Dr Gregory L Whiting, Electronic Materials and Devices Laboratory, Palo Alto Research Center, Palo Alto, CA, USA
Corresponding author: [email protected]
Traditional semiconductor device manufacturing is carried out through the selective removal of material using photolithography. In photolithography a geometric pattern is transferred from a master (mask) to a substrate through the use of a light-sensitive chemical (photoresist). This process is then cycled repeatedly to create complex structures. Extremely high resolution can be obtained this way (tens of nanometers), which translates into ever increasing performance. Furthermore, the reliability and predictability of the process enables the use of powerful design tools, which significantly reduce the amount of time it takes to develop new products. The ever cheaper and increasingly complex electronic devices that surround us are a testament to the success of this processing method. However, while these methods have been very successful, novel manufacturing platforms could enable a much wider application space to be addressed which would be both complimentary and disruptive to conventional semiconductor fabrication.
Recent developments have enabled the fabrication of electronic devices using additive printing techniques rather than subtractive photolithography. As a manufacturing method printing brings many benefits including processing over large areas at high speed or over curved surfaces. Using an additive method which places the material only where it is required greatly reduces the number of steps needed compared with a subtractive method where the material is deposited everywhere and then etched back into the required pattern. Printing also readily allows digital methods to be used (such as ink-jet), so that new layouts can be created directly from the design, enabling rapid prototyping and facile customization. Furthermore, printing should also enable manufacturing sites to be set up at a fraction of the cost of conventional semiconductor fabrication lines, allowing smaller, more diverse organizations to be involved in the manufacture of electronic components.
Printing Electronic Devices
Using a broad definition of printing and some ingenuity, effectively any pure material or composition can be deposited onto a surface in a pattern using one of the many printing techniques available. However, while examples can be shown for print-type processing from both the solid1,2 and vapor3 state, the printed electronics field has focused primarily on materials which can be printed from solution using standard techniques such as ink-jet,4 or gravure.5 In general these materials fall into one of two classes, they are either solutions of soluble compounds or suspensions of particles, and the development of soluble organic conductors and semiconductors as well as metal nanoparticles are the principal enabling technologies for printed electronics. These materials can typically be processed at relatively low-temperature (less than 150°C), enabling the use of low-cost plastic substrates, thus allowing for roll-to-roll type processing as well as mechanical flexibility as a feature of the completed device.
The printing of solutions onto a surface is a dynamic process and can be relatively complex. In addition to the properties of the printer itself, the substrate’s surface energy, solubility and roughness as well as the solutions’ surface tension, vapor pressure and solute chemistry all play important roles in controlling the resulting printed feature’s shape, position and structure. To reliably print electronic devices, these factors must all be carefully considered. Practically, this can be illustrated by the example shown in Figure 1,6 which describes an ink-jet printed field-effect transistor (FET), based on the organic semiconductor TIPS-pentacene.7 In order to achieve good quality printed features the surface energy of the underlying substrate must be carefully controlled. If the surface energy is too high then the drop will spread out, leading to a large feature size and low resolution, if the surface energy is too low then the drops will dewet, and break up (this situation in illustrated in Figure 1c). In this example a self-assembled monolayer (SAM) and a polymer film are used to modify the substrate and dielectric surfaces respectively to improve the quality of the printed conductive silver source, drain and gate contacts. Since the semiconductor in FETs must span a gap between two printed conductive features, it will experience a region of surface energy contrast, which can cause the solution to dewet selectively onto the region with higher surface energy, thereby altering the position of the material and potentially breaking connectivity across the source-drain gap. To avoid this problem, the silver contacts were functionalized with a solution processed strong electron acceptor (F4TCNQ), which presents a surface energy similar to that of the SAM modified substrate. Not only does this ensure continuity of the semiconductor between the source and drain, but also improves charge injection and extraction between the semiconductor and the contacts. This improves the device performance as illustrated by the transistor’s transfer characteristics, (Figures 1e & 1f), significantly increasing the field-effect mobility (μ).6,8 While this example illustrates the importance of controlling the properties of the substrate, equally important is control over the properties of the solution to influence film formation, for example to suppress the coffee-ring effect.9
Figure 1. Example of a printed field-effect transistor.6 The top equation (Young‘s relation) relates the surface tensions between the three phases, liquid/vapor (γlv), solid/vapor (γsv) and solid/liquid (γsl) and the contact angle that the drop makes on the surface (θ). a) Diagram of a field-effect transistor indicating the location of significant contact angles for solution processing [substrate surface contact angle (θSUB), source/drain surface contact angle (θSD) and dielectric surface contact angle (θDI). b) Table of contact angles for the indicated surfaces before and after modification. Two different types of printed silver are used, nanoparticle based (Ag-NP) and a soluble silver neodecanoate metal precursor ink (Ag-neo). c) Optical microscope image of a printed Ag-NP line on Cytop, inset shows a water droplet on the surface. d) Optical microscope image of a printed Ag-NP line on Parylene-N coated Cytop, inset shows a water droplet on the surface. e) Transfer characteristic showing both forward and reverse sweeps for a TIPS-pentacene FET with untreated Ag-neo electrodes. f) Transfer characteristic showing both forward and reverse sweeps for a TIPS-pentacene FET with F4TCNQ treated Ag-neo electrodes.
A driving application for printed electronics has been active-matrix backplanes, particularly for reflective displays.4 Fabrication of these devices using printing techniques is desirable as it is a potential route to large area, flexible displays. In order to drive a reflective display (such as an e-ink display) each pixel of the backplane contains a transistor and capacitor. When the transistor is switched on, the capacitor is charged, switching that region of the display between black and white. Other applications of printed FETs have also been shown, including integrated circuits such as complementary invertors,10 and ring oscillators11 as well as memory devices.12
In addition to field-effect transistors, many different types of electronic device can be made using solution processed materials and additive printing techniques. Examples include light-emitting diodes,13 solar cells,14,15 various sensors16,17 and batteries.18 Using printing these individual devices can then be readily integrated with other printed devices to create more complex systems. For example, printed sensors can be integrated with electronics and memory to record environmental conditions. If mobility is required the system could be powered from an integrated printed battery. This type of approach is analogous to the way that discrete elements are combined on a circuit board to create electronic devices. However, rather than using pre-formed elements which were created through a separate process, all of the components can be created and connected directly using the printer and a limited number of inks. Some examples of printed electronic devices are shown in Figure 2.
Figure 2. Various examples of printed electronic devices. a) Reflective display with e-ink media driven by an active-matrix backplane using transistors fabricated using an ink-jet printed organic semiconductor. b) Microscope images of a few pixels of an ink-jet printed active-matrix backplane based on printed silver conductors and printed organic semiconductors. c) An ink-jet printed active-matrix backplane.4 d) A segment of a shift register composed of complementary n- and p-type ink-jet printed organic transistors. e) A ring oscillator composed of complementary n- and p-type ink-jet printed organic transistors. f) A memory array composed of ink-jet printed organic transistors using a ferroelectric polymer dielectric.12 g) An ink-jet printed light sensor.16 h) A screen printed temperature sensor. i) Acoustic and pressure sensors.17 j) A stencil printed battery powering a green LED.18 k) the same stencil printed battery powering the LED after being folded over.
The performance of these low-temperature processed, solution printed devices is often lower than that of devices produced using conventional techniques for a number of reasons. Principally this is due to the types of materials that are used. Soluble organic semiconductors (both polymers and small molecules) are generally disordered and have weak intermolecular interactions that lead to thin-film field-effect mobilities lower than that of crystalline or polycrystalline silicon, but which can be similar to that of amorphous silicon (μ ≈ 1 cm2 V-1 s-1). Furthermore, the resolution of printing techniques is such that the smallest feature size and spacing are on the order of tens of micrometers, considerably larger than is achievable with photolithographic processes. As such printed electronic devices are currently suitable in application areas where large area, flexible processing is beneficial and high-performance is not required. As the performance of printed electronic devices is improved through the use of higher resolution printing techniques,19,20 higher mobility organic semiconductors,21 and inorganic semiconductors which can be processed from solution at lower temperatures,22 a larger application set will be addressed, further broadening the scope of this technology.
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