Eric Hudson

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...

Synergistic effects of vacuum ultraviolet radiation, ion bombardment, and heating in 193nm photoresist roughening and degradation

The roles of ultraviolet/vacuum ultraviolet (UV/VUV) photons, Ar+ ion bombardment and heating in the roughening of 193nm photoresist have been investigated. Atomic force microscopy measurements show minimal surface roughness after UV/VUV-only or ion-only exposures at any temperature. Simultaneous UV/VUV, ion bombardment, and heating to surface temperatures of 60–100°C result in increased surface roughness, and is comparable to argon plasma-exposed samples. Ion bombardment creates a modified near-surface layer while UV/VUV radiation results in loss of carbon-oxygen bonds up to a depth of ∼100nm. Enhanced roughness is only observed in the presence of all three effects.

Photoresist modifications by plasma vacuum ultraviolet radiation: The role of polymer structure and plasma chemistry

While vacuum ultraviolet (VUV) photon irradiation has been shown to significantly contribute to material modifications of polymers during plasma exposures, the impact of radiation-induced material alterations on roughness development during plasma processing has remained unclear. The authors have studied the interaction of the radiation of Ar and C4F8/Ar plasma discharges with 193 and 248 nm advanced photoresists (PRs). Optical filters were used to vary the radiation exposure wavelength range in the ultraviolet (UV) and VUV emission spectra. This enables clarification of the respective roles of plasma photon radiation wavelength and PR polymer structure on the chemical and structural changes produced in the materials. Chemical changes in polymer composition at the film surface and in the material bulk were determined by vacuum transfer x-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy. Morphological changes, film thickness reduction, and changes in surface and pattern morphology...

Studies of film deposition in fluorocarbon plasmas employing a small gap structure

A small gap structure was designed to examine surface chemistry aspects of film deposition for fluorocarbon (FC) plasmas produced using both inductively coupled plasma (ICP) and capacitively coupled plasma (CCP) systems. The small gap structure provides a completely shadowed region without direct ion bombardment. Neutrals diffuse into this region and form a fluorocarbon layer. The lack of ion bombardment increases the retention of the chemical structure of the FC film precursors. The surface chemistry of FC film deposited in this region is compared with film deposited in the region exposed to direct ion bombardment. For films deposited in the exposed region, x-ray photoelectron spectroscopy analysis shows that CF2 is the dominant chemical bond for pure C4F8 in both ICP and CCP systems. For C4F8∕Ar discharges, C–C bonding is dominant for the polymer formed in the ICP system, whereas CF2 species are dominant for films deposited in the CCP system. In the completely shadowed region, CF2 bonding is dominant fo...

Electron, ion and vacuum ultraviolet photon effects in 193 nm photoresist surface roughening

Low temperature plasma exposure of methacrylate-based 193 nm photoresist (PR) can result in enhanced surface roughening or smoothing, but mechanisms are poorly understood. We present measurements of 193 nm PR surface roughness following exposure to 1 keV electron beams in various combinations with positive ion and vacuum ultraviolet (VUV) photon irradiation. Electron beams will scission or cross-link 193 nm PR under low and high fluence exposure, respectively. When coupled to simultaneous ion/VUV photon irradiation, low fluence (scissioning) electrons amplify surface roughening while high fluence (cross-linking) electrons reduce surface roughness. These results further suggest that enhanced roughening of 193 nm PR is initiated by the synergistic interaction between an ion bombardment-induced carbon-rich surface layer (~2 nm) and a sicssioned bulk layer (~100 nm).

CF A 2Σ+–X 2Π and B 2Δ–X 2Π study by broadband absorption spectroscopy in a plasma etch reactor: Determination of transition probabilities, CF X 2Π concentrations, and gas temperatures

Broadband absorption spectroscopy was applied to study the CF A 2Σ+–X 2Π and B 2Δ–X 2Π transitions in a plasma etch reactor. We report a previously unobserved band, which is assigned as CF A 2Σ+–X 2Π (3,0). This band is significantly broadened by predissociation, and we estimate the average collision-free lifetime of the CF A 2Σ+ v′=3 level to be 0.30±0.08 ps. Experimental relative oscillator strength measurements, together with ab initio calculations, Rydberg–Klein–Rees-based wave functions and experimental lifetimes were used to calculate a full set of transition probabilities for the CF A 2Σ+–X 2Π and B 2Δ–X 2Π bands. The maximum observed number densities of CF X 2Π were ∼2×1013 cm−3 with sensitivity to measure to 1010 cm−3. The excited state and ground state temperatures were determined by comparing the spectra to simulations. The ground state rotational temperature was 450±30 K and the vibrational temperature was 850±80 K near the substrate surface. The CF B 2Δ excited state rotational temperatures a...

Effect of Cu contamination on recombination of O atoms on a plasma-oxidized silicon surface

In the dual damascene microelectronics integration scheme during the last stage of plasma etching of dielectrics down to underlying Cu layers, Cu is sputtered onto the reactor walls and is believed to cause a drift in etching rates. For photoresist etching in an O2-containing plasma, a drop in etching rate suggests that Cu could cause a decrease in the O-atom concentration in the plasma, due perhaps to an increase in the O recombination rate on the chamber walls. We therefore studied the effects of traces of Cu on O recombination on an oxygen plasma-conditioned surface, using the spinning wall technique. With this method, a cylindrical substrate, here coated in situ with sputter-deposited Si and then oxidized in an O2 plasma, is rotated past skimmers, allowing the surface to be periodically exposed to the plasma and an Auger electron spectrometer with a pressure gauge in a differentially pumped chamber. Between plasma exposures, the sample could also be dosed with Cu from an evaporation source in a differ...

Effects of vacuum ultraviolet photons, ion energy and substrate temperature on line width roughness and RMS surface roughness of patterned 193 nm photoresist

We present a comparison of patterned 193 nm photoresist (PR) line width roughness (LWR) of samples processed in a well characterized argon (Ar) inductively coupled plasma (ICP) system to RMS surface roughness and bulk chemical modification of blanket 193 nm PR samples used as control samples. In the ICP system, patterned and blanket PR samples are irradiated with Ar vacuum ultraviolet photons (VUV) and Ar ions while sample temperature, photon flux, ion flux and ion energy are controlled and measured. The resulting chemical modifications to bulk 193 nm PR (blanket) and surface roughness are analysed with Fourier transform infrared spectroscopy and atomic force microscopy (AFM). LWR of patterned samples are measured with scanning electron microscopy and blanket portions of the patterned PRs are measured with AFM. We demonstrate that with no RF-bias applied to the substrate the LWR of 193 nm PR tends to smooth and correlates with the smoothing of the RMS surface roughness. However, both LWR and RMS surface roughness increases with simultaneous high-energy (≥70 eV) ion bombardment and VUV-irradiation and is a function of exposure time. Both high- and low-frequency LWR correlate well with the RMS surface roughness of the patterned and blanket 193 nm PR samples. LWR, however, does not increase with temperatures ranging from 20 to 80 °C, in contrast to the RMS surface roughness which increases monotonically with temperature. It is unclear why LWR remains independent of temperature over this range. However, the fact that blanket roughness and LWR on patterned samples, both scale similarly with VUV fluence and ion energy suggests a similar mechanism is responsible for both types of surface morphology modifications.

Predicting synergy in atomic layer etching

Atomic layer etching (ALE) is a multistep process used today in manufacturing for removing ultrathin layers of material. In this article, the authors report on ALE of Si, Ge, C, W, GaN, and SiO2 using a directional (anisotropic) plasma-enhanced approach. The authors analyze these systems by defining an “ALE synergy” parameter which quantifies the degree to which a process approaches the ideal ALE regime. This parameter is inspired by the ion-neutral synergy concept introduced in the 1979 paper by Coburn and Winters [J. Appl. Phys. 50, 5 (1979)]. ALE synergy is related to the energetics of underlying surface interactions and is understood in terms of energy criteria for the energy barriers involved in the reactions. Synergistic behavior is observed for all of the systems studied, with each exhibiting behavior unique to the reactant–material combination. By systematically studying atomic layer etching of a group of materials, the authors show that ALE synergy scales with the surface binding energy of the bu...

Sensitive End-Point Detection for Dielectric Etch

A method is described for detecting the initial exposure of the stop layer underlying a patterned dielectric film. The bias compensation signal from the power supply of the ceramic electrostatic chuck displays a step increase when the etch front reaches the underlayer The end-point signal strength is essentially independent of exposed oxide area over the range ∼0.01 to 1%. End point is detected just as the underlayer is exposed, significantly earlier than that for optical emission end point. The proposed origin of the bias compensation end-point signal is the change in resistance between wafer substrate and plasma as holes are etched through the dielectric film. This model correctly predicts which etch applications will produce an end-point signal, including standard via etch and contact etch. Dual damascene via etch also produces a useful end-point signal, apparently due to lowered resistance of the silicon nitride stop layer during etch. The use of bias compensation end point in a commercial wafer production facility is described.