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Description  |
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This invention relates to the use of an inductively excited plasma as a
large area homogeneous source of ions.
Reactive ion etching (RIE) is a dry etching technique used predominantly in
semiconductor wafer fabrication. The technique is to ionise a gas or gas
mixture to generate a plasma containing excited and ionised, atomic or
molecular species which will react with the material to be etched. The
sample, part of which is to be etched, is placed in the vicinity of or
within the plasma, and it is biased in such a way that the ionidised
species impact the surface to be etched. This may be accomplished by
connecting the substrate to a negative potential, generally termed the
bias, when the ions, which are mainly positively charged, will be
attracted to the substrate. However, since the substrate is usually
insulating a source of radio frequency power is commonly used, such that
ions and electrons will be alternately attracted to the substrate and
though in equilibrium a mean voltage is usually established, the surface
continues to be bombarded so long as the R.F. power is applied.
Reactive ion etching may be accomplished using a single source of R.F.
power applied to the electrode supporting the substrate with the remainder
of the apparatus or a second electrode grounded. This, while being simple
has the disadvantage that the bias power and the plasma intensity, hence
the number of ions which are available to bombard the surface are directly
related and so cannot be independently varied.
Various techniques have been used in an attempt to overcome these
limitations, particularly some employing microwave power to generate the
plasma. This does allow higher plasma densities and will extend the
pressure range down to about 1.times.10.sup.-4 Torr. However it is
complex, expensive and somewhat difficult to shield against unwanted
radiation. By contrast this invention now utilises an inductively coupled
R.F. plasma generator to provide the source of ions while the substrate is
biased as previously described.
Broadly stated from one aspect the present invention consists in an
apparatus for use in treating semiconductor wafers or other work pieces by
active ion technique or by chemical vapour deposition, comprising a
"vacuum" chamber having means for connection to an evacuating pump and to
a source of the selected gases, an induction coil surrounding part of the
chamber and connected to an alternating current supply to create a plasma
within the chamber containing the gas mixture to be ionised, a support for
the work piece within the chamber, and means for creating an electric
field between the support and part of the chamber where the plasma exists.
Preferably the apparatus includes means for controlling or adjusting the
electric field. Furthermore it also preferably includes means for
controlling or adjusting the supply to the induction coil. The invention
provides an independent relationship between the plasma and the electric
field and the apparatus preferably includes means for adjusting or
controlling independently the electric field and the electric supply to
the induction coil.
The invention has three specific identified areas of application, dry
etching, chemical vapour deposition and sputter deposition. The invention
also consists in a method of treating a semiconductor wafer or other work
piece by an active ion technique or by chemical vapour deposition in which
the work piece is placed in a "vacuum" chamber which is evacuated to a low
pressure, a selected gas is introduced under control, a gas plasma is
created within the chamber by means of an external induction coil, the
plasma intensity is controlled or adjusted by controlling the supply to
the coil, and ionised particles are caused to move onto the work piece by
creating an electric field within the chamber between the plasma zone and
the work piece.
Inductively excited plasmas have a number of features which are
advantageous in this application:
1. The coil which induces the plasma may be outside the vacuum environment
if desired so avoiding any contamination of the plasma.
2. The ion density within the plasma is uniform over a large fraction of
the excitation coil diameter.
3. The inductive excitation is power efficient by virtue of the inherent
electron trapping mechanism.
4. Also due to the trapping, plasma excitation at pressures as low as 0.001
Pa (10.sup.-5 Torr) is possible.
5. Also due to the trapping, high ion densities in the region of 10.sup.17
per M.sup.3 are possible at low pressures.
6. The plasma produced is at low energy. This is evidenced by low electron
temperature and mean ion energies and the plasma potential is low.
7. The ion density is directly related to the applied r.f. excitation
power, and is independent of pressure over a wide range. This allows
precise control of ion density and hence ion flux extracted.
8. The inductive excitation is inherently simple to implement in practice;
consequently it is reliable and comparatively inexpensive.
The invention may be performed in various ways and two embodiments will now
be described by way of example with reference to the accompanying
drawings, in which:
FIG. 1 is a diagram illustrating the basic principle of the invention,
FIG. 2 is a diagramatic illustration, partly in section, of a reactive ion
etching apparatus according to the invention, and
FIG. 3 is a similar diagramatic illustration of another form of apparatus
according to the invention.
FIG. 1 shows diagramatically how excitation and ion extraction is achieved.
A coil 1 is fed with R.F. power, in a vacuum environment with a gas
pressure of the order of 0.1 Pa (10.sup.-3 Torr). A plasma 3 is generated,
and an electrode 2 maintained at a negative potential will extract a flux
of ions 4 towards the electrode.
In the example of FIG. 2 the apparatus comprises a "vacuum" chamber 10
having a connection 11 to a vacuum pump and a further connection 12 to a
source of gas to be ionised such as carbon tetrafluoride, chlorine, or
silicon tetra-chloride. The support or table 13 within the chamber
positions the substrate 14 and is connected via a terminal lead 15,
insulated from the chamber wall, to an external electric supply 16, the
other terminal of which is earthed. At the opposite end of the chamber an
opposing electrode 17 is connected by a lead 18, insulated from the
chamber wall, to earth. Thus an electric field is established between the
electrode 17 and the substrate support 13 and this field can be adjusted
by controlling the electric supply source 16.
Surrounding the chamber wall 10 is an induction coil 20, the axis of the
coil extending in the direction of the field between the electrode 17 and
the support 13. The coil is connected to an external R.F. alternating
current supply 21 and means may be provided for supplying a coolant
through the hollow tube 22 which constitutes the coil 20. Thus a heavy
R.F. current can be supplied to the coil so as to create a gas plasma in
the zone 23, as a result of which ionised atomic or molecular particles
are caused to move towards the work piece and make impact at high speed
against the wafer to be etched.
It will be seen that the electric supply to the coil 20 can be varied as
required by controlling the source 21 and this is independent of the
electric field created between the electrode 17 and the support 13 under
the influence of the electric supply 16. An effective, though not
essential, means of powering the excitation coil 20 is to connect a
capacitor 24 in parallel with the coil of such a value that a parallel
resonant circuit is formed at the R.F. supply frequency. The R.F. source
21 is connected to this circuit with a transformer 25 to match the supply
impendence to that of the coil and capacitor circuit. This gives efficient
power transfer into the coil up to frequencies in the low megahertz range:
at higher frequencies a conventional phi network is moro suitable.
Reactive ion etching may be carried out on this apparatus by introducing a
reactive gas or gases while pumping to maintain a suitable pressure in the
10 to 10.sup.-3 Pa (10.sup.-1 to10.sup.-5 Torr) range. Examples of
reactive gases are chlorine, silicon tetrachloride, boron trichloride,
tetrafloromethane, trifloromethane, and oxygen. The gas(es) are selected
depending upon the material to be etched and the exact results desired.
the plasma is excited first at the required level, then the bias is
established, again to the required level. The completion of the etch may
be determined by a number of existing techniques e.g. plasma emission
spectrometry, laser interferometry or mass spectrometry. In certain cases
an inert gas is also introduced to modify the etch characteristics e.g.
Argon, Helium, Xenon.
Sputter etching, also called ion etching, may be performed in the same
apparatus and manner except that an inert gas is used and the lower part
of the pressure range. A particular advantage of this apparatus in this
application is that the ion energy may be kept low, i.e. less than 100 eV,
but the ion flux increased so maintaining a practical etch rate without
recourse to high ion energies which will damage certain samples.
Chemical vapour deposition (CVD) may be carried out in the same apparatus
except that a heater 26 is generally used to raise the substrate
temperature. Chemical vapour deposition consists in introducing certain
gases into the chamber containing the sample to be coated and causing the
gases to react to form a compound which is deposited onto the substrate.
In practice most useful coatings require that the substrate be heated in
order to promote the reaction or alternatively the gas be energised by
means of a plasma to promote the reaction; often a combination of moderate
heating and plasma activation is used. In microcircuit fabrication often
the processing temperature must be limited to a level where even with
plasma activation the film properties are poor. The use of low energy ion
bombardment of the surface during the process will supply surface energy
to greatly improve the film quality; it is important that the ion energy
be kept low while the flux is adjusted to give the desired results. High
energy bombardment will degrade most films. The apparatus proposed here is
well suited to this type of processing.
For sputter deposition an adaptation of the previously described apparatus
is used, as shown in FIG. 3. The grounded electrode 17 is replaced by a
target 27, that is a disc of the material to be sputtered, and this is
connected to a fluid cooling circuit 28. The target is connected to a
power supply 29 which may be d.c. if the target is made from an
electrically conducting material, but must be R.F. if the target is of
insulating material. The substrate 14 is positioned on the electrode 13 as
before. The substrate electrode 13 may be grounded or connected to a power
supply depending upon the process requirements, and a substrate heater 26
may be used as for the CVD application.
In the sputtering process an inert gas, typically Argon, is introduced into
the apparatus and the excitation coil is energised. The target 27 is then
biased negatively by a d.c. or r.f. source as necessary, which causes the
target to be bombarded with ions and the ion energy is increased to
suitable level. Typical values are 100-400 A/m.sup.3 flux at energies of
200 to 600 eV. At these levels the target material will be sputtered and
will deposit on the internal surfaces of the system including the
substrate 14. Where the substrate electrode is connected to ground the
film will be formed by neutral target atoms. In order to improve the film
quality ion bombardment may be required as with CVD and this is introduced
by applying R.F. power to the substrate electrode just as is done for CVD
films. Some compounds can be deposited by reactive sputtering, e.g.
silicon dioxide, silicon nitride, titanium nitride. This process uses an
addition of the volatile component to the inert gas and in this way
reforms the material on the substrate. The material sputtered is in some
instances the compound, e.g. silicon dioxide and in some cases the pure
metallic material, e.g. titanium. The use of substrate bias to give some
ion bombardment improves the film quality for some materials.
The advantages which may be obtained by the invention include the
following:
1. The inductive excitation technique does not interact with biasing system
or systems used within the plasma or surrounding region except through the
plasma itself. Consequently ion flux or fluxes can be extracted, their
energy and density adjusted independently subject only to the plasma
properties.
2. High ion current densities can be extracted at low energies, e.g. some
50 A/m.sup.2 at 25 eV using some 8KW/m.sup.3 of excitation power.
3. The ion current extracted is uniform over a large fraction of the coil
diameter.
4. The system has a wide operating pressure range at least 10.sup.-3 to 4
Pa (10.sup.-5 to 3.times.10.sup.-2 Torr) and potentially larger.
5. The ion current is independent of pressure over a wide range limited at
high pressure by a short electron mean free path hence scattering below
the ionisation energy and at low pressure by long electron mean free path
hence loss from the trapping mechanism. By adjustment of the excitation
drive conditions the limits may be moved either towards lower or higher
pressures for any specific application.
6. The system is inherently simple to implement and efficient in comparison
with other potential techniques.
7. When using this invention a process may be started, with the plasma
excited, by gradually increasing the bias from zero to the required value.
Similarly on completion the bias may be turned gradually down to zero.
This is not possible where the excitation and bias are provided by the
same source as the plasma is only maintained above a certain power. This
is most important when processing semiconductor circutis. Transient
voltages are caused when switching on the bias at a high level, these can
damage the thin insulating layers in the circuits rendering them useless.
8. The technique may easily be implemented under very clean conditions. The
excitation coil and electrode may be outside a quartz envelope, the
substrate support may be completely covered by the substrate, or may even
be outside the envelope. The bias, using 13 MHz r.f. power, can be induced
through a quartz wall.
The frequency range for the inductive excitation is preferably of the order
of 100 KHz-13.6 MHz. Particular frequencies which are effective in
practice are 220 KHz, 450 KHz, 2 MHz and 13.56 MHz.
The excitation mechanism on which the invention depends is believed to
result from the fact that when a magnetic field with cylindrical symmetry
acts on a low pressure gas, electrons are constrained to move in circles
in planes perpendicular to the axis of symmetry of the field. Electrons
with radii of gyration significantly less than the extent of the field are
trapped within the field volume. In practice there are end losses but a
suitable geometric configuration will minimise these. If the magnetic
field is now made to oscillate the electrons will be accelerated as the
field increases and their radius of gyration will decrease. Conversely
they will be decelerated during periods of decreasing field. An
alternating field is used in practice so there are two periods of zero
field per cycle when the electrons are not trapped. However, this is the
time of minimum energy so that a suitable choice of frequency and
amplitude will minimise the losses during this period. Further, if the
field amplitude is made sufficiently high the peak electron energy will
exceed the gas ionisation energy, hence the plasma density will increase
each half cycle until equilibrium is established. Since the losses are
low, at equilibrium the plasma density will be high and the excitation
will be effective at low pressures.
The losses which do occur are seen to be at the ends and to a lesser extent
at the periphery of the field volume. The ionisation takes place
predominantly where the peak field strength is highest, that is close to
the excitation coils. These two effects combine to give a plasma which has
radical uniformity over most of the diameter of the excitation region.
Ions can be extracted from such a plasma as a uniform flux onto an
electrode biased negatively with respect to the plasma. Extraction along
the axis of symmetry is generally used in order to exploit the natural
uniformity across this axis. The ion flux extracted is space charge
neutralised by electrons from the plasma; this is an additional advantage
of the technique.
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