Built-In Self-Test for MEMS Tunable Capacitor

Xiaochun Yu

Project

Defects and local variations that occur during the fabrication of MEMS devices will lead to some of them working improperly or unreliably. It is therefore important to identify these compromised parts during the test stage. This project aims at detecting bad parts due to defects and/or unacceptable local process variations in MEMS tunable capacitors. (See Figure 1.) Previous work here at Carnegie Mellon University pursued a similar goal for MEMS accelerometers by electrically testing its symmetry [2]. Similarly, we examine the symmetry of the tunable capacitors for detecting defects and local process variations in this MEMS structure since:

• It is very improbable for defects or local variations to occur symmetrically so it is likely that a defect or local process variation will be identifiable by inspecting symmetry.

• It is extremely difficult to examine the micro-mechanical structures directly. However, the mechanical structure in a MEMS device manifests in the electrical signals it produces.

• The MEMS tunable capacitors themselves are symmetric in some sense and their multiplicity within an RF system makes this approach even more viable.

Selected Highlights

A voltage signal proportional to the relative symmetric difference of the structures' capacitances is produced by the following two-step process:

Step 1: For each of the two MEMS tunable capacitors in an RF system, we obtain a voltage signal proportional to the relative symmetric difference between the right- and left-side capacitances. This is accomplished by inserting an NMOS switch symmetrically between the left- and right-hand sides of the comb fingers shown in Figure 1.

Step 2: We generate a single-end voltage output that is proportional to the difference of the signals produced in Step 1.

Both steps above are accomplished by creating a circuit that uses principles similar to switched-capacitor circuits. Figure 2 shows the change in output voltage as a function of the difference between the left- and right-side capacitances when both capacitors are tuned to their minimum value. One side is assumed to be free of defects and local variations while the other has its capacitance varied between zero and 450fF. (Note the nominal value of one side is 200fF.) It is evident from Figure 2 that the circuit can detect a wide range of capacitance change and that the sensitivity and linearity is quite good when capacitance is within 20% of its 200fF nominal value. A similar plot results when the capacitors are tuned to their maximum value. From the magnitude of the output signal, we can determine to what extent the MEMS structure is symmetric and thus, we are able to electrically determine whether defects or local process variations occur within the MEMS tunable capacitors.