As we have discussed many times before, one of the important aspects of 3-D technology process flow is how you handle wafer thinning. A typical process flow for temporary bonding involves the carrier wafer and/or the device wafer being coated with an adhesive, bonding of the device and handle wafers, processing of the wafers and then removal of the carrier, hopefully without ever having to handle the thinned wafer.

In all options there are two main components: (1) the materials used as temporary adhesive; and (2) the automated equipment used to bond and debond the wafer. Material suppliers have all been trying to increase the thermal stability of their materials to allow for higher temperatures during the thinning and backside processing operations. Either glass or silicon carrier (handle) wafers can be used for the temporary bonding process. There will be a larger bow when using glass with silicon due to the inherent CTE mismatch. Resultant bowing can lead to handling difficulties.

There are three main solutions available where the equipment and the temporary adhesive are offered as a unified process. These are from EVG/Brewer, 3M and TOK. I thought we’d take a quick look at these three options.

EVG/Brewer Science

For more about the temporary bonding technology Brewer Science has been working on with EV Group, see PFTLE, “3-D IC at the WLP Conference,” Nov. 17, 2008.

The adhesive the companies are using is Brewer Science's WaferBOND HT materials. HT-250 can reportedly support processing up to 220°C without debonding. It bonds at 180-225°C and debonds at 240-260°C as described below.

Wafers are transferred to the bond chamber of the EVG 850TB, where they are bonded under vacuum at elevated temperatures. For <20 µm wafer topographies, a single spin-coating process on the device wafer is sufficient. For topographies >20 µm, spin-coating both device and carrier is recommended. For even greater topographies, spray coating has been used. This is shown in the figure below.

Following temporary bonding, the wafer stack undergoes required backside processing, which will depend on the chosen process flow. Finally, the thinned device wafer is debonded from the carrier wafer.

A thermo-mechanical “slide-off” technique is used to debond the device wafer from the handle. The wafer stack in the debonding module is held on both sides by a compliant vacuum chucking system. This protects the back side of the device wafer while it is heated to the softening point of the temporary adhesive. When debonding temperature is reached, the two wafers are slid apart. Debond for HT-250 is carried out at temperatures of 180-220°C.

After separation, the thin device wafer is transferred to a cleaning chamber. Solvent is dispensed on the device wafer. As the polymer dissolves, the wafer is sprayed with additional solvent to further cleaning. An end-effector supports the wafer throughout until it is transferred to the film-frame carrier or other output formats. The figure below shows the overall process flow, starting with wafer stack debonding, cleaning and unloading the thin wafer into output formats.

3M

The 3M wafer support system process flow is shown below. The system offers both the equipment and consumables necessary for mounting and demounting the device wafer.

In the 3M system, a glass plate is used to support the wafer through the backgrinding process. A UV-curable liquid adhesive is used as the bonding agent between the wafer and the glass plate. After the backgrinding process, the thinned wafer is transferred onto dicing tape, and the support glass is removed by laser debonding of the adhesive-glass interface, using a light-to-heat conversion (LTHC) layer (shown below). The adhesive can then be removed from the wafer, leaving behind less residue than seen with typical backgrinding tapes, demounting and removing adhesive from the wafer.

The 3M system provides a rigid, uniform support surface to minimize stress on the wafer as silicon is removed. For wafer thicknesses of 50-100 µm, faster grinding speeds and higher pressures can be used, resulting in shorter process times. The adhesive also protects the wafer edges so there is less chipping.

Tokyo Ohka Kogyo (TOK)

TOK is a major Japanese player in materials and equipment for microelectronics. TOK has also come out with equipment and materials for temporary bonding for 3-D TSV technology. They supply the bond and debond process equipment as well as the support plate (temporary substrate) and the temporary adhesive. TOK says that its adhesive, TZNR-A, is stable for 4+ hr at 240°C. Debonding is achieved by adhesive dissolution through the perforated support plate. Their full Zero Newton process flow is shown in the figures below.

The next few blogs will be covering presentations and news from the IEEE/ECA Electronic Component Technology Conference (ECTC) being held in San Diego next week.

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