Shot noise in electron-beam lithography and line-width measurements.
Shot noise in electron-beam lithography and line-width measurements.
复制标题
电子束光刻和线宽测量中的散粒噪声。
作者：
P. Kruit;S. Steenbrink
电子束光刻广泛用于纳米科学和技术，用于制造半导体工业的掩模，并在有限的规模上用于无掩模光刻：即直接在芯片上写入图案。我们预计，后一种应用将在今后几年扩大。控制写入结构的尺寸在半导体工业中是必不可少的。对于目前正在开发并将在十年结束时投入生产的45 nm一代，根据应用，对线宽的控制要求在1.5和5 nm之间。影响线宽控制的因素之一是照射抗蚀剂的电子数的统计。这种效应使得线边缘粗糙，或者换句话说，缺乏对抗蚀剂边缘的局部位置的控制。这一点早已得到承认，并经常讨论。最近，我们开发了一个分析模型的线边缘位置的变化，我们将说明和扩展本文。该模型，支持Monte Carlo模拟，表明线宽的变化是成反比的剂量用于照明的抗蚀剂。这使得不可能通过开发超灵敏抗蚀剂来增加光刻生产量。对于以30 nm的典型分辨率写入的45 nm特征，30 μ C/cm 2的抗蚀剂在45 nm长的线段上给出3 nm的线宽变化。线宽通常在经调整的临界尺寸扫描电子显微镜（CD-SEM）中测量。该测量需要比光刻步骤的结果更精确，因此要求通常为亚纳米。除了避免系统误差的所有问题之外，这种测量还遭受统计变化，这是由用于测量的有限数量的电子造成的。在本文中，我们将得出一个类似的模型，用于在光刻步骤中的散粒噪声效应的变化的估计。其中一个结论是，对于线宽的最精确测量，不建议为了最佳分辨率而调整CD-SEM。最好允许电子束的探针尺寸更大，因为这可能伴随着更大的电流，从而降低噪声水平。
Electron-beam lithography is used extensively in nanoscience and technology for making masks for the semiconductor industry and, on a limited scale, for maskless lithography: that is, writing the patterns directly on the chip. We expect the latter application to extend in the years to come. Control of the dimensions of the written structures is essential in the semiconductor industry. For 45 nm generation, which is presently under development and should reach production at the end of the decade, the required control over the line width is between 1.5 and 5 nm, depending on the application. One of the factors of influence on the line-width control is the statistics in the number of electrons illuminating the resist. This effect gives line edge roughness, or in other words a lack of control over the local position of a resist edge. This has long been recognized and often discussed. Recently, we developed an analytic model for the line edge position variation, which we shall illustrate and expand in this paper. The model, supported by Monte Carlo simulations, demonstrates that the line-width variation is inversely proportional to the dose used for the illumination of the resist. This makes it impossible to increase the lithography throughput by developing ultrasensitive resists. For 45 nm features written with a typical resolution of 30 nm, a 30 microC/cm2 resist gives 3 nm line-width variation over line segments of 45 nm long. The line width is usually measured in an adapted critical dimension scanning electron microscope (CD-SEM). This measurement needs to be more precise than the result of the lithography step, so the requirements are typically sub-nm. Apart from all the problems to avoid systematic errors, this measurement also suffers from statistical variations, resulting from the finite number of electrons used for the measurement. In this paper we shall derive an estimate for that variation with a similar model as used for the shot noise effect in the lithography step. One of the conclusions is that for the most precise measurements of the line width it is not advisable to tune the CD-SEM for the best resolution. It is better to allow a larger probe size of the electron beam because that can be accompanied by a much larger current and thus a decrease in the noise level.