H. S. Ko

Piezoresistive characteristics of MWNT nanocomposites and fabrication as a polymer pressure sensor

Polyimide (PI)-carbon nanotube composites were fabricated by in situ polymerization using multi-wall carbon nanotubes (MWNT) as fillers. The composite film was characterized by some analytical instruments to ensure its structure and good dispersion of the MWNTs in the composites. The electrical resistivity of this composite was found to vary significantly with both the temperature and the stress in the material. The PI-MWNT composites possess a very linear piezoresistive nature which can be used as a good pressure sensor material, provided with proper temperature compensation. Fabrication of a micropolymer pressure sensor using this nanocomposite sensing material is demonstrated and sensor performance is evaluated. The sensor has a higher sensitivity than a polysilicon sensor, rapid response, and is thermally stable. The sensor is suitable for mass production, and can be widely applied or integrated in a microfluidic system or biochip where pressure information is required.

Novel fabrication of a pressure sensor with polymer material and evaluation of its performance

Fabrication of a micro, essentially polymer pressure sensor is presented. Both the sensor cavity and the sensor diaphragm were made of SU-8 which can be readily spun coated on the substrate at the desired thickness and patterned by lithography. The thickness of the diaphragm and the height of the sensor cavities, allowing deformation of the diaphragm, can be readily varied from a few to hundreds of microns by spin coating different thickness of the SU-8 layer. This allows fabrication of a cavity with much greater height and measurement of pressure with a much wider range. However, the sensor material, which can readily sense deformation of the diaphragm, is a piezoresistive film. This has precluded the possibility of fabricating the cavity and diaphragm first, which is a low temperature process, and then depositing the piezoresistive sensor on the above, which is a high temperature process. The fabrication strategy has to be reversed, i.e., starts with the high temperature process of depositing the sensor layer and then the low temperature process. The fabrication process is simple. The fabricated sensor has been evaluated and has a higher sensitivity than that of the polysilicon sensor, has rapid response, and is thermally stable. The sensor is reliable and can be widely applied or integrated in the microfluidic system or biochip where pressure information is required.

Bonding of a complicated polymer microchannel system for study of pressurized liquid flow characteristics with the electric double effect

Liquid flow characteristics with the electric double-layer (EDL) effect are studied in a microchannel made by polymer materials. The microchannel is integrated with arrays of a polymer pressure sensor for measurements of the local pressure distribution inside the channel. The bonding of this complicated microchannel requires special caution not only to ensure good contact between the channel walls and the cover plate leading to correct height of a channel but also to avoid liquid leakage at higher pressure. A bonding technique is developed in this work, which can be readily applied in a complicated lab-on-chip system for the study of micro-fluid behavior. The completed channel system with arrays of micro pressure sensors is used to study both the entry and fully developed flow of pure water. The accurate measurements of the pressure distribution along the channel clarify many of the conflicting issues reported in the microchannel flow system. It is found that the friction factor of liquid flow in the microchannel agrees well with the prediction accounting for the aspect ratio of the channel. The EDL effect on the local pressure drop, which may frequently occur in the liquid flow system, is also discussed and presented.

Fabrication and design of a heat transfer micro-channel system by a low temperature MEMS technique

This work describes a low temperature fabrication technique for a micro-channel configuration that is integrated with an array of temperature sensors and micro-heaters. This channel configuration can be used to study the micro-scale heat transfer process in the channels. The low temperature fabrication process enables the use of low thermal conductivity material for forming the channel walls. This can provide a uniform heat flux boundary condition due to adequate insulation for reducing both the heat loss from the channel to the surrounding ambient and the streamwise conduction. In addition, the wall roughness of the micro-channel is minimized by a special wet etch process. Therefore, more accurate local heat transfer data along the channel can be obtained, which provides entry length heat transfer information for the first time in the literature. Design consideration and fabrication techniques used in this study are explained. Final measurements for validation of the fabricated heaters and sensors are presented. The paper also presents averaged heat transfer which is compared with available data. Discrepancies between different works can be clarified.

Flow characteristics in a microchannel system integrated with arrays of micro-pressure sensors using a polymer material

The use of a polymer material to fabricate a complicated microchannel system that is integrated with arrays of pressure sensors is presented. This channel can allow either gas or liquid to flow through for basic microfluidic studies. This developed fabrication process is much simpler, and is almost the reverse of the surface micromachining process usually used for the microchannel system. This allows for the use of SU-8 to fabricate, by lithography, the diaphragm and the cavities used for these pressure sensors and the channel. The height for the diaphragm, the cavities and the channel can be readily varied from few to hundreds of microns by spin-coating different thicknesses of the SU-8 layer. This will allow a much wider measurement range and flow conditions. More detailed design and developed fabrication techniques are presented. The pressure distributions are measured inside the channel and are compared with the analysis. The results obtained in this work clarify some of the arguments which existed in the literature.

Microwave bonding of MWNTs and fabrication of a low-cost, high-performance polymer pressure sensor

This paper describes the fabrication of a simple, low-cost pressure sensor that can be readily mass produced. Microwave-induced heating is used to bond a multiwall carbon nanotube (MWNT) network to a poly(ethylene terephthalate) substrate that serves as a pressure diaphragm. The MWNT network can be patterned with a damascene process and used as the sensor material. The pressure diaphragm with the MWNT network can be bonded with any flexible substrate pre-drilled with a cavity that allows a deflection of the diaphragm. Design and fabrication considerations for the sensor are discussed and its performance is demonstrated and evaluated. The sensor is thermally stable and has a much higher sensitivity and gauge factor than polysilicon sensors. In addition to the simple fabrication process, the sensor can be widely applied and integrated into microfluidic systems or biochips where pressure information is required.

A Novel Fabrication for Pressure Sensor with Polymer Material and Its Characteristic Testing

In the current fabrication of pressure sensor, both the sensor cavity and the sensor diaphragm were made of SU-8 which can be readily spun coat on the substrate at desired thickness and patterned by lithography. The thickness of the diaphragm, and the height of the sensor cavities, allowing deformation of diaphragm, can be readily varied from few to hundreds of microns by spin coat different thickness of SU-8 layer. This allows fabrication of cavity with much greater heights and measurement of pressure with much wider range. However, the sensor material used for the pressure sensor is the polysilicon doped with a high concentration of boron, which can readily sense the deformation of a diaphragm. This has precluded the possibility of fabricating the cavities and diaphragm first - which is a low temperature process, and then depositing the polysilicon sensor on the above - which is a high temperature process. Fabrication strategy has to be reversed, i.e., starts with the high temperature process of depositing the doped polysilicon layer and then the low temperature process.

Micro pressure sensor fabrication without problem of stiction for a wider range of measurement

The paper presents a novel fabrication process for single or arrays of micro pressure sensors. The fabrication process is almost the reverse of the surface micromachining process used for the pressure sensor. This allows the use of SU-8 to form a cavity that can be much deeper for pressure measurement. Thus, the sensor can provide a much wider range of pressure measurement. The fabrication process has a complete absence of diaphragm stiction. In addition, arrays of pressure sensors can be readily made and integrated into a complicated micro system. More detailed design and fabrication techniques developed for this sensor are presented.

Fabrication challenges for a complicated micro-flow channel system at low temperature process

This paper will present fabrication challenges for a complicated micro channel system at low temperature process by MEMS techniques. This channel will be integrated with an array of temperature sensors and a set of heaters for the purpose of study on the micro-scale heat transfer inside. The heat transfer results may provide a clue whether the microchannel cooling process can be used to solve the future cooling problem encountered in an extremely high power density CPU chip. Design and fabrication challenges encountered in this processes are discussed. A final measurement for the validation of the heaters and the sensors fabricated and a study of the heat transfer coefficient distributions inside the micro channel are also presented.

The study of polymer pressure sensor by nanocomposites with MWNT and it's characteristics testing

Polyimide (PI)-carbon nanotube composites were fabricated by in situ polymerization using multi-wall carbon nanotubes (MWNT) as fillers. The composite film was characterized by some analytical instruments to ensure its structure and good dispersion of the MWNTs in the composites. The electrical resistivity of this composite was found to vary significantly with both the temperature and the stress in the material. The PI-MWNT composites possess a very linear piezoresistive nature which can be used as a good pressure sensor material, provided with proper temperature compensation. Fabrication of a micropolymer pressure sensor using this nanocomposite sensing material is demonstrated and sensor performance is evaluated. The sensor has a higher sensitivity than a polysilicon sensor, rapid response, and is thermally stable. The sensor is suitable for mass production, and can be widely applied or integrated in a microfluidic system or biochip where pressure information is required.