Page 141 - Sami Franssila Introduction to Microfabrication

120 Introduction to Microfabrication

11.1 WET ETCHING advantages over tanks. Single-wafer tools are akin to
photoresist spinners, and in a sense, they are spray tools
Wet etching mechanisms fall into two major categories:
too. However, processing acts on the wafer topside only.
The heating of wet process tanks uniformly is no easy
metal etching:
task, because highly reactive and corrosive chemicals
◦
are used at high temperatures (e.g., 180 C boiling
electron transfer Me (s) −→ Me n+ (aq) + ne − ◦
nitric acid to etch nitride, or 120 C peroxo sulphuric
insulator etching: acid for cleaning, known as Piranha). The materials
of the tanks and heaters must be compatible with the
acid–base reaction SiO 2 + 6HF −→ process: in chemical, thermal and mechanical respects.

Teflon and quartz are often used in the most demanding
H 2 SiOF 6 (aq) + 2H 2 O
applications, but both are expensive materials and
The rate limiting steps in etching are similar to those difficult to machine. Polypropylene is used for less
encountered in CVD (Chapter 5): critical applications, while stainless steel is the material
for solvent tanks.
1. The surface reaction is slow, and it determines Temperature uniformity depends on stirring and
the rate. convective heat transfer. This is not trivial because
2. The surface reaction is fast, and rate is determined by stirring can affect the etch process in other ways too: it
etchant availability (transport of reactant by diffusion can enhance reactant supply, reaction product removal
and convection). or heat removal from an exothermic reaction.
Heating will result in higher etch rates, but there are
Surface reaction–limited processes exhibit activation practical limitations: resist (or other masking material)
energies of 30 to 90 kJ/mol. The rate increases with
increasing etchant concentration and it is insensitive to Table 11.1 Wet etchants for photoresist masked etching
stirring. Crystal planes can etch differently in surface
◦
reaction–limited etching. Aluminum etching in H 3 PO 4 SiO 2 NH 4 F:HF (7:1) BHF, 35 C
is surface reaction–limited: Al 2 O 3 dissolution is the SiO 2 NH 4 F:CH 3 COOH:C 2 H 6 O 2 (ethylene
rate-determining step, with 54 kJ/mol activation energy. glycol):H 2 O (14:32:4:50)
Transport-controlled reactions are characterized by poly-Si HF:HNO 3 :H 2 O (6:10:40)
activation energies of 4 to 25 kJ/mol. Their rate increases Al H 3 PO 4 :HNO 3 :H 2 O (80:4:16),
with agitation and stirring because more reactant is water can be changed to acetic acid
being brought to the vicinity of the surface. Furthermore, Mo H 3 PO 4 :HNO 3 :H 2 O (80:4:16)
all crystal planes etch at the same rate, which is W, TiW H 2 O 2 :H 2 O (1:1)
Cr Ce(NH 4 )NO 3 : HNO 3 :H 2 O (1:1:1)
natural because the reaction is not surface-limited.
Cu HNO 3 :H 2 O (1:1)
Silicon etching in a HF:HNO 3 mixture is limited by
Ni HNO 3 :CH 3 COOH:H 2 SO 4 (5:5:2)
HF diffusion through the product layer. The activation
Ti HF:H 2 O 2
energy is 17 kJ/mol.
Au KI:I 2 :H 2 O; KCN:H 2 O
11.1.1 Wet etching tools
Table 11.2 Wet etchants for other applications
Wet processing comes in three major variants: tank (bath),
spray tool and single-wafer processor. The tank is, for SiO 2, PSG HF (49%) sacrificial layer removal
example, a quartz vessel with heating and temperature (>1 µm/min)
control. It is filled with water and chemicals and the SiO 2 DHF, dilute HF, usually 1%, for removing
wafers are immersed in liquid for the required time, and native oxide (ca. 10 nm/min)
then transferred to similar tanks for rinsing. Spray tools <Si> KOH (10–50%) anisotropic crystal
handle a cassette (or cassettes) but instead of immersion, plane-dependent etch ◦
liquid is sprayed from stationary nozzles on rotating Nitride H 3 PO 4 boiling at 160 – 180 C, CVD oxide
mask
wafer cassette(s). After the first spraying, the process Si HNO 3 :HF:CH 3 COOH various compositions,
continues with either another chemical or DI-water spray rate > 10 µm/min possible
and nitrogen drying in the same vessel. Fresh mixing Pt, Au HNO 3 :HCl (1:3) ‘aqua regia’
of chemicals and lower liquid volumes are spray tool