Jordon Gilmore and Rodrigo Martinez-Duarte
Multiscale Manufacturing Laboratory
Department of Mechanical Engineering
Wet etching is the earliest and perhaps the most well-documented method for fabricating microfluidic networks. Using glass or silicon as substrate, researchers have been able to create a number of micro-scaled geometries including channels, wells, suspended structures, valves and pumps –. Etching techniques can be classified into isotropic and anisotropic based on the directionality of the etching agent. In isotropic etching, the etching rate is the same in all directions. This can be beneficial when targeting to machine circular channel cross sections or when profile control is not the primary objective. However, this isotropy limits depth and resolution of micromachined profiles . For example, deep narrow grooves are not possible with isotropic etching. Anisotropic etching methods have controlled etching rates based on the attack rate between a specific etchant and the substrate. For example, the different crystallographic planes in single crystal silicon substrates etch differently when reacted with potassium hydroxide (KOH) or EDP (ethylene diamine pyro-catechol). In this case, components with depth and wall dimensions (i.e. micro-wells) that are given by the geometric relations between the crystallographic properties can be fabricated. These geometries have found wide application in inkjet printing nozzles and pressure sensors. However, the ability to construct circular channels and other shapes is limited with this technique , . Figure 1 illustrates the differences between isotropic and anisotropic etching strategies as well as the complete fabrication process when patterning a substrate. Note the requirement of a photoresist mask to mask the attack of etchant to the substrate.
|Figure 1: Schematic comparing anisotropic and isotropic etching techniques in the left and right respectively. Courtesy of Prof. Marc Madou |
Etching techniques can be further classified into wet or dry etching. Wet etching techniques are largely isotropic in nature, with many researchers taking advantage of the fast etch rates (6 µm/min), relative low cost and simplicity of etching protocol (dip the sample into an etching bath), and channel depths up to 40-50 µm . Applications such as DNA amplification and analysis via electrophoresis  have been accomplished using microchannels up to 100 µm in width using this simplified method. However, the isotropic nature of etching limits resolution of these microstructures to 3 µm or greater. Other work has been done to create anisotropic etching of silicon using wet etchants such as KOH, EDP, or TMAH. Xu and coworkers used an anisotropic wet etching technique with KOH to create pyramid shaped holes that served as an alignment gap for laser-induced fluorescence detection of DNA in a microfluidic electrophoresis application . KOH wet etching was also used in combination with several other fabrication methods in a high-precision bio-separation application where anisotropic structures formed nanoscale sieving devices for particle separation, Figure 2, .
Reactive ion etching (RIE, Figure 2) is a dry etching technique that combines the advantages of the chemical and physical processes available when the workpiece is exposed to an ionized gas. RIE is good to create highly selective, relatively anisotropic geometries. This process allows for the manufacture of complex, high aspect ratio microfluidic components using ion bombardment to selectively prime the surface for reaction. Reactive ions are accelerated toward the surface of the substrate using electric fields to reduce the energy required for reaction of the substrate with the environment. Starting in the mid-1990s, groups such as Shaw and colleagues  and industrial researchers from Robert Bosch GmbH  began developing techniques such as Single Crystal Reactive Etching and Metallization (SCREAM) and Deep Reactive Ion Etching (DRIE, Figure 2), respectively. Both of these techniques involve the combinations of anisotropic and isotropic dry etching, however, DRIE includes the use of a photoresist polymer layer as a protective barrier to side walls during the bombardment of inert Argon (Ar+) ions. This allows for the creation of high aspect ratio features (>30:1) and etch rates of 2 to 3 µm/min . The SCREAM process is similar in that a protective oxide is added to guard sidewalls during ion etching, but the conclusion of the process includes a final isotropic etch and metallization of the remaining sidewalls . Devices made through this method focus on the suspended silicon beams or columns in sensor applications where high aspect ratios (> 50:1) and high suspension span (> 5 mm) are required .
|Figure 2: Schematic of the fabrication process for both the planar and vertical anisotropic nanofluid-filter array (ANA) devices. (a), (i-ii) the shallow and deep regions are etched into the Si by photolithography and RIE. KOH etching is used to create buffer access holes (not shown). (iii) A thermal oxide layer is added to provide electrical isolation between the Si and the buffer solution. (iv) The device is then sealed by a Pyrex wafer. (b), (i-ii) the narrow and wide regions are fabricated via photolithography, DRIE, and wet anisotropic KOH etching. (iii) The thermal oxide layer is added to decrease nanofilter gap size. (iv) Lastly, a uniform layer of PECVD oxide is deposited to seal the ANA device. Reprinted with permission from Nature Publishing. .|
While many types of structures are possible with etching techniques, including suspended structures, the major drawback of etching in general has been the need to close the microfluidic network through some bonding strategy. The need to bond two materials together (i.e. Si/Si, Si/glass, glass/glass) has proven difficult due to the need to introduce either additional heating and pressure (thermal bonding), additional heating and voltage application (anodic bonding), additional materials such as gold (eutectic bonding) or an adhesive (adhesive bonding). With all of these strategies there are considerations such as surface cleanness, additional equipment requirements, or introduction of surface imperfections that complicate the manufacturing process or limit the effectiveness of the device .