Claus Villringer

Silicon-on-insulator slot-waveguide design trade-offs

Silicon-on-insulator slot-waveguide structures are designed and analysed numerically. We present our theoretical investigation of field confinement factors and effective nonlinear areas for different waveguide structures in order to find optimized geometrical dimensions. It is shown that a slot-waveguide with a height of 220 nm, a slot width of 180 nm and a silicon rail width of 180 nm provides a five times higher field confinement in the cladding region compared to conventional strip-waveguides which explains the high sensitivity of slot-waveguide based label-free bio-sensors.

Novel Ring Resonator Combining Strong Field Confinement With High Optical Quality Factor

Slot waveguide ring resonators appear promising candidates for several applications in silicon photonics. Strong field confinement, high device tunability, and low power consumption are beneficial properties compared with strip waveguides. Slot waveguide ring resonators suffer, however, from rather low optical quality factors due to optical losses. This letter proposes and experimentally demonstrates a novel concept based on a partially slotted ring and a strip-to-slot mode converter. An exceptional high quality factor of ~105 has been measured.

Design Optimization of Silicon-on-Insulator Slot-Waveguides for Electro-optical Modulators and Biosensors

An approach for design optimization of the geometrical parameters of silicon-on-insulator slot-waveguides for electro-optical modulators and biosensors is presented. Theoretical investigations of field confinement factors and effective nonlinear areas for different slot-waveguide structures are critically analyzed and thoroughly calculated. With our simulation results we explain the high efficiency of electro-optical modulators and the enhanced sensitivity of biosensors compared to strip-waveguides. The influence on the effective refractive index, field confinement factor, and effective nonlinear area of the slot width and the silicon rail width were investigated.

Partially slotted silicon ring resonator covered with electro-optical polymer

In this work, we present for the first time a partially slotted silicon ring resonator (PSRR) covered with an electro-optical polymer (Poly[(methyl methacrylate)-co-(Disperse Red 1 acrylate)]). The PSRR takes advantage of both a highly efficient vertical slot waveguide based phase shifter and a low loss strip waveguide in a single ring. The device is realized on 200 mm silicon-on-insulator wafers using 248 nm DUV lithography and covered with the electro-optic polymer in a post process. This silicon-organic hybrid ring resonator has a small footprint, high optical quality factor, and high DC device tunability. A quality factor of up to 105 and a DC device tunability of about 700 pm/V is experimentally demonstrated in the wavelength range of 1540 nm to 1590 nm. Further, we compare our results with state-of-the-art silicon-organic hybrid devices by determining the poling efficiency. It is demonstrated that the active PSRR is a promising candidate for efficient optical switches and tunable filters.

Hybrid-Waveguide Ring Resonator for Biochemical Sensing

This paper proposes a hybrid-waveguide ring resonator for on-chip biochemical sensing. Consisting of a low-loss strip-waveguide and a highly sensitive slot-waveguide integrated in a silicon photonic platform, it combines advantages of both waveguide types. In this way, it provides the unique feature to increase the sensitivity while maintaining low optical losses. Thus, this resonator structure may represent a promising alternative approach for future integrated biochemical sensing applications. This is suggested by a theoretical analysis, involving numerical simulation of the hybrid-waveguide ring resonator and an optimization of the slot-waveguide structure with regard to light-analyte-interaction. It is demonstrated that the hybrid-waveguide concept may overcome limitations in terms of overall resonator sensitivity, which is described by a figure of merit, connecting the optical losses with the resonator sensitivity.

Characterization and modeling of Fabry–Perot ultrasound sensors with hard dielectric mirrors for photoacoustic imaging

A Fabry-Perot ultrasound sensor with nonhygroscopic dielectric mirrors made out of Ta2O5 and SiO2 for use in photoacoustic tomography is described. The sensor offers flat frequency response up to 36 MHz, low noise-equivalent pressure (70 Pa), and near-omnidirectional response up to 20 MHz as well as optical transparency for near-infrared illumination. A numerical model was developed to predict its frequency response, and the results were validated experimentally. An image of the human palm was acquired to demonstrate in vivo imaging capabilities.

Development of tunable Fabry-Pérot polymer film sensors for parellelised photoacoustic signal acquisition (Conference Presentation)

Fabry-Perot (FP) polymer film sensors exhibit small element sizes, high acoustic sensitivity, transparency and flat frequency response to enable high resolution 3D photoacoustic (PA) imaging in backward mode. However, conventional raster scan interrogation can result in slow data acquisition (several min for 3D images) compared to parallelized piezoelectric detector arrays. To address this limitation, parallelization using a camera-based readout of FP sensors is investigated. This approach requires the optical thickness of the polymer spacer to be sufficiently uniform over the scan area to obtain high acoustic sensitivity for all active elements. Since the deposition of passive polymer layers with sufficient homogeneity of thickness is challenging, the use of electro-optically (EO) or piezoelectric (PE) tunable polymer film spacers is investigated. The spacers are sandwiched between two dielectric mirrors and transparent electrodes to form an FP sensor. In this work, spin coated guest-host systems consisting of EO chromophores (2-methyl-4-nitroaniline) embedded in a PMMA matrix, and thermally evaporated PE film spacers (PVDF) were examined. Both systems were electrically poled using a corona discharge. The optical transfer function, the transmission spectrum of the excitation passband from 600 nm to 1100 nm and the tuning range of the FP sensors were determined. Furthermore, the detection of PA waves was demonstrated. Tunable FP sensors in conjunction with camera-based interrogation techniques have the potential to provide 3D image acquisition times on the order of seconds.

Parallelised photoacoustic signal acquisition using a Fabry-Perot sensor and a camera-based interrogation scheme

Tomographic photoacoustic (PA) images acquired using a Fabry-Perot (FP) based scanner offer high resolution and image fidelity but can result in long acquisition times due to the need for raster scanning. To reduce the acquisition times, a parallelised camera-based PA signal detection scheme is developed. The scheme is based on using a sCMOScamera and FPI sensors with high homogeneity of optical thickness. PA signals were acquired using the camera-based setup and the signal to noise ratio (SNR) was measured. A comparison of the SNR of PA signal detected using 1) a photodiode in a conventional raster scanning detection scheme and 2) a sCMOS camera in parallelised detection scheme is made. The results show that the parallelised interrogation scheme has the potential to provide high speed PA imaging.

Very high aspect ratio through silicon via reflectometry

Through Silicon Via (TSV) technology is a key feature of new 3D integration of circuits by creation of interconnections using vias, which go through the silicon wafer. Typically, the highly-selective Bosch Si etch process, characterized by a high etch rate and high aspect ratio and forming of scallops on the sidewalls is used. As presented in this paper, we have developed an experimental setup and a respective evaluation algorithm for the control and monitoring of very high aspect ratio TSV profiles by spectroscopic reflectometry. For this purpose square via arrays with lateral dimension from 3 to 10 μm were fabricated by a Bosch etch process and analyzed by our setup. By exploiting interference and diffraction effects of waves reflected from the top and bottom surfaces as well as from the side walls of the TSV patterns, the measurements provided etch depths, CD values and scallop periods. The results were compared with data obtained by a commercial wafer metrology tool. Aspect ratios of up to 35:1 were safely evaluable by our setup.

Development of graphene process control by industrial optical spectroscopy setup

The successful integration of graphene into microelectronic devices depends strongly on the availability of fast and nondestructive characterization methods of graphene grown by CVD on large diameter production wafers [1-3] which are in the interest of the semiconductor industry. Here, a high-throughput optical metrology method for measuring the thickness and uniformity of large-area graphene sheets is demonstrated. The method is based on the combination of spectroscopic ellipsometry and normal incidence reflectometry in UV-Vis wavelength range (200-800 nm) with small light spots (~ 30 μm2) realized in wafer optical metrology tool. In the first step graphene layers were transferred on a SiO2/Si substrate in order to determine the optical constants of graphene by the combination of multi-angle ellipsometry and reflectometry. Then these data were used for the development of a process control recipe of CVD graphene on 200 mm Ge(100)/Si(100) wafers. The graphene layer quality was additionally monitored by Raman spectroscopy. Atomic force microscopy measurements were performed for micro topography evaluation. In consequence, a robust recipe for unambiguous thickness monitoring of all components of a multilayer film stack, including graphene, surface residuals or interface layer underneath graphene and surface roughness is developed. Optical monitoring of graphene thickness uniformity over a wafer has shown an excellent long term stability (s=0.004 nm) regardless of the growth of interfacial GeO2 and surface roughness. The sensitivity of the optical identification of graphene during microelectronic processing was evaluated. This optical metrology technique with combined data collection exhibit a fast and highly precise method allowing one an unambiguous detection of graphene after transferring as well as after the CVD deposition process on a Ge(100)/Si(100) wafer. This approach is well suited for industrial applications due to its repeatability and flexibility.