Symposium highlights 300-mm fab issues
By George D. Miller
San Francisco — Though many semiconductor market forecasts cite a delay in the migration from 200- to 300-mm wafer fabrication, significant R&D strides are being made. In the area of contamination control, for example, such efforts include development of an ultraclean pumping system and high-reliability gas distribution system for 300-mm wafer fabrication; the use of chromium-rich oxide passivation and fluorine passivation for contamination-free delivery of reactive and corrosive gases; and the use of tunable diode laser absorption spectroscopy as a technique for monitoring contamination.
These and other developments were described in July at a symposium on contamination-free manufacturing for semiconductor processing conducted at the Semicon West 98 exposition in San Francisco. The program was chaired by Sowmya Krishnan, Ph.D., of Ultra Clean Technology, and Ahmed A. Busnaina, Ph.D., of Clarkson University. The symposium was one of 70 technical offerings by SEMI, the Semiconductor Equipment and Materials International organization.
The ultraclean pumping system and high-reliability integrated gas distribution system were described as part of a presentation on contamination-free manufacturing for 300-mm wafer processing made by Tadahiro Ohmi, Ph.D., of Tohuku University. Dr. Ohmi is also president of the Institute of Basic Semiconductor Technology Development (Ultra Clean Society) in Japan.
According to Dr. Ohmi and his co-authors, the accuracy, productivity and reproducibility of 300-mm wafer manufacturing will “drastically improve” through use of vertically integrated cluster tools that they developed. These tools include high-density microwave plasma sources and process equipment (see accompanying story page 53) as well as the pumping system and gas-distribution system.
Ultraclean pumping system
The researchers` goal in developing a pumping system was to achieve high speed in a maintenance-free and contamination-free pump. The pump they developed is capable of suppressing impurity back-diffusion, byproduct deposition, corrosion and oil degradation, according to the team. (See Figure, page 1.)
The system is maintained at a temperature of approximately 150 degrees Celsius to suppress byproduct deposition on the inner wall. The gas-contacting surface of the system is passivated to prevent corrosion. Impurity back-diffusion during standby is suppressed by adding N2 or Ar gas into the chamber. Oil degradation is prevented by feeding the gas into the outlet of the backing pump. A turbomolecular pump withstands high gas flow and maintains pumping speed. A gradational lead screw pump, used for the backing pump, features constant pumping speed.
Compared with conventional turbomolecular pumps, the new pump features narrow blade clearance to maintain molecular flow at high pressure. Researchers demonstrated that the impurity level of the chamber could be reduced by several orders of magnitude from when the chamber is kept under ultrahigh vacuum if gas flow of 100 standard cubic centimeter or more is maintained.
Researchers also developed a gradational lead screw vacuum pump, because a backing pump is required to have a higher pumping speed and to maintain that speed constant from atmospheric pressure to a low pressure of approximately 10-3 Torr. A conventional dry pump keeps its pumping speed to only about 1 Torr. The new pump achieves a pumping speed of 3600 liters/min with a small pumping system. By changing the pitch and the angle of the screw from the inlet to the outlet, the compression ratio and the pumping speed can be improved. To increase the compression ratio, a small amount of oil is injected near the outlet and is circulated. Researchers confirmed that the impurity back-diffusion decreases with increases in the inlet pressure, i.e., the N2 flow rate. At several mTorr, the gas flows into the transition flow region. Therefore, back-diffusion can be suppressed by the N2 stream through molecular collisions, and the proper gas-flow rate needed to accomplish this is only about 10 sccm. Hence, impurity back-diffusion from both the turbomolecular pump and the screw pump can be suppressed by continuously flowing gas from the chamber. The gas flow rate for the chamber must always be maintained at greater than 100 sccm to suppress back-diffusion contamination from the gas pumping system.
Gas distribution system
In addition to the ultraclean pumping system, researchers addressed the issue of gas distribution systems for 300-mm wafer fabrication. Such systems must be further integrated than they are today because equipment becomes larger as wafer sizes increase, according to the researchers. High reliability (quick response, quick gas replacement, and error-free operation) was another of their objectives.
The team developed a metal gasket type of fitting. It is divided into two parts: one for sealing, and the other for resisting mechanical stress. When the fitting is tightened, the sealing portion first contacts the gasket, and then the inside flat surface, outside flat surface, and stopper are contacted gradually. The fitting achieves high reliability, according to researchers, even at low torque.
Other parts of this gas distribution system are an electric valve and a flow controller. The valve addresses the inability of previous valves to operate correctly while changing between purge gas and process gas and during operation at high speed.
A characteristic of the new valve is that it uses electromagnetic force for operating drive instead of air pressure. The valve operated 10 times faster below 0.01 sec. than a conventional valve. By using this valve, say researchers, it is possible to distribute gas with high speed, high reliability, high throughput and no backward flow.
The flow controller they developed controls flow rate not by temperature distribution but by pressure, overcoming the sensor capillary clog of previous devices because a sensor capillary is not used. Researchers report that they can realize correct flow control from several cc to several liters by changing the inner diameter of the orifice.
More on gas delivery
High-purity gas delivery was also the topic of another presentation at the Semicon symposium. Dr. Sowmya Krishnan, director of technology development at Ultra Clean Technology in Menlo Park, CA, described advances in the area of corrosive gas delivery that meet the requirements of defect-free 300-mm processing. Her conclusion is that the use of CrP components for reactive gas delivery has advantages of reduced contamination, reduced maintenance and longer uptime cycles due to the faster dry-down property and inert chemical composition of CrP. For corrosive gas delivery, Chromium-rich oxide passivation (CrP) offers corrosion resistance, longer system lifetime and reduced metal contamination, according to Krishnan. For the delivery of highly reactive fluorine for excimer laser lithography, fluorine passivation (FP) systems provide stable and reliable gas delivery.
Corrosive gas delivery poses problems of corrosion, undesired side reactions, varying gas concentrations, particle generation and metal contamination. Several semiconductor processes use reactive gases like phosphine, arsine, silanes, etc. These gases react with moisture or trace gas impurities in the gas delivery system resulting in their decomposition, or undesirable side reactions due to contamination. Delivery of fluorine gas with electropolished systems poses problems of concentration variations and contamination.
CrP and FP are passivation techniques for gas delivery systems geared to address these problems. Chromium passivation on a stainless steel surface results in a 100-percent chromium oxide layer with a thickness of 200 to 500 Angstroms.
Krishnan characterized and compared the corrosion resistance and dry-down properties of CrP and electropolished tubing samples. SEM analyses of CrP weldments exposed to HCl and Cl2 showed no penetration of chlorine into the bulk of the film. The weld bead and the heat-affected zones retained the chemical composition of the surface before and after exposure to the corrosive gases.
Both the CrP and electropolished tubing were characterized for moisture dry-down properties using an atmospheric pressure ionization mass spectrometry (APIMS). Moisture challenge was set at 200 ppb and the zero gas was set at 200 ppt in nitrogen. Samples were baked at 200 degrees Celsius for five hours prior to the dry-down experiment. CrP exhibited 60% faster moisture dry-down compared to the electropolished tubing for levels of 10 ppb and 1 ppb.
FP tubing and electropolished tubing samples were characterized for reactivity to fluorine gas. Researchers conducted particle testing on FP sample tubing. Research data showed that F gas is consumed by the electropolished surface after exposure. In contrast, the FP surface causes no depletion in the F gas concentration. Particle data showed no particles shed by this film at atmospheric pressure. Krishnan concludes that the use of FP components can ensure stable delivery of fluorine for excimer laser lithography.
The use of modular components is being considered in manufacturing gas delivery systems with the goal of cost reduction. Modular gas delivery systems consist of top-mounted components bolted to substrate blocks with seals and screws. With the use of standardized components, sealing methods and base block architectures, it would be possible to reduce manufacturing costs and engineering design cycle times for such systems. Currently, however, there exist several sealing methods such as the C-seal, Z-seal, and W-seal that could replace the VCR sealing method. Additionally, several configurations of the base block architecture exist. All of these are significant roadblocks to standardization and cost reduction. Finally, 300-mm processing requires the use of some liquid sources that are not easily handled by the modular design.
Measuring moisture contamination
Among other presentations made at the Semicon symposium was one on the measurement of moisture contamination. Dr. James J.F. McAndrew, senior scientist at Air Liquide in Countryside, IL, concludes that tunable diode laser absorption spectroscopy (TDLAS) can be used for in situ contamination monitoring to improve equipment effectiveness and avoid yield loss in semiconductor manufacturing. The importance of such measurements will increase with increasing wafer size and decreasing device dimensions, McAndrew says.