M. van Soestbergen

Modeling ion transport through molding compounds and its relation to product reliability

Nowadays highly filled epoxy molding compounds are used as material for encapsulation of microelectronic devices. These molding compounds always contain a very low concentration of ionic impurity. In addition ionic species can originate from chemical processes inside the encapsulation. In the presence of an electrical field ions will migrate through the encapsulation, which might eventually result in failures, such as corrosion or dendrite growth. Although these failures are well-known they still lack a knowledge based description of their failure mechanism. Therefore a model describing the transport of ions might be useful to give more insight into these failures. However, calculating the transport of ions is numerically very challenging since it requires a multi-physics model on a multi-time and length-scale. Besides, the notion of a maximum ion concentration due to volume constraints opposed by the molding compound increases the complexity of the mathematical framework even further and results in a model that is very difficult to solve. In this paper we discuss several simplified models for the transport of ionic species that might be used to model their corresponding failure mechanisms. Further, we show the conductivity of molding compounds as a function of temperature and discuss how this accelerates the transport of ions.

Electrical characterization of plastic encapsulations using an alternative gate leakage test method

The supply current of plastic encapsulated microelectronic devices in the presence of a high potential source can increase abnormally due to parasitic gate leakage. According to reliability qualification standards, stress during a parasitic gate leakage test is applied by a corona discharge at a thin tungsten needle placed a few centimeters above the devices under test. The gate leakage sensitivity factor obtained from this test lacks any physical basis and is therefore not believed to be useful. Here we show that this sensitivity factor can be replaced by a physical model for charge transport through the encapsulation material. The model is used to explain why devices encapsulated by a molding compound with a low volume resistivity of 6 times 1011 Ohm-cm, at high temperature, 150degC, are more prone to fail the test on an increased current, compared to devices encapsulated by a compound having a high resistivity of 4 times 1013 Ohmtimescm at the same temperature. Furthermore, we discuss an alternative test setup where the potential difference between two parallel electrodes sandwiching the devices is used as the source of stress. It is suggested in literature that this setup yields identical results as the current setup. However, using both setups on the same product did not result in an equal outcome, which indicates that both tests do not trigger the same failure mechanism to the same extent.

Measuring the through-plane elastic modulus of thin polymer films in situ

Due to the ongoing increase of the transistor density on a chip, industry has replaced the silicon oxide dielectric layers, traditionally used in the back-end interconnect stack, by low-K polymer films with a thickness down to several hundred nanometers. The use of these polymer dielectric films has introduced new failure modes. To have a better understanding of these failures, knowledge of the mechanical properties is necessary. Due to surface effects, the material properties of thin films may differ in the in-plane and trough-plane direction. Most techniques available for measuring these properties are only capable of obtaining the in-plane modulus. To have an in situ measurement of the through-plane modulus, a parallel plate capacitor (PPC) under hydrostatic pressure is used in combination with an interdigitated electrode (IDE) to capture the change in dielectric constant. Since it is believed to be mechanical isotropic, a benzocyclobutene (BCB) film is used to provide a reference measurement. The through-plane elastic modulus and change in permittivity for a 1 lm thick film sandwiched by two aluminum electrodes on a silicon wafer are reported. Two circular PPCs and four IDEs were tested at a pressure of 0, 5, 7.5 and 10 MPa. An initial relative dielectric constant of the film of 2.66 ± 0.05 was obtained. This yields a change in constant equal to 1.241 · 10 � 4 ± 2.1 · 10 � 5 per MPa pressure at room temperature. The through-plane modulus showed a linear elastic behavior equal to 4.73 ± 0.46, 4.11 ± 0.39 and 3.64 ± 0.31 GPa for 20� ,5 0� and 75 � C, respectively. The modulus at room temperature is in good agreement with the values found in literature. � 2007 Elsevier Ltd. All rights reserved.

Measuring In-Thickness Mechanical Properties of Sub Micron Polymer Dielectric Films

The ongoing miniaturization of microelectronics has led to low-K polymer dielectric films with a thickness of several tens of nanometers. These thin polymer films generally show time dependent material properties, that are different in lateral directions and in-thickness direction. Most techniques available for measuring the mechanical properties of thin films are only capable of obtaining the in-plane modulus. To have an in-situ measurement of the in-thickness viscoelastic modulus, a parallel plate capacitor under hydrostatic pressure is used. An interdigitated electrode is used to capture the change in dielectric constant under pressure. As a first estimation, a BCB (benzocyclobutene) film was used. The in-thickness elastic modulus and change in permittivity for a 1.3 mum thick BCB (Cyclotenetrade 4022) film sandwiched between two alumina electrodes on a silicon wafer are reported to be 4.76plusmn0.42, 3.81plusmn0.26 and 3.16plusmn0.15 GPa for 20deg;, 50 deg; and 70deg; C respectively

Development of an elaborate simulation tool for electrochemical failures in microelectronic packages

The ever increasing complexity and function integration of microelectronic products in combination with the decreasing design margins, the decreasing time-to-market, and ever increasing gap between technology advance and fundamental knowledge opposes a severe challenge for the microelectronics industry to meet the quality, robustness, and reliability requirements of their products. In order to meet these requirements, the reliability of microelectronic products is traditionally assessed using tests at elevated external stimuli, such as temperature, ambient humidity and applied voltage. Recently, the perspective of reliability assessments has shifted towards an approach referred to as knowledge-based qualification, where costumer requirements and operational conditions are translated to stress tests conditions using computer simulations for failure mechanisms and reliability data from corresponding products under comparable conditions. While in the past years simulations tools to predict water absorption and (thermo-)mechanical stresses in packages have been developed, there are no generally accepted simulation tools to predict the effect of electrochemical processes on the performance of products. However, simulation tools that are capable of modelling the electrochemical processes at the interior of packages are indispensable instruments to rigorously study failures due to, e.g., the corrosion of bondpads or the growth of dendritic deposits at metallizations. In this talk a model for the transport of ionic species coupled to a relation for the electrochemical charge transfer rate at electrode is presented. We show results of this model for realistic two-dimensional structures and compare the results with experimental data. We will show that the experimental and model results agree well each other. Additionally, we will show that the model we present can be unequivocally incorporated in the current thermo-mechanical simulation models. Finally, we will address future trends and discuss the perspectives of elaborate simulation tools for the prediction of microelectronics reliability.

Modeling electrochemical migration through plastic microelectronics encapsulations

Plastic encapsulations will absorb moisture in humid environments due to their hydrophilic nature, this in combination with the inherent ionic contamination of the plastic will result in an electrolyte. This electrolyte might pose several reliability issues for the package and the encapsulated microelectronic circuit, such as electrochemical migration and bond pad corrosion. The corresponding failure mechanisms are associated with ionic currents through the package and electrochemical processes at e.g. the leads or bond wires. In this paper we present a generic mathematical framework for modeling ionic currents and electrochemical processes. We will apply this framework to electrochemical migration of metal between the leads of a plastic encapsulation, and show results for the transient formation of migration fluxes through the plastic encapsulation.

Mechanical Properties of Polymer Dielectric Films at ElevatedTemperatures

The miniaturization of microelectronics has led to the use of polymer dielectric films with a thickness less than a micrometer. The use of polymer dielectric films has introduced new failure modes. To have a better understanding of these failures, knowledge of the mechanical properties is necessary. The through-plane elastic modulus and change in permittivity for a 1 mum thick Cyclotenetrade 4022 film sandwiched by two aluminium electrodes on a silicon wafer are reported. Two circular parallel plate capacitors and four interdigitated electrodes where tested at a pressure of 0, 5, 7.5 and 10 MPa. An initial relative dielectric constant of the film of 2.66plusmn0.05 was obtained. This yields a change in constant equal to 1.241middot10-4plusmn2.1middot10-5 per MPa pressure at room temperature. The through-plane modulus showed a linear elastic behaviour equal to 4.73plusmn0.46, 4.11plusmn0.39, 3.64plusmn0.31 and 3.56plusmn0.32 GPa for 20deg, 50deg, 70deg and 100deg C respectively. The modulus at room temperature is in good agreement with the values found in literature