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     

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 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 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     

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.     

Process windows for low-temperature Au wire bonding

The process windows are presented for low-temperature Au wire bonding on Au/Ni/Cu bond pads of varying Au-layer thicknesses metallized on an organic FR-4 printed circuit board (PCB). Three different plating techniques were used to deposit the Au layers: electrolytic plating, immersion plating, and immersion plating followed by electrolytic plating. Wide ranges of wire bond force, bond power, and bond-pad temperature were used to identify the combination of these processing parameters that can produce good wire bonds, allowing the construction of process windows. The criterion for successful bonds is no peel off for all 20 wires tested. The wire pull strengths and wire deformation ratios are measured to evaluate the bond quality after a successful wire bond. Elemental and surface characterization techniques were used to evaluate the bond-pad surfaces and are correlated to wire bondability and wire pull strength. Based on the process windows along with the pull strength data, the bond-pad metallization and bonding conditions can be further optimized for improved wire bondability and product yields. The wire bondability of the electrolytic bond pad increased with Au-layer thickness. The bond pad with an Au-layer thickness of 0.7 μm displayed the highest bondability for all bonding conditions used. The bondability of immersion bond pads was comparable to electrolytic bond pads with a similar Au thickness. Although a high temperature was beneficial to wire bondability with a wide process window, it did not improve the bond quality as measured by wire pull strength.     

Thickness dependent mechanical behavior of submicron aluminum films

Mechanical behavior of thin metallic films has been investigated on aluminum films deposited on a flexible polyimide substrate. Aluminum thin films exhibit a higher tensile strength than bulk aluminum. As film thickness decreases from 480 to 60 nm tensile strength increases from 196 to 408 MPa. These mechanical behaviors are correlated with the microstructure and its evolution with the thickness of aluminum thin films. Films are consisted of fine columnar grains and average grain size increases monotonically with the film thickness. The volume fraction of (111)-textured grains increases and the dispersion of texture axis becomes narrow as the film thickens. The relative contributions of the film thickness, grain size, and texture to the strength of aluminum thin films are estimated using an empirical strengthening model. The result indicates that the high strength of aluminum thin films is due largely to their small grain size, followed by the strengthening due to the film thickness and texture.     

Surface topographical characterization of silver-plated film on the wedge bondability of leaded IC packages

Strong bond between gold wire and silver-plated leadframe is significantly crucial for maintaining either bondability or reliability during integrated circuit (IC) manufacturing process and IC application in the fields. This study investigated the surface and grain structure of the silver-plated film on the copper leadframe in terms of surface roughness test by atomic force microscopy (AFM), thickness measurement by nano-indentation, and grain structure by transmission electron microscopy (TEM). The characteristics of the silver-plated film surface and structure, e.g., surface roughness, film thickness and grain structure impacted the bonding quality which was verified by the full factorial design of experiment (DOE) wedge pull test. Implications of AFM, TEM and DOE results showed that more smooth and soft crystal surface topography and fewer grain boundaries of silver-plated film provided the higher pull strength which could improve the yield of wedge bondability.     

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.     

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.     

Effects of Cu/Al intermetallic compound (IMC) on copper wire and aluminum pad bondability

Copper wire bonding is an alternative interconnection technology that serves as a viable, and cost saving alternative to gold wire bonding. Its excellent mechanical and electrical characteristics attract the high-speed, power management devices and fine-pitch applications. Copper wire bonding can be a potentially alternative interconnection technology along with flip chip interconnection. However, the growth of Cu/Al intermetallic compound (IMC) at the copper wire and aluminum interface can induce a mechanical failure and increase a potential contact resistance. In this study, the copper wire bonded chip samples were annealed at the temperature range from 150/spl deg/C to 300/spl deg/C for 2 to 250 h, respectively. The formation of Cu/Al IMC was observed and the activation energy of Cu/Al IMC growth was obtained from an Arrhenius plot (ln (growth rate) versus 1/T). The obtained activation energy was 26Kcal/mol and the behavior of IMC growth was very sensitive to the annealing temperature. To investigate the effects of IMC formation on the copper wire bondability on Al pad, ball shear tests were performed on annealed samples. For as-bonded samples, ball shear strength ranged from 240-260gf, and ball shear strength changed as a function of annealing times. For annealed samples, fracture mode changed from adhesive failure at Cu/Al interface to IMC layer or Cu wire itself. The IMC growth and the diffusion rate of aluminum and copper were closely related to failure mode changes. Micro-XRD was performed on fractured pads and balls to identify the phases of IMC and their effects on the ball bonding strength. From XRD results, it was confirmed that the major IMC was /spl gamma/-Cu/sub 9/Al/sub 4/ and it provided a strong bondability.     

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.     

Wirebonding at higher ultrasonic frequencies: reliability and process implications

Higher-frequency ultrasonics have been utilized to improve the bondability of difficult substrates, i.e., substrates that would not bond or that bonded poorly using conventional ultrasonics (nominally at 60 kHz). A systematic study of the influence of higher-frequency ultrasonics on bond strength and the bondability of various substrates is reported. The studies were carried out using two essentially identical thermosonic ball bonding machines, one bonding at nominally 60 kHz and the other at 100 kHz. The only differences between the bonding machines were the ultrasonic generators’ operating frequency and the transducer horns. Key to the study was the ability to make the bonding experiments as controlled, repeatable, and independent of all variables (except frequency) as possible. Control techniques included setting the electronic flame-off to produce consistently sized free-air balls; monitoring the ultrasonic voltage and current waveforms; and picking force, dwell, energy, and substrate heat settings that would allow strong bonds to be formed at both frequencies. Wirebonds (ball bonds) in this study were evaluated primarily by the ball bond shear test. Statistical methods were used to determine whether the differences in the means and variances between comparable samples sets (one bonded at 60 kHz and the other bonded at 100 kHz) were significant. Results of our studies indicate that significant differences exist between bonding at nominally 60 kHz and bonding at 100 kHz. In particular, we describe effects associated with (1) the ball shear strength before and after thermal aging (temperatures up to 200 °C) for both 60- and 100-kHz bonds, (2) the influence of substrate-metallizations combinations on the geometry and strength of the bonds at the different frequencies, and (3) the sensitivity and control of the overall bonding processes.     

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.     

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.     

A new approach in free air ball formation process parametersanalysis

The shape and size of gold wire ball formation deeply affects the quality of wire bonding. It not only affects the bond-ability of the first bond (ball bond), but also affects the possibility of processing low loop height bonding for thin packaging [such as thin small outline package (TSOP) and thin quad flat package (TQFP)] and high input/output (I/O) fine pitch packaging such as ball grid array. The parameters which affect the gold wire ball formation include: 1) tail length left after second bond; 2) type and shape of capillaries used; 3) material characteristics of gold wire; 4) supplied voltage, current, and time of electrical flame-off (EFO) unit; 5) gap between tail and electrode plate; and 6) relative position between capillary and electrode plate. In this paper, experiments were conducted to find the effect of these parameters on ball formation. Taguchi method together with neural network is applied in this research to find the best parameters setting for gold wire ball formation. It can then be used for wire bonding process parameter adjustment and process monitoring. It can also be used as reference for the development of wire bonders     

A new bonding-tool solution to improve stitch bondability

A new bonding-tool solution is proposed to improve stitch bondability by creating a new surface morphology on the tip surface of a wire-bonding tool (capillary). The surface has relatively deep lines with no fixed directions. This new capillary has less slipping between the wire and the capillary tip surface and provides better coupling effect between them. Experiments of wire bonding on unstable lead frames/substrates, alloyed wire (2N gold wire) bonding, and copper wire bonding were carried out to confirm the effect of the new capillary on the stitch bondability. The experimental results are promising and have proved that the use of the new capillary could improve the bondability of the stitch bond and minimize the occurrence of short tail defects and non-sticking on lead during bonding.     

Increasing bondability and ball-shear force of gold balls thermosonic bonding to flex substrates by depositing a nickel layer

To improve the bondability and ball-shear force of gold balls that are thermosonically bonded to copper electrodes over flex substrates, a nickel layer was deposited on the surface of the copper electrodes to increase their rigidity. A silver layer was then deposited on the nickel layer to prevent oxidation of the copper electrodes during the thermosonic bonding process. This nickel layer was expected to enhance the rigidity of copper electrodes over the flex substrates, increasing the thermosonic bonding efficiency of gold balls to copper electrodes over the flex substrates.Deposition the nickel layer on the copper electrodes improved the elastic modulus of the flex substrates, indicating that the nickel layer is effective in enhancing the rigidity of copper electrodes over the flex substrates. The bondability and ball-shear force of gold balls that are thermosonically bonded to copper electrodes increases with the thickness of the nickel layer given fixed bonding parameters. One hundred percent bondability and high ball-shear force can be achieved when gold balls are thermosonically bonded to copper electrodes with the deposition of a 0.5 μm-thick nickel layer. Herein, the ball-shear force was higher than that specified in JEDEC standards. Furthermore, gold balls that were thermosonically bonded to copper electrodes with a nickel layer had a large bonded area with an extensive scrape, while gold balls that were thermosonically bonded to copper electrodes without a nickel layer had a blank surface morphology. This experimental result was similar to that of tests of the elastic modulus of flex substrates, similarity can be used to explain that the effectiveness of the nickel layer in increasing the rigidity of copper electrodes, increasing the bonding efficiency at the bonding interface between gold balls and copper electrodes during thermosonic bonding process. After ball-shear test, a layer that was stuck on the ball bond was observed at the location of fracture of the ball bonds for gold balls they were thermosonically boned on copper electrodes with 0.5 μm-thick nickel layer. This observation implies that the ball-shear force of the gold balls that were bonded on the copper electrodes exceeded even the adhesive force of the layers that were deposited on the copper electrodes.The deposition of a 0.5 μm-thick nickel layer on copper electrodes over flex substrates improved the rigidity of the copper electrodes; the ultrasonic power could be propagated to the bonding interface between the gold balls and the copper electrodes, increasing the bondability and ball-shear force.     

High temperature reliability of aluminium wire-bonds to thin film, thick film and low temperature co-fired ceramic (LTCC) substrate metallization

Reliable interconnects are essential for microelectronic systems intended for long life times in harsh environment applications. Intermetallic growth accelerates as the temperature increases, and the material system must be carefully selected to avoid mechanically and/or electrically weak connections. The dominating chip metallization is aluminium, and aluminium wire-bonding is therefore recommended to obtain a mono-metallic system at chip level. A suitable substrate metallization compatible with aluminium wire-bonds at high temperatures (HT) should therefore be found.Test substrates with low temperature co-fired ceramic (LTCC) silver conductors plated with nickel/gold, gold and aluminium thin film, gold thick film, and silver thick film plated with copper/nickel/gold have been manufactured. Wedge/wedge aluminium wire-bonding were performed with 25 μm aluminium wire on the substrates before they were subjected to long term ageing at temperatures up to 250 °C for 6-12 months. Bond-pull strength and electrical resistance were measured during ageing on selected components.The present work shows that long term reliable aluminium wire-bonds for 250 °C operation is feasible both with thin film, thick film and LTCC substrate technology. For the screen-printed conductors, a plating system with nickel is necessary. Aluminium wire bonded to gold thin film displays reliable long term high temperature performance for gold thicknesses up to ∼1 μm.