Charged particle detector

Details Charged particle detector Charged particle detector

1998-12-30

2001-05-22

Arroyo, Teresa M. (Department: 2881)

Radiant energy

With charged particle beam deflection or focussing

With detector

C250S310000, C250S3960ML, C250S505100

Reexamination Certificate

active

06236053

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to charged particle detectors and more particularly but not exclusively to detectors that form part of, or are used with, electron microscopes, and the configuration of the detectors within the electron microscopes.
BACKGROUND OF THE INVENTION
An electron microscope of the prior art is shown in FIG.
1
. In this arrangement a beam of electrons is passed down a central passage following the line of arrow
10
. This beam is the primary beam that strikes a sample
12
. Particles of different types return from the sample and may follow a range of paths. It is to be noted that, throughout the present specification the term “return” is used in relation to particles traveling from a sample broadly in the opposite direction to the primary beam and includes primary particles that strike the sample and bounce off as well as secondary particles that are emitted from the sample after being struck by particles of the primary beam.
Arrows
14
and
16
show two exemplary paths that returning electrons may take. Those electrons taking path
14
strike detector
18
. In the example shown the detector is an MCP or micro-channel plate detector and comprises a micro-channel plate but it may also be a pair of MCPs or a triple stack of MCPs
18
a
and an anode
18
b.
An MCP detector is shown because several of the problems given below are specific to MCP or MSP (micro-sphere plate) detectors. However the broad principle of the invention applies to any kind of detector that can be used in an electron microscope.
Electrons that follow path
16
, however, do not strike the MCP and thus cannot be detected because a detector cannot be placed in the path of the primary beam. Furthermore any electrons that follow a path leading to dead area
20
cannot be detected either and the reason for this is as follows: The MCPs
18
must be held at a high voltage in order to attract particles and cause particle multiplication effects. Such voltages are sufficiently high to interfere with the primary beam
10
on its way to the sample and therefore the central passageway has to be shielded from the voltage. This is achieved by using a shielding tube
22
, the walls of which are relatively thick. It would on the face of it be attractive to make the central passage long and thin, however a certain amount of interaction between the beam and the walls always occurs, resulting in deflection of the beam. Thus a minimum limit on the diameter of the central passageway of a significant fraction of the length of the tube is generally adhered to in order to minimize the interaction. As the length of the tube cannot be reduced below the thickness of the detectors
18
this sets a limit on how small the central passageway can be made.
In addition to the dead area caused by the central passageway itself that part of the detector immediately adjacent to the tube walls
22
cannot be charged to the voltage necessary to work, since it will cause a voltage breakdown at the wall
22
, which is at a different potential. Therefore the dead area
20
, in which no detection is possible, is relatively large and is particularly problematic in some applications in which important information is carried by those electrons that return from the sample in trajectories close to the axis of the microscope.
A further problem with the standard arrangement is that particles that return directly from the sample, which may include both primary and secondary electrons, occupy a range of energies, from those so weak that they cannot be detected to those so strong that they cause damage to the detector. The detector has certain energies within which detection efficiency is a maximum and not all electrons can be manipulation to lie simultaneously within that range.
In addition the MCP is exposed to being struck by pollutants, dust particles and the like, which can shorten the life span of the MCP by, for example, coating the active surfaces of the micro-channels so that particle multiplication is impeded.
A further difficulty is that the MCP
18
is limited by a relatively low saturation current.
FIG. 2
shows a characteristic of input against output current for a series of different gains and it will be seen that no matter what the gain, an absolute saturation value of around 1&mgr;A applies.
SUMMARY OF THE INVENTION
It is an object of the invention to alleviate the above problems.
According to a first aspect of the present invention there is provided a device for directing a primary particle beam at a sample and detecting particles in accordance with the emergence of particles from the sample. The device comprises a source for generating the primary beam and a passage leading from the source to the sample and having a first end towards the source and a second end towards the sample and wherein, alongside the passage, is placed at least one particle detector having a face for receiving particles to be detected, characterized in that at the first end of the passage is placed a cover suitable for generating secondary electrons and having a central aperture for admitting the primary beam and in that the at least one particle detector is orientated with the face for receiving particles towards the cover.
The primary particles beam may be focused within the aperture and shielding may be provided around the passage. The primary particle beam is preferably kept away from the shielding. The detector may be placed adjacent the shielding and may be provided with a voltage necessary for carrying out detection across the whole of the detector including that portion of the detector adjacent the shielding.
The cover is preferably sufficiently thin that interaction between the primary beam and walls of the aperture is minimal. The cover may be made of BeCu, which is a material that has a high yield of secondary electrons.
According to a second aspect of the invention there is provided a detector for charged particles having a face for receiving charged particles, a particle multiplier plate located at the face, a reflector located behind the plate with respect to the face and a grid anode located between the plate and the reflector.
Preferably the plate, the grid anode and the reflector are all charged and the grid anode is charged to a voltage substantially higher than the plate, and the reflector is charged to a volume moderately lower than the grid anode.
The reflector is made of a material suitable for producing secondary electrons, for example BeCu or conductive diamond.


REFERENCES:
patent: 4958079 (1990-09-01), Gray
patent: 5945672 (1999-08-01), Knowles et al.

Affiliated with

Also associated with

Rating

Charged particle detector does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Charged particle detector, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Charged particle detector will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2562445