Hole Sidewall Research Articles

Generation of Fast Neutral Beams by Ion Neutralization in High-Aspect-Ratio Holes: A Particle-in-Cell Simulation Study

In this paper, a particle-in-cell simulation of ion and neutral-beam extraction through a grid with high-aspect-ratio holes in contact with plasma was developed. Particular emphasis was placed on plasma molding over the holes, ion neutralization along the sidewall of the holes, and the energy and angular distributions of the residual ions and fast neutrals in the beam downstream of the holes. The target application was the generation of fast neutral beams for future charge-free microelectronics manufacturing. The energy and angular distributions of ions at different locations along the hole sidewall showed that ions which neutralize on the top section of the surface of the hole are “bad,” in the sense that these ions yield divergent neutral beams of relatively low energy. Ions that neutralize along the bottom section of the hole sidewall are “good,” in the sense that these ions yield neutral beams that are less divergent and retain more of the energy of the parent ions. A three-grid ion-extraction/neutralization system is proposed to increase the fraction of good ions and produce a higher quality neutral beam.

Carrier transport through a dry-etched InP-based two-dimensional photonic crystal

The electrical conduction across a two-dimensional photonic crystal (PhC) fabricated by Ar/Cl2 chemically assisted ion beam etching in n-doped InP is influenced by the surface potential of the hole sidewalls, modified by dry etching. Carrier transport across photonic crystal fields with different lattice parameters is investigated. For a given lattice period the PhC resistivity increases with the air fill factor and for a given air fill factor it increases as the lattice period is reduced. The measured current-voltage characteristics show clear ohmic behavior at lower voltages followed by current saturation at higher voltages. This behavior is confirmed by finite element ISE TCAD™ simulations. The observed current saturation is attributed to electric-field-induced saturation of the electron drift velocity. From the measured and simulated conductance for the different PhC fields we show that it is possible to determine the sidewall depletion region width and hence the surface potential. We find that at the hole sidewalls the etching induces a Fermi level pinning at about 0.12 eV below the conduction band edge, a value much lower than the bare InP surface potential. The results indicate that for n-InP the volume available for conduction in the etched PhCs approaches the geometrically defined volume as the doping is increased.

Characteristics of Cobalt Films Deposited by Using a Remote Plasma ALD Method with a CpCo(CO)2 Precursor

impurity content of the Co films was minimized using a H2 plasma in the process pressure range between 0.1 and 2 Torr at a plasma power of 300 W. The carbon and the oxygen contents of the Co films were about 7 at.% and below 1 at.%, the detection limit, respectively. The Co films showed a very uniform surface with a root-mean-square (RMS) roughness of 1.51 ˚ as determined by using an atomic force microscopy (AFM) analysis. The Co films deposited on contact holes, about 0.12 µm wide and 1.8 µm deep, showed excellent conformal coverage. The compositions of the Co films on the tops and the sidewalls of the contact holes were examined with Auger electron spectroscopy (AES), and the results showed nearly identical compositions.

Detailed analysis of the influence of an inductively coupled plasma reactive-ion etching process on the hole depth and shape of photonic crystals in InP∕InGaAsP

The authors report on the fabrication of photonic crystals in the InP∕InGaAsP∕InP material system for applications at telecommunication wavelengths. To achieve low optical loss, the photonic crystal holes must demonstrate smooth sidewalls and should be simultaneously deep and cylindrical. The authors present the etching process of these structures based on a Cl2∕Ar∕N2 chemistry with an inductively coupled plasma reactive-ion etching system. A systematic analysis is provided on the dependency of the hole sidewall roughness, depth, and shape on the process parameters such as etching power, pressure, and chemical composition of the plasma. They found that a low plasma excitation power and a low physical etching are beneficial for achieving deep holes, whereas for the nitrogen content in the plasma, a delicate balance needs to be found. Nitrogen has a negative impact on the hole shape and surface roughness but is capable of preventing underetching below the mask by passivation of the sidewalls. With the authors’ process more than 4μm deep holes with low conicity have been demonstrated.

Description of brittle failure of non-uniform MEMS geometries

The description of probability of failure of polysilicon micromachined components with complex geometries using a single set of Weibull parameters was investigated. Strength data from both uniform tension and from twelve non-uniform specimen geometries with central perforations were employed. These perforations allowed for twelve different combinations of stress concentration factors and radii of curvature. Two methods were applied to determine the Weibull parameters: in the first method, only the strength data from uniform tension specimens were used to determine the Weibull modulus and the material scale parameter. In the second approach, the strength data from all non-uniform tension geometries were used to calculate the material scale parameter and the Weibull modulus using the maximum likelihood method. The non-uniform stress state in each perforated specimen was taken into account through an elasticity finite element model and the use of the integral form of the Weibull probability function. Using the first method, an analysis considering active flaw populations at the top specimen surface or the specimen sidewalls indicated that the active flaw population is not the same at all scales: for 1–3 μm radius perforations and small stress concentration factor ( K = 3) the active flaw population was located at the specimen top surface, e.g. surface roughness, which, as the analysis indicated, was also the case for uniform tension specimens. For higher stress concentration factors (nominal K = 6 and 8) the analysis indicated that the active flaw population was located at the hole sidewall surface. As a result, for a given material the geometry of the specimen and the local state of stress determine the active flaw population from all flaws generated during fabrication and processing. Therefore, it is important that all potential failure modes be considered when extrapolations to other geometries and loadings are made using the Weibull probability function.

Effects of fluorocarbon gas species on electrical conductivity and chemical structure of deposited polymer in SiO2 etching processes

High-aspect-ratio SiO2 contact-hole etching is one of the key processes in the fabrication of ultralarge-scale integrated devices. However, there are many serious problems, such as charge-buildup damage, etching-stop, and microloading effects. Charge accumulation in high-aspect-ratio contact holes during etching is one of the main causes of these problems. In SiO2 etching using fluorocarbon gases, it is well known that fluorocarbon film is deposited on the underlayer surface and sidewall of contact holes. It is expected that such deposited fluorocarbon polymer will exert a great influence on the etching characteristics and charge accumulation in SiO2 contact holes. Therefore, it is necessary to measure the conductivity of the sidewall surfaces of contact holes with deposited fluorocarbon polymer. We made a monitoring device on a silicon wafer to evaluate the sidewall current of SiO2 contact holes and determined the relationship between the chemical structure and electrical conductivity of the fluorocarbon films deposited in the contact holes as a function of fluorocarbon gases and incident ion flux. We found that the electrical conductivity of the sidewall surface in SiO2 contact holes depends on the chemical structure of the deposited fluorocarbon polymer. It was also clear that the chemical structure of the deposited fluorocarbon polymer depended on nature of the radical species and ion flux incident on the etching surface. These results indicate that by controlling the chemical structure of the deposited fluorocarbon polymer one may be able to mitigate the influence of charge accumulation.

Hole depth- and shape-induced radiation losses in two-dimensional photonic crystals

A phenomenological model is adopted to study three-dimensional radiation losses in two-dimensional (2D) photonic crystals (PhC) deeply etched in a step-index waveguide. In the case of low vertical refractive index contrast, losses are modeled in a 2D approach and out-of-plane scattering is translated into an effective imaginary index in the air holes. The case of truncated-cone holes is analytically solved. Depth and shape contributions to losses are extracted showing that the verticality of the hole sidewalls is a fundamental parameter. In order to validate the model, the case of a 2D triangular lattice etched through an InP/GaInAsP slab waveguide is studied both theoretically and experimentally.

Electrical conductivity of sidewall-deposited fluorocarbon polymer in SiO2 etching processes

High-aspect-ratio SiO2 contact hole etching is one of the key processes in the fabrication of ultralarge scale integration devices. However, there are many serious problems, such as charge-build-up damage, etching-stop, and microloading effects. Charge accumulation in high-aspect-ratio contact holes during etching is one of the main causes of these problems. In SiO2 etching using fluorocarbon gases, it is well known that a fluorocarbon film is deposited on the underlayer surface and sidewall of contact holes. It is also predicted that an increase in the conductivity of the fluorocarbon film deposited on the SiO2 sidewall markedly reduces the charging potential in the high-aspect-ratio contact holes because of the high conductivity of the deposited polymer. To clarify these phenomena, it is necessary to obtain quantitative measurements of the conductivity of the sidewall surfaces in contact holes. Therefore, we fabricated the sensing device using semiconductor production technology, and monitored sidewall current of SiO2 contact holes on the silicon wafer. The relationship between the chemical structure and sidewall current of the fluorocarbon films deposited in contact holes is investigated. We have found that highly cross-linked and unsaturated fluorocarbon polymer causes high sidewall conductivity in SiO2 contact holes. It is suggested that the deposited fluorocarbon film reduces the charging potential during the SiO2 etching process.

Study of the SiO2-to-Si3N4 etch selectivity mechanism in the presence of polymers in fluorocarbon plasmas

The polymerization effects in 27/2 MHz capacitively coupled fluorocarbon plasmas during high selectively subquarter-micron contact hole etching, with the film structure of silicon dioxide (SiO2)/silicon nitride (Si3N4)/silicide (TiSix), are investigated. A high etch selectivity chemistry is first used to remove SiO2, then Si3N4 is removed with another chemistry with high selectivity of Si3N4-to-TiSix. To obtain good etch selectivity of SiO2-to-Si3N4, fluorocarbon plasmas containing a high carbon to fluorine (C/F) ratio is usually employed. Polymerization readily occurs in chemistries having a high C/F ratio. These unsaturated polymers are reactive and stick easily to contact hole sidewalls and bottoms, creating thick polymer films. While the chemistry used to etch Si3N4 has shown acceptable selectivity as measured on a patterned singular film, fluorine from the polymer will severely degrade the etch selectivity of Si3N4-to-TiSix on fully processed wafers. Process developments started with a design of experiment on real production wafers in order to understand the roles of fluorocarbon polymer during the Si3N4 etching. Scanning electron microscope analysis of this polymer film and its effects are included as well as the electrical parametric testing data of contact resistances. Different polymer removing methods to restore the Si3N4-to-TiSix etch selectivity back were initiated and will be briefly discussed. While this investigation pertains to contact etch, this phenomenologically observed result could be applied to any highly selective etching of oxide versus nitride.

Self-limiting MBE growth and characterization of three-dimensionally confined nanostructures on patterned GaAs (311)A substrates

The formation of triangular-shaped dot-like (TD) structures grown by molecular beam epitaxy on GaAs (311)A substrates patterned with square- and triangular-shaped holes is compared. On substrates patterned with square-shaped holes, TD structures are formed via the pinch-off of two symmetrically arranged {111} planes which develop freely in the regions between the holes on the original substrate surface, while the (111)A sidewalls of the as-etched holes develop a rough morphology during growth. The evolution of the rough (111)A sidewalls is eliminated on substrates patterned with triangular shaped holes resulting in similar TD structures with highly improved uniformity over the entire pattern. Spectrally and spatially resolved cathodoluminescence spectroscopy reveals the lateral variation of the quantum-well confinement energy in the TD structures generating distinct lateral energy barriers between the top portion and the nearby smooth regions with efficient radiative recombination. Formation of TD structures provides a new approach to fabricate three-dimensionally confined nanostructures in a controlled manner.

Mechanism of Etch Stop in High Aspect-Ratio Contact Hole Etching

The mechanism of etch stop in contact hole etching has been studied. It was found that in high aspect ratio holes, even though the incident ions lose charge due to collision with the sidewall, they are able to bombard the bottom of the hole maintaining their high energy. It was also confirmed that the redeposition of sputtered species from the fluorocarbon polymer on the hole sidewall induces the etch stop at the bottom of the high-aspect hole. Furthermore, it was observed that etch stop occurs at higher aspect ratios for the same hole diameter in oxide films with higher boron and phosphorous dopant concentrations. This is explained by the effective removal of etch-inhibiting carbon species due to the release of more oxygen at a higher etch rate in highly doped oxide film. In conclusion, the etch stop in a high-aspect-ratio hole is determined by the balance between the effects of high-energy-species bombardment and etch inhibition of carbon species.

Kidney and anti-kidney vortices in crossflow jets

Water tunnel experiments were conducted to examine the effect of hole exit geometry on the near-field characteristics of crossflow jets. Hole shapes investigated were round, elliptical, square, and rectangular, all having the same cross-sectional area. Laser-induced fluorescence (LIF) and particle image velocimetry (PIV) were used.The vorticity around the circumference of the jet was tracked to identify its relative contributions to the nascent streamwise vortices, which evolve eventually into kidney vortices downstream. The distinction between sidewall vorticity and that from the leading and trailing edges, though blurred for a round hole, became clear for a square or a rectangular hole. The choice of non-circular holes also made it possible to reveal the unexpected double-decked structures of streamwise vortices and link them to the vorticity generated along the wall of the hole.The lowermost vortex pair of the double-decked structures, located beneath the jet, is what we call a ‘steady’ vortex pair. This pair is always present and has the same sense of rotation as the kidney vortices. The origin of these lower-deck vortices is the hole sidewall boundary layer: as the jet emanates from the hole, the crossflow forces the sidewall boundary layer to roll up into nascent kidney vortices. Here, hole width sets the lateral separation of these steady sidewall vortices.The vortices comprising the upper deck ride intermittently over the top of the ‘steady’ lower pair. The sense of rotation of these upper-deck vortices depends on hole geometry and can be the same as, or opposite to, the lower pair. The origin of the upper deck is the hole leading-edge boundary layer. This vorticity, initially aligned transverse to the crossflow direction, is realigned by the entrainment of crossflow momentum and thus induces a streamwise component of vorticity. Depending on hole geometry, this induced streamwise vorticity can be opposite to the lower-deck vortex pair. The opposing pair, called the ‘anti-kidney’ pair, competes with the nascent kidney-vortices and affects the jet lift-off. The hole trailing-edge boundary layer can likewise be turned toward the streamwise direction. In this case, the turning is caused by the strong reverse flow just downstream of the jet.In the present range of parameters, all hole boundary layer vorticity, regardless of its origin along the hole circumference, is found to influence the kidney vortices downstream.

An experimental study of cutting fluid effects in drilling

Experiments were designed and conducted on aluminum alloys and gray cast iron to determine the function of cutting fluid in drilling. The variables examined included speed, feed, hole depth, tool and workpiece material, cutting fluid condition, workpiece temperatures and drill geometry. The experimental responses under consideration were quantitative as well as qualitative. The quantitative responses included drilling torque, thrust force, hole quality (diameter, perpendicularity, taper and cylindricity), surface finish and built-up edge, whilst the qualitative responses were chip morphology and inspection of hole side-walls. The results indicate that the cutting fluid does have an effect on the surface finish, and that the cutting fluid interacts with other variables to have an effect on hole quality. The evidence suggests that the mechanism of the cutting fluid is lubricating at the margins of the drill.

Analyses of the chemical topography of silicon dioxide contact holes etched in a high density plasma source

High aspect ratio sub-half-micron contacts were etched in SiO2 using a high density C2F6 plasma generated by an inductively coupled, low pressure reactor. Process development studies have shown that high aspect ratio contact holes can be open in thick dielectric layers with a good selectivity to the underlying silicon. Minimization of reactive ion etching lag is obtained by using high bias power conditions, which on the other hand induce a degradation of the photoresist, described as a graphitization of the resist. The degradation induces considerable roughness at the edges of the resist structures which leads to the creation of striations in the silicon dioxide as the pattern transfer in the SiO2 proceeds. X-ray photoelectron spectroscopy (XPS) studies have allowed analysis of high aspect ratio SiO2 contact holes. Using appropriate flood gun conditions, a complete separation of the XPS peaks originating from the resist mask and from the fluorocarbon polymer deposited on the bottom of the contact holes is possible. The XPS analyses show, in particular, that the fluorination of the polymers on the bottom of the contact holes strongly increases with the aspect ratio of the contact hole, and that the fluorination of the polymers deposited on the contact hole sidewalls is even greater.

Step coverage comparison of Ti/TiN deposited by collimated and uncollimated physical vapor deposition techniques

The step coverage and grain structure of Ti/TiN films sputtered with and without a collimator in a contact/via hole structure were compared using transmission electron microscopy, microelectron diffraction, and Auger electron spectroscopy with field emission microanalysis capability on cross-section samples. The holes have widths ranging from 0.40 to 0.75 μm and aspect ratios from 2.3 to 1.4, respectively. It was found that collimated Ti/TiN films demonstrate reasonably uniform coverage along the sidewall and at the bottom of holes. Collimated Ti bottom coverage is sufficient to assure good contact resistance. In contrast, uncollimated Ti/TiN films demonstrate an overhang on the top sidewall of holes and show decreased coverage when approaching the bottom corner along the sidewall or along the bottom of holes. Film property differences at different regions of a hole using either collimated or uncollimated deposition techniques will also be discussed.

Pulsed-sheath ion dynamics in a trench

Plasma ion dynamics following the application of a negative voltage pulse to a two-dimensional trench geometry are computed using a two-fluid model. The potential structure in the sheath both focuses and bunches ions, creating an ion density pulse that travels down the sidewall of the trench. It may be possible to exploit this phenomenon to improve ion implantation in the sidewalls of trenches and holes when using plasma source ion implantation.

Utilizing Magnetic Resonance Imaging Logs, Open Hole Logs and Sidewall Core Analyses to Evaluate Shaly Sands for Water-Free Production: ABSTRACT

Utilizing Magnetic Resonance Imaging Logs, Open Hole Logs and Sidewall Core Analyses to Evaluate Shaly Sands for Water-Free Production: ABSTRACT

Gas-assisted etching with focused ion beam technology

Abstract The role of focused ion beam (FIB) systems in device failure analysis has been well documented 1 . FIB etching is used to cross-section features and to prepare TEM samples with better than 0.1 micron (μm) accuracy 2 , to open probe holes for mechanical and electron beam probing of circuitry, and to rewire circuits by cutting or adding connections. FIB is used to image cross sections with either secondary ions or secondary electrons, to measure metal grain size distributions through channeling contrast, and to measure process control parameters. System limitations include a relatively slow removal rate by beam sputtering of large volumes of material. Also, redeposition of sputtered material on the sidewalls of holes limits the achievable sidewall angle, and thus limits the aspect ratios of holes to approximately 6:1. With decreasing geometry sizes and multiple planarized metal layers, device modifications require cuts and interconnect holes of up to 10:1 aspect ratio, which up to now have been difficult with FIB. We will discuss recent progress in using gas-assisted etching (GAE) to enhance the FIB etching, to sharpen the etch profile, and to etch high aspect ratio holes. The hardware and GAE process will be briefly described. We will discuss the relative etch rates and selectivity of GAE with typical device materials, such as aluminum (Al), tungsten (W), silicon dioxide (SiO 2 ) and silicon (Si), using two halogen-based etchant gases, xenon difluoride (XeF 2 ) and chlorine (Cl 2 ). Images of sidewall profiles with and without GAE will be compared, and we will demonstrate several applications of GAE for e-beam and optical analysis.

Submicron 3-Dimensional Structure Observation by Cyclotron SEM(Scanning Electron Microscope)

Experimental and numerical approaches have been taken to the study of the brightness of scanning electron microscope(SEM) images for bottom surfaces of deep holes and trenches by Cyclotron SEM. Secondary electrons emitted from the bottom are efficiently collected as an image signal by a couple of the following combinational effects. One is an electrostatic converging lens due to accumulated negative charges on the side wall of holes and trenches. The other is a strong magnetic field above the specimen surface generated by an immersion-type objective lens. The former plays the role of reducing the number of electrons which collide with the side wall. The latter binds diverged secondary electrons leaving from the specimen surface and introduces electrons into a detector.

Observation of anomalous high resistance to implanted areas caused by reactive ion etching (RIE)

Oxide etching for contact hole definition and sidewall spacer formation is normally performed using fluorocarbon gas mixtures in a plasma reactor. A significant amount of over-etching must be allowed to compensate for uneven oxide thickness and poor process uniformity. During this time underlying silicon material is subjected to energetic particle bombardment which under severe circumstances can induce polymer deposition and radiation damage. Recent results indicate that contacts made to n + and/or p + areas, defined by reactive ion etching, suffer from high resistance which can lower circuit yield. In view of the foregoing, experiments were performed to investigate this phenomena as well as to find a solution whereby the problem of high contact resistance could be circumvented. Reactive ion etching (RIE) was performed in an hexode etcher using CHF 3, O 2 gas mixture at 50 mtorr. Plasma etching was performed in a single wafer etcher using a gas mixture of CHF 3, C 2F 6 and He at 1.5 torr. The as-etched samples were analysed using spreading resistance profiling (SRP), RBS-channeling measurements, ESCA and 4-point probe techniques. These measurements were used to investigate both the presence and the electrical activity of the implanted dopants. The nuclear reaction 1H( 15N, αγ) 12C at 6.38 MeV was employed to profile the hydrogen depth distribution. Cross sectional TEM was performed in order to characterize the extent of the damage. Contact chains defined by dry processing were used for electrical evaluation. It is shown that the silicon surface can be restored after RIE via an in situ low power etch with NF 3. The effectiveness of such a procedure was verified by contact resistance and thermal wave measurements.