Piezoelectric Thin Film Resonator
A method of producing a polycrystalline film of a metal compound, AlN or ZnO, on a substrate with a non-zero mean tilt of the c-axis relative to the surface normal of the substrate, is disclosed. The method comprises deposition of crystallites of said compound onto the substrate that has a suitable surface for crystal growth with tilted c-axis relative to the surface normal by operating a deposition system designed for sputtering of metal atoms from a target, in a gas mixture of inert gas and nitrogen or oxygen, respectively, at a process pressure such that the mean free path of sputtered metal atoms is comparable or larger than the target to substrate distance, the geometry of the deposition system being such that there exists at least one arbitrary area on the substrate where the distribution of the depositing flux is asymmetric relative to the surface normal at that area. A piezoelectric shear wave resonator comprising the polycrystalline film, especially for use in a liquid medium for mass loading and/or viscosity measurements and/or pressure measurements, and particularly for use as a biosensor or a pressure sensor, is also described
1 - 22 . (canceled)
23 . A method of producing a polycrystalline film of a metal compound on a substrate with a mean non-zero tilt of the c-axis relative to the surface normal of the substrate, the substrate comprising a carrier wafer and has a suitable surface for crystal growth with a tilted c-axis relative to the surface normal of the substrate,
the method comprises the steps:
i) a nucleation step, performed by depositing crystallites of said compound onto the substrate in order to nucleate, or to create conditions for the nucleation of, crystal cones with a tilted c-axis relative to the surface normal, wherein the nucleation step is performed under conditions where sputtered atoms experience substantial gas phase collisions causing thermalization and randomization of the deposition flux, and
ii) a growth step, performed by depositing atoms from the metal compound onto the crystallites to obtain said polycrystalline film, wherein the growth step is performed under conditions reducing significantly gas phase collisions, causing that the deposition flux at any particular point on the substrate is directional.
24 . The method according to claim 23 , wherein the growth step is performed by operating a deposition system designed for sputtering of atoms from a target comprising the metal of the compound.
25 . The method according to claim 24 , wherein said sputtering is performed in a gas mixture atmosphere comprising an inert gas and nitrogen or oxygen, respectively, at a process pressure such that the mean free path of sputtered metal atoms is comparable or larger than the target to the substrate distance.
26 . The method according to claim 25 , wherein the geometry of the deposition system being such that there exists at least one arbitrary area on the substrate where the distribution of the depositing flux is asymmetric relative to the surface normal at that area.
27 . The method according to claim 23 , wherein the metal compound is AlN.
28 . The method according to claim 23 , wherein the metal compound is ZnO.
29 . The method according to claim 23 , wherein the metal compound has a wurtzite structure.
30 . The method according to claim 23 , wherein the substrate that has a suitable surface for crystal growth with a tilted c-axis relative to the surface normal of the substrate is selected from the group consisting of
a) a substrate that has a surface crystal structure lacking a hexagonal symmetry, such as Molybdenum, Aluminum, Platinum, Tungsten, or Tantalum,
b) a substrate that has a surface crystal structure similar to that of any particular AlN crystal face other than the c-face,
c) a substrate that has a non-zero surface micro-roughness,
d) a substrate onto which a very thin polycrystalline film of less than 200 nm thickness and a relatively low degree of texture is deposited,
e) a substrate onto which a very thin polycrystalline film of less than 200 nm thickness and having a preferred non-(002) texture with circular symmetry is deposited, and
f) a substrate onto which an arbitrary but very thin amorphous film of less than 200 nm thickness is deposited and having a non-zero surface micro-roughness.
31 . The method according to claim 30 , wherein the thin polycrystalline film in d) and e) is an AlN film.
32 . The method according to claim 30 , wherein the thin polycrystalline film in d) and e) is a ZnO film.
33 . The method according to claim 30 , wherein the thin polycrystalline film in d) and e) is a wurtzite structure metal compound film.
34 . The method according to claim 30 , wherein the thin polycrystalline film in e) is deposited through a spirally shaped blind, the blade of which is titled under a specific angle towards the center of the wafer.
35 . The method according to claim 34 , wherein the blind is centered directly over the substrate and is in relative rotational motion with the substrate.
36 . The method according to claim 30 , wherein the non-zero micro-roughness surface in c) is obtained by etching or polishing.
37 . A piezoelectric shear wave resonator comprising a polycrystalline film of a metal compound, selected from the group consisting of AlN and ZnO and wurtzite structure metal compounds, produced by the method according to claim 23 .
38 . A piezoelectric shear wave resonator according to claim 37 , comprising at least two electrodes in proximity to the film to provide excitation means.
39 . The piezoelectric shear wave resonator according to claim 37 , for use in a liquid medium for mass loading and/or viscosity measurements and/or pressure measurements.
40 . The piezoelectric shear wave resonator according to claim 37 , for use as a biosensor and/or pressure sensor.
41 . A piezoelectric shear wave resonator comprising a polycrystalline film of a metal compound with a mean non-zero tilt of the c-axis relative to a surface normal of a substrate, the resonator being produced by the method according to claim 23 .
42 . A piezoelectric shear wave resonator according to claim 41 , wherein the piezoelectric film has a distinct tilted texture with the mean tilt of the c-axis varying roughly in the interval 10 to 50 degrees over the substrate.
43 . A piezoelectric shear wave resonator according to claim 41 , wherein the piezoelectric film has a distinct tilted texture with the mean tilt of the c-axis varying roughly in the interval 28 to 32 degrees over the substrate.
44 . In-vivo pressure sensor comprising a piezoelectric shear wave resonator according to claim 39 .
45 . In-vivo pressure sensor comprising a piezoelectric shear wave resonator produced according to claim 23 .