US20050006683A1 - Capacitor and method for fabricating ferroelectric memory device with the same - Google Patents
Capacitor and method for fabricating ferroelectric memory device with the same Download PDFInfo
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- US20050006683A1 US20050006683A1 US10/899,171 US89917104A US2005006683A1 US 20050006683 A1 US20050006683 A1 US 20050006683A1 US 89917104 A US89917104 A US 89917104A US 2005006683 A1 US2005006683 A1 US 2005006683A1
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Images
Classifications
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L28/00—Passive two-terminal components without a potential-jump or surface barrier for integrated circuits; Details thereof; Multistep manufacturing processes therefor
- H01L28/40—Capacitors
- H01L28/55—Capacitors with a dielectric comprising a perovskite structure material
- H01L28/56—Capacitors with a dielectric comprising a perovskite structure material the dielectric comprising two or more layers, e.g. comprising buffer layers, seed layers, gradient layers
-
- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10B—ELECTRONIC MEMORY DEVICES
- H10B53/00—Ferroelectric RAM [FeRAM] devices comprising ferroelectric memory capacitors
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
- H01L27/02—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers
- H01L27/04—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers the substrate being a semiconductor body
- H01L27/10—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers the substrate being a semiconductor body including a plurality of individual components in a repetitive configuration
- H01L27/105—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers the substrate being a semiconductor body including a plurality of individual components in a repetitive configuration including field-effect components
-
- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10B—ELECTRONIC MEMORY DEVICES
- H10B53/00—Ferroelectric RAM [FeRAM] devices comprising ferroelectric memory capacitors
- H10B53/30—Ferroelectric RAM [FeRAM] devices comprising ferroelectric memory capacitors characterised by the memory core region
Definitions
- the present invention relates to a semiconductor device; and, more particularly, to a capacitor for use in a semiconductor device and a method for fabricating a having the same.
- a nonvolatile ferroelectric random access memory is expected to be a promising memory device in near future due to some advantages of saving information even if a power is off and competing with the prior DRAM device in terms of an operation speed.
- the nonvolatile memory device using the ferroelectric material inputs a signal by controlling a polarization direction controlled by applying an electric field, wherein a remnant polarization formed after the electric field is cut off is used for saving a digital signal 1 or 0 .
- SBT SrBi 2 Ta 2 O 9
- PZT Pb(Zr, Ti)O 3
- a new ferroelectric material such as Bi 4-x La x Ti 3 O 12 (BLT) and Bi 4 Ti 3 O 12 (BTO) have advantageous properties such as a superior reliability of the SBT and a low crystallization temperature of the PZT including a strong polarizability.
- FIG. 1A shows a cross sectional view illustrating a conventional ferroelectric memory device.
- an isolation layer 12 predetermining an active region is formed on a semiconductor substrate 11 .
- a stacked structure of a gate oxide layer 13 and a word line 14 is formed on the semiconductor substrate 11 thereafter.
- Source/drain regions 15 A and 15 B are formed on both right and left sides adjacent to the word line 14 .
- a first interfacial insulation layer 16 is formed on a transistor containing the word line 14 and the source/drain regions 15 A and 15 B.
- a bit line 18 is connected to one source/drain region 15 A through a bit line contact 17 contacting to one source/drain region 15 A by passing through the first interfacial insulation layer 16 .
- a second interfacial insulation layer 19 containing the bit line 18 is formed on an entire upper area of the transistor and a storage node contact 20 is formed thereon, wherein the first interfacial insulation layer 16 and the second interfacial insulation layer 19 are penetrated together in order to make the storage node contact 20 connected to the other source/drain region 15 B.
- a lower electrode 21 which should be connected to the storage node contact 20 is formed. Also, a planerized isolating insulation layer 22 for a separation of the lower electrode 21 from an adjacent electrode encompasses the lower electrode 21 . Next, a ferroelectric layer 23 covers the insolating insulation layer 22 and the lower electrode 21 , wherein the ferroelectric layer 21 uses the BTO or BLT and is formed only on a cell region.
- An upper electrode 24 is formed on the formed ferroelectric layer 23 .
- a capacitor structure shown in FIG. 1A is named as a merged top plate (MTP).
- the BTO or BLT capacitor has a disadvantage that the BTO or BLT ferroelectric material is operated at a high speed in a high temperature ambience, a data fatigue occurs generally and drastically, so that it causes a device data failure. Furthermore, the data fatigue brings about deoxidization of titanium (Ti) ions. More specifically, the Ti ion deoxidization is caused by a charge impact externally applied. In addition, the Ti ion deoxidization causes some defects, which are concentrated or diffused into an electrode interface, and consequently, those defects suppress a dipole formation. From this, the polarizability of the BTO or BLT is reduced. Eventually, the film impaired with such defects within its interface is prone to damage because a conductor phenomenon increases an applied electric field to the interface.
- Ti titanium
- FIG. 1B is a graph showing a result of a reliability test for BLT film fabricated by a conventional method, wherein a horizontal axis represents a switching cycle, a vertical axis denotes a polarization value (P* ⁇ P ⁇ circumflex over ( ) ⁇ ), and +dP and ⁇ dP mean a positive polarization value and a negative polarization value, respectively.
- FIG. 1C is a graph showing an attenuation range of the reliability in a numerical value, wherein an horizontal axis denotes a test temperature and a vertical axis describes a polarization value (P* ⁇ P ⁇ circumflex over ( ) ⁇ ).
- a remnant polarization is drastically reduced at about 125° C. ( ⁇ , ⁇ ). More precisely, the reliability is drastically reduced at a high switching cycle point when data is rapidly used for a long time. In addition, the reliability is reduced at the high switching cycle point even at a room temperature.
- the polarization value is drastically reduced as a measurement temperature gets high. More precisely, the polarization value (p* ⁇ p ⁇ circumflex over ( ) ⁇ ) is abruptly reduced from about 18 ⁇ C/cm 2 to less than about 10 ⁇ C/cm 2 .
- an object of the present invention to provide a method for fabricating a ferroelectric memory device capable of suppressing a defect generation caused by a charge impact.
- the ferroelectric memory device including: a semiconductor substrate on which a transistor is formed; a lower electrode on the interfacial insulation layer, which is connected to a source/drain region of the transistor through a contact penetrating the interfacial insulation layer; an isolating insulation layer on the interfacial insulation layer, which has a planerized surface exposing a surface of the lower electrode and encompasses the lower electrode; a ferroelectric layer covering the isolating insulation layer and lower electrode; an oxygen vacancy compensation layer on the ferroelectric layer compensating an oxygen vacancy caused by a deoxidization of a composition of the ferroelectric layer; and an upper electrode formed on the oxygen vacancy compensation layer.
- the method for fabricating the ferroelectric memory device including: forming the interfacial insulation layer on the semiconductor substrate; forming a stack pattern of the lower electrode and a hard mask in order on the interfacial insulation layer; forming the isolating insulation layer on an entire surface having the stack pattern; planerizing the isolating insulation layer until exposing a surface of the hard mask; removing the hard mask by using a liquid chemical; forming the ferroelectric layer on an entire surface having the lower electrode exposed after the hard mask is removed; forming the oxygen vacancy compensation layer on the ferroelectric layer; forming a conductive layer for the upper electrode on the oxygen vacancy compensation layer; and pattering the conductive layer and the oxygen vacancy compensation layer consecutively.
- FIG. 1A is a cross-sectional view illustrating a conventional ferroelectric memory device
- FIG. 1B is a graph showing reliability measurement result of a Bi 4-x La x Ti 3 O 12 (BLT) layer fabricated by employing a conventional method;
- FIG. 1C is a graph showing an attenuation range of the reliability for the conventional BLT layer in a numerical value
- FIG. 2 is a cross-sectional view illustrating a ferroelectric memory device having a capacitor with a merged top plate (MTP) structure in accordance with the present invention.
- MTP merged top plate
- FIGS. 3A to 3 F are cross-sectional views illustrating a method for fabricating the ferroelectric device illustrated in FIG. 2 ;
- FIG. 4A is a graph showing a reliability measurement result for the BLT layer in accordance with a preferred embodiment of the present invention.
- FIG. 4B is a graph showing an attenuation range of the reliability, in a numerical value, for BLT layer in accordance with the preferred embodiment of the present invention.
- FIG. 2 is a cross-sectional view illustrating a ferroelectric memory device containing a merged top plate (MTP) structure in accordance with preferred embodiment of the present invention.
- MTP merged top plate
- a capacitor of the ferroelectric memory device will be explained.
- a lower electrode deposited sequentially with an iridium layer 41 A, an iridium oxide layer 42 A, and a platinum layer 43 A is planerized while the lower electrode is encompassed by an isolating insulation layer 46 A.
- a ferroelectric layer 47 is formed on the planerized lower electrode and the isolating insulation layer 46 A.
- an oxygen vacancy compensation layer 48 and an upper electrode are deposited on the ferroelectric layer 47 .
- the oxygen vacancy compensation layer 48 using a metal oxide layer such as ruthenium (Ru) or iridium (Ir) oxide serves to compensate a defect, i.e. oxygen vacancy, caused by deoxidization of titanium (Ti) within a Bi 4-x La x Ti 3 O 12 (BLT) or Bi 4 Ti 3 O 12 (BTO) that is used to form the ferroelectric layer 47 .
- the oxygen vacancy compensation layer 48 has a thickness of about 10 ⁇ to about 1000 ⁇ , wherein the oxygen vacancy compensation layer 48 is deposited at a temperature ranging from about 100° C. to about 700° C. and at a pressure of about 0.1 mtorr to about 10 torr by using a chemical vapor deposition (CVD) or an atomic layer deposition (ALD) technique.
- CVD chemical vapor deposition
- ALD atomic layer deposition
- the BLT or BTO is used for forming the ferroelectric layer 47 , the ferroelectric layer 47 has a thickness of about 50 ⁇ to about 3000 ⁇ , and one or a few layers selected out of such materials as platinum (Pt), iridium (Ir), ruthenium (Ru), tungsten (W), iridium oxide layer, ruthenium oxide layer, tungsten nitride layer, and titanium nitride layer are used to form the upper electrode 49 .
- platinum platinum
- Ir iridium
- Ru ruthenium
- W tungsten
- iridium oxide layer ruthenium oxide layer
- tungsten nitride layer tungsten nitride layer
- titanium nitride layer titanium nitride layer
- HDP high density plasma
- BPSG Boro-Phospho-Silicate-Glass
- BSG Boro-Silicate-Glass
- PSG Phospho-Silicate-Glass
- a transistor containing source/drain regions 35 A and 35 B, a gate oxide layer 33 , and a word line 34 is formed on the semiconductor substrate 31 , wherein a device isolation layer 32 is formed in advance on the semiconductor substrate 31 before forming the transistor.
- An upper surface of the semiconductor substrate 31 is covered with a first interfacial layer 36 , and a bit line contact 37 is connected to one of the source/drain region 35 A by passing through the first interfacial layer 36 .
- a second interfacial insulation layer 39 is formed on the first interfacial insulation layer 36 , and both of the first and second interfacial insulation layers 39 are penetrated throughout the above structure so as to connect a storage node contact 40 to the other source/drain region 35 B.
- the storage node contact 40 is connected to the iridium layer 41 A used for the lower electrode, wherein the storage node contact 40 is formed sequentially with a poly silicon plug, titanium silicide, and titanium nitride.
- FIGS. 3A to 3 F show cross-sectional views illustrating a method for fabricating the ferroelectric memory device illustrated in FIG. 2 .
- the device isolation layer 32 for separating devices is formed on the semiconductor substrate 31 in order to predetermine an active area.
- the gate oxide layer 33 and the word line 34 are sequentially formed on the active area of the semiconductor substrate 31 .
- both sides of the word line 34 on the semiconductor substrate 31 are implanted with an impurity in order to form the source/drain regions 35 A and 35 B.
- a spacer may be formed at both lateral sides of the word line 34 , although it is not illustrated in the aforementioned drawings.
- the source/drain regions 35 A and 35 B for a light doped drain (LDD) structure can be formed. More precisely, the spacer is formed at the both lateral sides of the word line after forming the LDD area by carrying out a low concentration impurity implantation using a mask, and the source/drain regions 35 A and 35 B adjacent to the LDD area are formed by performing a high concentration impurity implantation using a mask into the word line 34 and spacer.
- LDD light doped drain
- a bit line contact hole exposing one source/drain region 35 A formed by etching the first interfacial insulation layer 36 through the use of a contact mask (not shown in FIG. 3A ) is formed after the first interfacial insulation layer 36 is deposited on the semiconductor substrate providing the transistor and planerized thereafter.
- the bit line contact 37 buried in the bit line contact hole is formed, wherein the bit line contact 37 can be a tungsten plug formed by an etch back process or a chemical mechanical polishing (CMP) process after depositing the tungsten.
- CMP chemical mechanical polishing
- a conductive layer for the bit line is deposited on an entire surface and patterned so as to form the bit line 38 connected to the bit line contact 37 , and the second interfacial insulation layer 39 including a bit line is deposited and planerized.
- a storage node contact hole exposing the other source/drain region 35 B is formed by etching both of the second interfacial insulation layer 39 and the first interfacial insulation layer 36 with a storage node contact mask not shown is buried in the storage node contact hole thereafter.
- the storage node contact 40 has a multi layer structure constructed sequentially with a poly silicon plug, a titanium silicide layer, and a titanium nitride layer not shown.
- titanium nitride layer is used to form an ohmic contact between the poly silicon plug and the lower electrode
- the titanium silicide layer used for a diffusion barrier layer functions to suppress a reciprocal diffusion.
- the diffusion barrier layer is subsequently planerized by performing a CMP process.
- a titanium silicide layer is formed after annealing a deposited titanium layer and plugging a tungsten layer. Subsequent to the titanium silicide layer formation, a titanium nitride layer acting as a barrier metal is formed and planerized thereafter by performing a CMP process.
- a iridium (Ir) layer 41 A, an iridium oxide layer (IrO 2 ) 42 A, and a platinum (Pt) 43 A are deposited in order, wherein the Ir, IrO 2 , and Pt are used as the conductive layer used as the lower electrode on the second interfacial insulation layer 39 having the storage node contact 40 .
- those layers mentioned above have a thickness of about 200 ⁇ to about 2000 ⁇ .
- a titanium nitride layer acting as a hard mask which is used for patterning the lower electrode on the Pt layer 43 A is deposited, and a patterned photo-resist layer 45 predetermining the lower electrode on the titanium nitride layer 44 is also formed.
- the titanium nitride layer 44 has a thickness of about 50 ⁇ to about 1000 ⁇ and shows a superior slope property when a patterning process is carried out on the lower electrode.
- the titanium nitride layer 44 is etched and patterned by using the patterned photo-resist mask.
- the patterned photo-resist mask 45 is removed, and the pt layer 43 A, the iridium oxide layer 42 A, and the iridium layer 41 A are all patterned in order by using the patterned titanium nitride layer 44 .
- the lower electrode having a triple layer stack structure is formed.
- the triple layer stack structure is constituted with Ir layer 41 A used as an oxygen barrier, the IrO 2 layer 42 A used as a glue layer, and the Pt layer 43 A.
- the Pt layer 43 A which is one of the elements of the lower electrode can be substituted for a ruthenium (Ru) layer, an iridium (Ir) layer (Ir), a tungsten (W) layer, or a tungsten nitride layer.
- the iridium (IrO 2 ) oxide layer 42 A which is used for the glue layer, can be substituted for a ruthenium oxide layer, or a tungsten oxide layer.
- the Ir layer 41 A which is used as the oxygen barrier layer, can be also substituted for a ruthenium (Ru) layer.
- the remnant titanium layer is denoted as reference numeral, 41 A.
- a third interfacial insulation layer 46 including the lower electrode is formed on an entire surface area until a space between neighboring lower electrodes is completely filled up.
- the third interfacial insulation layer 46 uses one of such materials as a HDP oxide layer, BPSG, BSG, or PSG, and it has a thickness of about 1000 ⁇ to about 10000 ⁇ .
- an isolating insulation layer 46 A encompassing the lower electrode is formed by planerizing the third interfacial insulation layer 46 until a surface of the titanium nitride layer 44 A is exposed. At this time, the Pt layer 43 A is not damaged even by an excessive CMP process because the titanium nitride 44 A is capping the Pt layer 43 A.
- the remnant titanium nitride layer 44 A on the platinum layer 43 A is removed by soaking it into a SC-1 chemical solution.
- the isolating insulation layer 46 A is also removed easily by the SC-1 chemical solution because it is also an oxide layer.
- a portion of the isolating insulation layer 46 A is removed until the surface of the platinum layer 43 A is exposed.
- the SC-1 chemical solution is a solution of HCl, H 2 O 2 , and H 2 O and the solution is vigorously reactive with any metal layer. Therefore, the isolating insulation layer 46 A is also removed easily.
- a remnant isolating insulation layer 46 A′ has a structure which insulates and encompasses the neighboring lower electrodes.
- the structure such that the isolating insulation layer 46 A′ encompasses the lower electrode overcomes difficulties in applying a masking process and a planerizing process caused by a step difference existing in the capacitor.
- the structure can prevent a short circuit between the lower electrode and the upper electrode.
- the BLT or BTO ferroelectric layer having a thickness of about 50 ⁇ to about 3000 ⁇ is deposited after finishing a CMP process, and the oxygen vacancy compensation layer 48 is formed on the ferroelectric layer 47 .
- the ferroelectric layer 47 goes through a sequence of a nucleation, a nucleus growth, and crystallization.
- a rapid thermal anneal (RTP) process is used for the nucleus growth.
- the RTP process is usually performed at a temperature of about 400° C. to about 800° C., and a ramp-up rate ranges from about 80° C./sec to about 250° C./sec.
- the crystallization is carried out by a typical furnace annealing process at about 500° C. to about 800° C. using one of such ambient gases as O 2 , N 2 O, N 2 , Ar, Ne, Kr, Xe, and He.
- the ferroelectric layer 47 having a thickness of about 50 ⁇ to about 3000 ⁇ is formed by a spin coating process or a liquid source mixed chemical deposition (LSMCD) process.
- LSMCD liquid source mixed chemical deposition
- the oxygen vacancy compensation layer 48 compensates a defect, that is, an oxygen vacancy generated by deoxidization of titanium within the BLT or BTO used as the ferroelectric layer 47 .
- a metal oxide layer such as a ruthenium (RuO x ) oxide layer or iridium (IrO x ) oxide layer is usually used for forming the oxygen vacancy compensation layer 48 .
- RuO x ruthenium
- IrO x oxide layer is usually used for forming the oxygen vacancy compensation layer 48 .
- a substitution between the oxygen of ruthenium oxide layer or iridium oxide layer and the defect caused by the titanium deoxidization generated by an externally applied charge impact occurs due to the use of the oxygen vacancy compensation layer 48 . Consequently, the titanium can obtain a stable structure without any defect. Furthermore, a fatigue phenomenon can be also stabilized.
- the oxygen vacancy compensation layer 48 has a thickness of about 10 ⁇ to about 1000 ⁇ and is deposited by a CVD process or an ALD process at a temperature of about 100° C. to about 700° C. at a pressure of about 0.1 mtorr to about 10 torr.
- a conductive layer for the upper electrode 49 is formed on the oxygen vacancy compensation layer 48 .
- the conductive layer is photo-resisted and etched so that the upper electrode 49 is formed.
- the oxygen vacant compensation layer 48 is etched and remained between the upper electrode 49 and the ferroelectric layer 47 .
- the conductive layer is formed by using a CVD, physical vapor deposition (PVD), ALD, or plasma enhanced atomic layer deposition (PEALD) process.
- the conductive layer consists of one or a few materials selected among platinum (Pt), iridium (Ir), ruthenium (Ru), tungsten (W), an iridium oxide layer, a ruthenium oxide layer, a tungsten nitride layer and a titanium nitride layer.
- the capacitor formed in accordance with the present invention is called a MTP capacitor.
- the lower electrode of the MTP capacitor is encompassed by the isolating insulation layer.
- the isolating insulation layer 46 A prevents a loss of the ferroelectric layer 47 caused by plasma during an etching process. Consequently, an effect of the defect within the ferroelectric layer 47 is abruptly reduced.
- the lower electrode having the triple layer stack structure constructed with sequential deposition of the oxygen barrier layer, the glue layer and the metal layer is suggested. It is still possible to form a lower electrode having a single metal layer, wherein the single metal layer is one of such materials as Pt, Ru, Ir, W, and WN.
- SBT SrBi 2 Ta 2 O 9
- SBTN SrBi 2 (Ta 1-x , Nb x ) 2 O 9
- FIG. 4A is a graph illustrating a result of a reliability measurement for the BLT layer formed in accordance with the present invention.
- a horizontal axis denotes a switching cycle and a vertical axis represents a polarization value (P* ⁇ P ⁇ circumflex over ( ) ⁇ ).
- Reference symbols, +dP and ⁇ dP mean a positive polarization value and a negative polarization value, respectively.
- FIG. 4B shows an attenuation range of the reliability in a numerical value.
- a horizontal axis represents a measurement temperature and an vertical axis denotes a polarization value (P* ⁇ P ⁇ circumflex over ( ) ⁇ ).
- the preferred embodiment of the present invention is reliable even at a high switching cycle when data is rapidly used.
- the remnant polarization is slowly reduced as the measurement temperature is increased, and eventually, the remnant polarization value is minimized at about 18 ⁇ C/cm 2 to about 10 ⁇ C/cm 2 .
- the remnant polarization value is drastically decreased at about 125° C. ( ⁇ , ⁇ ) as the measurement temperature is increased.
- the remnant polarization value is slowly decreased at the same condition as described above. Accordingly, the present invention shows that the oxygen vacant compensation layer between the BLT layer and the upper electrode improves the reliability of the ferroelectric memory device.
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Abstract
The present invention provides a ferroelectric memory device capable of suppressing a defect generation due to a charge impact and a method for fabricating the same. The ferroelectric memory device includes: a semiconductor substrate on which a transistor is formed; a semiconductor substrate structure having a transistor; a lower electrode formed on an interfacial insulation layer and connected to a source/drain region of the transistor; an isolating insulation layer on the interfacial insulation layer; a ferroelectric layer covering the isolating insulation layer and lower electrode; an oxygen vacancy compensation layer being formed on the ferroelectric layer and compensating an oxygen vacancy caused by deoxidization of a composition of the ferroelectric layer; and an upper electrode formed on the oxygen vacancy compensation layer.
Description
- The present invention relates to a semiconductor device; and, more particularly, to a capacitor for use in a semiconductor device and a method for fabricating a having the same.
- With using a ferroelectric material for a capacitor used in a semiconductor memory device, a development of the device has been progressed in order to overcome a refresh limit of a prior dynamic random access memory (DRAM) device and use a large quantity of memory. A nonvolatile ferroelectric random access memory (FeRAM) is expected to be a promising memory device in near future due to some advantages of saving information even if a power is off and competing with the prior DRAM device in terms of an operation speed.
- The nonvolatile memory device using the ferroelectric material inputs a signal by controlling a polarization direction controlled by applying an electric field, wherein a remnant polarization formed after the electric field is cut off is used for saving a
digital signal 1 or 0. - Usually, SrBi2Ta2O9 (SBT) or Pb(Zr, Ti)O3 (PZT) film is used as a ferroelectric material for the FeRAM device. The ferroelectric material has a dielectric constant for a few hundreds to a few thousands at a room temperature and two types of stable remnant polarization status. Therefore, it is expected to apply a ferroelectric material film to the nonvolatile memory device.
- In addition, a new ferroelectric material such as Bi4-xLaxTi3O12 (BLT) and Bi4Ti3O12 (BTO) have advantageous properties such as a superior reliability of the SBT and a low crystallization temperature of the PZT including a strong polarizability.
-
FIG. 1A shows a cross sectional view illustrating a conventional ferroelectric memory device. - As shown, an
isolation layer 12 predetermining an active region is formed on asemiconductor substrate 11. A stacked structure of agate oxide layer 13 and aword line 14 is formed on thesemiconductor substrate 11 thereafter. Source/drain regions word line 14. - In addition, a first
interfacial insulation layer 16 is formed on a transistor containing theword line 14 and the source/drain regions drain region 15A through abit line contact 17 contacting to one source/drain region 15A by passing through the firstinterfacial insulation layer 16. - A second
interfacial insulation layer 19 containing the bit line 18 is formed on an entire upper area of the transistor and astorage node contact 20 is formed thereon, wherein the firstinterfacial insulation layer 16 and the secondinterfacial insulation layer 19 are penetrated together in order to make thestorage node contact 20 connected to the other source/drain region 15B. - A
lower electrode 21 which should be connected to thestorage node contact 20 is formed. Also, a planerizedisolating insulation layer 22 for a separation of thelower electrode 21 from an adjacent electrode encompasses thelower electrode 21. Next, aferroelectric layer 23 covers theinsolating insulation layer 22 and thelower electrode 21, wherein theferroelectric layer 21 uses the BTO or BLT and is formed only on a cell region. - An
upper electrode 24 is formed on the formedferroelectric layer 23. A capacitor structure shown inFIG. 1A is named as a merged top plate (MTP). - However, the BTO or BLT capacitor has a disadvantage that the BTO or BLT ferroelectric material is operated at a high speed in a high temperature ambiance, a data fatigue occurs generally and drastically, so that it causes a device data failure. Furthermore, the data fatigue brings about deoxidization of titanium (Ti) ions. More specifically, the Ti ion deoxidization is caused by a charge impact externally applied. In addition, the Ti ion deoxidization causes some defects, which are concentrated or diffused into an electrode interface, and consequently, those defects suppress a dipole formation. From this, the polarizability of the BTO or BLT is reduced. Eventually, the film impaired with such defects within its interface is prone to damage because a conductor phenomenon increases an applied electric field to the interface.
-
FIG. 1B is a graph showing a result of a reliability test for BLT film fabricated by a conventional method, wherein a horizontal axis represents a switching cycle, a vertical axis denotes a polarization value (P*−P{circumflex over ( )}), and +dP and −dP mean a positive polarization value and a negative polarization value, respectively.FIG. 1C is a graph showing an attenuation range of the reliability in a numerical value, wherein an horizontal axis denotes a test temperature and a vertical axis describes a polarization value (P*−P{circumflex over ( )}). - Referring to
FIG. 1B , a remnant polarization is drastically reduced at about 125° C. (▴, ▾). More precisely, the reliability is drastically reduced at a high switching cycle point when data is rapidly used for a long time. In addition, the reliability is reduced at the high switching cycle point even at a room temperature. - Referring to
FIG. 1C , the polarization value is drastically reduced as a measurement temperature gets high. More precisely, the polarization value (p*−p{circumflex over ( )}) is abruptly reduced from about 18 μC/cm2 to less than about 10 μC/cm2. - It is, therefore, an object of the present invention to provide a method for fabricating a ferroelectric memory device capable of suppressing a defect generation caused by a charge impact.
- In accordance with an aspect of the present invention, there is provided the ferroelectric memory device, including: a semiconductor substrate on which a transistor is formed; a lower electrode on the interfacial insulation layer, which is connected to a source/drain region of the transistor through a contact penetrating the interfacial insulation layer; an isolating insulation layer on the interfacial insulation layer, which has a planerized surface exposing a surface of the lower electrode and encompasses the lower electrode; a ferroelectric layer covering the isolating insulation layer and lower electrode; an oxygen vacancy compensation layer on the ferroelectric layer compensating an oxygen vacancy caused by a deoxidization of a composition of the ferroelectric layer; and an upper electrode formed on the oxygen vacancy compensation layer.
- In addition, there is provided the method for fabricating the ferroelectric memory device, including: forming the interfacial insulation layer on the semiconductor substrate; forming a stack pattern of the lower electrode and a hard mask in order on the interfacial insulation layer; forming the isolating insulation layer on an entire surface having the stack pattern; planerizing the isolating insulation layer until exposing a surface of the hard mask; removing the hard mask by using a liquid chemical; forming the ferroelectric layer on an entire surface having the lower electrode exposed after the hard mask is removed; forming the oxygen vacancy compensation layer on the ferroelectric layer; forming a conductive layer for the upper electrode on the oxygen vacancy compensation layer; and pattering the conductive layer and the oxygen vacancy compensation layer consecutively.
- Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, in which:
-
FIG. 1A is a cross-sectional view illustrating a conventional ferroelectric memory device; -
FIG. 1B is a graph showing reliability measurement result of a Bi4-xLaxTi3O12 (BLT) layer fabricated by employing a conventional method; -
FIG. 1C is a graph showing an attenuation range of the reliability for the conventional BLT layer in a numerical value; -
FIG. 2 is a cross-sectional view illustrating a ferroelectric memory device having a capacitor with a merged top plate (MTP) structure in accordance with the present invention. -
FIGS. 3A to 3F are cross-sectional views illustrating a method for fabricating the ferroelectric device illustrated inFIG. 2 ; -
FIG. 4A is a graph showing a reliability measurement result for the BLT layer in accordance with a preferred embodiment of the present invention; and -
FIG. 4B is a graph showing an attenuation range of the reliability, in a numerical value, for BLT layer in accordance with the preferred embodiment of the present invention. - Hereinafter, an inventive ferroelectric capacitor and a method for fabricating a ferroelectric memory device having the same will be described in detail referring to the accompanying drawings.
-
FIG. 2 is a cross-sectional view illustrating a ferroelectric memory device containing a merged top plate (MTP) structure in accordance with preferred embodiment of the present invention. - Firstly, a capacitor of the ferroelectric memory device will be explained. As shown, a lower electrode deposited sequentially with an
iridium layer 41A, aniridium oxide layer 42A, and aplatinum layer 43A is planerized while the lower electrode is encompassed by anisolating insulation layer 46A. Aferroelectric layer 47 is formed on the planerized lower electrode and the isolatinginsulation layer 46A. Subsequently, an oxygenvacancy compensation layer 48 and an upper electrode are deposited on theferroelectric layer 47. - Herein, the oxygen
vacancy compensation layer 48 using a metal oxide layer such as ruthenium (Ru) or iridium (Ir) oxide serves to compensate a defect, i.e. oxygen vacancy, caused by deoxidization of titanium (Ti) within a Bi4-xLaxTi3O12 (BLT) or Bi4Ti3O12 (BTO) that is used to form theferroelectric layer 47. The oxygenvacancy compensation layer 48 has a thickness of about 10 Å to about 1000 Å, wherein the oxygenvacancy compensation layer 48 is deposited at a temperature ranging from about 100° C. to about 700° C. and at a pressure of about 0.1 mtorr to about 10 torr by using a chemical vapor deposition (CVD) or an atomic layer deposition (ALD) technique. - The BLT or BTO is used for forming the
ferroelectric layer 47, theferroelectric layer 47 has a thickness of about 50 Å to about 3000 Å, and one or a few layers selected out of such materials as platinum (Pt), iridium (Ir), ruthenium (Ru), tungsten (W), iridium oxide layer, ruthenium oxide layer, tungsten nitride layer, and titanium nitride layer are used to form theupper electrode 49. - One of such materials as a high density plasma (HDP) oxide layer, Boro-Phospho-Silicate-Glass (BPSG), Boro-Silicate-Glass (BSG), or Phospho-Silicate-Glass (PSG) is used to form the isolating
insulation layer 46A. - Next, a transistor containing source/
drain regions gate oxide layer 33, and aword line 34 is formed on thesemiconductor substrate 31, wherein adevice isolation layer 32 is formed in advance on thesemiconductor substrate 31 before forming the transistor. An upper surface of thesemiconductor substrate 31 is covered with a firstinterfacial layer 36, and abit line contact 37 is connected to one of the source/drain region 35A by passing through the firstinterfacial layer 36. A secondinterfacial insulation layer 39 is formed on the firstinterfacial insulation layer 36, and both of the first and second interfacial insulation layers 39 are penetrated throughout the above structure so as to connect astorage node contact 40 to the other source/drain region 35B. Herein, thestorage node contact 40 is connected to theiridium layer 41A used for the lower electrode, wherein thestorage node contact 40 is formed sequentially with a poly silicon plug, titanium silicide, and titanium nitride. -
FIGS. 3A to 3F show cross-sectional views illustrating a method for fabricating the ferroelectric memory device illustrated inFIG. 2 . - As illustrated in
FIG. 3A , thedevice isolation layer 32 for separating devices is formed on thesemiconductor substrate 31 in order to predetermine an active area. Thegate oxide layer 33 and theword line 34 are sequentially formed on the active area of thesemiconductor substrate 31. - Next, both sides of the
word line 34 on thesemiconductor substrate 31 are implanted with an impurity in order to form the source/drain regions word line 34, although it is not illustrated in the aforementioned drawings. As a result, the source/drain regions drain regions word line 34 and spacer. - A bit line contact hole exposing one source/
drain region 35A formed by etching the firstinterfacial insulation layer 36 through the use of a contact mask (not shown inFIG. 3A ) is formed after the firstinterfacial insulation layer 36 is deposited on the semiconductor substrate providing the transistor and planerized thereafter. In addition, thebit line contact 37 buried in the bit line contact hole is formed, wherein thebit line contact 37 can be a tungsten plug formed by an etch back process or a chemical mechanical polishing (CMP) process after depositing the tungsten. - A conductive layer for the bit line is deposited on an entire surface and patterned so as to form the
bit line 38 connected to thebit line contact 37, and the secondinterfacial insulation layer 39 including a bit line is deposited and planerized. - A storage node contact hole exposing the other source/
drain region 35B is formed by etching both of the secondinterfacial insulation layer 39 and the firstinterfacial insulation layer 36 with a storage node contact mask not shown is buried in the storage node contact hole thereafter. Furthermore, thestorage node contact 40 has a multi layer structure constructed sequentially with a poly silicon plug, a titanium silicide layer, and a titanium nitride layer not shown. Herein, titanium nitride layer is used to form an ohmic contact between the poly silicon plug and the lower electrode, the titanium silicide layer used for a diffusion barrier layer functions to suppress a reciprocal diffusion. Herein, the diffusion barrier layer is subsequently planerized by performing a CMP process. - There is another method for forming the
storage node contact 40. A titanium silicide layer is formed after annealing a deposited titanium layer and plugging a tungsten layer. Subsequent to the titanium silicide layer formation, a titanium nitride layer acting as a barrier metal is formed and planerized thereafter by performing a CMP process. - Continuously, a iridium (Ir)
layer 41A, an iridium oxide layer (IrO2) 42A, and a platinum (Pt) 43A are deposited in order, wherein the Ir, IrO2, and Pt are used as the conductive layer used as the lower electrode on the secondinterfacial insulation layer 39 having thestorage node contact 40. At this time, those layers mentioned above have a thickness of about 200 Å to about 2000 Å. - As a next step, a titanium nitride layer acting as a hard mask which is used for patterning the lower electrode on the
Pt layer 43A is deposited, and a patterned photo-resistlayer 45 predetermining the lower electrode on thetitanium nitride layer 44 is also formed. At this time, thetitanium nitride layer 44 has a thickness of about 50 Å to about 1000 Å and shows a superior slope property when a patterning process is carried out on the lower electrode. - The
titanium nitride layer 44 is etched and patterned by using the patterned photo-resist mask. - As shown in
FIG. 3B , the patterned photo-resistmask 45 is removed, and thept layer 43A, theiridium oxide layer 42A, and theiridium layer 41A are all patterned in order by using the patternedtitanium nitride layer 44. Eventually, the lower electrode having a triple layer stack structure is formed. Herein, the triple layer stack structure is constituted withIr layer 41A used as an oxygen barrier, the IrO2 layer 42A used as a glue layer, and thePt layer 43A. - The
Pt layer 43A, which is one of the elements of the lower electrode can be substituted for a ruthenium (Ru) layer, an iridium (Ir) layer (Ir), a tungsten (W) layer, or a tungsten nitride layer. The iridium (IrO2)oxide layer 42A, which is used for the glue layer, can be substituted for a ruthenium oxide layer, or a tungsten oxide layer. TheIr layer 41A, which is used as the oxygen barrier layer, can be also substituted for a ruthenium (Ru) layer. - Usually, a portion of the
titanium nitride layer 44 used for the hard mask is removed, and thus, therest titanium layer 44 with a decreased thickness becomes a remnant layer on theplatinum layer 43A when the lower electrode is patterned. Hereinafter, the remnant titanium layer is denoted as reference numeral, 41A. - Next, a third
interfacial insulation layer 46 including the lower electrode is formed on an entire surface area until a space between neighboring lower electrodes is completely filled up. At this time, the thirdinterfacial insulation layer 46 uses one of such materials as a HDP oxide layer, BPSG, BSG, or PSG, and it has a thickness of about 1000 Å to about 10000 Å. - As described in
FIG. 3C , an isolatinginsulation layer 46A encompassing the lower electrode is formed by planerizing the thirdinterfacial insulation layer 46 until a surface of thetitanium nitride layer 44A is exposed. At this time, thePt layer 43A is not damaged even by an excessive CMP process because thetitanium nitride 44A is capping thePt layer 43A. - Next, as shown in
FIG. 3D , the remnanttitanium nitride layer 44A on theplatinum layer 43A is removed by soaking it into a SC-1 chemical solution. Herein, the isolatinginsulation layer 46A is also removed easily by the SC-1 chemical solution because it is also an oxide layer. Eventually, a portion of the isolatinginsulation layer 46A is removed until the surface of theplatinum layer 43A is exposed. For detailed explanation, the SC-1 chemical solution is a solution of HCl, H2O2, and H2O and the solution is vigorously reactive with any metal layer. Therefore, the isolatinginsulation layer 46A is also removed easily. - Consequently, a remnant isolating
insulation layer 46A′ has a structure which insulates and encompasses the neighboring lower electrodes. - Similarly, the structure such that the isolating
insulation layer 46A′ encompasses the lower electrode overcomes difficulties in applying a masking process and a planerizing process caused by a step difference existing in the capacitor. In addition, the structure can prevent a short circuit between the lower electrode and the upper electrode. - Referring to
FIG. 3E , the BLT or BTO ferroelectric layer having a thickness of about 50 Å to about 3000 Å is deposited after finishing a CMP process, and the oxygenvacancy compensation layer 48 is formed on theferroelectric layer 47. - Herein, the
ferroelectric layer 47 goes through a sequence of a nucleation, a nucleus growth, and crystallization. Technically, a rapid thermal anneal (RTP) process is used for the nucleus growth. The RTP process is usually performed at a temperature of about 400° C. to about 800° C., and a ramp-up rate ranges from about 80° C./sec to about 250° C./sec. The crystallization is carried out by a typical furnace annealing process at about 500° C. to about 800° C. using one of such ambient gases as O2, N2O, N2, Ar, Ne, Kr, Xe, and He. Theferroelectric layer 47 having a thickness of about 50 Å to about 3000 Å is formed by a spin coating process or a liquid source mixed chemical deposition (LSMCD) process. - The oxygen
vacancy compensation layer 48 compensates a defect, that is, an oxygen vacancy generated by deoxidization of titanium within the BLT or BTO used as theferroelectric layer 47. A metal oxide layer such as a ruthenium (RuOx) oxide layer or iridium (IrOx) oxide layer is usually used for forming the oxygenvacancy compensation layer 48. A substitution between the oxygen of ruthenium oxide layer or iridium oxide layer and the defect caused by the titanium deoxidization generated by an externally applied charge impact occurs due to the use of the oxygenvacancy compensation layer 48. Consequently, the titanium can obtain a stable structure without any defect. Furthermore, a fatigue phenomenon can be also stabilized. - The oxygen
vacancy compensation layer 48 has a thickness of about 10 Å to about 1000 Å and is deposited by a CVD process or an ALD process at a temperature of about 100° C. to about 700° C. at a pressure of about 0.1 mtorr to about 10 torr. - With reference to
FIG. 3F , a conductive layer for theupper electrode 49 is formed on the oxygenvacancy compensation layer 48. The conductive layer is photo-resisted and etched so that theupper electrode 49 is formed. Concurrently, the oxygenvacant compensation layer 48 is etched and remained between theupper electrode 49 and theferroelectric layer 47. - The conductive layer is formed by using a CVD, physical vapor deposition (PVD), ALD, or plasma enhanced atomic layer deposition (PEALD) process. In addition, the conductive layer consists of one or a few materials selected among platinum (Pt), iridium (Ir), ruthenium (Ru), tungsten (W), an iridium oxide layer, a ruthenium oxide layer, a tungsten nitride layer and a titanium nitride layer.
- As mentioned, the capacitor formed in accordance with the present invention is called a MTP capacitor. The lower electrode of the MTP capacitor is encompassed by the isolating insulation layer. The isolating
insulation layer 46A prevents a loss of theferroelectric layer 47 caused by plasma during an etching process. Consequently, an effect of the defect within theferroelectric layer 47 is abruptly reduced. - In the preferred embodiment of the present invention, the lower electrode having the triple layer stack structure constructed with sequential deposition of the oxygen barrier layer, the glue layer and the metal layer is suggested. It is still possible to form a lower electrode having a single metal layer, wherein the single metal layer is one of such materials as Pt, Ru, Ir, W, and WN.
- In addition, SrBi2Ta2O9 (SBT) or SrBi2(Ta1-x, Nbx)2O9 (SBTN), which contains tantalum, is also used to form the oxygen
vacant compensation layer 48 and shows an identical capability compared with the BLT or BTO in the embodiment of the present invention. -
FIG. 4A is a graph illustrating a result of a reliability measurement for the BLT layer formed in accordance with the present invention. A horizontal axis denotes a switching cycle and a vertical axis represents a polarization value (P*−P{circumflex over ( )}). Reference symbols, +dP and −dP mean a positive polarization value and a negative polarization value, respectively.FIG. 4B shows an attenuation range of the reliability in a numerical value. Herein, a horizontal axis represents a measurement temperature and an vertical axis denotes a polarization value (P*−P{circumflex over ( )}). - As shown, there is no change of a remnant polarization at a room temperature as well as at a temperature of about 125° C. (▴, ▾) although the switching cycle is high. In short, the preferred embodiment of the present invention is reliable even at a high switching cycle when data is rapidly used.
- Referring to
FIG. 4B , the remnant polarization is slowly reduced as the measurement temperature is increased, and eventually, the remnant polarization value is minimized at about 18 μC/cm2 to about 10 μC/cm2. - According to the prior art, the remnant polarization value is drastically decreased at about 125° C. (▴, ▾) as the measurement temperature is increased. However, according to the present invention, the remnant polarization value is slowly decreased at the same condition as described above. Accordingly, the present invention shows that the oxygen vacant compensation layer between the BLT layer and the upper electrode improves the reliability of the ferroelectric memory device.
- While the present invention has been shown and described with respect to the particular embodiments, it will be apparent to those skilled in the art that many changes and modification may be made without departing from the spirit and scope of the invention as defined in the appended claims.
Claims (14)
1. A ferroelectric memory device, comprising:
a semiconductor substrate structure having a transistor;
a lower electrode formed on an interfacial insulation layer and connected to a source/drain region of the transistor through a contact by passing through the interfacial insulation layer;
an isolating insulation layer on the interfacial insulation layer, the isolation insulation layer having a planarized surface exposing a surface of the lower electrode and encompassing the lower electrode;
a ferroelectric layer covering the isolating insulation layer and lower electrode;
an oxygen vacancy compensation layer being formed on the ferroelectric layer and compensating an oxygen vacancy caused by deoxidization of a composition of the ferroelectric layer; and
an upper electrode formed on the oxygen vacancy compensation layer.
2. The ferroelectric memory device as recited in claim 1 , wherein a metal oxide layer is used to form the oxygen vacancy compensation layer.
3. The ferroelectric memory device as recited in claim 2 , wherein a ruthenium oxide layer or iridium oxide layer is used to form the metal oxide layer.
4. The ferroelectric memory device as recited in claim 1 , wherein the oxygen vacancy compensation layer has a thickness ranging from about 10 Å to about 1000 Å.
5. The ferroelectric memory device as recited in claim 1 , wherein, one of materials as a high density plasma (HDP) oxide layer, boro phospho silicate glass (BPSG), Boro phospho silicate BSG and phosphor PSG is used to form the isolating insulation layer.
6. The ferroelectric memory device as recited in claim 1 , wherein the lower electrode sequentially includes a glue layer, an oxide barrier layer and a metal layer.
7. The ferroelectric memory device as recited in claim 6 , wherein an iridium layer, an iridium oxide layer, and a platinum layer are used to form the glue layer, the oxygen layer, the metal layer, respectively.
8. A method for fabricating a ferroelectric memory device, comprising the steps of:
a) forming an interfacial insulation layer on a semiconductor substrate;
b) forming a stack pattern of a lower electrode and a hard mask on the interfacial insulation layer;
c) forming an isolating insulation layer on an entire surface having the stack pattern;
d) planarizing an isolating insulation layer until exposing a surface of the hard mask;
e) removing the hard mask by using a liquid chemical;
f) forming a ferroelectric layer on an entire surface having the lower electrode exposed after the hard mask is removed;
g) forming an oxygen vacancy compensation layer on the ferroelectric layer;
h) forming a conductive layer for an upper electrode on the oxygen vacancy compensation layer; and
i) patterning the conductive layer and the oxygen vacancy compensation layer consecutively.
9. The method as recited claim 8 , wherein one of materials as SrBi2(Ta1-x, Nbx)2O9 (SBTN), SrBi2Ta2O9 (SBT), Bi4Ti3O12 (BTO), and Bi4-xLaxTi3O12 (BLT) is used in order to form the ferroelectric layer.
10. The method as recited in claim 8 , wherein the oxygen vacancy compensation layer is a metal oxide layer deposited at a temperature of about 100° C. to about 700° C. and at a pressure of about 0.1 mtorr to about 10 torr by employing a chemical vapor deposition (CVD) or an atomic layer deposition (ALD) technique.
11. The method as recited in claim 10 , wherein the metal oxide layer is a ruthenium oxide layer or iridium layer.
12. The method as recited in claim 8 , wherein the oxygen vacancy compensation layer has a thickness ranging about 10 Å to about 1000 Å.
13. The method as recited in claim 8 , wherein the isolating insulation layer is planarized until exposing the surface of the hard mask by performing a chemical mechanical polishing (CMP) process.
14. The method as recited in claim 8 , wherein one of materials as a HDP oxide layer, BPSG, BSG and PSG is used to form the isolating insulation layer.
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- 2002-12-27 KR KR10-2002-0085054A patent/KR100476375B1/en not_active IP Right Cessation
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- 2003-07-08 US US10/613,993 patent/US6812042B2/en not_active Expired - Lifetime
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2004
- 2004-07-27 US US10/899,171 patent/US20050006683A1/en not_active Abandoned
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US20080111173A1 (en) * | 2005-06-17 | 2008-05-15 | Fujitsu Limited | Semiconductor device and method for manufacturing the same |
US7928479B2 (en) * | 2005-06-17 | 2011-04-19 | Fujitsu Semiconductor Limited | Semiconductor device and method for manufacturing the same |
US20110165702A1 (en) * | 2005-06-17 | 2011-07-07 | Fujitsu Semiconductor Limited | Method for manufacturing semiconductor device |
US8614104B2 (en) | 2005-06-17 | 2013-12-24 | Fujitsu Semiconductor Limited | Method for manufacturing semiconductor device |
WO2013066495A1 (en) * | 2011-09-09 | 2013-05-10 | Innovent Technologies | Coated crucible and method of making a coated crucible |
CN109037231A (en) * | 2017-06-08 | 2018-12-18 | 爱思开海力士有限公司 | Semiconductor devices and its manufacturing method including ferroelectric layer |
US10580783B2 (en) | 2018-03-01 | 2020-03-03 | Sandisk Technologies Llc | Multi-tier three-dimensional memory device containing differential etch rate field oxides and method of making the same |
Also Published As
Publication number | Publication date |
---|---|
US6812042B2 (en) | 2004-11-02 |
KR20040058683A (en) | 2004-07-05 |
KR100476375B1 (en) | 2005-03-17 |
US20040124453A1 (en) | 2004-07-01 |
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