Chapter 1 MEMS Technology
1.3 Parylene as a Microstructure Material
1.3.1 Introduction to Parylene
chip. It is understandable that the more components to be integrated on a chip the more difficult and likely lower-yield the process will be. Well, the multichip module (MCM) approach [38] is surely an alternative to be used for complicated systems. The major advantage of it is that the fabrication process for each chip will be much simpler and each component of the system can be designed or replaced individually with maximum flexibility. However, the MCM approach also introduces complexities such as microfluidic coupling between chips. A major goal of this thesis is to demonstrate with real examples (microchip HPLC systems) that reliable MEMS processes can be designed to achieve extensive components integration for LOC applications.
1.3 Parylene as a Microstructure Material
Figure 1-8 shows the parylene deposition procedures, the involved chemical processes, and the instrument. The process starts with placing parylene dimer (di-para- xylylene), a stable compound in granular form, into the vaporizer. The substrate to be coated with parylene is put into the deposition chamber. The whole system is then pumped down to medium vacuum (8-10 mTorr). The dimer is heated in the vaporizer and sublimed into vapor at around 170 °C. The dimer vapor enters the pyrolysis furnace that is maintained at 690 °C, where the dimers are cleaved into identical monomers (para- xylylene). In the room-temperature deposition chamber, the monomers reunite on all exposed surfaces in the form of polymers (poly-para-xylylene). Deposition chamber pressure is around 23 mTorr. Additional components of the system include a mechanical vacuum pump and associated cold trap to take away extra monomer vapor. Although parylene N structure is used in Figure 1-8, the deposition process is almost identical for all three common types of parylene, except for some slight differences in pyrolysis temperature and deposition pressure.
Typical parylene coating thickness in a single deposition run ranges from 0.1 to 20 microns. Sub-micron parylene deposition is trickier because thickness uniformity can be a concern. In general, the coating thickness is in proportion to the amount of dimer used. Chamber condition including cleanness, base pressure, and temperature fluctuation can result in a slight deposition rate change. The normal deposition rate of parylene C under a deposition vapor pressure of 23 mTorr is about 5 µm per hour. The deposition rate is directly proportional to the square of the monomer concentration in the chamber and inversely proportional to the absolute temperature of the substrate on which parylene will be coated [39].
CH2 CH2 n Di-para-xylylene
(Dimer)
para-xylylene (Monomer)
Poly(para-xylylene) (Polymer) CH2 CH2 CH2 CH2 CH2 CH2
CH2 CH2 (a)
(c) (b)
(a) Vaporizer:170oC
(b) Pyrolysis: 690oC
(c)Deposition 25 0C
Cold Trap -70oC
Vacuum Pump
Figure 1-8: Parylene deposition system and the involved chemical processes.
Since parylene thin film is deposited using chemical vapor deposition (CVD), the deposited film is highly conformal. As mentioned earlier, parylene is an excellent barrier to gas and moisture. Compared with PDMS (polydimethylsiloxane), which is another popular material for microfluidic devices, the gas permeability of parylene is more than four orders of magnitude smaller and moisture permeability is ten times smaller.
Parylene is extremely inert to most chemicals and solvents used in chemical or biological laboratories. Manufacturer’s study [40] shows that solvents have a minor swelling effect on parylene N, C, and D with a 3% maximum increase in film thickness. The swelling is completely reversible after the solvents are removed from parylene by vacuum drying.
Furthermore, parylene is biocompatible (USP Class VI), which makes it a good candidate material for making long-term human implants or biomimetic components.
As for mechanical properties, parylene has an elongation break of 200%. This mechanical flexibility makes it a durable material for composing MEMS microfluidic and actuation devices. Parylene is also an excellent electrical and thermal insulator. The electrical breakdown voltage of a 1-µm-thick parylene layer is over 200 volts. The room temperature thermal conductivity of parylene C (0.84 mW/cm-K) is only three times as big as static air (0.30 mW/cm-K). In terms of optical properties, parylene is pretty much transparent in the visible light range. However, parylene begins to absorb light with wavelength shorter than 280 nm significantly, which limits some of its UV applications.
The difference in chemical structures between parylene N, C, and D as shown in Figure 1-7 actually results in different thin-film properties. For example, parylene N has the lowest gas/moisture permeability. However, parylene N also has the lowest deposition rate which is not favorable in a mass production process. In terms of choosing a micromachining material from the three parylene species, parylene C has a good combination of electrical and physical properties plus a low permeability to moisture and other corrosive gases. Parylene C also has higher deposition rate than parylene N, and D.
Therefore, parylene C is often chosen to fabricate microstructures. In our lab, parylene C was chosen 99% of the time to build devices and the deposition parameters have been well characterized so to guarantee good thin film qualities which include film transparency, thickness uniformity/accuracy, and adhesion properties to assorted substrates. Parylene C was as well used exclusively in this thesis to build devices.
Detailed electrical, mechanical, thermal, optical, and other properties can be found on a parylene vendor’s website [39]. A list of selected properties for parylene N, C, and D is shown in Table 1-4.
Table 1-4: Properties for parylene N, C, and D.