To Improvements in Spectrophotometry

To improvements in spectrophotometry. . .

Andrew R. Hind

oward Cary launched Ap- Various optical components (or achieved using two matched de- plied Physics Corp. (later external optics) are also necessary tectors. The latter employs an op- H known as Cary Instru- to transfer the light from the tical chopper system to alternate ments) in 1947, releasing the first source, to the sample, and finally the source between sample and commercial recording UV-VIS to the detector. In recording (or reference. It is generally accepted spectrophotometer, the Cary 11, scanning) spectrophotometers, a that chopper-based systems ex- only a year later. His philosophy, double-beam design is generally hibit the higher accuracy of the performance combined with flexi- used to permit measurement with two, due mainly to the require- bility, was a simple one that, fu- continuous change in wave- ment that the detectors be per- eled by a desire to provide cutting- length. In this type of instrument, fectly matched in the first in- edge scientific researchers with both a sample beam and a refer- stance.2 As such, the majority of cutting-edge instrumentation, led ence beam (blank) are monitored high-end spectrophotometers are to the release of the first commer- continuously, so that absorbance based on the double-beam princi- cially available high-end UV-VIS- (or transmittance) can be calcu- ple (Figure 1). NIR (near-infrared) spectropho- lated at each wavelength. In this Thus the aims of high-perfor- tometer, the Cary 14, in 1954. The way, source fluctuations and drift mance UV-VIS-NIR spectropho- opening paragraph of his land- are also minimized. tometer design are fourfold: mark U.S. patent application of From a design perspective, a • A stable, broad-band source the same year1 made his inten- spectrophotometer is a complex provides the high radiant tions abundantly clear: “This in- combination of optics, mechanics, power necessary over the wave- vention relates to photometry, and and electronics engineered to give length region of interest • A monochromator system provides wavelength scanning and The tradition continues with the latest generation the high resolution, high light of high-end Cary UV-VIS spectrophotometers. throughput, and low stray light required for accurate and pre- cise absorbance measurements more particularly to improve- high photometric performance over • A low-noise, high-sensitivity ments in spectrophotometry.” a particular absorbance and wave- detection system offers photo- This statement was to set the trend length range. Photometric perfor- metric accuracy and stability for subsequent generations of Cary mance is determined by a number over the wavelength range spectrophotometers, garnering a of factors, but is largely dependent measured reputation for high performance on the monochromator—the heart • The external optics efficiently and flexibility in the process. In of any spectrophotometer. A mono- transfer light, splitting it into 1966, Cary Instruments merged chromator consists of an entrance sample and reference beams, with Varian Instruments (Palo slit, a dispersion device such as a focusing these beams in the Alto, CA). Thirty-six years later, grafting or prism, and an exit slit. sample compartment, and then the tradition continues with the Ideally, it produces monochromatic refocusing them onto the de- latest generation of high-end Cary light; however, in practice, a band tector(s). UV-VIS spectrophotometers. of light is always produced. A sec- Engineered correctly, the result ond monochromator can be used is a spectrophotometer that dis- Spectrophotometer design: to further enhance performance by plays all of the hallmarks of a high- Basic concepts allowing increased light through- performance instrument (Figure 2). put, reducing aberrations, and pro- Key differentiators include spectral A spectrophotometer is used for viding low stray light levels. resolution, wavelength accuracy measuring absorbance or trans- Double-beam spectrophotome- and precision, photometric accu- mittance as a function of wave- ters are operated using a beam sep- racy and precision, low stray light length. The key components of arated in space or a beam sepa- and noise, high stability, and wide any spectrophotometer include a rated in time. The former involves linear dynamic range. At the fore- light source, dispersion device, splitting the beam between sam- front of current spectrophotometer sampling area, and detector(s). ple and reference with detection design, and winner of the Scientific

32 DECEMBER 2002 Figure 1 Original double-beam design proposed by Howard Cary, U.S. patent no. 3,022,704.

Computing and Instrumentation longer wavelengths, 2002 Reader’s Choice Award for lead sulfide (PbS) and Spectroscopy Systems, the new- indium gallium ar- generation Cary UV–VIS–NIR spec- senide (InGaAs) detec- trophotometers combine the re- tor management per- nowned Cary design philosophy mits measurement with the latest in mechanical, elec- into the NIR, further trical, and optical engineering. increasing flexibility. Featuring the opti- At the leading edge cal stability of an independently mounted, Redesigned from the ground up, research-grade optical the spectrophotometers incorpo- bench, the Optical Iso- rate the most recent advances in lation System shields hardware, firmware, and software. the monochromator Only the Cary optical layout, de- from external influ- signed for high light throughput ences, guarding against Figure 2 New-generation Cary 5000 UV-VIS-NIR and optimum sample compart- mechanical, electronic, 1 spectrophotometer, incorporating the Cary Optical Iso- ment beam geometry, has been re- and atmospheric inter- lation System, PbSmart™ detector management, and tained in the form of the original ferences. To minimize accessory LockDown™ mechanism. optics casting. In combination vibrational noise, the with the Optical Isolation System stiffened optics casting is coupled to a vibrationally isolated mounting sys- and state-of-the-art electronics, the tem, and floats independently of the spectrophotometer cover and sample Cary’s out-of-plane double Littrow compartment. Engineered to meet U.S. military specifications, the internal monochromator design minimizes instrument covers improve grounding and substantially reduce electronic photometric noise and stray light, and electromagnetic interference. In conjunction with a sealed and purge- and supplies very high resolution able optics path for reduced risk of atmospheric contamination, the heart of without sacrificing light through- the spectrophotometer is completely isolated from external influences. In- put. Extending this performance to ternal vibration reduction further complements the Optical Isolation Sys-

AMERICAN LABORATORY 33 IMPROVEMENTS IN SPECTROPHOTOMETRY continued

Cary deep-UV spectrophotometer is suitable for both deep UV metrology and R&D.

High performance Photometric linearity deter- mines how accurately a spectrophotometer measures absorbance with increasing optical density or concentration. If an instrument has poor linearity, calibration curves may deviate from linearity at high absorbance levels, reducing the photometric range of the instrument and accuracy of high optical density measurements. The “addition of filters” technique provides a straightforward means of demonstrating the photometric linearity and range of a spectro- Figure 3 Photometric range and linearity in the UV-VIS. The inset compares the photometer. The addition of two spectral addition of the filters to their combined measurement, a difference of less blue filters (Figure 3) shows the than 8 × 10 –8 %T. wide photometric range and linearity of new-generation Cary tem, providing extremely low noise improved sensitivity of the InGaAs spectrophotometers in the UV-VIS spectrophotometry. detector in the NIR (up to 1800 (>9 abs). The inset in Figure 3 Powered by the latest genera- nm). Now in its second genera- compares the sum of the two fil- tion of fast microprocessors, new tion, the InGaAs detection system ters measured individually with detector control algorithms intel- provides excellent signal-to-noise the spectrum of the two measured ligently optimize the performance performance. The low noise and together, and shows a difference of the PbS NIR detector in real high sensitivity of the InGaAs de- of less than 8 × 10–8 %T between time. This affords a significant im- tection system are further en- the two. provement in noise and linearity hanced by the Cary Optical Isola- Along with linearity, photomet- performance from this mature de- tion and Peltier detector cooling ric accuracy defines the ability of a tector technology, and takes PbS systems, and result in improved spectrophotometer to measure an detector performance to a new detection limits and increased absorbance that can be directly re- level. Combined with the latest in scan rates, giving cleaner spectra lated to a standard solute of known Peltier thermoelectric cooling, this with better resolution in less time. concentration. Photometric accu- approach provides very high per- The spectrophotometers can racy and linearity are vital wher- formance across applications as also provide photometric perfor- ever quantitative measurements diverse as quantifying the out-of- mance in the deep UV spectral re- are concerned, since the ability to band blocking characteristics of gion down to 140 nm. Developed measure the correct photometric bandpass filters and measuring specifically for use by the semicon- response for a given concentration the high transmission of next- ductor industry, the instrument is is essential for any analytical mea- generation fiber optic materials. well suited to measurement at the surement. Of similar importance is laser wavelengths currently used the range over which the spectro- Flexibility and responsiveness, or in integrated circuit (IC) manufac- photometer remains linear. This is functionality turing processes (248.4, 193.4, and known as the linear dynamic range 157.6 nm). The research-grade, and is defined as the concentration In keeping with the company’s double-beam, deep-UV spectro- range over which absorbance and tradition of providing application- photometer breaks the vacuum concentration remain directly pro- specific solutions, the world’s first UV barrier for the characterization portional to each other. A wide lin- InGaAs double-beam UV-VIS-NIR of silicon wafers, photoresists, and ear dynamic range permits the spectrophotometer was developed microlithography optical compo- measurement of a variety of sam- with demanding photonics and nents such as stepper optics, allow- ple concentrations (optical densi- telecommunications applications ing transmission and reflectance ties) and reduces sample prepara- in mind. This pioneering work measurements from 140 to 260 tion (dilution) requirements. The aimed to take advantage of the nm. Combining sampling flexibil- quantitative analysis of aqueous dramatically reduced noise and ity with high performance, the potassium permanganate (Figure 4)

34 DECEMBER 2002 a scientists and engineers have en- sured that Cary spectrophotometers have become the benchmark for performance, flexibility, and reliability in this field. The new generation of Varian Cary spectrophotometers continues in the same vein, providing the performance necessary for those push- ing the edge of technology with their measurement requirements. The spectrophotometer series enables a wide variety of physical and optical measurements. A ver- satile sample compartment and accessory range permit a wide range of sample types, and very high optical performance enables measurement at the highest limits of performance. From solids and liquids to gases; from powders to b films to wafers; from the smallest, high-technology optical filter at the center of a laser guidance system to large sheets of glass for sky- scraper windows, the spectrophotometers provide the versatility and flexibility demanded by diverse applications requiring the ability to analyze a vast array of sample types using a wide variety of techniques. In the words of Howard Cary, “For investigators who on occa- sion must push a spectrophotome- Figure 4 Quantitative analysis of aqueous potassium permanganate. ter to the very limits of its performance capability to obtain the information they need, and yet further demonstrates the photo- (AR) coatings, while the inherent have an instrument that is adapt- metric accuracy and range of the linearity and dynamic range of the able to many different applica- spectrophotometers. Measurement instruments ensures analytical ac- tions,”3 Cary spectrophotometers at 555 nm permits determination curacy and eliminates the need for are the natural choice. from 0.1 to 500 ppm without dilu- unnecessary and time-consuming tion, and the plot of absorbance sample and standard dilutions. versus concentration highlights References the wide linear dynamic range (r 2 = The next generation 1. Cary HH. U.S. pat no. 3,022,704, 0.999) of the instruments. Feb 27, 1962 (application no. Permitting measurement in ex- For more than half a century, 411,650 filed Feb 23, 1954). cess of 8 abs, the spectrophotometers the Cary name has been synony- 2. Kennedy JH. Analytical chemistry: principles, 2nd ed. Philadelphia, are capable of measuring the most mous with high-performance PA: Saunders College Publishing, challenging samples. Whether col- spectrophotometry in the ultravi- 1990. lecting kinetics data in situ without olet, visible, and near-infrared re- 3. Cary Instruments. Cary 17. A New dilution, or measuring turbid, gions of the electromagnetic spec- Spectrophotometer for Every Ap- highly scattering life science sam- trum. The fact that an early Cary plication. Monrovia, CA: Cary, 1970. ples without the need for an inte- makes its home at the Smith- grating sphere, the spectrophotome- sonian Institute in Washington, ters deliver the performance and DC, bears testament to the role Dr. Hind is UV/VIS/NIR Sales Support flexibility required. Their wide pho- played by Howard Cary in the de- Manager Europe, Varian Ltd., 28 tometric range accommodates the velopment of modern analytical Manor Rd., Walton-on-Thames, Surrey highest optical density filters and instrumentation. Contributions KT 12 2QF, U.K.; e-mail: andrew. the lowest reflectance antireflection from subsequent generations of [email protected].

AMERICAN LABORATORY 35