Reverse Sputtering Parameters: At What Bias Voltage Does the Deposited Film Get Re-sputtered Away?
Will the already deposited film be knocked off when the bias voltage is high?
The answer is yes.
This phenomenon is known as reverse sputtering (or re-sputtering) in PVD.
Once the bias voltage exceeds a critical threshold, ion energy becomes sufficiently high to dislodge film atoms already deposited and eject them back into the gas phase. The deposition rate drops accordingly. In severe cases, negative deposition occurs - the film grows thinner over time.
What is Reverse Sputtering?
During PVD deposition, two processes simultaneously take place on the workpiece surface:
Film growth: Atoms sputtered from the target travel toward the substrate and build up the coating.
Film etching: Ions accelerated by bias bombard the workpiece and knock off already deposited atoms.
Therefore, the net deposition rate follows this relationship:
Net Deposition Rate = Deposition Rate − Re-sputtering Rate
Under normal operating conditions, the deposition rate exceeds the re-sputtering rate, so the film thickness increases. As bias voltage rises, re-sputtering intensifies. Eventually, the rate of atoms being removed can surpass the deposition rate.
Let us first introduce the fundamental concept: sputtering yield.
Sputtering yield refers to the average number of atoms ejected when one high-energy ion strikes a material. For example, when 500 eV argon ions bombard chromium (Cr), the sputtering yield ranges roughly from 0.6 to 0.8 Cr atoms per ion. Generally, higher ion energy generates a higher sputtering yield. Consequently, higher bias voltage accelerates the removal rate of material from the workpiece surface.
The critical bias voltage differs for various materials:
Pure metals (Cr, Ti, etc.)
50–100 V: Normal deposition
200–300 V: Noticeable re-sputtering begins
300–500 V: Deposition rate approaches zero
Above 500 V: Net etching may occur
Pure metals are most susceptible to re-sputtering.
Nitrides (TiN, CrN, etc.)
Nitrides feature stronger chemical bonding, making atoms harder to dislodge. Thus, their critical bias voltage for significant re-sputtering is higher.
Below 200 V: Stable normal deposition
300–500 V: Obvious reduction in deposition rate
500–800 V: Approaching the critical threshold for net material removal
DLC (Diamond-Like Carbon)
DLC is a special case. Many ta-C (tetrahedral amorphous carbon) processes employ pulsed bias at -800 V up to -1500 V. However, thanks to the pulsed mode and moderate average ion energy, stable deposition can still be maintained. Severe reverse sputtering only emerges at even higher ion energies.
Why Moderate Re-Sputtering Is Beneficial
Many newcomers mistakenly regard re-sputtering as an adverse effect, which is incorrect.
In PVD, substrate bias partially relies on mild re-sputtering to improve coating quality. Loosely bound atoms, weakly adhered particles and unstable microstructures are preferentially sputtered away. Only densely bonded, stable structures remain. The mechanism resembles sieving sand: loose grains are removed, leaving a compact framework.
Accordingly, controlled mild re-sputtering is a key factor for film densification.
Problems Arising from Excessive Re-Sputtering
Issues occur once re-sputtering becomes too intense:
Reduced deposition rate
This is the most direct observation. Even with unchanged target power, film thickness builds much slower, as newly deposited material is continuously etched away.
Shift in alloy composition
Take TiAlN as an example: aluminium is easier to re-sputter than titanium. Aluminium content in the coating gradually decreases, leading to a final composition deviating from the design target.
Disturbed crystal structure
Excessively high bias subjects the formed crystal lattice to persistent ion bombardment. Consequences include drastically elevated intrinsic stress, increased defects, damaged grain structures, brittle coatings and even film cracking.
Practical Method to Determine the Optimal Bias Voltage in Production
Bias voltage sweeping is the standard approach.
Keep target power, substrate temperature, working pressure and gas flow constant; only adjust bias voltage. Characterize deposition rate, hardness, residual stress and coating composition for each condition.
In most test results, coating hardness rises continuously at first. After reaching a certain point, the deposition rate drops sharply. This turning point marks where severe reverse sputtering initiates.
The optimal bias voltage is typically set slightly below this threshold. It delivers high coating density while avoiding excessive re-sputtering.
A Common Misconception
Many technicians assume higher bias voltage equals a more advanced process. This is a misunderstanding.
Substrate bias can be analogized to a road roller: insufficient pressure fails to compact the pavement; moderate pressure yields a smooth, solid surface; excessive pressure scrapes the road surface apart.
Summary
Reverse sputtering is an inherent physical phenomenon. Excessively high bias enables high-energy ions to re-etch already deposited coatings.
Moderate re-sputtering facilitates film densification. However, excessive re-sputtering causes slower deposition, compositional drift, increased stress and structural degradation.
Bias voltage is not superior at higher values. A mature process strikes the balance: sufficient ion bombardment to densify the film, without eroding the deposited coating.
