Inkjettable conductive adhesive for use in microelectronics and microsystems technology

Ink jet is an accepted technology for dispensing small volumes of material (50–500 picolitres). Currently traditional metal-filled conductive adhesives cannot be processed by ink jetting (owing to their relatively high viscosity and the size of filler material particles). Smallest droplet size achievable by traditional dispensing techniques is in the range of 150 μm, yielding proportionally larger adhesive dots on the substrate. Electrically conductive inks are available on the market with metal particles (gold or silver) <20 nm suspended in a solvent at 30–50 wt%. After deposition, the solvent is eliminated and electrical conductivity is enabled by a high metal ratio in the residue. Some applications include a sintering step. These nano-filled inks do not offer an adhesive function. Work reported here presents materials with both functions, adhesive and conductive. This newly developed silver filled adhesive has been applied successfully by piezo-ink jet and opens a new dimension in electrically conductive adhesives technology.The present work demonstrates feasibility of an inkjettable, isotropically conductive adhesive in the form of a silver loaded resin with a two-step curing mechanism: In the first-step, the adhesive is dispensed (jetted) and precured leaving a ‘dry’ surface. The second step consists of assembly (wetting of the 2nd part) and final curing.     

Adhesion improvement of silicon/underfill/polyimide interfaces by UV/ozone treatment and sol–gel derived hybrid layers

Adhesion and mechanical reliability improvement is an important issue for flexible electronics due to weak bonds between silicon/underfill/polyimide interfaces. These interfaces are bonded with weak hydrogen and ester bonds which are vulnerable to humidity. Therefore, in this study, adhesion and reliability of silicon/underfill/polyimide interfaces are enhanced by using UV/Ozone treatment and sol–gel derived hybrid layers. In order to examine the effectiveness of those surface treatment methods, double cantilever beam (DCB) test and subcritical crack growth test were applied to accurately measure the adhesion energy and subcritical crack growth rate. The results showed that the adhesion and reliability against humidity were enhanced by more than 300% and 1000% when both surface treatment methods were applied. Also, the adhesive failure path was altered to mixed mode failure of both cohesive and adhesive failure paths.     

Adhesion properties of inverted polymer solarcells: Processing and film structure parameters

We report on the adhesion of weak interfaces in inverted P3HT:PCBM-based polymer solar cells (OPV) with either a conductive polymer, PEDOT:PSS, or a metal oxide, molybdenum trioxide (MoO₃), as the hole transport layer. The PEDOT:PSS OPVs were prepared by spin or spray coating on glass substrates, or slot-die coating on flexible PET substrates. In all cases, we observed adhesive failure at the interface between the P3HT:PCBM with PEDOT:PSS layer. The adhesion energy measured for the solar cells made on glass substrates was about 1.8 J/m², but only 0.5 J/m² for the roll-to-roll processed flexible solar cells. The adhesion energy was insensitive to the PEDOT:PSS layer thickness in the range of 10–40 nm. A marginal increase in adhesion energy was measured with increased O₂ plasma power. Compared to solution processed PEDOT:PSS, we found that thermally evaporated MoO₃ adheres less to the P3HT:PCBM layer, which we attributed to the reduced mixing at the MoO₃/P3HT:PCBM interface during the thermal evaporation process. Insights into the mechanisms of delamination and the effect of different material properties and processing parameters yield general guidelines for the design of more reliable organic photovoltaic devices.     

Strength of Ta–Si interfaces by molecular dynamics

Miniaturization of electronic devices leads to nanoscale structures in the near future. In these length scales the technological choice for metalization and interconnect material seems to be copper mainly because of its low electrical resistance and resistivity against electromigration. In copper metalization a barrier layer between copper and silicon is needed to prevent diffusion. Tantalum seems to be the most common barrier metal. We use a modified embedded atom potential and molecular dynamics to study the energy, structure, and strength of Ta–Si interfaces. The interfacial energy has a negative correlation between the strength of the interface. We propose that mixing on the interface has an important role in interface strength.     

Correlation between localized strain and damage in shear-loaded Pb-free solders

A digital image correlation (DIC) algorithm was employed to measure microscopic strain-field evolution in shear-loaded model solder interconnections made out of a number of Sn-based alloys. Four different solder alloys studied were Sn–36Pb–2Ag, Sn–3.8Ag–0.7Cu (SAC), Sn–3.3Ag–3.82Bi, and Sn–8Zn–3Bi. The measured strain fields were correlated with damage observed at the scale of the sample, and at microscopic length scales.Local strain differs significantly from applied global strain and has been shown to depend on the geometry of the samples as well as the microstructure (on a grain level) of the solder.Strain fields in all solder interconnections were found to localize near but not at the solder–substrate interface and along grain boundaries in the solders. The eventual failure path as observed on the scale of the sample (parallel to the two solder–substrate interfaces with a cross-over from one interface to the other somewhere in the connection) showed a good correlation with measured strain fields in all interconnections.In contrast to the similarity on a macroscopic scale, on a microscopic scale the failure mechanisms were observed to be material specific.     

Adhesion improvement of Epoxy Molding Compound – Pd Preplated leadframe interface using shaped nickel layers

The weak adhesion between the Epoxy Molding Compound (EMC) and Pd Preplated leadframes (Pd PPF’s) often causes delaminations and reduces the reliability of integrated circuit. This paper reports on a practical method of dramatically improving the adhesion between EMC and Pd PPF’s using electroplating of shaped nickel layers. Button shear tests indicate that the adhesions between the EMC and three different shaped PPF’s are 100%, 160%, 169% higher than that of conventional PPF’s. The mechanical interlocking effect caused by increased surface roughness is the major reason for the improved adhesion as well as for the failure mode transition from adhesive failure to cohesive failure.     

Degradation by Sn diffusion applied to surface mounting with Ag-epoxy conductive adhesive with joining pressure

Shear and tensile tests were carried out on joints made with an isotropic conductive adhesive (ICA) consisting of Ag and epoxy: Sn–Pb plated components mounted on printed boards and tensile bars consisting of Sn plated Ni and Cu joined under various joining pressures. After 150 °C–100 h, shear strength degraded to 72% and Sn in the plating diffused slightly to the Ag fillers in the ICA for the component mounting. High joining pressure increased the initial tensile strength and volume percentage of Ag fillers in the ICA. After 150 °C–100 h, tensile strengths for all joining pressures degraded on average to 36% of the initial strengths. In the case of low joining pressures, an Ag–Sn intermetallic was formed at only the Ag–Sn contact points of the Sn/ICA interface; leading to reproducibility of the component mounting. The difference of degradation ratios between the mounted components and tensile bars could be explained by the offset of the Sn–Pb area at the component/ICA interface.     

Modeling early failure in integrated circuit interconnect

A model based on the random electron–atom scattering is developed to characterize the effects of defects and grain sizes on electromigration caused failure in confined sub-micron metal interconnect lines. Our study shows that lines at sub-micron widths with a more uniform microstructure exhibit a greater consistency in time to failure. Taking mean time to failure and dispersion in time to failure as criteria, the simulator predicts that grain sizes in the 0.03–0.05 μm range are optimal for 0.125 μm wide Al alloy lines. We also argue that the early failure mechanism associated with the missing metal defects is eliminated by using a homogeneous, fine-grained material. The uniformity of the structure results in a mono-modal failure distribution and contributes to increasing the built-in reliability of the interconnect lines.     

Modeling the silicon–hafnia interface

We report first-principles calculations of the structure and electronic properties of several different silicon–hafnia interfaces. The structures have been obtained by growing HfO_x layers of different stoichiometry on Si(1 0 0) and by repeated annealing of the system using molecular dynamics. The interfaces are characterised via their electronic and geometric properties. Moreover, electronic transport through the interfaces has been calculated using finite-element-based Green's function methods. We find that oxygen always diffuses towards the interface to form a silicon dioxide layer. This results in the formation of dangling Hf bonds in the oxide, saturated by either Hf diffusion or formation of Hf–Si bonds. The generally poor performance of the interfaces suggests that it is important to stabilise the system with respect to oxygen lattice diffusion.     

Application of time resolved emission techniques within the failure analysis flow

As the number of transistors and metal layers increases, traditional fault isolation techniques are less successful in exactly isolating the failing net or transistor to allow physical failure analysis. One tool to minimize the gap between global fault isolation – by means of emission microscopy or laser based techniques (TIVA, OBIRCH) – and physical root cause analysis is Time Resolved Emission (TRE). This paper presents two case studies illustrating the application of TRE within the failure analysis flow to generate a reasonable physical failure hypothesis.     

Adhesion study of tetra methyl cyclo tetra siloxanes (TMCTS) and tri methyl silane (3MS)-based low-k films

Chemical vapor deposited (CVD) low-k films using tri methyl silane (3MS) precursors and tetra methyl cyclo tetra siloxanes (TMCTS) precursors were studied. Films were deposited by means of four processes, namely, O₂, O₂ + He process and CO₂, CO₂ + O₂ process for 3MS and TMCTS precursors, respectively. Interfacial adhesion energy (G_c), of low-k/Si samples, as measured by a 4-point bending test displayed a linear relationship with film hardness and modulus. Fractography studies indicated two possible failure modes with the primary interface of delamination being either at low-k/Si or Si/epoxy interface. In the former, once delamination initiated at the low-k/Si interface, secondary delamination at the Si/epoxy and epoxy/low-k interfaces was also observed. Films with low hardness (<5 GPa) displayed a low G_c (<10 J/m²) with an adhesive separation of Si/epoxy, epoxy/low-k, and low-k/Si interfaces. Whereas, films of high hardness (>5 GPa) displayed interfacial energies in excess of 10 J/m² with separation of Si/epoxy and epoxy/low-k interfaces, thus indicating excellent adhesion between the Si and low-k films. Films with high hardness have less carbon in the system causing it to be more “silicon dioxide” like and exhibiting better adhesion with the Si substrate.     

Origin of electron mobility enhancement in (1 1 1)-oriented InGaAs channel metal-insulator-semiconductor field-effect-transistors

Electrical and physical characteristics of the Al₂O₃/InGaAs interfaces with (1 1 1)A and (1 0 0) orientations were investigated in an attempt to understand the origin of electron mobility enhancement in the (1 1 1)A-channel metal-insulator-semiconductor field-effect-transistor. The (1 1 1)A interface has less As atoms of high oxidation states as probed by X-ray photoelectron spectroscopy. The electrical measurements showed that energy distribution of the interface traps for the (1 1 1)A interface is shifted toward the conduction band as compared to that for the (1 0 0) interface. Laterally-compressed cross-section transmission electron microscopy images showed that the characteristic lengths of the interface roughness are different between the (1 1 1)A and (1 0 0) interfaces. The contributions of the Coulomb and roughness scattering mechanisms are discussed based on the experimental results.     

基于CZM法QFN器件热冲击载荷下的界面脱层研究

引进内聚力模型（CZM）法,利于有限元软件MSC．Marc对热冲击栽荷下QFN器件各材料界面之间的脱层开裂情况进行了研究。并分析了不同Diepad厚度对器件脱层失效的影响。结果表明：脱层开裂均发生在各个界面的两端,并逐渐沿着界面向里扩展,Diepad与芯片粘结剂之间的界面最容易发生脱层开裂;Diepad厚度对器件的脱层开裂影响较大,增加Diepad厚度能较大幅度的提高QFN器件的抗脱层开裂能力。     

Prediction of interfacial delamination in stacked IC structures using combined experimental and simulation methods

Interfacial delamination is an often-observed failure mode in multi-layered IC packaging structures, which will not only influence the yield of wafer processes, but also have direct impact on the packaging reliability. The difference in coefficient of thermal expansion, together with thermal and thermal–mechanical loading are the main driving forces for interfacial delamination. First of all, this type of delamination is considered as a mixed mode of failure at the material interfaces. Hence, at least two stress components are needed to predict its occurrence. However, due to the singular stress field at the interface, one could hardly obtain the correct stresses at the interface. Therefore, a combined experimental–numerical method is used to investigate the initiation and propagation of the interface delamination. The purpose of the experimental shear and tensile tests is to measure the critical loads, at which delamination initiates. Then, a Finite Element (FE) model is constructed to convert the critical load into critical failure data for further numerical investigation. The FE model is so constructed that it reproduces the geometrical configurations of the tests. Due to the singular stress distribution at the interface, the calculated local stresses will be both mesh and residual-stiffness dependent. The influences of the FE parameters on the interface stresses are studied. After that, a progressive failure approach is, in combination with a group of failure criteria and the estimated local critical stresses, applied to predict the initiation and propagation of the delamination between epoxy mould compound and the passivation layer in the Integrated Circuit (IC) for three different package structures. The present method and the obtained results are valuable to determine design rules for IC packaging structures.     

Bond strain and defects at interfaces in high-k gate stacks

The performance and reliability of aggressively-scaled field effect transistors are determined in large part by electronically-active defects and defect precursors at the Si–SiO₂, and internal SiO₂–high-k dielectric interfaces. A crucial aspect of reducing interfacial defects and defect precursors is associated with bond strain-driven bonding interfacial self-organizations that take place during high temperature annealing in inert ambients. The interfacial self-organizations, and intrinsic interface defects are addressed through an extension of bond constraint theory from bulk glasses to interfaces between non-crystalline SiO₂, and (i) crystalline Si, and (ii) non-crystalline and crystalline alternative gate dielectric materials.     

Structural and optical properties of AlN/Si system

Aluminum nitride (AlN) is a wide band gap III–V semiconductor material which is often used for optical applications. Thin films of aluminum nitride were deposited by ion beam sputtering in an Ar–N₂ atmosphere on Si (1 0 0). For film preparation, the N₂ flow was kept at 5 sccm and the ratio of N₂ and Ar was 4:1. The films have been characterized by Grazing Incidence X-ray Diffraction (GIXRD), X-ray Reflectometry (XRR), Atomic Force Microscopy (AFM) and optical spectroscopy. GIXRD shows that the structure of the as-deposited sample of AlN is hexagonal. It is observed that neither the ion-beam-induced dissociation of the nitride film nor the enhanced nitrogen diffusion across the interface takes place after Au ion irradiation. XRR was used to determine the thickness of the films. The reflectance of the irradiated films increases in the range 200–280 nm. UV–vis spectra were taken in Kubelka Munk (KM) units for as-deposited and irradiated samples. The band gap was calculated for both types of samples, which shows that the band gap of irradiated films of aluminum nitride decreases due to the increase in metal content at the surface. AFM confirms that the roughness of aluminum nitride increases by irradiation.     

100 μm Pitch flip chip on foil assemblies with adhesive interconnections

In this work the reliability of flip-chip-on-flexible substrate packages with electrically conductive adhesive as first level interconnections is studied. The pitch of the interconnections ranges from 300 to 100 μm with prospects to smaller pitches. The Physics-of-Failure approach is used to determine the nature of such an interconnection and hence the factors that will influence the performance of these packages. The results indicate that moisture is a more important stress factor than temperature. In particular cyclic exposure to high and low moisture levels may lead to degradation of the electrical interconnection. As failure mechanism, reduction of the compressive force that holds the interconnection together is proposed. Further, the proper combination of materials––based on their in- and out-diffusion rates for water––determines the resistance of the packages to reflow-soldering.     

Molecular simulation on the material/interfacial strength of the low-dielectric materials

In this paper, the material stiffness of amorphous/porous low-k material and interfacial strength between amorphous silica and low-k have been simulated by the molecular dynamics (MD) methods. Due to the low stiffness of the low-k material, the interfaces which include this material are critical for the most delamination and reliability issues around the IC back-end structure. MD simulation technique is applied to elucidate the crack/delamination mechanism at these critical interfaces. However, due to the amorphous nature of the low-k material (e.g., SiOC:H), the atomic modeling technique of the amorphous/porous silica is first established. Through the experimental validation, the accuracy of this amorphous modeling technique is obtained, and the results show that this algorithm can represent the trend of the mechanical stiffness change due to different chemical composition of low-k material. A novel interfacial modeling technique, which model the status of chemical bonds at interface during the delamination loading, is developed. Afterward, the simulation of the mechanical strength of the amorphous silica/SiOC:H interface, is implemented. The simulation depicts that the existence of the strong Si–O covalent bond will significantly enhance the adhesive strength of the interface. Instead of the covalent bond at interface, the simulation results also reveal the multiple atomic scaled crack path within the material during the interfacial delamination. Hence, improving the material stiffness of the soft low-k material and preventing the pore at interface can increase the adhesive strength of the silica/low-k interfacial system.     

Templates for LaAlO3 epitaxy on silicon

As direct epitaxy of crystalline LaAlO₃ on silicon has not been realized yet, we investigated the use of a template between the high-κ and the substrate. We performed calculations in the Density Functional Theory framework for two possible templates: a Sr_0.5O monolayer and a 0.5 nm thick γ-Al₂O₃(0 0 1) layer. We firstly found that in the Sr_0.5O monolayer case, care must be taken for the LaAlO₃ starting sequence in order to expect good band offsets with silicon. In the γ-Al₂O₃ case, a more complex engineering of the interface is needed. Nonetheless, we found stable interfaces and a surface reconstruction in agreement with experimental observations. Moreover, these interfaces exhibit insulating properties and insight calculations for a Si–γ-Al₂O₃–LaAlO₃ superstructure lead us to a 1.9 eV conduction band offset.     

Interface delamination in plastic IC packages induced by thermal loading and vapor pressure - a micromechanics model

A micromechanics model and an associated computational scheme are proposed to study interface delamination in plastic integrated circuit (IC) packages induced by thermal loading and vapor pressure. The die and die-pad are taken as elastic materials, while the die-attach and molding compound are taken as elasto-visco-plastic materials. The interface between molding compound and the die-pad is characterized by a cohesive law. The key parameters of this law are the interface strength and interface energy. The vapor-induced pressure along the interface is incorporated by way of a micromechanics model. Parametric studies are conducted to understand interface properties and vapor pressure effects on interface delamination. Under purely thermal loading, both weak and strong interfaces are highly resistant to interface failure. However, the combined effects of thermal loading and vapor pressure arising from moisture trapped within the interface can cause total delamination at the interface. Once delamination has initiated at a weak interface, no significant increase in thermal loading and vapor pressure is required for the delaminated zone to grow to a macro-crack and subsequently to catastrophic failure referred to as popcorn cracking. The critical factors controlling the occurrence of popcorn cracking are the interface adhesion strength and interface vapor pressure.