SE542905C2

SE542905C2 - Method for determining a pressure at a sample surface

Info

Publication number: SE542905C2
Application number: SE1851529A
Authority: SE
Inventors: Anders Nilsson; Peter Amann
Original assignee: Scienta Omicron Ab
Priority date: 2018-12-07
Filing date: 2018-12-07
Publication date: 2020-09-15
Also published as: CN113169016A; SE1851529A1; EP3891775A1; WO2020117125A1

Abstract

A method is described for monitoring a sample pressure at a sample surface (Ss) of a sample (1) placed in a sample region (2), which sample surface (Ss) is facing an aperture (3) in a wall (6) separating the sample region (2) from a low-pressure chamber (4) which is vacuum pumped. The method comprises the steps of determining a relationship between the sample reference pressures at the sample surface (Ss) and the chamber reference pressures in the low-pressure chamber (4), arranging (103) the sample (1) with the sample surface (Ss) facing the aperture (3) at a distance from the aperture (3), providing (104) a sample pressure, measuring (105) a chamber pressure inside the low pressure chamber (4), and determining (106) the sample pressure using the measured chamber pressure and the determined relationship between the sample reference pressures and the chamber reference pressures.

Description

1METHOD FOR DETERMINING A PRESSURE AT A SAMPLE SURFACE TECHNICAL FIELD The present invention is related to a method for determining a pressure at a sample surface. Morespecifically the present invention re|ates to a method for determining a pressure at a sample facing anaperture in a wall separating the sample region from a low-pressure chamber which is vacuum pumped.BACKGROUND ART ln the prior art systems for Ambient Pressure Photoemission Spectroscopy (APXPS) and AmbientPressure Photo Emission Spectroscopy (APPES) a high pressure is provided at a sample while radiatingthe sample to provide, e.g., photoelectrons or electrons originating from Auger processes. Thephotoelectrons are collected in an electrostatic lens system which is differentially pumped. Theelectrostatic lens system focuses the electrons to an entrance to a measurement region. To enable ahigh vacuum in the electrostatic lens system the aperture into the electrostatic lens system has to be small. ln prior art APPES is performed in three ways; 1) a sample is put in a chamber and the whole chamberis raised to ambient pressures in the mbar range, this is known as the backfill approach; 2) is a variantof the backfill approach where different chambers are used for different set of experiments and thechambers are exchanged, hence this method is called the exchangeable chamber approach; and 3) anin situ gas cell encapsulate the sample with the front aperture of the analyser. All these tree versionscould be operated in flow mode, where the gas is let in and simultaneously pumped out via an outlet or the gas is only pumped out via the front cone and the pumping arrangement of the analyser.

A variant of the gas cell method is described in J. Knudsen et al. ”A versatile instrument for ambient pressure x-ray photoelectron spectroscopy: The Lund cell approach", Surface Science 646 (2016) 160-169. Here Knudsen et al. describes an alternative ambient pressure cell in which a gas flow is directedat the sample. Gas outlets are arranged surrounding the aperture used for collecting electrons to the detector.

When performing APPES it is necessary to have the sample arranged close to the aperture. The reasonfor this is that the mean free path for electrons is short for high pressures. As an example, a desirable distance between the sample surface and the aperture is 30 um for XPS in carbon monoxide at a 2pressure of 1 bar, since the mean free path for 10 keV electrons in carbon monoxide at 1 bar pressureis about 30 um. Thus, a distance of 30 um would enable a reasonable part of the photoelectrons topass into the aperture. For such distances between the sample and the aperture it is very difficult tomeasure or predict the pressure at the sample surface. This is especially true when the pressure at the sample is provided by a flow of gas directed towards the sample surface.

The general prior art of pressure estimation originates from Ogletree et al. "Rev. Sci. lnstrum. (2002)73, 3872", where the pressure profile between the sample chamber and electrostatic lens chamberthrough an aperture is estimated using a simple analytical function. The pressure profile is alsodiscussed in H. Bluhm, "J. Electron. Spectrosc. Relat. Phenom, 177 (2010), 71-84, and in J. Kahk et al."J. Electron. Spectrosc. Relat. Phenom, 205 (2015) 57-65”. ln said articles it is estimated that thepressure at the sample surface is 95 % of the pressure measured in the sample chamber with adistance of 1 aperture diameter between the sample surface and the aperture and 98 % of thepressure measured in the sample chamber with a distance of 2 aperture diameters between thesample surface and the aperture. Kahk et al. have also calculated that the pressure at the samplesurface is varying with pressure. The higher the pressure the more accurate the pressure reading for 1diameter distance, but for low pressures 2 diameters will be more accurate. Knudsen et al. makes anestimation of the pressure at the sample based on a measurement of the pressure in the pressure cellat a distance from the sample surface and on theoretical calculations. According to Knudsen et al., thedeviation in pressure at the sample compared to the measurement position is less than 4 times, which they find acceptable.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for determining the pressure at a samplesurface arranged facing an aperture into a volume in which the pressure is lower than the pressure at the sample surface, which method is an alternative to the methods according to the prior art.

Another object of the present invention is to provide a method for determining the pressure at asample surface facing an aperture into a volume in which the pressure is lower than the pressure at thesample surface and where the pressure at the sample surface is provided by a flow of gas directed towards the sample surface.

Another object of the present invention is to provide a method for monitoring the pressure at a samplesurface facing an aperture into a volume in which the pressure is lower than the pressure at the samplesurface and where the pressure at the sample surface is provided by a flow of gas directed towards the sample surface. 3Another object of the present invention is to provide a method to measure the pressure at a samplesurface when the pressure at the sample surface is higher than the pressure of the surrounding environment such as, e.g., a chamber in which the sample is situated.

At least one of these objects is achieved with a method according to the independent claims.

Further advantages are achieved with the features of the dependent claims.

According to a first aspect of the present invention a method is provided for monitoring a samplepressure at a sample surface of a sample placed in a sample region, which sample surface is facing anaperture in a wall separating the sample region from a low-pressure chamber which is vacuum pumped.The method is characterized in that the method comprises the steps of a) providing a gas with a number of different sample reference pressures in the sample region, b) measuring the resulting chamber reference pressure inside the low-pressure chamber for each oneof the sample reference pressures, to determine a relationship between the sample reference pressuresand the chamber reference pressures, c) arranging the sample with the sample surface facing the aperture at a distance from the aperture, d) providing a gas with a sample surface pressure at the sample surface, e) measuring a chamber pressure inside the low pressure chamber, f) determining the sample surface pressure using the measured chamber pressure and the determinedrelationship between the sample reference pressures and the chamber reference pressures, wherein the pressure in the low pressure chamber is lower than each one of the sample surface fefefenCe pfeSSUfeS.

The calibration process is dependent on the type of gas used. This procedure may therefore be conducted for each individual gas separately.

The sample may be arranged in a vacuum chamber. ln such a case, the sample surface reference pressures may be provided by providing different static pressures in the vacuum chamber.

The sample reference pressures are known pressures which are applied as static pressures at least in thesample region. The sample surface pressure is provided by providing gas to the sample surface. lf thesample surface is arranged close to the aperture the aperture affects the pressure at the sample surface.However, the inventors have realised that the pressure inside the low pressure chamber is affected bythe pressure close to the aperture. Thus, for a specific pressure inside the low pressure chamber thepressure outside the low pressure chamber at the aperture is the same both when a sample is arranged close to the aperture and when no sample is present. 4The aperture may have a largest dimension in the plane of the end surface being smaller than 1 mm. Thelargest dimension of the aperture in the plane of the end surface is preferably smaller than 300 um andmay be smaller than 100 um. For circular apertures the largest dimension is equal to the diameter of theaperture. A small aperture is necessary to be able to have a small distance between the aperture andthe sample, which is desirable for a photon spectrometer at high sample pressures. The relationshipbetween the diameter of the aperture and the distance between the aperture and the sample surfacehas been discussed above. Thus, the distance between the aperture and the sample surface should notbe smaller than the diameter of the aperture. Thus, for a very small distance between the sample surface and the aperture, a very small diameter of the aperture is necessary.

The distance between the sample and the aperture should preferably be kept at no more than 3 times the diameter of the aperture for the method to function as good as possible.The low-pressure chamber may be an electrostatic lens for focusing electrons.

For the method to function properly the pressure should decrease in the direction outwards from thevolume between the sample and the aperture, when the sample surface pressure is applied. For samplesurface pressures above 1 bar the sample may be placed in ambient pressure, presuming that gas canescape into said ambient pressure. However, for lower sample surface pressures the sample and the wallshould be arranged in a chamber, which is vacuum pumped to provide a pressure gradient outwards from the volume between the aperture and the sample surface. ln steps b) and e) a pressure less than 1 mbar, preferably less than 102 bar, and most preferred less than103 is maintained in the low-pressure chamber. The latter pressures are suitable for an electrostatic lens. ln step a) each one of the different sample reference pressures may be higher than 10 mbar. lt isimportant that the sample reference pressures are considerably higher than the chamber pressures in the low-pressure chamber.

The method may comprise another step before step a) of arranging a test container enclosing the sampleregion and the end wall so that the inside of the low-pressure chamber is in fluid communication withthe inside of the test container through the aperture, wherein the sample reference pressures in thesample region are achieved by providing the sample reference pressures in the test container. With sucha test container, the different sample reference pressures may be provided more easily compared to thecase that an entire vacuum chamber has to be filled. Also, the use of a test container enables the use ofpressures higher than 1 bar. Pressures above 1 bar are not possible to apply in ordinary vacuum chambers due to their construction. This is due to the fact that vacuum chambers are designed only for sustaining a low pressure inside. The application of an over pressure in the vacuum chamber may lead to, e.g., breaking of the windows in the vacuum chamber.

The method may comprise the step of providing in the end surface at least one gas outlet, which isarranged to direct gas from a gas supply device into a volume between the end surface and the samplesurface, and wherein the sample surface pressure is provided by supplying gas from said gas supplydevice to said at least one gas outlet. With such a gas outlet may also the sample reference pressures beprovided. lt is primarily for the case with such gas outlets the invention finds its application as thismethod of providing the sample surface pressure results in a local high-pressure region. ln this case it isnot possible to use any estimation used in the prior art as the methods according to the prior art rely on measurements of the pressure at a remote location.

The method may comprise the steps of measuring at predetermined time intervals the pressure insidethe low-pressure chamber, and controlling in a closed loop the gas supply device in such a way that thepressure inside the low-pressure chamber is kept constant, thereby keeping the sample surface pressure at the sample surface constant. This is an effective way of controlling the sample surface pressure.

The distance between the sample surface and the aperture 3 is maintained at less than 1 mm, preferably less than 300 um. These distances are suitable for APPES.

According to a second aspect of the present invention a computer program is provided for monitoringa sample surface pressure at a sample surface of a sample placed in a sample region. The samplesurface is facing an aperture in a wall separating the sample region from a low-pressure chamberwhich is vacuum pumped. A sample surface pressure may be provided with a gas supply device, andthe pressure inside the low-pressure chamber may be measured with a first pressure measuringmeans. Said computer program comprises instructions which, when executed by at least one processorcause the at least one processor to carry out the steps of - receiving from a second pressure measuring means a number of different sample reference pressurevalues, - receiving from the first pressure measuring means a chamber reference pressure value, indicating thepressure in the low-pressure chamber, for each one of the sample reference pressures, to determine arelationship between the sample reference pressures and the chamber reference pressures, - after receiving a signal that a sample has been arranged with the sample surface facing the apertureat a distance from the aperture, controlling the gas supply device to provide a sample pressure, - receiving from the first pressure measuring means a chamber pressure value, indicating the pressure inside the low-pressure chamber, 6 - determining the sample surface pressure using the chamber pressure value and the determined relationship between the sample reference pressures and the chamber reference pressures.

The discussions above for the first aspect of the invention apply also to the computer program according to the second aspect of the invention.

The computer program may also comprise instructions which, when executed by at least one processorcause the at least one processor to carry out the steps of- before the step of receiving a number of different sample reference pressure values, controlling the gas supply device to supply gas to provide the number of different sample reference pressures.

This makes the control of the process more automatic, and hence the process is made more suitable for use in an industrial environment with a minimum of manual operation.

The computer program may also comprise instructions which, when executed by at least one processorcause the at least one processor to carry out the steps of - receiving a desired sample surface pressure value, - controlling in a closed loop the gas supply device to provide a gas flow which results in a sample surface pressure which is equal to the desired pressure value.

For such a computer program to function efficiently it is desirable if the gas flow is directed to the volume between the aperture and the sample.

Thus, the computer program provides automatic pressure setting and monitoring possible.

Alternatively or additionally when the distance between the sample surface and the aperture may becontrolled with a positioning system, the computer program may also comprise instructions which,when executed by at least one processor cause the at least one processor to carry out the steps of - receiving a desired sample surface pressure value, - controlling in a closed loop the positioning system (Ps) such that a sample surface pressure which is equal to the desired pressure value is achieved.

Thus, a specific distance may be set if the gas flow is well controlled.

According to a third aspect, a computer-readable storage medium is provided which carries a computerprogram for monitoring a sample surface pressure at a sample surface of a sample, according to the second aspect of the invention. 7In the following preferred embodiments of the invention will be described with reference to thedrawings. The drawings are not drawn to scale. Similar features in the different drawings will be denoted by the same reference numerals.BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows an analyser arrangement according to an embodiment of the present invention and in which the method according to the present invention may be implemented.

Fig. 2 shows in more detail the end wall and the aperture in the arrangement shown in Fig. 1, wherein a test chamber has been arranged at the end wall.Fig. 3 is a flow diagram over a method according to an embodiment of the present invention.DETAILED DESCRIPTION Fig. 1 shows partly in cross section an arrangement 100 for collecting charged particles from a samplesurface Ss of a particle emitting sample 1 and determining at least one parameter related to thecharged particles. The arrangement 100 comprises a sample holder 10, for holding the sample 1 in asample region 2. The arrangement also comprises a low-pressure chamber 4, comprising an aperture 3,in a wall 6 separating the sample region 2 from the low-pressure chamber 4, wherein the aperture 3 isarranged facing the sample surface Ss of the sample 1 placed in the sample holder 10, in order tocollect charged particles from the sample surface Ss into the low-pressure chamber 4. A heater 18 isalso arranged on the sample holder 10 and is arranged to heat the sample 1. The arrangement alsocomprises a positioning system Ps for controlling the position of the sample holder 10 and, thus, thedistance d between the sample surface Ss and the aperture 3. The arrangement also comprises avacuum chamber 11, which encloses the aperture 3, the sample holder 10 and the positioning systemPs. The arrangement also comprises first pump means 12 for vacuum pumping of the low-pressurechamber 4. Furthermore, the arrangement 100 comprises at least one gas outlet 5 arranged to directgas into a volume between the wall 6 and the sample surface Ss, and gas supply device 20 for providinga flow of gas from said at least one gas outlet 5, to provide a sample pressure. The arrangement 100also comprises first pressure measuring means G1 for measuring the pressure inside the low-pressurechamber 4, and a control unit CU. The control unit CU is connected to the first pressure measuringmeans G1 and to the positioning means Ps. The control unit comprises an input 19 for an input signal, which may be used to control the positioning system Ps and/or for input of other information.

The low-pressure chamber 4 is an electrostatic lens system comprising a first end 16, at which the aperture 3 is arranged, and a second end 37 at which an aperture 8 is arranged. The lens system 13 is 8arranged to form a particle beam from charged particles, emitted from the sample surface Ss andentering through the aperture 3 at the first end 16, and to transport the charged particles to the secondend 17. The arrangement 100 further comprises a measurement region 3 for determining at least oneparameter related to the charged particles emitted from the sample surface Ss of the particle emittingsample 1. The measurement region 3 comprising an entrance 8 allowing at least a part of said particlesto enter the measurement region 3. The second end 37 is arranged at the entrance of the measurementregion 3. The electrons enter the measurement region 3 through an entrance 8 and electrons enteringthe region between the hemispheres 25 with a direction close to perpendicular to the base plate 7 aredeflected by an electrostatic field applied between the hemispheres 25, and those electrons having akinetic energy within a certain range defined by the electrostatic field will reach a detector arrangement 9 after having travelled through a half circle.

The first pump means 12 for vacuum pumping in the low-pressure chamber 4, is arranged to maintain apressure in the interval between 104 to 102 mbar, preferably lower than 2x10'3 mbar. A low pressure isnecessary for many applications such as when the low-pressure chamber is an electrostatic lens arrangedto focus electrons into an electron beam. The gas supply means 20 for providing a constant flow of gas is typically arranged to provide a sample surface pressure of 10 mbar to 1 bar and beyond.

The vacuum chamber 11 is vacuum pumped by a separate second pump means 22. The backgroundpressure in the vacuum chamber 11 is typically maintained at 102 to 1 mbar. The sample surface pressureis thus a local pressure in the volume between the sample surface Ss and the aperture 3. A secondpressure measuring means G2 is arranged to measure the pressure in the vacuum chamber 11. Thesecond pressure measuring means is connected to the control unit CU. The first pump means 12 and the second pump means 22 may be controlled by the control unit CU.

Fig. 2 shows in larger detail the sample and a test container 21 for realisation of a method according toan embodiment of the present invention. ln Fig. 5 four gas outlets 5 are arranged symmetrically aroundthe aperture 3, of which only two are shown in the cross section in Fig. 2. The length axis L is shown toextend through the aperture 3 essentially perpendicular to the end surface S of the wall 6. A testcontainer 21 is arranged covering the end wall 6, the gas outlets 5 and the aperture 3. A gas line 23 isconnected to the test container 21 and may be used to provide gas to the test container. Alternatively, gas may be provided to the test container through the gas outlets 5.

Fig. 3 shows a flow diagram over a method according to an embodiment of the present invention. ln afirst step 101 a number of different sample reference pressures are provided in the sample region 2.This may be achieved either by providing the sample reference pressures in the vacuum chamber. The pressures may be achieved in the vacuum chamber 11 by supplying gas from the gas supply device 20 9 through the gas outlet 5. Due to the volume of the vacuum chamber this might take some time. Analternative method is to arrange a test container 21 at the end wall 6, which test container 21 coversonly the gas outlets 5, and the aperture 3. Such a test container 21 may, due to its small size, be filledto the desired pressure considerably faster than the vacuum chamber 11. The sample referencepressures may of course also be provided with a gas line 23 (Fig. 2) which is connected to the testcontainer 21. ln case the vacuum chamber 11 is used for providing the sample reference pressures, thesecond pump means 22 is of course shut off during the provision of the sample reference pressures.The second pressure measuring means G2 is used to measure the pressure in the vacuum chamber 11or the test container 21. ln a second step 102 the resulting chamber reference pressure inside the low-pressure chamber 4 is measured for each one of the sample reference pressures, to determine arelationship between the sample reference pressures and the chamber reference pressures. ln a thirdstep 103 the sample 1 is arranged with the sample surface Ss facing the aperture 3 at a distance fromthe aperture 3. The distance between the aperture and the sample 1 is typically set to between 10 umand 1 mm, but may be a few millimetres or even less than 100 um. ln a fourth step 104 a samplesurface pressure is provided. The sample surface pressure is preferably provided by supplying gas fromthe gas supply device 20 through the gas outlet(s) 5. ln a fifth step 105 a chamber pressure is measuredinside the low-pressure chamber 4. Finally, in a sixth step 106 the sample surface pressure isdetermined using the measured chamber pressure and the determined relationship between the sample reference pressures and the chamber reference pressures.

A desired sample surface pressure value may be input on the input 19. The control unit CU may bearranged to measure, at predetermined time intervals the pressure inside the low-pressure chamber 4with the first pressure measuring means 12. The control unit CU may also be arranged to control thegas supply device to decrease or increase the flow of gas through the gas outlet(s) 5 in order to maintain a chamber pressure which corresponds to the desired sample surface pressure.

A computer program may run on the CPU for monitoring the sample surface pressure at the samplesurface Ss of the sample 1. The computer program comprises instructions which, when executed by atleast one processor cause the at least one processor to carry out the steps of - receiving from a second pressure measuring means G2 a number of different sample referencepressure values, - receiving from the first pressure measuring means G1 a chamber reference pressure value, indicatingthe pressure in the low-pressure chamber 4, for each one of the sample reference pressures, todetermine a relationship between the sample reference pressures and the chamber referencepressures, - after receiving a signal that a sample 1 has been arranged with the sample surface Ss facing the aperture 3 at a distance from the aperture 3, controlling the gas supply device 20 to provide gas with asample surface pressure at the sample surface Ss, - receiving from the first pressure measuring means G1 a chamber pressure value, indicating thepressure inside the low-pressure chamber 4, - determining the sample surface pressure using the chamber pressure value and the determined relationship between the sample reference pressures and the chamber reference pressures.

The computer program may also comprise instructions which, when executed by at least one processorcause the at least one processor to carry out the steps of before the step of receiving a number ofdifferent sample reference pressure values, controlling the gas supply device 20 to supply gas toprovide the number of different sample reference pressures, and to receiving a desired pressure valueon the input 19. The CPU may then control, in a closed loop, the gas supply device 20 to provide a gasflow which results in a sample surface pressure at the sample surface Ss which is equal to the desiredpressure value. lt is also possible that the CPU and control unit CU controls the first pump means 12 and the second pump means 22.

The computer program can also comprise instructions which, when executed by at least one processorcause the at least one processor CPU to carry out the steps of - receiving a desired pressure value on the input 19, - controlling in a closed loop the positioning system (Ps) such that a sample surface pressure at the sample surface Ss, which is equal to the desired pressure value, is achieved.

This provides the possibility to keep a fixed distance.

The above described embodiments may be amended in many ways without departing from the scope of the invention, which is limited only by the appended claims.

Claims

1. A method for monitoring a sample pressure at a sample surface (Ss) of a sample (1) placed in a sampleregion (2), which sample surface (Ss) is facing an aperture (3) in a wall (6) separating the sample region(2) from a low-pressure chamber (4) which is vacuum pumped, characterized in that the methodcomprises the steps of a) providing (101) gas with a number of different sample reference pressures in the sample region (2),b) measuring (102) the resulting chamber reference pressure inside the low-pressure chamber (4) foreach one of the sample reference pressures, to determine a relationship between the sample referencepressures and the chamber reference pressures, c) arranging (103) the sample (1) with the sample surface (Ss) facing the aperture (3) at a distance fromthe aperture (3), d) providing (104) gas with a sample surface pressure at the sample surface, e) measuring (105) a chamber pressure inside the low pressure chamber (4), and f) determining (106) the sample surface pressure using the measured chamber pressure and thedetermined relationship between the sample reference pressures and the chamber reference pressures,wherein the pressure in the low pressure (4) chamber is lower than each one of the sample reference pfeSSUfeS.

2. The method according to claim 1, wherein in steps b) and e) a pressure between less than 1 mbar,preferably less than 102 bar, and most preferred less than 103 is maintained in the low-pressure chamber (4).

3. The method according to claim 1, wherein in step a) each one of the different sample reference pressures is higher than 10 mbar.

4. The method according to claim 1, comprising the step before step a) of arranging a test container (21)enclosing the sample region (2) and the end wall (6) so that the inside of the low-pressure chamber (4)is in fluid communication with the inside of the test container (21) through the aperture (3), wherein thesample reference pressures in the sample region (2) are achieved by providing the sample reference pressures in the test container (21).

5. The method according to any one of claims 1 to 4, comprising the step of providing in the end surface(S) at least one gas outlet (5), which is arranged to direct gas from a gas supply device (20) into a volumebetween the end surface (S) and the sample surface (Ss), and wherein the sample surface pressure is provided by supplying gas from said gas supply device (20) to said at least one gas outlet (5).

6. 26. The method according to claim 5, comprising the steps of- measuring at predetermined time intervals the pressure inside the low-pressure chamber (4), and- controlling in a closed loop the gas supply device (20) in such a way that the pressure inside the low- pressure chamber (4) is kept constant, thereby keeping the pressure at the sample surface (Ss) constant.

7. The method according to any one of the preceding claims, wherein the distance between the sample surface (Ss) and the aperture 3 is maintained at less than 1 mm, preferably less than 300 um.

8. A computer program for monitoring a sample pressure at a sample surface (Ss) of a sample (1)placed in a sample region (2), which sample surface (Ss) is facing an aperture (3) in a wall (6) separatingthe sample region (2) from a low-pressure chamber (4) which is vacuum pumped, wherein a sample pressure may be provided with a gas supply device (20), and wherein the pressureinside the low-pressure chamber (4) may be measured with a first pressure measuring means (G1),wherein said computer program comprises instructions which, when executed by at least oneprocessor cause the at least one processor to carry out the steps of - receiving from a second pressure measuring means (G2) a number of different sample referencepressure values, - receiving from the first pressure measuring means (G1) a chamber reference pressure value,indicating the pressure in the low-pressure chamber (4), for each one of the sample referencepressures, to determine a relationship between the sample reference pressures and the chamberreference pressures, - after receiving a signal that a sample (1) has been arranged with the sample surface (Ss) facing theaperture (3) at a distance from the aperture (3), controlling the gas supply device (20) to provide a gaswith a sample surface pressure at the sample surface, - receiving from the first pressure measuring means (G1) a chamber pressure value, indicating thepressure inside the low-pressure chamber (4), and - determining the sample surface pressure using the chamber pressure value and the determined relationship between the sample reference pressures and the chamber reference pressures.

9. The computer program according to claim 8, which also comprises instructions which, whenexecuted by at least one processor cause the at least one processor to carry out the steps of- before the step of receiving a number of different sample reference pressure values, controlling the gas supply device (20) to supply gas to provide the number of different sample reference pressures.

10. The computer program according to claim 8 or 9, which also comprises instructions which, whenexecuted by at least one processor cause the at least one processor to carry out the steps of - receiving a desired sample surface pressure value, 3- controlling in a closed loop the gas supply device (20) to provide a gas flow which results in a sample surface pressure which is equal to the desired pressure value.

11. The computer program according to claim 8 or 9, wherein the distance (d) between the samplesurface (Ss) and the aperture (3) may be controlled with a positioning system (Ps), wherein thecomputer program also comprises instructions which, when executed by at least one processor causethe at least one processor to carry out the steps of - receiving a desired pressure value, - controlling in a closed loop the positioning system (Ps) such that a sample surface pressure which is equal to the desired pressure value is achieved.

12. Computer-readable storage medium carrying a computer program for monitoring a sample pressure at a sample surface (Ss) of a sample (1), according to claim 8, 9 or 10.