Rotating target for extreme ultraviolet source with liquid metal

Abstract

It is an extreme ultraviolet (EUV) light source and has a vacuum chamber with a rotating target assembly therein. The rotating target assembly has an annular groove with a distal wall relative to the rotation axis. The distal wall has a porous region. With a molten metal layer on the distal wall of the annular groove in the rotating target assembly, the rotating target assembly is rotated to form a target by centrifugal force.

Description

Technical Field

[0001] [Cross - Reference to Related Applications] This application claims priority based on U.S. Provisional Patent Application No. 63 / 350868, filed on June 10, 2022, the disclosure of which is incorporated herein by reference.

[0002] This disclosure relates to extreme ultraviolet light sources.

Background Art

[0003] In next - generation projection lithography, when mass - producing integrated circuits (ICs) with a structural size of 10 nm or less, extreme ultraviolet (EUV) radiation in the range of 13.5 ± 0.135 nm corresponding to the effective reflection region of a multilayer Mo / Si mirror is used. Controlling the IC to be defect - free is an important part of the metrology process. A general trend in lithographic production is a shift from time - consuming and costly IC inspections in mass production to the analysis of lithographic masks. If there are defects in the mask, they are projected onto a silicon substrate with photoresist and defects appear on the printed chip. The mask in EUV lithography is a Mo / Si mirror, on which a topological pattern formed of a material that absorbs radiation with a wavelength of 13.5 nm is added. The most efficient method for the mask inspection process is performed at the same wavelength as the actinic radiation, which is radiation with a wavelength matching the operating wavelength of the lithography. Such scanning with radiation having a wavelength of 13.5 nm enables defect detection with a resolution better than 10 nm. Making the lithographic mask defect - free throughout their production and entire operating cycle is a challenge in EUV lithography. The creation of devices for diagnosing lithographic masks and their high - brightness actinic sources is a priority in EUV lithography development.

[0004] One example design involves scattering a rotating target coated with liquid tin or other metals with very low melting points, such as In, Pb, Ga, Cd, Bi, or Li, or a combination thereof, on the inner wall of a rotating drum. This rotating target can then be used as an EUV light source. Surface waves are generated with each laser pulse due to the interaction between the laser pulse and the liquid metal surface. These waves interfere with each other due to the high-speed rotation of the drum (e.g., over 1000 rpm) and the high-speed repetition rate of the laser-generated plasma (e.g., over 10 kHz).

[0005] These surface waves cause instability in the position of the driving laser relative to the focused spot on the liquid metal surface. As a result, the laser beam dimensions at the interaction point with the target fluctuate, and consequently, the laser beam intensity fluctuates. Consequently, the in-band conversion efficiency, which is determined by the deviation of the actual laser intensity from the optimal laser intensity, fluctuates, and the EUV energy fluctuates with each pulse. The brightness of the laser source is determined by the ratio of EUV energy to the radiating surface area, and therefore fluctuates due to the small depth of focus (Rayleigh length) of the focusing lens.

[0006] Estimates suggest that the wave speed is lower than the linear speed of the drum. Therefore, a wave generated by a single pulse cannot cause surface disturbance with the next laser pulse. At high rotational speeds (e.g., reaching 200 Hz), wave propagation to complete a full rotation of the drum is only about 5 ms. Surface disturbance caused by waves occurs after the drum has completed its full rotation. Interference between waves originating from multiple pulses can generate high-amplitude waves, which may be a cause of instability in EUV energy and brightness.

[0007] The rotating drum has a distal wall (relative to the rotation axis) and may also have a proximal wall. The distal wall is covered with liquid metal. The proximal wall is designed to reduce the scattering and / or evaporation of liquid metal within the vacuum chamber caused by laser pulses. If evaporated liquid metal accumulates on the surface, it can cause problems with the laser source's operation. The liquid metal thickness in the interaction zone can be several millimeters (e.g., 2-3 mm). Reducing this thickness below the minimum value may result in increased liquid metal scattering due to laser pulses. This is determined by the shock wave propagation in the liquid metal. Conversely, it may also be necessary to reduce the thickness to increase the effects of friction and viscosity, thereby attenuating the amplitude of the propagating wave. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] U.S. Patent Application Publication No. 2019 / 0115184 [Overview of the project] [Problems that the invention aims to solve]

[0009] Therefore, improved systems and methods are needed. [Means for solving the problem]

[0010] The first embodiment provides a system comprising a vacuum chamber and a rotating target assembly having an annular groove with a wall distal to the rotation axis. The rotating target is disposed within the vacuum chamber. A porous region is provided in the distal wall.

[0011] The rotating target assembly can be provided with a proximal wall facing a distal wall, thereby forming its annular groove.

[0012] This system can incorporate a rotating system coupled to its rotating target assembly. This rotating system can be configured to rotate the rotating target assembly around its axis of rotation.

[0013] An entrance window and an exit window, or an entrance window and an optical system, can be provided in the vacuum chamber. The proximal wall of the annular groove can be configured such that a line of sight is provided between the distal wall and the entrance window and exit window or optical system during the laser pulse.

[0014] This system can incorporate a laser light source configured to direct the laser beam towards the distal wall.

[0015] This system allows for the placement of molten metal within the annular groove. This molten metal can then be placed on the aforementioned porous region.

[0016] The porous region can have holes with a diameter of less than 1 mm.

[0017] The porous region can have a thickness of 1 to 5 mm from the distal wall towards the annular groove.

[0018] The porous region can be created from titanium, stainless steel, aluminum, or molybdenum.

[0019] The thickness of the porous region can be made to vary across the distal wall.

[0020] A second embodiment provides a method. In this method, a molten metal layer is disposed on the distal wall of an annular groove within a rotating target assembly, and the rotating target assembly is rotated in a vacuum chamber to form a target by centrifugal force. A porous region is provided in the distal wall. A pulsed laser beam is directed through the entrance window of the vacuum chamber. The pulsed laser beam is irradiated onto the target on the distal wall. The generated short-wavelength radiation beam is directed from the target.

[0021] The proximal wall of the annular groove can be configured such that a line of sight is provided between the distal wall and both the incident window and the exit window or between the incident window and both the optical system during the aforementioned aiming.

[0022] The molten metal can be disposed on the porous region.

[0023] The porous region can be made to have pores with a diameter of less than 1 mm.

[0024] The porous region can be made to have a thickness of 1 to 5 mm facing into the annular groove.

[0025] The porous region can be made of titanium, stainless steel, aluminum, or molybdenum.

[0026] The porous region can be positioned beneath the surface of the target during the aforementioned rotation.

[0027] The molten metal layer can be made to exhibit a depth greater than the height of the porous region along a direction perpendicular to the axis of rotation of the rotating target assembly during the aforementioned rotation.

[0028] The aforementioned short-wavelength radiation beam can be directed through the exit window of the vacuum chamber or through the optical system within the vacuum chamber.

[0029] For a more complete understanding of the nature and objects of the present disclosure, reference should be made to the following accompanying drawings in conjunction with the detailed description set forth below.

Brief Description of the Drawings

[0030] [Figure 1] It is a cross-sectional view of a system embodiment according to the present disclosure. [Figure 2] It is a cross-sectional view of an embodiment of a part of the rotating target assembly. [Figure 3]This is a cross-sectional view of another embodiment of a portion of the rotating target assembly. [Figure 4] This is a cross-sectional view of another embodiment of a portion of the rotating target assembly. [Figure 5] This is a cross-sectional view of another embodiment of a portion of the rotating target assembly. [Figure 6] Figure 5 is a top view along AA of the rotating target assembly of the embodiment shown. [Figure 7] This is a flowchart of the method for disclosure in this case. [Modes for carrying out the invention]

[0031] While the subject matter described in the claims is illustrated by specific embodiments, other embodiments, including those not providing all of the benefits and features described herein, also fall within the technical scope of this disclosure. Various structural, logical, processing step-by-step, and electronic modifications can be made without deviating from the technical scope of this disclosure. Thus, the technical scope of this disclosure is defined solely by reference to the claims set forth in a separate section.

[0032] The present invention describes a laser-produced plasma (LPP) target for an EUV source in which the inner surface of a rotating drum is covered with a liquid metal (e.g., tin). In the embodiments of the present disclosure, EUV performance stability (i.e., pulse-by-pulse stability of EUV inband brightness and energy) is improved by reducing waves generated on the target surface. The designs of the present disclosure can attenuate these waves so that a smooth surface for interaction between the focused laser and the target can be provided after at least one full rotation.

[0033] Figure 1 is a cross-sectional view of system 100. The system has a vacuum chamber 101, within which a rotating target assembly 102 is provided. The rotating target assembly 102 has an annular groove 109, which is provided with a distal wall 110 and a proximal wall 111 with respect to the rotation axis 104. The distal wall 110 has a porous region 112. The porous region 112 extends from the surface of the distal wall 110 into the annular groove 109 (i.e., along the X direction perpendicular to the rotation axis 104) and may have a thickness of 1 to 5 mm. The rotating target assembly 102 can be made of aluminum, titanium, their alloys, or other materials.

[0034] Depending on the design of the rotating target assembly 102, the proximal wall 111 may be rotatable or stationary. In some embodiments, the proximal wall 111 is omitted, and only the distal wall 110 is provided on the rotating target assembly 102. Even without the proximal wall 111, the annular groove 109 can be measured relative to a member in the center of the rotating target assembly 102, or a circular groove can be used instead.

[0035] The rotation system 103 is coupled to the rotation target assembly 102. The rotation system 103 rotates the rotation target assembly 102 around the rotation axis 104. The rotation system 103 allows rotation to be transmitted to the rotation target assembly 102 using a shaft. The rotation system 103 can be an electric motor or other mechanism.

[0036] The vacuum chamber 101 may be provided with an entrance window 107 and an exit window 108. The proximal wall 111 of the annular groove 109 may be configured such that a line of sight is provided between the distal wall 110 and the entrance window 107 and the exit window 108 during the laser pulse. The laser light source 105 is configured to direct the laser beam 106 toward the distal wall 110. The liquid metal on the distal wall 110 is the target for the laser beam 106.

[0037] Although Figure 1 shows the system with an emission window 108, the system 100 can also be equipped with an optical element inside a vacuum chamber 101 for focusing EUV radiation. In this design, the emission window 108 may not be present.

[0038] A molten metal (shown in another figure) is disposed within the annular groove 109. This liquid metal can be tin, other low-melting-point metals, or low-melting-point alloys. Besides Sn, other metals that may be included include In, Pb, Ga, Cd, Bi, Li, or combinations thereof. In some examples, the molten metal is disposed on the porous region 112, for example, on the surface or within the pores of the porous region 112. By rotating the rotating target assembly 102 around the rotation axis 104, the molten metal can be retained on the distal wall 110. This prevents direct influence from the laser beam 106 onto the porous region 112.

[0039] The porous region 112 can be a sponge-type or fired metal insert. The porous region 112 can also be formed directly inside or on the surface of the rotating target assembly 102. The porous region 112 can extend around the entire circumference of the rotating target assembly 102, or it can extend to a portion of the periphery of the rotating target assembly 102. The height of the porous region 112 (along the Y direction) can extend from the base of the annular groove 109 to the top of the molten metal or the top of the distal wall 110. The height of the porous region 112 can also be limited to less than the entire distance from the base of the annular groove 109 to the top of the molten metal or the top of the distal wall 110. The thickness of the porous region 112 (along the X direction) can be uniform or variable across the surface of the annular groove 109.

[0040] The porous region 112 may have holes with a diameter of less than 1 mm. The porous region 112 can be made of titanium, stainless steel, molybdenum, aluminum, or other metals or metal alloys. By configuring the thickness of the molten metal layer inside the annular groove 109 during rotation (i.e., along the X direction) to be sufficiently thick while minimizing it, the porous region 112 can be prevented from being affected by the laser beam 109. This can help preserve the porous region 112. Its molten metal thickness can be experimentally selected and can be 0.5 to 1 mm, corresponding to the laser crater depth. Because it is an effective thickness, no scattering occurs due to the interaction between the laser pulse (or the shock wave generated by the pulse) and the liquid metal permeating the porous region 112. By reducing the thickness of the liquid metal and increasing the roughness of the porous region 112, the attenuation of the waves generated by the laser pulse can be helped.

[0041] In addition to preventing scattering, the porous region 112 can act as a reservoir for the liquid metal when it is cut using the laser beam 106. The liquid metal can be stored in the pores of the porous region 112.

[0042] The rotating target assembly 102 can be disc-shaped. However, the shape of the rotating target assembly 102 can also be wheel-shaped, low-profile multifaceted prism-shaped, or other shapes.

[0043] The liquid-phase targets used in the embodiments disclosed herein, in contrast to solid-phase targets, help ensure the reproducibility of the target surface. This enhances the pulse-to-pulse stability of the emission characteristics of its short-wavelength radiation source. Long-term stability of the short-wavelength radiation source can be achieved through the continuous circulation, renewal, and replenishment of the liquid metal. The use of a laser-produced plasma of metal (e.g., tin) ensures that the short-wavelength radiation source is highly bright and efficient. This can be applied to the 13.5 nm operating wavelength of EUV lithography. The rotating target assembly 102 can limit the outflow of debris particles through it, thereby improving the cleanliness of the short-wavelength radiation source and reducing the consumption of the target material.

[0044] The laser light source 105 can generate short laser pulses (e.g., less than 100 ns). According to one embodiment, the wavelength of the laser can be set to 1 μm to 10 μm. By using the synchronization system in conjunction with the laser light source 105, the irradiation of the surface of the rotating target assembly 102 can be made to follow a line of sight. Since the reflected continuous signal of the auxiliary laser radiation modulated by the marker can be detected by the photodetector, the rotation angle of the annular groove 109 when initiating the main pulsed laser can be set to one that provides a line of sight connecting the interaction zone and the incident and exit windows 107, 108 via the proximal wall 111.

[0045] In one example, minute droplets of target material passing into the opening of the proximal wall 111 can be reverse-extruded into the annular groove 109 under the action of centrifugal force. That is, the plasma-forming material of the target can be prevented from leaving the annular groove 109, thereby extending the lifespan of the radiation source without refilling.

[0046] Figure 2 is a cross-sectional view of a partial embodiment of the rotating target assembly 102. This embodiment does not have a proximal wall 111. The porous region 112 is a region in which liquid metal 113 is impregnated into a porous material (e.g., sponge or fired product). The thickness of the liquid metal 113 relative to the distal wall 110 (i.e., the volume of the liquid metal 113) can be reduced compared to a design without the porous region 112. The liquid metal 113 is held on the distal wall 110 by centrifugal force during the rotation of the rotating target assembly 102.

[0047] Figure 3 is a cross-sectional view of another embodiment of a portion of the rotating target assembly 102. The proximal wall 111 has an entrance aperture 115 and an exit aperture 114 for the laser beam 106, as shown. There is also a cover 116 between the distal wall 110 and the proximal wall 111. The cover 116 may help to keep droplets of liquid metal 113 within the desired area. In the embodiment of Figure 3, the distal wall 110 is inclined with respect to the proximal wall 111. That is, the distal wall 110 does not meet the base of the rotating target assembly 102 at a perpendicular angle.

[0048] In some cases, waves may propagate through the liquid metal 113 and be reflected by the solid surface, leading to scattering. Wave propagation within the liquid metal 113 is reduced by the porous region 112. The porous region 112 also prevents scattering and surface unevenness of the liquid metal 113. Scattering can cause the generation of tiny droplets around the EUV and laser tunnels, which can gradually clog them. A more uniform distribution of liquid metal 113 (i.e., a uniform surface) can reduce vibrations in the rotating target assembly 102. A uniform distribution reduces vibrations in the rotating target assembly 102, stabilizing the position of the target surface relative to the laser focus spot and improving EUV stability.

[0049] According to the embodiments disclosed herein, the velocity of the surface wave generated by the interaction between the laser pulse constituting the laser beam 106 and the liquid metal 113 can be estimated based on the Korteweg-de-Vries equation, which describes waves on shallow water under a gravitational field. The propagation velocity c=(gh) is obtained from the long-wavelength solution. 1 / 2 Given the following, in the case of the rotating target assembly 102, the acceleration of free fall is the centrifugal acceleration V 2 It is replaced by / R, where V is the linear velocity of the drum surface with radius R. In this case, the velocity of the surface wave is c = V(h / R). 1 / 2 This becomes equal to c<V when h<R, so the wave generated by the laser pulse will move out of the focus zone at velocity V, while the generated wave will propagate more slowly. For example, if V=100m / s, R=80mm and h=2mm, then c=16m / s. However, when the rotating target assembly 102 completes its full rotation (e.g., 5 milliseconds at 12000 rpm), the wave may strike the laser unless it has settled. The amplitude of the wave may initially be in the millimeter range (e.g., 0.05~0.2mm), and unless attenuated by the porous region 112, it may remain unchanged or even be amplified, like a tsunami.

[0050] Figure 4 is a cross-sectional view of another embodiment of a portion of the rotating target assembly 102. A stationary shield 118 is used within its irradiation zone and can function as part of or as a whole of the proximal wall 111. There is a gap 119 between the rotating target assembly 102 and the stationary shield 118. An entrance aperture 115 and an exit aperture 114 can be drilled into the stationary shield 118 and aligned with the laser beam and the EUV optical system. In some examples, the entrance aperture 115 and the exit aperture 114 are conical. The stationary shield 118 may be separate from the rotating member of the target, and synchronization between the stationary shield and the cover 116 and / or between the stationary shield 118 and the base of the rotating target assembly 102 is not required.

[0051] In some cases, the thickness of the porous region 112 can be set to a thickness that varies across the distal wall 110, as shown in Figures 5 and 6 according to one example. The waves can be attenuated using a grooved rotating target assembly 102. Segments 117 extend from the distal wall 110 as shown in the figure, thereby forming grooves. These grooves can be made to a depth that avoids scattering interaction between the laser and the liquid metal 113. The segments 117 can create a barrier, and a minimum thickness of liquid metal 113 can be placed above the segments 117. This barrier can attenuate and reduce the waves. Since a small thickness of molten metal may be present in the space between the grooves, the waves will pass through surfaces of different depths. In shallow regions, the waves will be slowed down, and their amplitude will be reduced due to viscosity.

[0052] Uniform distribution of the liquid metal 113 can be achieved by properly filling the rotating target assembly 102. The laser beam 106 can be synchronized with the groove position using an encoder and external triggering.

[0053] The grooves can be filled with liquid metal 113, creating a thin layer on top of the segment 117. Since the thickness of the liquid metal 113 on top of the segment 117 relative to the distal wall 110 can be as small as 0.1-0.2 mm or less, surface waves can be efficiently attenuated there. The porosity allows for the generation of additional viscous friction.

[0054] According to one embodiment, the segment 117 can be formed within the porous region 112. Because it is a porous material, it can offer the advantage of equalizing the thickness distribution of the liquid metal 113 due to the high rotation speed of the rotating target assembly 102.

[0055] According to the embodiments disclosed herein, waves generated by the interaction between laser pulses and the liquid metal surface can be attenuated. As a result, pulse-to-pulse EUV stability can be improved with respect to both in-band energy and brightness. Since instability related to vibration and other disturbance sources is reduced, the thickness distribution of the liquid metal can be made more uniform. As a result, improvements in the target surface position and stabilization of the source brightness are also achieved.

[0056] Figure 7 is a flowchart of method 200. In method 201, the target is formed by centrifugal force by rotating the vacuum chamber within the rotating target assembly with the molten metal layer positioned on the distal wall of the annular groove within the rotating target assembly. A porous region is provided in the distal wall. The molten metal can be located on and within this porous region. The porous region can have holes with a diameter of less than 1 mm and a thickness of 1 to 5 mm toward the annular groove.

[0057] In 202, a pulsed laser beam is directed through the entrance window of the vacuum chamber. In 203, the pulsed laser beam is irradiated onto a target (e.g., liquid metal) on the distal wall. In 204, the short-wavelength radiation beam generated by the irradiation is directed through the exit window of the vacuum chamber or through the optical system within the vacuum chamber. By appropriately configuring the proximal wall of the annular groove, a line of sight can be provided between the distal wall and both the entrance and exit windows during the directing process. The proximal wall can be positioned as a stationary shield or can have a gap between it and adjacent rotating members.

[0058] The porous region can be positioned below the target surface during rotation. The depth of the molten metal layer can be made greater than the height of the porous region along the direction perpendicular to the rotation axis of the rotating target assembly during rotation. Wave propagation is reduced, resulting in a more uniform liquid metal distribution.

[0059] To produce a high-temperature laser-generating plasma with a strong light output belonging to the short-wavelength spectrum from ultraviolet to soft X-ray band, the laser radiation power density of the laser beam 106 on the target must be 10 10 ~10 12 W / cm 2 Therefore, the length of the laser pulse should be set to 100 ns to 0.5 ps.

[0060] Any number of pulsed or modulated lasers may be used to generate the laser beam 106. The laser light source 105 can be solid, fiber, disk, or gas discharge type. The average laser radiation power of the laser beam 106 can be in the range of 10W to approximately 1kW or more, and the laser beam 106 can be focused on a small focal spot on the target, for example, one with a diameter of approximately 100μm.

[0061] The laser pulse repetition frequency can be set to 1kHz to 10MHz. Within this range, increasing the pulse repetition rate and decreasing the emitted laser energy can reduce the scattering of debris particles.

[0062] The vacuum chamber is then pumped using an oil-free pump system. -5 ~10 -8 The gas can be removed to below bar, thereby removing gaseous components such as nitrogen and carbon that may interact with the target material.

[0063] The vacuum chamber can be filled with a buffer gas (e.g., H2, He, or Ar) that has high short-wavelength radiation transmittance to protect the optical system from debris generated by the plasma.

[0064] The liquid metal can be kept in a molten state using an induction heating system configured to maintain it within an optimal temperature range through temperature stabilization of the liquid metal.

[0065] While this disclosure has been described in relation to one or more specific embodiments, other embodiments of this disclosure can be made without deviating from the technical scope of this disclosure, as you can see. That is, this disclosure is limited only by the attached claims and their reasonable interpretation.

Claims

1. It is a system, Vacuum chamber and A rotating target assembly having an annular groove with a wall distal to the axis of rotation, A system comprising the rotating target disposed within the vacuum chamber and having a porous region in its distal wall.

2. The system according to claim 1, wherein the rotating target assembly has a proximal wall opposite the distal wall, and the annular groove is formed by them.

3. A system according to claim 1, further comprising a rotation system coupled to the rotation target assembly, wherein the rotation system is configured to rotate the rotation target assembly about a rotation axis.

4. A system according to claim 1, wherein the vacuum chamber comprises an entrance window and an exit window, or an entrance window and an optical system.

5. The system according to claim 4, wherein the proximal wall of the annular groove is configured such that, during a laser pulse, a line of sight is provided between the distal wall and both the incident window and the exit window, or between the incident window and the optical system.

6. A system according to claim 1, further comprising a laser light source configured to direct a laser beam towards the distal wall.

7. The system according to claim 1, further comprising molten metal disposed within the annular groove.

8. The system according to claim 7, wherein the molten metal is disposed on the porous region.

9. The system according to claim 1, wherein the porous region has holes with a diameter of less than 1 mm.

10. A system according to claim 1, wherein the porous region has a thickness of 1 to 5 mm from the distal wall toward the annular groove.

11. The system according to claim 1, wherein the porous region is made of titanium, stainless steel, aluminum, or molybdenum.

12. The system according to claim 1, wherein the porous region has a thickness that varies transversely across the distal wall.

13. It is a method, With a molten metal layer positioned on the distal wall of an annular groove within its rotating target assembly, the rotating target assembly is rotated within a vacuum chamber to form a target by centrifugal force, provided that its distal wall is provided with a porous region. A pulsed laser beam is directed through the entrance window of the vacuum chamber. The pulsed laser beam is irradiated onto the target on the distal wall, and The generated short-wavelength radiation beam is directed from the target. method.

14. A method according to claim 13, wherein the proximal wall of the annular groove is configured such that, during the deflection, a line of sight is provided between the distal wall and both the entrance window and the exit window, or between the entrance window and the optical system.

15. A method according to claim 13, wherein the molten metal is disposed on the porous region.

16. A method according to claim 13, wherein the porous region has holes with a diameter of less than 1 mm.

17. A method according to claim 13, wherein the porous region has a thickness of 1 to 5 mm inward toward the annular groove.

18. A method according to claim 13, wherein the porous region is made of titanium, stainless steel, aluminum, or molybdenum.

19. A method according to claim 13, wherein the porous region is positioned below the surface of the target during the rotation.

20. A method according to claim 13, wherein during the rotation, the molten metal layer has a depth greater than the height of the porous region in a direction perpendicular to the rotation axis of the rotation target assembly.

21. A method according to claim 13, wherein the short-wavelength radiation beam is directed through an exit window of the vacuum chamber or through an optical system within the vacuum chamber.

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