Written By Matt Hughes – President – Semicore Equipment, Inc.
Published: 24 November 2014
Sputtering is the thin film deposition manufacturing process at the core of today’s semiconductors, disk drives, CDs, and optical devices industries. On an atomic level, sputtering is the process whereby atoms are ejected from a target or source material that is to be deposited on a substrate – such as a silicon wafer, solar panel or optical device – as a result of the bombardment of the target by high energy particles.
The verb “To Sputter” comes from the Latin word Sputare meaning to “To emit saliva with noise.” While the word sputtering sounds funny to those who associate it with stammering and speech impediments, in 1970 Peter J. Clarke changed the course of history when he developed the first “Sputter gun” that catapulted the semiconductor industry by enabling the accurate and reliable deposition of materials on an atomic level using a charged plasma stream of electrons and ions in a vacuum environment.
The sputtering process begins when a substrate to be coated is placed in a vacuum chamber containing an inert gas – usually Argon – and a negative charge is applied to a target source material that will be deposited onto the substrate causing the plasma to glow.
Free electrons flow from the negatively charged target source material in the plasma environment, colliding with the outer electronic shell of the Argon gas atoms driving these electrons off due to their like charge. The inert gas atoms become positively charged ions attracted to the negatively charged target material at a very high velocity that “Sputters off” atomic size particles from the target source material due to the momentum of the collisions. These particles cross the vacuum deposition chamber of the sputter coater and are deposited as a thin film of material on the surface of the substrate to be coated.
Sputtering only takes place when the kinetic energy of the bombarding particles is extremely high, much higher than normal thermal energies in the “Fourth state of nature” plasma environment. This can allow a much more pure and precise thin film deposition on the atomic level than can be achieved by melting a source material with conventional thermal energies.
The number of atoms ejected or “Sputtered off” from the target or source material is called the sputter yield. The sputter yield varies and can be controlled by the energy and incident of angle of the bombarding ions, the relative masses of the ions and target atoms, and the surface binding energy of the target atoms. Several different methods of physical vapor deposition are widely used in sputter coaters, including ion beam and ion-assisted sputtering, reactive sputtering in an Oxygen gas environment, gas flow and magnetron sputtering.
Because ions are charged particles, magnetic fields can be used to control their velocity and behavior. John S. Chapin is credited with inventing the first planar magnetron sputtering source with a patent filed in 1974. While conventional diode sputtering can deposit extremely thin films down to the atomic scale, it tends to be slow and most effective with small substrates. The bombardment of the substrate can also create overheating or damage to the object to be coated.
Magnetron sputtering deposition uses magnets behind the negative cathode to trap electrons over the negatively charged target material so they are not free to bombard the substrate, allowing for faster deposition rates.
The most common magnetron sputter cathode/target shapes are circular and rectangular. Rectangular magnetrons are most commonly used in larger scale “In-line” systems where substrates scan linearly past the targets on some type of conveyor belt or carrier. Circular sputtering magnetrons are more commonly found in smaller scale “Confocal” batch systems or single wafer stations. Read More…
Reactive Sputtering is the process of adding a gas to the vacuum chamber that undergoes a chemical reaction before coming into contact with the materials to be coated. Gases like Nitrogen or Oxygen which are normally stable and inert under normal circumstances become ionized and reactive in the plasma environment as a result of the high energy collisions.
When this happens, the gas can react chemically with the target material cloud and create a molecular compound which then becomes the thin film coating. For example, a silicon target reactively sputtered with oxygen gas can produce a silicon oxide film, or with nitrogen can produce a silicon nitride film which are at the heart of the semiconductor industry. Read more…
Co-Sputtering is where two or more target materials are sputtered at once in the vacuum chamber and is often used with Reactive Magnetron Sputtering to produce thin films that are compounds such as alloys or composites.
It is widely used in the optical and architectural glass industries. By utilizing Reactive Co-Sputtering of two target materials such as Silicon and Titanium with dual Magnetron Sputtering, the refractive index or shading effect of the glass can be carefully and precisely controlled on applications ranging from large scale surfaces such as skyscraper architectural glass to sunglasses. It is also widely used producing solar panels. Read more…
Types of Sputtering Power Sources
There are several different types of power sources used to bombard the target material to sputter the atoms – including DC and RF Sputtering, Pulsed DC, MF, AC and the newly evolving HIPIMS sputtering techniques.
DC or Direct Current Sputtering is the simplest and most frequently used with electrically conductive target materials like metals because it is easy to control and relatively low cost in power consumption. When possible, DC Sputtering can be a relatively inexpensive, cost effective solution for coating a wide range of decorative metal coatings. Read more…
However, DC Sputtering has limitations when it comes to dielectric target materials – coatings which are non-conducting insulating materials that can take on a polarized charge. Examples of common dielectric coating materials include Aluminum Oxide, Silicon Oxide and Tantalum Oxide.
During DC Sputtering, the gas in the vacuum chamber becomes ionized. As a result, positive ions are produced which accumulate on the surface of the target face giving it a positive charge. This dielectric buildup of a positive charge over time can terminate the discharge of sputtering atoms.
Several methods have been developed to alternate or pulse the sputtering power source to “clean” or neutralize the target surface and prevent it from developing a positive charge.
RF or Radio Frequency Sputtering alternates the electrical potential of the current at radio frequencies to avoid a charge build up. By alternating the current in this manner, each phase of the cycle has the effect of reversing the buildup when the current is only flowing continuously in one direction. As with DC Magnetron Sputtering, RF Magnetron sputtering coaters increases the growth of the thin film by increasing the percentage of target atoms which become ionized. Read more…
Pulsed DC Sputtering is where the target is bombarded with powerful voltage spikes to clean the target face and prevent the buildup of a dielectric charge. These voltage spikes which clean the target surface are usually set at frequencies ranging from 40 to 200 KHz. Read more…
HIPIMS or High Power Impulse Magnetron Sputtering is a newly evolving sputtering technique which also uses a high current voltage spike to greatly increase the ionization of the sputtering target. Compared to traditional sputtering processes, ionized atoms in HIPIMS systems have significantly higher energies capable of producing very dense thin film coatings. Read more…
MF or Mid Frequency AC Sputtering is usually used for depositing non-conductive thin film coatings. Two cathodes are used with an AC current switched back and forth between them which cleans the target surface with each reversal of the current. Read more…
Matt Hughes is President of Semicore Equipment Inc, one of the world’s leading suppliers of high performance PVD coating equipment including RF, DC and Pulsed DC, HIPIMS and AC Sputtering Systems.
What is Sputtering? Video Script
Exactly how does the sputtering process work?
First your coating materials are placed on a magnetron in a solid form called a target. For highly pure coatings you need a clean environment with only materials of your choosing.
This is why the chamber is evacuated, to remove almost every molecule from the chamber. Then the chamber is backfilled with a process gas.
Which gas is selected is based on the type of material to be deposited; Common process gasses include argon, oxygen, and nitrogen.
Now the conditions are ready to begin the process. A negative electrical potential is applied to the target material to be sputtered which is the magnetron cathode – and the positive anode or ground is the chamber body.
This electrical potential will cause free electrons to accelerate away from the magnetron. When these electrons collide with a process gas atom they strip the gas atom of an electron creating a positively charged process gas ion.The positively charged ion is accelerated toward the magnetron.
This ion carries enough energy with it to “knock off” or “sputter” some of the magnetrons target material. Target material will then collect on surfaces in the path that the magnetron is directed. This is how “sputtered” material collects on your substrate.
The light from the plasma is created when the ions recombine with free electrons into a lower energy state. Positively charged ions recombine with free electrons to create a neutral atom again.
The plasma glow is created when the ions recombine with free electrons into a lower energy state. When a free electron recombines with an ion it has a voltage; the ion needs less voltage, so this ‘excess voltage’ is let off as light. The light is the plasma glow that is seen during processing.
This thin film deposition process continues at a constant rate until the desired thickness is achieved and the power is removed from the cathode.
This amazing atomic reaction known as “sputtering” is what makes Semicore a leader in custom vacuum equipment.
Semicore Equipment, Inc. is a leading worldwide supplier of sputtering equipment for the electronics, optical, solar energy, medical, automotive, military and related high technology industries. Please allow our support staff to answer any questions you have regarding “What is sputtering?” and how to implement the best equipment and techniques for your specific needs – whether it be DC, RF, Pulsed DC or HIPIMS equipment – by contacting us at email@example.com or calling 925-373-8201.
DC electrical current typically in the -2 to -5 kV range is applied to the target coating material that is the cathode or point at which electrons enter the system known as the negative bias. A positive charge is also applied to the substrate to be coated which become the anode. The electrically neutral argon gas atoms are first ionized colliding with the target which eject atoms off into the plasma – a hot gas?like state consisting of roughly half gas ions and half electrons that emits the visible plasma glow… Read More
RF Sputtering can be used for the coating of dielectric or insulative materials that can take on a charge that results in arcing in the vacuum chamber with convention DC Sputtering. However, RF Sputtering deposition rates are slower than DC Sputtering rates and have higher power costs and so is usually used on smaller substrates to be coated. … Read More
Compared to conventional DC Sputtering, arcing can be greatly decreased or even eliminated by pulsing the DC voltage in the 10–350?kHz range with duty cycles in the 50–90% range. A Pulsed DC electrical current typically in the few hundreds of volts range is applied to the target coating material. the voltage is either turned off or reversed with a low voltage short duration cycle to “cleanse” the target of any charge buildup… Read More
By pulsing the target coating material with short bursts of very high voltage energy – with a length of ~100 µs on the order of kW·cm-2 but with a relatively short duration or “Duty time” of less than 10% — allows for a large fraction of the sputtered target material to be ionized in the plasma cloud without overheating the target and other components of the system. The target has a chance to cool during the predominant “Off duty” time which results in a low average cathode power of 1–10 kW which helps maintain process stability… Read More