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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to device processing and, in particular, to
semiconductor processing.
2. Art Background
The etching of a first material without unacceptably removing or damaging a
second is often required in processes such as semiconductor device
fabrication procedures. For example, it is desirable in certain
situations, such as in the production of appropriately configured gates,
to remove a region of silicon and/or metal silicide without causing
unacceptable removal of an underlying or adjacent region of a silicon
oxide, e..g, silicon dioxide. Processes such as plasma etching and
reactive ion etching employing a chlorine-containing gas are often
utilized to accomplish these results. In these techniques, a
chlorine-containing gas is typically introduced in proximity to the body
to be etched, and a plasma is established in the gaseous medium by
applying r.f. power between electrodes. Typically, the substrate rests on
the powered electrode, and the DC electric field associated with this
electrode directs the energetic entities produced in the plasma (e.g.,
ionized molecular fragments, ionized molecules, and ionized atoms) towards
the substrate and, through various mechanisms, causes removal of the
impacted material.
A variety of etching apparatus geometries and processing conditions has
been employed in the dry etching of materials such as silicon. The
specific configuration and etching conditions are generally chosen to
yield etching characteristics tailored to the particular semiconductor
device fabrication application. For example, a hex reactor, e.g., an
apparatus disclosed in U.S. Pat. No. 4,298,443, issued Nov. 3, 1981 (which
is hereby incorporated by reference), and illustrated in the Figures of
this patent, is capable of processing a large number of substrates during
one etching procedure. This reactor includes a hexagonally shaped cathode
contacting the substrates and typically a grounded outer shell that
functions as the second electrode. A plurality of substrates is positioned
on each face of the hexagonal cathode. Thus, for example, if 4 substrates
are placed on each face, it is possible to process 24 substrates during
one etching procedure. Alternatively, parallel plate reactors, i.e.,
reactors having a cathode and anode each formed by a plate whose major
surfaces are held in a parallel configuration, have been advantageously
employed in less demanding applications to provide suitable simultaneous
etching of 4 to 6 substrates. In a third type of reactor, one substrate
covers essentially the entire r.f. driven electrode, and a second
electrode, e.g., a parallel plate or vessel component, is provided.
While in many situations etching involving plasma-generated energetic
entities is advantageously employed, it is not without associated
difficulties. For example, the use of a plasma often leads to the
deposition of contaminating materials on the substrate surface. These
contaminating materials such as metals from the reaction vessel or
substrate holder, e.g., aluminum, either degrade device properties or
hinder subsequent processing procedures. Various measures have been
employed to avoid such contamination. For example, in the case of a hex
reactor, a tray surfaced with a material, e.g., a polymer such as a
polyarylate, is positioned on each face of the hex cathode with openings
through which the substrates are inserted. Thus, the substrates contact
the underlying electrode while remaining exposed to the plasma
environment.
Although as presently practiced, dry etching yields excellent results with
limited shortcomings, new applications have produced further, yet
unsatisfied, demands. For example, there are many applications of emerging
importance that require the removal of materials such as silicon with the
effect on adjacent, e.g., underlying or coplanar, materials such as
silicon dioxide substantially reduced from that which has been previously
achieved. For typical etching systems, selectivity, i.e., the rate of
etching of the desired region relative to underlying or unmasked adjacent
regions of different compositions, is not greater than 30 to 1. However,
as packing density in electronic devices, e.g., integrated circuits,
increases, many situations are evolving which require selectivities of at
least 50, preferably at least 70, and most preferably at least 100 to 1.
For example, in the etching of TaSi.sub.2 /polycrystalline silicon
composite gates, selectivity on the order of 100 to 1 is required to
assure that the thin oxide, less than 250 Angstroms thick, which is used
as an etch stop will not be totally removed.
Despite the substantially increasing desire for higher selectivity, the
adjusting of dry etching apparatus configuration and processing conditions
to achieve such results has not been reported. Indeed, in the dry etching
of materials employed in semiconductor devices, often the adjusting of
conditions or configurations to achieve one result causes a substantial
problem in a second unrelated etching characteristic. Thus, although there
is a desire for selective, plasma dry etching procedures, i.e., procedures
involving a gas plasma with selectivities greater than 50, such techniques
have not as yet been reported.
SUMMARY OF THE INVENTION
The selectivity produced for materials etched by chlorine entities is
significantly increased by substantially attenuating the AC electric field
present in regions laterally adjacent to the substrate deposition surface
and thus confining the plasma to the region directly over the substrates.
For example, by AC coupling the substrate surroundings to ground, the
selectivity of chlorine plasma etching between silicon and silicon dioxide
is increased to as high as 150 to 1. By attenuating the AC field in all
etching chamber surface regions subjected to the gas in which the plasma
is struck, extremely uniform etch rates across the entire substrate are
achieved. Additionally, if the substrate surroundings are not DC coupled
to ground, it is possible, by adjusting the DC potential, to enhance this
etch uniformity even further. Thus, the adjusting of electrical field
conditions such as by AC coupling to ground does not degrade other
properties, e.g., etch uniformity, associated with the etching procedure.
BRIEF DESCRIPTION OF THE DRAWING
FIGS. 1-6 are illustrative of conditions and configurations affecting the
procedure.
DETAILED DESCRIPTION
The invention is not limited to a particular plasma etching apparatus
geometry. For example, excellent selectivity is achievable in both
parallel plate and hex configurations. Irrespective of the particular
geometry employed in the plasma etching apparatus, to achieve high
selectivity, the plasma should be confined essentially to the region
overlying the substrate deposition surface. (Although for pedagogic
purposes the disclosure will be in terms of the etching of one substrate,
the same disclosure is equally applicable to multiple substrate etching by
the confinement of the plasma for each substrate to the region over this
substrate. Thus, for example, if three substrates are etched, then there
would be three distinct plasma regions, with one of these regions confined
over each substrate.) In the context of this invention, the region
overlying the substrate etching area is the spatial region defined by
normals to all points on the substrate etching surface. The plasma is
suitably confined if on average it does not extend a distance more than 10
percent of the substrate effective diameter beyond the boundaries of this
substrate etching region. (The substrate effective diameter is the
diameter of a circle having the same area as that of the substrate.) This
confinement criterion is satisfied if at least 70 percent of the plasma
emitted light in the wavelength range 3900 to 8000 Angstroms emanates from
points within the desired confinement volume. Confinement is achieved by
limiting AC electric field outside the region in which it is desired to
confine the plasma. A variety of expedients is available for suitably
limiting the AC field. For example, the surfaces surrounding the substrate
are capacitively coupled to ground so that the capacitance to ground is
significantly, e.g., at least 5 times, greater than the capacitance
between this surface and the r.f. driven cathode. In this embodiment, the
AC field is attenuated because these two capacitances act as a voltage
divider between the r.f. cathode and ground. The large capacitance to
ground has the smaller impedance. Thus, the surfaces surrounding the
substrate are significantly closer to ground potential than the r.f.
cathode. This low potential results in correspondingly low electric
fields.
Capacitive coupling can be accomplished in a variety of ways. For example,
if a grounded region contacts the back surface of the metal reactor tray
over a large area, then the capacitive coupling of the tray to ground will
be much greater than the capacitive coupling of the tray to the r.f.
driven cathode. (In this case, the tray will also be DC coupled to
ground.)
Alternatively, to attenuate the AC electric field, it is possible to
surround the substrate with a thick dielectric material, e.g., a material
with a thickness greater than 0.125 inch, preferably thicker than 0.5
inch, for compositions with dielectric constants above 2.5. In this
configuration, the surface of the dielectric which is exposed to the
reactor volume is only weakly capacitively coupled to the r.f. driven
cathode and capacitively coupled to ground only through the plasma. The
dielectric material attenuates the AC field by, for example, (1)
increasing the distance from the source of the field to the surface
subjected to the etchant gas, (2) by dielectric dissipation of the field
through movement of dipoles in the dielectric, and (3) through a voltage
divider effect where the capacitance of the plasma is sufficiently large
to substantially decrease the potential and thus the field at the surfaces
surrounding the substrate.
When expedients such as capacitively grounding the environment of the
substrate or insulating it with a thick dielectric are performed, the
plasma is confined to regions essentially directly over the substrates.
Regions between the substrates are not subjected to the plasma fireball.
Thus, material between the substrates is not eroded by mechanisms such as
sputtering, and contamination of the substrates by surrounding materials
is substantially decreased. Even more significantly, as discussed,
selectivity is substantially increased. Although the exact mechanistic
explanation for the increase in selectivity is not precisely known, it is
contemplated that the confined plasma discharge substantially enhances the
production of molecular ions at the expense of atomic ions. Thus, in the
case of a chlorine-containing plasma, the presence of ions such as
C1.sub.2.sup.+ is substantially increased at the expense of the
concentration of ions such as C1.sup.+.
As discussed, by confining the plasma to the region above the substrate,
excellent selectivity is achieved. With confinement alone, the etch rate
across the substrate is somewhat non-uniform, e.g., etch rates vary by up
to 35 percent. It is, however, possible to eliminate this non-uniformity.
Uniformity is achieved by ensuring that all surfaces in the reaction
chamber subjected to the etchant gas are AC field controlled in the same
manner as required for the substrate environment by the confinement
criterion, i.e., that the plasma is removed from these surfaces a distance
of at least 20 percent of the substrate effective diameter. For example,
the metal bell jar container, 25 in FIG. 1, forming the reaction chamber
is capacitively coupled to ground by, for example, providing large-area
conductive paths to ground. Although this expedient yields the desired
results, the metal bell jar is a possible source of contamination. Thus,
it is advantageous to internally coat it with a continuous glass layer
that is sufficiently thin to suitably attenuate the electric fields, e.g.,
a glass layer on the order of 0.06 inch in thickness. The AC grounding
attenuates the AC field and leads to etch rate variations across the
substrate no greater than 15 percent. Although AC grounding of the metal
bell jar has been found to be an expeditious procedure for achieving
suitable attenuation, any other means for satisfying the AC field
attenuation criterion, such as those described in conjunction with plasma
confinement, is adequate.
Uniformity is further improved, although not as dramatically, by adjusting
the DC bias of the substrate environment to the DC bias level of the
substrate. A variety of expedients is available for adjusting the DC
potential level of the substrate environment. For example, this variation
is achieved by utilizing a variable DC power supply with a continuous
conductive path from the supply to the substrate environment.
The elimination of the non-uniformity is significant not only in
conjunction with the inventive improvement in selectivity, but also
significant in other configurations where the environment of the substrate
is not at DC ground or where AC fields at the surface of the reaction
chamber are not suitably controlled. Most significantly, uniformity, i.e.,
a mean square deviation in etch rate across the substrate of less than 4
percent, is achieved without adversely affecting selectivity.
The following examples are illustrative of the invention:
EXAMPLE 1
A silicon substrate measuring 3 inches in diameter having its major surface
in the (100) crystallographic plane was cleaned and oxidized at 1000
degrees C. in oxygen to yield a 1000-Angstrom thick silicon dioxide layer.
Four thousand Angstroms of polycrystalline silicon was deposited onto this
silicon dioxide layer utilizing a low pressure chemical vapor deposition
(LPCVD) procedure with a deposition gas of silane and with the substrate
heated to a temperature of approximately 600 degrees C. (This LPCVD
procedure is fully described in "Low Pressure CVD Production Processes for
Poly, Nitride and Oxide," by R. S. Rosler, Solid State Technology, Vol.
20, page 63 (April 1977).)
The sample was placed in position 5 of FIG. 2 on the driven electrode, 6,
of a reactive ion etching apparatus. The driven electrode, 6, had an
overlying Teflon (a registered trademark of E. I. duPont deNemours & Co.,
Inc.) sheet, 7. The addition of an aluminum plate, 9, overlying the Teflon
sheet was inserted to allow capacitive grounding through wing, 8, of this
sheet. Finally, an Ardel (a registered trademark of Union Carbide
Corporation) sheet, 11, 1/8th-inch thick, overlaid the aluminum plate. A
counterelectrode, 10, which was 18 inches in diameter, was positioned
approximately 4 inches from the exposed major surface of the sample. The
etching chamber was evacuated to a pressure of approximately
1.times.10.sup.-5 Torr. A Cl.sub.2 flow rate of 20 sccm was established
through the reactor chamber. The vacuum pumping speed was then diminished
so that the Cl.sub.2 pressure in the chamber increased to 20 mTorr. An
r.f. power of 9 watts was applied to the driven electrode at a frequency
of 13.56 MHz. The resulting etch rate of the exposed polycrystalline
silicon layer, as measured with a laser interferometer, was 200
Angstroms/minute. After the polycrystalline silicon layer was removed, as
indicated by the interferometer, etching was continued into the exposed
oxide for 30 minutes. This etching resulted in an oxide removal rate of
2.2 Angstroms/minute which, when compared with the polycrystalline silicon
etch rate, yielded a selectivity of 90 to 1.
EXAMPLE 2
The procedure of Example 1 was followed except two separate runs utilizing
14 watts and 20 watts, respectively, of applied r.f. power were performed.
These runs yielded a polycrystalline silicon etch rate of 400 and 457,
respectively, and an oxide etch rate of 6.2 and 10.6, respectively, and
thus a respective selectivity of 65 to 1 and 43 to 1.
EXAMPLE 3
The procedure of Example 1 was followed except the substrate was placed on
a pedestal attached to a driven electrode, and only a 1/8th-inch thick
Ardel sheet, 15, overlaid this electrode, as shown in FIG. 3. As can be
seen in Table 1, various r.f. powers were applied yielding selectivities
significantly lower than those obtained for the corresponding etching
conditions in the previous Examples.
TABLE 1
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R.F. Poly-Si Oxide Selectivity
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9 W 80 l.4 57:1
14 W 133 3.3 40:1
20 W 210 8.4 25:1
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EXAMPLE 4
Samples with a polycrystalline silicon and silicon dioxide layer were
prepared, as described in Example 1. These samples were placed on the
driven electrode of a hexagonally configured cathode RIE reactor. (This
reactor is described in U.S. Pat. No. 4,298,443, issued Nov. 3, 1981.) The
reactor was modified, as shown in FIG. 1, to have an aluminum tray, 17,
with Ardel outer plates, 18, having cutouts of a size slightly larger than
the individual substrates contacting grounded plate, 19. The grounded
plate was set off from the driven electrodes, 20, and pedestals, 21,
utilizing Teflon insulating spacers, 22. The bell jar, 25, was grounded
and had an internal glass coating that was 0.09 inch in thickness. Each
substrate position not occupied by a sample having both a polycrystalline
silicon and silicon dioxide layer was occupied by a silicon substrate
covered with resist to avoid exposure of the aluminum pedestal. The
chamber was evacuated to a pressure of approximately 1.times.10.sup.-5
Torr. A molecular chlorine flow through the chamber was established at a
rate of 40 sccm. The pumping speed of the vacuum pump was then decreased
so that the pressure in the chamber rose to approximately 20 mTorr. An
r.f. power of approximately 70 watts at a frequency of 13.56 MHz was
applied to the driven electrode. The etch rates of the polycrystalline
silicon and silicon dioxide were measured as described in Example 1. The
polycrystalline silicon etched at a rate of approximately 96
Angstroms/minute, the silicon dioxide etched at a rate of approximately
0.94 Angstrom/minute, and thus the selectivity was approximately 90 to 1.
The profiles obtained as the etching reached the polycrystalline
silicon/silicon dioxide layer interface are shown in FIG. 5 as an
indication of the achieved etch rate uniformity.
EXAMPLE 5
The procedure of Example 4 was followed except the r.f. power was
approximately 100 watts, and the bell jar was entirely a glass
composition. A grounded grid of aluminum enclosed this glass bell jar. The
relatively non-uniform profiles obtained as the etchant reached the
interface between the polycrystalline silicon and silicon dioxide layers
are shown in FIG. 4.
EXAMPLE 6
The procedure of Example 4 was followed except the hexagonal cathode was
only one tier high, as shown in FIG. 6, and thus accepted only one
substate per face of the hexagonal electrode. Additionally, the grounding
metal plate was not present, and only a tray of Ardel, 22, 0.5 inch in
thickness, surrounding the substrate and abutting the hexagonal cathode
was utilized. The applied power was 24 watts, and the initially
established chlorine flow rate was 30 sccm. The polycrystalline silicon
etch rate was 230 Angstroms/minute, the silicon dioxide etch rate was 3.1
Angstroms/minute, and thus the selectivity was approximately 74 to 1.
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