Low voltage drive ferroelectric capacitor
A method of forming a low-voltage drive thin film ferroelectric capacitor includes the steps of depositing a ferroelectric and platinum thin film dielectric layer over a bottom electrode, annealing the dielectric layer, wherein a nanocomposite layer is formed including nanoparticles of platinum and forming a top electrode over the dielectric layer. An integrated circuit is also provided including a ferroelectric capacitor. The capacitor includes a bottom electrode formed over a substrate and a ferroelectric and platinum thin film nanocomposite dielectric layer formed over the bottom electrode, wherein the nanocomposite layer includes nanoparticles of platinum. A top electrode is formed over the dielectric layer.
This application is a divisional application of U.S. patent application Ser. No. 10/313,776 entitled “Method of Forming a Low Voltage Drive Ferroelectric Capacitor” filed Dec. 6, 2002, now U.S. Pat. No. ______, the entirety of which is hereby incorporated by reference herein.
FIELD OF THE INVENTIONThe present invention relates to integrated circuits and more particularly to integrated circuits including ferroelectric capacitors and method of manufacturing the same.
BACKGROUND OF THE INVENTIONFerroelectric materials such as lead zirconate titanate (PZT) are utilized in forming ferroelectric capacitors. These capacitors are then used in the formation of ferroelectric RAM (FRAM) devices due to the capacitors' electrical properties of retention, read/write endurance, and speed of the write cycle.
As memory integrated circuits become more dense, low voltage operation becomes increasingly more important. Traditional methods of improving the polarization switching property of ferroelectric materials, such PZT, focus on changing the composition of the PZT by adding soft dopants such as Vanadium, Lead or other dopants. Lanthanum (La) doping has also been proposed as described in B. Yang, et al., “Low Voltage Performance of Pb(Zr,Ti)O3 Capacitors Through Donor Doping,” Applied Physics Letters, Vol. 71(24), Dec. 15, 1997. However, this approach has only pushed the driving voltage of the polarization switching down to between 1.5 to 2.0 volts. This, in turn, restricts the required voltage drive of FRAM devices to that range.
More recently, it was reported in N. Duan et al., “Enhancement of Dielectric and Ferroelectric Properties by Addition of Pt Particles to a Lead Zirconate Titanate Matrix,” Applied Physics Letters, Vol. 77(20), Nov. 13, 2000, that large amounts of platinum particles in a PZT matrix can result in improvements in the dielectric and ferroelectric properties of a PZT composite. A powder PZT/Pt composite was prepared by conventional solid state reaction by sintering of mixed powders, and dense compacts were formed from the composite for testing. The PZT/Pt composite material proposed by the article, however, is not a thin film. A thin film generally has a thickness that ranges from 1.0 nanometer to 1 micron. (K. N. Tu, et al., Electronic Thin Film Science: For Electronical Engineers and Materials Sciences, p. 1 (Macmillan Publishing Co. 1992). Rather, the PZT/Pt composite is a bulk material that contains micron-sized or larger particles of platinum in the PZT matrix. The bulk material is not suitable for forming ferroelectric capacitors in integrated circuits or FRAM devices. In ULSI technology, a ferroelectric film thicker than 1 μm (10,000 Å) in an integrated circuit has several disadvantages, including: (1) high cost; (2) difficult integration when a device is scaled down; and (3) difficulty in achieving low voltage applications.
Therefore, there remains a need for an improved ferroelectric capacitor, particularly a thin film ferroelectric capacitor, with improved polarization switching characteristics under low voltage drive and a method of manufacturing the same.
SUMMARY OF THE INVENTIONA method of forming a low-voltage drive thin film ferroelectric capacitor includes the steps of depositing a ferroelectric and platinum thin film dielectric layer over a bottom electrode, annealing the dielectric layer, wherein a nanocomposite layer is formed including nanoparticles of platinum and forming a top electrode over the dielectric layer. An integrated circuit is also provided including a ferroelectric capacitor. The capacitor includes a bottom electrode formed over a substrate and a ferroelectric and platinum thin film nanocomposite dielectric layer formed over the bottom electrode, wherein the nanocomposite layer includes nanoparticles of platinum. A top electrode is formed over the dielectric layer.
Both the remnant polarization and dielectric constant of the thin film are significantly enhanced by the formation of platinum nanoparticles. A capacitor having low voltage drive of 1 volt or lower with high remnant polarization, such as at least 7 μC/cm2, can be achieved while repressing fatigue degradation and exhibiting no significant coercive field strength or leakage current affects. Low voltage drive FRAM devices may be fabricated using the nanocomposite thin film ferroelectric capacitor structure and fabrication method.
The above and other features of the present invention will be better understood from the following detailed description of the preferred embodiments of the invention that is provided in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGSThe accompanying drawings illustrate preferred embodiments of the invention, as well as other information pertinent to the disclosure, in which:
An integrated circuit including an improved ferroelectric capacitor, such as an integrated circuit including a FRAM memory device including ferroelectric capacitors, is described herein along with a method of manufacturing the same. The thin film nanocomposite ferroelectric capacitor described hereafter includes nanoparticles of a metal such as platinum and exhibits improved polarization switching and fatigue degradation characteristics.
As used herein, a “nanocomposite” is a composite of a first material with nanoparticles of a second material. There is no significant chemical interaction between the first and second materials and the composition can be distinguished by using a simple physical analysis method such as Scanning Electron Microscopy (SEM) or Transmission Electron Microscopy (TEM). By “nanoparticle” it is meant a particle having dimension between about 10 to 900 Å.
Referring first to
As shown in
Alternatively, the bottom electrode of the capacitor is then patterned and formed directly over the titanium layer 14. The bottom electrode may be formed from platinum or other conductive material such as tantalum, titanium, iridium, etc. In one exemplary embodiment, the bottom electrode includes a platinum layer 15 having a thickness of approximately 150 nm deposited over titanium layer 14. A Lanthanum Nickel Oxide (LaNiO3 or “LNO”) layer having a thickness of approximately 100 nm is then sputter deposited or otherwise formed over the platinum layer 15. It is believed that the use of LaNiO3 layer in the bottom electrode of the capacitor helps to facilitate the crystallization of the deposited ferroelectric thin film (described below) at lower temperatures and to repress the fatigue problem associated with PZT films, i.e., decay in the switching polarization after numerous switching cycles. Lower growth temperatures, in turn, conserve thermal budget and avoid the overgrowth of platinum clusters. If the clusters become larger than nanoparticles, the electrical properties of the thin film may be affected.
Referring to
The PZT-Pt dielectric layer 18 should be deposited such that the platinum is relatively uniformly distributed through the PZT material. In an exemplary embodiment, the layer 18 is deposited by sputtering (such as PVD sputtering, magnetron sputtering, e-beam sputtering, etc.), preferably by a co-sputtering process. The layer 18 may also be deposited using chemical vapor deposition (CVD), including plasma enhanced CVD (PECVD) or low pressure CVD (LVCVD), laser ablation or sol-gel process.
After the layer 18 is deposited, a rapid thermal anneal process is employed to crystallize the film 18 wherein nanoparticles of platinum are fixed in the PZT matrix. The annealing temperature depends upon the ambient type and annealing method. For example, a conventional furnace anneal may be used at a temperature between about 250-750° C., and preferably around 550° C., for a period of 3-10 minutes or longer, depending upon the temperature, and preferably in an oxidization ambience. A rapid thermal anneal between about 30-90 seconds at about 450-850° C. may also be employed. Still further, a pulsed laser anneal for a short time period, e.g., less than 30 seconds, at a relatively high temperature, e.g., greater than 700° C., may also prove effective for nanocomposite formation. In some cases, an inert gas may be used to dilute the ambient during oxidation while hydrogen gas may be used to control nanoparticle size. The appropriate pressure range is dependent on what oxidant and equipment are selected. It is anticipated that the total pressure may range from a few mTorr to 1.0 atm. The nanoparticle size is primarily controlled by process temperature and process time. The preferred target size is around 30 nm or less, but a range from approximately 10-70 nm should also provide reasonable polarization values in the nanocomposite ferroelectric film. The density of the metal nanoparticle in the nanocomposite dielectric material is less than 5.0×1013 per cm2. An in-situ stress sensor may be used to provide confident metrology for ensuring the size of the nanoparticles during nanoparticle formation, since film stress of the desired nanoparticle film is different from a non-formed or over-formed film.
As shown in
Therefore, an exemplary capacitor structure 22 may include, from top electrode to bottom electrode the following structures: Pt/PZT-Pt/LNO/Pt or Pt/LNO/PZT-Pt/LNO/Pt, with the latter structure exhibiting better leakage current characteristics as described below. After the top electrode 20 is formed, conventional process steps may be employed to complete the FRAM device, including forming and patterning metalization layers so that the memory cells can be accessed and passivating the structure.
A nanocomposite ferroelectric thin film of PZT-Pt where PZT was the majority and Pt the minority was formed on a LaNiO3 electrode in a co-sputtering system 100 diagrammatically illustrated in
As shown in
The above-described process was used to form a thin film Pt/PZT-Pt/LNO/Pt nanocomposite ferroelectric capacitor having 3.4% Pt, a thin film Pt/PZT-Pt/LNO/Pt nanocomposite ferroelectric capacitor having 3.4% Pt, and a thin film Pt/LNO/PZT-Pt/LNO/Pt nanocomposite ferroelectric capacitor having 4.0% Pt. These capacitors were tested as described below and compared against a pure thin film PZT ferroelectric capacitor.
The P-E (remnant polarization (Pr) versus electric field) of the fabricated Pt/PZT-Pt/LaNiO3/Pt capacitors was measured to show the polarization change of the ferroelectric material with an applied electric field. These measurements were made with an RT66A Standardized Ferroelectric Test System available from Radiant Technologies, Inc. with a virtual ground mode. For purposes of clarity, a brief discussion of remnant polarization is provided. The direction of the electric dipole of a ferroelectric material can be changed by applying an electric field. The polarization remains when the electric field is removed. This polarization is referred to as the “remnant polarization.” To eliminate the remnant polarization, an external opposite electric field (coercive field strength, Ec) is required. For practical use, the Ec of the ferroelectric material should be smaller than its breakdown field. Like ferromagnetic materials, a displacement-electric field (D-E) hysteresis loop exists. The minimum driving voltage is the minimum applied voltage required to make the ferroelectric domains align in the same direction and a remnant voltage should be observed. The remnant polarization allows the capacitor to store “1” and “0” states, with the positive polarization representing a “0” state and the negative polarization representing a “1” state.
The P-E curves of the fabricated capacitors (the pure PZT capacitor and the 3.4% and 4.0% platinum PZT-Pt thin film nanocomposite capacitors) are shown in
The capacitors made with the thin film PZT-Pt nanocomposites indicate higher remnant polarization (Pr) than the pure PZT capacitor for conventional driving voltages of 1.5-3 volts. It is also apparent from the figures that the switching performance (i.e., representing a switch from a “0” to a “1” state and vise versa) of the nanocomposite thin film PZT-Pt capacitors under low voltage drive (e.g., 1 volt or less) is much better than the low voltage drive performance of the pure PZT capacitor. This is particularly evident from the 2PR verses driving voltage plot of
Per the above, a nanocomposite thin film ferroelectric capacitor is provided and a method of manufacturing the same. A ferroelectric capacitor having low voltage drive of 1 volt or lower with high remnant polarization of at least 7 μC/cm2 can be achieved while repressing fatigue degradation and exhibiting no significant coercive field strength or leakage current affects. Low voltage drive FRAM devices may be fabricated using the nanocomposite thin film ferroelectric capacitor structure.
It is also believed that the above-described nanocomposite structure may enhance the piezoelectric and pyroelectric properties of the ferroelectric thin film. In addition to the use of the above-described nanocomposite thin film in forming ferroelectric capacitors and FRAM devices, it is believed that the nanocomposite thin film may be useful for membrane switches in piezoelectric electro-micromechanical systems and for uncooled infrared detectors and thermal sensors
Still further, it is believed that the Pt nanoparticles clusters induce lattice coherent compression in the thin film after thermal treatment, i.e., the increasing of lattice c/a (lattice constant ratio of c-axis to a-axis) of PZT or even SBT (Strontium Bismult Tanatalate). This phenomena was discovered by applicants using X-ray diffraction. This phenomena is illustrated in
Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly to include other variants and embodiments of the invention that may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.
Claims
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12. An integrated circuit including:
- a ferroelectric capacitor, comprising: a bottom electrode formed over a substrate; a ferroelectric and platinum thin film nanocomposite dielectric layer formed over said bottom electrode, wherein said nanocomposite layer includes nanoparticles of platinum; and a top electrode formed over said dielectric layer.
13. The integrated circuit of claim 12, wherein at least said bottom electrode includes a LaNiO3 layer.
14. The integrated circuit of claim 12, wherein said dielectric layer is a PZT-platinum thin film.
15. The integrated circuit of claim 14, wherein said PZT-platinum thin film is less than approximately 10% platinum.
16. The integrated circuit of claim 12, wherein said dielectric layer has a thickness of less than approximately 2000 Å.
17. The integrated circuit of claim 12, wherein said ferroelectric capacitor is formed over a CMOS structure formed on said substrate.
18. The integrated circuit of claim 12, wherein substantially all of said platinum nanoparticles have dimensions less than about 30 nm.
19. The integrated circuit of claim 12, wherein said top and bottom electrodes each include a LaNiO3 layer.
20. (canceled)
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25. An integrated circuit including a ferroelectric memory device, comprising:
- a substrate having formed thereon a plurality of address transistors;
- a plurality of ferroelectric capacitors each formed over a respective one of said address transistors, each of said capacitors including: a bottom electrode formed over said substrate; a dielectric layer including a ferroelectric and platinum thin film formed over said bottom electrode, wherein said nanocomposite layer includes nanoparticles of platinum; and a top electrode formed over said dielectric layer.
26. The integrated circuit of claim 25, wherein said bottom electrode includes a layer LaNiO3.
27. The integrated circuit of claim 25, wherein said dielectric layer is a PZT-platinum thin film.
28. The integrated circuit of claim 27, wherein said PZT-platinum thin film is less than approximately 10% platinum.
29. The integrated circuit of claim 27, wherein said dielectric layer has a thickness of less than approximately 2000 Å.
30. The integrated circuit of claim 27, wherein substantially all of said platinum nanoparticles have dimensions less than about 30 nm.
31. An integrated circuit including a plurality of ferroelectric capacitors each ferroelectric capacitor comprising:
- a bottom electrode including a LaNiO3 layer formed over a layer of platinum;
- a dielectric layer including a ferroelectric and platinum thin film formed over said bottom electrode, wherein said nanocomposite layer includes nanoparticles of platinum; and
- a top electrode including a layer of platinum formed over said dielectric layer.
32. The integrated circuit of claim 31, wherein the top electrode of each capacitor further comprises a LaNiO3 layer formed between said platinum layer and said dielectric layer.
33. The integrated circuit of claim 32, wherein said dielectric layer has a thickness of less than approximately 2000 Å and wherein substantially all of said platinum nanoparticles have dimensions less than about 30 nm.
34. The integrated circuit of claim 33, wherein said bottom electrode is formed over a layer of titanium.
35. The integrated circuit of claim 31, wherein said dielectric layer has a thickness of less than approximately 2000 Å and wherein substantially all of said platinum nanoparticles have dimensions less than about 30 nm.
36. The integrated circuit of claim 35, wherein said bottom electrode is formed over a layer of titanium.
37. (canceled)
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Type: Application
Filed: Oct 18, 2005
Publication Date: Feb 23, 2006
Inventors: Yuan-Chieh Tseng (Kaohsiung), Chao-Hsiung Wang (Hsinchu), Tai-Bor Wu (Hsinchu)
Application Number: 11/253,178
International Classification: H01L 29/00 (20060101);