Sidewall Roughness Measurement Research Articles

Investigation for Sidewall Roughness Caused Optical Scattering Loss of Silicon-on-Insulator Waveguides with Confocal Laser Scanning Microscopy

Sidewall roughness-caused optical loss of waveguides is one of the critical limitations to the proliferation of the silicon photonic integrated circuits in fiber-optic communications and optical interconnects in computers, so it is imperative to investigate the distribution characteristics of sidewall roughness and its impact upon the optical losses. In this article, we investigated the distribution properties of waveguide sidewall roughness (SWR) with the analysis for the three-dimensional (3-D) SWR of dielectric waveguides, and, then the accurate SWR measurements for silicon-on-insulator (SOI) waveguide were carried out with confocal laser scanning microscopy (CLSM). Further, we composed a theoretical/experimental combinative model of the SWR-caused optical propagation loss. Consequently, with the systematic simulations for the characteristics of optical propagation loss of SOI waveguides, the two critical points were found: (i) the sidewall roughness-caused optical loss was synchronously dependent on the correlation length and the waveguide width in addition to the SWR and (ii) the theoretical upper limit of the correlation length was the bottleneck to compressing the roughness-induced optical loss. The simulation results for the optical loss characteristics, including the differences between the TE and TM modes, were in accord with the experimental data published in the literature. The above research outcomes are very sustainable to the selection of coatings before/after the SOI waveguide fabrication.

Application of focused ion-beam sampling for sidewall-roughness measurement of free-standing sub-μm objects by atomic force microscopy.

In the present study, a free-standing object-sampling technique for microelectromechanical systems (MEMS) is developed to measure their sidewall surface roughnesses by atomic force microscopy (AFM). For this purpose, a conventional focused ion beam (FIB) sampling technique widely used for cross-sectional transmission electron microscope specimen preparation was applied. The sub-nm-order roughness parameters were quantitatively measured for sidewalls of Si-bridge test samples. The roughness parameters were compared before and after H2 annealing treatment, which induced smoothing of the surface by migration of the Si atoms. The reduction in the surface roughness by a factor of approximately one-third with 60-s H2 annealing was quantitatively evaluated by AFM. The present study confirms that the developed FIB-AFM technique is one potential approach for quantitatively evaluating the surface-roughness parameters on the oblique faces of free-standing objects in MEMS devices.

Focused Ion Beam-Atomic Force Microscopy Technique for Sidewall Roughness Measurement of Free-Standing Objects with Sub-μm Size

Focused Ion Beam-Atomic Force Microscopy Technique for Sidewall Roughness Measurement of Free-Standing Objects with Sub-μm Size

Atomic Force Microscopy Sidewall Imaging with a Quartz Tuning Fork Force Sensor.

Sidewall roughness measurement is becoming increasingly important in the micro-electromechanical systems and nanoelectronics devices. Atomic force microscopy (AFM) is an emerging technique for sidewall scanning and roughness measurement due to its high resolution, three-dimensional imaging capability and high accuracy. We report an AFM sidewall imaging method with a quartz tuning fork (QTF) force sensor. A self sensing and actuating force sensor is fabricated by microassembling a commercial AFM cantilever (tip apex radius ≤10 nm) to a QTF. The attached lightweight cantilever allows high-sensitivity force detection (7.4% Q factor reduction) and sidewall imaging with high lateral resolution. Owing to its unique configuration, the tip of the sensor can detect sidewall surface orthogonally during imaging, which reduces lateral friction. In experiments, sidewalls of a micro-electro-mechanical system (MEMS) structure fabricated by deep reactive ion etching process and a standard step grating are scanned and the sidewall roughness, line edge roughness and sidewall angles are measured.

Direct measurement of sidewall roughness on Si, poly-Si and poly-SiGe by AFM

In this paper the effect of the commonly used HBr/Cl2 chemistry for dry etching on the line-edge roughness (LER) of photoresist patterned single crystalline Si (sc-Si), polycrystalline Si (poly-Si) and poly-Si0.2Ge0.8 sidewalls was characterized. Measurements were done by means of atomic force microscopy in combination with an elaborated sample preparation technique that allowed the LER at different depths of the sidewall to be measured. Samples were patterned by I-line lithography and etching was performed at an RF power of 200 W using HBr/Cl2 (30/10 sccm) plasma. For sc-Si the photoresist and Si sidewalls had an LER of 0.8–1.4 nm and 1.5–2 nm, respectively. For poly-Si and poly-SiGe the photoresist sidewall roughness was, respectively, increased to 1.5–3 nm and 2–3.5 nm due to light scattering from the rough surface of the polycrystalline materials. The poly-Si film had a sidewall roughness of 3–4 nm. Poly-SiGe sidewall exhibited larger roughness with an LER of 5–12 nm which was not transferred from the photoresist. The results show that for sc-Si and poly-Si the sidewall roughness mainly originates from the photoresist process and little additional roughening is caused by the HBr/Cl2 etching. However, for poly-Si0.2Ge0.8 the LER is considerably increased from that of the photoresist indicating that the HBr/Cl2 etching is the main contributor to the LER.

Sidewall roughness measurement inside photonic crystal holes by atomic forcemicroscopy

We present a measurement technique to quantify sidewall roughness inside planar photoniccrystal (PhC) holes. Atomic force microscopy is used to scan hole cross-section profiles. Byfitting a circle onto each scan line and subtracting this circle from the measurement data, aquantitative value for the deviation from the ideal cylindrical hole shape is extracted. Weinvestigate the sidewall roughness of InP-based PhC holes depending on the nitrogencontent of the semiconductor etching plasma. The existence of a trade-off between holeundercut and surface roughness by optimizing the flux of nitrogen during the plasmaetching of the PhC holes is confirmed. We further quantify with this technique the influenceof the direct-writing of octagons instead of circles by electron-beam lithography on themeasured roughness.

Waveguide sidewall roughness measurement on full wafers by SEM‐based stereoscopy

We present a technique aiming at sidewall roughness measurement on integrated optical silica waveguides using a scanning electron microscope. The technique uses the principles of stereoscopy to retrieve sidewall topography. Practical implementation details and first results are provided.

Direct measurement of sidewall roughness of polymeric optical waveguides

Sidewall roughness (SWR) of fluorinated polyether waveguides fabricated using reactive ion etching in pure oxygen gas was directly measured using atomic force microscopy (AFM). We confirm that SWR is not the replicate of line edge roughness (LER) of the waveguides. Statistical information such as standard deviation of roughness, autocovariance function (ACF), and autocorrelation length (ACL) have been obtained from AFM measurements. The ACL varies in the similar manner as SWR along the depth of the waveguide, and both are dependent on the etch depth. The depth dependence can be explained by the change in the arrival dynamics of etchant ions in a mechanism involving both shadowing and first-order reemission effects.

Correlation of atomic force microscopy sidewall roughness measurements with scanning electron microscopy line-edge roughness measurements on chemically amplified resists exposed by x-ray lithography

As critical dimensions of resist features shrink, roughness of the features may contribute significantly to the variation in critical dimension. Measuring and understanding the causes of this roughness will become increasingly important with smaller sizes. To date, mainly two techniques have been used to measure the roughness: atomic force microscopy (AFM) and scanning electron microscopy (SEM). Topdown SEM measurements provide an easy and expedient measure of the variation in the profile of the resist feature. These measurements are often called “line-edge roughness” (LER). AFM measurements are considerably more time consuming, but provide information on the entire sidewall surface of the resist, rather than just the profile in line-edge roughness. Our recent AFM measurements on the positive-tone resist APEX-E and UV5 have shown that the sidewall roughness of the resist is depth dependent; resist near the substrate is smoother than resist at the top surface of the resist. For instance, APEX-E may have a roughness on the order of 3 nm rms near the substrate, which may increase to 7 or so nm rms at the top of the resist line. This article will correlate measurements made by both AFM and SEM and explore how the depth-dependent roughness in AFM data may affect the magnitude of the roughness measured by topdown SEM.

Correlation Length Measurement of Sidewall Roughness of Dry Etched Facet and Its Effect on Reflectivity

Dry etching is an important tool to fabricate various semiconductor photonic devices. The roughness of etched sidewalls should be avoided as it reduces the scattering loss. We present a spatial frequency analysis of the sidewall roughness of dry etched facets processed by reactive ion etching. A characteristic parameter corresponding to a correlation length is estimated to be ∼0.5 μm. In addition, its effect on reflectivities of etched reflectors is discussed.

Three-dimensional electron probe roughness analysis of InP sidewalls processed by reactive ion beam etching

A quantitative three-dimensional measurement of sidewall roughness of InP etched by chlorine-based reactive ion beam etching (RIBE) is presented. An electron probe surface roughness analyzer using four secondary electron detectors was employed. The minimum value of average sidewall roughness under the optimized etching condition was as small as 1 nm, where the etching condition was an ion extraction voltage of 400 V and a Cl2 gas pressure of 1.2×10−3 Torr. It is found that the etched sidewall roughness can be reduced by lowering ion extraction voltage with a relatively higher gas pressure.

Measurement of Sidewall Roughness of InP Etched by Reactive Ion Beam Etching

We have quantitatively measured the sidewall roughness of InP etched by reactive ion beam etching (RIBE) for the first time. An electron probe surface roughness analyzer was employed. The minimum value of average sidewall roughness appeared to be 1 nm and the peak-to-peak roughness appeared to be 6 nm. The etching conditions were an ion extraction voltage of 400 V and a Cl2 gas pressure of 1.2× 10-3 Torr. It is found that the etched sidewall roughness is decreased by lower-ion-extraction-voltage etching in a constant gas pressure, as well as by higher-gas-pressure etching with a constant ion extraction voltage.

Measurement of Sidewall Roughness by Scanning Tunneling Microscope

A scanning tunneling microscope (STM) is a highly effective tool for observing a microfabricated pattern. However, it is difficult to measure sidewall roughness using a conventional STM because of the restriction of the tip shape and one-dimensional servo system. The main objective of this study is to develop a sidewall roughness characterization tool. The electron-beam deposition method is applied to preparing a novel STM tip shape. A two-dimensional servo system, with a subnano-vibration mode to provide vibrations below 1 nm for x- and z-directions to a tip during scanning, has been developed for sidewall roughness measurement.

Guideway sidewall roughness measurements on an automated guideway transit (AIRTRANS) system

Guideway sidewall roughness measurements on an automated guideway transit (AIRTRANS) system