1. A 3" Dia. High Efficiency Diffractive Corrector for
Head Up Display
S. Blit, B. Kobrin and Y. Malkin
Holo-Or Ltd., POBox 1051, Rehovot 76114, Israel
Abstract
A 3" diameter refractive-diffractive element is presented. The element is
used as a corrector in the optical system of a head up display at 545 nm.
Average grating density of the design was 6 cycles/mm peaking to 10
cycles/mm. A 4 mask 14 level design was utilized, having a theoretical
efficiency of 98.3%. The diffractive pattern was etched by RIE on a plano-convex
fused silica lens, and then AR coated. Mask design and
manufacturing process considerations are given and discussed. Tolerances
and fabrication errors are analyzed. Micron mask misalignment and duty
cycle errors, etch depth error of and uniformity were achieved
over the entire area, yielding a total efficiency of the element of >96%.
Diffraction measurements were made and compared to the theoretical
calculations. The design and fabrication results prove diffractive elements
in the visible a viable solution to correct aberrations in high quality optical
systems such as head up displays.
1. Introduction
The ability of diffractive optical surfaces to alter phase fronts together with
their strong dispersion make them alternatives to the optical designer to
utilize in optical systems. However, utilization of the theoretical potential
require manufacturing technologies to be developed where the effects of
tolerances would not interfere with the functionality of the elements. There
are several methods for manufacturing circular symmetric diffractive
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2. surfaces: diamond turning machining, photolithography and etching, either
of continuous proffles or of multilevel stepwise profiles, [1] [2] . Diamond
turning macliming is able to reconstruct accurate diffractive surfaces, both
in proffle shape and in surface quality. However, due to the limit of the
diamond tip size, and the inability to machine glasses, it is practical only
for IR optics. The photolithographic methods are applicable also in the
visible and even UV wavelengths, but the theoretical performance is
tougher to meet as the wavelength becomes shorter. In both methods
manufacthring tolerances are to be driven to the limit to achieve practical
performance. In most studies the efficiency is cited as the figure ofmerit of
the diffractive optics. The highest reported efficiencies are 94% for binary
element and 96% for a direct write element, quoted for microlenses or two
dimensional elements such as fan-out gratings [2] [3]. Most of the reports
of recent years quote efficiencies of 80 to 90% for diffractive microoptics.
Manufacturing tolerances and their effect on the performance are reviewed
by Cox et al.[4]. The most critical parameter in a binary process is mask
alignment. Misalignment of the order of 1% of the period (zone ) width
already causes a sharp drop in efficiency. A discussion of experimental
tolerances is given by Fersti et al. [3]. The best reported values for
misalignment in microlens manufacturing are about 0.5 micron.
In comparing performance of diffractive elements, the term "efficiency" is
regularly used. Never the less, when the DOE are implemented in imaging
systems, one has to consider the distribution of energy which is not
diffracted to the desired order, which creates ghost images, reduce contrast
by scattering etc.
Manufacturing a 3" dia. diffractive corrector for the visible (0.545 micron)
imposes a challenge since the optical demands of the HUD application are
high: wave front accuracy across the element of tenth wave, ghost images
below 1%, and very low scattering. Those demands were met using
multilevel photolithographic manufacturing process, minimizing fabrication
errors. Hereby we present the results together with manufacturing
considerations. It is shown that with fairly standard equipment macro
binary DOE in the visible can be manufactured which meets optical system
demands.
2. Design of the diffractive surface
The optical design of the system [5] defines the substrate, material and
physical surface, and phase front correction at a particular wavelength.
The zone frequency along the radius of the element is depicted in Fig. 1.
374 / SPIE Vol. 2426

3. 10: I
7. —
E 6
I
1 2 3O 3 40
Radial coordinctie (mm)
Fig I : Zone frequency of the corrector
Given the zone density to achieve the phase correction and assuming
achievable tolerances it was decided to utilize a 4 mask scheme having 14
levels. Calculating the diffractive power distribution using a linear grating
approximation, the diffraction order to be utilized is +1 having 98.3% of
the diffracted power, with next higher orders of +15 -13, having 0.4% and
0.6% of the power, correspondingly. Higher orders are below 0. 1 % each.
The method to design the masks is described in details in ref. [6]. In the
mask design care should be given to the alignment marks geometry since
all four masks have to be aligned with one another with sub micron
accuracy. Masks were fabricated with standard Ebeam machines with
having 0.2 micron resolution and micron positioning accuracy
across the mask.
3. Fabrication of the diffractive surface
Fabricating a large diffractive surface on a 3" macro lens using standard
equipment for electronic wafer photolithography imposes some specific
problems to be addressed: a) holding and fixing a piano-convex lens
during spinning, contact mask alignment and etching processes. The most
critical process is the mask alignment; this was done using a Karl Suss
model MJB3 mask aligner. It is vital to maintain high parallelism between
tl1e mask and the substrate surface, at a separation of less than 20 microns.
Moreover, the mask aligner mechanism and the lens holder have to
maintain constant lateral position of lens surface during the alignment
SPIE Vol. 2426 / 375

4. procedure and transition from alignment to contact for exposure. b)
Visibility of the alignment marks on low reflectivity highly polished ftised
silica surface. For increasing the contrast of the alignment marks critical
features CCD based monitoring system was applied together with chrome
evaporated alignment marks on the substrate. The last process included
chrome coating, resist patterning and chrome wet etching through
windows in the resist. c) Optimization of repeatable etching conditions for
thick (104 5mm ) lenses at sufficiently high etching rate maintaining high
uniformity and surface quality. Shipley photo resists were used throughout.
Fluoride based gas mixture was used for etching in a Reactive Ion Etching
system with a 1 70mm base electrode, automatic pressure control and
13.56 MHz RF-power generator (Oxford Plasma Technology, Model
Plasmalab ).The resist was stripped after each etching step and the profile
was measured in a stylus-based profilometer with a vertical accuracy of
5A. Besides, surface quality was inspected at each step by optical
microscopy. The finished surface diffractive performance was measured
using a doubledYAG laser beam, at various loci on the diffractive surface,
using the system described in Fig 2. The finished lens was AR coated and
was integrated in the HUD optics. Measurements of the spurious images
resulting from the higher orders were taken using a photometer.
DOE
O.532
power
Diffracted Orders
Fig.2: Measurement system for diffraction performance.
4. Results and discussion
Fig. 3 depicts profilometer traces of the diffractive surface. With the lateral
resolution of the instrument, the multilevel cross-section looks laterally
"flawless". Etching depth error and unifonnity as measured with the
profilometer are given in table 1.
376 /SP!E Vol. 2426

5. RH
SCAH' 6€iBuM
ø6-B8-94 SPEED : LOW
lee 200 OO
CUR: 566G A t 239uM
ct.IR:—,3c35 A e 22ø,M
Fig. 3 : Profilometer traces
VERT*-18,971 fl
HORI2:—19M
______ IB ,000
______ 8008
_____ 6088
_____ 4,800
_____ 2080
______ 0
_____ -2,008
______ -4, 000
_____ -b 808
40€i 500 688
M CURSOR 220
SLOAt1 DEKTAJ< II
Step Mask# çpth Error Uniformity
1 M4
2 —
M3 +6%
3
M2 +6%
4 Ml 0
It can be seen from the table that during all steps the uniformity and depth
error, which are not independent, were below 7%. No correlation was
found as to the location on the diffractive surface and the resulted
accuracy. Fig. 4 gives a diagram of the lateral fabrication enors in each
step, as measured with a microscope.
M3O.5
M2=o.8
Mt 1.0
—
R:
— —---——
:r ': —- H .. J__
—- -- — - —
Table 1 : Depth error and uniformity
M3o.8 A
41 =O.5
M3 =0.8
M2=0.8
Ml =0.8
M3==1.0
M2=1.0
Ml =0.8
Fig. 4: Optical microscopy measurements of pattern errors (p.)
SPIE Vol. 2426 1377

6. It should be noted however that optical microscopy measures the
combined image of misaligmnent, duty cycle error and waIl
perpendicularly. Hence, in order to isolate the misalignment error, the
optical measurement has to be correlated with profilometer traces. It can
be seen from the diagram that the total errors are below 1 micron. Since
both duty cycle and wall perpendicularity errors are "scalar" and
misalignment error has a direction, by comparing the pattern errors at
opposite locations with respect to the center of lens, it can be deduced that
the alignment errors are below 1 micron.
Diffractive performance data is drawn on Fig. 5 . for the highest zone
frequency area of 9 cy./mm. It can be seen that the diffracted power
distribution agrees with theory. The high orders are all below 1 % each of
the main +1 order.
. •••'•
.. ; lOOOLEI:i
—
A—Theory
F
-
TAT±[E1LL 1iJ4W !I!
Et 1
100
10 z
0.1
—100 —80 —60 —40 —20 0 20 40 60 80 100
ORDER NUMBER
Fig.5: Difftacted power disribution at 9cy./inm
The distribution gives also distinct peaks/orders where theory predicts
zeros. These orders are below 0.2% of the main peak and result from
fabrication errors. The total efficiency of the diffraction grating at the point
of measurement of Fig. 6 was measured to be 95% corresponding to a
greater value for the complete diffractive surface. Measurements of the
CRT pattern in the HUD system yielded a value of 0.5% each for the two
spurious images resulting from the higher orders.
cii
378 ISPIE Vol. 2426

7. 5. Conclusion
A 3 " diameter multilevel diffractive corrector at a wavelength of 0.545
micron on fused silica has been fabricated. The element exhibited high
efficiency (>96% ), low higher orders (<0.7% ) and no scattering which
makes it adequate as a component to be used in a high demand optical
system such as a head up display. The photolithography/etching
fabrication process is suitable for quantity production. Thus, large aperture
diffractive elements in the visible may join the arsenal of tools of the
optical designers to efficiently correct aberrations in high quality optical
systems.
Acknowledgment: We wish to thank Mr. Yoel Blumen.feld of ElOp Ltd.
for helpful discussions and assistance in system performance
measurements.
6. References
[1] M. Curcio: 'Diamond machining of infrared optics utilizing two-axis
machine technology", SPIE, v.306, 105(1981).
[2] P. Langlois, H. Jerominec, L. Leclerc, J. Pan: "Diffractive optical
elements fabricated by laser writing and other techniques", SPIE v.1751,
2(1992).
[3] M. Fersti, B. Kuhlov, et a!.: "AR coated arrays of binary lenses for
interconnection networks at 1.5 micron", SPIE, v.1992, 90(1993).
[4] J.A.Cox et a!.:" Diffraction efficiency of binary optical elements", SPIE
v.1211, 116(1990).
[5] Engineering Drawing, El-Op Electrooptics Ltd.
[6] J. Kednii, I Grossinger: "Diffractive Optical Elements", US patent
5,073,007 and US patent 5,227,915.
SPIE Vol. 2426 / 379