Abstract:
Unfavorable interactions of ferroelectric dielectric layers with silicon, intermetallic dielectrics, and other materials in metal-oxide semiconductor devices have discouraged the use of ferroelectric memory devices. This invention provides a zirconium titanate barrier layer with high insulating and low leakage characteristics. The barrier layer is not reactive with silicon or other materials used in metal-ferroelectric-semiconductor devices. These thermally stable layers should facilitate the integration of ferroelectric materials into memory and other semiconductor devices.

Description:
FIELD OF THE INVENTION 
     This invention relates to barrier layers for semi-conductor devices, specifically to barrier layers for ferroelectric memory devices. 
     BACKGROUND OF THE INVENTION 
     Ferroelectric memories (FeRAM) combine advantages of both nonvolatile read only memory (ROM) and high speed random access memory (RAM). Two types of ferroelectric memory have been proposed in the prior art. One type, one transistor and one capacitor ( 1 T- 1 C) memory, has a structure similar to that of the conventional dynamic random access memory. Such prior art memories are described, for example, in U.S. Pat. No. 5,767,541 to Hanagasaki, incorporated herein by reference. Fabrication of the  1 T- 1 C ferroelectric memory is relatively easy because the ferroelectric capacitor stacks are separated from the complementary metal-oxide semiconductor (CMOS) transistor layers by a thick interlayer dielectric film. However, while the ferroelectric medium in the capacitor is separated from CMOS transistor layers, there remain several critical integration related issues affecting the reliability of FeRAM. The interaction of a ferroelectric, such as PbZr 1−x Ti x O 3  (PZT), capacitor with intermetallic dielectrics (IMD) and damage to the ferroelectric capacitor stacks due to hydrogen attack during IMD and subsequent metal and plug deposition are the most serious problems in the manufacture of high density FeRAM. Therefore, an encapsulating barrier layer between the PZT capacitor and IMD layer with high insulating and low leakage characteristics is necessary. At present, several metal oxides, such as TiO 2 , have been tested for this application; however, their performance is not satisfactory for high density FeRAM. 
     Another type of ferroelectric memory is the one transistor ( 1 T) field effect type. Such devices are described, for example, by U.S. Pat. No. 3,832,700 to Wu, et al, incorporated herein by reference. At present, there is no reliable fabrication process available for this type of ferroelectric memory. Such a process would involve direct deposition of a ferroelectric layer on a silicon substrate in order to create a metal-ferroelectric-semiconductor field effect transistor (MFS FET). However, fabrication of a MFS FET incorporating PZT is extremely difficult because of the interaction of the PZT layer with Si and the formation of a low dielectric constant SiO 2  layer between the PZT layer and the Si substrate. For example, diffusion of oxygen through the layers of the memory device to the silicon substrate may result in the formation of an SiO 2  layer between the substrate and the adjacent ferroelectric layer, disrupting electrical communication between the substrate and the PZT layer. Lead atoms may subsequently leach out of the ferroelectric layer into the newly formed glassy SiO 2  layer, disrupting the network of silica bonds in the SiO 2  layer and further altering the electrical performance of the transistor. Several attempts have been made to fabricate MFS FETs using various barrier layers such Si 3 N 4 , CeO 2 , and Y 2 O 3 ; however, none of them have produced satisfactory and reproducible results. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention is a method of preventing detrimental chemical and electrical interaction between two materials, especially a ferroelectric material and an adjacent material in a multi-layered electrical device. The method comprises disposing an electrically insulating, thermally and chemically stable barrier layer between the ferroelectric material and the adjacent, non-ferroelectric material. In one embodiment, the barrier layer restricts current flow and oxygen diffusion between the ferroelectric material and the non-ferroelectric material. In addition, the barrier layer is not reactive with the non-ferroelectric material. In this embodiment, the device may include a ferroelectric transistor, and the non-ferroelectric layer may be a semiconductor substrate. In another embodiment, the device may include a ferroelectric capacitor, and the barrier layer may protect the capacitor from damage during formation of the non-ferroelectric layer. In either of these embodiments, the barrier layer may comprise a mixed transition metal oxide which may in turn comprise ZrTiO 4  or SrTiO 3 , and the ferroelectric material may comprise PbZr 1−x Ti x O 3 , SrBi 2 Ta 2 O 9 , and BaSr x Ti 1−x O 3 . The barrier layer should have a high dielectric constant, preferably at least 10, more preferably at least 20, and even more preferably at least 30. 
     In another aspect, this invention is a multi-layer electrical device comprising a ferroelectric and a non-ferroelectric material and an electrically insulating, thermally stable barrier layer disposed between them. The multi-layer electrical device may be produced according to the method described above. 
     To form a barrier layer according to this invention, a high dielectric constant barrier layer disposed between the ferroelectric layer and Si substrate is preferred. The barrier layer should also exhibit low leakage current, a high breakdown voltage, and a low diffusivity for oxygen and other atoms comprising the ferroelectric material. For both  1 T and  1 T- 1 C devices, the layer should also exhibit high thermal and chemical stability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1A is a cross-sectional view of a  1 T- 1 C FeRAM device. 
     FIGS. 1B-1G are cross-sectional views of a  1 T- 1 C FeRAM device at various stages of production. 
     FIG. 2 is a cross-sectional view of a one transistor FeFET memory device. 
    
    
     DETAILED DESCRIPTION 
     In one preferred embodiment, the invention employs thermally stable dielectric zirconium titanate (ZrTiO 4 ) films as barrier layers to solve the problems described above. Zirconium titanate thin films have a high dielectric constant (ε˜30) and very low leakage current and are thermally stable up to 700° C. In addition, zirconium titanate layers are known to be excellent oxygen barriers. The similar compositions of zirconium titanate and PZT materials prevent detrimental chemical and electrical interactions between the two materials. These films can be easily deposited by either metal organic chemical vapor deposition (MOCVD) or reactive sputtering. MOCVD is a preferred method for the manufacture of high density FeRAM in order to achieve conformal coverage. Therefore, zirconium titanate thin films are excellent candidates for the barrier layer for both  1 T- 1 C and  1 T FeRAMs. Zirconium titanate barrier layers inhibit interactions of PZT with IMD layers in  1 T- 1 C devices and minimize formation of SiO 2  layers at the interface with the Si substrate in  1 T FeRAM devices. In addition, the barrier layer is not reactive with other adjacent layers in the device. Other mixed transition metal oxides, for example, SrTiO 3  and some other perovskite materials, can also be used to produce the barrier layers of the invention. These materials can also serve as barrier layers for other ferroelectric materials, for example, SrBi 2 Ta 2 O 9  and BaSr x Ti 1−x O 3 . The barrier layer thus formed is thermally and chemically stable, especially with respect to oxide formation, and has a high dielectric constant, preferably at least 10, more preferably at least 20 and even more preferably at least 30. 
     A  1 T- 1 C device  10  according to the invention is shown in FIG.  1 A. The memory device  10  comprises transistor  17  and ferroelectric capacitor  23  deposited in layers on a silicon or other semiconductive substrate  12 . A barrier layer  27  is disposed over the capacitor  23 . Techniques for the manufacture of prior art  1 T- 1 C devices are well known in the art. One skilled in the art will recognize that such techniques can be easily adapted to produce a  1 T- 1 C device according to the invention. The substrate  12  is processed to provide a source  14  and drain  16  region (FIG.  1 B). A dielectric  18  and metal gate  19  are disposed between the source  14  and drain  16  to form transistor  17  (FIG.  1 C). An interlayer dielectric  20  (ILD) is deposited, covering the transistor  17  and the substrate  12 , and plugs  21  and  22  are disposed within the ILD to contact the source  14  and drain  16 , respectively (FIG.  1 D). The materials for a bottom electrode, usually platinum, a ferroelectric layer, and a top electrode, also usually platinum, are laminated sequentially on the surface. The ferroelectric layer preferably comprises PZT. These layers are then etched together to form ferroelectric capacitor  23  comprising a bottom electrode  24 , ferroelectric layer  25 , and a top electrode  26  (FIG.  1 E). Barrier layer  27  is then deposited around at least the exposed surfaces of the capacitor. However, while it is only necessary to dispose the barrier layer  27  over the capacitor, if the barrier layer is deposited using standard techniques (e.g., MOCVD, see below), then it will cover not only the capacitor  23  but the other exposed surfaces of the device, as shown in FIG.  1 F. Portions of the barrier layer  27  may be selectively removed to re-expose these other surfaces. In FIG. 1G, for example, most of the barrier layer  27  has been left intact, but a section has been removed to enable electrical communication between plug-to-plug contact  34  and plug  21 . 
     As noted above, ZrTiO 4  is preferred for the barrier layer  27 . The barrier layer may be deposited via MOCVD or reactive sputtering. MOCVD is preferred to achieve conformal coverage of the sides as well as the upper surface of the capacitor. If necessary, the barrier layer can be etched following deposition to achieve a desired geometry. The device is completed according to prior art techniques. An intermetallic dielectric  28  (IMD) is deposited covering the barrier layer (FIG.  1 F). The plug-to-plug contact  34  and a third plug  36  are disposed within the dielectric layers  20  and  28  to contact the plug  21  and top electrode  26 , respectively (FIG.  1 G). The plug-to-plug contact  34  and third plug  36  are connected to a word line  32  and bit line  30 . Other insulating layers and conductors are disposed in the device according to techniques well known in the art (FIG.  1 A). 
     A  1 T device  40  according to the invention is shown in FIG.  2 . Barrier layer  48  is disposed between a silicon or other semiconductive substrate  12  and ferroelectric layer  50 . Deposition techniques for prior art  1 T devices are well known in the art. One skilled in the art will recognize that such techniques can be easily adapted to produce a ferroelectric FET according to the invention. The substrate  12  is processed to form the source and drain regions  14  and  16 . Barrier layer  48  is disposed between the source and drain regions  14  and  16 . The barrier layer preferably comprises a metal oxide and more preferably comprises ZrTiO 4 . Again, the barrier layer may be deposited by MOCVD or reactive sputtering. 
     Ferroelectric layer  50 , preferably comprising PZT, and gate electrode  52  are deposited sequentially over the barrier layer. The layers can be masked and etched individually or together to expose the source and drain or achieve a desired geometry. Conductors must also be provided to connect the drain and source regions  16  and  14  and the gate electrode  52  to external circuitry. For example, source and drain electrodes  54  and  56  may be deposited on source and drain regions  14  and  16 . The barrier layer  48  and ferroelectric layer  50  may extend beyond the drain and source regions, as shown in FIG.  2 . 
     The methods described above for the production of  1 T- 1 C and ferroelectric FET devices are meant to be exemplary and not limiting. The methods for deposition of the barrier layers in the devices above can be modified as the production techniques for memory devices are changed. For example, the barrier layer can be masked and etched after deposition if necessary. Alternatively, it can be etched in combination with other layers deposited before or afterwards. It is also evident that the techniques described above can be used to produce a plurality of transistors or other memory devices on a single semiconductor wafer. 
     Other embodiments of the invention will be apparent to those skilled in the art from a consideration of the specification or practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope of the invention being indicated by the following claims.