Nitrogen implantation of Mo gate was used to fabricate MOS capacitors and CMOS transistors. Initial studies demonstrate that the work function of Mo is sensitive to nitrogen implantation energy. Mo with (110) orientation exhibits a high work function, making it suitable for bulk p-MOSFET gate electrodes. Nitrogen implantation can be used to lower the Mo work function, making it suitable for n-MOSFET gate electrodes. A gate work function reduction of 0.42 eV was achieved for the n-FETs on CMOS wafers. With further optimization, this single metal gate technology may potentially replace conventional poly-Si gate technology for CMOS and can also be used for multiple-VTtechnologies.

An adjustable work function technology using Mo gate for CMOS devices

The dependence of metal and polysilicon gate work-functions on the underlying gate dielectric in advanced MOS gate stacks is explored. We observe that the metal workfunctions on high-/spl kappa/ dielectrics differ appreciably from their values on SiO/sub 2/ or in a vacuum. We also show the first application of the interface dipole theory on the metal-dielectric interface and obtained excellent agreement with experimental data. Important parameters such as the slope parameters for SiO/sub 2/, Si/sub 3/N/sub 4/, ZrO/sub 2/, and HfO/sub 2/ are extracted. In addition, we also explain the weaker dependence of n+ and p+ polysilicon gate workfunctions on the gate dielectric. Challenges for gate workfunction engineering are highlighted. This work provides additional guidelines on the choice of gate materials for future CMOS technology incorporating high-/spl kappa/ gate dielectrics.

Effects of high-/spl kappa/ gate dielectric materials on metal and silicon gate workfunctions

This paper highlights and experimentally verifies a new source of random threshold-voltage (V_th) fluctuation in emerging metal-gate transistors and proposes a statistical framework to investigate its device and circuit-level implications. The new source of variability, christened work-function (WF) variation (WFV), is caused by the dependence of metal WF on the orientation of its grains. The experimentally measured data reported in this paper confirm the existence of such variations in both planar and nonplanar high-k metal-gate transistors. As a result of WFV, the WFs of metal gates are statistical distributions instead of deterministic values. In this paper, the key parameters of such WF distributions are analytically modeled by identifying the physical dimensions of the devices and properties of materials used in the fabrication. It is shown that WFV can be modeled by a multinomial distribution where the key parameters of its probability distribution function can be calculated in terms of the aforementioned parameters. The analysis reveals that WFV will contribute a key source of V_th variability in emerging generations of metal-gate devices. Using the proposed framework, one can investigate the implications of WFV for process, device, and circuit design, which are discussed in Part II.

Grain-Orientation Induced Work Function Variation in Nanoscale Metal-Gate Transistors—Part I: Modeling, Analysis, and Experimental Validation

This paper investigates the process, device, and circuit design implications of grain-orientation-induced work function variation (WFV) in high-k/metal-gate devices. WFV is caused by the dependence of the work function of metal grains on their orientations and is analytically modeled in the companion paper (part I). Using this modeling framework, various implications of WFV are investigated in this paper. It is shown that process designers can utilize the proposed models to reduce the impact of WFV by identifying proper materials and fabrication processes. For instance, four types of metal nitride gate materials (TiN and TaN for NMOS devices and WN and MoN for PMOS devices) are studied, and it is shown that TiN and WN result in lower V_th fluctuations. Moreover, device engineers can study the impact of WFV on various types of classical and nonclassical metal-gate CMOS transistors using these analytical models. As an example, it is shown that, for a given channel length, single-fin FinFETs are less affected by WFV compared to fully depleted SOI and bulk-Si devices due to their larger gate area. Furthermore, circuit designers can benefit from the proposed modeling framework that allows straightforward evaluation of the key performance and reliability parameters of the circuits under such V_th fluctuations. For instance, an SRAM cell is analyzed in the presence of V_th fluctuations due to WFV, and it is shown that such variations can result in considerable performance and reliability degradation.

Grain-Orientation Induced Work Function Variation in Nanoscale Metal-Gate Transistors—Part II: Implications for Process, Device, and Circuit Design

In this letter, the physical and electrical properties of physical vapor deposited (PVD) hafnium nitride (HfN) is studied for the first time as the metal gate electrode for advanced MOS devices applications. It is found that HfN possesses a midgap work function in tantalum nitride (TaN)/HfN/SiO/sub 2//Si MOS structures. TaN/HfN stacked metal-gated MOS capacitors exhibit negligible variations on equivalent oxide thickness (EOT), leakage current, and work function upon high-temperature treatments (up to 1000 /spl deg/C), demonstrating the excellent thermal stability of HfN metal gate on SiO/sub 2/. Our results suggest that HfN metal electrode is an ideal candidate for the fully depleted SOI and/or symmetric double gate MOS devices application.

Physical and electrical characteristics of HfN gate electrode for advanced MOS devices

Mo metal gate p-MOSFETs with several advanced gate dielectrics were fabricated A suitable p-MOSFET work function was achieved and good device characteristics were obtained in all cases Thermodynamic stability of Mo on Si/sub 3/N/sub 4/, ZrO/sub 2/ and ZrSiO/sub 4/ was verified by good carrier mobility agreement with the universal mobility model

Xin Guo

Papers

Molybdenum metal gate MOS technology for post-SiO/sub 2/ gate dielectrics