Ingrid De Wolf

Micro-Raman spectroscopy to study local mechanical stress in silicon integrated circuits

Local mechanical stress is currently an important topic of concern in microelectronics processing. A technique that has become increasingly popular for local mechanical stress measurements is micro-Raman spectroscopy. In this paper, the theoretical background of Raman spectroscopy, with special attention to its sensitivity for mechanical stress, is discussed, and practical information is given for the application of this technique to stress measurements in silicon integrated circuits. An overview is given of some important applications of the technique, illustrated with examples from the literature: the first studies of the influence of external stress on the Si Raman modes are reviewed; the application of this technique to measure stress in silicon-on-insulator films is discussed; results of measurements of local stress in isolation structures and trenches are reviewed; and the use of micro-Raman spectroscopy to obtain more information on stress in metals, by measuring the stress in the surrounding Si substrate is explained.

Stress measurements in silicon devices through Raman spectroscopy: Bridging the gap between theory and experiment

The different steps that have to be taken in order to derive information about local mechanical stress in silicon using micro‐Raman spectroscopy experiments, including theoretical and experimental aspects, are discussed. It is shown that the calculations are in general less complicated when they are done in the axes system of the sample. For that purpose, the secular equation is calculated in the axes system [110], [−110], [001], which is important for microelectronics structures. The theory relating Raman mode shift with stress tensor components is applied using two analytical stress models: uniaxial stress and planar stress. The results of these models are fitted to data from micro‐Raman spectroscopy experiments on Si3N4/poly‐Si lines on silicon substrate. In this fit procedure, the dimensions of the laser spot and its penetration depth in the substrate are also taken into account.

A comprehensive model to predict the charging and reliability of capacitive RF MEMS switches

Reliability issues currently hamper the commercialization of capacitive RF MEMS switches. The most important failure mode is parasitic charging of the dielectric of such devices. In this paper we present an improved analytical model that enables us to calculate and understand the effect of insulator charging on the behavior of capacitive RF MEMS switches, and to describe the way they fail, and their reliability. Emphasis is placed on a shift of the pull-out voltage to predict failures. Tests with capacitive RF MEMS switches have been performed that validate the most important features of the model.

Fabrication and reliability testing of Ti/TiN heaters

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.

A physical model to predict stiction in MEMS

One of the most important reliability problems in micro-electromechanical systems (MEMSs) is stiction, the adhesion of contacting surfaces due to surface forces. After reviewing the known physical theory, and the measurement method commonly used to investigate stiction, we present a model that can be used to investigate the sensitivity of MEMS to stiction. It quantitatively predicts the surface interaction energy of surfaces in contact. Included in the model is the roughness of the contacting surfaces and the environmental conditions (humidity and temperature). This is done by describing the surface interaction energy as a function of the distance between the surfaces. This distance is not a unique number, but rather a distribution of distances. It is shown that, if we know this distribution, we can calculate the surface interaction energy. The model is suitable for the prediction of forces due to capillary condensation and molecular interactions.

Stress measurements in Si microelectronics devices using Raman spectroscopy

The application of micro-Raman spectroscopy for the measurement of local stress in silicon microelectronics samples is discussed. Practical issues of concern for local stress measurements using micro-Raman spectroscopy are dealt with. Copyright

On the physics of stiction and its impact on the reliability of microstructures

Stiction is a very well-known failure mode in micro-electromechanical systems (MEMS). In this paper, a short overview is given of the important physical aspects of stiction in microsystems. The surface forces causing stiction are treated in some detail, and ways of preventing stiction failures are reviewed from a reliability point of view. A statistical calculation method to predict stiction failures is reviewed as well, and a more fundamental analysis of the prediction of stiction failures during normal operation is outlined. Degradation mechanisms in micro-electromechanical systems (MEMS) able to cause stiction due to a lowering of the restoring force, such as creep and charging of dielectrics, are also discussed. It is difficult to conduct accelerated testing on adhesion, but it is shown that mechanisms causing a change in the restoring force can be accelerated. Therefore, some stiction failure modes can well be investigated by accelerated testing.

Study of damage and stress induced by backgrinding in Si wafers

In this paper, a profound study of the subsurface damage induced by backgrinding Si wafers is presented. It is shown that a thin amorphous layer (30–80 nm) is generated during backgrinding. Below the amorphous layer, there is a polycrystalline zone. Its thickness (about 0.5 μm) is obtained from Raman spectroscopy measurements. Below that layer, there is a strained crystalline zone. Its stress can be roughly calculated from warpage measurements of the wafer. The stress is also measured directly by Raman spectroscopy. A new analytical model is established to study the stress propagation with depth. The model fits the Raman spectroscopy measurements very well. The measurements show that the stress is reduced to zero after dry etching of the wafers.

Strain determination in silicon microstructures by combined convergent beam electron diffraction, process simulation, and micro-Raman spectroscopy

Test structures consisting of shallow trench isolation (STI) structures are fabricated using advanced silicon (Si) technology. Different process parameters and geometrical features are implemented to investigate the residual mechanical stress in the structures. A technology computer aided design homemade tool, IMPACT, is upgraded and optimized to yield strain fields in deep submicron complementary metal–oxide–semiconductor devices. Residual strain in the silicon substrate is measured with micro-Raman spectroscopy (μ-RS) and/or convergent beam electron diffraction (CBED) for large (25 μm) and medium size (2 μm), while only CBED is used for deep submicron STI (0.22 μm). We propose a methodology combining CBED and technology computer aided design (TCAD) with μ-RS to assess the accuracy of the CBED measurements and TCAD calculations on the widest structures. The method is extended to measure (by CBED) and calculate (by TCAD) the strain tensor in the smallest structures, out of the reach of the μ-RS technique....

A low frequency electrical test set-up for the reliability assessment of capacitive RF MEMS switches

The reliability and lifetime testing of capacitive RF MEMS switches one by one at the intended signal frequency range (GHz) is very expensive because RF equipment is monopolized for a large amount of time. Testing a statistically significant number of devices in this way is therefore impractical. Furthermore testing the switches in a harsh environment is difficult. We have developed a new, low frequency measurement set-up to address these issues. In this paper, the principle of operation is described, as are measurements revealing the following failure modes of RF MEMS switches: stiction of the bridge of the devices under test due to charging, and breakdown of the dielectric. We also show that the system can be used to monitor other characteristics, such as the rise- and fall-times and incomplete pull-in of the bridge of the switch when actuated.