U.S. patent application number 13/341352 was filed with the patent office on 2013-01-03 for raman spectrometer for monitoring traces of dissolved organic and inorganic substances.
This patent application is currently assigned to IsoSpec Technologies, LP. Invention is credited to Jacek Borysow, Manfred Fink, Philip Varghese.
Application Number | 20130003055 13/341352 |
Document ID | / |
Family ID | 45532253 |
Filed Date | 2013-01-03 |
United States Patent
Application |
20130003055 |
Kind Code |
A1 |
Borysow; Jacek ; et
al. |
January 3, 2013 |
RAMAN SPECTROMETER FOR MONITORING TRACES OF DISSOLVED ORGANIC AND
INORGANIC SUBSTANCES
Abstract
A compact, ultra-sensitive, inexpensive NIR spontaneous Raman
spectrometer is presented. High sensitivity is achieved by the use
of a multi-pass cell configuration combined with the electromotive
properties of silicon crystal surface. A thin layer of silicon
oxide chemisorbs molecules, which stick to its surface without
altering their spectroscopic signatures. This new Raman
spectrometer may be used to detect less than 40 ng (.apprxeq.0.5 n
mol) of ammonium nitrate deposited on the silicon surface with the
signal-to-noise ratio better than 50 during 0.1 s recording time
and for an illuminated area of 2.times.10.sup.-8 m.sup.2. These
results may be useful in many areas, for example the foundation of
an extended project to record the dissolved NO.sub.3.sup.- ions in
a large river such as the Mississippi.
Inventors: |
Borysow; Jacek; (Atlantic
Mine, MI) ; Fink; Manfred; (Austin, TX) ;
Varghese; Philip; (Austin, TX) |
Assignee: |
IsoSpec Technologies, LP
|
Family ID: |
45532253 |
Appl. No.: |
13/341352 |
Filed: |
December 30, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13078777 |
Apr 1, 2011 |
8111394 |
|
|
13341352 |
|
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Current U.S.
Class: |
356/301 |
Current CPC
Class: |
G01N 21/65 20130101;
G01J 3/44 20130101; G01J 3/02 20130101; G01J 3/0208 20130101 |
Class at
Publication: |
356/301 |
International
Class: |
G01J 3/44 20060101
G01J003/44 |
Claims
1. A Raman spectrometer comprising: a laser source emitting a laser
beam; a lens having a lens focal length and being disposed
approximately one lens focal length away from a silicon substrate
and operable to focus said laser beam onto said silicon substrate
at an angle at least approximately 60 degrees from a line normal to
said silicon substrate, wherein said focused laser beam at least
partially reflects from said silicon substrate and diverges toward
a first concave mirror; said first concave mirror having a first
mirror focal length and being disposed approximately two first
mirror focal lengths away from said silicon substrate, said first
concave mirror operable to focus said laser beam onto said silicon
substrate, wherein said focused laser beam at least partially
reflects from said silicon substrate and diverges toward a second
concave mirror; said second concave mirror having a second mirror
focal length and being disposed approximately two second mirror
focal lengths away from said silicon substrate, said second concave
mirror operable to focus said laser beam onto said silicon
substrate, said silicon substrate, said first concave mirror, and
said second concave mirror together comprising a multi-pass Raman
cell; at least one lens operable to collect Raman light from said
silicon substrate and provide an image of said Raman light to a
spectrograph operable to separate said Raman light according to
wavelength; and a filter operable to remove at least a portion of
Rayleigh scattered light from said Raman light.
2. The Raman spectrometer of claim 1, wherein said laser source
comprises a laser diode.
3. The Raman spectrometer of claim 2, wherein said laser diode
comprises a grating-locked laser diode.
4. The Raman spectrometer of claim 3, wherein said angle is at
least approximately 70 degrees.
5. The Raman spectrometer of claim 4, wherein said angle is at
least approximately 80 degrees.
6. The Raman spectrometer of claim 3, wherein said filter comprises
a holographic notch filter.
7. The Raman spectrometer of claim 6, wherein said holographic
notch filter has an optical density of at least 5 for said Rayleigh
scattered light.
8. The Raman spectrometer of claim 3, wherein said filter comprises
an atomic vapor absorption filter.
9. The Raman spectrometer of claim 8, wherein said atomic vapor
absorption filter comprises a rubidium absorption filter.
10. The Raman spectrometer of claim 3, further comprising a Dove
prism operable to rotate said image of said Raman light.
11. The Raman spectrometer of claim 3, wherein said spectrograph
further comprises a chilled CCD camera.
12. The Raman spectrometer of claim 11, wherein said chilled CCD
camera is coupled to a programmed computer operable to analyze said
Raman light.
13. The Raman spectrometer of claim 3, wherein said silicon
substrate further comprises a layer of silicon dioxide operable to
chemisorb a chemical species for analysis.
14. A method for detecting a chemical species, said method
comprising: providing a silicon wafer having said chemical species
on a surface thereof; focusing a laser beam onto said surface;
repeatedly reflecting said laser beam between said surface, a first
concave mirror, and a second concave mirror, wherein said surface,
said first concave mirror, and said second concave mirror comprise
a multi-pass Raman cell; collecting a combination of Raman light
and Rayleigh light scattered by said chemical species; removing at
least a portion of said Rayleigh light from said combination via a
filter, thereby producing isolated Raman light; directing said
isolated Raman light into a spectrograph; and analyzing said Raman
light.
15. The method of claim 14, further comprising the step of
producing said laser beam via a laser diode.
16. The method of claim 15, wherein said laser diode comprises a
grating-locked laser diode.
17. The method of claim 16, wherein said grating locked laser diode
comprises a grating-locked laser diode in a Littrow
configuration.
18. The method of claim 17, wherein said filter comprises a
holographic notch filter.
19. The method of claim 17, wherein said filter comprises an atomic
vapor absorption filter.
20. The method of claim 20, wherein said atomic vapor absorption
filter comprises a rubidium absorption filter
Description
FIELD
[0001] Provided is a Raman spectral analyzer to measure the
scattered light from a multi-pass Raman cell. More specifically, a
Raman spectral analyzer capable of measuring trace amounts of
compounds on a silicon wafer is provided.
BACKGROUND
[0002] Raman scattering is a type of inelastic scattering of
electromagnetic radiation, such as visible light, discovered in
1928 by Chandrasekhara Raman. When a beam of monochromatic light is
passed through a substance some of the radiation will be scattered.
Although most of the scattered radiation will be the same as the
incident frequency ("Rayleigh" scattering), some will have
frequencies above ("anti-Stokes" radiation) and below ("Stokes"
radiation) that of the incident beam. This effect is known as Raman
scattering and is due to inelastic collisions between photons and
molecules that lead to changes in the vibrational and/or rotational
energy levels of the molecules. This effect is used in Raman
spectroscopy for identifying and investigating the vibrational and
rotational energy levels of molecules. Raman spectroscopy is the
spectrophotometric detection of the inelastically scattered
light.
[0003] "Stokes" emissions have lower energies (lower frequencies or
a decrease in wave number (cm.sup.-1)) than the incident laser
photons and occur when a molecule absorbs incident laser energy and
relaxes into an excited rotational and/or vibrational state. Each
molecular species will generate a set of characteristic Stokes
lines that are displaced from the excitation frequency (Raman
shifted) whose intensities are linearly proportional to the density
of the species in the sample.
[0004] "Anti-Stokes" emissions have higher frequencies than the
incident laser photons and occur only when the photon encounters a
molecule that, for instance, is initially in a vibrational excited
state due to elevated sample temperature. When the final molecular
state has lower energy than the initial state, the scattered photon
has the energy of the incident photon plus the difference in energy
between the molecule's original and final states. Like Stokes
emissions, anti-Stokes emissions provide a quantitative fingerprint
for the molecule involved in the scattering process. This part of
the spectrum is seldom used for analytical purposes since the
spectral features are weaker. However, the ratio of the Stokes to
the anti-Stokes scattering can be used to determine the sample
temperature when it is in thermal equilibrium.
[0005] The Stokes and anti-Stokes emissions are collectively
referred to as spontaneous Raman emissions. Since the excitation
frequency and the frequency of the Stokes (and anti-Stokes)
scattered light are typically far off the excitation of any other
component in the sample, fluorescence in near infrared (NIR)
wavelengths is minimal. The sample is optically thin and will not
alter the intensities of the Stokes emissions (no primary or
secondary extinctions), in stark contrast to infrared
spectroscopy.
[0006] Raman spectroscopy is a well-established technology to
determine the presence of trace compounds down to very low (e.g. n
mol/liter) levels. With Raman analysis, absolute densities can be
determined, the sparse spectra minimize interferences, and
overtones and combination lines are strongly suppressed.
[0007] However, conventional Raman analyzers tend to lack the
desired sensitivity, require an extensive integration time, be too
large, and/or be too costly for widespread use. Thus, there is a
need in the art for a relatively inexpensive, compact Raman
spectrometer capable of improved sensitivity and integration
times.
[0008] Laser-based techniques capable of detecting very small
traces of inorganic compounds have been recently reported in the
literature. However, these instruments generally require the use of
tunable lasers and special environments like a vibration-free
setting. Often the experimental setups are so sophisticated that
they can be operated only by Ph.D. level personnel.
[0009] A novel approach is presented here. Raman spectroscopy is
often used for identification and quantization of a mixture of
chemical species with high selectivity. In a typical Raman
experiment, a laser is used as an excitation source. Scattered
light is collected and sent to a grating spectrograph connected to
a detector, typically a charge-coupled device (CCD). Elastically
scattered (Rayleigh) light is rejected by a narrow atomic vapor
filter.
[0010] There are many Raman systems on the market today; however,
they tend to suffer from the same drawbacks. Raman cross sections
are extremely small; therefore, only dense materials (solids or
liquids) in sufficiently large quantities can be routinely detected
by these instruments. Raman spectrometers capable of detecting low
densities of gaseous substances have been reported in elaborate
intra-cavity laser setups, but these techniques require
sophisticated frequency stabilization and can be achieved today
only in state-of-the-art laboratories, without much hope for
deployment in the field.
SUMMARY
[0011] We have recently presented a Raman spectrometer capable of
differentiating isotopes of hydrogen at densities as low as
5.times.10.sup.13 cm.sup.-3 (see J. Borysow and M. Fink, "NIR Raman
Spectrometer for Monitoring Protonation Reactions in Gaseous
Hydrogen," J. Nucl. Mat, 341: 224-230 (2005)). The high sensitivity
was achieved using a multi-pass cell in conjunction with an atomic
vapor Rb absorption filter, which eliminates the Rayleigh,
scattered light. Similar sensitivities are possible for molecules
dissolved in transparent liquids such as water. However, the vapor
pressures of many organic solids such as polycyclic ether, natural
products, or nitrates at room temperature are significantly lower
than this detection limit. Therefore, that spectrometer may have
difficulty detecting these compounds.
[0012] The design we present here takes advantage of the ability of
the oxidized silicon wafer surface to attract via electrostatic
forces a large variety of organic and inorganic molecules. The
presence and the rates of adsorption and desorption of organic
molecules (especially hydrocarbons) have been studied because they
may cause serious problems in the advanced electronics fabrication
processes.
[0013] However, even today relatively little is known about the
interactions of the vapors of many inorganic compounds with silicon
surfaces. When the sticking coefficients for these species on
silicon wafers are as high as for most organic hydrocarbons,
SiO.sub.2 is an extremely efficient collection element. The Raman
spectrometer design described here may reach detection limits of n
mol/liter for ammonium nitrate (NH.sub.4NO.sub.3 or AN), the
primary compound used in this study.
[0014] Our approach may lead to the development of systems that can
measure a large variety of molecules in short order and calibrated
to an absolute scale over a very large dynamic range. Traces of AN
deposited on the silicon wafer surface have been recorded to
demonstrate the performance of our novel Raman set-up. The
measurements of the densities of AN in a river, such as the
Mississippi, can be very useful for the evaluation of data
collected in research projects which focus on the environment.
[0015] Nitrate compounds are routinely used as fertilizers. An
appreciable amount is transferred by the weather to the local
tributaries. Our Raman spectrometer is rugged and affordable enough
that one could, for example, equip a high school in every district
on the river with a Raman instrument and ask the students to record
the daily changes of AN for extended time periods to establish
seasonal and temporal variations.
[0016] Additional advantages will be set forth in part in the
description which follows, and in part will be obvious from the
description, or may be learned by practice of the aspects of the
disclosure as described herein. The advantages can be realized and
attained by means of the elements and combinations particularly
pointed out in the appended claims. It is to be understood that
both the foregoing general description and the following detailed
description are exemplary and explanatory only and are not
restrictive of the aspects of the disclosure, as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The features, nature, and advantages of the disclosed
subject matter will become more apparent from the detailed
description set forth below when taken in conjunction with the
accompanying drawings, wherein:
[0018] FIG. 1 is a schematic drawing of one embodiment of the
multi-pass Raman cell of the present disclosure;
[0019] FIG. 2 shows a spectrum of ammonium nitrate measured in
accordance with the present disclosure; and
[0020] FIG. 3 shows two spectra of ammonium nitrate demonstrating
the advantage of using the multi-pass configuration of the present
disclosure.
DETAILED DESCRIPTION
[0021] The present disclosure may be understood more readily by
reference to the following detailed description, examples,
drawings, and claims, and their previous and following description.
However, before the present devices, systems, and/or methods are
disclosed and described, it is to be understood that this
disclosure is not limited to the specific devices, systems, and/or
methods disclosed unless otherwise specified, as such can, of
course, vary. It is also to be understood that the terminology used
herein is for the purpose of describing particular aspects only and
is not intended to be limiting.
[0022] As used in the specification and the appended claims, the
singular forms "a," "an" and "the" include plural referents unless
the context clearly dictates otherwise. Thus, for example,
reference to an "analyzer" can include two or more such analyzers
unless the context indicates otherwise.
[0023] Ranges may be expressed herein as from "about" one
particular value, and/or to "about" another particular value. When
such a range is expressed, another embodiment includes from the one
particular value and/or to the other particular value. Similarly,
when values are expressed as approximations, by use of the
antecedent "about," it will be understood that the particular value
forms another embodiment. It will be further understood that the
endpoints of each of the ranges are significant both in relation to
the other endpoint, and independently of the other endpoint.
[0024] As used herein, the terms "optional" or "optionally" mean
that the subsequently described event or circumstance may or may
not occur, and that the description includes instances where said
event or circumstance occurs and instances where it does not.
[0025] Reference will now be made in detail to certain embodiments
of the disclosure, examples of which are illustrated in the
accompanying drawings. Wherever possible, the same reference
numbers are used throughout the drawings to refer to the same or
like parts.
[0026] A Raman apparatus in accordance with the present disclosure
is illustrated in FIG. 1. The main components are as follows. Raman
apparatus 10 includes laser diode 12 tuned to the D2 line of
rubidium (Rb) near 780 nm. Light from laser diode 12 is directed
into multi-pass cell 13, which is formed by concave mirrors 14 and
16. In one embodiment, mirrors 14 and 16 are 50.2-mm diameter
concave mirrors with a 100.0 mm radius of curvature, and they may
be made out of BK7 glass or any other suitable material. The
concave mirrors are shown to be separated by a distance of about
200 mm. One of ordinary skill will recognize that multi-pass cell
13 may be formed by any suitable combination of mirror focal length
and separation. In one embodiment, the nominal reflectivity of the
mirrors at normal incidence is better than 99.99%. Wafer 18 forms
the third component of multi-pass cell 13. In one embodiment, wafer
18 may be a polished, N-doped silicon wafer with a native layer of
oxide. However, one of ordinary skill will recognize that the
doping of the silicon wafer is irrelevant to the present
disclosure. P-doped or undoped wafers may also be used without
departing from the spirit of this disclosure. The oxide layer is
useful for chemisorbing the species to be measured.
[0027] Condenser 20 concentrates the Raman scattered light from
wafer 18 and passes it through a filter to remove the Rayleigh
scattered light. In this embodiment, the filter used is holographic
notch filter 22, which has an optical density of at least
approximately 5 at the Rayleigh wavelength. In order to detect
different compounds, the filter may be swapped out with a different
filter or atomic vapor (e.g. Rb) absorption cell as appropriate for
the wavelength desired. The Raman light then passes through
collecting lens 24, which focuses it down to spectrograph 28, which
in this embodiment is a 0.275-m spectrograph. If necessary, the
image of the focused light may be rotated by Dove prism 26 in order
to bring it parallel to the entrance slit of spectrograph 28.
Spectrograph 28 is coupled to CCD camera 30, which may be cooled to
reduce noise. CCD camera 30 is coupled to computer 32 for data
recording.
[0028] As shown in FIG. 1, laser diode 12 may be set up in a
grating-locked configuration to ensure that its output has a
desirable narrow bandwidth. In this embodiment, the tunable laser
diode system is composed of an approximately 6 cm laser cavity is
shown with a laser diode (SHARP GH0781JA2C, power of 120 mW in this
embodiment) coupled to a collimating lens, with diffraction grating
40 providing the optical feedback. In this embodiment, grating 40
has 1200 grooves/mm blazed at 750 nm and is mounted in the Littrow
configuration with the 0-order used as the laser output. Mirror 42
mounted at a right angle next to the diffraction grating
compensates for horizontal beam displacement caused by grating
rotation during tuning. The temperature of the laser diode may be
kept constant by a thermoelectric cooler (not shown). Once tuned to
the frequency of the rubidium D2 line, the laser drift may be less
than 1.0 GHz per hour without an active frequency locking
mechanism.
[0029] A portion of the laser light may be picked off by beam
splitter 43 and then used for frequency tuning. The fluorescence
monitored by the infrared viewer from rubidium reference cell 44
may be used to tune the laser to Rb D2 line. The mode structure may
be monitored by scanning confocal Fabry-Perot interferometer 46,
which has a 7.5 GHz free spectral range and a finesse of 200. The
laser diode may be powered by a standard commercial current source.
The estimated power of the monochromatic light in this embodiment
is about 40 mW. The laser light may be polarized vertically to the
table plane.
[0030] In actual operation of the system, beam splitter 43 and the
other components used for frequency tuning may be omitted once the
system has reached a stable state.
[0031] Lens 48, a 120-mm focal length convex lens is placed such
that it focuses the laser onto the silicon surface at a nearly
grazing angle. The beam then diverges and is re-focused to nearly
the same place several times by mirrors 16 and 14. After each
subsequent reflection, the angle of incidence at the silicon
surface decreases together with the fraction of laser light
entering the silicon (i.e. refracted light). The refracted laser
light is essentially completely absorbed in the silicon. The
estimated increase of the light intensity at the target in the
multi-pass configuration and the consequent increase in Raman
signal is nearly a factor of 5 compared to a single pass. Silicon's
index of refraction at 780 nm is assumed to be equal to 3.70, and
the well-known Fresnel equations may be to compute the intensity of
the reflected light as a function of the angle of incidence.
[0032] The Raman scattered light is collected by condenser 20,
which in this embodiment is a multi-element uncoated condenser lens
made of borosilicate crown glass, with f/#0.7 and with a back focal
length of about 25 mm. Lens 24, a 50-mm diameter and 200-mm focal
length lens, images the scattering volume to the entrance slit of
spectrograph 28, a 0.275-m Turner-Czerny spectrograph in this
embodiment. The light image is rotated by 90.degree. in this
embodiment by Dove prism 26 before entering the spectrograph. This
arrangement images approximately 20 micron.times.0.8 mm of a
focused laser beam area on the silicon wafer onto the 160
micron.times.6 mm entrance slit of the spectrograph. The imaging
optics match closely the f/#4 number of the spectrograph. The
overall magnification of the collection optics is about 8. The
grating used in all measurements has 600 grooves/mm and is blazed
at 1.0 micron. The resulting resolution is 0.2 nm (or 3.2
cm.sup.-1). A back-illuminated, cooled (243 K) Hamamatsu CCD array
with 1024.times.256 24-micron pixels with a well capacity of
300,000 electrons per pixel may be used as a light detector. The
dark electron count at 243 K in this embodiment is about 10
electrons per pixel per second. Most spectra were taken with an
exposure time between 0.01 s and 1.0 minute.
[0033] To prepare the wafer for testing detection of AN, solid AN
may be dissolved in water or methanol with a concentration of 43
g/liter. Then a pre-measured drop of the solution, approximately
1/37 ml, is applied to the silicon to cover a surface area of about
1.7.times.1.7 cm.sup.2. The resulting surface density of AN is
2.9.times.10.sup.22 molecules/m.sup.2 or about 1000 monolayers of
NH.sub.4NO.sub.3, calculated from the specific density of 1.725
gr/cm.sup.3. The area where the laser beam interacts with the
NH.sub.4NO.sub.3, with micro crystalline AN is estimated to be
2.0.times.10.sup.-8 m.sup.2. We assume that the laser is operating
in TEM00 mode and characterized by the Gaussian intensity
distribution. Assuming a uniform distribution of AN on the surface
of silicon, we conclude that the excitation laser light interacted
with about 4.5.times.10.sup.14 molecules or 0.5 n mol of AN.
[0034] The pre-measured, dissolved AN used in this embodiment has
advantages for characterizing the sensitivity of the apparatus;
however, because of the propensity for molecules to stick to the
SiO.sub.2 surface, the apparatus may also be used by simply passing
a vapor over the SiO.sub.2. This allows the device to be used in
the field to detect trace amounts of airborne compounds without
putting them into a solid or a liquid phase.
[0035] The representative Raman spectrum of deposited AN on the
silicon surface and obtained with our apparatus is shown in FIG. 2,
with Raman shift in cm.sup.-1 on the x-axis and arbitrary units on
the y-axis. The spectrum was taken in open air at room temperature
of about 295 K and relative humidity near 70%. The spectrum shown
in FIG. 2 is not corrected for response function of the
spectrograph and CCD camera. The identification of the nitrate
spectral lines 60 and 62 is according to known references for phase
III of AN. The unfiltered remnants of Rayleigh line at 780 nm were
outside the active area of the CCD detector, but spectral line 64
corresponds to the silicon wafer. The background was subtracted to
bring the baseline to zero counts.
[0036] The advantage of using a multi-pass configuration is
demonstrated in FIG. 3, with Raman shift in cm.sup.-1 on the x-axis
and arbitrary units on the y-axis, spectrum 70 was taken with a
single laser pass with mirror 16 removed. Spectrum 80 was obtained
with a double pass configuration with mirror 16 in place and mirror
14 removed. As shown, nearly a 70% increase in the magnitude of
Raman signal from AN after the second pass was evident. The
increments in Raman line intensity from subsequent passes decreases
rapidly with increasing angle of incidence for each subsequent pass
due to the decrease in intensity of the reflected laser light.
[0037] The estimated signal-to-noise ratio for the strongest line
in FIG. 2, spectral line 60, is better than 50. Based on our
estimates there were approximately 1000 layers of AN on the surface
of the silicon. This translates to a signal-to-noise ratio of 1 (a
value usually defined as the detection limit) for 20 layers of
NH.sub.4NO.sub.3.
[0038] Under one embodiment of experimental conditions (very short
integration times) the shot noise determines the noise level. Thus
increasing observation time from 0.01 s to 1 minute lowers the
noise level by factor of 70 and brings the detection limit to about
1/4 of a monolayer of deposited AN on the silicon.
[0039] The present disclosure provides a simple, relatively
inexpensive Raman spectrometer as a monitor for traces of molecular
species attached to the surface of silicon. We used AN as an
example of the capabilities of the spectrometer. AN is a common
component of fertilizers, and this disclosure demonstrates that
with this spectrometer the sub-monolayer densities of molecules can
be detected. It should be straightforward to extend this research
to dissolved NO.sub.3.sup.-. The substrate may be replaced by a
gold plated disk with plasma discharged deposited SiO.sub.2 (50
nm). This will significantly increase the laser power in the
multi-pass cell. A known amount of solvent may be deposited on a
SiO.sub.2 substrate, the solvent will evaporate, and the remaining
NO.sub.3.sup.- density will be recorded. The unit can be routinely
calibrated with AN.
[0040] It will be apparent to those skilled in the art that various
modifications and variations can be made in the present disclosure
without departing from the scope or spirit of the disclosure. Other
embodiments of the disclosure will be apparent to those skilled in
the art from consideration of the specification and practice of the
disclosure disclosed herein. It is intended that the specification
and examples be considered as exemplary only, with a true scope and
spirit of the disclosure being indicated by the following
claims.
* * * * *