Charged particle beam instrument

A charged particle beam instrument capable of reducing the spread of the probe diameter while maintaining the probe current constant. An electrical current Id is detected by a detection aperture to create a feedback signal. The feedback signal is supplied to a condenser lens control and to an objective lens control via a signal adjuster. The objective lens control portion controls the objective lens such that the charged particle probe is in focus.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a charged particle beam instrument, such as an electron probe microanalyzer or a scanning electron microscope.

2. Description of the Related Art

In a charged particle beam instrument, such as an electron probe microanalyzer or a scanning electron microscope, a charged particle beam emitted from a charged particle beam source is accelerated and focused onto a specimen by a condenser lens system and an objective lens. As the charged particle beam hits the specimen, X-rays and secondary particles are produced, and these are detected.

In this kind of instrument, the current of the charged particle probe made to hit the specimen is stabilized.FIG. 1is a diagram schematically illustrating this probe current-stabilizing function. A charged particle beam CB produced by a charged particle beam source (not shown) and accelerated is sharply focused onto a specimen3by a condenser lens system1and an objective lens2.

A detection aperture4is located between the condenser lens system1and the objective lens2and detects an outer portion of the charged particle beam. The output signal from the detection aperture4is amplified by a feedback device5and supplied to a control portion6for the condenser lens system1for adjusting the probe current.

The control portion6adjusts the strength of the condenser lens system1according to the magnitude of a reference signal and the magnitude of the output signal from the feedback device5. A feedback loop is formed in this way. Therefore, the current of the charged particle beam probe P impinging on the specimen3can be kept constant in principle if the current density of the charged particle beam does not vary.

To establish negative feedback (i.e., to prevent positive feedback as described in Japanese Patent Laid-Open No. 183044/1989), an aperture for limiting peripheral portions of a charged particle beam exiting from the condenser lens system is placed ahead of the detection aperture4as described in Japanese Technical Review 82-7798. This aperture is omitted in FIG.1.

The detection aperture4can also be designed to act also as an objective aperture for controlling the probe current and the divergence angle of the probe.

As mentioned previously, where negative feedback is applied to the condenser lens system1, if the exciting current supplied to the condenser lens system1is varied so as not to vary the probe current, the position of the focal point of the condenser lens system1automatically changes from the state indicated by the solid line to the state indicated by the broken line. It is now assumed that some change occurs in the charged particle beam source and that the probe current should vary from Ipby ΔIp. However, the negative feedback varies the distance between the detection aperture4and the focal point, thus maintaining the probe current Ipconstant.

In spite of this, an adjustment of the condenser lens system1moves the focal position of the charged particle probe P on the specimen out of the specimen surface by Δb. The spread portion Δd1pof the probe diameter due to the feedback adds to the final probe diameter dp.

It is assumed that the objective lens2has an object distance of a and an image distance of b. If the focal distance fOLof the objective lens2is constant, the following relation holds:
db/da=−M2
where M (=b/a) is the magnification of the objective lens. Therefore, when the object distance varies by a small distance of Δa, the image distance deviates by Δb, which is given by:
Δb=−M2·Δa

That is, the image distance deviation Δb can be reduced by combining the lenses so as to reduce the magnification M (=b/a). It can be seen, however, that the deviation Δb cannot be reduced to any desired small value, because the number of lenses is finite, and because the microscope column has a finite length.

On the other hand, in an instrument equipped with a charged particle beam source of low brightness, the final probe diameter dpis not thin. Therefore, the spread Δd1pof the probe diameter due to negative feedback presents no serious problems. In contrast, emission of a charged particle beam from a charged particle beam source of high brightness (e.g., field emission, electron emission, such as Schottky emission, and ion emission due to field ionization or electrolytic dissociation) can produce a quite thin final probe diameter dp. Consequently, the spread Δd1pof the probe diameter due to negative feedback can no longer be neglected.

Furthermore, in a charged particle beam source of high brightness, the emission current tends to vary. This increases the amount of correction made by negative feedback. This, in turn, increases the spread Δd1pof the probe diameter, thus increasing the amount of defocus.

A charged particle beam source of high brightness is adopted to obtain a small probe diameter. This object cannot be achieved due to the spread Δd1pof the probe diameter, which, in turn, is caused by negative feedback that is used to obtain a stable probe current.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a charged particle beam instrument capable of reducing the spread of the probe diameter greatly while maintaining the probe current constant.

This object is achieved by a charged particle beam instrument that has a charged particle beam source for producing a charged particle beam having a probe current, a first focusing means for focusing the charged particle beam and varying the probe current of the charged particle beam impinging on a specimen, a second focusing means for varying the degree of focus of the charged particle beam impinging on the specimen, a first control portion for controlling the first focusing means, and a second control portion for controlling the second focusing means. This charged particle beam instrument is characterized in that it is equipped with a means for detecting a part of the current of the charged particle beam from the charged particle beam source to thereby produce a detected signal, controlling the control portion for the first focusing means to maintain constant the probe current of the charged particle beam impinging on the specimen, and controlling the control portion for the second focusing means according to the detected signal to adjust the focus of the charged particle beam.

Other objects and features of the invention will appear in the course of the description thereof, which follows.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention is hereinafter described in detail by referring to the accompanying drawings.FIG. 2shows the fundamental structure of a charged particle beam instrument in accordance with the present invention. Like components are indicated by like reference numerals in various figures includingFIG. 1used to describe the prior art structure. Those components which have been already described will not be described below.

Referring toFIG. 2, a detection aperture4detects an electrical current Id. A feedback device5converts the detected current Idinto a voltage and creates a signal for feedback (referred to as the “feedback signal” herein). The output signal from the feedback device5is fed to a condenser lens control portion6and to an objective lens control portion8via a signal adjuster7. The objective lens control portion8compares the feedback signal whose amplitude is adjusted by the signal adjuster7with an objective lens control signal and controls the strength of the objective lens2according to the result of the comparison.

To establish this negative feedback, an aperture (not shown inFIG. 2) is mounted between the condenser lens system1and the detection aperture4to limit peripheral portions of the charged particle beam exiting from the condenser lens system1.

The operation of the instrument of the construction described thus far is next described. Let VCL(=VCLO) be an input signal applied to the condenser lens control portion6before applying negative feedback to it. Let VOL(=VOLO) be an input signal applied to the objective lens control portion8. The following signals corresponding to these input signals are applied to the condenser lens system1and the objective lens2, respectively.
ICL=ρCL·VCL
IOL=ρOL·VOL
where ρCLand ρOLare constants. Where the lenses are of the electrostatic type, ICLand IOLcorrespond to voltages. Where the lenses are of the magnetic type, they correspond to exciting currents.

Negative feedback is applied by the detection aperture4, the feedback device5, the condenser lens control portion6, and the condenser lens system1such that the current Iddetected by the detection aperture Idis kept substantially constant if the detected current Idincreases monotonously when the probe current Ipincreases. The conditions under which negative feedback is established are described, for example, in the above-cited Japanese Patent Laid-Open No. 183044/1989. The operation is described in further detail.

First, a state in which negative feedback is not applied is discussed. It is assumed that some change has occurred in a particle source, resulting in the following changes:
IpO→IpO+ΔIp
Id0→Id0+ΔId

It is first assumed that a signal ΔVCLis added to the condenser lens control portion6to return the probe current to its original value IpOand that the position of the focal point of the condenser lens system1has been varied thereby. Then, a signal ΔVOLis applied to the objective lens control portion8to prevent the degree of focus of the particle probe from being varied by the change Δa in the object distance. These signals ΔVCLand ΔVOLshould be applied to the condenser lens control portion6and the objective lens control portion8, respectively, by some method to maintain constant the degree of focus of the particle probe while maintaining constant the probe current and the detected current.

A state in which negative feedback is applied is next discussed. Under the presence of negative feedback, some cause on the side of the charged particle beam attempts to vary the probe current Ipand the detected current Id. However, these are kept almost constant because of negative feedback applied by the feedback device5to the condenser lens control portion6.

If the probe current Ipand the detected current Idare kept constant, the signal applied to the condenser lens control portion6from the feedback device5should be equal to the above-described ΔVCL, as can be seen from the description provided above. To obtain normal negative feedback operation, the amplification degree of the feedback device5with respect to the signal from the detection aperture4is designed to have a sufficiently large value. Therefore, this signal ΔVCLis created.

On the other hand, the signal created by negative feedback is ΔVCL. This signal is amplified or attenuated, and the signal adjuster7is so operated that the resulting signal is equal to the aforementioned signal ΔVOL. The operation of the signal adjuster7may be determined according to the state of the operating charged particle beam instrument (e.g., the energy E of the particle beam, the probe current Ip, and the image distance b of the objective lens2).

The signal adjuster7may be so designed that its output is in proportion to the input signal ΔVCL(linear output). If necessary, the following nonlinear calculations may be involved.
(ΔVCL)n(n=0, 1, 2, . . . )
sin(n·kΔVCL)(n=0, 1, 2, . . . ; k is a constant)

The general fundamental structure and the principle of operation have been described thus far. A more specific example is described below by referring toFIG. 3, which shows a modification of the feedback device5and a modification of the signal adjuster7, it being noted that the feedback device5and the signal adjuster7are shown in FIG.2.

In the configuration ofFIG. 3, if an instruction for start of stabilization of the probe current is given from the outside, the current Id=IdOof the charged particle beam detected by the detection aperture4is converted into a signal voltage by a current-to-voltage converter11immediately before negative feedback is applied to the condenser lens system1.

The output signal V1d=V1d0from the current-to-voltage converter11is applied to an A/D converter12that converts an analog signal into a digital signal. The A/D converter12sends data AD1d0corresponding to the V1d0to the control portion13. The control portion13saves this data and sends data DA1d0to a D/A converter14that converts a digital signal to an analog signal.

The output from the D/A converter14remains the same as the output V1d0from the current-to-voltage converter11immediately before application of negative feedback until a next instruction for start of stabilization of the probe current is given. Then, the output V1dfrom the current-to-voltage converter11and the output V1d0from the D/A converter14are applied to an adder15, which, in turn, produces the difference ΔV1d(=V1d−V1d0) between them.

The output from the adder15is applied to an amplifier16whose gain can be set to a sufficiently large value A1. Immediately before application of negative feedback, the difference ΔV1d=0. When negative feedback is subsequently applied to the condenser lens system1in practice, a switch19is turned on. Because of the setting to the sufficiently large gain A1, output ΔVCL=A1·ΔV1dis delivered.

It is obvious that ΔVCL=0 holds immediately after application of negative feedback. Then, a switch20is turned on. The signal is applied to an amplifier17whose gain can be set to A2. The output from the amplifier17is applied to a D/A converter18. The output from the amplifier16is multiplied by a factor of A2·r2, where r2is a signal ratio indicated by data DA2from the D/A converter18. The following signal is delivered from the D/A converter18:
ΔVOL=A2·r2·ΔVCL

The signal ΔVCLobtained in this way is added to the condenser lens control portion6, while ΔVOLis applied to the objective lens control portion8.

If some change occurs in the charged particle beam source, and if the probe current is not stabilized, the probe current Ipand the detected current Idshould vary as follows. However, because of the action of negative feedback applied to the condenser lens system1, the currents Ipand Idcan be kept substantially constant.
Ip0→Ip0+ΔIp
Id0→Id0+ΔId

The relation of the objective lens current signal VOL, which maintains the degree of focus of the particle probe, to the condenser lens control current VCLis given by:
VOL=F(VCL)
We have:
dVOL/dVCL=F′(VCL)
A variation for maintaining the probe current constant is given by:
VCLO→VCL0+ΔVCL
A variation for maintaining the degree of focus for the signal variation ΔVCLis given by:
VOLO→VCLO+ΔVOL
If the variation ΔVCLin VCLis infinitesimal, the variation ΔVOLin ΔVOLcan be found, using the aforementioned differential coefficient F′(VCL), from:
ΔVOL=F′(VCLO)ΔVCL

That is, the variation ΔVOLis in proportion to the variation ΔVCLif the signal ΔVCLis small. Accordingly, if the instrument is so set up that the gain A2of the amplifier17and the signal ratio r2of the D/A converter18satisfy the relation:
A2·r2=F′(VCLO)
then the degree to which the particle probe is focused can be kept constant by the use of the signal ΔVCLthat is employed to stabilize the probe current.

While one embodiment of the present invention has been described thus far, the invention is not limited to this embodiment. Rather, various changes and modifications are possible. For example, in the above embodiment, a part of the charged particle beam is used as a detected signal. If the degree to which the particle probe is focused is varied by negative feedback other than the negative feedback using the detection aperture4, the signal for the negative feedback may be adjusted, and this adjusted signal may be applied to the control portion for the condenser lens system and to the control portion for the objective lens.

As an example, a variation Ale in the emission current Iein a particle beam emission source is detected, and negative feedback is applied to the extraction voltage Vex. In this case, the varying signal Ieis adjusted and used.

In the above-described embodiment, a signal for adjusting the focal distance of the objective lens is added to the lens. It is to be noted that the invention is not limited to this scheme. For example, a signal for adjusting the focal distance of the lens may be added to a control portion for a focus-adjusting auxiliary lens located between the detection aperture4and the objective lens8or to a control portion for a control lens for adjusting the aperture angle of the beam incident on a specimen. Also, in this case, the object of the present invention can be accomplished. That is, a slight amount of defocus affects the final probe diameter greatly. In contrast, a slight deviation from the optimum aperture angle does not affect the final probe diameter.

The configuration shown inFIG. 3can be applied to the system shown inFIG. 5of the above-cited Japanese Patent Laid-Open No. 183044/1989. An example of its application is now described. X-axis detection electrodes arranged symmetrically around a charged particle beam produce output currents IX1and IX2, respectively. Y-axis detection electrodes perpendicular to the X-axis detection electrodes produce output currents IY1and IY2, respectively. Amounts of signals indicating the amounts of shifts of the charged particle beam from the optical axis in the X- and Y-axes are given by:
VX=(X1−X2)/(X1+X2)
VY=(Y1−Y2)/(Y1+Y2)
These amounts of signals are calculated by an arithmetic unit or the like. An amount of signal corresponding to the magnitude of the beam current is given by:
VT=X1+X2+Y1+Y2
Then, these signals of these amounts are converted into digital signals immediately before start of application of negative feedback. The obtained signals are converted into analog signals to find reference signals VX0, VY0, and VT0. Signals VX, VY, and VTare detected immediately after start of application of negative feedback. The differences between these signals VX, VY, and VTand the reference signals VX0, VY0, and VT0are given by:
ΔVX=VX−VX0
ΔVY=VY−VY0
ΔVT=VT−VT0
These differential signals are found.

Finally, these are amplified by a sufficiently large factor and used as signals for negative feedback. That is, ΔVXand ΔVYare used as signals for correcting beam shifts in the X- and Y-directions, respectively. ΔVTis used as a signal for correcting the probe current and as a signal for correcting defocus where the probe current is corrected in this way.

In this example of application and in the case ofFIG. 3, negative feedback is described using symbols of digital switches that are opened and closed. Instead, analog switches may be used. That is, a signal for negative feedback may be applied gradually. Similarly, an amplifier whose gain is increased gradually may be used. In this case, even if the differential signals ΔV1d, ΔVX, ΔVY, and ΔVTare finally amplified with extremely large degrees of amplification, the signal system will not be saturated. Furthermore, stable negative feedback is possible.

As described thus far, in the present invention, a part of a charged particle beam is detected. In response to the detected signal, a first lens is controlled to maintain constant the current of the charged particle beam made to hit a specimen. In response to the detected signal, a second lens is controlled to adjust the focus of the charged particle beam. Therefore, the current of the charged particle probe directed to the specimen can be kept constant at all times without defocus.

In another embodiment of the invention, a signal ΔVCLused when negative feedback is applied to the control portion for the first lens according to the detected signal is produced by amplifying the difference between V1dand V1d0(ΔV1d=V1d−V1d0) while maintaining the signal V1d0corresponding to the current detected immediately before application of negative feedback. The signal V1dcorresponds to the current detected after the start of negative feedback. A signal ΔVOLproportional to the feedback signal ΔVCLsupplied to the control portion for the first lens is fed to the control portion for the second lens. Consequently, stabilization of the focus of the charged particle probe can be accomplished with a simple structure. When negative feedback is started, the signal for feedback starts at 0. Therefore, the dynamic range of the signal for feedback can be made wide.

Having thus described my invention with the detail and particularity required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.