Electron temperature is one of the most informative and basic parameters in plasma physics because electrons are not only involved in excitation, dissociation, and ionization of atoms and molecules but also govern the chemical reactions inside the plasma. The kinetic temperature of the free electrons is usually related to the electronic excitation temperature of the bound electrons in an atom because excitation processes governing the distribution of excited states are mostly caused by the free electrons. Excitation temperature is widely utilized in high pressure plasmas, and especially, the methods for electron temperature determination at an atmospheric pressure are frequently based on the measurement of excitation temperature due to their close relationship.
One of the most prevalent and versatile means of measuring important plasma parameters in low temperature plasmas is electrical probes. Use of these probes is, however, occasionally limited owing to difficulties in analyses under some circumstances, perturbation of the plasmas etc. Optical emission spectroscopy (OES) can also determine electron temperature based on appropriate equilibrium models. However, it is sometimes not straightforward to obtain information corresponding to collision cross section of all species and to determine electron temperature directly from the spectral lines due to cumbersome interpretation using complex equilibrium models especially in processing plasmas.
On the other hand, excitation temperature measurement, which is directly evaluated from atomic emission lines, is sometimes more efficient to characterize the plasma in low pressure plasmas as well as in high pressure plasmas. When the electron density is higher than the critical one given by the Griem criterion, the local thermodynamic equilibrium (LTE) is satisfied and excitation temperature is close to electron temperature. Although it would be difficult to expect excitation temperature is equal to electron temperature in laboratory or industrial processing plasmas because they seldom satisfy the LTE condition, elucidating the relationship between the two physical parameters will be very instructive especially for plasmas in which electron temperature measurement is difficult.
Large-area capacitive discharges driven at frequencies higher than the conventional 13.56 MHz have recently attracted much attention in both plasma physics and in the semiconductor and display industries as processing plasma sources. At large plasma sources where the plasma dimension becomes comparable with the wavelength of the driving radio frequency (rf), physical phenomena such as standing wave and skin effects are found to occur, which do not take place at lower driving frequencies. These electromagnetic phenomena tend to diminish the degree of plasma uniformity. At large-area, therefore, achieving high spatial uniformity of the plasma and measuring this with high accuracy have become important research issues. As one of the most prevalent and versatile means of measuring spatial uniformity, OES is a useful diagnostic tool, but it inevitably involves chord-integrated measurements along the line of sight. Hence, either inversion or tomographic reconstruction is required to obtain spatially-resolved local information inside the plasma.
In this letter, spatially-resolved local measurement is introduced by reconstructing plasma emission using the Phillips-Tikhonov (P-T) regularization method for tomography system and the convolution theorem for lens optics system. With the spatially-resolved plasma emission, the excitation temperatures by both Tomography and lens optics systems are performed and compared with the electron temperature obtained by Langmuir probe. Furthermore, the optical diagnostics based on the reconstructed plasma emission is found to be a processing monitoring after applying tomography to the industry processing plasmas. We think this result may be useful for plasma diagnostics in which the conventional Langmuir probe method may not be readily applicable, such as in super large area plasma sources as in LCD processing plasmas.
대면적화 또는 초고주파화 되어가는 플라즈마 소스는 플라즈마 물리 분야뿐만 아니라 반도체 및 디스플레이 산업과 같은 공정 플라즈마에서도 큰 관심을 받고 있다. 특히 2 m 이상의 초대면적 LCD 패널 제작 및 태양전지 제조와 같은 분야에서는 대면적 플라즈마 소스가 더욱 요구되고 있다. 이렇게 플라즈마 면적의 대형화 및 높은 구동 주파수가 인가되는 환경에서는 플라즈마 내에 생성되는 전기장이 전극의 위치에 따라 균일하지 않게 되며, 또한 전기장의 시간적 변화에 의해 유도되는 자기장의 효과도 무시할 수 없게 된다. 이러한 플라즈마에서는 종래의 고주파(13.56 MHz)가 인가된 플라즈마에서 나타나지 않았던 정상파 효과(standing wave effect) 및 초고주파 표피 효과(skin depth effect)와 같은 전자기적 효과가 발생되고, 이러한 전자기적 효과는 플라즈마 공간균일도를 저하시킨다. 이와 같이 대면적화 또는 초고주파화 되어가는 플라즈마 소스에서 박막 증착 및 에칭과 같은 공정 플라즈마에서의 정확한 진단이 이루어지기 위해서는 플라즈마에 주는 섭동을 최소화하는 실시간 진단이 필수적이다. 따라서 본 학위논문에서는 대면적화 또는 초고주파화 되어 가는 공정 플라즈마에서 공정 모니터링에 활용될 수 있는 광 진단법을 개발하는 것에 초점을 두었다.
