US 11,885,979 B2

3

array of columns of target material of a sub-micron height
and different aspect ratios (with each aspect ratio being
smaller than 11 in one case, smaller than 9 in a related
implementation, smaller than 7 in yet another case, smaller
than 5 in a related embodiment, and between about 3 and
about 11 for various columns in the array in yet another
embodiment). The aspect ratio of a respective column is
defined as a ratio of a height to a spatial extent of a column
in plane transverse (usually perpendicular) to the direction
along which the height is defined. Generally, the cross-
section of a column in such transverse plane is defined by a
polygon and/or a closed curve differentiable at any point of
the curve. In any of the above-identified embodiments, the
array of columns is generally structured as a periodic array
al period defined—depending on the case and/or
on the location of the group of columns within a boundary
of such array—within a range from about 50 nm to about
500 nm. Furthermore, in substantially any implementation,
the method may be additionally enhanced by at least one of
the following steps: a) delivering a first vapor of a first
precursor material into the ALD chamber during a time-
window of a first duration to form a solid monolayer of
Hf[(CH,) ,N];,O on a surface of a chosen substrate while
generating a fluid by-product (CH,),NH(dimethylamine)
substantially simultaneously with formation of such solid
monolayer; b) removing a portion of the vapor of the first
precursor material that has not reacted with the surface of the
target material, as well as the fluid by-product from the ALD
chamber by purging the ALD chamber with nitrogen; c)
delivering a second vapor of liquid (in one specific case,
such liquid may be water) into the ALD chamber during a
time window of a second duration for create a monolayer of
the target material on the surface; d) removing excess of the
second vapor by purging the ALD chamber with nitrogen:
and e) repeating steps a) through d) multiple times to
accumulate a layer of the target material with pre-deter-
mined thickness. Such enhanced method may be addition-
ally structured to satisfy at least one of the following
conditions: i) the step of delivering the first vapor of the first
precursor material may include delivering the vapor of
Hf{(CH,),N], (TDMAH); ii) the target material may be
the fluid by- -product may be a gaseous substance;
and iv) the step of removing the portion of the vapor of the
first precursor material may include removing the first
precursor material from the ALD chamber until the remain-
ing amount of such vapor is below detectable limits achiev-
able with the use of electronic circuitry configured to
recognize the present of the first precursor material in
operation of the ALD chamber.

Embodiments of the present invention additionally pro- 5

vide a method for fabricating an all-dielectric metasurfac
optical device that includes at least one of a polarization-
independent metalens, a polarization-independent metaholo-
gram, and a polarization-independent Airy beam generator.

On any case, such device is formatted to have optical 5

‘transmittance of at least 50% at a wavelength of 364 nm or
shorter and, in a specific case—or at least 70%). The method
utilizes hafnium oxide to conformally fill an open volume,
formed in a patterned layer of an electron-beam resist
material, with the use of an atomic layer deposition process
conducted at a temperature not exceeding 105° C.

BRIEF DESCRIPTION OF THE DRAWINGS

‘The invention will be more fully understood by referring
to the following Detailed Description of Specific Embodi-
ments in conjunction with the Drawings, of which:

20

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&

5s lars of target material of varying diameters. Viewing angle:

4

FIGS. 1A, 1B, 1C, 1D, 1B, and provide illustrations to
implementations of the idea of the invention. FIG. 1A:
Schematic representation of a metasurface unit cell, showing
a high-aspect-ratio pillar of target material having height H,
an elliptical cross-section (with principle axes’ lengths D,
and D,), and a rotation angle 0. The pillar is formed on a
SiO, substrate, and multiple pillars are arranged in a square
lattice or array with a sub-wavelength lattice spacing P. As
discussed, specific optical functions are achieved by varying
D,, D2, and as a function of nanopillar position(s) within
the lattice. FIG. 1B provides a schematic representation of a
specific embodiment of a low-temperature ALD cycle, con-
figured according to the idea of the invention, which utilizes
a TDMAH precursor, a H,O reactant, and a process tem-
perature of T,=95° C. FIG. 1C illustrates a refractive index
n and an extinction coefficient k of as-deposited film of the
target material, measured with spectroscopic ellipsometry.
Values of n at the three pre-determined operational wave-
lengths are denoted by star-marks. Dashed line indicates
position of the bandgap E, of the target material. FIG. 1D
provides a scanning electron micrograph (SEM) of details of
a fabricated polarization-independent metalens designed for
operation at %,-325 nm. The structure of the metalens
exhibits a lattice of $00-nm tall, circularly-shaped nanopil-

52°. FIG. 1E: SEM of details of a fabricated spin-multi.
plexed metahologram designed for operation at 4=266 nm,
showing a lattice of 480-nm tall, elliptically-shaped nano-
pillars of target material of varying in-plane cross-sections
and rotation angles. Viewing angle: 52°. The nanopillars are
coated with a layer of Au/Pd alloy (~5 nm thick) to suppress
charging during imaging. FIG. 1F provides optical micro-
graphs of full a metalens (in a top panel) and a spin-
multiplexed metahologram (in a bottom panel), correspond-
ing respectively, to metasurfaces described in FIGS. 1D and
IE. Scale bars: 100 um.

FIGS. 2A, 2B, and 2C illustrate implementations of
polarization-independent near-UV metalenses. FIG. 2A is a
schematic representation of focusing by metalens (of two
designs, L,, or L595) of normally-incident plane-wave at
o=364 nm and 325 nm, respectively. FIGS. 2B and 2C
demonstrate cross-focus cuts and intensity distributions in
the focal plane, as measured for metalenses [3,4 and L525.
respectively. Theoretically predicted cross-focus cuts are
plotted for reference. Scaie bars: 1 jm.

FIGS. 3A, 3B, and 3C illustrate embodiments of polar-
ization-independent -UV self-accelerating beam genera-
tors. FIG. 3A provides a schematic representation of the
generation of a self-accelerating beam by metasurfaces
(B3g, and B35) from normally-incident plane-waves at
2o=364 nm and 325 nm, respectively. FIG. 3B plots mea-
sured transverse defection in different z planes (ranging
from 2.5 mm to 5.5 mm, with an increment of 0.5 mm) for
illumination waves at respective operational wavelengths of
49-364 nm and 325 nm. Error bars denote one standard
deviation of the measured data. The targeted beam trajec-
tory, d=9 72, is shown for reference. FIG. 3C illustrat
array of measured and computed xy-plane intensity dis
butions (normalized) at different z planes for both of Bye,
and B35 devices at their designated wavelengths of opera-
tion. Each distribution is displayed over an equal square area
of side length 120 um, but shifted along the —xy direction
as a function of increasing z, such that the center of the main
lobe maintains an invariant position within each image.

FIGS. 4A, 4B illustrate embodiments of polarization-
independent near- and deep-UV metaholograms structured
according to designs Hye4s Has, and Hyg. FIG. 4A is a



U.S. Patent Jan. 30, 2024 Sheet 12 of 16 US 11,885,979 B2

tym

FIG. 12 FIG. 14

FIG. 13A FIG. 13B



US 11,885,979 B2

11
operation at which a given metasurface is fabricated (in the
examples of FIGS. 1D, and 1E: 400 nmsHs550 um).

Accordingly, embodiments of the invention provide a
method for fabricating a structured electromagnetic meta-
surface containing a sub-wavelength-scaled surface pattern.
Such method includes utilizing an ALD chamber to confor-
mally fill an open volume formed in a patterned layer of a
resist material with a target material with the use of an ALD
process conducted at a temperature not exceeding 105° C. to
define a material preform. (The used target material devoid
of any of silicon, silicon-containing compounds, titania,
gallium nitride, and niobium pentoxide). The method further
includes a step of removing the resist material with a solvent
to expose the sub-wavelength-scaled pattern structure of the
target material. Here, the resulting pattern structure is
dimensioned to operate as at least one of refractive, di
tive, birefringent, and resonant optical elements at a wave-
length defined in a deep-UV range of an electromagnetic
spectrum. In a specific case, the process of conformally
filling includes depositing hafnium oxide with a Tetrakis
(dimethylamino)Hafnium (TDMAH) precursor and/or over-
coming boundaries of the open volume at a top surface of the
patterned layer of the resist material to form a protrusion of
the target material above the top surface of the patterned
layer. In the latter case, the process additionally includes a
step of back-etching such protrusion with ion milling to
yield a substantially planar top surface of the material
preform.

Tn one implementation, the method includes at least one
of the following steps: (a) delivering a first vapor of a first
precursor material into the ALD chamber in a during a
time-window ofa first duration to form a solid monolayer of
Hf](CH,).N],0 on a surface of a chosen substrate while
generating a fluid by-product (CH,);NH(dimethylamine)
simultaneously with formation of said solid monolayer; (b)
removing a portion of the vapor of the first precursor
material that has not reacted with the surface of the target
material and the fluid by-product from the ALD chamber as
a result of purging the ALD chamber with nitrogen gas; (c)
delivering a second vapor of liquid into the ALD chamber
during a time window of a second duration for create a
monolayer of the target material on the surface; (d) remov-
ing excess of the second vapor by purging the ALD chamber
with nitrogen; and (e) repeating steps a) through d) multiple
times to accumulate a layer of the target material having a
pre-determined thickness on the surface.

Ic

Non-Limiting Examples and Implementations

Initially, as discussed below, we demonstrated the fabri- 5

cation of lenses, self-accelerating beam generators, and
holograms (based on polarization-independent metasurfaces
with nanopillars of in-plane circular cross-sections) that
operate at near-UV wavelengths of 364 nm and 325 nm

(which wavelengths correspond to emission lines of an s

argon-ion and a helium-cadmium laser, respectively) with
efliciencies (measured in terms of optical transmittance) up
to 72%. Further exploiting the high patterning fidelity of the
newly-devised processing technique and leveraging the neg-
ligible optical loss of the as-deposited Hf0, dielectric mate-
rial across most of the ultraviolet regime, critical dimensions
of metasurfaces were scaled to realize polarization-indepen-
dent holograms operating at a deep-UV wavelength of 266
nm (which wavelength corresponds to the emission line of
an optical parametric oscillator pumped by a nanosecond
Q-switched Nd:YAG laser), moreover with relatively high
efliciencies (or optical transmittance values, greater than

&

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12

50%, and even greater than 60% in different cases). Finally,
by opening up the design space with the three degrees of
freedom (parameters 0, D,, and D,, see FIG. 1A) provided
by elliptically-shaped nanopillars compared to the only,
single degree of freedom (parameter D, see FIG. 1A)
allowed by circularly-shaped nanopillars, we realize spin-
multiplexed metasurfaces that impart independent phase
shift profiles to light emerging from the device, under
illumination with left-handed circularly-polarized (LCP) or
right-handed circularly-polarized (RCP) light, respectively,
The implemented self-accelerating beam generators and
spin-multiplexed metaholograms operate at UV wave-
lengths of 364 nm and 266 nm, respectively, with eflicien-
cies up to 61%. For the purposes of this disclosure and
appended claims, and unless expressly defined otherwise,
the self-accelerating beam is defined as a propagation-
invariant optical wave a main intensity lobe of which
propagates along a predefined trajectory while the wave
itself is resilient to perturbations applied thereto and does
not diffract upon propagation. (One of the best known in the
art self-accelerating beams is the Airy beam that propagates
along a parabolic trajectory while preserving its amplitude
structure substantially unchanged during such propagation.)

Examples of Polarization-Independent Ultraviolet
Metasurfaces

Each of the examples of implemented polarization-inde-
pendent metasurfaces (those configured as a lens, a self-
accelerating beam generator, and a hologram) was structured
as a square lattice or array of Hf, cylindrical nanopillars.
The diameters of these pillars vary as a function of the pillar
position within the lattice/array. Each nanopillar is config-
ured to act as a truncated dielectric waveguide with top and
bottom interfaces of low reflectivity, through which light
propagates with transmittance and phase shift controlled by
the pillar height H, pillar diameter D, and lattice spacing
(that is, spatial separation between and among the constitu-
ent pillars) P. For each targeted operational wavelength (the
non-limiting examples: 2-364 nm, 325 nm, and 266 nm),
a corresponding pillar height (H=550 nm, 500 nm, and 400
nm, respectively) and sub-wavelength lattice spacing
(P=200 nm, 190 nm, and 150 nm, respectively) were chosen
in specific non-limiting implementations, along with a range
of pillar diameters that yield phase shifts varying over a full
range of 2x, while maintaining a relatively high and constant
transmittance ([50 nm, 160 nm}, [50 nm, 150 nm], and [50
‘nm, 110 nm], respectively). The detailed design procedure is
elaborated in Supplementary Information, Section IV.
Example 1: As a first demonstration of polarization-
independent UV metasurfaces, two 500-um-diameter, polar-
ization-independent metalens designs, [54, and Lys, of
substantially equal numerical apertures NA=0.6 (corre-
sponding to focal lengths f=330 jum), were implemented for
focusing ultraviolet light at respective free-space wave-
lengths 4,=364 nm and 325 nm (see the schematic of FIG.
2A). Single-mode focusing of an incident onto the metalens
200 plane wave L was achieved by implementing the
radially-symmetric phase shift function g(x, y, Ao)=mod
(2a/bo) G-Vx?+y74+$?), 22), where f is the focal distance
normal to the plane of the lens 200 (along the z direction),
x and y are in-plane distances along orthogonal directions
from the center of the lens 200, and normal incidence is
assumed, As shown in FIGS, 2B, 2C, each intensity distri-
bution measured at the metalens focal plane (see also
Supplementary Information, Section V) revealed a substan-



U.S. Patent Jan. 30, 2024 Sheet 3 of 16 US 11,885,979 B2

FIG. 2C

FIG. 2B

200

FIG. 2A


US 11,885,979 B2

5

schematic representation of the holographic image projec-
tion/formation by a generalized metahologram, under nor-
mally-incident plane-wave illumination at an operational
wavelength. FIG. 4B includes left and right image panels.
The left panel displays three theoretical (targeted) holo-
graphic images calculated to be generated by metaholo-
grams Hye4s Hyo53 OF Hogs at 4-364 nm, 325 nm, and 266
mn, respectively. The tight panel displays three empirically
measured holographic images projected by the implemen-
tations of the metaholograms Hy¢4, Hs35, and Hyg5 at the
same respective wavelengths in the 7-40 mm plane.

FIGS. 5A, 5B, 5C, 5D, 5E, and SF address embodiment
of a spin-multiplexed near-UV self-accelerating beam gen-
erator, FIGS. 5A, 5B provide schematic representations of
the generation of a self-accelerating beam by the spin-
multiplexed metasurface, B,,.°””", under normally-incident
plane-wave having either left-hand circular polarization,
LCP, or right-hand circular polarization, RCP, respectively,
at the operational wavelength 4-364 nm. FIG. 5C provides
a plot of measured -transverse defection in different z planes
(ranging from 2.5 mm to 4.5 mm, with an increment of 0.5
mm) for LCP illumination of By,” at the operational
wavelength. Error bars denote one standard deviation of the
measured data. The targeted beam trajectory, d=16 27, is
shown for reference. FIG. 5D illustrates measured transverse
defection in different z planes (ranging from 4.5 mm to 10.5
mm, with an increment of 1.5 mm) for RCP illumination of
Baggs” at the operational wavelength. Error bars denote one
standard deviation of the measured data. The targeted beam
trajectory, d=2.25z°, is shown for reference. FIG 5E, SF are
panels illustrating measured xy-plane intensity distributions
(normalized) at different z planes for the device at its
designated wavelength of operation under either LCP illu-
mination or RCP illumination, respectively. Each distribu-
tion is displayed over an equal square area of side length 140
jm, but shifted along the —xy (FIG. 5E) or xy (FIG. 5F)
direction as a function of increasing z, such that the center
of the main lobe maintains an invariant position within each
image.

FIGS. 6A, 6B, 6C, and 6D illustrate embodiments of
spin-multiplexed near- and deep-UV metaholograms. FIG.
6A provides two schematic representations of the holo-
graphic image projection by the spin-multiplexed metaholo-
gram H,,,°” under LCP or RCP illumination at 4..=364 nm,
respectively. FIG. 6B includes two measured holographic
images projected by the metahologram H,,3"”” in the 2-40
mm plane under LCP illumination (top image) and RCP
illumination (bottom image). FIG. 6C provides two sche-
matic representations of the holographic image projection by

the spin-multiplexed metahologram Hy5¢?"” under LCP and

RCP illumination at 25=266 nm, respectively. FIG. 6D
includes two measured holographic images projected by the
metahologram Hy¢,” in the z=40 mm plane under LCP
illumination (top image) and RCP illumination (bottom
image).

FIG. 7 is a plot of measured transmittance of a 500-1m-
thick, UV-grade fused silica wafer.

FIGS. 8A, 8B illustrates measured and best-match-mod-
eled Psi and Delta curves for ellipsometric characterization
of a low-temperature ALD-deposited HfO,. The legend is
common to FIGS. 8A and 8B.

FIG. 9 presents plots of measured refractive index n and
extinction coefficient k of HfO, films deposited by ALD at
95°C. and 200° C., respectively.

FIGS. 10A, 10B, 10C are scanning electron micrographs
(viewing angle: 20°) of selected areas of polarization-inde-
pendent metasurfaces designed for operation at A,=364 nm.

20

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s

5s free-space wavelength design (A,

6
FIG. 10A: Metalens L.5,,. FIG. 10B: Self-accelerating beam
generator By,,. FIG. 10C: Metahologram Hy,4-

FIG. 11A, 11B, 11C present scanning electron micro-
graphs (viewing angle: 20°) of selected areas of polariza-
tion-independent metasurfaces designed for operation at
4-325 nm. FIG. 11A: Metalens L555. FIG. 1B: Self-
accelerating beam generator B35. FIG. 11C: Metahologram.
Hy2s-

FIG. 12 is a scanning electron micrograph (viewing angle:
20°) of selected areas of polarization-independent metaholo-
gram designed for operation at 44=266 nm, Hy5¢.

FIGS. 13A, 13B are scanning electron micrographs
(viewing angle: 20°) of selected areas of spin-multiplexed
metasurfaces designed for operation at 4,=364 nm. FIG.
13A: Metahologram H,,,’””. FIG. 13B: Self-accelerating
beam generator Byg,’?”.

FIG. 14: Scanning electron micrograph (viewing angle:
20°) of selected areas of spin-multiplexed metahologram
designed for operation at 4o=266 nm, Baga?”

FIGS. 15A, 15B, 15C illustrate transmission intensity T
and phase shift @ for an incident light of free-space wave-
length 4..=364 nm (FIG. 15A), 325 nm (FIG. 15B), and 266
nm (FIG, 15), as a function of cylinder diameter D. For each
64 nm, 325 nm, and 266
nm), a corresponding cylinder height (H-550 mm, 500 mm,
and 400 nm, respectively), sub-wavelength lattice spacing,
(P=200 nm, 190 nm, and 150 nm, respectively), and maxi-
mum value of pillar diameter (D,,,.,=160 nm, 150 nm, and
110 nm, respectively) are chosen. For ease of display, the
phase shift for pillar arrays with diameter D=S0 nm is set to
zero for each design.

FIGS. 16A, 16B, 16C illustrate metasurface phase shift
profiles, (354'(X, y. Ao), Psas'(X, Ys ho), and P266."(X, ¥s Ao)s
designed to produce a “NIST” holographic image for nor-
mal-incidence, plane-wave illumination at 4)~364 nm (FIG.
16A), 325 nm (FIG. 16B), and 266 nm (FIG. 16C). Imple-
mented with the use of polarization independent metaholo-
grams, Hy6,, Hyos, and Hy5o, respectively).

FIG. 17 provides a panel of plots representing computed
and measured xy-plane intensity distributions (normalized)
at different z planes for device B,,,”’”" at its designated
wavelength of operation under LCP illumination. Each
distribution is displayed over an equal square area of side
length 140 um, but shifted along the —xy direction as a
function of increasing z, such that the center of the main lobe
maintains an invariant position within each image.

FIG. 18 is a panel of plots illustrating computed and
measured xy-plane intensity distributions (normalized) at
different z planes for device B5,.°”” at its designated wave-
length of operation under RCP illumination. Each distribu-
tion is displayed over an equal square area of side length 140
jm, but shifted along the xy direction as a function of
increasing z, such that the center of the main lobe maintains
an invariant position within each image.

FIGS. 19A, 19B depict metasurface phase shift profiles,
Prcazcr? (KY. ho) and Pse4.ecr?!"(&, Ys ho), designed to
project a “NIST” (FIG. 19A) and “NJU” (FIG. 19B) holo-
graphic images to the 2-40 mm plane, under normally
incident, plane-wave illumination at 45=364 nm (imple-
mented by spin-multiplexed metahologram Hy,:””).

FIGS. 20A, 20B are targeted holographic images in the
z=40 mm plane, numerically computed assuming an ideal
metahologram realization having a unity transmittance. The
targeted image of FIG. 20A has a phase shift
profile s6arcr%"?"(X. Ys Ao) of FIG. 19A. The targeted
image of FIG. 20B has a phase shift profile @36s,n0r"7?!"(X.



US 11,885,979 B2

17

or elliptical Hf, posts with smooth and straight side-wall
profiles (thanks to the resist templating process), of height
varying from 400 nm to 550 nm (depending on the specific
metasurface), and aspect ratios varying, from about 3 to
about 11.

Additional Details of Specific Implementation of a Low-
Temperature TDMAH-based HfO ALD Process. In step 1
of the ALD cycle, TDMAH vapor (Hi[CH,),N],) was
pulsed into the ALD chamber for a duration t,=1 s, reacting
with the dangling O—H bonds on the hafnium-coated
surface to create a new solid monolayer of Hf[(CH,).N]0.
and generate the gas by-product (CH;),NH (dimethylam-
ine). In step 2, high-purity nitrogen (N,) gas was flowed for
a duration t,=75 s to fully remove any un-reacted TDMAH
vapor and dimethylamine byproduct from the chamber. In
step 3, water vapor was pulsed into the chamber for a
duration t,=60 ms, reacting with the Hf[(CH,)N],0 to
create a monolayer of HfO, on the surface. Finally, in step
4, the excessive water vapor as well as the Dimethylamine

reaction byproduct were completely (that is, to a level below 2

the level of sensitivity of the appropriate composition-
detecting electronic circuitry) removed from the chamber by
N, purging for a duration 1,75 s.

Section II

Spectroscopic ellipsometry characterization of low-tem-
perature ALD-deposited HO. A 200-nm-thick Hf, film
was grown by low-temperature ALD on a silicon wafer
coated with a 300-nm-thick thermal oxide layer. The film’s
optical properties were characterized by reflection-mode
spectroscopic ellipsometry using the interference enhance-
ment method, at three different angles of incidence (55°, 65°,
and 75°) with respect to the normal to the plane of the HfO,
layer, The dielectric function of the HfO, was modelled by
a Tauc-Lorentz oscillator. The measured and best-match
modeled Psi () and Delta (A) curves (FIGS. 8A, 8B)
display close correspondence, as evidenced by a low mean-
squared-error for the fit (MSE-6.686). The corresponding
curves for the extracted values of refractive index n and
extinction coeflicient k are plotted in FIG. 1C.

Section IIT

Comparison of UV optical properties of ALD-deposited
HfO, at 95° C. and 200° C. The UV optical properties
(characterized by spectroscopic ellipsometry) of HfO,
deposited by TDMAH/water-based ALD using the new,
low-temperature process (process temperature: T,=95° C.,
TDMAH pulsing time:

pulsing time: t,=60 ms, N, purging time: t,=75 s, and
deposition rate: 0.110 nm/cycle) were compared to those
resulting from deposition using a standard process (process
temperature: T,=200° C., TDMAH pulsing time: t,=250 ms,
N, purging time: t,=12 s,
purging time: t,-12 s, and deposition rate: 0.106 nm/cycle),
Over the UV range, the two films exhibit (FIG. 9) virtually
identical extinction coefficients k (where k=O for 42220
nm), and refractive indices n that differ only slightly (where
values for the film deposited at 95° C. were at most
lower by ~0.03 than those for film deposited at 200° C., at
any given wavelength in that range).

Section IV

Design of the polarization-independent metasurfaces. The
transmittance, T, and induced phase shifi, ¢, of an array of

=I s, Nj purging time: t,-75 s, H,O 5

H,0 pulsing time: t,-60 ms, N, $

&

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18

cylindrical pillars of diameter D, height H, and lattice
spacing P (generally ranging from about 50 nm to about 500
‘nm, in different examples), under plane-wave normal illu-
mination at wavelengths 4-364 nm, 325 nm, and 266 nm,
respectively, were computed using the finite-difference-
time-domain (FDTD) simulations with periodic boundary
conditions. Given a constant lattice spacing P over the entire
metasurface, the function of the cylindrical pillars was to
provide a relative propagation phase shift of UP tO @ yar
min=2, between the phase @,,,, induced by the smallest-
diameter pillar array (having a small filling factor compared
to P, yielding a through-array propagation (refractive) index
Nyn@l, close to the refractive index of air) and the phase
naz induced by the largest-diameter pillar array (having a
larger filling factor compared to P, yielding a through-array
propagation (refractive) index n,,,,~2.1, close to averaged
index of refraction n of HfO, in the UV regime). This
requirement sets a lower limit on possible values of H of
Hynin=Po! Omax—Dmin a+

Other constraints help to limit the extent of parameter
space necessary to explore during the metasurface design
process: (1) to avoid (shin, prevent from happening) dif-
fraction of transmitted light (and, in particular, of a substan-
tially planar optical wavefront incident onto the metasur-
face) in operation of the metasurface, the upper limit of P,
Pav is chosen such that P,,..<Ao; (2) to stay within the
process tolerance of the Damascene-type lithographic pro-
cess, the minimum value of D, D,,,,. is chosen sucl
Dyun=50 nim; (3) to maintain the mechanical stability of the
resist template, the maximum possible value of D is chosen
as (P-40 nm). For each targeted free-space operation wave-
length , different combinations of H, P are surveyed
subject to the above requirements, along with the additional
constraints that: (1) (D)-@(D,,,) span at least the full
range of [0 2z], as D increases between D,,,,, and a value
Dmax2(P-40 nm), where @(Dyax)=2, and (2) the transmit-
tance T maintains a high and relatively constant value as D
varies over the same range. Sets (H, P, Dyyiy, ANd Dyay)
satisfying all those conditions are obtained for each of the
targeted free-space operation wavelength, 4,=364 nm, 325
nm, and 266 nm, of (550, 200, 50, 160). (500, 190, 50, 150),
and (400, 150, 50, 110), respectively, where all values are
expressed in nanometers (FIGS. 15A, 15B, 15C).

For each implemented polarization-independent metasur-
face device, the pillar height H and lattice spacing P are
chosen based on the metasurface operating wavelength ho.
The diameter of each nano-cylinder over the metasurface
plone is chosen such that the induced phase-shift g(D. x..
od (G""(x,, ¥,), 2), where @”” is the required phase-
shit profile for each metasurface, D is the diameter of the
nanopillar, and (x, y.) is the center position of each cylinder.

With the knowledge of this technical description, the
skilled artisan will readily appreciate that embodiments of
the invention provide a metasurface that includes an optical
substrate and a spatially-periodic two-dimensional array of
cylindrical pillars oriented on the optical substrate substan-
tially normally to the optical substrate, the cylindrical pillars
formed from a target material that is devoid of any of silicon,
silicon-containing compounds, titania, gallium nitride, and
niobium pentoxide. (Here, a spatial period of the array is
substantially constant across an area of the optical substrate
occupied by the array while a) different cylindrical pillars
have different diameters to form areas of the array with
different filling factors and b) heights of the cylindrical
pillars in the array approximately equal or exceed a free-
space operational wavelength chosen within a deep-UV
region of the electromagnetic spectrum such that the meta-



US 11,885,979 B2

9

elliptical in-plane cross-sections (FIG. 1A), densely arrayed
on a transparent UV-grade fused silica substrate of low
refractive index. (As shown in FIG, 7, the 500-tum-thick,
UV-grade fused silica wafer utilized as a metasurface sub-
strate, possessed high optical transmittance in excess of 90%
at each wavelength over the wavelength range of interest,
about 266 nm to about 364 nm.)

The choice of HfO, as a target material—a material most
commonly exploited for its high static dielectric constant as
transistor gate insulator in complementary metal oxide semi-
conductor (CMOS) integrated circuits—was guided by the
promise of both a large refractive index (n>2.1 for 4.j<400
nm) and a wide bandgap E,=5.7 eV (4j=217 nm) corre-
sponding to a wavelength well within the deep-UV spectral
region, thereby leading to a negligible extinction coefficient
(K=0) at wavelength(s) 2,24,. Though the requirement of
nanopillar dimensions of wavelength-scale height several
hundred nanometers), subwavelength-scale in-plane circle
diameter or ellipse minor axis (~few tens of nanometers),
and vertical sidewalk suggest that pattern transfer with a
directional dry-etching technique such as reactive ion etch-
ing would be optimal, it was empirically verified that a
suitable dry-etch chemistry for HfO, (a material commonly
patterned by non-directional wet chemical etching) could
not be identified. Instead, an embodiment of the process for
pattering structured around the use of Damascene lithogra-
phy for HfO, metasurface fabrication was implemented,
Specifically, a foundation of the processing method was
turning on first patterning the resist template using one of
available processes (for example an electron beam lithog-
raphy, or e-beam lithography, or EBL); then conformally
filling the open volumes/voids of the resist template with the
material of choice (the target material) using atomic layer
deposition, ALD—this step including the over-coating or
over-deposition of the material of choice above the upper
level of the open void(s) to ensure the spatial continuity of
the material of choice within the void(s) and complete filling
of the voids with the material of choice. The over-coated
portion of the target material above the upper level of the
so-filled void(s) was then back-etched with the use of an
Arion milling procedure, followed by removing the
volume(s) of remaining resist in the resist template with
solvent (to leave the required high-aspect-ration nanopil-
lars).

Preservation of or maintaining the physical/structural
integrity of the resist template (that is, preventing the resist
template from changing its shape or form during the fabri-
cation process), however, was empirically proven to be
frustrated and not ensured when conventionally-defined
Damascene lithography was used. Normally, maintaining
the structural integrity of the used resist required use of a
plasma-free thermal ALD process characterized by a process
temperature, T,, that is lower than the glass transition
temperature (reflow temperature) of the utilized resist, T,.
along with a process chemistry having by-products that are

not corrosive to the resist. Therefore, as a skilled artisan will s

now readily appreciate, fulfilling both process tolerance
requirements ruled out the use of common Hf precursors
such as Tetrakis(ethylmethylamino)Hafnium (TEMAH), for
which the minimum T,(~150° C.) is significantly greater
than the T, of common electron beam (e-beam) resists; or
Hafnium Chloride (HiCI,), for which the reaction by-prod-
uct (HCI) attacks the resist. To address these problems, a
new solution was required and found in utilizing Tetrakis
(dimethylamino)Hafnium (TDMAH) as an alternative Hf
precursor for thermal ALD of high-optical-quality Hf03,
that has a T,, below that of the T, of common e-beam organic
resists (such as ZEP, for which T,~105° C.). Furthermore, to

20

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10

avoid the risk of incomplete reaction cycles and physical
condensation of precursors associated with low-temperature
ALD (yielding films having defects and voids, and hence,
degraded sub-bandgap optical properties, such as reduced
refractive index n and finite extinction coefficient k), an
existing ALD process using TDMAH and H,O precursors
and operating at T,=200° C. was additionally modified (as
schematically illustrated in FIG. 1B by (1) decreasing the
process temperature to a temperature below 105° C. (in one
non-limiting specific case, T,-95° C. was set); (2) increas-
ing the TDMAH pulsing time, t,, from 0.25 s to 1 s, to
enable a complete reaction with the OH monolayer resulting
from the previous cycle; and (3) increasing N, purging
times, t, and t,, from 12 s to 75 s, to ensure a substantially
full removal (that is, removal down to the level below the
detection level of the available detecting electronic circuitry)
of excessive precursors and reaction by-products (see addi-
tional details in the Supplemental information, Section I
below). As revealed by spectroscopic ellipsometry measure-
ments (FIG. 1C; see also Supplementary information, See-
tion II below), hafnium oxide films deposited using the so
judiciously-modified low-temperature ALD process are
characterized by a high refractive index (n>2.1) and negli-
gible optical loss (k~0), over an UV wavelength interval 220
nmsh,<380 nm spanning the full mid- and near-UV ranges,
as well as more than half of the deep-UV range. In FIG. 1C,
values of n at the three operational wavelengths targeted in
this example are denoted by star-shaped marks. Dashed line
indicates position of HfO, bandgap E,. The measured wave-
length dependences of n and k closely match those of a film
grown using the 200° C. reference ALD process (Supple-
mentary Information, FIG. $3), demonstrating that the opti-
cal quality of the deposited HfO, can be maintained at
significantly lower ALD process temperatures with choice of
a suitable Hf precursor and proper adjustment of pulsing and
purging times. Such non-trivial modifications to the con-
ventional ALD process have not been considered before in
related art, and allowed the continued use of the ZEP-type
resist traditionally employed in EBL for generating a resist
template.

As a result, utilization of the ZEP resist and the novel
low-temperature TDMAH-based ALD process for deposi-
tion of the target material (in the described example
hafnium oxide), the proposed modified fabrication process
was applied to yield substantially defect-free metasurfaces
each exhibiting large arrays of densely packed Hf0, nano-
pillars on a UV-grade fused silica substrate. FIGS. ID, 1E,
and 1F provide illustrations to typical results of the
employed novel processing methodology. In this non-limit-
ing example, FIG. 1F contains two optical micrographs —the
upper of a full metalens and a lower of a spin-multiplexed
metahologram, while enlarged views of contents of these
metasurfaces are shown, respectively, in FIGS, 1D and 1E.
‘As can be seen, constituent nanopillars have uniform height,
circular (FIG. 1D) or elliptical (FIG. 1E) in-plane cross
sections, and are characterized by straight, vertical and
smooth sidewall (see also FIGS. 10A, 10B, 10C, 11A, 11B,
11C, 12, 13A, 13B, and 14). The nanopillar rotation angle
and two principal axes’ lengths in the plane of the metasur-
faces (respectively, 0, D,, and D,, where 0-0 and D,=D,=D
in the case of a circular cross-section, in reference to FIG.
1A), vary as a function of nanopillar position at the corre-
sponding metasurface (with0s@sxand 50 nms(D,, D,)s160
nm) depending on the particular optical function imple-
mented by the corresponding metasurface. The nanopillar
height H varies depending on the operational wavelength for



U.S. Patent

FIG. 21B

FIG. 21A

Jan. 30, 2024

Sheet 16 of 16

-

x (um)

FIG. 22B

x (um)

FIG. 22A

US 11,885,979 B2

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U.S. Patent Jan. 30, 2024 Sheet 8 of 16 US 11,885,979 B2

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US 11,885,979 B2

23

distributions of the generated self-accelerating beams in
selected z-planes beyond the metasurface were recorded
using custom-built imaging system including an NA-0.75
objective and an EMCCD camera, under LCP and RCP
illumination, respectively. Specific z-planes (ranging from
2.5 mm to 4.5 mm, with an increment of 0.5 mm, for LCP
illumination, and ranging from 4.5 mm to 10.5 mm, with an
increment of 1.5 mm, for RCP illumination) were addressed
using a stage which translates the metasurface relative to the
camera, along the direction of the laser beam. A 500-ym-
diameter aperture was placed behind the metasurface sub-
strate to completely separate the generated self-accelerating
beam from directly transmitted light leaking around the
edges of the metasurface (which occupied a square area of
side length 330 jm). The physical size of the captured
intensity profile was determined by comparison to the image
of the area occupied by the metasurface.

‘The experimental SAB generated by the device, under
LCP and RCP illumination, respectively, exhibited a diffrac-
tion-free character with xy-plane intensity distributions
similar to the targeted intensity distributions (FIGS. 17 and
18), numerically computed using the angular spectrum rep-
resentation method that is known in related art, assuming an
ideal metasurface realization having both the designed phase
shift profile g” and an unity transmittance T.

Section XII
Implementation of spin-multiplexed _metahologram
Hy¢3"". A spin-multiplexed metahologram, Hy54”"”, occu-
pying a square area of side length 330 jim, was implemented
for operation at 4-364 nm, Different phase profiles (FIGS.
19A, 19B), Ps6,rc7 PK, Yo bo) ANA P4,nc7 "(Ye
Ag). all based on the Gerchberg-Saxton algorithm, were
calculated for projecting a holographic “NIST” image (for
LCP illuminating light) and “NJU” image (for RCP illumi-
nating light), all located in the z~40 mm plane, with an offset
of y=-3 mm (FIGS. 20A, 20B),

Anonmally incident, 364-nm continuous wave (CW) laser
beam (beam diameter: ~5 mm) was used to illuminate the
metahologram, and a linear polarizer and a quarter-wave
plate (with a center wavelength of 355 nm) were used in
addition to contro! the handedness of the illuminating light.
An EMCCD camera was placed in the hologram formation
plane located 40 mm beyond the metahologram to directly
record the projected holographic images under LCP and
RCP illumination, respectively. A 500-jm-diameter aperture
was placed behind the metahologram substrate to com-
pletely separate the generated holographic image (projected

with a 3-mm lateral offset) from directly transmitted light 5

leaking around the edges of the metasurface (which occu-
pied a square area of side length 330 um).

Section MIL.

Implementation of spin-multiplexed _metahologram
Hy¢6°”. Two different phase shift profiles over 320-um-
square areas (FIGS. 21A, 21B), 2¢6,2077""(% Ys ho) and
Iawo.rcr "(Xs Ys hq), Were calculated using the Gerch-
berg-Saxton algorithm to achieve projection of distinct
holographic images under normal-incidence, plane-wave
illumination at 4-266 nm, namely, images forming indicia
“deep” and “UV”, respectively, in the z=40 mm plane, with
an offset 3 mm (FIGS. 22A, 22B),

A normally incident, 266-nm pulsed laser (pulse
duration: ~5 ns, repetition rate: 10 Hz, beam diameter: ~6
mm) was used to illuminate the spin-multiplexed metaholo-

20

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24

gram Hyg¢”. In addition, a linear polarizer and a quarter-
wave plate (with a center wavelength of 266 nm) were
employed to control the handedness of the illuminating
light. A piece of fluorescent white paper was placed in the
hologram formation plane located 40 mm beyond the meta-
hologram and imaged with a custom-built imaging system
including a lens and a CCD camera. Holographic images
were recorded for device H5,.”" under LCP and RCP
illumination, respectively. Five hundred images were
recorded and averaged, in each case, with and without laser
illumination to subtract background and dark counts from
the recorded image as well as to reduce random image noise.
A 500-um-diameter aperture was placed behind each meta-
surface substrate to completely separate the generated holo-
graphic image (projected with a 3-mm lateral offset) from
directly transmitted light leaking around the edges of the
metasurface (which occupied a square area of side length
320 um). The physical size of the captured holographic
image was determined by comparison to the image of an
object of known size placed in the plane of the fluorescent
white paper screen.

References throughout this specification to “one embodi-
ment,” “an embodiment,” “a related embodiment,” or simi-
lar language mean that a particular feature, structure, or
characteristic described in connection with the referred to

5 “embodiment” is included in at least one embodiment of the

present invention. Thus, appearances of the phrases “in one
embodiment,” “in an embodiment,” and similar language
throughout this specification may, but do not necessarily, all
refer to the same embodiment. It is to be understood that no
portion of disclosure, taken on its own and in possible
connection with a figure, is intended to provide a complete
description of all features of the invention.

For the purposes of this disclosure and the appended
claims, the use of the terms “substantially”, “approxi-
mately”, “about” and similar terms in reference to a descrip-
tor ofa value, element, property or characteristic at hand is
intended to emphasize that the value, element, property, or
characteristic referred to, while not necessarily being exactly
as stated, would nevertheless be considered, for practical
purposes, as stated by a person of skill in the art. These
terms, as applied to a specified characteristic or quality
descriptor means “mostly”, “mainly”, “considerably”, “by
and large”, “essentially”, “to great or significant extent”,
“largely but not necessarily wholly the same” such as to
reasonably denote language of approximation and describe
the specified characteristic or descriptor so that its scope
would be understood by a person of ordinary skill in the art
In one specific case, the terms “approximately”, “substan-
tially”, and “about”, when used in reference to a numerical
value, represent a range of plus or minus 20% with respect
to the specified value, more preferably plus or minus 10%,
even more preferably plus or minus 5%, most preferably
plus or minus 2% with respect to the specified value. As a
non-limiting example, two values being “substantially
equal” to one another implies that the difference between the
two values may be within the range of +/-20% of the value
itself, preferably within the 10% range of the value itself,
more preferably within the range of +/-5% of the value
itself, and even more preferably within the range of +/-2%
or less of the value itself

The use of these terms in describing a chosen character-
istic or concept neither implies nor provides any basis for
indefiniteness and for adding a numerical limitation to the
specified characteristic or descriptor. As understood by a
skilled artisan, the practical deviation of the exact value or
characteristic of such value, element, or property from that
stated falls and may vary within a numerical range defined



US 11,885,979 B2

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US 11,885,979 B2

1

LOW-LOSS METASURFACE OPTICS FOR
DEEP UV

CROSS-REFERENCE TO RELATED,
APPLICATIONS

This patent application claims priority from and benefit of
the U.S. provisional patent application No. 62/956,875 titled
“Systems, Device, Materials and Methods for Low-Loss
Metasurface Optics” and filed on Jan. 3, 2020. The disclo-
sure of the above-identified provisional patent application is
incorporated herein by reference.

STATEMENT REGARDING FEDERALLY
SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under
contract 70NANB14H209 awarded by the National Institute
of Standards and Technology. The government has certain
rights in the invention.

TECHNICAL FIELD,

The invention relates generally to a system, device, mate-
rials, and methods for metasurface-based optics and, in
particular, to metasurface-based optical devices and systems
configured to operate not only in the ultraviolet (UV) portion
of the electromagnetic spectrum, but also in a deep-UV
portion of it. Methodologies of fabrication of such systems
are judiciously defined to be compatible with CMOS.

RELATED ART

The term “optical metasurface” (interchangeably referred
to below simply as “metasurface”) is typically used in
related art to refer to a material interface that has structures
on it that impose particular phase shifts on light interacting
with this material interface (that is, which is either trans-
mitted through or reflected off the interface). In some cases,
the optical metasurface may include a planar array of
subwavelength electromagnetic structures that emulate the
‘operation of a conventional, bulk refractive, birefringent, or
diffractive optical component (such as, in non-limiting
examples, a lens, a waveplate, or a hologram) through
individually tailored amplitude, phase, or polarization trans-
formations imposed by the metasurface on incident light
with which such metasurface interacts. Several dielectric
materials such as amorphous Si, polycrystalline Si, titanium
dioxide or titania (TiO,), and gallium nitride (GaN) have

been used to realize metasurfaces operating at frequencies of 5

infrared and/or visible light. The development of metasur-
faces for applications in the ultraviolet (UV) portion of the
electromagnetic spectrum, however, has been and remains
to-date impeded at least by the scarcity of dielectric mate-

rials that both are characterized by low optical loss (at s

corresponding frequencies) and amenable to high-aspect-
ratio nanopatterning. To date, the design of metasurfaces for
operation in the near-UV (UV-A; free-space wavelength
range: 315 nm <h,<380 nm; energy range: 3.26 eVsEy=3.94
eV) has been attempted using niobium pentoxide (Nb20.).
demonstrating the operation down to an operation free-space
wavelength of 4,355 nm. Crystalline Si has been used to
realize metasurfaces operating at 2.=290 nm?!, a wave-
length falling within the mid-UV (UV-B; 280 nmsios315
nm; 3.94 eVsE,s4.43 eV); in this demonstration, however,
the device efficiencies were recognized to be critically
limited by severe absorption loss associated with illumina-

20

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tion frequencies above the bandgap of Si (E,#1.1 eV). In
both of the above-mentioned studies, the device function-
alities that were realized in practice were restricted to
hologram generation and beam deflection. And yet, the UV
region (especially the deep UV region) of the optical spec-
trum remains a technologically important spectral regime
hosting diverse application in lithography, imaging, spec-
troscopy, time-keeping, and quantum information process-
ing—to name just a few—where other important optical-
wavefront-shaping functionalities that can be empowered by
optical metasurfaces (such as high-numerical-aperture
focusing and structured beam generation, for example) have
not been achieved yet.

Meanwhile, metasurfaces able to operate at even higher
frequencies, such as within the deep-UV range (defined and
referred to as longer wavelength portion of UV-C; 190
nmzhos280 nm; 4.43 eVsE,s6.53 eV), have not been
realized due to multiple recognized challenges that include
the identification of a dielectric material that has a suitably
low optical absorption coefficient in that spectral range and
the lack of nano-fabrication fabrication processes that would
allow for successful patterning of such (not identified yet)
material into high-aspect-ratio nanostructures (here, the
available nano-fabrication techniques fall short in achieving

5 this goal, as a skilled artisan well knows).

‘A need therefore remains to enable high-performance
dielectric metasurfaces that operate over a broad UV range,
including within the record-short, deep-UV portion of the
optical spectrum, and that perform multiple required optical-
wavefront-shaping operations or functionalities.

SUMMARY

Embodiments of the invention provide a method for
fabricating a structured electromagnetic metasurface con-
taining a sub-wavelength-scaled surface pattern. The
method includes a process (conducted in an atomic layer
deposition, ALD, chamber) of conformal filling an open
volume—that has been created or patterned in a layer of a
resist material—with a chosen target material with the use of
an ALD process that is conducted at a temperature not
exceeding 105° C., to define a material preform (here, the
target material is selected to not include of any silicon by
itself (outside of a compound or combination with another
element), titania, gallium nitride, and niobium pentoxide).
The method also includes a step of removing the resist
material with a solvent to expose a pattern structure of the
target material so dimensioned on a sub-operational-wave-
length scale as to define as at least one optical element
including any of refractive, diffractive, birefringent, and
resonant optical elements. (The operational wavelength is
predetermined in a deep-UV range of an electromagnetic
spectrum.) The method may include patterning a positive
tone electron-beam resist material (to form the open volume
in it) and/or defining the conformally filling step of the
process by depositing hafnium oxide with a Tetrakis(dim-
ethylamino)Hafnium (TDMAH) precursor. Alternatively or
in addition, the conformally filling action may include
overcoating boundaries of the open volume at a top surface
of the patterned layer of the resist material to form a
protrusion of the target material above the top surface of the
patterned layer, and the addition of a back-etching step (at
which step such protrusion is removed with ion milling to
yield a substantially planar top surface of the material
preform). In practically any implementation of the method,
the process of removing the resist material may be structured
as soaking the material preform in a solvent to expose an



US 11,885,979 B2

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tially circularly-symmetric focal spot, characterized by a
cross-section that closely matched the intensity distribution
theoretically predicted for a diffraction-limited lens of
numerical aperture NA=0.6 and given by the Airy disk
function I(x)=[2J,(AVAP’, where J, is the Bessel function of 5
the first kind of order one, and A=2nNAx/h. The focusing
efficiencies, defined as the ratio of the optical power of the
focused spot to the total power illuminating the metalens,
were (55.1742.56) % (for lens Lg,) and (56,281.37) % (for
lens L495). The cited uncertainties represent one standard
deviation of the measured data.

Example 2 illustrates creating of polarization-independent
metasurfaces configured to transform a normally-incident,
plane wave L into a diffraction-free output beam propagating
along a curved trajectory, ie., a self-accelerating beam
(SAB). Here, two designs,(B3g, and B55), for a sy m=
square SAB generator were implemented to operate at the
respective wavelengths of 364 nm and 325 nm, see FIGS.
3A, 3B.). The SAB generator design and operation are»
conveniently described using a Cartesian coordinate system
in which the constituent metasurface 300 is located in the
z=0 plane and the Ist xy quadrant, with one corner posi-
tioned at the origin. The implemented SAB for each targeted
free-space wavelengths 29364 nm and 325 nm, was char-
acterized by a L-shaped wave-packet of main lobe centered ~
on the trajectory y=x=—az”, where a=9 m (in other words,
originating from (0, 0, 0), propagating in the +z direction in
a curved trajectory confined to the plane y=x, with a height
above the surface given by z=Vd/a, where d=Ixi=lyl is the
lateral displacement). The targeted SAB was generated by
implementing a phase-shift profile @%(x. y, A)=mod (

s

8

8

8x

3
wy

2m) in the metasurface. The measured lateral displacement
values d(z) awere observed to closely match, in each case,
the calculated values based on the targeted trajectory (FIG.
3B: see also Supplementary Information, Section VI). As
illustrated in FIG. 3C, the experimental SAB generated by
each implemented device exhibited substantially diffraction-
free characteristics with xy-plane intensity distributions
similar to the intensity distributions numerically computed
using the angular spectrum representation method (assum-
ing an ideal metasurface realization having both the
designed phase shift profile o” and unity transmittance T).
The measured efficiencies, defined as the ratio of the total
optical power of the SAB in the z=5 mm plane to the total
power illuminating the metasurface, were (46.75+2.31) %
(for design By,,) and (67.42+4.43) % (for design B33).
These efficiencies compared favorably to that reported in
related art TiO,-based self-accelerating beam generator
operating at visible frequencies (Fan et al., in Nano Lett.
2019, 19, 1158-1165; incorporated herein by reference).
Example 3: As a final demonstration of polarization-
independent UV metasurfaces, three metaholograms,
denoted Hy¢4, Hy25, and Hage and operating at three respec-
tive UV wavelengths 1,=364 nm, 325 nm, and 266 nm were 6
fabricated (see the schematic of FIG. 4A). Each metaholo-
gram 400, which occupied a square area of side length 270
}m, was mapped to a Cartesian coordinate system in which
the constituent metasurface was positioned in the 7=0 plane
and the Ist xy quadrant, with one corner at the origin. The 6
Gerchberg-Saxton algorithm (Optik, vol. 35, No. 2, pp.
237-246, 1972; incorporated herein by reference) was

3

14

employed to calculate the required phase shift profile "56,
(X,Y, Ags OM 505(X. Ys Ag)s and OM a66(X. Ys Ag) for producing
a holographic image “NIST” formed in the =40 mm plane,
under normal-incidence, plane-wave illumination L (see
Supplementary information, Section VID. An additional
offset of y=-3 mm was added to avoid overlap of the
generated holographic image with the residual directly trans-
mitted beam. The images projected by metaholograms H4¢,,
Hy95, and Hye were measured (Supplementary Information,
Sections VIII and IX) and now displayed in FIG. 4B, right
panel. Each of the experimental holographic images faith-
fully replicated the shape of the corresponding target image
(illustrated in the left panel of FIG. 4B). The target images
were numerically computed assuming an ideal metaholo-
gram realization having both the designed phase shift profile
o” for a given operation wavelength and unity transmittance
T. In addition, the speckle patterns filling the shapes of the
measured images projected by metaholograms Hy,, and
Hy, presented numerous similarities to those of the corre-
sponding target images; the as-measured holographic image
projected by metahologram H.¢, did not offer the possibility
of such a comparison due to the employed fluorescence
transduction characterization scheme, which washed out the
details of the speckle patterns. The measured efficiencies for
metaholograms Hy,, and Hs, defined as the ratio of the
total optical power of the holographic image to the total
power illuminating the structure, are (62.99+4.14) % and
(71.78+2.06) %, respectively. The measured efficiency for
metahologram Hage, defined as the ratio of the total fluo-
rescence power of the holographic image to the fluorescence
power of light illuminating the structure (Supplementary
Information, Section IX), were (60.6742.60) %. These efli-
ciency values were comparable to those of recently reported
TiO,-based metaholograms operating in the visible portion
of the optical spectrum (see Devlin et al. in PNAS, Sept. 20,
2016, vol 113, no. 38, pp 10473-10478; published at www.p-
nas.org/cgi/doi/10.1073/pnas.1611740113; incorporated
herein by reference).

Examples of Spin-Multiplexed Ultraviolet
Metasurfaces

While metasurfaces operating to switch between distinct
optical outputs, such as different holographic images, or
differently oriented beams, under the control of fundamental
optical state of the input beam (such as polarization), or a
spatial feature of the input beam (such as angle of incidence)
have been demonstrated, the related art has not presented
any methodology to-date to implement spin-multiplexed UV
metasurfaces structured to switch between distinct outputs
depending on the handedness of input light (left-hand cir-
cularly polarized-LCP or right-hand circularly polarized-
RCP). Embodiments of the current invention made such
metasurfaces possible for the first time. The detailed design
procedure is elaborated in Supplementary Information, Sec-
tion X.

Example 4: In particular, utilizing the proposed process-
ing methodology, an embodiment of a polarization-depen-
dent, spin-multiplexed self-accelerating beam generator
operating at 4g=364 nm and referred to as Byg,'”" was
realized. In reference to the schematic illustration of FIGS.
5A and 5B, the embodiment 500 of the B3.,"”” self-accel-
erating beam generator was configured to generate SABs
following different trajectories under the control of the
handedness of circularly-polarized incident light. The spin-
multiplexed SAB generator, which occupies a square area of
side length=330 um, was referenced to a Cartesian coordi-


11885979B2
a2) United States Patent (0) Patent No.: US 11,885,979 B2
Zhang et al. (45) Date of Patent: Jan. 30, 2024
(54) LOW-LOSS METASURFACE OPTICS FOR (52) US. Ch
DEEP UV cpc . G02B 3/08 (2013.01); C23C 16/042
(2013.01); €23C 16/40 (2013.01); C23C
(71) Applicants: University of Maryland, College Park, 16/405 (2013.01):
College Park, MD (US); Government (Continued)
of the United States of America as (58) Field of Classification Search
represented by the Secretary of None
Commerce, Gaithersburg, MD (US) See application file for complete search history.
(72) Inventors: Cheng Zhang, Gaithersburg, MD (US); (56) References Cited
Shawn Divitt, Gaithersburg. MD (US); .
Wengqi Zhu, Vienna, VA (US); Amit U.S. PATENT DOCUMENTS
Kumar Agrawal, Rockville, MD (US); 2018/0341090 A1* 11/2018 Devlin GO3F 7/40
Henri Lezee, Silver Spring, MD (US) 2019/0301025 A1* 10/2019 Akselrod C23C 14/021
(73) Assignees: UNIVERSITY OF MARYLAND, THER PI ATION
COLLEGE PARK, College Park, MD OTHER PUBLICATIONS
(US); GOVERNMENT OF THE Hausmann et al. “Atomic Layer Deposition of Hafinium and Zirco-
UNITED STATES OF AMERICA AS nium Oxides using Metal Amide Precursors” Chem Mater. 14,
REPRESENTED BY THE 4350-4358 (2002) (Year: 2002).*
SECRETARY OF COMMERCE, (Continued)
Gaithersburg, MD (US)
. . ; Primary Examiner — Jose 1 Hernandez-Kenney
(*) Notice: Subject to any disclaimer, the term of this (74) Avtorney, Agent, or Firm — Quarles & Brady LLP;
patent is extended or adjusted under 35. -Yakoy Sidorin °
USC. 154(b) by 141 days.
(57) ABSTRACT
(21) Appl. No.: 17/136,277
High-performance optical-metasurface-based components
(22) Filed: Dee. 29, 2020 configured to at frequencies of UV light and, in particular, in
deep UV range and performing multiple optical-wavefront-
(65) Prior Publication Data shaping functions (among which there are high-numerical-
US 2021/0208312A1 Jul, 8, 2021 aperture lensing, accelerating beam generation, and holo-
gram projection). As a representative material for such
components, hafnium oxide demands creation and establish-
Related U.S. Application D ment of a novel process of manufacture that is nevertheless
iclated U.S. Application Data based on general principles of Damascene lithography, to be
(60) Provisional application No. 62/956,875, filed on Jan, _ compatible with existing technology and yet sufficient for
3, 2020. producing high-aspect-ratio features that currently-used
materials and processes simply do not deliver. The described
(51) Int. CL invention opens a way towards low-form-factor, multifunc-
GO2B 3/08 (2006.01) tional ultraviolet nanophotonic platforms based on flat opti-
€23C 16/40 (2006.01) cal components and enabling diverse applications including
(Continued) (Continued)

Hy Pucging
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a8 'Old

V8 “Old


U.S. Patent Jan. 30, 2024 Sheet 9 of 16 US 11,885,979 B2

0.95
8.90
—
0.85
0.80
200 250 300 350 400
Ag (nm)
FIG. 7
comme #195 °C ALD
we ome EZOO MS ALO
wena J GBS AC ALD)
wwe om R{2OO °C ALD)
Sos. 02 x
28 0.0

FIG. 9


US 11,885,979 B2

27

further comprising back-etching said protrusion with ion
milling to yield a substantially planar top surface of the
material preform.

5. The method according to claim 1,

wherein said removing the resist material includes soak-
ing the material preform in a solvent to expose an array
of columns of target material of a sub-micron height
and different aspect ratios, each aspect ratio smaller
than 11, an aspect ratio of a respective column defined
as a ratio of a height to a transverse dimension thereof.

6. The method according to claim 1, wherein said remov-
ing the resist material includes soaking the material preform
in a solvent to expose an array of columns of target material
of sub-micron height and aspect ratios of less than 9.

7. The method according to claim 1, wherein said remov-
ing the resist material includes soaking the material preform
in a solvent to expose an array of columns of the target
material of sub-micron height and aspect ratios of less than
7.

8. The method according to claim 1, wherein said remov-
ing the resist material includes soaking the material preform
in a solvent to expose and array of columns of the target
material of sub-micron height, the array including different
columns
a range from about 3 to about 11.

9. The method according to claim 1, wherein said remov-
ing the resist material includes soaking the material preform
in a solvent to expose an array of columns of target material
of a sub-micron height and different aspect ratios wherein
the array of columns is a spatially-periodic array with a
spatial period having a value within a range from about 50
nm to about 500 nm.

10. The method according to claim 1, comprising at least
one of the following steps:

a) delivering a first vapor of a first precursor material into
the ALD chamber in a during a time-window of a first
duration to form a solid monolayer of Hi[(CH,)N],O
ona surface of a chosen substrate while generating a
fluid by-product

(CH,),NH(imethylamine) simultaneously with forma-
tion of said solid monolayer;

aving corresponding different aspect ratios within ,

28

b) removing a portion of the vapor of the first precursor
material that has not reacted with the surface of the
target material and the fluid by-product from the ALD
chamber as a result of purging the ALD chamber with

5 nitrogen gas;

c) delivering a second vapor of liquid into the ALD
chamber during a time window of a second duration for
create a monolayer of the target material on the surface;

d) removing excess of the second vapor by purging the
ALD chamber with nitrogen; and

10 @) repeating steps a) through d) multiple times to accu-
mulate a layer of the target material having a pre-
determined thickness on the surface.

11. The method according to claim 10, wherein at least
one of the following conditions is satisfied:

15) said delivering the first vapor of the first precursor
material includes delivering the vapor of Hf[(CH)2N],
(IDMAH);

id target material is HfO,;

oo id liquid is water;

iv) said fluid by-product is gas; and
y) said removing the portion of the vapor of the first
precursor material include removing the first precursor
material from the ALD chamber down to below detect-
able limits achievable with the use of electronic cir-
cuitry configured to detect the present of the first
precursor material and utilized in operation of the ALD
chamber.

12. A method for fabricating an all-dielectric metasurface
optical device including at least one of a polarization-
© independent metalens, a polarization-independent metaholo-
gram, a polarization-independent Airy beam generator, said
device having optical transmittance of at least 50% at a
wavelength of 364 nm or shorter, the method comprisin;

as part of an atomic layer deposition process conducted at

a temperature not exceeding 105° C., conformally
filling an open volume, formed in a patterned layer of
an electron-beam resist material, with hafnium oxide
material.

13. The method according to claim 12, wherein the optical
4° transmittance of the device is at least 70%.

30

+e eee


US 11,885,979 B2

25

by an experimental measurement error that is typical when
using a measurement method accepted in the art for such
purposes.

For example, a reference to an identified vector or line or
plane being substantially parallel to a referenced line or
plane is to be construed as such a vector or line or plane that
is the same as or very close to that of the referenced line or
plane (with angular deviations from the referenced line or
plane that are considered to be practically typical in related
art, for example between zero and fifteen degrees, preferably
between zero and ten degrees, more preferably between zero
and 5 degrees, even more preferably between zero and 2
degrees, and most preferably between zero and 1 degree).
For example, a reference to an identified vector or line or
plane being substantially perpendicular or normal to a
referenced line or plane is to be construed as such a vector
or line or plane the normal to the surface of which lies at or
very close to the referenced line or plane (with angular
deviations from the referenced line or plane that are con-
sidered to be practically typical in related art, for example
between zero and fifteen degrees, preferably between zero
and ten degrees, more preferably between zero and 5
degrees, even more preferably between zero and 2 degrees,
and most preferably between zero and 1 degree). As an
example, the use of the terms “substantially flat” or “sub-
stantially planar” in reference to the specified surface
implies that such surface may possess a degree of non-
flatness and/or roughness that is sized and expressed
commonly understood by a skilled artisan in the specific
situation at hand. Other specific examples of the meaning of
the terms “substantially”, “about”, and/or “approximately”
as applied to different practical situations may have been
provided elsewhere in this disclosure.

With the benefit of this disclosure, a skilled artisan readily
appreciates that—given the negligible extinction coeflicient
of the target material (in discussed non-limiting examples—
HfO, deposited with the use a low-temperature-ALD pro-
cess according to the idea of the invention deposited Hf0,
down at wavelengths as low as that corresponding to the
bandgap of the target material (1¢~217 nm), and due to the
inherent high patterning fidelity of the Damascene-type
process—metasurfaces formed according to the idea of the
invention can be structured to operate at substantially shorter
wavelengths than those discussed above. Moreover, experi-
mental demonstration of broader range of device function-
alities in the deep-UV regime other than hologram projec-
tion should be possible by using a continuous-wave light
source and an appropriate direct imaging system.

The constituent nanostructured elements of the metasur-

faces were formed of hafnium oxide—a CMOS-compatible, 5

wide-bandgap, low-loss UV-transparent, high-refractive-in-
dex dielectric material deposited using a judiciously-defined
low-temperature atomic layer deposition (ALD) and pat-
tered using high-aspect-ratio, resist-based lithography

technique. With the above identified new material and s

developed fabrication technique, the metasurfaces designed
for operation at the three representative UV wavelengths of
364 nm, 325 nm, and 266 nm were practically implemented.
The metasurfaces were configured to perform a variety of
optical functions (and, specifically, high-numerical-aperture
lensing, accelerating beam generation, and hologram pro-
jection, including under spin control for the last two appli-
cations). This achievement opens the way for low-form-
factor and multifunctional photonic systems based on UV
flat optics, and suggests promising applications in photoli-
thography, high-resolution imaging, UV spectroscopy and
quantum information processing and enabling applications

26

such as atom trapping, fluorescence imaging, and circular
dichroism spectroscopy in a compact form factor.
Features of the specific implementation(s) of the idea of

the invention is described with reference to corresponding
5 drawings, in which like numbers represent the same or
similar elements wherever possible. In the drawings, the
depicted structural elements are generally not to scale, and
certain components are enlarged relative to the other com-
ponents for purposes of emphasis and understanding. It is to
be understood that no single drawing is intended to support
a complete description of all features of the invention. In
other words, a given drawing is generally descriptive of only
some, and generally not all, features of the invention. A
given drawing and an associated, portion of the disclosure
containing a description referencing such drawing do not,
generally, contain all elements of a particular view or all
features that can be presented is this view, for purposes of
simplifying the given drawing and discussion, and to direct
the discussion to particular elements that are featured in this
drawing. A skilled artisan will recognize that the invention
may possibly be practiced without one or more of the
specific features, elements, components, structures, details,
or characteristics, or with the use of other methods, com-
ponents, materials, and so forth. Therefore, although a
particular detail of an embodiment of the invention may not
be necessarily shown in each and every drawing describing
such embodiment, the presence of this detail in the drawing
may be implied unless the context of the description requires
30 otherwise. In other instances, well known structures, details,
materials, or operations may be not shown in a given
drawing or described in detail to avoid obscuring aspects of
an embodiment of the invention that are being discussed.

The invention as recited in claims appended to this
disclosure is intended to be assessed in light of the disclosure
as a whole.

20

What is claimed i
1. A method for fabricating a structured electromagnetic
metasurface containing a sub-wavelength-scaled surface
pattern, the method comprising:
in an atomic layer deposition (ALD) chamber, confor-
mally filling an open volume formed in a patterned
layer of a resist material with a target material with the
use of an ALD process conducted at a temperature not
exceeding 105° C. to define a material preform, the
target material being devoid of any of silicon, titania,
gallium nitride, and niobium pentoxide; and

removing the resist material with a solvent to expose the
sub-wavelength-scaled pattern structure of the target
material, wherein the pattern structure is dimensioned
to operate as at least one of refractive, diffractive,
birefringent, and resonant optical elements at a wave-
length defined in a deep-UV range of an electromag-
netic spectrum.

2. The method according to claim 1, comprising pattern-
ing the resist material that is a positive tone electron-beam
resist material to form the open volume therein.

. The method according to claim 1, wherein said con-
60 formally filling includes depositing hafnium oxide with a
Tetrakis(dimethylamino)Hafnium (TDMAH) precursor.
4. The method according to claim 1,
wherein said conformally filling includes overcoating
boundaries of the open volume at a top surface of the
patterned layer of the resist material to form a protru-
sion of the target material above the top surface of the
patterned layer, and

&

&



US 11,885,979 B2

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U.S. Patent

(pozyeuucrl j

painseayy

sana g7g sty “Hy

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US 11,885,979 B2

7

Y, ho) of FIG. 19B. A normally-incident plane-wave illumi-
nation at 45-364 nm was assumed.

FIGS. 21A, 21B illustrate metasurface phase shift pro-
files, Proocer?'"(%, Ys ho) and Pros,nce P(X, Ys Ao)»
designed to project a “deep” (FIG. 21A) and “UV” (FIG.
21B) holographic images to the z~40 mm plane, under
normally-incident plane-wave illumination at 4,266 nm
(implemented by spin-multiplexed metahologram H.6.”"”).

FIGS. 22A, 228 illustrate targeted holographic images in
the z-40 mm plane, numerically computed assuming, i)
normally-incident plane-wave illumination at ,=266 nm, ii)
an ideal metahologram realization having a unity transmit-
tance, and iii) FIG. 22A: a phase shift profile ()63,.cp"""
(x. y. Ap) shown in FIG. FIG. 21B or FIG. 22B: a phase shift
profile P266,ecp?"(K. Y. ho) Shown in FIG. 21B.

Generally, the sizes and relative scales of elements in
Drawings may be set to be different from actual ones to
appropriately facilitate simplicity, clarity, and understanding
of the Drawings. For the same reason, not all elements
present in one Drawing may necessarily be shown in
another,

DETAILED DESCRIPTION.

The skilled artisan is well aware, therefore, that conven-
tional target materials (such as silicon, silicon-based mate-
rials, titania, gallium nitride, niobium pentoxide, to name
just a few) falls short of providing a material base for
formation of metasurface optical elements—both because
none of these materials simultaneously possess a suitably
low optical absorption coefficient at free-space wavelengths
within the deep UV and lend themselves to patterning of
nanostructures with the required high-aspect ratios and
sidewall straightness.

Embodiments of the present invention solved this prob-
Jem by employing a target material that is free from (devoid
of, does not include) any of the above-identified materials
conventionally utilized for generating an optical metasur-
face for operation at deep UV wavelengths. Here, the
following convention is used: UV-A range, or Near-UV:
315-400 nm; UV-B range, or Mid-UV: 280-315 nm; UV-C
range: 100-280 nm; Deep-UV: 190-280 nm. In at least one
implementation, the target material included hafnium oxide
(HfO,).

The use of the target material of choice, however, brought
to light a different set of practical problems that manifest, in
particular, in inapplicability of wet chemical etching for
producing optical metasurfaces formed of this material (such
non-directional etching simply cannot produce the required

high-aspect ratio structural features) and in absence (lack of ‘5

availability) of chemical compositions that would be suit-
able for directional dry-etching. These shortcoming inevi-
tably limited the remaining choices of processing to a
methodology of fabrication that generally follows the logic
of a Damascene lithographic process.

‘To implement the embodiments of the present invention,
s, however, could not be used
in its conventionally-recognized form—and had to
be judiciously modified in a rather subtle fashion to over-
come yet another a practical problem caused by simultane-
‘ous presence of several limitations on fabrication process.

The simultaneous limitations referred to above were
imposed by, on the one hand, a) geometrical dimensions (of
the final metasurface structure to be formed) to enable such
structure to be optically-operable in the deep-UV region,
and, at the same time, b) inability of conventionally-used
resist and solvent formulations to maintain spatial integrity

“as is’

20

&

&

s

8

of the structured resist templates filled and/or overcoated
with the target material at temperatures of conventionally-
performed ALD process(es). In particular, the former
demands that physical integrity of spatial features formed in
a resist template with high spatial resolution provided by
electron beam lithography, ELB (a few nanometers across,
for example, 4 nm or so and/or a few nanometers half-pitch
extent for periodic spatially-dense features—for example, 8
nm or so) be preserved during the following plasma-free
atomic-layer deposition, ALD, process. The latter, on the
other hand, begs for identification and utilization of such
combination of resist material(s) and ALD process in which
the glass transition temperature (reflow temperature) of the
resist is higher than the temperature of the ALD process to
be used, as well as the use of solvents that are substantially
not corrosive to the utilized resists.

Embodiments of the invention addressed this problem as
well, by utilizing an alternative, an unconventional Hf
precursor for thermal ALD of high-optical quality HfO,
while, at the same time, at least modifying the commonly-
used ALD process with the use of such precursor to have the
process temperature to that generally below 105° C. (sub-
stantially the lowest glass-transition temperature of a known.
organic electron-beam resist) and/or modifying durations of
the ALD sub-processes to ensure the completion of chemical
reactions and/or full removal of excessive precursor and the
ALD-reaction by-products.

Accordingly, some implementations) of the present inven-
tion utilize target materials (the materials used for fabrica-
tion of optical metasurface-based components and devices)
that lend themselves as optical nano-antennas operating on
the scale of a UV and/or deep-UV spectral regimes, and have
such materials arranged on a surface of a chosen substrate
(which, in one case, may be a planar surface). In substan-
tially any implementation, a specific dielectric target mate-
rial is used, for example hafnium oxide (HfO,). In other
embodiments, other dielectric materials may be suitable. In
certain example embodiments, dielectric materials are
selected to have a low extinction coefficient and a relatively
large refractive index as compared with materials used in the
visible, near-UV, or mid-UV spectral ranges, as well as
material compatibility with CMOS processing.

Some specific embodiments of the present invention
enable optical elements configured to perform multiple
functions (such as optical multiplexing, for example, which
conventional UV optical technology cannot easily provide).
In various examples of embodiment described below, a
spin-multiplexed hologram projection is demonstrated that
operates at 364 nm (near-UV range) and 266 nm (deep-UV
range), as well as spin-controlled accelerating beam genera-
tion at 364 nm.

For the purposes of this disclosure and appended claims,
and unless expressly defined otherwise, the term “electro-
magnetic metasurface” (and, in particular, “optical metasur-
face”) is defined as and used to refer to a planar array of
artificially shaped material structures of subwavelength
dimensions in height, width, and packing distance, that
collectively emulate the operation of a conventional refrac-
tive, birefringent, or diflractive optical component (such as,
but not restricted to, a lens, waveplate, grating or hologram),
through individually tailored amplitude, phase, or polariza-
tion transformations of an incident electromagnetic wave
(light).

Contriving the Material Choice and Fabrication
Methodology

In one non-limiting implementation, realized metasurface
devices included of HfO, nanopillars of either circular or


US 11,885,979 B2

19

surface is configured to operate, in transmission of light at
the operational wavelength, as at least one of a refractive
optical element, a diffractive optical element, a birefringent
optical element, and a resonant optical element. In at least
one of the implementations, a cylindrical pillar in the array
may be dimensioned as an elliptic cylinder and the spatial
period does not exceed the operational wavelength to not
have light at the operational wavelength, incident onto the
metasurface, diffract upon transmission through the meta-
surface. In substantially any embodiment, a minimum diam-
eter of different diameters of the cylindrical pillars has a
value of about 50 nm while a maximum diameter of such
diameters has a value of about 40 nanometers smaller than
the spatial period such that a first area of the array (the one
with the smallest filling factor available across the metasur-
face) has a first effective refractive index for said light
transmitted therethrough while a second area of the array
(the one with the largest filling factor available across the
metasurface) has a second effective refractive index for such

light transmitted through the metasurface. (Here, the first 2

effective refractive index has a value of about one and the
second effective refractive index has a value of about 2.1.)
in practice, any embodiment of such metasurface can be
dimensioned to induce a first phase shift for light that
propagates through a first auxi area of the array char-
acterized by the smallest available filling factor and a second
phase shifi for said light that propagates through a second
auxiliary area of the array characterized by the largest
available filling factor (here, the first phase shift is about
zero while the second phase shift is about 2z. In at least one
case, the first area of the array and the first auxiliary area of
the array substantially coincide with one another and/or the
second area of the array and the second auxiliary area of the
array substantially coincide with one another.

Section V

Characterization procedure for metalenses Ls, and L525.
A continuous wave (CW) laser beam (diameter: =5 mm) was
used to illuminate either metalens L,,, (using wavelength
207364 nm) or metalens L525 (using wavelength 4.=325
nm) at normal incidence, yielding a focused spot in the focal
plane of the metalens. The intensity distribution of the image
in this plane was captured using a custom-built imaging
system including an NA=0.75 objective and an BMCCD
camera. The magnification of the system, characterized at
each wavelength by translating the focal spot within the field
of view of the objective using a calibrated stage, was
measured to be 420 and 617 for 49-364 and 325 nm,

respectively. The physical size of the focal spot projected by 5

metalenses L34, and L325 was then derived based on the
CCD pixel size and magnification calibrated for respective
operating wavelengths.

Section VI

Characterization procedure for erating beam
generators B3¢q and B35. A continuous wave (CW) laser
beam (diameter: ~5 mm) was employed to illuminate either
self-accelerating beam generator B35, (using wavelength
27364 nm) or generator B33, (using wavelength 4.32.5
nm) at the normal incidence. The intensity distributions of
the generated self-accelerating beams in selected z-planes
beyond the metasurface were recorded using custom-built
imaging system including an NA=0.75 objective and an
EMCCD camera. Specific z-planes were addressed using a
stage which translated the metasurface relative to the cam-

0

3

&

s

20

era, along the direction of the laser beam. A 500-jm-
diameter aperture was placed behind each metasurface sub-
strate to completely separate the generated self-accelerating
beam from directly transmitted light leaking around the
edges of the metasurface (which occupied a square area of
side length 270 jm). The physical size of the captured
intensity profile was determined by comparison to the image
of the area occupied by the metasurface.

Section VII

Devising phase shift profiles for metaholograms and Hy.
Hya5 Hago- The Gerchberg-Saxton algorithm was employed
to calculate the respective metasurface phase shift profiles,
364%, ¥s bo )s Pra5(X Ys ho), and 256K, ¥; ho), required
to produce a holographic “NIST” image at three different
operation wavelengths 4)=364 nm, 325 nm, and 266 nm
(shown in FIGS. 16A, 16B, and 16C and implemented,
respectively, by metaholograms Hyg, Hyo5, and Hyg)

Section VIII

Characterization procedure for metaholograms Hy, and
H,,5. A continuous wave (CW) laser beam (diameter: ~5
mm) was used to illuminate either metahologram Hyo,
(using wavelength 4-364 nm) or metahologram Hy, (us-
ing wavelength 4-325 nm) at normal incidence. An
EMCCD camera was placed in the hologram formation
plane located 40 mm beyond the metahologram, to directly
record the projected holographic image. A 500-um-<diameter
aperture was placed behind the metasurface substrate to
completely separate the generated holographic image (pro-
jected with a 3-mm lateral offset) from directly transmitted
light leaking around the edges of the metasurface (which
occupied a square area of side length 270 um)

Section IX

Characterization procedure for metahologram Hy65- A
normally incident, 266-nm pulsed laser light (pulse
duration: ~5 ns, repetition rate: 10 Hz, beam diameter: ~6
mm) was used to directly illuminate the polarization-inde-
pendent metahologram H,,,. A piece of fluorescent white
paper was placed in the hologram formation plane located
40 mm beyond cach metahologram and imaged with a
custom-built imaging system including a lens and a CCD
camera. Holographic images were recorded for device Hogg
under linearly-polarized illumination. 500 images each were
recorded and averaged, in each case, with and without laser
illumination to subtract background and dark counts from
the recorded image as well as to reduce random image noise.
A 500-um-diameter aperture was placed behind each meta-
surface substrate to completely separate the generated holo-
graphic image (projected with a 3-mm lateral offset) from
directly transmitted light leaking around the edges of the
metasurface (which occupies a square area of side length
270 um). The physical size of the captured holographic
image was determined by comparison to the image of an
object of known size placed in the plane of the fluorescent
white paper.

To verify that the utilized fluorescent transduction scheme
is linear, images were recorded under illumination with
different laser powers. The ratio of the fluorescence power
integrated over the holographic image to that integrated over
the image of the directly transmitted beam was found to be



US 11,885,979 B2

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US 11,885,979 B2

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15
nate system in which the constituent metasurface is located
in the 2=0 plane and the Ist xy quadrant, with one corner
positioned at the origin. Two distinct phase shift profiles,
9 (x, y, Ag)=mod (

Se cle
mm ews?

2) and p*P(x, y, Ag)=mod (

8 3
St ras(a- wt se-
xo 5 ( JE+U-y

2n), were targeted for device operation, in order to yield
SABs exiting the metasurface from opposite corners and
following different trajectories, y=x=—d,=-1.6 2°, and
(y-D=(x-N=d,=2.25 27, under LCP and RCP illumination,
respectively (as shown by 510 and 512 in FIGS. 5A, 5B),
The measured lateral displacement values, d,(z) and d,(z),
were observed to closely match, in each case, the calculated
values based on the targeted trajectory (illustrated by respec-
tive plots of FI
ated by the device exhibited diffraction-free characteristics
with icy-plane intensity distributions (shown in FIGS. 5E,
5F) similar to the targeted intensity distributions (discussed
in Supplementary Information, Section XD. The targeted
distributions, in turn, were numerically computed assuming
an ideal metasurface realization having both the designed
phase shift profile @’” (@*) and unity transmittance T.
The measured efficiency under LCP illumination [or RCP
illumination, respectively]), defined as the ratio of the total
optical power of the SAB in the z=4.5 mm [or z=10.5 mm]
plane to the total power illuminating the metasurface, was
(38.42+1.95) % [or (61.90+2.03) %]. The efficiency of the
beam generator under LCP illumination, that was lower than
that demonstrated under RCP illumination, could be attrib-
uted to challenges associated with implementing a phase
shift profile of higher spatial gradient.

Example 5 demonstrates a spin-controlled metahologram
operating at the same near-UV wavelength of 364 nm. In
reference to FIG. 6A (displaying two related images), the
implementation 600 of a 330-ym-square metahologram,
Hy,'?"", located in the 2=0 plane, was designed to project a
holographic “NIST” image (for LCP illuminating light) and
“NJU" image (for RCP illuminating light) at 1,=364 nm, all
located in the xy-plane at 2=40 mm, with an offset of y=-3.
Corresponding phase shift profiles are illustrated by plots
discussed in Supplementary Information, Section XII, FIGS.
19A, 19B). Both experimentally captured holographic
images shown in FIG. 6B correctly replicate the shape of the
corresponding targeted images computed from the designed
phase profiles, including some fine grain details (as dis-
cussed in Supplementary Information, Section XII, FIGS.
20A, 20B). The measured efficiencies, defined as the ratio of
the total optical power of the holographic image to the total
power illuminating the metahologram, were (54.02+2.22) %
(under LCP illumination) and (53.76+2.42) % (under RCP
illumination), respectively.

Example 6: Yet in another related embodiment 650, a
spin-multiplexed metahologram, Hy", occupying a
square area of side length of 320 um, was demonstrated to
operate at the deep-UV wavelength of 266 nm. The device,
located in the 2=0 plane, was designed to project, at 49=266
nm, a holographic “deep” image for LCP illumination and a

5C, 5D). The experimental SAB gener- 2:

s

8

8

5

s

3

16

holographic “UV” image for RCP illumination, where the
targeted location for each of these images was defined as
7=40 mm plane with a lateral offset of y=-3 mm (see FIG.
6C). Corresponding phase shift profiles are plotted in
Supplementary Information, Section XIIL, FIGS. 21A, 21B).
In practice, each of the experimental holographic images,
displayed in panels of FIG. 6D, faithfully replicated the
shape of the corresponding target image (Supplementary
Information, Section XIII, FIGS. 22A, 22B), including
subtle details of the chosen font, such as linewidth variation
and serif. The measured efficiencies, defined as the ratio of
the total fluorescence power of the holographic image to the
fluorescence power of light illuminating the structure, were
(58.9541.95) % under LCP illumination, and (61.23+1.49)
% under RCP illumination.

A person of ordinary skill in the art readily appreciates
that metasurface-based systems containing a combination of
one or more of the above-described metasurface devices can
be produced with embodiments of the invention. Therefore,
as disclosed in the above Examples, embodiments of the
invention provide a method for fabricating an all-dielectric
metasurface optical system (that includes at least one of a
polarization-independent metalens, a polarization-indepen-
dent metahologram, and a polarization-independent Airy
beam generator; with devices in such system having optical
transmittance of at least 50% at a wavelength of 364 nm or
shorter). The method utilizes hafnium oxide to conformally
fill an open volume; formed in a patterned layer of an
electron-beam resist material, using an atomic layer depo-
sition conducted at a temperature not exceeding 105° C. In
a specific implementation of the method, the utilization of
hafnium oxide according to an idea of the invention results
in a metasurface device with optical transmittance of at least
70%.

Supplemental Information
Section I

Additional Details of Specific Implementations of a Meta
surface Fabrication Process. As the first step in the meta-
surface fabrication process, 500-pm-thick, double-side-pol-
ished UV-grade fused silica substrate was vapor-coated (at
150° C.) with an adhesion-enhancing monolayer of hexam-
ethyldisilizane (HMDS). A layer of ZEP 520 A resist was
spin-coated onto the substrate, followed by baking on a hot
plate at 180° C. for 10 minutes. The spin speed was adjusted
to yield a resist thickness varying between 400 mm and 550
mm (as characterized by spectroscopic ellipsometry),
depending on the specific metasurface design. To suppress
charging during the electron beam (e-beam) lithography, a
20-nm-thick Al layer was thermally evaporated onto the
ZEP layer (deposition rate: 0.1 nm/s). The ZEP-resist tem-
plate was then defined using e-beam lithography (acceler-
ating voltage: 1.00 KV; beam current: 0.2 nA), followed by
Al layer removal (AZ 400K 1:3 developer: 2 minutes; DI
water: | minute) and resist development (hexyl acetate: 2
minutes; isopropyl alcohol: 30 seconds). Deposition of HfO,
to a thickness of 200 nm (deposition rate: 0.11 nm/cycle)
was then performed using the low-temperature ALD process
detailed immediately below. The deposition thickness was
chosen to provide a substantial over-coating of all e-beam
patterned resist features and yield a quasi-planar top surface.
Following the ALD, the newly-formed HfO, layer was
back-etched to the resist top surface using argon (Ar) ion
milling (HfO, mill rate: =0.4 nm/s). Finally, the remaining
resist was removed by soaking in a solvent, yielding circular



US 11,885,979 B2
Page 2

lithography, imaging, spectroscopy, and quantum informa-
tion processing,

13 Claims, 16 Drawing Sheets

(51) Int. CL.

€23C 16/04 (2006.01)
GO2B V/118 (2015.01)
€23C 16/455 (2006.01)
(52) 1
. €23C 16/45527 (2013.01); C23C 16/45553
(2013.01); G02B 1/118 (2013.01)
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US 11,885,979 B2

21

invariant as a function of incident laser power, over the full
range of power levels used for the experiment.

Section X

Design of spin-multiplexed metasurfaces. The spin-mul-
tiplexed metasurfaces implemented in this work imparted
independent phase shift profiles on an incident beam
depending on its handedness, namely left-handed circularly
polarized (LCP) beam of light and right-handed circularly
polarized (RCP) beam of light. Let the LCP and RCP states
of polarization of incident light be represented by Jones
Vectors IL) =[_,'] and IR) =[,'], respectively, and the wave
transformation characteristics of the spin-multiplexed meta-
surface be described by a Jones Matrix J(x, y).

I(x, y) then satisfies the pair of equations

(aoe = Ja YL) aw
PL) = 0%, »R) @

It where @,(x, y) and ©,(x, y) denote the phase shift
profiles of the emerging right-handed and left-handed light,

for left-handed and right-handed illumination light wave- ,.

fronts, respectivel
The Jones Matrix J(x, y) can then be written as:

8)

Such a Jones Matrix can be realized using uniaxial
(geometrically birefringent) nanostructures providing differ-
ent phase shifts along their two orthogonal principle axes
(here denoted as I and If)’. In this study, we use HfO,
nanopillars of in-plane elliptical cross-sections to provide
such birefringent functionality. To implement the Jones
Matrix of Equation (3), the phase shifts experienced by the
constituent linear polarization components of the circularly-
polarized incident light decomposed along principal axes I
and II must satisfy 8(x, y)=[9,(x. yHPa(x, y)I/2, and 8,/(x.
y)=-R+L9,(x, YH2(X, y)/2, respectively. In addition, the
orientation angle@(x, y) of the cylinder’s principal axis IT
must satisfy:

ux,

110% YI“Male, yA @

To design spin-multiplexed metasurfaces operating at
9o=364 nm, the transmittance and phase shift for propaga-
tion of 364-nm-wavelength light, linearly-polarized either
(1) parallel to one principle axis I (T, and A,), or (2) parallel
to the other principle axis II (T, and A,) of an array of
elliptical HfO, pillars (of orthogonal principle axis lengths
D, and D,, height H, and lattice spacing P) were computed
using the finite-difference-time-domain (FDTD) simulations
with periodic boundary conditions. For a given pillar height
H and lattice spacing P, D, and D, are iteratively varied to
identify orthogonal principle axis combinations simultane-
ously leading to |A,-A,l=nand T,=T>, in other words,
half-wave-plate-like operation. To confine the search to a
computationally reasonable parameter space, the targeted
values of A, were restricted to eight discrete, equally spaced
values spanning the full 0 to 2m range (modulo 2m), required
for arbitrary hologram implementation. The above con-
straints were found to be achievable with eight different
nanopiliar arrays, each of which were composed of elliptical
nanopillars (denoted as C,*** to C,*¢") having uniform

s

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8

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22

height H=500 nm, uniform spacing P=330 nm, and discrete
principle axis combinations, (D,, D,), expressed in nano-
meters, of (70, 190), (80, 240), (90, 260), (100, 270), (190,
70), (240, 80), (260, 90), and (270, 100), respectively (where
half of the set is mathematically degenerate under an in-
plane coordinate system rotation by 90°). The simulated
orthogonal principal axis transmittance combinations, (T,,
T.), were (93.4%, 62.5%), (93.1%, 68.19%), (95.6%, 61.4%),
(89.1%, 51.5%), (62.5%, 93.4%), (68.1%, 93.1%), (61.4%,
95.6%), and (51.5 %, 89.1%) for arrays composed exclu-
sively of pillars C,**" to C,**", respectively. The simulated
orthogonal principal axes phase shift combinations, (A,, A),
were (0.82, 2m), (1.35m, 0.41n), (1.582, 0.67m), (1.87m,
0.87n), (2x, 0.82x), (0.417, 1.35n), (0.67R, 1.58), and
(0.87, 1.877) for the corresponding arrays.

To implement each spin-multiplexed metasurface operat
ing at 364 nm (metahologram H,,,°”" and self-accelerating
beam generator B3,,°”), the required phase shifts induced
along the two orthogonal principle axes of an elliptical pillar
of center position (X.. Yo) OlXe Yel and O/[X.. Yel. were
calculated as O/fx,, yJ=mod((P, (Xo YHPa%» ¥)/2, 2m)
and O[x.. ¥el=mod (-T+(1(X.. YI+OXe. Yo)V/2, 2m),
respectively. 6, was then compared to each element ,.,; of
a discretized reference phase set 0,={m, 1.25, 1.5m,
1.75mr, 2,0.252, 0.25m, 0.52, 0.751}, in order to identify
the closest element to C (in other words, the index i such
that 10,,;-0,|$0.125n is satisfied). The resulting index i then
identifies the specific nanopillar Ce {C,>TM, . .., Cg°"}
providing the optimal choice for metahologram constituent
nanopillar at position (x,, y,). For each of such choice, the
orientation angle @[x,, y,] of the cylinder’s principal axis IL
is set to O[x,, yJ-mod((,(X_- ¥)- res ¥-)V/4, 2m)

Following the same design procedure, the spin-multi-
plexed metasurfaces operating at A,=266 nm can be imple-
mented using a pillar library which consists of eight distinct
elliptical nanopillars (denoted as C,°° to C,7°°) having
uniform height H=480 nm, uniform spacing p=160 nm, and
eight discrete principle axis combinations, (D,, D,),
expressed in nanometers, of (118, 58), (126, 62), (126, 70),
(118, 50), (58, 118), (62, 126), (70, 126), and (118, 50)
respectively (where half of the set is mathematically degen-
erate under an in-plane coordinate system rotation by 90°).
The simulated orthogonal principal axis transmittance com-
binations, (T,, T,), were (93.4%, 99.5%), (94.4%, 98.5%),
(94.8%, 94.9%), (97.8%, 96.6%), (99.5%, 93.4%, (98.5%,
94.4%), (94.9%, 94.8%), and (96.6%, 97.8%) for arrays
composed exclusively of pillars C,76° to C,°°, respectively.
The simulated orthogonal principal axis phase shift combi-
nations, (A,, A,), were (1.02, 27m), (1.217, 0.26n), (1.54m,
048m), (1.742, 0.8%), (2m, 1.02n) (0.26n, 1.21) (0.48x,
1.54m), and (0.87, 1.74) for the corresponding arrays. The
two independent phase shift profiles associated with the
spin-multiplexed metahologram, Hyg”, were imple-
mented by choosing, at each position of the metasurface, the
optimal nanopillar C,7°°e {C,2%, .. . Cs2°°}, according to
the algorithm outlined in earlier part of this section.

Section XT

Characterization procedure for spin-multiplexed self-ac-
celerating beam generator B.,,,. To characterize the spin-
multiplexed self-accelerating beam generator Byg;°”", a
normally incident, 364-nm continuous wave (CW) laser
beam (beam diameter: =5 mm) was used to illuminate the
device, and a linear polarizer and a quarter-wave plate (of
center wavelength 355 nm) were used in addition to control
the handedness of the illuminating light. The intensity



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