Electric lamp and discharge devices: systems – Discharge device load with fluent material supply to the... – Plasma generating
Reexamination Certificate
2002-10-25
2004-07-06
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Discharge device load with fluent material supply to the...
Plasma generating
C315S111210, C315S111910, C315S039650, C315S039690, C313S231310, C118S7230AN
Reexamination Certificate
active
06759808
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to stripline microwave applicators particularly for creation and maintenance of mini and micro microwave (plasma) discharges. The apparatus and methods described are directed toward efficiently creating and precisely controlling very small microwave discharges (plasmas). These discharges have typical physical dimensions, d, that are less than a millimeter and as small as a few tens of microns. The free space wavelength, &lgr;, of microwave energy (300 MHz-30 GHz) varies from one meter to one centimeter and thus &lgr; is much greater than d throughout the entire microwave frequency spectrum. In particular, the present invention relates to an apparatus wherein the stripline conductors that couple microwave energy are transverse to the microwave discharge, and preferably with a container for generating the plasma so that the plasma extends beyond the stripline excitation region.
(2) Description of Related Art
It is also well known that a condition for the existence of a plasma discharge is that d>(6-10)&lgr;
DE
where
λ
DE
=
743
T
e
n
e
Cm
,
T
e
is the electron temperature in volts, and n
e
is the electron density in electrons per cm
3
. This criteria implies that to produce very small plasmas (discharges) high densities and low electron temperatures are desirable. For example to create 100 micron size microwave plasmas the Debye length, &lgr;
DE
, must be approximately 10-15 microns. If T
e
~4 volts then n
e
≳10
12
cm
−3
. If d~10 microns and if T
e
~1 volts, then n
e
≳10
14
cm
−3
. Thus very small discharges require low electron energies and high charge densities, and are as a result very intense discharges that have very high absorbed power densities (W/cm
3
). Despite the required high power densities the total absorbed power of these discharges is very low, i.e. of the order of a few watts or less.
The high density n
e
requirement of these very small microwave plasmas implies that n
e
>>n
c
where n
c
is the critical density. The critical density n
c
is defined as the density where, f, the excitation frequency is equal to the plasma frequency, f
pe
. That is when
f=f
pe
=8980
{square root over (n
c
)}Hz
where n
c
is in units of cm
−3
. Very small plasmas require very high electron densities, n
e
. Thus n
e
>>n
c
. Therefore, the microwave plasma will be over dense, and as a result the electromagnetic energy will not freely propagate through the discharge, but will exist in a thin discharge surface layer equal to about the skin depth, &dgr;
c
, where
δ
c
=
(
2
mν
eff
ω
μ
o
e
2
n
e
)
1
/
2
=
c
ω
pe
(
2
ν
eff
ω
)
1
/
2
However, for very small discharges &dgr;
c
>d.
This condition indicates that higher excitation frequencies will more readily produce higher density discharges. The required high densities also impose conditions on discharge pressure. To readily achieve the required high densities it is desirable to operate these discharges at moderate pressures (≳Torr) to higher pressure environments (one or more atmospheres) where high species densities are available and where &ngr;
eff
/&ohgr; is greater than one thereby insuring some electromagnetic energy penetration within the discharge.
Mechanisms of coupling energy into microwave discharges vary with pressure. At low pressure the effective electron collision frequency, &ngr;
eff
, is much less than &ohgr;. Thus energy coupling takes place primarily via stochastic heating and resonant wave/collisionless heating mechanisms. These mechanisms include such phenomena as electrons impinging on the oscillating sheath edge and wave particle interactions that occur in electroacoustic wave/surface wave plasma interactions. As the pressure is increased &ngr;
eff
increases and thus electromagnetic/discharge coupling takes place via an electron collisional process, i.e. ohmic heating.
Early microwave discharge experiments demonstrated the formation of relatively small discharges with dimensions of a few mm or larger and with microwave absorbed power levels of a few Watts or more (Fritz, R., M. S. Thesis, Michigan State University, East Lansing, Mich. (1978); and Asmussen, J., Thesis, University of Wisconsin (1967); Asmussen, J., et al., Appl. Phys. Letters 11, 324-326 (1967); Asmussen, J., et al., IEEE Trans. Electron Devices, ED-16, 19-29 (1969)). During the period of these experimental investigations it was envisioned that the practical application of microwave discharges required discharges with typical dimensions of several centimeters or more. Thus research efforts were directed toward development of applicator coupling techniques that created and maintained large volume, high density discharges with dimensions of 8.0-40 cm. These efforts resulted in a variety of microwave discharge configurations such as those described in Asmussen, J., et al., IEEE Trans. In Plasma Science, PS-25, 1196-1221 (1997); and Popov, G., High Density Plasma Sources, Chapter 6, Noyes Pub. (1996)) and in U.S. Pat. Nos. 4,507,588; 4,585,688; 4,630,566; 4,727,293 and 5,081,398 to
Asmussen.
Using this technology with 2.45 GHz excitation, large volume discharges were created strategically locating the bounded plasma volume within the applicator. The optimal location of the discharge volume allowed the discharge to be exposed to a relatively large region (in comparison to the excitation wavelength) of applied electromagnetic field. Additionally the applicator had to be adjustable to enable first the ignition of the discharge and then the efficient matching of high power (100-thousands of watts) into the high density plasma. Then these applicator/discharge configurations were scaled up by decreasing the excitation frequency to 915 MHz. These techniques were successful in creating uniform microwave plasmas over a pressure regime of a few millimeters to over 200 Torr with dimensions of 10-35 cm.
However, the applicator technologies that were developed to create large discharges, are not optimal for the formation of small discharges. If the excitation frequency is raised the waveguide and cavity applicators become smaller and thus become more difficult to fabricate.
One method of producing high density discharges is by the use of rf inductive plasma coupling via planar or helical coils (Lieberman, M. A., et al., “Principles of Plasma Discharges and Materials Processing,” John Wiley and Sons, (1994)). Inductive coupling results in the noncapacitive power transfer to the charged species of this discharge, thereby achieving a low impressed voltage across all plasma sheaths at electrode and wall surfaces. These high density plasma sources are typically excited by 13.56 MHz rf energy and are capable of producing large 10-40 cm diameter discharges with densities in excess of 10
12
cm
−3
. Thus n
e
>>n
c
and &lgr;>>d and their behavior can be understood by quasistatic electromagnetic analysis. These discharges represent an important method of electromagnetic/plasma excitation, i.e. quasistatic inductive excitation.
Recently, Hopwood et al has scaled these inductive planar discharges down to very small dimensions (Yin, Y., et al., “Miniaturization of Inductively Coupled Plasma Sources,” IEEE Trans. Plasma Science, 27, 1516-1524 (1999)). See also U.S. Pat. No. 5,942,855 to Hopwood for a small plasma generator. Small planar coils of 5-15 mm diameter were fabricated and were excited with 100-460 MHz rf energy. These small discharges demonstrated the ability of inductive coupling at high frequencies to sustain small high density plasmas. Microfabrication techniques were used to fabricate the small planar inductive coils. However, these experiments indicated that as rf frequency was increased the coupling efficiency decreased. Small plasmas required high plasma densities which in turn require high excitation frequencies (~1-5 GHz) and the fabrication of smaller inductive coils that must operate
Asmussen Jes
Grotjohn Timothy A.
Wijaya Andy
Board of Trustees of Michigan State University
McLeod Ian C.
Philogene Haissa
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