WO2008152132A2

WO2008152132A2 - Apparatus and method for performing secondary ion mass spectroscopy

Info

Publication number: WO2008152132A2
Application number: PCT/EP2008/057477
Authority: WO
Inventors: Wilfried Vandervorst
Original assignee: Interuniversitair Microelektronica Centrum Vzw (Imec)
Priority date: 2007-06-15
Filing date: 2008-06-13
Publication date: 2008-12-18
Also published as: WO2008152132A3

Abstract

An apparatus and method for performing Secondary Ion Mass Spectroscopy (SIMS) analysis of a sample are disclosed. The apparatus and method include a gas field ion source as the source of a first ion beam directed to a sample to sputter particles, which are optionally ionized by a laser beam and accelerated in a secondary ion beam towards a mass spectrometer. This apparatus and method allow obtaining SIMS characterization with a lateral resolution less than about 1nm.

Description

APPARATUS AND METHOD FOR PERFORMING SECONDARY ION MASS SPECTROSCOPY

Field of the invention

[0001] The present invention relates to methods and apparatuses for destructive diagnostics of substrates. In particular the present invention relates to methods and apparatuses for performing secondary ion mass spectroscopy.

Background

[0002] Due to the scaling of semiconductor technology, the vertical as well as the lateral dimensions of semiconductor devices, manufactured in this technology, shrink. Currently semiconductor devices are being manufactured in 65nanometer (nm) semiconductor technology, while 32nm and 23nm semiconductor technologies are under development. Corresponding semiconductor devices will have minimal lateral dimensions in respectively the 65nm, the 32nm and in the 23nm range. Therefore it is important to be able to analyze with a high resolution the composition of the layers and substrates used to manufacture such semiconductor devices or only parts thereof, such as highly doped junction regions, the channel region, the gate stack, etc .

[0003] One method to analyze the composition of a solid sample is Secondary

Ion Mass Spectroscopy (SIMS). This analytical technique uses a first or primary ion beam directed in vacuum to the area of the sample wherein the composition of the sample is to be determined. Upon impact on the surface of the sample, these first or primary ions will amorphize this surface and become implanted in the sample. Some particles will be ejected from within this area due to the impact of the first ion beam. These particles can be neutral or charged. Neutral particles can be ionized by means of a laser beam. Using an electrical field, ions of one polarity type, i.e. either positively or negatively charged, are extracted from the sputtered area and accelerated towards a mass spectrometer. These extracted ions form a secondary ion beam. Depending on their mass and energy these secondary ions will follow different paths in the magnetic field of the mass spectrometer, which allows sorting them by mass and energy. By counting the secondary ions in the respective paths using an ion detector, one can obtain information about which molecules, elements or isotopes thereof are present in the sample within the sputtered area. By selecting the dose and the energy of the first ion beam, one can either confine the interaction between the first ion beam to the first monolayer of the sample, i.e. so-called static SIMS, or move gradually into the bulk of the sample by continuous sputtering of material, i.e. so-called dynamic SIMS. [0004] The vertical resolution of the SIMS measurement technique can be improved by reducing the energy of the first ion beam. Typically this vertical resolution is in the range of 20 to 30nm, but can be improved by methods as proposed in United States Application US 6 809 317. Here it is suggested to add a reactive gas mixture to the chamber of the SIMS apparatus. The interaction between the first ion beam and the reactive gas mixture will lead to a controlled and limited removal speed of material of the sample at the place of impact.

[0005] However the lateral resolution of the SIMS measurement technique depends on the diameter of the first ion beam, being typically in the micro-meter range. This lateral resolution can be improved by increasing the energy of the first ion beam however at the expense of the vertical resolution. The lateral resolution of the state-of-the-art SIMS measurement tools is 40nm or more, which is more than the lateral dimensions of state-of-the art sub-45nm semiconductor devices.

[0006] Hence there is a need for an apparatus and method that allow performing

Secondary Ion Mass Spectroscopy with improved lateral resolution. [0007] Moreover there is a need for an apparatus and method that allow performing Secondary Ion Mass Spectroscopy with a lateral resolution in the nanometer range or less, without jeopardizing the vertical resolution of this technique.

Summary of Various Inventive Aspects

The invention is related to an apparatus and method as disclosed in the appended claims. Certain inventive aspects relate to an apparatus for performing secondary ion mass spectroscopy. The apparatus comprises a vacuum chamber containing a sample holder for holding a sample, an ion source for providing a first ion beam, means for directing the first ion beam to a predefined location on the sample for sputtering particles from the sample, means for extracting and accelerating sputtered charged particles, and means for performing a mass spectroscopy measurement on the extracted particles. According to embodiments of the invention, the ion source is a gas field ion source. The first ion beam preferably has a spot size less than about 10 Angstrom. The gas of the ion source is preferably selected from the group of Argon, Neon, Xenon, Cesium or Oxygen.

Brief description of the drawings [0008] Exemplary embodiments of the invention are illustrated in referenced figures of the drawings. The embodiments and figures disclosed herein must be considered illustrative rather than restrictive. The drawings described are only schematic and are non- limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes and need not to correspond to actual reductions to practice of the invention. Same numerals are used to refer to corresponding features in the drawings. [0009] Figure 1 illustrates a schematic view of a secondary ion mass spectroscopy apparatus according to one embodiment.

[0010] Figure 2 illustrates an ion source as used in a secondary ion mass spectroscopy apparatus according to one embodiment. [0011] Figure 3 shows a flowchart illustrating a method for operating a secondary ion mass spectroscopy apparatus according to one embodiment. [0012] Figure 4 illustrates the operation of a prior art helium ion microscope apparatus.

Detailed description of Certain Embodiments

[0013] Figure 1 illustrates a schematic view of a secondary ion mass spectroscopy apparatus 14 according to one embodiment. This apparatus comprises a vacuum chamber 15 able to contain a sample 6, preferably the sample being placed on a sample holder or stage 7. The vacuum chamber further comprises a gas field ionization source 2 for providing a first beam 4 of ions to a predefined location on the surface of the sample 6. This first ion beam 4 will be focused and deflected by an electrostatic column 3, comprising electromagnetic lenses for focusing the ion beam 4 towards the sample 6. . Optionally magnetic lenses 5 (fig. 2) are used in column 3 to manipulate the beam 4 and for deflecting the ion beam 4 to scan the surface of the sample 6 under investigation. Optionally a laser beam source 18 is provided to direct one or more laser beams 17 to ionize uncharged particles 16 originating from the sample surface. The vacuum chamber further comprises an extractor electrode 30 for extracting ions 19 from the sputtered area and to accelerate them towards a mass spectrometer 8. These extracted ions form a secondary ion beam 20. Depending on their mass and energy these secondary ions 19 will follow different paths in the magnetic field of the mass spectrometer 8, which allows sorting them by mass and energy. By counting the secondary ions in the respective paths using an ion detector, one can obtain information about the composition of the sample 6 within the area scanned by the first ion beam 4.

[0014] As shown in figure 2, the gas field ionization source 1 comprises a gas source 2 of atoms 13, a tip 10 and an extractor electrode 11. When applying an electrical field through voltage source 12, typically in the range of about 2-4 v/m, between the tip 10 and the extractor electrode 11, field ionization of the gas 13 ambient in between the tip 10 and the extractor electrode 11 takes place. Typically a voltage in the range of 5kV to 3OkV is applied between the tip 10 and the extractor electrode 11 to generate an electrical field capable of ionizing the gas ambient. The atoms become ionized 21 and will be extracted by the extractor electrode 11 to form an ion beam 4. The atoms are selected to have a mass sufficient to sputter the sample 6. Preferably the heavier atoms are used to allow sputtering of the sample such that particles are ejected from the sample surface. For example Ne ,Xe ' Ar⁺, O²⁺, Ga⁺ or Cs⁺ ions can be used. The ions 21 in the first ion beam 4 will have an energy in the range of 20OeV to lOOkeV, preferably 20OeV to 50keV. The thus created first ion beam will have an energy dispersion of less than 2eV, preferably less than IeV, even as low as 0.4eV. The brightness of the source 1 is in the order of 1 10⁹ A/cm²sr . The spot size of the first ion beam is in the range of 10 to 5 Angstrom.

[0015] A computer control system 9 is typically used to control (see dotted lines) the operation of various elements of the SIMS apparatus 14, such as a voltage source 12 for generating the electrical field between tip 10 and extractor electrode 11, the electrostatic column 3, the sample holder 7, the vacuum and temperature conditions of the gas field ion source 1 , the light source 18, the mass spectrometer 8 and the means for sending to and receiving information to and from the computer control system 9. This computer 9 typically comprises an input unit for inputting data, an output unit for outputting data, a storage unit for storing data and algorithms to be used upon the data sent to and/or data received from the SIMS apparatus 14 and a control unit to control the operating of the SIMS apparatus 14.

[0016] Although the general operation of a SIMS apparatus is known, the flow chart of figure 3 illustrates a method for operating a secondary ion microscope apparatus according to one embodiment. A sample 6 having a surface is placed in the vacuum chamber 15 of the SIMS apparatus 14. A first ion beam 4 is created in the gas field ion source 1 by field ionization of a predefined gas 13 by providing the gas 13 to the ion source 1 and forming a predefined electrical field between the tip 10 and the extractor electrode 11 of the ion source 1. This first ion beam 4 is then directed with predefined energy and concentration to the sample 6 to incident within the area of interest thereof. The first ion beam 4 is focused and deflected by the electrostatic column 3 to scan the surface area of interest. The first ion beam 4 will sputter particles from this area. These particles can be neutral 16 or charged 19. Neutral particles 16 can be ionized by means of one more laser beams 17 generated by a light source 18. Using an electrical field created by an extractor electrode 30 ions 19 of one polarity type, i.e. either positively or negatively charged, are extracted from the sputtered area and accelerated towards a mass spectrometer 8. These extracted ions form a secondary ion beam 20. Depending on their mass and energy these secondary ions will follow different paths in the magnetic field of the mass spectrometer 8, which allows sorting them by mass and energy. By counting the secondary ions in the respective paths using an ion detector, one can obtain information about which molecules, elements or isotopes thereof are present in the sample within the sputtered area. By selecting the dose and the energy of the first ion beam, one can either confine the interaction between the first ion beam to the first monolayer of the sample, i.e. so-called static SIMS, or move gradually into the bulk of the sample by continuous sputtering of material, i.e. so-called dynamic SIMS. [0017] The gas field ion source used in the SIMS apparatus of the invention can be a source as used in the known Helium ion microscopes using gas field ionization. Compared to the well-known electron microscopes, such helium ion microscopes provide higher source brightness, a smaller beam spot size and hence a better lateral resolution and a smaller excitation volume in the sample under study.

[0018] Figure 4 shows a schematic cross-section of a prior art Scanning He ion microscope (SHeM) 40 comprising: a vacuum chamber comprising a helium ion source 41 for providing a beam 4 OfHe⁺ ions, an electrostatic column 3 for focusing this He⁺ ion beam 4 towards a sample 6 which needs to be analyzed, a detector 42 for responding to He⁺ ions reflected from the sample 6 and/or to secondary electrons created at the sample 6 by the He⁺ ion beam. The sample 6 is placed on a sample holder or stage 7 which can transport the sample when loading the sample 6 to or from the vacuum chamber or move the sample 6 to the desired position within the vacuum chamber. The electrostatic column 3 comprises electromagnetic lenses 5 for focusing the He⁺ ion beam 4 with predefined energy and concentration towards the sample 6. The electrostatic column is also able to deflect the He⁺ ion beam 4 to scan the surface of the sample 6 under investigation according to a predefined pattern. This way the predefined sample area of interest can be rastered by the He ion beam 4 such that substantially the whole area of interest is investigated. The helium source 41 comprises a source of helium atoms, a tip 10 and an extractor electrode 11. When applying 12 an electrical field, typically in the range of about 2-4 V/m, between the tip 10 and the extractor electrode 11, field ionization of the gas ambient in between the tip 10 and the extractor electrode 11 takes place. Typically a voltage in the range of 5kV to 3OkV is applied between the tip 10 and the extractor electrode 11 to generate an electrical field capable of ionizing the gas ambient. The helium atoms become ionized and will be extracted by the extractor electrode 11 to form a He⁺ ion beam 4. A computer control system 43 is typically used to control (see dotted lines) elements of the He ion microscope 40, such as voltage source 12 for generating the electrical field between tip 10 and extractor electrode 11, the electrostatic column 3, the sample holder 7, the vacuum and temperature conditions of the microscope 40, the detector 42 and means for sending to and receiving information from the computer control system 43. [0019] J. Morgan et al discloses in "An introduction to Helium Ion Microscope", published in Microscopy Today, July 2006, a He ion microscope using a gas field ionization source. This publication is hereby incorporated by reference in its entirety. In particular, sections 4 and 5 of this publication disclose the use of a gas field ionization source having a particular tip design to provide a helium ion beam with a spot size as small as 10 to 5 Angstrom. By designing the tip 11 such that the number of tip sites sharing the Helium gas is reduced, the current and current density of the ion beam is increased and can now range from IfA up to 10OpA. A high intensity in the range of 1.10⁹ A/cm²sr is obtainable. The energy spread of the beam can be less than 2eV, even as low as 0.4Ev.

[0020] Various other gas field sources can be used in the SIMS apparatus of the invention. V.N. Todare discloses in "Quest for high brightness, monochromatic noble gas ion sources" in J. Vac. Sci Techno. A 23(6), Nov/Dec 2005 various ways of generating noble gas ions using field ionization of a noble gas. In particular section III.A. discusses various types of gas field ion sources. This publication is hereby incorporated by reference in its entirety. [0021] Likewise United States application 2006/0284092, hereby incorporated by reference in its entirety, discloses a Scanning Transmission Ion Microscope using such a helium ion source. [0022] Although these analytical measurement techniques employ the helium ion beam 4 created by helium gas field ion sources to obtain an ion beam with improved lateral resolution, they don't provide the capability of analyzing material originating from the surface or the bulk of the sample. A SIMS apparatus and method according to the present invention provides an improved lateral resolution without jeopardizing the vertical resolution as the energy of the first ion beam 4 is not used to improve this lateral resolution as a gas field ion sources provides a smaller excitation volume in the sample compared to an electron beam as the energy spread of the ion beam is minimized.

[0023] The foregoing description details certain embodiments of the invention.

It will be appreciated, however, that no matter how detailed the foregoing appears in text, the invention may be practiced in many ways. It should be noted that the use of particular terminology when describing certain features or aspects of the invention should not be taken to imply that the terminology is being re-defined herein to be restricted to including any specific characteristics of the features or aspects of the invention with which that terminology is associated. [0024] While the above detailed description has shown, described, and pointed out novel features of the invention as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the device or process illustrated may be made by those skilled in the technology without departing from the spirit of the invention.

Claims

1. An apparatus for performing secondary ion mass spectroscopy, comprising :

• a vacuum chamber (15) containing a sample holder (7) for holding a sample (6),

• an ion source (1) for providing a first ion beam (4),

• means (3) for directing the first ion beam (4) to a predefined location on the sample

(6) for sputtering particles (16, 19) from the sample (6), • means (18) for extracting and accelerating sputtered charged particles (19), and

• means (8) for performing a mass spectroscopy measurement on the extracted particles

(19), characterized in that the ion source (1) is a gas field ion source.

2. The apparatus according to claim 1, wherein the ion source (1) is adapted to produce a first ion beam (4) having a spot size less than about 10 Angstrom.

3. The apparatus according to claim 1 or 2, wherein the gas (13) of the ion source (1) is selected from the group of Argon, Neon, Xenon, Cesium or Oxygen.

4. The apparatus according to any one of claims 1 to 3, further comprising a laser light source (18) arranged to perform an ionization of sputtered particles.

5. A method for performing a surface analysis of a sample, comprising the following steps : placing a sample (6) having a surface, in a vacuum chamber (15), directing a first ion beam (4) to a location on said surface, thereby causing sputtering of particles (16,19) from the sample (6), - accelerating ions (19) resulting from said sputtering, to form a secondary ion beam (20) directed towards a mass spectrometer (8), and performing a mass spectrometry analysis on said ions, characterized in that said first ion beam is produced by a gas field ion source (1).

6. The method according to claim 5, wherein said first ion beam (4) has a spot size less than about 10 Angstrom.

7. The method according to claim 5 or 6, wherein the gas (13) of the ion source (1) is selected from the group of Argon, Neon, Xenon, Cesium or Oxygen.

8. The method according to any one of claims 5 to 7, further comprising a step of ionizing sputtered particles using at least one laser beam, prior to the step of accelerating ions towards a mass spectrometer.