Wednesday, February 13, 2013

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.

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.

1 comment:

  1. Another consideration for a flexible glass product, if the display image has any polarization dependence, as does an LCD, the stress on the glass from bending may result in a non-uniform pattern of birefringence, spoiling the displayed image.


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