Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-12-27
2002-12-17
Niebling, John F. (Department: 2812)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
Reexamination Certificate
active
06495840
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a hybrid scanning type of ion-implanting apparatus and ion-implanting method which magnetically sweep an ion beam and mechanically scan a target, and more particularly to means capable of sweeping an ion beam in a wide variety of energies and ion species and shortening an ion-implanting time to improve the throughput of the device. In this specification, the magnetic reciprocative scanning of the ion beam is referred to as “sweep or sweeping”, and the mechanical reciprocative scanning of the target is referred to “scan or scanning”.
2. Description of the Related Art
Ion implantation, it is important to implant ions into a target (e.g. wafer) with good uniformity. This is particularly important when the ion implantation is adopted in a semiconductor manufacturing process. As the case may be, it is desired to irradiate the target with the ion beam scanned in parallel.
A conventional art of the ion-implanting apparatus proposed to fulfill the above-mentioned demand is shown in FIG.
2
. This apparatus basically has the same structure as described in Japanese Patent Unexamined Publication No. Hei. 8-115701(JP-A-8-115701).
The ion-implanting apparatus includes an ion source
2
for drawing an ion beam
4
, a mass separation magnet
6
for selecting a specific ion species drawn therefrom, an accelerator tube
8
for accelerating or decelerating the ion beam derived therefrom, a mass separation magnet
10
(also referred to as “energy separation magnet”) for selecting the ion species with a specific energy from the ion beams
4
derived therefrom, a sweeping magnet
12
for sweeping the ion beam derived therefrom in an X-direction(for example, horizontal direction) under a magnetic field, and paralleling magnet
14
for bending the ion beam
4
derived therefrom again to scan the ion beam
4
in parallel in cooperation with the sweeping magnet, i.e. making the ion beam
4
in parallel to a Z-axis which is a movement direction of the ion beam
4
. The ion species is defined by the mass number and the valence of the ion.
The ion beam
4
derived from the paralleling magnet
14
is applied to a target (e.g. wafer)
20
held in a holder
8
of a scan mechanism
16
. Referring to
FIG. 3
, the scan mechanism
16
mechanically scans the target
20
within a sweep region
4
a
of the ion beam
4
in a Y-direction (e.g. vertical direction) perpendicular to the above X direction. Due to a cooperation between the scanning of the target
20
and the sweep of the ion beam
4
, the ion can be implanted into the entire surface of the target
20
with good uniformity.
The ion-implantation in this ion-implanting apparatus can be controlled by a control circuit including an implanting control device
26
as shown in FIG.
4
.
The desired ion species, beam energy, beam current (beam quantity) and implanting quantity are set by a man-machine interface
28
and supplied to an implanting control device
26
.
The implanting control device
26
calculates the number of times of scanning of the target
20
for realizing the aimed implanting quantity by the aimed beam current on the basis of these items of set information, and controls the scan mechanism
16
through a scan control unit
32
to realize it. The scan mechanism
16
converts a control signal supplied from the scan control device
26
into a signal for driving a motor in the scan mechanism
16
.
The initial value of the scan speed of the target
20
is constant, and made variable during implantation by the implanting control device
26
according to a change in the beam current of the ion beam
4
which is implanted. Specifically, as the beam current decreases, the scan speed is decreased. Inversely, as the beam current increases, the scan speed is increased. In order to implement this, the beam current I
B
of the ion beam which is being implanted is measured all the time by a dose Faraday
22
(see
FIG. 3
also) arranged on the side of the upstream of the target
20
. The measured beam current I
B
is supplied to the implanting control device
26
via a current converter
24
.
The sweep current I(t) which drives the sweeping magnet
12
results from the shaping of a triangular wave. This waveform shaping is carried out by the implanting control device
26
through a known method described in Japanese Patent Unexamined Publication No. Hei. 9-55179(JP-A-9-55179). For example, using a multi-point beam monitor not shown arranged upstream and downstream side of the target
20
, the beam current density distribution in the X direction on the target
20
is estimated, and the triangular wave is shaped so that the distribution approaches constant. Concretely, the triangular wave is shaped so that the sweep speed of the ion beam
4
is decreased at the position where the current density in the current density distribution is desired to be increased whereas the sweep speed is increased at the position where the current density is desired to be decreased, thereby forming a sweep signal S(t). The sweep signal is amplified by a driving amplifier
30
and is supplied to the sweeping magnet
12
as a sweep current I(t).
In the prior art ion-implanting apparatus, the sweep frequency of the ion beam
4
is fixed. The minimum number of times of scanning of the target
20
is also fixed in order to assure the constant uniformity of implanting.
More specifically, generally, assuming that the scan speed in the Y direction of the target
20
is fixed, the implanting uniformity for the target
20
is improved as the sweep frequency of the ion beam
4
becomes high. Namely, as described above, the scan speed of the target
20
is fixed when the beam current of the ion beam
4
is fixed. Therefore, when viewed from the target
20
, as shown in
FIG. 5
, the ion beam is implanted while it draws the locus in zigzag. Therefore, it can be easily supposed that when the sweep frequency of the ion beam
4
becomes low, the area which was not implanted by scanning the target only once is produced.
In an actual implantation, as the case may be, drawing of the ion beam
4
stops momentarily because of discharging in a beam drawing portion (drawing electrode system) of an ion source
2
. At this time, the change in the beam current I
B
of the ion beam
4
is measured by the dose Faraday
22
described above, and a command of stopping the scanning of the target
20
is issued from the implanting control device
26
. This intends to prevent the area of the target not implanted from being generated. However, the change in the beam current I
B
is detected later than the real change by a delay time t
o
(see
FIG. 6
) of one period at the maximum in the sweep of the ion beam
4
. The reason is as follows. When the beam current I
B
changes at the instant the ion beam
4
passes the dose Faraday
22
, this change cannot be measured by the dose Faraday
22
until the ion beam returns to the dose Faraday
22
again. This also applies to the detection at the time of recovery of the ion beam.
In addition, since the scan mechanism
16
has inertia, the scanning of the target
20
cannot be stopped within a zero time. At the time of the recovery of the ion beam
4
, the scan speed cannot also be increased to a rated speed within a zero time. An example of the change in the scan speed of the target
20
during that time is shown in FIG.
6
. In this example, the stopping time of the ion beam
4
is set for 0.1 sec.
For the above reason, when the drawing of the ion beam
4
is stopped for an instant, the non-uniformity of ion implantation in the Y-direction on the target occurs. An example of the distribution of the quantity of the implanted ion when the phenomenon shown in
FIG. 6
occurs is shown in FIG.
7
. It shows the example where the target
20
is scanned once, and discharge has occurred once when the ion beam
4
stands at the center of the target
20
in the Y-direction. In this example, the uniformity of implantation is lowered(deteriorated) to 1.156%.
Incidentally, it should be noted that the unif
Hamamoto Nariaki
Matsumoto Takao
Finnegan Henderson Farabow Garrett & Dunner LLP
Niebling John F.
Nissin Electric Co. Ltd.
Stevenson André C.
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