IEEE Consumer Electronics Magazine - September 2018 - 12

Resonant
Micromirror

FIGURE 4. A one-dimensional electrostatic micromirror (STM,
from [14]).

-
Voltage

(a)

Magnetic
Field
Current

(b)

Piezo
Actuators

(c)

FIGURE 5. The structure of micromirrors with three different actuation schemes: (a) electrostatic [14], (b) magnetic [17], and (c)
piezoelectric [18].
12 IEEE Consumer Electronics Magazine

september 2018

MOEMS scanning micromirrors can replace motorized
mechanical lidars at a fraction of the cost. Several such solutions are under development, but none has hit the market yet.
Among design specifications for lidars, a large angular FOV
is desirable to reduce the number of lidars to explore the full
360° around the vehicle. Systems with a single 360° rotating
mirror can be used to achieve an all-around view, but they are
bulky and require an unappealing roof mount to avoid blind
angles. The advantage of mechanical scanners versus cheaper
solid-state scanners is the wider scanning angle. In singleaxis MEMS micromirrors, angular deflection is due to the
oscillating motion of the deformable mirror suspensions,
which act as torsional springs (Figure 4). Mirrors with a true
360° scanning range have been developed [15], where twodimensional (2-D) actuation kinematics gives a full rotation
around the axis of the incident laser beam. Two-axes (2-D)
MEMS micromirrors are also actively developed, but their
main envisioned application is currently for image projectors
[16]. These 2-D mirrors include a gimbal to decouple the two
rotation axes; therefore, their structure is complex and bulky,
limiting the maximum operating frequency. More compact,
gimballess solutions are in development, but mechanical
interference between axes is a problem.
In automotive lidars, the required vertical FOV is typically small, and, thus, it can be obtained by a linear vertical
array of one-dimensional micromirrors instead of a single
but more complex 2-D mirror. To maximize the oscillation
angle, mirrors can be operated at or near their mechanical
resonance frequency. In this regime, the deflection is proportional to the mechanical quality factor, which can be boosted
by operation in a vacuum. Vacuum packaging, however, has
the problems of cost and reliability. Size is a crucial parameter for MEMS mirrors, affecting the collected laser power
and, thus, the maximum range (ADASs require a range higher than 100 m). Large diameters come with a series of technology drawbacks: the mirror actuation is more challenging,
and it is harder to achieve a good optical flatness, which is
required to avoid deformation of the beam. Published scanning micromirror diameters range from 2 to 7 mm [15]. At
this size, resonance frequencies scale from tens of kilohertz
to 1 kHz or less. For successful long-range MEMS lidars, an
increase of the mirror size is one of the key issues. The type
of MEMS actuation also has an impact on the mirror structure and performance. Electrostatic actuation is a common
choice in MEMS. In this approach, deflection is obtained by
applying a voltage between two plates, one of which is
attached to the mirror.
Electrostatic actuation does not scale well with size. To
increase the torque, the plates are made of dozens of tiny
interdigitated structures or comb fingers [Figure 5(a)] [14].
Despite this, actuation voltages of tens or even hundreds of
volts are not uncommon [16]. Magnetic actuation, where a
torque is generated by an external magnetic field over a current flowing into coils placed on the mirror surface [Figure 5(b)] [17], is characterized by low voltages but high
currents (tens to hundreds of milliamps). The coil increases

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