Vertical Etch Profile Research Articles

Cryogenic DRIE processes for high-precision silicon etching in MEMS applications

Cryogenic deep reactive ion etching (Cryo DRIE) of silicon has become an enticing but challenging process utilized in front-end fabrication for the semiconductor industry. This method, compared to the Bosch process, yields vertical etch profiles with smoother sidewalls not subjected to scalloping, which are desired for many microelectromechanical systems (MEMS) applications. Smoother sidewalls enhance electrical contact by ensuring more conformal and uniform sidewall coverage, thereby increasing the effective contact area without altering contact dimensions. The versatility of the Cryo DRIE process allows for customization of the etch profiles by adjusting key process parameters such as table temperature, O2 percentage of the total gas flow rate (O2 + SF6), RF bias power and process pressure. In this work, we undertake a comprehensive study of the effects of Cryo DRIE process parameters on the trench profiles in the structures used to define cantilevers in MEMS devices. Experiments were performed with an Oxford PlasmaPro 100 Estrelas ICP-RIE system using positive photoresist SPR-955 as a mask material. Our findings demonstrate significant influences on the sidewall angle, etch rate and trench shape due to these parameter modifications. Varying the table temperature between −80 °C and −120 °C under a constant process pressure of 10 mTorr changes the etch rate from 3 to 4 μm min−1, while sidewall angle changes by ∼2°, from positive (<90° relative to the Si surface) to negative (>90° relative to the Si surface) tapering. Altering the O2 flow rate with constant SF6 flow results in a notable 10° shift in sidewall tapering. Furthermore, SPR-955 photoresist masks provide selectivity of 46:1 with respect to Si and facilitates the fabrication of MEMS devices with precise dimension control ranging from 1 to 100 μm for etching depths up to 42 μm using Cryo DRIE. Understanding the influence of each parameter is crucial for optimizing MEMS device fabrication.

Plasma-based etching approach for GEM detector microfabrication at FBK for X-ray polarimetry in space

Gas Electron Multiplier (GEM) detectors are crucial for enabling high-resolution X-ray polarization of astrophysical sources when coupled to custom pixel readout ASIC in Gas Pixel Detectors (GPD), as in the Imaging X-ray Polarimetry Explorer (IXPE), the Polarlight cubesat pathfinder and the PFA telescope onboard the future large enhanced X-ray Timing and Polarimetry (eXTP) Chinese mission. The R&D efforts of the IXPE collaboration have resulted in mature GPD technology. However, limitations in the classical wet-etch or laser-drilled fabrication process of GEMs motivated our exploration of alternative methods. This work focuses on investigating a plasma-based etching approach for fabricating GEM patterns at Fondazione Bruno Kessler (FBK). The objective is to improve the aspect ratio of the GEM holes, to mitigate the charging of the GEM dielectric which generates rate-dependent gain changes. Unlike the traditional wet-etch process, Reactive Ion Etching (RIE) enables more vertical etching profiles and thus better aspect ratios. Moreover, the RIE process promises to overcome non-uniformities in the GEM hole patterns which are believed to cause systemic effects in the azimuthal response of GPDs equipped with either laser-drilled or wet-etch GEMs. We present a GEM geometry with 20 μm in diameter and 50 μm pitch, accompanied by extensive characterization (SEM and PFIB) of the structural features and aspect ratios. The collaboration with INFN Pisa and Turin enabled us to compare the electrical properties of these detectors and test their performance in their use as electron multipliers in GPDs. Although this R&D work is in its initial stages, it holds promise for enhancing the sensitivity of the IXPE mission in X-ray polarimetry measurements through GEM pattern with more vertical hole profiles. The outcomes of this study have the potential to advance the current technological platforms and improve the capabilities of future space-based X-ray polarimetry missions.

Low global warming C5F10O isomers for plasma atomic layer etching and reactive ion etching of SiO2 and Si3N4

AbstractPlasma atomic layer etching (ALE) processes for SiO2 and Si3N4 and reactive ion etching (RIE) processes for SiO2 with hole patterns were developed using C4F8 and the low global warming potential gases of perfluoroisopropyl vinyl ether (PIPVE) and perfluoropropyl vinyl ether (PPVE). The ALE windows of SiO2 and Si3N4 were in the range of 50.0–57.5 V for all precursors. Etch per cycle of SiO2 was determined to be 5.5 Å/cycle (C4F8), 3.3 Å/cycle (PIPVE), and 5.4 Å/cycle (PPVE), all lower than that of Si3N4. PPVE reduced global warming emissions by 49%, demonstrating better vertical etch profiles in RIE compared to C4F8.

GaN damage-free cyclic etching by sequential exposure to Cl2 plasma and Ar plasma with low Ar+-ion energy at substrate temperature of 400 °C

Damage-free atomic layer etching (ALE) of GaN was demonstrated using a cyclic process in which the chlorinated layer formed by Cl2 plasma exposure was removed by exposure to Ar plasma with energy-controlled ions when the substrate temperature was maintained at 400 °C. The layer chlorinated at 400 °C by Cl2 plasma exposure was found to be thinner than that chlorinated at 25 °C. Therefore, in the case of an Ar+-ion energy of 70 eV, the “ALE synergy” parameter, which quantifies the degree to which a process approaches the ideal ALE regime, decreased from 86% at a substrate temperature of 25 °C to 24% at a substrate temperature of 400 °C. A substrate temperature of 400 °C promoted etching even at the lower ion energy of 40 eV, thereby resulting in a higher ALE synergy of 62%. The vertical etching profile with no degradation of photoluminescence near the band edge on the etched surface was then observed. The proposed high-temperature ALE method is promising for realizing a vertical pattern profile via damage-free etching of GaN.

Al0.7Sc0.3N butterfly-shaped laterally vibrating resonator with a figure-of-merit (kt2·Qm) over 146

This work presents the laterally vibrating Lamb wave resonators (LVRs) based on a 30% aluminum scandium nitride (Al0.7Sc0.3N) thin film with three interdigited transducer pairs operating in the S0 mode. In order to reduce the anchor loss, perfect matched layer-based finite element analysis simulations are utilized to design and optimize the device. Thanks to the high quality AlScN using magnetron sputtering with a single alloy target, vertical etching profile, and designed device structure, 1-μm-thick Al0.7Sc0.3N-based LVRs with high performance are fabricated. The resonator equivalent electric parameters are extracted utilizing the modified Butterworth–Van Dyke model. The best Al0.7Sc0.3N LVR achieves an electromechanical coupling coefficient (kt2) of 9.7% and a loaded quality factor (Qr) of 1141.5 operating at approximately 305 MHz. The same resonator shows a motional quality factor (Qm) of 1507.2, resulting in a high figure-of-merit (FoM = kt2 · Qm) of 146.2. A 1.8 MHz tuning range is measured for an Al0.7Sc0.3N LVR by applying DC voltage in the range of −40 to 40 V due to the ferroelectric property of high Sc doping in Al0.7Sc0.3N. With the high FoM, Qr, Qm, and low motional resistance (Rm), the Al0.7Sc0.3N-based LVRs show strong potential in applications of radio frequency communications and piezoelectric transducers.

Optimization of Metal-Assisted Chemical Etching for Deep Silicon Nanostructures.

High-aspect ratio silicon (Si) nanostructures are important for many applications. Metal-assisted chemical etching (MACE) is a wet-chemical method used for the fabrication of nanostructured Si. Two main challenges exist with etching Si structures in the nanometer range with MACE: keeping mechanical stability at high aspect ratios and maintaining a vertical etching profile. In this work, we investigated the etching behavior of two zone plate catalyst designs in a systematic manner at four different MACE conditions as a function of mechanical stability and etching verticality. The zone plate catalyst designs served as models for Si nanostructures over a wide range of feature sizes ranging from 850 nm to 30 nm at 1:1 line-to-space ratio. The first design was a grid-like, interconnected catalyst (brick wall) and the second design was a hybrid catalyst that was partly isolated, partly interconnected (fishbone). Results showed that the brick wall design was mechanically stable up to an aspect ratio of 30:1 with vertical Si structures at most investigated conditions. The fishbone design showed higher mechanical stability thanks to the Si backbone in the design, but on the other hand required careful control of the reaction kinetics for etching verticality. The influence of MACE reaction kinetics was identified by lowering the oxidant concentration, lowering the processing temperature and by isopropanol addition. We report an optimized MACE condition to achieve an aspect ratio of at least 100:1 at room temperature processing by incorporating isopropanol in the etching solution.

Development of Micron Sized Photonic Devices Based on Deep GaN Etching

In order to design and development efficient III-nitride based optoelectronic devices, technological processes require a major effort. We propose here a detailed review focussing on the etching procedure as a key step for enabling high date rate performances. In our reported research activity, dry etching of an InGaN/GaN heterogeneous structure was investigated by using an inductively coupled plasma reactive ion etching (ICP-RIE). We considered different combinations of etch mask (Ni, SiO2, resist), focussing on the optimization of the deep etching process. A GaN mesa process with an etching depth up to 6 µm was performed in Cl2/Ar-based plasmas using ICP reactors for LEDs dimen sions ranging from 5 to 150 µm². Our strategy was directed toward the mesa formation for vertical-type diode applications, where etch depths are relatively large. Etch characteristics were studied as a function of ICP parameters (RF power, chamber pressure, fixed total flow rate). Surface morphology, etch rates and sidewall profiles observed into InGaN/GaN structures were compared under different types of etching masks. For deep etching up to few microns into the GaN template, we state that a Ni or SiO2 mask is more suitable to obtain a good selectivity and vertical etch profiles. The optimized etch rate was about 200nm/min under moderate ICP conditions. We applied these conditions for the fabrication of micro/nano LEDs dedicated to LiFi applications.

Plasma Parameters and Etching Characteristics of SiOxNy Films in CF4 + O2 + X (X = C4F8 or CF2Br2) Gas Mixtures.

In this work, we carried out the study of CF4 + O2 + X (X = C4F8 or CF2Br2) gas chemistries in respect to the SiOxNy reactive-ion etching process in a low power regime. The interest in the liquid CF2Br2 as an additive component is motivated by its generally unknown plasma etching performance. The combination of various diagnostic tools (double Langmuir probe, quadrupole mass-spectrometry, X-ray photoelectron spectroscopy) allowed us to compare the effects of CF4/X mixing ratio, input power and gas pressure on gas-phase plasma characteristics as well as to analyze the SiOxNy etching kinetics in terms of process-condition-dependent effective reaction probability. It was found that the given gas systems are characterized by: (1) similar changes in plasma parameters (electron temperature, ion current density) and fluxes of active species with variations in processing conditions; (2) identical behaviors of SiOxNy etching rates, as determined by the neutral-flux-limited process regime; and (3) non-constant SiOxNy + F reaction probabilities due to changes in the polymer deposition/removal balance. The features of CF4 + CF2Br2 + O2 plasma are lower polymerization ability (due to the lower flux of CFx radicals) and a bit more vertical etching profile (due to the lower neutral/charged ratio).

Microscale pattern etch of 4H–SiC by inductively coupled plasma

SiC is commonly used in the field of high temperature, high frequency, high power and radiation-resistant semiconductor devices and ultraviolet photodetectors due to its excellent photoelectric characteristics. Plasma etching is one of the key processes in the fabrication of SiC devices. However, it is difficult to obtain a smooth and vertical etch profile due to the subtrench effect when the conventional inductively coupled plasma (ICP) is used to etch SiC with small-size patterns (such as 1 μm holes or trenches), thus affecting the subsequent process and device performance. To study microscale pattern etch of 4H–SiC, we used different mask and etch gas systems for ICP etching. The etch profile was characterized using SEM. When the chlorine gas was used, there was no subtrench but a very inclined angle between the SiC bottom and sidewalls, which is hard to improve. When we used photoresist (PR) mask–HBr system, sidewall deposition happened. SiO2 mask–HBr/Ar system could help to etch different angles but has a limitation angle of up to 80°. Compared to above systems, the etching profile was much more vertical when HBr/SF6/O2 was used. After many rounds of process debugging, we obtain a good etching profile with flat and vertical sidewalls, no subtrench in the bottom and no damage of the SiO2 mask. Our work shows a good way of microscale pattern SiC etching by ICP.

The effect of reactive ion etch (RIE) process conditions on ReRAM device performance

The recent surge of research on resistive random access memory (ReRAM) devices has resulted in a wealth of different materials and fabrication approaches. In this work, we describe the performance implications of utilizing a reactive ion etch (RIE) based process to fabricate HfO2 based ReRAM devices, versus a more unconventional shadow mask fabrication approach. The work is the result of an effort to increase device yield and reduce individual device size. Our results show that choice of RIE etch gas (SF6 versus CF4) is critical for defining the post-etch device profile (cross-section), and for tuning the removal of metal layers used as bottom electrodes in the ReRAM device stack. We have shown that etch conditions leading to a tapered profile for the device stack cause poor electrical performance, likely due to metal re-deposition during etching, and damage to the switching layer. These devices exhibit nonlinear I–V during the low resistive state, but this could be improved to linear behavior once a near-vertical etch profile was achieved. Device stacks with vertical etch profiles also showed an increase in forming voltage, reduced switching variability and increased endurance.

Ultradeep electron cyclotron resonance plasma etching of GaN

Ultradeep (≥5 μm) electron cyclotron resonance plasma etching of GaN micropillars was investigated. Parametric studies on the influence of the applied radio-frequency power, chlorine content in a Cl2/Ar etch plasma, and operating pressure on the etch depth, GaN-to-SiO2 selectivity, and surface morphology were performed. Etch depths of &amp;gt;10 μm were achieved over a wide range of parameters. Etch rates and sidewall roughness were found to be most sensitive to variations in RF power and % Cl2 in the etch plasma. Selectivities of &amp;gt;20:1 GaN:SiO2 were achieved under several chemically driven etch conditions where a maximum selectivity of ∼39:1 was obtained using a 100% Cl2 plasma. The etch profile and (0001) surface morphology were significantly influenced by operating pressure and the chlorine content in the plasma. Optimized etch conditions yielded &amp;gt;10 μm tall micropillars with nanometer-scale sidewall roughness, high GaN:SiO2 selectivity, and nearly vertical etch profiles. These results provide a promising route for the fabrication of ultradeep GaN microstructures for use in electronic and optoelectronic device applications. In addition, dry etch induced preferential crystallographic etching in GaN microstructures is also demonstrated, which may be of great interest for applications requiring access to non- or semipolar GaN surfaces.

Dry etching of palladium thin films in high density plasmas of CH3OH/Ar, C2H5OH/Ar, CH4/Ar, and CH4/O2/Ar gas mixtures

High density plasma etching of Pd thin films masked with TiN films was performed using CH3OH/Ar, C2H5OH/Ar, CH4/Ar, and CH4/O2/Ar gas mixtures. The etch rates of the Pd films and TiN masks in all the gas mixtures decreased whereas the etch selectivities increased. The etch profiles of the Pd films etched under C2H5OH/Ar gas were better than those obtained with CH3OH/Ar gas. Addition of O2 gas to the CH4/Ar gas mixture considerably improved the etch profiles of the Pd films. CH4, Ar, and O2 were found to play a critical role in obtaining a vertical etch profile with smooth sidewalls. Energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy revealed the formation of polymeric layers and PdOx compounds on the sidewalls and the film surface. Good etch profiles with a high degree of anisotropy were achieved with the use of C2H5OH/Ar and CH4/O2/Ar gas mixtures.

SiO2 nanohole arrays with high aspect ratio for InGaN/GaN nanorod-based phosphor-free white light-emitting-diodes

Vertically aligned InGaN/GaN nanorod (NR)-based phosphor-free light emitting diodes (LEDs) using SiO2 nanohole patterns are demonstrated. The highly ordered SiO2 nanoholes were realized on a 2 μm-thick n+GaN template by a two-step dry etching process. The use of C4F8/O2/Ar plasma chemistries under the low pressure is found to greatly enlarge the bottom diameter of each hole, exhibiting high aspect ratio (AR ∼ 9) and vertical etch profile (∼89°). SAG technique was used to define the height of the GaN NRs while the width is determined by the trimethylgallium flow rate and growth temperature. An LED structure consisted of three-pairs of InGaN/GaN quantum well and AlGaN electron blocking layer on the sidewall of the nanorod in a core-shell structure. The wavelengths were successfully tuned by controlling pitches of the rods, which was caused by the different growth rate and indium incorporation of conformally overgrown InGaN multiquantum wells. At the operating current density of 1.5 A/cm2 (65 mA), NR-based single-chip phosphor-free white LEDs with the dimension of 630 × 970 μm2 show highly stable white emission characteristics which are attractive for future solid-state lighting and full-color display applications.

Dry etching of SiC using Ar/F2 plasma and XeF2 plasma

We investigated the SiC dry etching process using Ar/F2 plasma and XeF2 plasma. We carried out optical observation of Ar/F2 plasma and XeF2 plasma. The dominant etching species were different between Ar/F2 plasma and XeF2 plasma. The etching rates of SiC were approximately 100 nm/min at 25 sccm and 200 W for Ar/F2 plasma and 45 nm/min at 2.5 sccm and 100 W for XeF2 plasma. Vertical etching profiles and a smooth etched surface were obtained. The average roughness of the etched bottom surface was 1 nm, which satisfied the requirements for optical device fabrication. We believe that the proposed etching process using F2 of zero-global-warming-potential gases is very simple and useful for fabricating optical devices and micro-electromechanical systems (MEMSs).

Effect of process parameters on sidewall damage in deep silicon etch

Sidewall damage caused in deep reactive ion silicon etch was investigated by varying etch cycle time, bias power, etch pressure and SF6 flow rate using the Bosch process in a uniquely designed, inductively coupled plasma reactor. The effects of these process parameters on the etch profile and sidewall angle were also studied for high density metal–insulator–metal capacitor structure. By choosing the proper etch cycle time of 2 s, it was observed that the sidewall damage was very sensitive to these etch process parameters. As bias power increased, the sidewall damage increased gradually. Especially, at the bias power of 500 W, a dual bowing shape with severe sidewall damage was seen, which might be due to a combination of two mechanisms: the formation of a redeposition region and a secondary ion etch effect. With increasing pressure, the sidewall damage was not always located in a specific depth range but distributed along almost the whole trench sidewall. An etch pressure below 80 mTorr was favorably recommended for reducing the extent of the sidewall damage. In addition, we found that an appropriate SF6 flow rate was also very beneficial to the realization of a smooth trench sidewall when it was controlled within an appropriate range. Based on these investigations, an acceptable etch condition could be selected to achieve a nearly vertical etch profile as well as a smooth sidewall surface.

Inductive couple plasma reactive ion etching characteristics of TiO2 thin films

Changes in the inductively coupled plasma reactive ion etching characteristics of TiO2 thin films in response to the addition of HBr, Cl2 and C2F6 to Ar gas were investigated. As the HBr, Cl2 and C2F6 concentration increased, the etch rate increased; however, the etch profile degree of anisotropy followed a different trend. As HBr concentration increased, the greatest anisotropic etch profile was obtained at 100% HBr, while the greatest anisotropic etch profile was obtained at concentrations of 25% when etching was conducted under C2F6 and Cl2. Field emission scanning electron microscopy revealed that 25% C2F6 generated the greatest vertical etch profile; hence, etch parameters were varied at this concentration. The effects of rf power, dc-bias voltage and gas pressure on the etch rate and etch profile were also investigated. The etch rate and degree of anisotropy in the etch profile increased with increasing rf power and dc-bias voltage and decreasing gas pressure. X-ray photoelectron spectroscopy analysis of the films etched under a C2F6/Ar gas mixture revealed the existence of etch byproducts containing F (i.e. TiFx) over the film. CxFy compounds were not detected on the film surface, probably due to contamination with atmospheric carbon.

Vertical directionality-controlled metal-assisted chemical etching for ultrahigh aspect ratio nanoscale structures

High aspect ratio nanoscale features are becoming increasingly important in a wide range of applications. In this paper, the authors describe the concepts of vertical directionality-controlled metal-assisted chemical etching (V-MaCE) and demonstrate resolution and fabrication capabilities for V-MaCE in the context of x-ray diffractive optics. The authors fabricate x-ray grating structures having a period of 50 nm, 25 nm lines, and 25 nm spaces, with a thickness of 1.26 μm, and achieve an aspect ratio of ∼50:1. In order to maintain the vertical etching profiles of these deep structures, the authors demonstrate the design and usage of adjacent, outrigger, gratings to balance the electron-hole concentration.

Angled etching of (001) rutile Nb–TiO2 substrate using SF6-based capacitively coupled plasma reactive ion etching

We demonstrated the plasma etching of a (001) rutile Nb–TiO2 substrate using SF6 plasma. A vertical etching profile and a smooth etched surface were obtained. In addition, the angled etching of a (001) rutile Nb–TiO2 substrate was achieved in a conventional SF6 reactive ion etching system for the first time. The etching angle was determined by the angle of the groove of the holder. We believe that our simple dry etching technique is suitable for the formation of a three-dimensional photonic crystal with complete photonic bandgap (CPBG) and optical waveguides.

Deep reactive ion etching of in situ boron doped LPCVD Ge0.7Si0.3 using SF6 and O2 plasma

► Fabrication of microelectromechanical resonator is demonstrated by DRIE of boron doped Ge 0.7 Si 0.3 . ► Etch profile control by oxygen flow for doped Ge 0.7 Si 0.3 . ► Increased etch selectivity towards oxide with an increase in temperature. ► Suppressed etch rate with increased boron concentration in Ge 0.7 Si 0.3 alloy. This paper reports on deep reactive ion etching (DRIE) of in situ highly boron doped low pressure chemical vapor deposited Ge 0.7 Si 0.3 alloy in SF 6 and O 2 plasma. The effect of RF power, SF 6 flow, O 2 flow and temperature on the etch rate of Ge 0.7 Si 0.3 films with a boron concentration of 2.1 × 10 21 atoms/cm 3 is investigated. Optimized conditions for a combination of a vertical etch profile and a high selectivity towards PECVD oxide are reported. The effect of boron doping concentration on the etch rate is also investigated. The etch rate is found to decrease with an increase in the dopant concentration. The developed SF 6 and O 2 based DRIE recipes are applied to fabricate GeSi microresonators.

SF6-Based Deep Reactive Ion Etching of (001) Rutile TiO2 Substrate for Photonic Crystal Structure with Wide Complete Photonic Band Gap

We demonstrated the fabrication of a photonic crystal structure of a (001) rutile TiO2 substrate by deep reactive ion etching (RIE) using SF6 plasma. A vertical etching profile and a smooth etched surface, which satisfy the requirements for optical device application, were obtained. We think that this proposed process is useful for the microfabrication of TiO2-crystal-based optical devices, such as photonic crystals with a complete photonic band gap (CPBG) and optical waveguides.