Wet Processing Application Capabilities Provided by Modutek

Modutek provides a complete line of wet processing equipment to semiconductor manufacturing plants and research facilities. The company’s wet process stations support all common cleaning and etching applications. Modutek works with customers to continuously improve wet processing performance and reduce contamination and particle counts to a minimum. Wet process applications supported by Modutek equipment include the following:

Standard Clean 1 (SC1 Clean)

The SC1 Clean process uses a mixture of ammonium hydroxide and hydrogen peroxide in water to remove organic material and other non-metallic contaminants from the surface of the silicon wafer. Additional cleaning steps are required if metallic particles have to be removed.

RCA Clean

RCA Clean is a two-step process in which the first step, immersing the silicon wafer in a mixture of ammonium hydroxide and hydrogen peroxide, is similar to SC1 Clean. The second step bathes the silicon wafer in a mixture of hydrochloric acid and hydrogen peroxide. This step removes metallic contamination and particles from the wafer surface, leaving it ready for further process steps.

SPM Process Clean

SPM Clean stands for cleaning with a sulfuric peroxide mixture and the method bathes silicon wafers in a solution of sulfuric acid and hydrogen peroxide. SPM Clean is also known as Piranha Clean and it removes heavy organic material contamination such as photoresist from the surface of the silicon wafer.

KOH Etch

KOH etching uses a potassium hydroxide solution for anisotropic etching of the silicon wafer. The process is safe and reliable and produces precise etching that can be controlled by varying the KOH concentration and the temperature. KOH etch is one of the most common etching applications.

Nitride Etch

Silicon nitride masks are etched with phosphoric acid to create highly selective etch masks where silicon dioxide masks can’t be used. While silicon dioxide masks resist etching with KOH, for example, sometimes long etching times mean that the etching selectivity versus silicon may not be enough and silicon nitride masks are used instead.

Aluminum Etch

Aluminum layers are deposited on silicon wafers to create conducting paths between the semiconductor components on the wafer. The aluminum layers have to be etched so that only the desired conducting paths are left. Etching is performed under vacuum using a variety of etchants, including phosphoric and nitric acids. The vacuum eliminates the hydrogen bubbles created in a chemical reaction with the etchants.

Modutek Equipment That Supports These Wet Processing Applications

A complete line of wet process equipment has to include chemical handling systems, stations that can support the different cleaning and processing methods and automation where needed. Modutek has chemical delivery, pumping and neutralization systems; different types of baths, tanks and rinsers; as well as fume hoods, scrubbers and dryers. The company can advise customers on the different options for a wet process line and deliver suitable equipment.

For chemical handling, Modutek can offer chemical delivery systems, pumps carts, lift stations and chemical collection systems. The systems include bulk storage or local storage container options and, once collected, the chemicals can be neutralized.

Processing and cleaning station options include quartz baths and Teflon tanks. Rotary wafer etching systems are available as are vacuum metal etchers and stainless steel solvent stations. Baths can be sub-ambient, ambient or temperature controlled. Stations can be manual, semi-automatic or fully automated. In each case, controls and automation deliver a precise and reliable setting of variables and excellent repeatability of process steps.

Modutek’s extensive experience in developing wet processing equipment and on-going efforts to develop advanced process systems provide customers with superior performance with lower cost of ownership. For a free quote or consultation on selecting the right equipment for your manufacturing process contact Modutek at 866-803-1533.


Isotropic and Anisotropic Silicon Wet Etching Processes

The silicon wet etching of monocrystalline wafers produces microscopic structures that are used in micromechanical devices and semiconductor components. Areas of the silicon wafer not to be etched are protected by masks made of materials such as silicon dioxide or silicon nitride. The exposed areas of the silicon are etched when the wafer is immersed in a chemical bath.

In isotropic etching, an isotropic etchant such as hydrofluoric acid etches the silicon equally in all directions. This means that the wafer is etched directly downwards and also sideways under the mask. The resulting cavity has rounded corners and edges and is larger than the opening in the mask.

For anisotropic etching, the anisotropic etchant such as potassium hydroxide (KOH) etches with different speeds in different directions. This means that the etch rate in the downward direction can be faster than in the sideways directions under the mask. When properly designed, the anisotropic etch can produce cavities with straight sides and less undercutting of the mask.

More details on isotropic and anisotropic silicon wet etching can be found in the following documentation:

How Anisotropic Etching Works

In monocrystalline silicon wafers, the silicon atoms are arranged in a crystal lattice. The crystal has planes which have different atom densities. For example, the 111 plane in silicon is a diagonal plane that has a different atom density than the vertical 100 or 110 planes. As a result, the etching speed for certain etchants differs according to the plane of the silicon crystal through which etching takes place.

To design a successful anisotropic etching process, the etching speed of the etchant and the orientation of the crystal planes have to be coordinated. For example, if the aim is to etch a deep cavity with straight walls, the crystal planes with a slow etching speed have to be oriented along the cavity walls while the plane with a high etching speed should form the bottom. Rather than producing etched cavities with rounded walls and edges, anisotropic etching can create clearly defined straight-walled cavities along the silicon crystal planes.

Controlling the Etching Speed to Obtain Specific Structures

The silicon wafer masks define where etching can take place but the depth of the etched cavity and its shape can be determined by choosing the right etchant and controlling the etching rate. Isotropic etching is often used to create larger features in the initial stages of silicon wafer processing while anisotropic etching can produce straight-edged microstructures in the finishing stages.

Beyond the choice of the type of etching used, the etch rate depends on the concentration of the etchant and the etching solution temperature. Once the etch rate is determined for an etchant concentration at a specific temperature, the masked wafer is immersed in the etchant just long enough to produce the size of cavity required. In each case the precise concentration has to be mixed and the temperature has to be maintained at the target level. Subsequent wafers can be processed exactly the same way for reproducible results.

Differences between Isotropic and Anisotropic Silicon Wet Etching

While isotropic etching is harder to control precisely and hydrofluoric acid is difficult to handle, isotropic etching is faster than anisotropic etching. As a result it is often used for large geometries for which etching speed is an important factor. Anisotropic KOH etching is more precise but it depends on the orientation of the silicon crystal planes. If the wafer crystal orientation doesn’t match the requirements of the process, the resulting cavity will not have the desired characteristics.

Silicon wet etching equipment such as Modutek’s Teflon tanks support both types of processes and they come in temperature controlled and ambient versions. For processes dependent on temperature for controlling etch rate, the tank temperature controllers provide rapid and accurate heating. Concentration can be maintained through supplementation of de-ionized water and custom tank sizes are available if needed. For a free consultation or quote on selecting the right silicon wet etching equipment for your etching process contact Modutek at 866-803-1533.