Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
1999-02-16
2001-10-09
Anderson, Bruce C. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S397000
Reexamination Certificate
active
06300642
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an ion implantation apparatus for use in manufacturing semiconductors, and more particularly to a method of monitoring operation of Faraday cups in an ion implantation apparatus.
BACKGROUND OF THE INVENTION
An ion implantation apparatus used in manufacturing semiconductors implant impurities in targets such as silicon or gallium arsenide substrates. Such ion implantation apparatuses extract an ion beam from an ion source, accelerate the ion beam to a desired implantation energy, and direct the ion beam at a target to implant the ions in the target. The concentration of the impurity being introduced into the target can be established by measuring the current of the ion beam using a Faraday cup and integrating the current over time to determine the dose. For current integrated circuit manufacturing processes, high energy implant processes that can freely control impurity profile in the interior of silicon substrates are increasingly important. To achieve high energy implants, implantation apparatuses commonly use tandem acceleration principles for accelerating ions to high energy and implanting them in silicon substrates. Tandem acceleration principles are well known and are described in U.S. Pat. No. 3,353,107, which is hereby incorporated herein its entirety. Tandem acceleration techniques produce a negative ion beam by combining a positive ion source and a charge exchange cell, or by using a sputter type negative ion source. The negative ion beam is directed toward an accelerator terminal that is at high positive voltage and accelerated to the terminal voltage. Electrons are then stripped from this accelerated negative ion beam by passing the beam through a gas or thin foil, to convert the beam into a positive ion beam. The positive ion beam is accelerated again to ground potential from the accelerator terminal maintained at high positive potential and acquires its final energy.
An example of an actual apparatus that uses this tandem principle is a Genus Inc. Model G1500 high energy ion implantation apparatus.
FIG. 1
shows the model G1500 modified by omitting a pre-acceleration tube. U.S. Pat. No. 4,980,556 further describes such ion implantation devices and is hereby incorporated by reference herein in its entirety. In this apparatus, a hot-cathode PIG (Penning Ion Gauge) ion source
1
produces positive ions that are extracted as a beam by impressing a high positive voltage on ion source
1
. The extracted positive ion beam collides with magnesium vapor when passing through a charge exchange cell
2
which is adjacent the extraction electrode system. In the collisions, some of the positive ions in the positive ion beam pick up two electrons from the magnesium vapor and are converted to negative ions.
After passing through the charge exchange cell
2
, a 90-degree analyzing magnet
3
analyzes the beam according to the charge-to-mass ratio of the ions so that only the desired negative ions are injected into a tandem accelerator
5
. A quadrapole magnetic lens (Q-lens)
4
at the entrance aperture part of the low-energy acceleration tube
6
of the tandem accelerator
5
receives the mass-analyzed negative ion beam and focuses the beam to create a beam waist at the center of a stripper canal
7
which is in tandem accelerator
5
. At the same time, a high positive voltage accelerates the negative ion beam towards stripper canal
7
.
When this accelerated negative ion beam passes through the stripper canal
7
, ions lose orbital electrons in collisions with nitrogen gas in stripper canal
7
and are converted again into positive ions. At this time, the energy of the collisions determines the distribution of charge states of the ions in the beam. In particular, higher energy collisions produce more multi-charged ions. The positive ion beam thus obtained is directed towards ground potential from the tandem accelerator terminal, and further accelerates while passing through a high-energy acceleration tube
8
.
The beam thus having its final energy receives further focusing by a quadrapole lens
9
. An analyzing magnet
10
selects ions having the desired charge state and directs the selected ions into a process chamber
11
containing a target (for example, wafer or substrate).
The ion implantation apparatus of
FIG. 1
generally includes two Faraday cups (or cages). The first Faraday cup is between analyzing magnet
3
and Q-lens
4
and measures the beam current for set up the beam. The second Faraday cup is within the process chamber
11
and measures the implant dose by measuring the beam current. Hereinafter, the first Faraday cup is called the “injector Faraday cup” and the second Faraday cup is called “disk Faraday cup”.
FIG. 2
illustrates a beam current
21
striking a disk
23
at a radius
24
. Disk
23
holds a wafer
25
and rotates to scan beam
21
across wafer
25
. Disk also moves up and down to change the radius at which beam
21
crosses wafer
25
. A disk Faraday cup
22
, which is behind disk
23
, samples beam current
21
each time disk
23
rotates so that beam current
21
passes through a gap in disk
23
. At the end of a scan across wafer
25
, beam
21
is at radius
27
and no longer impacting upon wafer
25
, and injector Faraday cup
26
, which is upstream of spinning disk
23
, moves in position to measure the upstream current.
In the above-described ion implantation apparatus, the injector and disk Faraday cups have bias rings that are electrically connected in series. The bias rings have a bias voltage that suppresses capture of electrons by the respective Faraday cups. Capture of electrons can lead to errors in measurement of the current and the implantation dose, resulting in failure of an integrated circuit being manufactured. However, the known ion implantation apparatus is unable to monitor whether both bias rings are appropriately biased.
SUMMARY OF THE INVENTION
An embodiment of the present invention provides an ion implantation apparatus that monitors the operation of Faraday cups to prevent an ion overdose or underdose. The apparatus includes a controller for controlling overall functions of the apparatus and generating an optical power control signal, a first Faraday cup for optimally setting up a beam current, a second Faraday cup for measuring an implant dose, an interface for converting the optical power control signal into an electrical power control signal, and a Faraday bias circuit for generating a bias voltage to be supplied to the first Faraday cup in response to the electrical power control signal. The first Faraday cup has a first bias ring for receiving a bias voltage, and the second Faraday cup has a second bias ring electrically connected to the first bias ring. The bias voltage suppresses capture of secondary electrons. The apparatus further includes a loopback device in communication with the second bias ring. The loopback device detects whether a current flows through the second bias ring, and if the current does not flow properly, generates an interlock signal to the controller. At the same time, a driver circuit activates an alarm in response to the interlock signal, and stops the apparatus' operation.
According to another aspect of the invention, a method for monitoring operation of a Faraday cup positioned in an ion implantation apparatus includes providing a bias voltage to a bias ring of the Faraday cup, detecting whether a current flows through the bias ring, generating an interlock signal to a controller of the apparatus if the current does not flow properly, and stopping operation of the apparatus in response to the interlock signal.
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patent: 5319212 (1994-06-01), Tokoro
patent: 5468966 (1995-11-01), Elmer et al.
patent: 5554926 (1996-09-01), Elmer et al.
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patent: 5977553 (1999-11-01), Oh et al.
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patent: 402051837A (1990-02-01), Non
Cho Yeon-Ha
Go Seok-Ho
Lee Joon-Ho
Yun Jae-Im
Anderson Bruce C.
Heid David W.
Samsung Electronics Co,. Ltd.
Skjerven Morrill & MacPherson LLP
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