Thin film binary and ternary oxides are a major component of many electronic devices. The specific properties required of the film are determined by the specific applications, which determine the material ( e.g., SiO2 , HfO2, or TiO2) and the method of deposition, physical vapor deposition (PVD) or atomic layer deposition (ALD.
Some of the more important material properties for electronic applications are composition, dielectric constant, refractive index, bandgap, breakdown field, and leakage. Many of these properties are affected by the phase and microstructure of the film, as well as point defects such as vacancies and interstitials.
The phase of the film includes whether it is amorphous or crystalline. If crystalline, the phase is the lattice structure, which may be cubic, tetragonal, orthorhombic, etc. Under microstructure it is important to know whether the film is single crystalline or polycrystalline. If it is crystalline, the grain size and preferred orientation of the grains are determined. For many devices, it is also important to know properties of interfaces such as the barrier height between a metal electrode and the oxide conduction/valence band edges.
Characterizing the oxide’s material properties requires a wide range of metrology techniques.
- X-ray fluorescence (XRF) , energy dispersive x-ray spectroscopy (EDS) and x-ray photoelectron spectroscopy (XPS) are used for composition.
- X-ray diffraction (XRD) and transmission electron microscopy (TEM) provide valuable information on the structure.
- Refractive index can be measured using ellipsometry.
- Bandgaps can be determined from wavelength dependent ellipsometry or absorption using spectrophotometers.
- Dielectric constants and leakage need electrical measurements on Capacitor stacks; barrier heights can be determined using internal photoemission (IPE) on the same stacks.
By combining the ability to deposit different types of oxides and simultaneously measuring various material properties, it is possible to identify relationships between properties that can be used to select a particular material for a specific application. An example in the figure below shows the breakdown electrical field strength of 7nm ALD deposited films ranging from Al2O3 to TiO2, plotted against the measured static dielectric constant.
The breakdown strength Ebd decreases with dielectric constant κ and our measurements indicate that it is accurately described by the relation
Ebd = 25.4 κ-0.46
Based on this relationship, in an application where the capacitor is used as an energy storage device and the maximum voltage is not a constraint, SiO2 films are the best candidate. On the other hand, if voltages are low (as in advanced node DRAM and logic) materials with higher dielectric constants such as ZrO2 and HfO2 are the best solution.
IMI has investigated many different oxides. We illustrate further IMI capabilities by focusing on work done on transition metal oxides for different applications, with particular emphasis on the use of Hafnium Oxide and Zirconium Oxides in semiconductor technology.
The key material requirements for the dielectric for DRAM include a high dielectric constant with a low leakage at thicknesses less than 10nm. Since the capacitor structures have nm dimensions (but large aspect ratios (%):1 or more), the only deposition technique is ALD.
IMI has investigated ZrO2 based films and optimized them for successive generations of DRAM. A wide range of ALD precursors to provide optimum performance and manufacturability were investigated. For future nodes, the stringent leakage requirements require a deeper understanding of the leakage mechanisms.
Key parameters are the barrier height between the metal electrode and the dielectric, as well as the density of defects. IPE was utilized to directly measure the barrier height. Combined with simulation of the leakage due to vacancies, this allows optimization of the overall capacitor stack and anneal process. The effect of dopants and thin interfacial layers are being investigated as part of our ongoing research,
Resistive RAM (ReRAM)
The cells for this type of future NVM utilize the unique properties of metal oxides that have undergone a soft electrical breakdown called “forming”. The resistance of the cell can be toggled between a high resistance state and a low resistance state by applying a positive or negative pulse respectively.
IMI evaluated a wide variety of metal oxides and was the first to show that a HfO2 film of the right composition has all the desired characteristics needed for future NVM cells. In combination with appropriate electrodes, the cells can be programmed at low power and show good endurance and retention. Simulation of the defect kinetics during forming and programming, also allowed the optimization of the electrical operation in an actual memory array.
Thin films composed of mixtures of HfO2:ZrO2 have been shown to have excellent ferroelectric properties. They retain bulk polarization after the application of a critical electric field, and the direction of the polarization can be switched by reversing the field. By varying the exact composition of the two components, it is possible to tune the exact properties of the film.
Pure HfO2 films are paraelectric , while pure ZrO2 are Antiferroelectric. By choosing the right electrodes (both bottom and top), composition and annealing conditions, record-setting polarization numbers were achieved. These films can be used in a NVM cell, either as a capacitor in series with a switching transistor, or as part of the gate stack in a single transistor. This allows for much smaller cell sizes than the current commercial FeRAM cells, which use much thicker PZT oxides.
By incorporating transition metal oxides with high dielectric constants, such as HfO2 as the gate oxide of MOS transistors, it is possible to aggressively shrink the feature size of the transistors, while controlling the gate leakage.
In order to meet the stringent requirements of performance, reproducibility, and reliability, the properties of the HfO2 has to be finely tuned. A deep understanding of the defects generated in the films during deposition and post processing, and interaction with the metal gate, is critical.
With current Finfets, step coverage on high aspect fins is also very important and ALD is the preferred method of deposition. IMI has done intensive work on ALD processes, including assessment of different precursors, to develop a suite of characterization techniques to fully evaluate the films. By combining experimental IPE results, leakage data, and threshold voltage measurements with simulations of defect density, the optimum conditions for transistor performance can be determined.
While different applications may ostensibly use the same metal oxide, the specific needs require tailoring of the properties. This is done at IMI by rapid design of Design of Experiments guided by sophisticated material and device simulations, and an extensive database of material properties.