1. What is sputtering deposition?
Sputter deposition is a method of depositing thin films by sputtering material from a “target”. Sputtering is a term used to describe the mechanism in which atoms are ejected from the surface of a material when that surface is stuck by sufficiency energetic ions generated by low-pressure gas plasma, usually an argon plasma. The sputtering takes place at a much lower temperature than evaporation. Sputtered atoms ejected into the gas phase in vapor form are not in their thermodynamic equilibrium state and tend to condense on all surfaces in the vacuum chamber including the substrate.
2. Why are thin films important?
There are many useful applications for thin films. We desire to produce properties in a material that are often conflicting in nature if we use one homogeneous material. Semiconductor devices, for example, are fabricated on a thin layer deposited on a semiconductor substrate. The integrated electronic circuits depend on the confinement of electrical charges, which relies on the interfaces between different materials with different electronic properties. There are also many occasions when the properties demanded an engineering application involve features that are different for the surface than that are for the bulk. Depositing metal films and patterning them on the surface are relatively easy to secure electrical connection between semiconductor devices.
3. How does a sputtering system work?
Generally, in sputtering systems ions are generated and directed at a target, then ions sputter targets atoms. The ejected atoms are transported to the substrate and atoms condense and form a thin film. Below image shows the simple schematic of a sputtering system.
4. What are the advantages of sputtering systems compared to other thin films deposition methods?
Several methods are currently used for deposition of thin film layers. Physical vapor deposition methods (PVD) are using low pressure, therefore, these methods reduce particulates and provide purer film qualities. The main categories of PVD processing are vacuum deposition (evaporation), sputter deposition, arc vapor deposition, and ion plating.
Sputtering is a versatile method, it gives us the capability to use scalable substrates, and it provides good stability and control over the deposition rate. Our sputtering systems are capable to place samples in different sizes and shapes on the substrate.
High purity targets are available and interchangeable for sputtering systems, therefore, various types of materials, alloys and compounds can be sputtered and deposited. Although, sputtering target provides a stable, long-lived vaporization source.
5. What are the applications of sputtering?
Sputtering has many well-known applications, such as the production of clean surfaces, deposition of metallic and insulating thin films, and analysis of surfaces and growth of active layers in devices2. Thin film deposition allowed the development of many kinds of thin film electronic devices including thin film transistors (TFTs), surface acoustic devices, high-precision resistors, solar cells, magnetic and/or optical memory, liquid crystal display (LCD) and the plasma display, and a variety of sensors and actuators.
Today, lots of thin films are widely used not only for information devices but also for energy and environmental systems like ecological buildings.
Sputter deposition is widely used to deposit thin film metallization on a semiconductor material, coatings on architectural glass, and reflective coatings on compact discs (CDs), and for magnetic films, dry film lubricants, hard coatings (tools, engine parts), and decorative coatings3.
Below table gives typical thin film materials used for these applications 4.
6. What is DC sputtering?
Several sputtering systems are used for the deposition of thin films. Among these sputtering systems, the basic model is the DC diode sputtering system. The other sputtering systems are improved systems of DC diode sputtering4.
7. What is RF sputtering and how does it work?
In the RF-sputtering system, the thin films of the insulator are sputtered directly from the insulator target. In RF sputtering, it is easier to keep plasma going, it can operate at lower Ar pressures (1-15 mTorr) so there will be fewer gas collisions and therefore more line of sight deposition.
8. What is reactive sputtering?
Reactive sputtering is defined by the reaction between atoms sputtered from a metal target and reactive gas molecules diffused from a discharge gas on the substrate to produce compound thin films. The most essential process is the two-dimensional collision of metal atoms and reactive gas molecules on the substrate. When the reaction occurs on the substrate, the deposited film becomes a metal or oxide which is greatly affected by the deposition rate and the oxygen gas pressure. There are two different kinds of reactive sources: one is gas source and the other is solid source.
For the gas sources reactive sputtering, the target is a nominally pure metal, alloy, or mixture of species. The gas source is a pure reactive gas or an inert-gas-reactive-gas mixture. The reactive gas comprises ingredient elements. The ingredients make desired compound thin films. The reactive sputtering is realized by standard sputtering equipment, for instance DC (or/none) magnetron sputtering or RF (or/none) magnetron sputtering. Reactive sputtering deposition is widely used in industry. Reactive sputtering is a simple process for the deposition of the compound thin films 4.
Table 2: Reactive Gases Used in Reactive Sputtering, Some kinds of the reactive gas are poisonous. Obtain expert advice on using these before the deposition 4.
Typically, a problem in dc reactive sputter deposition is preventing the “poisoning” of the sputtering target by the formation of a compound layer on its surface. Poisoning of a target surface greatly reduces the sputtering rate and efficiency. This problem is controlled by using an appropriate sputtering configuration (dual cathode, pulse power, etc.) at a high sputtering rate and controlling the availability of the reactive gas, such that there will be enough reactive species to react with the film surface to deposit the desired compound, but not so much that it will unduly poison the target surface. 3
9. What is pulsed DC sputtering?
During the reactive deposition process, regions on the target adjacent to the racetrack become coated up, or ‘poisoned’ with an insulating layer of the reactive product (e.g. Al2O3, TiO2, SiO2, etc.). The poisoned regions charge up until breakdown occurs in the form of an arc. Arc events cause severe problems during deposition. Each event disrupts the reactive process control system and can lead to the ejection of a droplet of target material, which may cause a defect in the growing film. Furthermore, the power supply will momentarily shut down to attempt to quench the arc, thereby reducing the deposition rate. Thus arcs are detrimental to the structure, properties and composition of the coating and can lead to damage to the power supply.
The problems associated with DC reactive sputtering of dielectric materials were largely overcome through the introduction of the pulsed magnetron sputtering (PMS) process in the early 1990s. During pulsed sputtering, the target potential is periodically switched either to ground (unipolar mode) or to a positive potential (bipolar mode), at frequencies in the range 20-350 kHz. The most common mode of operation is the asymmetric bipolar one where, during the ‘pulse-off’ phase, the voltage is reversed to a magnitude equivalent to approximately 10% of the average voltage during the ‘pulse-on’ phase.
Figure 3:Micro-particles formation during DC sputtering process6.
10. I have a specific target material. How do I determine which type of power supply to use: an RF power supply or DC supply?
It’s certainly straightforward to determine if you need to use RF; you will need a simple ohm meter. Place both ohm meter leads anywhere on the target surface. If your meter reads infinity (for example, a pure alumina target will read infinity), your process requires RF power. On the other hand, if your ohm meter has a reading other than infinity, use an AC or DC power supply.
11. How to choose between diode and magnetron sputtering?
Your priorities for sputtering speed, film quality, and target utilization determine the best choice between diode and magnetron sputtering.
Diode sputtering applications produce better film uniformity, as well as 100% target usage. However, the sputtering rate is much slower than the magnetron methods. Magnetron sputtering applications have a high rate, but use a maximum of only 50% of the target. Following the shape of the magnetron, the target material is consumed in an oval shape (called a racetrack), leaving the remaining material untouched7.
Figure 5: Film quality produced by straight-DC power (left) vs. pulsed-DC power (right)
Figure 6: Film quality produced by straight-DC power (left) vs. AC power (right) 8
Table 3. Power supply selection matrix7.
12. How to sputter ferromagnetic materials?
Sputtering deposition of ferromagnetic materials (Fe, Ni, Co, Permalloy, etc) of normal thickness have high magnetic permeabilities which traps the flux from the source’s normal magnet set. However, reducing the target’s thickness causes it to saturate allowing the magnetic field to penetrate. A nickel target’s permeability is such that a target thickness of 0.100 (or less) saturates. An iron target, with its much higher permeability, must be approximately 0.002 to saturate in the normal magnet set’s field. Sputtering very thin targets brings it own problems: first the power must be very low to prevent target melting and burn-through; second, thin targets tend to bend away from the cooling well’s surface exacerbating the chances of melting and burn-through; and third there may be no obvious signs when burn-through occurs and the cooling well’s surface starts to sputter. For these reasons, it is strongly recommended the customer use high-strength magnet sets when sputtering ferromagnetic materials.
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