精仪学院 微纳加工 期末考试 论文翻译 注入.docx

ABSTRACT
A 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 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 purpose 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. Introduction
A photomask is an essential component for semiconductor manufacturing and microfabrication. 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 coating. 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 etching
technique. Finally, the remaining photoresist layer is removed through a stripping process.
Attempts have 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。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 [3–5]. FIB milling technology has 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，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 fabrication 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 details
Investigation 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 emission-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 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 time. 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 discussion
The key issue in the FIBDW technology 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 predefined
localized 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 method
The 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 will open the ion beam. Blue determines the dwell time per pixel. If blue is set to be 0, the corresponding dwell time of a pixel will be 100 ns. If blue is set to be 255, the corresponding maximum dwell time is used. The dwell time corresponding to the pixels is linearly interpolated on the basis of the blue component value between the dwell time of 100 ns and the user-defined maximum value.
FIB系统以图案的方式接受点位图，每个像素点上离子束的停留时间根据位图的色置调整。位图文件应保存为24位位图格式。每个像素包含红绿蓝成分。绿色决定离子束是否关闭。G值大于零时离子束打开。蓝色决定每个像素的停留时间。如果B值为0，这个像素相对的停留时间为100ns。如果蓝设置为255，使用相对最大的停留时间。停留时间与像素点B值呈线性关系，最小值为100ns，最大值由用户设定。
As a result, the ion beam at the white position has longer dwell time than the one in the black position. Fig. 3 shows the bitmap of a Chinese poem and its milled pattern fabricated by FIB direct milling.
因此，白色点处离子束的停留时间长过黑色点出。图3为诗的位图以及聚焦离子束位图加工法获得的图案
3.2. Astigmatism correction
Firstly, in order to fabricate nanostructures patterns using FIBDW, detection and correction of the ion beam’s astigmatism are crucial issues. When there is astigmatism, the shape of the focused ion beam spot would change from a circle to an ellipse, as shown in Fig. 4(a). There might be four different orientations of the ellipse that are formed due to the imbalance of electrostatic forces that originate from the octopole electrodes [10].
The FIB machine has the capability to correct astigmatism by using stigmators equipped in the ion beam column. The stigmator consists of eight radially oriented electrostatic deflection elements.
By applying combinations of voltages to these deflectors, asymmetrical forces can be exerted on the beam to change its cross-sectional shape. The beam shape becomes circular when balanced stigmating forces are exerted by the octoples。
像散修正
首先，为了用FIBDW加工纳米结构图案，离子束像散的检测和修正的关键问题。像散存在，聚焦离子束点的形状会从圆形变为椭圆，图4。根据八级电极引起的静电力不平衡，椭圆可能有四种不同的朝向。
用安装在FIB上的消像散器，FIB机器有能力纠正像散。消除像散器由八个放射状静电方向导向板构成。用导向板结合电压，对离子束施加不对称力，改变其形状。当被施加修正像散的力，离子束形状成为圆形。
Unfortunately, the voltage offsets cannot be displayed on the operation screen. The FIB astigmatism is often judged by the naked eye of the operators according to their observation of the sample micrograph during FIB imaging, which fails to eliminate the astigmatism accurately and this is also highly dependent on experience.
Here the FIB milling dot arrays method was used to check the shape of the beam spot qualitatively. The shape of the milled dots would reflect the ion beam shape. Dots in elliptical shape would indicate that an astigmatic ion beam has been formed due to the existence of surplus stigmation, as shown in Fig. 5. Then the astigmatism correcting process should be performed accurately until the beam shape becomes circular in theory.
Therefore, it is necessary to perform the FIB astigmatism calibration process when the machine has been restarted or the beam parameters are changed.
然而，电压补偿不能再操作屏幕上显示出来。像散常常是由操作者裸眼观察FIB微图像样品来判断，这使精确消除消散及其依赖经验。
本文用FIB铣削点阵列方法定性检查点光束点形状。铣削点的形状反映了离子束形状。椭圆型电表明，离子束因存在纠正过度而像散，图5。像散纠正过程可以精确进行。
因此，当机器重新打开或离子束参数改变，需进行FIB像散校准过程。
3.3. Beam overlap control
Line edge roughness is very important for the photomask fabrication. Beam overlap control is critical for fabricating smooth mask edges in the FIBDW process. For the bitmap patterning method, the beam overlap can be controlled by means of defining the bitmap pixel density, milling size, beam parameters, etc. Under the same FIB parameters, nanodots array and nanolines array can be fabricated by changing different bitmap pixel densities, as shown in Fig. 6. The ion beam parameters are 30 kV, 50 pA and 1 ms dwell time, respectively, and the fabricated size is 10 mm  10 mm. The beam overlap will increase for the larger beam current because the beam diameter increases with the beam current.
边缘粗糙度对光掩膜非常重要。离子束重叠控制对FIBDW加工光滑膜边缘很关键。对位图加工方法，离子束重叠可由规定位图像素密度，铣削大小，束参数的方法控制。相同的FIB参数下，可以以改变位图像素密度的方法加工纳米点阵列，图6。FIB参数为： 30 kV, 50 pA 和1 ms停留时间，加工大小为10um乘10um。束重叠会因束电流增加而增加，因为束直径会因束电流增加而增加。
3.4. Dwell time
After numerous experiments, it was found that the dwell time definition is vital for the nano-mask fabrication. The FIB dwell time can be varied from 100 ns to 4 ms in the FIB system. If the beam dwell time is set to be too small (less than 1 ms), the FIB scanning loops will increase dramatically, which will result in unwanted ion milling. Fig. 7(a) shows the dots array results milled by FIB with 1 ms dwell time. It is obvious that the areas between adjacent dots are also milled, which clearly reflects the FIB milling trace. Thus nano-photomasks cannot be fabricated using FIBDW with dwell time less than 1 ms.
If the beam dwell time is chosen to be too large, redeposition will be a serious problem during the FIBDW process. As the sample material is sputtered away, some of it is redeposited in the volume that is being sputtered. In normal mechanical machining, the buildup of machined material is avoided using liquid or air streams that carry the chips away. However, this is not practical for FIB
milling in vacuum. For a small dwell time (fast repeated scans), some fraction of the Ga ions are used to sputter away most of the redeposited material formed in the last loop. For a large dwell time (slow scans), the redeposited material cannot be removed completely, as shown in Fig. 8(a). It was found that the redeposition effect would greatly degrade the fabrication results if the beam dwell time is larger than 400 ms for the nano-photomask fabrication.
经多次试验检验，停留时间对纳米光掩膜至关重要。FIB停留时间为100ns至4ms。如果离子束停留时间过小，FIB扫描圈急剧增加，导致多余离子铣削。图7展示了1ms停留时间FIB点阵列。很明显，相邻点之间的区域也被铣掉了，这清晰地反映了铣削轨迹。因此，纳米光掩膜不能用停留时间1ms以下的FIBDW制造。
如果离子束停留时间过长，再沉淀会成为FIBDW加工的严重问题。样品材料飞溅，一些沉积在了被铣削掉的地方。在普通加工中，用液体或气流带走切下来的屑。但是，Fib不能。短暂的停留时间中，Ga溅射粒子大部分将被真空系统抽走。随着加工深度的增加，被溅射的原子会不可避免地沉积在孔的侧壁表面，如8所示。如果停留时间大于400ms，再沉淀效应极大程度损坏加工结果。

3.5. Photomask results by FIBDW
By controlling the FIB fabrication parameters mentioned above accurately, nano-photomasks with larger than 32 nm line width can be well fabricated using the FIBDW method, as shown in Fig. 9.
As the FIBDW bitmap patterning method shows high resolution and flexibility, nano-photomasks with different patterns can be fabricated using this method, examples are circular patterns, Siemens star patterns and optical proximity effect correction (OPC) photomasks, as shown in Figs. 10 and 11, respectively.
Therefore, the FIBDW method provides an effective approach for fabricating nano-photomasks.
通过精确控制上文所述FIB参数，线宽大于32nm的纳米光掩膜可以用FIBDW加工，如图9。聚焦离子束直写位图加工法显示了极高的分辨率和灵活性，不同形状的纳米光掩膜可以用这种方法加工，有圆形，星形，光学临近效应矫正光膜为例，如图。

3.6. Slits orientation influence study
Height differences of the photomask patterns should be strictly controlled. For photomasks patterned with two perpendicular slits, there will be remarkable height differences for the two different orientated slits if the bitmap is chosen ‘‘d’’ or ‘‘L’’ shape in the FIBDW method, as shown in Fig. 12(a), (c), and (e). The FIB scan direction is set horizontally here. The FIB redeposition effect and milling restriction at the positions for the slits vertical to the scan direction (area B) in FIBDW will be larger than that of the parallel slits (area A). This will result in height differences for the slits with different orientations, which is the reason why slits oriented differently are not in the same focus in the SEM image, as shown in Fig. 12(c).
By tilting the ‘‘d’’ shape bitmap 45度 to the ‘‘V’’ shape, the orientation of the two slits relative to the FIB scan direction is nearly the same. The height difference can also be well minimized
光掩模形状高度差异被严格控制。对于两个垂直纹路的膜，如果位图被选择为d型L型的FIBDW方法，两个方向纹路高度差异大，图12ace
将d型倾斜45度为v型，两个方向的纹路与Fi的方向几乎一样。高度差异被缩减到最小

4. Conclusions
The FIBDW bitmap patterning method has been put forth to fabricate nano-photomasks. The FIBDW parameters such as beam dwell time, beam astigmatism, beam overlap and slits orientation as well as their influence on the mask fabrication results have been extensively studied. It was found that the FIB dwell time needs to be controlled precisely to overcome the unwanted ion milling effect and the redeposition effect in the FIBDW process. The dot array milling approach is proposed for inspecting and correcting the beam astigmatism. By controlling the FIB fabrication parameters carefully, nano-masks with 32 nm line width have been successfully fabricated using this method.
本文提出了FIBDW位图加工法制造纳米光掩膜。广泛地研究了FIBDW参数如停留时间，离子束像散，重叠以及纹路方向和他们对光膜结果的影响。经证实，应精确控制FIB停留时间，以避免多余铣削和再沉淀效应。点阵列方法可以检测和纠正离子束像散。通过精确控制参FIB加工参数，32nm线宽的掩膜用这种方法成功制造。