Cutting-Edge LCDs: Your Metal Oxide Questions Answered [Updated]

[Updated February 13, 2013 because metal oxide backplane technology has improved since the original post was published. See question 7.]

There has been a huge amount of interest and discussion around new LCD backplane technologies, particularly about metal oxide. Following on from my first post on the subject last week, I thought it might be useful to answers

some of the questions I’ve been hearing most often.

1. Amorphous Silicon (a-Si) has been the dominant transistor backplane technology for displays the last 20 years. Why are new technologies necessary?

Changes are being driven primarily by the demand for higher resolution and faster refresh rates. The most important transistor parameter is electron mobility. Electron mobility of a-Si is very low (around 1cm2/Vs) and is at the edge of the physical ability to support high refresh rates such as 240Hz for high definition television. (Just in case you need a reminder, as this graphic shows, each transistor is basically an on/off switch that controls each red/green/blue subpixel and 240Hz refers to 240 switches per second.)

2. What other backplane technologies are alternatives to amorphous silicon?

The other transistor backplane technologies are low temperature polysilicon (LTPS) and metal oxide (MO). Both have advantages over a-Si in terms of electron mobility: typically 5 to 10cm2/Vs for metal oxide and 60 to 100 for LTPS.

cross-section of a single thin film transistor
cross-section of a single thin film transistor
3. Why is it called metal oxide?

Metal Oxide refers to the material that forms the active channel of the transistor (see illustration). A promising metal oxide transistor material is indium gallium zinc oxide (IGZO). As IGZO is a metal compound, it is deposited by physical vapor deposition (PVD), sometimes called sputtering. Compare this with the plasma enhanced chemical vapor deposition (PECVD) technique used to deposit both a-Si and LTPS layers.

4. Why not just use LTPS over metal oxide if it has better electron mobility?

There are other characteristics of LTPS that make it less advantageous compared to metal oxide. LTPS has higher manufacturing costs due to the longer manufacturing sequence: extra masking steps are required and an additional laser anneal step that crystalizes the silicon. That last anneal step is particularly challenging because the LTPS laser annealing machines aren’t available for very large size glass (above 3m2) so display makers can’t take advantage of the cost savings that they usually achieve. Making more displays at the same time by using larger glass substrates reduces the manufacturing cost per square meter of glass.

5. What’s special about Applied Materials’ PECVD technology for a metal oxide backplanes?

The IGZO transistor channel for metal oxide is susceptible to degradation if it is contaminated by hydrogen atoms. In order to protect the IGZO layer it must be surrounded by a very low hydrogen content dielectric film such as SiO2. Applied Materials has developed high performance SiO2 films that are used to surround the transistor channel as a hydrogen barrier. The machines that we make to deposit these films are impressively large, as you can see from this photograph.

6. What consumer products work best with metal oxide backplanes?

Large area high resolution LCD TVs and 3D TVs are good examples. 3-D technology used today creates the illusion of depth by actually displaying two different sets of images. 3-D glasses allow each eye to see only one set of images at a time and your brain then translates the dual inputs into a single image with depth. Having to interleave two sets of images halves the apparent refresh rate, so you need twice the actual refresh rate to avoid unpleasant motion artifacts in the picture. So, a high definition 3D TV with a 240Hz refresh rate is actually being driven at 480Hz. Metal oxide transistor backplanes are a promising technology for large size LCD TVs as the electron mobility that I mentioned before is enough to enable refresh rates of 240Hz or 480Hz. Also, the metal oxide manufacturing process can take advantage of the latest, largest glass size (up to 9m2), which is one of the reasons metal oxide is less expensive than LTPS.

LCDs for tablet PCs are another great candidate. In addition to faster refresh rates, metal oxide technology reduces power consumption and can contribute to pixel size reduction. That’s a great combination because it allows display makers to fabricate beautiful, high-resolution displays with limited impact on battery life.

7. What displays are not well suited for metal oxide?

OLED for smart phones and tablet PC’s are not a good candidate for metal oxide. Because each pixel of an OLED emits light directly, without the need for a backlight, each pixel requires two transistors. One is used to switch the pixel on an off, and one to control the current fed to the pixel. The problem is that metal oxide transistors aren’t stable enough for this application. This causes unpleasant pixel-to-pixel variations that can be visible to the viewer. You can compensate for the signal variation with an additional transistor, but there is not enough room on small displays to squeeze in three metal oxide transistors per pixel. That’s why OLED screens for mobile applications use more stable LTPS backplanes instead.

[Update: metal oxide manufacturing technology has improved since this post was published and it may now be possible to use metal oxide transistor backplanes for OLED displays. Not just for small, mobile displays, either: Sony and Panasonic exhibited a 56-inch 4K OLED TV with a metal oxide backplane at CES 2013. LG is also beginning to ship 55-inch OLED TVs with metal oxide backplanes. New IGZO deposition technology that we announced late last year is helping the industry to make the switch (press release).]

Did you find these answers useful? If so, or if I’ve raised new questions, please let me know in the comment section below.

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