Page 164 - Sami Franssila Introduction to Microfabrication

13

Thermal Oxidation

Silicon dioxide, SiO 2 , is probably a more important doped material, and the higher the oxygen pressure, the
material in silicon technology than silicon itself: while higher the rate.
GaAs and Ge have higher electron mobilities than Thin oxides, such as CMOS gate oxides, Flash mem-
silicon, and enable potentially faster devices; they do ory tunnel oxides and dynamic random access memory
not have native oxides that protect their surfaces, and (DRAM) capacitor oxides are of the order of 1 to 20 nm.
neither do stable, thick oxides exist. Silicon dioxide has These oxides are grown in dry oxygen at 850 to 950 C.
◦
functions as capacitor dielectric and isolation material, in Thin oxides also have many auxiliary and sacrificial
which case the oxide forms a part of the finished device. roles: a thin oxide under nitride relieves stresses caused
But oxides are used intermittently many times during by the nitride film. Thicker oxides are used for device
silicon processing as a masking material for diffusion isolation and as masking layers for ion implantation,
or etching, and as a cleaning method to reclaim perfect diffusion and etching steps. They are usually 100 to
silicon surface. 1000 nm thick, and grown by wet oxidation.

13.1 OXIDATION PROCESS 13.2 DEAL–GROVE OXIDATION MODEL
Silicon is easily oxidized: a native oxide of nanome- A model for oxide growth has been put forth by
tre thickness grows on the silicon surface in a couple Deal and Grove. It is a phenomenological macroscopic
of hours or days, depending on surface conditions, and model that does not assume anything about the atomistic
similar thin oxides form easily in oxygen plasma or in mechanisms of oxidation. Oxygen diffusion through the
oxidizing wet treatment. These oxides are, however, lim- growing oxide and chemical reaction at the silicon/oxide
ited in their thickness and they are not stoichiometric interface are modelled with the classical Fick diffusion
SiO 2 . Deposited CVD oxides are used in some appli- equation and chemical rate equation (Figure 13.2).
cations where low temperatures are absolutely neces- Oxidation is modelled as if the boundaries were
sary, but superior silicon dioxides are grown in 800 to stationary (which is a reasonable assumption because
◦
1200 C, Figure 13.1. Two basic schemes are used: wet oxidation is slow). The diffusion equation for oxygen is
(aka. steam) and dry oxidation. 2 2
0 = D(d C/dz ) (13.1)
Wet oxidation: Si (s) + 2H 2 O (g) −→
where C is the oxygen molar concentration (in units
SiO 2 (s) + 2H 2 (g) mol/m ), subject to the boundary conditions
3
Dry oxidation: Si (s) + O 2 (g) −→ SiO 2 (s)
C = C s z = 0 (13.2)
Thermal oxidation is a slow process: dry oxidation at at the SiO 2 surface and
◦
900 C for 1 h produces ca. 20 nm thick oxide and wet
oxidation for 1 h produces ca. 170 nm. Exact values −D(dC/dz) = R z = Z (13.3)
are dependent on silicon crystal orientation: oxidation
rate of <111> is somewhat higher than that of <100> at the SiO 2 /Si interface, where R is the reaction rate at
2
silicon; highly doped silicon oxidizes faster than lightly the interface (in units mol/m s).

Introduction to Microfabrication Sami Franssila
 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)