US20100012998A1 - Flash memory device with stacked dielectric structure including zirconium oxide and method for fabricating the same - Google Patents

Flash memory device with stacked dielectric structure including zirconium oxide and method for fabricating the same Download PDF

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US20100012998A1
US20100012998A1 US12/568,591 US56859109A US2010012998A1 US 20100012998 A1 US20100012998 A1 US 20100012998A1 US 56859109 A US56859109 A US 56859109A US 2010012998 A1 US2010012998 A1 US 2010012998A1
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dielectric layer
dielectric
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Seung-Ryong Lee
Moon-Sig Joo
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SK Hynix Inc
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Hynix Semiconductor Inc
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    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof  ; Multistep manufacturing processes therefor
    • H01L29/40Electrodes ; Multistep manufacturing processes therefor
    • H01L29/401Multistep manufacturing processes
    • H01L29/4011Multistep manufacturing processes for data storage electrodes
    • H01L29/40114Multistep manufacturing processes for data storage electrodes the electrodes comprising a conductor-insulator-conductor-insulator-semiconductor structure
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof  ; Multistep manufacturing processes therefor
    • H01L29/40Electrodes ; Multistep manufacturing processes therefor
    • H01L29/41Electrodes ; Multistep manufacturing processes therefor characterised by their shape, relative sizes or dispositions
    • H01L29/423Electrodes ; Multistep manufacturing processes therefor characterised by their shape, relative sizes or dispositions not carrying the current to be rectified, amplified or switched
    • H01L29/42312Gate electrodes for field effect devices
    • H01L29/42316Gate electrodes for field effect devices for field-effect transistors
    • H01L29/4232Gate electrodes for field effect devices for field-effect transistors with insulated gate
    • H01L29/42324Gate electrodes for transistors with a floating gate
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof  ; Multistep manufacturing processes therefor
    • H01L29/40Electrodes ; Multistep manufacturing processes therefor
    • H01L29/43Electrodes ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/49Metal-insulator-semiconductor electrodes, e.g. gates of MOSFET
    • H01L29/51Insulating materials associated therewith
    • H01L29/511Insulating materials associated therewith with a compositional variation, e.g. multilayer structures
    • H01L29/513Insulating materials associated therewith with a compositional variation, e.g. multilayer structures the variation being perpendicular to the channel plane
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof  ; Multistep manufacturing processes therefor
    • H01L29/66Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/66007Multistep manufacturing processes
    • H01L29/66075Multistep manufacturing processes of devices having semiconductor bodies comprising group 14 or group 13/15 materials
    • H01L29/66227Multistep manufacturing processes of devices having semiconductor bodies comprising group 14 or group 13/15 materials the devices being controllable only by the electric current supplied or the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched, e.g. three-terminal devices
    • H01L29/66409Unipolar field-effect transistors
    • H01L29/66477Unipolar field-effect transistors with an insulated gate, i.e. MISFET
    • H01L29/66825Unipolar field-effect transistors with an insulated gate, i.e. MISFET with a floating gate
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof  ; Multistep manufacturing processes therefor
    • H01L29/66Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/68Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
    • H01L29/76Unipolar devices, e.g. field effect transistors
    • H01L29/772Field effect transistors
    • H01L29/78Field effect transistors with field effect produced by an insulated gate
    • H01L29/788Field effect transistors with field effect produced by an insulated gate with floating gate
    • H01L29/7881Programmable transistors with only two possible levels of programmation

Definitions

  • the present invention relates to a method for fabricating a semiconductor device; and more particularly, to a flash memory device with a dielectric layer having a high k-dielectric constant and a method for fabricating the same.
  • a distance between devices decreases as the integration scale of semiconductor devices improves. Accordingly, it becomes difficult to perform a gap-filling process and thus, the devices are often hardly isolated.
  • ONO oxide-nitride-oxide
  • IPD inter-poly dielectric
  • a property of a capacitor for storing electric charges may be degraded. Accordingly, a high k-dielectric thin layer having a small thickness and a high k-dielectric constant is used as the IPO layer or the IPD layer.
  • FIG. 1 is a cross-sectional view illustrating a conventional flash memory device.
  • a tunnel oxide layer 12 is formed over a substrate 11 , and a floating gate 13 is formed over the tunnel oxide layer 12 .
  • a dielectric structure 14 is formed over the floating gate 13 .
  • the dielectric structure 14 is formed by sequentially stacking a first silicon oxide (SiO 2 ) layer 14 A, a zirconium oxide (ZrO 2 ) layer 14 B, and a second silicon oxide (SiO 2 ) layer 14 C.
  • a control gate 15 is formed over the dielectric structure 14 .
  • the ZrO 2 layer may be crystallized at a predetermined thickness or greater while forming a thin layer. Accordingly, along a grain boundary, current is likely to leak or a defect may be generated. As a result, device reliability may be degraded.
  • the present invention provides a flash memory device using a zirconium oxide (ZrO 2 ) layer having a high k-dielectric constant as a dielectric layer, a low capacitive equivalent thickness (CET), a low leakage current level, and improved reliability, and a method for fabricating the same.
  • ZrO 2 zirconium oxide
  • a dielectric structure disposed between a floating gate and a control gate of a flash memory device including: a first dielectric layer; a third dielectric layer having a k-dielectric constant substantially the same as that of the first dielectric layer; and a second dielectric layer disposed between the first dielectric layer and the third dielectric layer, having a greater k-dielectric constant than that of the first and third dielectric layers and formed by alternately and repeatedly stacking a plurality of aluminum oxide (Al 2 O 3 ) layers and a plurality of zirconium oxide (ZrO 2 ) layers.
  • Al 2 O 3 aluminum oxide
  • ZrO 2 zirconium oxide
  • a flash memory device including: a substrate; a tunnel oxide layer formed over the substrate; a floating gate formed over the tunnel oxide layer; a first dielectric layer formed over the floating gate; a second dielectric layer formed by alternately and repeatedly stacking a plurality of Al 2 O 3 layers and a plurality of ZrO 2 layers over the first dielectric layer and having a greater k-dielectric constant than that of the first dielectric layer; a third dielectric layer formed over the second dielectric layer and having substantially the same k-dielectric constant as that of the first dielectric layer; and a control gate formed over the third dielectric layer.
  • a method for forming a dielectric structure disposed between a floating gate and a control gate of a flash memory device including: forming a first dielectric layer; forming a second dielectric layer in a stack structure that has a greater k-dielectric constant than that of the first dielectric layer by alternately and repeatedly stacking a plurality of Al 2 O 3 layers and a plurality of ZrO 2 layers over the first dielectric layer; and forming a third dielectric layer that has substantially the same k-dielectric constant as that of the first dielectric layer over the second dielectric layer.
  • a method for fabricating a flash memory device including: forming a tunnel oxide layer over a substrate; forming a floating gate over the tunnel oxide layer; forming a first dielectric layer over the floating gate; forming a second dielectric layer in a stack structure that has a greater k-dielectric constant than that of the first dielectric layer by alternately and repeatedly stacking a plurality of Al 2 O 3 layers and a plurality of ZrO 2 layers over the first dielectric layer; forming a third dielectric layer that has substantially the same k-dielectric constant as that of the first dielectric layer over the second dielectric layer; and forming a control gate over the third dielectric layer.
  • FIG. 1 is a cross-sectional view illustrating a conventional flash memory device
  • FIG. 2 is a cross-sectional view illustrating a dielectric structure of a flash memory device in accordance with an embodiment of the present invention
  • FIG. 3A is a graph of a capacitive equivalent thickness (CET) that varies depending on various stack structures
  • FIG. 3B is a graph of leakage current level that varies depending on various stack structure
  • FIG. 4 is a simplified diagram illustrating an atomic layer deposition (ALD) method to form an Al 2 O 3 /ZrO 2 /Al 2 O 3 /ZrO 2 /Al 2 O 3 structure in accordance with an embodiment of present invention
  • FIG. 5 is a cross-sectional view illustrating a flash memory device using the dielectric structure shown in FIG. 2 ;
  • FIGS. 6A and 6B are Weibull plots illustrating time-to-breakdown (TBD) distributions in accordance with the embodiment of the present invention.
  • FIG. 2 is a cross-sectional view illustrating a dielectric structure of a flash memory device in accordance with an embodiment of the present invention.
  • the dielectric structure 100 of the flash memory device includes a first dielectric layer 101 ; a third dielectric layer 103 having substantially the same k-dielectric constant as the first dielectric layer 101 ; and a second dielectric layer 102 disposed between the first dielectric layer 101 and the third dielectric layer 103 .
  • the second dielectric layer 102 has a greater k-dielectric constant than the first dielectric layer 101 and the third dielectric layer 103 , and is formed by alternately and repeatedly stacking an aluminum oxide (Al 2 O 3 ) layer 102 A and a zirconium oxide (ZrO 2 ) layer 102 B.
  • the dielectric structure 100 has a stack structure formed by sequentially stacking the first dielectric layer 101 , the second dielectric layer 102 and the third dielectric layer 103 . Particularly, these alternately and repeatedly stacked Al 2 O 3 layer 102 A and the ZrO 2 layer 102 B will be denoted as “AZAZA structure”.
  • the first dielectric layer 101 and the third dielectric layer 103 individually include a silicon oxide (SiO 2 ) layer, and the second dielectric layer 102 is a stacked AZAZA structure.
  • the dielectric structure 100 becomes an OAZAZAO structure.
  • the Al 2 O 3 layer 102 A contacts with the first dielectric layer 101 and the third dielectric layer 103 as shown in the AZAZA structure to reduce silicon diffusion.
  • the Al 2 O 3 layer 102 A can prevent silicon from the first dielectric layer 101 and from the third dielectric layer 103 from diffusing to the corresponding ZrO 2 layer 102 B by being interposed between the first dielectric layer 101 and the ZrO 2 layer 102 B and between the third dielectric layer and the ZrO 2 layer 102 B.
  • the OAZAZAO structure in this embodiment of the present invention has a stack structure which additionally includes the Al 2 O 3 layers 102 A between the SiO 2 layers and the ZAZ structure.
  • a k-dielectric constant of the ZrO 2 layer 102 B is not decreased due to the additionally formed Al 2 O 3 layers 102 A.
  • the OAZAZAO structure includes two more Al 2 O 3 layers than the ZAZ structure and thus, the ZrO 2 layer 102 B can have substantially the same k-dielectric constant as the ZAZ structure although the ZrO 2 layers 102 B are thinly formed. Accordingly, since the ZrO 2 layers 102 B can be thinly formed, the ZrO 2 layers 102 B is not likely to be crystallized while forming the ZrO 2 layers 102 B.
  • the ZrO 2 layer 102 B is stacked twice and the Al 2 O 3 layer 102 A is stacked three times. It should be appreciated that stacking more of the Al 2 O 3 layers could be undesirable. For instance, if the Al 2 O 3 layer 102 A is stacked four times, the number of the ZrO 2 layers 102 B to be stacked is decreased to one. That is, if a proportion of the ZrO 2 layers 102 B is decreased (i.e., from two layers to one layer) in the stacked structure, a capacitive equivalent thickness (CET) and a leakage current level are increased. Accordingly, a desirable stack structure to obtain a low CET and a low leakage current level is the OAZAZAO structure.
  • CET capacitive equivalent thickness
  • FIG. 3A is a graph of a capacitive equivalent thickness (CET) that varies depending on various stack structures
  • FIG. 3B is a graph of leakage current level that varies depending on various stack structures.
  • the stacked dielectric structures compared with each other in FIGS. 3A and 3B are a ZAZ structure, an AZAZA structure, an AZAZAZA structure and a ZrO 2 layer.
  • the ZrO 2 layer has the lowest CET and the ZAZ structure has the highest CET.
  • the AZAZA structure has a larger CET than the AZAZAZA structure but a lower CET than the ZAZ structure.
  • the ZrO 2 layer has the highest leakage current, and the AZAZA structure has the lowest leakage current.
  • the AZAZAZA structure has higher leakage current than the ZAZ structure.
  • the AZAZA structure has a larger CET and a lower leakage current than the AZAZAZA structure and the ZAZ structure.
  • the second dielectric layer 102 with the AZAZA structure shown in FIG. 2 can be formed through an atomic layer deposition (ALD) method or a chemical vapor deposition (CVD) method.
  • ALD atomic layer deposition
  • CVD chemical vapor deposition
  • FIG. 4 is a diagram briefly illustrating an ALD method to form an AZAZA structure in accordance with an embodiment of the present invention.
  • the ALD method is generally performed by repeating a unit cycle until an atomic layer with a desirable thickness is obtained.
  • the unit cycle includes supplying a source gas to a wafer loaded into a chamber, supplying a purge gas thereto, supplying a reaction gas thereto, and supplying a purge gas thereto.
  • an aluminum (Al) source gas is supplied to a chamber where a wafer is loaded and then, absorbed into a surface of the wafer.
  • the Al source gas includes Al(CH 3 ) 3 .
  • a purge gas is supplied into the chamber to purge out non-adsorbed parts of the Al source gas remaining inside the chamber.
  • the purge gas includes an inert gas selected from a group consisting of argon (Ar), helium (He), nitrogen (N 2 ), or a combination thereof.
  • a reaction gas is supplied into the chamber.
  • the reaction gas includes one selected from a group consisting of ozone (O 3 ), oxygen (O 2 ) plasma, and water (H 2 O).
  • O 3 ozone
  • O 2 oxygen
  • H 2 O water
  • a reaction between the Al source gas and the reaction gas is induced to deposit an Al 2 O 3 layer. Accordingly, the Al 2 O 3 layer is formed over the surface of the wafer in an atomic unit.
  • a purge gas is supplied into the chamber to purge out non-reacted parts of the reaction gas and byproducts.
  • the purge gas includes an inert gas selected from a group consisting of Ar, He, H 2 , or a combination thereof.
  • the Al 2 O 3 layer with a desirable thickness can be deposited by repeating the unit cycle including supplying the Al source gas, supplying the purge gas, supplying the reaction gas, and supply the purge gas. Theses sequential steps are denoted with reference letters T 1 , T 2 , T 3 , and T 4 in FIG. 4 .
  • a zirconium (Zr) source gas is supplied to a chamber where a wafer is loaded and then, adsorbed into a surface of the wafer.
  • the Zr source gas includes one selected from a group consisting of Zr(O-tBu) 4 , Zr[N(CH 3 ) 2 ] 4 , Zr[N(C 2 Hs)(CH 3 )] 4 , Zr[N(C 2 H 5 ) 2 ] 4 , Zr(tmhd) 4 , Zr(OiC 3 H 7 ) 3 (tmhd) and Zr(OtBu) 4 .
  • the above listed materials are known as a precursor.
  • a purge gas is supplied into the chamber to purge out non-adsorbed parts of the Zr source gas remaining inside the chamber.
  • the purge gas includes an inert gas selected from a group consisting of Ar, He, N 2 , or a combination thereof.
  • a reaction gas is supplied into the chamber.
  • the reaction gas includes one selected from a group consisting of O 3 , O 2 plasma and H 2 O.
  • a reaction between the Zr source gas and the reaction gas is induced to deposit a ZrO 2 layer. Accordingly, the ZrO 2 layer is formed over the surface of the wafer in an atomic unit.
  • a purge gas is supplied into the chamber to purge out non-reacted parts of the reaction gas and byproducts.
  • the purge gas includes an inert gas selected from a group consisting of Ar, He, N 2 , or a combination thereof.
  • the ZrO 2 layer with a desirable thickness can be deposited by repeating the unit cycle including supplying the Zr source gas, supplying the purge gas, supplying the reaction gas, and supplying the purge gas. These sequential steps are denoted with reference letters T 11 , T 12 , T 13 , and T 14 in FIG. 4 .
  • the Al 2 O 3 layers and the ZrO 2 layers are alternately deposited through the ALD method to form the AZAZA structure. Meanwhile, the depositing of the Al 2 O 3 layers and the ZrO 2 layers uses the same purge gas and reaction gas except for the source gas. Accordingly, the Al 2 O 3 layers and the ZrO 2 layers can be alternately deposited only by changing the source gas at the same ALD chamber.
  • FIG. 5 is a cross-sectional view illustrating a flash memory device using the dielectric structure shown in FIG. 2 .
  • the flash memory device includes a substrate 201 , a tunnel oxide layer 202 over the substrate 201 , a floating gate FG 203 , a first dielectric layer 204 over the floating gate FG 203 , a second dielectric layer 205 having a k-dielectric constant greater than the first dielectric layer 204 formed over the first dielectric layer 204 by alternately and repeatedly stacking an Al 2 O 3 layer 205 A and a ZrO 2 layer 205 B, a third dielectric layer 206 formed over the second dielectric layer 205 and having substantially the same k-dielectric constant as the first dielectric layer 204 , and a control gate CG 207 over the third dielectric layer 206 .
  • the tunnel oxide layer 202 includes a SiO 2 layer, and the floating gate FG 203 and the control gate CG 207 includes a polysilicon layer.
  • the first dielectric layer 204 and the third dielectric layer 206 are SiO 2 layers, and the second dielectric layer 205 has an AZAZA structure, which is a stack structure of the Al 2 O 3 layer 205 A, the ZrO 2 layer 205 B, the Al 2 O 3 layer 205 A, the ZrO 2 layer 205 B, and the Al 2 O 3 layer 205 A.
  • the tunnel oxide layer 202 is formed over the substrate 201 completed with a well formation process and a channel ion-implantation process and other related processes.
  • the tunnel oxide layer 202 is formed through one of a thermal oxidation method and a CVD method.
  • the floating gate 203 is formed over the tunnel oxide layer 202 .
  • the floating gate 203 includes the polysilicon layer.
  • the first dielectric layer 204 is formed over the floating gate 203 .
  • the first dielectric layer includes the SiO 2 layer formed through an ALD method or a CVD method.
  • a dielectric layer having a greater k-dielectric constant than the first dielectric layer 204 is formed over the first dielectric layer 204 . That is, an AZAZA structure is formed by alternately and repeatedly stacking the Al 2 O 3 layer 205 A and the ZrO 2 layer 205 B over the first dielectric layer 204 . When the Al 2 O 3 layer 205 A and the ZrO 2 layer 205 B are alternately and repeatedly stacked, the Al 2 O 3 layer 205 A contacts with the first dielectric layer 204 or the third dielectric layer 206 . The third dielectric layer 206 is formed over the second dielectric layer 205 . The third dielectric layer 206 includes the SiO 2 layer as similar to the first dielectric layer 204 . Accordingly, the high k-dielectric second dielectric layer 205 formed between the low k-dielectric first dielectric layer 204 and the low k-dielectric third dielectric layer 206 gives rise to an OAZAZAO structure.
  • the control gate 207 is formed over the third dielectric layer 206 , and the control gate 207 includes the polysilicon layer.
  • the dielectric structure of the flash memory device between the floating gate 203 and the control gate 207 is a stack structure of the first dielectric layer 204 , the second dielectric layer 205 , and the third dielectric layer 206 , more particularly, of an OAZAZAO (SiO 2 /Al 2 O 3 /ZrO 2 /Al 2 O 3 /ZrO 2 /Al 2 O 3 /SiO 2 ) structure.
  • the OAZAZAO structure is formed with a thickness ranging from approximately 50 ⁇ to approximately 200 ⁇ .
  • the OAZAZAO dielectric structure can prevent silicon from diffusing to the ZrO 2 layer 205 B by interposing repeatedly the Al 2 O 3 layer 205 A between the ZrO 2 layer 205 B before depositing the third electric layer including the SiO 2 layer. Accordingly, a ZrSiO layer cannot be formed, and a k-dielectric constant of the ZrO 2 layer cannot be reduced.
  • FIGS. 6A and 6B are Weibull plots illustrating time-to-breakdown (TBD) distributions in accordance with an embodiment of the present invention.
  • FIG. 6A shows the TBD distribution when a positive polarity stress is exerted
  • FIG. 6B shows the TBD distribution when a negative polarity stress is exerted.
  • the positive polarity stress voltage level is approximately +14.5 V
  • the negative polarity stress voltage level is approximately ⁇ 14.5 V.
  • a ZAZ structure seems to exhibit a TBD distribution obtained when a bi-modal breakdown process is applied under a stress voltage of approximately ⁇ 14.5 V.
  • the AZAZA dielectric structure has a much longer lifetime than other dielectric structures. Accordingly, comparing with other dielectric structures, reliability of the AZAZA dielectric structure can be improved.
  • a dielectric structure of a flash memory device uses an OAZAZAO (SiO 2 /Al 2 O 3 /ZrO 2 /Al 2 O 3 /ZrO 2 /Al 2 O 3 /SiO 2 ) structure and thus, it is possible to obtain the flash memory device having a low CET level and a low leakage current level.
  • OAZAZAO SiO 2 /Al 2 O 3 /ZrO 2 /Al 2 O 3 /ZrO 2 /Al 2 O 3 /SiO 2
  • impurities such as silicon cannot diffuse into a grain boundary of a ZrO 2 layer and thus, reliability can be improved.

Abstract

A dielectric structure disposed between a floating gate and a control gate of a flash memory device includes: a first dielectric layer; a third dielectric layer having a k-dielectric constant substantially the same as that of the first dielectric layer; and a second dielectric layer disposed between the first dielectric layer and the third dielectric layer, having a greater k-dielectric constant than that of the first and third dielectric layers and formed by alternately and repeatedly stacking a plurality of aluminum oxide (Al2O3) layers and a plurality of zirconium oxide (ZrO2) layers.

Description

    CROSS-REFERENCES TO RELATED APPLICATIONS
  • The present application is a divisional of U.S. application Ser. No. 11/582,705, filed Oct. 17, 2006, which claims priority to Korean Application No. KR 2006-0036924, filed Apr. 24, 2006, which are incorporated herein by reference.
  • BACKGROUND OF THE INVENTION
  • The present invention relates to a method for fabricating a semiconductor device; and more particularly, to a flash memory device with a dielectric layer having a high k-dielectric constant and a method for fabricating the same.
  • When using a conventional oxide-nitride-oxide (ONO) structure as a dielectric structure (i.e., an inter-poly oxide (IPO) layer or an inter-poly dielectric (IPD) layer) existing between a floating gate (FG) and a control gate (CG), a distance between devices decreases as the integration scale of semiconductor devices improves. Accordingly, it becomes difficult to perform a gap-filling process and thus, the devices are often hardly isolated.
  • Furthermore, as a size of the device has been reduced, a property of a capacitor for storing electric charges may be degraded. Accordingly, a high k-dielectric thin layer having a small thickness and a high k-dielectric constant is used as the IPO layer or the IPD layer.
  • Recently, a process of using a high k-dielectric thin layer with an OZO structure, formed by sequentially stacking an oxide layer, a zirconium oxide (ZrO2) layer and an oxide layer has been suggested to overcome the aforementioned limitations.
  • FIG. 1 is a cross-sectional view illustrating a conventional flash memory device.
  • A tunnel oxide layer 12 is formed over a substrate 11, and a floating gate 13 is formed over the tunnel oxide layer 12. A dielectric structure 14 is formed over the floating gate 13. The dielectric structure 14 is formed by sequentially stacking a first silicon oxide (SiO2) layer 14A, a zirconium oxide (ZrO2) layer 14B, and a second silicon oxide (SiO2) layer 14C. A control gate 15 is formed over the dielectric structure 14.
  • However, in the case of using the ZrO2 layer, the ZrO2 layer may be crystallized at a predetermined thickness or greater while forming a thin layer. Accordingly, along a grain boundary, current is likely to leak or a defect may be generated. As a result, device reliability may be degraded.
  • BRIEF SUMMARY OF THE INVENTION
  • The present invention provides a flash memory device using a zirconium oxide (ZrO2) layer having a high k-dielectric constant as a dielectric layer, a low capacitive equivalent thickness (CET), a low leakage current level, and improved reliability, and a method for fabricating the same.
  • In accordance with one embodiment of the present invention, there is provided a dielectric structure disposed between a floating gate and a control gate of a flash memory device, including: a first dielectric layer; a third dielectric layer having a k-dielectric constant substantially the same as that of the first dielectric layer; and a second dielectric layer disposed between the first dielectric layer and the third dielectric layer, having a greater k-dielectric constant than that of the first and third dielectric layers and formed by alternately and repeatedly stacking a plurality of aluminum oxide (Al2O3) layers and a plurality of zirconium oxide (ZrO2) layers.
  • In accordance with another embodiment of the present invention, there is provided a flash memory device, including: a substrate; a tunnel oxide layer formed over the substrate; a floating gate formed over the tunnel oxide layer; a first dielectric layer formed over the floating gate; a second dielectric layer formed by alternately and repeatedly stacking a plurality of Al2O3 layers and a plurality of ZrO2 layers over the first dielectric layer and having a greater k-dielectric constant than that of the first dielectric layer; a third dielectric layer formed over the second dielectric layer and having substantially the same k-dielectric constant as that of the first dielectric layer; and a control gate formed over the third dielectric layer.
  • In accordance with a further embodiment of the present invention, there is provided a method for forming a dielectric structure disposed between a floating gate and a control gate of a flash memory device, including: forming a first dielectric layer; forming a second dielectric layer in a stack structure that has a greater k-dielectric constant than that of the first dielectric layer by alternately and repeatedly stacking a plurality of Al2O3 layers and a plurality of ZrO2 layers over the first dielectric layer; and forming a third dielectric layer that has substantially the same k-dielectric constant as that of the first dielectric layer over the second dielectric layer.
  • In accordance with still another embodiment of the present invention, there is provided a method for fabricating a flash memory device, including: forming a tunnel oxide layer over a substrate; forming a floating gate over the tunnel oxide layer; forming a first dielectric layer over the floating gate; forming a second dielectric layer in a stack structure that has a greater k-dielectric constant than that of the first dielectric layer by alternately and repeatedly stacking a plurality of Al2O3 layers and a plurality of ZrO2 layers over the first dielectric layer; forming a third dielectric layer that has substantially the same k-dielectric constant as that of the first dielectric layer over the second dielectric layer; and forming a control gate over the third dielectric layer.
  • BRIEF DESCRIPTION OF THE DRAWINGS
  • The above features of the present invention will become better understood with respect to the following description of the exemplary embodiments given in conjunction with the accompanying drawings, in which:
  • FIG. 1 is a cross-sectional view illustrating a conventional flash memory device;
  • FIG. 2 is a cross-sectional view illustrating a dielectric structure of a flash memory device in accordance with an embodiment of the present invention;
  • FIG. 3A is a graph of a capacitive equivalent thickness (CET) that varies depending on various stack structures;
  • FIG. 3B is a graph of leakage current level that varies depending on various stack structure;
  • FIG. 4 is a simplified diagram illustrating an atomic layer deposition (ALD) method to form an Al2O3/ZrO2/Al2O3/ZrO2/Al2O3 structure in accordance with an embodiment of present invention;
  • FIG. 5 is a cross-sectional view illustrating a flash memory device using the dielectric structure shown in FIG. 2; and
  • FIGS. 6A and 6B are Weibull plots illustrating time-to-breakdown (TBD) distributions in accordance with the embodiment of the present invention.
  • DETAILED DESCRIPTION OF THE INVENTION
  • Hereinafter, detailed descriptions on certain embodiments of the present invention will be provided with reference to the accompanying drawings.
  • FIG. 2 is a cross-sectional view illustrating a dielectric structure of a flash memory device in accordance with an embodiment of the present invention.
  • The dielectric structure 100 of the flash memory device includes a first dielectric layer 101; a third dielectric layer 103 having substantially the same k-dielectric constant as the first dielectric layer 101; and a second dielectric layer 102 disposed between the first dielectric layer 101 and the third dielectric layer 103. The second dielectric layer 102 has a greater k-dielectric constant than the first dielectric layer 101 and the third dielectric layer 103, and is formed by alternately and repeatedly stacking an aluminum oxide (Al2O3) layer 102A and a zirconium oxide (ZrO2) layer 102B. The dielectric structure 100 has a stack structure formed by sequentially stacking the first dielectric layer 101, the second dielectric layer 102 and the third dielectric layer 103. Particularly, these alternately and repeatedly stacked Al2O3 layer 102A and the ZrO2 layer 102B will be denoted as “AZAZA structure”.
  • The first dielectric layer 101 and the third dielectric layer 103 individually include a silicon oxide (SiO2) layer, and the second dielectric layer 102 is a stacked AZAZA structure. Thus, the dielectric structure 100 becomes an OAZAZAO structure.
  • In more detail about the second dielectric layer 102, the Al2O3 layer 102A contacts with the first dielectric layer 101 and the third dielectric layer 103 as shown in the AZAZA structure to reduce silicon diffusion. Specifically, the Al2O3 layer 102A can prevent silicon from the first dielectric layer 101 and from the third dielectric layer 103 from diffusing to the corresponding ZrO2 layer 102B by being interposed between the first dielectric layer 101 and the ZrO2 layer 102B and between the third dielectric layer and the ZrO2 layer 102B.
  • Accordingly, the OAZAZAO structure in this embodiment of the present invention has a stack structure which additionally includes the Al2O3 layers 102A between the SiO2 layers and the ZAZ structure. In one embodiment of the present invention, a k-dielectric constant of the ZrO2 layer 102B is not decreased due to the additionally formed Al2O3 layers 102A.
  • Furthermore, for the number of stacked Al2O3 layers, the OAZAZAO structure includes two more Al2O3 layers than the ZAZ structure and thus, the ZrO2 layer 102B can have substantially the same k-dielectric constant as the ZAZ structure although the ZrO2 layers 102B are thinly formed. Accordingly, since the ZrO2 layers 102B can be thinly formed, the ZrO2 layers 102B is not likely to be crystallized while forming the ZrO2 layers 102B.
  • As shown in the OAZAZAO structure (or stacked structure), the ZrO2 layer 102B is stacked twice and the Al2O3 layer 102A is stacked three times. It should be appreciated that stacking more of the Al2O3 layers could be undesirable. For instance, if the Al2O3 layer 102A is stacked four times, the number of the ZrO2 layers 102B to be stacked is decreased to one. That is, if a proportion of the ZrO2 layers 102B is decreased (i.e., from two layers to one layer) in the stacked structure, a capacitive equivalent thickness (CET) and a leakage current level are increased. Accordingly, a desirable stack structure to obtain a low CET and a low leakage current level is the OAZAZAO structure.
  • FIG. 3A is a graph of a capacitive equivalent thickness (CET) that varies depending on various stack structures and FIG. 3B is a graph of leakage current level that varies depending on various stack structures. The stacked dielectric structures compared with each other in FIGS. 3A and 3B are a ZAZ structure, an AZAZA structure, an AZAZAZA structure and a ZrO2 layer.
  • As shown in FIG. 3A, the ZrO2 layer has the lowest CET and the ZAZ structure has the highest CET. The AZAZA structure has a larger CET than the AZAZAZA structure but a lower CET than the ZAZ structure.
  • As shown in FIG. 3B, the ZrO2 layer has the highest leakage current, and the AZAZA structure has the lowest leakage current. For reference, the AZAZAZA structure has higher leakage current than the ZAZ structure.
  • From the results shown in FIGS. 3A and 3B, the AZAZA structure has a larger CET and a lower leakage current than the AZAZAZA structure and the ZAZ structure.
  • The second dielectric layer 102 with the AZAZA structure shown in FIG. 2 can be formed through an atomic layer deposition (ALD) method or a chemical vapor deposition (CVD) method.
  • Hereinafter, a deposition method of the AZAZA structure through the ALD method will be examined.
  • FIG. 4 is a diagram briefly illustrating an ALD method to form an AZAZA structure in accordance with an embodiment of the present invention.
  • The ALD method is generally performed by repeating a unit cycle until an atomic layer with a desirable thickness is obtained. The unit cycle includes supplying a source gas to a wafer loaded into a chamber, supplying a purge gas thereto, supplying a reaction gas thereto, and supplying a purge gas thereto.
  • In more detail of an Al2O3 layer deposition, an aluminum (Al) source gas is supplied to a chamber where a wafer is loaded and then, absorbed into a surface of the wafer. The Al source gas includes Al(CH3)3.
  • A purge gas is supplied into the chamber to purge out non-adsorbed parts of the Al source gas remaining inside the chamber. The purge gas includes an inert gas selected from a group consisting of argon (Ar), helium (He), nitrogen (N2), or a combination thereof.
  • A reaction gas is supplied into the chamber. The reaction gas includes one selected from a group consisting of ozone (O3), oxygen (O2) plasma, and water (H2O). A reaction between the Al source gas and the reaction gas is induced to deposit an Al2O3 layer. Accordingly, the Al2O3 layer is formed over the surface of the wafer in an atomic unit.
  • A purge gas is supplied into the chamber to purge out non-reacted parts of the reaction gas and byproducts. The purge gas includes an inert gas selected from a group consisting of Ar, He, H2, or a combination thereof.
  • As described above, the Al2O3 layer with a desirable thickness can be deposited by repeating the unit cycle including supplying the Al source gas, supplying the purge gas, supplying the reaction gas, and supply the purge gas. Theses sequential steps are denoted with reference letters T1, T2, T3, and T4 in FIG. 4.
  • Depositing a ZrO2 layer based on the ALD method will be examined hereinafter.
  • A zirconium (Zr) source gas is supplied to a chamber where a wafer is loaded and then, adsorbed into a surface of the wafer. The Zr source gas includes one selected from a group consisting of Zr(O-tBu)4, Zr[N(CH3)2]4, Zr[N(C2Hs)(CH3)]4, Zr[N(C2H5)2]4, Zr(tmhd)4, Zr(OiC3H7)3(tmhd) and Zr(OtBu)4. The above listed materials are known as a precursor.
  • A purge gas is supplied into the chamber to purge out non-adsorbed parts of the Zr source gas remaining inside the chamber. The purge gas includes an inert gas selected from a group consisting of Ar, He, N2, or a combination thereof.
  • A reaction gas is supplied into the chamber. The reaction gas includes one selected from a group consisting of O3, O2 plasma and H2O. A reaction between the Zr source gas and the reaction gas is induced to deposit a ZrO2 layer. Accordingly, the ZrO2 layer is formed over the surface of the wafer in an atomic unit.
  • A purge gas is supplied into the chamber to purge out non-reacted parts of the reaction gas and byproducts. The purge gas includes an inert gas selected from a group consisting of Ar, He, N2, or a combination thereof.
  • As described above, the ZrO2 layer with a desirable thickness can be deposited by repeating the unit cycle including supplying the Zr source gas, supplying the purge gas, supplying the reaction gas, and supplying the purge gas. These sequential steps are denoted with reference letters T11, T12, T13, and T14 in FIG. 4.
  • The Al2O3 layers and the ZrO2 layers are alternately deposited through the ALD method to form the AZAZA structure. Meanwhile, the depositing of the Al2O3 layers and the ZrO2 layers uses the same purge gas and reaction gas except for the source gas. Accordingly, the Al2O3 layers and the ZrO2 layers can be alternately deposited only by changing the source gas at the same ALD chamber.
  • FIG. 5 is a cross-sectional view illustrating a flash memory device using the dielectric structure shown in FIG. 2.
  • The flash memory device includes a substrate 201, a tunnel oxide layer 202 over the substrate 201, a floating gate FG 203, a first dielectric layer 204 over the floating gate FG 203, a second dielectric layer 205 having a k-dielectric constant greater than the first dielectric layer 204 formed over the first dielectric layer 204 by alternately and repeatedly stacking an Al2O3 layer 205A and a ZrO2 layer 205B, a third dielectric layer 206 formed over the second dielectric layer 205 and having substantially the same k-dielectric constant as the first dielectric layer 204, and a control gate CG 207 over the third dielectric layer 206.
  • The tunnel oxide layer 202 includes a SiO2 layer, and the floating gate FG 203 and the control gate CG 207 includes a polysilicon layer.
  • The first dielectric layer 204 and the third dielectric layer 206 are SiO2 layers, and the second dielectric layer 205 has an AZAZA structure, which is a stack structure of the Al2O3 layer 205A, the ZrO2 layer 205B, the Al2O3 layer 205A, the ZrO2 layer 205B, and the Al2O3 layer 205A.
  • A method for fabricating the flash memory device shown in FIG. 5 will be examined hereinafter.
  • The tunnel oxide layer 202 is formed over the substrate 201 completed with a well formation process and a channel ion-implantation process and other related processes. The tunnel oxide layer 202 is formed through one of a thermal oxidation method and a CVD method.
  • The floating gate 203 is formed over the tunnel oxide layer 202. The floating gate 203 includes the polysilicon layer.
  • The first dielectric layer 204 is formed over the floating gate 203. The first dielectric layer includes the SiO2 layer formed through an ALD method or a CVD method.
  • A dielectric layer having a greater k-dielectric constant than the first dielectric layer 204 is formed over the first dielectric layer 204. That is, an AZAZA structure is formed by alternately and repeatedly stacking the Al2O3 layer 205A and the ZrO2 layer 205B over the first dielectric layer 204. When the Al2O3 layer 205A and the ZrO2 layer 205B are alternately and repeatedly stacked, the Al2O3 layer 205A contacts with the first dielectric layer 204 or the third dielectric layer 206. The third dielectric layer 206 is formed over the second dielectric layer 205. The third dielectric layer 206 includes the SiO2 layer as similar to the first dielectric layer 204. Accordingly, the high k-dielectric second dielectric layer 205 formed between the low k-dielectric first dielectric layer 204 and the low k-dielectric third dielectric layer 206 gives rise to an OAZAZAO structure.
  • The control gate 207 is formed over the third dielectric layer 206, and the control gate 207 includes the polysilicon layer.
  • The dielectric structure of the flash memory device between the floating gate 203 and the control gate 207 is a stack structure of the first dielectric layer 204, the second dielectric layer 205, and the third dielectric layer 206, more particularly, of an OAZAZAO (SiO2/Al2O3/ZrO2/Al2O3/ZrO2/Al2O3/SiO2) structure. The OAZAZAO structure is formed with a thickness ranging from approximately 50 Å to approximately 200 Å.
  • Comparing the OAZAZAO dielectric structure with the OZAZO dielectric structure, the OAZAZAO dielectric structure can prevent silicon from diffusing to the ZrO2 layer 205B by interposing repeatedly the Al2O3 layer 205A between the ZrO2 layer 205B before depositing the third electric layer including the SiO2 layer. Accordingly, a ZrSiO layer cannot be formed, and a k-dielectric constant of the ZrO2 layer cannot be reduced.
  • Furthermore, in an OZAO structure or an OZO structure, as impurities such as silicon diffuse into a grain boundary of the ZrO2 layer 205B, a leakage current is likely to increase and device reliability may be degraded. However, in the OAZAZAO structure, the above described limitations can be reduced by repeatedly interposing the Al2O3 layer 205A between the ZrO2 layers 205B. Thus, the leakage current can be decreased and the device reliability can be improved.
  • FIGS. 6A and 6B are Weibull plots illustrating time-to-breakdown (TBD) distributions in accordance with an embodiment of the present invention. FIG. 6A shows the TBD distribution when a positive polarity stress is exerted, and FIG. 6B shows the TBD distribution when a negative polarity stress is exerted. Herein, the positive polarity stress voltage level is approximately +14.5 V and the negative polarity stress voltage level is approximately −14.5 V.
  • A ZAZ structure seems to exhibit a TBD distribution obtained when a bi-modal breakdown process is applied under a stress voltage of approximately −14.5 V. The ZAZ structure seems to have small (with β=approximately 1.25) and large (with β=approximately 2.0) TBD distribution components together.
  • In more details about FIGS. 6A and 6B, the AZAZA dielectric structure has a much longer lifetime than other dielectric structures. Accordingly, comparing with other dielectric structures, reliability of the AZAZA dielectric structure can be improved.
  • According to this embodiment of the present invention, a dielectric structure of a flash memory device uses an OAZAZAO (SiO2/Al2O3/ZrO2/Al2O3/ZrO2/Al2O3/SiO2) structure and thus, it is possible to obtain the flash memory device having a low CET level and a low leakage current level.
  • Furthermore, impurities such as silicon cannot diffuse into a grain boundary of a ZrO2 layer and thus, reliability can be improved.
  • While the present invention has been described with respect to certain embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.

Claims (10)

1. A non-volatile memory device, comprising:
a floating gate provide over a substrate;
a control gate provided over the floating gate; and
a dielectric structure provided between the floating gate and the control gate, the dielectric structure including:
a first dielectric layer;
a second dielectric layer formed over the first dielectric layer and having a k-dielectric constant greater than that of the first dielectric layer, the second dielectric layer including a plurality of aluminum oxide (Al2O3) layers and a plurality of zirconium oxide (ZrO2) layers; and
a third dielectric layer formed over the second dielectric layer.
2. The device of claim 1, wherein a first Al2O3 layer and a last Al2O3 layer contact the first dielectric layer and the third dielectric layer, respectively,
wherein the third dielectric layer has a k-dielectric constant substantially the same as that of the first dielectric layer.
3. The device of claim 1, wherein a first Al2O3 layer and a third Al2O3 layer contact the first dielectric layer and the third dielectric layer, respectively, and
wherein the second dielectric layer comprises the first Al2O3 layer, a first ZrO2 layer, a second Al2O3 layer, a second ZrO2 layer, and the third Al2O3 layer.
4. The device of claim 3, wherein the first dielectric layer and the third dielectric layer comprise a silicon oxide (SiO2) layer.
5. The device of claim 4, wherein a total thickness of the dielectric structure from the first dielectric layer to the third dielectric layer ranges from approximately 50 Å to approximately 200 Å.
6. A flash memory device, comprising:
a substrate;
a tunnel oxide layer formed over the substrate;
a floating gate formed over the tunnel oxide layer;
a first dielectric layer formed over the floating gate;
a second dielectric layer over the first dielectric layer and formed by alternately stacking a aluminum oxide (Al2O3) layer and a zirconium oxide (ZrO2) layer, so that the second dielectric layer has at least five different layers, the second dielectric layer having a greater k-dielectric constant than that of the first dielectric layer;
a third dielectric layer formed over the second dielectric layer; and
a control gate formed over the third dielectric layer.
7. The flash memory device of claim 6, wherein the Al2O3 layers and the ZrO2 layers of the second dielectric layer are stacked such that a first Al2O3 layer and a third Al2O3 layer contact the first dielectric layer and with the third dielectric layer, respectively, and
wherein the third dielectric layer having a k-dielectric constant substantially the same as that of the first dielectric layer.
8. The flash memory device of claim 6, wherein the Al2O3 layers and the ZrO2 layers of the second dielectric layer are stacked such that a first Al2O3 layer and a last Al2O3 layer contact the first dielectric layer and with the third dielectric layer, respectively,
wherein the third dielectric layer having a k-dielectric constant substantially the same as that of the first dielectric layer, and
wherein the second dielectric layer comprises the first Al2O3 layer, a first ZrO2 layer, a second Al2O3 layer, a second ZrO2 layer, and the last Al2O3 layer.
9. The flash memory device of claim 8, wherein the first dielectric layer and the third dielectric layer comprise a SiO2 layer.
10. The flash memory device of claim 9, wherein a total thickness of the first dielectric layer, the second dielectric layer, and the third dielectric layer ranges from approximately 50 Å to approximately 200 Å.
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