Rocktech

Capacitive touch screens are made of single or multiple layers of material that are coated with a conductor such as Indium Tin Oxide. A protective cover seals the assembly off from the environment.

When another electrical conductor, like a bare fingertip or a stylus, touches the surface, an electric circuit is completed at that location. Sensors embedded in the glass then detect the location of the flow of current, which is then registered as a touch event.

This is different from the way that resistive touch technology works, where physical pressure is involved.
Capacitive touch technology can be divided into:

Surface Capacitive

Transparent conductive coating is on the base glass sheet, and glass protective coating is placed over it. Electrodes are placed on the four corners. The same phase voltage is imposed to the electrodes on the four corners, then a uniform electric field will be forming over the panel. When a finger touches on the panel, electrical current will flow from the four corners through the finger. Ratio of the electrical current flowing from the four corners will be measured to detect the touched point. The measured current value will be inversely proportional to the distance between the touched point and the four corners.

Features:

◆ Surface capacitive technology is suitable for large size monitors.

◆ Surface capacitive sensor can respond to light touch, and no pressure force is needed for detection

◆ Visibility is high because structure is only one glass layer.

◆ Surface capacitive is structurally tough as it is made of one sheet of glass.

◆ Surface capacitive does not get affected by moist, dust, or grease.

◆ Parallax is minimized in surface capacitive.

◆ Surface capacitive has high resolution and high response speed.

◆ Basically, surface capacitive can detect touches by fingers only. It does not detect input by gloved hand. Some surface capacitive touch screens may detect touches by thin-gloved hand, but they do not support the combination use of bare finger and gloved finger. Some surface capacitive touch screens support pen writing, but they usually does not support combination use of finger touch and pen writing.

◆ Surface capacitive technology does not support multi-touch.

◆ Surface capacitive touch screen is likely to be affected by noise. Recently, tolerance for noise has been improved with various methods such as noise shielding.

Projected Capacitive

Projected capacitive touch panels are often used for smaller screen sizes than surface capacitive touch panels. The internal structure of these touch panels consists of a substrate incorporating an IC chip for processing computations, over which is a layer of numerous transparent electrodes is positioned in specific patterns. The surface is covered with an insulating glass or plastic cover. When a finger approaches the surface, electrostatic capacity among multiple electrodes changes simultaneously, and the position were contact occurs can be identified precisely by measuring the ratios between these electrical currents.

There are two types of sensing methods in projected capacitive technology. They are GRID type and wire sensing type. GRID type will be introduced here. Human body is conductive since it contains a lot of water. When a finger comes close to the patterning of X and Y electrodes, a capacitance coupling will occur between the finger and the electrodes. The capacitance coupling makes the electrostatic capacitance between the X and Y electrodes change. The touch sensor detects touched points as it checks where on the electrode lines the electrostatic capacitance changed.

Features:

◆ Projected capacitive supports multiple touches, thus supports various elaborate inputs.

◆ Projected capacitive has relatively long life because it has no moving parts in operation.

◆ Projected capacitive has high durability.

◆ Sensitivity of the sensor can be adjusted. If sensitivity is adjusted to high level, the touch screen can be operated over a cover glass or cover plastic sheet. These cover sheets provide additional durability, environmental resistance, and a lot of flexibility in design.

◆ If sensitivity is increased, projected capacitive can be operated with gloved fingers.

◆ Projected capacitive touch screen is excellent at optical property.

◆ Projected capacitive responds to light touch. No pressure force is needed for detection.

◆ Projected capacitive requires an advanced technology to measure electrostatic capacitance and achieve precise locational information from it. Unlike resistive technology, it does not work simply by connecting a touch screen with a controller sourced from somewhere. A projected capacitive touch screen and controller need to be designed together.

◆ Projected capacitive is susceptible to electrical noise due to its detection mechanism. Noise from LCD is especially influential to the sensor. Recently, various methods are developed to improve tolerance for noise.

◆ Projected capacitive requires fine pattering, thus takes high processing cost.

Self-Capacitance

Self-Capacitance is based on measuring the capacitance of a single electrode with respect to ground. When a finger is near the electrode, the human-body capacitance changes the self-capacitance of the electrode. In a self-capacitance touch screen, transparent conductors are patterned into spatially separated electrodes in either a single layer or two layers. When the electrodes are in a single layer, each electrode represents a different touch coordinate pair and is connected individually to a controller. When the electrodes are in two layers, they are usually arranged in a layer of rows and a layer of columns; the intersections of each row and column represent unique touch coordinate pairs. However, self-capacitance touch-screen controllers do not measure each intersection; they only measure each row and column;

This works well when only a single finger is touching the screen. For example, As shown in the below picture, a single-finger touching location X2,Y0 can be sensed accurately by measuring all the X electrodes and then all the Y electrodes in sequence. Measuring individual electrodes rather than electrode intersections is the source of one of the major disadvantages of two-layer self capacitance touch screens – the inability to unambiguously detect more than one touch. Two fingers touching in locations X2,Y0 and X1,Y3 produce four reported touch points. However, this disadvantage does not eliminate the use of two finger gestures with a self-capacitance touch screen. The secret is in software – rather than using the ambiguous locations of the reported points, software can use the direction of movement of the points. In this situation it does not matter that four points resulted from two touches; as long as pairs are moving away from or toward each other (for example), a zoom gesture can be recognized.

Mutual Capacitance

Mutual Capacitance is a more common type of pro-cap today, which allows an unlimited number of unambiguous touches, produces higher resolution, is less sensitive to EMI, and can be more efficient in its use of sensor space. Mutual capacitance makes use of the fact that most conductive objects are able to hold a charge if they are very close together. If another conductive object, such as a finger, comes close to two conductive objects, the charge field (capacitance) between the two objects changes because the human-body capacitance “steals” some of the charge.

In a mutual-capacitance touch screen, transparent conductors are always patterned into spatially separated electrodes in two layers, usually arranged as rows and columns. Because the intersections of each row and column produce unique touch-coordinate pairs, the controller in a mutual-capacitance touch screen measures each intersection individually (see right Fig). This produces one of the major advantages of mutual-capacitance touch screens – the ability to sense a touch at every electrode intersection on the screen.
Because both self-capacitance and mutual capacitance rely on the transfer of charge between human-body capacitance and either a single electrode or a pair of electrodes, this method of capacitive sensing is most commonly called “charge transfer.”

Optical donding

Optical bonding is a type of integration service that is used to laminate the touch screen (or other rigid material) to the top surface of the LCD. The bond is created by dispensing and then curing a form of optically clear adhesive between the touch screen and display forming a permanent bond with no air gap between the two components. There are many benefits to the optical bonding method of integration vs. traditional gasket attach methods, one of the most notable being the enhanced optics and improved impact strength.

Advantages:

◆ Improved transmission by removing internal reflections between the LCD and touch sensor glass.

◆ Improved viewing in bright ambient light conditions.

◆ Less power required to make the backlight brighter in high ambient or even normal viewing conditions.

◆ Can improve mechanical performance.

◆ Can provide better impact, shock and vibration resistance for units that require a rugged environment.

◆ Elimination of any dust, dirt, or moisture that can collect between the sensor and display surfaces.

◆ Low haze and low yellowing fill material.

TFT Display (TN Film)

TFT (Thin-Film Transistor) Displays are active-matrix LCDs with full RGB color screens. These screens feature bright, vivid colors and have the ability to show fast animations, complex graphics and crisp custom fonts. TFTs are perfect displays for providing a rich user interface for all types of products. While typically used in consumer devices like personal DVD players and handheld devices, TFTs are also well suited for industrial application.

TFTs are Active-Matrix LCDs that have tiny switching transistors and capacitors. These tiny transistors control each pixel on the display and require very little energy to actively change the orientation of the liquid crystal in the display. This allows for faster control of each Red, Green and Blue sub-pixel cell thus producing clear fast-moving color graphics.

The transistors in the TFT are arranged in a matrix on the glass substrate. Each pixel on the display remains off until addressed by applying a charge to the transistor. Unlike conventional Passive-Matrix displays, in order to activate a specific pixel, the corresponding row is turned on and a charge is sent down the proper column. This is where only the capacitor at the designated pixel receives a charge and is held until the next refresh cycle. Essentially, each transistor acts as an active switch. By incorporating an active switch, this limits the number of scan lines and eliminates cross-talk issues.

The main problem with TN Film technology is that viewing angles are pretty restrictive, especially vertically, and this is evident by a characteristic severe darkening of the image if you look at the screen from below. Contrast and colour tone shifts can be evident with even a slight movement off-centre, and this is perhaps the main drawback in modern TN Film panels. Some TN Film panels are better than others and there have been improvements over the years to some degree, but they are still far more restrictive with fields of view than other panel technologies.   

MVA ― Multi-domain Vertical Alignment
MVA (Multi-domain Vertical Alignment) displays can offer wide viewing angles, good black depth, fast response times, and good color reproduction and depth. Each pixel within a MVA type TFT consists of three sub-pixels (Red, Green and Blue). Each of these sub-pixels is divided further into two or more sub-pixels, where the liquid crystals are randomly lined up due to the ridged polarized glass. When a charge is applied to the transistor, the crystals twist. With these crystals being randomly placed, it allows the backlight to shine through in all different directions keeping the intended color saturation retained while giving the display a 150deg. viewing angle.

How it works:

1. Light is generated from a backlight source, typically LED. Light is generated as close to white spectrum.

2. Driver ICs will logically control to activate pixels on or off.

» Inactive LCD pixels will block light

» Active pixels will open with the direction of the light to let it pass through.

3. Top Circular polarization is added to enhance contrast

4. Color is added through a color filter to all sub-pixels (R,G,B)

IPS ― In-Plane Switching

In-Plane Switching (IPS) TFTs were developed to improve on the poor viewing angle and the poor color reproduction of TN TFT panels at that time. The crystal molecules move parallel to the panel plane instead of perpendicular to it. This change reduces the amount of light scattering in the matrix, which gives IPS its characteristic wide viewing angles and good color reproduction. Because of its wide viewing angle and accurate color reproduction (with almost no off-angle color shift), IPS is widely employed in high-end monitors aimed at professional graphic artists.

The name In-Plane Switching comes from the crystals in the cells of the IPS panel lying always in the same plane and being always parallel to the panel’s plane (if we don’t take into account the minor interference from the electrodes). When voltage is applied to a cell, the crystals of that cell all make a 90-degrees turn. By the way, an IPS panel lets the backlight pass through in its active state and shutters it in its passive state (when no voltage is applied), so if a thin-film transistor crashes, the corresponding pixel will always remain black, unlike with TN matrices.

IPS (In-Plane Switching) displays provide consistent, accurate color from all viewing angles without blur or grayscale inversion. IPS displays show clear images with fast response time, and no halo effect is produced when touched. Each pixel within an IPS type TFT consists of three sub-pixels (Red, Green and Blue). Each sub-pixel has a pair of electrodes to control the twisting of the Liquid Crystals. Unlike TN type TFTs where the electrodes are on opposing plates, the electrodes in an IPS TFT are on only one of the glass plates (i.e. in the same plane). When voltage is applied to the electrodes, all the Liquid Crystal molecules align in parallel with that plane and allow light to pass through to the polarizers and RGB color filters. In effect, TN displays force the Liquid Crystal molecules perpendicular to the glass which blocks some light from coming out at wide angles, while IPS displays keep the Liquid Crystal molecules in line to allow light through at all angles.

LTPS

Low-temperature polycrystalline silicon (LTPS) is polycrystalline silicon that has been synthesized at relatively low temperatures (~650 °C and lower) compared to in traditional methods (above 900 °C). LTPS is important for display industries, since the use of large glass panels prohibits exposure to deformative high temperatures. More specifically, the use of polycrystalline silicon in thin-film transistors (LTPS-TFT) has high potential for large-scale production of electronic devices like flat panel LCD displays or image sensors.

Polycrystalline silicon (p-Si) is a pure and conductive form of the element composed of many crystallites, or grains of highly ordered crystal lattice. In 1984, studies showed that amorphous silicon (a-Si) is an excellent precursor for forming p-Si films with stable structures and low surface roughness. Silicon film is synthesized by low-pressure chemical vapor deposition (LPCVD) to minimize surface roughness. First, amorphous silicon is deposited at 560–640 °C. Then it is thermally annealed (recrystallized) at 950–1000 °C. Starting with the amorphous film, rather than directly depositing crystals, produces a product with a superior structure and a desired smoothness. In 1988, researchers discovered that further lowering temperature during annealing, together with advanced plasma-enhanced chemical vapor deposition (PECVD),could facilitate even higher degrees of conductivity. These techniques have profoundly impacted the microelectronics, photovoltaic, and display enhancement industries.

LTPS is needed for:

1. Circuits on glass – integrated drivers and scanners and multiplexer – reduces use of external IC and connectors to panel

2. Much smaller TFT ⇒ larger aperture ratio – mobile applications

3. Much more stable than a-Si under high current loading (OLED needs current to drive)

Transflective mode

Transflective LCDs combine elements of both transmissive and reflective characteristics. Ambient light passes through the LCD and hits the semi-reflective layer. Most of the light is then reflected back through the LCD. However some of the light will not be reflected and will be lost. Alternately a backlight can be used to provide the light needed to illuminate the LCD if ambient light is low. Light from the backlight passes through a semi-reflective layer and illuminates the LCD. However as with ambient lighting some of the light does not penetrate the semi-reflective layer and is lost.

Transflective LCDs are used in devices that will operate in a wide variety of lighting conditions (from complete darkness to full sunlight). Under dim lighting conditions transflective LCDs offer visual performance similar to transmissive LCDs, whilst under bright lighting conditions they offer visual performance similar to reflective LCDs. However this performance is a tradeoff because the transflective mode is less efficient due to some light loss.

A simple transflective display is shown as below, in which there are two regions, T and R respectively. The cell gap in two regions are different, dT = 2*dR. This is to maintain the reflection and transmission from two regions are the same intensity, and give same color reproduction, because in the T region, light only goes through the LC layer once, while in the R region, light passes through twice.

Reflective film
Integrate such film (like polarizer) at the front and backside of TFT panel.

Full viewing angle
Full viewing angle film
Integrating the film on the front of TFT-panel

Sunlight readability
1.Pure transflective TFT panel integrating the reflective layer on the pixel, in-cell.
2.Transmissive TFT panel with high brightness-High brightness LEDs + Bright enhancement film.