Gas Chopping Research Articles

Anisotropic plasma etching of Silicon in gas chopping process by alternating steps of oxidation and etching

In the present work a two-stage process for deep anisotropic etching of Silicon based on alternating steps of etching in SF6 plasma and passivation of Silicon surface by oxidation in O2 plasma is proposed. This developed Ox-Etch process has advantages of being free from deposited polymers on sidewalls and does not use cryogenic equipment. Etched structures show anisotropic profile with vertical scalloped walls similar to Bosch and STiGer processes. Selectivity of Silicon etch process with respect to SiO2 mask is up to 120:1. The investigations of etching results depending on process parameters are presented. Main control parameters for Ox-Etch process defining the etch rate and degree of anisotropy are the durations of etch and oxidation steps.

Large area fast-AFM scanning with active “Quattro” cantilever arrays

In this work, the fabrication and operation of an active parallel cantilever device integrating four self-sensing and self-actuating probes in an array is presented. The so called “Quattro” cantilever system is controlled by a multichannel field programmable gate array (FPGA) controller. The integrated cantilever devices are fabricated on the basis of a silicon-on-insulator wafer using surface micromachining and gas chopping plasma-etching processes [I. W. Rangelow, J. Vac. Sci. Technol., A 21, 1550 (2003)]. The unique design of the active cantilever probes provides both patterning and readout capabilities [Kaestner et al., J. Micro-Nanolithogr. MEMS 14, 031202 (2015)]. The thermomechanical actuation allows the individually operation of each cantilever in static and dynamic modes. This enables a simultaneous atomic force microscopy operation of all cantilevers in an array, while the piezoresistive read-out of the cantilever bending routinely ensures atomic resolution at a high imaging speed. The scanning probe lithography capabilities of the active cantilevers are based on the utilization of a Fowler–Nordheim field emission process of low-energy electrons (20–50 eV) for direct writing maskless lithography. The cantilever in the Quattro active cantilever array have a pitch of 125 μm (tip-to-tip distance), which allows an image size of 0.5 × 0.2 mm to be acquired within a single scan with 0.2 nm resolution in the vertical direction. Using parallel imaging, an effective scanning speed of 5.6 mm/s is achieved. The multichannel, scalable controller architecture allows four FPGA channels to scan and collect data simultaneously. A data buffer of 128 Mbits for a single frame of 4096 × 1024 pixels is applied. The designed data transfer system allows a packet size of 128 pixels to be transmitted within less than 10 μs, respectively. Thus, the entire image frame is transferred in less than 280 ms, which exceeds the required throughput in the practical cases like critical dimension-metrology and inspection. In this article, the authors are presenting the concept of the system, which combines imaging, metrology, and lithography capabilities with a low-cost of ownership. In this context, the authors are investigating the throughput capability, reproducibility, resolution, and positioning accuracy of the Quattro active cantilever system.

Charging effect simulation model used in simulations of plasma etching of silicon

Understanding the consequences of local surface charging on the evolving etching profile is a critical challenge in high density plasma etching. Deflection of the positively charged ions in locally varying electric fields can cause profile defects such as notching, bowing, and microtrenching. We have developed a numerical simulation model capturing the influence of the charging effect over the entire course of the etching process. The model is fully integrated into ViPER (Virtual Plasma Etch Reactor)—a full featured plasma processing simulation software developed at Ilmenau University of Technology. As a consequence, we show that local surface charge concurrently evolves with the feature profile to affect the final shape of the etched feature. Using gas chopping (sometimes called time-multiplexed) etch process for experimental validation of the simulation, we show that the model provides excellent fits to the experimental data and both, bowing and notching effects are captured—as long as the evolving profile and surface charge are simultaneously simulated. In addition, this new model explains that surface scallops, characteristic of gas chopping technique, are eroded and often absent in the final feature profile due to surface charging. The model is general and can be applied across many etching chemistries.

Gas chopping etching process for InP based nanostructures with high aspect ratios

The authors report a novel etching process for the fabrication of nanostructured III-V semiconductors with ultra-high aspect ratios. For the very first time, a time multiplexed etching process (gas chopping process) for an InP based material system has been successfully developed and demonstrated. The repetition of a Cl2/H2/Ar based etching step together with passivation and removal steps ensures very deep etching with straight walls without general restrictions by the etch depth. The newly developed process is very promising for the fabrication of deeply etched feedback gratings for lasers or nanophotonic devices requesting high aspect ratios, like lasers with surface defined distributed Bragg reflector/distributed feedback gratings or deeply etched photonic crystals. This new process allows the formation of vertical profiles with smooth surfaces and reduced footing effect. Using this gas chopping process for InP, a high mask selectivity of 15, good etching rate of 95 nm/min, and record values of the aspect ratio up to 41 could be demonstrated.

Low bias reactive ion etching of GaAs with a SiCl4∕N2∕O2 time-multiplexed process

An inductively coupled plasma reactive ion etching process was developed for transferring patterns from a thin intermediate mask consisting of Ni or SiNx into GaAs. Smoothed out etch floors and sidewalls can be achieved under an approximately 200V bias by switching between an anisotropic etch phase and a deposition phase by gas chopping. The deposition of Si–O based film that protects the sidewall from chlorine attack is promoted by the addition of O2 to an SiCl4∕N2 gas mixture. The total achieved etch depth was approximately 20μm in this work, but the process can in principle be adopted for a larger depth and other chlorine-etchable III-V semiconductors. SiCl4 is shown to act as a suitable deposition gas as well as an etch gas.

Integrated framework for the flux calculation of neutral species inside trenches and holes during plasma etching

An integrated framework for the neutral flux calculation inside trenches and holes during plasma etching is described, and a comparison between the two types of structure in a number of applications is presented. First, a detailed and functional set of equations for the neutral and ion flux calculations inside long trenches and holes with cylindrical symmetry is explicitly formulated. This set is based on early works [T. S. Cale and G. B. Raupp, J. Vac. Sci. Technol. B 8, 1242 (1990); V. K. Singh et al., J. Vac. Sci. Technol. B 10, 1091 (1992)], and includes new equations for the case of holes with cylindrical symmetry. Second, a method for the solution of the respective numerical task, i.e., one or a set of linear or nonlinear integral equations, is described. This method includes a coupling algorithm with a surface chemistry model and resolves the singularity problem of the integral equations. Third, the fluxes inside trenches and holes are compared. The flux from reemission is the major portion of the local flux at the bottom of both types of structure. The framework is applied in SiO2 etching by fluorocarbon plasmas to predict the increased intensity of reactive ion etching lag in SiO2 holes compared to trenches. It is also applied in deep Si etching: By calculating the flux of F atoms at the bottom of very high aspect ratio (up to 150) Si trenches and holes during the gas chopping process, the aspect ratio at which the flux of F atoms is eliminated and etching practically stops is estimated.

The influence of reactant transport on the profiles of gas chopping etching processes: a simulation approach

A two-dimensional profile simulator [B.E. Volland et al., J. Vac. Sci. Technol. B 20 (6) (2002) 3111-3117] was applied to the simulation of the etching of high aspect ratio trenches into silicon by means of a gas chopping etching technique. Reactant transport is modeled by a two-dimensional diffuse reflection model in case of neutrals and by direct line-of-sight shadowing in case of ions. Good agreement of the simulated with the experimental profiles was observed for a wide range of aspect ratios and for different etching to deposition cycle timings. Variation of the reactant transport parameters of the simulation model leads to simulated profiles with nearly no RIE-Lag, which has to be experimentally confirmed in the future.

The influence of reactant transport on the profiles of gas chopping etching processes: a simulation approach

A two-dimensional profile simulator [B.E. Volland et al., J. Vac. Sci. Technol. B 20 (6) (2002) 3111–3117] was applied to the simulation of the etching of high aspect ratio trenches into silicon by means of a gas chopping etching technique. Reactant transport is modeled by a two-dimensional diffuse reflection model in case of neutrals and by direct line-of-sight shadowing in case of ions. Good agreement of the simulated with the experimental profiles was observed for a wide range of aspect ratios and for different etching to deposition cycle timings. Variation of the reactant transport parameters of the simulation model leads to simulated profiles with nearly no RIE-Lag, which has to be experimentally confirmed in the future.

Profile simulation of gas chopping based etching processes

A simulation program based on a phenomenological surface etching reaction model and on a reactant transport model including shadowing and diffuse particle reflection at the sidewalls was developed to investigate the dependence of the etching rate and profile quality of gas chopping deep reactive ion etching processes on process parameters and sample temperature. The simulations are in good agreement with the experimental results. The dependence of the heating characteristics on the geometry (area, thickness) of teeny microelectromechanical systems devices or membrane-like silicon samples during microstructuring by means of plasma etching was investigated using finite element simulations. It was found that membrane-like samples are considerably heated unless the membrane area is sufficiently small (<≈5 mm φ) and/or is sufficiently thick (≈500 μm).

Plasma etch models based on different plasma chemistry for micro-electro-mechanical-systems application

The suitability of different plasma etch models based on various plasma chemistry has been evaluated for the fabrication of micro-mechanical structures and micro-electro-mechanical systems. Different etch models have been described for silicon etching based on fluorine and chlorine chemistry and the mechanisms involved in SiO 2 etching. Conventional planar reactive ion etching systems have been utilized for the etching of SiO 2 and silicon based on fluorine and chlorine etch models. Fluorine containing gases such as CHF 3 in combination with Ar have been used for SiO 2 etching and achieved nearly vertical sidewalls with smooth bottom surface. Gas mixtures such as SF 6/O 2 at low substrate temperatures and Cl 2/BCl 3 have been used for silicon etching and sidewall passivation techniques have been employed to achieve vertical sidewalls. The usefulness of chlorine chemistry using BCl 3 gas chopping technique for the fabrication of high aspect ratio structures has been demonstrated.

High aspect ratio silicon tips field emitter array

Micromachined high aspect ratio field emitter areas have been realised to obtain a stable and uniform emission current. Dry etching based on the gas chopping method was used to etch the 42 μm high ultra sharp emitter tips in single-crystal silicon. Diamond-like carbon (DLC) films of 25–100 nm thickness were deposited from hydrocarbon precursor gas on the Si tips applying the inductively coupled plasma enhanced chemical vapour-deposition (ICPECVD) technique. The partial stabilisation of emission current vs. time due to oxygen or nitrogen plasma treatment could be related to surface passivation and/or the formation of a stable DLC/nanolayer on the emitter surface. It is shown that DLC-coated emitters provide comparable good long-term emission stability — the emission current was found to be almost stable during a 3 week operation cycle.

Dry etching-based silicon micro-machining for MEMS

The aim of this work is to demonstrate the “dry” etching-based micro-fabrication technologies in the manufacturing of single-crystal silicon for micro-electro/(optical)-mechanical-systems (ME(O)MS). The ME(O)MS technology is a very fast-growing industry based often on the same silicon technology as integrated circuits. The process of plasma dry etching is quite simple straightforward and can be considered as the key technology in the micro-machining of silicon. The most important motivation for this work is advancement of the micro-machining industry. The basic aspects of pattern transfer of the prepared resist-patterns by plasma etching into the substrate, considering plasma chemistry, gas flow or gas chopping, and substrate temperature will be presented. Special attention will be given to discuss the physical and chemical phenomena which are involved in the generation of features with high aspect ratios.

Fabrication and electrical characterization of high aspect ratio silicon field emitter arrays

Micromachined high aspect ratio field emitter areas have been realized to obtain stabile and uniform emission current. Dry etching based on gas chopping method was used to etch the 42-μm-high ultra-sharp emitter tips in single-crystal silicon. Diamond-like carbon (DLC) films of 25–100 nm thickness were deposited from hydrocarbon precursor gas on the Si tips applying inductively coupled plasma enhanced chemical vapor deposition (ICPECVD) technique. Partial stabilization of emission current versus time due to oxygen or nitrogen plasma treatment could be related to surface passivation and/or formation of stable DLC/nanolayer on the emitter surface. It has been shown that DLC-coated emitters provide a comparable good long-term emission stability—the emission current did almost not decrease during a 46 h operation cycle. Summarizing we can say that high aspect ratio field emitter areas seem to have favorable field emission properties in respect to long-term stability.

Fabrication of open stencil masks with asymmetric void ratio for the ion projection lithography space charge experiment

For the ion projection lithography space charge experiment [P.W.H. de Jager et al., J. Vac. Sci. Technol. B 17, 3098 (1999)] a suitable stencil test mask has been realized and used. This article addresses the stress engineering and fabrication process of this specific test mask with openings in the range of some few 100 nm up to some 100 μm. This large difference in stencil pattern dimensions causes reactive ion etching (RIE) lag (dependence of the etch rate on feature size and pattern density) during the fabrication process and considerable stress variations across the membrane field. The solution to these problems was by (i) implementing novel doping technologies to create variable thickness areas on the membrane serving as reinforcement and stress relief structures, and (ii) adjusting the design to feature sizes and pattern densities to the same order of magnitude for the e-beam lithography and RIE processing steps. The etching of the openings through the 3 μm thick Si membrane was done by inductively coupled plasma etching with gas chopping techniques. These methods as devoloped for this specific test mask can be used to improve the stencil mask technology in general.

Dry etching with gas chopping without rippled sidewalls

In the last years, silicon micromachining techniques based on high aspect ratio reactive ion etching with gas chopping have been developed. There the gas flow of etching and deposition gas precursors is chopped which results in controllable sidewall passivation and high anisotropy. However, the rippled sidewalls are a serious limit for various applications. We report on the development of a novel gas chopping etching technique (GCET) process in order to achieve a smooth (rippled free) sidewall surface. As the direct etch mask, we used a 1 or 2-μm-thick resist layer, which was lithographically patterned. The novelty of the process consists in the replacing of the isotropic etching step by an anisotropic etching step. In this way we omit the main source for sidewall ripples. GCET combined with inductively coupled plasmas and fluorine chemistry provide very high etch rates and good control of the sidewall slope. These techniques also can be applied to conventional reactive ion etching equipment with Cl or F based plasma chemistry. However, the techniques used in this study have lower selectivity (in range of 30) than the conventional GCET. A SiON/Si selectivity as high as 50 has been achieved.

Fabrication of high aspect ratio structures using chlorine gas chopping technique

This article reports on the development of dry etching technology which will offer the potential to fabricate high aspect ratio structures in silicon for Micro Electro Mechanical Systems (MEMS) fabrication, using chlorine chemistry. In this study, we report about our investigations on the etching mechanisms using gas chopping technique. Chlorine with BCl3, as chopping gas for scavenging was the basis for deep trench etching in silicon. A conventional planar RIE (Reactive Ion Etching) reactor has been used. We optimised the process parameters as per our reactor's configuration to have compromise for best selectivity, anisotropy and high etch rates, which was obtained at BCl3 gas chopping time of 2 to 4 minutes, with DC-bias of −200 to −300 Volts.

Silicon stencil masks for masked ion beam lithography proximity printing

Silicon membranes with 2 μm to 6 μm thickness and ≈ 10×10 mm2 mask field have been fabricated with the help of electrochemical etch stop techniques. The Si foil was coated with 0.3 μm thick PECVD Si3N4. Shaped electron beam lithography was done in ARCH (OCG) positive resist. RIE etching into the nitride layer was done with CHF3/Ar/SF6. Silicon trench etching was based on Cl2/Ar/BCl3 plasma chemistry implementing gas chopping. Ion beam proximity printing of the Silicon stencil mask structures was done with 55 keV Helium ions into 0.4 μm thick AZ PN114 negative resist using the Alpha ion projector of the Society for the Advancements of Microelectronics in Austria in the MIBL (Masked Ion Beam Lithography) mode. Pattern transfer of a mask feature of less than 100 nm diameter (25:1 aspect ratio in the stencil mask) could be demonstrated even for a mask to wafer gap of 1 mm. The prospects of fabricating large area (> 100×100 mm2) Silicon stencil masks for MIBL printing of gate levels for FieldEmitterDisplays (FEDs) andMicro-Electro-Mechanical-Systems (MEMS) is discussed.

Enhancement of photoacoustic signals from condensed materials in the presence of volatile liquids: Influence of optical absorption coefficient, particle size, length of the gas phase and chopping frequency

The influence of the optical absorption coefficient, the particle size, the chopping frequency, and the length of the gas phase on the recently observed enhancement of photoacoustic signals in the presence of saturated vapors of ether has been investigated. Amorphous selenium has been used as the standard. The particle size plays a crucial role in understanding the enhancement as a function of optical absorption coefficient or chopping frequency. The enhancement is more when the optical absorption length is greater than particle size. The frequency dependence of the photoacoustic signal in the presence of ether is different from that in air when the thermal diffusion length of the solid is greater than the particle size. A qualitative interpretation of the results based on the ‘‘adsorbed’’ piston effect has been made.