This article is a brief review of dry etching as applied to pattern transfer, primarily in silicon technology. It focuses on concepts and topics for etching materials of interest in micromechanics. The basis of plasma-assisted etching, the main dry etching technique, is explained and plasma system configurations are described such as reactive ion etching (RIE). An important feature of RIE is its ability to achieve etch directionality. The mechanism behind this directionality and various plasma chemistries to fulfil this task will be explained. Multi-step plasma chemistries are found to be useful to etch, release and passivate micromechanical structures in one run successfully. Plasma etching is extremely sensitive to many variables, making etch results inconsistent and irreproducible. Therefore, important plasma parameters, mask materials and their influences will be treated. Moreover, RIE has its own specific problems, and solutions will be formulated. The result of an RIE process depends in a non-linear way on a great number of parameters. Therefore, a careful data acquisition is necessary. Also, plasma monitoring is needed for the determination of the etch end point for a given process. This review is ended with some promising current trends in plasma etching.

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A survey on the reactive ion etching of silicon in microtechnology

Surface science aspects of etching reactions

Physical and ion‐enhanced chemical etch yields are shown to be a linear function of the square root of the ion energy down to the etching threshold for self‐sputtering of metals, sputtering of metals by noble gas ions, sputtering of Si and SiO2 by noble gas and reactive ions, and ion beam enhanced chemical etching of Si. The threshold energy must be taken into account for a quantitative description of etch yields even at intermediate ion energies. The relationship between the dependence of etch yields on ion energy and incident angle is also discussed.

Universal energy dependence of physical and ion-enhanced chemical etch yields at low ion energy

A review of particle-solid processes pertinent to modelling plasma-wall interactions is presented, and sets of recommended data are given. Analytic formulas are used where possible; otherwise, data are presented in the form of tables and graphs. The incident particles considered are e−, H, D, T, He, C, O, and selfions. The materials include the metals aluminum, beryllium, copper, molybdenum, stainless steel, titanium, and tungsten and the nonmetals carbon and TiC. The processes covered are light ion reflection, hydrogen and helium trapping and detrapping, desorption, evaporation, sputtering, chemical effects in sputtering, blistering caused by implantation of helium and hydrogen, secondary electron emission by electrons and particles, and arcing.

Data Compendium for Plasma-Surface Interactions

Energy and angular distributions of sputtered particles

Argon‐ion assisted etching of silicon by molecular chlorine has been investigated. The masses and some kinetic energy distributions of the neutral particles emitted from the surface have been determined by mass spectrometry and time‐of‐flight spectra. It is found that an important part of the silicon‐containing particles consists of SiCl and SiCl2 molecules which have kinetic energies in the eV region. The observations exclude a simple evaporation of SiCl4 at the target temperature. A tentative model—consisting of a collision cascade like process parallel to thermal evaporation at the substrate temperature induced by a reduction of the effective surface binding energy during a sputtering event—is given to explain the observed kinetic energy distributions qualitatively.

Argon‐ion assisted etching of silicon by molecular chlorine

The sputtering of Si by 3‐keV Ar+ ions under simultaneous exposure to a beam of XeF2 gas has been investigated. The masses and some energy distributions of the neutral particles emitted from the target have been determined by mass spectrometry and time of flight. We found the following Si‐containing ions: Si+, SiF+, SiF+2, and SiF+3. The energy distributions of the sputtered particles cannot be explained by thermal desorption at the target temperature but can be interpreted as a collision cascadelike mechanism with a low surface binding energy.

Mass and energy distribution of particles sputter etched from Si in a XeF2 environment

Condensed gas layers of H2O, NH3 and CO at 15–20 K have been bombarded by 6 keV H+2 and 3 keV He+ and Ar+ ions. Mass spectra of the neutral species sputtered from these layers have been measured. There is a substantial yield of products which originally were not in the target material, and which have thus been formed in chemical reactions induced by the ion bombardment. The relative yields of some of the products have been found to increase with decreasing incident ion mass. This is mainly attributed to the larger amount of energy deposited by electronic stopping in such situations. From CO a nonvolatile residue is left after ion irradiation. From a layer of H2O frozen on top of the CO-residue H2CO was detected.

Reactive sputtering of simple condensed gases by keV ions II: Mass spectra

The reaction of Si with Cl2 alone and under simultaneous exposure to an Ar+ ion beam is investigated as a function of target temperature and Ar+ ion energy using mass spectrometry and time‐of‐flight studies. The main products of the thermal reaction are SiCl4 at low and SiCl2 at higher temperatures. The main products of combined exposure are atomic Si and Cl and molecular SiCl and SiCl2. The contributions of atoms and molecules are of a comparable order of magnitude. It appears that the main fraction of the products of the combined exposure results from sputtering and not from stimulation of the thermal reaction path by ion bombardment. The results strongly suggest that a modification of the top atomic layers of the Si, from which the products are sputtered, by incorporation of significant quantities of Cl plays a dominant part in the reaction mechanism.

Ion‐assisted etching of silicon by molecular chlorine

Condensed gas layers of H2O, NH3 and CO have been bombarded by 3–6 keV H+, H+2, He+ and Ar+ ions. Kinetic energy distributions of the sputtered neutral particles have been measured using a time-of-flight technique. The mechanism of ejection of the sputtered products in nearly all cases appears to involve momentum transfer in the final steps, as is inferred from the collision cascade-like energy spectra. The effective surface binding energies observed are much lower than the sublimation energies. Nevertheless most sputtered species have energies far in excess of those expected for evaporation at the ambient temperature. Only H2 from NH3 shows evaporation as the main desorption mechanism under irradiation by light ions.

R.A. Haring

Papers

Argon‐ion assisted etching of silicon by molecular chlorine

Mass and energy distribution of particles sputter etched from Si in a XeF2 environment

Reactive sputtering of simple condensed gases by keV ions II: Mass spectra

Ion‐assisted etching of silicon by molecular chlorine

Reactive sputtering of simple condensed gases by kev ions III: Kinetic energy distributions