Donald Higgins, Brady Cole, MKS Instruments, Andover, Massachusetts
John Tracy, Ricky Ruffin, Axcelis Technologies, Rockville, Maryland
There is a benefit in adding water vapor to some sub-atmospheric wafer processing applications, including photoresist strip, metal etch, and rapid thermal processing. However, systems for delivering precise amounts of water vapor into a low pressure or vacuum environment have been limited by low reliability, poor stability, large footprint, and inadequate flow capacity. Now, a new module integrates vaporization and flow control into a compact package that connects directly to a fab deionized water supply. This system has demonstrated excellent process results on a commercially available photoresist strip system.
Several wafer fabrication processes exhibit improved results when water vapor is one of the constituent process gases, particularly sub-atmospheric and vacuum-based processes. For example, water vapor is widely used in metal etch to passivate aluminum surfaces in situ during resist removal after etch. This helps remove chlorine residues and has almost eliminated metal corrosion problems . Also, water vapor is now used in RTP systems to grow high-quality silicon oxides .
We have shown how water vapor added to process gas as a source of hydrogen in plasma photoresist strip can improve both strip rate and selectivity in fluorine-based processes for removing etch residues. Such residues can be completely removed without affecting exposed dielectric, barrier, and antireflective coating layers.
Conventional plasma dry strip processes for polymer residue removal have used fluorine chemistries such as CF4. The free fluorine in a CF4 plasma is effective at reacting with post-implant and post-etch polymer residues either to volatilize them or improve their solubility. The aggressive reactivity of the free fluorine can, however, cause undesired loss of polysilicon, silicon dioxide, or nitride layers, so precise control of these processes is critical.
Adding water vapor to CF4 plasma chemistries has produced improved ash rate at low and high process temperatures, while greatly reducing substrate loss. Benefits to deep via residue removal have also been observed.
Water vapor flow control, vaporization
Considering the process advantages discussed above, a current challenge is reliable and cost-effective delivery of precise amounts of water vapor from a liquid source to a process chamber. Deionized water is highly corrosive, especially at the elevated temperatures (e.g., 120-140°C) required for vapor delivery. Also, complex components such as mass flow controllers (MFCs) must operate reliably at these temperatures. Current system designs generally consist of a heated reservoir for generating water vapor, heated plumbing, and a high-temperature MFC for regulating vapor flow.
The conventional approach to water vapor delivery is limited for several reasons. Most importantly, corrosion and high-temperature operation can degrade reliability and generate particles. Second, system components (the vaporizer, MFC, and electronics) are physically separate from each other and therefore require added space. Finally, the maximum flow rates provided by these systems are limited to 1-2slm, which is too low for many processes, 300mm processes in particular.
Figure 2. I-line resist removal with and without water vapor addition in a 120°C process. (The different data point markers simply refer to right and left chamber results and are not significant.)
We have been involved with the development of a new subsystem module (Fig. 1) that integrates all the functions of water vaporization and flow control a vaporizer, a vapor MFC, control electronics, and a power supply into a small-footprint box (~72 in2) that requires only a standard MFC interface for operation. The module connects directly to a DI water supply and provides a precisely controlled flow of water vapor to a process without need for reservoirs or additional external control elements.
The new vapor delivery module has full scale flow ranges of 500sccm, 1000sccm, or 3000sccm, with an accuracy of ±3.0% of full scale and repeatability of ±0.2% of full scale. Setpoint settling time, per Semi Standard E17-91, is <2 sec. Water vapor purity, which was verified by ICP-MS chemical analysis of condensed vapor, shows no significant metallic contamination (see table).
Dry strip, residue removal
By performing design of experiments (DOEs) both in-house and at production wafer-processing facilities, we evaluated our vapor delivery module on Axcelis Technologies FusionGemini ES and FusionES3 dry strip and residue removal systems. In particular, the use of water vapor as a process gas additive has proven to be valuable for use with fluorine-based residue removal processes, especially for via sidewall polymer removal over nitride or oxide layers.
We coated wafers with 1.2µm of i-line resist and measured subsequent ash rates with and without the addition of 2% water vapor to a CF4O2 plasma at 120°C and 1.25 torr (Fig. 2). Water vapor addition to a 120°C process increased the ash rate an average of 60%, from ~20Å/sec to 32Å/sec (6340Å removed in 200 sec). The percent nonuniformity was unchanged between the two sets of conditions. Clearly, water vapor addition enhanced the ash rate at low temperature with CF4O2.
We also conducted experiments using a more aggressive 140°C and 1.5 torr CF4O2 plasma process with resist-coated HMDS-primed wafers having 1500Å of PECVD TEOS oxide; here we wanted to characterize the effects of water vapor addition on oxide loss during residue removal. Oxide loss without water vapor in the process averaged 154Å/min compared to <5Å/min with the addition of 5% water vapor. Similar results were obtained for photoresist on titanium nitride (TiN), where water vapor in the process reduced TiN loss from >1100Å/min to below accurate measurement capabilities.
Figure 3. Photoresist strip rate vs. temperature in various oxygen-free plasmas, with and without water vapor addition.
Water vapor was also added to oxygen-free plasma with and without CF4O2 at 200-320°C (our tested range). Water vapor addition to the oxygen-free plasmas resulted in higher ash rates across the range of temperatures (Fig. 3).
The SEMs in Fig. 4 show the effectiveness of water vapor addition to a CF4 plasma for photoresist residue removal from vias; our test process here was a CF4O2 plasma with water vapor at 110-160°C. The SEMs reveal that the TiN layer below the via was left intact, and no residue remained on top or sidewalls.
Figure 4. Surface condition following etch residue removal by plasma stripping in a FusionGemini ES with a CF4O2 plasma followed by a 5-min wet clean. Contact holes are etched to TiN under TEOS oxide, revealing no attack of the TiN layer, and all sidewall polymer was removed.
Before-and-after SEMs (Fig. 5) illustrate etch residue removal after metal etch using a CF4O2 plasma with higher CF4 content and water vapor (10-15% CF4 by volume); these SEMs reveal complete polymer removal with no damage to underlying structures.
In our work, we used mass spectrometer analysis to show that CF4 plasma with added water vapor is rich in free hydrogen. This chemistry provides an abundance of active species that can be used to reduce polymers, such as azo-coupled resin and sensitizer used in positive photoresist. Also, the presence of hydrogen suppresses the formation of free fluorine, possibly explaining the reduction in oxide and nitride attack that was observed.
Figure 5. Etch sidewall residue a) before and b) after photoresist stripping from aluminum metal lines, showing no damage to the underlying TiN barrier layer.
Overall, we found that adding water vapor to fluorine-containing plasmas using an accurate vapor delivery subsystem provides benefits in etch residue removal from deep vias and post-metal-etch residue removal, and minimizes the etching of thin underlying films. Water vapor is a valuable chemical species to be used for challenging residue removal applications.
The Vapor-on-Demand Module (VoDM) methodology is a natural extension of well-established pressure-based flow technologies. Because it provides reduced footprint, improved reliability, and expanded flow range, the VoDM is an effective means of water vapor generation for a range of wafer processing applications. It has been adopted by Axcelis as the vapor delivery component of a water vapor delivery system for use in residue removal applications on the FusionGemini and FusionES3 photoresist strip systems. It is also being tested on some of the newest metal etch and RTP tools for simple, cost-effective water vapor delivery.
Baratron is a registered trademark and Vapor-on-Demand Module (VoDM) is a trademark, both of MKS Instruments.
- S. Tabrez, et al., "Controlling Corrosion, Particles in Metal Etch-and-Strip Cluster Tool Systems," Micro Magazine, October 1998.
- R. Sharangpani, et al., "Steam-based RTP for Advanced Processes," Solid State Technology, Vol. 41, No. 10, October 1998.
- Commercially available on the MKS Vapor-on-Demand Module (VoDM).
For more information, contact firstname.lastname@example.org, John.Tracy@axcelis.com.