Thorium-free cathode for short arc discharge lamp as well as preparation method and application of thorium-free cathode

Abstract

The invention belongs to the technical field of electric light source materials and manufacturing, and discloses a thorium-free cathode for a short arc discharge lamp and a preparation method and application of the thorium-free cathode. The preparation method of the thorium-free cathode for the short-arc discharge lamp comprises the following steps: S1, mixing raw materials and carrying out drying treatment; s2, the dried mixed powder is subjected to reduction treatment in a reducing atmosphere, and tungsten alloy powder is obtained; s3, the tungsten alloy powder is subjected to compression molding, sintering and vertical sintering, and a cathode blank is obtained; s4, the cathode blank is forged, and a cathode base body is obtained; and S5, a groove with an inverted triangular cross section is machined in the middle section of the cathode base body in the direction of the center axis of the cathode base body, and the thorium-free cathode for the short arc discharge lamp is obtained. According to the preparation method of the thorium-free cathode for the short-arc discharge lamp, through collaborative optimization of a material system and structural design, comprehensive improvement of the electron emission performance, the thermal stability and the service life of the cathode is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of electric light source materials and manufacturing technology, and particularly relates to a thorium-free cathode for a short-arc discharge lamp and a preparation method and application thereof. BACKGROUND

[0002] As a high-brightness and high-directional point light source, the short-arc discharge lamp has an irreplaceable role in the field of precision optics, and can efficiently and stably emit characteristic light spectrum (i-line) with a wavelength of 365 nm. The specific band is highly consistent with the photosensitivity curve of positive and negative photoresists widely used in the manufacture of semiconductor and micro-electro-mechanical systems (MEMS), and is the basis of optical energy for accurately transferring micron or even nanometer patterns from a mask to a silicon wafer surface. Therefore, such a light source has become the standard exposure light source of key equipment such as i-line step-and-scan photolithography machines and contact / proximity photolithography machines to meet the stringent requirements of these devices in terms of brightness, uniformity and stability.

[0003] In the complex internal structure of the short-arc discharge lamp, the cathode, as the source of electron emission, is the core element that determines the discharge characteristics, light efficiency and stability. The performance of the cathode material, especially its electron emission capability, thermal stability and ion bombardment resistance, directly determines the stability of the discharge, the luminous efficiency and the overall service life of the discharge lamp. At present, the industry generally adds a certain mass fraction of thorium dioxide (ThO2) as an electron emission activator to a high-melting-point and high-strength tungsten (W) substrate to prepare a thorium-tungsten cathode. The addition of thorium dioxide can form a dispersed distribution of thorium atoms or related defect structures in the tungsten lattice, effectively reducing the surface work function of the material. This doping can increase the thermal electron emission efficiency of the cathode at high temperature by about an order of magnitude compared with a pure tungsten cathode, making the discharge process easier to start, the arc more concentrated and stable, thereby bringing higher luminous efficiency, better stability and longer effective working life of the lamp source.

[0004] However, the thorium-tungsten cathode with thorium dioxide as the activator has the following technical problems in actual application and subsequent processing: (1) Thorium is a naturally radioactive element, although its radioactivity is relatively low, but it makes the cathode material in all links such as powder metallurgy preparation, electrode machining, lamp source packaging, use and maintenance, and waste disposal, need to be included in the management category of radioactive substances, which not only increases the special protection requirements of the production site, the cost of employee health monitoring and the complex waste disposal process, but also puts forward higher requirements for the environmental protection compliance of the product; (2) Due to the rigid constraints of radiation safety control, relevant standards and specifications at home and abroad all impose strict upper limits on the content of thorium in the cathode, which limits the technical path of simply increasing the thorium doping amount to continuously improve the cathode emission current density, reduce the working temperature or prolong the service life.

[0005] As semiconductor manufacturing technology continues to evolve towards smaller nodes and higher precision, increasingly stringent requirements are being placed on the power, stability, and lifespan of short-arc discharge lamps used in lithography machines. Simultaneously, global demands for green, safe, and environmentally friendly production processes are becoming increasingly stringent. Therefore, against this backdrop, developing a novel cathode material system with excellent electron emission performance, good thermodynamic stability, and long lifespan, along with its scalable manufacturing process, while avoiding radioactive hazards, has become an urgent and significant technical challenge for driving the development of high-end short-arc discharge lamp technology and meeting the needs of next-generation semiconductor manufacturing and precision optical applications. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a thorium-free cathode for short arc discharge lamps, its preparation method, and its application.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention provides a method for preparing a thorium-free cathode for a short-arc discharge lamp, comprising the following steps: S1. Mix the raw materials and dry them; the raw materials include tungsten powder and dopant; the mass of the dopant is 1%-5% of the total mass of the raw materials; the dopant includes the following components in parts by mass: 0.05-0.4 parts of carbonaceous material, 0.05-0.15 parts of zirconium oxide, and 0.9-4.5 parts of rare earth oxides; S2. The dried mixed powder is reduced under a reducing atmosphere to obtain tungsten alloy powder; S3. Press the tungsten alloy powder into shape, and then sinter and sinter by vertical melting to obtain a cathode blank; S4. The cathode blank is forged to obtain the cathode substrate; S5. A groove with an inverted triangular cross-section is machined along the central axis of the middle section of the cathode substrate to obtain the thorium-free cathode for the short arc discharge lamp.

[0008] This invention provides a method for preparing a thorium-free cathode for short-arc discharge lamps. Through synergistic optimization of the material system and structural design, it achieves a comprehensive improvement in the cathode's electron emission performance, thermal stability, and service life. Firstly, in terms of material composition, this invention uses a specific proportion of rare earth oxides as the key electron-emitting material, replacing radioactive thorium dioxide. This effectively reduces the work function of the cathode material surface, promotes thermionic emission, and thus ensures an electron emission efficiency comparable to that of thorium-containing cathodes. Secondly, by introducing specific amounts of carbonaceous materials and zirconium oxide, this invention further synergizes with rare earth oxides to form a functionally complementary doping system. This system acts as a dispersed reinforcing phase distributed within the tungsten matrix. During high-temperature sintering and subsequent operation, it enhances the high-temperature strength and creep resistance of the cathode matrix, suppressing deformation and loss of the cathode under high-temperature arc conditions. Furthermore, it pins grain boundaries, refines grains, improves the overall high-temperature structural stability of the cathode material, delays the evaporation of tungsten and active rare earth components, and maintains the stability of the microstructure of the working surface. Furthermore, the unique inverted triangular trench structure design of this invention can effectively guide the concentrated heat flow generated when the cathode tip is working to diffuse along the trench to the rear, significantly increasing the effective heat dissipation surface area, slowing down the thermal evaporation rate of tungsten and active components (especially low work function active materials), reducing high-temperature ion sputtering erosion, and fundamentally extending the effective working life of the cathode.

[0009] In a preferred embodiment of the method for preparing the thorium-free cathode for the short-arc discharge lamp of the present invention, in step S1, the carbonaceous material includes carbon powder and / or graphite; and / or, the rare earth oxide includes cerium oxide and / or yttrium oxide.

[0010] Preferably, in step S1, the carbonaceous material includes graphite; and / or, the rare earth oxide includes yttrium oxide.

[0011] In a preferred embodiment of the method for preparing the thorium-free cathode for short-arc discharge lamps according to the present invention, in step S1, the mass of the doping material is 2%-4% of the total mass of the raw materials; and / or, the doping material comprises the following components in parts by mass: 0.25-0.4 parts of carbonaceous material, 0.1-0.15 parts of zirconium oxide, and 1.65-3.5 parts of rare earth oxides.

[0012] Preferably, in step S1, the mass of the doped material is 3% of the total mass of the raw materials; and / or, the doped material comprises the following components in parts by mass: 0.4 parts of carbonaceous material, 0.1 parts of zirconium oxide, and 2.5 parts of rare earth oxides.

[0013] In a preferred embodiment of the method for preparing the thorium-free cathode for the short arc discharge lamp of the present invention, in step S2, the temperature of the reduction treatment is 600℃-800℃ and the time is 100min-120min.

[0014] Preferably, the reduction treatment is performed at a temperature of 600°C for 110 minutes.

[0015] In a preferred embodiment of the method for preparing the thorium-free cathode for the short arc discharge lamp of the present invention, in step S3, the sintering temperature is 1200℃-1500℃ and the time is 30min-50min; and / or, the vertical melting sintering temperature is 2000℃-2200℃ and the time is 40min-60min.

[0016] Preferably, in step S3, the sintering temperature is 1400°C and the time is 40 min; and / or, the vertical melting sintering temperature is 2000°C and the time is 60 min.

[0017] In a preferred embodiment of the method for preparing the thorium-free cathode for the short arc discharge lamp of the present invention, in step S5, the depth h of the trench is 1mm-1.2mm, the included angle β of the apex is 20°-40°, and the bottom width a is 0.42mm-0.8mm.

[0018] Preferably, in step S5, the depth h of the trench is 1.2 mm, the included angle β of the apex is 20°, and the bottom width a is 0.42 mm.

[0019] Secondly, the present invention provides a thorium-free cathode for a short arc discharge lamp prepared by the aforementioned preparation method.

[0020] Thirdly, the present invention provides a short arc discharge lamp including a thorium-free cathode for the short arc discharge lamp.

[0021] Compared with existing technologies, the beneficial effects of this invention are as follows: The method for preparing the thorium-free cathode of the short-arc discharge lamp of this invention completely abandons thorium dioxide, which has natural radioactivity, and uses non-radioactive rare earth oxides as electron emission materials, fundamentally eliminating the radioactive hazards and environmental pollution risks in the production, use, and waste disposal processes, and meeting the requirements of green manufacturing and sustainable development. Secondly, the thorium-free cathode prepared by the method of this invention maintains a low electron work function and good electron emission capability comparable to that of thorium-tungsten cathodes, while possessing excellent high-temperature resistance and structural stability through composite doping and structural reinforcement. Its high-temperature strength, ablation resistance, and thermal shock resistance are effectively improved, thereby ensuring the reliable operation of the cathode in harsh arc environments. Meanwhile, the method for preparing the thorium-free cathode of the short-arc discharge lamp of this invention, by processing inverted triangular grooves on the surface of the cathode substrate, can effectively guide the concentrated heat flow generated during cathode tip operation to diffuse backward along the grooves. This achieves active control of the cathode operating temperature field, significantly improving heat dissipation efficiency and effectively suppressing thermal evaporation and loss of the cathode material. Consequently, the short-arc discharge lamp exhibits superior durability in maintaining light output, and its overall service life is effectively extended. Furthermore, the method for preparing the thorium-free cathode of the short-arc discharge lamp of this invention, based on a stable material system and an efficient heat dissipation structure, significantly improves the heat resistance and electron emission stability of the cathode during operation. This is reflected in the discharge lamp as reduced light output fluctuations and enhanced reliability, thereby improving the overall performance of the light source. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the middle section of the cathode of the thorium-free cathode for the short-arc discharge lamp of the present invention; Figure 2 This is a schematic cross-sectional view of the thorium-free cathode trench for the short-arc discharge lamp of the present invention. Detailed Implementation

[0023] To better illustrate the objectives, technical solutions, and advantages of this invention, the invention will be further described below with reference to specific embodiments. Those skilled in the art should understand that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0024] The following description, in conjunction with specific embodiments, illustrates the practical effects of the present invention.

[0025] Unless otherwise specified, the experimental methods used in the examples are conventional methods; the materials, reagents, equipment, etc. used are all commercially available unless otherwise specified.

[0026] Example 1: This embodiment provides a method for preparing a thorium-free cathode for a short-arc discharge lamp, including the following steps: (1) Raw material ratio: Weigh the raw materials according to the following mass percentages: 95% tungsten powder and 5% total doping material; wherein the doping material contains the following components in the following mass percentages: 3.5 parts cerium oxide, 0.35 parts carbon powder and 0.15 parts zirconium oxide.

[0027] (2) Mixing and drying: Mix the above raw materials evenly and dry them at 180°C for 9 hours to ensure that the powder is dried evenly.

[0028] (3) Reduction treatment: Under a hydrogen atmosphere, the dried powder is reduced at 800℃ for 100 min to obtain tungsten alloy powder.

[0029] (4) Forming and sintering: Tungsten alloy powder is pressed into shape and then sent to a sintering furnace for sintering at 1200℃ for 50 minutes. Then it is placed in a vertical melting machine for vertical melting sintering at 2200℃ for 40 minutes to obtain cathode blank.

[0030] (5) Forging: The cathode blank is forged to form the cathode substrate.

[0031] (6) Grooving: In the middle section of the cathode (e.g.) Figure 1 As shown, grooves are engraved along the central axis, and the cross-section of the grooves is an inverted triangle (as shown). Figure 2 As shown in the figure, specifically, the diameter of the middle section of the cathode is 12 mm, the depth h of the inverted triangular groove is 1 mm, the included angle β of the apex is 30°, and the bottom width a is 0.54 mm.

[0032] (7) The thorium-free cathode prepared in the above steps is used to prepare a 4500W short arc discharge lamp A.

[0033] Example 2: This embodiment provides a method for preparing a thorium-free cathode for a short-arc discharge lamp, including the following steps: (1) Raw material ratio: Weigh the raw materials according to the following mass percentages: 97% tungsten powder, 3% total doping material; wherein the doping material contains the following components in the following mass percentages: 2.5 parts yttrium oxide, 0.4 parts graphite, and 0.1 parts zirconium oxide.

[0034] (2) Mixing and drying: Mix the above raw materials evenly and dry them at 150°C for 10 hours to ensure that the powder is dried evenly.

[0035] (3) Reduction treatment: Under a hydrogen atmosphere, the dried powder was reduced at 600℃ for 110 min to obtain tungsten alloy powder.

[0036] (4) Forming and sintering: Tungsten alloy powder is pressed into shape and then sent to a sintering furnace for sintering at 1400℃ for 40 minutes. Then it is placed in a vertical melting machine for vertical melting sintering at 2000℃ for 60 minutes to obtain cathode blank.

[0037] (5) Forging: The cathode blank is forged to form the cathode substrate.

[0038] (6) Grooving: In the middle section of the cathode (e.g.) Figure 1 As shown, grooves are engraved along the central axis. The cross-section of the grooves is an inverted triangle. Specifically, the diameter of the middle section of the cathode is 12 mm, the depth h of the inverted triangle groove is 1.2 mm, the included angle β of the apex is 20°, and the bottom width a is 0.42 mm.

[0039] (7) The thorium-free cathode prepared in the above steps is used to prepare a 4500W short arc discharge lamp B.

[0040] Example 3: This embodiment provides a method for preparing a thorium-free cathode for a short-arc discharge lamp, including the following steps: (1) Raw material ratio: Weigh the raw materials according to the following mass percentages: 98% tungsten powder and 2% total doping material; wherein the doping material contains the following components in the following mass percentages: 1.85 parts cerium oxide, 0.05 parts graphite and 0.1 parts zirconium oxide.

[0041] (2) Mixing and drying: Mix the above raw materials evenly and dry them at 200℃ for 8 hours to ensure that the powder is dried evenly.

[0042] (3) Reduction treatment: Under a hydrogen atmosphere, the dried powder was reduced at 700℃ for 120 min to obtain tungsten alloy powder.

[0043] (4) Forming and sintering: Tungsten alloy powder is pressed into shape and then sent to a sintering furnace for sintering at 1500℃ for 30 minutes. Then it is placed in a vertical melting machine for vertical melting sintering at 2100℃ for 50 minutes to obtain cathode blank.

[0044] (5) Forging: The cathode blank is forged to form the cathode substrate.

[0045] (6) Grooving: In the middle section of the cathode (e.g.) Figure 1 As shown, grooves are engraved along the central axis. The cross-section of the grooves is an inverted triangle. Specifically, the diameter of the middle section of the cathode is 12 mm, the depth h of the inverted triangle groove is 1.1 mm, the included angle β at the apex is 40°, and the bottom width a is 0.8 mm.

[0046] (7) The thorium-free cathode prepared in the above steps is used to prepare a 4500W short arc discharge lamp C.

[0047] Comparative Example 1: This comparative example uses a thorium-tungsten cathode to prepare a 4500W short-arc discharge lamp.

[0048] Test example: This test case performs the following performance tests on the short-arc discharge lamps of the above embodiments and comparative examples to systematically evaluate and compare the photoelectric performance and lifespan characteristics of short-arc discharge lamps with different cathode materials.

[0049] (1) Illuminance maintenance rate (light decay) test Test method: A 4500W short-arc discharge lamp is installed in a test lamp box. The test probe of the illuminance meter is set at a fixed distance from the light outlet of the test lamp box. After the short-arc discharge lamp is lit and runs stably for 30 minutes, its initial illuminance value is immediately measured and defined as 100%. Then, the lamp is kept running continuously at its rated power, and the illuminance value is monitored at fixed intervals. The cumulative lighting time (unit: h) corresponding to the short-arc discharge lamp illuminance dropping to 94% is recorded. This time directly reflects the cathode material's ability to resist electron emission attenuation and is a key indicator for measuring lamp life.

[0050] (2) Cathode loss test Test method: After completing the illuminance maintenance rate (light decay) test when the illuminance of each lamp drops to 94%, the lamps are cooled to room temperature, the short arc discharge lamps are removed, and the maximum loss depth (unit: mm) of the cathode tip along the axial direction is measured and recorded using a digital measurement projector. This data directly characterizes the cathode material's resistance to evaporation and ion sputtering under the high temperature of the electric arc.

[0051] The test results are shown in Table 1.

[0052] Table 1. Test results of short-arc discharge lamp ignition time and cathode loss in the test examples of this invention. Illuminance Instability Test Test method: Install a 4500W short-arc discharge lamp in a test lamp box. Set the illuminance meter probe at a fixed distance from the light outlet of the test lamp box. After lighting and running stably for 30 minutes, measure the illuminance value every 3 minutes for a total of 10 measurements. Take the average illuminance value and calculate the illuminance instability value of the light source according to the following formula: I stab =(I max -I min ) / I avg ; Among them, I avg I is the average of 10 illuminance measurements. max I is the maximum value among 10 illuminance measurements. min The minimum value among the 10 illuminance measurements is shown in Table 2.

[0053] Table 2. Test results of the illuminance instability values ​​of the short-arc discharge lamps in the test examples of this invention. As shown in Table 1, the illuminance of the thorium-tungsten cathode short-arc discharge lamp in Comparative Example 1 decreased to 94% of the initial illuminance after 200 hours of operation. In contrast, the illuminance of the short-arc discharge lamps (A, B, and C) manufactured with the thorium-free cathode of this invention only decreased to 94% of the initial illuminance after 210 hours, 218 hours, and 215 hours, respectively, with an illuminance maintenance rate improved by 5%-9%. This data clearly indicates that the thorium-free cathode of this invention has superior resistance to electron emission attenuation and extends the service life of the short-arc discharge lamp. Furthermore, the cathode wear of the short-arc discharge lamps manufactured with the thorium-free cathode of this invention (0.27-0.28 mm) is at the same level as that of the thorium-tungsten cathode short-arc discharge lamp (0.26 mm). This proves that this invention has successfully prepared a novel cathode material with ablation resistance comparable to or even slightly superior to that of the thorium-free cathode while completely eliminating radioactive thorium. Furthermore, as shown in Table 2, the illuminance instability values ​​(0.97%-1.12%) of the short arc discharge lamps A, B, and C prepared by the thorium-free cathode of the present invention are all lower than those of the thorium-tungsten cathode of Comparative Example 1 (1.22%). This means that the emission active material distribution of the thorium-free cathode of the present invention is more uniform and consistent, avoiding the problems of arc drift and flicker caused by premature depletion or uneven emission in some areas, and has better luminous stability.

[0054] Therefore, this invention has successfully developed a high-performance, long-life, and highly stable cathode for thorium-free short-arc discharge lamps. Its comprehensive photoelectric performance is superior to that of traditional thorium-tungsten cathodes, while also solving the problem of radioactive hazards. It provides a more advanced and reliable cathode technology solution for high-end short-arc discharge lamps (such as photolithography, searchlights, and special lighting).

[0055] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a thorium-free cathode for a short-arc discharge lamp, characterized in that, Includes the following steps: S1. Mix the raw materials and dry them; the raw materials include tungsten powder and dopant; the mass of the dopant is 1%-5% of the total mass of the raw materials; the dopant includes the following components in parts by mass: 0.05-0.4 parts of carbonaceous material, 0.05-0.15 parts of zirconium oxide, and 0.9-4.5 parts of rare earth oxides; S2. The dried mixed powder is reduced under a reducing atmosphere to obtain tungsten alloy powder; S3. Press the tungsten alloy powder into shape, and then sinter and sinter by vertical melting to obtain a cathode blank; S4. The cathode blank is forged to obtain the cathode substrate; S5. A groove with an inverted triangular cross-section is machined along the central axis of the middle section of the cathode substrate to obtain the thorium-free cathode for the short arc discharge lamp.

2. The method for preparing a thorium-free cathode for a short-arc discharge lamp as described in claim 1, characterized in that, In step S1, the carbonaceous material includes carbon powder and / or graphite; and / or, the rare earth oxide includes cerium oxide and / or yttrium oxide.

3. The method for preparing a thorium-free cathode for a short-arc discharge lamp as described in claim 2, characterized in that, In step S1, the carbonaceous material includes graphite; and / or, the rare earth oxide includes yttrium oxide.

4. The method for preparing a thorium-free cathode for a short-arc discharge lamp as described in claim 1, characterized in that, In step S1, the mass of the doped material is 2%-4% of the total mass of the raw materials; and / or, the doped material comprises the following components in parts by mass: 0.25-0.4 parts of carbonaceous material, 0.1-0.15 parts of zirconium oxide, and 1.65-3.5 parts of rare earth oxides.

5. The method for preparing a thorium-free cathode for a short-arc discharge lamp as described in claim 4, characterized in that, In step S1, the mass of the doped material is 3% of the total mass of the raw materials; and / or, the doped material comprises the following components in parts by mass: 0.4 parts of carbonaceous material, 0.1 parts of zirconium oxide, and 2.5 parts of rare earth oxides.

6. The method for preparing a thorium-free cathode for a short-arc discharge lamp as described in claim 1, characterized in that, In step S2, the reduction treatment is performed at a temperature of 600℃-800℃ for a time of 100min-120min.

7. The method for preparing a thorium-free cathode for a short-arc discharge lamp as described in claim 1, characterized in that, In step S3, the sintering temperature is 1200℃-1500℃ and the time is 30min-50min; and / or, the vertical melting sintering temperature is 2000℃-2200℃ and the time is 40min-60min.

8. The method for preparing a thorium-free cathode for a short-arc discharge lamp as described in claim 1, characterized in that, In step S5, the depth h of the trench is 1mm-1.2mm, the included angle β of the apex is 20°-40°, and the bottom width a is 0.42mm-0.8mm.

9. A thorium-free cathode for a short-arc discharge lamp prepared by the preparation method according to any one of claims 1-8.

10. A short-arc discharge lamp, characterized in that, Including the thorium-free cathode for short-arc discharge lamps as described in claim 9.

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