Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 3M

Design for test is an important responsibility of design engineers for MEMS and microsystems

Following major tasks are involved:

• Design for testing

  • Set the testing strategy, e.g., identifying testing points.
  • Establish range of acceptable Device Performance:
    • Proper PASS/FAIL limits for test results.
    • A proper balance between Quality (being too lenient)

• Performance tests:

  • Parametric testing
  • Testing during assembly.
  • Burn-in and final testing
  • Self testing
  • Testing during use.

Parametric Testing

• For inspecting key components during and after the fabrication.
• Requires the definition of parameters, e.g., film resistances, surface stress/strain for such testing.
• Requires proper selection of test points on the workpiece.
• Parametric test structures are attachged to the workpiece for the testing.

Example 1: The van der Pauw sheet resistance test structure.

Pass current to pad 2 & 3

Measure voltage across pad 1 & 4

The surface resistance in the area is:

$R_c = \frac{V_{1.4}}{I_{2.3}}$

Example 2: Parametric test structures for measuring tensile strain.

The compressive strain responsible for the bucking of the thin beam is:

$L_c = \sqrt{\frac{t^2 \pi^2}{3 \epsilon}}$ where $L_c$ = length of the beam, t= thickness

Example 3: Parametric test structure for measuring both tensile and compressive strains.

Beam electrodes are connected and anchored on the workpiece at shallow angles.

Associated tensile or compressive strains can be correlated to the measured capacitances from these beam electrodes.

Example 4: Parametric test structure using resonator for monitoring surface stresses.

• Change of stiffness of springs due to change of stresses in attached workpiece leads to change of resonant frequencies.
• Resonant frequency of the resonator can be generated by electrical stimulator.
• Shifting of resonant frequencies in the resonator can be related to the surface stresses in the workpiece.

Testing during Assembly:

For two purposes:

• To determine which device components are good enough for further packaging into devices.
• To monitor the yield of the packaging process.

Example 1: Texas instruments digital micromirror device with 0.5 to 1.5 million electrostatically actuated mirrors at 16µm * 16µm

• Mirrors offer 0 or 1 signals on the reflected intensities.
• The open center in the array shows the CMOS beneath that supplies voltage to rotate the mirrors for reflecting lights.
• Mirrors are tested for reflecting lights at increasing voltage supplies by the CMOS.
• Dies with mirror fails to perform are rejected.
• Further inspection on mirror functions after dies are assembled.