Nondestructive Characterization of Fusion and Plasma Activated Wafer Bonding Using Mesa and Recess Structures

Abstract

We present two methods for characterization of wafer bonding. They are based on recess and mesa bond test structures with various shapes, measurement of unbonded regions using scanning acoustic microscopy (SAM), and image analysis. The first method maps locally the surface energy across the bonded wafers using the measured deformations around these structures and the finite element method (FEM). The FEM analysis is supported by analytical modeling. The second method uses the measured bonding probabilities of 10-19 nm deep recess bond test structures in investigation of surface interactions and in determination of the average of the surface energy at the wafer level. The present methods and proposed optimized test structures allow the evaluation of surface cleans without destructive, off-line methods such as the crack-opening method, which is employed as a reference. The methods are utilized in the investigation of the effect of O 2 and N 2 plasma activation and the dilution and temperature of Standard Clean 1 on Si/SiO 2 direct bonding. The results from both methods correlate with each other. The bond strength of the annealed wafers is observed to increase in the order 1) O 2 plasma, 2) standard SC1 at 65 • C, 3) N 2 plasma, and 4) dilute SC1 at 45 • Wafer bonding is an essential component of the semiconductor industry.1-3 It allows wafer level packaging, 3D integration of diverse devices, and manufacturing of silicon-on-insulator (SOI) wafers needed as substrates for microelectromechanical systems (MEMS) and high performance microelectronic and photonic devices.1 Perhaps the most common bonding process used, especially for SOI production, is fusion wafer bonding which usually follows the procedure:First the wafer surfaces are prepared in a cleaning treatment and then the wafers are joined together in vacuum at room temperature. The final bond strength is obtained during annealing at high temperatures. In plasma-activated wafer bonding 3,4 the wafers are pretreated in plasma. This allows high bond strengths to be obtained at lower annealing temperatures.Wet cleaning is an essential step required before fusion bonding of hydrophilic silicon wafers can succeed. The chemistry of Standard Clean 1 (SC1) is a convenient and highly effective method. Numerous early studies such as Refs. 5-8 all used SC1 as part of the standard wafer cleans before wafer bonding. There are three main objectives of the SC1 clean. Firstly, the importance of a hydrophilic surface terminated with hydroxyl groups, readily provided by a basic solution such as SC1, to bonding has long been noted by Maszara et al.,9 and elucidated in detail by Tong and Gösele.8 Secondly, the SC1 clean removes organic contaminants which are known to interfere with bonding reactions.6 Finally, the SC1 clean is very effective at removing particles from wafer surfaces as described in detail by Itano and Kezuka.10 As an additional criteria, roughening of the surface can be minimized with proper dilution of the cleaning solution.9,11 The SC1 clean meets these requirem...