Improvement of Wafer Bonding Processing for HB-LED with Low-Temperature-Grown Compound Semiconductors as Adhesive Materials

A wafer bonding process using low-temperaturegrown (LTG) compound semiconductor as the bonding agent has been demonstrated. The optical and electrical properties of bonded samples have been characterized. This method provides several advantages, including high optical transparent, orientation-free, and low bonding temperature, which are desirable for high-brightness LED applications. INTRODUCTION The wafer bonding technology has been extensively used for orange and red color highbrightness LED application. [1-5] In an LED structure, spontaneous emission is assumed isotropic and approximately half of the light goes toward substrate. Those photons would easily be absorbed if the substrate has a smaller bandgap than the active region. In order to achieve highbrightness LED, bottom distributed Bragg reflectors [6] or metal layers [4-5] can be incorporated in the LED structure to reflect the light back to the surface and prevent it from entering the absorbing substrate. Alternatively, a transparent-substrate LED structure, achieved by direct wafer bonding, also enhances the light extraction because of the transparent substrate, which has a higher bandgap energy and does not absorb the light emitted from the active region. [1-3] To date, using metal layer as a bottom reflector and using transparent substrate are the two most common approaches to fabricating high-brightness LEDs in high-volume manufacturing. Having a high bandgap energy of 2.2 eV, GaP is an excellent material to be used as a transparent substrate material for orange and red LEDs. [1-3] However, it is relatively difficult to integrate materials such as GaAs with GaP by using the direct wafer bonding technique, because GaP has a high melting point. It usually requires high temperatures over 750°C for atoms to interchange through the interface and hence to bond the two wafers. Consequently, this high bonding temperature process may affect the doping profile and the emission wavelength of the active region. [7] There are additional concerns with the direct wafer bonding. For example, it has been found that electrical conduction through the interface depends on the orientation and/or misalignment between bonded wafers. Non-ohmic characteristics were observed across the bonded interface with two misaligned n-type substrates (InGaP and GaP layers). [8] On the other hand, perfect alignment of two ntype wafers results in an ohmic I-V dependence through the bonded interface. EXPERIMENTS AND DISCUSSION In this study, we focus on developing a new bonding agent which can be used for fabricating high-brightness transparent-substrate LEDs. This potential bonding agent is low-temperaturegrown (LTG) compound semiconductors. By applying this new LTG material as an adhesive layer, it provides several advantages over the current direct wafer bonding technology for high-brightness LED, such as low bonding temperatures, and high optical transmission through the bonding interface. Additional advantage includes the recrystallization in the bonding layer. It is found that the recrystallized polycrystalline structure does not have a specific or preferred crystal orientation. This lack of specific orientation in the recrystallized bonding layer completely disrupts the crystal orientation and direction relationship between the two bonded substrates or wafers. As a result, the misorientation/misalignment dependence of nonohmic current conduction encountered in the conventional direct wafer bonding can be reduced or even eliminated. [8] Two main material systems: (Ga,As) and (Ga,P) were investigated in this study. The microstructure and V/III incorporation ratio of the LTG materials were studied with TEM and Auger electron spectroscopy. The crystallinity of the as-grown LTG materials, amorphous or polycrystalline, is controlled by the group V overpressure during growth. Figure 1 shows the dependence between V/III atomic ratio and phosphorous overpressure of the LTG (Ga,P). Figure 1. V/III incorporation ratio in low-temperature-grown (Ga,P) materials as a function of phosphorous overpressure during growth. With a decreasing phosphorous overpressure, less phosphorous atoms were incorporated in the film and the microstructures of (Ga,P) materials changed from amorphous to polycrystalline (dash line indicates the border between amorphous and polycrystalline). The LTG materials are used as adhesive materials to bond two GaP wafers. Before bonding, the wafers coated with LTG material were cleaved in 1cm×1cm pieces and the surfaces were cleaned by standard degreasing solvents and a short HCl dip to remove organic contamination and surface oxides. Two clean LTG material-coated surfaces were then brought together and placed in a stainless steel jig. Pressure is applied and the whole fixture is then placed in an open-tube furnace. Bonding is performed by heating the wafers in N2 ambient at different temperatures for different times. The results show that two n-type GaP substrates, each with a LTG (Ga,P) material on top were strongly bonded face-to-face at 600 °C for several hours. When using LTG (Ga,As) as the bonding agent, either amorphous or polycrystalline, two GaP substrates could be bonded at even lower temperature at 400 °C for 1hr. This annealing condition for amorphous LTG (Ga,As) bonding agents can yield sufficient bonding strength for further processing. The further decrease from 600 °C to 400 °C in bonding temperature and reduction in annealing time from several hours to one hour are significant. TEM shows that along the bonded interface recrystallization of the LTG layer has taken place during high-temperature annealing and hence helped two wafers bond together, as shown in Figure 2. Figure 2. The dark field TEM image shows that two GaP substrates were bonded with the polycrystalline LTG (Ga,As) grown at As overpressure of 3.3×10 torr. The optical light transmission and currentvoltage (I-V) characteristics of the bonded samples were also characterized. Polycrystalline LTG (Ga,As) material grown at 3.3×10 torr shows the highest light transmission ratio, compared with other LTG (Ga,As) materials. In addition, the sample bonded with such LTG (Ga,As) material of as thin as 60 Å shows light transmission of 99%, as shown in Figure 3 Figure 3. The light transmission ratio of bonded samples with different thickness of the LTG polycrystalline (Ga,As) layers (As: 3.3×10 torr) as the bonding agents. With a decreasing thickness of the bonding agent to 30 Å, more light (~99%) can transmit through the bonding interface (60Å). 0 20 40 60 80 100