Ion implantation apparatus and a method of monitoring high energy neutral contamination in an ion implantation process

High energy neutral contamination in an ion implanter can be caused by beam ions neutralised as they are temporarily accelerated at an electrode before being decelerated again to the desired implant energy. This occurs for example in the decel lens arrangement which includes an electrode at a relatively high negative potential to provide the required focusing. The level of this contamination is monitored by measuring the current drain on this negative field electrode.

FIELD OF THE INVENTION 
This invention relates to ion implantation apparatus and in particular the 
monitoring of high energy neutral contamination in ion implantation 
processes. 
DISCUSSION OF PRIOR ART 
Ion implantation is one of the standard processes employed in the 
manufacture of integrated circuit devices to modify the electrical 
properties of defined regions of a substrate of semiconductor material by 
doping these regions with a selected concentration of impurity atoms. The 
technique involves generating a beam of a preselected specie of ions and 
directing the beam towards a target substrate. The depth of the ion 
implant depends inter alia on the energy of the ion beam at the point of 
implantation at the substrate, that is the implant energy. 
A typical implantation apparatus comprises a vacuum chamber, which may have 
a number of intercommunicating compartments. One compartment may comprise 
an ion source in which ions of the required specie are formed. Typical ion 
species used for implantation are boron (B.sup.+), phosphorous (P.sup.+) 
and arsenic (As.sup.+). Ions from the ion source are extracted at an 
extraction energy selected to provide efficient operation of the ion 
source. Ions at the extraction energy are then transported through a 
flight tube at a constant transport energy. In the flight tube, the ions 
are passed through a mass selector, typically taking the form of a magnet 
and associated mass selection slits. Ions of a selected mass then emerge 
from the flight tube at an exit aperture, still at the transport energy. 
For efficient extraction from the ion source and subsequent transport 
through the flight tube, the extraction energy and subsequent transport 
energy of the ions is typically between 10 and 20 keV. At lower extraction 
energies extraction efficiencies tend to fall off and at higher energies 
more powerful and/or physically larger magnets are required. 
In prior art ion implantation apparatus, implant energies up to 160 keV or 
greater have been required and it has been standard practice to accelerate 
the ions emerging from the flight tube to these higher implant energies. 
This procedure has been called "post mass selection acceleration" or 
"post-accel" for short. 
More recently, there has been a growing requirement for implantation at 
lower energies, even below 10 keV. This has led to the suggestion of post 
mass selection deceleration processes ("post-decel" for short). 
A problem with all ion implantation processes is energy contamination. In 
order to provide the desired structure of doped material in the substrate 
being treated, it is important that the implanting ions have a predictable 
and controlled energy. A problem arises with implantation apparatus if 
ions in the ion beam become neutralised, e.g. by electron exchange with an 
atom of residual gas in the vacuum chamber, before they reach their final 
implant energy. Once neutralised, the atom of the desired implant specie 
is no longer effected by the accelerating and decelerating fields and will 
therefore continue at its previous energy. If the ion is neutralised when 
travelling in a direct line to the target substrate, the neutralised atom 
will implant in the target substrate at the energy it had immediately 
after becoming neutralised. 
Where implantation apparatus is operated with post-decel, contamination 
with high energy neutrals is a particular problem, resulting from ions in 
the beam being neutralised between when the ions emerge from the magnetic 
field and are travelling in a direct line to the target substrate and when 
the ions have been fully decelerated by the subsequent decelerating 
electric field. 
It is known that the number of neutrals formed in the ion beam is dependent 
on the residual gas pressure within the vacuum chamber, which directly 
effects the number of electron exchange collisions resulting in ions being 
neutralised. However, some residual gas pressure is desirable since 
collisions between the beam ions and residual gas atoms also produce free 
electrons in the region of the ion beam which serve to reduce the space 
charge of the ion beam, which in turn reduces the tendency of the ion beam 
to "blow up" due to the mutual repulsion of the positively charged ions in 
the beam. 
A particular problem of high energy contamination on the target substrate 
can arise when the ion beam is temporarily accelerated on emerging from 
the exit aperture of the flight tube before being subsequently decelerated 
down to the implant energy. Any ions neutralised whilst at the temporary 
higher energy will, of course, continue to implant in the target substrate 
at this higher energy. Since this energy of contaminating neutrals is even 
higher than the transport energy, the effect on the implanted substrate 
can be more serious. 
Ions emerging from the exit aperture of the flight tube will be temporarily 
accelerated to a higher energy if there is present a field electrode 
between the exit aperture and the target substrate which is at a negative 
potential difference relative to the flight tube. Such a negative field 
electrode may be used as an electron suppression electrode designed to 
prevent electrons in the ion beam emerging from the flight tube from being 
drawn out of the beam in the flight tube by the deceleration field which 
reduces the energy of the ions down to the implant energy. However, a 
negative field electrode at a relatively low potential, several kilovolts 
more negative than the potential of the flight tube, may be employed in 
applications where the field electrode, in combination with other 
electrodes, is intended to provide a focusing field opposing the tendency 
of the ion beam to blow up through space charge effects before reaching 
the target substrate. An arrangement which uses such a relatively low 
potential field electrode is disclosed in United Kingdom Patent 
Application No. 9522883.9. 
SUMMARY OF THE INVENTION 
According to one aspect of the present invention, a method of monitoring in 
an ion implantation process high energy neutral contamination of an ion 
beam caused by beam ions neutralised as they are temporarily accelerated 
at a field electrode before being decelerated again to the desired implant 
energy, comprises the step of monitoring the current drain on the field 
electrode to indicate the flow of said neutralised ions to the target. It 
has been discovered that the current drain on this field electrode 
provides a good indication of the amount of contamination of the target 
substrate, especially at energies up to the maximum energy to which beam 
ions are accelerated at the field electrode. This may be explained by 
appreciating that a beam ion is neutralised by electron exchange with a 
residual gas atom in the vacuum chamber. This causes the residual gas atom 
to become ionised but typically to remain at a low energy. These low 
energy residual gas ions are then attracted to the field electrode where 
they are neutralised producing a corresponding current drain on the 
electrode. As a result, this electrode current drain provides a direct 
measure of the energy contamination on the target substrate at energies 
above the transport energy of ions through the flight tube. 
The method of the invention may include the step of precalibrating said 
field electrode current drain as a measure of said neutralised ion flow. 
In a preferred arrangement, the method includes the steps of comparing said 
monitored current drain with a threshold value selected to correspond to a 
maximum tolerable level of said high energy neutral contamination, and 
providing an alarm signal if said monitored current exceeds said threshold 
value. Alternatively, the monitored current drain may be integrated during 
an implantation process run and the integrated value may be compared with 
a threshold function during the course of the run to generate an alarm 
signal. 
A system interlock may be operated to halt the implantation process in 
response to the alarm signal. 
In another aspect, the present invention provides ion implantation 
apparatus comprising a vacuum chamber and within the vacuum chamber: 
a holder for a target substrate to be implanted, 
a source of ions, 
a flight tube to carry a beam of ions from said source at a transport 
energy, said flight tube having an exit aperture and a mass selector to 
select only ions of a desired mass for delivery in a beam at said 
transport energy from said exit aperture of the flight tube, 
a first voltage potential supply connected to provide a deceleration field 
between said exit aperture and said substrate holder to decelerate ions in 
the beam to an implant energy at the holder, 
a field electrode located between said exit aperture and said substrate 
holder, 
a second voltage potential supply connected to provide a potential 
difference between said field electrode and said exit aperture which 
temporarily accelerates ions in the beam above said transport energy, 
a current meter to provide a signal indicating the current drain on said 
field electrode, 
and means responsive to said current drain signal to provide an indication 
of the quantity of accelerated neutral particles in the beam leaving the 
suppression electrode. 
Said means responsive may include calibration means which is precalibrated 
to be responsive to said current drain signal to provide a signal 
indicating said neutral particle flow. 
In one embodiment, said means responsive includes means for comparing said 
monitored current drain with a threshold value selected to correspond to a 
maximum tolerable level of said high energy neutral contamination, and 
means providing an alarm signal if said monitored current drain exceeds 
said threshold value. The apparatus may include a system interlock 
operable to halt the implantation process in response to said alarm 
signal. 
In a preferred embodiment, said exit aperture of the flight tube has a 
screening element at the flight tube potential, and said field electrode 
is located adjacent to said screening element. A deceleration electrode 
may be included adjacent to and downstream of said field electrode, said 
deceleration electrode being connected to be at substantially the 
potential of the target holder. 
The arrangement of the present invention is most useful when said second 
voltage potential supply is arrange to bias said field electrode 
negatively relative to said exit aperture by at least 5 kV. In practice, 
said second voltage potential supply may be arranged to bias said field 
electrode negatively relative to said deceleration electrode by between 5 
kV and 40 kV. Conveniently the field electrode is held at a fixed voltage 
relative to said deceleration electrode of between -5 kV and -30 kV, more 
particularly between -15 kV and -30 kV. In a particular arrangement, the 
field electrode is held at about -25 kV relative to said deceleration 
electrode.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to FIG. 1, an ion implanter 1 comprises an ion beam generator 3 
for generating a beam of ions, a magnet 5 adjacent the ion beam generator 
for resolving spatially the beam ions according to their mass, an ion 
selector 7 disposed adjacent the analysing magnet 5 for selecting a specie 
of ions to be implanted into a target substrate and for rejecting other 
ions in the spatially resolved beam from the magnet, an electrode assembly 
9 disposed adjacent the ion selector 7 for controlling the final energy of 
the ion beam before implantation, a support 11 spaced from the electrode 
assembly 9 for supporting a target substrate 12 to be implanted with beam 
ions and an electron generator 13 disposed between the electrode assembly 
9 and the substrate support 11 for introducing electrons into the ion beam 
near the target surface to neutralise the beam and wafer surface. An ion 
beam collector 14 is positioned downstream of the substrate support 11 
which serves as a beam stop and ion current detector for dosimetry 
measurements. 
In more detail, the ion beam generator 3 comprises an ion source 15 
including an arc chamber 17 having an exit aperture 19 formed in the front 
face thereof. A pair of extraction electrodes 21,23 are spaced from the 
exit aperture 19 for extracting ions from the arc chamber and forming an 
ion beam 25. The extraction electrode 21 which is closest to the exit 
aperture 19 of the arc chamber serves as a suppression electrode to 
prevent electrons forward of the beam generator from flowing to the arc 
chamber. 
A flight tube 27 is positioned between two poles (only one shown) of the 
mass analysing magnet 5 for receiving the ion beam from the beam generator 
3 and for controlling the energy (the transport energy) of the ion beam 
during its passage between the poles of the magnet 5, which is determined 
by the potential difference between the flight tube 27 and the ion source 
15. In this particular embodiment, the magnetic field strength of the 
analysing magnet and the energy of the ion beam through the magnet are 
chosen so that ions having an appropriate mass are deflected through 
approximately 90.degree. and the flight tube 27 is configured accordingly, 
with the exit aperture 55 of the flight tube 27 being approximately 
orthogonal to the entrance aperture 29. 
The ion selector 7 comprises a series of discrete elements 35, 39, 41 and 
43 which are spaced apart along the beamline 45 and define a series of 
apertures which, in combination, select ions of the correct mass to be 
implanted in the target substrate while rejecting other spatially resolved 
ions which pass through the analysing magnet 5. In this particular 
embodiment, the ion selector 7 comprises a plate electrode 35 which 
rejects most of the unwanted ion species from the magnet, a pair of 
elements 39, 41, which together define a variable width mass resolving 
slit which passes only the selected ion specie, and a further element 43 
which defines the height of the ion beam. However, the number of mass 
resolving elements and their configuration may be varied. 
The ion selector assembly is housed in a chamber 47 between the magnet and 
the electrode assembly 9 forming an extension of the flight tube 27. The 
mass resolving chamber wall 49 comprises a part 51 which extends in the 
direction of the beamline and defines a generally cylindrical envelope, 
and a transverse part 53 adjacent the cylindrical part 51 which 
constitutes a plate electrode disposed transverse to the beam line and 
defines the exit aperture 55 through which the beam can pass, the aperture 
55 being adjacent the final element 43 of the ion selector 7. 
In this particular embodiment, a vacuum port 57 is formed in the chamber 
wall 49 near the analysing magnet 5 which is connected to a vacuum pump 59 
for evacuating the chamber 47, although in another embodiment this vacuum 
port may be omitted. 
A screening assembly 52 is positioned between the exit aperture 55 of the 
mass resolving chamber 47 and the electrode assembly 9 to reduce 
penetration of the electric field from the electrode assembly 9 into the 
mass resolving chamber 47 through the exit aperture 55.The screening 
assembly 52 comprises a cylindrical electrode 54, and a field defining 
electrode 56. The cylindrical electrode 54 is arranged coaxially with the 
exit aperture 55 and with one end 58 positioned adjacent and connected to 
the transverse part (or front end) 51 of the mass resolving chamber wall 
49. The cylindrical electrode 54 extends forward of the mass resolving 
chamber 47 and may have an inwardly extending radial flange 60 formed near 
or at the other end of the cylindrical electrode 54 to provide additional 
screening. 
The field defining electrode 56, which may or may not be used, comprises a 
circular plate with a square aperture 62 formed in the centre thereof. The 
field defining electrode 56 is mounted within and supported by the 
cylindrical electrode 54 and is positioned about midway between the ends 
of the cylindrical electrode 54 (although this may vary) and transverse to 
beam line 45. The square aperture 62 tapers gently outwards towards the 
electrode assembly 9. In this example, the width of the square aperture is 
about 60 mm. The cylindrical electrode and the field defining electrode 
may each be made of graphite or other suitable material. 
The electrode assembly 9 for controlling the implant energy of the ion beam 
is situated just beyond the screening assembly 52, and comprises a field 
electrode 61 and a deceleration electrode 65. The field electrode 61 
defines a rectangular aperture 63 adjacent and substantially coaxial with 
the exit aperture of the screening assembly 52. The deceleration electrode 
65 is disposed generally transverse to the beamline 45 and defines a 
further aperture 67 through which the ion beam can pass, this further 
aperture 67 being disposed adjacent the field electrode aperture 63. The 
field electrode and the deceleration electrode may each be made of 
graphite or other suitable material. 
In this embodiment, the electron injector 13 comprises a plasma flood 
system which introduces low energy electrons into the ion beam near the 
target. The plasma flood system includes a guide tube 69 through which the 
ion beam can pass from the deceleration electrode aperture 67 to the 
target substrate 12, and which both maintains the low energy electrons in 
the vicinity of the ion beam and screens the portion of the ion beam 
between the deceleration electrode aperture and the wafer from stray 
electric fields. 
The ion implanter further comprises an ion source voltage supply 71 for 
biasing the ion source, a suppression electrode voltage supply 73 for 
biasing the suppression electrode 21, a flight tube voltage supply 75 for 
biasing the flight tube 27 and the mass resolving chamber 47 and the other 
extraction electrode 23, a field electrode voltage supply 77 for biasing 
the field electrode 61 of the electrode assembly 91, and a plasma flood 
voltage supply 79 for biasing the electron confining electrode 69. The 
deceleration electrode 65, the target substrate support 11 and the 
substrate 12 are maintained at ground potential, which facilitates 
handling of the target substrate, simplifies the support assembly, and 
serves as a convenient reference potential for the other electrodes. 
A method of operating the ion implanter to implant ions at low energy will 
now be described with reference to a specific example for illustrative 
purposes only. 
The ion implantation energy is determined by the potential difference 
between the substrate 12 and the ion source 15. As the substrate is 
maintained at ground potential, the ion source voltage supply 71 is biased 
positively with respect to ground by an amount which determines the ion 
implantation energy. For example, for a 2 keV implant, the ion source 
voltage supply is biased to +2 kV. The transport energy of the ion beam 
through the analysing magnet 5 and the mass resolving chamber 47, which is 
also referred to as the extraction energy of the ion beam, is determined 
by the potential difference between the ion source 15 and the flight tube, 
which is controlled by the flight tube voltage supply 75. Thus, for 
example, to transport the ion beam at an energy of 10 keV through the 
flight tube, the flight tube is biased at -10 kV relative to the ion 
source or -8 kV relative to ground. The ion beam is transported with 
substantially constant energy through the analysing magnet and different 
ionic species within the ion beam are resolved spatially by the magnet 
according to their mass. The spatially resolved beam then passes into the 
mass resolving chamber, where the beam first passes through a predefining 
aperture defined by the plate electrode 35 closest to the analysing magnet 
5. The plate.sub.-- electrode 35 acts as a course, first stage filter for 
the spatially resolved beam and blocks a proportion of the spatially 
resolved ion species which are not required in the implant. The second and 
third elements 39 and 41 spaced from the analysing magnet 5, and which are 
displaced axially from one another along the beamline, define a variable 
width mass resolving slit 42, whose position can be varied in a direction 
transverse to the beamline, for selecting from the filtered beam the ion 
species to be implanted. 
As an example, in a boron implant the spatially resolved beam leaving the 
analysing magnet may contain BF.sub.3, BF.sub.2, BF, B and F ions and the 
molecular and boron ions will contain either isotope of boron, .sup.10 B 
and .sup.11 B. Thus, for a "boron 11" implant the predefining element 35 
and the mass resolving elements 39, 41 will filter out all ionic species 
except .sup.11 B. 
As the beam traverses the mass resolving chamber, the energy of the beam is 
maintained constant, in this example 10 keV. The 10 keV mass resolved beam 
46 passes through the exit aperture 55 of the mass resolving chamber 47, 
through the screening assembly 52 to the electrode assembly 9. 
A potential is applied to the field electrode 61 so that the field 
electrode is negative relative to the mass resolving chamber 47. The 
magnitude of the potential applied to the cylindrical electrode 61 is 
sufficient to establish an electrostatic focusing field in the region 
between the final aperture 67 of the grounded deceleration electrode 65 
and the aperture of the flange 60 of the screening electrode 54. The 
inventors have found that a potential on the field electrode of between -5 
kV and -30 kV with respect to the potential of the deceleration electrode 
65 is sufficient to establish the required focusing field at the final 
aperture 67 to maintain the beam ions within the beam between the final 
aperture 67 and the target substrate. However, preferably the field 
electrode 61 is held at -25 kV. 
As the flight tube and the mass resolving chamber are at -8 kV for an 
implant energy of 2 keV, the field electrode 61 is biased to a potential 
lower than the potential of the flight tube and serves to prevent 
electrons in the mass resolving region from being drawn to the 
deceleration electrode 65, which would destroy space charge neutralisation 
in this region causing beam expansion and loss of current. 
In the present example, as the mass resolved beam 46 approaches the field 
electrode 61, the beam is briefly accelerated above the transport 
(extraction) energy of 10 keV to an energy defined substantially by the 
potential difference between the ion source 15 and the field electrode 61, 
typically 25 keV. The beam passes through the field electrode aperture 63 
and is then decelerated to substantially the required implant energy in 
the gap between the field electrode aperture 63 and the final aperture 67 
of the deceleration electrode 65. At the same time, a net focusing force 
is applied to the ion beam in the region between the flange 60 and the 
field electrode 61, in the region between the field electrode 61 and the 
deceleration electrode 65, and just beyond. 
The ion beam then passes into the region between the final aperture 67 and 
the target substrate. In this region, the ion beam is transported to the 
substrate at substantially the required implant energy. Expansion of the 
now, low velocity beam is minimised by flooding the beam with low energy 
electrons, by means of the plasma flood system 13. The plasma flood system 
also minimises surface charging of the target substrate during ion 
implantation and simultaneously reduces the potential of the ion beam, 
again to minimise the extent to which the beam expands before reaching the 
substrate. 
As the ion beam travels from the ion source 3 to the target substrate 12, 
ions in the beam can be neutralised by picking up an electron. Between the 
ion source 3 and the region of the plasma flood system 13, the usual 
mechanism for neutralising beam ions is electron exchange with a residual 
gas atom. Once a beam ion is neutralised it is no longer effected by 
magnetic or electric fields. Thus, any neutral atoms formed before beam 
ions are travelling in direct line of sight to the target substrate will 
continue to fly in the direction of the beam ion when neutralised and will 
be absorbed in the flight tube or the mass selection chamber 47 and will 
not emerge through the mass selecting elements 41 and 42 in the beam 
emerging through the exit aperture 55. However, any beam ions neutralised 
once they are travelling in direct lines to the target substrate will 
continue with the energy at neutralisation to implant in the substrate at 
this energy. 
Beam ions neutralised before emerging from the cylindrical electrode 54 
will have the transport energy, typically 10 keV. These neutral atoms will 
produce energy contamination in the substrate at this energy, penetrating 
further than the ions at the desired implant energy, typically 2 keV. 
However, a more serious energy contamination occurs at a higher energy due 
to ions which are neutralised as they are temporarily accelerated past the 
field electrode 61. Such neutral atoms formed in this region may have 
energies up to 25 keV. 
In the example of the present invention shown in FIG. 1, the quantity of 
ions being neutralised at this higher energy as they are temporarily 
accelerated past the field electrode 61 is monitored by monitoring the 
current drain on the field electrode 61. 
As explained previously, beam ions which are neutralised as they pass the 
field electrode 61 experience electron exchange with a residual gas atom, 
resulting in a low energy residual gas ion which is positively charged. 
Such low energy residual gas ions are attracted to the negative potential 
on the field electrode 61, at which they are discharged to contribute to 
the current drain on the electrode 61. 
In FIG. 1, a current meter 80 provides a signal on a line 81 indicative of 
the current drain on the power supply 77 maintaining the field electrode 
61 at the desired potential. The current drain signal on the line 81 is 
supplied to a calibration and utilisation unit 82. 
In the simplest arrangement the calibration and utilisation unit 82 may be 
a threshold detector which has been set to generate an alarm signal if the 
signal on the line 81 indicates the current drain on the electrode 61 
exceeds a predetermined threshold value. This predetermined threshold 
value would correspond to a maximum tolerable level of high energy 
contamination in the ion beam being delivered to the target substrate 12. 
This predetermined maximum current drain value would be determined 
empirically. The alarm signal could be an audible or visual alarm on the 
implanter apparatus, but more preferably would take the form of an 
electronic signal supplied to the implanter system controller. 
FIG. 2 illustrates in very general form the system controller of a typical 
ion implantation apparatus. The implantation apparatus itself is indicated 
generally by the box 90 and the system controller is shown generally by 
the box 91 represented in dashed lines. The system controller receives a 
number of system parameter signals from the implanter 90 on lines 92 and 
delivers to the implanter a number of system control output signals on 
lines 93. In FIG. 2, the line 81 supplying the signal indicative of the 
current drain on the field electrode 61 is shown supplying the signal from 
the implanter 90 to the system controller 91. Within the controller 91, 
the signal on line 81 may be delivered to a calibration unit 94. In the 
calibration unit, the current drain signal may be scaled or otherwise 
converted so as to provide a representation of the level of high energy 
contamination of the ion beam being delivered to the target substrate. The 
factors used in the scaling or conversion functions are determined 
empirically in a precalibration process. 
Alternatively, a precalibration process may be used to determine threshold 
levels to indicate maximum tolerable levels of contamination in the beam. 
If the current drain signal on the line 81 is determined in the calibration 
unit 94 to show a level of high energy contamination in excess of the 
tolerable maximum, an alarm signal is generated as supplied to alarm unit 
95, which may provide visual or audible alarm signals to machine 
operators. 
In addition, signals from the calibration unit 94 are supplied to 
compensatory control and/or interlock unit 96 within the system 
controller. In one arrangement, this unit is arranged to respond to 
signals from the calibration unit 94 indicating an excessive level of high 
energy contamination by operating system interlocks to halt the 
implantation process. 
In other arrangements, the unit 96 may be arranged to respond to high 
levels of current drain on the electrode 61 by adjusting other system 
parameters with a view to controlling the level of high energy neutral 
contamination in the beam. For example, the beam current may be reduced or 
the implant suspended to allow the vacuum in the field electrode region to 
recover. 
Although the example described above relates to a post-decel implanter with 
a beam focusing field electrode, the invention may also be used in other 
arrangements where beam ions are accelerated past a field electrode.