Microbridges that constitute the superconducting-transition sensory elements of electron-diffusion-cooled hot-electron bolometers can now be fabricated in increased yield and quality. This improvement is made possible by a fabrication process that notably includes the use of a self-aligned mask during a critical etching step.
Some explanation of the underlying principles is necessary to give meaning to a description of the fabrication process. Electron-diffusion-cooled hot-electron bolometers are undergoing development for use as mixers that could operate at carrier and local-oscillator frequencies in the terahertz range, putting out signals at intermediate frequencies (IFs) in the gigahertz range. The most likely initial applications of such mixers would occur in radio-astronomy receivers. A prototype of the present electron-diffusion-cooled hot-electron bolometers was described in "Diffusion-Cooled Hot-Electron Bolometer Mixer" (NPO-19719), NASA Tech Briefs, Vol. 21, No. 1 (January 1997), page 12a.
The microbridge in a device of the present type is made from a film of niobium 10 nm thick. The length and width of the bridge are typically of the order of 100 nm (see Figure 1). The film material and dimensions are chosen to obtain the required superconducting-transition temperature (about 6 K) and to allow electron diffusion to be the dominant cooling mechanism. At the ends of the bridge, the strip of niobium widens into pads that make contact with relatively thick overlying gold pads, which define the length of the bridge and provide both heat sinking and electrical contact to external circuitry.
Like other microelectronic devices, a device of this type must be fabricated in a lithographic process. The need to ensure that the niobium contact pads extend under the full areas of the gold contact pads can give rise to difficulty in lithographic alignment. The present fabrication process (see Figure 2) was developed to solve this alignment problem.
The devices are fabricated, variously, on quartz or silicon wafers, depending upon the circuits in which they are to be used. The wafers are cleaned with organic solvents, then mounted in a vacuum processing chamber on an aluminum disk that keeps them near room temperature during processing. The chamber is evacuated, the substrates are cleaned by an argon-ion beam, then the substrates are coated with niobium 10 nm thick by magnetron sputtering. Then gold is deposited by thermal evaporation to a thickness of 15 nm to prevent oxidation of the niobium during subsequent processing steps.
In the subsequent steps, device features larger than 1 µm are formed by optical lithography, and the smaller ones by electron-beam lithography. Initially, the gold-coated wafers are optically patterned with alignment marks. Then in preparation for electron-beam lithography using a bilayer-stencil technique, the wafers are coated with two layers of poly(methylmethacrylate) of different molecular weights. The bilayer-stencil technique is used because it provides smoother edges for metal lift-off than does a single-layer technique.
The width of the bridge in each device is defined in the first electron-beam-beam-lithography step. After electron-beam exposure and development, gold is deposited to a thickness of 25 nm. After lift-off, what remains of this deposit are gold mask lines 50 to 200 nm wide and several microns long.
The width of the bridge is defined in the second electron-beam-lithography step. Two 110-nm-thick gold pads (destined to become the contact pads) separated by a gap 50 to 300 nm long are positioned over the ends of the gold mask line.
The resulting composite of gold pads and the mask line constitutes a self-aligned mask for use in reactive-ion etching to remove the surrounding, unmasked portion of the gold-coated niobium film to form the niobium microbridge while leaving the masked portion of the niobium film intact under the full areas of the gold pads. This etch is performed in a two-step procedure that leaves the niobium microbridge still coated with some gold.
Then by use of a combination of optical and/or electron-beam lithography depending on the specific design, gold antenna structures or other circuit elements are formed in contact with the gold contact pads, and the remaining gold coat on the microbridge is removed by reactive-ion etching. Finally, all devices thus fabricated are separated from each other by dicing the wafers.
Prototype devices for use in quasi-optical receivers for operating frequencies of 1.2 and 2.5 THz have been fabricated by this method. Tests of these devices have shown that they can be fabricated within reasonable tolerances for microbridge lengths and widths as small as 80 nm.
This work was done by Bruce Bumble and Henry LeDuc of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.comunder the Electronic Components and Circuits category, or circle no. 104 on the TSP Order card in this issue to receive a copy by mail ($5 charge).