Low-K

In semiconductor 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 and transistors) 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.

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Low-κ Materials

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. ^[1]

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, ^[2] Aurora from ASM International N.V.. ^[3] 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. ^[4] Novellus Systems' Coral 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, ^[5] Aurora 2.7 and Aurora ULK. ^[6] 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. ^[7] Other spin-on organic low-K include polyimide, polynorbornenes, Benzocyclobutene, PTFE.

Porous SiLK

By introducing pores into the SiLK resin, the dielectric constant value can be lowered to 2.2. ^[8]

Spin-on silicone based polymeric dielectric

There are two kinds of silicone based polymeric dielectric materials, hydrogen silsesquioxane (HSQ) and methylsilsesquioxane (MSQ).

See also

• High-k

References

1. ^ http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V0W-44MFR55-G&_user=961305&_coverDate=01%2F31%2F2002&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000049425&_version=1&_urlVersion=0&_userid=961305&md5=343451401b32a10c9c5be652692bcb0b
2. ^ http://www.appliedmaterials.com/products/producer_black_diamond_4.html
3. ^ http://www.reed-electronics.com/semiconductor/article/CA528453
4. ^ http://www.fabtech.org/content/view/2079/74/
5. ^ http://www.appliedmaterials.com/products/producer_black_diamond_4.html
6. ^ http://www.reed-electronics.com/semiconductor/article/CA528453
7. ^ http://www.dow.com/silk/
8. ^ http://www.dow.com/silk/silky/feature.htm

• Nasa on Low-k
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