US3866044A

US3866044A - Apparatus for determining concentration profile of an element

Info

Publication number: US3866044A
Application number: US438696A
Authority: US
Inventors: J Evan Grund
Original assignee: NUCLEAR EQUIPMENT CORP
Current assignee: NUCLEAR EQUIPMENT CORP
Priority date: 1974-02-01
Filing date: 1974-02-01
Publication date: 1975-02-11
Anticipated expiration: 1992-02-11

Abstract

Apparatus for determining the distribution of an element in a specimen from x-rays emitted when the specimen is scanned with an electron beam. A ramp generator operates in synchronization with the beam scan to produce a ramp signal having a level corresponding to the position of the beam on the specimen. The ramp signal is applied to an analog gate, and this gate is opened to produce output pulses in response to x-rays corresponding to the selected element. The height of each output pulse corresponds to the level of the ramp signal and therefore to the position of the beam when the selected element is detected. The output pulses are applied to a pulse height analyzer or other suitable recording or display device to provide an indication or record of the distribution of the selected element.

Description

United States Patent [191 Grund [111- 3,866,044 [451 Feb. 11, 1975 APPARATUS FOR DETERMINING CONCENTRATION PROFILE OF AN ELEMENT Primary Examiner]ames W. Lawrence Assistant Examiner-C. E. Church Attorney, Agent, or Firm-Flehr, Hohbach, Test, Albritton & Herbert [75] Inventor: J. Evan Grund, Hayward, Calif.

[73] Assignee: Nuclear Equipment Corporation, [57] I ABSTRACT San Carlos Apparatus for determining the distribution of an ele- [22] Filed; F b,1,1974 ment in a specimen from x-rays emitted when the specimen is scanned with an electron beam. A ramp [211 App]' 438696 generator operates in synchronization with the beam scan to produce a ramp signal having a level corre- 521 US. Cl 250/310, 250/397 spending to the position of the beam on the specimen- [Sl] Int. Cl. H01j 37/26 The p Signal pp to an analog gate and this [58] Field of Search 250/310, 3 l 1, 397, 307, gate is opened to Produce l Pulses in response to 250 309 x-rays corresponding to the selected element. The height of each output pulse corresponds to the level of 5 R f r Cited the ramp signal and therefore to the position of the UNITED STATES PATENTS beam when the selected element is detected. The output pulses are applied to a pulse height analyzer or gg j f other suitable recording or display device to provide e an indication or record of the distribution of the selected element.

9 Claims, 1 Drawing Figure o +V I y l 27 23 I l f3l l l 24 l7 I l6 I V I l RAMP 22 I l P. H. A. '2 J GENERATOR l I I3 I BLANKING SIGNAL APPARATUS FOR DETERMINING CONCENTRATION PROFILE OF AN ELEMENT BACKGROUND OF THE INVENTION This invention pertains generally to x-ray systems for analyzing the chemical composition of a specimen and more particularly to apparatus for determining the distribution of an element in a specimen.

Electron column devices such as electron microscopes have been used Widely in the visual examination of structures too fine to be resolved with ordinary light operated microscopes. In such instruments, a beam of electrons is directed toward a specimen, and in one such instrument known as a transmission electron microscope the beam is transmitted through the specimen to a flourescent screen or photographic plate or film where the image is observed or further processed, as desired. In a scanning electron microscope, the electron beam is intercepted by -the specimen, and the reflected or reemitted beam is processed to provide the desired image or record. In a transmission electron microscope, scanning of the specimen can be effected by moving the stage by which the specimen is carried. In a scanning electron microscope, the specimen is mounted on a stage, and scanning is effected by deflection of the beam. The image is frequently displayed on the screen of a cathode ray tube which is scanned in synchronism with the electron beam in the column device. v

In recent years, electron column devices have found wide use in analyzing the chemical composition of a specimen. The electron beam impinging upon a specimen excites x-rays having energy levels which are characteristic of the elements of which the specimen is composed. The x-rays are utilized to provide three principal types of information about the specimen: elemental analysis, x-ray mapping of an element, and onedimensional concentration profile of an element.

In elemental analysis, characteristic x-rays from a specimen are measured and sorted according to energy to determine the elements in a specimen and the relative concentrations thereof. While this technique provides both qualitative and quantitative information about the elements, it provides no information about the distribution of the elements in the specimen.

X-ray mapping provides a pictorial representation of the distribution of an element on the surface of the specimen. The picture is generated by analyzing the xrays produced by the beam impinging upon the specimen and displaying dots on a cathode ray tube or other suitable device at locations corresponding to the points on the specimen where x-rays corresponding to the selected element are produced. While this technique provides information about the location of an element in a specimen, it does not provide sufficient information about the concentration of the element at different locations.

A concentration profile indicates the concentration of a selected element along a one-dimensional line on a specimen. In techniques heretofore utilized in concentration profiling, the electron beam is moved slowly along the line by either beam deflection or stage movement, and the x-rays produced by the impinging beam are monitored by a detector which produces an output pulse whenever an x-ray corresponding to the selected element is encountered. In one prior art approach, the

detector output is fed to a rate meter which produces a DC voltage corresponding to the pulse rate, and this voltage is recorded by a strip chart recorder. In another prior art technique, the output of the detector is applied to a pulse height analyzer with a special multiscaling capability. This special capability enables the pulse height analyzer to record or display the count rate as a function of time to provide the desired concentration profile.

The concentration profiling techniques of the prior art have certain disadvantages. They require special instrumentational capabilities often not found in standard electron column devices and pulse height analyzers. Most scanning electron microscopes are not equipped with an automatically driven stage, and many pulse height analyzers do not have a multiscaling capability. With the rate meter and strip chart recorder approach, the scan and the chart move at linear proportional speeds, and these speeds are limited by the recorder and rate meter. For example, the scan may require 200 seconds, and many electron microscopes are incapable of scanning this slowly. Variations in the intensity of the electron beam produce inaccuracies, particularly at slow scanning rates. With the multiscaling approach, the beam scan and the multiscaling must be started simultaneously to assure proper alignment between the output and the scan, and this synchronizing generally must be done manually. Moreover, the electron beam and the multiscaling must move linearly with time, or the output will be distorted.

SUMMARY AND OBJECTS OF THE INVENTION The invention provides concentration profiles with a conventional electron column device and pulse height analyzer without special capabilities. A ramp generator operates in synchronization with the scan of the column device to produce a ramp signal having a level corresponding to the position of the beam on the specimen. The ramp signal is applied to an analog gate, and this gate is opened to produce output pulses in response to x-rays corresponding to the selected element. The height of each output. pulse corresponds to the level of the ramp signal and therefore to the position of the beam when the'selected element is detected. The output pulses are applied to a pulse height analyzer or other suitable recording or display device to provide an indication or record of the distribution of the selected element.

It is in general an object of the invention to provide new and improved apparatus for determining the concentration profile of an element in a specimen.

Another object of the invention is to provide apparatus of the above character utilizing readily available components such as a conventional electron column device and a standard pulse height analyzer.

Additional objects and features of the invention will be apparent from the following description in which the preferred embodiment is set forth in detail in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is a block diagram of one embodiment of apparatus according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As illustrated in the drawing, the apparatus comprises an electron column device such as an electron microscope, a scanning electron microscope or an electron microprobe of known design. In the device, a specimen 11 is supported by a stage 12, and a predeter- The scanning can be effected either by deflection of the beam, as indicated, or by movement of the stage.

The line on the specimen is scanned repeatedly, and at the outset of each scan, a timing or sync signal 16 is provided. As illustrated, this signal is a negative-going pulse having a leading edge which defines the end of one scan and a trailing edge which defines the beginning of the next.

The timing signal from electron column device 10 is applied to the reset input of a ramp generator 17. This generator produces a ramp signal which increases at predetermined rate from an initial level such as zero volts. The ramp signal is reset to its initial level at the outset of each scan, and the level of the ramp signal corresponds to the position of beam 14 on the specimen. As discussed more fully hereinafter, the rate at which the ramp signal increases determines the width of the profile display, and in the preferred embodiment the rate is adjusted to provide a full-scale display.

The output of ramp generator 17 is connected to one input of an analog gate 21 which, in the embodiment shown, comprises

operational amplifiers

22 and 23 and a controlled switching device 24. The output of the ramp generator is connected to one input of operational amplifier 22, and an adjustable voltage source 26 is connected to the other input of this amplifier. As will appear more fully hereinafter, source 26 serves as a baseline control for setting the quiescent DC output voltage level when gate 21 is open or passing the ramp signal. A similar source 27 connected to the input of amplifier 23 serves as a pedestal control for setting the DC level of the output signal when the analog gate is closed or not passing the ramp signal.

The outputs of

amplifiers

22 and 23 are connected to the input terminals of switching device 24. The output of the switching device is connected to the input of a pulse height analyzer 31, and the switching device serves to apply the output of one of the amplifiers to the input of the pulse height analyzer in accordance with a control signal.

Pulse height analyzer 31 is an instrument of conventional design which is sometimes called a multichannel analyzer. lt sorts incoming pulses according to height, counts the number of pulses of each height, stores the counts in digital form, and displays the counts for different heights in the form of a histogram in which the information for different heights is displayed in discrete channels along an axis. If desired, another type of display device or recording device can be used in place of or in addition to the pulse height analyzer for displaying, storing or further analyzing and reducing the data. Suitable devices for this purpose include magnetic tape recorders, punched paper tape recorders, strip chart recorders, printers, film, and cathode ray tubes.

An x-ray detector and selector 36 monitors the x-rays which are produced when electron beam 14 impings upon specimen 11 and produces output pulses 37 in response to x-rays corresponding to a selected element whose concentration profile is to be determined. The x-ray detector and selector comprises a detector of known design for producing voltage pulses having levels corresponding to the energy levels of the x-rays from the specimen. It also includes a selector commonly known as a single channel analyzer for delivering output pulses in response to voltage pulses having a level corresponding to the selected element. Alternatively, if desired, the x-ray detector and selector can be of the wavelength-dispersive type utilizing a crystal for selecting the x-rays from the desired element. As illustrated, output pulses 37 are positive-going pulses.

The output of x-ray detector and selector 36 is connected to one input of an AND gate 38. Timing signal 16 is applied to a second input of gate 38, and this gate has a third input to which a negative-going blanking signal 39 can be applied, if desired-The output of AND gate 38 is connected to the control input of switching device 24 in analog gate 21.

Operation and use of the invention to determine the distribution of an element in a specimen can be described briefly. Electron column device 10 is set to repeatedly scan a desired line on the specimen, and x-ray detector and selector 36 is set to produce output pulses 37 in response to x-rays corresponding to the selected element. The x-ray detector requires a finite amount of time to process signals, and the scan rate is preferably set such that the beam does not move more than about 1% of its total travel during the processing time of the detector, or the display will be displaced to an extent which may not be acceptable in some applications. With a conventional wavelength-dispersive detector, for example, scanning rates as fast as 1 microsecond can be used without applicable displacement.

Ramp generator 17 produces a ramp signal having a level corresponding to the position of the electron beam on the specimen. Baseline control 26 is set so that the output data begins in the outermost channel at one side of the pulse height analyzer, and the ramp rate is set to provide a full-scale display in which the data ends in the outermost channel at the other side of the pulse height analyzer. In the absence of x-rays corresponding to the selected element, switching device 24 is in the position shown, and the output of analog gate 21 is a DC voltage having a level set by pedestal control 27.

When an x-ray corresponding to the selected element is detected, AND gate 38 delivers an output pulse 41 to the control input of switching device 24, conditioning analog gate 21 to pass the ramp signal from generator 17. The switching device momentarily connects the output of amplifier 22 to the input of pulse height analyzer 31, producing an output pulse 42 having a width corresponding to the width of pulse 41 and a height corresponding to the level of the ramp signal. This height also corresponds to the position of the beam at the time the x-ray is detected.

Pulses 42 are sorted by height in the pulse height analyzer, and the pulses of different heights are counted. The pulse height and count information is then displayed to provide a concentration profile of the element along the desired line in the specimen. Since the line is scanned repeatedly and the data is displayed as it is accumulated, the operator can stop the accumulation when a satisfactory signal-to-noise ratio has been obtained in the display.

AND gate 38 prevents the generation of spurious output signals during retrace intervals. During such intervals, timing signal 16 has a low logic level, which inhibits the passage of pulses through gate 38 and prevents analog gate 21 from passing the ramp signal. If desired, a blanking signal 39 can be applied to the third input of AND gate 38 to inhibit the delivery of output pulses 42 at other times. 7

The invention has a number of important features and advantages. It permits the distribution of an element to be determined with a standard electron column device and a standard pulse height analyzer. The invention will determine the concentration profile along any line the column device is capable of scanning, and special instrument capabilities such as flexible beam scanning, stage motion or multiscaling are not necessary. Once the apparatus is calibrated to a given rate of scan, all control is automatic and the operator is not required to start multiple devices simultaneously. The repeated scanning at fast rates tends to average out inaccuracies due to variations in beam intensity and detector nonlinearity. Accuracy is high because the output data is stored digitally in the pulse height analyzer and the actual number of x-ray counts is known.

It is apparent from the foregoing that new and improved apparatus for determining the distribution of an element in a specimen has been provided. While only the presently preferred embodiment has been described, as will be apparent to those familar with the art, certain changes and modifications can be made without departing from the scope of the invention as defined by the following claims.

I claim:

1. ln apparatus for analyzing a specimen to determine the distribution of an element therein: means for scanning a portion of the specimen with an electron beam at a predetermined rate to produce x-rays having energy levels correspondingto the elements in the portion of the specimen which is scanned, x-ray selector means for providing output signals in response to x-rays having energy levels corresponding to a selected element, means for providing a ramp signal having a level corresponding to the position of the electron beam on the specimen, means responsive to the ramp signal and the output signals from the x-ray selector means for delivering output pulses having levels corresponding to the levels of the ramp signal when the output signals are received, whereby the level of each output pulse corresponds to the position of the element giving rise thereto, and display means responsive to the output pulses for indicating the distribution of the selected element in the specimen.

2. The apparatus of claim 1 including means for scanning the specimen repeatedly along a predetermined line and for resetting the ramp signal to an initial level at the beginning of each scan.

3. The apparatus of claim 1 wherein the means for delivering output pulses comprises an analog gate.

4. The apparatus of claim 1 further including gate means responsive to the timing signal for inhibiting delivery of the output signals to the analog gate means during retrace intervals between successive scans.

5. In apparatus for determining the distribution of a predetermined element in a specimen: an electron column device for scanning the specimen repeatedly along a predetermined line and providing a timing signal at the start of each scan, means responsive to x-rays emitted by the scanned portion of the specimen for delivering output signals in response to x-rays having energy levels corresponding to the predetermined element, a ramp generator for producing a ramp signal which increases from an initial level at a predetermined rate, said ramp signal being reset to the initial level in response to the timing signal whereby the level of the ramp signal corresponds to the position of the electron beam on the specimen, analog gate means responsive to the ramp signal and the output signals from the x-ray responsive means for delivering output pulses having levels corresponding to the levels of the ramp signal when the output signals are received, the level of each output pulse corresponding to the position of the element giving rise thereto, and pulse height analyzer means responsive to the output pulses for giving a visual display of the distribution of the predetermined element in the specimen.

6. ln apparatus for determining the distribution of a predetermined element in a specimen from signals pr0- duced by repeatedly scanning a portion of the specimen with an electron beam and delivering an output signal whenever the element is detected, the start of each scan being accompanied by a timing signal: a ramp generator for producing a ramp signal which increases at a predetermined rate from an initial level, means for applying the timing signal to the ramp generator for resetting the ramp signal to the initial level at the outset of each scan, whereby the level of the ramp signal corresponds to the position of the beam on the specimen, and analog gate means responsive to the output signals for passing the ramp signal to produce output pulses having heights corresponding to the positions of the beam when the predetermined element is detected.

7. The apparatus of claim 6 further including pulse height analyzer means responsive to the output pulses for giving a visual display of the distribution of the prestorage means for storing the output pulses.

Claims

1. In apparatus for analyzing a specimen to determine the distribution of an element therein: means for scanning a portion of the specimen with an electron beam at a predetermined rate to produce x-rays having energy levels corresponding to the elements in the portion of the specimen which is scanned, x-ray selector means for providing output signals in response to x-rays having energy levels corresponding to a selected element, means for providing a ramp signal having a level corresponding to the position of the electron beam on the specimen, means responsive to the ramp signal and the output signals from the x-ray selector means for delivering output pulses having levels corresponding to the levels of the ramp signal when the output signals are received, whereby the level of each output pulse corresponds to the position of the element giving rise thereto, and display means responsive to the output pulses for indicating the distribution of the selected element in the specimen.

6. In apparatus for determining the distribution of a predetermined element in a specimen from signals produced by repeatedly scanning a portion of the specimen with an electron beam and delivering an output signal whenever the element is detected, the start of each scan being accompanied by a timing signal: a ramp generator for producing a ramp signal which increases at a predetermined rate from an initial level, means for applying the timing signal to the ramp generator for resetting the ramp signal to the initial level at the outset of each scan, whereby the level of the ramp signal corresponds to the position of the beam on the specimen, and analog gate means responsive to the output signals for passing the ramp signal to produce output pulses having heights corresponding to the positions of the beam when the predetermined element is detected.

7. The apparatus of claim 6 further including pulse height analyzer means responsive to tHe output pulses for giving a visual display of the distribution of the predetermined element of the specimen.

8. The apparatus of claim 7 further including gate means responsive to the timing signal for inhibiting delivery of the output signals to the analog gate means during retrace intervals between successive scans.

9. The apparatus of claim 6 further including data storage means for storing the output pulses.