Surface Activation Using Remote Plasma for Hydrophilic Bonding at Elevated Temperature

Abstract

Remote plasma activation is presented here as a surface technique allowing hydrophilic wafer bonding at elevated temperatures. Prebonding remote plasma treatment resulted in a marked improvement over other techniques, increasing bonding energy for both the initial van der Waals bond and the final covalent bond. Increased bonding energy is reported over a variety of bonding and annealing conditions. Bond energies of 2.4 J/m 2 were attained for Si-Si pairs annealed at 275°C. Increased bonding energy enabled elevated-temperature ͑180°C͒ Si-quartz bonding. Results focus on plasma parameter variations of exposure time and gas species and the effect on surface energy.Wafer bonding is the preferred method used in producing siliconon-insulator ͑SOI͒ substrates for the semiconductor industry. Much work 1,2 is currently focused on combining SOI technology with strained Si. Strained SOI ͑SSOI͒ combines numerous SOI advantages, such as reduced parasitics and absence of latch-up along with the enhanced performance of strained Si. This allows us to use larger devices that exhibit deep submicrometer attributes such as higher speeds and lower power dissipation. Larger devices can be fabricated using standard SOI techniques; they are also highly reliable and have a high yield. SSOI would allow 130 nm node reliability and fabrication ease to be combined with the speeds expected from a 32 nm node. High channel mobility and isolation can be obtained with a strained silicon layer on top of an insulator or a quartz substrate. The work presented here is part of a larger research program of elevated temperature silicon-quartz bonding to produce strained silicon on quartz ͑SSOQ͒. 3,4 We are working to produce a silicon layer which is strained within its elastic, damage-free region. Our research is attempting to use elevated temperature silicon-quartz bonding to produce low-strain-level SOI. We have extensively reported 5-7 the benefits of low-level strain.Plasma activation is commonly used in wafer bonding to attain high bonding energy at low bonding and annealing temperatures. 8,9 Conventional plasma sources produce both ionized and reactive neutral species in the same chamber; remote plasma sources deliver only reactive species to the process chamber. 10, 11 We observed remote plasma pretreatment to be increasingly beneficial at longer exposure times. We report a remote plasma surface activation technique which enables higher bonding energy at elevated temperatures, for both silicon-silicon bonding as well as silicon quartz bonding, extending the practical bonding temperature range in hydrophilic wafer bonding. The plasma is generated remotely, away from the surfaces, avoiding sputtering and surface damage.Silicon-silicon wafer bonding was investigated first. Siliconsilicon wafer bonding is not hampered by debonding ͑wafer pair separation͒ due to coefficient of thermal expansion ͑CTE͒ mismatch and so this was the easiest way to monitor the hydrophilic bonding process. A range of experimental variables were determined prior to attemptin...

A Review of Hydrophilic Silicon Wafer Bonding

“…They followed this with a more detailed paper in 2007. 112 Wafers are activated by the use of O 2 , N 2 and Ar radicals created in an Astron remote RF plasma generator. As shown in this second paper the system is arranged with an electrostatic filter to remove any charged particles and allow only radicals to reach the wafer surfaces.…”

Low Temperature Wafer Bonding

“…But, this short time exposure "rule" is not completely true using different plasma generation source such as MW (Micro-wave) plasma [1]. Moreover, using remote or down stream plasma, bonding energy seems to increase with the plasma exposure time [10]. The difference in physical effect between the different plasma types could explain these behaviors but careful analysis has to be performed.…”

Room Temperature Si/Si Wafer Direct Bonding in Air

“…A 100 pim thick razor blade is inserted between the bonded pairs, and the crack propagation length is measured using a real-time infrared (IR) camera. And the bonding energy (y) is calculated based on following equation [13].…”