Chlorine-based Reactive Ion Etching Research Articles

Single crystal diamond blazed diffraction gratings and Fresnel microlens arrays with improved optical performance by high-resolution 3D laser lithography and pattern transfer by dry etching

We present a method to fabricate high precision 3D micro-optical components in single crystal diamond, and we demonstrate and experimentally characterize Fresnel microlens arrays and blazed diffraction gratings. The fabrication process is based on high-resolution 3D dip-in laser lithography and pattern transfer by chlorine-based reactive ion etching with a low diamond:photoresist selectivity (1:15) to obtain smooth etched surfaces with Ra < 2 nm. The fabricated microlenses have a focal length that deviates by 4% from the target value, while the blazed diffraction gratings have a maximum relative efficiency of 66% in the first order.

Fabrication of wafer-scale nanopatterned sapphire substrate through phase separation lithography

A phase separation lithography (PSL) based on polymer blend provides an extremely simple, low-cost, and high-throughput way to fabricate wafer-scale disordered nanopatterns. This method was introduced to fabricate nanopatterned sapphire substrates (NPSSs) for GaN-based light-emitting diodes (LEDs). The PSL process only involved in spin-coating of polystyrene (PS)/polyethylene glycol (PEG) polymer blend on sapphire substrate and followed by a development with deionized water to remove PEG moiety. The PS nanoporous network was facilely obtained, and the structural parameters could be effectively tuned by controlling the PS/PEG weight ratio of the spin-coating solution. 2-in. wafer-scale NPSSs were conveniently achieved through the PS nanoporous network in combination with traditional nanofabrication methods, such as O2 reactive ion etching (RIE), e-beam evaporation deposition, liftoff, and chlorine-based RIE. In order to investigate the performance of such NPSSs, typical blue LEDs with emission wavelengths of ~450 nm were grown on the NPSS and a flat sapphire substrate (FSS) by metal–organic chemical vapor deposition, respectively. The integral photoluminescence (PL) intensity of the NPSS LED was enhanced by 32.3 % compared to that of the FSS-LED. The low relative standard deviation of 4.7 % for PL mappings of NPSS LED indicated the high uniformity of PL data across the whole 2-in. wafer. Extremely simple, low cost, and high throughput of the process and the ability to fabricate at the wafer scale make PSL a potential method for production of nanopatterned sapphire substrates.

Submicron-patterning of bulk titanium by nanoimprint lithography and reactive ion etching

Nanopatterns on titanium may enhance endosseous implant biofunctionality. To enable biological studies to prove this hypothesis, we developed a scalable method of fabricating nanogrooved titanium substrates. We defined nanogrooves by nanoimprint lithography (NIL) and a subsequent pattern transfer to the surface of ASTM grade 2 bulk titanium applying a soft-mask for chlorine-based reactive ion etching (RIE). With respect to direct write lithographic techniques the method introduced here is fast and capable of delivering uniformly patterned areas of at least 4 cm2. A dedicated silicon nanostamp process has been designed to generate the required thickness of the soft-mask for the NIL–RIE pattern transfer. Stamps with pitch sizes from 1000 nm down to 300 nm were fabricated using laser interference lithography (LIL) and deep cryogenic silicon RIE. Although silicon nanomachining was proven to produce smaller pitch sizes of 200 nm and 150 nm respectively, successful pattern transfer to titanium was only possible down to a pitch of 300 nm. Hence, the smallest nanogrooves have a width of 140 nm. An x-ray photoelectron spectroscopy study showed that only very few contaminations arise from the fabrication process and a cytotoxicity assay on the nanopatterned surfaces confirmed that the obtained nanogrooved titanium specimens are suitable for in vivo studies in implantology research.

“Chlorine-based Reactive Ion Etching Process to Pattern Platinum for MEMS Applications”

AbstractWe have developed platinum (Pt) deposition and chlorine-based Reactive Ion Etch (RIE) processes that are needed to deposit, pattern and embed electrodes deep within a MEMS process flow. Various combinations of chlorinebased gases were tested to find the optimum gas mixture for RIE. A 1: 0.4 mixture ratio of pure chlorine to argon and 100-150 Watts of RF microwave power were found to be optimum conditions for the RIE of platinum metal. The addition of argon gas to chlorine was found to contribute to the anisotropic etching of platinum, obtaining vertical shaped sidewall patterns. A simple model of the platinum etching mechanism is proposed. Following the plasma enhanced formation of platinum and chlorine ions, volatile products of platinum chlorides were formed and driven away at elevated temperature. As a demonstration of our RIE process, micron-sized platinum patterns with vertical sidewalls were fabricated. Etch chemistry was investigated using ToF SIMS analysis. This etch technique will be useful to device developers intending to use the unique properties of platinum metal as an electrode that is deeply embedded within a MEMS process flow.

Low-noise, Low-dark-current GaN Diodes for UV Detectors

ABSTRACTWe report mesa-isolated Schottky barrier photodetectors fabricated on n-GaN. Single-element detectors were constructed from nitride epilayers grown by gas source molecular beam epitaxy (GSMBE) on Si(111). Chlorine-based reactive ion etching was used to form two-level mesas. The detectors were front-illuminated through 100 Å Pd semitransparent Schottky contacts on the upper mesas; ohmic contact on the lower mesas was made using standard Ti/Al/Ti/Au metallurgy. Silicon dioxide grown by plasma-enhanced chemical vapor deposition provided both surface passivation and electrical isolation. The dark current of an 86 × 86 µm2 single-element detector is 2.10 × 10−8 A/cm2 at –2 V bias, and the zero-bias noise power density at 1Hz is as low as 9 × 10−29 A2/Hz. In addition, we present preliminary results for p-n diodes fabricated from epilayers grown on sapphire. The dark current of a 50 × 50 µm2 single-element detector is 1.6 × 10−6 A/cm2 at –2 V bias.

Chemical and Complementary Role of Fluorine in a Chlorine-Based Reactive Ion Etching of GaN

Chemical and Complementary Role of Fluorine in a Chlorine-Based Reactive Ion Etching of GaN

Chemical and Complementary Role of Fluorine in a Chlorine-Based Reactive Ion Etching of GaN

Conventional reactive ion etching of epitaxially grown GaN on (0001) sapphire substrate has been investigated using a basic chemistry of SiCl4, Ar and SF6. Photoresist or plasma-deposited SiNx were used for masking. The influence of gas flow, pressure and rf-power on the etch rate and morphology were studied. A maximum etch rate of 430 nm/min was obtained at an rf power of 300 W. Very smooth surfaces and reasonable etch rates (±100 nm/min) were obtained using the same chemistry at a lower rf-power of 105 W (dc bias of ±290 V). The chemical and complementary roles of chlorine and fluorine will be demonstrated.

Reactive Ion Etching of Co–Zr–Nb Thin Film Using BCl3

Chlorine-based reactive ion etching (RIE) of a high-permeability Co–Zr–Nb film has been carried out. The etching rate was only about 10 nm/min in pure Cl2 in the temperature range in which anisotropic ion etching was dominant. Addition of BCl3 to the Cl2 resulted in an increase in the etching rate at low temperature. The activation energy in BCl3–Cl2 RIE was about 0.5 eV, whereas that in pure Cl2 RIE was 1.0 eV. The optimum BCl3 content was around 30 vol.%. The etching rate reached 750 nm/min at 1.2 W/cm2 RF power density and 300° C etching temperature. This improvement is due to the heavier mass and the higher ionization rate of BCl3 than those of Cl2. The ratio of the side etching rate to the depth-etching rate was about 30%, when the RF power density was 1.2 W/cm2 in BCl3–Cl2 RIE. The side etching was suppressed by both an increase in the RF power and addition of SiCl4 or SiCl4–O2 to the BCl3–Cl2. Anisotropy was promoted by both an increase in the ion energy and sidewall protection by Si or Si-oxide. An etching rate of 200 nm/min at 150° C and 2 W/cm2 RF power density in BCl3–Cl2–SiCl4 RIE has been achieved. The etching selectivity vs SiO2 was 20 and the side-etching rate was only 2% of the depth-etching rate.

Study of sidewall passivation and microscopic silicon roughness phenomena in chlorine-based reactive ion etching of silicon trenches

Sidewall passivation layers produced in the formation of Si trenches by HCl/O2/BCl3 reactive ion etching have been characterized by angle-resolved x-ray photoemission measurements and secondary electron microscopy. Electron-shading effects observed at grazing electron emission angles and electrostatic charging of insulating portions have been used to differentiate photoemission contribution from (i) the trench sidewall film (moderately insulating), (ii) the oxide mask (highly insulating), and (iii) the Si trench bottom (conductive). The sidewall passivation film for this process is found to be a silicon oxide. Only minor amounts of chlorine are incorporated in this film. The thickness of the sidewall passivation layer depends on the width of the Si trench and ranges from ≂0.1 μm for narrow trenches to ≂0.7 μm for wide Si trenches for our conditions. The Si taper formed in this trench etching process is produced by the simultaneous formation of the sidewall film which protects the underlying Si from etching. It is also shown that microscopic silicon pillars (‘‘black silicon’’ or ‘‘grass’’) form by micromasking under process conditions where large areas of unmasked silicon are exposed. The microscopic Si pillars are protected by the same sidewall passivation film as the trench sidewalls. Islands of black silicon are observed only in large open silicon areas and separated from oxide masked areas by a characteristic distance which is free of silicon pillars. This roughness-free distance is of the order of the mask thickness employed. Ion scattering from the sloped mask sidewalls and sputtering of involatile micromasking material can explain this observation. Our results are consistent with a mechanism in which the processes that cause the formation of the beneficial sidewall film also initiate and control the production of undesirable silicon roughness in large unmasked Si regions.

Effects of chromium on the reactive ion etching of steep-walled trenches in silicon

Formation of vertical-walled, high-aspect-ratio, narrow trenches in silicon is often required in submicron VLSI semiconductor processing. Present trench etching processes suffer from undercutting of the mask and sloped or bulged sidewalls; and when using chlorine-based reactive ion etching (RIE), there is frequently enhanced etching at the bottom corners of the trench. We report here the effects of chromium on the RIE of silicon. We found that, compared to using a SiO2 mask only, using a mask consisting of a chromium layer on top of SiO2, in a Cl2:SiCl4:N2 etching gas mixture, improves the trench sidewall angle relative to the vertical from 8° to 2°, and almost eliminates the enhanced etching at the bottom corners. Furthermore, we found that, by etching a sample partially covered with chromium in a Cl2:SiCl4:N2 gas mixture, the trench shape in the areas not covered with chromium depends on how far the trench is from the edge of the chromium layer. Those areas, within 1 cm of the edge of the chromium layer, showed quite steep sidewall angles that degraded (to 8°) at greater distances (few cm). Similarly, the size of the grooves due to the enhanced etching at the bottom corners is reduced as the trench gets closer to the edge of the chromium layer. It appears that the beneficial effect of the chromium is an indirect chemical action, based on the suppression of the concentration of neutrals or the formation of an inhibitor layer, rather than direct physical masking.