Multifunctional Materials Enable Single-Layer Temporary Bonding And Debonding

Many new wafer-level packaging (WLP) technologies involve the processing of thin wafers that must be mechanically supported during the manufacturing flow. These technologies include fan-out wafer-level packaging (FOWLP), fan-in wafer-level chip-scale packaging (FI-WLCSP), 3-D FOWLP, 2.5-D integration with interposer technology, and true 3-D IC integration using through-silicon via (TSV) interconnects.

Temporary bonding is proving to be a reliable method for handling and processing thin device wafers. In the basic process, a device wafer is temporarily adhered to a rigid carrier wafer using a polymeric bonding material, whose properties largely control the stability of the entire structure during thinning and other processing. Temporary bonding/debonding (TB/DB) technologies have been widely developed and adapted for use in many WLP applications, as shown in Figure 1.

Figure 1. TB/DB can be implemented for a variety of WLP wafer and substrate applications.

Developmental advancements in material designs have resulted in the creation of multifunctional materials, which are a relatively new area for WLP. These materials have been designed to be thermally curable and to prevent material reflow of the bonding layer at higher temperatures, while still remaining wet cleanable without the use of harsh chemicals and long process times. In addition, their bonding temperature is less than 200°C, and they can be used for laser debonding at commercially available laser wavelengths.

Benefits of multifunctional materials
With the increase in processing steps and the emergence of increasingly advanced technologies, the use of multifunctional materials will become integral to the future of TB/DB. One approach to multifunctional material design incorporates adhesive and laser release attributes in one material layer. Although this is similar to a thermal release material, it has greater thermal capabilities due to its ability to be cured and undergo laser debonding.

Combining a curable adhesive and laser release layer into one material significantly reduces both processing time and the number of steps required to bond wafer pairs. Curable adhesive single-layer systems offer a number of technical benefits, including:

• Access to higher temperatures with less material flow from the curable layer;
• Strong adhesion in high-stress applications, where wafers can delaminate or spontaneously debond when using multilayer mechanically debonding systems such as FOWLP;
• Lower wafer stress and warpage due to fewer material interfaces within the bonded wafer pairs, causing less potential mismatch of materials’ coefficient of thermal expansion (CTE).

Some challenges with this concept stem from concerns regarding cleanability of a curable layer and potential laser damage to the device. Wet-cleaning a curable layer (usually highly solvent-resistant due to being crosslinked) requires harsh solvent-based solutions, which may cause damage due to their chemical content, required cleaning time, and/or high temperature requirements.

As with any material that utilizes laser release techniques, this raises concerns about damage from laser energy penetrating to the device. However, multifunctional materials address this issue in two ways: they offer high absorbance of the laser energy at all commercially available laser tool wavelengths; and they can be utilized as a thicker film since they also act as the bonding layer.

By overcoming these WLP challenges related to such parameters as cleanability, laser damage and bonding temperature, new multifunctional materials and processes will contribute to minimizing cost of ownership while helping drive future advancements.