Oxide charges induced by plasma activation for wafer bonding K. Schjùlberg-Henriksen a,* , M.M. Visser Taklo b , A. Hanneborg a , G.U. Jensen b a Physics Department, University of Oslo, P.O. Box 1048, Blindern, 0316 Oslo, Norway b SINTEF Electronics and Cybernetics, Forskningsveien 1, P.O. Box 124, Blindern, 0314 Oslo, Norway Received 24 April 2002; received in revised form 22 August 2002; accepted 9 September 2002 Abstract Plasma activated wafer bonding is a low-temperature process for joining similar and dissimilar materials. We have measured oxide degradation caused by a plasma activation process intended for bonding of silicon wafers. The ®xed oxide charge increased by 2:9  10 10 cm 2 and the interface trap density saturated at 7:2  10 10 cm 2 eV 1 after plasma activation, independent of gate geometry. The increase in interface trap density was reversed by a forming gas anneal. Bonding experiments were performed with wafers that were subjected to forming gas anneal between plasma activation and wafer mating. We measured a fracture surface energy of 0.3 Jm 2 , compared to 0.7 Jm 2 for samples that had not been subjected to the anneal. Pull tests yielded a lower limit for the mean bond strength in both set of samples in the 3±5 MPa range. Contact angle measurements showed the plasma activated wafers to be slightly less hydrophilic after forming gas anneal, but still much more hydrophilic than before plasma activation. Our results indicate that Q f and D it may be reduced without eliminating the bonding capability of the surface. With appropriate process optimisation, strong plasma activated wafer bonding without oxide damage can be feasible. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Plasma; Wafer bonding; Oxide charge; Bond strength; Contact angle 1. Introduction Plasma activated bonding (PAB) is a chemical free room- temperature process for wafer joining. With PAB, high bond strengths can be obtained without the 1000 8C anneal required for conventional fusion bonding [1]. Moreover, the bonded interfaces have been shown to form hermetic seals [2]. Successful PAB has been reported using O 2 ,N 2 , NH 3 and Ar plasmas [3,4], but the bonding mechanism is under debate and subject to substantial research [5±9]. Nevertheless, plasma activated bonding is already exploited commercially in the fabrication of silicon-on-insulator (SOI) wafers by the Genesis Process TM [10]. Recent research demo- nstrates that the process is a promising method for bonding of dissimilar materials [11]. The process is also interesting for bonding wafers with temperature-sensitive structures, like diffused layers and metal patterns which can not sustain the high annealing temperatures of fusion bonding. If PAB is to be applicable for low-temperature wafer-level packaging of microsystems, damage caused by the plasma activation must be avoided. The question of plasma-induced damage is particularly important for silicon dioxide, like the gate oxide of metal-oxide-semiconductor (MOS) devices. Bjeletich et al. [12] found that their plasma activation produced an increase in oxide charge. Moreover, they found indications that the plasma activation applied during SOI wafer fabrication was deleterious to BJT and PMOS tran- sistors manufactured on those SOI wafers. The objective of our work was to assess the damage induced in the oxide by a standard plasma activation process. Furthermore, we investigated if the damage could be repaired by a process modi®cation that would not eliminate the bonding capability of the wafers. We found a forming gas annealing process which showed promising results when performed between plasma activation and wafer mating. The fracture surface energy was reduced by 50% in the forming gas annealed wafers, but bonding did still take place. With appropriate process optimisation, plasma acti- vated bonding without oxide damage can be feasible. 2. Experimental MOS-capacitors were used for monitoring the oxide properties throughout the experiment. The capacitors were Sensors and Actuators A 102 (2002) 99±105 * Corresponding author. Present address: SINTEF Electronics and Cybernetics, Forskningsveien 1, P.O. Box 124, Blindern, 0314 Oslo, Norway. Tel.: 47-2206-7732; fax: 47-2206-7350. E-mail address: kshe@sintef.no (K. Schjùlberg-Henriksen). 0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0924-4247(02)00380-1