Processing of thin wafers becomes more and more important for a variety of different products and applications. Thin wafer handling and processing is performed by temporary bonding to a rigid carrier wafer. The rigid carrier wafer gives mechanical support during wafer thinning and backside processing. After backside processing the thin wafer is debonded from the carrier wafer and attached to a dicing tape on film frame.

EV Group provides production equipment for temporary bonding and debonding since 2001. We have the largest install base and are the clear market share leader. EV Group has more than 20 years experience in wafer bonding. Figure 1 shows the generic process flow for thin wafer processing with temporary bonding to a carrier wafer. The starting point is a device wafer with complete front-end processing on the frontside of the wafer. This device wafer is bonded to a carrier wafer with its frontside in the bond interface. After bonding the first step is back-thinning of the wafer. Usually back-thinning is a multistep process consisting of mechanical back-grinding and subsequent stress relief etching and polishing. After back-thinning the backside of the device wafer can be processed using standard wafer fab equipment. The carrier wafer gives mechanical support and stability and protects the fragile wafer edge of the thin wafer. Finally, when all backside processing is done, the wafer gets debonded, cleaned, and transferred to a film frame or to other output formats.

Temporary bonding and debonding is an enabling technology for wafer-level processing of thin wafers. The main advantages of temporary bonding and debonding using a carrier wafer are:

Compatibility with standard fab equipment
The bonded wafer stacks literally mimic a standard wafer. The geometry of the bonded stack can be tailored in such a way that the resulting geometry is in accordance with SEMI. This brings the advantage that standard wafer processing equipment can be used without any modification. There is no need for special end-effectors, wafer chucks, cassettes, or pre-aligners. No downtime at all is required to switch between processing of standard thick wafers and temporarily bonded thin wafers.

Compatibility with existing process lines
With the addition of only two pieces of equipment, the temporary bonder and the debonder, a complete process line or even fab becomes able to process thin wafers.

Compatibility with existing processes
The mechanical and thermal properties of the bonded wafer stack are very similar to a standard thick wafer. This enables the use of existing wafer processing recipes, which have been proven and qualified for standard wafers.

Compatibility with future process flows
The user has the full flexibility to change the processing sequence and the individual process steps for backside processing. After temporary bonding the device wafer is securely protected against mechanical damage. Furthermore, adding process steps or modifying the process flow does not impact the cost of ownership for thin-wafer processing.

Compatibility with product roadmaps
For many devices and products, the roadmaps lead to even thinner wafers in the future. With temporary bonding the entire backside processing becomes independent of the wafer thickness. Reducing the wafer thickness does not require any modifications or adjustments to the processing equipment.

There are different reasons for the need to go to thinner wafers.

Enabling technology
For some devices a thin die is an enabling technology, e.g., backside illuminated image sensors or stacked chips with high interconnect density. Through-silicon vias (TSVs) usually require thin dies in order to limit the real estate consumption for the TSVs. For a given aspect ratio of the via, a thinner die results in lower area consumption, therefore thin wafer processing enables small pitch and high density TSVs.

Reduced form-factor
For handheld applications such as smartphones the chips have to be smaller and smaller to fit into modern designs. TSVs enable smaller packages but require thin dies in order to keep the TSV aspect ratio in a reasonable and economical range.

Higher performance
For many devices a thin die enables higher device performance. The simplest application is cooling from the backside through the bulk of the die. A thinner die allows better heat transfer and cooling performance, which, depending on the device, enables either higher speed or higher currents and voltages.
TSVs usually result in higher bandwidth and speed, reduced power consumption and a better signal-to-noise ratio. The manifold of advantages of TSVs has been discussed in greater detail in [1].

Reliable and breakage-free processing of thin wafers allows a significant cost reduction. A lower aspect ratio of the TSVs enables higher speed and throughput of the TSV processing (etching or drilling, passivation, plating or filling, etc.).
In many cases it is necessary to process the dies on the backside prior to packaging. In these cases wafer-level processing is very attractive due to the cost advantages of wafer- vs.-die processing and due to the abundance of equipment and field-proven process technology on wafer level.

[1] Banqiu Wu et. al (edt.), 3D IC stacking technology, McGraw Hill 2011