Mikael Broas

Structural and chemical analysis of annealed plasma-enhanced atomic layer deposition aluminum nitride films

Plasma-enhanced atomic layer deposition was utilized to grow aluminum nitride (AlN) films on Si from trimethylaluminum and N2:H2 plasma at 200 °C. Thermal treatments were then applied on the films which caused changes in their chemical composition and nanostructure. These changes were observed to manifest in the refractive indices and densities of the films. The AlN films were identified to contain light element impurities, namely, H, C, and excess N due to nonideal precursor reactions. Oxygen contamination was also identified in the films. Many of the embedded impurities became volatile in the elevated annealing temperatures. Most notably, high amounts of H were observed to desorb from the AlN films. Furthermore, dinitrogen triple bonds were identified with infrared spectroscopy in the films. The triple bonds broke after annealing at 1000 °C for 1 h which likely caused enhanced hydrolysis of the films. The nanostructure of the films was identified to be amorphous in the as-deposited state and to become n...

Reliability assessment of a MEMS microphone under mixed flowing gas environment and shock impact loading

In this work the reliability of a Micro-Electro-Mechanical Systems (MEMS) microphone is studied through two accelerated life tests, mixed flowing gas (MFG) testing and shock impact testing. The objective is to identify the associated failure mechanisms and improve the reliability of MEMS devices. Failure analyses are carried out by using various tools, such as optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS). Finite element analysis is also conducted to study the complex contact behaviors among the MEMS elements during shock impact testing. The predicted failure sites are in agreement with the experimental findings.

Correlation of gate leakage and local strain distribution in GaN/AlGaN HEMT structures

Abstract GaN/AlGaN HEMT structures are observed to undergo a reversible, drastic change in the leakage current when covered with an additional polymer passivation layer. The polymer layer induces a stress on the HEMT structures, which initiates material migration processes and the formation of structural defects, influencing the electrical performance. Local strain measurements were performed in the semiconductor, at the critical HEMT gate electrode, to evaluate the impact of the stress on the Schottky gates. The strain distributions in the structures were measured with nanobeam electron diffraction from electron-transparent samples at cross sections and longitudinal sections at the positions of high leakage currents. A variation of the strain distribution underneath the gate electrode was detected in a cross-sectional sample. On the contrary, only minor differences in the strain values were measured in the longitudinal sections at different photoemission sites. Finally, localized metal interdiffusion was detected at the sites with the highest photoemission intensities.

Reliability assessment of MEMS devices — A case study of a 3 axis gyroscope

Recent technological breakthroughs in Micro-Electro-Mechanical Systems (MEMS) technologies have enabled significant cost reductions of MEMS gyroscopes and they are being increasingly employed in new application areas such as portable consumer electronics. Reliability assessment of MEMS assemblies is, however, more challenging than that of conventional IC assemblies: reliability characterization of MEMS must be made while the devices are in a functional state and the large number of small structural features requires new approaches for their physical failure analyses. In this paper we explore these challenges with a case study of MEMS gyroscopes, which are increasingly being employed in handheld consumer as well as automotive applications. Reliability of the gyroscopes will be characterized under elevated temperature and humidity (85°C/90%RH), and under high-G shock impact loading (up to 35 kG). The board assemblies were characterized for (i) maximum deceleration tolerance and (ii) fatigue lifetime under lower shock impact loads. Under the temperature-humidity characterization failures associated with absorption of moisture in the polymeric materials of the MEMS package showed early failures in 37 % of the samples while remainder of the samples survived 150 days of exposure. The shock impact characterization showed that the mean lifetime of the gyroscope assemblies depends significantly on the orientation of the impact load. Furthermore, package failures were produced at much higher decelerations (above 8 kG) than electrical failures of the device (at about 4 kG). Finite element model was established to predict the failure sites and the model correlated well with experimental observations. Several internal failure modes were e identified: fractured comb arms, fractured comb fingers, stuck MEMS elements, and chipped corners and edges of the elements caused by internal collisions. Transient failures were commonly observed under all testing conditions.

Blistering mechanisms of atomic-layer-deposited AlN and Al2O3 films

Blistering of protective, structural, and functional coatings is a reliability risk pestering films ranging from elemental to ceramic ones. The driving force behind blistering comes from either excess hydrogen at the film-substrate interface or stress-driven buckling. Contrary to the stress-driven mechanism, the hydrogen-initiated one is poorly understood. Recently, it was shown that in the bulk Al-Al2O3 system, the blistering is preceded by the formation of nano-sized cavities on the substrate. The stress- and hydrogen-driven mechanisms in atomic-layer-deposited (ALD) films are explored here. We clarify issues in the hydrogen-related mechanism via high-resolution microscopy and show that at least two distinct mechanisms can cause blistering in ALD films.

Chemically Stable Atomic-Layer-Deposited Al2O3 Films for Processability

Atomic-layer-deposited alumina (ALD Al2O3) can be utilized for passivation, structural, and functional purposes in electronics. In all cases, the deposited film is usually expected to maintain chemical stability over the lifetime of the device or during processing. However, as-deposited ALD Al2O3 is typically amorphous with poor resistance to chemical attack by aggressive solutions employed in electronics manufacturing. Therefore, such films may not be suitable for further processing as solvent treatments could weaken the protective barrier properties of the film or dissolved material could contaminate the solvent baths, which can cause cross-contamination of a production line used to manufacture different products. On the contrary, heat-treated, crystalline ALD Al2O3 has shown resistance to deterioration in solutions, such as standard clean (SC) 1 and 2. In this study, ALD Al2O3 was deposited from four different precursor combinations and subsequently annealed either at 600, 800, or 1000 °C for 1 h. Crystalline Al2O3 was achieved after the 800 and 1000 °C heat treatments. The crystalline films showed apparent stability in SC-1 and HF solutions. However, ellipsometry and electron microscopy showed that a prolonged exposure (60 min) to SC-1 and HF had induced a decrease in the refractive index and nanocracks in the films annealed at 800 °C. The degradation mechanism of the unstable crystalline film and the microstructure of the film, fully stable in SC-1 and with minor reaction with HF, were studied with transmission electron microscopy. Although both crystallized films had the same alumina transition phase, the film annealed at 800 °C in N2, with a less developed microstructure such as embedded amorphous regions and an uneven interfacial reaction layer, deteriorates at the amorphous regions and at the substrate–film interface. On the contrary, the stable film annealed at 1000 °C in N2 had considerably less embedded amorphous regions and a uniform Al–O–Si interfacial layer.

Atomic layer deposition of AlN from AlCl3 using NH3 and Ar/NH3 plasma

The atomic layer deposition (ALD) of AlN from AlCl3 was investigated using a thermal process with NH3 and a plasma-enhanced (PE)ALD process with Ar/NH3 plasma. The growth was limited in the thermal process by the low reactivity of NH3, and impractically long pulses were required to reach saturation. Despite the plasma activation, the growth per cycle in the PEALD process was lower than that in the thermal process (0.4 A vs 0.7 A). However, the plasma process resulted in a lower concentration of impurities in the films compared to the thermal process. Both the thermal and plasma processes yielded crystalline films; however, the degree of crystallinity was higher in the plasma process. The films had a preferential orientation of the hexagonal AlN [002] direction normal to the silicon (100) wafer surface. With the plasma process, film stress control was possible and tensile, compressive, or zero stress films were obtained by simply adjusting the plasma time.

Galvanic corrosion of structural non-stoichiometric silicon nitride thin films and its implications on reliability of microelectromechanical devices

This paper describes a reliability assessment and failure analysis of a poly-Si/non-stoichiometric silicon nitride thin film composite structure. A set of poly-Si/SiNx thin film structures were exposed to a mixed flowing gas (MFG) environment, which simulates outdoor environments, for 90 days, and an elevated temperature and humidity (85 °C/95% R.H.) test for 140 days. The mechanical integrity of the thin films was observed to degrade during exposure to the chemically reactive atmospheres. The degree of degradation was analyzed with nanoindentation tests. Statistical analysis of the forces required to initiate a fracture in the thin films indicated degradation due to the exposure to the MFG environment in the SiNx part of the films. Scanning electron microscopy revealed a porous-like reaction layer on top of SiNx. The morphology of the reaction layer resembled that of galvanically corroded poly-Si. Transmission electron microscopy further clarified the microstructure of the reaction layer which had a comp...

Methods for reliability assessment of MEMS devices — Case studies of a MEMS microphone and a 3-axis MEMS gyroscope

Despite the fact that MEMS devices have become common in many electronic products, methods for their reliability assessment are still undeveloped. One significant difference to the reliability assessment of conventional electronic component boards is that MEMS devices require a stimulus or means to measure an output in order to monitor their health while the MEMS assemblies are exposed to loadings. Challenges are faced particularly when the instruments to perform the stimulus or measurements will also be exposed to the loading condition. Furthermore, for MEMS devices simple functional/not-functional failure criteria are often not sufficient and health monitoring during loading must cover several characteristics, each of which have their own application specific acceptance limits. Solutions to these challenges are discussed with the help of two case studies: i) a MEMS microphone and ii) a 3-axis MEMS gyroscope. The number of different failure modes in MEMS devices is also large and some of the failures are transient, such as those caused by temporary sticking of moving parts. The small length scale and complexity of the MEMS structures together with the fact that many of the failure modes are transient make the employment of new methods for their failure analyses necessary. Methods of failure analyses, the role of Finite Element Modeling in failure analyses, and some typical failure modes are also discussed.

Galvanic corrosion of silicon-based thin films: A case study of a MEMS microphone

MEMS microphones are widely utilized in commercial applications such as mobile phones and laptops. Microphones cannot be sealed completely from the atmosphere since sensing requires propagation of sound waves to the sensing element. Therefore, the sensing element is vulnerable to airborne impurities and humidity. The prevalent material of choice for MEMS microphones is silicon, which is generally considered reliable and inert material in normal atmospheric conditions, although micron-scale silicon is known to be susceptible to delayed fracture failures under cyclic stresses. Especially humidity has been shown to affect its fatigue properties. Furthermore, the fabrication history of the device can affect the mechanical performance of the devices significantly. Hence, there exist large discrepancy for the fatigue limit and fracture strength of micron-scale silicon in the literature. In addition, there is little information on how harsh environments can affect the reliability of silicon-based thin films in MEMS applications. Therefore, harsh environmental tests were conducted on MEMS microphones. The silicon-based sensing elements of the microphones were observed to degrade during the testing. Failure analysis was conducted employing microscopy and chemical analysis techniques. The degree of degradation was evaluated qualitatively with nanoindentation, and finite element method was employed in explaining the influence of the observed degradation.