Chapter 3 High-Pressure Microfluidic Channel Technology
3.3 High-Pressure Parylene Microfluidic Channel Technology
pressure. Type II: After patterning the sacrificial photoresist and before the top parylene deposition, a self-aligned DRIE trench (with a mushroom anchoring space in the bottom [17]) lying right next to the sacrificial photoresist can be created. This trench structure provides a larger equivalent parylene/silicon contact area and a mechanical locking mechanism from the mushroom structure, which effectively enhance the top parylene adhesion to the silicon substrate (Figure 3-3). Testing results have shown the pressure capacity of this trench-anchored parylene channel is around 700 psi [2]. The failure modes for the trench-anchored channels can be either parylene peeling from the trenches or parylene-film-bursting at defected locations.
Channel space
Roughened silicon
DRIE trench
(a) (b)
Figure 3-2: Anchored-type parylene channel. (a) Roughening-anchored channel, (b) trench-anchored channel.
Figure 3-3: Cross-sectional pictures of a trench-anchored parylene channel [2].
3.3.2 Embedded-Type Parylene Microfluidic Channel
The design concept of the embedded-type parylene channel is to build the channel space inside the silicon substrate so to obtain high pressure-capacity of the channel.
Different from the buried channel technology (BCT) mentioned in 3.2.5, the channel top surface is at the same height as the silicon wafer surface. This channel configuration allows easy integration of optical sensing such as laser-induced fluorescence detection or electrical sensing such as capacitively-coupled contactless conductivity detection (C4D) through the top surface of the channel.
Parylene
Channel
Silicon 20 µm
(a)
(b)
Figure 3-4: Basic embedded-type parylene channel. (a) Fabrication process flow for the basic embedded-type parylene channel, (b) fabricated basic embedded-type parylene channel.
The simplest way to fabricate an embedded-type parylene channel is shown in Figure 3-4 (a). Thermal oxide is first grown on the silicon substrate. The oxide layer is then patterned with buffered HF to create openings for XeF2 etching. XeF2 is used to
etch silicon through the oxide opening and create the channel cross-section. Finally, a parylene layer is deposited to seal the oxide opening. Figure 3-4 (b) shows the fabricated channel cross-section [18]. The fabrication process for the basic embedded-type parylene channel is extremely simple. However, the produced channel configuration does have unavoidable drawbacks. First, the channel cross-section is not highly symmetric compared with most commercial LC columns. This unsymmetry can contribute to band- broadening in the chromatographic process due to eddy diffusion or flow distribution.
Second, it is more difficult to fabricate a well-defined stationary-phase particle filter using this technology (to be discussed more in chapter 5).
To overcome the issues encountered in the basic embedded-type parylene channel, a more sophisticated process has been developed as shown in Figure 3-5. The fabrication process starts with creating trenches on the silicon substrate using DRIE (Figure 3-5 (a)).
The width between trenches defines the initial channel width. The first parylene layer is then deposited, which fills up the trenches. Oxygen plasma is then used to etch back the first parylene layer until the silicon surface reveals. The trenches, however, will still be filled with the first parylene coating. The second parylene layer (around 2 µm in thickness) is then deposited. The second parylene is then patterned with oxygen plasma to create openings for the latter XeF2 etching process (Figure 3-5 (b)). XeF2 is used to etch away silicon through the parylene opening and create the square-like channel cross- section (Figure 3-5 (c)). The depth of the channel can be monitored during XeF2 etching for precise cross-section control. Finally, the third parylene layer is deposited to seal the parylene opening (Figure 3-5 (d)). Figure 3-6 then shows the fabricated channel cross- section [19].
The pressure capacity of the embedded-type parylene channel has been tested to be higher than 1,000 psi without fracture [18]. Moreover, due to the highly planarized chip surface, a direct-top-clamping to the chip with a glass plate can be easily applied to enhance channel pressure capacity to thousands of psi, which will be discussed more in chapter 5.
Photoresist
Parylene1 Parylene2
Silicon
Parylene3 (a)
(b)
(c)
(d)
Figure 3-5: Fabrication process flow for the advanced embedded-type parylene channel.
Figure 3-6: SEM of the advanced embedded-type parylene channel.
50 µm
3.3.3 Comparisons
Both anchored-type and embedded-type parylene channels have their unique advantages and disadvantages, which make them suitable for different application needs.
For example, the anchored-type channel allows the fabrication of in-channel electrodes that are exposed to the solvents. These electrodes can serve the purposes of electrochemical/conductivity sensing of analytes, electrolysis, or ionization of solvents [2, 20]. On the other hand, the in-channel electrodes of the anchored-type channel will always be covered by the third parylene coating and therefore the electrodes will not be in direct contact with the analytes or solvents. The embedded-type channel does have higher pressure capacity than the anchored-type channel, especially when the glass-plate- clamping technique is applied to the planarized chip surface. The embedded-type channel has a square-like cross-section while the anchored-type channel normally has a rectangular cross-section that can contribute to band-broadening and degrade the chromatography performance. Characteristics of each channel technology will be further discussed in chapters 4 and 5, respectively.