By Ankit Malik, Senior Research Fellow, Pursuing PhD, Indian Institute of Science
Electronic devices have travelled a long journey witnessing their inception in semiconductors during 1947 when John Bardeen, William Shockley and Walter Brattain demonstrated the first point-contact transistor at Bell Labs. This lead to significant dimensional miniaturisation of electronic devices as well as their power requirement reduced. Fairchild Semiconductors was the first one to fabricate a monolithic integrated circuit (IC) using the planar process. Since then, the boom of IC miniaturisation has changed the entire industry. The current industrial ability is capable of processing electronic devices down to the 5nm level, as available in the Apple M1 processing unit, which has 16 billion transistors. The strong base of silicon electronics has actualised applications in almost every area at present. Silicon electronics has manifested as a rigid system that lacks flexibility and therefore doesn’t fit well in wearability and flexibility applications.
This attribute of flexing with functional chips retaining their electronics can be realised using organic materials. There have been few products possessing such abilities, such as the recently launched Samsung Galaxy Z Flip. The applicability of organic nanoelectronics is not limited to electronic processing units only. Organic Field-Effect Transistors (FET) possess a solid ability to be utilised as sensing units. Using organic FET as sensors is currently an active topic of research and have potential due to the tailoring ability of the organic material for a particular sensing type. As per Allied Market Research, the organic electronics market is expected to grow to $159.11 billion by 2027. One of the most popular organic electronics products, which has seen immense success, has been the OLED (Organic Light Emitting Diode) television sets. OLED televisions are known for their phenomenal contrast and production of black. The black pixels on the screen do not consume any power compared to previous generation LCD (Liquid Crystal Display) television. Apart from power consumption, they are relatively thinner hence enhancing the portability.
Discussing power requirements, the most demanded source of power has been solar energy for its unlimited availability subject to solar energy irradiated in a given geographical region. The solar cells market has seen new developments, such as flexible solar cells and organic solar cells. Their cost of fabrication and large-area processing has been a significant catalyst in propelling the horizons of solar cell research. As per the theoretical calculations, organic photovoltaics has the potential to surpass silicon ones. However, the power conversion efficiency and stability of organic photovoltaics are yet limited to 10-15% compared to 20-25% in the case of silicon photovoltaics. Yet academia and industrial research is investing resources due to their potential ability.
The manufacturing of electronics has had a strong base for several decades and has seen manganous success, which can be inferred based on Apple Inc becoming a trillion-dollar company, which depended on the power of semiconductor products. However, the case of fabrication of organic electronics has different challenges. One of the primary challenges is regarding miniaturisation of these devices, which gets limited due to several factors such as selective area deposition of organic material in the active region. Competing with silicon fabrication success which surpassed sub 10nm node, the retention of organic material within such miniature active ragions is still a challenging task.
The potential deposition methods of organic materials being researched are spin coating via solution processing, evaporation, chemical vapour deposition (CVD) and Molecular Layer Deposition (MLD). The evaporation process works on a simple principle of thermally heating a material to its evaporating temperature, but this also requires a very high vacuum to have a uniform deposition on the intended area, which is non-conformal. Apart from this, the yield of the evaporation process is relatively low since the evaporation flux deposits the material inside the entire vacuum chamber, and the target area is relatively small; therefore, there is material wastage.
The conformality issues are addressed by the technique of molecular layer deposition (MLD). This process requires precursors to create an organic material on the surface of the target substrate, has lower vacuum requirements, and gives excellent control on film thickness. However, the availability and existence of precursors are pending challenges to be addressed. The primary limiting attribute of this process is its film growth rate, which slows down the device fabrication process. The issue of time of material deposition is remarkably addressed by spin coating, which involves dropping an organic material dissolved in its appropriate solvent on the intended substrate and spin coating the system at high rpm (revolutions per minute).
There is film formation post-spin coating, generally followed by heating the substrate-organic material stack to evaporate the solvent, leaving only the organic material. The film thickness control is governed by the rpm used, and hence it is a relatively easy processing technique. The drawback poised here is in the coverage of the organic material over the entire substrate covering the unintended areas. Also, the role of solvent and evaporation rate due to heating affects the organic material’s electronic properties, which is a complex parameter to control. Henceforth, the material deposition methodologies have their advantages split amongst different processing techniques as mentioned above. An ideal process would be covering all pros of these techniques and leaving behind the cons.
The aspects of organic material deposition play a critical role in fabricating electronic devices due to its effect on electronic properties, cost, and yield. Another decisive player is the usage of patterning techniques which define the features of organic electronic devices. The commercial lithography technique employed is UV (ultraviolet) photolithography, which has been one of the most critical processes in semiconductor manufacturing for patterning.
Apart from UV photolithography, electron beam lithography has a significant role in the research and development of these devices. UV lithography works on the principle of illuminating a pattern available on a physical photo-mask, from where UV light passes from open slots and gets blocked from opaque spaces. This slot design is as per the requirement of pattern type required. The shadow of this photo-mask projects on the substrate under process and gets patterned by the help of a photo-sensitive material called a photoresist. Electron beam lithography works on the principle of focussing a beam of electrons ejected by an electron gun and controlling the beam motion to scribe the electron-beam sensitive photoresist.
The pattern is available as a CAD (Computer-aided design) file which gets scribed serially using electron beams. Both these lithography techniques involve the device’s exposure under preparation to UV light and highly energetic electron beams. The incident radiation has a potent ability to affect the organic material in organic device fabrication. The control over damage caused to the organic material is another major challenge in device fabrication. These methods are essential to achieve miniaturisation to have higher functionality in less area, therefore increasing the density of devices.
Device fabrication processes involve exposure of the substrate to several chemicals, which is a necessary integral part of the chain of operations involved. Before the exposure of these chemicals to the organic material, it requires assessment for its effect on organic material. The organic material can be etched, wholly removed or even modified during the chemical treatment step.
As discussed briefly here, the hurdles in organic electronics fabrication are under intense research by academia and industries like Samsung and LG.
The major foreseeable applications such as disposable electronics, edible electronics, biocompatible electronics and many more drive the zeal for addressing the fabrication hurdles. Several research groups are investing their resources in this domain, such as Organic Nanoelectronic Lab (ONE Lab) and Mayank Shrivastava’s Devices Lab (MSD Lab) lead by Prof. Praveen C. Ramamurthy and Prod. Mayank Shrivastava, from the Indian Institute of Science, Bengaluru, India. The future of this domain has witnessed significant success in the television industry, as mentioned earlier, and other segments of organic electronics shall thrive with time.
