Plastic logic's transistor technology

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Plastic Logic Technology Overview

Beginnings

Over the last three decades organic electronics, which is electronics based on carbon rather than silicon, has been extensively researched. In 1998 Professor Richard Friend’s group at the University

of Cambridge, UK published a seminal work using an organic transistor to drive an organic light emitting diode1. Two years later Plastic Logic was founded to develop and commercialize the successes of the work done by Professor Friend, Professor Henning Sirringhaus and their teams at

the Cavendish Laboratory.

Plastic Logic soon focused its activity on transistor arrays for displays. Organic materials are typically flexible, lightweight and robust. Plastic Logic decided to exploit these attributes by developing its arrays on a plastic base which would then allow any final display to be lighter and more robust than

equivalent silicon-‐based products.

At the same time, teams of researchers began pushing for high-‐quality materials that would meet the rigorous demands of a commercial environment. Several leading materials companies started to put serious effort into refining their materials for use in this new application space and Plastic

Logic developed close relationships with many industrial research teams to guide their work and exploit the results at the earliest opportunity.

By mid-‐2004 Plastic Logic had developed small area displays with relatively low resolution which were extremely robust, as evidenced by the photographs in Figure 1 a .

Figure 1 b: Demonstrating the robustness of Plastic Logic’s displays – note the small bend radius

Just over a year later, in late 2005, the company had progressed its technology to large area displays

with much higher resolution as shown in Figure 2.

1 Sirringhaus et al, Science (1998) Vol 280 page 1741-‐1744

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Figure 2: An example of Plastic Logic’s displays in late 2005

By early 2007, Plastic Logic had identified a site for its manufacturing facility in Dresden, Germany and had begun the factory build. Ideally placed in the heart of Silicon Saxony, Plastic Logic has

drawn a high-‐caliber team with extensive manufacturing experience from the surrounding region, where many silicon manufacturing facilities are based. The teams in Dresden and Cambridge worked closely together to ensure that the transfer of the technology from lab to fab would be as smooth

and as efficient as possible.

Only eighteen months later the Dresden manufacturing facility opened its doors and began producing flexible displays on a scale never seen previously in the organic electronics community.

Figure 3: a) Aerial image of Plastic Logic’s manufacturing facility in Dresden Germany and b) showing the size of the motherplates used in the factory.

In parallel the company has been ramping its product development, marketing, and business development and activities in the US to ultimately complete the transition of Plastic Logic from a

small R&D company, spun out of academia, to a product-‐based organization with the facilities and know-‐how to take technologies from the lab bench to mass market.

An Introduction to Plastic Logic Technology

Now that the field of organic electronics is firmly on its journey to industrial maturity it is important to step back and recognize the key components in taking a small scale, academic activity and scaling it to the realities of a commercial environment. A balance between device performance and ease of

manufacture must be struck for commercial success.

For the past 10 years, Plastic Logic has been at the forefront of this progression— taking its own organic transistor technology from a lab bench to a high tech manufacturing environment— and is

a) b)

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therefore well placed to discuss the considerations from both the research and manufacturing perspectives.

This document will give an overview of the key considerations which frame the transistor’s

performance and manufacturing considerations based on Plastic Logic’s learnings.

Transistors

Transistors are formed from three electrodes, a dialectric and a semiconductor. The electrodes control the current flow by way of the voltage applied to them. The semiconductor is the material through which the current flows. A schematic is shown in Figure 4.

Figure 4: A generic top-‐gate transistor in cross-‐section.

A good transistor is analogous to a good water tap.

1) When you turn the tap on, water soon starts to flow and as you turn it on a little more the

water flows faster until it is soon flowing very fast. Similarly for a transistor, the current, which is a flow of electric charge, should begin to flow once a small voltage is applied and as you increase the voltage the current should increase until you have a surfeit of current for

your application. 2) When you turn the tap off, it shouldn’t allow any water to leak out. Similarly a transistor

should not allow current to flow when it is off.

In the vast majority of display applications the transistors use silicon as the semiconductor because it is a well-‐established technology that can provide ample current to drive the LCD, OLED, electrophoretic or whichever other screen technology is being used. However, silicon has its

drawbacks in terms of cost, ease of device manufacture and fragility. In these areas organic electronics offer an advantage. Made primarily from materials which can be processed from solution, the transistors are inherently simpler and cheaper to manufacture. Even though today

silicon can have higher performance than organic semiconductors, there are many applications where the performance advantage of silicon is not required and where an organic electronics solution is more cost effective.

The key metric of semiconductor performance is mobility. This is effectively a measure of the speed

at which the charge can flow in the semiconductor. The required mobility is dependent on the application. The faster the application, the higher the mobility needed. For a television, the picture changes rapidly and hence the mobility required is high. Where the image changes more slowly,

such as in an e-‐reader, the mobility can be much lower.

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The typical mobility of crystalline silicon is on the order of 1000cm2/Vs but many applications use poly-‐crystalline silicon (mobility >50cm2/Vs ) or amorphous silicon (mobility ~0.5cm2/Vs) as the

performance is still adequate but the cost of manufacture is greatly reduced.

Within organic transistors there is also a mobility range available. Pentacene, which is a crystalline material, can achieve mobilities of 10cm2/Vs but it is difficult to process on any meaningful scale. At the other end of the spectrum, fully amorphous polymer devices are simple to manufacture. They

can be made and driven in air, without encapsulation, and have a whole host of attributes which are extremely desirable in a manufacturing context, but they can only reach mobilities of around 0.05cm2/Vs. Nevertheless this is still sufficient for a number of applications. For example,

electrophoretic displays, which are used to make e-‐paper and use reflected light rather than an internal backlight, can be successfully driven with mobilities in this range. The mobility values of various semiconductors are summarised in Figure 5.

Figure 5: Mobility levels of various semiconductors.

Much is made in the academic literature about high mobility devices and often this is the metric which denotes whether or not a device is a success. However, the highest mobility devices are often

made in nitrogen environments using toxic or expensive solvents and using processes which are slow and inherently small scale. Such devices are of no use in commercial products. Consistent devices are needed, made from materials which are easy to manufacture on a large scale, at a sensible cost,

with good reproducibility and which are easy to process in air. This is often forgotten in the quest for headline mobility values. Fortunately, over the last few years there has been increasing effort on parameters other than mobility. Now that materials manufacturers are becoming more

acclimated with industrial requirements, materials are starting to appear which are closer to pentacene in performance whilst retaining many of the desirable processing attributes of the amorphous materials. This development will open up display applications beyond electrophoretic

into LCD and OLED displays. Additionally this advancement will enable organic electronics use in a number of non-‐display applications such as logic.

Source: A. Salleo

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Mobility isn’t the only factor which determines the current that is available. The size of the transistor is also important. If the transistor is large enough then a high current can be achieved even with a

low mobility.

In practice, the space available for the transistor is usually limited. For example, a laptop screen is backlit and the light must pass through the transistor array to the user. The transistor is not transmissive and hence needs to be as small as possible if the front-‐of-‐screen performance is not to

be impaired. In an electrophoretic application, where reflected light is used, the size of the transistor will not affect the user experience and this substantially relaxes the size constraint, allowing the devices to be much larger and consequently allowing the transistor mobility to be much

lower. There are still limits however. For example, in active matrix displays at least one transistor is required to drive each pixel. Therefore, in a display with a resolution of 200 pixels per inch all of the requirements for the pixel need to fit within a space 127 µm x 127 µm in size.

In an ideal transistor the current would begin to flow once a small voltage has been applied to the device to turn it on. Usually, however, there is a resistance preventing current flow when the voltage begins to be applied. This resistance is caused by poor physical or electrical contact between

the semiconductor and the electrode, known as contact resistance, and/or by the bulk of the semiconductor hindering the charge as it travels to the semiconductor/dielectric interface where the

charge flow occurs.

In order for current to flow the voltage must be increased to overcome the resistance. The size of the resistance is especially important in mobile applications because the greater the voltage that is required to obtain a useful current, the quicker the battery will run down. It is therefore desirable to

minimize any resistance as far as possible, by appropriate choice of materials and careful consideration of the cleaning methods and device processing methods employed.

It is also wasteful if a high voltage is needed to turn the transistor off as this also requires power which will shorten the battery run time. Thus it is preferred if the transistor is off with no significant

current flow when no voltages are applied. Additionally, a high current flow with only minimal voltage increase is optimum so the device should switch from off to on with only a small applied voltage.

The materials choice for each of the components of the transistor (source, drain, gate,

semiconductor and dielectric) can have significant implications for its performance and the relative ease that charge can flow. The source and drain electrodes must be chosen so that charge can flow easily from the source through the semiconductor to the drain when the transistor is on. The

dielectric must also be carefully chosen as the wrong dielectric can reduce the device mobility by several orders of magnitude which would render the device worthless. Plastic Logic has long realized the importance of the dielectric choice and has extensive experience in matching the

dielectric to the semiconductor. Materials suppliers are now also seriously investigating the dielectric selection to provide the combination of dielectric and semiconductor to device companies rather than just providing the semiconductor, which was previously the case.

From this discussion it is clear that, when designing a transistor for the mass market, mobility is only

part of the story. The ease at which the device can be turned on and off is also important and depends on the choice of materials. However, there are yet further considerations when the leap is

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made from the individual transistor to the active matrix array for a display application. For example, in arrays, device uniformity is key. It is expected that devices will perform similarly to one another,

otherwise visual differences may be observable in the resultant display. Operational stability is also required so that the array continues to function predictably throughout its life, with all of the individual devices aging consistently regardless of how they have been driven.

Active Matrix Arrays for Display Applications

Active matrix arrays consist of a series of transistors laid out in a grid. The isolated gate line shown

in Figure 4 is extended to connect all transistors in the same row and the source line in Figure 4 is extended to connect all the transistors in the same column. This allows each transistor to be uniquely addressed. These arrays can then be used to drive display media, for example,

electrophoretic media (such as E Ink), LCD or OLED. In the simplest architecture, each pixel within the display is controlled by one transistor and if the transistor is switched on then the pixel will switch and otherwise will not switch. A schematic is shown in Figure 6a with the display pixels

overlaid in Figure 6b.

Figure 6: a) A transistor array and b) Display pixels overlaying the transistor array

Voltage is applied to the first gate line and concurrently each source line in parallel, this is then repeated with the second gate line and so on until all the transistors have been addressed and all

the pixels are on or off as required for the image. Because the millions of transistors within the array are addressed one row at a time, any one transistor is only addressed for a very short period. In the example in Figure 6b, voltages are applied to turn on the TFT at the Source-‐2 Gate-‐2

intersection (S2G2) and change the associated pixel to its on state, which is white, and then applied to S4G3 and finally S2G4 to change their pixel colors to white. The remaining transistors are left in their off state and the pixels remain black.

LCD color displays use this basic principle and then use color filters distributed in a pattern across

the display to give red, green and blue pixels as well as white ones. This methodology can also be used for reflective technologies although there are also other device architectures that can be employed.

The gate lines and source lines running across and down the transistor array can form transistors

other than those in the array if the array is poorly constructed. These unwanted transistors, called parasitics, can cause the display pixels to turn on when they should be off. It is important that

a) b)

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careful consideration is given to where connections are routed and how the devices are built up so that parasitic devices are avoided. Plastic Logic has extensive knowledge in array design to minimize

the impact of parasitic devices.

Parasitic transistors are not the only source of unwanted current. Transistors within the array can also leak current to one another so it is important to ensure there is no path for the current to travel between neighboring devices.

While we have focused on transistors, these are not the only devices within the array and the other

components must not be neglected. During the time that the transistor is not being addressed the charge it produced during the address time needs to be retained until it is next addressed. This is achieved by the use of a storage capacitor which comprises two plates separated by a dielectric. The

drain pad of the transistor makes up one of the plates of the capacitor. The cross-‐section is shown in Figure 7.

Figure 7: Cross-‐section of TFT and capacitor combination

The metric for the capacitor is known as capacitance. The capacitance is a measure of the ability of the capacitor to store charge and is determined by the capacitor’s area, the separation of the plates,

and a measure of the dielectric known as the dielectric constant.

For any given capacitance the area of the capacitor can be reduced if the dielectric constant is increased. As space is at a premium within the array it would be ideal to have a dielectric with a high dielectric constant so that the capacitor can be as small as possible. Unfortunately most organic

transistors have relatively small dielectric constants, when compared to inorganic transistors, and consequently the capacitor structure is often larger than would ideally be the case. The competing requirements of the transistor and the capacitor present one of the problems that has to be

addressed for success in the displays market. There are several routes to solve the problem, all of which present challenges.

1) The transistor could be shrunk to allow more space for the capacitor, although this will

increase the mobility requirement.

2) The dielectric used could have a high dielectric constant so that the capacitor can be small,

but this will impair transistor performance.

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3) A capacitor and the transistor could be processed such that they use different dielectrics from one another, although this will certainly add complexity and consequently cost to the

system.

As was previously stated, materials suppliers have recently started to realize that the dielectric must also be carefully optimized to match the semiconductor and in so doing maximize the transistor mobility. Materials manufacturers also need to extend this thinking and realize that the transistor is

not the only component in the array and that developing a semiconductor that could work with high dielectric constant materials would be very desirable.

The array structure is complicated further because the bottom capacitor plate (the drain pad) needs to be in direct contact with the display media. As shown in Figure 7, the drain pad is underneath all

of the other layers so it therefore has to be brought to the top of the stack. This is achieved by adding an interlayer dielectric, making a hole in the stack of layers and adding a metal or polymeric conductor on top to effectively move the bottom capacitor plate from the bottom of the stack to the

top.

A generic repeat unit in the active matrix array would therefore be:

Figure 8: a) Plan view of a generic repeat unit in an active matrix array, b) cross-‐section of repeat unit.

Thus once the transistors are incorporated into a real-‐world application there are many aspects

which must be considered and not just the design and performance of the transistor itself. This is true not only in displays but also in non-‐display applications such as sensors or RFID.

Non-‐Display Applications

Transistors can either be p-‐type or n-‐type depending on whether they are turned on by applying negative voltages or positive ones. For display applications an active matrix array can be produced

using transistors which are either all p-‐type or all n-‐type. Logic circuits, however, are most efficient if both n-‐type and p-‐type transistors are available.

To date the vast majority of organic transistors are p-‐type because p-‐type semiconductors are the most advanced in terms of our understanding and also in terms of the key performance metrics such

as mobility. However n-‐type transistors would bring many advantages even into the displays space.

a) b)

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A display requires drivers in order to address the pixels correctly and in Plastic Logic’s case all of the driving electronics which surround the active matrix array are made from silicon. Some of this could

be replaced by organic transistors if both p and n-‐type materials were available. This would allow the advantages of organic materials, namely ease of processing, cost and robustness to be utilized in more of the system. Some companies are beginning to seriously develop n-‐type materials and Plastic

Logic is actively engaged in the testing and development of these materials to ensure they reach commercial viability as soon as is practicable.

In addition to n-‐type devices, Plastic Logic has also given significant consideration into how the devices are constructed so that unwanted capacitances and currents can be removed. Plastic Logic’s

IP portfolio extends broadly over high-‐resolution printing methods, where sub-‐micron channel lengths have been demonstrated, and fine-‐feature patterning techniques, both of which help to reduce parasitics and improve the device speed.

As the transistor mobility improves for commercially viable devices in both p and n-‐type devices, and

deposition methods enable fine features and low parasitics, it becomes possible for organic electronics to move into other application areas such as RFID, Sensors, ASIC, and smartcards.

Reliability

Of paramount importance when discussing any commercial application is the reliability of the electronic components in the product and the reliability of the process used to make them. The

product will not be a commercial success if the transistors stop working when they are exposed to heat, light, water, or wear out after being operated for a few weeks. In the case of displays this would create ‘dead’ pixels which remain permanently off and in logic circuits it would prevent the

circuit from operating correctly and cause the product to fail. While it would be desirable for the transistors to always perform the same way in all environments and all operating conditions this is

unrealistic. Temperature and moisture will change the device behaviour not only in organic devices but in silicon and other semiconductors too. Additionally, as with most things, extensive use will cause degradation over time. When designing a product it is important to investigate the operation

of the devices in a range of environments and under a range of operating conditions which are specific to the application in question. The changes to the device performance caused by varying these factors can then be accounted for in the design of the devices such that it doesn’t cause a

difference in the visual performance of the display or the operation of the logic circuit. When completing such a design it is important to remember that it is the performance of the worst transistor that is of most interest. The worst transistor in the display must still be functional at the

end of the product life and therefore the worst transistor dictates the pixel design. If the product is to reach its full potential and thereby maximize revenue for the manufacturer, uniformity across all the devices within the display is key.

The importance of uniformity is also clear when considering how the products will be tested to

ensure they are fit for purpose. It is impossible to fully test every device in every product and therefore it is important that the transistor behaviour is consistent and predictable such that a basic test will show whether the product will work as expected.

Again this highlights the importance of using materials which can be easily mass-‐produced and are

well understood so that the variability between devices is minimized.

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It is not only variability in the materials which can cause variation in the device performance. Variability in the process can have the same effect and hence the manufacturing process needs to be

robust and repeatable. An unreliable process will reduce yield, increase cost and make forecasting product availability difficult. This needs to be considered at the outset, in the initial device design, as a complicated and intricate process will be harder to maintain than a straightforward and simple

one.

Manufacturing

The requirements and intricacies of manufacturing are worthy of a document in their own right. Here, a couple of examples are used to give a flavor of some of the considerations involved in the transition from a lab-‐based environment to a manufacturing one.

When moving from an R&D environment to manufacturing every minutia has to be validated and

understood. Issues that affect a couple of displays in the lab could wipe out whole batches of displays in a factory, which would be extremely costly. Thus it is important to understand all the parameters so that issues can be rectified quickly with minimal impact on production.

As an example, one major consideration is display build time. In a lab, where displays are being

processed one at a time, tight time constraints can be accommodated. For example, if one layer cannot be exposed to air for more than an hour or one clean or treatment process wears off after ten minutes, then displays can be moved from one station to another quickly in order to

accommodate this criterion. In a manufacturing facility however, such tight time constraints cause complexity because displays are usually processed in relatively large batches using automated equipment, meaning that any one display must wait for all the other displays ahead of it before it

goes through a particular process. Any delay could potentially push large numbers of displays beyond the allowable time between process steps. Consequently any time criticalities need to be

fully understood, not only so batches are processed through genuinely critical steps within the allotted time but also so perfectly good batches are not scrapped for failing to meet an arbitrary time constraint.

A second issue in moving from the lab to manufacturing is how to scale the processing of flexible

substrates to a size not previously used in industry. The manufacturing of organic electronic devices on flexible substrates is still in its infancy. Equipment suppliers are used to sheet fed, glass based products and their tools are designed with rigid, inflexible substrates in mind. Plastic Logic

addressed this conundrum by laminating its flexible substrate to glass so that it could be processed as if it were glass. This minimized the equipment modifications, and removed the challenge from each and every tool supplier, who might each have different, and potentially mutually exclusive,

ways of addressing the issue, and moved it squarely back to Plastic Logic. This allowed Plastic Logic to develop unrivalled expertise and competency in the handling and processing of flexible substrates and their lamination to glass and facilitated a deep understanding of how the substrate is affected

by factors such as temperature, chemicals and humidity, which is invaluable information not only at the lamination stages but for all of the other processing steps.These examples help to show that to successfully progress out of the lab and into a factory there are not only scientific considerations,

such as how the transistor degrades or whether devices are uniform, but also practical considerations, which are every bit as important and which must also be addressed.

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Conclusion

In a commercial environment it is not enough to design a transistor purely on the basis of high mobility. The optimum transistor is the one which can be processed simply, affordably, consistently,

and which has a performance that is sufficient for the task in hand. Additionally the requirements of the other components of the system, for example the capacitor in the display, must also be accounted for right at the outset of the design. This ensures that the design optimizes the system

rather than any individual component. In designing a system, Plastic Logic understands the balance that must be struck between the myriad of influencing factors, and this is critical to commercial success.

Plastic Logic has unrivalled expertise in developing organic electronics for consumer products and in

such a rapidly changing technology environment it is vital to remain at the forefront of research and development for early integration of new features and hence is a competitive advantage. Plastic Logic is devoting significant resources to the integration of a compatible color technology and

optimum front-‐of-‐screen performance. Plastic Logic is also focused on the continued development of the p-‐type transistors in its array, using materials with similar performance to amorphous silicon. For further cost benefit and feature enhancement it is also developing n-‐type transistors which will,

when integrated successfully, expand the functionality of organic electronics beyond the transistor array and into the surrounding logic circuits.

In Plastic Logic the research teams are highly aligned with the manufacturing engineers to procure suitable equipment that can meet the challenges of mass manufacture, both in Dresden and in

Plastic Logic’s planned second manufacturing facility in Russia. Close alignment ensures rapid inclusion of new advances into the end product.

This work will ensure that Plastic Logic continues to advance its technology platform for the future.

Plastic Logic Inc. Headquarters 650 Castro Street, Suite 500 Mountain View, CA 94041 USA Phone: +1 (650) 584-2100 Fax: +1 (650) 584-2101