Lights-out MEMS Manufacturing
09/01/2006
BY SHARI FARRENS AND JAMES HERMANOWSKI, SUSS MicroTec, Inc.
It is well-known in semiconductor manufacturing that fast process feedback leads to better process control, which results in higher yield. In-line and in-situ metrology evolved over the past several years and are now available for commercial production as MEMS companies move into foundries. Delays in process feedback due to off-line metrology reduce yields, drive product costs higher, and delay time-to-market. Integrated overlay alignment metrology for wafer-to-wafer bonding provides a pathway for “lights-out” manufacturing with full piece of mind.
Wafer-bonding processes cover a variety of approaches including anodic, direct/fusion/SOI, and thermocompression (glass frit, eutectic, metal, and adhesive), which allow for bonding a variety of material and substrate combinations. The trend in all wafer bonding manufacturing is toward tighter alignment accuracy in all application areas. Industry standard techniques involving viscous flow of interlayer materials, such as glass frit and eutectic metals, are primarily used in applications with alignment specifications >5µm. However, it is possible to extend these techniques to 2µm with proper process development and continuous monitoring. Advanced bonding techniques that eliminate viscous flow and reduce overall thermal budgets result in alignment capabilities below 1 µm. To maintain this performance in production requires accurate and immediate metrology feedback.
Approaches to Bond Alignment Metrology
Typical wafer-bonding manufacturing processes use off-line alignment metrology systems for process control. Examination of the alignment occurs during back-end-of-line processes when dicing reveals the bond interface. The first step in the metrology flow is wafer alignment, clamping into a bond fixture, and transport to the bonder. Next, temperature, force, and pressure are applied to form the permanent bond. Final alignment results are measured off-line using various in-house metrology tools. They vary according to materials and methods. If the process shifts out of control, the entire lot of wafers is scrapped since detection is delayed for several additional manufacturing steps.
 Figure 1. In-line and real time wafer bonding metrology. |
Figure 1 represents an in-situ metrology model, available on cluster equipment, that allows for pre- and post-bond monitoring of alignment accuracy and user-defined algorithms for “go” and “no-go” processing without operator intervention. This system allows for measurement of post-bond results after each processed wafer, or at a user-defined frequency within the lot. Different optics can be used for initial alignment and the post-bond metrology step. For example, the wafer-to-wafer alignment can be done using a backside alignment (BSA) scheme with visible light, while the metrology can be measured using infrared (IR) illumination.
The user establishes process control set points to accept or reject wafer pairs at each step. Marginal wafers (such as those with bad alignment marks) can be excluded from being permanently bonded via pattern recognition software. Rejected product can be re-aligned manually and continued in the process or sent to reject cassettes. Tool alarms will alert the operator whenever user intervention is required. The in-line metrology system also facilitates fast tool installations and maintenance because of immediate data collection and feedback.
In-line metrology does not affect production throughput because a single bond-alignment (BA) module can be configured to conduct initial wafer alignment and measure post-bond overlay in a multi-tasking configuration. In most cases, the aligner is not the bottleneck of the cluster platform and multi-tasking the aligner for metrology is a next obvious step toward improved yield and reduced scrap.
Enabling “Lights-out” Production
Many wafer-bonding processes require slow temperature ramps or long anneal times resulting in cycle times of 20-45 minutes per wafer. Therefore, 25 bonded pairs can take 12 hours to process. An automated cluster tool with in-line metrology allows hands-free production and computerized process control. The machine can stop production if the final process results drift out of specification. This enables the machine to run for long periods of time without human intervention, and is dubbed “lights-out” production. Process control set points established by the engineer can accept or reject wafer pairs during post-bond metrology. The wafers can be flagged or sent to a reject cassette, or the tool can be alarmed and stopped to prevent additional wafers from being introduced into the process.
Overlay Measurement Methods
Alignment schemes that allow the user to visualize the targets and align them include transparent substrate alignment (TSA), BSA, inter-substrate alignment (ISA), and IR. TSA is used when one wafer is transparent and the alignment marks from that substrate can be observed simultaneously with the opaque wafer below - a system much like standard mask alignment. BSA involves an initial image capture of the front-side alignment marks on the first wafer and a subsequent alignment of backside marks on the second wafer to stored images of the upper substrate. IR imaging is used to peer through both wafers simultaneously and is most often used during the inline metrology step (Figure 2). In the ISA method, optics are inserted between the two substrates to be aligned. The objectives image both the upper and lower alignment marks simultaneously and in real-time (Figure 3). Each method has trade-offs in terms of accuracy and compatibility with various substrate types (Table 1).
 Figure 2. IR alignment principle. High contrast is achieved by double side polish on the upper wafer and proper design of the alignment marks and materials choices. |
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 Table 1. Critical Comparison of Wafer to Wafer Bond Alignment Methods |
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Baseline Capability of the Metrology System
After bonding, the wafers’ front sides are permanently attached. TSA or BSA methods are used for post-bond metrology in any application using a transparent substrate, such as glass or sapphire, and IR is used exclusively for face-to-face alignment marks buried at the bond interface. To evaluate the reliability of an in-line metrology system, a systematic error analysis was completed for each step in the alignment process flow.
 Table 2. Effects of various target shapes and settings on target position repeatability. |
In this case, system repeatability was calculated by training a target and then allowing the system to measure its location over time, obtaining a baseline for tool capability. Different targets were then used to determine if specific targets afforded higher repeatability. Each target style was tested with the IR vs. visible metrology methods. Finally, focus and illumination settings were adjusted to extremes to determine the process window for the contrast settings. Table 2 summarizes 150 measurements for each experiment. In production the system reports and records a fitness parameter that is measured between the actual target and the trained target for success criteria.
Summary
In-line, real-time alignment metrology is an important aspect of quality, high-volume wafer-bonding production. A unique approach to wafer-bond alignment metrology allows 24/7 production with minimum intervention, and the ability to self-regulate process drifts. The metrology system uses pattern-recognition hardware, and software, compatible with a variety of target designs. The baseline repeatability of the measurement system has been measured under various conditions and found to be acceptable for a variety of applications. The 0.1µm, 3 sigma alignment measurement capability is critical for high-end aligned bonding applications approaching micron-level registry. Reproducibility in the system enhances development efforts for bonding technology development.
SHARI FARRENS, chief scientist, and JAMES HERMANOWSKI, director of technical support, may be contacted at SUSS MicroTec, Inc. 228 Suss Drive Waterbury Center, VT 05677; 802/244 5181; E-mail:[email protected], [email protected]