Wafer bonding is an enabling technology for materials integration in microelectronics, optoelectronics, and MEM systems . Although most published work focuses on forming a permanent bond between two wafers, a growing aspect of bonding involves the use of temporary or controlled bonding, so called "post-it" style bonding. Surface nano-texturing could be a way to engineer the bond to a prescribed strength. Recently, we described the surface textures produced by low energy Ar ion sputtering under various experimental conditions. An important feature is that nanometer scale ripples can be formed, with a wave vector either parallel or perpendicular to the projected direction of ion beam. The surface topography is characterized by atomic force microscopy (AFM), and topography parameters are extracted as functions of incidence angle, ion energy, and dose .In 2003, we will focus on the correlation of the delamination energy (measured by the crack opening method) with the topography parameters.
A polymer wafer bonding and laser lift-off process has been developed and implemented to integrate GaN heterostructures and functional devices with dissimilar substrates. The integration process takes advantage of the high quality of GaN growth on sapphire and the complementary functionality of the receptor substrates. The process developed in this study uses an SU-8 photo-polymer both as a bonding material and as a delamination material for layer transfer. SU-8 was spun onto an optically transparent handle wafer (e.g., quartz or glass) and then bonded to the GaN-on-sapphire donor wafer. The pre-exposure bake (PEB) temperature has an effect on the Tg or the maximum working temperature of the resist. In this case, 200-300 C PEB was chosen to achieve good SU-8 bonding above 160 C. UV curing of the SU-8 through the backside of the handle wafer could be performed after PEB to yield higher bonding strength at low temperatures. However, the bonding strength of UV-cured samples degraded very quickly at temperatures above 170-200 C. This was expected to be a result of crosslinking, which prevented the relaxation of the resist from residue internal thermal stresses. Laser lift-off of the GaN from the sapphire substrate was then performed with a 38 ns-KrF laser pulse (248 nm wavelength) transmitted through the backside of the sapphire with a fluence of 500-600 mJ/cm2, yielding a GaN/SU-8/silica glass final structure. For the double-transfer experiments, the GaN/SU-8/silica glass structure obtained was then bonded to a Si substrate using the Pd/In transient-liquid-phase method. Laser ablation or heat treatment above Tg of the UV-uncured resist was used to separate the GaN film from the silica-glass substrate.
Recent developments in layer transfer processes for multi-material integration relies greatly on the “paste-and-cut” approach. Exemplified in both thermal ion-cut , mechanical ion-cut , and ELTRAN® processes for SOI wafers, as well as laser liftoff transfer of GaN onto Si , the “paste-and-cut” approach consists of (1) the bonding of two materials' units and (2) separating the bonded system along a prescribed layer other than the bonded interface. Our work investigates a low temperature layer transfer process using a mechanical ion-cut process consisting of direct wafer bonding (Si/Si and Si/SiO2) and crack-initiated separation at the ion implantation region. Experimentally measured strengths of bonded Si/Si and Si/SiO2 interfaces, and of the hydrogen-implanted cut layer indicate that layer transfer occurs when the strength of bonding interface is greater than that of the implanted layer. In summary, the mechanical ion-cut renders three modes of separation depending on relative interface strengths. The schematics illustration of the three modes and corresponding images for the Si/SiO2 system are shown in Figure 1. The process temperature for partial mechanical transfer starts as low as 105°C for Si/Si and 170°C for Si/SiO2, compared to 400-600°C for the conventional thermal ion cut.
Figure 1: Schematic illustration of (a) three separation modes for an implanted Si-SiO2 pair and (b) images of corresponding separation surfaces.
One of the major challenges in realizing fluorescence-based lab-on-a-chip or micro-total analysis systems (m-TAS) by heterogeneous integration of the necessary components such as light sources, microfluidic channels, optical filters, and photodetectors, is the direct integration of the light source into the system. The ability to integrate multi-color narrow-band sources that are spatially distributed so as to operate in the near field would provide functionality on a chip that is currently only feasible with benchtop instruments.
We have developed a novel pixel-to-point transfer process to integrate GaN light-emitting diodes with photodetector chips and thin-film band-edge filters. This integration process was enabled by the double transfer technique  (Figure 1) which we previously developed for integration of GaN light-emitting diode (LED) arrays with silicon substrates. The pixel-to-point transfer process solved problems of pixel pick-up from the source wafer and pixel registration to the target system. The transfer was accomplished by (1) temporarily bonding the LED pixel to a specially designed pick-up rod with sapphire substrates facing upward using Super Glue®, (2) removing the sapphire substrates using laser lift-off, and (3) permanently bonding the LED pixel to the designated area in the pre-fabricated silicon photodiodes using Pd-In transient-liquid-phase bonding.
Using the pixel-to-point transfer process, we are now fabricating a prototype light-source/detector chip based on fluorescence devices to evaluate the performance of the integrated biochips. A GaN LED with peak emission at 463 nm will be used to excite 515 nm fluorescence from FluoSpheres® carboxylate-modified yellow-green fluorescent microspheres (40 nm in diameter).
Figure 1: General scheme of double transfer process