ROB SIMPSON, JOE DANKO, and MICHAEL WATTS
For some time, darkfield wafer inspection has been recognized as a powerful tool in maximizing yields, particularly for product lines that require chemical mechanical planarization (CMP) during fabrication. Now, advances in laser technology have improved the practical implementation of this testing method, allowing it to keep pace with the demands of state-of-the-art wafer processing.
Initially, this improvement was a result of the general evolution of visible laser technology to all-solid-state platforms. More recently, as laser manufacturers such as Spectra-Physics Lasers (Mountain View, CA) recognized the importance of this application, they have worked with system designers to develop laser sources whose performance and reliability are tailored precisely to the needs of wafer inspection. With increased reliability and high power capacity, advances in laser technology have enabled Inspex (Billerica, MA) to remedy a deficiency in the field and launch a fast on-line patterned wafer-inspection system.
Darkfield wafer inspection is particularly well suited to looking for submicron defects and particulates at various points in the manufacturing cycle, such as on patterned or coated wafers. This has become an increasingly important need, in part because of the widespread use of CMP at several stages during wafer fabrication. Chemical mechanical planarization was developed to replace spun-on glass techology, because the move toward increased vertical stacking of levels warranted higher levels of planarization.
During CMP, wafers are planarized using a polishing pad and an abrasive slurry. The technique can be performed after four or more of the typical fabrication processes, including deposition of oxide, polysilicon, and certain metal layers. Unfortunately, if these CMP processes are not tightly monitored, they can induce surface defects such as microscratches and chatter marks. Also, inadequate cleaning may leave small abrasive particles behind on the surface. These tiny polishing scratches and particulates are extremely difficult to detect using conventional brightfield microscopy, but can be observed with a well-designed darkfield system.
In darkfield inspection, a CCD camera and detection optics observe a small area of the wafer surface illuminated by a collimated laser beam. The detection optics and CCD are set up at normal incidence, and the laser illuminates from a larger incident angle. Thus, if the wafer surface is perfectly smooth, none of the directly reflected laser light reaches the camera, hence the name darkfield imaging
However, if the surface contains imperfections or particles, these will scatter light into the detection optics and show up as bright features against a dark background (see figure). The entire wafer surface is inspected by automated translation of the wafer, which is mounted on a motion stage.
Of course, the surface-pattern features themselves will also scatter/diffract light into the detection optics. Darkfield inspection can nonetheless differentiate defects from regular features using several techniques. For example, in a high-density memory device, optical spatial filtering can significantly reduce scatter from repetitive patterns in dense surface features. Foremost, however, is the redirection of the strong diffraction orders, originating from these features, away from the CCD. In high-performance systems, this is accomplished via an adjustment of the laser beam's incident angle and a rotation of its plane of incidence. In general, scatter is very sensitive to small changes in the angle of illumination. For a specific product, at a given process level, there is generally an optimum illumination approach that will maximize the scatter ratio of signal (from defects, scratches, or particulates, for example) to background features. Moreover, the technique is equally applicable to both high-density memory and logic devices.
Impact of laser performance
The original illumination source for this application was the helium-neon laser, which produces a low-power (20-30 mW) red beam. To support on-line inspection of every wafer lot, faster systems with shorter signal-integration times were required. This, in turn, necessitated higher-intensity scatter signals, generated in part with diode-pumped solid-state lasers, which can easily produce several hundred milliwatts of output power and have the added advantage of a shorter emission wavelength. These are two-stage lasers in which a high-power diode laser provides the pumping power for a solid-state laser crystal. Early versions of these lasers offered lifetimes of only a few thousand hours, in part because of epoxy adhesive that secured internal components but outgassed onto critical optics and forced a choice between gas lasers with long laser life and slow wafer inspection, or faster inspection with solid-state lasers but more frequent downtime.
The trade-off stemmed from the fact that scatter in general is extremely sensitive to even the slightest change in the angle of illumination. It also limited the performance and cost-effectiveness of compact darkfield systems that enabled optimization of defect detection by adjusting the angle and plane of incidence. For multiple inspection tools to yield the same results on any given wafer, a high alignment tolerance must be maintained on the illumination optics, which is compounded in systems that offer illumination-angle adjustment because the optics are more complex than the simple optics in fixed angle inspection systems.
By partnering together, laser manufacturers and producers of inspection systems have now eliminated this issue by increasing laser lifetime as well as significantly reducing the mean time to repair. To increase lifetime, these all-solid-state lasers have been redesigned, with primary emphasis on long-term reliability rather than on the more-esoteric performance parameters needed for other laser applications. The result has been a new family of rugged, epoxy-free, completely sealed lasers. Ongoing, indefinite life tests at Spectra-Physics Lasers are approaching 20,000 hours with no dropouts. To date, none of the lasers, introduced in 1997, have failed in the field.
Detailed analysis of these long-life lasers indicated that a leading cause of long-term failure is diode-laser deterioration. In the latest lasers, these diodes are now operated at a significantly derated power level to maximize their lifetime. Furthermore, the lasers were designed so that these diode-laser modules were physically remote from the main laser head. The diode output is coupled into the head using simple fiberoptics, which allows the diode-laser module to be field-replaced without affecting the pointing direction of the output beam from the main laser head. This eliminates the need to realign the illumination optics and has reduced downtime for this repair from up to two days to less than a half-hour.
Rob Simpson and Joe Danko are with Inspex/Hamamatsu, 47 Manning Rd, Billerica, MA 01821; [email protected] and [email protected]. Michael Watts is with Spectra-Physics Lasers, 1330 Terra Bella Ave., Mountain View, CA, 94039-7013; [email protected].