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Dynamic Reverse Pulsing – What About Duty Cycle?
Posted October 16, 2023 by Gayatri Rane
A previous blog showed that Dynamic Reverse Pulsing (DRP) mode offers several benefits to reactive sputtering of SiO2 through the reduction of substrate heat load by about 12% and delivered a 10% increase in deposition rate when compared to conventional bipolar pulsed (BP) dual-magnetron sputtering. DRP mode halves the power applied to each magnetron and shares the pulsing with an explicit anode, as opposed to bipolar mode in which the two targets alternate as cathode and anode with a 50/50 duty cycle (Figure 1). DRP mode maintains a high duty cycle such that the polarity is reversed on the anode only for a short time, about 5 to 30% with the goal being to sufficiently discharge target electrical charge buildup.
Figure 1: DRP vs Bipolar Pulsed Mode
Studies indicate lower plasma spread, resulting in reduced heating, towards the substrate with DRP has been attributed partly to the effect of the magnetic field on electron movement and plasma confinement. In Bipolar (BP) mode, where the targets alternate as anode and cathode, the anode is magnetic so that electrons are magnetically shielded and can reach the anode only along field lines. The secondary electrons emitted from the cathode are, therefore, concentrated in the cathode vicinity increasing the plasma density in front of the substrate. In contrast, in the case of DRP mode, if the non-magnetic anode intersects with the magnetic field line of the target, it will effectively collect the electrons so that they are not lost to the plasma and thus do not contribute to substrate heating1.
The roles of both the duty cycle and the anode are of particular interest when it comes to DRP mode. We studied the effect of different duty cycles on process and film properties. Tests were performed in an industrial drum coater with dual rotary targets and an anode placed between the targets, as shown in Figure 2.
Figure 2. Industrial drum coater (left) and the process door (right) with the dual rotary targets and anode used for the tests
Experimental set-up: Two synchronized Advanced Energy Ascent® SMS/DMS AP power supply units operating at 6 kW on each side (i.e., totaling 12 kW) with a pulsing frequency of 80 kHz were used for depositing 200 nm SiO2 films from Si targets in an Ar/O2 atmosphere. Duty cycles (i.e., the ON times of Cathode/Anode) of 80/20, 70/30, 60/40 and 50/50 were employed.
Figure 3. (Left) Hysteresis curves obtained in voltage regulated mode for reactive sputtering of Si with DRP configurations at different duty cycles. (Right) Discharge voltage/current for the film depositions with different duty cycles in DRP mode
Figure 3 (left) shows the hysteresis curves obtained with voltage regulation for the different duty cycles. Reducing the on time on the cathodes yielded a hysteresis curve shift to higher voltages (lower current). Beneficially, lower O2 flow was required to achieve an equivalent working process condition in the curve (indicated for 80% position in the transition region with arrows). Films deposited at these equivalent working points yielded a similar refractive index and composition. However, with a reduction in duty cycle, the power-normalized dynamic deposition rate decreases (7%) and the substrate heat load increases (3%) as shown in Figure 4.
Figure 4. 200 nm SiO2 film properties as a function of DRP duty cycle
It should be noted that with DRP50, the heat load is still lower and the DDR is still higher when compared to those with BP mode (as discussed in our previous blog). However, with decreasing duty cycle the films appear to get smoother, and the surface asperities flatten and become larger as illustrated in the AFM images in Fig. 5
Figure 5: Figure 3. AFM images (stretched z-scale on top and non-distorted z-scale in the bottom row) of SiO2 films deposited with different duty cycles in the DRP mode.
To summarize, the experiments demonstrate how changing the duty cycle can allow process engineers to optimize deposition conditions and film properties.
And what about the anode? We will discuss this in an upcoming blog, so keep following us for more interesting tests and results as we work towards understanding DRP mode further.
References
Figure 1: DRP vs Bipolar Pulsed Mode
Studies indicate lower plasma spread, resulting in reduced heating, towards the substrate with DRP has been attributed partly to the effect of the magnetic field on electron movement and plasma confinement. In Bipolar (BP) mode, where the targets alternate as anode and cathode, the anode is magnetic so that electrons are magnetically shielded and can reach the anode only along field lines. The secondary electrons emitted from the cathode are, therefore, concentrated in the cathode vicinity increasing the plasma density in front of the substrate. In contrast, in the case of DRP mode, if the non-magnetic anode intersects with the magnetic field line of the target, it will effectively collect the electrons so that they are not lost to the plasma and thus do not contribute to substrate heating1.
The roles of both the duty cycle and the anode are of particular interest when it comes to DRP mode. We studied the effect of different duty cycles on process and film properties. Tests were performed in an industrial drum coater with dual rotary targets and an anode placed between the targets, as shown in Figure 2.
Figure 2. Industrial drum coater (left) and the process door (right) with the dual rotary targets and anode used for the tests
Experimental set-up: Two synchronized Advanced Energy Ascent® SMS/DMS AP power supply units operating at 6 kW on each side (i.e., totaling 12 kW) with a pulsing frequency of 80 kHz were used for depositing 200 nm SiO2 films from Si targets in an Ar/O2 atmosphere. Duty cycles (i.e., the ON times of Cathode/Anode) of 80/20, 70/30, 60/40 and 50/50 were employed.
Figure 3. (Left) Hysteresis curves obtained in voltage regulated mode for reactive sputtering of Si with DRP configurations at different duty cycles. (Right) Discharge voltage/current for the film depositions with different duty cycles in DRP mode
Figure 3 (left) shows the hysteresis curves obtained with voltage regulation for the different duty cycles. Reducing the on time on the cathodes yielded a hysteresis curve shift to higher voltages (lower current). Beneficially, lower O2 flow was required to achieve an equivalent working process condition in the curve (indicated for 80% position in the transition region with arrows). Films deposited at these equivalent working points yielded a similar refractive index and composition. However, with a reduction in duty cycle, the power-normalized dynamic deposition rate decreases (7%) and the substrate heat load increases (3%) as shown in Figure 4.
Figure 4. 200 nm SiO2 film properties as a function of DRP duty cycle
It should be noted that with DRP50, the heat load is still lower and the DDR is still higher when compared to those with BP mode (as discussed in our previous blog). However, with decreasing duty cycle the films appear to get smoother, and the surface asperities flatten and become larger as illustrated in the AFM images in Fig. 5
Figure 5: Figure 3. AFM images (stretched z-scale on top and non-distorted z-scale in the bottom row) of SiO2 films deposited with different duty cycles in the DRP mode.
To summarize, the experiments demonstrate how changing the duty cycle can allow process engineers to optimize deposition conditions and film properties.
And what about the anode? We will discuss this in an upcoming blog, so keep following us for more interesting tests and results as we work towards understanding DRP mode further.
References
Gayatri Rane
Advanced Energy
As an R&D scientist at AE, with past experience in sputtering and thin film analysis, I am involved with the research activities of the Customer Solutions Lab in Karlstein am Main, Germany.
More posts by Gayatri Rane