3D chip stacking with C4 technology

In order to achieve the shorter circuit design of multiple chips, three-dimensional (3D) packaging technologies with through silicon vias were developed to achieve high performance, low power consumption and small packaging size. In this paper, a PCB (Printed Circuit Board) processing compatible structure of three-dimensional chip stacking with low cost and easy fabrication will be shown. 3D and through Si via connections were formed by UV laser drilling technology. Laser drilling is a non-contact manufacture method and laser beam with high energy can be focused to a small spot (15 mum beam diameter) for material ablating and removing without mask used. Several processes are the keys to accomplish 3D stacking, such as wafer thinning process, through silicon via forming process, dielectric layer forming process, metallization process, and inter chips bonding process. By integration of the mentioned key processes, a 3D chip stacking structure with 10 layers was carried out. The thickness of chip was 100 mum. Daisy chain pattern was designed for the electrical measurement of 3D stacking structure. The testing results show that the resistance of multi- chip stacking structure is about 0.056 Omega/cm. Some reliability test, such as temperature cycling test and pressure cooker test were also done. These testing results verified this PCB processing compatible 3D chip stacking technology with low cost is a reliable structure for 3D SiP (System in Packaging) module application.

As device-scaling challenges increase, three-dimensional (3D) integrated circuits (ICs) are receiving more attention for system performance enhancements, due to their higher interconnect densities and shorter interconnect lengths. However, because of the limited contact area and the higher circuit density, the cooling of 3D ICs is more challenging. In order to assess appropriate cooling solutions for 3D chip stacks in various uses, we need better understanding of the total thermal resistance of 3D chip stacks. This calls for precise thermal resistance measurements and thermal modeling for each component of a 3D chip stack. A 3D chip stack is composed of interconnections, silicon substrates, back-end-of-the-line (BEOL), front-end-of-the-line (FEOL) and in this study, the thermal resistance of interconnections is the primary focus because interconnections are regarded as one of the thermal resistance bottlenecks of a 3D chip stack. With regard to the thermal resistance measurements of interconnections, Yamaji et al. found it difficult to measure the thermal resistance of interconnections with the laser-flash method and pointed out that care was necessary for uniform temperature distribution in the sample when using the laser-flash method on heterogeneous specimens, such as stacked chips with interconnections. Considering this concern, we use a steady-state method for the thermal resistance measurements of the interconnections. The thermal resistance of 200μpitch-C4 (Pb97Sn3) joined samples is measured and the thermal conductivity of C4 is derived to be 18 – 24 W/mC. Also the thermal resistance of a silicon with various interconnection pitches and diameters is modeled and the relationship of thermal resistance to interconnection pitch and diameter is obtained. The thermal resistance reduction by underfill with various interconnection pitches and diameters is also studied.

In order to determine appropriate cooling solutions for 3D chip stacks in various cases, it is important to have a better understanding of the total thermal resistance of a 3D chip stack. For this purpose, precise thermal resistance measurements and modeling of each component of a 3D chip stack are important. The thermal resistance of interconnection is considered to be one of the thermal resistance bottlenecks of a 3D chip stack. In this study, a steady-state thermal resistance measurement method is employed for the thermal resistance measurement of interconnection. The thermal resistance of the 200 μm pitch C4's (Pb97Sn3) jointed samples are measured and the thermal conductivity of C4's is derived to be 18–24 W/mK. With regard to the thermal resistance of a silicon substrate, the thermal resistance of a silicon substrate with various interconnection pitches and diameters has already been modeled by considering the concentrated heat flow to interconnection, as presented in ICEP 2008.[15] Based on the modeled data, the thermal resistance reduction by underfill with various interconnection pitches and diameters is also studied.

Increasing demand for new and more advanced electronic products, with smaller form factor and superior functionality and performance, while reducing the cost, has driven the semiconductor industry to develop more innovative and emerging advanced packaging technologies. 3D packaging, using z-axis through silicon via (TSV) stacking concept has been and continues to be investigated by many of the semiconductor manufacturers and research institutes and is believed to be one of the most promising, if not the most promising concept that could successfully address the limitations of today's packaging technologies. There is a continuous increase of interest in development and study of applicability of this new chip stacking approach to existing and future devices; currently, more than fifty companies worldwide are involved in some sort of 3D TSV packaging developments. There are several steps involved in 3D chip stacking using TSV technology. Each of these steps requires different techniques, materials and processes, applications that have to be well understood and integrated in order to successfully be applied. This paper will address various electrodeposition processes applied to form 3D interconnects, from TSV deposition, which is the main metallization step that stays at the hearth of this new technology, to other metallization steps for additional connections, such as bumping and redistribution layers, already well known and applied processes in current wafer level packaging (WLP) applications. Advantages and difficulties associated with each of these technologies, with more focus on 3D vertical integration using TSV copper interconnect, including the approaches taken to overcome them, will be presented. Deposition conditions for bumping were optimized in order to enable higher deposition rates. For other through- mask applications, such as redistribution lines (RDL), the complexity of the pattern, deposition rate, chemical formulation and process parameters were found to have a significant effect on within-die (WID) thickness distribution. In case of WLP structures of thick metal and high aspect ratios, seed layer uniformity proved to be essential for achieving void-free deposition. Void-free structures, especially for high aspect ratio vias seeded with dry methods, such as physical vapor deposition (PVD) or chemical vapor deposition (CVD), seem difficult to obtain. New approaches, such as seed layer repair (SLE) or direct on barrier electrodeposition (DOB) can provide superior uniformity and coverage of the conductive layer, necessary for a uniform nucleation of copper electrodeposition process along the entire sidewall of the via, at significantly lower costs. This paper will present TSV structures obtained with wet chemical processes applications for both seed layer and via filling. Besides seed layer uniformity, additional factors were identified to also be critical for successfully filling deep vias. They are: via profile, wettability of vias, superior performance of chemistries and high performance equipment. By selecting the most appropriate deposition methods and optimizing the chemical formulations together with the process parameters, void-free TSV structures can be obtained.

Three-dimensional (3D) stacking integration is offering many product benefits to SoC and memory: performance enhancements, product miniaturization and cost reduction. Besides, image sensors featuring 3D stacking of a specialized image sensor layer on the top of a deep submicron digital CMOS have just come to the market. The objective of this forum is to present applications and details of process integration, device techniques, circuits and system featuring 3D stacking integration. This will start with an overview of 3D stacking ICs, followed by a system perspective with scaling the memory wall. The next two talks will discuss challanges for power reduction with wide memory bandwidth, and performance gains through advanced packaging and chip stacking. This is followed by two talks covering challenges for foundry-specific issues and impact on device performance. The last two talks highlight how to integrate an imaging device on an SoC layer: technical issues and phenomenon of 3D stacked image sensor products, and evolution of 3D integration for imaging systems.

Three dimensional (3D) integration technology, involving the vertical stacking of multiple chips using through-silicon vias (TSVs), has emerged as a promising solution to improve the performance of microelectronic devices. However, the complex structures and the consequent increase in power density exacerbate the challenge of thermal management in the device including multiple chips. In order to ensure the reliability of 3D stacked chips during operation, it is important to have a better understanding of the thermal distribution of a 3D chip stack. In this paper, a detailed analysis of 3D IC packaging from thermal reliability perspective is presented, performed by finite element simulations. The effect of the variation of structure-related parameters on steady-state temperature profiles in the stack has been analyzed, including TSVs diameter and pitch, the thickness of SiO 2 , as well as the thermal conductivity of under fill. The research provides useful insights for thermal management of 3D IC packaging.

With the increased demand of functionality in electronic device, three dimensional integration circuits technology together with downscaling of interconnection pitch present an important role for the development of next generation electronics. In the current types of interconnects, solder micro bumps have received much attention due to its low cost in material and process. For fine pitch solder micro bump interconnections, selection of under bump metallurgical material is a crucial issue because the UBM structure/material will show a significant influence on the reliability performances of the solder micro bump joints. However, which UBM structure/material for fine pitch solder micro bump joint presents the better reliability properties is not concluded yet until now. In this study, the effect of UBM structural/material on the reliability properties of lead-free solder micro interconnections with a pitch of 30μm was discussed. The chip-on-chip test vehicle with solder micro bump interconnections having a diameter of 18 μm was adopted to evaluate the effects of UBM structure/material. There were more than 3000 micro bumps with Sn2.5Ag solder material on both the silicon chip and carrier. In this study, three types of UBM were selected on the silicon carrier: they were single copper layer with a thickness of 8μm; the Cu/Ni layer with the thickness of 5μm/3μm and the Cu/Ni/Au layer with the thickness of 5μm/3μm/0.5μm. The UBM was electro-plated on the Al trace and then the Sn2.5Ag solder with a thickness of 5 μm was deposited. For the silicon carrier with the bump structure of Cu/Sn and Cu/Ni/Sn, the silicon test chip with solder micro bump of Cu/Sn was used for chip bonding. The test chip having the solder micro bump of Cu/Sn and Cu//Ni/Sn was used for bonding with the silicon carrier having the Cu/Ni/Au UBM. In chip stacking process, we adopted the fluxless thermocompression process for each type of micro joints. After chip bonding process, the fine gap between bonded chips was filled by capillary type of underfill. The influence of underfill on the reliability of solder micro bump interconnects with various combinations of UBM structures were estimated also. After the assembly process, temperature cycling test (TCT), high temperature storage (HTS) and electromigration test (EM) were performed on the chip-stacking module to assess the effect of UBM structure/material on the reliability of solder micro bump interconnections. The results of reliability test revealed that only Cu/Sn/Au/Ni/Cu micro joint could not pass the TCT and HTS of 1000 cycles. After reliability test, the Cu/Sn/Cu joints showed the evidently microstructural evolution while the Cu/Ni/Sn/Cu joints did not show apparent microstructure change among all the types of micro joints and still revealed the most contents of residual solder within the joint. On the other hand, the existence of Au layer upon the UBM caused the complicated interface reaction between solder and UBM, which presented a negative effect during long-term reliability performance. The reliability results also displayed that the introduction of underfill could apparently enhance the reliability of micro joint under mechanical evaluation. From the results of EM reliability test, all the types of the micro joints showed excellent electromigration resistance under current stress of 0.08A at an ambient temperature of 150°C irrespective of the types of UBM. This superior property was attributed to the microstructure change transformed from the solder joint to the IMC joint within the solder micro bump interconnections during electromigration test. All the results of reliability test illustrated that the selection of UBM structure/material well influenced the reliability performance of fine-pitch solder micro bump interconnections in 3D chip stacking.

3D chip stacking with through silicon vias (TSVs) has been identified as one of the major technologies for achieving higher silicon packaging density and shorter interconnect. The test vehicle presented in this paper is a 3D chip stack package. Each layer of the test vehicle has two silicon flip chips mounted at the bottom of a silicon interposer with solder bumps. The flip chip has the equivalent dimensions and pad patterns as commercial memory chips. The interposer, with multiple interconnect TSVs for electrical connection and a central TSV for underfill dispensing, can function as a logic chip or as a redistribution chip in a real application. The assembly steps of the test vehicle include conductive adhesive filling for the interconnect TSVs, bonding two bumped flip chips on an interposer (to form a single layer), vertical stacking of the single layers and underfill dispensing. For the filling of the interconnect TSVs, an auger-dispensing method is first adopted to overfill the interconnect TSVs, followed by removing the excessive adhesive beyond the interconnect TSVs by squeegeeing. A jet valve continuously dispenses free dots of an underfill encapsulant into the central TSVs. The central TSVs function as an entrance for underfill dispensing and an uninterrupted point-source to provide fluid for each layer. The free dots form a capillary flow to fill the under-chip spaces of the test vehicle. The usage of TSVs rather than chip edges eliminates the presence of a wide edge reservoir, resulting in smaller ‘keep-out’ area occupation on the substrate.

3D-TSV chip staking becomes the ultimate 3D chip stacking technology because of its smallest size and weight, best electrical performance, and potentially lower cost. There have been several 3D-TSV chip to chip vertical interconnection methods such as copper-copper bonding and solder interconnect. In the solder 3D-TSV interconnect, copper/solder double bump structure has been used to interconnect TSV bumps followed by underfill dispensing. However, solder joint electrical shortage between neighboring bumps and underfill dispensing at the very thin 3D-TSV chip stack becomes very challenging issues. In this study, the copper/eutectic solder bump bonding method using pre-applied NCAs was investigated as an alternative 3D-TSV interconnection method. The non-conductive polymer adhesive was pre-applied on 3D-TSV wafers with copper/eutectic solder bumps as a film format resulting in no solder joint electrical shortage problem between neighboring bumps and no extra underfill process. Specially designed NCA materials have been developed to satisfy the polymer resin flow characteristics, solder flux function, fast curing speed, and 3D chip stacking process compatibility. In addition, simple and fast thermo-compression bonding processes have been developed to finish the solder joining and NCA curing at 250 ° C within 10 seconds. The electrical interconnections mechanism of 3D-TSV stacked chips using copper/solder double bumps and newly developed pre-applied NCAs were investigated and 3D-TSV vertical interconnect using NCAs was successfully demonstrated. The electrical interconnection through the arrays of the bumps between two 3D-TSV test chips showed excellent reliability test results, high temperature storage test (HTST) of 200 hours, pressure cooker test (PCT) of 24 hours, and thermal cycle (T/C) of 200 cycles. As a summary, it was proven that the one step metal/polymer hybrid bonding was very efficient 3D-TSV vertical interconnection method.

As the demands of functionality and performance for electronic products increase, three-dimensional chip stacking with high-density I/O has received much attention. For high density interconnections packaging, solder micro bumps are adopted extensively. However, its process temperature is high during chip stacking process. High bonding temperature would be easy to lead chip damage and chip warpage. For diminishing the thermal damage resulted from the high bonding temperature during chip stacking, we used the anisotropic conductive film as an intermediate layer to bond the chips. In this paper, a new type of ACF with Ni/Au-coated polymer arrayed particles was adopted for assembling a chip stack module with a micro bump pitch of 30μm. The reliability of the chip stack assembled by such novel material was evaluated and estimated also. The chip-to-chip stack module having more than 3000 I/Os with a pitch of 30μm was used as the test vehicle. The structure of Cu/Ni/Au micro bump was chosen and fabricated on both the silicon chip and substrate. The silicon chip was bonded onto the silicon substrate using the arrayed-particles ACF material after ACF lamination process. The optimized lamination conditions and the effects of bonding pressure and temperature were evaluated and determined by considering the particle deformation, electrical performance and adhesive flow phenomenon. After optimizing the lamination and bonding parameters, the reliability of the assembled C2C module was evaluated by Pre-condition test, TCT and THST. Cross-sectioned inspection of micro joints by scanning electron microscopy, observation of interface between adhesive and silicon by scanning acoustic tomography, and adhesion test of ACF film after bonding and precondition were conducted to determine the failure modes of the ACF joining. After precondition test, less than 15% of daisy-chain resistance variation was found. The ACF joints with stable electrical resistance could be obtained by such kind of novel material. Also, no any obvious ACF delamination could be observed. The adhesion strength did not show any degradation after precondition test. The reliability test results revealed that the assembled C2C module by the arrayed-particles ACF showed the acceptable reliability performance in TCT and THST. The results of failure analysis displayed that the connectivity of ACF joints was damaged by induced thermal stress coming from the mismatch of CTE between adhesive matrix and conductive particles during environmental testing. This study presented that the arrayed-particles anisotropic conductive film adopted had great potential and could be applied for the 3D chip stacking assembly with fine pitch interconnects.

3D integration consists of 3D integrated circuit (IC) packaging, 3D Si integration, and 3D IC integration. They are different and in general the through-silicon via (TSV) separates 3D IC packaging from 3D Si/IC integrations since the latter two use TSV but 3D IC packaging does not. 3D Si integration and 3D IC integration are different. 3D IC integration stacks up the thin chips with TSV and microbump, while 3D Si integration stacks up thin wafers with TSV alone (i.e., bumpless). TSV is the heart of 3D Si/IC integrations and is the focus of this investigation. Also, the state-of-the-art, challenge, and trend of 3D integration will be presented and examined. Furthermore, supply chain readiness for high volume manufacturing (HVM) of TSVs is discussed.

Multi-layer 3D chip stacking by a surface-tension-driven self-assembly technique is demonstrated. After multi-layer self-assembly, memory chips having Cu-SnAg μbump and Cu-TSVs are bonded on a substrate by thermal compression to confirm electrical joining between them. In addition, we investigate the impacts of wetting properties of chip/substrate surfaces, μbump shapes, and μbump layout on alignment accuracies of self-assembly. Good electrical characteristics are obtained from the TSV-μbump daisy chains in the stacked memory chips.

For the thermal management of three-dimensional (3D) chip stack, its thermal resistance needs to be clearly understood. In this study, 3D stacked test chips are fabricated, which are implemented with PN junction diodes for temperature sensors and diffused resistors for heating. At SemiTherm2011, the equivalent thermal conductivity of the interconnection, including BEOL (Back-End-Of-the-Line, wiring layer) is experimentally obtained to be 1.6W/mC and this time, we measure the thermal effect of Cu TSVs and it is experimentally supported that as the Cu TSV area ratio increases, the thermal conductivity of chip with TSVs in the vertical direction increases, on the contrary, that in the horizontal direction decreases. Also, the transient thermal measurement is performed and its result is compared with steady state measurement result. Further, the thermal capacitance measurement of 3D stacked test chip with hot spot heating is performed, which is essential to determine the transient thermal performance of 3D chip stack.

The rapid increase in on-chip heat fluxes and package heat density, accompanying the migration to nanoelectronics and 3D chip stacks, has placed thermal management squarely on the critical path for advanced product development. Innovative, cost-effective cooling techniques, combined in a synergetic way with more conventional approaches, must be developed if the benefits of the Moore's Law progression are to be realized. Following a brief discussion of the industry roadmap for IC packaging and review of chip package thermal management options, attention will turn to the application of solidstate thermoelectric refrigeration and the thermal characteristics of direct immersion cooling in dielectric liquids. The presentation will close with a discussion of the research challenges associated with the commercial implementation of on-chip thermoelectric coolers and direct liquid cooling.

Cooling of a planar 2D IC chip utilizes heat transfer from a face of the chip though a heat sink. In case of a 3D IC chip stack, the individual chip faces are not available for mounting conventional heat sinks. Mounting the heat sinks on the ends is feasible, but the heat flow paths for the interior chips from the junction to the heat sink become longer. Further, multiple heat sources present along the heat flow paths in stacked chips may create localized hot spots which exceed the allowable junction temperatures. While 2.5D integration in complex ICs where individual layers are mounted on another base die called an interposer can alleviate the heat dissipation issues, it cannot deliver the benefits of monolithic 3D ICs due to the planar distance between the chips or chiplets over the interposer. Introducing interlayer cooling with microchannels and introducing fins in the coolant flow paths extend the thermal dissipation capability of a 3D stack; however this is often accompanied with taller microchannels that lead to longer lengths of through-silicon-vias (TSVs). Placement of TSVs, microchannels walls and fins present conflicting design requirements. Therefore co-design and innovative approaches are seen as critical before widespread commercialization of 3D ICs becomes a reality. An overview of the available cooling options for 3D ICs and their performance evaluation are presented in this chapter.