconstrued as Kirkendall voiding, by cross sectional analysis.

Bond pad surface quality for reliable wire bonding

The impact of fluoride and AlO x contaminants on the quality of wire bonding (Au to Al welding) has been measured. No or minor impact of these contaminants has been found with a standard ball shear test. Despite an accelerated lifetime test, the shear strength remains above the required specification limits. Intermetallic coverage, the percentage of the interface between the bond ball and the bond pad where welding resulted in intermetallic diffusion, gives a better picture of the quality decay, but this method is too elaborate for production control and specification limits are unclear.

Titanium layers on aluminum bond pads: Characterisation of thin layers on rough substrates

AbstractA big challenge in the semiconductor industry is to control surface contamination, especially on bond pads, since they might take influence on bond wire adhesion and reliability behavior. The impact of fluorine and organic contamination on wire bonding is already well documented.1 Nevertheless, the influence of metal contamination has not yet been investigated in detail.As titanium contamination has been found on bond pad surfaces, owing to some manufacturing process steps, this presentation concerns preliminary investigations on titanium‐contaminated aluminum structures.Titanium layers of different thicknesses in the nanometer range have been deposited by means of physical vapor deposition, either directly after aluminum deposition, or after an air break, in order to study their influence on wire bond quality in a systematic manner.Depth profile measurements have been carried out by means of TOF‐SIMS. In order to study the oxide and the titanium distribution, those measurements have been performed in positive and negative modes.To overcome the big challenge to distinguish between the thicknesses of thin layers in the nanometer scale, depth profiling parameters were optimized regarding signal intensities.This work deals with our analytical results and shows the possibility to clearly identify nanoscale layers and interdiffusion areas between different materials by enhanced depth resolution. In the near future, we will quantitatively investigate the impact of different titanium contaminations on bond pads on the adherence of Al‐wedge bonds. Copyright © 2010 John Wiley & Sons, Ltd.

Impact of Thermal Cycling on Sn-Ag-Cu Solder Joints and Board-Level Drop Reliability

The electronic packaging industry uses electroless nickel immersion gold (ENIG) or Cu-organic solderability preservative (Cu-OSP) as a bonding pad surface finish for solder joints. In portable electronic products, drop impact tests induce solder joint failures via the interfacial intermetallic, which is a serious reliability concern. The intermetallic compound (IMC) is subjected to thermal cycling, which negatively affects the drop impact reliability. In this work, the reliability of lead-free Sn-3.0Ag-0.5Cu (SAC) soldered fine-pitch ball grid array assemblies were investigated after being subjected to a combination of thermal cycling followed by board level drop tests. Drop impact tests conducted before and after thermal aging cycles (500, 1000, and 1500 thermal cycles) show a transition of failure modes and a significant reduction in drop durability for both SAC/ENIG and SAC/Cu-OSP soldered assemblies. Without thermal cycling aging, the boards with the Cu-OSP surface finish exhibit better drop impact reliability than those with ENIG. However, the reverse is true if thermal cycle (TC) aging is performed. For SAC/Cu-OSP soldered assemblies, a large number of Kirkendall voids were observed at the interface between the intermetallic and Cu pad after thermal cycling aging. The void formation resulted in weak bonding between the solder and Cu, leading to brittle interface fracture in the drop impact test, which resulted in significantly lower drop test lifetimes. For SAC/ENIG soldered assemblies, the consumption of Ni in the formation of NiCuSn intermetallics induced vertical voids in the Ni(P) layer.

The Characterization of Al Bond Pad Surface Treatment in Electroless Nickel Immersion Gold (ENIG) Deposition

This study reports a number of experiments that were designed to characterize aluminum bond pad surfaces prior to electroless nickel immersion gold (ENIG). In the ENIG process, aluminum bond pads need special treatment to achieve successful nickel deposition and provide reliable interconnection of under bump metallurgy in advanced packaging. During this treatment process, the aluminum pad was cleaned, activated and then coated with a layer of zinc. Systematic study was carried out to determine the best parameters, through multiple and various exposure times of the zincation process and zincation solution concentration effect on the Ni/Au surface roughness and aluminum dissolution rate on the bond pad during multiple zincation process. The ball shear strength was evaluated between eutectic 37Pb/63Sn solder ball and under bump metallurgy (UBM) interfaces across multiple zincation process. Scanning Electron Microscope (SEM), Energy Dispersive X-Ray (EDX), Atomic Force Microscopy (AFM), Focused Ion Beam (FIB) and ball shear tester were used as analytical tools. The results suggest that the multiple zincation process consistently produces a smoother surface of ENIG UBM and consequently provides a better shear strength.

Fundamentals of thermo-sonic copper wire bonding in microelectronics packaging

Fine copper wire bonding is capable of making reliable electrical interconnections in microelectronic packages. Copper wires of 0.8–6 mil diameter have been successfully bonded to different bond pad metallized and plated substrate materials such as Al, Cu, Ag, Au and Pd. The three metallurgical related factors; solid-solubility and diffusion of dissimilar contact metals, oxide film breakage and plastic deformation of asperities play a critical role in the bonding. Plastic deformation of an asperity is the most significant factor one has to consider to attain good bonding. Soft aluminum metal (30–40 VHN), with a lower % asperity threshold deformation is easier to wire bond than harder metallic surfaces (Ni, W, Mo, Cr, Co, Ta) of 150–500 VHN. Good adhesion of wire bonding is achieved for the surface roughness (Ra) of 0.01–0.15 μm and 0.02–0.6 μm of bare and plated surfaces respectively. It is rationalized that the application of ultrasonic energy principally breaks the oxide film and deform the asperities, while a compressive force increases the proximity of asperities. Hence wire welds to bond pad surface by molecular attraction and inter diffusion. Storage of copper ball bonds at 175 °C for 100–1,000 h forms copper aluminide at the interface. EDAX and Auger analysis reveal 22 at% Al + 78 at% Cu composition of the aluminides and Cu3Al2 empirical formula is calculated, which, does not match with any of the reported copper aluminides. Hardness of the copper ball bonds and stitch bonds are higher than wire exhibiting work hardening of the bonds on processing.

Comparative performance of gold wire bonding on rigid and flexible substrates

This paper reports comparative performance of wire bondability of electrolytically plated Au/Ni/Cu bond pads on rigid FR-4 and bismaleimide trazine (BT) PCBs, as well as flexible polyimide (PI) substrate. The metallization surfaces were treated with plasma to study the effect of bond pad surface cleanliness on wire bondability. Process windows were constructed as a function of bonding temperature and bond power for the individual substrate materials. Significant improvements of wire pull strength and process window were noted after plasma treatment with a substantial reduction in minimum bonding temperature from 120°C to 60°C for both the rigid and flexible substrates. The minimum bond power required to produce successful bonds decreased with increasing bonding temperature. At a bonding temperature of 120°C, the process window for the flexible substrate was wider than the rigid substrates. The wire bondability and wire pull strength of rigid substrates decreased with increasing bonding temperature above 120°C due to softening of the substrate which adversely affected the effective bond force and the transmission of ultrasonic energy. In contrast, the wirebonding performance of the flexible substrate remained stable at 120°C or above because the thermo-mechanical properties of flexible PI substrate were rather insensitive to temperature. The process windows of flexible substrates with and without stiffener showed similar bondability.

The Effects of Multiple Zincation Process on Aluminum Bond Pad Surface for Electroless Nickel Immersion Gold Deposition

This paper reports the effects of a multiple zincation process on the Al bond pad surface prior to electroless nickel immersion gold deposition. The study of multiple zincation comprises the surface topography and morphology of the appearance of the Al bond pad. In addition, by comprehension of the effects of the multiple zincation process, the study includes investigating the Al dissolution rate and adhesion strength between eutectic a 37Pb∕63Sn solder ball and an under bump metallurgy (UBM) interface. Scanning electron microscopy, energy dispersive x-ray, atomic force microscopy, focused ion beam, and an Intellectest STORM series FA1500 shear tester were used as analytical tools in this study. The results suggest that the first zincation process follows the contour of the initial bond pad. The second zincation produces a slightly better surface appearance with a smooth and fine Zn crystallite. The Zn crystallites become a continuous film with the deposits looking like an island formation after the third zincation. The smooth surface of the third zincation, as an effect of multiple zincation, is later transferred to Ni and Au surfaces. The smooth surface of the UBM leads to a better shear strength with only a minimum Al dissolved.

Effect of plasma treatment of Au-Ni-Cu bond pads on process windows of Au wire bonding

The wire bondability of Au-Ni-Cu bond pads with different Au plating schemes, including electrolytic and immersion plates, are evaluated after plasma treatment. The plasma cleaning conditions, such as cleaning power and time, are optimized based on the process window and wire pull strength measurements for different bond pad temperatures. Difference in the efficiency of plasma treatment in improving the wire bondability for different Au plates is identified. The plasma-cleaned bond pads are exposed to air to evaluate the recontamination process and the corresponding degradation of wire pull strength. The changes in bond pad surface characteristics, such as surface free energy and polar functionality, with exposure time are correlated to the wire pull strength, which in turn provides practical information about the shelf life of wire bonding after plasma cleaning.

Process windows for low-temperature Au wire bonding

The process windows are presented for low-temperature Au wire bonding on Au/Ni/Cu bond pads of varying Au-layer thicknesses metallized on an organic FR-4 printed circuit board (PCB). Three different plating techniques were used to deposit the Au layers: electrolytic plating, immersion plating, and immersion plating followed by electrolytic plating. Wide ranges of wire bond force, bond power, and bond-pad temperature were used to identify the combination of these processing parameters that can produce good wire bonds, allowing the construction of process windows. The criterion for successful bonds is no peel off for all 20 wires tested. The wire pull strengths and wire deformation ratios are measured to evaluate the bond quality after a successful wire bond. Elemental and surface characterization techniques were used to evaluate the bond-pad surfaces and are correlated to wire bondability and wire pull strength. Based on the process windows along with the pull strength data, the bond-pad metallization and bonding conditions can be further optimized for improved wire bondability and product yields. The wire bondability of the electrolytic bond pad increased with Au-layer thickness. The bond pad with an Au-layer thickness of 0.7 μm displayed the highest bondability for all bonding conditions used. The bondability of immersion bond pads was comparable to electrolytic bond pads with a similar Au thickness. Although a high temperature was beneficial to wire bondability with a wide process window, it did not improve the bond quality as measured by wire pull strength.

Investigation of defect on copper bond pad surface in copper/low k process integration

In this work, inspection tools and surface analysis instruments were used to inspect and to analyze the defects at copper bond pads fabricated with copper/low k dual damascene deep submicron interconnect process integration. The defects at level are believed to be responsible for metal peeling at the Ta + Al and copper interface observed during chip wire bonding operation. The analysis results of the trace defects’ chemical composition show that the trace defects are the remainder of dielectric materials of passivation layer that is deposited on the top of the chip for protection. Copper oxide is also found to be present at the copper bond pads surface. A clear copper bond pad surface could be obtained using optimized dielectric pad window opening plasma etching conditions with suitable level plasma etching power and some overetch, improved photoresist stripping with oxygen and wet clean recipe with some chemicals. A clear copper bond pad surface will contribute to obtainment higher adhesion and lower contact resistance at Ta + Al and copper pad interface.

Direct gold and copper wires bonding on copper

The key to bonding to copper die is to ensure bond pad cleanliness and minimum oxidation during wire bonding process. This has been achieved by applying a organic coating layer to protect the copper bond pad from oxidation. During the wire bonding process, the organic coating layer is removed and a metal to metal weld is formed. This organic layer is a self-assembled monolayer. Both gold and copper wires have been wire-bonded successfully to the copper die even without prior plasma cleaning. The ball diameter for both wires are 60 μm on a 100 μm fine pitch bond pad. The effectiveness of the protection of the organic coating layer starts from the wafer dicing process up to the wire bonding process and is able to protect the bond pad for an extended period after the first round of wire bond process. In this study, oxidization of copper bond pad at different packaging processing stages, dicing and die attach curing, have been explored. The ball shear strength for both gold and copper ball bonds achieved are 5 and 6 g/mil 2 respectively. When subjected to high temperature storage test at 150 °C, the ball bonds formed by both gold and copper wire bond on the organic coated copper bondpad are thermally stable in ball shear strength up to a period of 1440 h. The encapsulated daisy chain test vehicle with both gold and copper wires bonding have passed 1000 cycles of thermal cycling test (−65 to 150 °C). It has been demonstrated that orientation imaging microscopy technique is able to detect early levels of oxidation on the copper bond pad. This is extremely important in characterization of the bondability of the copper bond pad surface.