Specimen analysis with ion and electrom beams

Abstract

A method and apparatus employing ion and electron beams for chemically analyzing a specimen. A specimen is mounted on a movable platform in an evacuated chamber and irradiated with an ion beam over a predetermined area of interest to liberate secondary ions. The secondary ion spectrum is analyzed with a mass filter and display unit to provide a spectral distribution. Ions having a particular mass-to-charge ratio are selected for spatial distribution analysis and the mass filter is tuned to the selected mass-to-charge ratio. The filtered beam of secondary ions passed through the mass filter is detected by an ion detector which generates a signal representative of secondary ion abundance at that mass-to-charge ratio. The ion detector output signals are used to control the intensity or deflection of a CRT beam. An independently generated electron beam is scanned over the specimen area irradiated by the ion beam and the CRT beam is swept in synchronism with the scanned electron beam. The electron beam, scanned over the ion irradiated specimen area, modulates the secondary ion yield at the point where both the electron beam and the ion beam are coincident on the specimen. The resulting display is a two dimensional spatial distribution map of the species in the specimen to which the mass filter is tuned.

Description

BACKGROUND OF THE INVENTION

This invention relates to specimen analysis by means of beams of charged particles. More particularly, this invention relates to an improved technique for investigating the chemical composition of a specimen by secondary ion analysis.

Chemical analysis of a specimen using the technique of secondary ion emission is known. In a typical arrangement, e.g., that disclosed a U.S. Pat. No. 3,517,191 to Liebl issued Jan. 23, 1970, a specimen to be investigated is placed on a platform and bombarded with a primary beam of ions from a suitable ion source. The primary beam is scanned in raster-like fashion over the entire area of interest. The interaction of the bombarding beam with each elemental area of the specimen results in the ejection of secondary ions characteristic of the chemical composition of the specimen in that area. These secondary ions are collected and filtered by means of a mass analyzer which permits only ions having a specific mass-to-charge ratio to be transmitted therethrough. The filtered secondary ions encounter an ion detector which provides an output signal whose magnitude is representative of the intensity of the ion beam incident thereto. This output signal is typically used to control the intensity of a cathode ray tube, or CRT, scanning beam which is scanned in synchronism with the ion beam. The resulting display on the face of the CRT thus provides a spatial distribution plot of ion density as a function of location on the specimen.

While this conventional technique has been found to provide useful results in some applications, it suffers from several disadvantages. For example, the spatial resolution of the chemical specimen constituents is a function of the fineness of the primary ion beam. The degree of resolution of the primary ion beam is directly dependent upon the complexity and size of the beam generation ion optics and deflection circuitry. In general, the finer the resolution desired, the more complicated, cumbersome and costly the ion optics and circuitry must be. In actuality, the cost of specimen analysis increases inordinately with the fineness of the resolution desired and, in many cases, is so great as to render the gathering of much-needed data economically prohibitive.

A further disadvantage inherent in known ion beam analysis techniques is the lower limit of resolution. The minimum primary ion beam diameter practically obtainable with known systems is about 1 micron. Thus, where a finer resolution than that obtainable with this lower limit is required, ion beam analysis cannot furnish meaningful data and other techniques must be employed, usually with less satisfactory results.

Efforts to overcome these and other disadvantages inherent in known secondary ion analysis techniques have not met with wide success to date.

SUMMARY OF THE INVENTION

The invention comprises a method and apparatus for providing a spatial map of secondary ion emission which provides a resolution which is substantially finer than that hitherto obtainable. In the preferred embodiment, a beam of primary ions from a source is directed onto the surface of a specimen, with the primary ion beam covering the entire area of interest. An electron beam from an electron-optical column is line scanned over the same area to interact with the secondary ions liberated from the specimen area by the primary ion beam. The secondary ions are passed through a mass filter tuned to ions of a preselected mass-to-charge ratio, and the ions transmitted therethrough are detected by an ion detector, the output of which is used to control the intensity or deflection of the beam of a CRT display. The CRT beam is scanned synchronously with the electron beam to provide a spatial distribution map of secondary ion intensity as a function of electron beam position.

As the electron beam scans those regions of the specimen which contain unconcentrated amounts of the element corresponding to the preselected secondary ions, a substantially constant background level of secondary ion current is produced which is attributable to the steady state emission of secondary ions from the total area irradiated by the primary beam. However, as the electron beam scans those regions of the specimen containing concentrations of the same element, a pronounced modulation in secondary ion current is produced. The resulting spatial distribution map exhibits regions of markedly different intensity corresponding to those portions of the specimen containing concentrations of the species under investigation. By tuning the mass filter to secondary ions having different mass-to-charge ratios, both qualitative and quantitative data relating to several different elements may be obtained from the same or different specimens. Since the spatial resolution of the resulting distribution map is dependent upon the size of the scanned electron beam, which is typically two orders of magnitude smaller than the smallest obtainable ion beam, maps exhibiting an extremely fine resolution are obtained with the invention.

For a fuller understanding of the nature and advantages of the invention, reference should be had to the ensuing detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagramatic view of a preferred embodiment of a system constructed according to the invention;

FIG. 2 is a schematic diagram illustration the distribution frequency of elements of a specimen; and

FIG. 3 is a schematic diagram illustrating the principle of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawings, FIG. 1 illustrates a preferred embodiment of a system constructed according to the invention. A housing 10 shown partly broken away provides a main specimen chamber 11 which can be evacuated by means of a vacuum pump 12 and valved conduit 13. Mounted within chamber 11 is a specimen platform 14, commonly termed a stage, which provides a support surface for a specimen 15 to be investigated. Stage 14 is provided with suitable mechanical connections indicated by broken lines 16-20 for enabling independent movement of stage 14 in the x, y, and z directions, and tilting and rotation of stage 14 about the x-y plane and the z axis, respectively, in response to the manipulation of control elements 21-25. Elements 16-25 may comprise any suitable known mechanical arrangement for imparting the desired motion to stage 14. Since several such arrangements are known, details thereof have been omitted for clarity.

Mounted within chamber 11 is a source of primary ions 26, which may comprise a duo plasmatron ion source, a diode source, an RF source, or the like. Ion beam source 26 is powered by a conventional ion gun power supply 27 which provides beam generation, focusing and alignment control signals. Ion gun 26 is so arranged in chamber 11 that the beam of primary ions 28 generated by gun 26 is emitted an an angle .alpha. between the beam axis and the x-y reference plane. If desired, ion gun 26 may be provided with a suitable mechanical arrangement for adjusting the angle .alpha..

The optimum value for the angle a depends primarily on the primary ion beam energy, the nature of the specimen under investigation and the type of analysis desired, and can best be determined on an empirical basis for a given application. In general, shallow angles are preferred to a minimum of about 10.degree., although values of a up to a limit of about 85.degree. can provide good results.

The nature of the primary ion beam used in practicing the invention also depends upon the nature of the sample under investigation, the desired ion beam energy and the type of analysis desired. Some examples of suitable primary ion beams are argon, cesium, the halogens, helium, nitrogen and oxygen. Other types will occur to those skilled in the art.

Also mounted within chamber 11 is a conventional quadrupole mass filter 33 powered by a mass filter control unit indicated by reference numeral 34 which comprises a conventional RF generator 35 and DC power supply 36.

Mass filter 33 may be operated in two modes: a narrow filter mode in which filter 33 is turned by the combination of control signals from control unit 34 to transmit only ions having a given mass-to-charge ratio, and a spectral mode in which the narrow transmission window normally provided during narrow filter mode is cyclically varied over a preselected range of mass-to-charge ratios.

A secondary ion detector 38 which may be a conventional electron multiplier, is mounted at the secondary ion beam output of mass filter 33, and is powered by a conventional ion detector power supply 39.

The output of ion detector 38 is coupled via an amplifier 40 to the beam intensity control element 41 of a standard cathode ray tube 42 and also to fixed contact 43 of a switch 44. A conventional CRT power supply 45 provides beam generator signals to CRT 42.

Mounted on housing 10 is a conventional electron-optical column 46 powered by a standard column control unit 47. Column control unit 47 provides beam generation, beam focusing and beam alignment control signals to electron optical column 46.

A sweep generator 48 provides raster type beam deflection scanning signals which are coupled to the deflection coils of electron-optical column 46. The deflection scanning signals from sweep generator 48 are also coupled to a first pair of contacts 51, 52 of a double-pole, double-throw switch 50. The alternate pair of contacts 53, 54 are coupled to the output signals from RF generator 35 and the blade contact 55 of switch 44. As indicated by broken line 56, blade 55 of switch 44 is mechanically linked to a pair of moveable blades 57, 58 of switch 50 so that blade 55 closes when blades 57, 58 are moved to the left-hand position of FIG. 1. Blades 57, 58 are coupled to the deflection control elements of CRT 42. Thus, the deflection of the display beam of CRT 42 is alternately controlled by sweep generator 48, and the output signals from RF generator 35 and ion detector 38, depending on the position of switch 50.

Preferably, the elements designated by reference numerals 10-14, 16-25, 42, 45, 46, 47 and 48 comprise conventional scanning electron microscope (SEM) elements, such as those found in the AUTOSCAN model Scanning Electron Microscope available from Etec Corporation of Hayward, Calif. Since these elements, and the other elements noted above as conventional, are all well-known, further details thereof have been omitted to avoid prolixity.

In operation, a specimen 15 is mounted on stage 14 and maneuvered to the desired target position by elements 16-25, after which chamber 11 is evacuated by pump 12 and valve 13. Ion gun power supply 27 is then activated to cause ion gun 26 to generate a beam of primary ions 28 of the desired size and intensity. Mass filter control unit 34 and ion detector power supply 39 are energized to activate mass filter 33 and ion detector 38. CRT power supply 45 is activated and switch 50 is placed in the left-hand position (not illustrated).

Mass filter 33 is now operated in the spectral mode and the filter control signals generated by RF generator 35 and the output signals from ion detector 38 are coupled via switch contacts 43, 53, 54 and blades 55, 57, 58 to the deflection elements of CRT 42 so that the CRT beam is swept in the horizontal direction as a function of mass-to-charge ratio and in the vertical direction as a function of the beam intensity.

The resulting display shown in FIG. 2 is a spectral plot of intensity vs. mass-to-charge ratio of the secondary ions emitted from the area of specimen 15 subjected to the primary ion beam. The location and the intensity of peaks 59 i give the relative distribution of various species of elements or compounds in the sample 15. From this display, a particular species of interest is selected for spatial distribution analysis.

To begin the spatial distribution analysis, mass filter control unit 34 is adjusted to operate mass filter 33 in the narrow filter mode. In this mode only those ions, whether positive or negative, having the preselected mass-to-charge ratio are permitted to pass to ion detector 38. Switch 50 is next moved to the right-hand position illustrated in FIG. 1 and synchronous beam scanning is initiated by actuating sweep generator 48 and column control unit 47. Thus, in the spatial distribution mode, the electron beam generated by electron-optical column 46 and the CRT beam simultaneously scan the target area of interest and the CRT face, respectively, while the output of secondary ion detector 38 modulates the intensity of the CRT beam.

FIG. 3 illustrates the variation of detected secondary ion current with the electron beam position as the electron beam scans a line element 60 of the specimen target area. In this figure, the portion of specimen 15 subjected to the primary ion beam is indicated by the closed path denoted by reference numeral 61. The shaded regions denoted by reference numerals 62, 64 represent regions having a concentration of the species of interest while the remaining unshaded area bounded by 61 is devoid of such concentrations. Assuming the electron beam is swept from left to right as viewed in the figure, as the electron beam scans along line 60 from point a to point b, the intensity of the secondary ion current is substantially constant. As the electron beam crosses the boundary of concentrated region 62 at point b of line 60, the intensity of the detected secondary ion current rapidly decreases to a minimum and remains at this minimum level until the beam emerges from concentrated region 62 at point c of the line. As the electron beam completes the sweep of line 60 from point c to point d, the level of the secondary ion current increases to the original level and remains substantially uniform.

As the electron beam scans along line 60 of the specimen, the CRT beam is synchronously swept by sweep generator 48 along a corresponding line 60 (FIG. 1) and modulated in intensity in accordance with the output from ion detector 38 amplified by amplifier 40. The result is a swept line element 60' whose intensity varies as a function of the concentration of the species of interest in the corresponding scanned line 60 of the specimen. Thus, as the electron beam is rastered over the entire target area of the specimen 15, CRT 42 displays a pictorial representation of the target area, which exhibits those regions having a concentration of the species under investigation as well-defined dark regions 62', 64' on a lighter background. If desired, this intensity variation may be reversed by providing an inverter at the output of amplifier 40, so that regions 62, 64 appear light on a dark background.

In the above discussed example, regions 62, 64 are assumed to have a substantially uniform concentration of the particular species under investigation. As will be apparent to those skilled in the art, if the concentration is not uniform, the resulting display will exhibit intensity variations in regions of concentration which correspond identically to the variation in concentration over the specimen target area.

Once the spatial distribution of the particular species of interest has been obtained, mass filter 33 may be tuned to pass secondary ions of a different mass-to-charge ratio and the process may be repeated.

Since the resolution of spatial distribution maps obtained in accordance with the invention is primarily a function of the scanning electron beam size, extremely fine resolution on the order of 100 Angstrom units can be obtained herewith. Further, since the primary ion beam need only be focused to the maximum width of the target area, typically on the order of 100 microns, extremely simple and inexpensive primary ion beam generation and focussing circuits can be employed without sacrificing the resolution of the system.

While the above provides a complete disclosure of the preferred embodiment of the invention, various modifications, alternate constructions and equivalents may be employed without departing from the true spirit and scope of the invention. For example, if desired, mass filter 33 may be operated in a combined spectral and narrow filter mode in which the narrow pass band of the filter is shifted at a slow rate compared to the synchronous scanning rate of the electron beam and the CRT beam to provide a composite image of the spatial distribution of two or more species of interest. Similarly, mass filter 33 may be operated in a discrete combined spectral-narrow filter mode in which the narrow pass band of this element is shifted discretely in accordance with the initially obtained spectral plot of intensity versus mass-to-charge ratio of the specimen under investigation, with the stepping rate being substantially less than the scanning rate of the electron beam. Further, if desired, the various analog data signals obtained with the preferred embodiment, such as the sweep signals from mass filter control unit 34 and sweep generator 48 and the intensity control signals from ion detector 38 and amplifier 40 may be converted to digital form and recorded or stored in a computer memory for further quantitative and qualitative analysis. Therefore, the above description and illustrations should not be construed as limiting the scope of the invention which is defined by the appended claims.

Claims

What is claimed is:

1. A method of analyzing a specimen for elemental constituents comprising the steps of:

a. directing a beam of primary ions to a target area of said specimen to produce secondary ions;

b. scanning said irradiated target area with an electron beam; and

c. detecting the variation of secondary ion current with position of said electron beam.

2. The method of claim 1 further including the step of filtering said secondary ions to permit detection of secondary ions having substantially the same preselected mass-to-charge ratio.

3. The method of claim 1 wherein said step (c) of detecting includes the steps of:

i. synchronously scanning said electron beam and the imaging beam of a cathode ray tube; and

ii. modulating said CRT beam in accordance with said variation of said secondary ion current.

4. The method of claim 1 wherein said step (b) of scanning is preceded by the step of determining the spectral distribution in mass-to-charge ratios of said secondary ions emanating from said irradiated target area.

5. The method of claim 4 wherein said step of determining includes the steps of:

i. sweep filtering said secondary ions to permit detection of ions having varying mass-to-charge ratios;

ii. scanning the imaging beam of a cathode ray tube synchronously with said step of sweep-filtering;

iii. detecting the variation of said secondary ion current with mass-to-charge ratio; and

iv. modulating said cathode ray tube beam in accordance with said variation of said secondary ion current.

6. The method of providing a spatial distribution plot of constituent elements of a specimen comprising:

a. generating a beam of primary ions;

b. irradiating a target area with said beam of primary ions to liberate secondary ions from said target area;

c. generating beam position signals;

d. scanning said irradiated target area with an electron beam in accordance with said beam position signals;

e. scanning a recording element with said beam position signals in synchronism with said electron beam scanning;

f. generating control signals representative of the variation of secondary ion current with position of said electron beam relative to said target area; and

g. applying said control signals to said recording element to record said variations.

7. The method of claim 6 wherein said step (f) of generating includes the step of passing said secondary ions through a mass filter tuned to reject substantially all secondary ions hot having a preselected mass-to-charge ratio so that only control signals representative of the variation of secondary ions having said preselected mass-to-charge ratio are generated.

8. A system for analyzing a specimen for elemental constituents comprising:

means for directing a beam of primary ions onto a target area of said specimen to produce secondary ions;

means for scanning said irradiated target area with an electron beam; and

means for detecting the variation of secondary ion current with position of said electron beam.

9. The system of claim 8 wherein said detecting means includes filter means for preventing detection of secondary ions not having a preselected mass-to-charge ratio.

10. The system of claim 8 wherein said detecting means includes means for synchronously scanning said electron beam and a recording element and means for modulating said recording element in accordance with said variation of said secondary ion current.

11. The system of claim 8 further including means for determining the spectral distribution in mass-to-charge ratios of said secondary ions liberated from said irradiated target area.

12. The system of claim 11 wherein said spectral distribution determining means includes:

means for sweep filtering said ions to permit detection of ions having varying mass-to-charge ratios;

means for scanning said recording element synchronously with said sweep filtering means;

means for detecting the variation of secondary ion current with mass-to-charge ratio; and

means for modulating said recording element in accordance with said variation of said secondary ion current.