Plasma etching installation

Details Plasma etching installation Plasma etching installation

2000-11-22

2003-03-11

Mills, Gregory (Department: 1763)

Adhesive bonding and miscellaneous chemical manufacture

Differential fluid etching apparatus

With radio frequency antenna or inductive coil gas...

C118S7230IR, C118S7230AN

Reexamination Certificate

active

06531031

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a plasma processing system
BACKGROUND INFORMATION
Plasma processing systems of this kind, which use inductively coupled plasma sources, are suited, in particular, for deeply etching silicon at very high etching rates, employing the method described in German Patent DE 42 41 045, and are widely known. A simple and proven arrangement is made up of an ICP coil (ICP=“inductively coupled plasma”), which is wound around a plasma volume and is fed by a high-frequency a.c. voltage. In the plasma volume, the high-frequency currents flowing through the ICP coil induce a high-frequency, magnetic alternating field, whose electric curl (rotational or vortex) field, in turn, excites the plasma in accordance with the law of induction (rotE=−∂B/∂t). The applied high frequency has values of between 600 kHz and 27 MHz; a frequency of 13.56 MHz is usually used.
In the method known from German Patent DE 42 41 045, a plasma source, preferably having inductive high-frequency excitation, is used to liberate fluorine radicals from a fluorine-supplying etching gas, and passivation gas (CF
2
)
x
from a Teflon-forming monomer, the plasma source generating a highly dense plasma having a relatively high density of ions (10
10
-10
12
cm
−3
) of a low energy, and the etching and passivation gases being alternately used. The ionic energy, which accelerates the generated ions toward the substrate surface, is likewise relatively low, i.e., between 1-50 eV, preferably 5-30 eV. In the description,
FIG. 2
of this patent shows a typical asymmetrical supplying of an ICP coil of such a plasma source, as known from the related art. In the simplest case, it is composed of only one single winding around a reactor in the form of a vessel (tank or chamber) made of ceramic material, having a diameter of, for example, 40 cm. One coil end is grounded; the other coil end is fed with the high-frequency a.c. voltage, and described as “hot”, because at this coil end, very high voltages of, for example, 1000-3000 volts build up, which are typical for the amplitude of the supplied high-frequency high voltage.
The capacitors C
2
and C
3
, likewise shown in
FIG. 2
, are used to adapt the impedance of an asymmetrical 50 &OHgr; coaxial cable output of a high-frequency incoming supply to the impedance of the asymmetrically operated ICP coil (so-called “matchbox” or “matching capacitors”). The capacitor C
4
is connected in parallel to the ICP coil and, together with the matching capacitors, produces the resonance condition.
The consequence of supplying the plasma source in the known asymmetrical and inductive fashion is that the asymmetry is also projected onto the plasma that is produced. On the average, depending on the intensity of an occurring capacitive coupling, this lies above earth potential by a few up to tens of volts. Thus, one coil end of the ICP coil is at earth potential (0 V) and the opposite “hot” coil end is at a high high-frequency voltage of up to a few thousand volts. As a result, strong electrical fields are induced in the plasma, in particular at the “hot” coil end, through the ceramic vessel wall of the reactor, which, in turn, produce displacement currents in the plasma, through the ceramic vessel wall. In this case, one speaks of the already mentioned “capacitive coupling”, while the actual production of plasma is an inductive mechanism, i.e., one based on time-variable magnetic fields.
For the most part, the capacitive coupling causes a current to flow from the supplied, i.e., “hot” coil end, through the ceramic vessel wall of the reactor, into the plasma. Since the average plasma potential fluctuates near earth potential, to which the “cold” coil end is also fixed, and since the potential difference between the plasma and the “cold” coil end is too small to allow the displacement current to flow off (discharge) again across the ceramic vessel wall to the coil, this current flow is not able to flow off (discharge) to the grounded coil end. Thus, the displacement current must discharge (flow off) from the region of the “hot” coil end into the plasma, again out of the plasma, across a ground that is in direct contact with the plasma. In known methods heretofore, this is essentially the substrate electrode, which, for example, as a substrate, bears a wafer, and which is operated via a separate high-frequency supplying at a low negative DC bias potential of 1-50 V with respect to the plasma. Therefore, it is able to directly take up the mentioned displacement currents. However, this leads to inhomogeneities in the plasma-processing method in question, across the substrate surface and, thus, partially to considerable profile deviations when etching in individual regions.
In addition, the strong electrical fields occurring on one side, due to the asymmetrical supplying, distort the position and density distribution of the produced plasma, which is shifted away from the middle of the reactor and is displaced, for example, toward the “hot” coil end. In this case, one speaks of a so-called “bull's eye shift”, since the inhomogeneity of the plasma is projected in the shape of an eye onto the wafer used as a substrate, and since this “eye” shifts away from the middle of the wafer toward the wafer rim.
A first measure for improving the process homogeneity and for avoiding the “bull's eye shift” is a process known from German Patent DE 42 41 045, as described in the unpublished Application DE 197 34 278.7. An aperture construction is proposed, which, due to an expanded ionic recombination zone at the inner wall of a metal cylinder mounted on an aperture, homogenizes the ionic flow toward the substrate across the substrate surface in question. It does this by incorporating an ionic loss mechanism in the outer region of the plasma attaining the substrate. It also recenters the plasma and partially shields electrical fields coming from the source region of the plasma, on the way to the substrate.
A further measure, which diminishes profile deviations of etched structures on the substrate or a wafer, occurring, in part, as the result of electrical interference fields, is proposed in the unpublished Application DE 197 363 70.9, in which a so-called “parameter ramping” is used.
In the same way as the “hot” coil end, i.e., the “cold” coil end, the grounded coil end in the related art, is a problem zone, because this end is the location of minimal feeding in (injection) or feeding out (coupling out) of displacement current, due to the capacitive coupling into the produced plasma. Moreover, in known methods heretofore, the corresponding “cold” voltage feed point of the ICP coil, which is in communication with the “cold” coil end, must be grounded with great care, since, in the active ambient environment of the plasma reactor, vertically flowing currents, in particular, must be avoided at all costs, i.e., currents flowing from the ICP coil down to the grounded housing. Such vertical currents, in other words currents that do not flow in parallel to the coil plane defined by the ICP coil, result in a time-variable magnetic field that is tilted by 90° and that has corresponding electrical induction effects over the electric curl (rotational or vortex) field. This leads to considerable local disturbances in the plasma, which are manifested, in turn, in profile deviations (pocket formations, negative etching-edge slopes, mask rim undercuts).
Another interference effect that is known in related-art methods and is caused by high voltages at a “hot” coil end, entails sputtering deposition at this end, on the inside of the reactor side wall, by ion bombardment, i.e., by positively charged ions accelerated by strong electrical fields toward the chamber wall. In the process, sputtered wall material can also attain the wafer or the substrate and have a micro-masking effect there. The consequence, as is generally known, is the formation of silicon needles, micro-roughness, or silicon particles. Since the sputter-type ablation of the reac

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