Precision Manufacturing: A New Technology for Microdevice Fabrication
Using computer-aided design, complex 3-D microdevices can be fabricated by electroplating multiple, patterned layers.
|Figure 1. Instant Masking provides a template for patterned deposition (a). A blanket of material is rapidly deposited (b). The layer is planarized (c). The process is repeated to create multiple layers (d). A selective chemical etch removes sacrificial material.
(Click to enlarge)
The world is miniaturizing. Whether the application is medical implants or laboratory equipment that handles biological fluids, miniaturization is a recurring theme for advancing the state of the art and making new breakthroughs in medical applications. This article examines a new manufacturing technology that has been developed to address the growing need for miniaturized devices and components on a millimeter or microscopic scale.
Electrochemical fabrication (EFAB) is a solid, free-form fabrication technology that can create complex, miniature three-dimensional (3-D) metal structures that are impossible or impractical to make using other technologies such as electrical discharge machining (EDM), laser machining, or silicon micromachining. This article describes how the new technology process works, compares it with other technologies, and discusses potential applications.
How EFAB Works
The automated EFAB process creates metal structures by electroplating multiple, independently patterned layers. The process is similar in concept to rapid prototyping techniques, such as stereolithography, in that multiple patterned layers are stacked to build structures. But unlike stereolithography, EFAB is a batch process suitable for volume production of fully functional devices, not just models and prototypes. Furthermore, EFAB provides greater accuracy than stereolithography.
It also defines material layers by an in situ patterning method called Instant Masking. The mask consists of an insulator patterned on an anode (see Figure 1a). The masking patterns a substrate by simply pressing the mask against the substrate, electrodepositing material through apertures in the insulator, and then removing the mask from the substrate. The result is a layer that has been rapidly deposited and patterned in a single step (see Figure 1b).
The masking process is much faster than photolithography, which makes it possible to fabricate devices with a dozen or more completed layers in one day. With this technology, each level of part build is composed of both structural and sacrificial material. The block of sacrificial material in which devices are temporarily embedded serves as a mechanical support of structural material. Additional material can be deposited over the entire layer without constraint. Therefore, the use of sacrificial material eliminates all geometrical restriction, allowing the structural material on a layer to overhang—and even be disconnected from—that of the previous layer. Such geometrical freedom also makes possible monolithically fabricated assemblies of discrete, interconnected parts. This process eliminates the need for subsequent bonding or assembly steps.
|Figure 2. Microdevices generated from 3-D CAD data.
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To fabricate a multilayer device, the geometries of the layer cross sections are automatically determined based on the desired 3-D geometry, and one or more instant masks are generated that include all of the unique cross sections of the device. The process consists of the following steps, repeated on every layer: 1) selectively depositing the first material by masking, 2) blanket depositing second material, and 3) planarizing. The complete process flow is shown in Figure 1. In Figure 1a, the first material is patterned onto a substrate, producing a patterned layer. In Figure 1b, the second material has been blanket deposited over the first material so that it contacts the substrate in those regions not covered with the first material. Then, as shown in Figure 1c, the entire two-material layer is planarized to achieve precise thickness and flatness. After repetition of this process for all layers, the embedded multilayer structure shown in Figure 1d is etched to yield the desired device as shown in Figure 1e.
This precision manufacturing technology can be used to form structures from any electrodepositable metal or alloy, with the only constraint being that the accompanying sacrificial metal can be selectively etched after the layers are formed. Other materials including insulators will eventually be introduced to enhance the capabilities of the process.
The Best of Micro- and Macro-Machining Technologies
The precision technology allows miniature and microdevices to be generated from 3-D computer-aided design (CAD) data. The system uses data from any standard CAD package to determine device cross sections. These cross sections are used to fabricate the mask tooling. The masks are then used to rapidly create the devices. Figure 2 shows a variety of microdevices and microstructures that have been fabricated and the corresponding CAD file used to generate them. The CAD interface makes it easier for a designer to develop devices using EFAB without any specific expertise in microfabrication processes.
The EFAB process is designed to combine the advantages of traditional machining technologies with those of lithography-based semiconductor microfabrication. With precision machining technologies such as laser machining or EDM, it is possible to machine a wide range of geometries quickly and cost-effectively with a single stand-alone machine tool. The processes are fast and cost-effective for prototyping, but also scalable to production.
In terms of miniature and microdevices, precision machining techniques usually reach their limits when parts must be smaller than millimeter-scale. Also, when complex internal geometries or assemblies are required, machining and assembling individual components may be prohibitively difficult. When the geometric complexity or precision of the part is increased, the time and cost to make an individual part increases.
|Figure 3. A wide range of microdevices with complex features can be fabricated simultaneously on the same substrate using EFAB. Microstructures such as those shown are impractical with other manufacturing processes.
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Lithography-based microfabrication technology, such as silicon micromachining commonly used to make micro-electromechanical systems (MEMS), has a different set of advantages and disadvantages. Silicon micromachining can provide smaller devices and more-intricate detail than other machining processes. For a given process, flow costs scale with area regardless of design detail. As a result, very complex integrated devices, assemblies, and mechanisms are possible.
Silicon micromachining is also a batch process, allowing thousands of devices to be fabricated simultaneously on a single substrate, which dramatically lowers costs at high volumes. Unfortunately, this technology has serious drawbacks as well. Silicon micromachining is not accomplished with a single machine tool. It typically requires a large cleanroom with a multimillion dollar equipment set consisting of several dozen separate machines.
Prototyping in silicon micromachining is expensive and slow, typically taking 8–12 weeks or more for a single fabrication run. The processes are exotic to the extent that designers of silicon micromachined devices typically require a PhD in electrical engineering. Silicon micromachining processes are also severely restricted in terms of the geometries that can be created. Devices are generally limited to simple, two-dimensional extruded shapes in three or fewer bonded layers. Devices are so small (often less than 100 µm in any dimension) that they are fragile and difficult to package and interface with the outside world. Although silicon micromachining technology is well suited for electrical circuits, it is extremely difficult to fabricate practical mechanical and electromechanical devices. The cost and complexity are prohibitive for most companies and all but the largest applications.
EFAB technology combines many of the advantages of precision machining and microfabrication technology. Like a computer numerical control machine tool, the EFAB process is performed on a single, fully automated system that builds complete devices from start to finish. The process is fast and flexible, which allows a wide range of devices to be fabricated. Complex structures, internal geometry, and preassembled structures can be fabricated. The process can be used by any design engineer familiar with 3-D CAD.
Like semiconductor-based manufacturing, EFAB is a batch process capable of building thousands of devices (depending on size) simultaneously. Because it requires only a single, rapid tool, it is more practical for prototyping and product development than silicon micromachining. The costs are driven primarily by geometric volume rather than by the level of detail or precision. Therefore, the process is an effective means for mass-producing devices with a high degree of precision and detail. EFAB can produce parts with feature sizes below 0.001 in., and tolerances better than 0.0001 in. Such precision is difficult to obtain with precision machining technology. The EFAB process combines the speed, ease of use, and flexibility of machine tools with the precision, detail, and volume scalability of silicon-based micromachining. However, the EFAB process can accommodate parts a few millimeters in size larger than is generally practical with silicon-based micromachining.
Figure 3 shows microdevices with complex and varied features fabricated simultaneously on the same substrate.
Like a machine tool, the EFAB process is suitable for a wide range of applications. The examples shown in Figure 2 include radio-frequency (RF) electronic components such as high-Q inductors and transmission lines, optical components, microfluidic networks, and mold tooling for plastic devices. The technology can be applied to virtually any MEMS application in which metal materials are desirable. It is also suitable for purely mechanical applications.
The EFAB process is well suited for a variety of medical applications in which miniaturization is required, such as precise microsurgical tools in sufficient volume for disposability. It also has potential applications in endoscopes, or fixtures used to assist in microsurgery.
A variety of medical applications for electrical devices can also be fabricated. It can be used to fabricate specially shaped miniature probes or electrodes for applications such as pacemakers, deep brain stimulation, or chronic pain management. The process could also be used to fabricate high-Q RF inductors for inductively coupled implanted devices such as sensors.
Although EFAB is primarily a metal fabrication technology, it is also broadly applicable to a range of plastic devices. By using it to fabricate miniature plastic molds, high-precision mold tooling can be made quickly from 3-D CAD, with intricate features and complexity that would be difficult with conventional machining.
As long as the trend toward miniaturization continues, new approaches to fabricating microdevices will be required. The EFAB process is based on the concept that microscopic devices can be fabricated without the need for cleanrooms full of sophisticated equipment and teams of highly trained engineers.
With this electrochemical process, micromachining is no more challenging than conventional machining. Flexible, stand-alone, automated tools can generate virtually any device needed directly from CAD, and the implications for medical applications are vast. The EFAB process is designed to help manufacturers take a leap forward in realizing the full potential of miniaturization in manufacturing.
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