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Feature-Scale Modeling of Low-Bias SF 6 Plasma Etching of Si
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We present a novel resonant MEMS sensor for the detection of particles with dielectric properties in liquid environments. The sensor consists of a piezoelectrically excited silicon microcantilever with integrated tailored electrodes on the surface that accumulate particles due to dielectrophoresis. By monitoring the mass-induced shift of the cantilever-related resonance frequency, the accumulated particle mass during the measurement cycle is deduced. We evaluated the mass-sensing performance by exposing the sensor to four artificially aged lubricating oils, thus containing different particle concentrations due to the oxidative degradation of the oil constituents. The measured resonance frequency shift showed a clear correlation to the degree of oil oxidation and is used to determine the accumulated mass by applying an automated particle detection scheme. The measured mass responsivity and limit of detection are in the range of 13 Hz/ng and 3.5 ng, respectively. Compared to standard laboratory methods, such as infrared spectroscopy, the analysis of the sensor response with respect to the oil quality shows that even subtle changes of the oil condition can be detected. These results pave the way for on-line monitoring of oil degradation in industrial applications, such as needed for the condition-based and predictive maintenance of lubricated machinery.
The characteristics of isotropic etching of silicon in a purely inductively coupled SF6 plasma are quantitatively studied. Since the etch results are strongly dependent on mask features, the authors investigated both large area and narrow trench etch characteristics. Circles of diameter 500 μm were used as a proxy for unpatterned surfaces and etched for different durations to establish the material etch rate and surface roughness. The average etch rate using the chosen recipe was found to be 2.27 μm/min. Arrays of narrow trenches ranging from 8 to 28 μm were also etched to analyze the effect of trench size on etch rate and degree of anisotropy. The etch rate of the trenches was found to strongly decrease with decreasing trench width. The results demonstrate that isotropic SF6 etch can be readily used as a replacement for more exotic silicon vapor phase etch chemistries such as XeF2.
Optical resonators are essential for fundamental science, applications in sensing and metrology, particle cooling, and quantum information processing. Cavities can significantly enhance interactions between light and matter. For many applications they perform this task best if the mode confinement is tight and the photon lifetime is long. Free access to the mode center is important in the design to admit atoms, molecules, nanoparticles, or solids into the light field. Here, we demonstrate how to machine microcavity arrays of extremely high quality in pristine silicon. Etched to an almost perfect parabolic shape with a surface roughness on the level of 2 Å and coated to a finesse exceeding F = 500,000, these new devices can have lengths below 17 µm, confining the photons to 5 µm waists in a mode volume of 88λ3. Extending the cavity length to 150 µm, on the order of the radius of curvature, in a symmetric mirror configuration yields a waist smaller than 7 µm, with photon lifetimes exceeding 64 ns. Parallelized cleanroom fabrication delivers an entire microcavity array in a single process. Photolithographic precision furthermore yields alignment structures that result in mechanically robust, pre-aligned, symmetric microcavity arrays, representing a light-matter interface with unprecedented performance.
Semiconductor device dimensions have been decreasing steadily over the past several decades, generating the need to overcome fundamental limitations of both the materials they are made of and the fabrication techniques used to build them. Modern metal gates are no longer a simple polysilicon layer, but rather consist of a stack of several different materials, often requiring multiple processing steps each, to obtain the characteristics needed for stable operation. In order to better understand the underlying mechanics and predict the potential of new methods and materials, technology computer aided design has become increasingly important. This review will discuss the fundamental methods, used to describe expected topology changes, and their respective benefits and limitations. In particular, common techniques used for effective modeling of the transport of molecular entities using numerical particle ray tracing in the feature scale region will be reviewed, taking into account the limitations they impose on chemical modeling. The modeling of surface chemistries and recent advances therein, which have enabled the identification of dominant etch mechanisms and the development of sophisticated chemical models, is further presented. Finally, recent advances in the modeling of gate stack pattering using advanced geometries in the feature scale are discussed, taking note of the underlying methods and their limitations, which still need to be overcome and are actively investigated.
We focus on a surface evolution problem where the surface is represented as a narrow-band level-set and the local surface speed is defined by a relation to the direct visibility of a source plane above the surface. A level-set representation of the surface can handle complex evolutions robustly and is therefore a frequently encountered choice. Ray tracing is used to compute the visibility of the source plane for each surface point. Commonly, rays are traced directly through the level-set and the already available (hierarchical) volume data structure is used to efficiently perform intersection tests.
Chemical etching of silicon by fluorine atoms in the absence of ion bombardment is reviewed. Controversies on the identity of etching products and reaction probabilities are discussed. Attempts are made to explain the apparent presence of SiF2 as a primary product in many studies, dating back to 1980, but not in others, including those of Harold Winters from as early as 1979. Reported estimates of reaction probabilities (here defined as the probability of removing a Si atom from the substrate per incident F atom) vary by a factor of 2000. When these values, with some corrections and reasonable adjustments, are plotted as a function of F atom flux, most of them fall on a “universal curve” that reveals a large (∼30-fold) decrease in the reaction probability with increasing F flux, from 0.03 at a F flux 1012 cm⁻² s⁻¹ to 0.001 at a flux of 1020 cm−2 s⁻¹. These values were extracted from beam experiments with F atoms generated from cracking of F2, including those by Harold Winters, from isotropic etching in plasma experiments (both in-plasma and downstream) with F2, CF4/10%O2, and NF3 feed gases, as well as from molecular dynamics simulations. Reaction coefficients derived from chemical etching rates in SF6 plasmas do not follow this trend, however, suggesting a large enhancement in the F reaction probability (∼20 to 100-fold at F fluxes of 10¹⁸–1019 cm−2 s⁻¹), due to the presence of sulfur.
Fluorine atoms etch silicon with a rate, R F(Si) = 2.91±0.20×10-12T1/2n F e-0.108 eV/kT Å/min, where n F (cm-3) is the atom concentration. This etching is accompanied by a chemiluminescent continuum in the gas phase which exhibits the same activation energy. These phenomena are described by the kinetics: (1) F (g) +Si surf →SiF 2(g) , (2) SiF 2(g) +F (g) →SiF* 3(g) , (3) SiF 2(g) +F 2(g) →SiF* 3(g) +F (g) , (4) SiF* 3(g) →SiF 3(g) +hν continuum where formation of SiF 2 is the rate‐limiting step. A detailed model of silicon gasification is presented which accounts for the low atomic fluorine reaction probability (0.00168 at room temperature) and formation of SiF 2 as a direct product. Previously reported etch rates of SiO 2 by atomic fluorine are high by a constant factor. The etch rate of SiO 2 is R F(SiO 2 ) = (6.14±0.49)×10-13n F T1/2e-0.163/kT Å/min and the ratio of Si to SiO 2 etching by F atoms is (4.74±0.49)e-0.055/kT.
Samples of 53 materials that are used or potentially can be used or in the fabrication of microelectromechanical systems and integrated circuits were prepared: single-crystal silicon with two doping levels, polycrystalline silicon with two doping levels, polycrystalline germanium, polycrystalline SiGe, graphite, fused quartz, Pyrex 7740, nine other preparations of silicon dioxide, four preparations of silicon nitride, sapphire, two preparations of aluminum oxide, aluminum, Al/2%Si, titanium, vanadium, niobium, two preparations of tantalum, two preparations of chromium, Cr on Au, molybdenum, tungsten, nickel, palladium, platinum, copper, silver, gold, 10 Ti/90 W, 80 Ni/20 Cr, TiN, four types of photoresist, resist pen, Parylene-C, and spin-on polyimide. Selected samples were etched in 35 different etches: isotropic silicon etchant, potassium hydroxide, 10:1 HF, 5:1 BHF, Pad Etch 4, hot phosphoric acid, Aluminum Etchant Type A, titanium wet etchant, CR-7 chromium etchant, CR-14 chromium etchant, molybdenum etchant, warm hydrogen peroxide, Copper Etchant Type CE-200, Copper Etchant APS 100, dilute aqua regia, AU-5 gold etchant, Nichrome Etchant TFN, hot sulfuric+phosphoric acids, Piranha, Microstrip 2001, acetone, methanol, isopropanol, xenon difluoride, HF+H2O vapor, oxygen plasma, two deep reactive ion etch recipes with two different types of wafer clamping, SF6 plasma, SF6+O2 plasma, CF4 plasma, CF4+O2 plasma, and argon ion milling. The etch rates of 620 combinations of these were measured. The etch rates of thermal oxide in different dilutions of HF and BHF are also reported. Sample preparation and information about the etches is given.
The field of plasma etching is reviewed. Plasma etching, a revolutionary extension of the technique of physical sputtering, was introduced to integrated circuit manufacturing as early as the mid 1960s and more widely in the early 1970s, in an effort to reduce liquid waste disposal in manufacturing and achieve selectivities that were difficult to obtain with wet chemistry. Quickly, the ability to anisotropically etch silicon, aluminum, and silicon dioxide in plasmas became the breakthrough that allowed the features in integrated circuits to continue to shrink over the next 40 years. Some of this early history is reviewed, and a discussion of the evolution in plasma reactor design is included. Some basic principles related to plasma etching such as evaporation rates and Langmuir–Hinshelwood adsorption are introduced. Etching mechanisms of selected materials, silicon, silicon dioxide, and low dielectric-constant materials are discussed in detail. A detailed treatment is presented of applications in current silicon integrated circuit fabrication. Finally, some predictions are offered for future needs and advances in plasma etching for silicon and nonsilicon-based devices.
A Thorough Update of the Industry Classic on Principles of Plasma Processing The first edition of Principles of Plasma Discharges and Materials Processing, published over a decade ago, was lauded for its complete treatment of both basic plasma physics and industrial plasma processing, quickly becoming the primary reference for students and professionals. The Second Edition has been carefully updated and revised to reflect recent developments in the field and to further clarify the presentation of basic principles. Along with in-depth coverage of the fundamentals of plasma physics and chemistry, the authors apply basic theory to plasma discharges, including calculations of plasma parameters and the scaling of plasma parameters with control parameters. New and expanded topics include: Updated cross sections Diffusion and diffusion solutions Generalized Bohm criteria Expanded treatment of dc sheaths Langmuir probes in time-varying fields Electronegative discharges Pulsed power discharges Dual frequency discharges High-density rf sheaths and ion energy distributions Hysteresis and instabilities Helicon discharges Hollow cathode discharges Ionized physical vapor deposition Differential substrate charging With new chapters on dusty plasmas and the kinetic theory of discharges, graduate students and researchers in the field of plasma processing should find this new edition more valuable than ever.
High-quality optical cavities with wavelength-sized end mirrors are important to the growing field of micro-optomechanical systems. We present a versatile method for calculating the modes of diffraction limited optical cavities and show that it can be used to determine the effect of a wide variety of cavity geometries and imperfections. Additionally, we show these calculations agree remarkably well with FDTD simulations for wavelength-sized optical modes, even though our method is based on the paraxial approximation.
The dependence of etch rate on the quantity of material being etched, often referred to as the loading effect, for plasma
etching is analyzed quantitatively with the aid of some simplifying assumptions. The etch rate is related to the quantity
of material being etched through three phenomenological parametersβ, τ, and
, which are related to the affinity of the etching material for the active species created in the plasma, the lifetime of
the active species, and the volume generation rate of the active species, respectively. It is shown analytically that the
reciprocal of the etch rate, when
identical wafers are etching simultaneously, is directly proportional to
. Data are presented for the plasma etching of Si in a
plasma which confirm the analytical prediction. The significance of loading with respect to the measurement of etch rate,
end‐point detection, and the interpretation of the temperature dependence of etch rate is discussed. A brief discussion of
the physical meaning of the phenomenological parameters and their relation to process parameters subject to control such as
power, pressure, and temperature is also included.
Micromachining arbitrary 3D silicon structures for micro-electromechanical systems can be accomplished using gray-scale lithography along with dry anisotropic etching. In this study, we have investigated the use of deep reactive ion etching (DRIE) and the tailoring of etch selectivity for precise fabrication. Silicon loading, the introduction of an O2 step, wafer electrode power, and wafer temperature are evaluated and determined to be effective for coarsely controlling etch selectivity in DRIE. The non-uniformity and surface roughness characteristics are evaluated and found to be scaled by the etch selectivity when the 3D profile is transferred into the silicon. A micro-compressor is demonstrated using gray-scale lithography and DRIE showing that etch selectivity can be successfully tailored for a specific application.
Seismic Analysis (Chap. 20) Computational Geometry (Chap. 19) Computer Vision (Chaps, 16, 17) Optimality and Control (Chap. 20) Fluid Mechanics (Chap. 18) Combustion (Chap. 18) Materials Sciences (Chap. 18) Semiconductor Manufacturing (Chap. 21) This book is an introduction to level set methods and Fast Marching Methods, which are powerful numerical techniques for analyzing and computing interface motion in a host of settings. They rely on a fundamental shift in how one views moving boundaries, rethinking the natural geometric Lagrangian perspective and replacing it with an Eulerian partial differential equation perspectives. The resulting numerical techniques are used to track three-dimensional fronts that can develop sharp corners and change topology as they evolve. The book begins with an overview of the two techniques, and then provides an int
Feature-Scale Model of Si Etching in SF
- R J Belen
- S Gomez
- M Kiehlbauch
- D Cooperberg
Reactions of Fluorine Atoms with Silicon, Revisited, Again
- V M Donnelly
Microfabrication of 3D Silicon MEMS Structures using Gray-Scale Lithography and Deep Reactive Ion Etching
- C Waits