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Etching (microfabrication)
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Etching is used in microfabrication to chemically remove layers from the surface of a wafer during manufacturing. Etching is a critically important process module, and every wafer undergoes many etching steps before it is complete.
For many etch steps, part of the wafer is protected from the etchant by a "masking" material which resists etching. In some cases, the masking material is photoresist which has been patterned using photolithography.
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Etching is used in microfabrication to chemically remove layers from the surface of a wafer during manufacturing. Etching is a critically important process module, and every wafer undergoes many etching steps before it is complete.
For many etch steps, part of the wafer is protected from the etchant by a "masking" material which resists etching. In some cases, the masking material is photoresist which has been patterned using photolithography. Other situations require a more durable mask, such as silicon nitride.
Figures of merit
If the etch is intended to make a cavity in a material, the depth of the cavity may be controlled approximately using the etching time and the known etch rate. More often, though, etching must entirely remove the top layer of a multilayer structure, without damaging the underlying or masking layers. The etching system's ability to do this depends on the ratio of etch rates in the two materials (selectivity).
Some etches undercut the masking layer and form cavities with sloping sidewalls. The distance of undercutting is called bias. Etchants with large bias are called isotropic, because they erode the substrate equally in all directions. Modern processes greatly prefer anisotropic etches, because they produce sharp, well-controlled features.
Etching media and technology
The two fundamental types of etchants are liquid-phase ("wet") and plasma-phase ("dry"). Each of these exists in several varieties.
Wet etching
The first etching processes used liquid-phase ("wet") etchants. The wafer can be immersed in a bath of etchant, which must be agitated to achieve good process control. For instance, buffered hydrofluoric acid (BHF) is used commonly to etch silicon dioxide over a silicon substrate.
Different specialised etchants can be used to characterise the surface etched.
Wet etchants are usually isotropic, which leads to large bias when etching thick films. They also require the disposal of large amounts of toxic waste. For these reasons, they are seldom used in state-of-the-art processes. However, the photographic developer used for photoresist resembles wet etching.
As an alternative to immersion, single wafer machines use the Bernoulli principle to employ a gas (usually, pure nitrogen) to cushion and protect one side of the wafer while etchant is applied to the other side. It can be done to either the front side or back side. The etch chemistry is dispensed on the top side when in the machine and the bottom side is not affected. This etch method is particularly effective just before "backend" processing (BEOL), where wafers are normally very much thinner after wafer backgrinding, and very sensitive to thermal or mechanical stress. Etching a thin layer of even a few micrometres will remove microcracks produced during backgrinding resulting in a the wafer having dramatically increased strength and flexibility without breaking.
Anisotropic wet etching
Some wet etchants etch crystalline materials at very different rates depending upon which crystal face is exposed. In single-crystal materials (e.g. silicon wafers), this effect can allow very high anisotropy, as shown in the figure.
Several anisotropic wet etchants are available for silicon. For instance, potassium hydroxide (KOH) can achieve selectivity of 400 between <100> and <111> planes. Another option is EDP (an aqueous solution of ethylene diamine and pyrocatechol), which also displays high selectivity for p-type doping. Neither of these etchants may be used on wafers that contain CMOS integrated circuits. Both of them etch aluminium, commonly used as a metallization (wiring) material. KOH introduces mobile potassium ions into silicon dioxide, and EDP is highly corrosive and carcinogenic. Tetramethylammonium hydroxide (TMAH) presents a safer alternative, although it has even worse selectivity between <100> and <111> planes in silicon than does EDP.
Plasma etching
Modern VLSI processes avoid wet etching, and use plasma etching instead. Plasma etchers can operate in several modes by adjusting the parameters of the plasma. Ordinary plasma etching operates between 0.1 and 5 torr. (This unit of pressure, commonly used in vacuum engineering, equals approximately 133.3 pascals.) The plasma produces energetic free radicals, neutrally charged, that react at the surface of the wafer. Since neutral particles attack the wafer from all angles, this process is isotropic.
The source gas for the plasma usually contains small molecules rich in chlorine or fluorine. For instance, carbon tetrachloride (CCl4) etches silicon and aluminium, and trifluoromethane etches silicon dioxide and silicon nitride. A plasma containing oxygen is used to oxidize ("ash") photoresist and facilitate its removal.
Ion milling, or sputter etching, uses lower pressures, often as low as 10-4 torr (10 mPa). It bombards the wafer with energetic ions of noble gases, often Ar+, which knock atoms from the substrate by transferring momentum. Because the etching is performed by ions, which approach the wafer approximately from one direction, this process is highly anisotropic. On the other hand, it tends to display poor selectivity. Reactive-ion etching (RIE) operates under conditions intermediate between sputter and plasma etching (between 10-3 and 10-1 torr). Deep reactive-ion etching (DRIE) modifies the RIE technique to produce deep, narrow features.
Common etch processes used in microfabrication
Etchants for common microfabrication materials | Material to be etched | Wet etchants | Plasma etchants |
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| Aluminium (Al) | 80% phosphoric acid (H3PO4) + 5% acetic acid
+ 5% nitric acid (HNO3) + 10% water (H2O) at 35–45 °C | Cl2, CCl4, SiCl4, BCl3 | | Indium tin oxide [ITO] (In2O3:SnO2) | Hydrochloric acid (HCl) + nitric acid (HNO3) + water (H2O) (1:0.1:1) at 40 °C | | | Chromium (Cr) |
- "Chrome etch": ceric ammonium nitrate ((NH4)2Ce(NO3)6) + nitric acid (HNO3)
- Hydrochloric acid (HCl)
| | | Gold (Au) | Aqua regia | | | Molybdenum (Mo) | | CF4 | | Organic residues and photoresist | Piranha etch: sulfuric acid (H2SO4) + hydrogen peroxide (H2O2) | O2 (ashing) | | Platinum (Pt) | Aqua regia | | | Silicon (Si) | Nitric acid (HNO3) + hydrofluoric acid (HF) | CF4, SF6, NF3 Cl2, CCl2F2 | | Silicon dioxide (SiO2) | Hydrofluoric acid (HF) Buffered oxide etch [BOE]: ammonium fluoride (NH4F) and hydrofluoric acid (HF) | CF4, SF6, NF3 | | Silicon nitride (Si3N4) | 85% Phosphoric acid (H3PO4) at 180 °C (Requires SiO2 etch mask) | CF4, SF6, NF3 | | Tantalum (Ta) | | CF4 | | Titanium (Ti) | Hydrofluoric acid (HF) | BCl3 | | Titanium nitride (TiN) | Nitric acid (HNO3) + hydrofluoric acid (HF) SC1 | | Tungsten (W) | | CF4SF6 | |
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