Method for creating a tempered hollow structure in a substrate using a processing light beam

Abstract

In a method for producing a tempered hollow structure in a substrate, particularly in a mirror substrate of an EUV projection exposure system or other semiconductor technology system, a substrate (12) consisting of a substrate material is first provided. The tempered hollow structure (22) is then incorporated into the substrate (12) by successively focusing a processing light beam (34) onto processing locations (44) where the tempered hollow structure (22) is to be formed. This modifies or removes the substrate material at the processing locations (44). According to the invention, residues (74) in the tempered hollow structure (22) are broken up by inserting a lance (76), which can be in the form of a wire or a tube, into an already produced tempered hollow structure (22) or into the substrate (12) during the production of a tempered hollow structure (22).

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The invention relates to a method for producing a tempering cavity structure in a substrate using a processing light beam. The substrate can, in particular, be a mirror substrate for a mirror of an EUV projection exposure system or another semiconductor technology system. 2. Description of the state of the art

[0002] Microlithographic projection exposure systems are used in chip manufacturing to transfer patterns on a mask onto a photoresist previously applied to a wafer. To do this, the mask is illuminated with light and imaged in a reduced size onto the light-sensitive layer. In EUV projection exposure systems, the light has a wavelength between approximately 5 nm and approximately 30 nm; commercially available systems use light with a wavelength of 13.5 nm.

[0003] However, there are no optical materials with sufficiently high transmittance for such short wavelengths. Therefore, in EUV projection exposure systems, the lenses typically used for longer wavelengths are replaced by mirrors, and the mask also contains a pattern of reflective structures.

[0004] Providing mirrors for EUV projection exposure systems is technologically challenging. The mirror substrate consists of a substrate material, typically glass, e.g., titanium-doped quartz glass such as ULE®, or a glass-ceramic. A coating is vapor-deposited onto the mirror substrate that reflects the EUV light and consists of a multitude of thin double layers with varying refractive indices.

[0005] Even with such complex coatings, the mirrors' reflectivity for EUV light rarely exceeds 70%, and even then only for light that hits the reflective coating perpendicularly or at angles of incidence of a few degrees. The portion of the EUV light not reflected by the coating is absorbed in the mirror substrate, where it leads to significant heating, due to the high power of the EUV light sources used. Even when glass-ceramics with low thermal expansion coefficients are used, the heating can lead to intolerable deformation of the mirrors.

[0006] The developers of other semiconductor technology systems, such as mask or wafer inspection systems, are also confronted with the same problems, since they also use mirrors that heat up under the influence of radiation.

[0007] It was therefore proposed to provide the mirror substrates with cooling channels through which water or another fluid flows during operation, thus dissipating heat. The cooling channels have a cross-section of approximately 1 mm 2 and ideally run at a defined distance below the reflective coating.

[0008] An overview of the methods known to date for creating cooling channels is contained in the applicant's application DE 10 2021 214 310.5. Particularly promising are methods in which a processing light beam is successively focused on processing locations where cooling channels are to be created. This modifies the substrate material at the processing locations so that it can be removed by chemical processing in a subsequent step. Alternatively, the substrate material can also be removed directly by the processing light beam; any resulting residues are then rinsed away with a rinsing fluid.

[0009] However, it has been shown that the machining results are occasionally unsatisfactory because the walls of the cooling channels are not smooth but irregularly shaped.

[0010] Unevenly formed tempering cavity structures can be caused by gas bubbles and other inclusions in the mirror substrate. If the processing light beam encounters a gas bubble on its way to the processing location, for example, this acts like a small lens that deflects the incoming processing light beam. The deflected light usually no longer reaches the processing location, which is why the radiation intensity required for the desired modification or ablation of the substrate material may no longer be available there. If this results in less material being modified or ablated at the processing location, this leads to undesirable cross-sectional changes in the tempering cavity structure. In the worst case, the process is aborted because no more material is modified or ablated at all.

[0011] The situation is similar for inclusions that absorb the processing light and thus weaken the intensity at the processing location.

[0012] Similar problems arise in other contexts as well. In order to keep a substrate at a precisely defined temperature, a channel created in the substrate can also be used for heating. The substrate does not have to be part of a mirror, but can, for example, carry a sensitive sensor that must be located in a precisely defined position. If the substrate is exposed to high and time-varying thermal stress, e.g. as a result of fluctuating electrical power losses from neighboring components or radiation that is only incident intermittently, this can lead to changes in the shape of the substrate and thus to undesirable changes in the position of the sensor. Hollow structures through which a cooling or heating fluid flows can also be used with such substrates to keep the substrate at a constant target temperature.

[0013] In the following, we will therefore generally refer to hollow tempering structures, even though these will often be cooling channels.

[0014] The substrate does not need to be transparent to visible light. For example, hollow structures can be created in silicon, which is opaque to visible light, if the processing light beam has a wavelength of around 2 µm. SUMMARY OF THE INVENTION

[0015] The object of the invention is to provide a method for producing a tempering hollow structure in a substrate, in which the tempering hollow structure has a more uniform cross-section than in known methods.

[0016] This object is achieved by a method according to claim 1.

[0017] It has been shown that in many cases, residues of substrate material remain in the cooling cavity structures, protruding from the interior surface into the cooling cavity structure, similar to stalactites on a cave wall. If such residues exceed a certain size, functional impairments cannot be ruled out. In extreme cases, the residues are so large that the cooling cavity structure is completely sealed by a structure that, although thin, is impermeable to the cooling or heating fluid flowing through it.

[0018] According to the invention, the remaining residues in the tempering cavity are broken up by inserting a lance into an already created tempering cavity or into the substrate during the creation of a tempering cavity. This either directly destroys the residues to a large extent or damages them to such an extent that they can be removed in the same or a subsequent processing step. The removal process can also be performed mechanically, e.g., with the aid of a rinsing fluid or with the aid of an element conveying the rinsing fluid. Alternatively, a fluid can be used to chemically dissolve and carry away the broken-up residues.

[0019] The mechanical breaking can be achieved, for example, with the help of a pressurized fluid that is introduced into an existing tempering cavity structure that is just being formed.

[0020] As already mentioned, according to the invention, a lance is inserted into an already created tempering cavity structure or into the substrate during the creation of a tempering cavity structure. If a free end of the lance encounters an interfering residue during insertion, the momentum of the moving lance breaks it off and possibly crushes it. This allows even complete closures of the tempering cavity structures to be broken open. Simulations have shown that the force typically required to break off a residue is on the order of 0.1 N.

[0021] The remaining residues form protrusions with a maximum height h of 15 µm < h < 50 µm relative to the surrounding and otherwise smooth inner surface. Compared to typical temperature control channel widths of approximately 2 mm, this is a very small value, so that the fluid flow through the protrusions is only slightly disrupted. In this context, a smooth inner surface is defined as one with a maximum roughness depth Rt of less than 5.0 µm and preferably less than 3.0 µm.

[0022] In a subsequent (possibly second) treatment step using a chemically active and preferably liquid medium, the remaining crushed residues can be further removed and rounded. This allows the maximum height h of the protrusions to be reduced to values ​​of 15 < h < 25 µm. Such shallow protrusions generally do not impair the functionality of the substrate during subsequent use.

[0023] The lance can be a separate tool, e.g., a thin but sufficiently stiff wire, designed solely to break up the residues. If the tempering cavity is a channel with a uniform diameter and slight bends, a lance can be used whose diameter matches the diameter of the channel—i.e., one with the same shape but slightly smaller dimensions. This allows the residues to be broken off so that only very short stumps remain.

[0024] Alternatively, a (possibly steerable) hose can be used as a lance. This hose is inserted manually or with the aid of a tracking unit into the already created tempering cavity structure, or into the substrate during the creation of a tempering cavity structure, to dispense a chemical rinsing or processing fluid. A rinsing fluid is preferably used once the processing light beam has removed the substrate material.

[0025] If the substrate material is merely modified by the processing beam, a subsequent chemical processing step using a chemical processing fluid, such as an etching fluid or another chemically active medium, is required. The stiffness of the tube can be adjusted by, among other things, the material selection, the tube diameter, the tube wall thickness, and the pressure of the chemical processing fluid within the tube.

[0026] If the lance is a hose, the momentum can also be transferred to the residue indirectly via a flushing fluid. In this case, the flushing fluid exits the hose at sufficiently high pressure and tears the residue from the inner surface of the cavity. Direct contact of the hose with the residue is not necessary in this case. Since the hose does not have to directly touch the residue, it can be comparatively soft, which facilitates navigation in the cavity.

[0027] Due to their greater mass, water or another liquid is particularly suitable as a flushing fluid. However, air or another gas (or mixture) could also be used.

[0028] Preferably, the fluid exits the hose in pulses. The pressure surges generated in this way increase the momentum transfer to the residues, causing them to break off faster and more reliably.

[0029] Regardless of the type of lance, it is advisable to move it alternately back and forth while removing the residue. This allows the lance or a fluid escaping from it to repeatedly impact a residue, gradually removing it.

[0030] Mechanical disruption can be performed after the cooling hollow structure has been created. Some processing methods allow for the residues to be mechanically disrupted during the creation of the cooling hollow structure. Such processing methods are described in patent application DE 10 2023 205 563.5, filed with the German Patent and Trademark Office on June 14, 2023, entitled "Method for incorporating cooling hollow structures into a mirror substrate of a mirror for an EUV projection exposure system, and processing system therefor." In these processing methods, a tube is inserted into the substrate while the processing light beam ablates or modifies the substrate material. This tube can be used as a lance in the manner described above and can mechanically disrupt the newly formed residues either through its movement or through the ejection of a fluid. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In the following, exemplary embodiments of the invention are explained in more detail with reference to the drawings. In these drawings: Fig. 1 a mirror for an EUV projection exposure system in a schematic cross-section; Fig. 2 a mirror substrate into which cooling channels are inscribed using a processing light beam; Fig. 3a to 3d show several phases of a machining process in which residues in a previously created cooling channel are broken off with a lance inserted into the cooling channel; Fig. 4 a variant in which the residue is not broken off directly by the lance but indirectly via an outflowing fluid; Fig. 5 a cooling channel with a protrusion remaining after breaking off the residue; and Fig. 6 the cooling channel according to the Fig. 5 after an additional chemical processing step. DESCRIPTION OF PREFERRED EMBODIMENTS 1. Structure of an EUV mirror

[0032] The Fig. Figure 1 shows a schematic cross-section of a mirror, designated overall by 10, for an EUV projection exposure system or other semiconductor technology system. Mirror 10 can be arranged in the illumination system or in the projection lens.

[0033] The mirror 10 comprises a mirror substrate 12, which in the illustrated embodiment is made of a glass ceramic. Suitable glass ceramics are offered under the trade names Clearceram® or Zerodur® and have the property of having a very low coefficient of thermal expansion at the operating temperature of the mirror 10. The mirror substrate 12 is monolithic, i.e., it consists of a single, continuous piece of material that was not obtained by joining individual pieces.

[0034] The mirror substrate 12 has a precisely machined support surface 14, the curvature of which determines the optical properties of the mirror 10. The support surface 14 of the mirror substrate 12 bears a coating 16 that predominantly reflects incident EUV light 18. As can be seen in the enlarged section C, the coating 16 consists of several double layers 20 applied to the support surface 14. The coating 16 can have a reflection coefficient of more than 70% for perpendicularly incident EUV light 18.

[0035] The non-reflected portion of the UV light 18 penetrates the mirror substrate 12 and is absorbed there, predominantly near the carrier surface 14. As a result of this absorption, the mirror substrate 12 heats up, particularly near the areas of the carrier surface 14 exposed to the EUV light 18. Since the thermal expansion coefficient of the substrate material is not identical to zero and is also temperature-dependent, the heating can lead to changes in the shape of the mirror substrate 12, which affect the optical properties of the mirror 10. However, due to the extremely tight specifications in semiconductor technology systems, changes in the optical properties of the mirrors are not tolerable or, at best, only negligible.

[0036] In order to minimize temperature fluctuations and the resulting changes in shape of the mirror substrate 12, several cooling channels 22 are incorporated into the mirror substrate 12. Typically, the cooling channels 22 run close to and parallel to the carrier surface 14. The cooling channels 22 in the Fig. However, the arrangement of the cooling channels 22 shown in Figure 1 is only an example and may vary in real systems. For example, only a single cooling channel 22 may be provided, which meanders through the mirror substrate 12 or part of it.

[0037] During operation of the semiconductor technology system, water or another cooling fluid flows through the cooling channels 22. The water absorbs the heat introduced by the EUV light 18 and removes it from the mirror substrate 12. For this purpose, the cooling channels 22 are connected to a pump unit 24 and a cooling unit 26. The pump unit 24 draws the water from the cooling channels 22 and directs it via a return line 28 to the cooling unit 26. There, the water is cooled to its target temperature before flowing through the cooling channels 22 again. 2. Creating the cooling channels

[0038] The Fig. Figure 2 shows the mirror substrate 12 in a modified orientation prior to the material-removing creation of the carrier surface 14 shown in dashed lines. Above, in a modified orientation of the mirror substrate 12, a processing device 30 is schematically shown, with which the cooling channels 22 are inscribed into the mirror substrate. The processing device 30 has a light source 32 that generates a processing light beam 34. The light source 32 is preferably a high-power laser that generates ultrashort pulses. The processing light beam 34 passes through a focusing lens 36 and, in the illustrated embodiment, can then be directed onto the mirror substrate 12 at different angles using an adjustment device 38.For this purpose, the adjustment device 38 in the illustrated embodiment comprises a tiltable mirror 40, which is additionally arranged to be linearly movable parallel to the process direction indicated by 42. The process direction 42 runs in the longitudinal direction of the cooling channel 22 and thus corresponds to the direction along which the modification or removal process proceeds.

[0039] If the mirror 40 is tilted as well as moved linearly, the direction of the beam axis of the processing light beam 34 can be changed without significantly changing the location of the focus 44. In the Fig. In Figure 2, various tilt and movement positions of the mirror 40 are indicated by dashed lines. In these positions, only the direction of the beam axis changes, while the focus 44 remains virtually unchanged. To ensure that the focus 44 remains exactly at the same location, the focusing lens 36 must also be moved along the optical axis.

[0040] In order to be able to process material at all desired locations in the mirror substrate 12, the relative arrangement between the mirror substrate 12 and the processing device 30 must be variable. For this purpose, for example, the mirror substrate 12 can be moved linearly in all three spatial directions using a traversing table (not shown), as indicated by a multiple arrow 45. It is often expedient for the mirror substrate 12 to be movable not only linearly but also tiltable about at least one axis.

[0041] The traversing table and the adjustment device 38 are then controlled by a control device 46 such that the processing light beam 34 is successively focused on all processing locations at which cooling channels 22 are to be created.

[0042] Alternatively, the relative arrangement between the mirror substrate 12 and the processing device 30 can of course also be changed by moving the processing device 30.

[0043] At the focus 44 created by the focusing lens 36, the intensity of the processing light beam 34 is so high that irreversible modifications of the substrate material occur. Depending on the process, substrate material can be removed in such a way that it can be washed away in a subsequent process step. However, modifications to the substrate material that merely result in it being able to be selectively removed from the mirror substrate 12 in a subsequent chemical processing step are also possible. In both cases, the location of the focus 44 determines where a cooling channel 22 is created in the mirror substrate 12.

[0044] To create a cooling channel 22 with a sufficiently large cross-section, the machining light beam 34 traverses the entire cross-section at a specific axial position according to a predetermined pattern. This process is then repeated at adjacent axial positions until the cooling channel 22 has the desired axial dimension. 3. Removing residues

[0045] The Fig. 3a shows a previously created cooling channel 22 in the mirror substrate 12. However, the inner surface 72 of the cooling channel 22 is not completely smooth, but rather has a residue 74. In the illustrated embodiment, this has the shape of a thorn-like protrusion that extends from one side of the otherwise smooth inner surface 72 of the cooling channel 22 almost to the opposite side of the inner surface 72. The residue 74 thus significantly impedes the flow of a cooling fluid.

[0046] The residue 74 was caused by a gas bubble 52 located near the surface 50 of the mirror substrate 12. The shadowing 54 forming behind the gas bubble 52 causes the processing light beam 34, with a beam axis 48 running along a standard direction, to be unable to reach the substrate material in the region of the residue 74.

[0047] To remove the residue 74, a lance 76 is pushed into the cooling channel 22, as shown in Fig. 3b. In the illustrated embodiment, the lance 76 is a thin but comparatively stiff wire. The wire is just soft enough to follow curves in the cooling channel 22 and thus allow the lance to advance to the residue 74. If the free end 78 of the lance 76 hits the residue 74, as shown in the Fig. 3b, the force exerted by the lance 76 causes the residue 74 to break off, cf. Fig. 3c. If the residue 74 does not break off immediately or only a small piece of it, it may be useful to move the lance 76 back and forth several times, as shown in the Fig. 3c is indicated by a double arrow 79.

[0048] Due to the mechanical action of the lance 76, the broken-off part of the residue 74 is usually broken down into smaller pieces, as is the case with Fig. 3d shows.

[0049] The Fig. Figure 4 shows a variant in which the lance 76 is not designed as a wire, but as a hose. The hose is connected to a pressure source (not shown), which ensures that water 82 or another liquid emerges from the open end 78 of the hose under pressure surges, as shown in the Fig. 4. In this variant, the residue 74 is not torn off by direct contact with the lance 76, but by the water 82 escaping from the hose under pressure.

[0050] The Fig. 5 shows a section of the Fig. 3c after breaking off the residue 74. As a rule, the fracture edge will not lie exactly on the inner surface 72 of the cooling channel 22. This leads to a protrusion 80 remaining even after removing the residue 74 with the aid of the lance 76, which protrudes from the otherwise smooth inner surface 72 of the cooling channel 22. The maximum height h of the protrusion 80 is, however, small and lies between 15 µm and 50 µm, as shown in the Fig. 5 is indicated.

[0051] If the mechanical removal with the aid of the lance 76 is followed by a chemical processing step, the maximum height h of the elevations 80 can be reduced to values ​​between 15 µm and 25 µm, cf. Fig. 6.

[0052] The inner surface 72 of the cooling channel 22 is therefore not completely smooth when using the lance 76, but has an elevation 80 with a maximum height h in the specified value ranges at locations where a gas bubble 52 or another inclusion has hindered the creation of the cooling channel 22 using the processing light beam 34. The gas bubble 52 is located between the surface 50 of the mirror substrate 12 and the cooling channel 22. The Fig. 5 and Fig. 6 The distance between the surface 50 and the cooling channel 22, designated by a, is typically between 20 mm and 30 mm. 4. List of reference symbols 10 mirrors 12 Mirror substrate 14 Carrier surface 16 Coating 18 EUV light 20 double shifts 22 Cooling channel 24 pump unit 26 Cooling unit 28 Return line 30 processing equipment 32 light source 34 processing light beam 36 Focusing lens 38 Adjustment device 40 tilting mirrors 42 Process direction 44 Focus 45 Travel table 46 Control device 48 beam axis 50 surface 52 Gas bubble 54 Shading 72 interior surface 74 behind 76 Lance 78 free end of the lance 79 Double arrow 80 Survey 82 Water QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2021 214 310.5

[0008] DE 10 2023 205 563.5

[0030]

Claims

[1] Method for producing a tempering hollow structure in a substrate, in particular in a mirror substrate for a semiconductor technology system, comprising the following steps: a) providing the substrate (12) consisting of a substrate material; b) incorporating the tempering hollow structure (22) into the substrate (12) by successively focusing a processing light beam (34) onto processing locations (44) at which the tempering hollow structure (22) is to be created, whereby the substrate material is modified or removed at the processing locations (44); characterized by that residues (74) in the tempering cavity structure (22) are broken up by inserting a lance (76) into an already produced tempering cavity structure (22) or into the substrate (12) during the production of a tempering cavity structure (22). [2] Method according to claim 1, characterized by that the tempering hollow structure is a channel (22). [3] Method according to claim 1 or 2, characterized by that the broken-up residues (74) are removed with the aid of a rinsing liquid. [4] Method according to claim 1 or 2, characterized by that the broken up residues are chemically dissolved with the help of a fluid. [5] Method according to one of the preceding claims, characterized by that the mechanical breaking up of the residues (74) is effected by a pressurized fluid (82). [6] Method according to one of the preceding claims, characterized by that the residues (74) are broken off or crushed by the impulse of the moving lance (76). [7] Method according to one of claims 1 to 6, characterized by that the lance (76) is designed as a wire. [8] Method according to claim 7, characterized bythat the tempering hollow structure is a channel (22) and the diameter of the wire is adapted to the diameter of the channel, but has slightly smaller dimensions. [9] Method according to one of claims 1 to 6, characterized by that the lance (76) is designed as a hose which releases a liquid (82). [10] Method according to claim 9, characterized by that the processing light beam (34) modifies the material and the hose (76) releases a chemical processing fluid, in particular an etching fluid. [11] Method according to claim 9, characterized by that the processing light beam (34) removes the material and the hose releases a rinsing liquid. [12] Method according to claim 9 with reference back to claim 5, characterized by that the hose (76) releases a pressurized fluid (82) which breaks up the residues. [13] Method according to claim 12, characterized bythat the fluid is a liquid, in particular water (82). [14] Method according to one of claims 12 or 13, characterized by that the fluid exits the hose (76) in a pulsed manner. [15] Method according to one of the preceding claims, characterized by that the lance (76) is moved alternately back and forth in the tempering cavity structure. [16] Method according to one of the preceding claims, characterized by that after or during the insertion of the lance (76) the tempering hollow structure (22) is rinsed with a chemically active medium, whereby the residues (74) are further removed.

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