Masking Material Research Articles

Study of Tip-Induced Ti-Film Oxidation in Atomic Force Microscopy Contact and Non-Contact Mode

Local anodic oxidation of metals using scanning probe techniques is mostly used for fabrication of isolated gates. The high-resistance oxide created in such a manner divides a thin metallic film into isolated regions. Thetip-induced metallic oxide has not so far been used in nanolithography processes as a masking material. The aim of this contribution is to study the technological potential of a TiO x mask prepared by the local anodic oxidation of a Ti film. Such a mask can be used to complete a nanotechnology process using atomic force microscopy, which can be easily combined with standard optical lithography techniques. We have found that the insulating properties of the oxides prepared in contact and non-contact modes differ strongly - they represent an energy barrier of 200 meV and 400 meV, respectively.

Optical properties of thin film CrxNy–CrOxNy multilayer films for 157 nm optical lithography

Cr x N y and CrxOyNz thin films and multilayer CrxNy–CrxOyNz thin films have been investigated as a binary masking material for 157 nm lithography. The chemical compositions of commercial photolithography masks were determined to be a bilayer structure of Cr0.85N0.15 with a top anti-reflection layer of graded CrxOyNz with a total thickness of ∼100 nm. To simulate the commercial structures, CrxNy and CrxOyNz thin films were reactively rf sputter deposited from a metal chromium target in various Ar–N2 and Ar–N2–O2 environments and their optical properties examined. After replicating the standard ∼100-nm-thick bilayer structure, the process was scaled to ∼50 and 80 nm by: (1) thinning the CrxNy layer and (2) by thinning both the CrxNy and CrxOyNz layers proportionally. The vacuum ultraviolet optical properties of the thinned masking materials were evaluated and the 80 nm features were determined to have the desired optical density >4 and the reflectivity of <15%. Modeling of the optical properties agreed well with the experimental results.

Super-Resolution Near-Field Structure with Alternative Recording and Mask Materials

Using a variety of recording materials, such as terbium–iron–cobalt (TbFeCo), tungsten (W), silver (Ag) and silver–zinc (Ag–Zn), the characteristics of super-resolution near-field structure (super-RENS) disks with the reactive diffusion (RD) recording mechanism were investigated. As a new mask material, WOx, which has a high transition temperature, was applied to a super-RENS disk. A combined W recording layer and WOx mask layer achieved much higher carrier-to-noise ratio (CNR), and better thermal stability, compared with AgOx light-scattering-type super-RENS disks.

Measuring small samples at the ISIS muon source

Studying samples appreciably smaller than the muon beam spot size, using conventional masked holders, is complicated by the detection of positrons from the muons stopped in the masking material. A technique of suspending samples in the beam as a method of overcoming these inherent difficulties has been developed; muons not implanted in the sample are now able to fly-past and their contribution is removed from the measured signal. The implementation of this method on the EMU spectrometer at ISIS is described in this paper. Details of modifications to equipment to enable this type of operation and the results obtained from several test experiments are reported. Practical details associated with carrying out experiments using suspended samples are also considered. Although greatly reduced, background counts are still present in the data; experiments investigating the source of these counts and future improvements to the apparatus are considered.

Optimizing vacuum ultraviolet attenuated phase shift masking materials

Extensive vacuum ultraviolet (VUV) optical data have been collected for sputter-deposited thin films of masking material compounds for use at 157 nm. The use of VUV optical data and effective media modeling has lead to 27 possible material candidates for further study. These composite material films consist of a silicon dioxide host combined with various oxides and nitrides, generally of refractory metals. By adjusting material stoichiometry through control of deposition parameters (target composition ratios, pressure, power, flow rates, etc.), films have been tailored for specific optical properties. By using extinction coefficient/refractive index relationships for 180° phase shift, 4%–25% transmission, and <15% reflectivity, evaluation of the materials is possible. Additional detailed analysis of three specific composite materials is shown for use as attenuated phase shift masking at 157 nm. In each case, silicon dioxide is the host material within the film and the various metal oxides provide the appropriate absorption. The use of fluoride film etch stop materials is also described as a method to improve etch selectivity.

A new masking method for protecting silicon surfaces during anisotropic silicon wet etching

A room-temperature silicon masking approach based on the exposure of silicon to CHF 3 -based plasma is explored. This plasma treatment leads to ultra-thin (2–5 nm) films that consist of a fluorocarbon top layer and a sub-oxide lower layer and are appropriate for anisotropic wet etching masks. The mask resistance to anisotropic wet-etchants is studied as a function of film preparation parameters. Defect evolution is examined for two key film preparation conditions. Masks explored compare favourably with common masking materials such as SiO 2 or Si 3 N 4 in terms of achievable patterns and processing options compatible with standard silicon integrated circuit technology. In addition, the new masking method can be applied when sidewall-only wet etching of mesa patterns is desired.

Low-pressure chemical vapour deposition growth of epitaxial silicon selective to silicon nitride

The selective epitaxial growth of silicon through windows in a masking material has a wide range of applications for silicon based heterostructures. For many device structures, silicon deposition selective to silicon nitride is required. A novel low-pressure chemical vapour deposition growth process has been developed for epitaxial silicon growth which is selective to silicon nitride. The influence of pressure on the epitaxial quality was investigated. Substrate wafers were heated using a resistive graphite element at 850 °C. The gas chemistry consisted of a combination of SiH 4 and SiH 2Cl 2 in a carrier gas stream of H 2 in the ratio 5:1:20, respectively. Selective growth has been demonstrated at pressures from 50 mTorr to 2 Torr on 4 in. wafers, which were patterned with a series of silicon nitride bars.

The application of nanosecond-pulsed laser welding technology in MEMS packaging with a shadow mask

Abstract A nanosecond-pulsed laser bonding process with a shadow mask for MEMS packaging applications has been successfully demonstrated. YAG Surelit II laser with pulse duration of 4–6 ns, a wavelength of 355 nm and a focal diameter of 1 mm is used to provide the bonding energy for glass-to-silicon bonding using a 4 μm thick indium layer as the bonding material. The optimal bonding parameters are found experimentally when the laser energy is between 8 and 22 mJ and multiple laser shots are used. Furthermore, a regular white paper with pre-defined patterns has been successfully used as the masking material to achieve selective heating and bonding. Simulation results show that localized heating can be achieved by using the nanosecond laser. One micro-second after the laser power is applied, the temperature at the indium/silicon interface drops from 2500 to 760 °C and drops further to 43 °C after 1 ms. This nanosecond laser power is believed to provide suitable energy and time for localized heating and bonding applications for MEMS.

Silicon layer transfer using wafer bonding and debonding

Two experiments were performed that demonstrate an extension of the ion-cut layer transfer technique where a polymer is used for planarization and bonding. In the first experiment hydrogen-implanted silicon wafers were deposited with two to four microns low-temperature plasma-enhanced tetraethoxysilane (TEOS). The wafers were then bonded to a second wafer, which had been coated with a spin-on polymer. The bonded pairs were heated to the ion-cut temperature resulting in the transfer of a 400 nm layer silicon. The polymer enabled the bonding of an unprocessed silicon wafer to the as-deposited TEOS with a microsurface roughness larger than 10 nm, while the TEOS provided sufficient stiffness for ion cut. In the second experiment, an intermediate transfer wafer was patterned and vias were etched through the wafer using a 25% tetramethylammonium hydroxide (TMAH) solution and nitride as masking material. The nitride was then stripped using dilute hydrofluoric acid (HF). The transfer wafer was then bonded to an oxidized (100 nm) hydrogen-implanted silicon wafer. After ion-cut annealing a silicon-on-insulator (SOI) wafer was produced on the transfer wafer. The thin silicon layer of the SOI structure was then bonded to a third wafer using a spin-on polymer as the bonding material. The sacrificial oxide layer was then etched away in HF, freeing the thin silicon from the transfer wafer. The result produced a thin silicon-on-polymer structure bonded to the third wafer. These results demonstrate the feasibility of transferring a silicon layer from a wafer to a second intermediate “transfer” or “universal” reusable substrate. The second transfer step allows the thin silicon layer to be subsequently bonded to a potential third device wafer followed by debonding of the transfer wafer creating stacked three-dimensional structures.

Relationship between deprotection and film thickness loss during plasma etching of positive tone chemically amplified resists

Positive tone chemically amplified (CA) resists have demonstrated the sensitivity, contrast, and resolution necessary to print state-of-the-art subwavelength features using 248 nm and more recently 193 nm lithography. These materials are also being considered for printing sub-100 nm features with 157 nm and next-generation lithography technologies such as extreme ultraviolet and electron beam projection lithography. The basis for solubility differential and image formation in these resists is the acid catalyzed deprotection of labile protecting groups of an inherently base soluble polymer. The deprotection is effected by the photochemical generation of strong acid during the exposure process. Such acid-catalyzed deprotection reactions can also occur in unexposed resist areas when etched in a plasma. This can be due to UV exposure, high-energy ion bombardment, elevated substrate temperatures, or interaction of the resist surface with plasma species to form acidic moieties. Deprotection has been associated with resist mass loss and film shrinkage during plasma etching, leaving inadequate masking material for the entire etch step. In this article, we report the film thickness loss of several unexposed CA resists as a function of etch time in a variety of plasmas and correlate these data with film composition, monitored by Fourier transform infrared spectroscopy. These results are compared with theoretical predictions based on generally accepted deprotection mechanisms. Our findings indicate that the “acidic” nature of certain plasmas such as Cl2/O2 can result in deprotection in the resist film, even in the absence of a photoacid generator. Additionally, the data suggest that the nature of the resist polymer and, in turn, the identity of the deprotection products directly influence resist mass loss and etch rate linearity, both of which can be controlled by careful selection of resist materials.

Thick porous silicon formation using implanted mask technology

Surface micromachining is an established microtechnology. The process is only limited by sacrificial layer thickness and sometimes a disturbing surface topology. This paper describes an innovative surface micromachining technology. Standard surface micromachining allows layer thicknesses of a few micrometer. Using porous silicon as sacrificial layer, it is possible to create any layer thickness up to 100 μm. Thick porous silicon sacrificial layers are used to combine the advantages of standard surface micromachining with the advantages of bulk micromachining. The problems resulting from surface topology are eliminated by using ion implanted masks. Based on different porous silicon formation mechanisms for n- and p-type silicon, it is possible to use n-implanted layers as masking material during the anodization of p-type silicon, resulting in a plane surface without any steps which would be generated using a hard surface mask. For this masking technology, no additional masking layers are required. For free-standing membrane generation it is possible to deposit, e.g. a PECVD-layer on top of the porous silicon layer. A complete process flow has been developed for thick porous silicon layers up to 100 μm. The use of this sacrificial layer technology for thermally isolated gas sensor membrane fabrication and the detailed process parameters will be presented.

Effect of Etch Mask and Etching Solution on InP Micromachining to Form V-Grooves

Realization of low-cost and reliable packaging techniques to couple the optical fiber with the photodetector has become a critical area of research today. In the present work, we have developed a simple technique to automatically align the fiber with the detector by micromachining the InP substrate (used for fabricating the p-i-n detector) and using the resulting V-groove for accurate positioning of the fiber. A masking material has been deposited on the back surface of the wafer and patterned by photolithography to open a window exactly aligned to the p-i-n detector realized on the top surface. InP has been etched anisotropically through this window to a depth of more than 350 μm. This has resulted in a V-groove through which the fiber has been inserted and held in position. Three different etch mask materials viz. Ti, Cr, and SiN have been tried and their suitability to withstand such long time etching has been investigated. and etch solutions have been used to etch InP. It has been found that while the choice of the etching solution determines the angle of the V-groove as well as the surface finish, the mask material dictates the amount of undercut. Ti as the mask material with etch solution has been found to give the best results for this particular application. © 2001 The Electrochemical Society. All rights reserved.

Effects of Mask Materials on Near Field Optical Nanolithography

AbstractSimulations have been performed to study the effects of different masking materials (Cr, Au, W and Si) on near field optical contact lithography. For near field amplitude masks it is found that Cr is an excellent material at blue/UV wavelengths, due to its small skin depth. Au is too transmissive to be considered. W has a small skin depth like Cr, although its dielectric behaviour at blue/UV wavelengths leads to image reversal phenomena that may be undesirable. Image reversal is even more pronounced for Si. For Evanescent Interferometric Lithography (EIL) - a technique that relies on surface-plasmon-induced resonant enhancement of the evanescent fields beneath a grating - Cr and W are again superior to Au, and significant intensity enhancements are predicted beyond the grating cutoff. For Si the resonant enhancements can be very large and narrow band, which may be of great advantage. For example, near field exposure intensity approximately 100 times the incident intensity is predicted for a 270 nm period silicon grating illuminated at a wavelength less than 10 percent higher than cutoff.

Selectivity Control of Mask Material for XeF<sub>2</sub> Etching Using UV light Exposure

Selectivity Control of Mask Material for XeF<sub>2</sub> Etching Using UV light Exposure

Shape deterioration of mesa structures in post-CMOS anisotropic etching of silicon microsensors: an experimental study

In the present study, the shape deterioration problem encountered in post-CMOS silicon anisotropic etching using Plasma Enhanced CVD (PECVD) silicon nitride masking layer has been investigated. Large shape deterioration was observed at every convex corner of silicon mesa structures formed with some kinds of PECVD-SiN masking layer. The cause of the problem has been clarified through experiments. The fragile property of the masking material was an important factor on the resulting shape deterioration problem. It was found from the experimental results that compressive intrinsic stress or strong adhesion of masking material is necessary to obtain a desirable shape of mesa structures. In addition to PECVD-SiN masking layer, sputtered SiO 2 layer was also evaluated for its suitability as masking layer in post-CMOS micromachining. It has been found that sputtered SiO 2 masking layer performs well for post-CMOS anisotropic etching of silicon mesastructures/microsensors.

The basic materials of masking

Tomorrow will bring new masking materials, products, and applications. The finishing industry will become increasingly sophisticated, and the trend of buying from masking sources that provide the best combination of quality, delivery, technological expertise, tooling support, customization capability, price, and customer service will continue.

An Investigation of the Reduction in Tensile Strength and Fatigue Life of Pre-Corroded 7075-T6 Aluminum Alloy

In aging aircraft, the synergetic interaction between corrosion and fatigue has been shown to reduce the life expectancy of aluminum alloys. The objective of this study was to quantify the effects of corrosion, in terms of mass loss per unit area, on the static strength and fatigue life of 7075-T6 aluminum alloy. This was an experimental study in which test specimens were corroded in a laboratory environment. The corrosion process was accelerated by use of a corrosion cell. Test specimens were cut from flat sheets of aluminum and covered with masking material to restrict corrosion to a confined area. After testing, the fatigue life, ultimate tensile strength (UTS), and hardness of the specimens were observed to drop significantly with small amounts of corrosion. After the initial decrease, the UTS was observed to decrease linearly with increasing corrosion levels. The fatigue life of the specimens decreased in an inverse exponential fashion as mass loss per unit area increased. The hardness values of the corroded surfaces were also observed to drop. The topology of the pits and the related subsurface damage produced areas of high stress concentration resulting in the immediate reduction of UTS and fatigue life of the specimens. Subsurface corrosion damage was responsible for the reduction in hardness.

Optimized technology for the fabrication of piezoresistive pressuresensors

In this work a technology for the fabrication of piezoresistivepressure sensors is presented, based on the use of silicon BESOI (bondedand etch-back silicon on insulator) wafers. The main purpose of theproposed technology is the optimization of the thin silicon diaphragmdefinition process that is one of the most critical steps in thefabrication of silicon pressure sensors. The buried silicon oxide layer ofthe BESOI wafers is used as an automatic etch stop of the siliconanisotropic etching, making it possible to obtain very precise control ofthe sensor diaphragm thickness. In addition, the type and thickness of thelayers acting as masking materials on the backside of the wafers have beenoptimized in order to get a high-yield process. The experimental resultsobtained when using the proposed technology are presented and discussed.

Mask materials for powder blasting

Powder blasting, or abrasive jet machining (AJM), is a technique inwhich a particle jet is directed towards a target for mechanical materialremoval. It is a fast, cheap and accurate directional etch technique forbrittle materials such as glass, silicon and ceramics. The particle jet (whichexpands to about 1 cm in diameter) can be optimized for etching, while themask defines the small and complex structures. The quality of the maskinfluences the performance of powder blasting. In this study we tested andcompared several mask types and added a new one: electroplated copper. Thelatter combines a highly resistant mask material for powder blasting with thehigh-resolution capabilities of lithography, which makes it possible to obtainan accurate pattern transfer and small feature sizes (<50 µm).

Nitrogen-doped diamond films selected-area deposition by the plasma-enhanced chemical vapor deposition process

Abstract The nitrogen-doped diamond films have been successfully synthesized by using urea as the nitrogen source. Selected-area deposition of diamond nuclei was formed by using a SiO 2 layer as the masking material. Diamond pads, around 9 μm in diameter, were obtained when the N-doped diamond films were deposited on these patterned diamond nuclei using the chemical vapor deposition process. An emission current density as high as 200 μA/cm 2 , with a turn-on field of around 8 V/μm, was obtained. However, the diamond emitters broke down easily, which is ascribed to the localized melting of the substrate materials surrounding the diamond pads.