In the manufacture of semiconductor devices, etching refers to the technique of selectively removing material from a thin film on a substrate (with or without a preexisting structure on its surface) and forming a pattern of that material on the substrate through this removal. The pattern is defined by a mask that is resistant to the etching process, the creation of which is described in detail in photolithography. Once the mask is in place, the material not protected by the mask can be etched by wet chemical or “dry” physical methods. Figure 1 shows a schematic of this process.
Historically, until the advent of VLSI and ULSI technology, wet chemical methods played an important role in etch pattern definition. However, as device feature sizes decreased and surface topography became increasingly stringent, wet chemical etching gradually replaced dry etching technology. This change is mainly due to the isotropic nature of wet etching. As shown in Figure 2, wet etching removes material in all directions, which can lead to differences between the feature size defined by the mask and the feature size replicated on the substrate. Compared with larger feature sizes, VLSI and ULSI designs require much more precise correlation between mask and pattern feature size. Additionally, aspect ratios (ratios of depth to width) in advanced devices have increased, and achieving these ratios requires the ability to etch materials anisotropically using directional etching stainless steel techniques. Figure 3 provides a schematic that helps understand isotropic vs. anisotropic feature generation and directional etching. The ultimate application of wet etching in advanced processes has suffered, perhaps because many of the newer materials used in device fabrication do not have readily available wet chemistries for etching. These issues have combined to limit wet etching techniques to almost exclusively cleaning rather than etching applications.
Only devices with relatively large feature sizes, such as some MEMS structures, continue to be wet etched. Surface cleaning, anisotropic feature generation, and directional etching are discussed in detail below. The ultimate application of wet etching in advanced processes has suffered, perhaps because many of the newer materials used in device fabrication do not have readily available wet chemistries for etching. These issues have combined to limit wet etching techniques to almost exclusively cleaning rather than etching applications. Only devices with relatively large feature sizes, such as some MEMS structures, continue to be wet etched. Surface cleaning, anisotropic feature generation, and directional etching are discussed in detail below. The ultimate application of wet etching in advanced processes has suffered, probably because many of the newer materials used in device fabrication do not have readily available wet chemistries for etching. These problems have combined to make wet etching techniques almost exclusively useful for cleaning, rather than etching applications.
Only devices with relatively large feature sizes, such as some MEMS structures, continue to employ wet etching. Surface cleaning is discussed in detail below. The practicality of wet etching in advanced processes may be due to the fact that many of the newer materials used in device fabrication do not have readily available wet chemistries for etching. These problems have combined to make wet etching techniques almost exclusively useful for cleaning, rather than etching applications. Only devices with relatively large feature sizes, such as some MEMS structures, continue to employ wet etching. Surface cleaning is discussed in detail below. The practicality of wet etching in advanced metal etching product and processes may be due to the fact that many of the newer materials used in device fabrication do not have readily available wet chemistries for etching. These problems have combined to make wet etching techniques almost exclusively useful for cleaning, rather than etching applications. Only devices with relatively large feature sizes, such as some MEMS structures, continue to employ wet etching. Surface cleaning is discussed in detail below.
Anisotropic etching uses a range of techniques, which are referred to as “dry” etching. These techniques are commonly used for etching in VLSI and ULSI device fabrication, and they will be the only methods discussed in detail in this section. Dry etching can remove material by physical means such as ion bombardment, accompanied by ejection of the material from the substrate or by chemical reactions that convert the substrate material into volatile reaction products that can be pumped away. Dry etching techniques include the following common methods (whether the etching process is chemical etching, physical etching, or a combination as described in parentheses):
- Isotropic radial etching (chemical)
- Reactive ion etching (chemical/physical)
- Sputter etching (physical)
- Ion milling (physical)
- Ion beam assisted etching (physical)
- Reactive ion beam etching (chemical/physical)
All dry etching techniques are performed under vacuum conditions, and the pressure determines the nature of the etching phenomenon to some extent.
Table 1, taken from Wolf and Tauber, shows the relative pressure ranges and general characteristics of different etching methods. Although there are many specific variations in the equipment and process characteristics used for etching, we will limit our discussion to a brief description of the process basics and the three main etching methods identified in Table 1.
Basic Etching Process
In-depth discussions of the basics of plasma etching can be found in many textbooks (Wolf and Tauber, Sze), and the interested reader is referred to these sources. Here, we provide only the simplest description of the basic principles of plasma generation. During the plasma etching process, many physical phenomena are at work. When a strong electric field is generated in the plasma chamber using electrodes (in the case of DC or RF excitation) or waveguides (in the case of microwaves), this electric field accelerates any available free electrons, thereby raising their internal energy (free electrons present in any environment are generated by cosmic rays, etc.). The free electrons collide with atoms or molecules in the gas phase, and if the electron transfers enough energy to the atom/molecule in the collision, an ionization event will occur, producing a positive ion and another free electron. Nevertheless, collisions that deliver insufficient energy for ionization can still deliver enough energy to produce stable but reactive neutral species (i.e., molecular radicals). When sufficient energy is supplied to the system, a stable gas-phase plasma containing free electrons, positive ions, and reactive neutral ions will be generated.
In a plasma etching process, atomic and molecular ions and/or reactive neutral ions from a plasma can be used to remove material from a substrate by physical or chemical routes or by a mechanism that employs both. Purely physical etching is accomplished by accelerating positive atomic ions, typically ions of heavy noble elements such as argon, toward a substrate using a strong electric field (Figure 4). This acceleration imparts energy to the ions, and when the ions impact the substrate surface, their internal energy is transferred to atoms in the substrate. If enough energy is transferred, the substrate atoms are ejected into the gas phase and are pumped out by the vacuum system. The incident ion is neutralized in the collision, and since it is a gas, it desorbs into the gas phase to be re-ionized or pumped out of the system.
Chemical etching differs from physical etching in that it employs a chemical reaction between reactive neutral species generated within the plasma and the substrate. The most common type of chemical etching involves halide chemistry, where chlorine or fluorine atoms are the active agents in the etching process. A representative chemistry for etching processes is silicon etching using NF 3. The sequence of chemical reactions in this etching process is:
NF 3 dissociates in the plasma to produce highly reactive atomic fluorine radicals. These radicals react with silicon in the substrate to produce silicon tetrafluoride SiF 4, which is a volatile gas that can be pumped away. In this way, silicon is etched from the substrate. As with wet etching, chemical etching is isotropic and non-directional (Figure 5). The reason for this is that the adhesion coefficient of the reactive neutrals is relatively low, so most collisions with the substrate surface do not result in etching, but rather in the reactive neutrals simply desorbing back into the gas phase. This phenomenon leads to a non-uniform etching process within the etched features and ultimately to isotropy in etching.
Most etching techniques used in modern device manufacturing combine aspects of physical and chemical etching. In processes such as reactive ion etching (RIE), directional etching is achieved by applying a bias to the substrate, which accelerates ion species from the plasma towards the substrate surface. There they interact with the surface and the reactive neutrals to produce volatile products that can be pumped away (Figure 6). The ion energies in RIE are much lower than those employed in physical etching techniques, and ion bombardment effects are negligible. The transfer of ion energy to the surface can enhance directionality by improving the adsorption of reactants on the bombarded surface (the incoming ions create high-energy defects where they preferentially adsorb and react) and by enhancing the desorption of byproducts (the transfer of incoming ion energy to the reaction products causes their desorption from the surface).
