Research On Growth Mechanism And Surface Roughness Control Of GDP Film For Laser Fusion Target

Glow discharge hydrocarbon polymer(GDP)film is an amorphous network structure.As it has advantages such as a low average atomic number,compact structure,no crystal boundary,and finished surface,GDP films are in favour of suppressing the Rayleigh-Taylor instability(RT instability)in the process of implosion of laser inertial confinement fusion(ICF)experiments.It also has the advantages of good infrared permeability,which is benefit to the infrared heating and optical measurement.Based on these advantages,GDP thin film has become the most preferred ablation layer material for ICF targets.As research continues,much rigorous requirements about surface roughness of GDP target have been proposed in ICF.For instance,the root-mean-square roughness must be lower than 20 nm as the wall thickness of GDP target is over 100 ?m.In the research fields of roughness controlling,evolution of morphology,and formation of defects,there is a big gap between us and American.So far,the research on GDP films mainly focus on simple optimization of process parameters and technical exploration.There is short of systemic researches on the surface roughness of GDP films.The evolution of defects and growth mechanism of GDP films is still not clear.It is known that the surface topography and roughness depend on the interaction process between plasma and the substrate during the growth of GDP films,which is closely related to the state parameters of the plasma.However,there is short of systemic researches on the plasma properties in C4H8/H2 discharging.As mentioned above,some research on surface roughness and growth mechanism of GDP film is urgently needed.In this paper,the relationship between plasma properties of C4H8/H2 discharging and process of GDP film growth has been studied based on the plasma diagnostic.Based on this reasearches,we have revealed the evolution mechanism of surface roughness and morphology of GDP film,and found the way to control the surface roughness of GDP film with large thickness.The main research finds are as follows:First,the GDP films are prepared by plasma enhanced chemical vapor deposition(PECVD),using C4H8 and H2 as the reaction gas and exciting gas,respectively.The effect of technology parameters on C4H8/H2 plasma properties is investigated based on the in situ diagnosis by means of the combination of mass spectrum(MS)and Langmuir probe.It is found that:(a)At low rf power,the CH4/H2 plasma is mainly composed large mass ionic fragments,and second polymerization happens in plasma region.With the RF power increasing,the ionization ratio,ion energy,self-bias voltage,floating potential Vf,plasma potential Vp,and electron density increase.(b)With working pressure increasing,the ionization ratio decrease.The ionic fragments are mainly small-mass ions at low working pressure.(c)As the C4H8/H2 gas flow ratio increases,ionization ratio increases first and then decreases.The ionization ratio and ion energy reach the maximum at C4H8/H2=0.4/10.Second,based on the results of the plasma diagnostic,the relation between the plasma characteristics and the GDP film's properties such as the chemical composition component,surface morphology and surface roughness with the change of technical parameters have been also discussed to investigate the growth mechanism of GDP films.The result shows that large mass groups are apt to polymerize as clusters,which will increase the surface roughness.While it tends to form smooth surface as small mass groups are deposited.Higher energy of plasma fragments are benefit to diffusion and migration of species on the surface of GDP films.While over high energy will lead to excessive etching on the film surface,which results in the hole defects.What's more,high deposition rate means that there is no enough relaxation time for groups to migrate on the film surface.Then dome and hole defects appear on the film surface.Subsequently,the best technique parameters are determined as RF power of 30 W,working pressure of 10 Pa and C4H8/H2 gas flow ratio of 0.4/10.Third,based on above,Ar ion etching has been proposed to control the growth process and reduce the surface roughness of GDP films.MS is employed to investigate the difference of composition and ion energy distribution between C4H8/H2 plasma and C4H8/H2/Ar plasma.Meanwhile,we investigate the chemical component,surface morphology and surface roughness of GDP film prepared with and without Ar ion etching.In addition,the Ar ion intermittent etching technology has been first applied in preparation of GDP film,which greatly reduces the surface roughness of GDP film.The results indicate that:(a)Both of the ion energy and ionization ratio increase with the Ar ion etching.The suppression of Ar ions on second polymerization reactions is in favour of lowering the surface roughness of GDP films.(b)The effects of Ar ion etching and the intermittent/etching cycle on chemical structure,surface morphology and roughness of the GDP film are studied.The investigation releases that there are less defects like domes,holes and bumps observed on GDP film surface and the surface roughness decreases by using Ar ion etching.By modulating the intermittent/etching cycle,the surface roughness(Rq)of 80 ?m hick GDP film has been successfully decreased from 220 nm to 34 nm at the intermittent/etching cycle of 60 min/15 min.At last,based on this researches,it is concluded that the etching effect,surface migration and deposition rate are the three important competitive factors in the evolution of the surface morphology and roughness of GDP films.To sum up,based on the in situ diagnostic of plasma,great progress in surface roughness controlling has been obtained in the preparation of GDP films with thick wall.The surface roughness is close to the design requirements of NIF.It provides reliable experimental data and theoretical guidance for GDP thin film process optimization.It is expected to obtain GDP targets satisfying the ignition requirements.