Patent Application
20080197750
The present invention relates to a method of producing a polycrystalline film of AlN or ZnO for shear mode piezoelectric thin film resonators, such as biosensors or pressure sensor, e.g. in-vivo pressure sensors, especially for use in a liquid medium for mass loading and/or viscosity measurements and/or pressure measurements.
Deposition processes for highly textured (c-axis oriented) thin AlN or ZnO films have recently been developed by a number of companies for the fabrication of high frequency electroacoustic devices, most notably resonators, bandpass filters, etc using both longitudinal bulk and surface acoustic modes. A preferred method of film deposition is reactive sputtering since it is characterized by excellent uniformity as well as sufficiently high deposition rates. In the majority of cases, highly c-axis oriented films are desired to optimize both the electromechanical coupling kt and the Q value of the device. The degree of orientation is normally quantified by the full width half maximum (FWHM) value of the rocking curve corresponding to the (0002) X-ray diffraction (XRD) peak. Small FWHM values indicate highly oriented crystallites with the c-axis being almost normal to the surface, resulting in improved device performance since such devices normally utilize the thickness excited longitudinal mode. The latter exhibits a maximum electroacoustic coupling when the c-axis is parallel to the direction of wave propagation and at the same time parallel to the polarization particle displacement), i.e. when the c-axis is normal to the film surface. A typical example of such devices is the thin film bulk acoustic resonator (FBAR).
Gas sensors based on the FBAR are readily fabricated since the loss in Q of the longitudinal mode when the resonator is in contact with a gas is sufficiently small.
When such a FBAR is in contact with a liquid, however, Q decreases by more than an order of magnitude due to acoustic leakage into the liquid. In such cases it is desirable to utilize the thickness excited shear mode which exhibits acceptable (low) losses in Q when in contact with a liquid. C-axis textured films are not suitable for this type of resonator, since the electromechanical coupling of the shear mode is practically zero for well oriented films. In such cases, the c-axis of the AlN films must exhibit a non-zero mean tilt relative to the surface normal, which will allow the excitation of the quasi-shear mode [N. F. Foster, et al, IEEE Trans. on Sonics and Ultrasonics, Vol. SU-15, No. 1, January 1968, p. 28] in such films and hence making the latter suitable for the fabrication of biosensors operating in liquid media.
Wang et al [U.S. Pat. No. 4,719,383] have developed a method for the deposition of such tilted ZnO and AlN films by applying an additional electric field which is thought to promote tilted c-axis film growth during reactive sputter deposition. Thus, this method requires a specific hardware alteration of a standard reactive sputter system. Further, the additional ion bombardment of the growing film results in highly compressive stresses causing plate bowing for films thinner than 3 μm as reported by the authors.
Attempts have also been made to sputter deposit c-axis tilted ZnO films by placing the substrate at a 90° angle with respect to the plane of the target [M. Minakata et al., Japan. J. Appl. Phys. Vol. 12, 1973, p. 474]. This method, however, suffers from poor thickness and functional uniformity.
The present invention relates to shear mode piezoelectric thin film resonators and in particular to a method of producing the thin piezoelectric film included therein. The method comprises deposition of thin piezoelectric films with a non-zero mean tilt of the c-axis relative to the film normal. The deposition is done in a standard reactive magnetron sputter deposition system without any hardware modifications.
Most generally, the method consists of a two stage deposition process in the case where the surface onto which the piezoelectric film is to be deposited lacks a suitable crystallographic structure or alternatively roughness. Normally this surface is the surface of a conducting thin film serving as a bottom electrode in a device and which film together with a carrier wafer onto which it is grown comprise the substrate for the film growth. In cases where a bottom electrode is not needed, the carrier wafer itself represents the substrate. In the first stage (so called nucleation stage) deposition of the material is done under conditions where the sputtered atoms experience substantial gas phase collisions causing thermalization and randomization of the deposition flux. Thus, the film growth is said to be kinetically limited due to the limited surface diffusivity resulting from the relatively low kinetic energy of the condensing species. This results in a film with a relatively low texture, that is, it consists of small crystallites having a relatively broad crystallographic distribution.
During the second stage, which is used for all embodiments of the invention, the deposition process is operated under conditions reducing significantly gas phase collisions, which gives rise to two effects. The first one is that the deposition flux at any particular point on a substrate is directional (or anisotropic) arising from the asymmetry in the deposition flux emanated from the circular racetrack and received at the point under consideration. This, of course, is not valid for a small exclusion zone in the center of the substrate. The second effect is that the sputtered particles retain most of their original energy, which is of the order of half of the sublimation energy of the target material, and which energy is favorable for enhancing surface diffusion and subsequently crystal growth. The latter now proceeds under competitive growth conditions where crystallographic planes exhibiting relatively high surface binding energies as well as receiving largest flux, grow fastest. The resulting 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 excluding a small exclusion zone in the center of the latter. The mean tilt angle distribution over the substrate has a circular symmetry.
In a different embodiment, the first stage of the process may be omitted provided the substrate surface, has properties inhibiting (002) growth normal to the surface. This, for instance, may be a surface lacking hexagonal symmetry (assuming that the piezoelectric film has such a symmetry) or the surface has a sufficient microroughness which in turn promotes preferred crystallite growth along the net flux direction.
In either case, the resulting film has a pronounced tilted structure and is shown to have adequate properties for resonator operation in liquids.
Thus, the invention is directed to a method of producing a polycrystalline film of a metal compound, having specifically a wurtzite crystallographic structure such as AlN ZnO, BeO, BN, AlBN, etc 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 with an optional electrode film deposited onto it, comprising the steps of depositing crystallites of said compound onto the substrate that has a suitable surface for crystal growth with tilted c-axis relative to the surface normal of the substrate by operating a deposition system designed for sputtering of atoms from a target comprising the metal of the compound, 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, 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.
The target comprising the metal of the compound may be composed of the metal as such or of the metal compound itself.
The proportion of inert gas, such as argon, in the gas mixture is not critical as long as there is sufficient reactive gas, i.e. nitrogen or oxygen, present in the gas mixture atmosphere for growing a stoichiometric compound.
In an embodiment of the invention the metal compound is AlN, and in another embodiment it is ZnO, or BN, or BeO, or AlBN.
In a preferred embodiment of the method of the invention the substrate that has a suitable surface for crystal growth with tilted c-axis relative to the surface normal of the substrate is selected from the group consisting of
The randomly textured polycrystalline film in d) is preferably an AlN film or a ZNO film.
The non-zero micro-roughness surface in c) is preferably obtained by etching or polishing unless it already has this property as grown (a priori).
The present invention is also directed to a piezoelectric shear wave resonator comprising a polycrystalline film of a metal compound, selected from the group consisting of AlN and ZnO, produced by the method of the invention, and at least two electrodes in proximity to the film to provide excitation means.
In an embodiment the piezoelectric shear wave resonator of the invention is for use in a liquid medium for mass loading and/or viscosity measurements and/or pressure measurements, e.g. for use as a biosensor or a pressure sensor.
a. Schematic illustration of an arbitrary substrate with a relatively smooth surface, AlN nuclei with varying orientation of the c-axis relative to the surface normal and direction of the c-axis of an arbitrary nucleus.
b. Schematic illustration of initial film growth onto a substrate, surface with a relatively small microroughness.
a. The near resonance behavior of the conductance of a shear wave resonator when the bottom electrode is in contact with various concentrations of glycerol.
b. Series resonance shift of a shear wave resonator when the bottom electrode is in contact with various concentrations of glycerol, measured relative to the resonance frequency in pure water.
The following is a detailed description of a preferred embodiment of the present invention. Other embodiments will be readily apparent to those skilled in the art.
The method of the present invention for depositing thin film piezoelectric materials suitable for use as a thickness excited piezoelectric acoustic shear wave resonator will be described with reference to the drawings. The resonator comprises a piezoelectric film of a material having 6 mm symmetry as well as two electrodes on either side of the film in intimate contact with the latter. The c-axis of the piezoelectric film exhibits a non-zero mean tilt with respect to the normal to the film surface.
By the method of the invention, the piezoelectric film is sputter deposited in a standard reactive planar magnetron sputtering system where an Al or AlN target is sputtered in a gas mixture of an inert gas (e.g. Ar) and nitrogen while the sputtered flux is condensed onto a substrate. The discharge can be any of the following three types, direct current (DC), pulsed DC or radio frequency (RF). The magnetic field configuration can be both balanced or unbalanced.
Unique to the method of the present invention is the two stage deposition process, nucleation and growth stages, respectively. The purpose of the first stage is to nucleate, or alternatively to create conditions for the nucleation of, crystal cones (columns) with a tilted c-axis relative to the surface normal. This can be done in a variety of ways as illustrated in the examples below. At this stage there exists no particular preferred nuclei orientation, rather than all orientations may be possible as illustrated schematically in
During the second stage, crystallite growth at any particular area on the substrate proceeds predominantly along the direction of the mean flux, the latter being defined as the vector sum of all individual fluxes emanated at all elementary areas on the target surface and received at the substrate, i.e. substrate, area in question as illustrated in
Thus, the preferred mean tilt lies in the range from 28 to 32°, but a broader mean tilt range of 10 to 50° have also been shown to be applicable when embodying the present invention.
A target of 99,999% pure Al, 15 cm (6 inch) in diameter, was sputtered in a pulsed DC planar magnetron sputtering system Von Ardenne-CS 730. No modifications to the latter were made. A standard highly polished 10 cm (100) Si wafer was placed onto the wafer holder such that the wafer surface was parallel to that of the target and the wafer centered directly under the target. The distance between the wafer and the Al target was about 5.5 cm. No external heating of the wafer holder was applied.
Initially, the discharge was operated in a 99,999% pure Ar atmosphere at a process pressure of 0.7 Pa, gas flow of 100 cm3/min (sccm) and a discharge power of 1000 W for a period of 1 minute resulting in an Al electrode film of thickness 200 nm. The substrate was then taken out of the chamber and the Al electrode film was then patterned to define the bottom electrode geometry using a standard optical lithography process followed by subsequent dry etching in a fluorine based plasma. The resist was then stripped in a standard oxygen plasma reactor. The Al electrode film had a (111) texture with a FWHM of the rocking curve of about 1.6 degrees.
The substrate, i.e. the thus deposited Al electrode film together with the Si wafer, was reinserted into its original position in the process chamber. The discharge was then operated in a mixture of 99,999% pure nitrogen (45 cm3/min (sccm)) and 99,999% argon (15 cm3/min (sccm)), and a discharge power of 1200 W. Initially, during the nucleation stage (stage 1) the discharge was operated at a process pressure of 2.66 Pa for a period of 5 minutes resulting in a seed layer with a thickness of about 100 nm. Since the mean free path at this pressure is much smaller than the target-to-substrate distance, the sputtered Al atoms experience multiple scattering collisions in the gas phase resulting in a more or less isotropic deposition flux at the substrate surface (i.e. no preferred flux direction). In addition, the sputtered Al atoms are thermalized as a consequence of the scattering process so that when condensing onto the substrate surface they have an effective temperature approximately equal to that of the surrounding gas. Under these conditions the growth process is diffusion limited resulting in a film with an almost random crystallographic texture resembling that shown in
A target of 99,999% pure Mo, 15 cm in diameter, was sputtered in a pulsed DC planar magnetron sputtering system Von Ardenne-CS 730. Si wafers as in Example I were used. The discharge was operated in a 99,9990% pure Ar atmosphere at a process pressure of 0.8 Pa and a discharge power of 1000 W for a period of 5 minutes resulting in a Mo electrode film of thickness 200 nm and (110) texture with a FWHM of the (110) rocking curve of about 5 degrees. The Mo electrode film is then sputtered in an Ar plasma and an Ar pressure of 2.66 Pa by applying a 100 W RF power to the wafer holder for a duration of 5 minutes. This mild sputtering of the deposited Mo electrode film results in increasing the surface microroughness, schematically shown in
Owing to the microroughness of the Mo electrode film thus achieved, both the nucleation and the growth of the AlN film take place predominantly in the direction of the deposition flux at any particular area on the substrate resulting in a tilted c-axis film growth.
A target of 99,999% pure tungsten, 15 cm in diameter, was sputtered in a pulsed DC planar magnetron sputtering system Von Ardenne-CS 730. Si wafers as in Example I were used. The discharge was operated in a 99,999% pure Ar atmosphere at a process pressure of 0.8 Pa and a discharge power of 1000 W for a period of 1 minute resulting in a W electrode film of thickness 200 nm. Under these conditions the W electrode film has a rather smooth surface (a r.m.s. value in the order of 5 to 10 nm). The texture of the W electrode film was (110) with a rather large FWHM of the (110) rocking curve in the range 6 to 10 degrees. After patterning the W electrode film, the process then proceeded directly with stage 2 as described in Example I. Owing to the relatively poor texture of the W electrode film as defined by the FWHM of the (110) rocking curve both the nucleation and the growth of the AlN film take place predominantly in the direction of the deposition flux at any particular area on the substrate resulting in a tilted c-axis film growth.
The foregoing examples are intended for illustration and are not intended to limit the choice of materials or sputter deposition parameters or deposition geometry comprised by the scope of the present invention. For example, ZnO can equally well be used instead of AlN, in which case a Zn or a ZnO target will be sputtered in an oxygen containing atmosphere. Analogously, the method equally applies to BN, BeO AlBN, AlGaN, and in fact all similar materials having a wurtzite crystallographic structure.
The method disclosed inhere refers primarily to circular targets but for those skilled in the art it is obvious that the method is readily adapted to any other shape or form of the target such as linear magnetrons or even multiple magnetrons. A specific example could be the use of an array of mutually parallel linear magnetrons (see
The carrier wafer type is immaterial to the invention as it acts as a physical carrier, and hence any other carrier wafer, such as glass, etc, with a sufficient surface smoothness can be used. In specific cases, however, its crystallographic surface structure may play an important role in achieving the required crystallographic properties of the bottom electrode film.
Deposition parameters, such as substrate temperature, discharge power, electrode and magnetic field geometry, substrate bias, gas composition, etc may also be varied for further optimization of film properties, such as density, surface smoothness, stress, degree of inclination, film uniformity, etc, as is well known in the art of sputtering. Post deposition annealing may also be applied.
A key element of the present invention is the texture of the seed layer. As stated above, the seed layer deposited during stage I of the process has more or less a random texture. This condition is needed to guarantee that at every point on the substrate surface there is a nucleus with a suitable orientation of the c-axis which is more or less aligned with the flux direction at that point. Ideally however, the texture of the seed layer should have a preferred texture (say (103) in the case of AlN) with a circular symmetry, such that the nuclei have their c-axis tilted towards the center of the wafer under one and the same angle. Thus, such a texture of the seed layer, where the c-axis of the nuclei is roughly aligned with the mean flux direction at every point on the substrate, is most suitable for the growth during stage II of the process. In this context it is obvious to those skilled in the art that improvements of the texture of the seed layer can readily be done by optimizing the deposition parameters as well as by modifying the deposition geometry. A specific example of the latter is the introduction of a focusing system consisting, say, of a spirally shaped blind (see
Essential features of the present invention, as explained above, are two main elements, the properties of the interface between the bottom electrode film and the piezoelectric film (i.e. the seed surface or alternatively the seed layer) as well as the directionality of the deposition flux at the substrate surface. With respect to the first of these two elements, the surface of the bottom electrode film has to have a relatively poor crystallographic texture and/or have a relatively high surface roughness and/or lack a hexagonal symmetry. Further, the bottom electrode film may even consist of more than one layer, provided the top layer satisfies at least one of the above surface requirements. If none of these requirements is satisfied, then a thin seed layer that satisfies some or all of these surface requirements is first deposited onto the substrate (as in Example I above). The thickness of this seed layer may also be varied for optimal results. This, in fact, is the preferred embodiment of the present invention since in this case the deposition process is completely independent of the properties of underlying material, i.e. the bottom electrode film or the carrier wafer in cases where the latter is omitted.
Once a favorable nucleation surface is established, tilted growth is achieved through the directionality of the deposition flux at the wafer surface. This directionality is guaranteed by the process pressure, which is such that the mean free path of the sputtered particles is comparable or larger than the substrate-to-target distance. In this context both the process pressure and the substrate-to-target distance may be varied as is known in the art of sputtering as long as the mean free path criterion is satisfied. Further modifications may readily be done by optimizing the radius of the so called race track on the target by modifying the magnetic field configuration in view of reducing the exclusion zone at the center of the wafer as well as optimizing the structural and thickness uniformity of the piezoelectric film.
The topology of the piezoelectric shear wave resonator and its acoustic isolation from the substrate are not essential to the present invention and will not be described in detail. However, a typical piezoelectric shear wave resonator according to the present invention comprises a film of piezoelectric material, i.e. AlN or ZnO or generally a material having a wurtzite structure (based on the hexagonal close packed lattice, where every other atom is A or B, for example GaN), having its c-axis tilted with respect to the film normal, and at least two electrodes in proximity thereto to provide excitation means as illustrated schematically in
a shows the near resonance conductance of a resonator when the bottom electrode is in contact with four different concentrations of glycerol. It is noted that in this case an additional Au layer was deposited on the back side of the bottom electrode in view of further studies.
b shows the corresponding resonance frequency shift relative to the resonance in pure water where the x-axis denotes time intervals at which subsequent measurements are taken. The latter were done at room temperature without any compensation for temperature fluctuations. The temperature coefficient of the resonator was found to be −25 ppm/° C. in the range of 20° C. to 100° C.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0500647-3 | Mar 2005 | SE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SE2006/050041 | 3/23/2006 | WO | 00 | 10/22/2007 |
