WO2002078749A2

WO2002078749A2 - Atmospheric pressure rf plasma source using ambient air and complex molecular gases

Info

Publication number: WO2002078749A2
Application number: PCT/US2002/008752
Authority: WO
Inventors: Jaeyoung Park; Ivars Henins
Original assignee: University of California Berkeley; University of California San Diego UCSD
Current assignee: University of California Berkeley; University of California San Diego UCSD
Priority date: 2001-03-28
Filing date: 2002-03-21
Publication date: 2002-10-10
Anticipated expiration: 2003-09-28
Also published as: AU2002306800A1; WO2002078749A3

Abstract

An atomospheric pressure rf plasma processor in which a plasma is created between two electrodes using high frequency rf power, and a mixture of gases with a complex molecular gas as the majority component of the mixture of gases.

Description

ATMOSPHERIC PRESSURE RF PLASMA SOURCE USING AMBIENT AIR AND

COMPLEX MOLECULAR GASES

STATEMENT REGARDING FEDERAL RIGHTS This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S Department of Energy. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION Surface properties such as cleanliness, hardness and hydrophobicity is of vital importance in many industries, not the least of which is the semiconductor industry. Clean substrates and devices are imperative if high quality devices are to be manufactured. Additionally, other applications require that a material's surface be treated in a predetermined manner to effect the desired operation from the material . Currently, this cleaning or surface treatment is accomplished through a variety of methods. One of the most widely employed methods is low-pressure plasma processing. By utilizing the reaction of the plasma with selected feed gases, surface processing has been performed on organic films, fabrics, and semiconductor wafers. In these processes, the target material is immersed in the plasma, typically by placing the wor piece directly on an electrode. These discharge processes are effective because of the action of the radicals and ions produced in the plasma by collisions with electrons. Typically, the radicals and ions are more chemically active than the corresponding neutral gas species. Furthermore, ions are accelerated across the sheath region in the plasma, gaining kinetic energy. The kinetic energy contributed by these ions combined with their chemical reactivity enhances the desired chemical reaction. Unfortunately, however, these prior art processing methods require expensive vacuum systems in order to be effective, because the formation of stable, non-thermal, steady-state plasma is favored by low pressure. In addition, it is necessary to minimize gas- phase collisions within the sheath region to obtain a high kinetic energy of the ions.

To overcome this limitation by low-pressure plasmas, atmospheric pressure RF plasma discharge systems were developed. These inventions are U.S. Patent No. 5,961,772, issued to Gary S. Selwyn for "Atmospheric-Pressure Plasma Jet," U.S. Patent Application Serial No. 09/295,942, filed April 21, 1999, for "Large Area Atmospheric-Pressure Plasma Jet," and Provisional Patent application serial no. 60/243,380, filed October 25, 2000, for "Production of Stable, Nonthermal Atmospheric Pressure RF Capacitive Plasmas Using Gases Other Than Helium or Neon." In these demonstrations, the stable, non-thermal atmospheric pressure plasmas are produced by applying rf fields at 13.56 MHz or 27.12 MHz. Additionally, the use of noble gases such as helium or argon is deemed critical to prevent arcing.

The primary objective of the present invention, like the new discipline of low-pressure plasma processing, is to modify selected surfaces. This modification can include film deposition, contamination removal, surface material removal, known as etching, or changes in the physical state or property of the surface, known as surface modification. However, the present invention accomplishes this using complex gases that have heretofore been considered unlikely candidates for use in atmospheric pressure plasmas. This renders operation of such plasma processing much less expensive since expensive noble gases are not employed.

Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, an atmospheric pressure rf plasma processor wherein a plasma is created between two electrodes using high frequency rf power of greater than 27.12 MHz. In a further aspect of the present invention, and in accordance with its objects and principles, an atmospheric pressure rf plasma processor wherein a plasma is created between two electrodes using a mixture of gases, the improvement comprising using a complex molecular gas as a majority component of said mixture of gases. DETAILED DESCRIPTION This invention provides the production of stable, steady-state, non-thermal atmospheric pressure rf plasmas using complex molecular gas mixtures including ambient air. In particular, the present invention concerns the plasma source design that allows the use of a complex molecular gas system as a major portion (> 50% by volume) of the feedgas . Previously, the production of stable, steady-state, non-thermal atmospheric pressure plasmas was achieved by using various noble gases such as helium or argon as a majority feedgas to prevent arcing. With the use of ambient air as the majority discharge feedgas, this invention overcomes this limitation. Thus, this invention greatly simplifies the use of stable, steady-state, non- thermal atmospheric pressure plasmas by eliminating the need of a separate gas supply of rare and expensive noble gases. This improvement may be critical for the practical use of atmospheric pressure plasmas in many applications. Furthermore, this invention explicitly demonstrates the operation of non-thermal atmospheric pressure plasmas using complex molecular gas mixtures such as ambient air, carbon dioxide, and an acetylene and hydrogen mixture. Therefore, this invention provides the basis for the use of atmospheric pressure plasmas for many different applications that may require a particular feedgas mixture to Obtain desired reaction chemistry, without the complication of using a noble gas as the majority feedgas . This present invention uses high frequency rf fields to produce atmospheric pressure plasmas, that are steady- state, volumetric, homogeneous and non-thermal, but uses complex molecular gas mixtures, such as air, carbon dioxide, acetylene with hydrogen, as the majority (> 50% by volume) component of the feedgas . The plasma source consists of two closely spaced metal electrodes, one grounded and the other powered by a high frequency (-100 MHz) power supply. In the present embodiment, one of the electrodes is covered with a thin dielectric material . It was found that the dielectric cover on at least one of the electrodes is necessary to prevent arcing in an air plasma. However, in the case of carbon dioxide, the dielectric insulator is not needed to produce stable, steady-state plasmas at atmospheric pressure. It is noted that the electrical characteristics of the source are not affected by the use of a dielectric layer because the rf displacement current readily passes through the dielectric. The use of dielectric cover provides a significant advantage over a naked metal electrode because of the much wider variation of secondary electron emission coefficients in dielectrics compared to metals. Previously, one of the inventors herein found, using a computer discharge model, that the secondary electrons play an important role in discharge stability. However, it is in principle possible to use two bare metal electrodes to produce stable plasmas as long as the secondary electron emission characteristics of the metal electrodes satisfy the necessary stability conditions. In fact, this has been demonstrated in the case of carbon dioxide feedgas by producing stable plasmas with and without the dielectric cover.

The efficacious operation of this invention was demonstrated by using two planar aluminum electrodes with one of the electrodes covered with a thin (1.5 mm) sheet of boron nitride. The gap spacing between the two electrodes was between 0.1 cm and 0.4 cm, and the surface area was ~ 16 cm². With power inputs between 100 and 500 W from a 100 MHz rf power supply, stable, steady-state, non-thermal atmospheric plasmas were created using only ambient air. It is to be noted that the present embodiment produced stable plasmas using ambient air up to 900 torr inside a pressurized chamber. The same system has also produced a stable, steady-state, non-thermal plasma using either 100 % carbon dioxide (C0₂) gas or a gas mixture of acetylene and hydrogen at a gas pressure of up to 900 torr. Furthermore, a stable, steady-state, non-thermal plasma using 100 % carbon dioxide (C0₂) gas at a gas pressure of up to 850 torr without a dielectric cover has been produced between two aluminum electrodes.

Previously, the production of stable, steady-state, non-thermal atmospheric pressure plasmas has been achieved using various noble gases such as helium or argon as the majority feedgas (more than 50 % by volume) to prevent arcing. In addition, the frequency was limited to either 13.56 MHz or 27.12 MHz.

A major innovation of this invention is to use high frequency electric fields of ~ 100 MHz or more with common complex gases . This increase in frequency enhances the discharge stability, as discussed by several studies of rf α-mode discharges for gas lasers. In addition, the ability to control the secondary electron emission characteristics of the electrodes is also an important part of this invention. Based on these findings, it is expected that it will be possible to use even higher frequencies from a few hundred megahertz to even microwave frequencies, such as 2.45 GHz, to produce stable atmospheric pressure plasmas using complex molecular gases.

Once produced, atmospheric pressure air plasmas can be used in various materials processing applications since the air contains reactive gases like oxygen and nitrogen. It has been demonstrated the removal of photo-resist from a silicon wafer using air plasmas at an etch rate over 1.5 μm/min. The present invention is an improvement of rf α-mode plasma sources for gas lasers and Atmospheric Pressure Plasma Jet (APPJ) technology, combining the benefits of two existing technologies. By operating at atmospheric pressure, this invention increases the operating gas pressure of rf α-mode plasma sources for gas lasers. In addition, by using higher rf frequency (~ 100 MHz) and by controlling the secondary electron emission characteristics of the electrodes, this invention overcomes the limitation of existing APPJ technology which requires the use of noble gases as a major feedgas to prevent arcing. Therefore, this invention simplifies the plasma source design that can be used for a wide range of practical applications. In particular, the plasma source using ambient air is operable without having to utilize a pressurized gas supply system, which is both expensive and dangerous.

Indeed, the recent success of the APPJ technology in producing stable, non-thermal, steady state plasmas has not translated into a wide range of practical applications because helium is required in large quantities to prevent arcing. In comparison, the present invention changes the field of atmospheric pressure plasma technology radically in that stable, steady-state, non-thermal atmospheric pressure rf plasmas can be produced without the use of helium or argon as a majority species.

In particular, the success of using ambient air makes this invention operable anywhere and at any time with minimal set up, requiring only a rf power supply, electrode assembly and a simple fan. Potential applications may include, but are not limited to, various industrial manufacturing processes, medical and food sterilization, and cleaning of contaminated surfaces .

Additionally, by not needing stabilizing noble gases to produce stable, non-thermal, steady-state atmospheric pressure plasmas, the present invention provides a very attractive means to utilize the potential of atmospheric pressure plasmas for many industrial processes . To name only a few: semiconductor manufacturing, waste treatment, textile modification, and food and medical sterilization are some that may benefit from this technology.

The foregoing description of the embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

Claims

What is claimed is:

1. An atmospheric pressure rf plasma processor wherein a plasma is created between two electrodes using high frequency rf power of greater than 27.12 MHz.

2. An atmospheric pressure rf plasma processor wherein a plasma is created between two electrodes using a mixture of gases, the improvement comprising using a complex molecular gas as a majority component of said mixture of gases.

3. The atmospheric pressure plasma processor as described in Claim 1, wherein said high frequency rf power has a frequency of up to 100 MHz.

4. The atmospheric pressure plasma processor as described in Claim 1, wherein said high frequency rf power has a frequency of up to 2.45 GHz.

5. The rf plasma processor as described in Claim 2, wherein said complex molecular gas comprises ambient air.

6. The rf plasma processor as described in Claim 2, wherein said complex molecular gas comprises carbon dioxide .

7. The rf plasma processor as described in Claim 2, wherein said complex molecular gas comprises acetylene and hydrogen.