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.     

Oxidation of copper pads and its influence on the quality of Au/Cu bonds during thermosonic wire bonding process

To understand the copper oxide effect on the bondability of gold wire onto a copper pad, thermosonic gold wire bonding to a copper pad was conducted at 90–200 °C under an air atmosphere. The bondability and bonding strength of the Au/Cu bonds were investigated. The bondability and bonding strength were far below the minimum requirements stated in industrial codes. At elevated bonding temperature of 200 °C, the bondability and bonding strength deteriorated mainly due to hydroxide and copper oxide formation on the copper pad. Oxide formation occurred if no appropriate oxide preventive schemes were applied. At lower bonding temperature, 90 °C, poor bondability and low bonding strength were mainly attributed to insufficient thermal energy for atomic inter-diffusion between the gold ball and copper pad.Copper pad oxidation was investigated using an electron spectroscopy for chemical analysis (ESCA) and thermogravimetric analysis (TGA). An activation energy of 35 kJ/mol for copper pad oxidation was obtained from TGA. This implies that different mechanisms govern the oxidation of copper pad and bulk copper. Hydroxide and copper oxide were identified based on the shifted binding energy. Cu(OH)₂ forms mainly on the top surface of copper pads and the underlying layer consists mainly of CuO. The hydroxide concentration increased with increasing the heating temperatures. After heating at 200 °C, the hydroxide concentration on the copper pad surface was approximately six times that at 90 °C. Protective measures such as passivation layer deposition or using shielding gas are critical for thermosonic wire bonding on chips with copper interconnects.     

Effect of electronic flame off parameters on copper bonding wire: Free-air ball deformability, heat affected zone length, heat affected zone breaking force

Cu bonding wire is more and more used for interconnections to integrated circuits (ICs) to reduce cost and increase performance compared to Au wire. To eliminate underpad damage for Cu wire applications, it is worthwhile to reduce the hardness of the free-air ball (FAB). Short heat affected zone (HAZ) and high HAZ breaking load are often required for advanced microelectronics packaging in order to decrease the loop height and thereby the package thickness.Online measurements of deformability and HAZ breaking force at temperatures close to the bonding temperature of 220 °C are new tools used in this study to evaluate the effects of electronic flame off (EFO) current and firing time on the Cu FAB deformability and the HAZ length and tensile strength. FABs with 50 μm diameter formed from a 25 μm diameter Cu wire with a breaking load of 118.6 mN were used. EFO currents and firing times ranged from 40 to 250 mA and 0.11 to 0.90 ms, respectively. Average FAB deformability factors, HAZ breaking forces, and HAZ lengths were in the rounded ranges of 36.64–44.09% (with a deformation force of 0.60 N), 107.7–116.8 mN, and 167–215 μm, respectively. When produced with 250 mA current during 0.11 ms, the FABs are 7.01–7.89% more deformable than when produced with 45 mA during 0.9 ms, the HAZ breaking force is 7.53–9.37% higher, and the HAZ length is 7–90 μm shorter.     

PTCDA on Au(1 1 1), Ag(1 1 1) and Cu(1 1 1): Correlation of interface charge transfer to bonding distance

The electronic structure at the interfaces of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) and the metal surfaces Au(1 1 1), Ag(1 1 1) and Cu(1 1 1) was investigated using ultraviolet photoelectron spectroscopy (UPS). By combining these results with recent X-ray standing wave data from PTCDA on the same substrates clear correlation between the electronic properties and the interface geometry is found. The charge transfer between the molecule and the metal increases with decreasing average bonding distance along the sequence Au–Ag–Cu. Clear signatures of charge-transfer-induced occupied molecular states were found for PTCDA on Ag(1 1 1) and Cu(1 1 1). As reported previously by Zou et al. [Y. Zou et al., Surf. Sci. 600 (2006) 1240] a new hybrid state was found at the Fermi-level (E_F) for PTCDA/Ag(1 1 1), rendering the monolayer metallic. In contrast, the hybrid state for PTCDA/Cu(1 1 1) was observed well below E_F, indicating even stronger charge transfer and thus a semiconducting chemisorbed molecular monolayer. The hybridisation of molecular and Au electronic states could not be evidenced by UPS.     

Comparison of the copper and gold wire bonding processes for LED packaging

Wire bonding is one of the main processes of the LED packaging which provides electrical interconnection between the LED chip and lead frame.The gold wire bonding process has been widely used in LED packaging industry currently.However,due to the high cost of gold wire,copper wire bonding is a good substitute for the gold wire bonding which can lead to significant cost saving.In this paper,the copper and gold wire bonding processes on the high power LED chip are compared and analyzed with finite element simulation.This modeling work may provide guidelines for the parameter optimization of copper wire bonding process on the high power LED packaging.     

Comparison of the copper and gold wire bonding processes for LED packaging

Wire bonding is one of the main processes of the LED packaging which provides electrical interconnection between the LED chip and lead frame. The gold wire bonding process has been widely used in LED packaging industry currently. However, due to the high cost of gold wire, copper wire bonding is a good substitute for the gold wire bonding which can lead to significant cost saving. In this paper, the copper and gold wire bonding processes on the high power LED chip are compared and analyzed with finite element simulation. This modeling work may provide guidelines for the parameter optimization of copper wire bonding process on the high power LED packaging.     

Characterization of interfacial reaction and chemical bonding features of LaOx/HfO2 stack structure formed on thermally-grown SiO2/Si(1 0 0)

A stack structure consisting of ～1.5 nm-thick LaO_x and ～4.0 nm-thick HfO₂ was formed on thermally grown SiO₂ on Si(1 0 0) by MOCVD using dipivaloymethanato precursors, and the influence of N₂ annealing on interfacial reaction for this stack structure was examined by using X-ray photoelectron spectroscopy and Fourier transform infrared attenuated total reflection. We found that compositional mixing between LaO_x and HfO₂ becomes significant from 600 °C upwards and that interfacial reaction between HfLa_yO_z and SiO₂ proceeds consistently at 1000 °C in N₂ ambience.     

Experimental evidence and extraction of the electron mass variation in [1 1 0] uniaxially strained MOSFETs

In this paper, we investigate electron mobility enhancement in [1 1 0] uniaxially strained nMOSFETs with three different channel orientations on a [0 0 1] Si substrate. We have experimentally demonstrated that, for stress applied in a [1 1 0] direction, electrical results cannot be explained without considering that the in-plane mass m_t for the [0 0 1] two-fold valleys (Δ2) becomes anisotropic and varies with strain. For the first time, their values in the transport direction, perpendicular and parallel to an applied stress, have been extracted from electrical characterization of the MOS transistor thanks to an original technique. It has been found that the conduction mass of Δ2 along the standard [1 1 0] channel direction is reduced by a tensile uniaxial stress along [1 1 0] while it is increased by a tensile uniaxial stress along . These results reinforce several previous theoretical works.     

Microstructure, thermal analysis and hardness of a Sn–Ag–Cu–1 wt% nano-TiO2 composite solder on flexible ball grid array substrates

Sn-Ag-Cu composite solders reinforced with nano-sized, nonreacting, noncoarsening 1 wt% TiO₂ particles were prepared by mechanically dispersing TiO₂ nano-particles into Sn-Ag-Cu solder powder and the interfacial morphology of the solder and flexible BGA substrates were characterized metallographically. At their interfaces, different types of scallop-shaped intermetallic compound layers such as Cu₆Sn₅ for a Ag metallized Cu pad and Sn-Cu-Ni for a Au/Ni and Ni metallized Cu pad, were found in plain Sn-Ag-Cu solder joints and solder joints containing 1 wt% TiO₂ nano-particles. In addition, the intermetallic compound layer thicknesses increased substantially with the number of reflow cycles. In the solder ball region, Ag₃Sn, Cu₆Sn₅ and AuSn₄ IMC particles were found to be uniformly distributed in the β-Sn matrix. However, after the addition of TiO₂ nano-particles, Ag₃Sn, AuSn₄ and Cu₆Sn₅ IMC particles appeared with a fine microstructure and retarded the growth rate of IMC layers at their interfaces. The Sn-Ag-Cu solder joints containing 1 wt% TiO₂ nano-particles consistently displayed a higher hardness than that of the plain Sn-Ag-Cu solder joints as a function of the number of reflow cycles due to the well-controlled fine microstructure and homogeneous distribution of TiO₂ nano-particles which gave a second phase dispersion strengthening mechanism.     

Silicate formation at the interface of Hf-oxide as a high-k dielectrics and Si(0 0 1) surfaces

The composition and chemical bonding of the first atoms across the interface between Si(0 0 1) and the gate dielectrics determine the quality of gate stacks. An analysis of that hidden interface is a challenge as it requires high sensitivity in both elemental and chemical state information. We used synchrotron radiation (SR) based photoelectron spectroscopy and, in particular, X-ray absorption spectroscopy in total electron yield and total fluorescence yield at the Si2p and the O1s edges to address this issue. We report on results for Hf oxide prepared by ALD and compare to Pr₂O₃/Si(0 0 1). For Hf oxide thin films we find evidence for the silicate formation at the interface as derived from the characteristic features in the X-ray absorption spectra at the Si2p and the O1s edges. Resonant photoelectron spectroscopy is used to analyze the absorption band in detail. Following the resonant profiles of initial and final states we deduce from the resonant behaviour a charge donation via a Si-induced charge transfer.     

The influence of 1 nm AlN interlayer on properties of the Al0.3Ga0.7N/AlN/GaN HEMT structure

The sheet carrier concentrations, conduction band profiles and amount of free carriers in the barriers have been determined by solving coupled Schrödinger and Poisson equation self-consistently for coherently grown Al_0.3Ga_0.7N/GaN and Al_0.3Ga_0.7N/AlN/GaN structures on thick GaN. The Al_0.3Ga_0.7N/GaN heterojunction structures with and without 1 nm AlN interlayer have been grown by MOCVD on sapphire substrate, the physical properties for these two structures have been investigated by various instruments such as Hall measurement and X-ray diffraction. By comparison of the theoretical and experimental results, we demonstrate that the sheet carrier concentration and the electrons mobility would be improved by the introduction of an AlN interlayer for Al_0.3Ga_0.7N/GaN structure. Mechanisms for the increasing of the sheet carrier concentration and the electrons mobility will be discussed in this paper.