An MD&DI January 1997 Feature Article

MANUFACTURING

Microtechnology Opens Doors to the Universe of Small Space

Peter Zuska

Highly innovative microfabrication techniques have emerged from the laboratory environment during the last decade, creating a new method for developing and producing microstructures and tiny microsystems. What began about 20 years ago with the three-dimensional micromachining of silicon wafers has since become a technology that holds much promise for the medical device industry. Today, thousands of pressure or acceleration sensors can be batch processed from a single silicon wafer, and numerous applications for microfabrication techniques have been identified, including sensing or actuating principles for mechanical, optical, or fluidic functions. For instance, structures with micron features and tolerances in the submicron range are being used in optical systems as waveguides, switches, or connectors, and as read-write heads in miniaturized disk drives, and microstructured orifices are used for ink-jet printing and fuel injection applications.

The medical industry has certainly benefited from spinoffs from other industries. For example, disposable micromachined sensors are now being used to monitor blood pressure in a patient's IV line or to support treatment after a brain trauma. Among the medical applications that involve microfabrication and assembly techniques are drug-delivery systems that use micropumps or flow restrictors to precisely administer medicine over time, microneedles that are used as medical implants to stimulate nerves or as ultrasharp lancets for less-painful blood sampling, and micronozzles that are key in atomizing aerosols with droplets that are several microns in diameter and allow accurate control of metered dose inhalation. Additional applications are automated in vitro diagnostic systems that use disposable microstructures for defining capillary flow paths, mixing structures, reaction chambers, and structures that support their assembly. Flexible endoscopes or catheters developed for brain surgery also use microstructured components that can integrate multiple sensing and working functions. With these sorts of advances, it seems the real impact of new applications 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 UV lithography to newer techniques like high-aspect-ratio micromolding. Determining which method or combination of methods to use for a particular application depends on a variety of factors, several of which are listed in Table I (below) along with current micromachining techniques.

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

High-Aspect-Ratio Microreplication (HARM). HARM is a process that involves micromachining as a tooling step followed by injection molding or embossing and, if required, by electroforming to replicate microstructures in metal from molded parts. It is one of the most attractive technologies for replicating microstructures at a high performance-to-cost ratio. In this approach, a microstructured preform is defined in a polymer or soft metal and is replicated by electroforming into a tool insert. This insert, or an array of inserts, is then used in the succeeding molding step.

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

The most challenging features to manufacture with any technique are high-aspect-ratio microstructures with structural aspects that can be as small as a few microns in the two axes of the plane and up to several hundred microns deep. Molding an array of several hundred thousand posts or holes with a minimum post diameter of 2 µm and a structural height starting at 20 µm isn't an easy task. Doing it consistently in high volume while maintaining quality is the most challenging part, and success is mostly determined by the precision of the tool inserts.

LIGA. An important tooling and replication method for high-aspect-ratio microstructures is called LIGA, which is a German acronym for deep-etch lithography, electroforming, and molding. The technique employs x-ray synchrotron radiation to expose thick acrylic resist (polymethylmethacrylate) under a lithographic mask (see Figure 1 below). The exposed areas are chemically dissolved 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 be combined to generate a preform for the tool insert. These include laser ablation, multiple-step optical (UV) lithography, and mechanical micromachining, which includes electrodischarge machining (EDM) and diamond milling. EDM uses a spark erosion technique, while diamond milling uses highly accurate, preshaped diamond geometries. This mix of techniques provides the freedom to develop and design geometries for a wide range of customer-specific design requirements. Designs may include stepped features, parallel lines, and tapered or curved slopes. Additionally, special alignment helps and interconnecting bridging structures can be integrated to interface with conventional industrial assembling and handling techniques.

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

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

While silicon is a well-known material preferred in applications that combine its electrical performance as a semiconductor with its excellent mechanical properties (for instance, high tensile strength, hardness, elasticity, and low density), limiting factors can include lengthy processing times 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 brittleness of the material may limit its usefulness as well.

Laser Machining. The first use of lasers in industrial manufacturing processes began more than 25 years ago. Today, lasers are increasingly used for precise welding and cutting, and for structuring many polymers, metals, and especially hard materials. Recent technological advances have significantly improved laser performance, reliability, and cost. Better optics and the development of a line-narrowed microlithographic excimer laser, for example, have increased the precision and flexibility in combining various geometries. Applications for laser machining vary, ranging from uses in prototyping to drilling holes in flow restrictors and in catheters for liquid or material removal.

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

A good overview of laser manufacturing is provided in Ronald Schaeffer's article in the November 1996 issue of MD&DI. The article addresses the advantages associated with laser machining and also details the most common industrial lasers available: carbon dioxide (CO₂), solid-state (Nd:YAG), and excimer lasers. While these techniques are not specifically addressed here, they have been included in Table I.

APPLICATIONS

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

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

Some additional advantages of microfabrication result from the drastically reduced sample and reagent consumption, the low-power operation feasible with microfluid components, and the reduction of analysis time resulting from the short diffusion zones inside the miniaturized fluid system. Other developments include applications such as cell counting, cell separation, and the emerging field of DNA sequencing. DNA bases, for example, can be incorporated into microstructured capillaries where they are mixed with buffers, reagents, and a torturous post matrix to help filter the fragments. Electric current sent through a gel pulls the DNA fragments along, and different fragment sizes can then be separated by the speed of movement into visible groups. The fragments that are tagged with a visible dye can be finally distinguished with a closely located miniaturized spectrometer.

CONCLUSION

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

Photos courtesy of American Laubscher Corp.