Note: Descriptions are shown in the official language in which they were submitted.
CA 02625251 2008-03-07
Title
High performance micro-fabricated electrostatic quadrupole lens
Field of the Invention
This invention relates to mass spectrometry, and in particular to the
provision of a
miniature electrostatic quadrupole mass filter with high range, low noise and
high
sensitivity.
Background
Miniature mass spectrometers have application as portable devices for the
detection
of biological and chemical warfare agents, drugs, explosives and pollutants,
as
instruments for space exploration, and as residual gas analysers.
Mass spectrometers consist of three main subsystems: an ion source, an ion
filter,
and an ion counter. One of the most successful variants is the quadrupole mass
spectrometer, which uses a quadrupole electrostatic lens as a mass filter.
Conventional quadrupole lenses consist of four cylindrical electrodes, which
are
mounted accurately parallel and with their centre-to-centre spacing at a well-
defined
ratio to their diameter [Batey 1987].
Ions are injected into the pupil between the electrodes, and travel parallel
to the
electrodes under the influence of a time-varying hyperbolic electrostatic
field. This
field contains both a direct current (DC) and an alternating current (AC)
component.
The frequency of the AC component is fixed, and the ratio of the DC voltage to
the
AC voltage is also fixed.
Studies of the dynamics of an ion in such a field have shown that only ions of
a
particular charge to mass ratio will transit the quadrupole without
discharging against
one of the rods. Consequently, the device acts as a mass filter. The ions that
successfully exit the filter may be detected. If the DC and AC voltages are
ramped
1
CA 02625251 2008-03-07
together, the detected signal is a spectrum of the different masses that are
present
in the ion flux. The largest mass that can be detected is determined from the
largest
voltage that can be applied.
The resolution of a quadrupole filter is determined by two main factors: the
number
of cycles of alternating voltage experienced by each ion, and the accuracy
with
which the desired field is created. So that each ion experiences a large
enough
number of cycles, the ions are injected with a small axial velocity, and a
radio
frequency (RF) AC component is used. This frequency must be increased as the
length of the filter is reduced.
The sensitivity and hence the overall performance of a mass spectrometer is
also
affected by the signal level and the noise level. Noise arising from stray
ions is
conventionally reduced by the use of a grounded screen [Denison 1971]. The ion
transmission is clearly reduced as the size of the entrance pupil is
decreased. Efforts
have therefore been made to improve transmission in small quadrupoles, and it
has
been shown that significantly improved transmission at a given resolution can
be
obtained by reducing the effect of fringing fields at the input to the
quadrupole.
One effective method involves the use of a so-called Brubaker lens or Brubaker
pre-
filter, which consists of an additional set of four short, cylindrical
electrodes mounted
co-linearly with the main quadrupole electrodes. The Brubaker pre-filter is
excited
with the AC voltages (but not the DC voltages) applied to the main quadrupole
lens.
It is well known that a quadrupole excited only with AC voltages acts as an
all-pass
filter, so that the Brubaker pre-filter provides an ion guide into the main
quadrupole.
However, the delay in application of the DC voltage component results in a
reduction
in fringing fields and significantly improves overall ion transmission at a
given mass
resolution [Brubaker 1968; US 3,129,327; US 3,371,204].
In order to create the desired hyperbolic field, highly accurate methods of
construction are employed. However, it becomes increasingly difficult to
obtain the
required precision as the size of the structure is reduced [Batey 1987].
2
CA 02625251 2008-03-07
Microfabrication methods are therefore increasingly being employed to
miniaturise
mass spectrometers, both to reduce costs and allow portability.
Microfabricated devices are often fabricated on silicon wafers, because of the
range
of compatible deposition, patterning and etching processes that may be used.
However, the resistivity of silicon is inherently limited to that of intrinsic
material, and
the thickness of deposited insulating films is limited by the stress in such
films.
These restrictions have particular consequences for the performance of RF
devices
such as electrostatic quadrupole mass filters formed in silicon.
For example, a silicon-based quadrupole electrostatic mass filter consisting
of four
cylindrical electrodes mounted in pairs on two oxidised, silicon substrates
was
demonstrated some years ago. The substrates were held apart by two cylindrical
insulating spacers, and V-shaped grooves formed by anisotropic wet chemical
etching were used to locate the electrodes and the spacers. The electrodes
were
metal-coated glass rods soldered to metal films deposited in the grooves. [US
6,025,591].
Mass filtering was demonstrated using devices with electrodes of 0.5 mm
diameter
and 30 mm length [Syms et al. 1996; Syms et al. 1998; Taylor et al. 1999].
However,
the performance was limited by RF heating, caused by capacitative coupling
between co-planar cylindrical electrodes through the oxide interlayer via the
substrate. As a result, the device presented a poor electrical load, and the
solder
attaching the electrodes tended to melt. These effects restricted the voltage
and
frequency that could be applied, which in turn limited both the mass range (to
around
100 atomic mass units) and the mass resolution. While the substrate was
grounded,
the use of an incomplete screen also resulted in high noise levels, and the
devices
also suffered in low transmission rates.
In an effort to overcome these limitations, an alternative construction based
on
bonded silicon-on-insulator (BS01) was developed [GB 2391694]. BSOI consists
of
an oxidised silicon wafer, to which a second silicon wafer has been bonded.
The
3
CA 02625251 2008-03-07
second wafer may be polished back to the desired thickness, to leave a silicon-
oxide-silicon multi-layer.
In this geometry, the electrode rods were again mounted in pairs on two
substrates.
However, the electrodes were now retained by silicon springs etched into the
substrate of the BSOI wafer, while the device layer was used as a spacer. The
oxide
interlayer was largely removed, so that capacitative coupling between co-
planar
cylindrical electrodes via the substrate was greatly reduced. As a result, the
device
could withstand considerably higher voltages, and a mass range of 400 atomic
mass
units was demonstrated [Geear et al. 2005].
Despite these results, only partial screening was again possible. Furthermore,
it was
found that the transmission was again low, because of obstruction of the
entrance
pupil by the features such as springs and hooks mounting the cylindrical
electrodes.
These features also hampered the incorporation of auxiliary optics such as a
Brubaker pre-filter.
A further microfabricated quadrupole filter, described as a "square rods
quadrupole"
and based on a two-substrate assembly formed in silicon and mounting a set of
polygonal rods, has also been described [Silion and Baptist 2002; US
6,465,792].
However, it does not appear to have been demonstrated.
Because many applications of mass spectrometry require greater mass range,
there
is a need to provide a more effective solution to the problem of RF heating.
There is
therefore a need to provide such a solution and also a requirement for mass
spectrometer devices that are operable in conditions requiring low noise and
greater
sensitivity at a given resolution.
Summary
These and other problems are addressed by a mass spectrometer device in
accordance with an illustrative embodiment of the invention that eliminates
the use
of thin deposited oxide layers for electrical isolation in a microfabricated
electrostatic
4
CA 02625251 2015-10-22
quadrupole mass filter. A device in accordance with an illustrative embodiment
of the
invention also addresses the problem of incorporating both a grounded screen
and a
Brubaker pre-filter. Such benefits are provided by incorporating a mount for
the
quadrupole electrodes in which any silicon parts are physically separated and
attached to an insulating substrate.
In accordance with an illustrative embodiment of the invention there is also
provided a
method of aligning sets of cylindrical electrodes in the geometry of a
miniature
quadrupole electrostatic lens, which can act as a mass filter in a quadrupole
mass
spectrometer. The electrodes are mounted in pairs on microfabricated supports,
which
are formed from conducting parts on an insulating substrate. Complete
segmentation
of the conducting parts provides low capacitative coupling between coplanar
cylindrical electrodes, and allows incorporation of a Brubaker lens to improve
sensitivity at a given mass resolution. A complete quadrupole is constructed
from two
such insulating substrates, which are spaced apart by further conducting
spacers. The
spacers are continued around the electrodes to provide a conducting screen.
In accordance with another illustrative embodiment of the invention, there is
provided
a quadrupole lens formed from first and second microfabricated mounts. Each
mount
has an insulating substrate having formed thereon first and second mounting
members configured to receive a first electrode and a second electrode
respectively.
The first and second mounting members are physically distinct from one
another.
Each mount further includes at least one spacer, the at least one spacer
having a
height greater than the height of either the first or second mounting members.
Each
mounting member is formed from two support members, the support members being
physically distinct from one another and each of the support members is formed
as an
island on the insulating substrate. Each one of the islands are separated by
trenches
formed along longitudinal and transverse axes of the mount. The support
members of
each mounting member are located at first and second locations on the
substrate and
separated from one another by the at least one spacer, the at least one spacer
5
CA 02625251 2015-10-22
forming a shield located between the first and second locations such that each
one of
the first and second electrodes passes through the shield.
In accordance with another illustrative embodiment of the invention, there is
provided
a quadrupole mass spectrometer including a lens formed from first and second
microfabricated mounts. Each mount has an insulating substrate having formed
thereon first and second mounting members configured to receive a first set of
electrodes including first and second electrodes respectively. The first and
second
mounting members are physically distinct from one another. The mass
spectrometer
further includes a second set of four electrodes arranged in series with the
first set of
electrodes. The second set of electrodes are coupled to an RF supply only and
the
first set of electrodes are operable at both RF and DC voltages. The lens
further
includes a spacer located between the first and second mounting members such
that
a received electrode passes through the spacer.
In accordance with another illustrative embodiment of the invention, there is
provided
a microfabricated mass spectrometer formed from first and second
microfabricated
mounts. Each mount has an insulating substrate having formed thereon first and
second mounting members coupled to a first set of at least two electrodes. The
first
and second mounting members are physically distinct from one another. The mass
spectrometer further includes a second set of at least four electrodes
arranged in
series with the first set of electrodes. The second set of electrodes is
coupled to an
RF supply only and the first set of electrodes is operable at both RF and DC
voltages.
A lens further includes a spacer located between the first and second mounting
members such that a received electrode passes through the spacer.
In accordance with another illustrative embodiment of the invention, there is
provided
a quadrupole lens formed from first and second microfabricated mounts provided
in a
sandwich structure. Each mount has an insulating substrate. The quadrupole
lens
includes first and second electrodes defining a first pair of electrodes
received on first
and second physically distinct mounting members each mounting member formed
5A
CA 02625251 2015-10-22
from two physically distinct and electrically isolated support members located
at first
and second locations on the substrate. Each of the first and second mounting
members is configured to receive the first and second electrode respectively.
The first
and second mounting members are physically distinct from one another, the
first pair
of electrodes being operably coupled to a RE and DC voltage. The quadrupole
lens
also includes a second pair of electrodes aligned longitudinally with the
first pair of
electrodes and operably coupled to an RF voltage only. On forming the sandwich
structure the electrodes define a first and second quadrupole.
5B
CA 02625251 2008-03-07
These and other features of illustrative and exemplary embodiments will be
better
understood with reference to Figures 1 ¨ 9 which follow.
Brief Description of the Drawinqs
Figure 1 shows in section and in plan a microfabricated mount for an
electrostatic
quadrupole lens containing laterally segmented conducting parts on an
insulating
substrate, according to an embodiment of the present invention.
Figure 2 shows in an isometric view the mounting of cylindrical electrodes in
a
microfabricated mount, according to an embodiment of the present invention.
Figure 3 shows in a side view and in two sections the mounting of cylindrical
electrodes and the assembly of a complete microfabricated electrostatic
quadrupole
lens, according to an embodiment of the present invention.
Figure 4 shows the incorporation of an additional set of RF only electrodes in
the
geometry of a Brubaker lens, according to an embodiment of the present
invention.
Figure 5 shows in plan an arrangement providing all electrical connections to
a
microfabricated quadrupole on a single substrate, according to an embodiment
of
the present invention.
Figure 6 shows in section an arrangement providing all electrical connections
to a
microfabricated quadrupole on a single substrate, according to an embodiment
of
the present invention.
Figure 7 shows the main geometric parameters associated with the mounting of a
single cylindrical electrode, according to an embodiment of the present
invention.
Figure 8 shows in plan two substrates forming the mount for a miniature
electrostatic
quadrupole lens according to an embodiment of the present invention.
6
CA 02625251 2008-03-07
Figure 9 shows in section the assembly of a set of substrates forming the
mount for
a miniature electrostatic quadrupole lens according to an embodiment of the
present
invention.
Detailed description of the drawinqs.
The invention will now be described with reference to exemplary embodiments
which
are provided to assist in an understanding of the teaching of the invention.
While
features may be described with reference to one figure it will be understood
that
such features could be used with or replaced by the features described in
another
figure as it is not intended to limit the invention to the interpretation of
any one figure,
as modifications can be made without departing from the scope of the
invention.
Such scope is only to be limited as is deemed necessary in the light of the
appended
claims.
In Figure 1, an insulating substrate 100 is used to co-locate a variety of
features
formed in an additional layer of material that is either conductive or coated
in a
conductive layer. This additional layer may be fabricated or formed to provide
different features such as one or more supporting members or shields, as will
become apparent from the following description. Examples of suitable
insulating
substrate materials include glasses, ceramics and plastics. It will be
understood that
although any insulating material may be useful in the context of the teaching
of the
present invention that glasses are more suitable for the intended application
in mass
spectrometry because of their lower out-gassing rates under vacuum. Examples
of
suitable conducting materials include metals, and metal-coated semiconductors
and
insulators. Metal-coated silicon is of particular interest, since it may
easily be
structured using micro-fabrication processes such as photolithography and
etching.
However, metal structures may also be microfabricated by photolithography and
electroplating.
At either end of the substrate, two pairs of support members or features 101a,
101b
and 102a, 102b provide alignment for and electrical connection to a pair of
inserted
7
CA 02625251 2008-03-07
cylindrical electrodes. The combination of the support members and the
insulating
substrate form a microfabricated mount. Each of the pair of support members
provide collectively a mounting member for their respective inserted
electrode. Each
of the two electrodes have the same diameter, and will ultimately act as two
of the
four electrodes in an electrostatic quadrupole lens. It will be evident that
the
electrodes, when received within the support members are aligned parallel to
one
another along a longitudinal axis which is substantially perpendicular to the
Section
Lines A-A' or B-B'. In this way it may be understood that the substrate has a
longitudinal axis which is parallel to the electrodes and a transverse axis
which is
parallel to the Section Lines.
Mechanical alignment for the cylindrical electrodes which may be located in
and
supported by the support members 101a and 101b is provided using grooved
locating features 105a and 105b, and similar features 107a and 107b are
provided in
the elements 102a and 102b. Suitable features include V-shaped, U-shaped and
rectangular grooves, which may all be formed by microfabrication processes
such as
photolithography and etching. Suitable methods of attaching the cylindrical
electrodes include the use of conductive epoxy and solder. It will be
understood that
the grooved supports or recesses 105a, 105b provide a support for their
respective
electrodes at a first end of each electrode and the grooved supports or
recesses
107a, 107b provide support at a second end; each electrode has a length and is
supported at either end of that length.
In this embodiment the support members for each of the two electrodes are
electrically isolated from one another. To achieve this electrical isolation
between
adjacent supports, this embodiment provides for a physical separation or
trench 103,
106 to be provided between each of the adjacent supports 101a/101b and
102a/102b respectively. Each of the two trenches is formed in a direction
parallel to
the longitudinal axis of the electrodes. The formation of the trenches 103,
106
provides a physical separation between the adjacent supports which as they are
each located on the insulating substrate achieves the necessary electrical
isolation.
Electrical connections along the length of each of the support features 101a
and
101b is provided by the use of a conducting material, or by making their top
surfaces
8
CA 02625251 2008-03-07
104a and 104b conducting by a deposited film. Electrical isolation between the
features 102a and 102b is similarly provided by providing a physical
separation 106,
and electrical connections along the support features 102a and 102b are
provided
by the use of a conducting material or deposited film along their top
surfaces. By
coupling the electrodes to their respective locating features using a
conductive
material and having the upper surfaces of these features also conducting it is
possible to provide an electrical connection between the support features and
their
respective supported electrodes.
The separations or trenches 103 and 106 are desirably formed using
photolithographic or etching techniques and as such may be relatively large.
Consequently, it will be appreciated that the capacitance between elements
101a
and 101b and between elements 102a and 102b may be lower than using an
alternative method based on a thin deposited insulating layer. Further, it
will be
appreciated that very small currents will flow between the elements 101a and
101b
when the pair are excited by a radio frequency (RF) AC voltage. Consequently
the
arrangement will provide an electrical load more closely corresponding to an
ideal
capacitor, with reduced RF heating.
The trenches 103, 106 provide for longitudinal separation between the adjacent
supports. It is also possible to provide for transverse isolation, such that
each
electrode is supported at either end by electrically isolated support members
101a/102a and 101b/102b. Such transverse isolation is provided in the
arrangement
of Figure 1 by two transverse trenches 110a, 110b which extend in a direction
substantially transverse to the longitudinal axis of the inserted electrodes.
The
formation of both transverse and longitudinal trenches effectively forms the
individual support members 101a, 101b, 102a, 102b as islands on the substrate
100.
By isolating the support members in a transverse direction a gap is defined
within
which a shield may be provided. The shield serves to cover up portions of the
insulating substrate which if exposed to ions could possibly otherwise become
charged. As shown in Figure 1, between the two pairs of electrode mounting
9
CA 02625251 2008-03-07
features 101a, 101b and 102a, 102b is provided a further shielding feature in
the
form of a shield 108 containing a deep trench 109, which extends in a
longitudinal
axis substantially parallel to the intended location of the electrodes. The
trench 109
has side surfaces or walls 112a, 112b which are upstanding from a bottom
surface
111. The shield is also attached to the insulating substrate 100 but isolated
from the
electrode mounting features by the physical separations or trenches 110a,
110b.
Electrical connection over the surface of the shielding feature 108 is
provided by the
use of a conducting material, or by making the surfaces 111, 112a, 112b, 113a,
113b conducting by a deposited conducting film. The depth and width of the
trench
which will define the vertical position of the conducting surface 111 and the
lateral
positions of the conducting surfaces 112a, 112b are chosen so that these
surfaces
do not make electrical contact with the electrodes when the electrodes are
inserted
into the grooves 105a, 105b and 107a, 107b. As shown in Section A-A' and B-B'
of
Figure 1 and also the perspective view of Figure 2 upper surfaces 113a and
113b of
the shield are higher than upper surfaces 104a and 104b of the support
members.
By this it will be understood that the distance of the upper surfaces of the
shield from
the underlying substrate is greater than the distance of the upper surfaces of
the
support members from the underlying substrate.
Figure 2 shows how two cylindrical electrodes 200a, 200b are inserted into the
alignment grooves in the blocks 101a, 101b and 102a, 102b. It will be
understood
that the depth of the locating alignment grooves 101a, 101b and 102a, 102b is
less
than the depth of the trench 109 such that an electrode located in the
alignment
grooves will be suspended over the trench defined in the shield. By providing
a
suspension of the cylindrical electrodes at a distance from the trench 109
formed in
the conducting surface of the shielding element 108, it will be appreciated
that the
trench can then provide a conducting shield extending at least partly around
the
cylindrical electrodes.
It will be appreciated that the dimensions of the five main features 101a,
101b, 102a,
102b and 108, and the separations 103, 106, 110a and 110b may all be
accurately
outlined using photolithography, as may those of the subsidiary features 105a,
105b
and 107a, 107b and 109. It will also be appreciated that the relative heights
above
CA 02625251 2008-03-07
the insulating substrate of features such as104a, 104b, 113a, and 113b may
also be
accurately defined by etching to a known depth. Consequently, the overall
structure
may be formed with well-defined dimensions using processes well known to those
skilled in the art of micro-fabrication.
Figure 3 shows how a complete electrostatic quadrupole lens may be constructed
from combining two such assemblies 301a, 301b, which are stacked together face
to
face so that conducting surfaces 302a, 302b of their shielding elements align
and
abut and form a sandwich structure. It will be appreciated that the assembly
now
provides a means whereby four cylindrical electrodes 303a, 303b, 303c, 303d
may
be supported at either end by grooves in similar conducting features 304a,
304b,
304c, 304d, which are held by and isolated from each other by two insulating
substrates 305a, 305b which form outer surfaces of the sandwich structure. It
will
also be appreciated that the two insulating substrates 305a, 305b are
supported and
spaced apart by the two shielding features 306a, 306b.
With a suitable choice of dimensions, the assembly may therefore mount four
similar
cylindrical electrodes with their axes parallel and with their centres located
on a
square. Since the size of the square may be chosen appropriately compared with
the diameter of the electrodes, the overall assembly provides the geometry of
an
electrostatic quadrupole lens.
It will also be appreciated that the conducting features 304a, 304b, 304c,
303d
provide little obstruction in the space between the cylindrical electrodes,
which forms
the pupil of the quadrupole lens, so that the greater portion of the
electrodes may
provide a quadrupole field with low distortion. It will also be appreciated
that the
inner conducting surfaces 307a, 307b of the shielding features 306a, 306b,
which
correspond to the side walls of the trench 109 in Figure 2, can now fully
shield the
four cylindrical electrodes along the greater portion of their length.
It will be understood that while only one quadrupole configuration is shown in
the
exemplary embodiments heretofore described that multiple quadrupoles may be
constructed on the same substrate, in the form of a parallel array, to
increase the
11
CA 02625251 2008-03-07
overall ion flux and hence the sensitivity or that a serial array of multiple
quadrupoles
could also be formed on the same substrate. By providing a plurality of
quadrupoles
in parallel it is possible to increase throughput through the device whereas
the
provision of electrodes in series allows the fabrication of additional
features such as
for example a Brubaker lens or prefilter, as will be discussed below.
Figure 4 shows one method of combining an electrostatic quadrupole lens with a
Brubaker prefilter consisting of a RF-only quadrupole. Here each insulating
substrate
401 is extended to allow the incorporation of extra mounting features 402a,
402b for
a second pair of separate cylindrical electrodes 403a, 403b in addition to the
pair of
primary cylindrical electrodes 404a, 404b held in mounts 405a, 405b and 406a,
406b. The additional electrodes are aligned longitudinally with their
respective
primary cylindrical electrodes. Because the electrodes in a Brubaker prefilter
are
conventionally very short, a single set of mounting features holding the
cylindrical
electrodes at their midpoint will normally suffice. Again, suitable attachment
methods
include conductive glue and solder. It will be appreciated that the Brubaker
electrodes may be mechanically contiguous with but electrically isolated from
the
main quadrupole electrodes. In this case, the mounting method is further
simplified.
The short cylindrical electrodes 403a, 403b may be driven directly with the RF
voltages VAC1, VAC2 supplied to the long cylindrical electrodes.
Alternatively, they
may be driven from the long cylindrical electrodes via capacitors 407a, 407b
and
resistors 408a, 408b, which provide a means to couple the RF voltages VAC1,
VAC2 to the short cylindrical electrodes while ensuring that the DC voltage
applied
to the short cylindrical electrodes is substantially that of ground.
Figures 5 and 6 show in plan and in section how all of the electrical
connections to a
single quadrupole may be provided on the same substrate. This arrangement is
generally the most convenient for attaching bond wires to external circuitry.
The upper substrate 501a and the features thereon are narrower than the lower
substrate 501b, so that contacts to the cylindrical electrodes 502a, 502b and
to the
shield 503a, 503b on the lower substrate are freely exposed when the two
12
CA 02625251 2008-03-07
substrates are stacked together. This is achieved by providing the upper
substrate
with a smaller footprint than that of the lower substrate.
Contacts to the cylindrical electrodes 504a, 504b on the upper substrate are
routed
to pillars 505a, 505b, which are connected when the two substrates are stacked
together to additional features 506a, 506b on the lower substrate. Wire bonds
601a,
601b may then be attached to features 502a, 502b connecting to the lower
cylindrical electrodes. Similarly, wire bonds 602a, 602b may be attached to
features
506a, 506b connecting to the upper cylindrical electrodes, and wire bonds
603a,
603b may be attached to features 503a, 503b connecting to the shield.
It will be appreciated that in each case wire bonds are attached to features
existing
only on the lower substrate 501b, thus simplifying the wirebonding operation.
It will
also be appreciated that this connection scheme may be extended to provide for
connection to any additional similar electrodes, for example when a prefilter
is used.
Figure 7 shows in section how the main geometric parameters of the
microfabricated
quadrupole mount are restablished. Here, the grooved feature 701 supporting a
single cylindrical electrode 702 of radius re is shown.
Conventionally it is desired to hold the electrode at an equal distance s from
the two
axes of symmetry 703, 704 of the electrostatic field created by the quadrupole
assembly. The exact geometry is determined by the radius rip of a circle 705
that can
be drawn between the four electrodes. Past work has shown that a good
approximation to a hyperbolic potential is obtained from cylindrical
electrodes when
re = 1.148 ro [Denison 1971].
The value of s is then s = {re + ro}/21/2. If the distance between the two
contact points
706a, 706b of the cylindrical electrode 702 and the groove in the supporting
feature
701 is 2w, the height h between the contact points and the axis of symmetry
703 is h
= s + (re2 ¨ W2)112. Suitable choices of re, ro, s, w and h therefore allow
the geometry
of a quadrupole to be established.
13
CA 02625251 2008-03-07
Substrates of the type described may be constructed with micron-scale
precision by
microfabrication, using methods such as photolithography, etching, metal-
coating
and dicing. However, as will be apparent to those skilled in the art, there
are many
combinations of processes and materials yielding similar results. We therefore
give
one example, which is intended to be representative rather than exclusive. In
this
example, etched features are formed on silicon wafers, which are then stacked
together to form batches of complete substrates, which are then separated by
dicing.
Figure 8 shows how two sets of parts are formed on two separate silicon
wafers.
The first wafer 801 carries parts defining all features of the microfabricated
substrate
lying between the contact points 706a, 706b in Figure 7. Because these
features
desirably have the height h shown in Figure 7, the starting material is a
silicon wafer,
which is polished on both sides to this thickness. The wafer is patterned
using
photolithography to define the desired features (for example, the contact pad
802)
together with small sections of sprue (for example 803) attaching them to the
surrounding wafer (804).
The pattern is transferred right through the wafer using deep reactive ion
etching, a
plasma-based process that may etch arbitrary features in silicon at a high
rate and
with high sidewall verticality. The lithographic mask is removed, and the
wafer is
cleaned and then metallised, for example by RF sputtering. Suitable coating
metals
include gold.
The second wafer carries parts defining all features of the microfabricated
substrate
lying below the two contact points 706a, 706b in Figure 7. Because the depth
of
these features is not critical in determining the accuracy of the quadrupole
assembly,
the thickness "d" of this wafer must only be sufficient to allow the
cylindrical
electrode to be seated. The wafer is patterned twice, firstly to define
partially etched
features such as the electrode seating grooves (for example, 805) and the base
of
the conducting shield 806, and secondly to define fully etched features
outlining all
the main parts. Once again, features are attached by short sections of sprue
(for
example, 807) to the surrounding substrate 808.
14
CA 02625251 2008-03-07
The pattern is again transferred into the wafer using deep reactive ion
etching, so
that the partially etched features are etched to the sufficient depth de in
Figure 7 and
the fully etched features are transferred right through. Multilevel etching of
this type
may easily be performed using a multilevel surface mask, well known to those
skilled
in the art. The lithographic masks are removed, and the wafer is cleaned and
metallised. Suitable coating metals again include gold.
Figure 9 shows how the wafers are assembled into a stack forming a set of
complete
microfabricated assemblies. The upper wafer 801 is attached to the lower wafer
802,
which is in turn attached to an insulating substrate 901, for example a glass
wafer.
Suitable attachment methods include gold-to-gold compression bonding.
Rectangular dies comprising individual microfabricated substrates are then
separated using a dicing saw, for example by sawing along a first set of
parallel lines
902a, 902b, which separate all sections of sprue, and a second set of
orthogonal
parallel lines 903a, 903b.
Quadrupole assembles are completed by inserting cylindrical electrodes into
microfabricated substrates as previously shown in Figure 2, and then
assembling
two substrates as previously shown in Figure 3. Wirebond connections to
external
circuitry are then attached as previously shown in Figure 6.
It will be appreciated that the processes described above can be used to
construct a
microfabricated quadrupole containing the main features described, namely
electrically-isolated supports for cylindrical electrodes, a conducting shield
and a
Brubaker prefilter, the overall assembly having the correct geometrical
relationship.
However, it will also be appreciated that many alternative fabrication
processes can
achieve the same result.
For example, the lower silicon wafer may be replaced with a silicon-on-glass
wafer,
thus eliminating the need for the lower wafer-bonding step shown in Figure 9.
Alternatively, the two silicon wafers may be combined together into a single
layer,
which is multiply structured by etching to combine all the necessary features,
thus
CA 02625251 2008-03-07
eliminating the need for the upper wafer-bonding step shown in Figure 9. In
this
case, the precision needed to define the height h may be achieved using a
buried
etch stop, which may be provided using a bonded-silicon-on-insulator wafer.
It will also be appreciated that appropriate separation between the two
substrates
may be achieved by the use of separate inserted conducting objects, for
example
conducting blocks or cylinders, eliminating the need for the upper wafer in
Figure 9.
It will also be appreciated that the necessary conducting features may be
constructed from alternative materials such as metals. For example, an
insulating
wafer carrying a suitable set of conducting features may also be constructed
by
repetitive use of deep lithography to form a mould and electroplating to fill
the mould
with metal.
It will be appreciated that the glass may be structured by etching rather than
by
dicing. It will also be appreciated that the glass may be replaced with a
plastic. If the
plastic is photosensitive, it will be appreciated that it may be structured by
lithography.
It will be understood that what has been described herein is an exemplary
method of
aligning sets of cylindrical electrodes in the geometry of a miniature
quadrupole
electrostatic lens, which can act as a mass filter in a quadrupole mass
spectrometer.
The electrodes are mounted in pairs on microfabricated mounting members or
supports, which are formed from conducting parts on an insulating substrate.
Complete segmentation of the conducting parts provides low capacitative
coupling
between co-planar cylindrical electrodes, and allows incorporation of a
Brubaker
prefilter to improve sensitivity at a given mass resolution. A complete
quadrupole is
constructed from two such supports, which are spaced apart by further
conducting
spacers. The spacers are desirably continued around the electrodes to provide
a
conducting screen which may form a shield. The height of the spacer is greater
than
the height of the mounting members such that when two supports are brought
together it is contact between spacers provided on respective substrates that
defines
the separation between opposing substrates and ensures that electrodes that
are
16
CA 02625251 2008-03-07
located in a first mount are correctly spaced relative to electrodes located
within a
second mount. While such an exemplary embodiment is useful in an understanding
of the teaching of the invention it is not intended to limit the invention in
any way
except as may be deemed necessary in the light of the appended claims.
There are therefore many processes that achieve a similar objective.
Within the context of the present invention the term microengineered or
microengineering or microfabricated or microfabrication is intended to define
the
fabrication of three dimensional structures and devices with dimensions in the
order
of microns. It combines the technologies of microelectronics and
micromachining.
Microelectronics allows the fabrication of integrated circuits from silicon
wafers
whereas micromachining is the production of three-dimensional structures,
primarily
from silicon wafers. This may be achieved by removal of material from the
wafer or
addition of material on or in the wafer. The attractions of microengineering
may be
summarised as batch fabrication of devices leading to reduced production
costs,
miniaturisation resulting in materials savings, miniaturisation resulting in
faster
response times and reduced device invasiveness. Wide varieties of techniques
exist
for the microengineering of wafers, and will be well known to the person
skilled in the
art. The techniques may be divided into those related to the removal of
material and
those pertaining to the deposition or addition of material to the wafer.
Examples of
the former include:
= Wet chemical etching (anisotropic and isotropic)
= Electrochemical or photo assisted electrochemical etching
= Dry plasma or reactive ion etching
= Ion beam milling
= Laser machining
= Eximer laser machining
Whereas examples of the latter include:
= Evaporation
= Thick film deposition
= Sputtering
17
CA 02625251 2008-03-07
= Electroplating
= Electroforming
= Moulding
= Chemical vapour deposition (CVD)
= Epitaxy
These techniques can be combined with wafer bonding to produce complex three-
dimensional, examples of which are the interface devices provided by the
present
invention.
Where the words "upper", "lower", "top", bottom, "interior", "exterior" and
the like
have been used, it will be understood that these are used to convey the mutual
arrangement of the layers relative to one another and are not to be
interpreted as
limiting the invention to such a configuration where for example a surface
designated
a top surface is not above a surface designated a lower surface.
Furthermore, the words comprises/comprising when used in this specification
are to
specify the presence of stated features, integers, steps or components but
does not
preclude the presence or addition of one or more other features, integers,
steps,
components or groups thereof.
18
CA 02625251 2008-03-07
References
Batey J.H. "Quadrupole gas analysers" Vacuum 37, 659-668 (1987)
Denison D.R. "Operating parameters of a quadrupole in a grounded cylindrical
housing" J. Vac. Sci. & Tech. 8, 266-269 (1971)
Brubaker W.M. "An improved quadrople mass analyser" Adv. Mass Spectrom. 4,
293 (1968)
Brubaker W.M. "Mass filter with one or more rod electrodes segmented into a
plurality of insulated segments" US 3,371,204 Feb. 27 (1968)
Brubaker W.M. "Auxiliary electrodes for quadrupole mass filters" US 3,129,327
Apr.
14 (1964)
Taylor S., Tate T.J., Syms R.R.A., Dorey H.A. "Quadrupole mass spectrometers"
US
Patent 6,025,591 Feb. 15 (2000)
Syms R.R.A, Tate T.J., Ahmad M.M., Taylor S. "Fabrication of a microengineered
quadrupole electrostatic lens" Elect. Lett. 32, 2094-2095 (1996)
Syms R.R.A., Tate T.J., Ahmad M.M., Taylor S. "Design of a microengineered
quadrupole electrostatic lens" IEEE Trans. on Electron Devices TED-45, 2304-
2311 (1998)
Taylor S., Tunstall J.J., Leck J.H., Tindall R., Julian P., Batey J., Syms
R.R.A., Tate
T.J., Ahmad M.M. "Performance improvements for a miniature quadrupole with
a micromachined mass filter" Vacuum 53, 203-206 (1999)
Syms R.R.A. "Monolithic microengineered mass spectrometer" GB 2391694 Mar. 1
(2006)
Geear M., Syms R.R.A., Wright S., Holmes A.S. "Monolithic MEMS quadrupole mass
spectrometers by deep silicon etching" IEEE/ASME J. Microelectromech. Syst.
14, 1156-1166 (2005)
Sillon N., Baptist R. "Micromachined mass spectrometer" Sensors and Actuators
B83, 129-137 (2002)
Baptist R. "Miniature device for generating a multi-polar field, in particular
for filtering
or deviating or focusing charged particles" US 6,465,792 Oct 5 (2002)
19
