Helicon source as element of hybrid plasma system in facility for thin-film coating with controlled nanostructure
This paper presents the results of optimizing of inductive RF discharge with external magnetic field in the prototype of plasma system required for the development of hybrid sputtering system with magnetron and vacuum-arc deposition assisted by a stream of accelerated ions of high density. Considered range of external parameters of helicon discharge corresponds to the area, where the resonant excitation of linked helicon and oblique Langmuir waves is possible. The resonance allows to optimize the energy contribution in the discharge and to obtain a high plasma density. In addition, the penetration of RF fields into the plasma in resonance area allows to obtain an extended regions of uniform plasma. As the main parameter under optimization, the ion current in the processing chamber is chosen. The influence of structural characteristics of hybrid plasma system (antenna type, configuration of the magnetic system, the material of the structural elements of the plasma source) on the ion current is considered. The obtained results are used for the choice of operating mode of magnetron sputtering system with the helicon discharge.
Hybrid plasma system
and measurement technique
Scheme of the experimental facility is described in detail in . Experimental prototype of HPS (Fig.1) consisted of two cylindrical chambers of different diameters. The upper part of the plasma source is a gas discharge chamber made of quartz, which had a diameter of 10 cm and height of 25 cm. The lower part of the source is processing chamber, also made of quartz, which had a diameter of 46 cm and height of 30 cm. The solenoidal antenna or Nagoya III antenna was used for input of RF power. The antenna was located on the lateral surface of the gas discharge chamber at a distance of 12-16 cm from its upper end. The ends of the antenna were connected to the output of the automatic matching systems connected to AE Cesar 1310 RF generator with an operating frequency of 13.56 MHz and an output of 0-1000 watts. The Rogowski loop was used to measure the current in the antenna.
Two types of magnetic system were used. In the first case, two electromagnets located in the upper and lower parts of the processing chamber, allowed to create a uniform (within 7%) magnetic field up to 7 mT. In this case, a weakly diverging magnetic field appeared in gas discharge chamber. In the second case by means of additional coil a uniform magnetic field in the gas discharge chamber was created, while in the processing chamber a divergent field has appeared.
Diagnostic stand allowed to measure the power of RF generator delivered to an external chain, the current through the antenna, the RF voltage at the ends of the antenna, the emission spectrum of the plasma and spatial distribution of the luminescence intensity of the plasma. Standard probe method allowed to measure current-voltage characteristic of probes, the ion saturation current, concentration and energy distribution of electrons.
The experiments were carried out in the pressure range from 0.7 to 3.0 mTorr, with the output of the RF generator from 150 to 400 W and the working frequency of 13.56 MHz.
Helicon discharge parameters in the hybrid plasma system
Discharge experimental tests in the plasma source layout performed with two types of magnetic systems showed that the application of a magnetic field leads to significant changes in the length of a discharge in using both a solenoidal inductor and the Nagoya III antenna. In the absence of a magnetic field, a charge is concentrated in the top gas discharge chamber. An increase in the magnetic field strength at pressures below 1 mTorr first led to the emergence of plasma in the upper portion of the lower discharge chamber, then, in the case of a uniform magnetic field, the length of the intensely luminous part of the discharge in the lower chamber was beginning to grow, and finally the discharge ended with the lower flange by creating an extended plasma column (2a). In the case of a magnetic system, which creates a uniform magnetic field in the processing chamber, the diameter of the plasma column is approximately equal to the diameter of the upper discharge chamber. In using in a processing chamber of a divergent magnetic field the diameter of the plasma column increased. A change in the magnetic field configuration makes it possible to control the position of the plasma column and also turn it at an angle close to 90° (Fig.2).
Estimates show that the creation of the plasma column takes place at pressures, when the mean free path is greater than the geometric dimensions of the plasma source. Thus, qualitative results can be explained as follows. The current flowing through the antenna excites a discharge at the top of the gas discharge chamber, and plasma appears. An external magnetic field interferes with the movement of electrons across the magnetic field, and they mostly move along the field lines. If the electron mean free path is sufficiently large, the electrons go out from the upper chamber, and a discharge occurs in the lower chamber. For sufficiently large mean free paths and the external magnetic field density the discharge ends with the lower grounded flange. The motion of electrons across the magnetic field is constrained, so in the case of a uniform magnetic field, a plasma column sharply defined in the radial direction can be recorded. The curvature of the field lines results in a change in the electron trajectories and position of the plasma column. Pressure increases lead to a decrease in the mean free path and the disappearance of the plasma column. Thus, at pressures greater than 1 mTorr an extended plasma column is not created, and the length of the brightly glowing discharge decreases with increasing pressure.
Fig.3 shows the change in the axial distribution of the probe ion saturation current with the magnetic field strength measured in using the magnetic system of the first type, a solenoid inductor and Nagoya III antenna as well as the magnetic system of the second type and a solenoid coil. It can be seen that in the absence of a magnetic field, the discharge is concentrated at the top of the plasma source, as long as the magnetic field is increased in the case of the magnetic system of the first type the maximum ion current moves into the lower chamber of the plasma source, and in the magnetic fields exceeding 36 G, a discharge is localised at the bottom of the source. This effect is observed in all the above RF-generator power, and maximum values of the ion current increase in proportion to the input power. The greatest value of the ion saturation current is achieved by using a solenoidal antenna. Application of the magnetic system of the second type does not allow obtaining a high ion saturation current in the processing chamber.
Thus, the optimal design of the hybrid system layout is a two-chamber inductive plasma source equipped with a solenoidal antenna and a magnetic system that makes it possible to create in the processing chamber a magnetic field homogenous within 7% with the induction of up to 7 mT, and a slightly diverging magnetic field in the gas discharge chamber.
In the above inductive range of the external magnetic field (0-60 G) and RF generator power (600 watts), the inequality occurs:
where ωLi, ω, Ωe, ωLe are the ion plasma frequency, circular working frequency, electron cyclotron frequency and Langmuir frequency. According to the theoretical models of the RF inductive discharge placed in an external magnetic field , under the conditions (1) two interconnected waves, i.e. helicon-like, quasi-electrostatic and oblique Langmuir can be excited.
HF measurements of the longitudinal component of the magnetic field Bz have shown that in the magnetic fields of 28 G or more in the plasma source a partly travelling wave, the amplitude profile of which is shown in Fig.4, is really created. An increase in the induction of the external magnetic field leads to an increase in the number of half-waves n placed on the length of the plasma source (Fig.2b). The growth of the magnetic field is accompanied by an increase in the amplitude of the Bz field in the processing chamber.
The obtained results allow optimising the HPS design (Fig.6).
The installation consists of two parts. The main part is a metal chamber of the cylindrical shape with a diameter of 500 mm and a height of 350 mm. At the bottom of the chamber there is the turntable table for placing the processed samples. In testing the technological modes of HPS a couple of probes were mounted on the table in order to control the plasma parameters. To output signals to the recording equipment there is a special port. To carry out the spectrometric researches of plasma parameters, there are two optical inspection window placed strictly opposite each other above the table. On the sides of the chamber a magnetron and a vacuum arc source are installed. Below the main chamber there is an electromagnet which makes it possible to obtain the induction of the external magnetic field in the table area, up to 150 G.
In the upper part of the main chamber a cylindrical quartz helicon plasma source with a length of 250 mm and a diameter of 220 mm is mounted. Above the volume of the source is closed by a hollow glass flange and by metal flange with an opening providing access to the main plasma chamber at the bottom. Two electromagnets provide a magnetic field in the processing chamber. For excitation of the discharge, a solenoidal antenna placed on the outside of the quartz chamber is used. The ends of the antenna were connected through the matching system to the HF generator with an operating frequency of 13.56 MHz and an output power of 1000 watts.
To study the plasma homogeneity in the substrate 25 flat near-wall probes area were mounted on the table. 13 probes are parallel to the symmetry axis of the magnetron (the x axis) and 12 probes perpendicular thereto (the y axis). To measure the ion saturation current, a potential of 60 V negative in relation to the main chamber walls were supplied to the probes. The plasma radiation was transmitted through the light guide to the input of the MDR-41 monochromator, at the output of which the FEU-100 photomultiplier tube was placed. A signal from the photomultiplier tube was amplified and fed to the ADC built in the computer. The spectrum was scanned in the range of 400-700 nm. Measurements were carried out in the argon environment at pressures ranging from 0.2 Pa to 1.5 Pa.
Optimization of operation modes of HPS
Experimental investigations of discharge in HPS showed that the superposition of a homogeneous magnetic field leads to significant changes in the length of the discharge. Whit a magnetic field of about 40 Gauss, the discharge closes on the bottom flange, forming a long plasma column. The diameter of the plasma column is approximately equal to 20 cm. Fig.7 shows the results of measurement of radial distribution of ion saturation current along x and y axes obtained in the helicon discharge mode, in the magnetron mode and in the combined mode of the two discharges. As can be seen, the joint work of the two discharges leads to a substantial increase of ionic current whose value is higher than the sum of the values measured in separate modes for inductive helicon RF source and the magnetron. This conclusion is true in all the considered experimental conditions. Growth of the pressure from 0.3 to 0.7 Pa, as shown by the experiments, leads to substantial improvement in the uniformity of the radial distribution of ion current, however, its absolute value declines. Increasing of pressure to 1 Pa leads to a further decrease of ion current.
The results presented above indicate the impact of the magnetron on parameters of the hybrid sputtering system. The influence of the HS of plasma on the operation of the magnetron with a titanium target can be estimated on the basis of spectral studies of plasma. Fig.8 shows a part of the emission spectrum of the plasma, where intensive lines of TiI are localized, which was measured at a pressure of 0.3 Pa in the cases of operation of the magnetron and of joint operation of the magnetron and helicon source. The experiments showed that when only the magnetron operates, spectral lines of titanium are practically not identified due to small quantity of atoms in the discharge. However, in hybrid mode the emission intensity of spectral lines of titanium atoms increases that indicates the increase in their concentration (Fig.8). In addition, it was found that the intensity of the titanium lines in the area of the substrate, where should be placed the sample, is a little less than the intensity in the location of the magnetron.
The table presents the thicknesses of Al films evaporated on silicon substrate during the operation only of the magnetron (1), during of the simultaneous operation of the magnetron and helicon (2) and during simultaneous operation of the magnetron and Helicon (3) with the bias to the substrate. The time of sputtering in all three cases was the same. The thickness of the films was measured on the chips of the plates using SEM Supra 40.
As can be seen, during the joint operation of the magnetron and helicon the sputtering rate increases. This is because the additional helicon discharge, as shown by the probe measurements, increases the plasma density. This leads to an increase of the ion flow that bombarding the target of the magnetron and to an increase of the rate of her sputtering. Apparently, the bias to the substrate leads to an increase in the number of aluminum ions, which reach the substrate and are involved in the formation of the film. The latter contributes to further increasing of the growth rate of the film.
Studies of the samples surface showed significant differences in its morphology depending on sputtering mode (Fig.9).
The Al films were intended for use as a structural element of the anode layer for thin-film Li-ion batteries, in which the developed morphology of the surface layers have an important role. As can be seen, the surface of the three considered samples of the films has features of a polycrystalline structure with "large" blocks. These polycrystalline blocks are least pronounced in case of the joint operation of the magnetron and helicon discharge. The chip of the film sample, obtained in case of bias to the substrate, differs from other samples because it is more fine-dispersed, but with a larger effective area (Fig.10).
Thus, the experiments demonstrated that the assisting helicon discharge allows to increase the rate of magnetron sputtering. In addition, the obtained results indicate good prospects of the adjustment the film structure by changing parameters of the helicon discharge.
The investigation of the parameters of the helicon plasma source showed the promise of the chosen design of the HPS. The best solution is a double-chamber inductive source equipped with a solenoidal antenna and a magnetic system that allows to create uniform magnetic field of not less than 7 mT in the processing chamber and a weakly diverging magnetic field in the gas discharge chamber.
The study of plasma parameters in RF hybrid plasma system based on helicon or magnetron discharges, showed the mutual influence of two types of discharge, leading to an increase in the plasma density and in the concentration of sputtered atoms.
The project was financially supported by the Ministry of education and science of the Russian Federation. Agreement No. 14.576.21.0021 of 30 June 2014. The unique identifier of applied research (project) is RFMEFI57614X0021.