In thesemiconductor industry, the termhigh-κ dielectric refers to a material with a highdielectric constant (κ,kappa), as compared tosilicon dioxide. High-κ dielectrics are used insemiconductor manufacturing processes where they are usually used to replace a silicon dioxidegate dielectric or another dielectric layer of a device. The implementation of high-κ gate dielectrics is one of several strategies developed to allow further miniaturization of microelectronic components, colloquially referred to as extendingMoore's Law.
Sometimes these materials are called "high-k" (pronounced "high kay"), instead of "high-κ" (high kappa).
Silicon dioxide (SiO2) has been used as agate oxide material for decades. Asmetal–oxide–semiconductor field-effect transistors (MOSFETs) have decreased in size, the thickness of the silicon dioxide gate dielectric has steadily decreased to increase thegate capacitance (per unit area) and thereby drive current (per device width), raising device performance. As the thickness scales below 2 nm, leakage currents due totunneling increase drastically, leading to high power consumption and reduced device reliability. Replacing the silicon dioxide gate dielectric with a high-κ material allows increased gate thickness thus decreasing gate capacitance without the associated leakage effects.
The gate oxide in aMOSFET can be modeled as a parallel plate capacitor. Ignoring quantum mechanical and depletion effects from theSi substrate and gate, thecapacitanceC of this parallel platecapacitor is given by
where
Since leakage limitation constrains further reduction oft, an alternative method to increase gate capacitance is to alter κ by replacing silicon dioxide with a high-κ material. In such a scenario, a thicker gate oxide layer might be used which can reduce theleakage current flowing through the structure as well as improving the gate dielectricreliability.
The drain currentID for aMOSFET can be written (using the gradual channel approximation) as
where
The termVG − Vth is limited in range due to reliability and room temperature operation constraints, since a too largeVG would create an undesirable, high electric field across the oxide. Furthermore,Vth cannot easily be reduced below about 200 mV, because leakage currents due to increased oxide leakage (that is, assuming high-κ dielectrics are not available) andsubthreshold conduction raise stand-by power consumption to unacceptable levels. (See the industry roadmap,[1] which limits threshold to 200 mV, and Royet al.[2]). Thus, according to this simplified list of factors, an increasedID,sat requires a reduction in the channel length or an increase in the gate dielectric capacitance.
Replacing the silicon dioxide gate dielectric with another material adds complexity to the manufacturing process. Silicon dioxide can be formed byoxidizing the underlying silicon, ensuring a uniform, conformal oxide and high interface quality. As a consequence, development efforts have focused on finding a material with a requisitely high dielectric constant that can be easily integrated into a manufacturing process. Other key considerations includeband alignment tosilicon (which may alter leakage current), film morphology, thermal stability, maintenance of a highmobility of charge carriers in the channel and minimization of electrical defects in the film/interface. Materials which have received considerable attention arehafnium silicate,zirconium silicate,hafnium dioxide andzirconium dioxide, typically deposited usingatomic layer deposition.
It is expected that defect states in the high-κ dielectric can influence its electrical properties. Defect states can be measured for example by using zero-bias thermally stimulated current, zero-temperature-gradient zero-bias thermally stimulatedcurrent spectroscopy,[3][4] orinelastic electron tunneling spectroscopy (IETS).
Industry has employedoxynitride gate dielectrics since the 1990s, wherein a conventionally formed silicon oxide dielectric is infused with a small amount of nitrogen. The nitride content subtly raises the dielectric constant and is thought to offer other advantages, such as resistance against dopant diffusion through the gate dielectric.
In 2000,Gurtej Singh Sandhu and Trung T. Doan ofMicron Technology initiated the development ofatomic layer deposition high-κfilms forDRAM memory devices. This helped drive cost-effective implementation ofsemiconductor memory, starting with90-nmnode DRAM.[5][6]
In early 2007,Intel announced the deployment ofhafnium-based high-κ dielectrics in conjunction with a metallic gate for components built on45 nanometer technologies, and has shipped it in the 2007 processor series codenamedPenryn.[7][8] At the same time,IBM announced plans to transition to high-κ materials, also hafnium-based, for some products in 2008. While not identified, the most likely dielectric used in such applications are some form of nitrided hafnium silicates (HfSiON).HfO2 andHfSiO are susceptible to crystallization during dopant activation annealing.NEC Electronics has also announced the use of aHfSiON dielectric in their 55 nmUltimateLowPower technology.[9] However, evenHfSiON is susceptible to trap-related leakage currents, which tend to increase with stress over device lifetime. This leakage effect becomes more severe as hafnium concentration increases. There is no guarantee, however, that hafnium will serve as a de facto basis for future high-κ dielectrics. The 2006ITRS roadmap predicted the implementation of high-κ materials to be commonplace in the industry by 2010.
