Liquid Crystal Materials
 

Experimental material physics

5A1710

 

 

 

Daniel Lundström

Jan Yilbar

 

- Table of contents -

 

Chapter

 
 
1. Introduction and historical overview  
  

2. Different liquid crystal materials, briefly

 
 

2.1 STN and TN display materials

2.2 FLC display materials

2.3 PDLC display materials

2.4 ECB display materials

2.5 NCPT display materials

 

3. Physical LCD properties 3.1 Active matrix LCDs 3.1.1 Direct – Active addressing 3.2 Passive matrix LCDs

3.3 Twisted nematic devices

3.3.1 How polarizers work

3.3.2 Twisted nematic liquid crystal cells

3.4 Supertwisted nematic LCDs

3.5 Ferroelectric liquid crystals

3.5.1 What are ferroelectric liquid crystals ?

3.5.2 What are surface-stabilized ferroelectric liquid crystals ?

3.6 Polymer-dispersed LCDs

3.7 Ferroelectric liquid crystal textures

 

4. Status and prospects for the future for the different LCD materials 4.1 Twisted nematic LCDs

4.2 Fast supertwisted nematic LCDs

4.3 Vertically aligned nematic LCDs

4.4 Ferroelectric LCDs

 

5. Active or passive LCD displays ?

5.1 Active matrix (TFT) LCDs

5.2 Passive matrix LCDs

5.3 Comparison tables of passive matrix LCD technologies

 

6. Manufacturing of LCDs and TFTs

6.1 Manufacturing in general

6.2 Manufacturing equipment

6.2.1 PECVD

6.2.2 Sputtering

6.2.3 Lithography

6.2.4 Wet processing and cleaning

6.2.5 Dry etching

6.2.6 Drivers and packaging

 

7. Suppliers and markets

 

8. Some commercial products and prototypes


1. Introduction and historical overview

 

Liquid crystal materials were first discovered in 1888 by an Austrian botanist, F. Renitzer. However those liquid crystals were not suitable for any commercial usage, and it is only 25 years ago since the first material suitable for electronically driven displays, was developed. The first room-temperature nematic liquid crystal was observed in the late 1960s. Unfortunately this crystal had quite a short temperature range as it was affected by impurities. Occasionally in homologous series the temperature range could reach from –40 to +100 degrees Celsius. Unfortunately these mixtures were very unstable and they possessed a negative dielectric anisotropy not useful in the twist cell.

The major breakthrough came when cyanobiphenyl materials were discovered a few years later. The more stable phase had a large positive dielectric anisotropy as well as a strong birefringence nearly ideal for the twist cell.

During the 1970s and 1980s several liquid crystal compounds and phases were discovered, primarily by the industry, but also in several research programs on liquid crystal materials in colleges and universities around the world.

The ferroelectric chiral smecic (FLC) phase was discovered in 1975 and proved to have a unique form of ferroelectricity. The first display based on the FLC phase was actually patented in 1980. Another example of new liquid crystal phases also discovered during this intense research period, are the forms of polymer dispersions. Also a new effect, the electroclinic effect, was discovered during this period and is now being carefully studied for possible future display applications.

Lately several new materials has been discovered such as the retardation film which is extremely important for the supertwisted nematic (STN) and twisted nematic (TN) displays.

Most of this research is situated in Japan due to strong manufacturing capability and high research funds. The most frequently used liquid crystalline phase used today in display devices is the nematic phase. The phase is used both in the TN cell as well as in the active matrix (AM) TN cell. About 60 % of all nematic materials supplied by Merck-Japan, which is the biggest producer of such materials, goes to these applications. The active matrix TN cell is expected to grow substantially during the next 5 years as this technology dominates the manufacturing industry today. Other types of rapidly growing display types are the electrically controlled birefringence (ECB) and polymer-dispersed liquid crystals (PDLCs). Due to the relatively recent technology discovered in producing these cells, they have not yet reached a fully commercial usage.


2. Different liquid crystal display materials

 

2.1 STN and TN display materials

 

There has been considerable research over the past 20 years in the development of low-molecular-weight nematic compounds with improved characteristics such as lower viscosity, increased temperature range, larger birefringence and dielectric anisotropy. However the TN and STN cell is about to get as far as research can provide, and only minor changes like improvements of existing cells may be possible. Instead new cells and compounds are being examined such as the FLC phase and the PDLC material. These materials shows substantially improved features in certain areas such as speed and brightness.

Unfortunately the low response speed, lower power consumption and low cost still makes TN and STN materials suitable for slow displays. Some basic properties are compared in figure 2.1 and 2.2 below.

 
 
Properties and parameters For High Contrast For Fast response
     
Electric constant ratio K,,/K.. Large Small
Dielectric anisotropy, D e  Small  
Twist angle Large (220-260° ) Small
Birefringence, D n    
Viscosity   Low
Pretilt    
Resistivity   High
Cell spacing    
Threshold voltage    
Figure 2.1 - Nematic materials properties and display parameters for STN displays considering response time and contrast

 
 
Properties and par. Passive Matrix Active Matrix TFT Active Matrix MIM STN
         
Elec. Cons. ratio K,,/K..   Small    
Dielectric anisotropy, D e  Large Large Large  
Birefringence, D n -0.1 - 0.16 Low, ~0.08 - 0.1 High -0.15 - 0.18 ~0.12 - 0.15
Viscosity ~20 – 30 CST ~15 - 23 CST ~15 – 23 CST ~16 - 23 CST
Pretilt ~1°  ~2 - 3°  ~2 - 3°  5 - 10° 
Resistivity 1011 W cm 1013 - 1014 W cm 1013 - 1014 W cm 1012 W cm
Cell spacing 8 - 10 m m 5 - 7 m m 5 - 7 m m 4 – 7 m m
Threshold voltage 0.9 - 1.8 V 1.5 - 2.0 V 1.5 - 2.0 V 1.2 - 2.0 V
Voltage holdingratio   >98%    
 Figure 2.2 - Nematic materials properties and display parameters for a TN and STN cell

 

2.2 FLC display materials

 

The FLC display materials show improved switching times and bistability, however the latter feature permits the use of the LCD as a passive matrix followed by reduced display cost. This has made the commercial product very difficult to fabricate, also the small cell spacing makes it difficult to manufacture. These are easily destroyed by mechanical shock due to unstable molecular anchoring at the surface. The desired gray scale is also not yet achieved and currently Canon is working on the problem as well as with the temperature and viscosity sensitivity, which lowers the response time.

There are also several antiferroelectric materials under research in Europe and USA but as the contrast ratio isn’t to satisfactory yet, manufacturers consider using these materials unsafe until the problems are solved. Also worth mentioning are that the antiferroelectric materials have improved stability and this due to the soft layers.

 

2.3 PDLC display materials

These materials are relatively new on the market and their improved brightness (as no polarizers are needed) as well as their simplicity to fabricate, makes them very interesting for commercial purposes. They are most interesting for use in projection television, and this cause many companies to foresee their use in direct-view displays. The PDLCs require an active matrix in order to reach a high resolution, and because of this most research programs focus on the effort to lower the drive voltage and increase the resistivities required for the AM TFT. Both aqueous and non-aqueous polymers is being used for these applications. To reach the high resistivity needed, both the polymer and liquid crystal material must be of high purity and hysteresis may turn out be a problem. The most desired nematic materials are those with high dielectric anisotropy and birefringence. An example of this is the fluorinated tolans (see figure 2.3 below), which exhibits these high values.

Figure 2.3 – Fluorinated tolans

 

2.4 ECB display materials

Materials used for ECB LCDs are a very small part of the liquid crystal materials market and normally large dielectric anisotropy and birefringence materials are desired. Vertically aligned nematics (VANs) however require a negative dielectric anisotropy.

 

2.5 NCPT display materials

Displays based on these materials require a suitable bias voltage and possesses a bistable memory, needing only a passive matrix to work. It uses the light-scattering principle, providing a bright projection display without polarizers. This system has several advantages over the STN projection system: It does not degrade in the center of the picture as some STN projection systems do, manufacturing cost, according to Fujitsu, is lower and high definition is also possible. With these materials, it is now possible to manufacture systems with up to 7M pixels or even more.

A key material to the success of the NCPT is a chiral material that possesses a temperature-independent pitch length over a wide temperature range. The length of this pitch is approximately 1 micron in a cell with an inner electrode spacing of 5-6 microns. However some limitations in the contrast makes research setting high efforts in using polymer gel dispersions in the NCPT cell in order to eliminate this.


3.0 Liquid Crystal Properties

 

Liquid crystal displays (LCD’s) offer several advantages over traditional cathode-ray tube displays. LCD’s are flat, and they use only a fraction of the power required by CRT’s.

They are easier to read and more pleasant to work with for long periods of time than most ordinary video monitors. One should also now that there are several tradeoffs, such as limited view angel, brightness, and contrast, not to mention high manufacturing costs. As research continues, this limitations are slowly becoming less significant.

Today’s LCD’s come mostly in two flavors passive and active. The less expensive passive matrix displays trade off picture quality, view angel, and response time with power requirements and manufacturing costs. Active matrix displays have superior picture quality and viewing characteristics, but need more power to run and are much more expensive to fabricate.

Liquid crystal displays show great potential for the future and there are improvements to be made. The next question is what are the physical limits of this technology ? To answer this question we need to explain liquid crystal in general to determine what characteristics it has and what makes it so appropriate for use in displays. We need to examine in detail the two common kinds of liquid crystal displays passive and active matrix to see how each works.

What is liquid crystal? There are three common states of matter that we know about: solid, liquid and gas. Liquid crystal is a fourth "state" that certain kinds of matter can enter into under the right conditions. The molecules in solids exhibit both positional and orientational order, in other words the molecules are constrained to point only certain directions and to be only in certain positions with respect to each other. In liquids the molecules do not have any positional or orientational order, the direction the molecules point and positions are random.

The liquid crystal "phase" exists between the solid and liquid phase, the molecules in liquid crystal do not exhibit any positional order, but they do possess a certain degree of orientational order. The molecules do not all point the same direction all the time. They tend to point more in one direction over time than other directions. This direction is referred to as the director of the liquid crystal. The "amount" of order is measured by the order parameter of the liquid crystal. This order parameter is highly dependent on the temperature of the sample. See figure 3.1, a typical order vs. temperature relationship. Tc is the temperature of transition between the liquid crystal and the liquid states.

 

Figure 3.1 – Typical order vs. temperature relationship

 

Not all substances can have a liquid crystal phase. Molecules that tend to be candidates for having the phase are long and have a rigid central region and ends that are slightly flexible. Liquid crystals are essentially more like liquids than they are like solids. This is evident from the latent heat of transition (the amount of energy needed for a phase transition to occur).

Liquid crystals may be nematic, smectic or cholesteric, depending on the arrangement of the molecules they are also birefringent, meaning that it possesses two different indices of refraction. One index of refraction correspondents to light polarized along the director of the liquid crystal, and the other is for light polarized perpendicular to the direction. Nematic liquid crystal is a common type and has a thread-like formation. The molecular orientation (and hence the material’s optical properties) can be controlled with applied electric fields. See figure 3.2 and 3.3 below.

 

Figure 3.2 & 3.3 – nematic liquid crystal

 

Smectic liquid crystal has a soapy texture and are found at lower temperatures than the nematic. In the Smectic A phase, the molecules are orientated along the layer normal, while in the Smectic C phase they are tilted away from the layer normal. See figure 3.4 below.

 

 

Figure 3.4 – smetic phases.

 

Chiral nematic (or cholesteric) liquid crystal exhibits a twisted structure, the director rotates about an axis as you move through the material. This helical structure is exploited in making modern flat-panel displays. Of particular interest are tilted phases of chiral molecules , which possess permanent polarization’s and are ferroelectric. These ferroelectric liquid crystals respond mush more quickly to applied fields than nematics do and can be used to make fast, bistable electro-optic devises called Surface-Stabilized Ferroelectric Crystals (SSFLC’s). Chiral nematic liquid crystal is birefringent in a different way. Supposing that the helical structure is aligned with the direction of propagation of the light, circularly polarized light will travel through the crystal at different speeds depending on whether it is right-circularly polarized or left-circularly polarized (referring to the direction the polarization rotates around the axis of propagation ). This is called circular birefringence, and it is exhibited in chiral nematic liquid crystal because of the helical structure the molecules form.

The circular birefringence is highly wavelength dependent, so light of different colors gets modified in different amounts. Circularly polarized light can be thought of containing tow components, one of left- and one the right-circularly polarized light. If the light is linearly polarized, these components are equal. If we send linearly light through a chiral nematic liquid crystal, the component of circularly polarized light that matches the chirality of the crystal structure will travel faster than the other component, having the ultimate effect that its polarization will rotate faster with respect to the other component’s polarization. When the two components emerge, their polarization will again rotate at the same rate and the light will again be linearly polarized, but since one of the rotations got ahead of the other, the light will now be linearly polarized along a different angel. What the new angel is will depend on how far out of "circular phase" the two components were, which is directly dependent on thickness of the liquid crystal. This effect is called the optical activity (it is a measure of the change in polarization angel per unit thickness, and it is heavily wavelength dependent. See figure 3.5 below for visualisation of optical activity. The two arrows represent the directions of the polarization of the components of left and right circularly polarized light at a particular point in time. After passing through the liquid crystal (right), the two components sum to create linear polarization in a different direction.

Figure 3.5 - optical activity

 

3.1 Active matrix display

 

Active matrix liquid crystal displays are standard on most new laptop computers. Two properties of liquid crystal is used as tiny switches to turn picture elements (pixels) off and on. Fist the crystals are transparent but can alter the orientation of polarized light passing through them. Second, the alignment of their molecules (and their polarization properties) is changed by applying an electric field.

In a color display the liquid crystals are held between two glass plates or transparent plastics. These plates are usually manufactured with transparent electrodes, typically made of indium tin oxide, that makes it possible to apply an electric field across small areas of the film of liquid crystal.

The outsides are coated with polarizing filters. Only light with a perpendicular polarization can pass through these filters. (a). See figure 3.6 to the right.

Inside the plates are transparent electrodes and color filters, which form very small picture element regions called subpixels. A grouping of a red, a green and a blue subpixels defines the color that the pixel transmits. Fluorescent backlighting illuminates a display from the rear. In pixels that are off, light passes through the rear polarizing filter, the crystals (b) and the color filters, only to be blocked (absorbed) by the front polarizing filter. To the eye , these pixels appear dark. When a pixel is turned on, the liquid crystals reorient their position, and they in turn repolarize the light so that it can pass through the front polarizing filter (c).

The active matrix provides a superior method of electronically addressing (turning on ) an array of pixels. For an image to appear on screen, one row of pixels receives the appropriate voltage. At the same time, software in the computer dictates that voltage be applied to those columns holding active subpixels. Where an activated row and column intersect, a transistor turns on a subpixel electrode, generating an electrical field that controls the orientation of the liquid crystal. This process repeats sequentially for each of the rows in figure 3.6 above, an advanced display, which can take 16 to 33 milliseconds.

 

3.1.1 Direct - Active Addressing

 

In addition to the issues of display design concerning direct addressing there is a direct connection to every element in the display, which is good since we would have direct control over the pixels, but it is also bad because in large displays there could be thousands or even millions of pixels that would require separate connection’s. The method used in the vast majority of large modern displays is multiplexing. In this method, all the pixels across each row are connected together on the plate on one side of the liquid crystal film, and all the pixels in each column are connected on the opposite side. The rows are then "addressed" serially by setting all of the column voltages separately for each row and turning on the row voltages in sequence. This scheme leads to very low voltage selection ratios and, low contrast.

A new method called active addressing has been developed to tackle this problem. In this scheme, several rows are addressed simultaneously in different patterns corresponding to orthogonal functions.

 

3.2 Passive matrix displays

 

Perhaps the most common kind on the market today is the twisted nematic display. In this display, the liquid crystal molecules lie parallel to the glass plates, and the glass is specially treated so that the director of the crystal is forced to point a particular direction near one of the plates and perpendicular to that direction near the other plate. This forces the director to twist by 90° from the back to the front of the display, forming a helical structure similar to chiral nematic liquid crystals. Some chiral nematic crystal is added to make sure all of the twists go the same direction.

This thin film of twisted nematic liquid crystal is circularly birefringent. When linearly polarized light passes through, the optical activity of the material causes the polarization of the light to rotate by a certain angle. The thickness of the film, typically around 6 or 8 micrometers, can be controlled to produce a rotation of the polarization of exactly 90° for visible light. Therefore, when the film is placed between crossed polarizers, this arrangement allows light to pass through. However, when an electric field is applied across the film, the director will want to align with the field. The crystal will lose its twisted structure and, consequently, its circular birefringence. Therefore, linearly-polarized light entering the crystal will not have its polarization rotated (it is only rotated very slightly), so light will not be able to penetrate through the other polarizer. When the field is turned off, the crystal will relax back into its twisted structure and light will again be able to pass through. In some displays, the polarizers are parallel to each other, thus reversing the on and off states. If red, green, and blue colored filters are used on groups of 3 pixels, color displays can be created.

The twisted nematic system coupled with multiplexed addressing is used in many of today's so-called passive matrix LCD’s. Even though it is the most popular kind, it does have a number of disadvantages as well. First of all, the use of polarizers reduces the potential brightness since they allow less than half of the light incident on the display to pass through. The effective viewing angle of the display can be very small because the optical activity and the polarizers are tuned to work best only on light that is propagating perpendicular to the display. The voltage-brightness response curve is often not very sharp, leading to reduced contrast. The display is also affected by crosstalk where voltage meant for a certain pixel can leak through ``sneak paths'' to nearby pixels, causing a ghosting effect. And finally, switching speed of the liquid crystal is often not as high as might be desired---typically around 150 milliseconds. Lower switching speeds are necessary when doing multiplexing since one want the crystal to respond to voltages over the whole scanning cycle to reduce flicker, but such low speeds make passive matrix displays unusable for many applications (such as full motion video).

The degree of multiplexing in twisted nematic displays has a huge influence on the contrast of the display. The liquid crystal will respond to the average voltage applied to it over a certain period of time, depending on its viscosity. Assuming the liquid crystal responds to voltages over one frame period, we can calculate the average voltage felt by a pixel that is on and a pixel that is off. A row gets a voltage of 1 if it is being addressed and 0 if it is not. During each row pulse, the column voltages are set according to which pixels in that row are on or off. If a certain pixel is on, it receives a column voltage of -1, otherwise it is 0.

It is now possible to calculate the average voltage felt by a specific pixel assuming that other pixels in the same column have equal probability of being on or off. The calculation will show that the average voltage is of a 1/2 over a frame period. Since the voltage-brightness response curve of the crystal is not very steep these voltages will not lead to as high of a difference in brightness as would be possible if each pixel received either a full 2 volts or 0 volts continuously over the entire frame period. See figure 3.7 below for the response curve for lighttransmission vs. voltage.

 

Figure 3.7 - Response curve of a twisted nematic LCD. The range of voltage (and therefore brightness) felt by on and off pixels is much less than the full range possible.

 

Ideally, one would want the response curve to have near infinite slope between the average on and off voltages, but this is very difficult to achieve. A certain minimum ratio is needed to get good contrast. Since the ratio is a function of the number of rows, the more rows, the less contrast the display will have.

Active matrix displays solves this problem by putting a full switch at every pixel in the entire

Display (as described above). This special element ``actively'' drives the voltage to the pixel continuously over the frame period. Essentially, a thin-film transistor at each pixel allows the effects of the column voltages to be felt only by the row that is being addressed. Therefore, outstanding contrast is possible, and a faster liquid crystal substance can be used since it is not necessary for the pixel to respond to the average voltage over a whole frame period. Crosstalk is also cut to a minimum. With increased contrast and switching speed, active matrix displays are more than able to handle many kinds of information, including full-motion video which requires a minimum switching speed of 50 milliseconds.

We have now described the many limitations of passive matrix displays and how active matrix displays are able to over come those problems. However active matrix displays are extremely expensive to manufacture and the process of creating large sheets of thin-film transistors is far from perfect. Therefore, much research has been done to try to improve the characteristics of the more affordable passive matrix displays. In some very recent cases, the quality of passive matrix displays has gotten very close to that of active matrix displays.

 

3.3 Twisted nematic devices

 

The twisted nematic, invented by Schadt and Helfrich and first demonstrated by Fergason in 1971, represents the first successful application of liquid crystals. In this archetypal device, the liquid crystal is confined between crossed polarizers and its birefringence controlled electrically. This basic principle is explained in more detail below.

 

3.3.1 How polarizers work

 

When unpolarized light passes through a polarizing filter, only one plane of polarization is transmitted. Two polarizing filters used together transmit light differently depending on their relative orientation. See figure 3.8 for a complete review of polarizers.

Figure 3.8 - Polarizing filters in an isotropic medium (such as air). The system's optical throughput depends on the relative orientation of the polarizer and analyzer.

 

For example, when the polarizers are arranged so that their planes of polarization are perpendicular to each other, the light is blocked (Fig. 3.8a). When the second filter (called the analyzer) is parallel to the first, all of the light passed by the first filter is also transmitted by the second (Fig. 3.8b).

 

3.3.2 Twisted nematic liquid crystal cells

 

A Twisted Nematic cell is made up of :

See figure 3.9 for polarizer and analyser in a twisted nematic liquid crystal.

 

 

Figure 3.9. Twisted nematic device geometry. The polarizer and analyzer, which are arranged parallel to the director orientation at their adjacent glass plates, are oriented at 90 degrees to each other.

 

The surfaces of the transparent electrodes in contact with the liquid crystals are coated with a thin layer of polymer, which has been rubbed or brushed in one direction. The nematic liquid crystals molecules tend to orient with their long axes parallel to this direction. The glass plates are arranged so the molecules adjacent to the top electrode are oriented at a right angle to those at the bottom (Fig. 3.9a).

Each polarizer is oriented with its easy axis parallel to the rubbing direction of the adjacent electrode (so the polarizer and analyzer are crossed).

In the absence of an electric field, the nematic director undergoes a smooth 90 degree twist within the cell (hence the name "twisted" nematic liquid crystal). Unpolarized light enters the first polarizing filter and emerges polarized in the same plane as the local orientation of the liquid crystals molecules. The twisted arrangement of the liquid crystals molecules within the cell then acts as an optical wave guide and rotates the plane of polarization by a quarter turn (90 degrees) so that the light which reaches the second polarizer can pass through it. In this state the liquid crystal cell is transparent.

When a voltage is applied to the electrodes, the liquid crystal molecules tend to align with the resulting electric field E (Fig. 3.9b) and the optical wave guiding property of the cell is lost. The cell is now dark, as it would be without the liquid crystals present (as in Fig. 3.9a). When the electric field is turned off, the molecules relax back to their twisted state and the cell becomes transparent again.

For applications such as digital watches and calculators, a mirror is used under the bottom polarizer. With no voltage applied, ambient light passes through the cell, reflects off the mirror, reverses its path, and reemerges from the top of the cell, giving it a silvery appearance. When the electric field is on, the aligned liquid crystals molecules do not affect the polarization of the light. The analyzer prevents the incident light from reaching the mirror and no light is reflected, causing the cell to be dark. When the electrodes are shaped in the form of segments of numbers and letters they can be turned on and off to form an alphanumeric display. Passive displays such as these can function solely using ambient lighting, which makes them ideal for batteryoperated devices.

 

3.4 Super-twisted nematic LCDs

 

Twisted nematic displays rotate the director of the liquid crystal by 90° , but super-twisted nematic displays employ up to a 270° rotation. This extra rotation gives the crystal a much steeper voltage-brightness response curve and also widens the angle at which the display can be viewed before losing much contrast. With the sharper response, it is possible to achieve higher contrast with the same voltage selection ratio. Therefore, the degree to which multiplexing is possible is greatly increased. The largest common super-twist displays have up to 500 rows. See figure 3.10 for voltage vs. light transmission in super-twisted nematic LCDs.

Figure 3.10 - Response curve of a super-twisted nematic LCD compared with the response of an ordinary twisted nematic LCD. The steeper response exhibited by the super-twist display makes a wider range of brightness possible.

 

In dual-scan super-twist displays, two essentially separate displays are placed adjacent to each other and scanned simultaneously by separate electronics. This cuts the number of rows in each part by half. The reduced multiplexing in each half means higher selection ratios and consequently, even better contrast. In a double-cell super-twist display, there are two cells on top of each other with helical directions opposite each other. Since the circular birefringence is wavelength dependent as mentioned earlier , this provides a way to undo the wavelength disturbance created when passing through only one layer, thereby providing slightly improved contrast and a true black and white display. (as also mentioned earlier if red, green, and blue colored filters are used color displays can be created.

 

3.5 Ferroelectric liquid crystal

 

3.5.1 What are ferroelectric liquid crystals?

 

The Nematic and Smectic A liquid crystal phases are too symmetric to allow any vector order, such as ferroelectricity. The tilted smectics, however, do allow ferroelectricity if they are composed of chiral molecules. Figure 3.11 below shows the original ferroelectric liquid crystal, DOBAMBC, while figure 3.12 shows the modern compound W 314.

 


Figure 3.11 - DOBAMBC

 

Figure 3.12 - Modern compound, W 314

 

In the simplest case, the Chiral Smectic (Smectic C) , the average long molecular axis is tilted from the layer normal z by a fixed angle but the molecules are free to rotate on the so-defined tilt cone. The phase has a C2 symmetry axis perpendicular to both the molecular director and the layer normal. The molecules exhibit a net spontaneous polarization along this axis. The magnitude of the polarization depends on temperature, generally decreasing as the tilt angle goes to zero at the Smectic C – Smectic A phase transition. The following figure (3.13) shows the geometry of the chiral Smectic C phase.

 

Figure 3.13 – Geometry of chiral smectic C phase

 

Ferroelectric liquid crystals also exhibit a sponteous helixing of the polarization, so that over macroscopic distances (a few microns, ) the polarization averages to zero. Since the coupling of the polarization to applied fields is linear in the field, this means that Ferroelectric liquid crystals can be made to switch quickly (typically within a few microseconds) and in a bipolar manner. This makes Ferroelectric liquid crystals ideally suited to electrooptic applications. Ferroelectric liquid crystals are now included in several display technologies, the most popular of which use the surface-stabilized (SSFLC) geometry.

3.5.1 What are surface-stabilized ferroelectric liquid crystals? (SSFLC)

 

The molecular director in bulk ferroelectric liquid crystals adopts a helical structure, Noel Clark and Sven Lagerwall found in 1980 that by confining the ferroelectric liquid crystals material between closely-spaced glass plates (spaced closer than the ferroelectric helix pitch), the natural helix could be suppressed. This principle is illustrated in the polarized micrograph figure 3.14, where helix lines are largely absent in the thinner (upper right) part of the cell. Clark and Lagerwall found that the smectic layers were oriented approximately perpendicular to the glass. Furthermore, they discovered that such cells could be switched rapidly between two optically distinct, stable states simply by alternating the sign of an applied electric field.

Figure 3.14 – Polarized micrograph of ferroelectric LC

 

Figure 3.15 – Chevron SSFLC.

 

It has since been established by Clark's group that there are two commonly found layer geometries, called bookshelf and chevron, the latter being portrayed in figure 3.15 above. The electro-optic properties of an SSFLC depend strongly on the layer geometry as well as on the nature of the orienting properties of the bounding glass plates.

The surface-stabilized ferroelectric liquid crystal display shows great promise. It uses chiral smectic liquid crystal, a type in which the director rotates around a cone of directions 22.5° from the perpendicular of the layers. The hope is to make the film so thin that the director will not rotate. The planar layers are perpendicular to the glass plates of the display. The liquid crystal molecules have a permanent electric dipole perpendicular to the long axis of the molecule (the director). When a positive electric field is applied across the film, the dipole wants to align toward one of the glass plates. This makes the director point a particular direction that is parallel to the glass plates. When the opposite electric field is applied, the dipole will rotate 180° to point the other direction. The director will then be 45° away from where it used to be.

 

Figure 3.16 – Another polarized micrograph of ferroelectric LC

 

If the entering light is polarized along the director in one of these states, then no light will be able to penetrate a crossed polarizer on the other side. When the molecule is rotated, the polarization of the light will be at a 45 degree angle to the director. This means that the two components of the light, one parallel and one perpendicular to the director, will travel at different speeds through the liquid crystal. If the thickness can be controlled so that the component perpendicular to the director will be 180° out of phase from where it started, then the resulting light's polarization will be 90° rotated and will be able to penetrate through the crossed polarizer. See figure 3.17 below.

 

Figure 3.17 Modification of polarization by ferroelectric displays.

 

Before passing through the liquid crystal, the components of light polarized parallel and perpendicular to the director can be separated. After passing through the liquid crystal film, the perpendicular component is shifted in phase by 180° . The resulting linear polarization is rotated 90° from its original direction. Surface-stabilized ferroelectric displays have the distinct advantage that switching speeds are very high since electric field is necessary for both turning a pixel on and for turning it off. However, problems in development of this kind of display have persisted. The most difficult issue to resolve has been the orientation of the director throughout all the layers of the film.

 

3.6 Polymer-dispersed LCDs

 

Also under active development is the polymer-dispersed liquid crystal display. In this kind of display, very small bubbles of liquid crystal are produced inside a transparent polymer. If no field is applied, these bubbles of liquid crystal will take on many different orientations. Since at least one of the two indices of refraction of the liquid crystal and the index of refraction of the polymer must differ, there will be some reflection of incoming light. Now, if a field is applied, the director of the liquid crystal will align with the field. If the materials are carefully controlled so that the index of refraction of the liquid crystal for light polarized perpendicular to the director matches the index of refraction of the polymer, then light will propagate all the way through the material without being reflected at the bubbles.

 

Figure 3.18 Action of polymer-dispersed LCDs

 

Polymer-dispersed displays are very bright because polarizers are not needed. Also, they are easy to manufacture since the exact thickness of the film is not important. However, the characteristics of the display break down when it is viewed at an angle, and the voltage-brightness response curve is not very steep, so multiplexing ability is limited. Will liquid crystal displays ever look as good as a printed piece of paper? The answer is yes. Japan is one of the countries manufacturing Ferroelectric liquid displays to day, size 16 inch the approximated cost is about 35 000 Kr with an extremely high resolution and no flicker what so ever, the trade off is that it is only in black and whit and suffers from low contrast. There are others like Xerox at Palo Alto in California that have produced FLC’s with a resolution as good as a printed piece of paper.

 

3.7 Ferroelectric liquid crystal textures

 

The photomicrographs were obtained in transmission using polarized light:

 

Partially Helixed Cell

The right (reddish) part of the cell is about 20 microns thick, while near the (yellower) left edge a layer of liquid crystal only a few microns thick is trapped between a spacer sheet and one of the glass surfaces. Most of the cell is filled with helix lines oriented parallel to the smectic layers. In the upper left these lines are largely absent. This is a classic illustration of the principle of surface stabilization. The arrow shows the local layer orientation. [horizontal dimension ~200 microns]

 

Helixed Unwinding by Electric Field

When an electric field of only a few volts is applied to the partially helixed cell shown in the preceding i image, the liquid crystal molecules are reoriented such that the ferroelectric dipoles align along the field direction. The helix is thereby "unwound" and there are no helix lines remaining in the cell. Color variation is due to birefringence effects (the cell is viewed in polarized light). These arise because the smectic layer normal is not uniformly aligned everywhere, and partly because the cell thickness is non-uniform: the right (greenish) part of the cell is about 20 microns thick, while near the (yellower) left edge a layer of liquid crystal only a few microns thick is trapped between a spacer sheet and one of the glass surfaces. [horizontal dimension ~200 microns]

 

Defects at free surface of DOBAMBC [horizontal dimension ~200 microns]

 

Smectic A fan texture in HOBACPC [horizontal dimension ~200 microns]

 

 

Zig-zag lines in a chevron SSFLC [horizontal dimension ~100 microns]

 

 

Zig-zag lines in a chevron SSFLC. The cell is rotated to extinction. [horizontal dimension ~100 microns]

 

Zig-zags and domains in a chevron SSFLC. A negative field is applied and the yellow domains are growing. [horizontal dimension ~100 microns]

 

 Zig-zags and domains in a chevron SSFLC. A positive field is applied and the pink domains are growing. [horizontal dimension ~100 microns]

 

 Boat domains in a chevron SSFLC. A positive field is applied and the pink domains are growing with characteristic speedboat shapes. [horizontal dimension ~100 microns]

 

 

Focal conic domains in an SSFLC. Focal conic domains formed in the filling port of an SSFLC cell. The cell thickness is 4 microns and the material Merck ZLI-3654. [horizontal dimension ~500 microns]


 

Colorful zig-zags in an SSFLC (a). Colorful zig-zag lines an SSFLC cell. These defects are indicative of a reversal of the chevron layering direction in the cell. The cell thickness is 4 microns and the material Merck ZLI-3654. The smectic layer normal is approximately vertical. [horizontal dimension ~100 microns]

 

Colorful zig-zags in an SSFLC (b). Colorful zig-zag lines an SSFLC cell. These defects are indicative of a reversal of the chevron layering direction in the cell. The cell thickness is 4 microns and the material Merck ZLI-3654. The smectic layer normal is approximately vertical. [horizontal dimension ~100 microns]


4. Status and prospects for the future for the different LCD materials

 

4.1 Twisted nematic LCDs

 

The most widely used LCD type today is the TN LCD. They are the lowest cost LCDs to produce and use very low power in reflective mode. Areas in which TN LCDs may be used are watches, calculators, games and instruments. In any application where price, size and power are important, they will still be used even in the near future. However when multiple lines of data are required as well as higher contrast, long outdoor usage and better brightness, FSTN LCDs will continue to dominate the market.

 

4.2 Fast supertwisted nematic LCDs

 

FSTN LCDs are the best LCDs of choice when it comes to office automation applications. Performance is increasing as research continue and their cost also comes down. Sizes up to a 17 inch diagonal with a resolution up to 1024 lines is possible to produce and with a response time decreased to less than 100 msec, mouse operations on portable computers is now possible. A major disadvantage is the limited viewing angle compared to active matrix LCDs, but some improvements in retardation films now provide a better viewing angle and is believed to be even better in the near future. In the latest FSTN LCDs, response time under 50 msec is now possible and they may be used for limited video applications such as a slow scan videophone.

The main problems today are cross-talk, response time and viewing angle. Cross-talk appears as a shadowing on the screen and gets even worse as the multiplex ratio increases. For full video applications, the response time must be down to 20 msec and the viewing angle must be better if the product is to be used for more than one-person viewing.

Recently developed electronic drive circuits solves most of the response time problems and lower resistance transparent conductors reduce the cross-talk effect.

The prospects for the future considering FSTN LCDs is steady improvements and lowering costs. In the next few years FSTN LCDs will most probably dominate the market, however FSTN LCDs are still inferior to active matrix LCDs.

 

4.3 Vertically aligned nematic LCDs

 

Lately, VAN LCDs has achieved the impressive results of excellent viewing angles, up to 120 degrees, in full color. These are developed mainly by Toshiba and Stanley and are available up to a 14 inch diagonal size. Since the cell gap isn’t critical it is also easy to manufacture. Major disadvantages are the limited temperature range, low response time (~250 msec) and low transmission (~1.5-20% for color VAN). Due to the low transmission efficiency, use will be limited to AC-powered monitors with non-video applications cause of the slow response time. The future for these LCDs is unsure and they may only be used in special niche markets, if at all. Most probably they will be left behind as faster LCDs showing the same results in large viewing angles catch up with them.

 

4.4 Ferroelectric LCDs

 

FLCDs hold much in promise when it comes to commercialized applications, but one major disadvantage is the manufacturing problem due to a very small cell spacing, about 1.5 microns +/- 0.05 microns, which is a very severe tolerance. Monochrome and full color 15-inch diagonal ferroelectric LCDs are now under production and sizes up to 17 or even 19 inch diagonals may soon be possible to manufacture for office applications in the near future. The sensitivity to shock and vibration is believed to be solved and the contrast ratio is also improved as the limitation of defects increase. Sony is currently using SiO evaporation for alignment layers to improve uniformity and contrast ratio. They are also in lead when it comes to gray scale techniques to address the video requirement. The future for ferroelectric LCDs is still uncertain, but if a higher contrast and wider viewing angle is achieved, they may actually compete with active matrix LCDs.


5. Active or passive LCD displays ?

 

5.1 Active matrix (TFT) LCDs

 

A display made with active matrix TFT technology is a LCD that has a separate and independent transistor for each pixel on the display. Having a transistor at each pixel means that the current that triggers pixel illumination can be smaller and therefore it may be switched on and off more quickly. The pixel emit colored light which passes through a RGB (Red, Green, Blue) filter. The result of this is a screen with a very small response time and direct pixel addressing is also possible which makes the TFT LCD suitable for video and fast graphic applications. Se figure 5.1 for the basic layer construction of an active matrix LCD and figure 5.2 for a typical LCD screen layout.

 

 

Figure 5.1 – Basic layer construction for an active matrix LCD

 

Figure 5.2 - Typical LCD screen layout

 

5.2 Passive matrix LCDs

 

Each color pigment is turned on or off by supplying current through a timed operation method to electrodes oriented in length and width. TFT LCDs may be noted for their outstanding screen image, but STN LCDs certainly offer superior economy. STN is also known as passive matrix LCD technology. See figure 5.3 below for a schematic picture of the basic layers that build up a STN LCD.

 

 

Figure 5.3 – Basic layer construction for a passive matrix LCD

 

5.3 Comparison tables of passive matrix LCD technologies

 

In the two charts below (Table 5.4, 5.5) several comparisons are made between the faster, direct addressed active matrix TFT LCD and the passive matrix LCD. In order to compare them correctly, both active and passive matrix LCDs are run with multiplex addressing, this due to the fact that passive matrix LCDs aren’t able to use direct pixel addressing.

 
Type
TN LCD
STN LCD
VAN LCD
FLCD
PDLCD
Diagonal
Up to 17"
Up to 17"
Up to 17"
Up to 19"
Unlimited
Resolution (lines)
64 lines
400 lines
400 lines
1000 lines
8 lines
Contrast ratio (Direct drive)
> 100:1
> 100:1
> 50:1
> 50:1
> 10:1
Multiplexed
> 3:1
> 20:1
> 20:1
> 50:1
> 5:1
Response time (ON/OFF)
60ms
200ms
400ms
0.1ms
100ms
Suitable application
Video
mouse speed
quasi static
line address
mouse speed
Color* 
Poor
average
good
good
average
Viewing angle
Poor
average
good
good
-
Cost 
Low
moderate
moderate
high
low
Status
mass prod.
mass prod.
development
development
r&d stage
*Color achieved with absorptive color filters within the LCD

Table 5.4 - Comparison of various passive matrix LCD properties

 
 
Type
Passive
Active
Contrast
10-20
100+
Viewing angle
limited
wide
Gray scale
16
256
Response time 
100-200 ms
<50 ms 
Multiplex ratio
~480
>1000
Size
up to 19"
<14"
Manufacturability
simple
complex
Cost
moderate
High
 

Table 5.5 - Comparison of passive and active matrix LCDs

 

The main thing that limit the widespread of active matrix LCDs today is the high cost and complex manufacturability. In all other areas except size, active matrix LCDs win the comparison between the two LCD types. However it is believed that in the not so distant future, passive matrix LCDs might achieve the same results as active matrix LCDs when it comes to brightness and contrasts, which will make them take the lead in commercial products.


6. Manufacturing of LCDs and TFTs

 

6.1 Manufacturing in general

 

Most manufacturers use very similar TFT and LCD fabrication processes. An example of the TFT process is summarized in figure 6.1 below.

 

 

Figure 6.1– Summarized TFT process

 

Large custom buildings are being used for clean room facilities. About 4000-6000 square meters is not uncommon and the investment in such a building may easily reach up to 1 billion us dollars. Since the LCD manufacturing process demands a very clean atmosphere, several precautions has to be made. The processing plant is often ergonomically arranged with a process flow sequence. Different classes of dust free environments are necessary for the many steps. A lower class is used for large sputtering machines while a higher class is necessary for the different wet etching and lithography steps. A typical production from a processing plant at this size is about 20,000-40,000 10" VGA displays per month. Typical cycle times are as follows :

 

 

Since the substrates are very sensitive to shocks and vibrations, robots are often required to operate the substrate transferring steps. Figure 6.2 below describes the defects caused in the manufacturing.

 

Figure 6.2 – Manufacturing defects

 

6.2 Manufacturing equipment

 

6.6.1 PECVD

 

In order to deposit the a-Si and gate dielectric insulation layer in the deposition step, a PECVD vertical deposition system is used. The system have multiple vacuum chambers connected via gate valves. Substrates are loaded in the trays, often by an load/unload robot, on the front side and the substrates are transported into the first two vacuum chambers. These chambers contain quartz heater lamps to elevate the substrate temperature to about 350 degrees Celsius. Several deposition chambers follow the heating chamber as shown in figure 6.3. Each deposition layer requires different temperatures and therefore the many different chambers. After a complete cycle, 3 layers of respectively SiN, a-Si and SiN are deposited. Typical cycle time is about 10 minutes and a regular PECVD can load up to 8 substrates per chamber. Unfortunately mechanical cleaning is required of this system which lowers the productivity substantially.

 

Figure 6.3 – PECVD deposition system

 

6.2.2 Sputtering

 

After a 60 minutes long cooling and cleaning period, the substrates are transported into the sputtering system. The sputtering system are commonly used for data and scan metal lines as well as ITO. A typical ITO deposition system is shown in figure 6.4 below. Sputtering systems are the most time consuming systems in the entire process and a complete cycle takes up to several hours.

 

Figure 6.4 – ITO sputtering system

 

6.2.3 Lithography

 

In a multi-mask process, photolithography productivity is critical. The stitching accuracy is particularly important because significant errors can lead to gray scale shift across stitching boundaries and this can be quite visible. Improvements in the lithography stage are vital for future requirements on high-resolution displays.

 

6.2.4 Wet processing and cleaning

 

Cleaning processes are one of the key factors in achieving higher yield. More than 80% of the defects come from particles on the substrate, which are almost impossible to completely eliminate. Particles larger than 1 micrometer are subjects for the wet processing and cleaning system. After the coating of the polyamide alignment film, a very efficient freon cleaning process is currently used, but as these compounds are dangerous for the environment, alternative compounds and cleaning processes are being sought. The different cleaning processes and secondary effects are listed in figure 6.5.

 

 

Figure 6.5 – Different cleaning processes

 

6.2.5 Dry etching

 

Dry etching may provide better line-width control, but, because it is a single-plate process, it is extremely slow and rarely used. This result in TFT labs containing a large number of dry etchers if this technique is being used. Ferroelectric LCDs are commonly produced using the dry etching process.

 

6.2.6 Drivers and packaging

 

High quality packaging is essential for high-resolution LCDs. There are currently three different types of packaging : chip-on-board (COB), TAB (COF) and chip-on-glass (COG). The most frequently used type is the TAB which is suitable for the typical VGA LCD. Compact, thin LCD modules can be achieved as the TAB IC are positioned on the sides of the backlight. Figure 6.6 describes the comparison of packaging configuration.

 

Figure 6.6 – Comparison, different packaging configurations

 

Figure 6.7 below shows the different packaging types. As shown in the c section, the COG technique is to prefer as it is easier to assemble and provides easier testing once applied. Chip-on-glass is also used for high resolution LCDs. There are more than 120 output driver ICs available and these have been developed mostly by Japanese companies like Sharp, NEC and Hitachi.

 

Figure 6.7 – Different packaging types


7. Suppliers and markets

 

Current cost of one gram of nematic material ranges between $2.85 and $10.00 per gram, depending on the quality of the materials used. This is however a relatively small percentage of the total cost of an STN or TN AM thin-film transistor (TFT) display. Those displays used in pocket calculators and wristwatches use lower quality nematic materials or even other kinds of cells which are much cheaper than the TFT displays to produce. The TFT displays are most frequently used in laptop computers and other products that demand a better quality and higher speed. Some of the most complex and expensive nematic material mixtures includes several hundred compounds and may take up to a year to properly prepare, resulting in a very high price per unit.


8. Some commercial products and prototypes

 

14" Regular LCD screen

 

 

15" Ferroelectric LCD, front and side view

 


KTH, 1998-05-18 - Experimental Material Physics, Liquid Crystal Materials

Daniel Lundström, Jan Yilbar