Radiant energy – Source with charged plate-type detector
Reexamination Certificate
2001-05-24
2004-04-06
Lee, John R. (Department: 2881)
Radiant energy
Source with charged plate-type detector
C250S370110, C250S397000
Reexamination Certificate
active
06717146
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to design of electron detectors. Specifically, this invention relates to design of tandem microchannel plate and solid state electron detector having a high linearity of the amplification coefficient over a wide dynamic range of the input current signal.
2. Description of the Related Art
In a conventional scanning beam apparatus, a specimen under inspection is irradiated with a particle beam called a primary beam. For example, the irradiating primary particle beam can be an electron beam. The interaction of the primary particle beam with the specimen causes the specimen to emit electrons with kinetic energies ranging between zero electron-volts (eV) and the kinetic energy of the particles in the primary beam.
The electrons emitted by the specimen are classified according to their initial kinetic energies. The first group of electrons, with kinetic energies of up to 50 eV is called secondary electrons, or secondaries. The secondary electrons emitted by the specimen typically carry information about the topographical structure of the specimen.
The interaction of the primary beam with the specimen also causes the emission of a second class of electrons, called backscattered electrons. The backscattered electrons have energies ranging from 50 eV and up to the kinetic energy of the particles (electrons) in the primary beam and carry information about the topographical structure and the material composition of the specimen.
The secondary and backscattered electrons emitted by the specimen are collected using an electron detector. It should be noted that most of the existing electron detectors are capable of detecting only electrons with kinetic energies included in a predetermined detection energy range. In addition, the detection efficiency (the ratio of the number of detected electrons to the total number of secondary and backscattered electrons emitted from the specimen) generally increases with the increase of the electron energy. Accordingly, in order to detect the secondary and backscattered electrons with higher efficiency, it is advantageous to increase their kinetic energies. Typically, this is accomplished by accelerating the electrons in the electric field of the scanning beam apparatus. The aforementioned accelerating electric field can be produced by biasing the surface of the specimen and the surface of the electron detector, such as to create a suitable electric potential difference therebetween.
The aforementioned electron detector collects the electrons emitted by the specimen and generates an output electrical signal representative of the cumulative charge of the collected electrons, multiplied by the amplification factor of the detector. The electric signal produced by the electron detector is used in creating an image of the specimen. Depending on the nature of the electrons used in imaging (secondary or backscattered), the created image is indicative of the topographic and/or the material structure of the specimen. After the image of the area of the specimen irradiated by the primary beam spot is created using the secondary and/or backscattered electrons, the specimen is moved with respect to the irradiating primary electron beam so that the scanning beam apparatus can produce an image of the next area. The specimen can be moved in a continuous or stepwise manner.
Unfortunately, when the secondary and backscattered electrons emitted by the specimen are detected by a detector, the “transit time” between the arrival of the detected electron and the collection of the amplified signal at the other side of the detector can vary, sometimes substantially. A wide variation in the transit time results in the decrease of the frequency response of the detector. That subsequently decreases the scanning speed of the particle beam apparatus because the apparatus has to “wait” for the signal emitted by the irradiated spot of the specimen to clear the detector, before it can move on to scan the next spot. Because the width of the transit time distribution is proportional to the average electron travel time through the detector, it is advantageous to minimize the electron travel time by accelerating electrons in an electric field and/or by reducing the travel distance of the electrons.
Accordingly, in order to minimize the spread of the detector transit time in a scanning beam apparatus, the detector of the secondary and backscattered electrons preferably has a low profile so that it does not add significantly to the overall length of the electron travel path.
One type of electron detector which is presently used for detecting electrons in microcolunms is a microchannel plate (MCP) electron detector. The microchannel plate detector comprises a thin plate, typically manufactured from an insulating material, such as glass. For example, the plate can be a few hundred microns thick. The plate of the microchannel plate detector contains a plurality of thin, typically round channels, which pass through the bulk of the plate and connect the opposite faces of the plate. The inside surfaces of these channels are coated with a material having a good secondary electron emission coefficient. A potential difference is applied between the two faces of the microchannel plate to create an accelerating electric field inside the channels.
A typical microchannel plate detector can be used to amplify the input current signal with a gain of up to tens of thousands. The amplification factor of the microchannel plate detector has such a high value for the following reasons. First, as well known to persons of skill in the art, when an electron strikes a working surface of the microchannel plate detector, it releases additional electrons, the number of which goes up with the kinetic energy of the striking electron, to about 1 keV. To maximize the number of the released electrons, the working surface of the microchannel plate detector is manufactured of, or coated with a material having a good secondary emission coefficient. It will also be appreciated by those of skill in the art that before striking the working surface of the microchannel plate, the secondary and backscattered electrons emitted by the specimen under examination are accelerated by the electric field of the microcolumn to a few hundred eV, or more.
Second, the electrons released from the working surfaces of the microchannel plate travel through the channels of the detector being accelerated in the electric field created by a potential difference applied to the opposite faces of the microchannel plate. During their travel inside the channels of the microchannel plate detector, the electrons strike the inside surfaces thereof, releasing greater and greater numbers of additional electrons. These additional electrons are also accelerated and strike the walls, which results in the production of even greater numbers of electrons. Accordingly, the described avalanche-like electron production results in exceptionally high signal amplification in the microchannel plate detector.
Because the number of the electrons released in each collision is related to the energy of the striking electron, the amplification factor of the microchannel plate detector depends on the potential of the front face of the detector with respect to the specimen, and the potential difference applied to the opposite faces of the detector.
The electronic current signal amplified by the microchannel plate is collected by a collector electrode and measured by a current monitor. A 100 pA input signal can give a 1 microamp output current at a gain of 10,000. Due to the aforementioned exceptionally high gain of the microchannel plate detector, detection of the input signals down to 1 pA is routinely possible. A major drawback of the microchannel plate detector is that its output current is substantially limited. When the output current of the detector exceeds about 10% of the microchannel plate strip current, the gain of the microchannel plate decreases causing the amplification factor of the
Chang Tai-Hon Philip
Friedman Stuart L.
Yu Ming L.
Applied Materials Inc.
Gurzo Paul
Lee John R.
Sughrue Mion
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