| Floating gate memory dominates the market of nonvolatile memory now. The feature sizes of semiconductor devices scale down together with the increasing of the integrated density. When the technology node enters the22nm generation, floating gate memory technique will approach to its physical limitations. So, it is urgent to find a new type of nonvolatile memory. Charge trapping memory has a lot of advantages such as compatibility with semiconductor, low operating voltage, low power consumption and good endurance as a kind of promising new nonvolatile memory. Especially metal nanocrystals (NCs) memory has received great attention due to its unique advantages of high storage density, wide range choice of work function, no multi carrier confinement effect etc.Metal NCs memory consists of semiconductor substrate\tunneling layer\metal NCs\blocking layer\gate electrode. At present, the main preparation method of metal nanocrystal storage layer is high temperature annealing, which has disadvantages of poor controllability and repeatability. It is still a tough challenge to explore nanocrystal deposition technique with uniform distribution and reliable processing,.Atomic layer deposition (ALD), based on sequential self-limited and complementary surface chemisorptions reactions, is a novel thin film deposition technology. The self-restriction and self-saturation mechanism ensures large area uniformity and excellent three-dimensional conformality. In recent years, ALD shows increasing prospect in the fields of microelectronic and nanotechnology.In this thesis, we mainly adopt ALD technology as the metal NC memory cell fabrication method. FePt NCs, Pt NCs and Ir NCs storage layers was prepared by self-assembly and ALD, respectively. High k thin films of Al2O3and HfO2were deposited as tunneling layer and blocking layer by ALD. FePt, Pt and Ir metal NC memory cells have been fabricated. The effect of the processing parameters on NCs formation, arrangement, density, and structure has been investigated systematically. The memory effects have been characterized deeply. The impact of ALD high k tunneling and blocking layers on the properties of NC memory cells has also been studied. In addition, we also calculated the effect of Gd doping on the dielectric properties and bandgap structure of cubic HfO2by first principles. ALD Gd-Si-O thin films were explored to use as the storage layer in the defect type charge trapping memory.Main achievements are summarized as follows:1. The self-assembly process of FePt NCs by spin coating and dip coating was studied and compared. The monolayer FePt nanocrystal memory cells were prepared by the combination of self-assembly and ALD, and their structures and storage properties were characterized in depth. The monolayer FePt nanocrystals (NCs) can be self-assembled on H-terminated Si surface by spin coating, however high quality tunneling layer cannot be obtained by annealing of Si\FePt NCs\Al2O3composite structure with poor memory effect, therefore, this process is not suitable for the preparation of FePt NCs memory cells. Instead a hexagonally arranged monolayer FePt NCs with lattice constant of8nm and density of1.8×1012/cm2, can be self-assembled on ALD Al2O3by dip coating. After annealing at500℃for5min in O2atmosphere, the core-shell structure Feo.75Pt@Fe2O3is formed with fcc-Fe0.75Pt NC core and amorphous Fe2O3shell. The core-shell structure significantly improves the storage capacity of Si\Al2O3\FePt NCs\Al2O3\Pt memory cells. Memory window is increased from4.1V with unannealed FePt NCs to8.1V with annealed FePt NCs at the sweeping gate voltage of±8V. FePt NC memory cells show excellent endurance, but retention property is unideal, which attributes to the fact that FePt NCs position order is reduced after annealing at500℃although they are still discrete. The fabrication process needs further improvement.2. The effect of precursor temperature and cycles of ALD process on Pt NCs nucleation, size, and density has been investigated systematically. It is beneficial to adjust Pt NCs size and density by setting MeCpPtMe3source temperature at70℃. The formation of Pt NCs on Al2O3surface has been studied, following the nucleation incubation model. A nucleation delay of30～40ALD cycles is confirmed. Then the density and size of Pt NCs increase with the cycle number. Merging between Pt NCs dominates the growth after80cycles, the density reduces. The maximum density of Pt NCs with average size of3.9nm is approximately1.0×1012/cm2at70cycles. Two kinds Pt NC memory cells of Si\Al2O3\Pt NCs\Al2O3\Pt and Si\Al2O3\Pt NCs\HfO2\Pt were fabricated and compared. The appropriate tunneling layer thickness and high-k blocking layers are needed to achieve good storage properties. The distribution of voltage drops on tunneling and blocking layers can be easily changed by replacing Al2O3blocking layer with high-k HfO2layer, to obtain large memory window and good retention of Pt NC memory cells. Electrical measurements of Si\Al2O3\Pt NCs\HfO2\Pt show a larger memory window of6.6V at the sweeping gate voltage of±12V and～73%retention property after1×105s. According to the band alignment structure, HfO2blocking layer can increase voltage drop on tunneling layer, leading to the programming mechanism of FN tunneling at10V instead of direct tunneling for memory cells with Al2O3blocking layers. The programming/erasing mechanism can be tuned by choosing proper blocking layer materials, and the storage properties can be optimized.3. The substrate surface and its pretreatment have great influence on the nucleation of ALD Ir NCs. Ir cannot be directly deposited on H-terminated Si due to lack of active groups on H-Si. ALD-Al2O3surfaces have various active groups by different surface pretreatment. The ex situ nucleation of Ir NCs on ALD Al2O3is difficult because the OH group is less after the passivation of ALD-Al2O3in the air. Ir NCs can be easily in situ formed on Al2O3, ascribed to more active groups adsorbed on Al2O3pretreated by TMA or H2O pulse. And surface with Al(CH3)2groups is more active than surface with OH groups, leading to the higher density of Ir NCs on Al2O3. The formation of in situ ALD Ir NCs on Al2O3is similar to that of Pt NCs. A nucleation delay of40-50ALD cycles is also observed. Then the density and size of Ir NCs increase with the cycle number from50to90cycles. Merging between Ir NCs dominates the growth after100cycles, the density reduces and the size increases rapidly. The highest density of Ir NCs with the size of4.9nm is0.6×1012/cm2at90cycles. The compositional and structural analyses indicate that ALD Ir NCs are pure metallic Ir with fcc phase. Ir NCs memory cell show a memory window of4.2V at±10V and～68%retention property after1×105s. Compared to Ir NCs memory cells, Pt NCs memory cells with similar structures exhibit relatively better memory characteristics with higher storage density and retention, which is ascribed to easier Pt nucleation on ALD-Al2O3surface than Ir ones and deeper potential wells in Pt NCs memory cells.4. According to first-principles calculations,[(GdHf)2Vo]0complex defect with four sevenfold coordination Hf atoms around Vo, is formed in Gd doped HfO2. The Gd doping can stabilize the fcc HfO2phase, improve the dielectric constant, suppress the oxygen vacancies in HfO2. The d-d coupling of Hf5d and Gd5d antibond state electrons through an O atom (Gd-O-Hf) is confirmed theoretically, which increases the band gap. Gd doped HfO2ternary films with various Gd/Hf cycle ratios were deposited by ALD. With the increase of Gd content, the band gap increases, the conduction band offset increases first and then decreases, and the valence band offset decreases first and then increases. However,11.6%Gd doped HfO2films has the largest dielectric constant of13.3, which is still less than that of HfD2films, attributing to the impurity Si residuel in ALD Gd2O3films using Gd[N(SiMe3)2]3precursor. Si\Al2O3\Gd-Si-O\Al2O3\Pt charge trapping memory cells with ALD Gd-Si-O films as charge storage layers have been fabricated. Memory window is2.46V at the sweeping gate voltage of±8V, storage effect may come from the oxygen vacancies in Gd-Si-O films. The oxygen vacancies obviously decrease after annealing at600℃in oxygen, leading to no memory window. In addition, the ability of Gd-Si-O films to trap holes is significantly greater than that to trap electrons, and retention characteristics of memory cells at the programmed state is significantly better than that at the erased state, which is related to the fact the number of trapped electrons at the programmed state is less than that of lost electrons at the erased state.In conclusion, ALD is explored to fabrication of charge trapping memory, especially high density metal NCs memory. The related storage properties have been characterized deeply. This work helps to promoting practical applications of new charge trapping memories in the future.. |
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