Resonators are a major component in RF electronic products. They are used in a host of ways to filter radio signals. Modern and Future RF communications have placed high demands on the industry; requiring low power usage, wide array of applications and resistance to noise.
In this thesis, a discussion of the motivation for RF MEMS filters and basic theory is given with an explanation of the concepts of Q factor, piezoelectricity, acoustics theory, the major types of resonators (SAW, BAW, CMR or LAMB), apodization theory and techniques as well as design, simulation of CMR and BAW devices, testing and process development of aluminum nitride by RF reactive sputtering at RIT.
Finite element analysis was performed on a number of factors of aluminum nitride contour mode resonators (CMR) from piezoelectric film thickness, to electrode pitch, electrode thickness and electrode configuration; to understand the effects. First order and second order vibration modes were seen including symmetric S0, S1 and antisymmetric A0, A1 resonant modes in the pizeoacoustic devices and higher. A series of time dependent video simulations of SAW, BAW and LAMB wave resonators were also performed, perhaps the first of their kind.
The RF reactive sputtering deposition for aluminum nitride was developed at RIT by a fractional factorial experiment with the factors being RF power, nitrogen to argon flow rate ratios, changing the distance of the wafer to the platen from 5 to 4 cm, use of a aluminum, molybdenum or virgin silicon seed layer and chamber pressure. In nearly all cases it was found that an RF power of 1000W is the most important factor contributing to the ⟨002⟩ orientation. The decreasing of the target distance may inhibit a reaction mechanisms in the plasma resulting in a more amorphous deposition. It may be due to the increase in temperature resulting from the higher RF power that promotes the growth of ⟨002⟩ oriented aluminum nitride. A molybdenum seed layer tends to have a stronger ⟨002⟩ peak relative to aluminum and a chamber pressure of 3mT was found to exhibit a deposition that most favors the ⟨002⟩ oriented aluminum nitride.
It was found that molybdenum is not consumed in a wet etch of KOH. Molybdenum is oxidized during photo resist ashing. The Contact Vias were necessarily over retched in order to ensure complete removal of Al-N over the Bottom Electrode.
C-V measurements were done on the aluminum nitride to determine its quality, the measured extensional piezoelectric coefficient d 33 is -0.000108716 nm/V, which is -0.108716 pm/V lower than 8pm/V typically reported. The lower piezo electric coefficient measured as compared with typical values, may be due to low film density a result of the high power used in the RF reactive sputtering that was used to heat the platen to a high enough temperature to promote the?002?oriented growth of AlN.
A series of iterations were designed and S11 frequency response measured. The electrode overlap from 25 to 50 to 75μm, it does not appear to have an effect on the resonant frequency, but does increase the amplitude of the response at that die's given frequency. Increasing the anchor width from 5μm to 10μm to 20μm lowers the relative amplitude of the response therefore lowering the Q of the resonator. It may be that the increasingly wide anchor, increases the mechanical resistances within the device and thereby lowers the Q factor of the resonator. Increasing the number of electrodes increases the relative amplitude of the response. Increasing pitch from 5μm to 6μm seems to have a small effect on the resonant frequency of the devices, shifting them from 4.57 to 4.59 GHz. A quality factor was measured, with an anchor width of 5μm, pitch of 5μm, 24 electrodes and an electrode overlap of 75μm had a measured Q value of 98.8.
|Commitee:||Fuller, Lynn, Jackson, Michael, Pearson, Robert|
|School:||Rochester Institute of Technology|
|School Location:||United States -- New York|
|Source:||MAI 54/05M(E), Masters Abstracts International|
|Subjects:||Electrical engineering, Materials science, Acoustics|
|Keywords:||Aluminum nitride, Apodization, Filter, Micro-electromechanical systems, Process, Resonator|
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