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扫描电镜需要提供尽可能小的电子束直径,足够大的电子束流和尽可能大的工作距离。在很多情况下,物镜的球差作为主要象差限制了信噪比,而且球差系数随工作距离的加长而剧增。线性增长的场分布,在长工作距离的情况下,可以得到很小的球差系数。在磁透镜极靴中增夹铜片能够形成接近线性增长的场分布。文献采用增加铜片的方法,在扫描电镜上极靴中与主间隙S相距a_1处附加一子间隙S_1(图1),使工作距离L(从下极靴内表面起)为30mm,放大倍数M=0时球差系数降为原来DX5物镜的50％(表1中的DX5—1)。在此基础上,利用Munro程序进行计算,在不夹铜片的情况下,增大主间隙S,使S/D=1的DX5a物镜也可收到相近的效果。这里D=1/2(D_1+D_2)为上、下极靴内孔直径D_1及D_2的平均值。在上极靴中加一铜片的DX5a—1物镜在工作距离分别为15,20,30及40mm时球差系数可进一步减小到原DX5物镜的62,51,41及36％。这时色差系数略有增大,可用提高电源稳定度等方法来减小其影响。
SEM needs to provide the smallest electron beam diameter, a sufficiently large electron beam current, and the largest possible working distance. In many cases, the spherical aberration of the objective lens limits the signal-to-noise ratio as the primary aberration, and the spherical aberration coefficient increases dramatically with increasing working distance. With linear growth of the field distribution, a small spherical aberration coefficient can be obtained with a long working distance. In the magnetic lens pole piece by clip copper can form a nearly linearly increasing field distribution. The literature uses the method of adding copper sheet, in the scanning electron microscope pole shoe with the main gap S at a_1 additional subspace S_1 (Figure 1), so that the working distance L (from the lower pole shoe inner surface) is 30mm, magnification The spherical aberration coefficient at M = 0 is reduced to 50% of the original DX5 objective (DX5-1 in Table 1). On this basis, using Munro program to calculate, in the case of not clamping the copper sheet, increasing the main gap S, DX5a objective lens with S / D = 1 can also receive similar results. Here D = 1/2 (D_1 + D_2) is the average value of the inner hole diameter D_1 and D_2 of the upper and lower pole shoes. The DX5a-1 objective lens with a copper plate in the upper pole piece can further reduce the spherical aberration coefficient to 62, 51, 41 and 36% of the original DX5 objective lens at working distances of 15, 20, 30 and 40 mm respectively. At this moment the color difference coefficient increases slightly, can improve the power stability and other methods to reduce its influence.