Abstract:
A thin film structure includes a substantially single-phase, c-axis PGO film on an insulator for use in metal ferroelectric insulator semiconductor single transistor non-volatile memory applications. The PGO on insulator structure can also be used in capacitors, pyroelectric infrared sensors, optical displays, optical switches, piezoelectric transducers, and surface acoustic wave devices. In a preferred embodiment, the PGO film is deposited on a Zirconium Oxide insulator layer.

Description:
FIELD OF THE INVENTION 
     This invention relates to a single c-axis Lead Germanium Oxide (PGO) thin film on an insulator layer for non-volatile memory applications and methods of making the same and, more particularly, to a PGO thin film on a Zirconium Oxide (ZrO 2 ) insulator for ferroelectric random access memory (FeRAM) and dynamic random access memory (DRAM) applications. 
     BACKGROUND OF THE INVENTION 
     Lead Germanium Oxide (PGO or Pb 5 Ge 3 O 11 ) is a well-known ferroelectric material. Pure c-axis oriented PGO thin films have been successfully grown on Iridium (Ir), Platinum (Pt), Platinum/Iridium (Pt/Ir) and Iridium-Tantalum-Oxygen (Ir—Ta—O) conductive electrodes. These structures can be used for metal ferroelectric metal oxide semiconductor (MFMOS) single transistor memory applications. 
     Ferroelectric devices have also been manufactured with SrBi 2 Ta 2 O 9 (SBT). Unlike the ferroelectric material SBT, which exhibits ferroelectric properties with a polycrystalline structure, PGO only exhibits ferroelectric properties when it is c-axis oriented. However, it is rather difficult to grow single-phase c-axis PGO films on insulators because PGO films usually exhibit amorphous or polycrystalline structure. 
     Metal ferroelectric semiconductor field effect transistors (MFSFET) are an attractive alternative to MFMOS. However, a strong interaction between the PGO film and the Silicon precludes the fabrication of such devices using a PGO ferroelectric layer. Metal ferroelectric insulator semiconductor field effect transistors (MFISFET) have also been investigated because their structure is simpler than that of MFMOS structures. However, heretofore, good insulator films for fabrication of MFISFET devices, which include a PGO ferroelectric layer, have not been found. Accordingly, there is a need for finding a good insulator film that allows for the fabrication of MFISFET devices having a PGO ferroelectric layer. 
     SUMMARY OF THE INVENTION 
     The invention comprises a new thin film structure, including a single-phase c-axis PGO film on an insulator, such as ZrO 2 , for metal ferroelectric insulator semiconductor single transistor non-volatile memory applications. This PGO-on-insulator structure can also be used in capacitors, pyroelectric infrared sensors, optical displays, optical switches, piezoelectric transducers, and surface acoustic wave devices. 
     Accordingly, an object of the invention is to provide a MFIS transistor structure including a PGO ferroelectric film. 
     Another object of the invention is to provide a MFIS transistor structure including a ZrO 2  insulator film. 
     A further object of the invention is to provide a MFIS transistor structure including a PGO ferroelectric layer on an insulator layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of the inventive PGO ferroelectric film on an insulator film. 
     FIG. 2 is a graph of an X-ray diffraction spectrum of a PGO film on a ZrO 2  insulator film. 
     FIG. 3 is a graph of a high frequency Capacitance-Voltage (CV) measurement of a metal ferroelectric insulator semiconductor (MFIS) capacitor comprising Pt/PGO/ZrO 2 /Si. 
     FIG. 4 is a graph of a Capacitance-Voltage (CV) measurement of a Pt/PGO(180 nm)/Ir capacitor structure. 
     FIG. 5 is a graph of a leakage current (I-V) of a PGO/ZrO 2  film. 
     FIG. 6 is a schematic of a metal ferroelectric insulator semiconductor field effect transistor including a PGO layer on an insulator structure. 
     FIG. 7 is a flowchart of the method of making the PGO-on-insulator structure of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings, FIG. 1 shows a schematic of the inventive device. Device  10  includes a Semiconductor substrate  12  such as a Silicon substrate. An Insulator film  14 , such as ZrO 2 , is positioned on substrate  12 . A ferroelectric layer  16 , namely, a substantially single phase, c-axis Pb 5 Ge 3 O 11 (PGO) film, is positioned on insulator film  14 . A metal top electrode  18  is positioned on PGO film  16 . 
     A typical method for preparing such a structure may include the following steps. First, the semiconductor substrate is prepared using any state of the art process including isolation and well formation. Second, the insulator film, such as ZrO 2 , is deposited by any of the following means: physical vapor deposition (PVD); evaporation and oxidation; chemical vapor deposition (CVD); and atomic layer deposition. Any necessary post deposition anneal may include annealing in forming gas or Oxygen ambient at a temperature up to 800 degrees Celsius. Third, the PGO film is deposited by any of the following methods: spin-on; physical vapor deposition; CVD; metal organic CVD (MOCVD); chemical solution deposition (CSD); and laser ablation. Fourth, the metal gate electrode is deposited on the PGO layer by any means known in the art. Fifth, the contact and interconnect formations are made. 
     As an example, the following device was manufactured. ZrO 2  was sputtered-deposited onto a clean Silicon wafer by reactive sputtering of a Zirconium target in an Oxygen ambient. The film thickness  20 , as measured by spectroscopic ellipsometry, was 135 Angstroms. The thickness of the insulator layer typically will be greater than at least 20 Angstroms. The PGO thin film  16  was spin coated using lead acetate (Pb(OAc) 2  3H 2 O) and germanium isopropoxide (Ge(OPr′) 4 ) in 2-(2-ethoxyethoxy) ethanol solution (H(OC 2 H 4 OC 2 H 4 OC 2 H 5 ), heated with an air exposure until a deep red brown color. The Lead to Germanium ratio (Pb/Ge) was 5.25/3. The baking temperature was approximately 50 to 350 degrees Celsius for 30 seconds to 3600 seconds. The annealing temperature after each spin-on layer was approximately 400 to 550 degrees Celsius for 30 seconds to 3600 seconds. The final annealing temperature was approximately 450 to 600 degrees Celsius for five minutes to three hours. The thickness  22  of the PGO layer  16  was approximately 1600 Angstroms and typically will be in a range of 100 Angstroms to 5000 Angstroms. The phases of the deposited PGO layer were examined by X-ray diffraction. 
     FIG. 2 is a graph of an X-ray diffraction spectrum of a PGO film on a ZrO 2  substrate, as manufactured by the steps listed above. The x-axis represents two-times-theta (degrees) and the y-axis represents counts per second. The graph shows that substantially pure c-axis PGO was obtained on the ZrO 2  substrate, as shown by the peaks labeled ( 001 ), ( 002 ), ( 003 ), ( 004 ), ( 005 ) and ( 006 ). No other peaks are observed (except for minimal peaks at 29 degrees), which indicates that no reaction, or only a minimal reaction (as shown by the secondary phases at 29 degrees), occurred at the interface of the PGO and ZrO 2  layers. 
     FIG. 3 is a graph of a high frequency Capacitance-Voltage (CV) measurement of a metal ferroelectric insulator semiconductor (MFIS) capacitor comprising Pt/PGO/ZrO 2 /Si. The x-axis shows voltage and the y-axis shows Capacitance. In order to measure the memory windows of a PGO thin film on a ZrO 2  substrate, a Platinum (Pt) top electrode was deposited on the PGO surface with a shallow mask. The area of the top electrode was approximately 4×10 −4 cm 2 . The PGO film had a thickness of approximately 1600 Angstroms and the ZrO 2  layer had a thickness of approximately 130 Angstroms. The hysteresis in the C-V curve indicates a memory window of approximately 0.7 volts. The memory window typically will be between 0.1 to 3.0 volts. This is less than the 1.3 volt memory window for an 1800 Angstrom PGO film in a metal ferroelectric metal (MFM) structure. 
     FIG. 4 is a graph of a Capacitance-Voltage (CV) measurement of a Pt/PGO(180 nm)/Ir capacitor structure. The memory window shown is about 1.8 volts. FIG. 5 is a graph of a leakage current (I-V) of a PGO film on a ZrO 2  film. As shown by the graph, the leakage current though the PGO/ZrO 2  structure is very small, indicating that a good interface is maintained between the PGO, the ZrO 2  and the Silicon substrate. In particular, the leakage current typically is less than 1×10 −6  A/cm 2  at 100 KV/cm. 
     FIG. 6 is a schematic of a preferred embodiment of the composite PGO/insulator layered structure, namely, a metal ferroelectric insulator semiconductor field effect transistor (MFISFET) including a PGO film on an insulator layer. Device  30  includes a Semiconductor substrate  32  such as a Silicon substrate, including a source region  34  and a drain region  36 . An Insulator film  38  is positioned on substrate  32 . Insulator film  38  may comprise Zirconium Oxide (ZrO 2 ), Hafnium Oxide (HfO 2 ), silicates of Zirconium or Hafnium, or mixtures of the above. The insulator film  38  may also comprise Aluminum Oxide, Yttrium Oxide, Calcium Oxide, Lanthanum Oxide, Titanium Oxide (TiO 2 ), Tantalum Oxide (Ta 2 O 5 ), doped ZrO 2  or doped HfO 2 , Zr—Al—O, Hf—Al—O, Zr—Ti—O, Hf—Ti—and La—Al—O, and combinations thereof. The ferroelectric layer  40  comprises a single phase, c-axis Pb 5 Ge 3 O 11 (PGO) film, positioned on insulator film  38 . The ferroelectric PGO layer may be deposited by the method set forth in U.S. Patent Registration No. 6,190,925, issued on Feb. 20, 2001, assigned to Sharp Laboratories of America, Inc., and entitled Epitaxially Grown Lead Germanate Film and Deposition Method, wherein said patent is incorporated herein by reference. By use of the disclosed PGO deposition method as set forth in the above listed patent, a PGO layer can be deposited with a substantially c-axis crystalline orientation, i.e., more than a 70% c-axis orientation, but may be as high as 80%. In a preferred embodiment, the PGO layer will have approximately a 90% c-axis orientation or better. A metal top electrode  42  is positioned on PGO film  40 . The metal top electrode may comprise Platinum (Pt), Iridium (Ir), Tantalum (Ta), Ruthenium (Ru) or conductive oxides or alloys. 
     FIG. 7 is a flowchart of the method of making the PGO-on-insulator structure of the present invention. Step  50  comprises preparing the semiconductor substrate using any state of the art process including isolation and well formation. Step  52  comprises depositing the insulator film by any of the following means: physical vapor deposition (PVD); evaporation and oxidation; chemical vapor deposition (CVD); and atomic layer deposition. Step  54 , if required, comprises post deposition annealing of the insulator film in forming gas or Oxygen ambient at a temperature up to 800 degrees Celsius. Step  56  comprises depositing the PGO film by any of the following methods: spin-on; physical vapor deposition; CVD; metal organic CVD (MOCVD); chemical solution deposition (CSD); and laser ablation. Step  58  comprises depositing the metal gate electrode on the PGO layer by any means known in the art. Step  60  comprises making any required contact and interconnect formations. 
     Thus, a single-phase, c-axis PGO thin film on an insulator for non-volatile memory applications, and a method for making the same, has been disclosed. Although preferred structures and methods of manufacturing the device have been disclosed, it should be appreciated that further variations and modifications may be made thereto without departing from the scope of the invention as defined in the appended claims.