Fabrication of microchannels for use in micro-boiling experiments

Increased power densities in VLSI chips have led to a need to develop cooling methods that can cope with the increased heat produced by such chips. Currently one of the more attractive methods to meet this goal is through the use of two phase flow of a fluid as changing phase of the material allows high heat transfer rates for a low temperature change. To bring this technology to commercialisation a greater understanding of the underlying physics involved at the microscale is required as there is much debate within literature as to what occurs during two phase flow heat transfer at these scales. The work conducted as part of this thesis is a step towards improving the understanding of the mechanisms involved with this process. This thesis describes the fabrication of a novel microchannel structure, which can be used to experimentally characterise two phase heat transfer as it occurs. The final process reported for these microchannels structures provides the basis of a technology for the fabrication of microchannels with increased sensor densities. Two types of microchannel devices have been fabricated for this project. The first device of these was an array of parallel microchannels formed by the reactive ion etching (RIE) of silicon, which was then bonded with Pyrex glass. These microchannels were simple in that sensors were not integrated for local measurement. However the production of these devices incorporated fabrication techniques such as anodic bonding and inductively coupled plasma RIE that were essential to the fabrication of more complex devices. The second device built was a single microchannel that contained an integrated heater and several temperature sensors. The use of wafer bonding enabled the device to take full advantage of both bulk and surface micromachining technology as the placement of the temperature sensors on the channel floor would not be possible with conventional bulk micromachining. The initial microchannel structures demonstrated that wafer bonding could be used to fabricate novel devices, but they highlighted the difficulty of achieving strong anodic bonds due to the presence of dielectric films throughout the fusion bonded wafer stack used in the channel fabrication. To improve the performance of the device the process was optimised through the use of insitu, non-destructive test structures. These structures enabled the uniformity and strength of the bonds to be optimised through visualisation over the whole wafer surface. The integrated sensors enabled temperature measurements to be taken along the channel with a sensitivity 3.60 ΩK-1 while the integrated heater has delivered a controllable and uniform heat flux of 264 kWm-2.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Room-temperature fabrication of anodic tantalum pentoxide for low-voltage electrowetting on dielectric (EWOD)

This paper presents a robust anodic Ta2O5 dielectric as an alternative insulator for fabricating low-voltage electrowetting on dielectric (EWOD) systems. Previously reported low-voltage EWOD technologies require high-temperature processes ( > 435degC), which unlike this room temperature technology, are not compatible with standard copper and aluminum integrated circuit interconnect technology as well as polymer-based substrates. The anodized Ta2O5 forms a uniform pinhole free layer with a surface roughness (R a) of 0.6 nm. This robust film enables an ultrathin amorphous FluoroPolymer layer to be employed to reduce the EWOD driving voltage to 13 V. Both sub-20-nm Teflon-AF and CYTOP layers have been successfully coated on top of Ta2O5 with good adhesion. Applying voltages of 6-15 V significantly modified the contact angles of droplets in air on these samples (121deg to 81deg on Teflon-AF at 13 V and 114deg to 95deg on CYTOP at 6 V). Successful 14-V EWOD manipulation involving droplets being dispensed from a reservoir, their movement, followed by merging them together has been demonstrated using devices using a Teflon-AF + Ta2O5 dielectric

Test structure for characterizing low voltage coplanar EWOD system

This paper presents test structures designed for studying the relationship between the operating voltage and different electrode configurations and areas for coplanar electrowetting on dielectrics (EWOD) devices. New test structures have been designed and fabricated using anodic Ta2O5 dielectric and thin aFP (amorphous Fluoropolymer CYTOP from Asahi Glass Co., Ltd.). These test structures have been used to characterize the contact angle change, which is between 114deg and 81deg with an applied voltage of less than 20 V. This demonstrates that by modifying the coplanar architecture, the operating voltage can be reduced by a factor of two, compared to previously reported coplanar EWOD structures. Droplet manipulation on a coplanar EWOD system with this new design has been successfully demonstrated, with a driving voltage of 15 V

Anodic Ta2O5 for CMOS compatible low voltage electrowetting-on-dielectric device fabrication

This paper reports a CMOS compatible fabrication procedure that enables electrowetting-on-dielectric (EWOD) technology to be post-processed on foundry CMOS technology. With driving voltages less than 15 V it is believed to be the lowest reported driving voltage for any material system compatible with post-processing on completed integrated circuits wafers. The process architecture uses anodically grown tantalum pentoxide as a pinhole free high dielectric constant insulator with an overlying 16 nm layer of Teflon-AF®, which provides the hydrophobic surface for droplets manipulation. This stack provides a very robust dielectric, which maintains a sufficiently high capacitance per unit area for effective operation at a reduced voltage (15 V) which is more compatible with standard CMOS technology. The paper demonstrates that the sputtered tantalum layer used for the electrodes and the formation of the insulating dielectric can readily be integrated with both aluminium and copper interconnect used in foundry CMOS

The integration of EWOD and SAW technologies for improved droplet manipulation and mixing

This paper details the first reported integration of two advanced digital microfluidic technologies where 100 mum silicon cubes are transported with electrowetting on dielectric (EWOD) and the droplet then held with EWOD while the silicon cubes are mixed with another liquid using a surface acoustic wave (SAW). Together these two technologies provide a comprehensive lab-on-a-chip combination with well developed functionalities. These include droplet generation, splitting and transportation offered by EWOD with transportation, mixing and biosensing being potentially available with SAW. The fabrication of both EWOD and SAW structures on LiNbO3 substrates used low temperature Ta/Ta2O5/CYTOP layer deposition and patterning technologies, which enabled efficient transportation and mixing functions to be demonstrated

Demonstration of a wireless driven MEMS pond skater that uses EWOD technology

A silicon swimming robot or pond skating device has been demonstrated. It floats on liquid surfaces using surface tension and is capable of movement using electrowetting on dielectric (EWOD) based propulsion. Its dimensions are 6 × 9 mm and the driving mechanism involves first trapping air bubbles within the liquid onto the hydrophobic surface of the device. The air bubbles are then moved using EWOD, which provides the propulsion. The device employs a recently reported View the MathML source EWOD technology enabling a driving voltage of ≈15 V, which is low enough for RF power transmission, thus facilitating wire-free movement. A wired version has been measured to move 1.35 mm in 168 ms (a speed of 8 mm s−1). This low voltage-EWOD (<15 V) device, fabricated using a CMOS compatible process, is believed to be the world’s smallest swimming MEMS device that has no mechanical moving parts. The paper also reports results of EWOD droplet operation driven by wireless power transmission and demonstrates that such a wireless design can be successfully mounted on a floating EWOD device to produce movement

Wireless driven EWOD technology for a MEMS pond skater

A silicon swimming robot or pond skating device has been demonstrated. It floats on liquid surfaces using surface tension and is capable of movement using electrowetting on dielectric (EWOD) based propulsion. Its dimensions are 6 times 9 mm with a thickness of 380 mum. The driving mechanism involves the trapping of air bubbles within the liquid onto the hydrophobic surface of the device with the subsequent ejection using a recently reported Ta2O5 EWOD technology. The required driving voltage of ~15 V is low enough for RF power transmission, thus providing wire-free movement. A wired version has been measured to move 1.35 mm in 168 ms (a speed of 8 mm s-1). This low-voltage EWOD device, fabricated using a CMOS compatible process, is believed to be the worldpsilas smallest swimming MEMS device that has no mechanical moving parts. The paper also reports results of EWOD droplet operation driven by wireless power transmission and demonstrates that such a wireless design can be successfully mounted on a floating EWOD device

Extraction of sheet resistance and line width from all-copper BCD test structures fabricated from silicon preforms

Test structures have been fabricated to allow electrical critical dimensions (ECD) to be extracted from copper features with dimensions comparable to those replicated in integrated circuit (IC) interconnect systems. The implementation of these structures is such that no conductive barrier metal has been used. The advantage of this approach is that the electrical measurements provide a nondestructive and efficient method for determining critical dimension (CD) values and for enabling fundamental studies of electron transport in narrow copper features unaffected by the complications of barrier metal films. This paper reports on the results of tests which have been conducted to evaluate various extraction methods for sheet resistance and line width values from the current design.</p

Extraction of sheet resistance and line width from all-copper BCD test structures fabricated from silicon preforms

Test structures have been fabricated to allow electrical critical dimensions (ECD) to be extracted from copper features with dimensions comparable to those replicated in integrated circuit (IC) interconnect systems. The implementation of these structures is such that no conductive barrier metal has been used. The advantage of this approach is that the electrical measurements provide a nondestructive and efficient method for determining critical dimension (CD) values and for enabling fundamental studies of electron transport in narrow copper features unaffected by the complications of barrier metal films. This paper reports on the results of tests which have been conducted to evaluate various extraction methods for sheet resistance and line width values from the current design.</p