Working Group 5.11

Nanoforce Metrology for Tactile Sensors

• 3d-micro-probe
• Si-Cantilever sensors
• 3d-calibration device for the dynamical calibration of micro systems

3d-micro-probe

The increased industrial use of microsystems gives rise to novel requirements for quality assurance and for the associated instruments technology. For dimensional measurements on microstructures, only a few micro tracing systems have been available up to now. So, in cooperation with the Institute for Microtechnology of the Braunschweig Technical University and Carl Zeiss, a microprobe was developed which allows three-dimensional coordinate measurements on microstructures to be carried out.

Figure 1: Schematic diagram and photo of the 3D microprobe

The square ring diaphragm of the micro probe is manufactured by anisotropic etching of crystalline (100) silicon. In the centre of the diaphragm remains the boss (approx. 1 mm x 1 mm). The external dimensions of the chip are: 6.5 mm x 6.5 mm x 0.4 mm. The deflection of the probe pin occurring during the probing of measurement objects leads to a deformation of the boss diaphragm. Semiconductor resistors (the piezoresistive elements) integrated in the diaphragm are used to detect the stress resulting from the diaphragm deformation via resistance changes. Their functioning is similar to that of strain gauges, their sensitivity can, however, be increased by almost a factor of 100. Eight of the 24 resistors are each time interconnected to form Wheatstone bridges so that separate detection of the three probing directions x, y and z is possible. The piezoresistive elements are manufactured by diffusing phosphor (n-injection) into the wafer. The meander-shaped resistors (total length ≈ 4 mm, width ≈ 20 µm, depth ≈ 1.1 µm) show values of 1000 Ohm ± 20 Ohm. The resistors are interconnected on an intermediate layer on the back of the chip (see Figure 2).

Figure 2: Interconnection of eight piezoresistors each on the back of the chip to form a Wheatstone bridge

Bond connections establish the required connections for the bridge supply voltages Ub and the output signals U to the chip. Figure 3 shows the complete probe-change facility with 3D microprobe chip (however, without probe pin).

In addition to the measurement of coordinates, the probe allows forces in the range from only a few µN to some hundred mN to be measured. For this purpose, the probe is calibrated. Besides the deflection of the probing ball, both the output signals and the probing forces are simultaneously measured. A commercial compensating balance serves to measure the probing forces.

Figure 3: Probe-change facility, composed of carrier substrate with 3D microprobe chip (left) and support (right)

Table 1 summarizes the specifications of the 3D microprobe with integrated interconnection of the piezoresistors on the back of the diaphragm and with commercial probe pin and 300 µm ruby probing ball.

Table 1: Specifications of the 3D microprobe with commercial probing pin

* Detection bandwidth B = 2,3 mHz to 80 Hz