Nanoelectrospray as a direct writing method for electronic circuits

在电子工业中，光刻技术是用功能材料制作表面图案的标准技术。然而，由于与大量工艺步骤相关的成本，它的使用需要特殊的设施和数百万英镑的重大投资。并非所有的电子元件都需要光刻技术的分辨率。对平板显示器等使用低分辨率技术的设备的需求不断增长。因此，许多团体开发了喷墨打印机喷嘴作为数字书写技术。功能材料在液相中直接打印到表面上。这降低了图案的成本，提高了掩模设计和基板选择的对准精度和灵活性。先前的喷墨工作已经沉积了光刻胶和导电油墨，然而，这些特征的尺寸只有几十微米。该技术也受到堵塞困难和打印精度的限制。高成本的高分辨率光刻技术与低成本但低分辨率的喷墨打印技术之间存在着能力差距。该项目旨在发展电喷雾作为一种制造技术，具有直接书写的优点，但具有接近光刻的特征尺寸和放置精度。电喷雾已经改变了对大分子生物的分析，并经常用于筛选小分子药物。它已被用于生产用于生物支架应用的封装纳米颗粒和聚合物微纤维。使用标准的电喷雾必须克服流量控制的问题，因为需要液体泵或气体压力来驱动流量。为了获得微米级的特征，需要非常低的流量，目前还不能以足够高的精度控制流量，以允许按需滴注。相反，沉积头必须以高速扫过基板，否则沉积的液体会积聚成大的特征。纳米电喷雾在某些方面是最简单的电喷雾形式，其喷嘴的流量由施加的电压决定。其中一个好处是，通过选择正确的喷嘴和液体组合，很容易实现非常低的流量。我们已经确定了一种稳态振荡模式，其中射流周期性地形成然后松弛。这种脉动喷射发生在高频率，每次脉动喷射一个非常小和固定体积的液体。用这种方法可以制成微米大小的点，或者通过重叠这些点，可以打印出微米宽的连续线。沉积物比喷嘴直径小10倍。相比之下，喷墨打印的特点往往大于喷嘴直径。这解决了喷墨过程中遇到的喷嘴堵塞问题。概念验证研究喷射了广泛的液体，并展示了打印微米尺寸和放置精度的液滴的能力。提出的研究将旨在使用纳米电喷雾直接书写来打印光刻胶线和微米宽度的导电轨道。光刻胶的线条将使用扫描电镜进行检查，并使用硅蚀刻进行功能测试。微米宽的抗蚀剂线是这个项目的首要目标。这项工作还将测试打印导电轨道的可行性；一系列导电油墨将被评估。该项目的第二个主要目标是实现线宽和放置精度接近微米的低电阻率导电轨道。如果有效的话，这个项目的两个主要成果将为光刻提供廉价的替代品，其分辨率和多层精度比喷墨印刷要高得多。

In the electronics industry, photolithography is the standard technology used to pattern surfaces with functional materials. However, its use requires special facilities and significant multi-million pound investment due to the costs associated with the large number of process steps. Not all electronic components require the resolution of lithography. There is a growing demand for devices such as flat panel displays which can use lower resolution techniques. As a result, many groups have developed inkjet printer nozzles as a digital writing technology. The functional material is printed directly onto the surface in the liquid phase. This reduces the cost of the patterning and increases alignment accuracy and flexibility in mask design and substrate choice. Previous work in ink jets has deposited photoresist and conductive inks however, the size of the features is a few tens of micrometers. The technique is also limited by the difficulty of clogging and printing accuracy. There is a capability gap between the high resolution at high costs of photolithography and the low cost but low resolution of inkjet printing. This project aims to develop electrospray as a fabrication technology with the benefits of direct writing but with feature sizes and placement accuracy approaching that of photolithography. Electrospray has transformed the analysis of large biomolecules and is routinely used to screen small molecules for pharmaceuticals. It has been used for the production of encapsulated nanoparticles and polymer microfibers for bioscaffold applications. The use of standard electrospray would have to overcome the problem of flow control, as a liquid pump or gaseous pressure is needed to drive the flow. To obtain micrometer features requires very low flowrates, which can not currently be controlled with a sufficiently high accuracy to allow a drop on demand approach. Instead, the deposition head must be swept over the substrate at high speeds otherwise the deposited liquid builds up into large features. Nanoelectrospray is in some ways the simplest form of electrospray, where the flowrate from the nozzle is defined by the voltage applied. One of the benefits is that very low flowrates are easily achieved by selection of the correct nozzle and liquid combination. We have identified a steady state oscillating mode where a jet periodically forms and then relaxes. This pulsating spray occurs at high frequencies and each pulsation ejects a very small and fixed volume of liquid. In this way micrometer-sized dots can be made or, by overlapping these dots, micrometer wide continuous lines can be printed. The deposits are ~ 10 times smaller than the nozzle diameter. By contrast inkjet printed features are often larger than the nozzle diameter. This solves the problems of nozzle clogging experienced with the inkjet process. Proof of concept research sprayed a wide range of liquids and has demonstrated the capability to print droplets with micrometre size and placement accuracy.The proposed research will aim to use nanoelectrospray direct writing to print lines of photoresist and conductive tracks with micrometre widths. The lines of photoresist will be examined using SEM and functionally tested using a silicon etch. Micrometre wide lines of resist is the first aim of this project. The work will also test the feasibility of printing conductive tracks; a range of conductive inks will be assessed. The second principal aim of the project will be to achieve low resistivity conductive tracks with line widths and placement accuracy approaching a micrometre. If effective, the two main outcomes of this project will provide inexpensive alternatives to photolithography with a resolution and multi-layer accuracy much improved over inkjet printing.