Copper, silver, and gold share unique properties that offer numerous application possibilities. For instance, low resistivity and high electromigration resistance make them attractive as interconnect layers in integrated circuits and microelectronic devices, while the optical properties of their nano-scale structures allow fabrication of plasmonic devices. Etching or patterning techniques are required in order to fabricate nano-structures, devices, and circuits. Although liquid or vapor phase techniques can be applied, plasma or glow discharge methods are typically invoked due to the need to generate anisotropic nanometer pattern sizes. Group 11 metals (Cu, Ag, and Au) form few volatile compounds at temperatures below 150 • C, which limits the approaches that can be used to perform etching/patterning. Halogenated plasmas are widely used to etch metal layers; however, the low volatility of Cu, Ag, and Au halides precludes low temperature processes. Group 11 metals form hydrides readily in plasmas containing hydrogen species. Despite the thermodynamic instability of these metal hydrides, they appear to form at low (below room) temperature and can be desorbed from the etching surface by ion-or photon-assisted processes. Similarly, methylated metal etch products can be generated with hydrocarbon etching plasmas. As a result, etch rates above 12 ± 1 nm/min can be achieved, even when polymer forming plasmas (e.g., methane or other hydrocarbons) are used for patterning. These simple subtractive plasma etching approaches offer significant advantages relative to other vapor phase or liquid techniques in the manufacture of nano-scale devices and circuits.
Low temperature plasma-based subtractive etching of Ag and Au films was demonstrated and results compared to those of similar studies for Cu etching. Ag and Au were etched in H2 plasmas at 10°C at rates of 33 nm/min and 26 nm/min, respectively. These rates were higher than those reported for Cu etching in H2 plasmas. Such results suggest that metal reaction with hydrogen to form hydrides plays an important role in the plasma etch process for these Group 11 metals. Unlike Cu, etch rates of Ag and Au increased with an increase in etchant species (He and Ar) atomic mass. Comparison of Ag etch rates using H2, He, and Ar plasmas with those of Cu indicates that momentum transfer during the etch process plays a more significant role in Ag etching than in Cu etching. In addition, the higher etch rate of Au relative to Cu in an H2 plasma, indicates the importance of chemical reaction between H2 and Au in the Au etching mechanism.
Methane- (CH4) based subtractive plasma etching of Cu was investigated at low temperature. The Cu etch rate using a CH4 plasma (17 nm/min) was higher than that of an H2 plasma (13 nm/min) under the same plasma conditions, despite the fact that hydrocarbon deposition was evident with CH4. The higher Cu etch rate is ascribed to increased momentum transfer due to the heavier mass of CH+ and C+ relative to H+ and to the more thermodynamically stable etch products of CH3Cu or CH3CuH− relative to CuHx. Methyl-containing Cu etch product species were not detected by optical emission spectroscopy, most likely because Cu-CH3 was readily dissociated by energetic electron impact collisions after desorption from the Cu surface. Patterns generated in Cu displayed a sidewall angle of ∼80°, similar to that of H2 plasma patterning, but unlike the result in an H2 plasma, a photoresist mask was not significantly degraded during the etch process.
Plasma-assisted Ag and Au subtractive etching was investigated in CH4 plasmas at 10 °C. The etch rate of Ag (29 ± 2 nm/min) was higher than that observed for Cu (17 nm/min), while the Au etch rate (12 nm/min) was lower than that for both Ag and Cu. Etch rates of Ag and Au due to pressure variation decreased as pressure increased, analogous to Cu etch results. However, the specific plasma conditions under which hydrocarbon formation occurred on Cu, Ag, and Au depended upon the metal being etched as a result of variation in surface chemical reactivities. Comparison of etch results using glass slides and Si wafers as etch masks, confirmed the formation of volatile etch products for Cu and Au. Etch product removal for Au and Cu was enhanced by UV photons, while Ag etching showed no effect at wavelengths > 300 nm. These studies demonstrated that in CH4 plasmas, chemical components in the etch process are most important for Cu while Ag and Au etching is more dependent upon physical sputtering.
Thin film patterning in integrated circuit processing has traditionally been performed by subtractive processes. However, over the past 25 years, the inability to plasma etch copper (Cu) interconnect patterns has led to patterning of Cu by an additive process: the damascene approach. In the current work, a simple subtractive, low temperature hydrogen (H2) plasma etch process is described that permits an alternative method to damascene technology. Cu thin films were etched in an H2 plasma using inductively coupled plasma (ICP) reactors at temperatures between -150 oC and +100 oC. This process achieves anisotropic Cu features (~82o) with a pure H2 etch rate of ~13 nm/min. Both physical (ion bombardment) and chemical contributions (neutral species) are critical to the Cu etch process.
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