U.S. patent application number 13/871208 was filed with the patent office on 2013-09-12 for nanofabrication process and nanodevice.
This patent application is currently assigned to Cornell University-Cornell Center for Technology, Enterprise & Commercialization. The applicant listed for this patent is Cornell University-Cornell Center for Technology, Enterprise & Commercialization. Invention is credited to Michael Gaitan, Samuel Martin Stavis, Elizabeth Arlene Strychalski.
Application Number | 20130236698 13/871208 |
Document ID | / |
Family ID | 44062293 |
Filed Date | 2013-09-12 |
United States Patent
Application |
20130236698 |
Kind Code |
A1 |
Stavis; Samuel Martin ; et
al. |
September 12, 2013 |
NANOFABRICATION PROCESS AND NANODEVICE
Abstract
A nanodevice includes a substrate that has an elongated channel
with a plurality of nanoscale critical dimensions arranged as a
stepped gradient across a width of the elongated channel.
Inventors: |
Stavis; Samuel Martin;
(Montgomery Village, MD) ; Strychalski; Elizabeth
Arlene; (North Potomac, MD) ; Gaitan; Michael;
(North Potomac, MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Enterprise & Commercialization; Cornell University-Cornell
Center for Technology, |
|
|
US |
|
|
Assignee: |
Cornell University-Cornell Center
for Technology, Enterprise & Commercialization
Ithaca
NY
|
Family ID: |
44062293 |
Appl. No.: |
13/871208 |
Filed: |
April 26, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12625077 |
Nov 24, 2009 |
8435415 |
|
|
13871208 |
|
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Current U.S.
Class: |
428/156 |
Current CPC
Class: |
G03F 7/40 20130101; Y10T
428/24479 20150115; B82Y 30/00 20130101; B81B 1/006 20130101 |
Class at
Publication: |
428/156 |
International
Class: |
B81B 1/00 20060101
B81B001/00 |
Claims
1. A nanodevice comprising: a substrate including an elongated
channel having a plurality of nanoscale critical dimensions
arranged as a stepped gradient across a width of the elongated
channel.
2. The nanodevice as recited in claim 1, further comprising first
and second voltage control channels within the substrate, with the
elongated channel being located between the first and second
voltage control channels, and the first and second voltage control
channels are configured to generate an electric field in the
elongated channel along a direction that is varied between
perpendicular and parallel to the length of the elongated
channel.
3. The nanodevice as recited in claim 1, wherein the plurality of
nanoscale critical dimensions is heights of steps of the stepped
gradient, and the step heights are less than 100 nanometers.
4. The nanodevice as recited in claim 1, wherein the stepped
gradient includes at least two different depths.
5. The nanodevice as recited in claim 1, wherein the stepped
gradient includes at least 1,000 different depths.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 12/625,077 which was filed Nov. 24, 2009, and is
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] This disclosure relates to processes for patterning and
etching a substrate to form a complex three dimensional surface
topography defined by a plurality of nanometer scale critical
dimensions and devices manufactured using such processes.
[0003] Lithography (e.g., photolithography) is known and used for
fabricating nanofluidic devices, integrated circuits, and the like.
As an example, a typical nanofluidic device may include a fluidic
channel with a nanometer scale depth for the manipulation and
analysis of biomolecules, such as nucleic acids and proteins.
[0004] Currently, photolithography is one method that is used to
fabricate such nanofluidic channels. For instance, a photoresist
layer may be deposited onto a substrate and then exposed to a light
pattern created using a photomask. The portions of the photoresist
that are exposed to the light are either rendered resistant to a
developer (i.e., when a negative photoresist is used) or soluble in
the developer (i.e., when a positive photoresist is used). In
either case, the developer removes the portions of the photoresist
that are soluble to thereby expose the underlying substrate. The
exposed portions of the substrate are then etched to a nanometer
scale depth which may be enclosed to form a fluidic channel. Thus,
one iteration of applying the photoresist, exposing the photoresist
to the light pattern, and etching the substrate forms a mono-depth
channel in the substrate. Traditional lithography is therefore
planar with respect to the features formed in a single iteration.
Additional channels or channel depths can be formed using
additional iterations but require precise alignment of the
subsequent photomasks relative to the channels formed in prior
iterations. Features from different iterations must overlap to form
a continuous channel, while nanoscale alignment limitations can
result in inadvertent over- or under-etching of the overlapping
region that limits device design and functionality.
[0005] The inherent dimensional limitations on serial patterning
and alignment limit the geometry, number and size of the channel
depths that can be formed and prevent the fabrication of some
complex three dimensional surface features. Indeed, since the
utility of a nanodevice is in general proportional to its
complexity and dimensionality, current devices provide relatively
limited ability to manipulate biomolecules or other analytes of
interest.
SUMMARY OF THE INVENTION
[0006] An exemplary nanodevice that may be fabricated using a
disclosed nanofabrication processes includes a substrate having an
elongated channel that includes a plurality of nanoscale critical
dimensions arranged as a stepped gradient across the elongated
channel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The various features and advantages of the disclosed
examples will become apparent to those skilled in the art from the
following detailed description. The drawings that accompany the
detailed description can be briefly described as follows.
[0008] FIG. 1 illustrates an example of a nanofabrication
process.
[0009] FIG. 2 illustrates sequential views of selectively etching a
photoresist etch mask and substrate according to a nanofabrication
process.
[0010] FIG. 3 illustrates an example of a nanodevice having an
elongated channel with a stepped gradient across a width of the
channel.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0011] FIG. 1 illustrates an example of nanofabrication process 20
that may be used with a photoresist that is disposed on a substrate
to form a nanodevice. As will be appreciated from the following
description, the nanofabrication process 20 may be adapted to form
a variety of different types of nanodevices that are unavailable
using conventional techniques. In a few examples, the
nanofabrication process 20 may be used to form a nanofluidic
device, integrated circuit, nanomolding tool, resonator, or other
device that would benefit from the ability to form complex three
dimensional surface topographies defined by a plurality of
nanometer scale or nanoscale critical dimensions. As an example,
the terms "nanometer scale" or "nanoscale" may refer to a critical
dimension or characteristic dimension of up to about one-hundred
nanometers. In comparison, larger dimensions of up to one
micrometer may be referred to as "submicrometer" and dimensions
exceeding one micrometer and up to about one-hundred micrometers
may be referred to as "micrometer scale."
[0012] The exemplary nanofabrication process 20 includes an
exposure step 22, a developer step 24, and a transfer step 26. As
will be described, the transfer step 26 may optionally include the
action 28 of controlling an amount of oxygen gas in an etchant gas
mixture used to etch a photoresist and substrate. The following
description of the nanofabrication process 20 will be made with
reference to a substrate and a photoresist layer disposed on the
substrate. The type of substrate and photoresist materials may
vary, depending on the application. However, some examples may
utilize a fused silica substrate having a surface roughness of
approximately less than 5 angstroms and a polymeric photoresist. As
an example, the photoresist may be MEGAPOSIT SPR 700 1.2. The
photoresist may be applied in a known manner, such as by using a
spin coat technique. In some examples, the photoresist may be
applied at an angular acceleration of 8,000 revolutions per minute
and then baked at around 95.degree. C. for about 2 minutes. The
resulting photoresist thickness may be about 1070.+-.10
nanometers.
[0013] Turning first to the exposure step 22, a user of the
nanofabrication process 20 exposes the photoresist to a grayscale
radiation pattern of varied intensity. The term "grayscale" refers
to a controlled radiation intensity over some area of the pattern.
As an example, lower intensity radiation does not penetrate as
deeply into the photoresist as higher intensity radiation. Thus,
the pattern can be designed to imprint a complex three dimensional
topography (e.g., a surface pattern) into the photoresist from the
"top down."
[0014] In a further example of forming a grayscale radiation
pattern, a photomask having a diffractive pattern may be used. For
instance, the photomask may be formed on a transparent substrate
using known techniques. The substrate includes a pattern of opaque
areas, such as squares, disposed thereon such that the photomask
reduces incident radiation into a grayscale pattern.
[0015] In one specific example, a chromium-on-quartz photomask may
be used in conjunction with a reduction stepper as a diffraction or
spatial frequency filter. The photomask may be patterned with a
diffractive array of chromium squares of size s on a square lattice
of pitch p. The reduction stepper illuminates the photomask with
light of wavelength .lamda..sub.s and partial coherence parameter
.sigma..sub.s, and a lithographic lens projects the pattern onto
the photoresist with a reduction factor of 1/M.sub.s. With
appropriate selection of s and p, diffractive orders other than
zero are rejected by the lens aperture. As the zeroth diffractive
order determines only the amplitude of the image intensity,
individual elements within the diffractive arrays are not resolved,
and a grayscale of uniform intensity results. The stepper
resolution determines the critical aerial pitch per Equation (1)
below, while the diagonal spacing between adjacent elements in the
diffractive arrays on the photomask determines the critical square
size per Equation (2) below. Diffractive array pitches larger than
p.sub.c or squares smaller than s.sub.c will result in fluctuations
in aerial intensity as diffractive elements begin to resolve. When
Equations (1) and (2) are satisfied, the aerial image intensity of
a grayscale is represented per Equation (3) below, where I.sub.0 is
the incident illumination intensity.
p c ' = 1 1 + .sigma. s .lamda. s NA s ( 1 ) s c = p - p c 2 2 ( 2
) I GS ' = I 0 ( 1 - ( s 2 p 2 ) ) 2 ( 3 ) ##EQU00001##
[0016] In one example, a staircase function grayscale aerial
intensity pattern may be rendered with diffractive arrays of
chromium squares varying in size from s=1.37 to 2.24 micrometers on
a fixed pitch p=4.00 micrometers. The photomask may have a critical
dimension tolerance of 15 nanometers (absolute error), critical
dimension uniformity of 15 nanometers (maximum range) and an
address unit of 5 nanometers. In this case, the photomask creates
thirty different grayscale depths having an aerial width of 4.00
micrometers defined by diffractive arrays five square elements
wide. The aerial grayscale intensity I.sub.GS normalized by the
incident illumination intensity I.sub.0 is a function of square
size s. Many more grayscales depths can be rendered by varying the
diffractive array lattice structure, pitch, or element shape, or by
specifying a photomask with improved critical dimension tolerance
and uniformity. Non-planar nanofluidic structures with
submicrometer lateral dimensions could also be fabricated by
reducing the width of the diffractive arrays to one diffractive
element per unit pitch.
[0017] A calibration photomask may be used to characterize the
response of a particular type of photoresist to grayscale exposure.
For instance, the incident illumination intensity I.sub.0 is the
dose required to fully clear the photoresist during development. In
one example, an approximately linear response may occur over a
usefully large range and may simplify subsequent nanofabrication
process design.
[0018] After exposure, the substrate and the irradiated photoresist
are developed in the developer step 24. The type of developer used
may depend, for example, on the type of photoresist selected for
use. In this case, the developer removes the irradiated portions
and partially irradiated portions of the photoresist (i.e., a
positive photoresist). The non-irradiated portions are insoluble in
the developer and remain on the substrate. The developer thereby
forms a patterned topography in the photoresist. The patterned
topography corresponds to the pattern imprinted by the grayscale
radiation pattern and includes a plurality of nanoscale critical
dimensions. That is, the grayscale radiation pattern may be
designed to create a desired patterned topography in the
photoresist, with features having critical dimensions of nanoscale
size. The physical structure of a "critical dimension" may depend
on the type of feature but may include dimensions such as
photoresist film thicknesses, feature heights or depths, steps in
photoresist film thickness or feature height or depth, gradients of
smooth surfaces which are sloped or curved, and the like.
Generally, the critical dimension can be regarded as the smallest
geometrical dimension which can be formed.
[0019] Turning now to the transfer step 26, the plurality of
nanoscale critical dimensions of the patterned topography is then
transferred from the photoresist to the substrate. One premise of
this disclosure is that the nanofabrication process 20 provides the
ability to form a plurality of nanoscale critical dimensions that
comprise a complex three dimensional topography, in a substrate in
a single pattern transfer process without the need for multiple
patterning and etching cycles or alignment of photomasks as in
standard photolithography.
[0020] As illustrated in the progressive views of FIG. 2, the
photoresist 40 initially includes a patterned topography 41 having
the plurality of nanoscale critical dimensions 44 (steps in this
example). In this case, the plurality of critical dimensions 44
includes seven steps having nanoscale heights and arranged as a
stepped gradient from a shallowest depth to a deepest depth. A
depth 46, for instance, is less than the next, deeper depth 48 and
so on and so forth. In other examples, the patterned topography 41
may include fewer nanoscale critical dimensions 44 or more
nanoscale critical dimensions, or the topography may have a pattern
that is not a staircase structure. The depths from the surface of
the photoresist 40 may be nanoscale (in which case this is
considered to be a critical dimension) or submicrometer scale, and
the height or step size may also be nanoscale (in which case the
step size is considered to be a critical dimension).
[0021] The photoresist 40 and the substrate 42 are selectively
etched to transfer the plurality of nanoscale critical dimensions
44 to the substrate 42. One example etching process is isotropic
reactive ion etching. For instance, as shown in the middle
progression of FIG. 2, an etchant initially removes the thinnest
portion of the photoresist 40 to expose the underlying substrate
42. Once exposed, the etchant also removes the substrate 42 and
continues to remove the thicker portions of the photoresist 40 to
expose additional substrate 42 area. Thus, the etchant cuts deeper
into the initially exposed area of the substrate 42 than the area
that is last exposed to thereby transfer the plurality of nanoscale
critical dimensions 44 into the substrate 42, as in the bottom
progression. As an example, the etching may be ceased shortly after
the etchant removes the last step of the plurality of nanoscale
critical dimensions 44. The etching duration is selected such that
the thickest portions may not be completely removed.
[0022] The etching is controlled to effect transfer of the
nanoscale critical dimensions 44 in the patterned topography 41 of
the photoresist 40 into the substrate 42. As an example, the
etchant may be an etchant gas mixture that is designed to
selectively etch the photoresist and the substrate 42. In
comparison, the typical desire in traditional photolithograpy is to
limit the etching of the photoresist (e.g., high selectivity) in
order to protect the substrate from exposure. However, the etchant
gas mixture of the nanofabrication process 20 may be a relatively
low selectivity, multi-component mixture for etching the
photoresist 40 and the substrate 42. For instance, the etchant gas
mixture may include a first etchant primarily for etching the
photoresist 40 and a second etchant primarily for etching the
substrate 42. In one example, the etchant gas mixture may include
oxygen gas and a fluorinated gas, such as trifluoromethane gas. The
oxygen generally etches the photoresist 40, while the fluorinated
gas etches the substrate 42.
[0023] A user may control the amount of the oxygen gas in the
etchant gas mixture to establish a desirable etching ratio between
the substrate 42 and the photoresist 40 to transfer a patterned
topography having a plurality of nanoscale critical dimensions 44
in the photoresist 40. The patterned topography transfers as a
corresponding patterned topography having a plurality of nanoscale
critical dimensions 44b in the substrate 42. For instance, the
corresponding patterned topography having a plurality of nanoscale
critical dimensions 44b in the substrate 42 may be a down-scaled
transfer of the patterned topography of plurality of nanoscale
critical dimensions 44 in the photoresist 40. The amount of oxygen
in the etchant gas mixture is controlled to establish an etching
selectivity of about 0.35-0.65. The etching selectivity is a ratio
of an etching removal rate of the substrate 42 to an etching
removal rate of the photoresist 40. The flow rate of oxygen gas may
be controlled to achieve desired etching rates and selectivities.
In one example, the flow rate of the oxygen gas may be about 10-25
standard cubic centimeters per minute, while the flow rate of the
fluorinated gas may be around 50 standard cubic centimeters per
minute with an overall pressure of about 60 milliTorr. Given this
description, one of ordinary skill in the art will be able to
recognize other flow rates to suit their particular needs.
[0024] In the example of FIG. 2, the etching creates an elongated
channel 49 (e.g., extending perpendicular with regard to FIG. 2) in
the substrate 42, with the plurality of nanoscale critical
dimensions 44b arranged as a stepped gradient across the width of
the elongated channel 49. For instance, each step of the stepped
gradient may have a nanoscale depth with regard to the surface (as
represented by the dashed line) of the substrate 42 and/or a
nanoscale step size. The stepped gradient spans across the width of
the channel 49, which may be of a macroscale dimension. As an
example, a macroscale may be a dimension larger than nanoscale,
such as microscale, milliscale or larger. In this respect, the
etching can be controlled to produce desired nanoscale critical
dimensions of the steps. As an example, the steps may include a
depth range and/or step size across several scales from 10
nanometers to 0.6 micrometers.
[0025] In the illustrated example, the steps are generally
perpendicular, however, in other examples the corners of the steps
may be angled non-perpendicularly. In other examples, the gradient
may extend lengthwise along the elongated channel rather than
across the width. As shown, the elongated channel includes about
seven steps. However, in other examples, the nanofabrication
process may be used to form smaller, more discrete steps of the
stepped gradient, or even a smooth slope. For instance, in some
examples, a stepped gradient may include hundreds of steps or even
more than 1,000 steps. Additionally, some examples may have a
geometry containing no multiply etched regions between adjacent
disparate depths, which can result from two or more iterations of
traditional photolithography.
[0026] Different etching selectivities and durations may be used to
fabricate nanostructures with different depth profiles and depth
offsets from a single photomask. As an example, a less selective
etch within the above-given range may be used to make a "shallow"
stepped structure with a step size of about 11 nanometers, no depth
offset, and depths controlled from 11.+-.4 nanometers to 332.+-.4
nanometers (mean.+-.standard deviation) across a 120 micrometer
width of a channel. A more selective etch may be used to make a
"deep" stepped structure with a step size of about 19 nanometers, a
depth offset of approximately two-and-a-half steps, and depths
controlled from 64.+-.4 nanometers to 624.+-.5 nanometers across a
120 micrometer channel width. The measurements may be made using a
scanned probe surface profilometer. The less selective and more
selective etches may result in a root mean square surface roughness
value of about 3 nanometers and 2 nanometers, respectively.
[0027] In use, a cover may be provided over or around the channel
49 such that the enclosed channel 49 includes an inlet or inlets at
one end and an outlet or outlets at the other end for transporting
a material to be analyzed. The nanodevice may also include other
structures or components that function in cooperation with the
channel 49 for the purpose of facilitating movement of the material
through the channel or analyzing the material.
[0028] FIG. 3 illustrates one implementation of the elongated
channel 49. In this example, the elongated channel 49 is included
within a nanofluidic device 50. The elongated channel 49 includes
an inlet 52 at one end and an outlet 54 at the other end.
[0029] The nanodevice 50 further includes first and second voltage
control channels 56a and 56b arranged with the channel 49
therebetween. Lateral channels 58 extend between the voltage
control channels 56a and 56b and through the channel 49.
[0030] In use, conductive fluids such as electrolyte solutions
flowing through the voltage control channels 56a and 56b facilitate
generating an electric field 60 across the channel 49. As an
example, the voltage in the second voltage control channel 56b may
be greater than the voltage in the first voltage control channel
56a. The applied voltages cooperate with the lateral channels 58 to
create a voltage axial offset through the channel 49 that results
in an electric field 60 that is oriented in a direction that is
transverse to the lengthwise direction of the channel 49.
[0031] The electric field 60 facilitates electrokinetically
transporting materials through the channel 49 between the inlet 52
and the outlet 54. As an example, an axial component of the
electric field 60 is oriented along the lengthwise direction of the
channel 49 and functions to move material within the channel 49
towards the outlet 54. A lateral component of the electric field 60
that is oriented in a direction perpendicular to the lengthwise
direction of the channel 49 functions to drive the material toward
the shallow side of the channel 49. As can be appreciated, smaller
sized materials will be driven farther into the shallow end of the
channel 49 before interfering with the steps of the gradient, which
facilitates variably confining and manipulating the materials for
the purpose of analysis. In this example, the materials are
electrokinetically-driven through the channel; however, in other
examples, the materials may be hydrodynamically-driven, or the
like.
[0032] The elongated channel 49 and electric field 60 may be used
for many different purposes. As an example, the elongated channel
49 may be used for the separation and characterization of
nanomaterials, such as nanoparticles, biomolecules, or the like,
via the injection of an analyte into the channel such that
nanomaterials in the analyte are driven down the channel and across
the width of the channel into the shallow side. The steps of the
gradient of the channel exclude rigid nanoparticles by size within
spatially separate regions of the channel. A size distribution of
the nanoparticles may then be determined using fluorescence
microscopy of other applicable technique. Biomolecules or other
flexible nanomaterials may enter the shallow side of the channel
and then be transported, concentrated, separated, and organized by
the complex nanoscale confinement resulting from the plurality of
nanoscale critical dimensions of the channel.
[0033] Although a combination of features is shown in the
illustrated examples, not all of them need to be combined to
realize the benefits of various embodiments of this disclosure. In
other words, a system designed according to an embodiment of this
disclosure will not necessarily include all of the features shown
in any one of the Figures or all of the portions schematically
shown in the Figures. Moreover, selected features of one example
embodiment may be combined with selected features of other example
embodiments.
[0034] The preceding description is exemplary rather than limiting
in nature. Variations and modifications to the disclosed examples
may become apparent to those skilled in the art that do not
necessarily depart from the essence of this disclosure. The scope
of legal protection given to this disclosure can only be determined
by studying the following claims.
* * * * *