2009 International Conference on Electronic Packaging Technology & High Density Packaging (ICEPT-HDP) 978-1-4244-4659-9/09/$25.00 2009 IEEE

Low Temperature Cu-Sn Bonding by Isothermal Solidification Technology

Yibo Rong1, 2, Jian Cai1, 2, Shuidi Wang1, 2, Songliang Jia1, 2 1Tsinghua National Laboratory for Information Science and Technology (TNList), Beijing 100084, CHN

2Institute of Microelectronics, Tsinghua University, Beijing 100084, CHN Email: roberto_acmel@sina.com.cn, Tel: 86-10-62794957

Abstract

A low temperature wafer-to-wafer bonding technology for 3D packaging/integration based on Cu-Sn isothermal solidification (IS) technology is introduced in this paper. The fluxless bonding technique using Cu-Sn multilayer composites to produce higher re-melting temperature bonding layer is presented. The structure of the intermediate multi-layers and bonding patterns are designed, and the bonding process is optimized. The microstructure of bonding layer was investigated by SEM (Scanning Electronic Microscopy) and EDS (Energy Dispersive X-Ray Spectrometer). The compositions of the bonding layer show that there are intermetallic compounds (IMCs) with higher melting points. The bonding layers consist of Cu6Sn5 and Cu3Sn phases. High strength of bonding layer has been detected, with average shear strength of 37.5MPa.

Introduction With the development of microelectronic technology, 3D

packaging/integration has been paid a lot of attention. As one of the important interconnect methods, Through Silicon Via (TSV) is getting more and more attention. The major technologies of TSV involve via formation (mostly by deep reaction ion etching), via filling by electroplating and inter-wafer bonding. Wafer-to-wafer bonding would be critical for TSV technology, especially when there are multi-layer interconnections. Bonding layer formed should not melt again in further bonding process. It leads to the requirement of higher re-melting temperature bonding layer.

As one of the crucial processes for 3D packaging, research and development of wafer-to-wafer bonding technology have become hot in both academia and industry. There are different wafer-to-wafer bonding methods for 3D packaging, including anode bonding, epoxy bonding, glass frit bonding and solder bonding, etc. All of these bonding technologies have their own advantages and disadvantages. For example, solder alloy bonding would have a lower bonding temperature and has been one of the promising bonding methods. However, the melting point of traditional solder joints/bonding layer would have a re-melting temperature no higher than the bonding temperature. This can not satisfied multiple layer bonding. Thus, the bonding temperature limited the post-processing temperature. This also restricts devices applications due to temperature constraint.

Another solution for wafer-to-wafer bonding is a liquid-solid inter-diffusion method, which is also called as isothermal solidification (IS) technology in some literature. Bonding layers by IS technology could achieve higher re-melting temperature structure.

IS Technology Mechanism and Bonding Couple Selection The bonding structure of isothermal solidification (IS)

technology includes a diffusion couple consist of two metals. One metal is with relatively lower melting point but the other is a higher melting point metal. At a constant temperature slightly higher than the melting point of the lower melting temperature metal, reaction or/and diffusion would take place between the liquid phase of lower melting temperature metal and the solid phase of the higher one. The reaction products would be intermetallic compounds (IMCs) with higher melting point. This means the bonding layer would have a higher re-melting temperature. So IS process gives a quantum jump to the post-processing temperature of the compound or solid solution fabricated. Additionally, as bonding would be occurred in lower temperature, this could induce lower stress for bonding system and devices on wafer.

For diffusion couple design, the low melting point metal is the key issue. Only a few of metals have lower melting temperature. Considering environmental issues, only Ga, In, Sn, Bi could be used for bonding. Table 1 shows the melting points of these elements. The melting temperature of Ga is too close to room temperature and the melting temperature of Bi is a little bit high. So In and Sn would be the candidates for diffusion couple. However, In is too easy to be oxidized. Then, Sn is the final decision for the lower melting point metal in diffusion couple.

Table 1 Low melting point non-contaminated metals

Element Melting Point(C) Ga 29.8 In 156.6 Sn 232.0 Bi 271.3

For high melting point in diffusion couple, several metals

would be selected. The common metals would be Au, Ag, and Cu. These metals could form different IMC with Sn and thus would achieve higher re-melting bonding structure. Compared with Au-Sn system, Cu-Sn bonding system would be lower costs. And the reaction rate is also higher. So Cu-Sn system was chosen as bonding media in this work.

Fundamental of Cu-Sn Bonding To descript the fundamental of isothermal solidification, it

is necessary to take a look on the Cu-Sn binary phase diagram. The Equilibrium phase diagram of Cu-Sn System is shown in Fig. 1 [1]. There are many equilibrium phases in this diagram. Among them, Cu3Sn and Cu6Sn5 are considered to be two most important phases. Above 227C, with tin composition of 99.3wt.%, reaction occurs between copper and tin [2]. And above 232C, when tin becomes liquid, the

96

behnamHighlight

behnamHighlight

behnamHighlight

2009 International Conference on Electronic Packaging Technology & High Density Packaging (ICEPT-HDP)

reaction accelerates. For tin of 61 to 100wt.%, the alloy is a mixture of Sn and Cu6Sn5 with a melting point of 227C. The alloy with tin composition of 38 to 61wt.% consists Cu6Sn5 and Cu3Sn with a melting point of 415C. And if tin composition reduces below 38wt.%, the melting point may even increase to 713C [3].

Fig. 1 Equilibrium phase diagram of Cu-Sn System

In order to realize a stable higher re-melting temperature

of bonding structure, tin has been designed to be exhaust after reaction, while some copper would be left. The expected structure after bonding would be as those shown in Fig. 2, Cu/Cu3Sn/Cu6Sn5/Cu3Sn/Cu or Cu/Cu3Sn/Cu.

Fig. 2 Expected Cu-Sn Bonding Structures

If only Cu3Sn left after reaction, then

3S SCu Cu Sn Sn

W WCu Sn

=

(1) If only Cu6Sn5 left after reaction, then

5 6S SCu Cu Sn Sn

W WCu Sn

=

(2) S: Area of Bonding : Bump Height/Thickness W: Mole volume of Cu or Sn Hereby, this paper designed the thickness of Cu/Sn/Cu

multilayer as 5m/5m/5m before bonding.

Concerning the requirement of test, the figures of the top wafer and the bottom wafer are designed separately. Layout of the bonding pattern is shown in Fig. 3.

Fig. 3 Layout of bonding pattern

Experimental Dummy wafers with Cu/Sn bumps were fabricated in-

house. After carefully optimizing of fabricating process, excellent bump plating results are achieved. At a 260C process temperature, 4 inches wafer-level Cu-Sn bonding, in which joints were almost void-free, was realized, including an interface with a melting point of 415C. With further bonding and annealing steps, re-melting temperature of the joints could increase to 713C. Details of the whole process are as following.

Firstly, this work prepared the wafers for bonding. TiW/Cu was sputtered as seed layer. And copper and tin bumps were electroplated as the diffusion couple for bonding.

When the wafers are ready, they are rinsed in plasma environment. By this way, impurities and oxides could be removed from the surfaces. Then, two wafers are aligned in SUSS MA6, and transferred into SUSS SB6 for bonding.

To avoid bubbles left between bonding patterns, this work select to process the whole bonding in vacuum, rather than in N2 atmosphere. Heating until 160C firstly and pre-bonding was preformed for 5 minutes at this temperature. Wafers would be heated until 260C and bonded for 20 minutes at this temperature. The temperature of the bonding pair decreased to 200C and inlet N2 cutting down the cooling time. Besides, during the bonding process, pressure should be kept at 6kgf/cm2. The thermal-pressure curve of bonding is shown in Fig. 4.

97

2009 International Conference on Electronic Packaging Technology & High Density Packaging (ICEPT-HDP)

Fig. 4 Thermal-Pressure curve of bonding

Results and Discussion To evaluate the microstructure of the bonding layer, SEM

(Scanning Electronic Microscopy) is employed. Fig. 5 shows the SEM image of the cross-section of bonding intermetallic inter-layers. From this picture, there are three distinct phases existing in the joint between two copper phases. The EDS results of the three phases are listed in Tab. 2. The results show that the upper part of the bonding layer is Cu3Sn, the middle one is Cu6Sn5, and the bottom one is Cu3Sn.

Fig. 5 Cross-section of bonding inter-metallic inter-layers

Table 2 EDS results of intermetallic inter-layers IMC Component wt.% at.%

Cu3Sn(Up) Cu Sn

61.38 38.62

74.81 25.19

Cu6Sn5 Cu Sn

40.61 59.39

55.09 44.91

Cu3Sn(Down) Cu Sn

62.27 37.73

75.51 24.49

Dage Series 4000 Shear Tester was utilized to measure the

shearing strength of the bonding structure. The maximum shearing strength is as 47MPa and the minimum is as 33MPa. The average pressure is as 37.5MPa and the variance is 5.1MPa. Fig. 6 shows the surface of the bond after shearing

test. The image shows that peeling occurred between TiW and Si. This reveals the high reliability of the multi-layers of Cu-Sn bonding. Also from Fig. 6, we could figure out that the alignment accuracy during bonding is about 10m.

Fig. 6 Top-view of sheared bonding structure

Conclusion Based on Cu-Sn isothermal solidification technology,

bonding structures are designed and processes are optimized, Good bonding joints are achieved.

SEM results clearly show the microstructures of the joints consisting of Cu6Sn5 and Cu3Sn inter-metallic phases.

In summary, the fluxless bonding method based on Cu-Sn isothermal solidification (IS) technology was demonstrated in the paper. At a 260C process temperature, 4 inches wafer-level Cu-Sn bonding was realized, including an interface with a melting point of 415C. Studies on the bonds reveal the basic bonding mechanism, and demonstrate the feasibility of this theory.

Acknowledgments This work was supported by the National Basic Research

Program of China, 2006CB302703. The authors would appreciate Ms. Wenyan Yang, from the

National Key Lab of Tribology, Tsinghua University, for her help on SEM analysis. The authors would also appreciate for the help from MEMSensing Microsystems Co., Ltd and Suzhou Institute of Nano-tech and Nano-bionics, CAS.

References 1. N. Saunders, A. P. Miodownik and etc., Binary Alloy

Phase Diagrams, ASM International Metals Park, Ohio, USA, 1990: 1481.

2. S. Bader, W. Gust and H. Hieber, Rapid formation of intermetallic compounds by interdiffusion in the Cu-Sn and Ni-Sn systems, Acta metal, 1995, 34, pp. 539-557.

3. Chin C. Lee, Yi-Dhia Chen, High temperature tin-copper joints produced at low process temperature for stress reduction, Thin Solid Films, 1996, 286, pp. 213-218.

98

/ColorImageDict > /JPEG2000ColorACSImageDict > /JPEG2000ColorImageDict > /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 200 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 2.00333 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages true /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict > /GrayImageDict > /JPEG2000GrayACSImageDict > /JPEG2000GrayImageDict > /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 400 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 600 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.00167 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict > /AllowPSXObjects false /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile (None) /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName () /PDFXTrapped /False

/CreateJDFFile false /Description > /Namespace [ (Adobe) (Common) (1.0) ] /OtherNamespaces [ > /FormElements false /GenerateStructure false /IncludeBookmarks false /IncludeHyperlinks false /IncludeInteractive false /IncludeLayers false /IncludeProfiles true /MultimediaHandling /UseObjectSettings /Namespace [ (Adobe) (CreativeSuite) (2.0) ] /PDFXOutputIntentProfileSelector /NA /PreserveEditing false /UntaggedCMYKHandling /UseDocumentProfile /UntaggedRGBHandling /UseDocumentProfile /UseDocumentBleed false >> ]>> setdistillerparams> setpagedevice