Organic VPD shows promise for OLED volume production
07/01/2002
By J.K. Mahon, T. Zhou, Universal Display Corp., Ewing, New Jersey
Max Shtein, S.R. Forrest, Princeton University, Princeton, New Jersey;
M. Schwambera, N. Meyer, AIXTRON AG, Aachen, Germany
Overview
Moving rapidly from basic research into low-volume manufacturing, organic light-emitting device technology is widely recognized as the next major technology for flat panel displays. Organic light-emitting devices also have tremendous potential in a variety of other optoelectronics such as photodetectors, solar cells, and thin-film transistors, as well as the potential to revolutionize the general illumination market. For peak performance, many of these devices rely on the use of organic small-molecule materials that are typically deposited using vacuum thermal evaporation. Though such vacuum evaporation is finding application in first-generation organic light-emitting device manufacturing lines, an innovative deposition technology enabling numerous process advantages — organic vapor phase deposition — is being developed through a partnership between Universal Display Corp. (UDC), Princeton University, and AIXTRON AG.
Organic vapor phase deposition (OVPD) has the potential to offer large-area film uniformity, precise doping control, low particulate generation, and high source material utilization for flat panel displays (FPDs). The technology has also shown promise for high-resolution patterning for large-area, high-throughput organic electronic manufacturing. These advantages are being demonstrated in the first pilot line OVPD system, which consists of a gas/source cabinet and chamber module attached through a central robot platform to other process modules. One is a Tokki-designed and -manufactured metal electrode (cathode) deposition chamber. Also included are a robotic shadow mask attachment, a robotic substrate-flipping module, and an entry/exit to a "glove box" system.
Today, full-color OLED displays are fabricated in pilot line systems around the world and are increasingly moving into full-scale production. The basic process begins with the cleaning and patterning of an indium tin oxide (ITO)-coated substrate (typically glass or plastic) using photolithographic and wet etching techniques to form the bottom electrodes (i.e., the anodes).
The patterned ITO-coated substrate is then loaded into the organic deposition system where a number of sequential process steps occur. First, the substrate is cleaned using one of a number of plasma-enhanced techniques. Then, a series of organic layers, each ranging from 50-500Å in thickness, are deposited (Fig. 1a). In this stack, one or more of the layers is commonly doped with a small percentage of another organic material.
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Figure 1. OLED overview. a) Cross section of a full-color OLED display with subpixel size and pitches of 80 and 100μm, respectively, for ~100dpi display resolution. OLED device thickness is ~0.3μm. b) The deposition/patterning process shows how the substrate 1) is deposited and patterned with 2) red-, 3) green-, and 4) blue-emitting materials by translating a shadow mask from left to right prior to depositing 5) a metallic top contact or cathode.
To achieve a full-color display, red-, green-, and blue-emitting chromophores must be selectively deposited into pixels closely positioned side by side (Fig. 1b). The conventional way to do this is to deposit through a precisely aligned mechanical shadow mask spatially translated between red, green, and blue pixel positions. To finish the device, the top electrode, typically a bilayer of lithium fluoride and aluminum, is then deposited. Finally, the device is packaged to protect it from the environment.
 Figure 2. The conventional VTE process. |
The deposition of these organic small-molecule materials is typically accomplished using vacuum thermal evaporation (VTE) and mechanical shadow masks (Fig. 2). Organic materials are evaporated upward from point sources onto a downward-facing substrate in a high-vacuum (10-6 to 10-11) chamber.
On a small scale, this practice works well, but over large areas, film thickness uniformity, deposition rate, and dopant concentrations are difficult to control due to geometric limitations of line-of-sight evaporation from point sources. Additionally, a large portion of the organics ends up on the cold sidewalls and other surfaces of the vacuum chamber. Over time, these organics create thick coatings that flake off to contaminate the system. As a result, materials utilization is low, typically about 5%, and the system must be shut down frequently for cleaning. While significant engineering has gone into improving the effectiveness of shadow mask patterning in VTE, gravity creates additional challenges.
Novel OVPD processing
Several years ago, a research team led by Stephen R. Forrest at Princeton University demonstrated that OVPD could deposit high-quality organic films by using an inert carrier gas to precisely transfer the materials onto a cooled substrate in a hot-walled, low-pressure chamber. Early on, heterostructure OLEDs consisting of N,N-di-(3-methylphenyl)-N,Ndiphenyl-4,4-diaminobiphenyl (TPD) and aluminum tris(8-hydroxyquinoline) (Alq3), as well as optically pumped organic lasers consisting of rhodamine 6G doped into Alq3, were shown with device performance comparable to similar devices fabricated by VTE [1, 2]. More recently, polycrystalline pentacene films for thin-film transistors were grown by OVPD with record-setting field-effect mobilities >1.4 cm2/V-sec [3]. In addition to providing comparable, if not better, performance than VTE, the OVPD process offers other additional advantages, including finer process control, reproducibility, and scalability for cost-effective, full-scale manufacturing.
The original deposition apparatus developed at Princeton consists of an 11cm-dia. x 150cm-long Pyrex multibarrel glass chamber system with in situ temperature and thickness measurement capability shown schematically in Fig. 3a [4, 5]. The center of the chamber is positioned inside of a three-zone furnace used to establish a temperature gradient along the chamber axis. The desired organic source temperature is then attained by appropriate positioning of each source cell along the gradient within the chamber. In addition to the source temperature, carrier gas flow rates, which are regulated by mass flow controllers, and the chamber pressure, which is regulated between 0.01-5torr, are also used to control deposition conditions. Figure 3b illustrates how the vapor pressure of organics exiting the source varies with flow rate and temperature to effect two evaporation regimes. In the "equilibrium" evaporation regime, the flux of organic species is proportional to the vapor pressure and the carrier gas flow rate, but in the "kinetic" evaporation regime, the flux leaving the source is independent of the carrier gas flow. The ability to operate OVPD in either of these two regimes provides the ability to finely control the composition, structure, and quality of the organic thin films.
Elements of the OVPD tool
Based on this research tool and patented process, UDC, AIXTRON AG, and Princeton University jointly designed, developed, and recently installed a pilot-scale OVPD system (Fig. 4). The four key elements of the OVPD tool are 1) the external organic sources, 2) the showerhead configuration, 3) the OVPD chamber, and 4) the shadow mask capability.
 Figure 4. OVPD system configuration. |
Organic sources. The organic small-molecule materials are placed in separate, thermally controlled cells external to the OVPD chamber [6]. These organic materials typically have sufficiently high vapor pressures to evaporate at temperatures below 400°C, and can be evaporated from their ovens and transported in the vapor phase by a pre-heated inert carrier gas. The carrier gas — often nitrogen — transports these materials through heated lines into the showerhead. Once entrained in dilute concentrations in the carrier gas, each material can be reduced to below its evaporation temperature without condensing. This diluting effect offers additional process control capabilities useful not only for simple structures, but also for complicated ones. Therefore, the OVPD process should be less destructive to these sometimes thermally fragile organic compounds, and multiple materials with different evaporation temperatures can be combined. Additionally, because the sources are independent and external, there is no source-to-source contamination. As a result, multiple materials and layers can be reproducibly deposited in one chamber.
With three key computer-controlled process variables, i.e., source temperature, carrier gas flow rate, and chamber pressure, the concentration of organics in the carrier gas stream can be precisely controlled to achieve reproducible and precise film thickness over a wide range of deposition rates. Also, doping of one organic species into another can also be carefully controlled. In VTE, the dopant deposition rate, which is solely controlled by the source temperature, is subject to fluctuations due to inaccuracies associated with the thermal mass of the container and the material preparation. In contrast, in OVPD, the dopant deposition rate can be more directly regulated by carrier gas flow rate. Because gas flow rates can be adjusted with very short response times, desired doping ratios can be rapidly achieved. This is particularly important for the growth of organic light-emitting devices (OLEDs), where the precise introduction of small concentrations of luminescent dopants into thin layers of host material is required. The doping concentration of a red lumophore, Dcm2, doped into an Alq3 host, is shown in Fig. 5. The very low doping concentration (i.e., 0.5%) and the precise control over a range in concentrations (as demonstrated by the low scatter in data) are extremely difficult, and, indeed, nearly impossible to attain using VTE. This feature also means that continuously varying doping ratios can be achieved to build graded layers and a host of new device structures with graded-layer interfaces.
 Figure 5. OVPD doping control across a wide range by dopant source temperature at a set pressure. |
Showerhead. The organic sources pass through a small cylindrical plenum into an AIXTRON-proprietary, closed-coupled showerhead, which is designed to provide flow patterns that deposit materials uniformly across a substrate located only several centimeters below. All surfaces in the reactor are heated to prevent condensation, whereas the substrate is cooled to room temperature to facilitate condensation on itself. The plenum is kept small and the gas flows are controlled by high-precision mass flow controllers to obtain either sharp or graded-layer interfaces.
Chamber. The OVPD chamber has a flat or "pancake" geometry with the heated sources feeding in from the top for top-down deposition. The top-down configuration is possible, in part, because of the closed-couple showerhead design, and because the chamber walls are heated to minimize deposition buildup that can flake onto the substrate. As a result, materials only deposit where it is cold, i.e., onto the substrate, so that in principle, a materials utilization >50% is possible. Because the showerhead can be designed to maintain constant source-to-substrate distance over any size or configuration, this chamber design is easily scaleable for larger substrate sizes and shapes.
Shadow masking. OVPD and this specific chamber design also provide a conducive environment for shadow mask patterning. In this case, the mask can be placed on top of the substrate for better mask-to-substrate distance control. The mask thickness can be dictated by the desired pattern shape instead of the need for rigidity. As a result, precise, reproducible pixel profiles can be achieved. In fact, scanning electron microscope (SEM) images of the pattern of a 1000Å-thick film of Alq3 deposited through a 7μm opening by OVPD are comparable to those by VTE. The resulting deposited organic film better replicates the mask opening of 7μm. As a result, OVPD technology may also provide the foundation for high-resolution patterning for large-area, high-throughput OLED manufacture. The key features of OVPD are summarized in the table.
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Roadmap to the future
With the first pilot line OVPD system installation underway, preparations are being made to address the remaining challenges in this advanced organic film deposition technology for high-volume manufacturing. Building on the work done by the Princeton team on the first research system, the roadmap begins with a detailed process study to determine the effect of deposition conditions on a single organic thin film [4]. Specifically, the organic film surface morphology and thickness uniformity will be studied as a function of pressure, flow, and substrate temperature. From this data, it will be possible to determine the limits on deposition rate for a single thin film and apply the experience to the pilot line tool.
The next step will be the actual OLED device fabrication and test. Starting with the basic OLED structures already fabricated by VTE, a comparison of optical and operation lifetime performance will be made. As part of the evaluation, systematic experiments to evaluate the enhanced doping control afforded by OVPD will be undertaken.
Once the baseline OLED performance is proven, advanced OLED structures not practical with today's VTE will be built. These designs are based on interface grading and finely controlled mixed layer structures for reducing voltage, increasing efficiency, and extending device lifetime [7].
In parallel to the OLED performance evaluation, a series of experiments to identify the pixel resolution that can be achieved by OVPD deposition through a high-precision metal shadow mask are planned. The pilot line system is equipped with the capability for in situ attachment of shadow masks to the substrate prior to film deposition, allowing the study of the effects of process conditions and shadow mask design on high-resolution patterning of organic thin films by OVPD.
A series of experiments to determine the minimum pixel-to-pixel spacing and the overall uniformity of finely spaced pixels on a 6x6 in2 substrate will be run. This evaluation is critical to prove the applicability of this technology for high-resolution (>100dpi) monochrome and full-color OLED displays. As part of the study, such displays will be fabricated.
Finally, data will be gathered to evaluate the manufacturing advantages afforded by OVPD. As part of this effort, the source utilization efficiency, total actual cycle time, and additional cost of ownership parameters will be studied.
In addition to batch cluster operation, the OVPD process and the showerhead design support adaptation to continuous or "roll-to-roll" processing. Based on the adaptation of the Princeton research tool, where it was used to deposit an OLED stack on continuously translated ITO-coated plastic substrates (Fig. 6), the pilot line tool may be used to collect data for developing a design for pilot line roll-to-roll demonstration. Developing such process technology (for plastic, metal foil, and even paper) is a significant opportunity for an industry looking for truly innovative ways to cost-effectively produce organic electronics for a myriad of applications.
Conclusion
Based on development to date, the OVPD process is becoming a fundamental building block for bringing OLEDs and other organic electronics into future high-volume and throughput production.
Acknowledgments
The authors would like to acknowledge the contributions of co-authors Julie Brown from UDC; Max Shtein from Princeton University; and Gert Strauch, Michael Heuken, and Holger Jörgensen from AIXTRON AG. Pyrex is a registered trademark of Corning Glass.
References
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2. M. Baldo, M. Deutsch, et al., "Organic Vapor Phase Deposition," Adv. Mater., Vol. 10, pp. 1505, 1998.
3. Y.Y. Lin, D.J. Gundlach, S.F. Nelson, T.N. Jackson, "High-Mobility Pentacene-based Organic Thin Film Transistors," presented at the 55th Ann. Dev. Res. Conference, Ft. Collins, CO, 1997.
4. M. Shtein, H.F. Gossenberger, J.B. Benziger, S.R. Forrest, "Material Transport Regimes and Mechanisms for Growth of Molecular Organic Thin Films Using Low-Pressure Organic Vapor Phase Deposition," J. Appl. Phys., Vol. 89, pp. 1470, 2001.
5. E. Burrows et al., "Organic Vapor Phase Deposition: A New Method for the Growth of Organic Thin Films with Large Optical Nonlinearities," J. Cryst. Growth, Vol. 156, pp. 91, 1995.
6. G.B. Stringfellow, Organometallic Vapor-Phase Epitaxy, New York, Academic, 1989.
7. A.B. Chwang, R.Kwong, J.J. Brown, "Graded Mixed-layer Organic Light Emitting Devices," Applied Physics Letters, Vol. 80, pp. 725, 2002.
Janice K. Mahon received her BS in chemical engineering from Rensselaer Polytechnic Institute and her MBA from Harvard University. She has worked for SAGE Electrochromics, Chronar Corp., and FMC Corp. Mahon is VP of technology commercialization at Universal Display Corp., 375 Phillips Boulevard, Ewing, NJ 08618; ph 609/671-0980 ext. 206, fax 609/671-0995, e-mail [email protected].
Theodore Zhou received his AB in physics from Princeton University, and his ScM and PhD in physics from Brown University. He served as a post-doctoral fellow at the Institiute of Energy Conversion at the University of Delaware and the Microelectronics Research Center at Iowa State University, and worked for Solar Cells Inc. and Materials Research Corp. Zhou is principal scientist at Universal Display Corp.
Max Shtein received his BS in chemical engineering from the University of
California at Berkeley in 1998 and is currently a PhD candidate in chemical engineering at Princeton University, jointly advised by Prof.
S.R. Forrest and J.B. Benziger of the Electrical and Chemical Engineering Departments, respectively. His research interests are in
the science and technology of organic semiconductor materials and devices.
Stephen R. Forrest received his BA degree in physics from the University of California at Berkeley. He received his MS and PhD degrees in physics from the University of Michigan. Forrest has been professor of electrical engineering at the University of Southern California. He has also worked for AT&T Bell Laboratories. Forrest is now a professor of electrical engineering at Princeton University.
Markus Schwambera received his MS in electrical engineering from the Technical University, Aachen, Germany. He is department manager of OVPD equipment at AIXTRON AG.
Nico Meyer received his MS in chemistry and his PhD in organometallic chemistry from the Technical University, Aachen, Germany. He is an engineer at AIXTRON AG.