There are known metals with the right work functions, TiN for PMOS and Al for NMOS. The metal gate for NMOS transistors requires a work function close to the conduction band of Si (∼4.1 eV) and the PMOS transistor needs a metal gate with a work function close to Si valence band (∼5 eV). Details will be described in Section 2.2. The work function of this gate stack will be defined by the used metals, their thickness values and deposition conditions. The dummy gate will be removed after completion of all implant and high thermal budget processes and replaced by a metal gate electrode. For gate last approach, a polysilicon dummy gate is formed as in the classical SiO 2/polysilicon technology, and all process steps with high thermal budget will be performed with this dummy gate in place. The details of this approach are given in Section 2.1. The required work functions for NMOS and PMOS have to be set by careful optimization of thermal treatments or anneals. The work function of metals used in the gate stack is shifted toward mid-gap for temperatures above ∼500☌. This exposure to high temperature limits the material choice for the gate stack. In gate first technology, the complete gate stack is formed before gate patterning and has therefore to withstand the high thermal budget of all subsequent processes which are required for transistor formation, including dopant activation. In both integration schemes, getting the right threshold voltage for NMOS and PMOS devices is a challenge. Two different integration approaches for high-k metal gate have been developed and implemented in high-volume production: gate first and gate last the latter is also known as replacement gate approach. To avoid this effect, the gate electrode on top of the high-k dielectric must be a metal electrode. The work function difference between n and p-gate electrodes becomes very small. As a result, the Fermi level of p + polysilicon increases significantly and the Fermi level of n + polysilicon decreases, causing high threshold voltages. Direct contact of HfO 2 with polysilicon leads to oxygen and electron transfer through this interface. The introduction of this new material requires also a change in the material for the gate electrode. Therefore, the SiO 2 with dielectric constant k = 3.9 had to be replaced by a dielectric material with higher dielectric constant, a so-called high-k material such as HfO 2 (k = 20). The well-established SiO 2 dielectric became too leaky for this thickness range, since tunnel leakage became the dominating leakage path. The desired electrical thickness of the gate dielectric became less than 2 nm. This scaling of the gate length also requires a significantly thinner gate dielectric for gate control and to mitigate short channel effects. Today’s leading edge technologies have a gate length of below 20 nm, a shrink by a factor of 100 and almost in the range of gate oxide thickness from the early technologies. In the early years of MOSFET technology, typical gate length was around several micrometers and the thickness of the dielectric between the silicon and the gate electrode was above 10 nm. Doping of the polysilicon can tune the work function for N-type metal-oxide-semiconductor (NMOS) and P-type metal-oxide-semiconductor (PMOS) transistors accordingly. The Si/SiO 2-dielectric/polysilicon-electrode gate stack is optimized to fulfill these requirements. The work function difference between channel material and gate electrode should be small to ensure a low threshold voltage ( Figure 1). The required gate voltage to turn the transistor on (to form the inversion channel)-the threshold voltage Vt-is defined by the work functions of the transistor channel semiconductor and the gate electrode and by additional charges at the transistor channel-dielectric interface and distributed charges through the dielectric. The control of charges close to the silicon surface by an applied voltage to the gate electrode turns the transistor channel on and off. The basic principle of metal-oxide-semiconductor field-effect transistor (MOSFET) function has not been changed since the introduction of this transistor type ∼40 years ago.
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