498
`
`IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-29, NO. 4, APRIL 1982
`
`Advantages of Thermal Nitride and Nitroxide
`Gate Films in VLSI Process
`
`TAKASHI ITO, TETSUO NAKMURA, MEMBER, IEEE, AND HAJIME ISHIKAWA, MEMBER, IEEE
`
`Abstract-Thin gate Si02 films thinner than 200 A often deteriorate
`throughout developmental VLSI processes, including refractory metal
`or silicide gates and ion- or plasma-assisted processes. Thermal nitrida-
`tion of such Si02 films improves the MOS characteristics by producing
`surface protective layers
`against impurity penetration
`and by pro-
`ducing good interfacial characteristics. This fact indicates that a ther-
`mally grown silicon nitride film
`on a silicon substrate
`is the most
`promising candidate for a very-thin gate insulator. Experimental data
`show significant benefits from the nitride film for future VLSI devices.
`
`R
`
`I. INTRODUCTION
`ECENT requirements for producing thinner insulators ap-
`pear in the development of high-density random-access
`memories (RAM'S)
`[ l ] and electrically erasable read-only
`memories (E2PROM) [2] as well as projected micrometer- or
`submicrometer-range MOSFET's [3]. The 4- and 16-kbit
`RAM'S use Si02 films having a thickness of about 1000 A, and
`reduction of film thickness to 200 A has enabled development
`of 256-kbit RAM'S [4], [5]. The projected 1-Mbit RAM will
`require a film thickness of 100 A or less if the conventional
`one-transistor/cell structure is used.
`Furthermore, use of an MOS gate insulator thinner than that
`predicted from
`the constant
`electric-field scaling theory
`greatly reduces the so-called "second-order effects" appearing
`in downscaled MOSFET characteristics. These include short-
`channel effect, narrow-channel effect, drain-induced barrier
`lowering subthreshold current, and hot-electron trapping.
`Shift of a gate threshold voltage due to mobile ions or ionized
`traps released in a gate insulator becomes smaller as the in-
`sulator thickness becomes thinner. Alpha-particle disturbance
`the increased density of mobile
`may also be reduced by
`carriers induced by the thinner
`gate insulator. A dielectric
`constant insulator greater than Si02 gives the same benefits as
`thinner films. The integrity of such thin insulator films should
`be much greater than the relatively thick SiOz films currently
`in use because they will be used under more severe conditions.
`The properties of thin SiOz films are affected by various
`deteriorating effects throughout
`device production. The fol-
`lowing phenomena should be examined for producing reliable
`thin insulators of MOS gate grade in VLSI's:
`of
`1) Structural homogeneity and breakdown uniformity
`thermal Si02 fdms are influenced by
`the quality of silicon
`bulk and its surface conditions.
`2 ) Penetration of gate electrode material and impurities
`
`Manuscript received September 19, 1981; revised November 24, 1981.
`The authors are with Semiconductor Devices Laboratory, Fujitsu
`Laboratories, Ltd., 1015 Kamikodanaka, Nakahara, Kawasaki,
`Japan.
`
`211,
`
`0 2 4 6 0 2 4 , 6 0 2 4 6
`Bombardment time bin)
`(b)
`(C)
`(a)
`Fig. 1. (a) Auger-depth profiles of Si02, (b) nitroxide films formed at
`1200°C for 1 h, and (c) for 5 h.
`
`es-
`
`structural homogeneity,
`into thin Si02 films degrades
`pecially for reactive refractory metal or silicide gates.
`3) Charge accumulation on thin SiOz films, resulting from
`ion or plasma processes such as ion implantation and reactive
`ion etching, often leads to destructive breakdown of the oxide
`films.
`This paper discusses the inherent problems in producing re-
`liable thin insulators (less than 200 a thick) suitable for VLSI
`gate films. Special
`emphasis is placed on the advantages of
`thermally grown silicon nitride and thermally nitrided oxide
`(hereafter; nitroxide) fdms over thermal Si02 films.
`
`11. GROWTH OF THERMAL si02, NITROXIDE, AND
`THERMAL NITRIDE FILMS
`Because of the greater structural homogeneity of thin Si02
`fdms at higher temperatures, considerable efforts are being
`made to grow thin SiOz films in thermal oxidation of silicon
`at a temperature above 1000°C. Better controllability in
`film
`thickness may be achieved by using oxidizing species diluted
`with an inert gas [7] . We are not sure, however, whether such
`Si02 films produce state-of-the-art MOS structures. In this ex-
`periment, silicon wafers, p-type, CZ, (100) oriented, having
`resistivities ranging from 1 to 5 !i2 . cm were degreased, chemi-
`cally cleaned in a hot aqueous solution of N H 4 OH + H 2 0 2 ,
`and rinsed in deionized water. Wafers were
`then thermally
`oxidized in dry oxygen at lOOO"C, followed by annealing in
`pure nitrogen at the same temperature for 20 min. The SiOz
`film thicknesses, measured by ellipsometry, were close
`to
`100 A.
`The nitroxide process entails heating of a thermally oxidized
`silicon wafer in pure ammonia
`gas at elevated temperatures
`[8] -[lo]. This produces an oxynitride layer over the Si02
`film. The nitride fraction in
`the oxynitride layer ranges from
`10 to 50 percent depending on the nitridation conditions; it
`decreases from the film surface to the inner film. Fig. 1 shows
`Auger-electron depth profiles of Si02 and nitroxide films.
`
`0018-9383/82/0400-0498$00.75 0 1982 IEEE
`
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`
`
`
`I T 0 et al.: THERMAL NITRIDE AND NITROXIDE
`
`FILMS
`
`499
`
`The KLL transition peak height of oxygen in a 90-A-thick
`is
`SiOz film with respect
`to argon ion-bombardment time
`shown in Fig. l(a). After nitriding the film in pure ammonia
`gas at 1200°C for 1 h, a KLL nitrogen peak appears
`as the
`oxygen peak decreases
`(see Fig. l(b)). Oxygen atoms were
`eliminated from the surface of SiOz as
`volatile species: Oz,
`OH, SiO, etc. The oxygen peak further decreases for a sample
`of 5 h, especially on the film surface, accompanied by an in-
`crease in the number of nitrogen atoms
`(see Fig.
`l(c)). The
`film thickness change is slight throughout the process. Nitrida-
`tion predominantly occurs on the
`SiOz surface, not at the
`of nitrogen atoms
`Si-SiOz interface; although some amount
`was observed at the interface. This differs from previously re-
`ported experimental results of ammonia annealing where direct
`conversion of SiOz to nitride was not pointed out except at an
`Si-SiOz interface [l 11 , [X21 . The chief reason for this differ-
`ence is the amount of oxidizing species in
`the ammonia gas
`used. In general, water, the predominant impurity in ammonia
`dissociating into Si and Oz. Fully
`gas, prevents SiOz from
`purified ammonia gas, however, contributes to dissociation of
`Si-0 bonds in SiOz followed by conversion to nitride. Nitrid-
`ing species hardly penetrate to the Si-SiOz interface just after
`a nitroxide film with a barrier effect is formed.
`in a previously reported
`Thermal nitride films were grown
`, [14] .
`reactor for plasmaenhanced thermal nitridation [13]
`Chemically cleaned silicon wafers were mounted on Sic-coated
`carbon susceptors. Highly purified ammonia gas was
`intro-
`duced into a quartz tube at
`reduced pressures. A radio fre-
`quency (RF) power of
`400 kHz was
`applied to heat the
`induction-coupled susceptors while glow discharge was gener-
`ated around the wafers, producing NHz, NH radicals hydrogen
`radicals, and electrons. A 100-A-thick nitride film was grown
`at 1050°C and a pressure of
`130 Pa for 150 min. Except
`for the
`initial growth, the nitridation appeared to follow a
`diffusion-limited process where active nitrogen atoms pro-
`duced from the ammonia plasma played an
`important role.
`The low growth rate may allow better control of fdm
`thick-
`ness. An Auger-depth profile of the 100-A-thick nitride fdm
`was obtained accompanying argon ion bombardment
`at an
`uniform percentages of
`energy of 1 keV (see Fig. 2). Nearly
`nitrogen, silicon, and oxygen atoms were observed along the
`film. The percentage of mixed oxygen atoms was less than 5
`percent, except
`for the surface layer of water or oxygen
`adsorption.
`111. MOS BREAKDOWN FAILURE DUE TO CHARGE
`ACCUMULATION
`Fig. 3 shows a breakdown voltage histogram of thin-oxide
`film thickness of 90 A and Al
`MOS diodes having an SiOz
`gates. The diameter of each Al gate is 400 pm. Breakdown
`voltages were measured by applying a forward bias on a gate
`with respect to the substrate and were determined as the volt-
`age at which a current of 1 pA begins to flow. A state-of-the-
`art MOS process enables production of
`such intrinsic-line
`breakdown voltages with a
`fairly narrow distribution. With
`this process, it may be possible to produce MOS devices having
`an SiOz film with a thickness of less than 100 A.
`The same wafer as
`that shown in Fig. 3 was subjected to a
`
`0
`40
`20
`Bombardment time Cmin)
`Fig. 2. Auger-depth profile of a 100-A-thick thermal nitride film.
`
`UJ w
`a
`0
`
`60
`
`Sioe ,90 a
`
`A! gate,
`
`z
`
`20
`0
`15
`5
`IO
`
`C V )
`Breakdown -voltage
`thin oxide MOS diodes having 90-A-
`Fig. 3. Breakdown histogram of
`thick SiOz fiims and a 400-pm-diameter A1 gate electrodes.
`
`~
`
`0
`
`15
`5
`IO
`Breakdown voltage (V)
`Fig. 4. Degradation of breakdown voltages of the same MOS diodes as
`shown in Fig. 3 measured after RF oxygen plasma treatment.
`
`20
`
`RF power (W)
`Fig. 5. Failure rates of breakdown voltages of MOS diodes having SiOz
`and nitroxide films treated in RF oxygen plasma environment.
`
`barrel-type oxygen plasma reactor. The excellent integrity of
`the SiOz film was destroyed in the plasma environment (see
`Fig. 4). This result is reproducible with our experimental set-
`up. An RF power of 150 W and exposure time of 8 min are
`conventionally used for ashing photoresists. The cause
`of
`destruction is believed to be charge accumulation on the gate
`which acts as a charge collector in the plasma environment
`which generally contains
`several orders smaller amounts of
`ions than neutral species. When the gate potential exceeds the
`maximum. breakdown voltage of a 90-A-thick SiOz film, the
`MOS diodes are
`either destroyed or badly damaged. Such
`breakdown failure is observed not only in oxygen plasma, but
`in other ion- or plasma-assisted processes as well.
`Fig. 5 shows the failure rates of MOS breakdown voltages for
`in RJ? oxygen plasma. Cumulative
`SiOz and nitroxide fdms
`
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`500
`
`IEEE TRANSACTIONS ON ELECTRON DEVICE§, VOL. ED-29, NO. 4, APRIL 1982
`
`5 m%$ed
`qh.-Jw-jl!-]
`
`V
`
`layer
`
`Ntraxide Mos
`Si02 MOS
`(b)
`(a)
`Fig. 6. (a) Current distribution model of MOS diodes having Si02 and
`.(b) nitroxide films just before destructive breakdown.
`
`30 rnin
`
`z
`
`0
`
`5
`IO
`Dielectric strength (MVlcm)
`Fig. 7. Histogram of breakdown fields of MOS diodes having Si02 and
`nitroxide films annealed in a forming gas.
`
`MOS diodes having
`failure percentages were calculated for
`breakdown voltages of less
`than half those of the
`intrinsic
`values. All MOS diodes of a couple of wafers were measured
`for each test point. At an RF power of 100 W, almost all MOS
`diodes having 90-A-thick SiOz films were destroyed. Nitroxide
`MOS diodes formed at 1200°C for 1 and
`2 h demonstrate
`greatly improved yields. This may be attributable to retarda-
`tion of the destructive dielectric breakdown of nitroxide films.
`This breakdown retardation is explained as follows:
`Breakdown of an MOS diode is known to be a localized phe-
`. Current concentration occurs at the lowest
`nomenon [15]
`potential point which
`is formed by penetration of gate ma-
`an SiOz film.
`terial or adsorption of ionized impurities on
`Fig. 6 shows a schematic current distribution model just be-
`fore breakdown. The localized current density increases as the
`applied gate bias increases. When the density reaches the value
`allowable for the MOS diode, the diode is destroyed. Because
`the nitroxide film acts as a protective layer against impurity
`penetration [9], localized current concentration just before
`in an MOS diode having a nitroxide
`destruction is avoided
`film. This allows a high current flow without destruction.
`
`OF IMPURITIES
`IV. HIGH-TEMPERATURE PENETRATION
`Future MOS processes, including refractory metal or silicide
`gates, will require high-temperature forming-gas annealing. For
`example, mobile-ion density in an Mo-gate MOS structure can
`be reduced by two orders of magnitude compared to nitrogen
`annealing [16] . Stability of
`interfacial characteristics is
`further improved. Resistivities of such gate materials are re-
`duced by high-temperature forming-gas annealing [ 171 , [ 183 .
`Fig. 7 shows a histogram of MOS breakdown after annealing
`insulator films in 5-percent hydrogen over nitrogen at 1 100°C
`for 30 min. Conventional MOS diodes having SiOz films an-
`nealed under these conditions
`are usually badly degraded.
`These diodes initially had dielectric breakdown fields near lo7
`V/cm before annealing. This
`is believed to be caused by easy
`penetration of hydrogen and
`partial dissociation of silicon-
`oxygen bonds of Si02 in the deoxidizing environment. This
`
`ioS' 0
`
`ds ds
`a2 a4
`Surface potential (VI
`of MOS structures having 90-
`and
`Surface-state densities
`500-A-thick Si02 and nitroxide films.
`
`Fig. 8.
`
`Si02 film.
`process forms electrically conductive passes in an
`MOS diodes having a nitroxide film, however, retain the nearly
`intrinsic histogram of breakdown voltages, exhibiting no ini-
`tial short. This shows that the nitroxide film is more tolerant
`of high-temperature hydrogen annealing than an SiOz film.
`en-
`Penetration of boron atoms
`into an SiOz film is also
`hanced by high-temperature forming-gas annealing. The char-
`acteristics of an MOS structure having a boron-diffused Si02
`film become unstable due to carrier trapping [19] . Boron
`atoms easily reach
`the silicon substrate through an Si02 film
`[20]. This process results in a great Shift in the
`threshold
`voltage. For example, as the annealing temperature increased
`from 1000 to 1100°C for 30 min, flat-band votlages ( V F B ) of
`MOS structures having boron-doped poly-Si gates shifted from
`3.5 to 12.5 V, whereas the value change was slight at 900°C.
`MOS structures having nitroxide films showed no change in
`flat-band voltages under those annealing conditions. The sur-
`face protective layer of a nitroxide film acts as a barrier against
`boron penetration. The surface barrier of this type of film is
`also effective for arsenic- or phosphorus-doped gate materials.
`The advantages of a nitroxide film or a thermal nitride film
`having great diffusion barrier effectiveness enables fabrication
`of MOS devices having a very thin gate insulator fim.
`
`V. INTERFACIAL CHARACTERISTICS OF MOS STRUCTURES
`Fig. 8 shows a comparison of the
`surface-state densities
`(PIss) of Si02 and nitroxide MOS diodes having aluminum as
`the gate electrodes. These were measured by the conventional
`quasi-static capacitance-voltage technique after annealing MOS
`diodes in a forming gas at 450°C for 30 min. The broken lines
`indicate energy distribution of Nss for 90- and 500-A-thick
`SiOz films. The 90-8 curve is relatively high owing to inter-
`facial charges or electrically active defects between the Si02
`film and the AI gate.
`Such interface effects become more important as compared
`to bulk effects as the gate insulator film becomes thinner [21].
`Si02 film at 1200°C for 5 h re-
`Nitridation of a 90-A-thick
`duces the Nss by almost two orders of magnitude. The Nss of
`a 500-&thick nitroxide film decreases further,' exhibiting the
`minimum value of less than 10" cm-2 * eV-' obtained with
`the 500-A-thick unnitrided Si02 film having a poly-Si gate.
`Improved interfacial characteristics of the Si-SiOz interface
`are attributable to nitridation of the
`silicon interface. The
`further the nitridation proceeds, the lower Nss becomes. This
`indicates that thermal nitridation of bare silicon is a better ap-
`proach for obtaining improved
`silicon interfacial characteris-
`tics than the nitroxide process.
`
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`IT0 et al.: THERMAL NITRIDE AND NITROXIDE FILMS
`
`501
`
`- i o @ - -
`’\
`
`.-
`.
`Poly, Si gate
`
`-O.hj------
`
`0.4
`(V)
`Surface potential
`Fig. 9. Distribution of surface-state densities of an MOS diode having a
`poly-Si gate, a 75-A-thick nitride Tim, and a p-type (100) substrate.
`
`0.0
`
`
`m
`e
`L
`V
`
`n l + a .
`-
`~- .-
`-
`Fig. 11. Cross-sectional diagram of an n-channel
`FET with a thermal
`nitride gate insulator and a poly-Si gate.
`
`~
`
`5 ”1
`
`Leff=5.5 pm
`weff
`
`~
`
`SiOp ,86 &
`
`1
`
`’
`
`T h e r m 1 1 nitride.
`
`1
`
`
`
`I
`
`(b)
`(a)
`Fig. 10. Distributions of flat-band voltages of thermal nitride film MOS
`diodes having (a) poly-Si and (b) A1 gates.
`
`Fig. 9 shows a distribution of Nss with respect to surface po-
`tential of a diode having an arsenic-doped poly-Si gate, a 75-A-
`thick thermal nitride fdm, and a p-type (100) substrate. Nss is
`on the order of 10’O cm-? eV-’
`at the mid-gap of the silicon
`energy band, even when the
`insulator film is much thinner
`than 100 A. This experim.ental result coincides with the previ-
`ously mentioned observation, that
`is, as nitridation of an
`Si02 film proceeds, the silicon interfacial characteristics are
`improved.
`Fig. 10 shows the distribution of VFB over a wafer
`for
`Al gate diodes having 85-&thick thermal
`poly-Si gate and
`nitride films. The VFB is -0.98 and -0.89 V for the poly-Si
`and Al gate diodes, respectively. The deviation from the aver-
`age value is
`t0.016 and t0.020 V, respectively. These ex-
`cellent uninformities are due to the homogeneous structure of
`the nitride films and the very thin film thickness which make
`insulator charges less sensitive to V ~ B .
`
`VI. FET CHARACTERISTICS
`Thermal nitride films were used to fabricate insulated-gate
`field-effect transistors (IGFET’s). A cross-sectional diagram
`is shown in Fig. 11. A conventional
`of an n-channel FET
`n-channel silicon-gate MOS process was used by substituting a
`thermal nitride film for an SiOz
`film. The source and drain
`regions and the poly-Si gate were heavily doped with arsenic-
`ion implantation. The junction depth was 0.3 pm.
`voltage histogram of the
`Fig. 12 shows a gate breakdown
`fabricated FET’s measured after assembling them in dual-in-
`line packages (DIP’s). The FET’s have an effective channel
`length and width of
`5.5 and 18 pm, respectively. Each lead
`was directly bonded on pads of source, drain, and gate elec-
`trodes using a conventional ultrasonic technique. FET’s having
`86-A-thick Si02 f h s , fabricated for comparison, were dam-
`aged and showed a wide distribution of breakdown
`voltages.
`This degradation was caused by unintentional electrostatic
`
`-5
`0
`-10
`(V)
`Breakdown voltage
`Fig. 12. Histogram of gate breakdown voltages of ultrathin-insulator
`FET’s assembled in DIP’s by using ultrasonic lead bonding tech-
`nique. Thickness of SiOz and thermal nitride gates are 86 and 73 A,
`respectively.
`
`.- >
`U
`\ 5
`
`E cu t
`
`Fig. 13. (a) Characteristics of a half-micrometer channel length thermal
`its SEM photograph, where a 53-A-thick
`nitride gate FET
`and (b)
`thermal nitride film is used. Effective channel length and width are
`0.5 and 18 pm, respectively.
`
`discharge through the lead bonding process. Thermal nitride
`gate FET’s having 73-A-thick films were, however, hardly in-
`fluenced by the process, exhibiting almost the
`intrinsic value,
`even when the thickness of the nitride film was less than that
`of the SOz-gate FET’s. This result relates the strength of
`nitroxide films against charge
`accumulation destruction
`de-
`scribed in Section 111.
`Fig. 13 shows the characteristics of a half-micrometer
`channel length nitride gate FET and its SEM photograph. The
`FET has a 53-A-thick thermal nitride film whose range appears
`to be the theoretical limit for an MOS-type FET because the
`occurrence of direct tunneling current
`in-
`through the gate
`sulator begins. The
`drain characteristics exhibit very high
`transconductance (280 mmho/mm) and good current satura-
`tion. Such a thin nitride gate FET appears
`to be a good pros-
`pect for future VLSI devices.
`
`VII. SUMMARY
`Yield and reliability of thin Si02 films suffer from various
`deteriorating effects in VLSI processes, including refractory
`metal or silicide gates and plasma- or
`ion-assisted processes.
`
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`

`502
`
`
`
`IEEE TRANSACTIONS ON ELECTRON DEVICES,
`
`
`
`
`
`VOL. ED-29,
`
`NO. 4, APRIL 1982
`
`Influences of penetration of gate material or impurities and
`charge accumulation damages were studied in this experiment.
`Thin-oxide MOS characteristics, however, are greatly improved
`by a nitroxide film formed by thermal nitridation of an Si02
`film. This is because of a surface protective layer against con-
`tamination or impurity penetration and
`because of
`improve-
`ment of the
`Si-SiOz interfacial structure.
`This verifies the
`benefits of
`a thermal nitride
`film grown
`in pure ammonia
`plasma environment. This film appears to be the most promis-
`required for VLSI
`ing candidate for
`a thin-gate insulator
`circuits.
`
`ACKNOWLEDGMENT
`The authors would like to thank T. Misugi, Y. Fukukawa,
`M. Shinoda, K. Shirai, and M. Nakano for their encourage-
`ment and discussions, and T. Nozaki, H. Arakawa, and I. Kato
`for their helpful contributions to the work.
`
`REFERENCES
`
`I
`
`~
`
`__
`[ l ] H. H. Chao, R. H. Dennard, M. Y. Tsai, M. R. Wordeman, and
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`] W. Johnson, G. Perlegos, A. Renninger, G . Kuhn, and T. R.
`Ranganath, “A 16 Kb electrically erasable nonvolatile memory,”
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`[7] Y. Kamigaki and Y. Itoh, “Thermal oxidation of silicon in various
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`[8] T. Ito, T. Nozaki and H. Ishikawa, “Direct thermal nitridation of
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`[9] T. Ito, H. Arakawa, T. Nozaki, and H. Ishikawa, “Retardation of
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`J. Electrochem. Soc., vol. 127, p. 2248, 1980.
`[ l o ] M. L. Naiman, F. L. Terry, J. A. Burns, J. I. Raffel, and R..
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`vicesMeet., Dig. Tech. Papers, p. 562, 1980.
`[ 111 E. Kooi, J. E. vanlierop, and J. A. Appels, “Formation of silicon
`nitride at an Si-Si02 interface during local oxidation
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`Electrochem. SOC., vol. 123, p. 11 17, 1976.
`[ 121 S. P. Murarka, C. C. Chang, and A. C. Aolams, ‘Thermal nitrida-
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`T. Nakamura, and H. Ishikawa,
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`Micron Ex. 1027, p. 5
`Micron v. Godo Kaisha IP Bridge 1
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