In a modern plasma etching device, the plasma sheath potential is usually superposed by an externally-driven oscillating potential to control the bombarding ion energy. A cohesionless particle simulation is used to study the behavior of average kinetic energy of bombarding ions as a function of a wide range of sheath oscillation frequency ($ν{ τ_f}$$0,IV_{pi}$≤$V_{τf}$≤$I0V_{pi}$ where $V_{pi}$ is the ion plasma frequency). It is found that a resonance phenomenon between the ion transit motion and the sheath oscillation can yield a large enhancement of K from the average sheath potential energy. By considering the relative contribution of ions with different velocity in conventional time integrated ion energy distribution function, F, and the property of ion flux conservation inside plasma sheath region, we also obtain modified energy distribution function of ions bombarding at a given moment, f.
Based on a simplified $Cl_2$ plasma Si etching mechanism, we calculate the Si etching rate through a kinetic analysis of the role of the born-barding ions. In the kinetic analysis, ions bombarding a Si wafer are regarded as the sum of independent monoenergetic ion beams. The difference between the kinetic $ER_K$ and monoenergetic $ER_M$ anafysis is compared over rf frequency $ν{τ_f}$ range frmo 1 MHB to 20 $MH_Ζ$ν$0,IV_{pi}$≤$V_{τf}$≤$I0V_{pi}$ where $V_{pi}$ is the ion plasma frequency)under the typical low pressure high density plasma condition $n_e$ : 1×$10_{11_cm^-3}$, where ne is the plasma density). The investigation shows that the difference in the etching rate between the two approaches turns out maximum about 17% of the ERK value. We also investigate the elect of threshold energy on the Si etching mechanism, and explain the cause of a somewhat extraordinary etching rate result. The value of the low frequency criterion, yp, increases with plasma density, so the difference will be more considerable with a higher density plasma etching. One conclude that under the low rf frequency, kinetic analysis is required for a precise etching property examination with the low pressure high density plasma etching process.
We also review the role of ion bombardment on Si etching mechanism using fluerocarbon $C_nF_m$ gas and investigate the effect of the addition of hydrogen $H_2$ gas. For various fluerocarbo plasms Si etching process, it is observed that relatively thick (2-7nm) fluerocarbo layers exist on Si surface. In steady state, the etching rate and the surface modifications of Si do not change as a function of time. The contribution of direct impact of ion bombardment on the Si substrate to the etching mechanism is reduced with increasing fluerocarbo layer thickness. Therefore, the Si etching rate is controlled by a neutral etchantflux through the layer. But, the ions are also known to play an important role in the transport of Si etching precursor (ftuorine atom) through the layer. A model is developed that describes the etch kinetics through a fluorocarbon layer based on a fluorine diffusion transport mechanism. The model is based on the following two roles of the ions on the Si etching process. The first role is an enhancement in the diffusivity of fluorineatoms through the fluorocarbon layer and an enhancement in the reaction probability of fluorine in the fluorocarbon layer. In this case the fluorine is assumed to originate from the gas phase. The second role is ion fragmentation and dissociation of the fluorocarbon surface molecules. We also consider the change of plasma parameters and the density of the essential radicals for Si etching, such as F and $CF_x$ for pulse modulated ECR plasma using $CHF_3$ gas. The Si etching rate formula for the pulse modulated plasma is suggested. The Si etching rate result shows a satisfactory coincidence with the experimental result.
현대화된 플라즈마 공정장비에서는 플라즈마 sheath에 외부에서 rf bias를 걸어줌으로써 substrate에 입사하는 ion의 에너지를 증가시킨다. 충돌이 없는 sheath에 대해 시험입자 전산모사를 통해서 충돌하는 ion들의 평균에너지를 외부의 rf bias 진동수 ()에 따라 조사하였다. (). 플라즈마 sheath를 통과하는 ion과 sheath내의 전기장의 진동사이에 공명현상이 일어나 충돌하는 ion의 에너지 값이 (K)평균 sheath 포텐셜의 에너지보다 더 커지는 현상을 발견할 수 있었다. 또한 서로 다른 에너지를 가지는 입사하는 ion들의 상대적인 기여를 고려하여 전통적인 Monte-Carlo방법으로 얻을 수 있는 ion의 flux분포함수(F)로 부터 어떤 한 순간에 substrate에 입사하는 ion의 에너지 분포함수 (f)를 얻을 수 있는 방법을 제시하였다.
단순화 된 $Cl_2$플라즈마의 Si에칭 기작을 바탕으로, Si표면에 입사하는 ion들의 기여를 Kinetic한 관점에서 고려하여 에칭율을 구해보았다. 여기서는 입사하는 ion들의 기여를 독립된 현상으로 보고, 서로 다른 에너지를 가지고 입사하는 ion이 에칭기작을 일으키기 위해서는 임계에너지가 존재하게 되는데, 이러한 임계에너지가 에칭율에 미치는 영향을 조사해보았다. 이와같은 연구를 통해 우리는 낮은 rf bias 진동수 영역에서는 정확한 에칭율을 얻기위해서는 입사하는 ion을 kinetic 한 관점에서 다루어야 함을 알게되었다.
높게 선택비($SiO_2/Si$)를 얻기위해 많이 쓰이는 fluorocarbon($C_nF_m$)가스를 이용한 Si의 에칭기작에 대해 살펴 보았으며, 수소($H_2$)가스를 첨가할 때의 효과도 고려해보았다. Fuorocarbon플라즈마의 Si에칭은 표면에 두껍게 쌓이는 (2-7nm)$CF_x$ polymer표면에 충돌하는 ion의 에너지가 커질수록 더욱 원할해지므로, 이러한 수송모델을 바탕으로 bias전위차에 따른 애칭율의 변화도 살펴보았다. 또한 $CHF_3$가스를 사용한 시간변조된 (pulse modulated) 플라즈마의 경우 pulse특성에 따라 Si에칭에 영향을 주는 플라즈마 특성(전자온도, 플라즈마 밀도)과 주요 반응종들의 밀도를 관측하였고, 이를 통해 Si에칭율을 구해 보았다.