Process for cleaning surfaces with a low-pressure plasma

The invention relates to a method for cleaning treat- treatment of surfaces with a low-pressure plasma consisting of generates a base gas mixture at pressures below 100 mbar becomes.

For efficient cleaning of surfaces of materials of all kinds (Metallic, plastic, glass...) Are, for. Example, wet-chemical and Known halogen-hydrocarbon chemical processes. Out Environmental grounds - on the one hand with oils and greases ver polluted waste water and exhaust gases occur and on the other hand known, ozone depleting and global warming promotional Effect of HFCs and CFCs is delivered it - is, however, increasingly want this surface cleaning method to be replaced by more environmentally friendly methods. A Prinzi Pielle way to do this seems to be the treatment or Beauf to be Nominal workpieces with low-pressure plasmas, since thereby surface-cleaning effects. Among plasma is, inter alia, ions, radicals, electrons, and Photo NEN containing state of a gas to be understood that at example by low pressure conditions - pressure <100 mbar - and power supply -. eg microwaves - can be generated. The cleaning effect of the plasma due to the initiation and Support of chemical reactions as well as on the mechanical African initiating the detachment of particles from the workpiece.

In the art of soldering of printed circuit boards is addressed in DE-PS 39 36 955 these problems.

One problem with the cleaning with plasmas, however, that often different contaminants - in for example adhesively bonded fat layers on the one hand and chemically bonded oxides on the other hand - to be cleaned by a lowing workpiece must be removed. These are known somewhat of a base gas plasmas formed, z. B. from O₂ or CF₄ Plasmas formed, applied. Known and consistently is also the use of plasma gas mixtures of O₂ and CF₄, Often, and especially "mixed" with contaminated plant pieces, but contact these plasmas unsatisfactory Cleaning results on.

The object of the present invention therefore provide a highly effective, especially mixed Impurities-removing, plasma cleaning process specify.

This object is achieved by the fact that the Plasma for the cleaning treatment, starting from a Base gas mixture consisting of

0.5 to 25% by volume of O₂.80 to 0.5% by volume of H₂.10 to 65% by volume of one or more halogen hydrocarbons (HKW's) or SF₆ and0 to 40% by volume of Ar, N₂, Or He

is generated, wherein the plasma in a process step simultaneously an oxidation and reduction effect entfal tet. Preferred base gas mixtures for this purpose are in claim 2 indicated.

The proposed gas mixtures which a main element representing the present invention have a particularly favorable combination of effects on. To provide one of the provided oxygen content for the reduction of substances banned by Oxidation (cold oxidation) are removable. For example this concerns greasy or oily residues, as frequently in Production processes are deposited on workpieces. part as same direction of action, ie a chemical own reaction initiating and / or supporting action, the HKW- or SF6 shares described based gas, because in plasma from this highly reactive radicals are formed, which with various impurities, especially with long chained hydrocarbons may react. on the other hand in these compounds is due to the relatively large mass the molecules also a cleaning effect solely by mechanical given African shocks. This mechanical action mechanism also have the option example be admixed inert gases, which in addition to some extent by light emission (UV photons in particular Ar) related effects are present. To the other stabilizing di e inert gases an activated, burn the plasma in its effect. Finally, the water causes ingredient component a major portion of the reducing ability the plasma of the invention, therefore, bears principal responsibility to eliminate at any existing oxide layers.

Overall, therefore, be described with the base gas in the various applications, good cleaning results achieved, and moreover, - due to the low total flow amount of gas in this process - only minute amounts converted to environmentally harmful compounds or to be evaluated are released.

In an advantageous development of the invention Method is a fed to the respective application cut base gas within the specified limits defined and locally from the respective components mixed. The determination of the mixture of the base gas and in its ability to reduce the consequences or to oxidize ren depends in most essential of the type to remove the Contamination from. If z. B. a workpiece known he significantly covered with oxides, as is a strong reducing effect the plasma based on high hydrogen units in the base gas set, but are mostly adhesively bonded, removing oily impurities, then the center of gravity to lay the plasma effect on chemical and mechanical action, what high O₂ and HKW-play (or SF₆Shares) to is accomplished.

According to Applied halocarbons (HKW's) for example, CF₄, HCF₃ or H₂C₂F₄,

In a further variant of the invention, the base gas is also in the course of treatment is still in its cooperation tion varies and so z. B. initially amplified a removal of greasy-oily layers causes (oxidation) during in late ren treatment phases by increasing the reducing ability oxide removal is promoted.

An advantageous basic variant of the invention, however, that the base gas as ready-mixed gas delivery for is provided and used available. Here are locally no complicated gas mixing systems and process controls provide.

The application of the method is aimed predominantly at the cleaning workpieces or workpiece parts from a variety of Pure metals or alloys, such as steel, Brass, copper, bronze, aluminum, etc.

Based on the figure, the invention is hereinafter exemplified explained in more detail.

The figure shows a system configuration for implementing the The inventive method with mixed ability locally.

A treatment chamber <b>1</b>What a treat with the parts to be filled workpiece basket <b>2</b> is equipped, is on one side with a gas supply via a gas pipe <b>3</b> in connection. The gas supply is in each of three gas storage tanks <b>5</b>. <b>6</b>. <b>7</b>, with a gas mixer <b>8th</b> are connected to which in turn the gas pipe <b>3</b> followed. The gas storage vessel containing essential to the invention gases, namely oxygen, Hydrogen and tetrafluoromethane. Option way here Another reservoir additionally provided with inert gas will. On the other hand, the treatment chamber<b>1</b> at a pump <b>11</b> connected, for the production of the for Plasma cleaning required low pressure and generally for the extraction of exhaust gases is responsible.

On the output side, the pump <b>11</b> in addition to an exhaust purification facility <b>12</b>is, for. example, an activated carbon filter be closed. However, this is to force today, law regulations due to the only small, occurring Gas and pollutants not required in most cases. The harmful components of the exhaust gases consist of a hand of cleansed from the workpieces materials and other hand from the halocarbons of the base gas and whose resulting in process reaction products. In the treat- ment chamber <b>1</b> the plant is finally an energy source for plasma formation, such as a radio frequency wave generator <b>4</b>Disposed.

The operation of the plant shown is as follows:

To carry out a corresponding Reinigungsbehand ment's first define a suitable base gas mixture - In any case, however, has this corresponding inventions tion according to interpretation of both reducing and also oxidize Rende effect. then for the other determination initially the nature of the workpiece requiring cleaning - Metal, places possibly also plastic, ceramic or glass - as well as its Pollution - hydrocarbon compounds, greases, oils - Coatings - closer to define. Of which takes place starting the narrower definition of the appropriate base gas mixture, the essential considerations for the ultimate festival interpretation are briefly described below:

Assigns the surface to be cleaned a high contami pollution degree on of carbon compounds, so is a base gas mix - within the specified limits - with relatively high oxygen content and a relatively high content of a halogen hydrocarbon apply because especially at this Weighting of the shares effectively said contamination be removed. An example for this is in mix one

15% O₂, 60% CF₄, 5% H₂ and 20% Ar

Containing mixture. If, however, essentially an oxide layer to remove from a workpiece, so is especially reducing effect hydrogen portion of the base gas to emphasize mix. A base gas mixture for this target direction is recordable, from

1% O₂20% CF₄, 70% H₂ and 9% Ar.

Exemplary is now the cleaning of greasy and mostly oily polluted, but also of oxides are to be removed from Copper existing switch contacts are described.

These relatively small workpieces in large numbers in the treatment chamber shown in the figure <b>1</b> and the parts shown Basket <b>2</b> brought in. After closing the chamber <b>1</b> If the evacuation of the chamber with the suction pump <b>11</b>, Pumping the air atmosphere is carried out to next until a lower limit pressure value of approximately 0.5 mbar is reached. Subsequently, the supply of one of

10% O₂, 55% CF₄ and 35% H₂

containing basic gas mixture via line <b>3</b> Coming from gas mixer <b>8th</b> and the associated gas reservoirs <b>5</b>. <b>6</b>. <b>7</b>, turned switches. In the now following treatment phase is a gas comply with feed-gas output balance so that a Pressure level of about 0.5 to 2 mbar in said treatment chamber is maintained. It is high-frequency wave transmitter<b>4</b> to commission and operate with appropriate energy. Suitable values ​​for a treatment chamber volume of about 40 1 to 60 are about 500 to 1000 W at a frequency of about 500 to 1000 MHz. The gas mixture is thereby in the Adjusting members not shown drawing the gas mixer <b>8th</b> set and a flow regulator in the gas line <b>3</b> of the treatment chamber <b>1</b> supplied. Supply quantity and pumping performance are in accordance with a low-flow quantitative and appropriate the necessary pressure conditions set, in the present case about 20 to 60 Normal liters of gas per hour (liters Amount based on Normalbe conditions in terms of pressure and temperature) supplied Who the. With these provisos is a high-quality cleaning of obtained said treated.

Finally, it should again be noted that a lot number of materials and objects with the described The method optionally with addition of previous or downstream steps and high in compliance Demands on the environment are cleaned advantageous can. This is in particular by at various Claim profiles customizable basis gas mixture ensures.