Zongliang Cao

Single-material MEMS using polycrystalline diamond

The multi-material MEMS fabrication process often requires a larger number of masks making it more expensive as compared to single-material MEMS (SMM) technology. By varying the doping level in poly-C, semi-conducting, metallic and insulating (undoped) properties are achieved that are needed for poly-C SMM. However, the development of diamond-based SMM technology faces a number of challenges including (a) producing highly-insulating and highly-conducting poly-C films, (b) creating ohmic contacts and (c) patterning by dry etching of poly-C films grown on Si or SiO2. These challenges are addressed in this paper.

All-diamond micro-electrode arrays for neural recordings and diamond electrochemistry

This paper reports the successful fabrication and testing of all-diamond micro-electrode-arrays (MEAs) for use in neural recording and chemical detection applications using standard electrochemical methods. The MEAs have been tested through in vivo neural recordings. Also, electrochemical probes are reported using diamond as working and reference electrodes, achieving a potential window of about 4 V.

Thin film packaging process for MEMS device using polycrystalline diamond

This work reports two designs of a poly-C thin film packaging process, each including an encapsulated poly-C device. The 1st design uses boron-doped poly-C as electrical feedthroughs which can be embedded into the undoped, electrically insulating poly-C package. Access ports were opened along the package edge to release the thin film package and the device. Then, additional poly-C growth was used to seal the access ports. The 2nd design is based on the concept of using porous diamond to release the structures from the top of the package, thereby significantly reducing the release and sealing time of the package, without significantly affecting the device. A preliminary test regarding the packages fluidic hermeticity was performed to demonstrate that the poly-C thin film package has good fluidic hermeticity in an acidic environment. This is the first time that porous diamond created by RIE has been used in all-diamond thin film packaging including poly-C electrical feedthroughs, and a poly-C encapsulated device has been reported.

MEMS structures using polycrystalline diamond single-material micro technologies

Large band gap materials such as diamond (5.5eV) and AlN (6eV) offer the possibility of making MEMS structures out of a single material by varying the doping level to achieve the semi-conducting, metallic and insulating (undoped) properties needed in a typical MEMS structure. Polycrystalline diamond (poly-C), which has recently been used in the fabrication of BioMEMS, RFMEMS, and MEMS packaging, is inexpensive and retains many of the unique properties of single-crystal diamond. However, the development of diamond-based SMM technology faces a number of challenges including (a) producing highly-insulating and highly-conducting poly-C films, (b) creating ohmic contacts, and (c) patterning by dry etching of poly-C films grown on SiO2. The results presented in this paper, which addresses these issues for the first time, are expected to lead to SMM based MEMS structures and packaging.

Technology of single-material field-emission diode using polycrystalline diamond

There has been an increasing interest in polycrystalline diamond (poly-C) based field-emission (FE) devices due to their huge application [1] potential and lower cost. In order to permit (a) lower turn-on voltage, (b) high accuracy of anode to emitter spacing, and (c) well defined emitter area, the recent research focus has been on devices with a built-in anode. This paper takes the built-in anode technology to an entirely new level, the so-called single-material FE device (SMFD) technology. SMFD made of p-type poly-C is shown in Fig. 1. The single-material technology was originally developed for Single-Material Microsystems (SMM), which is being reported for the first time for SMFD. The SMM concept uses undoped (109Ω· cm), lightly-doped (p, 1-10Ω · cm) and highly-doped (p+, 10-3Ω· cm) poly-C as mechanical, sensor and interconnect material, respectively.

Optimization of Reactive Ion Etching of Polycrystalline Diamond for MEMS Applications

It has been found that diamond columns can be formed unintentionally in reactive ion etching (RIE) with O2 plasma even without a precoated metal layer. The experimental results indicate that the existence of these diamond columns prevents the effective removal of polycrystalline diamond (poly-C), also known as microcrystalline diamond. A three-step sequential RIE of poly-C thin film in CF4 (or CF4/Ar), O2, and H2 plasmas is developed using lithographically patterned Al acting as a hard mask to achieve a smooth-etched surface after the removal of poly-C. This etching technique can remove a thick poly-C layer very effectively. For the first time, this letter eliminates RIE-related damage to underlying substrate (specific to RIE of poly-C) to optimize the technology for single- and multi-material MEMS made from poly-C.

Fabrication and testing of CVD diamond single-material MEMS resonators with piezoresistive detection

Using piezoresistive detection and piezoelectric actuation, the fabrication and testing of polycrystalline diamond based single-material MEMS (SMM) resonators, with a resonant frequency of 48.2 KHz and a quality factor of 160 in air, is reported for the first time. A 0.6-µm-thick boron-doped poly-C with a resistivity of 9 Ω·cm is used as a piezoresistor. A 50-nm-thick highly-doped poly-C inter-layer, with a resistivity of 5×10⁻³ Ω·cm, was used between the metal and the piezoresistor to reduce the contact resistance. A 3-µm-thick undoped poly-C film, with a resistivity > 10⁹ Ω·cm, was used as an insulating as well as a structural material.

All-diamond integrated field emission device with a micro-tip array cathode

In order to permit (a) lower operating voltage, (b) high accuracy of anode to emitter spacing, and (c) well defined emitter area, a gated poly-C field emission device (FED) is reported. It uses undoped (ρ=10⁹ Ω·cm) as an insulation layer and highly-doped (ρ=10⁻³ Ω·cm) poly-C as electrodes and interconnects to create an integrated FED (IFED) with a micro-tip cathode array using dry-etching of poly-C. Fabrication and testing of an alldiamond IFED is reported for the first time.

Piezoresistive sensor technology for RFMEMS using p-type polycrystalline diamond

Design, simulation, fabrication and testing of p-type polycrystalline diamond (poly-C) piezoresistive RFMEMS is reported for the first time. The use piezoresistive detection in RFMEMS can lead to an output impedance in the ranges of 20 - 500 Ω and several MΩ for intra- and inter-grain piezoresistors, respectively. The inter-grain gauge factor of the poly-C film with a resistivity of 22 Ω · cm was estimated to be over 20. An Ohmic contact with a contact resistance of 5.21 MΩ was achieved by using a highly-doped poly-C interlayer between metal and lightly-doped piezoresistor.

Diamond micro and nano resonators using laser, capacitive or piezoresistive detection

Design, fabrication and testing of polycrystalline diamond (poly-C) micro and nano resonators are reported using laser, capacitive or piezoresistive detection. Important diamond MEMS fabrication issues, including micro-masking in dry etching, are also addressed. The resonators consisted of undoped poly-C (grown at 700degC or 780degC) cantilever beams with lengths, widths and thicknesses in the ranges of 40 - 2000 mum, 10 - 200 mum and 0.6 - 2 mum, respectively. The ploy-C piezoresistors, in-situ boron doped (p = 1 - 10 Omega*cm) at 700degC, had lengths, widths and thickness in the ranges of 24 - 42 mum, 3-12 mum and 0.5 mum, respectively. The early results of measurement using piezoelectric actuation and laser detection, in a vacuum chamber held at 10 s torr, are very encouraging.