Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Physical stress responsive
Reexamination Certificate
2000-05-16
2002-07-16
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Physical stress responsive
C438S113000
Reexamination Certificate
active
06420202
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of radio frequency (RF) electrical filters, frequency control elements and oscillators having resonators and, more particularly, to a process for structuring a thin film resonator to advantageously shape the mode of the acoustic resonator such that the electrical and acoustic performance of the resonator are enhanced.
2. Description of the Related Art
Thin film resonators (TFRs) are thin film acoustic devices which can resonate in the radio frequency to microwave range, for example, 0.5 to 5 Gigahertz (GHz), in response to an electrical signal. A typical TFR has a piezoelectric film between a first electrode and a second electrode which apply an electric field to the piezoelectric film. The film is made of a piezoelectric crystalline material, such as zinc oxide, aluminum-nitride (AlN) or other piezoelectric crystalline material, which exhibits a piezoelectric effect. The piezoelectric effect occurs when the piezoelectric material expands or contracts in response to an electric field applied across the piezoelectric material, for example by the first and second electrodes, or produces charge or current in response to mechanical stress or strain applied to the piezoelectric material. The mechanical resonance frequency of the film, is defined for a film of uniform thickness as the acoustic velocity (v) in the film divided by two (2) times the thickness (t) of the film or f
r
=v/2t. If an RF source is used to apply an alternating electric field of variable frequency to a piezoelectric film, as the frequency of the source matches the mechanical resonance frequency of the film, a pronounced enhancement in the mechanical motion of the piezoelectric film will occur. Correspondingly, the piezoelectric material will produce a maximum amount of current at this resonance frequency because of the mechanical motion which is large in magnitude. Piezoelectric film is useful as an element in electrical filters, oscillators, or frequency control circuits because it yields different amounts of current at different frequencies. If the resonating or transducing material is magnetostrictive or electrostrictive (i.e., CoFeTaZr alloys (cobalt iron tantalum zirconium) or PMN (Lead Magnesium Niobate)), electric filters can also be made using mechanical resonators because, as in piezoelectric material, an RF source will cause the resonating material to mechanically move.
Boundaries at the upper and lower surfaces of a piezoelectric material may be imposed to reflect acoustic waves excited in the material such that a sympathetic vibration (i.e., resonance) occurs at a desired frequency. For example, a material can be polished with two flat interfaces separated by just enough material (for example, half of a wavelength at the propagation velocity of sound through the material) such that the transit time of a wave back and forth in the material occurs at a desired period. Such resonators can be made from bulk piezoelectric crystals which are polished to the desired dimensions. However, there is a practical limit to the highest frequency achievable in this manner, which frequency occurs when polishing reduces the material to thicknesses so small that the material cannot be handled easily.
TFRs can be used at radio frequency (RF) because piezoelectric films can be made thin, for example at higher frequencies, such as 0.5-10 GHz, the piezoelectric film can be between 0.4 and 8 microns in thickness. Piezoelectric resonators needed for these higher frequency applications can be made by using techniques similar to those used to manufacture integrated circuits. The TFR structure can be formed on the substrate, such as a silicon (Si), Gallium Arsenide (GaAs) or other semiconductor substrate, for monolithic integration purposes, such as integration with active semiconductor devices. If the TFR has acoustic reflecting layer(s) as explained below, the acoustic reflecting layer(s) are formed on the substrate followed by the second electrode which is formed on the reflecting layer(s). If there are no acoustic reflecting layers, then the second electrode is formed on the substrate, for example using chemical vapor deposition (CVD) or sputtering. See, Kern & Vossen,
Thin Film Processes
, Vols. I and II, Wiley & Sons. The piezoelectric film is then formed on the second electrode, and the first electrode is formed on top of the piezoelectric film, for example using chemical vapor deposition (CVD) or sputtering.
If such a piezoelectric film is simply deposited on a silicon wafer, a large portion of the sound which is produced may leak into the silicon and be lost due to the poor acoustic reflection between the piezoelectric material and the silicon. In order to impose a good reflecting interface in this case, a portion of the silicon substrate is removed from under the device, yielding a membrane type TFR device. Alternatively, an acoustic mirror may be manufactured by repeatedly causing small reflections from many interfaces between different materials and ensuring all the reflections sum constructively. For example, in the resonator
10
shown in
FIG. 1
, as movement of a sound wave from the upper layer
11
of aluminum nitride (AIN) to the first layer
12
of silicon dioxide occurs (SiO
2
), some of the energy of the wave is reflected instead of completely passing through the first layer
11
, due to the different densities and sound propagation speeds of each layer. At the interface between the silicon dioxide layer
12
and the next aluminum nitride layer
13
, another reflection occurs. However, each time the sound wave is reflected, it is constructively summed with the reflected sound wave which exists in the upper layer
11
. To ensure that these reflections sum constructively, each mirror layer should have a thickness of exactly one quarter of the wave length of sound at the desired frequency. In this manner, if one half of the energy of the imposed wave is reflected at each interface, once the sound wave has passed through twelve layers, for example, all of the wave except 1 part in 1,000 of the sound wave will be reflected. This non reflected portion represents the amount of sound wave lost from the resonator.
To resonate at typical RF frequencies such as 2 GHz, an AlN film will be typically 2.5 microns thick, and the electrodes may be 300 microns across for an optimum match to a 50 Ohm circuit. Thus, the ratio of the material thickness to lateral dimension is small, and the sound energy is not laterally well confined. Detrimental interactions between different types of acoustic waves (modes) and the edge of the resonator occur, and large fields at the edge of the resonator may create unwanted motions in the resonator. Any of these effects remove energy from the desired vibration and degrade resonator quality.
SUMMARY OF THE INVENTION
The present invention is a process for lithographically shaping a resonator such that a predominance of acoustic energy is provided at desired locations within the resonator. For example, by providing a resonator which is shaped to be constructed thicker in its middle, the resonant mode shape becomes changed such that the resonant mode will also be greater at the middle of the resonator. The mode shape is seen when an instantaneous “snapshot” of a standing resonant wave is taken; it shows where a response amplitude of motion is large and where it is small. As a result of increasing the thickness of the resonator at the middle and thus changing the mode shape, the fractional amount of energy which is converted to unwanted vibrations by the edge of the resonator is reduced. This resonator lithographic shaping process can be used during batch-fabrication (i.e., where multiple thin film resonators are manufactured on a continuous silicon substrate of thin-film resonators which are used in high frequency applications.) In certain embodiments, the shaping is achieved using photolithography. Photo-definable resists can be positioned over areas of the substrate that are not meant to b
Barber Bradley Paul
Gammel Peter Ledel
Huggins Harold A.
Wong Yiu-Huen
Agere Systems Guardian Corp.
Dang Phuc T.
Darby & Darby
Nelms David
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