Isotropic Etching Process Research Articles

Fabrication of metal field emitter arrays for low voltage and high current operation

This article describes a new fabrication process for a metal field emitter to achieve low voltage and high current operation. The key element of the fabrication process is that the isotropic silicon etching and oxidation process used in silicon tip fabrication are utilized for gate hole size reduction and gate oxide layer formation. A reliable structure with a small gate hole can be easily obtained. Metal field emitter arrays were fabricated in order to demonstrate the validity of the new process and submicron gate apertures were successfully obtained from 2.25-μm-diam disk patterns defined by a conventional contact mask aligner. The required gate voltage to obtain an anode current of a 0.1 μA/tip was 54 V, and emission currents above 20 μA/tip were stable at a gate bias of 96 V without any disruption.

On the potential-dependent etching of Si(111) in aqueous NH 4F solution

The topography of p-Si(111) surfaces was investigated by STM after etching in concentrated NH 4F solution at cathodic, open circuit and anodic potentials for various durations. A significant influence of the etch potential as well as the etch rate on the resulting surface topography was found. The STM data were supplemented by I( V) curves and by Auger electron spectroscopy in dependence of the applied etch potential. The cathodic and open circuit etched samples show a smoothing of the surfaces due to anisotropic etching at step sites. For cathodically etched surfaces, the time taken to reach a smooth surface is approximately 10 2 times higher than for samples etched at the open circuit potential. In the case of anodic etching, the surfaces are roughened during etching and the formation of a pore structure is initiated on a rather short time scale. The results are discussed in terms of the potential-dependent competition of three different electrochemical processes. We identified a site-selective anisotropic etch process, being most significant at cathodic potentials, which leads to preferred etching of surface sites of local (100) geometry as compared to sites with local (111) symmetry. This process results in a smoothing of the (111) surfaces. A site-independent isotropic etch process has relative importance for anodic potentials. Finally, anodic oxidation dominates for the highest anodic potentials.

Development of three-dimensional microstructure processing using macroporousn-type silicon

The development of a micromachining technique for processing arbitrary structures with high aspect ratios in bulk silicon is presented. It is based on utilizing standard microelectronic processes and electrochemical macropore formation onn-type silicon in electrolytes containing hydrofluoric acid. This pore-etching technique allows us to produce very regular pore arrays with pore diameters and distances in the micrometer range and pore lengths up to wafer thickness. Samples with prefabricated pore arrays which differ in pore spacing, pore diameter and geometry are used as substrates for a micromachining process. The pores will facilitate the anisotropic etch profile which is required for the desired high aspect ratios although an isotropic etch process is used. Very deep microstructures with steep pore walls and aspect ratios of 10–15 are produced with this technique. It is shown that smaller pore array dimensions improve microstructure resolution.

Behaviour of a silicon spring fabricated by wire electro-discharge machining

A silicon spring/frame combination has been fabricated laterally out of a (100) silicon wafer, using wire electro-discharge machining (WEDM) in a one-dimensional measurement problem. After a thermal annealing step and an isotropic etching process the crystalline structure of the silicon wafer could be restored. Exposing the spring to over three-million working cycles yielded no detectable fatigue at all. A high-resolution X-ray diffractometer was used to measure the strain in the wafer. This is an accurate, non-destructive microscopic method newly applied in this field to obtain quantified results on the crystalline disorder. The method is applied to the microfabricated spring after annealing and etching steps.

Application of wet chemical selective etch techniques to the fabrication of thin silicon detectors

Anisotropic selective etching of silicon was applied to the fabrication of dE/ dx-detectors with a thickness between 20 and 40 μm using 〈100〉-oriented FZ-silicon wafers and KOH/H 2O-solution as an etchand. Main etch parameters have been investigated to demonstrate the advantage of anisotropic selective etching in comparison with the isotropic etch process. As an example the properties of 400 mm 2 ion-implanted and oxide passivated dE/ dx-detectors with anisotropically etched membranes are described. Finally, the fabrication possibilities of detectors with thicknesses up to 1 μm with high thickness accuracy across the detector area are discussed.

Emission characteristics and morphology of wet etched cathodes in p-type silicon

An isotropic wet etching process was used to generate arrays of field emitting, square-based pyramidal cathodes on (001), p-type silicon wafers. Their general morphology compared well with that found by Cade et al.,1 except that for cathodes with edges oriented along 〈110〉 directions, a ridgelike apex was formed. Transmission electron microscope examination of oxidized tips confirmed that tip sharpening occurs, improving the aspect ratio and producing two sharp cusps from a ridgelike apex and a single sharpened tip from a pointed apex. A modified Philips 505 SEM was used to make electrical measurements from the arrays. Measurements could be made over the range 1–2500 V with a current sensitivity of 10−13 A, current readings being taken over 6 orders of magnitude, in a vacuum of 3×10−7 Torr. Fowler–Nordheim plots exhibited characteristics typical of p-type silicon showing a plateau region which was eliminated by coating with metal. Oxidation sharpening was shown to improve emission and the lowest voltage at which electron emission occurred from a given tip was found to decrease with emission time.

Diamond membrane based x-ray masks

Diamond layers are of special interest for x-ray mask fabrication because of their excellent optical and mechanical properties. Using a plasma activated chemical vapor deposition process, 2–5 μm thick diamond layers were deposited and characterized. Microhardness and Young’s modulus are found to be very close to the literature values of bulk diamond. A standard isotropic etching process was applied for membrane fabrication. The optical transparency and the thickness homogeneity were measured. Concerning surface roughness, scanning electron micrographs and atomic force measurements were evaluated. After irradiation of diamond masks with synchrotron light, a slight increase of optical transparency was found.

Isotropic plasma etching of doped and undoped silicon dioxide for contact holes and vias

Isotropic plasma etching of doped and undoped oxides has been studied as an alternative to the ‘‘wet’’ isotropic step in the ‘‘wet–dry’’ contact etch approach. Czochralski grown p-type 〈100〉 100-mm-diam silicon substrates with either 0.1-μm low-temperature chemical vapor deposition oxide (LTO) and 0.5-μm borophosphosilicate glass (BPSG, 2.5 wt. % B and 7 wt. % P), or 0.6-μm thermally oxidized silicon were used. These substrates were masked with a resist pattern with contact holes whose diameters were between 10.0 and 0.75 μm. Subsequently, they were isotropically etched in a Matrix System One etcher. In this step 0.3 μm of the oxide was removed. The remaining oxide was anisotropically etched in an AME 8110 hexode etcher in a CHF3 / CO2 /He mixture at 72 mTorr and 1250 W. The isotropic oxide etch process was characterized using a multilevel statistical experimental design with pressure, 13.56-MHz rf power, and total gas flow (50% NF3 in He) as the variables. Optimized etch conditions resulted in an etch rate uniformity of less than ±3.0% (one sigma divided by the average for 9 points), a BPSG etch rate in excess of 0.3 μm/min, a thermally oxidized silicon etch rate of 0.12 μm/min, and a resist etch rate of <14 nm/min. The scanning electron miscroscope measurements showed a critical dimension variation of <0.1 μm total in the contact hole diameter at the LTO–silicon interface as compared to the contact hole diameter at the resist–BPSG interface before etch.

Plasma-Assisted Etching in Microfabrication

It has long been recognized that a molecular gas glow discharge is a prolific source of chemically active radicals. However, it was not until the last twenty years or so that glow discharges were used to provide active radicals for the etching of solid materials. The first major application of this type was the use of oxygen glow discharges to volatilize carbonaceous materials such as photoresists (1). In the mid-1960s this approach was extended to the etching of silicon and its compounds using glow discharges of fluorineand chlorine-containing gases (2). Beginning in the mid-1970s and continuing up to the present time (mid-1982), there has been an explosive growth in the use of reactive gas glow discharges for etching of solids. The driving force behind this growth has been the need for anisotropic or directional etching processes in microfabrication technologies-most notably in the elec­ tronics industry. Many lithographic patterns now have minimum feature sizes in the 1-3-J.Lm range and isotropic etching processes, either wet or dry, cannot maintain the dimensional control necessary to replicate these small features with acceptable yield. Associated with this rapid growth has been a proliferation of etching equipment, accompanied by a sometimes confusing terminology. Early work was performed with plasma ashing or plasma stripping equip­ ment originally intended for oxidative volatilization of carbonaceous layers. In these systems, the surface to be etched is immersed in the glow discharge without an applied electrical bias (i.e. the surface resides at the floating potential in the plasma). These so-called barrel usually etch isotropically; and hence, are unsatisfactory for most high resolution etching tasks. Recognition of the importance of energetic ion bombardment in obtaining etching directionality led to the use of planar systems (3, 4) in which energetic ion bombardment of the surface to be etched can be easily

Control of Plasma Etch Profiles with Plasma Sheath Electric Field and RF Power Density

In this paper it is shown that the plasma sheath electric field in which the etch sample is immersed controls the anisotropy of the etching of polysilicon films. These films are on wafers supported by the grounded electrode of a diode plasma etch reactor. The sheath electric field at the wafer supporting electrode is modified either by changing the rf power delivered to the plasma, or, at constant rf power, by application of d‐c voltage to the rf driven electrode, with similar consequences to the etch profiles. The absence of mask undercut and vertical etch walls characteristic of an anisotropic etch process are observed above a certain threshold power density‐sheath electric field, while below this threshold, the profiles become progressively more undercut until they are equal to the etch depth, as expected of isotropic etch processes. The experimental results presented in this paper, demonstrate these effects for a containing gas mixture at a total pressure of 65 Pa (0.5 Torr). A similar power density and pressure dependence of the etch anisotropy has been reported previously in different plasma etch gas under comparable conditions. These results will be shown to be explicable in terms of the increase in the anisotropy of the transport of ions for increasing sheath field.

Simulation of plasma‐etched lithographic structures

The resist development model has been extended to simulate etched images in semiconductor processing. The model has been used to simulate under a variety of conditions varying from conventional wet etching to newer directional etching processes such as dry etching. As compared to the conventional wet etching processes the dry etch technologies erode the resist mask more intensively during processing. The model has been confirmed using a number of experimentally alterable and lithography‐dependent conditions such as etch rates, resist profiles, resist thicknesses, etc. The effects of these on the final image shapes and the image sizes are predicted by simulations and aid the process definitions minimizing multivariate experiments. Isotropic etch processes such as those in the high‐pressure reactors are also modellable by this approach.