A new standard for better 3D chips
Challenge
The processing power of traditional silicon chips, or integrated circuits, is now reaching its limit. To address this, and the demand for smaller devices, manufacturers are introducing ‘3D’ chips where parts are assembled on top of each other. This additional level of architecture requires new types of measurements to verify that components are correctly aligned to ensure appliances function as intended. A commonly used method is ‘stylus profiling’ where the movement of a stylus over a surface maps its features, but this process can damage delicate parts and is relatively slow, unsuited to high-throughput manufacturing.
Optical measurements using 3D microscopes are an attractive alternative as they are non-contact, rapid and can measure larger areas but several problems require addressing before these can be routinely used. 3D chips contain ‘high-aspect-ratio’ components, which are tall but have a small surface area, and feature sharp edges that are hard to detect accurately using light microscopes. Furthermore, these chips contain sub-micron elements often at the resolution level of these instruments.
Accurate, traceable measurements require that these factors must be accounted for, but instrument calibration standards are not available, limiting this approach.the SI.
Solution
Within the EMPIR project Metrology for manufacturing 3D stacked integrated circuits the National Metrology Institute of Germany (PTB) developed a new standard using advanced electron-beam lithography and reactive ion etching techniques to enable a state-of-the-art characterisation of 3D microscopes. The surface contains nanometre sized, sharp-edged, circular concentric ‘rings’ which give the standard innovative design features. Made to precise dimensions with known measurement accuracies the spacing, or distribution frequency, between each ‘groove’ form a geometric pattern termed a ‘chirp’. This type of pattern is ideal for characterising a microscope’s ‘Instrument Transfer Function’ (ITF) which indicates the lateral resolving power of optical instruments for making 3D surface measurements. The rotational symmetric patterns of the rings are designed to detect angular-dependent asymmetries and their depth informs on an instruments performance at detecting the height of nanometre sized structures. As a result, optical measurements can be accurately assessed at the nanometre scale.
Impact
Following the end of the project the Zygo Corporation, a global leader in the design and manufacture of advanced optical measurement systems, used the standard to assess the performance of their products. These include a range of 3D microscopes, sensors and precision optical components and are used in a wide range of markets, from consumer electronics to the aerospace and biotechnology industries.
With its Nexview™ advanced optical surface profiler, the Company was able to measure the new standard’s challenging features within the specified accuracies. This has helped Zygo establish the best approach to measure features with dimensions approaching the resolution level of the most advanced optical microscopes, knowledge they can now pass on to their customers facing similar measurement issues. Zygo considers that the use of the standard helped them improve the results of fine-structure surface measurements and provided demonstrable data of the superior performance of their instruments relative to competitors.
Increased measurement precision for high-aspect-ratio components will allow a greater confidence in the use of optical microscope for the rapid, inline detection of defects in the 3D chips as well as in other industries that use submicron components.
- Category
- EMRP,
- Industry,
- Standardisation,