Experimental Study Of Plasma Characteristics In Continuous And Pulse-modulated Rf Inductively Coupled Ar/CF₄ Plasmas

Inductively coupled plasma?ICP?is widely used in microelectronics industry due to its high density,low electron temperature,large uniform area and good uniformity.With the development of semiconductor industry,the feature size of integrated circuits is decreased and the requirement of plasma source in etching is more stringent.There are many problems in the traditional continuous wave plasma etching,such as the damage of high energy particles to the base,abnormal etching caused by charge accumulation,etc.In order to address these problems,pulse-modulated RF inductively coupled plasma is proposed.Pulse-modulated ICP provides two controllable parameters:pulse frequency and pulse duty ratio,so that the plasma can be controlled more flexibly.Moreover,the sheath collapses during the glow period after the pulse,and negative ions and electrons can move to the bottom of the trench and accumulate positive charges during the etching process,thus improving the etching morphology.Due to the important application of CF₄ gas in etching,there are systematic researches on continuous wave RF inductively coupled CF₄ plasma.However,the research about the pulse-modulated RF inductively coupled Ar/CF₄ plasma is very limited.In this paper,VI-probe,Langmuir probe,Hairpin probe,spectrometer and other diagnostic tools are used to carry out experimental diagnostic research on the characteristics of continuous wave?CW?and pulse-modulated RF inductively coupled Ar/CF₄ plasma.The effects of pressure and input power on electron density,effective electron temperature and electron energy probability function?EEPF?of plasma were observed,and the evolution of their spatial distribution.It is found that in CW RF inductively coupled plasma,with the increase of the pressure,the electron density first increases and then decreases.The effective electron temperature gradually decreases,because the greater the pressure,the greater the loss of high-energy electrons.With the increase of power,the electron density at each radial positions increases linearly and the effective electron temperature remains almost unchanged.The electron density is the maximum at the center of the chamber and gradually decreases to the sidewall.In pulse-modulated RF inductively coupled plasma,input power has two peaks in the initial pulse.The first peak is determined by the characteristics of the RF power source,while the second peak is affected by the growth rate of electron density and the state of the matching network.The electron density increases rapidly at the initial pulse stage,then decreases slightly to reach a stable value,resulting in overshoot behavior,which is related to the balance between electron generation and loss rates.When the power is turned off,the electron density decreases to a small value.When the power is turned on,the relative light intensity increases rapidly,the overshoot behavior is also observed,and then drops to the steady state.When the power is turned off,the intensity decreases rapidly to zero.The effective electron temperature presents a peak at the beginning of the pulse,then drops to a stable value,and drops to a smaller value after the pulse is turned off.With the increase of pressure,the electron density increase more slowly,and the overshoot behavior gradually disappears.This is because the high excitation collision frequency causes the lower ionization rate,so that the high-energy electrons decrease.After the pulse is turned off,the electron density decreases rapidly at low pressure,but at high pressure,the electron density increases first and then decreases,and the rate of decrease is slow at high pressure.With the increase of power,the overshoot behavior becomes more obvious,because the power transmission efficiency increases with the power,and at the end of the previous pulse period,the electron density is almost the same,and the input power rises to the set value at the beginning of the pulse.Therefore,electrons absorb more energy at higher input power.Like continuous discharge,the electron density is the largest at the center of the chamber and gradually decreases with radial distance.With the increase of axial distance,the overshoot behavior gradually disappears and the response of electron density to input power gradually delays.The effective electron temperature decreases with the increase of axial distance.As the plasma is generated near the coil,it diffuses to the surroundings.In Ar/CF₄ discharge,the response of plasma to the change of input power is faster than that in Ar discharge,which is as a result of more ionization channels,larger ionization cross section,and lower ionization threshold.Therefore,it takes a short time to realize a balance between electron generation and loss rates in Ar/CF₄ discharge.