U.S. patent application number 11/207442 was filed with the patent office on 2006-12-28 for optimized grating based biosensor and substrate combination.
This patent application is currently assigned to SRU Biosystems, Inc.. Invention is credited to Stephen Schulz.
Application Number | 20060291779 11/207442 |
Document ID | / |
Family ID | 37567444 |
Filed Date | 2006-12-28 |
United States Patent
Application |
20060291779 |
Kind Code |
A1 |
Schulz; Stephen |
December 28, 2006 |
Optimized grating based biosensor and substrate combination
Abstract
A grating-based biosensor is disclosed where the biosensor is
constructed and arranged such that the grating lines of the sensor
align with an optical axis of a birefringent substrate, so as to
improve resonance peak uniformity. Methods of manufacturing
biosensors to provide alignment of the grating lines with the
optical axes of a birefringent substrate are also disclosed. One
embodiment uses a grating master wafer to form a grating on a
continuous web of substrate material. The grating master wafer is
rotated relative to the web until the lines of the grating in the
master wafer are in substantial alignment with the optical axis of
the web. A UV curable material is applied to the wafer and cured in
place to form the grating on the surface of the substrate web. With
a web of some preferred materials, such as PET film, one need only
determine the optical axis orientation once for a given web since
the optical axis orientation is essentially constant along the
length of the web.
Inventors: |
Schulz; Stephen; (Lee,
NH) |
Correspondence
Address: |
MCDONNELL BOEHNEN HULBERT & BERGHOFF LLP
300 S. WACKER DRIVE
32ND FLOOR
CHICAGO
IL
60606
US
|
Assignee: |
SRU Biosystems, Inc.
|
Family ID: |
37567444 |
Appl. No.: |
11/207442 |
Filed: |
August 18, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60693680 |
Jun 23, 2005 |
|
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Current U.S.
Class: |
385/37 |
Current CPC
Class: |
G01N 21/0303 20130101;
G01N 21/23 20130101; G01N 21/253 20130101 |
Class at
Publication: |
385/037 |
International
Class: |
G02B 6/34 20060101
G02B006/34 |
Claims
1. A biosensor comprising: a substrate, the substrate comprising a
birefringent material having two optical axes; and a grating
applied to the substrate, wherein the grating comprises features
arranged in a plurality of parallel lines and wherein the lines of
the grating are in substantial alignment with one of the optical
axes of the substrate.
2. The biosensor of claim 1, wherein the substrate comprises a
clear polymer film.
3. The biosensor of claim 2, wherein the film comprises a
polyethylene terepthalate (PET) film.
4. The biosensor of claim 1, wherein the grating comprises a
UV-curable material applied to the substrate using a grating master
wafer.
5. A method of manufacturing a biosensor, comprising the steps of:
a) feeding a web of birefringent substrate material to a station,
the web of substrate material having two optical axes; b) applying
a grating to the substrate material at the station, wherein the
grating comprises a plurality of parallel lines; wherein, in the
performance of step (b), the grating is applied to the substrate in
a manner whereby the lines of the grating are in substantial
alignment with one of the optical axes of the web of substrate
material.
6. The method of claim 5, further comprising the step of
determining an optical axis orientation of the web of substrate
material, and, before or during the performance of step b)
orienting a grating wafer for use in applying the grating to the
substrate material relative to the substrate whereby the grating
wafer is oriented in substantial alignment with the optical axis
orientation.
7. The method of claim 6, further comprising the grating is formed
of a UV curable material.
8. The method of claim 5, wherein the substrate material comprises
a clear polymer film.
9. The method of claim 8, wherein the clear polymer film comprises
a polyethylene terepthalate (PET) film.
10. The method of claim 5, wherein the method is performed
substantially continuously on a continuous web of substrate
material.
11. The method of claim 10, wherein the continuous web of substrate
material is formed by dividing a master web of substrate material
into a plurality of continuous longitudinal strips, each strip
forming a continuous web of substrate material.
12. The method of claim 6, wherein the step of determining the
optical axis orientation is performed at substantially the center
of the web of material.
13. A method of manufacturing a biosensor, comprising the steps of:
a) providing a continuous web of substrate material; b) determining
the orientation of an optical axis in the web of substrate
material; c) providing a grating master wafer having a plurality of
parallel lines; d) orienting the grating master wafer relative to
the web of substrate material such that the lines of the grating
master wafer are in substantial alignment with the optical axis; e)
forming a grating on the web of material using the grating master
wafer; and f) advancing the web of substrate material relative to
the grating master wafer and repeating step e).
14. The method of claim 13, wherein the grating is formed of a UV
curable material.
15. The method of claim 13, wherein the substrate material
comprises a clear polymer film.
16. The method of claim 15, wherein the clear polymer film
comprises a polyethylene terepthalate (PET) film.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority benefits under 35 U.S.C.
.sctn. 119(e) of U.S. Provisional application Ser. No. 60/693,680
filed Jun. 23, 2005, the entire content of which is incorporated by
reference herein.
BACKGROUND
[0002] A. Field of the Invention
[0003] This invention relates generally to grating-based
biochemical sensor devices, and methods of manufacture of such
devices. Such devices are typically based on photonic crystal
technology and are used for optical detection of the adsorption of
a biological material, such as DNA, protein, viruses or cells, or
chemicals, onto a surface of the device or within a volume of the
device.
[0004] B. Description of Related Art
[0005] Grating-based biosensors represent a new class of optical
devices that have been enabled by recent advances in semiconductor
fabrication tools with the ability to accurately deposit and etch
materials with precision less than 100 nm.
[0006] Several properties of photonic crystals make them ideal
candidates for application as grating-type optical biosensors.
First, the reflectance/transmittance behavior of a photonic crystal
can be readily manipulated by the adsorption of biological material
such as proteins, DNA, cells, virus particles, and bacteria. Each
of these types of material has demonstrated the ability to alter
the optical path length of light passing through them by virtue of
their finite dielectric permittivity. Second, the
reflected/transmitted spectra of photonic crystals can be extremely
narrow, enabling high-resolution determination of shifts in their
optical properties due to biochemical binding while using simple
illumination and detection apparatus. Third, photonic crystal
structures can be designed o highly localize electromagnetic field
propagation, so that a single photonic crystal surface can be used
to support, in parallel, the measurement of a large number of
biochemical binding events without optical interference between
neighboring regions within <3-5 microns. Finally, a wide range
of materials and fabrication methods can be employed to build
practical photonic crystal devices with high surface/volume ratios,
and the capability for concentrating the electromagnetic field
intensity in regions in contact with a biochemical test sample. The
materials and fabrication methods can be selected to optimize
high-volume manufacturing using plastic-based materials or
high-sensitivity performance using semiconductor materials.
[0007] Representative examples of grating-type biosensors in the
prior art are disclosed in Cunningham, B. T., P. Li, B. Lin, and J.
Pepper, Colorimetric resonant reflection as a direct biochemical
assay technique. Sensors and Actuators B, 2002. 81: p. 316-328;
Cunningham, B. T., J. Qiu, P. Li, J. Pepper, and B. Hugh, A plastic
calorimetric resonant optical biosensor for multiparallel detection
of label-free biochemical interactions, Sensors and Actuators B,
2002. 85: p. 219-226; Haes, A. J. and R. P. V. Duyne, A Nanoscale
Optical Biosensor: Sensitivity and Selectivity of an Approach Based
on the Localized Surface Plasmon Resonance Spectroscopy of
Triangular Silver Nanoparticles. Journal of the American Chemical
Society, 2002. 124: p. 10596-10604.
[0008] The combined advantages of photonic crystal biosensors may
not be exceeded by any other label-free biosensor technique. The
development of highly sensitive, miniature, low cost, highly
parallel biosensors and simple, miniature, and rugged readout
instrumentation will enable biosensors to be applied in the fields
of pharmaceutical discovery, diagnostic testing, environmental
testing, and food safety in applications that have not been
economically feasible in the past.
[0009] In order to adapt a photonic bandgap device to perform as a
biosensor, some portion of the structure must be in contact with a
liquid test sample. Biomolecules, cells, proteins, or other
substances are introduced to the portion of the photonic crystal
and adsorbed where the locally confined electromagnetic field
intensity is greatest. As a result, the resonant coupling of light
into the crystal is modified, and the reflected/transmitted output
(i.e., peak wavelength) is tuned, i.e., shifted. The amount of
shift in the reflected output is related to the amount of substance
present on the sensor.
[0010] The sensors are used in conjunction with an illumination and
detection instrument that directs polarized light into the sensor
and captures the reflected or transmitted light. The reflected or
transmitted light is fed to a spectrometer that measures the shift
in the peak wavelength.
[0011] The ability of photonic crystals to provide high quality
factor (Q) resonant light coupling, high electromagnetic energy
density, and tight optical confinement can also be exploited to
produce highly sensitive biochemical sensors. Here, Q is a measure
of the sharpness of the peak wavelength at the resonant frequency.
Photonic crystal biosensors are designed to allow a liquid test
sample to penetrate the periodic lattice, and to tune the resonant
optical coupling condition through modification of the surface
dielectric constant of the crystal through the attachment of
biomolecules or cells. Due to the high Q of the resonance, and the
strong interaction of coupled electromagnetic fields with
surface-bound materials, several of the highest sensitivity
biosensor devices reported are derived from photonic crystals. See
the Cunningham et al. papers cited previously. Such devices have
demonstrated the capability for detecting molecules with molecular
weights less than 200 Daltons (Da) with high signal-to-noise
margins, and for detecting individual cells. Because
resonantly-coupled light within a photonic crystal can be
effectively spatially confined, a photonic crystal surface is
capable of supporting large numbers of simultaneous biochemical
assays in an array format, where neighboring regions within
.about.10 .mu.m of each other can be measured independently. See
Li, P., B. Lin, J. Gerstenmaier, and B. T. Cunningham, A new method
for label-free imaging of biomolecular interactions. Sensors and
Actuators B, 2003.
[0012] There are many practical benefits for biosensors based on
photonic crystal structures. Direct detection of biochemical and
cellular binding without the use of a fluorophore, radioligand or
secondary reporter removes experimental uncertainty induced by the
effect of the label on molecular conformation, blocking of active
binding epitopes, steric hindrance, inaccessibility of the labeling
site, or the inability to find an appropriate label that functions
equivalently for all molecules in an experiment. Label-free
detection methods greatly simplify the time and effort required for
assay development, while removing experimental artifacts from
quenching, shelf life, and background fluorescence. Compared to
other label-free optical biosensors, photonic crystals are easily
queried by simply illuminating at normal incidence with a broadband
light source (such as a light bulb or LED) and measuring shifts in
the reflected color. The simple excitation/readout scheme enables
low cost, miniature, robust systems that are suitable for use in
laboratory instruments as well as portable handheld systems for
point-of-care medical diagnostics and environmental monitoring.
Because the photonic crystal itself consumes no power, the devices
are easily embedded within a variety of liquid or gas sampling
systems, or deployed in the context of an optical network where a
single illumination/detection base station can track the status of
thousands of sensors within a building. While photonic crystal
biosensors can be fabricated using a wide variety of materials and
methods, high sensitivity structures have been demonstrated using
plastic-based processes that can be performed on continuous sheets
of film. Plastic-based designs and manufacturing methods will
enable photonic crystal biosensors to be used in applications where
low cost/assay is required, that have not been previously
economically feasible for other optical biosensors.
[0013] The assignee of the present invention has developed a
photonic crystal biosensor and associated detection instrument. The
sensor and detection instrument are described in the patent
literature; see U.S. patent application publications U.S.
2003/0027327; 2002/0127565, 2003/0059855 and 2003/0032039. Methods
for detection of a shift in the resonant peak wavelength are taught
in U.S. Patent application publication 2003/0077660. The biosensor
described in these references include 1- and 2-dimensional periodic
structured surfaces applied to a continuous sheet of plastic film
or substrate. The crystal resonant wavelength is determined by
measuring the peak reflectivity at normal incidence with a
spectrometer to obtain a wavelength resolution of 0.5 picometer.
The resulting mass detection sensitivity of <1 pg/mm.sup.2
(obtained without 3-dimensional hydrogel surface chemistry) has not
been demonstrated by any other commercially available
biosensor.
[0014] A fundamental advantage of the biosensor devices described
in the above-referenced patent applications is the ability to
mass-manufacture with plastic materials in continuous processes at
a 1-2 feet/minute rate. Methods of mass production of the sensors
are disclosed in U.S. Patent application publication 2003/0017581.
As shown in FIG. 1, the periodic surface structure of a biosensor
10 is fabricated from a low refractive index material 12 that is
overcoated with a thin film of higher refractive index material 14.
The low refractive index material 12 is bonded to a substrate 16.
The surface structure is replicated within a layer of cured epoxy
12 from a silicon-wafer "master" mold (i.e. a negative of the
desired replicated structure) using a continuous-film process on a
polyester substrate 16. The liquid epoxy 12 conforms to the shape
of the master grating, and is subsequently cured by exposure to
ultraviolet light. The cured epoxy 12 preferentially adheres to the
polyester substrate sheet 16, and is peeled away from the silicon
wafer. Sensor fabrication was completed by sputter deposition of
120 nm titanium oxide (TiO.sub.2) high index of refraction material
14 on the cured epoxy 12 grating surface. Following titanium oxide
deposition, 3.times.5-inch microplate sections are cut from the
sensor sheet, and attached to the bottoms of bottomless 96-well and
384-well microtiter plates with epoxy.
[0015] As shown in FIG. 2, the wells 20 defining the wells of the
mircotiter plate contain a liquid sample 22. The combination of the
bottomless microplate and the biosensor structure 10 is
collectively shown as biosensor apparatus 26. Using this approach,
photonic crystal sensors are mass produced on a square-yardage
basis at very low cost.
[0016] The detection instrument for the photonic crystal biosensor
is simple, inexpensive, low power, and robust. A schematic diagram
of the system is shown in FIG. 2. In order to detect the reflected
resonance, a white light source illuminates a .about.1 mm diameter
region of the sensor surface through a 100 micrometer diameter
fiber optic 32 and a collimating lens 34 at nominally normal
incidence through the bottom of the microplate. A detection fiber
36 is bundled with the illumination fiber 32 for gathering
reflected light for analysis with a spectrometer 38. A series of 8
illumination/detection heads 40 are arranged in a linear fashion,
so that reflection spectra are gathered from all 8 wells in a
microplate column at once. See FIG. 3. The microplate+biosensor 10
sits upon a X-Y addressable motion stage (not shown in FIG. 2) so
that each column of wells in the microplate can be addressed in
sequence. The instrument measures all 96 wells in .about.15
seconds, limited by the rate of the motion stage. Further details
on the construction of the system of FIGS. 2 and 3 are set forth in
the published U.S. Patent application 2003/0059855.
[0017] All of the previously cited art is fully incorporated by
reference herein.
SUMMARY
[0018] A grating-based biosensor is disclosed where the biosensor
is constructed and arranged such that the lines of the grating are
aligned with one of the optical axes of a substrate sheet (e.g.,
PET film), so as to improve resonance peak uniformity. Such
alignment is maintained during the biosensor fabrication, for
example by rotating a grating master wafer relative to the axis of
the web of substrate material, and then forming the grating on the
surface of the substrate web using the master such that the grating
lines are in alignment with an optical axis of the substrate. The
operator measures substrate optical axis orientation prior to the
beginning of grating replication and then rotates the grating
master wafer so as to align the grating with the optical axis.
[0019] With a biosensor constructed in this configuration, light
with polarization important to the resonance phenomenon will not
undergo significant phase shift as it travels to or from the
grating. Such a biosensor has uniform and reliable resonance peak
quality.
[0020] In one embodiment, a biosensor is provided comprising a
substrate, such as a birefringent clear polymer film. One preferred
film is PET, however other selections are possible. The substrate,
which may be a birefringent film, comprises a material having an
optical axis. A grating is applied to the substrate. The grating
comprises features arranged in a plurality of parallel lines and
wherein the lines of the grating are in substantial alignment with
the optical axis of the substrate.
[0021] Methods of manufacturing biosensors to provide alignment of
the grating lines with the optical axes of a birefringent substrate
are also disclosed.
[0022] In one embodiment, a method of manufacturing a biosensor is
provided comprising the steps of:
[0023] a) feeding a web of substrate material to a station, the web
of substrate material having an optical axis;
[0024] b) applying a grating to the substrate material at the
station, wherein the grating comprises a plurality of parallel
lines; and
[0025] wherein, in the performance of step (b), the grating is
applied to the substrate in a manner whereby the lines of the
grating are in substantial alignment with the optical axis of the
web of substrate material.
[0026] In another embodiment, a method is provided of manufacturing
a biosensor which provides for continuous production of a
biosensor. The method comprises the steps of:
[0027] a) providing a continuous web of substrate material;
[0028] b) determining the orientation of an optical axis in the web
of substrate material;
[0029] c) providing a grating master wafer having a plurality of
parallel lines;
[0030] d) orienting the grating master wafer relative to the web of
substrate material such that the lines of the grating master wafer
are in substantial alignment with the optical axis;
[0031] e) forming a grating on the web of material using the
grating master wafer; and
[0032] f) advancing the web of substrate material relative to the
grating master wafer and repeating step e).
[0033] In addition to the exemplary aspects and embodiments
described above, further aspects and embodiments will become
apparent by reference to the drawings and by study of the following
detailed descriptions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] Exemplary embodiments are illustrated in referenced figures
of the drawings. It is intended that the embodiments and figures
disclosed herein are to be considered illustrative rather than
restrictive.
[0035] FIG. 1 is an illustration of a prior art biosensor
arrangement.
[0036] FIG. 2 is an illustration of a prior art biosensor and
detection system for illuminating the biosensor and measuring
shifts in the peak wavelength of reflected light from the
biosensor.
[0037] FIG. 3 is an illustration of an arrangement of 8
illumination heads that read an entire row of wells of a biosensor
device comprising the structure of FIG. 1 affixed to the bottom of
bottomless microtiter plate.
[0038] FIG. 4 shows a graph of measured illumination intensity (in
relative units) as a function of wavelength for a row of 12 wells
in a microtiter plate in the construction of FIG. 3 when a
substance to be tested is present in the unit cells. Each line in
the graph represents data for a separate well. A measured resonant
frequency peak of 852 nm is observed in each of the wells. In this
example, the grating and substrate optical axes have .about.25
degrees separation. A fixed polarizer in the detection instrument
is present which aligns perpendicularly to the grating so as to
create transverse magnetic mode polarization through the
device.
[0039] FIG. 5 shows a graph of intensity as a function of
wavelength for the same row in the microtiter plate cell where the
grating and substrate optical axes are substantially in alignment
in accordance with the teachings of this disclosure. In this
example, they have .about.2 degrees separation. The fixed polarizer
aligns perpendicularly to the grating. Note that the resonant
frequency peak for all of the unit cells in the row using the
construction in which the grating axis and substrate optical axis
are aligned is much sharper in FIG. 5 as compared to FIG. 4,
resulting in increased accuracy of the detection of the peak
wavelength value.
[0040] FIG. 6A and 6B show the arrangement of a grating master
wafer and the substrate sheet before, and after, respectively, the
alignment of the grating master wafer lines to the optical axis of
the substrate material web.
DETAILED DESCRIPTION
[0041] This disclosure describes a grating-based biosensor where
the grating lines of the sensor and the optical axes of the
sensor's substrate are substantially aligned. In practice, they
have a parallel or perpendicular orientation with respect to each
other, typically within a few degrees of each other or preferably
closer.
[0042] The biosensing technology described above in the background
section relies on accurately determining the spectral wavelength at
which resonance occurs when light reflects from the sensor's
grating structure. Resonance manifests as a narrow spectral peak.
The accuracy of peak position determination is proportional to the
slope of the peak shape. Hence, narrow and tall peaks improve
sensor sensitivity. This invention dramatically improves the
lot-to-lot consistency and spatial uniformity of the biosensor's
peak quality.
[0043] The detection instrument of the above-described published
applications of the applicant's assignee exploits an optical
resonance mode stimulated by light polarized with some vector
component perpendicular to the grating lines. The resonance
phenomenon reflects 100% of that component that has polarization
perpendicular to the grating lines. The literature refers to this
mode as the Transverse Magnetic (TM) mode. A TE mode exists,
orthogonally to the TM mode, but it has a much broader and less
useful resonance shape. A polarizer, in the instrument that
interrogates the sensor, separates the sharp TM resonance peak from
non-resonant, background light reflected at other
polarizations.
[0044] To achieve maximum peak intensity, the optical axis of the
instrument polarizer must align with the TM light reflected from
the grating and/or polarized incident light should have TM
polarization. Thus, in the ideal configuration, the polarizer axis
aligns orthogonally to the grating lines. In its most versatile
configuration, the detection instrument requires light to
interrogate the sensor from the bottom, specifically the light
travels twice through the sensor substrate. This invention allows
the ideal polarization condition to hold, over the entire surface
of the biosensor, while also allowing the use of a birefringent
(polarization changing) substrate material.
[0045] The applicant's assignee has pioneered a low cost, polymer
web based version of the resonant grating biosensor. The choice of
PolyEthyleneTerephthalate (PET) polymer film as the substrate
material (item 16 in FIG. 1) offers a number of advantages. For
example, PET has relatively high mechanical strength (modulus),
thermal stability (Tg), and chemical tolerance. Perhaps most
importantly, for optical biosensing and a number of other technical
applications, one can readily obtain PET with high optical clarity
and quality at low cost. PET has one significant disadvantage with
respect to use as a substrate for grating based biosensors. The two
dimensional stretching, that occurs during PET manufacturing,
results in a birefringent material. The magnitude of stretching and
hence the magnitude of birefringence, varies across the width of
the PET web manufacturing process. Stretching and birefringence
remain constant along the length (machine direction) of the PET
manufacturing process. This invention overcomes the problem of PET
birefringence in the manufacture of biosensors.
[0046] Most introductory optics texts treat the subject of
birefringence and birefringent materials. A brief summary is
provided here, and the interested reader is directed to the
textbooks for a more extensive analysis of the phenomenon.
[0047] A birefringent material has two optical axes. Polarized
light, in a birefringent material, has two components traveling at
different speeds. Light polarized along the high index, "slow" axis
travels slower than light polarized perpendicularly along the fast
axis. The refractive index (speed) difference along each axis then
introduces a phase difference between the two components. The phase
difference grows as the light travels further through the
birefringent material. At any given substrate location, the exiting
light has a composite polarization orientation and intensity that
is a function of the phase difference and magnitude of the two
components. In the case of PET film, these axes are approximately
orthogonal to each other. The relationship below gives the phase
difference in terms of number of wavelengths: [0048] Phase
difference in number of wavelengths=Dn*d/lambda where Dn represents
the substrate's birefringence, the refractive index difference
between the two optical axes of the substrate, d is thickness of
substrate and lambda is wavelength of the light. [0049] Equations
for elliptical polarization generally describe the amplitude and
orientation of light exiting a birefringent material.
[0050] The two substrate optical axes have a numerically small
difference in refractive index (.about.0.05 for PET). However, over
the optically large thickness of the substrate, the total lag
between light components, traveling along each axis, translates to
numerous wavelength periods. Small gradations in birefringence
magnitude (Dn) or substrate thickness d across the substrate (the
result of the PET manufacturing process) translate into large
gradations in polarization state. High spatial variability of
polarization orientation results in high variability in the
biosensor's peak quality. This variability occurs spatially over
the area of the biosensor and temporally with the use of different
sections of the PET manufacturer's master roll.
[0051] In a principal aspect of this disclosure, the biosensor is
constructed and arranged such that the alignment of the grating
lines with one of the PET optical axes is specified, during the
sensor fabrication. The alignment need not be exact, but ideally is
as close as can be reasonably attained consistent with
manufacturability and cost considerations. In this configuration,
light with polarization important to the resonance phenomenon will
not undergo significant phase shift as it travels to or from the
grating. Such a biosensor has uniform and reliable resonance peak
quality. To achieve such alignment, the operator in the
manufacturing line measures the substrate optical axis orientation
prior to the beginning of grating replication onto the substrate,
and once this orientation is determined then rotates the grating
master wafer to substantially align the grating formed in the
substrate with the determined optical axis orientation.
[0052] As shown in FIG. 4, if incident light is polarized in an
orientation that is between the substrate's two optical axes (a
condition that is typically present in the prior art), incident
light will experience birefringence as it travels upwards from the
bottom substrate surface towards the grating on the top surface in
the detector arrangement of FIG. 3. Spatial variation in material
properties (optical axis orientation) translates into polarization
variability at the grating. Polarization variability at the grating
leads to loss of resonance peak uniformity. A similar process
occurs as light reflects from the grating back through the
substrate.
[0053] However, as shown in FIG. 5, light polarized in alignment
(or close to alignment) with one of the substrates optical axes as
explained in this disclosure will not experience birefringence
because it "sees" only one refractive index. The result is sharper
detected peak resonance frequency and thus increased accuracy with
the sensor device.
[0054] A presently preferred process of manufacturing biosensors in
accordance with the principles of this disclosure will now be
explained. The process involves "printing" or replicating the
grating onto the substrate. The grating is constructed on the
substrate web as explained in the above-cited patent application
documents of the applicant's assignee. If the grating X and Y axes
align in a parallel and perpendicular manner to the substrate's
optical axes (which are also typically at right angles) then the
birefringent properties of the substrate do not affect resonance
peak quality. Hence, care is taken during manufacturing to
correctly orient the grating master wafer relative to the substrate
(e.g., by rotating the grating master wafer) such that when the
grating is applied to the substrate the desired alignment between
grating lines and optical axis is observed.
[0055] PET manufacturers typically produce 2M (2 meter) wide rolls
or webs of PET film. Optical axis orientation, with respect to the
web direction, varies across the width of the web but not in the
direction of the web. The 2M roll is cut into smaller rolls, each
0.2M in width. The biosensors are produced from the 0.2M wide
rolls. Hence, the process samples many sections across the width of
the master roll. This invention compensates for variability in
optical axis orientation across the width of the master roll since
the measurement of optical axes is made for each 0.2M roll and
alignment between grating and optical axes is observed on each 0.2M
roll.
[0056] In a preferred mode of practicing this invention, a
measurement is taken of the orientation of the substrate's two
perpendicular optical axes, at the center of the 0.2M web, with
respect to the web direction (or web edge). Then, the grating
patterning tool (master wafer) is rotated to align the grating with
one of the optical axes of the web. In general, it does not matter
which axis (fast or slow) the grating aligns with. Typical rotation
values range between 0 and 30 degrees at the web's center. The
grating is then formed and UV bonded to the substrate web using the
grating master wafer. Bonding occurs during UV curing. As the
polymer material hardens/crosslinks into the grating shape, it also
bonds to the PET substrate. The web is advanced or indexed and
another grating is formed and bonded to the web. The web advances
and the process repeats. When sensors are constructed as just
described, incident light polarized along the second orthogonal
axis (basically in a direction extending into the web) maintains
its TM orientation when incident on the grating.
[0057] The process optimizes alignment at the center of the 0.2M
web. Variation from the ideal occurs as the sampling point deviates
from the center. However, the effect of birefringence increases
approximately as the sine squared of the angle between an optical
axis and the incident light polarization. Also the 0.2M web has a
relatively small spatial rate of optical axis change across the
width of the web. These two points yield the result that optimizing
to web center gives excellent resonance peak uniformity over the
area of microplate based biosensors.
[0058] Beyond improving the uniformity and quality of measured
resonance peaks, implementation of the invention has allowed the
applicant's assignee to simplify its biosensor reading
instrumentation. Before the invention, sensor readers required an
adjustable polarizer to partially compensate for polarization
rotation induced by the substrate. This apparatus produced usable
resonance peaks for substrate rolls with less than .about.15
degrees of misalignment between the grating and substrate optical
axes. Approximately 40% of the rolls that the assignee uses have
optical axes with greater than 15 degrees of angle from the web
direction (old grating direction). After making sensors according
to this invention, it is possible to make full use of a substrate
inventory. Moreover, the prior adjustable polarizer may be replaced
with a much simpler fixed polarizer.
[0059] FIG. 6A shows a web 100 of PET film (sensor substrate
material) is advanced in a machine direction 102 to a grating
station during production of grating-based biosensors of the type
described in the above-referenced patent applications of the
applicant's assignee. The web 100 is 0.2M in width in this example.
The optical axis of the web material is measured beforehand by
placing a specimen of the web between crossed polarizers (two
polarizers with axes perpendicular to one another) and rotating the
web specimen relative to the crossed polarizers to identify the
orientation where a photoextinction occurs. The orientation of the
web to the crossed polarizers determines the orientation of the
optical axes relative to the edges of the web. The fast and slow
optical axes of the web specimen are shown as axes 104 and 106,
respectively, and are usually perpendicular to one another. The
axis 104 is offset from the direction of the web movement (and the
edge of the web) by angle .theta.. Note that in FIG. 6A, the axis
116 is aligned with the web direction of travel 102 instead of the
optical axis 104 or 106, a situation which is corrected in this
invention. A roll (not shown) is applied to the web 100 and rolls
the web over a silicon wafer grating master 100. The master forms a
rating 112 of UV cured material on the web 100. The grating 112
consists of a periodic surface of low index of refraction material
arranged in a plurality of rows and columns of units (or cells),
each of which has a multitude of grating elements 114 of the type
shown in FIG. 1 as reference 12. The rectangular arrangement of the
grating elements defines lines having a grating axis shown as
116.
[0060] As shown in FIG. 6B, manufacture of the biosensor at the
grating station is designed such that alignment is specified
between the grating axis 116 and the optical axis 104 (or 106) of
the web substrate 100. To achieve this, the master grating wafer
110 is rotated by an amount such that the grating axis 116 and the
optical axis 104 are substantially aligned with each other
(preferably within a few degrees of each other). A UV curable
material (e.g., liquid epoxy) is applied in droplet form to the
wafer grating master 110, the material is spread using pressure
from the roller behind the web 100, and then the material is UV
cured to form and bond the grating pattern 112 onto the PET web
100. This arrangement of the master 110 and grating 112 relative to
the web 100 is shown in FIG. 6B. Exact alignment of the two axes is
not required, but closer alignment is better. Now, both the optical
axis 104 and the grating axis 116 form the same angle .theta.
relative to the direction 102 of travel of the web. The web is
advanced, the cured epoxy grating+web is peeled away from the
master grating wafer 110, another grating is applied to the web 100
using the master grating wafer 110 as described above, and the
process repeats in a continuous fashion.
[0061] In a downstream station (not show), high index of refraction
material is deposited on the grating, and the grating 112 is cut
from the web 100. The grating is then bonded to the bottom of a 96
well bottomless microtitre plate with the individual cells 118
aligned with the individual wells of the microtitre plate. The
biosensor device is then ready for use e.g., with a detection
instrument as described in the above-referenced patent
applications.
[0062] While a number of exemplary aspects and embodiments have
been discussed above, those of skill in the art will recognize
certain modifications, permutations, additions and sub-combinations
thereof. For example, other tools or processes may be used to form
a grating on a substrate material than those described herein
without departure from the scope of this invention. It is therefore
intended that claims hereafter introduced are interpreted to
include all such modifications, permutations, additions and
sub-combinations as are within their true spirit and scope.
* * * * *