GB2038544A

GB2038544A - The minimisation of surface originating contamination in electron microscopes

Info

Publication number: GB2038544A
Application number: GB7943946A
Authority: GB
Original assignee: South African Inventions Development Corp
Current assignee: South African Inventions Development Corp
Priority date: 1978-12-20
Filing date: 1979-12-20
Publication date: 1980-07-23
Also published as: JPS5586058A; ZA787151B

Abstract

Surface originating contamination of a sample (18) in an electron microscope is minimised by indirectly irradiating the sample with low energy electrons supplied by electron gun (10) adjacent sample holder (20). The gun supplies electrons with energies between 50 eV and 500 eV, a beam current in the microamp range and a divergent beam with an angle of between 2 DEG and 15 DEG . The sample may be irradiated either before and/or during imaging of the sample. Neutralization of positive charge induced on the sample by secondary electron emission and subsequent non-attraction of hydrocarbons to the sample is considered. Also excess negative charge on the sample causes hydrocarbons to desorb more rapidly. <IMAGE>

Description

SPECIFICATION The minimisation of surface originating contamination in electron microscopes THIS INVENTION relates to an electron microscope and to a means and method for minimising surface originating contamination of a sample therein.

According to the invention there is provided an electron microscope which includes a sample holder in which a sample to be examined may be held; and a contamination minimising means for minimising surface originating contamination of the sample, comprising a generating means located adjacent the sample holder for generating low energy electrons and for indirectly irradiating the sample therewith.

Further according to the invention there is provided a method of minimising surface originating contamination of a sample in an electron microscope, which includes indirectly irradiating the sample with low energy electrons.

The electrons may have energies of suitable values. Preferably, the electrons may have energies between 50 and 500 eV. The applicant has furthermore found that electrons having energies of between 200 and 450 eV are most effective.

It is believed by the applicant that these low energy electrons neutralize a positive charge induced on the sample by secondary electron emission when the sample is imaged by a high energy primary beam of electrons. It is further believed that as a result of this neutralization hydrocarbons contaminating the surface are no longer attracted to that region of the sample illuminated by the primary beam.

It is also believed that the excess negative charge resulting from the low energy electron beam and incident on the sample surface will cause the hydrocarbons to desorb more rapidly therefrom.

The sample may be irradiated transversely with respect to the primary beam. The beam of low energy electrons may also be divergent, having an angle of between 2" and 15 .

Coveniently, the electrons may be provided by a suitable electron gun.

Although it is preferable to irradiate the sample whilst it is being imaged by the primary beam, the sample may first be irradiated and then imaged.

The sample may be irradiated with a suitable flux. The low energy electron beam may have a current in the microamp range.

Further, the sample may be irradiated for a suitable period of time of between 2 seconds and 600 seconds.

The invention is now described by way of an example, with reference to the accompanying drawings, in which: Figure 1 shows schematically a low energy anti-contamination electron (LEACE) gun and a sample in an electron microscope; Figure 2 shows a sectional view of the electron gun in more detail; and Figure 3 shows schematically a scanning electron microscope and the position of the electron gun.

Referring to Figs. 1 and 2, shown therein is a low energy anti-contamination electron gun 10. The gun 10 is a standard triode electron gun with a DC-heated tungsten hairpin filament 12, an anode 14, and a control electrode 16.

The gun 10 is suitably mounted to irradiate a sample 18, which is held in a sample holder 20, transversely with respect to the axis of a primary beam of the electron microscope, as shown in Figs. 1 and 3. Referring particularly to Fig. 3, the electron microscope 22 has a primary electron gun 24, an anode 26, a condenser lens 28, an intermediate lens 30, an objective lens 32, the sample holder 20 with the electron gun 10 adjacent thereto, and a transmitted electron detector 34.

Further as shown in the drawings, the gun 10 emits a diverging beam of electrons that cannot impinge directly on the sample due to the recessed shape of the holder 20.

Such a gun as that shown in the drawing has been mounted by the inventor in the Xray take-off port of a Philips EM-200 transmission electron microscope.

This gun was then operated with the filament 1 2 at a voltage of - 220 V with respect to the anode 1 4 which was connected to earth of the microscope's electrical system.

The gun 10 provided a current in the yA range.

Those skilled in the art will be aware that with the electron microscope referred to above the sample 1 8 is positioned in the magnetic gap between the pole pieces of its objective lens. Thus, it is necessary to switch off the objective lens when the sample is irradiated with electrons from the LEACE gun 1 0. Under these conditions the sample can be imaged with the first intermediate lens even though under rather poor resolution conditions. However, it has been found that it is not essential that the LEACE gun 10 should be on at the same time as the primary electron beam and that the sample may first be irradiated with the low energy electrons from the LEACE gun 10 and then be imaged with the primary beam and the standard objective lens, with dramatic decontamination effects.

Finally, it has been found that the degree of decontamination is dependent on the period of irradiation, if the beam current is not less than about 1 yA. Time periods of 5 to 10 minutes have been found to be effective.

1. An electron microscope which includes a sample holder in which a sample to be

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION The minimisation of surface originating contamination in electron microscopes THIS INVENTION relates to an electron microscope and to a means and method for minimising surface originating contamination of a sample therein. According to the invention there is provided an electron microscope which includes a sample holder in which a sample to be examined may be held; and a contamination minimising means for minimising surface originating contamination of the sample, comprising a generating means located adjacent the sample holder for generating low energy electrons and for indirectly irradiating the sample therewith. Further according to the invention there is provided a method of minimising surface originating contamination of a sample in an electron microscope, which includes indirectly irradiating the sample with low energy electrons. The electrons may have energies of suitable values. Preferably, the electrons may have energies between 50 and 500 eV. The applicant has furthermore found that electrons having energies of between 200 and 450 eV are most effective. It is believed by the applicant that these low energy electrons neutralize a positive charge induced on the sample by secondary electron emission when the sample is imaged by a high energy primary beam of electrons. It is further believed that as a result of this neutralization hydrocarbons contaminating the surface are no longer attracted to that region of the sample illuminated by the primary beam. It is also believed that the excess negative charge resulting from the low energy electron beam and incident on the sample surface will cause the hydrocarbons to desorb more rapidly therefrom. The sample may be irradiated transversely with respect to the primary beam. The beam of low energy electrons may also be divergent, having an angle of between 2" and 15 . Coveniently, the electrons may be provided by a suitable electron gun. Although it is preferable to irradiate the sample whilst it is being imaged by the primary beam, the sample may first be irradiated and then imaged. The sample may be irradiated with a suitable flux. The low energy electron beam may have a current in the microamp range. Further, the sample may be irradiated for a suitable period of time of between 2 seconds and 600 seconds. The invention is now described by way of an example, with reference to the accompanying drawings, in which: Figure 1 shows schematically a low energy anti-contamination electron (LEACE) gun and a sample in an electron microscope; Figure 2 shows a sectional view of the electron gun in more detail; and Figure 3 shows schematically a scanning electron microscope and the position of the electron gun. Referring to Figs. 1 and 2, shown therein is a low energy anti-contamination electron gun 10. The gun 10 is a standard triode electron gun with a DC-heated tungsten hairpin filament 12, an anode 14, and a control electrode 16. The gun 10 is suitably mounted to irradiate a sample 18, which is held in a sample holder 20, transversely with respect to the axis of a primary beam of the electron microscope, as shown in Figs. 1 and 3. Referring particularly to Fig. 3, the electron microscope 22 has a primary electron gun 24, an anode 26, a condenser lens 28, an intermediate lens 30, an objective lens 32, the sample holder 20 with the electron gun 10 adjacent thereto, and a transmitted electron detector 34. Further as shown in the drawings, the gun 10 emits a diverging beam of electrons that cannot impinge directly on the sample due to the recessed shape of the holder 20. Such a gun as that shown in the drawing has been mounted by the inventor in the Xray take-off port of a Philips EM-200 transmission electron microscope. This gun was then operated with the filament 1 2 at a voltage of - 220 V with respect to the anode 1 4 which was connected to earth of the microscope's electrical system. The gun 10 provided a current in the yA range. Those skilled in the art will be aware that with the electron microscope referred to above the sample 1 8 is positioned in the magnetic gap between the pole pieces of its objective lens. Thus, it is necessary to switch off the objective lens when the sample is irradiated with electrons from the LEACE gun 1 0. Under these conditions the sample can be imaged with the first intermediate lens even though under rather poor resolution conditions. However, it has been found that it is not essential that the LEACE gun 10 should be on at the same time as the primary electron beam and that the sample may first be irradiated with the low energy electrons from the LEACE gun 10 and then be imaged with the primary beam and the standard objective lens, with dramatic decontamination effects. Finally, it has been found that the degree of decontamination is dependent on the period of irradiation, if the beam current is not less than about 1 yA. Time periods of 5 to 10 minutes have been found to be effective. CLAIMS

1. An electron microscope which includes a sample holder in which a sample to be examined may be held; and a contamination minimising means for minimising surface originating contamination of the sample, comprising a generating means located adjacent the sample holder for generating low energy electrons and for indirectly irradiating the sample therewith.

2. An electron microscope as claimed in claim 1, in which the generating means is adapted to generate electrons with energies of between 50 eV and 500 eV.

3. An electron microscope as claimed in claim 2, in which the generating means is adapted to generate electrons with energies of between 200 eV and 450 eV.

4. An electron microscope as claimed in any one of the preceding claims, in which the generating means is an electron gun.

5. An electron microscope as claimed in any one of the preceding claims, which is adapted to supply an electron current in the microamp range.

6. An electron microscope as claimed in any one of the preceding claims, in which the generating means is adapted to generate a divergent beam of electrons.

7. An electron microscope as claimed in claim 6, in which the generating means is adapted to generate a beam of electrons having an angle of between 2" and 15 .

8. An electron microscope as claimed in any one of the preceding claims in which the generating means is positioned such that it supplies electrons in a transverse direction to a primary electron beam of the microscope.

9. A method of minimising surface originating contamination of a sample in an electron microscope, which includes indirectly irradiating the sample with low energy electrons.

10. A method as claimed in claim 9, in which the sample is irradiated with the electrons transversely with respect to a primary imaging beam of the microscope.

11. A method as claimed in claim 9 or 10, in which the electrons have energies of between 50 eV and 500 eV.

12. A method as claimed in claim 11, in which the electrons have energies of between 200 eV and 450 eV.

1 3. A method as claimed in any one of claims 9 to 12, in which the sample is irradiated with the electrons whilst it is being imaged.

14. A method as claimed in any one of claims 9 to 13, in which the sample is irradiated with the electrons before it is imaged.

1 5. A method as claimed in any one of claims 9 to 14, in which the electrons have a beam current in the microamp range.

1 6. A method as claimed in any one of claims 9 to 15, in which the sample is irradiated with the electrons for a time period of between 2 secs and 600 secs.

1 7. An electron microscope, substantially as described in the specification with reference to the accompanying drawings.

1 8. A method of minimising surface originating contamination of a sample in an electron microscope, substantially as described in the specification with reference to the accompanying drawings.