A Study of the Electrical Flame Off Process During Thermosonic Wire Bonding with Novel Wire Materials

Thermosonic ball bonding is the most popular method used to create electrical interconnects between integrated circuits (ICs) and substrates in the microelectronics industry. Traditionally gold (Au) wire is used, however with industry demands for lower costs and higher performance, novel wire materials are being considered. Some of these wire materials include Cu, insulated, and coated wires. The most promising of which being Cu wire. Some of the main issues with these wire materials is their performance in the electrical flame off (EFO) step of the wire bonding process. In the EFO step a ball called the free air ball (FAB) is formed on the end of the wire. The quality of the FAB is essential for reliable and strong ball bonds. In Cu wire bonding the hardness of the FAB and oxidation are the main issues. A hard FAB requires larger bonding forces and US levels to make the bond which increases the likelihood of damage to the IC. Excessive oxidation acts as a contaminant at the bond interface and can also influence the shape of the FAB. Shielding gases are required to reduce oxidation and improve FAB quality. This thesis focuses on the EFO process and the influence of EFO parameters and shielding gases on Au and Cu FABs. The primary focus of this thesis is to provide a better understanding of the EFO process in order to expedite the introduction of novel wire materials into industry. Several different experiments are performed on an automated thermosonic wire bonder with 25 µm Au and Cu wires to investigate the EFO process during ball bonding. The effects of EFO parameters on the hardness and work hardening of FABs and the effects of shielding gas type and flow rates on the quality of the FABs are determined. The EFO discharge characteristics in different shielding gases is also studied to better understand how the composition of the atmosphere the FAB is formed in influences the energy input via the EFO electrical discharge. Using the online deformability method and Vickers microhardness testing it is found that the EFO current (IEFO) and EFO time (tEFO) have a large influence on the hardness and work hardening of Au and Cu FABs. A harder FAB produced with a large IEFO and low tEFO will work harden less during deformation. The bonded ball will be softer than that of a FAB produced with a lower IEFO and higher tEFO. The online deformability method is found to be twice as precise as the Vickers microhardness test. An online method for characterizing the quality of FABs is developed and used to identify shielding gas flow rates that produce defective FABs. The EFO process for an Au wire and two Cu wire materials is investigated in flow rates of 0.2-1.0 l/min of forming gas (5 % H2 + 95 % N2) and N2 gas. All three of the most common FAB defects are identified with this online method. It is found that good quality FABs cannot be produced above flow rates of 0.7 l/min and that H2 in the shielding gas adds a thermal component to the EFO process. It is recommended that the gas flow rate be optimized for each new wire type used. The EFO discharge power is measured to be 12 % higher in a N2 gas atmosphere than in a forming gas atmosphere. The lower ionization potential of the forming gas leads to a higher degree of ionization and therefore lower resistance across the discharge gap. It was found that the discharge power does not determine the energy transferred to the wire anode because the Au FAB produced in forming gas has a 6 % larger diameter than that of the FABs produced in N2 gas. Other factors that effect the voltage of the EFO discharge include the controlled EFO current, the discharge gap, and the wire anode material

Process Quality Improvement in Thermosonic Wire Bonding

This thesis demonstrates the feasibility of methods developed to increase the quality of the crescent bond together with the tail bond quality. Low pull force of the crescent bond limits the usage of insulated Au wire in microelectronics assembly. Premature break of the tail which results in the stoppage of the bonding machine is one of obstacles to overcome for Cu wire. The primary focus of this thesis is to understand the tail and crescent bonding process and then to propose methodologies to improve thermosonic wire bonding processes when Cu and insulated Au wires are used. Several series of experiments to investigate the crescent and tail bonding processes are performed on auto bonders. Cu and insulated Au wires with diameters of 25mm are bonded on the diepads of Ag leadframes. For simplicity, wire loops are oriented perpendicular to the ultrasonic direction. It was found that the crescent bond breaking force by pulling the wire loop (pull force) with insulated Au wire is about 80 % of that of bare Au wire. A modification of the crescent bonding process is made to increase the pull force with insulated Au wire. In the modified process, an insulation layer removing stage (cleaning stage) is inserted before the bonding stage. The cleaning stage consists of a scratching motion (shift) toward to the ball bond in combination with ultrasound. Bonds are then made on the fresh diepad with the insulation removed from the contact surface of the insulated Au wire. This process increases the pull force of the crescent bond up to 26% which makes it comparable to the results obtained with bare Au wire. An online tail breaking force measurement method is developed with a proximity sensor between wire clamp and horn. Detailed understanding of tail bond formation is achieved by studying tail bond imprints with scanning electron microscopy and energy dispersive x-ray analysis. Descriptions are given of the dependence of the tail breaking force on the bonding parameters, metallization variation, and cleanliness of the bond pad. Simultaneous optimization with pull force and tail breaking force can optimize the Cu wire bonding process both with high quality and robustness. It is recommended to first carry out conventional pull force optimization followed by a minimization of the bonding force parameter to the lowest value still fulfilling the pull force cpk requirement. The tail bond forms not only under the capillary chamfer, but also under the capillary hole. The tail breaking force includes both the interfacial bond breaking strength and the breaking strength of the thinned portion of the wire that will remain at the substrate as residue. Close investigations of the tail bond imprint with scanning electron microscopy indicate the presence of fractures of the substrate indicating substrate material being picked up by Cu wire tail. Pick up is found on Au and Cu wires, but the amount of pick up is much larger on Cu wire. The effect on the hardness of the subsequently formed Cu free air ball (FAB) as investigated with scanning electron microscopy and micro - hardness test shows that Cu FABs containing Au and Ag pick ups are softer than those without pick up. However, the hardness varies significantly more with Au pick up. The amount of Au pick up is estimated higher than 0.03 % of the subsequently formed FAB volume, exceeding typical impurity and dopant concentrations (0.01 %) added during manufacturing of the wire

Mechanical and Tribological Aspects of Microelectronic Wire Bonding

The goal of this thesis is on improving the understanding of mechanical and tribological mechanisms in microelectronic wire bonding. In particular, it focusses on the development and application of quantitative models of ultrasonic (US) friction and interfacial wear in wire bonding. Another objective of the thesis is to develop a low-stress Cu ball bonding process that minimizes damage to the microchip. These are accomplished through experimental measurements of in situ US tangential force by piezoresistive microsensors integrated next to the bonding zone using standard complementary metal oxide semiconductor (CMOS) technology. The processes investigated are thermosonic (TS) Au ball bonding on Al pads (Au-Al process), TS Cu ball bonding on Al pads (Cu-Al process), and US Al wedge-wedge bonding on Al pads (Al-Al process). TS ball bonding processes are optimized with one Au and two Cu wire types, obtaining average shear strength (SS) of more than 120 MPa. Ball bonds made with Cu wire show at least 15% higher SS than those made with Au wire. However, 30% higher US force induced to the bonding pad is measured for the Cu process using the microsensor, which increases the risk of underpad damage. The US force can be reduced by: (i) using a Cu wire type that produces softer deformed ball results in a measured US force reduction of 5%; and (ii) reducing the US level to 0.9 times the conventionally optimized level, the US force can be reduced by 9%. It is shown that using a softer Cu deformed ball and a reduced US level reduces the extra stress observed with Cu wire compared to Au wire by 42%. To study the combined effect of bond force (BF) and US in Cu ball bonding, the US parameter is optimized for eight levels of BF. For ball bonds made with conventionally optimized BF and US settings, the SS is ≈ 140 MPa. The amount of Al pad splash extruding out of bonded ball interface (for conventionally optimized BF and US settings) is between 10–12 µm. It can be reduced to 3–7 µm if accepting a SS reduction to 50–70 MPa. For excessive US settings, elliptical shaped Cu bonded balls are observed, with the major axis perpendicular to the US direction. By using a lower value of BF combined with a reduced US level, the US force can be reduced by 30% while achieving an average SS of at least 120 MPa. These process settings also aid in reducing the amount of splash by 4.3 µm. The US force measurement is like a signature of the bond as it allows for detailed insight into the tribological mechanisms during the bonding process. The relative amount of the third harmonic of US force in the Cu-Al process is found to be five times smaller than in the Au-Al process. In contrast, in the Al-Al process, a large second harmonic content is observed, describing a non-symmetric deviation of the force signal waveform from the sinusoidal shape. This deviation might be due to the reduced geometrical symmetry of the wedge tool. The analysis of harmonics of the US force indicates that although slightly different from each other, stick-slip friction is an important mechanism in all these wire bonding variants. A friction power theory is used to derive the US friction power during Au-Al, Cu-Al, and Al-Al processes. Auxiliary measurements include the current delivered to the US transducer, the vibration amplitude of the bonding tool tip in free-air, and the US tangential force acting on the bonding pad. For bonds made with typical process parameters, several characteristic values used in the friction power model such as the ultrasonic compliance of the bonding system and the profile of the relative interfacial sliding amplitude are determined. The maximum interfacial friction power during Al-Al process is at least 11.5 mW (3.9 W/mm²), which is only about 4.8% of the total electrical power delivered to the US transducer. The total sliding friction energy delivered to the Al-Al wedge bond is 60.4 mJ (20.4 J/mm²). For the Au-Al and Cu-Al processes, the US friction power is derived with an improved, more accurate method to derive the US compliance. The method uses a multi-step bonding process. In the first two steps, the US current is set to levels that are low enough to prevent sliding. Sliding and bonding take place during the third step, when the current is ramped up to the optimum value. The US compliance values are derived from the first two steps. The average maximum interfacial friction power is 10.3 mW (10.8 W/mm²) and 16.9 mW (18.7 W/mm²) for the Au-Al and Cu-Al processes, respectively. The total sliding friction energy delivered to the bond is 48.5 mJ (50.3 J/mm²) and 49.4 mJ (54.8 J/mm²) for the Au-Al and Cu-Al processes, respectively. Finally, the sliding wear theory is used to derive the amount of interfacial wear during Au-Al and Cu-Al processes. The method uses the US force and the derived interfacial sliding amplitude as the main inputs. The estimated total average depth of interfacial wear in Au-Al and Cu-Al processes is 416 nm and 895 nm, respectively. However, the error of estimation of wear in both the Au-Al and the Cu-Al processes is ≈ 50%, making this method less accurate than the friction power and energy results. Given the error in the determination of compliance in the Al-Al process, the error in the estimation of wear in the Al-Al process might have been even larger; hence the wear results pertaining to the Al-Al process are not discussed in this study

XRD analysis of Cu-Al interconnect intermetallic compound in an annealed micro-chip

Cu-Al intermetallic compound (IMC) in Cu wire-Al bond pad interconnect interface is drawing attention of researches. However, due to thin IMC thickness, the characterizations of the IMC are limited to expensive and time consuming techniques. An evaluation is performed to use common X-Ray Diffraction (XRD) technique to identify the IMC in the Cu wired micro-chip samples in powder form. Existence of mixture of CuAl and CuAl2 was first confirmed by transmission electron microscope (TEM) and energy dispersive X-ray (EDX). In XRD analysis, peak correspond to CuAl phase is identified from measurement with slower scan configuration. The difficulty for IMC peak detection in diffractogram is due to low composition ratio of IMC relative to other materials available in the sample. KOH treatment for enhancing IMC peaks intensity does not work as expected as it etches the IMC as well

A Study on the Effect of Bond Stress and Process Temperature on Palladium Coated Silver Wire Bonds on Aluminum Metallization

In the past ten years, the increasing price of gold has motivated the wire bonding industry to look for alternative bonding wire materials in the field of microelectronics packaging. A new candidate wire to replace gold is palladium coated silver wire. In this thesis, the effect of the two specific process parameters “bond stress” and “process temperature” on the ball bonds made with the new candidate wire are investigated. Using 20 μm diameter wire and various level-combinations of these process parameter, ball bonds are produced according to a special accelerated optimization method to result in a target diameter of 46 ± 0.5 μm and target height of 16 ± 0.5 μm. Three different levels are used for each of the specific process parameters. After pre-selecting a few process parameters, the accelerated method determines the levels for the process parameters “impact force” and “electric flame-off current” with a 2×2 design of experiments. Then, the ultrasound parameter is maximized up to a level where a pre-selected ultrasonic deformation occurs to the bonds, maintaining the target bond diameter and height. The bond quality is measured by measuring the shear strength of the bonds. The results show that • the bond geometry is not affected by the bond stress, • the optimized specific process parameters vary by less than ~0.5 % when bond stress values are varied from 60 to 100 MPa, • the variations in optimized parameters are larger than ~3.0 % when the BT is changed from 100 to 200 ºC, • ball bonds achieve acceptable shear strength (> 120 MPa) when the values for both, bond stress and bond temperature, are high, • ultrasound level and shear stress interact, the higher shear stress the lower the ultrasound level required. An average shear strength of ~120 MPa is achieved with 11.4 % ultrasound, 100 MPa bond stress, and 200 ºC bond process temperature. In summary, a robust methodology is presented in this thesis to efficiently optimize the ball bonding process as demonstrated with the new candidate wire has a bondability similar to that of gold wire with only minor adjustment in the bonding process needed

Experimental Characterization Of Cu Free-Air Ball And Simulations Of Dielectric Fracture During Wire Bonding

Wire bonding is the process of forming electrical connection between the integrated circuit (IC) and its structural package. ICs made of material with low dielectric constant (low-k) and ultra low-k are porous in nature, and are prone to fracture induced failure during packaging process. In recent years, there is increasing interest in copper wire bond technology as an alternative to gold wire bond in microelectronic devices due to its superior electrical performance and low cost. Copper wires are also approximately 25% more conductive than Au wires aiding in better heat dissipation. At present, validated constitutive models for the strain rate and temperature dependent behavior of Cu free-air ball (FAB) appear to be largely missing in the literature. The lack of reliable constitutive models for the Cu FAB has hampered the modeling of the wire bonding process and the ability to assess risk of fracture in ultra low-k dielectric stacks. The challenge to FAB characterization is primarily due to the difficulty in performing mechanical tests on spherical FAB of micrometers in size. To address this challenge, compression tests are performed on FAB using custom-built microscale tester in the current study. Specifically, the tester has three closed-loop controlled linear stages with submicron resolution, a manual tilt stage, a six-axis load cell with sub-Newton load resolution for eliminating misalignment, a milliNewton resolution load cell for compression load measurement, a capacitance sensor to estimate sample deformation and to control the vertical stage in closed loop, a high working depth camera for viewing the sample deformation, and controllers for the stages implemented in the LabVIEW environment. FAB is compressed between tungsten carbide punches and a constitutive model is developed for Cu FAB through an inverse modeling procedure. In the inverse procedure, appropriate constitutive model parameter values are iterated through an automated optimization workflow, until the load-displacement response matches the experimentally observed response. Using the material properties obtained from the experiment, a macroscale finite element model for the impact and ulatrasonic vibration stages of wire bonding process is constructed to simulate (a) Plastic deformation of the Cu FAB at different time steps (b) Evolution of contact pressure (c) Phenomenon such as pad splash and lift-off. The deformations from the macroscale model are provided as input to a microscale model of the dielectric with copper vias as well as line-type heterogeneities. The microscale model is used to identify potential crack nucleation sites as well as the crack path within the ILD stack during wire bonding. The modeling provides insight into the relative amounts of damage accumulated during the impact and the ultrasonic excitation stages. In general, Bonding over Active Circuit (BOAC) has made wire bonding a considerable challenge due to the brittleness of the dielectric. Identifying and locating microscale fractures beneath the bond pads during wire bonding require extensive sample preparation and investigation for microscopic characterization. While simulations of fracture are an attractive alternative to trial and error microscopic characterization, the length scale of components involved in wire bonding varies from millimeters to nanometers. Therefore, constructing a finite element mesh across the model is computationally costly. Also, a multi-scale simulation framework is necessary. Such a modeling framework is also developed in this work to predict crack nucleation and propagation in wire bond induced failure

Effects of Ultrasound in Microelectronic Ultrasonic Wire Bonding

Ultrasonic wire bonding is the most utilized technique in forming electrical interconnections in microelectronics. However, there is a lacking in the fundamental understanding of the process. In order for there to be improvements in the process a better understanding of the process is required. The mechanism of the bond formation in ultrasonic wire bonding is not known. Although there have been theories proposed, inconsistencies have been shown to exist in them. One of the main inconsistencies is the contribution of ultrasound to the bonding process. A series of experiments to investigate the mechanism of bond formation are performed on a semi automatic wire bonder at room temperature. 25 µm diameter Au wire is ball bonded and also 25 µm diameter Al wire is wedge-wedge bonded onto polished Cu sheets of thickness 2 mm. It is found that a modified microslip theory can describe the evolution of bonding. With increasing ultrasonic power the bond contact transitions from microslip into gross sliding. The reciprocating tangential relative motion at the bond interface results in wear of surface contaminants which leads to clean metal/metal contact and bonding. The effect of superimposed ultrasound during deformation on the residual hardness of a bonded ball is systematically studied for the first time. An innovative bonding procedure with in-situ ball deformation and hardness measurement is developed using an ESEC WB3100 automatic ball bonder. 50 µm diameter Au wire is bonded at various ultrasound levels onto Au metallized PCB substrate at room temperature. It is found that sufficient ultrasound which is applied during the deformation leads to a bonded ball which is softer than a ball with a similar amount of deformation without ultrasound. No hardening of the 100 µm diameter Au ball is observed even with the maximum ultrasonic power capable of the equipment of 900 mW. In summary, the fundamental effect of ultrasound in the wire bonding process is the reciprocating tangential displacement at the bond interface resulting in contaminant dispersal and bonding. A second effect of ultrasound is the softening of the bonded material when compared to a similarly non-ultrasound deformed ball

INTERFACIAL DEGRADATION OF COPPER WIRE BONDS IN THERMAL AGING AND CYCLING CONDITION

Copper (Cu) wire bonds have become the dominant wire material used in microelectronic packages, having replaced gold (Au) in the majority of applications. Cost saving has been the key factor to drive this transition in wire bond material, although there are other advantages to Cu such as better electrical and thermal conductivity, reduced wire sweep during transfer molding and most importantly slower intermetallic compound (IMC) formation with Al (bond pad). Although IMC layers are much thinner than for Au-Al bonded joints, growth of second phase, Cu9Al4, due to exposure to high temperature leads to interfacial separation, which is exacerbated under thermal cycling condition ultimately leading to failure of the joint. Part I of this dissertation aims at addressing the effect of combined loading (thermal aging and cycling) on the reliability of Cu wire bonded devices using a unique long dwell thermal cycling profile that accelerates growth of different IMC phases (CuAl2 and Cu9Al4) and accelerates failure due to CTE mismatch between epoxy mold compound, die and Cu wire bond. Unlike many of the studies presented in literature, the test vehicle in this study are made of commercial off-the-shelf (COTS) parts, where a multitude of factors vary from one another, such as wire diameter, wire bond and bond pad characteristics, etc., the combination of which play a significant role in the life time of these devices and is not fully captured by first-principal models. Hence, a data-based life estimation method is developed, to aid in part selection based on initial bond characteristics. Critical parameters of wire bond that contribute to reliability are identified, the most significant of which is Al bond pad thickness, which controls the growth of IMC and influences time for Cu9Al4 IMC phase formation. Second part of this work is focused entirely on the Al bond pad thickness. Part II-A focuses on the qualitative comparison of pad thickness effect on the quality of initially formed bond through use of bond shear analysis and the effect of bond interface aging on bond shear analysis. Test vehicle consists of three pad thicknesses namely, 0.5 µm, 1 µm and 4 µm, over which Cu wirebonds with four different thermosonic bond recipes are made. Results from Part II-A provide guidelines for bond comparison using bond shear analysis. Part II-B focuses on the effect of bond pad thickness on the reliability of Cu wire bonds under isothermal aging at 175°C and 200°C for 1000 hours and 650 hours respectively. Test vehicle in this study consists of 0.675 µm and 3 µm pad thickness on silicon die in 20 leaded 5x5 QFN package. Wire bonds with one thermosonic bonding recipe are made on all the 90 packages used in the study. Electrical resistance and cross-sectional analysis are used to derive failure times, which is in turn used to build empirical relationship between pad thickness and time to failure. Result from this study shows longer time to failure for wire bonds on 3 µm pad compared to 0.675 µm pad due to delay in Cu9Al4 formation

3D packaging for integrated circuit systems

A goal was set for high density, high performance microelectronics pursued through a dense 3D packing of integrated circuits. A {open_quotes}tool set{close_quotes} of assembly processes have been developed that enable 3D system designs: 3D thermal analysis, silicon electrical through vias, IC thinning, mounting wells in silicon, adhesives for silicon stacking, pretesting of IC chips before commitment to stacks, and bond pad bumping. Validation of these process developments occurred through both Sandia prototypes and subsequent commercial examples