Martin P.M. Handbook of Deposition Technologies for Films and Coatings, Third Edition: Science, Applications and Technology
Подождите немного. Документ загружается.
Evaporation 171
Figure 4.25: Transverse electron beam gun.
Figure 4.26: Schematic representation of a Pierce gun.
172 Chapter 4
Figure 4.27: Cold cathode plasma electron beam gun.
from which electrons are extracted through a small aperture in one end. The cathode is
maintained at a negative potential, e.g. −5to−20 kV, relative to the workpiece and
remainder of the system, which are at ground potential. After evacuation of the
system, a low pressure of ionizable gas in the range of 10
−3
–10
−1
torr is introduced.
Depending on the high-voltage level, a long path discharge between the cathode and
other parts of the system will occur in the gas at a particular pressure. Ionizing
collisions in the gas then produce positive ions which are accelerated to the cathode,
causing electrons to be released from the cathode surface. Although the cathode may
heat up slightly owing to ion bombardment, no heating is required for electron
emission. Upon proper adjustment of cathode voltage and gas pressure, a beam mode
of operation is established, since interaction between the plasma inside and outside of
the cathode and the electric fields between cathode surface and plasma boundary will
largely confine electron emission to the end of the cathode and its interior. In argon, a
beam mode is supported at about 10
−2
torr with 5–10 kV. Beam currents range up to
3 A for a 3 inch diameter cathode in argon at 20 kV. With lighter gases, e.g. helium,
higher pressure to about 10
−1
torr will yield a beam mode in this same voltage range.
Beam current will vary with voltage and pressure control. More specific information is
Evaporation 173
Figure 4.28: Schematic of the hot hollow cathode electron beam gun.
given by Cocca and Stauffer [95]. The beam is self-collimating because of the
focusing effect of positive ions in the beam path and the electrostatic lensing action of
the aperture since it separates regions of different potential gradient. The beam is well
collimated, having a cross-section equal to that of the cathode aperture. Adjustment of
focus can be achieved to some extent by varying pressure and voltage, but external
focusing may also be used if desired, with magnetic or electrostatic lenses, as with
conventional election beams.
Hot hollow cathode discharge beam – the hollow cathode discharge beam applied to
vacuum processing has been reported by Morley [96] and differs in a number of
respects from the plasma beam. A schematic of the hollow cathode discharge beam is
shown in Figure 4.28. Here the cathode must be constructed of a refractory metal since
it operates at elevated temperatures. An ionizable gas, usually argon, is introduced into
the system through the tubular-shaped cathode. A pressure drop across the orifice in
the cathode provides a sufficient amount of gas inside the cathode to sustain the
plasma, which generates the beam.
A low-voltage, high-amperage direct current (DC) power source is utilized. When
radio-frequency (RF) power from a commercial welding starter is coupled to the gas, it
174 Chapter 4
becomes ionized and the plasma is formed. Continued ion bombardment of the cathode results
in heating of the cathode and increased electron emission. Ultimately, a high-current ‘glow
discharge’ will occur, analogous to that experienced in vacuum arc melting at higher pressures.
At this point, the discharge appears as a low power density beam ‘flowing’ from the cathode
aperture and fanning out in conical shape into the chamber. However, a parallel axial
magnetic field is imposed on the beam (as seen in Figure 4.28) which then forms a high
power density, well-collimated beam. The hollow cathode discharge beam is operationally
stable and efficient over the pressure range from 10
−4
to 10
−1
torr. A more detailed description
of physical aspects, operational characteristics, and cathode design has been given by
Morley [96].
4.6.6.3 Comparisons
Thermionic and plasma e-beam guns can be used equally well for evaporation. Focusing of the
beam spot is easier for the thermionic guns. The plasma guns have the advantage of being able
to operate at higher pressures, which can be important for gas scattering evaporation, reactive
evaporation, and ion plating.
4.6.7 Arc Evaporation
Definitions of arcs include: ‘A discharge in a gas or vapor that has a voltage drop at the
cathode of the order of the minimum ionizing or exciting potential of the gas or vapor’
(Karl T. Compton, Princeton University).
Berghaus [97] describes the use of arcs to form refractory compounds by reactive evaporation.
Since 1940, consumable and non-consumable vacuum arc melting processes have been
developed to melt and refine various reactive metals such as Ti, Hf, Zr, etc. More recently, arc
techniques have been used to deposit metals [98, 99] and refractory compounds, and even for
extraction of ions from the vacuum arc plasma for the deposition of metal films [100].
Wroe [101] in 1958 and Gilmour and Lockwood [102] suggested vacuum arcs as a source for
metallic coatings. The US patents to Snaper [103, 104] in 1971 and the Russian patents to
Sablev [105, 106] in 1974 set the stage for the commercial production of arc coatings, which
were achieved in the USSR around 1977–1978. The first commercial use of the arc
evaporation-deposition method was for TiN coatings deposited at low temperatures,
particularly for high-speed steel cutting tools by arc evaporation of titanium in a nitrogen
plasma. This follows on the heels of the activated reactive evaporation (ARE) process,
developed in 1971 for deposition of refractory compounds such as TiN using e-beam
evaporation techniques and discussed in Section 4.8.6. There is very extensive Russian
literature on vacuum arc coating technology and the reader can find a convenient source in
recent reviews by Sanders [107] and by Martin [108].
Evaporation 175
There are two types of cathodic arc systems: pulsed and continuous. In the pulsed devices, the
arc is repeatedly ignited and extinguished using a capacitor blank to supply the arc power
[102]. Pulsed arcs have the advantage of letting the target cool between the pulses. The
disadvantage is the decrease in steady-state coating rates.
The continuous cathodic arc can be random in nature or controlled. By the use of an insulating
ring, a random arc source can be constrained at the edge of the target, but allowed random
motion within that constraint. Random arc sources have the advantage of simplicity and
excellent target utilization because the entire target (except near the very edge) is utilized in
the arc of very large parts. The main disadvantage of random arcs is the formation of
macroparticles which may cause the resulting coating to be unsuitable in some applications.
Figure 4.29 shows that macroparticles are ejected at small angles with respect to the target
surface, and can therefore be minimized using appropriate shielding. Such a strategy has made
possible arc-produced decorative coatings where surface finish and optical peculiarity are of
concern.
Magnetic fields can be used to control the trajectories of the arcs. These fields can be used to
discourage the arc from leaving the desired portion of the target surface or can be used to
define a well-controlled path for the arc to follow, in the ‘steered arc’ devices. While the
mechanism is still the subject of some debate, it is clear, at least in the case of ceramic coatings
based on refractory metals, that steered arcs can produce coatings having extremely low or no
measurable macroparticle component.
Macroparticles can also be removed by the use of suitable filters, as discussed by Sanders
[107] and by Martin [109]. This is known as the filtered arc evaporation process. Other
Figure 4.29: Phenomena occurring at a discrete cathodic arc spot.
176 Chapter 4
Figure 4.30: Schematic of the anodic arc evaporation process.
strategies for macroparticles involve the production of diffuse arcs. In one case, the cathode is
contained in a crucible which is allowed to heat up to a temperature where the target material
has a substantial vapor pressure [110]. This causes a decrease in the arc voltage and current
density, the discharge becomes diffuse and macroparticles no longer form. The other approach
is the ‘anodic arc’ [111–113] (Figure 4.30). In this process the cathode initially supplies
electrons as well as ions until the anode heats up. At this point, with sufficient electron
emission, a diffuse arc forms on the hot anode target material which supplies the ions
necessary to sustain the discharge. The cathode material is not evaporated and the coating
material now emanates from the anode. There are no macroparticles formed. High deposition
rates (several micrometers per minute) are obtained for a variety of metals including Al, Ti, V,
Ca, Mn, Fe, Ni, Cu, Pd, Ag, Au, and Pt [111]. Since the substrate is left relatively cool, the
process makes it possible to produce adherent coatings on plastics at temperatures less than
70
◦
C, which makes this relatively new process a competitor for sputter deposition. Alloy
coatings such as stainless steel can be readily deposited with good stoichiometric transfer. For
example, Ni, Al, and stainless steel coatings less than 1 m thickness impart excellent
corrosion protection to iron [114].
One of the main advantages of arc deposition processes is the relatively high level of ionizing
atoms in the plasma. This makes it convenient to extract ion beams from the plasma and
deposit macroparticle free coatings entirely from the ion beam [107, 109].
Evaporation 177
4.7 Deposition Rate Monitors and Process Control
The properties of deposits are dependent on the control exercised during the process. The
thinner the deposit, the more critical the control of the operation.
4.7.1 Monitoring of the Vapor Stream
4.7.1.1 Ionization Gauge Rate Monitor
This device is very similar to a hot cathode ionization gauge and monitors the atom density in
the vapor phase by ionizing the vapors, collecting and measuring the ion current. Several
arrangements are shown in Figure 4.31.
Figure 4.31: Ionization rate monitor designs and arrangements: (A) after Schwarz; (B) after
Giedd and Perkins; (C) after Perkins; (D) after Dufour and Zega. (From Handbook of Thin Film
Technology [7]. © 1970, McGraw-Hill, Used with permission of McGraw-Hill Book Co.)
178 Chapter 4
Figure 4.32: Particle-impingement-rate monitors: (A) torsion-wire device (after Neugebaur); (B)
pivot-supported device (after Beavitt). (From Handbook of Thin Film Technology [7]. © 1970,
McGraw-Hill. Used with permission of McGraw-Hill Book Co.)
4.7.1.2 Particle Impingement Rate Monitors
The gauge, which is a cylinder suspended by a wire or riding on a bearing, is imparted a
momentum by the impinging particles which can be measured by the torsional forces. They
are illustrated in Figure 4.32.
4.7.1.3 Ion Current Monitor for Electron Beam-Heated Source
An e-beam-heated molten pool has a plasma sheath above it. Positive ions from the plasma
follow a very similar trajectory as the electrons with a slightly larger radius of curvature,
owing to their higher mass, and are beamed away from the molten pool by the same magnetic
field which bends the electrons towards the pool. Therefore an ion collector can be placed so
as to intercept this ion beam and the resultant ion current can be used in a feedback loop to
control the evaporation rate. Two manufacturers of e-beam guns have offered this option.
4.7.1.4 Spectroscopic Methods
Monitoring and control of the deposition rate can be done on the basis of mass spectrometry,
atomic absorption spectrometry, and electron emission impact spectrometry. Each of them
involves the choice of an appropriate materials-selective sensor. The principles, advantages,
and limitations of each of these are presented in a good review paper by Lu. The reader is
referred to this paper and the references cited therein [115].
4.7.2 Monitoring of Deposited Mass
4.7.2.1 Microbalances
There are various types of devices which measure a change in mass due to condensed atoms
based on elongation of a thin quartz-fiber helix, the tension of a wire, or the deflection of a
pivot-mounted beam. Examples are shown in Figures 4.33 and 4.34.
Evaporation 179
Figure 4.33: (A) Schematic drawing and (B) circuit diagram of a microbalance constructed from a
microammeter movement (Hayes and Roberts). (From Handbook of Thin Film Technology [7].
© 1970, McGraw-Hill. Used with permission of McGraw-Hill Book Co.)
Figure 4.34: Microbalance with torsion-fiber suspension and electromagnetic force compensation
at beam end (Mayer et al.). (From Handbook of Thin Film Technology [7]. © 1970, McGraw-Hill.
Used with permission of McGraw-Hill Book Co.)
180 Chapter 4
Figure 4.35: Oscillator crystal holders for deposition monitoring: (A) after Behrndt and Love; (B)
after Pulker. (From Handbook of Thin Film Technology [7]. © 1970, McGraw-Hill. Used with
permission of McGraw-Hill Book Co.)
4.7.2.2 Crystal Oscillators
The crystal oscillator monitor utilizes the piezoelectric properties of quartz. The resonance
frequency induced by an alternating current (AC) field is inversely proportional to crystal
thickness. In practice, the change in frequency of a crystal exposed to the vapor beam is
compared to that of a reference crystal. An example is shown in Figure 4.35.
4.7.3 Monitoring of Specific Film Properties
In preparing thin films, often only one property is of interest, e.g. optical or electrical.
Optical monitors measure phenomena such as light absorbance, transmittance, reflectance or
related interference effects during film deposition. An example is shown in Figure 4.36.
Using resistance monitors, the film thickness can be continuously monitored using in situ
resistance measurements as shown in Figure 4.37.
4.7.4 Evaporation Process Control
4.7.4.1 Thickness Control
Usually monitoring of an evaporation process is combined with the means to control film
deposition. Frequently, the only requirement is to terminate the process when the thickness or
a thickness-related property has reached a certain value. The simplest way is to evaporate a