Jong-Soo Cho

Reliability study of low cost alternative Ag bonding wire with various bond pad materials

There have been many studies on replacing Au bonding wire with other bonding wire materials, because the cost of Au has dramatically increased by approximately 2~3 times in recent years. When replacing part of the bond pad with a noble metal, Ag bonding wire is of particular interest due to its superior electrical properties, lower cost and similar mechanical properties as compared with Au. Ag bonding wire is thermosonically bonded to 3 kinds of bond pad (Al, Au and Pd) and aged at high temperature (175°C). Then, the bondability and interface reactions are characterized at each bond pad. In the case of Ag-Al bonding, 2 kinds of intermediate phases were observed and the composition ratios of Ag and Al in these phases were 4:1 and 2:1, respectively. After 300 hrs of aging, cracks were formed in these intermediate phases and ball-lift failure occurred. However, in the case of the noble metal bond pad, a solid solution was formed between the Ag wire and bond pad and no voids or cracks were formed. This shows the robust bonding characteristics. The diffusion layer was observed and the diffraction pattern was analyzed by TEM (Transmission Electron microscopy). The Au-Al bond reliability was also characterized by a comparative study. In this study, Ag bonding wire is proposed as an alternative to Au bonding wire for noble metal pads. Also, the thermal reliability is reviewed and the failure mechanisms are verified with various bond pads.

Pd effects on the reliability in the low cost Ag bonding wire

Recently, the application of Ag bonding wires in electronic devices has been attempted as alternative to Au bonding wires to reduce the material cost of Au. Nevertheless, Ag bonding wires have not been applied to devices due to interface corrosion problems between the Ag wire and Al pad during humidity reliability tests, such as the PCT (Pressure Cooker Test). As the technology for alloying Pd element in Ag wire has developed recently, the corrosive failure problem at the interface in the PCT has been improved significantly. This study examined the behavior of IMCs(intermetallic compounds) and interface corrosion between an Ag wire and Al metallization under humidity conditions (100%RH, 121C). The chemical compositions of the Ag wires tested were pure Ag, Ag — 1wt%Pd and Ag-3%Pd. These wires are bonded to Al and noble metal(Au, Pd) metallization using a thermo-sonic bonder. The interfaces were characterized by FIB(Focused ion beam), HRTEM (High Resolution Transmission Electron Microscope) and EDS (Energy Dispersive X-ray Spectroscopy) The findings show that, (1) the interfacial reliability between the Ag wire and Ag metallization was improved considerably. The interface corrosion was suppressed significantly as the Pd content was increased. (2) Ag wires on a noble metal(Au, Pd) pad have stable reliability in PCT.

Thermal reliability & IMC behavior of low cost alternative Au-Ag-Pd wire bonds to Al metallization

To reduce material cost of Au bonding wire, Au-Ag alloy wire had been tried to use in electronic device, in alternative for Au wire. Nevertheless, Au-Ag alloy wire had not been applied to device due to failure problem during high humidity reliability test, like PCT(Pressure Cooker Test) for a while. Recently, as the technology adding Pd element to conventional Au-Ag was developed, corrosive failure problem at interface between wire ball and Al pad (metallization) was solved in PCT. Finally, mass production of Au-Ag-Pd wire starts to grow up. The study on reliability and behavior of intermetallic compounds(IMC) have not been performed in terms of thermal Aging or HTST(High Temperature Storage Life Test) yet. This study investigates IMC & voids behavior between Au-Ag-Pd wire and Al metallization under thermal aged condition (175C). The chemical compositions of gold wires tested are 99.99% Au, Au-30wt.%Ag and Au-30wt.%Ag-5.5%wt.Pd, respectively. The wires are bonded on Al metallization by using thermosonic bonder, IMC behavior was characterized by FIB(Focused ion beam), TEM (Transmission Electron Microscope), EDS (Energy Dispersive X-ray Spectrometer) The findings reveal that, interfacial reliability between Au-Ag-Pd wire and Al metallization is considerably improved, compared with 4N Au or Au-30%Ag, Growth of IMCs as well as the voids formation are significantly suppressed.

The behavior of FAB (free air ball) and HAZ (heat affected zone) in fine gold wire

The trend of high integration and miniaturization of semiconductors has accelerated the development of gold bonding wire with smaller diameter. For the stable bonding of fine wire, it is important to characterize the wire with various diameters during the bonding process. To investigate this relationship, the experiments were done for the various sizes of wire diameter and FAB. The wire size and the FAB size were chosen for the test from 15 /spl mu/m to 25 /spl mu/m and from 1.4 WD (wire diameter) to 2.0 WD, respectively. The results showed that as the size of FAB became smaller, the size deviation of FAB increased and FAB itself was tilted to one side. When FAB was formed at the same parameter, the length of HAZ became shorter for the wire with the high temperature of recrystallization. It is also revealed that the length of HAZ decreased for the smaller size of FAB. This phenomenon is considered to be related to the beat generated during the FAB formation.

Effect of Pd addition on ultra-fine pitch Au wire/Al pad interface

Due to increasing cost of gold and technological demand for finer pitch application, Au bonding wires with finer diameter are needed. In such demands, Au bonding wire manufacturing companies produced ultra-fine Au wires with diameter of 12 μm (0.5 mil). However, interfacial reactions between ultra-fine Au wires and Al pads may cause serious reliability problems as wire diameter get smaller. Decrease in bond reliabilities may be caused by interfacial failures such as Kirkendall voids and oxidation of Au4Al IMCs. For the application of ultra-fine Au wires in real products, studies on interfacial reactions of ultra-fine Au wires/Al pads and effects of Pd addition on Au wire/Al pad interface which is previously proven to be enhancing bond reliabilities are necessary. In this study, interfacial reactions and failures of two types of 4N ultra-fine Au wires(12 μm)/Al pads with different bonding types were analyzed, and effects of Pd addition on interfacial reactions and failures are also investigated by using 3000, 6000, and 10000 ppm of Pd added Au wires. High temperature storage test (HTST) at 175D under air-ambient was performed to simulate an accelerated thermal exposure characteristic of device operation conditions. Intermetallic compound (IMC) phases and Pd behavior at the Au wire/Al pad interface were identified using various analytical tools. Ball pull test (BPT) was conducted to evaluate bond reliabilities depending on bonding types, wire compositions, and aging times. According to experimental results, two types of ultra-fine Au wires/Al pads showed different bonding reliabilities depending on initial inter-metallic coverage. It was found that initial intermetallic coverage determined the interface bond reliability because interfacial oxidation problem depended on initial intermetallic coverage. Addition of Pd on Au wires/Al pads significantly affected bond reliabilities depending on Pd contents. 3000 ppm of Pd added Au wires/Al pads resulted the best bond reliabilities and strongest resistance against the interfacial oxidation due to the Pd accumulation at Au4Al grain boundaries. However, 10000 ppm of Pd added Au wires/Al pads showed low bond reliabilities because separated Pd-rich island grains formed among Au4Al grains resulting in less protection of Au4Al from oxidation.

The role of serum CA-125 levels in early-stage epithelial ovarian cancer on preoperative CT and MRI

BACKGROUNDnWe sought to identify the role of serum CA-125 levels in early-stage epithelial ovarian cancer (EOC) on preoperative CT and MRI.nnnMETHODSnClinical data of 101 patients with early-stage EOC on preoperative CT and MRI were collected between January 2000 and December 2007. Clinical stage I (n=59) was defined as tumor limited to the ovaries with or without ascites, whereas clinical stage II (n=42) was defined as tumor within the pelvis with or without ascites. The primary endpoint was to investigate the efficacy of serum CA-125 levels for the prediction of advanced-stage disease, and secondary endpoints were to evaluate the accuracy of preoperative CT and MRI, and to examine the role of serum CA-125 levels as a prognostic factor for survival.nnnRESULTSnThe results of preoperative CT and MRI were concordant with no peritoneal implants outside the pelvis in 50/101 (50%) and no lymph node metastasis in 71/101 (70%) patients. The receiver operating characteristic curves showed that best cut-off values of serum CA-125 levels were 320 U/ml (71% sensitivity, 84% specificity) and 510 U/ml (67% sensitivity, 80% specificity) for the prediction of peritoneal implants outside the pelvis and lymph node metastasis. The serum CA-125 level (> or =320 U/ml) was a significant factor for the prediction of advanced-stage disease (adjusted OR, 7.43; 95% CI, 2.39-23.04). However, it was not an independent prognostic factor for survival.nnnCONCLUSIONSnSerum CA-125 levels may be very useful for the prediction of advanced-stage disease in early-stage EOC on preoperative CT and MRI.

Pd effect on reliability of Ag bonding wires in microelectronic devices in high-humidity environments

We investigated the effect of Pd concentration in Pd-doped Ag wires on the humidity reliability and interfacial corrosion characteristics between Ag wire and Al metallization. Additionally, we confirmed no corrosion problem between Ag wire and noble metal (Pd, Au) metallization, even after a pressure cooker test (PCT). The chemical composition of the tested Ag wires was pure Ag, Ag-1wt% Pd and Ag-3wt% Pd. These wires were bonded to Al and noble metal (Au, Pd) metallization using a thermo-sonic bonder. The interfaces were characterized by focused ion beam (FIB), high resolution transmission electron microscope (HRTEM) and energy dispersive X-ray spectroscopy (EDS). The interface corrosion of Pd doped Ag wires was significantly reduced as the Pd concentration in the Ag wires increased. Furthermore, the Ag wires on the noble metal (Au, Pd) metallization exhibited stable reliability during the PCT.

Analysis on interfacial failures of ultra-fine pitch wire with low inter-metallic coverage

Ultra-fine pitch Au wire has become inevitable in the electronic packaging industry due to decreasing electronic package dimension and increasing cost of gold. However, fine pitch Au wire with diameter less than 0.5 mil. (12 μm), bonded on Al pad, showed low inter-metallic coverage, and it caused distinctive interfacial failures: the oxidation of Au4Al and overgrowth of inter-metallic compound (IMC). In order to enhance the interfacial failure, addition of Pd was proposed due to its known effect as the diffusion barrier. In this study, 0 ppm, 3000 ppm, 6000 ppm, and 10000 ppm of Pd was added to 0.5 mil. Au wire as alloying element and subsequently aged under high temperature storage test (HTST) in air and vacuum condition at 175 °C. By the observation of Au/Al interface using scanning electron microscope (SEM) and reliability test using ball pull test (BPT), it was confirmed that major failure mode was the oxidation of Au4Al for air-aging and overgrowth of IMC for vacuum-aging; however, it was observed that the interfacial failures were affected by the addition of Pd. For air-aging, the oxidation rate was in order of 10000 ppm (fastest) > 0 ppm = 6000 ppm > 3000 ppm (slowest) of Pd showing that the interfacial failure due to the oxidation was affected by Pd contents. For vacuum-aging, higher Pd contents decreased overgrowth of IMC and number of ball-lift. Detailed effects of Pd for the oxidation rate were analyzed using transmission electron microscope (TEM). Au wire with 2500 ppm of Pd was analyzed as most oxide-stable Pd composition, and was observed to form segregation of Pd mass at the grain boundary of Au4Al, and the segregation protected Au4Al from the oxidation. However, the sample with 10000 ppm of Pd made separate Pd-rich grain, and it didnt prevent the oxidation and even caused quicker oxidation than the sample without Pd.