Authors: Nanu Brates, Renaud Richard, Jessie McCarthy, Vikram Singh
Laser-Driven Light Source to Optimize FTIR Performance
Energetiq’s Laser-Driven Light Source (LDLS®) uses a laser to excite and sustain a 100 µm – 320 µm diameter xenon plasma. The result is an extremely bright, broadband radiation emitter, covering the wavelength range from 170 nm to 2400 nm (DUV-NIR). Because it is laser-sustained, the brightness, spatial stability, and lifetime are not limited due to the use of electrodes, as is the case in traditional plasma sources, such as a xenon short-arc lamp.
Recently, Energetiq expanded the emission band of the LDLS to the mid-IR with the EQ-77C, pictured in Figure 1. This was achieved by designing a new light cell with different output window material, allowing transmission of the xenon plasma discharge broad spectrum from 350 nm up to 20 μm (VIS to mid-IR). The very small plasma of the EQ-77C produces high spectral radiance, about ten times higher than a thermal IR source (e.g., Globar®), as seen in Figure 2, and exhibits a high level of spatial stability.
The combination of high radiance and spatial stability allows light to be efficiently coupled into small apertures and small-diameter fibers. The small plasma also approximates a point source, enabling collimation with a low divergence angle, which is important for applications with long optical path lengths.
Figure 3 shows a Globar, the typical IR source currently used for FTIR. Fabricated from silicon carbide or silicon nitride, it is electrically heated to a temperature of 1100-1650K. Target applications for the new EQ-77C LDLS are:
- semiconductor metrology and process monitoring.
- optical coating process monitoring and control.
- gas analysis.
- S-SNOM microscopy.
Figure I: Energetiq’s new EQ-77C broadband light source
Figure II: Spectral radiance of LDLS with extended
mid-IR spectral output
Figure III: Silicon carbide Globar at 1300K
FTIR for Detecting Elemental Composition During Semiconductor Manufacturing
Fourier Transform Infrared (FTIR) reflectance spectroscopy is a preferred technique when characterizing film chemistry and composition due to its non-destructive nature and excellent sensitivity to molecular bonds and free carriers. While FTIR spectroscopy has been widely used in R&D environments, its application to mainstream production metrology and process monitoring on product wafers has historically been limited. These limitations have been eliminated via a series of continuous advances in FTIR technology, including the use of new broadband IR sources, new sampling optics, and comprehensive model-based analysis [1].
FTIR spectroscopy also offers real-time solutions for Epi thickness, trench depth measurements, and thin film such as high-k and low-k chemical composition. Through optical modeling of the transmission or reflectance spectra, information about the electronic structure and chemical composition may be obtained, which can then be used for process control and monitoring.
For thin film thickness in-line monitoring, the semiconductor industry has historically utilized the LDLS technology for spectral ellipsometry and reflectometry, especially in deposition equipment. This application has now expanded from front-end processes to back-end processes with the rapid adoption of advanced packaging. A relatively newer aspect to semiconductor manufacturing process control has been composition [1], and FTIR is gaining wide adoption. With costs of chip manufacturing rapidly increasing, the need to monitor both the thickness and composition of every wafer is critical. This is a key inflection for the adoption of the EQ-77C LDLS given its throughput benefits.
Figure IV: FTIR Layout for measuring chemical composition [2]
Summary
The measurement capabilities of FTIR spectroscopy for the hydrogen bonding in cell silicon nitride and amorphous carbon hard masks (ACHM) were previously demonstrated [2]. For cell silicon nitride, deconvolution of the spectra allowed differentiation between individual peaks corresponding to Si-N, Si-H, N-H, Si-O, and Si-OH bonds [2]. Further improvements to the FTIR measurement methodology could be realized with an LDLS-based IR source (in place of the SiN Globar). The combination of the higher radiance and spatial stability found in the LDLS allows light to be efficiently coupled into small apertures and increases SNR.
References
- Horiba. (n.d.). High-k dielectric with nanoscale thickness studied by spectroscopic ellipsometry and FTIR-ATR. Retrieved from
https://static.horiba.com/fileadmin/Horiba/Application/Information_Technology/Semiconductors/Display_technologies/High_k_dielectric_with_nanoscale_thickness_studied_by_Spectroscopic_Ellipsometry_and_FTIR-ATR.pdf - Frederick, Joshua et. al. Advanced FTIR optical modeling for hydrogen content measurements in 3D NAND cell nitride and amorphous carbon hard mask. Proc. SPIE 12955, Metrology, Inspection, and Process Control XXXVIII, 129550H (9 April 2024); https://doi.org/10.1117/12.3010524