Atmospheric Plasma Surface Cleaning for Advanced Nanotechnology
Update: 2011-03-02 04:06:38, Hits: 2000
One of the on-going challenges with ever shrinking feature sizes is to provide an effective yet economical cleaning solution that yields a properly treated surface that result in consistent and reliable adhesion and coating properties. There are numerous requirements for this in electronics and nanotechnology manufacturing. A new and innovative technology offers a paradigm shift from conventional cleaning methods.
Some of the applications include: lead frame pre-treatment prior to epoxy or eutectic die attach; bond pad pre-treatment to improve wire bond adhesion; fine pitch PC board cleaning prior to component assembly; flux printing and flip chip soldering (FCBGA, FPBGA); polyimide films and plastic surface treatments prior to screen printing or coating; glass treatment prior to advanced coatings; silicon wafer cleaning prior to backside gold; and carbon contamination removal in other nanotechnology applications.
The list is nearly endless with new process challenges discovered every day. This tool offers a cost-effective "pre-clean" solution that improves manufacturing yields and product quality while lowering production costs. Now, new emerging challenges exist in nanotechnology applications.
Some new emerging applications include solar cell production, both PV and organic; hydrogen fuel cells; ink jet technology in FPD production; MEMs; and DNA sample tray preparation.
Advanced cleaning processes like AP Plasma can help with many of these new technologies in developing cost-effective manufacturing solutions.
AP Plasma
Atmospheric Pressure Plasma (AP Plasma) is a radical new technology providing surface cleaning and pre-treatment of substrates. AP Plasma utilizes a novel approach to generating a plasma without the complexities, cost and potential for substrate damage normally associated with vacuum plasma or the handling, disposal and processing challenges associated with wet chemical cleaning. Plasma can be described simply as the 4th state of matter consisting of atoms, ions (charged species), radicals and electrons. Conceptually to create a plasma state, think of starting with a solid substance and continue to add energy and for each substance the state of matter moves from solid to liquid, then gas and finally with sufficient energy to a plasma state.
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| Polyimide contact angle improvement. |
Atmospheric pressure plasma is a plasma discharge system that requires no vacuum systems or special process chambers. Using specially designed electrode and power source techniques, the plasma can be easily generated at 1 atmosphere (760 Torr). So, it could be the best solution for the company which requires continuous in-line plasma processing for mass production.
Obtaining effective breakdown voltages at atmospheric pressure requires the use of special dielectric materials and electrode designs. Upon entering the space between the electrodes, the gas becomes ionized into active neutral radicals, ions and electrons when a carefully biased AC voltage is applied across the electrodes. The active neutral radicals that are created are very reactive with the density of these species as much as 100 to 1000 times greater than conventional vacuum plasma. These neutral radicals are very effective for cleaning and surface pre-treatment processes and they do not create any electrically induced surface damage issues. Furthermore, in AP Plasma no ions or charged particles reach the substrate surface; only the free radical neutrals in the plasma. Since neutrals are the reactive component for surface cleaning and treatment, there is no charge build-up or potential for ESD damage at the substrate surface. This is important since many products are sensitive to ESD and can be damaged if high energy charged ions were to contact the surface. In the AP Plasma process this is no longer a concern.
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| In-line reel-to-reel AP plasma process tool. |
Free radical oxygen is highly reactive and wants to either recombine with another oxygen atom to create the more stable diatomic oxygen molecule or will readily react with a carbon atom on the surface of the substrate in an isotropic non-directional form. This allows for the cleaning to be extremely effective in hard-to-reach places including high aspect ratio or blind cavity applications.
AP Plasma Results
There are several widely available analytical techniques that can be used to determine the effectiveness of the AP Plasma process on the substrate. Techniques such as AES and XPS-ESCA are quantitative techniques that can provide elemental insight into the "Before" and "After" processing conditions of the tested surface. This is valuable when trying to isolate a particular source of contaminant or when trying to define quantitatively, "How Much" is present. Examples here include understanding both the chemistry and surface activation states (valuable for pre-treatment conditioning prior to printing or coating when trying to create polar groups to promote improved adhesion).
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| Glass and silicon contact angle improvement. |
Oftentimes the complexity and sophistication of these analytical techniques are not required to monitor process performance. Therefore, another widely used and much simpler process control methodology uses a goniometer to measure the contact angle on the surface of the substrate before and after processing. For hydrophobic surfaces including glass, silicon, polyimide, PC board, plastics, oxides etc. it is easy to see significant differences in the pre- and post-processing values for the contact angle. Post AP Plasma processing contact angle values can provide real-time statistical process control (SPC) data to monitor the effectiveness of the manufacturing process. An excursion or deviation in the contact angle as an inline measurement can quickly highlight to the production floor that something in the process has changed. This will prevent manufacturing from further processing additional product with inherent quality issues until the problem can be investigated and resolved.
"Before" and "After" processing images of a water droplet and the improvement in the contact angle as the surface changes from a hydrophobic state to hydrophilic show dramatic differences. The goniometer measurement provides discrete numerical readouts that can be used as an analytical tool to measure and therefore monitor process performance.
Process Flexibility. Many of the substrates that require cleaning are not planar yet still require cleaning over a range of heights across the substrate. Fortunately, AP Plasma provides robust results with electrode spacing of 2-8mm providing a large process window to accommodate different heights on substrates. Experiments may be designed that vary the input power, gas flow and ratio and belt speed (time), and can be run to optimize the performance results for a particular substrate and application. An innovative new atmospheric pressure plasma cleaning and surface treatment process has been developed by Plasma Systems & Materials, a Korean company that is represented in North America by Ceiba Technologies. The process has applications in a wide variety of nanotechnology manufacturing and R&D environments. Cleanliness requirements and controlling surface properties will become even more important as device features continue to shrink. The AP Plasma application opens up a new realm of opportunities to improve process performance and repeatability while lowering on-going operating costs when compared to more complex processes like EUV, vacuum plasma and wet chemistry alternatives.
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