IEEE Solid-States Circuits Magazine - Winter 2022 - 81

creative combination of MEMS, optics,
and electronics through customized
technology. A nonlinear
DAC corrects for MEMS nonlinearity
and process variations, leading
to good linearity and experimental
performance of fast axial focusing.
Optogenetics is a technique that
involves the use of light to excite
or inhibit neurons that have been
genetically modified to express
light-sensitive ion channels. Stimulating
neurons optically, rather
than electrically, holds tremendous
promise due to the cellular selectivity
and high spatial and temporal
resolutions of the technique.
Recently, the interference properties
of electromagnetic waves have
been exploited to sculpt light into
single-cell resolution patterns without
needing to physically penetrate
the tissue region of interest. Having
the ability to rapidly switch the
patterns of incident light through
spatial light modulation will enable
us to scale optogenetic systems to
target thousands of neurons without
making direct contact with the
neural tissue, entirely preventing
cortical scarring.
Patterning light with the required
speed, resolution, and scale remains
a significant bottleneck in the capabilities
of optical neural interfaces.
High-precision optical interfaces
generally employ point scans or holography
to access individual neurons.
Scan-based approaches steer
a cell-sized spot of light to access
a single neuron and commonly utilize
separate lateral and axial scanning
elements, as shown in Figure 4.
Holography-based methods require
more extensive and precise patterning
tools, namely, spatial light modulators
(SLMs), to generate arbitrary
distributions of light targeting multiple
neurons per frame.
These systems are restricted by
the settling times of liquid crystal
materials, the state-of-the-art technology
used to build SLMs, limiting
their throughput and temporal resolution.
Piston motion MEMS micromirrors
are a promising alternative
that can serve as lateral and axial
scanner elements in a scan-based
system and replace the liquid crystal
elements in a full-scale SLM, enabling
parallel access to thousands of neurons
at single-cell precision and with
biologically relevant (<100-µs) settling
times. Such a system needs to
accommodate the complicated drive
requirements of these MEMS structures,
correcting for the inherent
nonlinearity in the electrostatic drive
of parallel plate actuators as well as
process variations causing deviation
from designed behavior.
In this work [5], a system combining
an ASIC and a custom MEMS micromirror
array (Figure 4) performs
high-speed focus tuning aimed toward
optogenetic stimulation. A micromirror
array consisting of more
than 23,000 mirrors wired as 32
concentric rings was driven by an
ASIC capable of correcting for the
actuation nonlinearity and global
process variations of the array. A reconfigurable
nonlinear DAC capable
of generating a wide variety of nonlinearities
compensates for the postmanufacturing
actuation behavior
of the micromirrors in a power- and
area-efficient manner. The system
was demonstrated to perform focus
tuning across a wide continuous
sweeping range spanning 22
distinctly resolvable depth planes,
with refresh rates greater than 12 kHz
and random-access capability. This
technology can be scaled using the
presented ASIC to realize a 200 ×
200 SLM with a >10-kHz refresh rate,
enabling a miniaturized chip-scale
holographic device at speeds that
are an order of magnitude higher
than presently available. This work
Lens
Lateral (xy)
Scanner
Axial (z)
Scanner
Laser Source
(a)
MEMS Array
ASIC Driver
Nonlinear DAC
Digital
Control
200 × 200 Pixel Array
5 mm
LVDS Rx
(b)
FIGURE 4: (a) A simplified optical diagram of a scan-based light delivery system, with the
axial and lateral scanners labeled. (b) The two components that constitute the high-speed
dynamic axial focusing system: a MEMS micromirror array and a driver ASIC (modified from
[5] and [9]).
IEEE SOLID-STATE CIRCUITS MAGAZINE WINTER 2022
81
5.4 mm
Analog Multiplexers and Amplifiers
Output Buffers
IEEE Solid-States Circuits Magazine - Winter 2022

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