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Materials for Microelectromechanical Systems 2-21

Bulk micromachining of diamond is more difficult than SiC because electrochemical etching tech-
niques have not been demonstrated. Using a strategy similar to that used in SiC, bulk micromachined dia-
mond structures have been fabricated using bulk micromachined Si molds [Bjorkman et al., 1999]. The
Si molds were fabricated using conventional micromachining techniques and filed with polycrystalline
diamond deposited by HFCVD. The HFCVD process uses hydrogen as a carrier gas and methane as
the carbon source. A hot tungsten wire is used to crack the methane into reactive species as well as to heat
the substrate. The process was performed at a substrate temperature of 850 to 900°C and a pressure of 50
mtorr. The Si substrate was seeded with diamond particles suspended in an ethanol solution prior to dep-
osition. After diamond deposition, the top surface of the diamond structure was polished using a hot iron
plate. The material removal rate was reported to be around 2 mm/hr. After polishing, the Si mold was
removed in a Si etchant, leaving behind the micromachined diamond structure. This process was used
to produce all-diamond high-aspect-ratio capillary channels for microfluidic applications [Rangsten
et al., 1999].
Surface micromachining of polycrystalline diamond thin films requires modifications of conventional
micromachining practices to compensate for the nucleation and growth mechanisms of diamond thin
films on sacrificial substrates. Early work in this area focused on developing thin-film patterning tech-
niques. Conventional RIE methods are generally ineffective, so effort was focused on developing selective
growth methods. One early method used selective seeding to form patterned templates for diamond
nucleation. The selective seeding process was based on the lithographic patterning of photoresist mixed
with diamond powders [Aslam and Schulz, 1995]. The diamond-loaded photoresist was deposited onto
a Cr-coated Si wafer, exposed and then developed, leaving a patterned structure on the wafer surface.
During the diamond deposition process, the photoresist rapidly evaporates, leaving behind the diamond
seed particles in the desired structural shapes, which then serve as a template for diamond growth.
A second process has been developed for selective deposition directly on sacrificial substrate layers.
This process combines a conventional diamond seeding technique with photolithographic patterning and
etching to fabricate micromachined diamond structures using SiO sacrificial layers [Ramesham, 1999].
2
The process can be executed in one of two approaches. The first approach begins with the formation of
a SiO layer by thermal oxidation on a Si wafer. The wafer is then seeded with diamond particles, coated
2
with photoresist, and photolithographically patterned to form a mask for SiO etching. Unmasked
2
regions of the seeded SiO film are then partially etched in BOE to form a surface unfavorable for dia-
2
mond growth. The photoresist is then removed and a diamond film is selectively deposited. The second
approach begins with an oxidized Si wafer, which is coated with photoresist. The resist is photolitho-
graphically patterned, and the wafer is then seeded, with the photoresist protecting select regions of the
SiO surface from the damage caused by the seeding process. The photoresist is removed, and selective
2
diamond deposition is performed. In each case, once the diamond film is patterned, the structures can be
released using conventional means. These techniques have been used to fabricate cantilever beams and
bridge structures.
A third method to surface-micromachine polycrystalline diamond films follows the conventional
approach of film deposition, dry etching, and release. The chemical inertness of diamond renders most
conventional plasma chemistries useless for etching diamond films. Oxygen-based ion beam plasmas,
however, can be used to etch diamond thin films [Yang et al., 1999]. The oxygen ion beam prohibits the
use of photoresist masks, so hard masks made from metals such as Al are required. A simple ion-beam,
etching-based, surface-micromachining process begins with the deposition of a Si N film on a Si wafer
4
3
and is followed by the deposition of a polysilicon sacrificial layer. The polysilicon layer is seeded with a
diamond slurry, and a diamond film is deposited by HFCVD. To prepare the diamond film for etching, an
Al masking film is deposited and patterned. The diamond films are then etched in the O ion beam plasma,
2
and the structures are released by etching the polysilicon with KOH. This process has been used to create
lateral resonant structures; although the patterning process was successful, the devices were not operable
because of a significant stress gradient in the film. With a greater understanding of the structure–property
relationships of diamond thin films, such problems with surface micromachined structures should be
solvable, thus enabling the successful fabrication of a new class of highly functional devices.

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