patent landscape and market segments of sintered silver...

37th International Electronic Manufacturing Technology Conference, 2016

Patent Landscape and Market Segments of Sintered Silver as Die Attach Materials in Microelectronic Packaging

*Kim S Siow 1,2 Mérigeault Eugénie3

1Institute of Microengineering and Nanoelectronics 2 Center for Collaborative Innovation

Universiti Kebangsaan Malaysia 43600 Bangi, Selangor, Malaysia

3 Questel 1 Boulevard de la Madeleine,

75001 Paris, France *Email: [email protected]

Abstract Sintered silver (Ag) is a porous silver attaching the semiconductor die to the microelectronic substrates. The market sizes for sintered Ag are explored, and the relevant patents are analyzed at macro and micro-levels. At the macro-level, patenting activity moves to Stage 2 of the technology life-cycle “s-curve”. In stage 2, patenting slows down before the next surge in patenting, likely to be spurred by the wider adoption of wide band-gap semiconductor. The low profitability of sintered Ag also coincides with the available market research reports on the three major market of this technology, i.e., power module, power discrete technology and consumer integrated circuits. However, the patent owners are not abandoning their patents applications. Only eleven entities are co-filing patents related to sintered Ag based on 350 patents and patent applications analyzed here. Such trend suggests the nascent characteristics and continuing investment in this technology. At the micro-level, patenting addresses the following issues. Firstly, pressure assisted sintering is necessary and more reliable than pressureless sintering but the former requires additional process controls and capital investments than the latter. The current state of the art of sintered Ag joint also favours sintering in an ambient environment that oxidizes the substrate. This oxidation poses a delamination risk, and a reliability concern to the microelectronic packages. Lastly, sintered Ag paste favours sintering on the Ag or Au-metallized substrate that represents an additional cost to the customers.

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1. Introduction Various forms of sintered silver (Ag) are used in printed

electronics, solar cells, semiconductor packages, corrosion probes and piezoelectric devices. The use of sintered Ag joints as die-attach materials for microelectronic packaging could be traced to the early patent on attaching the large area power thyristor on molybdenum substrate, filed by Siemens in the late 1980s [1]. Then, this sintered Ag technology was known as low temperature joining technology (LTJT) .

For the past two decades, the application and formulation of sintered Ag evolved together with the technology development in the packaging of the semiconductor devices. In the last ten years, sintered Ag was actively explored as a Pb-free die attach materials for the power electronic applications. Unlike current Pb-Sn solders used as die attach materials, sintered Ag paste did not form Cu-Sn intermetallics at the die attach joints which suffered from thermo-mechanical fatigue failure [2-4]. Sintered Ag joint

was also touted as one of the possible die-attach joints for the high power module when their operating temperature increased to beyond 200oC with the introduction of the wideband gap (WBG) semiconductors, such as SiC and GaN, to the market. Sintered Ag joints also possessed higher electrical and thermal conductivities than solder joints or electrically conductive adhesives because of the inherent high conductivities of pure Ag and absence of polymeric matrices in the sintered Ag joints [5, 6].

This review aimed to fill up several gaps in information in the development and adoption of sintered Ag joints as die attach materials, regarding the market size, technology life cycle (also known as s-curve), macro and micro views of patents filed in this area. We also updated our previous review on this topic of sintered Ag as die-attach materials by summarizing the latest package designs, processes and equipment, and formulation of Ag pastes used to produce these sintered Ag joints [7].

2. Market Segments and Growth Catalysts for Sintered Ag

Although sintered Ag technology, also known as LTJT, was reported in the literature since the late 1980s, sintered Ag pastes were mostly developed in-house because of its simple formulation. Furthermore, sintered Ag joint is considered a niche application that did not attract the attention of major materials suppliers. Recently, several growth catalysts related to the technological advancement and environmental law changed the market dynamics. Hence, the market sizes and segments of sintered Ag deserved further investigations.

Several market reports gave different projections on the market sizes of the sintered Ag paste. In the most recent projection, Yole Development estimated that the market size for silver sintering for power electronics to be worth USD110 million which might have included the market for supporting tools and equipment [8]. An earlier report from Fuji Keizai estimated the market size of sintered Ag paste to be worth 2 billion yen (equivalent to USD 17 million) in 2015 [9]. However, Fuji Keizai did not show their methodology nor elaborate the costing of their sintered Ag paste. Based on the assumed price of USD 3-5 per gram, the market size for sintered Ag paste was estimated to be between 3 and 6 metric tons per annum.

This market size could sustain the current interest in sintered Ag paste before wider adoption of WBG technology drove the market consumption to the next level. Another growth catalyst was the imminent ROHS directives to


implement Pb-free die attach in the power semiconductor package by 2019 [10]. Without any reliable Pb-free die-attach alternatives in the market, there would be an increasing demand for sintered Ag, if it could overcome the technology challenges mentioned in Section 4.

Our analysis showed that the following sectors can utilize sintered Ag technologies as die-attach materials: 1. Power module technology that attaches the silicon

and WBG semiconductor dies to the direct bond copper (DBC) or direct bond aluminium (DBA) substrates.

2. Power discrete technology that uses either the clip-bonding or wire-bonding to connect the semiconductor dies to the Cu lead frame.

3. Consumer integrated circuit (IC) technology that uses either printed circuit board or lead frame as their substrate. This IC technology is divided to mainstream IC or power IC technology (e.g. GaAs and GaN).

Besides, the micro-electromechanical systems (MEMS) devices also used Ag paste as die-attach material in their packaging. This nascent MEMS market was only reported in the scientific journal [11]. In this MEMS market, the sintered Ag paste bonded the pressure sensor to the substrate for applications in high-temperature and pressure environments such as oil-rigs or geothermal plants. Another nascent market was the power light emitted diodes (LED) that were reported in several publications in the literature [12, 13], and another few patents were filed by LED manufacturers [14, 15].

3. Patent Analysis of Sintered Ag We used Questel Orbit to search for patents and patent applications related to the entire value chain of sintered Ag filed by materials suppliers, equipment suppliers, semiconductor companies and end-customers. We manually sieved out the patents related to sintered Ag to finalize the patents and patent applications related to die-attach or “bonding” from our initial dataset that used the following algorithm in Orbit: (((H01L-2224/8384)/CPC AND (SILVER OR AG)/CLMS) OR ((SINTER+ SILVER OR SINTER+ AG)/BI/OBJ/CLMS AND (BOND+ OR PASTE)/BI/OBJ/CLMS)) NOT (SOLAR CELL? OR PHOTOVOLT+ OR PRINT+ ELECTRO+)/TI/OBJ. “H01L-2224/8384” refers to “bonding by sintering in semiconductor device” based on the Cooperative Patent Classification (CPC). In total, there were 350 patents and patent applications retrieved from patent offices in the USA, Russia, Germany, Korea, Japan, Taiwan, China, and European Patent Office. Hereafter, “patents and patent applications” are abbreviated as “patents (applications)”. In general, patent applications are examined by patent offices in each country before they are reclassified as “patents” when they are finally approved with amendments.

Figure 1 shows the number of patent families related to sintered Ag based on their first priority year. A patent family represents a single invention filed and extended in multiple geographic locations, resulting in several priority dates. Our approach of using first priority year eliminates the possibility of multiple counts in Fig. 1. The increasing

patenting clearly demonstrated the dynamism of this sintered Ag technology; companies were filing patents to protect their investment in this technology. The low number of “dead” patents within each year also suggested the continuing interests amongst the pioneers of this technology. Dead patents are patent applications that are abandoned at different stages of prosecution in the patent offices.

Fig. 1: Patent family counts for sintered Ag from 1995 till 2015 based on first priority year and their legal status.

We used the technology life cycle ‘s-curve’ to elucidate the development stages of sintered Ag shown in Fig. 1. While there are limitations on the use of s-curve as a tool for technology management, it does provide insights into the different stages of the technology development [16]. A drop in patent family counts signified the beginning of stage 2 in the s-curve (Fig. 2). This reduction was evident from 2012 to 2013 (Fig. 1). However, the number of patents (applications) in 2014 and 2015 were expected to surpass the figures in 2013 when the complete patent datasets were released after the mandatory black-out period of 18 months.

Fig 2: Different stages of technology life-cycle “s-curve”: 1 (research), 2 (growth), 3 (maturity) and 4 (decline) [17].


In an analysis of “s-curve”, the main parameter value (MPV) refers to the parameter which determines the performance of the technology. Unlike other technologies like Wi-Fi which has a clear parameter (i.e. coverage distance), sintered Ag faces the issue of selecting a suitable MPV because of incomplete public information. Cycles to failure or shear strength may seem to be a good choice, but such parameters depend on the joint thickness, joint area (i.e. die-sizes) and metallization schemes at interfaces which are often not reported in the public domain.

Hence, we use “die-size for pressureless sintered Ag” as a proxy MPV to show the advancement in this field because it is more technologically challenging to sinter a larger die without voids of less than 3-5 %, than a smaller die. In reports on pressureless sintered Ag, the die-size was between 2 and 16 mm2 (2006-2007) before it migrated to a footprint of 64 to 100 mm2 in the 2014-2015 [18-22]. Although these data were based on published papers which did not study the influence of die-sizes, they aimed to produce reliable Ag joints to demonstrate the viability and flexibility of this technology. This increase appeared to coincide with Stage 2 of the s-curve. Die sizes used for pressureless sintering was expected to be as large as those produced by pressure-assisted sintered Ag which currently could sinter die-size of 15 mm x 16 mm [23]. This migration will halt if the proliferation of WBG technology does not necessitate the use of large semiconductor dies.

Stage 2 was also characterized by the emergence of early adopters for this sintered Ag. Semikron Elektronik and Infineon Technologies were the technology leaders in producing power modules with sintered Ag as die-attach materials [23-25]. However, the profitability of the materials suppliers were still low because of the limited demand from other device makers.

Fig. 3 shows the top key patent applicants of sintered Ag as die-attach materials. Tianjin University was the only academic institution amongst the top 19 entities, followed by Virginia Polytechnic and Harbin Institute of Technology with two inventions each (not shown in Fig. 3). The rest of the top patent applicants were companies involved in manufacturing power electronics, power devices, LEDs and Ag paste suppliers.

Fig 3: Top patent applicants of inventions related to sintered Ag as die-attach materials in September 2016.

In addition, there were a total of 10 co-ownership of patents amongst the 350 patents (applications). For example, Bosch and Siemens co-owned 6 patents (applications) while Harima Chemicals, Panasonic and Toshiba shared two patents (applications) amongst them. This lack of co-ownership reflected the nascent stage of this technology or one of the collaborators, usually academic institution, was not included in the patent prosecutions. The latter is a common practice in the industry-academic collaboration.

The filing trend of the key patent applicants can be further refined based on their first priority year (Fig. 4). The highest number of inventions per year were filed by Bosch (2011, 2012) followed by Hitachi (2009). Several patents owned by Infineon, Nihon Handa, Danfoss, Hitachi, Toshiba, Fuji Electric and Ebara were considered as “sharked” inventions because these patents (applications) received more than 30% of their forward citations from the same assignee. Patents from these seven entities were currently being analyzed based on the doctrine of equivalents, i.e., same result, operation principles and functions of components in the paste formulation or processes, and their prosecution history to determine whether estoppel exist for each of them. This on-going analysis will be published elsewhere.

Fig. 4: Breakdown of top patents applicants in sintered Ag between 1996 and 2015. Blackened circles indicates “sharked” invention, i.e., a patent family that receives more than 30% of their forward citations from the same assignee.

Besides “sharked” inventions, the most cited patents (applications) in our dataset was a PCT patent application owned by Ebara, i.e., WO200426526, which was cited by 42 patent families in our dataset and additional 78 patent families outside our dataset. This Ebara patent application signified that sintered Ag is a platform technology which has applications beyond die-attaching in semiconductor packages. These forward citations included inventions in Ag, Cu, Ni, Sn, Al or solder paste formulations and related sintering processes, sinter equipment, production of nanoparticles, microelectronic package designs in power and LED devices. In non-semiconductor applications, the forward citations included turbine repair [26] and piezoelectric vibrating device [27]. Fig. 5 shows the rest of the top cited patent (applications) in our dataset. All these top cited patent (applications) were included within the


dataset retrieved earlier and would be discussed simultaneously in the following sections.

Fig. 5: The most cited patents (applications) amongst the data-set related to sintered Ag as die-attach materials. 4. Factors Affecting Sintered Ag Joints. Figure 6 shows the dependence of a sintered Ag joint on the design, process, equipment and formulation of Ag pastes. These factors influence the microstructures and interfacial properties of the Ag joints to possess the necessary mechanical, electrical and thermal properties to operate reliably at their designated environment.

Fig. 6: Factors affecting mechanical, electrical and thermal properties of sintered Ag joints [7].

4.1 Package Designs

The package design depends on various electrical, thermal and mechanical requirements set by customers of the device makers, power module makers and inverter makers, with less consideration to the introduction of a new technology like sintered Ag technology in the package. Sintered Ag is merely mentioned as one of the possible die-attach materials in most patents (applications). For example, several patents (applications) merely specified the use of Ag paste to connect the heat spreader to the substrate [28, 29] or as an intermediate layer in the fastener to hold the substrate or pins together in the power module [30]. Others used electrically insulating but thermally conductive spacer elements which were sinterable as part of their package designs [31]. Others increased the roughness of the DBC to an average roughness of 2 µm to 4 µm, before Ag plating to

sinter with the Ag pastes, to prevent delamination in this interface [32]. Of late, there is a shift in the design to take advantage of the high thermal performance of the sintered Ag joints, but such designs are still in their infancy [8]. For example, Texas Instruments embedded their semiconductor dies in the laminate body to pressure sinter with the contact pad of the substrate for their system-in-package design in a single step [33].

4.2 Processes and Equipment Process and equipment development complement the Ag

paste development to produce reliable sintered Ag joints. The primary processes to implement the sintered Ag joints are shown in Fig. 7.

Fig. 7: Main processes of producing sintered Ag joints [7].

In pressure-assisted sintering, the process steps involved

the application of the Ag pastes on either the substrate or wafer/dies or both surfaces, by stencil-printing or lamination. Then, moderate drying of Ag pastes provided the tackiness for subsequent die placement step. The drying condition was specified to have a flow rate of atmospheric air between 0.5 and 3 lt/min at the pressure of 4.0 x 10-4 Pa to 5.5 x 10-3 Pa within the temperature range of 100oC to 150oC for a duration of 30 to 60 mins [34]. In the case of over-drying, tacking agent were applied to increase positional accuracy and adhesion before pressure sintering [35].

Next, pressure sintering is an additional step which is not commonly used in microelectronic packaging. Innovations in this area face the challenges of ensuring good bonding without damaging the semiconductor dies while achieving throughput comparable to the conventional soldering or adhesive-based die-attaching process. Various designs of sinter press and inserts were used to reduce damage to the semiconductor dies during pressure sintering [7]. Others used gas, liquid, putty and gel to actuate this pressure during sintering [36, 37]. Another innovation used porous Ag foams of 25%- 50% porosity to absorb the uneven thickness of the chip-set during the pressure sintering, though this approach was attaching the DBC to the heat sink [38]. A similar approach could be used to attach the semiconductor dies to the DBC. Others provided more detailed descriptions of the pores in the Ag foams, i.e., ellipsoidal pores with inclination angle between 50o and 57o with the total porosity between 2 and 38 vol% [39].


As shown in Fig. 7, pressureless sintering is similar to the application of electrically conductive adhesive, but with an additional preheating stage to allow the solvents to escape from the Ag paste. The main challenge in pressureless sintering is to sinter large semiconductor dies in the absence of progress and proliferation in WBG technology for power devices. Large die limits the access of atmospheric oxygen to pyrolyze the binders in the Ag paste. Residual binders might pose a reliability risk though excessive oxidation of the substrate also heightened the delamination risk of the packaged semiconductor [40]. Therefore, some sintering process limited the percentage of oxygen to a level between 0.05 % and 0.15 % with an option to introduce methanoic acid to remove any excessive oxides [41]. LED dies did not face this residual binder and associated delamination issues because of their generally smaller die sizes of less than 25 mm2.

4.3 Formulation of Sintered Ag There are three main groups of Ag pastes namely,

micron-Ag, nano-Ag and hybrid Ag. They differ by their filler sizes and compositions, but share almost similar constituents. Nano-Ag paste relies on the nano-particle sizes, i.e., less than 100 nm, as the driving force to form the sintered Ag joints, while micron Ag paste relies on the Ag-based endothermically decomposable compounds like Ag carbonate or Ag oxalate to form reactive Ag nanoparticles to bond the adjacent micron-sized Ag fillers. Others specified these micron Ag fillers to have a centre-line average surface roughness (Ra) of 2-10 nm with the diameter range of 1 µm to 10 µm to achieve a higher density than smooth micron Ag fillers [42]. Hybrid Ag pastes consisted of micron and nano-Ag fillers which were tailored to achieve high density and good mechanical properties upon sintering.

Other additives were added to deoxidize the interfacial oxides of the substrates without the additional risk of residual binders remaining in the Ag joints. Instead of atmospheric oxygen, a local source of oxygen for pyrolyzation could also be incorporated as part of the binder systems [43]. Most patents addressed this interfacial failure of the faying surface as the sintered Ag paste could only form reliable joints on surfaces metallized with Ag, Au, palladium or platinum surfaces, though progress had been made in sintering on the bare Cu substrate. For example, Namics formulated their Ag paste to include glass frits containing the Ag20, V205, and Mo03 to form calcined films to bond directly on the bare Cu [44]. A summary of these different binders and solvents has been reported elsewhere [5]. The binders and solvents were tailored to be pyrolyzed at sintering temperature between 200oC to 300oC, though one patent suggested solvent with boiling temperature more than 300oC [45]. Besides pastes, sintered Ag joints could also be produced from Ag film or foils or preforms to achieve the same objectives of die-attaching as mentioned earlier in section 4.2 [46].

6. Conclusions and Recommendations In conclusion, sintered Ag is entering an exciting phase

of development with increasing investment in protecting this

technology from device maker, power module maker, inverter makers, materials suppliers, research institutes and universities. Detailed analysis of our dataset suggested significant developments in paste formulations, processes and equipment development to control the porosity, grain sizes and interfacial properties of the sintered Ag joints to produce reliable die-attach joints. As predicted in the technology life-cycle “s-curve”, the slight dip in current patenting activity would be followed by a second surge after the different stakeholders established a common standard and best manufacturing practices in producing reliable sintered Ag joints. One such effort is led by an iNEMI consortium to produce reliable high temperature and die attach joints [47].

Acknowledgment We acknowledge the financial support given by

Universiti Kebangsaan Malaysia Dana Impak Perdana DIP-2015-008 for this work. We also thank Wang SW (ON Semi), Shutesh K (On Semi) and Vemal RM (Robert Bosch) for their comments on this paper, and KSS’s former employers, colleagues, customers, materials suppliers for their support and insights.

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