Tech Briefs

Small, robust sensors can be fabricated on a variety of substrates.

The purpose of this article is to present additional information about the flow-velocity sensors described briefly in the immediately preceding article. As noted therein, these sensors can be characterized as artificial hair cells that implement an approximation of the sensory principle of flow-sensing cilia of fish: A cilium is bent by an amount proportional to the flow to which it is exposed. A nerve cell at the base of the cilium senses the flow by sensing the bending of the cilium. In an artificial hair cell, the artificial cilium is a microscopic cantilever beam, and the bending of an artificial cilium is measured by means of a strain gauge at its base (see Figure 1).

Figure 1. Artificial-Hair-Cell Flow Sensors shown in this scanning electron micrograph have several different widths as well as different heights ranging from 0.6 to 1.5 mm.

Figure 2 presents cross sections of a representative sensor of this type at two different stages of its fabrication process. The process consists of relatively- low-temperature metallization, polymer- deposition, microfabrication, and surface-micromachining subprocesses, including plastic-deformation magnetic assembly (PDMA), which is described below. These subprocesses are suitable for a variety of substrate materials, including silicon, some glasses, and some polymers. Moreover, because it incorporates a polymeric supporting structure, this sensor is more robust, relative to its silicon-based counterparts. The fabrication process consists mainly of the following steps:

  1. Figure 2. The Cantilever Remains Horizontal during most of the fabrication process. It is released by etching away the aluminum sacrificial layer, then raised to its perpendicular orientation by PDMA.
    A 0.5-μm-thick sacrificial layer of Al is deposited (by evaporation) and patterned on a substrate.
  2. A 5.8-μm-thick layer of a photodefinable polyimide is spun on and patterned photolithographically. The polyimide is cured in a 1-torr (≈133-Pa) atmosphere of N2 for 2 hours at a temperature of 350 °C (this is the highest temperature used in the fabrication process).
  3. A 750-Å-thick layer of NiCr, intended to serve as the electrically resistive transducer in the strain gauge, is deposited by electron-beam evaporation.
  4. A layer of Au/Cr 0.5 μm thick, from which the strain-gauge electrical leads and a bending hinge are to be formed, is deposited by evaporation.
  5. A portion of the Au/Cr layer also serves as a seed layer for electrodeposition of a 5-μm-thick layer of a highly magnetically permeable Fe/Ni alloy. Once this alloy has been deposited, the remaining unused Au/Cr is removed by lift-off.
  6. A 1.8-μm-thick polyimide film (omitted from the figure) is deposited to form a protective coat on the Fe/Ni alloy layer and the NiCr strain gauge.
  7. The workpiece is placed in a basic solution for more than a day to etch away the sacrificial layer of Al.
  8. The workpiece is rinsed, then placed in an electroplating bath. A magnetic field is applied to pull up on the Fe/Ni layer, thereby bending the cantilever upward at the hinge. While the magnetic field remains thus applied, Ni is electrodeposited onto the Au/Cr hinge to a thickness of ≈10 μm, thereby reinforcing the hinge and fixing the cantilever perpendicular to the substrate.

This work was done by Chang Liu and Jack Chen of the University of Illinois at Urbana-Champaign for Goddard Space Flight Center.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

University of Illinois at Urbana-Champaign
Office of Technology Management
319 Ceramics Building, MC-243
105 South Goodwin Avenue
Urbana, IL 61801

Refer to GSC-14812-1, volume and number of this NASA Tech Briefs issue, and the page number.

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