In semiconductor manufacturing, a low-κ is a material with a small dielectric constant relative to silicon dioxide. Although the proper symbol for the dielectric constant is the Greek letter κ (kappa), in conversation such materials are referred to as being "low-k" (low-kay) rather than "low-κ" (low-kappa). Low-κ dielectric material implementation is one of several strategies used to allow continued scaling of microelectronic devices, colloquially referred to as extending Moore's law. In digital circuits, insulating dielectrics separate the conducting parts (wire interconnects and transistors) from one another. As components have scaled and transistors have gotten closer together, the insulating dielectrics have thinned to the point where charge build up and crosstalk adversely affect the performance of the device. Replacing the silicon dioxide with a low-κ dielectric of the same thickness reduces parasitic capacitance, enabling faster switching speeds and lower heat dissipation.
The dielectric constant of SiO2, the insulating material used in silicon chips, is 3.9. This number is the ratio of the permittivity of SiO2 divided by permittivity of vacuum, εSiO2/ε0,where ε0 = 8.854×10−6 pF/μm. There are many materials with lower dielectric constants but few of them can be suitably integrated into a manufacturing process. Development efforts have focused primarily on three classes of materials:
Fluorine-doped silicon dioxide
By doping SiO2 with fluorine to produce fluorinated silica glass, the dielectric constant is lowered from 3.9 to 3.5.
Carbon-doped silicon dioxide
By doping SiO2 with carbon, one can lower the dielectric constant to 3.0,density 1400 and thermal conductivity of 0.39w/(m.k).
Porous silicon dioxide
Various methods may be employed to create large voids or pores in a silicon dioxide dielectric. Voids can have a dielectric constant of nearly 1, thus the dielectric constant of the porous material may be reduced by increasing the porosity of the film. Dielectric constants lower than 2.0 have been reported. Integration difficulties related to porous silicon dioxide implementation include low mechanical strength and difficult integration with etch and polish processes.
Porous carbon-doped silicon dioxide
By UV curing, floating methyl groups in carbon doped silicon dioxide can be eliminated and pores can be introduced to the carbon doped silicon dioxide low-κ materials.
Spin-on organic polymeric dielectrics
Polymeric dielectrics are generally deposited by a spin-on approach, such as those traditionally used to deposit photoresist, rather than chemical vapor deposition. Integration difficulties include low mechanical strength and thermal stability. Some examples of spin-on organic low-κ polymers are polyimide, polynorbornenes, benzocyclobutene, and PTFE.
Spin-on silicon based polymeric dielectric
There are two kinds of silicon based polymeric dielectric materials, hydrogen silsesquioxane (HSQ) and methylsilsesquioxane (MSQ).