TFT Technology

 

A display screen made with TFT (thin-film transistor) technology is a liquid crystal display (LCD), common in notebook and laptop computers, that has a transistor for each pixel (that is, for each of the tiny elements that control the illumination of your display). Having a transistor at each pixel means that the current that triggers pixel illumination can be smaller and therefore can be switched on and off more quickly.

TFT is also known as active matrix display technology (and contrasts with "passive matrix" which does not have a transistor at each pixel). A TFT or active matrix display is more responsive to change. For example, when you move your mouse across the screen, a TFT display is fast enough to reflect the movement of the mouse cursor.

 

Active matrix is a type of addressing scheme used in flat panel displays. In this method of switching individual elements (pixels), each pixel is attached to a transistor and capacitor actively maintaining the pixel state while other pixels are being addressed, in contrast with the older passive matrix technology in which each pixel must maintain its state passively, without being driven by circuitry.

The active matrix technology was invented by Bernard J. Lechner at RCA and first demonstrated as a feasible device using thin-film transistors (TFTs) by T. Peter Brody and his Thin-Film Devices department at Westinghouse Electric Corporation in 1974, and the term was introduced into the literature in 1975.

Given an m × n matrix, the number of connectors needed to address the display is m + n (just like in passive matrix technology). Each pixel is attached to a switch-device, which actively maintains the pixel state while other pixels are being addressed, also preventing crosstalk from inadvertently changing the state of an unaddressed pixel.

 

 

1. A TFT display is sharper and brighter than a common LCD display. It refreshes more quickly than a regular LCD display and shows motion more smoothly.

2. TFT displays use more electricity than regular LCD screens, so they not only cost more in the first place, but they are also more expensive to operate.

3. A TFT display is much higher quality than a regular LCD display. It is a better display in every way, except that it is more expensive.

 

TFT formation consists of several vacuum process steps, using  PECVD for deposition of a-Si and the gate dielectric insulation layer and sputtering equipment for data and scan metal lines as well as for ITO layers. A typical process step series is: Deposition of gate metal (Ta, Al, MoTa), patterning, anode oxidation Ta2O5, deposition of silicon nitride, patterning, deposition of a-Si for the electrode, patterning, deposition of source and data line (Ti, Al), patterning, deposition of pixel electrode (ITO), patterning, passivation, patterning. Some companies use pre-coated ITO substrates; thus, the first step is to pattern and etch the layer.

For high performance displays a poly-silicon deposition step is used instead of the a-Si deposition. The poly-silicon deposition is done under low pressure in a tube furnace. This furnace is similar to equipment used also in the semiconductor industry.

In order to structure the various layers, the listed patterning steps use common lithographic equipment like resist coaters, steppers and dry or wet etching equipment. Dry etching can provide much better line-width control, but wet etching is the faster and cheaper method because it is a batch process.