Field of the invention
The present invention is related to the realization of ferroelectric devices and in particular of a simplified bottom electrode stack for ferroelectric memory cells. More particularly, the invention is related to ferroelectric memory cells where the ferroelectric capacitor is positioned directly on top of a contact plug.
State of the art
FRAM (Ferroelectric RAM) is random access memory that combines the fast read and write access of dynamic RAM (DRAM) - the most common kind of personal computer memory - with the ability to retain data when power is turned off (as do other non-volatile memory devices such as ROM and flash memory). Because FRAM is not as dense (i.e., cannot store as much data in the same space) as DRAM and SRAM, it is not likely to replace these technologies. However, because it is fast memory with a very low power requirement, it is expected to have many applications in small consumer devices such as personal digital assistants (PDAs), handheld phones, power meters, and smart card, and in security systems. FRAM is faster than flash memory. It is also expected to replace EEPROM and SRAM for some applications and to become a key component in future wireless products.
The formation of a crystalline ferroelectric film typically requires high temperature treatment in oxygen ambient. The film can be prepared by different techniques, such as spin-on, physical vapor deposition (PVD), chemical vapor deposition (CVD), and metal organic chemical vapor deposition (MOCVD). MOCVD may be performed in a two-step process, wherein in a first step the ferroelectric film is deposited at lower temperature, and afterwards in a second step the ferroelectric film is crystallized at a higher temperature, e.g. a temperature higher than 400°C in oxygen ambient. Alternatively, MOCVD may be performed in a one-step process at a higher temperature in oxygen ambient, wherein deposition and crystallization of the ferroelectric film occur simultaneously.
Examples of ferroelectric materials include, but are not limited to SrBi2
(PZT) and (Bi, La)4
(BLT). All ferroelectric layers ultimately incorporate oxygen. This oxygen forms a part of the so-called "perovskite" crystal structure, which is typical for ferroelectric films. An example of a ferroelectric memory cell with an irridium bottom electrode is shown US6238932
, related to a specific process for depositing the bottom electrode. Document EP1096570
describes a method for producing a ferroelectric capacitor with a platinum bottom electrode. A problem with Pt is however that it is not a good barrier to oxygen diffusion.
Aims of the invention
A method of fabricating ferroelectric memory cells that avoids the oxidation of a metal electrode layer while forming a crystalline ferroelectric layer on top of it at elevated temperature in oxygen is desirable. Also desirable is a method of fabricating a ferroelectric memory cell wherein a crystalline ferroelectric film is formed on a conductive layer while preserving the conductive properties of this layer, or a method wherein a crystalline ferroelectric film is formed directly on an oxygen diffusion barrier layer. A method of forming a ferroelectric capacitor with a simplified bottom electrode stack is also desirable.
Summary of the invention
Accordingly, a method for forming a crystalline ferroelectric layer on a metal electrode in oxygen is provided that avoids the oxidation of the underlying metal electrode. The method comprises the crystallization of the ferroelectric layer in an atmosphere having a reduced oxygen partial pressure. In the method, the total pressure in the process chamber is controlled to prevent evaporation of metal or metal oxide compounds from the ferroelectric film as it forms. The oxygen partial pressure (pO2
) is kept sufficiently low so as to prevent the oxidation of the metal electrode, yet sufficiently high so as to prevent the reduction of the chemical elements constituting the ferroelectric film at the processing temperature. The oxidation of a metal electrode depends not only on the oxygen partial pressure, but also on the processing temperature and on the reduction potential of the metal electrode. The higher the metal reduction potential is, the higher the minimum temperature at which the metal oxidizes.
The method of preferred embodiments permits the use of a simplified bottom electrode barrier structure for stacked ferroelectric memory cells.
In an embodiment the bottom electrode comprises a single layer, which remains in its metallic form, is conductive and forms an oxygen diffusion barrier. In a preferred embodiment where Ir is the bottom electrode and SrBi2
(SBT) is the ferroelectric layer, simple stacks comprising SBT/Ir/contact plug and SBT/Ir/TiN/contact plug may be formed.
A method is provided as disclosed in appended claim 1.
In an aspect of the invention, where Ir is the bottom electrode and SrBi2
(SBT) is the ferroelectric layer, simple stacks comprising SBT/Ir/contact plug and SBT/Ir/TiN/contact plug may be formed. The process temperature ranges from about 600C to 800C, preferably from 650 to 750C. The partial oxygen pressure range log(pO2
) is from about -3.5 to about -1.
In an aspect of the invention, where Ir is the bottom electrode and (Bi, La)4
(BLT) is the ferroelectric layer, simple stacks comprising BLT/Ir/contact plug and BLT/Ir/TiN/contact plug may be formed. The process temperature ranges from about 600C to 800C, preferably from 650 to 750C. The partial oxygen pressure range log(pO2
) is from about -3.5 to about -1.
In an aspect of the invention, where Ru is the bottom electrode and Pb(Zr,Ti)O3
is the ferroelectric layer, simple stacks comprising PZT/Ru/contact plug and PZT/Ru/TiN/contact plug may be formed. The process temperature ranges from about 400C to 700C, preferably from 550C to 650C. The partial oxygen pressure range log(pO2
) is from about -9 to about -12, at a process temperature ranging from 575C to 625C.
A ferroelectric device is described, the ferroelectric device comprising at least a conductive top electrode, a conductive bottom electrode, and in between a ferroelectric layer, the conductive bottom electrode comprises a single substantially free of oxygen layer, being in direct contact with the ferroelectric layer, this single oxygen-free layer being conductive and forming a barrier to oxygen diffusion.
The ferroelectric layer may comprise Pb(Zr,Ti)O3
(PZT). The bottom electrode may comprise a single non-oxidised layer consisting of Ru. The bottom electrode can further comprise an adhesion layer comprising TiN.
The ferroelectric layer may comprise SrBi2
(SBT). The bottom electrode may comprise a single non-oxidised layer consisting of Ir. The bottom electrode can further comprise an adhesion layer comprising TiN.
The ferroelectric layer may comprise (Bi,La)4
(BLT). The bottom electrode may comprise a single non-oxidised layer consisting of Ir. The bottom electrode can further comprise an adhesion layer comprising TiN.
Short description of the drawings
Fig. 1 provides a cross sectional view of a memory cell showing a ferroelectric capacitor 9 stacked on a selection transistor 2.
Figure 2a provides a Log(pO2)-T diagram for the different metal/metal oxide elements in a SBT/Ir system.
Figure 2b provides a schematic graph showing the logarithmic of the partial oxygen pressure as function of the absolute temperature for a ferroelectric layer, an Ir bottom electrode layer, and a Pt bottom electrode layer.
Figure 2c provides a Log(pO2)-T diagram for the different metal/metal oxide elements in a PZT/Ru system.
Figure 3a provides stability curves for Ir and Bi, with experimental data points for Ir, IrO2, and SBT stability.
Figure 3b provides stability curves for Ir, Bi, La, with experimental data points for Ir, IrO2, and LBT stability.
Figure 4 provides a cross sectional SEM image of an Ir/TiN sample annealed at 700°C, 30 min in 100% O2. Lines delineating the different layers have been added to the micrograph.
Figure 5 provides a cross sectional SEM image of a Ir/TiN sample annealed at 700°C, 30 min in 10 ppm O2 in N2. Lines delineating the different layers have been added to the micrograph.
Figure 6 provides a cross sectional SEM image of 2 SBT/Ir/TiO2 samples annealed at 700°C, 30 min in 100 ppm O2 in N2. Lines delineating the different layers have been added to the micrograph.
Detailed description of the invention
As depicted in Figure 1, a ferroelectric memory cell typically comprises a ferroelectric capacitor 9
and a selection transistor 2.
The ferroelectric capacitor 9
comprises a stack of a conductive bottom electrode 10,
a ferroelectric film 11,
and a conductive top electrode 12.
The ferroelectric memory cell is programmed by applying an electrical signal to the conductive top and bottom electrodes across the ferroelectric film 11.
When an electric field is applied to a ferroelectric crystal, the central atom of the ferroelectric compound moves in the direction of the field. Internal circuits sense the charge required to move the atom. When the electric field is removed from the crystal, the central atom stays in position, preserving the state of the memory. The selection transistor 2
comprises a source 3
and a drain 4
junction formed in a semiconductor substrate 1 separated by a channel region 6.
A gate 5
is formed above the channel region 6.
The gate is covered with an insulating layer 7
which electrically isolates the selection transistor 2
from the ferroelectric capacitor 9.
In this insulating layer a opening 8 is formed to contact the ferroelectric capacitor 9
with the one of the electrodes of the transistor 2.
In the fabrication of a ferroelectric capacitor, the ferroelectric film 11
is sandwiched between a top electrode 12
and a bottom electrode 10.
In general the bottom electrode must fulfill several requirements. The parts of the bottom electrode exposed to the oxygen ambient are preferably stable in oxygen at high temperature or form a conductive oxide after exposure of the electrode material to an oxygen ambient. Suitable materials include noble metals such as platinum and conductive electrode materials such as IrO2
. In this manner, the bottom and top electrode layers remain conductive and an electrical signal can be conveyed to the ferroelectric film in order to program the memory cell..
In stacked ferroelectric memory cells, the ferroelectric capacitor 9
is preferably placed on top of a contact 8
in order to conserve area. The contact can be formed from a stack of layers. These layers, however, are not considered part of the bottom electrode 10
of the capacitor structure itself, as these contact layers 8
are used for contacts on the chip and to contact the bottom electrode 10.
The contact 8
connects the memory capacitor 9
with the selection transistor 2.
The contact 8
comprises, for example, a plug fill material 81
such as tungsten or, polysilicon, and can further comprise an adhesion layer 82
on top of the plug fill material 81.
This adhesion layer 82
can furthermore prevent interaction of material of the bottom electrode 10
with contact material 81.
e.g. prevent the formation of a silicide due to the interaction of Ir from the bottom electrode with Si of the plug fill material 81.
This adhesion layer consists of nitrides of Ti, Ta, Al or alloys thereof. The transistor and the contact are commercially available on chips. The characteristics of these elements as used in the "digital" or "logic" circuitry on chip must not be influenced by the formation of the memory cells in subsequent processing, however .The bottom electrode 10
is conductive when exposed to a high temperature oxygen containing ambient. The bottom electrode 10 forms a barrier to diffusion of oxygen from the oxygen containing ambient towards the underlying layers, such as the contact 8
in order to avoid oxidation of the materials used to form the contact as a non-conductive layer would then be formed. The bottom electrode 10 does not react, e.g. oxidize, the underlying layers, such as the layers of the contact 8.
Because of these additional requirements, multiple layers, each from a different material, are used to constitute the bottom electrode. Namely, the well-studied Pt electrode (which does not oxidize under typical process conditions) cannot be used alone because of its insufficient oxygen barrier properties. In contrast, it is known that IrO2
on top of TiN results in the formation of a TiO2
interfacial layer On the other hand, metal oxide films as IrO2
, which have good oxygen diffusion barrier properties, act as powerful oxidizers when in contact with a plug or with an adhesion material like TiN. Hence, the addition of a metallic Ir or Ru layer underneath the IrO2
layer to separate the IrO2
layer from the contact material 8
is required. Moreover, the use of layers comprising solely Ir or Ru, which are believed to be good oxygen barrier layers, is not possible because they oxidize in an uncontrolled way during the processing in oxygen of the ferroelectric layer deposited on top of them.
Typically, very complex bottom electrode-barrier structures are used to avoid oxygen diffusion to the plug during the processing of the ferroelectric layer as Pt/IrO2
/Ir(/TiN) where TiN is an additional layer which protects the contact plug from interaction with the electrode stack or improves adhesion. See, e.g., D. Jung et al., Technical Digest IEDM (International Electron Devices Meeting), San Francisco, CA, December 10-13, 2000, page 00-801
, paper 34.4.1. This TiN layer can be part of the contact 8
in the figure 1) or can be formed on top of the contact 8.
The large number of processing steps required to achieve a complex bottom electrode structure, besides being cost inefficient and environmentally unfriendly, imposes stringent requirements on the fabrication of stacked ferroelectric memory cells.
Description of a preferred embodiment of the invention
The following description and examples illustrate a preferred embodiment of the present invention in detail. Those of skill in the art will recognize that there are numerous variations and modifications of this invention that are encompassed by its scope. Accordingly, the description of a preferred embodiment should not be deemed to limit the scope of the present invention.
In conventional stacked ferroelectric memory cells, the use of an electrode layer comprising solely, for example, Ir or Ru is typically not feasible, even though these metals are expected to have good oxygen barrier properties. In fact, these metals oxidize in an uncontrolled way during processing at elevated temperature in oxygen of the ferroelectric layer deposited on top of them. The formation of the ferroelectric film and/or the crystallization of the ferroelectric film take place in a controlled environment, i.e., a process chamber.
A method for forming a crystalline ferroelectric layer is provided that avoids the oxidation of the underlying electrode, such that a simplified electrode/barrier structure can be employed. The method involves the crystallization of the ferroelectric layer in a reduced oxygen partial pressure. An advantage of this method is that a bottom electrode 10
can be formed comprising a non-oxidized layer in contact with the ferroelectric layer 11, while this non-oxidised or metallic layer is conductive and forms an oxygen diffusion barrier.
In the method of preferred embodiments, the total pressure in the process chamber is set so as to prevent evaporation of metal or metal oxide compounds from the ferroelectric film. For example, in a two-step process, a fixed total pressure of about 1 atm can be employed. During the second step of the two-step process, i.e. the annealing step, the partial oxygen pressure (pO2
) range is selected as a function of the annealing temperature such that the electrode metal (e.g., Ir or Ru) is not oxidized, thus defining the upper bound of the selected range, and such that the metal compounds constituting the ferroelectric layer do not undergo any chemical reduction, thus defining the lower bound of the selected range of the oxygen partial pressure. The method is also applicable for a one-step process wherein deposition of the ferroelectric film and crystallization of this film occurs simultaneously because of the higher deposition temperature and the presence of the oxygen in the process chamber.
The oxidation of a metal electrode is a function not only of the oxygen partial pressure, but also of the processing temperature and the reduction potential of the metal electrode. The higher the metal reduction potential is, the higher the minimum temperature at which the metal electrode oxidizes. Generally, annealing temperatures from about 400°C or lower to about 800°C or higher are preferred.
For a given metal, it is possible to determine from thermodynamic calculations a good estimate of the oxygen partial pressures as a function of temperature above which a metal oxide is stable and below which the reduced metal form is stable. These calculations can be based on Richardson-Ellingham diagrams that show the relative stability versus temperature for different metal oxides at 1 atm total pressure. See, e.g., "Thermodynamics in Material Science," R.T. De Hoff, McGraw Hill, Inc, 1993
. These data can be recalculated into an oxide stability curve (or metal oxide decomposition curve) in an oxygen partial pressure versus temperature diagram. Figure 2a provides such a diagram for the specific case wherein the bottom electrode is Ir and the ferroelectric material is SrBi2
(SBT). Although some approximations are made in the calculation of such curves, it is observed that IrO2
is less stable than any of the metal oxides forming the complex SBT oxide, as shown by the stability curve of Ir/IrO2
in Figure 2a, which lies above all the others. Bi2
is the least stable oxide constituting the complex SBT oxide, as shown by the stability curve of Bi/Bi2
in Figure 2a which lies above the ones for Ta/Ta2
and Sr/SrO. Figure 2b provides a schematic graph showing the logarithmic of the partial oxygen pressure as function of the absolute temperature for a ferroelectric layer, an Ir bottom electrode layer, and a Pt bottom electrode layer. As is depicted in the graph, Ir is more sensitive to oxidation than Pt as illustrated by the fact that the stability curve of Ir corresponds to a much lower oxygen partial pressure than the stability curve of Pt. A smaller process window of acceptable partial oxygen pressure conditions at a given temperature is available for Ir compared to Pt.
Figure 2c provides such a diagram for the specific case wherein the bottom electrode is Ru and the ferroelectric material is Pb(Zr,Ti)O3
Focusing only on the curves for Ir/IrO2
in Figure 2a, and based on improved approximations (see, e.g., CRC, 77th edition, CRC press; S.Y. Cha and H.C. Lee, Jpn. J. Appl. Phys, Vol. 38 (1999), page 1128
), the relevant curves are recalculated in Figure 3a. In Figure 3a at 700°C, which is the temperature nowadays, used for the crystallization of SBT films, there is a working window for pO2
where both Ir and Bi2
are stable. This window consists of the partial pressure of oxygen below the Ir/IrO2
(curve) and above the Bi/Bi2
curve. Figure 3b provides stability curves for Ir, Bi, La, with experimental data points for Ir, IrO2
, and LBT stability. As the stability curve for La is below the stability curve of Bi, Bi is more likely then La to form a metaloxide. For the LBT/Ir system the process window will be determined by stability curves of Ir and Bi.
It is generally preferred that the pO2
pressure range is above the pO2
present in bulk (i.e., so-called pure) N2
, as used in a typical semiconductor fabrication. If the process window requires a partial oxygen pressure below this practical limit, then industrial processing might be difficult. Figure 2c provides stability curves for Ru, Pb as used in a PZT/Ru system In this case the process window is below the pO2
present in bulk (i.e., so-called pure) N2
. Figure 2c provides such a diagram for the specific case wherein the bottom electrode is Ru and the ferroelectric material is Pb(Zr,Ti)O3
(PZT). In this case other gases are used or added to obtain such a low partial pressure of oxygen. Examples of such gasses are reducing gasses such as CO, as will be appreciated by a person skilled in the art. A process window ΔpO2 for a PZT/Ru system include a temperature range between 400°C to 700°C, preferably 550°C to 650°C. A process window for a PZT/Ru system include a oxygen partial pressure range of log (p02
) between -9 and -12. A process window for a SBT/Ir system include a temperature range between 650°C to 750°C. At a process temperature TΔpO2
, the oxygen partial pressure range ΔpO2 of log (p02
) is between -1.5 and -3.
Several experiments were performed to define such a working oxygen partial pressure window. The following conditions and materials were employed in the experiments: an SBT ferroelectric layer, an Ir bottom electrode layer, an Ir/TiO2
or Ir/TiN bottom electrode-adhesion structure, a crystallization temperature of 650° and 750°C, and an oxygen partial pressure during crystallization of 100% O2
, 10% O2
(prepared by mixing O2
gas), 100 ppm O2
(using a premixed gas mixture), 10 ppm O2
using 10% of a 100 ppmO2
gas mixture in 90% N2
, and 100% N2
(flow limited by the oxygen content of bulk N2
to approximately 0.07 ppm O2
or Ir/TiN stacks were annealed at a temperature between 650 and 750°C with different oxygen partial pressures. Note that the Ir/TiO2
stack is the product of the formation of an Ir layer on an already formed TiO2
bottom electrode layer. This TiO2
bottom electrode layer is not the product of the oxidation of a Ti layer during the formation of a PZT layer.
Experimental data points indicating the presence of Ir or IrO2
are reported in Figure 3. The stability of IrO2
at 10% O2
is confirmed at all of the reported temperatures by Auger spectroscopy. Scanning electron microscopy (SEM) investigations on Ir(10
) stacks annealed in 100% O2
at 700°C revealed large non-uniform IrO2
) grains, as depicted in Figure 4. Figure 5 shows that formation of IrO2
grains is not observed when Ir(10
) is annealed at the same temperature in 10 ppm O2
. Similar results are observed for annealing at 100 ppm O2
. An optimal window of temperature and partial pressure of O2
) for Ir may be determined from the data. Assuming an ambient pressure of 760 torr (1 atm), a pO2
exceeding that of a bulk N2
gas (containing approximately 0.07 ppm O2
) is preferred. However, in certain embodiments, a nonzero partial pressure of oxygen less than that observed in a bulk N2
gas as described above may be acceptable. The pO2
is preferably within the range at which Ir is stable, but less than the pO2
at which IrO is stable, at the specified temperature. A process window for a SBT/Ir system include a temperature range between 600°C to 800°C, preferably 650°C to 750°C. A process window for a SBT/Ir system include a oxygen partial pressure range of log (p02
) between -1.5 and -3. A process window for a SBT/Ir system include a temperature range between 650°C to 750°C, and a oxygen partial pressure range of log (pO2
) between -1 and -3.5 Particularly preferred conditions for Ir include a temperature of between about 667°C and 717°C, a total pressure of about 760 torr, and a partial pressure of from about 0.0532 mtorr and less than or equal to about 2.81 torr. If it is preferred to conduct the process at a temperature of less than about 667°C or greater than about 717°C, the partial pressure of oxygen may be adjusted up or down accordingly so as to remain at a nonzero value below the Ir stability limit (depicted by the smaller diamonds in Figure 3a) at that temperature. Preferred combinations of pO2
and temperature may also be determined for Ru using the same methodology once the stability limit is determined. The preferred combination will depend upon the particular metal selected.
An SBT layer 11
was deposited on top of the bottom electrode stacks. An SBT(11
) sample annealed in 1 atm O2
will show, a substantial amount of interfacial IrO2 (13)
to be formed. In contrast, for anneals at 10-100 ppm O2
, a substantially oxide free SBT(11
) interface is maintained, as shown in Figure 6. From these results, it can be concluded that the presence of an SBT layer does not prohibit the oxidation of the Ir electrode and that the low pO2
is responsible for the formation of a controlled SBT/Ir structure. For the SBT/Ir samples, no optically visible change in the SBT material occurred for anneals in the different pO2
ambients, except for the anneal at 750°C in a 100% N2
flow (the triangle in Figure 3a). At the latter condition, a gray metallic shine appeared at the surface of the structure. As expected from thermodynamical calculations, the Bi2
layer is expected to be reduced and, because of the high temperature, the Bi is expected to melt. From merely theoretical calculation, it is impossible to determine the exact position of the metal oxide stability curves shown in Figure 3a and 3b. A partially reduced form of Bi2
, e.g. BiO, may form at oxygen partial pressures where Bi2
is predicted by theoretical calculations to be stable. The existence of intermediate phases, such as fluorite or pyrochlore, is well known and they may influence the nucleation mechanism. In view of these considerations, the lower bound for pO2
is preferably controlled in such a way as to prevent reduction of the metal constituting the ferroelectric material.
The above-described method may be employed to fabricate ferroelectric capacitors of the type Ir/SBT/Ir with hysteresis characteristics comparable to those of a Pt/SBT/Pt ferroelectric capacitor. Annealing ferroelectric films in reduced oxygen pressure ambient has been proposed before, but always in order to improve the film morphology, structure, or texture. For example, anneals at a reduced partial pressure of oxygen in a N2
ambient have been proposed in order to provide improved PZT film texture and to lower film crystallization temperature. See Fujimori et al, "Low-temperature crystallization of sol-gel-derived Pb(Zr,Ti)O3 thin films" in the Japanese Journal of Applied Physics Vol 38, 1999, p5346-5349
, where a reduced partial oxygen pressure is used to promote outgassing of the solvents used during spin coating of the sol-gel. In other publications wherein it has been reported to anneal SBT films at reduced oxygen total pressure with the goal of improving film quality and lowering crystallization temperature, a large loss of Bi element was reported. See Ito, Jpn. J. Appl. Phys., Vol. 35 (1996), pp. 4925-4929
; and Ogata et al., Extended Abstracts of the 1997 International Conference on Solid State Devices and Materials, Hamamatsu, 1997, page 40-41
In contrast, reduced pO2
and a fixed total pressure in the method of preferred embodiments is employed to avoid the oxidation of the underlying electrode metal, and ultimately to permit the use of a simplified bottom electrode barrier structure for stacked ferroelectric memory cells.
In the preferred embodiment described in detail above, where Ir is the bottom electrode and SBT the ferroelectric layer, the following simple stacks can be prepared: SBT/Ir/contact plug, and SBT/Ir/TiN/contact plug. As depicted in the cross-sectional SEMs from investigations of oxygen-annealed Ir/TiN samples formed on a silicon wafer 1
as shown in Figure 4 and Figure 5, no oxidation of the TiN (81)
layer is observed for anneals at 1 atm total pressure and 100% O2
or 10 ppm O2
. These results demonstrate that Ir exhibits good oxygen barrier properties. Because of the low oxygen partial pressure employed in the crystallization process described above, layers comprising Ir alone can serve as electrode materials and barrier layers. Often in the formation of ferroelectric stacked memory cells, one or more additional layers, e.g. TiN, are placed between the electrode material and the contact plug in order to improve adhesion or to avoid material interaction.