In our last blog post, I wrote about some of the physics and materials science principles that go into the design and manufacture of liquid-crystal display (LCD) screens. The eponymous liquid crystals (LCs) in such a display have to be quite small in order to create a seamless image; as I mentioned in the last entry, the subpixels (the red, green and blue elements comprising a pixel, visible in Figure 1) can be smaller than a red blood cell. Each of these subpixels needs its own control system that interacts with the data drivers along the periphery of the screen. These power and control circuit components must therefore be even smaller than the LC elements, as they need to fit between those elements to allow backlight through. The design of such a circuit is a balancing act between larger traces with lower resistance and smaller traces that let more backlight through. In this blog entry I’ll cover the design and fabrication of these microscopic circuits. As before, for an on-hand example I’ve disassembled a legacy Sony Ericsson® S500i slider phone for analysis under our optical microscope.
Matrix Display Control Methods
Modern LCD screens use a matrix method of sequentially addressing and controlling each pixel by its row and column. Although older direct-drive LCD screens (like the seven-segment displays on an old digital clock) used an individual circuit for each pixel, that method quickly proved inadequate as engineers began looking to LCDs to create whole displays. For instance, even the low-resolution 240×320 display on the S500i would have required 76,800 traces to reach each pixel. Treating the display as a matrix cuts the number of required traces down to only 560. This makes image-writing slightly more complicated, as the control system cannot address every pixel simultaneously. Instead, each pixel receives its instructions for an instant and then holds its open or closed position while the control system moves across the rest of the screen. The pixel must be able to hold its charge while the data circuit is addressing other elements. Otherwise the pixels would deactivate before they refresh with data for the next image frame. The left half of the screen might blank out while the data circuit is still writing the right side.
At their simplest, LCD screens are controlled using a single thin-film transistor (TFT) and capacitor per subpixel driving circuit. An array of power line traces extend in one direction across the display and connect to the source electrode on each subpixel’s TFT. Running perpendicularly to the power lines are “gate lines” or “scan lines”, which provide current to the gate electrode on each TFT, ultimately controlling the voltage across the TFT’s source and drain electrodes. In short, the current in the gate line opens or closes the connection between the power line and the liquid crystal layer. As for the capacitor, in simple screens like the S500i it is stacked in series with the liquid crystal itself. While older “passive matrix” LCD screens used materials that would naturally hold onto their charge (to some extent) while unpowered, modern “active matrix” screens use the capacitor-TFT combination to deliver a sharper image and better image control. These elements are all shown in Figure 2, an optical micrograph of the S500i’s pixel and peripheral circuitry at 400x magnification. For a sense of scale, at that magnification a human hair might cover the width of two or three of those rectangular subpixels.
Layer Composition and Deposition
Since the pixels and their driver circuits are assembled on such a small scale, the designers use the same types of materials and methods used in semiconductor fabrication. The thin-film transistor (TFT) uses doped polycrystalline silicon layers in its construction. As the name implies, the poly silicon is deposited as a thin film on a glass substrate, usually using a plasma-enhanced chemical vapor deposition process (PECVD). In conjunction with the silicon, these circuits might use other semiconductor materials with different properties, such as molybdenum disulfide, to complete the transistor structures.
If the capacitor is a separate structure within the circuit, it can be formed in the levels above the TFTs, using two conductive plates sandwiching a dielectric layer. However, in the simpler circuit used in the S500i and many other LCD screens, the capacitor stacks in series with the liquid crystal. This saves space on the display, but adds the complication that the backlight must still be able to pass through the liquid crystal and out through the color filter and polarizing screen. For this situation, designers use materials called transparent conducting oxides (TCOs). The most common, indium tin oxide (ITO) is electrically conductuve, optically transparent, and like the semiconductors is easy to deposit as a thin-film. However, instead of a chemically-based process like PECVD, ITO requires a physical vapor deposition process such as sputtering.
On the top layer of the circuit structure are the gate line and power line traces. These traces use a metal like copper or silver. With their low resistances, these metals can conduct a charge across the width and height of the screen without noticeably dropping in voltage. These metals also have a lower melting point than the semiconductor materials below them, so they can be deposited onto the display without damaging the layers below. That deposition might be physical vapor sputtering as well, or simply by electroplating the surface as needed.
Aside from the obvious electrical engineering inherent in laying out the circuit, liquid-crystal display technology requires just as much materials science expertise in its design. The display requires materials that change their internal structure in response to an electric charge and others that are both transparent and conductive. Furthermore, most of these materials require different temperatures and techniques when building a circuit, so the fabrication must be as carefully planned as the physical architecture itself. Simple as they are compared to many other electrical components, LCD screens are a perfect way to illustrate the concepts and methods used in semiconductor device fabrication. The integrated circuits used in processors and memory chips are much smaller and more complex than an LCD, but they are fabricated with similar processes and require the same types of materials science engineering.