Processing, Characterization and Reliability of Silica Xerogel Films for Interlayer Dielectric Applications
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Fracture Toughness Measurements: Maintaining the mechanical integrity of multilayer coatings is an important requirement for successful integration in back end of the line processing. The materials must possess both cohesive and adhesive fracture toughness. Aging processes affect the microstructure (and hence the mechanical strength) of the film as dissolution and reprecipitation reactions cause neck formation between the particles. The time and temperature of the aging, the pH of the medium, and the solvent influence these reactions9. The modified-edge lift off test (m-ELT)10, 11 was used to quantify the toughness of the xerogel coatings. It consists of applying a thick backing layer (e.g. epoxy) to the test film. The test film is supported on a rigid substrate (Si). The backing layer materials must have a known stress temperature profile, higher fracture toughness than the test material and excellent adhesion to the test material. The wafer is diced so that 90° edges to the substrate are formed. The sample is then cooled until debonding is observed. If the backing layer material is much thicker than the test material, the stored energy is approximately that in the backing layer, so, the applied fracture intensity, KI, is given by the stress in the backing layer times the square root of half the backing 1/2
Fracture toughness(MPam )
Fracture toughness, (MPam 1/2)
0.35 Fracture Toughness vs. Aging time o For films (~ 70% porous) aged at 60 C
0.3 0.25 0.2 0.15
0.35 Films (porosity ~70%) aged for 17 hours
0.3 0.25 0.2 0.15 0.1 0.05 45
0.1 0
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Aging time(hours)
Figure 1: Fracture toughness of films aged at 60 ºC. Each data point represents a separate film. The whole wafer was broken into small pieces. The values represent the average toughness and the error bars are the standard deviation
50 55 60 Aging Temperature, oC
Figure 2: Fracture toughness of different xerogel films aged at different temperatures for aging time of 17 hours. The aging should be done at highest possible temperature to achieve the required strength
layer thickness. To perform the m-ELT tests, several films aged at different conditions were fabricated. The aging temperature was varied from 50 to 60 °C for a fixed aging time of 17 hours and the aging time was varied from 0.5-24 hours for a fixed aging temperature of 60 °C. Thus the effect of both aging time and temperature could be determined separately. They were then coated with 0.1 µm of PECVD oxide. This ensured that resist used in the test would not wick through the pores of the xerogel during curing. The multilayer structure was again annealed at 400 °C for 1 hour in vacuum. Before doing the m-ELT test, a scotch tape test was performed and all samples survived. An epoxy was applied to the xerogel and cured for one hour at 177 °C. In a separate experiment, the residual stress versus temperature was measured for the epoxy on a bare silicon substrate. The samples were then cleaved and observed for debonding as described above. Figure 1 shows the fracture toughness
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