III.1 Passive and active oxidation of silicon
In literature, high temperature oxidation of silicon has been studied on dense silicon wafer materials [Wag58, GAB66, LM62, GCG01, GJ01]. Two kinds of oxidation mechanisms, active or passive, can occur depending on the temperature and partial pressures of oxidizing species. Under inert atmosphere, the oxidizing species are dioxygen, O2(g), while under hydrogenated atmosphere the stable oxidizing species are water molecules, H2O(g) (Table III.1).
i. At low temperature and high oxygen pressure, passive oxidation of silicon occurs, which means that the formation of solid silica, SiO2(s), is promoted according to reactions (R1) or (R3) (equilibrium constant K1 or K3) depending on the atmosphere ii. At high temperature and low oxygen pressure, active oxidation occurs. It
corresponds to the formation of gaseous silicon monoxide, SiO(g), according to reactions (R2) or (R4) (equilibrium constant K2 or K4) depending on the atmosphere
Gas flow H2 atmosphere Inert atmosphere
Mechanisms and oxidation type (i.) and (ii.)
Si(s) + 2H2O(g) = SiO2(s) + 2H2(g)
(R1) passive
Si(s) + H2O(g) = SiO(g) + H2(g)
(R2) active
Si(s) + O2(g) = SiO2(s)
(R3) passive
Si(s) + ½ O2(g) = SiO(g)
(R4) active
Transition type Active – Passive (iii.) Si(s) + SiO2(s) = 2SiO(g) (R5)
12 5 R
SiO
5 P K
P (III.1)
Si(s) + SiO2(s) = 2SiO(g) (R5)
12 5 R
SiO
5 P K
P (III.1)
Passive – Active (iv.) SiO2(s) + H2(g) = SiO(g) + H2O(g) (R6)
6 1
O H H R
SiO 2 2
6 P P P K
P (III.2)
SiO2(s) = SiO(g) + ½ O2(g) (R7)
7 12 O 32 R
SiO 2
7 P P K
P (III.3)
Table III.1: Passive and active oxidation reactions of silicon under hydrogenated and inert atmospheres. Equilibrium constant are estimated from Malcolm and Chase Thermochemical data [MC98]. P° (~1atm) corresponds to the standard gas pressure.
Wagner [Wag58] was the first to predict the conditions of active and passive oxidation, under an inert gas flow, by considering two distinct cases (Table III.1):
iii. The first one in which the silicon surface is initially free from its oxide.
iv. The second one in which the silicon is initially covered with a continuous layer of silica.
In the first case (iii.), the competition between the formation of solid silica (R3) and gaseous silicon monoxide (R4) is discussed. The combination of both reactions gives reaction (R5) of which equilibrium partial pressure, PSiOR5, determines the transition between active and passive oxidation (Table III.1). As long as the effective partial pressure of silicon monoxide,PSiO, remains lower than the equilibrium partial pressure, PSiOR5 (Equation (III.1)), the silicon surface remains bare. But, once this partial pressure exceeds PSiOR5 (Equation (III.1)), a silica layer starts to develop.
In the second case (iv.), a protective silica layer perfectly covers the silicon surface. Silica cannot be reduced by silicon according to reaction (R5) in that case, since silicon monoxide cannot leave the Si(s)-SiO2(s) interface. Wagner considers that silica reduction occurs through reaction (R7). In the same way as discussed above, PSiOR7 (Equation (III.3)) determines the transition between passive and active oxidation.
Thermogravimetric experiments have been carried out by different authors to characterize the active to passive [GAB66] and passive to active [GCG01, GJ01] transitions for bulk silicon, but only under vacuum conditions. Actually, authors noticed that both transitions were controlled by reaction (R5) and that silica would not act as a diffusion barrier during the passive to active transition. Gulbransen and Janson [GJ01] assumed the presence of cracks in the SiO2(s) layer while Gelain et al. [GCG01] considered the formation of blisters by SiO(g) at the silicon-silica interface which cause the rupture of the silica layer. Healing of the silica layer by reaction (R1) is no longer possible after the transition and the system is shifted to the active oxidation state where vacuum conditions were thought to allow a rapid removal of the oxide.
The thermochemical approach of Wagner is extended to a stagnant hydrogenated atmosphere in Appendix A. Reactions (R5) and (R6) are considered as references to determine the temperatures of active to passive and passive to active transitions, TAWP and TPWA, respectively. During thermogravimetric analysis (TGA), the transition can be identified when the mass loss rate is nil as the mass flux of water equals that of silicon monoxide. The active to passive and passive to active transition temperatures are then called, TAWmP and TPWmA. Assuming a steady-state diffusion of oxidizing species in a furnace tube, these temperatures
can be determined in terms of a surrounding water vapor partial pressure, HO P 2 , in Equations (III.4) and (III.5) respectively, where Mj and Dj are respectively the molar mass and the molecular diffusion coefficient of the species j (Appendix A).
12 Wm
A P mol 5
O H O
mol SiO WR SiO
O H
2 5
2 P K T
D M
D
P M (III.4)
12 Wm
P A 6 12 12 H 12 mol
O H O
mol SiO WR SiO
O
H 2
2 6
2 2 P P K T
D M
D
P M (III.5)
In Figure III.1, the expected TAWmP and TPWmA are plotted as a function of the surrounding water vapor pressure. In the next section, these will be compared with TGA measurements as a reference for the elucidation of silicon powder oxidation mechanism.
Figure III.1: Expected active to passive (TAWmP, bare silicon surface, full line) and passive to active (TPWmA, silicon surface covered with silica, dashed line) transition temperatures as a function of the surrounding water vapor pressure from Wagner’s approach (Appendix A). Measured passive to active, TP*mA, as well as active to passive, TA*mP, transition temperatures and surrounding water vapor pressure (dot).
800 900 1000 1100 1200 1300 1400 1500
1E-3 0.01 0.1 1 10 100 1000 10000
T*mA P=T*mP A TWmP A=f( PWR
6
H2O )
Active oxidation
SiO2 PH2O (Pa)
Temperature (°C)
Si Si
Passive oxidation
TW
m A P
=f( P
WR5
H2O )
III.2 Elucidation of silicon powder oxidation mechanisms III.2.1 Experimental approach
Under atmospheric conditions, the water vapor pressure is largely higher than 5
2
R W
O
PH . On initially bare silicon surfaces, the formation of a native silica layer is favored until the silica layer reaches several angstroms and precludes the diffusion of oxidizing species towards the Si(s)-SiO2(s) interface. On single crystal wafers, layer-by-layer growth occurs from 0.2 to 1 nm in approximately 10 h time exposure and requires the coexistence of oxygen and water [MOH+90]. IGA measurements of the oxygen powder content allows to estimate the silica layer thickness,
SiO2
e , which is of the same order of magnitude (see section II.1.3 and Table II.1).
The silica mass fraction is estimated in Equation (III.6) depending on the specific surface area, SSABET (or the corresponding equivalent spherical particle size, 2a), the silica layer thickness,
SiO2
e , and the theoretical densities of silica and silicon,
SiO2
TD and