MIM actuators with thick PZT films and haptic devices using such actuators

Abstract

A process is disclosed for fabricating MIM structures using thick films of PZT material (10). The PZT material is deposited on a substrate (2, 4, 6, 8) by alternating deposition of a PZT slurry (10.1) and a PZT solution (10.2). Haptic devices using MIM actuators fabricated by such a process are also disclosed.

Description

[Technical field]

[0001] The present invention relates to the field of microsystems manufacturing, and in particular to the manufacture of piezoelectric devices obtained by depositing components on a substrate. The target application is haptic devices. [Background technology]

[0002] Ferroelectric capacitors on silicon substrates are typically fabricated as a MIM structure: a metallic bottom electrode, an insulating layer, and a metallic top electrode.

[0003] The electrodes, made for example of Pt or AgPd, are separated from each other by an insulating layer. x Ti 1-x )O3(PZT), where 0≦x≦1. The films are typically thin, with a thickness of about 1 μm, which is a typical average for depositions made by sol-gel spin coating or inkjet printing.

[0004] These MIM actuators can be used in haptic touch devices. The substrate can be excited at a frequency of about 300 Hz to provide haptic feedback to the user. Because these MIM actuators have a high capacitance, the power consumption of these devices is high (peak power of about 100 W).

[0005] Attempts have been made by the French company "Hap2U" to provide actuators with thicker ceramic-based PZT layers (about 500 μm). However, the use of ceramic involves an expensive deposition process of PZT (by gluing) on ​​a substrate, since ceramic cannot be deposited by cost-effective processes (sol-gel or inkjet printing, spin-coating, etc.). Furthermore, the thickness of the ceramic layer cannot be technically made thin, which may be necessary depending on the properties or functions desired for the film. Finally, the purpose of these thick layers is to modify the mechanical behavior (sensitivity and mechanical deflection) of the actuator.

[0006] Therefore, there is a need to provide a manufacturing method for realizing haptic actuators that maintains the mechanical properties of existing actuators while resulting in low power consumption. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention is directed to the technical problem of providing an actuator for a haptic device that provides reduced power consumption without compromising the properties in terms of deflection or sensitivity. [Means for solving the problem]

[0008] The present invention is directed to a method for fabricating a metal-insulator-metal actuator for a sensory device, comprising the steps of: (a) providing a substrate; (b) providing a lead zirconate titanate (PZT) solution; (c) preparing a PZT slurry containing the PZT solution, a nanopowder, and a surfactant; (d) depositing a layer of the PZT solution on the substrate; (e) depositing a layer of the PZT slurry on the layer of the PZT solution; (f) depositing at least one layer of the PZT solution on the layer of the PZT slurry; (g) repeating steps (e) and (f) to achieve a thick film of PZT material having an integrated thickness comprised of 1-10% of the thickness of the substrate and a thickness comprised of about 10-100 μm; and (h) depositing a metal patterned electrode on the thick film of PZT material.

[0009] It has been established by the inventors that the use of a PZT slurry containing nanoparticles allows for a capillary action, which improves the penetration of the PZT solution above the slurry. Thus, the solution is well distributed throughout the slurry. The improvement of the power consumption while maintaining the same deflection characteristics is believed to be the result of a combination of three things: (1) the penetration process; (2) the thickness ratio between the substrate and the PZT layer; and (3) the thickness of the PZT layer, which is about 10-100 μm. "About 10" is understood to be 9-12 μm and "about 100" is understood to be 95-105 μm. The penetration process makes it possible to achieve thick layers. It has been established by the inventors that the energy required to generate haptic feedback is inversely proportional to the thickness of the PZT layer (when under similar voltages). However, a layer that is too thick will have a non-negligible thickness compared to the substrate, which will change the mechanical behavior of the haptic device and therefore the optimal solution with the three features 1-3 above. In addition, it is recognized that the absolute thickness is a good tradeoff between reducing power consumption and the amount of time required to produce the actuator. It is also recognized that 10 μm thicker films achieved by different deposition processes do not yield the same properties (dielectric constant, loss tangent) and therefore do not maintain the same amount of deflection for the actuator.

[0010] According to an advantageous embodiment, the individual layers of PZT solution and PZT slurry are so thick that steps (e) and (f) are repeated about seven times to achieve a combined thickness of about 10 μm. These seven repetitions allow the single layer to be thinned sufficiently to dry quickly between two successive depositions and to achieve a layer of uniform thickness. Those skilled in the art will appreciate that about 70 repetitions may be required to achieve a thickness of about 100 μm.

[0011] According to an advantageous embodiment, four layers of PZT solution are deposited in succession for each performance of step (f), the cavities of the slurry can be completely filled by four layers of PZT, so adding more than four layers would compromise the effectiveness of the deposition process and create non-uniformities through the thickness of the PZT layer.

[0012] According to a preferred embodiment, step (e) comprises spin-coating the PZT slurry at a rotation speed of 2000-4000 rpm, preferably 3000 rpm, for 30-60 seconds, preferably 30 seconds.

[0013] According to a preferred embodiment, the deposition of each layer in step (f) comprises spin-coating the PZT solution at a rotation speed of 300-700 rpm, preferably 500 rpm, for a time of 20-40 seconds, preferably 30 seconds. Capillary action makes it possible to achieve a good distribution of the solution with this reduced rotation speed.

[0014] According to an advantageous embodiment, after the droplet of PZT solution is deposited, the spinner rotation is delayed for about 30 seconds. Capillary action operates within these seconds, allowing the solution to spread over the substrate, thereby maximizing penetration.

[0015] According to an advantageous embodiment, the surfactant is Triton X-100 or polyvinylpyrrolidone.

[0016] According to an advantageous embodiment, after step (f), at least one layer of the PZT solution is crystallized at 700° C. for approximately half an hour.

[0017] According to an advantageous embodiment, in step (g), the repetition of steps (e) and (f) results in a thick film of PZT material (10) having an integrated thickness comprised between 1 and 2% of the thickness of the substrate and comprised between about 10 and 20 μm.

[0018] According to a preferred embodiment, after all the PZT layers are deposited, the PZT film is annealed in a furnace at 700° C. for about 1 hour.

[0019] According to a preferred embodiment, the substrate is a platinized silicon substrate or a fused silica substrate.

[0020] The present invention also relates to a piezoelectric haptic device comprising an actuator with a thick PZT film prepared by the method according to any of the previous descriptions.

[0021] At the very small scale, thick films produced using alternating deposition of PZT slurry and PZT solution exhibit a unique grain structure, which can be confirmed using SEM or X-ray analysis, among others.

[0022] At infinitesimal scale, the haptic devices of the present invention are physically distinguishable from known haptic devices through both the thickness of the PZT layer and the power consumption for an equivalent voltage source.

[0023] According to an advantageous embodiment, the combined thickness of the PZT layers is about 10 μm, and at a frequency of 62 kHz and a peak-to-peak voltage of 40 V, the actuator consumes a power of about 350 mW. [Brief description of the drawings]

[0024] [Figure 1] FIG. 2 is a diagram illustrating the manufacturing process of the present invention. [Diagram 2] FIG. [Diagram 3] 13 is a table showing a comparative experiment. [Figure 4] 13 is a table showing a comparative experiment. [Figure 5A] FIG. 13 shows a comparative graph of PZT layers of similar thickness. [Figure 5B] FIG. 13 shows a comparative graph of PZT layers of similar thickness. [Figure 6] FIG. 1 illustrates capillary action. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] FIG. 1 is a schematic diagram of a manufacturing method according to the present invention.

[0026] In a preparation step 100, a substrate 102 is provided as well as a PZT solution 104. A PZT slurry 106 is prepared with PZT nanoparticles.

[0027] A first layer of a PZT solution is deposited on a substrate in step 200 .

[0028] After deposition of the PZT slurry layer (step 300), deposition of the PZT solution layer (step 400) can be repeated several times, for example four times.

[0029] The PZT slurry can be spin-coated at 3000 rpm for 30 seconds (step 310), then dried at 130° C. (step 320) and pyrolyzed on a hotplate at 350° C. (step 330). Crystallization can be performed at 700° C. for 60 seconds (step 340). The PZT solution is then spin-coated at 500 rpm, dried and pyrolyzed under the same conditions as the slurry (steps 410, 420, 430). After several (e.g., four) subsequent deposition-drying-pyrolysis cycles, crystallization (step 500) can be performed at 700° C. for 30 minutes in air. The aforementioned steps (300, 400, 500) for PZT deposition can be repeated several times (e.g., seven times) to achieve a desired integrated thickness (e.g., an integrated thickness of 10 μm).

[0030] Patterned metal electrodes (Pt) are then deposited (step 600) to ensure the desired function as an actuator.

[0031] The electrodes can be patterned using a shadow mask, and then 500 nm platinum electrodes can be DC sputtered at room temperature.

[0032] Figure 2 is a cross-sectional view (not to scale) of an actuator 1 realised using the method of Figure 1. The actuator 1 comprises a substrate (2, 4, 6, 8), in this example formed of a platinised silicon substrate. The layers indicated as 2, 4, 6, 8 represent, in succession, 0.65 μm Si, 500 nm SiO2, 20 nm TiO2 and 100 nm Pt.

[0033] This substrate is only an example: for example, layer 6 (TiO2) can be replaced with HfO2; and / or platinum (layer 8) can be replaced with any other conductor such as Mo, Au, ITO, etc; and / or layers 2, 4 (SiO2 / Si) can be replaced with fused silica glass.

[0034] On top of the first electrode 8 a thick layer of PZT 10 (about 10 μm) is deposited. The dotted lines indicate diagrammatically the various layers of PZT slurry and PZT solution that make up the thick layer of PZT 10. These are also shown on the right hand side of Fig. 2, referenced 10.1 and 10.2 as the various layers. In a preferred embodiment, two successive layers of PZT slurry 10.1 are separated by four layers of PZT solution 10.2.

[0035] A second electrode 12, approximately 500 nm thick, is deposited on top of the thick layer 10 of PZT.

[0036] Figure 3 represents a comparative example: the left column shows the results for an actuator with a PZT layer of 1 μm integrated thickness, while the right column shows the corresponding values ​​for an actuator with a PZT layer of 10 μm integrated thickness.

[0037] The comparison was carried out by applying a voltage of 40V peak-to-peak at a frequency of 62kHz. The vibrometer makes it possible to draw a 2D map of the displacement. This 2D map shows that both actuators have a vibration displacement of about 3 microns (plus or minus 1.5 microns above and below the rest position). It can thus be ensured that the mechanical behavior of the two actuators is similar.

[0038] However, the smaller the thickness of the PZT layer, the higher the measured capacitance and the higher the measured power consumption.

[0039] Thus, the haptic device of the present invention is physically distinguishable from known haptic devices through both the thickness of the PZT layer and the power consumption for an equivalent voltage source.

[0040] Figure 4 shows a comparative example of an actuator according to the invention (right column) compared to an actuator printed with only PZT slurry (Kwon, DOI: doi.org / 10.1016 / j.jcrysgro.2006.07.005). It can be seen that the dielectric constant is twice as high, while the loss tangent is 1 / 4.

[0041] Micrographs also show that the particle structure is different. The method of the present invention can be distinguished by, among other things, particle size, uniformity of solution distribution, and reduced number of cavities.

[0042] 5A and 5B are comparative graphs of PZT layers of similar thickness but prepared by different processes.

[0043] FIG. 5A shows the polarization as a function of voltage for a known actuator (Wang, DOI:10.1016 / j.jeurceramsoc.2011.12.013). A high Ec of 10 V / μm can be seen, indicating a large leakage current.

[0044] By comparison, Figure 5B shows the polarization as a function of voltage of an actuator prepared using the method according to the invention, where no leakage current is observed.

[0045] Figure 6 illustrates the observed capillary action: when a small drop of PZT solution 10.2 is deposited on a layer of PZT slurry 10.1, the drop penetrates and spreads evenly over the PZT slurry within a few seconds.

[0046] Without being bound by theory, the inventors have determined that this capillary action may explain the reduced power consumption without changing the actuator deflection.

[0047] The exemplary embodiments and various amounts and numbers presented above are provided to illustrate the invention. Those skilled in the art will understand that the scope of the invention is limited only by the appended claims, and that variations in dilution amounts, temperatures, or time durations in the various steps of the method do not depart from the scope of the invention. For example, variations of 10% to 20% in dilution ratios, step durations, temperatures, or spinner speeds may be used.

[0048] While the particular application described above relates to haptic devices, the invention may also provide advantages in other applications such as ferroelectric devices, non-volatile RAM, memories with pyroelectric readout, and piezoelectric applications using electrical cycling under high amplitude electric fields.

[0049] Furthermore, although the deposition process is exemplified as a related spin-coating process, alternative deposition processes are possible, such as inkjet printing.

Claims

1. 1. A method for fabricating a metal-insulator-metal actuator for a sensory device, comprising: (a) providing (102) a substrate (2, 4, 6, 8); (b) providing a lead zirconate titanate (PZT) solution (104); (c) providing a PZT slurry containing the PZT solution, nanopowder, and a surfactant (106); (d) depositing (200) a layer of PZT solution (10.2) on said substrate (2, 4, 6, 8); (e) depositing (300) a layer of PZT slurry (10.1) on said layer of PZT solution (10.2); (f) depositing (400) at least one layer of a PZT solution (10.2) on said layer of PZT slurry (10.1); (g) repeating steps (e) and (f) to achieve a thick film (10) of PZT material comprised of 1-10% of the thickness of said substrate and having an integrated thickness comprised of about 10-100 μm; (h) depositing (600) a metal patterned electrode (12) on the thick film (10) of PZT material; A method comprising:

2. 2. The method of claim 1, wherein the individual layers (10.1, 10.2) of PZT solution and PZT slurry are of a thickness such that steps (e) and (f) are repeated about seven times to achieve a combined thickness of about 10 μm.

3. 3. The method according to claim 1 or 2, wherein for each performance of step (f), four layers of the PZT solution (10.2) are deposited in succession.

4. The method according to any one of the preceding claims, wherein step (e) comprises spin-coating (310) said PZT slurry at a rotation speed of 2000-4000 rpm, preferably 3000 rpm, for a period comprised between 30 and 60 seconds, preferably 30 seconds.

5. 5. The method according to any one of claims 1 to 4, wherein the deposition of each layer in step (f) comprises spin-coating (410) said PZT solution at a rotation speed of 300 to 700 rpm, preferably 500 rpm, for a period comprised between 30 and 60 seconds, preferably 30 seconds.

6. 6. The method of claim 5, wherein after the droplet of PZT solution is deposited, the rotation of the spinner is delayed by about 30 seconds.

7. The method according to any one of claims 1 to 6, characterized in that the surfactant is Triton X-100 or polyvinylpyrrolidone.

8. The method according to any one of claims 1 to 7, wherein after step (f), the at least one layer of the PZT solution is crystallized (500) at 700°C for about half an hour.

9. 9. The method according to any one of claims 1 to 8, wherein in step (g), the repetition of steps (e) and (f) achieves a thick film (10) of PZT material having an integrated thickness comprised between 1 and 2% of the thickness of the substrate and a thickness comprised between about 10 and 20 μm.

10. The method of any one of claims 1 to 9, wherein the substrate is a platinized silicon substrate or a fused silica substrate.

11. A piezoelectric haptic device comprising an actuator with a thick PZT film prepared by the method of any one of claims 1 to 10.

12. 12. The piezoelectric haptic device of claim 11, wherein the combined thickness of the PZT layer is about 10 μm and the actuator consumes about 350 mW of power at a frequency of 62 kHz under a voltage of 40 V peak-to-peak.

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