Our goal is to develop novel, combinatorially-compatible measurement methods and metrologies, as well as comprehensive and consistent data sets, that will enable manufacturers of devices based on inorganic functional materials to select new materials more rapidly and intelligently. For example, the silicon microelectronics industry is currently materials limited; the traditional transistor and capacitor formation materials — silicon, silicon dioxide, and polysilicon — have been pushed to their undamental material limits, and continued scaling will require the introduction of novel materials.
Our approach is to develop a novel metrology for interrogating the property of interest in the combinatorial library film. A good example of the application of this approach is the advanced gate stack of the integrated circuit MOSFET device. The traditional gate stack layers (SiO2 gate dielectric and polycrystalline Si gate electrode) in current Si microelectronic devices must be replaced with a high dielectric constant (high-.) gate dielectric, and a metal gate electrode. We will develop combinatorial methodologies to: (1) fabricate compositionally graded thin film libraries of novel gate metal electrode/high-. gate dielectric/ substrate combinations (“gate stack” structures); and (2) measure the key electronic properties (e.g., work function) and thermal stability of such libraries. Comprehensive data sets of electronic properties as a function of composition will be generated for materials systems identified as high priority by the microelectronics industry. In addition, first-principles modeling techniques will be used to predict the electronic properties of such systems.
Impact and Customers:
We commissioned a state-of-the-art combinatorial tool capable of producing thin film libraries by reactive sputtering or pulsed laser deposition (PLD). Both chambers are equipped with multiple targets, allowing for the deposition of ternary films of metals and nitrides (by sputtering) and oxides (by PLD) with monolayer (0.5 nm) thickness control. At present, this tool produces library films for our thermoelectric materials and advanced MOS gate stack materials projects.
Combinatorial thin film tool
Thermoelectric materials have a major application for vehicular waste heat recovery, which could result in a 10% improvement in fuel efficiency, as well as a decrease in CO2 emissions. There is currently no screening tool to measure the Power Factor (equal to the square of the Seebeck coefficient multiplied by the electrical conductivity), an important indicator of energy conversion efficiency of thermoelectric thin films. If applied to combinatorial thin film libraries, it would accelerate material selection and optimization. In the example shown, the power factor is plotted as a function of composition for La and Sr substituted Ca3Co4O9 thermoelectric materials. Several hundred data points were automatically probed on this library film in about 2 hours. It can be seen that the power factor “sweet spot” is approximately at the composition (Ca2Sr0.7La0.3)Co4O9.
Power factor of La and Sr substituted Ca3Co4O9 thermoelectric materials
Another driver for our combinatorial project is the MOS gate stack (gate dielectric and gate metal electrode), which must be replaced by advanced materials if scaling is to continue. The microelectronics industry has identified HfO2, among other high-k gate dielectrics, as the leading candidate replacement materials for the SiO2 gate dielectric. However, dielectrics with dielectric constants even greater than HfO2 (k = 20), i.e., between the range of 50 and 100, are now needed. We have investigated the HfO2-TiO2-Y2O3 ternary oxide system, and determined that there are many compositions with dielectric constants between 50 and 100, notably TiO2-rich oxides containing about 30% Y2O3 and 15% HfO2.
We have also studied the other gatestack layer, the gate metal electrode. The selection of gate electrode materials to replace polysilcon is not as far along as for the gate dielectric case. Recently, we have focused our efforts on gate electrode
Dielectric constants of HfO2-TiO2-Y2O3
materials, specifically metalloids such as the Ta-C-N ternary system. We have developed high throughput, automated methods to measure current-voltage (I-V) and capacitance-voltage (C-V) properties of gate stack structures. From these measurements, the work function (φm) and leakage current density (JL) can be determined. Shadow masks are used during thin film deposition, so etching is not needed to produce capacitor pillars. Each capacitor is individually addressable;the properties of 700 capacitors can be measured in about 5 hours. Our work function results on the Ta-C-N system are the first reported comprehensive measurements of the dependence of work function on composition for this metalloid system deposited on high-κ dielectrics.
Work function as a function of position (i.e., composition) on a Ta-C-N library film