CN1284207C - 一种用于半导体封装的焊球的制备方法 - Google Patents

一种用于半导体封装的焊球的制备方法 Download PDF

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CN1284207C
CN1284207C CNB031424163A CN03142416A CN1284207C CN 1284207 C CN1284207 C CN 1284207C CN B031424163 A CNB031424163 A CN B031424163A CN 03142416 A CN03142416 A CN 03142416A CN 1284207 C CN1284207 C CN 1284207C
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陈正豪
肖国伟
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Hong Kong University of Science and Technology HKUST
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Abstract

本发明提出一种用于半导体封装的焊球的制备方法，包括以下步骤：a.采用镍-钒合金和浮脱工艺，在晶片上制备凸点下金属层和分别在凸点下金属层的相对侧处连接的长方形回流引导金属层；b.采用制备直径在75微米至350微米的焊球的印刷模板，所述印刷模板具有比凸点下金属层和回流引导层大的矩形开孔；c.使用印刷机，通过印刷模板在已经制备了凸点下金属层和回流引导金属层的晶片上印制焊膏；d.根据焊膏材料要求，在一定温度下熔化和回流焊膏来形成焊球。这样提高了模板印刷技术制备焊球的工艺控制性，减小了焊球间距，提供了用于不同尺寸和材料的，具有可靠性的倒装焊焊球。

Description

一种用于半导体封装的焊球的制备方法
技术领域
本发明与电子封装有关，是一种于半导体工艺过程中，在晶片或芯片上制备倒装焊凸点焊球的技术。
背景技术
随着集成电路芯片尺寸不断缩小，电子封装技术逐步向高性能、高密度、高输入/输出端口、低成本、小型化方向发展，新型封装技术在产业界越来越得到广泛应用。相对于表面贴装(SMT)、载带自动键合(TAB)、四边扁平封装(QFP)、针栅阵列(PGA)和焊球阵列(BGA)等封装技术，倒装焊技术能够满足电子产品的高性能、小外形、高速、高频的要求，使产品具有很好的电学性能和传热性能。在凸点倒装焊技术中，凸点是焊料通过一定工艺沉积在芯片互连金属层上，经过一定温度回流形成的金属焊球。在焊球下有防塌陷金属层，以保证焊料在回流时不会流动塌陷在芯片表面。然后，将表面植球的芯片翻转焊接在底板上，所形成的焊点提供了信号连接通道。
IBM公司在60年代首先开发了倒装焊技术和可控塌陷凸点(C4)技术，其中凸点制备技术是利用金属掩膜蒸发工艺完成的，由于成本很高、工艺复杂，这一技术的应用受到很大的限制。目前随着倒装焊技术的不断发展，许多种焊球的制备技术被开发出来获得应用，包括电镀凸点技术、激光植球技术、模板印刷技术等。其中，电镀凸点技术作为一种主要工艺被采用。不过，该技术需要应用蒸发或溅射工艺制备凸点下金属层，同时需要厚光胶工艺形成电镀掩膜和电镀图形。因此，电镀凸点技术依然有成本高、工艺复杂的问题。
模板印刷技术能够采用化学镀沉积凸点下金属层，取代真空溅射工艺。同时，使用模板印刷避免了使用厚光胶工艺，从而使工艺成本降低很多。在模板印刷工艺过程当中，焊膏(焊锡浆料)在印刷机上通过模板印刷在晶片上，通过一定温度曲线的回流形成焊球。
不过，使用模板印刷技术制备焊球目前依然存在许多问题。由于印刷工艺要求，焊膏体积一般比回流后焊球体积大40～60％，从而限制了焊球边间距的缩小。在制备小间距焊球时，工艺面临很大限制，成品率低，可靠性差。另外，由于半导体晶片中互连金属材料的不同，化学镀凸点下金属层也存在工艺控制问题，致使该金属层连接可靠性降低，影响焊球寿命。
发明内容
本发明通过新的工艺和设计方法，提高了模板印刷技术制备焊球的工艺控制性，减小了焊球间距，提供了用于不同尺寸和材料的，具有高可靠性的倒装焊焊球。本发明所采用的新工艺技术通过采用相应焊膏，可用于制备不同组分的铅-锡合金焊球、无铅的锡基焊球(如：铜-锡、锡-铜-银、锡-银、锡-铋合金)等。
本发明是用于在晶片或芯片上，在半导体集成电路前工序加工工艺完成后，在钝化开孔处互连金属层上，完成可用于倒装焊技术的焊球制备的系列工艺和与工艺相关的设计技术。本发明适用于并能够实现焊球边间距75微米以上，焊球直径在75微米至350微米的工艺需求。
本发明提出一种用于半导体封装的焊球的制备方法，包括以下步骤：a.采用镍-钒合金和浮脱工艺，在晶片上制备凸点下金属层和分别在凸点下金属层的相对侧处连接的长方形回流引导金属层；b.采用制备直径在75微米至350微米的焊球的印刷模板，所述印刷模板具有比凸点下金属层和回流引导层大的矩形开孔；c.使用印刷机，通过印刷模板在已经制备了凸点下金属层和回流引导金属层的晶片上印制焊膏；d.根据焊膏材料要求，在一定温度下熔化和回流焊膏来形成焊球。
本发明所采用的主要技术和设计有：
1.在晶片表面上制备凸点下金属层工艺：方法1：采用镍-钒合金，通过溅射或者蒸发工艺，在晶片表面，钝化开孔处金属层上制备凸点下金属层。该金属层厚度在300～1500纳米之间。然后，沉积30～80纳米金在镍-钒合金层表面。方法2：运用化学镀工艺沉积镍在晶片表面，钝化开孔处金属层上，其厚度在2500～6500纳米之间。然后，沉积30～80纳米的金在镍层表面。
2.所制备焊球直径与凸点下金属层直径比值在1.1～1.35之间，以良好控制焊球结构，实现小间距焊球工艺要求。
3.为减小焊球间距，凸点下金属层采用如图1或2结构设计。图1中凸点下金属层2为圆形，图2中凸点下金属层2为正方形。其中，长方形金属层称为焊料回流引导层1。长方形1宽度W与正方形边长或圆直径D比值在0.08～0.15之间，长方形长度L与正四方形边长或圆直径D比值在0～1.2之间。
4.根据凸点下金属层结构，在印刷模板3设计中，如图3所示，模板开孔4尺寸需要与晶片5上的凸点下金属层2和回流引导层1的尺寸匹配。如图4所示，模板开孔处4孔洞体积V2与模板厚度T和开孔面积有关，焊球体积V1与模板开孔处4孔洞体积V2比值在0.4～0.65之间。同时，为获得印刷工艺良好控制性，避免焊膏在模板壁上粘连，开孔处孔洞平均边长与模板厚度T比值在2.4～3之间。如图4(a)所示，当应用回流引导层1时，模板开孔处4呈长方形。如图4(b)所示，当没有应用回流引导层1时，模板开孔处4呈正方形。
5.模板印刷工艺完成后，根据焊料成分采用一定温度曲线进行回流工艺。在高温中焊料熔化，在表面张力作用下，焊料形成圆球状。长方形焊料回流引导层将有助于引导焊膏凝聚形成焊球，避免焊膏在回流过程中分离，影响焊球质量和体积，出现分离焊料。同时，由于长方形焊料回流引导层宽度校小，在表面张力作用下，回流过程中将只有少量焊料在该层金属上。这种设计有助于减小模板印刷焊球间距。
按照本发明所采用的技术和设计，现使用有关材料举一实施例，制备直径110微米左右铅锡合金焊球，其边间距140微米。现结合附图，详细说明工艺过程，所使用附图不是按照为正常比例绘制。
附图说明
图1为圆形凸点下金属层和焊料回流引导层俯视结构图。
图2为正方形凸点下金属层和焊料回流引导层俯视结构图。
图3为印刷模板开孔处斜视结构示意图。
图4为印刷模板开孔设计示意图，图4(a)为应用回流引导层模板设计示意图，图4(b)为无回流引导层模板设计示意图。
图5为制备镍-钒凸点下金属层和回流引导层应用的光胶工艺示意图，图5(a)为截面示意图，图5(b)为俯视示意图。
图6为图5(a)后和截面示意图，图6(a)为与图5(a)同一截面但采用镍-钒合金沉积后的截面图，图6(b)与图6(a)沿A-A截面在采用浮脱工艺(Lift-off process)后制备凸点下金属层的截面示意图。
图7为印刷模板设计尺寸和俯视图。
图8为模板印刷工艺流程截面示意图，图8(a)为刮板将焊膏通过模板印刷在晶片上截面示意图，图8(b)为焊膏印刷后截面示意图。
图9为焊膏回流后焊球截面示意图。
图中：
1-回流引导金属层                2-凸点下金属层
3-印刷模板                      4-印刷模板上印刷焊膏开孔处
5-晶片                          6-互连金属层
7-钝化层                        8-浮脱工艺中光胶
9-钝化层开孔处                  10-镍-钒合金和金层
11-印刷后的焊膏                 12-凸点焊球
13-印刷刮板
具体实施方式
将要制备的凸点下金属层2，如图1所示为圆形或图2为正方形。其中，在凸点下金属层2的两对向方向形成一对长方形金属层称为焊料回流引导层1。长方形焊料回流引导层1的宽度W与正方形边长(图2)或圆直径D(图1)比值为在0.08～0.15之间，长方形焊料回流引导层1的长度L与正四方形边长或圆直径D比值在0～1.2之间。
如图3所示，即在图8(a)中采用的印刷模板3是这样设计的，模板开孔4的尺寸需要与凸点下金属层2和回流引导层1的尺寸匹配。
如图4所示，模板开孔处4孔洞体积(V2)与模板厚度T和开孔面积有关，焊球体积(V1)与模板开孔处4孔洞体积(V2)比值在0.4～0.65之间。同时，为获得印刷工艺良好控制性，避免焊膏在模板壁上粘连，开孔处孔洞平均边长与模板厚度T比值在2.4～3之间。如图4(a)所示，当应用回流引导层1时，模板开孔处4呈长方形。如图4(b)所示，当没有应用回流引导层1时，模板开孔处4呈正方形。以下描述会以图4(a)作描述的基础。
模板印刷工艺是将焊球制备在已完成了的互连金属层6和钝化层7的晶片上。如图5-6，采用镍-钒合金和浮脱工艺(Lift-off process)制备凸点下金属层。并且为实现微细间距焊球工艺，按照图1或2结构设计光胶工艺掩膜版。
在晶片5上完成光胶8工艺后，如图5所示通过刻蚀形成直达钝化层7的开孔，从而在互连金属层6上形成光胶开孔9的图形。
如图6(a)所示通过溅射或者蒸发工艺，在光胶8上，沉积300～1500纳米镍-钒合金层和30～80纳米金层10。然后，使用浮脱工艺，去除在互连金属层6上原为钝化开孔9周围的光胶8和部分金属层10，这样余下在钝化开孔9处的金属层10，制备成镍-钒合金和金材料组成的凸点下金属层2和回流引导层1，如图6(b)所示。
考虑焊球尺寸要求，凸点下金属层2直径设计为85～95微米。为增加焊膏体积，达到设计焊球尺寸，如图7所示，长方形焊料回流引导层1的宽度W设计为8～10微米，长度L设计为80～100微米。
同样，考虑焊球尺寸和工艺要求，设计印刷模板。这里采用75微米厚度模板。考虑焊球中心间距为250微米，为避免焊膏在回流过程中出现连接，同时满足焊膏体积达到焊球尺寸要求，模板开孔处4采用长方形设计，如图7所示。长方形宽度设计尺寸可在130～150微米之间，长方形长度设计可在260～305微米之间。
如图8(a)所示，在印刷机上使用刮板13，将焊膏11印制在晶片5上，调整模板与晶片间间隙，印刷时压力等工艺参数，能够获得设计需要的焊膏体积。如图8(b)所示，印刷完成后移走模板，设计需要的焊膏11就印刷在晶片5上。
根据焊膏材料，确定回流温度曲线。在回流工艺中，焊膏中有机溶剂在高温中会挥发，焊膏体积将缩小。焊料熔化后，在表面张力作用下形成圆球状焊球12。长方形焊料回流引导金属层1将有助于焊膏凝聚形成焊球，保证工艺质量。同时，由于回流引导金属层1结构设计，凝聚在回流引导金属层1上面的熔化焊料将很少，如图9所示。

Claims (8)

1.一种用于半导体封装的焊球的制备方法，包括以下步骤：
a.采用镍-钒合金和浮脱工艺，在晶片上制备凸点下金属层和分别在凸点下金属层的相对侧处连接的长方形回流引导金属层；
b.采用制备直径在75微米至350微米的焊球的印刷模板，所述印刷模板具有比凸点下金属层和回流引导层大的矩形开孔；
c.使用印刷机，通过印刷模板在已经制备了凸点下金属层和回流引导金属层的晶片上印制焊膏；
d.根据焊膏材料要求，在一定温度下熔化和回流焊膏来形成焊球。
2.根据权利要求1所述的方法，其特征在于，制备凸点下金属层和回流引导金属层是通过沉积300～1500纳米镍-钒合金层和30～80纳米金层，然后运用浮脱工艺来形成。
3.根据权利要求2所述的方法，其特征在于，所制备焊球直径与凸点下金属层直径比值在1.1～1.35之间。
4.根据权利要求2所述的方法，其特征在于，回流引导金属层的宽度W与凸点下金属层正方形边长或圆直径D的比值在0.08～0.15之间，回流引导金属层的长方形长度L与凸点下金属层正方形边长或圆直径D的比值在0～1.2之间。
5.根据权利要求4所述的方法，其特征在于，设计相应光胶掩膜版，用于制备凸点下金属层和回流引导金属层的浮脱工艺。
6.根据权利要求1所述的方法，其特征在于，设计印刷模板开孔尺寸满足焊球体积V1与模板开孔处孔洞体积V2比值在0.4～0.65之间，模板开孔处孔洞平均边长与模板厚度比值在2.4～3之间。
7.根据权利要求1-6之一所述的方法，其特征在于，凸点下金属层的直径设计为85～95微米，各所述回流引导金属层设计为宽度W在8～10微米之间而长度L在80～100微米之间。
8.根据权利要求1-6之一所述的方法，其特征在于，模板开孔的宽度设计为在130～150微米之间，而模板开孔的长度设计在260～305微米之间。
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