February 2007
A Closer Look At LCDs

LCD monitors and TVs are the fastest-growing display technology in the world.
Here's everything you need to know about how they work and how they're made.


Photo courtesy of LG. Philips LCD

by Pete Putman, CTS, ISF

Available in all shapes, sizes, and resolutions, liquid crystal displays (LCDs) are everywhere these days — from the screen on your notebook computer and the big-screen HDTV in your family room to the airline information screens at the airport and electronic menus at your local fast food joint. In fact, it seems like everyone and their brother is either manufacturing or selling LCD TVs, which are the most popular LCD product category for consumers.

Some of the brands you probably immediately recognize like Samsung, LG, and Sharp. Others you may have heard of include Westinghouse, Vizio, Polaroid, and Syntax Olevia. Then there are the companies, such as Chi Mei Optoelectronics and AU Optronics, you might not know at all. Yet all of these names are important in the world of LCD displays — from manufacturing to retailing.

Long time coming
LCD technology isn’t new. In fact, the first discoveries of liquid-crystal birefringence — the ability to split beams of light into two polarized planes — were made in the 1880s in Austria. In the 1950s and 1960s, RCA Corp. performed detailed research into liquid crystals, investigating the possibility that they could be the basis of a new lightweight, low-power display technology.

In the 1970s, after RCA discontinued its efforts, Japanese companies, spearheaded by Sharp and Casio, took the lead in commercializing LCDs, including monochrome calculators and watches. Color LCD screens made their debut in the 1980s, followed by overhead projection panels and notebook computers in the early 1990s, and small, low-resolution televisions.

For years, the largest LCD TVs and monitors couldn’t exceed 30 inches without manufacturing sleight-of-hand, such as precision stitching of smaller panels to achieve larger sizes. The real breakthroughs came about the turn of the 21st century when the first one-piece, 40-inch diagonal LCD panels were introduced.

Today, single-cut LCD panels with diagonal sizes of 108 inches have been shown, and 1080p LCD HDTVs as large as 52 inches are available at retail for less than $4,000. To put things in perspective, Sharp’s 28-inch LCD monitor from 1999 — a breakthrough product at the time — was priced at $15,000!

In recent years, prices have dropped so fast on LCD HDTV products that it’s causing major headaches for well-known brands. What’s behind this downward price pressure? Lots and lots of manufacturing capacity, particularly in Taiwan and China. In less than a decade, LCDs have gone from an expensive niche technology to ubiquitous. Many analysts even predict they’ll kill off the venerable cathode-ray tube (CRT) in the near future.

How they work
LCDs aren’t particularly complex. Here’s how twisted nematic (TN) liquid crystals (commonly used in computer monitors and small electronics) switch light.

In an individual pixel, TN liquid crystals rotate in response to changes in voltages applied across the cell. When no voltage is present, the individual TN crystals float in a random fashion, and any polarized light passing through the cell is unimpeded. However, when the TN crystals start to align themselves with an increase in voltage, less polarized light can pass through. At full operating voltage, the TN crystals assume their characteristic alignment pattern, and virtually all rays of light are blocked from passing through the cell.

As a result, we wind up with a display that may appear to be “on” when it’s actually “off” and vice versa! As a real-world analogy, look at a window blind, which passes more or less light depending on how you align the individual slats.

Unlike plasma displays, which essentially operate the same way no matter who the manufacturer is, there are several different LC alignment modes. Some of these modes shutter light in an opposite manner from TN and its variations. All of them promise high image contrast and wide viewing angles — both of which have been a real challenge for LCD display manufacturers.

In addition to TN liquid crystals, there are variations called super twisted nematic (STN) and triple super twisted nematic (TSTN). While TN LC displays have been around for a long time, more sophisticated alignment structures have recently come into existence.

One is patterned vertical alignment (PVA), where the individual liquid crystals align in a vertical orientation when blocking light. As voltage is applied, the liquid crystals shift to a horizontal alignment, and the degree of that shift determines how much light passes through each cell. There’s also a super PVA variation coming to market (see sidebar below).

Another popular system is in-plane switching, where the liquid crystals align parallel to the surfaces of the cell when the pixel is off. As voltage is applied, the individual crystals rotate on axis up to 90 degrees while remaining parallel to the cell walls, passing light. A more advanced version of this LC alignment, known as super in-plane switching (S-IPS), is in wide use for consumer LCD HDTVs.

Yet a third alignment structure starts with the liquid crystals arrayed in a circle, standing vertically like a row of bowling pins. These LCs tip over in a circular pattern when energized. This system is known as continuous pinwheel alignment (CPA), because the individual LCs resemble a pinwheel or daisywheel when switched fully on.

Out of the oven
The secret to making LCDs is in the liquid crystal paste. This stuff, most of which comes from chemical giant Merck KGaA of Darmstadt, Germany, is a paste that contains those tiny little light shutters. (One industry veteran I interviewed for this article likened it to peanut butter.) Merck’s brand name for it is Licristal, and it’s available in numerous formulations, depending on the desired LC alignment.

For example, Merck makes six different LC formulations for small, low-voltage LCD screens, such as those found in PDAs, iPods, notebook PCs, and digital cameras. Four different recipes for IPS and S-IPS LCD fabrication are available, while there are five choices for PVA and ASV manufacturers.

The second most important part of the equation is the actual glass material used to form the support structures for the individual TFT-controlled cells and color filters, not to mention the polarizers. And the lion’s share of that glass comes from Corning, which developed a special fusion process to make suitable LCD glass (EAGLE2000) that is transparent, thermally stable over a wide range of temperatures, and very strong.

Corning makes LC glass in many locations around the world, including the United States, Japan, and Korea. To reflect the growing importance of LCD manufacturing in China, Corning has opened a facility in Taiwan and announced in December of 2006 that a new LCD glass facility would be constructed near Beijing.

The glass isn’t particularly thick — typically only 0.7 mm for large LCD fabrication lines (fabs, for short.) So extreme care must be taken when moving these “motherglass” sheets, some of which measure as large as 7.9 feet by 7 feet (Generation 8 fab dimensions)!

To see how these raw materials are made into a finished 42-inch LCD panel for a consumer HDTV set, let’s follow the finishing steps inside a Gen 7 LCD fab, such as LG.Philips LCD’s Paju, Korea facility. (LG.Philips defines Gen 7 “motherglass” size as 2,250 mm by 1,950 mm, or 7.4 feet by 6.4 feet by 0.028 inches.)

Step 1 – TFTs: The super-small thin film transistors (TFTs) used to control individual LC cells, or pixels, must be formed onto a glass substrate. Ultra-thin chemical films are microdeposited and shaped using chemical vapor deposition into semiconductors, with additional etching to apply electrodes made from an indium-tin-oxide (ITO) alloy.

There are three separate color (red, green, and blue) sub-pixels in every imaging pixel that must be formed, and this process requires four to five different masks, depending on the size of the panel. As small and transparent as the TFT is, it still blocks some light through the pixel. The ratio of unblocked to blocked area is known as the pixel’s aperture ratio.

Pixel density is measured in pixels per inch (PPI) — with 100 PPI a common resolution for large-screen LCD monitor panels. It’s possible to increase that density even more — LCD HDTVs with screens as small as 37 inches diagonally are widely available with full 1920x1080 pixel resolution. That’s a total of 6,220,800 sub-pixels!

Step 2 – Color Filters: While the TFTs are being formed in one building, another piece of glass the same size and thickness is being coated with color filters in another building nearby. These color filters — liquid pigments — are precisely microdeposited and rapidly cured. (Some LCD manufacturers are now experimenting with inkjet deposition of pigments.)

The choice of color pigments depends on the application, so an LCD panel destined to become a consumer TV set will have a different mix of red, green, and blue than a computer monitor. Color pigments are also chosen carefully with backlights in mind. One cold-cathode backlight unit (CCFL) may accentuate brightness and bluish colors at the expense of skin tones, while another is better suited for watching movies.

Step 3 – LC Paste: The color filter and TFT processes are parallel operations. Once both sides of the LCD module are complete, the two pieces must be bonded together with the desired LC compound inside. The liquid crystals can be injected into a finished LC cell, but a more common approach with large LCD glass is to apply the LC paste precisely to each TFT pixel before bonding.

This “one drop filling” technique was originally known as VALC, or vacuum aligned liquid crystals, first developed in LG.Philips LCD’s Gen 5 LCD fab. VALC sped up LC application for large displays by more than 50 times.

Once the LC paste is applied to each individual cell, a powerful epoxy bond holds the color filter arrays and TFT glass together, creating the physical pixel structures.

Step 4 – Final Assembly: At this point, the LCD is almost complete. All it needs is assembly and attachment of the polarizing filters, attachment of the backlight unit, which can be a cold-cathode or hot-cathode fluorescent lamp or even a light-emitting diode, and attachment of wiremold connectors and driver integrated circuits to all TFT electrodes. The LCD panel will also be installed into some sort of housing.

All told, there are six “layers” to a finished LCD panel — the backlight unit (BLU), then the first polarizer (which throws away half the light out of necessity), followed by the TFT array. Next comes the liquid crystal layer, bounded on the other side by the color filter glass. The second polarizer, closest to your eye, completes this liquid crystal sandwich.

It takes about one to two weeks at LG.Philips LCD’s Paju plant to produce a finished piece of “motherglass” after Steps #1 through #3.

What’s next?
New facilities like the LG.Philip’s Paju plant can perform Steps #1 through #3 without having to cut the “motherglass” sheet, unlike some other facilities. This speeds up production considerably.

Further process improvements will allow bigger glass cuts to be made. Sharp is now operating a Gen 8 fab in Shizuoka, Japan, from which it makes 46-inch and 52-inch LCD modules for use in HDTVs. Corning is the primary supplier of glass sheets for this facility, using a newly developed glass substrate (EAGLE XG), free of all heavy metals and halides, according to a company press release.

Could we see Gen 9 and 10 fabs? Certainly, but the only practical reason a manufacturer might go to those larger sizes is to increase capacity for and reduce the costs of existing panel sizes, not necessarily to make larger LCD panels and modules.

How big can they go? Back in the spring of 2006, LG.Philips LCD showed a 100-inch LCD HDTV from the Paju factory. That particular glass size would be very costly to commercialize, and the market economics just don’t make sense — HDTVs measuring less than 50 inches diagonally are where the money is these days.

 

Samsung’s PVA Technology
When it comes to liquid crystal alignment patterns, there’s more than one path to follow. Samsung SSI manufactures LCD monitors and HDTVs on its jointly owned Gen 7-1 and wholly owned 7-2 lines in Tangjeong, Korea, using vertically aligned (VA) liquid crystal compounds, also made by Merck KGaA.

The Samsung process is actually known as patterned-ITO vertical alignment (PVA), and it gets its name by the characteristic tilt of the individual liquid crystals as they move to shutter light. The crystals stand at 90-degree angles to the pixel walls when switched off, and tilt toward the horizontal when switched on. The effect resembles bowling pins falling over. The degree of tilt is what provides the variable light shutter.

A newer version of PVA, known as super patterned vertical alignment (S-PVA), is claimed to deliver higher contrast and wider viewing angles than PVA. It fits eight liquid crystals per pixel instead of the four used with PVA technology.

Samsung ‘s Gen 7 lines start with 1870 mm x 2200 mm (74 inch by 87 inch) motherglass to produce S-PVA LCD modules for TV, from 23-inch (24 per substrate), 26-inch (18 per substrate) and 32-inch (12 per substrate) to 40-inch (eight per substrate) and 46-inch (six per substrate).

 

Pete Putman is a contributing editor for Pro AV and president of ROAM Consulting, Doylestown, PA. Especially well known for the product testing/development services he provides manufacturers of projectors, monitors, integrated TVs, and display interfaces, he has also authored hundreds of technical articles, reviews, and columns for industry trade and consumer magazines over the last two decades. You can reach him at pete@hdtvexpert.com.