In the semiconductor industry, the term high-κ dielectric refers to a material with a high dielectric constant (κ, kappa), as compared to silicon dioxide. High-κ dielectrics are used in semiconductor manufacturing processes where they are usually used to replace a silicon dioxide gate 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 extending Moore's law. Sometimes these materials are called "high-k" (pronounced "high kay"), instead of "high-κ" (high kappa).
Need for high-κ materials Silicon dioxide (SiO2) has been used as a gate oxide material for decades. As metal–oxide–semiconductor field-effect transistors (MOSFETs) have decreased in size, the thickness of the silicon dioxide gate dielectric has steadily decreased to increase the gate capacitance (per unit area) and thereby drive current (per device width), raising device performance. As the thickness scales below 2 nm, leakage currents due to tunneling increase drastically, leading to high power consumption and reduced device reliability. Replacing silicon dioxide with a high-κ material allows a higher gate capacitance to be achieved without further reducing the physical thickness of the gate, thereby suppressing tunneling leakage while enabling continued scaling.
First principles
The gate oxide in a MOSFET can be modeled as a parallel plate capacitor. Ignoring quantum mechanical and depletion effects from the Si substrate and gate, the capacitance C of this parallel plate capacitor is given by
C = κ ε 0 A t {\displaystyle C={\frac {\kappa \varepsilon _{0}A}{t}}}
where
A is the capacitor area κ is the relative dielectric constant of the material (3.9 for silicon dioxide) ε0 is the permittivity of free space t is the thickness of the capacitor oxide insulator Since leakage limitation constrains further reduction of t, 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 the leakage current flowing through the structure as well as improving the gate dielectric reliability.
Gate capacitance impact on drive current The drain current ID for a MOSFET can be written (using the gradual channel approximation) as
I D , Sat = W L μ C inv ( V G − V th ) 2 2 {\displaystyle I_{D,{\text{Sat}}}={\frac {W}{L}}\mu \,C_{\text{inv}}{\frac {(V_{G}-V_{\text{th}})^{2}}{2}}}
where
W is the width of the transistor channel L is the channel length μ is the channel carrier mobility (assumed constant here) Cinv is the capacitance density associated with the gate dielectric when the underlying channel is in the inverted state VG is the voltage applied to the transistor gate Vth is the threshold voltage The term VG − Vth is limited in range due to reliability and room temperature operation constraints, since a too large VG 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) and subthreshold conduction raise stand-by power consumption to unacceptable levels. (See the industry roadmap, which limits threshold to 200 mV, and Roy et al. ). Thus, according to this simplified list of factors, an increased ID,sat requires a reduction in the channel length or an increase in the gate dielectric capacitance.
Materials and considerations Replacing the silicon dioxide gate dielectric with another material adds complexity to the manufacturing process. Silicon dioxide can be formed by oxidizing 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 include band alignment to silicon (which may alter leakage current), film morphology, thermal stability, maintenance of a high mobility of charge carriers in the channel and minimization of electrical defects in the film/interface. Materials which have received considerable attention are hafnium silicate, zirconium silicate, hafnium dioxide and zirconium dioxide, typically deposited using atomic 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 stimulated current spectroscopy, or inelastic electron tunneling spectroscopy (IETS).
… excerpt ends here. Continue reading the full article.
