Investigation On Characteristics Of High K Gate Materials And Devices Deposited By ALD

Through efforts of several decades, semiconductor devices have been dramaticallyscaled down to nano-scale in order to achieve higher device density and performance.As the technology node comes to45nm, the equivalent thickness of silicon oxide gatedielectric is only around1nm, which causes the intolerable gate leakage current, andmakes silicon oxide come to its fundamental physical limit. To reduce the substantialgate leakage current resulting from direct quantum mechanical tunneling across thedielectric layer, high-k dielectrics which can give large gate capacitances with dielectricfilms physically thicker than those of corresponding silicon oxide gate dielectrics areneeded. In this dissertation, high-k gate materials and devices are researchedsystematically, and the author’s major contributions are outlined at follows:1. The ALD deposition process is studied. The relationship between ALD processparameters and high k gate material is given. The research result shows that growth ratewill keep stable when the growth cycle is in a big value. Variation of depositionparameters and the type of oxide source will significantly influence the quality of high kgate material deposited by ALD, which mainly appear in impurities, high temperaturecharacteristic, atom chemical stoichiometry and flat band voltage. The aluminumpretreatment is done at the interface of HfO₂and SiO₂, aiming to tune the VFB. Theexperiment results show the fact that the aluminum pretreatment can effectively tune theVFBof high k devices.2. The bandgap of the high k dielectric is obtained from the VASE testing result,indicating that the bandgap of the high k dielectric will decrease with the increment ofthe thickness, and this trend will gradually disappear as the thickness increases.Compares to the H2O-based HfO₂, the bandgap of the O₃-based HfO₂is smaller, and thedeposition temperature has more influence on the O₃-based HfO₂than that of theH2O-based HfO₂. Based on the VASE and XPS testing results, the VBO and CBO ofHfO₂with different thickness are calculated, for which the accurate band structure ofHfO₂-Si system are obtained.3. The leakage current mechanism of high k gate device is investigated. Thefrequency dispersion effect of high k gate MOS device deposited by ALD is studied.The C-V curves show that the accumulation capacitances take on the frequency dispersion at high frequency. For MOS capacitor high k gate, different fabricationprocess and measurement equipments will cause parasitic effect. Therefore, anequivalent circuit model is proposed, and is finally proved to be effective in eliminatingthe frequency dispersion effect.4. The effects of different annealed temperatures on HfO₂gate dielectrics strainedSi MOS and HfO₂gate dielectrics strained SiGe MOS are analyzed. Compared HfO₂Si-based strained MOS to HfO₂Si MOS, it’s found that the gate leakage current in HfO₂strained Si MOS is the smallest and the one in HfO₂strained SiGe MOS is the highest,which is the effect of interfacial and the barrier at the oxide semiconductor interface.The SILC on HfO₂strained Si MOS and HfO₂strained SiGe MOS is also studied. It’slearned negative voltage stress have less effect on the MOS structures. And positivevoltage stress decrease the gate leakage current in both of HfO₂strained Si MOS andHfO₂strained SiGe MOS.5. Through the main scattering mechanism of optical phonon scattering andacoustic phonon scattering, the high field electron transport mechanism of HfO₂gate issystematically researched by ensemble Monte Carlo method. It is found that acousticphonon scattering rate will dramaticalyy increase when electron is in a high energy state,which can increase the optical phonon scattering rate. It will allow electrons keep thesystem energy balance even in the high field environment, which increase thebreakdown voltage. With the result of Monte Carlo simulation, the energy relaxationlength is researched. It is figured out for different initial energy and electric field.Finally, the average drift velocity when electrons get through HfO₂is also given.