Dry Etching vs Wet Etching: Everything You Need To Know

Etching is required to form a functioning microelectromechanical system (MEMS). Etching is a vital process in manufacturing microelectronic components, and each substrate undergoes an etching process before it is complete. During the etching process, masking material becomes resistant, as it protects the substrate from the etching reagent. In some cases, the masking material is a photoresist that has been developed through lithography. A more lasting mask, such as silicon nitride, is required in other cases. In this article, you'll get to know everything about dry etching vs wet etching, which are the two major etching processes that exist. Also, etching applications to semiconductors and PCBs will be discussed, as well as the advantages and differences that exist in dry etching vs wet etching.

What types of etching processes exist?

Etching technologies are frequently classified based on the phase of the reagent used. The liquid phase and the plasma or gaseous phase are the two most common etching reagents. 

However, when determining what type of etching to use, it is essential to consider two things:

1. Isotropy and Anisotropy: 

In isotropic etching, the etching rate is constant along with all directions; that is, the properties of a material are independent of the direction. Whereas Anisotropic etching removes material along a certain direction, it is direction-dependent[1].

2. Selectivity: 

Selectivity describes the relative etch rates between a mask (used for patterning) and the etch rate of the material of interest. The selectivity of most liquid and gas chemical etches is high, but they're generally isotropic, making etch rate control challenging [2]. Plasma etching is sort of controlled and may be tailored to be very anisotropic; however, achieving good selectivity is usually more challenging.

Dry Etching Process

Dry etching, also referred to as Plasma etching, is the process of removing a masked pattern of semiconductor material by bombarding it with ions. Typically, the ions are a plasma of reactive gases like oxygen, boron, fluorocarbons, chlorine, and trichloride. To get rid of portions of the fabric of the exposed surface, nitrogen, helium, argon, and other gases are sometimes used. The dry etching technology, unlike some wet chemical etchants/reagents utilized in wet etching, etches with a continuing etching rate in all directions (isotropy) or with a change in direction (anisotropy)[1]. 

Dry etching technology can be split into three separate classes called vapor phase etching, sputter etching, and reactive ion etching (RIE) [3].

During the vapor etching process, the substrate is placed inside a chamber, into which one or more gases are introduced. The fabric to be etched dissolves at the surface during a reaction with the gas molecules. Silicon etching with xenon difluoride (XeF2) and silica etching with fluoride (HF) are the two hottest vapor phase etching procedures, and they are both isotropic. When constructing a vapor phase process, it is vital to ensure that no bi-products from the reaction condense on the surface and interfere with the etching process.

Furthermore, the technologies utilized in the sputter etching process are fundamentally just like those used in sputter deposition. The key difference is that the substrate is bombarded with ions rather than the fabric target utilized in sputter deposition. Sputter deposition may be a thin film deposition process supporting the physical vapor deposition (PVD) phenomena of sputtering. This entails ejecting material from a "target" (a source) onto a "substrate" (a silicon wafer, for example).

Reactive ion etching (RIE)

In reactive ion etching(RIE), the etching characteristics - etch profile, etch rate, selectivity, uniformity, and reproducibility - may be accurately adjusted [4]. Both anisotropic and isotropic etch profiles are possible. As a result, the most essential technique in semiconductor manufacturing for structuring various layers is the RIE process, which is a chemical-physical etch procedure.

In RIE, the substrate is placed inside a reactor filled with various gases. A plasma is created in the gas mixture using an RF power source, shattering the gas molecules into ions. The ions are propelled towards the surface of the substance being etched, where they react, creating new gaseous material. This is referred to as the chemical part of reactive ion etching.

Because there are so many parameters to alter, developing dry etch procedures that balance chemical and physical etching is difficult. Without a chemical reaction, the ions can knock atoms out of the substance to be etched if they have enough high energy. The etching's anisotropy can be modified by adjusting the balance; because the chemical part is isotropic and the physical part is very anisotropic, the combination can produce sidewalls with forms ranging from rounded to vertical.

Deep RIE (DRIE) is a unique subclass of RIE that is rapidly gaining popularity. Etch depths of hundreds of microns can be produced with practically vertical sidewalls using this method. The fundamental technique is based on the so-called "Bosch process," which alternates two different gas compositions in the reactor and is named after the German company Robert Bosch, which submitted the initial patent[5].

The first gas composition forms a polymer on the substrate's surface, while the second gas composition etches it.

The physical part of the etching immediately sputters away the polymer, but only on the horizontal surfaces, not the sidewalls. The polymer builds up on the sidewalls and protects them from etching because it dissolves slowly in the chemical part of the etching. As a result, high aspect ratios of 50 to 1 can be reached during etching[6]. The method can easily etch a silicon substrate fully through etching rates, which are 3-4 times faster than wet etching.

Wet Etching

The most basic etching process is wet etching, also known as chemical etching. It entails the chemical removal of a substance using a liquid reactant. It might be a chemical that dissolves the etching substance or a chemical mixture that oxidizes the fabric first then dissolves the oxide. Chemical etching is accomplished by immersing the sample in an etchant solution container. All you need is a container filled with a liquid solution to dissolve the substance. The partially protected substrate is immersed in the solution, which then chemically etches the surface of the exposed substrate[7].

This is usually a stronger acid, such as hydrofluoric acid for a silicon substrate (the only one capable of interacting with the silicon dioxide layer that occurs naturally on silicon's surface), hydrochloric acid for a gallium arsenide substrate (the gallium reacts significantly with the chloride ion), or a weaker acid, such as citric acid, for a gallium arsenide substrate (the chloride ion has a strong reaction with gallium).

Wet etching is frequently used due to its selectivity. Many wet etches are highly selective about which materials they work with. The use of thinner masks and the ability to abruptly halt etching on the layer beneath the layer being etched are both possible with high selectivity. Unfortunately, because a mask is frequently used to selectively etch the material, there are complications. This necessitates using a mask that does not dissolve or at least etches far more slowly than the substance to be patterned.

Wet etchants are usually isotropic, which results in large bias when etching thick films. They also require the disposal of huge amounts of toxic industrial waste. Secondly, some single-crystal materials, such as silicon undergo anisotropic etching in certain chemicals. Anisotropic etching, contrary to isotropic etching, involves varied etch rates in various directions inside the fabric. Examples include the Si{111} crystal plane sidewalls which develop while etching a hole in a {100} silicon wafer in a chemical such as potassium hydroxide(KOH). Instead of a hole with rounded sidewalls, an isotropic etchant produces a pyramid-shaped hole[8]. The majority of wet etches are isotropic, which means they undercut the mask, resulting in features that are larger than the mask.

Isotropic Etching

All etching procedures with similar etch rates in all spatial directions are classified as isotropic etching. Production facilities use two different etching procedures within the primary categories of wet and dry etching. One of these is an isotropic etching. Isotropic etching is a chemical procedure that removes material from a substrate using an etchant solution, which is extensively employed in semiconductors. The etchants are often liquid, gaseous, or plasma-based, while liquid etchants such as buffered acid (BHF) are most ordinarily employed for silicon etching.

In contrast to isotropic etching, anisotropic etching employs different amounts of etching and reacts in specific directions. Isotropic etching etches in several directions within the substrate rather than in a single one. As a result of any horizontal component of the etch direction, undercutting of patterned portions and significant changes in device properties may occur. Wet chemical and dry etching methods are both used in isotropic etching. Isotropic etching may be unavoidable or desirable for process reasons.

Isotropic and Anisotropic - Image credits: Characterization of low pressure plasma-dc glow discharges (Ar, SF6 and SF6/He) for Si etching

Dry Etching vs Wet Etching

When it comes to the two major etching procedures, dry etching is a plasma-based etching process, whereas wet etching is a liquid-based process. Dry etching employs chemicals in the gaseous phase, whereas wet etching uses chemicals in the liquid phase. Wet etching techniques have the advantages of being quick and having high etch rates. High selectivity can be achieved using simple equipment baths or wet chemical sprays. Liquid solvents, corrosive leeches, and acids are common etching agents in wet processes. The process equipment is usually a chemistry-specific immersion tank or spray system.

Also, dry etching is far safer than wet etching in terms of safety. Wet etching is not safe since hazardous chemicals must be disposed of properly to avoid water contamination. The dry etching process is more exact than the wet etching process when it comes to precision. The etchant chemicals remove substrate material behind the masking material at the same rate as the bulk etch since wet etching is usually isotropic.

Wet etching necessitates a considerable quantity of etchant chemicals since the etchant chemical must be applied to the substrate material. Furthermore, because the etch chemistry loads with etched waste, the etchant chemicals must be replenished regularly to maintain the same starting etching rate. As a result, wet etching has comparatively significant chemical and disposal expenses. However, the ability to use several etch gases with highly diverse process settings within the same tool, with little to no hardware modification over time, is one among the advantages of dry etching.

This process is likewise carried out in a vacuum chamber away from the operator, resulting in a clean working environment. In comparison to wet etching, dry etching (e.g. plasma etching) chemistry disposal costs less and is easier to dispose of the by-products. Plasma etching has a few drawbacks: it's usually a batch operation that takes place in a vacuum chamber.

Popular Etching Applications You Should Know

Electrolytic and chemical etching has long been used as a foundational technology in electronics, printing industries, and precision machinery. Even though more modern technologies have rendered etching obsolete in some applications, etching is still a valuable processing tool. The examples below are just a few of the etching applications.

PCB Etching

The impedance requirements of various wires have gotten increasingly stringent as the print circuit board (PCB) business has grown, necessitating stricter regulation of wire width[9]. The quality of PCBs is improving in industrial manufacturing, and their reliability is increasing; however, the design process is becoming more and more diversified and excellent. The application of etching technology to PCB processing and assembly is growing in popularity. One of the most important procedures in the final processing of printed circuit boards(PCBs) is etching.

Excess copper is removed during this operation, revealing the ideal circuit layouts. The technique of removing undesired copper (Cu) from a circuit board is known as PCB etching. Unwanted copper is non-circuit copper that is removed from the board following the PCB design. The desired circuit pattern is created as a result. The base copper, also known as the start copper, is removed from the board during this procedure. Compared to electroplated copper, rolled and heat-treated copper is easy to etch off. Before beginning the etching process, a layout is created to ensure that the final result satisfies the designer's specifications.

The transfer of the designer’s desired image of the circuit onto a PCB is done by a process called Photolithography. This serves as a blueprint for determining which parts of the copper board must be removed. There are two separate techniques of etching the inner and exterior layers. The etch resist in the outer layer etching process is tin plating. The photoresist is the etched resist in the inner layer. Except for the circuitry that is protected by the tin plating that was applied during the prior treatment in PCB manufacture, all copper is removed during the PCB etching process. After the tin is removed and the copper is cleaned, the circuit is ready to continue to the next stage of production.

Semiconductor Etching

Currently, plasma etching is utilized to treat semiconducting materials for usage in electronics production. Small features are often etched into the surface of semiconducting materials to enhance efficiency or improve certain qualities when utilized in electrical devices. Plasma etching, for instance, is often used to make deep trenches on the surface of silicon for MEMS applications[8]. Plasma etching could play a big part in microelectronics production, according to this application.

Scientists are also looking into how to apply the method to the nanoscale. Other uses for hydrogen plasma etching, in particular, are intriguing. Hydrogen plasma etching has been demonstrated to be effective in eliminating sections of native oxides from semiconductor surfaces when employed in the etching process. Hydrogen plasma etching also produces a chemically clean and balanced surface that is suitable for a variety of applications.

Conclusion

Various etching processes have been reviewed and discussed systematically. The two basic types of etching procedures, dry and wet etching, are effective for removing surface materials and creating patterns on surfaces. Dry etching differs from wet etching in that wet etching employs liquid chemicals or etching agents, whereas dry etching uses plasmas or etching gases. Etching is a critical step in the fabrication of microelectronic components. Etching technology applications are found in PCBs and Semiconductors. 

The removal of unwanted copper from a circuit board is one of the most significant steps in the final processing of printed circuit boards (PCB). Plasma etching is also used to prepare semiconducting materials for use in electronics manufacturing. When used in electrical devices, little features are frequently etched onto the surface of semiconducting materials to improve efficiency or particular properties. However, scientists are investigating how to apply the process at the nanoscale, implying that there will be more etching applications in the future.

Reference

1. ByJu's Classes (2021). Difference between Isotropic and Anisotropic. Available at: https://byjus.com/chemistry/difference-between-isotropic-and-anisotropic/ [Accessed 2nd December, 2021]

2. Etch Overview for Microsystems (n.a) Southwest Centers for Microsystems Education. Available at: https://www.google.com/url?sa=t&source=web&rct=j&url=https://nanoscale.unl.edu/pdf/Etching_Overview_LM_PG.pdf&ved=2ahUKEwja6-O3us30AhVI6qQKHSMcAPsQFnoECC4QAQ&usg=AOvVaw2ZhDgFeyqTKPvFBZaX9rIc [Accessed 1st December, 2021]

3. MEMS and Nanotechnology Exchange (2020). Available at: https://www.mems-exchange.org/MEMS/processes/etch.html [Accessed 1st December, 2021]

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5. Bosch Today (2018). Bosch Global. Available at: https://assets.bosch.com/media/global/bosch_group/our_figures/pdf/bosch-today-2018.pdf [Accessed 1st December, 2021]

6. Yeom, J., et.al(2005). Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures. DOI: 10.1116/1.2101678

7. Jaeger, Richard C. (2002). "Lithography". Introduction to Microelectronic Fabrication (2nd ed.). Upper Saddle River: Prentice Hall. ISBN 978-0-201-44494-0.

8. Gad-el-Hak M (2002) The MEMS Handbook. CRC Press LLC, Boca Raton. Available at: https://scholar.google.com/scholar_lookup?title=The%20MEMS%20Handbook&publication_year=2002&author=Gad-el-Hak%2CM [Accessed 2nd December, 2021]

9. Gong, L.(2016) PCB Etching Technique and Analysis Solution. Available at: https://www.seeedstudio.com/blog/2017/03/16/pcb-etching/ [Accessed 3rd December, 2021]