This invention relates to nanotechnology and/or microelectronics, and specifically to solid state sensors and detectors which are particularly well suited for use in optoelectronic devices.
There is an increasing need for inexpensive, sensitive solid state gas sensors for applications such as pollution and toxic gas monitoring, homeland security, “lab-on-a-chip”, etc. It is known that materials, such as metal oxides, exhibit sensitivity to various gases because of their ability to adsorb these gases in their surfaces. Recently, solid state gas sensors have been proposed and fabricated which employ planar thin films of these metal oxides, such as In2O3, SnO2, Fe2O3, and ZnO, to detect and quantify various gases. Nanostructured materials, because of their inherent high surface area relative to the volume of material, have been reported to be ideal for sensing applications. However, the lack of a reasonable method to fabricate a useful, i.e., electrically measurable and MOS integratable, device using nanostructured materials, has prevented widespread use of these materials.
Martins et al., Zinc oxide as an ozone sensor, J. Appl. Phys. 96(3), 1398 (2004), describes the use of a UV bombarded ZnO film-on-glass as a sensor.
Gordillo et al., Effect of gas chemisorption on the electrical conductivity of ZnO thin films, Advances in Mat. Sci. and Tech. 1(1), 1 (1996), describes use of an annealed ZnO thin film as a detector for CO2, O2, H2 and CH4.
A method of fabricating a nanowire sensor device structure includes preparing a substrate, having a silicon base layer, a buried oxide layer in the silicon base layer in some instances, a top silicon layer on the buried oxide layer, and a doped well in the silicon base layer; forming a silicon island from the top silicon layer; etching the buried oxide layer; etching the buried oxide layer to undercut the silicon island; depositing a seed layer of polycrystalline ZnO over the silicon island, the buried oxide layer, the doped well and the silicon base layer; removing the polycrystalline ZnO from the top of the silicon island; growing ZnO nanostructures on the seed layer of ZnO; removing ZnO and extraneous nanostructures from the edges of the silicon islands; treating the ZnO nanostructures to sensitize the ZnO nanostructures to a desired application; depositing a layer of insulating material; patterning and etching the insulating material; and metallizing the nanowire device structure.
It is an object of the invention to provide a method of fabricating a nanowire sensor device.
Another object of the method of the invention is to provide a nanowire sensor which may be fabricated using conventional, state-of-the-art IC fabrication processes.
This summary and objectives of the invention are provided to enable quick comprehension of the nature of the invention. A more thorough understanding of the invention may be obtained by reference to the following detailed description of the preferred embodiment of the invention in connection with the drawings.
The method of the invention provides a protocol for fabricating a solid state gas sensor on a silicon-on-insulator (SOI) substrate which incorporates ZnO nanowire bridges. Selective growth of ZnO nanowire bridges is achieved using an ALD ZnO seed layer in some embodiments of the method of the invention. The method of the invention is compatible with standard microelectronic processing techniques and may be integrated with CMOS devices.
The steps of the method of the invention are shown in
Next, silicon surface 20 is coated with photoresist, patterned, and dry etched 24 to produce a silicon island 20a that is electrically insulated from the substrate, as shown in
A thin seed layer 30 of poly-crystalline ZnO is deposited 32 on the surface of the wafer using atomic layer deposition (ALD). It has been previously disclosed that ALD ZnO may be used as a seed layer for ZnO nanostructure growth in U.S. patent application Ser. No. 10/976,594, of Stecker et al., filed Oct. 29, 2004, for ALD ZnO Seed Layer for Deposition of ZnO nanostructures on a Si substrate, wherein a ZnO film was described as being used as a seed layer for ZnO nanostructure growth. In this implementation of the method of the invention, about 10 nm of ALD ZnO is deposited using 35 cycles of alternating pulses of a precursor, such as diethylzinc (DEZ), and H2O vapor, at a substrate temperature of between about 130° C. to 200° C., although any thickness of ZnO from between about 1 nm to 70 nm may be used. The ALD method of deposition is necessary to conformally coat the underside of the undercut island.
The ALD ZnO surface is coated with photoresist, patterned, and dry etched to remove ZnO 34. Dry etching is used so that the thin ALD coating remains under the island. The photoresist is then stripped and a patterned ZnO surface remains as shown in
ZnO nanostructure growth is next induced. For this embodiment, the structure of
To clean the edges of the structure, the horizontally oriented nanostructures and ZnO on the side(s) of silicon island 20a may be selectively removed with an optional dry etch 40, also referred to herein as structuring the nanostructures and the ZnO layer. The chemistry may be made selective to ZnO so that the top silicon layer is not eroded. The structure appears as in
The remaining steps are performed in order to make electrical contact to the two terminals of the device: layer 20 and layer 22. A coating of insulating material 42, such as SiO2 or Si3N4, etc., is deposited 44, patterned 46, filled 48 with metal 50, and then patterned again to arrive at the structure in
An alternative embodiment is shown in
A device constructed according to the method of the invention will work similarly to previously described planar devices in that exposure to gases will modify the surface of the ZnO so as to change the conductivity of the wires, which is measured between the two terminals. Changes in conductivity may be empirically tied to changes in gas concentration. Because of the increased surface area, any device constructed according to the method of the invention is much more sensitive than a planar device.
The other standard components of such gas sensors such as a resistive heater, temperature sensor, etc., may be fabricated around this device by standard IC fabrication methods which are well known in the industry. It is likely that the method of the invention will be functional and have utility when using other appropriate seed layers for other nanowire structures, such as ALD In2O3 for In2O3 nanowires, etc.
In another variation of the method of the invention, a structure 60, shown in
Another alternative embodiment is shown in
Thus, a method for fabricating nanowire sensor device structures has been disclosed. It will be appreciated that further variations and modifications thereof may be made within the scope of the invention as defined in the appended claims.