CHAPTER: IMPLANTER SYSTEMS
IMPLANTER CONCEPTS
In the early 1960s when the concept of ion implantation of semiconductors and other
materials for potential electronics and optoelectronics applications were being conceived
and considered, implanters began as simple laboratory ion beam systems built on tabletop vacuum systems, with or without mass separation. Various ion sources, acceleration
concepts, and mass separation systems were considered, designed, and implemented, and
their performance was studied. At least four previous implementations of ion beam
systems played roles in this development, viz., isotope separators, ion propulsion
systems, high energy physics beam delivery systems, including van de Graaff generators,
and neutron generators. Some of the earliest implanter implementations were based on
combinations of concepts and components taken from these existing systems. Most of the
interest at this time was for research to learn how the mechanical, optical, and electronic
properties of materials could be modified for a wide variety of potential applications, and
high ion current for volume production was not yet a significant, or certainly not yet a
realized concern.
Some early 'implanter' concepts were based on isotope separators, adapted to handle
single or multiple semiconductor-related materials at the target location. Some were small
van de Graff generators modified to handle 'dopant' gases and to accommodate
semiconductor materials and samples at the target. Some systems were built and sold that
were based on a neutron generator modified to generate and deliver ion beams of many
elements rather than neutrons (from hydrogen beams). This series of implanters, as well
as the van de Graaff and high energy physics concepts, performed mass separation on the
ion beam after it was accelerated to its final energy. For a short period of time, concepts
of mass separation at ground potential, and both ion source and target at high potential
were studied, but the difficulties of having the target at high potential, especially accurate
current or fluence measurement, ended further implementation of this concept.
After many successful results were achieved in the fabrication of controlled useful
modifications of the mechanical, optical, and electronic properties of many materials, it
became evident that ion implantation was likely to have many industrial/commercial
applications and that 'production' needs (high volume and low cost) would require higher
ion currents than could be delivered by most of the existing 'implanter' concepts. This
need resulted in the creation of 'implanter companies' and the important move into higher
ion currents and new concepts for creating uniformly implanted areas, first small and then
larger and larger, as the Si wafer industry, in parallel, developed the ability to provide
larger and larger Si wafers. This situation continues until today (2003), at which time 12inch Si wafers and 6-inch GaAs wafers are being engineered in large 'fabs (production
facilities) around the world. 'Medium current,' 'high current,' and 'MeV energy' implanter
industries thrive today.
The need for higher ion currents caused a significant change in basic implanter concept
and design. Space-charge expansion of positive ion beams (caused by the mutual
repulsion between particles of the same charge), which is increasingly serious with
decreasing ion velocity (energy), prevented high ion currents from being delivered
through mass separators, which required linear distance, and to target handling systems
located far from the ion source. Space-charge expansion of positive ion beams can be
significantly reduced by injection of electrons into the ion beam, but electric and
magnetic fields in and from other components of the beam handling systems removed
these charge-compensating electrons; mass separators and beam deflection systems are
two examples. Many implanter concepts employed electrostatic scanning of the ion beam
to achieve uniformity of implantation at the target. The nature of electrostatic scanning
removes electrons for the ion beam.
Thus two significant changes necessarily occurred in implanter concept, viz., mass
separation of the ion beam as close to the ion source as possible (immediately after
extraction from the ion source) and before acceleration to the full ion energy desired for
implantation, and mechanical scanning concepts for multiple targets. Thus the mass
separator was moved close to the ion source and the accelerator was placed after mass
separation, and sophisticated systems to mechanically scan many larger and larger wafers
at the target location caused the design of implanters to change, for example, for the 'end
stations' or wafer handling chambers to become large, complicated, and costly.
More recently, advantages of implanting certain elements more deeply into wafers and
materials developed in at least the Si device and circuit community, to the magnitude that
it became economical for implanter manufacturers to develop commercial high energy
implanters. High energy ion beams had existed for decades in the high energy physics
community, but the ion currents used in most of that community are orders of magnitude
less than the currents that are needed in the Si device and circuits industry.
The relatively large size of the Si device industry placed the primary emphasis on a few
selected chemical elements for ion beams, especially B, Si, P, and As. The developing
field of silicon-on-insulator caused a small industry to develop to provide fairly high
currents of oxygen for implantation into Si to create buried layers of Si02. The need for
these layers to be nearer and nearer to the surface as the lateral device dimensions (gate
length) decrease, is causing the energy of this oxygen implantation to decrease.
Plasma implantation concepts (for energies less than about 5 keV) were implemented to
create shallow junction devices in Si, and for various surface modification of materials,
for example, hardening. Applications for plasma implantation to create shallow depths
may continue to increase.
Thus today we see the energy capability of implanters changing in both directions (higher
and lower), as well as higher current, but higher current may become practically limited
because of the associated issues of target handling, and more efficient target systems may
become the issue. At present, members of the semiconductor device industry simply
purchase more and more implanters, and build more and more production facilities as
chip demand increases.
ONE SPECIFIC IMPLANTER IMPLEMENTATION
While progress has continued in the directions described above, driven primarily by the
increasing production needs of the Si device and circuits industry, the more modest needs
of the research sector allowed older concepts of implanter to meet those needs. The needs
of the research sector, in which many new materials (beyond Si and GaAs) are the subject
of study, emphasize versatility and the need to vary some parameters that are not so
important in Si circuit production. The ability to implant all elements of the periodic table
is one such specific need. The ability to implant at low fluence and high fluence, at low
temperature and at high temperature, at various angles of incidence, and into many
different materials and structures, places different requirements on the implanter.
These research needs within the former Hughes family of companies and sectors was
meet by the one instrument developed at (then) Hughes Research Laboratories in the late
1960s. This machine is still in routine use for many applications (2003). It represents the
basic concepts of implantation and is a relatively low cost implementation that uses
relatively low cost components, and could be assembled in a university or small business
that needs an interactive versatile research implantation capability. For these reasons, this
instrument is described in some detail below. It bears little resemblance to implanters
used in today's Si wafer production industry. It does implant nearly all elements into all
solid materials within a wide variety of conditions.
This machine is really an ion mass spectrometer modified to be an 'implanter.' The basic
ion energy range is from 5 to 1100 keV (using singly, doubly, or triply ionized ions), with
ion current capability decreasing with increasing energy (above 350 keV), and with
decreasing current capability below about 100 keV. The ion current capability also varies
with the chemical element (implant species) and can be controlled in the range from 1 nA
(for fluences in the 109 to 1010 cm-2 range) to 100 A delivered to the target for one
isotope (for fluences up to 1018 cm-2). The maximum total ion current for all masses from
the ion source is about 1 mA. The implanted area is relatively small, being easily variable
from almost zero to a maximum of 4 inches in diameter, which is limited by the diameter
of the beam line at the target chamber; it could be conveniently increased only somewhat.
This limitation is caused by the use of electrostatic scanning of the beam in two normal
directions (which also relates to the current limitation via the issue of beam space-charge
neutralization mentioned above). The basic concept involves mass separation of the beam
after full acceleration, and two-dimensional electrostatic scanning. These two concepts
cause the instrument to have a large footprint, but one for which all components can be
easily accessed, maintained, or modified for research purposes, and one that minimizes
energy and element contamination in implants. It might be meaningful to note that one
significant use of this machine has been to produce samples that serve as quantification
standards with accurate densities of single isotopes of nearly all chemical elements
implanted into nearly all solid materials. It has also been used extensively to make
channeled implants of all elements into the major crystallographic directions of many
crystalline materials. It has also been used as a Rutherford backscattering analysis
instrument for 600-keV He nuclei.
This 400-kV ion mass spectrometer is described in some detail in the following
This ion mass spectrometer and the associated gas supply manifolds, dry boxes for
element storage, control console, and laminar flow hoods at the targets (2) occupies a
floor space about 20 ft by 35 ft. A list of components follows, with technical and
philosophical discussion for some components.
High voltage terminal
The terminal is a lead-covered box that contains the ion source, six power supplies, (three
for the ion source and three for the extraction and focusing optics), six leak valves
(Granville-Phillips), four connected to four gas bottles (BF3, SiF4, PF3, and AsF3) and
two connected to lines from an external manifold, and 12 meters (six voltage and six
current) that are read across the high voltage via a closed circuit TV. The terminal is
interfaced to an exhaust vent system that never ceases to operate because it has its own
emergency power system. Terminal corners should all be well rounded so that electric
field lines are not concentrated at corners, which will result in corona, which will produce
ozone and which causes leakage from the power supply and limits the current and voltage
capability, as well as causing instabilities when operating at high voltage. This terminal
as used here cannot be used at voltages greater than about 360 kV on humid days and
about 320 on dry days (unless water vapor is supplied in its environment, which we do)
with the risk of leakage and corona, which cause instabilities in the voltage, which
translates into spatial instabilities in the ion beam at the target . For example, it is desired
to maintain stable separation between isotopes of Pb (206, 207, 208) at the mass defining
slit for hours.
High voltage power supply
The 400-kV, 3-mA power supply was custom built by Universal Voltronics (Model BAC
400-3-HRL), The ripple specification is 10-4, which is the same as for the magnet power
supply, providing a very spatially stable ion beam at the target. Associated with this
power supply and the terminal is a 115 VAC, 3kVA, 400-kV isolation transformer that
supplies power to the six lab power supplies in the terminal.
High voltage meter
A 300-kV precision voltage divider is used as a voltmeter for this system, up to 300 kV.
The guaranteed accuracy is to within 0.3 %, but it is designed to be within 0.1 %.
Mass separator
The mass separation magnet is a double-focusing constant current, 300 A, 40V (12 kW)
unit made by High Voltage Engineering [HVE Model HQV 412] with mass-energy
product ME = 200 @ 25° and ME = 63 at 45°. The system has beams lines at each of
these angles. The magnet power supply is 40V, 300A, with ripple of 10-4. This magnet is
normally used with currents less than 130 A; for operation at currents greater than about
100 A, for times longer than one hour, the magnet heats and must be externally cooled.
400-kV accelerators (2, a second one so that cleaning can be done without shutdown)
The ones used here consist of 40 Al plates, each with two o-rings and o-ring grooves,
plus 40 1-inch glass spacers, plus two end plates, drilled to mate the opposing fittings,
and three insulating rods to maintain compression of the accelerator stack and the
required vacuum. The unit is designed for 10 kV per Al plate, and a resistor of
appropriate characteristics is placed between each pair of plates (40). The ion optics of
this constant gradient linear accelerator is designed so that 10 kV/plate must be
maintained; thus when the voltage is reduced to less than 400 kV, the ground plane must
be moved toward the HV terminal to maintain the constant gradient (10 kV/plate). To
operate at 100 kV, for example, the ground plane must be moved from plate number 40 to
plate number 10. This is easily accomplished using a flexible braided grounding strap and
requires a few seconds. As the voltage on the accelerator is decreased, the velocity of the
ions decreases and space-charge expansion of the ion beam causes the diameter of the ion
beam to increase and the amount of current that can be delivered through the aperture at
the grounded end of the accelerator to decrease. Thus the current delivery capability
decreases as the ion energy is decreased from below about 100 kV on the accelerator. The
limitation can be eased by employing an accel-decel; the ions are first accelerated to a
higher potential to extract a higher current and then decelerated to the final lower
potential.
A room to enclose the ion accelerator, high voltage terminal, and high voltage power
supply
This room provides several safety functions, and should be designed accordingly. These
features should include a) at least two exhaust systems, one with intake at the floor to
exhaust ozone produced by corona at all corners and surfaces that are at high voltage, and
one with intake above the terminal and designed to remove any gases that might leak
from the internal gas handling systems or from the lines from external gas supplies, b) an
rf screen that completely surrounds the volume that contains the ion source and high
voltage power supply [This rf screen is needed to protect sensitive electronics that may
exist anywhere within at least 100 m of the system, because rf noise is generated during
operation.], c) Pb shielding to protect personnel from x-rays that are generated during
operation [The x-ray intensity increases at least linearly with voltage and ion current.]
The x-ray intensity can be several Roentgens per hour when the voltage is 300 kV and the
total current from the ion source is greater than 1 mA. The x-rays are produced where
electrons that are produced in the accelerator and are accelerated back up the accelerator,
strike materials, which are the plates that comprise the lenses in front of the ion source,
and the ion source exit plate.] d) personnel protection from the high voltage, and e) a
thick dielectric shield (wall) to isolate the surfaces that are at high voltage from the
electrons of the universe and provide a dielectric surface that does not easily terminate
electric field lines (created by the high voltage on the terminal and power supply
components). Such a room can have an access door that is interlocked with the high
voltage power supply so that if the door is opened when the high voltage is on the
terminal, the power supply is instantly 'crowbarred' to ground potential. Such an event
can be damaging to the power supply so this system should be carefully designed.
Gas leak valves
There should be a controlled gas leak valve on each gas supply cylinder that supplies the
ion source, and on each feed line from an external supply manifold, in order to control the
gas flow rate so that the source discharge density can be optimized to produce a well
focused ion beam. Often the beam current must be reduced by closing a mechanical slit
because in order to form a well focused ion beam, a certain beam current must be
extracted, and that current is usually relatively high. This becomes an issue when low
fluence implants are required.
Ion current integrator (and a replacement unit)
A standard current integrator designed to operate in the current range of interest, which
incur case is 1 nA to 2 mA, is required
Ion beam scanner power supply
Beam scanners should be designed to produce two well regulated and stable triangular
wave forms of prime number frequencies (but variable) of variable voltage that varies
with maximum voltage of the implanter. In our case, these voltages are from 50 V to 5
kV, which is so difficult to do with a single scanner that we use two scanners, one with
lower voltage capability and one with higher voltage capability.
Beam monitoring oscilloscopes
Two oscilloscopes are used to monitor in real time the x and y components of the scanned
ion beam that strikes the target and is being integrated. The displays on these scopes are
used to adjust steering of the beam, to adjust the ion source and ion optics voltages to
optimize the beam, to allow the scanning of the beam to be adjusted to guarantee proper
uniformity at the target, to determine the degree and quality of mass separation, to adjust
properly the neutral trap voltage (beam deflection after mass separation), and to detect
any small drifts in the physical position of the beam at the target that might be caused by
a variety of effects. These scopes are the focus of attention during setup and execution of
implants. Small adjustments of any of several voltages might be made during a long
(hours) implant based upon observations of these scope displays.
An x-y recorder to record mass spectra (the components of the ion beam)
The signal for the x direction is supplied from a Hall probe that is placed within the
region of the magnetic field produced by the mass separation magnet. The signal for the y
direction is supplied by the current that reaches the target. Higher resolution mass spectra
are achieved by placing a small hole in the center of the 'setup' target position. This
approach can be used to separate well the masses of all elements through U. One or two
picoammeters are used in recording mass spectra.
400-kV ion mass spectrometer components listing
20 ft by 35 ft of floor space, plus some for access and work space
magnet power supply and control unit (High Voltage Engineering model HQV-412 40
V/300 A)
constant current ME = 200 @ 25° and 63 @ 45°
self-contained di-ionized water cooling system for magnet and power supply 3 current
integrators
4 scanners (3, 4, 5 kV)
x-y recorder (mass spectra)
two picoammeters (mass spectra)
2 dc power supplies (-5 kV) with panel controls for x and y beam steering (deflection) 1
dc power supply (-5 kV) with panel controls for neutral trap beam deflection box to
control magnet-to-recorder signal amplitude
video monitor to display HV terminal meters
closed circuit TV camera system for above
4 ionization gauge controllers
4 ionization gauges to monitor pressure (beam line, terminal, 2 target chambers)
400 kV dc power supply (Universal Voltronics Corp. model BAC-400-3-HRL)
deceleration power supply (50 kV dc 5 or 10 mA)
automatic LN2 controllers and sensors
target bias power supplies (several 180 and 360 V dc 2 mA)
x and y beam steering plates
2 slits (one each beam line) for mass separation control and beam current limiting:
mechanically or electronically variable from closed to say 2 cm
2 beam shutters (one each beam line) pneumatic or electromagnetic
ionization gauges throughout system
ionization gauge controllers for above
mechanical pump exhaust line system (connected to all mp exhausts)
panels for valves and switches
one vacuum system vent for each target chamber and one for ion source changing (N2
gas is recommended for target chambers and Ar for the ion source)
elapsed time clock
scanner control panel
HV control panel
ion source vacuum gauge
laboratory LN2 delivery system
300 (or 400) kV precision voltmeter or divider (inside HV terminal room) (0.3%
accuracy) DVM panel meter for above
6 insulator control rods and drives for 6 HV terminal power supplies
6 insulator control rods and drives for 6 HV terminal leak valves
panel for above 12 control rods
current monitor
beam line shutter switch and open/closed position light
voltage regulator on power line to HV terminal
set of interchangeable ion sources [fluoride gases, dedicated SiF4, chloride/bromide,
replacements (spares)]
boxes of ion source replacement parts]\
W/Re wire for filaments
gas manifold storage box (see list of gases below)
vacuum systems, with all valves, TC and ionization gauges, traps, roughing pumps, and
control panels and switches
one 6-inch oil diffusion pump (ion source and accelerator) with backing
pump and
LN2 trap
two 8-inch cryopumps (one each beam line) and compressors (target
chambers)
200 or 400 liter/s ion pumps (one each beam line) and control power
supplies
one roughing pump for the gas manifold
gate valves above every vacuum system
one to isolate each beam line
one to isolate each target chamber
one to isolate accelerator and ion source
many high voltage vacuum feedthrus
two target chambers, one with LN2 cooling and one with heated target capability
target holders
one 8-position 2-inch wafer RT with clips to hold 6 pieces each position
one 6-position 3-inch wafer LN2 or RT with clips to hold 4 pieces each position
one 5-position 4-inch wafer LN2 or RT
one 6-position 2-inch diameter RT custom with many clips one 1-position 3-inch
wafer heated (300°C)
one 1-position 1-inch wafer within black-body radiatively heated oven
(500°C)
one goniometer with x and y translation plus two directions of accurately
controlled tilt for controlled angle implants or channeled implants (0°) and
Rutherford backscattering analysis capability (custom made)
various single-position research target holders, some with rotational
capability
to allow multiple fluence implants in a circle on a wafer, or to mount
samples
thick in the vertical direction
one bin of RBS electronics, detectors, etc.
heater power supplies, secondary particle suppression equipment, and temperature
monitoring equipment for heated targets
power supplies for the secondary particle suppression in the main target chamber
secondary particle suppression lenses and apertures in all target chambers
mechanical or powered slits for each beam line
pneumatic or electromagnetic shutters for each beam line controlled both manually and
electronically by the current integrator
one vertical laminar flow hood for each target chamber
one accelerator alignment mandrel
teflon tubing for lines between gas manifold and HV terminal (replace when arc causes
holes)
dry box with load lock and pair(s) of chemically inert gloves for storing and loading ion
source probes and ion source probe chemicals
dry nitrogen or Ar gas flow and control system for above dry box
supply of chemicals for ion source probe loading (see list below)
supply of ion source probe tubes (quartz) (>100)
dry box for storing ion sources
dry N2 or Ar gas flow and control system for ion source storage box
exhausted box for loading ion sources, containing work area for cleaning ion sources
gas manifold with exhaust to store gas bottles for ion source (>12)
gas manifold system with many valves to supply gases from storage box to ion source
in HV terminal
system of feed lines from gas storage box to HV terminal - inside a safety tube connected
with the exhaust system
computer for RBS and implantation range calculations
computer for control system, if desired
List of chemicals for ion source probes - to make ion beams of many elements – for
storage in the dry box described above
LiBr, BeCl2, NaBr, Mg, AlCl3, crystalline red P, crystalline yellow S (S is also stored in
the gas manifold as CS2), KBr, Ca, ScCl3, TiC13, VCl2, CrCl2, MnCl2, FeC12, CoCl2,
NiCl2, CuCl3, Zn, GaF3, Se, RbCl, Sr, YC13, ZrC14, NbCl5, MoC15, AgCl, Cd, InCl,
SnCl2 or SnC14, Sb, Te, some iodide for I, CsCI, Ba, LaC13, all lanthanide rare earth
chlorides (13), HfC14, TaC15, W from the ion source filament or from WF6, Hg, Tl,
PbC12, Bi, ThCl4, UCl4
List of gases to be available in lecture bottles and with on/off and gas flow control valves,
and stored in the gas manifold hood box described above (exhausted), located inside the
high voltage terminal near the ion source (esp. SiF4): 1H2, 2H2, He, BF3, 12CO, 13CO, 14N2,
15
N2, 1602, 1802, Ne, SiF4, PF3, CS2, Ar, GeF4, AsF3, Kr, Xe