A. Witvrouw

KU Leuven | ku leuven · Department of Mechanical Engineering

In laser powder bed fusion (LPBF), defects such as pores or cracks can seriously affect the final part quality and lifetime. Keyhole porosity, being one type of porosity defects in LPBF, results from excessive energy density which may be due to changes in process parameters (namely, laser power and scan speed) and/or result from the part’s geometry...

X-ray computed tomography (X-CT) plays an important role in non-destructive quality inspection and process evaluation in metal additive manufacturing, as several types of defects such as keyhole and lack of fusion pores can be observed in these 3D images as local changes in material density. Segmentation of these defects often relies on threshold m...

In laser powder bed fusion (LPBF), defects such as pores or cracks can seriously affect the final part quality and lifetime. Keyhole porosity, being one type of porosity defects in LPBF, results from excessive energy density which may be due to changes in process parameters (laser power and scan speed) and/or result from the part’s geometry and/or...

Recently, the use of novel CuCr1 surface-modified powder for reliable laser powder-bed fusion (LPBF) manufacturing has been proposed, enabling a broader LPBF processing window and longer powder storage life. Nevertheless, virgin CuCr1 powder is also LPBF processable, on the condition that a high-energy density is employed. In this work, we compare...

Channels and bores in metal components produced by laser powder bed fusion (LPBF) are internal features that are typically affected by defects such as dross and sag formation, dimensional errors and global deformations in different proportions. Such deviations from the ideal geometry may strongly limit the functionality of the channels, but are dif...

Surface quality of parts produced by laser powder bed fusion (LPBF) is often insufficient for their final application. Up-facing inclined surfaces are among the major challenges, due to the combination of elevated edges (edge effect) and the staircase effect. This work presents a novel methodology to enhance the quality of inclined surfaces using a...

The typical surface topography of parts produced by additive manufacturing differs from those produced by conventional manufacturing. During the powder bed fusion process, spatter particles may eject from the melt pool due to multi-physics phenomena. These spatter particles can have an impact on the part quality as large spatter particles that land...

Surface quality and geometrical precision of parts manufactured by laser powder bed fusion (LPBF) are often insufficient for their final application. This work presents insights into the LPBF process and shows some possible means of improving the quality of up- and down-facing surfaces, as well as elevated edges of the part. The presented results w...

The typical surface topography of parts produced by additive manufacturing differs from those produced by conventional manufacturing. During the powder bed fusion process, spatter particles may eject from the melt pool due to multi-physics phenomena. These spatter particles can have an impact on the part quality as large spatter particles that land...

Numerous challenges of additive manufacturing (AM) are tackled in the European Horizon 2020 project PAM^2 by studying and linking every step of the AM process cycle. For example, PAM^2 researchers from the design, processing and application side have collaborated in this work to optimise the manufacturability of metal AM parts using an improved Top...

Improving the manufacturability of AM parts and ensuring that their desired dimensions, quality and surface finish are reached is a common goal in the PAM^2 project. To achieve that goal, our research span from the investigation of the raw material of Laser Powder Bed Fusion (LPBF) to the final assessment of the fabricated part. In this seminar, we...

Additive metal manufacturing processes, such as laser powder bed fusion, still show difficulties when producing overhang features or internal structures such as channels or bores. Channels are often mutilated by sag defects and dross formation at their upper part, when the channel-axis is close to parallel to the base plate and in the particular ca...

This work investigates the effect of the powder particle size distribution on the surface finish of Maraging 300 specimens, produced by the Laser Powder Bed Fusion (LPBF) process. Although it is recognized that the initial powder morphological characteristics play an important role on LPBF part density, mechanical properties and surface quality, th...

Laser powder bed fusion (LPBF) enables producing very complex geometries compared to conventional subtractive manufacturing techniques. However, the geometrical accuracy of LPBF surfaces remains limited and is often insufficient for applications in high-end sectors such as mould making, medical or aircraft industry. The most common imperfections fo...

The additive manufacturing (AM) of injection molding inserts has gained popularity during recent years primarily due to the reduced design-to-production time and form freedom offered by AM. In this paper, topology optimization (TO) is performed on a metallic mold insert which is to be produced by the Laser Powder Bed Fusion (LPBF) technique. First,...

Metal additive manufacturing, despite of offering unique capabilities e.g. unlimited design freedom, short manufacturing time, etc., suffers from raft of intrinsic defects. Porosity is of the defects which can badly deteriorate a part’s performance. In this respect, enabling one to observe and predict the porosity during this process is of high imp...

This work presents the redesign of an injection molding metal insert to be prototyped by the Laser Powder Bed Fusion (LPBF) process. The case study has been topology optimized to minimize its total mass while keeping the maximum von Mises stress and maximum displacement under load condition below chosen thresholds. Particular attention has been giv...

The occurrence of keyhole porosity in Ti-6Al-4V parts manufactured by selective laser melting (SLM) has not been systematically quantified using post-process characterizations. The X-ray computed tomography (X-CT) technique is capable to analyse the size, shape, location, and number of keyhole pores in SLM parts. In this study, keyhole porosity in...

The occurrence of keyhole porosity in selective laser melting (SLM) has so far not been quantified systematically using X-ray computed tomography (X-CT). In this study, keyhole porosity in selective laser molten Ti-6Al-4V grade 23 parts was analyzed using post-process characterizations. Single tracks were produced with optimum, high volumetric ener...

Purpose The cooling process of polymer components fabricated by selective laser sintering (SLS) plays a vital role in determining the crystallinity, density and the resultant properties of the produced parts. However, the control and optimization of the cooling process remains challenging. The purpose of this paper is to therefore investigate the...

PAM^2, which stands for Precision Additive Metal Manufacturing, is a European MSCA project in which 10 beneficiaries and 2 partners collaborate on improving the precision of metal Additive Manufacturing. Within this project, research is done for each process stage of AM, going from the design stage to modelling, fabricating, measuring and assessmen...

Despite its many benefits, Selective Laser Melting's (SLM) relatively low productivity compared to deposition-based additive manufacturing techniques is a major drawback. Increasing the laser beam diameter improves SLM's build rate, but causes loss of precision. The aim of this study is to investigate laser beam focus shift, or “defocus” using a dy...

This study demonstrates that, after adequate scaling error compensation, industrial µ-CT could be a viable metrological technique to derive, with one measurement, primary characteristics of Additive Manufacturing metal powders used in Powder Bed Fusion processes. For distribution and shape analyses, special care must be employed on the ISO value de...

The paper demonstrates the very first CMOS integrated monolithic MEMS (Micro Electro Mechanical System) accelerometer with SiGeMEMS technology on top of TSMC 0.18 µm CMOS technology. The developed SiGeMEMS technology shows the ability for integration above any standard foundry process. This has allowed us to build a surface micromachined accelerome...

In this chapter, an extensive analysis, based on FE (Finite Element) simulations, on the structural design and optimisation of poly-SiGe based piezoresistive pressure sensors is presented. The considered pressure sensors consist of a deformable poly-SiGe membrane, fully clamped at its edges, and four poly-SiGe piezoresistors placed on top following...

This chapter describes the fabrication and testing of an integrated poly-SiGe-based piezoresistive pressure sensor directly fabricated above 0.13 \(\upmu \)m Cu-backend CMOS technology. This represents not only the first integrated poly-SiGe pressure sensor directly fabricated above its readout circuit, but also the first time that a poly-SiGe MEMS...

In this chapter, the realized stand-alone poly-SiGe pressure sensors (both piezoresistive and capacitive) are tested and evaluated. The tested pressure sensors were fabricated following the process flow described in Chap. 4. The chapter begins with a description of the employed measurement setup. In this work, the fabricated sensors are tested in t...

This chapter describes in detail the pressure sensor fabrication process. the maximum processing temperature of the complete sensor, including the poly-SiGe piezoresistors, is kept below 460 \(^{\circ }\text{ C }\) to enable above-CMOS integration. The developed process allows for the simultaneous fabrication of both piezoresistive and capacitive p...

This chapter describes the sealing of polycrystalline SiGe (poly-SiGe) surface micromachined cavities for above-CMOS pressure sensor applications. Two different sealing techniques involving thin-film deposition are investigated: direct sealing and sealing by using an intermediate porous layer. The sealing materials studied include Si-oxide and alum...

The final goal of this thesis was the fabrication of a CMOS-integrated piezoresistive pressure sensor. SiGe MEMS was selected as the technology platform in which to reach this goal. This dissertation provides all the steps for the successful design and fabrication of MEMS pressure sensors directly on top of their readout circuitry, allowing for sma...

In this introductory chapter, the motivation and main topic of this work are introduced. After a general introduction to microelectromechanical systems (MEMS), the different existing approaches for the integration of the MEMS with the driving and controlling electronics are presented. The benefits of using silicon germanium (SiGe) as MEMS structura...

Absorption spectroscopy in the ultraviolet-visible-near infrared (UV-Vis-NIR) wavelength region has been used to quantify the aggregation factor of single-walled carbon nanotubes (SWCNTs) in liquid media through a series of controlled experiments. SWCNT bundles are dispersed in selected solvents using a calibrated ultrasonicator, which helps in det...

As lateral dimensions of electromechanical devices are scaled down to length scales comparable to electron mean free paths, the influence of thickness effect on their properties becomes sine qua non. This paper presents a detailed study of thickness effect on the Young's modulus, residual stress, resistivity and Hall mobility of ultrathin_poly-Si(1...

For years monocrystalline silicon has been the dominant material for the fabrication of piezoresistive MEMS sensors thanks to its high gauge factor and excellent mechanical properties. Despite its lower gauge factor, polycrystalline silicon (poly-Si) offers several advantages over monocrystalline silicon for sensor applications. In poly-Si piezores...

This paper demonstrates for the first time the use of meta-materials (MTM) in bulk acoustic wave resonators (BARs) for sensing applications under atmospheric pressure conditions. Key is the observation that implementing square holes in the BARs modifies their specific surface area (area-to-volume ratio) and lowers their mass. We establish through s...

In this work minimizing the non-specific binding onto the sensor surface was carried out using different blocking agents to obtain a specific detection (1-125nM). Bovine Serum Albumin (BSA) is considered. However, its serious disadvantage is that it has been shown to cause stiction of the Micro-Electro-Mechanical Systems (MEMS). In addition, it doe...

Poly-SiGe can be used for monolithically integrating Micro/Nano-ElectroMechanical Systems (M/NEMS) with its driving circuitry in a MEMS-last approach. For these applications, it is important to have poly-SiGe films with a low tensile stress, a low resistivity and a high deposition rate. This paper presents a systematic procedure for the simultaneou...

A few properties of polycrystalline silicon germanium (poly-Si1−x Ge x ) films can be tailored by modulating the germanium incorporation. In this paper, the structural, mechanical and electrical properties of heavily doped ultrathin (~100 nm) poly-Si1−x Ge x films (0.84 ≤ x ≤ 0.88) fabricated by low-pressure chemical vapour deposition were investig...

We report a totally CMOS-compatible fabrication technique to assemble horizontally suspended single-walled carbon nanotube (SWCNT) resonators. Individual SWCNTs are assembled in parallel at multiple sites by a technique called dielectrophoresis. The mechanical resonance frequencies of the suspended SWCNTs are in the range of 20–35 MHz as determined...

Several classes of microelectromechanical systems (MEMS) are sensitive to the internal pressure inside their packages. From functionality and reliability point of view, knowing and monitoring this pressure is important. This paper demonstrates for the first time a new method to measure the internal pressure of the package and, in addition, its leak...

This paper reports on a new test structure that is designed for measuring adhesion forces that arise between contacting surfaces in microelectromechanical systems (MEMS). More specifically, the test structure allows one to measure the force that occurs when out-of-plane moving MEMS enter into mechanical contact with a bottom electrode. The test str...

A variable optical attenuator based on a 2D MEMS grating is described. The device is a perforated and suspended poly-SiGe membrane with fixed islands within the perforations. It specularly reflects light in the non-actuated state, whereas after actuation the membrane deflects downwards forming a grating which diffracts light in higher orders reduci...

This paper investigates the nonlinear effects introduced by the intermolecular forces on the static and harmonic behavior of electrostatically transduced nano-electromechanical (NEM) resonators using a lumped parameter model. We compute the pull-in parameters and study the full impact of the intermolecular forces on the spring softening effect, com...

This paper presents a design approach for Nano-ElectroMechanical Structures (NEMS) relays taking Casimir or van der Waals forces into account for the static and dynamic modeling of the devices. The goal is to design these structures using the Poly Silicon Germanium process, with several fabrication constraints on the gap, the structure thickness an...

Methods for manufacturing micromachined devices and the devices obtained are disclosed. In one embodiment, the method comprises providing a structural layer comprising an amorphous semiconductor material, forming a shielding layer on a first portion of the structural layer and leaving exposed a second portion of the structural layer, and annealing...

In an ohmic nano relay, the surface quality of the contact materials is crucial in determining the performance of the device and also its lifetime. An in depth analysis of the surface of an highly doped poly SiGe material, considered as a potential candidate for relay applications, is carried out. A new contact resistance model based on the contact...

This paper evaluates the performance characteristics of a new miniaturized lateral capacitive accelerometer with a high bandwidth as well as a high sensitivity, utilizing a low thermal budget SiGe MEMS technology. The accelerometer combines a 4 mu m SiGe structural layer thickness with a small capacitive sensing gap of 500 nm, leading to an improve...

This work demonstrates, for the first time, the use of poly-SiGe for the fabrication of both piezoresistive and capacitive pressure sensors at CMOS-compatible temperatures. Despite the low processing temperature (455 °C), a sensitivity of 4.6 mV/V/bar for a membrane of 200 × 200 μm2 is reached by piezoresistor design optimization. The possibility o...

We report a detailed study of dielectrophoretic assembly of single-walled carbon nanotube (SWCNT) devices intended for nano electromechanical systems (NEMS) applications. The process conditions of dielectrophoresis (DEP) are optimized to maximize the yield of devices with only one SWCNT bridging the electrode structures. The frequency of the applie...

Several SiGe Micro-Electro Mechanical (MEM) double clamped beam resonators with different lengths were chemically functionalized and electrically measured. The impact of each functionalization step, including oxidation, silanization, immobilization and blocking, on the resonators' performance was studied. The resonance frequency of these resonators...

In this paper, we present for the first time, an analytical study of the nonlinearity introduced by the Van der Waals (VdW) force and its effect on the harmonic behavior of NanoElectromechanical (NEM) resonators. A single degree of freedom lumped model for the NEM resonator under the influence of both VdW and electrostatic forces is used. For harm...

Adhesion caused by various forces can lead to either permanent or temporary stiction. This remains one of the most critical reliability issues for micro-electro-mechanical system (MEMS). In this paper, we present a study using a poly-SiGe micromirror array as a “stiction sensor” to investigate the stiction induced by outgassing during the glass fri...

An integrated poly-SiGe-based piezoresistive pressure sensor, which is directly fabricated above 0.13 m Cu-back-end CMOS technology, is presented. This represents not only the first integrated poly-SiGe pressure sensor directly fabricated above its readout circuit but also the first time that a poly-SiGe MEMS device is processed on top of Cu-back-e...

Three different measurement methods for the measurement of the pressure inside empty MEMS packages are demonstrated and discussed. They are based on measurements of capacitance, cap resonance or cap buckling. The capacitance measurement method is shown to be the most sensitive of the three methods for this application, with a pressure resolution of...

Grating light valve (GLV) display pixels are reflection type diffraction gratings consisting of electrostatically movable coplanar microbeams. Once actuated, the alternate movable beams deflect downwards which produces controlled diffraction of light creating bright and dark pixels in a display system. GLV displays provide a huge improvement in con...

The mechanical response of electrostatically-actuated MEMS to ESD stress leads to contact breakdown or to discharges across micro-gaps. This is the root cause of most MEMS failures under ESD stress. This paper discusses improvement of the intrinsic ESD robustness of SiGe MEMS from Class0 (;500V), through smart design variations and higher mechanica...

Abrasive blade dicing is the most common technique for die separation. In this work an alternative dry and non-abrasive die separation method, which is known as "Stealth dicing", is assessed for surface-sensitive MEMS (Micro Electro Mechanical Systems) wafers. The dicing performance and capability of the system is investigated on 200mm full thickne...

This paper presents a rigorous design approach for (NanoElectroMechanical Structures/Systems) NEMS relays taking generalized nano-forces, e.g., Casimir, van der Waals, into account for their static and dynamic modelling. Key in our approach is the proper definition of the critical pull-in and pull-out conditions as well as the derived non-latching...

This paper presents a wafer-level temporary packaging technique utilizing a chemical vapor deposited (CVD) poly-(p-xylylene) polymer Parylene film, and oxygen plasma etching. As a test case, released, unpackaged accelerometers fabricated in a SiGe MEMS above IC technology were coated with two different types of Parylene, Parylene N and Parylene C r...

This paper reports on a novel temporary 0-level packaging process for MEMS (Micro Electro Mechanical Systems), in particular MEMS for optical applications. In this process protective caps, with a heat decomposable and photo-patternable polymer sealing ring, are placed by flip-chip on the MEMS wafer. The resulting temporary packages are gross leak t...

This paper presents for the first time a study of different methods to seal SiGe surface micromachined cavities for above-CMOS MEMS applications. Four different sealing layers are proposed: sputter-deposited AlCu, sub-atmospheric pressure chemical vapour deposited Si-oxide and a porous microcrystalline-SiGe cover in combination with either high-den...

Thermal desorption spectroscopy (TDS) was used to study outgassing from polycrystalline SiGe (poly-SiGe), SiC and SiO2 films used for poly-SiGe-based MEMS thin film vacuum package technology. Primary desorption products were found to be H2, H2O and CO2. The CO2 outgassing could be correlated with CF4 plasma interface cleaning used for thick SiGe PE...