Plasma Technology for Advanced Devices
Silicon oxides, nitrides and advanced low k materials are etched with fluorocarbon based chemistries which provide good resist selectivities. Fluorocarbon deposition plays an important role in etching of dielectrics stopping on silicon based materials like contact etching. Several studies have shown that the silicon etch rate is directly proportional to the inverse of the fluorocarbon film thickness. Figure 1 shows results published by Oehrlein et al. (“Reactive Ion Etching Related Si Surface Residues and Subsurface Damage: Their Relationship to Fundamental Etching Mechanisms”; Gottlieb S. Oehrlein, Young H. Lee; J. Vac. Sci. Technol. A5 (1987) 1595) on the relationship between the silicon etch rate and the thickness of the fluorocarbon film thickness. Etch rate studies and XPS investigations show that the etch rate is constant and at a maximum value for film a film thickness between 0 and 5 Angstrom. As the film growths thicker, the etch rate starts to drop proportionally to 1/thickness and reaches zero for values above 20 Angstrom. These values will depend on the specific etch conditions. In the given example, a successful contact etch process would employ a gas mixture with 50% H2 to ensure infinite selectivity of the oxide etch to silicon. This implies that the oxide etch rate under the same conditions is not zero because the protective film is not formed on silicon oxide due to the presence of oxygen. If the degree of polymerization is further increased, the oxide etch will eventually also shut down.
Joubert et al. (“Fluorocarbon High Density Plasma. IV. Reactive Ion Etching Lag Model for Contact Hole Silicon Dioxide Etching in an Electron Cyclotron Resonance Plasma”; O. Joubert, G.S. Oehrlein, M. Surendra; J. Vac. Sci. Technol. A12 (1994) 665) proposed that there are three different etching regimes during SiO2 etching in high density plasmas: A - Fluorocarbon deposition, B - Fluorocarbon suppression, and C - Chemically enhanced etching (Fig. 2). In the fluorocarbon deposition regime, a relatively weak relationship between the deposition rate and the ion energy can be found. The process is very sensitive to the ion energy in the fluorocarbon suppression regime where the surface process switches rapidly from deposition to etch as the ion energy increases. The etch rate levels off as a function of ion energy in the regime of chemically enhanced etching.
In the follow-on paper of the series about fluorocarbon high density plasmas, Joubert at al. studied the impact of fluorocarbon polymer formation on the etching of high aspect ratio structures in silicon dioxide (“Fluorocarbon High Density Plasma. V. Influence of Aspect Ratio on the Etch Rate of Silicon Dioxide in an Electron Cyclotron Resonance Plasma”; O. Joubert, G.S. Oehrlein, Y. Zhang; J. Vac. Sci. Technol. A12 (1994) 658). Figure 3 shows that the etch depth decreases as a function of the contact hole size (left panel) and the etch rate decreases as a function of time and the polymerizing potential of the chemistry: Faster in C2F4 than in C3F6 than in CHF3. The etch rate stops first in highly polymerizing chemistries (C2F4) (right panel).
Figure 4 shows that there is a linear dependence of the etch rate as a function of the aspect ratio for different hole diameters. This means that the important variable in etching SiO2 contact holes is the aspect ratio (Aspect Ratio Dependent Etch – ARDE).
In the case of high aspect contact etching, the etch regime can be different in the dense and open areas (Fig. 5). Even if SiO2 is etched in the chemically enhanced etching regime in open areas, the etch in the high aspect ratio contacts can be in the fluorocarbon suppression or even deposition mode, which would mean etch stop in the dense areas. This behavior can be explained among others by charging effects which in high aspect ratio structures lead to a decrease of the ion current density and energy reaching the SiO2 surface. During the etch, the ion power density decreases (ion energy x ion density). The etching starts in the chemically enhanced etching regime, then moves to the fluorocarbon suppression regime (as the power density decreases), and then ultimately to the fluorocarbon deposition regime (inducing etch stop in high aspect ratio contact holes).
The classic experiment which provided direct evidence for the importance of charging effects was conducted by Sekine, Hayashi, and Kurihara (H. Hayashi, K. Kurihara, M. Sekine; Jpn. J. Appl. Phys. 35 (1996) 2488; K. Kurihara, M. Sekine; Plasma Sources Sci. Technol. 5 (1996) 121). They showed that the ion current density measured by mass spectrometry through a thick quartz plate in which capillary holes have been formed decreases very strongly as a function of the aspect ratio of the capillary holes. When the top of the quartz plate is covered by Cu, the amplitude of the ion current density loss decreases strongly: charging effects are minimized by the presence of a conductor covering the SiO2 surface (Fig. 6).