I I
`
`r
`
`CONFERENCE RECORD OF THE
`1994
`
`INTERNATIONAL DISPLAY
`RESEARCH CONFERENCE
`
`OCTOBER 10-13, 1994
`MONTEREY, CALIFORNIA
`
`Featuring
`International Workshops
`on
`Active-Matrix LCDs
`and
`Display Materials
`
`Sponsored by
`The Society for Information Display
`and
`The Advisory Group on Electron Devices
`
`In cooperation with
`The IEEE Electron Devices Society
`
`ChinaStar Ex.1006
`
`

`
`%
`
`I I I I M/
`
`LOW RESISTANCE GATE LINE EOR HIGH-RESOLUTION TFT/LCD DISPLAY
`
`Takatoshi Tsujimura. Hiroaki Kitahara and Aisuya Makila
`LCD Tech.Dev..Display Technology. Yamaio Lab., IBM Japan Ud.
`1623-14, Shimo-Tsuruma, Yamato-shi, Kanagawa-ken, Japan
`
`Pctcr Fryer and John Batcy
`Thomas J. Watson Research Center. IBM Corporation.
`Yorktown Heights, New York
`
`Abstract
`
`12"-XGA and 16"-SXGA LCD panels arc
`manufactured with low resistance Mo/Al gale line.
`Stable etching for thicker aluminum is achieved. Mo
`passivity current density is 0.06A/cmA2 and
`electromotive force of Mo-Al electric cell makes
`Mo passivity. The stability of Mo passivity depends
`on the amount of nitrous acid on the cathode
`electrode. As "Dip" etching makes nitrous acid drift
`away,Mo passivity easily degrade and taper shape of
`Al is created because of higher etching rate of top
`Mo.
`
`I.Introduction
`
`Recently, the demand of high resolution large-size
`TFT/LCD display is rapidly increasing. In order to
`make high resolution large-size display, aluminum
`is widely used to reduce the time constant of gate
`line. But aluminum has many weakpoints, such as,
`(1) Al is easily damaged by etchant. (2) Al causes
`many problems (hillock, blister) by heating. (3) Al
`has poor adhesion to glass or under coal. If the
`coverage of gate insulator is not enough, the etchant
`intrude into aluminum layer and aluminum gets
`damages. To avoid the lack of coverage of gate
`insulator. Kawamura et al' is reporting aluminum
`taper etching using photo resisl/AI structure, but the
`adhesion between photo resist and Al easily changes
`due to various condition such as humidity or time
`after baking. Photo resist/Mo/AI structure is better
`from this viewpoint, but this structure sometimes
`causes overhang of Mo and causes lack of coverage
`of gate insulator. We successfully suppress the
`overhang of Mo to make taper shape of Al by
`reducing the Mo passivity with "Dip" etching
`method and resolved the mechanism of Mo
`overhang.
`
`2.Structurc
`
`Fig. I shows the cross section of our gate line
`structure. As Al makes hillocks or blisters at high
`tcnipcrature, wc must make multi-layer structure
`
`using high-melling-point metal Mo and
`MT(Molybdcniim-Tanlalum alloy) on Al. Multi­
`layer structure also prevents from open line, if one
`metal gets to be open. Mo is also used to make taper
`of Al using the difference of etching rate between
`Mo and Al. As taper shape makes the coverage of
`gate insulator better, we can prevent from the
`intrusion of ITO etchant. Taper shape of Al also
`prevent from ESD (Electrostatic Discharge) from
`the top edge of Al gate line.
`
`SiOx
`
`MT(Molybdenut!SI^taUirn)
`
`g 7^59 Glass
`v - - ' COTnin
`\ Aluminum \Molvbdcnum
`
`SiOx
`
`-
`
`. -
`• v
`
`• v
`
`tA. J
`MOM
`
`MT
`
`Al
`
`Glass
`
`Fig.l Cross section of new Al clad gale structure
`
`3.Experiment
`3.1 Taper Etching Control
`
`in order to make taper shape of Al. we applied
`Mo/Al structure (500A/2000A). After making
`0.5%HF (Hydrogen Fluoride) cleaning, washed the
`Coming 7059 glass substrate in distilled water, and
`then dried in 1PA (Isopropyl Alcohol) vapor. After
`
`424
`
`

`
`,t.aning process. Al and Mo arc deposited in
`Tokuda SYSTEM 512 Sputtering System in the
`Kpobcre of Ar at I50C temperature. The
`•Ogling Condition of Al and Mo are
`**
`Arl50sccm(~0.5pa)
`Al:7kW.
`Arl50sccm{~O.5Pa)
`Mo:1.5kW.
`.\ficr spuiiering process, photo resist was coated
`patterned by aligner and developer. After
`-resist was developed, photo resist was baked
`Ff
`m 130C for 150 sec.
`,\fier making photo resist pattern of gate line, we
`|nei| wet etching with two different methods, that is
`dip etching and conventional spray etching.
`Pig.2 shows the etching time dependence of side
`etching length (measured by SEM) of Mo and Al.
`Wccan see that we get taper shape (Fig.3 (i)) if the
`side etching length of Mo is larger than that of Al in
`Fig 2. If side etching length of Mo is smaller than
`dial of Al. Al has overhang shape (Fig.3 (ii)) and we
`can not apply it as gate line etching.
`
`,^jK
`
`V
`
`JjSlMo
`
`-
`
`-y vV
`
`•-*"
`
`• Glass
`
`(0 TAPER shape made by DIP etching
`
`•
`ttv
`
`N
`
`w
`
`I
`S/0
`E
`I.
`.5 4
`1
`i:
`
`o
`
`•
`
`»
`
`100
`
`Taper
`
`ipiKMb)
`
`.Y(AI)
`.Y(\t))
`.•.ayAY(A»
`i—SHIAY(M))
`Acting time | sec | Overhang
`
`150
`
`XO
`
`Fig.2 Etching time dependence of side etching
`length (Measured by SEM)
`
`We can see that the etching rate of dip etching is
`larger than that of spray etching. And in case of
`spray etching, the etching rate of Mo is smaller than
`lhai of Al. and in case of dip etching, the etching
`rale of Mo is larger than that of Al. In the case of
`etching technique using both dip and spray, the
`aching rate is middle of dip etching and spray
`c,ching. and the etching rate of Mo is almost the
`wmcasAL
`% SEM measurement, we can see that passivity of
`Mo is created by spray etching. By this
`"toMiremem we can see thai dip etching is suitable
`10 make taper shape of Al constantly.
`®> defect analysis, the sample with non-lapcr-shape
`M many stripping points of SiOx or ESD
`but the sample with taper-shape Al had no
`MJch defects occurred.
`
`425
`
`%
`
`Mo
`
`• as AH
`
`Glass1
`
`(ii) OVERHANG shape made by SPRAY etching
`Fig.3 Etching method dependence of aluminum
`shape
`
`12jraper Etching Mechanism
`
`To know how "Dip" etching suppresses the
`passivity of Mo, we measured the electric potential
`dependence of Mo.
`As Mo and Al is adhered in the elchant.
`electromotive force exist between Mo and Al. To
`see if the electromotive force makes passivity of
`Mo, we measured polarization diagram of Mo.
`Polarization diagram of Mo shows that the passivity
`current density of Mo is 0.06 A/cmA2. (Fig.4) In the
`etchant with 3% nitric acid. Mo dissolves faster than
`Al, and Mo becomes anode. As the electomotive
`force between Mo and Al makes current more than
`0.06 A/cmA2. the passivity of Mo is made in the
`etchant.
`
`J
`
`

`
`0 02 |
`o
`-0.02
`-004
`406
`2, -008
`-01
`
`•0.12
`414
`416
`•018
`
`5
`V(Mo)[Vl
`Fig.4 Electric potential dependence of Mo
`dissolution current
`
`10
`
`13
`
`Then wc tried nitrous acid dependence of Mo
`passivity using Mo-Pt cell in the etchant. First wc
`applied 15V lo Mo and stopped applying voltage to
`see the degradation of Mo.(Fig.5) The electric
`potential of Mo passivity goes down just after
`stopping voltage, and the electric potential reaches
`to the passivity potential. The passivity potential
`goes on for 60 sec and then passivity degraded and
`the electric potential goes to the potential of Mo.
`
`i
`
`E 0.8 I 0.6
`I 0.4
`i 0.2 -—
`10
` —
`1-0.2
`
`—NO UREA
`UREA 300mg -
`_ — UREAfiOOmg
`— UREA 900mg
`'—UREA 1500mg
`_ —UREA JOOOrag^
`— UREA (OOOmg
`— UREA JOOQmg'
`
`-0.4
`
`0
`
`50
`150
`100
`Time afiet jpplying 15V Istcl
`
`200
`
`Fig.5 Urea dependence of Mo passivity
`
`Next we tried adding urea to the etchant to reduce
`the amount of nitrous acid. The nitrous acid is
`reduced with this formula.
`(NH2)2CO + 2HN02 -» 2N2 + €0,+ 3H20
`Wc measured passivity degradation using this
`etchant with urea. As the amount of urea increase in
`the etchant. the passivity of Mo degrades much
`faster, and when the amount of urea come to 0.05
`mol/l(1500mg in Fig.5) the passivity potential
`region disappeared. By this measurement, we can
`conclude that the Mo passivity easily degrade if the
`etchant is lack of nitrous acid because of Dip
`etching and the taper shape is made with Dip
`etching.
`
`_
`
`4.Gate Line Performance
`
`Fig-6 shows the gate line resistance.The value of
`12" XGA and 16" SXGA is the value in case of
`MT/Mo/Al structure. The value of 10"VGA is in
`case of conventional MT gate line. We can see that
`gate line of 12" XGA and 16" SXGA has very low
`resistance even if compared with VGA.
`
`9000
`v
`8000
`r*
`7000
`6000 -
`.2
`| £5000
`g |04000
`-
`J 3000 —
`- 2000
`tfl
`O
`1000
`o
`
`10nVGA
`
`12"XGA
`
`16" SXGA
`
`Fig.6 Gate Line Resistance
`
`Fig.7 shows the time constant of 12" XGA and 16"
`SXGA.The time constant is less than 1.5 sec in case
`of 12"XGA. and the time constant of 16"SXGA is
`almost the same as that of IG' VGA.
`
`-
`
`2
`1 1
`
`I
`
`U
`I
`=
`«J0 5 t -
`
`H o
`10" VGA
`
`12" XGA
`
`16" SXGA
`
`Fig.7 Time Constant of Gate Line
`
`S.Vield of gate line test pattern
`
`We made different kind of gate line structure by
`10" VGA lest patlem to see the yield of our gate
`line structure.
`Fig.8 shows the gate line structure dependence of
`Signal Open defects.
`The number of Signal open defects was 1/10 of the
`sample without taper etching. The yield of the
`sample of taper etchin
`ig was better even if compared
`with the thinner 1000,
`A A1 panel without taper
`etching.
`
`426
`
`

`
`AMhickness (A)
`
`Mo MT
`Sig.Open/
`lAl
`iAi
`Sig.line
`500 2000
`l000(w/o taper)
`9.8E-4
`500 2000
`2000(taper etch)
`1.3E-4
`500 3000
`2000(taper etch.)
`1.9E-4
`1.5E-3 ]
`5001 20001
`2000(w/o taper.)
`Fis.8 Gate line structure dependence of Signal
`Open defects
`
`-
`«*,
`" 1 H
`
`j r • t
`* * ^
`. - .
`
`6.Conclusions
`
`^ f
`
`"Vx" JP-M f —
`
`As polarization diagram of Mo shows that the
`passivity current density of Mo is 0.06A/cmA2, Mo
`makes passivity because of the electromotive force
`of Mo-Al electric cell.
`Dip etching makes nitrous acid on cathode
`electrode drift away.Because of the shortage of
`polarizer nitrous acid, the passivity of Mo easily
`degrade. As dip etching of Mo/Al urges the
`degradation of Mo passivity, the etching rate of Mo
`is faster than A1 and taper shape of A1 is stably
`made.
`The yield of A1 gate line depends mainly on the
`coverage of SiOx gate insulator. Dip etching of
`Mo/Al can make good taper shape and can make
`good coverage of gate insulator.
`12" XGA (Fig.9) and 16" SXGA LCD display is
`manufactured with taper etching using dip etching
`technique, and low resistance gate line is obtained,
`and the lime constant of gate line is low enough to
`drive TFT effectively.
`
`1
`
`J /
`
`Fig.9 12.3" XGA TFT/LCD Product (IBM Green
`PC)
`T.Acknowledgements
`
`The author wish to thank to Mr.S.Tsuji for TEM
`photograph.
`
`S.Rcferences
`
`[1] Kawamura et al., "An a-Si TFT Array for 15-
`in. Full-Color High Resolution LCD" Japan
`Dispiay92 (1992) 344
`
`'our suatcgi
`nwas>Ne\