Vahid Vahedi

Ion energy distributions in rf sheaths; review, analysis and simulation

We present a review and analysis of ion energy distributions (IED) arriving at the target of a radio frequency (rf) discharge. We mainly discuss the collisionless regime, which is of great interest to experimentalists and modellers studying high-density discharges in which the sheath is much thinner than in conventional reactive ion etching systems. We assess what has been done so far and determine what factors influence the shape of the IEDs. We also briefly discuss collisional effects on the IEDs. Having determined the important parameters, we perform some particle-in-cell simulations of a collisionless current-driven rf sheath which show that ion modulations in an rf sheath significantly affect the IEDs when ion/rf<1, where ion is the ion transit time and rf is the rf period.

Overview of atomic layer etching in the semiconductor industry

Atomic layer etching (ALE) is a technique for removing thin layers of material using sequential reaction steps that are self-limiting. ALE has been studied in the laboratory for more than 25 years. Today, it is being driven by the semiconductor industry as an alternative to continuous etching and is viewed as an essential counterpart to atomic layer deposition. As we enter the era of atomic-scale dimensions, there is need to unify the ALE field through increased effectiveness of collaboration between academia and industry, and to help enable the transition from lab to fab. With this in mind, this article provides defining criteria for ALE, along with clarification of some of the terminology and assumptions of this field. To increase understanding of the process, the mechanistic understanding is described for the silicon ALE case study, including the advantages of plasma-assisted processing. A historical overview spanning more than 25 years is provided for silicon, as well as ALE studies on oxides, III–V c...

Effect of chamber wall conditions on Cl and Cl2 concentrations in an inductively coupled plasma reactor

The effect of chamber wall conditions on the Cl and Cl2 concentrations in a Cl2 discharge was studied in an inductively coupled plasma reactor. Cl and Cl2 mole fractions were determined using optical emission spectroscopy in conjunction with actinometry, while the state of the reactor walls was monitored using a surface probe that enables detection of films and adsorbates that deposit on these walls. Prolonged exposure of the chamber walls to a Cl2 plasma increases the Cl concentration in the discharge. This increase is due to the decreasing recombination probability of Cl atoms on the walls which with time are covered with a thin SiO2 film. The source of the SiO2 is the quartz dielectric window which is sputtered by ion bombardment. A SF6/O2 plasma etches the SiO2 film from the chamber walls and restores the chamber walls to a “clean” state. The Cl concentration in the reactor with these two different states of the wall conditions, under otherwise identical plasma operating conditions, was dramatically d...

Maintaining reproducible plasma reactor wall conditions: SF6 plasma cleaning of films deposited on chamber walls during Cl2/O2 plasma etching of Si

Silicon oxychloride films deposited on plasma etching reactor walls during the Cl2/O2 plasma etching of Si must be removed to return the reactor to a reproducible state prior to etching the next wafer. Using multiple surface and plasma diagnostics, we have investigated the removal of this silicon oxychloride film using an SF6 plasma. In particular, a diagnostic technique based on the principles of multiple total internal reflection Fourier transform infrared spectroscopy was used to monitor the films that formed on the reactor walls. The silicon oxychloride film etching proceeds by incorporation of F, which also abstracts and replaces the Cl atoms in the film. If the SF6 plasma is not maintained for a sufficiently long period to remove all the deposits, the F incorporated into the film leaches out into the gas phase during the subsequent etch processes. This residual F can have undesirable effects on the etching performance and the wafer-to-wafer reproducibility. The removal of the silicon oxychloride fil...

Physical and numerical methods of speeding up particle codes and paralleling as applied to RF discharges

We demonstrate the means, both physical and numerical, for speeding up particle-in-cell (PIC) simulations of RF discharges. These include implicit movers, longer ion timesteps, lighter-mass ions, different weights for electrons and ions, and improved initial density profiles. By using these methods (singly or together) on Ar and O2 RF discharges we were able to achieve speedups of six to 30 times with single-processor machines. In electrostatic 1d3v PIC simulations of RF discharges, the field solve is typically less than 1% of the work load. Even for 2d3v PIC simulations, the field solve can be a small percentage of the work load, especially when fast Fourier transform methods are used to solve the field. Thus, we can obtain significant gains by just paralleling particle processing (e.g., pushing/accumulating) without paralleling the field solve. We applied this simple scheme to conduct 1d3v and 2d3v PIC simulations of Ar RF discharges on two- and four-CPU symmetric multiprocessor machines and on a distributed network of workstations. For a fixed number of grid points, the speedup for this parallel particle processing became more linear with increasing number of particles. The combination of single-processor methods and paralleling makes run times for PIC codes more competitive with other types of codes.

New diagnostic method for monitoring plasma reactor walls: Multiple total internal reflection Fourier transform infrared surface probe

Films and adsorbates that deposit on reactor walls during plasma etching and deposition affect the discharge properties such as the charged particle and reactive radical concentrations. A systematic study of this plasma–wall interaction is made difficult by a lack of diagnostic methods that enable one to monitor the chemical nature of the reactor wall surface. A new diagnostic technique based on multiple total internal reflection Fourier transform infrared (MTIR-FTIR) spectroscopy was developed to monitor films and adsorbates on plasma etching and deposition reactor walls with monolayer sensitivity. Applications of this MTIR-FTIR probe are demonstrated. Specifically, we use this probe to (i) detect etch products and films that deposit on the reactor walls during Cl2 plasma etching of Si, (ii) determine the efficacy of a SF6 plasma to clean films deposited on reactor walls during Cl2/O2 etching of Si, and (iii) monitor wafer-to-wafer etching reproducibility.

Deposition of silicon oxychloride films on chamber walls during Cl2/O2 plasma etching of Si

The chemical nature and deposition rate of the silicon oxychloride films deposited on the chamber walls during Cl2/O2 plasma etching of Si were investigated using multiple total internal reflection-Fourier transform infrared spectroscopy. The differences in the infrared spectra of films deposited under different etching conditions were quantified through the Si–O and OSi–Cl absorption band intensities and positions to determine the growth rate and composition of these films. The changes in the film’s deposition rate and composition with rf bias power and O2 flow rate gave insight into the deposition mechanism. Based on our experimental observations, we propose that the silicon oxychloride film is deposited through oxidation of SiClx molecules adsorbed on the reactor walls and suggest a kinetic expression for the film deposition rate. This kinetic expression may also be used judiciously for describing the silicon oxychloride deposition on the sidewalls of etched features in gate etching and shallow trench ...

Semiempirical profile simulation of aluminum etching in a Cl2/BCl3 plasma

A semiempirical profile simulator to predict topographic evolution during Cl2/BCl3 plasma etching of photoresist patterned Al lines has been developed. Given incident flux distributions, the profile simulator uses a combination of a particle based Monte Carlo algorithm and analytic ray-tracing algorithm for solving feature-scale ion and neutral flux transport, respectively. We use angular and energy distributions for reflected ions that are consistent with experimental observation and molecular dynamic simulations. Etch yields with energy and angular dependence are experimentally determined for physical sputtering and ion-enhanced etching. The spontaneous etch rate of A1 by chlorine and the spontaneous desorption rate of Cl from photoresist are estimated from experimental results. Sticking coefficients for etchant, chlorine, and depositor, CClx, and depositing flux are determined by fitting simulated profiles to experimental data. A semiempirical site-balance model is developed to compute the surface cove...

Relation between the ion flux, gas phase composition, and wall conditions in chlorine plasma etching of silicon

Transients in plasma composition and positive ion flux due to changing chamber wall conditions during Cl2 plasma etching of Si were studied using multiple plasma and surface diagnostics. In presence of Si and O containing species in the gas phase a glassy silicon oxychloride film coats the chamber walls over a time scale determined by the concentrations of the Si and O containing deposition precursors. This time scale can be a few minutes as in the case of Si etching with Cl2 plasma, where the concentration of silicon chloride etching products can be high, or hours as in the case of a Cl2 plasma maintained in absence of Si wafer, where the Si and O can only come from very slow etching of a quartz window. In either case, SiClx (1⩽x⩽4) and Cl concentrations in the gas phase and the total ion flux impinging on the wafer surface increase as the chamber walls are coated with this glassy film. The increase in SiClx and Cl concentrations are primarily due to lower loss probability of these species by recombinati...

An on-wafer probe array for measuring two-dimensional ion flux distributions in plasma reactors

In plasma etching processes, the spatial distribution of ion flux across the wafer surface determines the uniformity and profile evolution when etching is ion limited. We have designed and built a two-dimensional array of planar Langmuir probes on a 200 mm diameter silicon wafer to measure the radial (r) and azimuthal (θ) variation of ion flux impinging on the wafer surface in plasma etching reactors. Herein we demonstrate the use of this probe array to obtain two-dimensional ion flux distributions in Ar, Cl2, and Cl2/HBr/He discharges in an inductively coupled plasma reactor. The results obtained using the probe array are in good agreement with Langmuir probe measurements but also reveal azimuthal asymmetries, due to irregularities in chamber geometry such as the pumping port and radio frequency coil configuration, that cannot be detected using radially movable Langmuir probes. The probe array can also be used to investigate the spatiotemporal fluctuations of the ion flux in the 1–100 Hz range.