Advances in optical metrology allow for non-contact inline inspection with resolutions up to 1µm g

properties and of the physics involved in making the measurement. And it’s not just the instrument manufacturing process that needs to adapt, as the test, measurement and monitoring of semiconductor production using different materials will also require different standards. The European Metrology Programme

for Innovation and Research (Empir) was developed as part of Horizon 2020, the EU framework programme for research and innovation. It has been funded by members of the Euramet alliance, to support collaborative research between measurement institutes, academia and industry to address key challenges and ensure measurement science meets the future.

The alliance’s A new standard for better 3D chips highlighted that, to address the nearing limit of processing power and demand for smaller devices, manufacturers are assembling chips with parts on top of each other, or stacking. This additional level of architecture, of course, requires new types of measurement, for instance, to verify that components are correctly aligned for devices to function. In a case study published last year,

Euramet explained that, while optical analysis methods currently offer the best solution, the standards required for accurate measurements at this nanometre scale are not yet available. So, under the Empir

20 Electro Optics June 2021

project, the National Metrology Institute of Germany (PTB) developed a standard using advanced electron-beam lithography and reactive ion etching techniques to enable a state-of-the-art characterisation of 3D microscopes. The surface features nanometre sized, sharp-edged, circular concentric ‘rings’ made to precise dimensions with known measurement, the spacing, or distribution frequency, between each groove forms a geometric pattern that the PTB termed a ‘chirp’. The pattern is designed to be ideal for characterising a microscope’s instrument transfer function, 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 an instrument’s performance at detecting the height of nanometre-sized structures, allowing optical measurements to be accurately assessed at the nanometre scale. After the project optical measurement

systems provider Zygo used the standard to assess the performance of its product range, including 3D microscopes, sensors and precision optical components. With its Nexview advanced optical surface profiler, the company was able to measure the new standard’s challenging features within the specified accuracies.

The Nexview 3D imaging and measurement system was designed to provide interactive surface metrology for semiconductor analysis, with multiple measurements including flatness, roughness, large steps and segments, thin films, and steep slopes, with features’ heights ranging from <1nm up to 20,000μm. It features ISO 25178 surface measurement parameters and a streamlined design with no manual controls. Zygo believes that the use of the standard helped the company improve the results of fine-structure surface measurements. Product manager Eric Felkel said: ‘Increased measurement precision for high-aspect-ratio components will allow a greater confidence in the use of optical microscopes for the rapid, inline detection of defects in 3D chips, as well as in other industries that use submicron components.’

Close collaboration In the development of optical metrology systems for semiconductor production, it is important for providers to work closely with customers to understand their changing requirements. This is something Precitec’s Pichot believes is vital. ‘OEM customisation is very important to us. We work with customers and can customise a probe to adapt to the customer and their requirements.’ In terms of the types of technology

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