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RLE Progress Report Number 138
Chapter 1. Genosensor Technology Development
Chapter 1. Genosensor Technology Development
Academic and Research Staff
Dr. Daniel J. Ehrlich,' Dr. Mark A. Hollis, Dr. Dennis D. Rathman, Dr. Albert M. Young 2
Graduate Students
Jeffrey T. Chiou
1.1 Introduction
The primary objective of our cooperative work with
the Houston Advanced Research Center (HARC) is
to develop a novel method for automated, low-cost,
high-throughput DNA sequence analysis.
The
overall goal is to demonstrate laboratory prototypes
that provide a substantial increase in speed over
the conventional DNA sequencing methods now
used in the biomedical, pharmaceutical, and agricultural industries.
The basic approach being taken is depicted in
figure 1. In a hypothetical DNA sequencing test, a
solution of single-stranded "target" DNA strands of
identical but unknown sequence is washed onto a
specialized miroelectronic chip called a genosensor. The genosensor surface contains a large
array of test sites, each site containing short pieces
of single-stranded DNA known as "probes." These
probes are chemically attached to the site. All
probes in a given site are of like sequence, and the
sequence for each site is unique on the chip. The
target DNA strands will bond, or hybridize, very
strongly to probes containing their exact WatsonCrick complement, but much less strongly to probes
on other sites. The sites containing hybridized DNA
are identified via electronic sensing on the chip, and
this information is used by off-chip instrumentation
to reconstruct the sequence of the target strands.
MIT's role in this effort is to perform the design and
fabrication of the genosensor chips.
1.2 Development of Genosensor Arrays
for DNA Decoding
Sponsor
Houston Advanced Research Center
Contract HRC-HG00665-01
Project Staff
Dr. Daniel J. Erhlich, Dr. Dennis D. Rathman, Dr.
Mark A. Hollis
1.2.1 Genosersor Electronic-Detection
Principle
The simplest electrical measurement that can be
made at a test site to detect hybridization is probably a measurement of the change in local permittivity due to the addition of long target strands to
The complex permittivity E'-jE" of an
the site.
aqueous solution containing DNA exhibits a dispersion around a relaxation frequency which is a function of the size and conformation of the DNA
molecule.
A measurement of the capacitance
and/or conductance between two electrodes in the
solution over a range of frequency can therefore differentiate between a site that contains only short
probe strands and one that contains long target
From
strands hybridized to the probe strands.
these measurements the relative permittivity E', the
dielectric loss E", and the dissipation factor E"/E' can
be obtained for the cell.
The ideal electrode structure in a test well consists
of two parallel plates spaced so that the entire
volume between them is filled by the hybridized
DNA globules in aqueous solution. For the sizes of
target DNA envisioned, this spacing ranges from
approximately 200 to a few thousand angstroms. A
1 MIT Whitehead Institute for Biomedical Research, Cambridge, Massachusetts.
2
MIT Whitehead Institute for Biomedical Research, Cambridge, Massachusetts, and MIT Lincoln Laboratory, Lexington, Massachusetts.
365
Chapter 1. Genosensor Technology Development
GENOSENSOR SYSTEM
)SENSOR
INSTRUMENTATION
COMPUTER
"AGTCG"
Figure 1. Conceptual genosensor system.
DETECTION- ELEMENT CROSS SECTION
1 TI
Au
I
AQUEOUS DNA
,4---
Siz3 Y
2 A--SOLUTION
UNHYBRIDtZED
S102
Au
DNA PROBES
SI SUBSTRATE
Figure 2. Electrode design for a permittivity genosensor.
The unit cell shown is repeated many times across a test
well to form an interdigitated test structure with the top
Au electrodes connected to one access line and the
bottom Au electrodes to the other.
practical, easily fabricated structure that approximates this ideal is an interdigitated design shown in
figure 2. Fabricated by a combination of wet and
dry etching with metal liftoff, this design can
achieve the required spacings between the upper
and lower electrodes at their edges.
Devices
having either Au or Pt electrodes have been fabricated, with Pt preferred for most applications.
Figure 3 shows various aspects of completed genosensor devices.
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RLE Progress Report Number 138
1.2.2 Genosensor Fabrication Development
The primary emphasis of this grant is on the
process development and feasibility exploration for
the genosensor design of figure 2. For this chip,
conventional 2-pm photolithography, wet and dry
etching techniques, and electron-beam evaporation
and liftoff processes are used to fabricate an
interdigitated electrode structure in which the
opposing electrodes are separated vertically. A
second genosensor design has also been designed
and fabricated where the interdigitated electrode
structure has the electrodes separated, horizontally,
in the same plane, as shown in figure 4.
During the past year, significant progress has been
made in process development for both genosensor
types. Yield and reproducibility of 6x6 passive
arrays of various active-area sizes have in all cases
been dramatically improved to greater than 90
percent. Problems related to the lift-off of the Pt
device electrodes and interconnect lines have been
solved. As detailed in last year's report, electrical
measurements provided strong motivation to
develop a buried-interconnect, or insulated-lead,
process for both types of genosensor devices to
eliminate potentially spurious signals generated by
the uninsulated leads. The use of hard-baked
polyimide as a second-layer dielectric deposited on
the Pt electrode/interconnect structure was abandoned, because it produced inconsistent results due
Chapter 1. Genosensor Technology Development
to the apparent degradation of the layer after exposure to the DNA strand attachment chemistries.
For the planar electrode geometry of figure 4, the
Pt electrodes are placed directly on a Si3N4 layer.
This enables the use of a deposited CVD SiO 2 film
(deposition temperature 4250 C) as the coveredlead dielectric. Vias in the oxide to expose the
active device electrodes can easily be etched
without attacking the underlying nitride dielectric.
For the split-architecture electrode design illustrated
in figure 2, the use of CVD SiO 2 for the coveredlead dielectric becomes more difficult because the
bottom electrode sits on an etched SiO 2 surface.
However, the lower oxide is thermally grown and
has an etch rate in buffered HF that is approximately 10 times lower than the CVD oxide. This differential etch rate has enabled us to cover the
device interconnects and subsequently remove this
deposited oxide from the active device area without
significantly attacking the underlying thermal oxide
and affecting the Pt electrode adhesion. We were
prepared to incorporate spin-on glass as the
covered-lead dielectric, since the differential etch
rate compared to thermal oxide is much greater for
spin-on glass than for our CVD oxide. However,
this proved to be unnecessary.
For the 6x6 passive arrays, each device's
input/output lead is completely isolated from
adjacent devices. Ultimately, high-speed DNA
sequencing will require the use of very large arrays
of genosensor devices (e.g., 1024x1024). With
limited space per chip for input/output pins, these
large devices require the use of crossbar switching
arrays where the sensor sites are connected in
rows and columns, as shown in figure 5. The fabrication of such an array requires the development of
a two-level metal, two-level dielectric process. The
insulated-lead process described above is the first
element required for the development of a crossbar
(b)
t -.--00
-
m
I
(c)
(d)
Figure 3. Collection of photographs showing various views of a complete genosensor device. (a) Top view of the standard 100 pm X 100 pm single test cell. (b) Top view of the 6 X 6 passive array of test cells. (c) Scanning electron
micrograph (SEM) closeup of the interdigitated electrode fingers. (d) Genosensor chip in electronic package.
367
Chapter 1. Genosensor Technology Development
switching array architecture. If the interdigitated
electrodes and either the row or column feeds to
the active cell are patterned as the first metal layer,
the fabrication sequence of the crossbar switching
array has only one additional step compared to the
processing steps required for the buried-lead
device. The leads are first covered over with the
second-level dielectric (CVD oxide), and the active
device area and contact pads are exposed by viahole etching down to the first-level metal layer.
Then, the second layer of metal can be lifted off
and the device completed.
Figure 5 shows two views of the first 16x16
crossbar switching array genosensor, which was
designed and completed this year utilizing the
planar interdigitated device geometry. Also visible in
the figure, along the array diagonal, are lines of
open and short test structures for rf signal calibration. The 16x16 design layout and the
admittance parameters for the active cell were
determined from both experimental data and
numerical simulations, in combination with an analysis of how the ultimate impedance-bridge measurements are obtained.
During the past year, we have delivered 22 packaged devices, as well as several unpackaged chips
for DNA attachment experiments, to HARC for tests
for this project. The devices are presently being
characterized by HARC.
1.3 Microdetection Technology for
Automated DNA Sequencing
Sponsor
Genometrix, Inc.
Contract GMX-GH00776-04
Project Staff
Dr. Daniel J. Erhlich, Dr. Dennis D. Rathman, Dr.
Mark A. Hollis
The primary emphasis of this effort is to complement the program described above by optimizing
the electrode geometry for maximum sensitivity.
Initial work for this contract began with the
electrode configuration illustrated in figures 2 and 3.
Subsequently, both experimental studies and theoretical modeling have shown a need to design and
fabricate more aggressive electrode geometries,
which are now being developed as described
below.
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RLE Progress Report Number 138
Figure 4. Interdigitated electrode structure where the
electrodes are coplanar and non-self-aligned.
Measurements by our colleagues at HARC and
Baylor College of Medicine have shown that DNA
attachment is more easily achieved to SiO 2 or polysilicon than to Pt or Au. In fact, surface-attachment
densities of DNA probes on oxide-coated polysilicon
are typically one order of magnitude higher compared to the Pt electrodes of our conventional
permittivity-chip designs. The exact reasons for this
are not yet known but are probably related to (1)
the different linker chemistries used for attachment,
or (2) the strong ionic and polarization effects
observed at liquid-metal interfaces. During the past
year, we have completed and shipped several permittivity chips with polysilicon electrodes to
Genometrix and HARC for test. For these first
wafers, the polysilicon electrode fingers were patterned by dry etching using a conservative heightto-width aspect ratio for the electrode fingers.
In addition to the polysilicon coplanar design, two
very aggressive self-aligned or split-architecture
designs are being pursued using a Ti-Pt electrode
metallization. These devices feature submicrometerperiodicity finger spacings in both the vertical and
horizontal dimensions. The patterning of the
electrodes is being done by a combination of laser
interferometric lithography and reactive ion etching.
A scanning electron micrograph of one of these
devices is shown in figure 6. The electrode linewidth and spacing is 160 nm, while the vertical separation of the interdigitated electrodes is
approximately 200 nm. This geometry yields a 12.52
times improvement in the electrode density per cm
of device area. The fingers themselves are Si 3N4 to
a depth of 200 nm, where thermal oxide is
exposed.
Chapter 1. Genosensor Technology Development
I
Figure 5. 16x16 crossbar array genosensors. (a)(top) Two arrays on a 1-inch silicon chip. (b)(bottom) Closeup of
sensor sites.
369
Chapter 1. Genosensor Technology Development
Figure 6.
Submicrometer-periodicity genosensor. (a)(left) Overall view of sensor site. (b)(right) Closeup of electrode
geometry. Note the 0.32-pm periodicity.
The use of oxide at the finger bottoms enables a
slight undercut to be obtained by wet etching of the
underlying oxide. It is anticipated that linker chemistries designed to attach either to the closely packed
Pt electrodes or to the supporting dielectric posts
would enable a substantial increase in probe
density at the sensor site with a corresponding
increase in detection sensitivity. An alternative
submicrometer-spaced electrode geometry, shown
in figure 7, is an array of 160-nm-diameter holes
where the center-to-center spacing is 320 nm.
Here, a continuous metal sheet (lower) forms one
electrode and an upper layer of metal (perforated
with the holes) forms the other electrode, with the
CAVITIES
CLOSED CAVITIES
electrodes separated by a thin film of deposited
CVD oxide. The fabrication of this electrode structure is accomplished by an extension of the laser
interferometric technique where two perpendicular
exposures are used. This technique has been
greatly developed and refined through another
ongoing program to produce small-geometry gated
electron emitters for a variety of vacuummicroelectronics applications. The hole density for
this closely packed geometry is 109 per cm 2. As
with the linear array design, the intended purpose
of this aggressive approach is increased sensitivity.
A number of submicrometer devices are expected
to be delivered during the coming year.
OPEN CAVITIES
OPEN CAVITIES
0.1 pm
0.1 pm
Figure 7. Alternate submicrometer-periodicity genosensor. The upper metal electrode is perforated with holes to allow
DNA-probe attachment to the lower metal layer in the cavities.
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RLE Progress Report Number 138
Chapter 1. Genosensor Technology Development
1.4 Advanced MEMS for
High-Throughput DNA Sequencing
Sponsor
National Institutes of Health
Grant 1-R01-HG01389
Project Staff
Dr. Daniel J. Ehrlich, Dr. Albert M. Young, Jeffrey T.
Chiou
The goal of our program is to explore the opportunities that arise from using microelectromechanical
systems (MEMS) to improve the throughput of automated DNA sequencing. This program is an outgrowth of similar work in microfluidic biodevices.
Microelectromechanical systems technology is the
only readily available technology that offers the truly
significant improvements in lane density needed for
the Human Genome Project and the ability to
satisfy long-term sequencing needs. In order to
sequence a single human genome with 5x redundancy, we need to sequence at least 50,000,000
templates. To finish this project in three years
would require 150 state-of-the-art instruments
running continuously. Other technologies, such as
capillary electrophoresis, may eventually reduce the
number of instruments required to approximately
50. Although these methods may be considered
technically feasible, they are costly, timeconsuming, and fail to address parallelism, which is
the most important requirement for future DNA
sequencing applications.
In order to develop the technology and instrumentation needed to perform high-throughput DNA
sequencing, we developed a technology base
under ARPA funding covering the important subsystems used in DNA sequencing:
*
Injection of linear polyacrylamide gel matrices;
*
Piezoelectrically-driven
sample loading;
*
Microfabrication of separation components in
glass;
*
Extension of microfabrication
large-area plates; and
*
Laser-induced fluorescence detection.
droplet-injection
IColed
water bath
Figure 8. A schematic of the optical subsystem.
Fluorescently-tagged DNA is to be electrophoresed from
left to right. Important features to note are: (1) the microfabricated plates are held down against a thermal cooling
bath; (2) side illumination is used to excite all lanes
simultaneously, and (3) a prism is used to separate out
the four fluorophores corresponding to the bases A, C, G,
and T. The microfabrication design to be used is also of
some interest.
In conjunction with our previously developed
custom robotics hardware and software for XYZ
manipulation and the automation of sample
preparative processing being developed concurrently at the Whitehead Institute, we have generated a new design for MEMS-based DNA
sequencing instrumentation which has recently
been funded.
From a systems perspective, the basic instrument
will involve the following elements:
*
A DNA separation subsystem based on largearea MEMS fabrication technology;
*
A CCD-based optical detection subsystem with
interfacing to base-calling; and
*
Automated sample-loading robotics.
for
techniques to
371
Chapter 1. Genosensor Technology Development
Lateral confinement by
expansion of beam waist
//A
ZA
/A
DNA peaks
=lectrophoresed through
nicrofabricated channels
Vertical confinement 1by
optical waveguiding
4
/I
Multiple waveguides
illuminated simultaneously
Figure 9. An enlarged schematic of the design for the illumination zone. In this schematic, DNA peaks again are being
electrophoresed from left to right through the microfabricated cnannels. A small gap in the channels is fabricated to
allow the excitation beam to pass through all lanes without interference from the glass sidewalls. Since the beam will be
impinging on the structure at near-grazing incidence, the beam experiences a high degree of reflectivity from the gelglass interface, and optical losses in the vertical direction are low. Confinement in the lateral direction is obtained by
defocussing the waist out to a diameter of approximately 150 pm. Calculation of the beam confocal parameter shows
there is very little beam spreading for this defocussed case. However, the beam width is still tight enough to provide
good resolution during the readout without having to resort to deconvolution methods.
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RLE Progress Report Number 138