Effect Of Gas Pressure On Ion Properties Of Metal Target Radio-frequency And Very-high-frequency Magnetron Sputtering Discharge

Magnetron sputtering is an important thin film deposition technology.It is widely used in the preparation of metal films,semiconductor films,dielectric films,periodic modulation structures and so on.During the deposition of thin films,the ion energy and ion flux acting on the surface of the substrate have a great impact on the growth and properties of the films.Discharge pressure is an important factor affecting the ion properties of magnetron sputtering.From low pressure collision-free plasma to high pressure collision plasma,it involves the change of plasma heating mechanism.With the increase of discharge driving frequency,the discharge pressure also affects the ion collision in the sheath.Therefore,it has a more complex influence on the characteristics of ion energy and ion flux at the substrate.In this thesis,the effects of discharge pressure on the ionic properties at the substrate of metal target radio-frequency(RF)and very-high-frequency(VHF)magnetron sputtering were studied.Using three metal targets of Ag,Cu and Al,the radio-frequency(13.56 MHz,27.12 MHz)and very-high-frequency(60 MHz)magnetron sputtering discharge were carried out at the discharge pressure of 1.0 Pa,3.5 Pa,5.0 Pa,7.5 Pa and 10.0 Pa.The discharge voltage,sputtering power,ion energy and ion flux in the discharge process of magnetron sputtering were measured by RF V-I probe and retarding field energy analyzer,and the effects of discharge pressure on the ion properties of RF and VHF magnetron sputtering of Ag,Cu and Al targets were obtained.The electron density and electron temperature were measured by Langmuir probe,and the ratio of ion transit sheath time τi to RF period τRF was estimated,the relevant mechanism on the effect of discharge pressure on the variation between most probable ion energy(E)and the discharge voltage(V)is analyzed.The major results are as follows.(1)For the RF and VHF magnetron sputtering discharges of Ag,Cu and Al metal targets,the effect of discharge pressure on the distribution of ion energy at the substrate is basically the same.At lower discharge pressure,because there is a collision-free plasma,the ion velocity distribution function(IVDFs)shows a single peak distribution,and there are few weak peaks caused by collision in low-energy and high-energy regions.With the increase of discharge pressure,the collision effect enhanced,the weak peaks increase in the low-energy and high-energy regions of IVDFs.The most probable ion energy E(i.e.the energy corresponding to the main peak)obtained from IVDFs basically decreases linearly with the increase of discharge pressure.(2)For the RF and VHF magnetron sputtering discharges of Ag,Cu and Al metal targets,the effect of discharge pressure on the variation of the most probable ion energy E at the substrate with discharge voltage V is basically the same.At lower discharge pressure,the E-V relationship changes from the downward trend of 13.56 MHz magnetron sputtering discharge to the first downward and then upward trend of 27.12 MHz magnetron sputtering discharge,and further changes to the upward trend of 60 MHz magnetron sputtering discharge.At higher discharge pressure,the E-V relationship shows an upward trend,which is independent of discharge frequency.(3)According to the Langmuir probe diagnosis on the discharge plasma properties of Ag target RF and VHF magnetron sputtering,the possible mechanism of the effect of discharge pressure on E-V relationship is analyzed.At lower discharge pressure,the increase of discharge voltage leads to the increase of electron density and the decrease of electron temperature.As a result,the kinetic energy of metastable Ar atoms produced by electron collision decreases,and the energy of Ag+ions produced by Penning ionization(Ar*+Ag→Ag++Ar+e)between metastable Ar atoms and neutral Ag atoms in bulk plasma decreases.When the ions enter the sheath form the bulk plasma,for the 13.56 MHz and 27.12 MHz magnetron sputtering discharge,with the increase of discharge voltage,the ion transit sheath time τi decreases from multiple RF periods to several RF periods.As a result,the ions energy obtained by responding to time-averaged potential sheath decreases,and the energy obtained by transient sheath potential acceleration increases.The decreasing trend of ion energy in bulk plasma is offset by the increasing trend of ion energy in sheath,resulting in the E-V relationship from a downward trend to an upward trend.For the 60 MHz magnetron sputtering discharge,ions experience more RF cycles when passing through the sheath,and ions obtain energy mainly by responding to time-averaged potential sheath.Therefore,with the increase of discharge voltage,the sheath potential increases and the ions energy increases.The decreasing trend of ion energy in bulk plasma is completely offset by the increasing trend of energy in sheath,resulting in the monotonous increase of E-V relationship.With the increase of discharge pressure,the collision effect increases gradually.The electron collision ionization and heating process in bulk plasma have an important impact on the ion energy.Because the electron temperature decreases with the increase of discharge voltage,the ion energy decreases.When the ions in the bulk plasma pass through the sheath,they experience the energy gain of charge-exchange,collision energy loss and electric field acceleration in the sheath.The ion energy mainly depends on the time-averaged sheath potential.Therefore,the E-V relationship increases with the increase of discharge voltage.The above research results play an important reference role in the research of thin film materials deposited by RF and VHF metal target magnetron sputtering,the regulation of thin film structure and properties and the analysis of their relationship with discharge plasma properties.