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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a magnetic field-enhanced etch reactor
suitable for both plasma etching and reactive ion etching (RIE) mode
plasma etching and, to associated processes for etching semiconductor,
conductor and dielectric materials.
Over the last several years, the trend to ever greater device densities and
smaller minimum feature sizes and smaller separations in integrated
circuits has caused the IC fabrication technology to shift from wet
chemical etching to dry or plasma etching technology.
The art includes at least three types of plasma etching systems. FIG. 1
depicts a parallel plate plasma, multiple wafer chemical etching system
10, which includes a closed reaction chamber 11 with a connection 12 to a
vacuum pump for partially evacuating the interior of the chamber, and a
gas supply 13 for communicating the reactive gas to the chamber through a
valve-controlled conduit arrangement 14. The system 10 also includes an
energy source 16 which supplies RF energy to a cathode structure 17 and
utilizes a grounded anode 18. The wafers 19 are mounted on the grounded
anode 18 which extends in a parallel plate configuration relative to the
cathode 17. The connection to the vacuum pump is configured to draw the
reactive gases into the region between the anode 18 and the cathode 17 for
confining the reactive gas plasma formed by the RF energy supplied to the
cathode 17.
FIG. 2 depicts a parallel plate reactive ion etching mode, plasma etching
system 20, which also includes a substantially closed reaction chamber 21
with a connection 22 to a vacuum pump for partially evacuating the
interior of the chamber, a gas supply 23 for communicating the reactive
gas to the chamber through a valve-controlled conduit arrangement 24, an
RF power supply 26 which supplies RF energy to a cathode structure 27 and
a grounded anode 28. In contrast to plasma system 10, FIG. 1, in the
reactive ion etching system 20, the wafers 19 are mounted on the cathode
27, which is shielded from and separated from the anode 28.
FIG. 3 schematically illustrates another RIE mode etching system, 30,
which, like reactors 10 and 20, is available commercially from Applied
Materials, Inc. of Santa Clara, Calif. System 30 includes a cylindrical
reaction chamber 31, a hexagonal cathode 37 connected to an RF supply 36,
and an exhaust port 32 that is connected to a vacuum pump. The walls of
the reaction chamber 31 and the base plate 38 form the grounded anode of
the system. Gas supply 33 is communicated into the chamber 31 through port
34 and conduit 35 to a gas distribution ring 41 at the top of the chamber.
The parallel plate plasma system 10 is a relatively high pressure system,
operating in the pressure range of 100 millitorr to several torr, and thus
involves a substantial flow rate of reactive gases into the system. In
contrast, the reactive ion etching systems 20 and 30 are operated at low
pressures in the range of 1 to 100 millitorr and, thus, use substantially
lower gas flow rates. In the reactive ion etching systems 20 and 30,
activated ion species in the neighborhood of the cathode have high
inherent directionality normal to the cathode and the wafers mounted
thereon. By using high frequency RF energy at fairly substantial power
levels, the etch rates are increased in systems 20 and 30, despite the
relatively low concentration of activated species, because the momentum of
the ions bombarding exposed material regions on the wafer surface enhances
the chemical reaction between the activated species and the material to be
etched. Also, the highly directional mechanical ion bombardment etch
component dominates the more isotropic chemical component and imparts high
anisotropy to the etching characteristics of the system. Consequently, RIE
mode systems such as 20 are preferred for etching very small features such
as grooves and trenches in VLSI and ULSI circuits.
The following are important factors and requirements in the design and
selection of commercially useful RIE mode etching reactors. First, to
provide acceptable device manufacturing yields, the RIE mode reactor must
meet certain process requirements such as directionality, selectivity,
uniformity, throughput, low particulate levels, etc. Secondly, it is
desirable that the RIE mode etch reactor require little or no maintenance,
for example, by incorporating in-situ, self-cleaning capability. Other
desirable characteristics include the adaptability to factory and reactor
automation, small reactor size and low manufacturing cost.
The latter group of factors would tend to favor the use of single wafer
systems over batch-type systems, other factors being equal. Moreover,
single wafer systems are more convenient for process development (only one
expensive wafer is used for each process run) and do not present
within-batch, wafer-to-wafer uniformity problems.
However, the single wafer RIE systems typically must be operated at high
pressures (>200 mT) in both the plasma mode and in the RIE mode, to
increase the etch rate and throughput. Unfortunately, high pressure
operation decreases directionality and selectivity and makes it difficult
to meet the exacting requirements of VLSI and ULSI device manufacturing.
Thus, as is usually true, other things are not equal and in order to
obtain commercially viable high throughput as well as acceptable
directionality and selectivity characteristics, most RIE mode etchers,
including etchers 20 and 30 shown in FIGS. 2 and 3, are low pressure
batch-type reactors.
Referring to FIG. 4, there is shown a single wafer RIE mode etch reactor 40
which is an exception to the above-described state of the art. The
magnetic field-enhanced, RIE mode plasma etching system 40 is that
described in co-pending, commonly assigned U.S. Pat. No. 4,668,338,
entitled "Magnetron-Enhanced Plasma Etching Process", issued May 26, 1987,
in the name of inventors Dan Maydan et al. The system 40 is a modification
of the magnetic field-enhanced CVD deposition system disclosed in
co-pending, commonly assigned U.S. Pat. No. 4,668,365, entitled "Apparatus
and Method for Magnetron-Enhance Plasma-Assisted Chemical Vapor
Deposition", also issued May 26, 1987, in the name of inventors Foster et
al. The U.S. Pat. Nos. 4,668,338 and 4,668,365 are hereby incorporated by
reference. The RIE mode etch reactor 40 uses magnetic field-enhanced
etching to provide a relatively high etch rate despite the use of
relatively low pressure and, therefore, can provide high throughput
without sacrificing directionality and selectivity, or vice versa. The RIE
mode etch reactor 40 also decreases the inherent etch non-uniformity which
results from interaction between the magnetic field and the plasma in
magnetic-field enhanced RIE systems.
The system 40 includes a cylindrical stainless steel vacuum chamber 43. A
flanged cathode assembly 42 is mounted within the chamber 43 on insulating
posts (not shown). Typically, the cathode 42 is polygonal and has
non-magnetic reflector end sections 44A (FIG. 5) formed of conductive
non-magnetic material such as aluminum. Outer end sections 44B are formed
of material such as Maycor.RTM. insulating material. Power is supplied for
plasma operation by an RF system 46, typically a 13.6 MHz system, that
includes an RF power supply and a load matching network and is connected
to the cathode 42. Reactant gas is communicated to the interior of chamber
43 by one or more inlet tubes or ring manifolds 47 from a gas supply
system 48, of gas storage tanks/reservoirs 49--49.
Semiconductor wafers 55 are held by means 51, such as posts or clips, at
the side of the cathode. As indicated by arrows 52, the reactant gas flows
across the substrate surface, and then via one or more exhaust outlets 53
to a mechanical pump (not shown) via a vacuum valve and Roots blower.
Electromagnets 54 and 56, typically formed of copper coils, are
circumferentially positioned about the chamber 43 near the top and bottom
thereof. The electromagnets form north and south poles which are
reversible by reversing the coil current.
Referring further to FIG. 4, during RIE mode plasma etching operation of
the reactor system 40, a selected etching gas or mixture is inlet from the
gas supply through the inlet tubes 52--52 to the reaction chamber 43,
which is evacuated by the exhaust pump system. As shown in FIG. 5,
application of RF power from the power supply 46 creates a low pressure,
reactant gas discharge or plasma 57 of electrons, ions and disassociated
species in the vicinity of the semiconductor wafers 55. An electric field
E is formed across the plasma shield or dark space directed from the
positive potential etching plasma toward the surface 58 of the electrode
central section. This field accelerates electrons across the sheath away
from the electrode surface and accelerates positive ions across the sheath
toward the electrode and the wafer 55 to provide the directional ion
bombardment etch component which is characteristic of RIE mode plasma
etching.
The reversible magnetic field, B, FIG. 5, is applied to the chamber 43
parallel to the substrates 55 and perpendicular to the electric field, E,
to control the characteristics of the etch process. The electrons are
confined by the magnetic field lines, which prevent the electrons from
moving easily from the cathode face 58 to the anode 43. Also, the magnetic
and electric fields impart an E.times.B drift velocity to the electrons so
that they tend to drift and to move from point to point along the cathode
surface. The electrons are concentrated in a band having a net drift
velocity along the cathode and substrate. In conjunction with the end
reflectors 44A, the E.times.B drift velocity tends to confine the
electrons within the plasma.
As mentioned, there is an inherent uniformity problem due to with the
interaction between the magnetic field and the plasma in magnetic-field
enhanced reactors. The plasma density is higher downstream in the
E.times.B direction, providing a higher etch rate. To illustrate, and
referring to the FIG. 5 enlarged partial cross-sectional view of the
cathode 42 of etcher 40, FIG. 4, the etching rate is greater at the end or
side 58 of the wafer corresponding to the positive side of the magnetic
field. Reversing the current through the two cylindrical coils 54 and 56
(FIG. 4) reverses the direction of the magnetic field across the wafer, to
B'. This reverses the plasma flow so that the higher etching rate is
switched to the opposite end 59 of the wafer. By reversing the magnetic
field, the inherent etching non-uniformity is partially compensated in
that the etch rate and total etching are averaged across the wafer along
the direction of the static field.
Other magnetic field-enhanced RIE etchers attempt to minimize the etch
non-uniformity using different techniques. For example, one approach
mounts permanent magnets beneath the wafer to provide the magnetic field
and mechanically moves these magnets to "smear" the field. This approach
does not really solve the non-uniformity problem, has potential mechanical
problems and does not provide an adjustable magnetic field strength. A
second approach known to us also uses fixed permanent magnets to generate
the magnetic field and uses very low pressures to minimize non-uniformity.
To summarize the state of the art, presently, batch-type reactors such as
those 10, 20 and 30 described above are used in most commercial plasma
etching and RIE mode plasma etching reactors. The batch reactors process a
relatively large number of wafers at once and, thus, provide relatively
high throughput. However, single-wafer reactors have certain advantages,
described above, such as their adaptability to automation, small size, low
manufacturing cost, and their lack of wafer-to-wafer within-batch
uniformity problems, which make such reactors attractive, particularly for
etching large, expensive wafers such as 6 inch and 8 inch diameter wafers.
Unfortunately, in the past, inter-related problems with etch rates,
throughput, directionality/selectivity and within-wafer uniformity have
prevented full utilization of the potential advantages of single wafer
etches.
SUMMARY OF THE INVENTION
OBJECTS
It is one object of the present invention to provide a single wafer etch
reactor which provides both high etch rates and high etch uniformity, in
addition to meeting the other process requirements.
It is a further object of the present invention to provide such an etch
reactor which incorporates in-situ self-cleaning capability.
It is also an object to provide such a reactor which incorporates automated
internal wafer handling capability and is readily interfaced with external
wafer exchange robots.
These objects are exemplary, not exhaustive. Others will be evident from
the disclosure.
SUMMARY
The above objectives are achieved in an etch reactor which in one aspect
embodies a housing defining a vacuum chamber adapted for processing a
wafer comprising: an electrode assembly having a convex surface supporting
a wafer in a bowed configuration parallel to the electrode surface; a gas
distribution system including a gas manifold positioned closely adjacent
to the electrode surface for supplying reactive gas to the chamber; means
for applying RF energy to the chamber to generate a reactive etching gas
plasma from the reactive gas between the electrode and the manifold; and
means for applying an electrically-controlled D.C. magnetic field parallel
to the wafer-support surface of the electrode, selectively varied as to
magnitude and direction for providing uniform etching over a wafer
positioned on the electrode.
Preferably, the magnetic field is provided by two or more pairs of
electromagnets located on opposite sides of the chamber for providing
separate magnetic fields across the wafer and, computer means for
controlling the current in the individual electromagnets to independently
control the magnitude and angular orientation of the resultant magnetic
field vector. In particular, both the magnitude and/or direction of the
magnetic field provided by this paired electromagnet configuration can be
changed instantaneously. The field can be stepped about the wafer at a
slow rate of a few cycles per minute to provide uniform etching at high
pressures without eddy current loss.
In another aspect, gas is applied between the wafer and the electrode at
pressure greater than the chamber pressure to increase uniform thermal
conduction from the wafer to the electrode, which, preferably, is liquid
cooled. Clamping means is provided for resiliently clamping the wafer to
the electrode. Protective coatings or covers of material, such as, for
example, quartz, may be provided on the clamp ring and gas manifold. The
wafer support surface of the electrode has a dome-shaped or convex
curvature. As a result, when this wafer is clamped to the electrode, the
wafer is bent into the domed surface configuration of the electrode and
closely parallels the surface of the electrode. This controlled parallel
close spacing between the bowed wafer and electrode is mantained when the
heat transfer gas is applied between the wafer and the electrode. This
enables uniform heat transfer across the entire surface of the wafer to
the electrode and, consequently, uniform processing characteristics such
as etch rate across the wafer. This gas-enhanced liquid electrode cooling
permits the use of very high power densities, for the purpose of
increasing etch rate and plasma control, while maintaining the wafer at a
relatively low temperature to facilitate profile control and avoidance of
phenomenon such as black silicon.
Wafer exchange means comprising a group of movable pins extending through
bores in the electrode assembly is used to position the wafer on the
electrode assembly and remove the wafer.
The reactor also comprises a feed-through device for coupling the electrode
heat transfer cooling gas at low pressure to the RF-powered electrode
assembly without breakdown. The feed-through device comprises a housing
having a gas inlet adapted for receiving the cooling gas and a spaced gas
outlet connected to the electrode. The housing further includes a pair of
internal, closely spaced apertured plates transversely spanning the path
of the gas, the plate on the outlet side of the gas flow being connected
electrically in common with the pedestal and the inlet side plate being
connected to system ground.
In another aspect, heating means such as an electrical resistance heating
unit is mounted on the housing for providing controlled heating of the
inner chamber walls to prevent formation of wall deposits.
This combination of features including the electrically-controlled
multi-directional magnetic field, the temperature control of the cathode
and reactor walls and the use of protective materials such as the quartz
covers enables satisfaction of the above-identified design objectives. The
various and conflicting process requirements such as directionality,
selectivity and uniformity are met over a range of pressures, including
high pressures in a low maintenance in-situ self-cleaned single wafer
system. In particular, the electrically-controlled multi-directional field
and the use of special protective materials provide high directionality,
high selectivity and high uniformity during operation. The
electrically-controlled multi-directional field provides uniform etching
over a very broad pressure range of about 0.001 to 0.300 torr, which
includes high pressures, thereby permitting high rate etching without
sacrificing uniformity. This broad pressure range permits in-situ
self-cleaning. The temperature controlled surfaces and
electrically-controlled multi-directional magnetic field facilitate clean
operation and in-situ cleaning. The electrically-controlled
multi-directional field independently increases the etch rate and, in
combination with the high pressure operation capability, provides a
throughput-practical single wafer etcher.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other aspects and advantages of the present invention are
described in conjunction with the following drawing figures, in which:
FIGS. 1-3 schematically depict three types of plasma etching systems which
are available in the art;
FIG. 4 is a perspective view, partially in schematic form, of a
magnetically-enhanced RIE mode plasma etch system that is a progenitor of
the magnetically-enhanced RIE mode plasma etch system of the present
invention;
FIG. 5 is a vertical cross-section, partly in schematic, through the
wafer-holding electrode of the chamber shown in FIG. 4, illustrating B
reversal;
FIG. 6 is an isometric view, partially cut away, of a preferred embodiment
of the magnetic field-enhanced plasma reactor of the present invention;
FIG. 7 is a vertical cross-section, partly in schematic, taken along lines
7--7 in FIG. 6;
FIGS. 8-10 are sequential, schematized representations of the operation of
the wafer exchange system in positioning wafers on, and removing wafers
from the reactor pedestal;
FIG. 11 depicts an enlarged, vertical cross-section of the gas feed-through
system shown in FIG. 7;
FIG. 12 is a schematic representation of the system for generating and
controlling the electrically-controlled, quasi-static, multi-directional
D.C. magnetic field used in the present invention and a computer system
suitable for controlling overall operation of the present reactor; and
FIGS. 13A-13C schematically depict the clamped mounting of the wafer to the
convex or dome-shaped wafer support pedestal.
DETAILED DESCRIPTION OF THE INVENTION
Overview of Magnetron Etch Reactor 60
FIGS. 6 and 7 depict, respectively, an isometric view of the preferred
embodiment of the single wafer, magnetic field-enhanced plasma etch
reactor 60 of our present invention, and a vertical cross-section through
the reactor 60. The description here is directed primarily to RIE mode
plasma etching, but the reactor capability extends to plasma mode etching
as well.
Referring to FIGS. 6 and 7, the etch reactor system 60 of our present
invention comprises a housing 62, typically of non-magnetic material such
as aluminum, which has an octagonal configuration of outer walls 64 (as
viewed in horizontal cross-section). Circular inner wall 66 defines etch
chamber 68. As described more fully subsequently, the reactor system 60
also includes a unique gas- and liquid-cooled pedestal/cathode assembly 70
and a wafer exchange system 74 (FIGS. 8-10).
The wafer exchange system 74 includes vertically movable wafer lift fingers
79 which pick up a wafer 75 from an external manually held or operated
blade 76 which is inserted into the chamber or, preferably, from an
external load lock robot blade 76, and transfer the wafer to the cathode
72 for processing, then return the processed wafer to the robot blade for
removal from the chamber.
In addition, the wafer exchange system 74 integrally incorporates a wafer
clamp ring 78 with the wafer lift fingers 79. As described in detail
below, the design of the wafer exchange system 74 and the incorporation of
the associated wafer lift and clamping structures permit the use of a
one-axis robotic drive within the chamber. Furthermore, the operation of
the chamber robot requires that the external robot merely present the
wafer to a selected transfer position for exchange with the chamber robot.
Simplifying the demands on the external robot makes possible a relatively
simple robot, even when the robot is used in a multi-chamber load lock
system that serves a multiplicity of reactors. Such a robot, which uses
R-.theta. movement, is disclosed in co-pending commonly assigned U.S.
patent application Ser. No. 944,803, entitled "Multiple Chamber Integrated
Process System", concurrently filed, in the name of Dan Maydan, Sasson
Somekh, David N. K. Wang, David Cheng, Masato Toshima, Isaac Harai and
Peter Hoppe, which patent application is incorporated by reference in its
entirety (also called "referenced multi-chamber system").
Process gases are supplied to the interior of the chamber 68 by a gas
manifold 80 from a gas supply system 81 comprising one or more gas storage
reservoir/tanks. The gas supply system 81 communicates to the manifold 80
and chamber 68 via supply line(s) 82, which is coupled into the manifold
80 by inlet connection 84. The system includes an automatic flow control
system or other suitable control system which controls the flow rates of
the various etchant gases, carrier gases, etc., supplied to the chamber
68.
Vacuum is supplied to the chamber and spent gases and entrained products
are exhausted via annular exhaust chamber 90 communicating to exhaust port
92 which, in turn, is connected to a conventional vacuum pumping system
93. The exhaust flow is directed from the chamber 68 through holes 94 in a
horizontal annular plate 96 mounted about the upper periphery of the
cylindrical cathode assembly 70. The apertured plate 96 inhibits plasma
penetration into the annular exhaust chamber 90. This exhaust arrangement
facilitates uniform coverage and etching of wafer 75 by the reactant gas.
Control of the exhaust system can be by a capacitive conventional system
such as manometer sensor (not shown) which operates through a pressure
control system and D.C. motor to control the speed of the blower, or by
other conventional control systems.
As indicated by the arrows 102, 104, 106, 108 in FIG. 7, the gas
communicated to inlet 84 (arrow 100) is routed into the manifold 80 (arrow
102) and is then directed downwardly from the manifold (arrow 104),
forming an etching gas plasma in chamber process region 110 during
application of RF power, then flows over the wafer 75 and radially
outwardly across the wafer and into the annular exhaust chamber (arrow
106), then out the exhaust port 92 (arrow 108).
The above-mentioned RF power is supplied by an RF supply system 112 to the
reactor system 60 for plasma operation, i.e., to create an etching gas
plasma from the inlet gases in process region 110. This system 112
includes an RF power supply and a load matching network, and is connected
to the pedestal 72, with the chamber walls being at ground. That is, the
pedestal is the powered cathode. The RF power typically is supplied at a
high frequency, preferably about 13.6 MHz. However, the reactor system 60
can be operated at low frequencies of, for example, several kHz.
The use of a powered pedestal cathode 72 has the advantage of concentrating
the RF power and plasma on the surface area of the wafer and increasing
the power density across the wafer while reducing it everywhere else. This
ensures that etching takes place on the wafer only, reducing erosion in
other parts of the chamber and thus reducing possible wafer contamination.
Typically, power densities of about 2.5-3.5 watts/cm.sup.2 are and can be
used. As discussed below, these high power densities require cooling.
Preferably, the RF-powered cathode 72 is constructed to combine
gas-enhanced wafer-to-cathode thermal conductance and liquid cathode
cooling. However, the application of cooling gas such as helium to the
powered pedestal 72 at low pressure would ordinarily cause the cooling gas
to break down. The present reactor includes a unique gas feed-through 114,
FIG. 7, that supplies the gas to the high voltage electrode without
ionization.
Reactor 60 also comprises an improvement of the magnetic field-generating
system disclosed in FIG. 4 above. Referring to FIG. 6, the system can use
a number of paired electromagnets such as the four electromagnets 116,
118, 120 and 122, typically comprising copper coils, mounted in a
rectangular array, one each on alternating walls of the octagon-shaped
housing. The two coil pairs cooperatively provide a quasi-static,
multi-directional field which can be stepped or rotated about the wafer to
provide etch uniformity at high and low pressures. Also, the magnitude of
the field can be varied to select etch rate and decrease ion bombardment.
Electrically-Controlled Multi-Directional Magnetic Field Generator
FIG. 12 is a schematic representation of the system for generating and
controlling the quasi-static, multi-directional magnetic field used in the
present invention.
Referring primarily to FIG. 12 in addition to FIG. 6, the two coil pairs
116,118 and 120,122 form mutually perpendicular magnetic field vectors
B.sub.y and B.sub.x, respectively, which are generally parallel to the
pedestal/cathode 72 and the wafer 75. In the exemplary illustration shown
in FIG. 12, computer 113 applies control signals via lines 103, 105, 107
and 109 to conventional power supply systems 115, 117, 119 and 121 to
control the magnitude and direction of the currents supplied over
conductors 123, 125, 127 and 129, respectively, to the electromagnets 116,
118, 120 and 122. The associated current determines the direction and
magnitude of the field generated by each coil pair.
The perpendicular field vectors B.sub.y and B.sub.x generated respectively
by coil pairs 116,118 and 120,122 are defined by
B.sub.x =B cos .theta. (1),
B.sub.y =B sin .theta. (2).
Given the desired or required values of the field, B, and its angular
orientation .theta., the computer 113 can independently solve equations
(1) and (2) to obtain the associated magnetic field vectors B.sub.x and
B.sub.y which provide the desired strength of field and orientation and
then control application of the necessary electric currents in the coils
116-122 to provide these fields B.sub.y and B.sub.x.
Moreover, the angular orientation and magnitude of this DC magnetic field
can be independently altered as quickly or as slowly as desired by
changing the current in the coils. The time that the field is on at each
angular position and the direction of angular stepping may be varied as
well as the field intensity, since these parameters are solely a function
of changing the currents to the electromagnets and are readily controlled
by computer 113. Thus, the field can be stepped around the wafer using
selected orientation and time increments. If desired, the magnitude of the
resultant field B.sub..theta. can be changed as the process or reactor
configuration may require, or a constant field strength can be used. In
short, the electrical current-controlled system provides the versatility
of a fast or slow moving, constant or varying strength magnetic field of
constant or varied angular velocity. In addition, the orientation of the
field need not be stepped or changed sequentially, but can be
instantaneously switched from any given orientation (or field strength) to
another.
This versatility in independently controlling the direction and magnitude
of the D.C. magnetic field is distinct from existing commercially useful
rotating magnetic fields, which typically rotate at a fixed relatively
high frequency such as the standard rate of 60 Hertz. In addition, the
ability to "rotate" slowly, at a rate, for example, as low as 2 to 5
sec./revolution (12 to 30 cycles/min.) or slower avoids problems such as
the eddy current losses associated with the use of higher frequencies in
aluminum or metal chambers.
The previous reactor 40, FIG. 4, reverses the static magnetic field along
one axis. In contrast, reactor 60 effectively rotates the magnetic field,
preferably at the slow rate of, e.g., 2 to 5 sec./revolution, by the
simple expedient of sequentially changing the currents to the
electromagnet coils. This steps the magnetic field ab | | |
