Low Temperature Wafer Bonding Based on Copper Nanoparticle Sintering for 3D Interconnect Fabrication

Abstract

The current thesis is organized into 7 chapters. Chapter 1 offers a brief overview of the current research work and presents the project objectives, report structure and the relevant 3D integration challenges . The introductory section 1.1 provides the motivation for the current work by matching the current requirements in 3D integration with advantages offered by the proposed wafer bonding technique based on nanocopper sintering. The chapter continues with a description of the objectives in section 1.2 and is concluded by a brief outline of the thesis report given in section 1.3. Chapter 2 provides the theoretical background based on a literature review of state-of-the art wafer bonding techniques, surface interactions, wafer specifications and bond characterization methods. The first section focuses on identifying challenges associated with low temperature wafer bonding techniques applicable to the current research work. The surface preparation and wafer parameters relevant for this project are presented in section 2.2. Section 2.3 offers a brief explanation of the theory behind the characterization methods utilized to measure the mechanical and electrical parameters of the interconnect structures fabricated using nanocopper sintering. Chapter 3 presents the first experiment on die bonding based on surface activation using wet etching. The first section 3.1 describes the sample preparation and mechanical characterization based on die shear tests of silicon dioxide dies bonded using hydrofluoric acid (HF) etching. Section 3.2 gives an insight into the sample fabrication and characterization procedure based on bond strength and electrical measurements of dies bonded using partial etching of aluminum. The conclusions 3.3 provides an overview and evaluation of the obtained results and discusses possible improvements for future work. Chapter 4 describes the development, fabrication and characterization of the nanocopper interconnect patterning using lithographically defined screen printing. Section 4.1 contains an explanation of the sample fabrication procedure. The screen printing process is presented in section 4.2 with focus on the challenges encountered and the solutions found to optimize the nanocopper screen printing process quality. The photoresist lift-off process employed to pattern the screen printed nanocopper is described in section 4.3. The measurements and results are presented and analyzed in section 4.4. The concluding section 4.5 offers a summary of the results, provides explanations for the observed effects and gives an outlook for future improvements and applications. Chapter 5 describes the main experiment of this research project focused on the development of a wafer bonding technique for metallic 3D interconnect fabrication using nanocopper sintering. The investigation of fused nanocopper sheet resistance is presented in section 5.1. It includes the sample fabrication procedure, the description of the nanocopper fusing process parameters and finally the sheet resistance measurements with conclusions and explanations based on the obtained results. Section 5.2 describes the nanocopper-to-bulk Cu contact resistance experiment. After the sample fabrication procedure is explained, the details of the nanocopper fusing process are given, followed by the description of the contact resistance measurements. The fabrication procedure details are presented and possible optimizations of the technique utilized are discussed. The results are presented and analyzed, followed by explanations of the observed phenomena and conclusions. Section 5.3 describes the in-situ nanocopper fusing measurements. The sample fabrication of the nanocopper structures on a PCB are described along with the experimental setup and fusing parameters. The results are presented and conclusions are drawn based on the result analysis and observed nanocopper fusing effects. The transmission electron microscope (TEM) analysis of the nanocopper material is presented in section 5.4. Important results regarding the nanocopper particle structure are gathered and analyzed. The mass loss ratio variation with temperature is investigated using thermal gravimetric analysis (TGA) in section 5.5. The nanocopper fusing temperature point is estimated based on the TGA results. Section 5.6 describes the die-to-die bonding experiment using nanocopper sintering. After explaining the sample fabrication procedure, the die bonding parameters are given, followed by the qualitative measurements of the contact resistance. Scanning electron microscope (SEM) imaging of ion-milled cavities is employed to analyze the structure of the sintered nanocopper-bulk Cu interface. The porosity and changes in the nanocopper layer after the sintering process are measured. Die shear tests are utilized for evaluating the bond strength of dies bonded using nanocopper. The results are summarized and valuable conclusions are drawn based on the results analyzed. Section 5.7 describes the final experiment on wafer-to-wafer bonding using nanocopper sintering. The important data and conclusions gathered from the previous experiments are employed to optimize the wafer bonding process. After describing the sample preparation procedure, the bonding process is explained. Further sample processing details are given including top wafer removal for access to the test structure for electrical measurements. The contact resistance measurements are described and the results are discussed with explanations found for the observed effects. The sintered nanocopper structure and bond interfaces are analyzed based on SEM imaging. Using these results, modifications to the nanocopper internal structure during the sintering process are correlated with the mechanical and electrical properties of the fabricated interconnects. Finally, the results from the entire chapter are summarized and conclusions are established based on the observed effects. Possible improvements for optimizing the nanocopper interconnect fabrication and bonding process are provided. Chapter 6 gives a brief outline of the current thesis starting from the theoretical background and ending with recommendations for future work. The most important results obtained from the experiments conducted on interconnect fabrication and wafer bonding using nanocopper sintering are presented. The effects observed during the fabrication and characterization steps are summarized and explained, with focus on process optimization. Possible improvements to the developed nanocopper patterning and wafer bonding techniques are discussed by giving an outlook for future research work. The appendix chapter 7 provides extra theoretical background information on wafer bonding techniques, surface preparation procedures and wafer parameters.