A New Route to high-Performance Functional Films on Polymeric Web

功能薄膜是许多电子和光电子设备的基础，包括平板显示器、OLED、图像传感器、薄膜光伏太阳能电池等。对这些设备特别重要的是透明导电氧化物(TCO)薄膜，如铟锡氧化物(ITO)和铝掺杂氧化锌(ZAO)。预计到2010年，英国的功能电影市场将增长到234亿英镑。如果这些设备可以在非常大的生产量的卷筒到卷筒涂布机中直接沉积到聚合物网上，那么生产率将进一步大幅提高，新的市场将被打开。然而，在卷筒纸上沉积TCO薄膜带来了许多重大的技术挑战。与玻璃相比，聚合物网相对粗糙，容易大量放出气体，而且对温度很敏感。后者尤其是一个问题，因为为了优化TCO材料的光学和电学性能，通常需要进行沉积后的退火工艺(通常在500℃)。解决这个问题的一个潜在的解决方案是使用新开发的高功率脉冲磁控溅射(HIPIMS)技术来沉积涂层。这一过程包括在磁控溅射阴极上短时间内施加非常大的功率脉冲。峰值脉冲功率可以在兆瓦范围内，脉冲持续时间通常在80-160微秒量级，重复率在10秒到100秒赫兹的范围内。对HIPIMS(也称为高功率脉冲磁控溅射/HPPMS)系统的初步研究表明，使用该技术(与通常不到1%的溅射材料被电离的常规磁控溅射相比)，这种强烈的脉冲对溅射物种产生高度的电离(对于钛，高达70%)。HIPIMS中溅射物种的电离程度与阴极弧放电中产生的电离程度相当；然而，使用HIPIMS通常不能产生大颗粒。另一个重要的考虑因素是，由于占空比非常低(~1%)和长的关断时间，基片的总热负荷可以比传统的直流和脉冲直流溅射低得多(5-10倍)。因此，HIPIMS的潜力是利用高电离度来生产性能显著改善的薄膜，同时保持适当较低的基片整体温度(低于150℃)，从而允许涂覆各种基片材料。因此，HIPIMS技术的引入有可能使沉积在聚合物网上的功能薄膜(如TCO)的性能发生阶梯变化。该项目将提供首次在MMU的开发规模系统和牛津大学的工业中试规模的卷筒到卷筒涂布机中详细研究这种新的、复杂的沉积过程的机会。该项目的另一个关键内容将是对排放的性质进行详细研究。将使用一系列时间分辨诊断工具和成熟的光学成像技术来确定等离子体特征，例如物种浓度和温度的空间和时间演变及其对基质的功率负载。在柔性基板上沉积具有最佳性能的全致密TCO涂层的能力将是一项重大突破，将代表着卷筒纸涂层技术的重大进步。

Functional films underpin many electronic and opto-electronic devices, including flat panel displays, OLED's, image sensors, thin film photovoltaic solar cells, etc. Of particular importance to these devices are transparent conductive oxide (TCO) films, such as indium tin oxide (ITO) and aluminium-doped zinc oxide (ZAO). The UK market for functional films is expected to rise to 23.4B by 2010. Further substantial gains in productivity would be made, and new markets opened up, if the devices could be deposited directly onto polymeric web in very large throughput reel-to-reel coaters. However, the deposition of TCO films onto webs poses many significant technological challenges. In comparison to glass, polymeric webs are relatively rough, tend to outgas significantly and are thermally sensitive. The latter point particularly poses a problem, because it is generally necessary to perform a post-deposition annealing process (typically at 500 degC) in order to optimise the optical and electrical properties of TCO materials.One potential solution to this problem is to deposit coatings using the newly developed technique of high powered impulse magnetron sputtering (HIPIMS). This process involves the application of very large power pulses to magnetron sputter cathodes for short periods of time. The peak pulse power can be in the megawatt range and the pulse duration is typically of the order of 80-160 micro seconds, at repetition rates in the range of 10s to 100s of Hz. Initial studies of the HIPIMS (also referred to as high power pulsed magnetron sputtering / HPPMS) system have shown that this intense pulse creates a high degree of ionization (up to 70% for titanium) of the sputtered species with this technique (in contrast to conventional magnetron sputtering, where usually less than 1% of the sputtered material is ionized).The degree of ionization of the sputtered species in HIPIMS is comparable to that produced in cathodic arc discharges; however, with HIPIMS macroparticles are not normally produced. Another important consideration is that, due to the very low duty cycles (~1%) and long off times, the total heat load to the substrate can be very significantly (5-10 times) lower than in conventional DC and pulsed DC sputtering. Thus, the potential for HIPIMS is to harness the high degree of ionization to produce films with significantly improved properties, whilst maintaining a suitably low (sub-150 degC) substrate bulk temperature, allowing a diverse range of substrate materials to be coated. The introduction of HIPIMS technology, therefore, has the potential to provide a step-change in the performance of functional films, such as TCO's, deposited onto polymeric webs. This project will offer the first opportunity to study this new, complex deposition process in detail in both a development-scale system at MMU and an industrial pilot scale reel-to-reel coater at Oxford University. An additional key element of the project will be a detailed study of the nature of the discharge. Plasma characteristics such as the spatial and temporal evolution of the concentrations and temperatures of the species and their power loading of the substrate will be determined using an array of time-resolved diagnostic tools and well developed optical imaging techniques. The ability to deposit fully dense TCO coatings with optimised properties onto flexible substrates would be a major breakthrough and would represent a significant advancement in web coating technology.