Ahsan Mian

The van der Pauw stress sensor

Piezoresistive sensors fabricated on (100) and (111) silicon surfaces are capable of measuring from four to all six components of the stress state at a point on the surface of an integrated circuit die. Such resistor-based sensors have been successfully designed and fabricated on these wafer planes and have been used successfully for measurement of die stresses in electronic packages by many research teams. In this paper, classical van der Pauw (VDP) structures, traditionally used for sheet resistance measurement, are shown to provide more than three times the sensitivity of standard resistor sensors. A single four-terminal VDP device replaces two resistor rosette elements and inherently utilizes the high-accuracy four-wire resistance measurement method. Theoretical expressions are developed for the change in resistance of the VDP device as a function of the individual stress components resolved in wafer coordinate systems on both the (100) and (111) silicon surfaces, and it is predicted theoretically that VDP devices will exhibit more than three times higher sensitivity to stress than standard resistor sensors. Design, fabrication, and experimental characterization of VDP and resistor test structures are presented for both silicon surfaces, and numerical simulation is used to help resolve discrepancies between theory and experiment. Sources of experimental error are identified, and the 3.16 times sensitivity enhancement of the VDP device is confirmed.

In-situ stress state measurements during chip-on-board assembly

In this work, die stresses in wire bonded chip-on-board (COB) packages have been measured using special [111] silicon stress test chips. The test die incorporate an array of optimized eight-element dual polarity piezoresistive sensor rosettes, which are uniquely capable of evaluating the complete stress state (six stress components) at points on the surface of the die. Sensor resistance measurements were recorded before packaging, after die attachment, and throughout the encapsulant cure process. Using the appropriate theoretical equations, the stresses at sites on the die surface have been calculated from the raw sensor resistance data. Also, three-dimensional (3-D) nonlinear finite element simulations of the chip-on-board packages were performed, and the stress predictions were correlated with the experimental test chip data.

Characterization of the Temperature Dependence of the Pressure Coefficients of n- and p-Type Silicon Using Hydrostatic Testing

Piezoresistive stress sensors on the (111) surface of silicon offer the unique ability to measure the complete stress state at a point in the (111) material. However, four-point bending or wafer-level calibration methods can measure only four of the six piezoresistive coefficients for p- and n-type resistors required for application of these sensors. In this work, a hydrostatic test method has been developed in which a high-capacity pressure vessel is used to apply a triaxial load to a single die over the -25degC to+100 degC temperature range. The slopes of the adjusted resistance change versus pressure plots yield pressure coefficients for p- and n-type silicon that provide the additional information necessary to fully determine the complete set of piezoresistive coefficients.

Laser Micro-joining of Dissimilar and Biocompatible Materials

Micro-joining and hermetic sealing of dissimilar and biocompatible materials is a critical issue for a broad spectrum of products such as micro-electronics, micro-optical and biomedical products and devices. Today, biocompatible titanium is widely applied as a material for orthopedic implants as well as for the encapsulation of implantable devices such as pacemakers, defibrillators, and neural stimulator devices. Laser joining is the process of choice to hermetically seal such devices. Laser joining is a contact-free process, therefore minimizing mechanical load on the parts to be joined and the controlled heat input decreases the potential for thermal damage to the highly sensitive components. Laser joining also offers flexibility, shorter processing time and higher quality. However, novel biomedical products, in particular implantable microsystems currently under development, pose new challenges to the assembly and packaging process based on the higher level of integration, the small size of the devices features, and the type of materials and material combinations. In addition to metals, devices will also include glass, ceramic and polymers as biocompatible building materials that must be reliably joined in similar and dissimilar combinations. Since adhesives often lack long-term stability or do not meet biocompatibility requirements, new joining techniques are needed to address these joining challenges. Localized laser joining provides promising developments in this area. This paper describes the latest achievements in micro-joining of metallic and non-metallic materials with laser radiation. The focus is on material combinations of metal-polymer, polymer-glass, metal-glass and metal-ceramic using CO2, Nd:YAG and diode laser radiation. The potential for applications in the biomedical sector will be demonstrated.

ANALYSIS AND CHARACTERIZATION OF ADHESIVELY BONDED Mg-STEEL LAP JOINTS

Dissimilar material joints are of significant interest in automotive applications. An investigation was carried out to determine the peculiarities of an adhesively bonded Mg-steel system for lap shear configuration. Both experimental approach and computational method (FEA) were utilized to evaluate and analyze the Mg-steel bond. The adhesive used was Betamate 1480 — an epoxy based adhesive. The tests were done according to ASTM D 1002-99 method using MTS machine at room temperature. For computational analysis, finite element modeling techniques using ABAQUS processor was utilized. Failure modes were studied for different systems. Results were compared with Mg-Mg and steel-steel systems. It is observed that Mg-Mg balanced system (system with equal adherend or substrate thickness) failed either at interface (adhesive failure) or at substrate and system is flexible with lower failure load. While steel- -steel balanced system failed only at substrate and system is rigid with higher load and lower displacement. Mg-steel system provides flexibility in between them and only adherend failure (either out of plane Magnesium failure or steel-betamate in plane substrate failure) observed. Cohesive failure was not observed in any of the systems. For Mg-Mg, the shear stress distribution in the adhesive is poor (stress distribution is steeper) while for steel-betamate-steel it is much better. The FEA models were compared and rationale was forwarded to assess the failure modes observed in each case.Copyright

Effects Of Laser Parameters On The Mechanical Response Of Laser Irradiated Micro-Joints

This paper is devoted to the laser irradiated joints between glass and polyimide. To facilitate bonding between them, a thin titanium film with a thickness of approximately 0.2 μm was deposited on glass wafers using the physical vapor deposition (PVD) process. Two sets of samples were fabricated where the bonds were created using diode and fiber lasers. The samples were subjected to tension using a microtester for bond strength measurements. The failure strengths of the bonds generated using fiber laser are quite consistent, while a wide variation of failure strengths are observed for the bonds generated with diode laser. Few untested samples were sectioned and the microstructures near the bond areas were studied using an optical microscope. The images revealed the presence of a sharp crack in the glass substrate near the bond generated with the diode laser. However, no such crack was observed in the samples made using fiber laser. To investigate further the reasons behind such discrepancy in bond quality, three-dimensional uncoupled finite element analysis (FEA) was conducted for both types of samples. The transient heat diffusion-based FEA model utilizes the laser power intensity distribution as a time dependent heat source to calculate the temperature distribution within the substrates as a function of time.

JOINING CHALLENGES IN THE PACKAGING OF BIOMEMS

Micro-joining and hermetic sealing of dissimilar and biocompatible materials is a critical issue for a broad spectrum of products such as micro-electronical, micro-optical and biomedical products and devices. Novel implantable microsystems currently under development will include functions such as localized sensing of temperature and pressure, electrical stimulation of neural tissue and the delivery of drugs. These devices are designed to be long-term implants that are remotely powered and controlled. The development of new, biocompatible materials and manufacturing processes that ensure long-lasting functionality and reliability are critical challenges. Important factors in the assembly of such systems are the small size of the features, the heat sensitivity of integrated electronics and media, the precision alignment required to hold small tolerances, and the type of materials and material combinations to be hermetically sealed.Laser micromachining has emerged as a compelling solution to address these manufacturing challenges. This paper will describe the latest achievements in microjoining of non-metallic materials. The focus is on glass, metal and polymers that have been joined using CO2, Nd:YAG and diode lasers. Results in joining similar and dissimilar materials in different joint configurations are presented, as well as requirements for sample preparation and fixturing. The potential for applications in the biomedical sector will be demonstrated.Micro-joining and hermetic sealing of dissimilar and biocompatible materials is a critical issue for a broad spectrum of products such as micro-electronical, micro-optical and biomedical products and devices. Novel implantable microsystems currently under development will include functions such as localized sensing of temperature and pressure, electrical stimulation of neural tissue and the delivery of drugs. These devices are designed to be long-term implants that are remotely powered and controlled. The development of new, biocompatible materials and manufacturing processes that ensure long-lasting functionality and reliability are critical challenges. Important factors in the assembly of such systems are the small size of the features, the heat sensitivity of integrated electronics and media, the precision alignment required to hold small tolerances, and the type of materials and material combinations to be hermetically sealed.Laser micromachining has emerged as a compelling solution to address these m...

Effect of Laser Power and Scan Speed on Melt Pool Characteristics of Commercially Pure Titanium (CP-Ti)

Selective laser melting (SLM) is an additive manufacturing technique that creates complex parts by selectively melting metal powder layer-by-layer using a laser. In SLM, the process parameters decide the quality of the fabricated component. In this study, single beads of commercially pure titanium (CP-Ti) were melted on a substrate of the same material using an in-house built SLM machine. Multiple combinations of laser power and scan speed were used for single bead fabrication, while the laser beam diameter and powder layer thickness were kept constant. This experimental study investigated the influence of laser power, scan speed, and laser energy density on the melt pool formation, surface morphology, geometry (width and height), and hardness of solidified beads. In addition, the observed unfavorable effect such as inconsistency in melt pool width formation is discussed. The results show that the quality, geometry, and hardness of solidified melt pool are significantly affected by laser power, scanning speed, and laser energy density.

Effect of process parameters on hardness, temperature profile and solidification of different layers processed by direct metal laser sintering (DMLS)

In DMLS process objects are fabricated layer by layer from powdered material by melting induced by a controlled laser beam. Metallic powder melts and solidifies to form a single layer. Solidification map during layer formation is an important route to characterize micro-structure and grain morphology of sintered layer. Generally, solidification leads to columnar, equiaxed or mixture of these two types grain morphology depending on solidification rate and thermal gradient. Eutectic or dendritic structure can be formed in fully equiaxed zone. This dendritic growth has a large effect on material properties. Smaller dendrites generally increase ductility of the layer. Thus, materials can be designed by creating desired grain morphology in certain regions using DMLS process. To accomplish this, hardness, temperature distribution, thermal gradient and solidification cooling rate in processed layers will be studied under change of process variables by using finite element analysis, with specific application to T...

A High Sensitivity MEMS Pressure Sensor

The piezoresistive effect in a van der Pauw (VDP) stress sensor subjected to biaxial stress was considered. The VDP resistance equations were combined with the silicon piezoresistivity equations to yield relations for the change in resistance of a VDP sensor in terms of the applied state of stress. Then the sensitivity of the VDP sensor to biaxial stress was determined analytically and simulated numerically. The biaxial stress states considered were those for a circular diaphragm under pressure. The VDP sensitivities to biaxial stress were compared to the sensitivity for the conventional piezoresistive stress sensor. It is observed that the theoretical (based on analytical and numerical results) pressure sensitivity of the new VDP sensor is about three times greater than the conventional counterpart.