Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is
Reexamination Certificate
1999-09-10
2001-10-02
Meier, Stephen D. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
C257S014000, C333S187000
Reexamination Certificate
active
06297515
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an integrated circuit having a reflector for an acoustic thin film resonator as a portion of the integrated circuit and a method of fabricating the integrated circuit with the reflector and resonator.
2. Brief Description of the Prior Art
High-Q resonators are key components for radio frequency (RF) wireless communication equipment. Resonators are required for filter construction and for production of stable oscillators. A number of materials, such as piezoelectric, high dielectric and surface acoustic wave (SAW) can be used to build high-Q resonators. However, these materials are generally not compatible with silicon semiconductor processing techniques and therefore cannot be integrated into silicon integrated circuit fabrication processes. Recently, several thin film acoustic resonators (TFR) were examined as candidates for integrated resonators as referenced by K M. Lakin et al., “Stacked Crystal Filters Implemented With Thin Films”, 43
rd Annual Symposium on Frequency Control
, (1989) and C. W. Seabury et al., “Thin Film ZnO Based Bulk Acoustic Mode Filters”, 1997
IEEE MTT
-5
Digest
. These designs demonstrated the size advantages and the Q-factor suitable for silicon integration. However, these designs also required additional non-planar processing, such as, for example, a deep silicon etch to create an air gap under the resonator structure or use of a high stack of quarter wave reflectors to isolate the thin film acoustic resonators. Accordingly, these designs are not suitable candidates for use in conjunction with an integrated circuit fabrication process.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided an integrated circuit formed of silicon and piezoelectric materials which includes therein active and/or passive elements as well as TFRs using a porous silicon (ps)/silicon (s) acoustic quarter wave reflector that has a planar structure which is integrated into the silicon integrated circuit fabrication process. It has been found that porous silicon has a different acoustic impedance from to non-porous bulk silicon with a reduced impedance proportional to increased silicon porosity, apparently due to the air content of the pores. For example, porous silicon having 60 percent porosity or 40 percent of the density of bulk silicon was measured to have about one fourth the acoustic impedance of the bulk silicon. The porosity range for the porous silicon can be made to vary from about 30 percent to about 70 percent. This difference in acoustic impedance was exploited to quarter wave reflector design for an integrated TFR.
In order to fabricate such a reflector onto an integrated circuit in accordance with the present invention, starting with a silicon substrate of a first conductivity type, for example P+, a stack of multiple silicon layers with alternating N− and P+ dopants are deposited on the substrate. Next, a P+ diffusion is applied, preferably surrounding the region chosen for the resonator fabrication. A subsequent standard anodization process converts the P+ but not the N− layers surrounded by the P+ diffusion into porous silicon, thus it turns the silicon stack into an acoustic reflector structure. Selected P+ diffusion regions of predetermined size can also be converted into thick porous silicon retion at the same time to provide noise isolation for passive components integration. The layer thicknesses of the multiple silicon stack are precalculated by the sound velocity measured in porous silicon and bulk silicon to provide a desired quarter wave acoustic reflector. The number of layers is determined by the final acoustic impedance requirement which is calculated, layer by layer using the &lgr;/4 impedance transformer equation Z
o
=(Z
in
Z
out
)
½
where Z
o
relates to non-porous silicon or porous silicon at each successive layer. These layers can be deposited by standard epitaxial deposition techniques with the porous layers being formed, for example, in accordance with known prior art techniques of the type described, for example, in Ser. No. 60/068,922, filed Dec. 29, 1997 (TI-23664) and in an article entitled “The ‘Islands’ Method-A Manufacturable Porous Silicon SOI Technology” of E. J. Zorinsky et al.,
IEDM
86-431, 1986 IEEE, the contents of all of which are incorporated herein by reference. All active and/or passive devices including metallization for interconnects are subsequently formed in or on the substrate in standard manner and spaced from the stack other than required connections to the stack. Some or all of the passive elements can be formed over the selected porous silicon region. Alternatively, the surrounding region can be formed along with the opposite conductivity region and subsequently masked and anodized to form porous silicon in the surrounding region in accordance with known prior art techniques of the type described in the above cited copending application and article. The process then proceeds to form the resonator, such as, for example, by depositing zinc oxide, aluminum nitride, etc., in standard manner over the reflector. Since the reflector is built into silicon, there is provided a relatively planar integrated circuit structure. Also, since the resonator is disposed over the silicon structure, it will have a solid physical support, unlike air gap isolated resonators, to maintain the device integrity for package and assembly.
REFERENCES:
patent: 5367308 (1994-11-01), Weber
patent: 5821833 (1998-10-01), Lakin
patent: 5864261 (1999-01-01), Weber
patent: 5936150 (1999-08-01), Kobrin et al.
patent: 6107721 (2000-08-01), Lakin
patent: 6114635 (2000-09-01), Lakin et al.
Brady III Wade James
Hoel Carlton H.
Meier Stephen D.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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