Using Raman Spectroscopy for Detection of Leaks in Sealed Micro Structures

Many microdevices (such as accelerometers, sensors and mechanical parts) are composed of silicon fused with glass. Contamination is a large risk to these devices, as they are hypersensitive to a variety of environmental components, such as dust, dirt and humidity. Protection from this contamination is therefore provided by hermetically sealing within an area entirely filled with an inert gas (e.g., N2). If this seal is compromised, the instantaneous performance of the device is not affected, but the overall lifetime can be greatly reduced.

Within automobiles, micromachined accelerometers are used for airbag activation. When the accelerometers accelerate, a suspended component is moved. This movement produces an electrical signal which is proportional the amount of acceleration. The device sits upon a transparent borosilicate glass substrate, and is hermetically secured by a silicon cap. The accelerometer is also made within a dry nitrogen atmosphere to ensure that the atmosphere within the enclosure is chemically inert.

The best method for detection of leaks within hermetically sealed microdevices is through the use of Raman microscopy. By focusing the incident laser beam through the glass substrate , the Raman scattered protons can be measured, detecting contaminants contained within the cavity whilst maintaining the seal.

Schematic drawing of the experimental setup.

Figure 1. Schematic drawing of the experimental setup.

Experimental

The HoloLab™* 5000 Raman microprobe from Kaiser Optical Systems, Inc. was utilized to obtain the Raman spectra, with an incident laser of 532 nm and a power of 50 mW. It took 6 minutes to procure the results. Using a 50× objective on a microscope, the laser was focused through the borosilicate glass window, resulting in a confocal spot diameter upon the sample of 16 µm. To prevent background oxygen contamination of the spectra, the Raman microprobe was also purged with argon.

Raman spectra of two microdevice cavities

Figure 2. Raman spectra of two microdevice cavities. Chip #1 is uncontaminated; chip #2 is contaminated with oxygen.

Glass Pharmaceutical Vials

Another application of Raman spectroscopy is the analysis of headspace gases in pharmaceutical vials. Some drugs are broken down by CO2 and therefore require sealing within a N2 environment. The content of the headspace gases is initially determined to ensure quality-control. Usually, CO2 is used within Raman spectroscopy as it ensures a good Raman scatter. However, testing revealed that the concentration was minimal. Instead, the concentration of the O2 band can be compared to the N2 band, revealing the amount of air leakage. This is enabled due to the HoloPlex™ transmission grating from Kaiser Optical Systems, Inc., which grants the collection of the whole spectrum from a single attempt. Since there is normally 21% O2 in the atmosphere, this value is used to indicate contamination within the headspace.

Raman spectrum of air, with the O2 and N2 bands marked.

Figure 3. Raman spectrum of air, with the O2 and N2 bands marked.

Distillation Monitoring

Distillations can be supervised by frequently measuring gas components within the headspace of the column.

In a study, acetone was distilled from water. As the distillation began, there was an increase in volatile acetone which was boiled out of the water/acetone combination, leading to an increase in concentration within the headspace. This acetone then accumulated within the collection flask, resulting in a decrease in the headspace. After around 15 minutes, this acetone was totally removed from the water, and resulted in boiling of the water. This therefore leads to an increase in a corresponding Raman band. This study used an incident laser (90 mW, 532 nm), which was aimed at the glass distillation column. The Raman scatter was then measured scatter at 180o. This method can also be utilized on an industrial scale by applying an immersion optic. Additionally, the head of the probe and the analyzer can be remotely placed to protect them from the harsh environments of the distillation column.

Raman spectra of the column headspace as acetone is distilled from water.

Figure 4. Raman spectra of the column headspace as acetone is distilled from water.

Conclusion

Therefore, Raman spectroscopy can be applied to analyze headspace gases in clear capsules as it does not require contact with the sample, and so the capsule does not need to open. The concentrations of chemical species can also be quantified by measuring the areas of the spectral bands.

References:

  • Weber, W.H.; Zanini-Fisher, M.; Pelletier M.J., “Using Raman Microscopy to Detect Leaks in Micromechanical Silicon Structures,” Applied Spectroscopy, Vol. 51, No. 1, 1997, 123.
  • Gilbert, A.S.; Hobbs, K.W.; Reeves, A.H.; Jobson, P.P., “Automated headspace analysis for quality assurance of pharmaceutical vials by laser Raman spectroscopy,” SPIE, Vol. 2248, 1994.

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.

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