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科学研究领域

CO2激光器应用之加热浮区融化实现单晶生长

来源:本站    发布时间:2022/3/3 13:16:58    点击量:

CO2激光器加热浮区融化实现单晶生长

过去被半导体和通讯行业驱动,今天先进的计算材料科学需要更加灵活的制造技术在未来晶体材料合成上,如陶瓷,压电器件,超导体等。

背景:在历史上,重要的科学发现往往是偶然的。据说,在1916年,波兰化学家Jan Czochralski正在研究金属的晶体化,他偶然间把他的笔插入融化的锡里而不是墨水瓶,他拿出来发现是细锡丝。后来检测发现,细锡丝是单晶。十年后,在1948年,美国科学家贝尔实验室的Gordon TealJohn Little使用Czochralski的方法生长出了绝对纯净的锗和硅单晶,为现代的半导体晶圆制造奠定了基础。今天,90%以上的基于半导体的电子器件的材料都是由Czochralski的方法合成生产的。

然而,仅仅几年后,在1955年,Henry Theuerer发明了第一个浮动区技术。一个垂直放置的锗棒被拉着通过一个射频加热线圈参数的局部融化区。不使用坩埚,可以产生更高纯度的晶体,但直径只能限制在<20mm。到20世纪70年代,由于在光纤应用上的独特优势,通信业对生长小直径高纯度单晶氧化物的兴趣大大增加。浮区技术是一个优先选择,但由于射频加热氧化物的低效率,没有被应用起来。1972年,John Haggerty对这个问题提出了解决方案,用CO2激光器在光纤拉丝过程中来参数浮动区。Martin FejerRobert Feigelson1980年做这个方案做了进一步提升,建立了叫做激光加热基座生长LHPGLaser-Heated Pedestal Growth)技术,如图1所示。

 

 

Figure 1: Drawing of the CO2 LHPG technique.

近些年,计算材料科学已经对新材料合成方法提出更好的要求,杂志 the 2019 Material by Design Roadmap 第一次报道,理论预测的材料数量超过了实验已知的晶体学数据库里面的数量。因此,很多科学家在寻找灵活的制造方式来测试这些新的预期材料上面都有共同的兴趣。

应用:LHPG是用在工业和材料研究,尤其是对于高熔点材料,因为能灵活并有成本优势的制造单晶光纤。LHPG使用CO2激光器聚集光斑参数浮动熔融区。为实现这一目的,激光器光斑被引导到一个封闭的腔室,光束撞到一个反射锥,把光束转变成一个空心圆柱外形。然后,光束被引导到抛物面凹镜上,可以把光束聚焦到基座源上。

 

Figure 2: Drawing of the floating zone formation.

光纤拉丝有3个步骤,如图2所示。首先,聚焦激光在基座顶端产生一个很小的熔融区。第二,一个种子晶体引入到熔融区,在基座和种子晶体之间产生一个固体-液体交界面。第三,熔融区和在种子晶体处的固化可以通过持续的拉伸光纤来实现。这些受限控制条件,对在固化过程中保持微观结构有利,这可以使单晶持续生长。稳定的热力学条件,因此一个稳定型高的CO2激光器是成功的必要。实际上,很小的热量的波动会导致光纤直径的变化。这需要使用高稳定的CO2激光器,例如大通激光AccessLaserAL50ST,通过检测熔融区如图2.4。精密控制拉伸速率和激光器的功率可以使得光纤直径变化最小,可到小于1%。为保持熔融区稳定,根据材料、直径、拉伸速率等,激光器的功率需要从10W200W。单晶光纤直径从25μm1mm都可以实现。

概要:使用CO2激光器作为加热源跟其他方式相比有两个主要优势。首先,10.6μm的波长能够被很多相关氧化物很好的吸收。第二,激光很容易被聚焦,能产生局部高温和梯度升温。高温梯度能够使生长速度快60,mm/分代替mm/小时,并且可加工高熔点材料。这些包括对材料科学和工业届有兴趣的材料,如硅、蓝宝石、YAG、超导体、功能性陶瓷、铁、光和压电材料和共熔化合物。在生产高熔点单晶光纤的极大灵活性和成本优势使得LHPG成为一个有力的工具。

Single Crystal Growth with CO2Laser-Heated Floating-Zone

In the past driven by the semiconductor and telecommunications industries, it is today’s advances in computational materials science that require flexible manufacturing technologies for the synthesis of tomorrow’s crystalline materials such as ceramics, piezoelectrics, superconductors etc.

Background: As often in history, important scientific discoveries are made by accident. It is said that in 1916 the Polish chemist Jan Czochralski was investigating the crystallization of metals as he accidentally dipped his pen into molten tin instead of his inkwell and pulled out a thin tin filament.1 Later examination of the filament revealed that it was a single crystal. Decades later, in 1948, the American scientists Gordon Teal and John Little of Bell Labs used Czochralski’s method to grow extremely pure germanium and silicon single crystals, paving the way for modern semiconductor wafer production. As of today, over 90% of all semiconductor-based electronic devices are made from materials synthesized by Czochralski’s method.2

However, only a few years later, in 1955, Henry Theuerer developed the first floating zone technique. A vertically arranged germanium rod was pulled through a localized melt zone created by an RF heating coil. The absence of a crucible allowed the growth higher purity crystals, but of limited diameter (< 200 mm). By the 1970s, the telecommunications industry was increasingly interested in growing single crystal oxides of small-diameter and high-purity due to their advantageous material properties for fiber optical applications.3 The floating zone technique was the preferred choice but could not be applied due to inefficient RF heating of the oxides. A solution to this problem was presented in 1972 by John Haggerty, who used CO2 laser radiation to create the floating zone in a fiber drawing process. Further improvements by Martin Fejer and Robert Feigelson in 1980 helped to establish the so-called Laser-Heated Pedestal Growth (LHPG), as shown in Figure 1.4

In recent years, advances in computational materials science have led to high demand for methods to exploit synthesis of new materials. The 2019 Materials by Design Roadmap reports that for the first time, the number of theoretically predicted materials exceeds the number of experimentally known entries in crystallographic databases. Thus, a shared interest by many scientists includes flexible manufacturing methods to test these newly predicted materials.5

Figure 1: Drawing of the CO2 LHPG technique.

 

Figure 2: Drawing of the floating zone formation.

Application: LHPG is such a method used in industry and material research, notably for high melting point materials, because of its flexible and cost-effective fabrication of single crystal fibers. LHPG uses focused CO2 laser radiation to create a floating melt zone. For this purpose, the laser beam is guided into a closed chamber where it hits a reflaxicon which converts the laser beam to a hollow cylinder shape. The beam is then guided to a parabolic mirror which focuses the radiation over the pedestal source.4

The fiber drawing takes place in three steps, as shown in Figure 2. First, the focused laser radiation creates a small melt zone on top of the pedestal. Second, a seed crystal is introduced into the melt zone, creating solid-liquid interfaces at the pedestal and seed. And third, the melt zone and solidification at the seed are fed by the continuous pulling of the fiber. Under these con-trolled conditions, it is energetically favorable to maintain the seed’s microstructure during solidification, which enables the continuous growth of the single crystal. Stable thermodynamic conditions, and thus a stable CO2 laser source, are essential for success. In practice, however, small thermal fluctuations lead to a variation of the fiber diameter. This can be overcome by using a stabilized laser source, such as Access Laser’s AL50ST, and by monitoring the melt zone (Figure 2.4). Precise control of the pulling rate vf, vP, and the laser power PL can minimize the diameter variations to less than 1%.4 The laser power required to keep the melt zone stable varies from 10 to 200 W depending on the material, diameter, pulling rate etc. Single crystal fibers with diameters from 25 µm to 1 mm have been realized.

Bottom line: Using CO2 laser radiation as a heating source has two major advantages over other methods. First, the wavelength of 10.6 µm is well absorbed by many relevant oxides. And second, laser radiation can be easily focused, resulting in high local temperatures and gradients. The high temperature gradients enable 60 times faster growth — mm/min instead of mm/h — and the processing of materials with very high melting points. This includes materials of great interest to material science and industry such as silica, sapphire, YAG, superconductors, functional ceramics, ferro-, opto-, and piezoelectric materials, and eutectic compounds.4 The great flexibility in producing single crystal fibers of high melting point materials at low cost makes LHPG a powerful tool.

1. Pajaczkowska, A. (2001): Prof. Dr. Jan Czochralski – An Inventor. Newsletter of the German Association for Crystal Growth (73), 30. ISSN 2193-3758
2. Evers, J. et al (2003): Czochralski’s Creative Mistake: A Milestone on the Way to the Gigabit Era. Angewandte Chemie International Edition, 42(46), 5684-5698. DOI: 10.1002/anie.200300587
3. Rudolph, P. (2014): Handbook of crystal growth: Bulk crystal growth. Elsevier. ISBN 9780444633033.
4. Andreeta, M.R. et al (2010): Laser-heated pedestal Growth of oxide Fibers. In Springer Handbook of Crystal Growth. 10.1007/978-3-540-74761-1_13.
5. Alberi, K. et al (2019): The 2019 materials by design roadmap. Journal of Physics D: Applied Physics 52.1 (2018): 01300

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