ATMOC (2021-2024)

Designing efficient optical and optoelectronic products requires nanomaterials with specific properties and dimensions. In the optics and semiconductor industry, the characterization of suitable nanomaterials is a challenging task. It may require accurate data acquisition, modelling, and reliable uncertainty estimation.

The purpose of this project is to create a framework for measuring the optical properties of thin-film structures over a wide range of wavelengths from infrared to X-rays. The practical characterization methods comprise traceable reflectometry, Mueller ellipsometry, and scatterometry techniques. Inverse modelling and virtual measurement methods will also be advanced in this project.

The role of Metrology Research Institute (MRI) of Aalto University in this project is to contribute to the spectrophotometric measurements and analysis of thin film and test materials using Aalto’s gonioreflectometry and spectrophotometry facilities. Furthermore, MRI uses mathematical modelling to determine the optical constants and thickness of components of single layer thin film structures and multilayer complex nanostructures.

Single layer structure analysis includes pre-characterization of twelve SiO₂ and two Al₂O₃ thin film samples on Si substrates. The measurements were performed using the Sentech micro-reflectometer with a 100-micrometer beam size (shown in Figure 1). 

The micro-reflectometer operated in the spectral range of 310 to 930 nm. We used a dedicated software which utilizes transfer matrix method (TMM) to fit the measured reflectance to obtain the thickness of the samples. Based on the geometry of microscope-based reflectometers, we developed a method to consider the distribution of the incident angles within a cone, the bandwidth effect and possible thickness variations in the samples. Using this method is crucial to improve our fitting process especially for the samples with thicker layers. Figure 2 shows the improved fit for the 2 µm SiO₂ on Si sample.

Multilayer complex nanostructures, where one of the thin layers consists of vacuum or air, are called PillarHalls. It has been developed to test new conformal thin film materials (Figure 3).

The PillarHall sample, featuring a nominal air gap height of 100 nm, was subjected to reflectometry measurements using a Cary 7000 spectrometer within a wavelength range 550-1800 nm (Figure 4).

Then this measurement data were analyzed with dedicated MATLAB code. The fitted reflectance spectrum for the three-layer model, as depicted in Figure 5, revealed an air gap thickness of 86 nm. The PolySi layer has a rectangular distribution of thickness values between 1590 nm and 1610 nm within the beam size [1].

Thus, measurement results obtained by reflectometry in visible and infrared wavelength range provide sufficiently information for analyzing these thin-film layer structures, giving access for estimation of layers thicknesses. 

[1] Aleksandr Danilenko, Masoud Rastgou, Farshid Manoocheri, Jussi Kinnunen, Virpi Korpelainen, Antti Lassila, Erkki Ikonen, "Characterization of PillarHall test chip structures using reflectometry technique." Measurement Science and Technology 34, 096006, 7 pages (2023).

     https://doi.org/10.1088/1361-6501/acda54

Further information: Masoud Rastgou and Aleksandr Danilenko