NBTI
Encyclopedia
Negative bias temperature instability (NBTI) is a key reliability issue in MOSFET
s. It is of immediate concern in p-channel MOS
devices, since they almost always operate with negative gate-to-source voltage; however, the very same mechanism affects also nMOS transistors when biased in the accumulation regime, i.e. with a negative bias applied to the gate too. NBTI manifests as an increase in the threshold voltage
and consequent decrease in drain current and transconductance
. The degradation exhibits logarithmic dependence on time.
In the sub-micrometer devices nitrogen
is incorporated into the silicon gate oxide
to reduce the gate leakage current density and prevent the boron
penetration. However, incorporating nitrogen enhances NBTI. For new technologies (32 nm and shorter nominal channel lengths), high-K metal gate stacks are used as an alternative to improve the gate current density for a given equivalent oxide thickness (EOT). Even with the introduction of new materials like hafnium oxydes, NBTI remains.
It is possible that the interfacial layer composed of nitrided silicon dioxide is responsible for those instabilities. This interfacial layer results from the spontaneous oxidation of the silicon substrate when the HK is deposited. To limit this oxidation, the silicon interface is saturated with N resulting in a very thin and nitrided oxide layer.
It is commonly accepted that two kinds of trap contribute to NBTI:
The existence of two coexisting mechanisms created a large controversy, with the main controversial point being about the recoverable aspect of interface traps. Some author suggested that only interface traps were generated and recovered; today this hypothesys is ruled out. The situation is clearer but not completely solved. Some authors suggest that interface traps generation is responsible for hole trapping in the bulk of dielectrics. A tight coupling between two mechanism may exist but nothing is demonstrated clearly.
With the introduction of High K Metal gates, a new degradation mechanism appeared. The PBTI for Positive Bias Temperature Instabilities affects nMOS transistor when positively biased. In this particular case, no interface states are generated and 100% of the Vth degradation may be recovered. Those results suggest that there is no need to have interface state generation to trapped carrier in the bulk of the dielectric.
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
s. It is of immediate concern in p-channel MOS
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
devices, since they almost always operate with negative gate-to-source voltage; however, the very same mechanism affects also nMOS transistors when biased in the accumulation regime, i.e. with a negative bias applied to the gate too. NBTI manifests as an increase in the threshold voltage
Threshold voltage
The threshold voltage of a MOSFET is usually defined as the gate voltage where an inversion layer forms at the interface between the insulating layer and the substrate of the transistor. The purpose of the inversion layer's forming is to allow the flow of electrons through the gate-source junction...
and consequent decrease in drain current and transconductance
Transconductance
Transconductance, also known as mutual conductance, is a property of certain electronic components. Conductance is the reciprocal of resistance; transconductance, meanwhile, is the ratio of the current change at the output port to the voltage change at the input port. It is written as gm...
. The degradation exhibits logarithmic dependence on time.
In the sub-micrometer devices nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
is incorporated into the silicon gate oxide
Gate oxide
The gate oxide is the dielectric layer that separates the gate terminal of a MOSFET from the underlying source and drain terminals as well as the conductive channel that connects source and drain when the transistor is turned on. Gate oxide is formed by oxidizing the silicon of the channel to form...
to reduce the gate leakage current density and prevent the boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
penetration. However, incorporating nitrogen enhances NBTI. For new technologies (32 nm and shorter nominal channel lengths), high-K metal gate stacks are used as an alternative to improve the gate current density for a given equivalent oxide thickness (EOT). Even with the introduction of new materials like hafnium oxydes, NBTI remains.
It is possible that the interfacial layer composed of nitrided silicon dioxide is responsible for those instabilities. This interfacial layer results from the spontaneous oxidation of the silicon substrate when the HK is deposited. To limit this oxidation, the silicon interface is saturated with N resulting in a very thin and nitrided oxide layer.
It is commonly accepted that two kinds of trap contribute to NBTI:
- first, interface traps are generated. Those traps cannot be recovered over a reasonable time of operation. Some authors refer to them as permanent traps. Those traps are the same as the one created by Channel Hot Carrier. In the case of NBTI, it is believed that the electric field is able to break Si-H bonds located at the Silicon-oxide interface. H is released in the substrate where it migrates. The remaining dangling bond Si- (Pb center) contribute to the threshold voltage degradation.
- on top of the interface states generation some preexisting traps located in the bulk of the dielectric (and supposedly nitrogen related), are filled with holes coming from the channel of pMOS. Those traps can be emptied when the stress voltage is removed. This Vth degradation can be recovered over time.
The existence of two coexisting mechanisms created a large controversy, with the main controversial point being about the recoverable aspect of interface traps. Some author suggested that only interface traps were generated and recovered; today this hypothesys is ruled out. The situation is clearer but not completely solved. Some authors suggest that interface traps generation is responsible for hole trapping in the bulk of dielectrics. A tight coupling between two mechanism may exist but nothing is demonstrated clearly.
With the introduction of High K Metal gates, a new degradation mechanism appeared. The PBTI for Positive Bias Temperature Instabilities affects nMOS transistor when positively biased. In this particular case, no interface states are generated and 100% of the Vth degradation may be recovered. Those results suggest that there is no need to have interface state generation to trapped carrier in the bulk of the dielectric.