ETCHING ELECTRONIC MATERIALS

Etching is used extensively in material processing for delineating patterns, removing surface damage and contamination, and fabricating three-dimensional structures. Etching is a chemical process wherein material is removed by a chemical reaction between the etchants and the material to be etched. The etchant may be a chemical solution or a plasma. If the etchant is a chemical solution, the etching process is called wet chemical etching. Plasma-assisted etching is generally referred to as dry etching, and the term dry etching is now used to denote several etching techniques that use plasma in the form of low-pressure discharges.

2.4.1 Wet Chemical Etching

Wet chemical etching involves three principal steps:

1. The reactants are transported by diffusion2 to the surface to be etched.

2. Chemical reactions take place at the surface.

3. Reaction products are again transported away from the surface by diffusion.

2 Under some circumstances, reactions can be reaction-rate-limited rather than diffusion-rate-(mass-transport) limited.

ETCHING ELECTRONIC MATERIALS 23 Let us consider, as an example, etching of silicon. For silicon, the most commonly used etchants are mixtures of nitric acid (HNOs) and hydrofluoric acid (HF) in water or acetic acid (CH3COOH). Wet chemical etching usually proceeds by oxidation. Initially, silicon is oxidised in the presence of holes as follows:

Si + 2H+ > Si2+ + H2 (2.9)

Water dissociates according to the reaction

H2O > (OH)- +H+ (2.10)

The hydroxyl ions (OH)- recombine with positively charged silicon ions to form SiO2 in two steps:

Si2+ + 2(OH) > Si(OH)2 (2.11) and

Si(OH)2 > SiO2 + H2 (2.12) SiO2 dissolves in HF acid according to the reaction

SiO² + 6HF » H²SiF⁶ + 2H²O (2.13) where H2SiFe is soluble in water. The reactions of (2.9) to (2.13) may be represented with HNO3 by the following overall reaction:

Si + HNO3 + 6HF * H2SiF6 + HNO2 + H2O + H2 (2.14) The chemical solution used for gallium arsenide (GaAs) etching is a combination of hydrogen peroxide (H2O2) and sulfuric acid (H2SO4) dissolved in water. Dielectrics and metals are etched using the same chemicals that dissolve these materials in bulk form and involve their conversion into soluble salts or complexes. Generally, film materials will etch more rapidly than their bulk counterparts.

Etching processes are characterised by three parameters:

1. Etch rate 2. Etch selectivity 3. Etch uniformity

The etch rate is defined as the material thickness etched per unit time. Etch selectivity is a measure of how effective the etch process is in removing the material to be etched without affecting other materials or films present in the wafer. Quantitatively, etch selectivity can be expressed as the ratio between the etch rate of the material to be etched and etch-mask materials on the wafer. Table 2.5 lists the properties of different wet etchants for different materials.

2.4.2 Dry Etching

The basic concept of dry or plasma etching is very simple. A glow discharge is used to generate chemically reactive species (atoms, radicals, and ions) from a relatively inert

24 ELECTRONIC MATERIALS AND PROCESSING

Wet etchants used in etching some selected electronic materials Etchant composition

molecular gas. The etching gas is chosen so as to produce species that react chemically with the material to be etched to form a reaction product that is volatile. The etch product then desorbs from the etched material into the gas phase and is removed by the vacuum pumping system. The most common example of the application of plasma etching is in the etching of carbonaceous materials, for example, resist polymers, in oxygen plasma - a process referred to as plasma ashing or plasma stripping. In this case, the etch species are oxygen atoms and the volatile etch products are CO, CO2, and H2O gases.

In etching silicon and silicon compounds, glow discharges of fluorine-containing gases, such as CF4, are used. In this case, the volatile etch product is SiF4 and the etching species are mainly fluorine atoms. In principle, any material that reacts with fluorine atoms to form a volatile product can be etched in this way (e.g. W, Ta, C, Ge, Ti, Mo, B, U, etc.). Chlorine-containing gases have also been used to etch some of the same materials, but the most important uses of chlorine-based gases have been in the etching of aluminum and poly-Si. Both aluminum and silicon form volatile chlorides. Aluminum is not etched in fluorine-containing plasmas because its fluoride is nonvolatile.

The characteristic of etching processes, which is becoming more and more important as the lateral dimensions of the lithography become smaller, is the so-called directionality (anisotropy) of the etch process. This characteristic is illustrated in Figure 2.12 in which the lithographic pattern is in the x-y plane and the z-direction is normal to this plane. If the etch rates in the x and y directions are equal to the etch rate in the z-direction, the etching process is said to be isotropic (or nondirectional) and the shape of the sidewall of the etched feature will be as shown in Figure 2.12(a). Etch processes that are anisotropic or directional have etch rates in the z-direction and are larger than the lateral (x or y) etch rates. The extreme case of directional etching in which the lateral etch rate is zero (to be referred to here as vertical etch process) is shown in Figure 2. 12(b).

Plasma etching, as described in the preceding discussion, is predominantly an isotropic process. However, anisotropy in dry etching can be achieved by means of the chemical reaction preferentially enhanced in a given direction to the surface of the wafer by some mechanism. The mechanism used in dry etching to achieve etch anisotropy is ion bombardment. Under the influence of an RF field, the highly energised ions impinge on the surface either to stimulate reaction in a direction perpendicular to the wafer

ETCHING ELECTRONIC MATERIALS 25

(a) Isotropic etch (b) Vertical etch

Figure 2.12 Characteristic profile of an (a) isotropic and (b) vertical etching process surface or to prevent inhibitor species from coating the surface and hence reenhance etching in the direction perpendicular to the wafer surface. Therefore, the vertical sidewalls, being parallel to the direction of ion bombardment, are little affected by the plasma.

Figure 2.13 is a schematic diagram of a planar etching system, which comprises a vacuum chamber, two RF-powered electrodes, an etching gas inlet, and a pumping mechanism. The planar systems are also called parallel-plate systems or surface loaded systems. These systems have been used in two distinct ways: (1) the wafers are mounted on a grounded surface opposite to the RF-powered electrode (cathode) or (2) the wafers are mounted on the RF-powered electrode (cathode) directly. This latter approach has been called reactive ion etching (RIE). In this approach, ions are accelerated toward the wafer surface by a self-bias that develops between the wafer surface and the plasma. This bias is such that positively charged ions are attracted to the wafer surface, resulting in surface bombardment. It has been demonstrated that a planar etching system, when operated in the RIE mode, is capable of highly directional and high-resolution etching.

To illustrate the mechanisms involved in reactive ion etching, consider the example of poly-Si etched in chlorine plasma:

1. Ions, radicals, and electron generations:

(or C12) > nCl+(or C12+) + ne (2.15)

26 ELECTRONIC MATERIALS AND PROCESSING Etching-gas

inlet

Vacuum enclosure

Electrodes

Pumping port

Figure 2.13 Schematic cross section of a plasma-etching system 2. Etchant formation:

Energy supplied by electron

e + C12 > 2C1 + e (2.16)

3. Adsorption of etchant on poly-Si:

nCl (or C12) » Si (surface) -I- nC\ (2.17) 4. lon-bombardment-assisted reaction to form product:

Ion-bombardment

Si (surface) + nCl > SiCln(adsorbed) (2.18) Table 2.6 Etch gases used for various electronic materials

Material Gases

Crystalline Si CF4, CF4/O2, CF3, Cl, SF6/C1, C12/H2, and poly-Si C2C1F5/O2, SF6/O2, SiF4/O2, NF3, C2Cl3F5,

CCl4/He, Cl2/He, HBr/Cl2/He Si02 CF4/H2, C2F6, C3F8, CHF3

Si3N4 CF4/O2, CF4/H2, C2F6, C3F8, SF6/He Organic solids O2, O2/CF4, O2/CF6

Al BC13, CCl4, SiCl4, BC13/C12, CCl4/Cl2, SiCl4/Cl2

Au C2C12F4, C12

5. Product desorption:

SiCln (adsorbed)

DOPING SEMICONDUCTORS 27

SiCln (gas) (2.19)

The final gas product is pumped out of the etching chamber. Table 2.6 provides a list of etch gases used for dry-etching various electronic materials.