Sidewall-implanted dual-axis piezoresistive AFM cantilevers

for simultaneous detection of vertical and lateral forces

Benjamin Chui, Thomas Kenny

Stanford University, Terman 551, CA 94305-4021

H. John Mamin, Bruce Terris, Daniel Rugar

IBM Almaden Research Center, 650 Harry Road, CA 95120-6099

In certain areas of atomic force microscopy (AFM), there exists a need to simultaneously measure vertical and lateral forces. For example, in microtribology, it is sometimes necessary to measure the lateral frictional force acting on an AFM probe tip. Or, in AFM data storage (in which information is stored as submicron data pits on a spinning polycarbonate disk), lateral force measurements can be used for tracking. In this approach, the lateral force exerted by the data pit sidewall on the probe tip provides an indication of the relative misalignment between the pit and the tip. The measured lateral force can be used as an input into a negative-feedback servo controller to keep the probe on track.

We have designed and fabricated a novel silicon micromachined atomic force microscope (AFM) cantilever with two independent sets of piezoresistive sensors for simultaneous lateral and vertical force detection. The vertical force sensor is in the form of a flat, triangular cantilever with a piezoresistive boron-doped layer (fig. 1 (left)). Vertical deflection of the cantilever tip causes a change in resistance that can be translated to a z-axis deflection signal. The vertical force probe is physically connected to the base by several high-aspect ratio ribs which give the structure lateral compliance. Two of the ribs are made piezoresistive, forming a lateral force sensor that produces an x-axis deflection signal (fig. 1 (right)). During normal probe operation, forces at the cantilever tip will excite both the lateral and the vertical deflection modes, giving rise to synchronized x- and z-axis output signals.

Fig. 1: Dual-axis AFM cantilever: (left) vertical deflection mode, (right) lateral deflection mode.

Figure 2 (left) shows the electrical path along the vertical force probe. The two outer ribs are heavily boron-doped to provide high conductivity, allowing current to flow from the base to the piezoresistive triangular probe and back. Figure 2 (right) shows the electrical path along the lateral force sensor. The two middle piezoresistive ribs are electrically connected by a segment of a flat horizontal beam that doubles as part of the triangular probe.

Fig. 2: Electrical paths on dual-axis AFM cantilever: (left) vertical deflection sensor, (right) lateral deflection sensor.

The electrical paths described above require current to flow in a vertical plane in some parts of the cantilever and a horizontal plane in others. A novel ion implant technique was developed to make this possible, i.e. to produce electrically conducting vertical sidewalls as well as horizontal surfaces. In this technique, dopant ions are implanted at approximately 45 degrees to the vertical (fig. 3). This allows vertical and horizontal surfaces to be implanted at the same time and ensures electrical continuity at the interface. This oblique ion implant technique is a powerful fabrication method applicable to high-aspect-ratio MEMS structures in general.

Fig. 3: Oblique ion implant technique for high-aspect-ratio MEMS structures.

Figure 4 (left) shows a finished cantilever, made of single- crystal silicon. The triangular probe is 1.3 microns thick; the ribs are 1.3 microns thick and 10 microns tall. Preliminary testing indicates x- and z-axis piezoresistive sensitivities delta_R/R on the order of 0.1 to 0.25 ppm per Angstrom.

Fig. 4: Single-crystal silicon dual-axis AFM cantilever, with a close-up of the sharp tip.

Figure 5 shows a pair of AFM images, taken during the same scan in air, on a silicon sample with a grid of ridges 1600 Angstroms tall, 2 microns in width and 10 microns in pitch. The tip is scanned from left to right. The vertical force image (left) shows the topography of the sample (obtained by measuring the vertical deflection of the probe tip), while the lateral force image (right) is obtained by measuring the lateral deflection of the probe tip. The bright vertical bands along the left edges of the y-oriented ridges correspond to increased lateral deflection, implying that the probe tip is temporarily caught at the positive step edge of the ridge before climbing it.

Fig. 5: AFM images taken with dual-axis piezoresistive cantilever on a silicon sample with a grid of ridges: (left) vertical force image, (right) lateral force image.

Simultaneous AFM data storage readback and tracking has been demonstrated with the dual-axis cantilever. In this experiment, the cantilever is mounted on a modified compact-disk actuator. The position of the actuator, which can move in the x and z directions, is controlled by a two-channel servo loop. The piezoresistive x- and z- deflections measured by the vertical and lateral deflection sensors on the dual-axis cantilever serve as input to the servo loop. The probe tip is able to follow an intermittent groove on a spinning polycarbonate disk that emulates a data track.

The dual-axis piezoresistive cantilever fabrication technique described here (especially the oblique ion implant technique) is not limited only to AFM probe applications, but can be applied to a wide variety of piezoresistive sensors such as inertial, magnetic and fluidic sensors.

For more information please refer to the following references:

Thermomechanical data storage:

H.J. Mamin and D. Rugar, “Thermomechanical writing with an atomic force microscope tip,” Appl. Phys. Lett. 61, pp.1003-5, 1992.

H.J. Mamin, L.S. Fan, S. Hoen, D. Rugar, “Tip-based data storage using micromechanical cantilevers,” Sensors and Actuators A 48, pp. 215-9, 1994.

B.W. Chui, T.D. Stowe, T.W. Kenny, H.J. Mamin, B.D. Terris, and D. Rugar, “Low-stiffness silicon cantilevers for thermal writing and piezoresistive readback with the atomic force microscope,” Appl. Phys. Lett. 69, pp. 2767-9 (1996).

B.D. Terris, S.A. Rishton, H.J. Mamin, R.P. Ried, and D. Rugar, “Atomic force microscope based data storage: track servo and wear study,” Appl. Phys. A, to be published.

Dual-axis piezoresistive cantilevers:

B.W. Chui, H.J. Mamin, B.D. Terris, D. Rugar and T.W. Kenny, “A novel dual-axial AFM cantilever with independent piezoresistive sensors for simultaneous detection of lateral and vertical forces,” in Proceedings ofASME Int’l Mech. Engr. Congress and Exposition, 1997, DSC vol. 62 (Microelectromechanical Systems), pp.55-59.

B.W. Chui, H.J. Mamin, B.D. Terris, D. Rugar, T.W. Kenny, “Sidewall-implanted dual-axis piezoresistive cantilever for AFM data storage readback and tracking,” Proceedings of IEEE International Workshop on Microelectromechanical Systems (MEMS 98), Jan. 1998, Heidelberg, Germany.

B.W. Chui, H.J. Mamin, B.D. Terris, D. Rugar, T.W. Kenny, "Independent detection of vertical and lateral forces with a sidewall-implanted dual-axis piezoresistive cantilever," accepted for publication in Applied Physics Letters, Feb. 1998.

(Updated 1998)