New Micro Wiring Allows 100 Times More Current to Flow

2014/01/29 12:55
Tetsuo Nozawa, Nikkei Electronics
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A Japanese research team announced that it has formed a micro interconnection pattern with a minimum width of 0.5μm by using a composite material made from single-wall carbon nanotubes (CNTs) and copper (Cu).

Its maximum allowable current density is 100 times higher than that of Cu while its thermal expansion coefficient is 4.2-6.3ppm/K, which is close to that of silicon (Si), or 3.0ppm/K. So, if a large current is applied and temperature rises, it does not cause much deformation.

The team consists of researchers from Technology Research Association for Single Wall Carbon Nanotubes (TASC), Japan's National Institute of Advanced Industrial Science and Technology (AIST) and New Energy and Industrial Technology Development Organization (NEDO).

The research team plans to announce the latest results at nano tech 2014, a trade show that runs from Jan 29 to 31, 2014.

Cu strengthened by CNTs

TASC and AIST announced July 23, 2013, that they used a composite material made from single-wall CNTs and Cu to realize a 100 times higher maximum allowable current density than that of Cu and an electric conductivity equivalent to that of Cu. Maximum allowable current density is a value related to the strength of wiring when a large current is applied to it. And, in general, there is a trade-off between the value and electric conductivity.

For the composite material, both of the two values become high because Cu particles are grown, for example, inside the single-wall CNTs by electroplating to form a structure that looks like a reinforced concrete building.

Specifically, the maximum allowable current density of the new material is about 108A/cm2, which is 100 times higher than that of Cu (106A/cm2). However, at that time, TASC and AIST did not mention the method to form an interconnection pattern in a semiconductor manufacturing process.

This time, the research team used the material to form a micro interconnection pattern on a Si chip. One of the technologies that contributed to realizing it is the "Super Growth Method," which was developed by AIST, etc to manufacture single-wall CNTs having a high orientation. The method was used to make cloth-like single-wall CNTs, in which CNTs having the same orientation are intertwined with each other.

"We made it by arranging a catalyst in a linear shape," AIST said.

Each of the cloth-like single-wall CNTs is peeled off and placed on a Si chip. It is then patterned, and Cu is attached to it by electroplating. The interconnection made of the composite material showed almost the same electric conduction properties as those of a material made with a conventional method. Currently, the width of the wiring ranges from 0.5μm to several tens of micrometers, AIST said.

However, at this point, the orientation of the wiring is not necessarily the same as that of the single-wall CNTs. When the orientation of CNTs is different from the direction of current, electric characteristics greatly change. But AIST said, "It can be solved by using a different process for each orientation of the wiring."