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图 1表示激光背向湿式加工蓝宝石的加工装置图。采用红外光纤激光器(波长为1064nm,最大输出功率为20W,光斑直径为30μm)作为光源。利用激光诱导背向湿式刻蚀技术加工样件,样件为光学级c-面(0001)蓝宝石基片,其尺寸为50.8mm×430μm(直径×厚度),原始表面粗糙度为0.1nm。实验前需将蓝宝石基片进行预处理:将蓝宝石基片在丙酮和水中各进行超声清洗5min,再将样件进行干燥处理。
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在混合溶液的作用下,刻槽的不同区域中元素的原子数分数如表 1所示。元素的原子数分数由能谱扫描可得,从中可以看出,铜存在于沟槽的底部和边缘。在1064nm激光作用下,工作溶液产生光化学反应,在蓝宝石表面形成铜沉积层。这是因为激光透过蓝宝石衬底聚焦在与液体接触的交界面,通过激光诱导液体产生光化学反应,沉积层在蓝宝石衬底背面形成,并增强了蓝宝石对激光的吸收作用使得温度高于蓝宝石的熔点,从而导致材料的去除。
Table 1. Atomic number fraction in different regions of grooving by energy dispersive spectrometer
the atomic number fraction elements substrate edge of groove bottom of groove particles substrate of bottom of groove hole of bottom of groove O 0.5996 0.57 0.5702 0.5494 0.4532 0.364 Al 0.4004 0.3591 0.3898 0.13 0.543 0.6307 Cu 0 0.0202 0.018 0.1612 0.003 0.0026 total 1 -
在硫酸铜溶液中加入浓氨水,首先析出浅蓝色的碱式硫酸铜沉淀,氨水过量时此沉淀溶解,同时形成四氨合铜(Ⅱ)络离子。主要的化学反应的离子方程式如下:
$ \begin{array}{l} 2{\rm{C}}{{\rm{u}}^{2 + }} + 2{\rm{SO}}_4^{2 - } + 2{\rm{N}}{{\rm{H}}_3} \to \\ {\rm{C}}{{\rm{u}}_2}{\left( {{\rm{OH}}} \right)_2}{\rm{S}}{{\rm{O}}_4} \downarrow + {({\rm{N}}{{\rm{H}}_4})_2}{\rm{S}}{{\rm{O}}_4} \end{array} $
(1) $ \begin{align} & \text{C}{{\text{u}}_{2}}{{\left( \text{OH} \right)}_{2}}\text{S}{{\text{O}}_{4}}\xrightarrow{\text{irradiation}}2{{\left[ \text{Cu}{{\left( \text{N}{{\text{H}}_{3}} \right)}_{4}} \right]}^{2+}}+ \\ & \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 2\text{O}{{\text{H}}^{-}}+\text{SO}_{4}^{2-} \\ \end{align} $
(2) Cu2(OH)2SO4为热不稳定的金属盐,由下式中可以看出, 在激光作用时,碱式硫酸铜吸收激光热量,将激光热量转化至工作液体,使其发生热分解生成金属或其它相应的产物。
$ \begin{align} & \text{C}{{\text{u}}_{2}}{{\left( \text{OH} \right)}_{2}}\text{S}{{\text{O}}_{4}}\xrightarrow{\text{irradiation}}\text{CuO}\downarrow + \\ & \ \ \ \ \ \ \ \ \ \ \ \ \text{C}{{\text{u}}^{2+}}+\text{S}{{\text{O}}_{4}}^{2-}+{{\text{H}}_{\text{2}}}\text{O} \\ \end{align} $
(3) 由于(2)式在激光作用时Cu2(OH)2SO4的量减少, 使得其发生逆向反应,铜氨离子的含量降低,由于激光光斑较小,铜氨离子的降低量较少,随着加工时间的延长,pH值有所降低,这时不能加入pH调节剂,只需要适量地加入氨水进行补给, 恢复到原来pH值即可。
在以次磷酸钠为还原剂的条件下,还原剂的第1步反应为去氧反应:
$ {{\rm{H}}_{\rm{2}}}{\rm{PO}}_2^ - \to {\rm{HPO}}_2^ - + {\rm{H}} $
(4) 生成的H2PO2-和OH-反应生成H2PO3-并释放电子。
$ \text{HPO}_{2}^{-}+\text{O}{{\text{H}}^{-}}\to {{\text{H}}_{\text{2}}}\text{PO}_{3}^{-}+{{\text{e}}^{-}} $
(5) 水与Cu2+争夺电子发生下述反应:
$ {{\text{H}}_{\text{2}}}\text{O}+{{\text{e}}^{-}}\to \text{O}{{\text{H}}^{-}}+\text{H} $
(6) 上式中生成氢原子结合生成氢气。
$ {\rm{H}} + {\rm{H}} \to {{\rm{H}}_2} \uparrow $
(7) 激光诱导光化学反应的主要反应:
$ \begin{align} & 2{{\text{H}}_{\text{2}}}\text{PO}_{2}^{-}+2\text{C}{{\text{u}}^{2+}}+2\text{O}{{\text{H}}^{-}}\to \text{Cu}\downarrow + \\ & \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{2}{{\text{H}}_{\text{2}}}\text{PO}_{3}^{-}+{{\text{H}}_{2}}\uparrow \\ \end{align} $
(8) 而(4)式~(6)式构成了副反应:
$ 2{{\rm{H}}_{\rm{2}}}{\rm{PO}}_2^ - + {{\rm{H}}_{\rm{2}}}{\rm{O}} \to {{\rm{H}}_{\rm{2}}}{\rm{PO}}_3^ - + {{\rm{H}}_2} \uparrow $
(9) 产生副反应的原因是铜离子浓度过低,H2PO2-含量过高,使以下(10)式的反应得以进行。这时沉铜速率降低,溶液中大量析氢,造成工作液体的不稳定。在光化学反应过程中,次亚磷酸钠还会发生歧化反应析出磷;并且在激光辐射过程中存在蓝宝石基材脱氧反应,因而在具有催化活性的蓝宝石上获得的沉积层实际上是含有少量磷的Cu-Al-P合金沉积层,有关的化学反应方程式如下:
$ {{\rm{H}}_{\rm{2}}}{\rm{PO}}_2^ - + 2{{\rm{H}}^ + } \to {\rm{P}} + 2{{\rm{H}}_{\rm{2}}}{\rm{O}} $
(10) $ \begin{align} & {{\text{H}}_{\text{2}}}\text{PO}_{2}^{-}+\text{C}{{\text{u}}^{2+}}+3\text{O}{{\text{H}}^{-}}\to \text{Cu}+ \\ & \text{ }\ \ \text{HPO}_{3}^{2-}+{{\text{H}}_{2}}+\text{P}+2{{\text{H}}_{\text{2}}}\text{O} \\ \end{align} $
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工作液体在激光作用下,由于液体内部发生光化学反应生成气体以及液体受热蒸发生成的部分气体形成气泡, 将阻碍工作液体与蓝宝石基片下表面的接触,进而阻碍对激光的吸收;随着气泡产生周期变化,工作液体对激光的吸收呈周期性变化,造成由于材料吸收激光能量的周期性变化而使产生的热应力也成周期性变化,极易对蓝宝石基片产生热应力破坏,特别是工作液体对激光的吸收率受到液层厚度影响较敏感时,热应力破坏越严重。采用功率计对不同溶液层厚度处工作溶液的功率进行测量,测量装置的示意图如图 13所示。吸收系数计算如下所示:
$ A = 1 - \frac{{{P_t}}}{{{P_i}}} \times 100\% $
(12) 式中, Pt为透过工作液体的激光功率;Pi为入射激光功率;Pt/Pi×100%为透射率。计算过程中忽略工作液体的反射率。
图 14表示混合溶液,浓度c(CuSO4)分别为1.2mol/L, 0.9mol/L, 0.6mol/L的硫酸铜溶液等4种工作液体的透射率与不同液层厚度的关系。从图 14可以看出,硫酸铜水溶液对液层厚度的敏感程度相比混合溶液高,这也间接地反映了在硫酸铜水溶液中气泡对液体对激光吸收的影响较大。激光作用硫酸铜水溶液过程中气泡随激光脉冲成周期性产生,气泡最大直径接近1mm。每次气泡的产生过程也就是液层厚度的增加和减少过程,增加和减少范围为气泡最大直径1mm,但是1mm厚度对于硫酸铜水溶液对激光的吸收影响极大,液层厚度从0.2mm增加到1mm时,液层对激光的透射率从90%降到20%,此时硫酸铜水溶液对激光的吸收剧烈,使得蓝宝石基片温度剧增,热应力过大,容易产生崩边现象。混合工作液体对激光的吸收相对稳定,随液层厚度的增加,溶液对激光透射率缓慢降低,且混合工作液体对激光的吸收受气泡的影响并不明显,表明混合溶液的性质是相对稳定的。
Figure 14. Relationship between the transmittance of working solutions with different thicknesses of the solution layer
在相同条件下,采用硫酸铜水溶液和混合溶液对蓝宝石进行切割,如图 15所示。蓝宝石在混合溶液中的切割深度高于硫酸铜溶液的5倍左右,因此,在混合溶液的作用下,大大提高了切割效率,并能实现对蓝宝石的成形切割。图 16中展示了加工好的异形蓝宝石零件。
激光诱导背向湿式刻蚀过程中工作液体的研究
Study on working solution of laser-induced backside wet etching
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摘要: 为了使用低成本的红外光纤激光器对蓝宝石进行高效率高质量的加工, 采用激光诱导背向湿式刻蚀方法进行了理论分析和实验验证, 研制了一种新型的活性高且稳定性高的混合溶液, 硫酸铜的质量浓度为28g/L、次磷酸钠的质量浓度为40g/L、氨水的质量浓度为45g/L、pH值为12。在相同的加工条件下, 采用硫酸铜水溶液和混合溶液对蓝宝石进行切割。结果表明, 蓝宝石在混合溶液中的切割效率高出硫酸铜水溶液中的5倍左右; 采用混合溶液对蓝宝石进行成形切割, 加工出了高质量的异形蓝宝石零件。此研究对提高低功率激光作用下蓝宝石的加工效率有一定的指导意义。
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关键词:
- 激光技术 /
- 混合溶液 /
- 激光诱导背向湿式刻蚀 /
- 蓝宝石 /
- 成形切割
Abstract: In order to process sapphire with high efficiency and high quality by using low cost infrared fiber laser, laser-induced backside wet etching (LIBWE) was used. Theoretical analysis and experimental verification were carried out. New type of the mixed solution with high activity and stability was developed in which mass concentration of copper sulfate, sodium hypophosphite, ammonia was 28g/L, 40g/L, 45g/L respectively and whose pH value was 12. Under the same processing conditions, sapphire was cut in copper sulfate solution and the mixed solution. The results show that the cutting efficiency of sapphire in the mixed solution is about 5 times higher than that in copper sulfate solution. High-quality special-shaped sapphire parts have been processed with the mixed solution. This research has certain guiding significance for improving the processing efficiency of sapphire under the action of low power laser. -
Table 1. Atomic number fraction in different regions of grooving by energy dispersive spectrometer
the atomic number fraction elements substrate edge of groove bottom of groove particles substrate of bottom of groove hole of bottom of groove O 0.5996 0.57 0.5702 0.5494 0.4532 0.364 Al 0.4004 0.3591 0.3898 0.13 0.543 0.6307 Cu 0 0.0202 0.018 0.1612 0.003 0.0026 total 1 -
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