Xi’an Jiaotong University, China
Professor of Institute of Wide Bandgap Semiconductors
Progress of diamond substrate development
Diamond has many excellent properties, such as wide bandgap, high carrier mobility, high breakdown voltage, high thermal conductivity, superior mechanical strength, and chemical stability among the well-known materials. In this talk, large size diamond development and application will be presented. For heteroepitaxial single crystal diamond growth, preferred orientation Ir (001) film was deposited on sapphire substrate. Then bias enhanced CVD method was used to form diamond nucleation, on which a microwave plasma CVD（MPCVD） system was used to grow single crystal diamond. Then, tungsten atoms were introduced into MPCVD to grow high quality single crystal diamond on this sample. Thereafter, a laser machining technique was used to produce patterned trenches in diamond substrate, on which microchannels were achieved by epitaxial lateral overgrowth of diamond layer by MPCVD. In addition, we studied the enhanced heat spreading due to conduction followed by convective dissipation of a locally heated resistor mimicking a linear hot spot within electronic chips. The combined effect of conductive spreading and convective dissipation exhibited a significant cooling enhancement, which could be useful for GaN/diamond Composite Devices.
University of Bristol, United Kingdom
Heat Transport across Interfaces for the Optimization of Heat Sinking in Device Applications
Heterogenous integration of materials is a powerful approach to overcome drawbacks of individual materials while benefiting from their “good” material properties. A simple example is the recent integration of GaN with diamond, where GaN has excellent electronic properties but only medium-high thermal conductivity, while diamond has the highest bulk thermal conductivity (TC) known to mankind, but limited electronic properties. Integration of both materials therefore allows equally excellent electronic and excellent thermal properties for device applications. When two materials are integrated e.g. via bonding or direct growth of one material on the other, interfaces are created, which may contain microstructure. These interfaces are typically amorphous or in other cases separate interface layers need to be included to enable the two different materials to mechanically adhere. These interfaces create thermal boundary resistances (TBR), which is a material property describing how difficult it is for heat to transfer from one material to the other. Factors such as microstructure, but also mismatch of phonon properties (in the case of semiconductors) or electronic properties in the case of metals come into play for TBR. It is therefore critically important to assess heat transfer across their interfaces to avoid thermal bottlenecks resulting in excessive device temperatures though. Experimental techniques to assess thermal conductivity of materials and TBR between materials are reviewed in this presentation, with examples from GaN-on-Diamond, diamond to metal diamond composites.
Schmalkalden University of Applied Sciences, Germany
Professor for Autonomous Intelligent Sensors
Anodic Bonding a Low Temperature Bonding Method for Processed MEMS and CMOS Wafers
Wafer bonding is the process step to realize real three-dimensional microsystems with freely defined layer structures, free standing mechanical elements, hermetically sealed cavities and other 3D elements. There are various bonding technologies available with advantages and drawbacks in application and process integration. Out of this variety, anodic bonding is in general a very attractive low temperature bonding method. It is a direct joining process of glass containing mobile ions to various substrates like silicon (even coated with functional materials like oxide or Aluminum) or metals. Utilizing the progress in glass structuring and with adapted bonding conditions it can be used for CMOS-MEMS integrated wafers enabling very complex, multifunctional chip solutions especially for sensor, microfluidic and optical applications. Anodic bonding is characterized by a very good process stability and complete process control. This makes it very attractive for industrial production processes. However also this bonding process has its own challenges, which need to be addressed.
The unique properties of SiCN as bonding material for hybrid bonding
Direct Cu-SiCN hybrid bonding is successfully realized by using a thermal budget of 250 °C. The excellent results should be attributed to the tight control on the different processing steps but also to the properties of the SiCN dielectric used as bonding material.
Interface reactions and thermal transport in heterogeneous heterostructures
Low temperature bonding has been leveraged to produce a wide array of materials combinations. One relatively unstudied aspect of this technique is the modification of surfaces prior to bonding. These modified surfaces become buried interfaces and can impact the electrical or thermal transport across such interfaces. One form of surface modification is amorphization. Amorphous interfaces reduce electrical transport but recent reports have indicated that amorphous interfaces across a heterogeneous junction can improve thermal transport properties. Another form of modification is the introduction to a thin (few nm) metal film at an interface. The stability of the amorphous and / or metallic interface layers after annealing is not well known. We provide a few examples in semiconductor-based systems to address the stability of different, technologically important, interface combinations as a function of annealing. Our main goal is to be able to exploit these surface preparation conditions to engineer the interface properties to optimize device performance.
The understanding of the evolution of the interface structure is described through homogeneous bonded materials (Si-Si) in which annealing leads to the epitaxial recrystallization of the interface with the recrystallization front dependent on different growth planes. A dramatic change in electrical properties is likewise observed.
For heterogeneous bonded materials, the interfacial reconstruction is more complicated with the formation of more stable phases and interdiffusion observed. This is observed in GaN/Si interfacial structure with world record thermal boundary conductance highest for the un-annealed interface. In other systems in which the two phases are thermodynamically stable, the crystallization of the amorphous layer dominates. When a metal layer is present, this is similar to deposition of a metal layer. In these cases, the interface reaction will be based on free energies of formation of the metal and semiconductor. If a reaction is favorable – as is the case for Al/β-Ga2O3 – then reaction kinetics control the transformation which means that the β-Ga2O3 orientation determine the extent of the reaction.
Meisei University, Japan
Low temperature bonding by extension of the SAB concept
Die to wafer for photonic and 3D applications
Integrated transmitters incorporating lasers and modulators on silicon are of primary importance for all communication applications, and at the same time are the most challenging to manufacture due to the need of hybrid III-V integration. In order to introduce III-V materials in low cost silicon platform manufacturing, direct bonding approach could present a great interest due to the growth limitation of III-V hetero-epitaxial layers directly onto silicon. Furthermore, by using a 100 nm silicon dioxide layer between the silicon wave guide and the III-V active stack, direct bonding allows the optical coupling necessary to build active optical device. Nevertheless, the potential low cost model of silicon photonic is based only on the hypothesis that we are able to work on the full surface of the 200/300 mm SOI photonic wafer. III-V wafer direct bonding is not suitable to fulfill this requirement for two main reasons. First, the maximum diameter available for III-V wafers is limited to 150 mm up to now. Secondly, the III-V material is necessary only on the emitter and receiver areas which represent only a very little part of the device area. Therefore, the most part of the reported full III-V wafer is lost by the layer patterning on required areas. To overcome this limitation in term of wafer diameter and to limit the loss of a very expensive starting material we have developed at LETI a collective die direct bonding process. By replacing the initial die silicon holder by a more conventional tape on a dicing frame, our bonding process keep similar results in term of throughput, die placement accuracy and bonding rate. In addition, tape using has highly improved the bonding process in term of non-device design dependency, die thickness variability tolerance and simplicity of execution.
From Hot SAB bonding to organic hydrophilic bonding
Direct bonding is now a wild sprayed bonding technology with mass production for some applications. However, research studies are still mandatory in order to understand or optimize its chemical physical mechanism. For example, interesting new developments have been recently obtained on the link between direct bonding and organic compounds. Even if organic contamination is one of the main detrimental surface contamination, specific organic molecule types are shown to enhance drastically the bonding energy at low temperature. More than the adherence enforcement, this effect highlights the silica hydrolysis as a key phenomenon in silicon direct bonding mechanism. And even more than just a little amount of organic compound at the bonding interface, direct bonding can even appear between one or two full organic surfaces. This opens a large window for direct bonding applications keeping some direct bonding advantages, as bonding throughput for instance. Other interesting direct bonding developments allow now direct bonding to appear above room temperature. Indeed recent results, using SAB technology, allow us to bond materials with important dissimilar thermal expansion coefficients. This opens interesting leverage arms for internal stress control. Even if direct bonding is now a well-established technology, interesting developments still appear, widening always and always its application field.