ICP RIE
Overview
From Handbook of Silicon Based MEMS Materials and Technologies (Second Edition):
Reactive ion etching (RIE), also known as plasma etching or dry etching, and its extension deep reactive ion etching (DRIE) are processes that combine physical and chemicals effects to remove material from the wafer surface. Simultaneous ion bombardment-enhanced desorption, ion induced damage, spontaneous chemical etching, radical generation, film deposition and other processes contribute to etching performance. The etch rate of the RIE process is substantially higher than the etch rate of purely physical processes. Ions, however, are present in very minor quantities (0.001% of all species in the chamber) so the name RIE is a misnomer, but it is widely used. Compared to wet etching, the most important feature of RIE is the capability of directional (anisotropic) etching without relying on crystal planes of the material.
Plasma etch process variables (such as radio frequency (RF) power, pressure, etch gas flow rates, and temperature) can be varied to optimize etch rate, selectivity, sidewall angle or other responses. DRIE is an extension of RIE that enables high-rate etching of deep and narrow structures. Typically DRIE also offers better selectivity and process controllability than RIE. DRIE reactors differ from traditional RIE with their at least two RF generators for independent control of plasma generation and ion bombardment, lower-pressure range, fast mass flow controllers (MFCs) or fast-acting valves in pulsed processes and a wider temperature range from liquid nitrogen cooled platen (typically −120°C) to up to 400°C in compound semiconductor etching.
Etch Chemistries
Etchant phase, gas or liquid, has been used as a dividing factor: wet etching in liquids versus dry etching in gaseous environment, usually in vacuum [1]. Wet etching depends on the solubility of etch products, and dry etching on their volatility:
solid+gaseousetchant⇒volatileproducts
3Si(s)+2SF6(g)+2O2(g)⇒3SiF4(g)+2SO2(g)
Si(s)+2Cl2(g)⇒SiCl44(g)
Many halides, fluorides, chlorides, bromides and iodides are volatile, as are oxyhalides [2], [3]. Typical etch products include SiF4, SiCl4, NF3, WF6, AlCl3 when silicon or metals are etched with SF6, CF4, Cl2, BCl3, SiCl4. High boiling point (low volatility, or non-volatile) halides include AlF3 and CrF2 which necessitate chlorine plasma etching for etching aluminum and chromium. Due to the fact that volatility in vacuum under ion bombardment is not an unambiguous property, etchability of a material under certain conditions is always a matter of engineering judgment. While copper chlorides are definitely non-volatile, and copper cannot be etched by Cl2 gas, aluminum–copper alloys with up to 4% copper can be etched in Cl2. This is because ion bombardment can assist in desorption of loosely bound specie which would not otherwise be desorbed. Elevated temperature can be used to enhance desorption, but the use of photoresist mask often sets limits to this.
Silicon and its compounds can be etched in fluorine, chlorine, or bromine plasmas, with SiF4, SiCl4, or SiBr4 as the main reaction products [4], [5]. Toxicity and tighter safety requirements make chlorine and bromine gases less popular, but they are widespread in integrated circuit fabrication for the better profile and linewidth control in shallow etches. For DRIE applications there is an additional drawback: chlorine (and bromine) processes are inherently much slower than fluorine processes.
Oxide and nitride plasma etching is customarily done with CHF3, C2F6, C3F8 or c-C4F8. The ratio of carbon-to-fluorine is important to control selectivity to silicon [6]. High fluorine content results in silicon etching, while a more balanced carbon–fluorine ratio enables high selectivity against silicon. Oxide and nitride can be etched in SF6 and CF4, but such chemistries exhibit poor selectivity against silicon and 1:1 to 3:1 nitride to oxide selectivity.
In addition to etchant gases, additive gases are used: oxygen, hydrogen, argon, helium, nitrogen. They modify plasma chemistry and physics, thermal characteristics and surface chemistry. In SF6/O2 and CF4/O2 plasmas oxygen scavenging controls the amount of free fluorine [7]. Oxygen is an active ingredient in many etching processes, for example, photoresist stripping where CO2 and H2O are generated as photoresist is etched. Oxygen is also important for anisotropy mechanism: deposition of SiOxFy films is essential for sidewall passivation in silicon etching in SF6/O2 plasmas. Diamond (and diamond-like materials a-C:H, DLC) and practically all polymer etching processes use oxygen, possibly with argon or fluorine-containing gases.
Etch gases CHF3 or C4F8 lead to deposition of fluoropolymer films of the type (CF2)n which passivate sidewalls and minimize lateral etching. Etch rate and anisotropy are manifestations of balancing etching and deposition reactions properly as in the Bosch DRIE process. By tailoring the balance between erosion and deposition processes, RIE can produce perfectly vertical sidewalls, and the sidewall angle can be tailored over a wide range, from fully isotropic to positively tapered to negatively tapered (a.k.a., retrograde).
Helium (He) Backside Cooling (HBC)
The sample chuck provides He backside cooling to the wafer. This keeps the wafer cool enough during processing to prevent resist burning and improve resist selectivity. The wafer to chuck interface uses an O-ring like seal called a lip seal to prevent He from leaking into the chamber and changing the etch rate. The quality of backside cooling is estimated through the helium leak rate, which should be about 15 mtorr/min with this tool.