Jeremy Murray

Characterisation of electroplated NiFe films using test structures and wafer mapped measurements

Nickel-iron alloys have useful magnetic properties that are of interest to the MEMS industry, but the high stress levels that can develop during the fabrication process pose a real challenge. This paper addresses the characterisation of NiFe films using suspended rotating structures, electrical test structures and X-ray fluorescence measurements. An automated measurement system has been developed, which facilitates rapid wafer mapping to spatially compare stress levels at different stages of the fabrication process. This has been used, together with other wafer mapped parameters such as alloy composition, sheet resistance and layer thickness, to identify correlations and provide an increased understanding of the relationships between the different process control factors.

Fabrication and Measurement of Test Structures to Monitor Stress in SU-8 Films

SU-8 is an epoxy-based, negative photoresist that is widely used in the manufacturing of micromechanical systems. The polymer cross-linking that occurs during the photolithographic processing of SU-8 can result in high levels of stress in the patterned film. This has significant implications for the yield and reliability of SU-8 structures and needs to be understood if the material is to be integrated with other technologies. This paper describes micromechanical test structures that provide the opportunity to wafer map the stress in SU-8 at different stages of the process. The structures are fabricated in a thick layer of SU-8 and are subsequently released from the underlying substrate using a dry chemical vapor etch process. An automated optical measurement system has been built to allow rapid optical inspection of many thousands of test structures fabricated on 200 mm wafers. Initial results indicate significant tensile stress in the SU-8, which demonstrates a radial variation along with a dependence on the process conditions.

Quantitative wafer mapping of residual stress in electroplated NiFe films using independent strain and Young's modulus measurements

The uncontrolled development of stress within MEMS deposited and processed films can be detrimental for both device performance and reliability. This work focuses on combining the data from previously reported strain measurements obtained from mechanical test structures with new nano-indentation measurements of Youngs modulus on both micromachined films and cantilevers. Both strain and Youngs modulus data are then used to produce arguably the first quantitative wafer-level stress mapping of residual stress in micromachined materials. Results show significant local variation and possible correlation between Youngs modulus and percentage of iron in the film. The measured values for the two test wafers, namely Youngs modulus and residual stress, fall within the range of ~30 to ~180 GPa and ~50 to ~220 MPa, respectively. Youngs modulus measurements on cantilevers show a consistent ~20% difference with respect to traditional indentation measurements, suggesting that this setup may help reduce or remove the influence of the substrate.

Analysis of the performance of a micromechanical test structure to measure stress in thick electroplated metal films

Previously reported suspended microrotating test structures designed to measure the stress in thick layers of electroplated Permalloy (NiFe alloy) have been analysed using finite element modelling and compared with experimental measurements. These results have been used to optimise a stress sensor test structure and design a new mask, with an array of test structures specifically designed to wafer map the stress of thick nickel and Permalloy films. This is the first time these structures have been employed for determining spatial variation in film stress and the results of this characterisation are reported for nickel.

A wafer mapping technique for residual stress in surface micromachined films

The design of MEMS devices employing movable structures is crucially dependant on the mechanical behaviour of the deposited materials. It is therefore important to be able to fully characterize the micromachined films and predict with confidence the mechanical properties of patterned structures. This paper presents a characterization technique that enables the residual stress in MEMS films to be mapped at the wafer level by using microstructures released by surface micromachining. These dedicated MEMS test structures and the associated measurement techniques are used to extract localized information on the strain and Youngs modulus of the film under investigation. The residual stress is then determined by numerically coupling this data with a finite element analysis of the structure. This paper illustrates the measurement routine and demonstrates it with a case study using electrochemically deposited alloys of nickel and iron, particularly prone to develop high levels of residual stress. The results show that the technique enables wafer mapping of film non-uniformities and identifies wafer-to-wafer differences. A comparison between the results obtained from the mapping technique and conventional wafer bow measurements highlights the benefits of using a procedure tailored to films that are non-uniform, patterned and surface-micromachined, as opposed to simple standard stress extraction methods. The presented technique reveals detailed information that is generally unexplored when using conventional stress extraction methods such as wafer bow measurements.

Fabrication of test structures to monitor stress in SU-8 films used for MEMS applications

SU-8, an epoxy based negative photoresist, is widely used in the manufacture of micromechanical systems but can exhibit significant levels of stress build-up during processing. This paper describes micromechanical test structures that provide the opportunity to spatially characterise stress the in SU-8 at different stages of the process. The structures are fabricated in a thick layer of SU-8 and are subsequently released from the underlying substrate using a dry chemical vapour etch process. The initial results indicate that there is significant tensile stress in the SU-8, and that this demonstrates a radial variation along with a dependence on the process conditions.

Correlation of optical and electrical test structures for characterisation of copper self-annealing

The self-annealing properties at room temperature of electrochemically deposited copper have been investigated for the first time using test structures to simultaneously measure the spatial variation of sheet resistance and film stress with time. The phase changes associated with the self-annealing of copper on 200mm wafers have been observed over a period of 30 hours indirectly through these parameters, mapped across the wafer. In particular, this paper reports the influence of the electroplating current density on the resulting characteristics of the deposited copper.

Integration of Electrodeposited Ni-Fe in MEMS with Low-Temperature Deposition and Etch Processes

This article presents a set of low-temperature deposition and etching processes for the integration of electrochemically deposited Ni-Fe alloys in complex magnetic microelectromechanical systems, as Ni-Fe is known to suffer from detrimental stress development when subjected to excessive thermal loads. A selective etch process is reported which enables the copper seed layer used for electrodeposition to be removed while preserving the integrity of Ni-Fe. In addition, a low temperature deposition and surface micromachining process is presented in which silicon dioxide and silicon nitride are used, respectively, as sacrificial material and structural dielectric. The sacrificial layer can be patterned and removed by wet buffered oxide etch or vapour HF etching. The reported methods limit the thermal budget and minimise the stress development in Ni-Fe. This combination of techniques represents an advance towards the reliable integration of Ni-Fe components in complex surface micromachined magnetic MEMS.

Micromechanical test structures for the characterisation of electroplated NiFe cantilevers and their viability for use in MEMS switching devices

This paper presents the fabrication of a series of test devices designed to prove the viability of electroplated NiFe freestanding structures for use in magnetically actuated MEMS switches. Preliminary results show promising actuation responses and further release optimisation and testing will enable the quantitative measurement of the desired characteristics. In addition, this will potentially enable the mechanical characterisation of freestanding structures in other materials by means of magnetic actuation, simply by depositing small quantities of NiFe or other magnetic materials in convenient areas of existing devices.

Development of an Advanced System for Automated 200 mm Wafer Mapping of Stress Using Test Structures

Controlling and understanding the stress in materials is of major importance in the successful fabrication of MEMS devices. Failure to properly account for stress related effects can lead to the substrate warping and layer delamination, both of which are detrimental to the performance and reliability of components. Hence, it is desirable to have reliable and automated technology to spatially monitor both stress and strain on silicon wafers. This paper reports in detail an integrated measurement system that has been specifically designed to semiautomatically wafer map stress, strain and Youngs modulus. The measurement system is designed to determine the rotation of a test structure automatically and then calculate the strain. Youngs modulus is then determined using a nanoindenter running customised software and the combination of the two measurements from the same location is used to calculate and map the spatial stress in the film.