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溯源性是纳米测量中的基础性问题[12]。使用SEM对线宽进行测量与表征时,首先要对其进行溯源,由于电子显微镜的放大倍率的变化是非线性的,因此需要对不同放大倍率进行校准[13]。作为校准SEM放大倍率的标准物质必须满足:线边缘平直良好、量值统一、稳定性好且标称量值必须可溯源到一个正确的物理量值上[14-15]。周期性线宽标准物质是校准光学/电子显微镜放大倍率的合适标准[16]。
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一维铬自溯源光栅标准物质完全满足以上要求,该光栅是基于原子光刻技术制备的光栅结构。原子光刻技术又称为激光会聚原子沉积[17]。以铬原子为例,会聚激光波长为425.6 nm,对应Cr原子的共振跃迁能级(7S3→7P4),在会聚激光和原子的相互作用下会使得Cr原子沉积到样板上,由于沉积位置与激光驻波场波谷或波腹位置严格对应,可以形成周期高度准确可靠的一维光栅结构。一维铬自溯源光栅的节距值由Cr原子的跃迁频率决定,而原子的跃迁频率是一个自然常数,因此,形成的一维铬(Cr)原子光刻光栅结构是一种自溯源节距标准,且其光栅的周期为激光波长的一半,故一维铬(Cr)原子光刻光栅结构周期为212.8 nm。使用该方法制备的一维212.8 nm自溯源光栅具有极高的准确性与一致性,均在0.001 nm量级[8]。
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量值溯源体系如图2所示,其中左边为经典的纳米溯源体系,右边为基于自溯源光栅的量值溯源体系。
与经典的纳米量值溯源体系相比,基于自溯源光栅建立的纳米量值溯源体系使溯源链有效缩短,实现了扫描电子显微镜的校准和溯源,确保其测量结果的准确性和可比性,降低了量值传递过程中由于纳米标准样板与计量型微纳米测量仪器引入的溯源误差, 实现工业生产中微纳米测量仪器直接溯源到光波波长[18],并且作为物理性稳定的实物基准更易保存与运输,推动整个微纳米科技的发展。
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自溯源光栅周期十分准确且可直接溯源至“米”的定义,具有极高的样品均匀性与一致性,使用SEM对自溯源光栅进行图像采集和测量,将光栅实际值与SEM测量进行比值,得出该放大倍率下的校准因子,具体校准方法如下:
将自溯源光栅线条竖直方向与SEM扫描方向垂直,采集图像并记录放大倍率。如图3所示,以单个光栅左侧线边缘和右侧线边缘为选取参考点,依据JJF 1916—2021《扫描电子显微镜校准规范》,选取标准样图像上N (N≥5)个栅格结构,测量栅格间距L[19]为:
$$ L=({L}_{1}+{L}_{2})/2 $$ (1) 式中:L1、L2分别为左侧线边缘间距和右侧线边缘间距。连续进行5次测量取平均值。自溯源光栅的光栅周期值为D,则上述测量间距在光栅上的实际长度应为ND。则该放大倍率下的校准因子K为:
$$ \begin{array}{c}K=ND/L\end{array} $$ (2) 完整的校准过程需要在多个不同的放大倍率下依次进行,校准流程和上述相同。
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采用校准后的SEM对多层膜线宽样片进行测量,在不同放大倍率下,分别在样品的上、中、下三个位置进行测量,线宽定值以中间半高宽为纳米线宽。
LER是指线宽边缘部分的表面不光滑程度,其主要表现为线宽边缘上的高低起伏或波动。LWR是指在同一条线宽上多个测量点线宽的变化范围,主要反映线宽的均匀性和一致性。由于LER、LWR不随器件线宽的减小而减小,因此当线宽下降至纳米尺度时,LER、LWR的测量对于线宽标准物质的可靠性与均匀性检验至为重要。均方根(Root mean square, RMS)粗糙度参数能够在一定意义上反映刻线边缘形貌的均匀性[20],因此,使用RMS对LER与LWR进行量化表征。通过对LER、LWR的精确表征,可以对各种新型材料的性能、新设备研制、新方法的试验结果做出正确有效的评价[21]。
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LER通常使用形成线边缘轮廓的点与其拟合直线之间距离的标准差来描述,具体计算方法如图4(a)、(b)所示。首先选择一个长度为L的测量窗口,在测量间隔∆处测量N个边缘线位置xi和宽度wi,从边缘位置减去一条平均线边缘$ \overline{X} $,从宽度中减去局部线宽平均值$ \overline{W} $。因此,线边缘粗糙度的幅值参数均方根粗糙度值$ {\sigma }_ {\rm{LER}} $可表示为[22]:
$$ {{\sigma }}_{{{{{\rm{LER}}}}}}=\sqrt{\frac{\displaystyle\sum _{{i}=1}^{{N}}{\left({{x}}_{{i}}-\overline{{X}}\right)}^{2}}{{N}}} $$ (3) 同理,可得线宽度粗糙度的幅值参数均方根粗糙度值$ {\sigma }_{{\rm{LWR}}} $可表示为:
$$ \overline{W}=\frac{1}{N}\sum _{i=1}^{N}{w}_{i} $$ (4) $$ {\sigma }_ {\rm{LWR}}=\sqrt{\frac{\displaystyle\sum _{i=1}^{N}{\left({w}_{i}-\overline{W}\right)}^{2}}{N}} $$ (5) 图 4 (a) 线边缘示意图; (b) 局部线宽示意图
Figure 4. (a) Schematic diagram of line edge; (b) Schematic diagram of local line width
线边缘粗糙度幅值表征的两种量化方法LER与LWR是互为补充的。一般来说,$ {\sigma }_ {\rm{LER}} $值较大的线边缘结构,其$ {\sigma }_ {\rm{LWR}} $值不一定大;而$ {\sigma }_ {\rm{LWR}} $ 值较大的线边缘,其$ {\sigma }_ {\rm{LER}} $值可能很小。图5 (a)、(b)给出了这两种情况的图示说明。
图 5 (a) $ {\sigma }_ {\rm{LER}} $=0,而$ {\sigma }_ {\rm{LWR}} $较大的情况; (b) $ {\sigma }_ {\rm{LWR}} $ 很小,而$ {\sigma }_ {\rm{LER}} $较大的情况
Figure 5. (a) $ {\sigma }_ {\rm{LER}} $=0, while $ {\sigma }_ {\rm{LWR}} $ is large; (b) $ {\sigma }_ {\rm{LWR}} $ is very small, and $ {\sigma }_ {\rm{LER}} $ is large
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由于SEM对样本进行测量得到的是表示扫描线与采样点上高度值的灰度信息,因此可以使用图像处理技术对SEM测量图像进行分析来确定线边缘位置,进而提取出LER特征。具体测量分析如下:
1)获得SEM测量图样与滤波。将所得图像导入MATLAB软件中,采用中值滤波对扫描图像进行去噪处理 [21];
2) 确定线边缘位置。采用对图像处理较简单、影响较小的Sobel算子进行线边缘检测[21],检测后的图像进一步阈值化得到线边缘;
3)确定评定基准。提取出左、右边缘线点的像素值$ \left({x}_{li},{y}_{li}\right) $,$ \left({x}_{ri},{y}_{ri}\right) $,采用最小二乘拟合方法确定平均线边缘[23]。设拟合直线方程为:$ \overline{X}=ay+b $、a、b的计算公式如下:
$$ a=\frac{\overline{xy}-\overline{x}\cdot \overline{y}}{\overline{{y}^{2}}-{\overline{y}}^{2}},b=\overline{x}-a\overline{y} $$ (6) 其中,
$$ \overline{x}=\sum _{i=1}^{n}\frac{{x}_{i}}{n} \text{,} \overline{y}=\sum _{i=1}^{n}\frac{{y}_{i}}{n} \text{,} \overline{{y}^{2}}=\sum _{i=1}^{n}\frac{{y}_{i}^{2}}{n} \text{,} \overline{xy}=\sum _{i=1}^{n}\frac{{y}_{i}{x}_{i}}{n} $$ 4)提取LER、LWR特征;确定评定长度$ {L}_{cl} $、$ {L}_{cr} $,提取LER特征、LWR特征。
5)数据处理;采用RMS粗糙度计算公式对LER、LWR进行计算。
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文中使用国家一级标准物质一维212.8 nm 自溯源光栅对SEM进行校准,其任意两个自溯源光栅平均节距的差异为0.001 nm,不确定度为0.008 nm,其线边缘粗糙度较小且具有极高地均匀性和一致性。
校准仪器为蔡司Sigma 300热场扫描电镜,放大倍数为10~1000000×,在15 kV 加速电压下,分辨率为1.2 nm;在1 kV加速电压下,分辨率为2.2 nm,加速电压调整范围为0.002~30 kV,样品台最大行程X=125 mm,Y=125 mm,Z=50 mm,样品台的旋转角度T的范围为−10°~90°,样品台的倾斜角度R的范围为360°连续旋转。仪器放置于恒温房间中,环境温度(20±5) ℃,温度波动:≤1 ℃/h,相对湿度:≤75%。校准前电镜载物台保持水平,便于电子束垂直入射光栅,自溯源光栅样品静置于载物台30 min,使测试状态保持稳定。在选择校准位置时尽量避开光栅的坑洞、划痕、污染等位置。
在40、60、80 K×放大倍率下,SEM的比例尺转换分别为2.74、1.85、1.37 nm/pixel,使用自溯源光栅对SEM在固定加速电压3.00 kV下,选取图像上5个栅格结构,测量栅格间距L,以放大倍率为80 K×为例,SEM对栅格进行测量的示意图如图6所示,计算过程如2.3节所述。将40、60、80 K×放大倍率下的测量结果如表1所示。
图 6 SEM放大倍率为$ 80\;\mathrm{K}\times $下,光栅栅格选取示意图
Figure 6. The SEM magnification is $ 80\;\mathrm{K}\times $, the schematic diagram of grating grid selection
表 1 放大倍率为40,60,80 K× SEM测量结果(单位:nm)
Table 1. The magnification is 40, 60, 80 K× SEM measurement results (Unit: nm)
The ith measurement 40 K× 60 K× 80 K× $ {L}_{1} $ $ {L}_{2} $ $ \overline{{L}_{i}} $ $ {L}_{1} $ $ {L}_{2} $ $ \overline{{L}_{i}} $ $ {L}_{1} $ $ {L}_{2} $ $ \overline{{L}_{i}} $ i=1 1065.8 1052.1 1059.0 1061.1 1057.4 1059.3 1053.4 1063.0 1058.2 i=2 1063.0 1052.1 1057.6 1053.7 1055.6 1054.7 1054.8 1054.8 1054.8 i=3 1082.2 1068.5 1075.4 1061.1 1055.6 1058.4 1052.1 1057.5 1054.8 i=4 1046.1 1060.3 1053.2 1064.8 1064.8 1064.8 1053.4 1054.8 1054.1 i=5 1060.3 1049.3 1054.8 1074.1 1064.8 1069.5 1052.1 1054.8 1053.5 在SEM不同放大倍率下,连续进行5次测量,计算每次测量所到的校准因子,求出校准因子的平均值,计算其标准误差,绘制校准因子误差线,如图7所示。
图 7 SEM不同放大倍率下校准因子结误差线
Figure 7. SEM calibration factor junction error bars under different magnifications
由图7可知,同一放大倍率下,多次测量的所得的校准因子十分接近,这说明测量过程是可靠的,所得的校准因子具有高度的可靠性和准确性。通过计算得出,在40、60、80 K×放大倍率下校准因子K分别为1.0038、1.0025、1.0085。在不同放大倍率下,5个栅格间距的测量均值为1058.8 nm与自溯源光栅5个栅格间距1064 nm相差约为5.2 nm,这种差异会对样本特征的准确性产生影响,无法得到精确的尺寸或形貌信息。通过对SEM的校准,可以提高测量的准确性,减小测量误差,确保结果的可靠性和一致性, 以及测量结果的可追溯性,同时有效缩短了溯源链,降低了量值溯源过程中通过纳米标准样板与计量型微纳米测量仪器引入的溯源误差,为量值传递日渐趋于扁平化提供了一种可能。
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在相同环境条件下,采用校准后的SEM在不同放大倍率下,分别对名义值为500、200、100 nm的线宽样片的上、中、下3个位置进行测量。首先将线宽扫描图像导入软件中进行上升边沿和下降边沿的空间确定,得到模拟图像,以样品扫描图像的下边缘为底,上边缘为顶,以中部线宽为线宽测量值,线宽测量如图8所示。测量结果与和校准因子K 进行乘积得到SEM 测量值,测量5次并取其平均值,将不同位置、不同放大倍率下测量5次所得均值填入表2中。将不同区域、不同的位置线宽测量结果进行绘图,绘制结果如图9所示。
图 8 (a)线宽标准器模拟图样; (b) 中部线宽提取示意图; (c) 线宽测量示意图
Figure 8. (a) Simulation pattern of linewidth standard; (b) Central line width extraction map; (c) Schematic diagram of central linewidth extraction
表 2 线宽均值测量结果(单位:nm)
Table 2. Line width mean measurement results (Unit: nm)
Nominal line width 40 K× 60 K× 80 K× Average value Absolute error Standard deviation Up Middle Down Up Middle Down Up Middle Down 500 457.1 458.7 454.9 465.2 464.1 466.7 453.7 452.6 462.3 459.5 40.5 5 200 188.1 190.3 187.6 193.8 191.6 197.2 188.4 189.0 193.1 191.0 9 3 100 96.3 97.9 97.4 99.1 100.3 103.6 98.1 100.6 102.8 99.5 0.5 2.3 图 9 (a)~(c)分别为不同倍率下100、200、500 nm线宽测量结果; (d)线宽测量平均值
Figure 9. (a)-(c) are the measurement results of 100 nm, 200 nm, and 500 nm linewidth at different magnifications, respectively; (d) Average line width
由线宽测量结果可知,线宽名义值为500、200、100 nm的样片其实际测量值分别为459.5、191.0、99.5 nm。由图9(a)~(c)可知,名义值为500 nm线宽,在不同测量放大倍率下,不同测量位置其线宽测量均值的最大偏差为7.2 nm,其均值相对误差为8.1%,由于制备过程中受沉积时间、沉积环境等因素的影响,导致实际测量值与名义值之间存在较大的差异。名义值为 200 nm线宽,在不同测量放大倍率下,不同测量位置其线宽测量均值的最大偏差为6.2 nm,其均值相对误差为4.5%,测量值与名义值之间的偏差相对减小,名义值100 nm线宽,其相对误差仅为0.5%,具有较高的精确性。不同尺寸的线宽在不同测量放大倍率下,不同测量位置其线宽测量均值的最大偏差为1.9 nm。由图9 (d)可知,相同尺寸的线宽在不同测量放大倍率下,不同测量位置其线宽测量结果基本一致。说明Si/ SiO2多层膜线宽具有良好地样间一致性,这表明多层膜技术是相对稳定和可靠的。
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对名义线宽值为500、200、100 nm的样品在40、60、80 K×放大倍率下,上、中、下三部分进行测量,500、200、100 nm线宽值其左右评定长度保持一致,$ {L}_{cl} $=$ {L}_{cr} $,其评定长度分别为1045、 550、275 nm。按照上述流程与3.2节计算方法分别计算出$ {\sigma }_{L} $、$ {\sigma }_{R} $与$ {\sigma }_ {\rm{LWR}} $,将结果列于表3中。将同一线宽值在不同位置、不同放大倍率下的左、右线边缘粗糙度及线宽粗糙度进行数据可视化处理,如图10所示。
表 3 线边缘LER 、LWR测量结果(单位:nm)
Table 3. Measurement results of line edge LER and LWR (Unit: nm)
Lineweight value Roughness 40K× 60K× 80K× Average value Standard deviation Up Middle Down Up Middle Down Up Middle Down 500 $ {\sigma }_{L} $ 1.3 1.3 1.6 3.2 3.4 3.5 3.7 3.0 2.7 2.6 0.9 $ {\sigma }_{R} $ 2.2 2.4 2.9 2.8 3.5 2.3 4.1 2.6 2.0 2.8 0.6 $ {\sigma }_{LWR} $ 2.2 1.9 3.2 4.1 3.9 3.5 5.3 6.5 4.2 3.9 1.3 200 $ {\sigma }_{L} $ 1.3 1.5 1.3 3.3 2.6 2.7 3.4 3.0 3.1 2.5 0.8 $ {\sigma }_{R} $ 2.6 1.1 2.9 2.1 3.5 1.6 1.7 1.7 2.7 2.2 0.7 $ {\sigma }_{LWR} $ 2.4 1.5 3.2 4.1 4.2 2.4 3.9 4.0 3.7 3.3 0.9 100 $ {\sigma }_{L} $ 1.0 0.9 0.8 3.7 3.4 2.7 3.1 2.9 2.2 2.3 1.1 $ {\sigma }_{R} $ 2.4 1.0 0.9 3.6 3.5 1.3 3.1 2.5 2.3 2.3 1.0 $ {\sigma }_{LWR} $ 1.7 0.8 1.0 4.8 3.9 2.3 4.0 4.1 2.8 2.8 1.4 图 10 (a)~(c)分别为100、200、500 nm左、右线边缘粗糙度以及线宽粗糙度
Figure 10. (a)-(c) are 100 nm, 200 nm, 500 nm left and right line edge roughness and line width roughness respectively
由测量结果可知,测量结果具有较好的一致性,说明基于图像处理的方法得到的LER 测量结果是可靠的。使用多层膜沉积技术制备的名义值为500、200、100 nm的线宽样片,线边缘的均方根粗糙度值$ {\sigma }_ {\rm{LER}} $分别为2.7、2.35、2.3 nm,$ {\sigma }_ {\rm{LWR}} $分别为3.9、3.3、2.8 nm。由图9可知,不同尺寸线宽,在不同位置、不同放大倍率下测量的线边缘粗糙度波动范围相对较小,测量值相对一致,线宽变化小,$ {\sigma }_ {\rm{LER}} $标准差平均值为0.85,$ {\sigma }_ {\rm{LWR}} $标准差平均值为1.2,这说明该方法制备的线宽样片边缘较为平整,线宽分布相对均匀,具有良好的均匀性与一致性。反映了多层膜沉积技术能够较好地控制线宽的制备,使得样片中的线宽值相对稳定且具有良好地边缘特性。
Research on fine characterization technology of key parameters of line width of Si/SiO2 multilayer film
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摘要: 线边缘粗糙度(LER)和线宽粗糙度(LWR)是衡量线宽标准样片质量的重要指标。文中基于自溯源光栅标准物质的自溯源、高精密尺寸结构特性,提出了一种直接溯源型精确校准SEM放大倍率的方法,以实现SEM对线宽标准样片关键参数的测量与表征。利用校准后的SEM,对利用Si/SiO2多层膜沉积技术制备的线宽名义值为500、200、100 nm样片进行关键参数的测量,采用幅值量化参数的均方根粗糙度 RMS描述线边缘粗糙度与线宽粗糙度,并通过图像处理技术确定线边缘位置,对线宽边缘特性进行了精确表征。实验结果表明,名义值为500、200、100 nm对的线宽样片,其实测值分别为459.5、191.0、99.5 nm,$ {\sigma }_ {\rm{LER}} $分别为2.70、2.35、2.30 nm,$ {\sigma }_ {\rm{LWR}} $分别为3.90、3.30、2.80 nm,说明了多层膜线宽标准样片线边缘较为平整、线宽变化小、具有良好的均匀性与一致性。基于自溯源标准物质校准SEM的方法缩短了溯源链,提高了SEM的测量精度,实现了线宽及其边缘特性的精确表征,为高精度纳米尺度测量和微电子制造领域提供了计量支持。Abstract:
Objective As the key parameters of line width, line edge roughness (LER) and line width roughness (LWR) are important indicators of the quality of line width standard samples. The accuracy of LER and LWR is important for characterizing the reliability and uniformity of line width standard materials. Inspection is very important. Through the measurement and characterization of LER and LWR, the quality label technology level of line width standard samples can be effectively evaluated. Due to the problem of magnification in the measurement method of SEM, the measurement and characterization of LER and LWR have trays. Therefore, before using SEM to measure the line width, it is necessary to adjust the magnification of SEM with standard substances in advance. Methods With the self-traceable grating reference material as the standard of mass transmission (Fig.2), SEM is used to scan the self-traceable grating reference material, and the grating period measurement value of the self-traceable grating is obtained (Fig.3). It is compared with the actual grating period value, and the SEM calibration factor is obtained to realize the direct traceability and magnification calibration of the scanning electron microscope. The calibrated SEM is used to measure the different values of the multilayer film line width standard samples in different areas and different magnifications. The image processing technology is used to determine the position of the line edge and the average line edge based on the least squares fitting method. The root mean square roughness of the amplitude quantization parameter is calculated for LER and LWR (Fig.4). Results and Discussions The calibration of different magnifications of SEM is realized, and the calibration factors under different magnifications are obtained, which ensures the accuracy and traceability of the measurement results and shortens the traceability chain. The measurement results of line widths of different sizes are basically the same at different positions and different magnifications (Tab.2, Fig.8), the fluctuation range of line edge roughness is relatively small, the measured values are relatively consistent, and the change of line width is small (Tab.3, Fig.9); It shows that the edge of the line width sample is relatively smooth, the line width distribution is relatively uniform, and has good uniformity and consistency, which shows that the Si/SiO2 multilayer film deposition technology has the advantages in controlling the line width size and edge characteristics. Conclusions The SEM value traceability and magnification calibration method based on the self-traceable grating standard material shortens the traceability chain, reduces the traceability error introduced in the process of value traceability, improves the accuracy and reliability of SEM measurement, and provides a possibility for the flattening of the value transfer gradually. Through the measurement and analysis of line edge roughness and line width roughness, accurate characterization of line width and edge characteristics is achieved, and metrological support is provided for high-precision nanoscale measurement and microelectronics manufacturing fields. -
图 5 (a) $ {\sigma }_ {\rm{LER}} $=0,而$ {\sigma }_ {\rm{LWR}} $较大的情况; (b) $ {\sigma }_ {\rm{LWR}} $ 很小,而$ {\sigma }_ {\rm{LER}} $较大的情况
Figure 5. (a) $ {\sigma }_ {\rm{LER}} $=0, while $ {\sigma }_ {\rm{LWR}} $ is large; (b) $ {\sigma }_ {\rm{LWR}} $ is very small, and $ {\sigma }_ {\rm{LER}} $ is large
表 1 放大倍率为40,60,80 K× SEM测量结果(单位:nm)
Table 1. The magnification is 40, 60, 80 K× SEM measurement results (Unit: nm)
The ith measurement 40 K× 60 K× 80 K× $ {L}_{1} $ $ {L}_{2} $ $ \overline{{L}_{i}} $ $ {L}_{1} $ $ {L}_{2} $ $ \overline{{L}_{i}} $ $ {L}_{1} $ $ {L}_{2} $ $ \overline{{L}_{i}} $ i=1 1065.8 1052.1 1059.0 1061.1 1057.4 1059.3 1053.4 1063.0 1058.2 i=2 1063.0 1052.1 1057.6 1053.7 1055.6 1054.7 1054.8 1054.8 1054.8 i=3 1082.2 1068.5 1075.4 1061.1 1055.6 1058.4 1052.1 1057.5 1054.8 i=4 1046.1 1060.3 1053.2 1064.8 1064.8 1064.8 1053.4 1054.8 1054.1 i=5 1060.3 1049.3 1054.8 1074.1 1064.8 1069.5 1052.1 1054.8 1053.5 表 2 线宽均值测量结果(单位:nm)
Table 2. Line width mean measurement results (Unit: nm)
Nominal line width 40 K× 60 K× 80 K× Average value Absolute error Standard deviation Up Middle Down Up Middle Down Up Middle Down 500 457.1 458.7 454.9 465.2 464.1 466.7 453.7 452.6 462.3 459.5 40.5 5 200 188.1 190.3 187.6 193.8 191.6 197.2 188.4 189.0 193.1 191.0 9 3 100 96.3 97.9 97.4 99.1 100.3 103.6 98.1 100.6 102.8 99.5 0.5 2.3 表 3 线边缘LER 、LWR测量结果(单位:nm)
Table 3. Measurement results of line edge LER and LWR (Unit: nm)
Lineweight value Roughness 40K× 60K× 80K× Average value Standard deviation Up Middle Down Up Middle Down Up Middle Down 500 $ {\sigma }_{L} $ 1.3 1.3 1.6 3.2 3.4 3.5 3.7 3.0 2.7 2.6 0.9 $ {\sigma }_{R} $ 2.2 2.4 2.9 2.8 3.5 2.3 4.1 2.6 2.0 2.8 0.6 $ {\sigma }_{LWR} $ 2.2 1.9 3.2 4.1 3.9 3.5 5.3 6.5 4.2 3.9 1.3 200 $ {\sigma }_{L} $ 1.3 1.5 1.3 3.3 2.6 2.7 3.4 3.0 3.1 2.5 0.8 $ {\sigma }_{R} $ 2.6 1.1 2.9 2.1 3.5 1.6 1.7 1.7 2.7 2.2 0.7 $ {\sigma }_{LWR} $ 2.4 1.5 3.2 4.1 4.2 2.4 3.9 4.0 3.7 3.3 0.9 100 $ {\sigma }_{L} $ 1.0 0.9 0.8 3.7 3.4 2.7 3.1 2.9 2.2 2.3 1.1 $ {\sigma }_{R} $ 2.4 1.0 0.9 3.6 3.5 1.3 3.1 2.5 2.3 2.3 1.0 $ {\sigma }_{LWR} $ 1.7 0.8 1.0 4.8 3.9 2.3 4.0 4.1 2.8 2.8 1.4 -
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