Yiting Zhang

Control of ion energy and angular distributions in dual-frequency capacitively coupled plasmas through power ratios and phase: Consequences on etch profiles

Anisotropic etching, enabled by energetic ion bombardment, is one of the primary roles of plasma–assisted materials processing for microelectronics fabrication. One challenge in plasma etching is being able to control the ion energy-angular distributions (IEADs) from the presheath to the surface of the wafer which is necessary for maintaining the critical dimension of features. Dual frequency capacitive coupled plasmas (DF-CCPs) potentially provide flexible control of IEADs, providing high selectivity while etching different materials and improved uniformity across the wafer. In this paper, the authors present a computational investigation of customizing and controlling IEADs in a DF-CCP resembling those industrially employed with both biases applied to the substrate holding the wafer. The authors found that the ratio of the low-frequency to high-frequency power can be used to control the plasma density, provide extra control for the angular width and energy of the IEADs, and to optimize etch profiles. If the phases between the low frequency and its higher harmonics are changed, the sheath dynamics are modulated, which in turn produces modulation in the ion energy distribution. With these trends, continuously varying the phases between the dual-frequencies can smooth the high frequency modulation in the time averaged IEADs. For validation, results from the simulation are compared with Langmuir probe measurements of ion saturation current densities in a DF-CCP.

Control of ion energy distributions using phase shifting in multi-frequency capacitively coupled plasmas

Summary form only given. Anisotropic etching for microelectronics fabrication is accomplished by energetic ion bombardment in chemically enhanced sputtering. One challenge is being able to control the ion energy-angular distributions (IEADs) onto the surface of the wafer to selectively activate desired processes, which is advantageous for maintaining the critical dimension (CD) of features. Capacitive coupled plasmas (CCPs) powered by non-sinusoidal waveforms and or using multiple frequencies are strategies employed to provide flexible control of IEADs which produce high selectivity and uniformity. Varying relative voltages, powers and phases between multiple frequencies that differ by integer multiples have demonstrated potential control mechanisms for the IEADs and optimization of etching profiles. In this paper, we report on computational and experimental investigations of lEAD control in a dual-frequency CCP where the phase between the frequencies is used as a control variable. The rf frequency and its harmonic frequencies are both applied to the wafer substrate. Both symmetric and asymmetric CCPs are studied. The Hybrid Plasma Equipment Model (HPEM) was employed to predict plasma properties and obtain the harmonic contributions to the power applied to the same electrode. The ion and radical fluxes incident onto the surface are used as input to the Monte Carlo Feature Profile Module (MCFPM) with which profiles are predicted. The operating conditions are 5-100 mTorr in Ar and Ar/CF4/O2 gas under different frequency mixing and phase of integer multiple frequency drives. We find that by changing the phase between the applied rf frequency and its second harmonic, the Electrical Asymmetric Effects (EAE) is significant and can shift the dc self-bias.[I] When changing phases between the rf and its higher harmonics, the EAE becomes less effective and ion energy distributions spike at specific energies. Computed results for lEADs are compared with rf phase locked harmonic experimental results measured by Radio Frequency Ion Energy Analyzer.

Investigation of feature orientation and consequences of ion tilting during plasma etching with a three-dimensional feature profile simulator

Pattern transfer in microelectronics fabrication using plasma-assisted etching processes is being challenged by the three-dimensional (3d) structures of devices such as fin field effect transistors. Etching of 3d structures typically requires a longer over-etch time to clear material in corners, introducing additional selectivity challenges to maintain feature scale critical dimensions. Feature open area, orientation, aspect ratio, and proximity to other nearby structures can influence the outcome of the etch process. In this paper, the authors report on the development and application of a 3d profile simulator, the Monte Carlo feature profile model in the investigation of aspect ratio, and feature orientation dependent etching. In these studies, energy and angularly resolved reactant fluxes were provided by the hybrid plasma equipment model. Results from the model were validated with trends from experimental data. Using reactant fluxes from He/Cl2 and Ar/Cl2 inductively coupled plasmas, etching of two di...

Role of neutral transport in aspect ratio dependent plasma etching of three-dimensional features

Fabrication of semiconductor devices having three-dimensional (3D) structures places unprecedented demands on plasma etching processes. Among these demands is the frequent need to simultaneously etch features with a wide variety of aspect ratios (AR) on the same wafer. Many plasma etching processes exhibit aspect ratio dependent etching (ARDE)—different etch rates for features that have different aspect ratios, usually slower for larger AR. Processes subject to ARDE require over-etch to clear the larger AR features, which increases the need for high selectivity and low damage. Despite these issues, the physical processes which contribute to ARDE are not well understood. In this paper, results are discussed from a computational investigation on the root causes of ARDE during Ar/Cl2 plasma etching of Si, and, in particular, the role which neutral transport plays in this process. Parametric studies were performed varying neutral-to-ion flux ratios, surface recombination rates of atomic Cl, and neutral and io...

Space and phase resolved ion energy and angular distributions in single- and dual-frequency capacitively coupled plasmas

The control of ion energy and angular distributions (IEADs) is critically important for anisotropic etching or deposition in microelectronic fabrication processes. With single frequency capacitively coupled plasmas (CCPs), the narrowing in angle and spread in energy of ions as they cross the sheath are definable functions of frequency, sheath width, and mean free path. With increases in wafer size, single frequency CCPs are finding difficulty in meeting the requirement of simultaneously controlling plasma densities, ion fluxes, and ion energies. Dual-frequency CCPs are being investigated to provide this flexible control. The high frequency (HF) is intended to control the plasma density and ion fluxes, while the ion energies are intended to be controlled by the low frequency (LF). However, recent research has shown that the LF can also influence the magnitude of ion fluxes and that IEADs are determined by both frequencies. Hence, separate control of fluxes and IEADs is complex. In this paper, results from ...

Atomic layer etching of 3D structures in silicon: Self-limiting and nonideal reactions

Current (and future) microelectronics fabrication requirements place unprecedented demands on the fidelity of plasma etching. As device features shrink to atomic dimensions, the plasma etching processes used to define these devices must resolve these scales. By separating etching processes into cycles of multiple, self-limited steps, different physics processes which are closely coupled in traditional plasma etching can be largely decoupled and separately optimized. This technique, atomic layer etching (ALE), can ideally remove uniform layers of material with consistent thickness in each cycle. ALE holds the promise of improving uniformity, reducing damage, increasing selectivity, and minimizing aspect ratio dependent etching (ARDE) rates. The practical implementation of ALE depends on how close to ideal the system can be operated and the tolerance to nonideal conditions. In this paper, results are discussed from a computational investigation of the consequences of nonidealities in the ALE of silicon usin...

Origins of aspect ratio dependent etching in plasma materials processing

Summary form only given. During reactive ion plasma etching (RIE) of semiconductor materials, a strong correlation between etch rate and feature size, or more specifically feature aspect ratio (AR), is often observed. There have been several analytical models proposed to explain this aspect ratio dependent etch-rate (ARDE), also known as “RIE lag.”[1,2] While most analytical models only address individual causes of ARDE, the complexity of most etching reactions implies that several ARDE mechanisms may be at play. This complexity motivates the use of numerical models which are sensitive to all of the probable causes of ARDE to determine the dominant factor in any given etching reaction. To investigate processes leading to ARDE, a 3-dimensional voxel based Monte-Carlo etch profile simulator was developed. In this model, the etch feature is represented by mesh of cubic computational voxels. Energy and angular resolved fluxes of neutral radicals and ions are produced by a companion plasma equipment model. Pseudoparticles are randomly chosen from these distributions and directed to the surface. Each collision of a gas phase pseudoparticle with the profile then adds, removes or changes the identity of material voxels based on probabilities defined in a reaction mechanism. We investigated some of the complex interactions that may lead to ARDE for Cl2 plasma etching of Si and fluorocarbon plasma etching of SiO2. By tuning parameters that are uncontrolled in experiments, such as the sticking coefficients of etch products to sidewalls of the feature, we were able to isolate the causes of ARDE in several etch regimes. Even within a single reaction mechanism, the dominant cause of ARDE can change with etch geometry, processing conditions or even with etch depth. For instance, as the profile of a trench or via transitions from a flat etch front to a tapered profile, a transition which occurs at some AR for almost all RIE processes, the dominant cause of ARDE can abruptly shift from radical starvation to lack of ion energy flux. The dominant cause(s) of ARDE will be discussed for a range of operating configurations (experimentally accessible) and model parameters (only simulation accessible). For conditions where a single mechanism dominates we present scaling relations and test the applicability of analytical models.

Computational investigation of dual-frequency power transfer in capacitively coupled plasmas

Summary form only given. Dual frequency capacitively coupled plasmas provide the microelectronics fabrication industry flexible control, high selectivity and uniformity. The spatial variation of the phases, magnitude and wavelength of the high frequency (HF) rf bias will affect electron density, electron temperature, sheath thickness and ion transit time through the sheath. These variations ultimately affect the ion energy and angular distributions (IEADs) to the substrate which are of critical importance for anisotropic etching or deposition. To optimize the separate control of rates of ionization and IEADs, the HF should be significantly different than the low frequency (LF), which results in the LF being few MHz. For classical sinusoidal rf biases applied on the same electrode, the HF/LF harmonic currents can be distinguished by their Fourier transforms. Recently, nonsinusoidal bias waveforms are being applied in etching recipes to control etching speed and selectivity, which then complicates separating the HF from LF since both now have high harmonic contents. In this paper, we report on a computational investigation of the rf power absorption, power coupling control and IEADs in a CCP resembling those industrially employed with dual-frequency biases both applied to the wafer substrate. The Hybrid Plasma Equipment Module (HPEM) was employed to predict plasma properties and obtain the harmonic contributions of voltage waveforms applied to the same electrode. The operating conditions are 20-50 mTorr in pure Ar and Ar/C4F8/O2 gas mixtures under with 2 MHz + 60 MHz rf biases. The ratio of the HF/LF power can be used to control plasma density, and provide extra control for the width and energy of the IEADs.