No abstract

A new method of exposing silicon/semiconductor wafers to a mixture of radicals is described, in which these species are generated in an oxygen-rich gas discharge confined between a concentric pair of annular mesh electrodes surrounding the wafers. This approach allows the wafer surfaces to be treated without damage from the energetic ions, strong electric fields, and high UV fluxes associated with direct treatment by exposure to gas discharge plasmas. The process is compared with direct oxygen plasma activation for its latitude with respect to treatment duration, effect on wafer surface roughness and bond strength. Wider process latitude and reduced surface roughening are obtained for treatment by radicals compared with direct plasma exposure. Comparative analysis of treated and untreated silicon surfaces by X-ray photoelectron spectroscopy indicate that traces of fluorine present on the wafer surface before treatment are removed with great efficiency by the process.

Silicon-to-silicon fusion (or direct) pre-bonding is an important enabling technology for many emerging microelectronics and MEMS technologies. A silicon-silicon direct bond can be easily formed, where the wafer surfaces are highly flat and very clean (Tong and Gosele), however for practical structured MEMS devices, wafer bow and local roughness may be compromised such that it is no longer a trivial task to achieve a direct bond. Tooling has been developed to facilitate the in situ alignment and bonding of siliconto-silicon wafers in a vacuum chamber. The rate and direction of the bond propagation are controlled, thus minimising the occurrence of non-particle related voids. The tooling system also allows wafers with ''non-ideal'' surfaces or warped profiles to be bonded, by maximising the area across which bonding occurs and providing in situ annealing. The ability to anneal the wafers while maintaining clamping force creates attractive forces high enough to overcome the mechanical repulsive forces between the wafers and maintain a permanent bond. The tooling system can also be configured to give control over the bow or residual stress in the bonded pair, a factor that is critical in multi-stack direct wafer bonding.

Wafer bonding is a widely used step in the manufacture of Microsystems, and serves several purposes: • Structural component of the MEMS device. • First level packaging. • Encapsulation of vacuum or controlled gas. In addition the technology is becoming more widely used in IC fabrication for wafer level packaging (WLP) and 3D integration. It is also widely used for the fabrication of micro fluidic structures and in the manufacture of high efficiency LED’s. Depending on the application, temperature constraints, material compatibility etc. different wafer bonding processes are available, each with their own benefits and drawbacks. This paper describes various wafer bonding processes that are applicable, not only to silicon, but other materials such as glass and quartz that are commonly used in MEMS devices. The process of selecting the most appropriate bonding process for the particular application is presented along with examples of anodic, glass frit, eutectic, direct, adhesive and thermo-compression bonding. The examples include appropriate metrology for bond strength and quality. The paper also addresses the benefits of being able to treat the wafer surfaces in-situ prior to bonding in order to improve yield and bond strength, and also discusses equipment requirements for achieving high yield wafer bonding, along with high precision alignment accuracy, good force and temperature uniformity, high wafer throughput, etc. Some common problems that can affect yield are identified and discussed. These include local temperature variations, that can occur with anodic bonding, and how to eliminate them; how to cope with materials of different thermal expansion coefficient; how best to deal with out-gassing and achieve vacuum encapsulation; and procedures for multi-stacking wafers of differing thicknesses. The presentation includes infra-red and scanning acoustic microscopy images of various bond types, plus some examples of what can go wrong if the correct manufacturing protocol is not maintained.

A tool for performing surface activation using radicals (RAD activation), followed by the subsequent, in-situ, low temperature bonding of wafers, was presented previously (1). This paper reports on the further characterization of the (RAD) tool and the importance of water vapor in the activation / bond chamber. The tool uses the diffusion of radicals across the wafer surfaces to activate those surfaces. Because the activation is performed in-situ in an aligner-bonder, the activated surfaces do not get the opportunity to adsorb any atmospheric contaminants between activation and bonding and this results in more reproducible interfaces from bond to bond. Results for various bonded wafers are presented in terms of bond quality and bond strength. These results demonstrate the importance of the wafer separation during the activation process, in order to minimize the process time, and the need for a controllable / variable source of H 2 O in the activation gas in order to maximize bond strength. .