The ozone-DI based wet cleaning processes are increasingly being practiced for the removal of organic contamination, oxidation of silicon surfaces, and resist stripping 1-3 The ozone-DI processes offer advantages of less resource consumption and comparable wafer cleaning performance to the standard cleans processes (SPM, SC 1, and SC 2). 4-7. A new membrane-based contactor, described in this article, offers unique advantages for the production of bubble-free ozone-DI water. The key advantages of the contactor are smaller footprint, high transfer efficiency/recovery, rapid start-up, and improved cost of ownership 8.
All PFA Hollow Fiber Ozone Contactor Design
The contactor is fabricated using all PFA (perfluoroalkoxy) hollow fibers, potted into a PFA tube using PFA resin; the PFA end caps with appropriate fittings are bonded to the potted ends to form the fluid pathways. Figure 1 shows a typical shell side flow pattern in which the liquid flows (on the outer surface of the fiber) across shell side and the ozone gas flows inside the lumen of the fiber. The hydrophobic porous structure of the membrane allows ozone gas to diffuse through the membrane and dissolve in the water.
Ozone equilibrium solubility
The equilibrium solubility of ozone in water can be determined from Henry’s law, which relates equilibrium ozone concentration in liquid, X, to its pressure in the gas phase, P, P = HX
The Henry’s coefficient, H, is dependent on temperature and pH. 9 H=3.8×107[OH-]0.035 exp(-2428/T)
The values of Henry’s constant are available in literature10. Figure 2 shows the interface equilibrium ozone solubility (ppm) in water as a function of temperature (for ozone pressure of 0.11 atm). The ozone-DI output from a contactor will depend upon the system parameters.
(1) Fast Start Up
As seen in Figure 3, the high transfer rates of the hollow fiber contactor enable a rapid ramp up of the ozone-DI output in a matter of seconds, almost paralleling the ramp up of the ozone gas from the generator. This improves equipment efficiency of a wet tool enabling it to quickly respond to changes in the demand for ozone during wafer processing.
(2) Ozone concentration vs Flow Rate in DI
Figure 4 shows ozone concentration and pressure drop as a function of liquid flow rate, for both tube side and shell side flow contactors.
The high gas transfer rate produces a much higher level of ozone output in the shell side flow configuration. As would be expected, the ozone-DI concentration is higher at low DI flow rates (almost approaching the equilibrium values at <1 lit/min) caused by the high contact time between ozone and DI water. The DI pressure drop through the contactors is modest and much lower than other commercial contactors.
Boundary layer effect
Efficiency of an ozone-DI contactor is controlled by the diffusion of the dissolved ozone from the gas-liquid interface into the flowing liquid. The slow diffusivity in liquid builds up a “boundary layer”, which limits the overall transfer rate of ozone, and thus the productivity of a device.
The shell-side flow design promotes turbulence, which lowers the boundary layer impedance and results in higher ozone transfer rates. Using this technology, the ozone concentration achieved is more than double that of other conventional and membrane contactors.
Integration of modules in tools
The ozone contactor can easily be integrated into a variety of tools such as immersion baths, single wafer spin processors, and batch spray processors. In some applications the ozonated water is produced at a central location and then recirculated for delivery to individual tools at the point-of-use. In some designs, further ozone is added at the point-of-use. The contactors can be plumbed in either vertical or horizontal configuration. A vertical set up with the gas flow outlet at the bottom end would be preferred, as any moisture would be swept out of the system. The hydrophobic hollow fibers of tight pore structure prevent any water intrusion onto the gas side, eliminating the need for a water trap. During a shutdown it is preferred that both the liquid and the gas flows to the system be turned off. This will ensure optimum performance upon re-start.
Field Evaluations in Key Applications
In addition to the extensive in-house testing, the contactor has been field tested in the US, Europe, Japan and Asia for producing ozone-DI water with excellent performance, in a variety of applications.
The ozone concentrations required in the wet processing and cleaning applications are reported to be in the following range:[a] Oxide growth for hydrophilic surface (6-20 ppm bubble free dissolved ozone)
Si + 2O3 SiO2 + 2O2[b] Photoresist and surfactant removal (80-120 ppm dissolved ozone- bubbles)
(-CH2-)x + 3O3 CO2 + H2O+ 3O2[c] Organic contamination oxidation (5-20 ppm dissolved ozone) oxidizes adsorbed organics on wafer [d] Metallic contamination removal (1-30 ppm dissolved ozone)
1 Sixth International Symposium on Ultra Clean Processing of Silicon Surfaces 2002, September 16-18, 2002.
2 F. De Smedt, H. Vankerckhoven, S. De Gendt, M. M. Heyns and C. Vinckier, UCPSS, September 16-18, 2002.
3 E. D. Olsen, C. A. Reaux, W. C. Ma, J. W. Butterbaugh in Semiconductor International, August 2000.
4 Dr. Tadahiro Ohmi, “Advanced Surface Cleaning of Silicon and Glass Substrates,” Tutorial seminar, 8th International Symposium, SCP Global Technologies, May 21, 2001.
5 Dr. Martin Knotter, “Improvement in Photoresist Stripping Process Using Ozonated Water,” 8th International Symposium, SCP Global Technologies, May 23, 2001.
6 B. Parekh, “Ozone in Wet Cleans (Part I: Technology),” Applications Note MAL 126, Mykrolis corporation, Bedford, MA.
7 Bipin Parekh, Saksatha Ly, KS Cheng, “High Purity Ozone-DI for Advanced Cleans,” European Semiconductor, November 2002.
8 Ming-Chien Yang, E. L. Cussler, “Designing Hollow-Fiber Contactors,” AIChE Journal, November 1986, vol. 32, no. 11, pg. 1910.
9 John A. Roth, “Solubility of Ozone in Water,” Ind.Eng. Chem. Fundam 1981, 20, 137-140.
qp Handbook of Chemical Engineering, pp.2-125.
Note: This article was adapted and updated from a presentation at the 5th Scientific Meeting of CREMSI, ST Microelectronics University on November 14th and 15th, 2002