Different types of microdevices, including accelerometers, and a verity of mechanical components and sensors are fabricated using silicon fused with glass. Since these devices are vulnerable to contamination by humidity, dirt and dust, they are hermetically sealed with an inert gas like nitrogen. Although the performance of these devices is not affected in case of failure in this seal, their service life is considerably reduced.
Micromachined accelerometers consist of a suspended object which produces an electrical signal when displaced by acceleration. The device mounts on a clear borosilicate glass substrate with a hermetically sealed cavity by a silicon cap. Since the enclosure environment needs to be chemically inert, the fabrication of the device takes place in a dry nitrogen environment.
Raman microscopy is uniquely suited for the detection of leaks in these hermetically sealed microstuctures. Raman scattered light were collected and examined by focusing the incident laser beam through the glass substrate in order to determine contaminants in the cavity with no breaking of the seal.
Experimental Procedure and Results
This experiment used Kaiser Optical Systems’ HoloLab™ 5000 Raman microprobe (current version is RamanRxn1™ Microprobe) to collect Raman spectra using 532nm excitation wavelength, 50mW laser power and 6min acquisition time. The laser was focused through the borosilicate glass window using the 50× objective on the microscope to provide a confocal spot size of 16µm on the sample (Figure 1). Inert gas argon was used to purge the Raman microprobe to protect the spectra from the interference of background oxygen outside the cavity of the microdevice.
Figure 1. Schematic drawing of the experimental setup.
Raman spectra of the gaseous components of two different micromachined accelerometer chip cavities are presented in Figure 2. The spectrum for Chip #1 contains no oxygen peak, revealing that there is no leakage in the seal. The spectrum for Chip #2 shows a small oxygen peak at 1550 cm–1, revealing that the cavity is contaminated due to leakage in the seal.
Figure 2. Raman spectra of two microdevice cavities. Chip #1 is uncontaminated; chip #2 is contaminated with oxygen.
Glass Pharmaceutical Vials
Raman spectroscopy can be also used for headspace gas analysis in pharmaceutical vials. Since some drugs are deteriorated by carbon dioxide, they need to be sealed under a nitrogen environment. It is necessary to determine the composition of the headspace gases for quality control purposes. Although carbon dioxide is a good Raman scatterer, it was not resolved in this case due to its very low level. Nevertheless, the degree of air leakage into the vial can be determined by comparing the intensity of the oxygen band (1550 cm–1) to the nitrogen band (2330cm–1) (Figure 3). This is achievable due to the ability of Kaiser Optical Systems’ HoloPlex™ transmission grating to collect the entire spectrum in a single “shot.”
Figure 3. Raman spectrum of air, with the oxygen and nitrogen bands marked.
Distillation monitoring involves periodic headspace gas analysis of distillation columns. Raman spectra of the column headspace in the distillation of acetone from water are depicted in Figure 4. The acetone peak appears at ~2900 cm–1, whereas the peak at ~3600 cm–1 corresponds to water. On the onset of distillation, the acetone concentration in the headspace increases continuously until it gets deposited in the collection flask.
Figure 4. Raman spectra of the column headspace, as acetone is distilled from water.
As the fraction of acetone in the mixture decreases, its headspace concentration also decreases. The acetone has been distilled off completely after roughly 15min, which is revealed by increasing characteristic Raman band of water in the headspace. Here, by focusing the incident laser of 90mW power and 532nm wavelength through the glass distillation column, the Raman scattered light was collected at 180° and analyzed. Industrial scale monitoring of distillation is possible with Raman spectroscopy using an immersion optic. Remote operation of the probe head and analyzer at a distance from the adverse environment of the distillation column is possible.
Raman spectroscopy can perform headspace gas analysis in transparent containers without opening the container or making contact with the sample. The areas of the spectral bands represent the concentration of corresponding chemical species, thus enabling accurate quantitative analyses.
About Kaiser Optical Systems
Kaiser Optical Systems, Inc. is a world leader in spectrographic instrumentation and applied holographic technology. Principal products include Raman sensors and instrumentation, advanced holographic components for spectroscopy, and astronomy and ultra-fast sciences. Principal offices and the manufacturing facility are located in Ann Arbor, Michigan.
Their products and services are deployed throughout the world in such diverse applications as pharmaceutical and chemical manufacturing, nanotechnology, telecommunications, education, forensic science, deep-sea exploration, and astronomy. From particles smaller than a human hair to objects as large as planets, their products are providing their customers unique insights into both today’s as well as “age-old” questions.
Kaiser was founded in 1979 to meet the need for diffractive or holographic optics for the avionics market. Kaiser entered the spectroscopy market in 1990 with the introduction of the holographic notch filter. In 1993 Kaiser released their first Raman analyzer, the HoloProbe. In 2013, the company became part of the Endress+Hauser Group.
To better serve the European community, Kaiser opened a new subsidiary in Europe in 1998. Kaiser Optical Systems SARL is located in Lyon, France. Kaiser SARL supervises their distributor network within Europe.
This information has been sourced, reviewed and adapted from materials provided by Kaiser Optical Systems.
For more information on this source, please visit Kaiser Optical Systems.