Electrical Modelling





Electrical Model Based on Experimental Measurements


Basic assumptions:The model  is one-dimensional, based on the following assumptions:

- Ions respond only to the time-averaged electric field and the ion motion is collisional with a constant mean free path. This assumption limits the model application to total gas pressures higher than 200 mTorr.

- The time-averaged values of the electric field and the electron density in the RF sheath have been calculated assuming that the sheath-plasma boundary oscillates as ds(t)= dp(1+sinωt). Results for the sheath of the grounded electrode have been extracted by shifting the phase of solution of RF sheath by ωt=π.

- An ambipolar transport in the bulk has been assumed and although the electron distribution function is not expected to be Maxwellian, a use of electron temperature has been made for an initial estimation of the ambipolar dc field and the determination of ambipolar diffusion coefficient.

 


Model Inputs

The formulation of the model is not self-consistent in the sense that it requires the self-bias voltage, the discharge current, the ohmic part of the discharge impedance, and the spatial distribution of the ionization rate.



Model outputs

The output parameters of the model are the spatial distribution of electrons and ions, the distribution of the electric field, the power dissipation to ions and electrons, and the mean electron energy.



Model Formulation

- Ion transport in the sheaths


The transport of ions in the sheath is usually described as a collisionless or collisional, space charge limited motion. The charge conservation condition and the assumption of no ionization or recombination in the sheath impose the requirement of a constant ion flux. In reality, ion density gradually decreases while ion drift velocity increases, from the plasma sheath boundary towards the RF electrode:


image002      (1)



where, ni(x) the ion density and u+(x) the ion drift velocity.


The mobility solution of the Child-Langmuir law for a collisional sheath has been used to calculate the ion flux thus,  the ion mean free path λ+ have to be  much smaller than the sheath thickness. The values of the sheath electric fields are expected to be high enough in the conditions studied here, allowing thus to express the ion drift velocity in the sheath using the high-field mobility factor k


image004          (2)



By combining eq. 2 to Poisson’s equation (dE/dx=eni(x)/εο), the expression of ion current density (eq.1) is transformed to a relation of the form of Child’s law


image006          (3)




where E is the time averaged value of the sheath field at the position x, and Eo is the value of the electric field at the plasma-sheath boundary (x=0). The use of nonzero value of the field at the plasma-sheath edge is necessary to eliminate the singularity for the ion density in the collisional case while the value of Eo is determined by the relation Eo=TeD  describing the plasma sheath transition. According to eq. (3) for constant ion flux, the distribution of the time-averaged field in the sheath can be written as



image008          (4)



where d is the mean sheath thickness and Ed the field value at the electrode.


The distribution of the electric field can then be combined to Poisson’s equation to calculate the ion density distribution



image010          (5)


and the voltage distribution in the sheath



image012          (6)



Furthermore, the electrons in the sheath are assumed to be in Boltzmann equilibrium and have a Maxwellian distribution. Therefore, the electron density distribution is of the form:



image014          (7)



where ne(0)  and V(0) is the electron density and the voltage at plasma-sheath boundary.

The time-averaged values of the electric field and the sheath thickness, required for the solution of the set of equations (3-7), are calculated as follows: The mean values of the powered and grounded sheath lengths are found assuming a pure capacitive nature of the two sheaths. The power and grounded sheath reactive impedances can be distinguished, and the sheath lengths can be then calculated using:



image016          (8)



where Id is the total discharge current and f a factor related to the fraction of the total current conducted by the grounded electrode, with respect to the applied voltage for each frequency.


The values of the sheath fields Ed at x=d are calculated with the assumption that the sheath current is a displacement current



image018          (9)



For the values of the field at x=0 (sheath-bulk interface) the Godyak-Sternberg model for the pre-sheath is used and Eo is calculated by equalizing the velocity of ions injected from the plasma into the sheath with the velocity of ions in the sheath, in Eq. (3)



image020          (10)



The necessary electron temperature estimation results in this case from equation



image022          (11)



with Vp the plasma potential, and Io the zero-order modified Bessel function of the first kind.




Model Formulation

- Ion transport in the bulk



In order to relate the time-averaged values of the field and the charged particle densities in the sheaths to their values in the bulk, ambipolar diffusion dominated ion transport is considered:


image024          (12)



where S(x) is the time averaged ionization rate in space and Dais the ambipolar diffusion coefficient. The ambipolar diffusion coefficient Da, is determined by the electron temperature and the ion mobility:    


image026         (13)


Since, the values of the electric field in the bulk are expected to be rather weak, a low field mobility  is used instead of the high field mobility k used to describe ion drift motion in the sheaths. Thus, the ambipolar diffusion equation (12) is solved for dps < x < dgs , with the boundary conditions:



image028          (14)


where, dpd, dgd are the mean powered and grounded sheath lengths, Jip, Jig the ion conduction current densities, and Npd, Ngd the charge densities at bulk-powered and grounded sheath boundaries respectively. Furthermore, the distribution of the ambipolar electric field Eab is calculated using the equation



 image030          (15)

 

 

 

Model Formulation

- Power dissipation



The calculation of the ion flux density in both sheaths permits the estimation of the power consumed for ion acceleration



image032           (16)



The power consumed for electron heating can then be distinguished as the difference between the total power consumed in the discharge and the power consumed for ion acceleration



image034            (17)


In order to relate the different paths of power dissipation with the ohmic part of the discharge impedance, an in series equivalent circuit is assumed for the discharge.


The resistance related with ion acceleration in the powered and grounded sheath is calculated using the formulas



image036          (18)


The resistive component Rν related with electron-molecule collisions is then calculated using the expression



image038          (19)


where, R the total resistive component, is strictly related to the frequency at which the electrons relax their momentum



image040          (20)



where nAv , is the average plasma density and db the bulk length.


   

Model Formulation

- Electron heating and mean electron energy


The power consumed for electron heating Pe that has been calculated above represents the total power transferred to electrons. However, significant variations are expected in the spatial distribution of electron heating.  In order to follow the changes in the spatial variation of electron heating, the time-averaged power transferred per electron θ can be used. This parameter can be expressed as a function of the time averaged electric field and the electron-molecule collision frequency




image042          (21)



The changes in electron heating will be reflected in the value of the mean electron energy, resulting from the balance between the energy gained from the field and the energy lost per collision:




image044          (22)



where, the value of the effective (ohmic) part of the RF bulk electric field has been used:




image046          (23)



ue is the electron drift velocity and k is the mean energy loss factor.


Assuming a sinusoidal RF field and substituting electron drift velocity as



image048          (24)



where, φ=tan-1(ω/νm) is the angle by which the electron drift motion lags the field eq. (24) results in a differential equation of the form




image050          (25)



that has the solution




image052           (26)



where νe is the mean collision frequency for energy transfer, resulting from the product of k times νm. The term  image054  can be attributed to the mean electron energy that would be observed in a dc electric field of strength Eeff.


 


Model Results


The complete model formulation has been presented in the following work:


        E. Amanatides and D. Mataras
        J. Appl. Phys. 89, 1556 (2001) ©


    while the extension of the model to include reactive gases and mixtures was published in:


        E. Amanatides, D. Mataras, D. E. Rapakoulias
        J. Appl. Phys. 90, 5799 (2001) ©



Application of the model to H2 and SiH4/H2 discharges and the main results are shown below:




Electron Heating:  Spatial distribution of electron heating for four different frequencies - The conditions are: pure H2 0.5 Torr discharges and constant power dissipation of 40 mW/cm2. The model predicts an increase of electron heating and a shift of the peaks of electron heating towards the electrodes electrode with increasing frequency


electron heating






Electron Density: Spatial distribution of electron density for four different frequencies - The conditions are: pure H2 0.5 Torr discharges and constant power dissipation of 40 mW/cm2. The model predicts an increase of electron density and a shift of the peak of electron density towards the center of the discharge with frequency

 

electron density

 


 

 

 

Electron Energy: Variation of the space averaged mean electron energy during an RF cycle at four different frequencies - The conditions were pure H2 0.5 Torr discharges and constant power dissipation of 40 mW/cm2. The model predicts a drop of mean electron energy and a small time variation with frequency

 

electron energy

 


 

 

 Electric Field: Spatial distribution of the electric field intensity for four different frequencies - The conditions were pure H2 0.5 Torr discharges and constant power dissipation of 40 mW/cm2. The model predicts an drop of electric field intensity with frequency

 


electric field

 

 

 

 

 

 Charge Loss Rate: Ion flux towards both powered and grounded electrodes as a function of frequency - The conditions are: pure H2 0.5 Torr discharges and constant power dissipation of 40 mW/cm2. The model predicts a clear drop of the ion flux for the powered electrode and a slight decrease for the grounded electrode with frequency


 




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