Awschalom Group


The fabrication of very delicate mechanical cantilevers combined with sensitive displacement detection schemes has resulted in a number of remarkably powerful experimental techniques, including scanning force microscopies, mechanically detected magnetic resonance, and a new class of torque magnetometers. In general, the force sensitivity of these techniques can be improved by lowering the spring constant k of the cantilever (thereby increasing the displacement per unit force) and increasing the resonant frequency ν0 (decreasing the necessary averaging time). Since most semiconductors and metals have mass densities (ρ) and elastic moduli (E) within an order of magnitude of each other, the design parameters that afford the greatest opportunities for improvements are the physical dimensions of the cantilever. Specifically, since for a rectangular cantilever where t, l, w are the thickness, length, and width of the cantilever, one can achieve small k and large ν0 by simultaneously decreasing all the dimensions. High mechanical Q is also important for resonance detection. Though Q is difficult to calculate a priori, in the case of mechanical cantilevers, it is usually limited by air damping.

Typically, micron-scale cantilevers are fabricated from silicon, silicon oxide, or silicon nitride. Fabricating cantilevers from the GaAs/AlxGa(1-x) as materials presents challenges in designing new processes for the III-V chemistry. More importantly, it offers the advantages of integration with optical devices, magnetic systems, and strain sensing elements that utilize the piezoelectric properties of the GaAs to detect the cantilever displacement. Cantilevers fabricated from the III-V semiconductors have usually contained Al-rich layers (included as part of a laser structure), which simplify the fabrication of free mechanical structures by allowing the selective etching of the GaAs substrate out from under the Al-rich layers.

We have developed a process for making cantilevers from a single epilayer of GaAs grown by molecular beam epitaxy (MBE) on an AlAs etch stop layer on a [100] GaAs single crystal substrate. We have fabricated cantilevers 100 nm thick, within an order of magnitude of the thinnest cantilevers fabricated from silicon and much thinner than cantilevers previously fabricated from GaAs/AlGaAs. This process allows easy access to both sides of the cantilever by etching a window through the entire thickness of the GaAs substrate, unlike previous GaAs/AlGaAs cantilever processes.

To construct very thin cantilevers made of a single material (as opposed to GaAs/AlGaAs layers), we use MBE to grow a 100 nm thick GaAs epilayer on a 300 nm thick AlAs epilayer on a [100] GaAs substrate (Fig. 1a).

Figure 1. Outline of processing steps: a) MBE structure: 100 nm GaAs layer on 300 nm AlAs layer on a GaAs substrate. b) Cantilever shape is defined in GaAs epilayer by Cl2 RIE etch. c) Patterning of windows on the back of the chip, directly beneath the cantilever shapes. The heavy dashed lines indicate the profile of the spray etch, while the light dashed lines show the hidden surfaces of the chip. d) Wet etch through substrate to AlAs etch stop. e) Removal of AlAs etch stop with HF acid.

The GaAs epilayer will ultimately form the cantilever, and so its thickness determines that of the cantilever; the AlAs serves as an etch stop and a sacrificial layer. The lateral shape of the cantilever is defined by optical lithography of the photoresist spun on the epilayers. In the present case, the pattern is of the form of a window with a cantilever extending into the window. This pattern provides protection to the cantilever against damage in later fabrication steps as well as in actual use.

The pattern is etched into the GaAs epilayer by a Cl2 reactive ion etch (Fig. 1b). After removal of the photoresist mask, a fresh layer of photoresist is spun and baked on the now-patterned epilayers to afford them protection during the subsequent steps. Next, photoresist is spun and baked on the back of the chip. Using an IR mask aligner, windows are patterned in the back mask directly beneath the cantilever patterns in the epilayers. The windows in the back mask define the region of the substrate that will be removed by a spray etch in order to free the cantilever, and so this alignment is crucial (Fig. 1c). The GaAs substrate is then etched in a H2O-H2O2-NH4OH spray etcher. This etch stops at the AlAs epilayer. To remove the AlAs epilayer, a few drops of HF are placed on the exposed AlAs epilayer (Fig. 1e). At this stage, the cantilever is free from the semiconductor material; it is only bound by the wax to the spray etcher mount. The wax can be dissolved with acetone, and the chip removed. However, removing the cantilever intact from the acetone bath is made difficult by the surface tension of the acetone, which tends to break the cantilevers. This problem can be circumvented by the use of a high pressure CO2 critical point dryer.

Figure 2. (a) SEM photograph of a chip with 15 100-nm thick GaAs cantilevers, approximately 150 µm long and 50 µm wide. (b) A close-up of one of the 100-nm thick GaAs cantilevers.

Fig. 2 shows cantilevers fabricated by this process. The lateral dimensions are defined by the first photoresist mask, and the vertical dimension by the MBE grown GaAs epilayer (100 nm in this case). Here, the cantilevers are 150 µm long and 50 µm wide.

To measure their mechanical properties, the cantilevers were mounted in a laser interferometer, and driven with a piezoelectric crystal. For a 135 mm long, 30 mm wide and 100 nm thick cantilever at room temperature and pressure, we found a resonant frequency of 4.5 kHz and a Q of ~ 2. These values agree with our measurements of the spectral density of the thermal noise of the cantilever. This technique of measuring the thermal noise of the cantilever has the advantage over the driving approach in that the spectral weight of the thermal noise tends to select the "soft" (small k) modes and reject the large k modes, including the modes of the mounting itself, whereas the response of a mode to the piezo driving force is insensitive to the mode's k.

Because of the cantilevers' large aspect ratio and low mass, air damping strongly affects the Q (though other damping mechanisms may be present as well). The thermal noise of the same cantilever at a pressure of 250 mTorr shows ν0 has increased to 5.1 kHz and Q to ~ 9. From this value of the resonance frequency and Eq's (1), we find k ~ 10-4 N/m. This spring constant is lower than for any other GaAs cantilever of which we are aware. Given a DC displacement sensitivity of 0.1 nm per root Hz, this results in a DC force sensitivity of ~ 10-14 N per root Hz. Moreover, the Q of these cantilevers increases by orders of magnitude within the ultra high vacuum obtained at low temperatures.