Emerging Plastic Films for Flexible Electronics Substrates

Transcription

1 Emerging Plastic Films for Substrates Abstract Along with the other enabling technologies for flexible electronics, substrates must be designed to meet the needs of this new branch of microelectronics. The major design requirements for polymer flexible electronics substrates are discussed and 3 polymers that are currently being investigated are introduced as viable solutions. The Need for Flexible Substrates Flexible electronics may have the potential to revolutionize the way the world interacts with electronics; however, there are technological barriers slowing their widespread integration. One of the most fundamental difficulties in flexible electronic systems is the substrate. Ideally, such a material would present a unique balance of properties that would make it suitable for all applications. However, the perfect material is yet to be discovered and at present, compromises must be made and the choice of substrate depends on the application in question. In order to choose a substrate, there are multiple important material properties that must be considered. Stability The most obvious physical trait for a flexible substrate is of course flexibility. Specifically, this means substrate must be able to bend and not crack or lose its other properties. Ideally, the substrate could repeatedly bend without significant long-term degradation, although there are applications in which bending only once is sufficient (signage, permanent displays, structurally embedded systems). Along with bending, it must be robust. It cannot stretch i.e. horizontally deform under stress. This is often

2 difficult to isolate from the polymers ability to bend. While it is assumed that the inorganic layers within the system are themselves thin enough to bend, horizontal deformation of the substrate will undoubtedly cause the inorganics to crack. It is conceivable that with advances in conducting and semi-conducting polymers, eventually there may not need to be any inorganic layers at all, however this is a long way away. Beyond the external physical stresses there includes internal stresses such as thermal stability as well. The substrate is required to be able to withstand reasonable processing temperatures. This means that the melting temperature of the substrate must be sufficiently high. Additionally, the coefficient of thermal expansion (CTE) must be sufficiently low. If the film expands or shrinks (or both) too much under heating, the layers deposited on top (probably a mixture of inorganics that typically have low CTEs) are likely to crack or de-adhere just as they would from an external stress. Thermal stability is important primarily for fabrication reasons because the ability to achieve very low processing temperatures is still the subject of much investigation. Typical operating temperatures are fairly constant and unlikely to play a significant role in stressing the device, although there may be a significant temperature change when the device goes through power cycles. Additionally, there are certainly applications in which a very large range of operating temperatures is desirable, especially for outdoor, government, and scientific uses of flexible electronics. Permeability For polymers to completely replace inorganic substrates they must at least compete with all the material properties offered by inorganic substrates. The most difficult property to replicate is the very low level of permeability offered by inorganic 2/15

3 substrates. Because displays are one of the major driving forces for the development of flexible electronics, and especially with the invention of organic light emitting diodes (OLEDs) in particular, matching the barrier properties of inorganics has become an extremely important issue. OLEDs require environments extremely low in oxygen and water vapor or they will quickly degrade, and the substrate must prevent these from seeping into the device. Despite this however, older display technologies such as liquid crystal displays (LCDs) do not require such stringent barrier properties and are consequently easier to accommodate (although permeability is still of importance). It is also important to remember that there are many applications for flexible electronics that do not require displays. Consequently, substrates do not always need to be completely impenetrable such as the case of radio frequency identification devices (RFIDs) which are shaping up to be a very important application. In fact, some applications are likely to need semi-permeable substrates to allow sensors to operate such as smart bandages. That said, it is unlikely that non-display applications will be able to drive growth in flexible electronics the way that displays do and an impermeable substrate is more versatile because if permeability is desired pinholes can easily be engineered into it. Generally, it is desirable that a flexible substrate exhibit strong barrier properties against both air and moisture. Surface Properties Due to the very thin nature of thin film transistors (TFTs) it is imperative that the surface of the substrate be planar. Any peaks existing on the surface will poke through the thin layers and cause pinholes. This will ultimately lead to failing devices, lower yields, and higher prices. While it is desirable to have very smooth substrates, it is also 3/15

4 necessary to have them adhere strongly to the deposited layers to survive the stress of bending. It is also preferential that the substrate be resistant to solvents and chemicals typically used for etching the layers used to form the active devices being placed on the substrate. This is not always a necessity however, as temporary protective layers may be deposited on the substrate before certain processing steps. Opacity As has been discussed, displays are a major driving force in development of flexible electronic systems, and thus there is a need for optically transparent substrates for light emitting devices. This is most important for bottom-emissive displays where the devices emit their light back down through the substrate instead of up off the top of the device. However, a second substrate needs to be placed on top of the device for encapsulation so for any display type, there must be a transparent substrate layer. Additionally, it would be beneficial to use the same substrate for the top and bottom (potential cost savings) so it is important that high optical clarity be a requisite for flexible electronics substrates. Besides the clarity, the substrate should not exhibit birefringence. This is when index of refraction is dependant on the polarization of the light, which often results in multiple instances of the image behind the material being visible. This is much more of a problem for LCD technology because it is highly dependant on the polarization of light, while other display technologies are impartial to polarization. 4/15

5 Thermal Conductivity Eventually, thermal considerations must be made if high performance electronics are ever made flexible. Heat dissipation through the substrate without aid from an external heat-sync would be the ideal method for heat extraction in flexible electronics because of their thinness and possibly large surface area. It is desirable for the substrate to have a high thermal conductivity, however at this time it is not necessary yet and will not be discussed in any detail. Cost While pricing is not necessarily a material property, it is a major issue that must be addressed once all the other properties have been examined. While flexible electronics offer a wide variety of new options to users, they also offer the possibility of significantly reduced production costs. Rigid substrates have forced electronics production to follow a wafer based production scheme. Each wafer consisting of many integrated circuits (ICs) are processed individually, with the exception of some batch steps such as etching and chemical vapor deposition (CVD). With each wafer requiring some 40 process steps (this is dependant on the level of sophistication of the chip), this becomes a drawn out process that is both time consuming and costly. With so many steps, simply moving the wafer from machine to machine becomes a significant portion of the processing time. In display processing, the size of the substrate has been increased over the years in an attempt to maximize the number of displays that are produced at the same time. Unfortunately, the effective limit of the size substrate that can be reasonably handled is quickly approaching (if it has not indeed been met), and production costs cannot be significantly reduced by this strategy any longer. Current flexible electronics technology has been developed using 5/15

6 these traditional procedures, often by attaching the flexible substrate to a reusable rigid substrate (either a solid wafer or a firm ring to simulate a wafer), so that processing may be accomplished with the same machinery or very similar machinery as the past. This offers only a small cost benefit to the producer assuming the flexible substrate itself is cheaper than the traditional substrate. The real savings of flexible electronics comes from the ability to operate in a roll-to-roll scheme. This allows for a continuous stream of devices to be produced virtually without interruption (only to swap rolls). This is dependant on the ability to produce large rolls of substrate, and the ability to effectively and continuously readjust the process to the inevitable alignment issues that arise from the quickly running substrate. To summarize, a flexible substrate must be physically and thermally stable yet flexible, impermeable, smooth yet sufficiently adhesive, transparent, and above all economically viable. There are a number of materials that meet most of these requirements and could possibly be tuned to function as a flexible electronics substrate. Glass With these considerations in mind, a suitable material must be selected to form the substrate. Silicon has been used as the substrate for most traditional electronics for a long time because of its host of favorable properties (semi-conductivity, easily processed oxide, cost, etc.). With decades of experience with processing silicon it was convenient for the industry to use thin glass (mostly composed of SiO 2 ) as the substrate of choice for producing rigid displays such as TFT-LCDs. Glass meets nearly all the requirements of a flexible substrate. It is both physically and thermally stable and a suitably high melting temperature can be achieved (higher melting temperatures are typically more expensive 6/15

7 because it costs more to produce, so this must be considered). The CTE of display glass such as Corning Eagle 2000 is only 3.18 ppm, extremely close to that of silicon which is typically one of the layers deposited for TFTs [1]. However, this is a much lower CTE than many of the organic materials that are likely to be used in flexible electronics, especially OLEDs. Glass is an extremely good barrier. It has water vapor penetration levels on the order of g/m 2 -day [2]. This is a value so low it is nearly immeasurable and can essentially be considered zero [3]. Glass also has excellent surface properties and can be easily polished if necessary (Corning uses a downdraw process that produces thin glass substrates without the need for polishing). Display glass typically has a roughness value of less than 1 nm (RMS). Glass is also highly resistant to most of the chemicals used in processing. Very few chemicals can erode glass such as HF, which is used to etch SiO 2 in traditional electronics. Glass also has an extremely long history in optics, which again makes it very suitable as a transparent substrate for displays. Unfortunately, glass has some significant drawbacks for use in flexible electronics. It is difficult to produce very large sheets of glass that are thin enough. Glass only becomes flexible with thicknesses thinner than 200 microns while traditional display glass is typically only as thin as 500 microns. Using downdraw processing, films as thin as 30 microns can be produced, but it is limited by narrow width and reduced surface smoothness. Perhaps most importantly, with reduced thickness the price increases, and glass is not suitable for roll-to-roll processing. These combined effects make glass a less than ideal substrate for use in flexible electronics, especially low cost and high volume flexible electronics such as RFIDs. 7/15

8 Metal Metal substrates also meet many of the requirements imposed on flexible substrates. With a very high modulus of elasticity metal films such as stainless steel (200 GPa) are very unlikely to deform in roll-to-roll processing [4]. The CTE of stainless steel at ppm is higher than silicon, but roughly equal to the CTE of the metal interconnects likely to be found within the flexible electronics system. Stainless steel is typically considered completely impermeable. The surfaces of metals can be planarized to meet the surface requirements of a flexible substrate, and can be produced in large rolls facilitating future roll-to-roll manufacturing. Metal substrates have already been integrated into first generation flexible displays. 75 micron steel-foil is used by E Ink for their reflective displays because it meets their need for a lightweight and mechanically stable substrate that is compatible with existing fabrication processes (ie. high temperature fabrication) [5]. This allows for product development without the cost of developing a completely new production scheme to design the devices. In the long run once however it is likely that costs could be reduced by switching to a roll-to-roll processing scheme. The major drawback with a metal substrate is that it is completely opaque, and thus unsuitable for transmissive displays. Polymers Polymers have a long-standing history of use in microelectronics because they offer multiple advantages over traditional materials. Their first use as a substrate was for tape automated bonding (TAB) interconnects [6]. This is in some sense the precursor to reel-to-reel processing as the tape is fed off a roll by sprocket holes punched down the sides like a movie projector. Polymers have also previously been used as a dielectric and 8/15

9 planarizer in interconnects. The principal motivation for incorporating polymers into electronics is their low cost. Beyond this, they can be synthesized to have a wide variety of properties such as high melting temperature, high strength, flexibility or high optical transparency. Polymers are particularly suitable for roll-to-roll processing because of their ability to be highly flexible and lightweight. The low per unit cost of very large volume production in this manner could allow for the production of truly disposable flexible electronics. This would be the enabling technology behind the widespread implementation of RFIDs, smart bandages, and a multitude of low-cost low-performance devices. Another advantage of the light weight of plastic is that it allows for lighter displays for portable applications such as phones and clothing-embedded electronics. Unfortunately, for these dreams to become a reality some significant hurdles must be leaped in the development of polymer substrates. It is as yet impossible to combine all the desired properties into a single polymer and until the chemistry can be perfected, tradeoffs must be made. Difficulties Many polymers suffer from melting temperatures far below those used in traditional electronic processing thus making it impossible to deposit the device layers on them. This can be addressed in two ways: increase the melting temperature or reduce the temperature requirements. Both options prove to be difficult. The maximum temperature that the substrate is processable at is typically much lower than its melting point because when the polymer passes its glass transition temperature it typically becomes too stretchy. Although specialized high temperature polymers do exist and exhibit spectacular stability, they do not meet any of the other requirements imposed on a 9/15

10 flexible substrate (often a lack of flexibility and transparency). This makes it necessary to use polymers that balance thermal stability with the other properties appropriate for a substrate. While temperatures for modern silicon processing surpass 1000 C, new techniques such as laser processing can reduce to processing temperatures to as low as 150 C [7]. This is still a fairly high processing temperature for many polymers, however it is accommodating enough that polymers can be considered a viable material for substrates. The high permeability of polymers poses a serious problem for the reliability of flexible electronics. Exposure to oxygen and water will lead to long-term failure, and in the case of OLEDs it will lead to failure almost immediately. For OLED operation the rate of permeation for oxygen and water must be lower than 10-5 ml/m 2 -day and 10-6 g/m 2 -day (at 40% relative humidity) respectively [8]. This is an extremely low penetration rate as compared to the requirements for LCD technology that requires only 10-1 g/m 2 -day for water [2]. This is a problem that has to be completely resolved, because permeability is high in nearly all forms of polymer. In the past this has been solved by applying barrier layers to polymer films, such as a thin layer of Al [9]. Such metalized plastics are currently used in a wide variety of applications from food packaging to microelectronics. Unfortunately they do not offer the layer of protection necessary for OLEDs and they are not transparent. More recently it has been seen that deposition of a thin oxide layer can significantly improve permeation rates and maintain transparency [10]. As with other processing steps, care must be taken to maintain low temperatures; PECVD is a popular deposition technique for this reason. While the permeability can be reduced by multiple orders of magnitude it is still typically insufficient for OLEDs and 10/15

11 multiple layers must be stacked in order to achieve the low levels necessary. Reasonable levels of permeation have been accomplished by stacking layers of organic and inorganic materials so that the effect of pinholes and defects. With this method, OLEDs with polymer substrates have been shown to have lifetimes on the order of 10 3 hrs [11]. This is not perfect, but it is a step in the right direction, and may be suitable for first generation products. The CTE for polymers is typically much higher than that of inorganics (roughly 1 order of magnitude), however for OLEDs and other organic circuitry the CTE does not pose a problem. Despite these inevitable tradeoffs, (at least until a breakthrough in polymers research solves some of these difficulties) there are multiple polymers that are posed to be useful in certain areas of flexible electronics. Polyimide Polyimide (PI) (fig. 1) has been a part of the electronics industry for decades. Initially it was used as a dielectric material in interconnects, and the substrate for TAB technology. Polyimide is available commercially from DuPont as a product called Kapton. Its advantages include a very high glass transition temperature allowing for processing around 350 C, and a permeability competitive with many other untreated polymers [4,6]. Its use as a substrate in TAB exemplifies its potential for roll-to-roll processing. This makes it an excellent choice as a substrate from a stability and processing standpoint. Unfortunately, polyimide is not optically transparent, but yellow. This limits its uses in display 11/15

12 applications; however, for products such as RFIDs it could prove to be a very competitive option. With a CTE of 20ppm polyimide is very similar to other polymers [12]. Fig 1. Polyimide monomer. 12/15

13 Polytetraflouroethylene Polytetraflouroethylene (PTFE) (fig. 2) has been used in microelectronics for a long time. It is highly resistant to chemicals and wear and has often found its principal use in outer coatings for devices [6]. With a melting temperature in the mid 300 C range it allows for processing steps in the 250C range [4]. Unfortunatly PTFE suffers Because of its dielectric properties and its thermal stability it has recently been implemented by Endicott Interconnect as the substrate for their HyperBGA chip packages [13]. While the HypberBGA packaging is not a truly flexible application, it is not a completely rigid structure and suggests its potential for its Fig 2. PTFE monomer. a/en/1/17/teflon_structure.png continued use in flexible electronics. Polyethylenes Two polymers in the polyethylene family showing significant promise are polyethylene terephthalate (PET) (fig. 3) and polyethylene naphthalate (PEN) (fig. 4). PET is probably best known for being the plastic most soda bottles are made out of. PET and PEN films are commercially available from DuPont under the names Melinex and Teonex respectively. As compared to Fig 3. PET monomer. pedia/en/5/5f/pet.png polyimide films, they offer transparency, but at the expense of glass transition temperature. The glass transition temperature of these polyethylenes is only in Fig 4. PEN monomer. polymerspectro/pen.gif 13/15

14 the range of C [14]. While this limits the processability of these films, as mentioned earlier new techniques in processing can accommodate such temperatures [7]. While permeability is still a serious issue, barrier layers can be applied to these films allowing them to be usable in organic displays [11]. As is evidenced by its widespread use as a substrate for flexible electronics research, this seems to be the most compelling candidate for flexible electronics substrates for OLED displays. Conclusion While no one polymer satisfies all the requirements, there are options available that show considerable promise. Right now, the most suitable for the widest number of applications appears to be PET or PEN, however PI offers the allows for more options in terms of processing as long as transparency isn t a requisite. With all the research in the area of flexible electronics, it is very likely that in the relatively near future the difficulties discussed here will be addressed, allowing for completely flexible integrated systems. 14/15

15 References [1] Corning Incorporated, Corning LCD glass substrates, 2005 [2] B. A. MacDonald et al., Latest developments in polyester film for flexible electronics, presented by M. D. Poliks at Cornell University on March [3] A. Plichta et al., Flexible glass substrates, in Flexible Flat Panel Displays, Ed. G. P. Crawford, John Wiley & Sons, Ltd, 2005, pp [4] [5] M. McCreary et al., " Flexible active-matrix electronic ink display," Nature, vol. 423, 8 May, pp. 136, [6] D. S. Soane, Polymers in Microelectronics, New York: Elsevier, [7] M. Thompson, Laser processing of Si-TFT s on plastic: technology and lessons from FlexICs, presented at Cornell University on April [8] B. A. MacDonald et al., "Engineered films for display technologies," in Flexible Flat Panel Displays, Ed. G. P. Crawford, John Wiley & Sons, Ltd, 2005, pp [9] G. L. Graff et al., Barrier layer technology for flexible displays, in Flexible Flat Panel Displays, Ed. G. P. Crawford, John Wiley & Sons, Ltd, 2005, pp [10] A. G. Erlat, SiO x gas barrier coatings on polymer substrates: morphology and gas transport considerations, J. Phys. Chem. B, vol. 103, pp , [11] M. S. Weaver, Organic light-emmitting devices with extended operating lifetimes on plastic substrates, Applied Physics Letters, vol. 81, 16, 14 Oct., pp , 2002 [12] DuPont Kapton HN datasheet. [13] Endicott Interconnect kaging/hyperbga/index.html [14] DuPont Teijin Films, Teonex datasheet. 15/15