Modifying Light

Joseph A Castellano. American Scientist. Volume 94, Issue 5. Sep/Oct 2006.

From colorful cell-phone screens to large flat-panel televisions and electronic billboards, it’s impossible to get away from liquid-crystal displays these days. Found only in the odd calculator or clock screen just a quarter-century ago, LCDs are suddenly realizing their potential. The latest chapter in their history represents a triumph in manufacturing technology, which has finally made it possible to put these materials into widespread use. But the full story of LCDs is a long one marked by many twists and turns. Since the discovery of liquidcrystal materials in the 19th century, scientific and engineering advances have been regular and frequent, coming together in a series of breakthroughs. These advances have brought us into the age of the light-modifying flat-panel display, one of the most important technological achievements of the 20th century and a driving force behind the colorful new electronics of the 21st.

It is an age that has been hoped for since the beginning of that prior big innovation of the 20th century, television. The early development of liquid-crystal displays was indeed motivated by the limitations of the bulky light-emitting, cathode-ray color picture tube. As early as 1956, David Sarnoff, chief executive officer of RCA Corporation, foresaw a thin “TV-on-a-wall,” and in 1964 RCA Laboratories in Princeton, New Jersey, assembled a cadre of physicists, chemists and engineers to work on liquid crystals. Research at RCA and a number of universities and other companies planted the seed of what is now an industry with annual sales exceeding $100 billion.

The industrial development of LCDs coincided with another advance: the large-scale manufacturing of integrated circuits, which ultimately led to the development of the thin-film transistor, or TFT. This electronics component provided a way to control the display of complex information on screens precisely and rapidly. At the same time, the personal computer was being developed. By the mid-1980s, LCDs were appearing in laptop and notebook computers, which could not have been created without flat-panel displays. In addition, LCD technology made possible the introduction of a host of modern consumer and industrial products that we now take for granted: cellular telephones, appliances, automobile dashboards, airplane cockpit displays, medical instruments and numerous personal electronic items.

But to understand how LCDs developed from simple laboratory samples to complex meter-size, high-definition television displays, one must recall how the global economic landscape changed during the final decades of the 20th century When the first LCD was fabricated in 1965, the exchange of technology across national borders was limited by the political tensions of the Cold War. Japan was the only technologically advanced Asian nation accessible to electronics manufacturing. With the opening of China and the break-up of the Soviet Union, international commerce changed, enabling an increased flow of ideas and technology transfer throughout the world. Much of the LCD technology developed in Western Europe and the United States was adapted for mass production by engineers in Southeast Asia and Eastern Europe. Today the LCD industry is a story of international technological achievement.

A New State of Matter

Liquid-crystal displays use the electrical excitation of a liquid to modify the direction and intensity of light generated from behind it by a lamp. Liquid crystals are composed of organic molecules-based on compounds of carbon and hydrogen-that typically have long, rodlike shapes.

The history of this class of materials began in 1888 when liquid crystallinity, also called mesomorphism, was first observed and characterized by Austrian botanist Friedrich Reinitzer. He observed an unusual melting characteristic in the compound cholesteryl benzoate: Although the crystal melted at 145 degrees Celsius, the melted material was opaque instead of clear, as a normal, or isotropic, liquid would be. As he continued to heat the material further, the opacity disappeared sharply at 178 degrees. When the material cooled, the opaque state appeared again. It was apparent to Reinitzer that within this 33-degree range a unique state of matter existed.

Reinitzer’s discovery of an intermediate state between a crystalline solid and a normal liquid flew in the face of the centuries-old concept that matter existed in only three states: solid, liquid and gas. His observations were consequently viewed with skepticism by his peers until the work was duplicated by the German physicist Otto Lehmann in 1890. It was Lehmann who coined the termflussige Krystalk, which translates to fluid crystal or liquid crystal.

During the 1920s and 1930s, research on the synthesis of liquid-crystal materials and their light-modifying effects was conducted in France, Germany, Great Britain and Russia. French physicist Georges Friedel’s paper in 1922 reported detailed optical studies on the new materials and established the terms derived from Greek that are used to describe the material’s various phases. Friedel described liquid-crystal materials as mesophase, derived from the Greek word mesos, meaning intermediate or between. Friedel also identified three distinct mesophase types: smectic, nematic and cholesteric. Some liquid-crystal materials can experience several of these states at different temperatures.

The smectic mesophase is a turbid, viscous state similar to soap, smectos being Greek for soap. The nematic mesophase is also turbid, but mobile like any normal liquid. Nematos is Greek for thread, and there are threadlike structures in a nemaric liquid.

The third distinct phase, the cholesteric mesophase, acquires its moniker from the fact that it is primarily exhibited by esters of cholesterol. Naturally occurring cholesteric liquid crystals make up a significant portion of the plaque that coats human coronary arteries. Cholsteric liquid crystals have the same opaque, liquid-like properties as nematic ones, but different optical characteristics. They are often also called durai nematic, because these molecules possess chirality, meaning they tend to twist in either a left-handed or a right-handed direction so they can’t be superimposed on a mirror image of themselves. As a result, cholesteric liquid-crystal molecules arrange themselves in a helical configuration. Changes in temperature alter the pitch of the helix, resulting in changes in the color of light reflected from the surface of cholesteric liquid crystals.

Studies of the properties of nematictype liquid-crystal molecules were carried out in the years prior to World War II, but there was little effort to use these materials in displays. Shortly after the war ended, research in liquid crystals was again begun in earnest at university research laboratories all across Europe. Chemist George W. Gray synthesized many new materials that exhibited the nematic liquid crystalline state during the late 1940s at the University of Hull in England. His work led to a better understanding of how to design liquid-crystal molecules and increase the temperature at which the material functions. His book, Molecular Structure and the Properties of Liquid Crystals, published in 1962, quickly became the definitive work on the subject and stirred a renewed interest in the materials.

The First Display Devices

In 1962 Richard Williams, a physical chemist working at RCA Laboratories, began searching for a physical phenomenon that might lead to a display technology not involving vacuum tubes. He began experimenting with liquid crystals. The most readily available nematic liquid-crystal material was the compound p-azoxyanisole, which had a melting point of 116 degrees, so Williams set up his experiments on a heated microscope stage. He sandwiched the liquid crystal between transparent tinoxide electrodes that were coated on glass plates and placed the sample onto a microscope stage held at 125 degrees.

When he applied an electric field of about 1,000 volts per centimeter, he observed the formation of a regular pattern of long parallel stripes, which he called domains. Others would later refer to these as “Williams domains.” Williams concluded that this concept might be used in a display, but was aware that it would take a long time to develop. So Williams turned to other projects after introducing a colleague, George H. Heilmeier, to the field of liquid crystals. Heilmeier, an electrical engineer and physicist, became so intrigued with liquid crystals that he decided to devote his research to them.

Heilmeier ‘s first experiments, in 1964, involved dissolving small amounts of a “guest” dichroic dye into a “host” nematic liquid-crystal material. He and electronics technician Louis A. Zanoni used a microscope with a polarizing filter, which permitted light oriented in only one direction to pass through the sample. They aligned the molecules so that their long axes were parallel to the electric vector of the polarized light by simply rubbing the tin oxide-coated surface in one direction prior to assembling the sample, resulting in a sample that took on the color of the dissolved dye.

The liquid-crystal molecules had a strong permanent dipole moment operating along the long molecular axis-that is, they had a positive electric charge at one end and a negative charge at the other. Heilmeier postulated that this would enable the liquid-crystal molecules to align in the direction of an applied electric field and, in turn, to orient the dissolved dye molecules with their long axes perpendicular to the electric vector of the polarized light. This experiment revealed that color could be turned on and off with an electric field. Heilmeier coined the term guest-host effect to describe the phenomenon.

Soon after this discovery, Heilmeier, Zanoni and chemist Lucian Barton discovered that liquid crystals could be made to electrically switch from a transparent state to a highly scattering, opaque state. In the first experiments, a black background was used so that the cell appeared black at first. When a field was applied, the liquid became turbulent and scattered light, making the cell appear white. With higher field strengths, the brightness of the cell increased, indicating that a gray scale could be obtained.

Furthermore, the bulk of the scattered light was sent forward rather than backscattered. With a specular reflecting back electrode, such as an aluminum mirror, the forward radiation could be directed back through the liquid to the viewer, to obtain a reflective display as well as a transmissive one. Heilmeier later proved that the motion of ions through the liquid created the scattering centers. Heilmeier dubbed the phenomenon dynamic scattering and recognized the possibility that it could be used as a way to display information for a variety of applications, not only television. Thus, Heilmeier and his group fabricated and demonstrated the first working liquid-crystal displays in the mid-1960s.

Initial studies of the guest-host and dynamic scattering phenomena were conducted with nematic materials with high operating temperatures. In 1965, Heilmeier called upon Joel E. Goldmacher and myself, both organic chemists, to help develop liquid-crystal materials that could operate at room temperature and over a wide temperature range. After about six months of research into the synthesis of a variety of benzylidene aniline compounds (called anils for short), we prepared numerous small anils that contained two benzene rings, as a way to achieve both a low melting point and a wide nematic operating range that included room temperature.

The next step was to make mixtures of two nematic compounds that differed only in the number of carbon atoms that were chained together to make the long, rodlike structure of each molecule. This process significantly reduced the melting point, but the nematic-isotropic transition temperature of the mixture remained high. We tried adding a third compound to the mix, and in March 1966, we had a material with a nematic range of 22 to 105 degrees. Operation at room temperature was finally in sight.

Recognizing the potential of this new technology, RCA management classified the project as “company secret” until the work was made public at a news conference in 1968. The announcement sparked a worldwide effort to develop LCDs for a variety of applications. The first LCD digital watch was built by Optel Corporation in 1970.

Twisted Nematics

The dynamic-scattering mode had several inherent drawbacks. One was the voltage requirement of 12 to 15 volts, which mandated the use of tiny transformers in the first digital watches. Another was the amount of current needed due to ionic dopants dissolved in the liquid crystal. In addition, the mirrored back electrode used to maximize contrast created viewing angle limitations.

One possible replacement was HeiImeier’s guest-host effect, which operated at low voltage and did not require a mirror electrode. Another was the twisted-nematic structure, first described by French physicist Charles H. Mauguin in 1911. Mauguin sandwiched a nematic liquid-crystal compound between two glass plates that had been rubbed with paper. He referred to the rubbed surface as a “membrane” because he correctly theorized that material from the paper was retained at the surface and oriented the liquid crystal molecules in the direction of rubbing.

Maugin then twisted the front plate 90 degrees to the direction of the back plate and made observations of the sample between crossed light-polarizing crystals. He found that the plane of polarized light was rotated by 90 degrees as the light passed through the layers. He hypothesized that the orientation of the molecules changed continually from the lower plate to the upper one, creating a helical structure whose pitch is determined by the degree of rotation of the two plates. However, Mauguin made no mention of attempting to use electric or magnetic fields to change the orientation of the liquid-crystal molecules.

Wolfgang Helfrich, a physicist who joined RCA in 1967, became interested in Mauguin’s twisted-nematic structure and thought it might be used to create an electronic display. However, RCA showed little interest in any effect that used two polarizers because of the large amount of light absorption in the resulting device. Helfrich stopped exploring the concept, and RCA lost a great opportunity to develop what would become a valuable piece of intellectual property.

In 1970, Helfrich joined Hoffmann-La Roche in Basel, Switzerland, where he teamed up with Martin Schadt, a solidstate physicist. The two scientists were searching for a novel display concept and Helfrich suggested that they investigate the possibility of electro-optically switching Mauguin’s twisted-nematic structure. Schadt built a sample with electrodes arid a twisted version of a liquid-crystal material called PEBAB (p-ethoxybenzylidenep-aminobenzonitrile), which Helfrich had reported in prior studies at RCA. The RCA team had already developed roomtemperature mixtures of the compounds for guest-host color displays.

After extensive experimentation with different surface treatments, different cell gaps and different driving conditions, Helfrich and Schadt improved the surface alignment to the point where an optical effect became visible under a polarizing microscope. In a simple twisted-nematic display, the surfaces of the electrodes are coated with a polymer and buffed to orient the long polymer chains in one direction. This enables the liquid crystal molecules to align with their long axes parallel to the polymer chains on the surfaces, a condition known as homogeneous alignment. Polarizers are laminated to the outside surfaces of the device, with the front and rear polarization directions at 90 degrees to each other.

With no voltage applied (the OFF state), polarized light enters the front of the device and is redirected by the liquid crystal: The light follows the direction of the material’s twist and undergoes a 90-degree rotation. The polarized light is now aligned with the direction of the rear polarizer and can pass through it.

With an applied voltage (the ON state), the liquid crystal molecules are oriented parallel to the electric field, temporarily destroying the twisted structure in the bulk of the fluid. In this case, polarized light entering the cell is not rotated and is absorbed nearly completely by the rear polarizer. Thus, the ON state is black whereas the OFF state is clear. When the field is turned off, the twist structure is restored.

Schadt and Helfrich immediately filed a patent on the twisted-nematic display and several months later published the results of their experiments. In order to demonstrate the feasibility of the new effect for displays, Schadt fabricated a 4-digit display panel in 1972. This is believed to be the first, fully-functional twisted-nematic LCD ever made.

Meanwhile back in the United States, James Fergason at the Westinghouse Research Laboratories in Pittsburgh, along with Sardari Arora and Alfred Saupe at Kent State University’s Liquid Crystal Institute, were doing their own research on the twisted-nematic effect, as Fergason was also well aware of Mauguin’s work. The group built a working device in April of 1970. However, it was not until February 9,1971, that Fergason filed the first U.S. patent application. This was two months after the Swiss patent was filed and set the stage for a three-year legal confrontation that was settled out of court. In the end, all the parties received a share of what would become many millions of dollars in royalties.

Advanced Materials Emerge

The anil compounds in the first liquid crystal mixtures were susceptible to breakdown from exposure to water and alkaline materials. For this reason, any display device needed hermetic glass seals to keep moisture out. Manufacturers were also required to use a layer of silicon dioxide to prevent alkali-ion migration from the glass surfaces.

A major breakthrough occurred in 1972 when a team led by Gray developed a new type of cyanobiphenyls that could be mixed with liquid-crystal compounds to produce less reactive materials with broad operating temperature ranges and excellent electro-optical performance. In addition, they were less viscous, thereby providing faster response and relaxation times under electrical excitation. As a bonus, the materials had better light-transmission characteristics; they were essentially pure white compared with the slightly yellow color of previous compounds.

This led to the work of Ludwig Pohl, Rudolf Eidenshiak and their colleagues at Merck KgaA in Darmstadt, who discovered that compounds called cyanophenylcydohexanes were also nematic. These materials had even lower viscosity, more rapid response times and smaller light-scattering effects, while retaining the colorless qualities and other desirable device-related characteristics of the cyanobiphenyls, so they soon became widely used in LCDs.

Today’s liquid-crystal formulations are complex mixtures of these and other compounds as well as various additives that possess chirality. The preparation of liquid-crystal material has become a highly sophisticated science in itself. Materials are now tailored to achieve precise performance for specific applications.

Thin-Film Transistors

A high-information-content display consists of rows and columns of electrodes connected to drivers that supply voltage. The electric field scans the display row by row from top to bottom at 60 to 100 hertz. In this technique, known as multiplexing, the liquid crystal reacts to the average of the voltage over time instead of to each individual frame scan. When the proper voltage difference is generated across the row and column, the intersection is said to be selected.

However, the non-selected elements also receive some fraction of the voltage. Thus, the liquid-crystal molecules in non-selected elements are partially oriented, reducing the display’s contrast. This problem was ultimately solved by placing a thin-film transistor (TFT) at the intersection of each row and column. TFTs are directly deposited onto a glass plate in a grid format. The point where a row and column crosses forms one transistor, which acts like a switch to control the electric current sent to one cell of liquid-crystal material.

The primary method to write data to LCDs with TFTs is known as active matrix addressing. This technique makes the display hardware more complex by adding a switch to each picture element, or pixel. The advantage is that the switch can be turned on very rapidly, in a few microseconds.

This approach too has a long history. The use of active elements to drive dynamic scattering LCDs was first demonstrated in 1966 at RCA Laboratories by Bernard Lechner and his colleagues. However, T. Peter Brody and his team at the Westinghouse labs were the first to build working displays using TFTs in 1968. The first foray into displays by Brody’s group utilized not LCDs, but electroluminescence. It is believed that Brody was the first to coin the term active matrix, which he introduced into the literature in 1975.

By 1971 Brody knew that LCD technology looked promising as a candidate for his active-matrix addressing scheme. One year later, Fan Luo, a key member of Brody’s team, produced active-matrix circuits that had adequate performance to drive liquid crystals. In 1973 they built a 6-inch by 6-inch panel that had 120 by 120 pixels.

While Brody was paving the way for TFTs in LCDs, other groups were looking to use silicon as the semiconducting material for electrodes. By the late 1970s, silicon was deeply entrenched as the material of choice for transistors. However, the question remained as to whether thin films of silicon would have the ability to move electrons around at a rate that would make them practical for displays.

In addition, many questioned whether these devices would ever be practical for a color display, as a typical television screen would require about 920,000 individual TFTs, one at each red, green and blue sub-pixel. In other integrated circuit devices, a small number of defective chips can be discarded from a silicon wafer. But every one of the nearly one million TFTs must function for a single color LCD to operate.

A significant breakthrough occurred when a research group at the University of Dundee in Scotland and the Royal Signals and Radar Establishment in Malvern, England, discovered that hydrogenated amorphous silicon had performance characteristics suitable for a TFT in LCD panels. This report sparked the worldwide effort to develop TFT-LCDs based on silicon. Soon thereafter major programs were under way at some 13 manufacturing companies in Japan. In 1983, Shinji Morozomi and his colleagues at Suwa Seikosha Corporation in Japan demonstrated the first commercial color LCD television, with a 2-inch-diagonal twisted-nematic LCD driven by an active matrix of polycrystalline silicon TFTs.

Morozumi’s work surprised many scientists because it proved that the elusive TFT could be practical for a color LCD. Other groups worldwide intensified their work on TFTs for LCDs, but the major U.S. and European LCD developers were not prepared to make the large investments in resources needed to develop the required manufacturing technology. Consequently, from the mid-1980s mass production of TFT-LCDs developed and matured in Japan, Korea and Taiwan. After the turn of the 21st century, manufacturers in Korea began building multiple TFT-LCD displays on very large plates of glass. Plants being built today are handling plates up to 2,160 by 2,460 millimeters (7.08 by 8.07 feet).

Recent Advances

As color twisted-nematic TFT-LCDs became widely available for notebook and desktop computers, users began to complain that the readability of the display would degrade significantly at angles more than 10 degrees in any direction from a central viewing location. Sometimes the display would invert to a negative image at the off-axis viewing angles. Once again, scientists looked for ways to modify light to solve this problem.

In the twisted-nematic LCDs described thus far, the liquid crystal is sandwiched between a pair of electrodes, one on the back and one on the front glass substrate. In 1992, scientists at the Institut Angewandte Feskorperphysik in Freiburg, Germany, led by Guenter Baur, described the use of electrode pairs placed side by side on only the back substrate. This setup prevented liquid crystal molecules from aligning themselves at right angles to the glass plates, so there is less restriction on the light passing through the display, resulting in a wider viewing angle of TFT-LCDs.

Baur’s group coined the term inplane switching (IPS) to describe the method. Today many companies use IPS to produce high-image-quality color LCDs with viewing angles of 150 degrees or more.

More recently, scientists at the research laboratories of Samsung Electronics in Korea, faced with the need to build TFT-LCDs for very-large-screen television displays, developed the patterned vertically aligned or PVA mode. This method involves patterning certain shapes on both the upper and lower electrodes, enabling the molecules to align vertically, or homeotropically. The PVA technique is being used today to manufacture television displays with screen sizes of up to 100 inches in diagonal measurement.

Another major engineering development in the mass production of large-screen television displays was the one drop fill (ODF) technology, developed in 2001 by a team of 15 IBM scientists in Japan and the United States.

Previously manufacturers had used an edge-filling technique to produce LCDs, followed by a final epoxy sealing operation. The units had a small opening in the seal on one edge, which is placed downwards in a vacuum chamber. The air is removed and the units are lowered into a bath of liquid crystal material. When the system is opened, the difference in pressure between outside air and the display envelope caused the liquid to fill the unit completely. However, it would take more than 30 hours to fill a 22-inch-diagonal unit.

In the ODF method, the precise amount of liquid crystal is dispensed on the surface of one plate before a pair of plates is assembled and sealed together in a vacuum chamber. Although this sounds simple, there was a major hurdle to overcome: contamination of the liquid crystal where it made contact with uncured sealant. The IBM engineers solved this problem by formulating sealants that could be cured quickly by ultraviolet light and heat; they also developed techniques to dispense the precise amount of liquid crystal needed to fill the space between the plates for each size of display.

A 22-inch-diagonal display can be filled in just 1.5 hours using the ODF process. Over the last five years, other manufacturers have adopted the ODF process to manufacture large-screen television displays. Recently, LG.Philips demonstrated a 100-inch, high-definition color TFT-LCD, the world’s largest thus far.

State of the Art

The path from the development of LCDs as light-modification devices to the creation of a huge display industry provides an excellent example of how technology progresses from a laboratory experiment conducted by a few visionaries to hundreds of products that enhance the quality of life. Records of LCD manufacturing go back to 1973, when slightly more than 1 million units valued at $7 million were sold worldwide. By 2004, this number had reached 2.86 billion units valued at $47 billion.

The drive to reduce manufacturing costs and improve performance continues to provide exciting challenges for scientists and engineers engaged in LCD research. A number of research groups are working on the integration of driving circuits on the glass panel using amorphous or polycrystalline silicon transistors in place of external drivers that today require wiring to the rowand-column electrodes. Such fully integrated displays are expected to be on the market in the next few years.

The shift from fluorescent tubes to light-emitting diode (LED) devices for backlighting LCD panels is also receiving considerable attention. Backlighting a display with LEDs provides a number of advantages: a wide color gamut, high dimming ratio, tunable white point and long lifetime. However, once high-intensity LEDs age, their color and luminance levels of are not stable over a wide temperature range, so engineers are developing optical feedback control systems. Ki-Chan Lee and his colleagues at Samsung Electronics’ LCD Development Center in Korea recently demonstrated a system that used an amorphous silicon photoconductive sensor integrated onto the LCD panel.

Perhaps the most daunting challenge today is the development of TFT-LCDs on flexible plastic substrates. The successful solution to this problem would open the way to high-volume manufacturing by continuous roll-to-roll processing. Displays manufactured in this manner not only would be less expensive, but would also provide unique designs and new uses for both smalland large-screen bendable display products. The key to their production is TFTs made with organic semiconductors instead of silicon.

Mun Pyo Hong is leading a large team of scientists and engineers at Samsung Electronics and the Samsung Advanced Institute of Technology in Korea to achieve such a goal. This group recently fabricated a 15-inch diagonal color LCD that used organic TFIs based on the semiconducting material pentacene. The group has also demonstrated displays that used a flexible polymer substrate called polyefhersulfone. A fully functional, large-screen flexible LCD that combines these technologies could become a reality in the next five years.

Lessons for the Future

Multi-disciplinary research has proven to be vital to the creation of new technologies. Development of the first LCDs was due to a team effort that included researchers from chemistry, physics, engineering and optics. In addition, a long-term commitment and a large investment of resources were necessary to achieve products with the highest performance. It took almost 30 years for the LCD to reach its developer’s goal of a large-screen color television.

Another factor for future success is international cooperation. Scientists and engineers from around the world contributed to the creation of the LCD industry. It was also the result of numerous joint projects among corporations, government agencies and universities.

Finally, the shifting of LCD manufacturing to newer industrialized nations and the free exchange of technology and goods among trading-partner nations has been a great benefit to the world in general. It is my hope that this example of technology exchange and free trade will spread to other developing nations to improve world economic and political stability in the 21st century.