Recently, for sub 20nm scaled DRAM, capacitor structure has been changed to a metal-insulator-metal (MIM) structure in order to remove interfacial layers with low dielectric constant at the interface between polysilicon and dielectric material in SIS or MIS capacitors.
Due to the extremely small size of the capacitor, a three-dimensional structures such as trench hole/stack type are required for Gbit-scale DRAMs to obtain sufficient storage capacitance even though dielectrics with a high permittivity are used. Therefore, a ALD/CVD technique providing excellent conformality is required in order to fabricate the top and bottom electrodes as well as the dielectric films.
Ru and RuO2 are promising materials due to their good susceptibility to dry etching, low resistivity (Ru~7 μΩ-cm, RuO2~30 μΩ-cm), high chemical stability and high work function (Ru~4.7 eV, RuO2~5.1 eV) compared to the currently-used TiN electrode (~4.2 eV).
The use of rare earth ruthenium oxide materials, such as SrRuO3 (SRO), as electrodes or as interlayer between the electrode and high-k for ferroelectric and DRAM applications is mentioned in various roadmaps due to the good lattice matching with SrTiO3(STO) and relatively higher polarization property than elemental electrode.
Electrode materials for DRAM capacitors
Deposition of ALD-Ru Electrode with DER precursor
There are lots precursor for deposition of Ru film by ALD method, such as Ru(EtCp)2, Ru(thd)2, etc. Among them, we have studied the deposition method of Ru electrode with DER precursor (2,4-(Dimethylpentadienyl)(ethylcyclopentadienyl)Ru) by ALD method using liquid injection system(LDI). Unlike other conventional ALD system, such as bubbler-type, LDI uses heated vaporizer. The precursor is injected to vaporizer in liquid state and vaporized, then carrier gas deliver the vaporized precursor into reaction chamber.
Pure and conformal Ru electrode films are deposited.
Pulsed CVD growth of Ru/RuO2 with RuO4 precursor
RuO4 precursor has very simple structure with small molecular size, which makes it adoptable for deposition of Ru or RuO2 electrode films on complex-structured substrate. Over the temperature of 190ºC, the RuO4 precursor is thermally decomposed to RuO2 phase and self-limited growth behavior of ALD method is not shown. However, high volatility of RuO4 molecule makes it possible that precursor diffuses in short time and conformal film is deposited.
Phases of electrode films, Ru or RuO2, could be controlled by injection time of H2-reduction gas, RuO2 for shorter H2 injection time and Ru for longer time. RuO2 films deposited by thermal decomposition of RuO4 precursor are reduced to intermediate states, which are different with H2 injection time.
ALD growth of Ru electrode with RuO4 precursor
RuO4 precursor has been used for pulsed CVD growth of Ru or RuO2 electrodes but ALD growth behavior is also shown at low temperature. Under the decomposition temperature of RuO4 precursor, self-limited growth behavior which is a characteristic of ALD deposition is observed. Deposited Ru electrode has low resistivity (~20μΩ-cm) and low impurities.
ALD/CVD combined growth of SrRuO3 electrode
Strontium Ruthenate (SrRuO3, SRO) which has perovskite structure and high work function value is being studied for electrode materials for SrTiO3 dielectric film since it has good lattice matching with STO film.
Deposition sequence of SRO electrode consists of few subcycles that alternating ALD-SrO cycles and CVD-RuO2 cycles. Generally, deposition of ternary system (oxide with two cations) is really complicated since non-ideal ALD reactions could be shown between each layer. In case of SRO, ALD-SrO layer shown initial excessive growth behavior due to existence of RuO2 layer, which is not observed from the ALD method of only SrO layer. Controlling excessive growth is very important since it could degrade step coverage of electrode film. Improving the conformallity of SRO film is being studied in several ways.
Dielectric materials for DRAM capacitors
The technology road map for memory devices states that tox(EOT, equivalent oxide thickness) less than 0.5nm is necessary for the DRAMs with a design rule of 20nm. It is also noted that there are no known material solutions to serve this purpose. Reducing the thickness of the dielectric films with k values ~20-30 to achieve the required tox results in unacceptably high leakage currents.
Among the various dielectric films, TiO2 thin film in rutile phase exhibits a k value ~100 and therefore can be used for DRAMs design rule. Moreover, Al ions as an acceptor can be doped into the TiO2 films for decreasing the leakage current.
The ever-shrinking dimensions of DRAM cells with the increasing packing density have made the capacitor size increasingly smaller and currently-used ZrO2 dielectric will not be able to maintain necessary capacitance. Some high-k dielectrics like SrTiO3 have been avoided for dielectric materials of DRAM capacitors because of their complicated structures and compositions, however, it is time to face the difficulties and overcome them. (STO k~300, ZrO2 k~50)
Deposition of Al-doped TiO2 dielectric film with ALD method
TiO2 film has high dielectric constant, which varies with the phase of TiO2, 40 for anatase phase and ~100 for rutile phase. However, rutile phase with high dielectric constant is thermally stable at high temperature over 800℃, which is such high temperature for conventional ALD process.
Our group has reported that ALD-grown TiO2 film with O3 has rutile phase on Ru or RuO2 substrate because of formation of lattice-matching RuO2 interlayer. The coherence of crystallinity induces the in-situ crystallization of TiO2 into rutile phase.
Rutile TiO2 film has dielectric constant but higher leakage current is attended due to its small band gap(3.1eV) and n-type nature. To suppress the leakage current characteristic, Al2O3 layer is inserted between TiO2 layers. Controlling the concentration of Al atom varies the electrical characteristics. Doped Al atom which acts as acceptor suppresses leakage current and lowers tox under 0.5nm.
Acceptor-like dopant Al atoms increases Schottky barrier height(SBH) since the energy level of dopant site is under the oxygen vacancy level of TiO2.
Position of Al2O3 layer in ATO film also effects on electric characteristic of dielectric film. Lower leakage current properties are shown when Al2O3 layer is inserted near the electrode interface since leakage conduction mechanism of TiO2 film is Schottky emission conduction, which is sensitive to interface state.
Further tox scaling of TiO2 under 0.4nm is achieved by adopting RuO2 top electrode in place of Pt top electrode. Just like the bottom electrode, RuO2 top electrode has structural coherence with TiO2 dielectric film and could reduce the tox of interface layer.
Deposition of ALD-SrTiO3 dielectric film and controlling growth behavior
SrTiO3 (STO) dielectric film, which has higher dielectric constant, about 300, is also being studied for dielectric material for next generation DRAM capacitor. ALD method depositing STO film consists of several SrO cycles and TiO2 cycles. Those layers are deposited alternatively and the sub-cycle ratio is varied to gain STO film with stoichiometric cation ratio.
SrTiO3 film deposited on Ru substrate, which is a candidate of future DRAM electrode material, shows excessive growth of SrO layer since its oxygen-absorbing properties. The non-ideal excessive growth behavior could cause deviation of composition or thickness. Variation of the deposition process, such as temperature, precursor or inserting barrier layer has been studied for controlling growth behavior.
Insertion of barrier layers like Al2O3 or TiO2 prevent the oxygen absorption of Sr precursor from oxidized Ru substrate. Barrier layers of certain thickness suppress the diffusion of oxygen atom and excessive growth is not shown. However, materials for barrier layer has lower dielectric constant than STO film so that net dielectric constant of capacitor decreases.
Variation of ALD precursor is another possibility for controlling growth behavior since the cause of non-ideal growth is the reaction between substrate-oxide layer or oxide-oxide layer. Using highly reactive precursor could enhance growth rate but excessive growth is also easily shown. Therefore, selecting proper precursor is important for ideal growth of STO film.
Ab initio modeling of perovskite hetero-interface (SrTiO3/LaAlO3&CaTiO3/LaAlO3)
The recent growth techniques enabling the fabrication of the oxide hetero-structures with atomically abrupt interfaces between dissimilar materials allow us to investigate the physics of emerging phenomena arising at the interface. One of the striking examples is the recent discovery of two-dimensional electron gas (2DEG) at oxide interfaces: electrically conducting layer is formed at the insulating LaAlO3 (LAO) and SrTiO3 (STO) interfaces. We investigated the symmetry-dependent interfacial reconstruction from ab-initio calculations as a way of relieving the polar discontinuity at the interface which may coexist with the 2DEG. As model systems, we used the hetero-interfaces of LAO/STO and LAO/CTO, where the crystal structure of STO is cubic while that of CTO is orthorhombic. We found that only B-B interlayer distance increased at the LAO/STO(fig. c), while both A-A and B-B interlayer distances increased at the LAO/CTO which corresponds to the unit cell expansion at the interfacial region(fig. d). Moreover, the octahedral tilt was found to play a crucial role for the interfacial reconstruction and occurred in different ways depending on the crystal symmetry of the materials that consist of the interface. Our ﬁnding provides an insight not only to understand the fundamental physics of the emerging properties at the oxide interfaces but also to design unique functional interfaces.