Compact Spectroscopy Instrumentation: On-Board Smarts or External Computer?

Compact, array-based spectrometers continue to develop in response to advances in detector, microprocessor, storage, and display technology. At the present time, this class of instruments has essentially divided into two different product paths. So-called “smart” spectrometers are self-contained instruments with an on-board microprocessor, as well as data storage, analysis, and display capabilities. The latest of these instruments even offers battery operation for true portability. The other class of instruments, which comprises essentially a spectrometer engine, uses a host computer to provide all storage, analysis, and display functions.

In applications that involve both laboratory and field operation, it can be highly advantageous to have both types of instruments, provided they are based on a common optical platform or spectrometer engine. This commonality ensures robust transfer of the calibration model developed in the laboratory into the on-line or OEM measurements. This article briefly reviews the operation of these types of spectrometers and examines a semiconductor processing application that relies on a robust model transfer.

Array-based spectrometers

Figure 1 - An array-based spectrometer uses a compact optical arrangement with a stationary diffraction grating to enable simultaneous acquisition of data at multiple wavelengths, with no moving parts.

The basic components of a typical array-based spectrometer include an entrance aperture, collimating optics, fixed-position diffraction grating, focusing optics, and linear photodetector array (Figure 1). This configuration offers several advantages over the traditional scanning geometry, including compact size, immunity to vibration, and the ability to acquire all wavelength components of a spectrum simultaneously. The latter minimizes data acquisition time and makes the instrument relatively immune to transient noise effects such as a bubble passing through a flow cell sampling unit, since all data points are affected equally.

In analytical applications, the spectral data are used for sample identification or, more commonly, to derive quantitative analytical information about the composition of samples. This process can involve measuring the relative intensity of a few select spectral peaks for a more complex fit, such as PLS (partial least squares), on multiple data points. In the case of near infrared (NIR), this may require a complete spectral fit using chemometrics.

Whatever model/algorithm is used to extract the target information, in virtually every case the fit is specific to the spectrometer hardware. This is because the raw data depend on the spectrometer engine—the particular optics, grating, and detector array used, and the exact arrangement of these components. In a given application, it is thus critically important that all the instruments are based on a common spectrometer engine, which allows the same software-fitting model to be used confidently with all the instruments.

Self-contained versus computer-interfaced

Because repeatable data fitting requires consistent instrumentation, it would seem simple to conclude that identical instruments should be used at all stages of calibration and implementation for a given application. However, for many applications, this does not make sense from a cost standpoint. This is because the applications developer typically needs an instrument with greater flexibility and functionality, and hence higher cost, than the production line or OEM user.

To meet these diverse needs, today’s instrument providers offer both computer-interfaced spectrometers and smart, self-contained instruments equipped with functionality that goes well beyond simply dispersion and detection. These capabilities fall into four categories: data storage, data display, data analysis, and data transfer.

Data storage can take several forms. Traditionally, disk drives were the method of choice for data storage, but today’s technology offers solutions that draw much less power and occupy less space. Data storage now is typically offered in the form of built-in, solid-state memory, supplemented by removable flash memory cards or sticks, not unlike the arrangement in a digital camera of the consumer market.

For data display, liquid crystal display (LCD) screens are well suited for a compact and/or portable instrument. These are smaller, lighter, draw less power, and are more rugged than their cathode ray tube (CRT) predecessors. Furthermore, in addition to just displaying data, the use of smart touchscreen technology enables the user to interact with the system software through a graphical user interface (GUI). This provides simplified access to a wide range of data analysis and instrument control functions in a compact, economical package.

The incorporation of data analysis capabilities enables an operator to perform different types of measurements, including absolute measurements, optical density, background subtraction, and so forth. It also permits other mathematical manipulations of the data, such as addition or subtraction of spectra or individual data points or values. The software typically provides a choice of data formats, including SPC (GRAMS Standard) and CSV.

Data transfer can take several forms. Serial (RS232) interfaces are still common, but most high-performance instruments now offer a choice of a GPIB parallel interface as well as an Ethernet input/output and a USB link. These interfaces not only allow data transfer but also enable remote operation of the spectrometer from a host computer.

Figure 2 - Instruments based on the same spectrometer engine can offer quite different levels of functionality, as in the OSM-100 and OSM-400.

The diverse needs of the applications marketplace mean that instrument manufacturers now often offer two or more versions of the same spectrometer (Figure 2). The most basic version has virtually none of these higher-level functions and is intended for operation with a dedicated PC in the case of an end user, or by the system computer in the case of an OEM application. An example of this type of instrument is the OSM-100 (Newport Corp., Irvine, CA). Conversely, self-contained instruments can be based on the same spectrometer engine, but offer some or all of the above functions, and may even include battery operation for true portability. An example of this latter type is the OSM-400 (Newport Corp.).

Cobalt cap technology for high-density ICs

Figure 3 - In state-of-the-art integrated circuits, high current densities in the miniaturized copper interconnects can result in migration of the copper atoms. This can lead to voids and failed interconnects, lowering device reliability. The problem can be largely reduced by the use of a CoWP barrier layer. (Figure courtesy of Blue29.)

The innovative cobalt cap process, pioneered by Blue29 (Sunnyvale, CA) for use in the semiconductor industry, provides an excellent example of a process in which calibration must be maintained from laboratory to factory floor. As chip manufacturers strive to keep pace with Moore’s Law, the resultant miniaturization presents problems in both fabrication and device reliability. One of the main reliability issues in high-density circuits is failure of the copper interconnects that link the multiple circuit layers. Specifically, the thin cross-section of these interconnects translates into high current density. This causes a process called electromigration (EM) in which copper atoms move in the opposite direction to the flow of electrons. EM thereby causes further thinning of the interconnects, and higher current density, which in turn drives even faster EM. Eventually, this leads to the formation of voids in the copper and failure of the interconnect (see Figure 3). Copper atom migration mainly occurs via the copper dielectric interface. This results in the additional problem of modifying the k value (dielectric constant) of the dielectric layer.

To address this situation, the company has pioneered the use of a cobalt alloy as an alternative barrier layer sometimes referred to as a cobalt cap. Blue29 believes that its cobalt layer has two advantages. First, the Cu-Co metal–metal bond is theoretically stronger than a Cu metal–dielectric bond; thus the interface between the copper and the barrier layer prevents copper migration. In addition, the cobalt itself is very resistant to migration. Blue29 has also developed a proprietary wet-chemistry process that directly deposits this CoWP layer on top of exposed copper, without resorting to electrochemistry or additional masking.

Spectroscopic process monitoring of cobalt

In the Blue29 process, wafers are immersed in a proprietary mixture of chemicals in a wet process that results in catalytic deposition of cobalt onto the copper interconnects. Automation, high throughput, and minimum down time are critical to the success of new processes in the semiconductor industry. The cobalt cap system provides all three, in part due to automated, real-time monitoring of the multicomponent process solution. The company supplies the hardware (chemical and wafer handling) specific to each user’s needs and the hardware/software to monitor the chemistry in a seamless manner. This allows the wafer manufacturer to simply expose the wafers to the chemistry for a finite time, knowing it will produce a specific target thickness of CoWP.

Real-time monitoring of the process solution is critical because key components slowly become depleted with repeat use. The solution is automatically maintained by the periodic recharging of these components before eventual replacement. This ensures constant cobalt deposition characteristics. The company monitors the various components using several different techniques that are all interfaced to the main system controller for complete automation. The cobalt ion concentration is monitored by visible absorption spectroscopy, using a deuterium-tungsten light source and OSM spectrometers.

In Blue29’s laboratory, an OSM-400 is utilized along with a cuvette holder accessory to study captured samples from test platforms, and to optimize new systems before customer delivery. The OSM is equipped with a 2048-element charge-coupled device (CCD) array and a grating setup to disperse the 250–850 nm spectral window onto this array. In the final product, an OSM-100 with the same grating and CCD is integrated into the rack-mounted system controller. The spectrometer is integrated with a rugged housing and a 1 × 4-fiber switch. This allows the system controller to connect the spectrometer with four different fiber bundles. Two of these are connected to Z-shaped flow cells, in two separate process modules, to maximize system throughput. One of the bundles is connected to a flow cell in a reference sample of deionized water, and the fourth bundle is used to monitor the condition of the deuterium lamp for preventative maintenance purposes.

The use of both an OSM-100 and OSM-400 lowers the company’s development costs and also minimizes the cost of the final systems shipped to its customers. The OSM-400 is used by the process development engineers as a portable, shared instrument to measure captured samples. Its battery-enabled portability also permits its use in the production simulation environment to monitor new systems during test and assembly, avoiding the cost of multiple spectrometers, which would not be in constant use.

Conversely, the display, storage, and analysis functions would be completely superfluous in the production systems shipped to Blue29 customers. Here, one of the most important functions is an Ethernet input/output, since that enables simple integration with the main system computer. The OSM-100 provides this critical interface in a cost-effective package. Clearly it is a major advantage that both instruments are based on the same spectrometer engine, allowing robust transfer of calibration models.

Conclusion

The ready availability of powerful microprocessors and high-speed interfaces has resulted in two categories of compact optical spectrometers. Self-contained instruments now incorporate on-board data analysis, storage, and display functions, whereas computer-interfaced instruments rely on a host computer for all of these functions. The former are ideal for laboratory and at-line applications, and the latter are generally better suited to on-line and OEM applications. However, some applications can benefit from using both types of instruments, provided they are built on identical spectrometer engines.

The authors are with Newport Corp., 1791 Deere Ave., Irvine, CA 92606, U.S.A.; tel.: 949-437-9874; fax: 949-253-1800; e-mail: [email protected].

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