Low pressure plasma processing is used extensively in the semiconductor industry for modern technology. As a result, the understanding of plasma/material interactions is very important to the improvement and design of materials and treatment methods. This dissertation focuses on the understanding of interactions between individual plasma species and materials that are used in the fabrication of the interconnect in semiconductor devices, namely porous low-k dielectrics.

Proper insulation of the metal lines in the interconnect relies on maintaining the integrity of the insulator's dielectric properties. As the features of the interconnect are shifting to smaller dimensions, techniques used to implement lower dielectric permittivity have also lowered the material's resistance to plasma-induced damage. As a result, plasma processing of these materials have become a major challenge for future advances in interconnect technology. This study examines specific species generated from typical plasma discharges and their effect on porous low-k materials.

The first part of this dissertation studies the mechanism for photon-induced effects and demonstrates the influence of emission wavelength on carbon removal from carbon-doped oxide films. VUV photons emitted from the plasma were observed to break Si-C bonds. However, oxidizing gas species must be present in the background to cause carbon removal. Depending on gas chemistry, VUV photons of different wavelengths are emitted, which affects the depth of the damage penetration into the film. Shorter wavelength emissions are absorbed by the SiO2-like damage layer that is produced after carbon extraction, thereby preventing further removal of carbon.

The dissertation continues by examining the effects of photons and radicals individually by isolating the species to obtain separate exposures. By doing so, radicals and photons generated from O2 plasma were observed to remove carbon in different ways, creating different carbon profiles as a function of depth. 130 nm wavelength photons are fairly transparent through the material, leading to a gradual removal of carbon throughout the modification depth. In contrast, oxygen radicals are diffusion-limited, leading to removal of carbon occurring as a front. Modeling of damage effects by these species was performed in each case, and good predictions of their behavior were obtained. However, direct plasma exposures were observed to behave differently, exhibiting much less carbon removal than predicted by the model. Further experimentation found evidence that synergy between photon and radical species led to an effective decrease in the diffusivity of the modified material, reducing subsequent plasma damage.

Through fundamental study of plasma/material interactions, the role of photons and radicals in plasma-induced damage has been determined. As a result, treatment methods to reduce plasma damage based on inhibiting these species can be designed. These include plasma filtration techniques, densification by ion bombardment, and the use of plasmas with reducing chemistries. While the scope of this research has focused on interactions with dielectric thin films, these approaches to plasma interactions are relevant to other processes that rely on plasma processing.

Low temperature plasma-based processes are used extensively in many modern technologies. It is thus very important to understand plasma and surface interactions in order to improve plasma processes and design of functional materials. Applying a high vacuum beam system, this dissertation studies the fundamental mechanisms of plasma species-induced modification of materials for two critical applications: manufacturing of semiconductor devices and surface deactivation of infectious biomolecules.

Manufacturing of integrated circuits relies on well-controlled film patterning technology. This is currently achieved by photolithography followed with plasma etch. While critical dimension control is acknowledged to be a major challenge for future miniaturization of transistors and other functionalities, degradation and roughening of methacrylate-based 193 nm photoresist (PR) during plasma etch processes result in poor pattern transfer and decreased device performance. The first part of this dissertation addresses the effects of ion bombardment, vacuum ultraviolet (VUV) irradiation, electron exposure, and moderate substrate heating in 193 nm PR surface roughening.

150 eV Ar ion bombardment results in physical sputtering and formation of a carbon-rich layer at the PR surface. The thickness of this surface layer is about 2 nm. This ion-modified layer is expected to bear an intrinsic compressive stress, roughly a few GPa. In contrast, 147 nm VUV irradiation and 1 keV electron exposure both modify the PR film up to the penetration depth, ~100 nm. Enhanced PR surface roughening is only observed when a simultaneous exposure provides a combination of an ion-modified surface layer on top of a scissioned/softened bulk layer, either resulting from VUV irradiation or low fluence electron exposure. 2-methyl-2-adamantyl methacrylate, the leaving group of 193 nm PR, is especially sensitive to VUV exposure. The adamantyl leaving group is shown to be one of the main photolysis products. The loss and detachment of bulky adamantyl groups are highly correlated to the surface roughening of processed PR. These phenomena can be qualitatively explained by a bi-layer wrinkling mechanism. The results demonstrate that PR structure can couple to plasma etch processes and strongly alter the post-etch morphology.

The second part of this dissertation is motivated by the insufficiency of conventional sterilization methods against bacterial and protein residues. Such residues on the surface of medical instruments increase the risk of healthcare-associated infections for patients. Low temperature plasmas are promising alternative sterilization/deactivation methods. A thorough understanding of plasma and biological target interactions is required to align applications with scientific principles. Lipid A, the immune-stimulating region of lipopolysaccharide, is chosen to be the model molecule. Using a surface-sensitive human whole blood-based assay, the present study shows that VUV photons, oxygen and deuterium radicals can cause deactivation of lipid A film through different mechanisms. Similar to 193 nm PR studies, VUV photons are able to induce bulk modification of lipid A film up to the penetration depth of photons, ~200 nm. VUV photons primarily cleave ester linkages and lead to desorption of aliphatic chains. Loss of phosphate groups and the glucosamine backbone is also observed. In contrast, radicals react at the lipid A film surface, form volatile products, and lead to slow chemical etching. The etch yield of radicals is one order of magnitude lower than that caused by VUV-induced photolysis. In spite of its low etch yield, radical exposure strongly modifies lipid A film surface. This work contributes to the fundamental understanding of plasma interaction with biomolecules. The principle of this study is also relevant to the broader scope of plasma applications on biological targets, including cells and tissues, in the rapidly growing field of plasma medicine.

The semiconductor industry's continued trend of manufacturing device features on the nanometer scale requires increased plasma processing control and improved understanding of plasma characteristics and plasma-surface interactions. This dissertation presents a series of experimental results for focus studies conducted in an inductively coupled plasma (ICP) system. First novel "on-wafer" diagnostic tools are characterized and related to plasma characteristics. Second, plasma-polymer interactions are characterized as a function of plasma species and processing parameters. Complimentary simulations accompany each focus study to supplement experimental findings.

Wafer heating mechanisms in inductively coupled molecular gas plasmas are explored with PlasmaTemp^TM, a novel "on-wafer" diagnostic tool. Experimental wafer measurements are obtained with the PlasmaTempTM wafer processed in argon (Ar) and argon-oxygen (Ar/O₂) mixed plasmas. Wafer heating mechanisms were determined by combining the experimental measurements with a 3-dimensional heat transfer model of the wafer. Comparisons between pure Ar and Ar/O₂ plasmas demonstrate that two additional wafer heating mechanisms can be important in molecular gas plasmas compared to atomic gas discharges. Thermal heat conduction from the neutral gas and O-atom recombination on wafer surface can contribute as much as 60 % to wafer heating under conditions of low-energy ion bombardment in molecular plasmas.

Measurements of a second novel "on-wafer" diagnostic sensor, the PlasmaVolt^TM, were tested and validated in the ICP system for Ar plasmas varying in power and pressure. Sensor measurements were interpreted with a numerical sheath simulation and comparison to scaling laws derived from the inhomogeneous sheath model. The study demonstrates sensor measurements are proportional to the RF-current through the sheath and the scaling is a function of sheath impedance. PlasmaVolt^TM sensor measurements are proportional to the square root of the plasma density at the plasma-sheath interface, one-fourth root of the electron temperature, and one-fourth root of the RF bias voltage under conditions where the sheath is predominantly capacitive. When the sheath impedance becomes increasingly resistive, the sensor measurements deviate from the scaling law and tend to be directly proportional to the plasma density.

Vacuum ultraviolet (VUV) emissions in Ar ICPs are characterized and the chemical and physical modifications to 193 nm photoresist (PR) polymer materials processed in Ar ICPs are investigated. Fourier transform infrared (FTIR) transmission measurements as a function of VUV photon fluence demonstrate that VUV-induced bond breaking occurs over a period of time. A numerical model demonstrates that VUV photons deplete near-surface O-containing bonds, leading to deeper, subsequent penetration and more bond losses, while the remaining near-surface C-C bonds absorb the incident radiation and slow VUV photon penetration.

The roughening mechanism of blanket and patterned 193 nm PR samples are explored in a well characterized Ar ICP. FTIR and atomic force microscopy (AFM) analysis of plasma processed 193 nm PR suggests that ion-induced generation of a graphitized layer at high energies, combined with VUV bulk modification of 193 nm PR may initiate PR roughening. The roughness of blanket samples increases as a function of VUV fluence, ion energy, and substrate temperature. Line width roughness (LWR) measurements of patterned samples demonstrate a similar trend suggesting that LWR may correlate with surface roughness of patterns. The results are compared to PR studies previously conducted in an ultra-high vacuum beam system demonstrating that the vacuum beam system is a useful tool that can deconvolute and simplify complex plasma systems.

Vacuum Beam Studies of Ruthenium Etching

Ru is known to have two volatile oxidation products, RuO₃ and RuO₄, although the etch rate is negligible when Ru is exposed to an O₂ plasma discharge. The introduction of a small amount of additive gas, such as Cl₂, has been shown to increase the Ru etch rate sixfold. The reason for this dramatic shift in etching is poorly understood, primarily because it is difficult if not impossible to study plasma-surface interactions in a plasma environment. The unique capabilities of the beam system have made it possible to explore the mechanism of Ru etching. It has been shown that under 500 eV Ar+ ion bombardment, the addition of O radicals lowered the etch rate by a factor of 2.5. This process was relatively insensitive to temperature over the range studied (room temperature to ~175°C). It was also shown that O radicals alone spontaneously etched Ru at a very slow rate over the entire temperature range.

Statistical Analysis of Polysilicon Etching and Gate Profile Evolution in Dual-Doped Polysilicon Gates

Polysilicon gate etching for the 90nm lithography node and below requires extremely precise control of the gate CD and profile. Generally speaking, the current requirement for Gate CD control is that the 3 sigma should less than ~5 nm for all gates, including across the chip, across the wafer, wafer-to-wafer, lot-to-lot, and tool-to-tool variations. Similarly, for gate sidewall angle control, the 3 sigma angle variation should be less than ~1 degree, inclusive of all sources of variation. This is particularly challenging for technologies which employ dual-doped gates, since the chemistry and physics of the etching process induces a different profile evolution between gates with different doping.

The goal of this project was to identify a parameter space where the differences in gate profile evolution across different polysilicon dopant types were minimized. Blanket etch rates and patterned wafers were used to determine the effect of different gate etch process variables on the gate profile. The materials studied were undoped polysilicon and polysilicon that had been doped with P, As, Sb, and B. Prediction models were created for the blanket etch rate studies that were used to optimize the processing conditions and to propose some simple mechanisms that identify which species are adsorbed on the surface.

The emerging field of plasma biotechology studies the applications of the plasma phase of matter to biological systems. "Ambient-condition" plasmas created at or near room temperature and atmospheric pressure are especially promising for biomedical applications because of their convenience, safety to patients, and compatibility with existing medical technology. Plasmas can be created from many different gases; plasma made from air contains a number of reactive oxygen and nitrogen species, or RONS, involved in various biological processes, including immune activity, signaling, and gene expression. Therefore, ambient-condition air plasma is of particular interest for biological applications.

To understand and predict the effects of treating biological systems with ambient-air plasma, it is necessary to characterize and measure the chemical species that these plasmas produce. Understanding both gaseous chemistry and the chemistry in plasma-treated aqueous solution is important because many biological systems exist in aqueous media. Existing literature about ambient-air plasma hypothesizes the critical role of reactive oxygen and nitrogen species; a major aim of this dissertation is to better quantify RONS by produced ambient-air plasma and understand how RONS chemistry changes in response to different plasma processing conditions. Measurements imply that both gaseous and aqueous chemistry are highly sensitive to operating conditions. In particular, chemical species in air treated by plasma exist in either a low-power ozone-dominated mode or a high-power nitrogen oxide-dominated mode, with an unstable transition region at intermediate discharge power and treatment time. Ozone (O3) and nitrogen oxides (NO and NO2, or NOx) are mutually exclusive in this system and that the transition region corresponds to the transition from ozone- to nitrogen oxides-mode. Aqueous chemistry agrees well with to air plasma chemistry, and a similar transition in liquid-phase composition from ozone mode to nitrogen oxides mode occurs as the discharge power increases.

One prominent example of plasma biotechnology is the use of plasma-derived reactive species as a novel disinfectant. Ambient-air plasma is an attractive means of disinfection because it is non-thermal, expends a small amount of power, and requires only air and electricity to operate. Both solid surfaces and liquid volumes can be effectively and efficiently decontaminated by the reactive oxygen and nitrogen species that plasma generates. Dry surfaces are decontaminated most effectively by the plasma operating in NOx mode and less effectively in ozone mode, with the weakest antibacterial effects in the transition region, and neutral reactive species are more influential in surface disinfection than charged particles. Aqueous bacterial inactivation correlates well with ozone concentration, suggesting that ozone is the dominant species for bacterial inactivation under the condition of a low-power discharge. Alternatively, air plasma operating in the higher-power, nitrogen oxides-rich mode can create a persistently antibacterial solution. Finally, when near-UV (UVA) treatment follows plasma treatment of bacterial suspension, the antimicrobial effect exceeds the effect predicted from the two treatments alone, and addition of nitrite to aqueous solution, followed by photolysis of nitrite by UVA photons, is hypothesized as the primary mechanism of synergy.

The results presented in this dissertation underscore the dynamic nature of air plasma chemistry and the importance of careful chemical characterization of plasma devices intended for biological applications. The complexity of atmospheric pressure plasma devices, and their sensitivity to subtle differences in design and operation, can lead to different results with different mechanisms.