For KOH Etching What is Better Nitride or Thermal Oxide?
Postdoc asked the following:
"Do you have wafers with wet oxide? And what are the prices? Also do you have any idea whether the wet oxide could resist KOH etching?"
If the wet oxide is thick enough, then yes, however as you increase your etch temp and concentration (better silicon etch), you’ll etch the oxide very quickly. LPCVD Nitride is the suggested masking for KOH for this reason.
Reference #198736 for specs and pricing.
KOH Etching Explained
KOH etching is a popular chemical etching process for a wide range of materials. During this process, a Si substrate is exposed to a solution containing KOH. This solution alters the surface morphology of the Si substrate, making the surface smoother or rougher.
Anisotropic Si etching process
Anisotropic etching is a process in which a substrate is etched with an acid, such as KOH. The acid is mixed with a carrier gas, such as air, to etch Si on a substrate. The solution used in this process can be of two types: pure or surfactant-added. The concentration of the etching agent is determined by measuring the etch rate on different crystallographic planes. Other factors that influence etch rate include the shape of etched surfaces and the undercutting of mask corners.
The use of Triton X-100, a surfactant, has a positive effect on the etching rate. This surfactant adsorbs to the surface of silicon and reduces activation energy. Temperature is another factor that affects the etching rate on silicon.
One of the best choices for a masking layer in anisotropic etching is thermally grown oxide. Its crystalline plane can be patterned using photolithography or oxide etching using buffered HF. For high undercutting, 10% KOH is used. It is also important to note that the addition of surfactant does not affect the etch rates of the oxide layer. In addition, the selectiveness of the anisotropic Si etching process is high even at low temperatures.
Anisotropic Si etching in potassium hydroxide can also be used to fabricate high aspect ratio gratings. These gratings have higher aspect ratios, which can enable the use of high-energy x-ray sources. Anisotropic Si etching in potassium hydroxide has been used for this purpose and has proved to be effective in achieving deep uniform gratings.
One important concern when fabricating MEMS structures is etch selectivity. High selectivity allows for a longer exposure time, which is crucial for free-standing structures and deep cavities. This process is a cost-effective option for micro-scale fabrication. And since it is highly versatile, it is being used increasingly in MEMS fabrication.
Anisotropic Si etching is a common technique for fabricating microsystems. The technique is inexpensive, effective, and allows for the fabrication of sidewalls defined by crystal planes. In addition, it allows for the formation of sidewalls that are inclined at precise angles to the substrate surface.
Chemical etching with KOH is a process that removes layers of silicon from a semiconductor substrate. It works by exposing the semiconductor substrate to a solution of KOH and other chemicals. This solution can be modified with a variety of additives to increase its wettability and etch rate. The addition of NH2OH to the KOH solution can improve etching characteristics, such as undercutting.
The first step in the etching process is chemical oxidation, where hydrogen-terminated silicon (Si-H) is replaced by hydroxyl (Si-OH) atoms. NH2OH added to the KOH solution can change the hydrogen atom of the silicon backbond to a hydroxyl atom (Si-OH). This weakens the backbond, which releases the silicon atom, resulting in the Si(OH)4 product.
A two-step process has been used to achieve superhydrophobicity surfaces. In the two-step process, the aluminium substrate is chemically etched with KOH for 30 min, 45 min, or 60 min, and then etched with a lauric acid-ethanol solution for 30 min. This process is also suitable for producing superhydrophobicity surfaces.
Potassium hydroxide is a highly corrosive chemical that can etch silicon. However, the etching rate of Si crystal planes depends on the temperature and the concentration of KOH solution. A standard operating procedure can be used to determine the etch rates for different temperatures and plane profiles.
The results obtained from this process are quite similar to those obtained using the RIE method. As the nanostructures become larger, the difference in effective reflectance decreases. However, the etching time (t) increases, and the nanostructures grow more complex. After 15 min, the cone-like structures have multiple ridges along their sides.
The front surface reflectance of silicon samples decreases with formation of nanostructures. This phenomenon is often interpreted as the effective medium effect or the multiple reflection effect. For normal incidence, the scaling law is 1.3 + 0.1. Furthermore, the micro-scale roughness is unaffected by the nanostructures' lateral size. The lateral size of silicon nanostructures is typically measured in tens of nanometers after two min of etching and reaches micrometers after seventy minutes of etching.
Chemical etching with KOH can be used as a method for reducing the amorphous layer of silicon or GaN. This process can be performed using a modified plasma cleaner. During the process, GaN and Si samples are placed in a 25% KOH solution for 3 min at 130 degC.
The etching process requires a mask to control the depth and shape of the etched layers. Different masking options have different characteristics. For instance, tapered masks require a longer etching time, while open-ended masks are able to achieve a rounded surface. In addition, the mask's width is related to the diffusion of the reactants.
One of the best masking options for anisotropic etching is thermally grown oxide. This material can be patterned by photolithography or oxide etching in buffered HF. In addition, this material exhibits a high undercutting rate. Researchers studied the effects of adding 0.1% v/v Triton-X-100 on the etching rates of the oxide layer. While the surfactant did not significantly affect the etch rates, the oxide layer displayed a high selectivity even at low temperatures.
Low-concentration KOH is another option. It is inexpensive and can be used as a mask and etching solution for silicon micromechanical structures. In addition, the presence of a small amount of surfactant minimizes undercutting at convex corners. Moreover, the process is almost conformal to a silicon microstructure.
A thicker mask layer increases the etch time and thickness, but it also causes the resist to degrade. This option is not recommended for tapered micro-channels, because it results in a second lithographic step. Furthermore, a compromised mask will lead to uneven etching along patterned elements and make the structures fragile.
KOH etching is becoming more popular for creating microscopic structures in silicon. This process uses a solution of 20-30% potassium hydroxide to create cavities in a silicon wafer. It is a safe solution for silicon and is extremely repeatable, which makes it a popular choice for semiconductor fabrication facilities.
Modutek offers a wide range of wet bench process equipment to support KOH etching. Modutek heated tanks are perfect for KOH etching and are available in standard carrier and double-capacity sizes. They are also modular and can be easily integrated into a pre-existing wet etching station.
Si3N4 film is a useful option for masking during KOH etching. The HF solution can selectively etch a scratched Si3N4 mask, providing a gap for KOH deep etching. This technique can be used to achieve a large area texture pattern with a depth of several microns. This technique also enables a deep structure pattern to be fabricated on the silicon surface.
What are KOH Etching Applications
KOH etching is a process in which the surface of a silicon-based device is etched. It is useful in micromechanical applications, as the solution is highly inexpensive and can be used to fabricate silicon micromechanical structures. A small amount of surfactant is added to KOH solutions to reduce undercutting at convex corners.
The KOH concentration used for an etching process has a great impact on the end product. For example, a 10 wt% KOH solution produces a high degree of undercutting compared to a 5 wt% solution. The presence of atomic defects or impurities in the Si crystal can also influence the etching process. Additionally, boron doped Si can act as an etch stop. Nonetheless, KOH etching processes have many advantages over dry etching and other silicon wet etching processes.
The most common applications of KOH etching are in the fabrication of MEMS and NEMS. In these technologies, the etching rate must be high, as a low etch rate results in reduced productivity. Potassium hydroxide can be modified to modify its etching properties by adding hydroxylamine.
Another application of KOH etching is the anisotropic etching of Si surfaces in a solution of KOH with Triton X-100. This method reveals that increasing temperature and stirring improve etching rates in the 110-plane plane. Arrhenius plots reveal that the activation energies of Si (hkl) and Si (hh1) planes are lower than in pure KOH solutions.
The adsorption of surfactant molecules on the surface of the silicon substrate is thought to play a major role in modifying the rate of the silicon etching process. The presence of surfactant molecules impedes the interaction between OH ions and silicon, slowing down the reaction process.