Patent Application
20220073348
The present invention relates to a piezoelectric body of aluminum nitride doped with magnesium and a specific element together and a MEMS device using the same.
Devices utilizing the piezoelectric phenomenon are used in various fields. For examples, the devices have been increasingly used in portable devices, such as cellular phones, which are strongly desired to be made smaller in size and minimize power consumption. Examples of the devices may include a filter using a film bulk acoustic resonator (FBAR) called an FBAR filter.
The FBAR filter is a filter based on a resonator that utilizes a thickness-longitudinal vibration mode of a thin film exhibiting a piezoelectric response and can resonate in gigahertz band. This kind of FBAR filter with such characteristics has a low loss and can be operated in a wide band. Thus, the FBAR filter is expected to further contribute to the operation at high frequencies and the reduction in size and power consumption of portable devices.
Examples of a piezoelectric material of such a piezoelectric thin film used for FBAR may include scandium-doped aluminum nitride (see Patent Literature 1) and aluminum nitride doped with inexpensive magnesium and niobium (see Non-Patent Literature 1). In particular, scandium-doped aluminum nitride has a high piezoelectric charge constant, so it is expected to be used in a next generation of high frequency filters. Further, scandium-doped aluminum nitride is expected to be used in physical sensors such as pressure sensors, acceleration sensors, and gyro sensors, and various MEMS devices such as actuators.
However, since scandium (Sc) is an expensive rare earth element, there has been a problem in which the production cost of the piezoelectric bodies constituted by scandium-doped aluminum nitride (AlN) is higher than piezoelectric bodies constituted by other substances.
Further, the present inventor has found that doping an element such as Sc to aluminum nitride improves properties such as piezoelectric charge constant. The inventor has also found that higher concentrations of the elements to be doped tends to further increase the piezoelectric charge constant and other properties. However, the upper limit value of the concentration of a single element that can be doped in aluminum nitride is low. Thus, the piezoelectric bodies having the high piezoelectric charge constant and other properties cannot be produced as they are.
Further, the performance of the piezoelectric body used in a MEMS device needs to be evaluated against the performance index of each type of MEMS device. In particular, when piezoelectric bodies are used as actuators or sensors, piezoelectric charge constants such as the level of strain caused by applying a voltage, i.e. d33, and the voltage caused by applying a pressure, i.e. g33, need to be evaluated. Further, k2, or the conversion efficiency between electric energy and mechanical energy, also provides an important performance index. However, there is a problem in which no other kinds of aluminum nitride, including the one described in Non-Patent Literature 1, show performance indices equal or better than that of the aluminum nitride doped with scandium.
Further, calculation of the piezoelectric charge constant d33 requires obtaining the following: the stress e33 generated by applying an electric field to the piezoelectric body that is restrained so as not to be distorted; and the proportional constant C33 of distortion generated by applying a stress to the piezoelectric body. Further, calculation of the piezoelectric voltage constant g33 and the electromechanical coupling constant k2 requires the dielectric constant ε33 of the piezoelectric body in addition to the values for the aforementioned physical properties. A general piezoelectric thin film has a wurtzite crystal structure that is oriented in the c-axis direction, emphasizing the importance of the c-axis component of the piezoelectric performance index.
Thus, under the aforementioned circumstances, an object of the present invention is to provide a nitride piezoelectric body having a value indicating a higher performance index (at least any one of d33, e33, C33, g33, and k2) than aluminum nitride not doped with any element, and a MEMS device using the nitride piezoelectric body.
The term “higher performance index” described herein does not mean a larger numerical value of the performance index, but a superior performance index.
As a result of persistent intensive studies on the aforementioned problems, the inventor of the present invention has found that adding (doping) a specific element (a substituent element M) together with magnesium (Mg) to aluminum nitride (AlN) makes it possible to produce aluminum nitride having a value indicating a high performance index, thereby discovering the following innovative piezoelectric body.
A first aspect of the present invention for solving the aforementioned problems is a piezoelectric body represented by a chemical formula Al1-X-YMgXMYN where X+Y is less than 1, X is in a range of more than 0 and less than 1, and Y is in a range of more than 0 and less than 1. In the chemical formula, M represents any one of Cr, Mn, Fe, Co, Ni, Mo, Tc, Ru, Rh, Pd, Ag, W, Re, Os, Ir, Pt, and Au.
In this first aspect, the piezoelectric body can have a value indicating a higher performance index (at least any one of d33, e33, C33, g33, and k2) than aluminum nitride doped with scandium at the same concentration as that of the doped elements (magnesium and the substituent element M).
A second aspect of the present invention is the piezoelectric body described in the first aspect, wherein X+Y is 0.65 or less, X is in a range of more than 0 and less than 0.65, and Y is in a range of more than 0 and less than 0.65.
In this second aspect, the piezoelectric body can have a value indicating the higher performance index (at least any one of d33, e33, C33, g33, and k2) than the aluminum nitride doped with scandium at the same concentration as that of the doped elements (magnesium and the substituent element M).
A third aspect of the present invention is the piezoelectric body described in the first aspect, wherein X+Y is 0.375 or less, X is in a range of more than 0 and 0.1875 or less, and Y is in a range of more than 0 and 0.1875 or less.
In this third aspect, the piezoelectric body can have a value indicating the higher performance index (at least any one of d33, e33, C33, g33, and k2) than the aluminum nitride doped with scandium at the same concentration as that of the doped elements (magnesium and the substituent element M).
A fourth aspect of the present invention is the piezoelectric body described in the first aspect, wherein X+Y is 0.125 or less, X is in a range of more than 0 and 0.0625 or less, and Y is in a range of more than 0 and 0.0625 or less.
In this fourth aspect, the piezoelectric body can have a value indicating the higher performance index (at least any one of d33, e33, C33, g33, and k2) than the aluminum nitride doped with scandium at the same concentration as that of the doped elements (magnesium and the substituent element M).
A fifth aspect of the present invention is the piezoelectric body described in any one of the first to fourth aspects, wherein M is any one of Cr, Mn, Fe, Co, Ni, Mo, Tc, Ru, Rh, Pd, Ag, W, Os, Ir, Pt, and Au.
In this fifth aspect, the piezoelectric body can have a value indicating the further higher performance index (at least any one of d33, e33, C33, g33, and k2) than the aluminum nitride doped with scandium at the same concentration as that of the doped elements (magnesium and the substituent element M).
A sixth aspect of the present invention is the piezoelectric body described in any one of the first to fourth aspects, wherein M is Cr or Mn.
In this sixth aspect, the piezoelectric body has a further lower mixing enthalpy and is more readily produced than Al1-XScXN doped with scandium at the same concentration X (mol %) as the concentration (mol %) of the doped elements (magnesium and the substituent element M). Further, the aluminum nitride doped with the elements (magnesium and the substituent element M) has the lower mixing enthalpy than the aluminum nitride doped with scandium at the same concentration, making it possible to have a higher solid solution concentration of the elements (magnesium and the substituent element M) than for scandium. This allows for the provision of a piezoelectric body with a higher performance index than existing ones.
A seventh aspect of the present invention is a MEMS device using the piezoelectric body described in any one of the first to sixth aspects.
The “MEMS device” used herein is not particularly limited as long as it is a micro electromechanical system, and examples thereof may include physical sensors such as pressure sensors, acceleration sensors, and gyro sensors, actuators, microphones, fingerprint authentication sensors, and vibration power generators.
In this seventh aspect, these piezoelectric bodies having high values of the piezoelectric charge constant d33 have lower losses and are operatable in a wider band. Thus, using these piezoelectric bodies makes it possible to provide the MEMS device which can further contribute to the operation at high frequencies and the reduction in size and power consumption of portable devices.
Hereinafter, embodiments of piezoelectric bodies according to the present invention will be described with reference to the accompanying drawings. Note that the present invention is not limited to the following embodiments.
First, a description will be given of a simulation performed by the inventor using aluminum nitride constituted by only aluminum (Al) and nitrogen (N) (non-doped AlN). The simulation was performed by using a software known as VASP (Vienna Ab initio Simulation Package) in which a calculation method referred to as “first principle calculation” was employed. The first principle calculation described herein is a general term for an electronic state calculating method without using a fitting parameter or the like and a method capable of calculating an electronic state by using only the atomic number and a coordinate of each atom constituting a unit lattice, a molecule, or the like.
In the simulation of the present embodiment, a supercell of the non-doped AlN having a wurtzite crystal structure was used for the simulation. The supercell containing sixteen aluminum atoms and sixteen nitrogen atoms was obtained by doubling a unit lattice containing two aluminum atoms and two nitrogen atoms in the a-axis, b-axis, and c-axis directions. Then, the first principle calculation was performed to this AlN having the wurtzite crystal structure by simultaneously changing the atomic coordinate, the cell volume, and the cell shape, thereby calculating the electronic state of the non-doped AlN in a stable structure.
Table 1 shows values (calculated values) of the lattice constant in the a-axis direction, the lattice constant in the c-axis direction, and a ratio (c/a) of the lattice constant in the c-axis direction with respect to the lattice constant in the a-axis direction, calculated from the electronic state of the AlN in the stable structure obtained by the first principle calculation. Further, Table 1 also shows experimental values measured by actually forming a non-doped AlN film using a sputtering method and subjecting this AlN film to an X-ray diffraction method.
As shown in this Table, each calculated value has almost the same numerical value as the experimental value with the relative error of 1% or less. This result demonstrated that the simulation of the present embodiment was sufficiently reliable.
The following shows that, when aluminum nitride (AlN) is doped with magnesium (Mg) and the substituent element M (M represents any one of Cr, Mn, Fe, Co, Ni, Mo, Tc, Ru, Rh, Pd, Ag, W, Re, Os, Ir, Pt, and Au) together, the AlN can be doped with the more amount of the elements (Mg and the substituent element M) than if the AlN is doped with only the substituent element M.
As an example of this,
As is evident from this diagram, it is found that the mixing enthalpy of the AlN doped with Mg and Cr together is lower than the mixing enthalpy of the AlN doped with only Sc or Cr at the same concentration. That is, it is shown that dissolving the elements (Mg+Cr) in the AlN as a solid is more thermodynamically advantageous than dissolving Cr in the AlN as a solid, at the same concentration. This demonstrated that the AlN could be doped with the more amount (at higher concentrations) of the elements (Mg and Cr) than scandium.
Note that, in the present embodiment, the mixing enthalpy of the aluminum nitride (Al0.875Mg0.0625Cr0.0625N) doped with Mg and Cr together has been described as an example. However, the mixing enthalpy of the aluminum nitride doped with Mg and the substituent element M other than Cr (excluding Re) together is also similarly lowered. Thus, the AlN can be doped with the more amount (at the higher concentrations) of the elements (Mg+the substituent element M (excluding Re)) than scandium.
Next, a description will be given of a simulation using doped AlN in which aluminum nitride (AlN) is doped with magnesium (Mg) and the substituent element M together.
As shown in this diagram, the crystal structure of this doped AlN forms a wurtzite crystal structure in which one Al atom is substituted with a Mg atom and one Al atom is substituted with an atom of the substituent element M in a unit lattice containing sixteen Al atoms and sixteen nitrogen atoms. Here, when the total of the number of Al atoms, the number of Mg atoms, and the number of atoms of the substituent element M is set to 1, the number of Mg atoms is defined as X and the number of atoms of the substituent element M is defined as Y. In such a case, in the doped AlN used in this simulation, both the concentration X of Mg atoms and the concentration Y of the substituent element M are given as 0.0625. Note that these kinds of doped AlN can be actually produced by the production method described in the aforementioned Non-Patent Literature 1.
In the present embodiment, chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), molybdenum (Mo), technetium (Tc), ruthenium (Ru), rhodium (Rh), palladium (pd), silver (Ag), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), and gold (Au) were used as the substituent element M.
As is the case with the non-doped AlN, the electronic state of these kinds of doped AlN and the AlN doped with only Sc in the stable structure can be calculated by the first principle calculation. Then, values of the lattice constant in the a-axis direction, the lattice constant in the c-axis direction, and the lattice constant ratio c/a can be calculated from the electronic state.
Then, a small strain is forcibly applied to each crystal lattice of the non-doped AlN, the AlN doped with only Sc, and the doped AlN in the stable structure. As a result, a small change in the total energy caused by this operation makes it possible to calculate each of the piezoelectric stress constant e33, the elastic constant C33, and the dielectric constant ε33 of the non-doped AlN, the AlN doped with only Sc, and the doped AlN. That is, each of the piezoelectric stress constant e33, the elastic constant C33, and the dielectric constant ε33 of the non-doped AlN, the AlN doped with only Sc, and the doped AlN can be calculated by using the first principle calculation.
Table 2 shows the lattice constant c, the lattice constant a, the lattice constant ratio c/a, the piezoelectric stress constant e33, the elastic constant C33, and the dielectric constant ε33 of the non-doped AlN, the AlN doped with only Sc, and each kind of doped AlN thus obtained. Note that the higher numerical value of the piezoelectric stress constant e33 indicates the higher performance index. On the other hand, the lower numerical value of the elastic constant C33 indicates the higher performance index.
Further,
On the other hand, the following relational expression of mathematical formula 2 holds between the piezoelectric stress constant e33, the elastic constant C33, and the dielectric constant ε33 in the c-axis direction, and the electromechanical coupling constant k2. Further, each of the following relational expressions of mathematical formula 3 holds between the piezoelectric charge constant d33, the piezoelectric stress constant e33, and the elastic constant C33. Thus, when the piezoelectric charge constant e33, the elastic constant C33, the dielectric constant ε33, and the like of the non-doped AlN, the AlN doped with only Sc, and the doped AlN calculated as above are each substituted in these relational expressions, it becomes possible to calculate each of the electromechanical coupling constant k2, the piezoelectric charge constant d33, and the piezoelectric voltage constant g33 of the non-doped AlN, the AlN doped with only Sc, and the doped AlN. Note that the elastic constants C11, C12, and C13, and the piezoelectric stress constant e31 can be calculated in the same manner as the piezoelectric stress constant e33 and the elastic constant C33.
Next, Table 3 shows the mixing enthalpy, the electromechanical coupling constant k2, the piezoelectric charge constant d33, and the piezoelectric voltage constant g33 of the non-doped AlN, the AlN doped with only Sc, and each kind of doped AlN thus obtained. Note that the higher numerical values of the electromechanical coupling value k2, the piezoelectric charge constant d33, and the piezoelectric voltage constant g33 indicate the higher performance indexes.
Further,
Further, the piezoelectric charge constant d33, the piezoelectric stress constant e33, and the elastic constant C33 of the doped AlN different from those described in Table 3 were calculated using the same method as described above. The results are shown in Table 4.
These results demonstrated that the AlN doped with Mg and any one of Cr, Mn, Fe, Co, Ni, Mo, Tc, Ru, Rh, Pd, Ag, W, Re, Os, Ir, Pt, and Au together showed a value indicating the higher performance index (at least any one of d33, e33, C33, g33, and k2) than the AlN not doped with any atom.
Furthermore, these piezoelectric bodies exhibiting the high performance index have low losses and can be operated in a wide band. Thus, using these piezoelectric bodies makes it possible to provide a MEMS device which can further contribute to the operation at high frequencies and the reduction in size and power consumption of portable devices.
Note that the present embodiment has been described using an example in which the piezoelectric body represented by the chemical formula Al1-X-YMgXMYN has a value of X as 0.0625 and Y as 0.0625. However, the present invention is not limited thereto, and it only requires that X+Y is less than 1, X is in a range of more than 0 and less than 1, and Y is in a range of more than 0 and less than 1.
Further, it is preferable that, regarding these variables X and Y, X+Y is 0.65 or less, X is in a range of more than 0 and less than 0.65, and Y is in a range of more than 0 and less than 0.65. Having the variables within these ranges allows the piezoelectric body to be reliably produced. For example, as shown in
Next,
Further,
Further, it is more preferable that, regarding these variables X and Y, X+Y is 0.375 or less, X is in a range of more than 0 and 0.1875 or less, and Y is in a range of more than 0 and 0.1875 or less.
Further, it is particularly preferable that, regarding these variables X and Y, X+Y is 0.125 or less, X is in a range of more than 0 and 0.0625 or less, and Y is in a range of more than 0 and 0.0625 or less.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2019-030057 | Feb 2019 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2019/051419 | 12/27/2019 | WO | 00 |
