In the electronics industry, the main application of silicon dioxide (SiO

) is used as the gate oxide in the manufacture of semiconductor devices (MOSFETs) and as an insulation layer. With fast progress in integration density, the importance of thin-gate oxides with thicknesses less than 7 nm increases (1). Moreover, transistors are expected to use a gate dielectric with capacitance equivalent to 2–3 nm of SiO

. These trends require thickness and optical constants measurement techniques for such thin SiO

Techniques suitable for measuring thin insulating films on semiconductors are ellipsometry, X-ray photoemission spectroscopy (XPS), transmission electron microscopy (TEM), Rutherford backscattering, and electrical methods of capacitance-voltage (C-V). The electronic structure, properties of ultrathin gate oxides, and imaging of individual dopant atoms and clusters in bulk Si at the atomic scale have been investigated with TEM (2–4). Ellipsometry also has been used to determine the optical properties of SiO

 (5). It has a very high resolution and accuracy like C-V among these techniques. Moreover, the ellipsometric technique has been the most sensitive to oxide thickness as thin as 2 nm or less. When films are very thin, the optical pathlength is very small compared to the wavelength, so it becomes difficult to determine the index. In most work (6,7), bulk SiO

 index values are used and only the thickness is fit while considering that is difficult to determine simultaneously thickness and index. Existence of correlation between index and thickness for very thin films makes it not reasonable to use the refractive index of SiO

 bulk for a film with a thickness less than 10 nm, because it was known that the optical properties including the refractive index of ultrathin SiO

 films were different from those of thick films (8–10). 

 index values translate into errors in film thickness, but these thickness errors are usually only a fraction of a monolayer for native oxides. This level of error traditionally has been acceptable in semiconductor manufacturing. However, in modern circuits, the oxide films are becoming very thin, so effects of oxide index and interface layers are becoming important. How to model these thin layers and determine simultaneously the refractive index and thickness for more accurate results should be considered. However, the difference of ellipsometric parameters between two ultrathin films was so little that the deduced values of the refractive index and thickness were very sensitive to errors within ellipsometric parameters. In this article, we propose a scheme to simultaneously obtain more accurate refractive index and thickness of natural SiO

Variable angle spectroscopic ellipsometry (VASE) measures the changes in the polarization state of light as a function of the angle of incidence and wavelength when light is reflected from or transmitted through a sample. Details of the VASE technique are described elsewhere (11–13). In this article, VASE was used to determine the thickness and the refractive index of natural SiO

 thin films and also investigate the effectiveness of various optical models. 

The silicon substrates were silicon wafers with a <100> orientation. They were boron-doped, p-type with resistivity in the range 2~4 Ωcm. The oxides were naturally grown, "native" oxide films on the Si substrates. The films were assumed isotropic and homogenous for the ellipsometric analysis. 

The ellipsometric parameters Ψ and Δ are defined by tanΨ = |R

) are the complex reflection coefficients for p and s polarized parts, respectively (9). In this article, data were acquired with a VASE ellipsometer made by J.A. Woollam Co. (Lincoln, Nebraska). This rotating analyzer system was equipped with an autoretarder unit allowing Δ to be measured accurately over a full 360° range. Ψ and Δ were acquired at several angles of incidence ranging between 74° and 80° over the spectral range 220–1100 nm in steps of 10 nm. At each measured wavelength, Ψ and Δ data measured at multiple angles of incidence provided the possibility of determining more unknown parameters simultaneously. Optical modeling and data analysis were performed with the WVASE32 software package. 

The mean square error (MSE) was the evaluation of the match quality between measurement (exp) and model data (mod), and was defined according to the Levenberg–Marquardt algorithm as:  

where N was the number of measured Ψ and Δ pairs, M was the total number of real valued fitting parameters, and σ

 were the standard deviations on the experimental data. To deduce the most approximate estimates, it was required that 2N ≥ M and minimize the MSE. 

All measurements were taken at room temperature. Three kinds of unknown quantities need to be determined: the film thickness (

), the real part of the refractive index (

), and the imaginary part (extinction coefficient 

). Optimal use of the ellipsometric technique critically depended on the choice of the angle of incidence (

) and wavelength (λ). To determine an approximate unknown quantity, it was desirable to find a suitable region of spectral range and the angle or angles of incidence. It is difficult to know these selections precisely. The angle of incidence chosen around the Brewster's angle or the quasi-Brewster's angle (Δ = 90°) might enhance analysis sensitivity (14). For instance, Figure 1 shows two simulated Δ and Ψ space trajectories generated by the WVASE32 software package for two transparent films, SiO

, which had disparate refractive indices (approximately 1.47 and 2.00, respectively), and 2-nm film thickness on an Si substrate within the 850–1100 nm wavelength range and corresponding angles of incidence 75°, which was near Brewster's angle, and 70°, which was far away from Brewster's angle. It could be seen that two trajectories diverge as the angle of incidences approach Brewster's angle and converge for angles of incidence far away from the Brewster's angle. 

The spectral range for measurements was varied with the angle of incidence, because the Brewster's angle or quasi-Brewster's angle was dependent upon wavelength. The relationship between the wavelength and the angle of incidence that we obtained for the sample system when Δ = 90° is shown in Figure 2. The accurate results could not be obtained with a single angle of incidence over the full region from 220 nm to 1100 nm according to Figure 2, so that the data with variable angles of incidence and various ranges of wavelength should be acquired. 

There was no direct access to optical constants and the thickness of the film from ellipsometric measurements, so modeling was required to deduce the sample's properties from the measured ellipsometric parameters. A model is an idealized mathematical representation of the sample. To construct a model, one has to assume each layer's thickness, dielectric functions, and composition, respectively. If the model is not good enough, it is impossible to accurately deduce the sample's properties. Generally, it is good practice to start from the simplest model with the fewest parameters used in the process of fitting. Therefore, the simplest three phase model of an air-SiO

Table I:  Measurement conditions and deduced results with a simple model of the air-SiO2âSi system			

The optical constants of Silicon substrates were taken from the literature (15) and were not allowed to vary during the fitting. For the model of a single homogenous SiO

 could be deduced from ellipsometric parameters Ψ and Δ. Because the number of unknown parameters was 

 = 3, the number of angles of incidence was required to be 

 only using literature reference values for 

, which will be helpful to find a very good initial thickness for fitting. Then adding 

 to fit should help avoid abnormal results in 

. The spectral range was chosen by fitting 

 in a narrow spectrum. Once an acceptable fitting was achieved, the spectral range could be extended gradually until some regions appeared that could not fit to the data well. The measurement conditions and results are illustrated in Table I and Figure 3. 

The results obtained at 290 nm and 360 nm were abnormal, and were mostly influenced by the absorption peak of substrate Si. The ellipsometric parameters Ψ and Δ fit to the experimental data over the spectral range 220–1100 nm, and as an example, the part of 850–1100 nm is shown in Figure 3a. Table I shows that thicknesses of SiO

 were almost equal in seven spectral regions, and their relative errors were only within 4%. As a comparison, 

 were also fixed at the values taken from the literature (16), and the relative error of the thickness of SiO

) obtained in the same seven spectral regions was 7% and MSE was larger than 1.0. 

value (dotted line) compared with previously published data (16) (solid line). Their difference was |δ

| ≤ 0.07, but the large deduced extinction coefficient 

 value indicated the fit was not very good, although MSE was below 1.0, and the results were only approximate. Considering the natural SiO

 film was ultrathin and the interface layer between SiO

 and Si could not be neglected, we added an interface layer to configure a four phase model of the air-SiO

Table II:  Measurement conditions and deduced results with the model of the air-SiO2âSi system			

The thin interface between the dioxide and the substrate was modeled as 50% Si and 50% SiO

 using a Bruggeman effective medium approximation (EMA) (16) and is a commonly used approach to model the optical constants of intermixed layers. The percentage was fixed at 50% in order to reduce the number of unknown parameters. Simultaneously fitting the thickness and optical constants of the SiO

 ≥ 3. The measurement conditions and results are shown in Table II and Figure 4. 

Figure 4a shows the ellipsometric parameters Ψ and Δ fitted to the experimental data within the range 850–1100 nm. Figure 4b shows |δ

 ≤ 0.09. The relative errors of thickness decreased to below 3%, and the MSE was also less than that of the last model according to Table II. All of these indicated that the model was superior to the air-SiO

-Si system. Previous work by other authors employing ellipsometry also studied the interface layers between Si and SiO

 (8,17–19). The system and measurement errors brought the errors in 

 was quite close to zero. Moreover, all the measurements were taken in the open air, so the results can be influenced easily by the conditions of the surrounding air. For instance, the adsorption of water made it difficult to obtain 

 accurately. These tiny factors can cause noticeable influence because ellipsometry, particularly ellipsometric parameter Ψ, is quite sensitive to even tiny variations on surfaces. All these could be considered so that 

We also investigated the surface roughness layer above the SiO

 layer, namely, the air-surface roughness-SiO

-interface-Si system. Similarly, the surface roughness layer was modeled as a 50:50 mixture of oxide and void using the Bruggeman effective medium approximation (16). Fits were not improved. Furthermore, adding the thickness and optical constants (or the EMA layer fraction) of the surface roughness layer as the fitted parameters, the number of unknown parameters would be 9, and it required N ≥ 5. In the same way, the results obtained were also not good enough to show any improvement. 

These proved that the model could not be improved by considering a surface roughness layer. The reason might be that the amount of useable information in the Ψ and Δ data already has been exhausted in the thickness and index parameters of the oxide and interface layers. Adding additional fit parameters does not improve the fits. The model is not sensitive to additional parameters. 

The optical properties and thickness of natural SiO

 thin films grown on silicon substrates were investigated simultaneously with a VASE system by choosing different angles of incidence and wavelength ranges. Results are better than previous results, in which the refractive index of silicon dioxide layer was fixed to reference values. The simple model of the air-SiO

–Si system was proved for determining the approximate film thickness and refractive index over the range 220–1100 nm. More precise fittings were achieved by adding an interface layer between SiO

 and Si under the optimized conditions of angles of incidence and wavelength ranges. 

We gratefully acknowledge the National Natural Science Foundation of China and the Chinese Academy of Sciences for their support. 

 are with Institute of Mechanics, Chinese Academy of Sciences, Beijing, China. Yanyan Chen is also with Graduate School of the Chinese Academy of Sciences, Beijing, China. They can be contacted at: 

(1) A.C. Diebold, D. Venables, Y. Chabal, D. Muller, M. Weldon, and E. Garfunkel, 

(2) D.A. Muller, T. Sorsch, S. Moccio, F.H. Baumann, and G. Timp, 

(3) J.B. Neaton, D.A. Muller, and N.W. Ashcroft, 

(4) P.M. Voyles, D.A. Muller, J.L. Grazul, P.H. Citrin, and H.J.L. Gossmann, 

(5) G.L. Tan, M.F. Lemon, R.H. French, and D.J. Jones, 

(7) H. Reisinger, H. Oppolzer, and W. Hönlein, 

Articles

October 2006. The authors investigate the optical properties and thickness of natural SiO2 thin films grown on silicon substrates simultaneously with a VASE system by choosing different angles of incidence and wavelength ranges.

Refractive Index and Thickness Analysis of Natural Silicon Dioxide Film Growing on Silicon with Variable-Angle Spectroscopic Ellipsometry

The last column closed with an in and out calculation challenge. To reiterate: We consider an EI source sensitivity specified as 1 × 10

  C/µg of methyl stearate introduced into the source (this is a reasonable number for instruments of moderate performance). Detail-oriented readers immediately ask about the time associated with that sample introduction. Is it 1 s? 10 s? 100 s? These times represent the period over which the ion current produced is integrated. For methyl stearate and EI, the molecular ion of the compound at 

 298 usually is specified. Given the molecular weight and Avogadro's number, the actual fraction of neutral sample molecules converted into molecular ions can be calculated. Let's complete the calculation. Methyl stearate of molecular formula CH

 has a molecular mass of 298.51. Avogadro's number is 6.02 × 10

 . A microgram of methyl stearate, therefore, is 2 × 10

  molecules. The elementary charge has a value of 1.602176 × 10

  C; this is the charge, in coulombs, of a singly charged ion. The sensitivity of 1 × 10

  ions. Setting up the ratio yields the ratio of ions to molecules as 3.1 × 10

 . In words (with rounding), in an optimized EI source, one of every 3200 molecules in is represented by an ion at the detector. This is not yet an out value, because the sensitivity (as mentioned above) convolutes mass analyzer transmission with ionization efficiency. An estimate of mass analyzer transmission is about 10%, which leads to an ionization efficiency for the optimized EI source in which one molecule of about every 300 is ionized. A time of 1 s for production of this ion current is concordant with gas flows into and pumping speeds out of an EI source used for GC–MS. We'll save gas dynamics in relationship to ion sources for a future column and concentrate in the remaining space here on charged-particle physics. A less-than-1% ionization efficiency is not, on its face, impressive. This fact, perhaps, explains why sensitivity is most often specified for an instrument rather than for a source alone. The sensitivity value so quoted is improved by the high gain of the electron multiplier usually used as the detector. It is claimed that other ionization sources demonstrate a higher specific ionization efficiency than the generic EI source, which is constrained by its fundamental dependence upon the interaction of a gaseous molecule with an electron, and the sometimes difficult-to-control flow of molecules (gas dynamics) into and out of the source. A subtext to such arguments, reserved for a later time, is whether ionization sources operate as mass- or concentration-dependent devices.  

In our previous EI description, we listed some suggested methods through which the chances for that interaction between the gas-phase sample molecule and the electron might be increased — a more concentrated sample gas density, a more intense electron beam, or a longer path length over which the molecule–electron interaction might take place. We described the use of a source magnet that caused electrons to follow a spiral path in the course and effectively increase the path length. The use of such a source magnet was first described in a paper by Walker Bleakney of the University of Minnesota (Minneapolis, Minnesota) in 1929 (1). This was Bleakney's first scientific publication, and part of his doctoral work under Professor John T. Tate. The usual field strengths of 100–500 G are achieved with small, permanent magnets, usually fitted to the outside of the source housing itself. 

For his doctoral research work, Bleakney measured ionization potentials using MS. Upon his graduation, he moved to Princeton University (Princeton, New Jersey), and he became immersed in a research career in which he studied shock-tube measurements. Al Nier, a self-described "gadgeteer," began his own scientific career at the University of Minnesota about five years later (2), and concentrated on the use of MS for the determination of isotopic abundances. (Nier was also a doctoral student of John Tate, and his early papers, as with Bleakney, were published without the authorship of the research advisor.) In such determinations, the EI source must operate in a constant and reproducible fashion, and the inherent discrimination against ions of different masses must be delineated. Nier, with the aid of a superb departmental machinist, devised a stable EI source and a new sector-based mass analyzer, and applied the new instrument to the study of isotopes. These instruments, including their EI sources, are described in Nier's seminal 1940 and 1947 publications (3,4), part of his doctoral dissertation work. Nier spent most of his research career at Minnesota, and his pioneering contributions to the development of sector mass analyzers have been reviewed in a recent historical special feature (5). His contributions to EI source design are equally noteworthy, supplemented by the work of many others, notably Mark Inghram. The Nier–Inghram source is widely used with a Knudsen cell for generation of transient reactive species, and Inghram made his own contributions to the field of dating through the study of isotopes. 

The Nier EI source has been referred to as a "three-electrode source." Ions are formed within the source volume, held at an appropriate voltage relative to the electron-emitting filament; the source volume is the first electrode. The ions are promptly pushed out of the source by the voltage applied to the repeller plate (the second electrode). The third electrode is the source slit, which establishes the accelerating voltage difference between the source volume and the mass analyzer, resulting in extraction of the ions into the mass analyzer of the sector instrument. The EI source that Nier joined to the sector mass analyzer in his 1940 instrument is shown in Figure 1. Notice the use of glass insulators (hand-blown). Nier and other "gadgeteers" eventually sought to increase source efficiency, sometimes by increasing sample pressure in the source (termed a closed source as opposed to the open design used, for instance, in ionization gauges used to measure pressure). The electron entrance to the source volume (an in) could be restricted to a slit (to change the shape of the ion image formed within the source volume), or could (in later designs of the EI source) become a circular aperture to reduce the energy spread of the ionizing electrons. Concomitantly, for the same reasons, the ions' exit aperture (an out) could be modified in size and distance from the source slit, which also had the effect of decreasing the penetration of the high-magnitude acceleration voltage into the confines of the ion volume. The Nier ion sources are known by their high ion output relative to their compact size.  

These first- and second-generation EI sources were studied carefully with respect to their ability to accurately determine ionization energies and appearance energies, along with isotopic abundances. At the same time, however, versions of these same sources were being fitted onto the first analytical mass spectrometers (the CEC series) used for organic analysis, in which the requisite characteristics were noticeably different, and included sample clearance times, resistance to contamination, and ease of cleaning and reassembly. While the early rules for mass spectral interpretation for organic analysis were being established by the "chemists," many of Nier's physicist colleagues determined "repeller curves," which were plots of ion output from the source as a function of the repeller plate voltage (6). Not all of the ions in an EI source are formed in the same place with the same energy. The collection of ions represent an "object" characterized by space and energy of individual constituents (time is left out for now). As in a physically real object that we perceive through light optics, the collection of ions is an image for which our source should provide an accurate representation, without discrimination. For a source in which gas dynamics creates a more-or-less homogeneous distribution of molecules, the image is constrained by, and can be the same as, the physical volume intersected by the ionizing electron beam. "Imaging" ion sources have always focused our attention on delineation of the space and energy characteristics of the ion image to a higher dimension and a higher resolution. 

For EI sources used for isotopic abundance measurements, lack of (or known) discrimination based upon mass was especially important. Stated succinctly, each different ion mass produces a slightly different ion image in the source. Each different physical iteration of an EI source, and each change in the voltages applied to the electrodes within it, changed the image. Settling on the standard EI source design and a calibration scheme introduced by Nier, the EI source could be used to make measurements with an extraordinary degree of accuracy. 

Empirical determination of repeller curves, or ion optics, is both tedious and somewhat unsatisfying, because the underlying rules of physics are known. Scientists match theory with experiment, and so directed studies of the ion optical properties of EI sources were first accomplished using the techniques of ray tracing, followed by calculations using infinitely thin electrodes, and then more realistic calculations that allowed for thick electrodes of various shapes, as in the actual EI source. Predictably, the power of the computer was brought to bear on the problem. Werner (7) modeled an EI source of the basic Nier type, factoring in such subtleties as the effect of the source magnet (no significant effect found), the penetration of the magnetic field of the sector into the source (no effect found), and the space charge (an effect that varied in proportion with the electron beam current and the number of ions produced). A particularly instructive result of such a computation is illustrated in Figure 2, adapted from Werner's original paper. The bottom part of the figure highlights (for a given set of operational conditions) the ionization region, the ion extraction region, and an overlap of the two, which represents the ion image. 

We have concentrated here on the EI source modeled after the design of Nier, which has persevered for over 60 years in its basic form since its introduction in Nier's 1940 mass spectrometer. Specialized EI sources for specialized applications abound, and there are other eponymous EI sources such as the Brink ion source, which has itself been further modified (8). The modifications are designed to change the most basic parameters of the source operation — the ins of samples in a molecular beam, and the outs of ions generated from beam components as compared to background components. The patent literature, too, is replete with new sources and variants of existing sources. The patent claims are examined using the concept of ins and outs. Accurate measurements of ion abundances are still the focus in isotope ratio MS, and this applications group in particular has continued to advance the optimization of the design of the EI source. 

For use in combined GC–MS instruments, the optimized EI source design is found at the balance point between physical characteristics [of size, cost, operational stability, response time] and ionization efficiency, which affects the overall sensitivity of the instrument. Once the basics of the source ins and outs and their underlying physical phenomena are understood, the genuine advances and the insights of the past and current generations of "gadgeteers" can be accurately assessed. 

 has overtightened more filament posts, cracked more spacer ceramics, and misaligned more repeller plates on various EI sources than he will ever admit in any public forum. He once designed an EI source for operation in a preparative mass spectrometer to be placed on the lunar surface, where the molecular losses due to pumping disappear from the balance of ins and outs. Reach the author at 

 Volume 74 (National Academy of Sciences Press, Washington, D.C., 1998). 

(8) A. Amirav, A. Fialkov, and A. Gordin, 

Mass Spectrometry Forum

October 2006. Columnist Ken Busch continues his discussion of the El source and how source design and operational details directly affect source performance.

Electron Ionization: More Ins and Outs

While perhaps the most extensive such timeline to date, it is surely not complete. Sources for further information have been provided.  

(1642–1727) (Figure 1) shows that the white light from the sun could be dispersed into a continuous series of colors. He coined the word "spectrum." His apparatus, an aperture to define a light beam, a lens, a prism, and a screen, was the first spectroscope. He suggested that light was composed of minute corpuscles (particles) moving at high speed. 

(1629–1695) proposes the wave theory of light.  

(1698–1758) notes that the amount of light passing through a liquid sample decreases with increasing sample thickness.  

(1726–1753) of the University of Glasgow, Scotland, observes a bright yellow light emitted from a flame produced by burning a mixture of alcohol and sea salt. When the salt is removed, the yellow color disappears.  

(1728–1777) publishes his "Law of Absorption."  

(1745–1827) (Figure 2) uses his "perpetual electrophorus" device for producing static electric charges to spark various materials. He notes different colors with different materials. Eventually he is able to identify certain gases by the colors emitted when sparked. 

American astronomer and instrument maker 

(1732–1796) produces the first primitive diffraction grating with parallel hairs laid across two screws.  

(1766–1828) is the first to observe dark lines in the spectrum of the sun. 

(1787–1826) invents the transmission diffraction grating and makes a detailed study of the dark lines in the solar spectrum.  

(1800–1877) observes that different salts produce colors when placed in a flame. 

produces the first spark-emission spectroscope.  

(1825–1863) publishes a paper showing that the amount of light absorbed was proportional to the amount of solute in aqueous solutions.  

(1811–1899) (Figure 3) discover that spectral lines are unique to each element.  

discover the elements cesium and rubidium using their new technique of spectral analysis.  

Figure 3: Gustav Kirchoff (left) and Robert Bunsen.			

(1832–1919) (Figure 4) using the method of spectral analysis. 

The element indium is discovered by German professor of physics 

(1799–1882) and German metallurgical chemist 

(1824–1898), also by the method of spectral analysis.  

The element helium is discovered through its characteristic spectral lines in the spectrum of the sun. The discovery was made independently by French astronomer 

(1836–1920). It was named for the Greek term for sun, Helios. (Note: Lockyer is knighted shortly after this discovery. Also, he founded the journal 

(1814–1874) (Figure 5) publishes a detailed study of the wavelengths of solar spectral lines, expressed in units of 10

meters. This unit is now known as the angstrom (Å). He is considered one of the fathers of modern spectroscopy.  

(see 1868) (Figure 6) observes that the (complex) spatial structure of the atomic emission within the high-voltage spark-induced plasma is a function of the concentration of the emitting element. Furthermore, he shows that improved quantitation is possible by comparing the analyte emission with that of another element in the sample (an early example of "internal standardization").  

introduces the pneumatic nebulizer for transferring liquid samples into a flame.  

(1848–1901) (Figure 7)produces greatly improved (curved) diffraction grating using his new grating "ruling machine" at Johns Hopkins University  (Baltimore,Maryland).Gratings produced in his laboratory became the standard throughout the world.  

of Dublin conducts a systematic study of change in spectral line intensity with concentration. Later, he produces the first semiquantitative spectrographic analysis (determination of beryllium in cerium compounds). 

(1825–1898) (Figure 8), a Swiss high school teacher and adjunct university mathematics professor,  shows that the wavelengths of the visible spectral lines of hydrogen could be represented by a simple mathematical formula. These lines are now known as the Balmer series of hydrogen.  

(1854–1919) generalizes Balmer's formula to: 1/λ = 

(1845–1923) (Figure 9) discovers X-rays and experiments extensively to discern their properties. He is awarded the first Nobel Prize in physics for his discovery (1901).  

(1865–1943), Dutch physicist, observes splitting of spectral lines by a magnetic field. He receives the 1902 Nobel Prize in physics for his work.  

(1852–1908) discovers radioactivity.He shares the 1903 Nobel Prize in physics with Pierre and Marie Curie for their work on radioactivity.  

The electron is discovered by British physicist 

(1856–1940). He is awarded the 1906 Nobel Prize in physics for this discovery and his investigations on the conduction of electricity in gases.  

(1858–1947) introduces the quantum concept. He is awarded the 1918 Nobel Prize in physics. 

(Adam Hilger Ltd., London, UK) produces the first commercially available quartz prism spectrograph. 

First work on time-resolved optical emission spectroscopy is reported by 

. Their technique involves moving the photographic film in the focal plane of the spectrograph. 

(1874–1954) discovers ultraviolet series of hydrogen lines. They fit the Rydberg formula with 

(1879–1955) explains the photoelectric effect, for which he was awarded the Nobel Prize in 1921. His theory explains that when a photon strikes the surface of some materials, an electron is ejected. This same year, he publishes his "Special Theory of Relativity."  

(1877–1944) (Figure 10) discovers that each element has a characteristic X-ray and that the degree of penetration of these X-rays is related to the atomic weight of the element. He is awarded the Nobel Prize in 1917. 

(1878–1909) proposes his Combination Principle (also known as the Frequency Sum Rule), which notes that the spectral lines of any element include frequencies that are either the sum or difference of two other spectral lines.  

(1882–1945) (Figure 11) develops a device for detecting radioactivity ("Geiger counter").  

(1879–1960) suggests using crystals to diffract X-rays.He is awarded the Nobel Prize in 1914.  

October 2006. This timeline provides a short history of the experimental and theoretical development of atomic spectroscopy for elemental spectrochemical analysis.

A Timeline of Atomic Spectroscopy

The worldwide market for absorbance colorimeters was worth close to $90 million in 2005. Colorimeters also are referred to as photometers and should not be confused with other fundamentally different types of instruments that are also referred to as colorimeters. They are commonly found at the heart of water testing kits that can be configured for numerous applications. Although the market is mature, demand for the technique remains cogent. 

Colorimeters that measure the absorbance of solutions also are  referred to as photometers, and  generally are classified as a type of visible spectrophotometer. By passing a single wavelength of visible light through a sample solution, they can measure the amount of light absorbed by the sample, which is directly proportional to its concentration. Another type of instrument that also is referred to as a colorimeter measures the reflected light from a solid sample in order to simulate how it is seen by the human eye, but it is a fundamentally different type of instrument. 

Colorimeters for absorbance measurement often are sold as part of test kits for water analysis, which include reactive dyes for identification of specific compounds such as ammonia, bromine, and chromium, among many others. More than half of the market is composed of these reagents and other peripheral products. 

The market for absorbance colorimeters is highly concentrated, with the top three vendors accounting for more than two-thirds of the market share. However, there are still a fairly large number of small vendors in the market due to the relative simplicity of the technology and the low cost of the instruments themselves. Annual growth in demand will remain consistent in the low single digits for the foreseeable future. 

The foregoing data were extracted from SDi's market analysis and perspectives report entitled 

Edition Global Assessment Report, 9th Edition: The Laboratory Life Science and Analytical Instrument Industry

, September 2006. For more information, contact Stuart Press, Senior Consultant, Strategic Directions International, Inc., 6242 Westchester Parkway, Suite 100, Los Angeles, CA 90045, (310) 641-4982, fax: (310) 641-8851, 

Departments

The worldwide market for absorbance colorimeters was worth close to $90 million in 2005. Colorimeters also are referred to as photometers and should not be confused with other fundamentally different types of instruments that are also referred to as colorimeters. They are commonly found at the heart of water testing kits that can be configured for numerous applications. Although the market is mature, demand for the technique remains cogent.

Market Profile: Absorbance Colorimeters

 is not new and is normally intended to indicate the integration of optics and electronics to form a single measurement package. In the system described in this column, the term is extended to include the close-coupling of the measurement system to the sample via nonimaging optics. 

This column focuses on the spectral engine concept and its role as an integrated component into continuous in-line/on-line measurement systems. One obvious mode of integration is the combination of the spectral sensor within a process analytical measurement system, and this is discussed in terms of the New Sample Sensing Initiative (NeSSI) miniaturized sampling systems, pioneered by the Center for Process Analytical Chemistry (CPAC), which is the industry–academia consortium based at the University of Washington, Seattle. Other forms of integration can include combinations with other small-scale sampling systems and microfluidic devices. Examples of applications are provided ranging from water quality measurements to the compositional measurements of organic sample streams. 

In 2000, I discussed the concept of a small-scale "microspectrometer" based upon the integration of a spectrally separating optical filter, known as a linear varible filter (LVF), with a photodiode array (1). This device, known as the microPac, was developed and introduced by Optical Coatings Laboratories, Inc. (OCLI), now a division of JDS Uniphase (Santa Rosa, California). These systems featured visible and near-infrared (NIR) filter devices optically coupled to a commercial silicon photodiode array. At that time, the device was offered as a tool for instrument developers. For some, one issue was that there were a number of miniaturized spectrometer systems on the market that offered good performance at competitive prices. When viewed as an instrument platform, the microPac did not seem to offer any advantage over the existing microspectrometers, primarily because of cost and size issues. 

The role of LVF devices, also termed variable filter array (VFA) devices, has been pioneered by Wilks Enterprises (South Norwalk, Connecticut) (2) for infrared spectroscopy applications. Wilks has always taken the position that high resolution is not necessary for dedicated analyses and has demonstrated this with the VFA analyzer systems. The Wilks VFA instruments operate with a typical spectral resolution in the range 40–50 cm

 . Wilks has contended that most analyses can be performed at low resolution and that optical filters are adequate for such measurements. The microPac microspectrometer was similar in concept but had its focus on the visible and NIR and provided a spectral resolution (based upon spectral bandpass) in the range of 4–8 nm. Likewise, adequate measurements could be made in the visible and NIR with this degree of spectral resolution. 

When the original microPac was introduced, it was recognized that it should be viewed as something different than a traditional instrument; the biggest opportunity for this type of integrated device was for spectral sensing. Spectral sensing means a component that can be integrated into a measurement system for monitoring chemical composition, rather than a standalone instrument. In the broadest sense, spectral sensing can be performed by instruments; however, instruments are not necessarily convenient or cost effective for true sensing applications. In an ideal world, spectral sensing should be performed by small, low-cost dedicated devices that are fully integrated with sample handling, data acquisition, and data handling. Such systems ideally are compact, maybe being no more than 1–2 in. wide and a few inches in length. They should be in a format that can be introduced into a process without major impact on the operation of the process. Note that the word "process" can be taken in the broadest sense and can include traditional industrial production processes, but it also can include environmental, biological (medical), and consumer-related processes such as water condition or quality monitoring. 

The microPac microspectrometer did lend itself to that kind of thinking insofar as it was compact and was packaged with all of the electronics necessary to make it self-sufficient. However, it was not a fully integrated device and it did not include an energy–light source or a sampling interface. Furthermore, it was not immediately adaptable to specific applications. This article will discuss the development of the next-generation spectral sensor, an evolution in optoelectronic packaging and a full integration with electronics and sample interface. The device, termed a micro spectral sensor, from micro Spectral Sensors (Wilton, Maine), features a spectral engine that is designed to operate within a variety of platforms, the most important being handheld versions, with their own integrated sample dispensing, and on-line versions, which are designed for integration directly into a sample stream via a dedicated sampling system.  

The focus for this current article, by way of example, is an on-line version, configured to operate with the NeSSI (ISAS standard SP76) small-scale modular sampling system platform. A follow-up column early in 2007 will describe the unique handheld versions of the system. With such a system it is possible to configure the spectral sensor for highly targeted applications. However, with on-board "smarts," the overall system can provide multiple analyses from a single device by selection of a suitable spectral range and appropriate reagents (if required). 

The standard version of the micro spectral sensing system is based upon a full spectrum version, which provides spectral data ranging from the long-wave UV, through the visible, and into the NIR. The spectral engine concept has been designed to allow for highly targeted applications in which colorimetric or dedicated multiwavelength sensing is accommodated. The colorimeter version is a basic configuration and provides an output based on a color assessment from a standard RGB system. The third version, which is effectively a multichannel photometer, is a hybrid, which combines attributes of both the full spectral analyzer and the RGB system. The spectral range in this case is not fixed and is defined by the combination of sources and wavelength selection devices used to make up the system. As designed, such a system can cover the UV–Vis and NIR wavelength ranges. 

When discussing microspectral sensors it is necessary to put the terms 

 into concept. In the end, the meaning is often in the eyes of the beholder and is very context or application dependent.  

 there are a large number of "spectrometers" or "spectrographs" that have been miniaturized and are sold in "small boxes." In the strictest sense of the word, physically, these are true spectrometers. The term microspectrometer has not always done justice to the potential of these instruments because, to "purists," they are not perceived to be real spectrometers. However, in reality, most commercial microspectrometers have the performance capabilities of conventional instruments. Current commercial microspectrometers are available at prices ranging from $2000 and upward. Because of their size, microspectrometers have enabled a multitude of new applications and a broad range of final product formats. These commercial devices can provide high spectral resolution, down to subnanometer resolution, matching the performance of conventional, large high-resolution spectrographs. Commercial microspectrometers can handle most applications in atomic and molecular spectroscopy. 

 device discussed in this article is something very different from a traditional microspectrometer. The device is not intended to replace the role of microspectrometers; instead, it is intended to either complement microspectrometers or to fill roles not currently served by these instruments. Certainly, for nonchallenging applications, a microspectral sensor might provide a comparable function to a microspectrometer. However, it is important to identify the application and to understand the relative role of a sensing system versus that of an analytical device, such as a spectrometer. The spectral sensor described here has a nominal spectral resolution (based upon spectral bandwidth and digital sampling) of 8 nm. Many applications can be served by simply monitoring a change in status or composition. For such applications, all a sensor needs to do is identify a change within an environment, or a delta function. 

The key word to describe the microspectral sensing concept is integration and the formation of a spectral engine. In a traditional spectrometer or photometer, it is normal to consider the instrument separately from the sample, and often even the data handling. In a microspectral sensing system, sample and sensor are close-coupled to provide what one can describe as an efficient management of photons. Most instruments require either internal or external optics to direct the radiation–light from the source to the sample and from the sample back to the detector of the instrument. These can range from traditional mirror–lens optics to optical fibers, or combinations of both. No ancillary optics are required in the micro spectral sensing concept; instead, the energy is channeled via transparent guides into the measurement chamber and is returned after interaction with the sample via a retroreflective system through a matching guide to the detection system. The term 

 is used somewhat generically because, in fact, the detector is also the spectral measurement system. As mentioned earlier, different detection options are involved, depending upon the application and the wavelength region of interest. 

Currently, configurations covering the visible and NIR spectral regions are in use. Figure 1 is a generic representation of the key components of a microspectral measurement system (1). While these are the functional elements (source, sample, and detection system), they are not the full implementation of a spectral engine. The separation here between an instrument and a spectral sensor is the ability to control the entire process of data logging and spectral data extraction onboard the device by dedicated electronics (Figure 2). In such a system, the raw signal from the spectrally configured detector is acquired and conditioned into a spectral measurement space based on wavelength and spectral intensity (absolute or relative). 

Multiple microprocessors can be used to convert the conditioned spectral signal into analytically relevant results such as analyte concentration or overall chemical composition. The conversions can range from simple linear equations, such as a basic implementation of Beer–Lambert, to a multivariate, multiequation solution based on chemometrics. The final component of the electronics of such a spectral engine is the communication of the resultant analytical data, which can range from an onboard visual display to hardwired (serial or bus oriented) or wireless external communications. 

As indicated earlier, the overall spectral engine, by definition, includes the sample interface. An example, in generic form, is presented in Figure 3. In this generic representation, the sample is accommodated within an optical chamber or a light interaction cavity. This approach can be incorporated within a number of different platforms ranging from a portable handheld device to an integrated in-line/on-line process monitoring system. It is this latter approach that will be discussed in further detail. 

For some time now, there has been a focus in the process analytical community on the scaling down and miniaturization of sample systems used to interface process lines to instrumentation. One of the most visible and most concerted efforts has been the development of the NeSSI standard. Several manufacturers have developed products, in the form of packaged components and integrated systems, based upon the ISAS standard SP76 sampling platform. 

Figure 4 shows an early manifestation of NeSSI and this standard, where components are built upon a common base with a standard footprint forming what is known as the standard NeSSI substrate. NeSSI is now being accepted by many industrial end-users as a standardized approach to the interfacing of conventional analyzer systems to process lines. The expansion plans for NeSSI include a phase where the analyzer systems themselves are integrated into the backbone of the NeSSI substrate (NeSSI 3). To a traditional observer, this might be a leap in faith when one views traditional process analyzers. However, in the context of the microspectral sensing concept and the format discussed previously, it is easy to appreciate that NeSSI can become a process analyzer in which sample conditioning and measurement are performed by a single entity. 

With this in mind, one also can  look at a number of standard methods of analysis and implement these within a single package by combining the spectral sensor with sample conditioning steps that include the interaction with standard chemical reagents, indicated, in concept, by Figure 5. This approach can bring on-line many standard methods of analysis within a single analytical platform because of the ability to program the sensor for different methods and reagent combinations. A classical example here is the analysis, by spectral or colorimetric methods, of water for quality and potential contamination. Figure 6 provides data obtained directly from a prototype microspectral sensor system for the analysis of phosphate and chromium(VI) in water. These two are important contaminants and are critical analytes for water quality testing from natural water resources. They are just two of many examples of water quality measurements possible with integrated sampling–spectral sensor measurement systems. Note that a NeSSI platform is used to set the stage for the concept of an integrated measurement system. However, for dedicated applications such as water quality monitoring, the integration of the spectral sensor with a custom-designed reagent dosing unit (for multiple reagents) can provide an optimized measurement package. 

Many analyses do not require special chemical reagents to meet the requirements of a particular measurement. Many compounds are naturally colored, and so measurements can be made directly on materials containing such substances. Also, many additional analyses can be performed in the NIR. The short-wave NIR region has been used for many industrial measurements, with dedicated process analyzers, for chemical and petrochemical and for food and beverage applications. The microspectral sensor system is available in an NIR version, which can be used on its own or integrated into a NeSSI type of sampling system for on-line measurements of chemical composition and chemical–physical properties. Some examples are provided in Figures 7 and 8, where microspectral sensing can be used to monitor changes in composition, or to measure specific properties, such as alcohol content of fermentation products. 

This article has introduced a new concept of spectral measurement and a new approach to the miniaturization of spectrometers. As indicated, the devices are described best as spectral sensors rather than spectrometers. They are discussed in terms of a core spectral engine that can be integrated readily into a number of different platforms ranging from simple handheld devices to on-line monitoring systems. These devices can be used for on-line monitoring for traditional and nontraditional applications. 

The microspectral sensor concept not only provides miniaturization, where the critical components can be unobtrusively incorporated into a measurement system, but it also provides a cost reduction to the point where spectral measurements can be incorporated into everyday applications. These can range from applications at work to those at home, in industry, in the environment, in public places, and in places of medical care. Many of these are part of a long-term goal, and today they are one step closer to becoming reality. 

is principal consultant for Coates Consulting and is a partner in several companies that design optical/spectral sensing systems. 

is chief technical officer at Cadrai Technology Group (Bel Air, MD). He can be reached via email at: 

Molecular Spectroscopy Workbench

Guest author John Coates discusses a miniature spectral measurement system that operates as a full spectrum analyzer, a colorimeter, or a multichannel photometer in the form of a spectral engine.

Think Small: Low-Cost Optical Spectral Measurements for Chemical Sensing

HORIBA Jobin Yvon introduces two interactive software tools for use with the company's CCD-based ACTIVA ICP system. According to the company, the M.A.S.T.E.R. (Multiline Analysis Selection Tools for Enhanced Reliability) software allows users to perform multiline selection with appropriate sensitivity and free of spectral interferences, and the S.O.S. (Statistical Outlier Survey) software is used to confirm possible outliers providing a single reliable concentration result per element. 

PIKE Technologies' grazing angle option for its VeeMAX II ATR accessory is designed to aid in the analysis of monomolecular layers on silicon to obtain FT-IR spectra. The accessory reportedly provides high throughput to cut down on sampling time and facilitate detection of the monomolecular layers of silicon without utilizing a liquid nitrogen–cooled detector. Crystal options for the accessory include ZnSe, ZnS, and Si. 

A catalog from Andor Technology provides an overview of digital camera technology, confocal microscopy, and spectrographs, and offers an application section showing where the company's products are being used in the field. 

also includes a section on fundamentals and the theory of light. 

Glass Expansion's Helix is an O-ring-free nebulizer fitting that is designed to eliminate the chemical compatibility issues and regular replacement due to wear of a traditional Oring seal between the nebulizer and spray chamber of an ICP spectrometer. The fitting is made from an inert PTFE ferrule and a locking screw to seal the spray chamber and a positive stop to ensure that the nebulizer is correctly positioned. 

The Multimodal Multiplex Spectroscopy (MMS) Raman spectrometer from Ocean Optics is designed for analysis of liquids and solids for applications such as materials inspection, chemical identification, and forensics analysis. The spectrometer utilizes a coded wide-area aperture to sample and mathematically process up to 500 measurement channels simultaneously, which the company claims provides more signal without a proportional increase in noise. 

Perkin Elmer introduces the Clarus 600 gas chromatograph, designed for environmental contract, forensics, petrochemical, and chemical quality control laboratories. The GC system's oven reportedly can cool from 450 °C to 50 °C in approximately 2 min, which enables reduced analytical cycle times and improved sample throughput. 

Combined with Thermo's Evolution 300 Uvvis spectrophotometer, the Rapid Mixing accessory is designed to allow scientists to make kinetic measurements nearly 1000 times faster than manual reagant mixing. The accessory reportedly features a dead time of 8 ms and an acquisition rate of 50 points/s. The accessory is also equipped with a thermocouple and an automated 

Melles Griot introduces single-frequency, low noise, diode-pumped solid-state lasers designed to provide options to traditional aircooled krypton–argon ion lasers. The lasers produce up to 25 mW at 56 nm and up to 15 mW at 473 nm. They also reportedly have a nearly circular (<1.1:1 aspect ratio), linearly polarized beam with an M2 factor of less than 1.2. 

Spellman's CCM capacitor charging module is designed to power UV light sources for curing and sterilization, industrial and edical lasers, and ICP-MS applications. The module reportedly provides 3100 J/s at voltages as high as 4 kV, with a power density of 6.6 W/in. and a low voltage +24 V dc at 2.2 A output to power remote auxiliary circuitry. 

The Spectra-Physics Lasers division of Newport Corp. has introduced the Explorer Scientific, a low power intra-cavity tripled and actively Q-switched diode-pumped solidstate laser that delivers pulse energies of up to 120 ÂµJ in the UV (349 nm). The Class 3B UV laser reportedly can be operated at frequencies ranging from single-shot up to a 5-kHz pulse repetition frequency at variable pulse energies. 

Agilent's 5975B GC–MS system features an oil-free vacuum pump, fully integrated CTC PAL sample injectors, enhanced data analysis with a wide range of quantitative tools, and synchronous SIM/scan. The system also includes the company's SemiQuant feature, which is designed to provide a fast, automated estimate of the concentration of noncalibrated compounds in samples. 

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