New Technology Presentation Meetings of AIST
New plasma processe tehnologies appeared on the scene

September 20, 2018, “New Technology Presentation Meetings of AIST (National Institute of Advanced Industrial Science and Technology)” was held in JST (Tokyo). In this here, two oral presentations about plasma process are picked up.

Mr. Shirakawa (Flexible Electronics Research Center) introduced an original low temperature plasma sintering technology in this oral presentation.

Fig.1 Structure of oxygen pump and relationship of temperature and oxygen partial pressure in Cu sintering process1)

As you know, Nano size Ag is mainly used as a low temperature sintering type wiring material in flexible electronics category. However, low temperature sintering Cu is ideal for this application because of low process cost and anti-migration property. Of course, if Cu is annealed in atmosphere environment, it changes to CuO, which is an insulator. For this reason, Cu must be sintered in reductive atmosphere. In this time, the research group tried to form pure Cu wiring in low temperature and atmosphere process by making use of an original low temperature sintering plasma technology (Cool Sintering Process：CPS).

As figure 1, N2 gas inclusive of ultra-low partial pressure O2, which is generated by making use of solid electrolyte type oxygen pump is treated into atmosphere plasma state, and then, it is blown to the substrate. As a result, Cu film is sintered on the substrate.

Pic.2 Baseball cap with radio1)

Pic.1 Sintered Cu film by low plasma sintering1)

As the graph of figure 1, oxygen partial pressure is 10^-27 atmospheric pressure and under, CuO changes to metallic Cu in low temperature such as 180 ℃ and under by reduction phenomenon. If process temperature is less 180 ℃, the substrate is heated by the heater.

Picture 1 shows microscope photograph of sintered Nano size Cu by this process. Its grain size (diameter) was increased from 20 nm to 0.3 μm after sintering. And also, uniform film without void was gained. Furthermore, specific resistance of this Cu film was 2.6 μΩ･cm, which was 1.5 times as large as bulk value. This property (low resistance, uniformity, etc.) is not obtained by making use of the conventional technologies such as the light sintering process and atmosphere sintering type Cu paste. In fact, if this process and PEN film substrate are used instead of Nano size Ag paste and polyimide film substrate, total process cost can be reduced by approximate 1/10.

The research group has pilot-produced a baseball cap with radio by use of this process. Concretely, radio circuit was manufactured on PEN film by this low temperature process, and then, surface-mounted component was mounted on the substrate, as a result, a flexible radio was completed. Furthermore, it was inserted into flange part for wearable product. As a result, it's possible to operate radio, for example arrange of sound volume, dialing at put on cap state.

Low temperature and large size plasma is realized in high pressure environment by array

On the other hand, MR. Kim proposed a new plasma process for electronics devices.

Fig.2 Characteristics of plasma process2)

As figure 2, characteristics of plasma depend on process pressure strongly. For this reason, it's difficult to reduce temperature and enlarge treatment dimension in high pressure environment, on the other hand, it's difficult to increase throuphput in low pressure environment. The research group succeeded to develop a low temperature and large area plasma process in high pressure environment, to solve this trade-off relationship.

This basic concept is same as that of PDP. In short, microwave plasma is generated into relatively small space, and then, it was arrayed for large substrate. However, it's difficult to small by making use of the conventional microwave plasma process due to use of waveguide. Therefore, a micro strip technology was used.
As figure 3, a conductive micro strip pattern is formed in relatively small space. Plasma is generated by irradiation of microwave (wavelength 2.45 GHz). And also, as figure 4, this unit is arrayed, to be correspond to low temperature and large area process in high pressure environment. As figure 4, a few plasma processes for example, pre-treatment, plasma CVD, and post-treatment can be treated continuously by the Roll to Roll method. It's possible to correspond to wide substrate (200 �o width) in the prototype system. Of course, it's possible to manufacture mass production system by this concept.

Fig.3 Plasma generation technology by use of micro strip technology2)

Reference
1)Shirakawa：Forming of printed Cu wiring by making use of low temperature plasma sintering and baseball cap with radio, AIST New Technology Presentation Meetings!,pp.15-19 (2018.9)
2)Kim：Various advanced plasma process technologies for creation of new industries, AIST New Technology Presentation Meetings!,pp.pp.25-29 (2018.9)
※This title is translated arbitrarily by Stella News Site because of without English edition of presentation material.

Fig.4 Large scale example of high pressure microwave plasma2)