The worldwide market for Micro Electromechanical Systems (MEMS) has grown at an impressive rate over the past years and has become a true success story. First commercialised in the 1970s, MEMS products today are used in many high-tech industries — such as energy, consumer electronics, medical technology, defense and transportation — making MEMS a true cross-sectoral technology. This feature article will cast some light onto the R&D and manufacturing aspects of MEMS production by introducing some of the current and future requirements with regard to metrology — the science of measurement at high precision, resolution and repeatability.
As MEMS technology advances, many manufacturers demand an evolution of adequate metrology equipment to support them in their R&D efforts and to provide mass production quality control capabilities at the wafer level. Leading metrology suppliers, such as FRT, offer distinct know-how, as well as a comprehensive range of high-resolution, automatable measuring solutions dedicated especially to the needs of today’s MEMS industry.
Unflexible metrology solutions are inefficient
Because of the nature of MEMS, namely their great technical complexity at a very small scale in the micro and nano metre range, it is obvious that metrology tools with one or two highly specialised measuring tasks are inefficient. Instead, the industry increasingly seeks flexible and future-safe solutions that are able to perform the full host of measuring tasks for current and upcoming technology-nodes.
Most of today’s MEMS wafers contain ever-smaller structures that need to be measured. During production, wafers undergo many processing steps, which result in a variety of parameters that need to be determined. Therefore, it is not only pure measurement resolution that matters, but also the overall flexibility of what can be measured with the tool. Multisensor metrology tools fulfil these two requirements. They incorporate multiple high-performance technologies and can be adapted to special needs at any time, providing the ROI everyone in the industry is looking for.
Basic setup of a multisensor metrology tool
Multisensor metrology tools are available as manually-loaded or fully automated tools with robotic wafer handling and optimised wafer throughput. There are dozens of parameters that need to be measured during MEMS production. Some are manufacturer specific, others have been adopted from the semiconductor industry for example.
However, most of the parameters are based on the evaluation of 2D profile, 3D topography, roughness or film thickness and are calculated by complex software routines in an automated software environment. An example is the determination of membrane bow during the production of pressure sensors for automotive applications. Here, the metrology system automatically measures several profiles on the wafer, calculates the individual membrane bow on each die and passes the results to the production host.
Different measuring technologies available
The feature to be measured, the material-mix of the wafer and its overall surface properties determine which measuring technologies come to use. Common are combinations of topography measurement tools like confocal microscopy, scanning probe microscopy (e.g. AFM) or optical profilometry, and film thickness measuring techniques.
A new technology that has become more important, especially for MEMS manufacturers, is high-resolution laser interferometric vibrometers. They are used to measure dynamic properties and static displacements of micro-structures, MEMS and cantilevers at frequencies of up to
2 MHz and amplitude variations with sub-nanometre resolution.
The presented technologies work mostly in a non-contact, non-destructive way to protect the valuable wafers, which can cost several thousand dollars per piece depending on their state of processing.
Measurement of the third dimension
Looking for ever higher levels of functionality, circuit density and smaller footprints, developers have begun to head for the third dimension. They have started to stack individual chips onto one another and to interconnect them vertically with Through-Silicone-Vias (TSVs). As well as reduced space requirements, these 3D-packages offer better performance, because the interconnection length and therefore the electrical signal ways can be drastically shortened. Packaging with TSV is essential for new MEMS design.
Depending on the height of a 3D package, future TSVs can vertically extend over more than a hundred microns through the entire wafer stack, but at the same time be only a few micrometres in diameter. The proportion between depth to width can be described as the aspect ratio. Aspect ratio is one of the major benchmarks in TSV wafer metrology. At FRT, there are solutions to quickly measure pitch and depth of a via at aspect ratios of 1:20 and more (depending on wafer sample) as depicted on the measurement samples.
About the author:
Dr. Thomas Fries is founder and sole proprietor of FRT. Thomas has studied physics in Saarbrücken and Bonn, and received his doctorate degree in the field of surface physics and scanning probe microscopy. Before he founded FRT, Thomas started his professional career as head of the department of surface analysis in an SME and established scanning probe microscopy as a contract measuring service for the first time in Germany. Thomas is a renowned expert in surface physics and has published more than 60 papers in this field. He is known throughout the international scientific community and across the borders as an expert and therefore is an always welcome speaker on surface physics and metrology.
Email: info@frt-gmbh.com Web: www.frt-gmbh.com
