Coating processes – Direct application of electrical – magnetic – wave – or... – Electromagnetic or particulate radiation utilized
Reexamination Certificate
2002-08-14
2004-09-14
Niebling, John F. (Department: 2812)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electromagnetic or particulate radiation utilized
C427S488000, C315S248000
Reexamination Certificate
active
06790487
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to plasma assisted processes and apparatus, in particular for performing deposition and etching operations on semiconductor substrates. The invention is particularly directed to the performance of such processes in electrostatically shielded radio frequency (ESRF) plasma sources.
An ESRF plasma source is constructed and operated to generate a plasma in a processing, or plasma, chamber which contains an ionizable gas or a mixture of ionizable gases at a desired pressure. The plasma chamber is usually cylindrical and the gas pressure within the plasma chamber is typically of the order of 0.1 to 10 milliTorr (mT).
The ESRF plasma source typically includes, in addition to the plasma chamber, a radio frequency (RF) oscillator-amplifier that operates at a frequency typically in one of the ISM bands (e.g., 13.56 MHz or 27.12 MHz), a tapped helical or solenoidal coil that is driven by the oscillator-amplifier and surrounds the plasma chamber, and a metal electrostatic shield placed between the helical coil and the wall of the plasma chamber. The oscillator-amplifier includes, or is connected to, an impedance matching network and is typically capable of providing RF power well in excess of 1 kW to the helical coil. The RF power is coupled by the coil into the plasma established within the plasma chamber.
A highly simplified diagram of an ESRF plasma source is shown in FIG.
1
. The source is constituted essentially by an enclosure
2
within which a low pressure region containing an ionizable gas can be maintained. Enclosure
2
is surrounded by a grounded RF shield
4
made of a conductive material. The upper portion of enclosure
2
is surrounded by a helical or solenoidal coil
6
having one end grounded via shield
4
and its other end open-circuited. An electrostatic shield
9
is located between the helical or solenoidal coil
6
and the wall of enclosure
2
. Electrostatic shield
9
acts to reduce to acceptable levels the amount of RF radiation emanating from the source. RF power is delivered by means of an RF input
5
to helical coil
6
via a tap of coil
6
that is positioned along the length of coil
6
to optimize the ability of any impedance matching network to adjust as required to couple the RF power to the plasma effectively for the intended application under both start-up and run conditions. The portion of helical coil
6
between the tap and the ground end thereof is approximately equivalent, at the operating frequency f
0
, to a quarter wavelength transmission terminated in a short-circuit.
According to conventional practice in the art, electrostatic shield
9
is provided with a number, possibly 15 to 20, of narrow slots (not shown) which extend vertically parallel to the axis of enclosure
2
and are roughly coextensive, in the vertical direction, with the axial length of helical coil
6
. To function properly, electrostatic shield
9
must be provided with at least one well-designed ground connection, as shown in FIG.
1
. Preferably, a ground connection is provided at each end of shield
9
.
The plasma source shown in
FIG. 1
is completed by a substrate support, or chuck,
8
which supports a substrate, such as a semiconductor wafer, that is to be subjected to a deposition or etching procedure. It is to be understood that
FIG. 1
does not purport to illustrate the details of such a plasma source, which are already known in the art, and is simply intended to provide an understanding of the basic spatial relations among a plasma source, a power coupling coil, and a substrate support.
The load acting on the oscillator-amplifier is composed principally of helical coil
6
, electrostatic shield
9
and the plasma, but also includes various other intrinsic components. The impedance of this load is preferably resonant at the operating frequency f
0
. Due to the high degree of non-linearity of the plasma, frequency components at integral multiples of the fundamental drive frequency exist with very significant amplitudes. The frequency of each harmonic component may be expressed in the form
f
n
=nf
0
, (1)
where n is an integer greater than or equal to 1.
In practice, the electromagnetic energy in a typical ESRF plasma source will simultaneously include components at the fundamental frequency and at one or more of the harmonic frequencies given by equation (1). Through slight variations in the position of the RF tap connection of coil
6
, or through other circuit modifications, some control of the harmonic amplitudes is possible.
FIGS. 2A and 2B
are plots of the measured frequency spectrum for an inductively coupled plasma source operating at f
0
=13.56 MHz.
FIG. 2A
shows the relative amplitudes of the fundamental and harmonic frequency peaks above the −20 db line of
FIG. 2B
, while
FIG. 2B
shows the overall frequency spectrum.
FIGS. 2A and 2B
are reproductions of actual spectral analyzer printout. It is believed that the second peak to the right of the 40 MHz indicium in
FIG. 2B
is a result of a spurious output. It is obvious that significant frequency components are present at the fundamental frequency f
0
and at a number of the harmonic frequencies. It has been found that this is true for values of n less than or equal to about nine. A similar result will be obtained at f
0
=27.12 MHz.
Plasma chemistry is greatly affected by the so-called electron temperature of the electrons in an ESRF plasma source and it is known that electron temperature depends on the RF power absorbed by the plasma. It is also known that the electromagnetic energy coupled into the plasma in an ESRF plasma source is absorbed in a plasma surface layer having a thickness typically of the order of one centimeter for 10
12
electron-ion pairs /cm
2
and a drive frequency of 13.56 Megahertz This layer thickness is comparable to the skin depth of the RF frequency in the conductivity of the plasma. The absorption of electromagnetic energy in this surface layer is analogous to the well-known “skin effect” in metallic conductors. The surface layer thickness is approximately proportional to the inverse of the square root of the fundamental frequency.
More specifically, the electron temperature is in the region of high power density absorbed proportional to the RF power density absorbed by the plasma. That is, the electron temperature is proportional to the RF power density in the surface layer in which the RF power is absorbed.
It is known that electron temperature can be measured with the aid of Langmuir probes immersed in the plasma, by analysis of the optical emissions from the plasma, or by analysis of the microwave emissions from the plasma. Measurement by analysis of microwave emission from the plasma to determine the electron temperature has the advantage of being non-intrusive and of being usable with various reactive gases as desired that may interfere with the quality of the contact between the plasma and the probe.
Another plasma parameter that is important for practical applications is electron, or plasma, density. It is known that the electron density increases almost linearly with the RF power density in the plasma, i.e., the absorbed RF power divided by the total volume of the plasma. In contrast, the electron temperature is proportional to the RF power divided by the volume of the plasma surface layer.
Electron temperature and electron density both influence the results of plasma assisted processes in different ways. For example, the electron density directly affects the concentration of ionic and neutral species available to react at a wafer surface to produce the desired result. In general, a greater electron density produces a greater process throughput due to a greater deposition, etch, or cleaning rate. However, a plasma process may require an electron density less than some process-dependent value, because the excessive generation of energetic species such as energetic ions or ultraviolet photons may cause damage to the wafer or to semiconductor devices already fabricated o
Niebling John F.
Pillsbury & Winthrop LLP
Stevenson Andre′ C.
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