Beijing Sanhe Lian Technology Co., Ltd. is a domestic manufacturer of ion beam etching equipment, independently researching and developing SLH brand ion beam etching equipment series products. Beijing Sanhelian will give you a brief explanation of material etching: processing materials using the corrosive nature of widely used acids. Etching was used in ancient times for printing and copper plate carving, and is still a core technology in printed circuit board and semiconductor manufacturing today. This section introduces the principle and types of etching, and explains its relationship with the cleaning process as well as the precautions to be taken when etching.

Etching was used in ancient times for printing and copper carving, and even today, it plays a positive role as a key technology in printed circuit boards and semiconductor manufacturing. This article explains the principle and types of etching, and discusses its relationship with cleaning operations, as well as the precautions that must be taken during etching. 

There is a method of processing materials called "etching". Etching is a chemical processing method that utilizes the properties of acids, bases, and ions, and it seems to be not widely known. Etching is a technique commonly used in areas where the eye cannot detect. Here, we will discuss the principle of etching, the types of etching, and the materials to be etched.

What is etching?

Etching is a method of processing the surface of target materials (mainly metals, glasses, and semiconductors) by utilizing the corrosiveness of acids, bases, and ions to remove a portion of the surface and produce the desired shape.

The material etching process of Beijing Sanhelian processes materials by removing a portion of the material to obtain the target shape. Other methods include cutting and grinding, but etching is often used when processing with higher precision than these methods, and the amount of material removed is smaller than these methods.

Etching principle

The principle of etching is to first apply a durable substance called "resist" to the areas of the etched surface that do not need to be removed. Etching in this state will remove areas other than those where the resist has been applied. Afterwards, by removing the applied resist, patterns and designs can be formed on the surface of the etched workpiece.

The primitive invention of etching is surprisingly ancient; It is said that the first invention was made in Europe around 1500. Since then, etching has been widely used to make printing plates and artworks. In modern times, etching has evolved into photolithography technology that integrates photography into plate making. At present, the processing of printed circuit boards and semiconductor chips adopts a further developed version of this photolithography technology. Especially in the manufacturing of semiconductor devices, a technique called photolithography is used, which can be called the limit of photolithography technology, with an order of magnitude of nanometers (billionth of a meter).

One advantage of etching at Beijing Sanhe Lian Technology Co., Ltd. is that it can be carried out in batches with higher precision than cutting or grinding. Therefore, etching is commonly used in the manufacturing of printed circuit boards and semiconductor processing, as well as in the process operations of microelectromechanical systems (MEMS). In other words, it can be said that the development of printed circuit board manufacturing, semiconductor processing, and MEMS has driven the advancement of etching technology.

In contrast, etching is less suitable for processes with large removal volumes. Etching is essentially a process of lightly scraping surfaces. Therefore, in order to process materials into three-dimensional shapes by removing more important parts from them, different processing methods need to be studied.

In addition, as mentioned below, there are two systems for etching at Beijing Sanhe Lian Technology Co., Ltd.: wet etching using acidic and alkaline solutions, and waste liquid treatment must be considered. Similarly, for dry etching, which is carried out through ion beam bombardment, the fact that the equipment will be a large-scale project must be taken into account due to the need for a vacuum chamber.

Types of Etching

1. Wet etching and dry etching

Etching can be roughly divided into two techniques. One method is wet etching, which utilizes the corrosiveness of acidic and alkaline solutions to remove materials. Wet etching is widely used in applications such as printed circuit boards, glass decoration and processing, and copper engraving. It can also be used as an auxiliary means for semiconductor processing and MEMS processing.

Another method is dry etching, in which atoms on the surface of the workpiece are removed by ion beam for processing. This impression is similar to the feeling of ionized particles hitting the surface of a workpiece at high speed, causing atoms on the surface of the workpiece to fly. Dry etching is mainly used for semiconductor processing and MEMS processing. In recent years, with the increase of ion beam output power, dry etching has been applied to remove processes with larger volumes

2. Isotropic corrosion and anisotropic corrosion

In addition, an important difference in wet etching is the distinction between isotropic etching and anisotropic etching.
Isotropic etching means that etching reactions occur in all directions. For example, if the etching reaction starts in the area near the resist on the surface of the workpiece, a reaction will occur, in which corrosion advancing in all directions enters below the resist and chisels out the bottom of the resist. This is sometimes referred to as' undercutting '. In this case, when the etching is completed and the cross-section of the workpiece is observed, it will be processed into a U-shape or rotated 90 degrees laterally to form a C-shape with rounded edges. If the reaction rate is fast, isotropic etching will occur.

Anisotropic etching can perform edge confinement processes, but it will not occur unless the reaction rate slows down.
That is to say, isotropy or anisotropy is determined by the etching reaction rate. The combination of factors such as the concentration, type, and temperature of the etching solution, as well as the stirring of the solution, is crucial for controlling the reaction rate.

3. The mechanism of isotropic/anisotropic corrosion occurrence

From a microscopic perspective, a brief observation of the etching reaction at Beijing Sanhe Lian Technology Co., Ltd. reveals that the mechanisms of isotropic and anisotropic etching largely depend on the crystal structure of the etched material surface. For example, when etching silicon materials with hydrofluoric acid, the etching should be isotropic. However, whether etching is isotropic or anisotropic depends on which crystal plane appears on the surface in the cross-section of the lattice that constitutes the silicon material. If the number of atoms appearing on the surface is large among the atoms that make up the lattice, the etching will be isotropic; if the number is small, it will be anisotropic.

In addition, the strength of lattice bonds, i.e. their "permissibility" for etching, depends on the type of elements that make up the etching material. For example, compared to copper, silicon does not allow etching and requires strong acids such as hydrofluoric acid. Therefore, generally speaking, when strong reactive acids are used, etching will be isotropic, while when weak reactive acids are used, etching will be anisotropic.

In addition, in some cases, surfactants are added to etchants, where the production of isotropic or anisotropic etching can be controlled according to the type and concentration of additives.

In fact, it is advisable to use etching as an anisotropic process to achieve satisfactory edge defining patterns, but the efficiency will be low due to the need to reduce the reaction rate. On the contrary, attempting to increase the process speed will result in isotropic etching and ultimately lead to bottom cutting. How to overcome this obvious contradiction can be said to be the skill of etching engineers and technicians.

Etching agent composition and waste liquid treatment

The key points for wet etching at Beijing Sanhe Lian Technology Co., Ltd. are the etchant and its treatment. Firstly, let's take a look at the etching on copper.

Mainly used for etching copper and ferric chloride (FeCl ∝). When copper is placed in an aqueous solution of ferric chloride, it will dissolve in the solution. Then, iron and copper exchange to produce copper chloride (CuCl ₂) Although these are principles of chemical etching, due to the toxicity of copper chloride, the Pollutant Release and Transfer Register (PRTR) regulations (laws promoting understanding and management reform of designated chemical emissions into the environment) strictly limit the discharge of copper chloride into the environment. In other words, if copper chloride is not non-toxic, it cannot be flushed into the sewer.

This requires the treatment of waste liquid. Specifically, hydrated lime (calcium hydroxide: Ca (OH) ₂) is decomposed into copper hydroxide (Cu (OH) ₂) and calcium chloride (CaCl ₂) by being placed in the waste liquid In addition, when heated, copper hydroxide decomposes into copper oxide (CuO) and water (H ₂ O). Calcium chloride and copper oxide are harmless and can be flushed into the sewer in this state. These are the chemical principles behind the treatment of etchant waste liquid.

Similarly, hydrofluoric acid (chemical formula: HF) is used for wet etching of glass and silicon, but it is a highly toxic substance that requires careful handling. Therefore, it is crucial to treat the waste hydrofluoric acid solution as well. The common method for treating hydrofluoric acid waste liquid is to react it with hydrated lime or calcium chloride to produce calcium fluoride (CaF ₂) Calcium fluoride is harmless and can be flushed into the sewer.

In any case, the essence of wet etching process and waste liquid treatment is a chemical reaction. Therefore, different acids and bases were used. Many of these chemical products are harmful to ecosystems and human health, so it is important to treat them during work and waste disposal.