GIREESH CHANDRAN M

P2VLD120081

• John Atalla and Dawon Kahng fabricate working transistors and demonstrate the first successful MOS field-effect amplifier (1960).

Early CMOS processes used aluminium gates in the 1960-70’s

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• As the device is scaled down, manufacturers transitioned to use polysilicon as the gate material.

Fabrication processes after the initial doping required very high temperature annealing. Metal gates would melt under such conditions whereas polysilicon would not.

Vt of the MOSFET inversion layer is correlated with the work-function difference between the gate and the channel. Using metal would result in a higher Vtcompared to polysilicon. 3

• Recently, after 45nm (for Intel) gates are again made with metal in conjunction with high-k insulators. (HKMG technology)

Poly depletion effect

Poly-Si gates are not chemically stable in contact with high-k dielectrics

High gate resistance

Fig: Replacement of Poly-Si/SiO2 with high-k/metal gate 4

• The scaling of gate oxides thickness with decreasing channel lengths is necessary to maintain effective gate control.

• The thinning of gate oxides (SiO2) below 1.2 nm leads to increase in gate electric fields and gate tunnelling currents may increase significantly, leading to oxide breakdown.

Fig: Figure showing Tunnelling current

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• The combination of a high-κ dielectric material and a poly-Si gate is not suitable for high performance logic applications since the resulting high-κ/poly-Si transistors will be having degraded channel mobility .

• phonon scattering limit the mobility in high-κ devices.

Metal gate electrodes are effective for screening phonon scattering in the high-κ dielectric from coupling to the channel, when under inversion conditions.

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• For sub-micrometer regime which uses polysilicon gate, the gate and interconnect delay becomes a significant fraction of total device delay.

Metal gates have reduced sheet resistance

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• The primary requirement for a prospective metal gate technology is attaining correct workfunctions for setting symmetrical Vt for nMOS and pMOS

Fig: Vt vs Gate workfunction 8

• The selection of metal materials is not straight forward !!

• The effective metal workfunction could differ appreciably from the vacuum workfunction and could depend on other factors such as the underlying dielectric material.

• Several metal gate electrodes have been studied as a replacement for poly-Si, such as W/TiN, Mo, Ta, TaN, TiN.

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Work functions of various metal materials.

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• Mo with (110) orientation exhibits a high workfunction (5 eV),making it suitable for

p-MOSFET gate electrodes.

• Nitrogen implantation can be used to lower the Mo workfunction, making it suitable for n-MOSFET gate electrodes.

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Effect of metal gates on gate leakages• With same EOT, poly-Si and metal gated devices show similar gate leakage

in magnitude. However, at same capacitance equivalent thickness (CET) at inversion, metal gated device exhibits lower gate leakage than poly-Si gate. CET is thicker than EOT because of the poly depletion at the gate side.

Fig:Gate leakage of n-MOS in future-generation CMOS LSTP application using HfO2/SiO2 stack 12

High drain currents and reduced Sub-threshold slope

• The absence of poly depletion, results in a higher on-current in metal gate MOSFETs.Sub-threshold swing decreases when SiO2 is replaced with HfO2 dielectrics as shown below .It is found to further reduce with the replacement of poly-Si gate with TiN metal gate.

Fig:Sub-threshold swing for poly-Si gated and TiN gated structures 13

Low ON-resistance (Ro)

• The use of power MOSFETs in portable electronic devices requires low Ro to ensure high current carrying capability in small packages. Ro reduces as we move from poly-Si/SiO2 device to HfO2/TiN devices.

Fig: ON-resistance comparison of poly-Si/SiO2, poly-Si/HfO2 and TiN/HfO2 structures 14

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The challenges for the metal gate technology lies in gate material selection along with selection of high-κ gate and understanding and control of factors that influence workfunction at metal/dielectric interface.

The metal gate technology should ensurereliability i.e. the metal should not diffuse into the channel

Although many of the metal fabrication techniques have been explored but to develop the fabrication techniques which are compatible, simple and cost effective still remains a challenge with metal gate technology

Intel, which has successfully taken HKMG to production for two generations, calls it the biggest change to transistors since the 1970s !!

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• MOSFET structures in deep-submicrometer regime necessitate the replacement of conventional poly-Si gates with metal gates.

• Metal gates showed reduced SCE’s.

• Metal gates are more compatible with high-κ gate dielectrics than poly-Si gates.

• Large reduction in gate leakage and sub-threshold swing projects the high-k metal gate technology to be a strong alternative for future nano scale MOS devices.

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• National Technology Roadmap for Semiconductors (NTRS),1993

• Dual Work Function Metal Gate CMOS Technology Using Metal Interdiffusion Igor Polishchuk, Student Member, IEEE, Pushkar Ranade, Student Member, IEEE, Tsu-Jae King, Senior Member, IEEE,and Chenming Hu, Fellow, IEEE

• Gate Workfunction Engineering for Deep Sub-Micron MOSFET’s: Motivation, Features and Challenges

Farkhanda Ana, Najeeb-ud-din,ECE, National Institute of Technology, Srinagar, India.

• Engineering the metal gate electrode for controlling the threshold voltage of organic transistors Yoonyoung Chung, Olasupo Johnson, Michael Deal,YoshioNishi,Boris Murmann,and Zhenan Bao2,Department of Electrical Engineering, Stanford University, Stanford, California 94305

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