What is Silicon Etching?
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What is Silicon Etchant?
Silicon etchant refers to a substance or solution that is used to selectively remove or etch silicon from a substrate. Silicon etchants are used in the manufacturing of electronic devices, such as microprocessors, solar cells, and sensors.
There are many types of silicon etchants, each with different properties and characteristics. Some common types of silicon etchants include:
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Buffered Oxide Etchant (BOE): BOE is a mixture of hydrofluoric acid and ammonium fluoride that is commonly used to etch silicon dioxide (SiO2) and silicon nitride (Si3N4).
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TMAH (Tetramethylammonium Hydroxide): TMAH is a solution of tetramethylammonium hydroxide in water, which is commonly used to etch silicon.
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KOH (Potassium Hydroxide): KOH is a strong base that is commonly used to etch silicon wafers.
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HF (Hydrofluoric acid): HF is a highly corrosive acid that is commonly used to etch silicon, especially in the manufacturing of microelectromechanical systems (MEMS) and microfluidic devices.
The choice of etchant depends on the specific application and the desired properties of the resulting silicon surface.
Important Silicon Etching Terms
Below are a list of important terms associated with the silicon etching process:
- anisotropic etching
- silicon etching
- wafer etch
- etching machine
- etch processes
- etch etchant
- etch solution
- anisothopic silicon
- surface etch
- silicon wafers
- silicon nitride
- isotropic etchant
- silicon surfaces
- silicon substrates
- wafer surface
What is Silicon Etching?
If you are wondering what silicon etching is, you have come to the right place. This article will answer all of 
your questions about the process, from the equipment used to the process itself. If you are still not sure, we recommend reading up on the different types of etching to get a better understanding of the whole process. Listed below are some of the most common types of silicon etching. Once you understand these different types, you'll be better equipped to choose the right etching method for your application.
What is Anisotropic Silicon Etching?
Anisotropic silicon etching is an etching technique in which polymer is applied to a surface defined by an etched mask. During the etching process, the polymer is partially removed from the surface and is redeposited at a deeper location on the side walls. The etching gas contains between three and forty percent oxygen and sulfur or fluorine. The oxygen has virtually no effect on the etching process.
The process is characterized by the presence of Cu2+ and Cu, which have different reduction potentials. This lower reduction potential results in the anisotropic etching of the p-type silicon substrate. The crystallographic planes in the (100 and 111) planes determine the orientation of the etched surface. Achieving this level of crystal orientation helps predict the final etching structure. Anisotropic silicon etching begins at a defect-ridden Si surface, which has a low-quality surface. The process can etch regions without defects or pits by extending the reaction time.
The concentration of ammonium persulfate (AP) in the silicon etching solution is an important factor in controlling the roughness of the etched surface. An AP concentration of between 0.6 and two percent is deemed ideal for achieving a surface roughness of less than 0.1 mm. Any higher concentration results in a surface that is even rougher than before. For example, Figure 36 shows a surface of silicon etched in a solution of five wt.% TMAH.
Alkaline and Cu based acid etching are both effective in anisotropic silicon etched devices. A solution containing a metal such as Cu nanoparticles has lower reduction potential than alkaline etching solutions. Cu nanoparticles deposited on the surface of the silicon etching solution is effective at forming an inverted pyramid. The etching rate decreases in a decreasing order from (100) to (110).
In the process of anisotropic silicon etching, crystal plane orientation and etchant concentrations are coordinated so that the resulting cavity is shaped according to the requirements. This process yields cavities with a uniform thickness. The etching solution also increases the area of a contacted surface, which is why anisotropic silicon etching is so beneficial. Its application is widespread in semiconductor manufacturing and is the most cost-effective way to create circuit patterns.
The process of anisotropic silicon etching starts with a mask that contains a square shape pattern. Once the masks are fabricated, anisotropic etching is used to create a cavity with the desired depth. The masking layer can be either thermal oxide or CVD nitride. For oxide, KOH is not recommended. Instead, TMAH is preferred. A TMAH mask is the most suitable option for this type of silicon etching.
Anisotropic silicon etching is a process in which the underlying layers of the surface are etched. The process involves using a solvent called tetramethyl ammonium hydroxide. The etching rate depends on the concentration of the solvent. Higher concentrations of tetramethyl ammonium hydroxide result in a lower anisotropy than lower concentrations of the same substance.
In the case of Cu nanoparticles, oxygen should be added only during the etching steps. Oxygen in the deposition process would impair the formation of Teflon(r)-like polymers. Oxygen molecules are extremely reactive with silicon. Therefore, adding oxygen in the deposition would cause the polymerization of the radicals, rendering the process inefficient. A higher concentration of oxygen would also lead to a longer and thinner etching time.
What is Isotropic Silicon Etching?
The morphology of single crystalline silicon surfaces after isotropic silicon etching has been investigated by means of dynamic scaling theory. Results showed that the roughness of the etched surface was not self-affine, and that its local properties underwent a reversal between the two length scales. This morphology is exhibited after 120 s of etching, when the early unstable phase passes over into a stable phase.
Another method of anisotropic etching is based on O2 etching. It is fast, and is compatible with multi-wafer batch processing. Moreover, it provides both anisotropic and isotropic etching, depending on the process conditions. The process is also safer than halogen-based dry etchants and does not require expensive waste management systems. A major advantage of O2 etching is its ability to produce clean, non-toxic etching solutions.
Isotropic silicon etching also utilizes a bilayer mask, with a fixed mask and a receding mask. In this method, a layer of Ti is used as the fixed mask and a 10-nm Si layer as the receding mask. This method allows for the production of very small and highly precise structures. It is particularly useful in the field of optofluidics.
The chemical part of the isotropic silicon etching process involves the use of a polymer that dissolves slowly. The polymer protects the sidewalls of the silicon substrate from the etching process. In fact, polymer etching can yield high aspect ratio trenches (50:1) and a comparatively high etch rate (3-4 times higher than wet etching. The downside of isotropic silicon etching is a complex process requiring expensive equipment and hazardous gas disposal systems.
The process can also be used for micromachining MEMS. In the process, a 25 vol% mixture of fluorine and N2 is used to etch the silicon substrate. The exposure time of the silicon to the fluorine gas is long enough to form pits that are 10-50 um in diameter. If the F2 concentration is reduced to a sufficient level, however, the SiO2 does not etch and the resultant material is a micron-scale silicon surface.
In both the wet and dry etching techniques, the etch rate is controlled and can be varied from 1:1 isotropic to 1.8:1 anisotropic. In addition, the technique uses high temperatures to achieve etch rate uniformity across the wafer. The main advantages of both wet and dry Si etching techniques are that they are stiction-free and offer high etch rate uniformity.
Wet etching is the easiest and most common method. It involves immersing a silicon wafer in a liquid etchant bath. A chemical reaction removes the unprotected material. This process is usually isotropic, although anisotropic etching is used when you need to remove a circuit pattern. Wet etching is also often used to clean wafers and substrates.
Plasma-based deep etching is an advanced technology that combines multiple processes. Plasma-based deep etching, which was developed by Bosch, alternating plasma etching with vertical polymer wall coating. Initially, the etching speed was one millimeter per minute, but today's fastest machines deliver ten millimeters-per-minute rough etching. It is an excellent method of etching silicon, but needs careful attention.
Wet etching is also a highly selective process. Wet etching preferentially etch grain boundaries, crystallographic defects, and dislocations. Its high selectivity has allowed metallurgists to develop several wet etches. Some of these processes can even produce superhydrophobic surfaces. Aluminum alloy (2024Al) has been etched in NaOH solutions.
O2 flow rate has also been investigated. At O2 flow rates below 50 sccm, a smooth Silicon surface can be achieved. The ratio of vertical-to-lateral etch rate is also studied. At ten sccm, the ratio is completely isotropic, while at 30 sccm, it reaches a transition. This transition is a necessary step for achieving high device yields.