Takahito Suzuki

High Density Integration of Single Crystal Thin Film Devices by "Epifilm Bonding" Technology

High density integration of dissimilar material devices is one of the most interesting subjects in electronic and optical device technology. Recent our investigation has achieved successful three-dimensional integration of single crystal thin film light-emitting-diodes (LEDs) and CMOS IC drivers by intermolecular force; we call this technology “epifilm bonding (EFB)” technology [1]. Employment of the EFB technology will allow us to form planar wiring structure to connect integrated devices by semiconductor wafer process; this will enable us to manufacture much more compact and higher density integrated device systems. Increasing density of integrated device systems brings more heat into the integrated device system. Heat arising from the device active region of the single crystal thin film (epifilm) will transfer very rapidly when the bonding substrate has higher thermal conductivity. This paper focuses especially on investigation of epifilm bonding on diamond-like carbon (DLC) thin films that have higher thermal conductivities. No report has been found on the application of the DLC thin film in intermolecular force bonding. The point to apply the DLC film in EFB is modification of the DLC thin film surface to achieve higher bonding strength. Figure 1(a) shows the epifilms that are bonded on the DLC thin film by the EFB technology. The epifilm consists of AlGaAs double hetero structure for fabrication of LEDs. The DLC thin film was formed on the Si substrate. The stripe-shape epifilms were released from the GaAs substrate by chemical etching of a sacrificial layer between the epifilm layer and the GaAs substrate [1]. The released epifilms were pressed to bond on the DLC thin film at room temperature after plasma activation of the bonding surfaces. As shown in Fig. 1(a), the epifilms are well bonded on the DLC thin film. No void and no crack are observed on the bonded epifilms on the DLC thin film. The bonded epifilms on the DLC thin film were processed into small pieces (10 mx10 m) in order to test the EFB on the DLC thin film. Figure 1(b) shows the microscope image of the array of the small epiflms after etching of the bonded epifilms shown in Fig. 1(a). As shown in Fig. 1(b), the bonded epifilms are well processed into the array of the small epifilms of an area of 10 mx10 m. This indicates that the epifilms can be considerably well bonded on the DLC thin film by EFB. Characteristics of the epifilm LED array that was bonded on the DLC thin film by the EFB technology were measured. The current-power (I-P) characteristic of the epifilm LED on the DLC thin film was compared with that on the polyimide (PI) layer. (The PI is an example of lower thermal conductivity material.) The test result showed that the emitted light power of the epifilm LED on the DLC thin film was one order of magnitude higher or more than that on the polyimide layer at a high current density of 12.5 kA/cm. This indicates that the epifilm LED bonded on the DLC thin film having higher thermal conductive structure provides higher characteristics by preventing temperature increase at the active region of the epifilm LED. In summary, this paper proves that the single crystal thin film LEDs can be well bonded on the DLC thin film by the EFB technology. The structure of the epifilm devices bonded on the DLC thin film by EFB will allow us to manufacture much higher density integrated device system providing higher characteristics using single crystal semiconductor thin films of dissimilar materials.