U.S. patent application number 11/089165 was filed with the patent office on 2006-09-28 for probe for a scanning probe microscope and method of manufacture.
Invention is credited to Ami Chand, Kevin J. Kjoller, Nihat Okulan.
Application Number | 20060213289 11/089165 |
Document ID | / |
Family ID | 37033860 |
Filed Date | 2006-09-28 |
United States Patent
Application |
20060213289 |
Kind Code |
A1 |
Kjoller; Kevin J. ; et
al. |
September 28, 2006 |
Probe for a scanning probe microscope and method of manufacture
Abstract
A probe assembly for an instrument and a method of manufacture
includes a substrate and a cantilever having a length independent
of typical alignment error during fabrication. In one embodiment,
the probe assembly includes a buffer section interposed between the
substrate and the cantilever. The cantilever extends from the
buffer section and a portion of the buffer section extends beyond
an edge of the substrate. The portion of the buffer section is more
stiff than the cantilever. The corresponding method of producing
the probe assembly facilitates batch fabrication without
compromising probe performance.
Inventors: |
Kjoller; Kevin J.; (Santa
Barbara, CA) ; Chand; Ami; (Goleta, CA) ;
Okulan; Nihat; (Los Angeles, CA) |
Correspondence
Address: |
BOYLE FREDRICKSON NEWHOLM STEIN & GRATZ, S.C.
250 E. WISCONSIN AVENUE
SUITE 1030
MILWAUKEE
WI
53202
US
|
Family ID: |
37033860 |
Appl. No.: |
11/089165 |
Filed: |
March 24, 2005 |
Current U.S.
Class: |
73/866.5 ;
73/105 |
Current CPC
Class: |
G01Q 60/38 20130101;
B82Y 35/00 20130101 |
Class at
Publication: |
073/866.5 ;
073/105 |
International
Class: |
G01B 5/28 20060101
G01B005/28; G01D 21/00 20060101 G01D021/00 |
Claims
1. A probe assembly for a surface analysis instrument, the probe
assembly comprising: a substrate defining a base of the probe
assembly; a cantilever extending from the base and having a distal
end; wherein a length of said cantilever is independent of
alignment error during probe fabrication.
2. The probe assembly of claim 1, further comprising a buffer
section interposed between said substrate and said cantilever, said
cantilever extending from said buffer section.
3. The probe assembly of claim 2, wherein a portion of said buffer
section extends beyond an edge of said substrate, and wherein said
portion is more stiff than said cantilever, and wherein said buffer
section is made of one of silicon nitride and silicon oxide.
4. The probe assembly of claim 3, wherein said buffer section is at
least two times thicker than said cantilever.
5. The probe assembly of claim 2, wherein said buffer section is
made of the same material as said cantilever.
6. The probe assembly of claim 2, wherein a stiffness of said
buffer section is in at least about an order of magnitude greater
than a stiffness of said cantilever.
7. The probe assembly of claim 6, wherein a stiffness of said
buffer section is in a range of about 10 to 50 N/m.
8. The probe assembly of claim 2, wherein said cantilever has a
thickness less than about 1000 nm.
9. The probe assembly of claim 8, wherein said cantilever has a
thickness less than about 100 nm.
10. The probe assembly of claim 3, wherein said portion of said
buffer section is at least about 10 times as stiff as said
cantilever.
11. The probe assembly of claim 1, wherein said substrate is one of
silicon and glass, and wherein an operational resonant frequency of
said cantilever is in a range of about 300 to 1000 kHz.
12. The probe assembly of claim 1, wherein a length of said
cantilever is less than about 50 microns.
13. The probe assembly of claim 12, wherein the length is less than
about 10 microns.
14. A method of fabricating a probe assembly for a surface analysis
instrument, the method comprising: forming a probe of the probe
assembly, the probe including a cantilever; and wherein a length of
the cantilever is independent of an alignment error associated with
said forming step.
15. The method of claim 14, wherein said forming step includes
forming at least a portion of the probe assembly on a substrate and
dicing the substrate to release the probe assembly, wherein said
dicing step is responsible for the alignment error.
16. The method of claim 15, further comprising, producing a buffer
section; and wherein said forming step includes disposing a layer
of a cantilever material on the buffer section such that the
cantilever extends from the buffer section, and wherein said dicing
step is performed so that at least a portion of the buffer section
extends beyond an edge of the substrate.
17. The method of claim 16, wherein the buffer section is a layer
of material having a thickness substantially greater than a
thickness of the cantilever.
18. The method of claim 17, wherein the layer of material is
silicon oxide.
19. The method of claim 16, wherein a stiffness of said buffer
section is at least an order of magnitude greater than a stiffness
of said cantilever.
20. The method of claim 16, wherein said cantilever is less than
100 nm thick.
21. The method of claim 16, wherein a length of the cantilever is
less than 20 microns.
22. The method of claim 21, wherein a length of the cantilever is
less than 10 microns.
23. The method of claim 18, wherein the cantilever material is
silicon nitride, and wherein the substrate is one of silicon and
glass.
24. The method of claim 23, wherein said forming step includes
depositing the silicon nitride on the substrate, and the substrate
is a sacrificial silicon wafer.
25. The method of claim 14, wherein said forming step includes
performing an offset alignment technique to define the length of
the cantilever.
26. The method of claim 25, wherein said performing step further
includes, providing a sacrificial wafer, shaping a tip in the
wafer, then depositing a cantilever material on the wafer, wherein
said coupling step includes bonding the substrate to the cantilever
material; removing the wafer; and applying a layer of a material to
the substrate opposite the cantilever material.
27. The method of claim 26, wherein said shaping step includes
using a nanodot and plane overetch technique.
28. The method of claim 26, wherein said removing step includes
etching the wafer, and wherein said bonding step includes fusion
wafer bonding.
29. The method of claim 26, wherein the cantilever material and the
material layer are silicon nitride.
30. The method of claim 26, wherein said performing step includes
patterning the probe, etching the cantilever material and the
material layer, and then etching the substrate.
31. The method of claim 28, wherein said etching the substrate step
includes a KOH etch to the (111) plane.
32. The method of claim 26, wherein the probe has a length less
than 20 microns.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention is directed to a probe assembly for a
metrology instrument used to measure a property of a sample, and
more particularly, a probe assembly including a cantilever having a
short length to support high bandwidth operation, and configured
for ready batch fabrication.
[0003] 2. Description of Related Art
[0004] Scanning probe microscopes (SPMs), such as the atomic force
microscope (AFM), are devices which use a sharp tip and low forces
to characterize the surface of a sample down to atomic dimensions.
Generally, the tip of the SPM probe is introduced to the sample
surface to detect changes in the characteristics of the sample. By
providing relative scanning movement between the tip and the
sample, surface characteristic data can be acquired over a
particular region of the sample and a corresponding map of the
sample can be generated.
[0005] In an AFM, for example, in a mode of operation called
contact mode, the microscope typically scans the tip, while keeping
the force of the tip on the surface of the sample generally
constant. This is accomplished by moving either the sample or the
probe assembly up and down relatively perpendicularly to the
surface of the sample in response to a deflection of the cantilever
of the probe assembly as it is scanned across the surface. In this
way, the data associated with this vertical motion can be stored
and then used to construct an image of the sample surface
corresponding to the sample characteristic being measured, e.g.,
surface topography. Similarly, in another preferred mode of AFM
operation, known as TappingMode.TM. (TappingMode.TM. is a trademark
owned by the present assignee), the tip is oscillated at or near a
resonant frequency of the associated cantilever of the probe. The
amplitude or phase of this oscillation is kept constant during
scanning using feedback signals, which are generated in response to
tip-sample interaction. As in contact mode, these feedback signals
are then collected, stored and used as data to characterize the
sample.
[0006] The deflection of the cantilever in response to the probe
tip's interaction with the sample is measured with an extremely
sensitive deflection detector, most often an optical lever system.
In such optical systems, a lens is employed to focus a laser beam,
from a source typically placed overhead of the cantilever, onto the
back side of the cantilever. The backside of the lever is
reflective (for example, using metalization during fabrication) so
that the beam may be reflected therefrom towards a photodetector.
The translation of the beam across the detector during operation
provides a measure of the deflection of the lever, which again is
indicative of one or more sample characteristics.
[0007] One area of continuing SPM development relates to the speed
of operation. In this regard, the greater the resonant frequency of
the cantilever of the probe of the probe assembly the greater the
speed at which the SPM can be operated to acquire sample surface
data. One way in which high operational resonant frequencies, and
thus improved SPM imaging speed, can be facilitated is by using a
probe having a cantilever that is much shorter than the typical
length of about 100-400 microns. This is due to the fact that, with
a shorter lever, the instrument can be operated at a higher
resonant frequency with less noise. Therefore, keeping the same
spring constant, one can operate the SPM faster while obtaining
high integrity data given a greater signal to noise ratio.
Preferably, a probe having a cantilever that is less than 50
microns or even less than 20 microns is preferred for such
applications.
[0008] One significant drawback associated with using probes having
short cantilevers, however, is that for a number of reasons it is
very difficult to bulk manufacture probes having cantilevers with
such short lengths, i.e., in the sub-50 micron range. In most such
processes, the lever is formed, as well as the tip, using micro
fabrication techniques that require precise alignment of the
manufacturing tools (e.g., photolithography masks, etc.) and
precise processing of the probe components, including bonding a
substrate to the formed probe prior to dicing the substrate into
individual probe assemblies. In the latter regard, when producing
short levered probes, it is nearly impossible to accurately control
the dicing from the backside of the batch fabricated probe
assemblies given alignment inaccuracies in the process. This causes
an offset between the edge of the diced substrate and the tip or
distal end of the cantilever. This offset cannot be readily
controlled. As a result, probe assemblies having profiles such as
that shown in FIGS. 1A-1C may result. In particular, for example,
FIG. 1A illustrates a probe assembly 15 having a lever 16 with a
length about 30 microns, as desired. However, given alignment
inaccuracies and related complicating factors during fabrication,
the probe assembly 15' of the next batch may be formed such that
the glass substrate 18 bonded to the probe yields no cantilever,
such as that shown in FIG. 1B. Finally, in the next batch, as shown
in FIG. 1C, the probe assembly 15'' may have a cantilever 16' with
a significantly greater length, such as 60 microns. In sum, the
offset from the edge of the substrate to the tip upon bonding and
dicing the substrate, hereinafter called the "uncontrollable
offset", yields a cantilever having a length that cannot be
predictably controlled, assuming the cantilever is formed at
all.
[0009] In this regard, two specific types of probes employing
silicon nitride cantilevers are shown in FIGS. 2A (glass substrate)
and 2B (silicon substrate). In FIG. 2A, a probe assembly 20
includes a probe 21 having a silicon tip 22 and a silicon nitride
lever 24 that extends from a glass substrate 26. In this case, the
substrate 26 is bonded directly to the silicon nitride that forms
and defines the length of cantilever 24. As a result, when batch
fabricating such a probe, the uncontrollable offset, "O", as shown
in FIG. 2C, present when the probes are diced operates to limit the
manufacturer's ability to produce repeatable probe assemblies
having a short, for instance, sub-20 micron, length. More
particularly, standard mechanical dicing operations have inherent
alignment errors (typically, as much as tens of microns) that,
though acceptable for fabricating conventional probes, is
unacceptable for fabricating the type of short probes contemplated
herein. In FIG. 2B, a self-actuated probe assembly 30 includes a
probe 32 having an integrated actuator 34 and a base substrate 35
made of silicon. A cantilever 36 of probe 32 made of silicon
extends from actuator 34 defined by top and bottom gold electrodes
38, 40, respectively, and an active element 42, such as zinc oxide.
Here again the substrate 35 is bonded directly to the silicon
nitride layer, or electrode 38, that defines cantilever 36. In this
case, the individual probe assemblies are released with an
appropriate etch of the sacrificial silicon. Due to processing
limitations, performing this etch with sufficiently high precision
to cost-effectively define cantilevers 36 having repeatable lengths
in the sub-20 micron scale is generally impossible, as understood
in the art. In particular, in this case, the length of the
cantilever is typically defined by front-side and back-side etches
of the substrate. It is very difficult to control these etches to
define short (e.g., sub-20 micron) levers because of orthogonality
and parallax considerations, as understood in the art.
[0010] In view of the above, the art of scanning probe microscopy
was in need of a probe assembly having a short lever, i.e., less
than 20 microns, and a corresponding method of batch fabricating
the probe such that the length of its associated cantilever can be
precisely controlled and batch processed independent of inherent
alignment errors associated with fabrication processes in which the
probes are either diced or etched. This control of the length of
the cantilever should be realized without compromising the physical
properties of the probe. For instance, the spring constant must be
maintained so that the probe is capable of operating at high
bandwidth, thus allowing the SPM to perform high speed imaging.
[0011] Note that "SPM" and the acronyms for the specific types of
SPMs, may be used herein to refer to either the microscope
apparatus, or the associated technique, e.g., "atomic force
microscopy."
SUMMARY OF THE INVENTION
[0012] The preferred embodiment overcomes the drawbacks of prior
art systems by providing a probe that is manufactured so that its
length is independent of inherent alignment error(s) associated
with the process. In one embodiment, the probe includes a buffer
section intermediate the substrate and the cantilever, thus
eliminating the drawbacks associated with the uncontrollable offset
(caused by alignment error during dicing) present in known
techniques of batch fabricating probes. The buffer section allows
the length of the cantilever to be precisely defined, in the sub-20
micron range, and is sufficiently stiff to allow the cantilever
extending therefrom to freely oscillate at high resonant
frequencies. In an alternative in which the probe is etched from
the front and back sides of a substrate, the length of the
cantilever is independent of conventional alignment errors caused
by orthogonality and parallax issues during the etch fabrication
process.
[0013] According to a first aspect of the preferred embodiment, a
probe assembly for an instrument for imaging a sample includes a
substrate, a cantilever and a buffer section interposed between the
substrate and the cantilever. In this case, the cantilever extends
from the buffer section and, preferably, a portion of the buffer
section extends beyond an edge of the substrate and is more stiff
than the cantilever.
[0014] According to another aspect of the preferred embodiment, the
buffer section is made of silicon oxide and is at least two times
thicker than the cantilever.
[0015] In another aspect of this embodiment, the buffer section is
corrugated and made of the same material as the cantilever.
[0016] According to a still further aspect of the preferred
embodiment, the probe assembly includes a tip, and the corrugation
of the buffer section has a depth not limited by a height of the
tip which extends generally orthogonally from the cantilever.
[0017] In another aspect of this embodiment, a cantilever has a
thickness between about 10 nm and 1000 nm and the buffer section is
designed to be substantially stiffer than the cantilever.
[0018] In yet another aspect of this preferred embodiment, the
length of the cantilever is less than about 10 microns.
[0019] In an even still further aspect of the preferred embodiment,
an operational resonant frequency of the cantilever is in a range
of about 300 to 1000 kHz.
[0020] According to an alternate aspect of the preferred
embodiment, a method of fabricating a probe for an instrument
includes forming a probe assembly having a cantilever and a tip.
The method further includes producing a buffer section, from which
the cantilever extends. The method also includes bonding a
substrate to the buffer section and dicing the substrate so that at
least a portion of the buffer section extends beyond an edge of the
substrate.
[0021] In another aspect of this embodiment, the buffer section is
a layer of silicon oxide having a thickness substantially greater
than a thickness of the cantilever.
[0022] In yet another aspect of this preferred embodiment, the
buffer section is corrugated so as to define a plurality of
trenches having a depth selected to control the stiffness of the
portion.
[0023] According to a still further aspect of the preferred
embodiment, the forming step includes depositing silicon nitride on
a sacrificial silicon wafer. Preferably, the substrate is either
silicon or glass.
[0024] These and other objects, features, and advantages of the
invention will become apparent to those skilled in the art from the
following detailed description and the accompanying drawings. It
should be understood, however, that the detailed description and
specific examples, while indicating preferred embodiments of the
present invention, are given by way of illustration and not of
limitation. Many changes and modifications may be made within the
scope of the present invention without departing from the spirit
thereof, and the invention includes all such modifications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] A preferred exemplary embodiment of the invention is
illustrated in the accompanying drawings in which like reference
numerals represent like parts throughout, and in which:
[0026] FIGS. 1A-1C are schematic side elevational views of a prior
art probe, illustrating the lack of control over the position of
the tip, and thus the length of the cantilever;
[0027] FIG. 2A is a schematic elevational side view of a prior art
probe;
[0028] FIG. 2B is a schematic side view of a prior art probe having
an integrated actuator;
[0029] FIG. 3 is a series of schematic side elevational
illustrations showing the steps to batch microfabricate a probe
assembly according to a preferred embodiment, the bottom
illustration showing the probe having a short cantilever extending
from a thick, and thus stiff, buffer section;
[0030] FIG. 4 is a perspective view of a probe assembly according
to another preferred embodiment of the invention, the probe having
a short cantilever extending from a corrugated buffer section;
[0031] FIG. 4A is a partially broken away cross-sectional view of
the corrugated section of the probe assembly along line 4A-4A of
FIG. 4;
[0032] FIG. 5 is a series of schematic side elevational
illustrations showing the steps to batch microfabricate the probe
assembly of FIG. 4;
[0033] FIG. 6 is a top plan view of a probe assembly being
fabricated according to another preferred embodiment similar to the
probe produced as shown in FIG. 5, where the corrugation is not
limited by tip height;
[0034] FIG. 7 is a cross-sectional front view of the probe assembly
of FIG. 6, along line 7-7;
[0035] FIG. 8 is a side elevational view of the resultant probe
assembly fabricated as shown in FIG. 6;
[0036] FIG. 9 is a series of schematic side elevational
illustrations showing the steps to batch microfabricate the probe
assembly shown in FIGS. 6-8, the bottom illustration corresponding
to FIG. 8;
[0037] FIG. 10 is back elevational view of a probe assembly
fabricated according to the present invention, similar to the probe
of FIGS. 6-9 but having pyramid shaped corrugation, similar to the
probe of FIG. 4;
[0038] FIG. 11 is a perspective view of the probe assembly
illustrated in FIG. 10; and
[0039] FIG. 12 is a series of schematic side elevation
illustrations showing the steps to batch microfabricate a probe
assembly according to another preferred embodiment of the invention
using offset alignment, the bottom illustration showing a probe
having a controllable short cantilever.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0040] Turning initially to FIG. 3, the bulk production of a probe
assembly having a short, sub-20 micron cantilever, is illustrated
in a series of fabrication steps. Moving from top to bottom, the
process is initiated with a sacrificial silicon wafer 100 provided
in step A. Then, a buffer layer 102 is deposited on the back side
of wafer 100 in step B. Buffer layer 102 may be a silicon oxide or
a silicon nitride, for instance, and will interface the flexible
cantilever of the probe, described further below. Layer 102 is
designed to be much stiffer than the cantilever that will
ultimately extend therefrom, and facilitates batch fabrication of
the probe assembly by eliminating the uncontrollable offset that
can result when dicing the probes, and specifically the substrate
coupled to the probe during process. This stiffness is preferably
achieved by forming layer 102 to have a significantly greater
thickness than the layer that will become the cantilever. For
instance, the thickness of layer 102 can be about 1-2 microns, but
it can be much larger if desired. Overall, the stiffness of the
buffer layer 102 is preferably about an order of magnitude greater
than that of the cantilever.
[0041] Next, in step C the buffer layer 102 is etched to an
appropriate point 104 relative to an end point 105 of wafer 100 so
as to form a buffer section 106 and expose wafer 100 for forming a
pit 107 in step D to accommodate a tip of the probe. Notably, this
etch is performed using standard planar processing used, for
example, in semiconductor fabrication processes. As understood in
the art, such processes are highly precise. Then, a thin layer 108
of silicon nitride is disposed on buffer layer 102 and the exposed
portion of silicon wafer 100 in step E so as to form a cantilever
109 which will be released as part of the probe assembly in a later
step. This layer 108 of nitride is approximately 10-1000 nm thick
to provide a flexible cantilever 109 having an appropriate spring
constant. Preferably, the spring constants of the short cantilevers
of the preferred embodiments are similar to the spring constants of
conventional AFM probes to maintain performance. For example, the
resonant frequency of a standard TappingMode.TM. cantilever is
typically about 300 kHz. To do so, the thickness of the levers are
preferably reduced to accommodate their shorter length. Also in
step E, a thin layer of silicon nitride will be deposited in pit
107.
[0042] In the next step of the fabrication process, step F, a glass
(or silicon) substrate 110 is bonded to the silicon nitride layer
108 that defines cantilever 109 such that substrate 110 is coupled
to cantilever with at least a portion of buffer layer 102
intermediate the two. Preferably, the bonding is accomplished using
conventional bonding techniques. Thereafter, the probes are cut or
diced in step G. It is this step, as discussed previously, that is
difficult to control due to an inherent alignment error associated
with the mechanical equipment used to dice the substrate, i.e., it
is this error that results in what may be called an uncontrollable
offset, labeled "O" in step G. By ensuring that this portion of
buffer section 106 is stiff (together with the precise etch of step
C), the cantilever extending therefrom can be formed with a precise
length while maintaining the physical properties of the probe. More
particularly, given the use of buffer section 106, the dicing
operation has generally no impact on controlling the length of the
lever 109, the fixed end of the cantilever 109 having been defined
when etching the thick buffer layer 102 to form buffer section 106.
Finally, in step H, the resultant probe assembly 114 according to
this embodiment is completed by a silicon etch to remove the
silicon of the initial sacrificial wafer 100.
[0043] Again, given its thickness, the flexibility of buffer
section 106 is negligible compared to the flexibility of the
silicon nitride cantilever 109 (lever 109 having a spring constant
"k" preferably equal to a range of about 0.005 to 100 N/m). This
embodiment is particularly adapted for use with a glass
substrate.
[0044] Turning to FIG. 4, an alternate probe assembly 150 according
to the present invention includes a probe 151 and a buffer section
152 that is produced so as to have a corrugated shape. In this
case, buffer section 152 is formed by etching a base silicon wafer
and depositing, for example, a silicon nitride layer thereafter, as
described in further detail below in connection with FIG. 5. A
cantilever 154 is also produced via the deposition of the silicon
nitride and extends from section 152 so that it may flex about its
base 156 defining the interface between cantilever 154 and buffer
section 152. As usual, cantilever 154 includes a distal end 158
from which a tip 160 extends generally orthogonally for interacting
with a surface of a sample under test (not shown).
[0045] Corrugated buffer section 152 includes a plurality of
trenches 162 that operate to stiffen probe 150 in a region of the
probe assembly 150 defining the uncontrollable offset associated
with producing probes having short cantilevers, described
previously. As a result, the more flexible cantilever 154 extending
therefrom is defined in a region where the length of the lever can
be predictably controlled, and thus batch reproduced. In between
trenches 162 lie flat portions 164 that are adapted for ready
bonding of a glass or silicon substrate thereto.
[0046] Ideally, base 156 of cantilever 154 is rigidly fixed to
buffer section 152 such that, for instance, cantilever oscillation
in TappingMode.TM. does not include any corresponding movement of
buffer section 162. Therefore, buffer section 162 should be as
stiff as possible. In this regard, the depth of trenches 162 at
least partially defines the stiffness of buffer section 152.
However, given processing constraints associated with forming
trenches 162 on the same side of the wafer as tip 160, the depth of
trenches 162 is limited to the height of the desired tip 160.
Therefore, because tip heights are typically small, at least
smaller than trench depth that would provide ideal stiffness to
buffer section 152, this limitation may hinder ideal performance in
certain applications. In this regard, one possible alternative is
illustrated in FIGS. 6-9, discussed below. Overall, an
uncontrollable offset of probe assembly 150 is defined by
corrugated buffer section 152, which is not a factor in batch
fabricating the probe assemblies having short, sub-20 micron
cantilevers 154.
[0047] Turning to FIG. 5, the fabrication of probe assembly 150 is
illustrated as a series of process steps. Initially, at step A, a
sacrificial wafer 170 is provided. Then, at step B, the corrugated
buffer section is selectively patterned to produce the series of
trenches, as well as the probe tip at locations 172 and 174,
respectively. At this point, step C, a thin silicon nitride layer
176 is deposited to, inter alia, form the cantilever. Layer 176 has
a thickness of about 100 nm. Thereafter, a substrate 178 is bonded
to the silicon nitride according to known techniques at step D of
the process. At step E, the wafer is diced into individual probe
assemblies using standard alignment procedures. Notably, at this
point, an offset labeled "O", which again cannot be readily
controlled due to imperfect alignment of the mechanical dicing
apparatus relative to the probe (i.e., alignment error), is
defined. However, as noted in connection with the previous
embodiment illustrated in FIG. 3, the length of the cantilever is
not controlled by this dicing operation and thus the bulk
processing problem associated with the uncontrollable offset
present in standard techniques is avoided. The length of the
cantilever is instead defined in part by an end point of the
trenches, a known position relative to tip. Notably, a portion of
corrugated buffer section 152 at step "E" is shown in FIG. 4A,
wherein the silicon trenches include a thin layer of silicon
nitride 176.
[0048] Finally, at step F, the silicon wafer 170 is etched from the
front side to expose a lever 182 and a tip 184. Tip 184 is
preferably silicon nitride, and is defined by a thin shell. As a
result, a probe assembly 180 having a corrugated buffer section
186, a portion 190 of which extends beyond an edge of substrate
section 188 is produced. Cantilever 182 extends from buffer section
186 at a point "X" about which the cantilever can oscillate, as in
TappingMode.TM.. Again, the depth of the trenches of corrugated
section 186 is limited by the height of tip 184, as appreciated by
those skilled in the microfabrication art.
[0049] Turning to FIGS. 6-8, a probe assembly 200 fabricated from a
silicon wafer 202 is shown. Probe assembly 200 is similar to probe
assembly 180 shown in FIG. 5. However, in this case, a corrugated
buffer section 204 of probe assembly 200 is formed on a side of
wafer 202 opposite the side where a tip 206 of probe assembly 200
is formed, and thus buffer section 204 may have a greater range of
depths "d" (see FIG. 7), and thus a greater range of stiffnesses
than the probe assembly shown in FIG. 4. In addition, the
cross-sectional shape of the series of trenches 208 of buffer
section 204 is rectangular rather than pyramid-shaped.
[0050] Probe assembly 200 includes a cantilever 210 extending from
the rigid buffer section 204 at about a fixed point 205. With
further reference to FIGS. 6 and 8, cantilever 210 also includes a
distal end 207 opposite fixed end or point 205 which supports tip
206 that is used for interacting with a surface of a sample in
conventional fashion. Probe assembly 200 also includes a glass
substrate 212 upon dicing the wafer to form the individual probe
assemblies prior to releasing the probes from the wafer, described
in further detail below with respect to FIG. 9.
[0051] Preferably, as shown in FIGS. 7 and 8, trenches 208 are
formed in the silicon wafer 202 opposite the side used to form the
pit for tip 206. As a result, more design flexibility is realized
in terms of achievable stiffness of buffer section 204. The length
of the buffer section 204, defined at least in part by the bonded
and diced glass substrate 212 and a fixed end 205 of probe 202,
generally corresponds to what has been described earlier as the
uncontrollable offset "O" associated with known probe fabrication
techniques. Because the length of probe 200 is not controlled by
the dicing of the substrate 212, this offset is not the problem it
is in known techniques, and thus bulk fabrication of probe
assemblies 200 is facilitated.
[0052] To form probe 200 reference is made to FIG. 9. Similar to
the previous probes, the process starts with a sacrificial silicon
wafer 250 at step A. A wet or dry etch is then employed at step B,
for instance, from the back side of wafer 250, to form a corrugated
buffer section 251 defined by a series of trenches 252. Notably,
the length "1" of trenches 252 can be precisely controlled using
known techniques. Next, in step C, a pit 254 for forming a tip is
etched in wafer 250. A thin layer 255 of silicon nitride is then
deposited on the etched wafer in step D. Thereafter, a substrate
256 (glass or silicon) is bonded to trenches 252 of wafer (step E),
and then cut to form the individual probe assemblies in step F. An
uncontrollable offset associated with this step E is shown at "O"
for illustrative purposes. To the contrary, a length "L" of a
resultant cantilever 258 can be precisely controlled, again due to
the fact that the dicing of the probes does not control cantilever
length; rather, edges of wafer 250 and corrugated buffer section
251 define cantilever length, which may be on the order of less
than 20 microns, and preferably less than 10 microns. Finally, in
step G, the sacrificial silicon 250 is etched to release the
probes.
[0053] In an embodiment similar to that discussed immediately
above, a probe assembly 300 including a corrugated buffer section
302 is formed having a series of pyramid-shaped, as opposed to
rectangular, trenches 304 in cross section, as shown in FIGS. 10
and 11. The process steps are otherwise identical to that shown in
FIG. 9.
[0054] In a still further embodiment of bulk microfabricating a
probe with a short cantilever, an offset alignment technique is
employed in the process steps shown in FIG. 12, for an etch
released, rather than diced, probe. In step A, a sacrificial wafer
320 is provided. In step B, a pit 322 for defining a probe tip is
formed, for example, using nanodot and (111) plane overetch
techniques. In addition, a silicon nitride layer 324 is deposited.
Next, in step C, a substrate 326 is bonded to layer 324, for
example, using fusion wafer bonding. The sacrificial wafer is then
stripped using an appropriate etch, and a silicon nitride layer 328
is applied to the substrate in step D. In step E, an "offset
alignment" technique is employed, including initially etching the
silicon nitride from the front and back sides of the substrate to
points U and V, respectively, a non-planar process. Again, it is
this non-planar etching process that is difficult to control due to
orthogonality and parallax considerations (which cannot be readily
minimized) present during defining the length of the lever with the
etches. An alignment error (as much as tens of microns) between the
front and back side etches results, and thus short levers are
difficult to repeatedly produce, as understood in the art.
[0055] In this embodiment, etching is accomplished with precise
photolithographic patterning, for instance, to ultimately define
the length of the lever with the following etch of the substrate
326. In particular, using a KOH etch in step F to etch to the (111)
plane of the silicon substrate 326 at fifty-four point seven
degrees (54.7.degree.), as understood in the art, the alignment
error is substantially eliminated and the length of a cantilever
330 of a probe 329 produced thereby can be repeatedly produced in
bulk.
[0056] Although the best mode contemplated by the inventors of
carrying out the present invention is disclosed above, practice of
the present invention is not limited thereto. It will be manifest
that various additions, modifications and rearrangements of the
features of the present invention may be made without deviating
from the spirit and scope of the underlying inventive concept.
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