Micron-scale systems for materials-process analysis

Material-process driven industries have benefited for decades from spectroscopic technology. Whether for material identification or quality verification, samples are collected in all phases of production, from incoming materials to outgoing process byproducts. They are logged and sent to the lab for analysis by chemists, with results dispatched back to the manufacturing floor for further handling.

Newly available microelectromechanical systems (MEMS) technology helps streamline these quality control processes. Large stationary instruments can be replaced by lightweight, hand-held devices that enable non-technical users to perform analytic tasks at the point of production. The hand-held Phazir™ (see Figure 1) is such a tool.¹

As the name implies, MEMS are micron-scale machines. They are manufactured as chips (see Figure 2) using well-known semiconductor fabrication techniques, but unlike their siblings—the ubiquitous electronic chips—areas or `elements’ on the chip can be moved by applying voltage. In our configuration, groups of micro-elements can be actuated together to create an optically diffracting `pixel.’ With multiple pixels replicated along a MEMS chip, a programmable spectral mask can be created on the fly. By dispersing the spectrum across the MEMS chip, discrete sections can be `blocked’ by diffracting light while other parts can be reflected with no loss of intensity (see Figure 3).

Figure 1. Phazir handheld analyzer. Cross section (left) and actual device (right).

Figure 2. Packaged MEMS chip (left) and microscopic view of the MEMS chip (right).

Figure 3. Schematic representation of diffractive MEMS: left, unactuated; right, actuated.

DTS is a new technique in which the spectrum collected from a sample is dispersed across the diffractive MEMS chip (see Figure 4). Pixels can be set to control diffraction and reflection of light in their corresponding spectral regions. The reflected light is collected and measured by a single photodetector.

Figure 4. Digital transform spectrometer schematic.

Figure 5. Digital transform measurement for polystyrene.

An instrument with this configuration uses the MEMS chip to selectively measure the total power in desired spectral regions with a single detector rather than an expensive, power-hungry detector array, resulting in a device that is highly cost effective.

Probably the easiest approach for measuring the full spectrum would be to block it entirely and time-sequence through each pixel, replicating a scanning monochromator. While this approach is valid and can be useful in some cases, the signal measured is relatively low at 1/(the number of pixels), and for some applications results in poor signal-to-noise performance.

Instead, a digital transform is used. The programmability of the MEMS chip implements an encoding scheme, maintaining 50% signal throughput. The matrix (see Figure 5) represents a time-sequenced set of spectral masks created using the MEMS chip, in which individual ‘cells’ represent pixels. Green cells represent a reflected spectral region or pixel. White areas represent diffracted (and hence un-measured) spectral regions. Measuring total power with the single detector for each spectral mask (row) in the matrix yields a transform. When the transform is fully collected, it is multiplied by the matrix to reproduce the measured spectrum.

The Phazir is the first in a new generation of fully integrated near-infrared hand-held analyzers that incorporate all the components essential to deliver field applications. These include a MEMS-based DTS NIR spectrometer and a tungsten light source for illuminating the sample in the near-infared. A reflectance probe for solid and powder samples, and a transflectance probe for liquid measurement, are also included. The color display offers a friendly, graphic user interface and, in addition to built-in field calibration standards, an on-baord computer delivers processed, meaningful information as to material identification or sample concentration percentages. The unit operates on rechargeable batteries and has a USB port for software updates and log downloads.

As the first device of its kind, the Phazir offers a platform that can be used to develop and implement applications in a variety of industries. It can be used, for example, in raw materials inspection in the pharmaceutical, food and beverage, and petrochemical industries. Sorting tasks in recycling materials prior to chemical processing represent other applications, together with uses in film, layer, and paint thickness analysis. In agriculture, the Phazir can provide analysis of moisture, protein, oil or sugar content to indicate maturity and ripeness. In fraud investigation, it can be employed to identify counterfeit drugs, fabrics, and chemicals. Finally, in forensics it is capable of white powder analysis for narcotics identification and potentially represents a long-sought tool for on-the-spot crime scene materials identification.