Thomas Detzel

Influence of Buffer Carbon Doping on Pulse and AC Behavior of Insulated-Gate Field-Plated Power AlGaN/GaN HEMTs

Pulse behavior of insulated-gate double-field-plate power AlGaN/GaN HEMTs with C-doped buffers showing small current-collapse effects and dynamic RDS,on increase can accurately be reproduced by numerical device simulations that assume the CN-CGa autocompensation model as carbon doping mechanism. Current-collapse effects much larger than experimentally observed are instead predicted by simulations if C doping is accounted by dominant acceptor states. This suggests that buffer growth conditions favoring CN-CGa autocompensation can allow for the fabrication of power AlGaN/GaN HEMTs with reduced current-collapse effects. The drain-source capacitance of these devices is found to be a sensitive function of the C doping model, suggesting that its monitoring can be adopted as a fast technique to assess buffer compensation properties.

Stress, Sheet Resistance, and Microstructure Evolution of Electroplated Cu Films During Self-Annealing

Electroplated copper films are known to change their microstructure due to the self-annealing effect. The self-annealing effect of electroplated copper films was investigated by measuring the time dependence of the film stress and sheet resistance for different layer thicknesses between 1.5 and 20 ¿m. While the sheet resistance was found to decrease as time elapsed, a size-dependent change in film stress was observed. Films with the thickness of 5 ¿m and below decrease in stress, while thicker films initially reveal an increase in film stress followed by a stress relaxation at a later stage. This behavior is explained by the superposition of grain growth and grain-size-dependent yielding.

Novel temperature dependent tensile test of freestanding copper thin film structures

The temperature dependent mechanical properties of the metallization of electronic power devices are studied in tensile tests on micron-sized freestanding copper beams at temperatures up to 400 °C. The experiments are performed in situ in a scanning electron microscope. This allows studying the micromechanical processes during the deformation and failure of the sample at different temperatures.

Hydrogen-related influence of the metallization stack on characteristics and reliability of a trench gate oxide

Abstract We discuss influences of the metallization / passivation stack on the 30 nm thick gate oxide of a trench DMOS. A variation in the metallization stack directly influences the gate oxide lifetime, but also the transfer characteristics of the device and the interface trap density revealed by charge-pumping measurements. Surprisingly, a better anneal of the Si-SiO2 interface and the bulk-oxide, resulting in a smaller measured interface-trap density on virgin wafers, implies a reduced GOX reliability. These effects are attributed to the release of reactive hydrogen from PECVD deposited silicon-nitride layers.

Threshold voltage instabilities in D-mode GaN HEMTs for power switching applications

Threshold voltage instabilities observed in GaN HEMTs designed for power switching applications when submitted to either DC or pulsed testing are here presented and interpreted. Main results can be summarized as follows: i) two acceptor trap levels, characterized by two well distinct time constants, are present in the UID GaN channel and C-doped GaN buffer respectively and behave as electron and hole traps respectively; ii) the trapped charge is modulated by the high voltage biasing of the gate and drain terminals; iii) when empty, channel electron traps induce a negative threshold-voltage shift, while buffer hole traps induce a positive threshold-voltage shift; iv) when the device is pulsed from off- to on-state conditions, trap charge/discharge dynamics induces negative and positive threshold-voltage instabilities over distinct time scales.

Apparatus for measuring local stress of metallic films, using an array of parallel laser beams during rapid thermal processing

The novel apparatus described here was developed to investigate the thermo-mechanical behavior of metallic films on a substrate by acquiring the wafer curvature. It comprises an optical module producing and measuring an array of parallel laser beams, a high resolution scanning stage, a rapid thermal processing (RTP) chamber and several accessorial gas control modules. Unlike most traditional systems which only calculate the average wafer curvature, this system has the capability to measure the curvature locally in 30 ms. Consequently, the real-time development of biaxial stress involved in thin films can be fully captured during any thermal treatments such as temperature cycling or annealing processes. In addition, the multiple parallel laser beam technique cancels electrical, vibrational and other random noise sources that would otherwise make an in situ measurement very difficult. Furthermore, other advanced features such as the in situ acid treatment and active cooling extend the experimental conditions to provide new insights into thin film properties and material behavior.

Prediction of wafer bow through thermomechanical simulation of patterned hard coated copper films

Due to the large difference in the coefficients of thermal expansion of the materials used in advanced semiconductor manufacturing, the fabrication process of semiconductor chips leads to a wafer bow. In addition, the layers may pass through material phase changes, which generate an unwanted film stress, making the large accumulated wafer bow very difficult to handle. We have compared the wafer bow calculated from a thermomechanical finite element simulation with experiments, and developed new material models for copper and protecting layers. We have also shown how the patterning of the thin films reduces the overall wafer bow. The wafer bow has a linear dependency with respect to the coverage percentage, especially over the low coverage range. This study introduces useful hints and predicts a reduced wafer bow, which is already experimentally proved in the manufacturing process of a modern power semiconductor technology.

GaN virtual prototyping: From traps modeling to system-level cascode optimization

The present paper focuses on the system-level optimization of GaN technology for high voltage applications. We will show that a key requirement for the future success of the GaN technology is the full system-optimization achieved by a simultaneous optimization of technology, packaging and applications. We will also show that Virtual Prototyping (VP) becomes, in GaN technology, a fundamental tool that allows not only to have a fundamental understanding of the device properties but more importantly it allows to strongly link device optimization, technology and system-level performance. In the present paper we will describe our view on the system-level optimization of high voltage GaN technology and present detailed simulations and comparison with experiments for both normally on isolated GaN transistors and cascoded GaN devices in real switching applications.

Modeling of multi-temperature-cycle wafer curvature

A multi-temperature-cycle wafer curvature experiment was performed in order to study the temperature dependent material properties of copper films. The specimen consists of 20µm copper film electrochemically deposited on a silicon wafer. Focused Ion Beam and Electron Back-Scatter Diffraction were used to characterize the microstructure of copper films. Wafer curvature (wafer bow) measurements were performed using a multi-laser-beam wafer curvature system, in a well controlled thermal chamber and at high-speed data acquisition. A very pronounced hysteresis of the curvature-temperature curves was observed. An empirical model was developed which specifically describes the transition from elastic to plastic behavior at low strain. Following the approach of Voce and Kocks, the stress-strain curve of polycrystalline copper is modeled, where the saturation stress is considered as model parameter, and the temperature dependence of the plastic behavior is described by the activation free enthalpy of steady state creep. The model describes biaxial stress states as they occur in a wafer; the initial slope of the stress-strain curve is related to the elastic modulus of the material, and the model is applied to cyclic stress curves. It is demonstrated that in the temperature range from −50°C to 500°C the model can accurately describe the results of the multi-temperature-cycle (Multi-TC) wafer curvature experiment.

Modeling of stress evolution of electroplated Cu films during self-annealing

Electroplated Cu films are known to change their microstructure at room temperature due to the self-annealing effect. This recrystallization process results in a film-thickness-dependent stress evolution. Films with the thickness of 5µm and below decrease in stress with time, while thicker films reveal initially an increase in film stress followed by a stress relaxation at a later stage. This behavior is explained by the superposition of grain growth and grain size dependent yielding. Existing models have been used and improved to describe the mechanisms related to stress evolution. In general, the models proposed in this study provide a satisfactory description of the stress evolution of electroplated Cu films and the simulated results show good agreement with the experimental data. This gives the possibility to evaluate and predict mechanical behavior of electroplated Cu films at room temperature.