Contact-free and non-destructive fast doping profiling of semiconductors using terahertz radiation
by Jaime G贸mez Rivas, Marion Matters-Kammerer
Advanced semiconductor chips and devices combine multiple layers with different compositions (materials, dopant densities, and charge carrier mobilities) that define their performance. Retrieving the exact composition as a function of position and depth after growth of the layers is time-consuming, invasive and/or destructive, which leads to non-optimum production yields for the semiconductor industry and an important waste of resources by the disposal of full wafer鈥檚 batches with defective layers. To improve sustainability in semiconductor foundries, it is necessary to develop fast, contact-free, and non-destructive techniques for the retrieval of the composition in semiconductor multilayers and heterostructures. Therefore, we aim to introduce terahertz (THz) electromagnetic radiation for fast and non-destructive spatially-resolved tomography of complex multilayered semiconductors relevant for industry. THz doping tomography could be deployed with every wafer after growth and before entering the device production chain to control material quality and virtually leading to zero-faults in semiconductor growth.
Opportunity 鈥� THz Doping Tomography
The semiconductor industry relies on wafers with a precise composition of multiple layers that provide the desired performance to devices in applications in different domains (ICs, solid-state lighting, photovoltaics, energy storage, quantum technologies, etc.). These layers are grown in reactors that deposit the materials layer by layer by controlling their thickness and composition (materials, defects, dopant atoms, and dopant density) to control properties such as carrier mobility (conductivity) and lifetime. An example of a semiconductor layer stack for LEDs is shown in Figure 1. Despite the critical relevance of the composition of these layers for the final device performance, current techniques for retrieving this composition are time-consuming, invasive, and/or destructive.
Hence, with the introduction of novel, fast, and non-destructive techniques, such as THz doping profiling, for the determination of semiconductor composition and properties as a function of depth and lateral position, it will be possible to characterize every wafer after growth and before it enters the production chain. This characterization will suppress the waste of resources by immediately detecting growth faults. It will also avoid the expensive processing of devices with faulty semiconductors that will be discarded after production, and it will also simplify the testing protocols of the devices.
Roadblock - Desctructive Technology for Semiconductor Metrology
Current techniques for retrieving the composition and properties of as-grown semiconductor multilayers, such as C-V profiling or secondary ion mass spectroscopy, rely on removing layer by layer while measuring the composition and/or properties. These techniques are very slow and destructive. Moreover, they have poor lateral resolution. Other techniques, such as 4-probe or Hall-mobility measurements, require electrical contacts and probe only the surface of the semiconductor. Therefore, these techniques are applied to a very small fraction of the produced wafers to check that the growth process is delivering the desired materials. Obvious problems of this approach include the necessity of discarding all wafers produced between the tested ones when the last gives a defective structure. Also, potentially defective wafers can enter the production chain leading to non-working devices.
Research Hypothesis
Our research targets the development of THz doping tomography as a fast, contact-free, and non-destructive technique that can be used to characterize every wafer after growth. With this technique, we will be able to retrieve critical material properties as a function of depth and with millimeter to micrometer lateral resolution1.
THz radiation corresponds to electromagnetic radiation with a frequency in the range 0.1 to 10 THz (far-infrared radiation). THz radiation is low-energy (non-ionizing) radiation that penetrates most materials, but is attenuated/reflected by metals. This is the reason why THz radiation is becoming popular for security screening in airports. THz radiation also penetrates semiconductors, being attenuated by free-charge carriers. Therefore, it can be used to probe multilayered semiconductors and retrieve quantitative information of the dopant composition, carrier mobility, and carrier lifetimes as a function of depth and position.
To illustrate the potential use of THz radiation for semiconductor tomography, we have measured the transmission through the layer stack shown in Fig. 1 (measurements shown in Fig. 2 with the green curve). The blue curve in Fig. 2 illustrates the calculation of the expected THz transmission considering the manufacturer specifications for this wafer, while the red curve shows the calculation of the transmission using the real layer composition retrieved using standard destructive techniques (C-V profiling). The large discrepancies between the measurement and the expectations with the information provided by the manufacturer illustrate the potential impact that THz tomography could have on the semiconductor industry.
Solution, research questions, and timelines
The novelty of the THz doping tomography technique that we propose to develop includes the controlled optical pumping of free carriers. This pumping will enable the unambiguous determination of compositions and layer thickness without prior knowledge of the samples. To fully exploit the potential of THz radiation for semiconductor metrology, several research questions need to be addressed:
- What is the limit accuracy for the determination of doping profiles? The combination of multiple wavelengths for optical pumping of the semiconductor stack will reduce uncertainties [2024-2026].
- Can we measure atomically thin 2D semiconductors in a fast manner (less than 1s)? We have recently measured atomic bilayers of 2D semiconductors using THz radiation2,3, but the measurements still require long integration times to improve signal-to-noise ratios [2025-2027].
- What is the lateral resolution limit for the determination of doping profiles? The lateral resolution in THz spectroscopy is limited by the diffraction limit (millimeter resolution). To improve this resolution, it is necessary to develop robust near-field THz tomography with THz microprobes [2027-2030].
- How can THz doping tomography be integrated for in-line characterization of semiconductors in production chains? [2028-2030].
REFERENCES
1 鈥淭ime-Resolved Terahertz Time-Domain Near-Field Microscopy鈥�, Optics Express 26, 32118-32129 (2018).
2 鈥淗igh-Frequency Sheet Conductance of Nanolayered WS2 Crystals for Two-Dimensional Nanodevices鈥�, ACS Applied Nano Materials (2022).
3 鈥淭hickness-Dependent Auger Scattering in a Single WS2 Microcrystal Probed with Time-Resolved Terahertz Near-Field Microscopy鈥�, Optics Letters 48, 708-711 (2023).
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