Patent Publication Number: US-2022223608-A1

Title: Bilayer dielectric stack for a ferroelectric tunnel junction and method of forming

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 17/390,399, filed Jul. 30, 2021, which claims priority to U.S. Provisional Patent Application No. 63/063,840, filed Aug. 10, 2020; the disclosure of which are expressly incorporated herein, in their entirety, by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to semiconductor processing and semiconductor devices, and more particularly, to substrate processing methods for forming dielectric materials with selected polarization for capacitor devices. 
     BACKGROUND OF THE INVENTION 
     Ferroelectric tunnel junctions (FTJs) are candidate devices for application of artificial synapses in neural networks. FTJs utilize a thin ferroelectric layer sandwiched between two electrodes, which allows electron tunneling through the ferroelectric layer. The two different polarization states are used to alter the potential landscape and therefore change the tunneling transmission coefficient and give the possibility of exhibiting multi-level resistance values through nucleation and propagation of domains of opposed polarity. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention describe formation of a metal-ferroelectric-dielectric-metal capacitor device that can function as a novel ferroelectric tunnel junction compatible with standard semiconductor fabrication processes. The device includes a bilayer stack with a linear dielectric film and a ferroelectric film, where the device operation can rely on polarization reversal of the ferroelectric film and electron tunneling through the thin linear dielectric film. A relatively thick ferroelectric film can be used in the bilayer stack since the tunneling current is controlled by the thin linear dielectric film. 
     According to one embodiment, a method of forming a bilayer stack for a ferroelectric tunnel junction includes depositing a first metal oxide film on a substrate by performing a first plurality of cycles of atomic layer deposition, where the first metal oxide film contains hafnium oxide (HfO 2 ), zirconium oxide (ZrO 2 ), or both hafnium oxide and zirconium oxide The method further includes depositing a second metal oxide film on the substrate by performing a second plurality of cycles of atomic layer deposition, where the second metal oxide film contains hafnium oxide and zirconium oxide, and has a different hafnium oxide and zirconium oxide content than the first metal oxide film. The method further includes heat-treating the first and second metal oxide films, where a ferroelectric phase is formed in the second metal oxide film but not in the first metal oxide film. 
     According to one embodiment, a bilayer stack for a ferroelectric tunnel junction includes a first metal oxide film containing hafnium oxide, zirconium oxide, or both hafnium oxide and zirconium oxide, and a second metal oxide film containing hafnium oxide and zirconium oxide, where the second metal oxide film is ferroelectric and the first metal oxide film is a linear dielectric. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the invention. 
         FIG. 1  is a flowchart of an example method of manufacturing a bilayer stack according to an embodiment of the invention; 
         FIGS. 2A-2D  show schematic cross-sectional views of an example film structure containing a bilayer stack with a linear dielectric film and a ferroelectric film according to an embodiment of the invention; and 
         FIG. 3A-3D  schematically show gas flow diagrams for depositing metal oxide films according to embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS 
     The present disclosure repeats reference numerals in the various embodiments. This repetition is for the purpose of simplicity and clarity such that repeated reference numerals indicate similar features amongst the various embodiments unless stated otherwise. 
     Embodiments of the invention describe formation of a metal-ferroelectric-dielectric-metal capacitor device that can find application as a ferroelectric tunnel junction and is compatible with conventional semiconductor fabrication processes. The film structure of the capacitor device includes a bilayer stack containing a first metal oxide film and a second metal oxide film. The first metal oxide film is not ferroelectric but is linearly polarizable, and the second metal oxide film is ferroelectric. According to embodiments of the invention, formation of this film structure is achieved by depositing a bilayer stack of metal oxide films with different HfO 2  content and ZrO 2  content. The combination of the different metal oxide films allows for a thickness of the second metal oxide film to be greater than a thickness of the first metal oxide film. For example, a thickness of the first metal oxide film can be about 1.5 nm, or less. 
     According to one embodiment, schematically shown in  FIGS. 1 and 2A-2D , a method in flowchart  1  includes, in  100 , providing, in a process chamber, a substrate  2  containing base layer  200  and a first metal-containing electrode layer  205  on the base layer  200 . As shown in  FIG. 2A , the metal-containing electrode layer  205  may be in direct physical contact with the base layer  200 . In one example, the process chamber may be configured to perform atomic layer deposition (ALD) of a dielectric material on the substrate  2 . The base layer  200  may, for example, include a semiconductor material, including silicon, germanium, silicon germanium, silicon germanium carbide, gallium arsenic phosphide, gallium indium phosphide, silicon carbide, gallium arsenide, indium arsenide, or indium phosphide. In one example, the base layer  200  may have an epitaxial layer overlying a bulk semiconductor. Furthermore, the base layer  200  may include a semiconductor-on-insulator (SOI) structure. The first metal-containing electrode layer  205  can for example, contain titanium nitride (TiN), tantalum nitride (TaN), or other electrically conductive metal-containing layer and metal layers. 
     The method further includes, in  110 , forming a first metal oxide film  210  on the substrate  2  by performing a first plurality of cycles of ALD. The resulting first metal oxide film  210  has a chemical composition that may be described as mol % HfO 2  and mol % ZrO 2 . According to some embodiments, the first metal oxide film  210  may include a laminate of alternating HfO 2  and ZrO 2  layers, or a solid solution of a mixture of HfO 2  and ZrO 2 . 
       FIG. 3A-3D  schematically show gas flow diagrams for depositing metal oxide films according to embodiments of the invention. According to one embodiment, schematically shown in  FIG. 3A , the first plurality of cycles of ALD can include x number of cycles of sequential gaseous exposures of a Hf precursor, a purge gas, an oxidizer, and a purge gas to deposit a layer of HfO 2  on the substrate  2 . Each exposure of the Hf precursor and the oxidizer can be performed for a time period that results in a saturation exposure on a surface of the substrate  2  and the purge gas exposure removes unreacted reactants and by-products from the process chamber and prevents gas phase mixing of the Hf precursor and the oxidizer. Each cycle deposits one atomic layer or less of HfO 2 , and the x number of cycles may be selected in order to accurately control the HfO 2  layer thickness. Steric hindrance of ligands in the Hf precursor and the oxidizer, and a limited number of bonding sites, can limit the chemisorption on the substrate surface, and therefore the HfO 2  film growth per cycle can remain at less than one atomic layer. 
     According to one embodiment, schematically shown in  FIG. 3B , the first plurality of cycles of ALD can include y number of cycles of sequential gaseous exposures of a Zr precursor, a purge gas, an oxidizer, and a purge gas to deposit a layer of ZrO 2  on the substrate  2 . Each exposure of the Zr precursor and the oxidizer can be performed for a time period that results in a saturation exposure on a surface of the substrate  2  and the purge gas exposure removes unreacted reactants and by-products from the process chamber and prevents gas phase mixing of the Zr precursor and the oxidizer. Each cycle deposits one atomic layer or less of ZrO 2 , and the y number of cycles may be selected in order to accurately control the ZrO 2  layer thickness. Steric hindrance of ligands in the Zr precursor and the oxidizer, and a limited number of bonding sites, can limit the chemisorption on the substrate surface, and therefore the ZrO: film growth per cycle can remain at less than one atomic layer. 
     According to one embodiment, a hafnium zirconium oxide film may be deposited by sequentially performing x number of HfO 2  ALD cycles and y number of ZrO 2  ALD cycles in a supercycle that may be repeated n times to increase the number of alternating HfO 2  and ZrO 2  layers in the laminate that forms the first metal oxide film  210 . The gas flow diagram in  FIG. 3C  schematically shows the formation of a HfO 2  layer before the formation of a ZrO 2  layer on the HfO 2  layer. However, other embodiments contemplate the formation of a ZrO 2  layer before the formation of a HfO 2  layer on the ZrO 2  layer. 
     According to another embodiment, schematically shown in  FIG. 3D , a hafnium zirconium oxide film may be deposited where the first plurality of cycles of ALD can include n number of cycles of sequential gaseous exposures of a mixture of a Hf precursor and a Zr precursor, a purge gas, an oxidizer, and a purge gas. Each co-exposure of the Hf and Zr precursors and the oxidizer exposure can be performed for a time period that results in a saturation exposure. Each cycle deposits one atomic layer or less of a mixture of HfO 2  and ZrO 2 , and the n number of cycles may be selected in order to accurately control the film thickness. The composition of the first metal film  210 , which comprises a solid solution of a mixture of HfO 2  and ZrO 2 , may be selected by independently controlling the flow rates of the Hf precursor and the Zr precursor that form the mixture that is exposed to the substrate  2 . 
     According to embodiments of the invention, the first metal oxide film  210  is not ferroelectric but is linearly polarizable in the presence of an external electric field. A HfO 2  films and ZrO 2  films are not ferroelectric, and the lack of ferroelectricity in hafnium zirconium oxide films is due to a HfO 2  content or a ZrO 2  content that is below a threshold value needed for ferroelectric phase formation in the first metal oxide film  210  after deposition on the substrate  2  or after a subsequent heat-treating step at an elevated substrate temperature. According to one embodiment, the HfO 2  content or the ZrO 2  content can be less than about 25 mol %. In one example, the HfO 2  content can be between about 10 mol % and about 20 mol %, and balance ZrO 2 . In another example, the ZrO 2  content can be between about 10 mol % and about 20 mol %, and balance HfO 2 . In another example, the HfO 2  content can be less than about 10 mol %, and balance ZrO 2 . In another example, the ZrO 2  content can be less than about 10 mol %, and balance HfO 2 . 
     Following the deposition of the first metal oxide film  210  on the substrate  2 , an optional heat-treating process may be performed on the first metal oxide film  210  using a predetermined substrate temperature and time period. The heat-treating may be performed at a substrate temperature between about 400° C. and about 900° C., between about 200° C. and about 500° C., between about 200° C. and about 300° C., between about 300° C. and about 400° C., or between about 400° C. and about 500° C. In one example, the heat-treating may be performed at a substrate temperature of about 500° C., or lower. In one example, the heat-treating may be performed in the same process chamber as the deposition of the first metal oxide film  210 . In another example, the heat-treating may be formed in a different process chamber than the deposition of the first metal oxide film  210 . The heat-treating may be performed under vacuum conditions in the presence of an inert gas, for example argon (Ar) or nitrogen (N 2 ). 
     According to one embodiment, the first metal oxide film  210  may be heat-treated after one or more cycles of the atomic layer deposition, before deposition of the entire first metal oxide film  210 . Thus, the heat-treating may be performed before the entire first metal oxide film  210  has been deposited. 
     The method further includes, in  120 , forming a second metal oxide film  220  on the substrate  2  by performing a second plurality of cycles of atomic layer deposition (ALD). The resulting second metal oxide film  220  contains HfO 2  and ZrO 2  and has a chemical composition that may be described as mol % HfO 2  and mol % ZrO 2 . According to some embodiments, the second metal oxide film  220  may include a laminate of alternating HfO 2  and ZrO 2  layers, or a solid solution of a mixture of HfO 2  and ZrO 2 . The second metal oxide film  220  may be formed as described above for the first metal oxide film  210 , including as described in  FIGS. 3C and 3D . However, the second metal oxide film  220  has a different chemical composition than the first metal oxide film  210 . 
     According to embodiments of the invention, the second metal oxide film  220  has a HfO 2  content or a ZrO 2  content that is above a threshold value needed for ferrolectric phase formation in the second metal oxide film  220  after deposition on the substrate  2  or after a subsequent heat-treating step at an elevated temperature. Accordingly, the second metal oxide film  220  is ferroelectric. According to one embodiment, the HfO 2  content and the ZrO 2  content are both greater than about 25 mol %. Thus, the HfO 2  content can be greater than about 25 mol %, balance ZrO 2 , or the ZrO 2  content can be greater than about 25 mol %, balance HfO 2 . Examples include HfO 2  content:ZrO 2  content of about 30 mol %:about 70 mol %, about 40 mol %:about 60 mol %, about 50 mol %:about 50 mol %, about 60 mol %:about 40 mol %, or about 70 mol %:about 30 mol %. 
     Following the deposition of the second metal oxide film  220  on the substrate  2 , a heat-treating process is performed in  130  on the first and second metal oxide films  210 ,  220  using a predetermined substrate temperature and time period. The heat-treating establishes a ferroelectric phase in the second metal oxide film  220  but the first metal oxide film  210  remains a liner dielectric without a ferroelectric phase. The heat-treating may be performed at a substrate temperature between about 400° C. and about 900° C., between about 200° C. and about 500° C., between about 200° C. and about 300° C., between about 300° C. and about 400° C., or between about 400° C. and about 500° C. In one example, the heat-treating may be performed in the same process chamber as the deposition of the first and second metal oxide films  210 ,  220 . In another example, the heat-treating may be formed in a different process chamber than the deposition of the first and second metal oxide films  210 ,  220 . The heat-treating may be performed under vacuum conditions in the presence of an inert gas, for example argon (Ar) or nitrogen (N 2 ). According to one embodiment, the first metal oxide film  210  is not heat-treated prior to depositing the second metal oxide film  220  on the first metal oxide film  210 . 
     According to embodiments of the invention, the first metal oxide film  210  and the second metal oxide film  220  may be deposited on the substrate  2  in any order. In one example, as schematically shown in  FIGS. 2A-2D , the first metal oxide film  210  is deposited with direct contact with the first metal-containing electrode layer  205 , and, thereafter the second metal oxide film  220  is deposited with direct physical contact with an upper surface of the first metal oxide film  210 . In another example, the second metal oxide film  220  is deposited on the first metal-containing electrode layer  205 , and, thereafter the first metal oxide film  210  is deposited with direct physical contact with an upper surface of the second metal oxide film  220 . 
     According to one embodiment, the first metal oxide film  210  and the second metal oxide film  220  differ in HfO 2  content, ZrO 2  content, and film thickness. According to one embodiment, the difference in HfO 2  content, ZrO 2  content, and film thickness is easily achieved by the plurality of cycles of ALD described above and schematically shown in  FIGS. 3A-3D . In one example, the x number of HfO 2  ALD cycles and the y number of ZrO 2  ALD cycles in  FIG. 3C  form a laminate of alternating HfO 2  and ZrO 2  layers, where the number of HfO 2  ALD cycles relative to the number of ZrO 2  ALD cycles may be used to select the chemical composition. In one example, in  FIG. 3D , the relative flow rates of a Hf precursor and a Zr precursor in a precursor mixture may be used to achieve the desired chemical composition. Further, the same hafnium precursor, zirconium precursor, oxidizer, and purge gas may be used to deposit both the first metal oxide film  210  and the second metal oxide film  220 . This provides several advantages over other methods where different film contain different chemical elements. Some advantages include higher manufacturing throughput, processing in a single process chamber, and fewer different reactants. 
     In some embodiments, the first metal oxide film  210  may be deposited by any of the gas flow diagram in  FIGS. 3A-3D . Similarly, the second metal oxide film  220  may either be deposited by the gas flow diagram in  FIG. 3C  or by the gas flow diagram in  FIG. 3D . In one example, both the first and second metal oxide films  210 ,  220  may be deposited by the gas flow diagram in  FIG. 3C . In another example, both the first and second metal oxide films  210 ,  220  may be deposited by the gas flow diagram in  FIG. 3D . In yet another example, the first metal oxide film  210  may be deposited by the gas flow diagrams in  FIG. 3A or 3B  and the second metal oxide film  220  may be deposited by the gas flow diagram in  FIG. 3C or 3D . 
     After the heat-treating in  130 , a second metal-containing electrode layer  225  may be deposited on the second metal oxide film  220 . This is schematically shown in  FIG. 2D . The second metal-containing electrode layer  225  can for example, contain titanium nitride (TiN), tantalum nitride (TaN), or other electrically conductive metal-containing layer and metal layers. 
     Embodiments of the invention may utilize a wide variety of zirconium (Zr) and hafnium (Hf) precursors for the vapor phase deposition. For example, representative examples include: Zr(O t Bu) 4  (zirconium tert-butoxide, ZTB), Zr(NEt 2 ) 4  (tetrakis(diethylamido)zirconium, TDEAZ), Zr(NMeEt) 4  (tetrakis(ethylmethylamido)zirconium, TEMAZ), Zr(NMe 2 ) 4  (tetrakis(dimethylamido)zirconium, TDMAZ), Hf(O t Bu) 4  (hafnium tert-butoxide, HTB), Hf(NEt 2 ) 4  (tetrakis(diethylamido)hafnium, TDEAH), Hf(NEtMe) 4  (tetrakis(ethylmethylamido)hafnium, TEMAH), and Hf(NMe 2 ) 4  (tetrakis(dimethylamido)hafnium, TDMAH). In some examples, tris(dimethylaminocyclopentadienylhafnium (HfCp(NMe 2 ) 3 ) available from Air Liquide as HyALD™ may be used as a hafnium precursor and tris(dimethylaminocyclopentadienylzirconinum (ZrCp(NMe 2 ) 3 ) available from Air Liquide as ZyALD™ may be used as a zirconium precursor. The oxidizer may include an oxygen-containing gas, including plasma-excited O 2 , water (H 2 O), or ozone (O 3 ). 
     According to one embodiment, a bilayer stack of the first metal oxide film  210  and the second metal oxide film  220  may be used in a metal-ferroelectric-dielectric-metal capacitor device as schematically shown in  FIG. 2D . The bilayer stack includes the relatively thick ferroelectric layer of the second metal oxide film  220  and the thinner linear dielectric layer of the first metal oxide film  210  that controls the tunneling current of the device. 
     A plurality of embodiments for forming a metal-ferroelectric-dielectric-metal capacitor device that can find application as a ferroelectric tunnel junction and is compatible with conventional semiconductor fabrication processes have been described. The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. This description and the claims following include terms that are used for descriptive purposes only and are not to be construed as limiting. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above teaching. Persons skilled in the art will recognize various equivalent combinations and substitutions for various components shown in the Figures. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.