Optical Pyrometry Begins to Fulfill its Promise
Process chamber design and electronics developments are fulfilling optical pyrometry
Alexander E. Braun, Associate Editor -- Semiconductor International, 3/1/1998
Fiber optics-based pyrometry, which uses a fiber or a quartz or sapphire rod to measure temperature utilizing the light emitted by the hot wafer, is gaining acceptance in most processes taking place above room temperature. Wafer temperature is the most important process variable for many of today's production processes. This includes chemical vapor deposition (CVD), physical vapor deposition (PVD), rapid-thermal processing (RTP) and some etch processes. The move to 200 and now 300 mm demands accurate temperature measurement, particularly since these wafers are processed singly, while the smaller geometries going on these wafers require that temperature be measured in situ. One of the best, albeit not the simplest, ways to do this is by inserting a crystal rod or fiber in the process chamber to collect the hot wafer's emitted light.All single-wafer process tools have some kind of in situ optical temperature measurement system. This was unheard of two years ago, when there was considerable resistance to adopting the technology. This was well-founded. To make an optical pyrometer measurement, it is best to know the wafer's emissivity, and determining this posed problems. However, if the wafers are similar -- even if emissivity is unknown -- it is possible to get reproducible measurements without knowing the absolute temperature.
Optical pyrometry's implementation has been delayed because of the significant wafer emissivity problem in PVD, CVD and RTP; and optical pyrometry does not lend itself to retrofits. It is nearly impossible to install a fiber in an existing furnace and make it work. However, when the furnace is designed for it, the end result is superior. Even if optical pyrometry's results are not exceptional, it is still desirable, since the alternative is to operate without temperature information. As a user put it, "It is the difference between having very poor vision and being blind."
| Optical pyrometry's vision is not great. Many of the tools shipped in $1 million deposition systems provide a measurement capability of between ±3°C and ±15°C accuracy. Although not ideal, it is orders of magnitude better than having no knowledge about what the temperature is. Currently, a ±3°C accuracy is state-of-the-art, but this is improving. An obstacle to better accuracy is wafer emissivity, which is minimized by the chamber geometry. Background "noise" is another; that is, light originating from somewhere other than the hot wafer. This is a considerable problem for plasma systems, where the plasma generates a large amount of stray light, and in RTP. Yet another issue is wafer transmission, where the wafer becomes semitransparent, and the measurement system looks through it as well as at it. The latter is a low-temperature (100-500°C) measurement problem. |  Fiber optics-based pyrometry systems use a fiber or a quartz or sapphire rod to measure temperature using a hot wafer's emitted light. |
Fiber optic pyrometry field calibration is problematic. Calibration can be thrown off when a system is installed. Dirty optics is another issue -- it is difficult to clean the system inside a process chamber. Companies like Applied Materials (Santa Clara, Calif.), Luxtron (Santa Clara, Calif.) and Novellus (San Jose, Calif.) are exerting considerable efforts to solve field calibration problems, and major improvements will result.
Using an integrating amplifier, it will be possible to process smaller signals. Less light will be needed to make measurements, permitting systems to track temperatures down to 200°C instead of 400°C, today's bottom end. That will open up new processes -- some plasma-assisted CVDs will have better temperature measurement, and more etch processes will be monitored. PVD processing will be better tracked, an important capability as geometries and thermal budgets shrink.
As electronics get smaller and the amplifier and signal processing system are miniaturized further, the collection optics will be directly connected to the electronics. As these improved systems begin coming on-line this year, optical pyrometry will be well on its way to becoming the standard for in situ wafer processing temperature measurement.
