In-line Infrared Metrology for High-Volume Temporary Bonding Applications
By Markus Wimplinger, Daniel Burgstaller, Jürgen Burggraf, Thomas Wagenleitner, Thorsten Matthias and Paul Lindner [EV Group]
With end applications demanding ever higher performance of microelectronic devices while reducing power consumption and package size, the industry is preparing for the market introduction of 3D stacked devices based on through-silicon via (TSV) technology. The ability to reliably handle and process wafers with a thickness significantly less than 100µm plays a key role in enabling low-cost, high-volume manufacturing of TSVs with acceptable yield. Consensus has developed on the use of temporary bonding/debonding technology as the solution of choice for reliably handling thin wafers through backside processing steps. While the majority of device manufacturing steps on the wafer's front side are completed with the wafer still at full thickness, the wafer will be temporarily bonded onto a rigid carrier before thinning and processing of backside features.
Once the wafer reaches this temporary bonding step, it already represents a significant value, as it typically has gone through the entire front-end-of-line (FEOL) processing and may have seen numerous back-end-of-line (BEOL) processing steps. For this reason, inspection of wafers prior to non-reworkable process steps is of great interest. Within the context of temporary bonding, this consideration calls for in-line metrology prior to wafer thinning.
The ability to reliably thin wafers to an accurate thickness is extremely important for 3D TSV processing, as the thickness and thickness uniformity have a direct impact on processes related to forming the TSV interconnects, e.g., the via reveal process (for a via-middle process flow) or the via formation process (for a via-last process flow). Currently, state-of-the-art equipment and processes can achieve total thickness variation (TTV) values ranging from 2µm to 5µm, depending on wafer parameters and process conditions, with roadmaps demanding an improvement to values ranging from 1µm to 3µm for the same set of conditions. With decreasing wafer thickness, maintaining low TTV values will receive an even greater focus.
As the TTV of the thinned wafer will be directly influenced during grinding by the thickness and thickness uniformity of the support structure (consisting of the rigid carrier wafer and the adhesive bondline), inspection of those parameters prior to wafer thinning is of great interest. Defects and unbonded areas in the bondline are also cause for concern as they pose a high risk of wafer damage during downstream processing, as well as particle contamination of process equipment from the wafer damage.
Maintaining highest yield levels will be a key area of focus, as throughput of temporary bonding/debonding systems is improved in an effort to meet cost-of-ownership (CoO) targets. The cost of yield loss for such systems is plotted in Figure 1: for an assumed high-volume production environment with a system operating 350 days a year for an average of 20 hours per day at varying throughputs, the yield loss rate is 1/10,000. It is obvious that the cost of yield loss can easily reach six digits for high-value wafers. Tighter design rules, in terms of thin-wafer TTV, will likely increase the need for sophisticated in-line monitoring as a key element for keeping yield rates high and cost of yield loss low.
Figure 1: Value of yield loss per year for 1/10,000 yield loss rate
Those considerations have led to the development of a novel inspection module designed to enable installation in-line in the EVG 850TB platform. The module allows for measuring carrier wafer thickness and TTV, adhesive thickness and TTV, and bonded stack thickness and TTV, as well as detecting bonding voids in one single measurement run. The thickness data is collected using non-contact infrared (IR) and white-light (WL) interferometric sensors, making the solution fully compatible with both glass and silicon carriers. A unique stage allowing for high-speed scanning enables an inspection module throughput that supports 100% inspection of all processed wafers without compromising throughput of the automated temporary bonding system.
Figure 2 shows the general setup of the metrology solution. The wafer stack is firmly affixed to a chuck, with the carrier wafer facing the sensors. An IR interferometer detects the TTV and thickness data for both the carrier wafer and the adhesive bondline. Optionally, a white-light interferometer measures the stack height. To collect the information for the entire wafer surface, the sensors scan across the entire wafer surface with a recipe programmable resolution.
Figure 2: Generic setup of the in-line metrology system
An inherent characteristic of IR interferometer measurement is that the measured thickness value at any given point for any given layer will depend on the refractive index of the material creating the layer. This fact is advantageous in detecting bonding voids, as the refractive index of the air or vacuum inside the void will significantly differ from the refractive index of the adhesive. Therefore, when scanning across a bonding void, the thickness profile will show a steep slope between the bonded and the void area, enabling automatic voids detection. The data collected for each wafer can be visualized in various formats. One example is shown in Figure 3, which represents an image that shows the thickness values of the adhesive layer across the wafer as different color grades. The simulated bonding voids that were intentionally created in this test wafer are clearly visible.
Figure 3: Screen capture showing thickness values of the adhesive layer across the wafer.
Sophisticated software algorithms allow for fully automatic classification of the metrology data and automated, real-time pass/fail decision making. This allows for detecting process excursions in a very timely manner, thus greatly reducing the amount of material-in-process/material-at-risk. Furthermore, the data will assist in monitoring trends in process results, triggering corrective actions before any process drifts to an out-of-spec performance.
Wafers that have been marked as "fail" by the in-line metrology solution can subsequently be reviewed in detail offline using a dedicated defect review system, where the final pass/fail decision may be made by an operator. For this defect review process, the same sensor technology available for the in-line metrology system can be selected as an add-on to the EVG40NT inspection system platform.
Wafers that don't meet acceptance criteria following inspection may be reworked prior to thinning by debonding them off the carrier wafer and repeating temporary bonding, thus eliminating any yield loss due to incorrect thickness and TTV values of the carrier solution.