Method of irradiating an object by means of a charged particle beam, and device for performing the method

A low-energetic charged particle beam, involving only a slight space charge effect, can be obtained by high-frequency deflection of a charged particle beam before deceleration. Using a deceleration element provided with curved surfaces and a slit-shaped aperture, a focusing effect is obtained so that the charged particle beam is imaged along a linear path on a target surface. For implantation of a semiconductor substrate, a uniform and shallow implantation can be achieved at adequate speed by means of a low-energetic ion beam obtained in the above manner.

The invention relates to a method of irradiating an object by means of a 
charged particle beam, where a charged particle beam emitted by a source 
in a charged particle beam system is deflected in two mutually 
perpendicular scanning directions, extending transversely of the optical 
axis, by means of a first and a second deflection element situated along 
an optical axis. 
The invention also relates to a device for performing the method. 
BACKGROUND OF THE INVENTION 
A method and device of this kind are known from Yukinori Ochiai e.a., 
"Focused Ion Beam Technology", Solid State Technology/November 1987, pp 
75-78. 
The cited article describes a method where patterns are inscribed in a 
photoresist layer on a semiconductor substrate by means of a focused ion 
beam, or ions are implanted directly in the semiconductor substrate. To 
achieve this, an ion beam is generated in a charged particle beam system, 
for example by means of a liquid metal ion source, which beam is 
accelerated to some hundreds of keV by means of electrodes arranged along 
an optical axis. Using an electrostatic condensor lens, the ion beam is 
imaged in a focused manner in a mass-separation system in which mutually 
perpendicular electrostatic and magnetic fields transmit only ions of 
appropriate mass (ExB filter). An electrostatic stigmator corrects the ion 
beam for deviations in the cross-section which is circular in the ideal 
case. After having passed an objective lens which comprises a number of 
electrodes which are arranged so as to be spaced apart around the optical 
axis and which have a circular aperture, the ion beam is deflected across 
the semiconductor substrate by means of a deflection element. The diameter 
of the ion beam on the substrate then amounts to 0.1 .mu.m, and a beam 
current may amount to from 1 .mu.A to 100 .mu.A. The ion energy amounts to 
from 10 to 150 keV at the area of the substrate. 
For shallow implantation of ions, directly below the surface of the 
irradiated object, it is desirable to implant ions having a low energy 
(less than 1 keV). A problem encountered in the case of such low energies 
consists in that for current densities of approximately 100 .mu.A the beam 
diameter increases substantially due to the effect of the radial Coulomb 
interaction between the ions; this interaction is large for the given 
current at a comparatively low speed of the ions (the space charge 
effect). A drawback of a large beam diameter consists in that for uniform 
implantation of a substrate the beam must be deflected as far as beyond 
the substrate. For a substrate having a diameter of, for example 10 cm, in 
the case of a beam diameter of 5 cm a surface which is approximately four 
times greater than the surface of the substrate must be scanned. As a 
result, the duration of implantation is unnecessarily long, thus impeding 
efficient production of large numbers of implanted substrates. 
Furthermore, in the case of prolonged implantation the risk of 
contamination of the substrate is comparatively high. 
SUMMARY OF THE INVENTION 
It is inter alia an object of the invention to provide a method where 
efficient and uniform ion implantation in a substrate takes place at a 
comparatively low energy (less than 1 keV). It is another object of the 
invention to provide a device which is suitable for performing this 
method. 
To achieve this, a method of irradiating an object by means of a charged 
particle beam in accordance with the invention is characterized in that 
the charged particle beam is first periodically deflected in one of the 
scanning directions by means of the first deflection element, after which 
it is decelerated by means of a deceleration element. 
By periodically deflecting the charged particle beam at a comparatively 
high frequency before the beam is decelerated, a virtually moving source 
of charged particles is formed which is imaged on the substrate by the 
focusing device. The space charge of the beam between the first deflection 
element and the substrate thus decreases sufficiently to prevent an 
increase of the beam diameter to undesirable dimensions. 
A version of a method of irradiating an object by means of a charged 
particle beam in accordance with the invention is characterized in that 
the charged particle beam is deflected by the first deflection element 
with a frequency of between 1 and 10 MHz. 
Due to the high deflection frequency, the spatial distribution of the ions 
exhibits a large number of oscillations with respect to the optical axis 
at a given instant t, so that a length over which the ions are distributed 
between the deflection element and the deceleration element at the instant 
t is increased and the space charge is reduced in proportion. The 
deflection frequency cannot be arbitrarily increased, because the angle 
through which the particles are deflected with respect to the optical axis 
decreases as the frequencies become higher. 
A further version of a method of irradiating an object by means of a 
charged particle beam in accordance with the invention is characterized in 
that the first deflection element comprises an electrostatic deflection 
element to which a sinusoidal voltage is applied. 
At deflection frequencies in the order of magnitude of some MHz, a 
sinusoidal deflection signal can be readily realised in comparison with a 
delta voltage. 
A further version of a method of irradiating an object by means of a 
charged particle beam in accordance with the invention is characterized in 
that the object is rotated about an axis of rotation which has at least 
one point in common with the optical axis, the second deflection element 
receiving a periodic voltage which, for one half period, as a function of 
time, is substantially equal to an at least third-order polynomial 
containing exclusively odd exponents. 
By rotation of the target surface, a uniform implantation can be obtained 
when an adapted delta voltage across the second deflection device is used. 
The axis of rotation may be situated adjacent the optical axis or may 
intersect the optical axis at an angle. 
It is to be noted that a method of irradiating an object by means of a 
charged particle beam and utilizing a rotating target surface is known per 
se from European Patent Application EP 263032-A1. However, therein the 
target surface is rotated in order to facilitate a scanning motion 
thereacross. From the cited Patent Application it cannot be deduced that 
greater uniformity of implanted charged particles can be achieved by 
rotation of the target surface. 
A preferred version of a method of irradiating an object by means of a 
charged particle beam in accordance with the invention is characterized in 
that a semiconductor substrate is used as the object and an ion beam is 
used as the charged particle beam. 
The method can be used notably in the semiconductor industry where large 
numbers of semiconductor substrates are implanted. When a layer of 
semiconductor material is vapour deposited on the substrate (MBE) 
simultaneously with the implantation of low-energetic ions (for example, 
P, B or As), well-defined doping profiles can be obtained which contain 
only few contaminating oxygen atoms. 
A charged particle beam system comprising a source for emitting a charged 
particle beam and a first and a second deflection element which are 
situated along an optical axis and which serve to deflect the charged 
particle beam across a target surface in two mutually perpendicular 
scanning directions, in accordance with the invention is characterized in 
that the charged particle beam system comprises a deceleration element 
which is situated along the optical axis, the first deflection element 
being situated between the source and the deceleration element. 
As a result of the inclusion of a deceleration element and a deflection 
element which is situated between the source and the delay element in a 
charged particle beam system, the charged particle beam can be extracted 
from the source only at a comparatively high energy, thus ensuring a large 
current, after which the charged particle beam can be deflected so as to 
reduce the space charge, followed by deceleration to the required low 
energy. 
A preferred embodiment of a charged particle beam system in accordance with 
the invention is characterized in that between the first deflection 
element and the deceleration element there is included a focusing element 
for focusing the charged particle beam in a first focusing direction, the 
deceleration element having a focusing effect in a second focusing 
direction which extends perpendicularly to the first focusing direction. 
The focusing element images the ion beam on the target surface in a first 
focusing direction in a well-defined manner. A point-shaped beam is imaged 
as an ellipse on the target surface by the focusing device, for example a 
multi-pole magnetic lens. In cooperation with the deceleration element, 
the beam is imaged on the target in a well-defined manner (point-shaped) 
in two mutually perpendicular focusing directions. The deceleration 
element comprises, for example a number of conductors which are situated 
along the optical axis and which carry a different potential, each 
conductor being provided with a slit of adequate length for transmitting 
the beam during its high-frequency deflection. 
A further preferred embodiment of a charged particle beam system in 
accordance with the invention is characterized in that the deceleration 
element comprises at least two conductors, each of which is provided with 
a curved surface and a slit. 
Because the deceleration element of this kind has a focusing effect in two 
focusing directions, the focusing device can be dispensed with; this means 
a saving as regards space. An embodiment of the deceleration element, for 
example in the form of three curved plate-shaped electrodes, each of which 
is provided with a slit, already appears to have adequate focusing 
properties for imaging the beam on the target surface in a focused manner 
in two mutually perpendicular focusing directions. 
A further preferred embodiment of a charged particle beam system in 
accordance with the invention is characterized in that the second 
deflection element is to be connected to a voltage source for the supply 
of a voltage which varies periodically in time and which exhibits a 
variation in time, over one half period, which approximates an at least 
third-order polynomial containing exclusively odd exponents. 
The deceleration element decelerates the ion beam from, for example 30 keV 
to 0.5 keV. At the area of the last electrode of the deceleration element 
the charged particle beam has a comparatively low energy, so that at the 
area of the last electrode deflection can take place using low deflection 
voltages. When the last electrode of the deceleration element is 
constructed as two mutually insulated parts, an alternating voltage can be 
applied across these parts so that the charged particle beam is moved in a 
scanning direction which extends perpendicularly to the scanning direction 
in which the beam has been moved so as to reduce the space charge.

DESCRIPTION OF THE INVENTION 
FIG. 1a shows a known charged particle beam system 1 which comprises a 
source 5, for example an ion source, which is connected to a vacuum 
envelope 3 via an insulator 2. The ion beam emitted by the source 5 is 
accelerated to an energy of, for example 30 keV by means of an electrode 
7. A supply voltage source 9 which is connected to the source 5 carries a 
potential of 30 kV with respect to ground, the electrode 7 connected to 
the vacuum envelope 3 being grounded. Possibly in cooperation with further 
particle-optical elements not shown in the Figure, a magnet 11 images a 
cross-over 14 of the ion beam at the area of an aperture in a diaphragm 
15. The magnet 11 deflects the ion beam along the optical axis 13, so that 
only ions having the correct ratio of charge and mass pass through the 
aperture. The ion beam is imaged on a target surface 19 of a substrate 20 
by means of a focusing device 16. Using deflection elements 17 and 18, for 
example electrodes, the ion beam is deflected across the target surface 19 
in two mutually perpendicular directions, so that uniform implantation of 
ions can take place in the substrate 20. Low-energetic ions have a small 
penetration depth in the substrate 20 which comprises, for example a 
semiconductor substrate, resulting in well-defined and shallow doping 
profiles in the case of simultaneous deposition of an epitaxial layer 
thereon. At 0.5 kV, the penetration depth of B, P, AS or Sb in Si amounts 
to a few nanometers. In order to implant the substrate 20 with 
low-energetic ions with a sufficiently large current (100 .mu.A), a 
deceleration element can be arranged between the focusing element 16 and 
the substrate 20, which deceleration element carries a voltage of -0.5 kV 
with respect to the source 5. However, the beam diameter is then increased 
to undesirable dimensions. This has the drawback that, as appears from 
FIG. 2, the surface to be scanned by the ion beam is larger than the 
substrate for uniform implantation of the substrate 20 where the ion beam 
24 is deflected beyond the edges of the substrate. When a substrate having 
a radius r is scanned by an ion beam having a diameter r, the ratio of the 
surface scanned by the beam to the substrate surface is 4/1. In the case 
of implantation with a small current (1 .mu.A), the increase of the beam 
diameter due to the space charge is small, but the time required for 
implantation is too long. For a beam current of 1 .mu.A and an ion 
concentration of 10.sup.19 cm.sup.3 to be implanted in the substrate over 
a depth of 0.5 .mu.m, the duration for uniform implantation of a substrate 
having a diameter of 10 cm amounts to approximately 7 hours. 
FIG. 1b shows a charged particle beam system 1 in accordance with the 
invention which comprises a deceleration element 25. The ions emitted by 
the source 5 are deflected in the plane of drawing by the deflection 
element 17' at a high-frequency (1-10 MHz). The extreme beam positions are 
represented by the rays 23a and 23b. The source 5 carries a voltage of 30 
kV with respect to the electrode 7 and is connected to a supply voltage 
source 9' carrying a voltage of 0.5 kV with respect to ground. The 
electrode 7 is connected, via the vacuum envelope 3, to a supply voltage 
source 26 carrying -29.5 kV with respect to ground. Within a section 3' 
which is insulated from the vacuum envelope 3 there is arranged the 
deceleration element 25 for decelerating the ion beam to ground potential. 
The deceleration element comprises, for example three electrodes, the 
electrode 25a carrying a voltage of -29.5 kV, while the electrode 25b 
carries a voltage of -6 kV and the third electrode (18') is connected to 
ground. The deceleration element 25 shown in the Figure sharply focuses 
the ion beam on the substrate 20 in a first focusing direction, extending 
in the plane of drawing, as well as in a focusing direction extending 
perpendicularly to the plane of drawing. For well-focused imaging in the 
plane of drawing, the electrodes 25a and 25b of the deceleration element 
have a curved surface 28. If a focusing device which sharply focuses the 
ion beam in the focusing direction in the plane of drawing is inserted 
between the deflection element 17' and the deceleration element 25, the 
curvature of the electrodes 25a and 25b can be dispensed with. 
FIG. 3a shows that a virtual displacement of a cross-over 14' is realised 
by using the deflection element 17' between the cross-over 14' and the 
deceleration element 25. The image of the cross-over 14' then moves across 
the target surface 19 along a linear path. FIG. 3b shows that a real 
displacement of the cross-over is produced by arranging the deflection 
element 17' so that it precedes the cross-over 14', the image of the 
cross-over then moving across the target surface 19 along a linear path. 
FIG. 4 shows the path 27 on which the ions are present at an instant t when 
a high-frequency sinusoidal voltage is applied across the electrodes of 
the deflection element 17'. In the case of a maximum or minimum deflection 
voltage across the deflection element 17', ions moving without deflection 
along the optical axis 13 are deflected in the direction of the rays 23a 
and 23b. In the case of a non-periodic or low-frequency deflection at an 
instant t, ions are situated on a path which is given by one of the rays 
23a or 23b. Due to sinusoidal deflection of the ion beam, the ions which 
would be present on the path AB at an instant t in the case of 
non-periodic or low-frequency deflection, are distributed along the path 
ACB. For an energy of 30 keV, the velocity of phosphor ions amounts to 
4.4.times.10.sup.7 cms.sup.-1. At a deflection frequency of 2 MHz, the 
length of the path AB amounts to 22 cm. After the passage of the 
deceleration element 25, the velocity component in the direction of the 
optical axis has been reduced and a path DE amounts to 2.8 cm for an ion 
energy of 0.5 keV. Due to the sinusoidal deflection of the ion beam, the 
ions which would be present along the path DE at an instant t in the case 
of non-periodic or low-frequency deflection, are distributed along the 
path DFE. It will be evident that deceleration of the ion beam increases 
the space charge and that this effect can be counteracted by 
high-frequency deflection of the ion beam. However, the deflection 
frequency f.sub.s may not become arbitrarily high, because the angle 
.alpha. enclosed by the rays 23a and 23b with respect to the optical axis 
13 is proportional to sin (.pi.f.sub.s T)/.pi.f.sub.s T. Therein, T is the 
transit time of the ions through the deflection element. A first zero 
value of the angle .alpha. occurs when the deflection frequency f.sub.s 
equals 1/T. When the deflection element comprises two plate-shaped 
electrodes having a length of 4 cm, the angle .alpha. is still 
sufficiently large for space charge reduction for phosphor ions with an 
energy of 30 keV at a deflection frequency of less than 5 MHz. 
FIG. 5 shows the substrate 20 across which the ion beam is deflected along 
the line 30 by the deflection element 17'. Because the ion beam is 
sinusoidally deflected along the line 30, the implanted concentration will 
be higher at the ends of the line 30 than in the center where the speed of 
the ion beam is highest. Due to rotation of the substrate around its 
center, the lines of equal concentration form concentric circles. When the 
ion beam is deflected by the deflection element 18' in a direction 
perpendicular to the line 30, utilizing a modified delta voltage V(t) 
across the electrodes of the deflection element 18' whose variation is 
given in FIG. 6 for a period Td, the beam is deflected faster for 
positions r situated further from the center than for positions on the 
substrate with a smaller r. V(t) can then be represented as 
V(t)=Dt+Bt.sup.3 +Ct.sup.5. Uniform doping is thus achieved. 
FIG. 7a shows the deceleration element 25 which comprises the electrodes 
25a, 25b and 18', each of which is provided with a slit 31. The electrodes 
25a and 25b carry, for example a potential of -29.5 kV and -6 kV, 
respectively. As is shown in FIG. 7b, the 18' is subdivided into two 
mutually insulated parts which are separated by an insulator 33. A 
modified delta voltage having a frequency of, for example a few hundred Hz 
and an amplitude of a few hundred volts is applied to the parts of the 
electrode 18', via a voltage source 39. Because the ion beam has a 
comparatively low energy at the area of the electrode 18', a deflection of 
the ion beam can be realised by way of a low voltage. 
FIG. 7c is a plan view of the electrodes 25a and 25b. Assuming a curved 
surface of the electrodes with a radius of curvature R, for a position 
Y.sub.0 with respect to the optical axis 13 the new position Y of the 
surface 28 of the electrodes 25a and 25b is given by: Y=(Y.sub.0 +k.sup.2 
Y.sup.2 /8R.sup.2). Therein, k.sup.2 is the ratio of the potentials of the 
electrodes 25b and 25a. When the electrodes are curved in this manner, a 
suitable focusing effect is obtained for the deceleration element in the 
plane of the drawing, spherical aberration being only slight. In the plane 
perpendicular to the plane of the drawing of FIG. 7c, focusing can be 
adapted by modification of the dimension of the electrodes 25a and 25b in 
the direction of the optical axis 13 as well as by modification of their 
potential with respect to one another. 
It is to be noted that, even though the invention is notably attractive for 
ion implantation in semiconductor materials, it can be used equally well 
for methods requiring a low-energetic electron beam.