Metal working – Piezoelectric device making
Reexamination Certificate
2001-12-17
2003-12-16
Vo, Peter (Department: 3729)
Metal working
Piezoelectric device making
C029S594000, C029S609100, C029S847000, C174S257000, C216S067000, C310S324000, C310S364000, C333S187000, C333S193000
Reexamination Certificate
active
06662419
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to forming a film bulk acoustic resonator (“FBAR”) structure. More specifically, the present invention relates to the methods of forming a structure for a film bulk acoustic resonator having high-Q and low insertion loss.
BACKGROUND OF THE INVENTION
In some instances it is desirable to provide a radio frequency front-end filter. In the past ceramic filters and SAW filters have been used as front-end radio frequency filters. There are problems with SAW filters in that such filters start to have excessive insertion loss above 2.4 gigahertz (GHz). Ceramic filters are large in size and can only be fabricated with increasing difficulty as the frequency increases.
A basic FBAR device
100
is schematically shown in FIG.
1
. The FBAR device
100
is formed on the horizontal plane of a substrate
110
. A first layer of metal
120
is placed on the substrate
110
, and then a piezoelectric layer
130
is placed onto the metal layer
120
. The piezoelectric layer can be ZnO, AIN, PZT, any other piezoelectric materials. A second layer of metal
122
is placed over the piezoelectric layer
130
. The first metal layer
120
serves as a first electrode
120
and the second metal layer
122
serves as a second electrode
122
. The first electrode
120
, the piezoelectric layer
130
, and the second electrode
122
form a stack
140
. A portion of the substrate
110
behind or beneath the stack
140
is removed using back side bulk silicon etching. The back side bulk silicon etching is done using deep trench reactive ion etching or using a crystallographic-orientation-dependent etch, such as KOH, TMAH, and EDP. Back side bulk silicon etching produces an opening
150
in the substrate
110
. The resulting structure is a horizontally positioned piezoelectric layer
130
sandwiched between the first electrode
120
and the second electrode
122
positioned above the opening
150
in the substrate. The FBAR is a membrane device suspended over an opening in a horizontal substrate.
FIG. 2
illustrates the schematic of an electrical circuit
200
which includes a film bulk acoustic resonator
100
. The electrical circuit
200
includes a source of radio frequency “RF” voltage
210
. The source of RF voltage
210
is attached to the first electrode
120
via electrical path
220
and attached to the second electrode
122
by the second electrical conductor
222
. The entire stack
140
can freely resonate in the Z direction “d
33
” mode when the RF voltage at resonant frequency is applied. The resonant frequency is determined by the thickness of the membrane or the thickness of the piezoelectric layer
130
which is designated by the letter “d” or dimension “d” in FIG.
2
. The resonant frequency is determined by the following formula:
f
0
~V/2d, where
f
0
=the resonant frequency,
V=acoustic velocity of piezoelectric layer, and
d=the thickness of the piezoelectric layer.
It should be noted that the structure described in
FIGS. 1 and 2
can be used either as a resonator or as a filter. To form an FBAR, piezoelectric films, such as ZnO and AlN, are used as the active materials. The material properties of these films, such as the longitudinal piezoelectric coefficient and acoustic loss coefficient, are key parameters for the resonator's performance. Key performance factors include Q-factors, insertion loss, and the electrical/mechanical coupling. Currently, to manufacture an FBAR the piezoelectric film is deposited on a metal electrode using reactive sputtering. The resulting films are polycrystalline with a c-axis texture orientation. In other words, the c-axis is perpendicular to the substrate. This processing procedure has several problems.
An FBAR is formed as a piezoelectric layer sandwiched between two electrodes. Top and bottom electrodes are necessary for electrical outpur of the FBAR Therefore a bottom electrode is required. The starting layer or seed layer for the piezoelectric film deposition for FBAR has been limited to conductive materials. Any other non-conductive or single-crystal materials, which could induce very high-quality or single-crystal piezoelectric films, can not be used as the seed layer using conventional FBAR fabrication techniques.
When a piezoelectric film is sputtered onto a conductive metal, the initial layer of approximately 0.05 um of the sputtered film typically consists of a polycrystalline material with partially developed texture. This initial layer has poor piezoelectric effect. This degrades the overall film quality. This becomes a performance issue for high frequency FBARs having a resonance frequency of 10 GHz or above which has a piezoelectric film about 0.2 um thick.
Thus, there is need for an FBAR device and a method for producing an FBAR device that results in a single-crystal piezoelectric film. There is also a need for a method of fabricating an FBAR device that has good performance qualities and which uses a seed layer other than a highly conductive electrode. There is also a need for a fabrication technique where an initial sputtered layer of piezoelectric material can be removed since this layer may be polycrystalline and have poor piezoelectric effect.
REFERENCES:
patent: 5747857 (1998-05-01), Eda et al.
patent: 5884378 (1999-03-01), Dydyk
patent: 6349454 (2002-02-01), Manfra et al.
patent: 1073198 (2001-01-01), None
Krishnaswamy, S. V., “Film Bulk Acoustic Wave Resonator Technology”,Proceedings of the Ultrasonics Symposium, Honolulu, HI,(Dec. 4, 1990),529-536.
Ma Qing
Rao Valluri
Wang Li-Peng
Nguyen Donghai
Schwegman Lundberg Woessner & Kluth P.A.
Vo Peter
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