7.4 Interface states and oxide charges

Interface states and oxide charges play a crucial role in semiconductor devices, especially in MOS structures. These phenomena occur at the boundary between semiconductors and insulating oxide layers, significantly impacting electrical properties like threshold voltage and carrier mobility.

Understanding these states and charges is essential for optimizing device performance. From fast and slow interface states to fixed oxide charges and mobile ionic charges, each type affects device behavior differently. Engineers must consider these factors when designing and fabricating advanced semiconductor devices.

Types of interface states and charges

• Interface states and charges are crucial in understanding the behavior of semiconductor devices, particularly metal-oxide-semiconductor (MOS) structures
• These states and charges are located at or near the interface between the semiconductor (usually silicon) and the insulating oxide layer (typically silicon dioxide, SiO2)
• The presence of these states and charges can significantly influence the electrical properties of the device, such as threshold voltage, carrier mobility, and reliability

Fast and slow interface states

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• Fast interface states, also known as surface states, are electronic states that can quickly exchange charge with the semiconductor
• These states have time constants comparable to the operating frequencies of the device, typically in the range of nanoseconds to microseconds
• Fast interface states are usually attributed to structural defects or impurities at the Si-SiO2 interface
• They can capture or emit carriers, leading to changes in the device's electrical characteristics
• Slow interface states, on the other hand, have longer time constants and do not respond to high-frequency signals
• These states are often associated with deeper energy levels in the bandgap and can contribute to long-term instabilities in the device

Fixed oxide charges

• Fixed oxide charges are positive charges that are located within a few nanometers of the Si-SiO2 interface
• These charges are primarily due to structural defects in the oxide layer, such as oxygen vacancies or excess silicon atoms
• Fixed oxide charges are immobile and do not change their charge state under normal operating conditions
• The presence of fixed oxide charges can lead to a shift in the threshold voltage of the device, as they induce a negative charge in the semiconductor

Oxide trapped charges

• Oxide trapped charges are charges that are trapped in the bulk of the oxide layer, away from the Si-SiO2 interface
• These charges can be either positive or negative and are typically introduced during device fabrication or operation
• Oxide trapped charges can be generated by various mechanisms, such as ionizing radiation, hot carrier injection, or Fowler-Nordheim tunneling
• The trapping and detrapping of these charges can cause instabilities in the device characteristics, such as threshold voltage shifts and changes in the subthreshold slope

Mobile ionic charges

• Mobile ionic charges are positively charged ions, such as sodium (Na+) or potassium (K+), that can move within the oxide layer under the influence of an electric field
• These ions can originate from contamination during device fabrication or packaging materials
• The presence of mobile ionic charges can lead to long-term instabilities in the device, as they can drift towards or away from the Si-SiO2 interface depending on the applied bias
• Mobile ionic charges can cause hysteresis effects in the capacitance-voltage (C-V) characteristics of MOS devices

Origin and characteristics

Structural defects at Si-SiO2 interface

• The Si-SiO2 interface is not perfect and contains various structural defects that can give rise to interface states and charges
• These defects can include dangling bonds (unsatisfied atomic bonds), strained bonds, or atomic displacements
• The density and distribution of these defects depend on factors such as crystal orientation, oxide growth conditions, and post-oxidation treatments
• Structural defects at the interface can act as trapping centers for carriers, leading to the formation of interface states

Dangling bonds and impurities

• Dangling bonds are a common type of structural defect at the Si-SiO2 interface
• These bonds occur when silicon atoms at the interface have unsatisfied valence electrons, which can trap or release carriers
• Dangling bonds can be passivated by hydrogen atoms, reducing their impact on device performance
• Impurities, such as metal atoms or organic contaminants, can also be present at the interface and contribute to the formation of interface states and charges

Energy distribution in bandgap

• Interface states are distributed throughout the bandgap of the semiconductor, from the valence band edge to the conduction band edge
• The density of interface states is often expressed as a function of energy, known as the interface state density (Dit)
• The distribution of interface states in the bandgap can be non-uniform, with peaks or valleys at certain energy levels
• The energy distribution of interface states can influence the device's electrical properties, such as the threshold voltage and subthreshold slope

Capture and emission of carriers

• Interface states can capture or emit carriers (electrons or holes) from the semiconductor, depending on their energy level and the applied bias
• The capture and emission of carriers by interface states are governed by the Shockley-Read-Hall (SRH) recombination-generation process
• The capture and emission rates depend on factors such as the carrier concentration, the capture cross-section of the interface states, and the temperature
• The dynamic exchange of carriers between interface states and the semiconductor can lead to changes in the device's electrical characteristics, such as the capacitance and conductance

Charge neutrality level

• The charge neutrality level (CNL) is a fundamental concept in understanding the behavior of interface states
• The CNL represents the energy level at which the interface states change from being predominantly donor-like (positive when empty) to acceptor-like (negative when filled)
• The position of the CNL relative to the semiconductor's Fermi level determines the charge state of the interface states
• If the Fermi level is above the CNL, the interface states are more likely to be negatively charged, while if the Fermi level is below the CNL, the interface states are more likely to be positively charged
• The CNL concept is important for understanding the impact of interface states on the device's threshold voltage and other electrical properties

Impact on device performance

Threshold voltage shifts

• Interface states and charges can cause shifts in the threshold voltage of MOS devices
• Fixed oxide charges and interface states near the conduction or valence band edges can induce a net charge in the semiconductor, leading to a shift in the threshold voltage
• A positive shift in the threshold voltage can occur due to the presence of negative charges at the interface, while a negative shift can result from positive charges
• Threshold voltage shifts can affect the device's switching characteristics and may require compensation through process or design optimizations

Mobility degradation

• Interface states and charges can also degrade the mobility of carriers in the semiconductor channel
• Carriers moving in the channel can interact with the interface states through scattering mechanisms, such as Coulomb scattering or surface roughness scattering
• This interaction can reduce the effective mobility of the carriers, leading to a decrease in the device's current drive capability and transconductance
• Mobility degradation is particularly significant in devices with high interface state densities or in devices operating at low carrier densities (e.g., near the subthreshold regime)

Subthreshold slope degradation

• The subthreshold slope of a MOS device is a measure of how sharply the device turns on with increasing gate voltage in the subthreshold regime
• Interface states can degrade the subthreshold slope by contributing to the capacitance of the device in the subthreshold region
• The presence of interface states can lead to a stretching of the subthreshold slope, making it more difficult to turn the device off completely
• A degraded subthreshold slope can result in increased leakage current and reduced Ion/Ioff ratio, which is critical for low-power applications

Reliability issues

• Interface states and charges can also impact the long-term reliability of MOS devices
• The capture and emission of carriers by interface states can lead to charge trapping and detrapping, which can cause instabilities in the device's electrical characteristics over time
• Mobile ionic charges can drift under the influence of electric fields and temperature, leading to long-term threshold voltage shifts and other reliability issues
• The presence of interface states and charges can also enhance the sensitivity of the device to various stress mechanisms, such as hot carrier injection or bias temperature instability (BTI)
• Addressing reliability issues related to interface states and charges is crucial for ensuring the long-term performance and stability of MOS devices

Measurement techniques

Capacitance-voltage (C-V) methods

• Capacitance-voltage (C-V) methods are widely used to characterize interface states and charges in MOS devices
• C-V measurements involve applying a DC bias voltage across the MOS structure and measuring the capacitance as a function of the applied voltage
• Interface states can contribute to the capacitance of the device, leading to distortions in the C-V characteristics
• By analyzing the C-V curves, parameters such as the flatband voltage, threshold voltage, and interface state density can be extracted
• Various C-V techniques, such as high-frequency C-V, quasi-static C-V, and conductance methods, can provide complementary information about the interface states and charges

Charge pumping

• Charge pumping is a powerful technique for quantifying the interface state density in MOS devices
• In this method, a series of voltage pulses are applied to the gate of the device, causing the interface states to capture and emit carriers repeatedly
• The resulting substrate current, known as the charge pumping current, is proportional to the interface state density
• By varying the pulse amplitude, frequency, or rise/fall times, the energy distribution of the interface states can be probed
• Charge pumping is particularly useful for characterizing fast interface states that may not be easily detectable by other methods

Conductance methods

• Conductance methods are based on measuring the AC conductance of the MOS device as a function of frequency and applied bias
• Interface states can contribute to the conductance of the device through the capture and emission of carriers
• By analyzing the conductance spectra, the density and time constants of the interface states can be extracted
• Conductance methods are sensitive to interface states with time constants comparable to the measurement frequency, typically in the range of kHz to MHz
• These methods can provide information about the energy distribution and capture cross-sections of the interface states

Deep-level transient spectroscopy (DLTS)

• Deep-level transient spectroscopy (DLTS) is a technique used to characterize deep-level defects, including interface states, in semiconductors
• DLTS involves applying voltage pulses to the device and measuring the capacitance transients associated with the capture and emission of carriers by the defects
• By varying the temperature and pulse parameters, the energy levels, capture cross-sections, and densities of the defects can be determined
• DLTS is particularly useful for studying slow interface states and bulk traps that may not be easily detectable by other methods
• The technique can provide valuable information about the nature and origin of the defects, helping to optimize device fabrication and performance

Passivation and annealing

Hydrogen passivation

• Hydrogen passivation is a widely used technique to reduce the density of interface states in MOS devices
• During the passivation process, hydrogen atoms are introduced into the device, typically through a forming gas anneal (FGA) or a plasma hydrogenation step
• Hydrogen atoms can diffuse to the Si-SiO2 interface and bond with the dangling bonds, effectively passivating the interface states
• Hydrogen passivation can significantly reduce the interface state density, leading to improvements in device performance, such as reduced threshold voltage shifts and increased carrier mobility

Low-temperature annealing

• Low-temperature annealing is a post-metallization treatment used to improve the quality of the Si-SiO2 interface
• Annealing temperatures typically range from 300°C to 500°C, which is below the temperatures used for high-temperature processes such as oxide growth or dopant activation
• Low-temperature annealing can help to reduce the density of interface states and charges by promoting the rearrangement of atoms at the interface and the diffusion of hydrogen
• This treatment can also help to mitigate the effects of process-induced damage, such as plasma-induced damage during etching or deposition steps

High-temperature annealing

• High-temperature annealing, typically performed at temperatures above 600°C, can be used to improve the quality of the Si-SiO2 interface
• During high-temperature annealing, the increased thermal energy can promote the reduction of interface states and charges through various mechanisms
• These mechanisms include the desorption of impurities, the relaxation of strain at the interface, and the diffusion of oxygen to the interface to reduce oxygen vacancies
• High-temperature annealing can also help to densify the oxide layer, reducing the density of bulk oxide traps
• However, high-temperature treatments may not be compatible with all device fabrication processes, particularly those involving temperature-sensitive materials or structures

Nitridation of oxide

• Nitridation of the oxide layer is another technique used to improve the quality of the Si-SiO2 interface
• In this process, nitrogen atoms are incorporated into the oxide layer, typically through thermal nitridation or plasma nitridation
• The incorporation of nitrogen can help to reduce the density of interface states and charges by several mechanisms:
  • Nitrogen can passivate dangling bonds at the interface, reducing the density of interface states
  • Nitrogen can also reduce the diffusion of impurities, such as hydrogen or metal atoms, to the interface
  • The presence of nitrogen can alter the stress at the interface, which can influence the formation and distribution of interface states
• Nitridation can also improve the dielectric properties of the oxide layer, such as increasing the dielectric constant and reducing the leakage current
• However, excessive nitridation can lead to the formation of new defects or charges, so careful optimization of the nitridation process is necessary

Role in advanced devices

High-k dielectrics vs SiO2

• As device dimensions continue to shrink, the traditional SiO2 gate dielectric becomes increasingly problematic due to its high leakage current and reliability issues
• High-k dielectrics, such as hafnium oxide (HfO2), zirconium oxide (ZrO2), and aluminum oxide (Al2O3), have been introduced as alternative gate dielectrics to address these challenges
• High-k dielectrics have a higher dielectric constant than SiO2, allowing for a thicker physical thickness while maintaining the same capacitance, thus reducing the leakage current
• However, the introduction of high-k dielectrics also brings new challenges related to interface states and charges
• The interface between the high-k dielectric and the semiconductor (e.g., Si or Ge) can have a higher density of interface states compared to the Si-SiO2 interface
• The origin and properties of these interface states may differ from those at the Si-SiO2 interface, requiring new characterization and passivation techniques

Interface engineering strategies

• To mitigate the impact of interface states and charges in advanced devices, various interface engineering strategies have been developed
• One approach is to use an interfacial layer (IL) between the high-k dielectric and the semiconductor
• The IL, typically a thin layer of SiO2 or a nitride, can help to reduce the density of interface states and improve the interface quality
• Another strategy is to use surface passivation techniques, such as hydrogen passivation or nitridation, to reduce the density of dangling bonds and other defects at the interface
• Atomic layer deposition (ALD) has emerged as a key technique for depositing high-k dielectrics with precise thickness control and uniform coverage
• ALD can also enable the incorporation of dopants or other materials at the interface to modulate the properties of the interface states and charges

Impact on ultra-thin body devices

• Ultra-thin body devices, such as fully depleted silicon-on-insulator (FD-SOI) or nanowire transistors, are increasingly being used to continue device scaling
• In these devices, the impact of interface states and charges can be more pronounced due to the reduced volume of the semiconductor channel
• The presence of interface states and charges can lead to enhanced carrier scattering, reduced mobility, and increased variability in device characteristics
• The impact of interface states and charges on the electrostatics of ultra-thin body devices, such as the threshold voltage and subthreshold slope, can also be more significant compared to bulk devices
• Careful optimization of the interface properties, through techniques such as interface engineering or surface passivation, is crucial for achieving high-performance ultra-thin body devices

Challenges in nanoscale devices

• As device dimensions shrink to the nanoscale regime, the characterization and control of interface states and charges become increasingly challenging
• The small volume of the semiconductor channel can make it difficult to accurately measure the density and properties of interface states using conventional techniques
• The increased surface-to-volume ratio in nanoscale devices can also make them more sensitive to process-induced damage and environmental factors, such as ambient moisture or contamination
• The presence of quantum confinement effects in nanoscale devices can alter the energy distribution and properties of interface states, requiring new theoretical models and simulation tools
• The development of novel device architectures, such as gate-all-around nanowire transistors or 2D material-based devices, may introduce new types of interface states and charges that require further investigation and understanding
• Addressing these challenges will be essential for the continued scaling and performance improvement of nanoscale devices in the future.