The manufacturing, handling and control of micro and nano scale devices require the quantification of their geometrical
and mechanical properties. While the measurement of geometrical and size data is easily accessible by SFM and SEM
imaging equipment, mechanical characterization is a general problem for these objects. Different kinds of size effects
more often force material property determination directly on micro/nano objects. Therefore, new strategies for material
testing have to be developed. Displacements and their derivatives are two basic properties to be measured during testing
for many mechanical material properties. The authors make use of SFM and high resolution SEM imaging in order to
obtain spatially resolved displacement data over the scan area. Locally applied cross correlation algorithms are utilized
to compute displacement fields and the corresponding first order derivatives. Micrographs are captured subsequently for
different object load states. The established technique and measurement system (nanoDAC) is reviewed briefly. The
authors present different applications of the nanoDAC method establishing the characterization of micro/nano scale
material behaviour. Among the application fields are approaches to measure fracture mechanics criteria from crack
opening displacement (COD) fields, a method of measuring residual stresses in thin membranes and testing techniques
to measure Young's modulus and Poisson's ratios of thin foils and micro wires. The measurement of fracture mechanics
bases on linear elastic fracture mechanics. Measured by AFM, COD fields in the very vicinity of crack tips are used to
extract fracture toughness values. Stress determination on membranes utilizes the unique capability of focused ion beam
(FIB) equipment, which allows concurrent material milling and micrograph capture with high resolution. A Zeiss XBeam
system has been used to mill trenches and holes into membranes of semiconductor structures. Treated that way
stress release fields are determined from SEM micrographs. Taking into consideration reasonable stress hypotheses,
membrane stresses are calculated from the obtained deformation fields. With the presented methods the basis is
provided for an experimental reliability analysis of MEMS/NEMS and nanodevices.
Research results obtained for local stress determination on micro and nanotechnology components are summarized. It meets the concern of controlling stresses introduced to sensors, MEMS and electronics devices during different micromachining processes. The method bases on deformation measurement options made available inside focused ion beam equipment. Removing locally material by ion beam milling existing stresses / residual stresses lead to deformation fields around the milled feature. Digital image correlation techniques are used to extract deformation values from micrographs captured before and after milling. In the paper, two main milling features have been analyzed - through hole and through slit milling. Analytical solutions for stress release fields of in-plane stresses have been derived and compared to respective experimental findings. Their good agreement allows to settle a method for determination of residual stress values, which is demonstrated for thin membranes manufactured by silicon micro technology. Some emphasis is made on the elimination of main error sources for stress determination, like rigid body object displacements and rotations due to drifts of experimental conditions under FIB imaging. In order to illustrate potential application areas of the method residual stress suppression by ion implantation is evaluated by the method and reported here.
The paper comprises research results obtained for stress determination on micro and nanotechnology components. It
meets the concern of controlling stresses introduced to sensors, MEMS and electronics devices during different
micromachining processes. The method bases on deformation measurement options made available inside focused ion
beam equipment. Removing locally material by ion beam milling existing stresses / residual stresses lead to deformation
fields around the milled feature. Digital image correlation techniques are used to extract deformation values from
micrographs captured before and after milling. In the paper, two main milling features have been analyzed - through
hole and through slit milling. Analytical solutions for stress release fields of in-plane stresses have been derived and
compared to respective experimental findings. Their good agreement allows to settle a method for determination of
residual stress values, which is demonstrated for thin membranes manufactured by silicon micro technology. Some
emphasis is made on the elimination of main error sources for stress determination, like rigid body object displacements
and rotations due to drifts of experimental conditions under FIB imaging. In order to illustrate potential application
areas of the method residual stress suppression by ion implantation is evaluated by the method and reported here.
Proc. SPIE. 6175, Testing, Reliability, and Application of Micro- and Nano-Material Systems IV
KEYWORDS: Microelectromechanical systems, Polymers, Interfaces, Reliability, Atomic force microscopy, Finite element methods, Microelectronics, Digital image correlation, Scanning probe microscopy, Chemical elements
With the development and application of micro/nano electronic mechanical systems (MEMS, NEMS) for a variety of market segments new reliability issues will arise. The understanding of material interfaces is the key for a successful design for reliability of MEMS/NEMS and sensor systems. Furthermore in the field of BIOMEMS newly developed advanced materials and well known engineering materials are combined despite of fully developed reliability concepts for such devices and components. In addition the increasing interface-to volume ratio in highly integrated systems and nanoparticle filled materials are challenges for experimental reliability evaluation. New strategies for reliability assessment on the submicron scale are essential to fulfil the needs of future devices. In this paper a nanoscale resolution experimental method for the measurement of thermo-mechanical deformation at material interfaces is introduced. The determination of displacement fields is based on scanning probe microscopy (SPM) data. In-situ SPM scans of the analyzed object (i.e. material interface) are carried out at different thermo-mechanical load states. The obtained images are compared by grayscale cross correlation algorithms. This allows the tracking of local image patterns of the analyzed surface structure. The measurement results are full-field displacement fields with nanometer resolution. With the obtained data the mixed mode type of loading at material interfaces can be analyzed with highest resolution for future needs in micro system and nanotechnology.
Proc. SPIE. 6179, Advanced Sensor Technologies for Nondestructive Evaluation and Structural Health Monitoring II
KEYWORDS: Microelectromechanical systems, Gas sensors, Sensors, Ions, Reliability, Atomic force microscopy, Scanning electron microscopy, Ion beams, Digital image correlation, Scanning probe microscopy
Micro machined micro sensors for gas or flow detection based on physical behaviour of a special layer of a membrane have to fulfil high quality and reliability requirements especially in safety or security applications. Up to now, most of the research studies neglected mechanical issues related to reliability of these structures. In this sense, the study and characterization of the stress distribution on the membranes after fabrication and during their operative life is required. Thin films used in micromachined structures exhibit residual mechanical stress strongly dependent on the layer composition and the deposition process parameters. Often, a deposition of a multilayer is required, and this adds factors like abrupt transitions in thermal, elastic and plastic mismatch across the interfaces that have a direct effect on the resultant stress. Moreover, in operating conditions, a thermal stress originated due to the difference in the thermal expansion coefficient (CTE) of the membrane materials adds to the residual stress of the membrane. The resultant stresses can induce excessive deformation, fracture, delamination and microstructural changes in the material that can lead to the breaking of the structure during the fabrication stage or affect the behaviour of the final device. In the present work, the 2D deformation of a gas and a flow sensor membrane under different thermo-mechanical load states will be analysed by means of the digital image correlation (DIC) techniques based on scanning probe microscopy (SPM) data. With this technique which is introduced as the nanoDAC method (nano Deformation Analysis by Correlation) deformation fields can be determined with nanometer-accuracy. In addition ion milling by focused ion beam (FIB) technique is demonstrated at membrane specimens with residual stresses. Object deformations fields nearby the milling area are measured by fibDAC allowing the evaluation of very local residual stresses. Some principal experiments illustrate the feasibility of the chosen approach.
Micromachined microsensors for gas or flow detection based on physical behaviour of a special layer of a membrane have to fulfil high quality and reliability requirements especially in safety or security applications. For the reliability assessment a combination of simulative and experimental methods is usually carried out for the fully understanding of the thermo-mechanical behaviour. Due to the micromachining involved in the production of the sensor components the thermo-mechanical response of the layers are strongly dependent on process parameters. Therefore experimental methods for the 3D deformation detection are essential. In this paper experimental methods such as profilometry and scanning probe microscopy are tested for the evaluation of residual stresses and thermomechanical induced stress/strain fields.
With ongoing miniaturization from micro electronic mechanical systems (MEMS) towards nano electronic mechanical systems (NEMS), there is a need for new reliability concepts making use of meso-type (micro to nano) or fully nanomechanical approaches. For the development of theoretical descriptions and their numerical implementation on the basis of simulation tools experimental verification will be of major interest. Therefore, there is a need for measurement techniques with
capabilities of determination and evaluation of strain fields with very local (nanoscale)resolution. Following this challenge the authors developed the nanoDAC method (nano Deformation Analysis by Correlation) which enables the extraction of nanoscale displacement fields from scanning probe microscopy (SPM) images. Components of interest are thermomechanically loaded under the SPM and topography scans of the critical areas are taken at specific load states. The obtained images are analyzed by digital image correlation resulting into full-field displacement and strain fields. Due to the application of SPM equipment deformations in the micro-, nanometer range can be easily detected. The method can be performed on bulk materials, thin films and on devices i.e microelectronic components, sensors or MEMS/NEMS. Furthermore, the characterization and evaluation
of micro- and nanocracks or defects in bulk materials, thin layers and at material interfaces can be carried out. In combination with finite element simulations the application of the described
experimental method to sensor elements is a promising approach for reliability analysis of newly designed sensor architectures.
The authors present a new approach, fibDAC, which allows to measure and analyze deformation fields on stressed micro and nano components, which can be utilized for mechanical material characterization. The method bases on digital image correlation (DIC) algorithms applied locally to load state images captured from focused ion beam (FIB) equipment. As a result, deformation fields are determined, which occur due to loading of microsystem structures inside the focused ion beam system. A similar tool, called microDAC/nanoDAC, has been reported earlier and applies DIC techniques to SEM or AFM images. The advantages of the new fibDAC approach occur in the incorporation of specimen preparation like ion milling, ion beam surface polishing and DIC patterning as well as specimen loading by ion milling and DIC deformation measurement in a single equipment. Combining measured fields with finite element simulations or analytical solutions of the corresponding mechanical problem, relevant mechanical material properties can be evaluated. Corresponding object loading is accomplished either externally by testing modules designed for application inside the FIB equipment or by ion milling on the test specimen. As an example ion milling on specimens with residual stresses is demonstrated. Released in this way residual stresses cause object deformations nearby the milling area. Measured deformation fields by fibDAC allow to evaluate very local residual stresses. Some principal experiments illustrate the feasibility of the chosen approach. Features and challenges connected with this new method are discussed in some detail.
The achievement of reliability is a major task during the design process of microsystems (i.e. MEMS: mechanical-electrical microsystems). In this respect CAD (computer aided design) simulation methods play a major role in the dimensioning of mechanical structures. It can be observed that a pure CAD approach becomes difficult because of the complexity of these systems, which originates from the large variety of integrated materials and thus a diversity of the resulting failure mechanisms. Therefore strategies dealing with these uncertainties in reliability estimates need to be incorporated in the design process. The approach presented in this paper is based on the application of simulation and advanced deformation measurement methods named microDAC (micro deformation analysis by means of grey scale correlation) and nanoDAC. It is exemplified on different detail levels of the reliability assessment, with an emphasis on fracture. The first stage consists of a parametric simulation approach, which helps to develop design guidelines for the geometry. For a more absolute quantitative analysis and for material selection in a new design the mechanical properties need to be specified and evaluated with respect to reliability. Besides, the described systematics of reliability assessment needs a profound knowledge of the failure behavior, which is analyzed by the application of microDAC/nanoDAC techniques. In the prescribed way, it becomes possible to tackle mechanical reliability problems in early design phases.
The authors present a digital image correlation (DIC) tool, which allows to measure deformation fields on micro and nano system components under thermal and/or mechanical impact. Load state micrographs are used to extract displacement and strain fields. An earlier developed DIC concept for that purpose has been extended from SEM to AFM and FIB imaging. As a consequence ultimate measurement resolution can be achieved by AFM imaging. The advantages of the new focused ion beam (FIB) approach occur in the incorporation of specimen preparation (ion milling, ion beam surface polishing and DIC patterning), specimen loading by ion milling and DIC deformation measurement in a single equipment. The application of DIC techniques on AFM base is illustrated for the investigation of thermal deformations on microsystem structures as well as for the evaluation of microcracks from crack opening displacements. Some first results for residual stress release by ion milling with subsequent deformation field measurement are reported, too.
With the development of micro- and nanotechnological products such as sensors, MEMS/NEMS and their broad application in a variety of market segments new reliability issues will arise. The increasing interface-to-volume ratio in highly integrated systems and
nanoparticle filled materials and unsolved questions of size effect of nanomaterials are challenges for experimental reliability evaluation. To fulfill this needs the authors developed the nanoDAC method (nano Deformation Analysis by Correlation), which allows the
determination and evaluation of 2D displacement fields based on scanning probe microscopy (SPM) data. In-situ SPM scans of the analyzed object are carried out at different thermo-mechanical load states. The obtained topography-, phase- or error-images are
compared utilizing grayscale cross correlation algorithms. This allows the tracking of local image patterns of the analyzed surface structure. The measurement results of the nanoDAC method are full-field displacement and strain fields. Due to the application of
SPM equipment deformations in the micro-, nanometer range can be easily detected. The method can be performed on bulk materials, thin films and on devices i.e microelectronic components, sensors or MEMS/NEMS. Furthermore, the characterization and evaluation of
micro- and nanocracks or defects in bulk materials, thin layers and at material interfaces can be carried out.
The extraordinary mechanical properties of high strength aluminum alloys such as AA7075-T6 are caused by coherent nanoprecipitations. These nanoprecipitations generate local stress fields and interact with moving dislocations and propagating microcracks. In this paper, image correlation techniques are used to determine the local strain and stress field in the vicinity of fatigue crack tips during the loading of compact tension (CT) specimen. The fatigue crack tip was sharpened with decreasing fatigue loading after fatigue cracks initial appearance. Images of the crack tip were taken using atomic force microscopy/ultrasonic force microscopy (AFM/UFM) and white light interference microscopy (WLIM) before and after mechanical loading of the specimen. Both techniques are applicable for measuring the out-of-plane displacement during the loading process. In addition, image correlation techniques can be used to determine the in-plane displacement resulting from mechanical loading. This information is used to calculate the local stress intensity factor in the vicinity of the crack tips.
In order to modify material properties different kind of filler particles are added to polymer matrices. Miniaturization in electronics, MEMS and photonics applications forces to reduce the size of filler particles, even to submicron and nano scale dimensions. R&D processes as well as later production quality control demand suitable tools and procedures to characterize filler particles, e.g., within polymeric composites. The authors studied different AFM based methods of particle detection and imaging. The underlying purpose was to utilize stress state micrographs of composites with filler particles for deformation measurements. The foreseen digital image correlation technique (DIC) for highest resolution deformation analysis is briefly introduced. In order to understand the impact of filler particles on the mechanical behavior, particle identification and imaging as well as deformation measurement has to be performed on the same micrographs. Main emphasis in this work is made on different imaging modes realizable with scanning probe microscopy (SPM), which can be used to image and to characterize submicron and nano scale fillers. Additionally the influence of surface finishing before the SPM imaging is analyzed, mainly the impact of focused ion beam (FIB) polishing after mechanical polishing. The examined SPM methods for filler characterization are compared to alternative tools like FIB, SEM, AFAM and Laser Scanning Microscopy (LSM).
The rapid development of a wide variety of new devises in microelectronics, MEMS, NEMS and nano technology will lead to new challenges for their mechanical characterization and reliability assessment. Measurement of deformations and stresses in microscopic and even nanoscopic regions becomes a key issue. The authors make use of load state images captured by Atomic Force Microscopes (AFM) in order to measure object deformations. Out-of-plane deformation is determined from usual topography scans by computing surface profile differences. NanoDAC, a recently established approach, allows to meet these goals with regard to in-plane deformation. The method bases on cross correlations analysis performed on AFM scans, which are captured from thermally and/or mechanically loaded samples. Finally, local 3D displacement fields and in-plane strain fields are measured. A description of the basic principles and the capability of the technique are given. Furthermore, the authors demonstrate the potential of the mentioned method by its application to microcrack evaluation and the study of sensor and MEMS structure degradation. The first application corresponds to the measurement of crack opening displacement in the very vicinity of crack tips. As a consequence, fracture mechanics parameters are derived and allow to assess the defect with regard to possible crack propagation and component failure. This approach is used to study the influence of nanoscale material structures on crack behavior. The second example illustrates how the impact of thermal loading to the constitution of sensor or MEMS submicron layers is investigated by deformation analysis. The devices had been heated actively under the AFM. Degradation processes due to a severe thermal material mismatch were observed and monitored.