The following experiments investigate the role of pores in etching of porous SiCO low k materials. Slide 1 shows the SiCO etch rate as a function of etch chemistry and porosity. For etch chemistries with low polymerization such as mixtures of CF4 and Ar, the etch rate increases with higher porosity. The first data point in the graph represents 50 sccm CF4 and 400 sccm Ar and the second datapoint 50 sccm CF4 and 200 sccm Ar. The etch rate for the less diluted CF4 is higher for all three materials and the effect is strongest for the material with the highest porosity. Considering the accuracy of the etch rate measurements, it is reasonable to assume that the etch rate increase is caused by the lower density of the material and the removal rate of the actual SiCO material is constant for all three materials.
The third data point represents a mixture of 30 sccm CF4, 200 sccm Ar, and 10 sccm CH2F2. This chemistry readily forms fluorocarbon based polymers on the wafer surface and reactor walls. The etch rates for all three materials drop significantly. The etch rates for the materials with 30 and 40% pores are almost equal and the etch rate for the material with 50% pores is only 30% higher than that of the material with 50% pores. When the CH2F2 flow is increased to 20 sccm, the etch rate trend as a function of porosity reverses. The etch rate for the material with 50% pores is the lowest and close to the removal rate for Ar sputtering.
One possible explanation for this effect is that is the etch chemistry if sufficiently polymerizing and the etch process close to etch stop, the pores at the surface of the material offer additional adsorption places for the polymer precursors and that they can be filled with polymers.
Slide 2 shows results from XPS measurements for SiCO with 50% pores for Ar/CF4 and Ar/CF4/CH2F2. For both chemistries, a CFx layer is formed. The case of the CH2F2 added chemistry, the combined C and F signal comprise 96% of the total signal indicating the the CFx surface layer is quite thick.
This is confirmed by SEM cross sections of etched samples in slide 3. When the etching proceeds without etch stop in the case of CF4/Ar, significant surface roughness is induced on the top surface of the porous material. In the case of the highly polymerizing CH2F2 process, a thick fluorocarbon layer is formed on top of SiOC closing the pores and potentially leading to etch stop.
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