US20010041416A1 - Method of fabricating semiconductor device - Google Patents
Method of fabricating semiconductor device Download PDFInfo
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- US20010041416A1 US20010041416A1 US09/897,615 US89761501A US2001041416A1 US 20010041416 A1 US20010041416 A1 US 20010041416A1 US 89761501 A US89761501 A US 89761501A US 2001041416 A1 US2001041416 A1 US 2001041416A1
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- 239000004065 semiconductor Substances 0.000 title claims abstract description 18
- 238000004519 manufacturing process Methods 0.000 title claims description 19
- 239000003990 capacitor Substances 0.000 claims abstract description 32
- 239000000758 substrate Substances 0.000 claims abstract description 23
- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Chemical compound [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 claims description 48
- 238000005530 etching Methods 0.000 claims description 40
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- 238000000034 method Methods 0.000 abstract description 21
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- 239000010408 film Substances 0.000 description 45
- WFKWXMTUELFFGS-UHFFFAOYSA-N tungsten Chemical compound [W] WFKWXMTUELFFGS-UHFFFAOYSA-N 0.000 description 29
- 229910052721 tungsten Inorganic materials 0.000 description 20
- 239000010937 tungsten Substances 0.000 description 20
- 238000001312 dry etching Methods 0.000 description 19
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- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 13
- 239000001301 oxygen Substances 0.000 description 13
- 229910052760 oxygen Inorganic materials 0.000 description 13
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 9
- 239000000463 material Substances 0.000 description 9
- 238000012545 processing Methods 0.000 description 8
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 7
- 229910021420 polycrystalline silicon Inorganic materials 0.000 description 6
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- 230000000694 effects Effects 0.000 description 5
- 238000005229 chemical vapour deposition Methods 0.000 description 4
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- 229910052682 stishovite Inorganic materials 0.000 description 3
- 229910052905 tridymite Inorganic materials 0.000 description 3
- KDLHZDBZIXYQEI-UHFFFAOYSA-N Palladium Chemical compound [Pd] KDLHZDBZIXYQEI-UHFFFAOYSA-N 0.000 description 2
- 229910002785 ReO3 Inorganic materials 0.000 description 2
- 238000013459 approach Methods 0.000 description 2
- 238000007796 conventional method Methods 0.000 description 2
- 230000007547 defect Effects 0.000 description 2
- 238000000151 deposition Methods 0.000 description 2
- HTXDPTMKBJXEOW-UHFFFAOYSA-N iridium(IV) oxide Inorganic materials O=[Ir]=O HTXDPTMKBJXEOW-UHFFFAOYSA-N 0.000 description 2
- 229910052451 lead zirconate titanate Inorganic materials 0.000 description 2
- HFGPZNIAWCZYJU-UHFFFAOYSA-N lead zirconate titanate Chemical compound [O-2].[O-2].[O-2].[O-2].[O-2].[Ti+4].[Zr+4].[Pb+2] HFGPZNIAWCZYJU-UHFFFAOYSA-N 0.000 description 2
- 238000007254 oxidation reaction Methods 0.000 description 2
- YSZJKUDBYALHQE-UHFFFAOYSA-N rhenium trioxide Chemical compound O=[Re](=O)=O YSZJKUDBYALHQE-UHFFFAOYSA-N 0.000 description 2
- 238000004544 sputter deposition Methods 0.000 description 2
- 238000000992 sputter etching Methods 0.000 description 2
- 229910052715 tantalum Inorganic materials 0.000 description 2
- 238000012360 testing method Methods 0.000 description 2
- 206010010144 Completed suicide Diseases 0.000 description 1
- KJTLSVCANCCWHF-UHFFFAOYSA-N Ruthenium Chemical compound [Ru] KJTLSVCANCCWHF-UHFFFAOYSA-N 0.000 description 1
- JRPBQTZRNDNNOP-UHFFFAOYSA-N barium titanate Chemical compound [Ba+2].[Ba+2].[O-][Ti]([O-])([O-])[O-] JRPBQTZRNDNNOP-UHFFFAOYSA-N 0.000 description 1
- 229910002113 barium titanate Inorganic materials 0.000 description 1
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- NKZSPGSOXYXWQA-UHFFFAOYSA-N dioxido(oxo)titanium;lead(2+) Chemical compound [Pb+2].[O-][Ti]([O-])=O NKZSPGSOXYXWQA-UHFFFAOYSA-N 0.000 description 1
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- 238000009499 grossing Methods 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 238000009413 insulation Methods 0.000 description 1
- 229910052741 iridium Inorganic materials 0.000 description 1
- GKOZUEZYRPOHIO-UHFFFAOYSA-N iridium atom Chemical compound [Ir] GKOZUEZYRPOHIO-UHFFFAOYSA-N 0.000 description 1
- 238000002955 isolation Methods 0.000 description 1
- 238000001755 magnetron sputter deposition Methods 0.000 description 1
- 229910044991 metal oxide Inorganic materials 0.000 description 1
- 150000004706 metal oxides Chemical class 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
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- 229910000510 noble metal Inorganic materials 0.000 description 1
- 229910052763 palladium Inorganic materials 0.000 description 1
- 238000000059 patterning Methods 0.000 description 1
- OYLRFHLPEAGKJU-UHFFFAOYSA-N phosphane silicic acid Chemical compound P.[Si](O)(O)(O)O OYLRFHLPEAGKJU-UHFFFAOYSA-N 0.000 description 1
- 230000010287 polarization Effects 0.000 description 1
- 238000011084 recovery Methods 0.000 description 1
- 229910052707 ruthenium Inorganic materials 0.000 description 1
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- 230000002269 spontaneous effect Effects 0.000 description 1
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- VEALVRVVWBQVSL-UHFFFAOYSA-N strontium titanate Chemical compound [Sr+2].[O-][Ti]([O-])=O VEALVRVVWBQVSL-UHFFFAOYSA-N 0.000 description 1
- 239000010409 thin film Substances 0.000 description 1
Images
Classifications
-
- 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
-
- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10B—ELECTRONIC MEMORY DEVICES
- H10B12/00—Dynamic random access memory [DRAM] devices
- H10B12/01—Manufacture or treatment
- H10B12/02—Manufacture or treatment for one transistor one-capacitor [1T-1C] memory cells
- H10B12/03—Making the capacitor or connections thereto
- H10B12/033—Making the capacitor or connections thereto the capacitor extending over the transistor
Definitions
- the present invention relates to a semiconductor device having capacitors using a ferroelectric film such as ferroelectric nonvolatile memory of a dynamic random access memory (DRAM).
- a ferroelectric film such as ferroelectric nonvolatile memory of a dynamic random access memory (DRAM).
- ferroelectric materials have extremely large relative dielectric constants ranging from several hundreds to several thousands. Therefore, use of a thin film made of these ferroelectric materials for a capacitor dielectrics provides a capacitor of small area and large capacity suitable for large scale integration (LSI) devices. Also, the ferroelectric material has spontaneous polarization that can be inverted in direction by an applied electric field, thereby providing a nonvolatile memory.
- LSI large scale integration
- the conventional ferroelectric memory is fabricated by forming on an interlayer insulating film 144 with a bottom Pt electrode 145 , ferroelectric film 146 , and a top Pt electrode 147 in this order, thereby forming a ferroelectric capacitor.
- each of the layers is formed with an independent mask, which makes the memory cell area large because of critical dimension uniformity and alignment tolerance, thereby making it difficult to fabricate highly integrated memory devices.
- the conventional technique also involves a problem of thinning the interlayer insulating film 144 for the conventional technique repeats the patterning on it for forming the ferroelectric capacitors.
- a method was proposed as described in Japanese unexamined Patent Application No. 2-288368, in which a top electrode 158 , a ferroelectric film 157 , and a bottom electrode 156 are collectively dry-etched with the photoresist used as a mask as shown in FIG. 15.
- This method uses polysilicon for the top and bottom electrodes 158 and 156 , which are dry-etched with C 2 Cl 2 F 4 , SF 6 , and Ar gases.
- platinum cannot be converted to a highly volatile reaction product to be dry-etched. It was observed that, if platinum is dry-etched, a redeposited material forms a wall-shaped residue (hereinafter referred to as a platinum-contained deposit) on the capacitor side wall due to the low volatility. In this structure, the above-mentioned platinum-contained deposits short-circuit the top electrode 45 and the bottom electrode 43 .
- This object is achieved by setting the taper angle of the side wall of the ferroelectric film constituting the ferroelectric capacitor to less than 75 degrees to the main surface of the substrate on which the ferroelectric capacitor is formed. That is, the taper angle of the cross sidewall of the ferroelectric capacitor to the plane on which the bottom electrode is formed is set to a value not reaching 75 degrees or more.
- FIG. 13 there is shown a relationship between the taper angle of the cross side wall of the ferroelectric capacitor to the main surface of the substrate and short-circuit. It is assumed herein that a short-circuit has occurred when a leakage current density at an applied voltage of 3V became 10 ⁇ 5 A/cm 2 or higher.
- the etching is performed at nearly 90 degrees, so that, after etching of the platinum top electrode 45 , the platinum of the top electrode 45 redeposits to form a platinum-contained sidewall deposit 101 as shown in FIG. 10A.
- a sidewall deposit 102 composed of elements constituting the ferroelectric film 44 remains along the platinum-contained sidewall deposit 101 as shown in FIG. 10B.
- this sidewall deposit 102 is composed of the components of the ferroelectric film, the composition and crystal structure thereof are out of order, resulting in insufficient insulation.
- FIG. 10C during etching of the platinum bottom electrode 43 , this deposit 102 composed of the components of the ferroelectric film is mostly removed. However, the platinum-contained sidewall deposit 101 still remains. Further, the platinum-contained sidewall deposit 103 may also be formed from the platinum bottom electrode. Thus, in the prior-art technology, depositing of platinum on the sidewall short-circuits the bottom and top electrodes 43 and 45 of the capacitor.
- FIG. 13 the angle of the cross sidewall of the platinum bottom electrode, the ferroelectric film, and the top electrode to the main surface of the substrate is shown; however, it is not always necessary to set the cross sectional sidewall of the entire capacitor to less than 75 degrees.
- tilting the sidewall of only the ferroelectric film 44 relative to the main surface of the substrate by less than 75 degrees also provides an effect of preventing the platinum deposition from occurring. The effect can be made more conspicuous, however, by tilting together the sidewall of the platinum bottom electrode by less than 75 degrees.
- the top electrode 45 may be another rare metal such as iridium or ruthenium or a conductive oxide such as IrO 2 , RuO 2 , or ReO 3 . If platinum is not used for the top electrode 45 , the platinum-contained deposit is formed on the capacitor sidewall only when the platinum bottom electrode 43 is etched. As described above, tapering the capacitor side walls to the main surface of the substrate by less than 75 degrees prevents the short-circuit between the top electrode and the platinum bottom electrode.
- the angle of the cross sidewall of the ferroelectric capacitor to the bottom surface of the bottom electrode is determined by the angle of the etching mask sidewall to the bottom surface of the bottom electrode.
- tungsten is used for the etching mask.
- FIG. 11 shows a relationship between the sidewall taper angle of photoresist sidewall and resist baking temperature. Shown are test results obtained from two types of photoresists A and B. The results indicate that the sidewall taper angle gets larger as the baking temperature rises for both the photoresists.
- the photoresist A is composed of a material having a flat distribution over molecular weights of 100 to 30,000, while the photoresist B is composed of a material having a peak over molecular weights 2,000 to 3,000.
- a preferable result is obtained by setting the baking temperature to a range of 140° C. to 160° C.
- the method of controlling the sidewall taper angle by the resist baking temperature is also applicable to the case in which materials such as SiO 2 for which isotropic tapering is difficult is used for the etching mask.
- the angle of the tungsten sidewall to the bottom surface of the bottom electrode can be controlled by the over-etching time of tungsten.
- FIG. 12 shows a relationship between the over-etching time of tungsten and the angle of the tungsten sidewall to the main surface of the substrate. As the tungsten over-etching time is increased, line width becomes narrower, while the sidewall approaches vertical angle. A preferable result will be obtained when the tungsten over-etching time is set to a range of 5% to 10%.
- the ferroelectric sidewall is exposed to plasma, which may cause an etching damage, resulting in an increase in the leakage current on the sidewall.
- This problem is overcome by performing oxygen plasma processing after dry-etching of the bottom electrode and before etching the conductive diffusion barrier layer (hereinafter referred to simply as the diffusion barrier layer).
- FIG. 1 is a cross section illustrating a semiconductor device according to the present invention
- FIG. 2 is a cross section illustrating a process of fabricating a memory cell based on the present invention
- FIG. 3 is a cross section illustrating a fabrication process next to that of FIG. 2;
- FIG. 4 is a cross section illustrating a fabrication process next to that of FIG. 3;
- FIG. 5 is a cross section illustrating a fabrication process next to that of FIG. 4;
- FIG. 6 is a cross section illustrating a fabrication process next to that of FIG. 5;
- FIG. 7 is a cross section illustrating a fabrication process next to that of FIG. 6;
- FIG. 8 is a cross section illustrating a fabrication process next to that of FIG. 7;
- FIG. 9 shows characteristics curves indicating a relationship between the voltage and leakage current density of the capacitor according to the invention after the fabrication processes of FIGS. 6 through 8;
- FIG. 10A is a cross section illustrating sidewall deposits formed on the capacitor as observed after dry-etching the top electrode
- FIG. 10B is a cross section illustrating sidewall deposits formed on the capacitor as observed after dry-etching the ferroelectric film
- FIG. 10C is a cross section illustrating sidewall deposits formed on the capacitor as observed after dry-etching the bottom electrode
- FIG. 11 shows characteristics curves indicating a relationship between baking temperature and photoresist sidewall taper angle
- FIG. 12 shows characteristics curves indicating a relationship between over-etching time and tungsten mask taper angle
- FIG. 13 shows capacitor short-circuit test results
- FIG. 14 is a cross section illustrating a prior-art memory cell
- FIG. 15 is a cross section illustrating another prior-art memory cell.
- FIG. 16 is a cross section illustrating still another prior-art memory cell.
- FIGS. 2 through 8 there are shown cross sections illustrating a method of fabricating a memory of FIG. 1 in the order of main processes, the method being practiced as one preferred embodiment of the present invention.
- a switching transistor is formed by the conventional MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) forming process.
- MOSFET Metal Oxide Semiconductor Field-Effect Transistor
- reference numeral 21 indicates a p-type semiconductor substrate
- reference numeral 22 indicates an isolation dielectric
- reference numeral 23 indicates a gate oxide film
- reference numeral 24 indicates a word line that provides a gate electrode
- reference numerals 25 and 26 indicate phosphorous-doped n-type regions
- reference numeral 27 indicates an interlayer insulating film.
- a deposit of SiO 2 28 of about 600 nm in thickness is formed by the known CVD (Chemical Vapor Deposition) all over the transistor. Then, the formed deposit is reflowed at 850° C. to be etched back by about 300 nm, thereby smoothing steps caused by the word line.
- bit line 31 is formed.
- This bit line 31 is composed of a film stack made of metal suicide and polysilicon. This film stack is etched by the known photolithography and dry etching to a desired bit-line pattern.
- An insulating film 32 of silicon oxide film type such as BPSG (Boron-doped Phosphor-Silicate Glass) is deposited to planarize. It should be noted that this insulating film 32 needs to be thick enough for to planarize the substrate surface. In the present embodiment, the insulating film 32 was formed to a thickness of about 600 nm and planarized by etching back.
- a contact hole 33 of memory section is opened to provide access for the storage capacity section to come into contact with the substrate.
- polysilicon 41 was deposited by CVD to a thickness of about 350 nm. Then, by dry etching, the polysilicon 41 was etched back by the film thickness to fill the contact hole 33 as shown in FIG. 4.
- TiN is formed to a thickness of about 50 nm as a diffusion barrier layer 42 and then a bottom electrode 43 is formed.
- a Pt film about 200 nm thick was deposited as the bottom electrode 43 .
- the TIN of the diffusion barrier layer 42 is provided to prevent the platinum of the bottom electrode 43 and the oxygen from diffusing into the polysilicon 41 .
- a ferroelectric film 44 is formed.
- a lead zirconate titanate (Pb(Zr 0.5 Ti 0.5 )O 3 ) film was formed to a thickness of about 150 nm by reactive evaporation and then crystallized by heat treatment in oxygen atmosphere at 650° C.
- the ferroelectric film 44 may also be formed by high-frequency magnetron sputtering, Sol-Gel method, MOD (Metal Organic Decomposition), or CVD. Then, by sputtering, a Pt film about 50 nm thick was formed as a top electrode 45 and a tungsten 46 was deposited to a thickness of about 350 nm for the mask as shown in FIG. 4.
- the tungsten 46 is patterned by dry-etching with SF 6 , a photoresist 51 being used as the mask.
- the top electrode 45 is patterned by the sputter etching with the tungsten 46 used as the mask as shown in FIG. 6.
- the dry etching conditions were adjusted such that isotropic etching is provided, and the etching was performed such that the cross section of the tungsten 46 becomes a trapezoid, the angle of each of the sides thereof relative to the substrate being less than 75 degrees.
- microwave dry etching was used with the conditions that an SF 6 gas flow of 10 SCCM, a pressure of 2 mTorr, and a microwave power of 400 W.
- the cross section of the tungsten 46 departs from the trapezoid and approaches a rectangular, leaving projecting deposits on the tungsten 46 and the photoresist 51 at the sidewalls thereof.
- FIG. 10A after the photoresist 51 has been removed, platinum-contained projecting sidewall deposits 101 remain.
- FIG. 10B after the ferroelectric film 44 has been dry-etched, sidewall deposits 102 composed of the components of the ferroelectric film remain on the periphery of the platinum-contained projecting sidewall deposits 101 .
- the bottom electrode 43 is etched by sputter etching. It should be noted that 30% over-etching was performed in order to remove the platinum deposited on the sidewalls as shown in FIG. 7.
- Oxygen plasma is generated in the same chamber in which the above-mentioned etching was performed to recover the etching damages of the cross section of the ferroelectric film 44 .
- the oxygen plasma processing was performed at an oxygen flow of 25 SCCM, a pressure of 30 mTorr, an RF power of 150 W for three minutes as shown in FIG. 8. Then, by the dry etching with SF 6 , the diffusion barrier layer 42 and the remaining tungsten 46 were removed simultaneously to complete the ferroelectric capacitor of FIG. 1.
- the memory device is completed by performing wiring like an ordinary semiconductor memory chip.
- the increase in leakage current on the sidewalls and the decrease in breakdown voltage can be prevented at the same time, thereby allowing fabrication of microscopic ferroelectric memory cells suitable for high integration.
- the leakage of the ferroelectric capacitor can also be decreased by performing the oxygen plasma processing after etching the diffusion barrier layer 42 by the dry etching using SF 6 .
- this causes oxidization of the diffusion barrier layer 42 left under the bottom electrode 43 from the sides of the layer, resulting in peeling off of the bottom electrode 43 from the bottom electrode/diffusion barrier layer interface. This problem can be avoided if the oxygen plasma processing is performed before dry-etching the diffusion barrier layer 42 .
- FIG. 9 shows comparisons of the leakage current density and voltage characteristics of the ferroelectric capacitor obtained after the etching of the top electrode shown in FIG. 6, after the etching of the bottom electrode shown in FIG. 7, after the over etching, after the oxygen plasma processing of FIG. 8, and after the etching of the diffusion barrier layer.
- the leakage current density is on a order of 10 ⁇ 7 A/cm 2 .
- the platinum-contained deposits on the ferroelectric film sidewalls short-circuit the top and bottom electrodes. Removing these platinum-contained deposits by 30% over-etching decreases the leakage current density to an order of 10 ⁇ 5 A/cm 2 .
- this value is larger than that obtained after the top electrode etching by an order of magnitude or more because the ferroelectric film sidewalls are exposed to the plasma to cause oxygen defects.
- the leakage current density decreases to the generally same level as that observed after the top electrode etching. Etching of the diffusion barrier layer does not indicate an increase in the leakage current density either.
- the tungsten 46 that provides the mask is formed by isotropy dry etching into a trapezoid in cross section. It will be apparent that, as described above with reference to FIG. 11, the tungsten 46 maybe formed by anisotropic dry etching into a trapezoid in cross section after forming the photoresist 51 into a trapezoid in cross section by setting the baking temperature of the photoresist 51 to a range of 140° C. to 160° C.
- lead ziroconate tintanate is used for ferroelectric film 44 .
- the material for the ferroelectric film is not limited to lead zirconate titanate; also available are, by way of example, perovskite-type oxides such as lead titanate, strontium titanate, and barium titanate, solid solutions of these, and bismuth-type layer-structured ferroelectric oxides.
- TiN is used for the diffusion barrier layer 42 . It will be apparent that the same effect can be obtained by use of Ti or Ta or by stacking a plurality of materials selected from TiN, Ti, and Ta.
- etching the top electrode, the ferroelectric film and the bottom electrode with single photolithography process step does not cause short-circuit between the top and bottom electrodes, thereby allowing the fabrication of the memory cell of a small cell area suitable for highly integrated memory devices.
- Use of the memory cell according to the present invention can implement not only a high integrated DRAM (Dynamic Random Access Memory) and a highly integrated ferroelectric nonvolatile memory, but also a high-performance LSI (Large Scale Integration) in which these memory cells and a logic LSI are integrated on one chip and a field-programmable logic LSI that allows modification of wiring by the ferroelectric nonvolatile memory.
- DRAM Dynamic Random Access Memory
- LSI Large Scale Integration
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Abstract
Description
- This application is a Continuation application of Ser. No. 09/475,033, filed Dec. 30, 1999, which is a Divisional application of Ser. No. 08/755,602, filed Nov. 25, 1996, the contents of which are incorporated herein by reference in their entirety.
- The present invention relates to a semiconductor device having capacitors using a ferroelectric film such as ferroelectric nonvolatile memory of a dynamic random access memory (DRAM).
- Some ferroelectric materials have extremely large relative dielectric constants ranging from several hundreds to several thousands. Therefore, use of a thin film made of these ferroelectric materials for a capacitor dielectrics provides a capacitor of small area and large capacity suitable for large scale integration (LSI) devices. Also, the ferroelectric material has spontaneous polarization that can be inverted in direction by an applied electric field, thereby providing a nonvolatile memory.
- As described in Japanese unexamined Patent Application No. 5-90606 and referring to FIG. 14, the conventional ferroelectric memory is fabricated by forming on an
interlayer insulating film 144 with abottom Pt electrode 145,ferroelectric film 146, and atop Pt electrode 147 in this order, thereby forming a ferroelectric capacitor. However, in the conventional ferroelectric memory, each of the layers is formed with an independent mask, which makes the memory cell area large because of critical dimension uniformity and alignment tolerance, thereby making it difficult to fabricate highly integrated memory devices. The conventional technique also involves a problem of thinning theinterlayer insulating film 144 for the conventional technique repeats the patterning on it for forming the ferroelectric capacitors. - To solve the above-mentioned problems, a method was proposed as described in Japanese unexamined Patent Application No. 2-288368, in which a
top electrode 158, aferroelectric film 157, and abottom electrode 156 are collectively dry-etched with the photoresist used as a mask as shown in FIG. 15. This method uses polysilicon for the top andbottom electrodes - However, forming a ferroelectric film directly on polysilicon, a silicon oxide film of a low dielectric constant is formed at the interface. The silicon oxide film thus formed significantly deteriorates capacitor characteristics. To avoid this deterioration, it is necessary to use electrodes made of noble metals such as platinum and palladium or conductive oxides such as IrO2, RuO2, and ReO3.
- Of the above-mentioned electrode materials, platinum is considered best suited for the application. Therefore, in the memory cell forming process described in Japanese unexamined Patent Application No. 5-299601 collectively dry-etches a
top electrode 45, aferroelectric film 44, abottom electrode 43, and a conductivediffusion barrier layer 169 with the photoresist used as the mask as shown in FIG. 16. Use of such a structure can implement microscopic capacitors without losing their properties - Actually, however, platinum cannot be converted to a highly volatile reaction product to be dry-etched. It was observed that, if platinum is dry-etched, a redeposited material forms a wall-shaped residue (hereinafter referred to as a platinum-contained deposit) on the capacitor side wall due to the low volatility. In this structure, the above-mentioned platinum-contained deposits short-circuit the
top electrode 45 and thebottom electrode 43. - It is therefore an object of the present invention to provide a capacitor in which the top and bottom electrodes thereof will not be short-circuited when the top electrode, the ferroelectric film, and the bottom electrode are etched with single photolithography process step.
- This object is achieved by setting the taper angle of the side wall of the ferroelectric film constituting the ferroelectric capacitor to less than 75 degrees to the main surface of the substrate on which the ferroelectric capacitor is formed. That is, the taper angle of the cross sidewall of the ferroelectric capacitor to the plane on which the bottom electrode is formed is set to a value not reaching 75 degrees or more.
- Referring to FIG. 13, there is shown a relationship between the taper angle of the cross side wall of the ferroelectric capacitor to the main surface of the substrate and short-circuit. It is assumed herein that a short-circuit has occurred when a leakage current density at an applied voltage of 3V became 10−5 A/cm2 or higher. In the above-mentioned prior art, the etching is performed at nearly 90 degrees, so that, after etching of the
platinum top electrode 45, the platinum of thetop electrode 45 redeposits to form a platinum-containedsidewall deposit 101 as shown in FIG. 10A. After completion of dry-etching of theferroelectric film 44, asidewall deposit 102 composed of elements constituting theferroelectric film 44 remains along the platinum-containedsidewall deposit 101 as shown in FIG. 10B. Although thissidewall deposit 102 is composed of the components of the ferroelectric film, the composition and crystal structure thereof are out of order, resulting in insufficient insulation. Referring to FIG. 10C, during etching of theplatinum bottom electrode 43, thisdeposit 102 composed of the components of the ferroelectric film is mostly removed. However, the platinum-containedsidewall deposit 101 still remains. Further, the platinum-containedsidewall deposit 103 may also be formed from the platinum bottom electrode. Thus, in the prior-art technology, depositing of platinum on the sidewall short-circuits the bottom andtop electrodes - Referring to FIG. 13, it is clear that setting the angle of the cross sidewall of the platinum bottom electrode, the ferroelectric film and the top electrode to the main surface of the substrate to less than 75 degrees prevents the platinum deposits from being formed on the capacitor sidewall.
- In FIG. 13, the angle of the cross sidewall of the platinum bottom electrode, the ferroelectric film, and the top electrode to the main surface of the substrate is shown; however, it is not always necessary to set the cross sectional sidewall of the entire capacitor to less than 75 degrees. For example, tilting the sidewall of only the
ferroelectric film 44 relative to the main surface of the substrate by less than 75 degrees also provides an effect of preventing the platinum deposition from occurring. The effect can be made more conspicuous, however, by tilting together the sidewall of the platinum bottom electrode by less than 75 degrees. - It will be apparent that, instead of platinum, the
top electrode 45 may be another rare metal such as iridium or ruthenium or a conductive oxide such as IrO2, RuO2, or ReO3. If platinum is not used for thetop electrode 45, the platinum-contained deposit is formed on the capacitor sidewall only when theplatinum bottom electrode 43 is etched. As described above, tapering the capacitor side walls to the main surface of the substrate by less than 75 degrees prevents the short-circuit between the top electrode and the platinum bottom electrode. - The angle of the cross sidewall of the ferroelectric capacitor to the bottom surface of the bottom electrode is determined by the angle of the etching mask sidewall to the bottom surface of the bottom electrode. In the present invention, tungsten is used for the etching mask. When tungsten is etched by anisotropic dry etching, the angle of the tungsten sidewall to the bottom surface of the bottom electrode is determined by the angle of the photoresist side walls. FIG. 11 shows a relationship between the sidewall taper angle of photoresist sidewall and resist baking temperature. Shown are test results obtained from two types of photoresists A and B. The results indicate that the sidewall taper angle gets larger as the baking temperature rises for both the photoresists. The photoresist A is composed of a material having a flat distribution over molecular weights of 100 to 30,000, while the photoresist B is composed of a material having a peak over molecular weights 2,000 to 3,000. For the photoresists shown, a preferable result is obtained by setting the baking temperature to a range of 140° C. to 160° C. The method of controlling the sidewall taper angle by the resist baking temperature is also applicable to the case in which materials such as SiO2 for which isotropic tapering is difficult is used for the etching mask.
- When tungsten is etched by isotropic dry etching, the angle of the tungsten sidewall to the bottom surface of the bottom electrode can be controlled by the over-etching time of tungsten. FIG. 12 shows a relationship between the over-etching time of tungsten and the angle of the tungsten sidewall to the main surface of the substrate. As the tungsten over-etching time is increased, line width becomes narrower, while the sidewall approaches vertical angle. A preferable result will be obtained when the tungsten over-etching time is set to a range of 5% to 10%.
- However, when etching the ferroelectric capacitor such that the sidewall taper angle thereof becomes less than 75 degrees relative to the main surface of the substrate, the ferroelectric sidewall is exposed to plasma, which may cause an etching damage, resulting in an increase in the leakage current on the sidewall. This problem is overcome by performing oxygen plasma processing after dry-etching of the bottom electrode and before etching the conductive diffusion barrier layer (hereinafter referred to simply as the diffusion barrier layer).
- It should be noted that performing oxidization processing for etching damage recovery after etching TiN of the diffusion barrier layer oxidizes the TiN under the bottom platinum electrode to cause peel-off or the like trouble. The peel-off can be prevented from occurring by performing oxygen plasma processing before etching the TiN.
- These above and further objects and features of the invention will be seen by reference to the description, taken in connection with the accompanying drawings.
- In the drawings, forming a part thereof, in which like reference characters denote like parts in the various views.
- FIG. 1 is a cross section illustrating a semiconductor device according to the present invention;
- FIG. 2 is a cross section illustrating a process of fabricating a memory cell based on the present invention;
- FIG. 3 is a cross section illustrating a fabrication process next to that of FIG. 2;
- FIG. 4 is a cross section illustrating a fabrication process next to that of FIG. 3;
- FIG. 5 is a cross section illustrating a fabrication process next to that of FIG. 4;
- FIG. 6 is a cross section illustrating a fabrication process next to that of FIG. 5;
- FIG. 7 is a cross section illustrating a fabrication process next to that of FIG. 6;
- FIG. 8 is a cross section illustrating a fabrication process next to that of FIG. 7;
- FIG. 9 shows characteristics curves indicating a relationship between the voltage and leakage current density of the capacitor according to the invention after the fabrication processes of FIGS. 6 through 8;
- FIG. 10A is a cross section illustrating sidewall deposits formed on the capacitor as observed after dry-etching the top electrode;
- FIG. 10B is a cross section illustrating sidewall deposits formed on the capacitor as observed after dry-etching the ferroelectric film;
- FIG. 10C is a cross section illustrating sidewall deposits formed on the capacitor as observed after dry-etching the bottom electrode;
- FIG. 11 shows characteristics curves indicating a relationship between baking temperature and photoresist sidewall taper angle;
- FIG. 12 shows characteristics curves indicating a relationship between over-etching time and tungsten mask taper angle;
- FIG. 13 shows capacitor short-circuit test results;
- FIG. 14 is a cross section illustrating a prior-art memory cell;
- FIG. 15 is a cross section illustrating another prior-art memory cell; and
- FIG. 16 is a cross section illustrating still another prior-art memory cell.
- Now, referring to FIGS. 2 through 8, there are shown cross sections illustrating a method of fabricating a memory of FIG. 1 in the order of main processes, the method being practiced as one preferred embodiment of the present invention.
- First, as shown in FIG. 2, a switching transistor is formed by the conventional MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) forming process. In the figure,
reference numeral 21 indicates a p-type semiconductor substrate,reference numeral 22 indicates an isolation dielectric,reference numeral 23 indicates a gate oxide film,reference numeral 24 indicates a word line that provides a gate electrode,reference numerals reference numeral 27 indicates an interlayer insulating film. A deposit ofSiO 2 28 of about 600 nm in thickness is formed by the known CVD (Chemical Vapor Deposition) all over the transistor. Then, the formed deposit is reflowed at 850° C. to be etched back by about 300 nm, thereby smoothing steps caused by the word line. - Next, an opening is formed in the
SiO 2 28 so that a bit line can come into contact with the n-type region 25. The opening is made by known photolithography and dry etching techniques. Then, thebit line 31 is formed. Thisbit line 31 is composed of a film stack made of metal suicide and polysilicon. This film stack is etched by the known photolithography and dry etching to a desired bit-line pattern. - An insulating
film 32 of silicon oxide film type such as BPSG (Boron-doped Phosphor-Silicate Glass) is deposited to planarize. It should be noted that this insulatingfilm 32 needs to be thick enough for to planarize the substrate surface. In the present embodiment, the insulatingfilm 32 was formed to a thickness of about 600 nm and planarized by etching back. - Referring to FIG. 3, a
contact hole 33 of memory section is opened to provide access for the storage capacity section to come into contact with the substrate. On the insulatingfilm 32 and inside thecontact hole 33,polysilicon 41 was deposited by CVD to a thickness of about 350 nm. Then, by dry etching, thepolysilicon 41 was etched back by the film thickness to fill thecontact hole 33 as shown in FIG. 4. - By sputtering, TiN is formed to a thickness of about 50 nm as a
diffusion barrier layer 42 and then abottom electrode 43 is formed. In the present embodiment, a Pt film about 200 nm thick was deposited as thebottom electrode 43. The TIN of thediffusion barrier layer 42 is provided to prevent the platinum of thebottom electrode 43 and the oxygen from diffusing into thepolysilicon 41. Then, aferroelectric film 44 is formed. In the present embodiment, a lead zirconate titanate (Pb(Zr0.5Ti0.5)O3) film was formed to a thickness of about 150 nm by reactive evaporation and then crystallized by heat treatment in oxygen atmosphere at 650° C. for 30 seconds for obtaining theferroelectric film 44. It will be apparent that theferroelectric film 44 may also be formed by high-frequency magnetron sputtering, Sol-Gel method, MOD (Metal Organic Decomposition), or CVD. Then, by sputtering, a Pt film about 50 nm thick was formed as atop electrode 45 and atungsten 46 was deposited to a thickness of about 350 nm for the mask as shown in FIG. 4. - Referring to FIG. 5, the
tungsten 46 is patterned by dry-etching with SF6, aphotoresist 51 being used as the mask. After thephotoresist 51 has been removed, thetop electrode 45 is patterned by the sputter etching with thetungsten 46 used as the mask as shown in FIG. 6. In doing so, the dry etching conditions were adjusted such that isotropic etching is provided, and the etching was performed such that the cross section of thetungsten 46 becomes a trapezoid, the angle of each of the sides thereof relative to the substrate being less than 75 degrees. In the present embodiment, microwave dry etching was used with the conditions that an SF6 gas flow of 10 SCCM, a pressure of 2 mTorr, and a microwave power of 400 W. - If the dry etching is performed with high anisotropy, the cross section of the
tungsten 46 departs from the trapezoid and approaches a rectangular, leaving projecting deposits on thetungsten 46 and thephotoresist 51 at the sidewalls thereof. As shown in FIG. 10A, after thephotoresist 51 has been removed, platinum-contained projectingsidewall deposits 101 remain. As shown in FIG. 10B, after theferroelectric film 44 has been dry-etched,sidewall deposits 102 composed of the components of the ferroelectric film remain on the periphery of the platinum-contained projectingsidewall deposits 101. - When the
ferroelectric film 44 has been etched by use of a mixed gas composed of CF4 and Ar, thebottom electrode 43 is etched by sputter etching. It should be noted that 30% over-etching was performed in order to remove the platinum deposited on the sidewalls as shown in FIG. 7. - Oxygen plasma is generated in the same chamber in which the above-mentioned etching was performed to recover the etching damages of the cross section of the
ferroelectric film 44. In the present embodiment, the oxygen plasma processing was performed at an oxygen flow of 25 SCCM, a pressure of 30 mTorr, an RF power of 150 W for three minutes as shown in FIG. 8. Then, by the dry etching with SF6, thediffusion barrier layer 42 and the remainingtungsten 46 were removed simultaneously to complete the ferroelectric capacitor of FIG. 1. Although not shown, the memory device is completed by performing wiring like an ordinary semiconductor memory chip. - According to the above-mentioned processes, the increase in leakage current on the sidewalls and the decrease in breakdown voltage can be prevented at the same time, thereby allowing fabrication of microscopic ferroelectric memory cells suitable for high integration. The leakage of the ferroelectric capacitor can also be decreased by performing the oxygen plasma processing after etching the
diffusion barrier layer 42 by the dry etching using SF6. However, this causes oxidization of thediffusion barrier layer 42 left under thebottom electrode 43 from the sides of the layer, resulting in peeling off of thebottom electrode 43 from the bottom electrode/diffusion barrier layer interface. This problem can be avoided if the oxygen plasma processing is performed before dry-etching thediffusion barrier layer 42. - FIG. 9 shows comparisons of the leakage current density and voltage characteristics of the ferroelectric capacitor obtained after the etching of the top electrode shown in FIG. 6, after the etching of the bottom electrode shown in FIG. 7, after the over etching, after the oxygen plasma processing of FIG. 8, and after the etching of the diffusion barrier layer. After the top electrode etching, the leakage current density is on a order of 10−7 A/cm2. After the bottom electrode etching, the platinum-contained deposits on the ferroelectric film sidewalls short-circuit the top and bottom electrodes. Removing these platinum-contained deposits by 30% over-etching decreases the leakage current density to an order of 10−5 A/cm2. However, this value is larger than that obtained after the top electrode etching by an order of magnitude or more because the ferroelectric film sidewalls are exposed to the plasma to cause oxygen defects. When the oxygen defects on the ferroelectric film sidewalls are remedied by oxygen plasma processing, the leakage current density decreases to the generally same level as that observed after the top electrode etching. Etching of the diffusion barrier layer does not indicate an increase in the leakage current density either.
- In the present embodiment, the
tungsten 46 that provides the mask is formed by isotropy dry etching into a trapezoid in cross section. It will be apparent that, as described above with reference to FIG. 11, thetungsten 46 maybe formed by anisotropic dry etching into a trapezoid in cross section after forming thephotoresist 51 into a trapezoid in cross section by setting the baking temperature of thephotoresist 51 to a range of 140° C. to 160° C. - In the present embodiment, lead ziroconate tintanate is used for
ferroelectric film 44. It will be apparent that the material for the ferroelectric film is not limited to lead zirconate titanate; also available are, by way of example, perovskite-type oxides such as lead titanate, strontium titanate, and barium titanate, solid solutions of these, and bismuth-type layer-structured ferroelectric oxides. - In the present embodiment, TiN is used for the
diffusion barrier layer 42. It will be apparent that the same effect can be obtained by use of Ti or Ta or by stacking a plurality of materials selected from TiN, Ti, and Ta. - As described and according to the invention, etching the top electrode, the ferroelectric film and the bottom electrode with single photolithography process step does not cause short-circuit between the top and bottom electrodes, thereby allowing the fabrication of the memory cell of a small cell area suitable for highly integrated memory devices. Use of the memory cell according to the present invention can implement not only a high integrated DRAM (Dynamic Random Access Memory) and a highly integrated ferroelectric nonvolatile memory, but also a high-performance LSI (Large Scale Integration) in which these memory cells and a logic LSI are integrated on one chip and a field-programmable logic LSI that allows modification of wiring by the ferroelectric nonvolatile memory. It will be apparent that the effect of the present invention is by any means restricted to the memory cell of the above-mentioned embodiment; rather, the effect of the present invention extends to all semiconductor devices including LSIs for communications applications that use the ferroelectric capacitor.
- While the preferred embodiment of the present invention has been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the appended claims.
Claims (8)
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US09/475,033 US6338994B1 (en) | 1995-12-05 | 1999-12-30 | Semiconductor device and method of fabricating thereof |
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