Technology Modules

Technologies consist of technology modules. MEMS devices are developed based on existing, modified and new technology modules.

Among others, Fraunhofer IPMS offers the following technology modules:

Sacrificial Layer Technology for Surface MEMS

Two-axis micromirrors
© Fraunhofer IPMS
Two-axis micromirrors.

Using sacrificial layer technology, freely suspended and / or movable structures can be generated on the wafer surface. Typical applications include spatial light modulators, bolometers, and capacitive micromechanical ultrasonic transducers (CMUTs).

The functional structures are fabricated atop a sacrificial layer, which is removed at the end of the manufacturing process using isotropic, vapor-phase etching (HF, XeF2).

Monolithic Integration of MEMS and CMOS

Cross section of a pixel consisting of CMOS backplane and mirror
© Fraunhofer IPMS
Cross section of a pixel consisting of CMOS backplane and mirror (16 µm pitch).

The monolithic integration of MEMS structures atop CMOS circuits produces wafer-level, integrated systems. Fraunhofer IPMS develops and manufactures monolithically integrated MEMS. Our MEMS structures are fabricated using the same CMOS process from which the control electronics are produced. These MEMS structures can be built upon CMOS wafers fabricated at Fraunhofer IPMS, as well as at other CMOS fabs.

The main applications of monolithically integrated MEMS include functional matrix components such as spatial light modulators, bolometers, capacitive micromechanical ultrasonic transducers (CMUTs), and infrared thermopile arrays.

Spring Elements for Actuators

Detail of a spatial light modulator
© Fraunhofer IPMS
Detail of a spatial light modulator.

Using moving spring elements, analog deflections up to 4° are enabled in spatial light modulators (SLMs). To achieve deflections which are stable and repeatable for many cycles, the springs are made of amorphous materials. Extremely thin spring elements (100 nm), which are fabricated using a multi-layer actuator technology module, allow large deflections at low drive voltages.

The photo shows the detail of a spatial light modulator. The mirror in the lower left half of the image has been removed, so that the spring and the electrodes of the actuator are visible.

Highly Reflective Mirror Layers

White light interferometer image of deflected micromirrors
© Fraunhofer IPMS
White light interferometer image of deflected micromirrors.

Fraunhofer IPMS develops technological solutions for highly reflective mirrors, for wavelengths from deep UV, to the visible, and to the near-infrared, for spatial light modulators and MEMS scanners. Particular applications in the deep UV range require a planarity in the range of a few nanometers. Using multilayer technology, mirror surfaces can be engineered to achieve particular performance specifications, such as resonant frequency, and radiation stability.

Electrical Isolation in Silicon Layers

Trench isolation
© Fraunhofer IPMS
Trench isolation.

Using isolation trenches, electrically-isolated silicon regions can be fabricated. Using this technique, different electrical potentials can be applied in a single silicon layer. For example, isolation trenches are used to define drive lines in MEMS scanners, and are suitable for voltages up to 250 V.

Isolation trenches are etched into the silicon layer using deep reactive ion etching, with a positive sidewall profile, and finally filled with an appropriate dielectric material. For a silicon layer thickness of 75 µm, the smallest trench width is 2.5 µm.

Comb Structures in Silicon

Comb electrodes
© Fraunhofer IPMS
Comb electrodes.

Electrostatic comb drives, which are used for example in MEMS scanners, consist of parallel silicon fingers with different potentials. For effective comb drives, the finger spacing must be as small as possible, and virtually constant. These structures are generated using deep reactive ion etching (DRIE), with a vertical sidewall profile (90° ± 0.3°).

For a silicon layer thickness of 75 microns, the smallest trench width is 3 µm.

Silicon Membranes with Vertical Sidewalls

Array of high aspect ratio holes in Si
© Fraunhofer IPMS
Array of high aspect ratio holes in Si (50 µm diameter, 436 µm depth).

Using deep reactive ion etching (DRIE), high aspect ratio holes can be etched in silicon with near-vertical sidewalls. With an etch depth of 400 µm, hole diameters of greater than 50 µm are feasible. This technology module is used, for example, in the production of silicon membranes for infrared thermopiles, and for flow sensors.

Anisotropic Wet Chemical Etching of Silicon

50 µm deep silicon membrane for pressure sensor
© Fraunhofer IPMS
50 µm deep silicon membrane for pressure sensor (side view).

Using wet chemical etching with KOH or TMAH, cavities are etched into silicon, where the sidewall angle is 54.75°. Etch depth can be controlled with very high precision (for example, ± 3 µm at an etch depth of 500 µm). This method is used for example in the fabrication of membranes for silicon pressure sensors, and in the process technology for MEMS single-mirror scanners.

Definition of Structures in Deep Silicon Cavities

Metallization in silicon cavities
© Fraunhofer IPMS
Metallization in silicon cavities.

Through the fabrication of structures at depths up to 500 µm below the top wafer surface, functional elements can be realized in silicon cavities.

Structure widths depend on the cavity depth. The photo shows an example with a 100 µm wide metal structure in a 300 µm deep silicon cavity.

Application examples are interconnects and contacts in silicon cavities, and TSVs (Through Silicon Vias).

Precision Silicon Components

MEMS chip for a micro-spectrometer
© Fraunhofer IPMS
MEMS chip for a micro-spectrometer.

MEMS technologies offer the possibility to combine structures with dimensions less than 1 µm, with three-dimensional structures in the millimeter range. Using this technique, precision optical components such as apertures and shutters, and precision watch components, can be made out of silicon. As an example, the photograph shows the back of a MEMS chip, which consists of a 1D MEMS scanner, and additional structures which serve as apertures and an adjusting device, for coupling light to an optical fiber and a photosensor. This MEMS chip is used in a micro-spectrometer, developed at Fraunhofer IPMS.