Field of Invention
The present invention provides a plasma system, and more particularly it describes a system and method for producing large volume plasma with uniform high density.
Background of the Invention
Plasmas are used in various industrial applications, for a variety of purposes. Examples include, etching and deposition processes in microelectronics, plasma based depositions of tribological coatings, and plasma based surface modifications of various metals, alloys and polymers. Many of these applications require the generation of large plasma volumes of uniformly high density throughout the process chamber.
Existing systems and methods for producing large volume plasma have proved to be inefficient and are very difficult to scale to larger or smaller volumes. These limitations arise from the fact that the existing systems generate the plasma inside the process chamber generally in its central area, from where it is dispersed to the other parts of the chamber. The distribution of the plasma across the volume of the chamber is controlled by a spatial-magnetic field distribution coupled with its natural diffusion process. This magnetic field is created by means of external arrangement of magnets along the perimetry of the chamber. This specific arrangement of magnets creates and maintains the uniformity of the spatial plasma density. However, the effectiveness of this method is limited owing to the fact that the plasma is generated inside the chamber and the work pieces that are introduced into the chamber for the intended plasma treatment interfere with both, the generation and distribution of the plasma. In particular, these work pieces produce a "shadowing" effect that affects the homogeneity and uniformity of the plasma.
Therefore, there is a need for a system and method that can overcome these problems and provide a uniform distribution of large volume, high density plasma within the process chamber.
Object and Summary of the Invention
• The object of the present invention is to provide a system and method for producing large

volume plasmas of uniform high density.
• The proposed invention facilitates easy scaling to larger as well as smaller plasma volumes.
• It is also an object of the present invention to overcome the undesired shadowing effects caused by work pieces that are introduced in the plasma chamber.
• It is a further object of the invention to provide a fault tolerant and cost effective plasma treatment system.
To meet he above-mentioned objective, the present invention provides a plasma system comprising :
• a plasma process chamber having an even number of plasma injection ports arranged symmetrically in one or more vertical planes around the perimeter of the chamber walls, and
• a corresponding number of Compact Plasma Sources (CPS) coupled to said plasma injection ports such that each plasma source when energized injects plasma radially into the chamber and produces a magnetic field aligned along its injection axis, the magnetic field injection arrangement being such that the resultant magnetic field distribution in combination with the physically injected plasma volume distribution, results in an even distribution of plasma in the volume of the chamber,
Accordingly the invention provides a scalable, plasma generation system to produce uniform, high density plasma in large volumes consisting of multiple symmetrically distributed plasma sources with integrated magnets and microwave assembly producing a multidirectional, radially inwards plasma injection which is characterized in that screening of shadowing effect; best axial and radial confinement of plasma by means of three dimensional, multiple cusp magnetic field distribution yielding uniform plasma density; and adaptable system parameter configuration.
Brief Description of Drawings
These and other aspects of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings in which like designations are used to designate substantially identical elements.

Figure 1 illustrates the proposed design of a large volume, uniform density plasma generation system (fig 1B).
Figure 2 shows the multi-cusp magnetic field produced in a source plane in a plasma system using 4 compact plasma sources.
Figure 3 shows the improved cusp magnetic field in a source plane for a plasma system using 6 compact plasma sources.
Figure 4 shows a plot of the radial variation of plasma density in the case of a large volume, uniform, high-density plasma system with one source plane and 4 compact plasma sources measured with the probe placed in-line with the plasma source and symmetrically between the consecutive plasma sources.
Detailed Description of the Preferred Embodiments
The large volume, uniform high density plasma generation system comprises of a plurality of compact ECR plasma sources, preferably those described in our co-pending patent application No. 66/DEL/2006.
Figure 1B shows a vertically oriented large-volume, semi-cylindrical plasma processing chamber (111) of a diameter 'D' and length 'L' selected suitably for the intended application. Although the semi-cylindrical chamber shown in Fig. 1B is vertical, a horizontal orientation
is equally suitable for the present system. The arrangement has 'S' (S=l,2,3, ) equally
spaced, co-axial source planes (112,113,114,115), oriented perpendicularly to the longitudinal axis of the chamber (125). The spacing between the adjacent source planes is 'd'.
As shown in Fig 1A, each source plane has an even number (4,6,8, ...2N) of source ports (116,117,118,119,120) located symmetrically along the circumference of the source plane (112) on the outer surface of the chamber. "N" corresponds to the integral number of source port pairs in the source plane. The total number of source ports "2N", remains the same for all source planes in the system. Any two adjacent source ports (belonging to the same source plane) subtend an angle  = π/N at the center (126) of the source plane or longitudinal axis (125) of the process chamber (111). The chamber dimensions, in terms of diameter D and length L, the number of source planes S, the spacing between adjacent source planes d (= L/S) and the total number of source ports per source plane (2N), are determined by the

required plasma volume.
Compact Plasma Sources (CPS) such as Compact ECR Plasma (CEPS) Sources (116-120, 121-124) are mounted on the source ports. The compact ECR plasma sources contain 3 piece ring-type permanent magnets, which are configured to produce a radially oriented magnetic field extending inwardly or outwardly. In Fig.lA&B the CEPS that are configured to produce the 'IN' field configuration are labeled (117,119,123,124), while those configured to produce the 'OUT' field configuration are labeled (116,118,120,121,122). The 2N compact plasma sources in a source plane are arranged such that any two adjacent CEPS (within a source plane) have opposite magnetic field configurations.
Figs. 2 and 3 display the resultant magnetic field distribution in a source plane, corresponding to a cusp field configuration, for four (201-204) and six (301-306) CEPS respectively, per source plane. Such cusp field configurations (200, 300) facilitate the movement of the plasma to the central region of the chamber where the magnetic field is very weak. All the CEPS, barring the top and bottom CEPS (123,124) in each vertical plane, have identical (121,122) magnetic field orientations. The top and bottom source plane CEPS are arranged to have magnetic field orientations opposite to that of the other CEPS in the same vertical plane. This arrangement provides an effective plasma distribution mechanism along the chamber axis and at the extremities. The multi-cusp magnetic fields in each of the source planes combine to provide an overall uniform distribution of the plasma in the chamber volume.
The physical injection of the plasma towards the chamber axis further aids in physically moving the plasma towards the "relatively weak" magnetic field region. This multidirectional radial inflow of plasma from the periphery towards the axis of the plasma processing chamber counteracts density gradients arising from plasma diffusion and electrostatic repulsion effects. This arrangement is also effective in overcoming shadowing effects caused by the workpieces in the chamber. An increase in the number of source ports per source plane or the number of source planes enhances the cusp formation and improves the plasma distribution.
The invention also enables more efficient operation in the presence of the workpieces that exist in the chamber at the time of its use as it does not involve the introduction of any microwave energy directly into the chamber for the plasma generation. In conventional

methods the microwave radiation experiences interference from the metallic workpieces in the chamber and requires special arrangements to enable reasonable plasma generation. Even it is inefficient and a compromise in terms of plasma treatment effectiveness.
Figure 4 shows the scatter plot for actual plasma density values achieved for the system described above. The system has been experimentally implemented using a semi-cylindrical plasma processing chamber of height L=2.1m and diameter D=lm, with three source planes and four spacer sections to separate the source planes. The consecutive source planes are separated by distance d=45 cm. Each source plane can accommodate 4 or 6 compact ECR plasma sources. Suitable ports for vacuum pumping, plasma diagnostics, system monitoring etc are also provided. For practical applications, the system can be configured in several ways by varying the chamber dimensions, the number of source planes, the distance between the planes, the number of source ports or CEPS in each source plane, etc.
Figure 4 shows the radial plasma density profile for the above mentioned system configuration, (volume = 0.75 m3). The plasma density for Argon at 10-3 Torr pressure, measured at a distance of 9 cm from a compact ECR plasma source was found to be = 5 x 1011 cm-3. This yields a plasma density of about 4.5 x 1011 cm-3 near the center of the chamber. For the experiment purpose, all four CEPS were operating at about 2.45GHz; with about 550 Watts of microwave power. With a single source plane and six CEPS, the plasma density in the central region would be further enhanced along with a more uniform plasma filling.
While the present invention has been described with reference to certain preferred embodiments, it is not exclusively limited to the particular embodiments described herein. The proposed system is conceptually quite general and hence other variations and embodiments of the invention will occur to those skilled in the art. For example, the proposed system is assured to be equally efficient when implemented by replacing the compact ECR plasma sources with other types of plasma sources employing different energy coupling modes such as helicon plasma sources or inductively coupled plasma sources. Also the semi-cylindrical chamber geometry can be generalized by using any right prism shaped plasma processing chamber. The system described above can also be combined with other

conventional industrial processes such as deposition, sputtering, etc. to generate new industrial processes.
All documents cited in the description are incorporated herein by reference. The present invention is not to be limited in scope by the specific embodiments and examples, which are intended as illustrations of a number of aspects of the invention and any embodiments which are functionally equivalent are within the scope of this invention. Those skilled in the art will know, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. These and all other equivalents are intended to be encompassed by the following claims.

We Claim:
1. A plasma system comprising :
• a plasma process chamber having an even number of plasma injection ports arranged symmetrically in one or more vertical planes around the perimeter of the chamber walls, and
• a corresponding number of Compact Plasma Sources (CPS) coupled to said plasma injection ports such that each plasma source when energized injects plasma radially into the chamber and produces a magnetic field aligned along its injection axis, the magnetic field injection arrangement being such that the resultant magnetic field distribution in combination with the physically injected plasma volume distribution, results in an even distribution of plasma in the volume of the chamber,

2. A plasma system as claimed in claim 1, wherein said compact plasma sources are compact ECR plasma sources.
3. A plasma system as claimed in claim 1, wherein said CPSs are arranged to produce a multi-cusp magnetic field distribution extending in three dimensions,
4. A plasma system as claimed in claim 1, wherein said plasma process chamber is semi-cylindrical in shape with vertical or horizontal orientation,
5. A plasma system as claimed in claim 1, wherein said CPS comprise integrated permanent magnets which also produce the magnetic field injected into the chamber,
6. A plasma system as claimed in claim 3, wherein said CPS are arranged such that any two adjacent CPS in same source plane have opposite magnetic field polarity,
7. A plasma system as claimed in claim 3, wherein all said CPS belonging to various source planes but stacked in the same vertical plane, except top and bottom source planes, have the identical magnetic field polarity,
8. A plasma system as claimed in claim 3, wherein said top and bottom source planes
have their CPS arranged such that their magnetic field polarity is opposite to the polarity of the corresponding CPS stacked in interior source planes sandwiched between them,
9. A plasma system as claimed in claim 1, wherein said energizing microwave frequency falls within the UHF range of radio frequency spectrum, ITU band 9 or S-band of electromagnetic spectrum, in particular,
10. A plasma system, substantially as herein described with reference to the accompanying drawings.