Laser debonding enables advanced thin wafer processing
Thomas Uhrmann, EV Group, St. Florian, Austria, and RALPH DELMDAHL, Coherent GmbH, G??ttingen, Germany
An economically viable method for delivering throughput in fab equipment.
Thin wafers represent an important technological advance in achieving power devices with higher efficiency, as well as enabling the use of through-silicon-vias (TSV), a critical tool in greater device miniaturization. However, the mechanically delicate nature of thin wafers makes it largely impossible to handle them using existing process equipment and techniques. Temporary bonding of wafers to a thick carrier has emerged as a viable method for back thinning and subsequent backside processing. The processed thin wafers are then debonded from this carrier just prior to stacking.
There are several techniques for performing this debonding, including chemical, thermal, and laser-based methods. This article reviews the basics of the laser debonding process, and some of the practical considerations related to its implementation
Figure 1 provides a schematic of the key process steps for thin wafer processing using a temporary carrier wafer. Specifically, a wafer is front-side patterned, and then bonded to a carrier substrate. The wafer is then back-thinned, and back-side processing is performed. Finally, laser light is introduced from the carrier side (which is transparent at the laser wavelength), causing debonding of the wafer from the carrier.
|Figure 1. Schematic of the key process steps for thin wafer processing using a temporary carrier wafer.|
The most important advantage of laser debonding over other techniques is that it enables the use of polyimide-based temporary adhesives that can withstand exposure to temperatures as high as 400??C. This enables the bonded assembly to successfully survive the temperature cycling experienced in steps such as dopant activation after ion implantation. In contrast, most thermally or chemically activated temporary adhesives have difficulties tolerating temperatures above 200??C.
Because of this, laser debonding is most useful for IGBT and silicon-based power devices (MOSFETs, etc.) because these often require ion implantation and activation to create back-side drain contacts. However, for CMOS device wafers, active elements are typically all on the front side, and are thus completed before bonding to the carrier wafer. Furthermore, the eutectic bumps used on CMOS wafers will reflow when exposed to high temperatures so exposure to them is avoided anyway.
Unlike thermal or chemical debonding, which utilize silicon carriers, laser-induced debonding requires the use of glass carriers. In CMOS fabs, where ionic species are particularly undesirable, glass carriers are problematic to implement. Thus, laser debonding is likely to coexist along with other methods, with each having its own market niche.
In practice, the temporary adhesive for laser debonding is most commonly spin coated on to the wafer, which is then mated with the carrier. Bonding then occurs under pressure and elevated temperature. After thinning and back-side processing, the laser-initiated detachment occurs essentially at the glass/adhesive interface. After laser debonding, the glass substrate is lifted off the thinned wafer, leaving some residual adhesive, which is then removed using solvents.
Laser process considerations
The laser debonding process being developed at EVG (Fig. 2) is based on excimer lasers operating at either 308nm or 248nm. It's important to differentiate this cold process from earlier techniques based on infrared lasers, which penetrate far into the adhesive layer (and sometimes even beyond), and cause debonding through a thermal mechanism (e.g., heating). Oxide layers are put into the assembly to absorb this infrared light, but if these are imperfect and the laser light penetrates through, it can damage wafer structures. After infrared laser debonding, residual adhesive must be physically peeled off.
|Figure 2. This high volume production tool from EVG integrates wafer handling robotics together with modules for various processes, such as cleaning, debonding and film frame mounting.|
In contrast, the ultraviolet light emitted by excimer lasers is absorbed very near the glass/adhesive interface, penetrating in just a few hundred nanometers. Thus, it leaves the thin wafer entirely unaffected. Furthermore, the ultraviolet light from the excimer laser debonds through a primarily photochemical means by directly breaking chemical bonds in the adhesive polymer. This non-thermal process breaks down the temporary adhesive at the glass/adhesive interface. Depending upon the polymeric backbone of the temporary adhesive, the precise debonding mechanism may vary. Modern laser debondable adhesives are designed in such way to have an easy and reliable debond, where the carrier wafer can be just lifted off the thinned device wafer.
There are two basic approaches for implementing excimer laser-based debonding, namely, line scanning and step-and-repeat (see Fig. 3). In line scanning, the naturally rectangular output distribution of the excimer laser is reshaped into a thin line, which is focused on to the carrier/adhesive interface. The length of this laser line is slightly greater than the wafer diameter, and the width is typically around 200 ??m, depending on the laser output power. This line is then scanned over the surface of the wafer a single time in order to produce debonding.
|Figure 3. Two basic approaches for implementing excimer laser based debonding: line scanning and step-and-repeat.|
In step-and-repeat, a homogeneous laser square or rectangular field (typically about 5 mm on each side) is projected at the carrier/adhesive interface, and an exposure is made that is sufficient to cause debonding. Then, the wafer is indexed a distance corresponding to the spot height, and the process is repeated until the entire wafer surface is covered.
The mechanical simplicity of the line scan approach more readily lends it to higher throughput. However, it also typically requires a higher power laser because the light is spread over a larger area, thus lowering the energy density. Also, away from the wafer center, much of the laser energy is wasted (since the line goes off the edge of the wafer when the line is anywhere except the very center of the wafer).
Conversely, the step-and-repeat method requires less laser power, yet is still capable of reaching up to 40 wafers/hour throughput even at low laser pulse frequency of 20Hz. The necessary laser power for step-and-repeat also depends upon the number of laser shots utilized in each exposure. However, it is quite possible to achieve debonding with a single laser shot with a relatively modest power laser.
The minimum required laser power for debonding is also very dependent upon wavelength because of absorption in the glass carrier. Specifically, a typical glass carrier might absorb about 5% of the incident laser light at 308nm, while the absorption at 248nm could be 95%. Thus, nearly 20 times more laser power would be required at 248nm to achieve the same energy density at the glass/adhesive interface as with 308nm. There are also subtle differences in the specifics of the light/adhesive interaction between the two wavelengths. However, EVG has found that both wavelengths can be successfully employed.
Laser cost characteristics
Excimer lasers have long been used for microlithography, but it is important to realize that the types of sources optimum for laser debonding are completely different from those used for microlithography, possessing lower cost, smaller size, and different pulsing characteristics
Microlithography lasers output pulses with low energy, typically in the 20mJ per pulse range and operate at relatively high repetition rates, usually between 4 to 6kHz. These characteristics are desirable because they enable very precise total dosage control (by monitoring total delivered energy and varying the total number of pulses as needed).
|Figure 4. In this film frame mounter, the wafer is secured by tape to the frame prior to debonding. This enables the thin wafer to be kept flat and safely handled after debonding has been performed.|
Operating an excimer laser at such a high repetition rate translates directly into system complexity and cost. This is because the gas volume between the laser electrodes must be shifted between each pulse. Accomplishing this at a multi-kHz repetition rate therefore requires a relatively powerful and complex blower arrangement. Just as important, excimer laser tubes can only deliver a set number of pulses before they require complete replacement. So, operating at a higher repetition rate runs through this operational lifetime more quickly, necessitating expense for both replacement parts and maintenance downtime.
In contrast, the lasers used for debonding operate in almost exactly the opposite regime. Specifically, they produce relatively high per pulse energy, typically at the 500mJ per pulse level, while operating at repetition rates of only 10 to 200Hz.
This reduced repetition rate, together with a larger internal spacing between electrodes, simplifies the construction and operation of the laser, reducing its capital cost by typically an order of magnitude. Cost of ownership is also reduced: even when combined with three-shift operation, the low pulsing rate results in total pulse counts that are so low that laser tube replacement only occurs at intervals of two to five years.
Microlithography lasers are also optically very complex since stepper systems require very narrow (i.e., extremely monochromatic) laser light. In addition, many microlithography lasers now operate at 193nm, which requires the use of much more expensive optical components than longer wavelengths. And, of course, the beam shaping and projection objective optics used for microlithography (at any wavelength) are quite costly.
Again, none of this is the case with the excimer lasers used for debonding. No line narrowing or wavelength stabilization systems are required, and the beam delivery optics used for both line scanning and step-and-repeat systems are orders of magnitude simpler and less expensive than microlithography optics. Furthermore, operation at 248nm, and especially 308nm, allows the use of much more economical optical materials (e.g. fused silica), which do not have to be replaced frequently.
These simpler debonding excimer lasers are also much physically smaller than microlithography lasers with simpler infrastructure requirements. For example, the Coherent COMPexPro excimer laser family provides 20W of output at 308nm (maximum pulse energy of 500mJ, maximum repetition rate 50Hz), measures only 1682 x 375 x 793mm, and operates from either 110 or 220V standard, single phase power.
In conclusion, laser debonding represents an economically viable method that can deliver the throughput required for fab process equipment. The characteristics of its polymeric adhesives make it particularly advantageous over other debonding techniques in the manufacture of a power device, or any other components that require exposure to high temperature during manufacture.
Thomas Uhrmann (email@example.com) is business development manager at EV Group and RALPH DELMDAHL is product marketing manager at Coherent GmbH.
Solid State Technology | Volume 56 | Issue 1 | January 2013