Method for determining the pressure at the sample surface

Abstract

A method for monitoring the sample pressure at a sample surface (Ss) of a sample (1) placed in a sample area (2) is described, wherein the sample surface (Ss) faces a hole (3) located in a wall (6) separating the sample area (2) from an evacuated low-pressure chamber (4). The method comprises the following steps: determining a relationship between a sample reference pressure at the sample surface (Ss) and a chamber reference pressure in the low-pressure chamber (4); arranging the sample (1) so that the sample surface (Ss) faces the hole (3) at a distance from the hole (3) (103); providing a sample pressure (104); measuring a chamber pressure (105) in the low-pressure chamber (4); and determining a sample pressure (106) using the measured chamber pressure and the determined relationship between the sample reference pressure and the chamber reference pressure.

Description

Technical Field

[0001] The present invention relates to a method for determining the pressure at the surface of a sample. More specifically, the present invention relates to a method for determining the pressure of a sample facing a hole located in a wall separating the sample area from an evacuated low-pressure chamber. Background Art

[0002] In prior art systems for ambient pressure photoelectron emission spectroscopy (APXPS) and ambient pressure photon emission spectroscopy (APPES), a sample is irradiated while a high voltage is provided at the sample to provide photoelectrons or electrons, for example, from an Auger process. The photoelectrons are collected in an electrostatic lens system with varying degrees of evacuation. The electrostatic lens system focuses the electrons to the entrance of the measurement region. In order to achieve a high vacuum in the electrostatic lens system, the aperture leading to the electrostatic lens system must be small.

[0003] In the prior art, APPES is performed in three ways: 1) the sample is placed in the chamber and the whole chamber is raised to ambient pressure in the millibar range, which is called the backfill method; 2) a variation of the backfill method, in which the chamber is replaced so that different experimental groups use different chambers, so this method is called the replaceable chamber method; and 3) the in situ gas cell encloses the sample in the front hole of the analyzer. All of these tree layouts can operate in flow mode, in which the gas is extracted through the outlet at the same time as entering, or the gas is extracted only through the front cone of the analyzer and the suction device.

[0004] A variation of the gas cell approach is described in J. Knudsen et al., "Aversatile instrument for ambient pressure x-ray photoelectron spectroscopy: The Lund cell approach" (Published in Surface Science, 646 (2016) 160-169). There, Knudsen et al. describe an alternative ambient pressure chamber in which a gas flow is directed to the sample. The gas outlet is arranged around a hole for collecting electrons to a detector.

[0005] When performing APPES, the sample must be arranged close to the hole. The reason for this is that, in the case of high pressure, the mean free path of the electrons is short. For example, at a pressure of 1 bar, the ideal distance between the sample surface and the hole for XPS in carbon monoxide is 30 μm, because the mean free path of 10 keV electrons in carbon monoxide is about 30 μm at a pressure of 1 bar. Therefore, a distance of 30 μm will enable a reasonable portion of the photoelectrons to enter the hole. Due to this distance between the sample and the hole, it is difficult to measure or predict the pressure at the sample surface. This is especially true when the pressure at the sample is provided by a gas flow directed at the sample surface.

[0006] The general prior art of pressure estimation is derived from Ogletree et al., Review of Scientific Instruments (2002) 73, 3872, in which the pressure distribution between the sample chamber and the electrostatic lens chamber through the hole is estimated using a simple analytical function. This pressure distribution is also discussed in H. Bluhm, Journal of Electron Spectroscopy and Related Phenomena, 177 (2010), 71-84, and J. Kahk et al., Journal of Electron Spectroscopy and Related Phenomena, 205 (2015) 57-65. In the literature, when the distance between the sample surface and the hole is 1 hole diameter, the pressure at the sample surface is estimated to be 95% of the pressure measured in the sample chamber, and when the distance between the sample surface and the hole is 2 times the hole diameter, the pressure at the sample surface is estimated to be 98% of the pressure measured in the sample chamber. Kahk et al. calculated that the pressure at the sample surface varies with pressure. For a distance of 1 diameter, the higher the pressure, the more accurate the pressure reading, but for low pressures, a distance of 2 diameters will be more accurate. Knudsen et al. estimated the pressure at the sample based on pressure measurements in a pressure cell at a distance from the sample surface and theoretical calculations. They concluded that the pressure at the sample deviated by less than a factor of 4 compared to the measurement location, which they considered to be acceptable. Summary of the invention

[0007] It is an object of the present invention to provide a method for determining the pressure at a sample surface arranged facing an aperture leading to a volume in which the pressure is lower than the pressure at the sample surface, which method is an alternative to methods according to the prior art.

[0008] Another object of the present invention is to provide a method for determining the pressure at a sample surface, which sample surface faces a hole leading to a volume, the pressure in the volume is lower than the pressure at the sample surface, and wherein the pressure at the sample surface is provided by a gas flow directed toward the sample surface.

[0009] Another object of the present invention is to provide a method for monitoring the pressure at a sample surface, wherein the sample surface faces a hole leading to a volume, the pressure in the volume being lower than the pressure at the sample surface, and wherein the pressure at the sample surface is provided by a gas flow directed toward the sample surface.

[0010] Another object of the present invention is to provide a method for measuring the pressure at the surface of a sample when the pressure at the surface of the sample is higher than the pressure of the surrounding environment (eg, the chamber in which the sample is located).

[0011] At least one of these objects is achieved by a method according to the first aspect of the invention.

[0012] Further advantages are achieved through the features of other aspects of the invention.

[0013] According to a first aspect of the invention, there is provided a method for monitoring the sample pressure at a sample surface of a sample placed in a sample area, the sample surface facing a hole in a wall separating the sample area from a vacuumed low pressure chamber. The method is characterized in that the method comprises the following steps:

[0014] Step a) providing a gas with a plurality of different sample reference pressures in the sample area,

[0015] Step b) for each sample reference pressure, measuring the chamber reference pressure generated in the low pressure chamber to determine the relationship between the sample reference pressure and the chamber reference pressure,

[0016] step c) arranging the sample so that the sample surface faces the hole at a distance from the hole,

[0017] Step d) providing a gas having a sample surface pressure at the sample surface,

[0018] Step e) measuring the chamber pressure in the low pressure chamber,

[0019] step f) determining the sample surface pressure using the measured chamber pressure and the determined relationship between the sample reference pressure and the chamber reference pressure,

[0020] The pressure in the low-pressure chamber is lower than the reference pressure on the surface of each sample.

[0021] The calibration process depends on the type of gas used. Therefore, it can be performed separately for each gas.

[0022] The sample can be arranged in a vacuum chamber. In this case, a sample surface reference pressure can be provided by setting a different static pressure in the vacuum chamber.

[0023] The sample reference pressure is a known pressure that is applied as a static pressure at least in the sample area. The sample surface pressure is set by providing gas to the sample surface. If the sample surface is arranged close to the hole, the hole will affect the pressure at the sample surface. However, the inventors have realized that the pressure in the low-pressure chamber is affected by the pressure close to the hole. Therefore, for a specific pressure in the low-pressure chamber, the external pressure of the low-pressure chamber at the hole is the same when the sample is arranged close to the hole and when there is no sample.

[0024] The maximum dimension of the hole in the plane of the end surface may be less than 1 mm. The maximum dimension of the hole in the plane of the end surface is preferably less than 300 μm, and may be less than 100 μm. For a circular hole, the maximum dimension is equal to the diameter of the hole. A small hole must be used to be able to have a small distance between the hole and the sample, which is desirable for a photon spectrometer at high sample pressure. The relationship between the diameter of the hole and such a distance, which is the distance between the hole and the sample surface, has been discussed above. Therefore, the distance between the hole and the sample surface should not be less than the diameter of the hole. Therefore, for the case where the distance between the sample surface and the hole is very small, the diameter of the hole must be very small.

[0025] The distance between the sample and the well should preferably be kept no greater than 3 times the diameter of the well in order for the method to work as well as possible.

[0026] The low pressure chamber can be an electrostatic lens for focusing the electrons.

[0027] For the method to work properly, when the sample surface pressure is applied, the pressure should decrease in the direction outward from the volume between the sample and the pore. For sample surface pressures above 1 bar, the sample can be placed in the ambient pressure, assuming that the gas can escape into the ambient pressure. However, for lower sample surface pressures, the sample and walls should be arranged in an evacuated chamber to provide a pressure gradient outward from the volume between the pore and the sample surface.

[0028] In step b) and step e), a pressure of less than 1 mbar, preferably less than 10 -2 bar pressure, most preferably less than 10 -3 The latter pressure is suitable for electrostatic lenses.

[0029] In step a), each of the different sample reference pressures may be higher than 10 mbar. It is important that the sample reference pressure is much higher than the chamber pressure in the low pressure chamber.

[0030] The method may comprise, before step a), a further step of arranging a test container surrounding the sample area and the end wall so that the interior of the low pressure chamber is in fluid communication with the interior of the test container via an aperture, wherein a sample reference pressure in the sample area is achieved by providing the sample reference pressure in the test container. With such a test container, different sample reference pressures can be provided more easily than if the entire vacuum chamber had to be filled. Also, the use of the test container allows the use of pressures higher than 1 bar. Due to the structure of ordinary vacuum chambers, pressures higher than 1 bar cannot be applied in ordinary vacuum chambers. This is due to the fact that the vacuum chamber is designed only for maintaining a low pressure inside. Applying too much pressure in the vacuum chamber may lead to, for example, rupture of windows in the vacuum chamber.

[0031] The method may comprise the steps of providing at least one gas outlet in the end surface, the gas outlet being arranged to direct gas from a gas supply device into a volume between the end surface and the sample surface, and wherein the sample surface pressure is provided by supplying gas from the gas supply device to the at least one gas outlet. With such a gas outlet, a sample reference pressure may also be provided. The present invention is primarily applicable to situations with such a gas outlet, since such a method of providing the sample surface pressure produces a local high pressure region. In such a situation, no scheme used in the prior art can be used for estimation, and the present method is based on a pressure measurement performed at a remote location.

[0032] The method may include the following steps: measuring the pressure in the low pressure chamber at predetermined time intervals; and controlling the gas supply device in a closed loop so that the pressure in the low pressure chamber remains constant, thereby maintaining the sample surface pressure at the sample surface constant. This is an effective method for controlling the sample surface pressure.

[0033] The distance between the sample surface and the well 3 is kept less than 1 mm, preferably less than 300 μm. These distances are suitable for APPES.

[0034] According to a second aspect of the invention, a computer program is provided for monitoring the sample surface pressure at the sample surface of a sample placed in a sample area. The sample surface faces a hole in a wall that separates the sample area from a vacuumed low-pressure chamber. The sample surface pressure can be provided by a gas supply device, and the pressure in the low-pressure chamber can be measured by a first pressure measuring device. The computer program includes instructions that, when executed by at least one processor, cause the at least one processor to perform the following steps:

[0035] - receiving a plurality of different sample reference pressure values ​​from a second pressure measurement device,

[0036] - for each sample reference pressure, receiving from the first pressure measurement device a chamber reference pressure value representative of the pressure in the low-pressure chamber to determine a relationship between the sample reference pressure and the chamber reference pressure,

[0037] - after receiving a signal that the sample has been arranged with the sample surface facing the aperture at a distance from the aperture, controlling the gas supply device to provide a sample pressure,

[0038] - receiving a chamber pressure value from a first pressure measuring device, the chamber pressure value representing the pressure within the low pressure chamber,

[0039] - Determining the sample surface pressure using the chamber pressure value and the determined relationship between the sample reference pressure and the chamber reference pressure.

[0040] The above discussion on the first aspect of the invention also applies to the computer program according to the second aspect of the invention.

[0041] The computer program may further include instructions that, when executed by at least one processor, cause the at least one processor to perform the following steps:

[0042] - before the step of receiving a plurality of different sample reference pressure values, controlling the gas supply device to supply gas to provide a plurality of different sample reference pressures.

[0043] This makes the control of the process more automated, thus making the process more suitable for use in an industrial setting with minimal manual effort.

[0044] The computer program may further include instructions that, when executed by at least one processor, cause the at least one processor to perform the following steps:

[0045] - receiving the desired sample surface pressure value,

[0046] - Controlling the gas supply device in closed loop to provide a gas flow which generates a sample surface pressure equal to the desired pressure value.

[0047] In order for such a computer program to work effectively, a gas flow needs to be directed to the volume between the well and the sample.

[0048] Therefore, a computer program can provide automatic pressure setting and monitoring.

[0049] Alternatively or additionally, when the distance between the sample surface and the well can be controlled by a positioning system, the computer program may further include instructions that, when executed by at least one processor, cause the at least one processor to perform the following steps:

[0050] - receiving the desired sample surface pressure value,

[0051] - The positioning system (Ps) is controlled in closed loop to obtain a sample surface pressure equal to the desired pressure value.

[0052] Therefore, a specific distance can be set with the airflow being well controlled.

[0053] According to a third aspect, there is provided a computer readable storage medium carrying a computer program for monitoring a sample surface pressure at a sample surface of a sample according to the second aspect of the invention.

[0054] Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. The accompanying drawings are not drawn to scale. Similar features in different drawings will be represented by the same reference numerals. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 An analyzer device according to an exemplary embodiment of the present invention is shown and in which the method according to the present invention can be implemented.

[0056] Figure 2 Shown in more detail Figure 1 The end wall and the hole in the device are shown, wherein the test chamber is arranged at the end wall.

[0057] Figure 3 is a flow chart of a method according to an embodiment of the present invention. DETAILED DESCRIPTION

[0058] Figure 1A partial cross section of an apparatus 100 is shown, the apparatus being used to collect charged particles from a sample surface Ss of a sample 1 emitting particles and to determine at least one parameter related to the charged particles. The apparatus 100 comprises a sample holder 10 for holding the sample 1 in a sample region 2. The apparatus also comprises a low-pressure chamber 4 comprising a hole 3 in a wall 6 separating the sample region 2 from the low-pressure chamber 4, wherein the hole 3 is arranged to face the sample surface Ss of the sample 1 placed in the sample holder 10 so as to collect the charged particles from the sample surface Ss into the low-pressure chamber 4. A heater 18 is also arranged on the sample holder 10 and arranged to heat the sample 1. The apparatus also comprises a positioning system Ps for controlling the position of the sample holder 10 and thus controlling the distance d between the sample surface Ss and the hole 3. The apparatus also comprises a vacuum chamber 11, which surrounds the hole 3, the sample holder 10 and the positioning system Ps. The apparatus also comprises a first suction device 12 for evacuating the low-pressure chamber 4. Furthermore, the device 100 comprises at least one gas outlet 5 arranged to direct gas into the volume between the wall 6 and the sample surface Ss, and a gas supply device 20 for providing a gas flow from the at least one gas outlet 5 to provide the sample pressure. The device 100 further comprises a control unit CU and a first pressure measuring device G1 for measuring the pressure in the low pressure chamber 4. The control unit CU is connected to the first pressure measuring device G1 and the positioning system Ps. The control unit comprises an input 19 for inputting signals, which can be used to control the positioning system Ps and / or for inputting other information.

[0059] The low pressure chamber 4 is an electrostatic lens system, which includes a first end 16 and a second end 37, wherein an aperture 3 is arranged at the first end and an aperture 8 is arranged at the second end. The lens system 13 is configured to form a particle beam of charged particles emitted from the sample surface Ss and entering through the aperture 3 at the first end 16, and to transmit the charged particles to the second end 17. The device 100 also includes a measurement region 3 for determining at least one parameter related to the charged particles emitted from the sample surface Ss of the sample 1 emitting the particles. The measurement region 3 includes an inlet 8 for allowing at least a portion of the particles to enter the measurement region 3. The second end 37 is arranged at the inlet of the measurement region 3. Electrons enter the measurement region 3 through the inlet 8, and electrons entering the region between the hemispherical portions 25 in a direction close to perpendicular to the substrate 7 are deflected by the electrostatic field applied between the hemispherical portions 25, and electrons having a kinetic energy within a certain range defined by the electrostatic field will reach the detector device 9 after traveling through half a circle.

[0060] The first suction device 12 for evacuating the low-pressure chamber 4 is arranged to maintain the pressure in the low-pressure chamber at 10 -4 mbar to 10 -2 mbar, preferably below 2x10-3 For many applications (for example when the low pressure chamber is an electrostatic lens arranged to focus electrons into an electron beam), low pressure is necessary. The gas supply device 20 for providing a constant gas flow is usually arranged to provide a sample surface pressure of 10 mbar to 1 bar and higher.

[0061] The vacuum chamber 11 is evacuated by a separate second suction device 22. The background pressure in the vacuum chamber 11 is usually maintained at 10 -2 mbar to 1 mbar. Therefore, the sample surface pressure is the local pressure in the volume between the sample surface Ss and the hole 3. The second pressure measuring device G2 is arranged to measure the pressure in the vacuum chamber 11. The second pressure measuring device is connected to the control unit CU. The first suction device 12 and the second suction device 22 can be controlled by the control unit CU.

[0062] Figure 2 A sample and a test container 21 for implementing a method according to an embodiment of the invention are shown in more detail. Figure 2 In the embodiment, four gas outlets 5 are arranged symmetrically around the hole 3. Figure 2 Only two of them are shown in the cross section of FIG. 6 . The longitudinal axis L is shown extending through the hole 3 substantially perpendicularly to the end surface S of the wall 6. The test container 21 is arranged to cover the end wall 6, the gas outlet 5 and the hole 3. The gas line 23 is connected to the test container 21 and can be used to supply gas to the test container. Alternatively, the gas can be supplied to the test container through the gas outlet 5.

[0063] Figure 3 A flow chart of a method according to an embodiment of the present invention is shown. In a first step 101, a plurality of different sample reference pressures are provided in the sample area 2. This can also be achieved by providing the sample reference pressure in the vacuum chamber. The pressure in the vacuum chamber 11 can be achieved by supplying gas from a gas supply device 20 via a gas outlet 5. Due to the volume of the vacuum chamber, this may take some time. An alternative method is to arrange a test container 21 at the end wall 6 and to make the test container 21 only cover the gas outlet 5 and the hole 3. Due to the small size of the test container, such a test container 21 can be filled to the desired pressure much faster than the vacuum chamber 11. Of course, the sample reference pressure can also be provided by a gas line 23 ( Figure 2) is provided. In the case where the vacuum chamber 11 is used to provide a sample reference pressure, the second suction device 22 is of course cut off during the provision of the sample reference pressure. The second pressure measuring device G2 is used to measure the pressure in the vacuum chamber 11 or the test container 21. In the second step 102, for each sample reference pressure, the chamber reference pressure generated in the low-pressure chamber 4 is measured to determine the relationship between the sample reference pressure and the chamber reference pressure. In the third step 103, the sample 1 is arranged so that the sample surface Ss faces the hole 3 at a distance from the hole 3. The distance between the hole and the sample 1 is usually set to 10μm to 1mm, but can be several millimeters or even less than 100μm. In the fourth step 104, the sample surface pressure is provided. The sample surface pressure is preferably provided by gas supplied from the gas supply device 20 via one or more gas outlets 5. In the fifth step 105, the chamber pressure inside the low-pressure chamber 4 is measured. Finally, in a sixth step 106, the sample surface pressure is determined using the measured chamber pressure and the determined relationship between the sample reference pressure and the chamber reference pressure.

[0064] The desired sample surface pressure value may be input on the input 19. The control unit CU may be arranged to measure the pressure in the low pressure chamber 4 at predetermined time intervals using the first pressure measuring device 12. The control unit CU may also be arranged to control the gas supply device to reduce or increase the gas flow through the gas outlet 5 to maintain a chamber pressure corresponding to the desired sample surface pressure.

[0065] A computer program can be run on a CPU for monitoring the sample surface pressure at a sample surface Ss of a sample 1. The computer program includes instructions that, when executed by at least one processor, cause the at least one processor to perform the following steps:

[0066] - receiving a plurality of different sample reference pressure values ​​from the second pressure measuring device G2,

[0067] - for each sample reference pressure, receiving from the first pressure measuring device G1 a chamber reference pressure value representative of the pressure in the low-pressure chamber 4 to determine a relationship between the sample reference pressure and the chamber reference pressure,

[0068] upon receiving a signal that the 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 having the sample surface pressure at the sample surface Ss,

[0069] - receiving a chamber pressure value from the first pressure measuring device G1 , which chamber pressure value represents the pressure in the low-pressure chamber 4 ,

[0070] - Determining the sample surface pressure using the chamber pressure value and the determined relationship between the sample reference pressure and the chamber reference pressure.

[0071] The computer program may also include such instructions, which when executed by at least one processor cause the at least one processor to perform the following steps: before the step of receiving a plurality of different sample reference pressure values, control the gas supply device 20 to supply gas to provide a plurality of different sample reference pressures and receive the required pressure value on the input 19. Then, the CPU can control the gas supply device 20 in a closed loop to provide a gas flow, which results in the sample surface pressure at the sample surface Ss being equal to the required pressure value. The CPU and the control unit CU can also control the first suction device 12 and the second suction device 22.

[0072] The computer program may further include instructions that, when executed by at least one processor, cause at least one processor CPU to perform the following steps:

[0073] - receiving the desired pressure value at the input 19,

[0074] - The positioning system (Ps) is controlled in closed loop such that a sample surface pressure equal to the desired pressure value is obtained at the sample surface Ss.

[0075] This provides the possibility to maintain a fixed distance.

[0076] The embodiments described above can be modified in many ways without departing from the scope of the invention, which is limited only by the accompanying claims.

Claims

1. A method for monitoring the sample pressure at a sample surface (Ss) of a sample (1) placed in a sample area (2), said sample surface (Ss) facing a hole (3) located in a wall (6) separating said sample area (2) from an evacuated low-pressure chamber (4), characterized in that The method comprises the following steps: Step a) providing a gas (101) with a plurality of different sample reference pressures in the sample area (2), wherein each of the different sample reference pressures is higher than 10 mbar, Step b) for each of the sample reference pressures, measuring the chamber reference pressure generated in the low pressure chamber (4) to determine the relationship between the sample reference pressure and the chamber reference pressure (102), Step c) arranging the sample (1) so that the sample surface (Ss) faces the hole (3) (103) at a distance from the hole (3), wherein the distance between the sample surface (Ss) and the hole (3) is maintained at less than 300 μm, Step c1) providing at least one gas outlet (5) in the end surface (S), the at least one gas outlet being arranged to direct gas from a gas supply device (20) into a volume between the end surface (S) and the sample surface (Ss), Step d) providing a gas having a sample surface pressure at the sample surface (104), wherein the sample surface pressure is provided by supplying gas from the gas supply device (20) to the at least one gas outlet (5), Step e) measuring the chamber pressure (105) in the low pressure chamber (4), and Step f) determining a sample surface pressure (106) using the measured chamber pressure and the determined relationship between the sample reference pressure and the chamber reference pressure, Wherein, the pressure in the low-pressure chamber (4) is lower than the reference pressure of each sample.

2. The method according to claim 1, wherein: In the step b) and the step e), a pressure of less than 1 mbar is maintained in the low-pressure chamber (4).

3. The method according to claim 2, wherein: In the step b) and the step e), the low pressure chamber (4) is maintained at a pressure less than 10 -2 mbar pressure.

4. The method according to claim 3, wherein: In the step b) and the step e), the low pressure chamber (4) is maintained at a pressure less than 10 -3 mbar pressure.

5. The method according to any one of claims 1 to 4, comprising the following steps before step a): A test container (21) is arranged surrounding the sample area (2) and the wall (6) so that the interior of the low-pressure chamber (4) is in fluid communication with the interior of the test container (21) through the hole (3), wherein the sample reference pressure in the sample area (2) is achieved by providing the sample reference pressure in the test container (21).

6. The method according to any one of claims 1 to 4, comprising the following steps: - measuring the pressure in the low pressure chamber (4) at predetermined time intervals, and - Controlling the gas supply device (20) in a closed loop so that the pressure in the low pressure chamber (4) remains constant, thereby maintaining the pressure at the sample surface (Ss) constant.

7. The method according to claim 5, comprising the steps of: - measuring the pressure in the low pressure chamber (4) at predetermined time intervals, and - Controlling the gas supply device (20) in a closed loop so that the pressure in the low pressure chamber (4) remains constant, thereby maintaining the pressure at the sample surface (Ss) constant.

8. A computer program for monitoring the sample pressure at a sample surface (Ss) of a sample (1) placed in a sample area (2), said sample surface (Ss) facing a hole (3) located in a wall (6) separating said sample area (2) from an evacuated low-pressure chamber (4), in, The sample pressure can be provided by at least one gas outlet (5), the at least one gas outlet being arranged to direct gas from a gas supply device (20) into a volume between an end surface (S) and the sample surface (Ss), and wherein the pressure in the low pressure chamber (4) can be measured by a first pressure measuring device (G1), wherein the computer program comprises instructions that, when executed by at least one processor, the at least one processor performs the following steps: - controlling the gas supply device (20) to supply gas to the at least one gas outlet (5) to provide a plurality of different sample reference pressures, wherein each of the different sample reference pressures is higher than 10 mbar, - receiving a plurality of different sample reference pressure values ​​from a second pressure measuring device (G2), - for each of the sample reference pressures, receiving from the first pressure measuring device (G1) a chamber reference pressure value representing the pressure in the low-pressure chamber (4) to determine a relationship between the sample reference pressure and the chamber reference pressure, - after receiving a signal that the sample (1) has been arranged so that the sample surface (Ss) faces the hole (3) at a distance of less than 300 µm from the hole (3), controlling the gas supply device (20) to supply gas having a sample surface pressure to the at least one gas outlet (5) at the sample surface, - receiving a chamber pressure value from the first pressure measuring device (G1), the chamber pressure value representing the pressure in the low-pressure chamber (4), and - determining the sample surface pressure using the chamber pressure value and the determined relationship between the sample reference pressure and the chamber reference pressure.

9. The computer program according to claim 8, further comprising instructions that, when executed by at least one processor, cause the at least one processor to perform the following steps: - receiving the desired sample surface pressure value, - Controlling the gas supply device (20) in a closed loop to provide a gas flow which generates a sample surface pressure equal to the desired pressure value.

10. The computer program according to claim 8 or 9, wherein: The distance (d) between the sample surface (Ss) and the hole (3) can be controlled by a positioning system (Ps), wherein the computer program further comprises instructions which, when executed by at least one processor, cause the at least one processor to perform the following steps: - receive the desired pressure value, - Controlling the positioning system (Ps) in closed loop so as to obtain a sample surface pressure equal to the desired pressure value.

11. A computer readable storage medium carrying a computer program for monitoring sample pressure at a sample surface (Ss) of a sample (1) according to any one of claims 8 to 10.

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