Method for depositing highly doped aluminum nitride piezoelectric material

Abstract

The present invention relates to a method for producing doped wurtzite aluminum nitride piezoelectric thin film material, comprising the steps of providing a deposition apparatus, such as a pulsed laser deposition apparatus or a physical vapor deposition apparatus, with a target and a substrate, wherein the target material is a doped aluminum nitride composite and the doping element is a rare earth element, preferably scandium; depositing a first layer of the target material on the substrate by operating the deposition apparatus, wherein the kinetic energy of the deposited plasma particles is greater than a first threshold; and depositing a second layer of the target material on the first layer by operating the deposition apparatus, wherein the kinetic energy of the deposited plasma particles is less than a second threshold and the first threshold is greater than the second threshold.

Description

[Technical field]

[0001] The present invention relates to a method for producing doped wurtzite aluminum nitride piezoelectric thin film material, the method comprising the steps of providing a deposition apparatus, such as a pulsed laser deposition apparatus or a physical vapor deposition apparatus, with a target and a substrate, where the target material is a doped aluminum nitride compound and the doping element is a rare earth element (preferably scandium), operating the deposition apparatus to deposit a first layer of the target material on the substrate, where the kinetic energy of the depositing plasma particles is greater than a first threshold, and operating the deposition apparatus to deposit a second layer of the target material on the first layer, where the kinetic energy of the depositing plasma particles is less than a second threshold.

[0002] In the present invention, the kinetic energy of plasma particles is defined by the plasma temperature measured in electron volts (eV), which is a common factor of the thermal kinetic energy per particle. [Background technology]

[0003] The production of piezoelectric material in a pulsed laser deposition apparatus is known, for example, from the applicant's EP 3 340 327. From this publication it is also known to apply two or more layers of target material onto a substrate, using different process parameters for each layer.

[0004] EP 3340327 teaches using a low deposition rate for the first layer and a high deposition rate for the second and subsequent layers, which allows the second layer to have a less unstable crystal structure than piezoelectric material grown using only a high deposition rate.

[0005] When doped aluminum nitride composites (especially scandium aluminum nitride) are used to fabricate piezoelectric materials, it is known that increasing the scandium concentration also increases the piezoelectric response (Morito Akiyama et al., "Scandium Aluminum Nitride (Sc x Al1-x N) Effect of growth temperature and scandium concentration on the piezoelectric response of alloy thin films" (2009), Applied Physics Letters 95, 162107 (2009)).

[0006] However, when the doping concentration is increased to high values ​​(>20% scandium), undesirable features begin to appear on the film surface. These features can be seen using a scanning electron microscope (SEM) as triangular areas called inversion domains on the surface of the deposited layer. Inversion domains are known to adversely affect the properties of scandium aluminum nitride thin films. It is therefore important to limit the amount of inversion domains.

[0007] As the concentration of the doping element is further increased beyond a certain value (e.g., >43%), a dramatic decrease in the piezoelectric response is observed. This is due to a phase transition of scandium aluminum nitride to a non-polar rock-salt structure that no longer has piezoelectric properties. It is therefore desirable to maintain the wurtzite phase at as high a doping concentration as possible. The amount of inversion domains, and the concentration at which a dramatic decrease in the piezoelectric response occurs, depends on the interface between the substrate and the target material layer (see Simon Fitchner et al., "Identifying and overcoming the interface originating c-axis instability in highly Scenhanced AlN for piezoelectric micro-electromechanical systems," Journal of Applied Physics 122, 035301 (2017)).

[0008] It is known to perform pre-cleaning treatments (e.g. etching, milling or passivation) using physical vapor deposition on the surface of the wafer or electrode stack to which the scandium aluminum nitride thin film needs to be applied in order to modify the interface between the substrate and the target material layer. In addition, the first thin film may be applied using another target material (usually undoped or scandium-doped aluminum nitride with a lower scandium concentration than the second bulk layer). This first layer acts as a template to stabilize the wurtzite phase of the second bulk layer of highly doped scandium aluminum nitride. These steps require additional (expensive) equipment and slow down the thin film manufacturing process.

[0009] These additional steps allow scandium to be used as a doping element in concentrations up to about 30% in mass production facilities and up to about 43% in laboratory settings (see "Scandium Aluminum Nitride (ScAu)" by Morito Akiyama et al.). x Al 1-x N) Effect of growth temperature and scandium concentration on the piezoelectric response of alloy thin films" (2009), Applied Physics Letters 95, 162107 (2009)).

[0010] Theoretical calculations indicate that wurtzite scandium aluminum nitride is stable up to a concentration of 56% (see MA Moran, "ScGaN and ScAlN: emerging nitride materials" (2014), Journals of Materials Chemistry A, 2014, 2, 6042-6050; and Miguel A Caro et al., "Piezoelectric coefficients and spontaneous polarization of ScAlN" (2015), Journal of Physics: Condensed Matter, 27 (2015) 245901). However, such high concentrations up to 56% do not appear to be possible, at least at present, using cleaning treatments and / or low Sc-doped seed layers. Summary of the Invention

[0011] Therefore, it is an object of the present invention to reduce the above disadvantages while using a deposition system (eg, a pulsed laser deposition system or a physical vapor deposition system) for producing piezoelectric thin film materials from doped aluminum nitride composites.

[0012] This object is achieved by the invention comprising a method according to the preamble, characterized in that the first threshold is greater than the second threshold, so that the stress in the first layer is typically smaller than the stress in the second layer.

[0013] By depositing a first layer using the target material, the kinetic energy of the depositing plasma particles is greater than a first threshold (i.e., a high level) so that the likelihood of inversion domain formation due to the quality of the substrate is substantially reduced, and the substrate is actually clean and smooth. When a second layer of the target material is subsequently deposited, the kinetic energy of the depositing plasma particles is less than a second threshold (i.e., a low level), reducing the likelihood of inversion domain formation while allowing the second layer of the target material to grow under optimal conditions, and accordingly allowing high concentrations of doping elements to be used in the wurtzite aluminum nitride without depositing a first layer containing particles with a high level of kinetic energy.

[0014] A further advantage of the method according to the invention is that only one target material is used, which contributes to the quality of the produced piezoelectric thin film material and does not delay the process of replacing the target material for the formation of the first layer or the use of additional reactors, and also prevents the use of cleaning chambers which add costs to the production facility.

[0015] Furthermore, since the second layer of target material is applied directly onto the first layer of target material, there is no need to first move the substrate to a cleaning position, perform a cleaning process, and then move the substrate with the cleaned substrate surface or electrode surface to a position where another layer of target material is applied, so that the method of the present invention can be used in a single chamber, as compared to known physical vapor deposition methods in which three different chambers are used (for cleaning, for the first layer, and for the second layer using a different target material than the first layer).

[0016] In a preferred embodiment of the method according to the invention, a first layer of target material is deposited on a substrate without intermediate processing of the substrate, which may be a wafer, an optional passivation layer, or an electrode layer.

[0017] In the present invention, only the process parameters are changed between the first and second layers, and there is no need to physically move the substrate and / or target material to obtain the doped aluminum nitride thin film.

[0018] In a further preferred embodiment of the method according to the invention, the first threshold is greater than 90 eV, preferably between 100 eV and 160 eV. This results in a stress in the first layer of less than -400 MPa (megaparkar). Depositing a target material with plasma particles having such a high kinetic energy ensures an optimal surface for growing the second layer of target material, thereby allowing a high concentration of doping elements. Otherwise, many inversion domains would be formed and / or phase transitions would occur, which would not be possible, and the produced thin film material would lose its piezoelectric properties.

[0019] In a further preferred embodiment of the method according to the invention, the second threshold is less than 90 eV, preferably between 40 eV and 80 eV, which results in a thin film stress of the second layer typically between -400 MPa and +600 MPa. This relatively low plasma particle kinetic energy will ensure optimal growth of the second layer of target material and ultimately provide the thin film material with the desired piezoelectric properties.

[0020] The kinetic energy with which the plasma particles reach the substrate is directly related to the effects realized by the present invention, but practice has shown that it is difficult to accurately measure the kinetic energy of plasma particles. Currently, probes are used to measure the kinetic energy of plasma particles. Such probes must be placed close to the surface of the substrate. Furthermore, when the kinetic energy of plasma particles generated by a pulsed laser deposition apparatus is measured, the ultrashort pulse of the laser and the resulting plasma makes it even more difficult to reliably measure the kinetic energy.

[0021] It is therefore common practice to derive the kinetic energy of the plasma particles from the stress induced by them in the deposited layer, which can be precisely measured, for example, by a camber gauge.

[0022] In another embodiment of the method according to the invention in line with the above, the kinetic energy of the plasma particles is derived from the stress of the layer deposited by the plasma particles and is greater than a first threshold value during deposition of the first layer by maintaining the stress of the first layer below -500 MPa (preferably below -1000 MPa).

[0023] In another embodiment of the method according to the invention, the kinetic energy of the plasma particles is derived from the stress of the layer deposited by the plasma particles and is less than a second threshold value while depositing the second layer by maintaining the stress of the first layer greater than -500 MPa, preferably between -400 MPa and +600 MPa.

[0024] In a further embodiment of the method according to the invention, the thickness of the first layer of target material is 10-50 nm (nanometers). Exposing the wafer or electrode during deposition of such a thickness at high kinetic energy is sufficient to provide the necessary surface cleaning. Such a thickness is also sufficient to provide a high quality interface with the second layer, allowing a high concentration of doping elements without causing phase transformations.

[0025] In a further embodiment of the method according to the invention the concentration of the doping element is at least 20% of the target material. [Brief description of the drawings]

[0026] [Figure 1A] Steps of the method according to the present invention. [Figure 1B] Steps of the method according to the present invention. [Figure 1C] Steps of the method according to the present invention.

[0027] [Figure 2A] SEM image of the surface of a piezoelectric thin film material using conventional technology.

[0028] [Figure 2B] 1 is an SEM photograph of the surface of a piezoelectric thin film material produced using the method according to the present invention.

[0029] [Diagram 3] Diagram of the piezoelectric response versus the concentration of the doping element. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] 1A shows a schematic diagram of a pulsed laser deposition apparatus with a target 1 and a substrate 2. A pulsed laser 3 is directed at the target 1 such that a plasma plume 4 is generated. To deposit target material on the entire surface of the substrate 2, the laser beam 3 is, for example, scanned over the target 1 while the substrate 2 is rotated or moved linearly in two dimensions such that the plasma plume 4 can cover the entire surface of the substrate 2.

[0031] According to the invention, a first layer 5 of target material is deposited on the substrate 2 using a high kinetic energy greater than a first threshold (see FIG. 1B).

[0032] After the first layer 5 of target material is deposited, the parameters of the pulsed laser deposition apparatus are adjusted so that a second layer 6 of target material is deposited on the first layer 5 using a low kinetic energy below a second threshold (see FIG. 1C).

[0033] FIG. 2A shows an SEM photograph of the surface of a thin film material made of scandium aluminum nitride with a scandium concentration of 40% and deposited by a pulsed laser deposition system. During the deposition of the target material, the kinetic energy of the plasma particles was maintained at 60 eV. The resulting thin film stress is +300 MPa (tension). From the photograph it is clear that there are zones 10, so-called inversion domains. This leads to a degradation of the thin film properties, such as the piezoelectric response, despite the high concentration of scandium.

[0034] FIG. 2B shows an SEM photograph of the surface of a thin-film material produced by the method according to the invention. The scandium concentration is equal to that used for the thin-film material of FIG. 2A (i.e., 40%). However, a first layer of target material was deposited on the substrate provided with an electrode layer using a kinetic energy of the plasma particles of 150 eV. After about 10-50 nm of this first layer had been deposited, the parameters of the pulsed laser deposition apparatus were changed so that the kinetic energy of the plasma particles was reduced to 50 eV, close to the value used for the thin-film material of FIG. 2A. The resulting combined stress of the thin first layer and the thick second layer is +300 MPa (tension), similar to the stress of the film of FIG. 2A.

[0035] FIG. 2B clearly shows that the method according to the invention eliminates zone 10, thereby allowing the produced piezoelectric thin film material to optimally benefit from a high concentration of scandium.

[0036] FIG. 3 shows a plot of the piezoelectric response e31,f along the Y axis and the scandium concentration in aluminum nitride along the X axis.

[0037] Curve 20 shows the theoretical piezoelectric response as a function of scandium concentration by Miguel A Caro et al. (2015).

[0038] Data point 21 shows the results for the prior art method where physical vapor deposition is used as suggested by Simon Fitchner et al. (2017). Data point 21 no longer follows the theoretical curve 20 at approximately 35% scandium and the piezoelectric response is subject to the onset of inversion domains, or rocksalt phase formation in the thin film, shown in Figure 2A.

[0039] Data points 22 show the results of a method according to the present invention. In these data points 22, a first layer of scandium aluminum nitride target material was deposited using a plasma particle kinetic energy of 150 eV. A second layer of scandium aluminum nitride target material was then deposited using a plasma kinetic energy of 50 eV. It is clear from data points 22 that even up to 50% scandium in wurtzite aluminum nitride, the piezoelectric response is not significantly affected by phase transitions or inversion domains, and data points 22 are still higher than using known physical vapor deposition techniques.

Claims

1. 1. A method for producing a doped wurtzite aluminum nitride piezoelectric thin film material, comprising the steps of: Providing a deposition apparatus, such as a pulsed laser deposition apparatus or a physical vapor deposition apparatus, with a target and a substrate, the target material being a doped aluminum nitride composite and the doping element being a rare earth element, preferably scandium; depositing a first layer of the target material on the substrate by operating the pulsed laser deposition apparatus, wherein the kinetic energy of the depositing plasma particles is greater than a first threshold; depositing a second layer of the target material onto the first layer by operating the pulsed laser deposition apparatus, wherein the kinetic energy of the depositing plasma particles is less than a second threshold; The first threshold is greater than the second threshold.

2. 2. The method of claim 1 , A method wherein the first layer of the target material is deposited on the substrate without intermediate processing of the substrate.

3. 3. The method according to claim 1 or 2, The method according to claim 1, wherein the first threshold is greater than 90 eV, and is preferably between 100 eV and 160 eV.

4. The method according to any one of claims 1 to 3, The method according to claim 1, wherein the second threshold is less than 90 eV, and is preferably between 40 eV and 80 eV.

5. The method according to any one of claims 1 to 4, wherein the kinetic energy of the plasma particles is derived from the stress of the layer deposited by the plasma particles and is greater than the first threshold value during deposition of the first layer by maintaining the stress of the first layer below -500 MPa, preferably below -1000 MPa.

6. The method according to any one of claims 1 to 5, wherein the kinetic energy of the plasma particles is derived from the stress of the layer deposited by the plasma particles and is less than the second threshold value while depositing the second layer by maintaining the stress of the first layer greater than -500 MPa, preferably in the range of -400 MPa to +600 MMPa.

7. The method according to any one of claims 1 to 6, The method of claim 1, wherein the first layer of the target material is between 10 nm and 50 nm thick.

8. A method according to any one of claims 1 to 7, comprising: A method, wherein the concentration of the doping element is at least 20% of the target material.

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