Interfacial evolution and bond reliability in thermosonic Pd coated Cu wire bonding on aluminum metallization: Effect of palladium distribution

In this paper, the growth kinetics of Cu–Al intermetallic compounds formed during isothermal annealing of Pd–Cu wire bonds with different palladium distribution at 175 °C are investigated by electron microscopy and compared to bare Cu wire bonds. Transmission electron microscopy (TEM) was used to provide high resolution imaging of the Cu–Al IMCs in the as-bonded state and TEM-EDX used to analyze the concentrations of Pd at the bond interface in the as-bonded state. Cu–Al IMCs were found to grow thicker with increasing annealing duration. The growth kinetics of the Cu–Al IMCs were correlated with the diffusion process during thermal annealing. The IMC thickness for Pd–Cu wire bonds with Pd at the bond interface was found to be thinner as compared to that for Pd–Cu wire bonds with no Pd at the bond interface. Thus, the presence of palladium at the bond interface has slowed down the IMC growth. Nano-voids were found in the Pd–Cu wire bonds with Pd at the bond interface, but not in the Pd–Cu wire bonds with no Pd at the bond interface. The IMC growth rate for the Pd–Cu bonds with no Pd was found to be close to that for bare Cu for the initial annealing durations. Corresponding bond pull testing showed that Pd–Cu wire bonds containing Pd have best preserved the bond strength after 168 h aging at 175 °C due to the beneficial presence of Pd.     

Interface microstructure effects in Au thermosonic ball bonding contacts by high reliability wire materials

Palladium-doped and (Cu, Pt)-doped high reliability gold wires were used to form wire bond interconnects on aluminum IC metallization. By isothermal annealing of wire bond samples the formation of intermetallic Au–Al phases was stimulated. SEM/EBSD investigations of the phase regions exhibited significantly slower isothermal growth rates compared to a reference gold wire. Correlated TEM, STEM–EDXS and nanobeam diffraction analyses revealed that Pd is preferentially incorporated into the Au₈Al₃ intermetallic forming a new stable phase but additionally can obviously form a new Pd-rich ternary intermetallic. In comparison, Cu dopants are also accumulated into a new Al–Au–Cu phase while Pt is rather found agglomerating within grain boundaries and interfaces. These results suggest a diffusion barrier model that allows discussing how wire doping can affect the bond contact microstructure, thus increasing the lifetime of bond contacts.     

Effect of gas type and flow rate on Cu free air ball formation in thermosonic wire bonding

The development of novel Cu wires for thermosonic wire bonding is time consuming and the effects of shielding gas on the electrical flame off (EFO) process is not fully understood. An online method is used in this study for characterizing Cu free air balls (FABs) formed with different shielding gas types and flow rates. The ball heights before (H_FAB) and after deformation (H_def) are responses of the online method and measured as functions of gas flow rate. Sudden changes in the slopes of these functions, a non-parallelity of the two functions, and a large standard deviation of the H_FAB measurements all identify FAB defects. Using scanning electron microscope (SEM) images in parallel with the online measurements golf-club shaped and pointed shaped FABs are found and the conditions at which they occur are identified. In general FAB defects are thought to be caused by changes in surface tension of the molten metal during EFO due to inhomogeneous cooling or oxidation. It is found that the convective cooling effect of the shielding gas increases with flow rate up to 0.65 l/min where the bulk temperature of a thermocouple at the EFO site decreases by 19 °C. Flow rates above 0.7 l/min yield an undesirable EFO process due to an increase in oxidation which can be explained by a change in flow from laminar to turbulent. The addition of H₂ to the shielding gas reduces the oxidation of the FAB as well as providing additional thermal energy during EFO. Different Cu wire materials yield different results where some perform better than others.     

Theoretical and numerical analysis of the effect of constant velocity on thermosonic bond strength

A new effective method is presented in this study to investigate the effect of constant velocity on the thermosonic bond strength. The proper relationship between the bond strength and results of FE analysis was introduced with some related theoretic equations in solid state welding. A transient non-linear dynamic finite element framework was developed, and the strain rate sensitivity of the gold ball and pad was considered. The effective normal pressure and bond surface exposure were all calculated, thus the bond strength was estimated approximately. It can be found that the bond strength increases with the increase of constant velocity, and then decreases after it exhibits the maximum. In addition, the strength distribution in bond interface was also studied. At last, the application range of the present method was discussed.     

Microstructural study of copper free air balls in thermosonic wire bonding

Copper wires are increasingly used in place of gold wires for making bonded interconnections in microelectronics. In this paper, a microstructural study is reported of cross-sectioned free air balls (FABs) made with 23 μm diameter copper bonding wire. It was found that the FAB is comprised of a few columnar grains and a large number of fine subgrains formed within the columnar grains around the periphery of the FAB. It was determined that conduction through the wire was the dominant heat loss mechanism during cooling, and the solidification process started from the wire-ball interface and proceeded across the diameter then outward towards the ball periphery.The microstructure of the Cu ball bond after thermosonic bonding was investigated. The result showed that the subgrain orientations were changed in the bonding process. It is evident that metal flow along the bonding interface was from the central area to the bond periphery during thermosonic bonding.     

A concept to relate wire bonding parameters to bondability and ball bond reliability

The effects of wire bonding parameters on bondability and ball bond reliability have been investigated. Bondability is characterized by ball shear stress (ball shear force per unit area) and ball bond reliability by median time to failure during in-situ ball bond degradation measurements. By introducing the concept of a reduced bonding parameter (RBP), a combination of all bonding parameters, we are able to relate the bonding parameters to bondability and ball bond reliability. With the appropriate RBP, ball shear force, ball shear stress, andball bond reliability appear to be well-behaved functions of the RBP fora wide range of settings. This provides us with simple analytical tool for optimizing bonding parameter windows.     

Silver pick up during tail formation in thermosonic wire bonding process

Cu ball bonding processes are significantly less robust than Au ball bonding processes. One reason for this are higher variations observed, e.g. in the free-air ball (FAB) formation process. There is a strong influence the tail bond process has on subsequent FAB formation. Tail tips and bond fractographs made with Cu and Au wires are investigated using scanning electron microscopy. Au and Cu wires pick up Ag material (Ag pick up) from the Ag metallization of Cu leadframe diepads during wire tail breaking. Ag pick up by Cu wire is more dominant than that by Au wire. Ultrasonic friction energy is necessary in order for Ag material pick up to occur. The impact force plays an important role for Ag pick up; less pick up is observed with a higher impact force. The hardness of the free-air ball (FAB) with Ag pick up is reduced by up to 4% compared to that of a FAB made from a tail broken at the neck of a ball bond and therefore having no Ag pick up. No significant change of FAB diameter is observed in these two cases.     

In situ ultrasonic force signals during low-temperature thermosonic copper wire bonding

Ultrasonic in situ force signals from integrated piezo-resistive microsensors were used previously to describe the interfacial stick-slip motion as the most important mechanism in thermosonic Au wire ball bonding to Al pads. The same experimental method is applied here with a hard and a soft Cu wire type. The signals are compared with those obtained from ball bonds with standard Au wire. Prior to carrying out the microsensor measurements, the bonding processes are optimized to obtain consistent bonded ball diameters of 60 μm yielding average shear strengths of at least 110 MPa at a process temperature of 110 °C. The results of the process optimization show that the shear strength c_pk values of Cu ball bonds are almost twice as large as that of the Au ball bonds. The in situ ultrasonic force during Cu ball bonding process is found to be about 30% higher than that measured during the Au ball bonding process. The analysis of the microsensor signal harmonics leads to the conclusion that the stick-slip frictional behavior is significantly less pronounced in the Cu ball bonding process. The bond growth with Cu is approximately 2.5 times faster than with Au. Ball bonds made with the softer Cu wire show higher shear strengths while experiencing about 5% lower ultrasonic force than those made with the harder Cu wire.     

Surface oxide films on Aluminum bondpads: Influence on thermosonic wire bonding behavior and hardness

Using indentation testing, wire bond tests and electron microscopy, the influence of increased oxide films on Al metallization surfaces on the wire bonding behavior and hardness was investigated. Oxide film thickness values larger than about 20 nm obstruct the bond contact and resulted in a poor bonding quality. The presence of such films causes also a hardness increase which can be detected by current sensitive indentation test methods. Therefore, an improved indentation testing technique can be applied during the quality control of bondpad metallizations prior to wire bonding.     

Golden bump for 20 micron diameter wire bond enhancement at reduced process temperature

With the rapid development of advanced microelectronic packaging technologies, research on fine-pitch wire bonding with improved reliability is driven by demands for smaller form factors and higher performance. In this study, thermosonic wire bonding process with a 20 μm wire for fine-pitch interconnection is described. To strengthen stitch bonds made in a gold-silver bonding system when the bonding temperature is as low as 150 °C, ball bumps (security bump) are placed on top of the stitch bonds. The ball-stitch bond and bump forming parameters are optimized using a design of experiment (DOE) method. A comparison of pull test results for stitch bonds with and without security bumps shows a substantial increase of the stitch pull force (PF) due to the use of security bonds. By varying the relative position of the security bumps to the stitch bonds via wedge shift offset (WSO), a WSO window ranging from 15 to 27 μm results in stitch PF higher than 7 gf, which is equivalent to an increase in average stitch PF of 118%.     

High current bond design rules based on bond pad degradation and fusing of the wire

In this paper design rules for maximum current handling capability of gold bond wires are derived based on two failure mechanisms: (1) fusing of the wire; and (2) degradation of the interface between gold bond balls and the aluminum bond pads under high current/high temperature stress. For determination of the fuse current as a function of the length an analytical model is used to calculate the temperature and power distribution in the wire as a function of the position. The current level at which the melt temperature of gold is reached is the fuse current. The degradation mechanism under high current stress (up to 2.5 A) was studied by in-situ monitoring of the gold bond ball–aluminum interconnect contact resistance under high current stress at various temperatures and stress currents. The cumulative failure distributions were used to fit a model for lifetime as a function of current and temperature that shows an order of magnitude difference in lifetime between positive and negative current stress. Finally, fuse current and the lifetime model result in data-driven high current design rules for bond pad and wire.     

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.     

Development of a thermosonic wire-bonding process for gold wire bonding to copper pads using argon shielding

To improve the bondability and ensure the reliability of Au/Cu ball bonds of the thermosonic (TS) wire-bonding process, an argon-shielding atmosphere was applied to prevent the copper pad from oxidizing. With argon shielding in the TS wire-bonding process, 100% gold wire attached on a copper pad can be achieved at the bonding temperature of 180°C and above. The ball-shear and wire-pull forces far exceed the minimum requirements specified in the related industrial codes. In a suitable range of bonding parameters, increasing bonding parameters resulted in greater bonding strength. However, if bonding parameters exceed the suitable range, the bonding strength is deteriorated. The reliability of the high-temperature storage (HTS) test for Au/Cu ball bonds was verified in this study. The bonding strength of Au/Cu ball bonds increases slightly with prolonged storage duration because of diffusion between the gold ball and copper pad during the HTS test. As a whole, argon shielding is a successful way to ensure the Au/Cu ball bond in the TS wire-bonding process applied for packaging of chips with copper interconnects.     

Nickel-palladium bond pads for copper wire bonding

The semiconductor packaging industry is undergoing a step-change transition from gold to copper wire bonding brought on by a quadrupling of gold cost over the last 8 years. The transition has been exceptionally rapid over the last 3 years and virtually all companies in the industry now have significant copper wire bonding production. Among the challenges to copper wire bonding is the damage to bond pads that had been engineered for wire bonding with the softer gold wire. This paper presents an extensive evaluation of electroless NiPd and NiPdAu bond pads that offer a much more robust alternative to the standard Al pad finish. These NiPd(Au) bond are shown to outperform Al in virtually all respects: bond strength, bond parameter window, lack of pad damage and reliability.     

Cu wire bond parameter optimization on various bond pad metallization and barrier layer material schemes

The use of copper wire for semiconductor package assembly has been gradually gaining acceptance throughout the industry over the last decade. Although copper has several advantages over gold for wire bonding applications, the manufacturing difficulties using copper wire have made high volume, fine pitch copper bonding slow to materialize. In recent years with the spike in gold prices, copper wire has become even more attractive, and this has driven many studies on the topic.Due to the propensity for copper to work harden upon deformation, which occurs during the ball bonding process as the capillary tip smashes the ball into the bond pad, a high amount of stress is transferred into the bond pad structure. This can result in catastrophic defects such as dielectric cracking or pad cratering. The current study aims to quantify the level of underlying bond pad damage with respect to various bond pad metallization and barrier layer schemes. A first bond parameter optimization was completed on each experimental group. The results indicate that barrier layer structure and composition have a significant impact on the presence of pad cratering. The experimental group containing only TiN as the barrier material showed a high occurrence of cratering, while groups with Ti and TiW barrier metals showed no cratering, even if a TiN layer was on top of the Ti. The bond pad metal thickness, on the other hand, does not appear to play a significant role in the prevention of bond pad cratering. Metal thickness values ranging from 0.825 to 2.025 μm were evaluated, and none had bond pad cratering other than the group with TiN as the barrier metal. In addition to the first bond parameter evaluations with various bond pad and barrier metal combinations, the initial free air ball (FAB) optimization is discussed.     

Cu wire bond microstructure analysis and failure mechanism

In this study, copper wire bonding samples were aged at 205 °C in air from 0 h to 2000 h. It was found that the bonding of a Cu wire and an Al pad formed Cu₉Al₄, CuAl, and CuAl₂ intermetallic compounds, and an initial crack was formed by the ultrasonic squeeze effect during thermosonic wire bonding. The cracks grew towards the ball bond center with an increase in the aging time, and the Cl ions diffused through the crack into the ball center. This diffusion caused a corrosion reaction between the Cl ions and the Cu-Al intermetallic phases, which in turn caused copper wire bonding damage.     

Iterative optimization of tail breaking force of 1 mil wire thermosonic ball bonding processes and the influence of plasma cleaning

An online tail breaking force measurement method is developed with a proximity sensor between wire clamp and horn. The wire under the tensile load measures about 1.5 cm extending from the bond location to the wire clamp. To increase the sensitivity, the bondhead speed is reduced to 2 mm/s during breaking the tail bond. It takes roughly 10 ms to break the tail bond. The force resolution of the method is estimated to be better than 5.2 mN. An automatic wire bonder used to continuously bond up to 80-wire loops while recording the on-line proximity signals. All wires are directed perpendicular to the ultrasound direction. The tail breaking force for each bond is evaluated from the signal and shown automatically on the bonder within 2 min after bonding.Results are obtained for a typical Au wire and a typical Cu wire bonding process. Both wires are 25 mm in diameter and bonded on Ag plated diepads of standard leadframes at 220 °C. An average Cu tail breaking force of higher than 50 mN is obtained if the leadframe is plasma cleaned before the bonding with 100% Ar for 5 min. This result is comparable to that obtained with Au wire. The standard deviation of the Cu tail breaking force is about twice that obtained with Au wire. The tail breaking force depends on the bonding parameters, metallization variation, and cleanliness of the bond pad. The cleanliness of the bonding pad is more important with Cu wire than with Au wire.     

Bond reliability under humid environment for coated copper wire and bare copper wire

There is growing interest in Cu wire bonding for LSI interconnection due to cost savings and better electrical and mechanical properties. Conventional bare Cu bonding wires, in general, are severely limited in their use compared to Au wires. A coated Cu bonding wire (EX1) has been developed for LSI application. EX1 is a Pd-coated Cu wire to enhance the bondability.Bond reliability at a Cu wire bond under a humid environment is a major concern in replacing Au wires. The bond reliability of EX1 and bare Cu was compared in the reliability testing of PCT and UHAST (Unbiased HAST). The lifetimes for EX1 and the bare Cu in PCT testing were over 800 h and 250 h, respectively. Humidity reliability was significantly greater for EX1. Continuous cracking was formed at the bond interface for the bare Cu wire. Corrosion-induced deterioration would be the root cause of failure for bare Cu wires. The corrosion was a chemical reaction of Cu-Al IMC (InterMetallic Compound) and halogens (Cl, Br) from molding resins. EX1 improves the bond reliability by controlling diffusion and IMC formation at the bond interface. The excellent humidity reliability of the coated Cu wire, EX1 is suitable for LSI application.     

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