I used to be in the etching zone! Semiconductor etching.
(3) etching
The mechanism of etching can be divided into such processes as reactant approaching the surface, surface oxidation, surface reaction and product leaving the surface. Therefore, the whole etching includes two parts: the approach of reactants, the diffusion of products leaving, and the chemical reaction. The whole etching time is equal to the sum of diffusion and chemical reaction time. Which of the two takes a long time, the whole etching speed is also limited by it, so there are two kinds of etching, namely "reaction limited" and "diffusion limited".
1, wet etching
The most common etching method with the lowest equipment cost is shown in Figure 2- 10. There are three factors that affect the etching speed of the etched object: the concentration of etching solution, the temperature of etching solution, and the presence or absence of stirring. Qualitative, increasing the etching temperature and stirring can effectively improve the etching rate; But the effect of concentration is not clear. For example, etching SiO2 _ 2 with 49% HF is certainly better than BOE (buffer oxide etching; Hf: nh4f = 1: 6) is much faster; But 40% KOH corrodes silicon more slowly than 20% KOH! The formula selection of wet etching is chemistry major. For researchers who are not in this field, they must consult their colleagues in this chemistry major. An important concept of choosing wet etching formula is "selectivity", that is, the removal rate of etching object is related to other materials (such as etching mask; Etching the mask or the substrate bearing the processed film; The ratio of the corrosion rate of the substrate). An etching system with high selectivity should only etch the treated film without damaging the etching mask or the underlying substrate material.
(1) isotropic etching
Most wet etching solutions are isotropic, in other words, there is no obvious difference in corrosion speed in any direction of corrosion contact point. Therefore, once the pattern of the etching mask is defined, the exposed area is where the downward corrosion occurs; As long as the etching formula is highly selective, it should stop at the depth.
However, because any etching film has its thickness, when it is etched to a certain depth, the edge of the etching mask pattern gradually comes into contact with the etching solution, so the etching solution also begins to etch the bottom of the edge of the etching mask pattern, which is called undercut or lateral etching. The lateral error of the pattern caused by this phenomenon is the same order of magnitude as the thickness of the etched film. In other words, wet etching technology cannot be applied to precision machining technology with similar "sub-micron" line width!
(2) Anisotropic etching
The concept of "selectivity" in wet etching mentioned above is explained by the etching speed of different materials. However, since 1970, many articles about etching monocrystalline silicon by alkaline or organic solutions have been published in magazines such as Journal of Electrochemical Society, which are characterized by great differences in etching rates of different silicon crystal surfaces, especially
This chapter will be introduced in detail in the micro-machining of body shape.
2. Dry etching
Dry etching is a relatively new technology, but it was quickly adopted by the semiconductor industry. Which uses plasma to etch semiconductor thin film materials. Only when the vacuum degree is about 10 to 0.00 1 torr can the plasma be excited. The gas used in dry etching, or the bombardment quality is quite large or the chemical activity is extremely high, can achieve the purpose of etching.
Dry etching basically includes two etching mechanisms: ion bombardment and chemical reaction. Argon gas is used for people who prefer the "ion bombardment" effect, and the lateral erosion of the machining edge is very small. For those with "chemical reaction" effect, fluorine-based or chlorine-based gases (such as carbon tetrafluoride CF4) are used, and the excited plasma, that is, ionic groups with fluorine or chlorine, can react with chip surface materials quickly.
The dry etching method can directly use photoresist as an etching mask, without growing the semiconductor material of the mask separately. And its most important advantage is that it can give consideration to the advantages of minimum lateral corrosion and high etching rate, in other words, it is the so-called "reactive ion etching" (reactive ion etching; RIE) has met the requirements of "sub-micron" line width technology and is being widely used.