NMRC: Research Highlights - Microsystems & Transducers

Where research, analysis and clinical diagnostics now require large laboratories with skilled scientists to arrive at new results, in the future, complete, portable, user friendly biological and chemical analysis systems for many applications will emerge based on microsystems technology. In applications such as rapid disease diagnosis, gene expression analysis and proteomics, microsystems technology will offer breakthrough capabilities, decreasing the time and cost of the analysis significantly. Current microsystems products are ink-jet heads, accelerometers and pressure sensors. In the longer term, the overlap between the biological and microelectronics fields will be a major source of future microsystem innovations.

Microsystem capability requires a wide range of enabling technologies including microfluidic control and chemical separation, sensing and actuation functions for electrical, chemical and optical signals, miniaturised wireless communications technology, (for example used in personal portable healthcare products), advanced simulation and design capability and advanced packaging capability. A selection of results achieved during 1999 in NMRC's microsystems and transducer technology research is presented.

MicroHPLC on Silicon

NMRC has designed and fabricated a microHPLC (High Performance Liquid Chromatography) column from micromachined silicon and anodically bonded glass. Using this high pressure system, the chemical separation of a two component test mixture and pharmaceutical has been achieved in a time of less than two minutes. Further system integration has involved incorporating on-chip electrochemical detection and the amperometric detection of phenol has been demonstrated (see Figure 15). This will offer significant advantages in the field of separation science. In addition, a capillary electrophoresis system, which separates ionised species, is also under development.

Figure 15:- Detection of 50 mM Phenol using the microHPLC system with on-chip injection and detection.

Figure 16:- Experimental DNA biosensors for rapid diagnostics applications.

The exceptional specificity provided by the inherent ability of complementary DNA molecules to hybridise or duplex by base pairing presents the biotechnologist with a simple means of detecting target DNA sequences in a sample. Applications for this biosensor or "gene-chip" technology are numerous and far-reaching, including clinical diagnostics, gene expression monitoring, diagnosis of genetic disorders and forensic examination.

In contrast to the usual gene-chip technologies, a sensor developed at NMRC measures binding directly without the requirement for additional labelling steps. Hybridisation was detected from 100 picomoles to 1800 picomoles of the target DNA sequence. The NMRC system is sufficiently sensitive for the direct detection of the normally amplified products (see Figure 16). Further significant improvements are expected.

A new low operating voltage dosimeter based on our radiation sensitive MOSFET technology that can be applied to satellites, portable and personnel monitoring has been developed. This European Space Agency-sponsored device is the most sensitive device of this type reported in the literature. In addition NMRC participated in the radiation monitoring of an experiment designed to test the effect of radiation and space conditions on biological processes for the BIOPAN mission launched on a FOTON rocket (see figure). Evaluation of the results is ongoing.

Figure 18:- Fabricated 10*10 ultrasonic arrays of 100 micron circular devices

NMRC is collaborating with the Electrical Engineering Department in UCC to fabricate ultrasonic devices (see Figure 18). Narrowband, multi-element arrays and broadband, pseudo-piston structures have been fabricated. The integrated, surface micromachining technology yields low-cost, well-matched elements, a requirement for beam forming and steering arrays with application in clinical and bio-medical microsystems (applications such as fluid flow and imaging).

Figure 19:- Folded hinge resonator in a double polysilicon process after etch release

Portable, handheld and remote diagnostic instruments require data transfer between the unit and a host computer or network; the most flexible solution will be to use RF data transmission. NMRC is developing micromechanical resonators for wireless communication applications. Two technology approaches are being researched, the first as a baseline (see Figure 19), is a classical polysilicon technology. The second uses a proprietary low thermal-budget, oxide-metal post process and should show significantly improved high frequency performance due to its lower series resistance.

Figure 20:- Plastic flow-through cell with electrode on PCB and SPAD optical detector (centre-top).

NMRC and the Chemistry Department of UCC are working on the development of an electrochemiluminescent biosensor. This involves the deposition of specific recognition biochemistry (i.e. antigens or antibodies) on the surfaces of gold electrodes in a plastic flow-through cell. Three optical detection options were investigated, a commercial photomultiplier tube (PMT), a large area NMRC PIN photodiode and NMRC's single photon avalanche diodes (SPADs) (see Figure 20). The SPADs used in this structure exhibit dark counts of less than 10 counts/second at room temperature, which is the best reported in the literature for this type of structure.

A key development in the rapid evolution of microsystems components is the availability of suitable CAD tools that address the microsystem component design and subsequent system integration and packaging. NMRC has identified this as a strategic area and is building on the considerable expertise in-house in the use of CAD tools for semiconductor package design. Behavioural CAD models of a magnetic sensor package and a pressure sensor package have been developed (see Figure 21).

Figure 21:- Simulation of a micromachined silicon membrane for a packaged pressure sensor.

Figure 22:- 50 micron three layer fixed-fixed beam for mechanical parameter extraction.

The main transducer processes in NMRC rely on CMOS compatible surface- micromachined technology. These structures are critically dependent on the mechanical properties of the nanoscale thin films with which they are fabricated. The mechanical properties of structures fabricated using NMRC polysilicon and oxide-metal, surface micromachining processes are being extracted using test structures, which include cantilevers, fixed-fixed beams and diaphragms. The measured values will be correlated with blanket film measurements which is essential for proper calibration of design tools and optimisation of the processes involved (see Figure 22).

A number of stand-alone packaging concepts for transducers that require direct outside world contact with fluidic and optical media have been developed. A conventional IC package has been adapted for a chemical sensor (see Figure 23). A novel "flip-chip over hole" concept, requiring no wirebond interconnects, has been developed which is particularly suitable for fluid sealing and optical device alignment (see Figure 24).

Figure 23:- "Flip-chip over hole" prototype package for chemical sensing.

Figure 24:- Plastic QFP package with built-in receptacle for direct sensor chip access