Temporary bonding technology improves thin wafer handling
03/01/2004
The processing of compound semiconductors often requires back-thinning of the processed wafers or backside lithography. The poor mechanical properties of the wafers may result in a high ratio of wafer breakage even during substrate handling between two process steps. Compound materials such as GaAs, InP, SiC and others are expensive as basic materials and their value increases dramatically during processing. One solution to master the handling problems is reversible wafer bonding.
V. Dragoi, C. Schaefer, P. Lindner, EV Group, Schaerding, Austria
M. Wimplinger, S. Farrens, EV Group Inc., Phoenix, Arizona
Yield is one of the most important parameters for evaluation of industrial production. Various factors affect the yield for each specific process step. For some applications, the main yield-limiting factor is the substrate survival ratio after certain process steps.
Standard semiconductor processes typically start from bulk wafers (hundreds of microns thick) that are eventually thinned down during or at the end of process. In some cases, wafer thinning is a delicate step as the material may be very brittle (e.g., compound semiconductors).
A similar situation is encountered during processing of thin and ultrathin semiconductor wafers (thickness <200µm). Semiconductor devices are sometimes manufactured starting from thin substrates. Among the advantages offered by the use of thin substrates as starting material are mechanical flexibility of chips (for smart cards, chip cards, or smart labels applications), a gain in space (for stacked memory chips or ICs for mobile consumer products), a reduced package height (<100µm), or an increased functional integration (for power devices or RF devices applications).
Optoelectronic and integrated circuits rely on semiconductor materials and depend on similar manufacturing processes, but new applications pose challenges for handling device wafers.
Compound semiconductor wafers, or thin silicon wafers, generally have poor mechanical strength, making wafer handling difficult through the multistep processes involving cleaning, coating, etching, and thin-film deposition. The current trend is to use already thin wafers — tens of microns thick — as starting materials, which decrease the risk of damaging fully processed wafers by eliminating the final harsh and unclean mechanical processes (e.g., grinding and polishing).
One solution to master the handling problems is reversible wafer bonding (see Fig. 1). In this approach, the thin-silicon or brittle compound-semiconductor device wafers are bonded to a carrier substrate uniformly coated with a bonding agent. After bonding the device wafer to the carrier, further processing of the device wafer, such as backside lithography or back thinning, can easily be performed.
 Figure 1. Schematic process flow for reversible wafer bonding. |
A carrier substrate improves the mechanical strength, while an intermediate layer reliably bonds and further protects the active surface of the device wafer during grinding and polishing. This type of bonding is termed reversible because device and carrier substrates can be de-bonded after back-thinning and backside lithography have been completed. Sapphire, quartz, glass, or even Si wafers are commonly used as carrier substrates. Various materials are currently used as intermediate layers to form a reversible bond between device wafer and carrier wafer, including spin-on adhesives, laminated dry film, and laminated or spun-on wax. The typical total thickness variation (TTV) values for the wafer stack after applying the methods mentioned has to be <10µm.
Reversible wafer bonding using high-temperature wax with good TTV requires spin-coating of the layer in liquid phase. To accomplish this, wax is dispensed at elevated temperatures onto the wafer surface using customized heated dispense lines, special dispense pumps, and heated spinner chucks. The target thickness uniformity has to be better than ±4%. The graph in Fig. 2 shows the layer thickness uniformity measured on a 150mm diameter GaAs wafer, wax coated for the temporary bonding process. The uniformity improves further during the bonding process. After coating, the wafer is loaded to a bond chamber. The bond chuck inside the bond chamber is also heated.
 Figure 2. Wax layer-thickness uniformity measured in 15 points along the diameter of a 150mm GaAs wafer. |
After mechanical alignment, a light force is uniformly applied for better bonding results. When an interlayer of wax is used, the bond strength has to be great enough to withstand harsh mechanical processes like grinding and chemical mechanical polishing (CMP).
The choice of the wax type is predicated on the maximum temperature that can occur during the entire process flow. De-bonding can be performed by heating in proprietary de-bonder setups or by dissolving the wax layer in a solvent.
Dry film adhesives for reversible bonding are usually double-sided adhesive materials: one side is bonded to the carrier, while the second remains covered with a protective film, which is released only prior to bonding to the device wafer. The process involves two bonding steps: in the first step, the adhesive film sheet is bonded to the carrier wafer, and then the device wafer is bonded to the adhesive film on the carrier wafer.
At the end of the process — and depending on the type of film that is used — de-bonding can be performed by either UV-cure or heating in a special de-bonder. After de-bonding, the device wafer can be unloaded from the de-bonder using a special end-effector, or by mounting it directly on a dicing tape that is mounted on a film frame.
The two reversible bonding procedures described fulfill the main requirements imposed by the actual applications level:
- easy accommodation of different wafers and substrates sizes to provide flexibility and use of the existing production lines,
- reliable and secure wafer handling for multistep processes (20–40 steps),
- avoidance of wafer contamination (by using "clean" materials), and
- high throughput.
Integrated systems enabling the automated processes described are already installed in production facilities.
Viorel Dragoi is chief scientist at EV Group, DI Erich Thallner Strasse 1, 4780 Schaerding, Austria; e-mail [email protected].