Q2. What was used to quantify the concentration of hydrogen, carbon, oxygen, and phosphorous present?
Secondary ion mass spectroscopy (SIMS), referenced to known standards, was used to quantify the concentration of hydrogen, carbon, oxygen, and phosphorous present.
Q3. What is the reason for the lack of a textured columnar structure in thin-film?
The lack of a textured columnar structure, which is routinely observed in thin-film silicon [27],may be a result of the 900°C annealing used to dope the silicon with phosphorous which allows the residual stresses associated with growth of the film to relax.
Q4. What is the cause of the fatigue of structural silicon in ambient air?
Since no evidence of dislocation activity near the crack surface nor phase transformations in the vicinity of the notch root was detected, the fatigue of structural silicon films in ambient air is deemed to be associated with stress-corrosion cracking in the native oxide layer that has been thickened under cyclic loading (reaction-layer fatigue).
Q5. Why have silicon-based structural films emerged as the dominant material system for MEMS?
Silicon-based structural films have emerged as the dominant material system for MEMS because the micromachining technologies for silicon are readily adapted from the microelectronics industry and are compatible with fabrication*
Q6. What was the average crack growth rate?
Crack-growth rates were determined using a modified secant method applied over ranges of crack extension of 2 nm with 50% overlap of the previous calculation window; the average crack-growth rate was calculated based on a linear fit of the experimental data.
Q7. What is the nature of the fatigue and overload fractures?
Overload fractures: Overload fracture surfaces were (unambiguously) created by manually loading the fatigue test structure with a fixed (non-cyclic) displacement under an optical microscope; these conditions generate cracks that arrest due to the decreasing stress gradient associated with displacement-control in this geometry.
Q8. How many small cracks were observed in the native oxide?
By interrupting fatigue specimens prior to failure after testing at various stress amplitudes and examining them with HVTEM, several small cracks (on the order of tens of nanometers in length) were observed within the native oxide at the notch root (Fig. 9).
Q9. What is the standard micromachining process for this foundry?
This standard micromachining process for this foundry is based on the low-pressure chemical vapor deposition (LPCVD) of n+-type (resistivity, ρ = 1.9×10-3 Ω⋅cm) polycrystalline silicon [20].
Q10. What was the effect of the fatigue test on the crack length?
As noted above, in situ measurements of the change in natural frequency during the fatigue test were used to determine the crack length, a, as a function of time or cycles [26].
Q11. What does the absence of heat in the cantilever beam section mean?
The absence ofheating in the cantilever beam section clearly indicates that the enhanced notch-root oxidation is not thermally induced.
Q12. What is the role of thermal effects in the oxidation process?
As it is conceivable that the high loading frequency and induced currents may induce heating in the notch region, the role of thermal effects in the oxidation process was experimentally evaluated.
Q13. What is the way to suppress the formation of the native oxide?
An alternative strategy is to use selfassembled monolayer (SAM) coatings to suppress the formation of the native oxide with the expectation that such samples would not be susceptible to fatigue in air.
Q14. What is the correlation between stress amplitude and fatigue life in metals and silicon films?
Although arising from different mechanisms, the correlation between stress amplitude and fatigue life in metals and silicon films is ultimately due to the range of stable crack growth within the timeframe of the tests.
Q15. What is the difference between the crack growth rate and the life of a metal?
In metals, the gradual increase in life with decreasing stress amplitude is a direct consequence of the relatively wide range of stable crack growth, characterized by a low crack-growth rate exponent of m ≈ 3-5 in the Paris law, da/dN = ∆Km (where ∆K is the stress-intensity range).