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使用单探针进行等离子体诊断时,需要根据探针所测的飞行时间谱(time-of-flight spectra, TOF)画出其伏安特性曲线。图 3为朗缪尔探针测量的理想U-I曲线。其中横轴Vp表示探针所加偏压,纵轴Ip表示探针收集的等离子体电流,Ies为电子饱和流,Iis为离子饱和流,Vf为漂浮电位,Vsp为等离子体电势[24]。根据偏压的不同,整个曲线可以被分为离子饱和区(Vp < Vf)、过渡区(Vf < Vp < Vsp)、电子饱和区(Vp>Vsp)3个部分。当探针偏压为0V时,所测得的电流信号是离子电流与电子电流的叠加值,所以信号幅度相对很弱,且无法展现出等离子中各成分的真实特性。当探针被施加负偏压后,会对电子产生排斥并吸引离子,形成带正电的屏蔽鞘层,从而减少等离子体中电子电流的影响,此时离子电流大于电子电流,若所加偏压足够高,即可得到稳定的离子饱和流。同理,探针被施加正压后,会相应地减少离子电流的影响。
Figure 3. U-I curve collected by Langmuir probe[24]
通过离子饱和流Iis即可求得等离子体电子密度,计算过程中假设等离子体呈电中性,并且离子为弱电离:
$n_{\mathrm{e}}=n_{\mathrm{i}}=\frac{I_{\mathrm{is}}}{e A v_{\mathrm{i}}}$
(1) $v_{\mathrm{i}}=\frac{L}{t}$
(2) 式中,ne为等离子体电子密度,ni为等离子体密度,e为元电荷, A为探针表面积,vi为t时刻所对应离子速度且由TOF谱决定,L为探针距靶材距离。
过渡区中,探针电流Ip与鞘层电场(Vp-Vsp)有以下关系:
$I_{\mathrm{p}}=I_{\mathrm{es}} \exp \left[\frac{e\left(V_{\mathrm{p}}-V_{\mathrm{sp}}\right)}{k T_{\mathrm{e}}}\right]$
(3) $I_{\mathrm{es}}=\frac{e A \bar{v}_{\mathrm{e}} n_{\mathrm{e}}}{4}$
(4) $\bar{v}_{\mathrm{e}}=\sqrt{\frac{8 k T_{\mathrm{e}}}{\pi m}}$
(5) 将(3)式两边取对数:
$k T_{\mathrm{e}}=\frac{e\left(V_{\mathrm{p}}-V_{\mathrm{sp}}\right)}{\ln I_{\mathrm{p}}-\ln I_{\mathrm{es}}}$
(6) 式中, Te为电子温度,m为电子质量, ve为电子平均热速度,k为玻尔兹曼常量。由(6)式可知,kTe即为等离子体电子温度,其在数值上等于探针lnI-U曲线中过渡区斜率的倒数。
图 4为探针偏压为24V时,激光触发的探针TOF谱。通道1为法拉第筒信号,通道2为朗缪尔探针信号,通道4为激光信号。通道1是一个放置于靶材法向100mm处的法拉第筒测得的信号,所加偏压为-20V;通道2为探针电流过示波器内阻(50Ω)而产生的电压信号,持续时间约5μs,探针在法向方向距离靶材50mm,偏压为24V;通道4是光电探测器检测到的激光信号,脉宽约20ns。图中探针信号在与激光信号延时1μs之前有较为强烈的噪声干扰,而3.5μs之后信号幅度较小,信噪比较低。在数据处理时会引起较大的误差,所以数据处理时把这部分舍去。
朗缪尔探针诊断脉冲激光锡等离子体特性
Research on the characteristics of laser produced tin plasma by using Langmuir probe
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摘要: 激光作用锡靶等离子体极紫外光转换效率与等离子体特性密切相关。为了对等离子体特性进行诊断,设计了一种用于激光等离子体诊断的朗缪尔探针,取得了不同激光能量下产生的锡等离子体电子温度与电子密度的时间演化。结果表明,能量为58.1mJ的激光产生的等离子体峰值电子密度约为4.5×1011cm-3,最大电子温度为16.5eV,均随激光能量减少而降低,与发射光谱法所测的电子温度演化趋势一致。该研究为激光等离子体极紫外光源提供了一种新的简单快速诊断方法,有利于对激光等离子体的极紫外光源的参量进行优化。Abstract: The extreme ultraviolet (EUV) light conversion efficiency of the laser-produced tin plasma is closely related to the plasma characteristics. To diagnose the parameters of tin plasma, a Langmuir probe for laser-produced plasma diagnosis was designed. And the time evolution of electron temperature and electron density of tin plasma produced by different laser energies were studied. The results show that the peak electron density of the plasma is about 4.5×1011cm-3 with laser energy of 58.1mJ, and the maximum electron temperature is 16.5eV, which decreased with the reduction of laser energy. Moreover, the evolution trends of electron temperature measured by Langmuir probe and emission spectrometry are consistent. This study provides a new simple and rapid diagnostic method for laser-produced plasma EUV light source, which is beneficial to optimize the parameters of EUV light.
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Key words:
- laser physics /
- plasma diagnosis /
- Langmuir probe /
- extreme ultraviolet lithography
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Figure 3. U-I curve collected by Langmuir probe[24]
Figure 11. Spectroscopic diagnosis of laser produced Sn plasma and SnO2 plasma[22]
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