Systematic research on the performance of self-designed microwave plasma reactor for CVD high quality diamond

Xio-Jing Li,Shun Zhou,Gng Chen,D-Sen Wng,Ning Pei,Hi-Lin Guo,Feng-Ming Nie,Xu Zhng,Shi Feng

aInner Mongolia Metal Material Research Institute,Ningbo,315103,China

bXi'an Technological University,Xi'an,710021,China

cChangchun University of Science and Technology,Changchun,130022,China

Keywords:MPCVD Plasma reactor Electric field distribution Plasma density

ABSTRACT Optical grade diamond,in particular,high-end products,are mainly used for aerospace and defense purposes.How to manufacture them with higher quality and lower cost was critical technology.Systematic research on the performance of Self-designed plasma reactor with higher power for synthesizing diamond film was carried out.Microwave input power can reach up to 10kW,the plasma reactor has good adaptability for the deviation of microwave frequency f at 2.45 GHz±20 MHz.The secondary plasma could be eliminated by adjusting the working gas pressure and the cylinder adjustment structure,it is helpful to improve the deposition rate with less energy loss.The concentrated and steady plasma could be obtained device with higher gas pressure and relative lower microwave input power.Gas supply modes and inlet gas flow rate were optimized,which would be beneficial to synthesize the film with good quality.

1.Introduction

Diamond films posses a number of outstanding physical properties,including extreme hardness,high thermal conductivity and wide-band optical transparency.This makes diamond films attractive for a large number of applications.It has wide application in many high tech field aerospace,electronic information system,especially in military field,such as nuclear,infrared window material,laser window,missile dome.

Within the last 50 years,a wide variety of methods have been developed to deposit diamond under various conditions.The most common process for diamond growth is the chemical vapor deposition(CVD).Starting from the first publications until the latest results today,a range of different developments can be seen[1].Among the various techniques to deposit diamond films,microwave plasma chemical vapor deposition(MPCVD)has distinguished advantages,such as no pollution from electrodes,high plasma density and good quality films.

The plasma reactor is the most important device of MPCVD system.Plasma reactor with high plasma density environment is crucial for deposition of diamond thin film/coating with high speed.How to obtain higher plasma density?One of the important methods is to improve the microwave power,which is in favor of the excitation of the plasma.Processes with increased pressure,flow rate and applied power are the general tendency[1].As was reported previously[2,3],when operated in the 100-160 torr pressure regime this reactor has synthesized high quality and high growth rate CVD polycrystalline diamond.

To achieve high rate,one should choose the synthesis conditions in 2.45GHz MPACVD reactor with a high microwave power density(MWPD)in plasma maintained in continuous wave(CW)regime[4-6].It was found that for the same MWPD,the diamond film growth rate increased with higher gas pressure[6].Michigan State University[7,8]has done many research work on improved microwave plasma cavity reactor for diamond synthesis at high pressure and high power density,they synthesized single crystal diamond(SCD)under pressures from180 to 300 torr with absorbed power densities Pabsbetween 400 and 1000W/cm3.

In order to obtain more information of our previously designed plasma reactor for synthesizing polycrystalline diamond[9],the plasma characteristics of the plasma reactor,especially,the absorbed power densities Pabswas studied theoretically in this paper.Due to the unstability of the microwave power,frequency deviation may occur during MPCVD process,and it may cause the plasma abnormal ignition which has negative effects on the film quality.It is a problem that should be paid more attention during the structure design of plasma reactor.Hence,the sensitivity of the plasma reactor to microwave frequency was analyzed in this paper.

In addition,other working parameters which also have influence on the microwave energy and plasma density,such as microwave input power and gas pressure,were calculated too.And a suitable condition in which the secondary plasma ignition could be eliminated was found.The elimination of secondary plasma ignition is necessary to synthesize uniform diamond film with less energy loss.

2.Numerical simulation method

The plasma reactor of MPCVD system designed for film deposition has been reported in our previous papers[9,10].The plasma reactor has a cylindrical microwave cavity whose effective volume can be adjusted.Its ring shape quartz window is located under the substrate holder,and it is helpful to generate plasma with high temperature.Comprehensive numerical simulation methods were adopted,in order to be closed to the actual condition.The working gas flow in the cavity was analyzed by software Fluent[11].Both the electromagnetic field and plasma were simulated by the finite difference time domain(FDTD)method[12],using a code written in Matlab.

The time-varying electromagnetic fields inside the reactor were modeled by solving the following Maxwell's equations(Eq.(1)and Eq.(2))[13].

Where E and H are the electric and magnetic field,μ and ε are permeability and permittivity.In Eq.(3),J is electric current density induced in the plasma,and t is time.σis electrical conductivity.

The Maxwell's equations were discretized with a centred difference approximation in both the time and space domain,and the radial and axial components of the electric field,Erand Ez,and the tangential component of the magnetic field,Hφwere solved through the following equations(Eq.(4),Eq.(5)and Eq.(6)).

In these equations,σ is electrical conductivity,Δt is the time step,Δr and Δz are space steps,n,i and j are step and cell node numbers,respectively.

For Eq.(5),Special treatment has been done for the vertical efield(Ez)calculated on the center(r=0)axis,as Eq.(7),

During the FDTD simulation process,Δt,Δr andΔz were chosen properly,to meet the requirement of dispersive stability.Loop was set for more than 1000 times,which must be iterated until the result converges towards a self-consistent solution of the field and plasma distribution in the cavity.

The self-designed plasma reactor as shown in Fig.1,the microwave will enter the plasma reactor from the bottom of the cavity,the plunger can be adjusted for stable plasma.To model the plasma,the simple model developed by Funer et al.[14,15]was used.According to this model,the plasma density(electron density)ne will be linearly dependent on local electric field E[16].

The absorption power density Pabsis defined as the input absorbed microwave power divided by the plasma volume.Pabscan be expressed by Eq.(8)[17].

where ε is permittivity,E is electric field intensity.After getting the electric field intensity in the plasma region,using the iterative formula by solving Eq.(1)and Eq.(2),the absorbed microwave power density can be obtained.Using absolute value to get Pabs.

To simulate the microwave input,a planar sinusoidal excitation was imposed along the coaxial antenna,the electric field and magnetic field could be expressed as follows:

Here,E0is the amplitude of the excitation field,andωis the circular frequency.βis the phase-shift constant,ηis the characteristic impedance of the coaxial antenna.

εris the relative dielectric constant of the air,a and b is the radius of inner and outer coaxial antenna,respectively.

3.Results and discussion

3.1.The plasma reactor's sensitivity to microwave frequency

Electric field distribution and performance of plasma are sensitive to the microwave frequency f.Generally speaking,microwave frequency f used in industry is 2.45GHz,but it will fluctuate in actual deposition process.The sensitivity of the MPCVD device was analyzed under the following working condition,the microwave input power is 5 kW,gas pressure is 70 torr,microwave frequency f varies from 2.43 to 2.47 GHz,the change of electric field distribution and plasma inside the new plasma reactor were studied,the simulation results are shown in Fig.2 and Fig.3.

It can be seen from Figs.2 and 3 that,when microwave frequencychanges in the scope of 2.45GHz±20 MHz,the electric field strength and the corresponding plasma density distribution in new MPCVD installations are able to keep stability.

By contrast,plasma reactor with ellipsoidal cavity structure in literature[18]was calculated.It can be seen that,in the same working condition,electric field distribution(Fig.4)and plasma density(Fig.5)in ellipsoidal plasma reactor are very sensitive to microwave frequency,their characteristics change greatly when the frequency fluctuate near 2.45GHz.

In conclusion,the designed plasma reactor has obvious superiority in frequency sensitivity.The device has good adaptability for the deviation of microwave frequency,which is helpful to keep the plasma stable during the deposition process.

3.2.Influence of microwave input power on electric field intensity

To study the influence of microwave input power on the energy distribution in plasma reactor,the electric field intensity distribution was simulated under different microwave input power.The results were shown in Fig.6.

It can be seen from Fig.6 that,the electric field intensity get higher gradually,when the microwave input power increase from 800W to 8 kW,that means more microwave energy was transformed and focused on the center of the substrate holder.

3.3.Plasma characteristics

In order to study the performance of designed plasma reactor,the effects of plasma density and power absorption density were also studied.

As shown in Fig.7,results indicate that with the increase of microwave power input,i.e.,from 2.5 kW to 8 kW,the maximum electric field intensity and plasma density increase accordingly(Fig.7(a)).When microwave input power is 800W,the electric field strength is insufficient to maintainplasma,in this case,the gas can't be ionized,so the plasma won't be ignited,the plasma electron density is zero.

It can be seen from Fig.7(b)that,the absorption power density Pabsalso increases with microwave input power.It indicates that more energy concentrate on the center of substrate.When the microwave input power exceeds 6 kW,higher absorption power density Pabs(＞150 W/cm3)can be obtained,which helptoaccelerate the film deposition rate.When the microwave input power increases to 8 kW,the absorption power density Pabsreach up to 200 W/cm3.

If microwave input power keeps a certain value,the gas pressure becomes one of the most important technical parameters which influences the diamond film deposition rate.In this paper,the influence of different gas pressure was simulated in hydrogen atmosphere,with the microwave input power of 6kW.

Fig.8 shows the distribution of plasma density in the cavity when the gas pressure is 40,60,80 and 100 torr respectively.Hemispheric plasma is formed.It is gathered on the center of the deposition substrate and the diameter ranges from 55 to 65 mm.With the increase of gas pressure,the size of plasma hemisphere decreases gradually.In addition,when the gas pressure is relatively lower(e.g.,P=40,60 torr),the secondary plasma will occur at the tip of the adjustment structure which at the top of the cavity(Fig.8(a)and(b)).It would lead to energy loss because of the heating effect on the adjusting structure.And with the increase of gas pressure(P＞70 torr),secondary plasma disappear,and that is exactly what we need in diamond film deposition environment.

In addition,in order to analyze the condition of secondary plasma ignition in plasma reactor,the plasma performance under different working gas pressure and input power were studied.The analysis result as Fig.9 illustrated.

Two cases of plasma state can be seen in Fig.9.The stable zoneⅡand unstable zoneⅠare separated by the dividing line.The steady-state plasma was located in Stable zoneⅡ,where only one hemispheric plasma with higher plasma density concentrated on the substrate.However,unstable zoneⅠ,where the plasma was in unstable state,secondary plasma could be seen at the top of the cavity besides on the substrate.It will lead to energy lose.Hence,the plasma density on the substrate will decrease accordingly,and film deposition rate,too.

It can be seen that,the appearance of secondary plasma in MPCVD device mainly depends on the working gas pressure.When gas pressure is higher than 100Torr,even if the microwave input power of MPCVD device is more than 10kW,the secondary plasma won't be generated.

The different plasma state exist(Fig.10)in experimental researchand the modeling reliability was verified.The unstable and scattered plasma can be observed in the reactor(Fig.10(a)),where the secondary plasma generating from quartz ring window or the cylinder adjustment structure which at the top of the microwave cavity.The concentrated and steady plasma(Fig.10(b))could be seen when the device with higher gas pressure and relative lower microwave input power.That is the required condition for diamond film deposition.During MPCVD film process,it needs to improve working gas pressure properly,the secondary plasma could be eliminated by adjusting the working gas pressure and the cylinder adjustment structure.Unstable plasma has effects on the surface quality of the diamond film,the diamond coating on silicon substrate were obtained under different plasma state,as shown in Fig.11,the Non-uniform coating(Fig.11(a))and uniform diamond coating(Fig.11(b))were deposited under unstable and stable plasma,separately.Obviously,the surface quality in Fig.11(b)is better than Fig.11(a),it can be observed by naked eye in Fig.11(a),there is black cycle on the edge of diamond surface.

3.4.Working gas supply mode and velocity optimization

The working gas used in the experiment for diamond film deposition was pumped into the chamber through the top of the cavity.The working gas is a mixture of hydrogen(H2)and methane(CH4).Gas supply in different ways has great influence on the distribution of gas flow rate inside the microwave cavity.

Two different gas supply modes were designed in this study,as shown in Fig.12 In mode(I),the mixed working gas flowed into the chamber through the micro-holes on the top of the microwave cavity.The holes were arranged around the plunger,as the dashed line represents in Fig.12(a).In mode(II),the gas inlet is a hole located at the middle of the plunger(Fig.12(b)).In both mode(I)and mode(II),the inlet gas flow rate was assumed to be 1 m/s,2m/s,5 m/s,10m/s.The distribution of gas flow rate inside the cavity was simulated.

It can be seen in Fig.13 that in mode(I),the gas flow distribution varied with the change of inlet gas flow rate.When the inlet gas flow rate was 1m/s(Fig.13(a)),the gas flow over the substrate holder was very small,but it increased a lot when the inlet gas flow rate increased to 2 m/s.When the gas flow rate reach up to 5 and 10 m/s（Fig.13(c),(d)),the gas density above the substrate presents the tendency of decrease,almost no gas flows through the substrate center,and it is not good for plasma excitation here.

Fig.14 shows the case of mode(II).It can be seen that most of the gas flow was distributed over the substrate when the inlet flow rate was low.With the increase of the inlet gas flow,the gas flow over the substrate became greater and uniform.As we know,the gas will be consumed when the plasma was ignited,so a uniform gas flow is needed to deposit a uniform film.Both the inlet gas flow of 5 m/s and 10 m/s could ensure a homogeneous gas supply,but a lower flow rate could be controlled more easily,while a higher flow rate means a higher deposit rate.By comparing the simulation results,it could be concluded obviously that mode(II)is a better gas supply method.And the inlet flow rate could be chosen between 5 m/s to 10 m/s.

To demonstrate the effectiveness of the above conclusion,an experimental research was carried out,in which the supply mode(II)was adopted,and a gas velocity of 5m/s was chosen.The result indicated that uniform diamond film could be deposited on substrates successfully with these process parameters.

4.Conclusions

Based on numerical simulation,systematic theoretic calculation on the performance of self-designed microwave plasma reactor was carried out.The research results show that,the plasma reactor is relative less sensitive to frequency deviation.When microwave frequency varies in the range of 2.45 GHz±20 MHz,the electric field strength and the corresponding plasma density distribution in new MPCVD device are able to keep stability.The maximum electric field intensity,plasma density and absorption power density increased with the microwave input power.The appearance of secondary plasma in MPCVD device mainly depends on the working gas pressure and microwave input power.The secondary plasma could be avoided by using higher gas pressure and relative lower microwave input power.Which is helpful to improve plasma density environment and avoid energy lose.In all,the above systematic research results will provide valuable information for new plasma reactor design,and it's also helpful to experimental research in MPCVD technology.The supply way and inlet gas flow rate were optimized.The mode(II)is a better supply way,it is favorable for the formation of uniform film.And the proper inlet gas flow rate is in the range of 5-10 m/s.In all,the above systematic research results will provide valuable information for new plasma reactor design,and it's also helpful to experimental research in MPCVD technology.

Acknowledgements

This material is based upon work funded by the Inner Mongolia Natural Science Foundation under Grant No.2017MS0539.