MATERIAL DEPOSITION METHOD AND MICROSYSTEM THEREWITH OBTAINED

Abstract

A material deposition method comprising: providing a substrate; forming a film of HfO2 by chemical solution deposition, CSD, on the substrate; depositing a solution of PbTiO3 on the film of HfO2; depositing a layer of Pb(Zrx,Ti1-x)O3 on the seed layer, where Ox1; and forming interdigitated electrodes on the Pb(Zrx,Ti1-x)O 3 layer. Also a ferroelectric microsystem obtained by this deposition method. Experiments show an improved fatigue resistance for such a microsystem.

Claims

1-12. (canceled)

13. A material deposition method, said method comprising: providing a substrate; forming a film of HfO.sub.2 by chemical solution deposition on the substrate; depositing a seed layer of a solution of PbTiO.sub.3 on the film of HfO.sub.2; depositing a layer of Pb(Zr.sub.x,Ti.sub.1-x)O.sub.3 on the seed layer, where 0x1; and forming interdigitated electrodes on the Pb(Zr.sub.x,Ti.sub.1-x)O.sub.3 layer.

14. The method according to claim 13, wherein the film of HfO.sub.2 is formed by deposition of at least two layers, each layer having a thickness of about 15 nm and deposited by spin coating.

15. The method according to claim 14, wherein the spin coating operation is performed at a speed comprised between 2000 rpm and 4000 rpm, and for a duration comprised between 20 and 40 seconds.

16. The method according to claim 14, wherein the spin coating operation is performed at 3000 rpm, and for a duration of 30 seconds.

17. The method according to claim 14, wherein after each layer is formed, an operation of drying at 215 C. for 5 min is carried out.

18. The method according to claim 13, wherein after its deposition, the film of HfO.sub.2 is annealed in a furnace at 700 C. for 90 s.

19. The method according to claim 13, wherein the chemical solution of HfO.sub.2 is a solution of 0.25 M Hf-Acetylacetonate in propionic acid.

20. The method according to claim 13, wherein the seed layer is deposited by spin coating a precursor solution of PbTiO.sub.3 prepared using 2 methoxy-ethanol or 1-methoxy-2-propanol as a solvent and optionally acetylacetone as a modifier.

21. The method according to claim 13, wherein x=0.53.

22. The method according to claim 13, wherein the substrate is a fused silica substrate.

23. The method according to claim 13, wherein the substrate is a silicon substrate with interlayers of SiO.sub.2.

24. The method according to claim 13, wherein the substrate is a sapphire substrate.

Description

DRAWINGS

[0031] FIG. 1 is an exemplary cross-section of a microsystem device in accordance with various embodiments of the invention.

[0032] FIGS. 2 and 3 exemplarily show a comparison of fatigue experiments between a known device and the device of the invention in accordance with various embodiments of the invention.

DETAILED DESCRIPTION

[0033] FIG. 1 shows a cross-section (not to scale) of a microsystem 1. The microsystem 1 comprises a superposition of films on a substrate 2.

[0034] A HfO.sub.2 film 4 is deposited (directly) on the substrate 2. A PbTiO.sub.3 seed layer 6 is (directly) deposited on the HfO.sub.2 film 4. A PZT layer 8 is built on the seed layer 6. Electrodes 10 are formed on the PZT layer 8. None of the layers 2, 4, 6, 8 contains or is interposed with an electrode.

[0035] The substrate 2 can be a 500 nm thick Si wafer from Siegert Wafer GmbH.

[0036] The HfO.sub.2 passivation film can be made of at least two layers deposited by CSD using 0.25 M HfO.sub.2 solution (Hf-acetylacetonate in propionic acid). The substrate 2 can be heated at 350 C. on a hot plate for surface activation. Then the HfO.sub.2 solution can be spin coated at 3000 rpm for 30 seconds, followed by drying at 215 C. for 5 minutes. The operation can be repeated at least once to obtain a thickness of HfO.sub.2 film of 30 nm. Then the film can be annealed in a rapid thermal annealing furnace at 700 C. for 90 seconds.

[0037] The PbTiO.sub.3 (PT) seed layer 6 can be prepared as discussed extensively in Luxembourgish patent application LU101884, i.e., with 2 methoxy-ethanol or 1-methoxy-2-propanol as solvent and optionally acetylacetone as modifier.

[0038] A film of PZT can be deposited over the seed layer 6, in various instances preferably Pb(Zr.sub.0.53,Ti.sub.0.47)O.sub.3. The PZT film is deposited on the seed layer by spin-coating. Alternatively, the deposition can be made by inkjet printing, sputtering, Pulsed Laser Deposition, MOCVD, etc. Again, patent application LU101884 provides exemplary details of the preparation and deposition of the PZT film.

[0039] Lead(II) acetate trihydrate (99.5%, Sigma-Aldrich, USA), titanium (IV)-isopropoxide (97%, Sigma-Aldrich, USA) and zirconium (IV)-propoxide (70% in propanol, Sigma-Aldrich, USA) can be used as precursors in stoichiometric ratio with 2-methoxyethanol as solvent to prepare both the PT and PZT solution. The PT solution can be spin-coated onto the HfO.sub.2 layer at 3000 rpm for 30 s, followed by drying and pyrolysis at 130 C. and 350 C., respectively, on hot plates. Final crystallization can be performed at 700 C. for 60 s in a rapid thermal annealing furnace (ASMaster, Annealsys, France) at 50 C./s heating rate in air. The PZT solution is then spin-coated, dried and pyrolyzed following the same deposition steps. After a couple of (e.g., four) subsequent deposition-drying-pyrolysis cycles, crystallization can happen at 700 C. in air for 300 s at 50 C./s heating rate, resulting in 170 nm thick PZT films. The aforementioned steps for PZT deposition can be repeated three times to achieve 500 nm film thickness. This process can also be adapted to fabricate thicker layers of PZT up to 1.2 m.

[0040] Over the PZT layer are formed planar electrodes. In particular, interdigitated electrodes (IDE) can be formed, having fingers of 10 m of width and an inter-finger distance of about 10 m. IDEs are patterned by lift-off photolithography using a direct laser writing (MLA, Heidelberg Instruments). Platinum electrodes of 100 nm can then be DC-sputtered at room temperature. The IDE geometry is only schematically illustrated in FIG. 1. The exact geometry of the design (width of individual fingers, width of gap between fingers, number of fingers, size of contact pads at each end) will be chosen according to the intended application of the microsystem (in particular depending on the required cycling speed).

[0041] The microsystem of the invention constitutes a substantial improvement over the known systems. FIGS. 2 and 3 highlight this improvement. A cyclically varying external electric field was applied to the capacitor structure to change the electrical polarization. In the present example, a frequency of 100 Hz at field amplitudes of 150 kV/mm and 200 kV/mm, respectively, was applied. Further experiments confirm that an amplitude which is sufficient to induce polarization switching leads to the same conclusions (i.e., an amplitude equal to or greater than 75 kV/cm).

[0042] FIG. 2 shows the development of the ferroelectric polarization loops measured on the known MIM structure in a new condition and after 1 million cycles (dotted line).

[0043] FIG. 3 shows a similar chart for the IDE structure with HfO.sub.2 (CSD) layer according to the invention.

[0044] Both FIGS. 2 and 3 show comparable hysteresis properties during the first few cycles, indicating that the performance of the device with the IDE structure can compete with the performance of the conventional MIM structure.

[0045] After one million cycles, the MIM structure shows notable degradation. The most important parameter for ferroelectric applications, the remnant polarization at zero field, nearly vanishes in the system having the MIM structure. In contrast, the shape of the polarization hysteresis of the IDE structure (FIG. 3, dotted line) is only slightly affected by the million cycles, the device conserving substantially the same remnant polarization. Any device based on the capacitor with the MIM structure is therefore unusable after 10.sup.6 switching cycles, whereas a device based on the capacitor with the IDE structure and HfO.sub.2 (CSD) layer remains functional.

[0046] The results of FIGS. 2 and 3 are consistent throughout the various solicitation (frequency, amplitude and number of cycles). Also, the improvement in fatigue is independent from the presence of the PbTiO.sub.3 seed layer.

[0047] HfO.sub.2 deposited by another technology (e.g., atomic layer deposition) does not result in the same fatigue improvement.

[0048] It is therefore concluded that the deposition of HfO.sub.2 by CSD technique is responsible of the improvement of the IDE-made microsystem fatigue resistance.

[0049] The exemplary embodiments presented above and the various quantities and numbers are given to illustrate the invention. The person skilled in the art would understand that the scope of the invention is only limited by the appended claims and that variations in the amount of dilution, the temperatures or the time duration for the various steps of the method do not depart from the scope of the present invention. For example, variations of about 10% to 20% in the dilution ratios, the duration of the steps, the temperatures or the speed of the spinner can be used.

[0050] If the particular application cited above relates to ferroelectric field-effect transistors, the invention also provides advantages in other applications, such as non-volatile RAM, memories with pyroelectric readout, piezoelectric applications using electrical cycling under high-amplitude electric fields.