manufacturing, a Low-κ dielectric
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
) from one another. As components have scaled and transistors have gotten closer and 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. 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, the dielectric constant can be lowered to 3.0. Major products of carbon doped silicon dioxide include Black Diamond
from Applied Materials
from ASM International N.V.
. The Aurora is the low-K material used in Intel
90nm, 65nm and 45nm lines, while the Black Diamond controlled about 80% of low-K material market. Novellus Systems
also falls in this category.
Porous Silicon Dioxide
Various methods may be employed to create large voids or pores in a silicon dioxide dielectric. Air has a dielectric constant of roughly 1.0005, 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 group in carbon doped silicon dioxide can be eliminated and pores can be introduced to the carbon doped silicon dioxide low-K materials. Products in this category include Black Diamond II
, Aurora 2.7
and Aurora ULK
. The reported K value can be as low as 2.5.
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. SiLK from Dow Chemical is a well known example of low-K material in this category. Other spin-on organic low-K include polyimide, polynorbornenes, Benzocyclobutene, PTFE.
By introducing pores into the SiLK resin, the dielectric constant value can be lowered to 2.2.
Spin-on silicone based polymeric dielectric
There are two kinds of silicone based polymeric dielectric materials, hydrogen silsesquioxane (HSQ) and methylsilsesquioxane (MSQ).