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摘要:
2 μm~5 μm波段GaSb基VCSEL对大气检测技术有着重要的应用，但制备技术的不成熟严重制约着GaSb基VCSEL的发展。刻蚀工艺中出现的下切效应就是器件制备中存在的突出问题。针对上述问题，选择三种不同成分的磷酸系刻蚀液进行了对比性刻蚀实验，并通过台阶仪、扫描电子显微镜(SEM)测试观察了刻蚀速率和表面形貌。实验分析表明，浓度配比为1 mL:1 mL:0.6 g:10 mL的H3PO4:H2O2:C4H6O6:H2O刻蚀液具有良好的腐蚀效果，消除了以往腐蚀过程中出现的下切效应，且垂直形貌好，未出现钻蚀现象，晶片表面平整且光滑，且保持稳定的刻蚀速率0.62 μm/min，为激光器制备提供了良好的前期实验基础。
Abstract:2 μm~5 μm GaSb-based VCSEL is an ideal light source for atmospheric detection. However, the immaturity of its fabrication technology seriously hinders its development. The undercutting effect is the outstanding etch problem in its fabrication. In this paper, Etching characteristics of GaSb is investigated in detail by use of phosphoric acid plus tartaric acid solution. In order to compare them, we chose concentration ratio of H3PO4:H2O2:C4H6O6:H2O as 1 mL: 1 mL: 0.3 g: 10 mL, 1 mL: 1 mL: 0.6 g: 10 mL, and 1 mL: 1 mL: 1 g: 10 mL, respectively. The testing results from step profiler and scanning electron microscopy (SEM) were compared and analyzed. Etched GaSb in the solution with a concentration ratio of H3PO4:H2O2:C4H6O6:H2O=1:1:0.6:10 shows very good morphology. Undercutting effect was eliminated and a vertical side wall was obtained with no lateral etching. Etching rate is 0.62 μm/min. The perfect etch behavior of GaSb provides a good technical support for laser preparation.
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Key words:
- Gallium Antimonide /
- VCSEL /
- etching rate /
- surface morphology /
- tartaric acid
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2 μm5 μm mid-infrared vertical cavity surface emitting laser (VCSEL), featured with advantages of low power consumption, small divergence angle, no astigmatism circular spot, high modulation bandwidth, wavelength stability,low production cost, and high density ultra-small dimensional packaging, is an ideal light source for the molecular spectrum measurement, biochemical testing, laser radar, thermal imaging and medical diagnosis. For example, in the TDLAS (tunable laser diode absorption spectroscopy test) system for monitoring polluting gases CO, CH4, NH3 and HF, the use of VCSEL as a light source will greatly reduce the complexity and the cost for no beam shaping and easily coupled packaging, and shrink the size of the system. Furthermore, its modulation rate can reach several tens of Gbit/s, so it is considered to be the best alternative device of distributed feedback (DFB) laser in the future. As the GaSb material can cover the entire mid-infrared band, it is the best material system for the development of 2 μm5 μm mid-infrared VCSEL.
GaSb-based VCSEL has become a hot concern over the world because of its promising application. However the immaturity of its fabrication processes seriously hinders its development. E.g., owing to its easily oxidized properties that make etching GaSb not a easy thing, etch process is one of the most key technologies in preparing a GaSb-based VCSEL.
In the previous etching experiments, only the effect of hydrofluoric acid plus tartaric acid etching solution on the corrosion rate was explored, but the surface morphology of the substrate and the undercutting effect was not well analyzed. Etching characteristics of GaSb is investigated in detail by use of phosphoric acid plus tartaric acid solution. In order to compare them, we chose concentration ratio of H3PO4:H2O2:C4H6O6:H2O as 1 mL: 1 mL: 0.3 g: 10 mL, 1 mL: 1 mL: 1 g: 10 mL, and 1 mL: 1 mL: 0.6g: 10 mL, respectively. In order to prevent the corrosion rate of the viscous reaction product formed on the GaSb surface during the etching process, the sample is continuously immersed in the etching solution by a magnetic stirrer to assist in obtaining a constant etching rate.
The experimental results were characterized through scanning electron microscopy (SEM) and so on, and the testing results were compared and analyzed. Etched GaSb in the third group solution (H3PO4:H2O2:C4H6O6:H2O=1:1:0.6:10) shows very good morphology: undercutting effect was eliminated and a vertical side wall was obtained with no lateral etching. Etching rate is 0.62 μm/min. The perfect etch behavior of GaSb provides a good preliminary experimental basis for laser preparation.
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1. 引言
2 μm~5 μm中红外波段垂直腔表面发射激光器(vertical cavity surface emitting laser，VCSEL)具有功耗低、发散角小、无像散圆形光斑、调制带宽高、波长稳定性好、制作成本低、可实现超小型的高密度二维封装等优点，是分子光谱测量、生化检测、激光雷达及热成像、医学诊断和药物分析等领域迫切需求的理想光源[1-4]。GaSb是发展2 μm~5 μm波段VCSEL的最佳材料体系[5]。但由于锑化物材料的特殊性，其器件工艺相对于GaAs尚未成熟，尤其在刻蚀工艺上，通常用到的腐蚀液与材料反应时，会生成一种难溶于水也不溶于酸碱溶液的氧化物，堆积在材料表面影响到反应的进一步进行，并且会降低其刻蚀精度[6]，从而制约着GaSb基VCSEL的发展。因此，刻蚀工艺的研究对于锑化物激光器的发展显得尤为重要[7]。
湿法刻蚀是目前制备VCSEL中各种台面结构的最常用方法，对于台面形貌，腐蚀液的成分及配比是最主要的影响因素。在以往报道的GaSb基材料湿法刻蚀工艺中，典型的腐蚀液体系有：HF+H2O2+C4H6O6+H2O[8]，HCL+H2O2+C6H8O7+H2O[9]和H3PO4+H2O2+C4H6O6+H2O[10]。在盐酸系腐蚀液中，腐蚀液氧化性过强，对作为掩膜的AZ5214光刻胶的腐蚀性很大，横向钻蚀严重且刻蚀表面形貌差，非常不利于器件的制备[9]。用HF酸系腐蚀液腐蚀后表面虽然光滑，但腐蚀的边缘仍较毛糙[11]。而且，由于溶液配比问题，往往会出现下切效应，钻蚀现象，刻蚀后晶片表面粗糙。所以采用磷酸系腐蚀液的优势其一就是对材料的晶向有刻蚀选择性[10]，其二是磷酸与双氧水的合适配比可以使晶片表面的粗糙度有所改善，对实验的刻蚀有良好的效果。基于以上分析，本文选择三种不同配比的磷酸系刻蚀液进行了对比性刻蚀实验。并通过台阶仪，扫描电子显微镜(SEM)测试对刻蚀速率及表面形貌进行了分析。
2. 实验原理及方法
本实验中采用的是晶向为(100)n型掺杂的GaSb衬底。配制的腐蚀液选用的是浓度分别为85%的H3PO4，30%的H2O2，以及纯度为99.5%的酒石酸。磷酸可以实现各向异性刻蚀，其与双氧水的适当配比可以改善表面的粗糙度，经查阅相关文献，磷酸与双氧水的最佳配比为1:1[9]。针对GaSb表面难溶的氧化物薄膜，可以利用酒石酸或柠檬酸与其发生的络合反应来改善表面形貌和刻蚀作用[12, 13]。酒石酸作为络合剂，与堆积在GaSb表面的氧化物发生络合反应生成可溶的络合物[14]，保证反应的进一步进行。加入酒石酸作为络合剂的反应方程式[15]如下：
氧化反应：
2GaSb+6H2O2→Ga2O3+Sb2O3+6H2O, 络合反应：
Sb2O3+2C4H6O6→2[Sb(C2H4O6)(H2O)]+H2O+2H+. 氧化剂先与GaSb发生反应生成表面难溶Sb2O3，之后再与酒石酸根离子发生络合反应生成可溶于水的络合产物，期间不断搅拌使生成物离开材料表面，因而反应能够持续进行[16, 17]。且由于扩散限制机制[9]，只有当刻蚀过程中产生的氧化物与络合剂反应的速率大于氧化速率时，反应才能顺利进行，否则氧化产物过度堆积会致使腐蚀液无法与材料充分接触。因此湿法刻蚀中最为关键的问题是选择合适的络合剂。理论上，调整GaSb腐蚀液的各组分配比是可以影响其反应速率及刻蚀的陡直度，陡直度越接近于90°效果越好。保证各向异性刻蚀，即只有垂直刻蚀，没有横向钻蚀。这样才能精确地在被刻蚀的材料上制作出满足要求的几何图形。为了得到最优的刻蚀效果，在本实验中，通过调整络合剂酒石酸的剂量配置三种不同配比的磷酸系刻蚀液进行了对比性刻蚀实验，溶液浓度配比如表 1所示，本实验所采用的温度为室温(25 ℃)。
表 1. 腐蚀液浓度配比.Table 1. Concentration ratio of corrosive solution.No. Concentration ratio of corrosive solution P1 H3PO4:H2O2:C4H6O6:H2O=1:1:0.3:10 P2 H3PO4:H2O2:C4H6O6:H2O=1:1:1:10 P3 H3PO4:H2O2:C4H6O6:H2O=1:1:0.6:10 | Show Table
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刻蚀实验前首先对晶片进行光刻，光刻前用丙酮、乙醇溶液依次对样品进行清洗，然后用去离子水冲洗，氮气吹干。清洗后样品表面旋涂AZ5214光刻胶作为掩膜[18]，前烘设定热板温度为105 ℃，时间为60 s，待胶膜干燥且温度降到室温后，到紫外光刻机对样品进行曝光4.3 s。将曝光后的样品立即放到显影液中，时间为35 s，显影后冲洗吹干，最后在115 ℃的热板温度下坚膜5 min。由于实验是采取多组对比测试，为了保证每次刻蚀前实验样品的一致性，排除其他因素的干扰，将光刻后GaSb晶片用划片机解理成若干小片，放入洁净的容器中备用。在室温(25 ℃)下进行刻蚀，在刻蚀反应进行时，为防止GaSb表面上形成的粘性反应产物降低刻蚀速率，将样品通过磁力搅拌器连续搅拌，以助于获得恒定的刻蚀速率。刻蚀后用去离子水冲洗残留的腐蚀液，氮气吹干表面，使用丙酮溶液清洗去胶[19]。用台阶仪测试刻蚀深度及刻蚀表面情况，再用扫描电子显微镜(SEM)观测表面形貌。
3. 实验结果及分析
3.1 刻蚀速率
在制备GaSb基VCSEL的湿法腐蚀工艺中，腐蚀速率直接影响腐蚀深度的精度控制，湿法腐蚀速率的影响因素有很多，其中最主要的因素是腐蚀液的浓度和腐蚀温度。腐蚀速率的稳定可以制备性能良好的器件。在本实验中，温度恒定(25 ℃)，研究了腐蚀速率与腐蚀深度及时间的关系。
实验采取的腐蚀时间分别为60 s、90 s、120 s、150 s、80 s、210 s、240 s、270 s，共8个时间段。并用台阶仪测得了刻蚀深度，图 1为不同浓度配比溶液的时间与深度关系曲线。由图可知，溶液浓度为H3PO4:H2O2:C4H6O6:H2O=1 mL:1 mL:0.6 g:10 mL所得的曲线图深度与时间呈现较好的线性关系，腐蚀速率稳定，约为0.62 μm/min。当配比为1:1:0.3:10和1:1:1:10时，腐蚀深度与时间的线性效果较差，腐蚀深度难以控制。配比为1:1:0.3:10时，速率约为0.54 μm/min。而配比为1:1:1:10时，腐蚀速率不稳定，波动较大，速率达到1.9 μm/min，腐蚀速率太快，不利于工艺中对腐蚀深度的控制。
3.2 刻蚀形貌的研究
为了改善刻蚀表面形貌，采取了三种不同酒石酸浓度的腐蚀液进行实验研究以使表面形貌达到最优的效果。首先采用的腐蚀液的配比为H3PO4:H2O2: C4H6O6:H2O=1:1:0.3:10(方案P1)，如图 2(a)所示，通过台阶仪对其刻蚀深度及剖面轮廓测量，可以看出，刻蚀台阶出现明显下切效应。图 2(b)为SEM测得的其表面形貌，可以更直观地看到明显的下切效应，陡直度经测量为59°。由图 2可知，垂直刻蚀深度约为0.7 μm，下切约0.2 μm，图形横向减小尺寸误差约为0.42 μm，刻蚀效果不好且表面刻蚀不平整。所以，此浓度的酒石酸并没有很好地改善刻蚀后的GaSb衬底表面形貌。
图 2. P1刻蚀后的样品剖面图及刻蚀表面形貌Figure 2. Cross-sectional view and corrosion surface of sample with P1.通过以上实验，经分析是酒石酸的含量低导致的，所以重新调整，其他成分不变，加大酒石酸的含量，腐蚀液比例改为H3PO4:H2O2:C4H6O6:H2O= 1:1:1:10(方案P2)，测试结果如图 3所示。从台阶仪的测试曲线图 3(a)可以看出样品下切效应已有所改善，被刻蚀部分的表面平整性有所改善，但在SEM测试下(图 3(b))，陡直度为66°，台面侧壁出现了严重的横向过蚀现象。刻蚀深度为2.5 μm时，横向过蚀误差达到0.7 μm。这可能是由于腐蚀液中酒石酸浓度过量引起的[7]。
图 3. P2刻蚀后的样品剖面图及刻蚀表面形貌.Figure 3. Cross-sectional view and corrosion surface of sample with P2.综合以上两种情况，在实验中调整腐蚀液配比为H3PO4:H2O2:C4H6O6:H2O=1:1:0.6:10(方案P3)，测试结果如图 4所示。由图 4(a)可知，刻蚀台阶处的下切效应已基本消除，刻蚀表面平整。图 4(b)没有出现横向钻蚀，样品表面平整且光滑，其刻蚀形貌陡直度为87°，已接近垂直形态，其垂直刻蚀深度约为2.5 μm，只在刻蚀侧壁与底部连接处出现15°角的圆弧，基本实现了精确刻蚀。
4. 结论
对比研究了三种配比的磷酸系腐蚀液对GaSb的腐蚀规律，通过改变起络合作用的酒石酸含量，来优化GaSb材料的刻蚀形貌和速率。研究表明溶液配比为H3PO4:H2O2:C4H6O6:H2O=1:1:0.6:10时，刻蚀效果最好，既消除了以往腐蚀过程中出现的下切效应，又保证了刻蚀台阶具有较好的陡直度，且未出现钻蚀现象，刻蚀表面平整且光滑。在25 ℃条件下，腐蚀速率稳定为0.62 μm/min，可操控性好。
湿法刻蚀工艺是GaSb基VCSEL制造过程中非常重要的工艺之一，鉴于GaSb基材料具有特殊的材料特性，针对这种材料的台面结构、刻蚀介质和刻蚀方法都存在一定难度，由于近年来VCSEL的整体性能不断提升，其台面结构也向更加复杂精细的方向发展，对刻蚀工艺的各项性能指标都有了更高的要求。刻蚀图形精度的提高和高度垂直的侧壁可以很大程度地提高VCSEL的性能，使光输出功率和特征温度大幅度提高，阈值电流密度也会明显降低[9]。并且在漂去氧化硅、去除残留物、表层剥离以及大尺寸图形刻蚀应用等方面起着非常重要的作用[20]。
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图 1 不同浓度配比溶液中腐蚀深度与时间的关系曲线.
Figure 1. The depth vs. time under different concentration ratio solution.
图 2 P1刻蚀后的样品剖面图及刻蚀表面形貌
Figure 2. Cross-sectional view and corrosion surface of sample with P1.
图 3 P2刻蚀后的样品剖面图及刻蚀表面形貌.
Figure 3. Cross-sectional view and corrosion surface of sample with P2.
图 4 P3刻蚀后的样品剖面图及刻蚀表面形貌.
Figure 4. Cross-sectional view and corrosion surface of sample with P3.
表 1 腐蚀液浓度配比.
Table 1. Concentration ratio of corrosive solution.
No. Concentration ratio of corrosive solution P1 H3PO4:H2O2:C4H6O6:H2O=1:1:0.3:10 P2 H3PO4:H2O2:C4H6O6:H2O=1:1:1:10 P3 H3PO4:H2O2:C4H6O6:H2O=1:1:0.6:10
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1. 张哲，赵江赫，张铭，熊青昀，熊金平. GaSb半导体材料表面的化学蚀刻研究进展. 表面技术. 2019(01): 114-125 . 
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