Monolithic Si-Micropillar Loop Heat Pipe with 250 µm Thickness for High-Heat-Flux Semiconductor Cooling

Monolithic Si-Micropillar Loop Heat Pipe with 250 µm Thickness for High-Heat-Flux Semiconductor Cooling | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Monolithic Si-Micropillar Loop Heat Pipe with 250 µm Thickness for High-Heat-Flux Semiconductor Cooling Ai Ueno, Ryobu Nomura, Masaaki Hashimoto, Madoka Kurosaki, Munehiro Tada, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8551966/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 11 You are reading this latest preprint version Abstract An ultra-thin and all-silicon micropillar loop heat pipe (Si-LHP) was designed and fabricated as a passive two phase cooling device fully compatible with MEMS processing. The device employs Si-Si wafer level bonding to realize a monolithic silicon architecture, which has been difficult to achieve due to the strict flatness and stress requirements of ultra-thin Si-Si bonding. A one-dimensional steady-state model was developed to evaluate conductive heat leakage through the silicon substrate. Using water as the working fluid, the Si-LHP integrates periodic arrays with micropillars and grooves in the evaporator, and a branched microchannel network in the condenser, achieving a total thickness of 250 µm and a theoretical heat transport capacity of 15 W. Experiments demonstrated stable operation up to 10 W (10 W cm⁻²), with a 34°C temperature reduction at 9 W. These results demonstrate the feasibility of monolithic Si-based two-phase cooling devices for thermal management in future semiconductor chips and 3D integrated systems. Physical sciences/Energy science and technology Physical sciences/Engineering Physical sciences/Materials science Physical sciences/Physics Loop heat pipe Semiconductor cooling technology Si-Micropillar Two-phase flow Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction As semiconductor devices continue to advance in performance, localized heat density has been steadily increasing in recent years, and high levels of heat generation have become a common challenge in areas such as mobile processors and AI accelerators [ 1 , 2 ]. High-end smartphones can reach total heat generation levels near 15 W [ 3 , 4 ], creating a critical demand for efficient, miniaturized heat transport technologies. Conventional air cooling and single-phase liquid cooling approaches are increasingly limited by space, efficiency, and thermal resistance [ 5 – 7 ]. In addition, the adoption of three-dimensional integrated circuits (3D-ICs) and chiplet architectures further exacerbates thermal bottlenecks, because multiple active layers are stacked within a confined footprint [ 8 ]. Concurrently, rapid progress in flexible and soft electronics has created new requirements for conformable thermal management systems. Stretchable strain sensors, wearable biochemical sensors, and flexible logic circuits must maintain stable operation under deformation [ 9 – 12 ]. Integrating thermal management functionality into such devices necessitates thin, low-power, and high-efficiency cooling mechanisms. In this context, various passive thermal transport devices such as heat pipes (HPs), vapor chambers (VCs), oscillating heat pipes (OHPs), and loop heat pipes (LHPs) have been investigated for use in compact and thin electronic devices [ 4 , 13 – 20 ]. Among these technologies, LHPs exhibit high potential for efficient heat transport owing to their capillary-driven two-phase circulation mechanism [ 21 ]. Because LHPs can, in principle, decouple the evaporator from the condenser via closed loop channels, they are especially attractive for applications where the heat source and the heat sink cannot be co-located or where form factor constraints are stringent [ 22 ]. However, studies on LHPs applicable to miniaturized and thin-form-factor devices remain very limited. Traditionally, LHPs have been fabricated by connecting cylindrical metal pipes, a method that poses challenges for miniaturization. In particular, the machining limit of wicks and the difficulty of maintaining high hermeticity in the overall device system have hindered further size reduction [ 23 ]. Therefore, the introduction of MEMS technologies which enable micro structured wicks, sealed cavities, and chip-level integration, is essential for the realization of miniaturized LHPs. Microfabrication-based wicks, including re-entrant grooves, and porous silicon structures, have been reported to provide large capillary pressure and well-controlled permeability, indicating that MEMS platforms are promising for high–heat-flux two-phase devices [ 24 – 28 ]. Previous work demonstrated a polymer-based flexible LHP using a PDMS micropillar wick, achieving low thermal leakage and flexibility [ 29 , 30 ]. However, limited barrier performance and thickness control constrained the transport capacity (~ 3 W). To overcome these limitations, a monolithic all-silicon loop heat pipe (LHP) has been developed, integrating MEMS-fabricated micro-pillar wicks and hermetically sealed cavities within a single silicon substrate. This architecture combines the relatively high thermal conductivity of silicon with precisely engineered capillary structures, enabling ultra-thin, high–heat-flux two-phase heat transport suitable for chip-level thermal management. Furthermore, the use of a silicon platform facilitates co-integration with semiconductor chips and through silicon vias (TSVs), which is advantageous for future 3D-IC and heterogeneous integration schemes [ 8 ]. Methods Figure 1 -a) presents the design concept of the Si-LHP, targeting compatibility with three-dimensional integrated circuits (3D-ICs). Heat from the semiconductor chip is absorbed by the evaporator, where liquid water evaporates within a periodic micropillar array. The generated vapor flows through the vapor line toward the condenser, where it releases latent heat and condenses. The condensed fluid returns through the liquid line to the compensation chamber (CC) via capillary action, enabling continuous passive circulation. Device operation is sustained when the capillary pressure ( \(\:{P}_{cap}\) ) generated by the micropillar wick exceeds the total loop pressure loss, as expressed by Eq. ( 1 ) and Eq. ( 2 ). [ 30 ] $$\:{P}_{cap}=\frac{4\sigma\:cos\theta\:}{D(\frac{4}{\pi\:}{\left(\frac{L}{D}\right)}^{2}-1)}$$ 1 , where s , q , D, L are the surface tension, the contact angle, the micropillar diameter, and the center-to-center distance between micropillars. $$\:{P}_{cap}=\varDelta\:{P}_{pillars}+\varDelta\:{P}_{gr}+\varDelta\:{P}_{vl}+\varDelta\:{P}_{con}+\varDelta\:{P}_{ll}$$ 2 , where \(\:\varDelta\:{P}_{pillars},\varDelta\:{P}_{gr},\:\varDelta\:{P}_{vl},\:\varDelta\:{P}_{con},\:\varDelta\:{P}_{ll}\) are the pressure losses in the micropillar region, groove, vapor line, condenser, and liquid line, respectively. Design specifications were defined to satisfy anticipated next-generation cooling requirements: (1) heat transport capacity ≥ 10 W (10 W cm⁻²), (2) operating temperature below 100°C, and (3) effective transport length between 40–50 mm. To realize these goals, several design strategies were implemented. The evaporator employs a micropillar–groove periodic structure to promote thin-film evaporation and stabilize the liquid–vapor interface. Above the evaporator, a secondary wick composed of aligned micropillars ensures stable liquid replenishment under high thermal loads. The condenser adopts a branched channel layout instead of a serpentine configuration, effectively reducing flow resistance. A spacer layer prevents deformation of high-aspect-ratio channels, maintaining structural stability and consistent two-phase flow. Given the relatively high thermal conductivity of silicon (148 W m⁻¹ K⁻¹), conductive heat leakage was incorporated into the thermal model. A one-dimensional steady-state analysis was performed to determine the device specifications. The final device dimensions were 81 mm × 58 mm × 250 µm. Using water as the working fluid, the calculated maximum heat transport capacity reached 15 W, as shown in Fig. 1 -b). The specifications of the designed Si-LHP device are as follows: the evaporator measures 15 × 20 × 0.08 mm with an integrated groove of 10 × 0.3 × 0.08 mm, the compensation chamber (CC) is 31 × 32 × 0.08 mm, the vapor and liquid lines are 52 × 6 × 0.08 mm and 20 × 6 × 0.08 mm, respectively, the condenser is 40 × 5 × 0.08 mm, and the heat source measures 10 × 10 × 1.8 mm. The design of the micro-pillar region in the evaporator, which generates the capillary pressure critical to the device performance, follows the relationship shown in Eq. ( 1 ). As the ratio \(\:L/D\) decreases, the capillary pressure \(\:{P}_{\text{cap}}\) increases; however, the fabrication becomes more challenging. Considering this trade-off, the final micro-pillar dimensions were determined to be a center-to-center pitch of 30 µm, a pillar diameter D of 27 µm, and a height of 80 µm. The Si-LHP was fabricated using MEMS-compatible processes, as illustrated in Fig. 2-a). Deep reactive-ion etching (DRIE) formed the micropillars and flow channels. To enhance wettability with the working fluid, a SiO₂ coating was applied, reducing the Si–water contact angle from 86° to 26°, corresponding to a sixteen fold increase in capillary driving pressure. The upper and lower wafers were bonded through surface-activated bonding, forming a hermetically sealed structure. Finally, a Si–Si monolithic LHP with a thickness of 250 µm was successfully fabricated by mechanical grinding and polishing. Considering the stringent flatness, surface-roughness, and stress-tolerance requirements inherent to wafer level of Si–Si bonding, this achievement represents the thinnest LHP realized entirely within a monolithic silicon substrate to date. The filling ratio of the working fluid, water, was set to approximately 70%. As shown in Fig. 2-b), SEM observations confirmed that the micro-pillars which formed the finest structures fabricated in the MEMS process were generally consistent with the design dimensions. Using the measured pillar diameter of 27 µm and the contact angle of 26° between the hydrophilic SiO₂ surface and water, the maximum capillary pressure was estimated to be 13.9 kPa at room temperature under a heat load of 15 W. Figure 2-c) shows the measured height profile of the fabricated micro-pillars. The 3D image indicates that the micro-pillars are densely and uniformly arranged, and the boundary between the inner and outer regions of the flow channel is nearly vertical, confirming high etching accuracy. The measured pillar height was approximately 68 µm, which is approximately 12 µm shorter than the designed value of 80 µm. This difference arose because further etching could reduce the pillar diameter. Therefore, the etching depth was adjusted such that the micro-pillar diameter, which directly affects the capillary force, reached the designed value of 27 µm. Results Thermal performance testing was conducted under horizontal orientation using heat load tests (Fig. 3 ). The heat load increased from 0 W to 10 W in increments of 1 W, and the experiments were conducted under ambient conditions with the condenser cooled by natural convection. Heating was applied to the back side of the evaporator using a ceramic heater, and the temperature distribution on the device surface was observed using an infrared (IR) thermal camera. Figure 3 -a) and b) show the images of IR camera and the heat source temperature results with and without the working fluid. A slight temperature differential appeared at 4 W and became pronounced above 7 W, indicating the onset of vigorous two-phase transport. Stable heat transfer was observed up to 10 W, with a temperature reduction of 34°C at 9 W, demonstrating the system’s ability to maintain efficient phase-change cooling under high heat flux. For the device without working fluid, heat was conducted radially from the evaporator, forming concentric temperature contours. In contrast, the device filled with working fluid exhibited less heat leakage to the compensation chamber (CC), indicating more unidirectional heat transport from the evaporator to the vapor line. Furthermore, reducing the device thickness to 250 µm effectively suppressed heat leakage to the CC, likely enhancing the contribution of latent heat evaporation at the micro-pillars on the evaporator side. The condenser of the present LHP employs a branched flow channel design, which maintains heat dissipation performance while reducing pressure loss. At a heat load of 9 W, vapor flow was observed in both Branch 1 and Branch 2, demonstrating the effectiveness of the branched condenser configuration in the ultra-thin LHP. A comparison between measured and simulated steady-state source temperatures is shown in Fig. 3 -c). At the maximum heat input of 10 W, the experimental temperature exceeded the modeled prediction by approximately 50°C. Improvement of the liquid encapsulation and sealing processes is thus critical for enhanced performance consistency. Discussion Figure 4 shows the measured thermal resistance of \(\:{R}_{\text{s}\text{y}\text{s}}\) calculated from Eq. ( 3 ) using the temperature difference between the evaporator and condenser. $$\:{R}_{\text{s}\text{y}\text{s}}=\frac{{T}_{ec}-{T}_{conave}}{{Q}_{load\:}},\:$$ 3 where \(\:{T}_{ec}\) , \(\:{T}_{conave}\) and \(\:{Q}_{load\:}\) are temperature of evaporator case and average of condenser, and heat load. The thermal resistance remained approximately 7°C/W up to 6 W, showing a decreasing trend beyond 7 W, and reached a minimum value of 4.82°C/W at 9 W. The presence of the working fluid reduced the thermal resistance by up to about 50%, indicating the effectiveness of two-phase heat transport in the device. The results obtained validate the feasibility of wafer-integrated two-phase cooling using a silicon micropillar-based LHP architecture. The observed temperature drops and sustained operation up to 10 W confirm that the system can function as a capillary-driven cooling platform suitable for future high-heat-density applications. Conclusion An ultra-thin silicon micropillar loop heat pipe was designed and fabricated using MEMS-compatible techniques. A one-dimensional steady-state model considering thermal leakage due to the high thermal conductivity of silicon was developed, and a device capable of transporting up to 15 W of heat was designed using water as the working fluid. An ultra-thin fabrication process with a total thickness of 250 µm was established through deep reactive ion etching (DRIE), SiO₂ deposition, surface-activated bonding, and mechanical polishing using MEMS technology. Thermal performance testing confirmed stable heat transport of up to 10 W (10 W cm⁻²) and a maximum temperature reduction of 34°C at 9 W depending on the presence of the working fluid. These results demonstrate the potential of the proposed LHP as a compact, high-performance thermal management solution for semiconductor cooling and future three-dimensional integrated circuit (3D-IC) applications. The Si-LHP demonstrated represents a compact, integrable, and high-heat-flux cooling solution for next-generation semiconductor and 3D-IC devices. Declarations Author Contribution A.U., R. N. and H.N. developed the thermal model, designed the Si-LHP structure.A.U., M.K., M.H. and M.T. carried out the MEMS fabrication, performed the thermal experiments, and prepared the measurement data.A.U. and H.N. analyzed the experimental results and validated the thermal model.R.N. prepared all figures and drafted the initial manuscript.H.N. supervised the overall researchrevised and finalized the manuscript.**All authors reviewed and approved the final version of the manuscript. Acknowledgments This work was supported by JST-ALCA-Next Japan Grant Number JPMJAN24E2, and the Asahi Glass Foundation. Data Availability Included in the paper. All data supporting the findings of this study are available within the paper. References Rangarajan, S., Schiffres, S. N. & Sammakia, B. Engineering 26 185–197. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8551966","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":576667331,"identity":"a6cc6e57-e6e0-481a-8c84-7517b531e013","order_by":0,"name":"Ai Ueno","email":"","orcid":"","institution":"Nagoya University","correspondingAuthor":false,"prefix":"","firstName":"Ai","middleName":"","lastName":"Ueno","suffix":""},{"id":576667336,"identity":"08a39334-9c9c-40d9-9f12-dab657d22c97","order_by":1,"name":"Ryobu Nomura","email":"","orcid":"","institution":"Nagoya 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16:05:24","extension":"xml","order_by":13,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":54296,"visible":true,"origin":"","legend":"","description":"","filename":"b8bf3911af314a01ba3363c63d7219811structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8551966/v1/538ce0be1c01fe959217a4a7.xml"},{"id":100701107,"identity":"f05e9302-1914-47cb-aa40-33e3c587e157","added_by":"auto","created_at":"2026-01-20 16:01:30","extension":"html","order_by":14,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":63625,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8551966/v1/259f355521f5d747b5967f3d.html"},{"id":100701250,"identity":"838102ba-da76-4c16-ba12-7015c3dbd6d8","added_by":"auto","created_at":"2026-01-20 16:02:32","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":84783,"visible":true,"origin":"","legend":"\u003cp\u003ea) Schematic illustration of the designed silicon micropillar loop heat pipe (Si-LHP).\u003c/p\u003e\n\u003cp\u003eb) Analytical results of pressure losses and capillary force in each section of the device using water as the working fluid.\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8551966/v1/9cb0ea1cab3295dae66cf332.jpeg"},{"id":100701845,"identity":"6229cf0d-e946-4bb6-aa09-e031279363a1","added_by":"auto","created_at":"2026-01-20 16:07:00","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":100766,"visible":true,"origin":"","legend":"\u003cp\u003ea) Process flow of a Si-LHP device fabricated using MEMS-compatible processes. b) Overview of\u003c/p\u003e\n\u003cp\u003ea silicon monolithic device (250 µm thick) and the micro-pillar section. c) Measurement results of the micro-\u003c/p\u003e\n\u003cp\u003epillar height profile obtained using a digital microscope.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8551966/v1/c661de48c9bc743515ed4246.jpeg"},{"id":100701155,"identity":"38b98c80-27a3-4897-8f5e-e901da8ab42b","added_by":"auto","created_at":"2026-01-20 16:01:55","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":85178,"visible":true,"origin":"","legend":"\u003cp\u003ea) Thermal images and b) Comparison of heat source temperature with and without the working fluid obtained from heat load testing. c) Comparison between analytical and experimental results of heat source temperature in steady state.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8551966/v1/60e6adbd3f7173d0e11e6fa3.jpeg"},{"id":100701617,"identity":"a45b517a-13d8-48d7-add9-ccdceffbf0a4","added_by":"auto","created_at":"2026-01-20 16:05:22","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":32660,"visible":true,"origin":"","legend":"\u003cp\u003eEvaporator–condenser temperature difference and thermal resistance in the Si-LHP.\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8551966/v1/1747ac22d298f4b6a8ff338c.jpeg"},{"id":100710634,"identity":"fe09aa4a-6e5b-4389-b6fd-21c2743e0a49","added_by":"auto","created_at":"2026-01-20 17:55:50","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":706809,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8551966/v1/068ffe8e-de70-48e9-9890-193274ab5fdd.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Monolithic Si-Micropillar Loop Heat Pipe with 250 µm Thickness for High-Heat-Flux Semiconductor Cooling","fulltext":[{"header":"Introduction","content":"\u003cp\u003eAs semiconductor devices continue to advance in performance, localized heat density has been steadily increasing in recent years, and high levels of heat generation have become a common challenge in areas such as mobile processors and AI accelerators [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. High-end smartphones can reach total heat generation levels near 15 W [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], creating a critical demand for efficient, miniaturized heat transport technologies. Conventional air cooling and single-phase liquid cooling approaches are increasingly limited by space, efficiency, and thermal resistance [\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. In addition, the adoption of three-dimensional integrated circuits (3D-ICs) and chiplet architectures further exacerbates thermal bottlenecks, because multiple active layers are stacked within a confined footprint [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eConcurrently, rapid progress in flexible and soft electronics has created new requirements for conformable thermal management systems. Stretchable strain sensors, wearable biochemical sensors, and flexible logic circuits must maintain stable operation under deformation [\u003cspan additionalcitationids=\"CR10 CR11\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Integrating thermal management functionality into such devices necessitates thin, low-power, and high-efficiency cooling mechanisms.\u003c/p\u003e \u003cp\u003eIn this context, various passive thermal transport devices such as heat pipes (HPs), vapor chambers (VCs), oscillating heat pipes (OHPs), and loop heat pipes (LHPs) have been investigated for use in compact and thin electronic devices [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan additionalcitationids=\"CR14 CR15 CR16 CR17 CR18 CR19\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Among these technologies, LHPs exhibit high potential for efficient heat transport owing to their capillary-driven two-phase circulation mechanism [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Because LHPs can, in principle, decouple the evaporator from the condenser via closed loop channels, they are especially attractive for applications where the heat source and the heat sink cannot be co-located or where form factor constraints are stringent [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. However, studies on LHPs applicable to miniaturized and thin-form-factor devices remain very limited. Traditionally, LHPs have been fabricated by connecting cylindrical metal pipes, a method that poses challenges for miniaturization. In particular, the machining limit of wicks and the difficulty of maintaining high hermeticity in the overall device system have hindered further size reduction [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Therefore, the introduction of MEMS technologies which enable micro structured wicks, sealed cavities, and chip-level integration, is essential for the realization of miniaturized LHPs. Microfabrication-based wicks, including re-entrant grooves, and porous silicon structures, have been reported to provide large capillary pressure and well-controlled permeability, indicating that MEMS platforms are promising for high\u0026ndash;heat-flux two-phase devices [\u003cspan additionalcitationids=\"CR25 CR26 CR27\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e \u003cp\u003ePrevious work demonstrated a polymer-based flexible LHP using a PDMS micropillar wick, achieving low thermal leakage and flexibility [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. However, limited barrier performance and thickness control constrained the transport capacity (~\u0026thinsp;3 W). To overcome these limitations, a monolithic all-silicon loop heat pipe (LHP) has been developed, integrating MEMS-fabricated micro-pillar wicks and hermetically sealed cavities within a single silicon substrate. This architecture combines the relatively high thermal conductivity of silicon with precisely engineered capillary structures, enabling ultra-thin, high\u0026ndash;heat-flux two-phase heat transport suitable for chip-level thermal management. Furthermore, the use of a silicon platform facilitates co-integration with semiconductor chips and through silicon vias (TSVs), which is advantageous for future 3D-IC and heterogeneous integration schemes [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e-a) presents the design concept of the Si-LHP, targeting compatibility with three-dimensional integrated circuits (3D-ICs). Heat from the semiconductor chip is absorbed by the evaporator, where liquid water evaporates within a periodic micropillar array. The generated vapor flows through the vapor line toward the condenser, where it releases latent heat and condenses. The condensed fluid returns through the liquid line to the compensation chamber (CC) via capillary action, enabling continuous passive circulation. Device operation is sustained when the capillary pressure (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{P}_{cap}\\)\u003c/span\u003e\u003c/span\u003e) generated by the micropillar wick exceeds the total loop pressure loss, as expressed by Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) and Eq.\u0026nbsp;(\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:{P}_{cap}=\\frac{4\\sigma\\:cos\\theta\\:}{D(\\frac{4}{\\pi\\:}{\\left(\\frac{L}{D}\\right)}^{2}-1)}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e,\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003es\u003c/em\u003e, \u003cem\u003eq\u003c/em\u003e, \u003cem\u003eD, L\u003c/em\u003e are the surface tension, the contact angle, the micropillar diameter, and the center-to-center distance between micropillars.\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:{P}_{cap}=\\varDelta\\:{P}_{pillars}+\\varDelta\\:{P}_{gr}+\\varDelta\\:{P}_{vl}+\\varDelta\\:{P}_{con}+\\varDelta\\:{P}_{ll}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e,\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varDelta\\:{P}_{pillars},\\varDelta\\:{P}_{gr},\\:\\varDelta\\:{P}_{vl},\\:\\varDelta\\:{P}_{con},\\:\\varDelta\\:{P}_{ll}\\)\u003c/span\u003e\u003c/span\u003e are the pressure losses in the micropillar region, groove, vapor line, condenser, and liquid line, respectively.\u003c/p\u003e \u003cp\u003eDesign specifications were defined to satisfy anticipated next-generation cooling requirements:\u003c/p\u003e \u003cp\u003e(1) heat transport capacity\u0026thinsp;\u0026ge;\u0026thinsp;10 W (10 W cm⁻\u0026sup2;),\u003c/p\u003e \u003cp\u003e(2) operating temperature below 100\u0026deg;C, and\u003c/p\u003e \u003cp\u003e(3) effective transport length between 40\u0026ndash;50 mm.\u003c/p\u003e \u003cp\u003eTo realize these goals, several design strategies were implemented. The evaporator employs a micropillar\u0026ndash;groove periodic structure to promote thin-film evaporation and stabilize the liquid\u0026ndash;vapor interface. Above the evaporator, a secondary wick composed of aligned micropillars ensures stable liquid replenishment under high thermal loads. The condenser adopts a branched channel layout instead of a serpentine configuration, effectively reducing flow resistance. A spacer layer prevents deformation of high-aspect-ratio channels, maintaining structural stability and consistent two-phase flow.\u003c/p\u003e \u003cp\u003eGiven the relatively high thermal conductivity of silicon (148 W m⁻\u0026sup1; K⁻\u0026sup1;), conductive heat leakage was incorporated into the thermal model. A one-dimensional steady-state analysis was performed to determine the device specifications. The final device dimensions were 81 mm \u0026times; 58 mm \u0026times; 250 \u0026micro;m. Using water as the working fluid, the calculated maximum heat transport capacity reached 15 W, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e-b). The specifications of the designed Si-LHP device are as follows: the evaporator measures 15 \u0026times; 20 \u0026times; 0.08 mm with an integrated groove of 10 \u0026times; 0.3 \u0026times; 0.08 mm, the compensation chamber (CC) is 31 \u0026times; 32 \u0026times; 0.08 mm, the vapor and liquid lines are 52 \u0026times; 6 \u0026times; 0.08 mm and 20 \u0026times; 6 \u0026times; 0.08 mm, respectively, the condenser is 40 \u0026times; 5 \u0026times; 0.08 mm, and the heat source measures 10 \u0026times; 10 \u0026times; 1.8 mm. The design of the micro-pillar region in the evaporator, which generates the capillary pressure critical to the device performance, follows the relationship shown in Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). As the ratio \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:L/D\\)\u003c/span\u003e\u003c/span\u003edecreases, the capillary pressure \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{P}_{\\text{cap}}\\)\u003c/span\u003e\u003c/span\u003eincreases; however, the fabrication becomes more challenging. Considering this trade-off, the final micro-pillar dimensions were determined to be a center-to-center pitch of 30 \u0026micro;m, a pillar diameter \u003cem\u003eD\u003c/em\u003e of 27 \u0026micro;m, and a height of 80 \u0026micro;m.\u003c/p\u003e \u003cp\u003eThe Si-LHP was fabricated using MEMS-compatible processes, as illustrated in Fig.\u0026nbsp;2-a). Deep reactive-ion etching (DRIE) formed the micropillars and flow channels. To enhance wettability with the working fluid, a SiO₂ coating was applied, reducing the Si\u0026ndash;water contact angle from 86\u0026deg; to 26\u0026deg;, corresponding to a sixteen fold increase in capillary driving pressure. The upper and lower wafers were bonded through surface-activated bonding, forming a hermetically sealed structure. Finally, a Si\u0026ndash;Si monolithic LHP with a thickness of 250 \u0026micro;m was successfully fabricated by mechanical grinding and polishing. Considering the stringent flatness, surface-roughness, and stress-tolerance requirements inherent to wafer level of Si\u0026ndash;Si bonding, this achievement represents the thinnest LHP realized entirely within a monolithic silicon substrate to date. The filling ratio of the working fluid, water, was set to approximately 70%. As shown in Fig.\u0026nbsp;2-b), SEM observations confirmed that the micro-pillars which formed the finest structures fabricated in the MEMS process were generally consistent with the design dimensions. Using the measured pillar diameter of 27 \u0026micro;m and the contact angle of 26\u0026deg; between the hydrophilic SiO₂ surface and water, the maximum capillary pressure was estimated to be 13.9 kPa at room temperature under a heat load of 15 W. Figure\u0026nbsp;2-c) shows the measured height profile of the fabricated micro-pillars. The 3D image indicates that the micro-pillars are densely and uniformly arranged, and the boundary between the inner and outer regions of the flow channel is nearly vertical, confirming high etching accuracy. The measured pillar height was approximately 68 \u0026micro;m, which is approximately 12 \u0026micro;m shorter than the designed value of 80 \u0026micro;m. This difference arose because further etching could reduce the pillar diameter. Therefore, the etching depth was adjusted such that the micro-pillar diameter, which directly affects the capillary force, reached the designed value of 27 \u0026micro;m.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eThermal performance testing was conducted under horizontal orientation using heat load tests (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The heat load increased from 0 W to 10 W in increments of 1 W, and the experiments were conducted under ambient conditions with the condenser cooled by natural convection. Heating was applied to the back side of the evaporator using a ceramic heater, and the temperature distribution on the device surface was observed using an infrared (IR) thermal camera. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003e-a) and b) show the images of IR camera and the heat source temperature results with and without the working fluid. A slight temperature differential appeared at 4 W and became pronounced above 7 W, indicating the onset of vigorous two-phase transport. Stable heat transfer was observed up to 10 W, with a temperature reduction of 34\u0026deg;C at 9 W, demonstrating the system\u0026rsquo;s ability to maintain efficient phase-change cooling under high heat flux. For the device without working fluid, heat was conducted radially from the evaporator, forming concentric temperature contours. In contrast, the device filled with working fluid exhibited less heat leakage to the compensation chamber (CC), indicating more unidirectional heat transport from the evaporator to the vapor line. Furthermore, reducing the device thickness to 250 \u0026micro;m effectively suppressed heat leakage to the CC, likely enhancing the contribution of latent heat evaporation at the micro-pillars on the evaporator side. The condenser of the present LHP employs a branched flow channel design, which maintains heat dissipation performance while reducing pressure loss. At a heat load of 9 W, vapor flow was observed in both Branch 1 and Branch 2, demonstrating the effectiveness of the branched condenser configuration in the ultra-thin LHP.\u003c/p\u003e \u003cp\u003eA comparison between measured and simulated steady-state source temperatures is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003e-c). At the maximum heat input of 10 W, the experimental temperature exceeded the modeled prediction by approximately 50\u0026deg;C. Improvement of the liquid encapsulation and sealing processes is thus critical for enhanced performance consistency.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the measured thermal resistance of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{R}_{\\text{s}\\text{y}\\text{s}}\\)\u003c/span\u003e\u003c/span\u003e calculated from Eq.\u0026nbsp;(\u003cspan refid=\"Equ3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) using the temperature difference between the evaporator and condenser.\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:{R}_{\\text{s}\\text{y}\\text{s}}=\\frac{{T}_{ec}-{T}_{conave}}{{Q}_{load\\:}},\\:$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{T}_{ec}\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{T}_{conave}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{Q}_{load\\:}\\)\u003c/span\u003e\u003c/span\u003eare temperature of evaporator case and average of condenser, and heat load. The thermal resistance remained approximately 7\u0026deg;C/W up to 6 W, showing a decreasing trend beyond 7 W, and reached a minimum value of 4.82\u0026deg;C/W at 9 W. The presence of the working fluid reduced the thermal resistance by up to about 50%, indicating the effectiveness of two-phase heat transport in the device.\u003c/p\u003e \u003cp\u003eThe results obtained validate the feasibility of wafer-integrated two-phase cooling using a silicon micropillar-based LHP architecture. The observed temperature drops and sustained operation up to 10 W confirm that the system can function as a capillary-driven cooling platform suitable for future high-heat-density applications.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eAn ultra-thin silicon micropillar loop heat pipe was designed and fabricated using MEMS-compatible techniques. A one-dimensional steady-state model considering thermal leakage due to the high thermal conductivity of silicon was developed, and a device capable of transporting up to 15 W of heat was designed using water as the working fluid. An ultra-thin fabrication process with a total thickness of 250 \u0026micro;m was established through deep reactive ion etching (DRIE), SiO₂ deposition, surface-activated bonding, and mechanical polishing using MEMS technology. Thermal performance testing confirmed stable heat transport of up to 10 W (10 W cm⁻\u0026sup2;) and a maximum temperature reduction of 34\u0026deg;C at 9 W depending on the presence of the working fluid. These results demonstrate the potential of the proposed LHP as a compact, high-performance thermal management solution for semiconductor cooling and future three-dimensional integrated circuit (3D-IC) applications. The Si-LHP demonstrated represents a compact, integrable, and high-heat-flux cooling solution for next-generation semiconductor and 3D-IC devices.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eA.U., R. N. and H.N. developed the thermal model, designed the Si-LHP structure.A.U., M.K., M.H. and M.T. carried out the MEMS fabrication, performed the thermal experiments, and prepared the measurement data.A.U. and H.N. analyzed the experimental results and validated the thermal model.R.N. prepared all figures and drafted the initial manuscript.H.N. supervised the overall researchrevised and finalized the manuscript.**All authors reviewed and approved the final version of the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgments\u003c/h2\u003e \u003cp\u003eThis work was supported by JST-ALCA-Next Japan Grant Number JPMJAN24E2, and the Asahi Glass Foundation.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eIncluded in the paper. All data supporting the findings of this study are available within the paper.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eRangarajan, S., Schiffres, S. 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Eng.\u003c/em\u003e \u003cb\u003e272\u003c/b\u003e, 126249 (2025).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Loop heat pipe, Semiconductor cooling technology, Si-Micropillar, Two-phase flow","lastPublishedDoi":"10.21203/rs.3.rs-8551966/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8551966/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAn ultra-thin and all-silicon micropillar loop heat pipe (Si-LHP) was designed and fabricated as a passive two phase cooling device fully compatible with MEMS processing. The device employs Si-Si wafer level bonding to realize a monolithic silicon architecture, which has been difficult to achieve due to the strict flatness and stress requirements of ultra-thin Si-Si bonding. A one-dimensional steady-state model was developed to evaluate conductive heat leakage through the silicon substrate. Using water as the working fluid, the Si-LHP integrates periodic arrays with micropillars and grooves in the evaporator, and a branched microchannel network in the condenser, achieving a total thickness of 250 \u0026micro;m and a theoretical heat transport capacity of 15 W. Experiments demonstrated stable operation up to 10 W (10 W cm⁻\u0026sup2;), with a 34\u0026deg;C temperature reduction at 9 W. These results demonstrate the feasibility of monolithic Si-based two-phase cooling devices for thermal management in future semiconductor chips and 3D integrated systems.\u003c/p\u003e","manuscriptTitle":"Monolithic Si-Micropillar Loop Heat Pipe with 250 µm Thickness for High-Heat-Flux Semiconductor Cooling","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-20 13:16:02","doi":"10.21203/rs.3.rs-8551966/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"123228714443084562035680367242541556209","date":"2026-01-19T08:44:20+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-19T03:32:00+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"29398814031665425846974612222096804077","date":"2026-01-18T22:13:50+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"139712302250445754205610515737076945616","date":"2026-01-17T17:58:10+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"8558872143615382319713097014088540371","date":"2026-01-16T17:42:22+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"249317668922364650331569863509679083121","date":"2026-01-16T14:00:48+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-01-16T10:23:26+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-01-16T10:18:48+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2026-01-16T09:49:31+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-01-15T09:05:54+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2026-01-15T08:54:07+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"d36a60cf-2826-4010-a8a5-4a990f1fb834","owner":[],"postedDate":"January 20th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":61351068,"name":"Physical sciences/Energy science and technology"},{"id":61351069,"name":"Physical sciences/Engineering"},{"id":61351070,"name":"Physical sciences/Materials science"},{"id":61351071,"name":"Physical sciences/Physics"}],"tags":[],"updatedAt":"2026-05-19T14:08:14+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-20 13:16:02","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8551966","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8551966","identity":"rs-8551966","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}