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.     

Influence of surface contamination on metal/metal bond contact quality

The influence of surface cleanliness of Au/Ni coated multichip materials (MCMs), Ag plated Cu lead frames, and Al bond pads on semiconductor chips on the strength of Au wire bond contacts has been investigated. A clean surface is important for good adhesion in any kind of attachment process. Investigations by means of x-ray photoelectron spectroscopy have been performed on the bond substrates to determine the chemical composition, the nature as well as the thickness of the contamination layer. The influence of contamination on bond contact quality has been examined by pull force measurements, which is an established test method in semiconductor packaging industry for evaluating the quality of wire bonds. The results clearly show that a strong correlation between the degree of contamination of the substrate and pull strength values exists. Furthermore, a contamination thickness limiting value of 4 nm for Au and Ag substrates was determined, indicating good wire bond contact quality. The effect of plasma cleaning on wire bondability of metallic and organic (MCMs) substrates has been examined by pull force measurements. These results confirm the correlation between surface contamination and the strength of wire bond contacts for Au/Ni coated MCMs and Ag plated Cu lead frames. Atomic force microscopy measurements have been performed to determine the roughness of bond surfaces, demonstrating the importance of nanoscale characterization with regard to the bonding behavior of the substrates. Finally, bonding substrates used in integrated circuit packaging are discussed with regard to their Au wire bonding behavior. The Au wire bonding process first results in a cleaning effect of the substrate to be joined and secondly enables the change of bonding energy into frictional heat giving rise to an enhanced interdiffusion at the interface.     

Effects of metallization characteristics on gold wire bondability of organic printed circuit boards

Organic printed circuit boards (PCBs) with Au/Ni plates on bond pads are widely used in chip-on-board (COB), ball grid array (BGA), and chip-scale packages. These packages are interconnected using thermosonic gold wire bonding. The wire bond yield relies on the bondability of the Ni/Au pads. Several metallization parameters, including elemental composition, thickness, hardness, roughness, and surface contamination, affect the success of the solid state joining process. In this study, various characterization and mechanical testing techniques are employed to evaluate these parameters for different metallization schemes with varying Ni and Au layer thicknesses. The pull force of Au wires is measured as a function of plasma treatment applied before wire bonding to clean the bond pads. Close correlations are established between metallization characteristics and wire bond quality.     

Thermosonic bonding of gold wire onto silver bonding layer on the bond pads of chips with copper interconnects

A copper pad oxidizes easily at elevated temperatures during thermosonic wire bonding for chips with copper interconnects. The bondability and bonding strength of a gold wire onto a bare copper pad are seriously degraded by the formation of a copper oxide film. A new bonding approach is proposed to overcome this intrinsic drawback of the copper pad. A silver layer is deposited as a bonding layer on the surface of copper pads. Both the ball-shear force and the wire-pull force of a gold wire bonded onto copper pads with silver bonding layers far exceed the minimum values stated in the JEDEC standard and MIL specifications. The silver bonding layer improves bonding between the gold ball and copper pads. The reliability of gold ball bonds on a bond pad is verified in a high-temperature storage (HTS) test. The bonding strength increases with the storage time and far exceeds that required by the relevant industrial codes. The superior bondability and high strength after the HTS test were interpreted with reference to the results of electron probe x-ray microanalyzer (EPMA) analysis. This use of a silver bonding layer may make the fabrication of copper chips simpler than by other protective schemes.     

Thermosonic bonding of gold wire onto a copper pad with titanium thin-film deposition

A novel thermosonic (TS) bonding process for gold wire bonded onto chips with copper interconnects was successfully developed by depositing a thin, titanium passivation layer on a copper pad. The copper pad oxidizes easily at elevated temperature during TS wire bonding. The bondability and bonding strength of the Au ball onto copper pads are significantly deteriorated if a copper-oxide film exists. To overcome this intrinsic drawback of the copper pad, a titanium thin film was deposited onto the copper pad to improve the bondability and bonding strength. The thickness of the titanium passivation layer is crucial to bondability and bonding strength. An appropriate, titanium film thickness of 3.7 nm is proposed in this work. One hundred percent bondability and high bonding strength was achieved. A thicker titanium film results in poor bond-ability and lower bonding strength, because the thicker titanium film cannot be removed by an appropriate range of ultrasonic power during TS bonding. The protective mechanism of the titanium passivation layer was interpreted by the results of field-emission Auger electron spectroscopy (FEAES) and electron spectroscopy for chemical analysis (ESCA). Titanium dioxide (TiO₂), formed during the die-saw and die-mount processes, plays an important role in preventing the copper pad from oxidizing. Reliability of the high-temperature storage (HTS) test for a gold ball bonded on the copper pad with a 3.7-nm titanium passivation layer was verified. The bonding strength did not degrade after prolonged storage at elevated temperature. This novel process could be applied to chips with copper interconnect packaging in the TS wire-bonding process.     

Thermosonic gold wire bonding to laminate substrates with palladiumsurface finishes

As the laminate substrate industry moves from hot air solder level (HASL) finishes, alternate plating finishes are being proposed such as immersion gold/electroless nickel, electroless palladium, and electroless silver. This paper presents results of an evaluation of the thermosonic gold ball wire bondability of electroless palladium. Two palladium thicknesses, with and without a nickel underlayer, were evaluated from two vendors. In each case, a thin gold passivation layer was deposited by immersion plating over the palladium. The initial evaluation criteria included bondability (number of missed bonds and flame off errors), wire pull strength, standard deviation, bond failure mode, and visual inspection. The bonding window was determined by independently varying force (four levels), power (four levels), and time (two levels). The stage temperature was maintained at 150°C, compatible with BT laminate material. Different preconditioning environments such as a solder reflow cycle, high temperature storage (125°C) and humidity storage (85%RH/85°C) on initial bondability were also considered. Rutherford backscattering and Auger analysis were used to examine the surface finishes. The stability of the bonds was investigated by high temperature storage (125°C) with periodic electrical resistance and pull strength testing     

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.     

Evaluation of wire bonding performance, process conditions, and metallurgical integrity of chip on board wire bonds

Chip on board wire bonding presents challenges to modern wire bonding technology which include smaller, closely spaced wire bond pads; bonding to soft substrates without special processing and pad construction; and diverse first bond and second bond metallurgies. These challenges are addressed by extensive bonding accuracy tests, a design of experiments approach for optimizing wire bond process parameters, reliability testing, and detailed materials characterization of the metallurgical integrity of the wire bonds. The thermo-mechanical integrity of the wire bond interconnects was evaluated by wire pull and hot storage tests. Hot storage testing allowed for detection of samples with an electrolytic gold surface finish that was too thin, and exhibited a contamination-corrosion condition of the nickel under-plating. Other samples with an excessively thick, rough textured nickel under-plating layer exhibited poor wire bond-ability. The methodology of materials analyses of the metallurgy of the wire bond interconnects is described. The paper illustrates a wire bond lift technique that is used to inspect for cratering damage and the “area-uniformity” of gold aluminum intermetallics. An improved understanding of the wire bonding process was achieved by showing the dependence of the visual appearance of the wire bonds on wire bond process parameters.     

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.     

Study of nickel contamination on the ultrasonic bondability of gold bond finger of SD-48L packages

Contamination is a major barrier to the adhesion of solid-phase metal couples. If me-tallic impurities are present on a gold (Au) bonding surface, it can easily react with oxygen molecules in the atmosphere to form oxides. The oxides formed prevent intimate metal-metal contact which is important for bond formation. A study is carried out to investigate whether nickel (Ni) contamination can affect the ultrasonic bondability of Au bond finger of side-braze 48-lead ceramic package. Two sets of samples are used for this study. The first set comprising uncontaminated packages serves as a control group. The second set are Ni contaminated packages. The results of the study show that Ni contamination does not affect the ultrasonic bondability of the Au bond finger. A second study to ascertain the quality of the wire bonds however shows that the mean bond tensile strength of the Ni contaminated packages has weakened slightly.     

The development of Cu bonding wire with oxidation-resistant metal coating

Although Cu bonding wire excels over Au bonding wire in some respects such as production costs, it has not been widely used because of its poor bondability at second bonds due to surface oxidation. We conceived an idea of electroplating oxidation-resistant metal on the Cu bonding wire to prevent the surface oxidation. The electroplating of Au, Ag, Pd, and Ni over Cu bonding wire all increased bond strengths as expected, but it caused problematic ball shapes except Pd-plated Cu bonding wire. The wire could produce the same ball shape as that of Au bonding wire. It was also proved to have excellent bondability sufficient to replace Au bonding wire. That is, it excelled in bond strengths, defective bonding ratio, and wideness of "Parameter Windows". It also showed the same stability as Au bonding wire in reliability tests, while bonds of Cu bonding wire were deteriorated in a few of the tests. In short, the Pd-plated Cu bonding wire can realize excellent bonding similar to Au bonding wire, while having much lower production costs.     

表面特性对PBGA软金丝键合点可焊性的影响

一级封装中最流行的互连技术仍为丝焊。引线键合的效率主要依赖于受表面特性影响的键合点的可焊性。在最近的研究中,我们调查了表面特性对金-金超声压焊系统的影响。表面特性包括金层厚度,表面硬度和粗糙度、有机物杂质及金属杂质。对两个样本间的不同特性进行比较。确定金表面特性的粗糙度依赖于镍层的外形结构。焊料掩膜逸出气体对可焊性具有负面影响,等离子清洗能够有效地去除有机物杂质。金层中的杂质将导致不良的可焊性。     

Interconnecting to aluminum- and copper-based semiconductors (electroless-nickel/gold for solder bumping and wire bonding)

Electroless nickel and immersion gold plating technologies (e-Ni/Au) have traditionally been used almost exclusively within the electronics industry to create a solderable surface on substrate materials, e.g. laminate boards. Recent advances in these plating technologies, along with the inherent low costs associated with electroless plating processes, have enabled the extension of their utilization into a variety of semiconductor applications, e.g. wafer level pad metallization. This paper describes the electroless nickel and immersion gold processes for both aluminum- and copper-based semiconductors. The nickel plating bath is a hypophosphite-based solution and the gold bath is a cyanide-free sulfate-based solution. For aluminum-based integrated circuits a zincation process is used to initiate nickel growth, and for copper, palladium is used to catalyze the surface. Tight control of the chemistries, equipment, and run-time process variables are required to ensure repeatability. Thin film Auger analysis of the as-plated films shows well-defined layers of high purity gold and nickel/phosphorous. Adhesion of the e-Ni/Au layers was evaluated by measuring the load required to shear I/O pads plated with tall nickel bumps. Integrity of the nickel was further evaluated by subjecting the structures to multiple temperature cycles and test for pad shear strength. Results show no degradation in shear load or failure mode.The deposition of nickel and gold onto the I/O pad surfaces enables the subsequent use of both wire bond and flip chip (lead-based and lead-free alloys) interconnect methods. The integrity of gold wire bonds to the e-Ni/Au plated I/O pad was evaluated using ball shear, wire pull, and the corresponding failure analysis of each. Results show values well above product specifications, with wire pull failure modes in the wire and intermetallic failure in the ball shear studies. For flip chip applications, the e-Ni/Au layer was evaluated using stencil-printing technology to deposit several different solder alloys. In the current investigation, two test vehicles were successfully bumped with both 63Sn/37Pb and 90Pb/10Sn lead-based solder alloys, as well as the 95.5Sn/3.8Ag/0.7Cu lead-free alloy. In order to evaluate the compatibility of these alloys with the electroless nickel layer, solder bump shear tests were performed as a function of number of reflow cycles. Results show no degradation in shear load or failure mode among all three of the alloys tested, indicating no critical nickel consumption (i.e., excessive intermetallic growth) during reflow. Additional tests were performed comparing nickel under-bump-metallurgy (UBM) thicknesses of 1, 2 and 5 μm. Again, no critical nickel consumption was detected.     

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/mil² 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.     

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.     

Effect of Chromium–Gold and Titanium–Titanium Nitride–Platinum–Gold Metallization on Wire/Ribbon Bondability

Gold metallization on wafer substrates for wire/ribbon bond applications requires good bond strength to the substrate without weakening the wire/ribbon. This paper compares the ribbon bondability of Cr-Au and Ti-TiN-Pt-Au metallization systems for an optoelectronic application. Both Chromium and Titanium are used to promote adhesion between semiconductor substrates and sputtered gold films. However, both will be oxidized if they diffuse to the gold surface and result in the degradation of the wire/ribbon bondability. Restoring bondability by ceric ammonium nitrate (CAN) etch was investigated. Experiments were conducted to investigate the effect of Cr-Au and Ti-TiN-Pt-Au, annealing, and CAN etch processes, on 25.4times254 mum (1 times 10 mil) ribbon bonding. All bonds were evaluated by noting pull strengths and examining specific failure modes. The results show that there is no significant difference in bondability between Cr-Au and Ti-TiN-Pt-Au before the annealing process. At this point, excellent bond strength can be achieved. However, wire/ribbon bondability of Cr-Au degraded after the wafers were annealed. The experimental results also show that a CAN etch can remove Cr oxide, and that the improvement in wire/ribbon bondability of Cr-Au depends on the CAN etch time. It is further demonstrated that the same annealing process does not have a significant effect on the bondability of Ti-TiN-Pt-Au metallization on the same type substrate materials. Auger electron spectroscopy was used to investigate the causes of the difference in bondability between these two metallizations     

Role of process parameters on bondability and pad damage indicators in copper ball bonding

Cu wire bonding is one of the hottest trends in electronic packaging due to the cost and performance advantages of Cu wire over Au wire. However, there are many challenges to Cu wire bonding, one of which is the increased stress transmitted to the bond pad during bonding. This high stress is not desirable as it leads to pad damage or cratering in the Si under the pad. Another issue is pad splash in which the pad material is squeezed outside the bonded area, which in severe cases can cause Al pad thinning and depletion. To study the root cause of the increased stress, ball bonding is performed with Au and Cu wires using the same levels of ultrasound (USG), bonding force (BF), and impact force (IF). The bonding is performed on a bonding test pad with integrated piezoresistive microsensors and the in situ pad stress is measured in real time. The ultrasonic pad stress did not show any significant difference between the Au and the Cu ball bonding processes. This indicates that the cause of increased stress cannot be attributed to material properties such as hardness alone, and that the differences in bondability and bonding parameters required for the Cu process might be more influential. To achieve optimal bonding results in terms of shear force per unit area, the Cu process requires higher BF and USG settings, which are the main causes of pad damage. To understand the effect of bonding parameters IF, BF, and USG on pad stress, a detailed DOE is conducted with Cu wire. In addition to conventional bonding parameters, the effect of a non-zero USG level applied during the impact portion of the bonding (pre-bleed USG) is investigated. One of the findings is the reduction of pad damage when higher pre-bleed USG levels are used.     

The effect of thin film structure and properties on gold ball bonding

The gold ball bonding process is widely used for making interconnections between integrated circuit chips and package lead frames, yet the relationships between the wire/substrate materials properties and the bond formation processes are not yet well understood. While the creation of a metallurgical bond at the interface between the wire and substrate is required, the deformation of the wire and substrate also play an important role in bond formation. Bonding to thin film substrates is of particular interest, since thin films often exhibit mechanical behavior distinctly different from bulk materials. In the present study, a systematic investigation has been conducted to understand the effects of the structure and properties of aluminum thin films on the quality of gold ball bonds. A series of aluminum thin films was fabricated with systematic variations in hardness, roughness, thickness, and composition. Gold wires were ball bonded to these substrates, and the bondability and bond shear strengths were assessed. Metallographic sections of several of these specimens were made and examined in the scanning electron microscope. The results show that the film thickness has the most dominant effect on the bondability and bond strength; films that were 0.5 μm thick often exhibited low strength or poor bondability. Very hard films also gave poor results. Ultimately, these results can be used to predict the wire bond reliability expected from various types of thin film metallization.     

Reliability evaluation of BOAC and normal pad stacked-chip packaging using low-K wafers

This work evaluates the wire bondability and the reliability tests for the stacked-chip TFBGA wire bond packaging with the Sn–4.0Ag–0.5Cu lead-free solder ball. The bonding-over-active-circuit (BOAC) pad is the top test chip and the normal pad is the bottom test chip and is combined in the stacked-chip packaging. Both test chips are 90 nm low-K dielectric with five copper layers and one layer aluminum pad and a background ranging from 775 μm to 150 μm. According to the simulation results, the maximum normal stress of low-K layer for the BOAC pad is higher than that of the normal pad by 146.4%. However, the maximum shear stress of Cu metal layer for the BOAC pad is lower than that of the normal pad by 64.2%. To compare the bonding pad strength for the BOAC and normal pad low-K wafers, this work uses the simplified two-layer model to extract the effective mechanical properties of the two bonding pad structures. The effective average Young’s modulus of the normal pad and the BOAC pad are 86 GPa and 69 GPa, respectively. The test results indicate that the effective Young’s modulus of the normal pad exceeds that of the BOAC pad by 17 GPa. The wire bondability test of the ball shear and the wire pull test results are superior to the specification by 80% and 83.75%, respectively. All stacked-chip TFBGA packaging samples underwent reliability tests, including HAST, TCT, and HTST. All the wire bondability and reliability tests passed the specification for the BOAC pad and the normal pad low-K structures. Accordingly, this work shows that the proposed stacked-chip TFBGA packaging passes the wire bondability and the reliability tests. The proposed packaging improves the electrical performance, enhances the utility of the active chip area and saves chip area through the use of low-K and BOAC chips. Furthermore, the results show that the equivalent stiffness of the bonding pad structure can be used as the bondability and reliability test index for the chip.     

Reduction of ultrasonic pad stress and aluminum splash in copper ball bonding

Given the cost and performance advantages associated with Cu wire, it is being increasingly seen as a candidate to replace Au wire for making interconnections in first level microelectronics packaging. A Cu ball bonding process is optimized with reduced pad stress and splash, using a 25.4 μm diameter Cu wire. For ball bonds made with conventionally optimized bond force and ultrasonic settings, the shear strength is ≈140 MPa. The amount of splash extruding out of bonded ball interface is between 10 and 12 μm. It can be reduced to 3-7 μm if accepting a shear strength reduction to 50-70 MPa. For excessive ultrasonic settings, elliptical shaped Cu bonded balls are observed, with the minor axis of the ellipse in the ultrasonic direction and the major axis perpendicular to the ultrasonic direction. To quantify the direct effect of bond force and ultrasound settings on pad stress, test pads with piezoresistive microsensors integrated next to the pad and the real-time ultrasonic force signals are used. By using a lower value of bond force combined with a reduced ultrasound level, the pad stress can be reduced by 30% while achieving an average shear strength of at least 120 MPa. These process settings also aid in reducing the amount of splash by 4.3 μm.