Experimental Research On The Two-dimensional Distributions Of Capacitively Coupled Plasma Characteristics

Capacitively coupled plasmas have been extensively utilized in the microelectronics industry,due to the advantages of simple device structure and uniform discharge.In practical industrial applications,the change of plasma characteristics will affect the performance of material processing,such as dielectric etching and thin film deposition processes.Axial distributions of plasma parameters reflect electron dynamics in capacitively coupled discharges to a certain extent.The radial uniformity of material processing is directly related to radial distributions of plasma parameters.The electron power absorption in low-temperature plasma discharges is of vital significance to the generation of plasma,the maintenance of discharge,and the regulation of plasma characteristics.Hence,it is necessary to systematically study the two-dimensional spatial distributions of plasma parameters and electron power absorption modes in capacitive discharges.An approach combining optical emission spectroscopy with a collisional radiative model is used to study the two-dimensional spatial distributions of plasma parameters and electron power absorption modes in single-frequency and dual-frequency capacitively coupled argon plasma discharges.An optical fiber spectrometer is used to measure electron density and electron temperature at the discharge center and to calibrate the Ar emission intensities at 696.5 nm and750.4 nm monitored by a charge-coupled device（CCD）camera.The discharge structure of plasma is detected by the CCD camera.Then,the two-dimensional spatial distributions of electron density and electron temperature are calculated from the collisional radiative model.In Chapter 1,several most widely used low-temperature plasma sources and their applications are introduced.Then,we review research progress on the two-dimensional spatial distributions of plasma parameters and electron power absorption modes in capacitively coupled discharges.In Chapter 2,the experimental setup and the plasma diagnostic method are described in detail.Besides,a two-dimensional diagnosis technique based on the use of the CCD camera and optical bandpass interference filters is presented.In Chapter 3,we investigate the two-dimensional spatial distributions of plasma parameters and an α-γ mode transition induced by increasing radio-frequency（RF）power in single-frequency capacitively coupled Ar plasma discharges.The axial profile of Ar emission intensity is bimodal in shape.The axial profile of electron density is parabolic in shape and the position of the axial maximum electron density is close to the powered electrode.The axial distribution of electron temperature shows a saddle type shape and electron temperature is very uniform in the bulk plasma.At a lower RF power,the spatial distribution of electron density is more uniform.As RF power increases,the Ar emission intensity becomes more intense near the sheaths in front of the two electrodes.When RF power exceeds a threshold,the electron power absorption mode switches from the α mode to the γ mode.This is accompanied by a significant enhancement in electron density increases and a sharp reduction in electron temperature.In addition,the mode transition leads to that the axial uniformity of electron density becoming worse.When driving frequency is lower,the electron impact excitation process can occur in a larger discharge space,so the spatial distribution of Ar emission intensity is very uniform.As driving frequency becomes higher,the sheaths get thinner and the excitation is mainly concentrated near the plasma-sheath boundaries.The axial distribution of Ar emission intensity shows an obviously bimodal shape.The axial uniformity of electron density is better at a lower driving frequency.The radial uniformity of electron temperature is better at a higher driving frequency.In Chapter 4,the effects of external parameters on the two-dimensional spatial distributions of plasma parameters in dual-frequency capacitively coupled Ar plasma discharges are systematically studied.Compared with the single-electrode driven type in dual-frequency discharges,the absolute value of the DC self-bias in the two-electrode driven type is very small,so the discharge structure of plasma in the axial direction is more symmetric;the electron density in the two-electrode is higher,because of a higher coupled power and more effective electron heating.Compared with single-frequency discharges,the Ar emission intensity near the sheaths weakens with the addition of a low-frequency（LF）source in dual-frequency discharges.It means that there is a strong coupling effect between frequencies.As electrode gap increases,the Ar emission intensity and electron density near the sheath regions and in the bulk plasma decrease.Besides,the axial symmetry of the discharge structure becomes worse.For the single-electrode driven type in dual-frequency discharges,the absolute value of the DC self-bias voltage increases as gas pressure decreases or LF power and high-frequency（HF）power increase;as LF power increases or gas pressure decreases,due to the enlarged sheath thickness,the position of the maximum electron density in the axial direction shifts gradually towards the grounded electrode.However,HF power has a limited effect on the position of the maximum axial electron density.Therefore,for asymmetrically capacitive Ar plasma discharges,the position of the axial maximum electron density is related to the position of the maximum ionization rate,which is mainly determined by,instead of the DC self-bias voltage,the maximum sheath thickness.When the plasma is highly dense,it is the more local charged particle dynamics（a higher gas pressure）and/or the higher power deposition at the electrode center（a higher HF power）that lead to the center-peaked density profile.The radial distribution of plasma density is more uniform under a low plasma density situation（e.g.a lower gas pressure or lower HF power）.In Chapter 5,the influences of external parameters on the electron power absorption modes in dual-frequency capacitively coupled Ar plasma discharges are systematically demonstrated.At the initial stage of LF voltage increasing,the Ar emission intensity near the sheath regions weakens slowly while the Ar emission intensity in the bulk plasma enhances slowly.The electron density in the bulk plasma increases slightly,and the axial profile of electron density is relatively uniform.The electron temperature in the bulk region rises slightly,and the plasma discharge is operated at the α mode.As LF voltage rises further,the Ar emission intensity near the sheath regions enhances significantly,especially near the sheath in front of the electrode driven by the LF source.The Ar emission intensity within the bulk plasma also rises evidently,and the discharge structure of plasma in the axial direction becomes very symmetric.The plasma density in the bulk region increases rapidly,and the electron temperature in the bulk region decreases gradually.The plasma discharge is dominated by the γ mode.Besides,the discharge turns into the γ mode earlier at a lower LF frequency,a higher HF power,a lower electrode gap,or a higher gas pressure.