聚酰亚胺对聚酰亚胺/铜厚膜复合材料热应力演变的影响资源-CSDN文库

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聚酰亚胺（PI）/铜复合厚膜广泛用于晶圆级封装（WLP），一个常见的问题是由热应力引起的诸如空隙，分层，裂纹或晶圆翘曲之类的缺陷。与传统的刚性衬底/铜/钝化系统相比，PI对Cu施加了完全不同的边界约束，从而导致了特殊的应力演化，而对这种机理的理解还远远不够。构造了五组复合材料，以通过热循环下的原位晶圆翘曲测量研究PI对Cu膜中热应力演变的影响。与有限元分析相结合，发现虽然PI确实降低了应力对Cu膜应力的影响是违反直觉的。铜，会加剧室温下的整体晶圆翘曲。翘曲演变表明，由基材/ PI / Cu组成的复合材料承受适度的压缩应力，而裸PI膜在高温下完全应力松弛，表明Cu和PI相互抑制应力松弛。这表明在评估聚合物金属复合材料厚膜中的应力分布时应考虑相互影响。 ### 聚酰亚胺对聚酰亚胺/铜厚膜复合材料热应力演变的影响 #### 概述本文探讨了聚酰亚胺（Polyimide, PI）/铜（Cu）复合厚膜在晶圆级封装（Wafer Level Packaging, WLP）中的应用以及热应力引起的问题。在WLP过程中，由于不同材料之间的热膨胀系数差异，常常会导致诸如空隙、分层、裂纹或晶圆翘曲等缺陷。相比于传统的刚性衬底/铜/钝化系统，PI对Cu施加了完全不同的边界约束，进而导致了独特的应力演化行为。然而，目前对于这一机理的理解尚不充分。 #### PI对Cu膜热应力的影响为了深入研究PI对Cu膜热应力的影响，研究者构建了五组复合材料，并通过热循环下的原位晶圆翘曲测量进行实验探究。实验结果结合有限元分析显示了一个违反直觉的现象：尽管PI确实降低了Cu膜内的应力水平，但它却加剧了室温下的整体晶圆翘曲程度。 #### 应力演化机制通过对复合材料的观察，发现由基材/PI/Cu组成的复合材料在高温下承受着适度的压缩应力。相比之下，裸PI膜在相同温度条件下几乎完全应力松弛。这表明Cu和PI之间存在着相互作用，彼此抑制了对方的应力松弛过程。这一发现强调了在评估聚合物金属复合厚膜中的应力分布时，需要考虑材料间的相互影响。 #### 实验设计与方法 1. **复合材料制备**：研究团队首先制备了五组不同配置的PI/Cu复合厚膜。 2. **原位晶圆翘曲测量**：采用原位测量技术监测复合材料在热循环条件下的晶圆翘曲变化。 3. **有限元分析**：利用有限元方法模拟并分析复合材料的热应力演化过程，以便更好地理解实验结果。 #### 结果分析 - **应力降低与翘曲加剧**：尽管PI能够有效地降低Cu膜内的应力，但整个复合结构的晶圆翘曲现象却更加明显。这一现象提示了复合材料内部复杂的应力传递机制。 - **应力松弛行为**：高温条件下，裸PI膜表现出几乎完全的应力松弛特性，而含有Cu的复合材料则展现出稳定的压缩应力状态。这说明Cu的存在限制了PI的应力松弛，反之亦然。 - **相互影响的重要性**：Cu与PI之间的相互作用显著地影响了复合材料的整体性能，特别是其应力分布和晶圆翘曲行为。 #### 讨论与结论通过对PI/Cu复合厚膜的研究，我们不仅揭示了PI对Cu膜内热应力的独特影响，而且也发现了二者之间复杂的相互作用机制。这些发现对于优化WLP工艺中的材料选择和结构设计具有重要意义。未来的研究可以进一步探索不同PI材料以及Cu合金对复合材料热应力演化的影响，以期为提高WLP产品的可靠性和性能提供理论支持和技术指导。

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Inﬂuence of polyimide on thermal stress evolution in polyimide/Cu

thick ﬁlm composite

Heng Li

1,2

•

Gong Cheng

1,2

•

Gaowei Xu

•

Le Luo

Received: 27 February 2016 / Accepted: 15 April 2016 / Published online: 29 April 2016

Ó Springer Science+Business Media New York 2016

Abstract Polyimide(PI)/Cu composite thick ﬁlms are

widely used in wafer level packaging (WLP), and a com-

mon problem is the defects such as void, delaminati on,

crack or wafer warpage induced by thermal stress. Com-

pared with traditional rigid substrate/Cu/passivation sys-

tem, PI imposes quite a different boundary constraint on

Cu, resulting in a special stress evolution, and the corre-

sponding mechanism is far from fully understood. Five sets

of composites are constructed to investigate the inﬂuence

of PI on thermal stress evolution in Cu ﬁlm by means of

in situ wafer warpage mea surement under thermal cycling.

Together with ﬁnite element analyses, it’s counterintuitive

to ﬁnd that although PI indeed reduces the stress in Cu, it

exacerbates overall wafer warpage at room temperature.

Warpage evolution reveals that composites consisting of

substrate/PI/Cu sustains a moderate compressive stress

while bare PI ﬁlm is totally stress relaxed at high temper-

ature, indicating that Cu and PI restrains stress relaxation

reciprocally. It suggests that mutual effect should be con-

sidered when evaluating the stress distribution in polymer-

metal composite thick ﬁlms.

1 Introduction

Polymer/metal composite thick ﬁlms are widely used in

wafer level packaging (WLP). A common problem of

such composites is the thermal stress when undergo

packaging process in which the induced undesirable

thermal str esse s a nd correspond ing micros tr uctu r e evo-

lution might cause v oid, hillock, crack or interfacial

delamination in layered structures [1, 2]. Moreover, large

wafer warpage originated from Cu and PI deformation

poses a threat to handling of t he wafer and affects the

quality of sequential process steps such a s lithography [3–

5]. Stress evolution of Cu ﬁlms due to coble creep, power-

law creep, dislocation glide/climb, or grain growth has

been analyzed [6–17 ], and transient behavior and Bau-

schinger-like Eff ect also draw the attention of industry

people [ 12, 18].

The polymer has large inﬂuence on stress relaxation of

metals within composites due to sophisticated reaction

during curing process and thermal cycling [19–28]. Vari-

ous polymers, including polyimide resin, cyanate ester

resin, bismaleimide resin, benzoxazine resin and so on,

have been widely studied [29–33]. Typically, polyimide

(PI) is widely used in semiconductor industry for passi-

vation or dielectric layer. Due to dramatic elution of ther-

mal mechanical properties during curing process, large

stress may arise in the composites. Figure 1 depicts some

typical thermal stress induced defects in a 2P1 M (two PI

layers with one metal layer embedded) composite from our

research project, which is commonly used in redistribution

layer (RDL) in WLP. The defects include PI crevices

within one PI layer caused by intrinsic stress, delamination

at PI/Cu interface cause d by coefﬁcient of thermal expan-

sion (CTE) mismatch,and cracks at the interface of two PI

layers caused by shrinkage.

& Le Luo

leluo@mail.sim.ac.cn

State Key Laboratory of Transducer Technology, Shanghai

Institute of Microsystem and Information Technology,

Chinese Academy of Sciences (CAS), Shanghai 200050,

China

University of Chinese Academy of Sciences, Beijing 100049,

China

123

J Mater Sci: Mater Electron (2016) 27:8325–8331

DOI 10.1007/s10854-016-4841-6

In polymer/metal composites, the mechanism of stress

evolution is far from fully understood. Polymers usually

imposes a soft constrains, which is rather differ ent from

rigid constrains, such as Si wafer substrate or inorganic

passivation. In PI/Cu composites, the PI beneath the Cu

ﬁlm cannot prevent dislocations in Cu from gliding at PI/

Cu interface, which differs from Si wafer substrate in

constrained diffusional creep model [ 9 ]. Also, the PI coated

above Cu ﬁlm is not an effective passivation for surﬁcial

diffusion of Cu atoms when compared with other oxide or

nitride or metal passivation layer. Hence, some traditional

models, such as constrained diffusional creep, cannot

evaluate the stress evolution process of the composite

satisfactorily [9, 17, 34].

In this work, ﬁve sets of PI/Cu composites are investi-

gated by warpage measurement. The inﬂuence of PI on

wafer warpage, as well as its inﬂuence on stress distribu-

tion and evolution within Cu/PI composite thick ﬁlms are

experimentally analyzed.

2 Experimental

The stress evolution in PI/Cu composite during thermal

cycling is analyzed on a 4-inch 420 lm-thick double-pol-

ished (DSP) (100) silicon wafer substrate.

PI used in this work is solvent developable negative-

tone photo-deﬁn able polyimide (PDPI) HD-4100 from HD

microsystem, which is widely used in semiconductor

industry for passivation, RDL or dielectric layers. The

solvent is N-methyl-2-pyrrolidone. The impurity content of

chloride, sodium, potassium, copper, iron are less than

1.0 ppm respectively, and the total metals are less than

10.0 ppm, as shown in Table 1.

It is spin coated onto the DSP wafer at a speed of

2000 rpm, then cured in the oven under nitrogen protec-

tion. The temperature proﬁle of the curing process is shown

in Fig. 2, i.e., heating from room temperature (RT) to

375 °C with a temperature rate of 10 °C/min, followed by

cooling to RT gradually. To evaporate solvent adequately

and to control the residual stress, the PDPI HD 4100 is

isothermal annealed at 200 °C for 30 min and at 375 °C for

60 min. The isothermal annealing is important for the

chemical reaction and solution evaporation, otherwise

bubbles may arise and the cured ﬁlm may be imperfect.

The thickne ss of a single cured PI layer is10.10 ± 0.23 lm

in current work.

The Cu ﬁlm is electro-chemical deposited (ECD) on the

DSP wafer, with the thickness of 5 lm. The plating system

is from Technic (SEMCON

). Prior to Cu electroplating,

a 100 nmTi/200nm Cu seedlayer is sputtering deposited by

physical vapor deposition (PVD) in high vacuum

continuously.

As illustrated in Fig. 3, ﬁve sets of samples are prepared

on DSP wafers. In set A and set B, bare PI or Cu ﬁlm are

coated on the wafers. In set C * E, the PI/Cu composites

are also prepared. Speciﬁcally, in set C, the Cu ﬁlm is

electroplated on the wafer, and a PI layer is then spin

coated above the Cu ﬁlm and then cured. Such a PI layer is

referred as capping PI, and this sample is referred as Si/Cu/

PI. In set D, PI is ﬁrstly spin coated and cured onto the

Fig. 1 Thermal stress induced typical defects in PI/Cu composite.

a is an optical image from top view, with an inset schematically

illustrating the composites in RDL. b, c are SEM images of annealed

and annealed 2P1 M respectively

Table 1 The properties of

PDPI HD-4100 before and after

cured in current work

Solution properties Cured Film properties (typical data)

Solids content (%) 33.2 ± 5.0 Decomposition temperature 600 °C

Viscosity (cSt) 3060 ± 660 Initial (1 %) weight loss 430 °C

Flash point 93 °C Residual stress 34 MPa

Copper content 1.0 ppm max Dielectric constant (1 MHz) 3.36

Iron content 1.0 ppm max Surface resistivity (50 V) 3.3E16 X

Total metals 10.0 ppm max Volume resistivity (50 V) 2.4E16 X

glass transition 280 °C Dissipation factor (1 MHz) 0.001

Solvents N-methyl-2-pyrrolidone Dielectric strength 250 kV/mm

8326 J Mater Sci: Mater Electron (2016) 27:8325–8331

123

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