MEMS Technology and Capacitance-Based Wafer Measurement
Micro-electrical-mechanical systems (MEMS) are tiny devices that house electrical and mechanical components on a single silicon chip or integrated circuit (IC). They integrate mechanical structures with electronics that are normally fabricated with complementary metal-oxide-semiconductor (CMOS) technologies. In addition to miniaturization, the benefits of MEMS include expanded functionalities at lower cost and with improved performance and reliability.
With MEMS devices, critical physical dimensions range from smaller than one micron (µ) to larger than several millimeters (mm). In complex systems, multiple moving elements may be controlled by integrated electronics. Examples of the mechanical components in MEMS include tiny gears, pumps, valves, and turbines. The electrical components in MEMS technology have various uses and can support the creation of microsensors and microactuators that convert energy from one form to another.
For example, a MEMS microsensor can convert a mechanical signal such as pressure into an electrical signal that can be continuously monitored. In addition to pressure, MEMS technology is commonly used for sensing temperature, chemical species, and radiation. MEMS can also create micro-scale systems such as tiny motors, microphones, and speakers. Today, the smartphone is the most popular consumer device that is using MEMS technology. Other common applications include automobile airbags.
MEMS Fabrication and Wafer Flatness
MEMS devices are fabricated using the same IC batch processes as silicon semiconductors. With lithography, the result is a substantially two-dimensional or planar structure. To create the mechanical components in a MEMS device, different machining techniques can be used. For example, surface micromachining alternately deposits, patterns, and etches thin films onto the surface of the substrate to create thin mechanical structures. Bulk micromachining directly etches onto the substrate instead.
Regardless of the fabrication method, MEMS devices are substantially flat. Moreover, the flatness of the silicon wafer is directly related to fabrication quality and product yields. Flatness is also essential to ensure the success of wafer-bonding-based stacking in non-monolithic CMOS MEMS. Capacitance, the ratio of the change in an electric field to the corresponding change in its electrical potential (i.e., voltage), provides a highly precise and cost-effective way to make flatness measurements at the microscale.
Capacitance-Based Wafer Measurement
Capacitive sensing can measure the flatness bow, warp, and total thickness variation (TTV) of semiconductor materials. This form of non-contact measurement locates the wafer between two capacitive probes. The distance between the probes is known, and the gap between the wafer and the probes is obtained from the capacitance at each measurement point. Flatness measurements are fast, and complete mapping of the wafer shape can be accomplished quickly.
MTI Instruments of Albany, New York, provides several capacitance-based metrology systems for non-contact wafer measurements in MEMS.
The Proforma 300i is a manual metrology system that measures thickness, 5-point TTV, and bow.
The Proforma 300iSA is a semi-automated metrology system that performs full-surface scanning for thickness, TTV, bow, and warp.