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1、ABSTRACTA novel nano-photomask fabrication method using focused ion beam direct writing (FIBDW) is proposed to normalize the dwell time of each pixel of the ion beam location with respect to the contrast of designed bitmaps. The removal mechanism is studied to develop the fabrication process. It has
2、 been confirmed that beam dwell time, astigmation and overlap are the most effective parameters for achieving the features in nanoscale. An approach for dot array milling is proposed also for inspecting and correcting the beam astigmatism. Photomasks with line width of 32 nm are employed for the pur
3、pose of successful application of this novel method in this study.本文提出了一种用聚焦离子束直写的纳米光掩膜制造新方法，本方法可以根据设计好的位图调整每个像素点上离子束的停留时间。本文研究去除机理以发展制造工艺。经证实，离子束停留时间，像散和重叠是对达到纳米级产品最至关重要的参数。同时，提出了一种铣削点阵列的方法以成功应用本研究提出的新方法。1. IntroductionA photomask is an essential component for semiconductor manufacturing and microf
4、abrication. Currently, the photomask is fabricated using electron beam or laser lithography processes, which are very expensive and time consuming as they are multi-step processes 1. In the process, a Cr/CrO2 layer is formed by means of sputtering on a quartz substrate, followed by photoresist coati
5、ng. Firstly, a certain pattern is written using E-beam or laser lithography on the top surface of the photoresist layer. Then the exposed Cr/CrO2 layer is etched using either the dry or wet etchingtechnique. Finally, the remaining photoresist layer is removed through a stripping process.Attempts hav
6、e also been made to fabricate photomasks using femtosecond laser direct writing in recent years 2. However, it is very difficult to achieve photomasks with sub-micron line width.光掩膜是半导体和精密加工的必要成分。如今，光掩膜用电子束或激光蚀刻的方法制造，这一过程非常昂贵和耗时，因为它们是多步加工。在过程中，Cr/CrO2 层用喷射石英基层，然后覆盖光阻材料涂层的方法形成。首先，电子束或激光蚀刻在光阻层表面写入特定图案
7、。然后，暴露在外的 Cr/CrO2 层被干蚀刻或湿蚀刻技术蚀刻。最后，剩下的光阻层被剥离过程移除。今年，人们也尝试了用飞秒激光直接写入制造光掩膜，但是，次微米线宽难以完成光罩。Focused ion beam (FIB) technology has unique advantages in comparison with other micro/nano-machining technologies such as high resolution, maskless processing, and rapid prototyping 35. FIB milling technology ha
8、s become an important approach in micro/nano-machining for the various applications, such as photomask repair 6, fabrication of photonic crystals with sub-micron period 7, and configuration optimization of carbon nanotube probes 8.聚焦离子束技术比较其他微米、纳米加工技术，如 xx、xx、xxx，有着独特的优势。FIB 铣削技术引起多种应用，如光罩修复，制造光激性晶体
9、，成为了微米纳米加工的重要方法。In this paper, it is proposed to fabricate a nano-photomask using focused ion beam direct writing on Cr thin film, which is a single-step process and is comparatively cheaper and faster than the traditional lithography process. Influences of the FIBDW parameters on the nano-mask fabr
10、ication are also studied in detail本文提出了用离子束直写的方式在 Cr 薄膜制造纳米光罩，这是一步过程，相对传统的光刻方法便宜迅速。本文还研究了 FIBDW 参数如何影响纳米光罩制造。2. Experimental detailsInvestigation of FIB milling was carried out using the FIB/SEM dual-beam system (FEI). The dual-beam system is a complete nanotechnology laboratory combining a field em
11、ission-based scanning electron microscopy (FE-SEM) with ultra-high imaging resolution, as shown in Fig. 1. The resolution of the FE-SEM image can be 1.1 nm, and 7 nm for the FIB column. This system uses a focused Gallium ion beam working under an accelerating voltage ranging from 5 kV to 30 kV, and
12、a probe current ranging from 1 pA to 20 nA. The Cr film is coated on the fused quartz substrate using an evaporation method. The FIB milling process is performed using a precise pixel-by-pixel movement, as shown in Fig. 2. The time that the beam remains on a given target pixel is called the dwell ti
13、me. The distance between the centres of the two adjacent pixels is called pixel spacing. Pixel spacing should be small enough to allow a proper overlapping between the adjacent pixels so that a smooth uniform profile can be fabricated using FIB milling.关于 FIB 铣削的研究由 FIB/SEM 双光束系统进行。如图 1 双光束系统是纳米技术结合
14、超高分辨率的电场发射扫描电子显微镜（FE-SEM） 。FE-SEM 的分辨率可以到达 1.1nm，而 FIB 可达到 7NM。系统使用的是在 5-30kv 电压范围下的聚焦 Ga 离子束，探针电流为 1pA 至20nA。Cr 薄膜用蒸汽法覆盖在熔凝石英基层。如图 2 所示，FIB 铣削过程用精确的像素运动执行。离子束停留在一个指定点像素的时间叫做停留时间。两个临近像素中心之间的距离叫做像素间隔。像素间隔应小到可以允许相邻像素有合适的重叠，使 FIB 铣削可以制造光滑均衡的轮廓。3. Results and discussionThe key issue in the FIBDW technol
15、ogy is to operate a FIB with proper process parameters such as ion beam size, shape, and dwell time for removing a specified volume of material from a predefinedlocalized area 9. In this paper, the nano-photomask is fabricated using FIBDW with bitmap patterning method.聚焦离子束直写技术的关键问题是以合适的工艺参数操纵离子束，比如
16、离子束大小、形状、以及去除指定位置指定大小的材料所需的停延时间。本文以位图加工法用离子束直写加工纳米光掩膜。3.1. Bitmap patterning methodThe FIB patterning system allows importing a bitmap file as a pattern, for which the dwell time for each pixel of the ion beam location is normalized to the color values of the bitmap. A bitmap file must be saved as 24 bit bitmap files. Each pixel consists of a red, green and blue component (RGB). The green component determines whether the beam is blank or not. Any other values larger than 0 wi
