In the DC sputtering process by accelerating positive ions towards the target material (which is in negative potential) and colliding with its surface, due to the lack of electrical conductivity of the surface to move the charge, the positive charge accumulates on the surface of the target material. Due to this phenomenon, the tendency of the positive ions to move towards the target material is reduced and the sputtering process does not function properly.
In the DC sputtering process of the dielectric material, the inner wall of the vacuum chamber is also coated with non-conductive material and traps the electric charge. This phenomenon, called the disappearing anode, causes the electrical charges from inclined toward this nonconductive layer. Due to this phenomenon, mini and macro arcs are created during the deposition process, causing the atoms to be excluded unevenly from the target; these particulates are incorporated into the deposited films. The construction of high-tech systems requires thin films that have acceptable uniformity; the non-uniformity of the deposited thin film causes the thin films to fail. In addition to the above, arcs can damage the power supply.
RF power supplies have been suggested in order to avoid the problems posed for the DC sputtering of dielectric materials. The 13.56MHz power source is applied to the target material, but the impedance matching box must be used to transfer the maximum power. In this mechanism, the target material and substrate holder act as two electrodes. As the power is applied, the electrons oscillate with the applied frequency between the two electrodes. Because the mobility of ions is lower than that of electrons, the ions remain in the center of the two electrodes. In the positive half-cycle, the target material acts as an anode and absorbs electrons to one side, but due to the low mobility of ions, the negative electrode does not absorb much of the positive charge, and the same is true for the negative half-cycle. As a result, both electrodes are negatively charged relative to plasma. This negative bias causes the target material to not absorb electron in the positive half-cycle and only in the negative half-cycle does it tend to absorb positive ions to neutralize the negative charge on the surface. At this point, the target has a net negative DC bias, attracting the process gas ions and resulting in the sputtering and deposition of the target material. If the electrodes are symmetrical, the process will be completely symmetrical and neither of the two electrodes will have a negative bias relative to each other. In order to remove atoms from the surface of the target material and deposition them on the substrate, the target material must be the target point of positive ions, therefore, the size of the electrodes (target material and substrate) are considered differently. Because in this method of sputtering the power is divided between two electrodes, the effective power at the target material level is 50% of the applied power in spattering by DC method. As a result, the RF sputtering rate is lower than the DC spattering. Also, due to the high cost of RF power supplies and their impedance matching box and their complexity, the use of RF sputtering is not very popular.
Using a pulsed DC power supply is a strategy that has replaced the use of RF power supplies. In pulsed spattering, the power is applied to the target material for τon. At this time, called “on-time”, the negative voltage is applied to the target material by several hundred volts and at the end of this time the voltage switch to positive polarity with less amplitude (about 20 volts). The applied voltage remains at this voltage for τoff called “off-time”. This is also called “reverse time” (τrev) because of the polarity inversion of the applied voltage in the “off-time” interval. The surface of the dielectric material that is charged during “on-time”, discharges during the “off-time” period. The “on-time” duration must be short enough that the loads on the surface cannot cause arcs in this time interval and on the other hand, the “off-time” should be long enough for the sorted charges on the surface to be completely discharged during the “on-time” in order to avoid charge accumulation in sequential ‘on’ and ‘reverse’ cycles. Usually the “off-time” interval is 1/10 of the “on-time” interval. Furthermore, the duration of the ‘on-time’ and ‘reverse time’ determines the lowest pulsing frequency (fc = 1/τcycle) known as the critical frequency, fc, and the highest duty cycle for arc-free pulsed dc reactive sputtering. Pulse DC Sputtering Power supplies work in constant current mode. This way, when returning to the negative pulse, an initially high voltage rapidly accelerates ions to the target and re-establishes the current and deposition rate. Otherwise, the current takes a short time to build up and drop the plasma impedance, which was increased slightly during the positive pulse. The time which it takes for plasma to form and stabilize depends on a variety of factors, including: Pulse duration, pulse frequency, power and pressure. In pulses of lower length and greater frequency, the plasma build-up phase dominates and the pulsed magnetron sputtering pulse will operate in voltage mode. At longer pulses and at lower frequencies, the stationary plasma phase is predominant and the pulse magnetron operates in the current mode.
In October 2001, a method called the High-Power Impulse Magnetron Sputtering (HPIMS) was introduced that won the US patent award. It was shown that using high power (100 times higher than conventional power) results in a high percentage of ions. These ions not only result from gas ionizing, but also spattering materials are ionized at this power. The result of this high ionization plasma was the film deposition uniformly on the substrate. The used pulses were at least 0.1 kW to 1 MW with peak voltages of 0.5 kV to 5 kV. The pulses were shorter than 1 ms in duration and occurred at intervals of 10 ms up to 1000 s.
This method of sputtering has many advantages over other methods. Due to the high power of the pulses, 90% of the spattered particles are ionized, which are directed directly to the substrate by electric and magnetic fields. As a result, a high degree of control can be applied to the deposition process. The thin film deposited by this method will have a high density. For example, the reported density for the carbon thin film deposited by HPIMS is 2.7 g / cm3 while the reported density for the thin film deposited by DC magnetron sputtering is 2 g / cm3.
In 2005, a method was introduced that divided the pulses into two phases. In the first phase, plasma is produced by low ionization, and in the second phase plasma is upgraded to its final phase, which is the high percentage of ionization. This method reduced power consumption in HPIMS.
See the following resources for more information.
- Sputtering Sources, Matthew M. Waite, West Chester University of Pennsylvania, West Chester,Pennsylvania; S. Ismat Shah, University of Delaware, Newark, Delaware;David A. Glocker, Isoflux Incorporated, Rochester, New York
- Pulsed magnetron sputtering – process overview and applications, P. J. KELLY, J. W. BRADLEY, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, Vol. 11, No. 9, September 2009, p. 1101 – 1107
- Characterization of pulsed dc magnetron sputtering plasmas, A Belkind, A Freilich, J Lopez, Z Zhao, W Zhu and K Becker, New Journal of Physics 7 (2005) 90, doi:10.1088/1367-2630/7/1/090