U.S. patent application number 12/682608 was filed with the patent office on 2010-09-02 for sensor, a sensor array, and a method of operating a sensor.
This patent application is currently assigned to NXP B.V.. Invention is credited to Franciscus Widdershoven.
Application Number | 20100221846 12/682608 |
Document ID | / |
Family ID | 40251755 |
Filed Date | 2010-09-02 |
United States Patent
Application |
20100221846 |
Kind Code |
A1 |
Widdershoven; Franciscus |
September 2, 2010 |
SENSOR, A SENSOR ARRAY, AND A METHOD OF OPERATING A SENSOR
Abstract
In an example embodiment, there is a sensor for detecting
particles. The sensor comprises an electrode, a sensor active
region covering the electrode and the sensor active region is
sensitive-for the particles. A first switch element is operable to
bring the electrode to a first electric potential when the first
switch element is closed, and a second switch element is operable
to bring the electrode to a second electric potential when the
second switch element is closed. A detector is adapted to detect
the particles based on a change of the electric properties of the
sensor in an operation mode in which the electrode is brought to
the first electric potential and an operation mode in which the
electrode is brought to the second electric potential.
Inventors: |
Widdershoven; Franciscus;
(Eindhoven, NL) |
Correspondence
Address: |
NXP, B.V.;NXP INTELLECTUAL PROPERTY & LICENSING
M/S41-SJ, 1109 MCKAY DRIVE
SAN JOSE
CA
95131
US
|
Assignee: |
NXP B.V.
Eindhoven
NL
|
Family ID: |
40251755 |
Appl. No.: |
12/682608 |
Filed: |
October 7, 2008 |
PCT Filed: |
October 7, 2008 |
PCT NO: |
PCT/IB08/54102 |
371 Date: |
April 12, 2010 |
Current U.S.
Class: |
436/512 ;
422/82.01; 436/94 |
Current CPC
Class: |
G01N 33/5438 20130101;
Y10T 436/143333 20150115; C12Q 1/6825 20130101 |
Class at
Publication: |
436/512 ; 436/94;
422/82.01 |
International
Class: |
G01N 33/563 20060101
G01N033/563; G01N 33/50 20060101 G01N033/50; G01N 27/00 20060101
G01N027/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 12, 2007 |
EP |
07118382.6 |
Claims
1. A sensor for detecting particles, the sensor comprising an
electrode; a sensor active region covering the electrode and being
sensitive for the particles; a first switch element operable to
bring the electrode to a first electric potential when the first
switch element is closed; a second switch element operable to bring
the electrode to a second electric potential when the second switch
element is closed; a detector adapted to detect the particles based
on a change of the electric properties of the sensor in an
operation mode in which the electrode is brought to the first
electric potential and an operation mode in which the electrode is
brought to the second electric potential.
2. The sensor of claim 1, wherein the electrode is a sub-micron
electrode, particularly a nanoelectrode.
3. The sensor of claim 1, wherein the first switch element and the
second switch element are transistors, wherein the electrode is
coupled to a first source/drain region of a transistor forming the
first switch element and is coupled to a first source/drain region
of a transistor forming the second switch element; wherein the
first electric potential is coupled to a second source/drain region
of the transistor forming the first switch element; wherein the
second electric potential is coupled to a second source/drain
region of the transistor forming the second switch element.
4. The sensor of claim 1, wherein the sensor active region
comprises at least one capture probe adapted for hybridizing with
the particles.
5. The sensor of claim 4, comprising a self assembled monolayer
between the electrode and the at least one capture probe.
6. The sensor of claim 1, comprising a clock adapted for providing
the first switch element and the second switch element with clock
signals to operate the first switch element and the second switch
element to alternate between an operation mode in which the first
switch element is closed and the second switch element is
simultaneously open and an operation mode in which the first switch
element is open and the second switch element is simultaneously
closed.
7. The sensor of claim 1, wherein the detector is adapted to detect
the particles based on a net charge transferred between a node
providing the first electric potential and a node providing the
second electric potential during one or more cycles in which, in an
alternating sequence, the electrode is brought to the first
electric potential and in which the electrode is brought to the
second electric potential.
8. The sensor of claim 1, wherein the detector is adapted to detect
the particles based on a change of a capacitance in an operation
mode in which the electrode is brought to the first electric
potential and an operation mode in which the electrode is brought
to the second electric potential.
9. The sensor of claim 1, comprising a further electrode configured
to be kept at a fixed third electric potential.
10. The sensor of claim 1, manufactured in CMOS technology.
11. The sensor of claim 1, adapted as a biosensor, particularly
adapted as one of the group consisting of a single-molecule
biosensor, a capacitive biosensor, or an electrochemical
biosensor.
12. A sensor array, comprising an arrangement of a plurality of
sensors of claim 1.
13. The sensor array of claim 12, wherein the plurality of sensors
are arranged in rows and columns, wherein the first electric
potential is provided in common for at least two, particularly for
all sensors of a column and the second electric potential is
provided in common for at least two, particularly for all sensors
of a row.
14. The sensor array of claim 13, wherein clock signals of a clock
are provided in common for at least two, particularly for all
sensors of a row.
15. The sensor array of claim 13, wherein sensors in adjacent rows
are arranged upside down to one another to share one of the group
consisting of the first electric potential and the second electric
potential.
16. The sensor array of claim 13, wherein sensors in adjacent
columns are arranged flipped left/right to one another to share one
of the group consisting of the first electric potential and the
second electric potential.
17. The sensor array of claim 12, monolithically integrated in a
common substrate.
18. The sensor array of claim 17, wherein the first switch element,
the second switch element and the detector of the plurality of
sensors are arranged within the substrate; wherein the electrode
and the sensor active region of the plurality of sensors are
provided at a surface of the substrate; the sensor array further
comprising a moisture resistant structure at the surface of the
substrate between adjacent ones of the electrodes of the plurality
of sensors.
19. The sensor array of claim 13, comprising a selection unit
adapted for selecting one of the rows for sensing, wherein the
selection unit is further adapted for disabling all other rows from
sensing by opening the first switch element and the second switch
element of the all other rows.
20. The sensor array of claim 13, comprising a selection unit
adapted for selecting one of the rows for sensing, wherein the
selection unit is further adapted for disabling all other rows from
sensing and for closing the first switch element or the second
switch element of at least a part of the all other rows to thereby
provide one of the group consisting of a counter electrode
functionality and a reference electrode functionality.
21. The sensor array of claim 13, comprising a row periphery
circuit comprising a number of multiplexers adapted for gating the
rows.
22. The sensor array of claim 21, wherein the row periphery circuit
comprises five multiplexers for each pair of rows, the five
multiplexers being configured to provide clock signals (.PHI.T,
.PHI.D) to operate the first switch element and the second switch
element and to provide the first electric potential or the second
electric potential to the sensors of a respective pair of rows.
23. The sensor array of claim 13, comprising a column periphery
circuit adapted for gating the columns.
24. The sensor array of claim 13, comprising a calibration row
having one or more calibration units each of which being
constituted as each of the plurality of sensors but being free of
an electrode and of a sensor active region.
25. The sensor array of claim 12, wherein the detector of at least
a part of the plurality of sensors is adapted to perform a
self-referencing function by comparing a detection signal with an
average detection signal of at least a part of the other sensors,
particularly by comparing the detection signal with an average
detection signal of other sensors of a row.
26. The sensor array of claim 12, wherein the detector is adapted
to detect the particles in an operation mode in which the electrode
is statically brought to the first electric potential and is
statically decoupled from the second electric potential.
27. The sensor array of claim 13, wherein active regions and
polysilicon lines of the sensor array are formed by orthogonal
continuous stripes with contact holes to source/drain parts of the
active regions between every pair of adjacent polysilicon
lines.
28. The sensor array of claim 13, wherein a detector for measuring
transferred charge comprises a source follower transistor to
control the first electric potential on the columns, an integration
capacitor provided by a gate capacitance of a further transistor
that also serves as a read out transistor, a reset transistor, and
a selection transistor connected to a read line being part of a
read bus.
29. The sensor array of claim 13, wherein a detector for measuring
transferred charge comprises a source follower, a reset transistor,
a separate integration capacitor, and transistors of opposite
conduction type so that one of these transistors is operable as a
source follower to measure a voltage on the integration
capacitor.
30. The sensor array of claim 13, comprising at least one on-chip
digital circuit to accumulate or average consecutive measurements
performed on the same sensors, particularly performed by sensors of
a row, to thereby improve the signal to noise ratio.
31. A method of detecting particles using a sensor, the method
comprising bringing a sensor active region covering an electrode in
contact with the particles; bringing the electrode to a first
electric potential; subsequently bringing the electrode to a second
electric potential; detecting the particles based on a change of
the electric properties of the sensor, particularly of the
electrode, in an operation mode in which the electrode is brought
to the first electric potential and an operation mode in which the
electrode is brought to the second electric potential.
32. The method of claim 31, comprising operating the sensor so that
double sampling is used to reduce an effect of a low frequency
noise of a read out transistor.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a sensor.
[0002] Furthermore, the invention relates to a sensor array.
[0003] Moreover, the invention relates to a method of operating a
sensor.
BACKGROUND OF THE INVENTION
[0004] A biosensor may be denoted as a device which may be used for
the detection of an analyte that combines a biological component
with a physicochemical or physical detector component.
[0005] For instance, a biosensor may be based on the phenomenon
that capture molecules immobilized on a surface of a biosensor may
selectively hybridize with target molecules in a fluidic sample,
for instance when an antibody-binding fragment of an antibody or
the sequence of a DNA single strand as a capture molecule fits to a
corresponding sequence or structure of a target molecule. When such
hybridization or sensor events occur at the sensor surface, this
may change the electrical properties of the surface which can be
detected as the sensor event.
[0006] US 2004/0110277 discloses a bio-sensor comprising a sensor
cell matrix in which sensor cells are arranged into a matrix, a row
driver which supplies a specific voltage signal to a group of
sensor cells lined up in the row direction of the matrix, and a
column driver which supplies a specific voltage signal to a group
of sensor cells lined up in the column direction of the matrix.
Each sensor cell comprises a capacitance element consisting of a
pair of opposing electrodes with probe DNA molecules that react
selectively with target DNA molecules immobilized to their
surfaces, a transistor whose gate terminal is connected to the
capacitance element so that the current value that is output from
the drain terminal of this transistor is caused to vary in
accordance with the amount of the capacitance variation of the
capacitance element which is varied by the hybridization of the
DNA, and a switching element which supplies a voltage signal
supplied from the column driver to the current input terminal of
the transistor.
[0007] Conventional sensor chips may suffer from a signal to noise
ratio which may be too small.
OBJECT AND SUMMARY OF THE INVENTION
[0008] It is an object of the invention to provide a sensor having
a sufficiently large signal to noise ratio.
[0009] In order to achieve the object defined above, a sensor, a
sensor array, and a method of operating a sensor according to the
independent claims are provided.
[0010] According to an exemplary embodiment of the invention, a
sensor for detecting particles is provided, the sensor comprising
an electrode (which may form one plate of a capacitor), a sensor
active region (for instance comprising capture molecules) covering
the electrode (for instance being arranged in such a manner that
sensor events occurring at the sensor active surface have an impact
on the electric condition of the electrode) and being sensitive for
the particles, a first switch element operable to bring the
electrode to a first electric potential (at a first time) when the
first switch element is closed (that is when an electrically
conductive coupling between the electrode and a node providing the
first electric potential is enabled by the first switch), a second
switch element operable to bring the electrode to a second electric
potential (at a second time which may differ from the first time)
when the second switch element is closed (that is when an
electrically conductive coupling between the electrode and a
further node providing the second electric potential is enabled by
the second switch), and a detector adapted to detect the particles
based on a change of the electric properties of the sensor in an
operation mode in which the electrode is brought to the first
electric potential and an operation mode in which the electrode is
brought to the second electric potential (for instance, the amount
of the change may be dependent on the presence/absence or the
concentration of the particles, since an accumulation of (for
instance dielectric) particles in an environment of the sensor
active region may change the electric properties, particularly the
capacity, of the electrode).
[0011] According to another exemplary embodiment of the invention,
a sensor array is provided comprising an arrangement of a plurality
of sensors having the above-mentioned features. However, multiple
sensors may share the same electric potentials or sources of
electric potentials.
[0012] According to still another exemplary embodiment of the
invention, a method of detecting particles using a sensor is
provided, the method comprising bringing a sensor active region
covering an electrode in contact with the particles, bringing the
electrode to a first electric potential, subsequently bringing the
electrode to a second electric potential, and detecting the
particles based on a change of the electric properties of the
sensor in an operation mode in which the electrode is brought to
the first electric potential and an operation mode in which the
electrode is brought to the second electric potential.
[0013] The term "sensor" may particularly denote any device which
may be used for the detection of the presence/absence or even the
concentration of particles.
[0014] The term "biosensor" may particularly denote any device
which may be used for the detection of an analyte comprising
biological molecules such as DNA, RNA, proteins, enzymes, cells,
bacteria, virus, etc. A biosensor may combine a biological
component (for instance capture molecules at a sensor active
surface capable of detecting molecules) with a physicochemical or
physical detector component (for instance a capacitor having a
capacitance which is modifiable by a sensor event).
[0015] The term "(bio)sensor chip" may particularly denote that a
(bio)sensor is formed as an integrated circuit, that is to say as
an electronic chip, particularly in semiconductor technology, more
particularly in silicon semiconductor technology, still more
particularly in CMOS technology. A monolithically integrated
biosensor chip has the property of very small dimensions thanks to
the use of micro-processing technology, and may therefore have a
large spatial resolution and a high signal-to-noise ratio
particularly when the dimensions of the biosensor chip or more
precisely of components thereof approach or reach the order of
magnitude of the dimensions of biomolecules.
[0016] The term "biological particles" may particularly denote any
particles which play a significant role in biology or in biological
or biochemical procedures, such as DNA, RNA, proteins, enzymes,
cells, bacteria, virus, etc.
[0017] The term "sensor active region" may particularly denote an
exposed region of a sensor which may be brought in interaction with
a fluidic sample so that a detection event may occur in the sensor
active region. In other words, the sensor active region may be the
actual sensitive area of a sensor device, in which area processes
take place which form the basis of the sensing.
[0018] The term "substrate" may denote any suitable material, such
as a semiconductor, glass, plastic, etc. According to an exemplary
embodiment, the term "substrate" may be used to define generally
the elements for layers that underlie and/or overlie a layer or
portions of interest. Also, the substrate may be any other base on
which a layer is formed, for example a semiconductor wafer such as
a silicon wafer or silicon chip. Also a layer sequence may fall
under the term substrate as used herein. Such a layer sequence may
be formed on and/or in a substrate, that is may be a part
thereof.
[0019] The term "fluidic sample" or "analyte" may particularly
denote any subset of the phases of matter. Such fluids may include
liquids, gases, plasmas and, to some extent, solids, as well as
mixtures thereof. Examples for fluidic samples are DNA-containing
fluids, blood, interstitial fluid in subcutaneous tissue, muscle or
brain tissue, urine or other body fluids. For instance, the fluidic
sample may be a biological substance. Such a substance may comprise
proteins, polypeptides, nucleic acids, DNA strands, etc. The
analyte may particularly denote a substance that contains the
bio-molecules to be analysed (for instance, blood plasma, saliva,
urine, food samples, etc., usually after pre-processing).
[0020] The term "capture probe" may particularly denote a molecule
that can capture specific target molecules from an analyte.
[0021] The term "electrolyte" may particularly denote a substance
containing free ions that behaves as an electrically conductive
medium (for instance saline water).
[0022] The term "electrolytic capacitor" may particularly denote a
capacitor comprising or consisting of a metal electrode, coated
with an insulating layer (the dielectric), and an electrolyte
electrode. The electrolyte can be connected by another metal with a
conducting interface to the electrolyte.
[0023] The term "redox couple" may particularly denote molecules
that can exchange one or more electrons with the electrode
surfaces.
[0024] The term "SAM" may particularly denote a self-assembled
monolayer of organic molecules. A SAM may denote a surface
consisting of a single layer of molecules on a substrate. Self
assembled monolayers can be prepared simply by adding a solution of
the desired molecule onto the substrate surface and washing off the
excess.
[0025] According to an exemplary embodiment of the invention, a
sensor element is provided which has two switch elements adapted
for selectively coupling or decoupling an electrode in functional
contact with a sensor active portion provided between the two
switch elements to one of two different electric potentials (which
may be denoted as a transfer potential and a discharge potential).
The sensor active region and the electrode may together form a
capacitor like configuration (which may be completed by an
electrolyte electrode) in which the value of the capacity is
dependant on whether a sensor event takes place at the sensor
active region or not. Consequently, when first coupling such a
sensing capacitor with the first electric potential and afterwards
with the second electric potential, a net charge flow during such a
procedure which may be optionally repeated once or several times
may be a characteristic parameter indicative of a sensor element,
and therefore indicative of the qualitative or quantitative
determination of the particles. This or another electric parameter
may be detected by a detector (for instance by an amperemeter) and
may allow to detect a sensor event with high accuracy, even in a
scenario in which only a single biological molecule hybridizes with
a corresponding complementary capture probe immobilized on the
sensor active region.
[0026] Such an architecture may be particularly advantageous when
nanoelectrodes are employed which are manufactured sufficiently
small. For example, such nanoelectrodes can be made with dimensions
of 250 nm, 130 nm or less, and may for instance be realized as
sensing pockets having dimensions close to dimensions of biological
molecules to be detected. This may allow to obtain a significant
improvement of the signal-to-noise ratio. For instance, on a copper
nanoelectrode, a self-assembled monolayer (SAM) may be provided
which may be specifically designed to attach capture molecules such
as antibodies. The copper electrode may then serve, in combination
with a second electrode which can be another metallization layer of
the semiconductor layer sequence or which can be a counter
electrode which may be provided apart from the semiconductor layer
sequence, as a capacitor. Sensor events (such as hybridization
events between capture molecules immobilized on the SAM layer and
target molecules in the sample) may then modify the value of the
capacitance of the capacitor.
[0027] In the following, further exemplary embodiments of the
sensor will be explained. However, these embodiments also apply to
the sensor arrangement and to the method.
[0028] The electrode may be a sub-micron electrode. In other words,
the electrode may have linear dimensions in the order of magnitude
of a micrometer or less. Particularly, the electrode can be a
nanoelectrode, particularly can have dimensions in a range of
essentially one nanometer to essentially some hundred nanometers.
By providing electrodes with such a small dimension and
consequently with such a small area, the detection sensitivity may
be significantly improved, since in such a configuration already
few hybridization events or a single hybridization event at the
sensor active region may result in a measurable electric signal at
the electrode, in a configuration in which the switch elements are
operated to electrically couple the electrode alternatingly with
the first electric potential and the second electric potential.
[0029] The capacitance element may be a single electrode with an
area comparable to that of the cross-section of a via
(interconnect) plug in an advanced CMOS process. Small physical
dimensions may be advantageous for achieving single-molecule
resolution. The smaller the electrode size, the higher the relative
capacitance change as a result of a single-molecule capture. The
footprint area of a captured bio-molecule on the electrode area may
determine the corresponding capacitance change. All electrode area
that is not covered by the captured molecule may in fact act as a
parasitic capacitance in parallel to the capacitance change due to
the single-molecule capture. That is why the electrode area should
be a small as possible. A specifically appropriate electrode is as
small as a molecule, provided the electrode pitch is small enough
to ensure a reasonable surface coverage. The detectability of
single molecules is a matter of achieving high-enough
signal-to-noise ratio. In general, if all dimensions (except the
molecule size) scale with the feature size of the CMOS process node
(90 nm, 65 nm, 45 nm, etc.), the signal-to-noise ratio (which may
be the square of the signal amplitude divided by the variance of
the noise) is more or less proportional to the inverse of the sum
of the electrode capacitance and its parasitic parallel
capacitance. That is why the smallest possible electrodes are
appropriate for single-molecule detection. But scaling the
electrode size (for instance slightly or much) below the feature
size of the CMOS process node does not help anymore because then
the sensitivity saturates at a value determined by parasitic, while
the surface coverage keeps decreasing.
[0030] When the sensor active region comprises a nanoelectrode, the
dimensions of the electrode may be in the order of magnitude of
nanometers, for instance may be less than 300 nm, for instance may
be less than or equal to 250 nm, or may be less than or equal to
130 nm. The smaller the nanoelectrodes, the more sensitive the
resulting sensor pocket or planar sensor surface.
[0031] The nanoelectrode may comprise copper material, particularly
copper material being covered by a self assembled monolayer (SAM).
These materials may serve as oxidation protection layers or as
barrier layers or for enabling bonding of capture molecules,
thereby allowing to implement the relative sensitive material
copper which is highly appropriate due to its high electrical
conductivity and compliance with procedural requirements. Copper
material has chemically similar properties to gold which is
conventionally used in biosensing, but which has significant
disadvantages because it diffuses rapidly into many materials used
in silicon process technology, thereby deteriorating the IC's
performance, it is difficult to etch, and gold residues are hard to
remove in cleaning procedures. However, less preferred embodiments
of the invention may involve gold as well. Furthermore, materials
such as aluminium or the like may be used as well, and even gold
may be a less preferred example for such a material.
[0032] The first switch element and/or the second switch element
may be a transistor. Such a transistor may have a gate region and
may have two source/drain regions. The gate region of such switch
transistors may be coupled to clock signals operating the
transistor in a "high" or in a "low" operation mode, thereby
selectively rendering the channel region between the two
source/drain regions of a respective transistor conductive or not.
One of the source/drain regions of a respective one of the two
switch transistors is coupled to the respective first or second
electric potential, wherein the other two source/drain regions of
the two switch transistors are coupled to one another and to the
electrode, which may also be denoted as a capacitor plate of the
capacitor like sensor region. The transistors may be field effect
transistors, bipolar transistors, etc. The transistors may be
configured as an N-transistor or a P-transistor, for instance a
P-MOS or an N-MOS.
[0033] The sensor active region may comprise one or more capture
probes adapted for hybridizing with the particles. Such a capture
probe may be, for instance, one of the two strands of a DNA helix
and may have the property to specifically hybridize only with a
particle to be detected having a complementary sequence. Thus, a
highly specific sensor active region may be provided which may be
based on hybridization events between the capture probes and
specific particles.
[0034] According to an exemplary embodiment, a clock generator may
be provided for providing the first switch element and the second
switch element with clock signals to operate the first switch
element and the second switch element to alternate between an
operation mode in which the first switch element is closed (that is
to say is coupled to the first electric potential) and the second
switch element is simultaneously opened (that is to say is
decoupled from the second electric potential), and an operation
mode in which the first switch element is opened (that is to say is
decoupled from the first electric potential) and the second switch
element is simultaneously closed (that is to say is coupled to the
second electric potential). Therefore, the clock signals generated
by the clock unit (which may be controlled by or which may be a
CPU, central processing unit) allow to operate the two switch
elements complementary to one another to enable a non-overlapping
sequence of "coupling" and "decoupling" phases. Thus, the clock
signals provided with the two gates of the switch transistors may
be inverse to one another. This may ensure that, with low effort, a
reliable sequence of coupling/decoupling phases of the capacitor
sensor with one of the two electric potentials is ensured, and that
the pulsed or oscillating switching operation can be repeated
several times. By repeating such switching modes, a time average of
the detection signal may be obtained which may further allow to
improve the accuracy, since artefacts may be filtered out or
suppressed by such a repetition.
[0035] The detector may be adapted to detect the particles based on
a net charge transfer between a node providing the first electric
potential and a node providing the second electric potential during
one or more cycles in which the electrode is brought to the first
electric potential and in which the electrode is brought to the
second electric potential. As will be explained below (particularly
referring to the description of FIG. 1) in more detail, it has been
surprisingly found by the present inventors that the net charge
transferred in connection with the described switching procedure is
an accurate parameter allowing to qualitatively determine the
particle concentration.
[0036] The sensor may comprise a further electrode configured to be
kept at a fixed third electric potential (which may differ from the
first electric potential and/or from the second electric
potential). This fixed third electric potential may be an
electrolyte potential of an electrolyte into which the further
electrode is immersed. The constant third electric potential may be
maintained by a counter electrode which may also be immersed in an
electrolyte. Alternatively, the third electric potential may be
maintained by correspondingly controlling electrodes of other
sensors of a sensor array, as will be explained below in more
detail.
[0037] The sensor may be manufactured in CMOS technology. A CMOS
generation appropriate for manufacturing a specific sensor may be
dependent on the size of the electrode to be achieved. For example,
for single molecule biosensors, the manufacture of very small
electrodes may be favourable, resulting in the selection of an
advanced CMOS technology generation. If in another embodiment the
provision of larger electrodes is desired to immobilize a larger
number of capture probes thereon, a former CMOS technology may be
an appropriate choice.
[0038] The biosensor device may be monolithically integrated in a
semiconductor substrate, particularly comprising one of the group
consisting of a group IV semiconductor (such as silicon or
germanium), and a group III-group V semiconductor (such as gallium
arsenide).
[0039] The sensor may be adapted as a biosensor, particular as a
single molecule biosensor which is able to detect even the presence
of individual or single molecules. The biosensor may be based on a
capacitive measurement principle, and may be an electrochemical
biosensor.
[0040] Next, further exemplary embodiments of the sensor array will
be explained. However, these embodiments also apply to the sensor
and to the method.
[0041] The plurality of (for instance electrically interconnected)
sensors constituting the sensor array may be arranged in rows and
columns (that is to say in a matrix-like configuration). The rows
and columns may be arranged to be aligned perpendicular to one
another resulting in a rectangular or matrix-like pattern.
Alternatively, it is possible to arrange the sensors in rows and
columns forming a hexagonal pattern or the like.
[0042] In one embodiment, the first electric potential may be
provided in common for at least two, particularly for all sensors
of a column and the second electric potential may be provided in
common for at least two, particularly for all sensors of a row, or
vice versa. By taken this measure of applying a common electric
potential (such as an electric voltage) to more than one sensor at
the same time, a very efficient control of the entire system is
made possible, since the electric potential control effort may be
kept small.
[0043] The clock signals generated by a clock unit may be provided
in common for at least two, particularly for all sensors of a row.
This clock signal supply architecture may be advantageous since it
allows to implement only a very small number of clock generating
units in the sensor array by simultaneously supplying the clock
signals to multiple sensors at a time. This may also allow for a
proper synchronisation of the clock scheme for different
sensors.
[0044] Sensors in adjacent rows may be arranged upside down to one
another to share one of the group consisting of the first electric
potential and the second electric potential. In other words, in a
matrix like arrangement of the sensors, sensors of adjacent rows
(that is rows which are directly next to one another) may be mapped
to one another geometrically by using a horizontal mirror plane.
Such a configuration may allow two sensors in adjacent rows and in
the same column to share the same terminal for providing one of the
first and the second electric potentials, resulting in a very dense
and efficient configuration with a small number of control
lines.
[0045] Sensors in adjacent columns may be arranged alternately
left/right oriented to one another to share the first electric
potential and/or the second electric potential. In other words,
also sensors of adjacent columns may be arranged inverse to one
another, wherein a mapping of such sensors can be geometrically
obtained by a vertical mirror plane. Even this arrangement may
contribute to make the electric signal supply scheme even more
efficient.
[0046] According to an exemplary embodiment, the sensor array may
be monolithically integrated in a substrate. Such a substrate may
be a semiconductor substrate or any other substrate. It is also
possible that such a substrate is formed by a sequence of layers
provided on top of each other.
[0047] In such a configuration, the first switch element, the
second switch element and the detector of the plurality of sensors
may be buried within the substrate, that is may be provided beneath
a surface of the substrate, for instance may be arranged in one of
the lower lying layers of a layer sequence representing the
substrate. In contrast to this, the electrode and the sensor active
region of the plurality of sensors may be provided at or close to
the surface of the sensor array. Thus, the electrodes may be
exposed to a fluidic sample under analysis to enable a functional
interaction between the sensor active regions and the particles to
be detected. Furthermore, a spatial decoupling of the sensor
electrodes and the electronic members located deeply within the
substrate may further increase the accuracy, since undesired
cross-talk between sensor events and electronic control signals may
be suppressed by arranging the corresponding members sufficiently
far away from one another without significantly reducing the
density of the cell arrangement. For example, at least three
layers, particularly at least five layers, more particularly at
least eight layers may be located between the buried components and
the surface bound components.
[0048] Particularly, the sensor array may further comprise a
moisture resistant structure at the surface of the sensor array
between adjacent ones of the electrodes of the plurality of
sensors. By taken this measure, it may be securely prevented that a
liquid sample under investigation penetrates into the sensor array
which might disturb the electronic components embedded therein. By
providing such a moisture resistant structure, for example
fluorosilicate glass, the life time of the sensor array may be
improved.
[0049] The sensor array may comprise a selection unit adapted for
selecting one of the rows (at a time) for sensing, wherein the
selection unit may be further adapted for disabling all other rows
from sensing by opening the first switch element and the second
switch element of the all other rows. Therefore, the non active
rows may be simply biased to be non active, whereas a single row
may be activated at a time.
[0050] Alternatively, a selection unit may be provided which is
adapted for selecting one of the rows for sensing, but is further
adapted for disabling all other rows from sensing and for closing
the second switch element of at least a part of the all other rows
to provide a counter electrode functionality. Only the discharge
switch may be closed to include the corresponding electrodes in the
reconfigurable counter electrode. This is not possible with the
transfer switch because then the corresponding electrode would be
connected in parallel with the active sensors element in the same
row. In such a configuration, the electrodes which are presently
not used for sensing may be not simply made inactive, but may be
controlled to serve as a counter electrode to provide the sensor
array with a constant electric potential at a position where it is
coupled to an electrolyte. Therefore, the presently non-used
electrodes may be synergetically used as configurable counter
electrode members, which may make a separate counter electrode
dispensable and may promote the miniature manufacturability of the
sensor array.
[0051] The sensor array may further comprise a row periphery
circuit comprising a number of multiplexers adapted for gating the
rows. Particularly, such a row periphery circuit may comprise five
multiplexers for each pairs of rows, the five multiplexers being
configured to provide clock signals to operate the first switch
element and the second switch element and to provide the first
electric potential or the second electric potential to the sensors
of a respective pair of rows.
[0052] It is noted that the previously described aspect of the
invention can be implemented separately from the architecture
described in the independent claims, however can be combined with
any embodiment described herein. In other words, the previously
described aspect is an independent aspect of the invention, which
can be implemented without the other provisions disclosed herein.
According to such an aspect, a sensor array may be provided
comprising an arrangement of a plurality of sensors arranged in
rows and columns, the sensor array further comprising a row
periphery circuit comprising a number of multiplexers adapted for
gating the rows. The row periphery circuit may comprise five (or
another appropriate number of) multiplexers for each pair of rows,
the five multiplexers being configured to provide clock signals to
operate switch elements of the sensors and being configured to
provide electric potentials to the sensors of a respective pair of
rows.
[0053] By such a five multiplexer per row pair architecture, an
efficient supply of all control signals and use signals may be
ensured, and such a row periphery circuit may be employed with high
versatility in different fields, for instance in the field of
biosensors or also for controlling an array of memory cells.
[0054] The sensor array may comprise a column periphery circuit
adapted for gating the columns. Thus, in addition to a row
periphery circuit, a column periphery circuit may be implemented to
adjust and control the electric potentials supplied to the various
columns.
[0055] According to an exemplary embodiment, a calibration row may
be provided having one or more calibration units each of which
being constituted as each of the plurality of sensors but being
free of an electrode and a sensor active region. In other words,
such a calibration row may have sensors which are only void of the
electrode and the sensor active regions. However, all the other
components of a sensor may be present in such a calibration unit,
so that the measurement of a signal at such a calibration unit may
be a proper measure for an unspecific underground signal which is
detected by the other sensors as well. Since only the interaction
with the particles to be detected lacks for the calibration unit,
the use of a calibration row may allow to improve the accuracy of
the signals by allowing to calibrate the measured signals on the
basis of the underground signal determined by the calibration unit.
Alternatively two or more calibration rows may be operated
simultaneously to create a larger calibration signal (this may be
advantageous to compensate for the effect of the lacking electrodes
in the calibration rows). Additionally or alternatively to a
calibration row, it is also possible to provide a single
calibration cell or a calibration column.
[0056] By using an entire row comprising a plurality of calibration
units, an average over the individual calibration signals may be
calculated to further improve the accuracy of the calibration
parameters, since spatially dependent (for instance edge effects)
effects can be suppressed by taking such a measure.
[0057] The detector of at least a part of the plurality of sensors
may be adapted to perform a self referencing function by comparing
a detection signal with an average detection signal of at least a
part of the other sensors, particularly by comparing the detection
signal with an average detection signal of other sensors of a row.
For example, time drift effects may be suppressed by taking such a
measure, since an individual signal is not considered in an
isolated manner, but is compared with a time dependence of an
average detection signal of other sensors which may allow to
improve the signal to noise ratio and detect even the presence of a
single bio-molecule.
[0058] The detector may be adapted to detect the particles in an
operation mode in which the electrode is statically brought to the
first electrical potential and is statically decoupled from the
second electrical potential. In such an operation mode, no
switching has to take place, so that the clock signals may be
maintained at a constant level. In the presence of a time
independent signal, it is possible to influence the properties of
the capture molecules by such a signal.
[0059] The biosensor chip or microfluidic device may be or may be
part of a sensor device, a sensor readout device, a lab-on-chip, an
electrophoresis device, a sample transport device, a sample mix
device, a sample washing device, a sample purification device, a
sample amplification device, a sample extraction device or a
hybridization analysis device. Particularly, the biosensor or
microfluidic device may be implemented in any kind of life science
apparatus.
[0060] For any method step, any conventional procedure as known
from semiconductor technology may be implemented. Forming layers or
components may include deposition techniques like CVD (chemical
vapour deposition), PECVD (plasma enhanced chemical vapour
deposition), ALD (atomic layer deposition), electroplating, or
sputtering. Removing layers or components may include etching
techniques like wet etching, plasma etching, CMP (chemical
mechanical polishing), etc., as well as patterning techniques like
optical lithography, UV lithography, electron beam lithography,
etc.
[0061] Embodiments of the invention are not bound to specific
materials, so that many different materials may be used. For
conductive structures, it may be possible to use metallization
structures, silicide structures, polysilicon structures, or
conductive polymer structures. For semiconductor regions or
components, crystalline silicon may be used. For insulating
portions, silicon oxide or silicon nitride may be used.
[0062] The biosensor may be formed on a purely crystalline silicon
wafer or on an SOI wafer (Silicon On Insulator).
[0063] Any process technologies like CMOS, BIPOLAR, BICMOS may be
implemented.
[0064] The aspects defined above and further aspects of the
invention are apparent from the examples of embodiment to be
described hereinafter and are explained with reference to these
examples of embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0065] The invention will be described in more detail hereinafter
with reference to examples of embodiment but to which the invention
is not limited.
[0066] FIG. 1 illustrates a sensor according to an exemplary
embodiment of the invention.
[0067] FIG. 2 illustrates a cross-sectional view of a
metal-electrolyte capacitor formed by a metal bottom plate, a
self-assembled monolayer (SAM) dielectric, and an electrolyte top
plate, without (left) and with (right) a bio-molecule captured on
the SAM.
[0068] FIG. 3 shows an achievable sensitivity for capacitance
measurements with Agilent's Precision Impedance Analyser 4294A
being 10 fF at 0.5 Vrms oscillator level (where the 10-fF line
touches the 10% accuracy contour line).
[0069] FIG. 4 shows a capacitance input noise and resolution versus
conversion time of the Analog Devices "24-bit
Capacitance-to-Digital Converter with Temperature Sensor" ICs
AD7745 and AD7746.
[0070] FIG. 5 shows an array of nano-electrodes and corresponding
switch transistors with shared control and discharge lines
(horizontal) and transfer lines (vertical) according to an
exemplary embodiment of the invention.
[0071] FIG. 6 shows a schematic (left) and layout (right) of the
same part of a sensor array according to an exemplary embodiment of
the invention.
[0072] FIG. 7 to FIG. 11 each show a schematic (left) and layout
(right) of the same part of a sensor array according to an
exemplary embodiment of the invention.
[0073] FIG. 12 shows a cross-section along a column through the
nano-electrodes of a sensor array according to an exemplary
embodiment of the invention.
[0074] FIG. 13 shows a versatile row peripheral circuit with analog
multiplexer switches according to an exemplary embodiment of the
invention.
[0075] FIG. 14 shows a description of the signals of the row
peripheral circuit of FIG. 13 according to an exemplary embodiment
of the invention.
[0076] FIG. 15 shows a column peripheral circuit according to an
exemplary embodiment of the invention.
[0077] FIG. 16 shows a system architecture according to an
exemplary embodiment of the invention.
[0078] FIG. 17 shows an alternative system architecture according
to an exemplary embodiment of the invention.
[0079] FIG. 18 shows a system architecture of sensor array with
calibration rows according to an exemplary embodiment of the
invention.
[0080] FIG. 19 shows an alternative system architecture of a sensor
array with calibration rows according to an exemplary embodiment of
the invention.
DESCRIPTION OF EMBODIMENTS
[0081] The illustration in the drawing is schematic. In different
drawings, similar or identical elements are provided with the same
reference signs.
[0082] In the following, referring to FIG. 1, a biosensor 100
according to an exemplary embodiment of the invention will be
explained.
[0083] The biosensor 100 is adapted for detecting biological
particles (not shown in FIG. 1). The biosensor 100 comprises an
electrode 102 as a first capacitor plate of a capacitor denoted
with C in FIG. 1. A second capacitor plate is formed by an
electrolyte electrode 118 (for instance in a manner similar to FIG.
2). An electrolyte 119 is connected by a separate further electrode
(not shown) to connect it to an electrical potential V.sub.L.
[0084] A sensor active region 104 covers the electrode 102 and is
sensitive for the biological particles.
[0085] A first field effect switch transistor 106 is provided which
is operable to bring the electrode 102 to a first electric
potential V.sub.T when the first switch element 106 is closed. In
other words, when a clock signal .PHI..sub.T provided by a clock
unit 110 is at a "high" level, the channel of the transistor 106 is
electrically conductive, so that an electric coupling between the
source/drain regions of the first switch transistor 106 is enabled,
thereby directly coupling the electrode 102 to the electric
potential V.sub.T. During the coupling of the electrode 102 to the
first electric potential V.sub.T, a second clock signal .PHI..sub.D
supplied to a gate of a second switch field effect transistor 108
is at a "low" level, so that no electrically conductive coupling is
provided between the electrode 102 and a second electric potential
V.sub.D. In another operation mode, when the second switch element
108 is closed, the electrode 102 is coupled to the potential
V.sub.D and is simultaneously decoupled from the potential V.sub.T
by applying a "low" signal to the first field effect switch
transistor 106 at this time. The complementary clock signals
.PHI..sub.T and .PHI..sub.D are shown in diagrams 120, 140.
[0086] More particularly, the electrode 102 is coupled to a first
source/drain region of the first switch transistor 106 and is
coupled to a first source/drain region of the second switch
transistor 108. The first electric potential V.sub.T is applied to
a second source/drain region of the first switch transistor 106.
The second electric potential V.sub.D is applied to a second
source/drain region of the second switch transistor 108. The clock
signal .PHI..sub.T is applied to a gate of the first switch
transistor 106. The clock signal .PHI..sub.D is applied to a gate
of the second switch transistor 108.
[0087] Hybridization events between the biological particles and
the sensor active region 104 may be detected by a detecting unit
(not shown in FIG. 1) by determining or measuring a change of the
electric properties of the sensor 100 in an operation mode in which
the electrode 102 is brought to the first electric potential
V.sub.T and an operation mode in which the electrode 102 is brought
to the second electric potential V.sub.D. A modulation of such a
charge transfer may be effected or may be the result of a change of
the capacity C in the presence or absence of the particles.
[0088] FIG. 1 shows the sensor 100 in an operation mode at a first
time t1 at which the clock signal .PHI..sub.T is "low" and the
clock signal .PHI..sub.D is "high", so that a coupling between the
electrode 102 and the second electric potential V.sub.D is
activated, as indicated by an arrow 114, while the electrode 102 is
decoupled from the first potential V.sub.T. In contrast to this, at
a time t.sub.2 which is shown in FIG. 1 as well, the clock
.PHI..sub.T is "high" and the clock .PHI..sub.D is "low", so that
the electrode 102 is coupled to the first electric potential
V.sub.T, as indicated by an arrow 116, and is decoupled from the
second electric potential V.sub.D.
[0089] By performing the operation cycle shown in FIG. 1 once or
several times, a net charge flow may be determined which can be
taken as a basis for deriving information regarding the presence or
absence and even for the concentration of the particles in an
environment of the sensor active region 104. Thus, qualitative or
quantitative information about a sample under analysis may be
obtained.
[0090] Capture probes are immobilized on the electrode 102, forming
a part of the sensor active region 104 which may additionally also
comprise a self-assembled monolayer (shown and denoted with
reference numeral 202 in FIG. 2).
[0091] The clock unit 110 is adapted for providing the first switch
element 106 and the second switch element 108 with the clock
signals .PHI..sub.T and .PHI..sub.D to operate the first switch
element 106 and the second switch element 108 to alternate between
an operation mode in which the first switch element 106 is closed
and the second switch element 108 is simultaneously opened (t2) and
an operation mode in which the first switch element 106 is opened
and the second switch element is simultaneously closed (t1).
[0092] During this configuration, an electrolyte 119 may be kept at
a fixed third electric potential V.sub.L provided by a counter
electrode in electrically conductive contact with the electrolyte
119 into which the sensor active surface 104 is immersed.
[0093] The electrolytic capacitor C in FIG. 1 is drawn
schematically with the following assumptions: [0094] The first
electrode 102 is the metal plate; [0095] The dielectric 104 is the
sensor active region that is sensitive for biological particles. It
is drawn here as an empty space between the first electrode 102 and
a second electrode 118; [0096] The second electrode 118 is the
interface between the sensor active region 104 and the electrolyte
119. It comprises a self-assembled monolayer (SAM, if present) and
the so-called "diffuse double layer" in the electrolyte. The
diffuse double layer is the part of the electrolyte immediately
above the first electrode and the SAM (if present) where the
electric field penetrates. For an electrolyte with physiological
salt concentration it has a thickness of the order of magnitude of
1 nanometre. So the actual capacitance of the capacitor C is
determined by the series connection of the capacitance of the SAM
(if present) and the capacitance of the diffuse double layer.
[0097] The electrolyte 119 forms the electrically conducting path
between the second electrode 118 and the location (not shown) where
the electrical potential V.sub.L is connected.
[0098] Next, considerations regarding signal-to-noise ratio will be
explained.
[0099] FIG. 1 shows an exemplary configuration of the biosensor
100. First, the discharge switch transistor 108 is closed to
discharge the "bio-electrolytic" capacitor C to the discharge
voltage V.sub.D. After a subsequent opening of the discharge switch
108 the charge Q.sub.D on the capacitor C is
Q.sub.D=(V.sub.D-V.sub.L)(C+C.sub.P) (1)
[0100] where V.sub.L is the voltage of the liquid, and C.sub.P is
the parasitic capacitance in parallel to the capacitor C. Because
of the thermal noise of the series resistance of the discharge
switch 108, the charge on the discharged capacitor C fluctuates
from one discharged state to another. The variance of these
fluctuations, which may be denoted as "reset noise", is
.sigma..sub.Q.sub.D.sup.2=k.sub.BT(C+C.sub.P) (2)
[0101] where k.sub.B is Boltzmann's constant, and T is the absolute
temperature.
[0102] Subsequently the transfer switch transistor 106 is closed to
charge the capacitor C to the transfer voltage V.sub.T. After
subsequent opening of the transfer switch 106 the charge on the
capacitor C is
Q.sub.T=(V.sub.T-V.sub.L)(C+C.sub.P) (3)
[0103] Because of the thermal noise of the series resistance of the
transfer switch 106, the charge on the charged capacitor C also
fluctuates from one charged state to another. The variance of these
fluctuations is
.sigma..sub.Q.sub.T.sup.2=k.sub.BT(C+C.sub.P) (4)
[0104] The net charge Q transferred from the transfer terminal 106
(at voltage V.sub.T) to the discharge terminal 108 (at voltage
V.sub.D) after N discharge/transfer cycles is
Q=N(Q.sub.T-Q.sub.D)=N(V.sub.T-V.sub.D)(C+C.sub.P) (5)
[0105] Because the charge fluctuations of the discharged and
charged states are uncorrelated (they originate from different
uncorrelated noise sources) the variance of Q is
.sigma..sub.Q.sup.2=N(.sigma..sub.Q.sub.T.sup.2+.sigma..sub.Q.sub.D.sup.-
2)=2Nk.sub.BT(C+C.sub.P) (6)
[0106] The change in Q as a result of a change .delta.C in the
capacitor C, caused by the capturing of one or more bio-molecules,
is
.delta.Q=N(V.sub.T-V.sub.D).delta.C (7)
[0107] To be able to detect this capacitance change after N
discharge/transfer cycles the signal-to-noise ratio
( .delta. Q ) 2 .sigma. Q 2 = N ( V T - V D ) 2 ( .delta. C ) 2 2 k
B T ( C + C P ) ( 8 ) ##EQU00001##
[0108] should be high enough (the exact number depends on the
required detection error probability). In practice additional noise
sources of the circuit for measuring Q have to taken into account.
Therefore, (8) is an upper limit for the achievable signal-to-noise
ratio.
[0109] The maximum tolerable modulation voltage |V.sub.T-V.sub.D|
is limited by the dielectric reliability properties of C, that is,
the leakage current, degradation, dielectric breakdown, etc. of the
self-assembled monolayer (SAM). Therefore, for a given SAM and
fixed N, the strategy for maximizing the signal-to-noise ratio
depends on the use case.
[0110] Next, surface coverage fraction measurements will be
explained.
[0111] For simplicity, the effect of capturing a bio-molecule on
top of the SAM is described by the elimination of a small area
A.sub.f (the footprint of the captured molecule on the SAM) from
the total area A of the capacitor C (see FIG. 2). So the
capacitance change associated to a surface coverage fraction
.gamma. = KA f A ( 9 ) ##EQU00002##
[0112] of the SAM by captured bio-molecules is
.delta.C=-.gamma.C (10)
[0113] where K is the number of captured bio-molecules. At fixed
.gamma. the maximum signal-to-noise ratio
( .delta. Q ) 2 .sigma. Q 2 = N ( V T - V D ) 2 .gamma. 2 C 2 2 k B
T ( C + C P ) ( 11 ) ##EQU00003##
[0114] increases with increasing C. So for this use case C and,
consequently, its area A, should be as large as possible.
[0115] For a properly designed circuit the parasitic capacitor
C.sub.P is dominated by the parasitic capacitances of the two
switching transistors 106, 108 (mainly junction capacitances and
overlap capacitances between the gate electrodes and the
source/drain regions). For fixed series resistances of the
discharge switch 108 and transfer switch 106 every consecutive CMOS
process node (0.35 .mu.m, 0.25 .mu.m, 0.18 .mu.m, etc.) typically
has a smaller C.sub.P than its predecessor. But because C has to be
large it is not necessary to implement the circuit in a more
advanced CMOS generation than required for keeping C.sub.P small
compared to C, and for fitting the switching transistors 106, 108
in the area covered by C and its surrounding spaces to isolate it
from neighbouring capacitors. Therefore, biosensors 100 for
measuring surface covering fractions may be designed in "old" CMOS
processes (which may be an attractive opportunity to give old CMOS
fabs a second life).
[0116] Next, single-molecule biosensors will be discussed,
[0117] Biosensors that measure surface coverage fractions can be
used to measure average properties of ensembles of captured
molecules. Furthermore, their large electrodes areas require SAMs
with very low defect densities. Single-molecule biosensors may be
required to overcome these limitations. They offer the potential to
measure properties of individual bio-molecules. Furthermore,
because of their small electrode areas, a significant fraction of
functional electrodes can be obtained with SAMs with higher defect
densities (bad electrodes can be detected and pruned).
[0118] The capacitance change associated to capturing a single
bio-molecule is given by
.delta.C=-A.sub.fc.sub.0 (12)
[0119] where the surface capacitance density
c 0 = C A ( 13 ) ##EQU00004##
[0120] of C is a constant, determined by the properties of the
dielectric (the SAM) and electrodes (the metal plate and the
electrolyte). The associated maximum signal-to-noise ratio
( .delta. Q ) 2 .sigma. Q 2 = N ( V T - V D ) 2 A f 2 c 0 2 2 k B T
( C + C P ) ( 14 ) ##EQU00005##
[0121] increases with decreasing C and C.sub.P. Therefore, the
small feature sizes of advanced CMOS generations offer advantages
in realizing the smallest possible values of C and C.sub.P.
[0122] In a proper design, with nano-electrodes and switching
transistors designed using minimum feature sizes, C and C.sub.P
typically are of comparable value. Therefore, extending the CMOS
process with a dedicated processes option for making
sub-feature-size nano-electrodes only has limited advantages
because it does not simultaneously reduce the parasitic capacitance
C.sub.P and it does not reduce the area occupied by a sensor cell
(resulting in a reduction of the fraction of sensitive surface area
of the sensor). Therefore, scaling to the next more advanced CMOS
generation is the obvious approach for further increasing the
signal-to-noise ratio.
[0123] Concluding, capacitive biosensors for surface coverage
fraction measurement may have large electrodes and can be
implemented in old CMOS processes. Single-molecule biosensors may
have the smallest possible electrodes and may be implemented in
advanced CMOS generations.
[0124] In the following, some recognitions of the present inventor
will be explained based on which exemplary embodiments of the
invention have been developed.
[0125] Electronic biosensors are attractive because of their
potential compatibility with CMOS processes. This allows to
integrate the sensor electronics and additional features like an
electronic interface to the outside world, programmable functions,
and on-chip data storage and processing. In general such sensors
consist of one or more electrodes immersed in the analyte. The
analyte typically behaves like a liquid electrolyte. Capture probes
are attached to the electrode surface, either directly or with some
intermediate layer between the electrode surface and the capture
probes. Examples of such intermediate layers are SAMs and
dielectric layers, or combinations of the two.
[0126] Conventionally, sensor electrodes are much larger than the
size of the molecules they should detect and/or recognize. However,
scaling to nanometre-scaled electrodes may boost the performance of
biosensors.
[0127] In the following, conventional capacitive bio-sensing will
be mentioned.
[0128] FIG. 2 is a cross-sectional view of a metal-electrolyte
capacitor 200 comprising a metal bottom plate 204, a self-assembled
monolayer (SAM) dielectric 202, and an electrolyte top plate 206,
without (left) and with (right) a bio-molecule 208 captured on the
SAM 202.
[0129] The detection principle of a capacitive biosensor may be
based on measuring the capacitance of the electrolytic capacitor
200. The surface of the metal electrode 204 is covered with a thin
(about 2-nm thick) SAM 202 of organic molecules that serves as a
dielectric (FIG. 2, left). The electrode capacitance is
C=c.sub.0A (15)
[0130] where c.sub.0 is the capacitance surface density and A is
the electrode area. For typical alkane-thiol SAMs with thicknesses
about a nanometre the value of c.sub.0 is about 0.04 F/m.sup.2 (the
exact number may depend on details of the electrode surface like
its roughness on a nanometre-scale, on the composition and density
of the SAM, etc.). So for a nano-electrode 204 with an area of
0.015 .mu.m.sup.2 (a value that should be achievable in a 90-nm
CMOS process) C would have a value of about 0.53 fF (1
fF=10.sup.-15 F).
[0131] The surface of the SAM 202 is chemically functionalised in
such a way that it can capture bio-molecules 208. Relevant
bio-molecules 208 typically behave as dielectrics with dielectric
constants similar to that of the SAM material 202. Their size is in
the range of 5 nm to 20 nm. When such a bio-molecule 208 is
captured at the surface of the SAM 202 it replaces a certain volume
of electrolyte 206. In a simplified picture this event can be
modelled as a replacement of a column of conducting electrolyte 206
with footprint area A.sub.f by an insulating dielectric 208 (FIG.
2, right). Assuming that the height of the column is much greater
than the thickness of the SAM 202 (because typical bio-molecules
208 of interest are larger than the SAM 202 thickness), and
neglecting fringing of the electric field near the intersection of
the column wall and the SAM 202, the resulting change in the
electrode capacitance is approximately
.DELTA.C=-c.sub.0A.sub.f (16)
[0132] Assuming that A.sub.f is of the order of magnitude of the
square of the bio-molecule 208 size, typical values of |.DELTA.C|
can be expected in the range of 1-16 aF (1 aF=10.sup.-18 F).
[0133] Such a small capacitance change is way outside the
sensitivity range accessible with off-the-shelf high-end
capacitance meters. Even Agilent's Precision Impedance Analyser
4294A can only measure a capacitance of 10 fF with an accuracy of
10% at an oscillator voltage of 0.5 Vrms (see diagram 300 in FIG.
3). But this usually requires long integration times (seconds or
more) in a system that has to be carefully screened from
interference by external sources. Furthermore, the required
accurate calibration for parasitic capacitances may be practically
difficult to achieve for a capacitor of which one electrode
consists of a liquid (the electrolyte). And the high oscillator
voltage may cause unknown nonlinear effects at the electrodes; the
long integration times may cause problems with drift, 1/f-noise,
etc. But even if one succeeds, the result is just a single
capacitance measurement, done with a very expensive system.
[0134] Also recently presented high-resolution capacitance meter
ICs like the AD7745, AD7746 or AD7747 of Analog Devices cannot
measure the typical |.DELTA.C| caused by the capture of a single
bio-molecule 208 on the SAM 202 surface. With a conversion time in
excess of 100 ms the standard deviation of the capacitance noise is
4.2 aF (see table 400 in FIG. 4, lowest entry in 5.sup.th column).
Theoretically this would allow measuring a capacitance change of
about 10 aF with a reasonable signal-to-noise ratio. But this noise
figure applies to an excitation (modulation) voltage of
.+-.V.sub.DD/2. The exact value of the supply voltage V.sub.DD for
this particular case is not specified in the IC's datasheet. But
the lowest applicable supply voltage of the IC is 2.7 V, so the
excitation voltage must be at least 2.7 V top-top, which is much
too high for a SAM 202 with a thickness of about 2 nm.
[0135] Clearly the capacitance change caused by the capture of a
single bio-molecule on the SAM 202 surface is too small for
conventional equipment. Therefore the contributions of many
molecules have to be added to arrive at a larger capacitance change
that can be measured with a sufficient signal-to-noise ratio in a
reasonable time and at a low-enough modulation voltage (in the 100
mV range). For instance, depending on their size, 630-10,000
bio-molecules have to be captured for a capacitance change of 10
fF, a value which would just be resolvable with a reasonable
accuracy with Agilent's 4294A instrument. As a result, even the
lowest resolvable capacitance change always will be an average
property of a large ensemble of captured bio-molecules. As a result
of this averaging process a lot of information about the individual
molecules is lost. Especially for a heterogeneous ensemble,
consisting of a mix of multiple types of bio-molecules, the
measured capacitance change hardly contains any information about
the individual types of bio-molecules.
[0136] In the following, advantages of massive parallel
single-molecule detection will be explained.
[0137] Capacitance measurements with electrodes that capture large
quantities of bio-molecules give average properties of the captured
molecules. As a result, only single-molecule signals common to at
least a fraction of the captured ensemble are retained, while all
other single-molecule signals are averaged out. In this way the
signal-to-noise ratio of the common signals can be improved, but
all other information about the individual molecules gets lost.
Information theoretical considerations show that this is not
necessarily the best detection method. For example, variations in
the binding details of individual molecules may cause blurring of
features in the signals (for instance, inhomogeneous broadening of
oxidation/reduction peaks in current-voltage curves, of features in
impedance spectra, etc.). If all single-molecule signals could be
acquired individually then more reliable detection and/or
recognition of bio-molecules would be possible with statistical
data processing techniques. Averaging is just one of many possible
algorithms that can be applied to the data. But other algorithms
can be applied as well (for instance, correction for systematic
variations over the ensemble, classification of signals, pruning of
bad samples, calculation of correlations, etc.).
[0138] An appropriate electronic biosensor can measure all captured
bio-molecules individually. In this way the highest possible amount
of information can be extracted from the molecules. For this
purpose very small electrodes are needed. They should be placed in
a high-density array to achieve high sensitivity (roughly
proportional to the fraction of the array area that is sensitive to
captured molecules).
[0139] The challenge for making biosensors with high-density arrays
of individually accessible nano-scale electrodes is the proper
segmentation of the addressing, control and read-out electronics
into a local part that is repeated in every cell (nano-electrode
and local electronics) and a peripheral part that is shared by all
cells in a column or row.
[0140] Exemplary embodiments of the invention describe an
architecture for a high-density capacitive biosensor array that
implements such a segmentation in a very efficient way, and that
can operate at high speed and very low power consumption. With the
disclosed architecture sensors with single-molecule sensitivity can
be manufactured.
[0141] Apart from an efficient segmentation it may also be
important to consider power dissipation. In capacitive biosensor
arrays modulation voltages have to be applied to the electrodes or
to the counter electrode(s), and the AC currents induced in the
electrodes have to be measured. In straightforward array
architectures, where electrodes are selected with selection
switches, the AC voltages and/or currents have to be transported
through long row and/or column connection lines. This may lead to
cross-talk between neighbouring lines or to loss of sensitivity
because of large parasitic capacitances of the lines. Furthermore,
modulating the voltages of long lines with large parasitic
capacitance leads to high dynamic power dissipation. The
architecture of embodiments of the invention does not suffer from
all these drawbacks, and can be considered optimal in many
respects. Furthermore, it can be implemented in standard advanced
CMOS processes with only very minor process changes in a very last
stage of the processing.
[0142] Next, further exemplary embodiments of the invention will be
explained.
[0143] Embodiments of the invention may implement polished copper
nano-electrodes for single-molecule biosensors in advanced CMOS
processes. These copper nano-electrodes may serve as sub-micron
metal plates of electrolytic capacitors. The dielectrics of the
capacitors typically comprise or consist of SAMs, functionalized
with capture probe molecules. The electrolyte plates typically
comprise or consist of the analyte or a buffer solution above the
sensor surface. Capacitors according to this construction are
referred hereafter as "nano-electrode electrolytic capacitors".
[0144] The above described FIG. 1 shows a configuration of a basic
sensor principle according to an embodiment of the invention.
[0145] The node voltage V.sub.N of the metal plate 102 of a
nano-electrode electrolytic capacitor C is controlled by the two
NMOS switch transistors 106, 108, preferably of minimal dimensions
(to limit their parasitic capacitances to a minimum). The
electrolyte plate 118 of C is maintained at a fixed voltage
V.sub.L, supplied to the liquid electrolyte 119. The gate voltages
of the two switch transistors 106, 108 are controlled by the
non-overlapping transfer and discharge clock signals .PHI..sub.T
and .PHI..sub.D, respectively.
[0146] When .PHI..sub.D is "high", the capacitor's metal electrode
102 is discharged to the discharge potential V.sub.D (FIG. 1, t1).
After .PHI..sub.D is made "low" again, the transfer clock
.PHI..sub.T is made "high". Then the capacitor's metal electrode
102 is charged to the transfer voltage V.sub.T (FIG. 1, t2).
Finally, the transfer clock .PHI..sub.T is made "low" again.
Assuming that eventual transient peaks in V.sub.L (for instance, as
a result of the electrolyte series resistance) have faded out at
the end of the switching pulses, the net effect is the transfer of
a charge
Q=(C+C.sub.P)(V.sub.T-V.sub.D) (17)
[0147] from the transfer terminal (biased at V.sub.T) to the
discharge terminal (biased at V.sub.D), where C.sub.P is the total
parasitic capacitance of the V.sub.N-node (equation (17) is a
special case of equation (5) for N=1). This sequence is repeated
with a transfer frequency f.sub.T, resulting in an average transfer
current
I.sub.T=f.sub.TQ.sub.T (18)
[0148] In an array of cells 100, the averaging may be done
implicitly by the parasitic capacitance of the column line (the
line that connects the transfer terminal, see below). This
parasitic capacitance mainly consists of the sum of the parasitic
capacitances of the transfer switch transistors of all non-selected
cells connected to the same column line. For low frequencies
(compared to f.sub.T/2) the cell effectively behaves like a
resistor
R T = 1 f T ( C + C P ) ( 19 ) ##EQU00006##
[0149] The transfer current I.sub.T in principle is independent of
the DC-value of electrolyte potential V.sub.L. This allows biasing
the electrolyte at a convenient potential, for instance, where the
average leakage current through the capacitor C is zero, thereby
effectively eliminating net long-term electrochemical reactions at
the metal/SAM/electrolyte junction.
[0150] Next, sensor arrays according to exemplary embodiments of
the invention will be explained in more detail.
[0151] A single nano-electrode only has a very small area to
capture bio-molecules. However, to be able to capture many
bio-molecules in a short period of time, a large sensitive area may
be needed. Therefore, many cells, each comprising or consisting of
a nano-electrode electrolytic capacitor and two switch transistors,
may be arranged in a dense two-dimensional array. A high density of
cells may be achieved by sharing control, discharge and transfer
lines between neighbouring cells in the arrays (control lines are
the lines that control the gates of the switch transistors).
Because only the part of a cell that is covered by the
nano-electrode is sensitive, the fraction of insensitive area of
the cell should be made as small as possible. This is another
reason to use small switching transistors (apart from reducing
their parasitic capacitances).
[0152] FIG. 5 shows an array 500 of nano-electrodes 102 and
corresponding switch transistors 106, 108 with shared control and
discharge lines (horizontal rows 502) and transfer lines (vertical
columns 504).
[0153] In the array architecture of FIG. 5, the cells 100 are
arranged in orthogonal rows (each row comprising several control
and discharge lines 502; however, in the following, the rows may
also be indicated by reference numeral 502) and columns 504. The
cells 100 in the odd-numbered rows 502 are oriented upside down
with respect to the cells 100 in the even-numbered rows. This
allows sharing contact holes and discharge lines in the array
layout. All cells 100 in the same row 502 are controlled by the
same discharge clock signals .PHI..sub.D,m and transfer clock
signals .PHI..sub.T,m, where m is the row index. As a consequence,
all nano-electrode 102 electrolytic capacitors in a row 502 are
addressed simultaneously. Their transfer currents can be measured
via their respective column 504 lines I.sub.C,n, where n is the
column index. With this parallel operation a high detection
throughput can be obtained.
[0154] Selection of a particular row 502 may proceed by applying
the appropriate clock signals and discharge voltage at its control
(discharge and transfer clock) and discharge lines. The control
lines of the non-selected rows may be biased at alternative
appropriate control voltages, for instance, to disable these rows.
The entire array or any other subset of rows is scanned by
subsequently selecting the respective rows in an appropriate scan
sequence.
[0155] Next, an advantageous array layout will be explained.
[0156] To achieve a high fraction of active sensor array surface it
may be advantageous to choose a layout that is as dense as
possible. FIG. 6 to FIG. 11 show a dense layout 600, 700, 800, 900,
1000, 1100 that satisfies all baseline CMOS design rules.
[0157] FIG. 6 shows a schematic (left) and layout (right) portion
of the same part of a sensor array. Shown design layers: active
602, poly 604 and contact 606.
[0158] FIG. 7 shows a schematic (left) and layout (right) portion
of the same part of a sensor array. Shown design layers: contact
606 and metal-1 702.
[0159] FIG. 8 shows a schematic (left) and layout (right) portion
of the same part of a sensor array. Shown design layers: metal-1
702, via-1 802 and metal-2 804.
[0160] FIG. 9 shows a schematic (left) and layout (right) portion
of the same part of a sensor array. Shown design layers: metal-2
804 and via-2 902.
[0161] FIG. 10 shows a schematic (left) and layout (right) portion
of the same part of a sensor array Shown design layers: via-2 902
and metal-3 1002.
[0162] FIG. 11 shows a schematic (left) and layout (right) portion
of the same part of a sensor array Shown design layers: metal-3
1002) and via-3 1102 (defining the nano-electrodes).
[0163] Active 602 and poly lines 604 are implemented as orthogonal
straight lines of minimum possible width (FIG. 6). In the vertical
direction the poly line pitch and, consequently, the vertical cell
pitch, is limited by the minimum contact-to-poly distance.
Minimum-width metal-1 column lines and minimum-area metal-1 landing
pads for the connections of the nano-electrodes and the discharge
lines determine the horizontal cell pitch (FIG. 7). Discharge lines
are implemented in metal-2 804 (FIG. 8). The metal-3 layer 1002
(FIG. 9 to FIG. 11) is included to provide more freedom in the
layout of the peripheral and input/output circuits. The via-3
design layer 1102 of the baseline CMOS process is used here to
define the nano-electrodes (FIG. 11).
[0164] In the following, referring to FIG. 12, a monolithically
integrated sensor array 1200 according to an exemplary embodiment
of the invention will be explained in more detail.
[0165] FIG. 12 shows a cross-sectional view through the sensor
array 1200 according to an exemplary embodiment of the invention.
FIG. 12 shows a cross-section along a column, through the
nano-electrodes.
[0166] Up to and including metal-3 1002 the process is identical to
the original baseline CMOS process. The top-dielectric 1202
deviates from the low-K dielectric that typically is used at the
via-3 level 1102. Instead a moisture-resistant layer 1202, for
instance, fluorosilicate glass, is used to prevent penetration of
moisture into the layers below. Bond pad access holes (not shown in
FIG. 12) are defined in the moisture barrier (at the via-3 level
1102). The via-3 holes and the bond pad access holes are filled
simultaneously with a diffusion barrier and copper. A subsequent
CMP (chemical mechanical polishing) procedure defines the polished
surfaces of the nano-electrodes 1102 and the copper bond pads (not
shown).
[0167] FIG. 12 shows a p-well 1202 of a silicon substrate. The
various switch transistors are shown, more particularly their
source/drain regions 1204. Furthermore, a gate 1206 is shown.
Contact plugs 606 are shown as well. Furthermore, a first
metallization structure 702 can be seen. A first via 802 is
indicated as well. A second metal layer 804 is provided above the
first via layer 802. A second via layer 902 is provided above the
second metal layer 804. A third metallization layer 1002 is
provided above the second via layer 902. A third via layer 1102 is
provided above the third metallization layer 1002. A column bias
strap 1222 to connect the switching transistors to a charge
transfer column line is shown as well. Furthermore, row bias lines
1224 (the discharge lines) are indicated. A sense pad 1226 is
provided on a surface of the monolithically integrated structure
1200. The surfaces of the via-3 plugs 1102 are the sensitive areas,
i.e., the sense pads 1226. Beyond, moisture barriers 1202 are
provided between adjacent sense pads 1226.
[0168] Alternative embodiments allow creating even smaller cells.
For instance, flipping the odd columns 504 around their vertical
axis enables sharing the metal-1 landing pads 702 for the discharge
line connections of pairs of cells 100 in adjacent odd and even
columns 504. This creates some freedom to reduce the horizontal
cell pitch by re-optimizing the metal-1 layout 702 without
violating the baseline CMOS design rules.
[0169] Using self-aligned contacts that overlap with the
source/drain sidewall spacers of the switch transistors 106, 108
enables reducing the vertical cell pitch. To avoid violating the
metal-1 minimum-area design rule the horizontal cell pitch has to
be increased a bit. However, the resulting cell has a less
rectangular (more square) shape, which reduces the cell area a
bit.
[0170] Violating the metal-1 702 minimum-area design rule may be
used to reduce the horizontal cell pitch. This can be done, for
instance, by fine-tuning the metal-1 702 lithography procedure for
a smaller but fixed metal-1 702 landing pad area. Or the regular
metal-1 702 landing pads may be replaced by via-like holes, for
instance, by means of a double-exposure metal-1 702 litho-step or
by other methods known to persons skilled in the art.
[0171] Apart from smaller cell sizes other improvements may be
considered. For instance, violating the "enclosure of contacts by
active" design rule, for instance, by using borderless contacts,
may be used to reduce the width of the active lines. Although this
does not reduce the cell size, it does reduce the parasitic
capacitances between the poly lines and the source/drain junctions
of the switching transistors, which in turn increases the dynamic
range of the sensor and reduces its dynamic power dissipation.
[0172] Using separate discharge lines for odd and even rows may
have other benefits, although at the expense of a larger vertical
cell pitch. For instance, with separate discharge lines it is not
necessary to exclude at least one row form the reconfigurable
counter electrode described below.
[0173] Instead of via-3 1102 an alternative via level (for
instance, a via-4) may be used to implement the nano-electrodes
(and the bond pads). In this way more metal levels can be made
available for signal or power routing in the array or in the
peripheral electronics. Such an approach may be used, for instance,
to strap the poly clock lines by metal lines to lower their series
resistance.
[0174] Of course, combinations of optimizations and improvements
may be combined whenever desired.
[0175] Next, an array operation and a reconfigurable counter
electrode architecture will be explained in more detail.
[0176] As an example, the measurement of the capacitances in row 2
will be considered (see FIG. 5). Discharge and transfer clock
signals similar to those of FIG. 1 are applied at the control lines
.PHI..sub.D,2 and .PHI..sub.T,2, and the required discharge voltage
is applied at the discharge line V.sub.D,1. (The index of the
discharge lines identifies pairs of rows with shared discharge
lines instead of individual rows. So rows 2m and 2m+1 share
discharge line m.) In principle all other rows can be disabled by
biasing their control lines .PHI..sub.D,m and .PHI..sub.T,m
(m.noteq.2) at a low potential to switch off their discharge and
transfer switch transistors. This requires a separate counter
electrode to bias the electrolyte voltage at a voltage V.sub.L. In
the current context, the counter electrode denotes the electrode
that provides the main electrical contact to the electrolyte.
Although this is a feasible way of operating the sensor array 500,
it involves a couple of challenges:
[0177] 1. The counter electrode has to be placed external to the
sensor chip or it has to be integrated on a separate part of the
chip. The first option may make the system more vulnerable for
picking up interference signals from external sources like the
mains grid, mobile phones, radio stations, etc. The second options
may result in a larger chip area (unless the counter electrode can
be segmented into pieces that can be distributed over the
insensitive surface parts of the cells).
[0178] 2. If the counter electrode has a different material
composition or nano-scale structure than the nano-electrodes, the
measured transfer currents may drift as a result of aging of the
electrode/electrolyte junctions of the nano- and counter
electrodes, and as a result of drift in the temperature, salt
concentration or pH of the electrolyte.
[0179] 3. A sensor system with an external counter electrode may be
more complex than one with an integrated counter electrode. For
example, it may require at least 1 bond pad to connect the counter
electrode, which precludes, for instance, a pure system-on-chip
(SOC) sensor system without external parts.
[0180] These challenges can be overcome with an alternative biasing
scheme for the non-selected rows.
[0181] Again, as an example, the selection of row 2 will be
considered. Discharge and transfer clock signals similar to those
of FIG. 1 are applied at the control lines .PHI..sub.D,2 and
.PHI..sub.T,2, and the required discharge voltage is applied at the
discharge line V.sub.D,1. As before, all other transfer clock lines
.PHI..sub.T,m with m.noteq.2 are biased at a low potential to
switch off the corresponding transfer transistors 106. But now all
other discharge clock lines .PHI..sub.D,m with m.noteq.2 and
m.noteq.3 are biased at a high potential to switch on the discharge
transistors 108 of the corresponding rows 502. This connects their
nano-electrodes 102 to their respective discharge lines. In the
peripheral of the array 500 (not shown in FIG. 5) these discharge
lines V.sub.D,k with k.noteq.1 are all biased at the same reference
voltage V.sub.R, for instance, by means of addressable pass-gates.
In this way the nano-electrodes 102 of all non-selected rows 502,
except row 3, effectively are connected in parallel to constitute
one large reconfigurable counter electrode with exactly the same
composition and nano-scale geometry as that of the selected
nano-electrodes 102 in row 2. The nano-electrodes 102 of row 3 have
to be excluded from this reconfigurable counter electrode because
their discharge line is already biased at the discharge voltage for
row 2. Therefore the discharge transistors 108 of row 3 have to
switched off by applying a low voltage to the discharge clock line
.PHI..sub.D,3. Such a reconfigurable counter electrode has a couple
of advantages over a separate counter electrode:
[0182] 1. For an array of M rows 502 (m=0, 1, . . . , M-1) the
effective counter electrode area per selected cell is M-2 times the
nano-electrode 102 area. So for large M the contact impedance
between the counter electrode and the electrolyte is M-2 times less
than that of all selected nano-electrodes 102 in parallel. As a
result, the reconfigurable counter electrode effectively controls
the electrolyte voltage.
[0183] 2. After one complete row-scan through the whole array 500
the integrated net charge transport through all nano-electrodes 102
is zero, even if the leakage currents of selected nano-electrodes
102 are not exactly zero (this may happen, for instance, if the
reference voltage V.sub.R is not exactly equal to the time average
of the voltages of the selected nano-electrodes 102).
[0184] 3. Because the reconfigurable counter electrode consists of
a large amount of nano-electrodes 102 the effects of the captured
bio-molecules on the individual nano-electrodes 102 are averaged
into an overall effect that compensates for drift caused by the
changing average surface composition of the nano-electrodes
102.
[0185] Alternative algorithms to group a subset of non-selected
nano-electrodes 102 into reconfigurable counter electrodes can be
used as well. For instance, only odd-row 502 non-selected
nano-electrodes 102 may be used in reconfigurable counter
electrodes for odd selected rows 502, and only even-row
non-selected nano-electrodes 102 may be used for reconfigurable
counter electrodes for even selected rows 502. This approach would
effectively split the ensemble of all nano-electrodes 102 into two
completely independent sub-ensembles of odd- and even-row 502
nano-electrodes 102, respectively. Such an approach can be
advantageous for several reasons (for instance, increased symmetry
within each sub-ensemble), but at the expense of an effectively
doubled contact impedance between the counter electrode and the
electrolyte. Alternatively, only non-selected nano-electrodes 102
of rows 502 in a certain close environment of the selected row 502
may be used. This may be advantageous if external factors cause a
gradient in the nano-electrode 102 properties (for instance, during
measurements in a flowing electrolyte), but at the expense of an
even higher contact impedance between the counter electrode and the
electrolyte. Of course, the flexibility in constructing complicated
patterns of reconfigurable counter electrodes may be limited by the
architecture of the peripheral circuits for the selection of rows
502 and the routing of control- and discharge-line voltages or
signals.
[0186] Alternatively, in combination with an external counter
electrode the reconfigurable counter electrode can also be used as
an on-chip reconfigurable reference electrode to monitor the
potential of the electrolyte. This effectively turns the
reconfigurable counter electrode into a reconfigurable reference
electrode with similar properties as that of the currently selected
electrodes 102 and, consequently, with similar advantages as a
reconfigurable counter electrode (for instance, compensation for
drift caused by temporal changes in the composition, temperature,
etc. of the electrolyte, or by aging, ware-out, etc. of the SAM
layers). The measured electrolyte potential may be exported from
the sensor chip, for instance, to control the potential of the
external counter electrode.
[0187] Next, a row peripheral circuit according to an exemplary
embodiment of the invention will be explained.
[0188] In the following, row peripheral circuits for arrays with
the architecture of FIG. 5 will be described (that is, with shared
discharge lines V.sub.D,m for even and odd rows 2m and 2m+1).
Extension to alternative architectures (for instance, with separate
discharge lines for every row) are possible.
[0189] The discharge lines V.sub.D,m and the discharge and transfer
clock signals .PHI..sub.D,2m, .PHI..sub.T,2m, .PHI..sub.D,2m+1 and
.PHI..sub.T,2m+1 of the row pairs m=0, 1, . . . , M/2 (where M is
the number of rows) are controlled by a row peripheral circuit.
Such a circuit may comprise or consist of an address decoder to
select an even/odd row pair, and a signal gating circuit to switch
an appropriate discharge voltage and appropriate control signals to
the selected row pair. The architecture of a row address decoder
may be similar to that for use in memories. Various architectures
are possible for the signal gating circuits, depending on the
required flexibility.
[0190] For instance, in an embodiment simple MOS switches may be
used to directly connect the discharge line of the selected row
pair to a fixed discharge voltage, and all other discharge lines
(of the non-selected row pairs) to an alternative voltage or to
leave them floating. Simple logic gates may be used to select
either the even or the odd row of the selected row pair by applying
clock signals to the discharge and transfer clock lines of the
chosen row, and to disable the other row of the selected row pair.
The other rows of the non-selected row pairs can be either disabled
for operation with an external counter or reference electrode or
grouped into a reconfigurable counter or reference electrode
according to the aimed array operation mode.
[0191] FIG. 13 shows a versatile row peripheral circuit 1300 with
analog MUX switches 1302, wherein FIG. 14 shows a table 1400 which
describes the signal lines. In other words, FIG. 14 is a
description of the signals of row peripheral circuit 1300 of FIG.
13.
[0192] Thus, FIG. 13 shows an example of a very versatile gating
circuit 1300 consisting of five analog multiplexer (MUX) switches
1302 per row pair. The MUX switches 1302 of row pair m are
controlled by the row pair select line RPS(m) 1304 originating from
a row pair address decoder. The lines CT(2m), CD(2m), VD(m),
Cd(2m+1) and CT(2m+1) in FIG. 13 correspond to the lines
.PHI..sub.T,2m, .PHI..sub.D,2m, V.sub.D,m, .PHI..sub.D,2m+1 and
.PHI..sub.T,2m+1 in FIG. 5, respectively. Selecting row pair m puts
its five MUX switches 1302 in the upper position while all other
MUX switches 1302 (of the non-selected row pairs) remain in the
lower position. This allows to apply five independent wave forms to
the lines CT(2m), CD(2m), VD(m), CD(2m+1) and CT(2m+1) via the row
control bus lines TES, DES, VDS, DOS and TOS, respectively, and to
apply five alternative independent wave forms to the lines CT(2m'),
CD(2m'), VD(m'), CD(2m'+1) and CT(2m'+1) of all non-selected row
pairs, where m'=0, 1, . . . , M/2, m'.noteq.m, via the row control
bus lines TEN, DEN, VDN, DON and TON, respectively. In this way row
2m can be selected by applying appropriate clock signals to the
lines TES and DES, while row 2m+1 is disabled by applying
appropriate disabling voltages to the lines TOS and DOS.
Alternatively, row 2m+1 can be selected by applying appropriate
clock signals to the lines TOS and DOS, while row 2m is disabled by
applying appropriate disabling voltages to the lines TES and DES.
The array can be operated with an external counter or reference
electrode by disabling all non-selected rows by putting appropriate
disabling voltages on the lines TEN, DEN, VDN, DON and TON.
Alternatively, the array can be operated in a reconfigurable
counter or reference electrode mode by putting the counter or
reference electrode voltage on the line VDN, appropriate enabling
voltages on the lines DEN and DON, and appropriate disabling
voltages on the lines TEN and TON. The row control bus lines can be
connected directly to bond pads. Alternatively, they can be
connected to an on-chip wave form generation circuit
[0193] Next, a column peripheral circuit according to an exemplary
embodiment of the invention will be explained.
[0194] Typical values of the nano-electrode capacitance C in a
90-nm CMOS process are in the range of 0.5 fF. The parasitic
capacitance C.sub.P typically has about the same size as C. The
maximum transfer frequency f.sub.T is limited by the series
resistance of the poly clock lines of the array and by their
distributed load capacitance (mainly the gate capacitances of the
switching transistors). As a result, for typical arrays with about
256 columns the maximum value of f.sub.T is about 40 MHz. The
maximum amplitude |V.sub.T-V.sub.D| on the measurement node is
limited to about 0.2 V by the breakdown and ware-out properties of
the SAM and by parasitic electrochemical reactions that make take
place at the nano-electrodes. As a result, typical values of the
average charge transfer current I.sub.T is of the order of
magnitude of 8 nA. This current should be measured with a
resolution better than 8 pA to be able to resolve changes
|.delta.C| down to 1 aF caused by the capture of a single
bio-molecule.
[0195] Real time monitoring of capturing single bio-molecules on
the nano-electrodes requires a temporal resolution of about a
second or better, depending on the concentration of the
bio-molecules (if the capture event rate is too high the sample may
have to be diluted to reduce the concentration). For an array
consisting of 256 rows of nano-electrodes this means about 4 ms per
row or less, provided that the average charge transfer currents of
all columns are measured in parallel. To be able to do this with a
resolution better than 8 pA the measurements may be done
on-chip.
[0196] FIG. 15 shows a column periphery circuit 1500 having a Reset
voltage line 1502, a Reset_not line 1504, a Group select_not line
1506, a Read currents line 1508, a Reference voltage line 1510,
voltage clamps 1512, a Read bus 1514, a Read multiplexer 1516,
integration caps and read out portion 1518, and Reset MOSTs
1520.
[0197] Massive parallel on-chip measurement of the average charge
transfer currents can be done in multiple ways. In the embodiment
of FIG. 15, the transfer voltage on the column line is controlled
by the source follower T.sub.1 and the reference voltage line. The
drain current of the source follower T.sub.1 is integrated on the
gate capacitance of transistor T.sub.2 after resetting the gate
voltage by the reset switch T.sub.3. At the end of the integration
period the drain of T.sub.2 is connected to the read bus by closing
the read MUX switch T.sub.4. Now the read current (that is, the
drain current of T.sub.2, which is a measure of the charge
integrated on the gate capacitance of T.sub.2) can be measured via
the corresponding read bus line. Grouping of columns allows
multiplexing of read currents over the read bus lines. In that case
resetting preferably should also be done per group to arrive at
equal integration period for the groups. The read bus lines can be
connected directly to separate bond pads to measure the
corresponding read currents with off-chip read electronics.
Alternatively, the read bus lines can be connected to on-chip
buffer circuits or current-to-voltage converters that export the
converted analog signals from the chip via bond pads.
Alternatively, the read currents can be digitized by on-chip
analog-to-digital converters (ADCs) and exported from the sensor
chip via a digital bus.
[0198] At an average charge transfer current of 8 nA and an
integration time of 4 ms the gate capacitance would have to be
equal to 64 pF to limit the voltage swing on the gate capacitance
of T.sub.2 to about 0.5 V (a typical value for a supply voltage of
1.2 V). This would require a gate area of T.sub.2 of about 4500
square microns, which is comparable to the area of a bond pad. This
is very large because such a large transistor would be needed for
every column. This challenge can be solved by splitting the
integration period of 4 ms into multiple smaller integration
periods. For instance, for an integration period of 40 microseconds
a gate area of 45 square microns is required. However, shorter
integration periods correspond to wider signal bandwidth and,
consequently, higher noise. Therefore, multiple sequential
measurements performed with these shorter integration periods have
to be averaged, for instance, on an external computer or with
on-chip digital circuits.
[0199] In an alternative embodiment of a column periphery circuit
the PMOS transistor T2 in FIG. 15 is replaced by a separate
integration capacitor and a NMOS source follower transistor. After
resetting the voltage on the integration capacitor with the reset
transistor T3 the drain current of transistor T1 is integrated on
the integration capacitor. The voltage over the integration
capacitor is measured by the source follower transistor. The source
follower transistor may be selected by the selection transistor T4.
Alternatively, the selection transistor T4 may be replaced by a
NMOS transistor.
[0200] Next, calibration and self-referencing will be
mentioned.
[0201] Apart from wide-band noise that can be reduced by averaging
sequential measurements, the read current may also contain
low-frequency noise (often referred to as 1/f-noise) generated by
the DC currents flowing through transistors T.sub.1, T.sub.2 and
T.sub.4 during the integration period or during the read-out via
the read bus (the reset transistor T.sub.3 and the discharge and
charge transfer transistors of the selected cell do not generate
low-frequency noise if the reset, discharge and charge transfer
transients are allowed to decay sufficiently at the end of each
switching event). This low-frequency noise typically cannot be
reduced by averaging subsequent measurements because of its
1/f-like noise power spectral density. Instead, a calibration
measurement may be done. For this purpose calibration rows can be
used.
[0202] Calibration rows may have the same architecture as the
active rows, but without nano-electrodes connected to their
measurement nodes. As a result, their average charge transfer
current is determined only by the parasitic capacitances of their
measurement nodes. Because these parasitic capacitances remain
constant over time they can be used to generate reproducible
reference currents for the columns.
[0203] To suppress low-frequency noise, one or more calibration
rows may be selected simultaneously, and the total average charge
transfer current in every column is measured by means of the column
peripheral circuit. Preferably the number of simultaneously
selected calibration rows should be chosen in such a way that the
total calibration charge transfer current in a column is closest to
the charge transfer current of an active row (that is, a row with
connected nano-electrodes). Because the nano-electrode capacitance
C and the parasitic C.sub.P typically have about similar values,
typically two calibration rows have to be measured simultaneously
to generate a reference current comparable to the charge transfer
current generated by an active cell. If necessary, the reference
current can be fine-tuned by means of the charge transfer
frequency.
[0204] To be able to resolve the small capacitance changes
|.delta.C| caused by single-molecule capturing events at the
nano-electrodes, the measured capacitances of the individual
nano-electrodes can be compared to the average capacitance of a row
or set of rows. In this way systematic temporal drift in the
nano-electrode capacitances C, for instance, as a result of
gradually changing dielectric properties of the SAM layers, can be
cancelled (such drift components in general cannot be cancelled by
using a reconfigurable counter electrode because the total
capacitance of the selected nano-electrodes is much less than that
of a typical reconfigurable counter electrode.
[0205] In case of a source-follower column periphery circuit the
low-frequency noise of source follower transistor can be suppressed
further by employing a correlated double sampling strategy. After
measuring the voltage on the integration capacitor at the end of
the integration cycle the reset transistor T3 is closed to
discharge the integration capacitor. While the reset transistor is
still closed the voltage on the discharged integration capacitor is
measured again (a second time) to serve as a reference for the
first measurement. By subtracting the second measurement from the
first, the low-frequency noise of the source follower transistor
can be eliminated to a large extent. Such a correlated double
sampling measurement strategy can be combined with calibration
measurements like explained before.
[0206] In the following, a system-level architecture according to
an exemplary embodiment of the invention will be explained.
[0207] FIG. 16 shows a system-level sensor-architecture 1600
according to an exemplary embodiment of the invention.
[0208] The sensor array 500 is controlled by a row peripheral
circuit 1602, and the average charge transfer currents of the
columns are measured by a column peripheral circuit 1604. The row
peripheral circuit 1602 and the column peripheral circuit 1604
connect to a wave form generator (WG) and control block 1606 that
is connected to an input-output (JO) bus 1608. The IO bus 1608
inputs the addresses and other control signals and outputs the read
currents and other optional output signals. Alternatively, the wave
form generator may be off-chip.
[0209] FIG. 17 shows a system-level sensor-architecture 1700
according to another exemplary embodiment of the invention.
[0210] In FIG. 17, separate upper and lower column peripheral
circuits 1702, 1704 are provided for even and odd columns,
respectively. This architecture may be used to ease the layout of
the column peripheral circuit (this may be advantageous because the
column pitch typically is smaller than the row pitch).
[0211] FIG. 18 shows a system-level sensor-architecture 1800
according to still another exemplary embodiment of the
invention.
[0212] In FIG. 18, calibration rows 1802 are provided which occupy
part of the row address space. Additional measures may have to be
taken to be able to select more than one calibration row 1802
simultaneously.
[0213] FIG. 19 shows a system-level sensor-architecture 1900
according to yet another exemplary embodiment of the invention.
[0214] In FIG. 19, the calibration rows 1802 are implemented as a
separate part of the array that falls outside the row address space
500.
[0215] Although embodiments of the invention have been described
assuming NMOS switching transistors in the sensor array it is clear
that alternative embodiments based on PMOS switch transistors are
possible as well.
[0216] By sweeping the charge transfer frequency a spectral scan
can be made to measure frequency-dependent dielectric properties of
individual captured molecules.
[0217] Instead of operating the sensor array with clock signals to
translate nano-electrode capacitances into charge transfer
currents, the sensor may also be used to directly measure DC
currents of nano-electrodes by statically disabling the discharge
transistors and enabling the charge transfer transistors of the
selected row. This may be used to operate the sensor as a massive
parallel electrochemical biosensor, for instance to measure DC
currents generated by single-molecule enzymes or redox couples
captured on the nano-electrodes. Such enzymes or redox couple
molecules may be used as labels to detect bio-molecules.
[0218] By statically enabling the discharge transistors and
disabling the charge transfer transistors of the selected row, the
capturing of molecules on the selected nano-electrodes may be
influenced by applying an appropriate voltage on the discharge line
of the selected row pair (or row, in case of an architecture with
separate discharge lines for every row). During this process the
other rows may be used as reconfigurable counter electrodes. By
scanning through the rows the capturing of molecules may be
influenced on all rows of the sensor. This method may be extended
to individual nano-electrodes by applying the required bias
voltages via the charge transfer lines instead of the discharge
lines. For this purpose the column peripheral circuit may be
modified or extended in such a way that different voltages can be
applied to every individual column line. Such a way of operation
may be used, for instance to enhance the concentration of
positively or negatively charged molecules at the selected
nano-electrode surfaces to enhance or disable their capturing. For
instance, the capturing of negatively charged DNA oligomers (small
fragments of DNA) may be influenced this way.
[0219] In the following, advantages of exemplary embodiments of the
invention will be explained: [0220] Massive parallel
single-molecule detection [0221] Extracting maximum possible
information from ensemble of captured bio-molecules [0222] Temporal
resolution at single-molecule level to measure reaction kinetics
[0223] Manufacturability in standard CMOS process with minor BEOL
modifications [0224] Using discharge and charge transfer transistor
of the same conductivity type (both NMOS or both PMOS) allows to
make much denser cell layout than using transistors of opposite
conductivity type [0225] "Natural" scalable; benefiting form
Moore's Law [0226] Only one plate of the nano-electrode capacitors
is connected to the switching elements in the cell. The other plate
(the electrolyte) is shared. This enables the extremely compact
cell architecture. [0227] Ultra-low power dissipation. In the
sensor array dynamic power is only dissipated in the selected row.
All non-selected rows only "see" DC voltages (no dynamic power
dissipation) and all columns lines only carry very low DC currents.
[0228] Virtually no cross talk between adjacent column lines
because they effectively only carry DC currents [0229] Almost
perfect charge balancing possible with reconfigurable counter
electrode [0230] Reconfigurable counter and reference electrodes
have (almost) equal composition and history as active electrodes
[0231] No long signal paths with (on-chip) reconfigurable counter
electrodes: minimal pick-up of interference from external sources
(radio stations, mains, mobile phones, etc.) [0232] Full CMOS
biosensor allows embedding additional functions (A-to-D converter,
microcontroller, memory, etc.) at the lowest price (possibility to
design with CMOS library blocks or IP blocks, perhaps modified at
the highest metal levels)
[0233] Finally, it should be noted that the above-mentioned
embodiments illustrate rather than limit the invention, and that
those skilled in the art will be capable of designing many
alternative embodiments without departing from the scope of the
invention as defined by the appended claims. In the claims, any
reference signs placed in parentheses shall not be construed as
limiting the claims. The words "comprising" and "comprises", and
the like, do not exclude the presence of elements or steps other
than those listed in any claim or the specification as a whole. The
singular reference of an element does not exclude the plural
reference of such elements and vice-versa. In a device claim
enumerating several means, several of these means may be embodied
by one and the same item of software or hardware. The mere fact
that certain measures are recited in mutually different dependent
claims does not indicate that a combination of these measures
cannot be used to advantage.
* * * * *