new high-k material from Bell Labs
06/01/1998
New high-k material from Bell Labs
A group of Bell Labs researchers has developed a new zirconium-tin-titanium oxide material with a high dielectric constant that may become a contender for use in capacitor and gate oxide structures in future generations of devices.
The team says the material can be deposited using on-axis reactive sputtering with a composite target that would allow today`s PVD technology to be leveraged in the material`s use. Deposition by CVD, which would be a necessity for 3-D DRAM capacitor structures, should be feasible, said Lynn Schneemeyer, a Bell Labs technical staff member who worked on the project with colleagues Bruce van Dover and Robert Fleming.
An interesting aspect of the work was the use of a three-gun sputtering system, which deposited a film of continuously variable composition across a silicon substrate. One gun was used for each of the three metals, with oxygen supplied during deposition to create the oxide.
The quantity of each metal deposited at a given point on the substrate was determined by its distance from that gun, thus creating a wide range of compositions. The film`s electrical properties were checked at 4000 points across the substrate, and areas with interesting performance were analyzed for composition using a Rutherford backscatter technique.
The material identified as most promising by the group is an oxide of a compound containing 60% titanium, and 20% each of tin and zirconium. It has a dielectric constant (k) of 60, and a breakdown field of 4 mV/cm2. The ratio of the three metals was something of a surprise, said van Dover, since it differs from the composition currently used in making bulk dielectrics for devices like microwave dielectric resonators. The three-gun system allowed about 120,000 different compositions to be evaluated over three months.
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One issue for all advanced circuit materials is their ability to be integrated with standard interconnect metals. Schneemeyer said the group`s aim is to make new material compatible, but, "it`s still too early to tell." She added that the Bell Labs material has a dielectric constant more than twice as high as tantalum pentoxide, one of today`s leading high-k contenders, and that it also has some advantages over BST, which has received a good deal of attention for future use. Unlike BST, the new material does not require high-temperature processing or exotic electrode metallurgy, said Schneemeyer.
The material still needs significant development to be made production worthy; a Bell Labs spokesman declined to comment on internal timetables or possible licensing. The advent of high-k dielectrics is envisioned in the SIA Roadmap for DRAM capacitors in the next few generations, and for gate oxides a few years later (see figure below), to avoid the problems associated with using extremely thin layers of silicon dioxide, the traditional dielectric. - P.D.