Overview

In wet chemical etching, layers are generally removed isotropically, yet an anisotropic etch profile is particularly important for small structures. Dry etch processes are well suited for this, offering sufficient selectivity. These processes enable reproducible, uniform etching of most layers used in semiconductor manufacturing. Besides anisotropic etch profiles, isotropic ones can also be achieved. Despite the high equipment costs and sequential single-wafer processing, dry etching has established itself over wet chemical etching.

Dry etch processes are described by a small number of key parameters, which are also the target parameters of every process development effort.

Etch rate r: The etch rate describes the material removed per unit time and is expressed, for example, in nanometers per minute or Ångström per second.

$$r=\frac{Etch\ removal\ \Delta z}{Etch\ time\ \Delta t}$$

Anisotropy factor f: The anisotropy factor describes the ratio of the horizontal etch rate r_h to the vertical etch rate r_v.

$$f=1-\frac{r_h}{r_v}$$

For pattern transfer, strongly anisotropic processes are desired, i.e. etching only in the vertical direction, so that the resist mask is not undercut. For anisotropic etch processes, f → 1, and correspondingly, for isotropic processes, f → 0.

Selectivity S_jk: The selectivity S_jk between material j and material k describes the ratio of the etch rates of two materials, e.g. of the layer to be patterned (j) and the resist mask (k).

$$S_{jk}=\frac{r_j}{r_k}$$

Whether a high or a low selectivity is desired depends on the particular process. When patterning layers, the value should be as high as possible, i.e. the layer to be patterned is removed faster than the resist mask. In reflow etchback, the selectivity has to be 1 in order to planarize topographies uniformly.

Uniformity U: Uniformity describes how evenly the material removal occurs – across a single wafer, from wafer to wafer, and from batch to batch. It is expressed as the spread of the etch rate relative to its mean value.

$$U=\frac{r_{max}-r_{min}}{r_{max}+r_{min}}\cdot 100\,\%$$

Aspect ratio A: The aspect ratio is the ratio of etch depth z to feature width b. It describes how slender a trench or hole is.

$$A=\frac{z}{b}$$

As the aspect ratio increases, the etch rate decreases, because the etch species have more difficulty reaching the bottom of the structure and the reaction products are less readily transported away. This effect is known as aspect ratio dependent etching (ARDE), or RIE lag. Today it is the decisive limitation when etching deep structures, such as the memory holes in 3D NAND flash, which have aspect ratios exceeding 50:1. Typical profile defects in such etches include bowing (bulging widening in the upper region), twisting (deep holes tilting relative to one another), and microtrenching (excessive removal at the bottom corners).

Loading effect: The etch rate depends on the total exposed area: a large area to be etched consumes more reactive species, so the etch rate decreases. Macro-loading affects the entire wafer or batch, while micro-loading affects the immediate vicinity of a single structure.

Since the etch rate depends on the state of the equipment, the gas flow, and the loading, an etch process in manufacturing is not controlled by a fixed time, but by endpoint detection. For this purpose, a spectrometer monitors the optical emission of the plasma (optical emission spectroscopy, OES) – when a layer is etched through, the intensity of characteristic lines changes because different reaction products are formed – or an interferometer measures the remaining layer thickness. Once the endpoint has been detected, a short, defined overetch time follows to reliably remove residues across the entire wafer.