| Etching is a very important step in the integrated circuit (IC) manufacturing process because it is involved in almost every procedure and directly affects the performance and reliability of the semiconductor devices. Etching techniques include wet etching and dry etching.In the microelectronics industry, the traditional solution of hydrofluoric acid is widely used in IC fabrication process as "wet etchant" for silicon dioxide (SiO2) films. Wet etching of sacrificial layers often causes collapse of microstructures and ineffective etching in deep vias and induces damage for the devices with high aspect ratio structures, which are the most critical obstacles to the next generation microelectronic devices. One of the major disadvantages of wet etching is the magnitude usage of the corrosive and toxic materials, which may incur high cost and personal injury in handling the disposals and will pollute the environment. Dry etching also has some disadvantages, such as devices damage caused by plasma, lower etching selectivity, and expensive equipment and so on.As minimum feature sizes for integrated circuits continue to shrink into the deep sub-half micrometer region, nanoscale structure devices will come to have a much higher aspect ratio (height/width). The "dry" etching method we proposed which dissolves anhydrous HF/pyridine in supercritical carbon dioxide (ScCO2) system can solve the problem of traditional etching. Supercritical carbon dioxide possesses many particular properties, which general solvents do not have. Its density, salvation ability, viscosity, dielectric constant, diffusion coefficient, and other physical and chemical properties can be adjusted by changing the temperature and pressure. The almost zero surface tension of ScCO2 makes it easily penetrate into the vias and the trenches and has high etching ability to irregular and high aspect ratio devices. Moreover, scCO2 is non-corrosive and has no chemical reaction with silicon, thus it will not cause the loss of silicon. In addition to this, ScCO2 is also one kind of non-toxic, non-flammable, easily available, lower price, and chemical inert solvents. It also does not leave residues in the vias and trenches, can be recycled, and does not cause pollution to the environment.The behaviors of SiCO2 films etched in the HF/pyridine/scCO2 complex were studied in this work. The effects of HF concentration, temperature, pressure, and processing time on the etching rate were investigated. The morphology of the SiO2 surface and Si-SiO2 interface after etching were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis. The composition of the residues was confirmed through fourier transform infrared spectroscopy (FTIR). The etching mechanism of SiO2 films in HF/pyridine/scCO2 solutions was deduced.The major work and results are as follows:1. The dependence of SiCO2 etch rate on the HF concentration (0.5-5.0mM), temperature (40-75℃), pressure (10-25 MPa), and processing time (3-25 min) were investigated in HF/pyridine/scCO2 solutions in high-pressure chambers. The results show that the etch rate was increase with HF concentration at first and then level when the concentration reached 2.5 mM. The temperature has more strongly influence on the etch rate than the pressure.2. The surface morphology of SiO2 films after etching was confirmed by SEM and AFM images, which showed that there are some residues left on the etched film surface. The composition of these residues was analyzed by FTIR. The absorbance peaks of aromatic C-H bond at 2849 cm-1 and 2917 cm-1 and C=C bond at 610 cm-1 are the evidence of the presence of pyridine. However, the peaks of Si-F at 673 cm-1 and the lack of N-H peaks at 1433 cm-1 and 3327 cm-1 showed that the residues are the adduct of octahedral (Pry)2SiF4 other than (PyrH+)2SiF62-.3. The etching mechanism of SiO2 films in HF/pyridine/scCO2 system was deduced according to the above results. First, the complex ion of pyridine-H+HF2-was generated in a diluted HF/pyridine solutions. Subsequently, the chemical reaction of pyridine-H·HF2- with SiO2 occurred to produce (Pry)2SiF4 adduct.4. The amount of residues on SiO2 surface after etching in different HF concentration (0.5 mM and 1.0 mM) solutions was measured by FTIR. We found that the higher HF concentration, the more etch residue. The residues were removed using standard RCA procedure from SiO2 surface, and the SiO2 surface morphology was checked by SEM and AFM. The results showed that SiO2 surface roughness increased with HF concentration.5. The quality of Si-SiO2 interface after etching is crucial for the next step. The SiO2 films were etched completely in HF/pyridine/scCO2 and HF solution, respectively. After cleaning in RCA solutions, the AFM results showed that the RMS roughness of Si surface etched in HF/pyridine/scCO2 (0.691 nm) was comparable to that in conventional HF solution (0.337nm). There are no major detrimental effects on the Si surface due to ScCO2 treatment in terms of the Si-SiO2 interface roughness. |
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