,NANO-MASTER, INC.,Wet and Dry Etching 湿法和干法刻蚀,NANO-MASTER 那诺-马斯特中国有限公司 吴运祥 2017年8月16日,NANO-MASTER, INC.,各向同性和各向异性,When a material is attacked by a liquid or vapor etchant, it is removed isotropically (uniformly in all directions) or anisotropic etching (uniformity in vertical direction). The difference between isotropic etching and anisotropic etching is shown in Figure 1. Material removal rate for wet-etching is usually faster than the rates for many dry etching processes and can easily be changed by varying temperature or the concentration of active species.,NANO-MASTER, INC.,湿法刻蚀,Synonyms: chemical etching, liquid etching 湿法刻蚀定义： Wet etching is a material removal process that uses liquid chemicals or etchants to remove materials from a wafer. The specific patters are defined by masks on the wafer. Materials that are not protected by the masks are etched away by liquid chemicals. These masks are deposited and patterned on the wafers in a prior fabrication step using lithography. 2 A wet etching process involves multiple chemical reactions that consume the original reactants and produce new reactants. The wet etch process can be described by three basic steps. (1),NANO-MASTER, INC.,湿法刻蚀,Diffusion of the liquid etchant to the structure that is to be removed. (2) The reaction between the liquid etchant and the material being etched away. A reduction-oxidation (redox) reaction usually occurs. This reaction entails the oxidation of the material then dissolving the oxidized material. (3) Diffusion of the byproducts in the reaction from the reacted surface. 各向异性的湿法刻蚀： Liquid etchants etch crystalline materials at different rates depending upon which crystal face is exposed to the etchant. There is a large difference in the etch rate depending on the silicon crystalline plane.,NANO-MASTER, INC.,湿法刻蚀,In materials such as silicon, this effect can allow for very high anisotropy. Some of the anisotropic wet etching agents for silicon are potassium hydroxide (KOH), ethylenediamine pyrocatechol (EDP), or tetramethylammonium hydroxide (TMAH). Etching a (100) silicon wafer would result in a pyramid shaped etch pit as shown in Figure 2a. The etched wall will be flat and angled. The angle to the surface of the wafer is 54.7o. Figure 2c-d depicts scanning electron micrographs of (110)-oriented two-dimensional silicon walls with micro and nanoscale dimensions generated based on KOH based wet etching.,NANO-MASTER, INC.,湿法刻蚀,NANO-MASTER, INC.,湿法刻蚀,The relationship between mask dimensions, etch depth and the floor width is given in equation 1. 各向同性湿法刻蚀： For isotropic wet etching, a mixture of hydrofluoric acid, nitric acid, and acetic acid (HNA) is the most common etchant solvent for silicon. The concentrations of each etchant determines the etch rate. Silicon dioxide or silicon nitride is usually used as a masking material against HNA. As the reaction takes place, the material is removed laterally at a rate similar to the speed of etching downward.,NANO-MASTER, INC.,湿法刻蚀,This lateral and downward etching process takes places even with isotropic dry etching which is described in the dry etch section. Wet chemical etching is generally isotropic even though a mask is present since the liquid etchant can penetrate underneath the mask (Figure 2b). If directionality is very important for high-resolution pattern transfer, wet chemical etching is normally not used.,NANO-MASTER, INC.,干法刻蚀,Synonyms: plasma etching, gas etching, physical dry etching, chemical dry etching, physical-chemical etching 干法刻蚀定义： In dry etching, plasmas or etchant gasses remove the substrate material. The reaction that takes place can be done utilizing high kinetic energy of particle beams, chemical reaction or a combination of both.,NANO-MASTER, INC.,干法刻蚀,物理干法刻蚀： Physical dry etching requires high energy kinetic energy (ion, electron, or photon) beams to etch off the substrate atoms. When the high energy particles knock out the atoms from the substrate surface, the material evaporates after leaving the substrate. There is no chemical reaction taking place and therefore only the material that is unmasked will be removed. The physical reaction taking place is illustrated in Figure 3.,NANO-MASTER, INC.,干法刻蚀,化学干法刻蚀： Chemical dry etching (also called vapor phase etching) does not use liquid chemicals or etchants. This process involves a chemical reaction between etchant gases to attack the silicon surface. The chemical dry etching process is usually isotropic and exhibits high selectively. Anisotropic dry etching has the ability to etch with finer resolution and higher aspect ratio than isotropic etching. Due to the directional nature of dry etching, undercutting can be avoided. Figure 4 shows a rendition of the reaction that takes place in chemical dry etching. Some of the ions that are used in chemical dry etching is tetrafluoromethane (CH4), sulfur hexafluoride (SF6), nitrogen trifluoride (NF3), chlorine gas (Cl2), or fluorine (F2).3,NANO-MASTER,