Wafer Bonding: Key Enabling Technology for Advanced Semi Manufacturing

Historically developed for MEMS and SOI substrates, the wafer bonding technology is today becoming a key processing technology for a wide range of applications: MEMS, CMOS image sensors, LEDs, power devices, RF and advanced packaging, according to latest report from Yole Développement.

The wafer bonding market is a very complex one crossing different wafer sizes (from 2’’ to 12’’), different applications, and different bonding technologies such as adhesive, anodic, fusion, direct Oxide, eutectic, glass frit, and metal diffusion.

Wafer bonding is usually defined as a process that temporarily or permanently joins two wafers or substrates using a suitable process. Initially developed for MEMS and then SOI wafers, wafer bonding technology has shifted to non-mainstream IC applications over the last years.

“MEMS has been the first application where wafer bonders have been massively used. The wafer bonding step is mostly used to protect the MEMS sensitive element. And CMOS image sensors is also a very promising application for wafer bonders,” explained Dr. Eric Mounier, Project Manager at Yole Développement. Indeed, up to two different wafer bonding steps can be necessary for next-generation CMOS image sensors: one for back-side illumination (BSI) and the second for WLCSP.

But besides MEMS and CIS, wafer bonders can be also used for LEDs or power devices. Indeed, in a typical LED active region, spontaneous emission scatters photons in all directions. If the substrate material has a smaller band gap than the active region, approximately half of the light is absorbed in the substrate; significantly reducing device performance. So, one of the manufacturing solutions for photon loss involves bonding a wafer containing an array of devices to another wafer that provides both a reflective surface for maximum light extraction and a heat sink for thermal management. And of course, over the five past years, much attention has been given to this technology for 3D integration of memories for example.

Technology Trends
For MEMS, there is today a shift from glass frit for eutectic/metal-based bonding mainly to increase real estate by smaller bond frames. Metal direct bonding also gives good hermeticity and mechanical stability for many MEMS applications. For example, Nasiri process is using eutectic bonding of the MEMS directly on the aluminum layer of the CMOS wafer. This leads to smaller package footprints and package heights. STMicroelectronics’ latest 3-axis accelerometer (LIS3DH) also shows a different sealing technique compared to what is usually done: gold eutectic sealing allows a dramatic die size reduction.

For CMOS image sensors, the advent of the BSI technology has raised a competition between molecular bonding and adhesive bonding. Here, cost and final application will drive the technology final choice.

Yole Développement has estimated the wafer bonder to have big market growth for the next year. The growth will be driven small size wafer for LEDs and 12” wafer for 3D stacking and CIS. Although EV Group is a market leader in permanent bonding, the growth of the bonding equipment market is attracting challengers.

Yole Développement