TFT-LCD Technology

TFT-LCD Technology

Feb 17, 2017

TFT-LCD technology, detailed process

Working principle of TFT-LCD liquid crystal display

In thin film transistor liquid crystal display (TFT-LCD), the function of TFT is a switch tube. TFT is commonly used in three terminal devices. A semiconductor layer is formed on a glass substrate, and a source electrode and a drain electrode are connected at both ends. And through the gate insulating film opposite the semiconductor, the voltage applied to the gate to control the source and drain of the current.

For the display screen, each pixel can be viewed as a liquid crystal between the pixel electrode and the common electrode. What is more important is that it can be viewed as a capacitor from the point of view of electricity. The equivalent circuit is shown in figure 1. To j row I column pixel P (I, J) charge, it is necessary to turn on the switch T (I, J) on the signal line D (I) to apply the target voltage. When the pixel electrode is fully charged, even if the switch is off, the charge in the capacitor is preserved. The function of the data (column) driver is to apply the target voltage to the signal line, and the gate (line) driver acts as the switch on and off. Because the voltage added on the liquid crystal layer can be stored, the liquid crystal layer can work stably. This display voltage can also be re written in a short period of time through the TFT, therefore, even for high definition LCD, can also meet the image quality requirements.

The key to display the image is also the molecular orientation of the liquid crystal under the electric field. Through the alignment of the inside of the substrate, the liquid crystal molecules are arranged to produce the desired deformation to realize different display modes. Under the action of the electric field, the liquid crystal molecule has a change of orientation, and the intensity of the incident light changes through the liquid crystal layer by the cooperation of the polarizer. In order to achieve image display.

Simple matrix thin film transistor liquid crystal display with passive TN-LCD and STN-LCD is different, it has a thin film transistor in each pixel of the liquid crystal display (TFT), which can effectively overcome the crosstalk non gating, which can display static characteristics and scanning line LCD screen is independent of the number, thus greatly improving the image quality. The switching unit (i.e. TFT) characteristics, it is necessary to meet the low on state resistance, closed state resistance is very large this requirement.

The color display of thin film transistor liquid crystal display is realized by adding a layer of color filter on the front panel of the display. It requires the production of red, green, blue and black matrix for each pixel.

The main advantages of TFT-LCD LCD

With the maturity of TFT technology in the early 90s, the development of color LCD flat panel display, less than 10 years, the rapid growth of TFT-LCD into the mainstream display, which has its advantages are inseparable. Main features are:

(1) good use characteristics

The application of low voltage, low driving voltage, solid use safety and reliability is improved; flat, and thin, save a lot of raw materials and the use of space; low power consumption, the power consumption is about CRT display 1/10, left and right one percent reflective TFT-LCD even only CRT, save a lot of energy and TFT-LCD products; specification, size series, variety, flexible and easy to use, maintenance, update, easy to upgrade, long service life and many other features. Display range from 1 inches to 40 inches display all application range and large projection plane is full size display terminal; display quality from the most simple monochromatic character graphics to high resolution, high color fidelity, high brightness, high contrast, high response speed of the video display of various specifications; display has a direct type of projection type, perspective, also reflection type.

(2) good environmental protection

No radiation, no flicker, no harm to the health of users. In particular, the emergence of TFT-LCD electronic books, will bring the human paperless office, paperless printing era, lead to human learning, communication and record of civilization revolution.

(3) wide scope of application

It can be used normally in the temperature range from -20 DEG C to +50 DEG C, and the working temperature of TFT-LCD can be reduced to 80 degrees below zero. The utility model not only can be used as a mobile terminal display, but also can be used as a large screen projection tv.

(4) a high degree of automation in manufacturing technology

Large scale industrial production characteristics. TFT-LCD industry mature technology, large-scale production of finished product rate of 90% or more.

(5) TFT-LCD is easy to integrate and update

It is the perfect combination of large scale semiconductor integrated circuit technology and light source technology. At present, there are amorphous, polycrystalline and monocrystalline silicon TFT-LCD, there will be other materials in the future TFT, both the glass substrate and plastic substrate.

TFT-LCD manufacturing process

The following are the common sizes:display 4.3,tft lcd 3.2,tft lcd 2.8

Thin film transistor liquid crystal display (TFT-LCD) is usually processed in parallel with two processing processes, as shown in figure 1. The basic process is as follows:

The color filter layer through the front glass substrate, color filter substrate / process form precise arrangement.

The formation of thin film transistor liquid crystal thin film - Substrate Technology (TFT) and other electronic components used in display pixel array control. Each pixel generally corresponds to three thin film transistor liquid crystal (TFT), each pixel to control a common constitute a pixel of the color point". The film forming process uses CVD, Etch and PVD technology, which is similar to semiconductor manufacturing technology. This process should be repeated several times, continuous film can form a functional element (Figure 2).

The two substrates of intermediate injection liquid crystal material be made one.

The final assembly and backlight driving electronic components, manufacturing TFT-LCD module.

TFT-LCD LCD technology

The manufacturing process of thin film transistor liquid crystal display has the following parts: TFT array is formed on the TFT substrate; forming a color filter pattern and ITO conductive layer on the color filter substrate; with two substrates to form a liquid crystal box installation; peripheral circuit, backlight module assembly assembly.

1. process for forming TFT array on TFT substrate

TFT has realized the industrialization of the types include: amorphous silicon TFT (a-Si TFT), polycrystalline silicon TFT (p-Si TFT), monocrystalline silicon TFT (c-Si) several. Currently the most widely used is a-Si TFT.

The fabrication process of a-Si TFT is to first sputter the gate material film on the borosilicate glass substrate, and then form the grid pattern after the mask exposure, the development and the dry etching. Step exposure machine for general mask exposure. The second step is to use PECVD method for continuous film formation, SiNx film, non doped a-Si film, phosphorus doped n+a-Si film. Then, the a-Si pattern of the TFT part is formed by mask exposure and dry etching. The third step is to form a transparent electrode (ITO film) by sputtering, and then to display the electrode pattern by mask exposure and wet etching. The contact hole pattern of the fourth step gate extreme insulation film is formed by mask exposure and dry etching method. The fifth step is to sputter AL into the film, with mask exposure, etching the source, drain and signal line pattern of TFT. Finally, the protective film is formed by PECVD method, and then the film is etched by mask exposure and dry etching. The protective film is used to protect the electrode and the electrode and the display electrode. So far, the whole process is completed.

TFT array technology is the key of TFT-LCD manufacturing process, and the most part of equipment investment. The entire process requires a high level of purification (e.g., level 10).

2. process for forming a color filter pattern on a color filter (CF) substrate

The method for forming the colored part of the color filter comprises a dye method, a pigment dispersion method, a printing method, an electrolytic deposition method and an ink-jet method. At present, the pigment dispersion method.

The first step of the pigment dispersion method is to disperse the fine particles (average particle size less than 0.1 M) (R, G, B tricolor) in the transparent photosensitive resin. Then they were coated with R., G. and B color patterns in order to be coated, exposed and developed. The device is mainly used for coating, exposure and developing.

In order to prevent light leakage, usually with a black matrix at the junction of three RGB (BM). In the past, the single layer metal chromium film was formed by sputtering method, and now there is a new type of BM film or resin mixed carbon resin BM.

In addition, it is necessary to make a layer of protective film on the BM and form the IT0 electrode, because the substrate with the color filter is used as a front substrate of the liquid crystal screen and a rear plate with TFT to form a liquid crystal box. Therefore, we must pay attention to the problem of positioning, so that each unit of the color filter and the corresponding pixels of the TFT substrate.

3. liquid crystal box preparation process

Firstly, the polyimide film is coated on the surface of the upper substrate and the lower substrate respectively, and an orientation film can be formed according to the requirements through the friction process. Then, the sealant material is arranged around the TFT array substrate. At the same time, the silver paste is coated on the transparent electrode of the CF substrate. Then, the two substrates are aligned and bonded, so that the CF pattern is aligned with the TFT pixel pattern. In the printing sealing material, need to leave the injection port, in order to vacuum infusion liquid crystal.

In recent years, with the progress of technology and the increase of substrate size, in the manufacturing process of the box also has greatly improved, more representative is filling the crystal style changes, from the original into a box after perfusion to the ODF method, namely filling crystal and simultaneously into a box. In addition, the pad lining method is no longer the traditional method of spraying, but directly on the array using photolithography.

4. peripheral circuit, assembly backlight module assembly process

After the production process of the liquid crystal box is completed, a peripheral driving circuit is needed to be arranged on the panel, and then the polarizing plate is pasted on the surface of the two substrates. If the transmission type LCD. also install backlight.

Specifically as follows:

Array Process (array process)

Actual plant production process:

The process is divided into two parts, CF and TFT, as shown in figure:

TFT-LCD specific production process

One,TFT array substrate formation stage

TFT array the main cleaning process, film, and yellow light system board, after the etching process to form a pattern, and then according to the number of mask circulation process, in this cycle process should first be washed into the glass substrate sputter plating a layer of metal, and then the yellow light and etching process for forming gate, regional pattern, Sui after stripping photoresist glass substrate by cleaning, to film area of plasma assisted chemical vapor deposition forming machine serves as the active region, through a series of regional collaboration. TFT protective layer formed in thin film region after chemical vapor deposition of Yang Tai, then dug angle hole, then sputtering a layer of indium tin oxide film (ITO), and the yellow light and etching process for forming pixel area in this region pattern circulation process after TFT etching process for the main steps.

According to the structure of TFT gate, source, drain electrode deposition sequence, can be roughly divided into four categories, as shown in figure TFT, is currently the yield is to reverse the stacked structure, because it has the advantages of simple structure and easy fabrication is widely used by the manufacturer TFT, because the process is different Channel: 1) after the etching of TFT 2) after retaining TFT or called production process key step three layer structure of TFT.TFT is N after etching channel end of amorphous silicon. In order to form a gate control channel, which is generally a dry etching method, and the causes of the leakage current may in the dry channel when easy to cause the matter out of the deer residue.

Two, the formation stage of TFT-LCD

Complete all the processes in the TFT-LCD glass, with another blow with red, green and blue color filter glass, first alignment film brush, alignment treatment, coating spacer and frame glue and two pieces of glass on the next group, cutting the lobes edge grinding and chamfering, cleaning then, liquid crystal injection and sealing, and finally inspection and electrical test, one of the most important steps are as follows:

1.alignment treatment

The whole process is brush grinding processing, mainly in volume franella brush metal rollers grinding after firing the alignment film, liquid crystal molecules can advance towards a certain direction.

2,.the spread of the spacer

The purpose is to obtain a uniform thickness of the liquid crystal, it dispersed high density can be more uniform CELL GAP, if there is leakage when the spacer will reduce the quality and lower the degree of spacer dispersion can not even CELL GAP, will also affect the quality. So the spacer evenly spraying very important, at present with type spreading method is easier to control the density, the first to do with spacer, quasi solid combination on the rubber frame should be carried out after the CF and TFT.

3,.panel cutting

With TFT and CF combined with quasi sealed super hard steel roller cutting and fracturing way to get every piece of panel.

After the Sui of shape, size and shape of the narrow frame module, the development of thin, ultra hard steel can not meet the requirements after the decoration, laser cutting way.

4,.liquid crystal injection

I will use glass frame fixed on the machine, liquid crystal injection with straight type liquid crystal injection method, should pay attention to avoid spacer and broken spacer) together.

II first let the CELL internal vacuum, the CELL liquid crystal injection port into the liquid crystal tank, and then the vacuum for the action of nitrogen, the pressure difference between the inside and the outside of the liquid crystal will be injected into the CELL

The shape and arrangement of liquid crystal injection

Spacer broken: is injected into the liquid crystal, liquid crystal particles, two pieces of glass pressing crack.

Spacer aggregation: when injected into the liquid crystal, liquid crystal has a large and small particles, because the liquid crystal is too small in the middle of the tank will be rolling, a few come together to form a Spacer aggregation

5.visual inspection and electrical measurement

Check the LCD CELL project is the main liquid crystal injection after alignment inspection, early lighting inspection and final inspection and these checks are carried out by visual.

Three, LCD module formation stage

The process of the module includes the connection of the chip glass substrate, the printed circuit, the pressing and sealing, the shell and the backlight assembly and detection.

How liquid crystal technology works:

Liquid crystal through different arrangement mode to work, using multi area vertical alignment mode (MVA mode) and in-plane switching mode (IPS mode) to make the liquid crystal display horizontal viewing angle has reached 170 degrees. The MVA mode also shortens the response time to 20ms.

(a) TN+Film

From a technical point of view, TN+Film solutions is one of the most simple, TFT display manufacturers will be used to display the old LCD twisted nematic (TN:Twisted Nematic) technology, combined with TFT technology, and TN+Film technology. This technology is mainly through the display cover a special film, to expand the visual angle can be viewed from 90 degrees to about 140 degrees. TN+Film is the same as the standard TFT display is through the arrangement of liquid crystal molecules to achieve the control of the image, it covers the upper surface of a thin film to increase the visual angle. However, the relative weakness of the TFT display and slow reaction time these shortcomings have not changed. So TN+Film is not a good way to do this, in addition to its cost is not cheap, there is no merit.

(b) IPS (In-Plane Switching)

IPS is the abbreviation of In-Plane Switching, which means a flat switch, also known as Super TFT. First developed by Hitachi (Hitachi), NEC and Nokia are now using this technology to monitor. The technique is different from the twisted nematic display (TN-Film), which is different from the basic arrangement of the liquid crystal molecules, as shown in Figure 7, when the voltage is added to the liquid crystal molecules and the substrate parallel arrangement. The visual angle using the technology to display up to 170 degrees, the visual angle has the same cathode ray tube is, but this technology also has disadvantages: in order to allow the liquid crystal molecules are arranged parallel electrodes not like twisted nematic display (TN-Film), in the two layer of substrate are only on the low substrate layer on a direct result of this is the display brightness and contrast decreased significantly, in order to improve the brightness and contrast, only enhance the brightness of the backlight source. As a result, the reaction time and contrast with the ordinary TFT display is more difficult to improve. So the technology does not seem to be the best solution.

(c) MVA (Multi-Domain Vertical Alignment)

MVA multi area vertical alignment technology, is developed by Fujitsu (Fujitsu) company, from a technical point of view, it takes into account the visual angle and reaction time two aspects. Found a compromise solution. MVA technology makes the visual angle up to 160 degrees, although not as good as IPS can reach 170 degrees of visual angle, but it ` s still good, because the reaction time of this technology can provide better contrast and less.

The M generation in MVA means "multi-domains" - the meaning of multi area. As shown in Figure 8, the purple protrusions (protrusion) form the so-called regions. Fujitsu currently produces MAV monitors in general there are such 4 regions.

VA is the abbreviation of "vertical alignment", which means vertical arrangement. However, literally, there will be some misunderstanding, because the liquid crystal molecules are not as shown in the diagram (protrusion) is completely vertical. Please see Figure 8 schematic diagram shown in black. When an electric field is generated by an electric voltage, the liquid crystal molecules are arranged in parallel with each other, so that the backlight light source can pass through, and the light rays can diverge in all directions, thereby enlarging the visual angle.

In addition, MVA also provides faster response time than IPS and TN+Film technology, which is very important for good video recovery and residual visual effects. MVA LCD contrast has also been improved, but also with the visibility of the changes.

In the optical compensation bending technology (OCB) field series developed on the basis of full color (FSFC) LCD technology not only to cancel the account for 1/3 of the cost of the color filter (CF), can improve resolution of 3 times, the transmittance is increased 5 times, while simplifying the process, reducing the cost. With the development of color film technology and backlight technology, the color reproduction ability of TFT screen is even more than CRT. As the comprehensive performance of the main technical indexes of commodity display TFT screen in various display devices is the most outstanding, especially the improvement of mass production technology of TFT screen products, many varieties, multi series of product development space, the scope of application of nothing.

The following is a typical sequence of TFT screen:

Where VSYNC is the frame synchronization signal, VSYNC each issued 1 pulses, means that the new 1 screen video data began to send. While the HSYNC is a row synchronization signal, each HSYNC pulse indicates that the new 1 lines of video data are sent. The VDEN is used to indicate the effectiveness of the video data, VCLK is used to lock the video image data.

And in the frame synchronization and synchronization of the head and tail are must have a flyback time for VSYNC, for example, before the flyback time is (VSPW+1) + (VBPD+1), after the flyback time is also similar (VFPD+1); HSYNC. This is the original CRT display timing requirements due to the electron gun deflection takes time, but later became the actual industry standard, as well as the TFT screen appeared later in time on CRT compatible, was also used to control the timing.

Design of TFT-LCD driving circuit

The VGA interface signal is converted to the analog LCD display driver circuit, using ADI high performance DSP chip ADSP-21160 to achieve the main function of the drive circuit.

Hardware circuit design

AD9883A is a high-performance three channel video ADC can simultaneously achieve real-time sampling of RGB tricolor signal. The system uses 32 bit floating-point ADSP-21160 chip to process the data, can complete the real-time gamma correction, time base correction, image processing and optimization, to meet the performance requirements of the system. ADSP-21160 has 6 independent high speed 8 bit parallel link junctions, which are connected to the front end of the ADSP-21160 analog to digital conversion chip AD9883A and the back-end digital to analog conversion chip ADV7125. ADSP-21160 has a super Harvard architecture, support single instruction operand (SIMD) mode, using programming language to achieve the efficient compilation of real-time processing of video signal, not because of long time delay and data processing.

System hardware schematic diagram shown in figure 1. The system can complete the input and output by using different chain junctions. Analog video signal from AD9883A to complete analog to digital conversion. AD9883A is a three channel ADC, so the system can complete monochrome video signal processing, color video signal processing can also be completed. The sampled digital video signal is input to the ADSP-21160 through the chain intersection, and the output signal is output to ADV7125 after different processing. ADV7125 is a three channel DAC, can also be used to deal with color signals. The output video signal to the gray voltage generation circuit, the drive voltage required to drive the LCD screen. ADSP-21160 also has a universal programmable I/O flag pin, which can be used to accept external control signals, send control information to the system and its modules in order to make the whole system work stably and orderly. For example, ADSP-21160 provides the necessary control signals for the grayscale voltage generation circuit and the LCD screen. In addition, the system also set up a number of LED lights, used to visually indicate the system hardware and DSP internal procedures for each module working state.

Figure 1 Schematic diagram of system hardware

This design uses the flash boot method to load DSP program files, flash memory has a high cost performance, small size, low power consumption. This system is due to flash memory DSP program, but also save the corresponding lookup table data and display data in different parts of default gamma value, so the selection of ST large capacity M29W641DL, can save the program code, and can save the necessary data and information.

Figure 2 is the DSP and flash interface circuit. Because the use of 8 bit flash boot mode, so ADSP-21160 address line should use A20 to A0, the data line is D39 ~ 32, read and write flash memory and chip select signals were received corresponding pins.

Figure 2 DSP and Flash interface circuit

System function and Realization

This design uses ADSP-21160 to complete the gamma correction, time base correction, clock generator, image optimization and control signal generating function.

Gamma correction principle

In LCD, the image data of IC/LSIDAC drive signal of linear change, and electro-optic properties of liquid crystal is nonlinear, so to adjust the voltage of liquid crystal, liquid crystal display brightness to meet the linear, gamma (gamma correction). Gamma correction is an important process to realize the visual information of the original object or the original image. The use of look-up table to compensate the electro-optical properties of liquid crystal gamma correction method can make the liquid crystal display system has the ideal transfer function. The input and output curve of the LCD system is S. The function of the gamma table is to convert the ADC signal into an inverse S shape nonlinear transformation, and finally, the input and output curves of the liquid crystal display system can meet the actual requirements.

LCD gamma correction pattern as shown in Figure 3, the LCD electro-optic characteristic curve, the brightness of LCD curves and voltage conversion chart.

Figure 3 Schematic diagram of LCD gamma correction

Gamma correction implementation

This paper used scientific gamma correction processing technology, the digital video signals are respectively three color digital gamma correction (or correction of the simulated three color video signals were gamma). At the same time, the gamma correction is not lost, so that the full color display image is more vivid, more realistic, more clear.

A monochromatic gamma adjustment process as shown in Figure 4, the other two colors are the same. For example, ADSP-21160 is the first to look up the table to get the gamma adjustment coefficient (gamma value). Then, according to the gamma value and the input display data, the second look-up table is obtained. The first look-up table gamma value is input to the control module through the external control signal for the first time to check table. 8 digit display data signal table of numbers from 0 to 255 gray scale display data (gamma correction).

Figure 4 monochromatic gamma adjustment process

Image optimization

In order to improve the image quality, ADSP-21160 is designed to optimize the image effects and stunt module, many in the simulation processing can not work can be carried out in digital processing, for example, two-dimensional digital filtering, contour correction, frequency tuning, accurate color detail compensation matrix (linear matrix circuit), black correction, G correction, the Kong Lan correction gain adjustment, black level control and stray light compensation, contrast adjustment, these treatments are to improve the image quality.

Is the size of the digital effects, the video signal itself and position change of brightness and color change of signal digital processing, it can make the image into various shapes, on any screen zoom, rotation, which is unable to realize the simulation of the stunt. Filters can also be designed to filter out some of the interference signal and noise signal, so that the image clarity is higher, better reproduction of the original image. All the signals and data are stored in the DSP, which is generated by the clock module and control module.

Time base correction and control system

Due to the internal ADSP-21160 module functions and different time, there are some delay between each module, it needs digital time base correction, the final output data memory can be strictly aligned without overlapping or discontinuous information. Digital time base correction is mainly used for correcting the video signal line, field synchronization signal timing error. First of all, it will be a correction signal to the time base signal as a reference to memory. Then, the TFT-LCD time base signal as the reference can be read out, the time base error smaller video signal. At the same time it also added other features of the video signal can adjust brightness, chroma, saturation, at the same time, phase, wave load phase adjustment, and has the function of clock.

The control module is mainly responsible for the timing control logic circuit to drive the management and operation of various functional modules, such as display data memory management and operation, will be responsible for the display data and instruction transmission parameters in place, will be responsible for the contents of the register parameters into the corresponding display logic. The internal signal generator generates a control signal and an address, and generates a control signal according to the horizontal and vertical display and the blanking counter value. In addition, it can receive external control signals to achieve human-computer interaction, so that the circuit is more powerful, more flexible.

In addition, the ADSP21160 also designed the I2C bus control module, analog I2C bus to work, for the external I2C interface device with SCLK (serial clock signal) and SDA (two-way serial data signal). The operating state of the simulated I2C is shown in Figure 5 and figure 6.

Figure 5 serial port read / write timing

Figure 6 serial interface - typical byte transfer

System software implementation

In the software design, as shown in Figure 7, the Matlab software is used to calculate the correction value, and stored in the form of a look-up table, for the timing call. Power on the system, the first to complete a series of ADSP-21160 register settings, so that DSP can work correctly and effectively. When the ADSP-21160 receives an effective video signal, the gamma value is determined according to the external control information. In order to adapt to different TFT-LCD screen to display the video signal, the system can adjust the gamma value to adjust the display effect to the best. As shown in Figure 4, table look-up of previously stored files, to obtain the required correction value, and then waiting to handle the next step. According to the characteristics of the video signal and the needs of the user, the system can also achieve some image optimization and special effects, such as the two-dimensional digital filtering, contour correction, gain adjustment, contrast adjustment, etc.. These operations can be selectively used by the user. The use of ADSP-21160 can also achieve image flip, stagnation and other stunts. Finally, the digital time base correction, mainly used for correcting the video signal line, field synchronization signal timing error, the final output data memory can be strictly aligned without overlapping or discontinuous information. In addition to the above features, ADSP-21160 also provides the necessary control signals for the grayscale voltage generation circuit and the TFT-LCD screen in accordance with the timing control signal. In addition, the ADSP-21160 can also be set to drive the general configuration of the I/O pin LED lamp, display system operating state.

Figure 7 software flow chart