Photoalignment and photo-patterning of planar and homeotropic
liquid-crystal-display configurations
Hubert Seiberle*
Martin Schadt**
Abstract — Optical alignment and micro-patterning of the alignment of liquid-crystal displays (LCDs)
by linear photopolymerization (LPP) technology renders high-quality multi-domain twisted-nematic
(TN) and supertwisted-nematic (STN) displays with broad fields of view over wide temperature ranges
feasible. The prerequisites are the generation of photo-induced high-resolution azimuthal alignment
patterns with defined bias-tilt angles 0° ≤ θ ≤ 90°. For the first time, LPP-aligned single- and dualdomain vertically aligned nematic LCDs (VAN-LCDs) are presented. Dual-domain VAN-LCDs are
shown to exhibit broad fields of view which are further broadened by combining the displays with
LPP-aligned optical compensators made of liquid-crystal polymers.
Keywords — LCDs, LCD alignment, LCD patterning, LCD photo-alignment.
1
Introduction
The exploitation of the many possibilities to generate
images with different electro-optical effects in LCDs
requires progress in material sciences, semiconductor
development, and panel engineering in a synergistic manner. Combined efforts eventually resolve problems that are
as old as today’s field-effect LCD technology. A problem
from the very beginning of the technology – the restricted
angle of view of the TN effect1 – and its improvements in
recent thin-film-transistor (TFT) TN-LCDs is a good example. One approach to improve the field of view is the combination of specifically configured optical retardation films
with TN-LCDs such that the inherently non-uniform TN
optics becomes much more uniform.2
The recently discovered technology of photoaligning
liquid crystals, instead of mechanical brushing, offers an
elegant alternative to solving the viewing-angle problem.
Instead of adding external optical correction elements to
displays, photoalignment combined with photo-patterning
was shown to render sub-pixellation of field-effect LCDs
feasible.3-6 The resulting multi-domain displays resolve the
viewing-angle problem7,8 and its temperature dependence
at the core. Moreover, photoalignment combined with
photo-patterning opens up the possibility of making use of
well-known electro-optical effects in displays whose
molecular director configurations could not thus far be sufficiently controlled to render them competitive with TN- or
STN-LCDs, such as VAN configurations.
Essentials for achieving high-quality field-effect displays are well-defined boundary conditions between display
substrate(s) and liquid-crystal director(s) n$ which extend
into sub-micron dimensions. The critical parameters are the
quality of uniaxiality of the alignment, precisely defined tilt
angles Θ between n$ and display substrates, anchoring
energy, as well as the thermal and optical stability of the
aligning layers. Depending on the requirements of the specific molecular configuration used in the display, appropriate tilt angles have to be chosen from the full range 0° ≤ θ ≤
90°. Among the different approaches of photo-alignment
which cover photoisomerization, 9 LPP,10 and photo-decomposition,11 LPP photoalignment has been shown to be superior in terms of photosensitivity and thermal and light
stability, as well as in the generation of pretilt angles.3,4
In the following, we will review the performance of
LPP-aligned dual- and four-domain TN-LCDs. Moreover,
we show, for the first time, that LPP alignment with controlled, slightly off-axis bias tilt angles θ < 90° is feasible and can
be used to photoalign VAN-LCDs or hybrid aligned nematic
LCDs (HAN-LCDs). Since the cylindrical symmetry of a
perfectly vertically aligned LC-configuration (θ = 90°) leads
to dislocations in the nematic director field upon application of a driving voltage, such displays exhibit a patchy
appearance, poor contrast, and a poor field of view. This
problem has thus far prevented the production of directview LCDs based on vertically aligned LC configurations.
To circumvent the dislocation problem, Takeda et al.12
recently presented an elaborate wedge-shaped surface
topology that induces a four-domain VAN configuration
which, when combined with an external optical retarder,
remarkably improves the appearance of VAN-LCDs. In the
following we show that LPP-photoalignment and photo-patterning not only allow the precise control of small bias tilt
angles θ < 25° but also large bias tilts θ ≈ 86° < 90°, thus
rendering effective manufacturing of multi-domain VANLCDs feasible. Prerequisites are novel LPP materials
recently developed in our laboratories.
*Member, SID.
**Fellow, SID.
Revised version of a paper presented at the 18th Intl. Display Research Conf. (Asia Display ‘98), Seoul, Korea.
The authors are with ROLIC Research Ltd., 4123 Allschwil, Switzerland; telephone +41-61-487-22-22, fax -2299, e-mail: hubert.seiberle@rolic.ch.
 Copyright 2000 Society for Information Display 1071-0922/00/0801-067$1.00
Journal of the SID 8/1, 2000
67
FIGURE 1 — LPP-aligned dual-domain TN-LCD (Ref. 5).
FIGURE 2 — LPP-aligned four-domain TN-LCD (Ref. 3).
2
2.1
Planar photo-aligned LCDs
Multi-domain LPP-aligned TN-LCDs
Figure 1 shows a partially switched single pixel of a LPPaligned dual-domain TN configuration which was recently
implemented by ADT to realize the first 5-in. 320 × 240pixel TFT TN-LCD with very broad field of view for automotive navigation screens.5 The configuration allows the
use of a single photomask. Figure 2 depicts a partially
switched single pixel of an LPP-aligned four-domain TN
configuration which was chosen because it again allows the
use of a single photomask to generate the four-domain photoaligning pattern.3 In both TN configurations control of the
respective photo-induced bias tilt angles Θ at the two display boundaries are prerequisites for generating the
appropriate twist sense of the sub-TN helices in each pixel.
On a macroscopic scale, the optical anisotropies of the
differently aligned central directors n$ of the sub-helices
compensate each other (Fig. 2). Therefore, the angular
dependency of multi-domain LCDs is strongly reduced
compared with conventional single-domain displays.3-5
Figure 3 depicts the remarkably improved field of
view of the LPP-aligned dual- and four-domain TN-LCD
configurations of Figs. 1 and 2 compared with the singledomain TN-LCDs. Automotive dashboards require excel-
68
FIGURE 3 — Angular dependence of luminance for 20% vertical
transmission for three cases. Single-domain TN-LCD (top); dual-domain
TN-LCD (middle); four-domain TN-LCD (bottom). Gray-level increments
are 1/10 of the transmission in the vertical direction.
lent horizontal readability. The viewing characteristics of
the dual-domain TN-LCD ideally meets this requirement5
(Fig. 3).
The examples also show that the large variety of aligning patterns that can be generated by the LPP technology
can also be used to achieve preferred directional viewing
properties in LCDs.
3
3.1
Vertically photoaligned LCDs
Multi-domain LPP-aligned VAN-LCDs
Figure 4 shows a single pixel of the first homeotropically
photoaligned LCD with slightly tilted vertical boundaries.
We chose the dual-domain VAN configuration in Fig. 4 to
Seiberle and Schadt / LCD Photoalignment and photo-patterning
FIGURE 4 — Single pixel of a LPP-aligned dual-domain VAN-LCD with
integrated LPP/LCP compensators C; bias tilt angles θ = ±89°.
demonstrate (a) homeotropic photoalignment and (b)
photo-patterning of homeotropic alignment. The dualdomain VAN configuration combined with a planar optical
retarder on each substrate leads to a broad field of view for
the display. In analogy to our first photo-aligned multi-
FIGURE 5 — Angular luminance dependence at V20 according to Fig.
4. Top: LPP-aligned single-domain VAN-LCD. Bottom: LPP-aligned
dual-domain VAN-LCD.
F IGUR E 6 — An gu la r O F F- st a t e l u m in an ce d e p e n d en ce o f
non-compensated (top) and LPP/LCP-compensated dual-domain
VAN-LCD (bottom) according to Fig. 4. Gray-level increments are 1/10
of the maximum transmission of the top graph.
domain work with TN- and STN-nematic LCDs,3-5 the
dual-domain configuration in Fig. 4 was achieved by LPPphotoalignment and simultaneous cross-linking of the two
sub-pixel areas on substrate S1 with UV light incident from
opposite directions through a single photomask. The resulting reverse bias tilt angles θ = ±89° cause the central LC
directors to tilt in opposite directions upon application of a
voltage (Fig. 4). We used the low-viscosity, negative dielectric anisotropy (∆E = –3.5) LC mixture 8987 from ROLIC
for our VAN experiments. The VAN-LCD cell gap was chosen as d = 4.6 µm. From the optical anisotropy ∆n = (n e –
no) = 0.0956 of mixture 8987, a maximum optical retardation of the display can be achieved: δVAN = ∆nd = 0.44 > λ/2.
The function of the optical retarders C1 and C2 in Fig.
4 is to compensate the off-axis birefringence of the VANLCD for oblique angles of view in the OFF state of the
display. The chosen respective optical retardations are: ∆C1
= 200 + 200 nm and δC2 = 130 + 130 nm. Each of the
retarders C1 and C2 consists of two identical LPP-aligned
liquid-crystal-polymer (LCP) retarder films with crossed
optical axes. The in-plane optical retarders comprise
uniaxial positive-birefringent LCP materials. C1 and C2 are
aligned such that the optical axes adjacent to P1 and P2 are
perpendicular to the respective polarization directions. The
LPP/LCP technology allows us to directly coat the crossed
Journal of the SID 8/1, 2000
69
FIGURE 7 — Transmission-voltage dependence of single- and
dual-domain LPP-aligned VAN-LCDs.
LCP-retarder layers with the respective LPP-aligning layers
on top of each other without any mechanical aligning step.13
LPP/LCP-compensators exhibit a comparable wavelength dispersion to the monomeric liquid crystals in the
display. Therefore, a better optical match with the LCD
results compared with conventional, mechanically stretched
polymer compensators.
The graphs in Fig. 5 depict measurements of the
angular dependence of the luminance of partially switched,
LPP-aligned single- and dual-domain VAN-LCDs at 20%
vertical transmission. The respective maximum number of
gray levels is 10. The remarkably improved azimuthal angular luminance dependence of the dual-domain VAN-LCD is
evident (bottom of Fig. 5).
Figure 6 shows the effect of the compensators C1 and
C2 on the OFF-state leakage of the dual-domain VAN-LCD
in Fig. 4. From Fig. 6 it follows that C1 and C2 remarkably
decrease the OFF-state leakage at oblique angles of incident light. The strong synergies which result from combining mu lti-do main VAN con fi guratio ns with op tical
compensation result in exceptionally high contrast ratios
over a broad range of view under any display driving condition (Figs. 5 and 6).
Figure 7 shows the transmission vs. voltage dependence of LPP-aligned single- and dual-domain VAN-LCDs at
vertical light incidence. Except for the reduced maximum
transmission of the dual-domain display, which is due to dislocations separating its sub-pixels, the graphs are identical
(cf also Fig. 8).
Figure 8, from top to bottom, shows polarization
microscope photographs of the striped sub-pixel structure
of the partially turned-on LPP-aligned dual-domain VANLCD of Fig. 4 at 20% transmission (V20). The top photograph was taken at vertical light incidence. It shows the
uniform, defect-free gray scale of the sub-pixels which is
characteristic for the high optical quality of LPP alignment.3,4 The dislocation walls at the pixel boundaries are
caused by the opposite splay deformations of neighboring
sub-pixels (Fig. 4). The center and bottom photographs
show the microscopic optical appearance of dual-domain
sub-pixels upon tilting the display by either +20° or –20°
70
FIGURE 8 — Photographs of the (striped) subpixels of a partially
switched LPP-aligned dual-domain VAN-LCD. Top: vertical light
incidence; center and bottom: ±20° tilted.
around an axis perpendicular to the LC deformation plane
(Fig. 4). The consequent alternating black and white appearance of the sub-pixels and the shift of the two patterns
by one sub-pixel (cf. middle and bottom photo) are caused
by the periodically changing tilt configurations of the LC
layers in adjacent sub-pixels in our VAN-LCD. Figure 8
illustrates the high resolution and perfect uniformity of LPP
alignment and its efficiency to generate not only small, 3,4
but also very large bias tilt angles.
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Martin Schadt has an internationally recognized career of over 25 years from in the field
of liquid crystals and display technologies. Dr.
Schadt received his Ph.D. in solid-state physics
from the University of Basel, Switzerland, in
1967, followed by a two-year postdoctoral fellowship at the National Research Council in
Ottawa. Canada, on organic semiconductors. In
1970, Dr. Schadt joined Roche in Basel as a scientist in the Central Research Department.
Among numerous other key inventions, Dr. Schadt, together with Dr.
Helfrich, invented in 1970 the twisted-nematic effect on which today’s
field-effect liquid-crystal displays (LCDs) are based. He is a Fellow of
the Society for Information Display (SID), recipient of the Karl Ferdinand Braun Prize, the Aachener und Muenchener Preis für Technik und
angewandte Naturwissenschaften, and the Robert-Wichard-Pohl of the
German Physical Society. Dr. Schadt is Chief Executive Officer of ROLIC
Research Ltd.
Hubert Seiberle received his doctoral degree in
p o l y m er p h y si cs fro m t h e Un i v er si t y o f
Freiburg, Germany, in 1989, where he investigated molecular motions in mixtures of liquidcr ys t a l p o l ym e rs . In 19 90 he jo in ed t he
liquid-crystal department of at Hoffmann-La
Roche where he initially was involved in the
design of nematic liquid-crystal mixtures. Dr.
Seiberle is head of LPP-photoalignment device
research and development at ROLIC. He is
co-inventor on all basic LPP/LCP-device patents of ROLIC.
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