Sputtering mechanics
Use a simple DC sputtering process (glow discharge) as an example, the sputtering process is as the following
- A working gas (Ar typically) is ionized to create a plasma
- Ions in the plasma are accelerated to hit the target surface on the cathode
- Ar ions, as a typical working gas, knock atoms out of the target surface
- Target atoms are transferred to the substrate surface to form a thin film on substrate
However, the DC plasma density and sputtering efficiency are low, not efficient enough for a practical application. To solve this problem, magnetic field is introduced into the sputtering system.
Magnetron Sputtering
Benefits of Magnetron Sputtering (reference):
- Magnetic field confine plasma near the surface of target and increase plasma density
- Confine plasma near target to avoid damage to coated thin film on a substrate
- increase the efficiency of the initial ionization process; electorns travel to spiral along magnetic flux lines; generate more ions and sputter off more target atoms
- create the plasma at lower pressures
- Obtain plasma at low background gas pressure
- Reduce scattering of the sputtered atom by working gas atoms collisions
Electron drift currents in a magnetron
A electron, in homogeneous and static magnetic filed, travels in a helical way along magnetic filed line as below.
Under perpendicular electrical field and magnetic field, electrons experience constant force and travels in cyclical way with net drifting movement in x direction, perpendicular to both electrical and magnetic fields, as shown in below.
Therefore in magnetron sputtering, a racetrack etching pattern appears at target surface.
Reference 1; reference 2; reference 3
Traditional DC Sputtering has cost benefits, however, it is limited dielectric target materials are used or insulating materials are formed on substrate, e.g. Al2O3, although Al is conductive. The underlying problem is that during DC sputtering, positive working ions bombard a target and cause positively charging the surface of the target, which repels further positive ion bombarding the surface, resulting in the cessation of sputtering process.
Therefore, RF sputtering was introduced.
RF Sputtering
In RF (Radio Frequency) sputtering, RF power supply and matching network between in between cathode are used, instead of DC power supply.
In RF sputtering, cathode is biased in positive and negative alternatively as shown above. In positively biased period of time, positive charges on the surface of target are neutralized. In negatively biased period, sputtering occurs normally as in the DC case. Through this design, RF solves the difficulties of DC sputtering of insulating targets.
Comparison between DC and RF sputtering
| Sputtering type | DC sputtering | RF sputtering |
| Plasma formation | The plasma formation is limited to the cathode or target surface in DC magnetron sputtering | The plasma formation can extend in the vacuum chamber. |
| Sputtering rate | With decrease in secondary electrons over cathode (less ions knock target in RF sputtering), deposition rate is lower than DC method and higher power level is needed to increase deposition rate | Higher sputtering rate at same power |
| Arcing | Arcing is a big riskMay cause droplets in coating consequently | Eliminating charge build up on the cathode surfaceplasma arcing reduced or eliminateduniform coating layer deposition is possible. |
| Target erosion | Less plasma confined near target, decreasing the so called ‘Race Track Erosion’ on its surface, Target utilization is enhanced. | Plasma confined and more erosionTarget utilization yield is lower |
| Working pressure | Need higher working pressure to generate enough plasma density | Higher plasma currents in lower working pressure (1-15 mTorr)less collision between sputtered atoms and chamber moleculeslarger mean free path for target atoms |
| Heating effect | higher voltages should be applied in order to increase the sputtering rate, leading to more heating effect on the substrate | Less heat for same sputtering rate |
| Matching network, connectors and cables | Matching network is needed to effectively apply power in sputtering, typical value is 50 ohm (reference). RF current is transported on the skin or surface of the conductors and not through them, so special connectors and cables is needed for RF sputtering | Simple DC power cable No matching network needed |
| Cost | more complicated and expensive compared | Less expensive |
In general, sputter flux decreases with Increasing frequency since working gas ions are heavy and are practically immobilized at high frequency. Frequency of 13.56MHz with a bandwidth of 14KHz is typically used in sputtering industry where the ions have sufficient time (frequency is not too high) to transfer momentum to target.
To increase sputtering rate, and having the advantages of RF sputtering, pulsed DC at mid frequency is introduced.
Pulsed DC sputtering
The following is a typical waveform of a pulsed DC.
Duty of cycle
If the frequency of a pulsed DC is 100KHz, the time for each cycle is 10^6/(100X10^3)=10us (micr0-second). Pulse-on time (sputtering time)=8us. The duty of cycle is 8/(8+2)=80%.
Power supply options
- DC to Mid-frequency power supply (0-300KHz) for conductive targets and in reactive sputtering
- High frequency(600KHz to 27MHz), including RF power supply for dieelectrical targets
Dual Cathode sputtering at Mid frequency
In pulsed DC, sputtering occurs at on time. To further improve sputtering yield, dual cathode sputtering was introduced, where two magnetron targets are connected to a power supply that can oscillate the polarity of each target, as shown below. The polarity typically oscillates at Mid frequencies, e.g. 5 kHz and 150 kHz.
Reactive sputtering
Depending on design, coating materials may not be exactly same as target materials; or coating materials can be formed by reaction between target materials and gas introduced into sputtering chamber. It is also called reactive sputtering, e.g. SiOx can be deposited through reactive sputtering of silicon with Oxygen in the chamber.
In reactive sputtering, both the thin film formed and the target surface react with introduced reacting gases, typically O2 or N2. and two processes occur as the following:
- Target surface undergoes bombardment by ions and therefore both O2 or N2 and metal are sputtered
- Thin film on substrate also reacts with the depositing metal to form a metal oxide or Nitride
The stoichiometry of the deposited thin film can be tuned by the flux of reacting gases to adjust final film properties.
With ongoing of reactive sputtering, dielectric layer forms on the target surface, e.g. oxides or nitrides, which potentially leads to two issues target poisoning and arcing.
Target poisoning
Target poisoning is that target surface is gradually covered by dielectric as shown above, leading to decreasing sputtering rate and arcing.
Arcing
Most Common Causes of Arcing is accumulate dielectric layer on target surface and breakdown under high electrical field as addressed above. Contaminates landing on target surface and Inhomogeneity/Defect in target materials are also possible causes to arcing.
The consequence of arcing are:
- Drain plasma to feed the arc and lead to the failure of sputtering process
- Causes defects in both the thin film and target surface
Process control and modern power supply can reduce or eliminate arcing.