Harold M. Anderson
Department of Chemical and Nuclear Engineering
University of New Mexico
209 Ferris Engineering Center
Albuquerque, NM 87131 USA
Wavelength-modulated, infrared diode laser absorption spectroscopy (IRLAS) and full-spectrum CCD-based optical emission spectroscopy (OES) are two powerful new tools for plasma diagnostics. This presentation focuses on the use of these diagnostics in plasma etch processing used for semiconductor manufacturing. In particular, IRLAS is finding extensive use in measuring absolute concentrations of gas phase radicals formed in the oxide etch process performed with fluorocarbon high-density plasma (HDP) discharges. Knowledge of the absolute concentrations of reactant species (CFx) and product species (SiFx and COFx) formed in trace amounts (1011-1014 cm-3) during etching of oxide substrates is important for predicting the behavior of this complex plasma-surface interaction. These measurements are a critical element in a SEMATECH sponsored program for plasma etch model development, and are intended for validating computer simulation models of this complex process. Full-spectrum OES on the other hand is a more mature technology that is finding application in day-to-day semiconductor fab plasma processing operations like end-point and fault detection. With the development of CCD sensors and micro-machined diffraction gratings, it has been possible to build miniaturized spectrometers mounted on computer boards that allow rapid acquisition of the complete uv-visible optical emission spectrum of the plasma. Chemometric multivariate statistical analysis of this full-spectrum database in real time is made possible by integrating the spectrometer with a fast portable computer. In combination, these techniques have been shown to provide superior sensitivity for detecting process end-point in real-time and information for process fault detection. This capability is becoming more important to the semiconductor industry as the critical dimensions of the chip are approaching the extreme sub-micron (~0.1 mm) range and the exposed area of the thin film to be etched is dropping to under one-half percent of the wafer surface.
Figure 1. CF and CF2 assignments in IRLAS signal from 10 mTorr C2F6 discharge.
The discussion of IRLAS focuses on the ability to detect trace amounts of radical species important to the oxide etch process with only a single or double pass using frequency modulation second derivative (or 2f) spectroscopy. This aspect of the diagnostic tool design is critical since any plasma diagnostic must be non-perturbing to the plasma and be accomplished without radically altering the commercial etch tool chamber configuration. At best, commercial etch tools provide only one to two small diagnostic windows for viewing the plasma during operation. Practically speaking then, this limits the type of optical diagnostic data acquisition to emission or line-of-sight absorption. Also, since space in a semiconductor clean room is extremely expensive, the size or foot-print of the diagnostic tool must be compact. Solid-state diode lasers operating in the mid-IR range (3-12 mm) meet all these requirements and provide radical detection sensitivity down to number densities in the 1010-1011 cm-3 range given a few passes through the plasma chamber (30-40 cm diameter) and 2f derivative spectroscopy. An example spectrum is shown in Figure 1. The diode laser in the system employed in this study is cooled in a compact liquid nitrogen dewar mounted on a compact portable optical board. Commercially available temperature and current controllers along with a lock-in amplifier, oscilloscope and PC computer are all that are needed for data acquisition. Although not suited for routine day-to-day use in a semiconductor fab, this type of diagnostic tool has been used extensively for diagnostics studies of plasmas in semiconductor research facilities such as those found at SEMATECH, etch tool manufacturer development facilities and universities.
Much of this portion of the presentation will focus on diagnostics needed for improving the performance of next generation, inductively coupled, low pressure HDP discharges that are being manufactured to meet the coming need for extreme sub-micron etching. The reactors typically operate with pressures in the 5-10 mTorr range and generate plasmas with densities in the high 1011 cm-3 range. Radicals, formed by electron impact dissociation, drive the important surface reactions even though their number densities are only in the 1012-1014 cm-3 range. The CFx radicals used in oxide etching are extremely important to the process since they provide for the selectivity of the etch (i.e. the ability to stop etching the silicon underlying oxide film). This is one of the most critical etch steps in semiconductor manufacturing and is used to establish the contacts to IC devices. A thin (2-3 nm) CFx polymeric deposited film is shown to be crucial for regulating the etch during processing. Monitoring the concentrations of CFx radicals is in turn shown to be important to controlling the amount of polymer deposition.
Figure 2. TEOS-Nitride contact etch and the corresponding endpoint trace (note first small hump Trace is the Via endpoint of the process).
The OES sensor discussion focuses on the advantages of full spectrum CCD-based optical emission spectroscopy (OES) over monochromator- based systems and the design of a compact computer integrated sensor known as the EP-2000 system now manufactured by Cetac Technologies. Traditionally, monochromator based systems have been used to determine endpoint by monitoring one or two strongly emitting wavelengths. For exposed open areas of <1.0%, a more sensitive approach is required for the next generation of chips. CCD array detector based systems can provide a wealth of spectral information from a variety of potentially useful gas phase emitting species. In the case of particularly challenging applications such as reverse mask shallow trench isolation (STI) and contact etches, utilization of the full optical emission spectrum has been shown to provide tangible benefits. Production facility results regarding these and other demanding applications will be presented. The talk will largely focus on oxide etching in AMAT MXP and AMAT HDP platforms. Evolving Window Factor Analysis (EWFA) and Multiple Curve Resolution (MCR) are the principal multivariate techniques used in the analysis. They allow one to dynamically track the principal components of the oxide etch process. An example end-point signal for a Teos Nitride etch step is shown in Figure 2. EWFA is also shown to useful for automatic fault detection. MCR is used to depict the dynamic rate of formation (or depletion) of the principal chemical species in the plasma during the etch. By integrating the sensor within a computer and coupling the sensor to the plasma emission via optical fibers, one is able to collect and statistically analyze the full-spectrum data in a 1-2 hundred milliseconds. This allows for sampling at 5-10 Hz rate, which in terms of the oxide etch process is effectively real time. The compactness, ease of use, reliability and robustness of this type of optical sensor makes it extremely valuable to the semiconductor industry for day-to-day process control. Units are now in use at IBM, Motorola and other semiconductor manufacturing facilities.
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