标签: plasma

Key words: TFT, thin film transistor, key drive device of LCD (driving orientation of liquid crystal molecules) and AMOLED flat display (driving OLED) a-Si, amorphous silicon, very early technology of TFT in FPD LTPS, low temperature poly silicon, much higher electron mobility, particularly used in mobile display applications FPD, flat panel display CD, critical dimension CF, color filter, the 2nd glass in LCD ESA, electrostatic attraction ESD, electrostatic discharge LCD, liquid crystal display AMOLED, active matrix organic light emitting diode 1.Brief view on electrostatic problems in FPD manufacturing: Electrostatics-induced-electrical failures, particle contamination and process control errors Electrostatics in FPD manufacturing is very common seen and could induce remarkable impacts to the whole FPD industry. It mainly includes three portions listed as below. Firstly, electrostatics-induced-electrical failures of FPD products is the top concern, typically TFT array circuits formed on glass substrates. These electrical defects both includes functional failures caused by catastrophic damages which directly impact production yields, and latent failures (or called “wounded walking”, typically are those gate oxide dielectrics, interlayer dielectrics or passivation) which may not impact production yields throughout FPD manufacturing, however, the reliability of these FPD products would be degraded and may lead to lower robustness to ESD in the following manufacturing processes or earlier failure in reliability testing or even at end-users’ applications. Fig1. Typical structure of bottom gate a-Si TFT (left) and top gate LTPS TFT (right) Fig2. Typical electrical failure caused by electrostatics induced ESD: higher gate leakage at electrical testing The other part could be impacted by electrostatics is the photomask used as a key process tool in photo-lithography process-exposure equipment. Electrostatics could also lead to physical damages (typically Critical Dimension failures) to the large amount of isolated chrome patterns deposited on glass or quartz substrates. Photomasks with CD failures if input into exposure equipment for production would produce lithography errors and lead to failed TFT glasses at very high possibility. This would be a disaster to any FPD manufacturer. Secondly, electrostatics induced particle contaminations to FPD device (including TFT deposition layer contaminated, glass substrates contaminations) and processing materials (such as photo-resist contaminated, susceptor surface contaminated of CVD, PVD, Dry-etching equipment). The effect of electrostatic attraction (abbr. ESA) dominates this type of particle contamination where cleanroom environment especially for the mini-environment inside manufacturing equipment with over-more airborne particles. Fig3. Local electrostatics on glass substrate attract airborne particles to case contamination Fig4. Actual case of TFT array glass contaminated by particles Lastly, electrostatics could also impact on process control qualities. For example, poor uniformity of liquid crystal molecules filling in ODF (One Drop Filling) process. Another example is misalignment between CF glass and TFT glass in vacuum assembly process. 2.Electrostatic Devices and the Sensitivity to Electrostatics: TFT array pattern and photomask Regarding electrostatics induced electrical defects, TFT array patterns of FPD products formed on glass substrates and chrome patterns formed on glass or quartz substrates. TFT array patterns consists of large amount of multi-layer devices over one glass substrate. It’s sensitivity to electrostatics in manufacturing is quite similar with wafer fabrication processes of semiconductors. That is insulating dielectrics between metal lines or between metal and poly silicon are susceptible to be damaged by electrostatic impacts, and dielectric leakage and lower electrical reliability are the main failures. However, FPD products would be exposed to many more and high electrostatic impacts because of the insulating glass substrates. On the other hand, FPD TFT array patterns consists of many much longer metal lines which would attract the electric fields and amplify field strength. Therefore, electrostatics induced electrical defects of FPD manufacturing is much worse than wafer fabrication of semiconductor. Fig5. Typical electrostatics sensitive device model in TFT array patterns of a FPD product Fig6. Electrostatics-induced-electrical failures by ESD on a mother glass substrate at TFT array fabrication The sensitivity of FPD TFT array patterns to electrostatics could be described as below: Electrostatic sensitive device pattern of metal/polysilicon-dielectrics-metal structure, while exposed to electrostatic sources, there would produce an electric field applied across the dielectrics. The higher strength of the electric field applied across the dielectrics, the higher probability of the dielectrics to break down (higher mobility of electrons). An abrupt discharge current would flow through the dielectrics if the field strength exceeded the breakdown threshold of the dielectrics (eg. Common gate insulation layer of TFT is SiO2 or SiNx, the electric breakdown threshold is about 5-10 MV/cm), and finally a leakage path would form across the dielectrics which is the electrical leakage failure. The sensitivity of photomask to electrostatics is similar with FPD array patterns, since both have similar electrostatic sensitive device pattern, similar sensitivity mode to electrostatics and similar electrostatic sources in manufacturing. The major difference of photomask is chrome-air-chrome structure only which could be damaged by electrostatics. Fig7. Typical chrome pattern CD failure of FPD TFT photomask caused by electrostatics induced ESD 3.Process flow of FPD Manufacturing FPD manufacturing consists of three sections, the front-end TFT array fabrication (and CF fabrication for LCD), the mid-end cell assembly for LCD & OLED evaporation, encapsulation for AMOLED and back-end module assembly. TFT array fabrication almost share the same manufacturing processes with wafer fabrication of semiconductor. TFT array fabrication also consist of thin film process by CVD & PVD, photo process, wet etch & dry etch, and stripper and similar equipment but lower-class. However, TFT fabrication has more transferring processes by robot arms and long conveyors of rolling rollers which could produce many more electrostatics to impact FPD products. Fig8. Typical manufacturing processes of TFT array fabrication LCD cell consists of PI coating, PI baking, PI rubbing of both TFT glass and CF glass, seal dispense of TFT glass, spacer dispense of CF glass, ODF of TFT glass and vacuum assembly of LCD cell. All these processes would involve transferring glasses or pick and place of glasses which all are prone to accumulate electrostatics. Fig9. Principle manufacturing processes of LCD cell assembly OLED evaporation consists of many vacuum chambers to process and transfer glasses where highly electrostatics could be a common problem. Module assembly consists of several thin film attachment including Polarizers, protective films which could accumulate large amount of electrostatics in peeling and attachment operations. 4.Electrostatics in FPD Manufacturing Electrostatics in front-end and mid-end FPD manufacturing mainly comes in several conventional ways: Contact and separation tribo-electrification (or called tribo-electrification) between equipment part and FPD glasses, such as pad-glass in robot arm to transfer glasses from one location to another, roller-glass in rolling transfer glasses, stage-glass in glass processing on equipment stage/chuck/susceptor and rubbing cloth-PI on glass in PI rubbing process. Fig11. Contact and separation electrification (tribo-charging) of glass on an equipment stage Fluid charging (or called electrification) is another important way to accumulate electrostatics on TFT array glasses, including highly purified water jetting rinsing, spin drying after water rinsing, photo-resist slit coating, PI coating, liquid crystal molecules dropping in ODF, etc. Plasma (vacuum and atmosphere), Ultraviolet (Extreme-UV and Excimer-UV) exposure or irradiation could also produce and accumulate electrostatics, such as plasma of CVD & Dry-etcher, dry cleaning by atmosphere pressure plasma, dry cleaning by excimer UV, etc. It should be noted that the above ways to accumulate electrostatics could show variations by process setup or equipment individual condition. For example, the tribo-charging by contact-separation way on a vacuum stage of exposure equipment varies by different equipment settings. Vacuum pressure, vacuum holding time duration, glass lift-up speed, etc. each could produce tribo-charging amplitudes of glass substrates. Moreover, the contact-separation tribo-charging could get worse as the exposure equipment continue to process more and more glasses. Another example is the conveyor line of rolling rollers. The tribo-charging between glass bottom side and rollers would produce more electrostatics accumulated on glass bottom side as transferring over longer distance. Common rollers made with materials of Teflon, PVDF, UPE and stainless steel could produce varied tribo-charging amplitudes and accumulation speeds over rolling transferring. To resolve electrostatic problems in FPD manufacturing, many manufacturers attempted to enhance roller’s conductivity by adding different conductive additives. These modified rollers would also show different tribo-charging of glass substrates.

There was once a radio comedy program called The Goon Show when I was a kid in England. It was originally broadcast by the BBC Home Service from 1951 to 1960, which means it actually finished when I was three years old, so they must have broadcast repeats after that because I remember listening to it with my dad. It's hard to describe the Goon Show – the script was incredibly zany, the humour was surreal, and the programs were jam-packed with bizarre sound effects – you had to be there. Prince Charles loved it, and it was cited as a major influence by folks as diverse as the Beatles and the members of Monty Python . The key members of the cast were Spike Milligan , Harry Secombe , Peter Sellers , and Michael Bentine . As an aside, Spike Milligan and Harry Secombe first met and became friends while serving in the Royal Artillery during World War II. Both must have been young men in their early 20s at the time. Milligan's artillery unit accidentally allowed a large howitzer to roll off a cliff. Secombe, who sitting in a small wireless truck at the bottom of the cliff, describes what happened as follows: "Suddenly there was a terrible noise as some monstrous object fell from the sky quite close to us. There was considerable confusion, and in the middle of it all the flap of the truck was pushed open and a young, helmeted idiot asked 'Anybody see a gun?' " The "idiot" asking the question was Spike Milligan. Secombe immediately responded "What colour was it?" (This must have been one of the greatest comebacks of all time – it makes me grin just thinking of the two of them meeting in this way.)   From left to right, Sellers, Milligan, and Secombe   Two of the characters in the Goon Show were called Bluebottle and Eccles . A snippet from one of the programs in which Eccles is trapped in a cellar (as I recall) sticks in my mind to this day. It went something like this: BLUEBOTTLE: Why don't you open the door? ECCLES: Okay, I'll open... how do you open a door? BLUEBOTTLE: You turn the knob on your side. ECCLES: I haven't got a knob on my side. BLUEBOTTLE: On the door you idiot! Well, it made me laugh... but that's not what I wanted to talk about... I was thinking of the way televisions were and how you had to twiddle and fiddle with their knobs to make them work... and it was this that caused the Goon Show to pop into my mind. These days we're used to seeing incredibly thin, flat television screens in the form of LCDs (Liquid Crystal Displays), plasma displays, OLED (Organic Light-Emitting Diode) displays, and so forth. Also, the electronics systems inside today's television sets are based on digital logic implemented using solid-state silicon chips. The end result is that modern televisions are extremely reliable, the full-screen picture appears almost instantly when you power-up the TV, and the picture remains rock-steady... ...things weren't always this way... When I was a young lad, the display portion of the television was a huge phosphor-lined, heavy-duty glass vacuum tube called a Cathode Ray Tube (CRT). Meanwhile, the electronics systems inside the set were based on vacuum tubes (called valves in England), which – if you took the back off the set and peered inside – sort of looked like dimly-glowing light bulbs. As a result, there were certain characteristic features associated with the early television sets that would be unacceptable to a modern viewing audience, but were simply regarded as a "fact of life" in the early days... When you first turned the set on, for example, the picture didn't immediately appear. Instead, a small version of the image appeared in the centre of the screen, and over the course of a few seconds it gradually grew to fill the screen. (In my "mind's eye" I'm visualizing a black-and-white television picture because that's what we had, but the same thing happened with colour sets). Similarly, when you turned the television off, the picture didn't simply disappear. Instead, it gradually shrank towards the centre of the screen, growing smaller and smaller until – eventuality – you were left with only a bright white spot in the middle of the screen (this dot subsequently faded away into oblivion). But wait, there's more, because things began to deteriorate over time. After a few years in operation, your television set was no longer as "robust" as once it was. For reasons too complicated to go into here, the vacuum-tube based electronic systems began to "drift" such that the image might not exactly "fit" the screen. Thus, on the back of the set (almost invariably to be found in the most inaccessible location) there would be a collection of knobs that allowed you to adjust the width and height of the picture. Also, there would be a knob to move the entire picture up/down and another to move it left-right. (Since folks at that time were generally unused to any form of electronics (outside of things like radio sets and record players), the vast majority of users regarded these controls as being the equivalent to the cockpit of a commercial airliner.) In some cases – possibly depending on surrounding temperature and humidity, or possibly depending on your television's sheer contrariness – the picture might start to rotate in a vertical direction. This would begin with the picture gradually moving up (or down) the screen such that the upper portion of the image disappeared off the top of the display and reappeared at the bottom. (Imagine watching an episode of I Love Lucie where the cast's legs are walking around the upper half of the screen while their heads, shoulders, and torsos appear on the bottom.) If left unattended, the image would start to rotate from top to bottom, or vice versa, which was a bit like watching one of the dial's on an old-fashioned slot machine just after you had pulled its handle. Thus, there was also a knob to control the "vertical hold" on the back of the television set. The sad thing is that once you started playing with these knobs, you knew in your heart-of-hearts that "the end was nigh" for your television. Tweaking one knob seemed to subtly affect the other settings. Thus, although things might appear to be "tickety-boo" by the end of your current session, the next time you turned the set on the picture could well be "all over the place", thereby requiring you to perform a new, more radical set of "tweaks". And remember that these controls were on the back of the set, which meant you had to drag the whole thing away from the wall and squeeze behind it to be able to access the small, ill-marked, hard-to-reach knobs. Now imagine the frustration of "tweaking furiously" to achieve the "perfect picture", returning the television to its usual location, sitting back down in your seat ... only to see the picture start to roll around again! Before long, you were spending more time adjusting the picture than you were actually watching the television programs. Ah... the good old days... (grin)