Izentis designs and sells custom tools for bonding silicon and other MEMS substrates to carrier wafers. The main purpose of these tools is to temporarily bond wafers that have been diced into chips onto whole wafers for subsequent processing. Many tools, and in particular etching tools, require complete wafers to process. In a research environment wafers often need to be cleaved for inspection or other purposes, which makes further processing steps difficult. We have developed a bonding tool to create high quality bonds in a vacuum. We use crystalbond 555, a wax with a melting point around 54 degrees C, to bond chips to carriers. We have found that bonding the wafers on a hot plate at ambient pressure leaves voids, which can be detrimental when using a reactive-ion etcher. A vacuum bond significantly reduces the voids since all the voids are compressed when the bond is placed at ambient pressure. The resulting bonds lead to significantly higher quality etches.
The vacuum bonder is a vacuum chamber with a precision hot plate inside, and a claw to hold the sample. The carrier wafer is placed on the hot plate, and crytalbond is applied and melted. The chip is then held above the carrier by the claw while the chamber is pumped down to around 700 mTorr. Once at the desired pressure, the sample can then be pressed onto the carrier and bonded. After the bond the carrier and sample can be removed from the chamber, and the crytalbond will solidify completing the bonding process.
We can design a variety of different vacuum chambers, with hot plates and claws for different size wafers. The vacuum bonder shown here is 18 inches in diameter, and the claw is designed for chips up to 4 inches. Please contact us for more information and quotes on vacuum bonders.
100 Mhz ultrasound image of the bond for two different bonding approaches. To the left is a void-free bond done with crystalbond in a vacuum bonder. To the right is a bond done with crystalbond at ambient pressure, then placed in a belljar vacuum chamber at 200 Torr. The belljar has considerable voids in comparison to the vacuum bonder. (Image credit: Sonoscan Inc.)
100 MHz ultrasound image of the bond between the sample and the carrier. The white regions are voids which correspond to the failed etch regions. The ultrasound image was taken from the back. (Image credit: Sonoscan Inc.)
Table of temperature across various layers of different materials. The calculation is for a 100 mm wafer with 1 kW of heat, and it assumes steady state and 1-dimensional heat transfer. Bubbles or voids of air lead to very high temperature rises which precludes many etching processes. Silicon is an excellent conductor of heat and a full wafer thickness has a small temperature rise. Thin films of carbon which is similar to a lot of MEMS coatings will contribute to a temperature rise; however, it is still small compared to voids.