A SLIT VALVE ASSEMBLY FOR USE IN A VACUUM CHAMBER, FOR EXAMPLE IN A VACUUM CHAMBER OF A SUBSTRATE PROCESSING SYSTEM CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims priority of NL application 2033346 which was filed on 18 October 2022 and EP application 23160068.5 which was filed on 03 March 2023, and which are incorporated herein in their entirety by reference. BACKGROUND OF THE INVENTION [0002] In e.g. wafer substrate vacuum processing systems, it is common to arrange vacuum process chambers in a cluster, in-line, or a combination of cluster/in-line arrangements to process wafer substrates. These systems may process wafer substrates in single or batch substrate fashion. During processing, wafer substrates may be transferred to and from vacuum process chambers in which vacuum must be maintained or established. To allow access to the inside of the vacuum process chamber, and to enable vacuum operation, a substrate transfer port is formed in one of the side walls of the vacuum process chamber housing. [0003] Usually such substrate transfer port has the shape of a slit, which can be parallel aligned with the horizontal, in order to allow the wafer substrate to enter the vacuum process chamber. Hereto the substrate transfer port is opened and closed (e.g., sealed) by a slit valve door by means of an actuator. In the open position, the substrate transfer port is clear of the slit valve door and one or more wafer substrates may be transferred through the substrate transfer port. [0004] With one or more wafer substrates being positioned in the vacuum process chamber and the substrate transfer port being closed and sealed by the slit valve door, the vacuum process chamber is put under vacuum. In a common application, the slit valve door is pressed against the substrate transfer port by means of the actuator in order to maintain a proper sealing and the vacuum applied in the vacuum process chamber. [0005] Apart from stray particles (air molecules) entering the vacuum process chamber during these opening and closing actions, the seal of the slit valve door is continuously subjected to pressures, deformation forces and temperature changes, and the wear of the seal material causes seal particles to come loose from the seal. Such particles originating from the seal due to wear can be divided in ballistic particles and airborne particles, and both types of particles exhibit a different propagation behavior through the vacuum process chamber. It is clear, that such stray particles (e.g. stray air molecules) entering the vacuum process chamber may disrupt the substrate processes, such as e.g. photolithography or metrology processes being conducted on the wafer substrates. [0006] It is the object of the disclosure to prevent such stray particles from landing on the wafer substrates and adversely affecting or disrupting the substrate processes being performed therein. SUMMARY OF THE INVENTION [0007] In order to meet this objective a slit valve assembly for use in a vacuum chamber, for example in a vacuum chamber of a substrate processing system is proposed, wherein the slit valve assembly comprises a housing having sidewalls and at least one substrate transfer port formed therein, the housing having an interior volume defined by the sidewalls; a slit valve door disposed within the housing and positionable between an open position, wherein the slit valve door is clear of the substrate transfer port and a closed position, wherein the slit valve door abuts against and seals the substrate transfer port along a sealing circumference; an actuator coupled to the slit valve door and operable to move the slit valve door between the open and closed positions; and means in the form of stray particle interacting elements to reduce particle kinetic energy of stray particles. Such stray particles may originate from the slit valve door. Additionally, or alternatively, such particles may enter the interior volume through at least one substrate transfer port. [0008] The disclosure is directed to implementing stray particle interacting elements, which prevent any stray particle, in particular so-called ballistic particles, from arriving or landing on the wafer substrate. This is achieved through interaction with those stray particle, wherein the interaction (contact through collision, deflection, bouncing, or trapping) causes the stray particle to lose particle kinetic energy and thus propagation speed. The deceleration in propagation speed to nearly nil (cm/sec) will prevent any stray particle, in particular so-called ballistic particles coming loose from the door seal, from arriving or landing on the wafer substrate. [0009] In an example, the means in the form of stray particle interacting elements are mounted to the slit valve door, and in particular the means in the form of stray particle interacting elements are mounted to the slit valve door at a slit valve door side facing the substrate transfer port. In another example according to the disclosure, the means in the form of stray particle interacting elements are mounted to the slit valve door at a slit valve door side opposite the substrate transfer port. [0010] In particular, in that latter example, the means in the form of stray particle interacting elements comprise at least one wall element extending from the slit valve door towards the interior volume, wherein the at least one wall element is provided with a cavity facing the substrate transfer port. [0011] In a further advantageous example, the means in the form of stray particle interacting elements are mounted at a housing side wall at a side facing the interior volume, in particular the means in the form of stray particle interacting elements are mounted near at least one substrate transfer port. In a further detail of that latter example, the means in the form of stray particle interacting elements comprise at least one wall element extending from the housing side wall towards the interior volume, wherein a free end of the at least one wall element is facing the slit valve door at a slit valve door side opposite the substrate transfer port. [0012] In all examples according to the disclosure outlined above, a stray particle interacting surface of the means in the form of stray particle interacting elements may be provided with a sticky material, such as a polymer layer with high adhesion properties. Alternatively, a stray particle interacting surface of the means may be provided with a particle entrapment layer, such as mesh layer or mesh fabric. [0013] All these examples result in a modified slit valve assembly, wherein the interaction (contact through collision, deflection, bouncing, or trapping) causes the stray particle to lose particle kinetic energy and thus propagation speed, and thus prevent any stray particle, in particular so-called ballistic particles, from arriving or landing on the wafer substrate. BRIEF DESCRIPTION OF THE DRAWINGS [0014] The invention will now be discussed with reference to the drawings, which show in: [0015] Figure 1 an example of a slit valve assembly according to the state of the art; [0016] Figure 2 an example of a slit valve door of a slit valve assembly according to the disclosure; [0017] Figure 3 a further detail of an example of a slit valve gate of a slit valve assembly according to the disclosure; [0018] Figure 4 yet a further detail of an example of a slit valve gate of a slit valve assembly according to the disclosure; [0019] Figure 5 yet a further detail of an example of a slit valve gate of a slit valve assembly according to the disclosure. DETAILED DESCRIPTION OF THE DRAWINGS [0020] For a proper understanding of the invention, in the detailed description below corresponding elements or parts of the invention will be denoted with identical reference numerals in the drawings. [0021] Figure 1 depicts an example of a slit valve assembly according to the state of the art, which is denoted with reference numeral 100. In e.g. wafer substrate vacuum processing systems, it is common to arrange vacuum process chambers in a cluster, inline, or a combination of cluster/in-line arrangements to process wafer substrates. These systems may process wafer substrates in single or batch substrate fashion. During processing, wafer substrates 60 may be transferred to and from vacuum process chambers, here marked with reference numeral 200 in which vacuum must be maintained or established. To allow access to the inside of the vacuum process chamber 200, and to enable vacuum operation, a substrate transfer port 11 is formed in one of the side walls 12 of the vacuum process chamber housing 10. [0022] Usually such substrate transfer port 11 has the shape of a slit, which can be parallel aligned with the horizontal, in order to allow the wafer substrate 60 to enter the vacuum process chamber 200. Hereto the substrate transfer port 11 is opened and closed (e.g., sealed) by a slit valve door 20 by means of an actuator (actuator means) 30. In an example, the slit valve door 20 may be mounted to an actuator arm 30a, which can be displaced by the actuator 30 in a so-called L-motion. The L-motion exerted on the slit valve door 20 starts with a displacement in a horizontal direction away from the substrate transfer port 11, resulting in a disengagement of the slit valve door 20 from the substrate transfer port 11. In a subsequent displacement, the actuator arm 30a is retracted, or displaced in a downward vertical direction, and the slit valve door 20, now free from the substrate transfer port 11, is displaced in a likewise vertical manner and the substrate transfer port 11 is opened. [0023] In the open position, the substrate transfer port 11 is clear of the slit valve door 20 and one or more wafer substrates 60 may be transferred through the substrate transfer port 11 into the vacuum process chamber 200. With one or more wafer substrates 60 being positioned in the vacuum process chamber 200, the substrate transfer port 11 is closed, during which the slit valve door 20 is displaced is a similar L-motion, albeit in the reversed sequence. The actuator arm 30a is displaced by the actuator 30 in the opposite, upward direction, and the slit valve door 20 is pressed towards and against the substrate transfer port 11 in a sealed manner. [0024] The vacuum process chamber 200 is always under vacuum and the chamber upstream of the substrate transfer port 11 is under vacuum or under ambient pressure conditions. Accordingly, prior to the opening of the slit valve door 20 there is still a pressure difference across the substrate transfer port 11 causing stray particles to enter the vacuum process chamber 200. It is desirable to prevent stray particles (e.g. stray air molecules denoted with reference numeral 70) from entering the vacuum process chamber 200 and to prevent them from disrupting the substrate processes, such as e.g. photolithography or metrology processes being conducted on the wafer substrates 60. [0025] In a common application, the slit valve door 20 is provided with a door seal 20z applied at and round its outer valve door circumference. Upon closing of the substrate transfer port 11, the door seal 20z is pressed against the outer circumference 11z of the substrate transfer port 11 by means of the actuator means 30 in order to maintain a proper sealing and the vacuum applied in the vacuum process chamber 200. Apart from stray particles (air molecules) 70 entering the vacuum process chamber 200 during these opening and closing actions (due to the small pressure difference across the substrate transfer port 11), the door seal 20z of the slit valve door 20 is continuously subjected to pressures, deformation forces and temperature changes, and the wear of the seal material causes seal particles to come loose from the seal. Such particles originating from the seal due to wear can be divided in ballistic particles and airborne particles, and both types of particles exhibit a different propagation behavior through the vacuum process chamber. [0026] For the sake of clarity and explanation, such ballistic particles and airborne particles originating from the door seal 20z are also denoted with reference numeral 70. In fact, it is noted that throughout the specification of this patent application, reference numeral 70 can denote stray air molecules entering the vacuum process chamber 200 through the substrate transfer port 11 (from outside) and/or any ballistic and/or airborne particles released from the door seal 20z due to the repeating opening and closing actions of the slit valve door 20. [0027] It is the object of the disclosure to prevent such stray particles 70 from landing on the wafer substrates 60 present in the vacuum process chamber 200 and adversely affecting or disrupting the substrate processes being performed therein. [0028] In Figure 2 a first example of a slit valve assembly according to the disclosure is depicted. In a similar fashion as with the prior art example of Figure 1, such slit valve assembly is used in a vacuum chamber, for example in a vacuum chamber of a substrate processing system. The slit valve assembly of Figure 2 is denoted with reference numeral 100’ and comprises a housing 10 with sidewalls 12 and at least one substrate transfer port 11 formed therein. The sidewalls 12 of the housing 10 defined an interior volume, which functions as a vacuum process chamber 200. [0029] A modified slit valve door 20’ is disposed within the housing 10 and is positionable between an open position, wherein the slit valve door 20’ is clear of the substrate transfer port 11 and a closed position, wherein the slit valve door 20’ abuts with its door circumference against a port circumference 11z. The slit valve door 20’ is provided with a first valve door side 20a and a second valve door side 20b, opposite from the first valve door side 20a. The first valve door side 20a is facing the substrate transfer port 11 and the outside environment of the slit valve assembly 100’, and can also be marked as the outer valve door side 20a. The second valve door side 20b is facing the vacuum process chamber 200 and be mentioned as the inner valve door side 20b. [0030] The door circumference of the first, outer valve door side 20a of the slit valve door 20’ is provided with a door seal 20z, which abuts and seals the substrate transfer port 11 along the sealing circumference 11z positioned at the inner side of the side wall 12 facing the vacuum process chamber 200. [0031] As in the prior art example of Figure 1, an actuator 30 is coupled via an actuator arm 30a to the slit valve door 20’. In a similar fashion, known in the art, the actuator arm 30a can be displaced by the actuator 30 in the so-called L-motion outlined above. For opening, the slit valve door 20’ is displaced or disengaged in a horizontal manner away from the substrate transfer port 11 and subsequently displaced in a vertical direction, resulting in the substrate transfer port 11 to be opened. In the open position, the substrate transfer port 11 is clear of the slit valve door 20 and one or more wafer substrates 60 may be transferred through the substrate transfer port 11 into the vacuum process chamber 200. With one or more wafer substrates 60 being positioned in the vacuum process chamber 200, the actuator arm 30a is displaced by the actuator 30 in the opposite, vertical, upward direction, and subsequently the slit valve door 20’ is pressed (in a horizontal direction) against the substrate transfer port 11 in a sealed manner. Subsequently, with the vacuum process chamber 200 being under vacuum, several photolithography processes can be performed on the wafer substrates 60. [0032] In order to prevent stray particles 70 (either stray air molecules entering the vacuum process chamber 200 through the substrate transfer port 11 from the outside environment and/or any ballistic and/or airborne particles 70 released from the door seal 20z) from landing on the wafer substrates 60 present in the vacuum process chamber 200 and adversely affecting or disrupting the substrate processes being performed therein, the slit valve assembly 100’ of Figure 2 further comprises means in the form of stray particle interacting elements 50, which are structured to reduce the particle kinetic energy of those stray particles 70 entering the interior volume of the vacuum process chamber 200. [0033] The stray particle interacting elements 50 are aimed at preventing any stray particle 70, in particular so-called ballistic particles originating from the door seal 20z, from arriving or landing on the wafer substrate 60 present in the vacuum process chamber 200. This is achieved through interaction with those stray particles 70, wherein the interaction (contact through collision, deflection, bouncing, or trapping) causes the stray particle 70 to lose particle kinetic energy and thus propagation speed. Such stray particle with lost particle kinetic energy and propagation speed and thus incapable of arriving or landing on the wafer substrate 60 is denoted with reference numeral 70’ (depicting the imaginary propagation path of the trapped or slowed down stray particle). [0034] In a first and second example, the stray particle interacting elements 50 are mounted to the modified slit valve door 20’ according to the disclosure, and are denoted with reference numeral 501 and 502. [0035] In the first example, the stray particle interacting elements 501 are mounted to the modified slit valve door 20’ at the first, outer slit valve door side 20a facing the substrate transfer port 11. As shown in Figure 2 and in more detail in Figure 3, the stray particle interacting element 50 ₁ is formed as a shielding plate mounted at the first, outer slit valve door side 20a and between (surrounded by) the circumferential door seal 20z. Accordingly, between the shielding plate 501 and the circumferential door seal 20z a small intermediate gap 401 (Figure 3) is formed in which stray particles 70 (denoted as 701 in Figure 3) are trapped. Due to the repetitive collisions and bouncing interactions between the trapped stray particle 70 and the shielding plate 50 ₁ within the gap 40 ₁ , any trapped stray particle 70 ₁ will quickly lose particle kinetic energy and particle propagation speed. [0036] In another, second example according to the disclosure, the stray particle interacting elements are denoted with 50 ₂ and are mounted to the modified slit valve door 20’ at a slit valve door side opposite the substrate transfer port 11, in particular at a slit valve door side facing towards the vacuum process chamber 200. In particular, the stray particle interacting elements 502 are shaped as a wall element or plate element 502 that extends from the slit valve door 20’ into the inner volume formed by the vacuum process chamber 200. In the example of Figure 2 and 3, plate element 502 extends from the slit valve door 20’ in a direction into the vacuum process chamber 200, yet parallel to the inner surface of the side wall 12 of the housing 10 in which the substrate transfer port 11 is present. [0037] During opening and closing actions of the modified slit valve door 20’, the plate element 50 ₂ is displaced in the L-motion explained above, and it’s downward and upward vertical direction is close and parallel to the inner surface 12a of the side wall 12 of the housing 10 in which the substrate transfer port 11 is present. In that latter example, the stray particle interacting elements 502 comprise at least one wall element 502 extending from the slit valve door 20’ towards the interior volume 200. The wall element 502 is provided with a cavity 502-a facing the substrate transfer port 11 or more in particular facing the inner surface 12a of the side wall 12 of the housing 10 in which the substrate transfer port 11 is present. The inner surface 12a of housing side wall 12 is facing the vacuum process chamber 200. [0038] Together with the intermediate gap 402 (see Figure 3) formed between the wall element 502 and the inner surface 12a of the side wall 12 of the housing 10 in which the substrate transfer port 11 is present, the cavity 502-a functions as a trapping mechanism (trapping maze) for any stray particle 70 (denoted as 702 in Figure 3). Thus, e.g. any so-called ballistic particle 702 coming loose from the door seal 20z due to the repetitive opening and closing actions of the slit valve door 20’, will be trapped. Because of the repetitive collisions and bouncing interactions between the trapped stray particle 70 ₂ and the wall element 50 ₂ and the inner surface 12a of the side wall 12 of the housing 10 in which the substrate transfer port 11 is present, the stray particle 70 will quickly lose particle kinetic energy and particle propagation speed. Any arrival or landing of such decelerated stray particle 702 on the wafer substrate 60 present in the vacuum process chamber 200 is thus prevented, depicted as reference numeral 702’ in Figure 3, showing the imaginary propagation path of such ballistic particle 70 in the event it has not been trapped and decelerated with the assistance of the stray particle interacting element 502. [0039] The material from which the shielding plate 501 and/or the wall element 502 is manufactured may be the same material from which the slit valve door 20’ is manufactured. Alternatively, the shielding plate 50 ₁ and/or the wall element 50 ₂ can be an integral part of the slit valve door 20’. [0040] In a third example, the stray particle interacting elements are denoted with reference numeral 503 and are mounted at an inner surface of a housing side wall 12 facing the interior volume 200. As shown in Figure 2 and 3, the stray particle interacting elements 503 are mounted near the at least one substrate transfer port 11. In particular, the stray particle interacting elements 503 comprise at least one wall element 50 ₃ . A first wall part 50 ₃ -a of the wall element 50 ₃ extends from the inner surface 12a of the housing side wall 12 towards the interior volume of the vacuum process chamber 200. Additionally and as shown, a free wall end 50 ₃ -b of the at least one wall element 50 ₃ is formed around the slit valve door 20’, such that free end 50 ₃ -b faces the second, inner slit valve door side 20b of the slit valve door 20’ opposite the substrate transfer port 11 and facing the inner volume of the vacuum process chamber 200. [0041] Likewise, the spatially formed wall element 503 formed of the first wall part 503-a and the free wall end 503-b formed at an angle, preferably at a perpendicular angle with the first wall part 503- a, functions as a shielding hood or trapping enclosure for the slit valve door 20’. A small intermediate gap 40 ₃ is formed between the spatially formed wall element 50 ₃ and the slit valve door 20’. In the gap 40 ₃ any stray particle 70 (denoted as 70 ₃ in Figure 3) will become trapped. Due to the repetitive collisions and bouncing interactions between the trapped stray particle 703 and the spatially formed wall element 503 and the slit valve door 20’, the trapped stray particle 703 will quickly lose particle kinetic energy and particle propagation speed. [0042] The material from which the spatially formed wall element 50 ₃ (the first wall part 50 ₃ -a and the free wall end 50 ₃ -b) is manufactured may be the same material from which the housing wall 12 is manufactured, and alternatively can be an integral part of the housing wall 12. [0043] Alternatively, as shown in Figure 4, in a fourth example, the stray particle interacting elements are denoted with reference numeral 504 and are of a similar construction as the stray particle interacting elements 503. However, unlike the third example, in this fourth example the stray particle interacting elements 504 are mounted to a surface of the slit valve door 20’ which faces the interior volume 200. As shown in Figure 4, the stray particle interacting elements 50 ₄ are also mounted near the at least one substrate transfer port 11. In particular, the stray particle interacting elements 50 ₄ comprise at least one wall element 50 ₄ . A first wall part 50 ₄ -a of the wall element 50 ₄ projects from the slit valve door 20’ and away from the inner surface 12a of the housing side wall 12 towards the interior volume of the vacuum process chamber 200. The first wall part 504-a of the wall element 504 is connected directly with the slit valve door 20’ by means of mounting point 504-c. [0044] Additionally and as shown, a free wall end 504-b of the at least one wall element 503 is formed around the slit valve door 20’, such that free end 504-b faces the second, inner slit valve door side 20b of the slit valve door 20’ opposite the substrate transfer port 11 and facing the inner volume of the vacuum process chamber 200. As with the third example, the spatially formed wall element 50 ₄ formed of the first wall part 50 ₄ -a and the free wall end 50 ₄ -b formed at an angle, preferably at a perpendicular angle with the first wall part 504-a, functions as a shielding hood or trapping enclosure for the slit valve door 20’. A small intermediate gap 403 is formed between the spatially formed wall element 504 and the slit valve door 20’. In the gap 403 any stray particle 70 (denoted as 703 in Figure 4) will become trapped. [0045] The material from which the spatially formed wall element 50 ₄ (the first wall part 50 ₄ -a and the free wall end 50 ₄ -b) is manufactured may be the same material from which the housing wall 12 is manufactured, and alternatively can be an integral part (together with the mounting point 50 ₄ -c) of the housing wall 12. [0046] In all four examples according to the disclosure outlined above, an surface of the stray particle interacting elements 501-502(502-a)-503(503-a, 503-b)-504(504-a, 504-b) that can interact with a stray particle 70 may be provided with a layer of a sticky material 501-z, 502-z, 503-z and 504-z, respectively, such as a polymer layer with high adhesion properties. [0047] Alternatively, as shown in more detail in Figure 5, one or all stray particle interacting surfaces of the stray particle interacting elements 50 ₁ -50 ₂ (50 ₂ -a)-50 ₃ (50 ₃ -a, 50 ₃ -b) that interact with a stray particle 70 means can be provided with a particle entrapment layer, such as a mesh layer or a mesh fabric. The several examples of particle entrapment layers can be denoted with reference numerals 501- y, 502-y, and 503-y, respectively, depending on whether they are part of or mounted to the first, second or third example of the stray particle interacting elements 501-502(502-a)-503(503-a, 503-b)- 504(504-a, 504-b). For illustrative purposes, in Figure 5, only the particle entrapment layers 501-y and 503-y are shown, being part of the first and third example of the stray particle interacting elements 50 ₁ and 50 ₃ (50 ₃ -a, 50 ₃ -b). [0048] However, it should be noted that in a similar fashion the stray particle interacting elements 502 and 504 (the second example of the disclosure shown in Figure 2 and 3, and the fourth example of the disclosure shown in Figure 4) can be equally provided with a particle entrapment layer, such as a mesh layer or a mesh fabric. [0049] The several examples of the particle entrapment layers 501-y, 502-y, and 503-y can be constructed as a metal mesh material layer, with an open, three-dimensional structure. Any stray particle 70 entering or impacting the metal mesh material layer will be entrapment and will lose its particle kinetic energy and particle propagation speed, due to interaction, collision, or impact with the several mesh filaments of the metal mesh layer. [0050] Similarly, when using a mesh fabric as the particle entrapment layers 501-y, 502-y, and 503- y, the mesh fabric can be from a polyester, a nylon or spandex. Likewise, its open, three-dimensional structure entraps any stray particle 70 upon entering, causing loss of particle kinetic energy and particle propagation speed, due to interaction, collision, or impact with the several mesh filaments of the fabric. [0051] All these examples result in a modified slit valve assembly 100’, wherein the interaction (contact through collision, deflection, bouncing, or trapping) causes the stray particle to lose particle kinetic energy and thus propagation speed. The deceleration in propagation speed to nearly nil (cm/sec) will prevent any stray particle 70, in particular so-called ballistic particles coming loose from the door seal 20z, from arriving or landing on the wafer substrate 60. LISTING OF REFERENCE NUMERALS 100 slit valve assembly (state of the art) 100’ slit valve assembly (according to the disclosure) 200 vacuum process chamber 10 housing 11 substrate transfer port (state of the art) 12 housing side wall(s) 12a surface of housing side wall (facing the vacuum process chamber) 11z sealing circumference of substrate transfer port 20 slit valve door 20’ slit valve door (according to the disclosure) 20a first slit valve door side (facing the substrate transfer port) 20b second slit valve door side (facing the vacuum process chamber) 20z sealing circumference of slit valve door 30 actuator 30a actuator arm 40 ₁ -40 ₂ -40 ₃ trapping cavity (1 ^st -2 ^nd -3 ^rd example according to the disclosure) 50 ₁ -50 ₂ -50 ₃ -50 ₄ stray particles interacting element(s) (1 ^st -2 ^nd -3r ^d -4 ^th example according to the disclosure) 501 shielding plate (1 ^st example) 501-y particle entrapment mesh layer or fabric (1 ^st example) 501-z sticky surface layer (1 ^st example) 502 wall element (2 ^nd example) 50 ₂ -a cavity in wall element (2 ^nd example) 50 ₂ -y particle entrapment mesh layer or fabric (2 ^nd example) 50 ₂ -z sticky surface layer (2 ^nd example) 503 wall element (3 ^rd example) 503-a first wall part of wall element 503 (3 ^rd example) 503-b free end of wall element 503 (3 ^rd example) 503-y particle entrapment mesh layer or fabric (3 ^rd example) 503-z sticky surface layer (3 ^rd example) 50 ₄ wall element (4 ^th example) 50 ₄ -a first wall part of wall element 50 ₄ (4 ^th example) 50 ₄ -b free end of wall element 50 ₄ (4t ^h example) 504-c mounting point of wall element 504 (4 ^th example) 504-z sticky surface layer (4 ^th example) 60 wafer 70(70 ₁ -70 ₂ -70 ₃ ) stray particle(s) 70’(70 ₂ ’) neutralized stray particle (imaginary propagation path)