EVG® LowTemp™ ZoneBOND®

Thin wafer processing is a key technology for 3D-ICs as it enables safe and reliable processing of thin wafer on front- and backside. Temporary wafer bonding to a carrier enables the reduction of cost-of-ownership of Through Silicon Vias (TSVs) manufacturing for several reasons. Reduced wafer thickness enables reduced aspect ratios of the vias, which results in shorter cycle times and less cost for via etching and plating. In addition the silicon real estate consumption for the TSVs is reduced, because a thinner wafer enables smaller via diameters.

EVG® LowTemp™ ZoneBOND® is a revolutionary breakthrough in thin wafer processing. It enables room temperature debonding method, which is independent from the properties of the temporary adhesive. Thereby it enables a standardization of the debonding process and debonding equipment as it is material independent. The EVG® ZoneBOND® Open Platform enables a versatile supply chain with multiple adhesive suppliers. Manufacturing of the carrier wafer is integrated in the high throughput EVG850 temporary bonding/debonding production platform.

In the past the debonding method, the adhesive properties and the carrier properties were closely linked to each other. A revolutionary new temporary bonding and debonding technology, ZoneBOND®, breaks this link between debonding method and adhesive properties. With ZoneBOND® technology the debonding process is not at all a function of the adhesive any more – debonding has become a function of the carrier. Figure 1 shows the principle of the ZoneBOND® carrier. The ZoneBOND® carrier has two zones, which differentiate by the degree of adhesion between the adhesive and the carrier. The adhesion in the center zone is reduced, whereas full adhesion is at work in the edge zone. It is important to note that the surface of the device wafer does not have to be treated at all for ZoneBOND®, which makes the technology compatible with any kind of surface passivation. This is especially important with regards to assembly after thin wafer processing. Debonding methods which rely on surface modifications of the device wafer have the inherent risk of causing adhesion problems with the underfill material during die bonding.

Figure 2 shows the debonding principle using a ZoneBOND® carrier. During the first step, the Edge Zone Release (EZR®), the adhesive in the edge zone is dissolved. The center zone with the reduced adhesion is now the only connection between the thin device wafer and the carrier. The device wafer is separated from the carrier during the Edge Zone Debond (EZD®) step with a pure mechanical separation at room temperature. It is important to note that the actual separation happens between the adhesive layer and the carrier wafer. This means that the debonding is totally independent from the top passivation layer of the device wafer. The debonding is also totally independent from topography in the bond interface: spherical bumps or pillars; 35µm or 80µm feature size, even for stacked dies on an interposer – as the debonding process happens at the boundary between adhesive layer and carrier the topography does not have any impact on the debonding process.  During the EZD® step the bumps in the bond interface are embedded in the adhesive layer. No vertical or shear force is applied to the bumps during debonding, which eliminates the risk of bump damage. After debonding the device wafer is cleaned in a dedicated thin wafer cleaning module. Thermoplastic adhesives are removed residue free by solvent stripping. Solvent cleaning process has the advantage that it works well with spherical bumps – no force is applied to the bumps during adhesive layer removal.

Figure 3 shows the entire debonding process flow for ZoneBOND® as it is implemented in the EVG850DB production debonding system. The bonded wafer stacks are delivered in a FOUP to the system. First the Edge Zone Release step is performed as single wafer process. As a result of the EZR process a thin free standing wafer edge is created. Putting the wafer stack after the EZR process into a FOUP can result in edge chipping. Combining EZR and EZD in one system is a necessary step for high yield debonding. Then the wafer stack is mounted on a film frame. Performing the edge zone release process prior to film frame mounting allows using dicing tapes, which are not compatible with the solvents used for EZR.  This gives foundries and OSATs full freedom of choice for the dicing tape. The EZR cycle time depends on the adhesive properties as well as on the process technology. With optimized EZR process modules cycle times of less than 10min per module have been qualified for several adhesives, which were optimized for ZoneBOND®. For high volume production the EVG850DB is configured with up to 9 process modules. After debonding the device wafer has to be cleaned. During cleaning the dicing tape is protected from exposure to the cleaning chemistry. The thin wafers on film frame are unloaded into a cassette.

The analysis of the process sequence shows that the debonding is completely independent from the adhesives properties. This opens up a complete new field for adhesive engineering. Taking away the necessity to design the debonding process into the adhesive enables a higher focus on the other success criteria for a temporary adhesive. The adhesive has to be solid at low temperature to enable backgrinding without dimples. It has to withstand the thermal and chemical exposure during backside processing. After debonding the adhesive has to be cleanable according to the requirements of a wafer fab. There must not be any residue or modification to the device wafer surface in order to allow standard underfill processes during assembly. ZoneBOND® debonding works with all kind of materials: dedicated thermal release adhesives, dedicated laser release materials as well as dedicated solvent release materials. Debonding is now a standardized process independent from the specific temporary adhesive. This standardization is a major milestone in thin wafer handling.

 

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Figure 1: ZoneBOND® carrier schematics; the carrier surface of the center zone is treated such that the adhesion is reduced.


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Figure 2: ZoneBOND® Debonding Principle: during the Edge Zone Release (EZR) step the adhesive edge zone is dissolved; the thin device wafer is separated from the carrier during the Edge Zone Debond (EZD) step.

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Figure 3: Full ZoneBOND® debonding process sequence