RF Sputtering and DC Sputtering
The manufacturing processes development of devices such as sensors and integrated circuits (ASICs) and materials as semiconductors, dielectrics and metals with specific physical characteristics, is widely required and of great importance in all the projects we attend at the CIDESI Queretaro Microtechnologies Management (MTM).
Coating process of materials by magentron Sputtering
One of the apparatus of which the CIDESI Queretaro clean room owns, is a "Magnetron Sputtering" system, of the Angstrom Engineering brand, this system is used by some of the users in reasearch projects and integrated circuits and sensors manufacturing, see Fig. 1. It is important to know, that the coating technique known as "sputtering" or cathodic erosion, is found as one of the most novel methods to obtain materials with specific characteristics due its reproducibility, scalability and varsatility of processes that can be made in this system, because of this and more, it is the most spoiled of the metalization area in the CIDESI Queretaro clean room.
Fig. 1. The magnetron Sputtering system of the CIDESI Queretaro Clean Room is used in diverse research projects and technological development such as sensors and integrated circuits.
Magnetron Sputtering system and the material coating process
In a common coating process by "Sputtering", the main chamber, where the material coating is carried on, is evacuated to the high vacuum, commonly to a pressure lower than 2 x10^6 Torr. The Sputtering of the Queretaro clean room uses a mechanical pump and a turbo-molecular pump in order to reach this base pressure. After reaching this base pressure, the coating stage may begin, where a ionizing gas, in this case Argon, flows into the chamber. In this stage, pressure and flow of argon are regulated until reaching pressures, typically, in the order of mTorr, this argon gas, is the gas that is going to form the plasma. It is important to know that reaching a high vacuum inside of the chamber is significant to reduce the partial pressures of other non-desirable elements gases that may potentially pollute the materials that are being coated.
The Sputtering coating technique is considered as a vapour phase physical coating techniques. The physical principle of the sputtering system operation consists in the generation of plasma, which is obtained by applying a potential difference between a cathode and an anode, subsequently, electrons that are found in the Argon gas, that at this point are already inside of the main chamber, are accelerated in the opposite direction to the cathode, which causes collisions of these electrons against Argon athoms. These collisions generate electrostatic repulse and promote the ionization of Argon, due electron removal from the athoms of this gas. Commonly, the cathode is in contact with the material that is going to be coated and the anode, which is connected to the chamber of the sputtering working as electric ground. At this point, the positively charged ions are accelerated to the cathode, which is negatively charged, causing high energy collisions on the target surface, and each of these collisions makes that athoms from the material surface to be eroded with enough kinetic energy so these travel from the target and get coated into a substrate, all of this, in a patrial vacuum inside of the coating chamber. The ionizing gases, commonly Argon or Xenon, are used because of their high molecular weight. See Fig. 2.
Fig. 2. In this image an Aluminium coating and the Ar plasma are observed in the magnetron Sputtering of CIDESI Queretaro. In the coatings, the pressure, commonly in the mTorr range and the Ar flux are controlled and adjusted for an optimal coating.
Coatings and co-coatings of materials
The Sputtering of CIDESI Queretaro is able to perform continuos material co-coatings and coatings before breaking the vacuum, thanks to the 3 sources it counts, two of the sources are DC and a RF. This equipment allows to perform from a coating to three material coatings at the same time, or if the manufacturing requires it, coat more than a material consequently before opening the coating chamber and due it, not breaking the vacuum, which gives us incredible advantages and possibilities in the devices and sensors manufacturing. See Fig. 3.
Fig. 3. The coating system, that the CIDESI Queretaro clean room has, is able to perform coatings and co-coatings, thanks to two 1000W DC sources and a 600W RF.
We have dedicated this system to the coating of semiconductors, metals and some dielectric. Some of the materials the we have available for coating in the clean room magnetron deposition Sputtering are: ITO, ZnO, Al, Ti, TiN, Si3N4, WO3, Bi, MgF2, Cr, Ni y SiO2, entre otros, principalmente.
This capability of coating different materials allows us to work in the development of sensors that come from micro bolometers, magnetic sensors, glucose detection to the pollutant detection in air, among others.
Coating with DC and RF sources
The Sputtering apparatus of the CIDESI Queretaro clean room has two DC sources and one RF. The DC sources, frequently are used to perform conductive target or transparent conductive oxides coatings, one of the DC sources has a special configuration in the magnetron, esta configuración helps the plasma generation due it uses a strong magnetic field, and this makes it special to perform the magnetic material coating. The RF source is used for materials with isolating properties such as oxides, nitrides and ceramics, specifically these materials have the characteristic of a great impedance, which usually changes the applied power on the coating.
Fig. 4. In the image we observe the installation of a Nickel target, a material with magnetic properties, for magnetic material coatings, the DC source must have a special configuration in the magnetron in order to generate a strong magnetic field during the coating.
The three cannons that the CIDESI Queretaro clean room has, are equipped with a magnetron, this magnetron implements a strong magnetic field that confines the electrons inside the plasma close to the target surface. Having a magnetron in each of the sources of the Sputtering system allows to increase the material coating rate, further prevents the possible impact of these electrons on the material that is being coated, which in consequence is shown in the coating quality.
Material coatings with reactive atmosphere
Among the capacities of the sputttering coating systems, it exists the possibility to perform material oxidation and nitridation during the coating. The physical phenomena previously mentioned are obtained when reactive gases are introduced into the sputtering chamber during the thin film coating, these gases are commonly oxygen and nitrogen. Currently, in the CIDESI Queretaro clean room we are working in the sputtering system upgrade, this upgrade consists in making able the reactive atmosphere coating with oxygen, the target is to obtain oxygen partial pressures during the coating and to have a better control over the coating stoichiometry, mainly the dielectric and semiconductor oxides coatings.
What are the material coating characteristics of the magnetron Sputtering?
Besides the continuos coating and co-coating options of materials before breaking the vacuum, the apparatus has also the following capabilities:
- Spinning and warming of the sample holder up to 50rpm and 650°C respectively, see Fig. 4.
- Simultaneous coating of up to three 4 inches wafers.
- The coating may be controlled by software (PID control), where the coating rate is monitored in angstrom/second.
- The coated film thickness is monitored by inficon sensor.
- Finally, the apparatus is supported by a mechanical pump and a turbo molecular pump that allows it to reach base pressures up to 5x 10 ^-7 Torr, with the objective of avoiding the pollution of the materials that are being coated.
Fig. 5. In the image the inside of the chamber is observed, the lighting is due the sample holder is being conditioned for a coating with temperature.
On the metallization bay of the CIDESI clean room, we have an e-beam evaporation system, this system is excellent and with it we coat a wide range of of materials such as metals, dielectrics and semiconductors. Nevertheless, our Sputtering system differentiates of the e-beam system, because it does not need that the materials reach such high temperatures to melt, this advetages almost every material to be coated by Sputtering, and finally, the materials that are coated have very similar properties to the targets that are being worked.
Fig. 6. In the image a silicon wafer with a zinc oxide coating is observed, the zinc oxide is a very well-known semiconductor on the sensors and electronic devices manufacturing.