Electric heating – Metal heating – By arc
Reexamination Certificate
2000-03-08
2001-05-29
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121570, C219S121430, C315S111510
Reexamination Certificate
active
06239404
ABSTRACT:
This invention relates to plasma processing apparatus and apparatus in which ions are extracted from a plasma source. Plasma processing apparatus is used extensively in the fabrication of semiconductor devices and in many other processes in which the deposition of coatings or the etching of surfaces is required. Particularly when these processes are being carried out on a production scale, uniformity between batches becomes very important.
Generally such plasmas are generated in a vacuum chamber, and it is preferred that the means for generating the plasma is either located outside of the chamber or within a protective wall, in order to reduce the disturbance to the plasma potential when radio frequency power is applied. When the radio frequency power is inductively coupled into the plasma, the antenna used for this purpose is either located within the chamber and encased by a dielectric material or outside of the chamber adjacent to a portion of the wall which is constructed out of dielectric material. The antenna then effectively forms the primary of a transformer of which the secondary is the plasma within the source.
A plasma source may utilise one or more antennae, and if located outside of the source, each will be located adjacent to dielectric sections of the sides or the top. A single antenna may take the form of a flat spiral on the top of the source, or a multiple turn coil wound around the sides or the source, or a single turn coil located adjacent to either the top or sides of the source, or other forms not detailed here. The dielectric section of the source may be re-entrant with the antenna placed within the profile. When multiple antennae are used, they may be located adjacent to separate dielectric sections of the source, or wound separately, but adjacent to the same dielectric section.
In most inductively coupled plasma systems, the antennae are driven so that one end or termination of each antenna is grounded and the other end is connected with the output of the impedance matching unit. This is generally a very effective technique for producing a dense plasma, however, there is the disadvantage that a high RF voltage is applied to the driven end or termination of the antenna. This leads to capacitive coupling of RF power to the plasma near this end of the antenna, the magnitude of the voltage an the capacitive coupling steadily reducing towards the grounded end of the antenna. The inductive power coupling is effectively constant along the antenna and therefore there is a non-uniform overall coupling of power (inductive+capacitive) into the plasma as a function of position along the antenna. This has the potential for producing a plasma which is not spatially uniform, leading to reduced uniformity in the plasma processing application. Methods have been adopted in commercial plasma processing systems to reduce the capacitive coupling element and therefore to partially overcome this significant inhomogeneity. For example, the use of a grounded electrostatic shield between the antenna and the dielectric window, with appropriate slots to permit effective inductive coupling of power to the plasma. This is used, for example by Prototech Research Inc.
From one aspect the invention consists in plasma processing apparatus including a chamber, an antenna for inducing a plasma and an alternating current supply means for supplying power to the antenna via its terminations chracrerised in that the power supply means provides substanially 180° out-of-phase inputs on the terminations of the antenna.
The power supply means may include a power supply and a transformer fed by the power supply and the antenna may be connected across at least part of the secondary winding of the transformer, the invention being further characterised in that a tapping of the secondary winding is grounded such that the antenna is effectively grounded intermediate its ends.
With a centrally tapped secondary winding and a single coil antenna, the antenna is effectively or virtually grounded at its centre point and the voltage excursion of each end of the coil with respect to ground may be effectively halved when compared with the voltage on the driven end of the conventional coil antenna. Improved spatial uniformity is produced in the plasma, which in turn leads to improved spatial uniformity in the processing of the semiconductor wafer or other work piece. In addition to the increase in the spatial uniformity of the plasma, a degree of impedance matching between the plasma and the RF power supply may be obtained by a suitable choice of the relative numbers of turns on the primary and secondary windings of the transformer.
Although the invention is most conveniently utilised and understood in terms of a single coil antenna, the other coil configurations described above and multiple antenna may also be utilised. It will be understood that with coils having more complex geometry, such as a spiral, the tapping point may be offset to achieve an offset ‘grounding’ of the coil, although the desired ground point will be generally midway between the terminations of the antenna. The correct point can be determined empirically by adjusting the tapping point to substantially minimise the voltage excursions at the terminations of the antenna.
Other ways of achieving a substantially 180° out-of-phase power input on the terminations of the antenna can be utilised and may be more advantageous in certain circumstances. Thus the power supply means may include separate power supplies connected to respective inputs of the antenna and these power supplies may be linked by a phase lock loop to maintain the power supplies out of phase one to the other. This arrangement is particularly useful when high power is required and is able to operate over a wide range of frequencies. Preferably respective matching circuits are included between the power supplies and their respective antenna terminations.
In an alternative approach, the power supply means may included a power supply, a transformer and a matching circuit and the antenna may be earthed intermediate its ends. To achieve 180° out-of-phase inputs, the earthing needs to be at or adjacent the midway point between the terminations of the antenna, just as the effective or virtual grounding discussed above.
In the still further alternative, the power supply means may include a variable matching circuit for matching the plasma and impedance to the output impedance of the power supply whereby substantially 180° out-of-phase inputs to the formations are achieved.
From the still further aspect the invention consists in a plasma processing apparatus including a chamber, and antenna for inducing a plasma and power supply means characterised in that the power supply means induces a virtual or effective ground intermedate the terminations of the antenna.
From yet another aspect the invention consists in plasma processing apparatus including a chamber, and an antenna for inducing plasma in the chamber and an alternating current supply means for feeding the antenna characterised in that the power supply means includes a power supply and a transformer fed by the power supply, in that the antenna is connected over at least part of the secondary winding of the transformer and a tapping of the secondary winding is grounded such that the antenna is effectively grounded intermediate its terminations.
REFERENCES:
patent: 5147493 (1992-09-01), Nishimura et al.
patent: 5565074 (1996-10-01), Qian et al.
patent: 5573595 (1996-11-01), Dible
patent: 5965034 (1999-10-01), Vinogradov et al.
patent: 9-106979 (1997-04-01), None
Guibarra Edward
Lea Leslie Michael
Jones Volentine, LLC
Paschall Mark
Surface Technology Systems Limited
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