Microtechnology Opens Doors to the Universe of Small Space

An MD&DI January 1997 Feature Article

MANUFACTURING

Highly innovative microfabrication techniques have emerged from thelaboratory environment during the last decade, creating a new method fordeveloping and producing microstructures and tiny microsystems. What began about20 years ago with the three-dimensional micromachining of silicon wafers hassince become a technology that holds much promise for the medical deviceindustry. Today, thousands of pressure or acceleration sensors can be batchprocessed from a single silicon wafer, and numerous applications formicrofabrication techniques have been identified, including sensing or actuatingprinciples for mechanical, optical, or fluidic functions. For instance,structures with micron features and tolerances in the submicron range are beingused in optical systems as waveguides, switches, or connectors, and asread-write heads in miniaturized disk drives, and microstructured orifices areused for ink-jet printing and fuel injection applications.

The medical industry has certainly benefited from spinoffs from otherindustries. For example, disposable micromachined sensors are now being used tomonitor blood pressure in a patient's IV line or to support treatment after abrain trauma. Among the medical applications that involve microfabrication andassembly techniques are drug-delivery systems that use micropumps or flowrestrictors to precisely administer medicine over time, microneedles that areused as medical implants to stimulate nerves or as ultrasharp lancets forless-painful blood sampling, and micronozzles that are key in atomizing aerosolswith droplets that are several microns in diameter and allow accurate control ofmetered dose inhalation. Additional applications are automated in vitrodiagnostic systems that use disposable microstructures for defining capillaryflow paths, mixing structures, reaction chambers, and structures that supporttheir assembly. Flexible endoscopes or catheters developed for brain surgeryalso use microstructured components that can integrate multiple sensing andworking functions. With these sorts of advances, it seems the real impact of newapplications for microfabrication technology is just beginning to be realized.

TECHNIQUES

Microfabrication techniques are no longer limited to silicon machining.Techniques used today range from various types of laser machining to UVlithography to newer techniques like high-aspect-ratio micromolding. Determiningwhich method or combination of methods to use for a particular applicationdepends on a variety of factors, several of which are listed in Table I (below) alongwith current micromachining techniques.

Table I. Micromachining techniques with corresponding informationregarding their effectiveness in various practical applications. (Scale forcolumns 6-10: 1 = very effective; 5 = ineffective.)

High-Aspect-Ratio Microreplication (HARM). HARM is a process thatinvolves micromachining as a tooling step followed by injection molding orembossing and, if required, by electroforming to replicate microstructures inmetal from molded parts. It is one of the most attractive technologies forreplicating microstructures at a high performance-to-cost ratio. In thisapproach, a microstructured preform is defined in a polymer or soft metal and isreplicated by electroforming into a tool insert. This insert, or an array ofinserts, is then used in the succeeding molding step.

Products micromachined with this technique include fluidic structures suchas molded orifice plates for ink-jet printing and microchannel plates fordisposable assays used in various diagnostic applications. The materials thatcan be used are electroformable metals and plastics, including polysulfone,acrylate, polycarbonate, polyimide, and styrene. Additional materials thatcustomers may suggest need to be qualified by tests.

The most challenging features to manufacture with any technique arehigh-aspect-ratio microstructures with structural aspects that can be as smallas a few microns in the two axes of the plane and up to several hundred micronsdeep. Molding an array of several hundred thousand posts or holes with a minimumpost diameter of 2 µm and a structural height starting at 20 µm isn'tan easy task. Doing it consistently in high volume while maintaining quality isthe most challenging part, and success is mostly determined by the precision ofthe tool inserts.

LIGA. An important tooling and replication method for high-aspect-ratio microstructures is called LIGA, which is a German acronym fordeep-etch lithography, electroforming, and molding. The technique employs x-raysynchrotron radiation to expose thick acrylic resist (polymethylmethacrylate)under a lithographic mask (see Figure 1 below). The exposed areas are chemicallydissolved and, in areas where the material is removed, metal is electroformed,thereby defining the tool insert for the succeeding molding step.

Figure 1. The LIGA technique.

Other Combined Techniques. Other microreplication techniques can becombined to generate a preform for the tool insert. These include laserablation, multiple-step optical (UV) lithography, and mechanical micromachining,which includes electrodischarge machining (EDM) and diamond milling. EDM uses aspark erosion technique, while diamond milling uses highly accurate, preshapeddiamond geometries. This mix of techniques provides the freedom to develop anddesign geometries for a wide range of customer-specific design requirements.Designs may include stepped features, parallel lines, and tapered or curvedslopes. Additionally, special alignment helps and interconnecting bridgingstructures can be integrated to interface with conventional industrialassembling and handling techniques.

The molding process itself is also demanding since it is difficult to fillthe small, high-aspect-ratio features without leaving cavities. After theejection of the molded parts, an additional step can be added by electroformingmetal into cavities to replicate the structure in metal, or to defineelectrically conductive areas. Different from conventional tooling and molding,the microreplication techniques require a more extensive feasibility and designphase up front in order to avoid high costs for multiple redesigns.

Bulk and Surface Machining. Two micromachining techniques oftenused are bulk machining and surface machining. Bulk machining is a subtractiveprocess that uses wet anisotropic etching—which depends on the crystalorientation of silicon—or a dry etching method such as reactive ion etching(RIE). Materials typically used for wet etching are silicon and quartz, whiledry etching is generally used with silicon, metals, plastics, and ceramics.Typical features for sensing or fluidic structures that can be created usingbulk machining include geometries such as membranes, beams, holes, or grooves.In addition to bulk machining, surface machining—which is an additive processused to deposit several layers onto a silicon wafer, including sacrificiallayers that are then selectively etched—can be used to combine different layersto add sensing functions such as measuring temperature, magnetic fields, orpressure.

While silicon is a well-known material preferred in applications thatcombine its electrical performance as a semiconductor with its excellentmechanical properties (for instance, high tensile strength, hardness,elasticity, and low density), limiting factors can include lengthy processingtimes and the relatively high cost of the substrate if a large area is required.In some medical applications, such as with micropumps or valves, the brittlenessof the material may limit its usefulness as well.

Laser Machining. The first use of lasers in industrialmanufacturing processes began more than 25 years ago. Today, lasers areincreasingly used for precise welding and cutting, and for structuring manypolymers, metals, and especially hard materials. Recent technological advanceshave significantly improved laser performance, reliability, and cost. Betteroptics and the development of a line-narrowed microlithographic excimer laser,for example, have increased the precision and flexibility in combining variousgeometries. Applications for laser machining vary, ranging from uses inprototyping to drilling holes in flow restrictors and in catheters for liquid ormaterial removal.

The integration of internal halogen generators, higher pulse rates, andimproved corrosion-resistant materials have pushed the processing costs ofexcimer laser equipment to less than $1 for 250,000 laser pulses. A newapplication for ink-jet printing has just recently become available in which 100to 300 nozzles with hole diameters as low as 20 µm are drilled in a fieldof 8 X 20 mm with precisely defined hole shapes and submicron tolerances.

A good overview of laser manufacturing is provided in Ronald Schaeffer'sarticle in the November 1996 issue of MD&DI. The article addressesthe advantages associated with laser machining and also details the most commonindustrial lasers available: carbon dioxide (CO₂),solid-state (Nd:YAG), and excimer lasers. While these techniques are notspecifically addressed here, they have been included in Table I.

APPLICATIONS

The demand for diagnostic and analytical equipment that is smaller andperforms faster is a major growth area in today's industry that involvesmicrostructure technology. A fast and reliable response is essential, forinstance, to measure blood parameters in emergency situations and duringsurgery, and to test for drugs of abuse at a crime scene. In point-of-caresituations such as in a physician's office or in home-care applications, the useof on-site tests is currently limited because of the complexity of testprocedures or the limited reliability of existing disposable tests. This is whytests for infectious diseases, therapeutic drugs, drugs of abuse, immunology,allergy, and tumors are typically performed in centralized laboratories onadvanced instruments operated by skilled personnel.

The types of diagnostic testing mentioned above often require sophisticatedinstrumentation and multiple-step procedures, including sample dilution,variable incubation times, and wash steps. Most of these functions can beincorporated by microfabrication techniques that already have the ability toimprove the performance of disposable assays. New tests are currently beingdeveloped in which microreplicated capillary structures are used to improve theperformance of disposable assays for the exam- ination of human fluids.Extremely precise geometries allow for exact control of the volume flow, thetiming of reactions between the sample and predeposited reagents, the separationof cells, and the mixing of different substances.

This postage-stamp-size spectrometer is microreplicated in polymer as a complete optical bench and mounted on a chip.

Some additional advantages of microfabrication result from the drasticallyreduced sample and reagent consumption, the low-power operation feasible withmicrofluid components, and the reduction of analysis time resulting from theshort diffusion zones inside the miniaturized fluid system. Other developmentsinclude applications such as cell counting, cell separation, and the emergingfield of DNA sequencing. DNA bases, for example, can be incorporated intomicrostructured capillaries where they are mixed with buffers, reagents, and atorturous post matrix to help filter the fragments. Electric current sentthrough a gel pulls the DNA fragments along, and different fragment sizes canthen be separated by the speed of movement into visible groups. The fragmentsthat are tagged with a visible dye can be finally distinguished with a closelylocated miniaturized spectrometer.

CONCLUSION

Micromachining and microreplication techniques have been qualified forhigh-series production in many applications, providing the key to integratingthe intelligence of electronic circuits with sensing and actuating functions toproduce complete microsytems. The advantages are evident not only in thereduction of size, but also in the increase of functional performance andreliability, and a unit-cost reduction in high-volume batch processing. Becauseof high capital investments for expensive manufacturing equipment,microstructures or microsystems are successful in applications where theperformance-to-cost ratio of the system increases significantly and highproduction volumes offset the up-front investments. And as the medical anddiagnostic industry continues to benefit from breakthroughs in other applicationareas, it is expected to be one of the fastest growing areas formicromanufacturing.

Photos courtesy of American Laubscher Corp.

Peter Zuska is product manager for American LaubscherCorp. (Farmingdale, NY).

Copyright © 1997 Medical Device & Diagnostic Industry