GB2269934A - Mass spectrometer. - Google Patents

Abstract

In a secondary ion mass spectrometer arrangement a sample 9 in a vacuum chamber 3 is exposed to a sensitizing agent, eg. chlorine, supplied through inlet 31, the sensitised sample surface is then irradiated with an ion beam from a source 11 eg. argon ions of less than 50 eV, to remove substantially only a molecular mono layer of surface species bonded to the sensitizing agent, the sputtered species being subjected to mass analysis in a mass spectrometer 23. Preferably, the sputtered species are ionised and/or the sensitizing material is stripped from them in an ECR chamber 19, plasma generator, or using an electron beam or the like, before the magnetic sector mass spectrometer <IMAGE>

Description

SPECTROMETER The present invention relates to an apparatus and method for performing secondary ion mass spectrometry (SIMS).

SIMS is a technique which has long been used as a tool in semiconductor research for obtaining information about surface condition, eg. to detect surface impurities. Typically, the surface is irradiated with a relatively high energy ion beam and the ionised species sputtered from the surface are accelerated and then mass analysed using a conventional mass spectrometer (eg.

magnetic sector or quadrupole).

Trends towards ultra large scale integration of semiconductor devices are making it increasingly important to increase depth resolution. Ideally, it would be desirable to determine the characteristics of just one surface molecular monolayer at one time.

Recently, molecular layer etching has been described by Aoyagi et al (Appl.Phys.Lett. 60 (8) 1992, pp 968-970), using chlorine gas etchant to bond to a surface monolayer, followed by removal of the monolayer by means of ion beam irradiation.

We have now devised a new apparatus and method for performing SIMS capable of atomic level resolution.

Thus, the present invention provides a method of performing secondary ion mass spectrometry on a sample, the method comprising the steps of: placing the sample in a vacuum chamber; exposing a surface of the sample to a sensitizing agent; irradiating said surface of the sample with an ion beam to sputter substantially only a molecular monolayer of surface species bonded to said sensitizing agent; and subjecting the sputtered species to mass analysis.

The present invention further provides a secondary ion mass spectrometer apparatus comprising a vacuum chamber, means for exposing a sample to a sensitizing agent within the vacuum chamber, an ion source for irradiating the sample, and a mass analyser.

Preferably, after exposure of the sample to the sensitizing agent, the remaining sensitizing agent still present in the vacuum chamber is purged by continued pumping back to the original pressure.

The essence of the present invention is sensitize the surface monolayer by exposure to the sensitizing agent, such that the irradiating ion beam has sufficient energy to remove the monolayer whilst leaving the next monolayer intact on the surface. A preferred sensitizing agent is chlorine, although other suitable agents are other Group VII elements or simple molecules involving a Group VII element, eg. HC1.

The irradiating beam may use any ion which would be used for conventional SIMS and is therefore preferably argon, although any inert gas ions could be used instead.

The irradiating ion beam should have an average energy sufficient to remove the sensitized surface monolayer whilst not removing any material from the next monolayer. Therefore, it is preferable for this ion beam to have an average energy of less than 50eV.

In general, the species sputtered from the surface of the sample will have surface atoms and atoms or molecules of the sensitizing ions still bonded to them.

The species with bonded sensitizing agent must be ionised if it is to be enalysed in that state (although some may already be ionised). Alternatively, the sensitizing agent may be stripped from the surface atoms which are subsequently ionised. Such ionisation and/or stripping is conveniently performed using electron cyclotron resonance plasma discharge, electron irradiation or the like.

Conveniently, the sputtered species may be collected by means of a collector situated close to the irradiated surface, connected to the stripping means and mass analyser, which will normally have its own pumping system. It is preferred for the collector to be provided with automatic valve means for isolating it from the vacuum chamber during exposure of the sample to the sensitizing agent.

The present invention will now be explained in more detail by way of the following description of a preferred embodiment and with reference to the accompanying drawings, in which: Figure 1 shows a schematic of an apparatus according to the present invention; Figure 2 shows a timing sequence for operating the apparatus shown in Figure 1.

An apparatus 1 for performing atomic resolution mass spectrometry (ARMS) is shown in Figure 1. The apparatus comprises a vacuum chamber 3 connected to a first pumping system 5. Normally this pump is isolated by means of a value not shown. However, the low pressure in the chamber is maintained by means of a main pumping system (not shown).

The chamber contains a holder 7 for a sample 9. Above the sample holder is situated an ion source 11 for irradiating the sample with low energy argon ions. The sample holder is connected to a voltage source 13 for controlling the energy of the ion beam incident on the sample.

An ion collector 15 is located immediately above the sample and this is connected via a solenoid valve to an electron cyclotron resonance (ECR) chamber 19 which has an argon bleed 21. The ECR then connects with a magnetic sector mass spectrometer 23 in front of which is located an ion accelerator 25.

The mass spectrometer is provided with its own pumping system 27 and a detector (multiplier) 29. Except as mentioned below, the second pumping system 27 is isolated by means of a valve (not shown).

It should be noted that the vacuum chamber 3 is also provided with a bleed inlet 31 for chlorine gas.

After the sample has been placed on the holder in the chamber and the system pumped down to around 10 7 to ~g 10 9 torr, and the sample degassed ions, and surface oxide is removed using low energy such as a hydrogen plasma. The apparatus is operated in accordance with the timing sequence shown in Figure 2.

First, chlorine gas is admitted (A) to a partial pressure of about 10 4. torr. The sample is exposed to the chlorine for about 10 seconds, although the time is relatively uncritical since the chlorine only bonds to a monolayer on the sample surface. The chlorine supply is then switched off and excess chlorine is pumped off for about 10 seconds to 1 minute by means of he first pump (D). At this stage, solenoid valve 17 is closed. The solenoid value is optional but can be used to protect the mass analyser from chlorine whilst the sample is being exposed.

Next, the first pump is switch isolated, the argon ion source is actuated (B) and the accelerating voltage turned on to bombard the sample with ca 20eV argon ions. At the same time, the solenoid valve is opened and a monolayer of ions sputtered from the sample surface, still bonded to chlorine atoms, enters the collector. At this stage, the system is opened to the second pump 27.

The neutral molecules or atoms enter the ECR where the chlorine molecules are ionised or the chlorine is stripped-off from the sputtered surface atoms which are then ionised. The ionised molecules or atoms are then accelerated by the accelerator and are mass analysed by the mass spectrometer, eg. over a period of 10 seconds to 1 minute to ensure sufficient integration per m/e to achieve required sensitivity.

A quadrupole MBE pressure gauge having Ca3 second integration time, sensitive to partial pressures down to 10 3 torr is especially suited to this purpose.

At the end of the analysis cycle, the mass spectrometer pump is again isolated (D) and the first pump is again reconnected to clear the system (E).

Figure 3 shows an alternative embodiment of the foregoing system. In this drawings, the same reference numerals are used as in Figure 1 to designate like components.

Here, the first pump 5 is the main pumping system for the chamber. It is isolated from the system (when desired) by a first valve 33. A second valve 35 is for turning the chlorine supply on and off.

A quadruple MBE partial pressure mass analyser 37 performs the mass analysis. It is provided with an integral accelerator. Any argon ion source/plasma generator and combined ECR 39 is provided for ion bombardment, surface cleaning, sputterd species ionisation/stripping etc.

The collector used in the embodiment of Figure 1 is dispensed with here.

Figure 4 shows the timing sequence which generally corresponds to that shown in Figure 2 but also shows the sequence for the first valve 33.

In the light of this disclosure, modifications of the described embodiment as well as other embodiments, all within the scope of the present invention as defined by the appended claims, will now be apparent to persons skilled in the art.

Claims (14)

1. Method of performing secondary ion mass spectrometry on a sample, the method comprising the steps of: placing the sample in a vacuum chamber; exposing a surface of the sample to a sensitizing agent; irradiating said surface of the sample with an ion beam to sputter substantially only a molecular monolayer of surface species bonded to said sensitizing agent; and subjecting the sputtered species to mass analysis.

2. A method according to claim 1, wherein after exposure of the sample to the sensitizing agent, remaining sensitizing agent is purged from the vacuum chamber.

3. A method according to either preceding claim, wherein said sensitizing agent is chlorine.

4. A method according to any preceding claim, wherein said irradiation is effected with argon ions.

5. A method according to any preceding claim, wherein the average energy of the irradiating ion beam is less than 50eV.

6. A method according to any preceding claim, comprising the further step of stripping the sensitizing agent from the sputtered species and ionising the residual surface atoms before subjecting them to mass analysis.

7. A method according to any of claims 1-6, comprising the further step of ionising the sputtered species and subjecting them to mass analysis.

8. A method according to claim 6 or claim 7, wherein the stripping and/or ionising step is effected by passing the ions through an electron cyclotron resonance chamber or a plasma, or irradiating the ions with an electron beam.

9. A secondary ion mass spectrometer apparatus comprising a vacuum chamber, means for exposing a sample to a sensitizing agent within the vacuum chamber, an ion source for irradiating the sample, and a mass analyser.

10. An apparatus according to claim 9, further comprising means for stripping a sensitizing agent from species sputtered from a surface of the sample and ionising the residual surface atoms and/or ionising the sputtered species without stripping.

11. An apparatus according to claim 10, wherein said means for stripping/ionising comprises an electron cyclotron resonance chamber, means for generating a plasma, or an electron gun.

12. An apparatus according to any of claims 9-11, further comprising a collector for sputtered species.

13. A method of performing secondary ion mass spectrometry, the method being substantially as hereinbefore described with reference to the accompanying drawings.

14. A secondary ion mass spectrometer apparatus substantially as hereinbefore described with reference to the accompanying drawings.