RF-Sputtering principles

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Introduction

RF-sputtering is a suitable technique to fabricate optical planar waveguides and photonic microcavities operating in the visible and NIR regions. Sputtering techniques are widely used in industrial process because high quality films can be obtained at low temperature substrates. We have also demonstrated as the rf sputtering is a suitable technique for fabrication of dielectric microcavities and it is a cheap and versatile technique to deposit alternating layers of different materials with controlled refractive index and thickness. With these advantages, as well as the possibility to incorporating QCM, rf-sputtering process is a extremely appropriate candidate to fabricate high quality and homogeneous 1-D photonic crystals and planar waveguides.

S. Valligatla, A. Chiasera, S. Varas, N. Bazzanella, D.N. Rao, G.C. Righini, M. Ferrari, "High quality factor 1-D Er³⁺-activated dielectric microcavity fabricated by RF-sputtering", Optics Express 20 (2012), pp. 21214-21222, doi: 10.1364/OE.20.021214.

In a sputtering system, energetic ions (usually argon ions) from the plasma of a gaseous discharge bombard a target that is the cathode of the discharge. Target atoms are ejected and impinge on a substrate (the anode), forming a coating.

J.E. Mahan, “Physical Vapor Deposition of Thin Film”, John Wiley & Sons, Inc. (2000), ISBM 0-471-33001-9.

DC Sputtering

DC sputtering utilize a DC gaseous discharge. Ions strike the target (the cathode of the discharge), which is the deposition source. The substrate and the vacuum chamber walls may be the anode. the power supply is simply a high-voltage DC source.

RF Sputtering

In RF sputtering there are a cathode (the target) and a anode, in series with a blocking capacitor (C). The capacitor is part of an impedance-matching network that proves the power transfer from the RF source to the plasma discharge. The power supply is a high voltage RF source often fixed at 13.56 MHz. The blocking capacitor C is placed in the circuit to develop the all-important DC self-bias, and a matching network is utilized to optimize power transfer from the RF source to the plasma. RF-sputtering offers advantages over DC; in particular sputtering of an electrically insulating target become possible.

Magnetron

The magnetron is the desing of high-deposition-rare sputtering sources. The magnetron is a magnetically assisted discharge. As in the DC and RF sputtering arrangements there is a perpendicular (to the target surface) electric field. But in the Magnetron configuration a permanent magnet (or electromagnet) is added, to create lines of magnetic flux that are parallel to the surface of the target. The magnetic field concentrate and intensifies the plasma in the space immediately above the target, as a result of trapping of electrons near the target surface. The effect results in enhanced ion bombardment (without the increase the operating pressure) and sputtering rate for both DC and RF discharges.

D.L.Smith, "Thin-Film Deposition: Principles Practice", McGrawHill (1995), ISBN 0-07-058502-4.

DC Model Discharge

The cathode is that electrode which attracts cations (positive ions) from the plasma, while the anode attracts anions (the electrons). Most of the space between the electrodes is electric-filed-free. Two relatively high-field regions, the sheaths, separate the plasma body from the anode and the cathode.

RF Model Discharge

Place an insulator onto the cathode of a DC discharge would make the discharge inoperable because no current could flow through the insulator. The situation is quite different with RF, on average, no net current does ftow. The RF voltage is applied through an external blocking capacitar.

The input waveform is: VRF(t) = VRFSin(wt)

We will assume that the amplitude of VRF is on the order of 500 V (a). The current that would flow to the target if the blocking capacitar were not there is reported in (b).Because the plasma potential is quite cose to ground, the current density to the electrode alternates between Jion and -qz- as VRF changes sign. Clearly, the net current averaged over one period is not equal to zero.

In (c) is showed schematically the effect of the DC self-bias on the key potential difference, that of the target (cathode) minus that of the body of the plasma. The cathode potential is Vcat(t) = VRF(t) - Vdc.

Most of the time the potential of the target is below that of the plasma. In order to achieve zero net current over each RF cycle, it is necessary that during most of the cycle the électrode's potential be below Vr. The self-bias of the blocking capacitor thus develops because negative charge accumulates on the right-hand Vdc-->VRF which is the amplitude of the applied voltage. All this may be visualized with the help of (d), which shows the current flowing to the target with the self-bias. Averaging the current to zero means that the area under the curve (d) must be zero for one complete period in steady state. All the parameters are choosen to have a charging time of the blocking capacitor during the electron bombardment on the target not much greater than the RF period. The discharging time during the ion bombardment must be greater than the RF period. The ion bombardment occours for most of the period and the electron bombardment, that allow to neutralize che residual charge on the target, thank to the higher mobility of the electrons in respect to the heavier ions, can be faster.

J.E. Mahan, “Physical Vapor Deposition of Thin Film”, John Wiley & Sons, Inc. (2000), ISBM 0-471-33001-9.

Matching Network

Typical matching network for high-frequency rf plasma coupling.

Microbalances

The vibrating quartz crystal mass-deposition monitor, or quartz crystal microbalance, is one of the most powerful and widely used diagnostic instruments in thin-film technology. It uses the resonant crystalline quartz wafers that were developed for frequency control in radios and are used also for timing in computers and watches.

Crystalline quartz is piezoelectric, so a quartz wafer generates an oscillating voltage across itself when vibrating at its resonant frequency, and this voltage can be amplified and fed back to drive the crystal at this frequency. Electrical coupling is done with thin-film metal electrodes deposited on opposite faces of a thin quartz wafer having the proper crystallographic orientation. For deposition monitoring, one electrode is exposed to the vapor flux and proceeds to accumulate a mass of deposit. This mass loading reduces the crystal's resonant frequency, Wr. Comparison of the loaded Wr with the Wr0 of a reference crystal located in the instrument's control unit is used to calculate the mass of deposit.

D.L.Smith, "Thin-Film Deposition: Principles Practice", McGrawHill (1995), ISBN 0-07-058502-4.

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