Reactive ion etching
|Reactive ion etching|
|Equipment||List of RIE equipment|
Reactive ion etching (RIE) is a high resolution mechanism for etching materials using reactive gas discharges. It is a highly controllable process that can process a wide variety of materials, including semiconductors, dielectrics and some metals. One major advantage to RIE over other forms of etching is that the process can be designed to be highly anisotropic, allowing for much finer resolution and higher aspect ratios. For a detailed overview of RIE, please review the technology workshop.
Below is a general description of the RIE equipment at the LNF. For a complete list, please see list of RIE equipment.
The P5000 is a 3 chamber tool designed for production etching. Chambers A and B are configured for SiO2 and Si3N4 etching, where as chamber C is configured for polysilicon and amorphous silicon etching. Chambers B and C are restricted to CMOS clean devices where as chamber A is open to semi-clean devices.
STS APS DGRIE
The glass etcher excels at deep etching of fused silica but it also has a nearly vertical SiO2 etch. It's capable of very high bias powers enabling it to process difficult to etch materials.
LAM 9400 SE
The LAM 9400 SE is an ICP etcher configured with a wide range of gas chemistries. It is mainly used to etch polysilicon but can also etch SiO2, Si3N4, compound semiconductors, some metals, and organic materials.
Oxford Plasmalab System 100
The oxford is another ICP etcher that also has a cryogenic chuck. By cooling the sample down to -150°C nearly vertical etches are possible in certain materials.
The Plasmatherm is configured with a variety of gases so that it can etch a wide array of materials. Most recipes tend to have slow etch rates on the order of 200 Å/min which is ideal for very thin films. The tool also has few material restrictions to allow it to process as many things as possible.
Deep reactive ion etching
Deep reactive ion etching (DRIE), while often referring specifically to the Bosch process, generally is any RIE used to etch high aspect ratio (> 10:1) features. This may be simply a longer, well controlled RIE etch, or may use a specific process such as cryogenic etching or the Bosch process. For all DRIE equipment at the LNF, please refer to the list of DRIE equipment.
The STS Pegasus is a tool that utilizes the Bosch process for high aspect ratio etching of silicon. It uses SF6 for the etch step and C4F8 for passivation. It can achieve etch rates of up to 20 μm/min and aspect ratios up to 50:1.
STS Glass Etcher
The STS Glass Etcher is a DGRIE tool for high aspect ratio etching of silicon dioxide, glass, and fused silica.
The Oxford Plasmalab is an ICP RIE system capable of cryogenic etching. While not configured to etch silicon, it can etch compound semiconductors and perform a cryogenic etch of polymers.
Method of operation
Samples are first masked by one of many patterning processes. They are then placed into a vacuum chamber. Gases are introduced into the chamber and then activated by RF or microwave power to create a plasma consisting of a wide variety of reactive species, ions, and electrons. The reactive species are chosen for their ability to react chemically with the material being etched. A negative DC bias is induced at the substrate by the free electrons which accelerates the ions towards the sample surface. The energy imparted by these ions reaching the surface greatly enhances the effectiveness of the chemical reaction and provides directionality to the etch. Generally some form of passivating component is incorporated such that the etch proceeds only where energetic ions strike the surface.
Many of the same parameters used in plasma etching apply to RIE, including pressure, gas composition, and generator power. Of particular importance is the plasma generation method (commonly a parallel plate or ICP configuration), as they have different advantages depending on the material being etched.
A critical parameter specific to RIE is the DC bias applied to the sample, which directly affects the physicality of the etch. Many materials (e.g. SiO2) require high activation energy to react with the gases in the reactor. Other materials (silicon, in particular) may be etched using a passivating component which must be removed on the surface being etched. Still other materials have little to no reactivity and must be physically removed from the surface.
In most reactors, DC bias is not controlled directly, but will depend on the conductance of the plasma and the power applied to the sample.