Spectroscopic Diagnostics Of Dual-frequency Capacitively Coupled Plasma

Dual-frequency capacitively coupled plasma (DF-CCP) is one of the crucial components for etching in microelectronic manufacturing, and has been widely used in the next generation etching applications due to its simple structure and ability to control the plasma density.The plasma density should be determined by the higher frequency (HF) source and the ion bombarding energy in the sheath region should be controlled by the lower frequency (LF) source, thus the separate contron of plasma densityand ion energy can be realized in a DF-CCP. Dual-frequency capacitively coupled plasma overcomes the shortage of traditional single frequency source, and can obtain higher etching rate and uniformity.The advantages of producing large area uniform plasma, operating easily and simple structure with lower cost meet the requirements of the industrial production, DF-CCP source has been widely used in the next generation etching applications.The physical processes and various physical parameters dual-frequency capacitive coupled plasma source have influence on plasma etching, and it is necessary to be studied in detailed.Optical emission spectrometry (OES) and absorption spectrometry were used to study the characteristics with different experimental conditions in dual-frequency capacitive coupled plasma in this paper, which include electron temperature, gas temperature, plasma spatial uniformity and particle density that play important role in the application of plasma.The electron temperature and gas temperature can be obtained by studying the emission spectrum of plasma. The electron temperature (Te) in dual-frequency capacitively coupled argon plasma is determined by a modified Boltzmann plot technique. The results showed that the electron temperatures in our experimental conditions range from 2.3 eV to 6.9 eV, and discharge pressure, power and gas flow rate have obvious effect on the electron temperatures in a DF-CCP.Comparing experimental spectrum with the one fitted using LIFBASE software of CF B2Δ-X2∏(0,0), (1,1) and (2,2) bands, the rotational and vibrational temperature can be obtained. For the experimental conditions (P=50 mTorr,fH=60 MHz,fL=2.0 MHz, PL=50 W and PH=300 W), the rotational and vibrational temperatures for the B2A state are 620 K and 2150 K, respectively. These temperatures reflect more directly the population distributions resulting from the B2Δstate production mechanism because the B2Δstate radiative lifetime of 20 ns allows very few thermalizing collisions to occur. At the same time, absolute fluorine atom concentrations in dual-frequency capacitively coupled plasmas by optimizing the actinometry technique. The emission intensities of F (703.7 nm) and Ar (750.4 nm) were simultaneously measured by a spectrograph in order to calculate the absolute fluorine atom concentrations. And the electron temperature at the same experimental conditions was studied by using the optical emission spectrometry.The radial and axial spatial uniformity was studied by using optical emission spectrometry, which is very important for plasma etching technology. A home-made optical probe was used to determine the emission intensity of argon plasma in different position of two electrodes, and the distribution of radial density can be obtained by moving the optical probe in. The results showed that the radial spatial uniformity was affected obviously by discharge pressure and lower frequency source. In addition, the axial plasma density distribution can be determined by adjusting the height of a modified fiber placed outside the window of discharge chamber, which shows an arc distribution in the two electrodes, the center of the arc is near upper electrode (two radio-frequency sources are connected to upper electrode together). Discharge pressure and high frequency power have remarkable effect on the distribution of axial density, and increased discharge pressure and high frequency power will significantly increase the emission intensity of plasma. Increasing speed of near upper electrode is higher than that of near lower electrode. Low frequency power has little effect for radial plasma density, and increasing low frequency power will result in lower emission intensity of plasma in DF-CCP.Absorption spectroscopy was used to study CF2 number density in dual-frequency capacitive coupled plasma, and discharge gas is composed of pure CF4. The absolute concentration of CF2 radical in CF4 plasma can be determined by using the transmission spectrum in DF-CCP. The spectra of CF2 A(0,v',0)→X(0,0,0) (v'=4-7) band were used to calculate its concentratin in this paper. Background absorption, stability of Xe lamp and plasma should be considered carefully in calculated the concentration of CF2 radical. For the plasma density is low, repeated determination is needed to improve the signal-to-noise ratio. The concentration of CF2 radical ranges from1.4×1013 cm-3 to 5.4×1013 cm-3 in this paper. The increasing of discharge pressure and high frequency can increase the the concentration of CF2 radical in DF-CCP, especially for the effects of high frequency power. In order to improve the plasma density we can increase the power of high frequency source and discharge pressure in dual-frequency capacitively coupled plasma.