Etching Flashcards
plasma
- Ionized gas, specifically a self-contained part of the electrical discharge in gases featuring equal concentration of ions and electrons.
- Plasma contains electrically active species, but as a whole is electrically neutral
- Ions can be extracted from plasma to play variety of important functions in semiconductor processing.
plasma etching
- Dry etching in which semiconductor wafer is immersed in plasma containing etching species
- Chemical etching reaction is taking place at the same rate in any direction, i.e. etching is isotropic; can be very selective.
- Used in those applications in which directionality (anisotropy) of etching in not required, e.g. in resist stripping.
dry etching
- Etching process carried out in the gas-phase.
- Can be either purely chemical (plasma etching), purely physical (ion milling) or combination of both (Reactive Ion Etching, RIE).
etching
- Subtractive process in the course of which a solid is either dissolved in liquid chemicals (wet etching) or converted into gaseous compound (dry etching)
- One among key processes in semiconductor manufacturing.
Reactive Ion Etching, RIE
- Variation of plasma etching in which during etching semiconductor wafer is placed on the RF powered electrode.
- Wafer takes on potential which accelerates etching species extracted from plasma toward the etched surface.
- Chemical etching reaction is preferentially taking place in the direction normal to the surface, i.e. etching is more anisotropic than in plasma etching but is less selective.
- Leaves etched surface damaged; the most common etching mode in semiconductor manufacturing.
Deep reactive-ion etching (DRIE)
- A highly anisotropic etch process used to create deep penetration, steep-sided holes and trenches in wafers/substrates, typically with high aspect ratios . It was developed for microelectromechanical systems (MEMS), which require these features.
- Bosch process is the only recognized production technique.
Bosch process
- Technology used for DRIE
- Pulsed or time-multiplexed etching
- Alternates repeatedly between two modes to achieve nearly vertical structures.
1) A standard, nearly isotropic plasma etch. The plasma contains some ions, which attack the wafer from a nearly vertical direction. Sulfur hexafluoride [SF6] is often used for silicon.
2) Deposition of a chemically inert passivation layer. (For instance, C4F8 (Octafluorocyclobutane) source gas yields a substance similar to Teflon.)
- Each phase lasts for several seconds.
- The passivation layer protects the entire substrate from further chemical attack and prevents further etching. However, during the etching phase, the directional ions that bombard the substrate attack the passivation layer at the bottom of the trench (but not along the sides). They collide with it and sputter it off, exposing the substrate to the chemical etchant.
- Results in a large number of very small isotropic etch steps taking place only at the bottom of the etched pits.
ICP etching
Inductively Coupled Plasma
aka RIE
Tornado ICP
- ICP coil with three dimensional structures. The three dimensional Tornado ICP® coil enables the formation of highly uniform plasma.
Isotropic Etching
- (e.g. wet etching or chemical etching) is a method commonly used in semi-conductor technology to remove material from a substrate via a chemical process using an etchant substance.
- The etchant may be a corrosive liquid or a chemically active ionized gas, known as a plasma.
- Unlike dry etching, isotropic etching does not etch in a single direction, but rather etches horizontally as well as vertically into the surface of the substrate.
- The horizontal etching may result in undercutting of patterned areas and significant changes in device characteristics. Isotropic etching may occur unavoidably, or it may be desirable for process reasons.
- Cannot create the small, high aspect ratio structures that are required by modern integrated circuits (e.g. gate, tungsten contact and via holes, silicon trenches for STI).
Selectivity
- This is a measure of the difference in etch rate of the target film relative to the photoresist mask etch rate or the etch rate of the substrate under the target film. It is typically expressed as the ratio of the target film etch rate to the mask or substrate etch rate.
- Higher selectivity is always desirable.
- High selectivity in an etch process will help avoid unwanted etching of materials or structures other than the target film, which may result in unacceptable degradation in the electrical performance of the final circuit structures.
etching rate
- Etching Rate = Depth/Time, Time = Depth/Etching Rate, Depth = Time/Etching Rate
- For example, if the scribe exposed-area ratio for an 8 inch wafer is 30%, the scribe width is 30 nm, and the wafer thickness is 150 nm, then the etching rate will be about 16 nm/min., and you can dice one wafer in ten minutes.
anisotropy
- Property of being directionally dependent
- It can be defined as a difference, when measured along different axes, in a material’s physical or mechanical properties (absorbance, refractive index, conductivity, tensile strength, etc.)
- An example of anisotropy is the light coming through a polarizer. Another is wood, which is easier to split along its grain than against it.
isotropy
Uniformity in all orientations
ion milling
- etching process
- process in which high energy ions impinging on the surface of the solid cause ejection (sputtering) of the host atoms
- highly anisotropic and non-selective etching.
