CN103680940A - A method for improving anti-fatigue properties of a ferroelectric thin film with conductive oxides as bottom electrodes - Google Patents

A method for improving anti-fatigue properties of a ferroelectric thin film with conductive oxides as bottom electrodes Download PDF

Info

Publication number
CN103680940A
CN103680940A CN201310429686.1A CN201310429686A CN103680940A CN 103680940 A CN103680940 A CN 103680940A CN 201310429686 A CN201310429686 A CN 201310429686A CN 103680940 A CN103680940 A CN 103680940A
Authority
CN
China
Prior art keywords
cfo
film
thin film
ceramic target
ferroelectric thin
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
CN201310429686.1A
Other languages
Chinese (zh)
Inventor
张晓东
褚君浩
邢怀中
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Donghua University
Original Assignee
Donghua University
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Donghua University filed Critical Donghua University
Priority to CN201310429686.1A priority Critical patent/CN103680940A/en
Publication of CN103680940A publication Critical patent/CN103680940A/en
Pending legal-status Critical Current

Links

Images

Abstract

The invention provides a method for improving anti-fatigue properties of a ferroelectric thin film with conductive oxides as bottom electrodes. The method comprises the following steps: a metal oxide film/substratum substrate forms on a substratum through the utilization of a sputtering deposition thin-film technique; a ferroelectric thin film/metal oxide film/ substratum substrate further forms; then a layer of CFO is deposited again on the ferroelectric thin film; and finally, top electrodes are deposited on the CFO. According to the invention, on the basis that the conductive oxides are employed, and one layer of CFO thin film is plated above, so that the anti-fatigue properties of the ferroelectric capacitors are further obviously improved. The needed CFO plating layer is low in cost; the operation is simple; and the performance is stable. According to the invention, the fatigue properties of a ferroelectric storage can be substantially improved; a biggest obstacle against productization is removed; and the method has the advantages of being low in cost, high in reliability and long in usage life.

Description

The method of the ferroelectric thin film fatigue resistance that conductive oxide is hearth electrode is take in raising
Technical field
The present invention relates to a kind of method that the fatigue properties of ferroelectric thin-flim materials (especially lead zirconate titanate (hereinafter to be referred as PZT) ferroelectric thin-flim materials) are significantly improved.
Background technology
Ferroelectric material is the dielectric that a class has spontaneous polarization and can reverse under the effect in outfield, makes it to be very suitable for doing memory, and two state correspondences of its residual polarization 1 state and 0 state of memory.And can change storage state or come its polarized state of sensing, reading information by peripheral circuit by changing the direction in outfield.Ferroelectric material has good ferroelectric, piezoelectricity, pyroelectricity, electric light, acousto-optic and non-linear optical property, integrates power, heat, the function such as optical, electrical, has the incomparable superior function of other material.These special natures of ferroelectric material make it in many-sides such as supersonic changer element, micro electronmechanical coupled apparatus, high capacity capacitor (DRAM), ferroelectric memory (FeRAM), electrooptical shutter, light shutter device, imaging and display devices, all be with a wide range of applications and therefore the research of its technology of preparing and characteristic obtained paying attention to widely.The larger operating voltage of polarization reversal needs due to lumpy ferroelectric material, be difficult to integrate with standard semiconductor circuit, thereby only need the thin-film ferroelectric material of less operating voltage to obtain nearly ten years great attention, formed the focus of ferroelectric thin film research, simultaneously also driven ferroelectric thin film in other respects as pyroelectricity, piezoelectric effect, and the application of the effect such as electric light, acousto-optic.
Hinder at present ferroelectric memory commercial because have four.The one, tired (fatigue) problem, is characterized in: along with the increase of polarization reversal number of times, spontaneous polarization strength reduces gradually.The 2nd, memory keeps (retention) or claims the problem of aging (aging).The 3rd, polarization relaxation problem (polarization relaxiation), it shows as: once outfield remove, the logarithmic decrement that the remanent polarization of former direction can be in time.The 4th, trace (imprint) problem, its feature is: when ferroelectric capacitor is during at another state of the aging rear steering of state, ferroelectric capacitor seems to be still ready to rest on original state and causes electric hysteresis loop laterally asymmetric, with respect to voltage axis, have the skew of a voltage, this will cause " 0 " state or one state readout error.Wherein, the polarization fatigue of ferroelectric thin film is to limit the biggest obstacle of its application.Therefore, to the research of its fatigue properties and to manage to improve its fatigue resistance be the key that realizes its application.
In recent years, adopt in the world oxide electrode to replace traditional Pt electrode, to improve the fatigue resistance of ferroelectric thin film, as adopted YBa 2cu 3o 7-x, SrRuO 3, IrO 2, RuO 2, La xsr 1-xcoO 3, LaNiO 3(LNO) etc.LNO is also a kind of metal oxide of conduction, and its resistivity is 10 -3～10 -4Ω cm order of magnitude left and right, it has accurate cubic perovskite structure, and its lattice constant is 0.384nm, very approaching with PZT.
Summary of the invention
It is simple that the technical problem to be solved in the present invention is to provide a kind of method of operation, and stable performance is reproducible, can realize the fatigue properties of the ferroelectric capacitor using conductive oxide as hearth electrode are obtained to the method further obviously improving.
In order to solve the problems of the technologies described above, technical scheme of the present invention has been to provide a kind of raising and has take the method for the ferroelectric thin film fatigue resistance that conductive oxide is hearth electrode, it is characterized in that, step is:
Step 1, prepare sputtering target: prepare respectively conducting metal oxide ceramic target, ferroelectric thin-flim materials ceramic target and CFO ceramic target;
Step 2, employing sputter-deposited thin films technology utilization conducting metal oxide ceramic target sputtering sedimentation conductive metal oxide film on substrate form metal-oxide film/substrate base, conducting metal oxide ceramic target is changed to after ferroelectric thin-flim materials ceramic target, adopts sputter-deposited thin films technique sputtering sedimentation ferroelectric thin film on metal-oxide film/substrate base to form ferroelectric thin film/metal-oxide film/substrate base;
Step 3, ferroelectric thin-flim materials ceramic target is changed to after CFO ceramic target, adopts sputter-deposited thin films technique sputtering sedimentation CFO film on ferroelectric thin film, and cover another ferroelectric thin film/metal-oxide film/substrate base as contrasting with silicon chip.Finally, on two samples, deposit again top top electrode, obtain top top electrode/CFO film/ferroelectric thin film/metal-oxide film/substrate ferroelectric capacitor.
The present invention, on the basis that adopts conductive oxide as hearth electrode method, then adds plating one deck CFO film, makes the fatigue properties of ferroelectric capacitor obtain further significantly improving.Required CFO coating cost is low, simple to operate, stable performance.The advantages such as the present invention will improve the fatigue properties of ferroelectric memory greatly, clears away the biggest obstacle of commercialization, and it has with low cost, and reliability is high, and the life-span is long.
Accompanying drawing explanation
Fig. 1 (a) is the XRD diffraction spectrogram of CFO/PZT/LNO/STO and PZT/LNO/STO sample;
Fig. 1 (b) is the XRD diffraction spectrogram of CFO/PZT/LNO/LAO and PZT/LNO/LAO sample;
Fig. 2 (a) is the electric hysteresis loop P-V of CFO/PZT/LNO/STO and PZT/LNO/STO sample;
Fig. 2 (b) is the electric hysteresis loop P-V of CFO/PZT/LNO/LAO and PZT/LNO/LAO sample;
Fig. 3 (a) is the testing fatigue of CFO/PZT/LNO/STO and PZT/LNO/STO sample;
Fig. 3 (b) is the testing fatigue of CFO/PZT/LNO/LAO and PZT/LNO/LAO sample.
Embodiment
For the present invention is become apparent, hereby with preferred embodiment, and coordinate accompanying drawing to be described in detail below.
Embodiment 1
The present embodiment is by adopting conventional magnetron sputtering at single-crystal strontium titanate SrTiO 3on (being designated hereinafter simply as STO) substrate, in succession deposit LaNiO 3(being designated hereinafter simply as LNO) hearth electrode, pzt thin film and CoFe 2o 4(being designated hereinafter simply as CFO) film, last, plate top electrode Au in the above, its concrete preparation process is as follows:
The preparation of step 1, sputtering target: prepare respectively LNO ceramic target, PZT ceramic target and CFO ceramic target, its preparation can adopt conventional steps, in its present embodiment, its preparation process is respectively:
LNO ceramic target:
Use 99.9%La 2o 3and Ni 2o 3powder is pressed into after by the La of 1:1, Ni atomic ratio mixed grinding
Figure BDA0000384255620000031
block then 1100 ℃ of high temperature sinterings 3 hours, make LNO ceramic target.
PZT ceramic target:
By PbO, ZrO 2and TiO 2powder (is the volatilization loss of compensation Pb by required stoichiometric proportion, the excessive Pb that adds 10%wt) evenly mixing, compressing, finally at 900 ℃ of sintering, within 3 hours, form, it is of a size of φ 60 * 5mm, and the percentage Zr:Ti of constituent content is 20:80.
CFO ceramic target:
By CoO and Fe 2o 3powder is by compressing after the required even mixed grinding of stoichiometric proportion, finally, 1200 ℃ of high temperature sinterings 3 hours, makes CFO ceramic target.
Step 2, two monocrystalline STO that adopt (100) to be orientated are substrate.Substrate first, with the ultrasonic cleaning that hockets of ethanol, acetone, then carries out chemical cleaning with trichloroethylene, finally in vacuum chamber, peels off cleaning again with Ar Ion Beam Etching.Adopt sputter-deposited thin films technique, utilize LNO ceramic target, on substrate, sputtering sedimentation LNO film obtains LNO/STO substrate, and the process conditions of sputter-deposited thin films are as follows: base vacuum degree 5 * 10 -4pa, 400 ℃ of base reservoir temperatures, target-cardinal distance is from 7cm, r.f. power 80-100W, it is 7 * 10 that the thickness that obtains in-situ sputtering deposition LNO film is about 50nm resistivity -4Ω cm.
Step 3, change LNO ceramic target into PZT ceramic target, the same sputter-deposited thin films technique that adopts, sputtering sedimentation pzt thin film on LNO/STO substrate, obtains PZT/LNO/STO substrate, and the process conditions of sputter-deposited thin films are: sputter cavity base vacuum degree is extracted into 5 * 10 -4pa, meanwhile, by LNO/STO substrate heating to 650 ℃, then passes into argon gas and keeps air pressure in 1.6Pa left and right, carries out sputtering sedimentation pzt thin film, controls sputtering time, obtains the pzt thin film of the about 100nm of thickness.
Step 4, change PZT ceramic target into CFO ceramic target, the same sputter-deposited thin films technique that adopts, sputtering sedimentation CFO film on PZT/LNO/STO substrate (covering another PZT/LNO/STO substrate with silicon chip), finally on two samples, deposit again top top electrode Au, obtain Au/CFO/PZT/LNO/STO and Au/PZT/LNO/STO ferroelectric capacitor.The process conditions of sputter-deposited thin films are: sputter cavity base vacuum degree is extracted into 5 * 10 -4pa, meanwhile, is heated to 650 ℃ by sample, then passes into argon gas and keeps air pressure in about 1.6Pa, carries out sputtering sedimentation CFO film, controls sputtering time, makes on the pzt thin film of silicon chip, to cover the CFO film that the about 50nm of one deck is thick not covering.
The Au/CFO/PZT/LNO/STO obtaining by the present embodiment and Au/PZT/LNO/STO ferroelectric capacitor are compared, from Fig. 1 (a), can find out, on STO substrate, all crystallization goes out the pzt thin film of perovskite structure, and occurs (200) and (002) two peak.(200) peak is corresponding to a farmland in PZT, and (002) peak is corresponding to c farmland.Illustrate in pzt thin film that a farmland and c farmland coexist., added after CFO layer, crystallization goes out the CFO film of (004) orientation meanwhile.
On STO substrate, have, without CFO layer, the electric hysteresis loop of Au/CFO/PZT/LNO/STO and Au/PZT/LNO/STO sample provides at Fig. 2 (a).From Fig. 2 (a), find out have, basic identical without the electric hysteresis loop shape of CFO layer sample, residual polarization is almost identical.
Fig. 3 (a) provides the clean reverse-poled intensity of Au/CFO/PZT/LNO/STO and Au/PZT/LNO/STO structure ferroelectric capacitor and the relation (triangular wave that the pulse signal applying is 100Hz) of reversion number of times.From Fig. 3 (a), can find out, in the sample without CFO film (Au/PZT/LNO/STO), when continuous reversion 10 3after inferior, clean reverse-poled intensity starts fast-descending.And have in the sample of CFO film (Au/CFO/PZT/LNO/STO), almost without fatigue phenomenon.
Embodiment 2
The present embodiment is by adopting conventional magnetron sputtering at monocrystalline lanthanum aluminate LaAlO 3on (being designated hereinafter simply as LAO) substrate, in succession deposit LaNiO 3(being designated hereinafter simply as LNO) hearth electrode, pzt thin film and CoFe 2o 4(being designated hereinafter simply as CFO) film, last, plate top electrode Au in the above, its concrete preparation process is as follows:
The preparation of step 1, sputtering target: prepare respectively LNO ceramic target, PZT ceramic target and CFO ceramic target, its preparation can adopt conventional steps, in its present embodiment, its preparation process is respectively:
LNO ceramic target:
Use 99.9%La 2o 3and Ni 2o 3powder is pressed into after by the La of 1:1, Ni atomic ratio mixed grinding
Figure BDA0000384255620000041
Figure BDA0000384255620000042
block then 1100 ℃ of high temperature sinterings 3 hours, make LNO ceramic target.
PZT ceramic target:
By PbO, ZrO 2and TiO 2powder (is the volatilization loss of compensation Pb by required stoichiometric proportion, the excessive Pb that adds 10%wt) evenly mixing, compressing, finally at 900 ℃ of sintering, within 3 hours, form, it is of a size of φ 60 * 5mm, and the percentage Zr:Ti of constituent content is 20:80.
CFO ceramic target:
By CoO and Fe 2o 3powder is by compressing after the required even mixed grinding of stoichiometric proportion, finally, 1200 ℃ of high temperature sinterings 3 hours, makes CFO ceramic target.
Step 2, two monocrystalline LAO that adopt (100) to be orientated are substrate.Substrate first, with the ultrasonic cleaning that hockets of ethanol, acetone, then carries out chemical cleaning with trichloroethylene, finally in vacuum chamber, peels off cleaning again with Ar Ion Beam Etching.Adopt sputter-deposited thin films technique, utilize LNO ceramic target, on substrate, sputtering sedimentation LNO film obtains LNO/LAO substrate, and the process conditions of sputter-deposited thin films are as follows: base vacuum degree 5 * 10 -4pa, 400 ℃ of base reservoir temperatures, target-cardinal distance is from 7cm, r.f. power 80-100W, it is 7 * 10 that the thickness that obtains in-situ sputtering deposition LNO film is about 50nm resistivity -4Ω cm.
Step 3, change LNO ceramic target into PZT ceramic target, the same sputter-deposited thin films technique that adopts, sputtering sedimentation pzt thin film on LNO/LAO substrate, obtains PZT/LNO/LAO substrate, and the process conditions of sputter-deposited thin films are: sputter cavity base vacuum degree is extracted into 5 * 10 -4pa, meanwhile, by LNO/LAO substrate heating to 650 ℃, then passes into argon gas and keeps air pressure in 1.6Pa left and right, carries out sputtering sedimentation pzt thin film, controls sputtering time, obtains the pzt thin film of the about 100nm of thickness.
Step 4, change PZT ceramic target into CFO ceramic target, the same sputter-deposited thin films technique that adopts, sputtering sedimentation CFO film on PZT/LNO/LAO substrate (covering another PZT/LNO/LAO substrate with silicon chip), finally on two samples, deposit again top top electrode Au, obtain Au/CFO/PZT/LNO/LAO and Au/PZT/LNO/LAO ferroelectric capacitor.The process conditions of sputter-deposited thin films are: sputter cavity base vacuum degree is extracted into 5 * 10 -4pa, meanwhile, is heated to 650 ℃ by sample, then passes into argon gas and keeps air pressure in about 1.6Pa, carries out sputtering sedimentation CFO film, controls sputtering time, makes on the pzt thin film of silicon chip, to cover the CFO film that the about 50nm of one deck is thick not covering.
The Au/CFO/PZT/LNO/LAO obtaining by the present embodiment and Au/PZT/LNO/LAO ferroelectric capacitor are compared, from Fig. 1 (b), can find out, on LAO substrate, all crystallization goes out the pzt thin film of perovskite structure, and occurs (200) and (002) two peak.(200) peak is corresponding to a farmland in PZT, and (002) peak is corresponding to c farmland.Illustrate in pzt thin film that a farmland and c farmland coexist., added after CFO layer, crystallization goes out the CFO film of (004) orientation meanwhile.
On LAO substrate, have, without CFO layer, the electric hysteresis loop of Au/CF0/PZT/LNO/LAO and Au/PZT/LNO/LAO sample provides at Fig. 2 (b).From Fig. 2 (b), find out have, basic identical without the electric hysteresis loop shape of CFO layer sample, residual polarization is almost identical.
Fig. 3 (b) provides the clean reverse-poled intensity of Au/CFO/PZT/LNO/LAO and Au/PZT/LNO/LAO structure ferroelectric capacitor and the relation (triangular wave that the pulse signal applying is 100Hz) of reversion number of times.From Fig. 3 (b), can find out, in the sample without CFO film (Au/PZT/LNO/LAO), when continuous reversion 10 3after inferior, clean reverse-poled intensity starts fast-descending, and has in the sample of CFO film (Au/CFO/PZT/LNO/LAO) almost without fatigue phenomenon.

Claims (1)

1. further improve and take the method for the ferroelectric thin film fatigue resistance that conductive oxide is hearth electrode, it is characterized in that, step is:
Step 1, prepare sputtering target: prepare respectively conducting metal oxide ceramic target, ferroelectric thin-flim materials ceramic target and CFO ceramic target;
Step 2, employing sputter-deposited thin films technology utilization conducting metal oxide ceramic target sputtering sedimentation conductive metal oxide film on substrate form metal-oxide film/substrate, conducting metal oxide ceramic target is changed to after ferroelectric thin-flim materials ceramic target, adopts sputter-deposited thin films technique sputtering sedimentation ferroelectric thin film on metal-oxide film/substrate to form ferroelectric thin film/metal-oxide film/substrate base;
Step 3, ferroelectric thin-flim materials ceramic target is changed to after CFO ceramic target, adopt sputter-deposited thin films technique sputtering sedimentation CFO film on ferroelectric thin film, finally, on CFO film, deposit again top top electrode, obtain top top electrode/CFO film/ferroelectric thin film/metal-oxide film/substrate ferroelectric capacitor.
CN201310429686.1A 2013-09-18 2013-09-18 A method for improving anti-fatigue properties of a ferroelectric thin film with conductive oxides as bottom electrodes Pending CN103680940A (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
CN201310429686.1A CN103680940A (en) 2013-09-18 2013-09-18 A method for improving anti-fatigue properties of a ferroelectric thin film with conductive oxides as bottom electrodes

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
CN201310429686.1A CN103680940A (en) 2013-09-18 2013-09-18 A method for improving anti-fatigue properties of a ferroelectric thin film with conductive oxides as bottom electrodes

Publications (1)

Publication Number Publication Date
CN103680940A true CN103680940A (en) 2014-03-26

Family

ID=50318207

Family Applications (1)

Application Number Title Priority Date Filing Date
CN201310429686.1A Pending CN103680940A (en) 2013-09-18 2013-09-18 A method for improving anti-fatigue properties of a ferroelectric thin film with conductive oxides as bottom electrodes

Country Status (1)

Country Link
CN (1) CN103680940A (en)

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH042698A (en) * 1990-04-20 1992-01-07 Matsushita Electric Ind Co Ltd Thin film base and production thereof
CN1766158A (en) * 2005-08-25 2006-05-03 中国科学院上海技术物理研究所 The preparation method of low resistivity metal oxide lanthanum nickelate
CN101376600A (en) * 2008-09-26 2009-03-04 清华大学 Stack ferro-electricity /magnetic multiferrou magnetoelectric compound film with conductive oxide as buffer layer and preparation thereof
CN101956166A (en) * 2010-10-13 2011-01-26 上海师范大学 Method for preparing plumbum magnesium niobate-plumbum titanate ferroelectric film

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH042698A (en) * 1990-04-20 1992-01-07 Matsushita Electric Ind Co Ltd Thin film base and production thereof
CN1766158A (en) * 2005-08-25 2006-05-03 中国科学院上海技术物理研究所 The preparation method of low resistivity metal oxide lanthanum nickelate
CN101376600A (en) * 2008-09-26 2009-03-04 清华大学 Stack ferro-electricity /magnetic multiferrou magnetoelectric compound film with conductive oxide as buffer layer and preparation thereof
CN101956166A (en) * 2010-10-13 2011-01-26 上海师范大学 Method for preparing plumbum magnesium niobate-plumbum titanate ferroelectric film

Similar Documents

Publication Publication Date Title
Starschich et al. Evidence for oxygen vacancies movement during wake-up in ferroelectric hafnium oxide
Zhuo et al. Giant negative electrocaloric effect in (Pb, La)(Zr, Sn, Ti) O3 antiferroelectrics near room temperature
CN102157682B (en) One-phase ferroelectric film and preparing method thereof as well as effective resistance regulation mode
Whatmore Ferroelectric materials
Ko et al. High-stability transparent flexible energy storage based on PbZrO3/muscovite heterostructure
CN107275481B (en) A method of improving Ferro-RRAM switching current ratio
Zhao et al. Recoverable self-polarization in lead-free bismuth sodium titanate piezoelectric thin films
Kang et al. AC conductivity and dielectric properties of (Bi, Na) TiO3–BaTiO3 lead free ceramics
CN100365160C (en) Preparation method of low resistivity metal oxide lanthanum nickelate
US7888138B2 (en) Ferroelectric thin film device and method of manufacturing the same
Sharma et al. Investigation on the effect of ferrite content on the multiferroic properties of (1-x) Ba0. 95Sr0. 05TiO3–(x) Ni0. 7Zn0. 2Co0. 1Fe2O4 ceramic composite
CN111423231A (en) Ternary system relaxor ferroelectric film material and preparation method and application thereof
Feng et al. Enhancement of strain by electrically-induced phase transitions in BNKT-based ceramics
US20140290733A1 (en) Photoelectric conversion element and photovoltaic cell
CN101436597A (en) Ferro-electricity film capacitor for ferro-electric memory and preparation method thereof
US20080204970A1 (en) Transparent oxide capacitor structures
Oh et al. Optical study of domains in Ba (Ti, Sn) O3 ceramics
Zhang et al. Large remanent polarization and small leakage in sol–gel derived Bi (Zn 1/2 Zr 1/2) O 3–PbTiO 3 ferroelectric thin films
CN110527952A (en) A kind of barium titanate/nickel acid lanthanum ferroelectric superlattice material and preparation method thereof
Ha et al. Effects of ZnO on piezoelectric properties of 0.01 PMW–0.41 PNN–0.35 PT–0.23 PZ ceramics
US20200052190A1 (en) Method of manufacturing piezoelectric thin film and piezoelectric sensor manufactured using piezoelectric thin film
US7527690B2 (en) Ferroelectric ceramic compound, a ferroelectric ceramic single crystal, and preparation processes thereof
Dewi et al. The effect of heating rate on BaZrxTi1-xO3 thin film for x= 0.4 and x= 0.6 as capacitors
Lin et al. Properties of RF magnetron sputtered 0.95 (Na0. 5Bi0. 5) TiO3–0.05 BaTiO3 thin films
CN103680940A (en) A method for improving anti-fatigue properties of a ferroelectric thin film with conductive oxides as bottom electrodes

Legal Events

Date Code Title Description
PB01 Publication
PB01 Publication
C10 Entry into substantive examination
SE01 Entry into force of request for substantive examination
RJ01 Rejection of invention patent application after publication

Application publication date: 20140326

RJ01 Rejection of invention patent application after publication