US3532880A - Electrostatic charged particle analyzer having deflection members shaped according to the periodic voltage applied thereto - Google Patents

Description

J, DlMEFF 3,532,880

ED PARTICLE ANALYZER HAVING DEFLECTION MEMBERS SHAPED ACCORDING TO THE Oct. 6, 1970 ELECTROSTATIC CHARG PERIODIC VOLTAGE APPLIED THERETO I Filed Jan. 30, 1968 SOURCE DETECTOR INVENTOR. JOHN DIM E FF 6J4; ATTORNEY Oct. 6, 1970 J. DIMEFF 3,532,880

' ELECTROSTATIC CHARGED PARTICLE ANALYZER HAVING DEFLBCTION MEMBERS SHAPED ACCORDINJ TO THE PERIODIC VOLTAGE APPLIED THEEE'IO 1 Filed 'Jan. 30, 1968 2 Sheets-Sheet 2 FIG-4 I SOURCE INX'ENTOR.

JQH N DI M E FF I w 9.4. 5 I I6 [il souRcE DETECTOR ATTORNEY United States Patent 3,532,880 ELECTROSTATIC CHARGED PARTICLE ANA- LYZER HAVING DEFLECTION MEMBERS SHAPED ACCORDING TO THE PERIODIC VOLTAGE APPLIED THERETO John Dimelr, San Jose, Calif., assignor to the United States of America as represented by the Administrator of the National Aeronautics and Space Administration Filed Jan. 30, 1968, Ser. No. 701,732 Int. Cl. H01j 39/34 US. Cl. 250-413 8 Claims ABSTRACT OF THE DISCLOSURE The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for Governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION The present invention relates in general to the analysis of charged particles, and more particularly to a charged particle analyzer which permits simultaneous measurement of both the energy-to-charge ratio of the particles and the velocity of the particles.

In most presently-used forms of charged particle analyzers, such as plasma analyzers, mass spectrometers, and the like, the charged particles pass through one or more separators which deflect, intercept or bunch the particles by means of either an electrostatic or an electromagnetic force. Each separator is dilferentially sensitive to some combination of the mass, charge, and velocity of the charged particles and acts as a filter, allowing only those particles within a certain spectral window to impinge on a collecting plate at the exit of the filter. By using various combinations of such filters, it is, in principle, possible to analyze the particles on the basis of energy-to-charge (or momentum-to-charge) ratio and velocity as independent variables. Examples of such prior art analyzers include those in which:

(1) Particles are passed through a narrowly constrained region wherein they are acted upon by an electrostatic force steadily applied normal to the direction of motion of the particles and typically derived from a differential voltage applied between two closely spaced segments of concentric cylinders or spheres;

(2) Particles are passed through a substantially unconstrained space wherein their motions are influenced by a periodic, typically radio frequency, field applied by electrostatic means;

(3) Particles are passed through a substantially unconstrained space wherein their motions are influenced by a magnetic field steadily applied in a direction normal to the direction of motion of the particles;

(4) Particles are permitted to drift through a fieldfree drift region for a given period of time (time of flight); and

3,532,880 Patented Oct. 6, 1970 (5) Particles are passed in sequence through an elec trostatic field and an electromagnetic field.

These prior art devices are characterized by certain undesired limitations, including: inadequate definition of the particle characteristics; requirements for large voltages to produce adequate electrostatic forces; requirements for heavy magnets; poor resolution; requirements for high voltage, short duration pulses, and accurate timing devices; and fringing of the deflecting fields which introduce unwanted lens effects.

It is the principal object of the present invention to overcome one or more of these and other limitations or prior art analyzers.

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SUMMARY OF THE INVENTION In accordance with the present invention, a given periodically-varying voltage waveform is applied between electrostatic deflection members, with the shape of said deflection members being chosen so that only those particles simultaneously characterized by a given velocity and a given energy-to-charge ratio can pass through said deflection members.

DESCRIPTION OF DRAWING The various features and advantages of the invention will become more apparent upon a consideration of the following description, taken in connection with the accompanying drawing, wherein the same reference numeral is used for the same or similar element in the various figures and:

FIG. 1 is a partially-schematic cross-sectional view of a particle analyzer in accordance with the present invention;

FIG. 2 is a partially-schematic cross-sectional view of a modified particle analyzer in accordance with the present invention which permits more frequent detection of particles;

FIG. 3 is a partially-schematic cross-sectional view of a modified particle analyzer in accordance with the present invention having multiple sections;

FIG. 4 is a partially-schematic cross-sectional view of another modified particle analyzer in accordance with the present invention having multiple sections;

FIG. 5 is a partially-schematic cross-sectional view of a modified particle analyzer in accordance with the present invention in which the particle trajectories are circular; and

FIG. 6 is a partailly-schematic cross-sectional view of another modified particle analyzer in accordance with the present invention in which the particle trajectories are circular.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is based on the concept that charged particle analysis may be achieved by shaping electrostatic deflection members in accordance with the timedependent action of a periodic waveform applied to said members. Consider, for example, a simple sinusoidal voltage applied differentially between two conducting plates, said plates being machined with that particular curvature such that when the sinusoidal voltage is impressed thereon, the accelerating electrostatic field always acts perpendicular to the direction of particle motion. From the physical laws describing the motion of a charged particle acting under these constraints, said particle will follow a trajectory such that the angle 0 between a local segment of the trajectory and the initial direction of entry into the analyzer is given by the function:

a gang where:

=22 mv Ar m is the particle velocity,

q is the particle charge,

Ar is the separation between the plates.

AB is the peak amplitude of the periodically-varying voltage between the plates,

40 is the angular frequency at which the voltage between the plates varies, and

s is the arc length along with particle trajectory between the plates.

With the deflection plates shaped to conform with Equation 1 for a particular value of K and a particular valve of w/V, the energy-to-charge ratio 6/ q of the particles which pass through the passageway between the plates without interception is: 2) s E q 2A7" and the velocity of the particles which pass through the plates without interception is: Q

where:

S is the total arc length moved along the particle trajectory between the plates during one-half cycle of the periodic variation of the voltage between the plates.

It should be noted that particles which enter the passageway between the plates with these particular values of e/q and v remain perpendicular to the electric field lines because of the shape of the plates as derived from Equation 1, and, since there is no motion of these particles along the rfield lines, these values of e/q and v remain constant throughout the entire trajectory. Further, it is to be noted that the same pair of plates provides the filtering for both e/q and v, and thus performs the combined functions of both an electrostatic e/q analyzer and a time-of-fiight (v) analyzer in the same compact space.

Referring now to FIG. 1, there is shown a particle analyzer comprising conducting plates and 11 separated by a constant distance Ar to form a passageway therebetween for changed particles. An alternating voltage source 12 enerates a sinusoidal voltage at angular frequency m which is coupled through a grounded center tap transformer 13 to provide a difierential voltage across the plates 10 and 11 which varies at the frequency (to with a peak amplitude AE. A source 14 of charged particles, for example, positive ions of a substance being studied, directs the charged particles into the passageway between the plates 10 and 11 at right angles to the electric field lines generated by the voltage AE between the plates. The plates 10 and 11 are shaped so as to define a trajectory 15 therebetween which curves in accordance with Equation 1 in an approximately sinusoidal manner. Accordingly, only those particles with valves of e/q and v given by Equations 1 and 2, respectively, pass through the length S of the analyzer and are collected by a suitable particle detector 16. As is evident from these equations, the particle spectrum can be scanned with respect to 5/ q by varying the peak amplitude AE of the generator 12, and can be scanned with respect to v by varying the angular frequency w of the generator.

The analyzer of the present invention, as illustrated by the just-described embodiment of FIG. 1, can be seen to have the following advantages:

(1) A high resolution because of the close constraints on the particle trajectory;

(2) Moderate values of voltage required to effect the particle analysis;

(3) Elimination of weight and power requirements associated with magnetic field devices; and

(4) Elimination of stray magnetic fields associated with magnetic separators and stray (fringing) electric fields associated with electrostatic separators.

The embodiment of FIG. 1 is operable only on alternate half cycles of the generator 12. For example, in the case of positive ions, the desired particle analysis occurs only as to those ions which enter the analyzer at the beginning of the half cycle for which the voltage on plate 11 is negative with respect to the voltage on plate 10, the selected particles leaving the analyzer at the end of said half cycle after having travelled the distance S particles entering the analyzer at other times are intercepted by the plates 10 and 11.

FIG. 2 shows an embodiment of the invention which permits more frequent detection of particles. An electronic switching network comprising four diode rectifiers 20, 21, 22 and 23, is connected between the transformer 12 and the plates 10 and 11. When the voltage of the upper line 12a of the generator 12 is positive with respect to the lower line 12b, a positive voltage appears on the upper transformer line 13a and a negative voltage appears on the lower transformer line 13b. The positive voltage is passed through the diode 20 to the plate 10; and the negative voltage is passed through diode 23 to the plate 11. On the next half-cycle, when the voltage of the upper generator line 12a is negative with respect to the lower generator line 12b, a negative voltage appears on the upper transformer line 13a and a positive voltage appears on the lower transformer line 13b. The negative voltage is then passed through the diode 22 to the plate 11; and the positive voltage is passed through the diode 21 to the plate 10. Thus, it is seen that the plate 11 remains negative with respect to the plate 12 during both half-cycles of the generator 12 and, accordingly, particles which enter the analyzer at the beginning of each halfcycle are analyzed and passed to the detector 16.

The detection resolution of a particle analyzer can be enhanced by the addition of multiple sections in series along the particle stream. The analyzer of the present invention is particularly well adapted to such multiplication of analyzer sections, as shown in FIGS. 3 and 4.

In FIG. 3, the deflection plates are formed by adding to the analyzer 10, 11, of FIG. 2 a second section 10a, 10b, which is identical in configuration and is joined tangentially at the end of the section length S In the embodiment of FIG. 3, the curvature of the second section 10a, 10b is reversed so that the multiple sections continue a generally linear progression of the particle stream from the source 14 to the detector 16. Consequently, for analysis of positive particles, the polarity of the top plate 10a must be negative with respect to the bottom plate 11a. Such reversal in polarity is automatically provided in the embodiment of FIG. 3, by having ohmic continuity between plate portions 10 and 10m and between plate portions 11 and 11a. Under these conditions, those particles which enter the analyzer 10, 11, at the beginning of the half-cycle of the generator 12 during which the lower plate portion 11 is negative with respect to the upper plate portion 10 will then enter the analyzer 10a, 11a at the beginning of the next halfcycle of the generator 12 during which the upper plate portion 10a is negative with respect to the lower plate portion 11b. It is apparent that several analyzer sections may thus be joined together along the direction of the particle stream. Also, it is apparent that the frequency of detection in the embodiment of FIG. 3 can be increased by providing an electronic switch as is shown in FIG. 2.

In FIG. 4, the multiple-section deflection plates are also formed by adding to the analyzer 10, 11, of FIG. 2 a second section 10a, 1012 which is identical in configuration and is joined tangentially at the end of the section length S However, in the embodiment of FIG. 4, the curvature of the second section 10a, 10b, is continued in the same sense as the first section so that the multiple sections return the particle stream to the detector 16 which is now closer to the source 14. For analysis of positive particles, it is now required that the plate portion 11a remain negative with respect to the plate portion 10a. To accomplish this, an electronic switching network comprising four diode rectifiers 30, 31, 32 and 33, is connected between the transformer 12 and the plate portions 10, 10a, 11 and 11a, it being noted that in this embodiment plate portion 10 is in ohmic insulation with respect to plate portion 10a: and plate portion 11 is in ohmic insulation with respect to plate portion 11a. When the voltage of the upper generator line 12a is positive with respect to the lower generator line 1211, a positive appears on the upper transformer line 13a and a negative voltage appears on the lower transformer line 13b. The positive voltage is then passed through the diode 30 to the plate portion 10; and the negative voltage is passed through the diode 31 to the plate portion 11. On the next half-cycle, when the voltage of the upper generator line 12a is negative with respect to the lower generator line 12b, a negative voltage appears on the upper transformer line 13a and a positive voltage appears on the lower transformer line 13b. The positive voltage is then passed through the diode 33 to the plate portion 10a; and the negative voltage is passed through the diode 32 to the plate portion 11a. Thus, those particles which enter the analyzer 10, 11, at the beginning of the halfcycle of the generator 12 during which the lower plate portion 11 is negative with respect to the upper plate portion 10 will then enter the analyzer 10a, 11a, at the beginning of the next half-cycle of the generator 12 during which the plate portion 11a is also negative with respect to the plate portion 10a. It is apparent that further diodes may be provided as in FIG. 2 so that a second group of particles, entering the first section 10, 11 at the same time as the first group enters the second section 10a, 11a, may be analyzed, thereby increasing the frequency of detection in the embodiment of FIG. 4.

For compactness of instrumentation, it may be desirable to constrain the particles in substantially circular trajectories, rather than the approximately sinusoidal trajectories as in FIGS. 1 through 4. Such circular trajectories are provided in the embodiments of FIGS. and 6.

In FIG. 5, there is shown a two section analyzer. The first section has circular plates 40 and 41, and the second section has circular plates 40a and 40b of identical configuration. The two sections are joined tangentially and with reversed curvature as in FIG. 3. In the embodiment of FIG. 5, the periodically-varying voltage across the plates is generated by a square wave generator 42. Solution of the equations of motion shows that the trajectory 45 normal to the lines of force established between the plates and 11 by the square Waveform of generator 42 is circular, rather than sinusoidal as in the case of the sinusoidal waveform. Thus, the combination of the square wave generator with the circular deflection plates also results in a selection of a particular group of particles characterized by given values of 6/ q and v. It is apparent that the embodiment of FIG. 5 can be readily modified to provide several analyzer sections, to provide just a single section as in FIG. 1, to provide increased frequency of detection by means of an electronic switch as in FIG. 2, and to form two sections with a turned-around trajectory as in FIG. 4. v

In FIG. 6, the analyzer comprises plates 51 and 52 shaped so that the spacing therebetween, instead of being constant as in FIG. 1, varies sinusoidally along the particle trajectory 45. The generator 12 provides a sinusoidal waveform to the plates as in FIG. 1. Under these conditions, the trajectory 45 normal to the lines of force established between the sinusoidally spaced plates 51 and 52 by the sinusoidal waveform of the generator 12 is circular. It is apparent that the embodiment of FIG. 6 can readily be modified to provide increased frequency of detection by means of an electronic switch as in FIG. 2, and to provide multiple sections as in FIGS. 3 and 4.

Various other modifications will occur to those skilled in the art. For example: the analyzer may be combined with charge detection apparatus to provide additional information on the properties of the particles; the analyzer may be combined with a conventional analyzer in which a D-C voltage is applied between the plates, with appropriate modification of the plate shape to provide a partial modulation of the particle beam; and other specially-shaped plates may be used for particle trajectories as they would develop under the influence of other voltage waveforms. Moreover, it is apparent that the deflecting members may be thin plates, or they may be opposing parallel surfaces of metal blocks; or they may have shapes formed by the rotation of the plate crosssections about an axis of symmetry.

It is to be understood that modifications and variations of the embodiments of the invention disclosed herein may be resorted to without departing from the spirit of the invention and scope of the appended claims.

Having thus described my invention, what I claim as new and desire to protect by Letters Patent is:

1. A charged particle analyzer, comprising: means for generating a periodically varying voltage; and a pair of electrostatic deflection members having opposed surfaces defining a passageway for charged particles to be analyzed, said passageway having a shape corresponding to a portion of the shape of said periodically varying voltage, said voltage being applied across said deflection members to establish an electric field therebetween, and said opposed surfaces of said deflection members being shaped for establishing the direction of said electric field perpendicular to the trajectory of said charged particles throughout the entire length of said passageway.

2. A charged particle analyzer according to claim 1 wherein the voltage applied across said deflection members has a sinusoidal waveform with a peak amplitude AE and an angular frequency w, and said opposing surfaces are separated by a constant distance Ar and are shaped in accordance with the relationship:

where qAE mv Ar m is the mass of the particles to be analyzed,

q is the charge on the particles to be analyzed, and s is the arc length along said passageway,

whereby the analyzed particles passing through said passageway without interception are characterized by an energy-to-charge ratio 'e/ q given by:

e qAE q mv Ar and a velocity v given by:

where S is the total arc length moved along said particle trajectory during one-half cycle of the periodic variation of the voltage applied across said deflection members.

3. A charged particle analyzer according to claim 1 further comprising switching means for reversing the voltage applied across said deflection members at the beginning of each half-cycle of said periodically varying voltage, whereby the polarity of the electric field between said deflection members remains the same.

4. A charged particle analyzer according to claim 1 in which said passageway has at least two sections in series along the particle trajectory, the first of said sections containing the electric field which interacts with the same particles during the next half-cycle of said periodically varying voltage, said sections being of substantially the same configuration and being positioned tangentially along said particle trajectory.

S. A charged particle analyzer according to claim 4 wherein said second section is reversed in curvature with respect to said first section and adjacent deflection member portions of the two sections are in ohmic continuity.

6. A charged particle analyzer according to claim 4 wherein said second section continues with the same curvature as said second section and adjacent deflection member portions are in ohmic insulation, and further comprising means for applying a voltage across the deflection member portions of said second section at the beginning of alternate half-cycles of said periodically varying voltage which is of the same polarity as the voltage applied across the deflection member portions of said first section on the preceding half-cycles of said periodically varying voltage.

7. A charged particle analyzer according to claim 1 wherein the voltage applied across said deflection members has a square waveform, and said opposing surfaces are separated by a constant distance and shaped to provide a circular passageway therebetween.

8. A charged particle analyzer according to claim 1 wherein the voltage applied across deflection members has a sinusoidal waveform, and said opposing surfaces are separated by a distance which varies sinusoidally along the length of said particle trajectory, said trajectory being circular in shape.

References Cited UNITED STATES PATENTS 2,769,910 11/1956 Elings. 3,040,173 6/1962 Higatsberger. 3,061,720 10/1962 EWald. 3,441,732 4/ 1969 Gautherin et al.

WILLIAM F. LINDQUIST, Primary Examiner A. L. BIRCH, Assistant Examiner

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