91
Metal-Oxide-Semiconductor System
(SiO x ) at the boundary between the silicon and SiO x layer as shown in
Figure 3.5. Generally, Q f is positive and appears to arise from incomplete
silicon-to-silicon bonds and depends on the oxidation ambient, temperature and annealing conditions, and silicon orientation. Since the density of
atoms at the surface of a silicon crystal depends on the crystal orientation,
Q f is higher in <111> silicon than in <100> wafers. However, it is independent of the doping type and concentration in the silicon, oxide thickness,
and oxidation time. Q f can be minimized by annealing the oxide in an inert
ambient, such as Argon at a temperature in excess of 900°C. A typical value of
Q f for a carefully treated Si/SiO 2 system is about 1 × 10 10 cm –2 for the <100>
surface. Because of the low values of Q it and Q f , the <100> orientation is
preferred for silicon MOSFETs (metal-oxide-semiconductor field-effect
transistors).
3.2.2.3 Oxide-Trapped Charge
The oxide-trapped charge density, Q ot , is associated with defects in SiO 2 .
Q ot is located in traps distributed throughout the oxide layer. The oxide
traps are usually electrically neutral and are charged by introducing electrons and holes into the oxide through ionizing radiation such as implanted
ions, X-rays, and electron beams. The magnitude of Q ot depends on the
amount of radiation dose and energy and the field across the oxide during irradiation. Like Q it , these charges could be positive (trapped holes) or
negative (trapped electrons). Q ot resembles Q f in that its magnitude is not a
function of silicon surface potential and there is no capacitance associated
with it.
3.2.2.4 Mobile Ionic Charge
The mobile ionic charge density, Q m , is due to sodium (Na + ) or other alkali
ions that get into the oxide during cleaning, processing, and handling of
MOS devices. These ions move very slowly within the oxide; their transport depends strongly on the applied electric field (~1 MV cm –1 ) and temperature (30°C–400°C). Positive voltages push the ions toward the Si/SiO 2
interface while the negative voltages draw them toward the gate. A current is observed in the external circuit during ion drift. The drift of ions
changes the centroid of charge within the oxide layer, resulting in a shift
of the flat band voltage of MOS capacitor system and may cause an unexpected
device failure. Different approaches are used to reduce mobile ion contamination in gate oxide and mitigate the risk of mobile ionic induced device
failure [1,5].
The earlier described oxide charges cause an additional band bending
at the silicon surface of an MOS capacitor system and shift the value of V fb
caused by Φ ms as described in the following section.
Metal-Oxide-Semiconductor System
(SiO x ) at the boundary between the silicon and SiO x layer as shown in
Figure 3.5. Generally, Q f is positive and appears to arise from incomplete
silicon-to-silicon bonds and depends on the oxidation ambient, temperature and annealing conditions, and silicon orientation. Since the density of
atoms at the surface of a silicon crystal depends on the crystal orientation,
Q f is higher in <111> silicon than in <100> wafers. However, it is independent of the doping type and concentration in the silicon, oxide thickness,
and oxidation time. Q f can be minimized by annealing the oxide in an inert
ambient, such as Argon at a temperature in excess of 900°C. A typical value of
Q f for a carefully treated Si/SiO 2 system is about 1 × 10 10 cm –2 for the <100>
surface. Because of the low values of Q it and Q f , the <100> orientation is
preferred for silicon MOSFETs (metal-oxide-semiconductor field-effect
transistors).
3.2.2.3 Oxide-Trapped Charge
The oxide-trapped charge density, Q ot , is associated with defects in SiO 2 .
Q ot is located in traps distributed throughout the oxide layer. The oxide
traps are usually electrically neutral and are charged by introducing electrons and holes into the oxide through ionizing radiation such as implanted
ions, X-rays, and electron beams. The magnitude of Q ot depends on the
amount of radiation dose and energy and the field across the oxide during irradiation. Like Q it , these charges could be positive (trapped holes) or
negative (trapped electrons). Q ot resembles Q f in that its magnitude is not a
function of silicon surface potential and there is no capacitance associated
with it.
3.2.2.4 Mobile Ionic Charge
The mobile ionic charge density, Q m , is due to sodium (Na + ) or other alkali
ions that get into the oxide during cleaning, processing, and handling of
MOS devices. These ions move very slowly within the oxide; their transport depends strongly on the applied electric field (~1 MV cm –1 ) and temperature (30°C–400°C). Positive voltages push the ions toward the Si/SiO 2
interface while the negative voltages draw them toward the gate. A current is observed in the external circuit during ion drift. The drift of ions
changes the centroid of charge within the oxide layer, resulting in a shift
of the flat band voltage of MOS capacitor system and may cause an unexpected
device failure. Different approaches are used to reduce mobile ion contamination in gate oxide and mitigate the risk of mobile ionic induced device
failure [1,5].
The earlier described oxide charges cause an additional band bending
at the silicon surface of an MOS capacitor system and shift the value of V fb
caused by Φ ms as described in the following section.