- Eleena Banerjee
The continuous advancement of microelectronics in all fields of technology had become a basic fact of our life. Increasingly complex task is performed by computers requiring larger memory capacity and faster processing speeds. This leads the microelectronic industry to be driven to shrink the size of individual circuit on semiconductor chips to produce faster devices and more powerful integrated circuits and the successful development of microelectronics depends on the optimal design and processing of polymer material.
As devices tend to become smaller, the distance between electrically conducting interconnect lines decrease and below a certain point these start influencing each other when a voltage and current is applied. When two neighboring lines at different voltages are close enough a capacitor is formed. Thus, below a certain separation of interconnected lines the signal delay is dominated by capacitive resistance of the interconnected array. The potential increase of performance can no longer be exploited to its full extend under these conditions. In the above case the increased signal speed can be obtained in three way:
· Changing the layout and the ratio of width to thickness of the metal lines
· Decreasing the specific resistance of the interconnected metal
· Decreasing the dielectric constant of the insulating metal
Nobel metals like copper exhibit high conductivities but have to be separated from doped silicon since mobile metal impurities destroy the n-p ruction formed by doping. At room temperature only silver exhibits higher electrical conductivity than copper but the difference is only 5%. Therefore, no significant advancement is expected from this direction. The alternative is to use an insulating material with a lower dielectric material. Of all bulk materials poly(tetrafluoroethylene) exhibits the lowest dielectric constant of 2. Aliphatic polymers such as poly(ethylene) also exhibit low dielectric constant of 2.3.A polymer with dielectric constant of 2.5 would bring a increase in signal strength by a factor of 1.6, thus bringing a considerable advantage. But it is not possible to incorporate any polymer into a microchip. One of the most important factors that the new material must meet and which is hardest of the polymers is thermal stability.
High temperature polymers like aromatic polyamides, poly(aryl-ethers), poly (ether ketones), heteroaromatic polymers and fluoropolymers have been suggested for the use as inter metal dielectrics. Initially aromatic polyimides were the main focus but at a later stage the attention moved to the incorporation of fluorine to decrease the dielectric constant. Polyimides then with fluorinated substitutes were studied intensively, but other fluorinated polymers like poly (arylene ethers), poly(perfluoro-cyclobutene) and even amorphous PTFE-derivatives were studied for this application. Although non-fluorinated polymers were still being developed, for some time it looked as if fluorine would have to be used to achieve sufficiently low dielectric constants.
Recently, non-fluorinated polymers have again received increased interest since it is believed that the presence of fluorine and hydrogen in a polymer structure may produce HF during processing. The potential corrosion problems are considered too severe to take this risk. So far, no material has been found to fulfil all criteria. Heteroaromatic polymers, especially poly(benzoxazole)s are very promising as control is easy for this class of polymers. The slightly polar heterocyclic do not increase the dielectric constant dramatically. Values not much higher than 2.7 can be achieved, even with fluorine free poly(benzoxazoles). Poly(quinoline)s and PPQs even have the required thermal stability and will probably exhibit low dielectric constants after optimization of chemical structure. Dielectric constant below 2.5 will certainly be reached with porous materials as long as absence of fluorine and thermal stability above 400°C are maintained as key requirements. A larger number of polymers can become applicable if the processing temperature reduces to 350°C evolution won’t be a problem so perflorinated aliphatic polymers such as Teflon and Teflon AF processed from solution will allow bulk constants in the vicinity of 2 and even lower values may be obtained with porous material.
Therefore, with the advent and continuous advancement in both chemical technology and electronic components, polymers are playing a crucial role in bringing the world into our hands, growing more complex yet integrated in every iteration. In accordance with the Moore's law, every 20 years, we witness a great chance in technology. Thus, its time to wait and see where Science and technology takes on the next incredible journey.
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