Abstract
The electronics industry with all its branches such as semiconductors, organic-electronics and the photovoltaics industry, is continuously growing in terms of its economic as well as its technological influence, [1]. This trend is fostered by the ongoing integration of various electronic functionalities in fields such as energy generation & distribution, transport & mobility as well as in consumer goods. Over three decades, electronic and semiconductor products as well as processes were driven by Moores law, [2].
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The electronics industry with all its branches such as semiconductors, organic-electronics and the photovoltaics industry, is continuously growing in terms of its economic as well as its technological influence, [1]. This trend is fostered by the ongoing integration of various electronic functionalities in fields such as energy generation & distribution, transport & mobility as well as in consumer goods. Over three decades, electronic and semiconductor products as well as processes were driven by Moores law, [2]. This paradigm focused nearly exclusively on the miniaturization of structures and therefore on the increase of the number of transistors per chip area. While this paradigm was directly attributed to an increase in computing power, the industry’s scope broadened in recent years. With the introduction of the so-called More-than-Moore paradigm, the enhancement of functionality and cross-modality integration has become more important, [3]. The implementation of this approach has enabled the development of novel products where functionalities of the analog, digital as well as of the power domain were combined. Furthermore, sensors and other MEMS have been used to enhance these products, [4]. Opposed to traditional geometrical scaling of chips this is also known as performance scaling, [5]. The implementation of these ideas at the product level has enabled the transformation of traditional analog components from board level to package (SiP—system in package) or chip level (SoC—system on chip). This includes a significant increase of complexity in semiconductor manufacturing, [6, 7]. New and advanced processes have been developed to enable three-dimensional production and packaging. Additionally, novel concepts for the management of disturbances such as thermal and electro-magnetical influences have been developed. All these product and process developments drove research and applications of novel materials and material combinations, [8–11]. On the one hand, this enabled bold innovations in power semiconductors regarding performance and energy efficiency which are main drivers behind electro-mobility and sustainable energy, [12, 13]. On the other hand, the novel and advanced manufacturing technologies imposed great challenges towards quality assurance, reliability and therefore metrology, [14]. According to Leach et al., [15], the currently available metrology still lags behind advanced production regarding precision, measurement speed and cost scaling. Where processes such as lithography could significantly scale on a cost per unit basis, metrology hardly did. This was partly due to technological reasons but also partly due to high specialization of metrology approaches with a strong focus on single applications. On the technological side, the main challenges lie in the need for systems that deliver large measurement ranges with high precision (high-dynamic range) while maintaining fast speeds to ensure process-integrated operation. Appropriate metrology has to enable the determination of geometrical parameters like critical dimensions or surface profiles alongside functional parameters such as surface roughness or defect detection. A typical case for the requirements of a process-integrated metrology tool is given by the stack of a modern solar cell module, [15]. The cell itself consists of a number of active and barrier layers, deposited by thin-film processes, which require the layer thicknesses to be monitored in order to ensure performance. Interconnects or divides such as bus-bars, laser scribes and vias need to be observed regarding positions and dimensions. Optics with high aspect ratios on top of the cell increase the efficiency and must be measured with respect to their surface profile. Additionally, polymeric adhesives as well as a polymeric top laminate protect the compound against environmental hazards. Monitoring of the degree of cross-linking of these materials enables optimal performance over 20 and more years of operation, [16].
The main scope of this work is the development of a metrology system which is capable of meeting the main requirements posted by this exemplary application in terms of resolution, dynamic range, measurement speed and flexibility. More precisely, the metrology approach aims to measure surface profiles, thin-film layer thickness as well as the degree of cross-linking in polymers. A review of relevant research literature paved the way for the development of a novel approach based on the principles of low-coherence interferometry. The enhancement of the principle by the introduction of dispersion encoding is performed in order to facilitate higher resolutions while maintaining large measurement ranges, known as high-dynamic range metrology. This approach will be called DE-LCI. With regard to surface profilometry, the aim is to cover an axial measurement range of nearly 100 µm while having sub-nm resolution. Additionally, the capabilities of the novel approach regarding the characterization of thin-film thickness as well as polymeric cross-linking will be assessed. This work evaluates the necessary developments of the setup and the analysis algorithms facilitating all three possible measurement modes.
The design and implementation of the novel approach will furthermore focus on the robustness for thermal and mechanical influences to maintain the resolution during long measurements. Another design criterion is the preparation for process integration which can be translated to acquisition and data processing speed. This work will demonstrate efforts to capture large sections of a sample comparatively fast to conventional methods. Theoretical limits of measurement range and resolution will be investigated on a more detailed level. Also, the occurrence of these limits will be assessed under practical conditions. In particular, it will be investigated which transients influence the axial resolution and measurement range and which technological measures can be taken to partially decouple axial and lateral measurement ranges. Furthermore, this work will evaluate techniques to gather two- and three-dimensional information of surfaces and bulk materials without the need for mechanical scanning to reduce influences from movement and increase measurement speed.
In terms of the characterization of polymeric cross-linking, it is investigated how the wavelength-dependent refractive index can be used as a measure for cross-linking. Consequently, the resolution of the DE-LCI approach will be examined with regard to the refractive index. An exploration of the capabilities to resolve nm-sized layers of thin-film materials will conclude the work.
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Taudt, C. (2022). Introduction and Motivation. In: Development and Characterization of a Dispersion-Encoded Method for Low-Coherence Interferometry. Springer Vieweg, Wiesbaden. https://doi.org/10.1007/978-3-658-35926-3_1
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DOI: https://doi.org/10.1007/978-3-658-35926-3_1
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