256 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 5, NO. 4, DECEMBER 1996 Etch Rates for Micromachining Processing Kirt R. Williams, Student Member, IEEE, and Richard S. Muller, Life Fellow, IEEE Abstruct- The etch rates for 317 combinations of 16 ma- terials (single-crystal silicon, doped, and undoped polysilicon, several types of silicon dioxide, stoichiometric and silicon-rich silicon nitride, aluminum, tungsten, titanium, TVW alloy, and two brands of positive photoresist) used in the fabrication of microelectromechanical systems and integrated circuits in 28 wet, plasma, and plasmaless-gas-phase etches (several HP solutions, H3P04, HNO3 + H20 + N-F, KOH, Type A aluminum etchant, HZ 0 + HZ 02 + HF, H2 02, piranha, acetone, HF vapor, XeF2, and various combinations of SF6, CF4, CHF3, C12, 02, N2, and He in plasmas) were measured and are tabulated. Etch preparation, use, and chemical reactions (from the technical literature) are given. Sample preparation and MEMS applications are described for the materials. [193] I. INTRODUCTION HEM DESIGNING a new process to fabricate micro- ma&ined devices, the etch rate of each layer that is to be patterned must be known. While the etch rates of many etchants that target specific materials (e.g., thermally grown silicon dioxide in 5:l buffered hydrofluoric acid) are com- monly known, the etch rates of the masking and underlying films are frequently not quoted in the literature. This paper provides this information for 317 different combinations of 16 materials and 28 etches used in the micromachining of micro- electromechanical systems (MEMS) and in integrated-circuit processing. These etch-rate data, based on tests performed in the U. C. Berkeley Microfabrication Laboratory (Berkeley Microlab), are tabulated in Tables I and 11. The first sections of this paper describe the preparation and use of the wet and dry etches in Tables I and 11, listing chemical reactions and variation of etch rate with such factors as temperature and concentration, based on literature on the subject. Recognizing that there are many sources of etch-rate variation, brief lists of these sources are given at the end of these wet- and dry-etch sections. The succeeding sections describe the sample preparation and MEMS applications for each of the materials, the measurement techniques used, and finally discuss the data in the tables. 11. ?kE WET ETCHES A. Comparison of Wet and Dry Etches The etches in the tables are divided into wet and plasma and plasmaless-gas-phase (“dry”) etches. The advantages and disadvantages of wet and dry etching are well known 111, Manuscript received January 10, 1996; revised July 1, 1996. Subject Editor, K. Najafi. This work was supported by the Berkeley Sensor & Actuator Center. The authors are with the Berkeley Sensor & Actuator Center, University of Califomia at Berkeley, Berkeley, CA 94720-1770 USA. Publisher ltem Identifier S 1057-7157(96)08843-9. [2]; the most important for micromachining are as follows. Wet etching is usually isotropic (desirable in some cases), can have a selectivity that depends on crystallographic direc- tion, and can be very selective over masking and underlying layers. Plasma etching uses fresh chemicals for each etch (resulting in less chemical-related etch-rate variability) and can be vertically anisotropic (as well as isotropic), allowing the patterning of narrow lines. When removing a sacrificial layer in micromachining, wet etching has the disadvantage of capillary-force pulldown of free-standing structures [3]. This can be avoided by using a supercritical-liquid drying process [4] or by switching to a dry-etched sacrificial layer [5], [61. B. Wet-Etch Chemicals All of the chemical mixtures made in the Berkeley Microlab and discussed in the next section are by volume, with one noted exception. Conversely, those prepared and bottled by chemical-supply companies are by weight. Many of the chemicals used in wet etching are not sup- plied in pure form. Acetic acid is supplied pure and sulfuric acid nearly pure (96%), while other acids normally come in lower concentrations for various reasons. Phosphoric acid is a deliquescent solid at room temperature [7]. Above the 85% concentration at which it is supplied, it is very viscous and tends to oligomerize into polyphosphoric acids. Pure hydrofluoric acid has a boiling point of 19.5”C [7]. As supplied at 49% concentration, it has a greatly reduced vapor pressure, increasing personal safety and allowing room-temperature storage in unpressurized containers. Nitric acid is a liquid in the range near room temperature, but tends to decompose above the supplied concentration of 70%. Sulfuric [8] and acetic [9] acids are liquids that are completely miscible in water at room temperature at all concentrations to 100%. Hydrofluoric acid [lo] is also a completely soluble liquid below its boiling point. An extensive list of other wet etchants for a variety of semiconductors, metals, insulators, and other compounds has been compiled by Vossen and Kern [ 111. C. Information about Individual Wet Etches In this section, each etchant is listed by its name from Table I in italics, followed by its complete name, target material, notes on use, information on the reaction(s) that occur, if known from the technical literature, and major sources of etch- rate variation. For brevity, etchants with the same reactions (e.g., all HF solutions) are discussed together. The etchants are grouped by target material. Unless otherwise noted, all of the wet etchants are isotropic. 1057-7157/96$05.00 0 1996 IEEEWILLIAMS AND MULLER ETCH RATES FOR MICROMACHINING PROCESSING 257 , __ , a - a 5 - 3 3 - a L - L ! - 0 3 L 3 8 2 .l 3 N d a C -4 0 0 3 c a s Y 0 L 9 a - 0 3 2 o 5 3 , 0 a 0 n __ 3 a __ 0 - e 3 - a - 3 0 0 - 0 a N a 8 Y - - e 8 9 - - I 'I 4 I bS 9 0 It 9 ;St25 8 JOURNAL All wet etching was done at room temperature (about 20" C in the temperature-controlled Berkeley Microlab), unless otherwise indicated. All wet etching was done with fresh solutions, agitating occasionally. To remove the vapors created by the etchants, all wet etching was done under fume hoods. 1) Silicon Dioxide Wet Etchants: Notes: All of the silicon dioxide
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