通过在玻璃上旋涂的低温晶圆键合

Wafer bonding techniques have been developed to integrate two lattice mismatched materials and devices.1,2 The limitation of lattice matching on crystal growth can be alleviated by adopting a wafer bonding process into the device fabrication. Wafer bonding processes can be classifified as direct or indirect bonding depending on whether a third material is used as the bonding agent or not. Successful examples of direct wafer bonding include high brightness visible light emitting diodes, in which the (AlxGa12 x)0.5 In0.5P/GaAs active layer was bonded to transparent GaP substrates,3 and long-wavelength vertical-cavity surface-emitting lasers ~ VCSELs! where 1.3 or 1.55 m m GaInAsP/InP active regions were bonded to a GaAs/AlGaAs distributed Bragg reflflector ~ DBR! grown on a GaAs substrate.4,5 Direct wafer bonding is usually performed at temperatures higher than 650 °C under uniaxial pressure. Special treatments like chemicalmechanical polishing ~ CMP! and wet etching are required to ensure the surface flflatness, cleanliness and absence of surface oxides for the two wafers to be bonded together. In the case of indirect bonding, polymer or polyimide materials have been used as bonding agents to produce hybrid, optoelectronic devices such as smart pixels.6,7 Polymer materials are easy to apply and can be cured at temperatures as low as 300 °C. However, the thermal stability of polymers could be a problem if subsequent processing requires a heat treatment at elevated temperatures.

The bonding procedure started with sample cleaning. GaAs, InP, and Si samples were cleaved into 1 cm by 1 cm pieces and then cleaned sequentially by acetone, methanol, isopropyl alcohol and de-ionized water with ultrasonic agitation. Cleaned samples were blown dry by nitrogen and baked at 125 °C to drive out surface moisture. After cleaning, the SOG solution was spin coated to each sample. Two different SOG solutions were used in this study, siloxane and silicate SOG. The spin rate was set at 1200 rpm so that SOG remained liquid on sample surface for better bonding ability. Then two SOG coated wafers were brought to face-to-face contact immediately. Attached samples were placed between two thermally conductive graphite pads and uniaxially pressurized in the direction perpendicular to sample surfaces. The bonding pressure was defifined by dividing the weight applied on the sample over the sample size. The pressure ranged from 0.1 to 0.6 kg/cm2. As clamped by pressurized graphite pads, SOG bonded samples were heated and cured in an open tube quartz furnace in a nitrogen ambient. The temperature of the furnace was ramped up from 50 to 400 °C in 2 h and stayed at 400 °C for 1 h before ramping down to room temperature. The SOG bonding interface was inspected by cross sectional transmission electron microscopy ~ XTEM! . To prepare samples for XTEM, the SOG bonded wafers were fifirst sliced by a dicing saw to 3 mm by 1 mm pieces. Then the sample slice was mechanically polished from 1 mm to a thickness around 25 m m. The thickness of the sample was further reduced by ion milling before XTEM. During the XTEM sample preparation, the SOG bonding interface experienced a strong mechanical stress due mostly to the sample dicing, lapping and polishing processes.

All samples bonded with silicate SOG disintegrated during the XTEM sample preparation in this study. They tended to separate when cut by the dicing saw. Examining the detached samples under optical microscope, we have noticed that the remaining silicate SOG fifilm on wafer surface had an irregular surface profifile and randomly distributed cracks. The main reason for the separation of bonded wafers when experiencing mechanical stresses is due to the low cracking resistance of the silicate SOG fifilm as discussed below. The cracking resistance of a SOG fifilm was found to be closely related with the fifilm shrinkage after curing. The volume of silicate SOG shrinks as much as 15%.12 The internal stress caused by fifilm shrinkage results in a low cracking resistance and a weak bonding strength for the silicate SOG fifilm. This result indicates that the silicate SOG is not suitable for wafer bonding under the bonding conditions investigated. On the other hand, the fifilm shrinkage of siloxane SOG is only 4% and thus siloxane SOG has a higher cracking resistance than the silicate SOG.12 The following discussions are focused on the siloxane SOG bonding.

Fig1

bonding pressure was applied, the ultimate thickness of the SOG fifilm was determined by the bonding pressure. The relation of siloxane SOG fifilm thickness and the applied pressure was evaluated from the XTEM results as shown in Fig. 1. The thickness of SOG fifilm reduced dramatically with increasing pressure. The SOG fifilm was as thick as 3.15 m m when the bonding pressure was only 0.1 kg/cm2. When the pressure was 0.6 kg/cm2, the bonded SOG fifilm was only 25 nm thick. Thin interfacial SOG fifilm is generally preferred because it induces less material stress and thermal resistance to the bonded device than thick SOG fifilm. Samples bonded with a pressure higher than 0.3 kg/cm2, which corresponded to SOG fifilms thinner than 500 nm, were found to be more robust. The mechanism of SOG bonding was reported as the result of chemical and physical adsorption occurring at the SOG/substrate boundary.9 If the bonding pressure was too low, the semiconductor wafer may not be able to keep an intimate contact with the SOG fifilm and the interaction between SOG and wafer reduced. Thus, a bonding pressure larger than 0.3 kg/cm2 is found necessary to ensure strong bonding strength.