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Control of ion beam current density and profile for high current ion implantation systems
86
Nuclear
Instruments
and Methods
in Physics Research
B55 (1991) 86-89 North-Holland
Control of ion beam current density and profile for high current ion implantation systems Masayasu Tanjyo, Shuichi Fujiwara, Hiromichi Sakmoto
and Masao Naito
Nissin Eleciric Co., Ltd 47, Ukyo-ku, Kyoto 615, Japan
One solution for the problems of the local temperature increase and the charge-up of a wafer during high current ion implantation is to decrease the beam current density while maint~~ng the total beam current necessary on the wafer. To solve these problems, a method, that maintains the beam spatial profile on a wafer by controlling the normalized perveance and the pole face angle of the analyzing magnet for various beam currents, has been developed experimentally. As a result, under the experimental conditions of 1 to 8 mA of 35 keV As+, the beam uniformity, defined as [average current density]/[ maximum current density], became more than 0.2 and the beam profile on the wafer was maintained by using a suitable value of the perveance.
The local temperature increase and charge-up of a wafer caused by a high current ion beam can degrade the performance of ion implantation systems. Lower beam current density implantation is one solution. Recently a new methodology has been developed to control the spatial profile of the ion beam current density. This has been applied to the N&sin high current ion implantation system (PR-XOA [l]). This paper describes the experimental procedure, results from the use of this new method and its application to high current ion implantation.
One way of shaping the spatial profile of the beam is to make use of the fringing field of the entrance or the exit pole face of the analyzing magnet [3]. In the PR-80A system, the entrance pole face angle of the analyzing magnet is denoted by (Y and the exit pole face angle is denoted by ,B, as shown in fig. 1. In the experiments, the fi dependence was investigated thoroughly.
The spatial profile of the beam at the target is equivalent to the transverse current density profile of
2. I. Normalized perveance In all ion beam systems the perveance is a parameter that indicates the status of the extracted ion beams. The effects of the space charge downstream of the beam extraction electrodes are characterized by the poissance I7 = I,,,/V3/2,8 where I,,, is the extraction current and V is the extraction voltage. The perveance, defined by the Child-Langmuir equation for the one-dimensional parallel plate model, is PC = (4~~/9)(2e/~)“‘( A/d;), where d, is the gap width of the extraction electrodes, A is the area of the slit of the plasma electrode and the other notations are as defined in ref. [2]. We define the effective gap width d,( = d, + t) as d,, where t is the thickness of the extraction electrode. The normalized perveance is one of the important beam optics matching parameters to control the spatial profile and is characterized by P, = n/P,. When an ion species and a beam energy are fixed, the normalized perveance is where K is a constant. expressed by PN = K(I,,,/di), 0 1991 - Bsevier
2.2. Pole face angle
2.3. Beam uniformity
2. Experimental parameters
0168-583X/91,/$03.50
Therefore, the value of the normalized perveance can be adjusted by varing the extraction current and the gap width.
Science Publishers
Fig. 1. Schematic
B.V. (North-Holland)
of the beam line of the PR-8OA system.
hf. Tanjyo ef al. / Control of ion beam current density and profile the ion beam at the target. The beam uniformity is defined as .I* = .JAvJ.Jmx, where JAvE and JMM are the average and the rn~rnu~ beam current densities, respectively, within an area of 70 mm (width W) X 390 mm (height W) on the target. The criterion adopted was that JN > 0.2.
3. Apparatus Some representive elements of the beam optics of PR-80A are shown in fig. 1. The ion beam from a Freeman type ion source is extracted from a slit with dimensions 2 mm (W) and 60 mm (H). Typical beam currents on target are 1 to 8 mA of 35-50 keV As’ and I to 3 mA of 30-50 keV B+. The analyzing magnet has an optical radius of 500 mm and a 90” bending angle. The entrance pole face angle (CT) is normally set to + 20 o and the exit pole face angle (fi) to 0 O. In the beam line, there is an analyzing slit of 18 mm (W) x 110 mm (H) and, also, a shaping slit of 24 mm (W) x 110 mm (H). The distance between the ion sonce and the
\
14
16
18 de (mm)
Fig. 3. B’ 16
20
~
22
beam spatial profiles on target showing the P, dependence.
entrance of the analyzing magnet and between the exit of the analyzing magnet and the target are 500 mm and 1050 mm, respectively. The beam trajectories have one focal point in the x (horizontal) plane between the analyzing magnet and the target. The beam (spatial) profile monitor, “BPM”, is installed behind the disk, as shown in fig. 1.
4. Results 4.1. Controf of the spatial profile
0
0
14
16
18
de (mm)
20
Fig. 2. As+ beam spatial profiles on target showing the P, dependence.
Spatial profiles of beams of 35 keV AswCand. 30 keV B+ in the plane of the target are shown in fig. 2 and fig. 3, respectively. The following conclusions can be drawn: - Both As’ and Bf beams have similar profiles for a given value of P,. - When P, is smaller than 0.6, the spatial profile becomes broad, and it becomes narrower when P, is changed to larger values. II. REAL TIME PROCESSING
M. Tanjyo et al. / Control of ion beam current density and profile
b)
a)
Jr0.4
16 _
16
P,-l,l J-0.2
P,-RQ h
\
\u \. \
I\.
JrO.3
\. I
Jr0.3
“‘(u
1p”=oJ
1b=oJ
1
t----f
f-l“0
14
16
18
‘0
20
14
16
18
detmm)
de(mm)
Fig. 4. (a) Contours of the beam current on target, and (b) contours of the beam uniformity on target. The ion beam is As+. a
As’/35keV
I
- When P,
is between 0.6 and 0.8, the spatial profile has a single maximum (peak) and the current density is a maximum. With increasing P,, the spatial profile becomes a trapezoid. _ For P, larger than 0.8, the beam diverges and when P, = 1.0 the profile has two maxima (“two-peak distribution”). The relationship between the beam uniformity, JN, and and the target beam current, I,, is shown in fig. 4. From fig. 4, one can choose the parameters to give the optimum beam current and uniformity. For example, for an As+ beam with I, = 8 mA and JN > 0.2, values of d, = 16 mm and I,,, = 16 mA should be selected. 4.2. Determination
0212
2260
0129
IO
I 8.750
0.432
I 0"
-10"
O0
-10"
0
-2 0”
-30”
b) of p ib =
The focussing of the fringing field on the exit side of the analyzing magnet was investigated by studying the dependence of the beam transport efficiency and the spatial profile upon the parameter ,K Typical experi‘mental data are shown in fig. 5 from which it can be deduced that the optimum values of p are - 30 o for 35 keV As+ and -20” for 30 keV B+.
5. Conclusions The spatial profile of the beam on the target is determined and controlled by the normalized perveance ( PN). This is possible because the spatial profile of the
6
-20
-30”
Fig. 5. Beam spatial profile on target showing the /3 dependence: (a) 35 keV As+, and (b) 30 keV B+.
M. Tanjyo et al. / Control of ion beam current density and profile
extracted beam can be maintained during transport through the analyzing magnet and the slits. Therefore, by setting P,, which is achieved by setting the extraction current and the gap width, one can reduce the beam current density on the target and yet still transport the required high beam current. This is an effective method of suppressing the local temperature increase and charge-up on a wafer.
89
References [l] T. Kawai et al., these Proceedings (8th Int. Conf. on Ion Implantation Technology, Guildford, UK, 1990) Nucl. Instr. and Meth. B55 (1991) 443. [2] L. Lejeune, in: Advances in Electronics and Electron Physics , Suppl. 13C, ed. A Septier (Academic Press, 1983) p. 207. [3] H.A. Enge, in: Focussing of Charged particles, vol. 2, ed. A. Sepier (Academic Press, 1967) chap. 4.2.
II. REAL TIME PROCESSING
Nuclear
Instruments
and Methods
in Physics Research
B55 (1991) 86-89 North-Holland
Control of ion beam current density and profile for high current ion implantation systems Masayasu Tanjyo, Shuichi Fujiwara, Hiromichi Sakmoto
and Masao Naito
Nissin Eleciric Co., Ltd 47, Ukyo-ku, Kyoto 615, Japan
One solution for the problems of the local temperature increase and the charge-up of a wafer during high current ion implantation is to decrease the beam current density while maint~~ng the total beam current necessary on the wafer. To solve these problems, a method, that maintains the beam spatial profile on a wafer by controlling the normalized perveance and the pole face angle of the analyzing magnet for various beam currents, has been developed experimentally. As a result, under the experimental conditions of 1 to 8 mA of 35 keV As+, the beam uniformity, defined as [average current density]/[ maximum current density], became more than 0.2 and the beam profile on the wafer was maintained by using a suitable value of the perveance.
The local temperature increase and charge-up of a wafer caused by a high current ion beam can degrade the performance of ion implantation systems. Lower beam current density implantation is one solution. Recently a new methodology has been developed to control the spatial profile of the ion beam current density. This has been applied to the N&sin high current ion implantation system (PR-XOA [l]). This paper describes the experimental procedure, results from the use of this new method and its application to high current ion implantation.
One way of shaping the spatial profile of the beam is to make use of the fringing field of the entrance or the exit pole face of the analyzing magnet [3]. In the PR-80A system, the entrance pole face angle of the analyzing magnet is denoted by (Y and the exit pole face angle is denoted by ,B, as shown in fig. 1. In the experiments, the fi dependence was investigated thoroughly.
The spatial profile of the beam at the target is equivalent to the transverse current density profile of
2. I. Normalized perveance In all ion beam systems the perveance is a parameter that indicates the status of the extracted ion beams. The effects of the space charge downstream of the beam extraction electrodes are characterized by the poissance I7 = I,,,/V3/2,8 where I,,, is the extraction current and V is the extraction voltage. The perveance, defined by the Child-Langmuir equation for the one-dimensional parallel plate model, is PC = (4~~/9)(2e/~)“‘( A/d;), where d, is the gap width of the extraction electrodes, A is the area of the slit of the plasma electrode and the other notations are as defined in ref. [2]. We define the effective gap width d,( = d, + t) as d,, where t is the thickness of the extraction electrode. The normalized perveance is one of the important beam optics matching parameters to control the spatial profile and is characterized by P, = n/P,. When an ion species and a beam energy are fixed, the normalized perveance is where K is a constant. expressed by PN = K(I,,,/di), 0 1991 - Bsevier
2.2. Pole face angle
2.3. Beam uniformity
2. Experimental parameters
0168-583X/91,/$03.50
Therefore, the value of the normalized perveance can be adjusted by varing the extraction current and the gap width.
Science Publishers
Fig. 1. Schematic
B.V. (North-Holland)
of the beam line of the PR-8OA system.
hf. Tanjyo ef al. / Control of ion beam current density and profile the ion beam at the target. The beam uniformity is defined as .I* = .JAvJ.Jmx, where JAvE and JMM are the average and the rn~rnu~ beam current densities, respectively, within an area of 70 mm (width W) X 390 mm (height W) on the target. The criterion adopted was that JN > 0.2.
3. Apparatus Some representive elements of the beam optics of PR-80A are shown in fig. 1. The ion beam from a Freeman type ion source is extracted from a slit with dimensions 2 mm (W) and 60 mm (H). Typical beam currents on target are 1 to 8 mA of 35-50 keV As’ and I to 3 mA of 30-50 keV B+. The analyzing magnet has an optical radius of 500 mm and a 90” bending angle. The entrance pole face angle (CT) is normally set to + 20 o and the exit pole face angle (fi) to 0 O. In the beam line, there is an analyzing slit of 18 mm (W) x 110 mm (H) and, also, a shaping slit of 24 mm (W) x 110 mm (H). The distance between the ion sonce and the
\
14
16
18 de (mm)
Fig. 3. B’ 16
20
~
22
beam spatial profiles on target showing the P, dependence.
entrance of the analyzing magnet and between the exit of the analyzing magnet and the target are 500 mm and 1050 mm, respectively. The beam trajectories have one focal point in the x (horizontal) plane between the analyzing magnet and the target. The beam (spatial) profile monitor, “BPM”, is installed behind the disk, as shown in fig. 1.
4. Results 4.1. Controf of the spatial profile
0
0
14
16
18
de (mm)
20
Fig. 2. As+ beam spatial profiles on target showing the P, dependence.
Spatial profiles of beams of 35 keV AswCand. 30 keV B+ in the plane of the target are shown in fig. 2 and fig. 3, respectively. The following conclusions can be drawn: - Both As’ and Bf beams have similar profiles for a given value of P,. - When P, is smaller than 0.6, the spatial profile becomes broad, and it becomes narrower when P, is changed to larger values. II. REAL TIME PROCESSING
M. Tanjyo et al. / Control of ion beam current density and profile
b)
a)
Jr0.4
16 _
16
P,-l,l J-0.2
P,-RQ h
\
\u \. \
I\.
JrO.3
\. I
Jr0.3
“‘(u
1p”=oJ
1b=oJ
1
t----f
f-l“0
14
16
18
‘0
20
14
16
18
detmm)
de(mm)
Fig. 4. (a) Contours of the beam current on target, and (b) contours of the beam uniformity on target. The ion beam is As+. a
As’/35keV
I
- When P,
is between 0.6 and 0.8, the spatial profile has a single maximum (peak) and the current density is a maximum. With increasing P,, the spatial profile becomes a trapezoid. _ For P, larger than 0.8, the beam diverges and when P, = 1.0 the profile has two maxima (“two-peak distribution”). The relationship between the beam uniformity, JN, and and the target beam current, I,, is shown in fig. 4. From fig. 4, one can choose the parameters to give the optimum beam current and uniformity. For example, for an As+ beam with I, = 8 mA and JN > 0.2, values of d, = 16 mm and I,,, = 16 mA should be selected. 4.2. Determination
0212
2260
0129
IO
I 8.750
0.432
I 0"
-10"
O0
-10"
0
-2 0”
-30”
b) of p ib =
The focussing of the fringing field on the exit side of the analyzing magnet was investigated by studying the dependence of the beam transport efficiency and the spatial profile upon the parameter ,K Typical experi‘mental data are shown in fig. 5 from which it can be deduced that the optimum values of p are - 30 o for 35 keV As+ and -20” for 30 keV B+.
5. Conclusions The spatial profile of the beam on the target is determined and controlled by the normalized perveance ( PN). This is possible because the spatial profile of the
6
-20
-30”
Fig. 5. Beam spatial profile on target showing the /3 dependence: (a) 35 keV As+, and (b) 30 keV B+.
M. Tanjyo et al. / Control of ion beam current density and profile
extracted beam can be maintained during transport through the analyzing magnet and the slits. Therefore, by setting P,, which is achieved by setting the extraction current and the gap width, one can reduce the beam current density on the target and yet still transport the required high beam current. This is an effective method of suppressing the local temperature increase and charge-up on a wafer.
89
References [l] T. Kawai et al., these Proceedings (8th Int. Conf. on Ion Implantation Technology, Guildford, UK, 1990) Nucl. Instr. and Meth. B55 (1991) 443. [2] L. Lejeune, in: Advances in Electronics and Electron Physics , Suppl. 13C, ed. A Septier (Academic Press, 1983) p. 207. [3] H.A. Enge, in: Focussing of Charged particles, vol. 2, ed. A. Sepier (Academic Press, 1967) chap. 4.2.
II. REAL TIME PROCESSING