Cleaning parts and components is an extremely quality-critical production stage, especially in micro-technology. CO₂ snow-jet cleaning is a dry, environmentally-friendly process which has proved itself over a wide range of applications. Use of the inline-capable snow-jet technology means that reproducibly high cleanliness levels up to the sub-micron range can be achieved.

Whether dealing with microchips, molded interconnect devices (MIDs), or mechanical micro parts and components such as cogwheels, gears, and pumps—cleanliness is key to the success of micro manufacturing. Even the tiniest particles and/or filmy residue can cause malfunctions and product failures. Due to this, a very thorough cleaning process is required. The CO₂ snow-jet cleaning process can be achieved in a reproducible and efficient way. The fine carbon dioxide snow removes both particulate contaminations such as particles, swarf, dust, and filmy residue (e.g. oils, fats, and processing media) from almost all materials.

DRY, RESIDUE-FREE, AND GENTLE
In contrast to CO₂ dry ice jets, CO₂ snow-jet uses liquid carbon dioxide as a medium. It is fed from bottles or tanks and can be stored for an unlimited period, which means separate manufacture and storage of dry ice pellets is no longer necessary. The softness of the fine snow crystals also ensures that sensitive components and fine structures are cleaned without damaging the substrate.

The “heart” of the snow-jet technology is a patented cleaning head designed as a supersonic two-component ring nozzle. As the fluid carbon dioxide exits the nozzle, it expands to form a snow/gas mixture, which forms the core jet. In addition, compressed air is forced in as an air-jacket jet which accelerates the CO₂ snow to supersonic speeds. By combining thermal, mechanical, and sublimation functions an excellent cleaning result is achieved.

Thermal function
When the blasted agent hits the surface at around -78.5°C, the surface is quickly cooled and the dirt layer becomes cracked and brittle. The difference in heat expansion coefficients of material and dirt layer maximized the result.

Mechanical function
Due to the mechanical effect which is transferred to the surface by the CO₂ blasting agent, the dirt particles are detached from the substrate. The aerodynamic force of the carbon dioxide and the compressed air jet then blows away the dirt particles.

Sublimation function
The transition of the CO₂ from solid to gas produces a pressure wave. This results in a volume increase, which supports the two previous functions.

Since the carbon dioxide transitions to the gaseous phase the material to be cleaned is immediately dry. This negates the energy-intensive drying processes needed by conventional wet-chemical methods. No cleaning or rinsing residues remain, ensuring biocompatible products as in medical technology components.

REMOVAL OF ABLATION RESIDUES FOR LDS-MIDS
These benefits mean that the dry process is used in more and more cleaning applications for the production of micro-components. One application would be the removal of ablation residues in the manufacture of MIDs using the LDS technology (laser direct structuring). This process enables circuit layouts to be generated directly on complex, three-dimensional carrier structures. The advantages of LDS are: extended design opportunities, considerably lower component dimensions and weights, and faster alterations of electronic components: This leads to significantly lower costs for development and series production. The laser also creates more precise structures than conventional processes. A special additive in the thermoplastic materials is specific to the manufacture of LDS MIDs. The laser beam induces a physical-chemical reaction to activate this additive. It is then broken down in the polymer matrix during the process and acts as a catalyser in the subsequent reductive copper-plating. During laser structuring, active ablation residues (which are also metallized and can therefore cause problems) remain on the surface. These residues are removed using CO₂ snow-jet cleaning. In comparison with conventional cleaning, e. g. using ultrasonic or high-pressure water jets, this process simultaneously levels the roughened laser structures. The result is a simplified structure and internal connection system for LDS MIDs such as wire bonding, fitting with unhoused chips, and in flip-chip technology. In addition, the cleaning module can be easily integrated directly into the laser structuring system.

VERSATILE IN MICROCHIP PRODUCTION
CO₂ snow-jet cleaning has also proved itself in the classical production of microchips by photolithography. It is used in the production of magneto-resistive (MR) sensors for various applications at Sensitec GmbH. The company established CO₂ snow-jet technology in the manufacture of three-dimensional microchips for MR sensors as an effective and environmentally- friendly alternative to conventional wet-chemical cleaning. These often require costly rinsing processes with ultra-pure media. The CO₂ technology removes the so-called fences formed at the edges of the conducting paths after metallization. The next structures can then be applied to the silicon wafer.

Since even the smallest residue of fluid media can affect the function of micro-electronic components or adhesion of a protective coating, these also need to be reliably removed. CO₂ snow provides an economical, residue-free solution to cleaning and to improving the adhesion of protective coatings. At the same time, the process also enables the selective treatment of specific sections such as bond and contact areas.

The process also provides a solution in the packaging of microchips for RFID applications. Prior to assembly, the semiconductors are placed in a special magazine by a gripper. During this process the robot creates sufficient airflow which may cause lifting the extremely light chips out of the packaging. To prevent this, the chip is held down by a vacuum applied from the rear of the packaging. For this purpose a tiny hole must be drilled and the residues caused by this drilling removed. CO₂ snow-jet cleaning excels at this particular process.

So-called fences, which are created by a chemical
reaction in the photoresist at the edges due to sputtering
on the conductor paths, can be effectively and reliably
removed using the dry CO₂ snow-jet cleaning process.

LASER MATERIAL PROCESSING WITH INTEGRATED CLEANING
3-D laser removal enables the manufacture of very small, fine cavities and shapes. This type of material processing is used in the computer industry, medical technology, the automobile and aerospace industries, as well as other sectors moving toward miniaturization. Thanks to shorter processing times and lower labor requirements, the process offers considerable economic benefits in comparison with conventional technology. This is particularly exemplified in the manufacture of injection molds, electronic and semiconductor shapes, mold inserts, indexable inserts, and dies and prototypes.

A wide range of materials such as steel, aluminium, brass, hard metal, ceramics, graphite, and boron carbide can be processed using the laser. During the removal process, very fine residues such as slag or scale are produced and then removed by suction. With very fine shapes, especially those with steep walls, accumulated residues can reduce laser effectiveness resulting in rejects.

For residue removal Sauer GmbH | Lasertec integrates CO₂ snow-jet cleaning modules in its 3-D laser systems. These modules enable the required cleaning steps through the laser treatment process lasting several hours. Processing of the workpiece is carried out on a guiding machine fitted with an X and a Y axis in addition to a turntable. During the (approx.) 10 minute cleaning intervals, the guiding machine positions the workpiece in front of the cleaning head in accordance with a part-specific program. This integrated cleaning process allows extremely finely-shaped parts and even steep walls up to 90° (with a depth of up to 2 mm on certain materials) to be manufactured by the laser system.

During laser-writing on components, such as data matrix codes, smoke traces often remain. As this contamination can cause inaccurate identification of the code by the reading device, use of carbon dioxide snow is key to the success of the process.

MICRO-DEBURRING AND CLEANING IN ONE OPERATION
Another application is the simultaneous microdeburring and cleaning of workpieces such as cogwheels, gear and pump components, as well as medical technology parts. The deburring effect is firstly based on the mechanical effect triggered by the direct impact of the snow crystals on the chip. Secondly, the burr is broken by a turbulent flow causing bending stress. These processes take place in a split second. Cleaning and deburring are performed in the shortest possible time and a reproducible result is achieved.

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RESOURCE-SAVING, ENVIRONMENTALLYNEUTRAL, AND ENERGY-EFFICIENT
The carbon dioxide used for the CO₂ snow-jet cleaning is created as a by-product during manufacturing processes in the chemical industry. It is not extracted from fossil fuels, so cleaning with CO₂ saves resources and has no impact on the environment. In contrast to wet-chemical cleaning no chemicals are necessary, no wastewater is produced, and no solvents need to be disposed of. Moreover, the process is considerably more energy efficient as no energy is required for heating, distillation, or preparation of the cleaning media or for part drying. This makes CO₂ cleaning an alternative which is of considerable benefit both with regard to economic and ecological aspects.

MEETING REQUIREMENTS AND EASILY IMPLEMENTED
CO₂ snow-jet cleaning equipment is offered in a broad range. The machines and cleaning modules can be easily integrated into automated production lines, or set up as a separate solution. Systems for cleanrooms can be installed to suit customer requirements. It also comes with its own cleanroom system (MENV) and a specially adapted and patented extraction system.

In order to achieve optimum cleaning results, the process parameters are precisely matched to the actual application, the material properties, and the contamination to be removed.

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Doris Schulz studied Business Administration at the “Fachhochschule für Wirtschaft, Gestaltung und Technik,” Pforzheim, Germany. She worked with different companies in Public Relations. In 1995 Doris Schulz founded her own PR consultancy, and she has also worked as a freelance journalist. One of her specialties is the field of surface treatment. She can be reached at +49711854085 or via e-mail doris.schulz@pressetextschulz.de.