Using a Compact Spectrometer to Detect Spectral Peaks in a Plasma System

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Ocean Optics has a long history of support for semiconductor process equipment suppliers that investigate new materials and take on challenges related to plasma etching and deposition, overlay control, and cleaning. We have provided many spectrometer setups for plasma monitoring applications, with optical emission spectroscopy a technique commonly utilized for endpoint detection. For example, detection of a drop-off in certain plasma species can be an indicator in etching processes that action is needed to prevent damage to the wafer.

Recently, we evaluated several newer-model Ocean Optics spectrometers for the detection of oxygen and carbon tetrafluoride (CF₄) in a plasma system commonly used for deposition and etching. The results demonstrated that the ST, SR and HR series spectrometers are all capable of performing the measurements successfully, with the high-resolution HR4 and HR6 models providing the strongest combination of spectral response and clearly defined emission peaks for these plasma species.

Note: The measurements conducted for this post were carried out by our colleague Derek Guenther, the inspiration behind the feature “What’s Derek Doing?” -- a recurring feature that spotlights some of Derek’s latest efforts in applying spectral technology to various measurement challenges. 

Experimental Setup

Using optical emission spectroscopy techniques, we conducted measurements using the ST-UV and SR4 spectrometers, loosely defined as “multipurpose” spectrometers; and the HR4 and HR6, which are high-resolution spectrometers. All spectrometers were configured with a 25 µm slit, with the integration time set for 1 second, averaging set to 1, and boxcar averaging varying from 0-1. In some scenarios, the integration time was set for 200, 400 or 600 ms and then re-scaled in post-processing.

Using a system designed for wafer batch plasma etching and plasma-enhanced chemical vapor deposition (PECVD), we monitored the oxygen and CF₄ at flow rates of 99 and 50 SCCM, respectively. (SCCM is standard cubic centimeters per minute). Oxygen can be used to clean surfaces in the chamber or combined with other gases for etching; CF₄ is used to etch materials including silicon, silicon oxide and silicon nitride.

Each spectrometer was connected to a 400 µm optical fiber and combined with the appropriate fiber feedthroughs, flanges and industrial-grade accessories for interfacing with the chamber (Figure 1). Plasmas were measured at 50, 200 and 400 Watt power settings.

Figure 1. All measurements were conducted using an industry-standard plasma tool for deposition and etching. Note the spectrometer at the bottom right of the photo.

Results

All the spectrometers performed well, with the choice of model depending on various factors, including the plasma gases of interest, optical resolution requirements and thermal stability needs (e.g., in a process environment subject to temperature changes). We recommend consulting an Ocean Optics Application Scientist for guidance in selecting the optimal combination of spectrometer and accessories for your application.

In measuring the oxygen plasma at 50W – the lowest power – HR4 and HR6 yielded the best response and cleanest (more distinct) spectral peaks. While the SR and ST had comparable response, the ST spectral data presented cleaner peaks (Figure 2).

Figure 2. The HR series spectrometers demonstrated strong performance in measuring oxygen plasma at a low power setting.

With CF₄ measured at 400W – the highest power plasma setting for our measurements – one can more clearly observe the advantages of the HR4 and HR6 spectrometers compared with the other spectrometers (Figure 3). Having access to precise, process-ready HR series spectrometers, which are distinguished by large-bench optical resolution performance in a compact footprint, provides customers with options when integrating OES systems into their plasma monitoring processes.

Figure 3. In measuring carbon tetrafluoride plasma at 400W, we observed the strongest spectral performance over the widest wavelength range with the HR series spectrometers.

Another benefit of the HR series spectrometers for plasma monitoring is their spectral range, which extends from the UV to the Shortwave NIR. Most OES systems focus on the UV-vis range because that’s where the key activity to track is occurring (e.g., endpoint detection with the creation/loss of oxygen peaks), but these measurements may miss the NIR response. That’s significant because at low-power conditions, the system operator may not be able to observe the glow of the plasma through the chamber window. In those scenarios, the spectrometer can spectrally “see” the NIR peaks of the plasma, confirming their presence (Figure 4).

Figure 4. Even with a plasma at a low power setting, the HR6 spectrometer detected spectral response >700 nm.

Summary

Spectroscopy is a powerful technique for measuring plasma emission and controlling plasma-based processes. By tapping the optimum combination of spectroscopy hardware and know-how, equipment suppliers can continue to improve and refine the etching technologies they offer to the semiconductor industry.

The newest generation of Ocean Optics spectrometers and components – especially the HR series – are well suited for supplying the semiconductor industry with robust sensing solutions for industrial customers and integrators on the front lines of chip design and manufacturing.