Special K
The dielectric constant k, also called the relative permittivity, specifies the ability of a material to contain an electric field. It's analogous to the permeability of a magnetic material where the material concentrates magnetic lines of flux. Dielectric strength is the ability of a material to withstand voltage without breaking down. A high k and a high dielectric strength are desireable properties in a capacitor.
The metal-oxide field effect transistor (MOSFET) is the workhorse of most integrated circuits. MOSFETs act as amplifiers and switches in computer chips. The Pentium 4 has more than forty million transistors. The increasing circuit density of chips is accomplished by shrinking the transistors. At present, the densest chips are defined by a 0.18 μm lithographic process in which the minimum feature size, the width of the transistor "gate," is 65 nm. Dielectric materials are an important part of this process because they insulate the conductive gate from the rest of the transistor. The "field" part of a MOSFET is the electric field in this insulator. Silica (SiO2), with a dielectric constant of 3.9 is presently used as the gate insulator. One problem with the very thin silica used in MOSFETs is that it isn't a perfect insulator, and some electric current flows between the gate and other parts of the transistor. These leakage currents have begun to dominate the power budget in dense computer chips. Not surprisingly, high k materials have caught the attention of the semiconductor manufacturers.
In simultaneous announcements [1-5], IBM and Intel announced that they will use high-k dielectric in their next generation of chips with 45 nm gate width. Intel announced that it will replace silica with a hafnia (Hafnium oxide, HfO2) material. Intel says its hafnia will reduce leakage by a factor of ten. The high-k process will require a change in the gate material from polysilicon to a metal, but Intel did not reveal the metal composition. IBM did not reveal the materials used for either its dielectric or gate, but it would be surprising if they were not similar to Intel's materials. Advanced Micro Devices, a competitor to Intel, is a research partner with IBM, along with Sony and Toshiba. They are expected to implement this change, also. NEC announced earlier that it will use a high-k process.
Atomic Layer Deposition (ALD) has been proposed as one method for deposition of hafnia materials. Common hafnia precursors are hafnium tetrachloride, tetrakis(ethylmethylamino) hafnium, tetrakis(dimethylamino) hafnium, and tetrakis(diethylamino) hafnium. As all chemists know, zirconium is chemically similar to hafnium, so making these materials in a pure form is difficult.
References:
1. Rhonda Ascierto, "It's silicon, Jim, but not as we know it." (Computer Business Review Online)
2. "Breakthrough spurs microchip arms race." (Reuters)
3. Tony Smith, "Intel 45nm CPUs to use metal gates, high-k dielectric." (The Register)
4. Mark LaPedus, "Intel tips high-k, metal gates for 45-nm." (EE Times)
5. Mark LaPedus, "IBM and partners tip high-k, metal gates." (EE Times)