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Materials for Microelectromechanical Systems 2-11

As expected, the authors found that film stress is related to the concentration of dissolved gases in the
film and that annealed films tend to suffer from cracking. By using a thin Si N film in conjunction with
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the thick SiO film, conditions were found where a low-stress, crack-free SiO film could be produced.
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Two other materials in the SiO family are receiving increasing attention from MEMS fabricators, espe-
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cially now that the material systems have expanded beyond conventional Si processing. The first of these
is crystalline quartz. The chemical composition of quartz is SiO . Quartz is optically transparent and, like
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its amorphous counterpart, quartz is electrically insulating. However, the crystalline nature of quartz
gives it piezoelectric properties that have been exploited for many years in electronic circuitry. Like single-
crystal Si, quartz substrates are available as high-quality large-area wafers. Also like single-crystal Si,
quartz can be bulk micromachined using anisotropic etchants based on heated HF and ammonium flu-
oride (NH F) solutions, albeit the structural shapes that can be etched into quartz do not resemble the
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shapes that can be etched into Si. A short review of the basics of quartz etching and its applications to the
fabrication of a micromachined acceleration sensor can be found in Danel et al. (1990).
A second SiO -related material that has found utility in MEMS is spin-on-glass (SOG), which is used
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in thin-film form as a planarization dielectric material in IC processing. As the name implies, SOG is
applied to a substrate by spin coating. The material is polymer based with a viscosity suitable for spin-
coating, and once dispensed at room temperature on the spinning substrate, it is cured at elevated tem-
peratures to form a solid thin film. Two publications illustrate the potential uses of SOG in MEMS. In the
first example, SOG was developed as a thick-film sacrificial molding material to pattern thick polysilicon
films [Yasseen et al., 1999]. The authors reported a process to produce SOG films that were 20µm thick,
complete with a chemical–mechanical polishing (CMP) procedure and etching techniques. The thick
SOG films were patterned into molds that were filled with 10µm thick LPCVD polysilicon films, pla-
narized by selective CMP and subsequently dissolved in a HCl:HF:H O wet etchant to reveal the pat-
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terned polysilicon structures. The cured SOG films were completely compatible with the polysilicon
deposition process, indicating that SOG could be used to produce MEMS devices with extremely large
gaps between structural layers. In the second example, high-aspect-ratio channel-plate microstrucures
were fabricated from SOG [Liu et al., 1999]. The process required the use of molds to create the struc-
tures. Electroplated nickel (Ni) was used as the molding material, with Ni channel plate molds fabricated
using a conventional Lithographie, Galvanoformung, Abformung (LIGA) process. The Ni molds were
filled with SOG, and the sacrificial Ni molds were removed in a reverse electroplating process. In this case,
the fabricated SOG structures were over 100µm tall, essentially bulk micromachined structures fabri-
cated using a sacrificial molding material system.

2.5 Silicon Nitride

Si N is widely used in MEMS for electrical isolation, surface passivation, and etch masking and as a
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mechanical material. Two deposition methods are commonly used to deposit Si N thin films: LPCVD
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and PECVD. PECVD Si N is generally nonstoichiometric and may contain significant concentrations of
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hydrogen. Use of PECVD Si N in micromachining applications is somewhat limited because its etch rate
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in HF can be high (e.g., often higher than that of thermally grown SiO ) due to the porosity of the film.
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However, PECVD offers the potential to deposit nearly stress-free Si N films, an attractive property for
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many MEMS applications, especially in the area of encapsulation and packaging. Unlike its PECVD coun-
terpart, LPCVD Si N is extremely resistant to chemical attack, thereby making it the material of choice
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for many Si bulk and surface micromachining applications. LPCVD Si N is commonly used as an insu-
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lating layer to isolate device structures from the substrate and from other device structures because it is
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a good insulator with a resistivity of 1016Ω-cm and a field breakdown limit of 10 V/cm.
The LPCVD Si N films are deposited in horizontal furnaces similar to those used for polysilicon dep-
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osition. Typical deposition temperatures and pressures range between 700 and 900°C and 200 to 500 mtorr
respectively. A typical deposition rate is about 30Å/min. The standard source gases are dichlorosilane
(SiH Cl ) and ammonia (NH ). SiH Cl is used in place of SiH because it produces films with a higher
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