Plasma Hysteresis, a 50 year Conundrum, Has Been Finally Solved

- Expected to address imbalance in semiconductor deviceㆍequipment -

 

# It has been never easy to evaluate someone without considering his/her history at all. Surprisingly, it is also the case in the world of science when it comes to hysteresis: the influence of the history of a certain substance on its current state.

 

Dr. Hyo Chang Lee, a senior researcher from the semiconductor Integrated Metrology Team of KRISS, has revealed the cause of plasma hysteresis and succeeded in controlling it. The production rate is expected to speed up as the chronical quality degrading problem in the semiconductor/display process has finally been resolved.

 

* Plasma: ionized gases consisting of electrons, ions, and neutral gas (reactive radicals). From an electrical point of view, it is quasi-neutral and is characterized by collective behavior. It is also called “the 4th matter state” other than solids, liquids, and gas.

 

Plasma, which contains electrons, ions, and reactive radicals, has been broadly used in various industries, such as nuclear fusion, environment, aerospace, and bio and medical industry. Especially in recent years, it has shown great value in semiconductor and display manufacture fields, which are high-tech intensive aggregates, and it is used in most processes such as deposition, etching and cleaning of devices to improve the integration degree.

 

The seemingly perfect plasma, however, causes an unexpected problem. Each device process requires different conditions of plasma, and therefore the external parameter (power) of inductively coupled plasma equipment is controlled in the process to obtain the desired state. The problem is that there is hysteresis with change in the external parameter (power): as plasma property is varied depending on its history, it is not changed to the desired state.

 

Once plasma hysteresis takes place, it deals a critical blow to the performance of a device and its process yield ratio. Despite long plasma research, how this plasma hysteresis occurs remains an unresolved question in plasma physics. In industrial processes, thus, researchers and engineers had to repeat the trial and error and continue to adjust the settings until the desired state of the plasma and process results were achieved, resulting in the desired process results.

 

Since the late 1900s, a number of theoretical and experimental researches have been conducted to find the cause of plasma hysteresis, but a comprehensive description and a physical understanding of the main mechanism underlying its bistability and hysteresis remain unresolved conundrum in the plasma physics society.

 

Dr. Hyo Chang Lee first proved that by using the precise measurement technique the cause of hysteresis is the difference caused by the electron energy distribution within plasma. The achievement is the result of a series of modeling and experiments regarding the theories of plasma hysteresis that were established based on precise measurement-based research of KRISS.

 

Dr. Lee took one step further than just finding the cause. He has developed original control technology to use plasma in an optimized process in a stable manner by injecting specific inert gases or changing the external settings of production equipment.

 

Plasma control technology allows stable production of the semiconductor devices with high quality performance and even makes it possible to secure plasma source technologies to optimize equipment. Thanks to the achievement, the device process and equipment industries, which are two pillars of the semiconductor market, are expected to grow together.

 

The findings of this research were published as an invited review article in March in the international journal of applied physics, Applied Physics Reviews–IF: 13.667.

 

 

○ Plasma Hysteresis

 

Hysteresis is a phenomenon where the physical quantity of a certain matter is not determined only by physical conditions at a certain moment, and it is rather dependent on the history of the matter.

Plasma hysteresis has been widely detected, such as in magnetic material, elastic body, electronic devices, and plasmas.

Hysteresis occurring in inductively coupled plasma equipment: As the external variable (power) increases, plasma is transferred from low-density to high-density modes. However, even if the external variable is decreased again, plasma is formed rather in the area with relatively high density as it depends on its previous state.

Finding the root cause of plasma hysteresis and developing control technology are highly needed in consideration that plasma hysteresis has a huge impact on the quality of a device and the process yield ratio in semiconductor/display plasma processes.

 

○ Inductively Coupled Plasma (ICP)

 

Inductively Coupled Plasma (ICP) is a method to create plasma by using Faraday’s law (∇×E = -∂B/∂t).

The method has been used in various industries for more than 130 years, and the fields using it have been expanded since 1970, and they include plasma torch for air and water quality purification and steel process in the high/atmospheric pressure regimes and etching process equipment for semiconductor/display/solar cell manufacturing in the low pressure regimes. The application of the semiconductor/display manufacturing process is possible because of the accelerated ions from plasma and sheath allows anisotropic etching and the synergy effect between ions and reactive radicals exponentially improves the etch rate.

As the etch profile and the etch rate has been improved thanks to plasma technology, the degree of integration of device has also been improved, leading to the development of the next-generation, 3D-structured semiconductors, such as 3D NAND Flash Memory, 3D DRAM, and FinFET.

Inductively coupled plasma equipment has been recently used not only for etching but also for cleaning and deposition in semiconductor, display, and solar cell processes. Even the industries of configuration devices for nuclear fusion, spacecraft propellent, and air purification and energy have actively used ICP equipment.

 

 

 

▲ The Proof Results of Plasma Hysteresis (Measurement, Theory, Modeling)

 

 

▲ Dr. Hyo Chang Lee is measuring plasma.