By Luke Volpe
Director of Engineering
Metrigraphics Precision Components
Dynamics Research Corporation
Wilmington, Mass.

3D microstructures are electro-mechanical devices that bridge the gap between conventional manufacturing techniques and MEMS technology.

As the drive to build smaller and more complex electro-mechanical devices or microdevices continues, design engineers struggle to bridge the gap between conventional micromachining techniques such as laser, water jet, microEDM, micromilling and Micro Electro Mechanical Systems (MEMS). Specifically, design engineers are discovering that many products now in the design stage are too small to be built with these manufacturing techniques, and are not candidates for the MEMS technology because of size, cost, or material limitations. The problem is apparent for both the mechanical and electronic interconnects portion of microdevices and affects many application areas including medical, optical, space-based and semiconductor manufacturing equipment.

The Big Picture
Typically, “three-dimensional (3D) micros” are defined by dimensional relationships where the aspect ratio (height divided by the minimum feature width) is greater than one. Structures with aspect ratios of one or less than one are considered planar. They are usually, but not always, metallic structures with precisely controlled X, Y, and Z dimensions that may range from 0.002 mm to 0.500 mm. Depending on the application, the structure could be as simple as a doughnut-shaped flat washer or as complex as a multi-planed, magnetically actuated control device. If a structure is large enough to be formed by conventional machine-tool technology it is not considered a 3D microdevice. Also, MEMS, silicon-based devices with sub-micron dimensions and embedded electronics, are not considered 3D microdevices.

3D Micros are manufactured in several forms. They can be (1) free standing structures, or (2) sheets of linked structures that can be singulated, (3) structures that are adhered to rigid substrates, (such as glass or alumina) or (4) structures that adhere to flexible substrates (such as metal foils or plastics).

Applications for 3D microdevices include medical implants, optics, micro fluidics and space based systems. Medical applications include micro-capillary systems for controlled fluid transfer and micro-induction coils for radio frequency coupling devices. Such coupling devices can be used to transmit data from outside the body to an implanted receiver.

Optical applications include micro-lenses and laser mounts, while space-based applications include solid gold or copper foils with specific thicknesses and slit widths designed to filter out all but selected wavelengths.

The process is such that virtually any complex combination of structural feature shapes like curves, spirals and straight lines, can be linked together in the X, Y plane. The Z plane (thickness) can be formed as a single layer of electrochemically deposited metal of one thickness or several layers of varying thicknesses and X, Y,  and Z dimensions. The ability to control X,Y, and Z dimensions, independently and in multiple layers, makes it possible to form blind holes or recessed areas, channels, cantilevered beams, and spring structures. Essentially, the X, Y-plane dimensions are controlled by the UV (ultraviolet) photolithography and the Z dimensions are controlled by the plating thickness within the aspect ratio limits of the UV photoresist. It is this design freedom, within defined limits, that makes this technology attractive and enabling to design engineers.

The Process
The building process for 3D microdevices is based on three disciplines: Semiconductor and micron level photolithography, thin film metal processing (sputter deposition and removal), and Electro-chemical metal deposition (electroplating/electroforming). Each discipline takes advantage of recent developments in semiconductor photoresist chemistry, electrochemical metal deposition and traditional thin film sputtering and ion milling.

In its simplest form, the process consists of:
• Creating a photoresist “mold” of the intended structure on a previously prepared, electrically conductive substrate, typically metal-coated glass.
• The metal seed coating provides the electrical conductance for electroplating into the photoresist mold.
• Removal of the photoresist mold from the conductive substrate.
• Removal of the completed 3D Micros from the conductive substrate.

Following is a more detailed description of the process model:

First, create a smooth, flat, electrically conductive substrate to be used as a building platform or substrate. This is usually accomplished by sputter depositing a thin film seed layer (<5000 angstroms) of conductive metal onto a base carrier. The seed metal layer must bond well to the substrate, and the seed metal/air interface must be sufficiently active to induce and maintain a bond during the plating process.

Second, deposit and image photoresist using pre-defined photo mask. This process step defines the X, Y-plane dimensions and creates the mold into which the electroformed metal will be deposited. Issues to be considered when selecting a photoresist are minimum feature size, maximum aspect ratio (maximum thickness/minimum feature size), number of (Z) layers and required dimensional tolerances.

As a rule, minimum feature size is 0.002mm, maximum aspect ratio 5/1, and critical dimension (CD) tolerances are +/-0.001mm. However, these values and tolerances are limited with regard to the aspect ratios required. The higher the ratio, the greater the tolerance needed. Depending on the particular structure, feature sizes as small as 0.001mm and aspect ratios as high 10/1 are possible.

Third, Using the sputter deposited seed metal (step one) as the conductor, electrochemically deposit the desired metal into the photoresist mold created in step two above. Metal options include, but are not limited to, nickel (Ni), nickel cobalt (NiCo), pure gold (Au), hard gold, and copper (Cu). Selection of the metal to be used depends on the particular application. Pure gold is essential for implantable devices, but NiCo is most commonly used where hardness, surface finish, tensile strength, and spring qualities are critical. Various combinations of hard gold and copper are most common in devices requiring subsequent bondability or critical electrical characteristics.  The actual metal deposition process is based on traditional electrochemical technology that has been finely tuned to result in metal deposits that closely match application requirements.

Fourth and finally, remove the photoresist mold and in some cases separate the completed 3D microstructures from the base mandrel. This last step can be as simple as pealing the individual structures from the base or may require some chemical immersion. The ease of releasing the completed 3D microstructure from the base mandrel is related to the surface energy of the top layer of the seed metal. For example, the more active the surface energy the better the bond between the electroplated deposit and the seed metal and the more difficult it is to remove. If the surface energy is lower, there will be a poor bond between the electroplated deposit and the seed metal making it easier to remove the 3D microstructure.

The intent is to create a surface energy high enough to maintain adhesion of the electroplated material until completion of the plating process, but also low enough to insure easy removal from the base mandrel. This is accomplished by applying appropriate activation or passivation treatments as required for the material being electroplated.

Sometimes the intention is to leave the 3D Micros permanently bonded to the carrier.  In those cases, the unplated seed metal around the 3D Micros may be removed to electrically isolate each structure on the base mandrel.

Material considerations
There are a number of variables to consider when choosing the design material. Once it has been established that the intended device is a candidate for the 3D Micro process the next consideration should be the material. As stated above the most commonly used 3D Micro materials are Ni, NiCo, Au, hard gold, and Cu. For those structures intended for implanted medical devices, pure Au may be the only available option. Other noble metals may be considered but are typically more difficult to electroform.

Applications that require greater physical strength, hardness, or smoother surface finish (smaller grain structure) and good spring qualities would use NiCo. Electroformed NiCo is a laminar (as opposed to columnar) growth material. This laminar growth molecular structure gives NiCo the above stated unique characteristics.

For those applications requiring corrosion resistance, pure Ni or Ni overplated with gold are both acceptable options. It should also be understood that solid hard gold and pure gold are both excellent options for applications involving extreme corrosive conditions. Likewise, applications demanding high conductance and low impedance would consider copper or copper overplated with gold.

Application examples
An example of 3D Micro applications is fluid jetting. Possible structure configurations include aperture holes, both straight walled and funnel shaped down to 0.001mm. In the case of fluid jetting systems, funnel shaped, single hole apertures holes are used to control pressure and direction. (See Figure 2)

In another, more familiar application, ink jet printing, one 5mm x 10mm hole array may contain up to 300 precisely (+/-0.001mm) shaped and positioned nozzle holes. Such hole arrays are usually manufactured in 300mm square sheets with the arrays connected by easily parted connecting links. The ability to temporarily link thousands of 3D micros together during the manufacturing process aids handling and assembly.

On the medical front, high aspect ratio conductive coils have been built for RF coupling applications used in data transfer in and out of the human body and for other areas where remote data transfer is required. An advantage of building coils with this process is the 3D Micros’ square or rectangular cross section, which can carry larger current loads and increase signal power over traditional round wire wound coils with similar dimensions. (See Figure 3)

In a more complex example, the 3D Micros technology has been used to build multi-level freestanding magnetically actuated switching devices. These devices consist of unconstrained micro springs with 0.050 mm square cross section and a 0.150 mm second layer for structural stability. The material for the device, shown in figure 4, is hard gold, chosen because its modulus of elasticity matched the device requirement.

In a more common application, the process has been used to form 3D Micro springs used for a variety of IC chip probing systems. In this case, the material of choice was NiCo because of its structural strength and spring characteristics. The overall dimensions of the springs were 0.010 mm x 0.020 mm x 1.4 mm long with a precisely controlled 12.0 mm radius of curvature along the 1.4 mm length. Figure 5 shows a 3D Micro probe with 0.050 mm x 0.100 mm spring cross section.

Experience has taught that this technology is most effective when the product or device design engineers collaborate with the manufacturing engineers early in the design stages. Early, and sometimes frequent, design review meetings involving both engineering organizations may significantly improve device functions and reduce cost. Most of the devices used today do not replace a component in an existing device, but instead enable a generational improvement in a family of devices.

Advantages and Disadvantages
The 3D Microstructures process does not come without advantages and disadvantages. The process allows for freedom of design within the defined dimensional ranged previously stated. Consider the repeatable precision of the lithography-based process as it consistently reproduces dimensional tolerances in the +/-0.001-mm range.  In terms of cost, most devices are built on a 150 mm or 300 mm square carrier substrate. When selling at a per substrate process cost, smaller devices mean greater number of devices on a single processed sheet, thus lowering the substrate unit cost.

On the contrary, you may find that total thicknesses are usually limited to 0.250 mm (but in extreme cases, a 0.500-mm thickness is possible) and active electronic devices are not included. Although the end product can result in more devices at a lower unit cost, the original tooling can sometimes be costly.

The strength of this technology lies in its ability to manufacture simple and complex three-dimensional microstructures in the 0.002 mm to 0.500-mm realm with 0.001-mm tolerances in a production-manufacturing environment. Additionally, the economics of the process have been demonstrated in its ability to build many thousands of complex and precise structures on a single processed substrate.

Metrigraphics Precision Components
www.drc.com/metrigraphics/metrigraphics.htm