Co-packaged optics for high performance computing or other data center applications requires dense integration of silicon photonic integrated circuits (PICs) with electronic integrated circuits (EICs). This work discusses the impact of three-dimensional (3D) hybrid integration on the thermal performance of Si ring-based photonic devices in wavelength-division multiplexing PICs. A thermal finite element model of the EIC-PIC assembly is developed and calibrated with thermo-optic device measurements, before and after integration of an electrical driver on top of the PIC by means of microbump flip-chip bonding. Both measurements and simulations of the thermal tuning efficiency and crosstalk between silicon photonic devices show that the EIC can have a significant impact on the thermal performance of the integrated heaters in the PIC by acting as an undesired heat spreader. This heat spreading lowers the heater efficiency with 43.3% and increases the thermal crosstalk between the devices by up to 44.4% compared with a PIC-only case. Finally, it is shown that these negative thermal effects of 3D integration can largely be mitigated by a thermally aware design of the microbump array and the back-end-of-line interconnect, guided by the calibrated thermal simulation model.
We present an integrated reliability test methodology for electrostatic discharge (ESD) testing of micro-electromechanical systems (MEMS). It is shown that conventional ESD test methods for failure detection, like current and voltage waveforms during ESD stress and direct current leakage are insufficient for MEMS. A functionality-based approach using the mechanical response of the MEMS during ESD is needed to accurately and conclusively detect ESD failure in MEMS. A novel test setup with a probe-mountable human body model (HBM) tester is presented for this purpose. This setup can perform simultaneous measurements of MEMS out-of-plane displacement, HBM current and HBM voltage in the MEMS in situ during ESD stress. Using this setup, a few examples are demonstrated that show that traditional electrical characterization is overestimating the ESD robustness of MEMS devices. ESD testing of MEMS is performed at different pressures on RF MEMS actuators and show that more than one type of failure mechanism can occur due to ESD stress. ESD-induced charging and functionality degradation in RF MEMS actuators are also briefly discussed.
Jonathan Govaerts, Jo Robbelein, Chun Gong, Bartek Pawlak, Mario Gonzalez, Ingrid De Wolf, Frederick Bossuyt, Steven Van Put, Ivan Gordon, Kris Baert, Jan Vanfleteren
Imec has developed a new technology to integrate and interconnect back-contact solar cells into modules, based on
embedding cells in silicone on top of a glass substrate. This technology aims at an improved optical performance and
reliability (through the use of silicones and low-temperature metallization). One of the additional advantages is that the
technology is suitable for integrating very thin cells into modules: whereas standalone interconnection of such fragile
thin cells, e.g. tabbing and stringing, would significantly lower the throughput yield due to breakage, the cells are better
protected if they are embedded inside silicone.
The paper will first elaborate on the process flow, the background and motivation, advantages, drawbacks and
limitations, and technical aspects of the developed technology. Then it will present the results of the measurements on
the performance of functional solar cells processed into modules using this technology, discussing losses and loss
mechanisms. Then, the approach towards determining the reliability of the module will be presented, indicating how
imec aims at building up an ageing model, and elaborating the results on the failure mode and effect analysis, modeling,
characterization and reliability testing.
Ingrid De Wolf, Jeroen De Coster, Vladimir Cherman, Piotr Czarnecki, Stanislaw Kalicinski, Olalla Varela Pedreira, Sandeep Sangameswaran, Kris Vanstreels
In this paper various non-standard methods and instruments for the functional, yield and reliability analysis of MEMS
are discussed. Most of these methods are based on existing instruments, involving electrical, optical or mechanical
measurements. We present either alternative applications of existing techniques, new methodology for data extraction, or
adaptation/automation of the techniques for automatic chip or wafer level measurements.
This paper presents two distinct measurement systems that were custom-built for the parametric and functional yield
inspection of MEMS devices on wafer-level. Throughput as well as accuracy was optimized by using automatic feature
detection and data segmentation algorithms. Inaccuracies in stage positioning during scanning are compensated for by a
grid detection algorithm. The analysis of the measurement data is performed in parallel with the ongoing measurements.
The data analysis includes the detection, parameter extraction, analysis of failures or damage of a single device and the
final stitching of the results in order to obtain a visual mapping of the measured arrays. The performance of both systems
has been demonstrated using arrays of micromirrors as test vehicles.
This paper describes how a standard laser-Doppler vibrometer (LDV) system was modified and extended for long-term
reliability measurements on micro-electromechanical (MEMS) devices. Since scanning LDV measurements are
performed by successively pointing a laser beam at a predefined set of locations, it is of the utmost importance that no
movement of the MEMS device relative to the laser source occurs during the extended lifetime testing period. In the
proposed system, the inevitable drift is compensated in three dimensions. The Z-drift is compensated by a piezo-actuated
lens and an autofocus algorithm. The in-plane drift is detected using an image correlation technique, and the coordinate
system of the LDV tool is modified accordingly. In this manner the scan points of the LDV can track the drift of the
MEMS device. During a 400-hour testing period, measurements were performed every 15 minutes. A total in-plane drift
of about 30μm was observed. The tracking error was below the resolving power of the microscope.
Laser joining is a promising technique for wafer-level bonding. It avoids subjecting the complete MEMS package to a high temperature and/or the high electric field associated with conventional wafer-level bonding processes, using the laser to provide only localized heating. We demonstrate that a benzo-cyclo-butene (BCB) polymer, used as an intermediate bonding layer in packaging of MEMS devices, can be satisfactorily cured with a substantial reduction of curing time compared with an oven-based process by using laser heating. A glass-on-silicon cavity bonded with a BCB ring can be produced in few seconds at typical laser intensity of 1 W/mm2 resulting in a local temperature of ~ 300°C. Hermeticity and bond strength tests show that such cavities have similar or better performance than cavities sealed by a commercial substrate bonders which require a minimum curing time of 10 minutes. The influence of exposure time, laser power and pressure on degree of cure, bond strength and hermeticity is investigated. The concept of using a large area, uniform laser beam together with a simple mirror mask is tested, demonstrating that such a mask is capable of protecting the centre of the cavity from the laser beam; however to prevent lateral heating via conduction through the silicon a high conductivity heat sink is required to be in good thermal contact with the rear of the silicon.
Zero-level packaging, i.e. the encapsulation of the MEMS device at wafer level, is an essential technique for MEMS miniaturization and cost reduction. A large number of different capping and sealing materials and techniques can be used. However, the testing and qualification of this type of packaging of MEMS devices requires special techniques. A number of conventional and new characterization techniques for mechanical and hermeticity testing are presented, as well as an overview about outgasing measurements and reliability testing.
Residual stresses can significantly affect the performances of silicon micro-structures. The understanding of the residual stress growth during their processing is of great importance. However, the experimental tracing of the stress at the various stages of the machining is still almost impossible. Quantitative modelling of these problems is the alternative to provide guidelines for the minimization of the residual stresses. In this paper, we describe a technology computer aided design homemade tool, IMPACT. The mechanical models and the numerical implementation are presented. We give details about our methodology to calibrate and validate our implementation. The originality of this study lies in (1) the capability to simulate almost all the sources of stress taking into account of the complex rheological behaviors of the materials, (2) the experimental determination of the mechanical properties of various thin film materials and (3) the validation of the calculations by direct comparisons with measured deformations in the micro-structures.
This paper discusses some reliability issues that play a role for capacitive RF MEMS switches. We describe how these degradation mechanisms affect the functioning of the switches. Also the methodology that can be used to test capacitive RF MEMS switches, including some packaging aspects, and dedicated instrumentation required to perform these tests are discussed.
We report on the high-speed 3D imaging capabilities of a newly developed inspection tool for MEMS. The instrument is capable of performing an imaging operation for a complete image at once, which is a large advantage over scanning laser Doppler vibrometers and related equipment. This new instrument is able to `slow down' fringe movements when illuminating a MEMS device with a dedicated interferometer, by using a slow beat frequency between object excitation and reference beam excitation and averaging over a lot of phase images. It is used in such a way that an ordinary CCD camera can be used to obtain 3D images and movies of the periodic mechanical motion of MEMS devices, either in mechanical resonance when excited using a piezo actuator, or using electronic excitation with probe needles at any frequency required (up to the limitations of the waveform generators). Two modes of operation are possible: a mode in which slow deformations (seconds or more) can be monitored, and a mode in which fast periodic movements (100 Hz - 1 MHz) can be investigated. We show that this imaging technique is especially useful for the investigation of the mechanical behavior of MEMS, both to monitor the intended movement of a structure, and to have a close look at the erratic mechanical behavior of defective parts.
In this paper, the capabilities and problems if micro-Raman spectroscopy are discussed for measuring local mechanical stress, both in mono- crystalline and poly-crystalline silicon microstructures. The possibilities of this technique are demonstrated for two different MEMS: the crystalline Si membrane of a pressure sensor and a poly-crystalline Si beam. For both MEMS, an auto-focus Raman system was used. The stress in the membrane of the pressure sensor was investigated before and after bonding of the Si wafer containing the sensors to a glass substrate. This bonding resulted in an under-pressure in the sensor, deflecting the membrane inwardly. Raman spectra were measured from the top surface and the bottom surface of the membrane. This resulted in a map of the stress distribution. It indicates, for the top surface, tensile stress near the edges, compressive stress in the center, and hardly any stress at the corners. The stress in the poly-crystalline Si beam was measured using two different wavelengths of the laser beam. The results show a local tensile stress distribution along the length of the beam.
Although a lot of work has been done on understanding auto-adhesion (stiction) in MEMS, the effect of surface roughness has never been extensively addressed. In this paper, a model is presented, which describes the auto-adhesion interaction energy of micromachined surfaces in contact. Included in the model is the capillary force, in such a way that it can be readily extended to accommodate electrostatic and van de Waals forces as well, in combination with the roughness of the contacting surfaces. By investigating the effect of the height distribution of the surfaces, a term is derived for the surface interaction energy in different environment conditions as a function of the mean separation between the rough surfaces. To obtain an equilibrium distance between the surfaces, the repulsive part of the interaction is also modeled. The combination of these terms gives us the equilibrium distance between the surfaces and the corresponding surface interaction energy, thereby quantifying the effect environmental conditions have on auto-adhesion. The results of the model agree well with surface interaction energies in MEMS known from literature for different environmental conditions.
Micro-Nano Technology Visualization (MNTV) is critical to studies in MEMS reliability. The ability to see and characterize the microstructures and interfaces with high resolution at the microscale and nanoscale is invaluable. In this paper we present the motivation, paradigm and examples of visualization techniques applied to several aspects of surface micromachined polysilicon structures. High resolution cross-section imaging, using both a FIB/SEM and FIB/STEM, is used to acquire information on profile differences between fabrication facilities and grain size and orientation. The AFM is used to compare surface roughness on both sides (top and bottom surfaces) of thin film polysilicon after release etching. The data gathered will be extremely useful feedback for fabrication facilities in terms of process characterization and quality assurance. The data will also be useful for MEMS CAD tools where device and process models must be validated.
After a short introduction on the theory and instrumentation of Raman spectroscopy, its application for local stress and temperature measurements in semiconductor devices is discussed. Examples are given for silicon isolation structures, transistors, solder bumps and back-grinding. It is shown how the resolution can be improved by using an oil immersion objective and deconvolution techniques. Different imaging modes are discussed and their resolution is compared. Examples of 1D and of 2D scans are shown.
We present a new material for highly resistive heaters: thin Ti/TiN layers. Their resistivity is indeed comparable to the resistivity of NiCr, i.e. 50-100 micro-Ohn-cm. However, as opposed to the latter material, Ti/TiN is CMOS compatible and thus easier to incorporate in CMOS integrated MEMS processing. To test the reliability of thin Ti/TiN resistive heaters, both 5 nm Ti/30 nm TiN and 5 nm Ti/60 nm TiN heaters were fabricated. A thermal analysis shows a small temperature coefficient of resistivity. To test the reliability of such heaters at temperatures up to 300 degrees C, 1 micron wide Ti/TiN lines were biased using high currents. Both DC and pulsed DC current stressing resulted in very small deviations from the initial resistance for sintered and passivated heaters. The temperature uniformity over the heater line is investigated using Emission Microscopy.
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