L Solving the TFT LCD

Yield Pr~blem

Thin film transistar active matrix liquid crystaz display (TFT AM LCD81 are the "state of the art" in high performance color LCDs. Active matrix (AM) refers to an LCD pixel design that utilizes an active element. The active element in most designs is a transistor, deposited on the glass substrate using a thin film prmess. Thk pmr~ss ia very similar to semiconductor fabrication, and various materi- als are used to form the TlTs. By incorporating a switck (the TFI") at each pjxel, TFT LCDs offer better contrast ratios and brightness than passive matrix LCDs.

Tl;"r LCDs are currently a critical component of most color postable computers, hand held televisions a d other consumer products. By the year 2000, the market for TFT LCDs is expected to grow to greater than $4 billion.

However, the major Japanese electronics companies who have invested billions of yen in TFT LCD development have found that TFT LCD3 are very dim- cult ta mmufacture. Kelds of TFT LCDs have been reported

-.- tobeas1owaslW0. ksaregultof low yi& kd&s TFT LiCDs are

verg egpemive, For -pie, an average 10" mlor TFT LCD used h a taptop aornput~~ is p r i d at abut $1,500. hdustrg o h m believe the market demand For ~ ~ w i l l ~ ~ n l y 8 e a result of dePtzaaed prim. The s m ~ 10" TFT LCD mu& dl for lesa than $500 Idom it will appeal to a masa market.

The only way a n deums? their prim- k to h t i d y inmas3 thek yields and &ce their productim a&. A major obstacle in the effort to inmaw yields has b the k c e of i n - m auto- matic test equipment (ATE) cPe- signed sp&cally for TF7C M'Da

ATE designed for the fmal test of LCDs has been available Tor many years. Hawwer, repairing a completed LCD on which faults have been detected is not cost effective bemuse the cost of d i s ~ r n b l y , reg&, and teas- sembly exceeds the value of the repaired LCD. A more logical test atratem is to also test the LCD at the substrate stage. At this stage the TFTs and electrodes have been fabricated on the glass but no other material& have been added (see figure 1). TE a TFT LCD can be teatedat the substrate stage, the

manufacturer has the choice of repairing it at a reasonable coat or scrapping it hhre moxe value is added. Scrapping LCDs with faults at this stage is a major cost saw since it is estimated that the manufactur- er has added Iem than W a of the TET LCD's due. -fore, an i e - v ATE Wdes immediate yield i d mh;t

impmrnents to ?TI' LCD manuf-.

To ddms these problems, GenItad and Tokya Elecb.on Limitd CTEL) jointly developed the GTS-I. LCD Test and Measurement System. TEL, the world's leading manufac- t uter of semimnductar probers, developed the GZS- 1 Automatic P r o k and Substrate Handhg Subsystem. GenRad, the world's leading manufacturer of board t& systems, invented a patented transfer admittance test technique and devebpd the GTS-1 Tf& and M m m e f l t subeystem.

The key feature of the GTS- I System is the abilib to precisely measure TFT characteristics and therefore provide extensive pixel data bg.oad simple wfd This m m m t data is critical ta ~ n a n u f m m for t h e m m FM, the GTS-1

system utilizes this ptre1 rneamwnent data and soRware algorithms to provide detailed pixel diagnostiE idomt ion to the manufahrI

Since many faults can be repaired and the repair process is not rep- pixels but a d d y repairing pixel circuitryv the more b g m & information prwidd to the &achmm the mom &cient their repair p- Will be. h n d l x tSlis mtwmment data p d d m p- oonbw1 information ta the manufflctums which will m- abIe them to eEfect longer tam yield impmemmta. Lastly, mea- SlJJWnent d& provides d* charactmi& inforrnatkn tn the m u f m which will enable h e m to impme future TFT m designs

The GTS-1 System measures each pixel on the substrate by applying ac voltage to the pte or control line (mw) and measuring the resulting curent in the drain or data line (column). The ratia of wtput current to input voltage is admi- Y, the reciprocal of impedanoe, 2. It is a t r a d e r admittance maasure- ment becam the voltage 8wl current are measured with seferenm h a third point, the m m o n terminal or guard (see figure 21.

This is the W q u e u d in in-circuit m m e n t s b- cause impdmm from input or output to the common or guard Izh a d 2%) have, t h d d x no effect. The m e element to bemeasuredcanhisolatedky mm* dl shunt m n t - t o t h e m p o i n t .

'T LCD (.

9 GI-

- CObtRlls

The plarized material is applied tothe back and front ofthedisplay. fhehontpolarizerisshifted90dagrees from the back potarizer. Jhrs allows light to pass thoughthefisplay because theliqu~dcfystalsinthar n m a l state tmst the light 90 degrees. When voltage is applied between the pixels and the common electrode, the liqu~d crystals align with the electric field. Thts b l m s thelight mming thmvgh the dsplay becausethe liquid wystsls no longertwiwistthslight90 degrees as the light passes through the display.

Bemw an ac HI@

has both magnitude d phase, an ac admittance is mmplq having both in-phase Cd) and qtlddm Cimaginarjr) parts. For a capmitiye admit- tan~(a3iSthecaseFY = G + j o C w h e m G i s c o n d u h c e , C is a- @tan= and w = 211:

frequency. Thus, each m w m m ment gim two piem of hfonmkian, G and C. These quantities are 4 c o m p d to the admittanoes olr mod or capacitors and are megrmred in - 0 - S i m (1 13s = V(lo9 ohme)) and pim-Fgrads. N m - theless, these gmall guantitieE mn be memud very quickly using modem mgnal-pmxdg methods.

A ~ingle meamremerit of each pixel would detect m r a l faults, such as line shorts er opens, However, much more information can be obtained by making two measurements en each pixel, one when the TFT is on and one when it is off. Obviously, if there is no chmge in the admittame, the pixel is not functioning properly. By d y z i n g the size oft he change . -

the GTS-1 System can usually determhe why the pixel is not working properly.

TheTlTiatumdworoff try applying a dc bim to the gate h e as &ow11 in figure 3. This gives four pi- or data: EoR, QTT, Gon and Con which p d e &emiw Swmafion abut the pixel. The most W t i w indimtor of mne hits is AC =Con- Co£CIEth i s i aabno~

Flgure 1. AM T I T LC0 Operation Flgura 2. Transfer AdrnlZtanm Measuremnt

the pixel is abnormal and thus

'-. fad& A high W or AG i n d i m other fault types. There are very few types of faults that can not be dehctd fi~-t~u~abl~ these me rare and. if pmen, wwld probably have dekbble m* eiT&

Examples of the data for two trpe8 of f&dt8 &re shown in figure 4. It is obvious which pixel ia bad in both cases. Additional measurements are necessary to a m h b identify most fault types. These measurements can be made with one mum removed (see figure 31, at a Werent bias Iwel, or on an adjacent pixel.

The GTS-I System is a meapuremmt system as w d as a test system Individual pix& mpmitanm and conduchm mn be meamd epmtely or d d a t e d r m several memum- menta Higher rnemumnmt aaumcy iB obtained by making dower mearmrementa at opti- mum frequencies and by &ration of each ckbhr using internal impdance stan&.

The GTS-1 S-rn am a h measure the m c character- + of apixel or the rmbhte as a whob. For example, the TFT mnductanoe vs DCV turn-on char~rkticanhrneasuredto determine the threshold voltage.

Measuring the phase shift of admithce e. the panel area, calmed by line redstance and capautance, determines the RC time canstant along thme h ~ .

In mummy, the GTS-1 Sgatern enables TFT LCD manufactur- ers t o inmeme their yields and lower their pduction costs by detecting, locating and diagnos- ing fadts on TFT LCD glass substrates. Thie new dtagnmtic capability provides manufac- turers with data required to repais substrates. The GTS-1 Sygtem aha offers exkemiw measurement capabilities for process mntml and deaign chder izat ion of subtratm.

This design utilizes a sepatate storage capacitance bus, C, Other designs may also be tested.

Figure 3. Measurement of Slngle Plxel m,n

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