AVS2017 Session PS-ThM: Plasma Sources

Substrate cleaning of organics utilizes a range of technologies that includes wet processing, barrel ashers, and microwave driven downstream plasmas. In this work, we introduce a unique inductively coupled downstream source configuration to generate high density radical concentrations (>1.1E+17 cm^-3) but without high ion and photon fluxes typically found in conventional inductively coupled plasmas. Although the plasma discharge tubes are isolated from the treatment chamber, they deliver a large concentration of free radicals. The low ion and photon exposure significantly reduces the opportunity for damage to sensitive layers. This inductive plasma arrangement prevents local heating and charging on the wafer, and behaves similarly as a microwave downstream plasma. However, a higher oxygen radical flux promotes more efficient organic layer cleaning and/or removal of photoresists even when low thermal budgets are a constraint.

This work will first describe the High Density Radical Flux (HDRF) source and characterize its behavior in generating high radicals flow and low ions in local downstream, on the wafer surface. Second, several applications using the HDRF technology will be discussed. These applications will include cleaning of 30:1 aspect ratio (AR) silicon vias, removal of sacrificial layers in MEMS structures, low temperature photoresist removal, and surface smoothing of Bosch generated sidewalls using micro-isotropic etching. With a low local electrical potential, due to the limited ions present in the process chamber, the HDRF is particular efficient with 3D structures on the wafer (e.g. MEMS and other high AR features) where preventing ion shielding effects is important.

The use of non-sinusoidal Tailored Voltage Waveforms (TVWs) to excite a plasma process has proven to be a rich field. Performing deposition or etching using such TVWs been shown to provide (1) a greater degree of control over outcomes, (2) more understanding of those processes, and even (3) processes unachievable by any other means, such as electrode-selective deposition.

The dream design for a Tailored Voltage Waveform plasma source is one that can ensure that an exact version of a given waveform appears on the RF electrode, but without increasing the complexity and cost of the source far beyond that of a single frequency RF source (including matchbox). These requirements are challenging due to the multi-harmonic nature of TVW's; the matching network must simultaneously provide efficient impedance matching at multiple frequencies, and as the phase between harmonics matters, for certain systems the waveform appearing at the RF feedthrough will not be a scaled version of the one on the electrode.

We address these two challenges directly. For the multi-frequency impedance matching challenge, we present progress on the design and fabrication of a high-power multi-frequency matchbox. This system allows the semi-independent tuning of the matching condition at each harmonic. For the second challenge involving uncertainty in the waveform appearing at the electrode, we present results using the plasma properties themselves to eliminate potential sources of error in the waveform. This technique avoids the need for probes located within the vacuum chamber, optical access to the plasma, or limiting the waveforms to lower frequencies.

Remote plasma sources (RPS) are being used to achieve isotropic etching with high selectivity by avoiding charging, energetic ion bombardment and UV/VUV radiation using long distance and discriminating barriers between the RPS and the substrate. By using multiple plasma sources or multiple gas inlets at different locations, the reaction pathway can be optimized for producing desirable process radicals. NF₃ and HBr are frequently used sources of F and Br atoms, the main etchants of silicon-containing materials, by electron impact dissociative attachment and excitation. NF_x (x = 1 – 3) and HBr can exothermically react with other neutral species to produce F, Br and OH radicals, which also enables customizing the reaction pathway by flowing gases downstream of the RPS.

In this paper, we report on results from a computational investigation of an inductively coupled RPS having multiple gas inlets with the goal of determining strategies for selectively producing reactive fluxes. The investigation was performed using the plug flow mode of 0-dimensional model, Global_Kin and in 2-dimensions using the Hybrid Plasma Equipment Model (HPEM). With NF₃/N₂/O₂ mixtures flowed through the RPS from an upstream inlet, the dominant radicals flowing downstream are F and O formed through dissociative excitation and attachment of NF₃ and O₂. NO molecules were formed through endothermic reactions among N₂, N, O₂ and O species. With HBr injected downstream of the plasma source, mixing with the plasma produced radicals enable another level of selectivity. Due to lack of electrons and low gas temperature (~ 350 K) downstream, HBr reacts with F and O through exothermic reactions (HBr + F > HF + Br, HBr + O > OH + Br and HBr + OH > H₂O + Br) and the dominant downstream radicals transition from F and O to Br and HF. Vibrationally excited HF(v), a highly polar molecule, may be formed through reactions having a larger exothermicity than the vibrational quanta, and so may produce a significant flux of activation energy to the wafer

Work was supported by Samsung Electronics, DOE Office of Fusion Energy Science and the National Science Foundation.

[1] J A Thornton, J. Vac. Sci. Technol. 15 (1978) 171

[2] N. Brenning et al., Plasma Sources Sci. Technol. 25 (2016) 065024

[3] C Huo et al., Plasma Sources Sci. Technol. 22 (2013) 045005

Generation of high-density plasmas have been one of the main topics in science and technology of low-temperature plasma since high throughputs in material processing such as dry etching and thin-film deposition are achieved by high electron density which enhances chemical and physical processes in weakly-ionized plasma. When we use microwaves in plasma generation, there have been several methods proposed so far for high-density plasma, like electron-cyclotron-resonance plasma and surface-wave plasma.

Here we propose another scheme in which a magnetic metamaterial makes magnetic permeability in discharge space negative. Microwave propagation in simple discharge space with no objects and no external magnetic field is limited by cutoff density where electric permittivity or dielectric constant is down to zero. When magnetic metamaterial who has negative permeability is installed in the space, microwave propagation is possible beyond the cutoff density, with negative refractive index state with negative permittivity that indicates high electron density. Experimental observation confirmed existence of this scheme, and electron density was much higher than the cutoff density (approximately 7x10¹⁰ cm^-3 when microwave frequency is 2.45 GHz) [1]. The value of electron density has no limitation with smooth microwave propagation with large negative values of refractive index.

In addition to these advantages on generation of high-electron-density plasma, recent experimental studies have revealed that this scheme of plasma generation has roles of high energy storage and an efficient energy converter. From the estimations based on monitored electron density and temperature, the existing energy density in the plasma generation space with the metamaterial is by 3 orders of magnitude larger than that in microwave propagation in the free space. Such stored energy is efficiently converted into the second harmonic wave via nonlinear and asymmetry effects between plasma and metamaterial [2], as well as into chemical energy via enhanced dissociation of gas molecules. These facts indicate that this plasma source will contribute to novel functions that can be hardly established using other plasma generation schemes as well as the general function as a high-density plasma source for material processing.

[1] O. Sakai, Y. Nakamura, A. Iwai and S. Iio, Plasma Sources Sci. Technol. 25 (2016) 055019.

The realization of the separate control of the ion flux and ion energy distribution at the substrate in capacitively coupled radio frequency (RF) discharges is an important issue for various applications of plasma processing, ranging from plasma based etching and deposition procedures in the semiconductor industry to plasma assisted surface treatment of medical interest. In order to attain such independent control of the ion properties, the application of non-sinusoidal voltage waveforms (pulse-like, or saw-tooth-type, for instance), known as „tailored“ or „customized“ RF voltage waveforms for the excitation of capacitive RF discharges, has recently been introduced. Such waveforms differ from the conventionally applied sinusoidal voltages by exhibiting different absolute values of their positive and negative extrema. This control method is based on the electrically asymmetric plasma response, known as the Electrical Asymmtery Effect, observed when non-sinusoidal exciting voltage waveforms are applied, leading to the generation of a dc self-bias voltage in a geometrically symmetric discharge cell. The applications of tailored voltage waveforms, generated by using multiple harmonics of a base frequency (multi-frequency excitation), offer new possibilities for controlling plasma properties. Most of the systematic studies on capacitive RF plasmas excited by tailored voltage waveforms have been conducted so far in electropositive capacitive RF discharges. However, the applications usually require complex mixtures of reactive gases. For instance, oxygen is widely used in etching and thin film deposition techniques, CF₄ is also frequently applied to etch silicon and silicon-dioxide in microelectronics.