Thick film patterning by lift-off process using double-coated single photoresists
Highlights
► Double-coated single photoresists are used for lift-off process. ► Undercuts up to 15 μm are obtained for material deposition. ► The approach is demonstrated by a carbon nanotube transfer process.
Introduction
Lift-off method is commonly used for patterning materials due to its process simplicity, compatibility and good controllability. A typical lift-off sacrificial structure consists of an undercut layer below and a patterning layer on top. In the lift-off process, the maximal thickness of target deposition materials is dependent on the thickness of the undercut layer. This is because the thickness of the target materials cannot exceed the height of the undercut, otherwise the deposited materials on the patterning resist will be connected with the deposited materials at target position which will kill the lift-off process.
It is easy to pattern thin films within a thickness of a few hundreds of nano meters by lift-off process using the undercut offered by a commercial lift-off resist. However, for thick films, especially up to 5 μm or even thicker which is common in microelectromechanical systems (MEMS) and high-power applications [1], [2], [3], [4], [5], [6], it is difficult to find a lift-off resist to provide so thick undercuts. This makes the patterning of thick films an obstacle and brings a lot of complexity to micromachining processes [2], [5]. Chemical wet etching is normally applied as an alternative scheme to pattern thick materials [4], [5]. However, the etching process is anisotropic so it is very difficult to control. What is worse is that it is not always easy to find a proper etchant for a specific material while avoiding excess etching or contamination to other materials. Therefore, a few methods were proposed to solve this problem. Lee et al. [7] attached an opal diffuser onto the mask to diffuse and randomize the path of the incident ultraviolet (UV) light, so that a negative profile on the cross section of the photoresist is created for lift-off process. Depending on the photoresists used in this approach, they can make undercuts up to 10 μm. Nevertheless, the utilization of opal diffuser in this method limits its application. Chew et al. [8] optimized the process for dry film photoresists and demonstrated a profile angle less than 84° on the cross section of the photoresist with a thickness of 50 μm. Since the dry film method is very different from the wet photoresist spin-coating process, which is the most commonly used nowadays, it is not available everywhere.
In this paper, we propose a new method to pattern thick films by the lift-off process. Instead of using any special lift-off resists or extra auxiliary equipments, only a single commercially available photoresist and a conventional mask aligner are needed in this method.
Section snippets
Experiment
The fabrication process is illustrated as Fig. 1. Firstly, a clean silicon wafer is spin coated with a photoresist (Fig. 1(b)), which is then flood exposed under UV (Fig. 1(c)). This flood exposed first layer of photoresist acts as a lift-off undercut layer. By controlling the spinning speed and using different types of photoresists, the height of the undercut layer can be thus adjusted. Afterwards, another layer of the same photoresist is coated on top of the first layer and exposed to
Results and discussion
The pattern obtained from developed AZ 1512 is shown in Fig. 2(a), from which it can be seen that the pattern is extracted just as defined without deviation. About 1 μm thick In is then thermally evaporated onto the patterned Si wafer which is subsequently immersed into photoresist remover for lift-off. After the undesired material together with sacrificial layer (~ 1.4 μm according to the material datasheet) is removed, the pre-defined In pattern is retained on the Si substrate, shown in Fig. 2
Conclusions
In summary, a new method using a single commonly used photoresist for patterning thick materials by lift-off technique is developed to meet the strong requirement in micromachining processes. This method is demonstrated to be reliable and repeatable, and it is also compatible with other micromachining processes. SEM images show that undercut as high as 15 μm can be obtained for lift-off and very clean and neat patterns are retained after the lift-off process. The application of the thick
Acknowledgements
This work was supported by EU programs “Thema-CNT”, “Smartpower”, “Nanotec”, “Nanocom”, “Nanoteg”, “Mercure”, and the Swedish National Science Foundation (VR) under the project “on-chip cooling” using thermo-electrical device with the contract no: 2009-5042. This work was also carried out within the Sustainable Production Initiative and the Production Area of Advance at Chalmers. J.L. also acknowledges the support from the Chinese Ministry of Science and Technology for the International Science
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