Interlayer cooling potential in vertically integrated packages

The heat-removal capability of area-interconnect-compatible interlayer cooling in vertically integrated, high-performance chip stacks was characterized with de-ionized water as coolant. Correlation-based predictions and computational fluid dynamic modeling of cross-flow heat-removal structures show that the coolant temperature increase due to sensible heat absorption limits the cooling performance at hydraulic diameters ≤200 μm. An experimental investigation with uniform and double-side heat flux at Reynolds numbers ≤1,000 and heat transfer areas of 1 cm² was carried out to identify the most efficient interlayer heat-removal structure. The following structures were tested: parallel plate, microchannel, pin fin, and their combinations with pins using in-line and staggered configurations with round and drop-like shapes at pitches ranging from 50 to 200 μm and fluid structure heights of 100–200 μm. A hydrodynamic flow regime transition responsible for a local junction temperature minimum was observed for pin fin in-line structures. The experimental data was extrapolated to predict maximal heat flux in chip stacks having a 4-cm² heat transfer area. The performance of interlayer cooling strongly depends on this parameter, and drops from >200 W/cm² at 1 cm² and >50 μm interconnect pitch to <100 W/cm² at 4 cm². From experimental data, friction factor and Nusselt number correlations were derived for pin fin in-line and staggered structures.     

Seed bubbles trigger boiling heat transfer in silicon microchannels

The smooth channel surface of microsystems delays boiling incipience in heated microchannels. In this paper, we use seed bubbles to trigger boiling heat transfer and control thermal non-equilibrium of liquid and vapor phases in parallel microchannels. The test section consisted of a top glass cover and a silicon substrate. Microheater array was integrated at the top glass cover surface and driven by a pulse voltage signal to generate seed bubbles in time sequence. Each microheater corresponds to a specific microchannel and is located in the microchannel upstream. Five triangular microchannels with a hydraulic diameter of 100 μm and a length of 12.0 mm were etched in the silicon substrate. A thin platinum film was deposited at the back surface of silicon chip with an effective heating area of 4,500 × 1,366 μm, acting as the main heater for the heat transfer system. Acetone liquid was used. With the data range reported here, boiling incipience was not initiated if wall superheats are smaller than 15°C without seed bubbles assisted. Injection seed bubbles triggers boiling incipience and controls thermal non-equilibrium between liquid and vapor phases successfully. Four modes of flow and heat transfer are identified. Modes 1, 2, and 4 are the stable ones without apparent oscillations of pressure drops and heating surface temperatures, and mode 3 displays flow instabilities with apparent amplitudes and long periods of these parameters. The four modes are divided based on the four types of flow patterns observed in microchannels. Seed bubble frequency is a key factor to influence the heat transfer. The higher the seed bubble frequency, the more decreased non-equilibrium between two phases and heating surface temperatures are. The seed bubble frequency can reach a saturation value, at which heat transfer enhancement attains the maximum degree, inferring that a complete thermal equilibrium of two phases is approached. The saturation frequency is about a couple of thousand Hertz in this study.     

Application of photo-etching of polytetrafluoroethylene induced by high energy synchrotron radiation to LIGA

Polytetrafluoroethylene (PTFE) microstructures’ processing characteristics using X-ray photo decomposition and desorption are studied in the highest energy region (2–12 keV). While the exposed surface states are seen melting and boiling from the remaining bubble structure of the irradiated surface, basic photochemistry of PTFE is also same as previous reports. Surface modification of PTFE from hydrophobic to hydrophilic was investigated in order to bond PTFE sheets to a brass substrate. Then, we have a successfully fabricated Ni microstructure by LIGA process using PTFE photo-etching using high energy X-ray. Proposed LIGA using SR-photo-etching of PTFE can simplify the total process, and it can ensure that dimensional errors remain small due to swelling in developer and electroplating bath.     

High aspect ratio metal micro and nano pillars for minimal footprint MEMS suspension

Vertical nano and micro pillars perpendicularly rising from a substrate offer two lateral translatory–rotatory degrees of freedom. Electroforming allows their production as small footprint integrated suspension elements of micro to nano scale. This paper demonstrates the design of a novel inertial sensor concept with acceleration sensor and gyroscope function using only one inertial mass. Experimental results using UV Direct LIGA with AZ 125 nXT show the feasibility of a technology demonstrator with a copper micro pillar of 400 μm length and 40 μm diameter. Further work using x-ray Direct LIGA is scheduled for the production of the pillar with a length of 100 μm and a diameter of 3–6 μm. Fabrication concepts and pilot tests show promising possibilities for miniaturization towards nano scale pillars for minimal footprint suspension in MEMS.     

Micro injection molding for mass production using LIGA mold inserts

Micro molding is one of key technologies for mass production of polymer micro parts and structures with high aspect ratios. The authors developed a commercially available micro injection molding technology for high aspect ratio microstructures (HARMs) with LIGA-made mold inserts and pressurized CO₂ gasses. The test inserts made of nickel with the smallest surface details of 5 μm with structural height of 15 μm were fabricated by using LIGA technology. High surface quality in terms of low surface roughness of the mold inserts allowed using for injection molding. Compared to standard inserts no draft, which is required to provide a proper demolding, was formed in the inserts. To meet higher economic efficiency and cost reduction, a fully electrical injection molding machine of higher accuracy has been applied with dissolving CO₂ gasses into molten resin. The gasses acts as plasticizer and improves the flowability of the resin. Simultaneously, pressurizing the cavity with the gasses allows high replication to be obtained. Micro injection molding, using polycarbonate as polymer resins, with the aspect ratio of two was achieved in the area of 28 × 55 mm² at the cycle time of 40 s with CO₂ gasses, in contrast to the case of the aspect ratio of 0.1 without the gasses.     

Are the available boiling heat transfer coefficients suitable for silicon microchannel heat sinks?

The typical MEMS fabrication of micro evaporators ensures the perfect smooth wall surface that is lack of nucleation sites, significantly decreasing the heat transfer coefficients compared with miniature evaporators fabricated using copper or stainless steel. In the present paper, we performed the boiling heat transfer experiment in silicon triangular microchannel heat sink over a wide parameter range for 102 runs. Acetone was used as the working fluid. The measured boiling heat transfer coefficients versus the local vapor mass qualities are compared with the classical Chen’s correlation and other correlations for macro and miniature capillary tubes. It is found that most of these correlations significantly over-predict the measured heat transfer coefficients. New correlations are given. There are many reasons for such deviations. The major reason is coming from the perfect smooth silicon surface that lowers the heat transfer performances. New theory is recommended for the silicon microchannel heat sink that should be different from metallic capillary tubes.     

A Novel Benzocyclobutene-Based Device for Studying the Dynamics of Heat Transfer During the Nucleation Process

A novel microelectromechanical device has been developed to study the details of the heat transfer mechanisms involved at the nucleation site for the nucleate boiling process. This device enables quantifying the magnitude, time period of activation, and specific areas of influence of different mechanisms of heat transfer from the surface with a resolution several times greater than previously reported. This is achieved through the use of an array of embedded temperature sensors within a carefully designed dual-layer (silicon and benzocyclobutene) wall which allows for the accurate calculation of local heat flux, circumventing difficulties encountered when using existing methods. The sensors are radially distributed around the nucleation site. Heat is supplied to the wall by a thin film heater fabricated on the outer nonwetted surface. Single bubbles are generated at the center of the array while the temperatures and the bubble images are recorded with a sampling frequency of 8 kHz. The temperature data provided the necessary thermal boundary conditions to numerically calculate the surface heat flux with an unprecedented radial resolution of 22-40 mum. Fabrication, characterization, and the ability of the developed device to elucidate the heat transfer aspects of the nucleation process are demonstrated.     

Fabrication of electrostatic MEMS microactuator based on X-ray lithography with Pb-based X-ray mask and dry-film-transfer-to-PCB process

Lithographie Galvanoformung Abformung (LIGA) is a promising approach for fabrication of high aspect ratio 3D microactuator for dual-stage slider in hard disk drive. However, this approach involves practically challenging X-ray lithography and structural transfer processes. In this work, electrostatic MEMS actuator is developed based on a LIGA approach with cost-effective X-ray lithography and dry-film-transfer-to-PCB process. X-ray lithography is performed with X-ray mask based on lift-off sputtered Pb film on mylar substrate and photoresist application using casting-polishing method. High quality and high aspect ratio SU8 microstructures with inverted microactuator pattern have been achieved with the interdigit spacing of ~5 μm, vertical sidewall and a high aspect ratio of 29 by X-ray lithography using the low-cost Pb based X-ray mask. A new dry-film-transfer-to-PCB is employed by using low-cost dry film photoresist to transfer electroplated nickel from surface-treated chromium-coated glass substrate to printed circuit board (PCB) substrate. The dry film is subsequently released everywhere except anchor contacts of the electrostatic actuator structure. The fabricated actuator exhibits good actuation performance with high displacement at moderate operating voltage and suitably high resonance frequency. Therefore, the proposed fabrication process is a promising alternative to realize low-cost MEMS microactuator for industrial applications.     

Submicron polymer structures with X-ray lithography and hot embossing

This article describes the process chain for replication of submicron structures with varying aspect ratios (AR) up to 6 in polymethylmethacrylate (PMMA) by hot embossing to show the capability of the entire LIGA process to fabricate structures with these dimensions. Therefore a 4.7 μm thick layer of MicroChem 950k PMMA A11 resist was spin-coated on a 2.3 μm Ti/TiO _x membrane. It was patterned with X-ray lithography at the electron storage ring ANKA (2.5 GeV and λ _c ≈ 0.4 nm) at a dose of 4 kJ/cm³ using a Si₃N₄ membrane mask with 2 μm thick gold-absorbers. The samples were developed in GG/BDG and resulted in AR of 6–14. Subsequent nickel plating at 52°C resulted in a 200 μm thick nickel tool of 100 mm diameter, which was used to replicate slit-nozzles and columns in PMMA. Closely packed submicron cavities with AR 6 in the nickel shim were filled to 60% during hot embossing.     

Resin micromachining by roller hot embossing

The roller hot embossing is an efficient process of manufacture in which patterns are continuously transcribed on film, etc. Recently, the application of the embossing roll to the manufacturing processes of micro parts is paid attention. In this paper, we examined the development of the embossing roll with patterns of micron level and we tried to make the embossing roll mold by using the LIGA process. In this study, instead of producing embossing patterns directly on the roll surface, we fabricated a flexible thin mold with micro-patterns, which was then wrapped onto a cylinder to form an embossing roll, and tested the soft-mold roller hot embossing method. First, by optimizing UV exposure conditions of UV lithography, we prepared a resist pattern of numerous dots with a diameter of 10 μm, a sag height of 8 μm and a pitch of 20 μm. By Ni-electroforming this pattern, a 50 μm-thick thin mold was successfully fabricated. The 50 μm-thick mold was then wrapped onto a cylinder to form an embossing roll. In the roller hot embossing process, the 10 μm-diameter dot shape was successfully replicated on PET sheets.     

Fabrication of ceramic microcomponents and microreactor for the steam reforming of ethanol

Ceramics have several advantages over other materials in MEMS, such as heat resistance, hardness, corrosion resistivity even in harsh environments, chemical inertness for biological applications and catalytic activity of surfaces and so on. For these advantages ceramic microstructures will be very potentially useful in microsystem, especially in microreactor. A novel method for the high aspect ratio micro ceramic structures fabrication based on deep X-ray lithography and lost-mold technique is developed. By using this method, ceramic microreactors have been successfully fabricated. The ceramic microreactor consists of 14 identical microchannels in parallel, each with typical dimensions of approximately 300 μm in width, 400 μm in height and 20 mm in length. The Ni film catalyst with thickness of 300 nm is uniformly coated through sputtering process. The steam reforming of Ethanol into hydrogen on the ceramics microreactor was studied at temperatures between 500 and 700°C. The microreactors have been characterized by studying of C₂H₆O conversion, H₂ selectivity, and product stream composition.     

Study on the effect of heating plate thickness on the micro induction heater for thermal bubbles generation

This paper designed a micro planar induction heater for thermal bubbles generation. The micro heater which consists of a micro planar coil, a copper heating plate and a glass slide was fabricated with MEMS fabrication process. The relations between the heating performance and the thickness of heating plate were studied in the simulation with the software of COMSOL. The experimental system has been built and the experimental tests for thermal bubble generation have been also carried out with the prototype. The process of thermal bubbles generation was recorded by the video camera. The frequency of AC current applied in the experiment is 100 kHz and the heating time of 1 s. The experimental results indicated that the micro heater has the best heating performance with the heating plate thickness of 12–16 μm, and the minimum power for thermal bubbles generation was only 0.427 W. This micro heater can be applied to a variety of thermal bubble devices, such as micro actuator, micro ejector and micro pump.     

Numerical study of seed bubble-triggered evaporation heat transfer in a single microtube

High boiling incipience temperature and flow instabilities in silicon-based microchannels with smooth surface are challenging issues. This work numerically investigates the seed bubble-triggered evaporation heat transfer in a microtube, with a length of 5.0 mm and diameter of 106 μm. Acetone was the working fluid. Seed bubbles were assumed to be generated periodically at the microtube upstream. The fixed grid allocation technique was proposed to successfully perform the parallel computation via a set of computer core solvers. It is found that the seed bubble-guided heat transfer consists of a start-up stage and a steady operation stage. The start-up time equals to the residence time of the first seed bubble growing and traveling in the microtube. The seed bubble frequency is a key parameter to influence the performance. Low-frequency seed bubbles cause alternative flow patterns of liquid flow and elongated bubble flow, corresponding to the apparent spatial-time oscillations of wall and bulk fluid superheats. High-frequency seed bubbles result in quasi-stable elongated bubble flow, corresponding to quasi-uniform and stable wall and fluid superheats. There is a saturation seed bubble frequency beyond which no further performance improvement can be made. There are residual fluid superheats specifying the required minimum superheats to sustain the evaporation heat transfer between the two phases. Elongated bubbles with thin liquid films are responsible for the heat transfer enhancement. Contrary to wall temperatures, the transient local Nusselt numbers are slightly changed due to the fact that heat transfer is more closely related to the dynamic elongated bubble flow evolution within millisecond timescale in the microchannel. The heat transfer coefficients can be 2.0 to 3.5 times of that for the superheated liquid flow before seed bubble injections.     

Micro-channel flow boiling heat transfer of R-134a, R-236fa, and R-245fa

The rapid development of micro-thermal technologies has conveyed an increasing interest on convective boiling in micro-channels. Although there is general agreement that these systems may be able to dissipate potentially very high heat fluxes per unit volume, their heat transfer characteristics are still unclear and require investigation. The present study illustrates heat transfer data for flow boiling in a single micro-channel, for two channel diameters, namely, 510 and 790 μm, three fluids, namely, R-134a, R-236fa and R-245fa, mass velocities from 300 to 2,000 kg/m² s, and heat fluxes up to 200 kW/m². Stable flow boiling heat transfer data are analyzed through a parametric investigation, and are also confronted with measurements in the presence of two-phase oscillatory instabilities, which were found to significantly change the trends with respect to vapor quality.     

High throughput projection UV lithography of high-aspect-ratio thick SU-8 microstructures

This paper presents a method and an ultra-violet (UV) lithography system to fabricate high-aspect-ratio microstructures (HARMS) with good sidewall quality and nice dimension control to meet the requirement for industrial high throughput and high yield production of micro devices. The advantages, equipment, working principle of UV projection scanning exposure, and scanning exposure strategies are introduced first. Following the numerical simulation for the UV projection scanning exposure of thick SU-8 photoresist, experiment results are demonstrated for different exposure strategies. With Continually Changing Focus Projection Scanning (CCFPS), SU-8 microstructures with 860 μm high and 15 μm feature size are demonstrated. For microstructure with 866 μm height, 20 μm width, from the top layer to the bottom layer, the dimension can be controlled in the range of +0.7 to ?1.7 μm; also, the vertical sidewall angle can be controlled inside 90 ± 0.16°. It approves that the CCFPS exposure for HARMS can achieve much straighter and more vertical sidewall compared with UV contact print or UV projection exposure with focusing image on the resist surface or an optimized depth.     

Thermal stability of electrodeposited LIGA Ni–W alloys for high temperature MEMS applications

For thermally stable LIGA materials for high temperature MEMS applications LIGA Ni–W layers and micro testing samples with different compositions (15 and 5 at% W) were electrodeposited. In order to investigate the thermal stability the Ni–W layers were annealed at different temperatures (300–700°C) and for different durations (1, 4, 16 h). Their microstructure and micro-hardness were than analysed after annealing and compared with those of as-deposited states. The observed microstructures show, in comparison to pure LIGA nickel, a small grain growth and a relatively stable structure up to 700°C. The micro-hardness values of the LIGA Ni–W layers are higher than those of the pure LIGA nickel. The micro-hardness measurements for high W-content show in addition a low decrease of the hardness values with increase of the annealing duration. Tensile tests were carried out for each composition (5 and 15 at%). Ni–W shows higher strength (UTS) above 750 MPa and 1,000 MPa, respectively and lower ductility than pure nickel.     

Fabrication of high-aspect-ratio microstructures on planar andnonplanar surfaces using a modified LIGA process

Large surface areas (tens of square centimeters to square meters) covered with high-aspect-ratio microstructures (HARMs) have potential applications in a wide range of fields including heat transfer, adaptive aerodynamics, acoustics, catalysts, seal and bearing design, and composite materials. HARMs are typically hundreds of micrometers in height, with widths ranging from a few micrometers to tens of micrometers, and they can be manufactured from a variety of materials such as metals, polymers, and ceramics. Three of the barriers to extensive use of large HARM-covered surfaces are cost, nonplanarity of typical surfaces, and adhesion of the microstructures to the surface. A starting point for inexpensive reproduction of large arrays of HARMs is the plastic molding step of the LIGA micromanufacturing process. In order to address the latter two problems, the standard LIGA process was modified/extended. Free-standing polymer sheets, perforated with a pattern of high-aspect-ratio throughholes, were clamped to conductive substrates. The sheets provide a template for electrodeposition of nickel microstructures onto the target surface. This process makes it economically feasible to electroform metal microstructures directly onto large planar and nonplanar metal surfaces (cylinders)     

Comparison of etch characteristics of KOH,TMAH and EDP for bulk micromachining of silicon (110)

Bulk micromachining in Si (110) wafer is an essential process for fabricating vertical microstructures by wet chemical etching. We compared the anisotropic etching properties of potassium hydroxide (KOH), tetra-methyl ammonium hydroxide (TMAH) and ethylene di-amine pyro-catechol (EDP) solutions. A series of etching experiments have been carried out using different etchant concentration and temperatures. Etching at elevated temperatures was found to improve the surface quality as well as shorten the etching time in all the etchants. At 120°C, we get a smooth surface (Ra = 21.2 nm) with an etching rate 12.2 μm/min in 40wt% KOH solution. At 125°C, EDP solution (88wt%) was found to produce smoothest surface (Ra = 9.4 nm) with an etch rate of 1.8 μm/min. In TMAH solution (25wt%), the best surface roughness was found to be 35.6 nm (Ra) at 90°C with an etch rate of 1.18 μm/min. The activation energy and pre-exponential factor in Arrhenius relation are also estimated from the corresponding etch rate data.

     

Cylindrical coils created with 3D X-ray lithography and metallization

The demand for microactuators is increasing recently. The key technology to realizing practical microactuators is microfabrication process. In the production of microminiature components, the technologies for processing high-aspect-ratio structures are essential. As one of these technologies, the LIGA process is widely known. Our laboratory researches the LIGA process to three-dimensional microfabrication and established the cylindrical-microcoil production process. In this paper, we have fabricated the cylindrical-microcoil for the solenoidal electromagnetic type microactuator. We designed and analyzed microactuators, and fabricated and evaluated microactuator coils produced by the combination of three-dimensional X-ray lithography and level copper plating. We succeeded in creating threaded groove-shaped structures with 10 μm line width, 20 μm pitch, and aspect ratio of 5 on the surface of an acrylic pipe by means of three-dimensional X-ray lithography. As a measure to suppress void generation, which is one of the shortcomings of electrolytic plating processes, the sputtering apparatus and plating equipment were improved, a pretreatment process was additionally provided, and the actual electrolytic plating method was improved. As a result, a void-free metallic deposit could be formed on a thin coil line. The processing technology enables the formation of thin-wire coil lines whose current paths feature a large allowable current-carrying capacity, enabling the production of miniature, high-output microactuators.     

Forced convection boiling in a microchannel heat sink

Micromachining technology was utilized to fabricate a transparent microchannel heat-sink system by bonding glass to a silicon wafer. The micro heat sink consisted of a microchannel array, a heater, and a temperature sensor array. This integrated microsystem allowed simultaneous qualitative visualizations of the flow pattern within the microchannels and quantitative measurements of temperature distributions, flow rates, and input power levels. Boiling curves of temperature as a function of the input power were established. No boiling plateau was observed in the boiling curves, consistent with our previously reported data but different from results reported for macrochannel heat sinks. Three stable boiling modes, depending on the input power level, have been distinguished from the flow patterns. Local nucleation boiling was observed in microchannels with a hydraulic diameter as small as 26 μm at the lower input power range. At the higher input power range, a stable annular flow was the dominant boiling mode. Bubbly flow, commonly observed in macrochannels, could not be developed in the present microchannels. Consequently, no boiling plateau was detected in the boiling curves