Capacitors are elements used extensively in semiconductor devices for storing an electric charge. Capacitors essentially comprise two conductive plates separated by an insulator. The capacitance, or amount of charge held by the capacitor per applied voltage, is measured in farads and depends upon the area of the plates, the distance between them, and the dielectric value of the insulator. Capacitors are used in filters, in analog-to-digital converters, memory devices, and control applications, and many other types of semiconductor devices. For example, a dynamic random access memory (DRAM) cell includes a storage capacitor coupled in series with an access transistor. Data can be stored into and read out of the storage capacitor by passing charge through the access transistor and into the capacitor.
For DRAM capacitors, some key requirements for sub-70 nm technologies are low leakage current, low Equivalent Oxide Thickness (EOT), minimization of polysilicon depletion, adequate band offsets (for the dielectric), and thermal stability during subsequent processing. To achieve these requirements, the idea of using MIS (metal-insulator-silicon) or MIM (metal-insulator-metal) capacitors is known. A key challenge is to optimize the various interface properties and to use dielectrics with high capacitance. For applications involving gate electrodes, additional requirements include minimization of tunneling leakage current and gate resistance.
A number of high-dielectric constant materials are known for capacitors. Examples of high dielectric constant materials that have been proposed as capacitor dielectrics are tantalum pentoxide, titanium oxide, barium strontium titanate, and titanium oxide. To get a dielectric constant that is greater than 10, the prior art has focused on materials based on the HfuAlvSiwOxNy or LauAlvSiwOxNy systems. These materials are limited to a maximum dielectric constant of around 30.