CN106661672B - 具有埃厚的表面氧化物层的铜键合丝 - Google Patents

具有埃厚的表面氧化物层的铜键合丝 Download PDF

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CN106661672B
CN106661672B CN201580034280.0A CN201580034280A CN106661672B CN 106661672 B CN106661672 B CN 106661672B CN 201580034280 A CN201580034280 A CN 201580034280A CN 106661672 B CN106661672 B CN 106661672B
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copper
copper wire
silk
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CN106661672A (zh
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M.萨兰加帕尼
张兮
杨平熹
E.米尔克
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Heraeus Deutschland GmbH and Co KG
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Abstract

具有10至80微米的直径的铜丝，其中铜丝本体材料是≥99.99重量%纯铜或由10至1000重量ppm银和/或0.1至3重量%钯及补足100重量%的作为余量的铜构成的铜合金，其特征在于所述铜丝具有0.5至<6纳米的氧化铜薄周向表面层。

Description

具有埃厚的表面氧化物层的铜键合丝
本发明涉及具有仅0.5至<6纳米的氧化铜薄周向表面层的铜丝。
键合丝在电子和微电子应用中的使用是公知的现有技术状况。尽管键合丝一开始由金制成，但现今使用更便宜的材料，如铜。尽管铜丝提供非常好的电导率和热导率，但铜丝的键合具有其挑战性。此外，铜丝容易受丝的氧化的影响。
存在各种手段来防止铜键合丝发生表面氧化。实例包括用抗氧化贵金属的涂层、玻璃涂层或聚合物涂层涂覆铜丝。
用于引线键合（wire bonding）应用的未经涂覆的铜丝通常具有氧化铜表面层，其层厚度通常在例如6至20纳米的范围内。
在本文中使用术语“氧化铜”。其应该表示CuO和/或Cu2O。
由于大量的研发努力，申请人已经开发出具有10至80微米，优选15至50微米的直径和0.5至< 6纳米，优选0.5至4纳米，特别是0.5至3.5纳米的氧化铜薄周向表面层的铜丝。该铜丝本体材料（bulk material）是≥ 99.99 wt.-%（重量%）纯铜或由10至1000 wt.-ppm（重量ppm），优选100至300 wt.-ppm，特别是200至250 wt.-ppm银和/或0.1至3 wt.-%，优选0.5至2 wt.-%，特别是1.2至1.3 wt.-%钯及补足100 wt.-%的作为余量的铜构成的铜合金。
在本文中使用术语“铜丝”。为避免误解，其应该表示任何形式的铜丝，即作为未经加工的铜丝和已经加工的、即球形或楔形键合的（wedge-bonded）铜丝。
在本文中使用术语“0.5至< 6纳米的氧化铜薄周向表面层”。其应该表示该0.5至<6纳米的氧化铜薄周向表面层是氧化铜或其包含氧化铜。换言之，该0.5至< 6纳米的氧化铜薄周向表面层可以由氧化铜构成或其可包含例如在50至100重量%范围内的浓度的氧化铜。在一个实施方案中，其表现出在从丝外表面向丝中心的垂直方向上经所述0.5至< 6纳米的深度的氧化铜浓度梯度。这样的氧化铜浓度可以在丝外表面处高，例如90至100重量%氧化铜并经所述深度迅速下降到0重量%。
在下文中，“由10至100重量ppm银和/或0.1至3重量%钯及补足100重量%的作为余量的铜构成的铜合金”也简称为“铜合金”。
在本文中使用短语“补足100重量%的作为余量的铜”。其应该表示铜是该铜合金中的主要组分。为避免误解，这不应被理解为排除没有明确提到和由于现行技术条件而可能以某种方式进入铜合金中（例如由于在制造过程中的无意但不可避免的并入）的其它元素。换言之，这样的其它元素可能作为不可避免的杂质存在于合金中，但仅以例如> 0至100重量ppm的极微总量。在任何情况下，此类不可避免的杂质不是被有意添加或引入形成铜合金的组合物中。就此而言，短语“补足100重量%的作为余量的铜”表示补足100重量%的铜合金所缺失的重量%比例由铜加上所述不可避免的杂质（如果后者存在）构成。
该铜合金可通过金属合金领域的技术人员已知的常规方法制备，例如通过将铜和银和/或钯以所需比率熔融在一起。在该情况下，可以使用常规铜-银或铜-钯母合金（master alloy）。可以例如使用感应炉进行熔融过程并且在真空下或在惰性气体气氛下工作是有利的。所用材料可具有例如99.99重量%和更高的纯度级。通常将该铜合金熔体在室温模具中铸造，在其中所述熔体冷却并凝固。
可以通过飞行时间-二次离子质谱（ToF-SIMS；使用铋离子分析处于正极性的分子）测定或表征铜或铜合金丝的氧化铜表面层的性质和厚度。
也可以通过下列等效方法测量铜或铜合金丝的氧化铜表面层的厚度：
- 逐步使用Ar离子溅射深度剖析（depth profiling）一段时间直至观察到100%铜（Cu）。这种方法是技术人员公知的并且不需要进一步说明。
- 通过使用标准实践的深度剖析的X-射线光电子能谱（XPS）。这种方法是技术人员公知的并且不需要进一步说明。
- 序贯电化学还原分析（SERA）。对于SERA，可以使用来自ECI Technology的装置Surface-Scan QC200。ECI Technology在其网站
http://www.ecitechnology.com/sites/default/files/downloads/eci- surfacescan-qc200.pdfcan 上公开了细节，据此SERA可用作用于确定铜丝表面氧化物层厚度的公差的质量控制工具。基本上，使用法拉第原理通过使用大约8.5 pH的硼酸盐电解质溶解氧化铜层来测量氧化铜厚度。SERA能够与本体铜类似地分析铜丝表面上的Cu2O、CuO和Cu2S。SERA相对时间记录Cu2O、CuO和Cu2S的反应电位。在-0.4至-0.6 V、-0.8至-0.9 V、-0.9至-1.0 V、和-1.1至-1.2 V范围内的反应电位分别对应于Cu2O、CuO、Cu2S还原和H2析出。表面氧化物/硫化物的还原时间与层厚度(T)的存在成比例，根据法拉第方程计算：
其中I是以安培计的电流，t是以秒测量的还原时间，M是以克计的分子量，F是法拉第常数（96498库仑），n是电子数，S是以平方厘米计的被浸渍的铜段的表面积，且d是以克/立方厘米计的膜密度。
SERA方法是申请人优选的用于测量铜或铜合金丝的氧化铜表面层的层厚度的方法。
在一个特定实施方案中，本发明的铜丝在其表面，即在氧化铜表面层的顶部显示出游离碳（元素碳）和/或在从丝外表面向丝中心的垂直方向上经例如0.6至5纳米的深度的游离碳浓度梯度。这种游离碳浓度可以在丝外表面处高，例如90至<100重量%游离碳并经所述深度迅速下降到0重量%。游离碳的浓度梯度可能与上述氧化铜的浓度梯度重叠。
可以通过ToF-SIMS（使用铋离子分析处于正极性的分子）或通过XPS（通过使用标准实践的深度剖析）测定或表征所述游离碳梯度的深度
本发明还涉及用于制造本发明的铜丝的方法。本发明的铜丝可通过包括下列步骤的方法制造：
(a) 提供≥ 99.99重量%纯铜或铜合金的铜前体，
(b) 拉制所述前体直至达到10至80微米，优选15至50微米的丝的最终直径；
(c) 在90至96 vol.-%（体积%）惰性气体: 4至10体积%氢气气氛中将经拉制的丝在580至750℃的目标温度（峰值温度）下退火0.2至0.4秒，和
(d) 将经退火的丝在水中淬火，
其中体积%合计为100体积%。
在本发明的方法的步骤(a)中，提供≥ 99.99重量%纯铜或铜合金的铜前体。
通常，这样的铜前体是具有例如2至25毫米的直径和例如5至100米的长度的杆形式。这样的杆可以类似于或根据上文公开的用于制备铜合金的方法制造，即通过在适当的室温模具中铸造铜或铜合金熔体、接着冷却和凝固。
在本发明的方法的步骤(b)中，该铜前体在几个步骤中被拉制直至达到10至80微米，优选15至50微米的丝的最终直径。这样的拉制过程是本领域技术人员公知的。可以使用常规的钨和金刚石拉丝模（drawing die）并可以使用常规的拉丝润滑剂（drawinglubricant）辅助该拉制。通常，在步骤(b)完成后获得的铜丝具有层厚度为例如6至20纳米的氧化铜周向表面层。
在本发明的方法的步骤(c)中，在90至96体积%惰性气体: 4至10体积%氢气气氛中将该经拉制的丝在570至750℃的目标温度下退火（最终退火、带材退火（strandannealed））0.2至0.4秒，其中体积%合计为100体积%。
在≥ 99.99重量%纯铜丝的情况下，优选选择570至630℃的目标温度，而在铜合金丝的情况下，优选的目标温度是620至750℃，特别是650至710℃。
95体积%惰性气体: 5体积%氢气气氛是优选气氛。惰性气体可以是氮气和/或氩气；其通常是氮气。
通常通过将丝以给定速度拉过常规退火炉（通常为具有给定长度和指定温度分布（temperature profile）的圆柱管形式）来进行退火。在该情况下，可以指定并设置退火时间/目标温度参数。用90至96体积%惰性气体: 4至10体积%氢气混合物吹扫该退火炉。优选在位于43至125 min-1，更优选43至75 min-1，最优选50至63 min-1范围内的气体交换速率（=气体流量 [升/分钟] : 炉内体积 [升]）下进行吹扫。
在本发明的方法的步骤(d)中，将经退火的丝在水中淬火，该水在一个实施方案中可含有表面活性剂，例如0.01至1体积%的表面活性剂。在水中淬火是指立即或迅速，即在0.2至0.4秒内，将经退火的丝从其在步骤(c)中达到的目标温度冷却到室温。
已经发现，以其仅0.5至< 6纳米的氧化铜薄周向表面层为特征的本发明的铜丝非常适合用作引线键合应用中的键合丝，而在本发明的铜丝的上述特定实施方案中，其特别适用于这样的用途。
引线键合技术是技术人员公知的。在引线键合过程中，通常形成球形键合（第一键合点）和针脚式键合（stitch bond）（第二键合点，楔形键合）。在键合形成过程中，施加一定的力（通常以克为单位测量），辅以施加超声能（通常以mA为单位测量）。在引线键合过程中，施加的力的上限和下限之间的差值和施加的超声能的上限和下限之间的差值的数学乘积界定了引线键合工艺窗口（process window）：
(施加的力的上限 - 施加的力的下限) · (施加的超声能的上限 - 施加的超声 能的下限) = 引线键合工艺窗口。
引线键合工艺窗口界定了能够形成符合规格，即通过常规测试，如常规拉力测试、焊球剪切测试（ball shear test）和焊球拉拔测试（ball pull test）（仅举几例而言）的引线键合的力/超声能组合的范围。
对于工业用途，为了引线键合工艺鲁棒性，希望具有宽的引线键合工艺窗口。本发明的铜丝表现出对铜键合丝而言相当宽的引线键合工艺窗口。例如，直径20微米的本发明的铜丝表现出对球形键合而言例如60至130 mA·g和对针脚式键合而言例如2500至6000mA·g的引线键合工艺窗口。
无论丝直径和丝类型如何，即≥ 99.99重量%纯铜或铜合金，引线键合结果相同。进一步相信，仅0.5至< 6纳米的氧化铜薄周向表面层对本发明的铜丝的宽引线键合工艺窗口是关键的。可以说，< 6纳米的氧化铜周向表面层是达到良好的球形和针脚式键合的水平。
据信，在本发明的方法的步骤(c)和(d)的过程中现行的工艺参数的独特组合对获得具有其相当宽的引线键合工艺窗口的本发明的铜丝是必需的。所述工艺参数的优选组合是：570至750℃（在≥ 99.99重量%纯铜的情况下570至630℃；在铜合金的情况下620至750℃或更优选650至710℃）的目标温度0.2至0.4秒，在43至125 min-1，更优选43至75 min-1的气体交换速率下使用95体积%惰性气体 : 5体积%氢气混合物作为吹扫气体，和在0.2至0.4秒内快速冷却到室温。在工艺参数的下列组合下发现最佳结果：570至750℃（在≥ 99.99重量%纯铜的情况下570至630℃；在铜合金的情况下620至750℃或更优选650至710℃）的目标温度0.2至0.4秒，在50至63 min-1的气体交换速率下使用95体积%惰性气体 : 5体积%氢气混合物作为吹扫气体，和在0.2至0.4秒内快速冷却到室温。
在步骤(d)完成后，本发明的铜丝完成。为了充分获益于其宽引线键合工艺窗口，有利的是将其立即用于引线键合应用，即无延迟，例如在步骤(d)完成后不长于7天内。或者，为了保持铜丝的宽引线键合工艺窗口性质和为了防止其受到氧化或其它化学侵袭，通常在步骤(d)完成后立即，即无延迟地，例如在步骤(d)完成后< 1至5小时内将完成的铜丝绕卷并真空密封，然后储存以供进一步用作键合丝。在真空密封条件下的储存不应超过6个月。在打开真空密封后，铜丝应该在不长于7天内用于引线键合。
所有工艺步骤(a)至(d)以及绕卷和真空密封优选在洁净室条件（US FED STD209E洁净室标准，1k标准）下进行。
下列非限制性实施例例示本发明。
实施例
一般程序:
将99.99重量%纯度的铜杆在真空感应炉中熔融并连续铸造成8毫米直径的杆。在另一些实施例中，将少量Cu-10重量%Ag或Cu-15重量%Pd母合金添加到熔体中以铸造分别具有下列组成的Cu-Ag和Cu-Pd合金：
。
进一步地，将这些连续铸造杆在室温（25℃）下冷拉丝（cold wire drawn）。使用碳化钨拉模拉制粗丝（heavy wire），并使用金刚石拉模用于进一步缩细（reduction）。在四个步骤中以不同拉制速度进行拉制：以0.5 m/s从8 mm至4 mm和以1 m/s从4 mm至0.8 mm的粗丝拉制，以4 m/s从0.8 mm至0.16 mm的中间丝拉制，和以6 m/s从0.16 mm至0.02 mm的细丝拉制。通过使用水基润滑剂来辅助拉制。拉模收缩率（die reduction ratio）为对于粗丝（thick wire）（直径> 200微米）14%和对于细丝（直径< 200微米）8%。
合金化Cu-Ag和Cu-Pd丝分别在400℃和600℃下在45微米的直径下中间退火。99.99重量%纯铜丝不经中间退火。
经拉制的丝具有20微米的最终直径并表现出具有12纳米（平均）层厚度的周向氧化铜层。
最后通过使该丝通过常规退火炉和使用常规设备（包括卷线器（reel）、线轴、滑轮）来将该丝带材退火。退火时间为0.3秒。带材退火温度为600℃（纯铜）、630℃（Cu-Ag合金）和700℃（Cu-Pd合金）。用合成气体（N2:H2，95:5）以如表2中所示的气体交换速率吹扫该炉。
在离开炉后立即，即在0.3秒内，将热丝在去离子水中淬火。最后，将经退火的丝卷绕在50毫米直径的干净的经阳极化（经电镀）的铝线轴上，真空包装并储存。
经退火的99.99重量%纯铜丝的使用SERA测量的典型电化学记录显示-0.4至-0.6V的反应电位，表明在该丝表面上存在薄Cu2O层。不存在CuO和Cu2S。由该记录计算出Cu2O厚度在1.7至3纳米之间。
此外，ToF-SIMS和XPS揭示了该丝表面上的游离碳（没有碳化物的迹象）。ToF-SIMS深度剖析确认在1纳米深度前存在强碳并在大约5纳米深度处进一步下降到0。ToF-SIMS深度剖面证实游离碳很可能存在于氧化铜层顶部，因为直至1纳米观察到强的游离碳峰，并在1纳米后发现强的氧和Cu2O峰。
通过球形键合（第一键合点）到Al焊盘上和针脚式键合（第二键合点）到银引脚（lead finger）上，测试该丝。在反应性环境下通过用合成气体（N2:H2；95:5）吹扫形成无空气焊球（free air ball）（FAB）并将其键合到Al焊盘上以达到大于0.01 g/µm2（6.5 g/mil2）的强球形键合和大于8 g的焊球拉力。
下表2显示引线键合工艺窗口结果：
表2: 球形和针脚式键合（第一和第二键合点）工艺窗口
USG = 超声能
++++极好，+++非常好，++良好，+尚可。
所有本发明的丝样品4至8产生非常适合工业应用的工艺窗口。特别地，本发明的丝样品7表现出对于第一和第二键合点分别为119 mA·g和5950 mA·g的值。

Claims (19)

1.具有10至80微米的直径的铜丝，其中铜丝本体材料是≥ 99.99重量%纯铜或由10至1000重量ppm银和/或0.1至3重量%钯及补足100重量%的作为余量的铜构成的铜合金，其特征在于所述铜丝具有0.5至< 6纳米的氧化铜薄周向表面层。
2.权利要求1的铜丝，其特征在于其表现出从丝外表面起在垂直方向上经0.6至5纳米的深度的游离碳浓度梯度。
3.前述权利要求任一项的铜丝，其可通过包括下列步骤的方法获得：
(a) 提供≥ 99.99重量%纯铜或所述铜合金的铜前体，
(b) 拉制所述前体直至达到10至80微米的丝的最终直径；
(c) 在90至96体积%惰性气体:4至10体积%氢气气氛中将经拉制的丝在570至750℃的目标温度下退火0.2至0.4秒，和
(d) 将经退火的丝在水中淬火，
其中体积%合计为100体积%。
4.权利要求3的铜丝，
其中所述铜丝本体材料是≥ 99.99重量%纯铜且目标温度为570至630℃。
5.权利要求3的铜丝，
其中所述铜丝本体材料是所述铜合金且目标温度为620至750℃。
6.权利要求3的铜丝，
其中惰性气体:氢气的体积比为95 : 5。
7.权利要求3的铜丝，
其中所述惰性气体是氮气和/或氩气。
8.权利要求3的铜丝，
其中通过将所述丝拉过用惰性气体/氢气混合物以位于43至125 min-1范围内的气体交换速率吹扫的退火炉来完成退火。
9.权利要求8的铜丝，
其中所述气体交换速率为43至75 min-1。
10.权利要求8的铜丝，
其中所述气体交换速率为50至63 min-1。
11.用于制造权利要求1或2的铜丝的方法，其包括以下步骤：
(a) 提供≥ 99.99重量%纯铜或所述铜合金的铜前体，
(b) 拉制所述前体直至达到10至80微米的丝的最终直径；
(c) 在90至96体积%惰性气体: 4至10体积%氢气气氛中将经拉制的丝在570至750℃的目标温度下退火0.2至0.4秒，和
(d) 将经退火的丝在水中淬火，
其中体积%合计为100体积%。
12.权利要求11的方法，
其中所述铜丝本体材料是≥ 99.99重量%纯铜且目标温度为570至630℃。
13.权利要求11的方法，
其中所述铜丝本体材料是所述铜合金且目标温度为620至750℃。
14.权利要求11至13任一项的方法，
其中惰性气体 : 氢气的体积比为95 : 5。
15.权利要求11至13任一项的方法，
其中所述惰性气体是氮气和/或氩气。
16.权利要求11至13任一项的方法，
其中通过将所述丝拉过用惰性气体/氢气混合物以位于43至125 min-1范围内的气体交换速率吹扫的退火炉来完成退火。
17.权利要求16的方法，
其中所述气体交换速率为43至75 min-1。
18.权利要求16的方法，
其中所述气体交换速率为50至63 min-1。
19.根据权利要求1至10任一项的或通过根据权利要求11至18任一项的方法制成的铜丝在引线键合应用中的用途。
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