Learn About LCD TV and TFT LCD Displays

TFT LCD TV - What is TFT LCD?

History of TFT LCD

Liquid crystal was discovered by the Austrian botanist Fredreich Rheinizer in 1888. "Liquid crystal" is neither solid nor liquid (an example is soapy water).

In the mid-1960s, scientists showed that liquid crystals when stimulated by an external electrical charge could change the properties of light passing through the crystals.

The early prototypes (late 1960s) were too unstable for mass production. But all of that changed when a British researcher proposed a stable, liquid crystal material (biphenyl).

Today's color LCD TVs and LCD Monitors have a sandwich-like structure (see figure below).

What is TFT LCD?

TFT LCD (Thin Film Transistor Liquid Crystal Display) has a sandwich-like structure with liquid crystal filled between two glass plates. 

TFT Glass has as many TFTs as the number of pixels displayed, while a Color Filter Glass has color filter which generates color. Liquid crystals move according to the difference in voltage between the Color Filter Glass and the TFT Glass. The amount of light supplied by Back Light is determined by the amount of movement of the liquid crystals in such a way as to generate color.

TFT LCD - Electronic Aspects of LCD TVs and LCD Monitors

Electronic Aspects of AMLCDs

The most common liquid-crystal displays (LCDs) in use today rely on picture elements, or pixels, formed by liquid-crystal (LC) cells that change the polarization direction of light passing through them in response to an electrical voltage.

As the polarization direction changes, more or less of the light is able to pass through a polarizing layer on the face of the display. Change the voltage, and the amount of light is changed.

There are two ways to produce a liquid-crystal image with such cells: the segment driving method and the matrix driving method.
The segment driving method displays characters and pictures with cells defined by patterned electrodes.

The matrix driving method displays characters and pictures in sets of dots.

Direct vs. multiplex driving of LCD TVs. 

Marine Displays LCD Monitors Part 2

The segment drive method is used for simple displays, such as those in calculators, while the dot-matrix drive method is used for high-resolution displays, such as those in portable computers and TFT monitors.

Two types of drive method are used for matrix displays. In the static, or direct, drive method, each pixel is individually wired to a driver. This is a simple driving method, but, as the number of pixels is increased, the wiring becomes very complex. An alternative method is the multiplex drive method, in which the pixels are arranged and wired in a matrix format.

To drive the pixels of a dot-matrix LCD, a voltage can be applied at the intersections of specific vertical signal electrodes and specific horizontal scanning electrodes. This method involves driving several pixels at the same time by time-division in a pulse drive. Therefore, it is also called a multiplex, or dynamic, drive method.

Passive and Active Matrix LCDs

There are two types of dot-matrix LCDs.

Passive-matrix vs. active-matrix driving of LCD Monitors.          

In passive-matrix LCDs (PMLCDs) there are no switching devices, and each pixel is addressed for more than one frame time. The effective voltage applied to the LC must average the signal voltage pulses over several frame times, which results in a slow response time of greater than 150 msec and a reduction of the maximum contrast ratio. The addressing of a PMLCD also produces a kind of crosstalk that produces blurred images because non-selected pixels are driven through a secondary signal-voltage path. In active-matrix LCDs (AMLCDs), on the other hand, a switching device and a storage capacitor are integrated at the each cross point of the electrodes.

The active addressing removes the multiplexing limitations by incorporating an active switching element. In contrast to passive-matrix LCDs, AMLCDs have no inherent limitation in the number of scan lines, and they present fewer cross-talk issues. There are many kinds of AMLCD. For their integrated switching devices most use transistors made of deposited thin films, which are therefore called thin-film transistors (TFTs).

The most common semiconducting layer is made of amorphous silicon (a-Si).
a-Si TFTs are amenable to large-area fabrication using glass substrates in a low-temperature (300°C to 400°C) process.

An alternative TFT technology, polycrystalline silicon - or polysilicon or p-Si-is costly to produce and especially difficult to fabricate when manufacturing large-area displays.

Nearly all TFT LCDs are made from a-Si because of the technology's economy and maturity, but the electron mobility of a p-Si TFT is one or two orders of magnitude greater than that of an a-Si TFT.

This makes the p-Si TFT a good candidate for an TFT array containing integrated drivers, which is likely to be an attractive choice for small, high definition displays such as view finders and projection displays.

Structure of Color TFT LCD TVs and LCD Monitors

A TFT LCD module consists of a TFT panel, driving-circuit unit, backlight system, and assembly unit.

Structure of a color TFT LCD Panel:

1. LCD Panel
- TFT-Array Substrate
- Color Filter Substrate
2. Driving Circuit Unit
- LCD Driver IC (LDI) Chips
- Multi-layer PCBs
- Driving Circuits
3. Backlight & Chassis Unit
- Backlight Unit
- Chassis Assembly

It is commonly used to display characters and graphic images when connected a host system.
The TFT LCD panel consists of a TFT-array substrate and a color-filter substrate.

The vertical structure of a color TFT LCD panel.

The TFT-array substrate contains the TFTs, storage capacitors, pixel electrodes, and interconnect wiring. The color filter contains the black matrix and resin film containing three primary-color - red, green, and blue - dyes or pigments. The two glass substrates are assembled with a sealant, the gap between them is maintained by spacers, and LC material is injected into the gap between the substrates. Two sheets of polarizer film are attached to the outer faces of the sandwich formed by the glass substrates. A set of bonding pads are fabricated on each end of the gate and data-signal bus-lines to attach LCD Driver IC (LDI) chips

Driving Circuit Unit

Driving an a-Si TFT LCD requires a driving circuit unit consisting of a set of LCD driving IC (LDI) chips and printed-circuit-boards (PCBs).

The assembly of LCD driving circuits. 

A block diagram showing the driving of an LCD panel.

To reduce the footprint of the LCD module, the drive circuit unit can be placed on the backside of the LCD module by using bent Tape Carrier Packages (TCPs) and a tapered light-guide panel (LGP).

How TFT LCD Pixels Work 

A TFT LCD panel contains a specific number of unit pixels often called subpixels.
Each unit pixel has a TFT, a pixel electrode (IT0), and a storage capacitor (Cs).
For example, an SVGA color TFT LCD panel has total of 800x3x600, or 1,440,000, unit pixels.
Each unit pixel is connected to one of the gate bus-lines and one of the data bus-lines in a 3mxn matrix format. The matrix is 2400x600 for SVGA.

Structure of a color TFT LCD panel. 

Because each unit pixel is connected through the matrix, each is individually addressable from the bonding pads at the ends of the rows and columns.
The performance of the TFT LCD is related to the design parameters of the unit pixel, i.e., the channel width W and the channel length L of the TFT, the overlap between TFT electrodes, the sizes of the storage capacitor and pixel electrode, and the space between these elements.
The design parameters associated with the black matrix, the bus-lines, and the routing of the bus lines also set very important performance limits on the LCD.

In a TFT LCD's unit pixel, the liquid crystal layer on the ITO pixel electrode forms a capacitor whose counter electrode is the common electrode on the color-filter substrate.

Vertical structure of a unit pixel and its equivalent circuit 

A storage capacitor (Cs) and liquid-crystal capacitor (CLC) are connected as a load on the TFT.
Applying a positive pulse of about 20V peak-to-peak to a gate electrode through a gate bus-line turns the TFT on. Clc and Cs are charged and the voltage level on the pixel electrode rises to the signal voltage level (+8 V) applied to the data bus-line.

The voltage on the pixel electrode is subjected to a level shift of DV resulting from a parasitic capacitance between the gate and drain electrodes when the gate voltage turns from the ON to OFF state. After the level shift, this charged state can be maintained as the gate voltage goes to -5 V, at which time the TFT turns off. The main function of the Cs is to maintain the voltage on the pixel electrode until the next signal voltage is applied.

Liquid crystal must be driven with an alternating current to prevent any deterioration of image quality resulting from dc stress.
This is usually implemented with a frame-reversal drive method, in which the voltage applied to each pixel varies from frame to frame. If the LC voltage changes unevenly between frames, the result would be a 30-Hz flicker.
(One frame period is normally 1/60 of a second.) Other drive methods are available that prevent this flicker problem.

Polarity-inversion driving methods. 

In an active-matrix panel, the gate and source electrodes are used on a shared basis, but each unit pixel is individually addressable by selecting the appropriate two contact pads at the ends of the rows and columns.

Active addressing of a 3x3 matrix 

By scanning the gate bus-lines sequentially, and by applying signal voltages to all source bus-lines in a specified sequence, we can address all pixels. One result of all this is that the addressing of an AMLCD is done line by line.

Virtually all AMLCDs are designed to produce gray levels - intermediate brightness levels between the brightest white and the darkest black a unit pixel can generate. There can be either a discrete numbers of levels - such as 8, 16, 64, or 256 - or a continuous gradation of levels, depending on the LDI.

The optical transmittance of a TN-mode LC changes continuously as a function of the applied voltage.
An analog LDI is capable of producing a continuous voltage signal so that a continuous range of gray levels can be displayed.
The digital LDI produces discrete voltage amplitudes, which permits on a discrete numbers of shades to be displayed. The number of gray levels is determined by the number of data bits produced by the digital driver.

Generating Colors

The color filter of a TFT LCD TV consists of three primary colors - red (R), green (G), and blue (B) - which are included on the color-filter substrate.

How an LCD Panel produces colors. 

The elements of this color filter line up one-to-one with the unit pixels on the TFT-array substrate.
Each pixel in a color LCD is subdivided into three subpixels, where one set of RGB subpixels is equal to one pixel.
(Each subpixel consists of what we've been calling a unit pixel up to this point.)

Because the subpixels are too small to distinguish independently, the RGB elements appear to the human eye as a mixture of the three colors.
Any color, with some qualifications, can be produced by mixing these three primary colors.

The total number of display colors using an n-bit LDI is given by 23n, because each subpixel can generate 2n different transmittance levels.

TFT LCD - Fabricating TFT LCD

Fabricating Color TFT LCD Displays

The pressure to reduce the manufacturing cost of TFT LCD displays is as constant and intense as it is in the semiconductor industry. To increase productivity, IC makers continuously reduce the sizes of c-Si chips and transistors in order to increase the number of chips per wafer.

IC makers increase productivity by continuously reducing chip size and
increasing wafer size to increase the number of chips per wafer. 

But this strategy doesn't work for LCDs because the panel sizes users demand most get steadily larger, not smaller.
Still, by increasing the number of panels produced on a single substrate, the cost of TFT-array processes can be reduced.

The IC makers' size-reduction strategy doesn't work for direct-view LCDs, but
LCD manufacturers can still reduce the cost of TFT-array processes by
increasing the number of panels produced on a single substrate. 

This process requires that the size of the glass substrate be steadily increased so that the number of LCD panels fabricated upon it can increase.

For more panels to be put on a glass substrate, the substrate size must be
steadily increased - which requires the continual design and construction of
new generations of process equipment.

New generations of process equipment must be continually designed and built to achieve these increases.
The fabrication processes this equipment must implement will be described below.
We can assume that the display being fabricated is a color TFT LCD that uses an inverse-staggered-type a-Si TFT as the active-matrix switching element.

Fabricating the TFT array

The manufacturing process used to fabricate an a-Si TFT array is very similar to those used to fabricate c-Si semiconductor devices. The various steps, including cleaning, deposition of thin films, photolithography, and wet and dry etching of the thin films - are alsso very similar. The difference between the a-Si TFT process and the c-Si semiconductor process is that a semiconductor layer is deposited onto a glass substrate in the a-Si TFT process, while Si wafers are used as the substrate in the c-Si semiconductor process. Today, critical issues in the processing of TFT arrays include the development of a low-resistance gate-bus line, uniform and fine etching, and improved lithographic accuracy.
TFT-array technologies are aimed at achieving high precision, large aperture ratio, and low power consumption, in addition to large screen size.
AMLCD manufacturers are also competing to minimize the number of array processes by reducing the number of photo masks and simplifying the thin-film-formation and etching processes.

In the bottom-gate TFT-array fabrication process, the first layer consists of the gate electrodes and gate bus-lines, which can have one or two metal layers.
Some storage capacitors can be constructed by using a part of the gate electrode as an electrode of the storage capacitor - which is called the Cs-on-gate method - while other capacitors are constructed independent of a gate bus-line.

If the independent Cs lines are constructed simultaneously with the gate bus-lines using the same metal layer, there is no difference in the fabrication process between the Cs-on-gate method and the independent Cs bus-line method.
The processing of an a-Si TFT array is complex.

This flowchart outlines the processes for making an a-Si TFT array using a
bottom-gate TFT structure and an independent storage capacitor. 

After constructing gate and storage-capacitor electrodes with 2000-3000A of a metal such as aluminum, chromium, tantalum, or tungsten, a triple layer of silicon nitride and amorphous silicon is deposited by using plasma-enhanced chemical-vapor deposition (PECVD).

In the etch-back type of TFT structure, the triple layer consists of 4000A of SiNx, 2000A of a-Si, and 500 A OF n+a-si, which is deposited over the gate electrode in a continuous process, i.e., a process without a vacuum break.

For the etch-stopper type of TFT structure, 4000A of SiNx, 500A OF a-Si, and 2000A of n+a-si are deposited.
Let us look at the etch-back TFT fabrication process in more detail.

TFT Fabrication 

After defining the a-Si area by using photolithography and plasma dry etching, an ITO layer is deposited with a thickness of about 500A via sputtering.

Then, the pixel electrodes are patterned. About 2000A of metal is sputter deposited, while data bus-lines and TFT electrodes are patterned by photolithography.

Then the ohmic contact layer (n+a-Si) at the channel region is etched by dry etching using the source and drain electrodes as an etch-protect mask.
Finally, a protective 2500A SiNx layer is deposited by PECVD and contact windows are opened.

The etch-stopper TFT structure requires one more process step - a chemical vapor deposition (CVD) - than does the etch-back TFT structure.
For etch-stopper TFT fabrication, a n+a-Si layer is deposited separately after the top insulator of triple-layer (SiNx/a-Si/SiNx) is patterned.

The a-Si area is patterned and the n+a-Si layer at the top of etch-stopper is removed. The source and drain electrodes are formed using about 2000A of metal; then, about 500A of ITO is sputter deposited, and pixel electrodes are patterned.
A SiNx protective layer is then deposited by PECVD and, finally, the contact windows are opened.

Fabricating Color Filters

Color filters (CFs) can be made with either dyes or pigments, utilizing coloring method such as dyeing, diffusion, electro-deposition, and printing.

Color filters (CFs) can be made with either dyes or pigments, and can be
further divided by coloring method. 

There are several fairly common color-element configurations for LCDs.
Stripe is the most popular, followed by mosaic and delta.

Among the many combinations of configuration and types of CF fabrication methods, the color-resist method with stripe-type RGB arrangement is currently the most popular.

Between the blocks of color in the CF is a black matrix (BM) made of an opaque metal, such as chromium, which shields the a-Si TFTs from stray light and prevents light leakage between pixels.
A double layer of Cr and CrOx is used to minimize reflection from the BM.
The sputter-deposited BM film is patterned using photolithography.

For reduced cost and reflectivity, black resin made by diffusing C and Ti in photo resist - can be used as a BM material.
In the color-resist method, the primary color-filter patterns are formed by using a photolithography technique.

The color-resist is negative and made by diffusing pigment in a UV-curing resin, such as an acryl-epoxy resin, and by dissolving the resin in a solvent.
A red colored resist is spin-coated onto a glass substrate on which a BM has previously been formed.
The red pattern is then formed by exposing the red resist through a mask and developing it.

The process is repeated using the same mask with a shifted mask-align technique for green- and blue-colored resins.
A protective film is then applied, and 1500A of ITO for the TFT array's common electrode is sputter-deposited to finish the color filter.

Liquid-crystal Cell Process

The TFT-array and color-filter substrates are made into an LCD panel by assembling the two substrates together with a sealant, while the cell gap is maintained by spacers.

The TFT-array and color-filter substrates are made into an LCD panel by
assembling them with a sealant. 

The assembly is begun by printing a polyimide alignment film on a cleaned TFT-array, and then rubbing the surface of the film with a piece of cloth wound on a roller, which orients the polyimide molecules in one direction.

Similarly, alignment film is applied to the color-filter substrate, and this substrate is also rubbed.
After the rubbing process, a sealant is applied to the periphery of the TFT-array substrate. To form electrical connections from the common electrodes on the color-filter substrate to the TFT array, the TFT-array substrate is coated with a conducting paste around the periphery.

At the same time, spacers to control the cell gap are sprayed onto the color-filter substrate. (In some cases, spacers are sprayed on to the TFT-array substrate, and a sealant is applied to the color-filter substrate.)
The two substrates are then assembled after the sealant is pre-hardened.
The sealant is then hardened completely with heat and pressure.

Then, the assembled substrates are scribed using a diamond wheel and separated into individual cells, and the empty cells are filled with liquid crystal material by vacuum injection.

Finally, a sealing agent is used to seal the cell, and the polarizers are applied to both cell surfaces after a visual function test.

Assembling LCD Modules

Although critical for producing panels with the desired characteristics and price, the details of the manufacturing process for AMLCD panels are often of less immediate interest to the OEM purchasers of displays than are the details of the module assembly process.

This is so because it is the physical and electrical characteristics of the module that OEMs must deal with when integrating the display into products for end users.

The process flow for assembling a module using the tape-automated-bonding (TAB) method is conceptually straightforward, but it's not simple.

The process for assembling LCD modules(flow chart). 

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