State-of-the-art diffractive optics are fabricated using e-beam lithography and dry etching techniques to achieve multilevel phase elements with very high diffraction efficiencies. One of the major challenges encountered in fabricating diffractive optics is the small feature size (e.g. for diffractive lenses with small f-number). It is not only the e-beam system which dictates the feature size limitations, but also the alignment systems (mask aligner) and the materials (e-beam and photo resists). In order to allow diffractive optics to be used in new optoelectronic systems, it is necessary not only to fabricate elements with small feature sizes but also to do so in an economical fashion. Since price of a multilevel diffractive optical element is closely related to the e-beam writing time and the number of etching steps, we need to decrease the writing time and etching steps without affecting the quality of the element. To do this one has to utilize the full potentials of the e-beam writing system. In this paper, we will present three diffractive optics fabrication techniques which will reduce the number of process steps, the writing time, and the overall fabrication time for multilevel phase diffractive optics

Daschner, Walter

Kress, Bernard

Lee, Sing H.

Stein, Robert

Urquhart, Kris

Zaleta, David

English

NASA Technical Reports Server

N94- 17347
Diffractive Optics Fabricated by Direct Write Methods
with an Electron Beam.
Bernard Kress, David Zaleta, Walter Daschner, Kris Urquhart,
Robert Stein, Sing H. Lee.
Electrical and Computer Engineering Department
University of California at San Diego
La Jolla, Ca. 92093-0407
USA
1. Introduction.
State-of-the-art diffractive optics are fabricated using e-beam lithography and dry etching
techniques to achieve multilevel phase elements with very high diffraction efficiencies.
One of the major challenges encountered in fabricating diffractive optics is the small
feature sizes required (e.g. for diffractive lenses with small f-number). It is not only the
e-beam system which dictates the feature size limitations, but also the alignment systems
(mask aligner) and the materials (e-beam and photo resists). In order to allow diffractive
optics to be used in new optoelectronic systems, it is necessary not only to fabricate
elements with small feature sizes but also to do so in an economical fashion. Since price
of a multilevel diffractive optical element is closely related to the e-beam writing time
and the number of etching steps, we need to decrease the writing time and etching steps
without affecting the quality of the element. To do this one has to utilize the full
potentials of the e-beam writing system.
In this paper, we will present 3 diffractive optics fabrication techniques which will
reduce the number of process steps, the writing time and the overall fabrication time for
multilevel phase diffractive optics.
2. Conventional fabrication technioue.
Most multilevel phase surface relief structures are fabricated by utilizing n binary
amplitude masks in n standard photolithography processes to achieve 2 n relief levels [|].
A standard photolithography process cycle for a single mask consists of the following
steps (Fig.I):
1) Spin photo-resist on substrate
2) Place substrate in contact with chrome mask (or project mask pattern onto
substrate), and illuminate with U-V radiation.
3) Process substrate to remove resist in exposed regions (for positive resist).
4) Place substrate in ion milling machine to create relief patterns.
5) Remove old resist.
Conf. on Binary Optics, 1993
195
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Thus for a 16 phase level diffractive optics, 20 (=4x5) processing steps are required to
complete the fabrication. Typically, this method allows fabrication of structures down to
1 micron (mainly limited by mask aligner type and the number of re-alignments required
between masks). It is easy to see that this fabrication technique fs very laborious and time
consuming, and subjecft0 many processing errors, w_ic_ directly influence the cost Of
production. This is because this fabrication technique does not fully utilize the power and
capabilities of the e-beam writing system.
3. Various improvements on the fabrication of multilevel relief structures using e-
beam direct write technklues.
3.1) Direct write on e-beam resist.
In order to decrease the misregistrations between mask alignments, we have developed a
fabrication technique which does not rely On amplitude masks. The different binary
patterns a?e directly aligned and written on the e:beam resist On the s_strate in the e-
beam machirie_ Between each e-beam writing steps, a ph3tollthographic and an etching
step are involved (see Fig.2). Since the e-beam machine is capable of automatically
locking onto etched alignment marks and aligning the next pattern with respect to these
marks, the alignment misregistrations errors are decreased to 0.1 micron. This allows the
user to fabricate multilevel relief structures with features down to 0.5 micron. However,
even though the misregistrations errors and the feature sizes are decreased, this method
still requires long fabrication time because the substrate needs to be recoated with
chrome and entered in the e-beam n times (for 2 n relief levels).
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3.2) Direct write on e-beam sensitive photoresist
In order to cut down the processing time, we used a photoresist which is sensitive to
electron beams (Hoechst AZ-5214-E photoresist) in place of the e-beam resist. Fig.3
shows a comparison of the number of photolithographic processes used by each method.
Since the photo resist can be exposed by the e-beam writer and then directly used in the
etching process, the photolithographic steps between e-beam writing and ion milling are
eliminated.
3.3) Direct write with various e-beam spot sizes.
To reduce the e-beam writing time, we have utilized a variable e-beam spot size to write
fringes or shapes of different sizes. Since many diffractive optics patterns are composed
of e-beam shapes whose sizes vary considerably over the aperture, these patterns can be
196
written moreefficiently in time by increasingthe spotsizeto write coarserfeatures,and
decreasingit to write finer features.
Although thee-beamhasto run n different patterns for n spot sizes, the writing time can
be decreased considerably if the partitioning between large and finer features is made in
an efficient way. Fig. 4 shows a spherical on-axis zone plate whose shapes have been
partitioned into two different files to be written with two different spot sizes. Here, the
first spot size is of 0.1 micron and the second of 0.2 micron. The gain in writing time is
about 35%. The maximum spot size is limited by the maximum beam current (the larger
the spot size, the higher the current): for a spot size of 0.1 micron, the beam current has
to be set at least to 8 nA while for a spot size of 0.4 micron the beam current is only of
100 nA. More complex patterns can be further partitioned into more sub-patterns to be
written with spot sizes ranging from 0.025 to 0.8 micron. However, the amount of time
required to set the exact e-beam current for the intended spot size manually by the e-
beam operator when using more than two spot sizes has also to be considered.
3.4) Direct write with various e-beam dosages on analog e-beam resist.
In order to significantly decrease the fabrication steps and to enable the fabrication of
features down to 0.1 micron, we used a direct write method where the e-beam exposure
on an analog resist varies over the desired pattern. This method requires a single e-beam
run and a single etching process (with no photolithographic step) to fabricate a multilevel
phase CGH. In standard e-beam lithography, an e-beam exposure contains enough
electron dose in the exposed regions of the e-beam resist to fully clear the resist during
the development process (for a positive e-beam resist). If the electron dose and/or the
development process is reduced, it is possible for the e-beam resist to not fully develop.
This is possible since the solubility of the resist in the developer varies with the electron
dose, i.e. the higher the dose, the faster the resist dissolves in the developer. This allows
different thickness control of e-beam resist simply by varying the electron dose. Fig.5
shows the large decrease in fabrication steps when using this technique. The
misregistration errors are decreased to the e-beam misregisration in field addressing,
which is typically of 0.025 micron. Thus the fabrication of features down to 0.1 micron
is only limited by the resist development, and no more by the alignment accuracy.
Two types of e-beam substrates have been used; one for reflection mode and one for
transmissive mode diffractive optics. For reflection mode, a standard chrome on glass e-
beam plate was used. For transmissive mode, an e-beam plate with an optically
transparent layer of ITO (indium-tin-oxide) was used in place of the chrome. Two
positive analog e-beam resists have been successfully used: EBR-9 (Toray) and PMMA
(KTI 950K 9%). Since EBR-9 could not be coated thicker than approximately 500nm, its
use was limited to reflection mode diffractive optics. PMMA was used for thicker layers
(up to 2 microns thick), and was successfully applied to transmissive mode diffractive
optics. The amounts of EBR-9 and PMMA resist removed by the developer are shown in
Fig.6.
197
4, Experimental results.
Several CGHs have been fabricated by using the different e-beam direct write methods
described in this paper. Diffractive cylindrical and spherical lenses have been fabricated
using the direct write on e-beam resist discussed in section 3.1 (see Fig.T). A set of linear
gratings has been fabricated in ana[0g e-beam sensitive photo resist as discussed in
section 3.2, then etched successfully into glass (see Fig.8). A CGH for wavefront
transformation is under fabrication using the various spot sizes method as discussed in
secti0n 3.3. Several arrays of CGH i have also beeh fabricated using the direct write on
analog e-beam resist using 16 different e-beam dosages as discussed in section 3.4:Fig.9
shows a 128 by 128 lenslet array of f/5 lenses with 50 microns diameter. Fig.10 shows a
diffractive lens out of the 2 by 2 array performing the space semi variant shuffle
exchange optical interconnect. Fig.ll shows a CGH out of the 64 by 64 CGH array
performing the hypercube interconnection architecture; each CGH has been calculated by
the Gerchberg-Saxton iterative algorithm [2]. Two optical reconstructions are shown
when only one CGH out of the 64 by 64 CGHs is illuminated.
_, Conclusion.
We presented several new e-beam microlithographic fabrication techniques for the
fabrication of multilevel surface relief phase structures for diffractive optics. These
fabrication techniques have several advantages over the conventional fabrication
technique described in refl. For example, the direct write on e-beam resist decreases
significantly the misalignment errors which occur usually during the pattern transfers in
the mask aligner, and decreases the minimum feature sizes allowed for fabrication. The
e-beam sensitive photoresist contribute to reduction in processing time during the
photolithography process, and the variation of the e-beam spot size decreases the overall
e-beam writing time by writing the fractured patterns in a more efficient way. E-beam
direct write on analog e-beam resist with various e-beam dosages can generate a sixteen
phase level element in one exposure step and one processing step.
Acknowledgmenls
We would like to thank Jiao Fan and Peng Li for their contributions in the holographic
pattern fracture and the e-beam data file preparation.
References,
[! I G.J.Swanson, MIT technical Report 854, 17-24 (1989).
[21 RW. Gerchberg and W.O.Saxton, Optik 35, 237-246 (1972).
198
Amplitude mask #1
t 1
Photolithography
II 11111
I_ , ,, v, T ,,
Ion Beam Milling
Photoresist lifl off
Amplitude mask #2
Photolithography and mask
aligning
ll111111f
'v T T v T T T T_
Ion Beam Milling
Pholoresist lift off
Chrome [_ Photo-resist
i"'-'-1 Exposed reslst _ Substrata
Fig.l) Conventional fabrication technique to produce muhilevel relief phase structures.
E-beam writing
pattern #2
?----j
Photolithog raphy
(back side illumination)
E-beam writing
pattern #1
Photolithography
(back side illumination)
i , ' lllll
Ion Beam Milling
Photreslst lift-off
Ion Beam Milling
Pholresist lift-off
substrata _11 e-beam resist mmBg pholo resist I
ImmmB chrome _ exposed resists
Fig.2) Direct write on e-beam resist, overcoming the need of using optical mask aligner.
Pattern #2 is written first because it contains finer and shallower structures than
pattern#1.
199
1} E-beam writing
2) Resist developped,
chrome elched,
resist stripped off
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3) Photo-resist spinning and
Back side illumination
1) E-beam writing
111
2) Ion Beam milling
4) Ion Beam milling
l_ Chrome _ E-beam rsslst _ Photo-reslst I
I
Exp sed resists Substrete E-b4sm sensitlvel
photo-rsslst J
photo resist tiff off photo resist lift off
Fig.3) Comparison of processing steps (a) using regular e-beam resist, (b) using e-beam
sensitive photoresist. ,_ _ :
4.a) Original spherical lens (f= lmm, dimension= 150 microns x 150 microns).
4.b) Fine shapes written with 0.1 micron spot size. 4.c) Coarse shapes written with _.2 micron spot size.
Fig.4) Direct write with various e-beam spot sizes.
2O0
E-beam writing with different
dosages
Developement of e-beam resist
1 !
Reactive Ion Beam Etching
I iiiqlwm E-beam resist _Conductlve layer (ITO, chrome) 1"-"--7 Substrate I
Fig.5) Direct write with various e-beam dosages on analog e-beam resist.
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Fig.6) Depth vs. electron dose for a): EBR-9 Torray resist, and for b): PMMA resist.
Equation shows second order polynomial fit (correlation coefficient R).
201
Fig.7) Four phase level f 1 diffractive lens fabricated by the direct write on e-beam
resist, (method described in section 3.1).
Fig.8) Linear grating of 60 microns period fabricated with the e-beam sensitive
photoresist method (discussed in section 3.2) and etched into glass.
202
Fig.9) SEM microphotograph of a 128 by 128 lenslet array (f/5, diameter: 50 microns)
fabricated by direct write on analog e-beam resist using 16 different e-beam dosages.
203
@ ___ p_oto Area
i
Fig.lO) SEM microphotograph of a diffractive off-axis Fresnel zone plate out of a 2 by 2
space semi variant shuffle exchange interconnection architecture, fabricated by direct
write on analog e-beam resist using 16 different e-beam dosages.
204
i_. _ ...... 3--- • -_ - "• _ ----
b-
Fig.ll) 5EM microphotograph of a Fourier CGH out of a 64 by 64 array performing the
hypercube interconnection architecture, fabricated by direct write on analog e-beam
resist using 16 different e-beam dosages. The CGHs were calculated using the
Gerchberg-Saxton iterative qlgorithm. Two optical reconstructions are shown when only
one CGH out of the 64 by 64 array is illuminated.
205
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Diffractive optics fabricated by direct write methods with an electron beam

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