WO1996008836A1

WO1996008836A1 - An apparatus for generating a glow discharge

Info

Publication number: WO1996008836A1
Application number: PCT/GB1995/002012
Authority: WO
Inventors: Philip Charles Allen
Original assignee: Central Research Laboratories Limited
Priority date: 1994-09-16
Filing date: 1995-08-25
Publication date: 1996-03-21
Also published as: GB9418658D0

Abstract

An apparatus for generating an a.c. glow discharge comprises a chamber containing a gas, and a plurality of electrically conductive drive coils surrounding the chamber for creating a glow discharge inside the chamber by inductive coupling. The axes of the coils are not parallel, and the axes lie in parallel or coincident planes. Each coil is coupled to a source of A.C. power such that in operation the coils generate a resultant magnetic field vector in a plane. This arrangement can give the advantage that the resultant electric field vector is perpendicular to the plane of any electrically conductive workpiece having a surface in the same plane.

Description

AN APPARATUS FOR GENERATING A GLOW DISCHARGE

This invention relates to an apparatus for generating an a.c glow discharge comprising a chamber for containing a gas, and an electrically conductive coil system, the coil system being coupled to a source of a.c. power and being arranged to create a glow discharge inside the chamber by inductive coupling. Apparatus of this type is often used in the manufacture of integrated circuits.

Such apparatus may be used to dry etch materials or deposit materials on substrates by glow discharge deposition or plasma enhanced chemical vapour deposition. A description of typical apparatus may be found, for example, in the textbook "Thin Film Processes", edited by Vossen & Kern, (Academic Press, 1978) on page 339 where they describe a tube reactor surrounded by an electrode in the form of a helix having many turns. Although this apparatus has the potential advantage over capacitively coupled systems of higher available power densities, in general such systems are less effective with electrically conductive substrates to generate plasmas adjacent the substrate surface because there is a component of the electric field which is parallel to the conductive surface, which thus effectively provides a short circuit which tends to extinguishing the discharge close to the surface.

According to the invention there is provided an apparatus as defined in the first paragraph above characterized in that the coil system comprises a plurality of coils each having a respective axis, the axes of the coils are not parallel, the axes of the coils are in substantially parallel planes, and the magnetic fields produced by the coils in operation substantially overlap inside the chamber to form a resultant magnetic field vector substantially in a plane parallel to the substantially parallel planes..

This can give the advantage that the resultant electric field vector is substantially orthogonal to the given plane and so any electrically conductive body placed adjacent this plane will not extinguish the plasma. In general, the coils generate a resultant magnetic field vector which rotates in the given plane. This can provide optimum discharge uniformity.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which Figure 1 shows a first embodiment of an apparatus according to the invention

Figure 2 shows a circuit diagram of a power splitter used in the embodiment of Figure 1, Figure 3 shows a circuit diagram of a matching unit used in the embodiment of Figure 1,

Figure 4 shows a second embodiment according to the invention, and Figure 5 shows the instantaneous fields produced by the first embodiment,

In a first embodiment of the invention shown in Figure 1 an apparatus for generating a glow discharge comprises a chamber (8) for containing a gas (12), and a plurality of electrically conductive coils (6, 7) each having a respective axis, the coils arranged to create a glow discharge inside the chamber by inductive coupling. The axes of the two coils intersect at the centre of the chamber. The two axes therefore lie in a common plane. A work-piece (9) has an electrically conductive flat upper major surface made from aluminium which is made to lie adjacent and substantially parallel to the common plane. The chamber (8) is made of quartz and the coils are external to and surround the chamber. The chamber is cylindrical in shape with a diameter of 12cms and a height of 12cms. A vacuum pumping unit (11) communicates with the chamber to control the gas pressure in the chamber to approximately 12 Pa. The atmosphere in the chamber is provided by allowing a gas (12) to Ωow into the chamber through a gas inlet (10) at a controlled rate. The gas used in this example is boron trichloride, which is commonly used to etch aluminium in plasma etching systems.

The apparatus also comprises a source of ax. power (1). In the present example this is a Nordiko model RFG 1250 r.f. generator which can supply up to 1250 watts of 13.56MHz sinusoidal r.f. power. The output of this generator feeds a power splitter (2) which splits the ax. power into two parts, one part for coil 6 and one part for coil 7. The power for coil 7 passes first through a delay line (3) which provides a delay of 18ns (i.e. a quarter cycle at 13.56MHz) and then into a matching unit (4). The matching unit matches the input impedance of the coil to the output impedance of the power splitter to minimize reflected r.f. power. The power for coil 6 does not pass through a delay line, but passes through a matching unit (5) similar to (4). The coils in the present embodiment each comprise two turns of copper wire 2mm thick and 18cms in diameter. The coils each have a self inductance of 2.5μH and a resistance of 2Ω. The coils are arranged with their axes perpendicular to one another surrounding the chamber. A plasma deposition system having a similar arrangement of conductive coils is disclosed in EP 0 574 100, however in this apparatus the glow discharge is created by capacitive coupling between two parallel driving electrodes inside the chamber, and the external coils merely act as Helmholtz coils to generate a rotating magnetic field. In the present invention the driving coils which generate the glow discharge simultaneously generate a rotating magnetic field inside the chamber.

Figures 2 and 3 show the electrical circuits used in the power splitter and matching units respectively. The construction of such units will be familiar to those skilled in the art. Many alternative circuits may be used to achieve proper matching. The present circuits are an example of a specific circuit which will work for the embodiment of figure 1.

Figure 2 shows in more detail the power splitter (2) of Figure 1. The unit has an input (15) coupled to a 50Ω impedance coaxial cable (14) which is connected at its other end to the source of A.C. power. The division of power is achieved using a transformer arrangement. The two output cables also have an impedance of 50Ω. The transformer has a primary winding (18) and two secondary windings (19 and 20). The primary winding has 28 turns and each secondary winding has 20 turns, so that the input voltage is 2 times the output voltage in each cable. This arrangement prevents power reflected from either coil being passed to the other coil, and splits the incoming power (typically 400 watts) into two equal parts. One output cable (17) feeds a delay line (shown as 3 in Figure 1) comprising 4 meters of 50Ω cable which provides a delay at 13.56MHz of approximately a quarter of a cycle or 90°. The other output cable (16) feeds direcdy into a matching unit (shown as 5 in Figure 1). This matching unit is shown in greater detail in figure 3. The circuit comprises two variable tuning capacitors (21 and 22). The series capacitor 21 has a capacitance of approximately 25pF, whilst the capacitance of the parallel capacitor 22 is approximately 50pF. The capacitances are tuned to give minimum reflected power from the coil (6). The matching units (4 and 5 shown in figure 1) are identical and are both as shown in Figure 3.

In addition to reducing the reflected power, these matching units also introduce a small variable phase difference between the two coils (6 and 7 in Figure 1). Preferably the delay means is arranged to provide a variable delay so that impedance matching and phase difference tuning can be optimized separately.

In order to initiate the discharge in an apparatus such as that shown in Figure 1 it is often convenient to provide ignition means in the chamber. Such means may comprise two small electrodes which generate a high electric field for a short time to produce energetic ion-electron pairs to start the chain reaction to produce a sustainable discharge.

In operation, the driving coils 6 and 7 produce a high intensity plasma in the shape of a disc or annulus or toroid. The magnetic field generated by the two coils (if 90° out of phase) sums to produce a rotating magnetic field vector B in the common plane of the axes of the coils. If d e maximum magnetic field produced by the two coils is equal d e resulting rotating magnetic field is of constant magnitude. Figure 5 shows an instantaneous "snapshot" of the electric (25) and magnetic (26) field present in the apparatus during operation. The electric field generated is always orthogonal to the common (x,y) plane but changes magnitude and sign at any given point as the magnetic field rotates. Because there is no component of electric field in the common plane, an electrically conductive substrate may be placed in this plane or parallel to the plane and adjacent the discharge, without extinguishing the discharge.

A second embodiment of the invention is shown in Figure 4. In this embodiment, the single coils (6 and 7 in Figure 1) are replaced by two sets of coils (23 and 24). In the present example the sets of coils comprise two coils which each share a common axis. The two sets of coils are placed such that the two axes are orthogonal and intersect, thus forming a common plane as in the first embodiment above. In operation the two coils in each set are each fed with A.C. power of the same phase, but there is a 90° phase difference between the two sets of coils. Although the above examples have used orthogonal driving coils or sets of coils, the invention is not restricted in scope to the use of orthogonal coils. For example, tiiree coils or sets of coils may be used, the axis of each coil (or sets of coils) sharing a common plane and making an angle of 120° with its nearest neighbour. Such coils (or sets of coils) are supplied with A.C. power such that the phase difference between nearest neighbour coils (or sets of coils) is +/- 60°. In general such an arrangement works for n coils or sets of coils where n is an integer greater than 2, and where the axis of each coil or sets of coils makes an angle of approximately +/- [180/n]° with its nearest neighbours and where the phase difference between the A.C. power supplied to neighbouring coils or sets of coils has a phase difference of +/- approximately [180/n]°. These arrangements are, however, less advantageous as there will always be an interaction between driving coils or sets of coils which are not orthogonal. Such interactions will give rise to complications which will be appreciated by those skilled in the art. The orthogonal coils need not be exactly orthogonal.

The invention works with approximately orthogonal coils or sets of coils driven by power which is has a phase difference between coils or sets of coils of approximately 90°. The invention also works with non-orthogonal coils, but the complications referred to above must be taken into account when designing the driving system. If the phase difference between neighbouring coils or sets of coils is not close to [180/n]°, the magnitude of the rotating magnetic field will change as it rotates. This will also happen if different power densities are supplied to neighbouring coils or sets of coils.

Although in the above examples the coils are external to the walls of die chamber (which are made from a dielectric material), the coils may be arranged inside the walls of die chamber to produce the discharge. In the latter case the walls may be made of electrically conductive material. Although a low pressure carbon tetrachloride plasma has been described, different chamber pressures and gas compositions may be used whilst still falling within die scope of the present invention. For example, the glow discharge may be made in a mercury ambient in a quartz envelope for use as a UV light source, or in an internally phosphored glass envelope for use as a liquid crystal display backlight.

In apparatus used for such applications the gas pressure will be of the order of 100 Pa.

The apparatus of Figure 1, togetiier with means for holding a work-piece adjacent the glow discharge (not shown), may be used for plasma etching of silicon wafers, or plasma deposition onto stainless steel or silicon plates in the manufacture of solar cells. The means for holding the work-piece used can be, for example, any of a variety of trays or chucks which are used in commercially available plasma processing systems to hold silicon wafers, and which will thus be well known to one skilled in the art The apparatus may find application in any field where plasma conditioning or modification of a surface (whether electrically conductive or not) is desired. The gas used will comprise in general different gases and/or mixtures of gasses depending on the type of conditioning of the surface required.

If the resultant magnetic field generated by die plurality of coils in the chamber has a component which does not lie in a plane substantially parallel to die surface of the work piece, this component may be reduced by placing an electrically conductive annular electrode or loop of electrically conductive material (with the ends preferably shorted) in die plane of d e surface of d e work piece and surrounding the work piece.

If two coils are employed, die respective axes of the coils must by definition lie in parallel planes. However, if more tiian two coils are employed the axes need not lie in parallel planes in general geometric terms. The arrangement of the coils according to d e invention is such that the planes are substantially parallel, and d e planes are sufficiendy close to produce a resultant magnetic field which rotates substantially in a given plane inside the chamber.

The apparatus according to die invention creates a glow discharge or plasma which is substantially stationary witii respect to the chamber walls. In particular, die apparatus does not create a helicon or whisder wave which propagates form one part of die chamber to another as is disclosed for example in US 5,146,137.

In summary, an apparatus for generating a glow discharge comprises a chamber containing a gas, and a plurality of electrically conductive drive coils surrounding the chamber for creating a glow discharge inside die chamber by inductive coupling. The axes of die coils are not parallel, and d e axes lie in parallel or coincident planes. Each coil is coupled to a source of A.C. power such ti at in operation d e coils generate a resultant magnetic field vector in a plane. This arrangement can give the advantage tiiat d e resultant electric field vector is perpendicular to the plane of any electrically conductive work-piece having a surface in the same plane.

Claims

1. An apparatus for generating an ax. glow discharge comprising a chamber (8) for containing a gas, and an electrically conductive coil system (6, 7), the coil system being coupled to a source of ax. power (1) and being arranged to create a glow discharge inside the chamber by inductive coupling, characterized in that d e coil system comprises a plurality of coils (6, 7) each having a respective axis, die axes of the coils are not parallel, die axes of the coils are in substantially parallel planes, and the magnetic fields produced by d e coils in operation substantially overlap inside the chamber to form a resultant magnetic field vector substantially in a plane parallel to the substantially parallel planes.

2. An apparatus as claimed in claim 1, in which the substantially parallel planes are substantially coplanar.

3. An apparatus as claimed in claim 2 comprising two coils having substantially orthogonal axes, die source of ax. power being arranged to supply the coils with power having a phase difference between the coils of approximately 90°.

4. An apparatus as claimed in claim 1 in which each coil comprises a set of coils sharing the same respective axis, the source of ax. power being arranged to supply each coil in the set witii power having substantially d e same phase.

5. An apparatus as claimed in claim 2 comprising n coils where n is an integer greater dian 2, die axis of each coil making an angle of approximately [180/n]° with die axes of nearest neighbour coils, the source of ax. power being arranged to supply die coils witii power having a phase difference between nearest neighbour coils of approximately +/-[180/n]°.

6. An apparatus as claimed in claim 1 for conditioning die surface of a body, further comprising locating means to locate the surface in a plane substantially parallel to the substantially parallel planes inside the chamber adjacent the glow discharge.

7. An apparatus as claimed in any preceding claim in which the glow discharge is substantially stationary witii respect to the chamber.