Carbon dioxide meets the challenge of precision cleaning
05/01/1998
COVER ARTICLE
Carbon dioxide meets the challenges of precision cleaning
Thomas Kosic, ATS Eco-Snow Systems Inc., Livermore, California
Jeff L. Palser, ATS Eco-Snow Systems Inc., Cambridge, Ontario, Canada
Manufacturers of IC components and disk drives are evaluating advances in carbon dioxide (CO2) cleaning.
As device geometries shrink, the challenge of cleaning processed wafers, read/write heads, and substrates for magnetic media escalates. Conventional solvent-based cleaning methods are reaching the limit of their ability to remove submicron particles.
Generic CO2 cleaning, in various formats, has been around for a number of years. Supercritical CO2 is used for chemical extraction, such as decaffeination of coffee beans and seed oil extraction. CO2 pellet blasting removes thick-layer contaminants from large rugged surfaces. CO2 snow, a combination of solid and gaseous CO2, is attracting much attention as a cost-effective, environmentally friendly solution for the removal of submicron particulate and thin-film organic contamination from surfaces.
Early CO2 snow cleaning efforts used a simple valve-and-nozzle system that produced a crude snow spray and had limited success in removing surface contaminants. Molecular film and submicron particulate contamination, in particular, proved resistant to initial efforts, and, in many respects, the technology has only recently matured. It now offers an excellent alternative to cleaning by conventional methods.
We have optimized key elements in the technological evolution of CO2 cleaning. The CO2 Eco-Snow process, originally developed for cleaning space-bound optical systems at Hughes Aircraft Co., has been refined through an extensive R&D program over a five-year period. These efforts were in response to environmental imperatives, including reduced water/chemical usage and chlorofluorocarbon emissions, as well as the need to develop operational controls for accurate process repeatability. Initial R&D efforts centered on nozzle design to qualify the process for a broad spectrum of precision cleaning applications without damage to delicate substrates. After developing the nozzle design to deliver and control variable CO2 solid/gas generation combinations, the R&D effort then focused on a combination of process parameters and automated process equipment to optimize CO2 snow cleaning in a high-volume production environment. In early 1997, Automation Tooling Systems (ATS) of Cambridge, Ontario, purchased Eco-Snow from Hughes Aircraft Co.
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Figure 1. The impact of CO2 snow dislodges small particles that are then swept away in the stream of CO2 gas. The Eco-Snow ACS1500 can remove particles as small as 0.1 ?m.
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Figure 2. Close-up of Fig. 1.
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Figure 3. When CO2 snow hits a surface contaminated with an organic film, it forms a transient, dense-phase boundary layer, then rebounds off the surface carrying the contaminant with it. When the spent CO2 leaves the system, it sublimes completely, eliminating the need for drying and for disposal of waste solvents.
Eco-Snow CO2 cleaning mechanisms
Liquid CO2 maintains an ambient line pressure of 830 psi and will effectively clean surfaces of both submicron particulate and thin-film organics through two different mechanisms: momentum transfer and liquid phase solubilization.
Particle removal results from momentum transfer. A small aperture nozzle orifice sprays liquid CO2 that then expands and cools rapidly to generate a stream of nucleated CO2 particles or "snowflakes" in a highly directional gas stream. The mechanism of particulate removal is primarily a result of momentum transfer between the incident snow particles and surface contaminant particles (Figs. 1, 2). In conventional spray cleaning, the gas stream exerts a viscous drag on the submicron particles. This force, a function of particle diameter, is insufficient to overcome strong Van der Waals and electrostatic surface adhesion forces because the drag exertion decreases on submicron particles faster than the opposing forces. In the CO2 process, particle removal depends on collisional momentum transfer between incident snow particles and resident debris particles. This process is not dependent on particle size and shows efficient particle removal well into the submicron regime. The gas stream carries away the contaminants released from the surface. A micrometer adjuster can alter nozzle orifice parameters to create varying gas stream velocities and densities of snowflakes, depending on particulate size and levels of contamination.
Contaminant films removed by solubilization. The triple point defines the temperature and pressure where the gas, liquid, and solid phases of matter can coexist in equilibrium. This phenomenon provides both dry ice impact and liquid CO2 cleaning in a high-velocity gas stream. Liquid phase CO2 has excellent solvent properties. The strong solvating action created by a combination of nozzle orifice design and positioning relative to the surface being cleaned removes organic matter such as hydrocarbon-based grease and thin oil films. When optimally implemented, the snowflake impact pressure creates a transient boundary layer of liquid CO2 on the substrate surface that dissolves and absorbs the contamination at the interface (Fig. 3). The gaseous CO2 stream then sweeps the solvated contaminants away as frozen particles.
Cleaning performance
The dry, nonabrasive, and nondestructive nature of CO2 cleaning is ideal for the removal of submicron particulate and thin-film organic molecules from delicate silicon wafer and glass flat panel display (FPD) substrates. Additionally, the snow-cleaning process can be fine-tuned to remove post-etch veils and resist residues effectively, without damage to fine circuitry.
Post-etch wafer cleaning. CO2 snow cleaning successfully removes a post-etch "veil" on semiconductor devices. During reactive ion etching (RIE), milled material is deposited on the sidewalls of the photoresist pattern; post-etch ashing used to strip the photoresist cannot remove it. Current approaches for removing this residue (often referred to as veils or fences) involve wet chemistry or an additional etch step after the ashing process, but CO2 snow cleaning completely removes the residue from both submicron circular vias and conductive lines (Figs. 4, 5). Nozzle design is the key to this cleaning performance. The ability to control the snow spray velocity and density manually or automatically provides a cleaning mechanism that can be adjusted for different substrates.
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Figure 4. Via veil removal after RIE and photoresist ashing: a) before and b) after.
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Figure 5. RIE veil removal from submicron metal lines: a) before and b) after.
Magneto resistive (MR) head wafer ion milling redeposition. Ion milling on MR head wafers deposits a residue of milled material that is not readily removed by the wet processes used to strip photoresist (Fig. 6). This residue significantly affects device yield and process cycle rates. The Eco-Snow process can clean these submicron details with double-digit yield improvements.
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Figure 6. Removal of MR head wafer ion milling redeposition: a) before and b) after.
FPD. A single pass clean with CO2 snow removed 99.53% of the particulate contaminants =0.5 ?m on a 6 in. ? 6 in. FPD substrate.
Process optimization
Enhancements developed by Eco-Snow can optimize the cleaning process and minimize inherent issues such as moisture condensation, static charge buildup, and airborne recontamination. For example, a sub-Class 1 cabinet (a sealed cleaning chamber with loadlocks) can be configured for manual or automated access (Fig. 7).
Flooding the chamber with nitrogen during the cleaning cycle eliminates moisture buildup. Maintaining a circulating air stream of 100-200 ft/min in the cabinet provides a constant laminar airflow across the cleaning surface. Dislodged contaminants are carried away in the air stream and subsequently entrapped in a ultra-low penetration air (ULPA) filter. Part-grounding or high-throughput ion bar and ion spray techniques can eliminate electrostatic discharges. Component preheating or the introduction of a hot nitrogen gas stream in combination with the CO2 can eliminate excessive cold shock.
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Figure 7. Clean dry booth subclass 1 microenvironment.
Fast cleaning cycles
A significant advantage of the CO2 cleaning process lies in the potential for dramatic savings in cleaning cycle times compared to other processes. CO2 snow cleaning is extremely fast, often completing in seconds what other techniques require minutes to achieve. The almost instantaneous evaporation of the CO2 gas eliminates costly drying cycle time. Fully automated hard drive disk presputter cleaning cycle rates of 3 sec/part have been obtained with the CO2-snow cleaning process.
CO2 environmental performance
Environmental demands to reduce toxic chemical and water usage and the necessity of removing increasingly smaller particulate contamination create opportunities for the use of CO2. The following advantages make CO2 snow a superior alternative to solvent-based systems:
CO2 evaporates after use and leaves no residue. Contaminants can be filtered out prior to exhausting or recapturing used gas.
CO2 is safe to use and requires no special safety equipment for the operator other than safe product handling.
CO2 meets the requirements of the Montreal Protocol for being environmentally benign.
ATS Eco-Snow Systems has entered into a cooperative agreement with BOC Gases for the development of precision CO2 cleaning applications. Combining Eco-Snow cleaning technology with BOC`s high-purity CO2 supply capability allows the two companies to offer a comprehensive range of cost-effective, high-performance cleaning systems as complete packages.
Ownership and operational costs of a CO2 cleaning system are competitive with other cleaning technologies.
THOMAS KOSIC may be reached at ATS Eco-Snow Systems Inc., 4763 Bennett Drive, Livermore, CA 94550; ph 510/606-2000 ext 302, fax 510/371-0798, e-mail [email protected].