Oxide and Silicon Etch
Advantages of Dry Etching
The increased utilisation of vapor phase etching systems has come about due to smaller feature sizes, increased selectivities between materials and a requirement for more complex materials. These have appeared in the form of vapor phase etch systems which are capable of releasing devices with zero stiction effects resulting in a marked increase in yield.
Dry etching using vapor phase processes was developed as a means of moving existing wet etching processes into the vapor phase to improve performance and material compatibility. This has allowed for a substantial improvement in the etch, increasing yield and enabling the release of smaller and complex devices. The advent of vapor phase etching systems have allowed manufacturing groups to pass over wet etching options in favour of dry processes, which provide increased controls over the process conditions (pressure, gas, flow, temperature).
The two most common systems now available for dry etching are vapor phase oxide and silicon etch systems.
Oxide etching systems utilize anhydrous hydrogen fluoride (HF) (compared with HF acid) now supplied in the gas phase where further process parameters can be adjusted for optimum etch performance.
Silicon etching systems now exist with the use of xenon difluoride (XeF2), a white crystalline material, which is introduced to the material to be etched in a gaseous state. It subsequently reacts with silicon (bulk, poly and amorphous), and other specific materials to remove them and release the final device.
Oxide & Silicon Etch Based on HF and XeF2 Chemistries
What is HF?
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds. In this application the HF is transported to the process chamber in the gaseous state. Anhydrous (without water) HF is used, not HF acid.
How does HF etch silicon dioxide?
The reaction between silicon and HF is described as: »»
2HF + SiO2 → SiF4 + 4H2O
From the above equation it can be seen that a catalyst is required to initiate the reaction. Without the catalyst, the reaction will not occur. memsstar’s unique patented process uses H2O as the catalyst. Older technologies use alcohol as a catalyst to displace the water, but these processes are limited in their capability.
What are the advantages of HF etching?
memsstar’s dry anhydrous hydrogen fluoride etching is unique because it eliminates stiction in a single process. The patented process is compatible with the widest range of metals – especially Al/alloy and other metals commonly used in MEMS mirrors and electrical contacts.
What is XeF2?
Xenon difluoride is a powerful fluorinating agent with the chemical formula XeF2, and one of the most stable xenon compounds. Xenon difluoride is adense, white crystalline solid.
How does it etch silicon?
The XeF2 adsorbs and dissociates to xenon (Xe) and fluorine (F) on the surface of silicon. Fluorine is the main etchant in the silicon etching process. The reaction between silicon and XeF2 is described as: »» 2 XeF2 + Si → 2 Xe + SiF4
What are the advantages of XeF2 etching?
XeF2 is highly selective to silicon with respect to aluminum, photoresist and silicon dioxide. It has a relatively high etch rate and does not require ion bombardment or external energy sources in order to etch silicon. Since it is isotropic, large structures can be undercut.
How do you get the vapor into the chamber?
memsstar is unique in using a carrier gas method to flow HF or XeF2 into the
process chamber. Called “Controlled Continuous Flow,” this technique is
the bedrock for the process performance.
What makes Controlled Continuous Flow unique?
Traditionally, this process has employed ‘pulsed’ process techniques that limit the process pressure and gas flow control during the process. At memsstar, we developed the Controlled Continuous Flow technique to allow for more flexible process capability in terms of process window, while achieving superior process performance.
memsstar’s XERICTM Silicon Etch
Used for dry processing, the XERIC Silicon Etch system utilises vapor phase dry release etching with xenon difluoride providing continuous flow design for the etch material. The patented memsstar XERIC sacrificial vapor release XeF2 process is highly selective to a range of materials including SiO2, Si3N4. This process means that large undercuts of structures can be performed with no degradation in etch rate and a wide range of films including all forms of silicon can be etched.
Find out more about the XERICTM Silicon Etch.
memsstar’s XERICTM Oxide Etch
This is a patented process which is compatible with a wide range of metals utilising vapor phase dry release with anhydrous hydrogen fluoride (HF) and has market leading selectivities towards Si3N4. The XERIC Oxide Etch HF sacrificial release system offers a variety of benefits to companies who are engaged in MEMS manufacturing and development.
Find out more about the XERICTM Oxide Etch