Preparation of conductive HfN by post rapid thermal annealing-assisted MOCVD and its application to metal gate electrode

Abstract

Conductive hafnium nitride (HfN) with negligible carbon impurity (<0.1 at.%) was chemically synthesized for the first time by post-rapid thermal annealing (PRTA)-assisted metal organic chemical vapor deposition (MOCVD) method. The thermodynamic instability of N-rich hafnium nitride (Hf₃N₄) phase, which is considered to be the dominant phase in CVD deposition of hafnium nitride, was utilized for pure and metallic HfN synthesis. By integrating the PRTA-HfN film into MOS capacitor, the electrical properties of the PRTA-HfN film as metal gate electrode were studied. Well behaved electrical characteristics such as about 4.9 eV of effective work function, low leakage current and large reduction in SiO₂ equivalent oxide thickness (EOT), which was attributed to the combination of physical thinning of SiO₂ and formation of high-κ interfacial layer, suggest the potential capability of PRTA-assisted MOCVD in chemically synthesizing HfN metal gate electrode for pMOS devices application.

Introduction

As the continuous scaling of complementary metal–oxide–semiconductor (CMOS) devices, especially for high-performance and low-power CMOS application in the sub-0.1 μm technology, advanced high-κ dielectric and metal gate electrode will be required to replace traditional SiO₂ dielectric and poly-silicon gate electrode [1], [2], [3]. Replacement of poly-silicon with metal gate electrode is expected to reduce the gate capacitance degradation originating from the doped poly-silicon gate depletion. It also contributes to eliminate boron penetration from poly-silicon into channel region. Considering the possible physical damage to the gate dielectric during gate electrode deposition by physical vapor deposition (PVD), chemical synthesis technologies such as chemical vapor deposition (CVD) are also required.

Hafnium nitride (HfN) is a potential promising candidate as a metal gate electrode because of its low bulk resistivity of 33 μΩ-cm [4] and its expected superior compatibility with hafnium-based high-κ dielectrics such as HfO₂ [5]. Up to now, some works have been carried out to study the physical and electrical characteristics of HfN gate electrode using physical vapor deposition (PVD) [5]. Chemical synthesis of metallic HfN film, however, has been rarely reported because of the preferred growth of insulating N-rich Hf₃N₄ phase. We have already reported that HfN_1.39 film with low levels of C (<0.1 at.%) and O (∼2 at.%) impurities can be synthesized by metal organic chemical vapor deposition (MOCVD) using Hf[N(C₂H₅)₂]₄ precursor and NH₃ gas. The as-deposited films, however, behave insulating characteristic due to the formation of N-rich Hf₃N₄ phase [6], [7], [8]. The N content control to realize metallic HfN, in which N:Hf ratio should be unity, is very difficult by adjusting NH₃ concentration during the growth. For example, by decreasing the NH₃ flow to the lowest limit in our experimental setup, only insulating Hf₃N₄ phase was formed and there was no tendency to decrease N content in the film by lowering NH₃ concentration during deposition. Without adding NH₃ gas, we also found that metallic hafnium nitride with a lot of carbon, i.e., hafnium carbonitride (HfCN), can be formed [9], [10]. However, the lots of residual carbon in the HfCN film degrade the leakage characteristic of the HfCN gated MOS capacitors [11], indicating the necessity of further investigation for hafnium nitride. Recently, Kim reported that conductive hafnium nitride HfN_x film could be prepared by plasma-assisted atomic layer deposition (ALD) technique despite the trade-off between carbon impurity and electrical resistivity [12].

Considering the descriptions above, metallic and pure HfN is difficult to be synthesized directly by plasma-free CVD methods. As well known that, however, for the approaches involving the use of plasma, large number of energetic particles (ions, electrons) are likely to introduce physical damage to the gate dielectric (rough interface, metal ions penetrating into the dielectric). This may result in degraded reliability in MOS devices, especially for the sub-0.1 μm technology, suggesting the requirement for plasma-free growth methods. On the other hand, in terms of the evaluation on high pressure synthesis of Hf₃N₄ by Kroll [13], the phase boundary between metallic HfN and insulating Hf₃N₄ phases is accessed as a function of temperature and pressure. According to the calculated data, although the orthorhombic Zr₃N₄-type Hf₃N₄ with lowest energy configuration seems to be thermodynamically stable at zero pressure for temperatures up to about 1000 K, the error bar in calculation is large. This indicates that the Hf₃N₄ phase also shows a possibility of unstable feature in vacuum ambient with regard to decomposition into metallic HfN phase and N₂ over temperature of 1000 K. Based on the speculation on the thermodynamic characteristic of the Hf₃N₄ phase, ex situ high temperature process may be capable of modifying the structure and composition of the Hf₃N₄ film, and further chemically synthesizing metallic and pure HfN film. We have recently made a preliminary evaluation on this idea by using insulating PVD-HfN_1.44 films grown by reactive sputtering, and found that high temperature PRTA process is capable of converting insulating N-rich HfN_1.44, which is believed to mainly consist of Hf₃N₄ phase, into metallic HfN at about 1000 °C through phase transformation [14]. Although the film is still N-rich after PRTA process, the excess nitrogen is considered to be accommodated in the HfN crystal by occupying the intergranular and/or interstitial position ways.

In this work, the application of plasma-free high temperature PRTA process in chemically synthesizing HfN metal gate electrode using MOCVD was evaluated for the first time. The PRTA-HfN film shows much low level of carbon impurity compared with the HfN_x film formed by plasma-assisted ALD [12]. The structural evolution and electrical properties of the PRTA-HfN gated MOS stacks were studied.