Glass v. Plastic Substrates

A short Primer on CRT Longevity
The picture tube in a television was originally one of the longer lived components in a TV set. As the supporting electronics moved from tubes to solid state devices, the life of the tube was extended so that, like many glass products, it functions until you get sick of it and throw it out. In the function of a CRT, an electron beam hits phosphors deposited on the inside of a glass bottle. In addition to exciting the phosphors, the electron beam can also drive off some of the more electronegative elements in the glass. First to go is, of course fluorine, a powerful phosphor poison. This is why it is important to have negligible amounts of fluorine in the glass as it is a phosphor poison. Next is oxygen.

In the tube making process, one of the last steps is “flashing the getter”, essentially coating the inside of the tube with barium metal. Barium being one of the least electronegative elements, any liberated oxygen latches on to the barium before it can do harm in other areas. “Gettering“ found a new use in telecom electronics which get buried in the ground and expected to last for at least 40 years.

For CRTs, after 15 to 20 years or so, another failure mechanism creeps in; the surface of the glass is so depleted in oxygen that a metal layer forms blocking the blue light. This was known as browning. One of the final requests of the tube industry was for the glass industry to fix the browning issue. However, rather than ensuring an ever longer lifetime for a bunch of tubes that were probably going to be disposed of around the HDTV transition, the industry was convinced that a better glass chemistry was one that aided recycling instead. Industry attention was focused on the glass as it had essentially fixed its phosphor life issues by going to higher and higher voltage phosphors. The bigger the band gap between the excited and resting state, the fewer the species that can insert themselves and deactivate the phosphor.

So, CRTs had a virtual hermetic seal being constructed inside a glass bottle that had a substantial vacuum. What atmosphere there was inside the tube was essentially reducing. There was no opportunity to oxidize, chlorinate, fluorinate, or hydrate any of the internal components. The glass also provided precise dimensional stability. As I note in an earlier post, the medical industry was building 12K CRTs on conventional TV glass. Monitor glass, which was made to a more precise spec, would have been capable of much more. And, the industry relied on a high voltage/long lived emission to generate light. All told you had a high resolution display that was capable of functioning for years, literally until the glass wore out from oxygen depletion.

LCD Longevity
In LCD technology, since their commercialization in the consumer TV market, the product has evolved so rapidly that there are not any 20 or 30 year old LCD TVs sitting in consumers living rooms. However the glass does provide the same hermaticity. Given the decay rate of the CCFLs (light output from the cold cathode fluorescent lamps declines 50% in about the first 2 years) used in the first models, likely the original LCDs will outlast their lamps by quite a bit and likely be replaced rather than repaired. The glass also provides precise dimensional control for the LCD photolithography. The original LCD glass, 7059, was a bit soft and frothy when it was made. It had to undergo a compaction step before being used as an LCD substrate as the high temperature operations would cause it to shrink. Since then, progressively harder glasses have been introduced and compaction is no longer necessary.

So as in a LCD, as in a CRT, the glass provides a flat surface for the photolithography, a stable surface for mulit-step processing, and a hermetic seal against the usual culprits in device decay the tree most electronegative elements (oxygen, fluorine, chlorine) and the most mobile electron donor (hydrogen).

Flexible Electronics with Glass
Currently there is much talk about flexible electronics. There are actually two distinct flavors of this, displays that are truly flexible and displays that are merely curved but fixed. Of the curved displays, making an LCD with a spherical profile is probably very easy. Making a cylindrical LCD could be doable as well but considerably more difficult. Making an aspheric would be much much harder and probably could never be justified. Making the substrate for any of these or for some type of new display would be the least of the problems to be solved.

As to making a flexible display, all glass forming imparts a compression layer on the surface. As a result, all glass is flexible to a degree depending mainly on its thickness. The expansion of the glass on the outer surface of a bend has to be less than the natural surface compression; once the surface of the glass comes under tension rather than compression, there is crack propagation. Consequently, not only is the flexibility limited to thin glass but it also only can happen in 2 dimensions, making cylindrical shapes. Flexing in 3 dimensions concentrates the outer surface tension into a single point/ Flat glass could be reformed (sagged) into a cylinder changing the range of radii it can accommodate but still only a small range of change in curvature can happen without breakage.

It is also important that the outer surface be pristine as well as any small, preexisting surface irregularities can rapidly grow into cracks. This is why Corning coats the edges of its flexible glass with polymer. Coating right on the glass draw, as is done with optical fiber, ensures that the surface never picks up any contact checks. Some years ago, I suggested that they do this with all of their fusion drawn glass to fix their then yield issue. The yield issue got fixed in other ways and the company lost interest. For a flexible display, it may be critical to coat the entire outer surface if glass is to be used.

Flexible Electronics with a Polymer Substrate
If plastic is to be used, then the potential breakage problem goes away but several new problems emerge. Polymers cannot stand nearly the temperature range that glass can. As a result flexible display development has focused on printing techniques rather than traditional high temperature photolithography operations. Polymers are also not as dimensionally stable as glass and printing is not as high resolution as photolithography. Although the limit will have to be explored, polymer substrate displays will not be as high resolution as displays on glass though they may be more than adequate. Finally, not only do polymer displays not offer the hermaticity of glass but polymer films frequently have mold release on them, and tramp, highly mobile, highly electronegative species within them (particularly residue from the polymerization initiators), and are permeable to small ion gasses such as hydrogen. Polymer displays cannot be expected to last nearly as long as ones built on glass but depending on the application, 3 years may be enough.

Conclusion
So, net/net, if you bend the glass in a tight enough radius it’s still going to break. Polymer displays will ultimately be less resolution and shorter lived than displays built on glass but few products need a 20 year life or 4K resolution. It remains to be seen how good printing resolutions, and ultimately printed display resolutions, can be taken.

Chemistry and the Apple TV

Such may not be the case today but formerly at MIT two terms of calculus, or the equivalent, was required for any degree, even what passes for liberal arts there. Engineers had to take a 3rd term. Chemical engineers and materials science folk took 18.03 “differential equations”, EEs took 18.031, also differential equations but with a greater emphasis on linear systems. The difference in the math embodied much of the difference between those branches of engineering, EE’s general deal with linear systems, chemistry is decidedly non-linear.

In practice, what this frequently means is that product development is very different for chemicals and materials than for electronics. In electronics, in linear systems, you can generally reduce the product risk and speed the time to market by testing the subsystems independently. In chemical systems even if different chemistries only touch each other rather than are mixed, the interface itself is a new system that must be tested and examined. Ions can migrate across boundaries or even re-arrange themselves within a monolithic material changing or defeating the product performance. In the display industry, when displays were actually made in the US, much of the industry was composed of chemical and materials folk. Today, as displays are all purchased from Asia, the domestic industry is largely EEs that design end product and develop display specifications.

Motorola spent close to a billion dollars building a fab and developing a process to make Field Emissions Displays (FEDs). In the end, they discovered tramp elements were migrating from their substrate into the FED structure. Using a different substrate, a different glass, meant starting over completely in process development. The investment was written off. A CRT glass plant once had to throw away several full days (the plant ran 24/7) of production during a product shortage due to parts per billion of fluorine in the glass. Fluorine outgasses from the glass in CRTs and is a powerful phosphor poison as is copper. Westinghouse, the inventor of the active matrix LCD, was put out of the TV business by a parts per billion copper problem in their plant water reclamation system. TV tubes would go dark about 6 months after consumers took them home.

Reportedly, Apple recently hired the OLED expert from LG prompting speculation that the much rumored Apple branded TV set will be an OLED. In the recent release of the iPhone 5 Apple had issues with the anodized aluminum coating on the case and with the sapphire lens cover. The lens cover was an optics issue but one that would have been expected if the company had truly comprehended the difference between a glass which it is familiar with and a transparent ceramic as is sapphire, fundamentally not the same thing. Two of the iPhone 5 product issues stemmed from predictable issues due to the nature of the materials they were using. Motorola's FED problem could have been anticipated as well. After Corning sold its consumer products business, the cookware industry is now rediscovering the difference between tempered flint glass and aluminum-borosilicate which was formerly synonymous with the term Pyrex.

OLED technology has had a long gestation period due to stability issues with the material. I would not expect Apple to venture so far into new materials technology. On a LinkedIn thread regarding LCD stability and suitability as outdoor digital signage, one of the posters commented that OLEDs might provide a better solution. If any stability issues remain with OLEDs, 24/7 operation and the heat and light of an outdoor installation will certainly bring those to light more so than use as a consumer TV. I would expect a proving period in TVs, as well as a cost reduction period, before OLEDs start finding their way into outdoor signage. I don’t expect that proving period to involve an Apple branded OLED TV. If Apple comes to market with a TV, I expect it to be an LCD, a very good LCD, but an LCD none-the-less. I also expect that the Apple magic will be in how its used rather than just a good looking screen. When Tim Cook said, “When I go into my living room and turn on the TV, I feel like I have gone backwards in time by 20 to 30 years", he wasn't referring to the picture quality.

The Other part of SpectraVue

As I mentioned in the original blog posting, there were actually two parts to the SpectraVue invention, The viewing film and the channel waveguide. Although the viewing film died a quick death after AlliedSignal folded the venture, the industry was mightily impressed with the channel waveguide and the design was widely adopted both in backlights for LCD as well as general lighting. The channel waveguide is something of the reverse of the viewing film, using a cons structure to columnate light rather than disperse it. The channel waveguide was rather problematic at the time as it did such a good job columnating light that you could see each individual channel element shining right through the display. The solution that was most obvious, giving the beams from each element some distance to integrate ran contrary to the trend of thinner displays. There was no appreciable LCD monitor business at the time; everything was notebooks. Finer structures and applications that could tolerate some thickness lead to a revival of the technique and AlliedSignal, after buying then changing its name to Honeywell, had a good time suing those that had adopted the idea.

Assuming the channel waveguide structure could be mad fine enough, there is an application for it on the front of some displays as well as in the backlight. If the optical pathway is run in reverse, you have a very efficient structure for capturing all off axis light and dumping it into the wavegude at sub TIR angles. This would give a display with extremely narrow viewing angle, virtually head on only, but the captured ambient light could be redirected to the LCDs own backlight or otherwise used to power the display.

A 12K Display

In other locations, I discuss some benefits of leaving out the color filter from LCD designs for signage: lower thermal radiation absorption, no cf fading, a brighter image. There is, of course a 4th benefit to removing the color filter in that you automatically get 3X more addressable pixels. That could give you a 3K display if applied to what would otherwise be an HDTV resolution screen or a 12K display if applied to the new 4K format.

Corning used to supply the glass for a 19” 12K monochrome CRT in the 1980’s. The customer used Corning’s standard 19” CRT glass to build the display although only about ½ of the glass supplied actually was within their spec. The 12K benchmark was important to the customer as it was a replacement for medical X-ray film. Because of liability concerns, the electronic replacement had to have at least the resolution of what it was replacing. I am unsure what X-ray film resolution is, but my understanding is that standard portrait film is the equivalent of 8K. With the transition to flat panel technology and the requirement of multi- billion dollar fabs, the opportunity to build such displays went away, replaced in part by the ability to pinch and zoom.

With the widespread availability of imagers well in excess of 12K, the market for 12K devices might extend well beyond the medical device market. Again, “digital signage “ is a possibility but there are a variety of other workstations where people look at images where 12K could be useful. … certainly defense as well. I would expect that whether they be field sequential color or monochrome, the product orders for 12K displays will start showing up.